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 (B2P2, 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 B2P2 with CuX (X = Cl,
Br, I) results in the formation of bimetallic complexes of the formula
(B2P2)­Cu2X2 of two different
structure types, depending on the halide. The Cu­(I) cation [Cu­(B2P2)]+ can be accessed by direct metalation
of B2P2 with [Cu­(CH3CN)4]­[PF6] or by halide abstraction with Na­[BArF4] (ArF = 3,5-bis­(trifluoromethyl)­phenyl) with
concomitant expulsion of CuX from the bimetallic Cu2X2 complexes. Metalation of B2P2 with
AgCl results in the formation of the zwitterion Ag­(B2P2)Cl featuring a diphosphine Ag cation tethered to a chloroborate
anion. Metathesis of chloride for the noncoordinating [BArF4]− affords the cation [Ag­(B2P2)]+. The cations [Cu­(B2P2)]+ and [Ag­(B2P2)]+ 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(B2P2) and Ag­(B2P2), 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.