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Title:
NIOB AND TANTAL COMPLEXES AS CATALYSTS FOR OLEFIN POLYMERIZATION
Document Type and Number:
WIPO Patent Application WO/2014/135824
Kind Code:
A1
Abstract:
This invention relates to complexes formed by the combination of a metal (M) and various ligands (L). The metals (M) include niobium and tantalum. The complexes serve to catalyze the polymerization of hydrocarbons. The process comprises mixing the complex or a combination of the complexes with a co-catalyst and one or more monomers.

Inventors:
REDSHAW CARL (GB)
Application Number:
PCT/GB2013/051884
Publication Date:
September 12, 2014
Filing Date:
July 15, 2013
Export Citation:
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Assignee:
UNIV HULL (GB)
International Classes:
C07F9/00; B01J31/36; C08F10/02
Domestic Patent References:
WO2002090365A12002-11-14
Foreign References:
EP0874005A11998-10-28
Other References:
NONSEE NIMITSIRIWAT ET AL: "The Reversible Amination of Tin(II)-Ligated Imines: Latent Initiators for the Polymerization of rac -Lactide", INORGANIC CHEMISTRY, vol. 47, no. 12, 15 May 2008 (2008-05-15), pages 5417 - 5424, XP055088437, ISSN: 0020-1669, DOI: 10.1021/ic701671s
CONG WANG ET AL: "Synthesis and Characterization of Titanium(IV) Complexes Bearing Monoanionic [O - NX] (X = O, S, Se) Tridentate Ligands and Their Behaviors in Ethylene Homo- and Copolymerizaton with 1-Hexene", ORGANOMETALLICS, vol. 25, no. 13, 19 May 2006 (2006-05-19), pages 3259 - 3266, XP055088435, ISSN: 0276-7333, DOI: 10.1021/om060062j
CARL REDSHAW ET AL: "Highly Active, Thermally Stable, Ethylene-Polymerisation Pre-Catalysts Based on Niobium/TantalumImine Systems", CHEMISTRY - A EUROPEAN JOURNAL, vol. 19, no. 27, 16 May 2013 (2013-05-16), pages 8884 - 8899, XP055088440, ISSN: 0947-6539, DOI: 10.1002/chem.201300453
TANMOY KUMAR SAHA ET AL: "Imino phenoxide complexes of niobium and tantalum as catalysts for the polymerization of lactides, [epsilon]-caprolactone and ethylene", DALTON TRANSACTIONS, vol. 42, no. 28, 9 May 2013 (2013-05-09), pages 10304, XP055088897, ISSN: 1477-9226, DOI: 10.1039/c3dt50752a
DATABASE CA [online] CHEMICAL ABSTRACTS SERVICE, COLUMBUS, OHIO, US; NAKAYAMA, YUUSHOU ET AL: "Synthesis of bis(imino)pyridine complexes of group 5 metals and their catalysis for polymerization of ethylene and norbornene", XP002716752, retrieved from STN Database accession no. 2008:263688
TSURUGI HAYATO ET AL: "Carbon Radical Generation by d0 Tantalum Complexes with .alpha.-Diimine Ligands through Ligand-Centered Redox Processes", JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, ACS PUBLICATIONS, US, vol. 133, no. 46, 23 November 2011 (2011-11-23), pages 18673 - 18683, XP008165886, ISSN: 0002-7863, [retrieved on 20111007], DOI: 10.1021/JA204665S
Attorney, Agent or Firm:
MURGITROYD & COMPANY (165-169 Scotland Street, Glasgow G5 8PL, GB)
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Claims:
Claims

1. A complex formed by the combination of a metal (M) and a ligand (L),

wherein M = Niobium or Tantalum and L has the formula:

and wherein either

(i) R1 = H, halogen and R2 = substituted or unsubstituted benzyl, substituted or unsubstituted phenyl, or (Ci-C6)alkyl, or CF3;

or

(ii) R1 = H, (Ci-C6)alkyl and R2 = (Ci-C6)alkyl or CF3.

2. A complex according to Claim 1, wherein either

(i) R1 = CI and R2 = unsubstituted phenyl, Me or CF3, preferably unsubstituted phenyl or Me

or

(ii) R1 = cBu and R2 = Me. 3. A complex formed by the combination of a metal (M) and a ligand (L),

wherein M = Tantalum and L has the formula:

and wherein

R1 = (Ci-C6)alkyl, preferably fBu;

R2 = substituted or unsubstituted phenyl.

4. A complex having the formula:

where M = Niobium.

5. A complex having the formula:

where M = Niobium or Tantalum. 6. A process for preparing any of the complexes of Claims 1-5 comprising reacting a metal (M) chloride with a ligand (L).

7. A process for polymerizing hydrocarbons comprising mixing a complex or a combination of complexes according to Claims 1-5 with a co-catalyst and one or more monomers, preferably olefins, more preferably a-olefins, even more preferably ethylene.

8. A process according to claim 7, wherein the co-catalyst is an alkylating agent. 9. A process according to Claim 8 wherein the alkylating agent is an alkyl aluminium compound, preferably tri-isobutylaluminium (TIBA] or a chloro-alkyl aluminium compound.

10. A process according to Claim 9 wherein the chloro-alkyl aluminium compound is EtAlCfe (EADC), MeAlCl2 (MADC), Me2AlCl (DMAC), Et2AlCl, Me3Al2Cl3 (MASC) or Et3Al2Cl3, preferably MeAlCl2 (MADC), Me2AlCl (DMAC) or Me3Al2Cl3 (MASC), more preferably MeAlCl2 (MADC).

11. A process for polymerizing hydrocarbons comprising mixing a complex having the formula:

where M = Niobium or Tantalum

with a co-catalyst and one or more monomers, preferably olefins, more preferably a-olefins, even more preferably ethylene,

wherein the co-catalyst is tri-isobutylaluminium (TIBA] or a chloro-alkyl aluminium compound.

12. A process according to claim 11, wherein the chloro-alkyl aluminium compound is EtAlCl2 (EADC), MeAlCl2 (MADC), Et2AlCl, Me2AlCl (DMAC), Me3Al2Cl3 (MASC) or Et3Al2Cl3, preferably MeAlCl2 (MADC), Me2AlCl (DMAC) or Me3Al2Cl3 (MASC), more preferably MeAlCl2 (MADC).

13. A process according to Claims 7-12 additionally comprising a re-activator, preferably an oxidising agent, more preferably ethyl tricholoroacetate (ETA). 14. A process according to Claims 7-13, wherein the reaction or the mixing is carried out in a solvent, preferably in toluene or an alkane, such as hexane or heptane.

Description:
NIOB AND TANTAL COMPLEXES AS CATALYSTS FOR OLEFIN

POLYMERIZATION

[002] FIELD OF THE INVENTION

[003] The invention relates to metal complexes, compositions thereof, and their uses.

[004] BACKGROUND OF THE INVENTION

[005] Non-metallocene-based pre-catalysts for the oligomerization/polymerization of a-olefins continue to attract considerable academic and industrial interest. [1] Systems utilising a variety of transition metals, both early and late, have exhibited very high activities. Researchers have shown that both the catalytic performance and the resulting product properties can be tuned by manipulation of the ancillary ligands present. Despite these efforts, there still remain a number of 'untapped' catalytic areas in the transition metal series. [006] Recent research has been carried out on a number of group V complexes to study their catalytic behaviours. [2 ]

[007] In the case of group V metals, the lightest congener vanadium has exhibited significant activity, with the majority of these vanadium-based systems having appeared in the literature in the last five years or so. [1, 2] In the case of imine- based systems, stabilization can be brought about by the coordination of the imino nitrogen at vanadium, and this can be highly beneficial to the polymerization catalysis. [3] [008] Tantalum-based systems also have shown promise, particularly in the trimerization of ethylene to 1-hexene. [4]

[009] In the case of niobium, however, results have been disappointing. A recent review by Galletti and Pampaloni, and work by Patil, give overviews of niobium- based pre-catalysts for ethylene polymerization. [5] Highlights include the deployment of hydridotris(pyrazolyl)borate ligation, which imparts steric protection, thereby restricting catalyst decomposition, leading to activities in the region of 130 g/mmol.h.bar, [6a] bis(imino)pyridine niobium(III) pre-catalysts with activities of up to 70 g/mmol.h.bar, [6b] and more recently N,N-dialkylcarbamato pre-catalysts of the form [Nb(0 2 CNR 2 ) n ] (n = 3-5, R = Me, Et) with activities of 110 g/mmol.h.bar [6c] . Indeed, the highest activity reported to-date for a niobium based pre-catalyst is 151 g/mmol.hr.bar, which was achieved using a system based on a simple alkoxide ligand (see Figure 1, left hand side) [6d] . We have previously found that by employing diphenolate ligands, activities of the order of 90 g/mmol.h.bar can be attained [7a] (Figure 1, middle left), whereas the use of calix[n]arenes was less successful (Figure 1, middle right). [7b]

[0010] Furthermore, it is noteworthy that kinetic studies and quantum calculations have revealed that olefin association/dissociation is far faster for niobium than for tantalum in complexes of the type [(tBu3SiO)3M(ethylene)] (M = Nb, Ta). [8]

[0011] Given all of the above, the question becomes 'Can any combination ancillary ligand and co-catalyst impart high activity at niobium or tantalum?'

[0012] SUMMARY OF THE INVENTION [0013] Here we describe new families of polymerization catalysts/pre-catalysts based on niobium and tantalum centres possessing ligand sets containing the imine functionality (see Figures 2 and 3). It has been surprisingly found that the niobium complexes of various bi- and tri-dentate chelate ligands, in the presence of, for example, the chlorinated co-catalysts such as dimethylaluminium chloride (DMAC), methylaluminium sesquichloride (MASC) or methylaluminium dichloride (MADC), and the reactivator ethyl trichloroacetate (ETA), exhibit catalytic activities that are two orders of magnitude greater than any previously observed for reported niobium-based systems. We have extended this work to related tantalum precursors, and have surprisingly found that these, too, afford highly active systems. The effect of peripheral changes to a Schiff-base backbone has also been investigated and has been found to have a major effect on the observed ethylene polymerization activity. Representative molecular structures are presented herein, namely [V-MC (M = Nb (la), Ta (lc)), (lb), L 2 NbOCl 2 (NCMe) (2b), L 6 NbCl 4 (6a), L 6 TaCl 4 (6b), L 7 NbCl 4 (7a), L 8 NbCl 4 (8a), (L 8 H) + Ta Cl 5 (8c), together with the structure of the unusual coupled product 5.

[0014] In one aspect, this invention relates to complexes formed by the combination of a metal (M) and a ligand (L), wherein M = Niobium or Tantalum and L has the following formula (A)

and wherein either (i) R 1 = H, halogen and R 2 = substituted or unsubstituted benzyl, substituted or unsubstituted phenyl, or (Ci-C6)alkyl, or CF3; or (ii) H, R 1 = (Ci- C6)alkyl and R 2 = (Ci-C6)alkyl or CF3. Preferably M = Tantalum. The ligands of this invention are sometimes referred to as chelate ligands.

[0015] It has been noted that such complexes can be formed with a variety of structures depending upon the conditions under which the complex is prepared. Complexes comprising the metal (M) and ligand (L) as defined above include the following structures:

[0016] The predicted structure formed by the combination of metal (M) and ligand (L) is (a). However, the reaction of ligand L where R 1 = f Bu and R 2 = Me with TaCls (ie M = Ta) in refluxing toluene has resulted in the loss of the methyl group to give (b). It is thus expected that this structure could be formed by other combinations of M, R 1 and R 2 . [0017] Similarly, the zwitterionic structure (c) (ie positive charge on the nitrogen atom, negative charge on M) has been prepared by the reaction of ligand L where R 1 = CI and R 2 = Ph with TaCls (ie M = Ta) in a sealed reaction vessel and a reduced reaction time. It is also expected that this structure could be formed by other combinations of M, R 1 and R 2 .

[0018] The term (Ci-C6)alkyl is used in relation to this invention to refer to a linear or branched alkyl group having between 1 and 6 carbon atoms. A preferred (Ci- C6)alkyl group for R 1 is a branched alkyl, preferably tertiary butyl (ie f Bu). A preferred (Ci-C6)alkyl group for R 2 is a linear alkyl, preferably methyl (ie Me).

[0019] When R 2 is unsubstituted or substituted phenyl, the phenyl group may be substituted with 1, 2 or 3 substituents selected from halogen, -OH, and (Ci-C6)alkyl. Preferably, the phenyl group is unsubstituted (ie Ph).

[0020] Preferably the halogen in Ligand A is chlorine. Preferably R 2 is unsubstituted or substituted phenyl. Particularly preferred combinations of ) R 1 and R 2 groups for the complexes as defined above are as follows:

(i) R 1 = c Bu and R 2 = Me,

(ii) R 1 = CI and R 2 = unsubstituted or substituted phenyl, preferably unsubstituted phenyl,

(iii) R 1 = CI and R 2 = Me, and

(iv) R 1 = CI and R 2 = CF 3 .

A particularly preferred combination is R 1 = CI and R 2 = unsubstituted phenyl.

