Synthesis of ternary sulfide nanomaterials using dithiocarbamate complexes as single source precursors

We report the use of cheap, readily accessible and easy to handle di-isobutyl-dithiocarbamate complexes, [M(S2CNiBu2)n], as single source precursors (SSPs) to ternary sulfides of iron–nickel, iron–copper and nickel–cobalt. Varying decomposition temperature and precursor concentrations has a significant effect on both the phase and size of the nanomaterials, and in some instances meta-stable phases are accessible. Decomposition of [Fe(S2CNiBu2)3]/[Ni(S2CNiBu2)2] at ca. 210–230 °C affords metastable FeNi2S4 (violarite) nanoparticles, while at higher temperatures the thermodynamic product (Fe,Ni)9S8 (pentlandite) results. Addition of tetra-isobutyl-thiuram disulfide to the decomposition mixture can significantly affect the nature of the product at any particular temperature-concentration, being attributed to suppression of the intramolecular Fe(iii) to Fe(ii) reduction. Attempts to replicate this simple approach to ternary metal sulfides of iron–indium and iron–zinc were unsuccessful, mixtures of binary metal sulfides resulting. Oleylamine is non-innocent in these transformations, and we propose that SSP decomposition occurs via primary–secondary backbone amide-exchange with primary dithiocarbamate complexes, [M(S2CNHoleyl)n], being the active decomposition precursors.


Introduction
Since the properties of nanomaterials are highly dependent upon their size, shape, composition and architecture, then the development of reproducible and controlled synthetic methodologies presents a major challenge. 1 Our interest in nanomaterials focuses on the potential role of iron suldes in prebiotic chemistry, especially the reduction of CO 2 to biologically useful molecules believed to have occurred at the interface between hydrothermal uids and the primordial ocean. 2, 3 Thus it has been proposed that iron suldes found in the chimney cavities of hydrothermal vents 4 catalysed CO 2 reduction forming a primitive acetyl-CoA pathway similar to that in contemporary enzymes. [5][6][7] Greigite is structurally similar to the Fe 4 S 4 cluster sub-units found in ferredoxins 8 and its catalytic nature in CO 2 activation has been demonstrated, 9,10 while iron suldes have also been shown to catalyse CO 2 reduction. 11 Thus in a recent communication 9 we showed that nanoparticles of greigite can reduce CO 2 under ambient conditions into methanol, formic, acetic and pyruvic acid.
The greigite used for these experiments was prepared from the controlled hydrothermal decomposition of the dithiocarbamate complex [Fe(S 2 CN i Bu 2 ) 3 ] (1) in oleylamine. 12 Dithiocarbamate complexes 13 nd extensive use as single source precursors (SSPs) towards a range of nanomaterials [14][15][16][17][18][19][20][21][22] their utility stemming from their ease of synthesis, and the ability to tune the solubility, volatility and decomposition properties upon making simple changes to the amine substituents. A further advantage of the dithiocarbamate approach to SSPs is the realisation that this simple ligand forms stable complexes with all the transition metals. 12 Thus in seeking to develop our work on the catalytic properties of metal-sulde nanomaterials we sought to prepare a range of ternary sulde nanomaterials, primarily containing iron but also nickel. To do this we have used a range of simple air-stable di-isobutyldithiocarbamate complexes (Chart 1) and carried out decomposition studies in oleylamine under a range of conditions. This has successfully led to the synthesis of a number of phases of FeNi, FeCu and NiCo suldes which are described herein, although attempts to prepare FeZn and FeIn suldes by this method failed.

