and Z is a halogen and R1 and R2, which may be identical or different, are each hydrogen, an alkyl group, a mono- or polyhydroxylated alkyl group, an alkyl group substituted with an alkoxy or hydroxyalkoxy group, or an aminoalkyl group, the amino group of which is optionally substituted with one or two alkyl or hydroxyalkyl groups, all the above alkyl groups or moieties containing from 1 to 6 carb