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

Abstract

A new chromium­(V)–oxo complex, [CrV(O)­(6-COO–-py-tacn)]2+ (1; 6-COO–-py-tacn = 1-(6-carboxylato-2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane), was synthesized and characterized to evaluate the reactivity of CrV(O) complexes in a hydrogen-atom transfer (HAT) reaction by comparing it with that of a previously reported CrV(O) complex, [CrV(O)­(6-COO–-tpa)]2+ (2; 6-COO–-tpa = N,N-bis­(2-pyridylmethyl)-N-(6-carboxylato-2-pyridylmethyl)­amine). Definitive differences of these two CrV(O) complexes were observed in resonance Raman scatterings of the Cr–O bond (ν = 911 cm–1 for 1 and 951 cm–1 for 2) and the reduction potential (0.73 V vs SCE for 1 and 1.23 V for 2); this difference should be derived from that of the ligand bound at the trans position to the oxo ligand, a tertiary amino group in 1, and a pyridine nitrogen in 2. When we employed 9,10-dihydroanthracene as a substrate, the second-order rate constant (k) of 1 was 4000 times smaller than that of 2. Plots of normalized k values for both complexes relative to bond dissociation energies (BDEs) of C–H bonds to be cleaved in several substrates showed a pair of parallel lines with slopes of −0.91 for 1 and −0.62 for 2, indicating that the HAT reactions by the two complexes proceed via almost the same transition states. Judging from estimated BDEs of CrIV(OH)/CrV(O) (85–87 kcal mol–1 for 1 and 92–94 kcal mol–1 for 2) and the activation barrier in the HAT reaction of DHA (E a = 7.9 kcal mol–1 for 1 and E a = 4.8 kcal mol–1 for 2), the reactivity of CrV(O) complexes in HAT reactions depends on the energy level of the reactant state rather than the product state.

A Highly Reactive Chromium(V)–Oxo TAML Cation Radical Complex in Electron Transfer and Oxygen Atom Transfer Reactions

“…A product analysis of the xanthene oxidation by 2 -Sc revealed the formation of xanthone as a major product (∼80% yield) and [Cr V (O)­(TAML)] − as the decay product of 2 -Sc (Figure S21 in the Supporting Information). In conclusion, while 1 and 1 -Sc are inactive in H atom abstraction reactions, 2 -Sc shows a moderate reactivity in the C–H bond activation reactions (Figure S15b,d in the Supporting Information). , …”

Using Computational Chemistry To Reveal Nature’s Blueprints for Single-Site Catalysis of C–H Activation

“…Several experimental and computational studies on heme , and nonheme metal–oxo-like Fe­(IV)-oxo, ,, Ru­(IV)-oxo, Mn­(IV)-oxo, and Cr­(V)-oxo complexes with various substrates indicate a linear correlation between BDFE and the logarithmic transformation of the HAT reaction rate constant ( k C–H ). These systems that show strong linear correlations between log­( k C–H ) and BDFE C–H follow a Bell–Evans–Polanyi (BEP)-like correlation, because log­( k C–H ) represents the HAT barrier via an Arrhenius relationship. , Several studies report concerted but asynchronous transfer of the electron and proton for various metal-oxo systems, including Fe­(V)-oxo, Mo­(V)-oxo, Co­(III)-oxo, Ru­(IV)-oxo, Cu­(III)-O 2 CAr, and Mn­(IV)-oxo complexes.…”

“…Within this framework, the CPET mechanism is categorized as either synchronous, with equal contribution of PT and ET in the TS, or asynchronous, with either PT or ET being dominant. 141 Several experimental and computational studies on heme 114,143 and nonheme metal−oxo-like Fe(IV)oxo, 116,144,145 Ru(IV)-oxo, 146 Mn(IV)-oxo, 128 and Cr(V)oxo 147 148,149 Several studies report concerted but asynchronous transfer of the electron and proton for various metal-oxo systems, including Fe(V)-oxo, 150 Mo(V)oxo, 151 Co(III)-oxo, 142 Ru(IV)-oxo, 152 Cu(III)-O 2 CAr, 153 and Mn(IV)-oxo 154 complexes. The dominant role of PT in C−H homolytic bond cleavage was found to be important in all of these studies, thus underscoring the importance of the basicity of the metal-oxo species.…”

Oxidative versus basic asynchronous hydrogen atom transfer reactions of Mn(iii)-hydroxo and Mn(iii)-aqua complexes

“…Mechanisms of hydrogen atom transfer (HAT) reactions from organic substrates (SH) to metal–oxygen complexes, such as metal-oxo, 1–23 metal-hydroxo, 24–39 metal-(hydro)peroxo 40–44 and metal-superoxo, 45–54 have been investigated extensively, as summarized in Scheme 1. HAT proceeds via the transfer of both a proton and an electron which can be transferred simultaneously in a coupled fashion (concerted proton–electron transfer, CPET) or a stepwise fashion (proton transfer–electron transfer, PT/ET, or electron transfer–proton transfer, ET/PT) (Scheme 1).…”