FIELD

The presently disclosed embodiments related to processes for producing ammonia, hydrazine and organic amines by reduction of dinitrogen with transition metal containing organometallic compounds and complexes. BACKGROUND

Reduction of dinitrogen by cleavage of the relatively inert dinitrogen (N2) molecule resulting from its extremely strong N≡N triple bond, has represented a major challenge toward development of synthetic processes for commercially useful nitrogen compounds, using molecular dinitrogen, particularly under mild conditions. The currently used Haber-Bosch process for the synthesis of ammonia, which involves reaction of hydrogen and nitrogen at high temperatures and pressures, using an iron catalyst, requires pressures of 200-300 atmospheres and temperatures as high as 400-500°C and results in only 15% conversion. This low efficiency, along with the need to effect the reduction under milder conditions, has instigated intense research in nitrogen reduction since the early 1960's.

Although modifications of the Haber-Bosch process, such as, for example the Kellogg Advanced Ammonia Process (KAAP), have succeeded to some extent in increasing efficiency while lowering the temperatures and pressures for effecting the reduction reaction, such processes still require relatively high temperatures and high pressures (~ 40 atmospheres), and involve complex, expensive equipment. Therefore, there is continued interest in developing a catalytic system that can effect reduction of dinitrogen at low temperature and ambient pressure.

The metalloenzyme nitrogenase constitutes a unique biological nitrogen-fixing system capable of nitrogen triple bond cleavage at ambient temperatures and pressures. Nitrogenase catalyzes the reduction of molecular nitrogen to ammonia, together with the production of dihydrogen, under relatively mild conditions. Although the structure of the enzyme nitrogenase, which converts dinitrogen to ammonia at ambient conditions, from different bacteria, has been elucidated in atomic detail, the mechanism by which the enzyme binds to the substrate to effect the reduction still remains largely unknown. Efforts to mimic this unique biological system have therefore been unsuccessful.

Various transition metal complexes have been studied for catalytic activity to effect dinitrogen reduction, mainly due to the discovery of their presence in nitrogenase enzymes in biological systems. This is based on the principle wherein atmospheric nitrogen is "fixed" at normal temperatures and pressures by several natural organisms. The enzyme nitrogenase is responsible for this. Nitrogenase contains clusters of iron, molybdenum and sulfur (Fe/Mo/S). Although several artificial metal clusters have been made to generate such catalytic activity, no demonstrable activation of the N≡N triple bond has been seen in such complexes. This may be attributable to the high coordination number of the complexing metal (typically greater than 3) in these artificial metal cluster compounds, which may result in the metal not being in a highly activated state.

Transition metal complexes with lower metallic oxidation states, such as vanadium in the +2 and +3 oxidation state, have been studied partly due to their presence in one of the nitrogenases, and also due to their ability to produce ammonia and/or hydrazine when present under the appropriate conditions. Most of the work in this area has involved organometallic as well as deprotonated amine-ligands in aprotic media. Although it has been postulated Vanadium (III) may be present in the reduced state of the alternative nitrogenase that contains a VFe-cofactor, V3+ is not strong a reductant, thereby requiring strongly electron-donating ligands in order to become suitable for binding dinitrogen. Although V3+ is found in the literature to bind dinitrogen, there are no examples of V3+ effecting the reduction of dinitrogen to ammonia or hydrazine.

In addition, the strong basicity of such systems renders them prone to undesired side reactions, especially oxidation and proton abstraction. The use of low-valent vanadium complexes for nitrogen fixation at atmospheric pressure in the art is that by Becker et al. (J. Electroanal. Chem., Vol. 250, pp. 385-397, (1988)) which describes an electrolytic process for reduction of dinitrogen to ammonia using V2+ complexes with mixed O,N-donor ligands consisting of aromatic nitrogen containing heterocyclic hydroxyls. The process disclosed in Becker et al. however give low yields of ammonia (maximum yields of 7.54% and 15.2 % for V2+complexes with 2,3-dihydroxypyridine and 1,2-naphthalenediol ligands respectively, in methanol). A V2+catecholate complex disclosed by Shilov et al. (Kinet. Katal. 1972, 5, 1602 ; Nouv. J. Chim. 1982, 6, 245) in methanol, on the other hand, results in 0.75 mol NH3/ mol V. However, that process requires a reaction process of 15 atmospheres, thereby requiring high- pressure reactors and safeguards that render it expensive.

