Synthesis and structural studies of subicosahedral adjacent-carbon carboranes

Article abstract of DOI:10.1021/ja961112m

Synthesis and structural studies, employing combined NMR, X-ray crystallographic, and ab initio/IGLO/NMR methods, of a variety of new subicosahedral carboranes with adjacent cage carbons are reported. Acetonitrile-induced cage degradation of arachno-4,5-C₂B₇H₁₂^- gave nido-4,5-C₂B₆H₉^-(1^-) in nearly quantitative yield, which can then be protonated to give the neutral carborane nido-4,5-C₂B₆H₁₀ (1) in good yield. Both of these nido electroncount clusters are shown to have an arachno-type geometry, i.e. a six-membered open face. The nido-4,5-C₂B₆H₁₀ (1) hydroborated alkenes or alkynes which following deprotonation gave nido-7-R-4,5-C₂B₆H₈^- (2a^--c^-) ions. Both nido-4,5-C₂B₆H₉^- (1^-) and nido-4,5-C₂B₆H₁₀ (1) serve as useful precursors to other adjacent cage-carbon clusters. Thus, nido-4,5-C₂B₆H₉^- (1^-) reacted with BH₃·THF to give arachno-5,6-C₂B₇H₁₂^- (3^-) which a single-crystal X-ray diffraction study showed is the first carborane to adopt the n-B₉H₁₅ cage geometry. Thermal or chemical degradation of nido-4,5-C₂B₆H₁₀ (1) gave closo-2,3-C₂B₅H₇ (5) in good to moderate yields. The nido-4,5-C₂B₆H₉^- (1^-) was also found prone to lose a cage boron as evidenced by its reactions with (η-C₅H₅)Co(CO)1₂ and (η⁶-C₆Me₆)₂Ru₂Cl₄ which gave closo-3,1,2-(η-C₅H₅)CoC₂B₅H₇ (6) and closo-3,1,2-(η⁶-C₆Me₆)RuC₂B₅H₇ (7), respectively. NMR studies showed the nido-4,5-C₂B₆H₁₀ (1) was converted to arachno-4,5-C₂B₆H₁₁^- by reaction with LiEt₃BH, and an alkyl derivative, arachno-7-CH₃-4,5-C₂B₆H₁₀^- (4^-), was formed by reacting MeLi with nido-4,5-C₂B₆H₉^- (1^-) followed by protonation. The closo-2,3-C₂B₅H₇ (5) was also converted in high yields to the smaller nido carborane, nido-2,3-C₂B₄H₈, via reaction with TMEDA/H₂O, and to nido-3,4-C₂B₅H₈^- (8^-) by reaction with LiEt₃BH.

Full text of DOI:10.1021/ja961112m

J. Am. Chem. Soc. 1996, 118, 11423-11433
11423
Synthesis and Structural Studies of Subicosahedral
Adjacent-Carbon Carboranes
Joseph W. Bausch,*,† Derek J. Matoka,‡ Patrick J. Carroll,‡ and
Larry G. Sneddon*,‡
Contribution from the Departments of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323, and VillanoVa UniVersity,
VillanoVa, PennsylVania 19085-1699
ReceiVed April 4, 1996. ReVised Manuscript ReceiVed September 9, 1996X
Abstract: Synthesis and structural studies, employing combined NMR, X-ray crystallographic, and ab initio/IGLO/
NMR methods, of a variety of new subicosahedral carboranes with adjacent cage carbons are reported. Acetonitrile-
induced cage degradation of arachno-4,5-C2B7H12- gave nido-4,5-C2B6H9- (1-) in nearly quantitative yield, which
can then be protonated to give the neutral carborane nido-4,5-C2B6H10 (1) in good yield. Both of these nido electron-
count clusters are shown to have an arachno-type geometry, i.e. a six-membered open face. The nido-4,5-C2B6H10
-
(1) hydroborated alkenes or alkynes which following deprotonation gave nido-7-R-4,5-C2B6H8 (2a--c-) ions.
Both nido-4,5-C2B6H9- (1-) and nido-4,5-C2B6H10 (1) serve as useful precursors to other adjacent cage-carbon clusters.
Thus, nido-4,5-C2B6H9- (1-) reacted with BH3‚THF to give arachno-5,6-C2B7H12- (3-) which a single-crystal X-ray
diffraction study showed is the first carborane to adopt the n-B9H15 cage geometry. Thermal or chemical degradation
-
of nido-4,5-C2B6H10 (1) gave closo-2,3-C2B5H7 (5) in good to moderate yields. The nido-4,5-C2B6H9 (1-) was
also found prone to lose a cage boron as evidenced by its reactions with (η-C5H5)Co(CO)I2 and (η6-C6Me6)2Ru2Cl4
which gave closo-3,1,2-(η-C5H5)CoC2B5H7 (6) and closo-3,1,2-(η6-C6Me6)RuC2B5H7 (7), respectively. NMR studies
-
showed the nido-4,5-C2B6H10 (1) was converted to arachno-4,5-C2B6H11 by reaction with LiEt3BH, and an alkyl
derivative, arachno-7-CH3-4,5-C2B6H10- (4-), was formed by reacting MeLi with nido-4,5-C2B6H9- (1-) followed
by protonation. The closo-2,3-C2B5H7 (5) was also converted in high yields to the smaller nido carborane, nido-
-
2,3-C2B4H8, via reaction with TMEDA/H2O, and to nido-3,4-C2B5H8 (8-) by reaction with LiEt3BH.
Materials. THF, pentane, hexanes, and benzene were dried over
Na/benzophenone and freshly distilled before use. Acetonitrile, dried
over CaH2, then P2O5, and methylene chloride, dried over CaH2, were
distilled before use. Acidification of 1,8-bis(dimethylamino)naphtha-
lene (Proton Sponge) with a 1 M Et2O solution of HCl was used to
generate the hydrochloride salt. The arachno-4,5-C2B7H13,4 (η-
Introduction
The small carboranes (subicosahedra) were first synthesized
in the early 1960s by high temperature cage growth reactions
involving boranes and acetylenes.1 Because of the lower
stability of adjacent-carbon carboranes relative to isomerization
to their non-adjacent isomers, the high temperature reactions
yielded predominantly non-adjacent carbon products. Likewise,
the high temperature reactions typically gave low yields and
complex mixtures of products that then required extensive
purification. Gas-phase methods for small carborane synthesis
are now giving way to solution based procedures that, because
of their milder conditions, generally have much higher yields,
selectivities, and easier purification, allowing the isolation of
kinetic rather than thermodynamic products.2 In this paper we
report the synthesis and structural studies of a variety of new
subicosahedral adjacent-carbon carboranes via selective cage
degradation and insertion reactions.
6
C5H5)Co(CO)I2,5 and (η6-C6Me6)2Ru2Cl4 were prepared according to
literature methods. All other materials were obtained from Aldrich
and used as received.
Physical Measurements. 1H NMR at 200 or 500 MHz, 11B NMR
at 64.2 or 160.5 MHz, and 13C NMR at 50.3 or 125.7 MHz were
obtained on Bruker AF-200 or Bruker AM-500 spectrometers. All 11
B
chemical shifts are referenced to BF3‚O(C2H5)2 (0.0 ppm) with a
negative sign indicating an upfield shift. All 1H and 13C chemical shifts
were measured relative to internal residual protons or carbons from
the lock solvents and then referenced to Me4Si (0.0 ppm). Infrared
spectra were obtained on a Perkin-Elmer 1430 spectrophotometer and
can be found in Supporting Information.
K+nido-4,5-C2B6H9- (1-). A 100-mL three-neck round-bottom flask
equipped with a vacuum stopcock, septum, sidearm addition-funnel,
and stirbar was charged with 1.50 g (13.3 mmol) of freshly prepared
arachno-4,5-C2B7H13 under a N2 atmosphere. Following the in Vacuo
addition of THF (∼50 mL), an excess of KH was slowly added via the
sidearm at -50 °C until H2 evolution had ceased. The K+arachno-
Experimental Section
All manipulations were carried out using standard high vacuum or
inert-atmosphere techniques as described by Shriver.3
† Villanova University.
‡ University of Pennsylvania.
(3) Shriver, D. F.; Drezdzon, M. A. Manipulation of Air SensitiVe
Compounds, 2nd ed.; Wiley: New York, 1986.
X Abstract published in AdVance ACS Abstracts, November 1, 1996.
(1) See the following, and references therein: (a) Williams, R. E. In
Progress in Boron Chemistry; Brotherton, R. J., Steinberg, H., Eds.;
Pergamon Press: Oxford, 1970; Vol. II, pp 37-118. (b) Grimes, R. N.
Carboranes; Academic: New York, 1970. (c) Onak, T. In Boron Hydride
Chemistry; Muetterties, E. L., Ed.; Academic: New York, 1973; pp 349-
382. (d) Onak, T. In ComprehensiVe Organometallic Chemistry; Wilkinson,
G., Stone, F. G. A., Abel, E., Eds; Pergamon: Oxford, 1982; Chapter 5.4,
pp 411-458.
(4) (a) Herˇma´nek, S.; Jel´ınek, T.; Plesˇek, J.; Sˇt´ıbr, B.; Fusek, J. J. Chem.
Soc., Chem. Commun. 1987, 927-928. (b) Sˇt´ıbr, B.; Herˇma´nek, S.; Plesˇek,
J. Inorg. Synth. 1983, 22, 237-239.
(5) King, R. B. Inorg. Chem. 1966, 5, 82-87.
(6) Bennett, M. A.; Huang, T.-N.; Matheson, T. W.; Smith, K. A. Inorg.
Synth. 1982, 21, 74-76.
(7) Wrackmeyer, B.; No¨th, H. Chem. Ber. 1976, 109, 3480-3485.
(8) Jel´ınek, T.; Sˇt´ıbr, B.; Herˇma´nek, S.; Plesˇek, J. J. Chem. Soc., Chem.
Commun. 1989, 804-805.
