Synthesis, reactivity and structural studies of selenide bridged carboranyl compounds

Article abstract of DOI:10.1039/b610944f

Reaction of the lithium salt Li[1-R-1,2-closo-C₂B ₁₀H₁₀] with selenium under mild conditions, followed by hydrolysis gave the diselenide compound (1-Se-2-R-1,2-closo-C₂B ₁₀H₁₀)₂ in contrast to the well-reported mercapto compounds 1-SH-2-R-1,2-closo-C₂B₁₀H₁₀ obtained using a similar synthetic procedure. Details for the preparation and X-ray structural characterisation of the new compounds (2-Me-1,2-closo-C ₂B₁₀H₁₀)₂Se (1), (1-Se-2-R-1,2-closo-C₂B₁₀H₁₀)₂ (R = Me,2, Ph, 3) are specified. To further explore the mechanism of the dimerization reaction, the complex [Au(1-Se-2-Me-1,2-closo-C₂B₁₀H ₁₀)(PPh₃)] 4 was synthesized, confirming the existence of the intermediate Li[1-Se-2-R-1,2-closo-C₂B₁₀H ₁₀] at the early stages of the reaction before selenium oxidation. Theoretical calculations and cyclic voltammetry (CV) studies were carried out to compare the bonding nature of the sulfur and the selenium analog compounds. It was determined that diselenides have a higher tendency to reduce with respect to the disulfides and all chalcogen atoms were found to be positively charged. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b610944f

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www.rsc.org/dalton | Dalton Transactions
Synthesis, reactivity and structural studies of selenide bridged carboranyl
compounds†‡
Anna Laromaine,a Francesc Teixidor,a Raikko Kiveka¨s,b Reijo Sillanpa¨a¨,c Massimiliano Arca,d Vito Lippolis,d
a
Eula`lia Crespo§ and Clara Vin˜as*a
Received 31st July 2006, Accepted 14th September 2006
First published as an Advance Article on the web 5th October 2006
DOI: 10.1039/b610944f
Reaction of the lithium salt Li[1-R-1,2-closo-C2B10H10] with selenium under mild conditions, followed
by hydrolysis gave the diselenide compound (1-Se-2-R-1,2-closo-C2B10H10)2 in contrast to the
well-reported mercapto compounds 1-SH-2-R-1,2-closo-C2B10H10 obtained using a similar synthetic
procedure. Details for the preparation and X-ray structural characterisation of the new compounds
(2-Me-1,2-closo-C2B10H10)2Se (1), (1-Se-2-R-1,2-closo-C2B10H10)2 (R = Me,2, Ph, 3) are specified. To
further explore the mechanism of the dimerization reaction, the complex [Au(1-Se-2-Me-1,2-closo-
C2B10H10)(PPh3)] 4 was synthesized, confirming the existence of the intermediate
Li[1-Se-2-R-1,2-closo-C2B10H10] at the early stages of the reaction before selenium oxidation.
Theoretical calculations and cyclic voltammetry (CV) studies were carried out to compare the bonding
nature of the sulfur and the selenium analog compounds. It was determined that diselenides have a
higher tendency to reduce with respect to the disulfides and all chalcogen atoms were found to be
positively charged.
or zinc reagents is remarkably interesting. After being introduced
in an organic substrate, the organoselenium group can easily
be removed by selenoxide syn elimination9 and [2,3] sigmatropic
Introduction
Although the first organoselenium compound was prepared by
Wo¨hler and Siemens in 1847,1 only in the early 1970s did the
chemistry of organoselenium compounds became a versatile tool
in organic chemistry.2 After that, the organoselenium chemistry
developed rapidly, mainly in the area of selenocarbohydrates,
selenoamino acids, and selenopeptides. The selenium group can
be introduced in an organic substrate via both nucleophilic
and electrophilic reagents. Organoselenium anions are powerful
nucleophiles and are usually prepared “in situ” because of their
sensitivity to air oxidation.3 They can be prepared from diaryl
diselenides by reaction with reducing agents, of which NaBH4
is the most used,4 from insertion of elemental selenium into
lithium and Grignard reagents,5 and from diorganoyl diselenides
by reduction with alkali metals6 or alkali hydrides.7 Vinyl selenides
are also important reagents and intermediates in organic synthesis8
and among a large list of described transformations, the ability of
these compounds to participate in the formation of carbon–carbon
bonds by Ni-catalyzed cross-coupling reactions with magnesium
rearrangement.10 In addition, the carbon–selenium bond can also
be replaced by a carbon–hydrogen,11 carbon–halogen,12 carbon–
lithium,13 or carbon–carbon bond.14 Thus, in general, organose-
lenium species can be efficiently introduced, manipulated, and
removed in a variety of ways under mild conditions.
o-Carborane can be viewed as being similar to a benzene ring.
Some researchers consider that boron clusters have a pseudo-
aromatic character which parallels the benzene’s aromaticity.15
Furthermore the size of a carborane is comparable to a rotating
benzene. In the way that diaryldiselenides exist, dicarboranyldis-
elenides may also be produced. In this paper we report on the
synthesis of the first dicarboranyldiselenides, their characteriza-
tion by spectroscopic, X-ray diffraction techniques, and DFT
calculations, and we explore some of their reacting possibilities.
