Reactivity of Trimethylaluminum with Lanthanide Aryloxides:  Adduct and Tetramethylaluminate Formation

The reaction of various highly substituted lanthanide(III) and -(II) aryloxide complexes with trimethylaluminum (TMA) was investigated. The solvent-free, π-arene-bridged dimers [Ln(OAr^iPr,H)₃]₂, derived from the ortho-iPr₂-substituted aryloxide ligand OC₆H₃ⁱPr₂-2,6, form bis-TMA adduct complexes, Ln(OAr^iPr,H)₃(AlMe₃)₂, for the metal centers yttrium, samarium, and lanthanum. Homoleptic monomeric Ln(OAr)₃, featuring a large La center and sterically bulkier ortho-tBu₂-substituted aryloxide ligands, afford the mono-TMA adducts La(OAr^tBu,R)₃(AlMe₃) (R = H, Me). The hetero-bridged moieties “Ln(μ-OAr)(μ-Me)Al” of these adduct complexes are rigid in solution, while at ambient temperature the exchange of bridging and terminal aluminum methyl groups is fast on the NMR time scale. Monomeric Ln(OAr^tBu,R)₃ (R = H, Me, tBu) of the smaller rare-earth-metal centers yttrium and lutetium react with TMA to give mono(tetramethylaluminate) complexes of the type (Ar^tBu,RO)₂Ln[(μ-Me)₂AlMe₂]. The heteroleptic Cp*-supported complex (C₅Me₅)Y(OAr^tBu,H)₂ also produced a tetramethylaluminate complex, namely (C₅Me₅)Y(OAr^tBu,H)[(μ-Me)₂AlMe₂], in the TMA reaction. The solvated aryloxide complexes Ln(OAr)₂(THF)_x (x = 1, 2), featuring the divalent rare-earth-metal centers ytterbium and samarium, yield the bis-TMA adduct complexes Ln[(μ-OAr^tBu,R)(μ-Me)AlMe₂]₂. However, it was found that the generation of homoleptic hexane-insoluble [Ln(AlMe₄)₂]_n is an important reaction pathway governed by the size (oxophilicity) of the metal center (Yb ≫ Sm), the amount of TMA, the reaction period, and the substituents of the aryloxide ligand (OAr^iPr,H ≫ OAr^tBu,H > OAr^tBu,Me ≫ OAr^tBu,tBu). For the Ln(III) aryloxide complexes, peralkylated complexes Ln(AlMe₄)₃ were detected only in the presence of the least bulky ligand, OAr^iPr,H. Various mechanistic scenarios are depicted on the basis of the rare-earth-metal species identified, including byproducts such as [Me₂Al(μ-OAr)]₂, and of the interactivity of rare-earth alkoxide complexes with trialkylaluminum compounds known from the literature. The complexes Y(OC₆H₃ⁱPr₂-2,6)[(μ-OC₆H₃ⁱPr₂-2,6)(μ-Me)AlMe₂]₂ and Ln(OC₆H₃^tBu₂-2,6)₂[(μ-Me)₂AlMe₂] (Ln = Y, Lu) have been characterized by X-ray diffraction structure determinations.