Synthesis and Stability of Homoleptic Metal(III) Tetramethylaluminates

Abstract

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.

Rare Earth Arene-Bridged Complexes Obtained by Reduction of Organometallic Precursors

“…Efforts in this area were initiated by Pearce and Lappert, who reported Cp2M(-Me)2AlR2 (R = Me and M = Sc, Gd, Dy, Ho, Er, Tm, or Yb; R = Et and M = Sc or Y) (Ballard and Pearce, 1975;Holton et al, 1976), and were continued by Anwander (Fischbach and 14 Anwander, 2006), who developed homoleptic tetraalkylaluminate rare earth complexes Occhipinti et al, 2011) and bridging methylidenes (Dietrich et al, 2006b) and methylidynes (Dietrich et al, 2006a) as part of rare earth aluminum small clusters. Interestingly, the crystal structures of isostructural bridging hexacoordinate carbides, (…”

Mixed metal multinuclear Cr(III) cage compounds and coordination polymers based on unsubstituted phenolate: design, synthesis, mechanism, and properties

“…A First Coordination Polymer [Cr 2 (μ 3 -OPh) 2 (μ-OPh) 6 (μ-OH)(μ 3 -Li) 4 (μ 3 -Cl) 2 Li(THF) 6 ] n . Upon reaction of CrCl 2 with 4 equiv of LiOPh and water, a 1-D coordination polymer [Cr 2 (μ 3 -OPh) 2 (μ-OPh) 6 (μ-OH)(μ 3 -Li) 4 (μ 3 -Cl) 2 Li(THF) 6 ] n (2) was obtained.…”

“…Upon reaction of CrCl 2 with 4 equiv of LiOPh and water, a 1-D coordination polymer [Cr 2 (μ 3 -OPh) 2 (μ-OPh) 6 (μ-OH)(μ 3 -Li) 4 (μ 3 -Cl) 2 Li(THF) 6 ] n (2) was obtained. A possible second reaction equation (eq 1b) leads to in a first step the formation of A as a nonisolated intermediate compound.…”

“…[Cr(μ-OPh)(μ 3 -OPh) 4 Li 3 (THF) 3 ] 2 A: CrCl 2 (0.5 g, 4 mmol) was dried under a vacuum at 400°C during 1 h and added to a solution of PhOLi (24 mmol) in THF (24 mL, 1.0 M). After the solution was stirred for 30 min under reflux, the color of the mixture was deep blue.…”

“…Mixed metal alkoxide compounds of, e.g., d-block elements with alkali, 1 alkaline earth, 2 lanthanide, 3 and actinide 4 metal ions, are an attractive field of research due to their large panel of applications. Indeed, as a function of the metal ions and ligands, mixed metal compounds can be used in catalysis, 5,6 magnetism, 7,8 optoelectronics, 9,10 luminescence, 11−15 energy storage (battery), 1,16−18 or fuel cells.…”