Redox chemistry of dimethylplatinum(II) diimine complexes. Oxidatively induced Pt-Me transfer between transient platinum(III) cation radicals

The redox chemistry of the series of Pt(II) diimine complexes L2PtMe2 (1; L-2 = Ar-N= CRCR=N-Ar, where Ar/R = 4-MeC6H4/H (a), 4-MeOC6H4/H (b), 4-MeC6H4/Me (c), 4-MeOC6H4/Me (d)), with particular emphasis on the oxidation processes, has been studied in detail. As seen by cyclic voltammetry, 1a-d undergo two successive, reversible one-electron reductions at the diimine ligands and an irreversible, metal-centered one-electron oxidation. The oxidation of Ib has been investigated in some detail. Chemical oxidation of Ib with Cp2Fe+PF6- in acetonitrile yields a near 1:1 ratio of the corresponding Pt(II) and Pt(IV) cations L2Pt(NCMe)Me+ (2b) and fac-L2Pt(NCMe)Me-3(+) (3b). Controlled-potential electrolysis of Ib yields mixtures of 2b and 3b in a 1:1 ratio, as well as the cis,cis (4b) and one cis,trans (5b) isomer of the dicationic Pt(IV) complexes L2Pt(NCMe)(2)Me-2(2+). The percentage of the dications 4b and 5b depended on the electrode potential. A mechanism involving methyl group transfer between two transient Pt(III) intermediates L2PtMe2.+ is proposed to account for the generation of 2b and 3b, whereas further oxidation of the Pt(III) species at the electrode eventually provides 4b and 5b. The X-ray crystal structures of 1b and 3b(OTf-) have been determined. All Pt-Me bond distances in these two species are essentially identical, averaging 2.057(1) Angstrom.