Although the advantageous properties of a multi-metallic metal-cluster system over a mono-metallic one for substrate activation and the capability of multiple coordination of the substrate to the metal clusters and the multi-electron transfer between the substrate and the cluster were recognized, transition metal clusters in the art have been largely restricted to metal clusters containing carbonyl ligands. The use of metal-clusters as catalysts, especially of reduction of dinitrogen is unknown. U.S. Patent Nos. 6,037,459 and 6,117,407 to Cummings et al. disclose dinitrogen cleavage to produce ammonia by reacting dinitrogen with a low oxidation state transition metal complex comprising ligands that are sufficiently bulky such that dimerization does not occur. The Cummings et al. patents, therefore, disclose that metal-clusters wherein metal dimerization and trimerization occurs, is undesirable for dinitrogen reduction.

SUMMARY

Methods and processes for dinitrogen reduction are disclosed herein.

According to aspects illustrated herein, there are provided transition metal complexes comprising clusters of transition metals or "metal-clusters" that are effective in the dinitrogen reduction reactions. More specifically, the transition metal complexes effect reduction of dinitrogen to provide ammonia, hydrazine and organic amines at atmospheric pressure under relatively mild reaction conditions. The metal-cluster transition metal complexes effect dinitrogen reduction catalytically, and can therefore be regenerated or recycled (5 to 10 cycles) for repeated use. The metal-cluster transition metal complexes overcome the limitation of requiring non-catalytic amounts of previously metal complexes for effecting reduction of dinitrogen, by bringing together a plurality of metal atoms in close vicinity to effect multi -metallic activation of the substrate resulting from the cooperative action of a collection of metal centers, thereby providing the necessary reducing equivalents of the catalyst.

According to aspects illustrated herein, there are provided metal compounds capable of effecting reductive cleavage of the N≡N triple bond in dinitrogen (N2) and dinitrogen containing compounds or complexes to produce ammonia, hydrazine and organic amines. The transition metal compounds are preferably organometallic complexes comprising a transition metal selected from the group consisting of molybdenum, titanium, vanadium, niobium and tungsten and a plurality of ligands coordinated to the metal atom such that the oxidation state of the transition metal atom is no more than three. The term "oxidation number", as used herein, refers to the difference (positive or negative) between the number of electrons associated with an metal atom in a compound, namely the organometallic complex, as compared with an atom of the elemental metal.

According to aspects illustrated herein, there is provided an organometallic compound of Fomula (I) for activation and reduction of dinitrogen and dinitrogen containing compounds.

wherein M is a transition metal selected from the group consisting of Mo, Ti, V, Nb, and W;

X = halogen;

n = 2,3;

m = l - 3

p = l - 5

Ri is H or alkyl; and

R2 and R3 are each independently -(CH2)qR4-(CH)R5θ, wherein R4 and R5 are independently H or lower alkyl, and q = 1 to 3.

According to aspects illustrated herein, there are provided soluble, metal complexes that effect the formation of ammonia, hydrazine and organic amines from dinitrogen. The metal complex comprises a two or three coordinate, low oxidation state transition metal complex. The metal complex comprises a metal selected from the group consisting of molybdenum, titanium, vanadium, niobium and tungsten, and a plurality of organic and inorganic ligands coordinated to the metal such that the metal has a coordination number of no more than three. The organic ligands in the metal complexes are amino alcohols comprising one or more amine nitrogens with at least two hydroxyl (OH) groups that function as multidentate ligands that are capable of bridging a plurality of metal atoms (metal centers) to effect the formation of metal -cluster complexes. According to aspects illustrated herein, there are provided metal-cluster complexes comprising a plurality of transition metal atoms selected from the group consisting of molybdenum, titanium, vanadium, niobium and tungsten, wherein the metal-cluster is formed by bridging the transition metal atoms by one or more N,O,O'-donating multidentate ligands of the formula [NR4[(CH2)q (CH)R5O]2 that are coordinated to the transition metal atoms. Examples of N,O,O'-donating multi-dentate include, but are not limited to, bis-hydroxyl aminoalcohol ligands.

According to aspects illustrated herein, there is provided a process for the production of ammonia, hydrazine and organic amines from dinitrogen by contacting a solution comprising the metal complex with dinitrogen in the presence of a hydrogenation agent or a reducing agent under atmospheric pressures and at ambient temperatures. Without wishing to be bound by theory, it is proposed that a metal-dinitrogen complex which, is then reduced in the presence of the hydrogenation agent source to give ammonia (NH3) and/or hydrazine (N2H4). Preferred hydrogenation agents include, but are not limited to, hydrogen, metallic hydrides such as sodium and potassium hydride, complex hydrides such as sodium borohydride, potassium borohydride, sodium cyanoborohydride and lithium aluminum hydride.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows ammonia (FIG. l(a)) and hydrazine (FIG. l(b)) yields of the reaction of BHEEN and [VCIs(THF)3] as a function of the ligand-to-metal ratio. Four equivalents of NaH were used for each mol of BHEEN. The yields are expressed as moles produced per mol vanadium used.

