Protonation of the dinitrogen-reduction catalyst [HIPTN₃N] Mo^III investigated by ENDOR spectroscopy

Dinitrogen is reduced to ammonia by the molybdenum complex of L = [HIPTN₃N]^3- [Mo; HIPT = 3,5-(2,4,6-iPr₃C ₆H₂)₂C₆H₃]. The mechanism by which this occurs involves the stepwise addition of proton/electron pairs, but how the first pair converts MoN₂ to MoN - NH remains uncertain. The first proton of reduction might bind either at N_β of N ₂ or at one of the three amido nitrogen (N_am) ligands. Treatment of MoCO with [2,4,6-Me₃C₅H₃N]BAr ₄ [Ar = 2,3-(CF₃)₂C₆H₃] in the absence of reductant generates HMoCO⁺, whose electron paramagnetic resonance spectrum has greatly reduced g anisotropy relative to MoCO. ²H Mims pulsed electron nuclear double-resonance spectroscopy of ²HMoCO⁺ shows a signal that simulations show to have a hyperfine tensor with an isotropic coupling, a_iso(²H) = -0.22 MHz, and a roughly dipolar anisotropic interaction, T(²H) = [-0.48, -0.93, 1.42] MHz. The simulations show that the deuteron is bound to N_am, near the Mo equatorial plane, not along the normal, and at a distance of 2.6 Å from Mo, which is nearly identical with the (N _am)²H⁺-Mo distance predicted by density functional theory computations.