Elucidation of Mononuclear Fe Photocatalysts for Visible Light-Induced Bond Homolysis (VLIH)

Carboxylic acids are among the most abundant feedstocks for carbon-based products in organic chemistry. These starting materials can be readily activated in the presence of a Lewis Acidic metal via a visible light-induced bond homolysis (VLIH) process. Despite its promise as a sustainable, benchtop catalyst the design of new Fe VLIH catalysts has been thwarted due to the lack of well-characterized Fe photocatalysts. Many VLIH photocatalytic reactions are prone to catalyst speciation, making isolation and characterization challenging. Thus far, only dinuclear and trinuclear complexes have been isolated in the literature though none of them have proven to be catalytically competent. Herein, we elucidate the structures of mononuclear Fe carboxylates that have been previously proposed, yet are synthetically elusive in photodecarboxylative reactions. X-Ray crystallography and Ultra-Violet Visible (UV-Vis) spectroscopy support the formation of ((diethylenetriamine(dien))FeIII(OCOR)3 (R= t-Bu, CH3, CF3, and Ph) from (dien)FeIIICl3 in the presence of carboxylic acid and base. Experimental and density functional theory (DFT) studies demonstrate that these complexes in the presence of visible light are competent radical sources, facilitating the generation of both stabilized and non-stabilized radicals. Additionally, an (dien)FeII carboxylate was synthesized and demonstrated to effectively regenerate the (dien)FeIII photocatalyst. This report provides evidence for the active photocatalyst in an Fe photodecarboxylative Giese reaction and communicates the first structural evidence for a discrete ligated mononuclear Fe photocatalyst for VLIH. By studying the efficacy of photocatalyst formation, radical generation, and turn-over, generalizable strategies for the development of well-defined earth-abundant VLIH photocatalysts can be established.