Investigation of ligand electronic effects on organometallic complexes using photoelectron spectroscopy and mass spectrometry

International audienceOrganometallic chemistry is a key process in homogenous catalysis and gives access to a wide range of products and reactions. Through their steric and electronic effects, ligands are able to orientate the reactivity of the complexes and can thus influence the selectivity in the chemical transformations. Usually, the metal-ligand bond is depicted by the Dewar-Chatt-Duncanson model1,2 involving two opposite interactions, a σ-donor and a π-acceptor effect. Over the years, many experimental methods have been developed to measure these electronic effects, the most common being that of Tolman (Tolman electronic parameter), which is based on the A1-symmetrical CO-stretching frequency shift in carbonyl complexes.3 We have recently developed the use of new experimental gas-phase approaches to measure these effects, one based on photoelectron spectroscopy (PES) coupled to synchrotron radiation, and the other on mass spectrometry using activation by higher energy collision dissociation (HCD). Under the first method, the satisfying results obtained on model systems4 have encouraged us to apply this method to catalysts developed for hydrogenation reactions,5 and to probe the electronic interaction between the ligand and the metal centre. Results obtained for tricarbonyl iron complexes of the (Fe(CO)3L) type, with L being substituted cyclopentadiene ligands, are presented here. The photoelectron spectra provide access to the ionisation energies and state-selected Fe-CO dissociation energies. This method, as well as that of Tolman are however limited to organometallic complexes possessing probe ligands such as CO. Very few reliable experimental methods make it possible to determine the electronic properties of ligands in complexes that do not possess probe ligands. Therefore, in order to make a direct measurement of the ligand effect, we were interested in the study on gold (III) complexes of the type [(C^C)Au(NHC/PPh3)L]+ with L a differently substituted pyridine ligand. The effect of pyridine substitution on the enrichment of the metal center was evaluated by determining the dissociation energy M-L+ -> M+ + L by mass spectrometry. Rationalization of the experimental results is made using density functional theory and bond description methods to get information on the electronic structure of the complexes and thus on the metal-ligand interaction