Results and discussion
(i) Iron-nickel suldes Ternary iron-nickel sulde materials can take a number of phases, 23 most commonly (Fe,Ni) 9 S 8 (pentlandite, pentlandite structure) and FeNi 2 S 3 (violarite, inverse thiospinel structure), although a pyrite type structure (Fe,Ni)S 2 (bravoite) has also been reported. 24 There are few examples of the synthesis of nanomaterials of these phases. Bezverkhyy 4 ] was decomposed at 800 C. 26 As far as we are aware, there are no reports of either violarite or bravoite being synthesised in the nanoparticle regime.
In our rst attempt at preparing ternary iron-nickel sulde nanomaterials we used [Fe(S 2 CN i Bu 2 ) 3 ] (1) and [Ni(S 2 CN i Bu 2 ) 2 ] (2) as SSPs; decomposition of 2.5 mM quantities of each was carried out in oleylamine at 150, 180, 230, 260 and 280 C. Upon warming the oleylamine solution a number of temperaturedependent colour changes were observed, being similar to those previously reported in the decomposition of 1 alone, but occurring at ca. 10 C higher. 12 Thus the initially dark brown solution become suddenly clear and pale yellow at 85 C, then at 90 C rapidly went black. Samples were heated to the target temperature and held there for 1 h aer which the generated nanoparticles were isolated as black powders and analysed by PXRD ( Fig. 1). At 150 C the material generated is mostly amorphous, but by 180 C peaks for FeNi 2 S 4 begin to emerge and at ca. 230 C a mixture of FeNi 2 S 4 , Fe 7 S 8 and a-NiS is seen. Increasing the temperature to 260 C sees the emergence of peaks attributed to (Fe,Ni) 9 S 8 , and at 280 C the crystalline phase of the sample appears to be comprised of pure (Fe,Ni) 9 S 8 . This suggests that FeNi 2 S 4 (violarite) is metastable, with (Fe,Ni) 9 S 8 (pentlandite) being thermodynamically favourable.
The pure (Fe,Ni) 9 S 8 produced at 280 C was analysed by TEM and HRTEM being comprised of roughly hexagonal nanoparticles of 53 nm (SD 18 nm) average diameter (Fig. 2). These are signicantly smaller than nanoparticulate pentlandite prepared by Bezverkhyy (Av. 240 nm). 25 HRTEM analysis reveals a d-spacing of 3.55Å corresponding to the [220] lattice plane (3.57Å).
In recent work we found that decomposition of 1 and 2 alone were signicantly inuenced by addition of tetraisobutylthiuram disulde (3) leading to the increased stability of metastable phases and allowing access to the thiospinel phases Fe 3 S 4 and Ni 3 S 4 . 22 For 1 this was related to an intramolecular ligand-metal electron transfer leading to reduction to an Fe(II) product and extrusion of 3, the addition of which moves the equilibrium towards the Fe(III) species. 12 We thus investigated how the addition of 3 would affect the decomposition of mixtures of 1 and 2. Consequently all reactions were repeated with addition of ca. 4 equivalents of 3 (i.e. 10 mM ca. 2 eq. to each precursor). PXRD analysis revealed a similar trend to the samples prepared in the absence of thiuram disulde but with some notable differences (Fig. 3). Thus, while spectra are similar at low temperatures, at 230 C seemingly pure FeNi 2 S 4 is formed, that is without Fe 7 S 8 and a-NiS impurities seen in the absence of 3. This indicates that addition of 3 stabilises the thiospinel, FeNi 2 S 4 , phase possibly preventing formation of binary metal suldes. At 260 C the peaks attributed to FeNi 2 S 4 broaden and those of Fe 7 S 8 and a-NiS begin to appear, while at 280 C the sample is mainly Fe 7 S 8 and a-NiS, with some FeNi 2 S 4 remaining.
