Carbon insertion into arachno- 6,9-C2B8H14 via acyl chlorides. Skeletal alkylcarbonation (SAC) reactions: A new route for tricarbollides

Article abstract of DOI:10.1021/ic401293g

Reactions between arachno-6,9-C₂B₈H₁₄ (1) and selected acyl chlorides, RCOCl, in the presence of PS (PS = proton sponge , 1,8-dimethylamino naphthalene) in CH₂Cl₂ for 24 h at reflux, followed by in situ acidification with concentrated H ₂SO₄ at 0 C, generate a series of neutral alkyl and aryl tricarbollides 8-R-nido-7,8,9-C₃B₈H₁₁ (2) (where R = CH₃, 2a; C₂H₅, 2b; n-C ₄H₉, 2c; C₆H₅, 2d; 4-Cl-C ₆H₄, 2e; 4-Br-C₆H₄, 2f; 4-I-C ₆H₄, 2g; 1-C₁₀H₇, 2h; and 2-C ₁₀H₇, 2i). The best yields were achieved for aryl derivatives (80-95%) while the yields of the corresponding alkyl substituted compounds are lower (60-70%). These skeletal alkylcarbonation (SAC) reactions are consistent with an aldol-type condensation between the RCO group and open-face hydrogen atoms on the dicarbaborane 1, which is associated with the insertion of the carbonyl carbon atom into the structure of arachno-6,9-C ₂B₈H₁₄ (1) under elimination of three extra hydrogen atoms as H₂O and HCl. The reactions thus result in an effective R-tricarbaborane cross-coupling. Individual compounds of structure 2 have been purified by chromatography on a silica gel support, using hexane as the mobile phase (R_F = ～0.3). Deprotonation agents, such as NEt₃, NaOH, NaH, etc., convert tricarbaboranes 2 into the corresponding conjugated anions [8-R-nido-7,8,9-C₃B₈H ₁₀]^- (2^-) which were isolated as salts with suitable countercations (for example, Et₃NH⁺, Tl ⁺, NEt₄⁺, etc.). The compounds have been characterized by multinuclear (¹¹B, ¹H, and ¹³C) NMR spectroscopy, mass spectrometry, and elemental analyses. The structures of anions [8-R-nido-7,8,9-C₃B₈H ₁₀] (where R = C₆H₅, 4-I-C ₆H₄ and 1-C₁₀H₇; 2a^-, 2g^-, and 2h^-) and that of the neutral 8-(1-C ₁₀H₇)-nido-7,8,9-C₃B₈H₁₁ (2h) have been established by X-ray diffraction analyses.

Full text of DOI:10.1021/ic401293g

Article
pubs.acs.org/IC
Carbon Insertion into arachno-6,9‑C2B8H14 via Acyl Chlorides.
Skeletal Alkylcarbonation (SAC) Reactions: A New Route for
Tricarbollides
†
,†
†
‡
‡
̌
Mario Bakardjiev, Bohumil Stíbr,* Josef Holub, Zdenka Padelkova, and Ales Ruzicka
̌
̌
́
̌
̊
̌ ̌
†
̌
Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic, 250 68 Rez, Czech Republic
‡Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentska
Pardubice, Czech Republic
̌
́
573, 532 10
S
* Supporting Information
ABSTRACT: Reactions between arachno-6,9-C2B8H14 (1)
and selected acyl chlorides, RCOCl, in the presence of PS
(PS = “proton sponge”, 1,8-dimethylamino naphthalene) in
CH2Cl2 for 24 h at reflux, followed by in situ acidification with
concentrated H2SO4 at 0 °C, generate a series of neutral alkyl
and aryl tricarbollides 8-R-nido-7,8,9-C3B8H11 (2) (where R =
CH3, 2a; C2H5, 2b; n-C4H9, 2c; C6H5, 2d; 4-Cl-C6H4, 2e; 4-
Br-C6H4, 2f; 4-I-C6H4, 2g; 1-C10H7, 2h; and 2-C10H7, 2i). The
best yields were achieved for aryl derivatives (80−95%) while
the yields of the corresponding alkyl substituted compounds are lower (60−70%). These skeletal alkylcarbonation (SAC)
reactions are consistent with an aldol-type condensation between the RCO group and open-face hydrogen atoms on the
dicarbaborane 1, which is associated with the insertion of the carbonyl carbon atom into the structure of arachno-6,9-C2B8H14 (1)
under elimination of three extra hydrogen atoms as H2O and HCl. The reactions thus result in an effective R−tricarbaborane
cross-coupling. Individual compounds of structure 2 have been purified by chromatography on a silica gel support, using hexane
as the mobile phase (RF = ∼0.3). Deprotonation agents, such as NEt3, NaOH, NaH, etc., convert tricarbaboranes 2 into the
corresponding conjugated anions [8-R-nido-7,8,9-C3B8H10]− (2−) which were isolated as salts with suitable countercations (for
+
1
example, Et3NH+, Tl+, NEt4 , etc.). The compounds have been characterized by multinuclear (11B, H, and 13C) NMR
spectroscopy, mass spectrometry, and elemental analyses. The structures of anions [8-R-nido-7,8,9-C3B8H10]¯ (where R = C6H5,
4-I-C6H4 and 1-C10H7; 2a−, 2g−, and 2h−) and that of the neutral 8-(1-C10H7)-nido-7,8,9-C3B8H11 (2h) have been established by
X-ray diffraction analyses.
