Kinetic UV–Vis Spectroscopic and DFT Mechanistic Study of the Redox Reaction of [Os^VIIIO₄(OH)_n]ⁿ⁻(n= 1, 2) and Methanol in a Basic Aqueous Matrix

Abstract

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.

Mechanism of the Visible-Light-Promoted C(sp3)–H Oxidation via Uranyl Photocatalysis

“…Geometry optimization calculations show that the U = O bond elongates from 1.805 Å in UO 2 2+* to 1.888 Å in the TS 1–1 and further to 2.034 Å in the Int 1–1 and that the C–H bond stretches from 1.098 Å R 1–1 to 1.165 Å in TS 1–1 (see Section S1.3 in the SI for the detailed structural information). The free-energy barrier of this HAT step via uranyl photocatalyst (i.e., R 1–1 → Int 1–1 ) is 17.3 kcal/mol, which is lower by a few kcal/mol than that of the hydrogen extraction process via some transition-metal catalysts (e.g., [Ru­(Por)­(QC)], [Mn­(O)­(Por)­(Cl)], and [Os VIII O 4 (OH) n ] n − (QC = quinoid carbene, Por = porphyrinato dianion)). − …”

Ru(iv)–Ru(iv) complexes having the doubly oxido-bridged core with a bridging carbonato or hydrogencarbonato ligand

“…A number of studies on the electronic structures, catalytic oxidation reactions and model reactions with sMMO analogues with diiron centers, 2 or some with dinickel centers 3 have been reported in homogeneous systems, which are not limited to the rich chemistry of mononuclear metal–oxido species by, e.g. the 8 th group iron, 4 ruthenium, 5 and osmium 6 centers. However, reports on those with diruthenium centers are limited although, only recently, a light-driven methane monooxygenation with the dinuclear Ru( iv )–Ru( iv ) complex was reported.…”