H. Martin Dietrich

Synthesis and Stability of Homoleptic Metal(III) Tetramethylaluminates

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.

Yttrium Siloxide Complexes Bearing Terminal Methyl Ligands: Molecular Models for Ln-CH3 Terminated Silica Surfaces.

Surface organometallic chemistry (SOMC) on silica materials is a prominent approach for the generation of highly active heterogenized polymerization catalysts. Despite advanced methods of characterization, the elucidation of the catalytically active surface species remains a challenging task. Alkylated rare-earth metal siloxide complexes can be regarded as molecular models of respective covalently bonded alkylated surface species, primarily used for 1,3-diene polymerization. Here, we performed both salt metathesis reactions of [Y(MMe4 )3 ] (M = Al, Ga) with [K{OSi(OtBu)3 }] and alkylation reactions of [Y{OSi(OtBu)3 }3 ]2 with AlMe3 . The obtained complexes [Y(CH3 )[(AlMe2 ){OSi(OtBu)3 }2 ](AlMe4 )]2 , [Y(CH3 )[(AlMe2 ){OSi(OtBu)3 }2 ]-{OSi(OtBu)3 }], [Y{OSi(OtBu)3 }3 (μ-Me)Y(μ-Me)2 Y{OSi(OtBu)3 }2 (AlMe4 )], and [Y(CH3 )(GaMe4 ){OSi(OtBu)3 }]2 represent rare examples of organoyttrium species with terminal methyl groups. The formation and purity of the mixed methyl/siloxy yttrium complexes could be enhanced by treating [Y(MMe4 )3 ] with [K(MMe2 ){OSi(OtBu)3 }2 ]n (M=Al: n=2; M=Ga: n=∞). Complexes [K(MMe2 ){OSi(OtBu)3 }2 ]n were obtained by addition of [K{OSi(OtBu)3 }] to [Me2 M{OSi(OtBu)3 }]2 . Deeper insight into the fluxional behavior of the mixed methyl/siloxy yttrium complexes in solution was gained by (1) H and (13) C NMR spectroscopic studies at variable temperature and (1) H-(89) Y HSQC NMR spectroscopy.

High tetraalkylaluminate fluxionality in half-sandwich complexes of the trivalent rare-earth metals

Steric factors govern the formation of half-sandwich complexes (C5Me4R)Ln[N(SiHMe2)2]2 according to acid-base reactions utilising Ln[N(SiHMe2)2)3(thf)2 and substituted cyclopentadienes. Subsequent trimethylaluminium-promoted silylamide elimination produces the first half-sandwich bis(tetramethylaluminate) complexes (C5Me4R)Ln(AlMe4)2.

[LnIIAlIII2(alkyl)8]x: donor addition instead of donor-induced cleavage

(SmAl2Me8)x and (SmAl2Et8)x are obtained via a silylamide elimination reaction from Sm[N(SiMe3)2]2(THF)2 and excess AlR3 (R = Me, Et); (LnAl2Et8)x (Ln = Sm, Yb) react with THF, pyridine, and 1,10-phenanthroline to form the first donor adducts of homoleptic peralkylated Ln-Al heterobimetallic complexes.