CH (where R = polarized group, such as CN, COOMe, and
COMe):
INTRODUCTION
■
Tricarbaboranes are usually prepared by the insertion of one
more carbon atom into the structure of dicarbaboranes.1 L. G.
Sneddon’s group was the first to develop the concept of so-
called skeletal alkylcarbonation (SAC)2 reactions resulting in
the incorporation of the RC unit into the dicarbaborane cage.
Excellent examples of such reactions are those between nitriles
(RCN) and the [arachno-4,6-C2B8H12]− anion leading to high-
yield formation of the 10-vertex tricarbaboranes [6-R-nido-
5,6,9-C3B7H9]− (where R = alkyl or aryl) upon elimination of
NH3:3
−
[arachno‐4, 6‐C2B7H12 + RC2H
]
→ [6‐RCH2‐arachno‐5, 6, 7‐C3B7H11]−
(2)
In these reactions, only one of the alkyne carbon atoms has
been inserted into the cage to form the 10-vertex tricarbaborane
skeleton with three adjacent carbon vertexes.4
The best method for the synthesis of 11-vertex tricarbabor-
anes (tricarbollides) consists of the insertion of the cyanide or
isocyanide carbon into the structure of the [nido-5,6-C2B8H11]−
anion:5
−
[arachno‐4, 6‐C2B7H12 + RCN
]
→ [6‐R‐nido‐5, 6, 9‐C3B7H9]− + NH3
(1)
−
[nido‐5, 6‐C2B8H11 + HCN
]
In this case, the three extra hydrogen atoms of the
dicarbaborane substrate are capable of interrupting the nitrile
RCN triple bond.
→ [7‐(H2N)‐nido‐7, 8, 9‐C3B8H10]−
(3)
Another type of SAC reactions occurs in the interaction
between the [arachno-4,6-C2B7H12]− anion and alkynes RC
Received: May 23, 2013
Published: July 25, 2013
© 2013 American Chemical Society
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Article
Scheme 1. Tricarbollide Compounds from the SAC Cross-Coupling via Incorporation of the RC Unit into the Structure of
a c
arachno-6,9-C2B8H14 (1) (B = BH, C = CH or C) −
a
b
c
RCOCl, PS, CH2Cl2, reflux, 24 h, followed by conc H2SO4, 0 °C. Deprotonation. Conc H2SO4, 0 °C.
−
t
and the reactions appear to be restricted only by the availability
of the starting acyl chloride.
The reactions of path a are in accord with a simple
stoichiometry of eq 5 comprising the deprotonation of 1 along
with the elimination of HCl and H2O:
[nido‐5, 6‐C2B8H11 + BuNC
]
→ [7‐(tBuHN)‐nido‐7, 8, 9‐C3B8H10]−
(4)
These aminocarbonation processes, in which the amino
group has been retained on the cluster carbon, is typical for
nido clusters.6 The reason for RNH-retention is that the lone
extra hydrogen atom in [nido-5,6-C2B8H11]− is incapable of
interrupting completely the CN or NC bond in reactions
3 and 4. Further chemical transformations of the amino
derivatives generated in turn a long series of tricarbollide
compounds, inclusive of the parent nido compounds [7,8,9-
C3B8H11]−, 7,8,9-C3B8H12, and [7,8,10-C3B8H11]− and their
derivatives.5
arachno‐6, 9 ‐C2B8H14+RCOCl + 2PS
1
→ 1PSH+[8‐R‐nido‐7, 8, 9‐C3B8H10]− + PS·HCl + H2O
2−
(5)
As inferred from Scheme 2, these reactions are consistent
with a regiospecific insertion of the 3-electron carbyne RC≡
unit (skeletal alkylcarbonation, SAC) into the endo-skeletal area
of 1− identified by B5, C6, C9, and B10 vertexes under
extraction of all the three extra hydrogen atoms which are then
eliminated as H2O and HCl. It is also readily seen that the
reactions result in an effective cross-coupling between R and
The works by Brellochs at al.7 on degradative insertion of the
aldehyde carbon into the structure of the [6-HO-arachno-
B10H13]2‑ dianion and some metal−carbonyl C-insertion
reactions8 suggested that some specific CO carbon
incorporation procedures might be, in principle, applicable to
other cluster systems. This strategy has proved to be justified,
and just recently we reported2 our preliminary results on
reactions between acyl chlorides (RCOCl) and the [arachno-
6,9-C2B8H13]− anion in the presence of triethylamine. These
so-called skeletal alkylcarbonation (SAC) reactions resulted in a
unique incorporation of the CO carbon into the dicarbaborane
cage and formation of the symmetrical 8-substituted 7,8,9-
tricarbollides. Herein we would like to report full experimental
details on the recent “proton-sponge modification” of this novel
method which allowed for the synthesis of an extended series of
alkyl and aryl derivatives in improved yields.
