**4. Conclusions**

The coordination of two pyridylbenzoate ligands to cobalt(II) and zinc(II) metal centres formed isostructural {[Co(34pba)(44pba)]·DMF}n (**1**) and {[Zn(34pba)(44pba)]·DMF}n (**3**) compounds. Using an acetonitrile/water mixture instead of a DMF/ethanol solvent system led to a framework isomorphous to **1**, {[Co(34pba)(44pba)]·(C3H6O)}n (**2**) where acetonitrile had undergone hydrolysis and ketonization to produce the gues<sup>t</sup> acetone (C3H6O). These MOFs retain their phase and crystallinity (**1d** and **3d**) after the removal of gues<sup>t</sup> molecules under vacuum. Both **1d** and **3d** took up chlorinated and amine VOCs and showed potential selectivity in the sorption of binary chlorinated solvent mixtures, with preference for dichloromethane and chloroform. The activated MOFs had a higher sorption capacity for amine VOCs, which was attributed to their stronger intermolecular interactions with the framework. The sorption of chlorinated VOCs did not affect the crystallinity of the frameworks while some amine VOCs led to new phases in **3d** and amorphous phases in **1d**. The crystalline phase **1d** could be recovered from these amorphous phases on desolvation under vacuum. Characteristic solvatochromism was observed in **1d** on sorption of water or ammonia. The desorption of these two guests led to a new phase, which was reversible for both colour and crystallinity. The activation energy associated with the removal of DMF, H2O, and NH3 from MOFs **1d** and **3d** was determined and found to be comparable with previous systems studied. This study shows the potential of the synthesised MOFs having selectivity to take up gues<sup>t</sup> molecules and undergo colour changes depending on the chemical and physical properties of the gues<sup>t</sup> molecules. Therefore, studies on these MOFs for sensing and separation applications are ongoing.

**Supplementary Materials:** Supplementary data (crystal structure data, PXRD, thermal analysis, NMR) are available online at http://www.mdpi.com/2624-8549/1/1/9/s1. Crystallographic data for this paper have been deposited with the CCDC, accession numbers 1935229-1935234. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data\_request/cif or by emailing data\_request@ccdc.cam.ac.uk.

**Author Contributions:** Conceptualization S.A.B.; synthesis, crystallography, thermal analysis C.A.N.; assistance with crystallography and analysis S.C.Z. and C.L.O.; writing—original draft preparation C.A.N.; writing—review and editing S.A.B., S.C.Z. and C.L.O.; supervision S.A.B. and C.L.O.

**Funding:** This research was funded by the National Research Foundation of South Africa, gran<sup>t</sup> number 111699.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
