Abstract
The molybdenum trisamidoamine (TAA) complex [Mo] {[3,5-(2,4,6-i-Pr(3)C(6)H(2))(2)C(6)H(3)NCH(2)CH(2)N]Mo} carries out catalytic reduction of N(2) to ammonia (NH(3)) by protons and electrons at room temperature. A key intermediate in the proposed [Mo] nitrogen reduction cycle is nitridomolybdenum(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 and then e(-); e(-) and then H(+). In this study, the paramagnetic Mo(V) intermediate {[Mo]N}(-) and the [Mo]NH transfer product were generated by irradiating the diamagnetic [Mo]N and {[Mo]NH}(+) Mo(VI) complexes, respectively, with γ-rays at 77 K, and their electronic and geometric structures were characterized by electron paramagnetic resonance and electron nuclear double resonance spectroscopies, combined with quantum-chemical computations. In combination with previous X-ray studies, this creates the rare situation in which each one of the four possible states of [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 multireference-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 g(e). 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 the implications of our results for the chemical reactivity of these states are discussed.