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.

Automated summary

This study determined the interconversion rates of the syn- and anti-isomers of neutral molybdenum and tungsten imido alkylidene N-heterocyclic carbene complexes and found that they were significantly lower than in tetracoordinated Schrock catalysts. NMR investigations suggest that higher transition state Gibbs free energy values are due to the dissociation of an anionic ligand from a neutral pentacoordinated catalyst. The anti-isomer showed higher reactivity and productivity in both ring-opening metathesis polymerization and ring-closing metathesis reactions.