Metal−Purine Interactions: Homo- and Heterodinuclear Platinum(II) and/or Palladium(II) Complexes of 8-Thiotheophylline. Crystal Structures of [Pt(μ-TT)(dppm)]2·2DMSO and [(dppm)Pt(μ-TT)2Pd(dppm)]·7H2O

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Abstract
The reaction of cis-[MCl2(dppm)] (M = Pt(II) and Pd(II); dppm = bis(diphenylphosphino)methane) with H2TT and NaOH (H2TT = 8-thiotheophylline) yields neutral mononuclear [M(HTT-S(8))2(dppm)] complexes. Homodinuclear [M(μ-TT-N(7),S(8))(dppm)]2 (M = Pt(II) and Pd(II)) are prepared either by the reaction of [M(HTT)2(dppm)] with cis-[MCl2(dppm)] and NaOH or by the direct reaction of cis-[MCl2(dppm)] with Na2TT, prepared in situ from H2TT and NaOH. The crystal structure of the Pt derivative is reported: [Pt(μ-TT)(dppm)]2·2DMSO (3) crystallizes in the triclinic space group P1̄ with a = 12.949(3) Å, b = 13.009(3) Å, c = 22.980(5) Å, α = 96.89(3)°, β = 103.53(3)°, γ = 106.43(3)°, Z = 2, and R1 = 0.045. Heterodinuclear [(dppm)Pt(μ-TT)2Pd(L-L)] complexes are obtained by reaction of [Pt(HTT)2(dppm)] with [PdCl2(L-L)] in basic medium (L-L = dppm and 2,2‘-bipyridine). The crystal structure of [(dppm)Pt(μ-TT)2Pd(dppm)]·7H2O (5) is reported. The complex crystallizes in the orthorhombic space group P212121 with a = 16.560(5) Å, b = 17.063(5) Å, c = 24.428(5) Å, Z = 4, and R1 = 0.042. The structures of 3 and 5 are almost identical, by which 5 can be seen as an isomorphic substitution of one of the Pt(II) ions of 3 by a Pd(II) ion. The structures consist of dinuclear units having a pseudo-2-fold axis perpendicular to that defined by the metal atoms. The two metal atoms are bibridged by two μ-TT-N(7),S(8) ligands, in a head to tail arrangement. The square-planar coordination of the metal atoms is completed by a chelate dppm ligand. The steric repulsions between the bulky dppm ligands must be the main factor precluding metal−metal interaction in these compounds.

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