Osmium dithiophosphates. Synthesis, X-ray crystal structure, spectroscopic and electrochemical properties

Abstract

The reactions of ammonium salt of dialkyldithiophosphate ligands, (RO)₂PS₂ ^-NH₄⁺ (R=Me/Et) with (NH₄)₂Os^IVBr₆ in methanol solvent and under dinitrogen atmosphere result in one-electron paramagnetic tris complexes, {(RO)₂PS₂}₃Os^III (1a-1b) in the solid state. The molecular structure of one complex (1a) has been determined by single-crystal X-ray diffraction. It shows the expected pseudo-octahedral geometry with reasonable strain due to the presence of four-membered chelate rings. The reflectance spectra of the solid complexes display two broad bands in the range 552-484 nm and in the solid-state complexes exhibit one isotropic EPR signal at 77K. Although the complexes 1a-1b are found to be stable in the solid state, in solution state the complexes are transformed selectively into the diamagnetic and electrically non-conducting metal-metal bonded dimeric species [{(RO)₂PS₂}₃Os^III-Os^III{S₂P(OR)₂}₃]. The formation of dimeric species in the solution state is authenticated by the electrospray mass spectrum of one representative complex where R=Et (1b). In dichloromethane solution the complexes show two moderately strong sulfur to osmium charge-transfer transitions in the visible region and two strong ligand based transitions in the UV region. The complexes exhibit successive two oxidations correspond to Os^IV-Os^IV/Os^III-Os^III and Os^V-Os^V/Os^IV-Os^IV processes near 0.8 and 1.9 V versus SCE, respectively. One reductive couple corresponds to the Os^III-Os^III/Os^II-Os^II couple has been observed near -0.6 V. Electrochemically generated oxidized species [{(RO)₂PS₂}₃Os^IV-Os^IV{S₂P(OR)₂}₃]²⁺ display lowest energy ligand to metal charge-transfer transition near 550 nm which has observed to be reasonably red shifted as compared to that of the parent trivalent species. On the other hand electrochemically generated reduced species [{(RO)₂PS₂}₃Os^II-Os^II{S₂P(OR)₂}₃]^2- are found to be unstable even on coulometric time scale.