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

Abstract

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.

Spectroscopic capture of a low-spin Mn(IV)-oxo species in Ni–Mn3O4 nanoparticles during water oxidation catalysis

“…The non-periodic DFT calculation for the low-spin Mn(IV)oxo species resulted in a large and fairly isotropic value of hyperfine coupling constants |A| = [158, 143, 135] G, which is in good agreement with the experimental values of |A| = [121, 121, 132] G. Notably, the DFT calculation for the high-spin Mn(IV)oxo species yielded a much smaller value of the hyperfine tensor |A| = [44,43,49] G, which was intuitively anticipated for a highvalent metal center in a high-spin state. Thus, the experimentally observed large value of |A| supports the existence of a low-spin Mn(IV)-oxo species.…”

“…5 ). The g -values can be described in terms of the ratio of the sum of the vibronic coupling energy at the equilibrium distortion ( V vib ) and the solvent-dependent environmental energies affecting the orbital splitting ( V L ) to the spin–orbit coupling constant ( λ ) 39 , through the fictitious angle 2θ : where k is the d -electron delocalization factor (0 (fully delocalized) ≤ k ≤ 1 (free metal ion)) 40 – 43 . As shown in Fig.…”

“…where k is the d-electron delocalization factor (0 (fully delocalized) ≤ k ≤ 1 (free metal ion)) [40][41][42][43] . As shown in Fig.…”

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

“…In contrast of Cummins’ tris‐amidate Mo III complex 1 , [27] the steric bulk of the ligand in the case of 246 prevented the binuclear dinitrogen cleavage to the corresponding nitride. The full catalytic mechanism was determined via a combination of theoretical studies [187–189] and the spectroscopic [190] and structural characterization of reaction intermediates [191–193] . The key steps of the reaction could be identified by the stepwise addition of reactant to isolate reaction intermediates.…”

N2Reduction on Fe‐Based Complexes with Different Supporting Main‐Group Elements: Critical Roles of Anchor and Peripheral Ligands

“…While generally correlated ab initio calculations with multireference wavefunction theory (WFT) can reliably predict the electronic structures of such strongly correlated systems, density functional theory (DFT) can often provide useful information when electronic structures are carefully evaluated. In this work, comprehensive theoretical studies using DFT and complete active space self‐consistent field (CASSCF) methods were carried out to unveil the characteristic features of different anchor atoms, ranging from Lewis acid to Lewis base, of trigonal bipyramidal (XP iPr 3 )Fe (X = B, C, N) scaffold for the catalytic N 2 ‐to‐NH 3 reaction.…”