(2) Sˇt´ıbr, B.; Plesˇek, J.; Jel´ınek, T.; Basˇe, K.; Janousˇek, Z.; Herˇma´nek,
S. Boron Chemistry; Herˇma´nek, S., Ed.; World Scientific Publishing Co.
PTE, Ltd.: Singapore, 1987; pp 175-206 and references therein.
(9) Beck, J. S.; Quintana, W.; Sneddon, L. G. Organometallics 1988, 7,
1015-1016.
S0002-7863(96)01112-2 CCC: $12.00 © 1996 American Chemical Society

11424 J. Am. Chem. Soc., Vol. 118, No. 46, 1996
Bausch et al.
-
4,5-C2B7H12 solution was brought to room temperature and filtered,
structure of PSH+3- was established crystallographically with suitable
single crystals obtained by cooling a CH2Cl2/Et2O/heptane solution of
PSH+3- at -25 °C.
and the filtrate was transferred to a 250-mL two-neck round-bottom
flask equipped with a stirbar, vacuum stopcock, and septum. Following
vacuum transfer of CH3CN (∼50 mL), the vacuum stopcock was
replaced with a condenser and the solution refluxed under a N2
atmosphere. According to 11B NMR spectroscopy, the reaction was
complete within 24-36 h. Any insoluble material, if formed, was
removed by filtration. The volatiles were vacuum evaporated leaving
K+nido-4,5-C2B6H9- (1-) as a yellow solid, which according to NMR
was contaminated with N,N,N-Et3B3N3H3 (δ 11B 33.1 ppm).7 Washing
the solid with two 50-mL portions of benzene afforded 1.76 g (12.8
NMR Study of Reaction of 1 with LiEt3BH. A 25-mL two-neck
round-bottom flask equipped with a stirbar, vacuum stopcock, and
rubber septum was attached to the vacuum line and 0.15 g (1.52 mmol)
of nido-4,5-C2B6H10 (1) followed by ∼10 mL of CH2Cl2 were vacuum
transferred into the flask. The flask was maintained at -30 °C, and
under nitrogen, ∼1 equiv (1.6 mL) of a 1.0 M THF solution of LiEt3BH
was added via syringe. When brought to room temperature, a 11B NMR
spectrum (see Supporting Information) of the solution showed the
-
mmol, 96.6%) of K+nido-4,5-C2B6H9 (1-) as a pale yellow, slightly
air-sensitive solid.
- 8
-
arachno-4,5-C2B6H11 (∼60%) and nido-4,5-C2B6H9 (1-) (∼40%)
ions.
The Bu4N+ salt of nido-4,5-C2B6H9- (1-) was prepared by addition
of a THF solution of (n-Bu)4NBr to a THF solution of K+nido-4,5-
C2B6H9- (1-). This mixture was then stirred at room temperature, KBr
was removed by filtration, and toluene was added to the solution until
cloudiness appeared. The solution was cooled at 0 °C to obtain crystals
NMR Study of Reaction of 1- with Methyllithium. To a THF
solution of K+nido-4,5-C2B6H9- (1-) in a NMR tube equipped with a
rubber septum was added a slight excess of a 1.4 M Et2O solution of
CH3Li at 0 °C. The solution was heated at 40 °C for 1 h until its 11
B
2-
NMR spectrum showed the formation of arachno-7-CH3-C2B6H9
-
suitable for crystallographic studies. For Bu4N+nido-4,5-C2B6H9
(42-), then ∼1 equiv of PSH+Cl- was added to form arachno-7-CH3-
4,5-C2B6H10- (4-) as shown by 11B NMR (see Supporting Information).
closo-2,3-C2B5H7 (5) via Thermal Degradation. A 0.70-g (7.07
mmol) sample of nido-4,5-C2B6H10 (1) was slowly passed (by keeping
it at -22 °C) through a 350 °C “hot tube” reactor (a 15 in. Pyrex glass
tube of 1 in. diameter) connected to a vacuum line. The condensable
products were collected in a -196 °C trap, then fractionated through
a 0, -45, -78, and -196 °C series of traps. According to 11B NMR,
the -78 °C trap contained a 5:1 mixture of closo-2,3-C2B5H7 (5) and
closo-1,7-C2B6H8, but they could not be further separated using vacuum
fractionation. Based on this ratio the estimated yield of closo-2,3-
C2B5H7 (5) was 0.33 g (3.8 mmol, 54%). The -196 °C trap contained
closo-2,4-C2B5H7 and B2H6. Refractionation of this material gave 0.04
g (0.47 mmol, 6.6%) of closo-2,4-C2B5H7 in a -100 °C trap. The 0
and -45 °C traps contained small amounts of larger closo carboranes
(1,6-C2B7H9, 1,6-C2B8H10, m-C2B10H12) as shown by 11B NMR and
mass spectroscopy.
(Bu4N+1-): mp 92-93 °C; elemental analyses for Bu4N+1- gave larger
than normal deviations (Anal. Calcd: C, 63.51; H, 13.32, N, 4.11.
Found: C, 59.73; H, 12.75; N, 4.12) but the structure of Bu4N+1- was
determined crystallographically (Supporting Information Tables S2-
S7), and exact mass measurements of the protonated form (1, see below)
are consistent with the proposed formula.
nido-4,5-C2B6H10 (1). A 250-mL round-bottom flask fitted with a
vacuum stopcock and stirbar was charged with 1.19 g (8.7 mmol) of
K+nido-4,5-C2B6H9- (1-) under a N2 atmosphere. Pentane (∼35 mL)
was added by vacuum distillation. The pentane suspension of K+nido-
-
4,5-C2B6H9 (1-) was brought to -50 °C, and an excess of HCl gas
was expanded into the vacuum line and allowed to react with the
solution for ∼2 h. When the HCl addition appeared complete, the
solution was vacuum fractionated through a series of -20, -78, and
-196 °C traps. The nido-4,5-C2B6H10 (1) condensed in the -78 °C
trap as an air- and temperature-sensitive, white crystalline solid in an
optimized yield of 0.601 g (6.17 mmol, 70.0%). For nido-4,5-C2B6H10
closo-2,3-C2B5H7 (5) via Thermolysis of 2b. Dry dodecane (∼15
mL) was placed in a 50-mL vacuum flask equipped with a stirbar, then
0.15 g (1.52 mmol) of nido-4,5-C2B6H10 (1) and a ∼5-fold excess of
2-butyne were vacuum transferred to the flask. The solution was heated
at 50 °C for 1 h at which point a 11B NMR spectrum showed complete
formation of nido-7-(cis-2-but-2-enyl)-4,5-C2B6H9 (2b). The solution
was heated further at 110 °C for 1.5 h, and a 11B NMR spectrum of
this solution showed the presence of closo-2,3-C2B5H7 (5) and a broad
resonance (65 ppm) in the region typically found for alkyl boranes.
This solution was then fractionated through a -20, -50, -78, and
-196 °C series of traps. An unoptimized yield of 0.045 g (0.53 mmol,
34.9%) of closo-2,3-C2B5H7 (5) was isolated in the -78 °C trap. Exact
1
(1): mp ≈ -50 °C; exact mass calcd for 12C211B6 H10 100.1341, found
100.1349.
-
PSH+nido-7-(R)-4,5-C2B6H8 (2a--c-). In a typical reaction, a
one-neck vacuum flask fitted with a stirbar was charged with ∼2 mmol
of nido-4,5-C2B6H10 (1) and an excess of phenyl acetylene, 2-butyne,
or 1-hexene, respectively. The reaction was stirred until completion,
then 1.2 equiv of Proton Sponge was added. The precipitated salt was
filtered, then washed and/or recrystallized. The yields were as
follows: PSH+nido-7-(trans-2-phenylethenyl)-4,5-C2B6H8- (2a-) (83.2%),
PSH+nido-7-(cis-2-but-2-enyl)-4,5-C2B6H8- (2b-) (77.5%), PSH+nido-
-
7-(n-hexyl)-4,5-C2B6H8 (2c-) (41.3%). Details are given in the
Supporting Information. To establish the composition of 2a-, in a
separate reaction, after the formation of 2a, nido-7-(trans-2-phenyl-
ethenyl)-4,5-C2B6H9, was observed, the volatiles were then evaporated
in Vacuo and the remaining oily material dissolved in CH2Cl2. Exact
mass measurements on 2a confirm its composition: exact mass calcd
1
mass calcd for 12C211B5 H7 86.1013, found 86.1011. A small amount
of closo-1-(butenyl)-2,3-C2B5H6 condensed in the -50 °C trap together
with dodecane.
closo-3,1,2-(η-C5H5)CoC2B5H7 (6) and closo-3,1,2-(η6-C6Me6)-
RuC2B5H7 (7). In separate reactions, CH2Cl2 solutions of (η-
C5H5)Co(CO)I2 (0.684 g) and (η6-C6Me6)2Ru2Cl4 (0.342 g) were added
12
1
for 2a,
C
11B6 H16 202.1810, found 202.1831. Furthermore, the
10
similarity of the spectroscopic data of 2a- with that of the X-ray
characterized butenyl analog 2b- (below) provides additional structural
confirmation. In a separate experiment carried out in a 5-mm NMR
tube equipped with a Teflon stopcock, nido-7-(octyl)-4,5-C2B6H9 (2d)
carboranes were generated by reaction of ∼0.1 mmol of nido-4,5-
C2B6H10 (1) and ∼0.7 mL of octenes (a mixture of 1- and 2-octenes)
at 60 °C for 1 h. 11B NMR showed only the formation of nido-7-
(octyl)-4,5-C2B6H9 (2d) carboranes.
