Importance of the Reactant-State Potentials of Chromium(V)-Oxo Complexes to Determine the Reactivity in Hydrogen-Atom Transfer Reactions.
Authors of this article are:
Kotani H, Kaida S, Ishizuka T, Mieda K, Sakaguchi M, Ogura T, Shiota Y, Yoshizawa K, Kojima T.
A summary of the article is shown below:
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 ( Ea = 7.9 kcal mol-1 for 1 and Ea = 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.
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