Synthesis and characterisation of the complete series of B-N analogues of triptycene

Article abstract of DOI:10.1039/c4dt00442f

The reaction between the bisborate Li₂[o-C₆H ₄(BH₃)₂] and 2 equivalents of an appropriate pyrazole derivative (Hpz^R) in the presence of Me₃SiCl yields o-phenylene-bridged pyrazaboles HB(μ-pz^R) ₂(μ-o-C₆H₄)BH (3a-3e; Hpz^R = 4-iodopyrazole (3a), 4-(trimethylsilyl)pyrazole (3b), 3,5-dimethylpyrazole (3c), 3,5-di(tert-butyl)pyrazole (3d), 3,5-bis(trifluoromethyl)pyrazole (3e)). The synthesis approach thus provides access to uncharged B-N triptycenes bearing (i) functionalisable groups, (ii) electron-donating or -withdrawing substituents and (iii) pyrazole rings of varying steric demand. Treatment of p-R*C ₆H₄BBr₂ with the potassium tris(pyrazol-1-yl) borates K[HBpz₃] or K[p-R*C₆H₄Bpz ₃] yields cationic pyrazolyl-bridged pyrazaboles [p-BrC ₆H₄B(μ-pz)₃BH]Br ([4a]Br) and [p-R*C₆H₄B(μ-pz)₃Bp-C ₆H₄R*]Br (R* = Br ([4b]Br), I ([4c]Br), SiMe₃ ([4d]Br)), which can be regarded as full B-N analogues of triptycene. The B-H bonds of 3b and [4a]Br are unreactive towards tBuCCH even at temperatures of 80 °C, thereby indicating an appreciable thermal stability of the corresponding B-N cage bonds. Most of the cage compounds are sufficiently inert towards water to allow quick aqueous workup. However, NMR spectroscopy in CD₃OD solution reveals degradation of 3b or [4a]Br to the corresponding pyrazoles and o-C₆H₄(B(OCD₃) ₂)₂ or p-BrC₆H₄B(OCD ₃)₂/B(OCD₃)₃. The diphenylated species [4b]Br is significantly more stable under the same measurement conditions; even after 76 d, most of the material degrades only to the stage of the syn/anti-pyrazaboles p-BrC₆H₄(CD₃O)B(μ- pz)₂B(OCD₃)p-C₆H₄Br (11a/11b). A derivatisation of [4c]Br with nBu₃SnCCtBu through Stille-type coupling reactions furnishes the alkynyl derivative [p-tBuCCC₆H ₄B(μ-pz)₃Bp-C₆H₄CCtBu]Br ([4e]Br). Larger B-N aggregates are also accessible: treatment of the tetrakisborate Li₄[1,2,4,5-C₆H₂(BH ₃)₄] with 4 equivalents of Hpz^R in the presence of Me₃SiCl leads to the corresponding B-N pentiptycenes 14a-14d (Hpz^R = 3,5-bis(trifluoromethyl)pyrazole (14a), 4-(trimethylsilyl) pyrazole (14b), 3,5-dimethylpyrazole (14c), 3,5-di(tert-butyl)pyrazole (14d)). the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00442f

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Synthesis and characterisation of the complete
series of B–N analogues of triptycene†
Cite this: Dalton Trans., 2014, 43,
8241
Ömer Seven, Sebastian Popp, Michael Bolte, Hans-Wolfram Lerner and
Matthias Wagner*
The reaction between the bisborate Li2[o-C6H4(BH3)2] and 2 equivalents of an appropriate pyrazole
derivative (HpzR) in the presence of Me3SiCl yields o-phenylene-bridged pyrazaboles HB(μ-pzR)2(μ-o-
C6H4)BH (3a–3e; HpzR = 4-iodopyrazole (3a), 4-(trimethylsilyl)pyrazole (3b), 3,5-dimethylpyrazole (3c),
3,5-di(tert-butyl)pyrazole (3d), 3,5-bis(trifluoromethyl)pyrazole (3e)). The synthesis approach thus pro-
vides access to uncharged B–N triptycenes bearing (i) functionalisable groups, (ii) electron-donating or
-withdrawing substituents and (iii) pyrazole rings of varying steric demand. Treatment of p-R*C6H4BBr2
with the potassium tris(pyrazol-1-yl)borates K[HBpz3] or K[p-R*C6H4Bpz3] yields cationic pyrazolyl-
bridged pyrazaboles [p-BrC6H4B(μ-pz)3BH]Br ([4a]Br) and [p-R*C6H4B(μ-pz)3Bp-C6H4R*]Br (R* = Br
([4b]Br), I ([4c]Br), SiMe3 ([4d]Br)), which can be regarded as full B–N analogues of triptycene. The B–H
bonds of 3b and [4a]Br are unreactive towards tBuCuCH even at temperatures of 80 °C, thereby indicat-
