Ab initio study of the protic conversion of an allene into an η2-vinyl complex of Re, and on their structure, bonding and redox behaviour

Abstract

Ab initio quantum-chemical methods at the RHF/UHF and MP2 levels of theory were applied to the investigation of the structure, bonding, reactivity and electrochemical behaviour of a rhenium(I)-bis(diphosphine) η²-allene complex and of the corresponding η²-vinyl product derived from protonation, by using the models trans-[ReCl(η²-H₂CCCH₂)(PH₃)₄] and trans-[ReCl{η²-C(CH₂)CH₃}(PH₃)₄]⁺. Full geometry optimization of the models was carried out and the electrostatic potential distributions and MO compositions were calculated, allowing not only the interpretation of the co-ordination bonds of the η²-allene and the η²-vinyl ligands and of the relative oxidation potentials of their complexes and study of the cathodically induced dehydrogenation of the η²-vinyl species to regenerate the parent allene complex, but also the investigation of the mechanism of protonation of the allene complex. The calculations were also performed on the possible singly and doubly protonated complexes, and the results indicate that the protic conversion of the η²-allene into the η²-vinyl complex conceivably occurs via chloro-protonated-allene and -vinyl intermediates (rather than via initial metal protonation), i.e.trans-[Re(ClH)(η²-H₂CCCH₂)(PH₃)₄]⁺ and trans-[Re(ClH){η²-C(CH₂)CH₃}(PH₃)₄]²⁺, which were not identified in a previous stopped-flow spectrophotometric study, thus supporting an unprecedented role, in the protonation reaction, of the chloro-ligand which does not behave as a mere spectator or trans-stabilizer.