Two-Electron-Two-Proton Transfer from Colloidal ZnO and TiO2 Nanoparticles to Molecular Substrates.

Transfers of multiple electrons and protons are challenging yet central to many energy-conversion processes and other chemical and biochemical reactions. Semiconducting oxides can hold multiple redox equivalents. This study describes the 2e-/2H+ transfer reactivity of photoreduced ZnO and TiO2 nanoparticle (NP) colloids with molecular 2e-/2H+ acceptors, to form new O-H, N-H, and C-H bonds. The reaction stoichiometries were monitored by NMR and optical spectroscopies. Faster 2e-/2H+ transfer rates were observed for substrates forming O-H or N-H bonds, presumably due to initial hydrogen bonding at the oxide surface. Chemically reduced ZnO NPs stabilized by Na+ or Ca2+ also engage in 2e-/2H+ transfer reactivity, showing that protons transferred in these processes are inherent to the oxide nanoparticles and do not exclusively stem from photoreduction. These results highlight the potential of ZnO and TiO2 for multiple proton-coupled electron transfer (PCET) reactions.

Stereodynamic Quinone-Hydroquinone Molecules That Enantiomerize at sp3-Carbon via Redox-Interconversion.

Since the discovery of molecular chirality, nonsuperimposable mirror-image organic molecules have been found to be essential across biological and chemical processes and increasingly in materials science. Generally, carbon centers containing four different substituents are configurationally stable, unless bonds to the stereogenic carbon atom are broken and re-formed. Herein, we describe sp3-stereogenic carbon-bearing molecules that dynamically isomerize, interconverting between enantiomers without cleavage of a constituent bond, nor through remote functional group migration. The stereodynamic molecules were designed to contain a pair of redox-active substituents, quinone and hydroquinone groups, which allow the enantiomerization to occur via redox-interconversion. In the presence of an enantiopure host, these molecules undergo a deracemization process that allows observation of enantiomerically enriched compounds. This work reveals a fundamentally distinct enantiomerization pathway available to chiral compounds, coupling redox-interconversion to chirality.

Theoretical Insights into Proton-Coupled Electron Transfer from a Photoreduced ZnO Nanocrystal to an Organic Radical.

Proton-coupled electron transfer (PCET) at metal-oxide nanoparticle interfaces plays a critical role in many photocatalytic reactions and energy conversion processes. Recent experimental studies have shown that photoreduced ZnO nanocrystals react by PCET with organic hydrogen atom acceptors such as the nitroxyl radical TEMPO. Herein, the interfacial PCET rate constant is calculated in the framework of vibronically nonadiabatic PCET theory, which treats the electrons and transferring proton quantum mechanically. The input quantities to the PCET rate constant, including the electronic couplings, are calculated with density functional theory. The computed interfacial PCET rate constant is consistent with the experimentally measured value for this system, providing validation for this PCET theory. In this model, the electron transfers from the conduction band of the ZnO nanocrystal to TEMPO concertedly with proton transfer from a surface oxygen of the ZnO nanocrystal to the oxygen of TEMPO. Moreover, the proton tunneling at the interface is gated by the relatively low-frequency proton donor-acceptor motion between the TEMPO radical and the ZnO nanocrystal. The ZnO nanocrystal and TEMPO are found to contribute similar amounts to the inner-sphere reorganization energy, implicating structural reorganization at the nanocrystal surface. These fundamental mechanistic insights may guide the design of metal-oxide nanocatalysts for a wide range of energy conversion processes.