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

摘要

The reactions oiammonium salt of dialkyldithiophosphate ligands. (RO)_2PS_2-NH_4+ (R= Me/Et) with (NH_4)_2OsIVBr_6 inmethanol solvent and under dinitrogen atmosphere result in one-electron paramagnetic tris complexes. {(RO)_2PS_2}_3OS~(III) (1a-1b) in the solid state. The molecular structure of one complex (la) 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 77 K. Although the complexes la-Ib 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)_2PS_2}_3OS~(III)-OS~(III){S_2P(OR)_2}J. The formation of dimeric species in the svlution state is authenticated by the electrospray mass spectrum of one representative complex where R = Et (Ib). 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 /OSIV -OSIV processes near 0.8 and 1.9 V versus SCE. respectively. On~ reductive couple corresponds to the OS~(II)I-OS~(III)/Os~(II)-OS~(II) couple has been observed near -0.6 V. Electrochemically generated oxidized species [{(RO)_2PS_2}_3OsIV_OsIV{S_2P(OR)_2}3r~+ display lowest energy ligand to metal charge-transfer transition near 550 nm which has observed to be reasonably red shifted as compared to jthat of the parent trivalent species. On the other hand electrochemically generated reduced species [{(Ro)_2PS_2}_3Os~(II)-Os~(II){S_2P(OR)_2}_3]~(2-) are found to be unstable even on coulometric time scale.