Conformation of Heterocycles Controlled by the Existence of Unusual C−H···X Hydrogen Bonds:  Syntheses and Structure Determination of Aluminum Aryloxides

The reactions of AlMe₃ in diethyl ether with 1 molar equiv of 2,2‘-methylenebis(4-methyl-6-tert-butylphenol) (MMBP-H₂), 2,2‘-methylenebis(4,6-di-tert-butylphenol) (MDBP-H₂), and 2,2‘-ethylidenebis(4,6-di-tert-butylphenol) (EDBP-H₂) afford series of four-coordinate monomeric aluminum aryloxides, MeAl(O∼O)(OEt₂), 1−3 (1, (O∼O) = MMBP; 2, (O∼O) = MDBP; 3, (O∼O) = EDBP). In THF, 1 molar equiv of EDBP-H₂ reacts with AlMe₃ to provide the THF-coordinated complex MeAl(EDBP)(THF) 4. However, in the absence of a coordinating solvent, the reaction of EDBP-H₂ with AlMe₃ yields the dimeric complex [MeAl(μ-EDBP)]₂ (5). Complex 5 further reacts with Et₄NCl, Et₄NBr, and Ph₃PO to afford the corresponding monomeric ionic complex [Et₄N][MeAl(EDBP)(X)] (6, X = Cl; 7, X = Br) and the neutral complex [MeAl(EDBP)(OPPh₃)] (8), respectively. Complexes 1, 2, 4 and 6−8 are subjected to X-ray structure analyses, and the solid state structures reveal that the conformations of the eight-membered heterocycles are governed by the formation of the unusual C−H···X hydrogen bonds.