Mechanistic Insights into C–H Oxidations by Ruthenium(III)-Pterin Complexes: Impact of Basicity of the Pterin Ligand and Electron Acceptability of the Metal Center on the Transition States

Abstract

A ruthenium(II) complex, [Ru(dmdmp)Cl(MeBPA)] (2) (Hdmdmp = N,N-dimethyl-6,7-dimethylpterin, MeBPA = N-methyl-N,N-bis(pyridylmethyl)amine), having a pterin derivative as a proton-accepting ligand, was synthesized and characterized. Complex 2 shows higher basicity than that of a previously reported Ru(II)-pterin complex, [Ru(dmdmp) (TPA)](+) (1) (TPA = tris(2-pyridylmethyl)amine). On the other hand, 1e(-)-oxidized species of 1 (1OX) exhibits higher electron-acceptability than that of 1e(-)-oxidized 2 (2OX). Bond dissociation enthalpies (BDE) of the two Ru(II) complexes having Hdmdmp as a ligand in proton-coupled electron transfer (PCET) to generate 1OX and 2OX were calculated to be 85 kcal mol(-1) for 1OX and 78 kcal mol(-1) for 2OX. The BDE values are large enough to perform H atom transfer from C-H bonds of organic molecules to the 1e(-)-oxidized complexes through PCET. The second-order rate constants (k) of PCET oxidation reactions were determined for 1OX and 2OX. The logarithms of normalized k values were proportional to the BDE values of C-H bonds of the substrates with slopes of -0.27 for 1OX and -0.44 for 2OX. The difference between 1OX and 2OX in the slopes suggests that the transition states in PCET oxidations of substrates by the two complexes bear different polarization, as reflection of difference in the electron acceptability and basicity of 1OX and 2OX. The more basic 2OX attracts a proton from a C-H bond via a more polarized transition state than that of 1OX; on the contrary, the more electron-deficient 1OX forms less polarized transition states in PCET oxidation reactions of C-H bonds.

Concerted Proton–Electron Transfer Reactivity at a Multimetallic Co4O4 Cubane Cluster

“…Conversely, no apparent correlation exists with the p K a of organic substrates (Figure S4), ruling out a p K a driven mechanism. Instead, the linear correlation implicates a concerted H-atom abstraction pathway, and this observation is consistent with other metal complexes that react via a CPET mechanism. − Additionally, large differences between the p K a values of 4 and C–H substrates (Δp K a > 30) and between the E ° values of 2 (Co III/IV couple) and C–H substrates (Δ E ° > 0.6 V) suggest that discrete proton or electron transfer is not thermodynamically accessible, disfavoring stepwise proton transfer-electron transfer (PTET) and electron transfer-proton transfer (ETPT) pathways. Taken together, these data strongly suggest that H-atom abstraction by 2 occurs by a CPET mechanism.…”

Importance of the Reactant-State Potentials of Chromium(V)–Oxo Complexes to Determine the Reactivity in Hydrogen-Atom Transfer Reactions

“…The α values (0.91 for 1 and 0.62 for 2) suggest that the transition states of the HAT reactions by 1 and 2 should be rather late and product-like. 31,33 In order to clarify the comparative energy levels between 1 and 2 in the DHA oxidation, a plausible energy diagram is proposed as shown in Scheme 3. It is important to consider the energy level of Cr IV (OH) species as an initial product in the stepwise HAT reaction, which is also related to that of Cr III species at the final product state.…”

“…The slope of the plot was determined to be 0.91 for 1 and 0.62 for 2 based on the Evans–Polanyi equation (eqs , ): , where Δ H is related to the BDE values of C–H bonds of substrates and O–H bonds of Cr V (O)/Cr IV (OH) and α is a standard value for the thermochemistry of the overall HAT reactions. The α values (0.91 for 1 and 0.62 for 2 ) suggest that the transition states of the HAT reactions by 1 and 2 should be rather late and product-like. , …”

Mechanistic Insight into Concerted Proton–Electron Transfer of a Ru(IV)-Oxo Complex: A Possible Oxidative Asynchronicity

“…Among a series of M n + (O) complexes, high-valent Ru-oxo complexes have also been intensely investigated as active species in substrate oxidation reactions, including C–H oxidation. − It has been revealed that C–H oxidation of cumene by Ru IV (O) complexes is initiated by hydrogen atom abstraction, followed by an oxygen-rebound process to afford cumyl alcohol in CH 3 CN. , In the course of HAT from the C–H bond to a Ru IV (O) complex, the CPET pathway has been accepted in light of arguments based on the Bell–Evans–Polanyi principle; , however, the influence of protons on the asynchronicity of CPET from a C–H bond to a Ru IV (O) complex has yet to be scrutinized.…”