Coordination mode of some polypyridyl ligands towards the platinum(II) complexes [Pt(dppf)(H2O)2](CF3SO3)2 (dppf=1,1′-bis(diphenylphosphino)ferrocene) and [Pt(Me2SO)2Cl2].: Crystal structure of the seven-membered chelate ring complex [Pt(dpq)Cl2] (dpq=2,3-di(2-pyridyl)quinoxaline)

A series of novel platinum(II) complexes were isolated through the reaction of the polypyridyl ligands 2,3,7,8-tetra(2-pyridyl)pyrazino[2,3-g]quinoxaline (tpq), 2,3-di(2-pyridyl)pyrazine (dpp), 2,3-di(2-pyridyl)quinoxaline (dpq) and 6,7-dimethyl-2,3-di(2-pyridyl)quinoxaline (ddpq) with the heterobimetallic complex [Pt(dppf)(H2O)(2)](CF3SO3)(2) or with the dimethyl sulfoxide complex [Pt(Me2SO)(2)Cl-2]. The di-platinum complex [(Pt(dppf)}(2)(tpq)](CF3SO3)(4) and the mono-platinum compounds [Pt(dppf)(L)](CF3SO3)(2) (L = dpp, ddpq) and [Pt(L)Cl-2] (L = dpq, ddpq) were synthesized. As shown by H-1 NMR spectroscopy, in these complexes the bridging tpq and the mononucleating ligands dpp, dpq and ddpq present an uncommon bidentate chelate mode; they are bonded to the Pt(II) centres only by the means of their pyridyl nitrogen atoms. This behaviour was corroborated through the X-ray crystallographic study of the [Pt(dpq)Cl-2] derivative. The redox properties of the three new dppf-based complexes, in dichloromethane solution, show in all cases that they undergo ferrocene-based (dppf) oxidations and that the primary cathodic processes are tpq-centered in the tpq complex and platinum(II)-centered in the dpp and ddpq complexes. (C) 2000 Elsevier Science S.A. All rights reserved.