--B-- wherein R' stands for a group of the following formula ##STR23## wherein R.sup.11 and R.sup.12 denote, independently of one another, ahalogen atom, for example a fluorine, chlorine or bromine atom, a straight chained C.sub.1 -C.sub.3 -alkyl group, for example a methyl or ethyl group, or a straight chained C.sub.1 -C.sub.3 -alkoxy group, for example a methoxy or ethoxy group.