Interconversion Rates and Reactivity of Syn- and Anti-rotamers of Neutral and Cationic Molybdenum and Tungsten Imido Alkylidene N-Heterocyclic Carbene Complexes

The interconversion rates of the syn- and anti-isomers of the neutral molybdenum and tungsten imido alkylidene N-heterocyclic carbene (NHC) complexes of the general formula [M(NR)(CHCMe2R')(NHC)(OR'')2] (M = Mo, W; R = adamantyl, 2,6-Me-2-C6H3, 2,6-iPr(2)-C6H3, 2,6-Cl-2-C6H3, 2-tBu-C6H4; NHC = 1,3-diisopropylimidazol-2-ylidene (IiPr), 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-dicyclohexylimidazol-2-ylidene (ICy), and 1,3-dimesitylimidazol-2-ylidene (IMes); R' = Me, Ph; R'' = CMe(CF3)(2), CMe2(CF3), C(CF3)(3), C(6)F5, SO2CF3) and of the cationic complex [Mo(N-2,6-Me-2-C6H3)(CHCMe2Ph)(IMes)(SO3CF3)(CD3CN)(+) B(3,5-(CF3)(2)C6H3)(4)-] were determined by irradiating solutions of the catalysts with 366 nm UV light followed by recording the back-isomerization of the anti- to the syn-isomer. Both the rate of anti to syn and syn to anti interconversion, k a/s and k s/a were found to be orders of magnitude lower than in tetracoordinated, neutral Schrock catalysts of the general formula [Mo(NR)(CHCMe2R')(OR'')(2)]. Accordingly, the values for the Gibbs free energy of the transition state, Delta G double dagger, are significantly higher for both neutral and cationic molybdenum and tungsten imido alkylidene NHC complexes than for Schrock catalysts. NMR investigations strongly suggest that these higher Delta G double dagger values are attributable to dissociation of an anionic ligand from a neutral pentacoordinated catalyst that precedes interconversion. As with Schrock catalysts, the anti-isomer proved to be the more reactive isomer, in both ring-opening metathesis polymerization and ring-closing metathesis (RCM) reactions, allowing for higher productivities, expressed as turnover numbers, in RCM.