Small boranes, carboranes, and heterocarboranes

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 (BNMe 2 ) 6 and the green closo-hexaborane (BNEt 2 ) 6 is observed. When a 1:1 mixture of Me 2 NBCl 2 and (Me 2 N(Cl)B) 2 is dehalogenated, a small amount of colorless crystals of the planar diamond-shaped tetrabora-bicyclo-butane ((Me 2 N) 2 BB−BNMe 2 ) 2 is obtained. Its MO analysis reveals that eight framework electrons are used to form two 3c,2e σ bonds, one 4c,2e π bond, and one 4c,2e σ bond along the edges. A new approach to reactive closo-dicarbapentaboranes involves the hydroboration of dichloroboryl-tbutylacetylene with HBCl 2 leading to Me 3 CCH 2 C(BCl 2 ) 3 . On heating, it is transformed into the closo-dicarbapentaborane (Me 3 CCH 2 C) 2 (BCl) 3 , in which the chlorine atoms may be substituted to give new derivatives. By reacting B 2 Cl 4 with C 5 Me 5 -SiMe 3 the nido-1-borane-2,3,4,5,6-pentacarbahexa- borane(6) is stabilized by the Lewis acid BCl 3 to give (MeC) 5 B−BCl 3 . 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 C 5 H 5 − . 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.