Synthesis and characterization of new oligomeric and polymeric complexes based on the [Cu(II)(bpca)](+) unit [Hbpca = bis(2-pyridylcarbonyl)amine]

Five novel bpca-based Cu(II) polynuclear coordination compounds [Hbpca = bis(2-pyridylcarbonyl)amine] were prepared using the [Cu(bpca)(H2O)(2)](NO3)center dot 2H(2)O (1) building block and characterized by single crystal X-ray diffraction. We have also isolated and characterized two new crystal forms of the starting species, with lower water contents. Three of the new products are dinuclear complexes obtained by reacting 1 with different rigid or flexible spacer ligands: [Cu-2(bpca)(2)(H2O)(2)(bipy)](NO3)(2)center dot 6H(2)O (2) (bipy = 4,4'-bipyridine) and [Cu-2(bpca)(2)(H2O)(2)(bpete)](NO3)(2)center dot xH(2)O (3) [bpete = (E)-1,2-di(pyridin-4-yl)ethane] are linear dumbbell-like species with Cu center dot center dot center dot Cu separations of 11.075 and 13.275 angstrom, respectively. The third dinuclear compound, [Cu-2(bpca)(2)(H2O)(2)(bpx)](NO3)(2)center dot 8H(2)O (4) [bpx = 1,4-bis((1H-pyrazol-1-yl)methyl)benzene], with the flexible bpx ligand, assumes an unusual S-shaped conformation and shows a quite shorter Cu center dot center dot center dot Cu contact of 6.869 angstrom only. We have also obtained a chiral 1D neutral polymeric complex, [Cu-3(bpca)(2)(bipy)(3)(NO3)(4)]center dot 6H(2)O (5), that shows a central linear -Cu-bipy-Cu- chain, with all these Cu atoms connected to two lateral [Cu(bpca)(NO3)(2)] groups on two opposite sides by means of bipy spacers. An unprecedented type of Cu(II) neutral trinuclear complex, [Cu-3(bpca)(2)(H2O)(2)(NO3)(2)] (6), was obtained which has a centrosymmetric structure with two external [Cu(bpca)(NO3)(2)] units chelating on a central copper atom via the two pairs of carbonyl groups of the bpca ligands. The central metal is octahedral with two axial water molecules, while the two lateral Cu atoms are in square pyramidal geometry; the Cu center dot center dot center dot Cu separation is 5.205 angstrom. The magnetic properties of 6 have been rationalized through a ferromagnetic coupling between the central metal ion and the peripheral ones which are coupled by a smaller antiferromagnetic interaction. DFT calculations have been also performed in order to give a better insight into magnetic interactions.