[0021] In another aspect, this invention relates to complexes formed by the combination of a metal (M) and a ligand (L), wherein M = Tantalum and L has the formula (B)

and wherein R 1 = (Ci-C6)alkyl, preferably f Bu; R 2 = substituted or unsubstituted phenyl. Preferably, R 1 = f Bu and R 2 = unsubstituted phenyl. The predicted structure for the combination of metal (M) and Ligand B is as follows:

[0022] In a further aspect, this invention relates to complexes having the following formula (3):

where M = Niobium. Complexes where M = Tantalum are also described. Complex (3) may be formed by the combination of the metal (M) and a ligand L, wherein L has the formula (L3):

where M = Niobium or Tantalum. Complex (4) may be formed by the combination of the metal (M) and a ligand L, wherein L has the formula (L4):

[0024] In a further aspect, this invention relates to complexes having the following formula (5):

where M = Niobium or Tantalum. Preferably, M = Niobium. Complex (5) may be formed by the combination of the metal (M) and a ligand L, wherein L has the formula (L5):

[0025] This invention also relates to processes for preparing any of the complexes defined above comprising reacting a metal (M) chloride with a ligand (L). Preferred metal (M) chlorides include MC1 5 , M0C1 3 , MC1 4 (THF) 2 and [MC1 3 (DME)] (THF = tetrahydrofuran, DME = 1,2-dimethoxyethane).

[0026] Where the process is for preparing complex formed by the combination of a metal (M) and ligand (L) wherein M = Niobium or Tantalum and L has the formula (A) or (B) as defined above, the metal (M) chloride is preferably MCI5. It is preferred that the reaction is carried out in a solvent, preferably toluene. The reaction may be carried out under reflux. [0027] Where the process is for preparing a complex as defined by formula (3) above, the metal (M) chloride is preferably MCl4(THF)2, more preferably NbCl4(THF)2. It is preferred that the reaction is carried out in a solvent, preferably THF. The reaction may be carried out under reflux. The ligand for preparing a complex as defined by formula 3 preferably has the following formula L3 :

[0028] Where the process is for preparing a complex as defined by formula (4) above, the metal (M) chloride is preferably [MCl3(DME)] . It is preferred that the reaction is carried out in a solvent. The ligand for preparing a complex as defined by formula 4 preferably has the following formula L4:

[0029] Where the process is for preparing a complex as defined by formula (5) above, the metal (M) chloride is preferably [MCl3(DME)], more preferably [NbCl3(DME)]. It is preferred that the reaction is carried out in a solvent, preferably THF. The reaction may be carried out under reflux. The ligand for preparing a complex as defined by formula 5 preferably has the following formula L5:

[0030] This invention also relates to a process for polymerizing hydrocarbons comprising mixing a complex or a combination of complexes as defined above with a co-catalyst and one or more monomers, preferably olefins, more preferably a- olefins, even more preferably ethylene. In some embodiments, the process may comprise more than one type of monomer, ie a co-polymerisation. Preferred monomers for co-polymerisation include ethylene and 1-hexene.

[0031] In relation to this invention, the term "co-catalyst" is used to mean a substance which reacts with the complex or combination or complexes (which are sometimes known as the "pre-catalyst") to form a catalyst (sometimes referred to as an "active catalyst"). These co-catalysts may have a number of functions. They can act as scavengers and reduce the amount of any impurities which would reduce the activity of the catalyst. The co-catalyst is preferably an alkylating agent. Such alkylating agents may be capable of forming weakly coordinating anions. In the case of chlorinated co-catalysts, they probably form M-C1-A1 containing clusters which are the active species. As a result, in some circumstances chlorinated co-catalysts have been found to work better than non-chlorinated co-catalysts such as MAO. [0032] As mentioned above, the co-catalyst can be a non-chlorinated co-catalyst or, preferably, a chlorinated co-catalyst. It is preferred that the co-catalyst is an alkyl aluminium compound, preferably tri-isobutylaluminium (TIBA] or methylaluminoxane (MAO], or a chloro-alkyl aluminium compound. The chloro-alkyl aluminium compound is preferably EtAlCl 2 (EADC], MeAlCl 2 (MADC), Me 2 AlCl (DMAC), Et 2 AlCl, Me 3 Al 2 Cl 3 (MASC) or Et 3 Al 2 Cl 3 , more preferably MeAlCl 2 (MADC), Me 2 AlCl (DMAC) or Me 3 Al 2 Cl 3 (MASC), even more preferably MeAlCl 2 (MADC).

[0033] In some embodiments, the process for polymerizing hydrocarbons may comprise adding a re-activator during the mixing step. In relation to this invention, the term "re-activator" is used to mean a substance that reacts with a complex or combination of complexes (which are sometimes known as the "pre-catalyst") and/or the co-catalyst during or after they have catalysed the polymerisation reaction in order to convert them to the form they were in (eg oxidation state) prior to catalysing the polymerisation reaction. Preferably the re-activator is an oxidising agent. It is believed that, when the co-catalyst is added, the oxidation state of the metal is reduced from the +5 state down to the +2 state. Calculations have suggested that the +2 state is inactive for catalysis polymerisation, and so the oxidising agent is understood to act to oxidize the metal to the +3 and +4 oxidation state so that it can re-enter the catalytic cycle. A preferred re-activator is ethyl tricholoroacetate (ETA).

[0034] The process of polymerizing hydrocarbons is preferably carried out in a solvent, more preferably in toluene or an alkane, such as hexane or heptane.

[0035] In a further aspect, this invention relates to compositions comprising a complex or a combination of complexes as defined above and a co-catalyst as defined above.

[0036] Also described is a compound having the following formula (L5):

[0037] DETAILED DESCRTIPTION OF THE INVENTION

[0038] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1. shows previously reported niobium pre-catalysts and their activities.

Figure 2. shows ligand systems employed in this study. Figure 3. shows complexes synthesized in this study.

Figure 4. shows a view of a molecule of a) L^NbC (la) and hydrogen-bonded acetonitrile molecule, and b) L^NbOCbfNCMe) (lb). The general atom numbering scheme for all molecules is shown in Figure 4(a); in subsequent figures, only the coordinated atoms and marker atoms are labelled. Thermal ellipsoids are drawn at the 50 % probability level. In the case of lb, solvent molecules in the lattice and the hydrogen atoms on the disordered coordinated acetonitrile have been omitted for clarity. Compound lc is isostructural with la.

Figure 5. shows a view of a molecule of L 2 NbOCl2(MeCN) (2b) with coordinated MeCN molecule. Thermal ellipsoids are drawn at the 50 % probability level.

Figure 6. shows a view of a molecule of 5, indicating the atom numbering scheme. Thermal ellipsoids are drawn at the 50 % probability level. Selected bond lengths (A) and angles (°): Nb— N(l) 2.279(2), Nb— N(21) 2.217(2), Nb— 0(31) 1.9076(18), Nb— 0(4) 1.708(2) , Nb— Cl(5) 2.3806(9), Nb— Cl(6) 2.5781(8); N(l)— Nb— N(21) 69.67(8), N(l)— Nb— 0(31) 142.01(9), N(21)— Nb— 0(31) 72.35(8), N(21)— Nb— Cl(5) 165.18(7), N(l)— Nb— 0(4) 86.58(9), N(l)— Nb— Cl(6) 83.11(6), 0(31)— Nb— Cl(5) 104.79(6), 0(4)-Nb-Cl(6)

169.17(7).

Figure 7. shows views of molecules of 6a, 7a and 8a. The images for

6a, 7a and 8a are of one of the two essentially identical, independent molecules of L 6 NbCl4, L/NbC and L 8 NbCl4 respectively. Compound 6b is isostructural with 6a. Thermal ellipsoids are drawn at the 50 % probability level.

Figure 8. shows a comparison of bond lengths and torsion angles of structures 6a, 6b, 7a and 8a. Labels (1) and (2) indicate the two independent molecules in each of these structures. Rotation of the bottom phenyl ring about the N-C bond is also shown.

Figure 9. shows a view of a molecule of 8c with a hydrogen-bonded acetonitrile molecule. Thermal ellipsoids are drawn at 50% probability level. Selected bond lengths (A) and angles (°): Ta— 01 1.904(2), Ta— Cl(5) 2.3510(10), Ta— Cl(6) 2.3576(10), Ta— Cl(4) 2.3579(10), Ta— Cl(3) 2.3630(10), Ta— Cl(7) 2.3911(10); 0(1)— Ta— Cl(4) 176.71(7),

Cl(6)— Ta— Cl(4) 87.66(3), Cl(3)-Ta-Cl(6) 176.95(3), Cl(5)-Ta-Cl(7) 178.85(3), N(21)-C(31) 1.297(4); hydrogen bond: N(21)-H(21) 0.86, H(21)-N(41) 2.15, N(21)-H(21)-N(41) 157.7.

Figure 10. shows an x-ray photoelectron survey spectrum of S4a, S4b, S7b and S7c. Nb3c and Ta4c energy windows are shown inset. The Cls peak was used to calibrate the spectra.

Figure 11. shows the effect of varying the co-catalyst on pre-catalysts lb, 2b, 3a, and 4a. (For conditions, see table 3, run 6 vs 8 (lb), runs 27 vs 29 (2b), table 4, runs 34 vs 35 (3a)) Figure 12. shows the effect of R 1 and R 2 on the Activity of Nb/Ta Schiff Bases (for conditions, see table 5, run 59).

Figure 13. shows complex 12, product from ligand transfer to Al.

Figure 14. shows consumption profiles of S4a, S7b and S7c with EADC as co-catalyst and ETA as re-activator. Ethylene uptake is recorded after premixing of co-catalyst and pre-catalyst.

[0039] Summary of experimental work [0040] Reaction of MC1 5 or M0C1 3 with the imidazole-based pro-ligands L X H, 3,5- tBu2-2-OH-C 6 H2-(4,5-Ph 2 -i//-)imidazole, or oxazole-based L 2 H, 3,5-tBu 2 -2-OH- C6H 2 (i//-phenanthro[9,10-d])oxazole, following work-up, afforded the octahedral complexes [ΜΧ(ΙΛ 2 )], where MX = NbCl 4 (L 1 , la; L 2 , 2a), NbOCl 2 (NCMe) (L 1 , lb; L 2 , 2b), TaCl 4 (L 1 , lc; L 2 , 2c), TaOCl 2 (NCMe) (L 1 , Id). Treatment of the ct-diimine ligand L 3 (2,6-/Pr 2 C6H 3 N=CH)2,with MC1 4 (THF) 2 (THF = tetrahydrofuran, M = Nb, Ta) afforded [MC1 4 (L 3 )] (M = Nb, 3a; Ta, 3b). The reaction of [MC1 3 (DME)] (DME = 1,2- dimethoxyethane, M = Nb, Ta) with the bis(imino) pyridine ligand L 4 , 2,6-(2,6- afforded known complexes of the type [MC1 3 (L 4 )] (M = Nb, 4a; Ta, 4b), whereas the 2-acetyl-6-iminopyridine ligand L 5 , 2 -(2,6- on reaction with the niobium precursor afforded the coupled product {[2-Ac-6-(2,6-/Pr 2 C 6 H3N=(Me)C)C 5 H3N]NbOCl2}2 (5). Reaction of MCls with the Schiff-base pro-ligands L 6 H-L 10 H, [3,5-(R 1 ) 2 -2-OH-C 6 H 2 CH=N(2-OR 2 - C 6 H 4 )], (L 6 H: R 1 = tBu, R 2 = Ph; L7H: R 1 = tBu, R 2 = Me; L 8 H: R 1 = CI, R 2 = Ph; L 9 H: R 1 = CI, R 2 = Me; L 10 H: R 1 = CI, R 2 = CF 3 ) afforded the complexes [MC1 4 (L 6 10 )], 6a-10a, (M = Nb) and 6b-9b, (M = Ta). In the case of 8b, the corresponding Zwitterion was also synthesised, namely [Ta " Cls(L 8 H) + ].MeCN 8c. Unexpectedly, reaction of L7H with Tads in refluxing toluene led to removal of the methyl group and formation of the trichloride 7c [TaCl3(L 7 Me )]; conducting the reaction at room temperature led to the expected methoxy compound 7b. The use of chloro-alkyl aluminium reagents, such as dimethylaluminium chloride (DMAC) and methylaluminium dichloride (MADC), as co-catalysts, optionally in the presence of the re-activator ethyl trichloroacetate (ETA) generates thermally stable catalysts with, in the case of niobium, catalytic activities of up to two orders of magnitude greater than previously observed. The effect of steric hindrance and electronic configuration on the polymerization activity of these tantalum and niobium pre-catalysts was investigated. Spectroscopic studies ( ! H NMR, 13 C NMR, Ή-Ή and ! H- 13 C correlation) into the reaction of 4a/4b and either MAO or AlMe3/ [CPh3] + [B(C6Fs)4] " were consistent with the formation of a diamagnetic cation of the form [L 4 AlMe2] + . This cationic aluminium complex, in the presence of MAO, was not capable of the ROMP (ring opening metathesis polymerization) of norbornene, whereas the systems 4a/4b with MAO were active. Parallel Pressure Reactor (PPR) based homogeneous polymerization screening using pre-catalysts lb, lc, 2a, 3a, 6a, in combination with MAO, revealed only moderate-to-good activity for ethylene homo-polymerization and ethylene/1- hexene co-polymerization. Molecular structures are reported for complexes la-c, 2b, 5, 6a, 6b, 7a, 8a and 8c.

[0041] Synthesis and characterization of the Imidazole /Oxazole Pre-Catalysts (la-d. 2a-c):

[0042] The ligands L X H and L 2 H (Figure 2) were synthesized from the reaction of 3,5-di-tert-butyl-2-hydroxybenzaldehyde and either benzyl (1,2-diphenylethane- 1,2-dione) or 9,10-phenanthrenequinone in glacial acetic acid in the presence of ammonium acetate following the reported procedure. [9] These ligands have been combined with the metal precursors NbCls (to afford la, 2a), TaCls (lc and 2c), NbOCl3 (lb and 2b) or TaOCl3 (Id); the oxychloride complexes were somewhat more air sensitive than the tetrachloride complexes. The pre-catalysts can be readily recrystallized from saturated solutions of acetonitrile, and representative molecular structures have been determined and are shown in Figures 4 and 5; principal dimensions are collated in Tables 1 and 2. In all the structures, the niobium (or tantalum) centre is present in a distorted octahedral environment, although that for 2b may be better described as square-pyramidal with the acetonitrile molecule occupying a 'second sphere' position rather than the sixth coordination site trans to the oxo group [Nb— N(6) 2.558(2) A]; the niobium centre lies 0.3540(6) A out of the 0(1), N(21), Cl(4), Cl(5) mean-plane.

