Cage-closing reactions of the nido-carborane anion 7,9-C2B9H12- and derivatives; formation of neutral 11-vertex carboranes by acidification

Article abstract of DOI:10.1039/b203920f

Reactions of the nido-carborane salts (Bu₄N)(nido-7,9-R,R′-7,9-C₂B₉ H₁₀) (R,R′ = H; R,R′ = Me; R,R′ = Ph; R = Ph, R′ = Me) with H₂SO₄ at ambient temperature give the 11-vertex closo-carbora

Full text of DOI:10.1039/b203920f

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؊
Cage-closing reactions of the nido-carborane anion 7,9-C2B9H12
and derivatives; formation of neutral 11-vertex carboranes by
acidification†
Mark A. Fox,* Andrew K. Hughes and John M. Malget
Chemistry Department, Durham University Science Laboratories, South Road, Durham,
UK DH1 3LE
Received 23rd April 2002, Accepted 5th July 2002
First published as an Advance Article on the web 15th August 2002
Reactions of the nido-carborane salts (Bu4N)(nido-7,9-R,RЈ-7,9-C2B9H10) (R,RЈ = H; R,RЈ = Me; R,RЈ = Ph;
R = Ph, RЈ = Me) with H2SO4 at ambient temperature give the 11-vertex closo-carboranes 2,3-R,RЈ-2,3-C2B9H9
in high yields (R,RЈ = H; R,RЈ = Me; R,RЈ = Ph; R = Ph, RЈ = Me). Halogenated derivatives of closo-carboranes,
10-X-2,3-C2B9H10 (X = Cl, I), are formed from reactions of salts containing 1(6)-X-7,9-C2B9H11Ϫ anions with
H2SO4, and 4-F-2,3-(p-FC6H4)2-2,3-C2B9H8 is formed from (Bu4N)(3-F-7,9-(p-FC6H4)2-7,9-C2B9H9) and F3CSO3H.
Elimination of the ethoxide and fluoride substituents from the cage occurs during acidification of salts containing
10-OEt-7,9-C2B9H11Ϫ and 10-F-7,9-(p-FC6H4)2-7,9-C2B9H9Ϫ by F3CSO3H, resulting in the closo-carboranes
2,3-C2B9H11 and 2,3-(p-FC6H4)2-2,3-C2B9H9 as major products respectively. A mixture of B-Me-2,3-C2B9H10
isomers is obtained from salts containing the 8-Me-7,9-C2B9H11Ϫ and 10-Me-7,9-C2B9H11Ϫ anions with H2SO4.
B-Me-2,3-C2B9H10 is suggested to be fluxional in solution with 4-Me-2,3-C2B9H10 as the major component.
Protonation of the fluxional anion 10(11)-endo-Me-7,9-C2B9H11Ϫ with acetic acid gives a neutral nido-carborane
11-Me-2,8-C2B9H12 rather than closo-B-Me-2,3-C2B9H10. Molecular geometries of 11-vertex closo-carboranes
and B-methyl-nido-carboranes are determined by the combined ab initio/GIAO/NMR method at the
GIAO-B3LYP/6-311G*//MP2/6-31G* level of theory.
Introduction
Base-induced deboronation of the three isomers of closo-
C2B10H12 generates three of the possible isomers of the
mono-anions nido-C2B9H12Ϫ, which have long been known to
generate the neutral nido carboranes C2B9H13 on acidification.
Two well-characterised isomers 7,8-C2B9H13 and 2,9-C2B9H13
Ϫ
are formed from their respective mono-anions 7,8-C2B9H12
Ϫ 1–3
and 2,9-C2B9H12
,
but the neutral carborane ‘7,9-C2B9H13’
Ϫ
produced by protonation of 7,9-C2B9H12 has not been
adequately characterised.4 Since the protonation reactions
introduce B–H–B bridges, and not C–H–B bridges, one
argument against the existence of 7,9-C2B9H13 is the lack of a
B–B bond available on the open face to accommodate the
second bridging hydrogen.2
The 11-vertex closo-carborane, 2,3-C2B9H11, is formed in
36% yield by the elimination of hydrogen in the reaction of
Cs(7,9-C2B9H12) with polyphosphoric acid at elevated tem-
peratures (Scheme 1).4 High yields of the C-phenyl derivative
2-Ph-2,3-C2B9H10 were obtained from Cs(7-Ph-7,9-C2B9H11)
by the same method.4 Acidification of the 7,8-anions 7,8-R,RЈ-
Ϫ
Scheme
1
The reactions of nido-7,9-C2B9H12 1a and nido-7,8-
Ϫ
C2B9H12 with acid and the synthesis of closo-2,3-C2B9H11 2a showing
the cage numbering. Exo-hydrogens are omitted for clarity.
Ϫ
7,8-C2B9H10 results in protonation to generate neutral nido-
2,3-Me2-2,3-C2B9H6.8 The only boron substituted 11-vertex
closo-carboranes to be formed by acidification are the two-cage
carboranes11,12 4-R-2,3-RЈ2-C2B9H8 (R = carboranyl, RЈ = H
or Me) generated from Cs(10-R-7,9-RЈ2-7,9-C2B9H8) and
2-Ph-2,3-C2B9H10 from Me4N[PhC2B9H10(OH)].4
If the substituent in the nido anion is at B10 then acid-
promoted elimination of the unique hydrogen on the open face,
and formation of two B–B bonds between B8 and B10/B11
to form the closo-carborane, will place the substituent at B4
(see Scheme 1 for cage numbering), as is found for the neutral
two-cage carboranes.11 The closure appears to be facile – the
mechanism has been discussed for the oxidative closure of
7,9-C2B9H112Ϫ to give 2,3-C2B9H11.13
carboranes 7,8-R,RЈ-7,8-C2B9H11, which undergo thermolysis
at 100 ЊC to give C-substituted closo-carboranes 2,3-R,RЈ-
2,3-C2B9H9.5–7
Boron substituted 11-vertex closo-carboranes are limited to
those containing hydroxyl and carboranyl groups. Oxidation of
2,3-Me2-2,3-C2B9H9 generates the hydroxy carboranes, 4-HO-
2,3-Me2-2,3-C2B9H8 and 4,7-(HO)2-2,3-Me2-2,3-C2B9H7.8–10
Bromination of the latter compound affords 10-Br-4,7-(HO)2-
† Electronic supplementary information (ESI) available: detailed NMR
data for 8; selected bond lengths for II and VI; rotatable 3-D molecular
structure diagrams of optimised geometries I to XXXVII in CHIME
format. See http://www.rsc.org/suppdata/dt/b2/b203920f/
DOI: 10.1039/b203920f
J. Chem. Soc., Dalton Trans., 2002, 3505–3517
This journal is © The Royal Society of Chemistry 2002
3505

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In this paper we report our investigation of the presumed
intermediate C2B9H13 formed on acidification of various salts
of 7,9-C2B9H12Ϫ. Conversion to closo-2,3-C2B9H11 occurs in the
reaction of (Bu4N)(7,9-C2B9H12) with conc. H2SO4 at ambient
temperature. We apply this simple acidification method using
H2SO4 or F3CSO3H to the many carbon- and boron-substituted
derivatives of nido-7,9-C2B9H12Ϫ, obtained by deboronation of
meta-C2B10H12 derivatives.3,14–17
(D3CCN)-2,3-C2B9H11 3 was formed by addition of dry
CD3CN to a sample of pure 2,3-C2B9H11 in an NMR tube.
Syntheses of 2-R-3-RЈ-2,3-C2B9H9 (R,RЈ ؍ Me 2b, Ph 2c;
R ؍ Ph, RЈ ؍ Me 2d)
Under nitrogen at 0 ЊC (ice bath), a stirred suspension of
(Bu4N)(nido-7,9-Me2-7,9-C2B9H10) 1b (0.81 g, 2 mmol) in
benzene (25 ml) was treated with conc. H2SO4 (30 ml). After the
solid had dissolved and gas evolution ceased (1 h), the benzene
layer was separated. After drying (MgSO4), benzene was
removed under reduced pressure. Sublimation of the residue at
50 ЊC/0.1 mmHg gave closo-2,3-Me2-2,3-C2B9H9 2b (0.27 g,
83%). Similar yields were obtained for closo-2,3-Ph2-2,3-
C2B9H9 2c and closo-2-Ph-3-Me-2,3-C2B9H9 2d and from
(Bu4N)(nido-7,9-Ph2-7,9-C2B9H10) 1c and (Bu4N)(nido-7-Ph-9-
Me-7,9-C2B9H10) 1d respectively. The diphenyl derivative 1c
required heating to 40 ЊC for complete conversion.
We also discuss the isomeric mixture of closo-B-Me-2,3-
C2B9H10 formed from the acidification of salts containing the
Ϫ
Ϫ
.
known18 anions 8-Me-7,9-C2B9H11 and 10-Me-7,9-C2B9H11
A unique example of an open 2,8-C2B9 cage, neutral nido-7-Me-
2,8-C2B9H12, is obtained by acidification of the fluxional anion
Ϫ 19–21
10(11)-endo-Me-7,9-C2B9H11
.
This 2,8-C2B9 fragment is
found in many closo-metallacarboranes with a metal atom
occupying the twelfth vertex.22
Geometries of the air-sensitive 11-vertex closo-carboranes
formed in this study were determined by multinuclear NMR
spectroscopy and confirmed where possible by the reliable ab
initio/GIAO/NMR method.23 In addition 11-vertex geometries
of B-methyl-nido-carboranes (where only the neutral carborane
nido-11-Me-2,7-C2B9H12 is structurally determined24) were
deduced by these methods.
Syntheses of 10-X-2,3-C2B9H10 (X ؍ Cl 2e, I 2f )
Under nitrogen at 0 ЊC (ice bath), a 1 : 2 mixture of
(Bu4N)(nido-1-Cl-7,9-C2B9H10) 1e and (Bu4N)(nido-6-Cl-7,9-
C2B9H10) 1g (0.82 g, 2 mmol) suspended in benzene (25 ml) was
treated with conc. H2SO4 (30 ml). After the solid had dissolved
and gas evolution ceased, the benzene layer was separated.
After drying (MgSO4), benzene was removed under reduced
pressure. The residue (0.3 g) was dissolved in benzene-d6 and a
sample of the solution was shown by NMR spectroscopy to
contain closo-10-Cl-2,3-C2B9H10 2e of high purity – estimated
to be 95% on the basis of its 1H and 13C NMR spectra where the
proton and carbon peaks corresponding to a Bu4Nϩ group are
also present. Attempts to purify the halogenated carborane by
sublimation or crystallisation proved unsuccessful.
From the related iodide salts (Bu4N)(nido-1-I-7,9-C2B9H10) 1f
and (Bu4N)(nido-6-I-7,9-C2B9H10) 1h using the same procedure,
10-I-2,3-C2B9H10 2f was obtained in similar purity. NMR data
for 10-I-2,3-C2B9H10: δB (C6D6) Ϫ3.1 (d, B5,6), Ϫ4.3 (d, B4,7),
Ϫ7.3 (d, B11), Ϫ7.3 (d, B8,9), Ϫ12.8 (d, B1), Ϫ18.2 (s, B10);
δH 4.84 (C2,3H), 3.45 (B11H), 3.09 (B4,7H), 2.51 (B8,9H), 2.28
(B5,6H), 0.03 (B1H).
