Alcohol and secondary amine complexes of tri-tert-butylaluminium: enhanced stability through intramolecular hydrogen bonding

Reaction of Al(Bu-t)(3) with between 1 and 2 equivalents of HOCH2CH2CH2NMe2 allows for the isolation of the Lewis acid-base complex, (Bu-t)(3)Al[O(H)CH2CH2CH2Me2] 1, which undergoes alkane elimination above 45 degrees C to yield [(Bu-t)Al(mu-OCH2CH2CH2NM2)](2) 2. Compound 2 is also formed directly when 2 equivalents of Al(Bu-t)(3) react with 1 equivalent of HOCH2CH2CH2NMe2. The molecular structure of 1 shows an Al-O bond distance comparable to that found in the bridging alkoxide compounds 2 and [(Bu-t)(2)Al(mu-OPrn)](2) 3, suggesting that the Al-O ... H unit may be considered analogous to a bridging alkoxide unit, Al(mu-OR)Al, as a consequence of a significant contribution from the zwitterionic alkoxide(-)/ammonium(+) form made possible by a strong intraligand hydrogen bond. The kinetics of the conversion of 1 into 2 have been studied. A large activation energy and positive deuterium isotope effect are consistent with breaking of the hydrogen bond during the transition state. The reaction of (Bu-t)(3)Al(NMe3)4 with ethanol yields [(Bu-t)(2)Al(mu-OEt)](2) 5. The reaction of Al(Bu-t)(3) with HN(Me)(CH2)(n)NMe2 (n = 3 or 2) yields the stable Lewis acid-base adducts (Bu-t)(3)Al[NH(Me)CH2CH2CH2NMe2] 6 and (Bu-t)(3)Al[NH(Me)CH2CH2NMe2] 7,1 respectively. The molecular structures of compounds 1-3, 6 and 7 have been confirmed by X-ny crystallography. The implications of the structures and stabilities of compounds 1, 6 and 7 are discussed with respect to the protonolysis reaction of aluminium alkyls with Bronsted acids (HX) and a new intermolecular elimination mechanism is proposed.