Cobalt complexes with multi-dentate N-donor ligands: Redox, X-ray photoelectron spectroscopic and theoretical study

Five CoII complexes were prepared with different multi-dentate N-donor ligands (5-Nitro-1,10-phenanthroline, 1,10-phenanthroline, 2,2 ':6 ',2 ''-terpyridine, 2,2 '-dipyridyl or 4,4 '-dimethoxy-2,2 '-bipyridine). Complexation was confirmed with the blue-shift of stretching frequency of the C--N moiety of the ligands as measured by ATR FTIR and by XPS spectra. The average XPS binding energies (BE) of the main and satellite structures of Co 2p3/2 photoelectron lines for CoII were ca. 780.3 eV and 785.0 eV respectively. The diffusion controlled formal reduction potential E0 ' of the CoII/III redox couple measured in aqueous medium ranged between 0.08 and 0.58 V versus NHE. Ihe CoII/I redox couple measured in DMF ranged between between-1.1 and-1.7 V vs HFc/HFc+. Directly proportional relationships were established between the E0 ' of both the CoII/III and CoII/I redox couples and the BE of the main CoII 2p3/2 photoelectron lines and DFT calculated energies and charges. These re-lationships all illustrate the electronic influence of the different N-donor ligands on the effective charge of the central cobalt(II) ion the ligands are coordinated to, can be quantised by the different experimental redox and XPS BE values and DFT calculated electronic descriptors.