A kinetic study of [OsO(4)] reduction by aliphatic alcohols (MeOH and EtOH) was performed in a 2.0 M NaOH matrix at 298.1 K. The rate model that best fitted the UV-VIS data supports a one-step, two electron reduction of Os(VIII) (present as both the [Os(VIII)O(4)(OH)](-) and cis-[Os(VIII)O(4)(OH)(2)](2-) species in a ratio of 0.34:0.66) to form the trans-[Os(VI)O(2)(OH)(4)](2-) species. The formed trans-[Os(VI)O(2)(OH)(4)](2-) species subsequently reacts relatively rapidly with the cis-[Os(VIII)O(4)(OH)(2)](2-) complex anion to form a postulated [Os(VII)O(3)(OH)(3)](2-) species according to: cis-[Os(VIII)O(4)(OH)(2)](2-) + trans-[Os(VI)O(2)(OH)(4)](2-) (k+2) <−> (k-2) 2[Os(VII)O(3)(OH)(3)](2-). The calculated forward, k(+2), and reverse, k(-2), reaction rate constants of this comproportionation reaction are 620.9 ± 14.6 M(-1) s(-1) and 65.7 ± 1.2 M(-1) s(-1) respectively. Interestingly, it was found that the postulated [Os(VII)O(3)(OH)(3)](2-) complex anion does not oxidize MeOH or EtOH. Furthermore, the reduction of Os(VIII) with MeOH or EtOH is first order with respect to the aliphatic alcohol concentration. In order to corroborate the formation of the [Os(VII)O(3)(OH)(3)](2-) species predicted with the rate model simulations, several Os(VIII)/Os(VI) mole fraction and mole ratio titrations were conducted in a 2.0 M NaOH matrix at 298.1 K under equilibrium conditions. These titrations confirmed that the cis-[Os(VIII)O(4)(OH)(2)](2-) and trans-[Os(VI)O(2)(OH)(4)](2-) species react in a 1:1 ratio with a calculated equilibrium constant, K(COM), of 9.3 ± 0.4. The ratio of rate constants k(+2) and k(-2) agrees quantitatively with K(COM), satisfying the principle of detailed balance. In addition, for the first time, the molar extinction coefficient spectrum of the postulated [Os(VII)O(3)(OH)(3)](2-) complex anion is reported.
This combined experimental and computational study builds on our previous studies to elucidate the reaction mechanism of methanol oxidation by Os VIII oxido/hydroxido species (in basic aqueous media) while accounting for the simultaneous formation of Os VII species via a comproportionation reaction between Os VIII and Os VI . UV−Vis spectroscopy kinetic analyses with either CH 3 OH or the deuterated analogue CD 3 OH as a reducing agent revealed that transfer of α-carbon−hydrogen of methanol is the partial rate-limiting step. The resulting relatively large KIE value of approximately 11.82 is a combination of primary and secondary isotope effects. The Eyring plots for the oxidation of these isotopologues of methanol under the same reaction conditions are parallel to each other and hence have the same activation enthalpy [Δ ⧧ H°= 14.4 ± 1.2 kcal mol −1 (CH 3 OH) and 14.5 ± 1.3 kcal mol −1 (CD 3 OH)] but lowered activation entropy (Δ ⧧ S°) from −12.5 ± 4.1 cal mol −1 K −1 (CH 3 OH) to −17.1 ± 4.4 cal mol −1 K −1 (CD 3 OH). DFT computational studies at the PBE-D3 level with QZ4P (Os) and pVQZ (O and H) basis sets provide clear evidence to support the data and interpretations derived from the experimental kinetic work. Comparative DFT mechanistic investigations in a simulated aqueous phase (COSMO) indicate that methanol and Os VIII first associate to form a noncovalent adduct bound together by intermolecular H-bonding interactions. This is followed by spin-forbidden α-carbon−hydrogen transfer (not O−H transfer) from methanol to Os VIII by means of HAT, which is found to be the partial rate-limiting step. Without the organic and inorganic fragments dissociating from each other during the entire stepwise redox reaction (in order to avoid formation of highly energetically unfavorable monomer species), the HAT step is followed by PT and then ET before the final product monomers formaldehyde and Os VI dissociate from each other. DFT-calculated Δ ⧧ H°is within 5 kcal mol −1 of the experimentally obtained value, while the DFT Δ ⧧ S°is three times larger than that found from the experiment.
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