Scheme 2. Proposed Aldolization Mechanism for the
Insertion of the Acyl Chloride RC Unit between the C6 and
C9 Positions in [arachno-6,9-C2B8H13]− (1−) (B = BH, C =
CH or C)
RESULTS AND DISCUSSION
■
Syntheses. Treatment of the arachno-6,9-C2B8H14 (1)
dicarbaborane9 with excess acyl chlorides (RCOCl) in the
presence of 2 equiv of PS (proton scavenger) in refluxing
CH2Cl2 for 24 h, followed by adding concentrated H2SO4 at 0
°C (Scheme 1, path a), led to the formation of a series of
neutral tricarbollides 8-R-nido-7,8,9-C3B8H11 (2). The reaction
seems to be of general character, as tested for RCOCl’s of
varying R substituents (R = n-alkyls with 1−5 carbon atoms,
aryls C6H5, 4-X-C6H4 (X = Cl, Br, and I), and naphthyl isomers
1-C10H7 and 2-C10H7). The yields of compounds 2 were in the
ranges 60−70% and 80−95% for alkyls and aryls, respectively,
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Article
the carborane cage of 1 which is then leaving the coupling
process enriched in one more carbon vertex.
Although no mechanistic studies have so far been done, it
can be reasonably supposed that the reaction starts with
acylation of anion 1− at B5 in the endo sphere to form the
neutral acyl derivative endo-5-(RCO)-arachno-6,9-C2B8H13 (1a)
(see Scheme 2) which then extracts the endo hydrogen atom
from the cage C(6)H2 group to form the aldolization product
1b. Entailing H2O elimination followed by C-insertion is then
leading directly to the formation of the neutral tricarbollide 2.
All the 8-R-nido-7,8,9-C3B8H11 (2) compounds can be
deprotonated (Scheme 1, path b) by dissolution in 10%
aqueous NaOH, followed by precipitation of the solution with a
+
suitable countercation (for example Tl+, NEt4 , etc.), to
generate very stable anions of general constitution [8-R-nido-
7,8,9-C3B8H10]− (2−). However, the most convenient way for
ready preparation of these anions is based on treatment of
compounds of structure 2 with NEt3 in dichloromethane, which
leads to quantitative formation of the corresponding Et3NH+
salts. These represent a very stable storage form of tricarbollides
that can be converted easily into any other salt on dissolution in
aqueous NaOH, followed by precipitation with a suitable
countercation. Na+ salts can be also generated via reaction of
compounds 2 with NaH in Et2O (for R = aryls) while refluxing
THF must be employed for R = alkyls, apparently due to lower
acidity of the bridging hydrogen.
Figure 2. ORTEP representation of the molecular structure of
+
HNEt3 [8-(4−I-C6H4)-nido-7,8,9-C3B8H10]− (2g−) anion at 50%
probability level. Selected bond lengths (Å) and angles (deg):
cg(C1−C2−B3−B4−C5)−N1 3.365(3), I1−C15 2.102(3). Open
face: C7−C8 1.529(4), C7−B11 1.621(5), C8−C9 1.519(4), C9−
B10 1.624(4), B10−B11 1.724(5); C8−C7−B11 111.6(3), C7−C8−
C9 109.6(2), C8−C9−B10 111.2(2), C7−B11−B10 103.3(2), C9−
B10−B11 104.0(2). Interbelt distances: mean C−B, 1.733(4), mean
B−B 1.789(4). Lower belt: mean B−B 1.766(5), mean B1−B
1.779(5). The B−H and C−H bond distances and angles fall within
usual limits.
Structural Studies. The structures of a pair of the anionic
[8-(1-C10H7)-nido-7,8,9-C3B8H10]− (2h−) and the neutral 8-(1-
C10H7)-nido-7,8,9-C3B8H11 (2h) compounds have already been
established unambiguously by X-ray diffraction analyses, as
reported in the preliminary communication.2 The structure of
the parent (unsubstituted) anion [nido-7,8,9-C3B8H11]− has
also been determined just recently by our group.10 In this work
we are adding the structures of the anionic phenyl derivatives
for anions 2d− (mean 1.532 Å) and 2g− (mean 1.524 Å) with
the equivalent bond vectors in the parent anion (mean 1.515
Å) reveals only a slight lengthening of these distances due to
aromatic substitution. The C(7)−C(8)−C(9) angles in anions
2d− and 2g− (109.7(2)° and 109.6(2)°, respectively)
approximate the ideal pentagonal angle, while the correspond-
ing angle in the parent anion is, at 111.00(13)°, slightly larger.
Compound 2g− crystallizes in the monoclinic space group P21/
c with four molecules within the unit cell without
intermolecular hydrogen bonding. However, the short contact
between NH and the upper rim of the carborane core is the
reason for forming a layered structure shown in Figure 3.
NEt4 [8-C6H5-nido-7,8,9-C3B8H10]− (2d−) and HNEt3 [8-(4−
I-C6H4)-nido-7,8,9-C3B8H10]− (2g−) shown in Figures 1 and 2,
respectively. The data confirm again the overall Cs-symmetry
and 8-substitution on the central carbon vertex of the
pentagonal open face. Comparison of cluster C−C distances
+
+
Figure 1. ORTEP representation of the molecular structure of
+
NEt4 [8-(C6H5)-nido-7,8,9-C3B8H10]− (2d−) anion at 40% probability
level. Selected bond lengths (Å) and angles (deg): open face, C7−C8
1.536(3), C7−B11 1.631(4), C8−C9 1.528(3), C9−B10 1.630(4),
B10−B11 1.727(5); C8−C7−B11 111.3(2), C7−C8−C9 109.7(2),
C8−C9−B10 111.3(2), C7−B11−B10 103.6(2), C9−B10−B11
104.0(2). Interbelt distances: mean C−B, 1.730(4), mean B−B
1.786(4). Lower belt: mean B−B 1.765(4), mean B1−B 1.779(4). The
B−H and C−H bond distances and angles fall within usual limits.