Carboranyl selenium chemistry is in its infancy. Up to now
research in which Se is bonded to the carborane cluster has been
restricted to the coordination chemistry of [Se2C2B10H10]2−,16 and
[SeC2B10H11]−.17
aInstitut de Cie`ncia de Materials de Barcelona, Campus U.A.B., 08193,
Bellaterra, Spain
bDepartment of Chemistry, P.O. Box 55, FIN-00014, University of Helsinki,
Finland
Results and discussion
cDepartment of Chemistry, University of Jyva¨skyla¨, FIN-40531, Finland
Synthesis and spectroscopic studies
dDipartimento di Chimica Inorganica ed Analitica, Universita` degli Studi di
Cagliari, S.S. 554 Bivio per Sestu, 09042, Monserrato (CA), Italy
Although the use of organoselenols as reagents is a drawback in
synthesis owing to difficulties in their preparation and handling
since they are very volatile, bad-smelling, toxic and air-sensitive
compounds, they can not be disregarded. This is why the
synthesis of the synthetically important selenol 1-SeH-2-R-1,2-
closo-C2B10H10 (R = Me, Ph) was the first target in this research.
We assumed that the synthetic procedure would be similar to the
† The HTML version of this article has been enhanced with colour images.
‡ Electronic supplementary information (ESI) available: Optimized bond
lengths, angles, and dihedral angles (Table S1); selected NBO-charges
and Wiberg bond indexes (Table S2); HOMO–LUMO Kohn–Sham
eigenvalue diagram for (2-Me-1,2-closo-C2B10H10)2S (5), (1-S-2-Me-1,2-
closo-C2B10H10)2 (6), (2-Me-1,2-closo-C2B10H10)2Se (1), (1-Se-2-Me-1,2-
closo-C2B10H10)2 (2), I2, IBr, and Br2 (Fig. S1). See DOI: 10.1039/b610944f
§ E. Crespo is enrolled in the UAB PhD program.
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related thiol 1-SH-2-R-1,2-closo-C2B10H10 (R = Me, Ph).18 Thence
the Li[2-R-1,2-closo-C2B10H10] (R = Me, Ph) salt was prepared by
direct metalation of 1-R-1,2-closo-C2B10H11 with nBuLi (1 : 1 ratio)
in diethyl ether followed by the addition of finely ground selenium
powder. It was expected that 1-SeH-2-R-1,2-closo-C2B10H10 would
be obtained after acidification of the medium but attempts to trap
or detect these species were unsuccessful (Scheme 1). The addition
of hydrochloric acid and the subsequent work-up gave yellow
solids. 77Se NMR solution analysis of the Me substituted solid in
CDCl3 has shown a resonance at 389 ppm. No resonance for Se–H
hydrochloric acid in the final stage, almost one equivalent of
selenium as a red amorphous species, which rapidly turns gray
and shiny (≈95% recovery after centrifugation after 30 min)
1
separated from the reaction mixture. The 11B{ H} NMR analysis
of the reaction solution after metallation of 1,2-closo-C2B10H12
and selenium addition, clearly shows the presence of a species
different from Li[1,2-closo-C2B10H11], probably Li[1-Se-2-H-1,2-
closo-C2B10H11], that goes back to the starting 1,2-closo-C2B10H12
after protonation via deselenation. This result is in contradiction
with a recent publication17 that reports the synthesis and isolation
of the moisture- and air-stable selenol 1-SeH-1,2-closo-C2B10H11
by using the same method described by us for 1-SH-1,2-closo-
C2B10H11. However, our results agree with Zakharkin et al. that
reported that selenide compounds oxidize easily with oxygen.21
The formation of the diselenide (1-Se-2-R-1,2-closo-C2B10H10)2
(R = Me, Ph) compounds seems therefore to be caused by the
oxidative work up. As it was thought that the proton from the hy-
drochloric acid solution was the oxidizing agent, protonation was
tried using either H2O or deoxygenated H2O, however, in all cases
stable diselenide (1-Se-2-R-1,2-closo-C2B10H10)2 (R = Me, Ph)
compounds were obtained. In order to prove that Li[1-Se-2-R-1,2-
closo-C2B10H10] was the first step of the reaction and confirm that
selenium oxidation takes place irrespective of using either acid or
H2O, a complexation reaction was performed. To a solution of 1-
methyl-o-carborane in dry diethyl ether was added n-BuLi and se-
lenium powder. Once the black selenium disappeared, [AuClPPh3]
was added and a solid precipitated (Scheme 2). The yellowish
solid [Au(1-Se-2-Me-1,2-closo-C2B10H10)(PPh3)] was characterized
1
was observed in the 1H NMR spectra. Besides, the 11B{ H} NMR
spectra were different from the starting compounds. Diselenides
formation, (1-Se-2-R-1,2-closo-C2B10H10)2 (R = Me 2, Ph 3), was
then postulated.
1
1
by elemental analysis, IR, 1H-, 11B-, 31P{ H}- and 13C{ H}-
NMR spectroscopy. The IR displayed m(B–H) bands in the range
2590–2561 cm−1 which is consistent with the closo fragment. In
Scheme 1 Synthesis of diselenide species (1-Se-2-R-1,2-closo-C2B10H10)2
(R = Me, Ph).
