Synthesis and characterization of four dicyanamide bridged copper(II) complexes with N2O donor tridentate Schiff bases as blocking ligands

Four copper(II) complexes [Cu(L1)(dca)]n (1), [Cu(L2)(dca)(H2O)]2 (2), [Cu(L3)(dca)]n (3) and [Cu(L4)(dca)]n (4), where HL1 = 2-[(2-aminoethylimino)methyl]-4-nitrophenol, HL2 = 2-[(2-(methylamino)ethylimino)methyl]-4-nitrophenol, HL3 = 2-[(2-(dimethylamino)ethylimino)methyl]-4-nitrophenol and HL4 = 2-[(2-(diethylamino)ethylimino)methyl]-4-nitrophenol are tridentate Schiff base ligands, were synthesized and characterized by elemental analysis, IR and UV–Vis spectroscopy and single-crystal X-ray diffraction studies. Complex 1 and 4 are 1D helical, complex 2 is dimeric and complex 3 is 1D linear chain. X-ray single crystal structure analyses reveal that in complexes 1, 2 and 4, dicyanamide (dca) anions act as a purely μ-1,5-bridging ligand whereas in case of complex 3, two consecutive dca units serve as 1,3-bridging ligand while the third one exhibits end-to-end 1,5-bridging mode. Copper(II) centers in case of two complexes (1 and 3) exhibit square pyramidal geometry as depicted by Addison parameter (τ) values whereas metal centers of complexes 2 and 4 depict distorted octahedral and square pyramidal geometry respectively. The role of weak forces like lone pair⋯π/π⋯π interactions in influencing the self-assembly process appears to be of importance. Such types of interactions leading to supramolecular network in the four complexes are described here.