-
dropwise to 1.53 and 1.02 mmol of K+nido-4,5-C2B6H9 (1-),
respectively, dissolved in ∼20 mL of THF. After stirring overnight,
the mixtures were filtered and separated by column chromatography
to give closo-3,1,2-(η-C5H5)CoC2B5H7 (6) (20 mg, 6.2% yield) and
closo-3,1,2-(η6-C6Me6)RuC2B5H7 (7) (56 mg, 20.5% yield). For the
dark red-orange colored closo-3,1,2-(η-C5H5)CoC2B5H7 (6): mp 124-7
°C; exact mass calcd for 12C711B559Co1H12 210.0736, found 210.0742;
Anal. Calcd: C, 40.20; H, 5.78. Found: C, 39.92; H, 5.70. For the
-
arachno-5,6-C2B7H12 (3-). A solution of BH3‚THF (1 mL of a
yellow solid closo-3,1,2-(η6-C6Me6)RuC2B5H7 (7): mp 150 °C dec;
12
1.0 M THF) was slowly added via syringe to 0.12 g (0.88 mmol) of
K+nido-4,5-C2B6H9- (1-) dissolved in ∼20 mL of THF at 0 °C. After
warming to room temperature, the volatiles were removed to give 0.13
exact mass calcd for
C
11B5101Ru1H25 350.1465, found 350.1455.
14
Anal. Calcd: C, 48.25; H, 7.23. Found: C, 47.97; H, 7.32.
NMR Study of Reaction of 5 with LiEt3BH. To a NMR tube
equipped with a Teflon stopcock was vacuum transferred ∼0.1 mmol
of closo-2,3-C2B5H7 (5) and ∼0.7 mL of THF, then via syringe an
excess of a 1.0 M THF solution of LiEt3BH was added. A 11B NMR
spectrum (see Supporting Information) showed complete conversion
-
g (0.86 mmol, 97.7%) of K+arachno-5,6-C2B7H12 (3-) as a slightly
air-sensitive, pale yellow solid.
-
The PSH+arachno-5,6-C2B7H12 salt (PSH+3-) was prepared by
addition of ∼10 mL of a CH2Cl2 solution of PSH+Cl- (0.22 g, 0.88
-
-
mmol) to a ∼10 mL THF solution of K+arachno-5,6-C2B7H12 (3-)
to nido-3,4-C2B5H8 (8-), as evidenced by comparison with the
-
(0.12 g, 0.79 mmol), followed by ∼10 mL of Et2O. Filtration removed
the KCl, and addition of heptane then precipitated the PSH+3-. The
literature values9 for nido-3,4-Et2C2B5H6 and the ab initio/IGLO
calculated shifts.

Studies of Subicosahedral Adjacent-Carbon Carboranes
J. Am. Chem. Soc., Vol. 118, No. 46, 1996 11425
nido-2,3-C2B4H8. A 25-mL two-neck round-bottom flask equipped
with a stirbar, vacuum stopcock, and septum was charged with ∼10
mL of dodecane and ∼2 mL of tetramethylethylenediamine (TMEDA).
Following the vacuum transfer of ∼2 mmol of closo-2,3-C2B5H7 (5)
(measured by gas volume), the solution was brought to room temper-
ature. A 11B NMR spectrum (Table 1) then showed complete formation
of a TMEDA adduct, presumably nido-6-(TMEDA)-3,4-C2B5H7 (9).
The solution was then maintained at ∼15 °C and ∼0.5 mL of degassed
H2O was added by syringe. The solution was stirred vigorously while
the volatiles were continuously vacuum fractionated through -78 and
-196 °C traps. After ∼3 h, 0.012 g (0.154 mmol, 77.0%) of nido-
2,3-C2B4H8 had been trapped at -196 °C. The product was character-
ized by comparison of its spectroscopic data with literature values.10
X-ray Crystallographic Studies. Details of the collection, solution,
and refinement of the crystallographic data for PSH+2b- and PSH+3-
are in the Supporting Information. The structural determination of
PSH+1- is described in the earlier communication11 and in the
Supporting Information.
Computational Methods. The details of the methods employed
have been given elsewhere.12 The NMR chemical shifts were calculated
using the IGLO method13,14 employing Huzinaga Gaussian lobe
functions15 with specific basic sets described previously.12 B2H6 is the
primary reference for the 11B NMR chemical shifts and the δ values
were converted to the BF3‚O(C2H5)2 scale using the experimental value
of δ(B2H6) ) 16.6 ppm.16 13C shifts are referenced to the experimental
standard, tetramethylsilane (TMS). Because of the satisfactory agree-
ment between the calculated and experimental values, the IGLO
calculated 13C NMR data have been included in Table 2; however, it
should be noted that previous studies17,18 indicate the levels of theory
employed in this study may not always be suitable for the accurate
calculation of cage carbons in carboranes. Comparisons of the
Figure 1. Proton spin-decoupled 160.5-MHz 11B NMR spectra of (a)
-
nido-4,5-C2B6H9 (1-) and (b) nido-7-(trans-2-phenylethenyl)-4,5-
-
C2B6H8 (2a-).
from the acetonitrile-induced cage degradation reaction of the
conjugate anion of arachno-4,5-C2B7H13:
1
calculated and available experimental H NMR data have not been
included because 1H shifts currently cannot be accurately calculated.19
The GIAO-MP2 electron correlated chemical shift calculation as
implemented by J. Gauss20 was carried out using the ACESII program
package21 employing the tzp Ahlrichs basis set22 for C and B, which
includes one set of d polarization functions. H is described by a
double-ú basis set.
The cage degradation observed in this reaction is different
than the reaction of CH3CN with the isomeric arachno-6,8-
Results and Discussion
C2B7H12- anion, which results in the monocarbon cage insertion
- 23
-
The new carborane anion nido-4,5-C2B6H9 (1-) was pre-
product nido-6-CH3-5,6,9-C3B7H9
.
This difference in reac-
tivity is probably a consequence of the fact that the 4,5-isomer
contains two BH2 groups while the 6,8-isomer has a single
BH2.24 The CH3CN appears to cause the base-induced cleavage
of BH3 from one of the BH2 groups of the 4,5-isomer with
subsequent rearrangement of the resulting CH3CN•BH3 adduct
to 1,3,5-Et3B3N3H3.25
pared in high yields, as described in the Experimental Section,
(10) Onak, T.; Drake, R. P.; Dunks, G. B. Inorg. Chem. 1964, 3, 1686-
1690.
(11) Kang, S. O.; Bausch, J. W.; Carroll, P. J.; Sneddon, L. G. J. Am.
Chem. Soc. 1992, 114, 6248-6249.
(12) Bausch, J. W.; Rizzo, R. C.; Sneddon, L. G.; Wille, A. E.; Williams,
R. E. Inorg. Chem. 1996, 35, 131-135.
(13) See: Kutzelnigg, W.; Fleischer, U.; Schindler, M. In NMR,
Principles and Progress; Diehl, P., Fluck, E., Gu¨nther, H., Kosfeld, R.,
Seelig, J., Eds.; Springer-Verlag: Berlin, 1990; Vol. 23, pp 165-262 and
references therein.
(14) For examples of the IGLO method applied to boron compounds,
see footnote 17 in: Diaz, M.; Jaballas, J.; Tran, D.; Lee, H.; Arias, J.; Onak,
T. Inorg. Chem. 1996, 35, 4536-4540.
(15) Huzinaga, S., Gaussian Basis Sets for Molecular Calculations;
Elsevier: New York, 1984.
The 11B NMR spectrum of 1- (Figure 1a) shows four doublet
resonances in 2:1:2:1 ratios, indicating the presence of a cage
mirror plane, with each boron containing one terminal hydrogen.
1
The H NMR spectrum of 1- is likewise consistent with Cs
cage symmetry, showing four B-Ht, one C-H, and a bridge-
hydrogen resonances in ratios of 2:2:2:1:1:1. The 13C NMR
spectrum of 1- shows a single resonance at 108.4 ppm (Table
1).
(16) Onak, T. P.; Landesman, H.; Williams, R. E.; Shapiro, I. J. Phys.
Chem. 1959, 63, 1533-1535.
(17) Bu¨hl, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1992, 114, 477-
491.
-
A C2B6H9 carborane is an 8-vertex, 20-skeletal-electron,
nido-cluster system and, on the basis of skeletal electron
counting rules,26 would be expected to adopt an open-cage
structure based on a tricapped trigonal prism missing one high-
coordinated vertex (Figure 2a). However, the only structurally
characterized 8-vertex nido-cluster with this geometry is nido-
(18) (a) Bu¨hl, M.; Gauss, J.; Hofmann, M.; Schleyer, P. v. R. J. Am.
Chem. Soc. 1993, 115, 12385-12390. (b) Schleyer, P. v. R.; Gauss, J.;
Bu¨hl, M.; Greatrex, R.; Fox, M. A. J. Chem. Soc., Chem. Commun. 1993,
1766-1768.
(19) Bu¨hl, M.; Schleyer, P. v. R. In Electron Deficient Boron and Carbon
Clusters; Olah, G. A., Wade, K., Williams, R. E., Eds., Wiley: New York,
1991; Chapter 4, pp 113-142.
(23) (a) Kang, S. O.; Furst, G. T.; Sneddon, L. G. Inorg. Chem. 1989,
28, 2339-2347. (b) Kang, S. O.; Sneddon, L. G. In Electron Deficient
Boron and Carbon Clusters; Olah, G. A., Wade, K., Williams, R. E., Eds.;
Wiley: New York, 1991; pp 195-213.
(24) The structures of the arachno-4,5-C2B7H12- and arachno-6,8-
C2B7H12-- anions, and a possible mechanism for the formation of nido-4,5-
C2B6H9 (1-), will be discussed in a subsequent paper. Bausch, J. W.,
Sneddon, L. G., unpublished results.
(20) (a) Gauss, J. Chem. Phys. Lett. 1992, 191, 614-620. (b) Gauss, J.
J. Chem. Phys. 1993, 99, 3629-3643.
(21) ACESII: Stanton, J. F.; Gauss, J.; Watts, J. D.; Lauderdale, W. J.;
Bartlett, R. J. Int. J. Quantum Chem., Quantum Chem. Symp. 1992, 26,
879-894.
(22) Scha¨fer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571-
2577.
(25) Emele´us, H. J.; Wade, K. J. Chem. Soc. 1960, 2614-2617.

