CHIRAL TRANSITION METAL COMPLEXES: SYNTHESIS, CHARACTERIZATION, APPLICATIONS.
Creators: name: FUSE', MARCO
Publisher: name: Universit� degli Studi di Milano
Year: 2015

description: The main target of this thesis was the investigation of different aspects of the chirality at the metal centre. In particular we studied stereo-specific synthesis of octahedral transition metallic complexes or their resolution in enantiomeric pure form, in order to evaluate the influence of the chirality at the metal centre in different fields such as: catalysis, emission or interaction with chiral environment. The use of chiral-at-metal complexes in catalysis is somewhat less common than that of other chiral organometallic architectures. Via stereospecific reaction, we were able to obtain ruthenium(II) complexes of general formula [Ru(NN)(PP)Cl2] in optically pure form, where NN was a chiral diamine and PP was a chelating or two mono phosphines. These complexes, after the activation into the hydride species, were active catalysts for the reduction of ketones via asymmetric transfer hydrogenation. We observed that, in complexes where two chiral ligands were simultaneously present, the third chirality that located at the metal centre was prevalent in the definition of the configuration of the products. In order to introduce the planar chirality, we synthesised different complexes of the types [h6-arene-Ru(NN)Cl], however in the most cases intractable complex mixture of stereoisomers were obtained. This results prompted us to explore different techniques able to describe complex mixtures of complexes with several type of chirality involved. VCD in tandem with DFT calculation is powerful technique for the determination of the absolute configuration and conformational analysis; despite new results concerning transition metal complexes are coming out, their application in the study of metallorganic compounds are not common. The strength of IR and VCD spectroscopy lies in the fact that the spectra of a chiral molecule contain sufficient stereochemical details to be consistent with only a single absolute configuration and an unique solution-state conformation, or distribution of conformations, of the molecule; moreover compared with ECD the DFT calculation is more reliable. For our purpose we selected three common motifs in organometallic chemistry never investigated by VCD spectroscopy. We decided to investigate the carbon monoxide as the first chromophore; carbon monoxide is one on the most common ligand for transition metal complexes; the carbonyl complexes found application as catalysts, precatalysts or stoichiometric reagents in many different reactions. Therefore we synthesised and resolved a series of of heteroleptic ruthenium (II) cyclopentadienyl complexes. The second chromophore was a metal-hydride, which has important role in catalytic reduction. We were able to synthesis one Ru(II) hydride complex as single enantiomer that was completely and whose metal chirality was highlighted by VCD. The third class of compounds was the cyclometallated iridium(III) complexes that due to their photophysical properties have been extensively studied in the last years, founding application in several fields. Here the chromophore was constituted by two orthometalled phenylpyridine coordinated at the metal in helical shape. In this latter case, our interest was also adressed to study the photophysical properties of the different diastereomers and in order to verify if the emission could be influenced by the diastereomeric nature of the complexes. In spite of that in solution no differences were highlighted, we found that the different packing of the enantiomers and stereoisomers produces aggregates with different emitting properties. By VCD spectrometry integrated with DFT calculation we were able to identify the characteristic features of the different stereogenic centre present in the complexes. In the CO and Ir(III) complexes we also investigated the origin of the VCD sign. VCD spectroscopy proved to be a valuable and powerful technique when applied to metallorganic chromophores, moreover the results of these investigations appear in the literature and here for the first time.