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-Me ₃C₆H₂, 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 D_2d 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.