Complexes of Strong Bidentate Lewis Acids Derived from 2,7-Bis(1,1-dimethylethyl)fluorene-1,8-diol

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Abstract
Treatment of 2,7-bis(1,1-dimethylethyl)fluorene-1,8-diol (3) with 2 equiv of TiCl4 converts the two OH groups into OTiCl3 groups and thereby yields bis(trichlorotitanium phenoxide) 7, a structurally well-defined reagent that holds two sites of strong Lewis acidity in close proximity. Bidentate Lewis acid 7 forms a crystalline 1:2 complex with 4,4‘-dimethylbenzophenone. An X-ray crystallographic study revealed that each atom of titanium binds only one molecule of ketone to form an unusual pentacoordinate adduct with an approximately trigonal bipyramidal geometry. Treatment of fluorenediol 3 with 2 equiv of Al(CH2CH3)3 does not yield the expected bis(diethylaluminum phenoxide) 8, but rather its dimer 9. Formation of diverse reagents 7 and 9 from the same precursor demonstrates that the strategy of converting organic compounds with suitably oriented hydroxyl groups into the corresponding metal alkoxides is a particularly versatile and effective way to make strong, structurally well-defined multidentate Lewis acids.