INTRODUCTION The selective functionalisation of short alkanes such as methane, available in natural gases, is a topic of current interest. Processes have been developed to convert gaseous hydrocarbons into highly valuable chemicals that serve as platform substrates for further applications. However, due to their non-polar and volatile nature, the specific activation of C-H bonds remains a challenging task. To address this challenge, hydrogen atom transfer (HAT) processes have emerged as a powerful tool. Small molecules such as chlorine radicals or oxygen-centred alcoholate radicals act as electrophilic openshell species with a high propensity for abstraction of hydrogen atoms from electron-rich C-H bonds within the range of their bond-dissociation energy. The resulting carbon-centred radicals undergo subsequent transformations and functionalisation reactions. One of the most efficient strategies for generating reactive radical species is ligand-to-metal charge transfer (LMCT). Among the various metals capable of undergoing chargetransfer chemistry, cerium stands out as an appealing option due to its abundance in the Earth's crust and suitable redox window, featuring oxidation states of +3 and +4. Recently, the groups of Schelter and Zuo have developed cerium-based photosensitizers and impressive methodologies involving the formation of coloured Ce 4+ -ligand complexes through the reaction of CeCl3 and chloride salts or alcoholate ligands, respectively. Upon excitation by visible or near UV-light, these complexes undergo oxidative accumulation, yielding chlorine or alkoxy radicals. These electrophilic species selectively abstract hydrogen atoms from complex molecules, and the resulting nucleophilic carbon-centred radicals are successfully trapped by derivatives of di-tert-butyl (azodicarboxylate), hypothesized to close the catalytic cycle of Ce 3+ /Ce 4+ . However, in some conditions where both CeCl3 and short alcohols were used as the catalytic system, the nature of the eventual active hydrogen atom transfer (HAT) species remains unresolved and is still under discussion despite numerous mechanistic investigations conducted by both groups (Scheme 1). Scheme 1. Different opinions on the reactive species in cerium LMCT chemistry. Jeevarajan et al. demonstrated that benzoic acids can function as hydrogen-atom acceptors in the presence of a hydrogen atom donor such as alcohols. Upon transfer of a hydrogen atom, radical hexa-dienyl radicals are formed, which are detectable by electron paramagnetic resonance (EPR) spectroscopy. Glorius and coworkers developed a strategy for the selective transformations of tertiary C-H bonds using benzoic acid or appropriate salts thereof as precursors for highly reactive electrophilic hydrogen atom transfer reagents. Through single electron transfer processes, the benzoic acid anion is oxidised by an Ir 4+ photocatalyst, which undergoes reductive quenching. Instead of the decarboxylative degradation observed in copper chemistry, the resulting radical can effectively abstract hydrogen atoms from nucleophilic C-H bonds, exhibiting high reaction rates (Scheme 2, A). To expand our understanding of HAT-active ligands, we conducted screenings across various compound classes in search of additional potential ligands capable of LMCT activation with Ce 4+ , aiming to gain deeper insights into the nature of charge transfer processes. Drawing inspiration from the single-electron transformation achieved by an excited-state redox couple of Ir 4+ and tetrabutylammonium benzoate for the selective trifluoromethyl-thiolation of complex C-H precursors, we selected this established system from the Glorius group as a benchmark reaction for cerium-catalysed transformations via hydrogen atom transfer (Scheme 2, B). Scheme 2. Selective C-H functionalisation of tertiary C-H bonds using benzoic acid salts as HAT catalysts. A) Approach from the Glorius group: applying single-electron transfer chemistry with an Ir(IV) catalyst, B) our approach: extending the scope for LMCT-active ligands forming complexes with Ce(IV). RESULTS AND DISCUSSION We initiated our investigations by conducting the test transformation outlined in Scheme 4.3. Isopentyl benzoate (1) underwent a reaction with N-(trifluoromethyl) thio-phthalimide (2) using a catalytic system comprising of CeF4 (5 mol%) and tetrabutylammonium benzoate (3) (5 mol%) in 1 mL of dry acetonitrile, under irradiation with 395 nm light (2.6 W) for 24 h at 25 °C. Evaluation of the reaction via F-NMR spectroscopy revealed the formation of 26% of the desired product, accompanied by the fading of the initial yellow colour of the reaction mixture upon exposure to the high-power LED. Notably, the reaction failed to proceed in the absence of light, without the cerium salt or when both conditions were lacking (Table , entries b-d). Additionally, in the absence of benzoic acid derivatives, product 4 was not formed (Table , entry e). CeF4 was selected due to its low likelihood of generating fluorine radicals upon irradiation with near UV or visible light and consequently, this choice helped to prevent competition between two potential hydrogen atom abstraction agents, as demonstrated by the work of Zhiwei Zuo. Scheme 3. Benchmark reaction for the regio-selective trifluoromethyl-thiolation of branched esters.