Cobalt clusters reaction with carbon monoxide

Finally, cobalt carbide-carbonyl clusters have recently been isolated through a two-step synthesis. First of all, the well-known Co3(CO)9CCl is prepared from Co2(CO)8 and CCh, and then the hexanuclear carbide dianion [Co6(CO)i5C]2- is obtained in good yields (9) by further reaction with Xa[(, o(CO)4] in diisopropylether [see Eqs. (18) and (19)]. Further redox condensation between [Oo6(CO)i5C]2-and Co4(CO)i2 [see Eq. (11)] gives the square antiprismatic octanuclear cluster [Cdo8(Cdt )i8 C]2- (13). Both these carbide derivatives, as well as all of the other cobalt high nuclearity clusters, are sensitive to air and react with carbon monoxide at atmospheric pressure. 

The cobalt carbonyls are prepared by the disproportionation reaction of [Co2(CO)g] in the presence of Lewis bases or by the reduction of cluster cobalt carbonyls with the alkali metals. The iridium compounds are obtained during reduction of [Ir4(CO)i2] with sodium in ether solution. The rhodium carbonyls are usually synthesized by reduction of [Rh2Cl2(CO)4] or [RhClg] " with carbon monoxide in basic medium or by nucleophilic attack of bases on the carbonyl group of carbonyl clusters (see preparation of [M4(CO)i2] and [M6(CO)i6]). 

The chemical reactivity of cobalt cluster anions, Co (n = 2-8), toward 02, N2, and CO have been investigated using a flow tube reactor (226). The reactivity was found to be in the order 02 > CO > N2 the least reactive ligand N2 only reacted with C07 and Cog. The primary reaction of oxygen was the removal of one or two cobalt atoms from the cluster. Carbon monoxide reacts by multiple additions giving saturation limits shown in Table V. 

A trinuclear cobalt(I) complex, PhCCo3(CO)9, can also catalyse the reduction of nitro compounds in the presence of hydroxide ion at room temperature under a normal pressure of CO [49]. Satisfactory results were obtained under phase transfer conditions. The catalyst and the aromatic nitro compounds were dissolved in benzene under carbon monoxide and an aqueous solution of sodium hydroxide containing cethyltrimethylammonium bromide was added. At a substrate/cat =10 ratio, ca. 60-80 % of amine was obtained in a 18 h reaction. The reaction also proceeded in a homogeneous phase (methanol-water, methanol, dioxane-water) but with lower conversions (less than 45 %). Cobalt complexes such as MeCCo3(CO)9 and MeCo(CO)4 were also active, but less effective. At the end of the reaction, the catalyst was recovered only in part (ca. 15 %). In the organic phase, an IR absorption at 1891 cm, attributable to [Co(CO)4] anion, was observed. Strangely enough, the preformed [Co(CO)4] anion has not been tested as catalyst. The active species was supposed to be the hydride cluster anion reported in Scheme 6. 

Bimetallic phase-transfer-catalysis is a process whereby a reaction that occurs using two different metal complexes, does not proceed in the absence of either metal species, or proceeds only at reduced rate. An apparent system of this class has been reported, in which Co2(CO)g and [RhCl(l,5-hexadiene)]2 mutually increased their reactivity when used as catalysts for the conversion of nitrobenzene to aniline in a biphasic system (benzene, aqueous NaOH, dodecyltrimethylammonium chloride) in a carbon monoxide atmosphere [73]. However, another member of the same research group later showed [74] that the apparent bimetallic promotion was due to the fact that the alkylammonium salt used as a phase-transfer agent actually inhibited the activity of the active rhodium complex (apparently a cluster, which is active in the absence of both the alkylammonium salt and the cobalt compound) by rendering it insoluble. The added Co2(CO)g reacts with the alkylammonium salt to generate... 

In the case of carbonyl metal clusters, the presence of free carbon monoxide notoriously affects the reduction processes inducing condensations as well as degradation of the clusters species. Thus the reduction of the tetranuclear cobalt cluster 04(00)4 with lithium, sodium, or potassium in which the final product is the hexanuclear cluster [Co6(CO)i5] is an illustrative example of the complexity of such reactions. 

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