Potential Precursors for Terminal Methylidene Rare-Earth-Metal Complexes Supported by a Superbulky Tris(pyrazolyl)borato Ligand

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 SiMe3X (X=Cl, I) gave [Tp(tBu,Me)LnX(2)] in high yields. The addition of only one equivalent of SiMe3Cl 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=CH2Ph, CH2SiMe3) through salt metathesis led to the monomeric mixed-alkyl derivatives [Tp(tBu,Me)LuMe(CH2SiMe3)] and [Tp(tBu,Me)LuMe(CH2Ph)], respectively, in good yields. The SiMe4 elimination protocols were also applicable when using SiMe3X 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.