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

Abstract

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 i Pr have been synthesized using Cu precursors and bis(imidazolium) salts. All of these compounds, namely, [Cu 2 (IMe CNC ) 2 ](PF 6 ) 2, 1; [Cu 2 (IEt CNC ) 2 ](PF 6 ) 2, 2; [Cu 2 (I i Pr CNC ) 2 ](PF 6 ) 2, 3; [Cu(IMe C^N^C )](PF 6 ), 4; [Cu(IEt C^N^C )](PF 6 ), 5; and [Cu(I i Pr C^N^C )](PF 6 ), 6, have been characterized by 1 H and 13 C NMR spectroscopies, elemental analysis, solution conductivity, and electrochemical studies. Single crystal X-ray structures were obtained for all complexes except 1 . 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 5, 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 5 . 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 1 – 3 but 1:1 electrolyte for 4 – 6, 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 1 – 3 . In contrast, the reversible Cu(II)/Cu(I) couples of 4 – 6 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.