EPR and ³¹P ENDOR Characterization of Pseudo-Jahn-Teller Dynamics and N₂ Activation in Functional Nitrogenase Models,...

The nominally trigonal, pseudo-Jahn-Teller (PJT)-active, S = 1/2 N₂-bound complexes, P₃^EM(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 P₃^EM(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). P₃^SiFe(N₂), P₃^BFe(N₂)^-, and P₃^BCo(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, P₃^CCo(N₂)⁺ exhibits a static PJT distortion directed toward a phosphine bonded to a d_x2-y2 SOMO lobe, leading to the exceptional isotropic ³¹P coupling a_iso(³¹P) = +250 MHz, leaving two phosphines, a_iso(³¹P) ≈ -20 MHz. Importantly, minimization of metal 'doming' out of the P₃^CCo(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.