Experimental
1
NMR spectra including 2D 11B–11B{1H} COSY and H{11B
1
selective} were recorded on Varian Unity 300 (299.9 MHz H,
96.2 MHz 11B and 75.4 MHz 13C), Varian Mercury 200 (188.2
MHz 19F) or Varian Inova 500 (160.3 MHz 11B) instruments. 1H
NMR spectra were referenced to residual protio impurity in the
solvent (CHD2CN, 1.95; C6D5H, 7.15 ppm). 13C NMR spectra
were referenced to the solvent resonance (CD3CN, 118.2, 1.3;
C6D6, 128.0 ppm). 11B and 19F NMR spectra were referenced
externally to Et2OؒBF3 in Et2O δ = 0.0 ppm and neat CFCl3
δ = 0.0 ppm respectively. Boron-11 peak assignments were
obtained with the aid of 2D 11B–11B{1H} COSY spectra where
possible. Proton peaks corresponding to boron-bound hydro-
gens were assigned on the basis of selective boron-decoupled
proton (1H{11B selective}) spectra. Unless otherwise stated, no
extraneous resonances due to impurities of any kind were
observed in the various NMR spectra of products. Carboranes
7,8-C2B9H13,25 (Me3NH)(nido-7,9-C2B9H12),26 (Bu4N)(nido-7,9-
C2B9H12),14 (Bu4N)(nido-7,9-Me2-7,9-C2B9H10),15 (Bu4N)(nido-
7-Ph-9-Me-7,9-C2B9H10),15 (Bu4N)(nido-7,9-Ph2-7,9-C2B9H10),15
(Bu4N)[nido-10-F-7,9-(p-FC6H4)2-7,9-C2B9H9],15 (Bu4N)[nido-
3-F-7,9-(p-FC6H4)2-7,9-C2B9H9],15 (Bu4N)[nido-1-(6-)-Cl-7,9-
C2B9H11],16 (Bu4N)[nido-1-(6-)-I-7,9-C2B9H11]16 and (Bu4N)-
(nido-10-OEt-7,9-C2B9H11)3,27 were prepared by literature
methods. Anhydrous monoglyme and diglyme (both Aldrich)
were used as received. Deuterated acetonitrile, CD3CN, was
dried over CaH2 and vacuum distilled.
Synthesis of 4-F-2,3-( p-FC6H4)2-2,3-C2B9H8 2i
Under nitrogen at 0 ЊC (ice bath), a vigorously stirred suspen-
sion of (Bu4N)[nido-3-F-7,9-(p-FC6H4)2-7,9-C2B9H8] 1i (1.16 g,
2 mmol) in benzene (20 ml) was treated with F3CSO3H (10 ml).
After the solid had dissolved and gas evolution ceased (2 h), the
benzene layer was separated. After drying (MgSO4), benzene
was removed under reduced pressure. The residual oil (0.55 g,
81%) was identified as 4-F-2,3-(p-FC6H4)2-2,3-C2B9H8 2i by
multinuclear NMR spectroscopy. Like 2e and 2f, the purity of
the oil 2i was at least 90% with the same impurity observed
in its proton and carbon NMR spectra. 19F NMR: observed
δF (C6D6) Ϫ112.2 (m, FC6H4), Ϫ161.4 (broad singlet, BF);
calculated δF Ϫ95.7 (FC6H4), Ϫ96.2 (FC6H4), Ϫ153.5 (BF).
CAUTION: Benzene is a suspected carcinogen and exposure
should be minimised. The reactions described here must be
performed with suitable containment. The alternative toluene
cannot be separated from closo-C2B9H11 on sublimation.
Toluene also undergoes sulfonation28 giving organic impurities
which cannot be separated from the desired closo-carborane
product.
Acidification of 10-F-7,9-( p-FC6H4)2-7,9-C2B9H9؊ 1j
Under nitrogen at 0 ЊC (ice bath), a suspension of (Bu4N)-
[nido-10-F-7,9-(p-FC6H4)2-7,9-C2B9H8] 1j (1.16 g, 2 mmol) in
benzene (20 ml) was treated with F3CSO3H (10 ml). After the
solid had dissolved, the benzene layer was separated. After dry-
ing (MgSO4), benzene was removed under reduced pressure.
Vacuum sublimation of the residue gave an off-white air-
Synthesis of 2,3-C2B9H11 2a
Under nitrogen, a vigorously stirred and cooled (0 ЊC ice bath)
suspension of (Bu4N)(nido-7,9-C2B9H12) 1a (3.75 g, 10 mmol)
in benzene (100 ml) was treated with conc. H2SO4 (100 ml).
After the solid had dissolved and gas evolution ceased (2 h), the
benzene layer was separated and dried (MgSO4). Benzene was
removed under reduced pressure to give a residue. Sublimation
of the residue at 50 ЊC/0.1 mmHg gave a white air-sensitive
solid closo-2,3-C2B9H11 2a (0.98 g, 74%). The adduct 10-
sensitive solid, 2,3-(p-FC6H4)2-2,3-C2B9H9 (0.46 g, 72%) 2j. 19
F
NMR: observed δF (C6D6) Ϫ112.2 (FC6H4); calculated δF Ϫ96.2
(FC6H4).
Acidification of 10-EtO-7,9-C2B9H11؊ 1k
Under nitrogen at 0 ЊC (ice bath), a stirred suspension
of (Bu4N)(nido-10-EtO-7,9-C2B9H11) 1k (0.42 g, 1 mmol) in
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J. Chem. Soc., Dalton Trans., 2002, 3505–3517

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benzene (20 ml) was treated with 10 ml of F3CSO3H (or conc.
H2SO4). 11B NMR spectroscopy on an aliquot of the benzene
solution revealed only 2,3-C2B9H11 2a.
Synthesis of 7-Me-2,8-C2B9H12 7
A solution containing a 4 : 1 mixture of Kϩ(nido-7,9-C2B9H12)
and Kϩ(nido-10-OEt-7,9-C2B9H12) from the deboronation3 of
1,7-C2B10H12 with KOH in ethanol was subjected to azeotropic
distillation with toluene using a Dean–Stark apparatus to
remove water and ethanol, precipitating the carborane salts.
The resulting solid was filtered and dried in vacuo. A solution of
this white solid (0.3 g) in THF (30 ml) was refluxed with an
excess of NaH (1 g) for 12 h. The resulting solution was filtered,
and an aliquot of the filtrate was examined by 11B NMR
spectroscopy and shown to contain Na2C2B9H11.29 The filtrate
was cooled to Ϫ80 ЊC and MeI (2 ml, 0.064 mol) was added.
The temperature was raised to 0 ЊC and THF was removed
under reduced pressure, finally leaving the residue in vacuo at
0 ЊC for 5 h. At 0 ЊC the oily residue was dissolved in diethyl
ether (10 ml) and slowly treated with HCl in diethyl ether (10 ml
of 2 M solution). An aliquot was removed and the solvents
removed under reduced pressure to give an oil which was
dissolved in benzene-d6 for NMR spectroscopy. Detailed NMR
studies on this sample showed neutral carborane 7-Me-2,8-
C2B9H12 7 along with minor organic impurities. Attempts to
purify the carborane by sublimation resulted in isolation
of arachno-1,3-C2B7H13 8.4 See ESI† for detailed NMR data
for 8.
Synthesis of 11-Me-2,7-C2B9H12
4
Under nitrogen, a stirred solution of 7,8-C2B9H13 (12.3 g, 0.064
mol) in a 1 : 1 mixture of diethyl ether and benzene (400 ml)
was treated dropwise with BuLi (64 ml of 2.5 M in hexanes,
0.16 mol). An aliquot of the solution was then examined by
11B NMR spectroscopy to ensure complete conversion to
29
Li2C2B9H11 and the solvents were removed under reduced
pressure. The oily residue was dissolved in 300 ml of THF and
cooled to 0 ЊC. MeI (8 ml, 0.256 mol) was added dropwise to the
solution.
Working at 0 ЊC, an aliquot of the THF solution was trans-
ferred into a Youngs’ NMR tube, THF was removed under
reduced pressure, and CD3CN added to the residue to record
multinuclear NMR spectra for the anion 11-Me-2,7-C2B9H11Ϫ 5.
At 0 ЊC THF was removed under reduced pressure and the
residue was treated with heptane (200 ml) and acetic acid
(200 ml). The solution was allowed to warm to room temper-
ature and water (400 ml) was added. The heptane layer
was separated and washed with distilled water (4 × 200 ml). The
organic layer was dried (MgSO4), and the heptane removed
under reduced pressure. The oily residue was distilled at 80 ЊC/
0.01 mmHg to give 6.5 g (68% yield) of 11-Me-2,7-C2B9H12 4 as
a clear oil which solidified on cooling to Ϫ30 ЊC.
Syntheses of B-Me-2,3-C2B9H10 isomers 2l–n
Under nitrogen a stirred suspension of (Me3NH)(8-Me-7,9-
C2B9H11) 1l (2.08 g, 10 mmol) in benzene (30 ml) was slowly
treated with conc. H2SO4 (10 ml) and stirred for 1 h. The
benzene layer was separated then dried (MgSO4) and the
solvents were removed under reduced pressure. The oily residue
was distilled at 50 ЊC/0.01 mmHg to give a clear air-sensitive
oil (1.08 g, 74%) identified by NMR as a 10 : 3 : 1 mixture of
4-Me-2,3-C2B9H10 2l, 1-Me-2,3-C2B9H10 2m and 8-Me-2,3-
C2B9H10 2n.
Synthesis of 8-Me-7,9-C2B9H11؊ 1l
A sample of 11-Me-2,7-C2B9H12 4 (1.5 g, 0.1 mol) in diethyl
ether (30 ml) was treated with neat Me3N (3 ml) and the result-
ing precipitate isolated by filtration. The air-sensitive white
solid (Me3NH)(8-Me-7,9-C2B9H11) 1l (1.85 g, 89%) was washed
with water (5ml) then diethyl ether (2 × 10ml) and finally dried
in vacuo.
The same mixture of closo-B-Me-2,3-C2B9H10 isomers
was obtained using an identical procedure with (Me3NH)-
(10-Me-7,9-C2B9H11) 1m instead of (Me3NH)(8-Me-7,9-
C2B9H11) 1l.
Synthesis of 9-Me-7,8-C2B9H11؊ 6
A THF (10 ml) solution of 11-Me-2,7-C2B9H12 4 (1.5 g,
0.1 mol) was added to a stirred suspension of NaH (1.5 g,
excess) in THF (25 ml). The solution was filtered and THF was
removed under reduced pressure to leave a semi-solid. This
residue was treated with H2O (25 ml) and then a saturated
solution of Me3NHCl in water (25ml) to give a yellow oil. The
oil was separated from the aqueous layer, and triturated with
anhydrous diethyl ether to give an off-white solid. The
hygroscopic but air-stable solid (Me3NH)(9-Me-7,8-C2B9H11)
6 (1.62 g, 78%) was dried in vacuo.
Reduction of 4-F-2,3-( p-FC6H4)2-2,3-C2B9H8 2i by NaBH4
An excess of NaBH4 (0.5 g, 13 mmol) was added to a solution
of 4-F-2,3-(p-FC6H4)2-2,3-C2B9H8 2i (0.4 g, 1.1 mmol) in dry
diglyme (20 ml). The solution was slowly heated to 70 ЊC and
maintained at this temperature for 2 h. A sample of the solution
was shown to contain the known anion 10-F-7,9-(p-FC6H4)2-
7,9-C2B9H8Ϫ 1j by 11B and 19F NMR spectroscopy.15
Synthesis of 10-Me-7,9-C2B9H11؊ 1m
Reduction of Me-2,3-C2B9H10 isomers 2l–n by NaBH4
Under nitrogen (Me3NH)(7,9-C2B9H12) (3.87 g, 0.02 mol) was
dissolved in 1 : 1 mixture of diethyl ether and benzene (100 ml).
BuLi (20 ml of 2.5 M in hexanes, 0.05 mol) was added dropwise
with stirring. An aliquot of the solution was examined by
11B NMR and shown to contain Li2C2B9H11.29 The solvents
were then removed under reduced pressure. The oily residue
was dissolved in THF (100 ml) and cooled to 0 ЊC before MeI
(2.5 ml, 0.08 mol) was added dropwise.
Working at 0 ЊC, a sample of the THF solution was taken
into a Youngs’ NMR tube and the solvent was removed under
reduced pressure. CD3CN was added to the residue to obtain
multinuclear NMR data for the anion nido-10-endo-Me-7,9-
C2B9H11Ϫ 1n.
The solution was allowed to warm to room temperature
over 2 h and the solvents were removed under reduced pressure.
The oily residue was slowly treated with Me3NHCl in water.
The resulting oil was separated from the aqueous layer, and
triturated with diethyl ether to give an air-sensitive off-white
solid of (Me3NH)(10-Me-7,9-C2B9H11) 1m (1.95 g, 47%).
