Reaction of o-carboranes with sterically demanding N-heterocyclic carbene: Synthesis and structural characterization of 1:1 adducts

Article abstract of DOI:10.1039/c2dt31765f

Several nido-carborane-carbene 1:1 adducts were prepared in very high yields from the reaction of o-carboranes with 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene in dry THF at room temperature. Single-crystal X-ray analyses reveal that they are zwitterionic salts consisting of a nido carborane cage and imidazolium moiety that are linked by a five-coordinate boron atom. They are inert toward carbenes, but sensitive toward moisture and water, leading to the formation of deboration products, nido-C₂B₉ ions. These results shed light on the deboration reaction mechanism of o-carboranes.

Full text of DOI:10.1039/c2dt31765f

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PAPER
Reaction of o-carboranes with sterically demanding N-heterocyclic carbene:
synthesis and structural characterization of 1 : 1 adducts†
Fangrui Zheng and Zuowei Xie*
Received 2nd August 2012, Accepted 30th August 2012
DOI: 10.1039/c2dt31765f
Several nido-carborane–carbene 1 : 1 adducts were prepared in very high yields from the reaction of
o-carboranes with 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene in dry THF at room temperature.
Single-crystal X-ray analyses reveal that they are zwitterionic salts consisting of a nido carborane cage
and imidazolium moiety that are linked by a five-coordinate boron atom. They are inert toward carbenes,
but sensitive toward moisture and water, leading to the formation of deboration products, nido-C2B9 ions.
These results shed light on the deboration reaction mechanism of o-carboranes.
temperature gave 1 : 1 adducts 3 in 86–92% isolated yield as
colorless crystals (Schemes 1 and 2). They were stable toward
Introduction
2− 1
o-Carborane (o-C2B10H12) can be reduced to nido-C2B10H12
,
excess 2 due to steric reasons, which is very different from steri-
cally less hindered NHCs.8 However, complexes 3 were moisture
sensitive, leading to the formation of deboration products 4 upon
exposure to air, for example, 4a (Scheme 3), whereas the 1 : 2
adducts were air- and moisture-stable.8 It was noted that both
m-carborane and p-carborane showed no reactivity toward 2
even under forced reaction conditions. Various cage B or C sub-
stituted carboranes were well compatible with the reaction as
shown in Scheme 2. For B(3) substituted ones, 2 attacked the
B(6) position owing to steric reasons.
4− 2
and arachno-C2B10H12
, be deprotonated to LiC2B10H11 or
Li2C2B10H10,3 react with electrophile (E+) to give B-substituted
4
carboranes o-C2B10H12−nEn and with nucleophiles to afford
deboration species nido-C2B9H112−.5 In the latter reaction, it is
proposed that the nucleophile first attacks the most electron
deficient cage boron that is bonded to both cage carbon atoms,
followed by an attack of the second equivalent of the nucleophile
on the same boron atom to remove one vertex from the o-carbo-
rane framework.6 Small N-heterocyclic carbenes (NHCs) have
recently been reported to either abstract a proton from the cage
C–H unit to generate ionic liquids (imidazolium salts of the car-
borane monoanion)7 or attack the cage boron atom to form 1 : 2
adducts of nido-carborane–carbene8 as shown in Scheme 1.
In view of the role of NHCs in the stabilization of low-
valence boron compounds,9 boryl radicals,10 and borenium
ions,11 we wondered if a sterically demanding NHC, 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene, could stabilize the 1 : 1
adducts of carborane–carbene, a class of anticipated intermedi-
ates in the formation of 1 : 2 adducts. We report herein the syn-
thesis and structural characterization of 1 : 1 nido-carborane–
carbene adducts.
Complexes 3 were fully characterized by various spectro-
scopic data and high-resolution mass spectrometry. The 1H
NMR spectra were in good agreement with the formulae corres-
ponding to 1 : 1 carborane–carbene adducts. The 11B NMR
spectra exhibited
a
pattern of 1 : 2 : 3 : 2 : 2 for 3a,
1 : 1 : 1 : 1 : 1 : 1 : 1 : 2 : 1 for 3b, 1 : 2 : 2 : 1 : 1 : 1 : 2 for 3c,
1 : 2 : 2 : 1 : 1 : 1 : 2 for 3d, 1 : 2 : 2 : 1 : 1 : 2 : 1 for 3e,
1 : 2 : 2 : 1 : 1 : 1 : 2 for 3f, 1 : 1 : 1 : 1 : 1 : 1 : 2 : 1 : 1 for 3g,
1 : 2 : 1 : 2 : 1 : 1 : 2 for 3h, 1 : 2 : 1 : 2 : 4 for 3i, and 1 : 2 : 1 : 2 : 4
for 3j, respectively. The chemical shifts of the unique bridging
B atoms fell in the range 15.9–27.8 ppm with JBH = 90–115 Hz,
suggesting that this boron still bonds to an exo-hydrogen atom.
