prov:value
| - A compound as claimed in claim 1, wherein R3 and R4 are identical or different and are a hydrogen atom, a fluorine, chlorine or bromine atom, a C1 -C4 -alkyl group which may be halogenated, a C6 -C8 -aryl group, a --NR2 15, --SR15, --OSiR3 15, --SiR3 15 or --PR2 15 radical in which R15 is a chlorine atom, or a C1 -C3 -alkyl group or a C6 -C8 -aryl group.
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