A new mixed-ligand copper pentaborate with square-like, rectangle-like and ellipse-like channels formed via hydrogen bonds

A new mixed-ligand copper pentaborate, [Cu(en)2(C5H9NO)][B5O6(OH)4]2·C5H9NO (en is ethylenediamine) (1), with novel framework has been synthesized under mild solvothermal condition and characterized by FT-IR, elemental analyses, powder X-ray diffraction and TG–DTA. The structure consists of isolated polyborate anion [B5O6(OH)4]−, copper complex cation [Cu(en)2(C5H9NO)]+ and one C5H9NO molecule. The [B5O6(OH)4]− units are connected to one another through hydrogen bonds to form an interesting three-dimensional (3D) supramolecular framework with alternate square-like channels and rectangle-like channels along the c-axis as well as ellipse-like channels with 16-membered boron rings along the a-axis which are the largest channels in the pentaborate reported so far. The copper complex cation contains two kinds of ligands, en and C5H9NO, forming a square pyramid with an O atom as the vertex.