The first example of an iron(III) photosensitizer for photoredox catalysis

Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light

The scarcity of 2nd and 3rd row transition metals such as ruthenium, iridium, rhodium or osmium, coupled with a global desire for sustainable chemistry has led to the resurgence of research centered on earth-abundant metal-based photosensitizers. In particular, luminescent complexes of iron are often considered a ‘holy grail’ because of iron’s high abundance in the earth’s crust, low toxicity, and low environmental impact. In this collaborative study between UCLouvain, ULB, Friedrich-Alexander-Universität Erlangen-Nürnberg, Universidad de Buenos Aires, University of North Carolina at Chapel Hill (UNC) and Brookhaven National Laboratory (BNL) , we have used [Fe(phtmeimb)2]+ (FePS+, phtmeimb = {phenyl[tris(3-methyl-imidazolin-2-ylidene)]borate}, an Fe(III) based photosensitizer reported in 2019 by Wärnmark et al., that exhibits an excited-state with a lifetime of ~2.2 ns in CH3CN. This lifetime is sufficient to initiate diffusional bimolecular excited-state reactivity. We have determined experimental conditions that allow to enable excited-state reactivity of FePS+ towards sacrificial electron donors such as triethylamine (TEA) or N,N-dimethylaniline (DMA), hence generating a geminate radical pair of the formally reduced iron photosensitizer and the corresponding oxidized sacrificial electron donor, i.e. {FePS;TEA•+}. The kinetic rate constant for this electron transfer step was determined by femtosecond transient absorption as ket = 2.3 x1010 M–1s–1, approaching a value expected for a diffusion limited reaction. We have then quantified the efficiency with which the geminate radical pair dissociates (cage-escape yields) and have found that cage-escape yields could be controlled by the nature of the solvent. Indeed, these cage-escape yields were small in polar solvent such as acetonitrile or dimethylformamide but reached high values in dichloromethane. Experiments performed in CH2Br2 or in CH3CN/MeI mixtures as well as with added TBAPF6 allowed to propose that these large cage-escape yields originated from a combination of increased state-mixing due to the heavy-atom effect, and electrostatic repulsion between the reduced iron photosensitizer and the oxidized electron donor due to solvent dielectric effects. The monoreduced [Fe(phtmeimb)2] photoproduct was then used to perform a benchmark dehalogenation reaction, relevant for organic synthesis and environmental applications, that operated with catalytic yields that exceed 90%. Importantly, the iron photosensitizer exhibited enhanced stability compared to the prototypical photosensitizer [Ru(bpy)3]2+ and could be recycled with 88% yields. A quantitative description of the catalytic mechanism was obtained using a combination of spectroscopic tools that included femtosecond and nanosecond transient absorption, time-resolved infrared spectroscopy as well as density functional theory.

Authors : Akin Aydogan, Rachel E. Bangle, Alejandro Cadranel, Michael D. Turlington, Daniel T. Conroy, Emilie Cauët, Michael L. Singleton, Gerald J. Meyer, Renato N. Sampaio, Benjamin Elias and Ludovic Troian-Gautier

J. Am. Chem. Soc. 2021, 143, 38, 15661–15673

https://pubs.acs.org/doi/abs/10.1021/jacs.1c06081