NITROGEN CONTAINING METAL HYDROXIDE COMPLEXES

The present invention relates to a process for preparing novel amine-metal hydroxide complexes.

Metal nanoparticles have many different applications in areas such as catalysis, optoelectronics and biotechnology. Various techniques are known for their formation, including chemical reduction of metal salts and electrochemical methods. Stabilisers, such as ligands, polymers and surfactants are often used in an effort to reduce nanoparticle agglomeration.

One example of the use of ligands to stabilise the surface of nanoparticles is gold nanoparticles stabilised with thiols, as formed by the Brust method (Brust et al., J. Chem. Soc. Commun., 1994, p. 801). This method makes use of a phase transfer agent to enable the reduction of chloroauric acid in toluene in the presence of a thiol to produce stable nanoparticles (with diameters in the range of from 1 to 3 nm) that are thermally stable, air-stable and may be handled as a dry powder or in solution when dissolved in a non-polar solvent.

More recently stabilised nanoparticles have been made making use of long chain alkylamines in place of thiols using various methods including a one-pot aqueous synthesis (Aslam et al., J. Mat. Chem., 2004, 14, 1795) and a two-phase aqueous/organic synthesis in the presence Of NaBH ₄ (Leff et al., Langmuir, 1996, 12, 4723). The former method results in nanoparticles of greater than or equal to 9 nm in diameter, whilst the latter method has been shown to result in gold chloride ions being present at the surface of the nanoparticles (Kumar et al. Langmuir, 2003, 19, 6277).

We have found, very surprisingly, that by carrying out a two-phase partitioning reaction with the salt of one or more metal halides in the presence of a base and an organic solvent system containing an amine, that novel, substantially halide-free amine-metal hydroxide complexes can be formed.

According to one aspect, the invention provides a process for making a substantially halide-free amine-metal hydroxide complex, which process comprising partitioning an aqueous solution of the hydrogen chloride or sodium chloride salt of one or more metal halides, in the presence of a base, with an organic solvent system containing an amine, wherein the metal of the metal halide is selected from the group consisting of gold, silver, copper, nickel, platinum and palladium.

Herein the term "substantially halide-free" means that the amine-metal hydroxide complex comprises less than 2 wt% halide ions, for example less than 1 wt% halide ions.

As described above, the amine-metal hydroxide complex comprises one or more metal selected from the group consisting of gold, silver, copper, nickel, platinum and palladium, different metals being suited to different applications. However, this invention is particularly suited to the preparation of amine-gold hydroxide complexes and products that may result therefrom.

In one embodiment of the invention the amine of the amine-metal hydroxide complex is a primary or secondary amine comprising one or more functional groups selected from the group consisting of hydrogen, linear or branched C ₁ -C ₁₀ alkyl,

C ₃ -C ₁₂ cycloalkyl, C ₄ -C ₂₀ bicycloalkyl, C ₅ -C ₂₀ polycy cloalkyl, hydroxyl, linear or branched C ₁ -C ₁₀ hydroxyalkyl, halogen, linear or branched C ₁ -C ₁₀ haloalkyl, linear or branched C ₁ -C ₁₀ perhaloalkyl, amino, linear or branched C ₁ -C ₁₀ alkylamino, linear or branched C ₁ -C ₁₀ dialkylamino, phenyl, aryl, heteroaryl, carboxyl, linear or branched

C ₁ -C ₁₀ alkoxycarbonyl, acyl, linear or branched C ₁ -C ₁₀ alkylcarbonyl, benzoyl, aroyl, substituted aroyl, butoxycarbonyl, C ₁ -C ₁₀ alkoxy, linear O-acetyl, O-benzyl,

O-substituted benzyl, O-tetrahydropyranyl, O- linear or branched C ₁ -C ₁₀ alkylacetyl,

O-benzoyl, O-aroyl, linear or branched C ₁ -C ₁₀ (C ₁ -C ₅ alkoxy) alkyl, formyl, acetyl, nitrile, nitro and amido, or wherein neighbouring functional groups form part of a further aromatic ring, optionally including one or more hereto atoms selected from N,

O, further optionally substituted with any of the preceding substituents. Commonly

the amine of the amine-metal hydroxide complex comprises one or more functional groups selected from the group consisting of linear or branched C ₁ -C ₁₀ alkyl, C ₃ -C ₁₂ cycloalkyl, C ₄ -C ₂₀ bicycloalkyl and C _S -C ₂₀ polycycloalkyl, most commonly linear or branched C ₁ -C ₁₀ alkyl, and in specific embodiments when the amine of the amine-metal hydroxide complex comprises hydroxyisoamylamine, hydroxycyclo- hexylamine, 3,3'-dimethylbutylamine, hexylamine or pentylamine.

