Compounds of formula (I) and salts are provided: wherein R.sup.4 is fluoro, R.sup.5 is selected from hydrogen, halogen, cyano, C.sub.1-6alkyl, C.sub.1-6alkyl substituted with one or more fluorine atoms, C.sub.1-6alkoxy, and C.sub.1-6alkoxy substituted with one or more fluorine atoms; and R.sup.6 is selected from halogen, cyano, C.sub.1-6alkyl, C.sub.1-6alkyl substituted with one or more fluorine atoms, C.sub.3-6cycloalkyl, C.sub.3-6cycloalkyl substituted with one or more fluorine atoms, C.sub.1-6alkoxy and C.sub.1-6alkoxy substituted with one or more fluorine atoms, and Q is hydrogen or C.sub.1-6alkyl. The compounds are expected to be useful for therapy, for example in the treatment of psychotic disorders and cognitive impairment.