Redox routes to arenechromium complexes of two-, three- and four-electron alkynes; structure and bonding in paramagnetic [Cr(CO)L(η-RCCR)(η-arene)]+

Abstract

X-Ray structural studies on the redox pair [Cr(CO)₂(η-PhCCPh)(η-C₆Me₅H)]^z (z = 0 and 1) show that one-electron oxidation of the neutral complex results in a shortening of the Cr–C_alkyne bonds and a lengthening of the Cr–C(O) bonds, consistent with depopulation of a HOMO antibonding with respect to the metal–alkyne interaction. Oxidation leads to an increase in the substitutional lability of the Cr–CO bonds so that [Cr(CO)₂(η-RCCR)(η-C₆Me₆)]⁺ (R = Ph or C₆H₄OMe-p) reacts with Lewis bases to give [Cr(CO)L(η-RCCR)(η-C₆Me₆)]⁺ {L = CNXyl, P(OMe)₃ and P(OCH₂)₃CEt}, X-ray studies on which show a rotation of the alkyne to align with the remaining Cr–CO bond. ESR spectroscopic studies on [Cr(CO)L(η-RCCR)(η-C₆Me₆)]⁺ show delocalisation of the unpaired electron onto the alkyne ligand, consistent with its description as a three-electron donor. The cations [Cr(CO)L(η-RCCR)(η-C₆Me₆)]⁺ undergo both one-electron reduction and oxidation, and chemical oxidation of [Cr(CO){P(OCH₂)₃CEt}(η-p-MeOC₆H₄CCC₆H₄OMe-p)(η-C₆Me₆)]⁺ with AgPF₆ gives the dication [Cr(CO){P(OCH₂)₃CEt}(η-p-MeOC₆H₄CCC₆H₄OMe-p)(η-C₆Me₆)]²⁺. Thus the two-electron alkyne of [Cr(CO)₂(η-RCCR)(η-C₆Me₆)] is converted into the four-electron alkyne of [Cr(CO)L(η-RCCR)(η-C₆Me₆)]²⁺ by an ECE (E = electrochemical, C = chemical) process in which all of the intermediates have been fully characterised.