Under certain reaction conditions the amine used to form the amine-metal hydroxide complex may reduce the metal present in the reaction mixture. The amine is generally present in excess within the reaction mixture and therefore the oxidised form of the amine may or may not form part of the amine-metal hydroxide complex formed. Therefore in one embodiment of the invention, the amine-metal hydroxide complex comprises an imine and reduced metal ions, wherein the imine comprises an oxidised form of the amine.

In one embodiment of the invention the base is selected from the list consisting of sodium hydroxide, sodium carbonate, calcium hydroxide, potassium hydroxide and tetrabutylammonium hydroxide. The use of the base enables the pH of the solution to be adjusted so that it is in the range of from 4 to 14, optionally in the range of from 9 to 12.

Usually the amine present in the organic solvent system will be in excess when assessed in relation to the partitioning reaction as a whole. The ratio between the amine and the salt of the one or more metal halides will be in the range of from 25:1 to 5:1, commonly from 12:1 to 8:1.

In one embodiment of the invention the organic solvent system comprises a substantially non water-miscible solvent, for example one or more selected from the group consisting of toluene, dichloromethane, diethyl ether, xylene, hexane, pentane, mesitylene and isomers thereof.

The invention also embodies the complexes obtainable by a process as described above. Such complexes may be reduced to form substantially halide-free metal nanoparticles, a process known as metallisation. The inventors have found that the complexes obtainable by a process described above may be reduced at low temperatures, for example at a temperature of 5 °C or less. Metallisation occurs at especially low temperatures when the amine comprises one or more functional groups selected from the group consisting of C ₁ -C ₁₀ alkyl, C ₃ -C ₁₂ cycloalkyl, C ₄ -C ₂₀ bicycloalkyl and C ₅ -C ₂₀ polycycloalkyl, more particularly from linear or branched C ₁ -C ₁₀ alkyl, and in specific embodiments when the amine comprises hydroxyisoamylamine, hydroxycyclohexylamine, 3,3'-dimethylbutylamine, hexyl- amine or pentylamine.

The substantially halide-free metal nanoparticles produced as described above are particularly small with greater than 80% of the nanoparticles in the range of from 2 to 5 nm in diameter. Additionally, these nanoparticles are redispersable in solvents such as toluene, xylene and isomers thereof, chlorinated solvents, glycol ethers, alcohols and water / alcohol mixtures.

Substantially halide-free metal films may be produced using substantially halide free metal nanoparticles as described above applied by spin coating, K-bar, dip coating, spraying, aerosol, pyrosol, misting, brushing, screen printing or ink-jet printing a substrate. The low metallisation temperature of the substantially halide free metal nanoparticles enables the formation of said substantially halide-free metal films at unusually low temperatures, for example at a temperature of less than or equal to 20 ⁰ C, thus enabling films to be formed at lower temperatures than used conventionally.

In order that the invention may be more fully understood the following Examples are provided by way of illustration only and with reference to the accompanying drawings, in which:

Figures Ia and Ib are TEM images of gold-nanoparticle stabilised with hexylamine, prepared using a reducing agent (Figure Ia) or a photochemical reactor (Figure Ib); Figures 2a and 2b are graphs showing the particle size distribution of gold nanoparticles stabilised with hexylamine (particles size in nm vs counts), prepared using a reducing agent (Figure 2a) or a photochemical reactor (Figure 2b); and

Figures 3 a, 3b and 3 c are AFM pictures of gold films on PET substrate made at 150 ⁰ C by spin coating with a gold hexylamine complex (Figure 3a), K-bar with a gold hexylamine complex (Figure 3b) or K-bar with a gold pentylamine complex (Figure 3 c).

