Whereas a number of homoleptic metal(III) tetramethylaluminates M(AlMe(4))(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(AlMe(4))(3) (Ln = Ce, Tm, Yb) have been prepared and fully characterized in the present work, in addition to Ln(OCH(2)tBu)(3)(AlMe(3))(3) (Ln = Sc, Nd) and [Eu(AlEt(4))(2)](n). At ambient temperature, donor-free hexane solutions of Ln(AlMe(4))(3) of the Ln(3+)/Ln(2+) redox-active metal centers display enhanced reduction to [Ln(AlMe(4))(2)](n) with decreasing negative redox potential, in the order Eu ≫ Yb ≫ Sm. Whereas Eu(AlMe(4))(3) could not be identified, Yb(AlMe(4))(3) turned out to be isolable in low yield. All attempts to prepare the putative Sc(AlMe(4))(3), featuring the smallest rare earth metal center, failed.

The fundamental reactivity of amorphous [YMe3] n was representatively examined:  toward GaMe3 as a Lewis acid, 9-fluorenone as a carbonyl substrate, and tetramethyldisilazane (HN(SiHMe2)2) as a Brønsted acid. The products obtained from the 3-equiv reactions were spectroscopically and X-ray crystallographically identified. Y(GaMe4)3 shows Y- - -Ga distances of 3.0393(4) and 3.0502(3) Å, which are significantly shorter than the Y- - -Al distances in Y[AlMe4]3[Al2Me6]0.5 (av 3.068 Å). The homoleptic alkoxide [Y(OC14H11)3] x bearing sterically demanding 9-Me-fluorenoxy ligands documents high methyl group transfer economy via the 1,2-addition reaction of [YMe3] n to 9-fluorenone; it was obtained in single-crystalline form in a minor byproduct, the asymmetric dimer [Y(OC14H11)2(μ-OC14H11)]2(9-fluorenone). The previously elusive unsolvated complex {Y[N(SiHMe2)2]3} also shows a dimeric molecular composition, {Y[N(SiHMe2)2]2[μ-N(SiHMe2)2]}2, featuring asymmetrically bridging silylamide ligands and Y- - -SiH multi-β-agostic interactions in the solid state [(Y- - -Si)min 3.0521(7) Å, (Y- - -H)min 2.41(3) Å, ν(Si−H)agostic 1931 cm-1].

The structure−reactivity relationship of the rare-earth metal aryl(alk)oxide-promoted coordination polymerization of isoprene was investigated using binary initiating systems Ln(OR)3(AlMe3) x /Et2AlCl (Ln = La, Nd, Y). Depending on the degree of the rare-earth metal aryl(alk)oxide prealkylation (x = 1, 2, 3), such discrete trimethylaluminum (TMA) adduct complexes of rare-earth metal alkoxide and aryloxide components displayed a distinct initiating capability. The heterobimetallic bis-TMA adducts Ln(OAr i Pr)3(AlMe3)2 and tris-TMA adducts Ln(OCH2 tBu)3(AlMe3)3 (Ln = La, Nd) produced highly reactive initiators, whereas the mono-TMA adducts Ln(OAr t Bu)3(AlMe3) were catalytically inactive. The highest reactivities and stereoselectivities (>99% cis) were obtained for a n Ln:n Cl ratio of 1:2. The alkoxide-based tris-TMA adducts gave narrower molecular weight distributions than the aryloxide-based bis-TMA adduct complexes (M w/M n = 1.74−2.37 vs 2.03−4.26). A plausible mechanistic activation/deactivation scenario of the formation of the catalytically active/inactive species is presented.