A compound of general formula (I): ##STR1## wherein R1 is hydrogen, alkyl or optionally-substituted aralkyl; R2 is hydrogen or alkyl; Y is hydrogen, halogen, nitro, C1-4 alkyl, C1-4 alkoxy or optionally substituted amino, and n is 1 or 2; each of the groups Y being the same or different when n is 2 and Y being other than hydrogen or halogen when R1 and R2 are both hydrogen; or a salt or metal comp