FIG. 2 shows plots of hydrazine (FIG. 2(a)) and ammonia (FIG. 2(b)) yields as mol produced per mol V for the reaction of MDEA and [VCl2(tmeda)2].

FIG. 3 shows an electrospray mass spectrum of the transition metal complex [VCIa(BHEEN)3].3THF. The inset shows a simulated isotope pattern of a metal complex of the formula [C22H47NgNaV2Oo] resulting from coordination Of N2 with transition metal complex [V(t-BuDEA)3(N2)].3THF. FIG. 4 shows an electrospray mass spectrum of [VCt-BuDEA)3(N2)].3 THF resulting from coordination of the metal-cluster complex [VCl2(t-BuDEA)3].3THF with N2.

DETAILED DESCRIPTION

The presently disclosed embodiments provide metallic compounds, in particular, transition metal complexes for effecting the reduction of dinitrogen and compounds or complexes containing dinitrogen to produce ammonia, hydrazine and organic amines.

In an embodiment, the transition metal complexes are described by the formula:

wherein M is a transition metal selected from the group consisting of Mo, Ti, V, Nb, and W;

X = halogen;

n = 2,3;

m = l - 3

p = l - 5

Ri is H or alkyl; and

R2 and R3 are each independently -(CH2)qR4-(CH)R5θ, wherein R4 and R5 are independently H or lower alkyl, and q = 1 to 3.

In a preferred embodiment, the transition metal is vanadium having an oxidation state of +2 (represented as V(II)), and X is chloride (Cl), and m and n = 2.

In a preferred embodiment, the transition metal is vanadium having an oxidation state of +3 (represented as V(III)), and X is chloride (Cl), and m and n = 3.

The transition metal complexes comprise one or more organic ligand of the general formula:

(NRiR2R3)

wherein Ri is H or alkyl; and R2 and R3 are each independently -R4(CH2)q (CH)RsO, wherein R4 and R5 are independently H or lower alkyl, and q = 1 to 3. In an embodiment, the transition metal complexes are described by general formula (II):

[VCl2NR4[CCH2)C1(CH)R5O]2]P (II)

wherein R4 and R5 are independently H or lower alkyl, p = 1-5 and q = 1 to 3.

In a currently preferred embodiment, the transition metal complex of formula (II) for reduction of dinitrogen (nitrogen fixation) comprises N,O,O' -donating aminoalcohol ligands of the formula [NR4[(CH2)q (CH)RsO]2 that coordinated to the vanadium 2+ atom. Examples of N,O,O'-donating aminoalcohol ligands present in transition metal complexes of formula (II) include, but are not limited to, diethanolamine (DEA), N- methyldiethanolamine (MDEA), N-n-butyldiethanolamine (11BuDEA), N-t- butyldiethanolamine (1BuDEA), diisoproanolamine ((1PrOH)2NH) and N- methyldiisoproanolamine ((1PrOH)2NMe). The chemical structures of these ligands are shown in Scheme 1.

(a)-DEA (b)-MDEA (c)"BuDEA

(d)-'BuDEA (e)-('PrOH)2NH (0-('PrOH)2NMe

Scheme 1. Chemical structures N, O, O' -donating aminoalcohol ligands

The yields of ammonia and hydrazine from reduction of dinitrogen with transition metal complexes of formula (II) comprising ligands listed in Scheme 1 are shown in Table 1 below.

Table 1. Ammonia and hydrazine yields for dinitrogen reduction with Formula (II) complexes obtained by reaction of [VCl2(tmeda)2] with respective ligands (L).

(a) The reaction mixtures were analyzed for ammonia and hydrazine by the indophenol and 4-(dimethylamino)benzaldehyde tests, respectively, by known methods (Weatherhead, M. W. Anal. Chem. 1967, 39, 971; Watt, G. W.; Chrisp, J. D. Anal. Chem. 1952, 24, 2006).

(b) The yield was calculated from the percentage of electrons used for fixation per V [(3 * mol NH3 + 4 * mol N2H4)/ 3].

