Donald F. Gaines

Electrospray mass spectrometry of borane salts: The electrospray needle as an electrochemical cell

Two borane salts ([(Me)4N][B3H8] and Cs[B3H8]) were examined by electrospray mass spectrometry in the positive ion mode. Acetonitrile solutions provided the most informative spectra; the salts exhibited a remarkable degree of clustering under electrospray conditions, and virtually all signals corresponded to cationic cluster ions of the general formula {[cationm+]x[anionn−]y}(mx − ny)+. In contrast, methanol solutions of these salts produced only B(OCH3)4− cluster ions under otherwise identical conditions. 11B NMR analyses corroborate the identities of the methanol solution species that enter the electrospray source and the reaction product generated during the electrospray process.

9Be NMR of beryllaboranes and other beryllium compounds

Abstract 9 Be has proved to be a very observable NMR nucleus, exhibiting a range of linewidths and chemical shifts somewhat smaller than that of 11 B. A selection of 9 Be NMR data for beryllaboranes is correlated with data from other beryllium compounds to illustrate the analytical value of 9 Be NMR in the area of beryllium chemistry.

Studies of 2,5;6,10;8,10-tri-µ-hydro-nonahydro-nido-nonaborate(1–), [B9H12]–: preparation, crystal and molecular structure, nuclear magnetic resonance spectra, electrochemistry, and reactions

Studies of 2,5;6,10;8,10-tri-µ-hydro-nonahydro-nido-nonaborate(1–)[B9H12]– have involved new syntheses, which have led to crystals suitable for the determination of the X-ray crystal and molecular structure, through reactions of B9H13(SMe2) with [OCN]– or other bases. Crystallographic study of [N(PPh3)2][B9H12] reveals the structure of the anion to be that of a nido-nine-vertex cage, based on the parent bicapped square antiprism with one 5-connected vertex removed. The pentagonal open face is symmetrically bridged by three (µ-H) atoms (two involving the lowest connected boron), conferring effective Cs symmetry upon the polyhedron. Crystals are monoclinic, space group P21/n, with a= 26.759(10), b= 10.340(4), c= 27.044(5)A, β= 103.47(2)°, and Z= 8. Using 6 777 diffracted intensities recorded at 185 K on an Enraf-Nonius CAD4 diffractometer, the structure has been refined to R 0.0847, R′ 0.1216. The 11B, two-dimensional 11B(COSY), and 1H n.m.r. parameters all confirm the structural assignment and the n.m.r. spectra have been assigned unambiguously. Cyclic and a.c. voltammetry and coulometry of [B9H12]– in several solvents at Pt led to the electrochemical parameters and to an isomer-specific electrochemical synthesis of anti-[B18H21]–. Chemically, the [N(PPh3)2]+ salt of [B9H12]– reacted with HCl to give [B9H13Cl]–, whereas the [NBun4]+ salt yielded anti- B18H22; reactions with CF3CO2H gave primarily anti-[B18H21]–.

Boron insertion reactions : IV. the synthesis of 1- and 2-[(CH3)3MIV]-μ-[(CH3)2B]B5H7 (MIV = Si, Ge)

Abstract Anions derived from 1- and 2-[(CH3)3MIV]B5H8 by proton abstraction reaction with (CH3)2BCl to produce 1- and 2-[(CH3)3MIV]-μ-[(CH3)2B]B5H7 (MIV = Si, Ge). Spectroscopic evidence indicates that the (CH3)2B moiety occupies a bridging position between two boron atoms in the base of the pentaborane(9) pyramid. When the (CH3)3MIV substituent is in a 2-position it appears to be attached to a boron atom adjacent to the bridging (CH3)2B group. These new B5H9 derivatives do not undergo the expected rearrangement to a corresponding hexaborane(10) derivative, whereas μ-[(CH3)2B]B5H8 itself rearranges readily.

2-Substituted icosahedral monocarbon carboranes. Part 1. Synthesis via boron insertion

New 2-substituted icosahedral monocarbon carboranes have been synthesised by functionalised boron insertion. The compounds [NBun4][2-Ph-closo-1-CB11H11]1, [PPh4][2-(p-MeC6H4)-closo-1-CB11H11]2, [N(CH2Ph)Me3][2-F-closo-1-CB11H11]3, 2-Me3N-closo-1-CB11H114 and [PPh4][2-Cl(CH2)4O-closo-1-CB11H11]5 were characterised by 1H, 11B and 11B–11B correlation NMR spectroscopy. The crystal structure of 5 shows a cage distortion that is postulated to result from π interaction of the substituent with the cluster. Crystal data: triclinic, space group P, a= 10.672(2), b= 11.047(2), c= 14.203(3)A, α= 75.97(2), β= 87.84(2), γ= 83.22(2)°, Z= 2. Final R= 0.042 for 3951 reflections with I > 2σ(I).