EXAMPLE 1 Preparation of Au-hcxylaminc complex

Chloroauric acid (HAuCl ₄ , 41.80% Au, 1 eq, 4.22 mmoles) was stirred with deionised water (20 ml). The pH of the solution was adjusted to 10.5 with a solution of sodium hydroxide (1 M). The gold solution was then added to a round bottomed flask containing a solution of n-hexylamine (10 eq, 42.20 mmoles) and toluene (40 ml).

The solution mixture was stirred vigorously for 30 minutes. A two-layer separation occurred with an orange-red organic layer at the top and a yellow aqueous layer at the bottom. The aqueous layer was separated and washed once with toluene (30 ml). The combined organic layers were washed several times with deionised water. The organic phase solution was then filtered on a GF/C filter paper to remove any solid.

The solvent was then removed using a rotary-evaporator. A glassy brown-red solid was obtained: Mass of product obtained: 0.50 g

Au assay: 59.9 %

Elemental analysis: %C: 29.19, %H: 5.87, %N: 5.41, %C1: 0.63

TGA analysis: exotherm at 133 ⁰ C.

IR analysis (KBr/cm ¹ ): 3377-3135 (v N-H of coordinated amine), 2954, 2925 and 2854 (C-H aliphatic stretch)

¹³ C-NMR [(δ ppm), CDCl ₃ -solution]: 164.9 NH=CH-(CH ₂ ) ₄ -CH ₃ , 61.4 (H ₂ N-CH ₂ - of coordinated amine), 42.0 (H ₂ N-CH ₂ - of free amine), 35.8, 32.2, 26.9, 26.8 and 22.8 (-CH ₂ - from amines and imine), 14.0 (-CH ₃ from amines and imines). ¹ H-NMR [(δ ppm), CDCl ₃ -solution]: 0.8 (CH ₃ -), 1.3 (m, -CH ₂ -).

EXAMPLE 2 Preparation of Au-pcntylaminc complex

Chloroauric acid (HAuCl ₄ , 41.80% Au, 1 eq, 4.22 mmoles) was stirred with deionised water (20 ml). The pH of the solution was adjusted to around 10.5 with a solution of sodium hydroxide (1 M). The gold solution was then added to a round bottomed flask containing a solution of n-pentylamine (10 eq, 42.20 mmoles) and dichloromethane

(40 ml). The solution mixture was stirred vigorously for 30 minutes. A two-layer separation occurred with an orange-red organic layer at the top and a yellow aqueous layer at the bottom. The aqueous layer was separated and washed once with dichloromethane (30 ml). The combined organic layers were washed several times with deionised water. The solvent was then removed using a rotary-evaporator. A glassy brown-red solid was obtained:

Mass of product obtained: 0.42 g Au assay: 63.3 %

Elemental analysis: %C: 25.76, %H: 5.57, %N: 5.74, %C1: 0.83

TGA analysis: exotherm at 137 ⁰ C.

IR analysis (KBr/cm ¹ ): 3371-3138 cm ^"1 (v N-H of coordinated amine), 2954, 2924 and 2858 cm ^"1 (C-H aliphatic stretch) ¹³ C-NMR [(δ ppm), CDCl ₃ -solution] : 164.9 NH=CH-(CH ₂ ) ₄ -CH ₃ , 61.3 (H ₂ N-CH ₂ - of coordinated amine), 42.0 (H ₂ N-CH ₂ - of free amine), 35.0, 33.4, 30.4, 29.4, 28.2 and

22.5 (-CH ₂ - from amines and imine), 14.0 (-CH ₃ from amines and imines).

¹ H-NMR [(δ ppm), CDCl ₃ -solution]: 0.86 (t, CH ₃ -), 1.3 (m, CH ₂ -)

EXAMPLE 3 Preparation of Au-3,3'-dimethylbutylamine

Chloroauric acid (HAuCl ₄ , 41.80% Au, 1 eq, 1.04 mmoles) was stirred with deionised water (20 ml). The pH of the solution was adjusted to around 10.5 with a solution of sodium hydroxide (1 M). The gold solution was then added to a round bottomed flask containing a solution of 3,3'-dimethylbutylamine (10 eq, 10.40 mmoles) and dichloromethane (40 ml). The solution mixture was stirred vigorously for 30 minutes. A two-layer separation occurred with an orange-red organic layer at the top and a yellow aqueous layer at the bottom. The aqueous layer was separated and washed once with dichloromethane (30 ml). The combined organic layers were washed several times with deionised water. The solvent was then removed using a rotary- evaporator. A glassy brown-red solid was obtained Mass of product obtained: 0.22 g Au content (%): 54.4 %

Elemental analysis: %C: 30.79, %H: 6.26, %N: 5.64, %C1: 1.25

TGA analysis: exotherm at 155 ⁰ C.

