EPR AND ABSORPTION-SPECTRA OF SINGLY REDUCED MONONUCLEAR AND DINUCLEAR DIORGANOPLATINUM COMPLEXES OF HETEROCYCLIC ALPHA-DIIMINES

The diorganoplatinum(II) complexes [(bpy)Pt(Ph)2] (I), [(bpym)Pt(Ph)2] (II), [(mu-bpym)(Pt(p-tol)2)2] (III), and [(mu-bpym)-(PtAd2)2] (IV) (bpy = 2,2'-bipyridine; bpym = 2,2'-bipyrimidine; Ph = phenyl; p-tol = p-tolyl; Ad = 1-adamantylmethyl) were subjected to two successive one-electron reductions, and the one-electron reduction products were studied in situ by UV-vis-near-IR and EPR spectroscopy. The first reduction is at the heterocyclic ligand, in possible contrast to the metal-centered reductions in such closely similar species as [Pt(bpy)(py)2]2+ (py = pyridine) (V). We attribute this difference to the greater ligand field strength of carbanionic as opposed to neutral N-bound ligands. While the EPR signals of the paramagnetic species show sizable g anisotropy (1.92-2.02) in frozen solution due to the large spin-orbit coupling constant of the metal, the rhombic g pattern, the low isotropic values (g(iso) < 2), and the isotropic hyperfine coupling as observed and reproduced in the case of [(bpym)Pt(Ph)2].- all suggest a predominant localization of the unpaired electron at the alpha-diimine ligand.