Effect of intramolecular π-π and CH-π interactions between ligands on structure, electrochemical and spectroscopic properties of fac-[Re(bpy)(CO)3(PR3)]+ (bpy=2,2′-bipyridine; PR3 = trialkyl or triarylphosphines)

Intramolecular pi-pi and CH-pi interactions between the bpy and PR3 ligands of fac-[Re(bpy)(CO)(3)(PR3)](+) affect their structure, and electrochemical and spectroscopic properties. Intramolecular CH-pi interaction was observed between the alkyl groups on the phosphine ligand (R = Bu-n, Et) and the bpy ligand, and intramolecular pi-pi and CH-pi interactions were both observed between the aryl group(s) on the phosphorus ligand (R = p-MeOPh, p-MePh, Ph, p-FPh, OPh) and the bpy ligand, while no such interactions were found in the trialkylphosphite complexes (R = (OPr)-Pr-i, OEt, OMe). The intramolecular interactions distort the pyridine rings of the bpy ligand as long as 3.7 x 10(-2) Angstrom in crystals. Molecular orbital calculations of the bpy ligand suggest that this distortion decreases the energy gap between its pi and pi* orbitals. An absorption band attributed to the pi-pi* (bpy) transition of the distorted rhenium complexes, measured in a KBr pellet, was red-shifted by 1-5 nm compared to the complexes without the distorted bpy ligand. Even in solution, similar red shifts of the pi-pi* (bpy) absorption were observed. The redox potential E-1/2(bpy/bpy(.-)) of the complexes with the trialkylphosphine and triarylphosphine ligand are shifted positively by 110-120 mV and 60-80 mV respectively, compared with those derived from the electron-attracting property of the phosphorus ligand. In contrast with these properties, three nu(CO) IR bands, which are sensitive to the electron density on the central rhenium because of pi-back bonding, were shifted to higher energy, and a Re(I/II)-based oxidation wave was observed at a more positive potential according to the electron-attracting property of the phosphorus ligand.