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[BAr^F₄] (Ar^F = 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 [BAr^F₄]^- 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.