Oxidative halogenation of dinuclear N-heterocyclic dicarbene gold(I) complexes

Cl2, Br2, and I2 are effective agents for the oxidative addition of carbene-coordinated Au(I) centres in [Au2(MeIm-xylylene-ImMe)2](PF6)2. The resulting complexes depend on the type of xylylene bridge between the two carbene units. With m- and p-xylylene, the obtained products are always the Au(III)/Au(III) complexes [Au2X4(MeIm-m,p-xylylene-ImMe)2](PF6)2 (X = Cl, Br, I). By contrast, with the o-xylylene bridge only in the reaction with iodine the Au(III)/Au(III) complex is isolated as the unique compound, while with chlorine or bromine a mixture of two complexes, the usual Au(III)/Au(III) and the Au(II)/Au(II) [Au2X2(MeIm-o-xylylene-ImMe)2](PF6)2 (X = Cl, Br), is obtained. This different behaviour is attributable to a greater tendency of o-xylylene bridge to favour an approach of the gold centres compared to the m- and p- analogues.