Alkali metal isopropoxide

Double alkoxides with alkali metals A[M(OR)6] (Table 8) were formed from the reaction of the pentaalkoxides with alkali metal alkoxides. The double alkoxides of Mg, Ca, Sr and Ba with Nb and Ta have been synthesized in the presence of MgCl2 as a catalyst.169 Refluxing M(OPri)5 and A(OPri)3 (A = A1 or Ga) in isopropyl alcohol afforded double isopropoxides of the type [MA(OPr )8J and [MA2(OPr1)n]170. [NbTa(OMe)10] appears to be the first mixed transition metal alkoxide isolated.171 NMR showed it to be in dynamic equilibrium with the symmetrical M2(OMe)i0 dimers in solution, with close to random distribution of the three species. 

In a recent publication (184), alkali metal (7Li and 23Na) NMR spectroscopy confirmed that in solution as well as in the crystalline state, [LiTi(0-/-Pr)5]2 exists as a dimer with a rapid isopropoxide exchange, whereas a tight ion pair exists between Na+ and the Ti(0-/-Pr)5- anion. 

For a better understanding of many successful applications of the rare earth-alkali metal containing LnM3tris (binaphthoxide) complexes (LnMB, Ln=rare earth, M=alkali metal) to catalytic asymmetric synthesis, intense investigations also focused on the determination of the structure. It has been shown that these complexes, which can be readily prepared from the corresponding rare earth trichlorides and/or rare earth isopropoxides [5],possess a structure as presented in Fig. 2. This structure was supported by various NMR spectroscopic, MS spectrometric, X-ray crystallographic and other analytic investigations of a variety of LnMB complexes. 

These are catalysts that are formed by combining an alkyl sodium with sodium alkoxide and with an alkali metal halide. A typical, effective catalyst for polymerization of dienes consists of allyl sodium, sodium isopropoxide, and sodium chloride. The preparation of such a catalyst is carried out by combining amyl chloride with sodium and then reacting the product with isopropyl alcohol. After that, propylene is bubbled through the reaction mixture to form allyl sodium. The reactions can be summarized as follows ... 

The chemistry and structures of alkali metal alkoxides [125], enolates [126], and phenoxides [127] has been compiled in several comprehensive review articles. Simple alkali metal alkoxides such as methoxides, ethoxides, or isopropoxides, and simple thiolates (LiSMe, NaSMe, KSMe, etc.) crystallize in layered structures. Interesting supramolecular self-assembly is found for other base-free and solvated alkali metal alkoxides, phenoxides, and their sulfur analogs. Typical examples are summarized in Table 6.3. 

In spite of the strongly electropositive character of alkali metals, the importance of the inductive effect of the alkyl group was shown" in the comparative molar conductivities of sodium methoxide (92.0 mhos), ethoxide (45.0 mhos), isopropoxide (2.5 mhos) and tertiary butoxides (0.05 mhos) in their parent alcohols. Although the effect could be partially ascribed to the differences in dielectric constants of the alcohols, yet the sharp break between the conductivities of the isopropoxide and tert-butoxide could be expected to arise at least partially from the strong -K inductive effect of the tertiary butyl group, making the sodium-oxygen bond much less ionic in character. 

The cubane structure [M4(m3-OR)4] (Fig. 4.6) is favoured by the alkali metal tertiary butoxides (M = K, Rb, Cs) and thallium(i) alkoxides, and also related species such as [Me4Zn4(/ t-OMe)4]. Another common tetranuclear structure [M4(jti-OR)6(OR)6] (Fig. 4.7) involves one central octahedral metal bridged to three tetrahedral metals in a chiral configuration (D3). Crystalline aluminium isopropoxide was the first example established to have this structural type. ° Interestingly in the heterometallic complex [ErAl3(OPr )i2] the central position is occupied by the erbium ion in a distorted trigonal prismatic configuration. ... 

Boyle T.J., Bradley D.C., Hampden-Smith M.J., Patel A., ZUler WJ. Metrical comparison and alkali metal NMR spectroscopic investigation of a congener set of alkali metalated titanium(IV) isopropoxides X-ray stmctures of[NaTi(OCHMe2)5]oo and [KTi(OCHMe2)5]oc. Inorg. Chem. 1995 34 5893... 

Compared to the tetra-alkoxoaluminates, the corresponding hexa-alkoxoniobate and -tantalate derivatives of most of the metals are less stable. Of these two, the niobate products tend to disproportionate more readily this is reflected in the conductivity curves obtained in the titrations of niobium and tantalum pentaisopropoxide with alkali (Na, K) isopropoxides in isopropyl alcohol (see Fig. 3.2). 

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