[0043] Compounds la and lc are isostructural, exhibiting similar coordination and conformation dimensions, for example the difference in the imidazole ligand rotation about the C(12)— C(22) bond in la is -13.9° versus -14.1° in lc. The phenyl rings are rotated 83.2° about C(25)— C(251) and 2.9° about C(24)— C(241) from the imidazole ring in la; the corresponding angles in lc are 83.4 and 3.4°. The amino NH group is hydrogen bonded to a solvent (MeCN) molecule in both crystals.

[0044] The rotations of rings in lb are quite different, viz -19.2° about C(12)— C(22), 45.1° about C(24)— C(241) and 66.9° about C(25)— C(251). The NH group here forms a hydrogen bond with 0(3) of a neighbouring molecule, and thus links molecules in chains throughout the crystal; this is the only case of extensive linking of molecules in this series of compounds. There are four MeCN solvent molecules in the crystal lattice, one of which is coordinated to the niobium and is disordered over two distinct orientations. The fixed conformation of the phenanthrene group in compound 2b restrains rotation about the C(24)-C(241) and C(25)-C(251) bonds to 2.5 and 6.3° respectively; the phenolate ring is rotated -7.0° from the oxazole ring, giving a much more planar ligand than in the other crystals. There is no hydrogen bonding present in this structure.

[0045] Synthesis of a-diimine. bisiminopyridine and 2- acetyl- 6-iminopyridine Pre- catalysts (3a/b.4a/b.5):

[0046] The a-diimine ligand L 3 was reacted with MCl4(THF)2 in tetrahydrofuran to give the pre-catalyst 3a (Nb); however, the analogous reaction using tantalum did not lead to complexation, and subsequently 3b was synthesised as reported in the recent literature. [10] The bis(imino)pyridine ligand L 4 , on reaction with [MC1 3 (DME)] [6b], afforded 4a (Nb) and 4b (Ta). Attempts to form a mononuclear 2- acetyl-6-iminopyridine complex using the ligand set 2-Ac-6-(2,6- (L 5 ), resulted in formation of the coupled dinuclear complex 5, isolated in yields of ca 20 %. Whilst coupling of bis(imino)pyridine fragments is now well established, [3a] the present case is a unique example for the NNO ligand set, and reflects the ability of [NbCl3(DME)] to act as coupling agent. [11] The molecular structure of 5 is shown in Figure 6, with selected bond lengths and angles given in the caption. The structure is dimeric about an inversion centre, coupled through a bridging C— C bond [C(32)— C(32') 1.576(6) A]. There is an offset π interaction between ring C(ll) - C(16) and that of a symmetry related molecule.

[0047] Synthesis of Schiff base Pre-catalvsts (6a/b-10a/b):

[0048] The ligand systems L 6 10 H were synthesized by the condensation reaction of the corresponding aldehyde and o-methoxy-/o-phenoxy-/o-trifluoromethyl-aniline in high yields [ca 80 %), and then reacted with either NbCls (for 6a-10a) or TaCls (6b-9b). Unexpectedly, reaction of the ligand L 7 with TaCls in refluxing toluene led to loss of methyl chloride to afford the phenoxide (7c); similar Lewis acid assisted cleavage of ethers has been utilized in calixarene chemistry as a means of controlling the charge of the ligand set. [12] Demethylation here can be avoided by carrying out the synthesis at ambient temperature, which affords the expected complex 7b. In all cases, crystalline solids can be isolated as yellow/orange to red solids in moderate-to-good yields [ca 35 - 72 %). Single crystals of 6a, 6b, 7a and 8a suitable for X-ray crystallography were grown from their respective saturated acetonitrile solutions, and their structures were determined, Figure 7. For each of these samples, there are two independent complex molecules in the crystal. In every case, the pseudo-octahedral metal centre is bound to both the nitrogen and oxygen atoms of the Schiff-base ligand. The Nb— 0 and Nb— N bonds have typical dimensions [7a/b]. The OR 2 groups are located in similar positions relative to the metal centre. Principal dimensions are compared in Figure 8. Interestingly, repeating the preparation of 8b in a sealed reaction vessel over a shorter reaction time yielded the Zwitterionic species 8c (Figure 9), in which the iminium hydrogen atom was clearly identified in a N + -H--N(acetonitrile) hydrogen bond.

Table 1. Selected bond dimensions (A and °) in structures of the MC L complexes 6a, 6b, 7a and 8a, dimensions of one of the two essentially identical, independent molecules are quoted.

Compound la lc 6a 6b 7a 8a

M(l)- -0(1) 1.863(2) 1.8706(13) 1.8746(13) 1.886(12) 1.868(4) 1.896(3)

M(l)- -N(21) 2.267(3) 2.2507(16) 2.2715(16) 2.246(13) 2.308(5) 2.309(4)

M(l)- -Cl(3) 2.3576(9) 2.3532(6) 2.3640(6) 2.355(4) 2.3159(18) 2.3655(14)

M(l)- -Cl(4) 2.3092(8) 2.3116(5) 2.3224(5) 2.322(4) 2.3487(19) 2.3129(15)

M(l)- -Cl(5) 2.3083(9) 2.3136(5) 2.3143(6) 2.313(4) 2.301(2) 2.2785(15)

M(l)- -Cl(6) 2.3843(9) 2.3789(6) 2.3401(6) 2.339(4) 2.3871(17) 2.3423(16)

0(1)- -M(l)- -N(21) 79.09(9) 79.58(6) 78.90(6) 80.0(5) 78.89(18) 79.90(16)

0(1)- -M(l)- -Cl(4) 169.31(7) 170.83(5) 165.11(4) 166.7(3) 164.91(13) 164.46(12)

N(21)- -M(l)- -Cl(5) 171.36(7) 171.23(4) 173.60(4) 173.6(3) 174.88(14) 178.30(12)

Cl(3)- -M(l)- -Cl(6) 172.09(4) 172.89(2) 172.43(2) 173.25(16) 169.77(7) 168.36(5)

Table 2. Selected bond dimensions (A and °) in structures of the MCI2OL complexes.

Compound lb 2b

M(l)-0(1) 1.951(6) 1.9319(16)

M(l)— N(21) 2.234(4) 2.2727(18)

M(l)-0(3) 1.705(4) 1.6899(16)

M(l)— Cl(4) 2.3889(15) 2.3696(7)

M(l)— Cl(5) 2.3772(16) 2.3741(6)

M(l)— N(6) 2.493(5) 2.558(2)

0(1)-M(1)- -N(21) 78.3(2) 78.78(7)

0(1)-M(1)- -Cl(4) 154.24(19) 152.50(5)

N(21)— M(l)- -Cl(5) 163.66(12) 166.02(5)

0(3)-M(l)- -N(6) 177.38(18) 177.79(8) [0049] Silica Immobilisation

[0050] Compounds 4a, 4b, 7b and 7c were immobilised on pre-treated silica; the S1O2 had been heated to 350 °C under dynamic vacuum for 48 h, and gave the supported structures S4a, S4b, S7b and S7c. To calculate the amount of niobium or tantalum bound to the silica surface, X-ray photoelectron spectroscopy (XPS) analysis was employed. X-ray photoelectron survey spectra are shown in Figure 10.

[0051] Analysis of the resulting photoelectron spectrum of S4a (Figure 10) showed a Nb3c peak with two components at 206.9 and 209.6 eV. The resulting concentration of niobium was shown to be 2.92 % of the bulk sample. The presence of peaks at 399.7 eV (0.896 %) of the bulk sample) and 198.7 eV (1.07 %) correspond to Nls and C12p peaks . The photoelectron spectra of S4b, S7b and S7c each show two Ta4c peaks at 242.2 eV and 230.6 eV (1.13 % of the bulk sample), 241.7 eV and 230.1 eV (1.35 %), 241.9 eV and 230.6 eV (0.495 %) respectively. The spectra also confirmed the presence of C12p peaks with 0.729, 1.06 and 0.729 % of S4b, S7b and S7c assignable to chlorine. In each of the Ta based samples, the percentage of nitrogen could not be determined. All values were recorded within 0.1 eV.

[0052] Catalytic Screening

[0053] Homogeneous Catalysis

[0054] The pre-catalysts la-2c were screened for the polymerization of ethylene employing either MADC or DMAC as co-catalyst. In most cases, the activities can be described as very high, Table 3; only runs 1, 7 and 8, conducted at 20 °C, gave activities < 1000 g/mmol.hr.bar. Interestingly, pre-catalyst lb employing MADC exhibited enhanced activity (~8,900 g/mmol.h.bar) at elevated temperatures (80 °C, run 17). Runs 5 - 6 and 7 - 8 revealed a dependency of catalytic activity on the [Nb] :[Al] concentration, which was far more dramatic for MADC than for DMAC. In the absence of ethyltrichloroacetate (ETA), there was a drop-off in activity (run 1 vs 2). At 40 °C, using MADC as co-catalyst, the oxydichloride systems [e.g. lb, run 15) out-performed the tetrachlorides [e.g. la, run 3). Lifetime studies on lb were conducted using either DMAC (Table 3, runs 9 - 11) or MADC (runs 12 - 14) as co- catalyst (8000 equivalents) at 40 and 60 °C, respectively. In the case of DMAC, there was no appreciable drop-off in activities over 15 min, whereas for MADC, the activity had dropped by ca 50 % within 10 min. In both cases, there was a slight increase in the polydispersity with time. As expected (based on the chloride content and the observations using DMAC and MADC as the co-catalyst), the use of MASC resulted in activities intermediate between those of DMAC and MADC [e.g. runs 27 - 29), i.e. higher activities were associated with increased chloride content in the co- catalyst. Therefore, among the alkyl aluminium reagents, MeAlCb (MADC) is the preferred co-catalyst.

[0055] Furthermore, from the screening results, it was also evident that enhanced activity was achieved when employing ligand set L 2 H for either the niobium tetrachlorides [e.g. runs 3 vs 26), whereas comparable activities were observed for the tantalum tetrachlorides (run 19 vs 31). In the case of the oxydichlorides, activities for lb, (runs 4 - 17) are generally less than those for 2b (runs 27 - 29). Use of hexane as the polymerization solvent proved to be detrimental, with systems displaying only negligible-to-poor activity (results not included here).

Table 3. Results for selected ethylene polymerization runs for pre-catalysts la - 2c.

Pre-

Co- Temp Time Yi

Run [Al] eld

catalyst Activity*

catalyst /[Nb or Ta] M n d PDF

(°C) (min)

(μηιοΐ) PE (g)

1" la (0.5} MADC 8000 20 30 0.11 440 - - -

2 la (0.5} MADC 8000 20 30 0.287 1148 - - - la

3 MADC 8000 40 15 0.119 1900 - - -

(0.25}

lb

4 MADC 2000 20 15 0.2 1600 - - -

(0.25} lb

5 MADC 4000 20 15 0.379 3032

[0.25] - - - lb

6 MADC 6000 20 15 0.525 4200

(0.25) - - - lb

7 DMAC 4000 20 15 0.089 710

[0.25) - - - lb

8 DMAC 6000 20 15 0.09 720

(0.25) - - - lb

9 DMAC 8000 40 5 0.067 1610 1030000 371000 2.8 (0.25)

lb

10 40 20

(0.25) DMAC 8000 0.172 1030 775000 228000 3.4 lb

11

(0.25) DMAC 8000 40 30 0.386 1540 764000 164000 4.6 lb

12 60 5

(0.25) MADC 8000 0.228 5470 555000 146000 3.8 lb

13 (0.25) MADC 8000 60 10 0.202 2420 490000 200000 2.5 lb

14 60 30

(0.25) MADC 8000 0.615 2460 496000 109000 4.6 lb

15 MADC 8000 40 15 0.332 5310

(0.25) - - - lb

16 MADC 8000 60 30 0.615 2460

(0.25) - - - lb

17 MADC 8000 80 15 0.557 8910

(0.25) - - - lc

18 MADC 8000 30 15 0.16 2580

(0.25) - - - lc

19 MADC 8000 40 15 0.200 3200

(0.25) - - - lc

20 MADC 8000 50 15 0.309 4940

(0.25) - - - lc

21 MADC 8000 80 15 0.210 3360 307007 122141 2.5 (0.25)

Id

22 MADC 8000 20 15 0.388 6210

(0.25) - - -

Id

23 MADC 8000 40 15 0.272 4350

(0.25) - - -

Id

24 MADC 8000 60 15 0.098 1570

(0.25) - - -

Id

25 MADC 8000 80 15 0 0

(0.25) - - -

2a

26 MADC 8000 40 15 0.215 3180

(0.25) - - -

2b

27 171762

DMAC 8000 50 15 0.097 1552 767677 2.2 (0.25) 1

2b

28 MASC 8000 50 15 0.220 3520

(0.25) - - - 29 2b

MADC 8000 50 15 0.741

(0.25) 11860 - - -

2c

30 MADC 8000 20 15 0.205 3280 567002

(0.25) 50218 11.3

2c

31 MADC 8000 40 15 0.211 3376

(0.25) - - -

2c

32 MADC 8000 60 15 0.621 9936

(0.25) - - -

Conditions:! bar ethylene Schlenk tests carried out in toluene (100 ml) in the presence of ETA (0.05 ml), reaction was quenched with dilute HCl, washed with methanol (50 ml) and dried for 12 h. at 80 °C. a Without ETA. b g/mmol.h.bar c Weight average molecular weight. d Number average molecular weight. e Polydispersity index: M w /M n

[0056] For the tantalum complexes, e.g. lc and 2c, observed activities (runs 18 - 21 and 30 - 32) were similar to those of the related niobium-based pre-catalysts described above, though we note that direct comparison between runs 2 and 3 versus 18 - 21, given the differing conditions, is not straightforward. The tantalum systems showed a general increase in activity with increasing temperature up to 50 °C [e.g. for 2c, runs 30 - 32); further increases in the temperature (to 80 °C, run 21), led to slightly reduced activity consistent with the start of the decomposition of the active species under these more robust conditions. The oxydichloride tantalum complex Id was most active at 20 °C (6210 g/mmol.hr.bar, run 22), and unlike its niobium counterpart (runs 15 - 17), showed a decrease in activity with increasing temperature.

