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
HOCH
2
CH
2
CH
2
NMe
2
allows for
the isolation of the Lewis acid–base complex,
(Bu
t
)
3
Al[O(![[upper bond 1 start]](https://www.rsc.org/images/entities/char_e010.gif)
H)CH
2
CH
2
CH
2
N![[upper bond 1 end]](https://www.rsc.org/images/entities/char_e011.gif)
Me
2
] 1, which undergoes alkane
elimination above 45 °C to yield
[(Bu
t
)
2
Al(µ-OCH
2
CH
2
CH
2
NMe
2
)]
2
2. Compound 2 is also formed
directly when 2 equivalents of Al(Bu
t
)
3
react with
1 equivalent of
HOCH
2
CH
2
CH
2
NMe
2
. 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(µ-OPr
n
)]
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
)
3
Al(NMe
3
) 4 with ethanol yields
[(Bu
t
)
2
Al(µ-OEt)]
2
5. The reaction
of Al(Bu
t
)
3
with
HN(Me)(CH
2
)
n
NMe
2
(n = 3 or 2) yields the stable Lewis
acid–base adducts
(Bu
t
)
3
Al[NH(Me)CH
2
CH
2
CH
2
NMe
2
] 6 and
(Bu
t
)
3
Al[NH(Me)CH
2
CH
2
NMe
2
] 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.
H)CH
2
CH
2
CH
2
NMe
2
] 1, which undergoes alkane
elimination above 45 °C to yield
[(Bu
t
)
2
Al(µ-OCH
2
CH
2
CH
2
NMe
2
)]
2
2. Compound 2 is also formed
directly when 2 equivalents of Al(Bu
t
)
3
react with
1 equivalent of
HOCH
2
CH
2
CH
2
NMe
2
. 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(µ-OPr
n
)]
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
)
3
Al(NMe
3
) 4 with ethanol yields
[(Bu
t
)
2
Al(µ-OEt)]
2
5. The reaction
of Al(Bu
t
)
3
with
HN(Me)(CH
2
)
n
NMe
2
(n = 3 or 2) yields the stable Lewis
acid–base adducts
(Bu
t
)
3
Al[NH(Me)CH
2
CH
2
CH
2
NMe
2
] 6 and
(Bu
t
)
3
Al[NH(Me)CH
2
CH
2
NMe
2
] 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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