Kinetics and DFT Studies of Photoredox Carbon–Carbon Bond Cleavage Reactions by Molecular Vanadium Catalysts under Ambient Conditions

Abstract

Visible light assisted photocatalytic organic reactions have recently received intense attention as a versatile approach to achieve selective chemical transformations, including C-C and several C-X (X = N, O, S) bond formations under mild reaction conditions. The light harvesters in previous reports predominantly comprise ruthenium or iridium photosensitizers. In contrast, selective, photocatalytic aliphatic C-C bond cleavage reactions are scarce. The present study focuses on rationally designing V V oxo complexes as molecular, photoredox catalysts towards the selective activation and cleavage of a C-C bond adjacent to the alcohol group in aliphatic alcoholic substrates. We have employed kinetics measurements and DFT calculations to develop a candidate for the catalytic C-C bond activation reaction that is up to 7 times faster than our original vanadium complex. We have also identified a substrate where the C-C bond cleaves at rates 2.5-17 times faster, depending on the catalyst used. In order to better understand the effects of ligand modification on the thermodynamics and catalysis, DFT calculations were employed to reveal the orbital energies, the electronic transitions during the C-C bond cleavage, and the activation barriers. Our combined kinetics and computational studies indicate that the incorporation of electron withdrawing groups at select sites of the ligand are essential for the development of active and stable vanadium photocatalysts for our C-C bond cleavage reactions.

Conversion of Lignin Models by Photoredox Catalysis

“…[48] As such, the HOMO energy level of the complex could be stabilized, and the oxida-tion potential of the V V photocatalyst was increased. [48] As such, the HOMO energy level of the complex could be stabilized, and the oxida-tion potential of the V V photocatalyst was increased.…”

“…It was found that the redox of noninnocentl igands (i.e.,h ydrazonei midate) was essential for photocatalytic activity.I na follow-up study,alibrary of modified V V photocatalysts were developed, and stronge lectron-withdrawing groups such as NO 2 and Fw ere introduced at specific locations of the ligand at which the spin density was located. [48] As such, the HOMO energy level of the complex could be stabilized, and the oxida-tion potential of the V V photocatalyst was increased. In particular,t he complex having pentafluoro substituents on the imidate and one nitro group on the aryl hydrazone gave the greatest activity according to kinetics measurements, with areaction rate that was up to seven times faster than that of the originalc omplex.…”

Photoluminescence of Seven‐Coordinate Zirconium and Hafnium Complexes with 2,2′‐Pyridylpyrrolide Ligands

“…[57,58] While compounds with aromatic substituents show lifetimes below 200 ns with quantum yields < 1%,c omplexes with bulky alkyl substituents (R = adamantyl, tert-butyl) exhibit longer-lived emission with lifetimes and quantum yields of up to 15.1 msa nd 20 %, respectively.M ore recently,h ydrazone-imidate vanadium(V) oxo complexes with luminescence lifetimes of up to 9.9 ns in acetonitrile have been utilized in photocatalysis. [59,60] The exciteds tate was assigned as LMCT (O 2À !V V )v ia density functional theory.…”

Photoelectrochemical Cells for Artificial Photosynthesis: Alternatives to Water Oxidation

“…However, Reisner's photocatalysts contained the highly toxic Cd, whereas Stephenson's system involved expensive Ir complexes. Our team has been at the forefront of using earth‐abundant, less toxic vanadium photocatalysts to effect C−C bond cleavage in lignin model compounds (Figure a) that can be characterized more thoroughly . We observed a remarkably selevtive C‐C bond cleavage at the aliphatic aochols (red, bold bond in Figure b) mediated by a vanadium photocatalyst under ambient air .…”