Observation of a water–anion tape constructed from unusual trimeric and tetrameric water clusters embedded in a copper(II) complex

Unusual trimeric and tetrameric water clusters have been observed in a complex of [Cu(DBA)(phen)2·(H2O)4·(NO3)] (1), (HDBA = benzilic acid, phen = 1,10-phenanthroline), which was prepared under solvothermal conditions and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis (TGA), UV–Vis spectroscopy, powder X-ray diffraction and single-crystal X-ray diffraction. In 1, a butterfly-like water–nitrate trimer formed by two water molecules and one nitrate anion, as well as a cyclic tetrameric water cluster are observed. The uncoordinated carboxylate oxygen atoms of DBA ligands, water molecules and nitrate anions interact via hydrogen bonds and extend 1 into a water–anion–cation tape, which in turn join to form a double water–anion–cation tape via π···π stacking interactions involving neighboring pyridyl and phenyl groups of DBA ligands.