In addition, the proton coupled 11B NMR spectra showed one
singlet at −7.9 ppm corresponding to the BCl vertex in 3c, one
resonance at −14.2 ppm assignable to the BBr vertex in 3d,
one peak at −29.4 ppm attributable to the BI in 3e, one singlet at
−9.8 ppm corresponding to the BPh vertex in 3f, one resonance
at −19.8 ppm assignable to the BI vertex in 3i.
Results and discussion
Synthesis
Treatment of o-carborane with 2 equiv. of 1,3-bis(2,6-diisopro-
pylphenyl)imidazol-2-ylidene (2) in dry THF at ambient
Structure
Department of Chemistry and Center of Novel Functional Molecules,
The Chinese University of Hong Kong, Shatin, NT, Hong Kong, China.
E-mail: zxie@cuhk.edu.hk; Fax: (+852) 2603-5057
†CCDC 893415–893424 for 3a to 3j. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2dt31765f
The solid-state structures of complexes 3 were further confirmed
by single-crystal X-ray analyses and are shown in Fig. 1. Note
that as 3c, 3d and 3e are isostructural and isomorphous, only a
representative structure of 3c is shown in Fig. 1c. They have a
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Scheme 1
Scheme 2
12908 | Dalton Trans., 2012, 41, 12907–12914
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1,2-(CH2)3-1,2-C2B10H10,21 1,2-(CH2CHvCHCH2)-1,2-C2B10-
H10,22 and 1,2-[o-C6H4(CH2)2]-1,2-C2B10H10 were prepared
23
according to literature methods. Infrared spectra were obtained
from KBr pellets prepared in the glovebox on a Perkin-Elmer
1600 Fourier transform spectrometer. 1H, 13C{1H} and 11B
NMR spectra were recorded on a Bruker DPX 400 spectrometer
at 400, 100 and 128 MHz, respectively. All chemical shifts were
reported in δ units with reference to the residual solvent reso-
nances of the deuterated solvents for proton and carbon chemical
shifts, and to external BF3·OEt2 (0.00 ppm) for boron chemical
shifts. Elemental analyses were performed by the Shanghai Insti-
tute of Organic Chemistry, CAS, China.
Scheme 3
very similar nido cage structure with the NHC bound to the brid-
ging boron atom that is disconnected from both cage carbon
Preparation of μ-9,10,11-[2′-{1′,3′-[2′′,6′′-iPr2(C6H3)]2-1′,3′-
N2C3H2}]BH-7,8-C2B9H11 (3a). THF solution (5 mL) of 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene (2; 78 mg, 0.2 mmol)
was slowly added to a stirring solution of o-C2B10H12 (14 mg,
0.1 mmol) in THF (5 mL) at room temperature, and the mixture
was stirred overnight. The resulting light-yellow solution was
then concentrated to ca. 2 mL, to which hexane (10 mL) was
added. The precipitate formed was filtered off and recrystallized
from diethyl ether to give adduct 3a as colorless crystals (49 mg,
92%). 1H NMR (CD2Cl2): δ 7.53 (t, J = 7.8 Hz, 2H, C6H3), 7.31
(d, J = 7.8 Hz, 4H, C6H3), 7.14 (s, 2H, imidazolium NCH), 2.79
(s, 2H, cage CH), 2.62 (m, 4H, CH(CH3)2), 1.34 (d, J = 6.8 Hz,
12H, CH(CH3)2), 1.18 (d, J = 6.8 Hz, 12H, CH(CH3)2).
13C{1H} NMR (CD2Cl2): δ 146.0, 132.5, 131.3, 124.5, 123.2
(C6H3 & imidazolium NCH), 67.1 (cage C), 29.4 (CH(CH3)2),
25.7, 22.4 (CH(CH3)2), the imidazolium NCN carbon was not
observed. 11B NMR (CD2Cl2): δ 24.0 (d, J = 115 Hz, 1B), −2.7
(d, J = 166 Hz, 2B), −7.6 (d, J = 128 Hz, 3B), −18.3 (d, J =
179 Hz, 2B), −19.9 (d, J = 141 Hz, 2B). IR (KBr, cm−1): νBH
2567 (vs). HRMS: m/z calcd for C29H48B10N2 [M − H]+:
531.4737. Found: 531.4744.
atoms. The observed geometry is close to those found in nido-
12
C2B10H12·HNP(NMe2)3
and [1,3-tPe2-1,3-N2C3H3][12-(1′-
{1′,2′-C2B10H11})-7,8-C2B10H12] (Pe = pentyl).8 As shown in
Table 1, the B–C(carbene) bond distances (1.61–1.64 Å) in 3 fall
in the range 1.58–1.64 Å8,10b,13 which is normally observed in
boron–carbene adducts. The B(12)–C(7)/C(8) and B(12)–B(9)/B(11)
distances range from 2.54 to 2.58 Å and 1.94 to 2.01 Å, respect-
ively, showing small variations. The B(12)–B(10) distances of
1.71–1.74 Å are in the normal range observed in o-carborane
derivatives.3,4,14 These data show a slip distortion of the B(12)
atom towards the boron side of the C2B3 bonding face upon
binding with NHC, cleaving the two B–C bonds and stretching
the two B–B bonds. The structural parameters also indicate that
both cage C and B substituents do not have obvious effects on
the bonding interactions between the bridging B atom and the
open C2B3 five-membered ring. These results suggest that 3 are
best viewed as zwitterionic salts with the negative charge on the
cage and positive charge on the imidazolium ring.
