Electron transfer in organometallic clusters. XI. Redox chemistry of M3(CO)12(M = Ru, Os) and PPh3 derivatives; mechanism of catalysed nucleophilic substitution

Alison J. Downard, Brian H. Robinson, Jim Simpson, Alan M. Bond

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Abstract

The generality of a two-electron reduction process involving an {A figure is presented} mechanism has been established for M3(CO)12 and M3(CO)12-n(PPh3)n (M = Ru, Os) clusters in all solvents. Detailed coulometric and spectral studies in CH2Cl2 provide strong evidence for the formation of an 'opened' M3(CO)122- species the triangulo radical anions M3(CO)12 having a half-life of < 10-6 s in CH2Cl2. However, the electrochemical response is sensitive to the presence of water and is concentration dependent. An electrochemical response for "opened" M3(CO)122- 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 M3(CO)112- and M6(CO)182- were confirmed by a study of the electrochemistry of these anions in CH2Cl2; HM3(CO)11- is not a product. The couple [M6(CO)18]-/2- is chemically reversible under certain conditions but oxidation of HM3(CO)11- is chemically irreversible. Different electrochemical behaviour for Ru3(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)XRu3(CO)10]- at the electrode is stopped under CO or at low temperatures but Ru3(CO)12 is still trapped by reversible attack by X presumably as [η1-C(O)XRu3(CO)11]-. It is shown that electrode-initiated electron catalysed substitution of M3(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 Ru3(CO)12 is only specific and fast in ether solvents particulary THF. Metalmetal bond cleavage is the most important influence on the rate and specificity of catalytic substitution by electron or [PPN]-initiation. The redox chemistry of M3(CO)12 clusters (M = Fe, Ru, Os) is a consequence of the relative rates of metalmetal bond dissociation, metal-metal bond strength and ligand dissociation and in many aspects resembles their photochemistry.

Original languageEnglish
Pages (from-to)363-384
Number of pages22
JournalJournal of Organometallic Chemistry
Volume320
Issue number3
DOIs
Publication statusPublished - 17 Feb 1987

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