A spectroscopic (stopped-flow UV-Vis and ¹H NMR Evans method) and DFT thermodynamic study of the comproportionation reaction of [Os^VIIIO₄(OH)_n]ⁿ⁻ (n = 1, 2) and [Os^VIO₂(OH)₄]²⁻

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• 作者：Daniel M.E. van Niekerk ; Wilhelmus J. Gerber ; ^{wgerber@sun.ac.za}
• 关键词：Redox reaction ; Stopped-flow kinetics ; Thermodynamics ; Os complexes ; DFT
• 刊名：Inorganica Chimica Acta
• 出版年：2017
• 出版时间：30 January 2017
• 年：2017
• 卷：455
• 期：part_P1
• 页码：140-151
• 全文大小：1519 K
• 卷排序：455

文摘

From a mole ratio 1H NMR Evans method experiment it is found that, in a 2.0 M NaOH aqueous matrix, diamagnetic [OsVIIIO4(OH)n]n− (n = 1, 2) (of d0 electron configuration) and trans-[OsVIO2(OH)4]2− species (d2) react in a 1:1 mol ratio to form two paramagnetic OsVII oxido/hydroxido product species (d1). This result is further validated as the chemical reaction model that best fitted stopped-flow UV–Vis spectroscopy kinetic data is given by OsVIII+OsVI⇌kCOMkCOM2OsVII. From non-linear least squares fits of stopped-flow UV–Vis spectroscopy kinetic traces the comproportionation reaction rate constants, activation energies (forward: ΔH‡(obs), ΔS‡(obs) and ΔG‡(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 (ΔH°rxn(obs), ΔS°rxn(obs) and ΔG°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 OsVI and OsVII oxido/hydroxido complexes in a simulated aqueous phase (COSMO) yield that the thermodynamically most stable OsVI species is the singlet spin state trans-[OsVIO2(OH)4]2− complex and the thermodynamically most stable paramagnetic OsVII product species are a combination of trans-[OsVIIO3(OH)2]− and mer-[OsVIIO3(OH)3]2− species. Using the DFT results, the OsVI & OsVIII comproportionation reaction is now proposed to be [OsVIIIO4(OH)2]2− reacts with trans-[OsVIO2(OH)4]2− to yield two trans-[OsVIIO3(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, ΔH°rxn, ΔS°rxn and ΔG°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.