1
agreement with this, the 11B{ H}-NMR spectrum appears in the
The IR spectra display typical m(B–H) absorption at frequencies
above 2600 cm−1, characteristic of closo carboranes. The 1H-, 11B-
ranges −5.96 and −9.12 ppm, typical for a closo cluster. The
1
31P{ H}-NMR spectrum displays one signal at 37.4 ppm. Full
1
and 13C{ H}-NMR data of these compounds in CDCl3 are in
proof of complexation by [1-Se-2-Me-1,2-closo-C2B10H10]− was
obtained after good crystals for X-ray diffraction were grown from
a saturated solution in diethyl ether.
complete agreement with the diselenide proposed structures which
were later confirmed by X-ray crystallography. The MALDI-TOF-
MS spectra were recorded for (1-Se-2-Me-1,2-closo-C2B10H10)2
and (1-Se-2-Ph-1,2-closo-C2B10H10)2. Molecular ion peaks with
the right isotopic distribution were observed for both compounds
although in low intensity. The peak observed at m/z = 472
(M, 14%) for (1-Se-2-Me-1,2-closo-C2B10H10)2 corresponds to
1
11
the diselenide 12C6 H26
found at m/z = 237 (M/2, 100%) corresponds to symmetrical
fragmentation to 12C3 H13 78Se+. For compound (1-Se-2-Ph-
B
10B278Se2 whereas the highest peak
18
Scheme
2
Reaction of Li[1-Se-2-Me-1,2-closo-C2B10H10] with
1
11
B
10
[AuClPPh3].
1,2-closo-C2B10H10)2 the molecular ion peak at 597 (M, 2%) is
very weak but the one at m/z = 299 (M/2, 100%) corresponding
to symmetrical fragmentation is the highest. This could suggest
a weaker Se–Se bond in the latter diselenide either due to an
intramolecular Se · · · X interaction, or to the higher electron
withdrawing ability of the Ph ring that would cause depletion
of electron density from the Se–Se bond.
Preparation of 1-SH-1,2-closo-C2B10H11 is complicated by the
existing equilibrium, in certain solvents, between the unmetal-
lated, monolithiated, and dilithiated o-carborane species.19 In
dimethoxyethane, however, 1-SH-1,2-closo-C2B10H11 can be ob-
tained in 93% yield.20 We attempted the synthesis of 1-SeH-
1,2-closo-C2B10H11 in this solvent under the same conditions
as reported for 1-SH-1,2-closo-C2B10H11.19 Upon addition of
Treatment of the diselenide (1-Se-2-R-1,2-closo-C2B10H10)2 com-
pounds (R = Me, Ph) with a stoichiometric amount of Li[2-R-
1,2-closo-C2B10H10] (R = Me, Ph) gave the symmetrical selenide
(2-Me-1,2-C2B10H10)2Se only for R = Me (Scheme 3). The stable
(2-Me-1,2-closo-C2B10H10)2Se derivative was obtained as a yellow
crystalline solid. The byproduct Li[1-Se-2-R-1,2-closo-C2B10H10]
can be recovered as the starting diselenide by oxidation with water
(Scheme 4).
The 77Se NMR chemical shift is informative about the Se · · · X
non-bonding interactions because the resonance is downfield
shifted.22 The 77Se NMR chemical shift of (1-Se-2-Me-1,2-closo-
C2B10H10)2 (2) exhibits a single resonance at 389 ppm., which is
downfield with respect to the diaryldiselenides, naphthyldiselenide
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Scheme 3 Synthesis of selenide species (2-Me-1,2-closo-C2B10H10)2Se (1).
of 2·toluene has crystallographic two-fold symmetry with the
symmetry axis going through the midpoint of the Se–Se bond,
but 3 has only very approximate two-fold symmetry as mutual
orientations of the phenyl groups are markedly different (Fig. 3
and 4). Expectedly, in each compound, most of the corresponding
bond lengths and angles in both halves of the molecule are very
similar and only a few minor differences can be noticed (Tables 1
and 2).
Scheme 4 Recovery of (1-Se-2-Me-1,2-closo-C2B10H10)2 (2) from the
synthesis of 1.
and diphenyldiselenide that appear at 436 and 463 ppm
1
respectively.23 The H NMR spectrum of (1-Se-2-Me-1,2-closo-
C2B10H10)2 show two singlets, of equal intensity one for each methyl
group, at 2.08 and 1.86 ppm.
Molecular and crystal structures of (2-Me-1,2-closo-C2B10H10)2Se
(1), (1-Se-2-Me-1,2-closo-C2B10H10)2 (2), (1-Se-2-Me-1,2-closo-
C2B10H10)2·toluene (2·toluene), (1-Se-2-Ph-closo-1,2-C2B10H10)2
(3) and [Au(1-Se-2-Me-1,2-closo-C2B10H10)(PPh3)] (4)
Symmetry of the four non-complexed selenide and diselenide
compounds varies considerably. 1 and 2 have pseudo two-fold
symmetry with the pseudo axis going through the selenium atom
in the former compound and though the midpoint of the Se–Se
bond in latter compound (Fig. 1 and 2). The diselenide molecule
Fig. 3 A view of the diselenide molecule of (1-Se-2-Me-1,2-closo-
C2B10H10)2·toluene (2·toluene) showing 50% displacement ellipsoids. Hy-
drogen atoms are omitted for clarity. Superscript “a” refers to equivalent
position −x, y, −z + 1/2.
Fig. 1 A view of (2-Me-1,2-closo-C2B10H10)2Se (1) showing 50% displace-
ment ellipsoids. Hydrogen atoms are omitted for clarity.
Fig. 4 A view of (1-Se-2-Ph-1,2-closo-C2B10H10)2 (3) showing 50%
displacement ellipsoids. Hydrogen atoms are omitted for clarity.
