Compounds of the formula I ##STR1## wherein R.sub.1 represents a hydrogen or fluorine atom, and R.sub.2 represents an alkyl group containing 2 to 6 carbon atoms, or a cycloalkyl or cycloalkyl-alkyl group containing 5 to 8 carbon atoms, each of these groups being substituted by one or two hydroxy groups or an oxo group and, if desired, also by a 4-(m-trifluoromethylphenyl)-piperazino or 4-(3-trifluoromethyl-4-fluoro-phenyl)-piperazino group with the proviso that, when R.sub.1 represents a hydrogen atom, R.sub.2 does not represent a monohydroxyalkyl group, and their pharmaceutically acceptable acid addition salts are novel and have therapeutic use, notably in the region of the central nervous system and in the cardiovascular region.