Controlled Photocatalytic Hydrocarbon Oxidation by Uranyl Complexes

Abstract

Controlled, photocatalytic C−H bond activations are key reactions in the toolkits of the modern synthetic chemist. While it is known that the uranyl(VI) ion, [UVIO2]2+, the environmentally dominant form of uranium, is photoactive, most literature examines its luminescent properties, neglecting its potential synthetic utility for photocatalytic C−H bond cleavage. Here, we synthesise and fully characterise an air‐stable and hydrocarbon‐soluble uranyl phenanthroline complex, [UVIO2(NO3)2(Ph2phen)], UPh2phen, and demonstrate that it can catalytically abstract hydrogen atoms from a variety of organic substrates under visible light irradiation. We show that the commercially available parent complex, uranyl nitrate ([UVIO2(NO3)2(OH2)2]⋅4H2O; UNO3), is also competent, but from electronic spectroscopy we attribute the higher rates and selectivity of UPh2phen to ligand‐mediated electronic effects. Ketones are selectively formed over other oxygenated products (alcohols, etc.), and the catalytic oxidation of substrates containing a benzylic C−H position is particularly improved for UPh2phen. We also show uranyl‐mediated photocatalytic C−C bond cleavage in a model lignin compound for the first time.

Unprecedented Catalytic Behavior of Uranyl(VI) Compounds in Chemical Reactions

“…A recent exploration of hydrolytically as well as air‐stable uranyl phenanthroline complex, [UO 2 (NO 3 ) 2 (Ph 2 phen)] (Ph 2 phen = 4,7‐diphenyl‐1,10‐phenanthroline) due to Arnold et al shows the high degree of photocatalytic activity toward the oxidation of organic substrates via controlled C–H bond cleavage. [ 64 ] It has better selectivity and a higher rate of conversions than the commercially available [UO 2 (NO 3 ) 2 (H 2 O) 2 ] · 4H 2 O complex, attributed to ligand‐mediated electronic effects. The scope of this transformation has been extended to a range of substrates.…”

“…Substrates having benzylic C–H sites such as toluene, triphenylmethane, indane, and isochroman exhibit highest improvements in oxidation while using the UO 2 (NO 3 ) 2 (Ph 2 phen) as catalyst. [ 64 ] Mostly, ketones are preferentially produced over other oxygenated products.…”

Visible‐Light‐Enabled C−H Functionalization by a Direct Hydrogen Atom Transfer Uranyl Photocatalyst

“…Very recently, Jiang and co‐workers reported a selective and compatible oxidation of sulfides for the synthesis of sulfones and sulfoxides by using UO 2 (OAc) 2 ⋅ 2H 2 O as photocatalyst and molecular oxygen [17] . Concurrently, Arnold [18] and Takao [19] and their co‐workers demonstrated the photocatalytic oxygenation of hydrocarbons in the presence of a uranyl complex and visible‐light irradiation. In general, these remarkable, but few contributions highlight the novelty and importance of unexplored uranyl photochemistry.…”

“…Next, the effect of solventw as surveyed, and CH 3 CN was found to be the most suitable solvent (cf. entries 9a nd[17][18][19][20][21].S ubsequently,w ea ttempted to perform the reactionu nder air,a nd found that the result wasi nferior to that obtained under argon (cf. entries 9a nd 22).…”

Meet the Women of Catalysis

“…Polly has received a variety of awards and prizes including the RSC Wilkinson prize in 2018, the Lord Kelvin Medal 2017, which is the senior prize for the physical sciences awarded by the Royal Society of Edinburgh, the Seaborg Lectureship 2015 (UC Berkeley, USA), and the Royal Society′s Rosalind Franklin award in 2012. She made the film ′A Chemical Imbalance′, a call to action for simple changes to achieve equality of opportunity in science, and was awarded an OBE in the Queen's birthday honors in 2017 for her contributions to chemistry and women in STEM …”