Molecular conformations of the selenide and diselenide com-
pounds are very different. The C(13)–C(2) · · · C(22)–C(33) dihe-
dral angle is −151.5(2)◦ for 1 but the corresponding angles for
the diselenide bridged compounds 2, 2·toluene and 3 are 3.1(4),
8.65(15) and −12.7(4)◦, respectively. The second selenium atom
in the bridge is the reason for the differences. The observed
Fig. 2 A view of (1-Se-2-Me-1,2-closo-C2B10H10)2 (2) showing 50%
displacement ellipsoids. Hydrogen atoms are omitted for clarity.
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◦
˚
Table 1 Selected bond lengths (A), angles and torsion angles ( ) for (2-Me-1,2-closo-C2B10H10)2Se (1)
Se–C(1)
Se–C(21)
C(1)–C(2)
1.937(3)
1.933(3)
1.699(3)
C(21)–C(22)
C(2)–C(13)
C(22)–C(33)
1.712(3)
1.511(4)
1.514(4)
C(1)–Se–C(21)
Se–C(1)–C(2)
Se–C(21)–C(22)
Se–C(1)–C(2)–C(13)
Se–C(21)–C(22)–C(33)
C(2)–C(1)–Se–C(21)
110.11(11)
117.67(16)
117.52(17)
−10.9(3)
−12.2(3)
96.71(19)
C(1)–C(2)–C(13)
C(21)–C22)–C33)
118.9(2)
118.8(2)
C(22)–C(21)–Se–C(1)
C(13)–C(2) · · · C(22)–C(33)
97.07(18)
−151.5(2)
◦
◦
˚
˚
Table 2 Selected bond lengths (A), angles and torsion angles ( ) for (1-Se-
2-Me-1,2-closo-C2B10H10)2 (2), (1-Se-2-Me-1,2-closo-C2B10H10)2·toluene
(2·toluene) and (1-Se-2-Ph-1,2-closo-C2B10H10)2 (3). For 2·toluene, Se(2),
C(21), C(22) and C(33) relate to Se(1), C(1), C(2) and C(13) at equivalent
position −x, y, −z + 1/2
Table 3 Selected interatomic distances (A), angles and torsion angles ( )
for [Au(1-Se-2-Me-1,2-closo-C2B10H10)(PPh3)] (4)
Au–Se
2.4254(6)
3.4783(6)
P–C(20)
P–C(26)
2.2740(13) C(1)–C(2)
1.821(5)
1.806(5)
1.703(7)
1.508(7)
Au · · · Sea
Au–P
Compound
2
2·toluene
2.3026(3)
3
Se–C(1)
P–C(14)
P–Au–Se
1.955(5)
1.814(5)
174.82(4)
94.86(2)
97.40(15)
−1.9(6)
C(2)–C(13)
Se–C(1)–C(2)
C(13)–C(2)–C(1)
119.1(3)
118.1(4)
Se(1)–Se(2)
2.3012(8)
1.948(5)
1.953(5)
1.701(7)
1.702(7)
1.518(7)
1.516(7)
2.3038(7)
1.945(5)
1.949(5)
1.726(6)
1.741(7)
1.499(7)
1.504(7)
Se–Au · · · Sea
Au–Se–C(1)
Se–C(1)–C(2)–C(13)
Se(1)–C(1)
Se(2)–C(21)
C(1)–C(2)
C(21)–C(22)
C(2)–C(13)
C(22)–C(33)
1.9526(11)
1.6897(17)
1.5157(18)
Au–Se–C(1)–C(2)
107.2(3)
a Relative to equivalent position −x, −y, 1 − z.
Se(2)–Se(1)–C(1)
Se(1)–Se(2)–C(21)
Se(1)–C(1)–C(2)
Se(2)–C(21)–C(22)
C(1)–C(2)–C(13)
C(21)–C(22)–C(33)
C(1)–Se(1)–Se(2)–C(21)
C(2)–C(1)–Se(1)–Se(2)
C(22)–C(21)–Se(2)–Se(1)
Se(1)–C(1)–C(2)–C(13)
Se(2)–C(21)–C(22)–C(33)
C(13)–C(2) · · · C(22)–C(33)
102.23(15)
103.16(15)
117.6(3)
115.9(3)
118.8(4)
119.2(4)
99.1(2)
111.2(3)
109.8(3)
−5.8(6)
−4.4(6)
3.1(4)
101.94(4)
102.58(13)
103.54(14)
115.3(3)
115.4(3)
120.2(4)
120.2(4)
−98.9(2)
−113.7(3)
−116.3(3)
4.8(5)
The structure of [Au(1-Se-2-Me-1,2-closo-C2B10H10)(PPh3)] (4)
confirmed complexation of the [1-Se-2-Me-1,2-C2B10H10]− ligand.
In 4 the metal is almost linearly bonded to the phosphorous atom
of the bulky PPh3 group and the selenate ion with a P–Au–Se bond
angle of 174.82(4)◦ (Table 3). In the solid state, two [Au(1-Se-2-
Me-1,2-closo-C2B10H10)(PPh3)] units form a dinuclear complex,
115.58(8)
118.91(10)
101.94(7)
111.53(9)
−4.61(16)
8.65(15)
˚
where two Se · · · Au bridging contacts of 3.4783(6) A between two
1.6(5)
−12.7(4)
neighbouring molecules exist (Fig. 5). These contact distances are
˚
ca. 0.12 A shorter that the sum of the corresponding van der Waals
radii.26 The Au · · · Au distance of 4.0684(4) A indicates that there
˚
is no direct Au · · · Au interaction in the dinuclear unit similar to
that found in [Au(1-Se-2-H-1,2-closo-C2B10H10)(PPh3)].17
dihedral values are comparable with those of the corresponding
thioether and disulfide compounds.24 In the diselenide compounds
the mutual conformations are quite similar, e.g the absolute values
of the Cc–Cc–Se–Se torsion angles (Cc refers to cluster carbon) are
very close to each other varying only within 6.5◦.
