Catalysis of Organic..

502 Electroreductive Catalytic Ullmann ReactionsIX2 + Cu Δ + CuI 22 + 2e – + 2X –catalystFigure 1 Ullmann reaction (left) and electrochemical catalytic alternative (right).Results and DiscussionIn a typical experiment, the aryl halide was stirred in a specially constructedelectrochemical cell (Figure 2, left) containing Pd and Pt electrodes, using the ionicliquid [octylmethylimidazolium] + [BF 4 ] – as a solvent (8). This solvent combines twoimportant advantages: It is an excellent conductor and it can stabilise metalnanoparticles via an ion bilayer mechanism (9). The electrolysis was done using aconstant current intensity of 10 mA at 1.6 V (cf. E 0 = –0.83 V for Pd 0 → Pd 2+ + 2e – ).Pd anodePt cathodePt anodePd cathode[C 8 mim] + [BF 4 ] –solvent[C 8 mim] + [BF 4 ] –solventFigure 2 Photo of the electrochemical cell (left) and schematics showing thegeneration of Pd clusters using a Pd anode (middle) and the reverse case (right).Using PhI as the substrate, the reaction mixture turned from a light yellow solution toa dark brown suspension after 20 min. However, no conversion was observed by GCanalysis. We assumed that Pd 2+ ions, oxidised from the anode, were in turn reducedto adatoms at the Pt cathode and formed Pd 0 nanoparticles, ca. 11 nm in diameter(10). After 8 h, the PhI was totally consumed, giving 80% biphenyl and 20%benzene. Weighing the electrodes before and after the reaction showed difference of~ 2.5 mg in the Pd anode, equivalent to 0.1 mol% of the aryl halide substrate. Thiscorresponds to a TON of 1000 at least (assuming that all the ‘missing’ Pdparticipates in the catalysis).To further investigate the role of palladium nanoparticles in this system, weswitched the current between the two electrodes, so that now the Pd electrode wasthe cathode (Figure 2, right). The rationale behind this experiment was that in theory,