Experimental and computational investigation of C-N bond activation in ruthenium N-heterocyclic carbene complexes
Haller, L. J. L., Page, M. J., Erhardt, S., Macgregor, S. A., Mahon, M. F., Abu Naser, M., Velez, A. and Whittlesey, M. K., 2010. Experimental and computational investigation of C-N bond activation in ruthenium N-heterocyclic carbene complexes. Journal of the American Chemical Society, 132 (51), pp. 18408-18416.
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A combination of experimental studies and density functional theory calculations is used to study C-N bond activation in a series of ruthenium N-alkyl-substituted heterocyclic carbene (NHC) complexes. These show that prior C H activation of the NHC ligand renders the system susceptible to irreversible C-N activation. In the presence of a source of HCl, C-H activated Ru((IPr2Me2)-Pr-i)'(PPh3)(2)(CO)H (1, (IPr2Me2)-Pr-i = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) reacts to give Ru((IPrHMe2)-Pr-i)(PPh3)(2)(CO)HCl (2, IiPrHMe(2) = 1-isopropyl-4,5-dimethylimidazol-2-ylidene) and propene. The mechanism involves (i) isomerization to a trans-phosphine isomer, 1c, in which hydride is trans to the metalated alkyl arm, (ii) C-N cleavage to give an intermediate propene complex with a C2-metalated imidazole ligand, and (iii) N-protonation and propene/substitution to give 2. The overall computed activation barrier (Delta E-calcd(double dagger)) corresponds to the isomerization/C-N cleavage process and has a value of +24.4 kcal/mol. C-N activation in 1c is promoted by the relief of electronic strain arising from the trans disposition of the high-trans-influence hydride and alkyl ligands. Experimental studies on analogues of 1 with different C4/C5 carbene backbone substituents (Ru((IPr2Ph2)-Pr-i)'(PPh3)(2)(CO)H, Ru((IPr2)-Pr-i)'(PPh3)(2)(CO)H) or different N-substituents (Ru(IEt2Me2)'(PPh3)(2)(CO)H) reveal that Ph substituents promote C-N activation. Calculations confirm that Ru((IPr2Ph2)-Pr-i)'(PPh3)(2)(CO)H undergoes isomerization/C-N bond cleavage with a low barrier of only +21.4 kcal/mol. Larger N-alkyl groups also facilitate C-N bond activation (Ru(I(t)Elu(2)Me(2))'(PPh3)(2)(CO)H, Delta E-calcd(double dagger) = +21.3 kcal/mol), and in this case the reaction is promoted by the formation of the more highly substituted 2-methylpropene.
|Creators||Haller, L. J. L., Page, M. J., Erhardt, S., Macgregor, S. A., Mahon, M. F., Abu Naser, M., Velez, A. and Whittlesey, M. K.|
|Departments||Faculty of Science > Chemistry|
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