To probe whether the thermodynamically stable phase, (Fe,Ni) 9 S 8, could be accessed in the presence of 3 but at higher temperatures, a decomposition was performed at 300 C with addition of only 2 eq. of 3. PXRD of the resulting material showed the crystalline phase to be a ca. 1 : 1 mix of Fe 7 S 8 and a-NiS, with neither FeNi 2 S 4 or (Fe,Ni) 9 S 8 being present. This suggests addition of 3 prevents formation of (Fe,Ni) 9 S 8 , making binary metal sulde phases more thermodynamically favourable.
Analysing the FeNi 2 S 4 particles produced at 230 C by TEM (Fig. 4) showed them to be short rods embedded in hexagonal interlocking sheets with an average diameter of 36.5 nm (SD 21 nm), being slightly smaller than (Fe,Ni) 9 S 4 nanoparticles formed in the absence of 3 (average diameter 53.1 nm). HRTEM analysis of the nanoparticles reveals d-spacings of 3.29Å, corresponding to the [220] lattice plane in FeNi 2 S 4 (3.35Å).
We next varied precursor concentrations, equimolar amounts of 1 and 2 at 2.5, 5, 10, 20 and 25 mM were decomposed in oleylamine at 230 C. PXRD analysis reveals a trend in the phase of metal sulde produced with increasing concentration of precursors ( Fig. 5). At 2.5 mM the majority of the crystalline sample corresponds to Fe 7 S 8 , with some FeNi 2 S 4 and a-NiS, while at higher concentrations the crystalline phase is predominantly the thiospinel FeNi 2 S 4 , with some a-NiS   impurity. This is consistent with the concentration study performed on 1 12 where at low precursor concentrations (5 mM) Fe 7 S 8 was formed, and as the concentration was increased (up to 50 mM) the phase changed to the thiospinel, Fe 3 S 4 . When a similar study was performed on 2, changing concentration had no effect on nickel sulde phase; a-NiS being formed at all concentrations.
Experiments were repeated upon addition of four equivalents of 3 and PXRD analysis again shows a concentrationdependent phase change ( Fig. 6). At low concentrations pure violarite is formed, but upon increasing the majority of the crystalline material matches the pattern for (Fe,Ni)S 2 (bravoite, a nickelian pyrite) and for the 1 : 2 : 3 ¼ 20 : 20 : 80 mM sample the pattern matches pure (Fe,Ni)S 2 . Peaks are, broad suggesting that the nanoparticles are small, being consistent with the concentration study on the nickel complex 2 in the presence of 3. 22 In the ternary system, when the concentration is increased further, additional peaks at slightly higher 2q angles appear, indicative of the presence of FeS 2 (pyrite). This is interesting since in the binary iron sulde system, pyrite was not accessible at high concentrations, even aer addition of 3. It should also be noted that at higher concentrations, low angle (10-20 ) broad peaks are observed, possibly resulting from excess sulfur.
Nanoparticles produced at 20 mM 1-2 were analysed by TEM, being approximately spherical and with small crystallite size (average diameter 6.7 nm, SD ¼ 1.9 nm), as indicated by the broad peaks in the PXRD pattern. They closely resemble NiS 2 nanoparticles formed at high concentration of In summary, products of the mixed iron and nickel ternary sulde system were found to be highly dependent upon both the decomposition temperature and precursor concentration, with similarities to both the iron sulde and nickel sulde binary systems. The pentlandite phase (Fe,Ni) 9 S 8 , thiospinel phase FeNi 2 S 4 and pyrite phase (Fe,Ni)S 2 could be selectively produced by varying reaction temperature, precursor concentration and employing the tetra-isobutylthiuram disulde additive 3. We believe that this is the rst reported synthesis of the latter two phases in nanoparticulate form.

