EPR and P ENDOR Characterization of Pseudo-Jahn-Teller Dynamics and N Activation in Functional Nitrogenase Models, PM(N) (M = Fe, Co; E = Si, B, C).

The nominally trigonal, pseudo-Jahn-Teller (PJT)-active, = 1/2 N-bound complexes, , M = Fe, Co, with three in-plane phosphine ligands and axial donors, E = Si, B, C, include functional nitrogenase models that catalyze the reduction of N to NH. We applied EPR, P ENDOR spectroscopy, and DFT computations to characterize the PJT-induced distortions of four selected , revealing how the metal ion and axial ligand E together tune both PJT dynamics, as revealed by P ENDOR and N activation, as indicated by a decrease in N≡N stretching frequency, ν(N≡N). , and each exhibit a single P isotropic hyperfine coupling, revealing dynamic pseudorotation of the PJT distortion, producing averaged symmetry with equivalent phosphine ligands. Conversely, exhibits a static PJT distortion directed toward a phosphine bonded to a d SOMO lobe, leading to the exceptional isotropic P coupling (P) = +250 MHz, leaving two phosphines, (P) ≈ -20 MHz. Importantly, minimization of metal 'doming' out of the phosphine plane toward N imposed by C-Co bond 'inelasticity' yields the longest M-N bond and the least-activated N, as measured by ν(N≡N). Comparisons reveal an unrecognized correlation among PJT distortion, M-E bond elasticity, and N activation, providing guidelines for designing bioinspired N-reduction catalysts.