With the exception of diethanolamine (DEA), which does not provide a suitable

environment for the binding and/or reduction of the dinitrogen, the N,O,O'-donating

aminoalcohol ligands of Scheme 1 , including both ν-Substituted DEAs and DEAs with a methyl group on the same carbon as the oxygen, provide a suitable environment for fixation and result in reduction of dinitrogen to ammonia and hydrazine. This may be due to more favorable electronic and steric characteristics in these ligands, since they become better σ-donors and can, as a result, stabilize higher oxidation states of vanadium. Formula II complexes comprising ν-substituted DEA ligands give similar overall yields and are largely independent of the size of the alkyl group. However, introduction of a

methyl group adjacent to the oxygen atom as in the (1PrOH)2NH ligand causes a noticeable drop in the total yield of reduction products. An even larger drop is observed when methyl

groups are introduced on the nitrogen and next to both oxygen atoms, which may result in steric crowding around vanadium. Thus, the nitrogen may be bonded to one vanadium atom and the oxygen is bridging between the vanadium atoms. The hydrazine yield is much greater than the ammonia yield. This may be attributed to the higher electronic requirement for the production of ammonia, which is six electrons/ N2, compared to four electrons/ N2 for production of hydrazine. The total yields observed for the diethanolamines (DEAs) reach a maximum of 39% for MDEA, which is substantially higher than those obtained by prior art methods for reduction of dinitrogen with transitional metal complexes.

The highest yield of both products was obtained when a ligand-to-metal (L:M) stoichiometry of about 1 :1 is used. The product yields drop substantially when the ratio of either the ligand or the metal is increased from this optimal ratio. FIG. 1 shows the change product yields for varied L:M stoichiometrics (metal:ligand ratios). Without wishing to be bound by theory, it is postulated that when the ligand is present in excess of the metal, the ligands occupy more coordination sites at the metal, and thereby renders them unavailable for binding dinitrogen. The lack of access to the metal when the ligand is present in excess of the metal may, therefore, be responsible for the inability of complexes with high M:L ratios (excess ligand) to fix or dinitrogen and effect reduction.

In an embodiment, the transition metal complexes are described by general formula (III):

[VClzNR4[(CH2)q(CH)R5O]2]p (III)

wherein R4 and R5 are independently H or lower alkyl, z = 0 to 3, p = 1-5 and q = 1 to 3.

In a currently preferred embodiment, the transition metal complex of formula (III) for reduction of dinitrogen (nitrogen fixation) comprises N,0,O'-donating aminoalcohol ligands of the formula [νR4[(CH)q (CH)RsOH]2 that coordinated to the vanadium 3+ atom. Examples of N,O,O'-donating aminoalcohol ligands present in transition metal complexes of formula (III) include, but are not limited to, the ligands shown in Scheme 2.

(g)-DEA (h)-MDEA (ι)-'BuDEA

O)-(1PrOH)2NH (k)-BHEEN (I)-Me2-BHEEN

(m)-('PrOH)2EN (n)-('PrOH)2DMEN

Scheme 2. Structures of the amino alcohol ligands.

N,O,O'-donating aminoalcohol ligands shown in Scheme 2 in transition metal complexes of formula (III) are produced in-situ by reacting, [VCIs(THF)3] with the corresponding ligand following its deprotonation with a base in an aprotic solvent. The transition metal complex formed in-situ is reacted with dinitrogen to effect reduction, following which the reaction mixture is protonated with mineral acid. The yields of ammonia and hydrazine obtained from dinitrogen reduction by the transition metal complex of formula IH for various N,0,0' -donating aminoalcohol ligands (L) are shown in Table 2. Ammonia and hydrazine were analyzed by known test methods using indophenol and 4-(dimethylamino)benzaldehyde, respectively. The production of ammonia and hydrazine reaction products was also confirmed by electron spray mass spectrometry ES-MS of the 4-(dimethylamino)benzaldehyde derivatives prepared in absolute ethanol.

Table 2. Yields of ammonia and hydrazine obtained from dinitrogen reduction by Formula (III) complexes obtained by the reaction of [VCl3(THF)3] with respective ligands (L). *The yield was calculated based on the percentage of electrons used for fixation per vanadium [(3 * mol NH3 + 4 * mol N2H4)/ 2].

In all cases, the hydrazine yield is larger than the yield of ammonia. However, the total yield depends on the structure of the ligand. Introduction methyl groups adjacent to the oxygen atoms results in a significant drop in the total yields for both substituted diethanolamines and the BHEEN derivatives. Introducing substituents on the nitrogen in either case did not result in such a dramatic change. This is consistent with the oxygen atoms being bridged between the vanadium atoms. The total yields are generally less than 100% with the exception of BHEEN, where approximately four electrons are provided per vanadium atom, rather than two. The extra electrons are supplied by the two hydride ions which did not react with the NH protons. The same reaction with only two equivalents of NaH gave 0.055 mol NH3/ mol V and 0.37 mol N2H4/ mol V for a total yield of 82.2%. DEA, with one NH group, behaves similarly although the yield is far less than with BHEEN. The small extra yield observed for Me2BHEEN may be a result of the uncertainty of the NaH weight. The ligand: metal stoichiometry in the transition metal complex of formula (III) wherein z = 3, influences the nature and yield of ammonia and hydrazine from the dinitrogen reduction reaction. As seen in FIG. 2, at a lower L:M ratio, ammonia is the only product, although in low yields. The yields of both ammonia and hydrazine increase with increasing L:M ratios, reaching maximum yields at a ratio of approximately between 1 : 1.5 to 1 :2 (FIG. 2). The presence of excess metal with respect to the ligand therefore, tends to favor the formation of higher nuclearity compounds.