In summary, several 1 : 1 carborane–carbene adducts have
been prepared from the reaction of o-carboranes with sterically
demanding NHC. They are zwitterionic salts consisting of nido
carborane cage and imidazolium unit that are linked by a five-
coordinate boron atom. They do not react with an excess amount
of NHC, but are sensitive to moisture and water, leading to
the formation of deboration products, nido-C2B9 ions. These
observations are significantly different from those of small
carbenes.7,8 The results indicate that steric factor plays a very
important role in the reaction of o-carborane with carbenes. The
isolation of 1 : 1 and 1 : 2 adducts offers a better understanding
of the mechanism in the deboration process.
Preparation of 7-Me-μ-9,10,11-[2′-{1′,3′-[2′′,6′′-iPr2(C6H3)]2-
1′,3′-N2C3H2}]BH-7,8-C2B9H11 (3b). This complex was prepared
as colorless crystals from 1-Me-1,2-C2B10H11 (16 mg, 0.1 mol)
and 2 (78 mg, 0.2 mmol) in THF using the same procedure as
reported for 3a: yield 48 mg (87%). X-Ray-quality crystals were
grown from a saturated diethyl ether solution at room tempera-
1
ture. H NMR (CD2Cl2): δ 7.53 (t, J = 7.8 Hz, 2H, C6H3), 7.32
(m, 4H, C6H3), 7.15 (s, 2H, imidazolium NCH), 2.67 (m, 5H,
CH(CH3)2 & cage CH), 1.41 (s, 3H, cage CH3), 1.36 (d, J =
6.8 Hz, 12H, CH(CH3)2), 1.16 (d, J = 6.8 Hz, 12H, CH(CH3)2).
13C{1H} NMR (CD2Cl2): δ 146.0, 145.8, 132.6, 131.2, 124.4,
124.3, 123.4 (C6H3 & imidazolium NCH), 29.3 (CH(CH3)2),
26.5 (cage CH3), 25.7, 22.2, 22.1 (CH(CH3)2), the imidazolium
NCN and cage C atoms were not observed. 11B NMR (CD2Cl2):
δ 21.1 (d, J = 90 Hz, 1B), −0.3 (d, J = 115 Hz, 1B), −3.0 (d, J =
154 Hz, 1B), −7.2 (d, J = 102 Hz, 1B), −7.9 (d, J = 128 Hz,
1B), −11.5 (d, J = 141 Hz, 1B), −14.7 (d, J = 154 Hz, 1B),
−17.6 (d, J = 141 Hz, 2B), −19.9 (d, J = 128 Hz, 1B). IR (KBr,
cm−1): νBH 2554 (vs). HRMS: m/z calcd for C30H50B10N2
[M − H]+: 545.4908. Found: 545.4910.
Experimental
General procedures
All experiments were performed under an atmosphere of dry
argon with the rigid exclusion of air and moisture using standard
Schlenk or cannula techniques, or in a glovebox. All organic sol-
vents were refluxed over sodium benzophenone ketyl for several
days and freshly distilled prior to use. All chemicals were
purchased from either Aldrich or Acros Chemical Co. and
used as received unless otherwise noted. Compounds 1-Me-
1,2-C2B10H11,15 3-chloro-o-carborane,16 3-bromo-o-carborane,17
3-iodo-o-carborane,18 3-phenyl-o-carborane,19 9-iodo-o-carborane,20
Preparation of 3-Cl-μ-9,10,11-[2′-{1′,3′-[2′′,6′′-iPr2(C6H3)]2-
1′,3′-N2C3H2}]BH-7,8-C2B9H11 (3c). This complex was prepared
as light-yellow crystals from 3-chloro-o-carborane (18 mg,
0.1 mol) and 2 (78 mg, 0.2 mmol) in THF using the same
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Fig. 1 Molecular structures of (a) 3a, (b) 3b, (c) 3c, (d) 3f, (e) 3g, (f) 3h, (g) 3i and (h) 3j.
