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

Article abstract of DOI:10.1016/s0020-1693(97)05704-6

Dinuclear organoplatinum compounds R_nPt(bpym)PtR_n (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, ¹H NMR or EPR spectroscopy and UV-Vis spectroelectrochemistry. Oxidation state assignments based on these data could be made for the following compounds: R₂Pt^II(bpym)Pt^IIR₂, [R₂Pt^II(bpym^-I)Pt^IIR₂] ^?-, [R₂Pt^II(bpym^-II)Pt^IIR ₂]^2-, [Mes₂Pt^III(bpym)Pt^IIIMes₂] ²⁺, Me₄Pt^IV(bpym)Pt^IVMe₄, [Me₄Pt^IV(bpym^-I)Pt^IVMe ₄]^?-. The two-electron oxidation of MeS₂Pt(bpym)PtMes₂, i.e. the absence of a stable Pt(III)/Pt(II) mixed-valent state, confirms that the highest-lying occupied metal orbitals (d^z2) do not communicate via the π system of the bridging ligand. A crystal structure analysis of photoreactive Me₄Pt^IV(bpym)Pt^IVMe₄ was obtained: space group Pbca, a := 12.830(2) A?, b = 11.502(2) A?, c = 12.877(2) A?, V = 1900.3(5) A?³, Z=4. The structural refinement revealed elongated axial Pt-methyl bonds of about 2.13 A?. The structural features could be well reproduced by a DFT calculation of H₄Pt^IV(bpym)Pt^IVH₄ which also confirmed the σ 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.