A DFT Mechanistic Study of the trans-[Os^VIO₂(OH)₄]^2– and [Os^VIIIO₄(OH)_n]ⁿ⁻ (n = 1, 2 cis) Comproportionation Proton-Coupled Electron Transfer Reaction

Abstract

Herein, we present a DFT computational study of the trans-[OsO(OH)] and [OsO(OH) ] ( n = 1, 2 cis) comproportionation reaction mechanism that occurs in a basic aqueous matrix. The reaction pathway where [OsO(OH)] reacts with trans-[OsO(OH)] 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, -2.6 ± 1.6 cal mol K, and 11.1 ± 0.9 kcal mol and for the reverse reaction are -6.7 ± 1.0 kcal mol, -63.6 ± 3.4 cal mol K, and 12.2 ± 2.0 kcal mol, respectively, where at the PBE-D3 level for the forward reaction are 11.3 kcal mol, -9.8 cal mol K, and 14.2 kcal mol and for the reverse reaction are -11.8 kcal mol, -80.7 cal mol K, and 12.3 kcal mol, respectively) and consists of (i) formation of a (singlet spin state) noncovalent adduct, [Os═O···HO-Os], (ii) spin-forbidden, concerted electron-proton transfer (i-EPT) from the trans-[OsO(OH)] donor to the Os acceptor to form a second (triplet spin state) noncovalent adduct, [Os-OH···O═Os], (iii) separation of the Os monomers, and finally (iv) interconversion of the separated species to form trans-[OsO(OH)] and mer-[OsO(OH)] stereoisomer species. i-EPT from Os to the Os 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.

Kinetic UV–Vis Spectroscopic and DFT Mechanistic Study of the Redox Reaction of [OsVIIIO4(OH)n]n−(n= 1, 2) and Methanol in a Basic Aqueous Matrix

“…both the total Os VIII and methanol (CH 3 OH) concentrations, with a calculated rate constant (expressed as k 1 in this study) for the redox reaction, eq , of 0.29 ± 0.02 M –1 s –1 . Moreover, while Os VIII reduction is occurring, eq , the produced Os VI complex reacts with Os VIII via a comproportionation reaction, eq , to form Os VII ,, …”

“…The rate-determining step for the forward and reverse reactions depicted by eq (depicted with abbreviated symbols for clarity) has been thoroughly studied previously and was found to occur through concerted electron–proton transfer (EPT), which falls in a class of chemical reactions called proton-coupled electron transfer (PCET). ,− We suspect that the reduction of Os VIII with methanol, eq , may also fall under the PCET category. From a mechanistic point of view, we are interested to find out if hydrogen atom transfer (HAT) occurs directly from methanol to Os VIII and to determine which HAT, that is, O–H or α-C–H, is the rate-determining step of the redox reaction.…”

“…First, when Os VIII reacts with CD 3 OH to produce Os VI and formaldehyde, eq , it could be expected that from the time the reaction is initiated up until equilibrium is reached, the absorbance will only decrease in magnitude because the molar extinction coefficient of Os VI is considerably smaller than that of Os VIII at 370 nm. , However, the absorbance first increases and then decreases. This trend can be understood when taking into account that when Os VI is initially produced, eq , it reacts with Os VIII via a comproportionation reaction to form Os VII , eq , ,, which has a larger molar extinction coefficient than both Os VI and Os VIII complexes at 370 nm. , Furthermore, the mole ratio of Os VIII and Os VI is 1:1 when the absorbance maximum is reached and because the total Os concentration is the same for each kinetic run and the reaction eq is orders of magnitude faster than reaction eq , the absorbance at the maximum for each kinetic run remains the same . Therefore, apart from investigating the reduction of Os VIII with methanol, eq , we are in effect also performing an indirect kinetic mole ratio experiment, eq , akin to that reported previously. , It should also be noted that the rate of the Os VI and Os VIII comproportionation reaction is several thousand times faster than the rate of Os VIII reduction with methanol (CH 3 OH), , which implies that the comproportionation reaction, eq , is very close to equilibrium at all times as reaction eq progresses ( vide infra ).…”

“…Most recently, a comprehensive DFT computational study of the comproportionation reaction mechanism was conducted. The reaction pathway where [Os VIII O 4 (OH)] − reacts with trans -[Os VI O 2 (OH) 4 ] 2– via an intermediate-mediated concerted electron–proton transfer, i -EPT, yielded the best agreement with the experiment (Δ ⧧ H °, Δ ⧧ S °, and Δ ⧧ G ° 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).…”

“…Our previous investigations − regarding the redox chemistry of high-oxidation state oxido- and hydroxido-osmium species (Os VI , Os VII , and Os VIII ) in aqueous media paved the way for this experimental kinetic and DFT computational study to elucidate the most probable proton-coupled electron transfer (PCET) reaction mechanism of cis -[Os VIII O 4 (OH) 2 ] 2– reduction with methanol in a 2.0 M NaOH aqueous matrix. This work also aims to provide important insights regarding a particular structural requirement of the oxidized alcohol (presence/absence of an α-C–H hydrogen) for these kinds of metal complex and organic substrate PCET reactions to occur.…”