Experimental and Computational Investigation of C−N Bond Activation in Ruthenium N-Heterocyclic Carbene Complexes

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(IiPr2Me2)′(PPh3)2(CO)H (1, IiPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) reacts to give Ru(IiPrHMe2)(PPh3)2(CO)HCl (2, IiPrHMe2 = 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/Cl− substitution to give 2. The overall computed activation barrier (ΔE ⧧ calcd) 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(IiPr2Ph2)′(PPh3)2(CO)H, Ru(IiPr2)′(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(IiPr2Ph2)′(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(ItBu2Me2)′(PPh3)2(CO)H, ΔE ⧧ calcd = +21.3 kcal/mol), and in this case the reaction is promoted by the formation of the more highly substituted 2-methylpropene.