Radical-Polar Crossover in Bismuth LMCT Photocatalysis

Kermelj, Vid (author), Iskra, Jernej (mentor), König, Burkhard (comentor)

Ligand to metal charge transfer (LMCT) has emerged as a promising photochemical activation process for facilitating organic transformations. Recently, the LMCT properties of bismuth – an element renowned for its minimal toxicity and environmental sustainability – have garnered significant interest. Given bismuth's ability to adopt multiple oxidation states, the radical-polar crossover represents a viable alternative reaction pathway. This pathway is explored in this master's thesis, aiming to potentially unlock novel reactivity and explore its applications in green chemistry. The ability of bismuth to mediate ligand to metal charge transfer (LMCT) decarboxylation of benzylic carboxylic acids has been confirmed. The observed results corroborate existing literature on the radical-radical rebound of bismuth(II) species and carbon radicals. The ionic nature of this system was confirmed by successful trapping of the resultant carbocations using electron-deficient nitrogen nucleophiles. Bismuth(III) triflate was identified as the optimal bismuth source due to its superior facilitation of oxidation to a carbocation-like intermediate. The findings suggest that coordination of the nucleophile to the bismuth center is essential, leading to product formation through a reductive elimination mechanism. A significant competing reaction within the system is the radical-radical homocoupling of the formed carbon radicals. The pH of the system plays a pivotal role, with an excess of a base enhancing the formation of homocoupling products. This is attributed to multiple carboxylic acids coordinated to the bismuth’s center, which facilitates rapid homocoupling and impedes nucleophile coordination. Conversely, the release of protons during oxidative coupling decelerates the reaction. Dichloromethane was identified as the optimal solvent due to its high polarity and non-coordinating nature. The system has been rendered catalytic, with persulfate salts proving to be the most effective external oxidants. DFT calculations have been performed for a series of bismuth complexes with aryl acetic acids. The visualizations of molecular orbitals suggest that the HOMO orbital of LMCT-active complexes is centered on the π-orbital system of the pendant aryl residue. DFT-derived UV-Vis predictions show absorption variance for different ligand environments. UV-Vis measurements have shown a shift of LMCT towards longer wavelengths with increasing electron richness of the pendant aryl ring for a series of aryl acetic acid complexes with bismuth(III) triflate.

• Language: English
• Keywords: bismuth, LMCT, photocatalysis, radical-polar crossover, decarboxylation
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2024
• PID: 20.500.12556/RUL-160211
• COBISS.SI-ID: 205911299
• Publication date in RUL: 23.08.2024

Secondary language

• Language: Slovenian
• Title: Radikalsko-polarni preklop v bizmutovi LMCT fotokatalizi

Abstract

Prenos elektrona z liganda na kovino (angl. Ligand to Metal Charge Transfer – LMCT) je fotokemijski pojav, ki je v zadnjem času pridobil na popularnosti na področju fotokemijskih organskih pretvorb. Nedavno so LMCT lastnosti bizmuta – elementa, znanega po svoji nizki toksičnosti in okoljski trajnosti – vzbudile veliko zanimanja. Zmožnost bizmuta, da prevzame več oksidacijskih stanj, nakazuje na radikalsko-polarni preklop kot možno reakcijsko pot. Ta je v tem magistrskem delu raziskana z namenom, da bi se potencialno odklenile nove reaktivnosti in aplikacije v zeleni kemiji. Potrjena je bila sposobnost bizmuta, da z LMCT aktivacijo omogoči dekarboksilacijo benzilnih karboksilnih kislin. Pridobljeni rezultati potrjujejo obstoječo literaturo o radikalskem spajanju bizmutovih(II) zvrsti z ogljikovimi radikali. Ionska narava sistema je bila potrjena z uspešnim lovljenjem nastalih karbokationov z elektronsko revnimi dušikovimi nukleofili. Bizmutov(III) triflat je bil prepoznan kot optimalen vir bizmuta zaradi svoje najboljše zmožnosti oksidacije ogljikovega radikala do karbokationu podobnega intermediata. Ugotovitve nakazujejo, da je koordinacija nukleofila na bizmutov center ključna za tvorbo produkta prek mehanizma reduktivne eliminacije. Pomembna tekmovalna reakcija je radikalsko so-spajanje nastalih ogljikovih radikalov. pH sistema igra ključno vlogo, pri čemer presežek baze vodi do tvorbe dimernih produktov. Razlog je v večkratni koordinaciji karboksilnih kislin na en bizmutov center, kar omogoča hitro so-spajanje in ovira koordinacijo nukleofila. Nasprotno, sproščanje protonov med oksidativnim križnim spajanjem upočasni reakcijo. Diklorometan je bil prepoznan kot optimalno topilo zaradi svoje relativno visoke polarnosti in nekoordinirajoče narave. Sistem je bil narejen katalitičen, pri čemer so se persulfatne soli izkazale kot najučinkovitejši oksidant. DFT izračuni so bili narejeni za vrsto bizmutovih kompleksov z aril ocetnimi kislinami. Vizualizacije molekularnih orbital kažejo, da se HOMO orbitala LMCT-aktivnih kompleksov nahaja na π-orbitalnem sistemu arilnega obroča. DFT-napovedani UV-Vis absorpcijski spektri kažejo odvisnost glede na različno koordinacijo ligandov. UV-Vis meritve so nakazale premik LMCT absorpcije proti daljšim valovnim dolžinam z naraščajočo elektronsko gostoto arilnega obroča v seriji aril ocetnih kislin v kompleksih z bizmutovim(III) triflatom.

• Keywords: bizmut, LMCT, fotokataliza, radikalsko-polarni preklop, dekarboksilacija