The present invention provides a compound of formula (I) or a salt or solvate thereof, wherein: R<1> is hydrogen or C1-4alkyl R<2> and R<4> are each independently selected from C1-4 alkyl, [<11>C] C1 4alkyl, and [<18>F]-C1-4 fluoroalkyl provided that at least one of R<2> and R<4> is [<11>C] C1 4alkyl or [<18>F]-C1-4 fluoroalkyl and R<3> is halo. Such compounds having use for imaging central nervous system receptors.
