Protonation of the Dinitrogen-Reduction Catalyst [HIPTN3N]MoIII Investigated by ENDOR Spectroscopy
journal contributionposted on 17.01.2011, 00:00 by R. Adam Kinney, Rebecca L. McNaughton, Jia Min Chin, Richard R. Schrock, Brian M. Hoffman
Dinitrogen is reduced to ammonia by the molybdenum complex of L = [HIPTN3N]3− [Mo; HIPT = 3,5-(2,4,6-iPr3C6H2)2C6H3]. The mechanism by which this occurs involves the stepwise addition of proton/electron pairs, but how the first pair converts MoN2 to MoNNH remains uncertain. The first proton of reduction might bind either at Nβ of N2 or at one of the three amido nitrogen (Nam) 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, aiso(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 Nam, 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 (Nam)2H+−Mo distance predicted by density functional theory computations.