Synthesis and Reactivity of Heteroditopic Dicarbene Rhodium(I) and Iridium(I) 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 H₂ 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

Charge-delocalized κ2 C, N-NHC-amine complexes of rhodium, iridium, and ruthenium

The development of a novel set of complexes bearing an NHC-amine ligand (CNHC-NH2) is described. M(cod) complexes (M = Ir, Rh) and a Ru complex have been synthesized in which three different coordination modes of the ligand were established: monodentate, neutral bidentate, and anionic bidentate. The anionic bidentate coordination mode of the anionic C NHC-NH- ligand arises from deprotonation of the amine moiety of the neutral CNHC-NH2 ligand. Ligand deprotonation proved to be reversible for the Rh and Ir complexes, as was shown by subsequent treatment of the complexes with base and acid. The structural parameters of these differently coordinated ligands were examined, and it was shown that the conjugation of the aniline ring plays a major role in determining the ligand properties. Structural parameters derived from DFT calculations confirm delocalization of the anionic charge over the ligand framework, as is clear from a comparison of the (hypothetical) neutral bidentate complexes [M(cod)(κ2C,N-{CNHC-NH2})]+ with those of the (synthesized) monoanionic complexes [M(cod) (κ2C,N-{CNHC-NH})] (M = Rh, Ir). A similar trend in the structure and bond lengths of the aniline rings was found in the solid-state structure of the novel dimeric complex [(Ru(κ2C,N-{C NHC-NH})(κ2C,N-{CNHC-NH 2})Cl)2(μ-Cl)](PF6). The octahedral d 5 ruthenium(III) centers in this complex both contain a neutral bidentate CNHC-NH2 ligand as well as an anionic bidentate CNHC-NH- ligand. Quite remarkably, the complex is diamagnetic, arising from antiferromagnetic coupling of the two low-spin ruthenium(III) centers over the chloride linker. DFT calculations indeed confirm that the open-shell singlet electronic structure is most stable