Synthetic and structural studies on C-ethynyl- and C-bromo-carboranes

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Synthetic and structural studies on C-ethynyl- and C-bromo-carboranes†

Mark A. Fox,*^aAudrey M. Cameron,^aPaul J. Low,^aMichael A. J. Paterson,^aAndrei S. Batsanov,^a

Andre´s E. Goeta,^aDavid W. H. Rankin,*^bHeather E. Robertson^band Julien T. Schirlin^b

Received 12th December 2005, Accepted 3rd April 2006

First published as an Advance Article on the web 27th April 2006

DOI: 10.1039/b517538k

A high-yield preparation of the C-monoethynyl para-carborane, 1-Me₃SiC≡C-1,12-C₂B₁₀H₁₁, from

C-monocopper para-carborane and 1-bromo-2-(trimethylsilyl)ethyne, BrC≡CSiMe₃is reported. The

low-yield preparation of 1,12-(Me₃SiC≡C)₂-1,12-C₂B₁₀H₁₀from the C,C^ꢀ-dicopper para-carborane

derivative with 1-bromo-2-(trimethylsilyl)ethyne, BrC≡CSiMe₃, has been re-investigated and other

products were identiﬁed including the C-monoethynyl-carborane 1-Me₃SiC≡C-1,12-C₂B₁₀H₁₁and

two-cage assemblies generated from cage–cage couplings. The contrast in the yields of the monoethynyl

and diethynyl products is due to the highly unfavourable coupling process between 1-RC≡C-12-Cu-

1,12-C₂B₁₀H₁₀and the bromoalkyne. The ethynyl group at the cage carbon C(1) strongly inﬂuences the

chemical reactivity of the cage carbon at C(12)—the ﬁrst example of the ‘antipodal effect’ affecting the

syntheses of para-carborane derivatives. New two-step preparations of 1-ethynyl- and 1,12-bis(ethynyl)-

para-carboranes have been developed using a more readily prepared bromoethyne, 1-bromo-3-methyl-

1-butyn-3-ol, BrC≡CCMe₂OH. The molecular structures of the two C-monoethynyl-carboranes,

1-RC≡C-1,12-C₂B₁₀H₁₁(R = H and Me₃Si), were experimentally determined using gas-phase electron

diffraction (GED). For R = H (R_G= 0.053) a model with C_5vsymmetry reﬁned to give a C≡C bond

˚

distance of 1.233(5) A. For R = Me₃Si (R_G= 0.048) a model with C_ssymmetry reﬁned to give a C≡C

˚

bond distance of 1.227(5) A. Molecular structures of 1,12-Br₂-1,12-C₂B₁₀H₁₀, 1-HC≡C-12-Br-1,12-

C₂B₁₀H₁₀and 1,12-(Me₃SiC≡C)₂-1,12-C₂B₁₀H₁₀were determined by X-ray crystallography. Substituents

at the cage carbon atoms on the C₂B₁₀cage skeleton in 1-X-12-Y-1,12-C₂B₁₀H₁₀derivatives invariably

lengthen the cage C–B bonds. However, the subtle substituent effects on the tropical B–B bond lengths

in these compounds are more complex. The molecular structures of the ethynyl-ortho-carborane,

=

1-HC≡C-1,2-C₂B₁₀H₁₁and the ethene, trans-Me₃SiBrC CSiMe₃Br are also reported.

Introduction

Unlike the more familiar ortho- and meta-carboranes, the icosa-

hedral C₂B₁₀cage of para-carborane 1 has ﬁve-fold symmetry

through the axis of the cage carbon atoms (Fig. 1, X, Y = H).

The high symmetry of para-carborane derivatives, 1-X-12-Y-1,12-

C₂B₁₀H₁₀, make them almost ideal for investigating the effects

of various substituents on the cage geometry. For example, a

C-monosubstituted-para-carborane 1-X-1,12-C₂B₁₀H₁₁(i.e. Y =

H in Fig. 1) contains only two distinct boron centres and ﬁve

unique cage bonds (i, ii, iii, ii₂, i₂), while a C,C^ꢀ-disubstituted-

Fig. 1 Cage numbering in 1-X-12-Y-1,12-C₂B₁₀H₁₀. i, i₂= averaged C–B

para-carborane (X = Y in Fig. 1) would have only three distinct

‘polar-tropical’ bond, ii, ii₂= averaged B–B bond from ‘tropical’ B–B

bonds e.g. B2–B3 and iii = averaged ‘meridianal’ B–B bond e.g. B2–B7.

cage bonds.

As the cage carbon atoms are easily functionalised,^1,2there has

been much recent attention on the use of this highly symmetrical

cage as not only a vehicle to explore cage structures, but also as a

linker/building block for the preparation of molecular compounds

with liquid crystalline phase behaviour,^3,4polymers,^5,6optical

materials with large hyperpolarizabilities^7,8and with the general

ﬁeld of rigid molecular rods.^9–15The three-dimensional aromaticity

associated with these carborane cages has resulted in the robust

para-carborane bridge –CB₁₀H₁₀C– also being investigated for

its capacity to act as a conduit for electronic effects.^16–22In

^aChemistry Department, Durham University Science Laboratories, South

Road, Durham, UK DH1 3LE. E-mail: m.a.fox@durham.ac.uk

^bSchool of Chemistry, University of Edinburgh, West Mains Road,

Edinburgh, UK EH9 3JJ. E-mail: d.w.h.rankin@ed.ac.uk

† Electronic supplementary information (ESI) available: Cartesian coordi-

nates of the MP2/6-31G* optimized geometries of carboranes described

in this study. Tables of least-squares correlation matrix (× 100) for

GED structure reﬁnement of 5 and 6 and computed (HF/MP2/B3LYP)

geometrical parameters for 6. Figures for experimental and ﬁnal weighted

difference (experimental–theoretical) molecular scattering intensities for 5

and 6. See DOI: 10.1039/b517538k

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these latter contexts, ethynyl para-carboranes have particular

appeal, preserving the rigid linear structure and ﬁve-fold symmetry

associated with the cage, and offering a simple extension to (and

mediated through) the tangential C p-orbitals.

By way of synthetic convenience we have also investigated

the use of an alternate, more readily obtained bromo-alkyne,

1-bromo-3-methyl-1-butyn-3-ol 7 (Scheme 2(b)), in place of 1-

bromo-2-(trimethylsilyl) ethyne 2. The molecular structures of C-

substituted-para-carboranes determined here and elsewhere are

discussed in terms of the substituent effects on the structure of the

cage framework.

The interest in the general area of conjugated materials and

molecular species with well-deﬁned linear architectures, and in

the development of simple single-source CVD precursors for

technologically useful materials, has prompted us to investigate

the development of convenient syntheses of C-ethynyl-para-

carboranes. Recently we applied our usual carborane copper-

mediated coupling procedure^19,23to the reaction of para-carborane

1 with 1-bromo-2-(trimethylsilyl) ethyne 2, from which 1,12-

bis(trimethylsilylethynyl)-para-carborane 3 (X, Y = C≡CSiMe₃)

was isolated in low yield; subsequent desilylation of 3 affords the

parent species 1,12-bis(ethynyl)-para-carborane 4 (X, Y = C≡CH)

(Scheme 1).²⁴A similar low-yielding synthesis of 3 has also been

reported by Kaszynski and co-workers.²⁵Despite the low yield

of these preparative routes, compounds 3 and 4 have been used

as precursors to metal complexes (to investigate the potential

of the diethynyl cage as a conduit for electronic effects between

metal centres)^16,20and incorporated into rigid, rod-like molecular

architectures.^11,13

Results and discussion

Synthetic aspects

Monoethynyl-para-carboranes. The copper-mediated cou-

pling of 1 with one equivalent of 2 proceeded smoothly with 5

being isolated in very good yield (76%) (Scheme 2(a)). Compound

5 was converted to the ethynyl carborane 6 by treatment with

KOMe. Attempts to grow suitable crystals of carboranes 5 and 6

at low temperatures for X-ray crystallography were not successful.

However, both compounds were sufﬁciently volatile at ambient

temperature for their molecular structures to be determined by

gas-phase electron diffraction.

Here we report the high yield preparation of monoethynyl para-

carboranes, 5 (X = C≡CSiMe₃, Y = H) and 6 (X = C≡CH,

Y = H),²⁶using procedures similar to the syntheses of 3 and 4

(Scheme 2(a)). The molecular structures of these carboranes have

been determined by gas-phase electron diffraction (GED) studies.

In seeking to understand the contrasting yields observed in the

preparations of the bis(ethynyl) carborane 3 and monoethynyl

carborane 6, we have re-investigated the low yield preparation of

3 shown in Scheme 1. Several carboranyl side-products, including

C-bromo-C-ethynyl-para-carboranes and a plethora of products

derived from C–C coupling between two carborane cages have

been identiﬁed.

The great attraction of ethyne 2 as a reagent lies in the simple

and efﬁcient manner in which the trimethylsilyl protecting group

can be removed after the cage C–ethyne C bond forming step. The

established, multigram syntheses of 2 involve reaction of bromine

with MC≡CSiMe₃(M = MgBr, Li), prepared either by deprotona-

tion of trimethylsilylethyne 8, or obtained by selective nucleophilic

desilylation of bis(trimethylsilyl)ethyne 9 (Scheme 3).^27,28While

both procedures afford 2 in high yield provided that all the starting

ethyne was converted into its metal intermediate Me₃SiC≡CM

(M = MgBr 10 or Li 11), careful analysis of the product mixture

reveals the formation of tribromoethene 12, which is a liquid at

ambient conditions, by inadvertent bromination of 2.^29,30In the

Scheme 1

Scheme 2

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Scheme 3

case of preparations involving 9, both 12 and the dibromoethene

13 were obtained, the latter being formed by bromination of 9.³¹

Here we also synthesised 2 in good yields by a simpler

method using ethyne 8 with butyllithium followed by bromine.

An alternative preparation of 2, albeit in low yields, is described

here using silver nitrate and N-bromosuccinimide on ethyne 8.

This mild silver nitrate method was used to convert some terminal

alkynes into bromoalkynes in very good yields.³²The main

difﬁculties in the preparation of 2 are its stability and puriﬁcation.

Attempted puriﬁcations of 2 by multigram-scale distillations of

product mixtures containing 2 were not successful. Crude 2 was

used in the syntheses of 3 and 5.

The difﬁculties encountered in the puriﬁcation of 2 together

with the signiﬁcant cost and laborious syntheses of 8 and 9

led us to consider an alternative bromoethyne for use in the

preparation of C-ethynyl-para-carboranes. Multigram quantities

of 1-bromo-3-methyl-1-butyn-3-ol, 7, can be obtained from the

readily available ethyne 3-methyl-1-butyn-3-ol, by reaction with

an aqueous solution of hypobromite and puriﬁed by distillation.³³

We were pleased to discover that the new carborane, 1-(3-hydroxy-

3-methylbutynyl)-para-carborane (14) could be formed in good

yield from the copper-mediated coupling of para-carborane 1 and

1-bromo-3-methyl-1-butyn-3-ol 7 (Scheme 2(b)). Equally, the high

thermal stability of the para-carborane cage meant that 1-ethynyl-

para-carborane (6) could be obtained in good yield following

deprotection of carborane 14 upon reaction with NaOⁱPr in 2-

propanol at reﬂuxing temperature.