IR analysis: 3389-3117 cm ^"1 (N-H stretch), 2948, 2906 and 2867 cm-1 (C-H aliphatic stretch) ¹³ C-NMR [(δ ppm), CDCl ₃ -solution]: 163.4 (NH=CH-CH ₂ -C(CH ₃ ) ₃ ) 57.9 (H ₂ N-CH ₂ - of coordinated amine), 45.1 (H ₂ N-CH ₂ - of free amine), 49.4, 48.2, 44.6 and 38.3 (-CH ₂ - from amines and imine), 14.0 (-CH ₃ from amines and imines). ¹ H-NMR [(δ ppm), CDCl ₃ -solution]: 0.86 (m, (CH ₃ ) ₃ -).

EXAMPLE 4

Preparation of gold nanoparticles stabilised with hexylamine using a reducing agent

Chloroauric acid (HAuCl ₄ , 41.80% Au, 1 eq, 8.53 mmoles) was stirred with deionised water (40 ml). The pH of the solution was adjusted to around 10.5 with a solution of sodium hydroxide (1 M). The gold solution was then added to a round bottomed flask containing a solution of n-hexylamine (10 eq, 85.30 mmoles) and toluene (100 ml).

The solution mixture was stirred vigorously for 30 minutes. A two-layer separation occurred with an orange-red organic layer at the top and a yellow aqueous layer at the bottom. The organic layer was washed several times with deionised water. The organic phase solution was then filtered on a GF/C filter paper to remove any solid. The organic layer was transferred back to a round bottomed flask equipped with a thermometer. The solution was kept at temperature between 0 and 5 °C in an ice bath. Deionised water (20 ml) was added to the flask. An aqueous solution of sodium borohydride (1 eq, 8.53 mmoles) was added dropwise with vigorous stirring. The resulting dark brown purple organic layer was concentrated using a rotary evaporator. UV-visible spectrum: surface plasmon at 512 nm.

EXAMPLE 5

Preparation of gold nanoparticles stabilised with hexylamine using a photochemical reactor

Chloroauric acid (HAuCl ₄ , 41.73% Au, 1 eq, 14.9 mmoles) was stirred with deionised water (200 ml). The pH of the solution was adjusted to around 12 with a solution of sodium hydroxide (1 M). The gold solution was then added to a round bottomed flask containing a solution of n-hexylamine (15 eq, 223.50 mmoles) and toluene (150 ml). The solution mixture was stirred vigorously for 60 minutes. A two-layer separation occurred with an orange-red organic layer at the top and a yellow aqueous layer at the bottom. The aqueous layer was discarded and the organic layer was washed several times with deionised water. The organic solution was then filtered through a GF/C filter paper to remove any solid then transferred into a photochemical reactor using a 125 Watt mercury lamp. The solution was irradiated for 6 hours, when a dark brown solution was obtained. The solvent was reduced to a minimum using a rotary- evaporator and acetone was added to the concentrated solution. A brown solid precipitated out. The mixture was filtered and the solid washed with cold acetone and allow to air dry.

Mass of product obtained: 1.71 g

Au content (%): 64.0%

Elemental analysis: %C: 24.89, %H: 4.94, %N: 6.53, %C1: 0.69

TGA analysis: exotherm at 150 ⁰ C.

IR analysis: 3187 cm ^"1 (N -H stretch), 2956, 2924 and 2855 cm ^"1 (C -H aliphatic stretch), 1575 cm ^"1 (N-H bend).

¹³ C-NMR [(δ ppm), CDCl ₃ -solution]: 47.3 (H ₂ N-CH ₂ - of coordinated amine), 42.3 (H ₂ N-CH ₂ - of free amine), 31.9, 31.1, 27.1 and 22.7 (-CH ₂ - from amines), 14.0 (-CH ₃ from amines).

