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 iPr have been synthesized using Cu precursors and bis(imidazolium) salts. All of these compounds, namely, [Cu2(IMeCNC)2](PF6)2, 1; [Cu2(IEtCNC)2](PF6)2, 2; [Cu2(IiPrCNC)2](PF6)2, 3; [Cu(IMeC^N^C)](PF6), 4; [Cu(IEtC^N^C)](PF6), 5; and [Cu(IiPrC^N^C)](PF6), 6, 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 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.