An alternative approach to Al2O2 ring systems by unexpected cleavage of stable Al-F- and Si-O- bonds

The reaction of dimethyl aluminum fluoride (Me2AlF, DMAF) with 2,6-diisopropylphenol and triethylcitrate, respectively, leads to the products (RO)(6)Al4F2Me4 (R = 2,6-i-Pr2C6H3) (1) and (ROAlFMe)(2) (R = C(CH2COOEt)(2)(COOEt)) (2), containing Al2O2 ring systems. Both aluminum-mu-oxo fluorides have been structurally characterized. Fluorine exchange in the reaction of 2,6-diisopropylphenol with DMAF has been monitored using F-19 NMR spectroscopy. The proposed processes in solution are confirmed by the solid-state structure of 1. Compound 1 contains four aluminum centers forming three four-membered ring systems. The two outer cycles consist of bridging-OR groups, whereas the aluminum atoms in the central cycle are connected via fluorine atoms. Two of the aluminum atoms are 4-fold coordinated, and two are 5-fold coordinated. Compound 1 contains a rare four-membered Al2F2 ring system. Compound 2 exhibits a dimeric structure, with oxygen rather than fluorine atoms bridging the aluminum atoms. A further carboxy oxygen atom binds coordinatively to one aluminum atom in 2. The aluminum atoms are 5-fold coordinated. In search of an alternative approach to synthesizing Al2O2 ring systems, it has been found that neither DMAF nor trimethyl aluminum (Me3Al, TMA) reacts with (Me3Si)(2)O, whereas the reaction of diisobutyl aluminum hydride (i-Bu2AlH, DIBAH) with (Me3Si)(2)O leads to (i-Bu2AlOSiMe3)(2) (3). In compound 3 the aluminum centers in the four-membered Al2O2 ring system are only 4-fold coordinated. This is the first example of a structurally characterized aluminum product obtained from (Me3Si)(2)O under cleavage of the Si-O bond.