A spectroscopic (stopped-flow UV–Vis and 1H NMR Evans method) and DFT thermodynamic study of the comproportionation reaction of [OsVIIIO4(OH)n]n− (n = 1, 2) and [OsVIO2(OH)4]2−

Abstract From a mole ratio 1 H NMR Evans method experiment it is found that, in a 2.0 M NaOH aqueous matrix, diamagnetic [Os VIII O 4 (OH) n ] n − ( n  = 1, 2) (of d 0 electron configuration) and trans -[Os VI O 2 (OH) 4 ] 2− species ( d 2 ) react in a 1:1 mol ratio to form two paramagnetic Os VII oxido/hydroxido product species ( d 1 ). This result is further validated as the chemical reaction model that best fitted stopped-flow UV–Vis spectroscopy kinetic data is given by Os VIII + Os VI ⇌ k COM k COM 2 Os VII . From non-linear least squares fits of stopped-flow UV–Vis spectroscopy kinetic traces the comproportionation reaction rate constants, activation energies (forward: Δ H ‡ (obs) , Δ S ‡ (obs) and Δ G ‡ (obs) 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 , respectively; and reverse 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) and standard reaction energies (Δ H ° rxn (obs) , Δ S ° rxn (obs) and Δ G ° rxn (obs) are 17.1 ± 1.2 kcal mol −1 , 61.0 ± 4.3 cal mol −1  K −1 and −1.1 ± 2.5 kcal mol −1 , respectively) at 298.15 K were determined. Stopped-flow kinetic isotope experiments provide evidence that these redox reactions coincide with the transfer of a proton. A systematic DFT speciation study of Os VI and Os VII oxido/hydroxido complexes in a simulated aqueous phase (COSMO) yield that the thermodynamically most stable Os VI species is the singlet spin state trans -[Os VI O 2 (OH) 4 ] 2− complex and the thermodynamically most stable paramagnetic Os VII product species are a combination of trans -[Os VII O 3 (OH) 2 ] − and mer -[Os VII O 3 (OH) 3 ] 2− species. Using the DFT results, the Os VI & Os VIII comproportionation reaction is now proposed to be [Os VIII O 4 (OH) 2 ] 2− reacts with trans -[Os VI O 2 (OH) 4 ] 2− to yield two trans -[Os VII O 3 (OH) 2 ] − species and two hydroxide anions. The DFT-calculated standard reaction energies for this reaction at 298.15 K (e.g. with the PBE functional, Δ H ° rxn , Δ S ° rxn and Δ G ° rxn are 21.13 kcal mol −1 , 71.06 cal mol −1  K −1 and −0.06 kcal mol −1 , respectively) compare exceptionally well with the obtained experimental data.