Copper N-heterocyclic carbenes (NHCs) are an emerging area of focus for catalysis and other applications. Using a straightforward methodology, a new and highly modifiable tetradentate copper­(II) NHC complex was generated and characterized using X-ray crystallography, UV–vis and EPR spectroscopy, cyclic voltammetry, and ESI-MS. This copper­(II) NHC complex adopted a distorted 4-coordinate coordination mode and demonstrates a unique absorption spectrum for a copper­(II) species, but more interestingly, its redox properties indicate that it can readily access all three common copper oxidation states under atmospheric conditions. The tetradentate copper­(II) NHC complex was used to catalytically generate new C–N bonds from a series of phenylboronic acids and amines. Once this CEL methodology was refined, moderate to high yields were achieved using catalytic amounts of the copper­(II) complex to couple phenylboronic acids to a series of aniline derivatives. Substituted phenylboronic acids and anilines had minimal impact on the catalytic capabilities of this copper complex; however, there is some indication that steric interactions between catalyst and substrates may have an impact on efficient catalysis. The straightforward synthesis of this framework and the utilization of an inexpensive, first-row transition metal center in this system highlight the usefulness of copper­(II) NHCs as catalyst for cross-coupling reactions.

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The coupling of phenylboronic acids with poorly-activated imidazoles is studied as a model system to explore the use of copper-catalyzed Chan -Evans-Lam (CEL) coupling for targeted CÀ N bond forming reactions. Optimized CEL reaction conditions are reported for four phenanthroline-based ligand systems, where the ligand 4,5-diazafluoren-9-one (dafo, L2) with 1 molar equivalent of potassium carbonate yielded the highest reactivity. The substrate 2-nitroimidazole (also known as azomycin) has documented antimicrobial activity against a range of microbes. Here N-arylation of 2-nitroimidazole with a range of aryl boronic acids has been successfully developed by copper(II)-catalyzed CEL reactions. Azomycin and a range of newly arylated azomycin derivatives were screened against S. pneumoniae, where 1-(4-(benzyloxy)phenyl)-2-nitro-1H-imidazole (3d) was demonstrated to have a minimal inhibition concentration value of 3.3 μg/mL.

With the aim of constructing hydrogen‐bonding networks in synthetic complexes, two new ligands derived from cis,cis‐1,3,5‐triaminocyclohexane (TACH) have been prepared that feature pendant pyrrole or indole rings as outer‐sphere H‐bond donors.  The TACH framework offers a facial arrangement of three N‐donors, thereby mimicking common coordination motifs in the active sites of nonheme Fe and Cu enzymes.  X‐ray structural characterization of a series of CuI‐X complexes (X = F, Cl, Br, NCS) revealed that these neutral ligands (H3LR, R = pyrrole or indole) coordinate in the intended facial N3 manner, yielding four‐coordinate complexes with idealized C3 symmetry.  The N‐H units of the outer‐sphere heterocycles form a hydrogen‐bonding cavity around the axial (pseudo)halide ligand, as verified by crystallographic, spectroscopic, and computational analyses.  Treatment of H3Lpyrrole and H3Lindole with divalent transition metal chlorides (MIICl2, M = Fe, Cu, Zn) causes one heterocycle to deprotonate and coordinate to the M(II) center, giving rise to tetradentate ligands with two remaining outer‐sphere H‐bond donors.  Further ligand deprotonation is observed upon reaction with Ni(II) and Cu(II) salts with weakly‐coordinating counteranions.  The reported complexes highlight the versatility of  TACH‐based ligands with pendant H‐bond donors, as the resulting scaffolds can support multiple protonation states, coordination geometries, and H‐bonding interactions.