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Protonation of the Dinitrogen-Reduction Catalyst [HIPTN3N]MoIII Investigated by ENDOR Spectroscopy

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journal contribution
posted 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 MoNNH 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.