BASE METAL CATALYSTS WITH NHC-OXYGEN LIGANDS FOR CO2 VALORIZATION REACTIONS

One of the most important issues that concerns our society is sustainable development. In this context, one of the main challenges is to reduce the concentration of CO2 in the atmosphere, and one of the most promising approaches involves its capture from energy power plants waste. The utilization of captured CO2 as C1 building block for the synthesis of added-value compounds is very appealing, requiring however efficient catalysts, in order to avoid drastic reaction conditions and to get access to a plenty of different chemical transformations. For this purpose, organometallic metal complexes are great candidates. In the last years, a renaissance of the organometallic chemistry of Earth-abundant metals is taking place, favoured also by cheap price of the precursors, their biocompatibility, and the capability of undergoing single electron transfer (SET) processes. N-heterocyclic carbenes (NHCs) are excellent ligand candidates for the development of new homogeneous catalysts, as they demonstrated to form robust organometallic species and promote efficient catalysis. However, base metals are dominantly hard species (HSAB theory) and the soft nature of NHC ligands makes their coordination difficult in some cases. The introduction of hard donors in the ligand structure, like oxygen donor atoms, is an effective strategy to overcome this issue. In this thesis differently NHC-O hybrid proligands were synthesized, maintaining imidazolylidene-based NHC donors and varying the O-donor nature. The complexation of these proligands to abundant metals was achieved, leading to Mn(I/III), Cu(I/II) and Ni(II) complexes. In particular, the Mn(III) with bis(NHC)-bis(phenolate) ligands were isolated, giving pentacoordinate species with an halide apical ligand. The same complexation reaction with Mn(I) using bis(NHC)-bis(phenolate) proligands affords the coordination of the ligandin a bidentate bis(NHC) fashion, maintaining protonated the OH functionalities. Instead, in the case of Fe(III) and Cr(III), the coordination to the metal only involves the O-donors, presenting protonated NHC functionalities, and a coordination sphere saturated by two acetylacetonate ligands. In the case of Cu(II) the complexation was successfully achieved using also NHC-phenolate, NHC-benzyloxy, bis(NHC)-phenolate and NHC-carboxylate proligands, leading to a rich family of different NHC-O coordination complexes. In the case of Ni(II), stable complexes were also isolated using NHC-phenolate and pincer NHC-bis(phenolate) proligands. The performance of the synthetized complexes was explored as catalysts for different reactions, maintaining the focus on CO2 valorization ones. The Mn(III), Ni(II), Cr(III) and Fe(III) complexes were studied in the cycloaddition of CO2 with epoxides, achieving high conversions for terminal epoxides. A Mn(I) bis(NHC) complex was studied as electrocatalyst for CO2 electroreduction both in dry conditions and in presence of H2O as proton donor, reaching activities comparable with the most active Mn(I) complexes reported in literature. A series of bis(NHC) Mn(I) and of NHC-phenolate Cu(II) complexes were studied in the reductive fixation of CO2 with amines using hydrosilanes as reducing agent, a challenging 6-electron approach for the synthesis of added value methylamines. Mechanistic insights were gained for both systems, supporting a formic acid-involving pathway in case of Mn(I) complexes, and the formation of a NHC silane adduct using the Cu(II) complexes. In both case, the methylation of a scope of amines were achieved, allowing to work under 1 atm CO2 pressure using the Cu(II)-based catalysis. Finally, the Ni(II) complexes were evaluated as catalysts for the reduction of secondary amides using PhSiH3, a reaction that has received very limited attention in the scientific literature with this metal center.