A convenient cyanide-free one-pot synthesis of nido-Me3N-7-CB10H12 and nido-7-CB10H13-

Article abstract of DOI:10.1039/b200930g

Reaction of nido-^tBuH₂N-7-CB₁₀H₁₂ with Na₂CO₃ and (MeO)₂SO₂ in THF results in mono-methylation to give nido-^tBuMeHN-7-CB₁₀H₁₂, whilst prolonged reaction at elevated temperatures results in a quantitative yield of the trimethyl derivative nido-Me₃N-7-CB₁₀H₁₂, as a result of metathesis of the tert-butyl group. The ¹¹B NMR spectrum of nido-^tBuMeHN-7-CB₁₀H₁₂ is explored as a function of pH, demonstrating exchange with nido-^tBuMeN-7-CB₁₀H₁₂^-. Reaction of B₁₀H₁₄ with CyNC gives nido-CyH₂N-7-CB₁₀H₁₂, which is methylated by Na₂CO₃ and (MeO)₂SO₂ in THF to give nido-CyMe₂N-7-CB₁₀H₁₂. Deprotonation of nido-Me₃N-7-CB₁₀H₁₂ and nido-CyMe₂N-7-CB₁₀H₁₂ yields Na[nido-Me₃N-7-CB₁₀H₁₁] and Na[nido-CyMe₂N-7-CB₁₀H₁₁] respectively. Both trialkyl(amino)carboranes can be converted to Na[nido-CB₁₀H₁₃], itself a precursor to the poorly coordinating anion closo-CB₁₁H₁₂^-. The molecular structures of nido-^tBuMeHN-7-CB₁₀H₁₂ and nido-CyMe₂N-7-CB₁₀H₁₂, determined by single crystal X-ray diffraction, are reported.

Full text of DOI:10.1039/b200930g

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A convenient cyanide-free “one-pot” synthesis of nido-Me3N-7-
؊
CB10H12 and nido-7-CB10H13 †
Andrei S. Batsanov, Mark A. Fox,* Andrés E. Goeta, Judith A. K. Howard,
Andrew K. Hughes* and John M. Malget
Department of Chemistry, University Science Laboratories, South Road, Durham,
UK DH1 3LE
Received 24th January 2002, Accepted 1st May 2002
First published as an Advance Article on the web 29th May 2002
Reaction of nido-tBuH2N-7-CB10H12 with Na2CO3 and (MeO)2SO2 in THF results in mono-methylation to give nido-
tBuMeHN-7-CB10H12, whilst prolonged reaction at elevated temperatures results in a quantitative yield of the tri-
methyl derivative nido-Me3N-7-CB10H12, as a result of metathesis of the tert-butyl group. The 11B NMR spectrum of
Ϫ
.
nido-tBuMeHN-7-CB10H12 is explored as a function of pH, demonstrating exchange with nido-tBuMeN-7-CB10H12
Reaction of B10H14 with CyNC gives nido-CyH2N-7-CB10H12, which is methylated by Na2CO3 and (MeO)2SO2 in
THF to give nido-CyMe2N-7-CB10H12. Deprotonation of nido-Me3N-7-CB10H12 and nido-CyMe2N-7-CB10H12 yields
Na[nido-Me3N-7-CB10H11] and Na[nido-CyMe2N-7-CB10H11] respectively. Both trialkyl(amino)carboranes can be
converted to Na[nido-CB10H13], itself a precursor to the poorly coordinating anion closo-CB11H12Ϫ. The molecular
structures of nido-tBuMeHN-7-CB10H12 and nido-CyMe2N-7-CB10H12, determined by single crystal X-ray
diffraction, are reported.
the chemistry of metallamonocarboranes has been far less
Introduction
explored.9,10 We were interested in the possibility of preparing
By contrast with di-carbon carboranes, the chemistry of
high oxidation state early transition metal examples of closo-
mono-carbon carboranes is much less widely explored. Mono-
3Ϫ
MCB10H11, containing the trianionic nido-7-CB10H11 cluster
carborane anions,1 especially closo-CB11H12Ϫ, and alkylated2 or
as a ligand. First we needed to address the synthesis of a
halogenated3 derivatives, have received attention recently since
suitable nido-7-CB10H11 ϩ nnϪ precursor.
they are among the best super-weakly coordinating anions
available. These closo-clusters possess chemical, electrochemical
and thermal stability; the anionic charge is highly delocalised,
The literature synthetic routes to closo-CB11H12Ϫ and nido-7-
CB10H13Ϫ are intimately intertwined. Knoth originally prepared
Ϫ
Ϫ
closo-CB11H12 from nido-7-CB10H13 by thermolysis with or
and the clusters have found application in cationic metallocene
catalysis,4 stabilisation of both coordinatively unsaturated
cations5 and of strong Brønsted acids6 and the preparation of
ionic liquids.7
We have been exploring the synthesis, reactivity and struc-
tural studies of high oxidation-state early transition metal
complexes containing the closo-MC2B9H11 unit.8 By com-
parison with the wealth of chemistry of metalladicarboranes,
without BH3ؒNR3.11–13 The anion nido-7-CB10H13 is made by
Ϫ
deamination of the zwitterion nido-7-Me3N-7-CB10H13.11,14,15
Ϫ
An alternative preparative route to CB11H12 was reported
later by thermolysis of nido-7-Me3N-7-CB10H13 with BH3ؒNR3
to yield Me2NHCB11H11 which is easily methylated then
deaminated to form closo-CB11H12 .
Ϫ 16,17 The zwitterion nido-7-
Me3N-7-CB10H13 has generally been synthesized from either
acidification and methylation of Na2B10H13CN or methylation
of Cs2B10H13CN (Scheme 1).15–18 The salt Na2B10H13CN is
formed from the reaction of B10H14 with NaCN followed by
† Electronic supplementary information (ESI) available: rotatable 3-D
molecular structure diagrams of experimental structures of 3a and 6b,
and of MP2-optimized geometries I to IX in CHIME format. See http://
www.rsc.org/suppdata/dt/b2/b200930g/
Ϫ
acid,19 or by the reaction of B10H13 with NaCN, avoiding the
need to use an excess of cyanide.16 In these reactions the by-
Scheme 1 The reaction of nido-B10H14 with NaCN and subsequent methylation.
2624
J. Chem. Soc., Dalton Trans., 2002, 2624–2631
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b200930g

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product HCN and the handling of NaCN pose hazards which
are unacceptable in view of our departmental safety policy.
