Werner complexes : synthesis, structure and enclathration properties

Abstract

Crystal engineering methods were used to combine Werner complexes with one or more components in a single solid form, with well-defined stoichiometry in order to create Werner clathrates and investigate their selectivity towards similar xylene isomers. Two families of six hosts, zinc tetrahedral ZnCl2L2 [H1 (L=nicotinamide), H2 (L=isonicotinamide) and H3 (L=mixed ligands, nicotinamide and isonicotinamide)] and cobalt octahedral Co(NCS)2L4 [H4 (L=nicotinamide), H5 (L=isonicotinamide) and H6 (L=mixed ligands nicotinamide and isonicotinamide)], were designed and synthesised. They were used in the formation of three types of complexes upon the crystallisation process with guests and/or solvents. These are Werner clathrate and mixed ligand complexes. Three types of complexes were prepared in the same process of crystallisation with different solvents, giving ten crystal structures. These produced thereof two complexes made from the zinc tetrahedral complex H2c and H3c, six Werner clathrates H1•nic, H4•propOH, H4•nic•H2O, H5•2isonic, H5•2isonic•propOH and H6•H2O from both zinc and cobalt complexes and two mixed ligand cobalt complexes H7 and H8. In four crystal structures, nicotinamide or isonicotinamide self-included as guests to form Werner clathrate complexes which contained one or two independent enclathrated nicotinamide or isonicotinamide molecules. This occurred serendipitously, although this process has previously been reported. Werner clathrates and complexes were linked into a three-dimensional network via intermolecular interactions such as N-H•••O, O-H•••S and N-H•••Cl. However, an intramolecular interaction was observed in H4•propOH via N-H•••O which affected the flexibility in the torsion angles of H4•propOH complexes. The Werner clathrate complexes synthesised from H4 and H5 presented similar arrangement and channels in their packing diagrams, although they were not isostructural. H7 and H8 crystal structures resulted from the substitution of two nicotinamide ligands by two solvent DMSO and methanol molecules respectively. This exchange of ligands in H4 and H6 complexes occurred during the evaporation procedure and was facilitated by the stronger pKa value and dipole moment of the DMSO or methanol ligands, compared with those of nicotinamide. All structures were elucidated via single crystal X-ray diffraction (SCXRD), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Powder X-ray diffraction (PXRD) scrutiny was used to compare differences between hosts, starting material and crystal structures. The study of the zinc complexes (H1•nic, H2c and H3c) indicated the importance of the halide interactions revealed in the molecular structure and the importance of stabilising frameworks of the structure. Similarly, the thiocyanato sulfur provided interactions between N-H•••S and O-H•••S in H4, H5 and H6 which also provided a stabilising effect on the framework of these crystal structures. Although all six host Werner complexes were exposed to the xylene isomers, H6 differentiated towards ortho-xylene from a 50:50 ortho/meta-xylene mixture. The selectivity of ortho-xylene was confirmed by single crystal X-ray analysis and the complex properties were shown by TG, Hirshfeld surface and fingerprint analysis.