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EPR/ENDOR and Theoretical Study of the Jahn–Teller-Active [HIPTN3N]MoVL Complexes (L = N, NH)

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Roemelt,  Michael
Max-Planck Institut für Chemische Energiekonversion;
Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum;
Research Group Roemelt, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Neese,  Frank
Research Department Neese, Max Planck Institute for Chemical Energy Conversion, Max Planck Society;

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Citation

Sharma, A., Roemelt, M., Reithofer, M., Schrock, R. R., Hoffmann, B. M., & Neese, F. (2017). EPR/ENDOR and Theoretical Study of the Jahn–Teller-Active [HIPTN3N]MoVL Complexes (L = N, NH). Inorganic Chemistry, 56(12), 6906-6919. doi:10.1021/acs.inorgchem.7b00364.


Cite as: http://hdl.handle.net/21.11116/0000-0000-EB9A-A
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 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 the implications of our results for the chemical reactivity of these states are discussed.