Spectroscopic (UV/VIS, resonance Raman) and spectroelectrochemical study of platinum(II) complexes with 2,2′-bipyridine and aromatic thiolate ligands

A series of complexes [Pt(bpy)(4-XC6H4S)2] (bpy = 2,2′-bipyridine; X = NO2, H, MeO or Me2N) have been synthesized and characterised spectroscopically. All absorb moderately in the visible region; the molar absorption coefficients of the solvatochromic absorption band lie below the values for the corresponding dithiolate complexes, indicating a slightly distorted planar geometry and a smaller HOMO – LUMO overlap. Resonance Raman, cyclic voltammetric and UV/VIS spectroelectrochemical experiments were carried out to confirm the charge transfer-to-diimine character of the visible electronic transition(s) directed from a p(S)–π(Ph)/d(Pt) delocalised orbital manifold to the π1* LUMO of the 2,2′-bipyridine ligand. Consistent with this bonding situation: (i) the reduction of [Pt(bpy)(4-XC6H4S)2] is initially localised on the bpy ligand; for X = NO2 the second and third added electrons enter the vacant π* orbitals on the 4-NO2 substituents; (ii) the resonance Raman spectra of [Pt(bpy)(4-MeOC6H4S)2] show, besides the internal bpy modes, only a weak effect for intrathiolate (Ph–S) and ν(Pt–S) vibrations; and (iii) the oxidation is initially a one-electron process due to electronic interaction of the thiolate ligands through the platinum centre. The strongly thiolate-dependent oxidation potentials indicate a prevailing thiolate character of the HOMO.