Alkoxohydridoaluminates: multinuclear NMR study of sodium hydrido-(2-methoxyethoxo) aluminates

Abstract

23Na, 27Al, and 1H NMR spectra were obtained on benzene solutions of varying sodium alkoxohydridoaluminate compositions, Na +H x Al −(OR) 4− x , where x = 2 to 0, and OR = OCH 2CH 2OCH 3. An extensive series was prepared by titration of initial Na +H 2Al(OR) 2 −, (SDMA), x = 2, by 2-methoxyethanol, HOCH 2CH 2OCH 3. The NMR data presented enabled the refinement of the structure of SDMA, sodium dihydrido-bis(2-methoxyethoxo)aluminate, proposed formerly as an individual compound. The preparation of the analogous ‘mono-hydrido aluminate’, Na +HAl −(OR) 3, x = 1, as an individual compound, for reason of excellent selective reducing ability, failed. NMR data revealed that alcoholysis of the SDMA by 2-methoxyethanol (in molar ratio Na +H 2Al −(OR) 2: HOCH 2CH 2OCH 3 = 1:1) does not result in in the desired individual Na +HAl −(OR) 3, but in an equilibrium mixture of the Na + salt of H 2Al −(OR) 2, HAl −(OR) 3 and Al −(OR) 4 aluminate anions. The inherently ionic sodium alkoxohydridoaluminate 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 mixture of Na + salt of HAl −(OR) 3, HAl −(OR) 3 and Al −(OR) 4 aluminate anions via ion-paired predominantly trimeric aggregates. These different types of aggregation were recognized on the basis of apparent molecular weight and conductivity measurements on the system. Multinuclear NMR data were used for discussion of these different types of aggregate in more detail.