[0057] Studies using lb revealed higher molecular weights (>750,000) when employing DM AC as the co-catalyst (runs 9 - 11). Indeed, polymer samples from runs utilizing DMAC contained some insoluble gel, whilst a number of the samples resulting from the use of MADC also proved too insoluble for GPC analysis. Ή NMR and IR spectroscopic end-group analysis of the polymer showed no vinylic groups, rather data that were consistent with the polymer containing only saturated end groups. [13] EPR spectra, recorded upon addition of MADC (5 equivs), were generally uninformative. For example, for la (solid-state), there was a broad signal at g = 2.50 consistent with the presence of Nb(IV), but no hyperfine splitting was observed.

[0058] Particular high activities and/or thermal stabilities have been observed for lb, 2b and 2c when MADC is employed, especially when the reaction temperature is at 50 °C or higher. Table 4. Results for selected ethylene polymerization runs for pre-catalysts 3a - 5.

Pre-

Co- [Al] Temp Time Yield

un catalyst Activity" PDI £ catalyst /[Nb or Ta] (°C) (min) PE (g)

(μπιοΐ)

3a

33 DMAC 6000 20 15 0.034 540 418255 181371 2.3

(0.25}

3a

34 DMAC 8000 20 15 0.012 190

(0.25}

3a

35 MADC 8000 20 15 0.393 6290 357843 138021 2.5 (0.25}

3a

36 MADC 8000 60 15 0.16 2560

(0.25}

3a

37 MADC 8000 80 15 0.1 1600 230256 90761 2.5 (0.25}

3b

38 MADC 8000 20 15 0.400 6400

(0.25}

3b

39 MADC 8000 40 15 0.620 9920

(0.25}

3b

40 MADC 8000 60 15 0.470 7520

(0.25}

3b

41 MADC 8000 80 15 0.065 1040

(0.25}

4a

42 DMAC 6000 20 15 0.037 590

(0.25}

4a

43 DMAC 8000 20 15 0.069 1100

(0.25}

4a 107247

44 MADC 4000 20 15 0.130 2080 257327 4.2 (0.25}

4a

45 MADC 8000 40 15 0.212 3390

(0.25}

4a

46 MADC 8000 60 15 0.17 2720 182778 63572 2.9 (0.25}

4a

47 MADC 8000 80 15

(0.25}

4b

48 MADC 8000 20 15 0.352 5632

(0.25}

4b

49 MADC 8000 40 15 0.645 10320

(0.25}

4b

50 MADC 8000 60 15 0.128 2050

(0.25}

4b

51 MADC 8000 80 15

(0.25}

52 5 (0.25} MADC 8000 20 15 0.191 1530

53 5 (0.25} MADC 8000 40 15 0.109 870

54 5 (0.25} MADC 8000 60 15 0.133 1060

55 5 (0.25} MADC 8000 80 15 0.162 1100

Conditions:! bar ethylene Schlenk tests carried out in toluene (100 ml) in the presence of ETA (0.05 ml), reaction was quenched with dilute HCl, washed with methanol (50 ml) and dried for 12 h. at 80 °C. a g/mmol.h.bar. b Weight average molecular weight. c Number average molecular weight. d Polydispersity index: M w /M n [0059] Complex 3a was synthesized from reaction of the a-diimine ligand L 3 and NbCl4(THF)2; however, the analogous reaction using TaCl4(THF) 2 did not lead to complexation, and the synthesis of 3b was therefore carried out using the recently reported procedure. [14] Polymerization studies (Table 4) showed similar activities of 3a and 3b at room temperature, approximately 6000 g/mmol.hr.bar (runs 35 and 38). However the tantalum species showed increased stability at higher temperatures [e.g. 7520 vs 2560 g/mmol.hr.bar, runs 40 and 36). In the case of the known complexes 4a and 4b, use of tert-butyl modified methylaluminoxane afforded activities < 70 g/mmol.h.bar. [5b] Here, we found that use of 4a with MADC, in the presence of ETA, afforded activities as high as 3390 g/mmol.h.bar (run 45). When comparing use of co-catalysts MADC and DMAC for polymerization, it is clear that MADC is the co-catalyst of choice (Figure 11).

[0060] Particular high activities and/or thermal stabilities have been observed for 3b and 4b when MADC is employed.

Table 5. Temperature dependence of activity for pre-catalysts 6a/b - 10a.

Metal Temp Time

Run catalyst R 1 R 2 PE Activity" PDI d

Centre (°C) (Miiis)

(μηιοΐ) (g)

56 6a (0.25) tBu Ph Nb 20 15 0.054 860 552289 143005 3.9

57 6a (0.25) tBu Ph Nb 30 15 0.053 850 - - -

58 6a (0.25) tBu Ph Nb 40 15 0.074 1180 - - -

59 6a (0.25) tBu Ph Nb 50 15 0.129 2060 - - -

60 6a (0.25) tBu Ph Nb 80 15 0.197 3150 108189 41677 2.6

61 6b (0.25) tBu Ph Ta 50 15 0.509 8140 - - -

62 7a (0.25) tBu Me Nb 50 15 0.231 3700 - - -

63 7b (0.25) tBu Me Ta 20 15 0.36 5760 - - -

64 7b (0.25) tBu Me Ta 40 15 0.485 7760 - - -

65 7b (0.25) tBu Me Ta 50 15 0.456 7300 - - -

66 7b (0.25) tBu Me Ta 80 15 0.33 5280 - - -

67 7c (0.25) tBu - Ta 20 15 0.25 4000 1152510 402526 2.9

68 7c (0.25) tBu - Ta 40 15 0.327 5230 420862 48734 8.6

69 7c (0.25) tBu - Ta 50 15 0.923 14770 - - -

70 7c (0.25) tBu - Ta 80 15 1.034 16540 495251 205942 2.4

71 8a (0.25) CI Ph Nb 50 15 0.786 12580 100674 14680 6.9 72 8b (0.25) CI Ph Ta 50 15 1.263 20200 - - -

73 8c (0.25) CI Ph Ta 20 15 0.329 5264 - - -

74 8c (0.25) CI Ph Ta 40 15 0.274 4384 - - -

75 8c (0.25) CI Ph Ta 60 15 0.027 432 - - -

76 8c (0.25) CI Ph Ta 80 15 - - - -

77 9a (0.25) CI Me Nb 50 15 0.378 6050 - - -

78 9b (0.25) CI Me Ta 20 15 0.5 8000 321815 64181 5.0

79 9b (0.25) CI Me Ta 40 15 0.708 11330 - - -

80 9b (0.25) CI Me Ta 50 15 0.785 12560 - - -

81 9b (0.25) CI Me Ta 60 15 1.153 18450 - - -

82 9b (0.25) CI Me Ta 80 15 0.547 8750 70331 11642 6.0

10a

83 CI CF 3 Nb 20 15 0.092 1470 - - -

(0.25)

10a

84 CI CF 3 Nb 40 15 0.168 2690 - - -

(0.25)

10a

85 CI CF 3 Nb 60 15 0.113 1810 - - -

(0.25)

10a

86 CI CF 3 Nb 80 15

CO 251 - - - - -

Conditions: 1 bar ethylene Schlenk tests carried out in toluene (100 ml) in the presence of ETA (0.05 ml), reaction was quenched with dilute HCl, washed with methanol (50 ml) and dried for 12 h. at 80 °C. a g/mmol.h.bar. b Weight average molecular weight. c Number average molecular weight. d Polydispersity index: M w /M n

[0061] Schiff base ligands L 6 H - L 10 H were synthesized and reacted with NbCls and Tads to give pre-catalysts 6a/b - 10a. The polymerization system MADC/ETA, which had been shown in Tables 3 and 4 to be the most promising, was then used to assess these systems for polyethylene production. Niobium pre-catalysts 6a - 10a (Table 5) possessed similar activities to la - 5; however, the tantalum analogues were much more active. These Schiff-base type catalysts (6a/b - 10a, but not 8c) were active up to 80 °C (runs 60, 66 and 82), although a noticeable drop-off in activity for 9b was observed above 60 °C. The molecular weight of the polyethylene decreased with increasing temperature and an increase in PDI (runs 78 vs 82). Interestingly, catalyst system 9b had one of the highest activities of the pre-catalysts studied herein (run 81, activity ~18,450 g/mmol.h.bar). [0062] Modification of the R 1 and R 2 groups of the Schiff base backbone had a major influence on the polymerization activity of the metal centre involved. The catalysts with chloro groups at the R 1 position generally afforded higher activities than their tert-butyl counterparts, Figure 12.

[0063] Pre-catalysts 8a/b and 9a/b all have chloro groups in the R 1 positions; changing the R 2 groups from methoxy to phenoxy increased the activity of the catalyst system by a factor of two (runs 71 vs 77 and 72 vs 80). Changing the metal centre from niobium to tantalum also led to a pronounced increase in activity, e.g. for catalyst systems 7 (runs 62 vs 65), 8 (71 vs 72) and 9 (77 vs 80) the activity doubled, whereas for 6 (runs 59 and 61) the activity quadrupled. The tantalum pre-catalyst 7c gave an activity of 16,540 g/mmol.hr.bar (run 70) at 80°C, revealing increased thermal stability versus the related 7b (run 66). The highest activity obtained utilized pre- catalyst 8b with an activity in excess of 20,000 g/mmol.hr.bar.

[0064] Benchmark Catalyst

[0065] The known diphenolate niobium pre-catalyst 11, was tested for ethylene polymerization using 8000 equivalents MADC over a 15 minute polymerization run, Table 6. As expected from other pre-catalysts used in this study, the highest activity when using this co-catalyst (7940 g/mmol.hr.bar, run 87) was much improved over the previously reported conditions (8000 equivalents of co-catalyst versus 800) using co-catalyst DMAC (123 g/mmol.hr.bar, run 91). [7a]

[0066] Particular high activities and/or thermal stabilities have been observed for 7b, 7c, 8a, 8b and 9b when MADC is employed. Table 6. Results for selected ethylene polymerization runs for known pre-catalyst

11.

Pre-

Run [Al] Time

catalyst Co-catalyst Temp (°C) Yield PE (g) Activity"

/[Nb or Ta] (min)

(μπιοΐ)

87 11(0.25) MADC 8000 20 15 0.496 7940

88 11(0.25) MADC 8000 40 15 0.399 6380

89 11(0.25) MADC 8000 60 15 - -

90 11(0.25) MADC 8000 80 15 - -

91 11 (5.0) DMAC 800 50 60 0.615 123

Conditions: 1 bar ethylene Schlenk tests carried out in toluene (100 ml) in the presence of ETA (0.05 ml), reaction was quenched with dilute HC1, washed with methanol (50 ml) and dried for 12 h. at 80 °C. a g/mmol.h.bar.

[0067] Investigation of the active species

[0068] The niobium and tantalum 2,6-bis(imino)pyridine complexes 4a/4b were selected for in-depth study because of the available 'spectroscopic handles' in the ligand framework. Upon activation with modified methylaluminoxane (MMAO) or AlMe3/[CPh3] + [B(C6F5)4] " , each system was followed using NMR spectroscopy (Ή NMR, 13 C NMR, COSY NMR, HSQC NMR). Interaction of 4a with the activators, both at ambient and at lower temperatures (temperatures between -40 to 50 °C were investigated), led to the formation of a new diamagnetic complex 12 (Figure 13), the product of the ligand transfer from niobium to aluminium.

[0069] Besides 12, some minor species from the reaction of 4a were present in the solution. However, attempts to increase the concentration of these species (by variation of temperature, Nb/Al ratio, etc), and to characterize them by NMR spectroscopy were unsuccessful. Mixing of the parent bis(imino)pyridine L 4 with MAO also resulted in the formation of 12. The system 12/MAO showed no catalyst activity in norbornene ROMP-polymerization, whereas the system L 4 NbCl3/MAO was active in this reaction leading to complete conversion of 7 equivalents of norborene to a mixture (1: 1) of cis- and trans-ROMP norbornene polymers over 5 hours at room temperature.

[0070] NMR spectra of the catalyst systems L 4 NbCl 3 /MAO and L 4 NbCl3/AlMe3/ [CPh3] + [B(C6F5)4] " in chlorobenzene (see ESI for spectra) contain strong signals of the [L 4 AlMe2] + cation and very weak signals of unidentified Nb species. Hence, upon interaction with MAO or AlMe3/ [CPh3] + [B(C6Fs)4] " , more than 90 % of the starting Nb complex loses its bis(imino)pyridine ligand to afford the ion pair [L 4 AlMe 2 ] + [A] -.