Under nitrogen, a stirred slurry of NaBH4 in dry DME (40 ml)
was treated dropwise with a solution of B-Me-2,3-C2B9H10 2l–n
(0.73 g, 5 mmol) in dry DME (10 ml). After gas evolution had
ceased (1 h), the DME was removed under reduced pressure.
Water was added to the residue followed by a saturated aqueous
solution of Bu4NBr. The resulting precipitate was filtered off
and dried in vacuo. The white solid (1.31 g, 67%) was identified
by NMR spectroscopy as being the Bu4Nϩ salts of a 16 : 5 : 2 : 1
ratio mixture of 10-, 8-, 3- and 2-Me-7,9-C2B9H11Ϫ anions (1m,
1l, 1o, 1p respectively). Limited observed NMR data for 1p δH
0.02 (B–Me), Ϫ2.40 (Hµ). Calculated δB 2.2 (B2), Ϫ4.7 (B5),
Ϫ7.5 (B8), Ϫ20.6 (B3), Ϫ22.7 (B4), Ϫ24.0 (B11), Ϫ25.6 (B10),
Ϫ34.3 (B1), Ϫ36.3 (B6); δH 2.83 (B5H), 2.55 (B8H), 1.86
(B10H), 1.76 (B11H), 1.67 (B3H), 1.62 (B4H), 1.15 (B1H), 1.02
(C7H), 0.95 (C9H), 0.70 (B6H), 0.17 (CH3), Ϫ2.43 (Hµ). The
Ϫ
other possible isomers 6- and 1-Me-7,9-C2B9H11 have calcu-
lated δH values of Ϫ0.03 and 0.21 for B–Me and Ϫ2.20 and
Ϫ2.02 for Hµ respectively.
J. Chem. Soc., Dalton Trans., 2002, 3505–3517
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Computational section
All ab initio computations were carried out with the Gaussian
94 and 98 packages.30,31 The geometries discussed here were
optimised at the HF/6-31G* level with no symmetry con-
straints. Frequency calculations were computed on these
optimised geometries at the HF/6-31G* level for imaginary
frequencies. For 8-F-2,3-C2B9H10 XIII, an imaginary frequency
was unexpectedly found at this level but at the B3LYP/6-31G*//
B3LYP/6-31G* level no imaginary frequency was found.
Optimisation of these geometries was then carried out at the
computationally intensive MP2/6-31G* level and NMR shifts
calculated at the GIAO-B3LYP/6-311G* level. Theoretical 11B
chemical shifts at the GIAO-B3LYP/6-311G*//MP2/6-31G*
level listed in Table 1 and Table 2 have been referenced to B2H6
(16.6 ppm32) and converted to the usual BF3ؒOEt2 scale: δ(11B)
1
= 102.83 Ϫ σ(11B). The 13C and H chemical shifts were refer-
enced to SiMe4: δ(13C) = 184.81 Ϫ σ(13C); δ(1H) = 32.28 Ϫ σ(1H).
Theoretical 19F chemical shifts at the GIAO-B3LYP/6-311G*//
MP2/6-31G* level have been referenced to HF (214 ppm) and
converted to the usual CFCl3 scale: δ(19F) = 185.4 Ϫ σ(19F).33
Relative energies (Table 3) were computed at the MP2/6-31G*
level with ZPE (calculated at HF/6-31G*) corrections scaled by
0.89. Where appropriate, the non-hydrogen atoms of experi-
mental and calculated geometries were compared using the ofit
command of XP,34 which produces a r.m.s. deviation or misfit
value.
See ESI† for .xyz files containing Cartesian coordinates of
MP2-optimized geometries of I to XXXVII.
Results and discussion
No reaction occurs on adding acetic acid to a suspension of
(Bu4N)(nido-7,9-C2B9H12), 1a, in benzene. Using conc. H2SO4
at ambient temperature results in the evolution of gas and
formation of closo-C2B9H11, 2a (see Scheme 2). Careful
monitoring of the reaction by 11B NMR spectroscopy provides
no evidence for the formation of a ‘C2B9H13’ carborane inter-
mediate. We suggest that in the reported reaction of ‘7,9-
C2B9H13’ with 6-SB9H11, the starting carborane must be closo
2,3-C2B9H11 2a as it is found as the main ‘product’.35 The high
yields of closo-2,3-C2B9H11 2a obtained from the reactions
between salts of nido-7,9-C2B9H12Ϫ 1a and sulfuric acid suggest
this to be superior to the reported polyphosphoric acid reaction
(36% yield) and thermolysis of 7,8-C2B9H13 (22% yield).4
At ambient temperatures, acidification of the corresponding
Ϫ
C,CЈ-substituted derivatives nido-7,9-R,RЈ-7,9-C2B9H10 (R,RЈ
= Me 1b, Ph 1c; R = Ph, RЈ = Me 1d) gives the neutral carb-
oranes, closo-2,3-R,RЈ-2,3-C2B9H9 2b–d in good yields. The
reaction of 1c is sluggish and requires heating for 1 hour at 40
ЊC for complete nido–closo conversion, presumably due to the
electron-withdrawing and bulky phenyl groups. Similar yields
of closo-2,3-R,RЈ-2,3-C2B9H9 have been reported by acidific-
Scheme 2 The nido-to-closo conversions by acidification of 7,9-
C2B9H12Ϫ and derivatives carried out in this study (Ar = p-C6H4F).
Ϫ
ation of the 7,8 isomers nido-7,8-R,RЈ-7,8-C2B9H10 to give
nido-7,8-R,RЈ-7,8-C2B9H11 and subsequent thermolysis.4–7 Our
11B NMR data, listed in Table 1, agree with reported values for
C,CЈ-substituted derivatives,36 except for closo-2-Ph-3-Me-2,3-
C2B9H9 1d which was previously characterised in THF.7 The 1H
NMR data for closo-2,3-C2B9H11 2a are in agreement with
reported values.37
1i gave the expected isomer closo-4-F-2,3-(p-FC6H4)2-2,3-
C2B9H8 2i (Scheme 2c) with neat F3CSO3H. In addition to the
phenyl groups, the electron-withdrawing fluorine substituent
makes the bridging hydrogen on the open face less basic, thus a
superacid is required in place of conc. H2SO4 for the nido–closo
reaction to proceed.
Ϫ
Boron-substituted nido-clusters are of particular interest in
exploring the mechanism of cage rearrangement involved in the
nido–closo conversion. Reactions of isomeric mixtures of nido-
Acidification of nido-10-F-7,9-(p-FC6H4)2-7,9-C2B9H9 1j
and nido-10-EtO-7,9-C2B9H11Ϫ 1k with neat CF3SO3H resulted
in closo-2,3-(p-FC6H4)2-2,3-C2B9H9 2j and closo-2,3-C2B9H11
2a, as the major carborane products respectively (Scheme 2d
and Scheme 2e). Adding conc. H2SO4 to nido-10-F-7,9-(p-
Ϫ
1-X- and nido-6-X-7,9-C2B9H11 anions (X = Cl 1e/1g, I 1f/1h)
with H2SO4 gave air-sensitive closo-10-X-2,3-C2B9H10 2e and 2f.
A simple cage closure of the nido-7,9-C2B9H12Ϫ anion, by form-
ation of two new bonds B8–B10 and B8–B11, would result in
the B6 and B1 atoms being identical in the closo-cage, (i.e., B10
and B11 in the closo-skeleton numbering, Scheme 1). Likewise
Ϫ
FC6H4)2-7,9-C2B9H9 1j did not result in the formation of
any closo-carborane, even at elevated temperatures. Again it
appears that the proximity of the bulky electron-withdrawing
phenyl groups and the fluorine substituent increases the basicity
of the bridging hydrogen. The loss of the ethoxy and fluoro
Ϫ
the 3-substituted anion nido-3-F-7,9-(p-FC6H4)2-7,9-C2B9H9
3508
J. Chem. Soc., Dalton Trans., 2002, 3505–3517

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Table 1 Observed and calculated 11B, 13C and 1H NMR data for closo-carboranes discussed in this study. All experimental data recorded in C6D6
Exp. (11B, 13C)a
Calc.(11B, 13C)a
Exp. (1H)
Calc. (1H)
2,3-C2B9H11 2a
Ϫ4.2 (B4,5,6,7)
Ϫ8.5 (B8,9)
Ϫ10.2 (B10,11)
Ϫ15.8 (B1)
87.3
Ϫ5.8 (B4,5,6,7)
Ϫ9.7 (B8,9)
Ϫ10.2 (B10,11)
Ϫ18.9 (B1)
2.76 (B4,5,6,7H)
2.76 (B10,11H)
2.25 (B8,9H)
0.62 (B1H)
3.31 (B4,5,6,7H)
2.98 (B10,11H)
1.76 (B8,9H)
1.40 (B1H)
93.9
5.19 (CH)
5.74 (CH)
2,3-Me2-2,3-C2B9H9 2b
