Chemistry Reference
In-Depth Information
2.3
Alkoxides of p-Block Elements: Al, Ga, In, Tl
2.3.1
Aluminium
The structural chemistry of aluminium alkoxides has posed many problems, owing to
the tendency of aluminium to attain coordination numbers of 4, 5, or 6 depending on
the steric requirements of the ligands.
The first alkoxide to be structurally characterized was the crystalline tetranuclear
isopropoxide [Al
4
-OPr
i
6
OPr
i
6
] (Fig. 4.7)
53
which appears to be the thermodynami-
cally stable form. This molecule contains a central octahedral aluminium “chelated” by
three Al
-OPr
i
2
OPr
i
2
groups containing distorted tetrahedral aluminiums. This struc-
ture had been postulated twenty years earlier in order to distinguish it from the trimeric
form which was then believed to involve a ring structure with tetrahedral aluminium.
54
The crystal structure of the tetramer has been redetermined at low temperature confirming
the original structure with some improvement in the refinement.
55
The trinuclear molecule
[ClAlf
-OPr
i
2
AlCl
2
g
2
]
56
has an interesting structure containing one five-coordinated
Al (trigonal bipyramid) bonded to one terminal Cl and bridged by isopropoxo ligands
to two tetrahedrally coordinated Al atoms which are each bonded to two terminal chlor-
ides. The analogous structure [fPr
i
O
fAlf
-OPr
i
2
Al
OPr
i
2
g
2
] was proposed for the
aluminium isopropoxide trimer based on NMR and mass spectral data.
57
As expected
the structure of the tetraisopropoxo aluminate anion [Al(OPr
i
4
]
was shown to be tetra-
hedral with some distortion due to hydrogen bonding with the cyclohexylammonium
counter-ions.
58
Owing to steric hindrance of the
tert
-butoxo ligands aluminium
tert
-
butoxide gives a dimeric species [Al
2
-OBu
t
2
(OBu
t
4
] containing pseudo-tetrahedral
Al.
59
Another dinuclear species [Al
2
-NMe
2
-OBu
t
4
] was isolated as an interme-
diate in the reaction of [Al
2
(NMe
2
6
] with Bu
t
OH which gave [Al(OBu
t
3
.
Me
2
NH]
as the kinetically stable product. Interestingly the thermodynamically stable dimeric
[Al
2
(OBu
t
6
] obtained from the dimethylamine adduct does not react with the amine
to reform the adduct.
60
The structures of the dimeric dialkylaluminium derivatives of 1-
menthol and 1-borneol have been reported
61
(Table 4.3 pp. 244 - 245). The
tert
-butoxo
aluminium hydrides are worthy of comment. For AlH
2
(OBu
t
a tetranuclear cubane
structure [Al
4
3
-OBu
t
4
H
8
] with five-coordinated Al might have been anticipated,
but it was shown to be dimeric [Al
2
-OBu
t
2
H
4
] as was the monohydride [Al
2
-
OBu
t
2
OBu
t
2
H
2
]
.
62
The unsymmetrical dinuclear [(Bu
t
O
Bu
t
OO
Al
-
OBu
t
2
Al
mesal
2
] (where mesal D methylsalicylate
contains pseudo-tetrahedral and
distorted octahedral aluminiums bridged by
tert
-butoxo ligands with a terminal
tert
-
butylperoxo ligand attached to the four-coordinated metal.
63
The liquid dimeric
[Al
2
(OPr
i
4
(acac)
2
] slowly rearranges to a stable crystalline trimer, which has an
unsymmetrical structure containing two octahedral and one tetrahedral aluminium
[(acac)
2
Al
-OPr
i
2
Al
-OPr
i
2
Al(OPr
i
2
]
.
64
Some trimethylsiloxo compounds have
been structurally characterized. In [(Me
2
NH
2
C
fAl(OSiMe
3
4
g
] hydrogen bonding
between the counter-ion N-H protons and the siloxo oxygens causes some distortion
in the tetrahedral aluminium whilst the dimeric [Al
2
-OSiMe
3
2
(OSiMe
3
4
]hasa
similar Al
2
O
6
core structure to the
tert
-butoxide.
65
In the dimer [(acac)
2
Al
-OSiMe
3
2
Al
OSiMe
3
2
]
64
one Al is pseudo-tetrahed-
ral and the other pseudo-octahedral in preference to a more symmetrical structure
[Al
-OSiMe
3
2
acac
2
] containing equivalent five-coordinated metals. With the more
