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Pure Appl. Chem., 2003, Vol. 75, No. 9, pp. 1277-1286

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 Universität Heidelberg, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany

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.