Gaines and Bridges demonstrated that the reaction of
Na2B10H12ؒ2THF with CH2X2 (X = Br, I) in THF affords
NaCB10H13, however the low yield (25%) makes this route
uneconomical,20 and we have been unable to optimise the
reaction. Michl and co-workers have recently reported that the
cost-effective reaction of Na2B11H13 (generated in situ from
NaH and nido-B11H14Ϫ, itself available21 from NaBH4 and
workers,28 giving nido-7-RH2NCB10H12 (R = tBu 1a, Cy 1b). The
ammonium moiety in these zwitterions is readily deprotonated
yielding the anions nido-7-RHNCB10H12Ϫ (R = tBu 2a, Cy 2b).
Since the cyclohexyl derivatives 1b and 2b were not known prior
to this study, their NMR data are reported here.
As previously mentioned, a variety of procedures are avail-
able for the conversion of RH2N– to a RMe2H– group. We
find that 1a and 1b do not react cleanly with either MeI or
(MeO)2SO2 in aqueous basic conditions, these conditions con-
sistently result in the formation of significant quantities of
boric acid (11B NMR) resulting from degradation of the
carborane framework. Following these observations we felt it
necessary to readdress the synthesis of RMe2NCB10H12 by
methylation of 1a and 1b.
Ϫ
BF3ؒEt2O) with CHCl3 produces CB11H12 in 42% yields.22
Mixing sodium hydride with halocarbons is however not
recommended.23 The C-phenyl analogue, closo-PhCB11H11Ϫ has
recently been synthesised24 using a modification of Brellochs’
reaction of PhCHO with B10H14.25
Therefore we were interested to explore alternative, and more
importantly safer, methods for the synthesis of the zwitterion
Here we find that whilst Na2CO3 is not sufficiently basic
to effect complete deprotonation of the amine it does act as
an effective buffer during alkylation, allowing a “one-pot”
synthesis of dialkyl- (tBuMeHNCB10H12) (3a) and trialkyl-
ammonio- (Me3NCB10H12 5a and CyMe2NCB10H12 6b) charge-
compensated mono-carboranes depending on conditions.
At ambient or moderate (<50 ЊC) temperature, treatment of
tetrahydrofuran solutions of 1a with (MeO)2SO2 and Na2CO3
affords only the zwitterionic mono-methylated derivative,
tBuMeHNCB10H12, 3a (Scheme 3).
nido-RMe2NCB10H12 (R = alkyl) as precursor to closo-
Ϫ 11,14,15
CB11H12Ϫ, nido-7-CB10H13
,
metallamonocarboranes and
Ϫ
2-substituted derivatives26 of CB11H12
.
Monocarbon-carboranes nido-7-RH2NCB10H12 (R = Me,27
Et,27 nPr,15,27 tBu,27,28 Bz,12 Ph29) may be obtained from B10H14
and the respective isocyanide (Scheme 2), a reaction first
Our initial attempts to characterise 3a were complicated by
the non-reproducible nature of the 11B NMR spectrum, similar
to spectra (a)–(c) in Fig. 1. Since the related butyl derivative 1a is
Scheme
2
The reaction of nido-B10H14 with isocyanides and
subsequent methylation.
reported by Todd and co-workers in 1966.27 The exhaustive
N-alkylation of nido-7-RH2NCB10H12 to generate nido-7-
RMe2NCB10H12 has been achieved variously by: (i) reaction
with NaH in THF then with (MeO)2SO2 (50% yield for R =
Me);27 (ii) reaction with NaOH as an aqueous solution, treat-
n
ment with (MeO)2SO2 (high yield for R = Pr);15 (iii) reaction
using NaHCO3 with MeI in THF–H2O (high yield for R =
nPr).15 The parent carborane anion nido-7-CB10H13Ϫ is obtained
n
by the deamination of nido-7-RMe2NCB10H12 (R = Me, Pr)
with sodium metal or sodium hydride in refluxing THF.14,15
Thirty years ago, the cost of alkyl isocyanide RNC relative
to NaCN or hazards involved in their preparation30 were
undoubtedly impediments to the widespread adoption of
the isocyanide procedure. At present several isocyanides are
commercially available, including tert-butylisocyanide and
cyclohexylisocyanide (CyNC). Here we take advantage of these
compounds and report a synthetically expedient, high yield
(>95%) “one-pot” synthesis of nido-7-RMe2NCB10H12 (R =
Me, Cy) directly from B10H14 without the use of NaCN. We also
describe the molecular structures of nido-7-CyMe2NCB10H12
and the monomethyl derivative nido-7-tBuMeHNCB10H12.
Fig. 1 Selected 96.2 MHz 11B NMR spectra of tBuMeHNCB10H12 3a,
in equilibrium with tBuMeNCB10H12Ϫ 4a recorded as a function of pH.
a precursor to metallacarboranes,10,31 we explored the reaction
of 3a with Ta(NMe2)5 and obtained a 11B NMR spectrum, (e),
Ϫ 32
confusingly similar to that of nido-CB10H13
.
In order to understand this behaviour, we explored the 11B
NMR spectra as a function of pH (Fig. 1). In acidic solution,
the only species present is neutral 3a, which contains a chiral
tBuMeHN group and hence has C1 symmetry. The CB10
cage thus contains ten inequivalent boron atoms, and the 11B
NMR spectrum at pH 1.05 shows nine lines, with two lines
accidentally equivalent, and close to a third. Under these
Results and discussion
The reactions of decaborane, B10H14, with tert-butyl- and
t
cyclohexyl-isocyanide (RNC, R = Bu, Cy) were performed
according to the modified procedure reported by Stone and co-
J. Chem. Soc., Dalton Trans., 2002, 2624–2631
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Scheme 3 The reactions described in this work. Reagents: (i) (MeO)2SO2–Na2CO3 in THF, (ii) NaOH in CH3CN, (iii) HCl, (iv) Na.
1
conditions, the H NMR spectrum shows two B–H–B bridge
resonances, and the N–Me protons appear as a doublet due to
coupling to the N–H.
