Tetrameric aluminum isopropoxide

The close relationships in the structural features of homo- and heterometallic alkoxides can be exemplified by those of Al Al(0-i-Pr)4 3 (150) and Ln Al(0-/-Pr)4 3 (18). The molecular weights of the latter in benzene correspond to their empirical formulas. All of these compounds can be distilled in the range of 200-180°C/0.1 mm, with a lowering of the boiling point as was expected from increasing the covalent character which results from lanthanide contraction in the series. The tetrameric aluminum isopropoxide A1 A1(0-/-Pr)4 3 or A1(0-/-Pr)3 4, however, disproportionates and distills as a dimeric vapor around... 

Figure 2. The crystal structure of tetrameric aluminum isopropoxide, [Al(0-i-Pr)3]4 (150 and 198).

Aluminum isopropoxide is shown as a monomer in Scheme 14, but it is known to exist in several oligomeric forms (114). Freshly distilled Al(OIV)3 is mainly trimeric and is slowly transformed to a tetrameric form (109) in which a central octahedral aluminum atom is surrounded by three tetrahedral aluminum atoms (112). 

The stmcture of solid-state aluminum isopropoxide was determined at 25°C in 1979 by Turova et al. (23) confirming Bradley s assertion of a possible non-cyclic tetrameric form (24). Folting and coworkers studied crystalline aluminum isopropoxide by X-ray diffraction as well as proton and a1 NMR. They determined it to be a tetrameric material having the following formula, Al[(/trO Pr)2Al(0 Pr)2]3, which supports earlier stmctural proposals (24,25) and the spectroscopic data of other researchers (15,18). The tetrameric form of aluminum isopropoxide is depicted in Figure 2 where the octahedral, central Al... 

Figure 2 Tetrameric and trimeric forms of aluminum isopropoxide...

Cryoscopic and NMR investigation (14) indicates that starting from the pure dimer, aluminum isopropoxide undergoes a very fast equilibrium shift to a mixture containing mainly ftie trimeric form followed by a quite slow conversion to the tetrameric form. Conversely, starting from the purely tetrameric form, an equilibrium mixture is slowly obtained consisting of all three forms. 

The alkoxides of aluminum are polymeric with aluminum achieving four or six coordination. The adopted structure depends greatly on the bulkiness of the alkyl groups. For sterically large groups, for example, tertiary butyl, the preferred structure is a dimer (25). The isopropoxide, on the other hand, is trimeric (26) at elevated temperatures but tetrameric (27) at room temperature. ... 

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