Figure 3. Supramolecular architecture of 2g−, view along the b axis.
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Article
Table 1. NMR Assignments for the Neutral 8-R-nido-7,8,9-C3B8H11 (2) Derivatives
a
b
δ(1H) and δ(13C)
δ(11B)
μ-
H10,11
R, compd
R signals
CH7,9
C8
BH2,5
BH3,4
BH10,11
BH6
BH1
CH3, 2a
1.65(3), 22.8
2.93
,47.0
−,
−2.02 −0.1, 158,
−15.7, 171, −19.8, 147/49, −28.0, 149,
−33.5, 150,
75.7
2.58
1.90 1.82 0.78
1.15
C2H5, 2b
0.98(3), 1.90(2), 30.2, 12.4
2.98,
45.2
−,
−2.12 −0.2, 159,
−16.2, 171, −19.9, 144/46, −27.7, 146,
1.83 1.90 0.91
−340, 150,
81.2
2.59
1.25
n-C4H9, 2c 1.86(2), 1.43(2), 1.33(2), 0.92(3), 37.2,
36.9, 22.9, 14.2
2.72,
44.8
−,
−2.30 −0.5, 158,
−16.8, 169, −20.5, 146/54, −28.2, 147,
1.97 1.94 0.95
−34.4, 152,
78.9
2.71
1.28
C6H5, 2d 7.38(2), 7.34(3), 136.5, 139.0, 129.6(2),
3.29,
47.4
−,
−1.77 −0.3, 128,
−16.4, 171, −19.6, 146/45, −26.6, 147,
2.19 1.96 0.96
−33.3, 149,
128.2(2)
80.9
2.67
1.33
4-ClC6H5,
7.37(4), 135.6, 135.8, 130.1(2), 128.5
3.29,
47.5
−,
−1.73 −0.2, 162,
−16.4, 171, −19.4, 150/50, −26.4, 146,
2.14 1.90 0.97
−33.3, 149,
2e
79.8
2.66
1.33
4-BrC6H5,
7.50(2), 7.31(2), 135.7, 132.5(2),
130.3(2), 123.7
3.29,
47.3
−,
−1.75 −0.3, 162,
−16.4, 174, −19.4, 147/46, −26.4, 146,
2.16 1.93 0.97
−33.3, 149,
2f
79.9
2.66
1.33
4-IC6H5,
7.71(2), 7.17(2), 138.6(2), 136.3,
130.3(2), 95.4
3.29,
47.3
−,
−1.74 −0.3, 162,
−16.4, 171, −18.4, 157/46, −26.4, 150,
2.14 1.92 0.98
−33.3, 149,
2g
80.0
2.66
1.33
1-C10H7,
8.01−7.32(7), 130.9−125.4(10)
3.28,
49.5
−,
−1.65
1.7, 158,
2.88
−15.2, 171, −19.2, 147/37, −25.9, 147,
2.70 2.08 1.13
−33.3, 150,
2h
82.6
1.44
2-C10H7,
7.85−7.47, 129.3−125.6(10)
3.42,
47.6
−,
−1.66 −0.2, 162,
−16.2, 174, −19.5, 146/46, −26.4, 147,
−33.2, 150,
2i
81.1
2.71
2.21 2.01 1.00
1.37
a
b
In ppm relative to TMS, δ(13C) in italics, intensities other than 1 in parentheses, measured in CD3CN. In ppm relative to BF3·OEt2/1J(BH) in
Hz/δ(1H) for BH cluster units.
