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

Platinum(II) complexes (NN)PtMes(2), Mes = mesityl, were synthesized w ith the alpha-diimmine chelate ligands N N = 2,2'-bipyridine, 2,2'-bip yrazine, 2,2'- and 4,4'-bipyrimidine, 1,4,7,10-tetraazaphenanthrene, a nd dipyrido[3,2-a:2',3'-c]phenazine. The compounds can be reversibly r educed to EPR and UV/Vis/near-IR spectroelectrochemically detectable r adical 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, l eading to cations [(NN)Pt(III)Mes(2)](+), which are EPR-silent even at 4 Kbut exhibit the typical ligand-field transitions for planar low-sp in d(7) ions. The unusual persistence of the Pt(III) state results fro m an effective protection of the axial positions by two mesityl groups which also block oxidative addition reactions. In contrast, the compl exes (bpy)PtClMes and (bpy)Pt(o-CF(3)Ph)(2), o-CF(3)Ph = o-(trifluorom ethyl)phenyl, have better accessible axial positions and are no longer oxidized reversibly. In agreement with the spectroelectrochemical res ults for the singly oxidized and reduced states, the neutral precursor molecules are distinguished by low-lying MLCT excited states which gi ve rise to solvatochromic absorption and emission features. Comparison of the data from optical spectroscopy and from reversible one-electro n redox processes allowed us to determine the solvent-dependent contri butions from intra- and intermolecular reorganization following MLCT e xcitation.