12910 | Dalton Trans., 2012, 41, 12907–12914
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Table 1 Selected bond lengths (Å) in 3
Complex
C
NHC–B12
C7–C8
C7–B12
C8–B12
B9–B12
B11–B12
B10–B12
3a
3b
3c
3d
3e
3f
3g
3h
3i
1.611(3)
1.610(4)
1.641(8)
1.616(5)
1.618(5)
1.636(5)
1.609(10)
1.634(4)
1.622(3)
1.617(5)
1.503(4)
1.510(4)
1.525(8)
1.525(5)
1.526(5)
1.526(5)
1.511(13)
1.527(4)
1.527(3)
1.517(4)
2.566(4)
2.579(4)
2.559(8)
2.570(5)
2.557(5)
2.578(5)
2.549(12)
2.563(4)
2.537(3)
2.552(4)
2.566(4)
2.555(4)
2.559(8)
2.570(5)
2.557(5)
2.578(5)
2.541(12)
2.563(4)
2.554(3)
2.572(4)
1.969(4)
1.959(5)
1.978(6)
1.986(4)
1.979(4)
1.991(4)
1.958(11)
1.967(4)
1.970(4)
1.976(5)
1.979(4)
1.996(4)
1.978(6)
1.986(4)
1.979(4)
1.991(4)
2.012(11)
1.967(4)
1.939(4)
1.956(5)
1.718(4)
1.718(4)
1.728(9)
1.720(6)
1.713(6)
1.739(6)
1.728(11)
1.708(5)
1.734(4)
1.724(6)
3j
procedure as reported for 3a: yield 51 mg (89%). X-Ray-quality
crystals were grown from a saturated diethyl ether solution at
room temperature. 1H NMR (CD2Cl2): δ 7.54 (t, J = 7.8 Hz, 2H,
C6H3), 7.32 (d, J = 8.0 Hz, 4H, C6H3), 7.16 (s, 2H, imidazolium
NCH), 2.92 (s, 2H, cage CH), 2.58 (m, 4H, CH(CH3)2), 1.33 (d,
J = 6.8 Hz, 12H, CH(CH3)2), 1.18 (d, J = 6.8 Hz, 12H,
CH(CH3)2). 13C{1H} NMR (CD2Cl2): δ 145.9, 132.2, 131.4,
124.5, 123.4 (C6H3 & imidazolium NCH), 71.1 (cage C), 29.3
(CH(CH3)2), 25.7, 22.3 (CH(CH3)2), the imidazolium NCN
carbon was not observed. 11B NMR (CD2Cl2): δ 26.3 (d, J =
102 Hz, 1B), −2.2 (d, J = 179 Hz, 2B), −7.1 (d, J = 179 Hz,
2B), −7.9 (s, 1B), −11.3 (d, J = 115 Hz, 1B), −18.8 (d, J =
141 Hz, 1B), −19.8 (d, J = 128 Hz, 2B). IR (KBr, cm−1): νBH
2575 (vs). HRMS: m/z calcd for C29H47B10ClN2 [M − H]+:
566.4338. Found: 566.4342.
m/z calcd for C29H47B10BrN2 [M − H]+: 609.3842. Found:
609.3835.
Preparation of 3-I-μ -9,10,11-[2′-{1′,3′-[2′′,6′′-iPr2(C6H3)]2-
1′,3′-N2C3H2}]BH-7,8-C2B9H11 (3e). This complex was prepared
as colorless crystals from 3-iodo-o-carborane (27 mg, 0.1 mol)
and 2 (78 mg, 0.2 mmol) in THF using the same procedure as
reported for 3a: yield 60 mg (91%). X-Ray-quality crystals were
grown from a saturated diethyl ether solution at room tempera-
1
ture. H NMR (CD2Cl2): δ 7.54 (t, J = 7.8 Hz, 2H, C6H3), 7.32
(d, J = 8.0 Hz, 4H, C6H3), 7.17 (s, 2H, imidazolium NCH), 2.99
(s, 2H, cage CH), 2.58 (m, 4H, CH(CH3)2), 1.33 (d, J = 6.8 Hz,
12H, CH(CH3)2), 1.18 (d, J = 6.8 Hz, 12H, CH(CH3)2).
13C{1H} NMR (CD2Cl2): δ 145.9, 132.2, 131.4, 124.5, 123.4
(C6H3 & imidazolium NCH), 71.6 (cage C), 29.3 (CH(CH3)2),
25.7, 22.3 (CH(CH3)2), the imidazolium NCN carbon was not
observed. 11B NMR (CD2Cl2): δ 26.0 (d, J = 102 Hz, 1B), −1.2
(d, J = 128 Hz, 2B), −6.5 (d, J = 141 Hz, 2B), −8.4 (d, J =
128 Hz, 1B), −17.2 (d, J = 179 Hz, 1B), −18.8 (d, J = 141 Hz,
2B), −29.4 (s, 1B). IR (KBr, cm−1): νBH 2575 (vs). HRMS: m/z
calcd for C29H47B10IN2 [M − H]+: 657.3703. Found: 657.3700.
