Stabilizing the elusive ortho-quinone/copper(I) oxidation state combination through π/π interaction in an isolated complex

Abstract

The heterodinuclear compound [(PhenQ)Cu(dppf)](BF₄), PhenQ = 9,10-phenanthrenequinone and dppf = 1,1'-bis(diphenylphosphino)ferrocene, was identified structurally and spectroscopically (NMR, IR, UV-vis) as a copper(I) complex of a completely unreduced ortho-quinone. Crystallographic and DFT calculation results suggest that this stabilization of a hitherto elusive arrangement is partially owed to intramolecular π/π interactions phenyl/PhenQ. Intermolecular PhenQ/PhenQ π stacking is also observed in the crystal. According to DFT calculations, the π interactions are responsible for the considerably distorted coordination geometry at Cu^I with one short and one longer Cu-O and Cu-P bond, respectively, and with bond angles at copper ranging from 99° to 133°. Electrochemical reduction proceeds reversibly at low temperatures to yield an EPR spectroscopically characterized semiquinone-copper(I) species.