Routes to bis(ethynyl)-para-carboranes. Bis(trimethylsilyl-

ethynyl)-para-carborane 3 was isolated in only low yields (10–15%)

from the copper-mediated coupling of para-carborane 1 with two

equivalents of 1-bromo-2-(trimethylsilyl)ethyne 2 (Scheme 1).^24,25

Schemes 1 and 2(a) differ only in reactant ratios. The monoethynyl-

para-carborane 5, which could be prepared in high yield from

equimolar proportions of reagents (Scheme 2(a)), was also a

major product of reactions intended to form the bis(ethynyl)

carborane 3.

The C-bromo-C-ethynylcarboranes 15 and 16 and

bis(carboranes) 17–22 (Fig. 2) were identiﬁed as minor products

(see Experimental section). Metal–halogen exchange has been

Fig. 2 Minor products identiﬁed from the reaction of 1,12-C₂B₁₀H₁₂1 with bromotrimethylsilylethyne 2.

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observed with other attempted cage C–C bond formation reactions

involving C-copper-para-carborane with perﬂuoroheptyl iodide

and perﬂuorohexyl bromide to give C-halocarboranes.³⁴

Compound 20 is related to a structurally characterised derivative

featuring heptyne substituents.⁴It is known that the bis(para-

carborane) 17 can be prepared from 1 by sequential reaction

with butyllithium and copper(I) chloride.³⁵Clearly, this cage-

coupling reaction competes with the cross-coupling reaction of

the intermediate RC≡CCB₁₀H₁₀CCu with 2 under the reaction

conditions described in Scheme 1.

of the bis(ethynyl) carboranes 3 and 23 respectively suggest the

second coupling reaction rate between RC≡CCB₁₀H₁₀CCu and

BrC≡CR is much slower than the ﬁrst coupling reaction rate

between HCB₁₀H₁₀CCu and BrC≡CR. This contrasts with the

moderate-to-high yields of diaryl-para-carboranes from para-

carborane and two equivalents of iodoarenes using our copper-

mediated coupling methods where the second coupling reaction

rate is similar to the ﬁrst coupling reaction rate.¹⁹It is assumed

here that the ethynyl group RC≡C at cage C(1) deactivates the

cage carbon at C(12) signiﬁcantly enough to prevent coupling

with BrC≡CR from taking place to a degree.

We have also investigated the use of the 1-bromo-3-

methyl-1-butyn-3-ol

7

as an alternate ethynyl source for

We also carried out the copper-mediated coupling reaction be-

tween the monoethynyl-para-carborane 14 and the bromoalkyne

alcohol 7 in the hope that the C,C^ꢀ-bis(ethynyl)-para-carborane

23 would be formed. Instead we obtained the diyne 26 in good

yield (Scheme 5). This result demonstrates facile removal of the

acetone protecting group as well as a strong preference for ethyne–

ethyne C–C coupling between CuC≡CCB₁₀H₁₀CH and BrC≡CR

over the cage–ethyne C–C coupling between RC≡CCB₁₀H₁₀CCu

and BrC≡CR. We exploited this route to make the symmetrical

diyne 27 with para-carboranyl moieties at each end using oxygen

instead of the bromoalkyne for the ethyne–ethyne C–C coupling

step. The diyne 27 can also be considered as a ‘building block’ for

rigid-rod architectures.

the preparation of bis(ethynyl) carborane 4. The carborane

1,12-bis(3-hydroxy-3-methylbutynyl)-para-carborane (23) was ob-

tained from para-carborane 1 and 7 in a low yield following

the usual copper-mediated cross-coupling protocol (Scheme 4).

Several side products were also identiﬁed, including 1-(3-

hydroxy-3-methylbutynyl)-para-carborane 14 and a signiﬁcant

amountof 2,5-dihydroxy-2,5-dimethyl-3,5-octadiyne 24. The 1,12-

bis(ethynyl)-para-carborane (4) was obtained in good yield from

carborane 23 following removal of the protecting group with

sodium isopropoxide in reﬂuxing 2-propanol.

The signiﬁcant yields of the monoethynyl carboranes 5 and 14

(as well as the two-cage products and diynes) in the preparations

Scheme 4

Scheme 5

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Table 1 Geometrical parameters (r_h1) for 5^a,b

In the preparation of the long-known 1-ethynyl-ortho-carborane

28 the critical carborane–ethyne C–C bond forming reaction be-

tween bromoethyne 2 and a C-silyl-ortho-carborane was achieved

without a copper catalyst (Scheme 6(a)).³⁶We repeated the

preparation of 3, but without copper(I) chloride and pyridine.

However, instead of the desired trimethylsilylethynyl carboranes 3

and 5 the 1-bromo- and 1,12-dibromo-para-carboranes³⁷29 and

30 were obtained in good yield (Scheme 6(b)). C-Bromination

is favoured over the cage–ethyne coupling process for para-

carborane in the absence of a copper catalyst here. The molecular

structure of the dibromocarborane 30 was determined by X-ray

diffraction.

Parameter

GED

3.083(9)

MP2/6–31G(d)

p₁

rC(1) · · · C(12)

3.076

1.437

1.234

1.711

1.140

0.016

0.093

1.864

3.4

117.6

120.1

1.3

0.7

108.1

110.7

p₂

rC(1)C(13)

1.442(5)

1.227(5)

1.718(6)

1.145(3)

0.014(8)

0.113(8)

1.857(3)

3.5(8)

117.6(2)

119.0(9)

1.3(2)

1.1(4)

109.7(4)

108.6(9)

p₃

rC(13)C(14)

p₄

av[rC(1)B, rC(12)B]

av(av rBH, av rCH)

dif[rC(1)B, rC(12)B]

dif(avrBH, avrCH)

p₅

p₆

p₇

p₈

av[rC(14)Si(15), rC(16)Si(15)]

dif[rC(16)Si(15), rC(14)Si(15)]

av[∠HC(12)B, ∠C(13)C(1)B]

av[∠C(1)BH, ∠C(12)BH]

dif[∠C(12)BH, ∠C(1)BH]

dif[∠HC(1)B, ∠C(13)C(12)B]

∠C(14)Si(15)C(16)

∠SiCH

p₉

p₁₀

p₁₁

p₁₂

p₁₃

p₁₄

p₁₅

Structural aspects

This section describes the molecular structures of the monoethynyl

carboranes 5 and 6 determined by GED and the disubstituted

carboranes, 3, 4/16 and 30, by X-ray diffraction. The molecular

structures of C-substituted-para-carboranes reported here and

elsewhere are examined here for the inﬂuence of substituents on

the cage geometry. The molecular structure of 1-ethynyl-ortho-

carborane 28—where the cage carbon atoms are adjacent—is also

discussed.

◦

a

˚

Distances in A, angles in

.

^bSee text for parameter deﬁnitions.

˚

Table 2 Bond distances (r_a/A) and amplitudes of vibration (u/A)

obtained in the GED reﬁnement of 5

Number

Atom pair

r_a

Amplitude

Restraint

u₁

u₂

u₃

u₄

u₅

u₆

u₇

u₈

u₉

u₁₀

u₁₁

C(12)–H(12)

B(2)–H(2)

C(13)–C(14)

C(1)–C(13)

B(7)–C(12)

C(1)–B(2)

B(2)–B(7)

B(7)–B(8)

B(2)–B(3)

C(14)–Si(15)

Si(15)–C(16)

1.086(4)

1.199(6)

1.226(5)

1.442(5)

1.709(7)

1.719(6)

1.764(8)

1.793(7)

1.793(6)

1.840(5)

1.874(5)

0.082(4)

0.090

0.039

0.041(4)

0.082(5)

0.086

0.072

0.074(3)

0.054(5)

0.066(4)

Tied to u₁

Monosubstituted-para-carboranes. The model used for the

GED reﬁnement of 5 was based upon the geometry obtained from

the ab initio structure optimizations. The least-squares reﬁnement

of the structure resulted in an R_Gfactor of 0.048 (R_D= 0.026),

with optimized parameters listed in Table 1 and the molecular

structure shown in Fig. 3. A summary of ﬁnal bond distances

and amplitudes of vibration is recorded in Table 2. All ﬁfteen

geometrical parameters and twenty groups of amplitudes of

vibration were reﬁned. Flexible restraints were employed during

the reﬁnement using the SARACEN method.³⁸Altogether, seven

geometric and ﬁve amplitude restraints were employed and are

listed in Table 3. For a full list of the ﬁnal bond distances and

amplitudes of vibration and a table of the least-squares correlation

matrix for the structural reﬁnement see ESI.† The success of the

ﬁnal reﬁnement can be assessed on the basis of the molecular

scattering curves (see ESI†) and radial distribution curve, Fig. 4.

The structure of 5 was reﬁned with a model of C_ssymmetry

with local C_5vsymmetry for the carborane cage, using ﬁfteen

Tied to u₅

Tied to u₉

parameters (Table 4). Parameters p₁, p₂and p₃correspond to

C(cage)–C(cage), C(cage)–C(ethynyl) and C(ethynyl)≡C(ethynyl),

p₄deﬁnes the average of the bond distance between C(1)–B(2) and

C(12)–B(7), while p₆is the difference between these two distances.

The average of C–H and B–H distances is described by parameter

p₅, while p₇represents the difference between these two bond

distances. The average distance between C(14)–Si(15) and Si(15)–

C(16/17/18) and the difference are deﬁned by p₈and p₉respec-

tively. The average of angles H(12)–C(12)–B and C(13)–C(1)–B

Scheme 6

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Table 3 Flexible restraints used in GED reﬁnement for 5

is described by p₁₀and their difference is represented by p₁₃.

The angles C(12)–B(7)–H(7) and C(1)–B(2)–H(2) are deﬁned

by an average and difference (p₁₁and p₁₂), and p₁₄represents

the angle C(14)–Si(15)–C(16). The methyl group assumes C_3m

local symmetry as theoretical methods show no appreciable

distortion, and so the angles Si(15)–C–H are represented by a

single parameter p₁₅.

The model used for the GED reﬁnement of 6 was based upon

the optimized geometry obtained from the ab initio calculations

with C_5vsymmetry. The least-squares reﬁnement of the structure

resulted in an R_Gfactor of 0.053 (R_D= 0.029), with parameters

listed in Table 4 and the structure depicted in Fig. 5. Final bond

distances and amplitudes of vibration are summarized in Table 5.

