John B. Leach

Pentaborane(9) Derivatives, part III: Long-range homonuclear and heteronuclear magnetic resonance coupling in pentaborane(9) derivatives

Abstract Long-range heteronuclear coupling has been observed between apex alkyl proton nuclei and both the apex (5.4–7.5 Hz) and basal boron (ca. 0.8 Hz) nuclei in apex alkyl substituted pentaborane(9) derivatives. Basal methyl substituted pentaborane(9) derivatives have also been investigated, and it has been shown that homonuclear long-range coupling is observed between the methyl protons and the near bridge hydrogen (2.9–3.3 Hz) nuclei. Similar homonuclear coupling was also detected for basal unsubstituted pentaborane(9) derivatives between the basal terminal protons and the near bridge hydrogen nuclei (4.8–5.7 Hz). Also discussed is the evidence for possible nuclear quadrupole relaxation of the heteronuclear and homonuclear coupling of the basal boron nuclei to apex substituents and apex boron nuclei, respectively.

Proton magnetic resonance studies on methyl and chloro substituted diboranes

Abstract Detailed proton NMR analyses were made on the diboranes, B2H6, CH3B2H5, 1,1-(CH3)2B2H4, 1,2-(CH3)2B2H4, 1,1,2-(CH3)3B2H3, 1,1,2,2-(CH3)4B2H2 and ClB2H5. The bridge hydrogen chemical shift moves −0.40 ppm per methyl group on the diborane molecule and −1.1 ppm with a chlorine substituent. From 11B-decoupled proton spectra long-range homonuclear proton coupling could be observed in the methyl substituted diboranes: J H T −H μ = 7.5—8.7 HZ, J CH μ −H μ = 2.4−3.5HZ, J CH 3 −HT( gem ) =5.0—5.3 HZ, Both the terminal proton (on methyl and chloro diboranes) and the methyl proton chemical shifts are moved significantly upfield with substitution on the far boron atom.

Formation of silylcarbaboranes from 1,2-bis(trimethylsilyl)pentaborane(9)

By way of an apparent carbon-insertion process, 1,2-bis(trimethylsilyl)pentaborane(9) gives, on flash thermolysis, a number of C-silyl derivatives of the smallest known carbaborane, 1-H3Si-1,5-C2B3H3, 1-MeH2Si-1,5-C2B3H4, 2-Me-1-(H3Si)-1,5-C2B3H3, as well as an equilibrium mixture of 2- and 4-methyl derivatives of CB5H7. A total carbaborane yield of ca. 40 mol % has been obtained. Temperature-dependent n.m.r. studies on the B-methyl isomers of CB5H7 show a fast bridge-hydrogen tautomerism at ca. 100 °C. A substituent chemical-shift effect has been calculated for Me3Si and H3Si groups on all positions of the pentaborane(9) cage.

Organosilyl- and silylorgano-pentaboranes: monocarbahexaboranes from 2-[(chlorodimethylsilyl)methyl]pentaborane(9)

Bridge and terminally substituted pentaborane(9) derivatives of the type [(ClCH2)Me2Si] B5H8, have been prepared. Rearrangement of both the 1- and 2-terminally substituted pentaboranecompounds to the carbon-attached isomers, 1- and 2-[(ClMe2Si)CH2] B5H8, is accomplished with the use of AlCl3 as catalyst. Flash thermolysis of 2-[(ClMe2Si)-CH2] B5H8 yields SiMe2ClH (48%), B5CH9(4.2%), and B5CH7(9.4%) as the major volatile products. Alkylation of B5H9 with SiMe3(CH2Cl) in the presence of AlCl3, gives 1 -MeB5H8 and SiMe3Cl in quantitative amounts, whereas use of Si(CH2Cl)Cl3 as the alkylating agent gives only 1-[(Cl3Si)CH2]Br5H8.

Nuclear magnetic resonance evidence for bridge hydrogen tautomerization (and/or exchange) in monocarbahexaborane(7)

Temperature studies on monocarbahexaborane(7) show that the two high field doublets in the 11B n.m.r. spectrum taken at 30° collapse into a single doublet at 100°: possible mechanisms are suggested to account for this observation; all involve migration of the single bridge hydrogen.