Complexes of Strong Bidentate Lewis Acids Derived from 2,2‘-(1,3-Butadiyne-1,4-diyl)bis[phenol]

Treatment of 2,2‘-(1,3-butadiyne-1,4-diyl)bis[6-(1,1-dimethylethyl)-4-methylphenol] (1) with 2 equiv of Al(i-Bu)₃ converts the two hydroxyl groups into OAl(i-Bu)₂ groups, thereby transforming diphenol 1 into the corresponding bis(di-i-butylaluminum phenoxide) 8, which holds two strongly Lewis acidic atoms of aluminum in a potentially convergent orientation. Bidentate Lewis acid 8 forms a crystalline 1:1 adduct with 1,2-dimethoxyethane (DME). An X-ray crystallographic study revealed that the adduct is a linear oligomer in which the two Lewis acidic sites of reagent 8 each bind a basic oxygen atom from different molecules of DME. However, low-temperature NMR studies indicated that in solution a discrete 1:1 adduct 9 is favored, in which the two Lewis acidic sites of reagent 8 each bind one of the two basic sites in a single molecule of DME. Formation of adduct 9 provides an example of the recognition and binding of a multidentate Lewis base by a complementary multidentate Lewis acid. Addition of 2 equiv of TiCl₄ to diphenol 1 converts the hydroxyl groups into OTiCl₃ groups and produces the corresponding bis(trichlorotitanium phenoxide) 11, which forms an unusual 1:2 complex with CH₃COOC₂H₅. An X-ray crystallographic study of this complex established that each Lewis acidic atom of titanium binds only 1 equiv of CH₃COOC₂H₅ to form an unprecedented pentacoordinate adduct with a square-pyramidal geometry. Formation of diverse reagents 8 and 11 from the same precursor demonstrates that the strategy of converting organic compounds with suitably oriented hydroxyl groups into the corresponding metal alkoxides is a versatile and effective way to make multidentate Lewis acids.