A novel tetranuclear copper(II) complex [Cu4(Phen)4(NO3)2(H2O)2(adip)4/4(Hadip)4/2](NO3)2• 2H2O (H2adip＝adipic acid) was isolated from the reaction of 1,10-phenanthroline (phen), adipic acid and Cu(NO3)•6H2O in water solution, and characterized by elemental analysis, IR, UV, TG and single-crystal X-ray diffraction analysis. The complex crystallizes in the triclinic system, space group with a＝1.0146(2) nm, b＝1.0261(2) nm, c＝1.8285(4) nm, α＝91.66(3)°, β＝92.19(3)°, γ＝112.76(3)°, V＝1.7520(6) nm3, Z＝1, Dc＝1.639 g/cm3, C66H66Cu4N12O28, Mr＝1729.47, F(000)＝886 and μ＝1.294 mm－1. The final R1 and wR2 are 0.0447 and 0.1141 for 5672 observed reflections with I＞2σ(I), respectively. It was revealed that the adipic acid groups adopt two types (syn-syn) of binding modes, an adipate bridge and two coordinated protonated adipate (Hadip). The adipate bridge provides its four carboxylate O atoms to link two U-shaped subunits into a centrosymmetric dual U-shaped tetranuclear Cu(II) structure. Each U-shaped frame consists of two crystallographically distinct Cu(II) sites, in which each Cu(II) exhibits a distorted square pyramidal geometry with the basal plane completed by two N atoms from one phen, one O atom from the adipate bridge and the other from the Hadip ligand. While the apical O atom in one Cu(II) coordination sphere is from a coordinated water, that in the other Cu(II) sphere is from a nitrate anion. There are extensive hydrogen bonds and p×××p interactions in the crystal structure.

Key words: tetranuclear complex, copper(II) complex, crystal structure, adipic acid, flexible ligand