Solution and solid state coexistence of head-head and head-tail isomers in dimeric Pd(II) and Pt(II) complexes of the type [M-2(a-a)(2)(mu-L-(NN4)-N-3)(2)](2+) with a bridging triazolopyrimidine ligand and chelating bidentate diamines

Six palladium and one platinum compound containing dimeric dinuclear cations of the type [M-2(a-a)2(mu-7tpO-N-3,N-4)(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 H-1-and Pt-195-NMR data, both isomers are present in dmso-d(6) solution and display analogous stability. An exhaustive assignment of the H-1 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.