[0071] Hence, in the system L 4 NbCl3/MAO, only a minor portion of Nb converts into catalytically active Nb species, which escapes reliable characterization. The same is true for the tantalum congener. However, in the system L 4 TaCl3/MAO, only the formation of 12 was observed, and even traces of intermediates assignable to tantalum based ion-pairs were not detected.

[0072] Thus, the results obtained show that, for some metals (Nb, Ta), the bis(imino)pyridine ligand is very easily transferred to aluminium and the active niobium species is formed as a minor product.

[0073] Parallel Pressure Reactor screening

[0074] A number of the pre-catalysts described herein were subjected to homo- and hetero-geneous parallel pressure reactor screening (PPR) employing methylaluminoxane (MAO) as co-catalyst. [15] The results for ethylene polymerization using pre-catalysts lc and 6a are given in Table 7, whilst pre- catalysts lb, lc, 2a and 3a have been employed in the co-polymerization of ethylene with 1-hexene (Table 8). The supported catalysts S4a, S4b, S7b and S7c were subjected to both homo-polymerization (ethylene) using tri-isobutyl aluminium (TIBA) as co-catalyst, and co-polymerization (1-hexene and ethylene) using tri-isobutyl aluminium (TIBA) or ethyl aluminium dichloride (EADC) as co- catalyst (Table 9). The effect of the addition of ethyl trichloroacetate (ETA) was also investigated (Table 10).

[0075] Ethylene polymerization

[0076] Both pre-catalysts lc and 6a were moderately active for the polymerization of ethylene under the conditions employed. The lower results here are thought to be associated with the use of MAO as co-catalyst. Nomura et al. have proposed for vanadium-based catalyst systems/MAO the formation of discrete ion-pairs, which led to lower activities. [16] In contrast to the Schlenk line screening [e.g. runs 57- 60), an increase in temperature here resulted in an approximately 20 % reduction in the observed activity, runs 97 vs 98/99). Increasing the catalyst loading was detrimental to the observed activity, suggesting that the concentration of the active species could have an upper limit.

Table 7. Parallel Pressure Reactor Ethylene polymerization screening of pre- catalysts lc and 6a.

Pre-Catalyst

Run Yield (g) Time (hr) Activity" Temp (°C)

(μπιοΐ)

93 lc (0.4) 0.035 1.0 13.2 80

94 6a (0.1) 0.040 1.0 60.4 60

95 6a (0.15) 0.040 1.0 40.0 60

96 6a (0.2) 0.040 0.6 59.1 60

97 6a (0.2) 0.041 1.0 30.9 60

98 6a (0.2) 0.033 1.0 25.0 80

99 6a (0.2) 0.032 1.0 24.1 80

100 6a (0.25) 0.030 1.0 19.5 80

101 6a (0.3) 0.031 1.0 15.7 80

102 6a (0.35) 0.031 1.0 12.3 80

Conditions: 6.68 bar ethylene, lhr reaction time, co-catalyst: methylaluminoxane, 4000 equivalents, heptane as solvent; a g/mmol.h.bar. [0077] Ethylene /1-hexene copolymerization

[0078] For 1-hexene/ethylene co-polymerization, pre-catalysts lb, lc, 2a and 3a exhibited moderate activity when activated using MAO as co-catalyst. Pre-catalyst 3a displayed the highest activity at ~60 g/mmol.h.bar (Table 8, run 112) for a catalyst loading of 0.2 μηιοΐ, though the activity was far less (~15 g/mmol.h.bar) for increased catalysts loadings (< 0.4 μηιοΓ).

Table 8. PPR co-polymerization screening of pre-catalysts lb, lc, 2a and 3a.

Run Pre-catalyst (μπιοΐ) Yield (g) Activity" Temp (°C)

103 lb (0.2) 0.035 26.4 80

104 lb (0.3) 0.031 15.6 80

105 lb (0.35) 0.032 13.8 80

106 lb (0.4) 0.040 14.8 80

107 lc (0.2) 0.034 25.6 80

108 lc (0.3) 0.034 18.0 80

109 lc (0.35) 0.035 14.7 80

110 2a (0.2) 0.035 26.1 80

111 2a (0.25) 0.036 21.6 80

112 3a (0.2) 0.078 58.2 80

113 3a (0.3) 0.036 18.1 80

114 3a (0.35) 0.035 15.0 80

115 3a (0.4) 0.038 14.2 80

Conditions: 6.68 bar ethylene, lhr reaction time; for co-polymerizations, 54 μΐ. hexane was added, with co-catalyst methylaluminoxane, 4000 equivalents, heptane as solvent; a g/mmol.h.bar.

Table 9. PPR co-polymerization screening of pre-catalysts S4a, S4b, S7b and S7c using TIBA and EADC.

Pre-catalyst Co- Metal Content Ethylene

Run (mg) catalyst (μηιοΐ) yield (g) Activity" Uptake (psi)

116 S4a (0.4) TIBA 0.1262 0.0028 22.19 3.97

117 S4a (0.8) TIBA 0.2523 0.0073 28.93 6.26

118 S4b (0.4) TIBA 0.0250 0.001 40.04 5.65

119 S4b (0.8) TIBA 0.0499 0.0014 28.03 3.36 120 S4b (1.0) TIBA 0.0624 0.0013 20.82 9.31

121 S7b (0.4) TIBA 0.0296 0.0013 43.90 7.94

122 S7b (0.8) TIBA 0.0592 0.0014 23.64 3.05

123 S7b (1.0) TIBA 0.0740 0.0101 136.43 8.55

124 S7c (0.4) TIBA 0.0110 0.0011 100.35 4.12

125 S7c (0.8) TIBA 0.0219 0.001 45.61 2.29

126 S7c (1.0) TIBA 0.0274 0.0079 288.29 11.75

127 S4a (0.4) EADC 0.1262 0.0022 17.44 2.59

128 S4a (0.8) EADC 0.2523 0.0032 12.68 5.04

129 S4a (1.0) EADC 0.3154 0.002 6.34 5.04

130 S4b (0.8) EADC 0.0499 0.0012 24.03 10.38

131 S4b (1.0) EADC 0.0624 0.0016 25.63 5.04

132 S7b (0.4) EADC 0.0296 0.0009 30.39 5.80

133 S7b (0.8) EADC 0.0592 0.0014 23.64 10.23

134 S7b (1.0) EADC 0.0740 0.0013 17.56 13.89

135 S7c (0.4) EADC 0.0110 0.001 91.23 2.75

136 S7c (0.8) EADC 0.0219 0.0019 86.67 6.26

137 S7c (1.0) EADC 0.0274 0.0006 21.90 4.43

Conditions: 6.68 bar ethylene, lhr reaction time, 54 μΐ. hexane added, 4000 equivalents co-catalyst, heptane as solvent; a g/mmol.h.bar.

[0079] The supported pre-catalysts S4a, S4b, S7b and S7c were found to be inactive for the homo-polymerization of ethylene when using TIBA as co-catalyst (results not shown). However, these pre-catalysts were found to be active for the co- polymerization of 1-hexene and ethylene, though the runs gave complicated and unpredictable results (Table 9). In general, supported pre-catalysts S7c and S7b were the most active with either tri-isobutylaluminium (TIBA) or ethyl aluminium dichloride (EADC); however, drawing more specific conclusions is difficult given the poor reproduceability of the data. Repeated runs of each pre-catalyst are shown (rather then being averaged) in the table due to the lower reproduceability.

Table 10. Co-polymerization screening of pre-catalysts S4a, S4b, S7b and S7c using EADC/ETA.

Pre-catalyst Metal Content

Run (mg) (μηιοΐ) yield (g) Activity" Ethylene Uptake (psi) ETA:M

138 S4a (0.3) 9.46E-02 0.0013 13.74 6.10 1440

139 S4a (0.3) b 9.46E-02 0.0015 15.85 4.88 1440 140 S4a (0.8) 2.52E-01 0.0017 6.74 4.88 1440

141 S4b (0.8) b 4.99E-02 0.0015 30.03 3.97 1440

142 S4b (0.8) b 4.99E-02 0.0015 30.03 7.48 1440

143 S4b (0.3) 8.22E-03 0.0009 109.48 4.73 1440

144 S7b (0.3) 2.22E-02 0.011 495.27 10.2 360

145 S7b (0.3) b 2.22E-02 0.0045 202.61 6.41 360

146 S7b (0.3) b 2.22E-02 0.003 135.07 5.65 720

147 S7b (0.3) b 2.22E-02 0.0041 184.60 3.51 720

148 S7b (0.3) b 2.22E-02 0.0014 63.03 5.49 1440

149 S7b (0.3) b 2.22E-02 0.001 45.02 8.09 1440

150 S7b (0.3) b 2.22E-02 0.0015 67.54 7.63 1440

151 S7b (0.8) 5.92E-02 0.0012 20.26 5.95 1440

152 S7b (0.8) b 5.92E-02 0.0023 38.83 5.80 1440

153 S7c (0.3) 8.22E-03 0.0024 291.94 4.58 1440

154 S7c (0.8) b 2.19E-02 0.0019 86.67 4.73 1440

Conditions: 6.68 bar ethylene, lhr reaction time, 54 μΐ. hexane added, 4000 equivalents EADC as co-catalyst, heptane as solvent; a g/mmol.h.bar; b repeated run.

[0080] The activities of pre-catalysts S4a, S7b and S7c were monitored through their consumption of ethylene (Figure 14). The pre-catalysts were first allowed a long contact time with EADC and ETA before the addition of ethylene. The uptake of ethylene intially rises and then plateaus after approximately one minute, thus indicating that the catalyst is only active for the first minute of the polymerization and then is effectively dead for the remainder of the run.

[0081] Conclusion

[0082] The representative results of the ethylene polymerization screening show that the activities of the pre-catalysts la/b - 10a can be classed as very high, particularly when MADC is employed as co-catalyst, e.g. table 3, runs 6, 12, 20, 29, 32, table 4, runs 35, 39, 49 and table 5, runs 69 - 72 and 79 - 81. All the catalyst systems produced essentially linear, high molecular weight polyethylene (mp by DSC ca 142 °C). [0083] We have shown that, under homogeneous conditions, the combination of a niobium or tantalum pre-catalyst bearing an imine-based ligand set and the co- catalyst MeAlCl 2 (MADC) is capable, in the presence of ETA, of polymerizing ethylene with activities in excess of 11,000 g/mmol.h.bar for niobium and 20,000 g/mmol.h.bar for tantalum. High activity is maintained at elevated temperatures when the ligand set also contains a phenoxide moiety. In the case of niobium, such activities are two orders of magnitude greater than any previously reported systems. Use of Me 2 AlCl (DMAC) or Me 3 Al 2 Cl3 (MASC) as the co-catalyst also yields highly active systems; activities also increase with increasing chloride content in the co-catalyst. Under more robust industrial conditions, use of MAO as co-catalyst for either the polymerization of ethylene or co-polymerization of ethylene with 1- hexene resulted in moderately active systems.

[0084] Related ligand systems, the polymerization of other a-olefins, and the identification of the active species are currently under investigation.

[0085] Experimental section

[0086] All manipulations were carried out under an atmosphere of nitrogen using standard Schlenk and cannula techniques or in a conventional nitrogen-filled glove- box. Solvents were refluxed over an appropriate drying agent, and distilled and degassed prior to use. Elemental analyses were performed by the microanalytical services at the London Metropolitan University. NMR spectra were recorded on a Varian VXR 400 S spectrometer at 400 MHz or a Gemini at 300 MHz ( R) at 298 K; chemical shifts are referenced to the residual protio impurity of the deuterated solvent. IR spectra (nujol mulls, KBr/CsI windows) were recorded on Perkin-Elmer 577 and 457 grating spectrophotometers. [0087] The precursors NbCl 4 (THF) 2 [14], NbOCl 3 [17], NbCl 3 (DME) [5], TaOCl 3 (DME) [17], (TaCl 4 ) n [10] and TaCl 3 (DME) [13] were prepared by the literature methods. The ligands L X H - L 3 were prepared using literature procedures.

[9] All other reagents were obtained commercially and used as received. Known complexes 3b [10], 4a [6b], 4b [6b] and 11a [7a] were synthesized according to the literature methods.

[0088] NMR studies of active species

[0089] Modified methylaluminoxane (MMAO) was prepared by vacuum distillation of commercial MAO sample at 50 °C; MMAO contains 1% (mol.) of Al in the form of "free" AlMe 3 . Mixing 4a with MMAO in toluene leads to the formation of a dark residue, which destroys the resolution of the NMR spectra. The amount of this residue can be decreased by the addition of 1,2-difluorobenzene or chlorobenzene into the toluene solution. Therefore, NMR spectroscopic studies of the system 4a/MMA0 were conducted in chlorobenzene.

[0090] The reaction of 4a with AlMe 3 /[CPh 3 ] + [B(C 6 F 5 ) 4 ]- at relatively low Nb concentrations (10 ~3 - 10 ~2 M) can be monitored in toluene. At higher Nb concentrations (>10 ~2 M), a dark precipitate is formed as in the case of the 4a/MMA0 system. Therefore, for 13 C NMR and 2-D NMR experiments, where high Nb concentrations are needed, chlorobenzene was used as a solvent. Niobium species formed in toluene and in chlorobenzene were similar. The resulting Ή NMR spectra are collected in the ESI.

