New Ionic Half-Metallocenes of Neodymium
Résumé
The potential of lanthanide-based compounds in catalysis has been steadily increasing in the last few years, particularly in the field of polymerization reactions [1]. In this context, the half-metallocene framework has attracted much less attention than the classical metallocenes and the more recently developped post-metallocenes. Despite very promising results [2], their potential in polymerization has only been little explored, mostly because the half-lanthanidocene precursors have been until now rather difficult to prepare [3]. We have shown recently that borohydrido half-lanthanidocenes could be isolated by ionic metathesis [4], but alternative methods with better access to catalytically relevant species are still highly desirable. In this contribution, we will describe an unprecedented method which allowed us to succeed in the one-pot and high yield synthesis of a series of new half-metallocenes of neodymium of general formula [CpRNd(BH4)3]2[Mg(THF)6] (CpR is a cyclopentadienyl-type ligand). X-Ray structural determination shows that such complexes are ionic, the neodymium atom displays a tetragonal piano stool geometry, with three tridentate BH4 ligands. In combination with stoechiometric amounts of dialkylmagnesium, the title complexes afford very efficient single-site catalysts towards isoprene and styrene polymerization. The polymerization is quasi-living and highly stereoselective.