In
1
the C(1)–Se–C(21) angle of 110.11(11)◦ is near
to the value of 109.6(2)◦ found in (C5Me5)2Se (C5Me5
=
pentamethylcyclopentadienyl)25 and also very close to the value of
a tetrahedral angle. However, there are several short intramolec-
ular contacts of H atoms between the cages like H(5) · · · H(26)
˚
˚
˚
(2.54 A), H(5) · · · H(33C) (2.54 A), H(6) · · · H(25) (2.49 A)
˚
H(6) · · · H(26) (2.41 A). Expectedly, the Cc–S–Cc angle in the
corresponding thioether (2-Me-1,2-closo-C2B10H10)2S is opened
[113.14(15)◦] because of increased steric repulsion between the
cages.24
There are minor Cc–Cc bond length differences between the
methyl and phenyl substituted selenide and diselenide compounds.
In the 2-Me substituted compounds the distances vary from
˚
1.6897(17) to 1.712(3) A, while in 3 the distances are 1.726(6)
Fig.
5
A
view
of
[Au(1-Se-2-Me-1,2-closo-C2B10H10)(PPh3)]
˚
and 1.741(7) A. Even the differences are minor, the trend observed
(4) showing the formation of the centrosymmetric dinuclear
[{Au(1-Se-2-Me-1,2-closo-C2B10H10)(PPh3)}2] unit. Thermal displacement
ellipsoids are drawn at 50% level. Superscript “a” refers to equivalent
position −x, −y, 1 − z, and hydrogen atoms are omitted.
agrees with the observation that contribution of an aryl substituent
to the lengthening Cc–Cc bond is greater than that of the non-aryl
substituent.
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Theoretical calculations on (2-Me-1,2-closo-C2B10H10)2Se (1),
(1-Se-2-Me-1,2-closo-C2B10H10)2 (2), (2-Me-1,2-closo-C2B10H10)2S
(5), and (1-S-2-Me-1,2-closo-C2B10H10)2 (6)
dichloromethane for 2 and 6, respectively), indicating a higher
tendency to reduction of diselenides with respect to disulfides. In
addition, the compositions of the LUMO’s of 2 and 6 along with
the positive NBO-charges carried by the chalcogen atoms (Q =
+0.331 and +0.224 e for 2 and 6, respectively; see ESI, Table S2‡)
clearly account for their reactivity towards nucleophilic species,
thus explaining the formation of 1 (Scheme 3) and 5 by reaction
with Li(1-E-2-Me-1,2-closo-C2B10H10) (E = S, Se).24 As expected
on the basis of HSAB (hard–soft acid–base) theory, the softness
of the compounds, that can be evaluated from the HOMO and
LUMO eigenvalues (see ESI, Fig. S1‡) increases in the order 5 >
1 > 6 > 2, i.e. it increases on passing from E = S to E = Se, and
from dicarboranylchalcogenides to dichalcogenides. The charge
distribution and the conformation of the compounds prevent any
basic behaviour. In fact, in all compounds the chalcogen atoms
are remarkably positively charged, and the lobes of the p orbitals
of chalcogen species within the HOMO’s are not accessible due to
the steric hindrance of the R substituents (R = Me for 1, 2, 5, and
6) at the boron clusters.32
With the aim of getting a deeper insight into the nature of the bond-
ing in the title molecular systems, theoretical calculations have
been performed on 1, 2, and on the sulfur analogues (2-Me-1,2-
closo-C2B10H10)2S (5) and (1-S-2-Me-1,2-closo-C2B10H10)2 (6).24 All
calculations have been performed at density functional theory
(DFT)27 level using the well-known B3LYP three-parameter
hybrid functional,28 which consists of hybrid Becke exchange
functionals and Lee, Yang, and Parr correlation functionals. For
all atomic species, the full-electron Schafer, Horn and Ahlrichs
pVDZ basis sets29 have been used.30 Geometry optimisations
converge to the same conformations as those observed in solid
state structures (see ESI, Table S1‡). A comparison between
the optimized parameters and the corresponding structural ones
(Table S1‡) shows that a very good agreement holds. In particular,
though slightly overestimated, the optimized Cc–Cc, Cc–E and E–
E bond distances (E = Se for 1 and 2; E = S for 5 and 6) reproduce
the experimental values well (with average discrepancies of 0.01,
0.02, and 0.04 A, respectively; see ESI‡).31 The frontier molecular
˚
Experimental
orbitals (MO’s) are localized in all cases on the chalcogen bridging
atoms (Fig. 6 and 7 for 1 and 2, respectively). The HOMO’s consist
of non-bonding MO’s built up of the np orbitals of the chalcogen
atoms (n = 4 for 1 and 2; n = 3 for 5 and 6). The LUMO’s of 1 and 5
are antibonding MO’s with respect to the Cc–E bonds (E = Se and
S for 1 and 5, respectively), while those of 2 and 6 are antibonding
r MO’s with respect to the E–E bond. The LUMO composition of
2 and 6 is in agreement with the ability of dichalcogenides to give