The UV-Vis spectrum of a THF solution of the BHEEN reaction mixture shows a strong absorption centered at 256nm (ε > 10,000 M^cm" ). This, together with the absence of any peaks in the visible region of the spectrum due to metal d-d transitions, indicates that the vanadium is present in a +5 oxidation state in the product, rather than +4 which has one J-electron and thus at least one transition should be observed in the visible region of the spectrum depending on the symmetry of the complex. The 1H-NMR spectrum of the BHEEN reaction product (in DMF-d7) provides further evidence that vanadium is present in the +5 oxidation state. All the peaks appear in the 0-8 ppm range and only a small broadening is observed. In addition, the solvent peaks appear at the same positions expected in diluted solutions free of paramagnetic metal ions.

FIG. 3 shows an electrospray mass spectrum of the transition metal complex [VCl3(BHEEN)3].3THF. The inset shows a simulated isotope pattern of a metal complex of the formula [C22H47NgNaV2Oe] resulting from coordination of N2 with transition metal complex [V(t-BuDEA)3(N2)].3THF.

FIG. 4 shows an electrospray mass spectrum of [V(t-BuDEA)3(N2)].3THF resulting from coordination of the metal-cluster complex [VCl2(t-BuDEA)3].3THF with N2.

EXAMPLES

Example 1.

The ligands were purchased from Aldrich except "f", which was prepared as follows: (1PrOH)2NH (0.1 mol) was stirred with methyl iodide (0.1 mol) and K2CO3 (0.1 mol) in acetonitrile (200 ml) for 2 days. The solution was then filtered, acetonitrile pumped off, and the product extracted with CHCl3. It was then vacuum-distilled. Elemental analysis, calcd (found): C, 57.11 (56.83), H, 11.64 (11.60), N, 9.51 (9.53). ES- MS, 148.1 m/z (C7H17O2N). 1H-NMR (300MHz), δ 1.15 (d, CCH3), 2.26 (m, NCH), 3.78 (br, OH), 3.91 (m, OCH). IR (cπf1), 3395.8 (vs, br), 2967.8 (s), 2936.4 (s), 2879.7 (s), 2842.0 (s), 2791.6 (s), 1457.3 (s), 1419.6 (m), 1375.5 (m), 1328.7 (s), 1281.1 (s), 1214.0 (w), 1177.6 (w), 1124.5 (s), 1062.9 (s), 1035.0 (s), 951.0 (s), 839.2 (m), 788.8 (vw).

Example 2.

The ligand is first deprotonated in dry tetrahydrofuran using two equivalents of sodium hydride. [VCl2(tmeda)2] is added to the white suspension two hours later and the reaction is stirred for 20 hrs under nitrogen atmosphere to give a black solution. After driving the solvent off under vacuum, a solution of sulfuric acid in methanol is added to the residue. After stirring for 18 hrs (to assure completion), the reaction mixture is analyzed for ammonia and hydrazine. All the reactions were performed at room temperature and atmospheric pressure.

(tmeda = tetramethylethylenediamine)

Example 3.

The reactions were generally done by first deprotonating the ligands with NaH in tetrahydrofuran (THF) for two hours before adding acetonitrile and [VCl3(THF)3] under nitrogen atmosphere. The reaction was then stirred for 20 hrs, before driving off the solvents to dryness under vacuum. The next step was to add a methanol solution of sulfuric acid (for 18 hrs to assure completion). Both steps were done at room temperature and atmospheric pressure.

Example 4: Synthesis of [VCl3(BHEEN)3] .3THF

A [VCl3(BHEEN)3].3THF compound was obtained by reacting BHEEN with [VCl3(THF)3] in acetonitrile in the presence of NaH. The isolated product was characterized by electrospray Mass spectroscopy (ES) at a sample cone voltage of 15 V. M+ = 658.22.

All patents, patent applications, and published references cited herein are hereby incorporated by reference in their entirety. It will be appreciated that various of the above- disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.