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