Metal-pentaborane(9) derivatives: 2-[Co(CO)4]B5H8 and 2[(η5-C5H5)Fe(CO)2]B5H8

Abstract Reaction of 2-XB 5 H 8 (X  Cl, Br) with Naco(CO) 4 produces the transiently stable 2-[Co(CO) 4 ]B 5 H 8 . The similar 2-[(η 5 -C 5 H 5 )Fe(CO) 2 ]B 5 H 8 , which exhibits much greater thermal stability, is prepared by reaction of LiB 5 H 8 with (η 5 -C 5 H 5 )Fe(CO) 2 I. Reactions of CO 2 (CO) 8 with B 5 H 9 under a variety of conditions produce 2-[Co(CO) 4 ]B 5 H 8 along with an inseparable impurity that appears to be 1-[Co(CO) 4 ]B 5 H 8 .

Metastable and collision-induced fragmentation studies of all C4H12Si+· isomers; a systematic study of structure-reactivity relations

Metastable ion (MI) and collision-induced dissociation (CID) mass spectra have been recorded and compared for all nine C4H12Si+. isomers. The (Me)4Si+., t-BuSiH3+., s-BuSiH3+, and (Me)2EtSiH+. isomers have unique MI and CID mass spectra. The MI mass spectra, including the kinetic energy release values, of (Me)(i-Pr)SiH2+. and (Me)(n-Pr)SiH2+. are identical, which implies isomerization. MI data also suggest that a fraction of the n-BuSiH3+. ions rearrange into branched (Me)2EtSiH+. ions and a fraction of the n-BuSiH3+. ions rearrange into branched s-BuSiH3+. ions. A comparison with the isomeric C5H12+. pentanes reveals a crucial difference: H2 loss occurs for n-BuSiH3+., i-BuSiH3+., s-BuSiH3+., (Me)(n-Pr)SiH2+., (Me)(i-Pr)SiH2+., and Et2SiH2+., but not for any of the C5Hi12+. isomers. Generation of four- or five-membered silicon containing rings is suggested for H2 loss from the C4H12Si+. silanes.

Selective syntheses of bipentaboranes: 1,2′-(B5H8)2 and 2,2′-(B5H8)2

The Friedel–Crafts catalysed reaction of 2-Br-B5H8 with B5H9 produces 1,2′-(B5H8)2, whereas metathesis of KB5H8 with 2-BrB5H8 produces 2,2′-(B5H8)2.

Metastable and collision-induced fragmentation studies and thermochemistry of isomeric C4H11Si+ ions and their adducts with C4H12Si silanes

Metastable Ion (MI) and collision-induced dissociation (CID) mass spectra for all isomeric even-electron [C(4)H(12)Si - H](+) ions were recorded and compared. Deuterium labeling experiments indicated that most precursors give rise to silylium ions. Silylium ions with two or more methyl groups are found to lose C(2)H(4) after isomerization via a straightforward hydrogen transfer to the appropriate ethylsilylium ion. Similarly, all isomeric propyl- and butyl-containing silylium ions are found to lose C(2)H(4) by rearrangement preceding dissociation. In the CI source of the mass spectrometer many of the silylium ions are found to cluster with the parent neutral silane present in the source to give stable [M - H](+)+M adduct ions. The MI and CID spectra of these adduct ions were also obtained. In the MI spectra of all adducts, except i-BuSiH(3), only the starting silylium ion is observed. Under CID conditions generation of silylium ions dominates. Deuterium labeling studies show that this dissociation may be accompanied by some rearrangement, in particular for the adducts from i-BuSiH(3). High-pressure mass spectrometric clustering equilibrium measurements were also carried out to determine the enthalpies and entropies of binding of the silylium ions to the neutral silanes. These measurements yield insight into the effects of various alkyl group substitutions on the association thermochemistry in these adducts. Copyright 2000 John Wiley & Sons, Ltd.

Interactions of Metal Groups with the Octahydrotriborate(1−) Anion, B3H8-

During the last decade interest in metal-containing cluster molecules has increased dramatically. This interest is in large part due to a growing mass of evidence supporting the importance of metal-containing clusters as catalysts and reaction centers in biochemical and industrial systems. The use of preformed borane and carborane clusters in the synthesis of metal-borane and metalcarborane clusters provides unique access to the study of metals in clusters. The formation of clusters containing the B3H8 − anion as a bi-, tri-, or tetradentate ligand constitutes an increasingly revealing part of the overall area of metal-borane clusters.