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Abstract

The molybdenum trisamidoamine (TAA) complex [Mo] {[3,5-(2,4,6-i-Pr₃C₆H₂)₂C₆H₃NCH₂CH₂N]Mo} carries out catalytic reduction of N₂ to ammonia (NH₃) 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.