[0091] Synthesis ofVNbCh (la)

[0092] A solution of (1.0 g, 2.36 mmol) and NbCl 5 (1.1 eq., 0.69 g, 2.60 mmol) in 30 ml toluene was refluxed for 12 h. The reaction mixture was cooled to room temperature and volatiles were removed in vacuo. The solid residue was extracted into hot acetonitrile. After prolonged standing at room temperature, deep red plates of la.MeCN formed (0.82 g, 56 % yield). MS (E.S.): m/z: 658 [M] + , 587 [M + -2C1] + , 851 [M + -3C1] + . IR (Nujol mull, KBr) ν Μ /™ 3279 (NH, br), 2289(w), 2257(w), 1587(m), 1568 (w), 1508(m), 1402(s), 1365(m), 1325(w), 1146(s), 924(m), 906(w), 877(s), 845(m), 766(m), 732 (w), 712 (m), 698(s), 670(m). Ή NMR (C 6 D 6 ) : δ = 9.87 (s, 1H, NH), 7.60 (dd, 2H, /j = 2.03, J 2 = 15.76, ArH], 7.47 (m, 1H, Ar-H], 7 AS (d, 1H, / = 1.69, ArH], 7.39-7.35 (overlapping m, 3H, ArH], 7.30 (m, 3H, ArH], 7.17 (m, 2H, ArH], 2.00 (s, 3H, Ctf 3 CN), 1.56 (s, 9 H, C(Ctf 3 ) 3 ), 1.43 (s, 9H, C(Ctf 3 ) 3 ). Found: C, 53.09; H, 4.62; N, 4.34. C 2 9H 3 iCl 4 N 2 NbO requires C, 52.91; H, 4.75; N, 4.26 %.

[0093] Synthesis of ^NbOChfMeCN) (lb)

[0094] As described for la but using NbOCl 3 (1.1 eq., 0.28 g, 1.30 mmol) and L 1 !! (0.50 g, 1.18 mmol), affording orange prisms of lb.3 (MeCN) (0.41 g, 54 % yield). MS (E.S.): m/z: 602 [M-CH 3 CN+H] + , 551 [M + -Cl-0+H] + . IR (Nujol Mull, KBr) ν Μ /™ 3572(w), 3405(w), 3195(NH, m br), 2307(m), 2278(m), 1950(w), 1883 (w), 1810(w), 1617(s), 1604(s), 1589(s), 1575(m), 1523(m), 1500(m), 1404(s), 1364(s), 1201(m), 1184(m), 1150(m), 1126(s), 1072(s), 975 (Nb=0, m), 910(s), 890(m), 846(s), 769(m), 697(s), 668(m). NMR ((CD 3 ) 2 CO): δ = 7.88 (d, 1Η, / = 2.36, ArH], 7.57 (d, 4H, / = 7.01, ArH], 7 AO (overlapping m, 7H, ArH], 2.68 (s, 3H, Ctf 3 CN), 1.49 (s, 9H, C(Ctf 3 ) 3 ), 1.33 (s, 9H, C(Ctf 3 ) 3 ). Found: C, 57.69; H, 5.33; N, 6.45. C 3 iH 34 Cl 2 N 3 Nb0 2 requires C, 57.78; H, 5.32; N, 6.52%.

[0095] Synthesis of^TaCU (lc) [0096] As described for la but using TaCl 5 (1.1 eq., 0.47 g, 1.30 mmol) and L^H (0.50 g, 1.18 mmol), affording dark red plates of lc.MeCN (0.50 g, 57 % yield). MS (E.S.): m/z: 746.1 [M] + , 675.1 [M + -2C1] . IR (Nujol Mull, KBr) v max /cm 1 3283 (NH, s), 2291(w), 2259(w), 1597(m), 1509(s), 1403 (s), 1326(m), 1286(s), 1177(s), 1147(s), 1128(s), 971(m), 928(s), 906(m), 881(s), 846(s), 771(s), 734(m), 698(s). NMR (C 6 D 6 ): δ = 8.517 (s, ΙΗ,), 7.67-7.65 (overlapping m, 2H, ArH], 7.23-7.15 (overlapping m, 7H, Ar-H], 7.04 (s, 1H, ArH], 6.80 (m, 1H, ArH], 6.67 (m, 1H, ArH], 1.67 (s, 9H, C(Ctf 3 ) 3 ), 1.23 (s, 9H, C(Ctf 3 ) 3 ), 0.56 (s, 3H, Ctf 3 CN). Found: C, 46.78; H, 4.07; N, 3.81. C 2 9H 3 iCl 4 N 2 OTa requires C, 46.67; H, 4.19; N, 3.75%.

[0097] Synthesis of ^TaOCblMeCN) (Id)

[0098] As described for la but using TaOCl 3 (DME) (1.1 eq., 0.47 g, 1.30 mmol) and L ! H (0.50 g, 1.18 mmol), affording orange powder (0.49 g, 51 % yield). MS (E.S.): m/z: 590.8 [M + -2Me-2Cl] . IR (Nujol Mull, KBr) v max /cm 1 3210 (br m, NH), 2258(w), 1672 (m, C=N), 1596 (m), 1524 (m), 1364(m), 1199(w), 1177(w), 922 (str, Ta=0), 872 (str), 770 (str), 698 (str). Ή NMR (C 6 D 6 ): δ = 7.55 (m, 2H, ArH], 7.46 (s, 1H, ArH], 6.93 (m, 2H, ArH], 6.76 (m, 4H, ArH], 6.61 (m, 1H, ArH], 1.55 (s, 3H, Ctf 3 CN), 1.36 (s, 9H, C(Ctf 3 ) 3 ), 1.07 (s, 9H, C(Ctf 3 ) 3 ). Found: C, 50.28; H, 4.24; N, 3.87. C 2 9H 3 iCl 2 N 2 0 2 Ta (sample dried in vacuo for 12hr, loss of MeCN) requires C, 50.38; H, 4.52; N, 4.05 %.

[0099] Synthesis ofL 2 NbCh (2a)

[00100] As described for la but using NbCl 5 (1.1 eq., 0.35 g, 1.30 mmol) and L 2 H (0.50 g, 1.19 mmol), affording red/orange crystals (0.56 g, 72 % yield). MS (E.S.): m/z: 657 [M] + , 622 [M + -C1] + , 550 [M + -3C1] + . IR (Nujol Mull, KBr) v^/cra 2297(w), 2271(w), 1601(w), 1572 (w), 1515(m), 1364(m), 1316(w), 1291(w), 1035(s), 996(m), 958(s), 922 (m), 885(m), 851(m), 774(m), 752 (s), 731(m), 686(m).

NMR ((CD 3 ) 2 CO): δ = 8.90 (t, 2H, / = 8.42, ArH), 8.6 - 8.54 (overlapping m, 1H, ArH], 8.47 - 8.46 (overlapping m, 1H, ArH], 8.20 (d, 1HJ = 2.49, ArH], 8.10 (d, 1HJ = 2.48, ArH], 7.84 -7.68 (overlapping m, 3H, ArH], 7.50 (d, 1H, / = 2.49, ArH], 3.68 (s, 3H, Ctf 3 CN), 1.46 (s, 9H, C(Ctf 3 ) 3 ), 1.35 (s, 9H, C(Ctf 3 ) 3 ). Found: C, 53.12; H, 4.22; N, 2.24. C 29 H 28 Cl 4 NNb0 2 requires C, 52.99; H, 4.29; N, 2.13 %.

[00101] Synthesis ofL 2 NbOCb(MeCN) f2bl [00102] As described for la but using NbOCl 3 (1.1 eq., 0.20 g, 0.91 mmol) and L 2 H (0.35 g, 0.83 mmol), affording orange/yellow crystals of 2b (0.34 g, 64 % yield). MS (E.S.): m/z: 603 [M-CH 3 CN+H] + , 566 [M + -CH 3 CN-C1] + . IR (Nujol Mull, KBr) Vmax/cm- 1 2296(w), 2270(w), 1601(w), 1571(w), 1515(m), 1315(w), 1291(w), 1201(m), 1157(m), 958(Nb=0, s), 922 (m), 885(m), 851(m), 751(s), 731(s). Ή NMR ((CD 3 ) 2 CO): δ = 9.0 (t, 2H, / = 8.09, ArH), 8.6 (d, 1H, / = 7.83, ArH], 8.54-8.49 (overlapping m, 1H, ArH], 8.20 (dd, 1H, Ji = 0.46, J 2 = 2.45, ArH], 7.95-7.73 (overlapping m, 4H, ArH], 7.62 (d, 1H, / = 2.44, ArH], 2.68 (s, 6H, 2Ctf 3 CN), 1.54 (s, 9H, C(Ctf 3 ) 3 ), 1.44 (s, 9H, C(Ctf 3 ) 3 ). Found: C, 57.91; H, 4.58; N, 2.51. C 2 9H 2 8Cl 2 NNb0 3 (sample dried in vacuo for 12h, loss of MeCN) requires C, 57.83; H, 4.69; N, 2.33 %.

[00103] Synthesis ofL 2 TaCh i2cl

[00104] As described for la but using TaCl 5 (1.1 eq., 0.47 g, 1.30 mmol) and L 2 H (0.5 g, 1.19 mmol), affording red microcrystals (0.64 g, 65 % yield). MS (E.S.): m/z: 745.1 [M] + , 706.1 [M-C1] + . IR (Nujol Mull, KBr) ν Μ /™ 2291(w), 2276(w), 1604(w), 1582(m), 1516(m), 1312 (w), 1297(w), 1202 (w), 1157(w), 924(m), 877(m), 852 (w), 751(s), 732(m), 722 (m), 686(w), 570(m). Ή NMR (C 6 D 6 ): δ = 8.31 (d, 1H, / = 8.30, ArH), 8.24 (d, 1H, / = 7.82, ArH], 8.04 (d, 1H, / = 2.24 ArH], 7.84 (d, 1H, / = 7.08, ArH], 7.75 (d, 1H, / = 2.2 , ArH], 7.61 (d, 1H, / = 7.72, ArH], 7 AO - 7.21 (overlapping m, 4H, ArH], 1.62 (s, 9H, C(Ctf 3 ) 3 ), 1.26 (s, 9H, C(Ctf 3 ) 3 ). Found: C, 46.59; H, 3.71; N, 1.84. C 29 H 28 Cl 4 N0 2 Ta requires C, 46.73; H, 3.79; N, 1.88 %.

[00105] Synthesis fL 3 NbCU (3a)

[00106] NbCl 4 (THF) (1.1 eq., 0.39 g, 1.46 mmol) and L 3 (0.50 g, 1.32 mmol) were refluxed in THF 40ml for 12hr, and then dried in vacuo. The residue was washed with hexane (2 x 20ml) and recrystallized from MeCN to give 3a as an orange powder (0.55 g, 62 % yield). MS (E.S.): m/z: 576 [M-C1] + , IR (Nujol Mull, KBr) Vmax/cm- 1 2311(m), 2284(m), 1645(C=N, w), 1582(w), 1510 (m), 1320 (m), 1198(m), 939(m), 782(m), 760(m), 722 (w). Ή NMR (CDC1 3 ): δ = 7.34 (m, 2H, ArH), 7.24 (m, 2H, ArH), 7.19 (m, 2H, Ar-H), 6.41 (s, 2H, N=CH), 2.85 (m, 4H, CH(Me) 2 ), 1.26 (d, 12HJ = 6.61 Hz, C(Ctf 3 ) 2 ), 1.19 (d, 12HJ = 6.28, C(Ctf 3 ) 2 ). EPR (toluene, 298 K) ^iso: 2.01, _4iso: 6G. Found: C, 50.96; H, 5.86; N, 4.48. C 26 H36Cl 4 N 2 Nb requires C, 51.08; H, 5.94; N, 4.58 %.

[00107] Synthesis ofL 5 ?Nb?CUO? Γ5Ί

[00108] A solution of NbCl 3 (DME) (0.55 g, 1.88 mmol) and L 5 (1.00 g, 2.07 mmol) in THF (40 ml) was refluxed for 12 h. The reaction mixture was cooled to room temperature and volatiles were removed in vacuo. The residue was washed with hexane (45 ml χ 2) and extracted into hot acetonitrile. After prolonged standing at room temperature, yellow plates of 5.4MeCN formed. (0.12 g, 13 % yield). IR (Nujol Mull, KBr) ν Μ /™ 2725(w), 1643 (s), 1588(w), 1396(w), 1190(w), 929(w), 865(w), 722 (m), 622 (w). Ή NMR (CDC1 3 ): δ = 7.19-7.16 (overlapping m, 12H, ArH), 2.78 (m, 4H, -Ctf(Me) 2 , 2.36 (s, 6H, Ctf 3 CN), 1.25 (s, 6H, C//3C-O), 1.17 (d, 24H, C(Ctf 3 ) 2 ). Found: C, 50.25; H, 5.09; N, 5.43. C 42 H 52 Cl4N 4 Nb 2 04 requires C, 50.22; H, 5.22; N, 5.58 %.

