Adsorption of water induces a reversible structural phase transition and colour change in new nickel(ii) macrocyclic complexes forming flexible supramolecular networks

Abstract

Two novel 3D flexible supramolecular networks [Ni₂(tpmc)(μ-NO₃)](NO₃)₃(H₂O)₂[LiNO₃·3H₂O] (1) and [Ni₂(tpmc)(μ-NO₃)]₂[Ni₂(tpmc)(μ-NO₃)Li(NO₃)₂OH]₂(LiOH(H₂O)₃)₂(NO₃)₈(H₂O)₁₃·nH₂O (2) (tpmc, 1,4,8,11-tetrakis-(2-pyridylmethylene)-1,4,8,11-tetraaza cyclotetradecane) were synthesized and characterized by spectroscopic and X-ray techniques. Their single crystal X-ray diffraction analysis showed that, in both compounds, nickel(II) ions are in an octahedral environment, due to an unusual binding mode of nitrate, acting either as a tetradentate ligand in 1, or as a pentadentate ligand in 2. The blue compound 1 adsorbs water leading to the violet compound 2. Adsorption of water leads to a reversible crystal-to-crystal structural phase transition, from the monoclinic structure of 1 to the triclinic structure of 2, a phenomenon that was corroborated by XRPD. This phase transition is accompanied by a reversible reaction of ligand exchange responsible for the color-change observed in the compounds, a property which might be useful for the development of molecular sensors. Adsorption–desorption isotherms recorded at low and high pressures indicated that 1 has capacity for adsorption of gases, such as dinitrogen, dioxygen, or carbon dioxide. Besides, dynamic water vapour isotherms confirm the chemisorption, highly selective for water vapor. This is the first time a Ni(II)–tpmc complex has been observed to be involved in a reversible gas adsorption process; furthermore, herein, we report the first crystal structures of six-coordinated nickel(II) complexes with tpmc.