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 CrV(O) complexes were observed in resonance Raman scatterings of the Cr-O bond (v = 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.