Revealing axial-ligand-induced switching of spin states for controllable single electron transfer-based radical initiation

Abstract

Radicals are highly reactive for coupling reactions while the applications are normally limited by the uncontrollable initiation and chaotic conversions. Although transition metal-based single electron transfer (SET) shows potential for controllable radical initiation, the detailed mechanism is still insufficient, especially for the roles of spin state transition in SET-based radical initiation. Herein, with an Fe(III)-catalyzed thiol–ene click (TEC) reaction as an example, the axial-ligand-induced switching of transition metals' spin states was revealed to facilitate controllable SET-based radical initiation and the subsequent coupling reactions. Given the advantages of online monitoring by ambient mass spectrometry (AMS), the short-lived radical intermediates and their dynamic changes were explored. As demonstrated, initiated by the axial coordination of sulfhydryl with Fe(III)–porphyrin, the selective generation of a thiyl radical (RS˙) via SET was achieved. Besides, as another axial-ligand, O₂ in air was coordinated to Fe(III)–porphyrin, inducing the conversion of Fe(III) from a high spin (S = 5/2) to a low spin state (S = 1/2). This lowered the energy barrier for SET-based radical initiation, further facilitating the final selective coupling with the vinyl reactant. Upon revealing the axial-ligand-induced switching of the spin states by AMS and other examinations, rational design of transition metal catalysts would be promoted for efficient and highly selective radical reactions.