Ϫ3.2 (B4,5,6,7)
Ϫ7.0 (B8,9)
Ϫ10.4 (B10,11)
Ϫ11.7 (B1)
102.5
Ϫ4.6 (B4,5,6,7)
Ϫ6.3 (B8,9)
Ϫ11.6 (B10,11)
Ϫ15.7 (B1)
108.9 (C2,3)
28.2 (CH3)
2.86 (B10,11H)
2.75 (B4,5,6,7H)
2.43 (B8,9H)
0.65 (B1H)
3.24 (B4,5,6,7)
2.85 (B8,9H)
2.80 (B10,11H)
1.38 (B1H)
2.06 (CH3)
2.57 (CH3)
24.4
2,3-Ph2-2,3-C2B9H9 2c
Ϫ4.0 (B4,5,6,7)
Ϫ5.8 (B8,9)
Ϫ9.6 (B10,11)
Ϫ11.5 (B1)
140.6
130.3
129.8
129.2
110.2
Ϫ4.6 (B4,5,6,7)
Ϫ5.5 (B9)
Ϫ7.3 (B8)
Ϫ9.0 (B10,11)
Ϫ11.7 (B1)
Ϫ4.5 (B4,5,6,7)
Ϫ6.9 (B8,9)
Ϫ8.3 (B10,11)
Ϫ11.8 (B1)
148.7 (ipso C)
133.3 (ortho C)
132.0 (para C)
131.2 (meta C)
115.7 (C2,3)
Ϫ4.8 (B6,7)
Ϫ5.4 (B4,5)
Ϫ5.7 (B9)
3.06 (B4,5,6,7H)
3.00 (B10,11H)
2.73 (B8,9H)
1.50 (B1H)
7.27 (2H, d, 7)
7.02 (3H, m)
3.46 (B4,5,6,7)
3.12 (B8,9H)
2.93 (B10,11H)
2.13 (B1H)
7.43 (ortho CH)
7.25 (meta CH)
7.25 (para CH)
2-Ph-3-Me-2,3-C2B9H9 2d
3.07 (B4,5H)
2.97 (B10,11H)
2.81 (B6,7H)
2.70 (B8H)
2.52 (B9H)
1.11 (B1H)
3.45 (B4,5H)
3.29 (B6,7H)
3.03 (B8H)
2.95 (B10,11H)
2.91 (B9H)
Ϫ7.5 (B8)
Ϫ10.5 (B10,11)
Ϫ14.7 (B1)
1.81 (B1H)
141.9
130.3
129.8
129.2
108.5
104.9
150.6 (ipso C)
136.8 (para C)
136.6 (ortho C)
134.0 (meta C)
115.4 (CPh)
110.9 (CMe)
28.3 (CH3)
6.1 (B10)
Ϫ3.1 (B5,6)
Ϫ5.7 (B11)
Ϫ7.4 (B4,7)
Ϫ10.2 (B8,9)
Ϫ13.6 (B1)
86.2 (C2,3)
7.4 (B4)
0.2 (B7)
Ϫ6.6 (B8)
Ϫ8.0 (B11)
Ϫ9.5 (B9)
Ϫ10.0 (B6)
Ϫ10.4 (B1)
Ϫ12.0 (B10)
Ϫ18.8 (B5)
7.27 (2H, d, 7)
7.03 (3H, m)
2.08 (CH3)
7.72 (ortho CH)
7.51 (meta CH)
7.51 (para CH)
2.61 (CH3)
25.3 (CH3)
4.3 (B10)
1.2 (B5,6)
Ϫ5.8 (B11)
Ϫ5.8 (B4,7)
Ϫ8.8 (B8,9)
Ϫ10.8 (B1)
81.2 (C2,3)
10.2 (B4)
2.2 (B7)
Ϫ4.2 (B8)
Ϫ5.3 (B11)
Ϫ8.2 (B9)
Ϫ8.2 (B6)
Ϫ9.4 (B1)
Ϫ13.1 (B10)
Ϫ19.7 (B5)
164.3 (252)
164.0(252)
132.7
10-Cl-2,3-C2B9H10 2e
3.46 (B11H)
2.99 (B4,7H)
2.64 (B5,6H)
2.22 (B8,9H)
0.67 (B1H)
3.66 (B11H)
3.52 (B4,7H)
3.34 (B5,6H)
2.62 (B8,9H)
1.84 (B1H)
4.69 (C2,3H)
3.34 (B7H)
3.19 (B6H)
2.99 (B8H)
2.89 (B10H)
2.89 (B11H)
2.79 (B9H)
2.33 (B5H)
1.68 (B1H)
5.41 (C2,3H)
3.64 (B7H)
3.49 (B8H)
3.27 (B6H)
3.09 (B10H)
3.04 (B11H)
2.92 (B9H)
2.91 (B5H)
2.72 (B1H)
4-F-2,3-(p-FC6H4)2-2,3-C2B9H8 2ib c
174.8 (C3 para)
174.3 (C2 para)
145.5 (C3 ipso)
142.5 (C2 ipso)
138.4 (C2 ortho)
138.2 (C3 ortho)
121.5 (C3 meta)
121.2 (C2 meta)
115.5 (C3)
7.15 (m)
6.69 (m)
7.78 (ortho C3)
7.74 (ortho C2)
7.11 (meta C3)
7.07 (meta C2)
132.1
132.0
132.0
116.3 (21)
116.2 (21)
107.8
105.5
100.5 (C2)
2,3-(p-FC6H4)2-2,3-C2B9H9 2jc
Ϫ4.2 (B4,5,6,7)
Ϫ5.6 (B8,9)
Ϫ9.6 (B10,11)
Ϫ11.4 (B1)
164.3 (250)
132.0 (4)
131.8
116.1 (21)
109.1 (C2,3)
9.5 (B4)
Ϫ5.6 (B4,5,6,7)
Ϫ6.3 (B8,9)
Ϫ9.7 (B10,11)
Ϫ13.3 (B1)
174.7 (para C)
146.1 (ipso C)
138.3 (ortho C)
121.2 (meta C)
109.1 (C2,3)
6.8 (B4)
Ϫ1.1 (B7)
Ϫ8.6 (B11)
Ϫ8.8 (B8)
Ϫ8.9 (B6)
3.02 (B4,5,6,7H)
3.02 (B10,11H)
2.70 (B8,9H)
1.37 (B1H)
3.49 (B4,5,6,7H)
3.12 (B10,11H)
3.11 (B8,9H)
2.17 (B1H)
7.17 (dd)
6.67 (dd)
7.74 (dd)
7.08 (dd)
4-Me-2,3-C2B9H10 2l
2.88 (B7H)
2.77 (B11H)
2.72 (B10H)
2.62 (B6H)
2.58 (B5H)
2.35 (B8H)
2.21 (B9H)
0.80 (B1H)
3.38 (B7H)
3.14 (B6H)
3.14 (B5H)
2.97 (B10H)
2.95 (B11H)
2.83 (B8H)
2.69 (B9H)
1.49 (B1H)
2.7 (B7)
Ϫ7.4 (B8)
Ϫ8.6 (B11)
Ϫ8.6 (B6)
Ϫ9.2 (B9)
Ϫ10.0 (B10)
Ϫ11.0 (B5)
Ϫ13.9 (B1)
Ϫ9.7 (B10)
Ϫ10.3 (B9)
Ϫ10.4 (B5)
Ϫ17.0 (B1)
J. Chem. Soc., Dalton Trans., 2002, 3505–3517
3509

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Table 1 (Contd.)
Exp. (11B, 13C)a
Calc.(11B, 13C)a
Exp. (1H)
Calc. (1H)
4-Me-2,3-C2B9H10 2l
86.7 (C)
85.8 (C)
94.3(C3)
92.7(C2)
5.24 (CH)
4.80 (CH)
5.80 (C3H)
5.40 (C2H)
3.2 (CH3)
Ϫ1.9 (B4,5,6,7)
Ϫ5.3 (B1)
Ϫ8.6 (B10,11)
Ϫ12.4 (B8,9)
90.6 (C2,3)
0.0 (CH3)
1.3 (B8)
Ϫ3.4 (2B)
Ϫ4.7 (2B)
Ϫ9.5 (2B)
Ϫ9.5 (1B)
Ϫ18.5 (1B)
90.7
5.5 (CH3)
0.42 (CH3)
0.58 (CH3)
1-Me-2,3-C2B9H10 2m
Ϫ3.5 (B4,5,6,7)
Ϫ8.3 (B10,11)
Ϫ8.5 (B1)
Ϫ13.3 (B8,9)
97.6 (C2,3)
2.9 (CH3)
2.97 (B4,5,6,7H)
2.87 (B10,11H)
2.12 (B8,9H)
3.46 (B4,5,6,7H)
3.05 (B10,11H)
2.56 (B8,9H)
4.92 (C2,3H)
Ϫ0.50 (CH3)
5.48 (C2,3H)
Ϫ0.04 (CH3)
3.34 (B4,5H)
3.25 (B6,7H)
2.95 (B10,11H)
2.78 (B9H)
1.22 (B1H)
5.74 (C2H)
5.60 (C3H)
0.38 (CH3)
8-Me-2,3-C2B9H10 2nd
0.6 (B8)
Ϫ4.6 (B4,5)
Ϫ6.4 (B6,7)
Ϫ9.4 (B9)
Ϫ9.7 (B10,11)
Ϫ21.2 (B1)
96.3 (C2)
5.24 (CH)
5.02 (CH)
0.32 (CH3)
81.6
90.0 (C3)
0.9 (CH3)
a Underlined values are 13C shifts. b ‘C2’ in computed shifts corresponds to the phenyl group at C2. Uncertain peak assignments are shown in italics.
c Coupling constants (JCF; Hz) in parentheses for observed 13C data. d Proton peaks obscured by the major isomers 4-Me- and 1-Me-2,3-C2B9H10.
substituents upon formation of the closo-carboranes suggests
that EtOH and HF are formed as co-products respectively
rather than H2. Based on our findings, the reported acidification
of the salt (Me4N)[(7-Ph-7,9-C2B9H10(OH)] to give closo-2-Ph-
2,3-C2B9H10 indicates the formula of the starting anion to be
[10(or 11)-OH-7-Ph-7,9-C2B9H10Ϫ].4
Recently we obtained nido-11-Me-2,7-C2B9H12 4, (Me3NH)-
(nido-8-Me-7,9-C2B9H11) 1l and (Me3NH)(nido-9-Me-7,8-
C2B9H11) 6 (Scheme 3) as precursors to B-methylated tantalum
Scheme
4
Re-investigation of the reported acid-promoted cage
rearrangement of the anion nido-8-Me-7,9-C2B9H11Ϫ 1l.
to be pure Cs(nido-11-Me-2,7-C2B9H11) 5. Reaction of Cs(nido-
11-Me-2,7-C2B9H11) 5 with acetic acid (Scheme 4) gave nido-11-
Me-2,7-C2B9H12 4 as expected. The salt 5 rearranges slowly in
the solid state under nitrogen at ambient temperature over-
night to Cs(nido-8-Me-7,9-C2B9H11) 1l. No reaction occurred
on addition of acetic acid to the ‘carbons-apart’ salt 1l. This
investigation rules out a reversible cage rearrangement between
nido-11-Me-2,7-C2B9H12
4 and the anion nido-8-Me-7,9-
C2B9H11Ϫ 1l.40
Addition of H2SO4 to (Me3NH)(nido-8-Me-7,9-C2B9H11)
1l gave a 10 : 3 : 1 ratio mixture of B-monomethyl-closo-
carboranes, 4-Me-2,3-C2B9H10 2l, 1-Me-2,3-C2B9H10 2m and
8-Me-2,3-C2B9H10 2n (Scheme 5). A straightforward closure
mechanism of the 8-substituted anion would generate closo-1-
Me-2,3-C2B9H10 only. Acidification of the related anion nido-
Scheme 3 Formation of nido-B-methyl-carboranes from nido-7,8-
C2B9H13.
carboranes.38 As depicted in Scheme 3, the neutral carborane
4 was formed by reacting nido-7,8-C2B9H13 with BuLi to give
Li2C2B9H11 which then reacts with MeI to give Li(nido-11-Me-
2,7-C2B9H11) 5 followed by acetic acid to give nido-11-Me-2,7-
C2B9H12 4. Both the anions 1l and 6 were generated from
4. Detailed syntheses of these B-methyl nido-carboranes (which
are modifications of literature methods18) are given in the
Experimental section.
Ϫ
10-Me-7,9-C2B9H11 1m (see below) gave the same mixture of
B-monomethyl closo carboranes. As the same isomeric mixture
of B-monomethyl-closo-carboranes is obtained from acidific-
Ϫ
ation of salts of nido-8-Me- and nido-10-Me-7,9-C2B9H11
anions, this suggests either a common carborane intermediate
in both reactions or that the B-monomethyl carborane is
fluxional in solution.
