A Series of Dinuclear Copper Complexes Bridged by Phosphanylbipyridine Ligands: Synthesis, Structural Characterization and Electrochemistry

The phosphanylbipyridine ligands 6-(diphenylphosphanyl)-4,4-dimethyl-2,2-bipyridine (PPh2-Me-2-bipy, a), 4,4-di-tert-butyl-6-(diphenylphosphanyl)-2,2-bipyridine (PPh2-tBu(2)-bipy, b), and 6-(diisopropylphosphanyl)-2,2-bipyridine (PiPr(2)bipy, c) and the corresponding dinuclear copper complexes [Cu-2(-PPh2-Me-2-bipy)(2)(NCCH3)(2)](PF6)(2) (1), [Cu-2(-PPh2-tBu(2)-bipy)(2)(NCCH3)(2)](PF6)(2) (2), [Cu-2(-PiPr(2)bipy)(2)(-NCCH3)](PF6)(2) (3), and [Cu-2(-PiPr(2)bipy)(2){-CNCH(CH3)(2)}](PF6)(2) (4) were synthesized. The X-ray structures of 1-4 show that the complexes are dinuclear with the bidentate bipyridine coordinating to one copper atom and the phosphane moiety coordinating the other copper center. Complexes 3 and 4 possess short Cu-Cu distances with bridging acetonitrile and isocyanide ligands. The cyclic voltammograms of 1-4 were examined under N-2 and CO2. Under N-2, 1-3 show four quasi-reversible 1e(-) reductions, and under CO2, they show current enhancement at the second reduction. In comparison, complex 4 shows four irreversible reductions under N-2 and no current enhancement under CO2.