Molybdenum(VI) Nitrido Complexes with Tripodal Silanolate Ligands. Structure and Electronic Character of an...

In contrast to a tungsten nitrido complex endowed with a tripodal silanolate ligand framework, which was reported in the literature to be a dimeric species with a metallacyclic core, the corresponding molybdenum nitrides 3 are monomeric entities comprising a regular terminal nitride unit, as proven by single-crystal X-ray diffraction (SC-XRD). Their electronic character is largely determined by the constraints imposed on the metal center by the podand ligand architecture. ⁹⁵Mo nuclear magnetic resonance (NMR) and, to a lesser extent, ¹⁴N NMR spectroscopy allow these effects to be studied, which become particularly apparent upon comparison with the spectral data of related molybdenum nitrides comprising unrestrained silanolate, alkoxide, or amide ligands. Attempted nitrogen atom transfer from these novel terminal nitrides to [(tBuArN)₃Mo] (Ar = 3,5-dimethylphenyl) as the potential acceptor stopped at the stage of unsymmetric dimolybdenum μ-nitrido complex 13a as the first intermediate along the reaction pathway. SC-XRD, NMR, electron paramagnetic resonance, and ultraviolet-visible spectroscopy as well as magnetometry in combination with density functional theory allowed a clear picture of the geometric and electronic structure of this mixed-valent species to be drawn. 13a is formally best described as an adduct of the type [(Mo^[O])^+III-(μN)^-III-(Mo^[N])^+VI], S = ¹/₂ complex with (Mo^[O])^+III in the low-spin configuration, whereas related complexes such as [(AdS)₃Mo-(μN)-Mo(NtBuAr)₃] (19; Ad = 1-adamantyl) have previously been regarded in the literature as mixed-valent Mo^+IV/Mo^+V species. The spin population at the two Mo centers is uneven and notably larger at the more reduced Mo^[O] atom, whereas the only spin present at the (μN) bridge is derived from spin polarization.