ALKOXOHYDRIDOALUMINATES - MULTINUCLEAR NMR-STUDY OF SODIUM HYDRIDE-(2-METHOXYETHOXO) ALUMINATES

Na-23, Al-27, and H-1 NMR spectra were obtained on benzene solutions o f varying sodium alkoxohydridoaluminate compositions, Na+HxAl-(OR)(4-x ), where x = 2 to 0, and OR = OCH2CH2OCH3. An extensive series was pre pared by titration of initial Na+H2Al(OR)(2)(-), (SDMA), x = 2, by 2-m ethoxyethanol, HOCH2CH2OCH3. The NMR data presented enabled the refine ment of the structure of SDMA, sodium dihydrido-bis(2-methoxyethoxo)al uminate, proposed formerly as an individual compound. The preparation of the analogous 'mono-hydrido aluminate', Na+HAl-(OR)(3), x = 1, as a n individual compound, for reason of excellent selective reducing abil ity, failed. NMR data revealed that alcoholysis of the SDMA by 2-metho xyethanol (in molar ratio Na+H2Al-(OR)(2):HOCH2CH2OCH3 = 1:1) does not result in the desired individual Na+HAl-(OR)(3), but in an equilibriu m mixture of the Na+ salt of H2Al-(OR)(2), HAl-(OR)(3) and Al-(OR)(4) aluminate anions. The inherently ionic sodium alkoxohydridoaluminates are stabilized in solvents of low dielectric constant and low donation strength-in benzene-via different types of aggregate: SDMA via higher , weaker ionic aggregates, and mixtures of Na+ salt of HAl-(OR)(3), HA l-(OR)(3) and Al-(OR)(4) aluminate anions via ion-paired predominantly trimeric aggregates. These different types of aggregation were recogn ized on the basis of apparent molecular weight and conductivity measur ements on the system. Multinuclear NMR data were used for discussion o f these different types of aggregate in more detail.