the corresponding chalcogenides upon reduction. Worthy of note,
the eigenvalues of the LUMO’s of 2 and 6 are remarkably lower
than those of 1 and 5, respectively, indicating a higher tendency
to reduction for dichalcogenides (ESI, Fig. S1‡). Furthermore,
the cyclic voltammetry (CV) experiments provide a reduction
potential measured for 2 is remarkably higher than that measured
for the sulfur isologue 6 (Ered = −0.96 and −1.46 V vs. SCE in
Materials and methods
Commercial o-carborane, 1-methyl-o-carborane and 1-phenyl-o-
carborane were sublimed under high vacuum at 0.01 mmHg
prior to use. Solvents were placed under vacuum to eliminate
dissolved oxygen. A 1.6 M solution of n-butyllithium in n-hexane
was used as purchased. (2-Me-1,2-closo-C2B10H10)2S (5) and (1-
S-2-Me-1,2-closo-C2B10H10)2 (6) were synthesised as previously
reported.24 All reactions were carried out under a dinitrogen
atmosphere employing Schlenk techniques. Microanalyses were
performed by using a Perkin-Elmer 240 B microanalyser. IR
spectra were obtained as KBr pellets on a Nicolet 710-FT
spectrophotometer. The 1H-NMR (300.13 MHz), 13C{ H}-NMR
1
(75.47 MHz), 11B-NMR (96.29 MHz), 31P{ H}-NMR (121.5
1
MHz) and 77Se-NMR (57 MHz) spectra were recorded on a
Bruker ARX 300WB spectrometer. Chemical shift values for 1H-
NMR spectra were referenced to an internal standard of SiMe4 in
deuterated solvents. Chemical shift values for 11B-NMR spectra,
1
31P{ H}-NMR spectra and 77Se-NMR spectra were referenced
relative to external BF3·OEt2, 85% H3PO4 and SeMe2 respectively.
Chemical shifts are reported in units of parts per million downfield
from the reference, and all coupling constants are reported in
Hertz. The mass spectra for anionic species were recorded in the
negative ion mode using a Bruker Biflex MALDI-TOF-MS [N2
laser; kexc 337 nm (0.5 ns pulses); voltage ion source 20.00 kV
(Uis1) and 17.50 kV (Uis2)]. Electrochemical measurements were
performed in a standard one-compartment three-electrode cell
using a Voltalab10 PGZ100 potentiostat/galvanostat. A SCE
electrode was used as reference electrode and platinum plates (U =
0.2 cm) as working and counter electrodes. All measurements
were performed in dichloromethane with tetrabutylammonium
hexafluorophosphate 0.1 M as supporting electrolyte. Cyclic
voltammograms were recorded from −2.5 V to +2 V with a
scan rate of 100 mV s−1. Quantum-chemical DFT27 calculations
were carried out on the compounds 1, 2, 5, and 6, starting
from experimental structural data, with the well-known three-
parameter B3LYP28 functional using the commercially available
Fig. 6 Drawings of Kohn–Sham HOMO (a) and LUMO (b) calculated
for 1 at the optimized geometry (cutoff value = 0.05 e).
Fig. 7 Drawings of Kohn–Sham HOMO (a) and LUMO (b) calculated
for 2 at the optimized geometry (cutoff value = 0.05 e).
5244 | Dalton Trans., 2006, 5240–5247
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suite of programs Gaussian03.33 Calculations on I2, Br2, and
IBr were performed at the same level of theory for the sake of
comparing the HOMO eigenvalues of 1, 2, 5, and 6, with those
of the LUMO’s of the dihalogen/interhalogen species (see ESI‡).
Schafer, Horn, and Ahlrichs29 pVDZ basis sets were used for H,
B, C, S, and Se, while for halogens the LanL2DZ basis sets with
relativistic effective core potentials (RECP’s)34 were exploited.30
For all compounds NBO populations and Wiberg bond indexes
were calculated at the optimised geometries.35,36 The programs
GaussView 3.0,37 Molekel 4.3,38 and Molden 4.439 were used to
investigate the NBO charge distributions and MO composition.
refluxed for 2 h. The resulting mixture was then washed with water
(10 cm3) and 0.5 M Na2CO3 aqueous solution (2 × 15 cm3). The
organic layer was dried over MgSO4. The diethyl ether filtrate was
evaporated under vacuum to give a yellow solid. Recrystallization
from petroleum ether (bp 40–60 ◦C) provided crystals. Yield: 35 mg
(84.1%). Anal. Calcd for C6H26B20Se: C, 18.32; H, 6.66. Found: C,
18.35; H, 6.98%. FTIR (KBr): m (cm−1) 2955, 2923 (C–H); 2576
1
1
11
(B–H). H NMR (CDCl3): d 2.06 (s, Me), 2.02 (s, Me). H{ B}
NMR (CDCl3): d 2.43, 2.27, 2.19, 2.14 (m, BH), 2.06 (s, Me), 2.02
1
(s, Me). 11B NMR (CDCl3): d −1.13 (d, JBH = 154, 1B), −4.22
1
1
1
(d, JBH = 156, 1B), −8.93 (d, JBH = 121, 8B). 13C{ H} NMR
(CDCl3): d 69.24 (s, Cc), 50.99 (s, Cc), 25.42 (s, Me), 24.95 (s, Me).
MALDI-TOF (m/z): 393 (M, 2%), 377.60 (M − Me, 22.5%), 237
(M − Me − carboranyl, 100%).
