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)<sub>4</sub>, <b>1</b>, where Ad = 2-adamantylidene and Mes = 2,4,6-Me<sub>3</sub>C<sub>6</sub>H<sub>2</sub>, 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)<sub>4</sub> have been structurally characterized despite having been the subject of both experimental and theoretical interest. Complex <b>1</b> 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 <i>D</i><sub>2</sub><i><sub>d</sub></i> symmetry. Complex <b>1</b> makes a simple 1:1 adduct with <i>t</i>-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 <b>1</b> interacts with NCN group donor <i>dbabh</i>CN prior to NCN group transfer to form the terminal cyanoimide complex <b>1</b>-NCN. The calculated Mo−N bond dissociation enthalpy for <b>1</b>-NCN is 104 kcal mol<sup>-1</sup>, 30 kcal mol<sup>-1</sup> greater than that for the corresponding dissociation of NCN from cyanophosphiniminato NCNPMe<sub>3</sub>.