Cationic rare-earth-metal methyl complexes: a new preparative access exemplified for Y and Pr

Abstract

A new procedure for the generation of cationic methyl complexes of rare-earth metals has been developed for yttrium and praseodymium as two examples from the rare-earth series. The reactions of the rare-earth-metal tetramethylaluminates [Ln(AlMe(4))(3)] of yttrium and praseodymium with three differently sized crown ethers in thf, namely [12]crown-4, [15]crown-5 and [18]crown-6, respectively, led to the compounds [YMe([12]crown-4)(2)](2+)[AlMe(4)](-)(2) (1), [PrMe([12]crown-4)(2)](2+)[AlMe(4)](-)(2) (2), [YMe([15]crown-5)(thf)(2)](2+)[AlMe(4)](-)(2) (3), [Pr([15]crown-5)(2)](3+)[AlMe(4)](-)(3) (4), [YMe([18]crown-6)(thf)](2+)[AlMe(4)](-)(2) (5) and [PrMe([18]crown-6)(thf)(2)](2+) [AlMe(4)](-)(2) (6). With the exception of 4, all compounds contain molecular di-cations with a Ln-CH(3) unit bonded to one (3, 4, 6) or two (1, 2) crown-ether molecules. The compounds were characterised by determination of their crystal structures, by elemental analyses and in the case of the yttrium compounds by variable temperature NMR spectroscopy. The latter gives insights into molecular dynamic processes and methyl group exchange between cations and anions in solution.

CH Activation versus Yttrium–Methyl Cation Formation from [Y(AlMe4)3] Induced by Cyclic Polynitrogen Bases: Solvent and Substituent‐Size Effects

“…The methine proton of the ring results in a triplet at d = 2.45 ppm. As observed in the crown ether bound monomethyl yttrium complexes, [17] a rapid exchange process of the metal-bound methyl groups can be identified by two prominent broad signals at d = À1. 19 Crystal structure: Complex 2 crystallizes in the triclinic space group P1 with two molecules in the unit cell.…”

“…[4] The formation of dicationic monomethyl yttrium and praseodymium complexes in the reactions of the tris(tetramethylaluminates) [LnA C H T U N G T R E N N U N G (AlMe 4 ) 3 ] (Ln= Y, Pr) with several crown ethers in toluene and their subsequent crystallisation from THF [17] raised the question as to whether this reaction pathway is also transferable to reactions with N-alkyl-triazacyclohexanes.…”

“…Compound 2 is a cationic dimethyl yttrium complex with an aluminate unit as the anionic counterpart [17] 2 is one of the few methyl complexes in which the coordination sphere of the rare-earth metal is not saturated through donor solvent molecules.…”

“…The yttrium atom is coordinated as a pentagonal bipyramid; two methyl groups occupy the apical positions and five THF molecules span the equatorial plane, which results in a total coordination number of seven. The Y À C bond lengths are, again, surprisingly short (2) ) [17] and the identi- (2) ). [4] The yttrium-oxygen bond lengths compare well to those of other cationic yttrium complexes, [20,22] but are longer than the bond lengths in THF-solvated, crown ether based, monomethyl yttrium cations (2.370 (2) …”

“…With [12]crown-4 it was even possible to generate solventfree complexes. [17] In this contribution, we will discuss the frontier between C À H activation and cationic dimethyl complex formation in the reactions of [YA C H T U N G T R E N N U N G (AlMe 4 ) 3 ] with 1,3,5-trialkyl-1,3,5-triazacyclohexanes determined by the steric bulk of the alkyl moiety. Further, we will reveal a new, easy route to solvent-free cationic yttrium-dimethyl complexes.…”