An equilibrium occurs between the three possible isomers of
B-monomethyl 7-vertex closo-B-Me-2,4-C2B5H6 at 300 ЊC.41 On
this basis 11-vertex closo-B-Me-2,3-C2B9H10 isomers are likely
to be in equilibrium at ambient temperature. Cage rearrange-
ments in the 11-vertex compound are expected to be facile as an
Ϫ
The anion nido-8-Me-7,9-C2B9H11 1l is reported to form
nido-11-Me-2,7-C2B9H12 4 on addition of acetic acid.39 This
reaction involves an interesting rearrangement of the carborane
skeleton to accommodate both bridging hydrogens on the open
face. We re-examined the acidification of Cs(nido-8-Me-7,9-
C2B9H11) 1l. Spectroscopic characterisation of the caesium salt
formed by deprotonation of nido-11-Me-2,7-C2B9H12 4 shows it
3510
J. Chem. Soc., Dalton Trans., 2002, 3505–3517

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Table 2 Observed and calculated 11B, 13C and 1H NMR data for nido-carboranes discussed in this study, all data in CD3CN unless otherwise stated
Exp. (11B, 13C)a
Calc.(11B, 13C)a
Exp. (1H)
Calc. (1H)
nido-8-Me-7,9-C2B9H11Ϫ 1lb
2.4 (B8)
Ϫ6.2 (B2,5)
Ϫ20.1 (B3,4)
Ϫ23.1 (B10,11)
Ϫ33.1 (B1)
Ϫ37.8 (B6)
33.4 (C7,9)
3.5 (CH3)
Ϫ2.6 (B5)
Ϫ4.5 (B2)
Ϫ6.4 (B8)
Ϫ12.8 (B10)
Ϫ21.3 (B4)
Ϫ23.2 (B11)
Ϫ24.7 (B3)
Ϫ33.6 (B6)
Ϫ34.3 (B1)
36.2 (C)
0.3 (B8)
Ϫ5.8 (B2,5)
Ϫ20.4 (B3,4)
Ϫ26.0 (B10,11)
Ϫ34.0 (B1)
Ϫ38.2 (B6)
39.1 (C7,9)
5.3 (CH3)
Ϫ1.8 (B5)
Ϫ5.0 (B2)
Ϫ7.1 (B8)
Ϫ16.5 (B10)
Ϫ22.4 (B4)
Ϫ25.0 (B3)
Ϫ25.7 (B11)
Ϫ35.1 (B1)
Ϫ35.4 (B6)
42.3 (C9)
2.04 (B2,5H)
1.37 (B10,11H)
1.04 (B3,4H)
0.62 (B1H)
Ϫ0.11 (B6H)
Ϫ2.20 (Hµ)
0.94 (C7,9H)
0.02 (CH3)
2.17 (B5H)
2.08 (B2H)
1.91 (B8H)
1.38 (B11H)
1.01 (B4H)
0.92 (B3H)
0.56 (B1H)
Ϫ0.05 (B6H)
Ϫ1.92 (Hµ)
1.17(CH)
2.80 (B2,5)
1.81 (B10,11H)
1.68 (B3,4H)
1.29 (B1H)
0.62 (B6H)
Ϫ2.33 (Hµ)
0.97 (C7,9H)
0.00 (CH3)
2.96 (B5H)
2.84 (B2H)
2.55 (B8H)
1.86 (B11H)
1.65 (B4H)
1.51 (B3H)
1.28 (B1H)
0.65 (B6H)
Ϫ1.97 (Hµ)
1.12 (C9H)
1.05 (C7H)
0.01 (CH3)
2.87 (B5H)
2.83 (B2H)
2.50 (B8H)
1.82 (B11H)
1.68 (B4H)
1.66 (B10H)
1.28 (B1H)
0.66 (B6H)
Ϫ2.40 (Hµ)
1.10 (C9H)
1.07 (C7H)
Ϫ0.02 (CH3)
3.00 (B2,5H)
2.95 (B10,11H)
2.70 (B8H)
2.12 (B1H)
1.85 (B3,4H)
1.48 (B6H)
1.78 (C7,9H)
0.84 (CH3)
3.31 (B5H)
3.19 (B2H)
2.99 (B8H)
2.56 (B11H)
2.21 (B4H)
2.03 (B3H)
2.00 (B1H)
1.29 (B6H)
Ϫ0.75 (Hµ)
2.28 (CH3)
2.01 (C9H)
1.99 (C7H)
nido-10-Me-7,9-C2B9H11Ϫ 1mb
30.0 (C)
1.7 (CH3)
36.7 (C7)
4.5 (CH3)
1.10 (CH)
0.02 (CH3)
nido-3-Me-7,9-C2B9H11Ϫ 1oc
Ϫ3.5 (1B)
Ϫ3.5 (1B)
Ϫ4.0 (1B)
Ϫ13.4 (B3)
Ϫ21.3 (1B)
Ϫ23.1 (1B)
Ϫ25.6 (1B)
Ϫ33.4 (1B)
Ϫ35.2 (1B)
Ϫ3.4 (B2)
Ϫ3.8 (B5)
Ϫ5.2 (B8)
Ϫ14.9 (B3)
Ϫ22.2 (B4)
Ϫ25.8 (B11)
Ϫ28.2 (B10)
Ϫ34.3 (B1)
Ϫ36.7 (B6)
43.3 (C7)
Ϫ2.38 (Hµ)
38.0 (C9)
5.3 (CH3)
Ϫ0.03 (CH3)
2.49 (B10,11H)
2.32 (B2,5H)
2.05 (B8H)
1.43 (B1H)
1.21 (B3,4H)
0.76 (B6H)
1.95 (C7,9H)
0.66 (CH3)
2.59 (B2H)
2.33 (B5H)
2.16 (B8H)
1.95 (B11H)
1.36 (B3H)
1.14 (B4H)
0.93 (B1H)
0.52 (B6H)
0.30 (Hµ)
nido-10-endo-Me-7,9-C2B9H11Ϫ 1nd e f
Ϫ2.0 (B2,5)
Ϫ3.2 (B10,11)
Ϫ5.7 (B8)
Ϫ20.1 (B1)
Ϫ20.1 (B3,4)
Ϫ22.1 (B6)
46.0 (C7,9)
10.8 (CH3)
Ϫ2.5 (B2)
Ϫ2.7 (B2,5)
Ϫ5.5 (B10,11)
Ϫ6.9 (B8)
Ϫ20.2 (B1)
Ϫ20.4 (B3,4)
Ϫ22.4 (B6)
54.5 (C7,9)
17.0 (CH3)
Ϫ0.9 (B5)
Ϫ2.9 (B8)
Ϫ3.3 (B2)
Ϫ18.8 (B3)
Ϫ19.3 (B4)
Ϫ23.8 (B10)
Ϫ24.0 (B11)
Ϫ30.0 (B1)
Ϫ30.0 (B6)
119.9 (CN)
47.0 (C7)
nido-10-(CD3CN)-7,9-C2B9H11 3g
Ϫ4.2 (B5)
Ϫ4.8 (B8)
Ϫ18.2 (B10)
Ϫ20.2 (B3)
Ϫ22.7 (B4)
Ϫ22.7 (B11)
Ϫ33.3 (B1)
Ϫ33.3 (B6)
35.0 (C7,9)
1.95 (CH)
1.61 (CH)
43.3 (C9)
3.9 (CH3)
3.5 (B5)
nido-11-Me-2,7-C2B9H12 4h
1.6 (B5)
Ϫ0.1 (B11)
Ϫ7.7 (B9)
Ϫ9.0 (B3)
Ϫ9.0 (B8)
Ϫ22.5 (B10)
Ϫ25.3 (B1)
Ϫ29.0 (B6)
Ϫ32.8 (B4)
58.6 (C2)
3.19 (B5H)
2.52 (B9H)
2.48 (B8H)
2.29 (B3H)
2.12 (B1H)
1.66 (B10H)
1.34 (B6H)
1.23 (B4H)
Ϫ2.81(Hµ10,11)
Ϫ3.27 (Hµ 8,9)
2.29 (C2H)
1.18 (C7H)
0.14 (CH3)
1.56 (B6H)
1.25 (B4H)
1.25 (B3H)
1.18 (B8H)
1.00 (B5H)
0.95 (B10H)
0.84 (B9H)
0.84 (B1H)
Ϫ3.29 (Hµ)
1.85 (CH)
3.60 (B5H)
3.10 (B3H)
3.04 (B8H)
2.78 (B9H)
2.53 (B1H)
2.10 (B10H)
1.76 (B6H)
1.49 (B4H)
Ϫ2.82 (Hµ 10,11)
Ϫ3.31 (Hµ 8,9)
3.30 (C2H)
2.04 (C7H)
0.52 (CH3)
2.31 (B6H)
2.07 (B4H)
1.92 (B8H)
1.75 (B3H)
1.40 (B5H)
1.20 (B10H)
1.02 (B9H)
0.96 (B1H)
Ϫ3.13 (Hµ)
1.60 (C2H)
1.18 (C7H)
Ϫ4.1 (B11)
Ϫ9.5 (B9)
Ϫ10.4 (B3)
Ϫ10.9 (B8)
Ϫ22.7 (B10)
Ϫ26.4 (B1)
Ϫ31.8 (B6)
Ϫ33.6 (B4)
67.4 (C2)
36.7 (C7)
1.0 (CH3)
41.0 (C7)
2.7 (CH3)
nido-11-Me-2,7-C2B9H11Ϫ 5b
Ϫ7.1 (B11)
Ϫ15.1 (B6)
Ϫ17.0 (B4)
Ϫ17.6 (B8)
Ϫ20.3 (B9)
Ϫ22.2 (B3)
Ϫ24.8 (B10)
Ϫ26.6 (B5)
Ϫ44.1 (B1)
51.2 (C7)
Ϫ11.8 (B11)
Ϫ15.3 (B6)
Ϫ16.2 (B4)
Ϫ17.7 (B8)
Ϫ23.4 (B9)
Ϫ24.7 (B3)
Ϫ26.5 (B10)
Ϫ27.6 (B5)
Ϫ46.1 (B1)
47.4 (C7)
49.1 (C2)
40.6 (C2)
1.25 (CH)
J. Chem. Soc., Dalton Trans., 2002, 3505–3517
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Table 2 (Contd.)
Exp. (11B, 13C)a
Calc.(11B, 13C)a
Exp. (1H)
Calc. (1H)
nido-11-Me-2,7-C2B9H11Ϫ 5b
nido-9-Me-7,8-C2B9H11Ϫ 6b e
6.5 (CH3)
Ϫ2.5 (B9)
Ϫ10.0 (B11)
Ϫ14.4 (B6)
Ϫ17.7 (B3)
Ϫ18.9 (B5)
Ϫ18.9 (B4)
Ϫ27.4 (B2)
Ϫ33.2 (B10)
Ϫ38.0 (B1)
43.4 (C)
2.7 (CH3)
Ϫ4.6 (B9)
Ϫ14.4 (B11)
Ϫ15.6 (B5)
Ϫ17.4 (B6)
Ϫ19.3 (B3)
Ϫ20.6 (B4)
Ϫ26.2 (B2)
Ϫ34.5 (B10)
Ϫ39.4 (B1)
47.5 (C8)
0.08 (CH3)
1.90 (B11H)
1.68 (B3H)
1.27 (B6H)
1.22 (B5H)
1.00 (B4H)
0.83 (B2H)
0.40 (B1H)
0.00 (B10H)
Ϫ2.74 (Hµ)
1.72 (CH)
0.07 (CH3)
2.28 (B11H)
2.21 (B3H)
1.92 (B5H)
1.83 (B6H)
1.65 (B4H)
1.57 (B2H)
1.01 (B1H)
0.54 (B10H)
Ϫ2.87 (Hµ)
1.53 (C7H)
1.49 (C8H)
0.24 (CH3)
3.53 (B6H)
3.06 (B3H)
3.00 (B9H)
2.89 (B4H)
2.65 (B1H)
2.46 (B10H)
2.01 (B11H)
1.44 (B5H)
Ϫ1.90 (Hµ 7,11)
Ϫ3.48 (Hµ 9,10)
2.09 (C8H)
1.56 (C2H)
0.49 (CH3)
42.0 (C)
1.3
46.0 (C7)
4.7 (CH3)
1.66 (CH)
0.25 (CH3)
2.93 (B6H)
2.57 (B3H)
2.54 (B9H)
2.39 (B4H)
2.27 (B1H)
2.05 (B10H)
1.64 (B11H)
1.17 (B5H)
Ϫ2.18 (Hµ 7,11)
Ϫ3.47 (Hµ 9,10)
nido-7-Me-2,8-C2B9H12 7h j
Ϫ1.1 (B7)
Ϫ1.9 (B6)
Ϫ4.8 (B9)
Ϫ9.7 (B4)
Ϫ11.5 (B3)
Ϫ19.5 (B10)
Ϫ22.0 (B11)
Ϫ24.9 (B1)
Ϫ32.9 (B5)
Ϫ1.6 (B6)
Ϫ3.1 (B7)
Ϫ7.1 (B9)
Ϫ9.6 (B4)
Ϫ12.2 (B3)
Ϫ20.8 (B10)
Ϫ24.0 (B11)
Ϫ26.4 (B1)
Ϫ33.8 (B5)
43.9 (C8)
30.2 (C2)
2.7 (CH3)
0.14 (CH3)
a Underlined values are 13C shifts. b Unassigned 11B NMR data reported in ref. 18. c Proton and carbon peaks obscured by major isomers, 8-Me- and
Ϫ
10-Me-7,9-C2B9H11
.
d Unassigned 11B NMR data reported in ref. 19. e Averaging of calculated shifts. f Unassigned 11B NMR data reported in refs.
18 and 39. g 1H and 13C NMR peaks corresponding to the CD3CN ligand were not observed – they are probably hidden in peaks corresponding to
free CD3CN, calculated NMR shifts are for 10-(CH3CN)-7,9-C2B9H11 XXXV. h In C6D6. i Measured at 0 ЊC. j Proton peaks corresponding to
carboranyl C–H obscured by organic impurities, 13C NMR not recorded. Unassigned 11B NMR reported in ref. 20.
Scheme 6 Formation of the neutral nido-carborane by acidification of
a derivative of the nido-7,9-C2B9H12Ϫ anion.
followed by MeI (Scheme 6). This procedure is a modification of
the known alkylation route involving facile cage rearrange-
ments with one carborane intermediate observed by 11B NMR
spectroscopy.18–21 The intermediate has been proposed to be
Ϫ
Ϫ
nido-10-endo-Me-7,9-C2B9H11 1n, a 7,9-C2B9H12 anion with
a bridging methyl group in place of the bridging hydrogen.
