Studies on arene interactions in flexible pyrazolo[3,4-d]pyrimidine core based symmetrical 'propylene/Leonard linker' models: X-ray crystallographic evidence for disappearance of intramolecular stacking due to presence of chloro- or cyano-groups in place

Proton NMR analysis of three new symmetrical 'trimethylene linker' compounds 1,3-bis(4-chloro-6-methylsulfanyl-lH-pyrazolo[3,4-d]pyrimidin-1-yl)propane (6), 1,3-bis(4-cyano-6-methylsulfanyl-lH-pyrazolo[3,4-d]pyrimidin-1-yl)propane (7) and 1,3-bis(4,6-dimethylsulfanyl-lH-pyrazolo[3,4-d]pyrimidin-1-yl)propane (8) show intramolecular stacking in solution like parent compound 1,3-bis(4,6-dimethylsulfanyl-lH-pyrazolo[3,4-d]pyrimidin-1-yl)propane (1a). Surprisingly, X-ray crystallographic analyses of the first two compounds having chloro- (6) or cyano- (7) groups do not show intramolecular stacking due to arene interaction present in parent compound (1a). In contrast, X-ray crystallographic analysis of the third compound (8) having four electron donating methoxy groups in place of methylsulfanyl of parent compound (1a) shows presence of intramolecular stacking like parent compound (1a). Supramolecular structures of these three compounds (6-8), differing only in substituents, are stabilized by weak intermolecular π-π, Cl…π, S…π, S…Cl…N, C-H…Cl, C-H…N and C-H…S interactions.