Mechanism of cis-directed four-electron oxidation by a trans-dioxo complex of ruthenium(VI)

Article abstract of DOI:10.1021/ja00080a025

The reductions of trans-[Ru(VI)(tpy)(O)₂(H₂O)]²⁺ or trans- [Ru(VI)(tpy)(O)₂(H₂O)]²⁺ or trans-[Ru(VI)(tpv)(O)₂(CH₃CN)]²⁺ (tpy is 2,2':6',2''-terpyridine) by PPh₃,Ph₂PCH₂CH₂PPh₂ (dppe), or Ph₂PCH₂PPh₂ (dppm) occur by successive Ru(VI) → Ru(IV) and Ru(IV) → Ru(II) oxygen atom transfer steps. The products appear to be five-coordinated diphosphine oxide complexes of Ru(II). They subsequently undergo stepwise solvolysis to give the free diphosphine dioxides and [Ru(II)(tpy)(CH₃- CN)₃]²⁺. The kinetics of the individual redox steps were studied by stopped-flow/rapid-scan spectrophotometry. For PPh₃ as reductant, k(VI/IV)(20 °C, CH₃CN) = (2.28 ± 0.08) x 10⁶ M^-1 s^-1 (ΔH((+)) = 4.2 ± 0.8 kcal mol^-1; ΔS((+)) = -19 ± 4 eu) and k(IV/II(20°, CH₂CN) = 1.04 ± 0.03) x 10⁴ M^-1 s^-1 (ΔH((+)) = 5.9 ± 0.5 kcal mol^-1; ΔS((+)) = - 20 ± 3 eu). With dppe or dppm, Ru(VI) acts as a cis-directed four-electron oxidant. The first step, {Ru(VI) → Ru(IV)}, is first order in both oxidant and diphosphine with k(VI/IV)(20 °C, CH₃CN) ~4 x 10⁸ M^-1 s^-1 (dppe) to give trans-[Ru(IV)(tpy)(O)(O=P(PPh₂)CH₂CH₂PPh₂)(CH₃CN)]²⁺. In acetonitrile with no added water, the subsequent reduction of Ru(IV) to Ru(II) follows first-order kinetics with k(IV/II)(20 °C, CH₃CN) = 5 x 10¹ s^-1 for either dppe or dppm. By inference, the rate-limiting step is intramolecular isomerization of the remaining oxo group followed by rapid O- atom transfer. In acetonitrile 1.75 M in H₂O the initial Ru(IV) product is trans-[Ru(IV)(tpy)(O)(O=P(PPh₂)CH₂-CH₂PPh₂)(H₂O)]²⁺. The subsequent Ru(IV) → Ru(II) step is considerably slower, k(IV/II)(20 °C) = (6.20 ± 0.12) x 10^-2 s^-1. This reaction exhibits a substantial inverse solvent isotope effect, k(H₂O)/k(D₂O) = 0.18 ± 0.02, which arises from the transfer of a single proton on the basis of a mole fraction study. Isomerization is also rate limiting in this case, but the rate-determining step is intramolecular proton transfer.