11426 J. Am. Chem. Soc., Vol. 118, No. 46, 1996
Table 1. NMR Data
Bausch et al.
compd
nido-4,5-C2B6H9- (1-)
nucleus
11Ba
δ (multiplicity, assignment, J (Hz))
5.0 (d, B7,8, JBH 135), -15.0 (d, B2, JBH 129), -16.8 (d, B3,6, JBH 124), -36.1
(d, B1, JBH 157, JBB 20)
11B-11Bb crosspeaks: B7,8-B2; B7,8-B3,6; B2-B1; B2-B3,6
1H{11B}c 5.5 (C4,5H), 3.9 (B7,8H), 2.1 (B3,6H), 0.8 (B2H), -0.3 (B1H), -6.3 (BHB)
13Cd
11Be
108.4 (br, C4,5)
nido-4,5-C2B6H10 (1)
7.8 (br, B7,8), -7.3 (d, B1, JBH 176), -12.4 (d, B3,6, JBH 143), -26.6 (d, B2, JBH 146)
11B-11Bf crosspeaks: B7,8-B3,6; B7,8-B2; B1-B3,6; B1-B2; B3,6-B2
1H(11B)g
13Ch
6.2 (C4,5H), 3.0 (m, B7,8H), 2.6 (B3,6H), 2.0 (B1H), 1.2 (B2H), -3.4 (br, BHB)
120.0 (br, C4,5)
15.5 (br, B7), 3.2 (d, B8, JBH 142), -12.1 (d, B2, JBH 131), -13.7 (d, B6, JBH 125),
-15.5 (d, B3, JBH 122), -34.1 (d, B1, JBH 151)
nido-7-(trans-2-phenylethenyl)-
4,5-C2B6H8- (2a-)
11Bi
1H{11B}j,k 7.5-7.0 (Ph, dC-H), 6.6 (d, dC-H, JHH 18), 5.7 (CH), 5.65 (CH), 3.6 (BH),
2.0 (BH), 1.9 (BH), 1.2 (BH), -0.3 (BH), -4.8 (B-H-B)
nido-7-(cis-2-but-2-enyl)-
4,5-C2B6H8- (2b-)
11Bl
11Bl
11Bm
17.2 (B7), -1.0 (br, B8), -13.4 (d, B2, JBH 146), -15.8 (d, B6, JBH 156),
-17.8 (d, B3, JBHH 141), -36.4 (d, B1, JBH 143)
19.1 (br, B7), -1.3 (br, B8), -13.0 (d, B2, JBH 153), -15.7 (d, B6, JBH 161),
-18.1 (d, B3, JBH 139), -36.3 (d, B1, JBH 140)
40.2 (br, B7), -8.4 (d, B6, JBH 141), -13.3 (d, B1, JBH ≈ 130),
-16.9 (d, B3, JBH 156), -23.0 (d, B8, JBH ≈ 160), -25.3 (d, B2, JBH 145)
43.8 (s, B7), -8.2 (d, B6, JBH 140), -13.7 (d, B1, JBH 176), -16.6 (d, B3, JBH 154),
-23.3 (d, B8, JBH 153), -24.7 (d, B2, JBH 144)
nido-7-(n-hexyl)-4,5-C2B6H8- (2c-)
nido-7-(trans-2-phenylethenyl)-
4,5-C2B6H9 (2a)
nido-7-(cis-2-but-2-enyl)-4,5-C2B6H9 (2b) 11Bn
11B-11Bo crosspeaks: B1-B2; B1-B3; B1-B6; B2-B3; B2-B6; B2-B8; B3-B8; B6-B7
nido-7-(n-hexyl)-4,5-C2B6H9 (2c)
nido-7-(octyl)-4,5-C2B6H9 (2d)
arachno-5,6-C2B7H12- (3-)
11Bp
11Bq
11Br
49.2 (s, B7), -6.4 (d, B6, JBH 138), -11.7 (d, B1, JBH 181), -16.3 (d, B3, JBH 142),
-21.4 (d, B8, JBH 176), -24.2 (d, B2, JBH 158)
49.2 (s, B7), -6.4 (d, B6, JBH 142), -12.0 (d, B1, JBH 182), -16.4 (d, B3, JBH 147),
-21.4 (d, B8, JBH 156), -24.4 (d, B2, JBH 154)
-1.4 (d, B1, JBH 126), -7.7 (t, B9, JBH 115), -9.6 (d, B4,7, JBH 129), -43.5
(d, B3,8, JBH 118), -44.7 (d, B2, JBH 157)
11B-11Bs crosspeaks: B1-B2; B1-B3,8; B1-B4,7; B3,8-B4,7
1H{11B}t 5.5 (C5,6H), 2.5 (B4,7H), 2.3 (m, exo-B9H, br), 2.1 (s, B1H), 0.7 (t, endo-B9H, JHH 6),
0.1 (B3,8H), -0.7 (B2H), -1.4 (BHB)
13Cu
11BV
116.2 (br, C5,6)
arachno-7-CH3-4,5-C2B6H10- (4-)
6.6 (d, B6, JBH 128), 3.4 (d, B3, JBH 146), -20.2 (d, B2, JBH 141), -31.6 (d, B7, JBH 99),
-37.9 (t, B8, JBH 107), -57.7 (d, B1, JBH 170)
11B-11Bw crosspeaks: B1-B2; B1-B3; B1-B6; B2-B3; B2-B6; B2-B7; B2-B8; B3-B8; B6-B7
arachno-7-CH3-4,5-C2B6H92- (42-
closo-2,3-C2B5H7 (5)
)
11Bx
-3.0 (br, B6), -9.6 (br, overlap, B2,B3), -31.5 (br, B7), -45.3 (t, B8, JBH ≈ 90),
-58.9 (d, B1, JBH ≈ 120)
11Be
6.9 (d, B4,6, JBH 172), 3.1 (d, B5, JBH 157), -17.9 (d, B1,7, JBH 176)
1H{11B}y 6.9 (C2,3H), 4.4 (B4,6H), 3.9 (B5H), -0.1 (B1,7H)
13Cz
11Baa
11Bf
93.8 (br, C2,3)
closo-1-(butenyl)-2,3-C2B5H6
closo-3,1,2-CpCoC2B5H7 (6)
6.9 (d, B4,6, JBH 154), 2.9 (d, B5, JBH 160), -6.8 (s, B1), -25.0 (d, B7, JBH 180)
67.2 (d, JBH 156), 18.2 (d, JBH 166), -1.3 (d, JBH 177), -8.2 (d, JBH 155)
1H{11B}bb 9.4 (BH), 6.4 (CH), 5.3 (C5H5), 4.3 (BH), 2.8 (BH), 0.7 (BH)
13Ccc
11Bf
86.1 (C5H5), 66.8 (cage C, br)
closo-3,1,2-(η6-C6Me6)RuC2B5H7 (7)
57.7 (d, 148), 13.7 (d, 153), -2.6 (d, 172), -11.8 (d, 134)
7.4 (BH), 5.1 (CH), 4.2 (BH), 2.7 (BH), 0.5 (BH) (one peak obscured)
102.6 (s, Me6C6), 60.4 (br, cage C), 17.4 (s, Me6C6)
21.4 (d, B2, JBH 128), 18.9 (t, B6, JBH 116), 1.1 (d, B5,7, JBH 110), -34.0 (d, B1, JBH 158)
19.6 (d, B2, JBH 132), 15.7 (d, B6, JBH 139), 4.5 (d, B5,7, JBH 136), -31.7 (d, B1, JBH 169)
1Hbb
13Ccc
11Bx
11Bo
nido-3,4-C2B5H8- (8-)
nido-6-(TMEDA)-3,4-C2B5H7 (9)
a 160.5 MHz in THF-d8 (K+ salt). b 64.2 MHz in THF-d8 (K+ salt). c 500 MHz in THF-d8 (K+ salt). d 50.3 MHz in THF-d8 (K+ salt). e 160.5
MHz in CD2Cl2. f 64.2 MHz in CD2Cl2. g 200 MHz in C6D6. h 50.3 MHz in C6D6. i 160.5 MHz in CD3CN (PSH+ salt). j 500 MHz in CD3CN
(PSH+ salt). k Peaks for PSH+ not included. l 64.2 MHz in CH2Cl2 (PSH+). m 64.2 MHz decane. n 160.5 MHz in C6D6. o 64.2 MHz in C6D6. p 64.2
MHz in pentane. q 64.2 MHz in octenes. r 160.5 MHz in CD2Cl2 (PSH+ salt). s 64.2 MHz in CD2Cl2 (Bu4N+ salt). t 200 MHz in CD2Cl2 (Bu4N+
salt). u 125.7 MHz in CD2Cl2 (Bu4N+ salt). V 64.2 MHz in CD3CN. w 64.2 MHz in CH2Cl2 (PPN+ salt). x 64.2 MHz in THF. y 500 MHz in CD2Cl2.
z 125.7 MHz in C6D6. aa 64.2 MHz in dodecane. bb 200 MHz in CD3CN. cc 125.7 MHz in CD3CN.
(η-C5H5)2Co2SB5H7.27 Other crystallographically characterized
isoelectronic clusters, such as nido-B8H12,28 nido-(η6-C6Me6)-
FeMe4C4B3H3,29 and nido-(η-C5H5)CoPh4C4B3H3,30 have struc-
tures based on a 10-vertex bicapped square antiprism missing
two vertices, which is the same geometry expected for 8-vertex
arachno-clusters (Figure 2b). Thus, the question of which is
the preferred geometry for 8-vertex nido cages has been a
longstanding problem in cluster chemistry.26,f,31,32
We previously communicated11 a single-crystal X-ray study
(26) (a) Williams, R. E. Inorg. Chem. 1971, 10, 210-214. (b) Wade, K.
AdV. Inorg. Chem. Radiochem. 1976, 18, 1-66. (c) Williams, R. E. AdV.
Inorg. Chem. Radiochem. 1976, 18, 67-142. (d) Rudolph, R. W. Acc. Chem.
Res. 1976, 9, 446-452. (e) Williams, R. E. In Electron Deficient Boron
and Carbon Clusters; Olah, G. A., Wade, K., Williams, R. E., Eds.;
Wiley: New York, 1991; pp 11-93. (f) Williams, R. E. Chem. ReV. 1992,
92, 177-207.
