of the Periodic Table of the Elements; preferably R8 is hydrogen or a linear or branched, saturated or unsaturated C1-C20-alkyl; R9, equal to or different from each other, is a hydrogen atom or a linear or branched, saturated or unsaturated C1-C20-alkyl, C3-C20-cycloalkyl, C6-C20-aryl, C7-C20-alkylaryl, or C7-C20-arylalkyl radical, optionally containing one or more heteroatoms belonging to groups