Werner clathrates with enhanced hydrogen bonding functionality

Abstract

The role of hydrogen bonding to generate molecular frameworks for selective enclathration of selected, closely related alcohols, carbonyls and water by three Ni(II) isothiocyanate based Werner hosts was scrutinized and their generation of crystal structures elucidated. Structures of host:guest compounds of H1, Ni(NCS)2(nicotinamide)4, with five alcohols were compared with five guests with carbonyl functionalities. In both cases, host columns formed by interaction via hydrogen bonded amide tetramers. However, the hydrogen bond between the guest alcohol functional group and thiocyanato sulphur showed a more robust structure than those with intercalated carbonyl guests found in layers between sheets of host molecular bands. A predictable framework of H1 host led to architectures with analogous patterns. Physical properties differed according to their hydrogen bonding capabilities. Selectivity was achieved for the separation of a pair of alcohols (1-butanol and 2- pentanol) and a pair of carbonyls (3- and 4-methylcyclohexanone), both difficult to separate due to similar physical properties. The positive discrimination characteristics of host H1 towards one of a pair of alcohols and carbonyl isomers have given possibilities for future industrial prospects. Host H2, Ni(NCS)2(isonicotinamide)4, presented three different tubulate packing arrangements, with the guests (H2O, ethanol and self-included ligand isonicotinamide) occupying channels in the structure. Hydrogen bonding between host carbonyl oxygens and amides formed molecular frameworks into which the guest was included. Mixed-ligand host H3, Ni(NCS)2(nicotinamide)2(isonicotinamide)2 with inclusion of water and methanol, confirmed our findings of hydrogen bonded frameworks with the two different ligands in H1 and H2; nicotinamide ligands formed amide tetramers, whereas the isonicotinamide ligands formed a variety of discrete hydrogen bonds with neighbouring molecules. The host networks formed porous materials for inclusion, giving the advantage of ‘tunability’, and pores with different size and nature available to accommodate guest molecules and to separate isomers.