Addition of HCl to this intermediate at 0 ЊC was reported to
give nido-7-Me-2,8-C2B9H12 7, the only known example of a
nido-2,8-C2B9 cage, and a potential precursor to metalla-
carboranes with 2,1,8-MC2B9 cages, normally obtained by
cage rearrangements of the more common metallacarboranes
with 3,1,2-MC2B9 cages.22 Since this protonation reaction
appears to form a neutral nido carborane rather than a closo
carborane, we repeated this reaction and find that 7 is indeed
formed. However we have not yet succeeded in obtaining a pure
sample of 7, sublimation gave arachno-C2B7H13 8.4
Scheme
5
The synthesis of isomeric 11-vertex closo-B-methyl
carboranes, by acidification of nido-B-methyl carboranes, and their
subsequent reduction by hydride ion.
unfavourable seven-coordinate boron atom (B1) is present.
Cage fluxionality occurs in solutions of related 11-vertex closo-
compounds, B11H112Ϫ and CB10H11
.
Ϫ 42
The salt (Me3NH)(nido-10-Me-7,9-C2B9H11) 1m was pre-
pared by the reaction of (Me3NH)(nido-7,9-C2B9H12) with BuLi
3512
J. Chem. Soc., Dalton Trans., 2002, 3505–3517

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Table 3 Energies of MP2/6-31G* optimized geometriesk
Optimised geometries
ZPE/a.u.
MP2/a.u.
Relative energy/kcal molϪ1
2,3-C2B9H11
2,3-Me2-2,3-C2B9H9 II
2,3-Ph2-2,3-C2B9H9 III
I
105.83(0)
143.23(0)
214.87(0)
204.04(0)
179.06(0)
145.68(0)
101.51(0)
101.74(0)
101.36(1)
101.61(0)
100.54(0)
100.83(0)
100.53(0)
100.71(0)
124.60(0)
124.74(0)
124.50(0)
124.62(0)
199.87(0)
113.91(0)
120.16(0)
120.32(0)
120.11(0)
120.71(0)
120.03(0)
138.85(0)
139.50(0)
131.11(0)
129.77(0)
130.61(0)
130.80(1)
131.15(0)
131.15(0)
131.14(0)
131.12(0)
130.60(0)
130.56(0)
129.41(1)
129.93(0)
130.08(0)
139.26(0)
182.31(0)
182.85(0)
Ϫ305.43626l
Ϫ383.78069
Ϫ766.05928
Ϫ964.09598
Ϫ574.91985
Ϫ3103.41997
Ϫ404.51661
Ϫ404.51657
Ϫ404.51320
Ϫ404.50984
Ϫ764.51188
Ϫ764.51428
Ϫ764.51187
Ϫ764.51162
Ϫ344.61660
Ϫ344.61735
Ϫ344.61553
Ϫ344.61368
Ϫ1063.17600
Ϫ256.96314
Ϫ306.55322
Ϫ306.55349
Ϫ306.54590
Ϫ306.56935
Ϫ306.56158
Ϫ345.72519
Ϫ345.74847
Ϫ345.27001
Ϫ345.26794
Ϫ345.23381
Ϫ345.22479
Ϫ345.26788
Ϫ345.27092
Ϫ345.26988
Ϫ345.26800
Ϫ345.24407
Ϫ345.24404
Ϫ345.24225
Ϫ345.20939
Ϫ345.20490
Ϫ437.77583
Ϫ611.72394
Ϫ611.70712
2,3-(p-FC6H4)2-2,3-C2B9H9 IV
2-Ph-3-Me-2,3-C2B9H9 V
10-Br-4,7-(OH)2-2,3-Me2-2,3-C2B9H6 VI
1-F-2,3-C2B9H10 VII
0.0
0.2
3.5
4.1
1.5
0.0
1.7
1.2
0.7
0.0
1.8
2.8
4-F-2,3-C2B9H10
X
8-F-2,3-C2B9H10 XIII
10-F-2,3-C2B9H10 XVI
1-Cl-2,3-C2B9H10 VIII
4-Cl-2,3-C2B9H10 XI
8-Cl-2,3-C2B9H10 XIV
10-Cl-2,3-C2B9H10 XVII
1-Me-2,3-C2B9H10 IX
4-Me-2,3-C2B9H10 XII
8-Me-2,3-C2B9H10 XV
10-Me-2,3-C2B9H10 XVIII
4-F-2,3-(p-FC6H4)2-2,3-C2B9H8 XIX
1,3-C2B7H13 XX
m
7,8-C2B9H13
17.8
10.8
19.1
0.0
12.3
18.9
0.0
0.0
2.4
23.1
33.3
1.9
0.0
0.5
2.1
21.0
20.9
21.9
45.9
46.8
7,9-C2B9H13 XXI
2,7-C2B9H13 XXII
2,8-C2B9H13 XXIII
m
2,9-C2B9H13
11-Me-2,7-C2B9H12 XXIV
7-Me-2,8-C2B9H12 XXV
8-Me-7,9-C2B9H11Ϫ XXXII
10-Me-7,9-C2B9H11Ϫ XXXIII
10-endo-Me-7,9-C2B9H11Ϫ XXVIIa
10,11-µ-Me-7,9-C2B9H11Ϫ XXVIIb
1-Me-7,9-C2B9H11Ϫ XXVIII
2-Me-7,9-C2B9H11Ϫ XXXI
3-Me-7,9-C2B9H11Ϫ XXX
6-Me-7,9-C2B9H11Ϫ XXIX
9-Me-7,8-C2B9H11Ϫ (Hµ at B9,10) XXXIVa
9-Me-7,8-C2B9H11Ϫ (Hµ at B10,11) XXXIVb
9-Me-7,8-C2B9H11Ϫ (H at B10) XXXIVc
11-Me-2,7-C2B9H11Ϫ (Hµ at B9,10) XXVIa
11-Me-2,7-C2B9H11Ϫ (Hµ at B8,9) XXVIb
10-(MeCN)-7,9-C2B9H11 XXXV
3,4-µ-(OCH2CH2O)-7,9-Me2-7,9-C2B9H8Ϫ XXXVI
10,11-µ-(OCH2CH2O)-7,9-Me2-7,9-C2B9H8Ϫ XXXVII
0.0
11.0
k ZPE = zero point energy at HF/6-31G* level, NIMAG = number of imaginary frequencies. Non-SI unit employed 1 kcal molϪ1 = 4.184 kJ molϪ1
.
l See also ref. 49. m From ref. 3.
1
The structure of 7 is consistent with 1D 11B and H NMR
spectra – however eight other isomers of MeC2B9H12 also fit the
data. The molecular geometry of nido-7-Me-2,8-C2B9H12 was
previously proposed19,21 on the basis of cage rearrangement
mechanisms determined for the formation of the structurally-
known neutral nido-11-Me-2,7-C2B9H12.24 Here we find that
nido-7-Me-2,8-C2B9H12 is the only isomer compatible with the
cross peaks observed in the 2D 11B–11B{1H} COSY spectrum
of 7.
Unlike the nido-7,9-C2B9 to closo-2,3-C2B9 conversion,
the reverse process has been well documented prior to this
study, and the existence of a facile synthesis of closo-2,3-
C2B9H11 2a makes these reactions a viable route to substi-
tuted nido-7,9-clusters. Thus, closo-2,3-C2B9H11 2a reacts
Ϫ
with hydride ion to give nido-7,9-C2B9H12 1a and with
Lewis bases (L) to afford nido-10-L-7,9-C2B9H11,4,11,43,44
–
Scheme 7 Typical closo-11 vertex to nido-11 vertex conversions
dissolving closo-2,3-C2B9H11 in CD3CN gives the adduct nido-
10-CD3CN-7,9-C2B9H11 3 (Scheme 7). The reported 11B NMR
chemical shifts for closo-2-Ph-3-Me-2,3-C2B9H9 2d in THF are
likely to correspond to the adduct PhMeC2B9H9ؒTHF. The
reaction of the isomeric mixture of B-Me-2,3-C2B9H10 with
NaBH4 gave four products 10-Me 1m, 8-Me- 1l, 3-Me- 1o and
performed by the addition of Lewis bases or nucleophiles.
Addition of NaBH4 to the fluoro-carborane closo-4-F-2,3-
(p-FC6H4)2-2,3-C2B9H8 2i gave the monoanion 10-F-7,9-
Ϫ
(p-FC6H4)2-7,9-C2B9H9 1j with the fluorine located at the
open face as shown in Scheme 7. Salts of nido-3-F-7,9-R,RЈ-
Ϫ
Ϫ
2-Me-7,9-C2B9H11 1p in a 16 : 5 : 2 : 1 ratio (Scheme 5). A
7,9-C2B9H9 anions, obtained from the deboronation of
strong preference for the methyl group to be at the open face
is noted.
meta-carboranes closo-1,7-R,RЈ-1,7-C2B10H10 by Bu4NF, may
therefore be defluorinated by acidification and reduction twice
J. Chem. Soc., Dalton Trans., 2002, 3505–3517
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Ϫ
to yield nido-7,9-R,RЈ-7,9-C2B9H10 anions. Likewise nido-3-
ORЉ-7,9-R,RЈ-7,9-C2B9H9
Geometry optimisations of other C-substituted derivatives
2,3-R,RЈ-2,3-C2B9H9 (R,RЈ = Ph III, p-FC6H4 IV, R = Ph RЈ =
Me V) and comparison with the unsubstituted carborane 2,3-
C2B9H11 I49 revealed the influence of alkyl and aryl groups on
the closo-cage framework to be small (misfit values in the region
of 0.0163 to 0.0207 Å). By contrast the optimised geometries of
the halogenated carboranes closo-4-X-2,3-C2B9H10 (X = F X, Cl
XI, Me XII) show substantial distortion of the cage geometry.
The substituent causes a shift in the position of the unique
six-coordinate boron B1 away from the substituted boron (B4)
further lengthening the already long B–B bonds between B1
and B4/B7 like that found in the X-ray structure of the bromo
carborane.48 The misfit values by comparing these substituted
compounds with the parent carborane I are 0.0389, 0.0254 and
0.0182 Å for X = F, Cl and Me respectively. Optimised geom-
etries of 11-vertex closo-carboranes B-X-C2B9H10 (X = F, Cl,
Me) with the substituents at B1 (VII–IX), B8 (XIII–XV) or B10
(XVI–XVIII) have cage skeletons with little distortion com-
pared to closo-2,3-C2B9H11 I (misfit values of 0.0046–0.0163 Å).
The B-fluoro carborane, 4-F-2,3-(p-FC6H4)2-2,3-C2B9H8 2i is
synthesized here, and Fig. 2 shows the distorted closo-skeleton
Ϫ
anions, formed from meta-
carboranes with KOH in alcohol,45,46 would generate nido-7,9-
R,RЈ-7,9-C2B9H10Ϫ anions by this four-step route.
Computational studies
Fully optimised geometries at the electron correlated and
computationally intensive MP2/6-31G* level of theory can be
regarded as excellent representations of molecular structures
found experimentally in the gas phase and in solution for
carboranes. If 11B NMR shifts computed at various levels (e.g.,
IGLO, GIAO) from an optimised geometry (MP2/6-31G*) of
a carborane show a very good correlation with its experimental
solution-state NMR data then the optimised geometry is con-
sidered to be a good representation of its molecular structure in
solution.23
The molecular geometries of two 11-vertex closo-carboranes,
2,3-Me2-2,3-C2B9H9 2b and 10-Br-4,7-(HO)2-2,3-Me2-2,3-
C2B9H6, were determined by X-ray crystallography over
20 years ago.47,48 The difference in the cage skeleton between
these two compounds is marked, the cage in closo-Me2C2B9H9
2b is highly symmetrical whereas the brominated compound
has the unique seven-coordinate boron atom (B1) shifted away
from the hydroxy-substituted boron atoms (B4,7) to such an
extent that the framework appears to have an open face
geometry like that found in the nido-7,9-C2B9H12 anion.3
Ϫ
Nevertheless, geometric data are in broad agreement with the
optimised geometries II and VI computed at the MP2/6-31G*
level (see Fig. 1) of theory and their bond lengths are listed in
Fig.