Deprotonation of the ammonium nitrogen of 3a gives the
stable mono-anion, [tBuMeNCB10H12]Ϫ 4a, and the completely
different 11B NMR spectrum of 3a at pH 6.59 can be accounted
for by rapid reversible deprotonation of the ammonium nitro-
gen, exchanging 3a with the small amount (0.03% at pH 6.59)
of 4a present. Since the re-protonation can produce the enan-
tiomer of 3a, an apparent molecular mirror plane is introduced,
exchanging the two enantiomers of 3a. The mirror plane con-
tains cage atoms C7, B5 and B1 and relates B8,11, B9,10, B2,3,
and B4,6. The chemical shifts at pH 6.59 are appropriate aver-
ages of those at pH 1.05. Whilst the two enantiomers of 3a are
indistinguishable by NMR, rapid inter-conversion of the enan-
tiomers is observable by the introduction of a pseudo-mirror
1
plane. At pH 6.59, the H NMR spectrum shows only one B–
H–B resonance of relative integral 2, and the N–Me signal
appears as a singlet. In solutions more basic than pH 6.59, the
zwitterion 3a is progressively deprotonated to 4a, and the spec-
tra between pH 6.59 and 12.35 reflect the weighted average
chemical shifts of 3a and 4a.
One feature of 3a/4a is that the resonances assigned to B2,3
and B8,11 move in opposite directions on deprotonation, such
that at pH 9.31 the two resonances are co-incident (spectrum e).
Such a pH is readily achieved by the reaction of 3a with
Ta(NMe2)5. At pH 13.69 (not shown) the spectrum obtained
is that of pure anion 4a. The greatest change in chemical shift
on deprotonation is seen in the 11B NMR resonance assigned
to B5, the atom antipodal to the carbon atom and site of substi-
tution. Fitting the pH dependence of this chemical shift gives
the pKa of 3a as 10.12.
Fig. 2 The molecular structure of tBuMeHNCB10H12 3a, showing the
adopted cage numbering scheme in 50% probability ellipsoids, with
hydrogen atoms as arbitrary sized spheres.
hydrogen and the hydrogen atom on B5 with a H ؒ ؒ ؒ H contact
distance of 2.412 Å. When the B5–H5 and N12–H12 distances
are normalised to compensate for the systematic shortening of
hydrogen distances in X-ray structures, the geometry presented
in Fig. 3 is obtained, and is characteristic of such B–H ؒ ؒ ؒ
The molecular structure of 3a has been confirmed by single
crystal X-ray diffraction and appears in Fig. 2, with selected
bond lengths and angles in Table 1. A nido-CB10 cage with
two B–H–B bridges on the open face is substituted by a
NHMetBu group on the carbon atom, in a fashion which is
grossly similar to the structure of 1a.28 Given the zwitterionic
nature of 3a, one might expect some interaction between the
two charged parts of the molecule, and there is a intermolecular
N–H ؒ ؒ ؒ H–B dihydrogen bond33 between the ammonium
Fig. 3 The bond lengths (Å) and angles (Њ) defining the N–H ؒ ؒ ؒ H–B
dihydrogen bond between two molecules of 3a, calculated using
normalised hydrogen atom positions.
2626
J. Chem. Soc., Dalton Trans., 2002, 2624–2631

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Table 1 Selected bond lengths (Å) and angles (Њ) for experimentally
t
determined geometries of BuH2NCB10H12 1a,28 tBuMeHNCB10H12 3a
and CyMe2NCB10H12 6b and the MP2-optimized geometry (III) of
Me3NCB10H12 5a
1a
3a
6b
5a/III
N(12)–C(14)
N(12)–C(13)
N(12)–C(20)
N(12)–H(12)
N(12)–C(7)
C(7)–B(8)
C(7)–B(11)
B(8)–B(9)
B(9)–B(10)
B(10)–B(11)
1.562(2)
1.577(2)
1.503(2)
1.562(2)
1.509(3)
1.513(3)
1.501
1.502
1.502
0.93(2)
0.88(2)
1.508(2)
1.653(3)
1.655(3)
1.854(3)
1.881(3)
1.856(3)
1.531(2)
1.658(2)
1.659(2)
1.871(2)
1.901(2)
1.849(2)
1.550(2)
1.662(3)
1.667(3)
1.875(4)
1.892(4)
1.865(4)
1.527
1.658
1.658
1.837
1.896
1.837
N(12)–C(7)–B(8)
N(12)–C(7)–B(11)
B(8)–C(7)–B(11)
C(7)–B(8)–B(9)
B(8)–B(9)–B(10)
B(11)–B(10)–B(9)
C(7)–B(11)–B(10)
C(13)–N(12)–C(7)
C(13)–N(12)–C(14)
C(7)–N(12)–C(14)
C(13)–N(12)–C(20)
C(20)–N(12)–C(14)
C(20)–N(12)–C(7)
118.2(1)
120.1(1)
112.3(1)
109.6(1)
103.8(1)
103.2(1)
110.1(1)
122.1(1)
123.20(11)
113.51(10)
111.47(10)
110.12(11)
103.33(10)
102.78(10)
111.39(10)
113.25(10)
112.36(10)
117.43(10)
118.7(2)
117.8(2)
110.9(2)
110.7(2)
103.4(2)
102.9(2)
111.3(2)
110.1(2)
107.96(14)
110.30(14)
106.6(2)
109.8(2)
112.0(2)
116.6
116.6
110.6
111.1
103.1
103.1
111.1
112.8
108.4
107.1
107.1
108.4
112.8
Fig. 4 The molecular structure of CyMe2NCB10H12 6b, showing 50%
H–N structures, albeit with a long H ؒ ؒ ؒ H distance. A similar,
though considerably shorter (2.117 Å), dihydrogen bond
appears in the structure of 1a, using a B–H on the open face.
