Charge-Delocalized κ²C,N-NHC-Amine Complexes of Rhodium, Iridium, and Ruthenium

Abstract

The development of a novel set of complexes bearing an NHC-amine ligand (C NHC -NH 2 ) 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 C NHC -NH 2 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. ■ INTRODUCTIONThe use of carbenes as ligands is well developed in the field of transition-metal complex chemistry.1 In particular, the Nheterocyclic carbene (NHC) class of imidazol-2-ylidines, first reported in the 1960s, 2 have received their fair share of attention. Many successful examples of the application of NHCs in transition-metal-catalyzed transformations such as hydrogenations and metathesis reactions 3−5 have been reported. We recently reported on the synthesis of a conjugated NHC-amine ligand species where an NHC is conjugated with an aniline (C NHC -NH 2 ). 6 In this species, the primary amine is tethered to the NHC via an aromatic ring. The amine moiety of our C NHC -NH 2 ligand A (see Figure 1) can coordinate in a neutral or in an anionic fashion (when deprotonated). Monoanionic (bidentate) ligands have found widespread application in homogeneous catalysis. The most recognized are (pentamethyl)cyclopentadienyl (Cp, Cp*), acetylacetonate (acac), and diketimine (nacnac, where the two oxygen atoms of acac have been replaced by nitrogen-based moieties of the form NR). 7,8 Well-known examples of bifunctional ligands are ligands containing hydroxyl or oxime 9 groups and pincer-type ligands containing an anionic donor arm. 10,11 Previous work by Morris, involving a benzylic amine tethered to an NHC, has shown the ability of this ligand scaffold to display cooperativity, but this depended greatly on other factors such as the ancillary ligand (p-cym vs Cp*) and solvent assistance.12 Morris also explored the potential of iridium(I)-cod complexes containing neutral bidentate NHC-benzylic amine ligands, coordinated as seven-membered chelate rings to the metal center. Anilinebased C NHC -NH 2 ligands, forming neutral complexes containing conjugated anionic C NHC -NH − ligands bound to Rh, Ir, or

Phenoxy-Linked Mesoionic Triazol-5-ylidenes as Platforms for Multinuclear Transition Metal Complexes

“…Surprised by the solution behavior of L1 ·[Rh­(COD)­Cl], we searched for NMR data of similar rhodium-NHC complexes containing cyclooctadiene groups. On the basis of several literature reports, we found that these asymmetrical 1 H NMR patterns are characteristic for neutral square planar NHC rhodium­(I) complexes where the cyclooctadiene ligand occupies two coordination sites and the carbene and chlorine centers complete the metal coordination sphere. It was interesting finding that even amino- and quinoline-functionalized NHCs containing Rh­(COD)Cl fragments do not display coordination of the hemilabile moiety to the metal center as the etha four coordination of the COD ligand is preferred.…”

Ruthenium olefin metathesis catalysts featuring unsymmetrical N-heterocyclic carbenes

“…All the other structural parameters are in good agreement with those observed for other similar complexes. [32][33][34][35] The steric bulk, measured as the percentage of buried volume (%V Bur ) of the N-cyclohexyl/N-o-isopropylphenyl NHC ligands with syn or anti phenyl groups on the backbone, was extracted using the SambVca program developed by Cavallo, 36 Scheme 4 Synthesis of Rh complexes 39-42. using the crystal data of the corresponding Rh complexes 39 and 40. The %V Bur values are 29.7 and 29.9 for syn uNHC and anti uNHC ligands, respectively, and thus within the same level of significance.…”