Homo- and Heterobinuclear Cu and Pd Complexes with a Bridging Redox-Active Bisguanidino-Substituted Dioxolene Ligand: Electronic Structure and Metal-Ligand Electron-Transfer.

A new redox-active 4,5-bisguanidino-substituted o-benzoquinone ligand L is synthesized, which allows rational access to heterobinuclear complexes through the sequential coordination of two metals. In the examples discussed in this work, mononuclear Cu and Pd complexes are prepared in a first coordination step, and these complexes are then used as precursors to homobinuclear [CuII -L0 -CuII ] and heterobinuclear [PdII -L0 -CuII ] complexes. In the heterobinuclear complex, the PdII is coordinated by the softer bisguanidine side of L and the CuII by the harder dioxolene side (in line with the HSAB concept). The heterobinuclear complex is in a temperature-dependent equilibrium with its dimer, with two unsymmetrical Cu-Cl-Cu bridges. The redox-chemistry of the [CuII -L-CuII ] and [PdII -L-CuII ] complexes is studied. One-electron oxidation of both complexes was found to be quasi-reversible in CV experiments, and chemical one-electron oxidation was achieved with NO+ (SbF6- ). In the case of the homobinuclear complex [L(CuCl2 )2 ]+ , intramolecular ligand-metal electron-transfer, triggered by coordination of a CH3 CN solvent molecule, leads to a temperature-dependent equilibrium between the form [CuII -L0 -CuIII ] at low temperatures (with CH3 CN coordinated to the CuIII atom) and [CuII -L⋅+ -CuII ] at higher temperatures (without CH3 CN).