Steric Control at the Wingtip of a Bis-N-Heterocyclic Carbene Ligand: Coordination Behavior and Catalytic Responses of Its Ruthenium Compounds

Changing the N-substituents of a methylene-linked bis-NHC
ligand from n-butyl to bulky mesityl shifts ligand
coordination from normal/normal to normal/abnormal mode. The mesityl
wingtip groups afford [RuII(MesNHC­(CH2)­NHCMes)­(MesNHC­(CH2)aNHCMes)­(CH3CN)2]­[PF6]2 (1), in which one of the ligands
exhibits mixed C2/C4 binding to the same metal,
while the second ligand utilizes C2/C2 carbons
for metal coordination. On the contrary, the n-butyl
analogue leads to metal oxidation, affording a RuIII complex
[Ru­(nBuNHC­(CH2)­NHCnBu)2Cl2]­[PF6] (2) upon crystallization in air, where both ligands show normal
C2/C2 coordination. Thus, the resulting complexes
exhibit different structural and electronic characteristics, which
are further reflected in their catalytic responses. The catalytic
utilities of both compounds toward carboxylic acid addition onto terminal
alkyne are evaluated in this work. The abnormally bound 1 shows higher activity and better selectivity compared to all-normal
counterpart 2.