EPR and ³¹P ENDOR Characterization of Pseudo-Jahn–Teller Dynamics and N₂ Activation in Functional Nitrogenase Models, P₃EM(N₂) (M = Fe, Co; E = Si, B, C)

The nominally trigonal, pseudo-Jahn-Teller (PJT)-active, S = 1/2 N2-bound complexes, P3EM(N2), 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 N2 to NH3. We applied EPR, 31P ENDOR spectroscopy, and DFT computations to characterize the PJT-induced distortions of four selected P3EM(N2), revealing how the metal ion and axial ligand E together tune both PJT dynamics, as revealed by 31P ENDOR and N2 activation, as indicated by a decrease in N≡N stretching frequency, ν(N≡N). P3SiFe(N2), P3BFe(N2)-, and P3BCo(N2) each exhibit a single 31P isotropic hyperfine coupling, revealing dynamic pseudorotation of the PJT distortion, producing averaged C3 symmetry with equivalent phosphine ligands. Conversely, P3CCo(N2)+ exhibits a static PJT distortion directed toward a phosphine bonded to a dx2-y2 SOMO lobe, leading to the exceptional isotropic 31P coupling aiso(31P) = +250 MHz, leaving two phosphines, aiso(31P) ≈ -20 MHz. Importantly, minimization of metal 'doming' out of the P3CCo(N2)+ phosphine plane toward N2 imposed by C-Co bond 'inelasticity' yields the longest M-N2 bond and the least-activated N2, as measured by ν(N≡N). Comparisons reveal an unrecognized correlation among PJT distortion, M-E bond elasticity, and N2 activation, providing guidelines for designing bioinspired N2-reduction catalysts.