-
of Bu4N+nido-4,5-C2B6H9 (1-) (Figure 3 and Supporting
Information) that confirmed the gross arachno-type structure
and this report was the first structural confirmation of this
geometry for a non-metal polyhedral cage system. The Cs cage
symmetry observed in the solid state is also consistent with the
NMR data discussed above. The carbon atoms occupy adjacent
(27) Zimmerman, G. J.; Sneddon, L. G. J. Am. Chem. Soc. 1981, 103,
1102-1111.
(28) Enrione, R. E.; Boer, F. P.; Lipscomb, W. N. Inorg. Chem. 1964,
3, 1659-1666.
(31) (a) Grimes, R. N. AdV. Inorg. Chem. Radiochem. 1983, 26, p 72.
(b) Reference 1d, p 473.
(32) Studies of 8-vertex nido electron count carborane clusters have
indicated that the 6-membered open-face geometry is usually, but not always,
the preferred structure: J. W. Bausch, presentation at Loker Hydrocarbon
Institute Kimbrough Symposium, Los Angeles, CA, December, 1995.
(29) Micciche, R. P.; Briguglio, J. J.; Sneddon, L. G. Organometallics
1984, 3, 1396-1402.
(30) Zimmerman, G. J.; Sneddon, L. G. Inorg. Chem. 1980, 19, 3650-
3655.

Studies of Subicosahedral Adjacent-Carbon Carboranes
J. Am. Chem. Soc., Vol. 118, No. 46, 1996 11427
Figure 3. ORTEP drawing of the cage structure of Bu4N+nido-4,5-
-
C2B6H9 (1-). Selected observed bond distances (Å): B1-B2, 1.749
(7); B1-B3, 1.794 (7); B1-C4, 1.687 (7); B1-C5, 1.686 (6); B1-
B6, 1.805 (7); B2-B3, 1.792 (7); B2-B6, 1.833 (7); B2-B7, 1.715
(6); B2-B8, 1.710 (7); B3-C4, 1.566 (7); B3-B8, 1.699 (7); C4-
C5, 1.400 (7); C5-B6, 1.562 (7); B6-B7, 1.709 (7); B7-B8, 1.666
(7); B7-H78, 1.219 (35); B8-H78, 1.294 (35).
Figure 2. Derivation of open 8-vertex cage frameworks based upon
geometrical systematics: (a) removal of a high-coordinated vertex from
a 9-vertex tricapped trigonal prism to generate a 5-membered open-
face nido geometry; (b) removal of two high-coordinated vertices from
a 10-vertex bicapped square antiprism to generate a 6-membered open-
face arachno geometry.
positions on the puckered 6-membered open face with the single
bridge-hydrogen located at the B7-B8 edge. The 6-membered
open-face geometry of 1- is clearly demonstated by the C4-
B8 distance of 2.72 Å, and the remaining cage lengths (Figure
3 caption) are within values normally found in carboranes.
The ab initio calculated structure for nido-4,5-C2B6H9- (1-)
(Supporting Information) and the IGLO calculated shifts and
assignments33 (Table 2) are likewise consistent with the crys-
tallographically observed structure and both the experimental
11B NMR chemical shifts and 2D 11B-11B NMR spectrum. A
variety of input structures containing a 5-membered open face
were employed in the calculations, but each optimized to the
structure with a 6-membered open face (the C4-B8 distance is
calculated to be 2.74 Å at the MP2/6-31G* level of theory).
Onak has recently reported34 an isomer of 1-, nido-3,5-
C2B6H9-, and shown with ab initio/IGLO calculations that it
also has an arachno-type structure, but with the carbons located
in non-adjacent positions on the 6-membered open face. The
3,5-isomer is energetically favored over 1- by 4.5 kcal/mol
(Table 3), which agrees with the empirical rules of Williams.26e
The neutral carborane nido-4,5-C2B6H10 (1) was obtained in
70% optimized yield by protonation of a pentane suspension
of 1- with gaseous HCl at -78 °C to give a colorless, air-
sensitive liquid that slowly decomposes at room temperature.
Figure 4. Proton spin-decoupled 64.2 MHz 11B NMR spectra of (a)
nido-4,5-C2B6H10 (1); (b) nido-7-(cis-2-but-2-enyl)-4,5-C2B6H9 (2b);
and (c) nido-7-(octyl)-4,5-C2B6H9 (2d). Complete shift assignments are
given in Table 1.
(Table 3). Good agreement of the IGLO calculated 11B NMR
shifts of 1a with the experimental values is obtained by
averaging the shifts of the “static” structure to give the
“dynamic” values (Figure 5). A fluxional process which would
interconvert the two enantiomers shown below could involve
rapid bridge-proton rearrangements across the B3-B8, B7-
B8, and B6-B7 edges. This would account for both the
apparent mirror plane of symmetry present in the NMR spectra
of 1 and the broad nature of the B7-B8 resonance.
Although the 11B (Figure 4a), 1H, 13C, and 2-D 11B-11B NMR
spectra (Table 1) of nido-4,5-C2B6H10 (1) suggest Cs symmetry,
the ab initio/IGLO calculations for a Cs symmetry isomer (1b)
showed poor agreement with the experimental data (Figure 5).
Williams’ empirical rules26e do not favor structure 1b as it
contains two bridge-hydrogens across 5-coordinated borons
(B3,6). Instead, an asymmetrical arrangement of bridge-
hydrogens (1a) would be favored. The calculations indeed show
1a to be significantly lower in energy (28.3 kcal/mol) than 1b
(33) The value for B2 deviates 8 ppm from the experimental value. This
larger than normal difference may arise because the computational method
does not take into account solvation and counterion effects, which could
be significant in these anions.
(34) Onak, T.; Tseng, J.; Tran, D.; Herrera, S.; Chan, B.; Arias, J.; Diaz,
M. Inorg. Chem. 1992, 31, 3910-3913.
This rearrangement appears to have a low-energy barrier, as

11428 J. Am. Chem. Soc., Vol. 118, No. 46, 1996
Bausch et al.
Table 2. IGLO Data
optimized structure
nido-4,5-C2B6H9- (1-)
IGLO calculated NMR data (II′/MP2/6-31G*)
11B: 7.1 (B7,8), -7.2 (B2), -19.5 (B3,6), -37.8 (B1)
13C: 112.0 (C3,4)
nido-4,5-C2B6H10 (1a)
11B: 37.3 (B8), -6.8 (B1), -9.8 (B3), -13.1 (B7), -19.1 (B6), -22.9 (B2)
13C: 160.4 (C4), 94.4 (C5)
nido-4,5-C2B6H10 (1b)
11B: 34.4 (B1), 25.7 (B7,8), -2.5 (B3,6), -22.3 (B2)
13C: 139.7 (C4,5)
nido-7-CH3-4,5-C2B6H8- (2e-)
nido-7-CH3-4,5-C2B6H9 (2e)
nido-8-CH3-4,5-C2B6H9 (2f)
arachno-5,6-C2B7H12- (3-)
arachno-5,6-C2B7H13 (3a)
arachno-5,6-C2B7H13 (3b)
arachno-7-CH3-4,5-C2B6H92- (42-
arachno-7-CH3-4,5-C2B6H10- (4-)
closo-2,3-C2B5H7 (5)
11B: 16.9 (B7), 4.0 (B8), -5.0 (B2), -18.4 (B6), -21.0 (B3), -37.8 (B1)
13C: 114.2 (C5), 110.5 (C4), -0.9 (Me)
11B: 50.6 (B7), -7.6 (B1), -8.7 (B6), -16.8 (B8), -19.2 (B3), -21.6 (B2)
13C: 159.6 (C5), 91.0 (C4), 1.3 (Me)
11B: 34.9 (B7), -0.9 (B8), -1.0 (B1), -11.7 (B6), -19.0 (B3), -20.3 (B2)
13C: 162.4 (C4), 96.1 (C5), -8.3 (Me)
11B: 2.0 (B1), -8.2 (B9), -8.8 (B4,7), -44.8 (B3,8), -48.5 (B2)
13C: 114.2 (C5,6)
11B: 30.9 (B7), 28.1 (B4), 20.6 (B9), 17.1 (B8), -12.7 (B1), -17.1 (B2), -21.1 (B3)
13C: 109.7 (C6), 11.7 (C5)
11B: 15.4 (B1), -3.2 (B2), -8.6 (B9), -9.0 (B7), -18.5 (B8), -29.7 (B3), -30.1 (B4)
13C: 150.4 (C6), 125.8 (C5)
)
11B: -3.1 (B6), -4.4 (B2), -7.4 (B3), -35.6 (B7), -45.5 (B8), -62.0 (B1)
13C: 96.0 (C5), 92.9 (C4), 8.9 (Me)
11B: 9.6 (B6), 6.8 (B3), -21.2 (B2), -30.5 (B7), -36.4 (B8), -59.5 (B1)
13C: 92.7 (C5), 91.8 (C4), 2.5 (Me)
11B: 8.1 (B4,6), 1.0 (B5), -17.5 (B1,7)
13C: 87.0 (C2,3)
closo-1-CH3-2,3-C2B5H6
nido-3,4-C2B5H8- (8-)
11B: 9.0 (B4,6), -0.4 (B5), -7.0 (B1), -23.9 (B7)
13C: 87.2 (C2,3), -8.6 (Me)
11B: 23.5 (B2), 22.7 (B6), 3.2 (B5,7), -36.6 (B1)
13C: 109.3 (C3,4)
nido-6-NH3-3,4-C2B5H7
11B: 25.2 (B2), 9.8 (B6), 0.1 (B5,7), -30.9 (B1)
13C: 125.8 (C3,4)
11B and 1H NMR studies of 1 to -90 °C were unable to
sufficiently slow down this process to allow observation of a
static asymmetrical structure.
The NMR spectra for the carbons-apart isomer, nido-3,6-
C2B6H10,35,36 of 1 were originally interpreted as consistent with
either static-arachno or fluxional-nido structures. However, an
ab initio/IGLO/NMR study37 strongly favors a static structure.
