Cationic Methyl Complexes of the Rare-Earth Metals: An Experimental and Computational Study on Synthesis, Structure, and Reactivity

Synthesis, structure, and reactivity of two families of rare-earth metal complexes containing discrete methyl cations [LnMe((2-x))(thf)(n)]((1+x))+ (x = 0, 1; thf = tetrahydrofuran) have been studied. As a synthetic equivalent for the elusive trimethyl complex [LnMe(3)], lithium methylates of the approximate composition [Li(3)LnMe(6)(thf)(n)] were prepared by treating rare-earth metal trichlorides [LnCl(3)(thf)(n)] with 6 equiv of methyllithium in diethyl ether. Heteronuclear complexes of the formula [Li(3)Ln(2)Me(9)L(n)] (Ln = Sc, Y, Tb; L = Et2O, thf) were isolated by crystallization from diethyl ether. Single crystal X-ray diffraction studies revealed a heterometallic aggregate of composition [Li(3)Ln(2)Me(9)(thf)(n)(Et2O)(m)] with a [LiLn(2)Me(9)](2-) core (Ln = Sc, Y, Tb). When tris(tetramethylaluminate) [Ln(AlMe4)(3)] (Ln = Y, Lu) was reacted with less than 1 equiv of [NR3H][BPh4], the dimethyl cations [LnMe(2)(thf)(n)][BPh4] were obtained. The coordination number as well as cis/trans isomer preference was studied by crystallographic and computational methods. Dicationic methyl complexes of the rare-earth metals of the formula [LnMe(thf)(n)][BAr4](2) (Ln = Sc, Y, La-Nd, Sm, Gd-Lu; Ar = Ph, C6H4F-4) were synthesized, by protonolysis of either the ate complex [Li(3)LnMe(6)(thf)(n)] (Ln = Sc, Y, Gd-Lu) or the tris(tetramethylaluminate) [Ln(AlMe4)(3)] (Ln = La-Nd, Sm, Dy, Gd) with ammonium borates [NR3H][BAr4] in thf. The number of coordinated thf ligands varied from n = 5 (Ln = Sc, Tm) to n = 6 (Ln = La, Y, Sm, Dy, Ho). The configuration of representative examples was determined by X-ray diffraction studies and confirmed by density-functional theory calculations. The highly polarized bonding between the methyl group and the rare-earth metal center results in the reactivity pattern dominated by the carbanionic character and the pronounced Lewis acidity: The dicationic methyl complex [YMe(thf)(6)](2+) inserted benzophenone as an electrophile to give the alkoxy complex [Y(OCMePh2)(thf)(5)](2+). Nucleophilic addition of the soft anion X- (X- = I-, BH4-) led to the monocationic methyl complexes [YMe(X)(thf)(5)](+).