ing an appreciable thermal stability of the corresponding B–N cage bonds. Most of the cage compounds
are sufficiently inert towards water to allow quick aqueous workup. However, NMR spectroscopy in
CD3OD solution reveals degradation of 3b or [4a]Br to the corresponding pyrazoles and o-C6H4(B-
(OCD3)2)2 or p-BrC6H4B(OCD3)2/B(OCD3)3. The diphenylated species [4b]Br is significantly more stable
under the same measurement conditions; even after 76 d, most of the material degrades only to the
stage of the syn/anti-pyrazaboles p-BrC6H4(CD3O)B(μ-pz)2B(OCD3)p-C6H4Br (11a/11b). A derivatisation
of [4c]Br with nBu3SnCuCtBu through Stille-type coupling reactions furnishes the alkynyl derivative
[p-tBuCuCC6H4B(μ-pz)3Bp-C6H4CuCtBu]Br ([4e]Br). Larger B–N aggregates are also accessible: treat-
ment of the tetrakisborate Li4[1,2,4,5-C6H2(BH3)4] with 4 equivalents of HpzR in the presence of Me3SiCl
leads to the corresponding B–N pentiptycenes 14a–14d (HpzR = 3,5-bis(trifluoromethyl)pyrazole (14a),
4-(trimethylsilyl)pyrazole (14b), 3,5-dimethylpyrazole (14c), 3,5-di(tert-butyl)pyrazole (14d)).
Received 11th February 2014,
Accepted 25th February 2014
DOI: 10.1039/c4dt00442f
www.rsc.org/dalton
leads to void spaces in a supramolecular assembly, called the
“internal free volume” (IFV), which can be used to store small
Introduction
Triptycenes 1 (Fig. 1) are a class of organic compounds posses- guest molecules.1 On the other hand, the three triptycene
sing a three-dimensional paddle-wheel structure. Due to their blades have also been used to create interlocking molecular
rigid bicyclo[2.2.2]octatriene core, angles close to 120° are structures acting as rotors, gearings or brakes in molecular
maintained between the three o-phenylene ring panels, which machines.2,3 Triptycenes are therefore useful building blocks
leads to the formation of three equally sized compartments for the generation of sophisticated molecular architectures
lined with π-electron clouds. On the one hand, this unique with well-defined geometries.4 During the last few decades, a
geometry often frustrates space-efficient packing and thereby wide spectrum of applications has opened up, ranging from
materials5,6 and sensor science7 to host–guest5,8 and coordi-
nation chemistry.4,9
Further developments in these fields are critically depen-
dent on the availability of broadly functionalised triptycene
Institut für Anorganische Chemie, J.W. Goethe-Universität Frankfurt, Max-von-Laue-
Strasse 7, D-60438 Frankfurt (Main), Germany.
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
derivatives. To date, the Diels–Alder reactions between anthra-
cenes and p-benzoquinones or benzynes have offered the most
important methods for the assembly of triptycene scaffolds
(Scheme 1). With respect to functional-group compatibility, it
is therefore essential to be able to release benzyne intermedi-
ates in situ under mild conditions and with high efficiency;
†Electronic supplementary information (ESI) available: Experimental details
including assigned NMR data of 3c, 3d, 3e, K[8] (R = p-Me3SiC6H4; p-IC6H4), 14c
and 14d. Plots of the 13C{1H} NMR spectra of 3a, 3c, 3d, [4a]Br, [4c]Br, [4e]Br,
14b, 14c and 14d. Decomposition experiments of 3b, [4a]Br and [4b]Br in
CD3OD and [4d]Br in CH3OH. X-ray crystal structure analyses of 3c, 3d, 3e,
3f·OEt2 and 12a. CCDC 985503–985515. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c4dt00442f
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Fig. 1 Triptycene (1) and its B–N analogues [2]−, 3 and [4]+.