Preparations
Synthesis of (1-Se-2-Me-1,2-closo-C2B10H10)2 (2). To a solu-
tion of 1-methyl-o-carborane (100 mg, 0.63 mmol) in dry diethyl
ether at 0 ◦C was added n-BuLi (0.4 cm3, 0.63 mmol). The mixture
was stirred at this temperature for 30 min, then maintained at
room temperature for the same period, and cooled again to 0 ◦C,
at which point selenium powder (50 mg, 0.63 mmol) was slowly
added over a period of 30 min. The resulting solution was stirred
at 25 ◦C for 1 hour. Then 10 cm3 of water was added. The mixture
was thoroughly shaken, and the two layers separated. The organic
layer was dried over MgSO4. The filtrate was evaporated to give
a yellow solid. Yield: (131 mg, 88%). Crystals were obtained
from slow evaporation of an acetone solution for 2 and from
slow evaporation of acetone–toluene (1 : 1) for 2·toluene. Anal.
Calcd for 2, C6H26B20Se2: C,15.25; H, 5.55. Found: C, 15.26; H,
5.45%. FTIR (KBr): m (cm−1) 2936 (C–H); 2610, 2584, 2572, 2564,
2546 (B–H). 1H NMR (CD3COCD3): d 2.23 (s, Me), 2.10 (s, Me).
Synthesis of [Au(1-Se-2-Me-1,2-closo-C2B10H10)(PPh3)] (4).
To a solution of 1-methyl-o-ca◦rborane (78 mg, 0.49 mmol) in
dry diethyl ether (10 cm3) at 0 C was added n-BuLi (0.31 cm3,
0.49 mmol). The mixture was stirred at this temperature for 30 min,
then maintained at room temperature for the same period, and
cooled again to 0 ◦C, at which point selenium powder (39 mg,
0.49 mmol) was slowly added over a period of 30 min. The resulting
◦
solution was stirred at 25 C for 1 h. Then [AuClPPh3] (242 mg,
0.49 mmol) was added and stirred at room temperature overnight.
After filtering off a violet solid, the organic solution was dried
over MgSO4. The filtrate was evaporated to obtain a yellowish
solid. Yield 146 mg (43%). Crystals were grown from a saturated
solution in diethyl ether. Anal. Calcd for C21H28Au1B10PSe: C,
36.27; H, 4.06;. Found: C, 36.51; H, 4.29%. FTIR (KBr): m (cm−1)
2926, 2855 (C–H); 2590, 2575, 2573, 2561 (B–H); 1479, 1435, 1101,
743, 690, 535 (PPh3). 1H NMR: d 2.08 (d, 1JHH = 2.5, 3H), 1.92 (d,
11
1H{ B} NMR (CD3COCD3): d 2.64 (s, BH), 2.37 (s, BH), 2.23 (s,
Me), 2.35 (s, BH), 2.10 (s, Me). 11B NMR (CD3COCD3): d −1.9
11
1JHH = 2.5, 3H). 1H{ B} NMR: d 2.08 (d, 1JHH = 2.5, 3H), 1.92
1
1
1
1
(d, JBH = 145, 1B), −4.1 (d, JBH = 156, 1B), −8.3 (d, JBH
=
(d, JHH = 2.5, 3H), 2.77 (s, BH), 2.40 (s, BH), 2.2 (s, BH), 2.00
137, 8B). 13C{ H} NMR (CD3COCD3): d 79.7 (s, Cc), 66.6 (s, Cc),
24.7 (s, Me). MALDI-TOF-MS (m/z): 472 (M, 14%), 237 (M/2,
100%). 77Se NMR (CD3COCD3): d 389 (s, Se).
1
(s, BH). 11B NMR: d −5.96 (br s, 6B), −9.12 (d, 1JBH = 156, 4B).
1
1
31P{ H} NMR: d 37.4 (s, PPh3). 13C{ H} NMR: d 134.3 (d, 1JCH
=
14, Ph), 131.9 (s, Ph), 129.3(d, 1JCH = 12, Ph), 128.7 (s, Ph), 64.5
(s, Cc), 25.83 (s, Me).
Synthesis of (1-Se-2-Ph-1,2-closo-C2B10H10)2 (3). This com-
pound was prepared analogously to the method described for
(1-Se-2-Me-1,2-closo-C2B10H10)2, using 1-Ph-1,2-closo-C2B10H11
(100 mg, 0.45 mmol), n-BuLi (0.28 cm3, 0.45 mmol) and selenium
(36 mg, 0.45 mmol) as starting materials to afford a yellow solid.
Yield: 104 mg (77%). Good crystals for X-ray diffraction were
grown from an acetone solution by slow evaporation. Anal. Calcd
for C16H30B20Se2: C, 32.21; H, 5.07. Found: C, 31.95; H, 5.32%.
FTIR (KBr): m (cm−1) 2932, 2931 (C–H) (cm−1); 2603, 2569 (B–H).
1H NMR (CDCl3): d 7.49–7.28 (m, Ph, 10H), 3.00–0.9 (m, B–H,
X-Ray structure determinations of (2-Me-1,2-closo-C2B10H10)2Se
(1), (1-Se-2-Me-1,2-closo-C2B10H11)2 (2), (1-Se-2-Me-1,2-closo-
C2B10H10)2·toluene (2·toluene), (1-Se-2-Ph-1,2-closo-C2B10H10)2
(3) and [Au(1-Se-2-Me-1,2-closo-C2B10H10)(PPh3)] (4)
Single-crystal data collections were performed at −100 ◦C on a
Nonius KappaCCD diffractometer using graphite monochroma-
tized Mo-Karadiation. Totals of 6748, 6010, 4917, 7433 and 16157
reflections were collected for 1, 2, 2·toluene, 3 and 4 giving 3776
(Rint = 0.0371), 3700 (Rint = 0.0455), 2581 (Rint = 0.0322), 4390
(Rint = 0.0509) and 4651 (Rint = 0.0358) independent reflections,
respectively. Crystallographic data are presented in Table 4.
