Counter anion modulated variation of denticity of NNO donor Schiff base in copper(II) complexes: Isolation of a zwitterionic Schiff base as the metal complex

Three copper(II) complexes [Cu(HL)2(ClO4)]ClO4·H2O (1), [Cu(L)DMF]ClO4 (2) and [CuL(NCO)][Cu2L2(NCO)2] (3) [where HL = 2-[(2-Diethylamino-ethylimino)-methyl]-4-nitro-phenol, a tridentate Schiff-base] have been synthesized and characterized by elemental analysis, IR and UV–Vis spectroscopy and single-crystal X-ray diffraction studies. In complexes 2 and 3, the Schiff base is acting as a tridentate ligand. However, in complex 1, the Schiff base is present in its zwitterionic form. The denticity of the tridentate Schiff base has been modulated by the acidity of counter anions. Complexes 1 and 2 are monomeric. Copper(II) assumes square pyramidal geometry in 1 (Addison parameter (τ) = 0.15) and square planar geometry in 2. The asymmetric unit of complex 3 consists of two crystallographically different double end-on cyanate bridged dimers, containing square pyramidal copper(II) {Addison parameter (τ) = 0.157}. Complexes 1 and 3 crystallize in triclinic space group, P 1 ¯ with cell dimensions a = 11.3479(10), b = 12.5198(9), c = 13.6640(12), α = 94.299(7)°, β = 97.281(7)° and γ = 116.007(6)° (for 1) and a = 11.6149(8), b = 11.8145(9), c = 13.8741(13), α = 65.524(6)°, β = 73.373(6)° and γ = 63.857(5)° (for 3), whereas complex 2 crystallizes in monoclinic space group, P21/c with cell dimensions a = 7.5765(2), b = 19.6005(6), c = 14.5861(4) and β = 102.679(1)° (for 2). The weak forces like lone pair⋯π/π⋯π/anion⋯π interactions lead to various supramolecular architectures.