Cu(I) Complexes of Pincer Pyridine-Based N‑Heterocyclic Carbenes with Small Wingtip Substituents: Synthesis and Structural and Spectroscopic Studies

Six new Cu­(I) complexes with pincer N-heterocyclic carbene (NHC) ligands of the type 2,6-bis­(3-alkylimidazol-2-ylidene)­pyridine, I­(R)^CNC, and 2,6-bis­(3-alkylimidazol-2-ylidene)­methylpyridine, I­(R)^{C^N^C}, where R = Me, Et, and ⁱPr have been synthesized using Cu precursors and bis­(imidazolium) salts. All of these compounds, namely, [Cu₂(IMe^CNC)₂]­(PF₆)₂, <b>1</b>; [Cu₂(IEt^CNC)₂]­(PF₆)₂, <b>2</b>; [Cu₂(IⁱPr^CNC)₂]­(PF₆)₂, <b>3</b>; [Cu­(IMe^{C^N^C})]­(PF₆), <b>4</b>; [Cu­(IEt^{C^N^C})]­(PF₆), <b>5</b>; and [Cu­(IⁱPr^{C^N^C})]­(PF₆), <b>6</b>, have been characterized by ¹H and ¹³C NMR spectroscopies, elemental analysis, solution conductivity, and electrochemical studies. Single crystal X-ray structures were obtained for all complexes except <b>1</b>. The crystallographic data reveal a binuclear structure containing two Cu atoms at a close distance, 2.622–2.811 Å for all the complexes except <b>5</b>, which shows a unique mononuclear structure. Spatial syn arrangement of ethyl groups and extensive π–π stacking in the solid state accounts for the mononuclear structure of complex <b>5</b>. A pseudolinear coordination geometry about metal centers consisting of two Cu–carbene bonds, as well as weak Cu–pyridine interactions, exist among all the complexes independent of their ligand. Solution-state conductivity data reveal a dominant 1:2 electrolyte behavior for <b>1</b>–<b>3</b> but 1:1 electrolyte for <b>4</b>–<b>6</b>, consistent with the sustainable binuclear structure in solutions of Cu­(I)–I­(R)^CNC complexes. Cyclic voltammetry and differential pulse voltammetry studies reveal an irreversible and two quasi-reversible peaks for the one-electron oxidation of solvent-bound and solvent-free binuclear and mononuclear Cu-NHC species in complexes <b>1</b>–<b>3</b>. In contrast, the reversible Cu­(II)/Cu­(I) couples of <b>4</b>–<b>6</b> at potentials close to that of complexes with tripodal polydentate NHC scaffolds indicate the electronic and structural flexibility of I­(R)^{C^N^C} ligands to accommodate both Cu­(I) and Cu­(II) ions