All eleven geometrical parameters and eleven groups of amplitudes

of vibration were reﬁned. Flexible restraints were employed during

the reﬁnement using the SARACEN method. Altogether, six

geometric and three amplitudes restraints were employed, as listed

in Table 6. For a full list of the ﬁnal bond distances and amplitudes

of vibration and the table containing the least-squares correlation

matrix for the structural reﬁnement, see ESI.† The success of the

ﬁnal reﬁnement can be assessed on the basis of the molecular

scattering curves (see ESI†) and radial distribution curve, Fig. 6.

◦

˚

Deﬁnition

Value/A or

Uncertainty/A or

p₆

p₇

p₉

dif[rC(1)B, rC(12)B]

0.016

0.093

0.034

120.1

1.3

0.008

dif(avrBH, avrCH)

0.010

1.0

0.2

0.5

dif[rC(16)Si(15), rC(14)Si(15)]

p₁₁av[∠C(1)BH, ∠C(12)BH]

p₁₂dif[∠C(12)BH, ∠C(1)BH]

p₁₃dif[∠HC(12)B, ∠C(13)C(1)B]

p₁₄∠C(14)Si(15)C(16)

0.7

108.1

0.075

0.047

0.053

0.127

0.168

0.5

u₁

u₄

C(12)–H(12)

C(1)–C(13)

0.005

0.012

0.030

u₁₀C(14)–Si(15)

u₁₇B(2) · · · H(2)

u₁₃₀B(7) · · · Si(15)

Fig. 3 Molecular framework showing atom numbering for 5 for the GED

reﬁnement.

Fig. 5 Molecular framework showing the atom numbering for 6 for the

GED reﬁnement.

Eleven geometric parameters were used to describe structure 6

for the reﬁnement procedure, with a model of C_5msymmetry (see

ESI†). In analogy to 5 parameters p₁, p₂, p₃deﬁne the distances be-

tween the two C(cage) atoms [C(1) · · · C(12)], C(cage)–C(ethynyl)

[C(1)–C(13)] and C(13)–C(14), respectively. The average of the

bond distances C(1)–B(2) and C(12)–B(11) is deﬁned by parameter

Fig. 4 Experimental and difference (experimental–theoretical) radial

distribution curves, P(r)/r, for 5. Before Fourier inversion, the data were

multiplied by s × exp(−0.00002s²)/(Z_Si− f _Si)(Z_C− f _C).

Table 4 Geometrical parameters (r_h1) for 6^a,b

˚

Table 5 Bond distances (r_a/A) and amplitudes of vibration (u/A)

obtained in the GED reﬁnement of 6. Non-bonded distances are listed

in the ESI†

Parameter

GED

3.078(5)

1.431(5)

1.233(5)

1.716(3)

1.184(3)

0.026(9)

1.102(12)

MP2/6-311G(d)

p₁

p₂

p₃

p₄

p₅

p₆

p₇

p₈

p₉

p₁₀

p₁₁

rC(1) · · · C(12)

3.079

1.438

1.220

1.717

1.186

0.015

1.076

117.4

119.9

1.4

Number

Atom pair

r_a

Amplitude

Restraint

rC(1)C(13)

rC(13)C(14)

u₁

u₂

u₃

u₄

u₅

u₆

u₇

u₈

u₉

u₁₀

u₁₁

C(14)–H(14)

C(12)–H(12)

C(13)–C(14)

C(1)–C(13)

C(12)–B(7)

B(7)–B(8)

B(2)–C(1)

B(2)–B(3)

B(2)–B(7)

1.099(12)

1.100(12)

1.233(5)

1.430(5)

1.704(5)

1.789(6)

1.730(5)

1.798(6)

1.774(4)

1.181(3)

1.182(3)

0.0726

0.075

0.037(3)

0.040(5)

0.071

0.070

0.073

0.071

0.069(1)

0.087

(ﬁxed)

av[rC(1)B, rC(12)B]

av(rBH)

dif[rC(1)B, rC(12)B]

av(rCH)

av[∠HC(12)B, ∠C(13)C(1)B]

av[∠C(1)BH, ∠C(12)BH]

dif[∠C(12)BH, ∠C(1)BH]

dif[∠HC(12)B, ∠C(13)C(1)B]

Tied to u₉

117.2(1)

119.3(7)

1.3(1)

1.7(4)

0.6

B(2)–H(2)

B(7)–H(7)

Tied to u₃

◦

a

˚

Distances in A, angles in

.

^bSee text for parameter deﬁnitions.

0.087

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Table 6 Flexible restraints used in GED reﬁnement for 6

by the X-ray method) is shorter than the (calculated or GED)

internuclear distance.

◦

˚

Deﬁnition

Value/A or

Uncertainty/A or

A single crystal grown from an oil consisting of 6 with 4 and 16

present was studied by X-ray diffraction and proved to be a solid

solution of 4 and 16 (in a ca. 2 : 1 ratio). This solid is isomorphous

with pure 4 which crystallises in the similar triclinic lattice with

p₆

p₇

p₉

dif[rC(12)B, rC(1)B]

av(rCH)

av[∠C(1)BH, ∠C(12)BH]

0.014

1.086

1.200

0.013

0.001

0.020

0.009

0.001

0.003

0.010

0.007

0.009

p₁₀dif[∠C(1)BH, ∠C(12)BH]

˚

a = 7.287(2), b = 7.579(1), c = 10.953(3) A, a = 82.73(2),

p₁₁dif[∠HC(1)B, ∠C(13)C(1)B] 0.006

b = 72.96(2), c = 80.01(2)^◦(at 150 K).²⁴The latter structure

contains two symmetrically non-equivalent molecules, both lying

at crystallographic inversion centres. In the mixed crystal both sites

are shared by the molecules of 4 and 16, the latter being disordered

between two opposite orientations, related by the inversion centre.

The resulting picture is one of the ordered centrosymmetric cage

with a superposition of an ethynyl and a bromine substituents (in a

5 : 1 ratio) at positions 1 and 12. The degree of substitution at both

molecular sites is equal within experimental error, notwithstanding

different packing environments.

d₁

u₄

B(2)–B(11)

C(1)–C(13)

1.761

0.046

0.051

0.064

u₄₅C(1) · · · C(14)

u₁₃C(1) · · · C(12)

The molecule of dibromo-para-carborane 30 has crystallo-

graphic C_isymmetry (Fig. 7). Three dibromocarboranes have

been structurally characterised previously, 9,10-dibromo-meta-

carborane,³⁹9,12-dibromo-ortho-carborane⁴⁰and 1,12-dibromo-

ortho-carborane.⁴¹The latter molecule is very similar to 30 in

shape, but adopts entirely different crystal packing (in chiral space

group P4₁) due to C–H · · · Br hydrogen bonds.⁴²

Fig. 6 Experimental and difference (experimental–theoretical) radial

distribution curves, P(r)/r, for 6. Before Fourier inversion, the data were

multiplied by s × exp(−0.00002s²)/(Z_B− f _B)(Z_C− f _C).

p₄and the difference associated with these bond lengths is deﬁned

by p₆. The average hydrogen/boron bond distance between B(2)–

H(2) and B(7)–H(7) is described by p₅while p₇represents the

average hydrogen/carbon bond distance between C(12)–H(12)

and C(14)–H(14). Parameter p₈deﬁnes the average angle between

H(12)–C(12)–B and C(13)–C(1)–B while the difference between

these two angles is represented by p₁₁. The average of the angles

C(1)–B–H and C(12)–B–H is deﬁned by p₉and the difference

between them is p₁₀.

Disubstituted-para-carboranes. The X-ray structure of the

disubstituted carborane 3 has been determined at 100 K. The

molecule possesses crystallographic 2/m (C_2h) symmetry in the

space group C2/m. The inversion centre coincides with the centre

of the cage, the Si, C(1), C(2), C(3), C(4) and B(2) atoms and their

inversion equivalents lie in the mirror plane. The C(1)–C(2)–C(3)–

Si fragment is essentially linear. While the present work was in

progress, the same structure (studied at 173 K) was reported by

Kaszynski and co-workers.²⁵Apart from a slightly (and spuriously)

˚

Fig. 7 X-Ray molecular structure of 30. C1–Br bond length 1.916(2) A.

Primed atoms are generated by the inversion centre. Thermal ellipsoids are

drawn at the 50% probability level.

Discussion of molecular structures of C-substituted-para-

carboranes. Tables 7 and8 list the averaged bondlengths for para-

carborane and its derivatives 1-X-12-Y-1,12-C₂B₁₀H₁₀determined

by gas-phase electron and X-ray diffraction respectively. The C₂B₁₀

cage geometry in the crystal structure of 4 (X, Y = HC≡C)

˚

shorter C≡C bond, 1.193(3) A at 173 K against 1.204(2) A at

100 K, and a rotational disorder of one methyl group in each

SiMe₃moiety, the results are identical with ours. High-angle X-

ray reﬂections, which are better observed in a low-temperature

study, have larger contribution from the core electrons (which do

not shift), hence the apparent C≡C bond lengths in the X-ray

structures increase with the decrease of temperature.

˚

revealed the averaged tropical B–B bond (ii) to be 1.793(3) A. This

˚

is ca 0.015 A longer than the respective bonds (ii, ii₂) determined

for the parent para-carborane by X-ray crystallography^43,44and

gas-phase electron diffraction.^45,46The C–B bond length (i) of

˚

1.726(3) A in 4 was also longer (ca. 0.02 A) than those measured

for the parent para-carborane. The molecular structure of 3 (X,

Y = Me₃SiC≡C) was also determined here and elsewhere²⁵with a

similar cage geometry to 4.

˚

The difference in the C≡C bonds of 1.204(2) A for the molecular

˚

structure of 3 by X-ray diffraction and of 1.227(5) A for the

molecular structure of 5 by GED can be attributed to the fact

that the valent electrons of the acetylenic carbon atoms are

substantially shifted into the C≡C bond area, hence the distance

between electron density maxima (‘bond distance’ as measured

The molecular structure for 6 (X = HC≡C, Y = H) determined

by GED here gave the expected lengthening of the neighbouring

C–B and tropical B–B bonds on ethynyl group substitution at

˚

one cage carbon with values of 1.730(5) (i) and 1.799(5) A (ii)

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Table 9 MP2/6-31G* data for selected 1-X-12-Y-1,12-C₂B₁₀H₁₀carboranes

˚

X

Y

C · · · C/A

i/A

ii/A

iii/A

ii₂/A

i₂/A

C(1)-exo/A

C≡C(N)/A

Point group

Ref.