The reaction of tBuH2NCB10H12 1a with NaH/MeI at
probability ellipsoids, with hydrogen atoms as arbitrary sized spheres.
with the cage frameworks in the crystallographically deter-
mined geometries. The misfit values between the optimised
geometry III and the experimental geometries of 1a, 3a and 6b
are only 0.0132, 0.0128 and 0.0167 Å respectively for the heavy
cage atoms.
t
ambient temperature is reported to give BuMe2NCB10H12.10,27
Using Na2CO3 and (MeO)2SO2 at ambient temperatures, even
in the presence of a large excess of dimethylsulfate we find
no conclusive spectroscopic evidence for the formation of
tBuMe2NCB10H12. In contrast, at elevated temperatures 1a
appears to be converted quantitatively to Me3NCB10H12 5a
whereas under the same conditions 1b is readily converted to
CyMe2NCB10H12 6b. We find evidence for a carborane inter-
mediate along with trimethyl compound 5a in the 11B NMR
spectrum of the reaction mixture at early stages of the reaction
at elevated temperatures. The intermediate has 11B NMR
resonances at δ 1.6, Ϫ8.8, Ϫ12.2, Ϫ22.4, Ϫ26.1 and Ϫ30.3 ppm
and is tentatively identified as tBuMe2NCB10H12 based on
calculated 11B NMR data generated from its MP2-optimized
geometry (VII). These observations may be rationalised by
considering Me3Cϩ as a viable leaving group, whilst Cyϩ is
not. A similar reaction is observed in the methylation of the
isoelectronic tricarborane, 7-tBuH2N-7,8,9-C3B8H10, with NaH
and excess methyl iodide in glyme to yield 7-Me3N-7,8,9-
C3B8H10.34 All spectroscopic data for 5a were identical to those
previously reported.32,35 Considering spectroscopic data for 6b,
Calculated 11B NMR chemical shifts (at the GIAO-B3LYP/6-
311G* level) generated from the optimised geometries of III
Ϫ
and of the parent nido-7-CB10H13 (I) are in good agreement
with their respective experimental NMR shifts. The slightly
worse correlation for 5a with III can be attributed to the
rotation of the trimethyl(ammonium) group in solution, since
the calculated values represent the static geometry.
Deprotonation of the trialkyl derivatives 5a and 6b by NaOH
Ϫ
in acetonitrile gave the air-sensitive anions Me3NCB10H11 8a
Ϫ
and CyMe2NCB10H11 7b. Given that 5a is well-known, it is
surprising that no characterisation data have been reported
previously for the mono-anion 8a. There are two reports of
Na[Me3NCB10H11] being present as an intermediate in the
reaction of 5a and sodium hydride.11,35 One is in the formation
of closo-Me3NCB10H10 and the other in the formation of closo-
CB10H11Ϫ and the nido-8-HO-7-CB10H12Ϫ anions. Furthermore,
it is logical that the mono-anion would be an intermediate
in the deamination of nido-7-Me3N-7-CB10H12 by sodium
metal11,14,15 and in the double deprotonation of nido-7-Me3N-
1
the H NMR spectrum confirms retention of the cyclohexyl
group and addition of two methyl substituents, whilst the 11B
NMR spectrum is in accord with a nido-CB10 framework and
a mirror plane, consistent with conversion to a RЈR2N– sub-
stituent. The formulation of 6b was confirmed by a single
crystal X-ray diffraction study, and the molecular structure
appears in Fig. 4 with selected bond lengths and angles in
Table 1.
7-CB10H12 by NaH,14 butyllithium,26 and (Me N) C᎐NH.36
᎐
2
2
Reported 11B NMR chemical shifts for the salt [(Me N) C᎐
᎐
2
2
NH2ϩ]2[nido-7-Me3N-7-CB10H10]2Ϫ are identical to our data for
Ϫ
Me3NCB10H11 8a. Addition of iodine to a wet acetonitrile
solution of 8a gave closo-Me3NCB10H10, whose 11B NMR data
are in agreement with reported values.36 This implies that the
formation of the di-anion [nido-7-Me3N-7-CB10H10]2Ϫ is not
required prior to the conversion of 5a to closo-Me3NCB10H10 as
proposed previously.14,36
Now that the C-mono-, di- and tri-alkylamino zwitterions of
7-CB10H12Ϫ have been structurally characterised (as 1a, 3a, and
6b respectively) it is clear that the effect of different amino
groups on the cage framework is very small (Table 1). The
anion of 1a in the form of the NEt3Bzϩ salt was also crystal-
lographically characterised28 and again shows only slight geo-
metrical changes in the cage framework. The cage framework in
the optimised geometry (III) of the trimethyl(amino) carborane
5a at the MP2/6-31G* level of theory is in excellent agreement
As we have yet to isolate salts of the air-sensitive anions 7b
and 8a suitable for X-ray crystallography, geometry optimis-
ations of the trimethyl anion 8a were carried out at the MP2/6-
31G* level. Two minima were located for 8a, with a hydrogen
bridging B8–B9 (IV) and B9–B10 (V) respectively. The latter
minimum is 5.4 kcal molϪ1 lower than the former and is shown
in Fig. 5 along with selected bond lengths and angles. Since very
J. Chem. Soc., Dalton Trans., 2002, 2624–2631
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standard Schlenk and cannula techniques or in a nitrogen filled
glove box. When required, solvents were dried by prolonged
reflux over the appropriate drying agent, prior to distillation
and deoxygenation by freeze–pump–thaw processes where
appropriate. NMR solvents were vacuum-distilled from suitable
drying agents and stored under a dry nitrogen atmosphere.
tBuH2NCB10H12 1a was prepared according to Stone.28 Elem-
ental analyses were performed by the micro-analytical service
within this department. NMR spectra were recorded at ambient
temperature in d3-acetonitrile on the following instruments:
Varian Unity-300 and Varian Inova 500. 2D 11B–11B COSY,
1
1H{11B} and H{11B selective} spectra were recorded on the
Unity. All chemical shifts are reported in δ (ppm) and coupling
1
constants in Hz. H NMR spectra were referenced to residual
protio impurity in the solvent (D2HCCN, 1.95 ppm). 13C NMR
spectra were referenced to the solvent resonance (d3-D3CCN,
118.0 ppm). 11B NMR were referenced externally to Et2OؒBF3,
t
δ = 0.0 ppm. Both BuNC and CyNC were purchased com-
mercially (Aldrich) and used as received.
Syntheses
Fig. 5 Optimised geometry (MP2/6-31G*) V of the anion Me3-
Ϫ
CyH2NCB10H12 1b. This was prepared using B10H14 (0.50 g,
4.1 mmol) and CyNC (0.50 g, 4.5 mmol) according to the pro-
cedure reported by Stone for 1a.28 Yield 0.87 g, 92%. (Found
C 37.34; H 10.84; N 5.59. C7H25B10N requires C 36.34; H 10.89;
N 6.05%); δH 1H{11B}, Ϫ3.84 (m, 2H, µ-BH), 0.25 (s, 2H, BH
4,6), 0.95 (m, 1H, CH2), 1.09 (m, 7H, CH2, BH 1,9,10), 1.44
(m, 1H, CH2), 1.61 (m, 2H, CH2), 1.75 (m, 2H, CH2), 1.93
(s, 2H, BH 2,3), 2.00 (s, 2H, BH 8,11), 2.15 (s, 1H, BH 5), 3.11
(m, 1H, ipso), 6.19 (br s, 2H, NH); δC 72.3 (cage C), 59.5 (C1),
34.1 (C2,6), 26.4 (C4) 25.3 (C3,5); δB Ϫ1.0 (B5), Ϫ9.6 (B2,3),
Ϫ12.2 (B8,11), Ϫ22.5 (B9,10), Ϫ26.1 (B1), Ϫ32.3 (B4,6).