The 3,6-isomer is energetically favored by 22.5 kcal/mol over
1 (Table 3).
substituted B7 borons are sensitive to their exopolyhedral
substituents, with those of the olefinic derivatives, 2a and 2b,
coming at slightly higher field (40.2 and 43.8 ppm) than those
of the saturated derivatives, 2c and 2d (49.2 and 49.2 ppm).
The IGLO calculated 11B shifts for the model compound nido-
7-CH3-4,5-C2B6H9 (2e, Figure 6) correlate well with the
experimental data for 2a-d. Other structures also having the
C1 symmetry which were not excluded by the experimental data
were also computationally investigated. The nido-8-CH3-4,5-
C2B6H9 (2f) isomer is calculated to be only 2.2 kcal/mol higher
in energy than the 7-Me isomer 2e (Table 3). However, the
IGLO 11B calculated shifts (Table 2) for 2f correlate poorly with
the experimental values for 2a-d.
The nido-4,5-C2B6H10 (1) was found38 to readily hydroborate
under mild conditions a variety of alkynes and alkenes, including
phenyl acetylene, 2-butyne, 1-hexene, and octenes, to give
exopolyhedral alkenyl or alkyl B7-substituted nido-C2B6H9 (2a-
d) species:
As discussed above, the ab initio/IGLO calculations and their
experimental 11B NMR spectra are consistent with the structures
proposed for 2a-d, but the compounds proved to be too
thermally unstable to allow complete isolation and characteriza-
tion. However, treatment of 2a-c with Proton Sponge resulted
in deprotonation to form stable alkyl or alkenyl B7-substituted
-
nido-4,5-C2B6H8 ions (2a--c-) in moderate to high yields:
The 11B NMR spectra for 2a-d (see Figure 4 for spectra of 2b
and 2d) contain resonances at chemical shifts expected for a
“static” nido-4,5-C2B6H10 (Figure 5), with five doublets and one
downfield singlet of equal intensity. The chemical shifts of the
(35) Gotcher, A. J.; Ditter, J. F.; Williams, R. E. J. Am. Chem. Soc.
1973, 95, 7514-6.
-
For example, PSH+nido-7-(trans-2-phenylethenyl)-4,5-C2B6H8
(36) Reilly, T. J.; Burg, A. B. Inorg. Chem. 1974, 12, 1250.
(37) Bausch, J. W.; Prakash, G. K. S.; Bu¨hl, M.; Schleyer, P. v. R.;
Williams, R. E. Inorg. Chem. 1992, 31, 3060-3062.
(2a-) was isolated in 81% yield after treatment of 2a with Proton
Sponge. The 11B NMR spectra (Table 1) for 2a--c- are
similar, with each containing one singlet and five doublet
resonances in equal ratios, indicating C1 cage symmetry (see
Figure 1b for the spectrum of 2a-). The resonances also occur
(38) Presented in part at the Eighth International Meeting of Boron
Chemistry, Knoxville, TN, July, 1993. See: Bausch, J. W.; Carroll, P. J.;
Sneddon, L. G. In Current Topics in the Chemistry of Boron; Kabalka, G.
W., Ed.; Royal Society of Chemistry: Cambridge, 1994; pp 224-227.

Studies of Subicosahedral Adjacent-Carbon Carboranes
J. Am. Chem. Soc., Vol. 118, No. 46, 1996 11429
Table 3. Absolute (-au) and Relative (kcal/mol) Energies of Carboranes Calculated in This Study
b
c
optimized structure
sym
6-31G*//6-31G*
ZPE (NEV)a
Erel
MP2/6-31G*//MP2/6-31G*
Erel
nido-4,5-C2B6H9- (1-)
nido-3,5-C2B6H9
Cs
C1
229.06624
229.08139
81.58 (0)
81.54 (0)
9.5
0.0
229.93727
229.94434
4.5
0.0
-
nido-4,5-C2B6H10 (1a)
nido-4,5-C2B6H10 (1b)
nido-3,6-C2B6H10
C1
Cs
C2V
229.58307
229.54192
229.63438
89.58 (0)
88.31 (0)
90.58 (0)
33.3
56.0
0.0
230.45050
230.40348
230.48772
22.5
50.8
0.0
d
nido-7-CH3-4,5-C2B6H8- (2e-)
C1
268.10894
100.54 (0)
269.11481
nido-7-CH3-4,5-C2B6H9 (2e)
nido-8-CH3-4,5-C2B6H9 (2f)
C1
C1
268.62980
268.62760
108.50 (0)
108.66 (0)
0.0
1.5
269.63637
269.63304
0.0
2.2
arachno-5,6-C2B7H12- (3-)
Cs
255.47820
103.43 (0)
256.46771
arachno-5,6-C2B7H13 (3a)
arachno-5,6-C2B7H13 (3b)
arachno-4,5-C2B7H13
C1
C1
C1
255.97924
255.95692
256.00663
112.58 (0)
111.32 (0)
114.08 (0)
15.9
28.7
0.0
256.95331
256.94044
256.98849
20.7
27.7
0.0
arachno-7-CH3-4,5-C2B6H92- (42-
arachno-7-CH3-4,5-C2B6H10- (4-)
closo-2,3-C2B5H7 (5)
)
C1
C1
C2V
Cs
Cs
Cs
268.51614
269.25029
203.21493
242.26088
203.79929
259.39771
103.67 (0)
113.51 (0)
67.24 (0)
86.04 (0)
71.92 (0)
94.70 (0)
269.54414
270.26844
203.95708
243.14204
204.56006
260.32430
closo-1-CH3-2,3-C2B5H6
nido-3,4-C2B5H8- (8-)
nido-6-NH3-3,4-C2B5H7
a Zero-point energy and number of imaginary frequencies in parentheses. b Relative energy at the 6-31G*//6-31G* + ZPE (6-31G*) level; the
zero-point energies have been scaled by 0.89 as recommended.13 c Relative energy at the MP2/6-31G*//MP2/6-31G* + ZPE (6-31G*) level; the
zero-point energies have been scaled by 0.89 as recommended.13 d Reference 35.
Figure 6. (a) Calculated structure and IGLO 11B NMR data for nido-
Figure 5. Calculated structures and IGLO 11B NMR data for nido-
4,5-C2B6H10 (1): MP2(FULL)/6-31G* optimized geometries for the
asymmetric (1a) and symmetric (1b) isomers and comparison with the
experimental data.
-
7-CH3-4,5-C2B6H8 (2e-): MP2(FULL)/6-31G* optimized geometry
and comparison with the experimental data for 2a--c-. (b) Calculated
structure and IGLO 11B NMR data for nido-7-CH3-4,5-C2B6H9 (2e):
MP2(FULL)/6-31G* optimized geometries and comparison with the
experimental data for 2a-c.
-
at shifts similar to those of nido-4,5-C2B6H9 (1-) with the
exception that the substituted-B7 resonances are shifted to lower
field. Strong support for the proposed structures 2a--c- also
comes from the good agreement (Figure 6) of the ab initio/
IGLO calculated shifts on model compound nido-7-CH3-4,5-
underwent cage expansion by the addition of BH3‚THF to a
THF solution of 1- giving the new carborane anion K+arachno-
-
5,6-C2B7H12- (3-) in essentially quantitative yield.39 A C2B7H12
carborane is a 9-vertex, 24 skeletal electron, arachno-cluster
system and, on the basis of skeletal electron counting rules,26
would be expected to adopt a structure based on a octadeca-
hedron missing two vertices. Two frameworks generated using
this method are shown in Figure 7: the “iso” arachno structure
with a 6-membered open face, found for iso-B9H15,40 and the
-
C2B6H9 (2e-) with the experimental data for 2a--c-.
The structure of PSH+2b- was further confirmed by a
crystallographic study, but because of the poor quality of the
diffraction data, a satisfactory refinement could not be obtained.
Thus, while the gross cage geometry and syn hydroboration of
2-butyne were confirmed, the bond distances and angles are
unreliable. Details of the structural determination and an
ORTEP plot of PSH+2b- are given in the Supporting Informa-
tion.
(39) For examples of cage expansion by the addition of BH3 to borane
anions, see: (a) Geanangel, R. A.; Shore, S. G. J. Am. Chem. Soc. 1967,
89, 6771-6772. (b) Remmel, R. J.; Johnson, H. D., II; Jaworiwsky, I. S.;
Shore, S. G. J. Am. Chem. Soc. 1975, 97, 5395-5403. (c) Geanangel, R.
A.; Johnson, H. D., II; Shore, S. G. Inorg. Chem. 1971, 10, 2363-2364.
(40) (a) Dobson, J.; Keller, P. C.; Schaeffer, R. J. Am. Chem. Soc. 1965,
87, 3522-3523. (b) Dobson, J.; Keller, P. C.; Schaeffer, R. Inorg. Chem.
1968, 7, 399-402.
Both nido-4,5-C2B6H10 (1) and its conjugate anion, nido-4,5-
-
C2B6H9 (1-), were found to be useful precursors to smaller
-
and larger cage systems. The nido-4,5-C2B6H9 (1-) readily

11430 J. Am. Chem. Soc., Vol. 118, No. 46, 1996
Bausch et al.
Figure 9. Calculated structures and IGLO 11B NMR data for arachno-
5,6-C2B7H13 (3): MP2(FULL)/6-31G* optimized geometries for 3a and
3b.
for 3- and n-B9H15 showed that they are similar when
allowances are made for the different cage lengths found in 3-
due to the presence of the cage carbons. The ab initio calculated
structure for 3- (Supporting Information) and the IGLO
calculated chemical shifts and assignments (Table 2) are likewise
consistent with the crystallographically observed structure and
both the experimental 11B NMR chemical shifts and 2D 11B-
11B NMR spectrum.
Figure 7. Derivation of 9-vertex arachno cage frameworks based upon
geometrical systematics: removal of six- and five-coordinated vertices
(1 and 7) from an octadecahedron to generate a 7-membered open-
face “normal” arachno structure and removal of six- and four-
coordinated vertices (1 and 2) to generate the 6-membered open-face
“iso” arachno structure.
