Four new metal–organic complexes by tuning the spacer length of flexible bis-pyridyl–bis-amide ligands: Assembly, structures and properties

Four new metal–organic complexes based on four flexible bis-pyridyl–bis-amide ligands with different spacer length and 1,3-benzenedicarboxylic acid, namely [Ni(3-dpye)(1,3-BDC)(H2O)2] (1), [Ni(3-dpyp)(1,3-BDC)(H2O)]·H2O (2), [Ni(3-dpyb)(1,3-BDC)(H2O)2]·H2O (3), [Ni(3-dpyh)(1,3-BDC)(H2O)2]·H2O (4) [3-dpye = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane, 3-dpyp = N,N′-bis(3-pyridinecarboxamide)-1,3-propane, 3-dpyb = N,N′-bis(3-pyridinecarboxamide)-1,4-butane, 3-dpyh = N,N′-bis(3-pyridinecarboxamide)-1,6-hexane, 1,3-H2BDC = 1,3-benzenedicarboxylic acid], have been hydrothermally synthesized and structurally characterized by elemental analyses, IR, PXRD, TG and single crystal X-ray diffraction analyses. X-ray analyses reveal that the title complexes show three different two-dimensional (2D) coordination layers (for complexes 1, 3 and 4) and a one-dimensional (1D) polymer chain (for complex 2). In four Ni(II) complexes, the four flexible bis-pyridyl–bis-amide ligands only show a μ2-bridging mode. While the 1,3-BDC ligands adopt two kinds of coordination modes in these four complexes: a bis(monodentate) bridging mode with each carboxyl group in a μ1-η1:η0 mode in complexes 1, 3 and 4, a chelating-monodentate bridging mode with one carboxyl group in a μ1-η1:η1 mode and another carboxyl group in a μ1-η1:η0 mode in 2. Finally, the adjacent 2D layers or 1D chains are further extended into 3D supramolecular frameworks via hydrogen bonding or π–π stacking interactions. The spacer length and conformation of the bis-pyridyl–bis-amide ligands play an important role in the final structures of the title complexes. The fluorescent and electrochemical properties of complexes 1–4 have also been investigated.