(ii) Iron-copper suldes
Copper-iron sulde has many known phases including Cu 5 FeS 4 (bornite), CuFeS 2 (chalcopyrite) and CuFe 2 S 3 (cubanite) 27 and there are numerous reports on their synthesis, although only a few involving the decomposition of dithiocarbamate complexes. Thus, methods to synthesise nanoparticulate copper-iron suldes include; solvothermal and microwaveassisted decomposition of metal salts with sulfur sources, colloidal synthesis and laser ablation of bulk CuFeS 2 . 28 Two reports have previously detailed the use of dithiocarbamate SSPs for the synthesis of copper-iron sulde nanoparticles. Gupta utilised a 'hot-injection' method to prepare CuFeS 2 nanoparticles of ca. 12 nm diameter, injecting a mixture of [Cu(S 2 CNEt 2 ) 2 ] and [Fe(S 2 CNEt 2 ) 3 ] in oleylamine/ dichlorobenzene into a solution of sulfur in oleylamine/ trioctylphosphine at 180 C, 29 while Pang and co-workers synthesised CuFeS 2 nanoparticles of ca. 6 nm (ref. 30) upon injecting solution of NaS 2 CNEt 2 in dodecanethiol into a hot (140 C) solution of FeCl 3 and CuCl 2 oleic acid/dodecanethiol; dithiocarbamate complexes presumably being formed in situ. 30 Following on from our methodology described above, we decomposed equimolar amounts (2.5 mM) of brown [Cu(S 2 -CN i Bu 2 ) 2 ] (4) and [Fe(S 2 CN i Bu 2 ) 3 ] (1) in oleylamine at 230 C. The reaction was then repeated with the addition 4 eq. of thiuram disulde 3, and the resulting brown powders were analysed. No discernible colour changes occurred during these reactions, clear purple dispersions resulting in chlorinated  This journal is © The Royal Society of Chemistry 2019 Nanoscale Adv., 2019, 1, 3056-3066 | 3059 Paper solvents. PXRD analysis (Fig. 8) showed that the crystalline phase produced in both cases corresponded to CuFeS 2 (chalcopyrite). This is in accordance with previous literature. 29,30 TEM analysis revealed that addition of 3 had little effect on the size or morphology of the nanoparticles, which were roughly spherical ( Fig. 9). Average particle diameters were 11 nm (SD ¼ 4 nm) without 3, and 10 nm (SD ¼ 4 nm) with 3. HRTEM analysis (Fig. 9) revealed a d-spacing of 2.99Å, which matches the [112] plane (3.21Å, CuFeS 2 ).
These results are in accord with those of Gupta and coworkers 29 who generated similar nanomaterials at 180 C. Even upon increasing the temperature by 50 C we saw no signicant change in the nanoparticle size suggesting that during nanoparticle formation the duration of burst nucleation and the number of nucleation sites is not affected by temperature. and CH 3 (CH 2 ) 11 OSO 3 Na (sodium dodecyl sulphate) in water with added thioacetamide. 34 We used equimolar amounts (2.5 mM) of [Zn(S 2 CN i Bu 2 ) 2 ] (5) and [Fe(S 2 CN i Bu 2 ) 3 ] (1) in oleylamine at 230 C in the absence and presence of 4 eq. of thiuram disulde 3. As with previous decompositions involving 1, dark brown solutions suddenly became clear and pale yellow at ca. 85 C, and at 100 C quickly   turned black. Dark brown powders were isolated but PXRD analysis revealed that the crystalline phase corresponded to a mixture of iron sulde and zinc sulde phases (Fig. 10). Interestingly, in both the zinc sulde phase matched to the pattern for the sphalerite rather than the wurtzite phase. Notably the characteristic peak for ZnS (W) at 39.8 2q is absent. Peaks match well for ZnS (S) , and are not shied to lower angles, as previously observed for mixed iron and zinc sulde systems 32 suggesting that no ternary mixed-metal sulde has been formed. The reason for this is unclear, but possibly the precursors decompose at quite different temperatures, giving rise to separate nucleation events and forming binary metal sulde nanoparticles. The sample prepared in the absence of 3 appears to also have peaks for the iron sulde Fe 7 S 8 and the sample prepared with 3 contains peaks which match well to Fe 3 S 4 , the thiospinel phase of iron sulde. This is consistent with previous results reported on the iron sulde binary phase system.
TEM images support the PXRD analysis, clearly showing a two phase system (Fig. 11). Samples comprised large hexagonal particles (average diameter 32 nm, SD 16 nm) which resemble the iron sulde phases observed in the binary system, and thin nanowires (length ¼ 16 nm, SD 15 nm, width ¼ 2.9 nm, SD 0.8 nm) resembling zinc sulde material produced in the binary system.

