Bohuslav Čásenský

27Al NMR behavior of aluminum alkoxides

Abstract A series of aluminum oxides [Al(OR) 3 ] n was prepared and the 27 Al NMR spectra were measured at 52.13 MHz. The existence of three types of structures differing in the character of their spectra was proved: a dimeric form prevails with bulky R alkyls, a tricyclic tetrameric form dominates in sterically nondemanding n -alkyls and a linear trimeric form occurs in medium size α-branched alkyls. A cyclic trimer was not found in the compounds studied. Depending on the reaction conditions and temperature, two or more oligomers were found to coexist.

27Al NMR spectroscopy of triethylaluminium. A direct method to the determination of the proportion of monomer in solution

Abstract The sole 27 Al NMR signal of triethylaluminum (TEA) is shifted significantly to lower field with: (1) decreasing concentration; (2) increasing temperature; and (3) increasing polarity of the solvent; that is, in each case with an increase in the amount of the monomeric form. 27 Al NMR chemical shifts of Al 2 Et 6 and AlEt 3 are estimated as 153 ± 2 and 265 ± 10 ppm, respectively. By use of these and observed values, thermodynamic data K d , Δ H d and Δ S d were calculated for the dissociation of Al 2 Et 6 . The dependence of the monomer—dimer equilibria on the concentration of TEA in the solvents used indicated the participation of an “intermolecular” process in the exchange of bridging and terminal ethyl groups not only in aromatic solvents but also, in contrast to previous reports, in aliphatic hydrocarbons.

27Al NMR spectroscopy of ethylaluminum halides

Abstract 27Al NMR data for the dimeric compounds of the (Et3-nAlXn)2 type (n = 0-3; X = Cl, Br and I) have been determined. A characteristic U-shaped relationship between the 27Al NMR chemical shift and the value of n was found, together with a correlation between available 27Al nuclear quardrupole coupling constants (NQCC) and the δ(27Al) NMR values for four-coordinate Al atoms. On the bases of this correlation, it is concluded that the U-shaped relationship arises principally from the changes in the groud-state charge-distribution at Al nucleus.

27Al NMR study of the trimethylaluminum monomer-dimer equilibrium

Abstract The 27 Al NMR signal from trimethylaluminum (TMA) in solution is shifted to lower field with: (i) increasing temperature; (ii) decreasing concentration of TMA; and (iii), more markedly, the increasing solvating power of the solvent. This shift reflects an increase in the degree of dissociation of the dimer to the monomer. Thermodynamic data ( K d , Δ H d , Δ S d ) of the TMA dissociation have been calculated from the observed values of the NMR chemical shift by use of the shifts estimated for the separate dimeric (Al 2 Me 6 ) and monomeric (AlMe 3 ) species. In both solvents employed (n-heptane and mesitylene) the derived Δ H d values lie between 11.2 and 14.7 kcal/mol for Al 2 Me 6 , values in good agreement with the Δ H d value of 13.4 kcal/mol previously determined experimentally. (These experimental values are lower than previously estimated Δ H d values, which lay between 14.2 and 19.7 kcal/mol.) The 27 Al NMR spectra of TMA solutions recorded over the range of 25–100°C unambiguously proved the exchange of methyl substituents between bridge and terminal positions in the TMA dimer. In both aromatic and aliphatic solvents, this exchange occurs via the “intermolecular mechanism”, i.e. via the separated monomeric form of TMA; experimentally determined Δ H d values are in accord with this mechanism.

Alkoxohydridoaluminates. Part 2. The influence of alkyl substituents, solvent, and temperature on the molar fractions of components, equilibrium constants, and 27Al n.m.r. data for equilibrium mixtures containing Bu4N+AlH4-n(OR)n–

Molar fractions, equilibrium constants of mutual exchange, and 27Al n.m.r. data of compounds Bu4N+AlH4-n(OR)–n and the dependence of these on n(0–4); R (Me, Et, Nonyl, Pri, cyclo-C6H11 Ph, and But); solvent (benzene, tetrahydrofuran, 1,2-dimethoxyethane, diglyme); and temperature (23 and 70 °C) are presented. Predominant separation of cation from anion and, consequently, the slowest exchange, is found with Bu4N+AlH(OR)–3 and bulky R, in 1,2-dimethoxyethane at 70 °C. Such compounds, free of tetra-, tri- and di-hydrido components are expected to exhibit a very high reduction stereoselectivity proportional to the steric strain resulting from the presence of all three bulky alkoxy groups.

Sodium Dihydrido-bis(2-methoxyethoxo)aluminate (SDMA): Structure and Behaviour in Solutions

SDMA, according to 27AI NMR almost uniform in benzene, is increasingly disproportionated in the presence of donors in the following order: C6H6 Bu2O ≤ Et2O ≤ anisole < dioxane ≪ THF < monoglyme ≤ diglyme to the NaAlH4-x(OCH2CH2OCH3)x compounds; the structure in solutions is represented by an open-chain oligomer with four-coordinated Na and Al atoms and with Na+ autocomplexed by two bidentate -OCH2CH2OCH3 ligands belonging to two neighbouring Al Atoms.