Liliane G. Hubert-Pfalzgraf

Some aspects of homo and heterometallic alkoxides based on functional alcohols

Abstract Functional alkoxides find applications in chemistry as catalysts or for organic synthesis as well as in material science. They can provide stabilization toward hydrolysis, reduce oligomerization and, thus, give access to volatility, act as assembling ligands for the building up of mixed-metal species and to some extent control their stoichiometry. These aspects will be illustrated on non-silicon systems with alkoxide ligands bearing pendant ether, hydroxyl or amino functionalities, or unsaturated sites for organic cross-linking reactions. Fluoroalkoxides act often as functional groups via secondary M·xa0·xa0·F bonds to electrophilic metals such as barium and will, thus, also be considered.

Heterometallic alkoxides and oxoalkoxides as intermediates in chemical routes to mixed metal oxides

Abstract Metal alkoxides are often associated with more accessible precursors such as carboxylates, nitrates, β-diketonates in chemical routes to complex metal oxides. The molecular constitution of such solutions has been examined for systems related to ferroelectrics and based on niobium, titanium or zirconium alkoxides. A variety of oxo or non-oxo mixed-metal acetatoalkoxides have been characterized in the solid (X-ray on single crystals) as well as in solution (FT-IR, multinuclear NMR). Conversion from these molecules to the final material through hydrolysis-polycondensation has been estimated. The amorphous and crystalline powders have been characterized by thermogravimetric analysis and X-ray diffraction, respectively. Strategies for a control of the stoichiometry between the metals in the precursors are discussed. Homoleptic alkoxides Pb6Nb4O4(OEt)24 and BaTi4(OEt)18 were used as models for getting insight into chemical transformations.

Molecular precursors of bismuth oxides ; β-diketonates and alkoxides. Molecular structure of [Bi2(μ2,η1-OC2H4OMe)4(η1-OC2H4OMe)2]∞ and of Bi(OSiPh3)3(THF)3

Abstract Various routes to bismuth(III) alkoxides and β-diketonate derivatives have been investigated. Metathesis reactions of the halides and the appropriate sodium derivative offered [Bi(OtBu)3]∞ (1), [Bi2(OC2H4OMe)6]∞ (2), [Bi(OSiPh3)3]m and Bi(dpm)3 (dpmH = dipivaloylmethane) in good yields. The alkoxides display different physical properties; 1 has good volatility, while 2 is highly soluble. Complex 2 and Bi(OSiPh3)3(THF)3 (3) have been characterized by X-ray diffraction. Compound 2 shows a one-dimensional extended “skew” chain structure based on dimeric asymmetric Bi2(μ2,η1-OC2H4 OMe)4(OC2H4OMe)2 units. The alkoxo functionality leads to a distorted pyramidal tetragonal surrounding, but the ether functionality of the bridging groups provides further oxygen donors, and thus coordination numbers of seven and even eight are reached. Bi(OSiPh3)3(THF)3 is monomeric wi th a capped octahedron geometry around the metal. The thermal stability of the volatile precursors 1 and Bi(dpm)3 is in favour of the alkoxide, while the high solubility of 2 is attractive for sol-gel purposes.

Characterization of aluminium isopropoxide and aluminosiloxanes

Abstract Aluminium isopropoxide, in the form of a crystalline solid of melting point 127°C, has been shown by X-ray diffraction to be a tetrameric molecular species of formula Al[(μ-OiPr)2Al(OiPr)2]3, consistent with earlier proposals and spectroscopic data. A central aluminium achieves coordination number six via two bridging alkoxide groups from each of three Al(OiPr)4− groups. The resulting planar Al(μ2-O)2Al rings have widened (∼ 132°) angles at oxygen to increase the non-bonded Alue5f8Al distance. While the terminal OiPr groups show the shortest (1.70 A) Alue5f8O distances in the molecule, they are nevertheless strongly bent (∼ 140°) at oxygen. The 1H NMR spectrum of Al4(OiPr)12 is analysed based on the structure reported here. Characterization of aluminosiloxane derivatives Al(OiPr)3-x(OSiMe3)x (x = 1−3) by 1H and 27Al NMR, mass spectrometry and IR spectroscopy is also reported. For x = 1 and 2, these show co-existence of several oligomers. The effect of incorporation of OSiMe3 groups is to favour smaller oligomers.

Synthesis and molecular structure of Nd5(μ5-O)(μ3-OR)2(μ2-OR)6(OR)5(ROH)2 (R = Pri). The first example of a trigonal bipyramidal metal oxoalkoxide

Abstract Reaction between neodymium chips and isopropanol (Pri) led to the isolation of an oxoisopropoxide solvate Nd5(OPri)13(PriOH)2, which has been characterized by microanalysts and IR spectroscopy, as well as by X-ray diffraction. The molecule corresponds to a trigonal bipyramidal oxoalkoxide Nd5(μ5-O)(μ3-OR)2(μ2-OR)6(OR)5(ROH)2 (R = Pri), in which all metals achieve hexacoordination. The two alcohol molecules are linked to the same neodymium centre, thus leading to distortion of the M5O16 core.

