Amine-based tripodal metal complexes for applications in chiral sensing and reduction catalysis

In the recent years, within the group where this thesis has been carried out, several metal complexes based on the ligands tris(2-pyridylmethyl)amine (TPMA) and triphenolamine (TPA) were studied for their applications in catalysis and sensing. This PhD work aimed to extend the knowledge within this field through the synthesis of new tailored metal complexes and subsequent applications in chiral sensing and reduction catalysis. The introduction aims to cover recent significant advancements in the field of chiral assessment and enantiomeric excess (ee) determination. It discusses the crucial role chirality plays across various scientific disciplines, including pharmaceuticals, agrochemicals, and materials science. Then the core of the introduction focuses on different methods for chiral assessment and ee determination, discussing their principles of operation, strengths, limitations, and recent developments. In Chapter 1, a newly designed TPMA-based complex is introduced as the first stereodynamic probe capable of utilizing Fluorescence-Detected Circular Dichroism (FDCD) to detect carboxylate species. The TPMA scaffold was functionalized with an anthracene moiety, imparting fluorescence to the probe and enabling it to distinguish between enantiomers of various carboxylates. The probe's sensitivity was evaluated and compared to that of conventional CD measurements, demonstrating its ability to detect concentrations as low as sub-µM. Additionally, the fluorescence-based technique enabled ee analysis even in the presence of “chiroptical contaminants,” which is particularly important for applications in complex matrices, where multiple species can interfere with classical CD analysis. In Chapter 2, the electronic properties of the TPMA scaffold were modified introducing one, two or three electron-donating methoxy (or one, two or three electron-withdrawing chlorine) substituents in para position relative to the pyridine nitrogen. A series of six new cobalt complexes was then synthetized and the effect of the substitution on the CO2 Reduction Reaction (CO2RR) and Hydrogen Evolution Reaction (HER) was evaluated. A peculiar CO2RR reactivity was found for the two more electron-rich complexes bearing two and three methoxy substituents showed a lower activity. Spectroeletrochemical (SEC-IR) measurements and a deep computational investigation are ongoing to clarify the observed phenomena. In Chapter 3, a chlorine-substituted TPA ligand was utilized for the chiral sensing of amines. The in-situ formation of a zinc metal complex between the TPA ligand and the amine resulted in effective enantiodiscrimination and the generation of a detectable circular dichroism (CD) signal. The X-ray crystallographic analysis of the probe complexed with R-methylbenzylamine revealed a key ion-pair interaction that is critical for the recognition process. This expands the possible probe-analyte interactions exploitable for the chiral sensing with the stereodynamic probe. In Chapter 4, a novel methodology for determining enantiomeric excess (ee) independently of the substrate being analyzed is discussed. The temperature dependence of the CD signal of an "ideal" stereodynamic probe is described using the Boltzmann distribution, which accounts for the diastereomeric ratio between the species formed upon coordination of the chiral analyte to the probe. As expected, a decrease in the CD signal is recorded by increasing temperature accordingly to the progressive population of the two opposing helical conformations of the complex. Within the experimental temperature range investigable, a linear decrease was observed. Consequently, a new plot was developed to illustrate the variation in slopes and intercepts, showcasing its effective application for determining ee. Subsequently, the vanadium TPA probe developed by our group was tested. Despite the good agreement with the model, practical prediction of ee proved challenging with our system.