Electron transfer in organometallic clusters: XI. Redox chemistry of M 3(CO) 12(M = Ru, Os) and PPh 3 derivatives; mechanism of catalysed nucleophilic substitution

Abstract

The generality of a two-electron reduction process involving an ▪ mechanism has been established for M 3(CO) 12 and M 3(CO) 12− n(PPh 3) n (M = Ru, Os) clusters in all solvents. Detailed coulometric and spectral studies in CH 2Cl 2 provide strong evidence for the formation of an ‘opened’ M 3(CO) 12 2− species the triangulo radical anions M 3(CO) 12 −· having a half-life of < 10 −6 s in CH 2Cl 2. However, the electrochemical response is sensitive to the presence of water and is concentration dependent. An electrochemical response for “opened” M 3(CO) 12 2− is only detected at low concentrations < 5 × 10 −4 mol dm −3 and under drybox conditions. The electroactive species ground at higher concentrations and in the presence of water M 3(CO) 11 2− and M 6(CO) 18 2− were confirmed by a study of the electrochemistry of these anions in CH 2Cl 2; HM 3(CO) 11 − is not a product. The couple [M 6(CO) 18] −/2− is chemically reversible under certain conditions but oxidation of HM 3(CO) 11 − is chemically irreversible. Different electrochemical behaviour for Ru 3(CO) 12 is found when [PPN][X] (X = OAc −, Cl −) salts are supporting electrolytes. In these solutions formation of the ultimate electroactive species [μ-C(O)XRu 3(CO) 10] − at the electrode is stopped under CO or at low temperatures but Ru 3(CO) 12 −· is still trapped by reversible attack by X presumably as [η 1-C(O)XRu 3(CO) 11] −. It is shown that electrode-initiated electron catalysed substitution of M 3(CO) 12 only takes place on the electrochemical timescale when M = Ru, but it is slow, inefficient and non-selective, whereas BPK-initiated nucleophilic substitution of Ru 3(CO) 12 is only specific and fast in ether solvents particulary THF. Metalmetal bond cleavage is the most important influence on the rate and specificity of catalytic substitution by electron or [PPN]-initiation. The redox chemistry of M 3(CO) 12 clusters (M = Fe, Ru, Os) is a consequence of the relative rates of metalmetal bond dissociation, metal-metal bond strength and ligand dissociation and in many aspects resembles their photochemistry.