(iv) Attempts to prepare iron-indium suldes
There have been few reported syntheses of nanoparticulate FeIn 2 S 4 , and none using solvothermal methods. Related to this work, Wu et al. recently synthesised FeIn 2 S 4 microspheres using an indium dithiocarbamate precursor. 35 Equimolar amounts (2.5 mM) of pale yellow [In(S 2 CN i Bu 2 ) 3 ] (6) and dark brown [Fe(S 2 CN i Bu 2 ) 3 ] (3) were decomposed in oleylamine, being repeated upon addition of 3, and the resulting brown powders were analysed. PXRD patterns (Fig. 12) did not match that of the mixed iron indium sulde, FeIn 2 S 4 , being broad and indicative of low crystallinity. Peaks for Fe 3 S 4 were detectable in both, as well as for b-In 2 S 3 in the sample prepared with added 3. As with the iron-zinc sulde chemistry ternary mixed-metal suldes were absent. Interestingly, unlike the iron zinc sulde samples, peaks for pyrrhotite are not observed in the sample prepared without 3, but peaks matching Fe 3 S 4 are seen in both. This might suggest that the presence of the indium precursor 6 or indium sulde product b-In 2 S 3 helps to stabilise the iron sulde thiospinel.

(v) Nickel-cobalt suldes
Although the focus of our work with ternary chalcogenides is on mixed-metal iron suldes, many other combinations of mixedmetal sulde materials are possible. The nickel-cobalt sulde system has several possible phases including; NiCo 8 S 8 (pentlandite structure), NiCo 2 S 4 and CoNi 2 Co 4 (both thiospinel structure). Nanoparticles of NiCo 8 S 8 have been synthesised by Bezverkhyy et al. in a two-step process; rstly Co(NO 3 ) 2 and Ni(NO 3 ) 2 were reacted with Na 2 S in water and then the dried precipitate was heated with H 2 S/H 2 for 3 h at 300 C. 25 Urchinlike nanostructures of NiCo 2 S 4 were synthesised by Jiang and co-workers in a two-step process involving the hydrothermal decomposition of NiCl 2 and CoCl 2 with urea in an autoclave at 140 C, followed by a further reaction with Na 2 S at 160 C (ref. 36   being repeated upon addition of thiuram disulde 3, and the resulting brown powders analysed. PXRD analysis (Fig. 13) revealed no signicant shi in peaks between the two samples, indicating that 3 has no observable effect on the phase of ternary sulde produced. In addition, to elucidate whether the nickel or cobalt rich thiospinel structures had been formed (CoNi 2 S 4 or NiCo 2 S 4 respectively) both samples were analysed by energy-dispersive X-ray spectroscopy (EDX) and found to have atomic compositions of cobalt, nickel and sulfur of 1.76 : 1.60 : 4 and 1.52 : 1.32 : 4, respectively. Although the ratios of metal to sulfur vary slightly, they remain approximately 1.1 : 1 Co : Ni, indicating an intermediate (Ni,Co) 3 S 4 phase. The difference between the PXRD patterns for CoNi 2 S 4 and NiCo 2 S 4 is small; nickel and cobalt having very similar ionic radii and electronegativity, and the patterns obtained for the samples synthesised here closely match both.
TEM analysis revealed the addition 3 also had little effect on the size or morphology of the nanoparticles formed, which were roughly spherical (Fig. 14). The average particle diameters were 10 nm (SD ¼ 5 nm) for the sample prepared without 3, and 10 nm (SD ¼ 4 nm) with 3. Nanoparticles of CoNi 2 S 4 previously reported by Pang et al. 37 were reported to have 'quasi-spherical' morphology with an average diameter of approximately 8-15 nm, similar to those produced in the current study. HRTEM analysis (Fig. 13)

Summary and conclusions
Nanomaterials of iron-nickel sulde, iron-copper sulde and nickel-cobalt sulde were successfully prepared via the decomposition of mixtures of simple metal-dithiocarbamate complexes in oleylamine, and for iron-nickel mixtures, various sulde phases were accessible upon varying decomposition conditions. In contrast, attempts to synthesise iron-zinc sulde and iron-indium sulde via this method were unsuccessful, producing only binary metal suldes. This may be due to the precursor complexes decomposing at different temperatures, although this has not been fully established. We have previously shown that [Fe(S 2 CN i Bu 2 ) 3 ] (1) undergoes an intramolecular electron-transfer process at ca. 80 C in oleylamine to generate [Fe(S 2 CN i Bu 2 ) 2 ] and thiuram disulde (S 2 CN i Bu 2 ) 2 (3). 12 This also occurs during decompositions carried out here as shown by the sudden colour change from brown to yellow at ca. 80-90 C. Thus the decomposition mixtures contain an Fe(II) and not an Fe(III) dithiocarbamate complex. Consistent with this supposition, addition of 3 to the decomposition mixture has a signicant effect on the precise nature of the nanomaterials formed. We have also previously shown that decomposition of [Ni(S 2 -CN i Bu 2 ) 2 ] (2) in oleylamine occurs via an initial amide-exchange process to generate the primary amine derivative   [Ni(S 2 CNHoleyl) 2 ] 21,22 and we have evidence that [Fe(S 2 CN i Bu 2 ) 2 ] behaves in the same way. 12 Thus, while it appears that ironnickel suldes result from decomposition of [Fe(S 2 CN i Bu 2 ) 3 ] (1) and [Ni(S 2 CN i Bu 2 ) 2 ] (2), most likely it is [Ni(S 2 CNHoleyl) 2 ] and [Fe(S 2 CNHoleyl) 2 ] that are actually decomposing via extrusion of oleylNCS. We have less information about the precise nature of 4-7 in oleylamine but cannot rule out similar amideexchange reactions being important. Hence, a general comment about the decomposition of dithiocarbamate SSPs in oleylamine (and other amines) is that, while changes to the alkyl-substituents may appear to lead to different nanomaterial sizes and shapes, this is most likely related to the rate of the amide-exchange reaction rather than the relative strengths of the C-S bonds that are cleaved. This may account for the failed attempts to prepare iron-zinc and iron-indium suldes, in which we obtained only mixtures of binary suldes. Thus if amide-exchange (and subsequent decomposition) is much faster for [Fe(S 2 CN i Bu 2 ) 2 ] than either [Zn(S 2 CN i Bu 2 ) 2 ] or [In(S 2 -CN i Bu 2 ) 3 ] then it is easy to see how iron-suldes would be generated rst followed by a slower process to afford zinc or indium sulde. Consequently we are focusing our current efforts on the synthesis (and subsequent decomposition) of primary-amine dithiocarbamate derivatives, [M(S 2 CNHR) n ], since this should allow for far better control over the decomposition process (e.g. lower temperatures, non-amine solvents etc.) potentially providing access to a wider range of binary and ternary metal sulde nanomaterials.

