Synthesis, structure, and thermal behavior of discrete Co(II), Ag(I), and Pd(II) complexes with 2,3-bis(2-pyridyl)quinoxaline. Insight into coordination modes

The reactions of 2,3-bis(2-pyridyl)quinoxaline (bpq) with CoCl2·6H2O, Ag(CH3CN)4BF4, and PdCl2(C6H5CN)2 produce [CoCl2(bpq)]2·2CHCl3, [Ag(bpq)CH3CN]2(BF4)2·2CH3CN, and [PdCl2(bpq)], respectively. All the products are discrete 1:1 (metal:bpq) adducts, where the chelation mode of the bpq is dependent upon the metal atoms. The structure of [CoCl2(bpq)]2·2CHCl3 is a centrosymmetric Cl-bridged four-membered dimer, [Co2Cl2], in which the bpq is bonded to the cobalt(II) atom in an anisobidentate mode with the nitrogen donors of pyridine and pyrazine rings. For [Ag(bpq)CH3CN]2(BF4)2·2CH3CN, each bpq ligand connects two tetrahedral silver(I) ions in a tridentate mode, resulting in a cationic cyclic dimer. The structure of [PdCl2(bpq)] approximates to a molecular rocking chair with an isobidentate bpq through the nitrogen donors of 2-pyridyl rings. The compounds exhibit significant and characteristic relationships between the structures and their thermal properties. For [CoCl2(bpq)]2·2CHCl3, the solvate chloroform molecules are safely contained up to 144°C, but drastically evaporate above this temperature. The striking feature of [Ag(bpq)CH3CN]2(BF4)2·2CH3CN is that the skeletal cyclic dimer is basically retained after dissociation of the coordinated acetonitriles in the solid state.