54

F

H

Cl

H

3.040

3.047

3.054

3.056

3.057

3.067

3.069

3.073

3.076

3.078

3.096

3.099

3.105

3.142

1.702

1.703

1.708

1.716

1.711

1.716

1.711

1.718

1.719

1.711

1.720

1.724

1.718

1.719

1.720

1.784

1.781

1.784

1.790

1.785

1.790

1.778

1.784

1.785

1.775

1.776

1.780

1.784

1.783

1.774

1.761

1.762

1.761

1.760

1.761

1.758

1.760

1.761

1.762

1.761

1.760

1.759

1.761

1.781

1.780

1.781

1.790

1.780

1.779

1.780

1.779

1.778

1.777

1.784

1.783

1.774

1.703

1.702

1.703

1.701

1.716

1.702

1.703

1.702

1.701

1.718

1.719

1.720

1.363

C_5v

D_5d

C_5v

C_s

D_5d

C_s

1.751

1.445

1.512

1.445

1.392

1.439

1.437

1.524

1.507

1.533

1.438

1.437

1.506

C≡N

CO₂H

C≡N

OH

C≡CH

C≡CSiMe₃

Me

1.183

H

C≡N

47

59

H

1.221

1.234

C_5v

C_s

Ph

C_s

CB₁₀H₁₀CH

C≡CH

C≡CSiMe₃

Ph

H

C_s

C≡CH

C≡CSiMe₃

Ph

1.221

1.235

D_5d

C_s

respectively. The C–B and tropical B–B values of 1.704(5) (i₂) and

This is supported here by the XRD data in Table 8 and additional

MP2/6-31G* data in Table 9. The substitution effects on the C–B

(i, i₂) and tropical B–B (ii, ii₂) bonds were not discussed in the

reported study on the basis that these changes are insigniﬁcant—

with ethynyl and aryl groups as substituents we believe that they

are signiﬁcant. The average C–B bond length (i, i₂) invariably

increases on substitution at the cage carbon compared to the

parent para-carborane but is longest when the substituent is

an ethynyl, C₂Co₂cluster or aryl group. The substituent effect

on the tropical B–B bonds is interesting. These bonds (ii, ii₂)

lengthen signiﬁcantly with ethynyl, cyanide and bromo groups as

substituents but are unaffected by aryl and C₂Co₂cluster groups. It

is intriguing that the ethynyl group has the most notable effect on

the cage geometry overall and thus mirrors its signiﬁcant inﬂuence

on the low-yield syntheses of C,C^ꢀ-bis(ethynyl)-para-carboranes

observed here and elsewhere.^24,25

˚

1.790(5) A (ii₂) observed near the unsubstituted cage carbon are

similar to corresponding values in the parent carborane geometry.

For the molecular structure of 5 determined by GED, the two C–B

˚

bond distances were 1.725(6) (i) and 1.708(7) A (i₂) as expected

but the averaged tropical B–B bond lengths of 1.789(6) (ii) and

˚

1.792(7) A (ii₂) were similar. The higher e.s.d.s found in the GED-

determined structure of 5 suggest the latter bonds are not precise

enough to reveal a difference. The MP2-optimised geometry for

˚

6 shows a difference of only 0.005 A for the two distinct tropical

˚

B–B bonds whereas a difference of 0.016 A is determined for the

two different C–B bond distances (Table 9).

A point worthy of comment here is the variable C≡C bond

lengths observed in GED and XRD for the C-ethynyl-carboranes 4

and 6 respectively as well as from ‘gas-phase’ optimised geometries

˚

at different levels of theory. A difference of 0.05 A found between

4 and 6 is due to the different diffraction methods used. The X-ray

diffraction method underestimates the C≡C bond length—even

at lower temperatures—so this should be taken into account when

using the X-ray structure of 4 as a model for longer molecules. The

HF/6-31G*-optimised geometry of 4 has a C≡C bond length

Monoethynyl-ortho-carborane. In 1964, the ﬁrst C-ethynyl

carborane 1-HC≡C-1,2-C₂B₁₀H₁₁28, where the carbons are

adjacent in the cage, was reported,⁵⁵with its chemistry being

explored over the next twenty years.⁵⁶Compound 28 behaves as a

typical terminal alkyne, and oxidative coupling affords the carbo-

rane terminated diyne. Nickel-catalysed cyclo-trimerisation of 28

affords 1,2,4-tris(1,2-C₂B₁₀H₁₁)C₆H₃in good yield.⁵⁷This volatile

solid, which polymerises upon UV irradiation, is considered a

suitable precursor candidate to boron–carbide (B₄C) ceramics

via MOCVD.³⁶The chemistry of 1-ethynyl-para-carborane 6 is

expected to be similar to that of its ortho-carborane analogue

28. However the substantially more thermally/chemically stable

para-carborane cage with a reduced electron-withdrawing strength

relative to the ortho-carborane cage points to some novel chemistry

to be discovered with 6. Moreover, the boron–carbide ceramics

generated from the volatile ethynyl-para-carboranes would be

expected to be more robust than those generated from their ortho-

carborane analogues.

˚

of 1.185 A-similar to the X-ray structure—and, again, this is

not an appropriate model for longer molecules. The DFT and

MP2-optimised geometries with C≡C bond lengths of 1.210²²and

˚

1.221 A respectively are in reasonably good agreement with the

˚

observed GED value of 1.233(5) A for 6. Table 9 lists the MP2/6-

31G* data for selected para-carborane derivatives 1-X-12-Y-1,12-

C₂B₁₀H₁₀. Moreover, the cage geometries in the DFT and MP2

geometries are in better agreement with the X-ray data for 4 than

with the HF geometry.

Unfortunately as shown from the e.s.d.s in Table 7, the majority

of reported structures determined by GED for C-substituted-para-

carborane derivatives are not sufﬁciently precise to examine the

subtle effects on the cage geometries by substitution at the cage

carbon atoms. Nevertheless, the number of accurate molecular

structures on para-carborane derivatives determined by XRD has

grown rapidly in the last ﬁve years since the molecular structure of

4 was compared with just three other para-carborane derivatives.²⁴

Relevant XRD data for selected derivatives are given in Table 8.

A previous study⁴⁷on selected C-disubstituted-para-carboranes

at the MP2/6-31G* level of theory concluded that the C · · · C bond

distances are related to the induction effect of the substituent.

Monosubstituted ortho-carboranes often have rotationally dis-

ordered cages in the solid state, with the (unsubstituted) carbon

and boron atoms statistically mixed.⁴²However, the present X-

ray diffraction study shows that the molecule of 1-ethynyl-ortho-

carborane 28 (Fig. 8) is ordered in the crystal at 120 K. The C(2)

atom was clearly distinguishable from boron atoms both by the

electron density peak height and by bond distances. The ordering

3552 | Dalton Trans., 2006, 3544–3560

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to the high yields of monoethynyl-para-carborane from similar

procedures, are due to the inﬂuence of the C-ethynyl group

on the second cage C–alkyne C coupling process. The readily

prepared bromoalkyne 1-bromo-3-methyl-1-butyn-3-ol affords

monoethynyl- and bis(ethynyl)-para-carborane and is probably the

preferred reagent for economy and convenience; however the yield

of the disubstituted carborane is low from both bromoalkynes.

A convenient high-yield synthesis of bis(ethynyl)-para-carborane

is therefore still needed to more readily explore and exploit its

potential as a precursor to molecular rigid rods or supramolecular

assemblies and to metal complexes for investigations into the

electronic transmission of the carborane cage.

Gas-phase electron diffraction data for the two C-monoethynyl-

para-carboranes show subtle geometrical effects of the ethynyl

group on the highly symmetrical cage geometry and also provide

realistic C≡C bond lengths, which are underestimated by X-

ray diffraction data for related ethynyl carboranes, such as

the C-monoethynyl-ortho-carborane determined here and 1,12-

bis(ethynyl)-para-carborane studied elsewhere.²⁴The effect of

substituents on the para-carborane cage geometry is intriguing.

Both neighbouring cage C–B bonds and tropical B–B bonds are

lengthened signiﬁcantly by ethynyl groups compared to other

substituents.

Fig. 8 X-Ray molecular structure of 28. The C(1)–C(13) bond length is

1.441(1) A.

˚

of the C₂B₁₀cage may be attributed to close intermolecular

contacts between positively and negatively charged hydrogen

atoms,⁴²viz. C(14)–H · · · H–B(9) (x − 1, y, z − 1) and C(2)–

H · · · H–B(7) (x − 1/2, 1/2 − y, z − 1/2), with internuclear H · · · H

˚

distances of 2.25 and 2.45 A, respectively (estimated using C/B–

H bond lengths from GED). The intra-cage C(1)–C(2) bond is

known for its ‘elasticity’, its length being very sensitive to the

nature and even the orientation of the substituents.^58,59The C(1)–

˚

C(2) bond distance of 1.653(1) A in 28 is some 0.03 A longer than

the C(1)–C(2) bond length determined for the unsubstituted ortho-

carborane.^43,45,60However, 28 is the ﬁrst structurally characterised

monosubstituted ortho-carborane 1-R-1,2-C₂B₁₀H₁₁where the

C(1)–C(2) bond is not inﬂuenced by the orientation effect of the

substituent, as the ethynyl group is cylindrically symmetric.

Experimental

Reactions were carried out using standard Schlenk techniques

under dry high-purity nitrogen. THF and diethyl ether were

distilled from potassium and CH₂Cl₂from calcium chloride prior

to use. Pyridine was distilled after standing over potassium

hydroxide for three days. Bromine was dried by shaking with

an equal volume of concentrated H₂SO₄and then distilled.

Trimethylsilylethyne, bis(trimethylsilyl)ethyne and 1-methyl-3-

butyn-1-ol (Aldrich) were used as supplied. MeLi (1.4 M, diethyl

ether), pentane, MeLi·LiBr (1.5 M, diethyl ether), HCl, CdCl₂,

NaCl, Na₂SO₄, Na₂SO₃, NaHCO₃and MgSO₄(dried in hot oven)

were used as supplied. Copper(I) chloride was puriﬁed according

to a literature procedure.⁶³

NMR spectra were measured using Varian Unity-300 (¹H, ¹¹B,

¹³C), Bruker AM250 (¹H, ¹³C), Bruker Avance 400 (¹H, ¹¹B, ¹³C)

and/or Varian Inova 500 (¹H, ¹¹B) instruments. All chemical shifts

are reported in d (ppm) and coupling constants in Hz. ¹H NMR

spectra were referenced to residual protio impurity in the solvent

(CDCl₃, 7.26 ppm). ¹³C NMR spectra were referenced to the

solvent resonance (CDCl₃, 77.0 ppm). ¹¹B NMR spectra were

referenced externally to Et₂O·BF₃, d = 0.0 ppm. Infrared spectra

were recorded from KBr discs on Perkin Elmer 1600 series FTIR

or Perkin Elmer 1720X FTIR spectrometers. Mass spectra (MS)

were recorded on a VG Micromass 7070E instrument under E.I

conditions (EI) at 70 eV. Gas chromatography–mass spectra were

recorded on a Fisons VG Trio 1000 mass spectrometer coupled

directly to a Hewlett Packard 5890 Series II gas chromatograph

(Column: HP-1; 25 m; 0.25 mm I.D.; 0.32 lm. l m thickness).