Table 2. NMR Assignments for [8-R-nido-7,8,9-C3B8H10] (2−) Anions (Et3NH+ Salts)
a
b
δ(1H) and δ(13C)
δ(11B)
R, compd
R signals
CH7,9
C8
BH6
BH10,11
BH2,5
BH3,4
BH1
CH3, 2a−
1.28(3), 22.8
1.33,
−,
−18.0, ∼120,
−18.0, ∼120,
−19.8, ∼170,
−19.8, ∼170,
−46.3, 137,
38.5
46.3
0.81
1.03
1.48
1.12
0.02
C2H5, 2b−
1.53(2), 0.87(3), 30.2, 13.0
1.37,
36.3
−,
−17.6, ∼120,
−17.6, ∼120,
−20.3, ∼150,
−20.3, ∼150,
−46.8, 137,
44.5
0.83
1.34
1.47
1.15
0.07
n-C4H9, 2c− 1.47(2), 1.35(2), 1.29(2), 0.84(3), 38.0,
31.8, 23.4, 14.1
1.33,
−,
−17.7, ∼130,
−17.7, ∼130,
−20.1, ∼150,
−20.1, ∼150,
−46.8, 137,
37.0
50.9
0.81
1.03
1.48
1.12
0.01
C6H5, 2d−
7.19−7.09(5), 145.2, 128.7(2), 126.5,
1.92,
38.4
−,
−15.8, −, 1.03 −16.5, ∼140,
−19.6, ∼150,
−20.6, ∼135,
−44.3, 137,
126.2(2)
52.9
1.35
1.73
1.16
0.27
4-ClC6H5,
7.16(4), 141.1, 131.7, 128.6(2), 127.8(2)
1.89,
38.5
−,
−15.8, −, 0.92 −16.3, ∼140,
−19.4, ∼155,
−20.6, ∼140,
−44.1, 140,
2e−
52.2
1.33
1.69
1.14
0.29
4-BrC6H5,
7.29(2), 7.10(2), 141.6, 131.6(2),
128.19(2), 119.8
1.90,
39.5
−,
−15.5, −, 0.92 −16.3, ∼140,
−19.4, ∼155,
−20.6, ∼140,
−44.1, 137,
2f−
52.2
1.25
1.65
1.03
0.29
4-IC6H5,
7.49(2), 6.97(2), 142.3, 137.6(2), 128.3(2),
1.89,
39.4
−,
−15.8, −, 0.92 −16.3, ∼140,
−19.4, ∼155,
−20.6, ∼140,
−44.1, 141,
2g−
91.0
54.7
1.25
1.67
1.05
0.28
1-C10H7,
8.72−7.28(7), 142.6−124.9(10)
1.64,
42.9
−,
−16.0, ∼120,
−16.0, ∼120,
−18.4, 152,
−20.1, 168,
−45.3, 141,
2h−
54.5
1.07
1.41
1.35
2.00
0.26
2-C10H7,
7.75−7.35(7), 139.7−124.0(10)
2.10,
38.5
−,
−15.7, −, 1.09 −16.2, ∼140,
−19.4, ∼155,
−20.6, ∼140,
−44.2, 140,
2i−
53.2
1.32
1.32
1.92
0.36
a
In ppm relative to TMS, δ(13C) in italics, intensities other than 1 in parentheses, measured in CD3CN. The 1H resonances due to Et3NH+ omitted.
b
In ppm relative to BF3·OEt2/1J(BH) in Hz/δ(1H) for BH cluster units.
Table 3. NMR Assignments for Selected [8-R-nido-7,8,9-C3B8H10] (2−) Anions (Tl+ Salts)
a
b
δ(1H) and δ(13C)
δ(11B)
R, compd
R signals
CH7,9
C8
BH10,11
BH6
BH2,5
BH3,4
BH1
C6H5, 2d−
7.30−7.13(5), 140.8, 129.0(2), 127.4,
2.18,
40.0
−, 56.4
−9.2, 122,
−13.4, 134,
−17.2, ∼135,
−17.4, ∼150,
−40.3, 141,
126.6(2)
1.68
1.35
1.47
1.86
1.06
4-ClC6H5,
7.26(2), 7.21(2), 140.1, 132.5, 128.7(2),
2.19,
−, 54.9
−, 54.7
−, 54.7
−10.1, 122,
−13.7, 132,
−17.8, ∼153,
−17.8, ∼153,
−40.8, 140,
2e−
128.1(2)
39.8
1.64
1.31
1.42
1.81
0.96
4-BrC6H5,
7.34(2), 7.17(2), 140.7, 132.5, 131.7(2),
2.19,
39.4
−10.2, 119,
−13.9, 128,
−17.8, ∼146,
−17.8, ∼146,
−40.7, 140,
2f−
128.4(2)
1.62
1.29
1.40
1.80
0.93
4-IC6H5,
7.54(2), 7.06(2), 141.2, 137.8(2),
128.6(2), 91.8
2.18,
39.4
−10.0, 119,
−13.6, 128,
−17.8, ∼150,
−17.8, ∼150,
−40.7, 141,
2g−
1.40
1.25
1.67
1.05
0.28
a
b
In ppm relative to TMS, δ(13C) in italics, intensities other than 1 in parentheses, measured in CD3CN. In ppm relative to BF3·OEt2/1J(BH) in
Hz/δ(1H) for BH cluster units.
The 1-naphthyl derivative 2h has been thus far a single
structurally determined2 tricarbollide of the neutral nido-
C3B8H12 series. The main structural difference between this
compound and its conjugated base 2h− lies in the presence of
the μ-H10,11 hydrogen bridge,2 which causes considerable
elongation (0.125 Å) of the open face B(10)−B(11) distance.
