Hydrogen-bonded chains in isostructural 5-methyl-2-(4-methylphenyl)-7,8-dihydro-6H-cyclopenta[g]pyrazolo[1,5-a]pyrimidine, 2-(4-chlorophenyl)-5-methyl-7,8-dihydro-6H-cyclopenta[g]pyrazolo[1,5-a]pyrimidine and 2-(4-bromophenyl)-5-methyl-7,8-dihydro-6H-cyclopenta[g]pyrazolo[1,5-a]pyrimidine, and sheets of pi-stacked hydrogen-bonded chains in 2-(4-methoxyphenyl)-5-methyl-7,8-dihydro-6H-cyclopenta[g]pyrazolo [1,5-a]pyrimidine

5-Methyl-2-(4-methylphenyl)-7,8-dihydro-6H-cyclopenta[ g]pyrazolo[ 1,5-a] pyrimidine, C17H17N3, 2-(4-chlorophenyl)-5-methyl- 7,8-dihydro-6H-cyclopenta[g] pyrazolo[1,5-a] pyrimidine, C16H14ClN3, and 2-(4-bromophenyl)-5-methyl-7,8-dihydro-6H- cyclopenta[g] pyrazolo[ 1,5- a] pyrimidine, C16H14BrN3, are isostructural; in each compound, the molecules are linked into chains by a single C - H center dot center dot center dot pi(arene) hydrogen bond. Molecules of 2-(4-methoxyphenyl)-5-methyl- 7,8-dihydro-6H-cyclopenta[ g] pyrazolo[ 1,5- a] pyrimidine, C17H17N3O, are linked by a single C - H center dot center dot center dot N hydrogen bond into chains, which are themselves linked into sheets by a pi-pi stacking interaction.