Photoelectrocatalytic C–H halogenation over an oxygen vacancy-rich TiO2 photoanode

Photoelectrochemical cells are emerging as powerful tools for organic synthesis. However, they have rarely been explored for C–H halogenation to produce organic halides of industrial and medicinal importance. Here we report a photoelectrocatalytic strategy for C–H halogenation using an oxygen-vacancy-rich TiO2 photoanode with NaX (X=Cl−, Br−, I−). Under illumination, the photogenerated holes in TiO2 oxidize the halide ions to corresponding radicals or X2, which then react with the substrates to yield organic halides. The PEC C–H halogenation strategy exhibits broad substrate scope, including arenes, heteroarenes, nonpolar cycloalkanes, and aliphatic hydrocarbons. Experimental and theoretical data reveal that the oxygen vacancy on TiO2 facilitates the photo-induced carriers separation efficiency and more importantly, promotes halide ions adsorption with intermediary strength and hence increases the activity. Moreover, we designed a self-powered PEC system and directly utilised seawater as both the electrolyte and chloride ions source, attaining chlorocyclohexane productivity of 412 µmol h−1 coupled with H2 productivity of 9.2 mL h−1, thus achieving a promising way to use solar for upcycling halogen in ocean resource into valuable organic halides.

Photoluminescence spectra of TiO2-OV-T photoanodes, excitation wavelength 300 nm.  a The Cl − amount was calculated from the integrated area of corresponding IC peaks.

Supplementary Note 2
For PEC halogenation of toluene, the products over the TiO2-Ov-400 photoanode are the aromatic ring-chlorinated compounds with total selectivity of 77.9% at high toluene conversion (82.5%) ( Supplementary Fig. 29), including p-chlorotoluene and o-chlorotoluene selectivity of 33.9 and 44.0%, respectively (ratio of p/o = 1/1.3). Benzyl chloride was also observed but with much lower selectivity (12.7%). The ratio of p/o was determined by NMR measurement ( Supplementary Fig. 29c). When pure TiO2 (with oxygen vacancy) was used as the photoanode, the main product is benzyl chloride with selectivity of 75.5% at toluene conversion of 83.2% ( Supplementary Fig. 31). The above results suggest that the PEC chlorination of toluene over TiO2-Ov-400 and pure TiO2 may follow different mechanisms.
For PEC chlorination of toluene over TiO2-Ov-400 photoanode that obtains pchlorotoluene and o-chlorotoluene as the main products, we propose the reaction mainly follows an electrophilic substitution mechanism. As shown in Supplementary Fig. 32a, according to the previous reports 1 , Cl − can be oxidized by the photogenerated holes (h + ) over S51 photoanode to produce Cl2. The generated Cl2 is adsorbed on the oxygen vacancy of TiO2-Ov-400 and polarized to form TiO2-Ov-Cl δ− -Cl δ+ moieties due to the electron-withdrawing effect of oxygen vacancy as a Lewis acid, which was supported by the DFT calculations shown below.
The charge-positive Cl δ+ then acts as an electrophile to attack the benzene ring of toluene to form a π-complex, while the charge-negative Cl δ− remains on the oxygen vacancy to form a chlorinated anion (denoted as TiO2-Ov-Cl δ− ). Then, the π-complex evolves into a σ-complex, which is eventually converted to p-chlorotoluene or o-chlorotoluene by deprotonation (ratio of The resulting proton reacts with the TiO2-Ov-Cl δ− anion to form HCl and the oxygen vacancy on TiO2-Ov-400 is recovered. The adsorption and polarization of Cl2 on the oxygen vacancy of TiO2-Ov-400 was demonstrated by DFT calculation. According to the HAADF-STEM image (Fig. 4c), the exposed facet of TiO2-OV-400 is the (101)  Moreover, to understand the important role of oxygen vacancy for Cl2 adsorption and the electrophilic substitution reaction of toluene, we directly used chlorine gas (Cl2) as the chlorine source to investigate if Cl2 can be activated on the TiO2-Ov to obtain similar product selectivity.
We injected Cl2 into an aqueous solution containing 5 mmol toluene in the presence of TiO2-Ov-400 or TiO2 or without catalyst, with the following reaction performed in darkness for 15 min. As shown in Supplementary Fig. 33a, p-chlorotoluene and o-chlorotoluene were obtained using TiO2-Ov-400 as the photoanode demonstrating that the TiO2-Ov with oxygen vacancy can activate and polarize Cl2 with the following electrophilic substitution of toluene to obtain aromatic ring-chlorinated compounds. These results also reveal that Cl2 activation over oxygen vacancy of TiO2 can even occur under dark condition. In contrast, very small amount of aromatic ring-chlorinated products are observed over TiO2 (blue curve) or without catalyst (black curve) under the same reaction conditions. The formation of the products may come from S52 the spontaneous electrophilic substitution reaction between Cl2 and toluene but with much slower conversion than the catalytic reaction over TiO2-Ov catalyst. These comparison results show the important role of TiO2 with oxygen vacancy in facilitating Cl2 activation and following toluene chlorination via an electrophilic substitution reaction.
The catalytic results of PEC chlorination of methylnaphthalene over TiO2-Ov-400 can also be explained by the electrophilic substitution mechanism. 1-chloro-2-methylnaphthalene was observed as the main product by using TiO2-Ov-400 as the photoanode (Supplementary Fig. 30).
For PEC halogenation of toluene over pure TiO2 photoanode that obtains benzyl chloride as the main product, we propose that the reaction mainly follows a free-radical mechanism. As shown in Supplementary Fig. 32b, Cl − can be activated by the photogenerated holes (h + ) over TiO2 photoanode to chlorine radicals (Cl·) through direct single electron transfer (SET) 2 . In the reaction system, Cl2 can be formed also by dimerization of two Cl· or by the direct oxidation of two Cl − by h + over TiO2. The generated Cl· reacts with toluene to form a carbon-centered radical via C-H bond dissociation of the methyl group on toluene. Finally, the generated carboncentered radical reacts with Cl2 to form benzyl chloride and release the Cl· for the next cycle.
The important role of Cl2 for the free-radical reaction was then demonstrated by Cl2 experiment. We injected Cl2 into an aqueous solution containing 5 mmol toluene in the presence of TiO2-Ov-400 or TiO2 or without catalyst, with the following reaction performed under light irradiation for 15 min. As shown in Supplementary Fig. 33b, benzyl chloride was observed on the GC spectra for all the three reactions. This is attributed the generation of Cl· via homolytic cleavage of Cl2 under light irradiation that doesn't require catalyst. The formed Cl· then activates C-H bond of the methyl group of toluene to form carbon radical, which is followed by the free-radical chain reaction between the carbon radical and Cl2 to give benzyl chloride and Cl·. Therefore, we propose that the PEC halogenation of toluene over pure TiO2 photoanode to produce benzyl chloride possibly follows a similar free-radical mechanism, in which case the Cl2 is produced by direct oxidation of Cl − by the photogenerated holes (h + ) over TiO2 photoanode. Noted that p-chlorotoluene and o-chlorotoluene were also observed over TiO2-Ov-400, although with much lower selectivity than benzyl chloride, which may be due to the occurrence of electrophilic substitution reaction of Cl2 on oxygen vacancy of TiO2 discussed above.