Axial Shielding of 5d8 and 5d7 Metal Centers in Dimesitylplatinum Complexes with Unsaturated Chelate Ligands: Spectroscopic and Spectroelectrochemical Studies of Four Different Oxidation States

, und . Organometallics 14 (3): 1176--1186 (1995)


Pt(II) complexes (N-N)PtMes2, Mes = mesityl, were synthesized with the a-diimine chelate ligands N-N = 2,2'-bipyridine, 2,2'-bipyrazine, 2,2'-, 4,4'-bipyrimidine, 1,4,7,10-tetraazaphenanthrene, and dipyrido3,2-a:2',3'-cphenazine. The compds. can be reversibly reduced to EPR and UV/visible/near-IR spectroelectrochem. detectable radical complexes (N-N-I)PtIIMes2- and to dianions (N-N-II)PtIIMes22-. Reversible 1-electron oxidn. is also possible, leading to cations (N-N)PtIIIMes2+, which are EPR-silent even at 4 K but exhibit the typical ligand-field transitions for planar low-spin d7 ions. The unusual persistence of the Pt(III) state results from an effective protection of the axial positions by two mesityl groups which also blocks oxidative addn. reactions. The complexes (bpy)PtClMes and (bpy)Pt(o-CF3Ph)2, o-CF3Ph = o-(trifluoromethyl)phenyl, have better accessible axial positions and are no longer oxidized reversibly. In agreement with the spectroelectrochem. results for the singly oxidized and reduced states, the neutral precursor mols. 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 1-electron redox processes allowed one to det. the solvent-dependent contributions from intra- and intermol. reorganization following MLCT excitation. on SciFinder(R)

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