Crystal Engineering with Piperazinediones

Abstract

Non covalent interactions are valuable tools for crystal engineering. Hydrogen bonding often plays a central role for molecular association among possible non covalent interactions. Together with hydrogen bonding, arene-arene interactions and van der Waals interactions can control crystal assembly. Understanding non covalent interactions permits the design of molecules whose functionalities can interact non covalently so that molecules will pack in a predicted fashion. For two decades Prof. Mash's group has been studying crystal packing based on a piperazinedione core scaffold which can have three orthogonal non covalent interactions and lead to controlled three dimensional crystal packing. Alkoxy-substituted piperazinediones were previously studied for crystal packing and liquid crystal properties. It was found that alkoxy piperazinediones pack in three dimesions as designed and exhibit interesting thermochemical properties. Given that small changes in structure can cause large changes in packing and liquid crystal properties, the replacement of alkoxy groups with alkyl groups in molecules provides an opportunity to investigate the role of oxygen in crystal packing and liquid crystal properties. A series of alkyl piperazinediones was synthesized in a convergent way where an intermediate tetrabromide was converted into a series of tetra alkyne piperazinediones, then into tetra alkyl piperazinediones. This approach overcame limitations in the synthesis of alkoxy piperazinediones, where every target molecule requires 10 to 11 steps starting from 2,3-dimethylbenzene-1,4-diol (Scheme 2.1). Crystal structure analyses were done for three different piperazinediones. It appears that crystal packing of alkyl piperazinediones mimics that of alkoxy piperazinediones.