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

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 P1h with a ) 12.949(3) Å, b ) 13.009(3) Å, c ) 22.980(5) Å, R ) 96.89(3)°, â ) 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.