Visible-light-driven reversible shuttle vicinal dihalogenation using lead halide perovskite quantum dot catalysts

Abstract

Dihalogenation of alkenes to the high-added value vicinal dihalides is a prominent process in modern synthetic chemistry. However, their effective conversion still requires the use of expensive and hazardous agents, sacrificial half-reaction coupling or primary energy input. Here, we show a photocatalytically assisted shuttle (p-shuttle) strategy for redox-neutral and reversible vicinal dihalogenation using low-cost and stable 1,2-dihaloethane under visible light illumination. Energetic hot electrons from metal-halide perovskite QDs enable the challenging photocatalytic reactions. Ultrafast laser transient absorption spectroscopy have unveiled the energy matching of the hot electrons with the high reduction potential of 1,2-dihaloethane, via two consecutive photoexcitation process. Powered by the sustainable energy as the only energy input, our new catalytic system using metal-halide perovskite QDs for dibromination, dichlorination and even unexplored hetero-dihalogenation, shows good tolerance with a wide range of alkenes at room temperature. In contrast to homogeneous photocatalysts, chalcogenide QDs and other semiconductor catalysts, perovskite QDs deliver previously unattainable performance in photoredox shuttle vicinal dihalogenation with the turnover number over 120,000. This work provides new opportunities in visible-light-driven heterogeneous catalysis for unlocking novel chemical transformations.

CsPbBr3 Nanocrystals as Efficient Photocatalysts for Dehydrohalogenation: Toward Environmentally Friendly Trichloroethylene Synthesis

“…One of the C–Cl bonds in TCE undergoes photocatalytic activation and subsequently undergoes reductive dissociation to produce Cl – . This process aligns with a prior study by Li and co-workers, where it was elucidated that excitons generated through two-photon absorption result in the formation of a hot electron possessing a potential exceeding that of the conduction band of CsPbX 3 (−3.38 to −3.26 V with respect to vacuum). This hot electron is then capable of reducing TCE, which has a reduction potential of −2.45 V with respect to vacuum …”

“…Figure c illustrates the proposed adduct formed in our reaction that is responsible for the EPR signal. This proposal is based on a prior study by Li and co-workers, where a similar radical was successfully trapped using TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) . Therefore, the reaction proceeds via a radical pathway after C–Cl breaking, as substantiated by previous literature reports …”

“…(c) EPR spectrum of the photoreaction mixture in the presence of the radical trap DMPO. (d) Energetics comparison of CsPbBr 3 NCs and TCE, demonstrating that hot electrons generated upon second photon excitation drive the reduction reaction . (e) Proposed mechanism for the entire chemical reaction.…”

“…Thus, based on the details mentioned earlier and in previous reports, it can be ascertained that both the reducing (Pb 2+ ) and oxidizing (X – ) centers on the perovskite surface are crucial for organic transformation. It involves the consecutive utilization of e – and h + , thereby eliminating the need for an external hole scavenger. , …”

“…It involves the consecutive utilization of e − and h + , thereby eliminating the need for an external hole scavenger. 44,58 In a separate experiment, we tested the stability of the cationic framework by regenerating CsPbBr 3 NCs after the reaction. For this purpose, mixed CsLHP NCs with PL maxima at 434 nm were extracted from a photocatalytic reaction mixture comprising CsPbBr 3 NCs dissolved in TCE and treated with PbBr 2 50 (Section S5) to regenerate pure CsPbBr 3 NCs with PL maxima at 502 nm (Figure 6a(i,ii)).…”