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

The reactions of ammonium salt of dialkyldithiophosphate ligands, (RO)(2)PS 2-NH4+ (R = Me/Et) with (NH4)(2)(OsBr6)-Br-IV in methanol solvent and under dinitrogen atmosphere result in one-electron paramagnetic tris complexes, {(RO)(2)PS2}(3)Os-III (1a-1b) in the solid state. The molecular structure o f one complex (la) has been determined by single-crystal X-ray diffraction. It shows the expected pseudo-octahedral geometry with reasonable strain du e to the presence of four-membered chelate rings. The reflectance spectra o f the solid complexes display two broad bands in the range 552-484 nm and i n the solid-state complexes exhibit one isotropic EPR signal at 77 K. Altho ugh the complexes la-lb are found to be stable in the solid state, in solut ion state the complexes are transformed selectively into the diamagnetic an d electrically non-conducting metal-metal bonded dimeric species [{(RO)(2)P S2}(3)O-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 r epresentative complex where R = Et (Ib). In dichloromethane solution the co mplexes show two moderately strong sulfur to osmium charge-transfer transit ions in the visible region and two strong ligand based transitions in the U V 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. Electrochemicall y generated oxidized species [{(RO)(2)PS2)(3)Os-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 gener ated reduced species [{(RO)(2)PS2}(3)Os-II-Os-II(S2P(OR)(2)}(3)](2-) are fo und to be unstable even on coulometric time scale. (C) 2001 Elsevier Scienc e B.V. All rights reserved.