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

Abstract

A series of solvent‐free heteroleptic terminal rare‐earth‐metal alkyl complexes stabilized by a superbulky tris(pyrazolyl)borato ligand with the general formula [TptBu,MeLnMeR] have been synthesized and fully characterized. Treatment of the heterobimetallic mixed methyl/tetramethylaluminate compounds [TptBu,MeLnMe(AlMe4)] (Ln=Y, Lu) with two equivalents of the mild halogenido transfer reagents SiMe3X (X=Cl, I) gave [TptBu,MeLnX2] in high yields. The addition of only one equivalent of SiMe3Cl to [TptBu,MeLuMe(AlMe4)] selectively afforded the desired mixed methyl/chloride complex [TptBu,MeLuMeCl]. Further reactivity studies of [TptBu,MeLuMeCl] with LiR or KR (R=CH2Ph, CH2SiMe3) through salt metathesis led to the monomeric mixed‐alkyl derivatives [TptBu,MeLuMe(CH2SiMe3)] and [TptBu,MeLuMe(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 [TptBu,MeLnMeR/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.

Mixed Methyl Aryloxy Rare-Earth-Metal Complexes Stabilized by a Superbulky Tris(pyrazolyl)borato Ligand

“…Apparently, the CF 3 substituents in the peripheral 3- and 5-positions exhibit the least interactions with the ancillary ligand. Cone angles of similar complexes of lutetium were already reported to range from 277.1° to 280.9° . This compares well with the mixed methyl aryloxy rare-earth-metal complexes under study.…”

“…The Ln–Tp t Bu,Me entity exhibits one significantly elongated Lu–N­(pz) bond of 2.626(3) Å compared to the respective precursor, which nicely displays the coordination flexibility of the scorpionate ligand. The Ln–C­(Me) bond length of 2.338(4) Å of five-coordinate complex 4-Lu compares well with those of similar methyl complexes, e.g., Tp t Bu,Me LuMe 2 (2.364(3)/2.375(2) Å), but is slightly elongated compared with the methyl triflate complex Tp t Bu,Me LuMe­(OTf) (2.327(4) Å), as well as [(DippN) 2 C] 2 LuMe (2.314(3) Å) . Additionally, the Ln−O distance of 2.092(2) Å of 4-Lu is slightly elongated compared to the range observed for 4-coordinate Lu­(OC 6 H 2 t Bu 2 -2,6) 2 (AlMe 4 ) (1.999(3) and 2.006(3) Å) and homoleptic “3-coordinate” Lu­(OC 6 H 3 t Bu 2 -2,6) 3 (2.02 Å) …”

“…We have shown previously that scorpionate-supported rare-earth-metal methyl complexes Tp t Bu,Me LuMe 2 and Tp t Bu,Me Ln­(AlMe 4 )­Me give access to mixed methyl alkyl Ln­(III) complexes Tp t Bu,Me LuRMe (R = CH 2 SiMe 3 , CH 2 Ph) . Therefore, it was reasoned that the role of these versatile precursors might be expanded to selective protonolyses with phenols as well as salt metatheses with potassium phenolates, respectively.…”

“…Trofimenko’s unique Tp scorpionate ligands , seem especially suited for studying the reactivity of such [(L)­LnMe 2 ] dimethyl compounds. Only very recently, we reported on a series of different mixed alkyl lutetium complexes Tp t Bu,Me LuRMe with R representing alkyl (CH 2 SiMe 3 , CH 2 Ph) or weakly coordinating groups (chlorido, SO 3 CF 3 or N­(SO 2 CF 3 ) 2 ) . The latter complexes were obtained from Tp t Bu,Me LuMe 2 by applying SiMe 4 -elimination and salt-metathesis protocols.…”

“…We have shown previously that scorpionate-supported rare-earth-metal methyl complexes Tp tBu,Me LuMe 2 and Tp tBu,Me Ln(AlMe 4 )Me give access to mixed methyl alkyl Ln(III) complexes Tp tBu,Me LuRMe (R = CH 2 SiMe 3 , CH 2 Ph). 35 Therefore, it was reasoned that the role of these versatile precursors might be expanded to selective protonolyses with phenols as well as salt metatheses with potassium phenolates, respectively. We set out to treat Tp tBu,Me LuMe 2 , supposedly the most reactive because sterically least hindered, with various alkyl-substituted phenol derivatives HOAr (HOC 6 H 3 Me 2 -2,6, HOC 6 H 3 iPr 2 -2,6, HOC 6 H 3 (CF 3 ) 2 -3,5, HOC 6 H 2 tBu 2 -2,6-Me-4) (Scheme 1).…”