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

Abstract

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)₃Al[O([upper bond 1 start]H)CH₂CH₂ CH₂N[upper bond 1 end]Me₂] 1, which undergoes alkane elimination above 45 °C to yield [(Bu^t)₂Al(µ-OCH₂CH₂CH ₂NMe₂)]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)₂Al(µ-OPrⁿ)]₂ 3, suggesting that the Al–O · · · H unit may be considered analogous to a bridging alkoxide unit, Al(µ-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)₃Al(NMe₃) 4 with ethanol yields [(Bu^t)₂Al(µ-OEt)]₂ 5. The reaction of Al(Bu^t)₃ with HN(Me)(CH₂)_nNMe₂ (n = 3 or 2) yields the stable Lewis acid–base adducts (Bu^t)₃Al[N[upper bond 1 start]H(Me)CH₂CH₂ CH₂N[upper bond 1 end]Me₂] 6 and (Bu^t)₃Al[N[upper bond 1 start]H(Me)CH₂CH₂ N[upper bond 1 end]Me₂] 7, respectively. The molecular structures of compounds 1–3, 6 and 7 have been confirmed by X-ray 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 Brönsted acids (HX) and a new intermolecular elimination mechanism is proposed.

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