Polymorphs And Aurophilic Interactions In Colorless Crystals Of Au-2(Mu-1,2 Bis-(Diphenylarsino)Ethane)X-2, (X = Cl, Br, I)

Solutions containing the components Au+, dpae (dpae is 1,2 bis(diphenylarsino)ethane), and X- (X is Cl, Br, or I) can produce two different types of crystals with the composition Au-2(mu-dpae)X-2: colorless blocks and colorless needles. Crystallographic studies of these crystals show that they are polymorphs with different structural motifs. In the alpha-polymorphs, which are isostructural, individual molecules of Au-2(mu-dpae)X-2 form discrete dimers through two identical Au center dot center dot center dot Au contacts. In the beta-polymorphs, which each have unique crystallographic parameters, the Au2(mu-dpae)X-2 molecules assemble into polymeric chains through aurophilic interactions. The Au center dot center dot center dot Au contacts in the alpha-polymorph (3.1163(2), 3.1064(3), and 3.0842(2) angstrom for Cl, Br, I, respectively) are somewhat shorter than those in the beta-polymorph (3.1668(3), 3.1042(8), and 3.1046(2) for Cl, Br, I respectively). The systematic study we now report shows an increase in the strength of this aurophilic interaction for the alpha-form in the series X = Cl < Br < I, which is in good agreement with theoretical studies by Pyykko and his co-workers.