A series of 1-D zinc(II) coordination polymers with 3-D supramolecular networks: synthesis, structural investigation, and NBO analysis

Abstract

N,N′-Bis(pyridin-4-ylmethylene)naphthalene-1,5-diamine (L) acts as a bipyridine analogue linker ligand towards {Zn7(μ4-O)2(OAc)10}, {Zn2(NCS)2(OAc)2}, and {Zn(N3)2} nodes and allows construction of three new 1-D coordination polymers, the linear chain [Zn7(μ4-O)2(OAc)10(L)]n (1), [Zn(NCS)(OAc)(L)]n (2) in ladder-type geometry and the zigzag chain [Zn(N3)2(L)]n (3). Structural characterization reveals that in 1 acetate anionic ligands connect seven Zn(II) ions through the bridging coordination modes μ3-η1,η2 and μ2-η1,η1. The resulting heptanuclear node is located on an inversion center and therefore consists of four crystallographically distinct cations; their coordination spheres correspond to distorted octahedra or tetrahedra. The Zn(II) ions in polymer 2 exhibit distorted trigonal bipyramidal {ZnN3O2} coordination; μ2-η1,η1 coordinated acetate and terminal thiocyanate ligands lead to inversion-symmetric [Zn2(NCS)2(OAc)2] secondary building units (SBU), which are further linked by the N,N′-bipyridine analogue L. Terminal coordination of two anionic azide ligands and the bridging bipyridine L result in coordination polymer 3, in which the cations adopt distorted tetrahedral {ZnN4} coordination. In all crystalline solids 1–3, adjacent 1-D chains interact through π–π stacking and non-classical (C − H···O, C − H···π) hydrogen bonds, leading to 3-D supramolecular architectures. Differences in their 3-D arrangement are due to variations in the anionic co-ligands, subtle conformational differences in the semi-rigid linker and the variable coordination sphere about the zinc cations. Thermogravimetric investigations indicate differences in both thermal stability and decomposition mode. Natural bond orbital (NBO) analysis provides a convenient basis for investigating the intramolecular bonding interactions and delocalization effects in these molecular systems. Finally, solids 1–3 exhibit intense luminescence at room temperature.