EPR/ENDOR and theoretical study of the Jahn-Teller active HIPTN3NMo(V)L complexes (L = N− NH)

摘要

The molybdenum trisamidoamine (TAA) complex [>Mo] (=(3,5-(2,4,6-i-Pr3C6H2)2C6H3NCH2CH2N)Mo) carries out catalytic reduction of N2 to ammonia by protons and electrons at room temperature. A key intermediate in the proposed [>Mo] nitrogen reduction cycle is nitrido-Mo(VI), [>Mo(VI)]N: the addition of [e⁻/H⁺] to [>Mo(VI)]N to generate [>Mo(V)]NH might in principle follow one of three possible pathways: direct proton-coupled electron transfer; H⁺ first, then e⁻; e⁻ then H⁺. In this study, the paramagnetic Mo(V) intermediate {[>Mo]N}⁻ and [>Mo]NH transfer product were generated by irradiating the diamagnetic [>Mo]N, {[>Mo]NH}⁺ Mo(VI) complexes respectively, with γ-rays at 77 K, and their electronic and geometric structures were characterized by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) spectroscopies combined with quantum chemical computations. In combination with previous X-ray studies this creates the rare situation where each one of the four possible states of an [e⁻/H⁺] delivery has been characterized. Because of the degeneracy of the electronic ground states of both, {[>Mo(V)]N}⁻ and [>Mo(V)]NH, only multi-reference based methods such as the complete active space self-consistent field (CASSCF) and related methods provide a qualitatively correct description of the electronic ground state and vibronic coupling. The molecular g-values of {[>Mo]N}⁻ and [>Mo]NH exhibit large deviations from the free electron value ge. Their actual values reflect the relative strengths of vibronic and spin-orbit coupling. In the course of the computational treatment, the utility and limitations of a formal two-state model that describes this competition between couplings are illustrated, and implications of our results for the chemical reactivity of these states are discussed.