Molecular Solids from Symmetrical Bis(piperazine-2,5-diones) with Open and Closed Monomer Conformations.

The design, synthesis and solid state structures of a new class of xylylene-linked bis(1,4- piperazine-2,5-diones) are reported in an effort to extend the molecular framework of piperazine-2,5-diones. These compounds were derived from piperazine-2,5-dione as the core structure, synthesized via a new efficient route, and their crystal structures were determined. We examined the effects of side chain substitution on conformations of the linked bis-DKPs. Crystallization of 3,3'-[1,4-phenylenebis(methylene)]-bis[6-(hydroxymethyl)-1,4-dimethylpiperazine-2,5-dione] yielded molecular solids with an unusual network of "C"-shaped monomers held together by four intermolecular hydrogen bonds per asymmetric unit. Similarly, intermolecular interactions between the iodomethyl groups in 3,3'-[1,4-phenylenebis(methylene)]-bis[6-(iodomethyl)-1,4-dimethyl-piperazine-2,5-dione] result in the monomers adopting a "C"-shape in the solid state. Assembly of the monomers with side chains converted to methyl groups or tert-butyldimethylsilyl ethers, thereby lacking these stabilizing intermolecular interactions, results in an infinite array of "S"-shaped conformations. These results suggest that the interplay between the attractive intermolecular interactions and repulsive steric interactions of the substituents at the C6 and C6' positions of the diketopiperazine rings is important in determining the solid-state conformations of xylylene-linked bis(piperazine-2,5-diones).