A crystallographic and solid-state NMR study of 1,4-disubstituted 2,5-diketopiperazines

Abstract p-Disubstituted phenyldiketopiperazines 1 (R = H), 2 (R = NO2) and 3 (R = -N(CH3)2) were synthesized and characterized by NMR in solution and IR spectroscopy. The identity of the compounds was confirmed and their fragmentation analyzed by ESI-MS and HRMS spectrometry. X-ray single crystal structures revealed that the three compounds crystallize in space groups P21 (1), Pbca (2) and P21/c (3), respectively. The solid-state structures of 1-3 were further analyzed by a combination of solid-state NMR spectroscopy and GIPAW calculations. The NMR crystallography approach was used to analyze symmetry breaking of nearly centrosymmetric molecules in 1, and disorder of piperazine groups in crystal structure of 3.