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

The relationship between molybdaziridine hydride Mo(H)(Me 2 C NAr)(N[ i -Pr]Ar) 2 ( 1, Ar = 3,5-C 6 H 3 Me 2 ) 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 - d 3, 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 .