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NMR Spectroscopy. As assessed by multinuclear 11B, H,
and 13C NMR spectroscopy (see Tables 1−3), the structures of
all the remaining compounds isolated in this study are also in
good agreement with 8-substitution by alkyl or aryl
substituents. Because of the same Cs-symmetry, the NMR
characteristics resemble those reported earlier for the parent
nido tricarbollides [7,8,9-C3B8H11]− and 7,8,9-C3B8H12.5 As
shown in Tables 1−3, the 11B NMR spectra of compounds of
structures 2 and 2− exhibit striking similarities, indicating that
electronic effects exerted by individual alkyl and aryl
substituents do not affect chemical shifs at β- and γ-boron
positions of the cluster too much. The spectra of compounds 2
consist of 2:2:2:1:1 patterns of doublets assigned to boron
vertexes B2,5, B3,4, B10,11, B6, and B1 (reading upfield) on
the basis of [11B−11B] COSY NMR measurements.11 Table 2
also demonstrates that anions of constitution 2− (with
countercations other than Tl+) exhibit entirely different
1:2:2:2:1 sets of doublets, the differences being caused by the
absence of the open-face bridging μ-H10,11 hydrogen.5
the molecule. The method, based on carbon insertion via acid
chlorides, brings new synthetic features into the so far restricted
area of 11-vertex tricarbaboranes,1,5 being extremely flexible and
allowing for variations either in molecular shape or substituent
design. These synthetic tools may be exploited in cluster
engineering and B-cage based biochemistry, for example, by
varying substituents on carbon and boron positions in the
starting carborane 1 and R in the RCOCl reagent; other
tricarbaborane molecular shapes are expected from thermal
rearrangement reactions. Metal complexation procedures
involving the tricarbollide part of the molecule are expected
to provide a series of alkyl and aryl substituted 12-vertex
metallatricarbollides14 that parallel the pioneering work of
Sneddon’s group in the area of 11- and 10-vertex metal-
latricarbaborane chemistry.15 It is also obvious that halophenyl
substituted derivatives isolated in this study can be employed as
starting components for cross-coupling reactions on the
benzene ring.15g,16 Moreover, experiments involving bifunc-
tional aryl chlorides and development of procedures leading to
other substituted derivatives are in progress along with
extensive structural investigations. It is also reasonable to
expect that the SAC strategy can be applied also to other-than-
dicarbaborane cages, which may result in substantial extensions
and simplifications in the fields of general carborane and
heteroborane chemistry.
1
Moreover, as exemplified by the spectra of the phenylated
anions 2a−, 2b−, 2c−, and 2d−, those of their Et3NH+ (Table 2)
and Tl+ salts (Table 3) rather differ. The latter salts exhibit a
downfield shift (∼5 ppm) of the B10,11 and B1 resonances,
which points to (at least partial) coordination of the Tl+ ion to
the open pentagonal face in structure 2−. The Tl center would
then appear in a position antipodal to the B1 vertex: this type of
coordination reminds us of that observed in the structurally
similar compound Tl2[nido-C2B9H11].12 A similar effect was
also observed for Tl+ salts of 7-aminosubstituted derivatives of
the [7,8,9-C3B8H11]−anion.5 As shown in Tables 1−3, apart
from the downfield resonances attributed to the exoskeletal 8-R
EXPERIMENTAL SECTION
■
Materials and Methods. All reactions were carried out under
argon atmosphere, but subsequent operations, such as column LC,
were carried out in air. The starting carborane 1 was prepared
according to the literature.9 Dichloromethane and hexane were dried
over CaH2 and freshly distilled before use. Other chemicals were of
reagent or analytical grade and were used as purchased. Column
chromatography was performed on silica gel (Aldrich, 250−350
mesh), and the purity of individual fractions was checked on Silufol
sheets (silica gel on Al foils, detection I2 vapors, followed by AgNO3
spray). Mass spectra were recorded on a Thermo Finnigan LCQ Fleet
Ion Trap mass spectrometer. NMR spectroscopy was performed on a
Varian Mercury 400 instrument inclusive of standard [11B−11B]
COSY11 (all theoretical cross-peaks were observed) and 1H−
{11B(selective)}13 NMR experiments leading to complete assignments
of all resonances to individual cage BH units. Chemical shifts are given
in ppm to high-frequency (low field) of Ξ = 32.083 971 MHz
(nominally F3B·OEt2 in CDCl3) for 11B (quoted 0.5 ppm), Ξ =
substituent, the H NMR spectra of compounds 2 and 2−
1
consist of an intensity 2 singlet attributed to the cage CH7,9
unit; this position is being remarkably shielded (∼1.5 ppm) for
anions 2− when compared to their conjugated acids 2 that
exhibit also a typical high-field resonance (approx −2 ppm) due
to the μ-H10,11 bridging hydrogen. Table 3 also shows that
conversion into Tl+ salts is associated with a marked downfield
shift of the cage CH7,9 signals. Moreover, 1H−{11B-
(selective)}13 measurements have led to complete assignments
of all BH resonances to individual cage positions.
The 13C{1H} NMR spectra of compounds 2 and 2− exhibit
expectedly two broader (13C−11B coupling) 1:2 singlets
attributed to the cage C8 and C7,9 carbon vertexes along
with sets of resonances due to aliphatic and aromatic carbon
atoms. Within individual series of compounds, all the spectra
show striking similarities, similarly as the corresponding 11B
NMR spectra, as the electronic effects of individual aryl
substituents on the 13C nuclei do not significantly differ.
Deprotonation of compounds 2 results in remarkable shielding
of the C8 (∼28 ppm) and C7,9 positions (∼9 ppm), which
may be associated with partial localization of the negative
charge on the open C3B2 face. The 13C{1H} NMR spectra of
the corresponding Tl+ salts exhibit a slight deshielding effect in
comparison with their Et3NH+ counterparts. Mass spectra of
compounds 2 are entirely in agreement with the calculated m/z
values.
1
25.144 MHz for 13C (quoted 0.5 ppm), and Ξ = 100 MHz for H
(quoted 0.05 ppm), Ξ is defined as in ref 17, and the solvent
resonances were used as internal secondary standards.
