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

Abstract

Six 5,5′-azotetrazolate based supramolecular complexes of {[Co(en)₃]₂(AT)₃·3H₂O} (1), {[M(en)₂(trans-AT)₂](trans-H₂en)·2H₂O} (M = Ni, 2; Cu, 3), {[Ni(en)₂(trans-AT)](trans-AT)(trans-H₂en)·2H₂O} (4), {[Cu(en)₂(trans-AT)]·2H₂O} (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-AT²⁻ anions in 1–6 show four different coordinating modes. In 1–6, the unusual face-to-face AT²⁻–AT²⁻ π⁻–π⁻ stacking interactions with the separated interplanar center to center distances ranging from 3.32 to 4.0 Å were observed. The π⁻–π⁻ 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 × 10⁻⁵ mol L⁻¹ show photochromism that originates from the trans–cis isomerisation of AT²⁻ anionic ligands. The onion cell imaging with 1–6 as labels has also been reported.