On the mechanism of radical C-N coupling in type B semiconductor photocatalysis: A high-pressure study

The CdS and CdS/SiO2 photocatatyzed addition of 2,5-dihydrofuran (2,5-DHF) to azobenzene in methanol solution affords 3-(2,3-dihydrofuryl)hydrazobenzene (1) as the major and hydrazobenzene (2) as the side product. According to C-13 NMR and adsorption data, 2,5-DHF is adsorbed parallel to the surface through hydrogen bonding with [OH/SH] groups; adsorption constants of 18 and 30 L mol(-1) are obtained for CdS and CdS/SiO2, respectively. This enhanced effect of the silica support favors formation of the addition product 1. Different from that, azobenzene exhibits adsorption constants identical within experimental error (11126 and 1059 L mol(-1)) for both photocatalysts and adsorbs most likely at Bronsted acid centers. These results are corroborated by corresponding adsorption studies with SiO2. The much better adsorption of 2,5-DHF onto CdS when performed in aqueous suspension is rationalized by comparison of adsorbent and adsorptive polarities, as determined by Reichardt's dye. The absence of My addition products of methanol to a mutual dihydrofuryl radical cation suggests that oxidation by the reactive valence band hole occurs via dissociative electron transfer affording a proton and the dihydrofuryl radical. The latter undergoes C-N coupling with PhN-N(H)Ph, formed in a proton-coupled reduction by the reactive electron, to afford 1. Formations of 1 and 2 exhibit the same Arrhenius activation energies of 11 kJ mol(-1). The rates of both reactions rue decreased upon increasing the solvent viscosity by increasing pressure or by using various alcohols. From the average activation volume of 17 cm(3)mol(-1), obtained for 1 and 2 at pressures up to 120 MPa, it is postulated that radical diffusion in the solvent-solute surface multilayer rather than C-N bond formation is the rate-determining step.