Oxidation/reduction reactions at metal oxide surfaces are important to emerging solar energy conversion processes, photocatalysis, and geochemical transformations. Here we show that the usual description of these reactions as electron transfers is incomplete. Reduced TiO(2) and ZnO nanoparticles in solution can transfer an electron and a proton to phenoxyl and nitroxyl radicals, indicating that e(-) and H(+) are coupled in this interfacial reaction. These proton-coupled electron transfer (PCET) reactions are rapid and quantitative. The identification of metal oxide surfaces as PCET reagents has implications for the understanding and development of chemical energy technologies, which will rely on e(-)/H(+) coupling.

Colloidal ZnO nanocrystals, capped with dodecylamine and dissolved in toluene, can be charged photochemically to give stable solutions in which electrons are present in the conduction bands of the nanocrystals. These conduction band electrons are readily monitored by EPR spectroscopy, with g* values that correlate with the nanocrystal sizes. Mixing a solution of charged small nanocrystals with a solution of uncharged large nanocrystals, e-CB:ZnO–S + ZnO–L, causes changes in the EPR spectrum indicative of quantitative electron transfer from small to large nanocrystals. EPR spectra of the reverse reaction, e-CB:ZnO–L + ZnO–S, show that electrons do not transfer from large to small nanocrystals. Stopped-flow kinetic studies monitoring the change in the UV band edge absorption show that reactions of 50 μM nanocrystals are complete within the 5 ms mixing time of the instrument. Similar results are obtained for the reaction of charged nanocrystals with methyl viologen (MV2+). These and related results indicate that the electron transfer reactions of these colloidal nanocrystals are quantitative and very rapid, despite the presence of ~1.5 nm long dodecylamine capping ligands. These soluble ZnO nanocrystals are thus well-defined redox reagents suitable for electron transfer studies involving semiconductor nanostructures.

Nine new double-salt compounds have been prepared and six crystal structures are reported that demonstrate a general metathesis route to double-salt compounds with metallophilic interactions. The compounds contain [Pt(terpy)X]+ or [Au(bpy)X2]+ cations, Au(III) or Au(I) anions such as [AuBr4]- or [AuCl2]-, and are prepared in water and recrystallized from organic solvents. In all crystallographically characterized cases, there exist metallophilic interactions between cations and anions, demonstrating the power of this general route. In most cases, there exists an unbroken chain of metallophilic interactions through the crystal, forming single-atom-wide wires.

The 2,6-di-tert-butyl-4-methoxy-phenoxyl radical is shown to dimerize in solution and in the solid state. The X-ray crystal structure for the dimer, the first for a para-coupled phenoxyl radical, reveals a bond length of 1.6055(23) Å for the 4-4’ C–C bond. This is significantly longer than typical C–C bonds. Solution equilibrium studies using both optical and IR spectroscopies show that the Keq for dissociation is 1.3 ± 0.2 M at 20 °C, indicating a C–C bond dissociation free energy of −0.15 ± 0.1 kcal mol−1. Van’t Hoff analysis gives an exceptionally small bond dissociation enthalpy (BDE) of 6.1 ± 0.5 kcal mol−1. To our knowledge, this is the weakest BDE measured for a C–C bond. This very weak bond shows a large deviation from the correlation of C–C bond lengths and strengths, but the computed force constant follows Badger’s rule.