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(I ⁱPr₂Me₂)′(PPh₃) ₂(CO)H (1, IⁱPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) reacts to give Ru(I ⁱPrHMe₂)(PPh₃)₂(CO)HCl (2, I ⁱPrHMe₂ = 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)′(PPh ₃)₂(CO)H, Ru(IⁱPr₂)′(PPh ₃)₂(CO)H) or different N-substituents (Ru(IEt ₂Me₂)′(PPh₃)₂(CO)H) reveal that Ph substituents promote C-N activation. Calculations confirm that Ru(I ⁱPr₂Ph₂)′(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^tBu₂Me₂)′(PPh ₃)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.