Axial shielding of 5d8 and 5d7 metal centers in dimesitylplatinum complexes with unsaturated chelate ligands: Spectroscopic and spectroelectrochemical studies of four different oxidation states

Article abstract of DOI:10.1021/om00003a019

Platinum(II) complexes (N N)PtMeS₂, Mes = mesityl, were synthesized with the α-diimine 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^IIMes₂]^- and to dianions [(N N^-II)Pt^IIMes₂]^2-. Reversible one-electron oxidation is also possible, leading to cations [(N N)Pt^IIIMes₂]⁺, 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 block oxidative addition reactions. In contrast, 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 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.