REACTIONS OF THE DIBORON TETRAHALIDES B2CL4 AND B2BR4 WITH B5H9 - PREPARATION AND PROPERTIES OF THE (DIHALOBORYL)PENTABORANE DERIVATIVES 1-BBR2B5H8, 1-BCL2B5H8, 1-BF2B5H8, 1-BOCH3(2)B5H8, 1-BTERT-BU2B5H8, 1-BH2B5H8 AND SYNTHESIS OF (BCL2)3B5H6

The reactions of B2C14with excess B5H9yield 1-BC12B5H8(73%) while those of B2Br4generate l-BBr2B5Hg(80%). Ligand exchange of 1-BC12B5H8with excess BBr3forms l-BBr2B5H8(86%), that with Hg(CF3)2results in l-BF2B5Hg(96%), that with CH3OH generates l-B(OCH3)2B5H8(46%), and that with Li(t-Bu) prepares B(t-Bu)(Cl)B5Hg(23%) and B(t-Bu)2B5H8(20%). The relative thermal stabilities of these products are BF2B5Hg> BC12B5H8> BBr2B5H8> B(OCH3)2B5Hg> B(t-Bu)2B5Hg. All of these BX2B5H8compounds (X = F, CI, OCH3, t-Bu) decompose to form BX3and B5H9as the volatile products. Reactions of BC12B5H8with excess B2C14yield (BC12)3B5H6, a compound of limited thermal stability, but no evidence for further BC12substitution on the pentaborane cage was obtained. Reductions of BC12B5HSwith LiBH4in C6H5C1 or C6H4CI2form apparent equilibrium mixtures of 1:1′,2′-[B5H8][B2H5] and 1:1′-[B5Hg][B2H5]. One or both of these compounds decompose with the evolution of B2H6, B5H9, and coupled pentaborane cages (B5H7)m, where n can be at least as large as 8. The 11B NMR and mass spectrometric evidence from the last reaction is consistent with the initial dimerization of the hexaborane 1-BH2B5H8, which is rapidly followed by the formation of 1:1′-[B5Hg][B2H5], the cross product arising from the interaction of B2H6with (BH2B5Hg)2, and then isomerization of this heptaborane to 1:1′,2′′-[B5H8][B2H5]. © 1990, American Chemical Society. All rights reserved.