Solution and solid state coexistence of head–head and head–tail isomers in dimeric Pd(II) and Pt(II) complexes of the type [M2(a–a)2(µ-L-N3N4)2]2+ with a bridging triazolopyrimidine ligand and chelating bidentate diamines

Six palladium and one platinum compound containing dimeric dinuclear cations of the type [M2(a–a)2(µ-7tpO-N3,N4)2]2+ are presented, a–a representing a bidentate chelating amine and 7tpO− being the anionic form of the ligand 4,7-dihydro-7-oxo[1,2,4]triazolo[1,5-a]pyrimidine, which bridges the metal atoms through its nitrogen atoms in positions 3 and 4. Two linkage isomers are possible for such complexes, so called head–head and head–tail. According to 1H- and 195Pt-NMR data, both isomers are present in dmso-d6 solution and display analogous stability. An exhaustive assignment of the 1H signals has been made. The crystal structure of three of these compounds has been determined by single-crystal X-ray diffraction. For two of them, those with bispyrimidine as auxiliary ligand, the head–head and head–tail isomers coexist even in the solid state, both being present in the same crystal in a disordered scheme. This situation is explained by the almost symmetric external shape of the bridging ligand, which is expected to interact with neighbouring species in a similar way even if rotated 180 degrees.