Potential Precursors for Terminal Methylidene Rare-Earth-Metal Complexes Supported by a Superbulky...

A series of solvent-free heteroleptic terminal rare-earth-metal alkyl complexes stabilized by a superbulky tris(pyrazolyl)borato ligand with the general formula [Tp^tBu,Me LnMeR] have been synthesized and fully characterized. Treatment of the heterobimetallic mixed methyl/tetramethylaluminate compounds [Tp^tBu,Me LnMe(AlMe₄ )] (Ln=Y, Lu) with two equivalents of the mild halogenido transfer reagents SiMe₃ X (X=Cl, I) gave [Tp^tBu,Me LnX₂ ] in high yields. The addition of only one equivalent of SiMe₃ Cl to [Tp^tBu,Me LuMe(AlMe₄ )] selectively afforded the desired mixed methyl/chloride complex [Tp^tBu,Me LuMeCl]. Further reactivity studies of [Tp^tBu,Me LuMeCl] with LiR or KR (R=CH₂ Ph, CH₂ SiMe₃ ) through salt metathesis led to the monomeric mixed-alkyl derivatives [Tp^tBu,Me LuMe(CH₂ SiMe₃ )] and [Tp^tBu,Me LuMe(CH₂ Ph)], respectively, in good yields. The SiMe₄ elimination protocols were also applicable when using SiMe₃ X featuring more weakly coordinating moieties (here X=OTf, NTf₂ ). X-ray structure analyses of this diverse set of new [Tp^tBu,Me LnMeR/X] compounds were performed to reveal any electronic and steric effects of the varying monoanionic ligands R and X, including exact cone-angle calculations of the tridentate tris(pyrazolyl)borato ligand. Deeper insights into the reactivity of these potential precursors for terminal alkylidene rare-earth-metal complexes were gained through NMR spectroscopic studies.