Abnormal Carbene Complexes Research Articles

Normal to abnormal ItBu·AlH3 isomerization in solution and in the solid state.

A complex of an N-heterocyclic carbene ItBu (1,3-di-tert-butylimidazolin-2-ylidene) and aluminum hydride was observed to isomerize into an abnormal carbene complex aItBu·AlH3 (aItBu = 1,3-di-tert-butylimidazolin-4-ylidene) in a polar solvent, and, for the first time, in the solid state. ItBu·AlH3 and aItBu·AlH3 were structurally characterized by single crystal X-ray diffraction analysis. NMR studies and DFT computations indicate that the polarity of the solvent promotes the isomerization process. The possible pathways for the isomerization are discussed on the basis of the DFT computational studies.

Zincocene and dizincocene N-heterocyclic carbene complexes and catalytic hydrogenation of imines and ketones.

The N-heterocyclic carbene (NHC) adducts Zn(Cp(R))(2) (NHC)] (Cp(R) =C(5)HMe(4), C(5)H(4) SiMe(3); NHC=ItBu, IDipp (Dipp=2,6-diisopropylphenyl), IMes (Mes=mesityl), SIMes) were prepared and shown to be active catalysts for the hydrogenation of imines, whereas decamethylzincocene [ZnCp*(2)] is highly active for the hydrogenation of ketones in the presence of noncoordinating NHCs. The abnormal carbene complex [Zn(OCHPh(2))(2) (aItBu)](2) was formed from spontaneous rearrangement of the ItBu ligand during incomplete hydrogenation of benzophenone. Two isolated Zn(I) adducts [Zn(2)Cp*(2) (NHC)] (NHC=ItBu, SIMes) are presented and characterized as weak adducts on the basis of (13)C NMR spectroscopic and X-ray diffraction experiments. A mechanistic proposal for the reduction of [ZnCp*(2)] with H(2) to give [Zn(2)Cp*(2)] is discussed.

Rational Design of Ligand Precursors to Prepare Abnormal (Mesoionic) and Normal Carbene Complexes and Zwitterionic CX‐Type Palladacycles (X = C, N)

Abstract The preparation of a series of ligand precursors that feature imidazo[1,2‐a]pyridine and amido functional groups is reported. The reactions between the new ligand precursors, pyridine, and palladium acetate in DMF afforded monodentate PdII abnormal (mesoionic) carbene complexes. An isomeric PdII normal carbene complex with a formally identical steric environment to that of the abnormal carbene counterpart was also prepared. The isomeric pairs were characterized by X‐ray structural studies. Under different complexation conditions, in which PdCl2 was employed as metal precursor, a zwitterionic CC‐type palladacycle formed by the C–H activation at the ortho‐N‐phenyl and methylene carbon atoms was obtained. The use of a similar ligand precursor with an NH instead of an NMe group produced a zwitterionic palladalactam complex. In contrast to the previous work in the literature, owing to the rigidity of the fused heterocyclic ring, the palladalactam complex did not undergo intramolecular proton transfer through the coordinated N atom. Overall, we demonstrated that proper modification of the structures of ligand precursors and complexation conditions allowed us to obtain a full range of intriguing PdII complexes including normal and abnormal carbene complexes, and zwitterionic palladalactam and CC‐type palladacycles by means of A–H bond (A = N, C) activations.

Normal and abnormal carbene complexes derived from thiazole: Preparation and a preliminary investigation of their relative catalytic performance

Normal and abnormal carbene complexes derived from thiazole: Preparation and a preliminary investigation of their relative catalytic performance

Solvent influence in the formation of normal and abnormal carbene complexes in reactions of imidazolium salts with [Ir(H) 2(PPh 3) 2(OCMe 2) 2]BF 4

Solvent influence in the formation of normal and abnormal carbene complexes in reactions of imidazolium salts with [Ir(H) 2(PPh 3) 2(OCMe 2) 2]BF 4

On the Electronic Impact of Abnormal C4‐Bonding in N‐Heterocyclic Carbene Complexes

Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for the abnormal carbene complexes, specifically upon reaction with Lewis acids and in oxidative addition-reductive elimination sequences. These reactivities as well as analytical investigations using X-ray diffraction and X-ray photoelectron spectroscopy indicate that the C4 bonding mode increases the electron density at the metal center substantially, classifying such C4-bound carbene ligands amongst the most basic neutral donors known thus far. A direct application of this enhanced electron density at the metal center is demonstrated by the catalytic H(2) activation with abnormal carbene complexes under mild conditions, leading to a catalytic process for the hydrogenation of olefins.

1,2,3-Triazolylidenes as Versatile Abnormal Carbene Ligands for Late Transition Metals

The [3 + 2] cycloaddition of azides and acetylenes followed by nitrogen quaternization was applied for the generation of novel and highly modular triazolium salts. The selective substitution of the 1,3,4-substitution pattern presets such salts as precursors for a new class of abnormal carbene ligands, thus expanding the family of these high-impact ligands. Metalation of the triazolium salts is highly versatile and is illustrated by direct C-H bond activation as well as by applying a transmetalation protocol, thus providing access to Pd(II), Ru(II), Rh(I), and Ir(I) abnormal carbene complexes. The donor properties of these carbenes were analyzed by using Tolman electronic parameters and were found to be slightly stronger than those the most basic normal carbenes.