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(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.