Values of M show the isotope range ¹⁰B_nto ¹¹B_nincluding a ¹³C

contribution if observed. Elemental carbon and hydrogen analyses

were performed using Exeter Analytical CE-440 or Carlo Erba

Strumentazione EA Model 1106 instruments.

˚

The triple C(13)≡C(14) bond length in 28, 1.185(2) A, is

˚

consistent with the value of 1.194(2) A observed in the X-

ray structure of the diphenylated derivative, 1-PhC≡C-2-Ph-1,2-

C₂B₁₀H₁₀.⁶¹The MP2/6-31G* optimised geometry of 28 reveals a

˚

slightly shorter C(1)–C(2) bond (1.642 A) and a longer C≡C bond

˚

(1.221 A).

Spectroscopic aspects

The ¹¹B NMR peak value for boron atoms B(2)–B(6) on replacing

hydrogen at C(1) in para-carborane 1 with a substituent is shifted

to high frequency by some 2–3 ppm here and elsewhere.¹⁰The type

of substituent does not inﬂuence the value considerably. The longer

C–B(2)/(6) bond lengths on substitution of C(1) may well be linked

to the high frequency ¹¹B shift change observed in C-substituted-

para-carboranes compared to para-carborane itself. The C(1) shift

value can vary to higher or lower frequency as expected when H

is replaced by different substituents. On substitution of H at C(1),

the ¹³C peak shift for C(12) is changed to low frequency (antipodal

effect⁶²) in all cases and the shift depends on the type of substituent.

We would expect the shift difference for C(1) on substitution at

C(1) to be greater than for the antipodal carbon C(12) but the

opposite is found for 1-bromo-para-carborane 29. The observed

¹³C chemical shifts of 56.5 ppm corresponding to the antipodal

C(12) and 58.2 for the substituted C(1) in 29 is unusual when

compared with the ¹³C shift of 63.5 ppm for para-carborane.

Conclusions

The low yields of bis(ethynyl)-para-carborane from copper medi-

ated reactions of para-carborane with bromoalkynes, compared

1-Ethynyl-ortho-carborane 28 was prepared by a reported

procedure,⁵⁵and conﬁrmed by comparison of spectroscopic data

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with that reported elsewhere.³⁶Crystals of 28 suitable for X-

ray diffraction were obtained by slow sublimation at ambient

temperature and pressure. 1-Bromo-3-methyl-1-butyn-3-ol 7 was

prepared according to a literature procedure.³³7: NMR (CDCl₃,

Synthesis of 2-bromotrimethylsilylethyne 2 from

trimethylsilylethyne 8 via silver nitrate

Trimethylsilylethyne 8 (4.9 g, 0.05 mol) was dissolved in acetone

(20 ml) under nitrogen and added with silver nitrate (2.83 g, 0.167

mol). The solution containing a suspension of white solid was

stirred for 1 hour. A solution of N-bromosuccinimide (8.9 g,

0.05 mol) in acetone (100 ml) was added slowly to the stirred

solution and the reaction mixture was left to stir for 4 h. Diethyl

ether (50 ml) was added to the mixture and the insoluble white

solid was ﬁltered off. The ﬁltrate was washed with saturated

NaHCO₃solution (3 × 50 ml) and water (3 × 50 ml), then

dried over MgSO₄. The solution was concentrated using a rotary

evaporator to leave an oil (3.5 g) identiﬁed as a 3 : 1 mixture of

bromotrimethylsilylethyne 2 (ca. 29% yield) and ether. The crude

compound 2 was stored over molecular sieves under nitrogen

in a fridge at 0 ^◦C and used in the syntheses of C-ethynyl

carboranes.

1

298 K), d (¹H) 3.07 (1H, OH), 1.48 (6H, CH₃); d (¹³C{ H}) 84.5

(C≡CBr), 66.1 (COH), 42.6 (C≡CBr), 31.0 (CH₃).

Synthesis of 2-bromotrimethylsilylethyne 2 from

trimethylsilylethyne 8 via butyllithium

Trimethylsilylethyne 8 (4.9 g, 0.05 mol) was dissolved in Et₂O

(100 ml) under nitrogen. A solution of butyllithium in hexanes

(1.6 M, 160 ml, 0.21 mol) was added dropwise at 0 ^◦C and

stirred for 20 min before being warmed to r.t. for a further 1 h.

The solution was cooled to −78 ^◦C (acetone–dry ice) before

bromine (8 g, 0.21 mol) was added dropwise and the mixture

stirred for 45 min. The solution was allowed to warm to room

temperature giving a light brown transparent solution, which was

poured into ice-cold 6 M hydrochloric acid (50 ml). The layers

were separated and the dark yellow aqueous phase was extracted

with ice-cold pentane (3 × 50 ml). The organic extracts were

combined and concentrated using a rotary evaporator. The pale

yellow oil (5.6 g) was found to contain 89% of Me₃SiC≡CBr 2 (56%

yield), tribromotrimethylsilylethene 12 (5%) and hexanes (6%).

The crude compound 2 was stored over molecular sieves under

nitrogen in a fridge at 0 ^◦C and used in the syntheses of C-ethynyl

carboranes.

Synthesis of 2-bromotrimethylsilylethyne 2 from

bis(trimethylsilyl)ethyne 9²⁸

Bis(trimethylsilyl)ethyne 9 (34.5 g, 0.2 mol) was dried under

vacuum while stirring in a 1 L round bottom ﬂask immersed in

an ice bath before THF (110 ml) was added under nitrogen. A

solution of methyllithium in diethyl ether (1.5 M, 160 ml, 0.21

mol) at 0 ^◦C was added dropwise, and stirred at 0 ^◦C for 1 h

before being warmed to r.t. and stirred for a further 1 h. After

this time, examination of an aliquot of the mixture by ¹³C NMR

conﬁrmed that all the ethyne had converted into the lithium salt,

Me₃SiC≡CLi (the ¹³C chemical shift assigned to the methyl group

of the ethyne is shifted to low frequency by 1.7 ppm on lithiation).

The solution was cooled to −70 ^◦C before a solution of bromine

(33.6 g, 0.21 mol) in CH₂Cl₂(30 ml) was added dropwise and the

mixture stirred for 45 min. The solution was allowed to warm to

room temperature giving a light brown transparent solution, which

was poured into ice-cold 6 M hydrochloric acid (50 ml). The layers

were separated and the dark yellow aqueous phase was extracted

with ice-cold pentane (3 × 50 ml). The combined dark orange

organic extracts were washed successively with ice-cold 6M HCl

(3 × 25 ml), 20% aqueous CdCl₂(20 ml), 10% aqueous Na₂SO₃

(20 ml), saturated NaHCO₃(20 ml) and saturated NaCl (30 ml),

and then dried with MgSO₄for 48 hours. The ether solution

2^27,64

. Mass (EI) spectrum: typical bromine pattern at m/z

161–163 [163, M − CH₃, 100%], 176–178 [178, M, 2%], C₅H₉BrSi

requires: M = 177. IR: m(CH) 2960, 2904; m(C≡C) 2125; 1251; 867,

1

844 cm⁻¹. NMR (298 K, CDCl₃) d (¹H) 0.19 (CH₃); d (¹³C{ H})

86.9 (BrC≡CSi), 61.4 (BrC≡CSi), −0.3 (CH₃).

Synthesis of 2-bromotrimethylsilylethyne 2 from

trimethylsilylethyne 8 via Grignard reagent²⁷

A solution of BuMgBr was prepared from Mg (4.98 g, 0.2 mol)

and bromobutane (28.6 g, 0.2 mol) in diethyl ether (200 ml);

little Mg was left after the solution was heated at reﬂux point

for 1 h. Trimethylsilylethyne 8 (20.1 g, 0.2 mol) was added

dropwise to the stirred solution, the resulting semi-solid mass

was diluted with ether (200 ml) and stirred at room temperature

for 18 h by a mechanical stirrer. The solution was cooled to

−45 ^◦C (dry ice–acetone bath) and bromine (32.5 g, 0.2 mol)

was added dropwise—each drop decolourised as it reached the

solution—then warmed slowly to ambient temperature. Water

(65 ml) was added slowly and stirred until two phases appeared.

The ether layer was separated, washed with water and dried

over MgSO₄. The solvent was removed at 0 ^◦C under vacuum

(0.05 mmHg) to afford a light brown residue (36.3 g), which

was identiﬁed by proton and carbon NMR spectroscopy to

be a mixture of 2-bromotrimethylsilylethyne 2 (85%), tribro-

motrimethylsilylethene 12 (7%) and bromobutane (8%). The crude

compound 2 was stored over molecular sieves under nitrogen

in a fridge at 0 ^◦C and used in the syntheses of C-ethynyl

carboranes.

◦

was distilled at 0 C with a Vigreux column under high vacuum

(0.01 mmHg). The undistilled residue (23.6 g) was found to contain

93% of Me₃SiC≡CBr 2 (65% yield), tribromotrimethylsilylethene

12 (2%), trans-1,2-dibromo-bis(trimethylsilyl)ethene 13 (2%) and

THF (3%). The crude compound 2 was stored over molecular

sieves under nitrogen in a fridge at 0 ^◦C and used in the syntheses

of C-ethynyl carboranes.

Formation of tribromotrimethylsilylethene 12 and

trans-1,2-dibromo-bis(trimethylsilyl)ethene 13 from

bis(trimethylsilyl)ethyne 9

Bis(trimethylsilyl)ethyne 9 (23.5 g, 0.14 mol) was dried under

vacuum in a 1 L round bottom ﬂask immersed in an ice bath

before being dissolved in THF (200 ml), cooled to 0 ^◦C and

treated dropwise with methyllithium (ca. 0.5 M, 115 ml). The

3554 | Dalton Trans., 2006, 3544–3560

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solution was stirred at 0 ^◦C for 30 min and warmed to room

temperature and stirred for 30 min to give a colourless solution.

A solution of bromine (22.4 g, 0.14 mol) in a mixtu_◦re of CH₂Cl₂

(20 ml) and pyridine (23 ml) was prepared at −78 C (acetone–

dry ice) and warmed to room temperature before being added

to the THF solution at −78 ^◦C. The resultant opaque yellow

solution was stirred at −78 ^◦C for 30 min and gradually warmed

to room temperature to give a cherry red transparent solution and

poured into ice-cold 6 M hydrochloric acid (50 ml). The layers

were separated and the aqueous phase was extracted with ice-cold

pentane (3 × 50 ml). The combined dark brown organic extracts

were washed successively with ice-cold 6 M HCl (3 × 25 ml),

20 ml aqueous solutions of 20% CdCl₂, saturated NaCl, satu-

rated Na₂SO₄, saturated NaHCO₃and saturated NaCl and then

dried with MgSO₄. The yellow solution was vacuum distilled at

20 ^◦C/0.01 mmHg to remove solvents, then at 50 ^◦C/0.01 mmHg

to give 8.0 g (34%) of unreacted bis(trimethylsilyl)ethyne 9 and

at 150 ^◦C/0.01 mmHg to give 11.1 g of a 1 : 1 mixture of

tribromotrimethylsilylethene 12 (24%) and trans-1,2-dibromo-

bis(trimethylsilyl)ethene^65,6613 (24%). The 1 : 1 mixture was cooled

to 0 ^◦C. After two weeks large crystals of pure 13 were formed and

subjected to an X-ray crystallography study which conﬁrmed the

trans-conformation of the ethene. The mother liquor was vacuum

distilled at 180 ^◦C/0.01 mmHg to give a colourless viscous oil

identiﬁed as pure tribromotrimethylsilylethene 12.