The 3- anion is the third isomer of this carborane anion to
-
be reported. The two previous isomers,24 arachno-4,5-C2B7H12
and arachno-6,8-C2B7H12-, have structures based upon iso-
B9H15,40 but this new isomer arachno-5,6-C2B7H12- (3-) is the
first reported carborane analog of n-B9H15.41 The only other
reported analog of n-B9H15 is a ruthenaborane, arachno-(η6-
C6Me6)RuB8H14.42 Many attempts were made to protonate 3-,
with the desired product being the unknown arachno-5,6-
C2B7H13 (3) carborane having a n-B9H15 structure. In NMR
tube experiments, when either the Bu4N+ or PSH+ salt of 3-
was dissolved in CH2Cl2 and treated with HCl‚Et2O or
concentrated H2SO4 at -78 °C and slowly warmed to room
temperature, the only identifiable product was arachno-4,5-
C2B7H13.4
The results of ab initio calculations indicate two possible
structures for arachno-5,6-C2B7H13 (3a and 3b) (Figure 9) that
retain the n-B9H15 framework, with 3a 8.0 kcal/mol more stable
than 3b (Table 3). However, 3a is 20.7 kcal/mol less stable
than the “iso” framework isomer arachno-4,5-C2B7H13.43 It may
be that “normal” 3a or 3b is initially formed by the protonation
of 3-, but quickly isomerizes to the more stable arachno-4,5-
C2B7H13 “iso” isomer. Attempts to locate computationally an
isomer similar to 3a, but with B3-B4 bridge-hydrogen moved
to the B3-B9 edge, failed; this geometry optimized to isomer
3a.
Figure 8. ORTEP drawing of the cage structure of (PSH)+arachno-
-
5,6-C2B7H12 (3-). Selected observed bond distances (Å): B1-B2,
1.678 (6); B1-B3, 1.791 (6); B1-B4, 1.776 (5); B1-B7, 1.755 (5);
B1-B8, 1.767 (5); B2-B4, 1.801 (7); B2-B7, 1.764 (5); B3-B4,
1.862 (6); B3-B8, 1.747 (5); B3-B9, 1.807 (6); C5-B2, 1.698 (6);
C5-B4, 1.513 (6); C5-C6, 1.430 (5); C6-B2, 1.701 (6); C6-B7,
1.520 (5); B7-B8, 1.864 (6); B8-B9, 1.815 (6); B3-H39, 1.155(28);
B8-H89, 1.172 (30); B9-H39, 1.379 (23); B9-H89, 1.501 (30); B9-
H9a, 1.066 (27); B9-H9b, 1.224 (25).
NMR studies of the reaction of sodium or potassium hydride
with nido-4,5-C2B6H10 (1) showed the formation of nido-4,5-
-
C2B6H9 (1-). However, when LiBEt3H was employed, the
less common “normal” arachno structure with a 7-membered
open-face, found for n-B9H15.41 The NMR data for 3- (Table
1), which indicate Cs cage symmetry with equivalent cage
carbon atoms, strongly support the “normal” arachno 9-vertex
framework.
-
major product (∼60%) was the known8 arachno-4,5-C2B6H11
,
arising from hydride addition to the cage, along with nido-4,5-
-
C2B6H9 (1-) (∼40%).
A single-crystal X-ray diffraction study of the PSH+3- salt
(Figure 8 and Supporting Information) shows that arachno-5,6-
C2B7H12- (3-) does in fact have a “normal” arachno-framework,
with the carbon atoms (C5,6) occupying low-coordinated
adjacent positions on the 7-membered open face. Consistent
with the triplet resonance observed in the 11B NMR spectrum
(Table 1), the unique boron (B9) on the open face (formally
from the BH3‚THF) is a BH2 with two bridge-hydrogens, similar
to that found in n-B9H15.41 A comparison of the geometries
An NMR study of the reaction of a THF solution of K+nido-
-
4,5-C2B6H9 (1-) with ∼1 equiv of a 1.4 M Et2O solution of
CH3Li showed evidence of the initial formation of arachno-7-
(42) Bown, M.; Fontaine, X. L. R.; Greenwood, N. N.; Kennedy, J. D.;
Thornton-Pett, M. J. Organomet. Chem. 1986, 315, C1-C4.
(41) Dickerson, R. E.; Wheatley, P. J.; Howell, P. A.; Lipscomb, W. N.
J. Chem. Phys. 1957, 27, 200-209.
(43) The optimized structure for arachno-4,5-C2B7H13 at HF/6-31G* has
been reported: McKee, M. L. Inorg. Chem. 1994, 33, 6213-6218.

Studies of Subicosahedral Adjacent-Carbon Carboranes
J. Am. Chem. Soc., Vol. 118, No. 46, 1996 11431
Table 4. 11B NMR Data for closo-2,3-C2B5H7 and Alkyl
Derivatives
compd
B4,6
B5
B1,7
calcda for closo-2,3-C2B5H7
closo-2,3-C2B5H7 (5)
6.7
6.9
10.6
7.0
3.1
3.1
5.2
2.5
15 (1)
-17.4
-17.9
-12.1
-14.2
-25 (2)
b
closo-2,3-Me2C2B5H5
c
closo-2,3-Et2C2B5H5
“closo-2,3-C2B5H7” (1973)d
-2 (2)
a GIAO-MP2 method. b Reference 44. c Reference 45. d Reference
46.
the 11B NMR signals, the ratios of closo-2,3-C2B5H7 (5), closo-
2,4-C2B5H7, and closo-1,7-C2B6H8 were approximately 10:1:2.
The closo-2,3-C2B5H7 (5) was also produced by moderate
temperature thermolysis of solutions of the hydroborated
intermediates 2a-c:
Figure 10. Calculated structures and IGLO 11B NMR data for arachno-
2-
2-
7-CH3-4,5-C2B6H9 (42-) and arachno-7-CH3-4,5-C2B6H10 (4-):
MP2(FULL)/6-31G* optimized geometries and comparison with the
experimental data.
2-
CH3-4,5-C2B6H9 (42-), which following protonation with
-
PSH+Cl- gave arachno-7-CH3-4,5-C2B6H10 (4-):
For example, thermal degradation of nido-7-(cis-2-but-2-enyl)-
4,5-C2B6H9 (2b) was accomplished by heating at 110 °C in
dodecane solution.
A
11B NMR spectrum of this solution
showed the presence of closo-2,3-C2B5H7 (5), as well as a broad
resonance (∼65 ppm) in the range typically found for alkyl
boranes, which is consistent with the loss of a “RBH2” unit.
Vacuum fractionation gave closo-2,3-C2B5H7 (5) in an un-
optimized yield of 34.9% in a -78 °C trap as a low-melting,
volatile, air-sensitive solid and is thermally stable at room
temperature. The closo-2,3-C2B5H7 made in this way is,
according to NMR and mass spectrometry, of higher purity than
that produced by the gas phase flow system. By careful choice
of reagents and optimization of the isolation procedure, this
solution phase method may ultimately give higher yields of
closo-2,3-C2B5H7 (5) than the gas-phase method.
The closo-2,3-C2B5H7 (5) carborane was one of the few
remaining parent adjacent-carbon closo carboranes to be isolated
and unambiguously characterized. The first alkyl substituted
adjacent-carbon closo-C2B5- system, closo-2,3-Me2C2B5H5, was
isolated in low yield by Schaeffer in 197146 from the gas phase
reaction of octaborane and 2-butyne. Beck has reported47 the
synthesis of closo-2,3-Et2C2B5H5 and its 11B NMR data are
consistent with those for closo-2,3-Me2C2B5H5 (Table 4). In
1973,48 Schaeffer also reported isolation of a small amount of
impure material from the octaborane/2-butyne reaction which
mass spectroscopy indicated was of the formula C2B5H7. The
11B NMR spectrum contained three resonances at 15, -2, and
-25 ppm, in 1:2:2 ratios, which were tentatively assigned to
closo-2,3-C2B5H7. However, these resonances are significantly
different (Table 4) than those of the alkyl derivatives discussed
above, as well as those for compound 5. Thus, ab initio/IGLO
studies were undertaken to confirm the structural assignment
of 5.
The structure of the final product (4-) is strongly supported by
its 11B NMR spectrum (Table 1), which shows six resonances
(C1 symmetry) in similar chemical shift regions as the parent
arachno-4,5-C2B6H11-. The 2-D 11B-11B NMR spectrum of
the PPN+4- showed the expected crosspeaks, except B7-B8.
The 11B NMR spectrum for the initial product (42-) contains
five resonances in ratios of 1:2:1:1:1, with overlap of two peaks
likely. In the proton-coupled 11B NMR spectrum, the three
downfield resonances are broad, but the two upfield resonances,
at -37.9 and -57.7 ppm, are a triplet and a doublet, respectively
(Table 1). The likely structure for 42- is similar to 4-, but
without the bridge-proton. The ab initio/IGLO calculations for
-
arachno-7-CH3-4,5-C2B6H10 (4-) and arachno-7-CH3-4,5-
2-
C2B6H9 (42-) (Figure 10) are in good agreement with the
experimental values.
Vacuum thermolysis of nido-4,5-C2B6H10 (1), through a hot
tube heated at 350 °C, resulted in loss of BH3 and the production
of closo-2,3-C2B5H7 (5).
The closo-2,3-C2B5H7 (5) was isolated in a -78 °C trap in
∼65% yield. Smaller amounts of other closo carboranes (closo-
2,4-C2B5H744 and closo-1,7-C2B6H845 ) were also produced, with
the closo-1,7-C2B6H8 not separable from the closo-2,3-C2B5H7
(5) by vacuum fractionations. According to the integration of
(46) Rietz, R. R.; Schaeffer, R. J. Am. Chem. Soc. 1971, 93, 1263-
(44) (a) Williams, R. E.; Good, C. D.; Shapiro, I. Abstracts; 140th
Meeting of the American Chemical Society, Chicago, IL, Sept 1961; p 14N.
