Imidazolylidene Cu(II) Complexes: Synthesis Using Imidazolium Carboxylate Precursors and Structure Rearrangement Pathways

Abstract

Copper(II) complexes of type (NHC)CuX2 (X = OAc, Cl, Br, BF4, NO3) bearing monodentate N-heterocyclic carbenes (NHCs) were prepared by in situ decarboxylation of imidazolium carboxylates as a new synthetic methodology for Cu(II)-NHC complexes. In contrast to the classical deprotonation method, the decarboxylation protocol does not require anaerobic conditions and provides access to complexes with NHCs that are unstable as free carbenes such as N,N'-diisopropyl-imidazolylidene and N,N'-dimethyl-imidazolylidene. Spectroscopic evidence of the formation of the Cu-CNHC bond is provided by UV-vis and EPR, in particular by the 44 MHz carbene hyperfine coupling constant using a 13 C-labelled imidazolylidene ligand. A variation of the nature of the carbene N-substituents and the anions bound to the Cu(II) center is possible with this methodology. These variations strongly influence the stability of the complexes. Structural rearrangement and ligand reorganization was observed during recrystallisation, which are comprised of heterolytic Cu-CNHC bond dissociation for unstable NHC ligands as well as homolytic Cu-X bond cleavage and disproportionation reactions depending on the nature of the anion X in the copper complex.

Synthesis and Characterization of a Tetradentate, N-Heterocyclic Carbene Copper(II) Complex and Its Use as a Chan–Evans–Lam Coupling Catalyst

“…Due to the low number of copper­(II) NHC complexes that have been reported, most of which do not include a spectroscopic study, direct comparisons are limited. The d-to-d transitions occur at higher energy than those reported by van Koten and Albrecht, who reported a series of copper­(II) imidazolylidene based complexes with d-to-d transitions between 660 and 990 nm . Similar transitions in the UV have been observed for related copper­(I) NHC complexes. , The X-band EPR spectrum of 4 shows an axial signal with no resolved hyperfine structure (Figure inset).…”

“…Herein, we report the synthesis and characterization of a tetradentate pyridyl-based NHC complex using Cu­(OAc) 2 under atmospheric conditions. This complex adds to the growing number of NHC complexes using Cu­(OAc) 2 as both the metal source and the general base to generate Cu-NHC complexes with good success. , These complexes were characterized and then screened as CEL coupling agents using phenylboronic acids and various amines.…”

Highly Modular Piano-Stool N-Heterocyclic Carbene Iron Complexes: Impact of Ligand Variation on Hydrosilylation Activity

“…Iron NHC complexes are generally prepared from the air and moisture sensitive free carbene, which requires inert conditions. − Here, we demonstrate that the decarboxylation method offers an attractive alternative for the synthesis of these iron complexes. While this strategy has previously been applied predominantly for platinum group metals, its application to first-row transition metals is much rarer, mostly involves copper, and has never been applied to iron­(II) precursors so far. − The reaction of the imidazolium carboxylate 2a with [FeCpI­(CO) 2 ] induced the release of CO 2 and afforded the known complex 3a in similar yields as via the deprotonation route (43% vs 58%, Scheme ). Attractively, the metalation via decarboxylation proceeds in a distinct step that is not air or moisture sensitive and avoids the tedious isolation of the air-sensitive free carbene prior to transfer to the iron precursor.…”

Versatile Activity of a Copper(II) Complex Bearing a N4‐Tetradentate Schiff Base Ligand with Reduced Oxygen Species

“…The UV-vis spectrum of the complex supports the proposed geometry for the complex in solution, as it exhibits a broad low-intensity d-d band centered at 626 nm typical of Cu(II) in square-pyramidal geometry. [33] The absence of bands at longer wavelengths precludes the presence of species with trigonalbipyramidal metal coordination geometry. [34] Additionally, the complex exhibits a very strong absorption in the UV region where π-π* and ligand-to-metal charge transfer transitions overlap.…”