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.