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 [Ru<SUP>II</SUP>(<SUP>Mes</SUP>NHC(CH<SUB>2</SUB>)NHC<SUP>Mes</SUP>)(<SUP>Mes</SUP>NHC(CH<SUB>2</SUB>)aNHC<SUP>Mes</SUP>)(CH<SUB>3</SUB>CN)<SUB>2</SUB>][PF<SUB>6</SUB>]<SUB>2</SUB> (1), in which one of the ligands exhibits mixed C<SUB>2</SUB>/C<SUB>4</SUB> binding to the same metal, while the second ligand utilizes C<SUB>2</SUB>/C<SUB>2</SUB> carbons for metal coordination. On the contrary, the n-butyl analogue leads to metal oxidation, affording a Ru<SUP>III</SUP> complex [Ru(<SUP>nBu</SUP>NHC(CH<SUB>2</SUB>)NHC<SUP>nBu</SUP>)<SUB>2</SUB>Cl<SUB>2</SUB>][PF<SUB>6</SUB>] (2) upon crystallization in air, where both ligands show normal C<SUB>2</SUB>/C<SUB>2</SUB> 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