R10 is a halogen atom, a C1 -C10 -alkyl group or a C6 -C10 -aryl group, R5 and R6 are identical or different and have the meaning given for R3 and R4, with the proviso that R5 and R6 are not hydrogen, R7 is ##STR10## in which R11, R12 and R13 are identical or different and are a hydrogen atom, a halogen atom a C1 -C10 -alkyl group, a C1 -C10 -fluoroalkyl group, a C6 -C10 -aryl group, a C6 -C10 -f