| dc.description.abstract | "Saturated hydrocarbons constitute the main feedstocks for energy production but their oxidation to valuable fine chemicals is still very challenging. Therefore the design and optimization of new catalysts for the functionalization of inert C H bonds is a compelling goal to access more valuable chemicals through milder and more selective processes. High-valent late transition metal complexes have been widely invoked as powerful oxidants, taking inspiration from the active oxidants found in several enzymatic catalytic cycles in nature. A lot of focus has been directed on their synthetic counterparts, both containing first-row late transition metals, such as Ni and Cu, due to the overall dearth of high-valent oxidants containing these metals, and third-row transition metals such as Au, due to the lack of mechanistic insights on this class of oxidants. The main topics reported in the present thesis are therefore the design, characterization and mechanistic studies of high-valent late transition metal (Ni, Cu and Au) oxidants for the oxidative activation of substrates bearing C H bonds. As the first step, in Chapters 2 and 3 we investigate the reactivity of two AuIII complexes, differing only in the ancillary ligand: [Au(X)(terpy)](ClO4)2 (X = Cl, 1; X = OH, 2). These complexes were characterized in polar aprotic solvents (DMF, DMSO) by 1H NMR, electronic absorption spectroscopy, CV and XRD and they showed to oxidise substrates bearing weak C H bonds (DHA, CHD) and O H bonds at room temperature. The kinetics of these oxidation reactions were monitored showing that both complexes were reacting via a hydrogen atom transfer (HAT) mechanism, but 1 was a more competent oxidant with respect to 2. These results proved for the first time that AuIII complexes can be effective HAT oxidants and that AuIII halide adducts can be more effective oxidants than the AuIII oxygen ones.
In Chapter 4 we synthesized and characterized three late-transition metal porphyrin complexes, [Cu(T(OMe)PP)] (1), [Ni(T(OMe)PP)] (2) and [Au(T(OMe)PP)]+ (3). Cyclic voltammetry highlighted the presence of two reversible oxidation waves below 0.89V for 1 and 2, therefore both complexes were successfully oxidised to higher-valent species, 1OX and 2OX, by using phenoxythiinyl hexachloroantimonate. The electronic nature of 1OX and 2OX was elucidated by electronic absorption spectroscopy, EPR and infra-red spectroscopy to be porphyrin π-cation radicals, respectively CuII‧+ and NiII‧+ complexes. We reacted the more competent 2OX with hydrocarbon substrates and a kinetic analysis performed by monitoring the reaction via electronic absorption spectroscopy allowed us to determine that 2OX was performing hydrocarbon oxidation via a CPET mechanism.
In Chapter 5 we proposed the synthesis and characterization of novel tailored ligands to isolate the precursors of putative high-valent late transition metal oxidants. by enforcing a trigonal planar geometry around the metal ions. These IMAM (Imino Amido) ligands were designed to bear a strong anionic σ-donor as a deprotonated amide, a neutral iminic donor and tunable bulky substituents on the phenyl rings to ensure a low coordination number. We were able to achieve the desired ligands in a two-step synthesis, with different substituents on the aryl rings and characterized them by 1H NMR, 15N NMR, ATR-FTIR and XRD. During the first complexation attempts we observed that the acidity of the α-protons lead to their deprotonation in the presence of a base. Therefore we optimized the ligands by methylating that position, yielding the alkylated IMAM 3 (bearing isopropyl substituents on the phenyl rings). We then attempted the complexation with CoII, NiII and CuII where we reported interesting preliminary results: MS, 1H NMR and EPR suggested in fact the formation of mononuclear low-coordinate complexes with these metal ions." | en |
