Copper and Silver Complexes of a Redox-Active Diphosphine-Diboraanthracene Ligand.

Redox-active ligands and Z-type acceptor ligands have emerged as promising strategies for promoting multielectron redox chemistry at transition-metal centers. Herein, we report the synthesis and characterization of copper and silver complexes of a diphosphine ligand featuring a diboraanthracene core (B2 P2 , 9,10-bis(2-(diisopropylphosphino)phenyl)-9,10-dihydroboranthrene) that is capable of serving as both a redox reservoir and a Z-type ligand. Metalation of B2 P2 with CuX (X = Cl, Br, I) results in the formation of bimetallic complexes of the formula (B2 P2 )Cu2 X2 of two different structure types, depending on the halide. The Cu(I) cation [Cu(B2 P2 )]+ can be accessed by direct metalation of B2 P2 with [Cu(CH3 CN)4 ][PF6 ] or by halide abstraction with Na[BArF 4 ] (ArF = 3,5-bis(trifluoromethyl)phenyl) with concomitant expulsion of CuX from the bimetallic Cu2 X2 complexes. Metalation of B2 P2 with AgCl results in the formation of the zwitterion Ag(B2 P2 )Cl featuring a diphosphine Ag cation tethered to a chloroborate anion. Metathesis of chloride for the noncoordinating [BArF 4 ]- affords the cation [Ag(B2 P2 )]+ . The cations [Cu(B2 P2 )]+ and [Ag(B2 P2 )]+ exhibit quasireversible reduction events at ∼ -1.6 V versus the ferrocene/ferrocenium redox couple, and the thermally sensitive radicals that result from their reduction, Cu(B2 P2 ) and Ag(B2 P2 ), were characterized by EPR spectroscopy and, in the case of the latter, single-crystal X-ray diffraction. Electronic structure calculations suggest these neutral radicals are best described as zwitterions with reduction centered at the diboraanthracene core.