Molecular and electronic structures of bis-(o-diiminobenzosemiquinonato)metal(II) complexes (Ni, Pd, Pt), their monocations and -anions, and of dimeric dications containing weak metal-metal bonds

Two series of square planar, diamagnetic, neutral complexes of nickel(II), palladium(II), and platinum(II) containing two N,N-coordinated o-diiminobenzosemiquinonate(1-) pi radical ligands have been synthesized and characterized by UV-vis and H-1 NMR spectroscopy: [M-II(L-2(ISQ))(2)], M = Ni (1), Pd (2), Pt (3), and [M-II(L-3(ISQ))(2)] M = Ni (4), Pd (5), Pt (6). H-2[L-2(PDI)] represents 3,5-di-tert-butyl-o-phenylenediamine and H-2[L-3(PDI)] is N-phenyl-o-phenylenediamine; (L-ISQ)(1-) is the o-diiminobenzosemiquinonate pi radical anion, and (L-IBQ)(0) is the o-diiminobenzoquinone form of these ligands. The structures of complexes 1, 4, 5, and 6 have been (re)determined by X-ray crystallography at 100 K. Cyclic voltammetry established that the complete electron-transfer series consisting of a dianion, monoanion, neutral complex, a mono- and a dication exists: [M(L)(2)](z) = -2, -1, 0, 1+, 2+. Each species has been electrochemically generated in solution and their X-band EPR and UV-vis spectra have been recorded. The oxidations and reductions are invariably ligand centered. Two o-diiminobenzoquinones(0) and two fully reduced o-diiminocatecholate-(2-) ligands are present in the dication and dianion, respectively, whereas the monocations and monoanions are delocalized mixed valent class III species [M-II(L-ISQ)(L-IBQ)](+) and [M-II(L-ISQ)(L-PDI)](-), respectively. One-electron oxidations of 1 and trans-6 yield the diamagnetic dications {cis-[Ni-II(L-2(ISQ))(L-2(IBQ))](2)}Cl-2 (7) and {trans[pt(II)(L-3(ISQ))(L-3(IBQ))](2)}(CF3SO3)(2) (8), respectively, which have been characterized by X-ray crystallography; both complexes possess a weak M...M bond and the ligands adopt an eclipsed configuration due to weak bonding interactions via pi stacking.