_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Aluminium, gallium, indium and thallium
3. 9.4 Organohalides
While organoboron halides R 2 BX and RBX 2 are monomeric, like R,B or BX,, the
derivatives of Al, Ga and In associate through halogen bridges. Direct reaction
between aluminium and methyl or ethyl halides gives the sesquihalides.
2Al + 3RX ---> R,Al 2 X, (X= CI, Br, I; R = Me, Et. Ph).
The metal generally needs to be activated either with some preformed aluminium
alkyl or with iodine or AlCl,. Propyl and higher alkyl halides, however, give
mainly AICI, and hydrocarbons on reaction with aluminium. The sesquihalides
disproportionate
2R,Al 2 X, ~ R 2 AIX 4 + R 4 Al 2 X 2
Me,Al 2 X 3 reacts cleanly with salt, and the Me 4 Al 2 Cl 2 can be decanted from the
complex halide and then distilled.
2Me,Al 2 Cl, + 2NaCI ---> 2Na[MeAICI,] + Me.Ai 2 Cl 2
Another approach is by redistribution between R,Al and AlClr Halogen is a
better bridging group than methyl to aluminium. The structure of Me 4 Al 2 Cl 2
(Fig. 3.1) is very similar to that of the Al 2 Cl 6 dimer.
The fluoride [Me 2 AlF] 4 is tetrameric in the crystal. lt is isoelectronic with
[Me 2 Si0) 4 (p. 113) and has a very similar structure. The eight-membered ring is
puckered and the bond angles LAlFAllarge (146'). Four coordinate aluminium
is also present in alkoxides and amino- derivatives which result from partial
reaction of R,Al with alcohols or amines (p. 82 ). Cu ba ne type tetramers e.g.
(PhAlNPh) 4 are formed when triphenylaluminium is heated with an arylamine.
Ph,Al + PhNH 1 ---> (PhAINPh) 4 + 2PhH
Their structure should be contrasted with that of borazines. Other oligomers
(RAlNR')" (n = 6, 7, 8) have also been confirmed in other systems (Fig. 3.20).
M~ Me Me
\/ F /
AL--~ '.....,.Me
/ Al
F)146.1 \
Me~.\ /
131.2o! _,Al)~2.3 Al
Me" l
"F--, ',,
1.947 'Me
(a) Me
"" /
Ph Ph
Ph /N Al
~ /: /
Al , N-Ph
) 1/N"'-.
/ Ph Ph
N AL
/ ""
Ph (b) Ph
Fig. 3.20 Structures of some oligomeric organoaluminium compounds. (a) [Me 2 AIF] 4 ,
puckered ring; (b) (PhAINPh) 4 , cubane structure; (c) (MeAINMe) 7 , methyl
groups omitted. See also Fig. 3 .1.
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