Unusual π--π- stacking interactions between 5,5′-azotetrazolate(AT) anions in six AT based 3d metal photochromic complexes

Six 5,5'-azotetrazolate based supramolecular complexes of {[Co(en)(3)](2)(AT)(3)center dot 3H(2)O} (1), {[M(en)(2)(trans-AT)(2)](trans-H(2)en)center dot 2H(2)O} (M = Ni, 2; Cu, 3), {[Ni(en)(2)(trans-AT)](trans-AT)(trans-H(2)en)center dot 2H(2)O} (4), {[Cu(en)(2)(trans-AT)]center dot 2H(2)O} (5) and {[Cu(en)(2)(trans-AT)(0.5)](trans-AT)(0.5)} (6) (en = ethylenediamine) have been synthesized and characterized by single crystal X-ray diffraction analysis. The trans-AT(2-) anions in 1-6 show four different coordinating modes. In 1-6, the unusual face-to-face AT(2-)-AT(2-) pi(-)-pi(-) stacking interactions with the separated interplanar center to center distances ranging from 3.32 to 4.0 angstrom were observed. The pi(-)-pi(-) stacking, hydrogen bonding and electrostatic interactions are responsible for the stabilization of these 3D supramolecular structures. The aqueous solutions of 1-6 with the concentration of 2 x 10(-5) mol L-1 show photochromism that originates from the trans-cis isomerisation of AT(2-) anionic ligands. The onion cell imaging with 1-6 as labels has also been reported.