Formation of {[HIPTN3N]Mo(III)H}−by Heterolytic Cleavage of H2as Established by EPR and ENDOR Spectroscopy

Abstract

MoN₂ (Mo = [(HIPTNCH₂CH₂)₃N]Mo, where HIPT = 3,5-(2,4,6-i-Pr₃C₆H₂)₂C₆H₃) is the first stage in the reduction of N₂ to NH₃ by Mo. Its reaction with dihydrogen in fluid solution yields “MoH₂”, a molybdenum−dihydrogen compound. In this report, we describe a comprehensive electron paramagnetic resonance (EPR) and ^1/2H/¹⁴N electron nuclear double resonance (ENDOR) study of the product of the reaction between MoN₂ and H₂ that is trapped in frozen solution, 1. EPR spectra of 1 show that it has a near-axial g tensor, g = [2.086, 1.961, 1.947], with dramatically reduced g anisotropy relative to MoN₂. Analysis of the g values reveal that this anion has the Mo(III), [d_xz, d_yz]³ orbital configuration, as proposed for the parent MoN₂ complex, and that it undergoes a strong pseudo-Jahn−Teller (PJT) distortion. Simulations of the 2D 35 GHz ¹H ENDOR pattern comprised of spectra taken at multiple fields across the EPR envelope (2 K) show that 1 is the [MoH]⁻ anion. The 35 GHz Mims pulsed ²H ENDOR spectra of 1 prepared with ²H₂ show the corresponding ²H⁻ signal, with a substantial deuterium isotope effect in a_iso. Radiolytic reduction of a structural analogue, Mo(IV)H, at 77 K, confirms the assignment of 1. Analysis of the 2D ¹⁴N ENDOR pattern for the ligand amine nitrogen further reveals the presence of a linear N_ax−Mo−H⁻ molecular axis that is parallel to the unique magnetic direction (g₁). The ENDOR pattern of the three equatorial nitrogens is well-reproduced by a model in which the Mo−N_eq plane has undergone a static, not dynamic, PJT distortion, leading to a range of hyperfine couplings for the three N_eq's. The finding of a nearly axial hyperfine coupling tensor for the terminal hydride bound Mo supports the earlier proposal that the two exchangeable hydrogenic species bound to the FeMo cofactor of the nitrogense turnover intermediate, which has accumulated four electrons/protons (E₄), are hydrides that bridge two metal ions, not terminal hydrides.