Cryogenic and temperature-dependent photoelectron spectroscopy of metal complexes

Negative ion photoelectron spectroscopy (NIPES) coupled with electrospray ionisation (ESI) has been proven to be a powerful gas-phase spectroscopic tool for characterising electronic structures, chemical bonding of a variety of cluster anions, and corresponding neutral radicals involved in the condensed phase reactions and transformations. Since the acquisition of cryogenic and temperature-controlling capabilities, a broader range of research has been covered. This review summarises our recent investigations on metal complexes employing cryogenic ESI-NIPES that provides essential information towards understanding complicated condensed phase reactions, including hydrocarbon activations and electron transfer reactions, and affords spectroscopic perspective of highly reactive transient species and intimate redox pairs. Special attention has been drawn to connect gas phase photodetachment processes with solution phase redox reactions. Photodetachment of transition metal-EDTA complexes has been systematically investigated to model the sequential oxidation reactions of these species in solutions. For each series of homologous metal complexes, the obtained gas phase electron affinity (EA) is compared with the solution redox potential (E1/2) and the metal ionisation potential (IP) to emphasise their intrinsic correlations, with deviations being largely modulated by different degrees of ligand participations.