CDCl₃) d (¹¹B) −11.5 (5B, d, 168, B2-6), −15.2 (5B, d, 167, B7–

11

11); d (¹H{ B}) 2.61 (1H, C12H), 2.44 (5H, B2–6H), 2.22 (5H,

1

B7–11H), 0.06 (9H, CH₃); d (¹³C{ H}) 100.5 (C–Si), 84.5 (C≡CSi),

69.6 (cage CC≡C), 59.9 (cage CH), −0.5 (CH₃).

Preparation of 1-ethynyl-para-carborane (6) from (5)

A sample of Me₃SiC≡CCB₁₀H₁₀CH (5) (0.5 g, 2 mmol) suspended

in 5 ml of methanol was treated with 0.5 ml of methanolic

potassium hydroxide solution (0.1 g KOH, 5 g H₂O, 20 ml

methanol). The mixture was stirred at room temperature overnight

before being diluted with diethyl ether (40 ml), washed with water

(2 × 30 ml) and dried (magnesium sulfate). Evaporation of the

ether at room temperature gave an oil which was distilled at

0^◦/0.05 mmHg with the receiver cooled at −78 ^◦C. A distillate

(0.32 g, 91%), consisting of pure 1-ethynyl-para-carborane (6),

was obtained.

6. Mass (EI) spectrum: typical carborane pattern at 158–170,

[167, M − H, 100%], [168, M, 95%] C₄H₁₂B₁₀requires: M =

168. IR: m(ethynyl CH) 3305(m); (m(cage CH) 3063(m); m(BH)

2613(vs) cm⁻¹. NMR (298 K, CDCl₃) d (¹¹B) −11.6 (5B, d, 167, B2-

11

6), −15.1 (5B, d, 167, B7–11); d (¹H{ B}) 2.61 (1H, C12H), 2.41

1

(5H, B2–6H), 2.16 (5H, B7–11H), 2.05 (1H, C≡CH); d (¹³C{ H})

79.6 (C≡CH), 67.9 (cage CC≡C), 67.7 (C≡CH), 60.4 (cage CH).

Preparation of 1-(3-hydroxy-3-methylbutynyl)-para-carborane (14)

12. Mass (EI) spectrum: typical bromine pattern at m/z 137–

139, [Me₂SiBr, 100%], 335–340 [336, M, 19%], C₅H₉Br₃Si requires:

A stirred solution of para-carborane 1 (1.44 g, 10 mmol) in

monoglyme (45 ml) was treated with BuLi (2.5 M in hexanes,

4.4 ml, 11 mmol) at ambient temperature. Pyridine (8 ml) and

CuCl (1.13 g, 11.3 mmol) were added to the solution and the

mixture heated at reﬂux (30 min) then allowed to cool. A solution

of BrC≡CCMe₂OH (7, 2.0 g, 11.3 mmol) in monoglyme (10 ml)

was added dropwise to the vigorously stirred solution at 0 ^◦C,

and the mixture heated at reﬂux (30 min). The reaction mixture

was cooled, and diluted with Et₂O (400 ml). The oil obtained

after evaporation of the volatiles was passed through a silica

column (1 : 1 hexane–dichloromethane) affording para-carborane

1 (0.07 g, 5%) and 1-(3-hydroxy-3-methylbutynyl)-para-carborane

14 (1.67 g, 74%).

−1

=

M = 337. IR: m(CH) 2956, 2897; m(C C) 1531; 1251; 891, 846 cm .

1

NMR (298 K, CDCl₃) d (¹H) 0.39 (CH₃); d (¹³C{ H}) 130.7

=

(BrC CBr₂), 94.1 (BrC CBr₂), 0.5 (CH₃).

13. Mass (EI) spectrum: m/z 73 [Me₃Si, 100%], typical

bromine pattern 328–332, [330, M, 10%] C₈H₁₈Br₂Si₂requires:

M = 330. IR: 2971, 2956, 2896; 1251, 1242, 841 cm⁻¹. NMR

1

13

1

=

(298 K, CDCl₃) d ( H) 0.37 (CH₃); d ( C{ H}) 137.3 (C C), 0.8

(CH₃).

Preparation of 1-trimethylsilylethynyl-para-carborane (5)

A solution of para-carborane (1.44 g, 10 mmol) in monoglyme

(45 ml) was treated with BuLi (2.5 M in hexanes, 4.4 ml, 11 mmol).

Pyridine (12 ml) and CuCl (1.13 g, 11.3 mmol) were added and

the mixture heated at reﬂux (30 min), then allowed to cool while

BrC≡CSiMe₃2 was introduced (2.0 g, 11.3 mmol) and then

returned to reﬂux overnight. The cooled reaction mixture was

diluted with Et₂O (100 ml). After 4 h, the suspension was ﬁltered

and the ﬁltrate washed with dilute HCl (100 ml, 2.5 M), water

(2 × 200 ml) and dried over MgSO₄. The semi-solid obtained

after evaporation of the volatiles was dissolved in cyclohexane

and passed through a short column packed with chromatographic

silica. The cyclohexane was vacuum removed leaving a brown oil.

The oil was distilled at 30 ^◦C/0.01 mmHg to give crude para-

carborane 1 (0.18 g, 13%), then at 60 ^◦C/0.01 mmHg to give a

clear oil (1.82 g, 76% yield) identiﬁed as 1-trimethylsilylethynyl-

para-carborane 5.

14. Mass (EI) spectrum: typical carborane pattern at m/z 206–

214 [211, M − CH₃, 100%], 221–227, [226, M, 4%] C₇H₁₈B₁₀O

requires: M = 226. IR: m(OH stretch) 3321(br,s), 3242(br,s); m(cage

CH) 3060(w), 3051(m); m(methyl CH) 2983(s), 2932(m); m(BH)

2615(vs); m(C≡C) 2276(vw) cm⁻¹. NMR (298 K, CDCl₃) d (¹¹B)

11

−11.6 (5B, d, 168, B2-6), −15.2 (5B, d, 166, B7-11); d (¹H{ B})

2.63 (1H, C12H), 2.43 (5H, B2-6H), 2.19 (5H, B7–11H), 1.90 (1H,

1

OH), 1.38 (6H, CH₃); d (¹³C{ H}) 83.8 (cage C–C≡C), 78.7 (cage

C–C≡C), 69.0 (cage C–C≡C), 64.8 (COH), 59.8 (cage CH), 30.9

(CH₃). Found, C 37.8, H 8.1; C₇H₁₈B₁₀O requires: C 37.2, H 8.0%.

Preparation of 1-ethynyl-para-carborane (6) from (14)

A sample of HOMe₂CC≡CCB₁₀H₁₀CH (14) (0.45 g, 2 mmol)

suspended in 20 ml of 2-propanol was treated with 0.5 g of sodium

metal. The mixture was stirred at room temperature for 2 h, after

which time the sodium metal had been consumed and a cloudy

solution formed. The reaction mixture was diluted with diethyl

ether (40 ml), washed with water (2 × 30 ml), dilute aqueous KOH

5. Mass (EI) spectrum: typical carborane pattern at m/z 221–

228 [225, M − CH₃, 100%], 238–242, [240, M, 5%] C₇H₂₀B₁₀Si

requires: M = 240. IR: m(cageCH) 3063(m); m(methylCH) 2960(s),

2899(m); m(BH) 2617(s); m(C≡C) 2183(m) cm⁻¹. NMR (298 K,

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solution (2 × 30 ml), water (4 × 30 ml) and dried over magnesium

sulfate. Evaporation of t_◦he ether at room temperature gave an oil

which was distilled at 0 C/0.05 mmHg with the receiver cooled

at −78 ^◦C, to give 1-ethynyl-para-carborane (6) (242 mg, 72%).

If a large excess of sodium metal is used in this preparation,

ethenyl carboranes are formed in the undistilled material and

thus reduce the yield of the ethynylcarborane 7. The mixture of

ethenyl carboranes are identiﬁed by NMR and GC-MS as 1-(trans-

characterized as 1,12-bis(trimethylsilylethynyl)-para-carborane 3.

Recrystallization of this sample from ethanol gave crystals suitable

for X-ray crystallography.

Elution with diethyl ether afforded a fraction (0.6 g) containing

the two-cage carborane products 17–21. The ratio of products was

found to be 5 : 4 : 3 : 3 : 3 for 18 : 21 : 20 : 17 : 19 by GC-MS.

In order of volatility according to GC-MS:

19. Mass (EI) spectrum: typical carborane pattern at 296–

314 [309, M − H, 100%] C₆H₂₂B₂₀requires: M = 310. 17: Mass

(EI) spectrum: typical carborane pattern at 270–289, [285, M −

H, 100%], C₄H₂₂B₂₀requires: M = 286. 21: Mass (EI) spectrum:

typical carborane pattern at 403–410, [406, M − H, 14%], 386–395

[390, M − MeH, 100%] C₁₁H₃₀B₂₀Si requires: M = 407. 18: Mass

(EI) spectrum: typical carborane pattern at 379–385, [382, M − H,

6%], 363–371 [368, M − MeH, 100%] C₉H₃₀B₂₀Si requires: M =

383. 20: Mass (EI) spectrum: typical carborane pattern at 475–

482, [479, M, 5%], 459–468 [464, M − Me, 100%] C₁₁H₃₀B₂₀Si₂

requires: M = 479.

=

HOCMe₂CH CH)-1,12-C₂B₁₀H₁₁31 and 1-(Me₂C CHCH₂)-

1,12-C₂B₁₀H₁₁32.

31. Mass (EI) spectrum: 43 [MeCO, 100%] typical carborane

patterns at 209–215 [213, M − CH₃, 35%], 225–230 [228, M, 1%],

C₇H₂₀B₁₀O requires: M = 212. NMR (298 K, CDCl₃) d (¹¹B) −12.6

(5B, d, B2-6), −15.1 (5B, d, B7-11); d (¹H) 5.62 (1H, d, 15.2 Hz,

Me₂CCH), 5.43 (1H, d, 15.2 Hz, cage CCH), 2.64 (1H, C12H),

1

1.20 (6H, Me); d (¹³C{ H}) 141.4 (Me₂CCH), 125.0 (cage CCH),

81.8 (cage CCH), 70.3 (CMe₂), 58.5 (cage CH), 29.4 (CH₃).