Molecular precursors of alkaline earths: Synthesis and structure of [N(C2H4O)(C2H4OH)2]2Ba·2EtOH. The first structurally characterized barium alkoxide

Abstract The reaction between barium and triethanolamine in toluene/THF affords, after recrystallization in ethanol, the barium alkoxide [N(C2H4O)(C2H4OH2]2Ba·2EtOH, which has been characterized by X-ray crystallography. The tetradentate character of the triethanolamine molecule allows the barium atom to be octacoordinated in a monomeric Ba[(OC2H4)(OHC2H4)2N]2 unit. However, these units are associated by intermolecular hydrogen-bonding due to the ethanol lattice molecules, thus forming an extensive two-dimensional network. The hydrogen-bonding reduces the volatility of the precursor, but is favourable to the formation of gels by hydrolysis.

Praseodymium alkoxide chemistry: synthesis and molecular structure of [Pr4(μ4-O)2(μ3, η2-OR)2 (μ, η2-OR)4(μ, η1-OR)(OR)(OPMe3)]2 (R = C2H4OMe) and [Y4Pr(μ5-O)(μ3-OR)4(μ-OR)4(OR)5] (R = Pri)

Abstract Various routes to praseodymium alkoxides have been investigated. Compounds such as Pr 5 O ( OR ) 13 [ R = Pr i ( 1 ), Am 1 ], [ Pr 4 O 2 ( ONp ) 8 ] m (Np = neopentyl), [Pr(OC2H4NMe2)3]3 were obtained by reaction of praseodymium turnings with the corresponding alcohols. Alcoholysis reactions of the bis-trimethylsilylamide afforded [Pr4O(ONp)10]m, [Pr4O2(ONp)8]m, Pr3(OR)9(ROH)2 [R = But, Amt] and [Pr4O2(OC2H4OMe)8]m (2a), the latter derivatives illustrating the trend for the formation of oxo species. The compounds were characterized by elemental analysis. FT-IR and NMR. Their reactivity has been investigated, the mixed-metal species Pr4TiO(OPri)14 and Y4PrO(OPri)13 were obtained. Ligand exchange reactions afforded [Pr4O2(OR)3(OR′)5]m species (R ue5fb Pri, R′ ue5fb C2H4NMe2, R ue5fb SiMe3, R′ ue5fb Pri) and [Pr(OC2H4NMe2) (thd)2]2. X-ray structure analysis of the adduct obtained by reaction of [Pr4O2(OC2H4OMe)8]m with trimethylphosphine oxide reveals a centrosymmetric framework [Pr4(μ4-O)2(μ3,η2-OR)2(μ,η2-OR)4(μ,η1-OR)(OR)(OPMe 1053 0971 V 3 3]2 (2b) of eight octacoordinated praseodymium atoms, assembled by tetrahedral and square-planar oxo ligands. The Mue5f8O bond lengths spread over the range 2.246(6)–2.762(6) A and to follow the order Prue5f8OR (terminal)

Facile thermal desolvation of Ce2(OPri)8(PriOH)2: Characterization and molecular structure of Ce4(μ4-O)(μ3-OPri)(μ-OPri)4(OPri)8

Abstract Thermal treatment of Ce2(OPri)8(PriOH)2 yielded Ce4(μ4-O)(OPri)14 by desolvation under mild conditions. The tetranuclear oxoalkoxide has been characterized by elemental analysis, FT-IT, 1H NMR and X-ray diffraction. The structure is described as [Ce4(μ4-O)(μ3-OPri)4(μ-OPri)4(OPri)8]. The Ce4(μ4-O) core corresponds to a distorted flattened tetrahedron, with two hexa- and two hepta-coordinated metals. The Ceue5f8O bond lenghts spread over the range 2.03(2) to 2.54(1) A and follow the orcer Ceue5f8OR

Chemical routes to barium-cerium oxides: the quest for mixed-metal precursors. Molecular structure of Ba4Ce2(μ6-O)(thd)4 (μ3-OPri)8(OPri)2

Abstract Various synthetic routes to Baue5f8Ce mixed-metal species have been investigated No reaction was observed between Ce2(OPri)8(PriOH)2 (1) and barium isopropoxide. The enhanced acidity of the alcohol molecules of 1 has been used as a means to access to Baue5f8Ce species and to fix the stoichiometry between the metals. Ba2Ce4(OPri)20 (2) has been obtained. This species is unstable in the presence of isopropanol giving BaCe4(OPri)18 (3). The reaction between 1 2,2,6,6-tetramethylheptane-3,5-dione (thdH) and barium isopropoxide gives Ba4Ce2(μ6-O)(thd)4(μ3-OPri)8(OPri)2 (4) independently of the stoichiometry used for the reaction. The reactions between 1, barium isopropoxide and acetylacetone are more complex and different compounds, namely Ba2Ce2O(acac)2(OPri)8 (5) or BaCe2(acac)(OPri)9 (6) are formed depending on the experimental conditions. All compounds have been characterized by elemental analysis, FT-IR, 1H and 13C NMR. 4 was also characterized by X-ray diffraction. Hydrolysis-polycondensation reactions have been estimated for 4 and for 6.

Synthesis and molecular structure of Cd9(OC2H4OMe)18,2HOC2H4OMe, the first cadmium aggregate based on oxygen donor ligands

Abstract The alcoholysis reactions between Cd[N(SiMe3)2]2 and functional alcohols such as 2-methoxyethanol produced soluble cadmium alkoxides. Cd9(OC2H4OMe)18, 2HOC2H4OMe has been characterized by X-ray diffraction. The compound corresponds to a centrosymmetric compact aggregate of nine cadmium atoms, the metals being hexa- or pseudo-hexa-coordinated. The alcohol molecules are hydrogen bonded to the terminal alkoxide groups. 113Cd NMR established that the Cd9 cluster was retained in solution.