2
MP2-optimized molecular geometry of closo-4-F-2,3-
(p-FC6H4)2-2,3-C2B9H8 XIX for 2i. Selected bond lengths (Å), B1–C2
1.655, B1–C3 1.650, B1–B4 2.114, B1–B7 2.069, B1–B5 1.966, B1–B6
1.983, B4–F1 1.357, B4–B8 1.808, B5–B8 1.780, B5–B6 1.830.
calculated in the MP2-optimized geometry XIX of this com-
pound with selected bond lengths. The misfit values between
the cage skeleton in XIX and the highly symmetrical closo-
carboranes 2,3-(p-FC6H4)2-2,3-C2B9H9 IV and 2,3-C2B9H11
I
are respectively 0.0273 and 0.0326 Å.
Calculated relative energies of the isomeric B-Me-2,3-
C2B9H10 carboranes reveal the 4-isomer (XII) to be the most
stable followed by 1- (IX), 8- (XV) and the least stable 10-isomer
(XVIII). This energy order is also found for the B-methyl-closo-
carboranes B-Me-2,3-C2B9H10 where a 10 : 3 : 1 ratio mixture of
4- 2l, 1- 2m and 8- 2n isomers was obtained experimentally here.
The agreement between relative energies and the ratio of
isomers observed here points to fluxional cage rearrangements
taking place in closo-B-Me-2,3-C2B9H10. Relative energies of
closo-B-Cl-2,3-C2B9H10 isomers indicate the 4-isomer (XI) to be
the most stable followed by the 10-isomer (XVII). The chlorin-
ated carborane isomer closo-10-Cl-2,3-C2B9H10 2e obtained
here on the other hand does not appear to rearrange into the
more stable isomer. A different stability order is found for closo-
B-F-2,3-C2B9H10 isomers with the 1-isomer (VII) being more
stable than the 4-isomer (X). It seems that the methyl group, but
not halogens or carboranyl groups,11 aids cage rearrangement
of the closo-C2B9 skeleton.
Fig. 1 11-Vertex closo-carboranes whose optimised geometries have
been computed at the MP2/6-31G* level.
Geometry optimisation, at the MP2/6-31G* level, of the
neutral carborane nido-7,9-C2B9H13 XXI (see Fig. 3) revealed a
minimum containing an endo hydrogen at C9 and a hydrogen
bridging between B10 and B11. This geometry is lower in
energy than the known carborane 7,8-C2B9H13.3 An endo-
hydrogen at a cage carbon in nido-carboranes is unusual but not
without precedent, two are known experimentally, in the per-
the ESI.† Best fitting between the experimental and MP2-
optimized, VI, geometries of the bromo carborane showed
a misfit value of 0.0351 Å for heavy cage atoms only. This
demonstrates that optimised geometries of closo-C2B9H11 deriv-
atives at the correlated MP2 level are reasonable representations
of their experimental geometries.
3514
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Fig. 3 11-Vertex nido-carboranes whose optimised geometries have been computed at the MP2/6-31G* level. Transition state minima are shown in
square brackets.
alkylated derivatives of 2,4-C2B4H8 and 2,3,5-C3B3H7.50–52 The
geometries of the unknown compounds, 2,7- XXII and 2,8-
C2B9H13 XXIII, were also optimised. The 2,7-isomer is not as
stable as the known 2,9-isomer3 whereas the 2,8-isomer is the
most stable – all geometries differ only in the position of the
lower belt carbon. Although the existence of nido-7,9-C2B9H13
is not ruled out theoretically it is so far not isolable experi-
mentally as demonstrated in this study.
Best fitting between the experimental (X-ray)24 and optimised
(MP2 XXIV) geometries of nido-11-Me-2,7-C2B9H12 showed a
misfit value of only 0.0170 Å for the heavy cage atoms, indi-
cating a very good agreement. This gives confidence in the
accuracy of MP2-optimized geometries for 11-vertex B-
monomethyl carboranes discussed in this study. The good
agreement between X-ray and MP2-geometries for the parent
anion nido-7,9-C2B9H12Ϫ has been demonstrated recently.3
Deprotonation of the structurally known carborane nido-11-
Me-2,7-C2B9H12 removes one bridging hydrogen to give the
monoanion nido-11-Me-2,7-C2B9H12Ϫ. Three minima (XXVIa–
c) were located at the RHF/6-31G* level, each with the bridging
hydrogen at a B–B bond on the open face. The most stable
Fig. 4 MP2-optimized molecular geometry of nido-7-Me-2,8-C2B9H12
XXV for 7. Selected bond lengths (Å), B7–C8 1.635, C8–B9 1.664, B9–
B10 1.893, B10–B11 1.867, C2–B11 1.697, C2–B7 1.716, B7–C(Me)
1.587.
shows the MP2-optimized geometry of nido-7-Me-2,8-C2B9H12
XXV. Although the compound could not be isolated pure here
it is computed to be more thermodynamically stable than
11-Me-2,7-C2B9H12 XXIV.
Ϫ
geometry (XXVIa) of nido-11-Me-2,7-C2B9H11 contains the
bridging hydrogen opposite the carbon atom on the open face.
The structure of 11-Me-2,7-C2B9H11Ϫ depicted in the literature
with the bridging hydrogen next to the methyl substituted
boron (XXVIc) is not a true minimum and is not located at the
MP2/6-31G* level.
While nido-11-Me-2,7-C2B9H12 was structurally determined
over 20 years ago, the geometry of the isomer nido-7-Me-2,8-
C2B9H12 has not been described. It was predicted to be like nido-
11-Me-2,7-C2B9H12 differing only in the position of the cage
carbon in the lower belt.20,21 This proposed geometry, sup-
ported here by detailed NMR spectroscopy, is confirmed by
geometry optimisations and NMR shift calculations. Fig. 4
In the formation of nido-7-Me-2,8-C2B9H12 7, an inter-
mediate was observed by 11B NMR spectroscopy and proposed
Ϫ
to be 10,11-µ-Me-7,9-C2B9H11 1n.19–21 A bridging methyl
group is unusual in carborane chemistry. Geometry optimis-
Ϫ
ation of 10,11-µ-Me-7,9-C2B9H11 gave a transition state
minimum XXVIIb. A true minimum XXVIIa was located with
an endo-substituted methyl group at B10 (or 11) and is shown in
Fig. 5. The difference in energy between these minima is only
ca. 6 kcal molϪ1, thus methyl group fluxionality is expected.
This fluxionality explains the apparent mirror symmetry in
the observed NMR data for the anion. The molecular geometry
J. Chem. Soc., Dalton Trans., 2002, 3505–3517
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Table 4 Comparison of literature 11B NMR values with computed 11
NMR shifts for closo-10-Br-4,7-(OH)2-2,3-Me2-2,3-C2B9H6
B
Observed6
Calculated VI
16.1 (B4,7)
1.5 (B1)
Ϫ3.4 (B8,9)
Ϫ7.5 (B11)
Ϫ14.8 (B10)
Ϫ21.4 (B5,6)
14.9 (B4,7)
Ϫ1.4 (B1)
Ϫ2.8 (B8,9)
Ϫ7.8 (B11)
Ϫ8.6 (B10)
Ϫ22.3 (B5,6)
Table 5 Comparison of literature 11B NMR data for 3,4-µ-(OC6H4O)-
7,9-Me2-7,9-C2B9H8Ϫ together with calculated data for 3,4- XXXVI and
10,11-µ-(OC2H4O)-7,9-Me2-7,9-C2B9H8Ϫ XXXVII
Observed27
3,4-Isomer
XXXVI calculated
10,11–Isomer
XXXVII calculated
Fig. 5 MP2-optimized molecular geometry of nido-10-endo-Me-7,9-
C2B9H11Ϫ XXVIIa for 1n. Selected bond lengths (Å), C7–B8 1.666, B8–
C9 1.584, C9–B10 1.738, B10–B11 1.905, C7–B11 1.587, B10–C(Me)
1.665.
0.5 (B8)
Ϫ3.3 (B3,4)
Ϫ5.5 (B2,5)
Ϫ25.6 (B10,11)
Ϫ31.6 (B1)
Ϫ41.4 (B6)
Ϫ4.3 (B8)
Ϫ4.4 (B3,4)
Ϫ5.2 (B2,5)
Ϫ30.5 (B10,11)
Ϫ36.1 (B1)
Ϫ43.6 (B6)
Ϫ4.6 (B8)
Ϫ5.3 (B2,5)
Ϫ7.6 (B10,11)
Ϫ27.3 (B3,4)
Ϫ37.5 (B6)
Ϫ39.2 (B1)
of the nido-11-vertex trimethyl carborane anion 10-endo-
Ϫ
9,11-Me3-7,8-C2B9H9 containing an endo-methyl group like
in XXVIIa (Fig. 5) has been determined by X-ray
crystallography.53
This suggests that calculated NMR chemical shifts generated
from MP2-optimized geometries are not accurate enough for
brominated (and presumably iodinated) carboranes and
acetonitrile–carborane adducts at the GIAO-B3LYP levels of
theory. Poor agreement between observed and calculated NMR
shifts has been reported previously for borane–acetonitrile
adducts.56
Since in this study we have prepared several B-methyl-nido-
7,9-C2B9H11 anions, it was of interest to compare the relative
Ϫ
energies of these anions. As expected from its facile cage
rearrangement described above, the least stable anion is that
with the methyl group on the open face, 10-endo-Me-7,9-
Ϫ
C2B9H11 XXVIIa. The most stable anion is the 2-methyl
isomer XXXI followed by, in decreasing order of energy,
8- XXXII, 3- XXX, 6-XXIX, 10- XXXIII and 1- XXVIII isomer.
The preferred formation of the 10-methyl anion 1m over the
3-methyl isomer 1o on hydride addition to B-Me-2,3-C2B9H10
2l–n is therefore due to kinetic, not thermodynamic, factors.
The reported sequence of relative energies for B-F-nido-7,9-
Hydride addition to closo-4,7-(OC6H4O)-2,3-Me2-2,3-C2B9H7
has been reported to give nido-3,4-(OC6H4O)-7,9-Me2-7,9-
Ϫ
C2B9H8 rather than the 10,11-analogue on the basis of a 11B
NMR study.27 Comparison between computed 11B NMR data
for the model geometries nido-3,4-(OC2H4O)-7,9-Me2-7,9-
Ϫ
Ϫ
C2B9H8
XXXVI and nido-10,11-(OC2H4O)-7,9-Me2-7,9-
C2B9H11 isomers is similar to the methyl analogues, differing
Ϫ
only in the reversed order of the 6-and 10-isomers.16
C2B9H8 XXXVII and observed values supports the proposed
isomer (Tables 4 and 5). That hydride addition to closo-4,7-
(OC6H4O)-2,3-Me2-2,3-C2B9H7 gives the nido-3,4- rather than
the nido-10,11-isomer is believed to be due to the presence of
both substituents preventing an hydride attack at the boron in
the 4- or 7-position.
Consideration of the best fit between the optimised geom-
Ϫ 3
etries of 10-MeCN-7,9-C2B9H11 XXXV and 7,9-C2B9H12
,
reveals that the effect of the acetonitrile ligand on the cage
skeleton is limited to the substituted boron atom (B10), which is
shifted away from the centre of the cage resulting in longer
bonds to neighbouring atoms. The misfit value between these
geometries is 0.0300 Å for heavy cage atoms only.
Conclusions
Ϫ
In solution, the monoanion 7,8-C2B9H12 is known to be
fluxional with an unsymmetrical bridging hydrogen flipping
between the two B–B bonds on the open face.54 Two minima,
with the bridging hydrogen at B9–B10 XXXIVa and at B10–B11
XXXIVb, of similar energy were located for the methyl-
substituted anion 9-Me-7,8-C2B9H11Ϫ. The energy barrier to
bridging hydrogen fluxionality between the two geometries (i.e.,
via the 10-endo-H geometry XXXIVc) is estimated to be only
1.0 kcal molϪ1 and the cluster is therefore expected to be
fluxional in solution.
This study demonstrates that, in general, addition of acid to
salts containing derivatives of the nido-7,9-C2B9H12 anion
Ϫ
results in the elimination of hydrogen and formation of closo-
C2B9H11 derivatives at ambient temperature. The only exception
to the nido–closo conversion on acidification is the 10-endo-
Ϫ
Me-7,9-C2B9H11 anion which gives nido-7-Me-2,8-C2B9H12.
The molecular geometries of these unusual compounds are
confirmed by detailed NMR spectroscopy and computations.