[00109] Synthesis ofL 6 NbCh (6a)

[00110] As described for la but using NbCl 5 (1.1 eq., 0.37 g, 1.38 mmol) and L 6 H (0.50 g, 1.25 mmol), affording dark yellow/brown plates (0.42 g, 53 % yield). MS (E.S.): m/z: 635 [M] + , 598 [M-C1] + . IR (Nujol Mull, KBr) ν Μ /™ 1585(m), 1552(m), 1484(m), 1365(m), 1330(w), 1202 (w), 1176(w), 11579(w), 982 (w), 923 (w), 891(w), 874(m), 753 (m). Ή NMR (CDC1 3 ): δ = 8.44 (s, 1H, CH=N], 7.80 (d, 1H, / = 2.24, ArH], 7.50 (dd, ΙΗ, /j = 2.24, J 2 = 7.89, ArH], 7.36 - 7.30 (overlapping m, 3H, ArH], 7.25 -7.18 (overlapping m, 1H, ArH], 7.13 - 7.06 (overlapping m, 4H, ArH], 6.90 (dd, ΙΗ, /j = 1.29, ] 2 = 8.27, ArH], 1.52 (s, 9H, C(C// 3 ) 3 ), 1.34 (s, 9H, C(C// 3 ) 3 ). Found: C, 50.92; H 4.74; N, 2.18; C 27 H 3 oCl 4 NNb0 2 requires C, 51.05; H, 4.76; N, 2.20 %. [00111] Synthesis ofL 6 TaCU f6bl

[00112] As described for la, but using TaCls (1.1 eq., 1.02 g, 2.86 mmol) and L 6 H (1.0 g, 2.6 mmol), affording orange needles (0.96 g, 51 % yield). MS (E.I.): m/z: 724 [MH] + , 687 [MH-C1] + , IR (Nujol Mull, KBr) ν Μ /™ 1641(C=N), 1602(m), 1586(m), 1553(m), 1365(m), 1201(w), 1176(m), 925(w), 875(m), 753(m), 699(m). NMR (C 6 D 6 ): δ = 7.89 (s, 1H, CH=N], 7.66 (m, 2H, ArH], 7.14 (m, 2H, ArH], 6.98 (m, 2H, ArH], 6.82 (m, 3H, ArH], 6.72 (m, 2H, ArH], 1.47 (s, 9H, ArH], 1.08 (s, 9H, C(Ctf 3 ) 3 ). Found: C, 44.79; H 4.12; N 2.00. C 2 7H3oCl 4 N02Ta requires C, 44.84; H, 4.18; N, 1.94 %.

[00113] Synthesis ofUNbCU (7a)

[00114] As described for la but using NbCl 5 (1.1 eq., 0.87 g, 3.24 mmol) and L7H (1.0 g, 2.95 mmol), affording dark red plates (0.72 g, 42 % yield). MS (C.I.): m/z: 573 [M]-, 536 [M-Cl]-, 521 [M-Me-Cl]-, IR (Nujol Mull, KBr) ν Μ /™ 1589(w), 1557(w), 1325(w), 1218(w), 1202(w), 1174(m), 923(w), 869(w), 757(m), 722(w).

NMR (C 6 D 6 ): δ = 7.66 (s, 1H, CH=N], 7.61 (d, 1H, / = 2.29, ArH], 7.48 (d, 1H, / = 7.79, ArH], 7.00 (t, 1Η, / = 7.90, ArH], 6.77 (t, 1HJ = 7.67, ArH], 6.68 (d, 1Η, / = 2.30, ArH], 6.53 (d, 1Η, / = 8.41, ArH), 3.33 (s, 3H, OMe], 1.48 (s, 9H, C(Ctf 3 ) 3 ), 1.05 (s, 9H, C(Ctf 3 ) 3 ). Found: C, 45.94; H, 4.80; N, 2.33. C 2 2H28Cl 4 NNb02 requires C, 46.10; H, 4.92; N, 2.44 %.

[00115] Synthesis ofUTaCh f7bl

[00116] TaCls (1.1 eq., 1.16 g, 3.24 mmol) and L7H (1.0 g, 2.95 mmol) were stirred at room temperature in toluene (40 mL) for 12 h. The solvents were removed in vacuo and the product was recrystallized from MeCN giving an orange powder (0.72 g, 37 % yield). MS (E.I.): m/z: 661 [M] + , 624 [M-C1] + , 609 [M-Me-Cl] + . IR (Nujol Mull, KBr) ν Μ /™ 1641 (C=N, w), 1593(w), 1555(s), 1495(w), 1409(w), 1301(w), 1228(w), 927(w), 872(m), 751(m), 723(m). 1 H NMR (CDC1 3 ): δ = 8.35 (s, 1H, Ctf=N), 7.91 (d, 1HJ = 8.01, ArH], 7.80 (d, 1Η,/ = 2.38, ArH], 7.41 (d, 1HJ = 7.13, ArH), 7.38 (d, IHJ = 2.32, ArH), 7.02 (dd, 1H J = 3.28, 8.24, ArH), 6.93 (dd, 1H J = 3.05, 8.36, ArH), 3.29 (s, 3H, OMe), 1.30 (s, 9H, C(Ctf 3 ) 3 ), 1.27 (s, 9H, C(Ctf 3 ) 3 ). Found: C, 39.84; H, 4.14; N, 2.01. C 2 2H28Cl 4 N02Ta requires C. 39.96; H, 4.27; N, 2.12 %.

[00117] Synthesis of (L 7 Me lTaCh (7c)

[00118] As described for la, but using TaCl 5 (1.1 eq., 1.16 g, 3.24 mmol) and L7H (1.0 g, 2.95 mmol), affording a red powder (0.86 g, 44 % yield). MS (E.I.): m/z: 609 [M] + , 594 [M-Me] + , 574 [M-C1] + , 558 [M-Cl-0] + . IR (Nujol Mull, KBr) ν Μ /™ 1651 (C=N, w), 1605(s), 1549(s), 1481(s), 1382(s), 1365(m), 1326(w), 1284(s), 1218(m), 1188(w), 1174(w), 924(w), 852 (s), 751(m), 696(w), 667(m). Ή NMR (CDC1 3 ) : δ = 8.41 (s, 1H), 7.65 (d, IHJ = 1.92, ArH), 7.22 (m, 3H, ArH), 6.87 (t, 1HJ = 7.53, ArH), 6.65 (m, 1H,), 1.41 (s, 9H, C(Ctf 3 ) 3 ), 1.27 (s, 9H, C(Ctf 3 ) 3 ). Found: C, 41.12; H, 3.95; N, 2.33. C 2 iH25Cl 3 N0 2 Ta requires C. 41.30; H, 4.13; N, 2.29%.

[00119] Synthesis ofL 8 NbCU (8a)

[00120] As described for la, but using NbCl 5 (1.1 eq., 0.83 g, 3.07 mmol) and L 8 H (1.0 g, 2.79 mmol), affording a dark red powder (0.78 g, 47 % yield). MS (E.I.): m/z: 480 [M-Ph-Cl+H] + , IR (Nujol Mull, KBr) ν Μ /™ 1645(C=N, m), 1583(m), 1548(m), 1275(s), 1217(s), 964(m), 872 (s), 693(s). Ή NMR (C 6 D 6 ): δ = 7.53 (dd, 1H, / = 7.64, 1.84 , ArH), 7.35 (s, 1H, CH=N), 7.11 (overlapping m, 2H, ArH), 7.01 (overlapping m, 2H, ArH), 6.76 (overlapping m, 5H, ArH), 6.16 (d, IHJ = 2.36, ArH). Found: C, 38.59; H, 1.96; N, 2.45; C^H^CleNC^Nb requires C, 38.55; H, 2.04; N, 2.37 %.

[00121] Synthesis ofL 8 TaCU f8bl

[00122] As described for la, but using TaCls (1.1 eq., 1.10 g, 3.07 mmol) and L 8 H (1.0 g, 2.79 mmol), affording a yellow powder (0.86 g, 44 % yield). MS (E.I.): m/z: 679 [M] + , 644 [M-C1] + IR (Nujol Mull, KBr) ν Μ /™ 2723 (m), 2251(m), 1948(w), 1779(w), 1646 (C=N, s), 1582(s), 1547(w), 1297(m), 1206(m), 907(w), 871(w), 848(m), 755(m), 727(m), 693 (m). Ή NMR (C 6 D 6 ): δ = 8.81 (s, 1H, CH=N], 7.83 (s, 1H, ArH], 7.36 (s, 1H, ArH], 7.04 - 6.96 (m, 3H, ArH], 6.92 - 6.78 (m, 3H, ArH], 6.71 - 6.55 (m, 2H, ArH], 6.30 (d, 1Η, / = 7.37, ArH] . Found: C, 33.42; H, 1.71; N, 2.15. Ci 9 Hi2Cl 6 N02Ta requires C, 33.56; H, 1.78; N, 2.06 %.

[00123] Synthesis of(L 8 HlTaC f8cl

[00124] TaCls (1.1 eq., 1.10 g, 3.07 mmol) and L 8 H (1.0 g, 2.79 mmol) were refluxed in toluene (50 mL) for 8h in a sealed Schlenk tube; recrystallization from acetonitrile (30 mL) afforded yellow needles of 8c (0.72 g, 32 % yield). MS (E.I.): m/z: 679 [M-C1] + , 644 [M-2C1] + . IR (Nujol Mull, KBr) ν Μ /™ 2855(m), 2257(w), 1781(w), 1649 (C=N, s), 1583(m), 1297(m), 1206(m), 1175(w), 907(w), 871(w), 848(m), 756(m), 728(m), 693 (m), 674(w). Ή NMR (C 6 D 6 ): δ = 10.49 (br s, 1H, C=NH), 8.95 (s, 1H, CH=N], 7.86 (s, 1H, ArH], 7.03 - 6.98 (overlapping m, 6H, ArH], 6.62 (overlapping m, 2H, ArH], 6.48 (s, 1H, ArH], 6.31 (d, 1HJ = 7.99, ArH]. Found: C, 31.98; H, 1.74; N, 1.86. Ci 9 Hi 3 Cl7N0 2 Ta requires C, 31.85; H, 1.83; N, 1.96 %.

[00125] Synthesis ofL 9 NbCU (9a) [00126] As described for la, but using NbCl 5 (1.1 eq., 1.00 g, 3.71 mmol) and L 9 H (1.0 g, 3.38 mmol), affording a yellow powder (1.16 g, 65 % yield). MS (E.I.): m/z: 494 [M-C1] + , 478 [M-Me-Cl] + , 444 [M-Me-2C1] + , IR (Nujol Mull, KBr) ν Μ /™ 2738(w), 2362 (w), 1797(w), 1634(C=N, m), 1601(m), 1584(m), 1543(m), 1301(m), 1167(w), 898(m), 875(m), 753 (m), 723 (m), 633(m). NMR ((CD 3 ) 2 CO) : δ = 8.97 (s, 1H, CH=N], 7.80 (s, 1H, ArH], 7.71 (d, 1H, / = 2.67, ArH], 7.66 (s, 1H, ArH], 7.55 (t, / = 7.19, 1H, ArH], 7.23 (overlapping m, 2H, ArH]. Found: C, 31.59; H, 2.03; N, 2.52. Ci 4 HioCl 6 NNb02 requires C, 31.73; H, 1.90; N, 2.64 %.

[00127] Synthesis ofL 9 TaCU f9bl [00128] As described for la, but using TaCls (1.1 eq., 1.33 g, 3.71 mmol) and L 9 H (1.0 g, 3.38 mmol), affording a dark red powder (1.13 g, 54 % yield). MS (E.I.): m/z: 618 [M+H] + , 583 [M+H-C1] + , IR (Nujol Mull, KBr) ν Μ /™ 2723(w), 1791(w), 1633(C=N, m), 1583(m), 1542(w), 1493(m), 1317(m), 1303(m), 1183(m), 1168(m), 903(m), 840(m), 753(m), 730(m), 674(w). Ή NMR (C 6 D 6 ): δ = 8.45 (s, 1H, CH=N], 7.73 (d, lH, / = 8.45, ArH], 7.47 (m, 1H, ArH], 7.34 (dd, 1Η, / = 1.66, 7.88, ArH], 7.21 (d, lH, / = 7.51, ArH], 7.05 (m, 2H, ArH], 3.85 (s, 3H, OCH 3 ). Found: C, 27.13; H, 1.53; N, 2.19. Ci 4 HioCl 6 N02Ta requires C, 27.21; H, 1.63; N, 2.27 %.

[00129] Synthesis ofL w NbCh f lOal

[00130] As described for la, but using NbCl 5 (1.0 eq., 0.743 g, 2.75 mmol) and L 10 H (1.0 g, 2.75 mmol), affording a dark red powder (0.56 g, 35 % yield). MS (E.I.): m/z: 583 [M] + . IR (Nujol Mull, KBr) ν Μ /™ 2723(m), 1775(w), 1594(m), 1551(m), 1298(m), 1213(w), 1174(s), 914(w), 900(w), 878(m), 760(m), 723(m), 674(w). Ή NMR (C 6 D 6 ): δ = 7.34 (m, 1H, ArH], 7.09 (s, 1H, CH=N], 6.96 (m, 1H, ArH], 6.72 (overlapping m, 3H, ArH], 6.21 (d, 1H, / = 2.43, ArH]. 19 F NMR (C 6 D 6 ) δ: -56.4 (s). Found: C, 28.69; H, 1.13; N, 2.34. requires C, 28.80; H, 1.21; N, 2.40 %.

[00131] Typical Ethylene Polymerization Procedure

[00132] A typical polymerization run consists of flame drying a (250 mL) pyrex flask, which is then purged several times with ethylene gas at 1 bar pressure. This pressure is maintained throughout the polymerization run. 100 ml of dry, degassed toluene is added, as well as ethyltrichloroacetate (ETA, 0.05 mL, 0.36 mmol) and stirred for 10 minutes to allow for saturation. The correct temperature is achieved using a stirrer hotplate fitted with temperature probe and via a water bath. The co-catalyst is then added and then the pre-catalyst is injected as a toluene solution. The polymerization time was recorded from injection of pre-catalyst and left for the required time. The polymerization solution was quenched by transferring the reaction solution to a 500 mL beaker containing methanol and acidified water. The solid polyethylene was filtered and dried in an 80°C oven overnight.

