Synthesis and characterization of unsymmetrical-CuNHC complexes; connecting the electronic influence of substituted pyridine rings to copper(II)

This report describes the synthesis and characterization of a family of unsymmetrical copper(II) N-heterocyclic carbene (NHC) complexes. A series of tridentate NCN pincer ligands bearing unsymmetrical C-4 substitutions on one pyridine ring were prepared using a straightforward synthetic methodology. Substituents, including methoxy (OMe), methyl (Me), chloro (Cl), and trifluoromethyl (CF₃), were introduced to systematically evaluate their electronic effects on copper(II) coordination and reactivity. Ligand precursors and corresponding copper(II) complexes were characterized by multinuclear NMR spectroscopy ( 1 H, 13 C, 31 P, 19 F), electrospray ionization mass spectrometry (ESI-MS), X-ray crystallography, cyclic voltammetry, and UV–vis spectroscopy. Structural analysis indicates that variation of pyridyl substituents has minimal impact on the solid-state coordination environment. However, electrochemical and spectroscopic studies reveal measurable electronic effects in solution. Computational investigations show comparable Lewis acidity and related physical properties across the series, with largely invariant HOMOs but substituent-dependent variation in LUMO and LUMO + 1 energies. Notably, the CF₃-substituted complex exhibits distinct behavior, promoting solvent oxidation to generate copper(I) species and displaying promising Ullmann-type C N bond-forming reactivity. Overall, these results demonstrate that while carbene donation dominates reactivity, fine electronic tuning is achievable through controlled modification of the pyridyl donor. • Stepwise syntheses of unsymmetrical NHC pincer ligands reported providing a mechanism for fine tuning electronic effects in coordination complexes. • Pyridinyl donor effects impact electronics in solution, but largely do not impact the coordination structure. • Select C N coupling reactivity through CEL and Ullman reactions unaffected by electron donor and weak withdrawer substitution. • Deactivation of CF 3 substituted py in 5e leads to remarkably different physical and electronic properties. • Ground state electronic states in the series of unsymmetrical copper(II) NHC complexes unaffected by py donor properties, however excited state orbitals (LUMO and LUMO + 1) appear to be more influenced.