Deciphering light-triggered Mg2+-assisted FeCl3 ion-pairing: an efficient photo-oxidant for chemo-divergent functionalization of 3-arylidene indolines

Abstract

Iron-based photocatalysis has emerged as an appealing “green alternative” to precious-metal photocatalysis that relies on ligand-to-metal charge-transfer (LMCT) activation, facilitating advanced organic transformations. Here, we investigate the Mg²⁺-assisted photocatalytic behaviour of FeCl₃ that selectively converts 3-arylidene indolines into thermodynamically elevated trans-1,2-di(indol-3-yl)ethanes and substituted 3-acyl/alkyl indoles in a chemo-divergent manner. This redox protocol involves Mg²⁺-induced solvent-assisted autoionization/light (λ_max = 545 nm)-promoted oxidation-state modulation of iron(III) via LMCT, offering an in situ photoactive [(MeCN)₆Fe^II]²⁺[Fe^IIICl₄]₂⁻ (Fe-IP) ion-pair-precatalyst. Mechanistic investigation reveals that the combined non-covalent interactions of the Fe-IP with the substrate and subsequent photo-induced electron transfer (PET), i.e. both static and dynamic quenching as evidenced by the non-linear Stern–Volmer quenching experiment, facilitate the hydrogen-atom transfer (HAT) by LMCT-derived transient Cl˙ radicals—forming organic radical intermediates. This synergy of HAT with the in situ-produced reactive oxygen species (ROS)—accessed by the reversible dioxygen binding of the Fe(II)/Fe(III) ion pair—has diversified the functionalization of 3-arylidene indolines through chemo-divergent photodimerization, photooxygenation, and reductive photoisomerization reactions, which undoubtedly expands the realm of FeCl₃-mediated photochemistry.