The structures were solved by direct methods using the
SHELXS-97 program and least-squares refinements were per-
formed using the SHELX-97 program.40 In 2·toluene the solvent
molecule is disordered lying at the vicinity of an inversion
centre. Only the carbon atoms of the disordered toluene were
refined with isotropic displacement parameters, but the rest of
the non-hydrogen atoms of the five compounds were refined with
anisotropic displacement parameters. Hydrogen atoms of all com-
pounds were included in the calculations at fixed distances from
their host atoms and treated as riding atoms using the SHELXL-97
1
11
20H). H{ B} NMR (CDCl3): d 7.49–7.28 (m, Ph,10H), 2.56 (s,
BH, 4H), 2.44 (s, BH, 12H), 2.31 (s, BH, 4H). 11B NMR (CDCl3): d
−2.0 (d, 1JBH = 147, 2B), −8.9 (d, 1JBH = 141, 6B), −10.8 (d, 1JBH
=
1
171, 2B). 13C{ H} NMR (CDCl3): d 130.50, 130.03, 127.80, 126.50
(s, Caryl), 85.51 (s, Cc), 67.81 (s, Cc). MALDI-TOF-MS (m/z): 597
(M, 2%), 299 (M/2, 100%).
Synthesis of (2-Me-1,2-closo-C2B10H10)2Se (1). To a solution
of 1-methyl-o-carborane (17 mg, 0.11 mmol) in dry diethyl ether
at 0 ◦C was added n-BuLi (0.07 cm3, 0.11 mmol). The mixture was
stirred for 30 min at 0 ◦C and at room temperature for 30 min
more. Then a solution of (1-Se-2-Me-1,2-closo-C2B10H10)2 (50 mg,
0.11 mmol) in dry diethyl ether (15 cm3) was added. The slurry was
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Table 4 Crystallographic data for (2-Me-1,2-closo-C2B10H10)2Se (1), (1-Se-2-Me-1,2-closo-C2B10H10)2 (2), (1-Se-2-Me-1,2-closo-C2B10H10)2·toluene
(2·toluene), and (1-Se-2-Ph-1,2-closo-C2B10H10)2 (3) and [Au(1-Se-2-Me-1,2-closo-C2B10H10)(PPh3)] (4)
Compound
1
2
2·toluene
3
4
Formula
M
C6H26B20Se
393.43
C6H26B20Se2
472.39
C6H26B20Se2·C7H8
C16H30B20Se2
596.52
C21H28AuB10PSe
695.43
564.52
Crystal system
Space group
Monoclinic
P21/n (No. 14)
13.9969(3)
11.7558(5)
14.2199(5)
90
118.572(2)
90
2054.86(12)
4
Triclinic
Monoclinic
C2/c (No. 15)
23.4834(11)
9.5944(2)
13.8538(6)
90
119.4851(14)
90
2717.12(18)
4
Monoclinic
Pc (No. 7)
8.0368(4)
12.3095(5)
14.0307(7)
90
91.717(5)
90
1387.42(11)
2
Monoclinic
P21/n (No. 14)
7.93370(10)
18.8060(4)
17.7509(4)
90
91.4646(7)
90
2647.59(9)
4
¯
P1 (No. 2)
˚
a/A
7.1599(4)
7.8129(5)
21.7731(15)
95.738(2)
90.839(2)
115.038(3)
1095.80(12)
2
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
U/A
Z
T/◦C
−100
−100
−100
−100
−100
˚
k/A
0.71073
1.272
0.71073
1.432
0.71073
1.380
0.71073
1.428
0.71073
1.745
Dc/g cm−3
l/cm−1
18.16
0.0353
0.0745
33.65
0.0502
0.1199
27.27
0.0346
0.0739
26.75
0.0374
0.0690
70.02
0.0305
0.0638
R1a(Fo) [I > 2r(I)]
wR2b (Fo2) [I > 2r(I)]
ꢀ
ꢀ
ꢀ
ꢀ
2
1/2
a R1 = ꢀFo| − |Fc|/ |Fo. b wR2 = { [w(Fo − Fc2)2]/ [w(Fo2)2]}
.
default parameters. 3 crystallizes in a non-centrocymmetric space
group, and absolute configuration was determined by refinement
of Flack’s x parameter with a resulting value of −0.001(12).
CCDC reference numbers 616221–616225.
14 C. C. Silveira, A. L. Braga, A. S. Vieira and G. Zeni, J. Org. Chem.,
2003, 68, 662.
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For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b610944f
Acknowledgements
The authors are grateful to the MCyT for financial sup-
port (project MAT01-1575, MAT04-01108) and Generalitat de
Catalunya for 2001/SGR/0033.
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5246 | Dalton Trans., 2006, 5240–5247
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31 Calculated Wiberg bond indexes C–C: 0.643, 0.644, 0.634, 0.630 for 1,
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32 In order to verify the inability of the dicarboranyldichalcogenidesto
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ESI, Fig. S1†).
33 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
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40 G. M. Sheldrick, SHELX-97, University of Go¨ttingen, Germany, 1997.
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 5240–5247 | 5247
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