Tris(pyrazolyl)methanide Complexes of Trivalent Rare-Earth Metals

Three types of trivalent rare-earth-metal complexes supported by a monoanionic tris­(pyrazolyl)­methanide ligand were synthesized and structurally characterized, and the catalytic activity of the dialkyl derivatives for isoprene polymerization was investigated. Reactions of the lithium salt of tris­(3,5-dimethylpyrazolyl)­methanide LLi­(THF) with 1 equiv of ScCl₃(THF)₃, YCl₃, or LuCl₃ in THF provided the ion-pair complexes [LLnCl₃]­[Li­(THF)₄] (Ln = Sc (1), Y (2), Lu (3)). Dialkyl complexes LLn­(CH₂SiMe₃)₂(THF) (Ln = Y (4), Lu (5)) were prepared by salt metathesis of LLi­(THF) with 1 equiv of [Y­(CH₂SiMe₃)₂(THF)₃]­[BPh₄] or [Lu­(CH₂SiMe₃)₂(THF)₃]­[BPh₄] in toluene. Reaction of 5 with PhSiH₃ provided the unexpected alkylidene-bridged dinuclear complex L₂Lu₂(μ-η¹:η¹-3,5-(CH₃)­C₃HN₂)₂(μ-CHSiMe₃) (6). Complexes 1–6 were structurally characterized by single-crystal X-ray diffraction, showing that the tris­(pyrazolyl)­methanide ligand acts as a κ³-coordinating six-electron donor in all complexes. The dialkyl complexes catalyzed 1,4-cis polymerization of isoprene with high selectivity upon activation with borate and alkylaluminum.