Dual behavior of gold nanoparticles, as generators and scavengers for free radicals

Literature data available on the interaction of stable free radicals with gold nanoparticles (Au NPs) suggested the adsorption of stable-free radicals on the nanoparticles surface, and the possibility of exchange interaction between the unpaired electron from the free radicals and the conduction-band electrons of the metal [1, 2]. Au NPs, as well as gold salts, are able to catalyze different processes, like oxidation, C–C bond formation, additions, alkylations, etc. [3, 4]. Exchange reactions (the most convenient method to prepare functionalized nanoparticles) mechanism of the ligands involves also in the first step the formation of an organometallic reactive complex [5–7]. The mechanisms of such processes are still under consideration. Nonetheless, free radicals play a major role in the surface chemistry of metal nanoparticles. Our previous work demonstrates that Au NPs may abstract a hydrogen or a halogen atom from different substrates, yielding the corresponding short-lived radicals [8, 9]. However, there is little information about the final products derived from the free radicals formed on or near the metal surface. Till now, literature data suggest that stable-free radicals may stay adsorbed on the nanoparticles surface [1], while the short-lived radicals may react with the nanoparticles (as in the thiol exchange reaction occurs [8]). Aryl radicals may act as well as stabilizers in the synthesis of metal NPs, via a metal–carbon bond formation [10]. Short-lived radicals are very reactive species, and they can stabilize in very different ways (dimerization, abstraction of different atoms, radical ? radical reactions). The knowledge of the pathway followed by the radicals toward their final products may provide very useful information about catalysis by Au NPs. Our first aim was to use Au NPs as catalyst at room temperature in an Ullmann-type reaction (a well-known coupling reaction, performed in harsh conditions, which uses copper as catalyst; the mechanism involves an organocopper compound and does not follow a radical pathway [11]), using the classical reactants aniline and iodobenzene. First, we used the spin-trapping technique (see also supplementary data for more details) to test if, from aniline or from iodobenzene, phosphine-protected Au NPs [12] generate the corresponding short-lived radicals C6H5NH • and C6H5 , in the same way as for previous substrates [8, 9]. As expected, our results showed (Fig. 1) that, from both reactions, the corresponding free radicals were formed and trapped by a spin-trap (DMPO). The hyperfine coupling constants of the EPR spectra are in full agreement with the literature data [13], and the EPR spectra can be very well simulated (see supplementary data). The amount of free radicals formed (measured by double integral of the spectra) was up to 12%, as previous data showed [8]. Using an equimolecular mixture of aniline and iodobenzene, we expected as well the formation of diphenylamine, in a radical ? radical reaction (Scheme 1). Remarkably, no diphenylamine was found in the reaction mixture. An EPR spin-trapping test was also performed to check if the radicals are formed in the mixture of the two reactants, and the recorded spectra showed that a mixture Electronic supplementary material The online version of this article (doi:10.1007/s10853-008-2987-1) contains supplementary material, which is available to authorized users.