A Homoleptic Molybdenum(IV) Enolate Complex:  Synthesis, Molecular and Electronic Structure, and NCN Group Transfer To Form a Terminal Cyanoimide of Molybdenum(VI)

A monomeric molybdenum(IV) tetrakis enolate complex Mo(OC[Ad]Mes)4, 1, where Ad = 2-adamantylidene and Mes = 2,4,6-Me3C6H2, has been synthesized and characterized structurally by X-ray diffraction, chemically through NCN group-transfer reactivity, and computationally to investigate the origins of the observed structure that is intermediate between tetrahedral and square planar. No prior examples of Mo(OR)4 have been structurally characterized despite having been the subject of both experimental and theoretical interest. Complex 1 has a singlet ground state and thus a metal-based lone pair of electrons. The latter has been visualized with the aid of the electron localization function (ELF) and appears as a two-bladed propeller with D2d symmetry. Complex 1 makes a simple 1:1 adduct with t-BuNC that is trigonal bipyramidal with an axial isocyanide as demonstrated by X-ray crystallography. This trigonal bipyramidal 1:1 adduct has a triplet ground state and provides a model for the way in which 1 interacts with NCN group donor dbabhCN prior to NCN group transfer to form the terminal cyanoimide complex 1-NCN. The calculated Mo−N bond dissociation enthalpy for 1-NCN is 104 kcal mol-1, 30 kcal mol-1 greater than that for the corresponding dissociation of NCN from cyanophosphiniminato NCNPMe3.