General procedures
Unless otherwise stated, manipulations were performed under a dry, oxygen-free dinitrogen atmosphere using standard Schlenk techniques or in a MBRAUN Unilab glovebox. All solvents used were stored in alumina columns and dried with anhydrous engineering equipment, such that the water concentration was 5-10 ppm. All other reagents were procured commercially from Aldrich and used without further purication.

Physical measurements
1 H and 13 C{ 1 H} NMR spectra were obtained on either a Bruker Avance III 400 or Avance 600 spectrometers. All spectra were recorded using CDCl 3 which was dried and degassed over molecular sieves prior to use; 1 H and 13 C{ 1 H} chemical shis are reported relative to SiMe 4 . Mass spectra were obtained using either Micromass 70-SE spectrometer using Electron Ionisation (EI) or a Thermo Finnigan MAT900xp spectrometer using Fast Atom Bombardment (FAB) ionisation. Elemental analysis was carried using Elemental Analyser (CE-440) (Exeter Analytical Inc). PXRD were measured on a Bruker AXS D4 diffractometer using CuKa 1 radiation, patterns obtained being compared to database standards. For TEM characterisation a 4 ml droplet of nanoparticle suspension (CHCl 3 ) was placed on a holey carboncoated copper TEM grid and allowed to evaporate in air under ambient laboratory conditions for several minutes. TEM images were obtained using a JEOL-1010 microscope at 100 kV equipped with a Gatan digital camera. HRTEM measurements were collected using a Jeol 2100 (high resolution) TEM with a LaB 6 source operating at an acceleration voltage of 200 kv. Micrographs were taken on a Gatan Orius Charge-coupled device (CCD).

Synthesis and characterisation of molecular precursors
Na[S 2 CN i Bu 2 ]. 22 i Bu 2 NH (5.24 mL, 30 mmol) was added to NaOH (1.20 g, 30 mmol) in water (50 mL). To this mixture CS 2 (1.80 mL, 30 mmol) was added dropwise over 10 min and the mixture stirred for 1 h. The dithiocarbamate salt was then used in situ adding various equivalents to the respective metal salts being dependent upon the stoichiometry of the product.
[Fe(S 2 CN i Bu 2 ) 3 ] (1). 38 A solution of FeCl 3 (1.62 g, 10 mmol) in water (50 mL) was added dropwise over 5 min, whereupon a black precipitate formed. This mixture was vigorously stirred for 2 h, ltered, washed with water (3 Â 30 mL) and evaporated to dryness. The resulting black powder was dissolved in 100 mL of CH 2 Cl 2 and stirred with magnesium sulphate for 30  [In(S 2 CN i Bu 2 ) 3 ] (6). 41 A solution of InCl 3 (1.11 g, 5 mmol) in water (50 ml) was added dropwise over 5 min, whereupon a white precipitate formed. This mixture was vigorously stirred for 4 h, ltered, and then washed and dried, yielding a white powder. Large colourless crystals were obtained by slow evaporation from CH 2 Cl 2 . Yield 3.28 g, 90%. Anal. Calc. for C 27  [Co(S 2 CN i Bu 2 ) 3 ] (7). 42 A solution of CoCl 2 $6H 2 O (1.00 g, 4.20 mmol) in water (100 ml) was added dropwise over 5 min, whereupon a dark green precipitate formed. This mixture was vigorously stirred for 2 h, ltered, washed with water (3 Â 30 mL) and evaporated to dryness. The resulting green powder was dissolved in 100 mL of CH 2 Cl 2 and stirred with magnesium sulphate for 30 min, aer which the mixture was ltered and the ltrate dried in vacuo. Yield 1.83 g, 65%. Anal. Calc. for C 27

Decomposition studies
In a typical synthesis the dithiocarbamate complexes (2.5 mM) were added to oleylamine (20 mL) in a three-neck round bottom ask attached to a water condenser and evacuated and relled with nitrogen repeatedly for ca. 15 minutes. The solution was heated to 230 C for 1 h. The mixture was slowly cooled to room temperature, whereupon methanol (80 mL) was added with stirring. The mixture was centrifuged and the solution decanted leaving behind the resultant nanoparticles. This procedure was repeated three times and then the material was dried under vacuum.

Conflicts of interest
There are no conicts of interest to declare.