Hydrogen bonding interactions between ions: A powerful tool in molecular crystal engineering

Hydrogen bonding interactions are the strongest of the non-covalent interactions and are highly directional (hence transportable and reproducible). With respect to hydrogen bonds between neutral molecules the hydrogen bonding interactions between ions (inter-ionic hydrogen bonds) respond to additional energetic and topological constrains that depend on the convolution of the proton donor-proton acceptor interactions with the Coulombic field generated by the presence of ions. Directionality and strength are exploited in the design of molecular crystals, hence in molecular crystal engineering strategies. Molecular crystal engineering is the planning and utilisation of crystal-oriented syntheses for the bottom-up construction of functional molecular solids from molecules and ions. The success of crystal engineering strategies depends on the availability of robust and transferable interactions to glue together construction materials. This chapter is devoted to an important subset of non-covalent interactions, namely those involving hydrogen bonding and pi-stacking interactions between ions. Some relevant analogies and differences between organic-type intermolecular interactions and those in which metal atoms are involved will be outlined. Selected examples of the utilization of inter-ionic hydrogen bonding interactions in crystal reactivity will also be described.