**Solvatochromism and Selective Sorption of Volatile Organic Solvents in Pyridylbenzoate Metal-Organic Frameworks**

### **Christophe A. Ndamyabera, Savannah C. Zacharias, Clive L. Oliver and Susan A. Bourne \***

Centre for Supramolecular Chemistry Research, Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa

**\*** Correspondence: susan.bourne@uct.ac.za

Received: 20 July 2019; Accepted: 13 August 2019; Published: 15 August 2019

**Abstract:** Using cobalt(II) as a metal centre with di fferent solvent systems a fforded the crystallization of isomorphous metal-organic frameworks {[Co(34pba)(44pba)]·DMF}n (**1**) and {[Co(34pba)(44pba)]·(C3H6O)}n (**2**) from mixed 4-(4-pyridyl)benzoate (44pba) and 3-(4-pyridyl)benzoate (34pba) ligands. Zinc(II) under the same reaction conditions that led to the formation of **1** formed an isostructural {[Zn(34pba)(44pba)]·DMF}n framework (**3**). Crystal structures of all three MOFs were elucidated and their thermal stabilities were determined. The frameworks of **1**, **2**, and **3** were activated under vacuum to form the desolvated forms **1d**, **2d**, and **3d**, respectively. PXRD results showed that **1d** and **2d** were identical, consequently, **1d** and **3d** were then investigated for sorption of volatile organic compounds (VOCs) containing either chloro or amine moieties. Thermogravimetric analysis (TGA) and nuclear magnetic resonance (NMR) were used to determine the sorption capacity and selectivity for the VOCs. Some sorption products of **1d** with amines became amorphous, but the crystalline framework could be recovered on desorption of the amines. Investigation of the sorption of water (H2O) and ammonia (NH3) in **1d** gave rise to new phases identifiable by means of a colour change (solvatochromism). The kinetics of desorption of DMF, water and ammonia from frameworks **1d** and **3d** were studied using non-isothermal TGA. Activation energies for both cobalt(II) and zinc(II) frameworks are in the order NH3 < H2O < DMF, with values for the **1d** analogue always higher than those for **3d**.

**Keywords:** metal-organic frameworks; vapour sorption; solvatochromism; desorption kinetics
