Synthesis and Reactivity of Heteroditopic Dicarbene Rhodium(I) and Iridium(1) Complexes Bearing Chelating 1,2,3-Triazolylidene-Imidazolylidene Ligands

1,2,3-Triazol-5-ylidenes (tzNHC) have become a popular class of NHC ligands in homogeneous catalysis. Herein, we introduce chelate monovalent Rh- and Ir(cod) complexes bearing bidentate ligands that combine this tzNHC and an Arduengo-type NHC motif. The reactivity of these complexes with H-2 and CO gas has been investigated, leading to an interesting octahedral [Ir(tzNHC-CH2-NHC)(CO)(2)(H)(2)]OTf complex and [M(tzNHC-CH2-NHC)(CO)(2)]OTf complexes. The carbonyl stretching frequencies of the latter indicate that the ligand has stronger electron-donating properties than classic di-NHC ligands. The square planar rhodium and iridium NHC-tzNHC complexes have been applied in transfer hydrogenation employing isopropyl alcohol as the hydrogen donor, in which they show moderate activity (Ir > Rh) toward a range of ketones as well as for an aldehyde, an imine, and a diene. The new dicarbene complexes proved to be more active for this reaction than the analogues in which the triazolyl moiety coordinates through a nitrogen donor.