Small boranes, carboranes, and heterocarboranes*
W.Siebert, C.-J. Maier, A. Maier, P. Greiwe, M.J. Bayer, M. Hofmann,
and H. Pritzkow
Anorganisch-Chemisches Institut der Universit�t Heidelberg,
Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany
Abstract: Dehalogenation of 1,2-dichloro-diborane(4) derivatives
with Na/K alloy does not lead to planar cyclo-tetraboranes but
to the blue puckered diisopropylamino compound and to the yellow 2,2,6,6-tetramethylpiperidino-tetraboratetrahedrane
derivative, respectively. With smaller dialkylamino substituents, the
formation of orange-red cyclo-hexaborane (BNMe2)6
and the green closo -hexaborane (BNEt2)6
is observed. When a 1:1 mixture of Me2NBCl2 and
[Me2N(Cl)B]2 is dehalogenated, a small amount
of colorless crystals of the planar diamond-shaped tetrabora-bicyclo-butane
[(Me2N)2BB -BNMe2]2 is obtained.
Its MO analysis reveals that eight framework electrons are used to form
two 3c,2e sigma bonds, one 4c,2e pi bond, and one 4c,2e sigma bond along
the edges.
A new approach to reactive closo-dicarbapentaboranes involves
the hydroboration of dichloroboryl-tbutylacetylene with HBCl2
leading to Me3CCH2C(BCl2)3.
On heating, it is transformed into the closo-dicarbapentaborane
(Me3CCH2C)2(BCl)3, in which
the chlorine atoms may be substituted to give new derivatives.
By reacting B2Cl4 with C5Me5-SiMe3
the nido-1-borane-2,3,4,5,6-pentacarbahexa- borane(6)is stabilized
by the Lewis acid BCl3 to give (MeC)5B-BCl3.
Hydroboration of 3,4-bis(isopropylidene)-1,3-diborolanes leads to the
formation of nido-2,3,5-tricarbahexaboranes(7),which may be deprotonated
to give the corresponding anions isolobal with C5H5-.
Heterotricarbahexaboranes are obtained when a 3,4-bis(dichloroboryl)-2,5-hexadiene
derivative is reacted with heptamethyldisilazane and hexamethyl-disilthiane
to give the corresponding 1,2,5-azadiborolane and thiadiborolane. Their
hydroboration leads to the aza-and thia-nido-dicarbahexaboranes,
respectively.
*Lecture presented at the XIth International Meeting
on Boron Chemistry (IMEBORON XI), Moscow, Russia, 28 July - 2 August
2002. Other presentations are published in this issue,
pp. 1157-1355.