32. Mass (EI) spectrum: 69 [C₅H₉, 100%], typical carborane

patterns at 176–183 [181, M − C₂H₇, 63%], 176–183 [197, M −

CH₃, 55%], 209–214 [212, M, 13%] C₇H₂₀B₁₀requires: M = 212.

NMR (298 K, CDCl₃) d (¹¹B) −12.6 (5B, d, B2-6), −15.1 (5B, d,

B7-11); d (¹H) 4.88 (1H, t, 7.6 Hz, CH), 2.61 (1H, C12H), 2.29

(d, 7.6 Hz, 2H, CH₂) 1.66 (3H, s, trans-Me), 1.48 (3H, s, cis Me);

Treatment of the reaction mixture with a methanolic solution

of NaOMe removed the trimethylsilyl groups, and the resulting

mixture of volatile ethynyl carboranes. Analysis of the mixture by

GC-MS conﬁrmed the formation of the new compound 22.

22. Mass (EI) spectrum: typical carborane pattern at 403–410,

[406, M − H, 14%], 386–395 [390, M − MeH, 100%] C₁₁H₃₀B₂₀Si

requires: M = 407.

1

d (¹³C{ H}) 134.5 (CMe₂), 120.2 (CH), 84.7 (cage C–CH₂), 57.9

(cage CH), 36.8 (CH₂), 25.5 (trans Me), 17.6 (cis Me).

Identiﬁcation of other products (15–22) in the preparation of

Preparation of 1,12-bis(3-hydroxy-3-methylbutynyl)-

para-carborane (23)

1,12-bis(trimethylsilylethynyl)-para-carborane (3)²⁴

A solution of para-carborane 1 (2.88 g, 20 mmol) in monoglyme

(80 ml) was treated with BuLi (2.5 M in hexanes, 16.5 ml, 41 mmol).

Pyridine (12 ml) and CuCl (4.50 g, 45 mmol) were added and

the mixture heated at reﬂux point (30 min), and allowed to

cool before BrC≡CSiMe₃2 was introduced (7.96 g, 45 mmol)

and the mixture heated at reﬂux for a further 19 h. The cooled

reaction mixture was diluted with Et₂O (400 ml). After 4 h, the

suspension was ﬁltered and the ﬁltrate washed with dilute HCl

(250 ml, 2.5 M), water (2 × 500 ml) and dried over MgSO₄. The

semi-solid obtained after evaporation of the volatiles was puriﬁed

by column chromatography on silica. Elution with cyclohexane

gave a mixture of 1,12-bis(trimethylsilylethynyl)-para-carborane 3

(0.88 g, 13% obtained after trituration of the eluant with ethanol),

5 (19%) and 15 (2%); the latter compounds being obtained by

fractional distillation (85 ^◦C/0.05 mmHg) of the ethanol washings.

A solution of para-carborane 1 (2.88 g, 20 mmol) in monoglyme

(80 ml) was treated with BuLi (2.5 M in hexanes, 16.5 ml,

41 mmol). Pyridine (12 ml) and CuCl (4.50 g, 45 mmol) were

added and the mixture heated at reﬂux (30 min), allowed to cool

and BrC≡CCMe₂OH (7) introduced (7.33 g, 45 mmol) and the

reaction mixture heated at reﬂux for a further 3 h. The cooled

reaction mixture was diluted with Et₂O (400 ml). After 2 h, the

suspension was ﬁltered and the ﬁltrate washed with water (4 ×

50 ml) and dried over MgSO₄. The oil obtained after evaporation

of the volatiles was vacuum distilled at 40 ^◦C/0.05 mmHg to

◦

give para-carborane (0.21 g, 7%) and then at 70 C/0.05 mmHg

to yield an oil which solidiﬁed on cooling. The latter solid was

identiﬁed by GC-MS and NMR to contain a 1 : 2 mixture of 1-(3-

hydroxy-3-methylbutynyl)-para-carborane 14 and 2,5-dihydroxy-

2,5-dimethyl-3,5-octadiyne 24. This solid was puriﬁed by column

chromatography on silica with 1 : 1 hexane–dichloromethane as

eluent yielding carborane 14 (0.95 g, 21% based on carborane

used) and diyne 24 (0.73 g, 20% based on ethyne).

15. Mass (EI) spectrum: typical carborane pattern at 317–

321, [319, M, 16%], 300–307 [304, M − Me, 100%] C₇H₁₉B₁₀BrSi

requires: M = 319.

The distilled fraction of 5 was shown by GC–MS to contain

minor impurities of 3 (4%) and 15 (2%). Treatment of the oil

containing 5—with 3 and 15 as minor impurities—with base gave

6 and 4 along with carborane 16. A single crystal grown from this

mixture was studied by X-ray diffraction and proved to be a solid

solution of 4 and 16 (in a ca. 2 : 1 ratio).

24. Mass (EI) m/z 166 [M, 1%], 151, [M − CH₃, 9%], 43 [100].

C₁₀H₁₄O₂requires: M = 166. NMR (298 K, CDCl₃) d (¹H) 0.06

1

(CH₃); d (¹³C{ H}) 84.2, 66.5, 65.8, 31.0 (CH₃).

Sublimation at 180 ^◦C/0.05 mmHg of the residue from the

distillation gave a white solid identiﬁed as crude 1,12-bis(3-

hydroxy-3-methylbutynyl)-para-carborane (23). This was puriﬁed

by column chromatography on silica with dichloromethane as

eluent to give 23 (0.99 g, 16%).

16. Mass (EI) spectrum: typical carborane pattern at 242–250,

[247, M, 100%] C₄H₁₁B₁₀Br requires: M = 247.

◦

Further distillation of the residue at 190 C gave an oil which

23. Mass (EI) typical carborane pattern at m/z 288–297 [293,

M − CH₃, 100%], 305–310, [308, M, 5%] C₁₂H₂₄B₁₀O₂requires:

solidiﬁed on cooling. The solid (0.36 g, 5%) was isolated and

3556 | Dalton Trans., 2006, 3544–3560

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M = 308. IR m(CH) 2981(s), 2933(m), 2868(w); m(BH) 2617(vs);

m(C≡C) 2252(vw) cm⁻¹. NMR (298 K, CDCl₃) d (¹¹B) −12.0 (d,

saturated solution of NaHCO₃(2 × 30 ml) and water (4 × 30 ml).

The ether layer was subjected to a rotary evaporator leaving a

brown residue which was eluted through a short column of silica

using hexanes. The white solid obtained after removal of hexanes

was recrystallised from hexane at 0 ^◦C to afford 117 mg (28%) of

1,4-(1^ꢀ,1^ꢀꢀ-bis(para-carboranyl)-1,3-butadiyne 27.

11

169); d (¹H{ B}) 2.43 (s, 10H, BH), 1.77 (s, 2H, OH), 1.37 (s,

1

12H, CH₃); d (¹³C{ H}) 84.8 (cage C–C≡C), 78.4 (cage C–C≡C),

65.4 (cage C), 64.8 (COH), 30.8 (CH₃). Found, C 46.9, H 7.9;

C₁₂H₂₄B₁₀O₂requires: C 46.7, H 7.9%.

27. Mass (EI) spectrum: typical carborane pattern at m/z 330–

336, [334, M, 100%] C₉H₁₈B₁₀O requires: M = 334. IR: m(cage CH)

3058(m); m(BH) 2618(vs); m(C≡C) 2266(vw) cm⁻¹. NMR (298 K,

CDCl₃) d (¹¹B) −11.7 (5B, d, 168, B2-6), −15.1 (5B, d, 167, B7-11);

Preparation of 1,12-bis(ethynyl)-para-carborane (4) from (23)

A sample of HOMe₂CC≡CCB₁₀H₁₀CC≡CCMe₂OH (23) (0.62 g,

2 mmol) dissolved in 20 ml of 2-propanol was treated with 0.5 g

of sodium metal. The mixture was reﬂuxed at room temperature

for 2 h, by which time the sodium metal had been consumed.

The solution was diluted with diethyl ether (40 ml), washed with

dilute aqueous KOH solution (2 × 30 ml), then with water (4 ×

30 ml) and dried over magnesium sulfate. Evaporation of the ether

at room temperature gave a semi-solid which was sublimed at

20 ^◦C/0.05 mmHg. A sublimate (314 mg, 82%), consisting of pure

1,12-bis(ethynyl)-para-carborane (4),²⁴was obtained.

11

d (¹H{ B}) 2.66 (1H, C12H), 2.42 (5H, B2-6H), 2.20 (5H, B7–

1

11H); d (¹³C{ H}) 75.3 (cage C–C≡C), 67.7 (cage C–C≡C), 63.0

(C≡C–C≡C), 61.0 (cage CH). Found, C 28.2, H 6.4; C₉H₁₈B₁₀O

requires: C 28.7, H 6.6%.

Formation of 1-bromo-para-carborane 29 and

1,12-dibromo-para-carborane 30

A solution of para-carborane 1 (0.72 g, 5 mmol) in diehtyl ether

(20 ml) was treated dropwise with BuLi (2.5 M in hexanes,

4.1 ml, 11 mmol). The resulting cloudy solution was heated at

reﬂux (30 min), allowed to cool and BrC≡CSiMe₃(2) (2.0 g,

11 mmol) introduced and the reaction mixture returned to

reﬂux for a further 19 h. The solution was diluted with ether

(100 ml) and water (50 ml). The ether layer was separated, washed

with water and dried over MgSO₄, and the solvent removed

in vacuo to give an orange oil. The orange oil was distilled at

40 ^◦C/0.01 mmHg to give unreacted para-carborane 1 (0.08 g,

11%) then at 80 ^◦C/0.01 mmHg to give 1-bromo-para-carborane

29 (0.28 g, 25%). A further distillate (0.87 g, 57% yield) was

obtained at 180 ^◦C/0.01 mmHg and dissolved in chloroform. Slow

evaporation of the solvent resulted in large crystals of pure 1,12-

dibromo-para-carborane 30.

Preparation of 1-(5-hydroxy-5-methyl-1,3-hexadiynyl)-

para-carborane (26)

A stirred solution of ethynylcarborane 14 (1.13 g, 5 mmol) in

monoglyme (20 ml) was treated with BuLi (1.6 M in hexanes,

6.5 ml, 11 mmol) at ambient temperature. Pyridine (8 ml) and CuCl

(0.63 g, 6.4 mmol) were added to the solution and the mixture

heated at reﬂux for 30 min then allowed to cool. A solution of

BrC≡CCMe₂OH (7, 1.0 g, 5.2 mmol) in monoglyme (10 ml) was

added dropwise to the vigorously stirred solution at 0 ^◦C, and

the mixture heated at reﬂux for 30 min. The reaction mixture

was cooled, and diluted with Et₂O (400 ml). After ﬁltering off

the solids, the oil obtained from the ﬁltrate after evaporation of

the volatiles was passed through a silica column (1 : 1 hexane–

dichloromethane). The white solid obtained was recrystallised

from 2 : 1 hexane–dichloromethane at 0 ^◦C to afford 0.70 g (56%)

of 1-(5-hydroxy-5-methyl-1,3-hexadiynyl)-para-carborane 26.

