Green Chemistry approaches in catalytic organometallic processes and application to photovoltaic materials design

To attain sustainability, the area of "Green chemistry," which is still in its early stages of development, must work at the molecular level. The Twelve Principles make up the framework of Green Chemistry for the design of new chemical products and processes, applying to all the aspects of the process life-cycle, from the commonly employed raw materials to the efficiency and safety of the transformation, the toxicity and biodegradability of products and reagents. This “green” approach should be the milestone in conducting synthetic organic chemical processes and assessing their impact on the environment. In this Ph.D. thesis work, we have explored the possibility of studying catalytic organometallic processes following the application of some of the principles of Green Chemistry as well as green metrics. Furthermore, the design of new organic molecules for emerging and renewable technologies in photovoltaics such as Dye-Sensitized-Solar Cells, has been investigated together with their preparation using a sustainable approach, when possible. In the first part of this work, the use of Deep Eutectic Solvents (DESs) as alternatives to the common organic solvent is explored to carry out a palladium-catalyzed Miyaura borylation reaction. The procedure employs bis(pinacolato)diboron as a boron source and a catalyst prepared in situ from the readily available Pd2(dba)3 as precatalyst and the phosphine ligand XPhos. Reactions proceed well in different ChCl-based DESs, among which the best results were provided by environmentally friendly and biodegradable mixtures with glycerol and glucose. The protocol tolerates both electron-rich and electron-poor (hetero)aromatic substrates and can be run on different halides and pseudohalides. Furthermore, for several substrates, the catalyst loading can be reduced to 1.0 mol % without compromising the reaction yield. Moreover, we show that the borylation protocol in DES can be combined with a subsequent Suzuki-Miyaura cross-coupling reaction in a one-pot procedure, allowing access to various biaryl products and demonstrating its synthetic utility by preparing the precursors of two compounds with reported applications in the photovoltaics sector. Furthermore, two green metrics (E-factor and EcoScale) of the new one-pot procedure were calculated and compared with literature values to demonstrate the advantages of our procedure in terms of waste reduction, safety, and energy consumption. [J. Org. Chem. 2024, 89 (10), 6991–7003]. In the second part, the project was focused on the use of nickel catalysis as sustainable alternative to the palladium catalysis, to perform a single step difunctionalization of unactivated olefins, which are one of the most widely available feedstocks in organic synthesis. In particular, we studied a regiodivergent 1,n-dicarbofunctionalization of unactivated olefins enabled by a Ni-catalyzed radical relay that forges both C(sp3)-C(sp3) and C(sp2)-C(sp3) linkages, even at long-range. The development of this method was inspired the second and the ninth principle (#2 – Atom Economy, #9 – Catalysis) of green chemistry. Initial studies support an intertwined scenario resulting from the merger of an atom-transfer radical addition (ATRA) and a chain-walking event, with site-selectivity being dictated by a judicious choice of the ligand backbone. (Hetero)aryl and alkyl bromides as radical precursors were introduced on olefins of different nature through a regiodivergent single step. Some studies have been also carried out to learn more about the reaction mechanism. [J. Am. Chem. Soc. 2024, 146, 28624-28629]. In the third part, the design and synthesis of new organic photosensitizers for Dye-Sensitized Solar Cells (DSSCs) are reported. The aim is to exploit devices able to work in low-light conditions. [Materials 2023 Vol. 16]. For the synthesis of the new molecules, green and sustainable approaches taking advantage of catalytic and one-pot transformations are preferred. Three families of new D-π-A or D-A-π-A dyes of different nature are listed here: 1. The FD dyes, where a pentacyclic scaffold [1]benzothieno[3,2-b]thieno[2,3-f][1]benzothiophene is used as common donor group to prepare three new D-A-π-A photosensitizers modifying the π-A part. 2. The to-BDT dyes, which have a benzo[1,2-b:4,5-b’]dithiophene 1,1,5,5-tetraoxide as internal chromophore and a carboxylic acid as anchoring group, combined with five different donor groups. 3. The ProTT and Invert-ProTT families, prepared involving a pro-aromatic thienothiophene core as internal chromophore, functionalized with four donor groups and a common cyanoacrilic acid as anchoring group. Furthermore, the synthetic strategy allows the obtaining of two isomers in which the position of the donor and the acceptor groups is inverted (Invert-ProTT). All dyes show photoelectrochemical properties suitable for this application. Small DSSCs devices were fabricated using the three families of photosensitizers with iodine and copper electrolytes, evaluating the photovoltaic performances under LED illumination. The ProTT dyes afforded the best results, exhibiting an excellent overlap with the lamp emission spectra and, PCEs up to 12.1% were achieved with ProTT-2 and Cu(tmby)2TFSI1/2 electrolyte. In the end, ProTT-3 dye with I–/I3– is also used for a device scale-up (DSSC active area: 38.4 cm2) with efficiencies up to 1.83% under cold white LED illumination.