Preparation of 3-Br-μ-9,10,11-[2′-{1′,3′-[2′′,6′′-iPr2(C6H3)]2-
1′,3′-N2C3H2}]BH-7,8-C2B9H11 (3d). This complex was prepared
as colorless crystals from 3-bromo-o-carborane (22 mg, 0.1 mol)
and 2 (78 mg, 0.2 mmol) in THF using the same procedure as
reported for 3a: yield 56 mg (92%). X-Ray-quality crystals were
grown from a saturated diethyl ether solution at room temp-
Preparation of 3-Ph-μ-9,10,11-[2′-{1′,3′-[2′′,6′′-iPr2(C6H3)]2-
1′,3′-N2C3H2}]BH-7,8-C2B9H11 (3f). This complex was prepared
as colorless crystals from 3-phenyl-o-carborane (22 mg, 0.1 mol)
and 2 (78 mg, 0.2 mmol) in THF using the same procedure as
reported for 3a: yield 55 mg (90%). X-Ray-quality crystals were
grown from a saturated diethyl ether solution at room tempera-
1
erature. H NMR (CD2Cl2): δ 7.54 (t, J = 7.8 Hz, 2H, C6H3),
7.32 (d, J = 8.0 Hz, 4H, C6H3), 7.17 (s, 2H, imidazolium NCH),
2.97 (s, 2H, cage CH), 2.58 (m, 4H, CH(CH3)2), 1.33 (d, J = 6.8
Hz, 12H, CH(CH3)2), 1.18 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13
C
{1H} NMR (CD2Cl2): δ 145.9, 132.2, 131.4, 124.5, 123.4 (C6H3
& imidazolium NCH), 71.3 (cage C), 29.3 (CH(CH3)2), 25.7,
22.3 (CH(CH3)2), the imidazolium NCN carbon was not
observed. 11B NMR (CD2Cl2): δ 26.3 (d, J = 90 Hz, 1B), −1.9
(d, J = 128 Hz, 2B), −6.9 (d, J = 141 Hz, 2B), −10.3 (d, J =
128 Hz, 1B), −14.2 (s, 1B), −18.3 (d, J = 154 Hz, 1B), −19.4
(d, J = 128 Hz, 2B). IR (KBr, cm−1): νBH 2576 (vs). HRMS:
1
ture. H NMR (CD2Cl2): δ 7.55 (t, J = 7.8 Hz, 2H, C6H3), 7.33
(d, J = 7.8 Hz, 4H, C6H3), 7.30 (m, 2H, BC6H5), 7.16 (s, 2H,
imidazolium NCH), 7.14 (m, 3H, BC6H5), 2.94 (s, 2H, cage CH),
2.63 (m, 4H, CH(CH3)2), 1.35 (d, J = 6.8 Hz, 12H, CH(CH3)2),
1.19 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR (CD2Cl2):
δ 146.0, 132.8, 132.5, 131.3, 127.5, 127.4, 124.5, 123.3 (aryl &
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imidazolium NCH), 70.7 (cage C), 29.3 (CH(CH3)2), 25.7, 22.4
(CH(CH3)2), the imidazolium NCN carbon was not observed.
11B NMR (CD2Cl2): δ 23.5 (d, J = 90 Hz, 1B), −2.6 (d, J =
128 Hz, 2B), −7.3 (d, J = 141 Hz, 2B), −9.2 (d, J = 179 Hz,
1B), −9.8 (s, 1B), −17.7 (d, J = 141 Hz, 1B), −20.1 (d, J =
115 Hz, 2B). IR (KBr, cm−1): νBH 2580 (vs). HRMS: m/z calcd
for C35H52B10N2 [M − H]+: 607.5050. Found: 607.5060.
J = 7.8 Hz, 4H, C6H3), 7.13 (s, 2H, imidazolium NCH), 5.47 (s,
2H, CH2CHvCHCH2), 2.64 (m, 4H, CH(CH3)2), 2.45 (d, J =
16.9 Hz, 2H, CH2CHvCHCH2), 2.33 (d, J = 16.9 Hz, 2H,
CH2CHvCHCH2), 1.34 (d, J = 6.8 Hz, 12H, CH(CH3)2),
1.14 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR (CD2Cl2):
δ 145.9, 132.8, 131.3, 124.3, 123.4, 123.1 (C6H3, imidazolium
NCH
&
CH2CHvCHCH2), 34.8 (CH2CH2vCH2CH2),
29.4 (CH(CH3)2), 25.9, 22.0 (CH(CH3)2), the imidazolium NCN
and cage C were not observed. 11B NMR (CD2Cl2): δ 15.9 (d,
J = 115 Hz, 1B), −1.1 (d, J = 128 Hz, 2B), −7.6 (d, J = 128 Hz,
1B), −13.1 (d, J = 141 Hz, 2B), −17.6 (d, J = 115 Hz, 4B). IR
(KBr, cm−1): νBH 2533 (vs). HRMS: m/z calcd for C33H52B10N2
[M − H]+: 583.5050. Found: 583.5045.
