R3 and R4 are identical or different and are a halogen atom, a C1 -C10 -alkyl group, which can be halogenated, a C6 -C10 -aryl group or an --NR2 10, --SR10, --OSiR3 10, --SiR3 10 or --PR2 10 radical in which R10 is a halogen atom, a C1 -C10 -alkyl group or a C6 -C10 -aryl group,