Oxidation of CpRu(CO)(PMe3)H by 2/3, 1, and 2 electrons by the judicious choice of reaction conditions. Generation of a bridging hydride via the reaction between a 17-electron metal hydride cation radical and its conjugate base

Article abstract of DOI:10.1021/ja00025a020

The hydride complex CpRu(CO)(PMe₃)H (1) undergoes a chemically irreversibe oxidation at 0.34 V vs the ferrocene/ferrocenium couple. By the judicious choice of reaction conditions, the oxidation of 1 in acetonitrile may be directed towards a 2/3-, 1-, or 2-electron process. The room temperature oxidation of 1 with Cp₂Fe⁺PF₆^- in acetonitrile-d₃, leads to the virtually quantitative (by ¹H NMR) generation of [CpRu(CO)(PMe₃)]₂(μ-H)⁺ (5), HRu(CO)(PMe₃)(NCCD₃)₃⁺ (4-d₉), and cyclopentadiene in a 1:1:1 molar ratio, in accord with an overall 2/3-electron oxidation of 1. A trace of CpRu(CO)-(PMe₃)(NCCD₃)⁺ (2-d₃) was also observed. Dihydrogen complex CPRu(CO)(PMe₃)(η²-H₂)⁺ (3) was observed as an intertermediate in the reaction, and it reacted to form 4-d₉ and cyclopentadiene. Ferrocenium oxidation of 1 in the presence of 2,6-lutidine procceeded as a 1-electron process, yielding 5 and H⁺. In the presence of pyrrolidine, a 2-electron process ensued and resulted in the formation of 2-d₃ and H⁺. Coulometry and preparative-scale experiments performed in acetonitrile proceeded somewhat differently. The electrochemical oxidation was a 1-electron process in the absence of base, yielding mostly 2 and 4, whereas a 2 electron process took place in the presence of 2,6-lutidine or pyrrolidine, yielding 2. The key to explaining all of the results is that an initial proton-transfer reaction from 1^?+ to a base occurs. This transfer may take fee rapidlyfrom 1^?+ to pyrrolidine, but only slowly to 2,6-lutidine. Proton transfer from 1^?+ to the base yields the radical CpRu(CO)(PMe₃)^?. The latter results in the formation of 5 via combination with 1^?+ when 1^?+ concentrations are relatively high. This is the case when 1 or 2,6-lutridine functions as a base. Otherwise, a second oxidation of CpRu(CO)(PMe₃)? produces 2. Pyrrolidine effects the rapid deprotonation of the cation radical, effectively depleting the solution of 1^?+ and closing the pathway leading to 5.