R6 and R7 independently of one another particularly preferably represent hydrogen, in each case optionally mono- to hexa-fluoro- or -chloro-substituted C1 -C6 -alkyl, C3 -C6 -cycloalkyl, C1 -C6 -alkoxy, C3 -C6 -alkenyl or C1 -C6 -alkoxy-C2 -C6 -alkyl, represent phenyl which is optionally substituted once or twice by fluorine, chlorine, bromine, -C1 -C5 -halogenoalkyl-, C1 -C5 -alkyl- and/or C1 -C5