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(mu-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.