Gold(I)−Purine Interactions:  Synthesis and Characterization of Cyclic and Open Chain Polynuclear Gold(I) Complexes Containing Xanthine Derivatives and Bis(phosphine) as Bridging Ligands. Crystal Structures of [Au₂(μ-HX)(μ-dmpe)]·3H₂O and [Au₂(μ-TT)(μ-dmpe)]·H₂O (H₃X = Xanthine; H₂TT = 8-Mercaptotheophylline)

The reaction of [(AuBr)₂(μ-PR₂P(CH₂)_nPR₂)] (where R = CH₃ for n = 1; R = Ph for n = 1, 3, 4) with N-alkylxanthine and thioxanthine derivatives, containing two ionizable protons in close positions, yields, under basic conditions, neutral heterobridged dinuclear gold(I) ring complexes [Au(μ-L)₂(μ-PR₂P(CH₂)_nPR₂)], which have been investigated by means of ¹H- and ³¹P-NMR and FAB spectroscopies. Crystal structures of two of these complexes are reported. [Au₂(μ-HX)(μ-dmpe)]·3H₂O (1) (H₃X = xanthine; dmpe = 1,2-bis(dimethylphosphine)ethane) crystallizes in the monoclinic space group P2₁/n with a = 9.348(2) Å, b = 8.656(2) Å, c = 24.585(5) Å, β = 98.24(2)°, Z = 4, and R = 0.040. [Au₂(μ-TT)(μ-dmpe)]·H₂O (2) crystallizes in the monoclinic space group P2₁/n with a = 10.853(4) Å, b = 14.031(6) Å, c = 13.574(5) Å, β = 100.80(4)°, Z = 4, and R = 0.063. The structures of 1 and 2 are similar and consist of dinuclear nine-membered ring molecules, in which the two linear two-coordinate gold atoms are bridged on one side by a dmpe ligand and on the other side by a bidentate xanthinato dianion, with intramolecular Au···Au distances of 3.053(1) and 2.952 (2) Å, respectively. In the former, the coordination of the xanthinato ligand to the gold atoms takes place through the N3 and N9 nitrogen atoms whereas, in the latter, N7,S8-chelate coordination of the 8-thiotheophyllinato dianion occurs. The magnitude of the Au···Au separation is analyzed in terms of the twisting of the xanthine derivative ligand from the plane containing the gold(I) and phosphorus atoms. For n = 6 the steric requirements of the Au(μ-dpph)Au group prevents the formation of dinuclear ring complexes and open chain complexes are obtained. Finally, when the xanthine derivatives do not contain two close active coordination sites dinuclear open chain complexes are formed.