NCB10H11 8a with near Cs symmetry, corresponding to the lower of
two energy minima. Selected bond lengths (Å) and angles (Њ); C7–B8/
B11 1.641, B8–B9/B10–B11 1.767, B9–B10 1.870, C7–N12 1.538, N12–
C13/C20 1.495, N12–C14 1.496, B9/B10–H12 1.324; N12–C7–B8/B11
115.4, B8–C7–B11 117.3, C7–B8–B9/C7–B11–B10 106.0, B8–B9–B10/
B11–B10–B9 105.3, C7–N12–C13/C20 113.2, C7–N12–C14 105.4.
good fits were found between the experimental and optimised
geometries of the alkylated zwitterions here, the MP2-
optimized geometry (V) shown in the figure is very likely to be
found experimentally for 8a. Confirmation of this geometry is
shown by the excellent correlation between the calculated 11B
chemical shifts generated from the optimised geometry V and
those observed experimentally for 8a.
tBuMeHNCB10H12 3a. A tetrahydrofuran suspension (50 ml)
containing Na2CO3 (1.00 g), 1a (0.50 g, 2.4 mmol) and
(MeO)2SO2 (0.30 g, 0.23 ml, 2.4 mmol) was brought to 50 ЊC
Double deprotonation of 1a has been reported to give a
di-anion nido-7-tBuHN-7-CB10H112Ϫ whose 11B NMR chemical
shifts (Ϫ12.7, Ϫ17.4, Ϫ21.5 and Ϫ39.2 ppm)28 are similar to 8a,
supporting the suggested formulation. Calculations on nido-7-
tBuHN-7-CB10H112Ϫ with the hydrogen bridging B9–B10 (VIII)
and B8–B9 (IX) suggest that the bridging hydrogen in the
di-anion is likely to be at B9–B10 rather than at B8–B9, the
symmetrical geometry (VIII) is the more stable of the two by
and stirred until the reaction was complete as determined by 11
B
NMR (like spectrum (c) in Fig. 1, about 12 h). The suspension
was filtered and the residue washed with acetonitrile (2 ×
25 ml). The solvent was removed from the combined filtrates
under reduced pressure to afford a white solid. The solid was
triturated in diethyl ether (15 ml) and cooled (Ϫ30 ЊC) to give a
bright white precipitate that was isolated by filtration and dried
in vacuo to afford spectroscopically pure 3a. Yield 0.38 g, 72%.
(Found C 32.70; H 11.69; N 6.56. C6H25B10N requires C 32.85;
H 11.49; N 6.38%). Spectroscopic data (acid solution, pH < 6):
δH 1H{11B}, Ϫ3.82 (m, 1H, µ-BH), Ϫ3.65 (m, 1H, µ-BH), 0.50
(s, 2H, BH 4,6), 1.26 (s, 2H, BH 1,10), 1.31 (s, 1H, BH 9), 1.46
(s, 9H, CH3), 2.17 (s, 1H, BH 11), 2.33 (s, 2H, BH 2,3), 2.41 (s,
ca. 3 kcal molϪ1 at the MP2/6-31G* level. The carborane
2Ϫ
di-anion 7-Me3N-7-CB10H10
in the salt [(Me2N)2-
C᎐NH ] ϩ[nido-7-Me3N-7-CB10H10]2Ϫ is in fact the mono-anion
᎐
2 2
7-Me3N-7-CB10H11 8a based on identical 11B NMR chemical
Ϫ
shifts and the salt is likely to have the formula [(Me2N)2-
C᎐NH] Hϩ[nido-7-Me N-7-CB H ]Ϫ. Calculated 11B NMR
᎐
2
3
10 11
chemical shifts generated from the MP2-optimized geometry
(VI) of the di-anion 7-Me3N-7-CB10H102Ϫ are not in agreement
with reported values for the salt.
The trialkyl derivatives Me3NCB10H12 5a and CyMe2NCB10-
H12 6b are readily converted to NaCB10H13 (presumably via
their anions 8a and 7b respectively) by treatment with Na in
THF in accordance with the protocol described in the liter-
ature.11 We propose that for the synthesis of NaCB10H13, the
route using cyclohexylisocyanide is preferable due to its lower
cost.
3
2H, BH 5,8), 2.73 (d, 3H, CH3, J(HH) = 7 Hz); δC 73.9 (br,
cage C), 71.4 (CMe3), 39.6 (CH3), 26.6 (CCH3); δB 0.1 (B5),
Ϫ9.4 (B11), Ϫ10.0 (B2,3), Ϫ12.9 (B8), Ϫ21.8 (B9), Ϫ23.5
(B10), Ϫ25.7 (B1), Ϫ31.6 (B6), Ϫ32.1 (B4); (neutral solution,
pH ca. 7.0): δH 1H-{11B}, Ϫ3.75 (m, 2H, µ-BH), 0.47 (s, 2H, BH
4,6), 1.23 (s, 3H, BH 1,9,10), 1.43 (s, 9H, CH3), 2.22 (s, 2H,
BH 2,3), 2.32 (s, 2H, BH 8,11), 2.40 (s, 1H, BH 5), 2.69 (s, 3H,
CH3); δB 0.0 (B5), Ϫ10.1 (B2,3), Ϫ11.4 (B8,11), Ϫ22.9 (B9,10),
Ϫ25.9 (B1), Ϫ32.1 (B4,6); (buffered, pH ca 9.3): δH (1H–{11B}),
Ϫ3.81 (m, 2H, µ-BH), 0.44 (s, 2H, BH 4,6), 1.18 (s, 3H, BH
1,9,10), 1.38 (s, 9H, CH3), 2.17 (s, 2H, BH 2,3), 2.31 (s, 3H, BH
5,8,11), 2.57 (s, 3H, CH3); δB Ϫ0.8 (B5), Ϫ10.8 (B2,3,8,11),
Ϫ23.7 (B9,10), Ϫ26.1 (B1), Ϫ32.2 (B4,6).