[00133] Typical Parallel Pressure Reactor Polymerization Run

[00134] A pre-weighed glass vial with stirring paddles was sealed and purged with ethylene. 5 μηιοΐ of co-catalyst from a 100 mM heptane solution was added along with co-monomer (if required). Heptane was then added to reach a volume of 4000 in the reaction vessel and heated to 80 °C. The ethylene pressure was set to 92 psi (6.34 bar) and the catalyst (along with ETA) was added as a heptane slurry. The run was left stirring for 60 minutes and quenched with C0 2 (35 % in N 2 ). The glass vial was dried by vacuum centrifuge and weighed.

[00135] X-ray Photoelectron Spectroscopy Analysis [00136] A small amount (~mg) of powdered sample was pressed onto adhesive carbon tape. High resolution XPS core level measurements were performed with a VG Escalab 250 in LENNF, Leeds University, equipped with a conventional hemispherical sector analyser and controlled by a VGX900 data system. XPS experiments were carried out using a high intensity monochromated Al-Kct source (1486.6 eV) operated at 15 kV and 20 mA. V-2p, C-ls , Cl-2p and N-ls spectra were recorded using a pass energy of 20 eV. The energy scale of the spectrometer was calibrated to the Ag-3ds/ 2 peak at 368.3 eV. The binding energy scale was calibrated to the C-ls signal at 284.5 eV. High-resolution peak fitting was performed using CasaXPS software (version 2.3.15).

[00137] Crystallographic analyses

[00138] Crystals, under oil, were mounted on glass fibres and fixed in the cold nitrogen stream on a diffractometer. For compounds la, lb, lc, 2b, 5, 6a, and 7a, intensity data were measured, by thin-slice ω- and φ-scans, on an Oxford Diffraction Xcalibur-3/Sapphire3-CCD diffractometer, equipped with Μο-Κα radiation and graphite monochromator. Data were processed using the CrysAlisPro-CCD and -RED [18] programs. [00139] Data for compounds 6b, 8a and 8c were measured on an AFC12 (Right), Kappa 3-circle diffractometer with a Rigaku Saturn724+ CCD detector, molybdenum radiation and a confocal monochromator, at the National Crystallography Service at the University of Southampton, and were processed with the CrystalClear-SM Expert 2.0 r7 [19] programs in Southampton. [00140] The structures of all structures were determined by the direct methods routines in the SHELXS program [20a] or, for lb, with SIR-2004 [21] and refined by full-matrix least-squares methods, on F 2 's, in SHELXL [20b]. The non- hydrogen atoms in most structures were refined with anisotropic thermal parameters; however, the data for 6b were from a very small crystal and did not permit anisotropic refinement of the carbon atoms. Hydrogen atoms were included in idealised positions and their Uiso values were set to ride on the Ueq/Uiso values of the parent carbon or nitrogen atoms.

[00141] Crystal data and refinement results for the ten structures are collated in Table 11. [00142] Scattering factors for neutral atoms were taken from reference [22] . Computer programs used in this analysis have been noted above, and were run through WinGX [23] on a Dell Precision 370 PC at the University of East Anglia.

[00143] References 1] See for example (a] V. Busico, Dalton Trans., 2009, 8794. (b] D. Takeuchi, Dalton Trans., 2010, 311.

2] (a] C. Redshaw, Dalton Trans., 2010, 5595. (b] K. Nomura and W. Zhang, Chem.

Rev. 2011, 111, 2342. (c] J.-Q. Wu and Y.-S. Li, Coord. Chem. Rev. 2011, 255, 2303. (a] V. C. Gibson, C. Redshaw and G. A. Solan, Chem. Rev. 2007, 107, 1745. (b) C. Redshaw in: Olefin Upgrading Catalysis by Nitrogen-based Metal Complexes I, Eds. J. Campora and G. Giambastiani, Springer Science + Business Media B.V. 2011.

C. Andes, S. B. Harkins, S. Murtuza, K. Oyler and A. Sen, /. Am. Chem. Soc, 2001, 123, 7423.

(a] R. Galletti and G. Pampaloni, Coord. Chem. Rev. 2010, 254, 525. (b] Y. R. Patil in: Olefins Polymerization Reactivity of Niobium-Based Metal Complexes, US, Lambert Academic Publishing, 2011.

(a] J. Jaffart, C. Nayral, R. Choukroun, R. Mathieu and M. Etienne, Eur. J. Inorg. Chem, 1998, 425. (b) Y. Nakayama, N. Maeda and T. Shinono, Stud. Surf. Sci. Catal. 2006, 161, 165. (c) F. Marchetti, G. Pampaloni, Y. Patil, A. M. Raspolli Galletti, F. Renili and S. Zacchini, Organometallics, 2011, 30, 1682. (d] F. Marchetti, G. Pampaloni, Y. Patil, A. M. Raspolli Galletti and S. Zacchini, Polymer Science, 2011, 49, 1664. (e) F. Marchetti, G. Pampaloni, Y. Patil, A. M. Raspolli Galletti and M. Hayatifar, Polymer International, 2011, 60, 1722. (f) K. Mashima, Y. Nakayama, N. Ikushima, M. Kaidzu and A. Nakamura, /. Organomet. Chem. 1998, 566, 111.

(a] C. Redshaw, D. M. Homden, M. A. Rowan and M. R. J. Elsegood, Inorg. Chimica Acta, 2005, 358, 4067. (b] C. Redshaw, M. Rowan, D. Homden, M. Elsegood, T. Yamato and C. Perez-Casas, Chem Eur], 2007, 13, 10129.

K. F. Hirsekorn, E. B. Hulley, P. T. Wolczanski and T. R. Cundari, / Am. Chem. Soc. 2008, 130, 1183.

A. 0. Eseola, W. Li, R. Gao, M. Zhang, X. Hao, T. Liang, N. 0. Obi-Egbedi, and W.-H. Sun, Inorg. Chem. 2009, 48, 9133.

H. Tsurugi, T. Saito, H. Tanahashi, J. Arnold and K. Mashima, /. Am. Chem. Soc. 2011, 133, 18673.

E. J. Roskamp and S. F. Pedersen,/ Am. Chem. Soc. 1987, 109, 6551.

A. Zanotti-Gerosa, E. Solari, L. Giannini, C. Floriani, N. Re, A. Chiesi-Villa and C. Rizzoli, Inorg. Chimica Acta, 1998, 270, 298. J. F. O'Keefe, Rubber World, 2004. 230, 27.

S. F. Pedersen, J. B. Hartung, E. J. Roskamp, P. S. Dragovich, C. J. Ruffing and B. A. Klein, Inorg. Synth., 2007, 29, 119. a] V. Murphy, X. Bei, T. R. Boussie, 0. Brummer, G. M. Diamond, C. Goh, K. A. Hall, A. M. LaPointe, M. Lecerc, J. M. Longmire, J. A. W. Shoemaker, H. Turner and W. H. Weinberg, The Chemical Record, 2002, 2, 278. b] T. R. Boussie, G. M. Diamond, C. Goh, K. A. Hall, A. M. LaPointe, M. Leclerc, C. Lund, V. Murphy, J. A. W. Shoemaker, U. Tracht, H. Turner, J. Zhang, T. Uno, R. K. Rosen and J. C. Stevens, /. Am. Chem. Soc, 2003, 125, 4306.

K. Nomura and W. Wang, Macromolecules, 2005, 38, 5905.

F. Marchetti, G. Pampaloni and S. Zacchini,.Da/ton Trans., 2008, 7026.

Programs CrysAlisPro, Oxford Diffraction Ltd., Abingdon, UK (2010].

Programs CrystalClear-SM Expert 2.0 r7, Rigaku Corporation, Tokyo, Japan (2011].

G. M. Sheldrick, SHELX-97 - Programs for crystal structure determination (SHELXS] and refinement (SHELXL], Acta Cryst. 2008, A64, 112.

M.C. Burla, R. Caliandro, M. Camalli, B. Carrozzini, G.L. Cascarano, L. De Caro, C. Giacovazzo, G. Polidori and R. Spagna, SIR2004 - an improved tool for crystal structure determination and refinement,/. Appl. Cryst. 2005, 38, 381.

'International Tables for X-ray Crystallography' , Kluwer Academic Publishers, Dordrecht (1992]. Vol. C, pp. 500, 219 and 193.

L. J. Farrugia,/ Appl. Cryst, 1999, 32, 837.

L. Clowes, (2011], Group V pro-catalysts for the polymerisation of ethylene and ε- caprolactone. PhD thesis. Unversity of East Anglia. Compound la lb lc 2b 5

Formula C 29 H 31 CLN 2 NbO.C 2 H 3 N C 31 H 34 Cl 2 N 3 Nb0 2 .3 (C 2 H 3 N) C 29 H 31 CL,N 2 0-Ta.C 2 H 3 N C 31 H 31 Cl 2 N 2 Nb0 3 C 4 2 CLN 4 0 4 -Nb 2 .4(C 2 H 3 N)

Formula weight (g mol) 699.3 767.6 787.4 643.4 1 168.71

Crystal system Orthorhombic Orthorhombic Orthorhombic Triclinic Monoclinic

Space group Pbca Pbcn Pbca P-l P2,/c

Unit cell dimensions

a (A) 12.1825(3) 26.4887(6) 12.20751( 12) 9.2530(3) 17.3058(5) b (A) 19.4186(5) 14.1088(2) 19.4381(2) 9.6926(3) 10.4825(3) c (A) 26.5342(5) 20.7568(5) 26.5521(3) 16.9423 (4) 15.1775(5)

« 0 90 90 90 91.042(2) 90

β {°) 90 90 90 102.538(2) 93.632(3)

y 90 90 90 100.719(2) 90

6277.1 (3) 7757.3 (3) 6300.55(1 1) 1454.64(7) 2747.79(14) z 8 8 8 2 2

Temperature (K) 140(1) 140(1) 140(1) 140(1) 140(1)

Calculated density (Mg.m ! ) 1.480 1.314 1.660 1.469 1.413

Absorption coefficient (mm "1 ) 0.753 0.487 3.858 0.632 0.660

Transmission factors, max/min. 1.081 and 0.912 1.094 and 0.889 1.204 and 0.650 1.087 and 0.896 1.000 and 0.761

Crystal size (mm) 0.24 x 0.15 x 0.015 0.33 x O.18 x O.18 0.42 x 0.22 x 0.06 0.30 x 0.18 x 0.12 0.27 x 0.16 x 0.09 e(max) (°) 25.0 25.0 30.0 27.5 32.53

Reflections measured 79369 104864 1 19629 24291 57508

Unique reflections, Rm t 5518, 0.143 6819, 0.076 9179, 0.058 6659, 0.053 9513 , 0.106

Reflections with F 1 > 2o(F 2 ) 3062 5021 6566 5231 5835

Number of parameters 362 440 362 353 309

Ri, wR 2 [f i > 2o(i 2 )] 0.034, 0.041 0.064, 0.136 0.021, 0.037 0.032, 0.080 0.055, 0.11 1

Ri, wR 2 (ah data) 0.099, 0.047 0.089, 0.142 0.040, 0.039 0.046, 0.082 0.109, 0.129

GOOF (S) 0.743 1.197 0.893 0.983 0.967

Largest difference peak and hole (e A "3 ) 0.90 and -0.58 0.83 and -1.26 0.70 and -0.56 0.73 and -0.55 0.98 and -0.54

6a 6b 7a 8a 8c

C 27 H 3 „CL,N 02 C 27 H 3 oCl 4 N0 2 Ta 2(C 22 H 28 CUNNb-0 2 ).C 2 H 3 N C 19 H 12 C¾NNb02 Ci,H 13 Cl 7 N0 2 Ta.-C 2 H 3 N

635.23 723.27 1187.38 591.91 757.46

Triclinic Triclinic Triclinic Orthorhombic Monoclinic

P-l P-l P-l P2 1 2,2 1 P2 ! /n

12.1954(2) 12.1398(6) 13.0985(7) 10.868(4) 8.183(2)

12.6842(2) 12.6559(6) 13.7170(7) 11.314(4) 13.749(4)

18.5633(4) 18.4905(13) 17.7489(8) 35.538(10) 22.878(6)

89.5122(14) 89.558(6) 99.982(4) 90 90

86.343(2) 86.364(6) 101.996(4) 90 94.750(4)

86.3393(14) 86.546(6) 118.206(5) 90 90

2859.81(9) 2830.0(3) 2608.9(2) 4370(2) 2565.2(11)

4 4 2 8 4

140(1) 100(2) 140(1) 100(2) 100(2)

1.475 1.698 1.511 1.799 1.961

0.819 4.286 0.892 1.301 5.04

1.000 and 0.853 1.000 and 0.702 1.000 and 0.890 1.000 and 0.791 1.000 and 0.752

0.31 x O.22 x O.15 0.04 x 0.03 x O.01 0.3 x 0.2 x 0.01 0.38 x 0.04 x 0.02 0.40 x 0.07 x 0.03

30.0 ? 25.0 22.5 27.53 30.0

56330 27028 28059 27674 26828

16648, 0.047 9890, 0.071 6787, 0.083 9973, 0.051 7472, 0.043

13655 7902 5308 9191 6847

631 362 569 524 299

0.037, 0.079 0.087, 0.210 0.058, 0.119 0.048, 0.091 0.036, 0.062

0.050, 0.083 0.105, 0.216 0.082, 0.128 0.053, 0.094 0.042, 0.064

1.035 1.153 1.064 1.083 1.093

1.66 and -0.89 4.86 and -3.47 1.92 and -1.06 1.12 and -1.01 1.44 and -1.38

Table 11. Crystallographic data for compounds la-c, 2b, 5, 6a, 6b, 7a, 8a and 8c.