Coordination Chemistry of a Hemilabile Amino-Tethered N-Heterocyclic Carbene with Ruthenium(II)

The reaction of the amine-tethered N-heterocyclic carbene ligand 2,4,6-Me3C6H2NC3H2NCH2CH2NH-2,4,6-Me3C6H2 (Mes[CNH]) with Ru(CHPh)(PCy3)2Cl2 leads to the formation of Mes[CNH]Ru(CHPh)(PCy3)Cl2, which exists as a mixture of two isomers in a ratio of 7:1. While the major species was characterized by X-ray crystallography, the minor rotamer species could be characterized only in solution. Phosphine exchange kinetics and equilibrium variable-temperature measurements did not show much difference from benchmark Grubbs systems. Evidence for coordination of the amine tether was obtained by addition of pyridine, which generated the octahedral complex Mes[CNH]Ru(CHPh)(py)Cl2 with the tethered amine coordinated. Similarly, addition of PMe3 results in the formation of Mes[CNH]Ru(CHPh)(PMe3)Cl2, also having an octahedral structure with the pendant amine ligated. Attempts to benchmark the catalytic potential of the PCy3 derivative showed that it was far inferior to known Grubbs-type systems.