In conclusion, the preparation of Me3NCB10H12 and Cy-
Ϫ
Me2NCB10H12, precursors in the syntheses of nido-CB10H13
and closo-CB11H12Ϫ, may be achieved without the use of NaCN,
in high yield and at comparable cost. These results are of
particular relevance to the continued development of both
carborane and metallacarborane chemistry.
“One-pot” synthesis of Me3NCB10H12 5a. A cooled (water
bath) solution of B10H14 (5.00 g, 41 mmol) in benzene (100 ml)
t
was treated dropwise with BuNC (3.70 g, 4.62 ml, 45 mmol)
Experimental
All manipulations of air- and moisture-sensitive compounds
were performed on a conventional vacuum/nitrogen line using
and left to stir for 1–2 h and then brought to reflux until
all the decaborane was consumed as determined by 11B NMR.
All volatiles were removed under reduced pressure. The crude
2628
J. Chem. Soc., Dalton Trans., 2002, 2624–2631

View Article Online
Table 2 Crystal data for compounds 3a and 6b at 120 K
solids were dissolved in THF (50 ml) and treated with Na2CO3
(5.0 g) and (MeO)2SO2 (15.51 g, 11.64 ml, 123 mmol). The
mixture was brought to reflux and heating continued for 7–8 d.
After this time, the solution was filtered whilst warm and the
residue washed with warm acetonitrile (2 × 25 ml). The filtrates
were combined and volatiles removed under reduced pressure.
The resulting solid was washed with cold (0 ЊC) diethyl ether
(2 × 25 ml) to afford Me3NCB10H12 5a as a spectroscopically
pure white powder. Yield 7.24 g, 92 %. Spectroscopic properties
are in agreement with the literature; δH 1H-{11B} Ϫ3.50 (s, 2H,
µ-BH), 0.47 (m, 2H, BH 4,6), 1.20 (s, 1H, BH 1), 1.30 (s,
2H, BH 9,10), 2.26 (s, 2H, BH 8,11), 2.37 (s, 2H, BH 2,3), 2.51
(s, 1H, BH 5), 3.12 (s, 9H, NCH3); δB 1.0 (B5), Ϫ9.7 (B2,3),
Ϫ13.8 (B8,11), Ϫ22.4 (B9,10), Ϫ26.1 (B1), Ϫ32.9 (B4,6).
CAUTION! On a large scale (>10 g) we noted this reaction
can become violently exothermic following a short induction
period of 30–45 s. For large scale preparations we recommend
larger solvent volumes and slower addition (>2 h) of isocyanide
for more efficient cooling.
3a
6b
Empirical formula
Formula weight
Crystal system
Space group
a/Å
b/Å
β/Њ
C6H25B10N
219.37
C9H29B10N
259.43
Orthorhombic
P212121
9.200(4)
11.114(4)
Monoclinic
P2(1)/c
11.3418(14)
8.8099(11)
95.852(4)
13.7750(15)
1369.2(3)
4
0.050
15282
3403
0.0451
c/Å
15.636(6)
1599(1)
4
0.052
16479
2105
0.0949
1818
0.0428
0.1153
U/Å3
Z
µ(Mo-Kα)/mmϪ1
Reflections measured
Unique reflections
R(int)
Reflections with I ≥ 2σ(I )
R [I ≥ 2σ(I )]
wR(F 2), all data
2662
0.0496
0.1348
“One-pot” synthesis of CyMe2NCB10H12 6b. Using the
method described above for 5a, with CyNC (5.00 g, 45 mmol)
affords 6b. Yield 10.27 g, 96%. (Found C 42.02; H 11.14;
N 4.81. C9H29B10N requires C 41.67; H 11.27; N 5.40%);
δH 1H-{11B}, Ϫ3.72 (m, 2H, µ-BH), 0.52 (s, 2H, BH 4,6), 1.04–
2.07 (m, 9H, CH2, BH 1,9,10), 2.33 (s, 2H, BH 8,11), 2.39 (s,
2H, BH 2,3), 2.54 (s, 1H, BH 5), 2.96 (s, 6H, CH3), 3.59 (m,
trum of an aliquot was recorded. Addition of small portions
of aqueous HCl (approx. 1 M) was used to make the solution
less basic before recording the 11B NMR spectrum of an
aliquot. The 11B NMR chemical shifts of B5 were fitted to the
Henderson–Hasselbalch equation,37 in the following form:
1H, CH2); δC 85.0 (cage C), 77.5 (CH3), 51.9 (C1), 28.6 (C2,6
)
25.5 (C4) 24.9 (C3,5); δB 1.3 (B5), Ϫ9.3 (B2,3), Ϫ13.0 (B8,11),
Ϫ22.5 (B9,10), Ϫ25.4 (B1), Ϫ32.0 (B4,6).
Mono-anions. The following mono-anions were prepared by
agitation of wet d3-acetonitrile solutions of neutral precursor in
the presence of NaOH.
and calculating the chemical shift at each pH value using δcalc
=
Na[BuMeNCB10H12] 4a. δH (1H-{11B}), Ϫ3.90 (s, 2H, µ-BH),
0.36 (m, 2H, BH 4,6), 1.07 (s, 3H, BH 1,9,10), 1.23 (s, 9H,
CCH3), 2.04 (s, 2H, BH 2,3), 2.16 (s, 1H, BH 5), 2.25 (s, 2H, BH
8,11), 2.42 (s, 3H, NCH3); δC 78.2 (cage C), 63.9(CMe3), 37.8
(CH3), 27.6 (CCH3); δB Ϫ3.1 (B5), Ϫ9.8 (B8,11), Ϫ11.2 (B2,3),
Ϫ24.2 (B9,10), Ϫ26.4 (B1), Ϫ32.2 (B4,6).
[acid]δacid ϩ [base]δbase with δacid = 0 and δbase = 3.37 ppm. The
sum of the squares of the difference between observed and
calculated chemical shift at each pH value was minimised by
varying pKa to give pKa = 10.12 as the best fit. A similar fit to
the chemical shift of B9/B10 above pH 7 gives the same value
for pKa.
