Small-Molecule Activation by Molybdaziridine Hydride Complexes: Mechanistic Sequence of the Small-Molecule Binding and Molybdaziridine Ring-Opening Steps

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Abstract

The relationship between molybdaziridine hydride Mo(H)(Me2CNAr)(N[i-Pr]Ar)2 (1, Ar = 3,5-C6H3Me2) and its three-coordinate isomer Mo(N[i-Pr]Ar)3 (2) has been probed via an investigation of the coordination chemistry of 1 and Mo(N[t-Bu]Ar)3 (3) with 1-adamantylisocyanide (AdNC). One or two equivalents of AdNC react with 1 to form the adducts 2-AdNC and 2-(AdNC)2, respectively. One equivalent of AdNC coordinates to 3, forming 3-AdNC. Similarly, tert-butylisocyanide (t-BuNC) reacts with 1 to form 2-t-BuNC and 2-(t-BuNC)2 and with 3 to form 3-t-BuNC. An X-ray crystal structure of 2-(AdNC)2 reveals a trigonal bipyramidal core with a trans disposition of the isocyanide ligands (C−Mo−C, 172.8(4)°; Mo−C, 2.135(11) and 2.083(11) Å). The structure of 3-t-BuNC features a bent isocyanide ligand with a C−N−C angle of 137.8(7)°, and the compound has a solution IR stretch of 1759 cm-1, revealing Mo−C multiple-bond character. The fast reactions of AdNC with 1, 1-d3, and 3 were studied by stopped-flow spectrophotometry in a wide temperature range (−80 to 25 °C). A comparison of the reaction rates and activation parameters indicates that 2 is not an intermediate on the pathway from 1 to 2-AdNC, but rather that the molybdaziridine hydride opens upon AdNC binding in an associative process. The enthalpies of reaction to generate the compounds of interest were measured using solution calorimetry: 2-AdNC, −24.6 kcal·mol-1; 3-AdNC, −29.1 kcal·mol-1. The enthalpy of binding of the second equivalent of AdNC to generate 2-(AdNC)2 is −10.2 kcal·mol-1.

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