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

A new chromium(V)-oxo complex, [Cr^V(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 Cr^V(O) complexes in a hydrogen-atom transfer (HAT) reaction by comparing it with that of a previously reported Cr^V(O) complex, [Cr^V(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 Cr^V(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 Cr^IV(OH)/Cr^V(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 Cr^V(O) complexes in HAT reactions depends on the energy level of the reactant state rather than the product state.