Compounds of formula I are described herein which are agonists or partial agonists of the 2C subtype of brain serotonin receptors. or pharmaceutically acceptable salts thereof, wherein each of R1, R2, R3, R4, Rx, Ry, and n are as defined herein. Such compounds, and compositions thereof, are useful for treating a variety of central nervous system disorders such as schizophrenia.