Heteroleptic Cu(I) Bis-diimine Complexes of 6,6′-Dimesityl-2,2′-bipyridine: A Structural, Theoretical and Spectroscopic Study

A series of heteroleptic Cu(I) complexes containing 6,6'-dimesityl-2,2'-bipyridine and phenanthroline-, bipyridine-, and biquinoline-based ligands is studied. The HETPHEN strategy is utilized to synthesize the heteroleptic complexes, which are stable in solution. The X-ray crystal structures of the complexes are presented; the solid-state four-coordinate Cu(I) geometries are quantified by using the τ4 parameter. A feature of the crystal structures is the intramolecular π-stacking between the mesityl ring(s) and the diimine ligand; the phen-based complexes exhibit stacking between the phen ligand and one of the mesityl rings, creating a "Pac-Man" motif. On the other hand, the bpy-based complexes show different types of packing interaction, with both mesityl rings "clamping down" on the bpy based ligand to give π-stacking. Cyclic voltammetry is used to examine the redox chemistry of the complexes. The most positive potentials for the oxidation process are observed for the complexes with bulky substituents ortho to the coordination nitrogens atoms, i.e., 2,9-dimethyl-1,10-phenanthroline and 6,6'-dibromo-2,2'-bipyridine. The Cu(I) MLCT transitions of the complexes are investigated by resonance Raman spectroscopy in concert with TD-DFT calculations. The resonance Raman spectra of complexes containing substituted biquinolines are straightforward, in that vibrational bands of the biquinoline-based ligand are selectively enhanced over bpy(Mes)2 bands. This is consistent with the purple color of the complexes, due to the lower energy of the biquinoline-based LUMO compared to the bpy(Mes)2 LUMO. All the phen- and bpy-based complexes show enhancement of bpy(Mes)2 bands.