Fathoming Synthesis Strategies for Rare-Earth-Metal Trimethyl Compounds

Abstract The synthesis of alkali-metal-free compounds [LnMe3]n is scrutinized using two distinct protocols. The donor-promoted cleavage of homoleptic tetramethylaluminate complexes Ln(AlMe4)3 proves rare-earth metal (Ln) size-dependent and gives access to [LnMe3]n (small to midsized Ln(III)) and [LnMe3-x(AlMe4)x(OEt2)y] (large Ln(III)). Both compounds can be converted to homoleptic tetramethylgallates Ln(GaMe4)3 by addition of trimethylgallium. Crystallization attempts of [LnMe3-x(AlMe4)x(OEt2)y] via extended donor-solvent treatment produced structural snapshots of a diverse set of clusters featuring different stages of methyl group degradation. New homometallic coordination motifs are detected for carbides [(C4−)(Ln3+)4] (Ln = La, Nd) and [(C4−)(Gd3+)6] as well as methylidyne [(CH3−)(Nd3+)5]. Applying the salt-metathesis protocol (Me3TACN)LnCl3(THF)x (x = 0, 1)/MeLi the discrete monomeric 6-coordinate (Me3TACN)LnMe3 (Ln = Dy, Er, Tm) and 7-coordinate (Me3TACN)LnMe3(THF) (Ln = La, Ce) can be obtained (Me3TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane). Incomplete Cl/CH3 exchange to afford mixed methyl/chloride complexes (Me3TACN)LnMe3-xClx was observed for praseodymium, neodymium, and samarium. Crystal structures of most of the new complexes have been obtained and are discussed along with the findings from NMR studies.