Molecular and Electronic Structures of Bis-(<i>o</i>-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-) π radical ligands have been synthesized and characterized by UV−vis and 1H NMR spectroscopy: [MII(2LISQ)2], M = Ni (1), Pd (2), Pt (3), and [MII(3LISQ)2] M = Ni (4), Pd (5), Pt (6). H2[2LPDI] represents 3,5-di-tert-butyl-o-phenylenediamine and H2[3LPDI] is N-phenyl-o-phenylenediamine; (LISQ)1- is the o-diiminobenzosemiquinonate π radical anion, and (LIBQ)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 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 [MII(LISQ)(LIBQ)]+ and [MII(LISQ)(LPDI)]-, respectively. One-electron oxidations of 1 and trans-6 yield the diamagnetic dications {cis-[NiII(2LISQ)(2LIBQ)]2}Cl2 (7) and {trans-[PtII(3LISQ)(3LIBQ)]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 π stacking.