Unraveling the Key Features of the Reactive State of Decatungstate Anion in Hydrogen Atom Transfer (HAT) Photocatalysis

Abstract

The decatungstate anion [W10O32]4– is a widely used photocatalyst for promoting hydrogen atom transfer (HAT) reactions. The mechanism implicated in the activation of organic substrates, however, still needs to be clarified and has been claimed to involve an unknown relaxed excited state of triplet multiplicity, tagged wO. A subpicosecond investigation allowed us to follow early events leading to the chemically reactive species. A hot singlet excited state (S1 HOT) has been individuated through pump–probe experiments, yielding S1 by ultrafast decay (<1 ps). The reactive species wO arises from S1 in competition with decay to S0 (efficiency ca. 0.5) and has been detected spectroscopically by flash photolysis experiments, with peculiar absorption bands in the near-UV (370 nm) and visible (600–800 nm) regions. TD-DFT calculations demonstrated that excitation to S1 occurs through a ligand to metal charge transfer (LMCT) transition, involving a displacement of electron density from dicoordinated (bridging) oxygen to tungsten atoms. Population of wO ensues and involves a reorganization of the singly occupied orbital centered on oxygen (not tungsten) atoms. As a result, monocoordinated O centers acquire a partial radical character that well explains the known chemistry, essentially hydrogen atom transfer (HAT), and highlights the similarity with nπ* carbonyl triplets. This rationalization may help in devising other photocatalysts able to promote HAT processes from unactivated precursors.

Sunlight decatungstate photoinduced trifluoromethylations of (hetero)aromatics and electron-poor olefins

“…12-14 could be applied in this case as photoredox catalyst for the generation of the trifluoromethyl radical (Scheme 1c). In fact, TBADT was only sparsely used so far in the role of oxidant in the excited state 13,14 despite its E* RED is quite high (a value of + 2.44 V vs SCE was estimated) 15 and largely sufficient to promote the reaction even under sunlight irradiation.…”

Efficiency and Selectivity Aspects in the C–H Functionalization of Aliphatic Oxygen Heterocycles by Photocatalytic Hydrogen Atom Transfer

“…27 The different selectivity observed for the decatungstate anion compared with the tested POCs might be related to the different lifetimes of the relevant reactive states, where a short-lived species reacts with poor discrimination between the available sites. Indeed, whereas in the case of the decatungstate anion, the state responsible for the observed chemistry (named wO) has a lifetime of about 50-60 ns in acetonitrile, 28 the triplet states of the ketones employed here show much longer lifetimes, of the order of microseconds, 25a,c in turn leading to predominant or exclusive functionalization at the acetalic position.…”

Cooperative Polar/Steric Strategy in Achieving Site‐Selective Photocatalyzed C(sp3)−H Functionalization

“…However, in more complex molecules, as in the present cases, we believe that the S H 2‐type transition states should be greatly disfavored on account of the steric effects between decatungstate anion and the target C−H bond. The observed lack of reactivity in these cases is probably related to a difficult approach of the apical oxygens of decatungstate, recently proposed to be responsible for the H‐abstraction step, to the reacting site. Our results convinced us that a synergistic strategy based on polar and steric effects provides a promising approach for the realization of site‐selective C−H functionalization by radicals, and importantly the strategy would be much more promising for highly substituted molecules rather than simple molecules.…”