Determination of Stereoelectronic Properties of NHC Ligands via Ion Pairing and Fluorescence Spectroscopy

Abstract

We present an experimental method, which provides information on the steric and electronic properties of ligands in metal complexes. This approach is based on the equilibrium of ion-pairing of a cationic [(NHC)Ir(cod)]+ and a fluorescent bodipy-sulfonate (bdpSO3−). The close ion-pair of [(NHC)Ir(bdpSO3)(cod)] in toluene solution is weakly fluorescent, but dissociates into solvent-separated ion pairs upon exposure to slightly more polar solvents. This spatial separation leads to a very pronounced increase of the fluorescence. The separation into distinct ions depends on the polarity of the solvent, but more importantly also on the stereoelectronic properties of the NHC ligand. 26 different NHC ligands with varying steric and electronic properties were probed in 1,2-dichloroethane solvent. The electronic properties of the ligands were examined via the established descriptors (redox potential and ν(CO)). The systematic variation of NHC donation enables the deconvolution of electronic and steric contributions of the respective ligand.