AXIAL SHIELDING OF 5D(8) AND 5D(7) METAL CENTERS IN DIMESITYLPLATINUM COMPLEXES WITH UNSATURATED CHELATE LIGANDS - SPECTROSCOPIC AND SPECTROELECTROCHEMICAL STUDIES OF 4 DIFFERENT OXIDATION-STATES

Platinum(II) complexes (NN)PtMes(2), Mes = mesityl, were synthesized with the alpha-diimmine chelate ligands N N = 2,2'-bipyridine, 2,2'-bipyrazine, 2,2'- and 4,4'-bipyrimidine, 1,4,7,10-tetraazaphenanthrene, and dipyrido[3,2-a:2',3'-c]phenazine. The compounds can be reversibly reduced to EPR and UV/Vis/near-IR spectroelectrochemically detectable radical complexes [(N N-(I))Pt(II)Mes(2)](-) and to dianions [(NN-II)Pt(II)Mes(2)](2-). Reversible one-electron oxidation is also possible, leading to cations [(NN)Pt(III)Mes(2)](+), which are EPR-silent even at 4 Kbut exhibit the typical ligand-field transitions for planar low-spin d(7) ions. The unusual persistence of the Pt(III) state results from an effective protection of the axial positions by two mesityl groups which also block oxidative addition reactions. In contrast, the complexes (bpy)PtClMes and (bpy)Pt(o-CF(3)Ph)(2), o-CF(3)Ph = o-(trifluoromethyl)phenyl, have better accessible axial positions and are no longer oxidized reversibly. In agreement with the spectroelectrochemical results for the singly oxidized and reduced states, the neutral precursor molecules are distinguished by low-lying MLCT excited states which give rise to solvatochromic absorption and emission features. Comparison of the data from optical spectroscopy and from reversible one-electron redox processes allowed us to determine the solvent-dependent contributions from intra- and intermolecular reorganization following MLCT excitation.