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)

Abstract

Intramolecular π–π and CH–π interactions between the bpy and PR₃ ligands of fac-[Re(bpy)(CO)₃(PR₃)]⁺ affect their structure, and electrochemical and spectroscopic properties. Intramolecular CH–π interaction was observed between the alkyl groups on the phosphine ligand (R = ⁿBu, Et) and the bpy ligand, and intramolecular π–π and CH–π 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 = OⁱPr, OEt, OMe). The intramolecular interactions distort the pyridine rings of the bpy ligand as long as 3.7 × 10⁻² Å in crystals. Molecular orbital calculations of the bpy ligand suggest that this distortion decreases the energy gap between its π and π* orbitals. An absorption band attributed to the π–π*(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 π–π*(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 ν_CO IR bands, which are sensitive to the electron density on the central rhenium because of π-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.