Redox-Active Metallodithiolene Groups Separated by Insulating Tetraphosphinobenzene Spacers

Compounds of the type [(S₂C₂R₂)M(μ-tpbz)M(S₂C₂R₂)] (R = CN, Me, Ph, p-anisyl; M = Ni, Pd, Pt; tpbz = 1,2,4,5-tetrakis(diphenylphosphino)benzene) have been prepared by transmetalation with [(S₂C₂R₂)SnR'₂] reagents, by direct displacement of dithiolene ligand from [M(S₂C₂R₂)₂] with 0.5 equiv of tpbz, or by salt metathesis using Na₂[S₂C₂(CN)₂] in conjunction with X₂M(μ-tpbz)MX₂ (X = halide). X-ray crystallography reveals a range of topologies (undulating, chair, bowed) for the (S₂C₂)M(P₂C₆P₂)M(S₂C₂) core. The [(S₂C₂R₂)M(μ-tpbz)M(S₂C₂R₂)] (R = Me, Ph, p-anisyl) compounds support reversible or quasireversible oxidations corresponding to concurrent oxidation of the dithiolene terminal ligands from ene-1,2-dithiolates to radical monoanions, forming [(^-S^•SC₂R₂)M(μ-tpbz)M(^-S^•SC₂R₂)]²⁺. The R = Ph and p-anisyl compounds support a second, reversible oxidation of the dithiolene ligands to their α-dithione form. In contrast, [(S₂C₂(CN)₂)Ni(tpbz)Ni(S₂C₂(CN)₂)] sustains only reversible, metal-centered reductions. Spectroscopic examination of [(^-S^•SC₂( p-anisyl)₂)Ni(μ-tpbz)Ni(^-S^•SC₂( p-anisyl)₂)]²⁺ by EPR reveals a near degenerate singlet-triplet ground state, with spectral simulation revealing a remarkably small dipolar coupling constant of 18 × 10^-4 cm^-1 that is representative of an interspin distance of 11.3 Å. This weak interaction is mediated by the rigid tpbz ligand, whose capacity to electronically insulate is an essential quality in the development of molecular-based spintronic devices.