Abstract
In recent years, photoinduced chemical transformations have received great attention, providing several new opportunities to organic synthesis, within a green chemistry perspective. Electrophotocatalysis (EPC) is one of the most promising technological innovation in this field, as a successful synergic combination of homogenous photocatalysis (PC) and electrochemistry (EC). By employing clean energy (light and electricity derived from renewable sources) and by coupling photo and electrochemical steps within the same catalytic cycle, EPC is able to promote chemical reactions under milder conditions, increasing selectivity, without the use of sacrificial reagents or activating functional groups, lowering the environmental impact of chemical synthesis. Moreover, EPC is able to generate catalysts with excited-state redox potentials beyond those accessible via PC alone, or it allows lowering the applied potentials of conventional EC. Anyway, EPC is still at its infancy and only few organic dyes and a couple of archetypical transition-metal complexes have been tested as catalysts in unoptimized organic reactions. Indeed, the development of new catalysts specifically engineered for electrophotocatalytic applications is not yet reported and the use of transition-metal complexes, while pervasive in PC, is basically unexplored in EPC. HEPIrCOS will take advantage of the expertise of the project team to (i) computationally engineered cyclometalated iridium(III) complexes for EPC applications, (ii) fully characterize them, and (iii) test them in challenging organic reactions, as well as in key synthetic steps of industrial or pharmaceutical relevance. Iridium-based EPC can take advantage of the remarkable thermal, photoand electro-chemical stability of such complexes and of the highly tunable energy levels, that can be modulated with simple modifications on the cyclometalating or ancillary ligands. Furthermore, HEPIrCOS will go beyond the abovementioned objectives and is planning to test EPC in heterogeneous reactors, which is unprecedent. To do so, we are planning to (iv) anchor or support the iridium(III) catalysts directly onto the electrode. The impact of heterogenization can be tremendous: a more efficient population of the radical excited states, even at lower catalyst concentrations; an easier recovery of the reaction products, with simpler purification process and virtually no catalyst losses; the creation of an in-flow photoelectrochemical reactor for organic synthesis. As its final objective, HEPIrCOS will so aim to merge together 2 of the 4 key technological challenges expected to be faced by photochemistry for organic synthesis in the next years: electrophotochemistry and flow chemistry. An important impact in the field of green chemistry, catalysis and industrial chemistry is clearly envisaged in the long term, which is pivotal to reach the targets of the European Green Deal for making the European Union climate neutral in 2050.
Results achieved
The research activity of unibo research unit focused on three Ir(III) complexes, namely Ir(ppy)2(bpy) (ppy = 2-Phenylpyridine; bpy = 2,2'-Bipyridine), Ir(ppz)2(bpy) (ppz = 1-phenylpyrazole) and Ir(bpy)2(bpz) (bpz = 2,2'-bipyrazine), whose syntheses are known and were reproduced and optimised within the project. After purification, all complexes were subjected to systematic electrochemical and spectroelectrochemical studies in order the stability of their reduced forms and the corresponding optical signatures. Cyclic voltammetry performed before and after chronoamperometric experiments, combined with time-resolved UV–Vis spectroscopy, showed that upon reduction both Ir(ppy)2(bpy) and Ir(ppz)2(bpy) exhibit two absorption bands in the visible region, with a main feature at approximately 530 nm attributed to the reduced species. These results, together with preliminary cell experiments indicating that the reduced forms are more stable than the oxidized ones under the investigated conditions, motivated a focused investigation of reductive processes within the project. To translate these observations into a controlled electrochemical setup, an initial H-type cell with separated compartments and a Pt mesh working electrode was tested, reproducing the cuvette configuration. However, this setup proved suboptimal, as platinum significantly limited the accessible cathodic potential window. A systematic screening of working electrode materials (Pt, Au, and glassy carbon) in acetonitrile identified glassy carbon as the most suitable option, due to its substantially wider cathodic window. It was therefore selected for subsequent exhaustive electrolysis experiments. A comprehensive solvent screening was then carried out to identify media providing adequate electrochemical stability across the relevant potential range. Distilled acetonitrile and dichloromethane, dried DMSO, anhydrous THF (further purified), and commercial DMF were investigated by cyclic voltammetry, with particular emphasis on the cathodic compartment. Standard configurations proved inadequate for assessing the stability of the reduced species, due to diffusion effects and interference from products generated at the counter electrode. To overcome these limitations, a new separated-compartment electrochemical cell was developed, employing a glassy carbon plate as working electrode, a Pt mesh as counter electrode, and a silver reference electrode. Controlled-potential electrolysis of Ir(ppy)2(bpy), Ir(ppz)2(bpy), and Ir(bpy)2(bpz) led to a clear color change, confirming the formation of the reduced species. The subsequent oxidation kinetics were monitored by UV–Vis spectroscopy. These investigations identified DMSO as the most suitable solvent for the electrophotocatalytic processes targeted in the project across all operative units. The optimized electrochemical cell was further integrated with instrumentation available at the CNR unit to enable estimation of the lifetime of the excited states generated from the reduced complexes. Overall, these results provide a solid foundation for subsequent photochemical characterization and for defining the optimal conditions for electrophotocatalytic reactions involving Ir(ppy)2(bpy), Ir(ppz)2(bpy), Ir(bpy)2(bpz), and newly synthesized Ir(III) organometallic complexes.Project details
Unibo Team Leader: Isacco Gualandi
Unibo involved Department/s:
Dipartimento di Chimica Industriale "Toso Montanari"
Coordinator:
CNR - Consiglio Nazionale delle Ricerche(Italy)
Total Unibo Contribution: Euro (EUR) 64.210,00
Project Duration in months: 24
Start Date:
28/09/2023
End Date:
28/02/2026
