Synthesis, structure and properties of [Zn(dpa)(N3)2] and [Zn(dpa)(N3)(NO3)]2 (dpa=2,2′-dipyridylamine): composition tailored architectures

The synthetic details, characterization data and single crystal X-ray structural analyses of [Zn(dpa)(N3)2] (1) and [Zn(dpa)(N3)(NO3)]2 (2) (dpa=2,2′-dipyridylamine) are described. Structural study reveals that the zinc(II) centre in 1 is in a pseudotetrahedral ZnN4 environment ligated by two terminal azide N atoms and two pyridine N atoms of the chelator. The coordination polyhedron around each metal ion in dinuclear 2 is best described as a ZnN4O2 distorted octahedron linked by two end-on azido bridges. The mononuclear units in 1 are engaged in bifurcated intermolecular hydrogen bonding (N–H…N) between amine N of dpa and coordinated N of azide along with aromatic π–π stacking leading to a 2D sheet. In 2, the dinuclear units participate in intermolecular N–H…O hydrogen bonding between amine N of dpa and bound O of bidentate nitrate in combination with π–π interactions resulting to a 3D supramolecular continuum. 1 and 2 display high energy intraligand 1(π–π*) fluorescence and intraligand 3(π–π*) phosphorescence in glassy solutions (DMF at 77 K). A band (372 nm) observed for both 1 and 2 in their solid-state emission spectra is an excimeric emission arising due to aromatic π–π interaction. The role of dpa, a tailored diimine (–N  C | –NH– C |  N–) in 1 and 2 is discussed towards functionalisation on crystal engineering and luminescence behaviour.