Neutral 8-R-7,8,9-C3B8H11 (2) Compounds (R = CH3, 2a; C2H5, 2b;
n-C4H9, 2c; C6H5, 2d; 4-Cl-C6H4, 2e; 4-Br-C6H4, 2f; 4-I-C6H4, 2g; 1-
C10H7, 2h; 2-C10H7, 2i). A solution containing carborane 1 (250 mg, 2
mmol) and PS (900 mg, 4.2 mmol) in dichloromethane (or 1,2-
dichloroethane) (30 mL) was cooled to ∼0 °C, and the corresponding
RCOCl (∼5 mmol) chloride was then added in small portions under
stirring over 0.5 h. The cooling bath was then removed, and the
stirring continued for 24 h at reflux temperature. The mixture was then
treated with conc H2SO4 (∼2 mL, dropwise) under intensive cooling
and shaking at ∼0 °C. The organic layer was carefully separated from
the semiliquid materials sticking on the walls of the reaction flask. The
solution thus obtained was then evaporated after adding silica gel (∼5
g). The residual material was mounted onto the top of a column
packed with silica gel (∼2.5 cm × 20 cm) which was then eluted with
100% hexane. Combined fractions of RF (anal.) ∼0.25−0.3 were
evaporated to dryness to isolate typically 1.2−1.4 (60−70%) and 1.6−
1.9 (80−95%) mmol of the corresponding 8-R-7,8,9-C3B8H11 (2)
derivatives (R = alkyl and aryl, respectively). Akyl derivatives were
isolated as liquid or low-melting substances, while aryl compounds
were obtained as white crystalline solids that can be crystallized by
CONCLUSIONS
■
It has been established undoubtedly that the novel SAC
method for carbon insertion appears to be highly effective for
the preparation of tricarbollide compounds bearing alkyl and
aryl functionalities located symmetrically on the open face of
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Inorganic Chemistry
Article
slow evaporation of the hexane solution. For 2a: liquid; m/z (max)
calcd 148.18, found 148.18; for C4H14B8 (mw 148.71) calcd 32.30% C,
9.48% H, found 33.10% C, 9.32% H. For 2b: mp 30−31 °C; m/z
(max) calcd 162.20, found 162.21; for C5H16B8 (mw 162.74) calcd
36.90% C, 9.91% H, found 36.51% C, 9.61% H. For 2c: mp 28−29 °C;
m/z (max) calcd 190.23, found 190.24; for C7H20B8 (mw 190.79)
calcd 44.06% C, 10.57% H, found 42.93% C, 10.18% H. For 2d: mp 82
°C; m/z (max) calcd 211.19, found 211.20; for C9H16B8 (mw 210.78)
calcd 51.30% C, 7.65% H, found 51.71% C, 7.56% H. For 2e: mp 84
°C; m/z (max) calcd 244.16, found 244.17; for C9H15B8Cl (mw
245.15) calcd 44.09% C, 6.17% H, found 44.18% C, 6.25% H. For 2f:
mp 85.5 °C; m/z (max) calcd 289.11, found 289.09; for C9H15B8Br
(mw 289.60) calcd 37.33% C, 5.22% H, found 38.10% C, 5.34% H.
For 2g: mp 86 °C; m/z (max) calcd 336.09, found 336.10; for
C9H15B8I (mw 336.60) calcd 32.11% C, 4.49% H, found 30.91% C,
4.38% H. For 2h: mp 84 °C; m/z (max) calcd 260.21, found 260.25;
for C13H18B8 (mw 260.77) calcd 59.88% C, 6.96% H, found 60.10% C,
6.82% H. For 2i: mp 82 °C; m/z (max) calcd 260.21, found 260.23;
for C13H18B8 (mw 260.77) calcd 59.88% C, 6.96% H, found 60.14% C,
6.92% H. For NMR spectra see Table 1.
cage. The hydrogen atom of the N−H group was placed according to
the maxima on the Fourier difference map. Crystallographic data for
2d−: C17H35B8N, M = 339.94, monoclinic, P21/c, a = 9.8570(6) Å, b =
15.3321(9) Å, c = 13.8940(9) Å, β = 91.977(5)°, Z = 4, V = 2098.5(2)
Å3, Dc = 1.076 g cm−3, μ = 0.055 mm−1, Tmin/Tmax = 0.988/0.993; −11
≤ h ≤ 12, −18 ≤ k ≤ 19, −18 ≤ l ≤ 17; 15 241 reflections measured
(θmax = 27.4°), 15 075 independent (Rint = 0.0553), 3164 with I >
2σ(I), 235 parameters, S = 1.130, R1(obsd data) = 0.0763, wR2(all
data) = 0.1664; max, min residual electron density = 0.537, −0.477 e
Å−3. For 2g¯: C15H30B8IN, M = 437.78, monoclinic, P21/c, a =
9.3060(3) Å, b = 11.1300(8) Å, c = 20.1821(11) Å, β = 97.688(4)°, Z
= 4, V = 2071.6(2) Å3, Dc = 1.404 g.cm−3, μ = 1.544 mm−1, Tmin/Tmax
= 0.626/0.801; −12 ≤ h ≤ 11, −14 ≤ k ≤ 14, −26 ≤ l ≤ 24; 18 184
reflections measured (θmax = 27.5°), 18 128 independent (Rint
=
0.0265), 3892 with I > 2σ(I), 242 parameters, S = 1.144, R1(obsd
data) = 0.0402, wR2(all data) = 0.0829; max, min residual electron
density = 0.894, −0.615 e Å−3. Crystallographic data for structural
analyses have been deposited with the Cambridge Crystallographic
Data Centre, CCDC deposition nos. 898534 and 924312 for 2d− and
2g−, respectively. Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road, Cambridge CB2
1EY, U.K. (Fax: +44-1223-336033. E-mail: deposit@ccdc.cam.ac.uk or
www: http:// www.ccdc.cam.ac.uk.)
