6,6′-Ditriphenylamine-2,2′-bipyridine: Coordination Chemistry and Electrochemical and Photophysical Properties

A 2,2'-bipyridine with bulky triphenylamine substituents in the 6 and 6' positions of the ligand (6,6'-ditriphenylamine-2,2'-bipyridine, 6,6'-diTPAbpy) was generated. Despite the steric bulk, the ligand readily formed bis(homoleptic) complexes with copper(I) and silver(I) ions. Unfortunately, efforts to use the 6,6'-diTPAbpy system to generate heteroleptic [Cu(6,6'-diTPAbpy)(bpy)](+) complexes were unsuccessful with only the [Cu(6,6'-diTPAbpy)(2)](PF6) complex observed. The 6,6'-diTPAbpy ligand could also be reacted with 6-coordinate metal ions that featured small ancillary ligands, namely, the [Re(CO)(3)Cl] and [Ru(CO)(2)Cl-2] fragments. While the complexes could be formed in good yields, the steric bulk of the TPA units does alter the coordination geometry. This is most readily seen in the [(6,6'-diTPAbpy)Re(CO)(3)Cl] complex where the Re(I) ion is forced to sit 23 degrees out of the plane formed by the bpy unit. The electrochemical and photophysical properties of the family of compounds were also examined. 6,6'-diTPAbpy exhibits a strong ILCT absorption band (356 nm, 50 mM(-1) cm(-1)) which displays a small increase in intensity for the homoleptic complexes ([Cu(6,6'-diTPAbpy)(2)]+; 353 nm, 72 mM(-1) cm(-1), [Ag(6,6'-diTPAbpy)2]+; 353 nm, 75 mM(-1) cm(-1)), despite containing 2 equiv of the ligand, attributed to an increased dihedral angle between the TPA and bpy moieties. For the 6-coordinate complexes the ILCT band is further decreased in intensity and overlaps with MLCT bands, consistent with a further increased TPA-bpy dihedral angle. Emission from the (ILCT)-I-1 state is observed at 436 nm (tau = 4.4 ns) for 6,6'-diTPAbpy and does not shift for the Cu, Ag, and Re complexes, although an additional (MLCT)-M-3 emission is observed for [Re(6,6'-diTPAbpy)(CO)(3)Cl] (640 nm, tau = 13.8 ns). No emission was observed for [Ru(6,6'-diTPAbpy)(CO)(2)Cl-2]. Transient absorption measurements revealed the population of a (ILCT)-I-3 state for the Cu and Ag complexes (tau = 80 ns). All assignments were supported by TD-DFT calculations and resonance Raman spectroscopic measurements.

Automated summary

A bulky 6,6'-diTPAbpy ligand was synthesized and used to form homoleptic complexes with Cu(I) and Ag(I) ions, as well as interact with 6-coordinate metal ions. The steric bulk of the ligand affected the coordination geometry and photophysical properties of the complexes, showing changes in absorption and emission properties.