Stabilizing the Elusive ortho-Quinone/Copper(I) Oxidation State Combination through π/π Interaction in an Isolated Complex

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 CuI 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.