Mechanistic control of product selectivity. Reactions between cis-/trans-[Os(VI)(tpy)(Cl)2(N)]+ and triphenylphosphine sulfide

Article abstract of DOI:10.1021/ic991425t

Reactions between the Os(VI)-nitrido complexes cis- and trans-[Os(VI)(tpy)(Cl)₂(N)]⁺ (tpy is 2,2':6',2_-terpyridine) and triphenylphosphine sulfide, SPPh₃, give the corresponding Os(IV)-phosphoraniminato, [Os(IV)(tpy)(Cl)₂-(NPPh₃)]⁺, and Os(II)-thionitrosyl, [Os(II)(tpy)(Cl)₂(NS)]⁺, complexes as products. The Os-N bond length and Os-N-P angle in cis-[Os(IV) (tpy)(Cl)₂(NPPh₃)](PF₆) are 2.077(6) A and 138.4(4)°. The rate law for formation of cis- and trans-[Os(IV)(tpy)(Cl)₂(NPPh₃)]⁺ is first order in both [Os(VI)(tpy)(Cl)₂(N)]⁺ and SPPh₃ with k(trans)(25 °C, CH₃CN) = 24.6 ± 0.6 M^-1 s^-1 and k(cis)(25 °C, CH₃CN) = 0.84 ± 0.09 M^-1 s^-1. As found earlier for [Os(II)-(tpm)(Cl)₂(NS)]⁺, both cis- and trans-[Os(II)(tpy)(Cl)₂(NS)]⁺ react with PPh₃ to give [Os(IV)(tpy)(Cl)₂(NPPh₃)]⁺ and SPPh₃. For both complexes, the reaction is first order in each reagent with k(trans)(25 °C, CH₃CN) = (6.79 ± 0.08) x 10² M^-1 s^-1 and k(cis)(25 °C, CH₃CN) = (2.30 ± 0.07) x 10² M^-1 s^-1. The fact that both reactions occur rules out mechanisms involving S atom transfer. These results can be explained by invoking a common intermediate, [Os(IV)(tpy)(Cl)₂(NSPPh₃)]⁺, which undergoes further reaction with PPh₃ to give [Os(IV)(tpy)(Cl)₂(NPPh₃)]⁺ and SPPh₃ or with [Os(VI)(tpy)(Cl)₂(N)]⁺ to give [Os(IV)(tpy)(Cl)₂(NPPh₃)]⁺ and [Os(II)(tpy)(Cl)₂(NS)]⁺.