Synthesis and characterization of new oligomeric and polymeric complexes based on the [CuII(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).2H2O (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: [Cu2(bpca)2(H2O)2(bipy)](NO3)2.6H2O (2) (bipy = 4,40-bipyridine) and [Cu2(bpca)2(H2O)2(bpete)](NO3)2.xH2O (3) [bpete = (E)-1,2-di(pyridin-4- yl)ethane] are linear dumbbell-like species with Cu...Cu separations of 11.075 and 13.275 Å, respectively. The third dinuclear compound, [Cu2(bpca)2(H2O)2(bpx)](NO3)2.8H2O (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...Cu contact of 6.869 Å only. We have also obtained a chiral 1D neutral polymeric complex, [Cu3(bpca)2(bipy)3(NO3)4].6H2O (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, [Cu3(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...Cu separation is 5.205 Å. 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.