Synthesis, Characterization, and Spectroscopic Studies of 2,6-Dimethylpyridyl-Linked Cu(I)-CNC Complexes: The...

Six new Cu(I) complexes containing pincer ligands of the type 2,6-bis(3-alkyl/arylimidazol-2-ylidene) methylpyridine I(R/^R'Ar) ^ĈN̂C, where R = trifluoroethyl (TFE) and R' = 4-CF₃, 4-NO₂, 4-CN, 4-H, and 4-CH₃, have been synthesized. These complexes, namely, [Cu(I(TFE)^ĈN̂C)]PF₆, 1-TFE; [Cu(I^CF3Ar ^ĈN̂C]PF₆, 2-CF; [Cu(I^NO2Ar ^ĈN̂C)]PF₆, 3-NO; [Cu(I^CNAr ^ĈN̂C]PF₆, 4-CN; [Cu(I^HAr ^ĈN̂C)]₂(PF₆)₂, 5-H; and [Cu(I^CH3Ar ^ĈN̂C)]₂(PF₆)₂, 6-CH, were fully characterized by ¹H, ¹³C, and HMBC NMR spectroscopy, elemental analysis, electrochemical studies, and single-crystal X-ray crystallography. The crystallographic data revealed different structures and copper nuclearities for the complexes bearing aryl wingtips with electron-withdrawing (2-CF, 3-NO, and 4-CN) and electron-donating (5-H and 6-CH) substituents. The solution-phase conductivity measurements in acetonitrile revealed a mix-electrolyte behavior for these complexes, supporting the presence of both mono- and binuclear forms of each complex. The fast monomer-dimer equilibrium of the Cu-CNC complexes at room temperature is reflected in their simple ¹H NMR spectra in acetonitrile. However, both mono- and binuclear forms were identifiable in ¹H diffusion-ordered spectroscopy (DOSY) at low temperatures. The dynamic behavior of these complexes in solution was further examined by variable-temperature ¹H NMR (VT ¹H NMR) experiments, and the relevant thermodynamic parameters were determined. The process was also probed by one-dimensional rotating-frame Overhauser enhancement spectroscopy (1D ROESY) experiments to elucidate the coexisting species in solution. The 2,6-dimethylpyridyl-linked Cu-CNC complexes also presented a quasi-reversible Cu(II)/Cu(I) couple in cyclic voltammetry studies, wherein a clear influence of the aryl wingtips on the E_1/2 values was observed. Furthermore, the percent buried volumes (% V_bur) of the complexes were calculated, showing a similar steric hindrance around copper in all complexes. These findings support the importance of electronic effects, induced by the aryl wingtips, on the preferred coordination geometry, copper nuclearity, and redox properties of the Cu-CNC complexes.