Preparation of 6-I-μ-9,10,11-[2′-{1′,3′-[2′′,6′′-iPr2(C6H3)]2-
1′,3′-N2C3H2}]BH-7,8-C2B9H11 (3g). This complex was prepared
as colorless crystals from 9-iodo-o-carborane (27 mg, 0.1 mol)
and 2 (78 mg, 0.2 mmol) in THF using the same procedure as
reported for 3a: yield 60 mg (91%). X-Ray-quality crystals were
grown from a saturated diethyl ether solution at room tempera-
1
ture. H NMR (CD2Cl2): δ 7.55 (t, J = 7.8 Hz, 2H, C6H3), 7.34
Preparation
of
7,8-[o-C6H4(CH2)2]-μ-9,10,11-[2′-{1′,3′-
(3j). This
(m, 4H, C6H3), 7.20 (s, 2H, imidazolium NCH), 3.09 (s, 1H,
cage CH), 2.81 (s, 1H, cage CH), 2.68 (m, 2H, CH(CH3)2), 2.53
(m, 2H, CH(CH3)2), 1.40 (d, J = 7.2 Hz, 6H, CH(CH3)2), 1.38
(d, J = 7.8 Hz, 6H, CH(CH3)2), 1.18 (d, J = 6.8 Hz, 12H, CH-
(CH3)2). 13C{1H} NMR (CD2Cl2): δ 146.0, 145.7, 132.3, 131.5,
124.6, 124.5, 123.8 (C6H3 & imidazolium NCH), 29.5, 29.4
(CH(CH3)2), 25.9, 25.8, 22.9, 22.1 (CH(CH3)2), the imidazolium
NCN and cage C atoms were not observed. 11B NMR (CD2Cl2):
δ 23.8 (d, J = 90 Hz, 1B), −1.5 (d, J = 141 Hz, 1B), −3.6 (d,
J = 179 Hz, 1B), −5.0 (d, J = 141 Hz, 1B), −6.0 (d, J = 128 Hz,
1B), −15.9 (d, J = 141 Hz, 1B), −17.1 (d, J = 154 Hz, 2B),
−19.2 (d, J = 102 Hz, 1B), −19.8 (s, 1B). IR (KBr, cm−1): νBH
2574 (vs). HRMS: m/z calcd for C29H47B10IN2 [M − H]+:
657.3703. Found: 657.3706.
[2′′,6′′-iPr2(C6H3)]2-1′,3′-N2C3H2}]BH-7,8-C2B9H11
complex was prepared as light-yellow crystals from 1,2-
[o-C6H4(CH2)2]-1,2-C2B10H10 (24 mg, 0.1 mol) and 2 (78 mg,
0.2 mmol) in THF using the same procedure as reported for 3a:
yield 57 mg (90%). X-Ray-quality crystals were grown from a
1
saturated diethyl ether solution at room temperature. H NMR
(CD2Cl2): δ 7.57 (t, J = 7.8 Hz, 2H, C6H3), 7.34 (d, J = 7.8 Hz,
4H, C6H3), 7.16 (s, 2H, imidazolium NCH), 7.05 (m, 2H,
o-C6H4(CH2)2), 6.85 (m, 2H, o-C6H4(CH2)2), 3.20 (d, J =
16.5 Hz, 2H, o-C6H4(CH2)2), 2.84 (d, J = 16.5 Hz, 2H,
o-C6H4(CH2)2), 2.68 (m, 4H, CH(CH3)2), 1.32 (d, J = 6.8 Hz,
12H, CH(CH3)2), 1.14 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C
{1H} NMR (CD2Cl2): δ 145.8, 135.2, 132.8, 131.3, 130.5,
127.1, 126.3, 124.4, 123.6 (C6H3, imidazolium NCH &
o-C6H4(CH2)2), 38.9 (o-C6H4(CH2)2), 29.4 (CH(CH3)2), 25.9,
22.0 (CH(CH3)2), the imidazolium NCN and cage C were not
observed. 11B NMR (CD2Cl2): δ 17.8 (d, J = 90 Hz, 1B), 1.2 (d,
J = 128 Hz, 2B), −6.5 (d, J = 115 Hz, 1B), −11.2 (d, J =
141 Hz, 2B), −17.6 (d, J = 128 Hz, 4B). IR (KBr, cm−1): νBH
2527 (vs). HRMS: m/z calcd for C37H54B10N2 [M − H]+:
633.5224. Found: 633.5233.
Preparation of 7,8-(CH2)3-μ-9,10,11-[2′-{1′,3′-[2′′,6′′-iPr2(C6H3)]2-
1′,3′-N2C3H2}]BH-7,8-C2B9H11 (3h). This complex was prepared
as colorless crystals from 1,2-(CH2)3-1,2-C2B10H10 (18 mg,
0.1 mol) and 2 (78 mg, 0.2 mmol) in THF using the same pro-
cedure as reported for 3a: yield 50 mg (88%). X-Ray-quality
crystals were grown from a saturated diethyl ether solution at
room temperature. 1H NMR (CD2Cl2): δ 7.53 (t, J = 7.8 Hz, 2H,
C6H3), 7.32 (d, J = 8.0 Hz, 4H, C6H3), 7.15 (s, 2H, imidazolium
NCH), 2.67 (m, 4H, CH(CH3)2), 1.97 (m, 2H, CH2), 1.86 (m,
2H, CH2), 1.66 (m, 1H, CH2), 1.36 (d, J = 6.8 Hz, 12H,
CH(CH3)2), 1.28 (m, 1H, CH2), 1.15 (d, J = 6.8 Hz, 12H,
CH(CH3)2). 13C{1H} NMR (CD2Cl2): δ 145.8, 132.8, 131.2,
124.3, 123.6 (C6H3 & imidazolium NCH), 67.9 (cage C), 36.3
(CH2CH2CH2), 29.4 (CH(CH3)2), 26.9 (CH2CH2CH2), 25.8,
25.7, 21.9 (CH(CH3)2), the imidazolium NCN carbon was not
observed. 11B NMR (CD2Cl2): δ 19.0 (d, J = 102 Hz, 1B),
−1.35 (d, J = 141 Hz, 2B), −5.7 (d, J = 115 Hz, 1B), −13.2 (d,
J = 141 Hz, 2B), −16.7 (d, J = 141 Hz, 1B), −18.1 (d, J =
192 Hz, 1B), −19.3 (d, J = 128 Hz, 2B). IR (KBr, cm−1): νBH
2542 (vs). HRMS: m/z calcd for C32H52B10N2 [M − H]+:
571.5050. Found: 571.5043.
Preparation
of
[1′,3′-{2′′,6′′-iPr2(C6H3)}2-1′,3′-N2C3H3]-
[C2B9H12] (4a). H2O (0.1 mL) was added to a stirring solution
of 3a (53 mg, 0.1 mmol) in acetone (10 mL) at room tempera-
ture, and the mixture was stirred for 2 d. The solvent was
removed in vacuo and the resulting light-green oil was extracted
by diethyl ether (5 mL × 3). The ether solutions were combined
and concentrated to 10 mL. Complex 4a was obtained as color-
less crystals after this solution stood at room temperature over-
night (42 mg, 81%). 1H NMR (CD3COCD3): δ 9.82 (s, 1H,
imidazolium NCHN), 8.45 (s, 2H, NCHvCHN), 7.71 (t, J =
7.8 Hz, 2H, C6H3), 7.55 (d, J = 7.6 Hz, 4H, C6H3), 2.60 (m, 4H,
CH(CH3)2), 1.31 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.26 (d, J =
6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR (CD3COCD3): δ
146.1, 139.5, 133.1, 131.1, 127.4, 125.7 (C6H3, NCHN &
NCHvCHN), 29.9 (CH(CH3)2), 24.6, 23.8 (CH(CH3)2), the
cage C atoms were not observed. 11B NMR (CD3COCD3):
δ −10.0 (d, J = 128 Hz, 2B), −15.9 (d, J = 115 Hz, 2B), −16.7
(d, J = 128 Hz, 1B), −21.4 (d, J = 154 Hz, 2B), −32.3 (d, J =
128 Hz, 1B), −36.9 (d, J = 141 Hz, 1B). IR (KBr, cm−1): νBH
2529 (vs). Anal. calcd for C29H49B9N2: C, 66.60; H, 9.40; N,
5.36. Found: C, 66.01; H, 9.33; N, 4.99.
Preparation of 7,8-(CH2CHvCHCH2)-μ-9,10,11-[2′-{1′,3′-
[2′′,6′′-iPr2(C6H3)]2-1′,3′-N2C3H2}]BH-7,8-C2B9H11
(3i). This
complex was prepared as colorless crystals from 1,2-
(CH2CHvCHCH2)-1,2-C2B10H10 (20 mg, 0.1 mol) and 2
(78 mg, 0.2 mmol) in THF using the same procedure as reported
for 3a: yield 51 mg (86%). X-Ray-quality crystals were grown
from a saturated diethyl ether solution at room temperature.
