Effects of chlorine substituents upon the formation, reactivity and electrochemical properties of platinum(II) and platinum(IV) metallacycles

The reactions of [Pt2Me4(mu-SMe2)(2)] (1) with chlorinated ligands Me2NCH2CH2N=CHAr (Ar = C6Cl5 (2a); 2,3,6-C6H2Cl3 (2b); 2,3,5-C6H2Cl3 (2c); 2,4-C6H3Cl2 (2d); 3,5-C6H3Cl2 (2e) and 3-C6H4Cl (2f)) yield either cyclometallated [C,N,N'] platinum(IV) complexes [PtMe2Cl(Me2NCH2CH2N=CHR-C,N,N')], arising from C-Cl bond activation, or cyclometallated [C,N,N'] platinum(II) complexes [PtMe(Me2NCH2CH2N=CHR-C,N,N')], arising from C-H bond activation, followed by methane elimination. These processes occur at room temperature except for the formation of compound [PtMe{Me2NCH2CH2N=CH(3,5-C6H2Cl2)}] (4e) which is produced in refluxing toluene, since at room temperature cyclometallation of ligand 2e is not achieved. Compound [PtMe2 {Me2NCH2CH2N=CH(3,5-C6H3Cl2)-N,N')}] (3e), arising from coordination of the ligand to the platinum center, is obtained at room temperature. The reactions of 1 with ligands PhCH2N=CHAr (Ar = 2,3,6-C6H2Cl3 (2g) and 2,3,5-C6H2Cl3 (2h)) produce cyclometallated [C,NJ platinum(IV) complexes. The reactivities of the platinum complexes towards phosphines and methyl iodide have been studied. All complexes have been characterized by NMR spectroscopy and the X-ray crystal structure of [PtMe2Cl {Me2NCH2CH2N=CH(3,5-C6H2Cl2)-C,N,N'}] (4c) has been determined. The electrochemical properties of the compounds based on cyclic voltammetry have also been studied. While the first reduction step is nearly reversible for cyclometallated platinum(II) compounds, coordination complex 3e and cyclometallated platinum(IV) compounds exhibit an irreversible reduction wave. In all cases oxidation occurs in an irreversible manner. The processes involved and the influence of the chlorine substituents are discussed. (C) 1998 Elsevier Science S.A. All rights reserved.