Na[Me3NCB10H11] 8a. δH (1H-{11B}), Ϫ2.76 (s, 1H, µ-BH),
0.11 (s, 1H, BH 1), 0.18 (s, 2H, BH 9,10), 0.45 (s, 1H, BH 5),
0.92 (s, 2H, BH 4,6), 1.31 (s, 2H, BH 2,3), 1.36 (s, 2H, BH
8,11), 2.97 (s, 9H, NCH3); δC 93.1 (cage C), 56.3 (CH3);
δB Ϫ18.3 (B8,11), Ϫ19.7 (B4,6), Ϫ21.5 (B2,3), Ϫ27.1 (B5,9,10),
Ϫ40.8 (B1). Addition of iodine to the NMR solution gave
closo-Me3NCB10H10: δH (1H-{11B}), 1.65 (1H, BH), 2.00 (4H,
BH), 2.09 (5H, BH), 3.13 (9H, NMe3); δB Ϫ4.4 (1B), Ϫ11.0
(5B), Ϫ14.1 (4B).
X-Ray crystallography
Single crystal X-ray diffraction experiments were carried out at
120(2) K with a SMART 1K CCD area detector, using
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å).
The structures were solved by direct methods and refined
by full-matrix least squares against F 2 of all data, using
SHELXTL programs.38 Crystal data and experimental details
are listed in Table 2. For 3a, the XP command HIMP was used
to move H12 and H5 to give B–H and N–H distances of 1.21
and 1.03 Å respectively, prior to the calculation of distances
and angles in Fig. 3.
Na[CyHNCB10H12] 2b. δH (1H-{11B}), Ϫ3.77 (s, 1H, µ-BH),
0.35 (s, 2H, BH 4,6), 1.04–1.28 (m, 7H, CH2, BH 1,9,10), 1.60
(d, 1H, CH2), 1.70–1.83 (m, 2H, CH2), 1.83 (s, 2H, BH 2,3),
2.13 (s, 3H, BH 5,8,11), 2.94 (m, 1H, CH2); δC 72.3 (cage C),
59.5 (C1), 34.1 (C2,6), 26.4 (C4), 25.3 (C3,5); δB Ϫ3.5 (B5), Ϫ9.2
(B2,3), Ϫ11.6 (B8,11), Ϫ23.7 (B9,10), Ϫ27.0 (B1), Ϫ32.3
(B4,6).
CCDC reference numbers 178300 and 178301.
See http://www.rsc.org/suppdata/dt/b2/b200930g/ for crystal-
lographic data in CIF or other electronic format.
Na[CyMe2NCB10H11] 7b. δH (1H-{11B}), Ϫ3.62 (s, 1H,
µ-BH), 0.00 (s, 1H, BH 1), 0.20 (s, 2H, BH 9,10), 0.48 (s, 1H,
BH 1), 0.98 (s, 2H, BH 4,6), 1.36 (m, 4H, BH 2,3,8,11), 1.33 (m,
8H, CH2, BH 2,3,8,11), 1.96 (m, 6H, CH2,), 2.65 (s, 6H, NCH3);
δC 93.8 (cage C), 76.3 (CH3), 49.3 (C1), 27.8 (C2,6), 25.8
(C4), 25.1 (C3,5); δB Ϫ17.8 (B8,11), Ϫ18.7 (B4,6), Ϫ21.5 (B2,3),
Ϫ27.1 (B5,9,10), Ϫ39.9 (B1).
Computational section
All ab initio computations were carried out with the Gaussian
98 package.39 The geometries discussed here were optimised
at the HF/6-31G* level with no symmetry constraints. No
imaginary frequencies were found for these optimised
geometries at the HF/6-31G* level. Optimisation of these
geometries were then carried out at the computationally inten-
sive MP2/6-31G* level and calculated NMR shifts at the
GIAO-B3LYP/6-311G* level. The geometries of the parent
NMR data for 3a as a function of pH and determination of pKa
for 3a
Ϫ
anion CB10H13 (I) then H3NCB10H12 (II) and finally
A solution of 3a (100 mg) in wet CH3CN (5 ml) was made basic
by stirring with KOH pellets and the pH determined with
a calibrated Jenway 3310 pH meter before the 11B NMR spec-
Me3NCB10H12 (III) and related geometries were optimised.
Theoretical 11B chemical shifts at the GIAO-B3LYP/6-311G*//
MP2/6-31G* level,40 were referenced to B2H6 (16.6 ppm)41 and
J. Chem. Soc., Dalton Trans., 2002, 2624–2631
2629

View Article Online
Table 3 Zero point (HF/6-31G*) and total (MP2/6-31G*) energies and calculated 11B NMR chemical shifts of calculated structures
ZPE/kcal molϪ1 Total E/au Calculated 11B NMR chemical shifts/ppm
Code Formula
Ϫ a
I
CB10H13
116.98
138.55
196.14
186.81
Ϫ293.43946
Ϫ349.11596
Ϫ466.60675
Ϫ466.05454
0.2 (5), Ϫ12.6 (2,3), Ϫ15.2 (8,11), Ϫ24.8 (9,10), Ϫ27.2 (1), Ϫ32.8 (4,6)
1.0 (5), Ϫ8.2 (2,3), Ϫ17.0 (8,11), Ϫ21.6 (9,10), Ϫ25.8 (1), Ϫ33.1 (4,6)
4.0 (5), Ϫ10.8 (2,3), Ϫ17.4 (8,11), Ϫ22.0 (9,10), Ϫ25.5 (1), Ϫ33.9 (4,6)
Ϫ7.4 (10), Ϫ9.6 (5), Ϫ14.6 (11), Ϫ19.6 (3), Ϫ21.1 (2), Ϫ25.5 (6),
Ϫ27.6 (8), Ϫ35.2 (9), Ϫ35.5 (4), Ϫ38.0 (1)b
Ϫ17.8 (4,6), Ϫ21.5 (8,11), Ϫ23.8 (2,3), Ϫ28.2 (5), Ϫ28.2 (9,10), Ϫ41.5 (1)
Ϫ22.7 (9,10), Ϫ23.9 (5), Ϫ27.3 (4,6), Ϫ27.6 (2,3), Ϫ29.8 (8,11), Ϫ51.3 (1)
4.4 (5), Ϫ9.2 (2,3), Ϫ15.4 (8,11), Ϫ22.2 (9,10), Ϫ25.2 (1), Ϫ30.5 (4,6)
Ϫ16.3 (8,11), Ϫ19.5 (4,6), Ϫ19.6 (2,3), Ϫ31.7 (5), Ϫ31.8 (9,10), Ϫ42.1 (1)
Ϫ6.3 (11), Ϫ14.2 (10), Ϫ14.3 (3), Ϫ14.5 (5), Ϫ18.5 (2), Ϫ24.6 (6),
Ϫ27.1 (8), Ϫ33.9 (4), Ϫ36.3 (9), Ϫ39.2 (1)c
II
H3NCB10H12
III
IV
Me3NCB10H12
Me3NCB10H11Ϫ µ-H at 8,9
V
VI
VII
VIII
IX
Me3NCB10H11Ϫ µ-H at 9,10
186.80
176.80
254.01
Ϫ466.06309
Ϫ465.32250
Ϫ584.09319
Ϫ504.60258
Ϫ504.59800
2Ϫ
Me3NCB10H10
tBuMe2NCB10H12
tBuHNCB10H112Ϫ µ-H at 9,10 194.28
tBuHNCB10H112Ϫ µ-H at 8,9
194.28
a Literature:32 0.3 (5), Ϫ11.2 (2,3), Ϫ11.2 (8,11), Ϫ22.4 (9,10), Ϫ24.8 (1), Ϫ30.4 (4,6). b Pairwise averaging: Ϫ9.6(5), Ϫ20.4 (2,3), Ϫ21.1 (8,11),
Ϫ21.3 (9,10), Ϫ30.5 (4,6), Ϫ38.0 (1). c Pairwise averaging: Ϫ14.5 (5), Ϫ16.4 (2,3), Ϫ16.7 (8,11), Ϫ25.3 (9,10), Ϫ29.2 (4,6), Ϫ39.2 (1).
