Synthesis and redox properties of a phosphine-subsituted para-dioxolene and its bimetallic palladium complex

Oxidation of 2,5-bis(diphenylphosphino)-1,4-hydroquinone (8) with iodobenzene diacetate produces the corresponding bis(phosphine) substituted benzoquinone (9), the first phosphine-substitued quinone. Cyclic voltammetry studies reveal that the redox functionality of the quinone unit in 9 is retained, and the reduction potentials render this compound slightly more easily reduced than the parent p-benzoquinone. Reaction of the precursor hydroquinone 8 with Pd(hfac)2 affords a binuclear complex 10 with the hydroquinonate ligand bridging two Pd(hfac) substrates. The redox activity of the bridging dioxolene ligand is retained in complex 10, although there are significant changes in the redox potentials relative to those of the free quinone 9. Chemical oxidation of 10 with AgPF6 yields a persistent cationic complex 11, which, based on EPR and electronic spectroscopy, can be formulated as containing a bridging semiquinone ligand.Key words: p-quinones, phosphines, bridging ligands, redox properties.