Homo- and Heterodinuclear Ir and Rh Imine-functionalized Protic NHC Complexes: Synthetic, Structural Studies, and Tautomerization/Metallotropism Insights

The influence of the potentially chelating imino group of imine-functionalized Ir and Rh imidazole complexes on the formation of functionalized protic N-heterocyclic carbene (pNHC) complexes by tautomerization/metallotropism sequences was investigated. Chloride abstraction in [Ir(cod)Cl{C3 H3N2 (DippN=CMe)-κN3}] (1 a) (cod=1,5-cyclooctadiene, Dipp=2,6-diisopropylphenyl) with TlPF6 gave [Ir(cod){C3 H3N2 (DippN=CMe)-κ(2) (C2,Nimine )}](+) [PF6 ](-) (3 a(+) [PF6 ](-)). Plausible mechanisms for the tautomerization of complex 1 a to 3 a(+) [PF6 ](-) involving C2-H bond activation either in 1 a or in [Ir(cod){C3 H3 N2 (DippN=CMe)-κN3}2](+) [PF6 ](-) (6 a(+) [PF6 ](-)) were postulated. Addition of PR3 to complex 3 a(+) [PF6 ](-) afforded the eighteen-valence-electron complexes [Ir(cod)(PR3){C3 H3N2 (DippN=CMe)-κ(2) (C2,Nimine )}](+) [PF6 ](-) (7 a(+) [PF6 ](-) (R=Ph) and 7 b(+) [PF6 ](-) (R=Me)). In contrast to Ir, chloride abstraction from [Rh(cod)Cl{C3H3N2 (DippN=CMe)-κN3}] (1 b) at room temperature afforded [Rh(cod){C3 H3N2 (DippN=CMe)-κN3}2](+) [PF6 ](-) (6 b(+) [PF6 ](-)) and [Rh(cod){C3 H3N2 (DippN=CMe)-κ(2) (C2,Nimine )}](+) [PF6 ](-) (3 b(+) [PF6 ](-) ) (minor); the reaction yielded exclusively the latter product in toluene at 110 °C. Double metallation of the azole ring (at both the C2 and the N3 atom) was also achieved: [Ir2 (cod)2 Cl{μ-C3H2N2 (DippN=CMe)-κ(2) (C2,Nimine ),κN3}] (10) and the heterodinuclear complex [IrRh(cod)2 Cl{μ-C3H2N2 (DippN=CMe)-κ(2) (C2,Nimine ),κN3}] (12) were fully characterized. The structures of complexes 1 b, 3 b(+) [PF6 ](-) , 6 a(+) [PF6 ](-) , 7 a(+) [PF6 ](-), [Ir(cod){C3HN2 (DippN=CMe)(DippN=CH)(Me)-κ(2) (N3,Nimine )}](+) [PF6 ](-) (9(+) [PF6 ](-)), 10⋅ Et2 O⋅toluene, [Ir2 (CO)4 Cl{μ-C3H2N2 (DippN=CMe)-κ(2) (C2,Nimine ),κN3}] (11), and 12⋅2 THF were determined by X-ray diffraction.

NHC Backbone Configuration in Ruthenium-Catalyzed Olefin Metathesis

The catalytic properties of olefin metathesis ruthenium complexes bearing N-heterocyclic carbene ligands with stereogenic centers on the backbone are described. Differences in catalytic behavior depending on the backbone configurations of symmetrical and unsymmetrical NHCs are discussed. In addition, an overview on asymmetric olefin metathesis promoted by chiral catalysts bearing C₂-symmetric and C₁-symmetric NHCs is provided.

Enantioselectivity switch in copper-catalyzed conjugate addition reactions under the influence of a chiral N-heterocyclic carbene–silver complex

The reversal of enantioselectivity using the same catalytic system was achieved simply by changing the order of the addition of substrates.

Ruthenium olefin metathesis catalysts featuring unsymmetrical N-heterocyclic carbenes

New second generation ruthenium catalysts bearing unsymmetrical NHCs show different catalytic efficiencies depending on the size of the N-alkyl group (methyl or cyclohexyl) and on the backbone configuration.

Synthesis and catalytic activity of ruthenium indenylidene complexes bearing unsymmetrical NHC containing a heteroaromatic moiety

New ruthenium(ii) indenylidene catalysts were synthesized and used in olefin metathesis reactions in toluene under air, leading to high conversions and good selectivities.

Amino acids as chiral anionic ligands for ruthenium based asymmetric olefin metathesis

Several amino acid ligands were introduced into the Hoveyda-Grubbs 2nd generation complex by a facile anionic ligand exchange. The chiral pre-catalysts obtained displayed enantioselectivity in asymmetric ring-closing and ring-opening cross-metathesis reactions. Reduction of the lability of the carboxylate ligands was found to be cardinal for improving the observed enantiomeric product enrichment.

Cycloalkyl-based unsymmetrical unsaturated (U2)-NHC ligands: flexibility and dissymmetry in ruthenium-catalysed olefin metathesis

Structural features, chemical stabilities and catalytic profiles in olefin metathesis of a new library of low-cost cycloalkyl-based U₂-NHC Ru complexes were disclosed.

Unsymmetrical N-heterocyclic carbenes with a 1,1′-ferrocenediyl backbone

N-Substituents have only a marginal influence on the electronic properties, but a strong influence on the steric properties of ferrocene-based NHC ligands, which therefore offer great potential for rational fine-tuning.

Metal–ligand bifunctional reactivity and catalysis of protic N-heterocyclic carbene and pyrazole complexes featuring β-NH units

The metal–ligand bifunctional cooperation of protic N-heterocyclic carbene and pyrazole complexes bearing an NH unit at the position β to the metal is surveyed.