converted to the usual BF3ؒOEt2 scale: δ(11B) = 102.83 Ϫ σ(11B).
and K. Wade, J. Chem. Soc., Dalton Trans., 2000, 3519; M. A. Fox,
J. A. K. Howard, A. K. Hughes, J. M. Malget and D. S. Yufit,
J. Chem. Soc., Dalton Trans., 2001, 2263.
9 R. R. Rietz, D. F. Dustin and M. F. Hawthorne, Inorg. Chem.,
1974, 13, 1580; C. G. Salentine, R. R. Rietz and M. F. Hawthorne,
Inorg. Chem., 1974, 13, 3025; V. N. Lebedev, D. F. Mullica,
E. L. Sappenfield and F. G. A. Stone, J. Organomet. Chem., 1997,
536, 537; I. T. Chizhevsky, I. V. Pisareva, E. V. Vorontzov,
V. I. Bregadze, F. M. Dolgushin, A. I. Yanovsky, Y. T. Struchkov,
C. B. Knobler and M. F. Hawthorne, J. Organomet. Chem.,
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V. I. Bregadze, F. M. Dolgushin, A. I. Yanovsky, Y. T. Struchkov and
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10 V. N. Lebedev, D. F. Mullica, E. L. Sappenfield and F. G. A. Stone,
Organometallics, 1996, 15, 1669.
11 W. H. Knoth, Inorg. Chem., 1971, 10, 598.
12 W. H. Knoth, J. Am. Chem. Soc., 1967, 89, 1274.
13 S. F. Myasoedov, T. V. Tsimerinova, K. A. Solntsev and
N. T. Kuznetsov, Zh. Neorg. Khim., 1984, 29, 1421 translated as
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14 D. E. Hyatt, F. R. Scholer, L. J. Todd and J. L. Warner, Inorg. Chem.,
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The relative energy was computed at the MP2/6-31G* level with
ZPE (calculated at HF/6-31G*) corrections scaled by 0.89. The
root mean squared fitting method used for comparison of
experimental and theoretical geometries was carried out
using the ofit command in the xp program as part of the
SHELXTL package.38 The misfit value for heavy atoms between
Ϫ
the MP2/6-31G* optimised geometry (I) of CB10H13 and the
crystallographically determined structure42 of CsCB10H13 is
0.0155 Å. Zero point (HF/6-31G*) energies in kcal molϪ1 and
total energies in au (MP2/6-31G*), and calculated 11B NMR
chemical shifts (GIAO-NMR/6-311G*//MP2–6-31G*) are
given in Table 3.
See
http://www.rsc.org/suppdata/dt/b2/b200930g/
for
CHIME files containing Cartesian coordinates of MP2-
optimized geometries of I to IX.
Acknowledgements
We acknowledge the award of an EPSRC Senior Research
Fellowship (J. A. K. H.), an EPSRC Advanced Research
Fellowship (M. A. F.) and support from the ERDF Centre for
21st Century Materials at the University of Durham (J. M. M.).
15 W. H. Knoth, J. L. Little, J. R. Lawrence, F. R. Scholer and
L. J. Todd, Inorg. Synth., 1968, 11, 33.
ˇ
16 J. Plesˇek, T. Jelínek, E. Drdáková, S. Herˇmánek and B. Stíbr, Collect.
Czech. Chem. Commun., 1984, 49, 1559.
17 D. S. Wilbur, US Pat., 1995, 5679322.
18 F. R. Scholer and L. J. Todd, J. Organomet. Chem., 1968, 14, 261.
19 W. H. Knoth and E. L. Muetterties, J. Inorg. Nucl. Chem., 1961,
20, 66; K. Basˇe, N. W. Alcock, O. W. Howarth, H. R. Powell,
A. T. Harrison and M. G. H. Wallbridge, J. Chem. Soc., Chem.
Commun., 1988, 341.
20 A. N. Bridges and D. F. Gaines, Inorg. Chem., 1995, 34, 4523.
21 G. B. Dunks and K. P. Ordonez, Inorg. Chem., 1978, 17, 1514.
22 A. Franken, B. T. King, J. Rudolph, P. Rao, B. C. Noll and J. Michl,
Collect. Czech. Chem. Commun., 2001, 66, 1238.
23 Bretherick’s Handbook of Reactive Chemical Hazards, ed.
P. G. Urben, 5th edn., Butterworth-Heinemann, Oxford, 1995,
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24 T. Jelínek, C. A. Kilner, M. Thornton-Pett and J. D. Kennedy, Chem.
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25 B. Brellochs, Spec. Publ.–R. Soc. Chem., 2000, 253, 212.
26 F. S. Mair, A. Martin, J. H. Morris, G. S. Peters and M. D. Spicer,
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Peters and M. D. Spicer, J. Organomet. Chem., 1995, 494, 195.
27 D. E. Hyatt, D. A. Owen and L. J. Todd, Inorg. Chem, 1966, 5,
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28 J. C. Jeffrey, P. A. Jelliss, J. A. Karban, V. N. Lebedev and
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