1H NMR (CD2Cl2): δ 7.54 (t, J = 7.8 Hz, 2H, C6H3), 7.32 (d,
12912 | Dalton Trans., 2012, 41, 12907–12914
This journal is © The Royal Society of Chemistry 2012

View Article Online
Table 2 Crystal data and summary of data collection and refinement for 3a·(CH3CH2)2O, 3b, 3c, 3d, and 3e
3a·(CH3CH2)2O
33H58B10N2O
0.50 × 0.40 × 0.30
606.9
3b
3c
3d
3e
Formula
Crystal size/mm
Mr
C
C30H50B10N2
0.50 × 0.40 × 0.30
546.8
C29H47B10ClN2
0.40 × 0.30 × 0.20
567.2
C29H47B10BrN2
0.40 × 0.40 × 0.30
611.7
C29H47B10IN2
0.40 × 0.30 × 0.20
658.7
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/n
Orthorhombic
Pnma
Orthorhombic
Pnma
Orthorhombic
Pnma
a/Å
b/Å
10.715(1)
21.911(1)
17.309(1)
104.661(2)
3931.5(4)
4
10.286(4)
18.148(6)
21.421(8)
90.323(7)
3999(2)
4
19.542(4)
16.440(3)
10.733(2)
90
3448.2(11)
4
19.605(1)
16.427(1)
10.752(1)
90
3462.5(4)
4
19.826(1)
16.333(1)
10.784(1)
90
3492.0(2)
4
c/Å
β/o
V/Å3
Z
Dc/Mg m−3
1.025
0.908
1.093
1.173
1.253
Radiation (λ(Mo Kα))/Å
2θmax/°
0.71073
50.5
0.056
0.71073
50.5
0.048
0.71073
50.5
0.133
0.71073
50.5
1.209
0.71073
50.5
0.939
μ/mm−1
F(000)
1312
7108
419
1.040
R1 = 0.057
wR2 = 0.144
1176
7233
383
0.910
R1 = 0.067
wR2 = 0.178
1208
3241
200
1.003
R1 = 0.069
wR2 = 0.164
1280
3253
202
0.979
R1 = 0.046
wR2 = 0.118
1352
3280
202
1.044
R1 = 0.037
wR2 = 0.094
Observed reflections [I > 2σ(I)]
Parameters
Goodness of fit on F2
Final R indices [I > 2σ(I)]
Table 3 Crystal data and summary of data collection and refinement for 3f, 3g, 3h, 3i and 3j
3f
3g
3h
3i
3j
Formula
Crystal size/mm
Mr
C35H52B10N2
0.40 × 0.30 × 0.20
608.9
C29H47B10IN2
0.50 × 0.40 × 0.30
658.7
C32H52B10N2
0.40 × 0.30 × 0.20
572.9
C33H52B10N2
0.40 × 0.30 × 0.20
584.9
C37H54B10N2
0.50 × 0.40 × 0.30
634.9
Crystal system
Space group
Orthorhombic
Pnma
Monoclinic
Cc
Orthorhombic
Pbcm
Monoclinic
P21/n
Orthorhombic
Pbca
a/Å
b/Å
c/Å
21.29(3)
17.20(2)
10.674(2)
90
3909(9)
4
18.203(1)
12.930(1)
15.522(1)
98.656(1)
3611.4(3)
4
10.726(1)
18.193(2)
20.347(2)
90
3970.4(6)
4
11.269(1)
16.920(1)
19.229(1)
105.086(1)
3540.0(4)
4
18.164(2)
19.478(2)
22.210(2)
90
7858(1)
8
β/°
V/Å3
Z
Dc/mg m−3
1.035
1.211
0.958
1.097
1.073
Radiation (λ(Mo Kα))/Å
2θmax/°
0.71073
50.5
0.055
0.71073
50.5
0.908
0.71073
50.5
0.051
0.71073
50.5
0.058
0.71073
50.5
0.057
μ/mm−1
F(000)
1304
3668
230
1.045
R1 = 0.067
wR2 = 0.172
1352
5521
379
0.974
R1 = 0.052
wR2 = 0.157
1232
3709
209
0.998
R1 = 0.066
wR2 = 0.174
1256
6411
406
1.019
R1 = 0.059
wR2 = 0.146
2720
7122
447
1.005
R1 = 0.061
wR2 = 0.122
Observed reflections [I > 2σ(I)]
Parameters
Goodness of fit on F2
Final R indices [I > 2σ(I)]
X-Ray structure determination
Tables 2 and 3. Further details were deposited with Cambridge
Crystallographic Data Center, CCDC reference numbers
893415–893424 for 3a to 3j, respectively.
All single crystals were immersed in Paraton-N oil and sealed
under nitrogen in thin-walled glass capillaries. Data were col-
lected at 293 K on a Bruker SMART 1000 CCD diffractometer
using Mo Kα radiation (0.71073 Å). An empirical absorption
correction was applied using the SADABS program.24 All struc-
tures were solved by direct methods and subsequent Fourier
difference techniques and refined anisotropically for all non-
hydrogen atoms by full-matrix least-squares on F2 using the
SHELXTL program package.25 All hydrogen atoms were geo-
metrically fixed using the riding model, so that a detailed discus-
sion of the B-H distances was not warranted. Crystal data and
details of data collection and structure refinements were given in
Acknowledgements
This work was supported by grants from the Research Grants
Council of the Hong Kong SAR (Project No. 404610) and The
Chinese University of Hong Kong (Direct Grant 2060432).
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This journal is © The Royal Society of Chemistry 2012
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12914 | Dalton Trans., 2012, 41, 12907–12914
This journal is © The Royal Society of Chemistry 2012