A DFT Mechanistic Study of the trans-[OsVIO2(OH)4]2- and [OsVIIIO4(OH) n] n- ( n = 1, 2 cis) Comproportionation Proton-Coupled Electron Transfer Reaction.

Herein, we present a DFT computational study of the trans-[OsVIO2(OH)4]2- and [OsVIIIO4(OH) n] n- ( n = 1, 2 cis) comproportionation reaction mechanism that occurs in a basic aqueous matrix. The reaction pathway where [OsVIIIO4(OH)]- reacts with trans-[OsVIO2(OH)4]2- via an intermediate mediated concerted electron-proton transfer yielded the best agreement with experiment (Δ‡ H°, Δ‡ S° and Δ‡ G° experimental data for the forward reaction are 10.3 ± 0.5 kcal mol-1, -2.6 ± 1.6 cal mol-1 K-1, and 11.1 ± 0.9 kcal mol-1 and for the reverse reaction are -6.7 ± 1.0 kcal mol-1, -63.6 ± 3.4 cal mol-1 K-1, and 12.2 ± 2.0 kcal mol-1, respectively, where at the PBE-D3 level for the forward reaction are 11.3 kcal mol-1, -9.8 cal mol-1 K-1, and 14.2 kcal mol-1 and for the reverse reaction are -11.8 kcal mol-1, -80.7 cal mol-1 K-1, and 12.3 kcal mol-1, respectively) and consists of (i) formation of a (singlet spin state) noncovalent adduct, [OsVIII═O···HO-OsVI]3-, (ii) spin-forbidden, concerted electron-proton transfer (i-EPT) from the trans-[OsVIO2(OH)4]2- donor to the OsVIII acceptor to form a second (triplet spin state) noncovalent adduct, [OsVII-OH···O═OsVII]3-, (iii) separation of the OsVII monomers, and finally (iv) interconversion of the separated species to form trans-[OsVIIO3(OH)2]- and mer-[OsVIIO3(OH)3]2- stereoisomer species. i-EPT from OsVI to the OsVIII species was found to be the rate-determining step, which corroborated the experimental evidence (kinetic isotope effect) that the rate-determining step involves the transfer of a proton.