29. Mass (EI) typical carborane and bromine pattern at 218–

226, C₂H₁₁B₁₀Br requires: M = 223. IR: m(CH) 3062; m(BH)

2616 cm⁻¹. NMR (298 K, CDCl₃) d (¹¹B) −11.0 (5B, d, 172, B2-6),

11

−14.9 (5B, d, 168, B7-11); d (¹H{ B}) 2.73 (1H, C12H), 2.65 (5H,

26. Mass (EI) spectrum: m/z 43 [MeCO, 100%], typical

carborane patterns at m/z 230–237 [235, M − CH₃, 60%], m/z

246–252, [250, M, 11%] C₉H₁₈B₁₀O requires: M = 250. IR: m(OH

stretch) 3355(br,s); m(cage CH) 3068(m); m(methyl CH) 2981(s),

2932(m); m(BH) 2617(vs); m(C≡C) 2262(vw) cm⁻¹. NMR (298 K,

CDCl₃) d (¹¹B) −11.7 (5B, d, 168, B2-6), −15.2 (5B, d, 167,

1

B2-6H), 2.19 (5H, B7–11H); d (¹³C{ H}) 58.2 (CBr), 56.5 (CH).

30. Mass (EI) typical carborane and bromine pattern at 297–

306, C₂H₁₀B₁₀Br₂requires: M = 303. IR: m(BH) 2616 cm⁻¹. NMR

11

(298 K, CDCl₃) d (¹¹B) −11.0 (d, 174); d (¹H{ B}) 2.65 (BH); d

1

(¹³C{ H}) 54.8.

11

B7-11); d (¹H{ B}) 2.66 (1H, C12H), 2.45 (5H, B2-6H), 2.17

(5H, B7–11H), 1.72 (1H, OH), 1.48 (6H, CH₃); d (¹³C{ H}) 84.9

1

Gas-phase electron diffraction

(HOMe₂C–C≡C), 74.8 (cage C–C≡C), 68.3 (cage C–C≡C), 65.9

(C≡C–C≡C), 65.5 (COH), 63.6 (C≡C–C≡C), 60.6 (cage CH),

30.9 (CH₃). Found, C 42.0, H 7.4; C₉H₁₈B₁₀O requires: C 43.2, H

7.3%.

GED data for both substituted para-carboranes 5 and 6 were

collected at two different camera distances using the Edinburgh

apparatus.⁶⁷The data were recorded photographically on Kodak

Electron Image ﬁlms. Three ﬁlms from each distance were studied

for 6 whereas four and three plates at long and short distances,

respectively, were used for 5. The electron-scattering patterns were

converted into digital form using an Epson Expression 1600 Pro

ﬂatbed scanner with a scanning program as described elsewhere.⁶⁸

Electron wavelengths were determined from the scattering pattern

of benzene vapour recorded immediately after each carborane

run. The weighing points for the off-diagonal weight matrices,

correlation parameters, and scale factors for the two camera

distances are given in Table 10.

Preparation of 1,4-(1^ꢀ,1^ꢀꢀ-bis(para-carboranyl))-1,3-butadiyne (27)

A stirred solution of ethynylcarborane 14 (0.55 g, 2.5 mmol) in

monoglyme (10 ml) was treated with BuLi (1.6 M in hexanes,

3.25 ml, 5.5 mmol) at ambient temperature. Pyridine (3 ml) and

CuCl (0.63 g, 6.4 mmol) were added to the solution and the mixture

heated at reﬂux for 2 h then allowed to cool. Oxygen was bubbled

through the vigorously stirred solution at 0 ^◦C for 30 min. The

reaction mixture was diluted with Et₂O (80 ml) and washed with a

This journal is

The Royal Society of Chemistry 2006

Dalton Trans., 2006, 3544–3560 | 3557

©

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Table 10 GED data analysis parameters for 5 and 6

The structures were solved by direct methods (combined with

Patterson method for 13 and 4/16) and reﬁned by full-matrix

least squares against F²of all data, using SHELXTL software

package.⁷⁴See Table 11 for crystallographic data.

5

6

Camera distance/mm

260.92

96.63

261.00

95.26

The molecule of 13 possesses a crystallographic inversion centre

T

_sample/K

_nozzle/K

400

413

1

20

40

424

426

2

348

358

1

20

40

365

385

2

=

at the midpoint of the C C bond and an approximate 2/m (C_2h)

Ds/nm⁻¹

symmetry. This is the ﬁrst crystal structure of a symmetrically

s

_min/nm⁻¹

80

substituted trans-dibromoethene, in which the C C moiety is not

=

sw₁/nm⁻¹

sw₂/nm⁻¹

100

258

300

0.4106

100

288

334

0.4759

disordered between two mutually perpendicular orientations⁷⁵and

thus its geometry can be accurately determined. The alkene C(1)

atom is essentially planar, though much distorted from the regular

trigonal, coordination.

129

150

0.4968

0.784(4) 0.828(11)

6.02 6.02

131

152

0.4991

0.755(4) 0.716(9)

6.02 6.02

s

_max/nm⁻¹

Correlation parameter

Scale factor, k

Electron wavelength/pm

CCDC reference numbers 292697–292701.

For crystallographic data in CIF or other electronic format see

DOI: 10.1039/b517538k

The data reduction and structure analyses were performed using

the standard program ed@ed,⁶⁹employing the scattering factors

of Ross et al.⁷⁰

Theoretical methods

The starting parameters for the r_h1reﬁnement were taken

from the theoretical geometry optimised at the MP2(fc)/6-31G*

level for 5 and MP2(fc)/6-311G* for 6. For both compounds a

theoretical Cartesian force ﬁeld was obtained with B3LYP/cc-

pVDZ⁷¹and converted into a force ﬁeld described by a set of

symmetry coordinates using the program SHRINK.⁷²It must be

noted that no major differences were seen between the Pople basis

set and Dunning’s type in the Cartesian force ﬁeld.

All ab initio computations were carried out with the Gaussian

98 package.⁷⁶All geometries discussed here were optimised at the

HF/6-31G* level of theory with symmetry constraints. Frequency

calculations at HF/6-31G* levels of theory were carried out and

conﬁrmed that the symmetry assumed for all compounds, were

local minima. Optimisation of these geometries were then carried

out at the MP2/6-31G* level of theory. Cartesian coordinates of

the MP2/6-31G* optimized geometries in Table 9 are in the ESI.†

Geometry optimizations for compounds 5 and 6 were carried

out using C_sand C_5msymmetry, respectively. Three different

methods were used; HF and two methods including electron

correlation, B3LYP and MP2. For these methods, basis sets

ranging from 6-31G* to aug-cc-pVDZ were used. Diffuse and

polarisation functions were also added to the basis sets. The

resultant geometrical parameters are given in ESI.† Frequency

calculations at HF/6-31G*, B3LYP/6-31G* and B3LYP/cc-

pVDZ levels of theory allowed the nature of the stationary

points to be determined, conﬁrming that the observed C_5vand

C_sstructures are true minima on the potential energy surfaces.

Crystallography

Single-crystal X-ray diffraction experiments were carried out on

Bruker AXS 3-circle diffractometers with a 1 K CCD or (for 28) an

APEX CCD area detector, using graphite monochromated Mo-

¯

˚

K_aradiation (k = 0.71073 A) from a sealed tube or (for 28) a 60 W

R

Mo-target microfocus Bede Microsource^ꢀX-ray generator with

glass polycapillary X-ray optics. Reﬂection intensities of 28, 30

and 4/16 were corrected for absorption by semi-empirical method

based on Laue equivalents, using the SADABS program,⁷³and

for 13 by numerical integration based on crystal face indexing.

Table 11 Crystal data for 3, 13, 28, 30, and solid solution of 16 in 4

3

13

28

30

4/16

1

3

Formula

M

C₁₂H₂₈B₁₀Si₂

336.62

Monoclinic

21.734(3)

7.535(1)

6.937(1)

90

107.77(1)

90

1081.8(3)

100

C2/m (#12)

2

0.16

6764

1554

C₈H₁₈Br₂Si₂

330.22

Orthorhombic

6.4980(4)

11.101(1)

18.698(2)

90

C₄H₁₂B₁₀

168.24

Monoclinic

7.0131(4)

20.751(1)

7.0409(4)

90

98.85(1)

90

1012.5(1)

120

P2₁/n (#14)

4

0.05

12934

3055

C₂H₁₀B₁₀Br₂

302.02

Monoclinic

7.0941(4)

7.1764(4)

10.851(1)

90

104.00(1)

90

536.02(7)

120

P2₁/n (#14)

2

7.50

6070

1495

²C₆H₁₂B₁₀· C₄H₁₁B₁₀Br

3

210.31

Crystal system

Triclinic

7.2696(8)

7.5132(8)

10.932(1)

83.05(1)

73.66(1)

80.34(1)

563.1(1)

120

˚

a/A

˚

b /A

˚

c/A

a/^◦

b /^◦

c /^◦

V/A

T/K

90

3

˚

1348.8(2)

120

Pbca (#61)

4

6.15

15332

1802

0.052

0.020

0.050

¯

Space group (no.)

Z

P1 (#2)

2

l/mm⁻¹

1.22

3946

1773

0.050

0.071

0.228

Reﬂns measured

Reﬂns unique

R_int

0.031

0.083

0.027

0.046

0.134

0.024

0.022

0.055

R(F), I > 2r(I)

wR(F²), all data

3558 | Dalton Trans., 2006, 3544–3560

This journal is

The Royal Society of Chemistry 2006

©

View Article Online

27 H. Buchert and W. Zeil, Spectrochim. Acta, 1962, 18, 1043.

28 J. A. Miller and G. Zweifel, Synthesis, 1983, 128.

Acknowledgements

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The authors wish to thank the EPSRC for ﬁnancial support. We

also thank Dr Sarah Hinchley for scanning the ﬁlms and for

considerable assistance in preparing data for publication and Mr

Thomas E. Baines for support in the synthetic work carried out

here.

33 M. J. Dabdaub, A. C. M. Baroni, E. J. Lenardao, T. R. Gianeti and

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34 A. G. Douglass, S. Pakhomov, B. Reeves, Z. Janousek and P. Kaszynski,

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3560 | Dalton Trans., 2006, 3544–3560

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Synthetic and structural studies on C-ethynyl- and C-bromo-carboranes

DOI: 10.1039/b517538k

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