Synthesis and Stability of Homoleptic Metal(III) Tetramethylaluminates

Whereas a number of homoleptic metal(III) tetramethylaluminates M(AlMe4)3 of the rare earth metals have proven accessible, the stability of these compounds varies strongly among the metals, with some even escaping preparation altogether. The differences in stability may seem puzzling given that this class of metals usually is considered to be relatively uniform with respect to properties. On the basis of quantum chemically obtained relative energies and atomic and molecular descriptors of homoleptic tris(tetramethylaluminate) and related compounds of rare earth metals, transition metals, p-block metals, and actinides, multivariate modeling has identified the importance of ionic metal−methylaluminate bonding and small steric repulsion between the methylaluminate ligands for obtaining stable homoleptic compounds. Low electronegativity and a sufficiently large ionic radius are thus essential properties for the central metal atom. Whereas scandium and many transition metals are too small and too electronegative for this task, all lanthanides and actinides covered in this study are predicted to give homoleptic compounds stable toward loss of trimethylaluminum, the expected main decomposition reaction. Three of the predicted lanthanide-based compounds Ln(AlMe4)3 (Ln = Ce, Tm, Yb) have been prepared and fully characterized in the present work, in addition to Ln(OCH2tBu)3(AlMe3)3 (Ln = Sc, Nd) and [Eu(AlEt4)2]n. At ambient temperature, donor-free hexane solutions of Ln(AlMe4)3 of the Ln3+/Ln2+ redox-active metal centers display enhanced reduction to [Ln(AlMe4)2]n with decreasing negative redox potential, in the order Eu ≫ Yb ≫ Sm. Whereas Eu(AlMe4)3 could not be identified, Yb(AlMe4)3 turned out to be isolable in low yield. All attempts to prepare the putative Sc(AlMe4)3, featuring the smallest rare earth metal center, failed.