ICIQ’s Pericàs group, in a collaboration with Prof. Timothy Noël and Dr. Paola Riente (Eindhoven University of Technology) have provided key insights into the chemical nature of the true photocatalyst involved in the Bi2O3-driven atom-transfer radical addition (ATRA) reaction. Their work is published in a recent Nature Communications paper.
In 2014, ICREA Professors Miquel Pericàs and Emilio Palomares together with former postdoctoral researcher Dr. Riente, published a paper in the Angewandte Chemie International Edition about pioneering research in organic transformations in mild reaction conditions using Bi2O3 and visible light as a sustainable alternative to other transition metals. Bi2O3 has become popular as a photocatalyst to drive light-induced organic transformations due to its low price, non-toxicity, solid nature, high availability, and visible light response. Moreover, in some cases, it can replace the use of metal complexes based on expensive and non-abundant ruthenium and iridium transition metal photocatalysts.
The researchers set out to unravel the atom transfer radical addition (ATRA) reaction between diethyl bromomalonate (DEBM) and 5-hexen-1-ol as a reaction model. As the reaction progresses, the mixture evolves from a suspension to a yellowish transparent solution. This quickly caught the researcher’s attention, as Bi2O3 isn’t soluble in organic solvents.
“We envisaged that the interaction of Bi2O3 with some component of the reaction would form a homogeneous bismuth-based intermediate species under irradiation, and work as the true photocatalyst of the reaction,” explains Dr. Riente, first author of the paper.
Reaching out to Dr. Mauro Fianchini, a theoretical postdoc working in the Pericàs group, the team devised a theoretical model that would go on to help elucidate that the catalytically active species involved in photocatalytic processes where Bi2O3 is used are actually closely related to pure BiBr3 or BiBr3– based complexes. In the presence of dimethyl sulfoxide (DMSO) or dimethylformamide (DMF), Bi2O3 transforms into BiBr3– based complexes, photocatalytic species able to absorb light, ultimately triggering the formation of the required alkyl radical in ATRA and alkylation reactions.
Pushing this idea forward, the researchers performed calculations of solvate complexes where DMSO coordinated with BiBr3 to find the ideal candidate. Combining these computational insights with the structural information provided by X-ray diffraction, the team cracked the puzzle, finding that the active photocatalytic species is a complex salt of bismuth hexabromide; A mixture composed of [(BiBr6)]3− octahedral anions balanced by [(CH3)3S]+ cations and [(CH3)3S]Br.
“This is a good foundation. This research is the basement of the “house” and, looking ahead, we will start building the walls and put a roof on by proposing the mechanisms behind the solvation of the precatalyst and the ATRA activation of organic substrates of interest.” Says Dr. Fianchini.
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