The reported nido–nido conversion on acidification of 8-Me-
7,9-C2B9H11Ϫ is found to be incorrect.
Table 1 and Table 2 reveal very good agreement between
1
Ϫ
experimental and calculated 11B, 13C and H NMR shifts in
nido-7,9-C2B9H12 anions with substituents at boron atoms
the majority of nido-and closo-carboranes discussed here. At
the GIAO-B3LYP/6-311G* level of theory, the calculated
shifts were generated from MP2-optimized geometries. The
not at the open face gave the expected substituted carboranes
on acidification based on simple cage closure. By contrast,
anions with electron withdrawing fluorine, hydroxy and ethoxy
substituents on the open face lead to removal of the substit-
uent, as HF or EtOH, and formation of closo-carboranes.
Salts containing anions with methyl groups substituted at
boron produced a mixture of B-Me-2,3-C2B9H10 isomers on
acidification. A facile cage rearrangement equilibrium possibly
exists for B-Me-2,3-C2B9H10 at ambient temperature.
Ϫ
two carborane monoanions, 10-endo-Me-7,9-C2B9H11 1n and
Ϫ
9-Me-7,8-C2B9H11 6, are fluxional in solution based on
comparison of their observed and calculated NMR data.
Recently the accuracy of the GIAO/NMR method has
been demonstrated to be poor for carboranes containing
bromine or iodine using optimized geometries at the B3LYP
level of theory.55 Comparison between the observed and
computed shifts for 10-(MeCN)-7,9-C2B9H11 3/XXXV and
10-Br-4,7-(HO)2-2,3-Me2-2,3-C2B9H6 (Tables 4 and 5) here
shows the substituted boron shifts to be in error by ca. 6 ppm.
Conversion of Bu4Nϩ salts containing derivatives of the nido-
Ϫ
7,9-C2B9H12 anion into their Naϩ salts can be achieved by
acidification then reduction as shown here. Unlike their alkali
Ϫ
metal salts, Bu4Nϩ salts of derivatives of the nido-7,9-C2B9H12
3516
J. Chem. Soc., Dalton Trans., 2002, 3505–3517

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29 M. A. Fox, A. K. Hughes, A. L. Johnson and M. A. J. Paterson,
anion are easily obtained by deboronation of their respective
meta-carboranes. The sodium salts are expected to be suitable
precursors to metallacarboranes with non-adjacent cage
carbon atoms.
J. Chem. Soc., Dalton Trans., 2002, 2009.
30 Gaussian 94, Revision E.2, M. J. Frisch, G. W. Trucks,
H. B. Schlegel, P. M. W. Gill, B. G. Johnson, M. A. Robb,
J. R. Cheeseman, T. Keith, G. A. Petersson, J. A. Montgomery,
K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz,
J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara,
M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong,
J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox,
J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon,
C. Gonzalez, and J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 1995.
31 Gaussian 98, Revision A.9, M. J. Frisch, G. W. Trucks,
H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
V. G. Zakrzewski, J. A. Montgomery, Jr., R. E. Stratmann,
J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin,
M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi,
B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski,
G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski,
J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox,
T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong,
J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle, and
J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 1998.
32 T. P. Onak, H. L. Landesman and R. E. Williams, J. Phys. Chem.,
1959, 63, 1533.
33 T. Onak, M. Diaz and M. Barfield, J. Am. Chem. Soc., 1995, 117,
1403.
34 SHELXTL-NT, Version 5.1, Bruker Analytical X-ray Instruments
Inc., Madison, WI, USA, 1998.
35 W. R. Pretzer and R. W. Rudolph, Inorg. Chem., 1976, 15, 1779.
36 F. R. Scholer, R. Brown, D. Gladkowski, W. F. Wright and
L. J. Todd, Inorg. Chem., 1979, 18, 921.
37 W. Jarvis, W. Inman, B. Powell, E. W. DiStefano and T. Onak,
J. Magn. Reson., 1981, 43, 302.
38 M. A. Fox, J. A. K. Howard, A. K. Hughes, J. M. Malget and
D. S. Yufit, J. Chem. Soc., Dalton Trans., 2001, 2263.
39 L. I. Zakharkin, G. G. Zhigareva, V. A. Antonovich, A. I. Yanovskii
and Yu. T. Struchkov, Russ. J. Gen. Chem., 1986, 1823.
40 A reversible cage rearrangement has been reported for a B-methyl
11-vertex tricarbaborane: A. M. Shedlow and L. G. Sneddon,
Collect. Czech. Chem. Commun., 1999, 64, 865.
41 A. P. Fung and T. Onak, J. Am. Chem. Soc., 1977, 99, 5512; B. Oh
and T. Onak, Inorg. Chem., 1982, 21, 3150.
Acknowledgements
We acknowledge the award of an EPSRC Advanced Research
Fellowship (M. A. F.) and thank the ERDF Centre for
21st Century Materials at the University of Durham for funding
(J. M. M.).
References and notes
1 R. A. Wiesboeck and M. F. Hawthorne, J. Am. Chem. Soc., 1964, 86,
1642.
2 J. Plesˇek and S. Herˇmánek, Chem. Ind. (London), 1973, 381.
3 M. A. Fox, A. E. Goeta, A. K. Hughes and A. L. Johnson, J. Chem.
Soc., Dalton Trans., 2002, 2132.
4 F. N. Tebbe, P. M. Garrett and M. F. Hawthorne, J. Am. Chem. Soc.,
1968, 90, 869.
5 F. N. Tebbe, P. M. Garrett and M. F. Hawthorne, J. Am. Chem. Soc.,
1964, 86, 4222.
6 F. R. Scholer, R. Brown, D. Gladkowski, W. F. Wright and L. J.
Todd, Inorg. Chem., 1979, 18, 921.
7 J. D. Hewes, M. Thompson and M. F. Hawthorne, Organometallics,
1985, 4, 13.
8 G. D. Mercer and F. R. Scholer, Inorg. Chem., 1973, 12, 2102.
9 G. D. Mercer and F. R. Scholer, Inorg. Chem., 1974, 13, 2256.
10 G. D. Mercer, J. Lang, R. Reed and F. R. Scholer, Inorg. Chem.,
1975, 14, 761; D. Gladkowski and F. R. Scholer, J. Organomet.
Chem., 1975, 85, 287.
11 D. A. Owen and M. F. Hawthorne, J. Am. Chem. Soc., 1969, 91,
6002.
12 M. F. Hawthorne and D. A. Owen, J. Am. Chem. Soc., 1968, 90,
5912.
13 V. Chowdhry, W. R. Pretzer, D. N. Rai and R. W. Rudolph, J. Am.
Chem. Soc., 1973, 95, 4560.
14 M. A. Fox, W. R. Gill, P. L. Herbertson, J. A. H. MacBride, K. Wade
and H. M. Colquhoun, Polyhedron, 1996, 15, 565.
15 M. A. Fox and K. Wade, Polyhedron, 1997, 16, 2517.
16 M. A. Fox and K. Wade, J. Organomet. Chem., 1999, 573, 279.
17 M. A. Fox, J. A. H. MacBride and K. Wade, Polyhedron, 1997, 16,
2499.
42 E. L. Muetterties, E. L. Hoel, C. G. Salentine and M. F. Hawthorne,
Inorg. Chem., 1975, 14, 950.
18 S. P. Knyazev, V. A. Brattsev and V. I. Stanko, Dokl. Chem. (Engl.
Transl.), 1977, 299; S. P. Knyazev, V. A. Brattsev and V. I. Stanko,
Dokl. Chem. (Engl. Transl.), 1977, 323.
19 S. P. Knyazev, V. A. Brattsev and V. I. Stanko, Russ. J. Gen. Chem.,
1978, 2398.
20 S. P. Knyazev, V. A. Brattsev and V. I. Stanko, Dokl. Chem. (Engl.
Transl.), 1978, 373.
21 V. Brattsev, Spec. Publ.–R. Soc. Chem., 2000, vol. 253, 205.
22 G. Barbera, S. Dunn, M. A. Fox, R. M. Garrioch, B. E. Hodson,
K. S. Low, G. M. Rosair, F. Teixidor, C. Viñas, A. J. Welch and
A. S. Weller, Spec. Publ.–R. Soc. Chem., 2000, vol. 253, 329.
23 M. Bühl and P. v. R. Schleyer, J. Am. Chem. Soc., 1992, 114, 477;
M. Bühl, J. Gauss, M. Hofmann and P. v. R. Schleyer, J. Am. Chem.
Soc., 1993, 115, 12385; P. v. R. Schleyer, J. Gauss, M. Bühl,
R. Greatrex and M. A. Fox, J. Chem. Soc., Chem. Commun., 1993,
1766; M. A. Fox, R. Greatrex, M. Hofmann and P. v. R. Schleyer,
J. Organomet. Chem., 2000, 614–615, 262.
24 Yu. T. Struchkov, M. Yu. Antipin, V. I. Stanko, V. A. Brattsev,
N. I. Kirillova and S. P. Knyazev, J. Organomet. Chem., 1977, 141,
133.
25 G. G. Hlatky and D. J. Crowther, Inorg. Synth., 1998, 32, 229.
26 M. F. Hawthorne, D. C. Young, P. M. Garrett, D. A. Owen,
S. G. Schwerin, F. N. Tebbe and P. A. Wegner, J. Am. Chem. Soc.,
1968, 90, 862.
43 L. J. Todd, A. R. Siedle, F. Sato, A. R. Garber, F. R. Scholer and
G. D. Mercer, Inorg. Chem., 1975, 14, 1249.
44 H. C. Kang, S. S. Lee, C. B. Knobler and M. F. Hawthorne, Inorg.
Chem., 1991, 30, 2024.
45 W. R. Pretzer, D. A. Thompson and R. W. Rudolph, Inorg. Chem.,
1975, 14, 2571.
46 A. J. Welch and A. S. Weller, J. Chem. Soc., Dalton Trans., 1997, 1205.
47 C.-C. Tsai and W. E. Streib, J. Am. Chem. Soc., 1966, 88, 4513.
48 M. E. Leonowicz and F. R. Scholer, Inorg. Chem., 1980, 19, 122.
49 P. v. R. Schleyer and K. Najafian, Inorg. Chem., 1998, 37, 3454.
50 B. Wrackmeyer, H.-J. Schanz and W. Milius, Angew. Chem., Int. Ed.
Engl., 1997, 36, 75.
51 M. Hofmann, M. A. Fox, R. Greatrex, R. E. Williams and
P. v. R. Schleyer, J. Organomet. Chem., 1998, 550, 331.
52 A. Fessenbecker, A. Hergel, R. Hettrich, V. Schäfer and W. Siebert,
Chem. Ber., 1993, 126, 2205.
53 N. I. Kirillova, M. Yu. Antipin, S. P. Knyazev, V. A. Brattsev,
Yu. T. Struchkov and V. I. Stanko, Bull. Acad. Sci. USSR, Div.
Chem. Sci. (Engl. Transl.), 1979, 2289.
54 M. A. Fox, A. E. Goeta, J. A. K. Howard, A. K. Hughes,
A. L. Johnson, D. A. Keen, K. Wade and C. C. Wilson,
Inorg. Chem., 2001, 40, 173.
55 C. Viñas, G. Barbera, J. M. Oliva, F. Teixidor, A. J. Welch and
G. M. Rosair, Inorg. Chem., 2001, 40, 6555.
27 L. J. Todd, A. R. Siedle, F. Sato, A. R. Garber, F. R. Scholer and
G. D. Mercer, Inorg. Chem., 1975, 14, 1249.
28 A. Streitweiser and C. H. Heathcock, Introduction to Organic
Chemistry, Collier Macmillan, London, 2nd edn., 1981, p. 818.
56 M. Hofmann and P. v. R. Schleyer, Inorg. Chem., 1998, 37, 5557;
M. Hofmann and P. v. R. Schleyer, Inorg. Chem., 1999, 38, 652;
P. N. Condick, M. A. Fox, R. Greatrex and D. L. Ormsby, Spec.
Publ.–R. Soc. Chem., 2000, vol. 253, 179.
J. Chem. Soc., Dalton Trans., 2002, 3505–3517
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