Electronic and molecular structure of 2,2'-bipyrimidine-bridged bis(organoplatinum) complexes in various oxidation states. Radical-bridged diplatinum species and the absence of a Pt(III)/Pt(II) mixed-valent intermediate

Dinuclear organoplatinum compounds RnPt( bpym)PtRn (R = Me = methyl, n = 2, 4; R = Mes = mesityl, n = 2; bpym = 2,2'-bipyrimidine) were prepared and characterized. The neutral compounds and their neighboring oxidation states were studied using cyclic voltammetry, H-1 NMR or EPR spectroscopy and UV-Vis spectroelectrochemistry. Oxidation state assignments based on these data could be made for the following compounds: R2PtII(bpym)(PtR2)-R-II [R2PtII(bpym(-I))(PtR2)-R-II](.-), [R2PtII(bpym(-II))(PtR2)-R-II](2-), [Mes(2)Pt(III)(bpym)Pt(III)Mes(2)](2+), Me4PtIV(bpym)(PtMe4)-Me-IV, [Me4PtIV(bpym(-I))(PtMe4)-Me-IV](.-). The two-electron oxidation of Mes(2)Pt(bpym)PtMes(2), i.e. the absence of a stable Pt(III)/Pt(II) mixed-valent state, confirms that the highest-lying occupied metal orbitals (d(Z)2) do not communicate via the pi system of the bridging ligand. A crystal structure analysis of photoreactive Me4PtIV(bpym)(PtMe4)-Me-IV was obtained: space group Pbca, a = 12.830(2) Angstrom, b = 11.502(2) Angstrom, c = 12.877(2) Angstrom, V = 1900.3(5) Angstrom(3), Z = 4. The structural refinement revealed elongated axial Pt-methyl bonds of about 2.13 Angstrom. The structural features could be well reproduced by a DFT calculation of H4PtIV(bpym)(PtH4)-H-IV which also confirmed the sigma character of the HOMO. Of the ligand-reduced systems, the Pt(II) species exhibits a higher degree of g anisotropy than the Pt(IV) species, Low-lying excited states of the species generated were assigned through information from vibrational structuring, solvatochromic behavior and from (spectro) electrochemical results. (C) 1997 Elsevier Science S.A.