[8-R-7,8,9-C3B8H10]− (2−) Anions. For Et3NH+ salts, details follow.
A solution of the corresponding compound 2 (reaction scale 1 mmol)
in dichloromethane (15 mL) was treated dropwise with Et3N (110 mg,
1.1 mmol) under cooling at ∼0 °C. The mixture was then evaporated
to dryness at room temperature and the residual white solid dried for 4
h at room temperature to obtain the corresponding Et3NH+[8-R-7,8,9-
C3B8H10]− salt as white crystals in quantitative yield. For Tl+ and
Et4N+ salts, details follow. A compound of structure 2 (reaction scale 1
mmol) was dissolved in 0.1 M aq NaOH (20 mL) under shaking, and
the solution thus obtained was precipitated by adding 1 M aq TlNO3
or Et4NCl. The white precipitates were isolated by filtration, washed
with water (3 × 20 mL), and vacuum-dried to obtain the
corresponding salts in practically quantitative yield as white crystalline
solids. For metathesis of Et3NH+ salts, details follow. An arbitrary
Et3NH+ salt of anion 2− (reaction scale 1 mmol) was dissolved in 0.1
M aq NaOH (20 mL) under shaking, and the Et3N thus evolved was
evaporated in vacuo. The residual aqueous solution of the Na+ salt was
precipitated as described to obtain the corresponding Tl+ (or Et4N+)
salts that were vacuum-dried. For 2a− (Et3NH+ salt): for C10H29NB8
(mw 249.9) calcd 48.06% C, 11.70% H, found 49.10% C, 11.90% H.
For 2b− (Et3NH+ salt): for C11H31NB8 (mw 263.93) calcd 50.06% C,
11.84% H, found 50.21% C, 11.69% H. For 2c− (Et3NH+ salt): for
C13H35NB8 (mw 291.98) calcd 53.47% C, 12.08% H, found 52.83% C,
11.84% H. For 2d− (Et3NH+ salt): for C15H31B8N (mw 311.90) calcd
57.76% C, 10.02% H, found 58.10% C, 10.15% H. For 2e− (Et3NH+
salt): for C15H30B8NCl (mw 346.34) calcd 52.02% C, 8.73% H, found
51.83% C, 8.81% H. For 2f− (Et3NH+ salt): for C15H30B8NBr (mw
390.80) calcd 46.10% C, 7.74% H, found 45.53% C, 7.51% H. For 2g−
(Et3NH+ salt): for C15H30B8NI (mw 437.80) calcd 41.15% C, 6.91%
H, found 40.83% C, 6.68% H. For 2h− (Et3NH+ salt): m/z (max)
calcd 260.25, found 260.24; for C19H33B8N (mw 361.96) calcd 63.05%
C, 9.19% H, found 62.80% C, 9.28% H. For 2i− (Et3NH+ salt): m/z
(max) calcd 260.25, found 260.25; for C19H33B8N (mw 361.96) calcd
63.05% C, 9.19% H, found 63.10% C, 9.37% H. For NMR spectra of
the Et3NH+ and selected Tl+ salts see Tables 2 and 3, respectively.
X-ray Crystallography. The X-ray data for white crystals of
anions 2d− and 2g− (NEt4+ and HNEt3+ salts) were obtained at 150 K
using Oxford Cryostream low-temperature device and a Nonius
KappaCCD diffractometer with Mo Kα radiation (λ = 0.710 73 Å), a
graphite monochromator, and the ϕ and χ scan mode. Data reductions
were performed with DENZO-SMN.18 The absorption was corrected
by integration methods.19 Structures were solved by direct methods
(Sir92)20 and refined by full matrix least-squares based on F2
(SHELXL97).21 Hydrogen atoms could be mostly localized on a
difference Fourier map. However, to ensure uniformity of treatment of
crystal structures, they were recalculated into idealized positions
(riding model) and assigned temperature factors Uiso(H) =
1.2Ueq(pivot atom) or of 1.5Ueq for the methyl moieties with C−H
= 0.96, 0.97, and 0.93 Å for the methyl, methylene, and aromatic
hydrogen atoms and 1.1 Å for B−H and C−H bonds in the carborane
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: stibr@iic.cas.cz. Phone: +420-2-6617-3106. Fax: +420-
2-2094-1502.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was supported by the Grant Agency of the Czech
Republic (Project P207/11/0705). We also thank Ing. Z.
̌
́
Haj
̌
́
kova
́
and Dr. V. Sicha for MS measurements, and Drs. P
́
Svec and R. Olejnik for their help in earlier stages of this work.
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