Copper clusters, reactions with carbon monoxide

In 1943, Hieber and Lagally reported that the reaction of anhydrous rhodium trichloride with carbon monoxide at 80°C, under pressure, and in the presence of silver and copper as halogen acceptors, gave a black crystalline product which, on the basis of elemental analysis, was formulated as Rh4(CO)n 75). The exact nature of this compound was established 20 years later by Dahl using three-dimensional X-ray analysis which led to its reformulation as Rh6(CO)i6 53). This discovery can be regarded as the birthday of the chemistry of high nuclearity clusters. 

The hexanuclear carbonyl Rh6(CO)ie was first prepared by treating anhydrous rhodium trichloride with carbon monoxide at 200 atm for several hours in the presence of a halogen acceptor such as cadmium, copper powder, silver, or zinc at temperatures of 80-230°.i At temperatures of 50-80°, the main product was Rh4(CO)i2. Optimum yields (80-90 %) of Rh6(CO)i6 are obtained by allowing methanolic solutions of rhodium trichloride trihydrate to react with carbon monoxide at 40 atm and 60°. Recently, however, there have been reports - of new high-yield syntheses of rhodium cluster carbonyls which require only ambient pressures of carbon monoxide. Chini and Martinengo have obtained Rh6(CO)i6 and Rh4(CO)i2 in high yield from the reaction of Rh2(CO)4Cl2 with carbon monoxide at atmospheric pressure and room temperature. 

With respect to the thermodynamic stability of metal clusters, there is a plethora of results which support the spherical Jellium model for the alkalis as well as for other metals, like copper. This appears to be the case for cluster reactivity, at least for etching reactions, where electronic structure dominates reactivity and minor anomalies are attributable to geometric influence. These cases, however, illustrate a situation where significant addition or diminution of valence electron density occurs via loss or gain of metal atoms. A small molecule, like carbon monoxide,... 

When the cluster [Pd4(C0)4(02CMe)4]-2MeC02H was dissolved in styrene and treated with methanol under an atmosphere of carbon monoxide, methyl cinnamate was formed.516 This reaction was also believed to occur by an alkoxycarbonyl route. The reaction became catalytic when [Pd(OAc)2] (106) was used in presence of NaOAc and a stoichiometric amount of copper(II) as reoxidant for the palladium(O) formed. Stille and co-workers have investigated this reaction, sometimes called carboalkoxylation, in detail. A basic difference between this reaction and the hydroesterification described above is that the oxidative nature of carboalkoxylation permits double functionalization of a double bond. Thus (E)- and (Z)-2-butene react readily with CO and methanol in the presence of a catalytic amount of PdCl2 and a stoichiometric amount of CuCl2 to give methyl 3-methoxy-2-methylbutanoate (equation 124).517,518... 

The high initial activity of the fresh alumocopperchromium catalyst in the oxidation of carbon monoxide is due to Cufll) cations within the copper chromite and in CuO surface clusters (Fig 1) Centres with chromium cations in the highest oxidation degrees are bound to the support surface and solid solutions of the aluminate type. These contain cations of bivalent copper and are less active in the CO oxidation reaction. It was shown by IRS and XPS that during the use of this catalyst in CHG, as well as during the unsteady-state oxidation reaction/ a partial reduction of Cu(II) to Cu I occurs (Fig, 2), which leads to a decrease in catalyst activity in the oxidation of the carbon monoxide. In the fresh alumomagnesiumchromium catalyst, the activity for carbon monoxide is determined by the centres containing chromium cations in... 

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