¹ H-NMR [(δ ppm), CDCl ₃ -solution]: 0.88 (-CH ₃ ), 1.29 (-CH ₂ -) ₄ , 1.72 (-NH ₂ ) and 3.05 (CH ₂ -NH ₂ ).

EXAMPLE 6

Preparation of Au-hydroxycyclohcxylaminc complex

Chloroauric acid (HAuCl ₄ , 41.73% Au, 1 eq, 21.2 mmoles) was stirred with deionised water (100 ml). The pH of the solution was adjusted to around 10.5 with a solution of sodium hydroxide (1 M). The gold solution was then added to a round bottomed flask containing a solution of cyclohexylamine (10 eq, 212.0 mmoles) and toluene (200 ml). The solution mixture was stirred vigorously for 40 minutes. A two-layer separation occurred with an orange organic layer at the top and a yellow aqueous layer at the bottom. The aqueous layer was separated and washed twice with toluene (50 ml). The combined organic layers were washed several times with deionised water then filtered through a GF/C filter paper to remove any solid. The solvent was then reduced to a minimum using a rotary-evaporator. Acetone was added to the concentrated solution. An orange solid precipitated out. The mixture was filtered and the solid washed with cold acetone and allow to air dry. Mass of product obtained: 0.18 g Au content (%): 55.0%

Elemental analysis: %C: 29.43, %H: 5.29, %N: 5.45, %C1: 1.49 TGA analysis: exotherm at 151 ⁰ C. IR analysis: 3394 and 3106 cm ^"1 (N-H stretch), 2919 and 2853 cm ^"1 (C-H aliphatic stretch), 1567 cm ^"1 (N-H bend).

¹³ C-NMR [(δ ppm), CDCl ₃ -solution]: 50.3 (-CH-), 42.0 (-CH ₂ - in α position), 36.8 (-CH ₂ - in α position), 34.4 (-CH ₂ - in para position) 27.07 (-CH ₂ - in β position), 25.7 (-CH ₂ - in β position).

¹ H-NMR [(δ ppm), CDCl ₃ -solution]: 0.31 to 0.92 [m, (-CH ₂ )- and (-NH ₂ )] and 1.96 [m, (-CH-)]

EXAMPLE 7 Preparation of Au-hydroxyisoamylamine complex

Chloroauric acid (HAuCl ₄ , 41.73% Au, 1 eq, 21.2 mmoles) was stirred with deionised water (100 ml). The pH of the solution was adjusted to around 10.5 with a solution of sodium hydroxide (1 M). The gold solution was then added to a round bottomed flask containing a solution of isoamylamine (10 eq, 212.0 mmoles) and toluene (200 ml). The solution mixture was stirred vigorously for 40 minutes. A two-layer separation occurred with an orange organic layer at the top and a yellow aqueous layer at the bottom. The aqueous layer was separated and washed twice with toluene (50 ml). The combined organic layers were washed several times with deionised water then filtered through a GF/C filter paper to remove any solid. The solvent was then reduced to a minimum using a rotary-evaporator. Acetone was added to the concentrated solution. An orange solid precipitated out. The mixture was filtered and the solid washed with cold acetone and allow to air dry. Mass of product obtained: 0.37g Au content (%): 10.0% Elemental analysis: %C: 21.00, %H: 4.67, %N: 4.72, %C1: 1.09 TGA analysis: exotherm at 137 ⁰ C.

IR analysis: cm ^"1 3379 and 3186 cm ^"1 (N-H stretch), 2950, 2868 cm ^"1 (C-H aliphatic stretch), 1569 cm ^"1 (N-H bend).

¹³ C-NMR [(δ ppm), CDCl ₃ -solution]: 59.5 (H ₂ N-CH ₂ - of coordinated amine), 44.7 (-CH ₂ )-, 39.9 (H ₂ N-CH ₂ - of free amine), 26.4 (-CH-) and 22.5 (-CH ₃ ). ¹ H-NMR [(δ ppm), CDCl ₃ -solution]: 0.85 to 1.05 [m, (-CH ₃ -), (-CH ₂ -) and (-CH-)] and 1.65 [m, (-CH ₂ -NH ₂ )].