A CdSO₄‑Type 3D Metal–Organic Framework Showing Coordination Dynamics on Cu²⁺ Axial Sites: Vapochromic Response and Guest Sorption Selectivity

Abstract

A unique 2-fold interpenetrated CdSO₄ coordination network of the formula {[Cu₂(4-pmpmd)₂(CH₃OH)₄(opd)₂]·2H₂O}_<i>n</i> [4-pmpmd = <i>N</i>,<i>N</i>′-bis­(4-pyridylmethyl)­phenyldiimide; opd = <i>o</i>-phthalic acid] has been synthesized and characterized by IR spectra, thermogravimetric (TG) analyses, elemental analyses, and single crystal and powder X-ray diffraction methods. The metal–organic framework (MOF) exhibits reversible dehydration and rehydration in a single-crystal-to-single-crystal (SC–SC) process. Moreover, the dehydrated material, having coordinatively unsaturated Cu²⁺ sites, can encapsulate CH₃OH molecules with a color change, again in a reversible SC–SC fashion, and shows selective adsorption of CO₂ over N₂ and H₂. This feature of obvious color variation induced by the presence of small hydroxylic molecules is highly promising for detecting hydroxylic molecules through a simple sensing mechanism. In addition, the MOF selectively interacts with hydroxylic guests and shows sorption selectivity for water, methanol, ethanol, and <i>n</i>-propanol over benzene guests. Notably, this compound shows complete selectivity in adsorption for <i>n</i>-propanol over 2-propanol owing to the effect of shape exclusion