(b) Onak, T. P.; Gerhart, F. J.; Williams, R. E. J. Am. Chem. Soc. 1963,
85, 3378-3380.
(45) Williams, R. E.; Gerhart, F. J. J. Am. Chem. Soc. 1965, 87, 3513-
3515.
1265.
(47) (a) Beck, J. S.; Kahn, A. P.; Sneddon, L. G. Organometallics 1986,
5, 2552-2553. (b) Beck, J. S.; Sneddon, L. G. Inorg. Chem. 1990, 29,
295-302.
(48) Rietz, R. R.; Schaeffer, R. J. Am. Chem. Soc. 1973, 95, 6254-
6262.

11432 J. Am. Chem. Soc., Vol. 118, No. 46, 1996
Bausch et al.
that there appears to be a significant “anti-podal” effect53 of
the R group at B1 upon B7, since the B7 shift (-25.0 ppm) is
over 8 ppm upfield from B7 in closo-2,3-C2B5H7 (-17.9 ppm).
The nido-4,5-C2B6H9- (1-) anion, like the neutral nido-4,5-
C2B6H10 (1), is also prone to loss of a cage boron, as exemplified
by its reactions with transition metal complexes. For example,
-
separate reactions of K+nido-4,5-C2B6H9 (1-) with CpCo-
(CO)I2 and (η6-C6Me6)2Ru2Cl4 gave closo-3,1,2-(η-C5H5)-
CoC2B5H7 (6) and closo-3,1,2-(η6-C6Me6)RuC2B5H7 (7), re-
spectively.
Figure 11. Calculated structure and IGLO and GIAO-MP2 11B NMR
data for closo-2,3-C2B5H7 (5) together with 64.2-MHz 11B NMR
spectra: (a) proton spin-decoupled and (b) proton spin-coupled.
The compositions of 6 and 7 were established by elemental
analyses and exact mass measurements. Their 11B NMR spectra
(Table 1), which each show four doublets in 1:2:1:1 ratios, are
nearly identical to those found for the previously structurally
characterized closo-1,2-(CH3)2-3,1,2-(η-C5H5)CoC2B5H5.54
NMR studies also showed that closo-2,3-C2B5H7 (5) readily
undergoes cage expansion and degradation reactions. For
Ab initio calculations49 show that only two isomers of C2B5H7
are energetically favored: closo-2,3-C2B5H7 and closo-2,4-
C2B5H7. Calculations by Schleyer et al.16 for closo-2,4-C2B5H7
agree very well with the experimental data for that isomer. Our
ab initio/IGLO calculated 11B NMR shifts for closo-2,3-C2B5H7
(Figure 11) of 8.1, 1.0, and -17.5 ppm are in excellent
agreement with the experimental values observed for 5 of 6.9,
3.1, and -17.9 ppm, respectively.50 GIAO-MP2 NMR chemical
shift calculations51 on closo-2,3-C2B5H7 (5) (Figure 11) likewise
gave even better results (Table 4): each boron and carbon shift
is within 0.5 ppm of the experimental value! On the other hand,
the 11B chemical shifts assigned by Schaeffer to his proposed
closo-2,3-C2B5H7 (15, -2, -25 ppm) correlate poorly with the
IGLO and GIAO-MP2 calculated values. Thus, we conclude,
based on the comparisons in Table 4, that compound 5 reported
herein, and not the product reported by Schaeffer, is definitively
characterized as closo-2,3-C2B5H7.52
-
example, reaction with LiEt3BH gave nido-3,4-C2B5H8 (8-):
(9)
This 8- ion is the parent derivative of the known nido-3,4-
Et2C2B5H6-, made in similar fashion by cage opening of closo-
2,3-Et2C2B5H5 with hydride ion.9 The ab initio optimized
structure for 8- (with Cs symmetry, Supporting Information)
contains a 5-membered open face with a BH2 unit, similar to
- 9
the crystallographically determined nido-3,4-Et2C2B5H6
. The
spectrum for 8- is similar to that of nido-3,4-Et2C2B5H6-, but
the B1 resonance in the Et2 derivative appears significantly
further downfield at -21.3 ppm. The 11B NMR spectra and
- 55
assignments (Table 1) of 8- and nido-3,4-Et2C2B5H6
cor-
In the thermal degradation of 2b, a small amount of an
additional compound was also formed, which according to the
11B NMR and GC-MS data combined with ab initio/IGLO
relate well with the IGLO calculated results and demonstrate
that ethyl substituents do indeed cause a downfield shift of the
B1 resonance.
Grimes has previously shown that closo-1-M-2,3-R2C2B4H4
metallacarboranes are easily “decapitated” by treatment with
wet TMEDA to give nido-1-M-2,3-R2C2B3H5 complexes.56 An
theoretical calculations is closo-1-(butenyl)-2,3-C2B5H6. Its 11
B
NMR spectrum indicated B1 substitution with the shifts of the
observed four resonances (Table 1) in good agreement with the
ab initio IGLO calculated shifts (Table 2) of the model
compound closo-1-CH3-2,3-C2B5H6. It should also be noted
(53) Bu¨hl, M.; Schleyer, P. v. R.; Havlas, Z.; Hnyk, D.; Heˇrma´nek, S.
Inorg. Chem. 1991, 30, 3107-3111 and references therein.
(54) Zimmerman, G. J.; Sneddon, L. G. Acta Crystallogr 1983, C39,
856-858.
(49) See: McKee, M. L. J. Am. Chem. Soc. 1988, 110, 5317-5321 and
references therein.
(50) In ref 38, the figure showing the comparison of the IGLO calculated
values for 5 versus the experimental values was mislabeled. The assignments
for B5 and B4,6 were accidentally reversed.
-
(55) The II//MP2/6-31G* calculation of nido-3,4-Et2C2B5H6 (Cs sym-
metry) gives 11B NMR chemical shifts in satisfactory agreement with the
experimental values:9 assignment (cald, obsd), B1 (-29.8, -21.3), B2 (21.0,
24.1), B5,7 (1.4, 4.8), B6 (20.5, 19.3). Bausch, J. W., Sneddon, L. G.,
unpublished results.
(51) For examples applied to carboranes, see ref 17.
(52) Recently, Greatrex and Fox observed that the 11B NMR data reported
by Grimes in 1971 for a compound believed at that time to be closo-C3B5H7
closely matches the data reported herein for closo-2,3-C2B5H7. See: Fox,
M. A.; Greatrex, R. J. Chem. Soc., Dalton Trans. 1994, 3197-3198.
Thompson, M. L.; Grimes, R. N. J. Am. Chem. Soc. 1971, 93, 6677-6679.
(56) (a) Grimes, R. N. Pure Appl. Chem. 1991, 63, 369-372. (b) Grimes,
R. N. In Electron Deficient Boron and Carbon Clusters; Olah, G. A., Wade,
K., Williams, R. E., Eds., Wiley: New York, 1991; Chapter 11, pp 261-
285.

Studies of Subicosahedral Adjacent-Carbon Carboranes
J. Am. Chem. Soc., Vol. 118, No. 46, 1996 11433
analogous reaction of 4 with excess TMEDA, then H2O, gave
nido-2,3-C2B4H8 in 77% yield.
been developed via cage degradation and expansion reactions.
Of special significance are (1) the first isolation and character-
ization of the parent closo-2,3-C2B5H7 (5) carborane, (2) the
structural confirmations of arachno-type geometries for nido-
10
-
4,5-C2B6H9 (1-) and nido-4,5-C2B6H10 (1), and (3) the
synthesis and structural confirmation of the “normal” n-B9H15
-
structure for arachno-5,6-C2B7H12 (3-).
Acknowledgment. Dedicated to Robert E. Williams on the
occasion of his 70th birthday. At the University of Pennsylvania
the support of the National Science Foundation is gratefully
acknowledged. We thank the Bochum group for permission to
use the IGLO program.
(10)
When the progress of the reaction was monitored by 11B
NMR spectroscopy, the formation of an initial TMEDA adduct
was observed. The addition of 2 electrons to the cage, provided
by the TMEDA, should result in cage opening to give a nido
7-vertex structure, nido-6-exo-(TMEDA)-3,4-C2B5H7 (9). In
support of this proposed structure, the 11B NMR spectrum (Table
1) of 9 contains four doublets in a 1:1:2:1 ratio, similar to the
Supporting Information Available: Details of the prepara-
tion of PSH+2a-, PSH+2b-, and PSH+2c-; tables listing
infrared data (S1); details of structural determination of
Bu4N+1- (S2-S7), PSH+2b- (S8-S12), and PSH+3- (S13-S18),
Cartesian coordinates of the optimized geometries (S19), and
comparison of calculated and experimental geometries for nido-
3,4-Et2C2B5H6- (S20); figures showing comparison of calculated
and experimental geometries for 1- (S1) and 3- (S2), optimized
geometry for 8- (S3), ORTEP for PSH+2b- (S4), 11B NMR
spectra of nido-4,5-C2B6H10 (1) with LiEt3BH (S5), nido-4,5-
C2B6H9- (1-) with MeLi (S6) followed by PSH+Cl- (S7), and
closo-2,3-C2B5H7 (5) with LiEt3BH (S8) (40 pages). See any
current masthead page for ordering and Internet access
instructions.
isoelectronic 8- ion, and the previously structurally characterized
- 47b
nido-3,4-Et2C2B5H6
,
nido-6-Me3P-3,4-Et2C2B5H5,47b and
- 57
nido-6-(Me3P+-CH2)-3,4-Et2C2B5H5
.
Further support for the
structure of 9 comes from ab initio/IGLO calculations (Table
2) on nido-6-NH3-3,4-C2B5H7, which showed satisfactory agree-
ment with the experimental values (Table 1).
In summary, simple methods for the synthesis of a variety
of adjacent carbon carboranes in moderate to high yields have
(57) Su, K.; Fazen, P. J.; Carroll, P. J.; Sneddon, L. G. Organometallics
1992, 11, 2715-2718.
JA961112M