[Cu(en){B6O7(OH)6}].3H2O (1) (en = 1,2-diaminoethane), obtained as a crystalline solid in low yield (31%) after prolonged standing of an aqueous solution initially containing [Cu(en)2](OH)2 and B(OH)3 (1:7 ratio), was characterized by thermal analysis (TGA/DSC), 11B NMR and IR spectroscopy, powder XRD, and single-crystal XRD studies, and magnetic susceptibility measurements. The single-crystal X-ray diffraction revealed that the oxidoborate complex is a 1D coordination polymer with the hexaborate(2-) ligand bridging two hexacoordinate Cu(II) centers, in an alternating a fac-tridentate (κ3-O) and monodentate (κ1-O) arrangement. Cu-O coordination bonds and extensive H-bonding networks promote and stabilize the self-assembly of [Cu(en){B6O7(OH)6}].3H2O from the Dynamic Combinatorial Libraries of available reactants. [Cu(en){B6O7(OH)6}].3H2O is thermally decomposed to CuB6O10 in air at 700 °C.