Our group has a long-standing interest in exploiting the
B–N/C–C isosterism for the facile synthesis of complex mole-
cular frameworks.13–16 This approach takes advantage of the
fact that B–N adduct bonds tend to form spontaneously in
essentially quantitative yields whereas the isoelectronic C–C
single bonds are usually much harder to establish.17,18 The
general concept of B–N/C–C isosterism is very well applicable
for the preparation of triptycene analogues (cf. compounds
[2]−, 3 and [4]+; Fig. 1).
In a previous communication, we have already reported on
the reaction between the Me2S adduct of 9,10-dihydro-9,10-
diboraanthracene (6; Scheme 1) and pyridazine, which gives a
soluble B–N congener of triptycene.19 The synthesis approach
also works faithfully for phthalazine (benzo[d]pyridazine) or
B-substituted 9,10-dihydro-9,10-diboraanthracenes as the start-
ing materials. If lithium pyrazolides are used instead of pyrid-
azine, ionic compounds of type Li[2] are obtained (Scheme 1;
Fig. 1).19
While the third member of the family of B–N triptycenes,
compound 3 (Fig. 1), has been unknown, scattered reports of
cationic [4]+-type species can be found in the literature. Most
of the times, the latter molecules have been observed as side
products of the complexation of Lewis-acidic transition metal
ions with tris(pyrazol-1-yl)borate ligands.20 Two early reports
of targeted syntheses do, however, also exist: Trofimenko pre-
pared [4]PF6 (R = R′ = Et) in yields of 22% through the reaction
of pyrazolide ions with EtB(OTos)2 and subsequent salt meta-
thesis (HOTos = toluenesulfonic acid); he also mentioned the
relationship between [4]+ and triptycene.21 In a later report,
Niedenzu et al. described the synthesis of derivatives of [4]+
carrying methyl groups at the pyrazolyl rings. The boron atoms
were either equipped with one hydrogen atom each or with
one hydrogen atom and one ethyl group.22 The synthesis
approach relied on the reaction of tris(pyrazol-1-yl)borate
anions (cf. [8]−; Scheme 1) with trigonal boranes containing
two good leaving groups.
Scheme 1 General synthesis methods of all-carbon triptycenes and of
the complete series of B–N triptycenes. General reaction conditions: (i)
Et2O, rt, between 12 h and 15 h. (ii) CH3CN-toluene or toluene, between
rt and 120 °C, between 8 h and 24 h.
good results are usually obtained through aprotic diazotisation
of anthranilic acids with amyl nitrite10 or upon treatment of
(phenyl)[2-(trimethylsilyl)phenyl]iodonium triflate with an
appropriate fluoride source.11 Even though various halo-
genated 1,2-bis(trimethylsilyl)benzenes have recently been
made available,12 which are suitable starting materials for the
synthesis of corresponding halogenated hypervalent iodine
benzyne precursors, the crucial role of the benzyne intermedi-
ates in triptycene syntheses still constitutes a bottleneck on
the way to novel triptycene-containing functional units.
The purpose of this paper is to provide high-yield syntheses
and full characterisations of selected examples of 3- and [4]+-
type molecules together with compounds possessing extended
B–N pentiptycene-type architectures. Special emphasis will be
placed on the development of C-halogenated derivatives,
which possess promising potential as future building blocks
of (supra)molecular structures. Moreover, we will investigate
and compare the stabilities of the bicyclo[2.2.2]octatriene
cores of [2]−, 3 and [4]+ in order to explore the scope and limit-
ations of these molecules as triptycene substitutes.
8242 | Dalton Trans., 2014, 43, 8241–8253
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Cationic [4]+-type B–N triptycenes are accessible starting
from readily available potassium tris(pyrazol-1-yl)borates27,28
(K[8]; cf. the ESI† for detailed information) and dibromo-
(organyl)boranes (Scheme 1). This synthesis approach provides
Results and discussion
Synthesis and NMR-spectroscopic characterisation of 3- and
[4]+-type molecules
Neutral 3-type compounds were prepared from the o-phenyl- access to unsymmetrically (R = H, R′ = p-BrC6H4 ([4a]Br);
ene-bridged bisborate Li2[7]23 and 2 equivalents of an appro- Fig. 2) as well as symmetrically B-functionalised compounds
priately substituted pyrazole (Scheme 1). The first two B–N (R = R′ = p-BrC6H4 ([4b]Br), p-IC6H4 ([4c]Br) or p-Me3SiC6H4
bonds assemble spontaneously at room temperature through a ([4d]Br); Fig. 2). The combination of R = R′ = (substituted)
Brønsted acid–base reaction with liberation of H2. Subsequent phenyl with 3,5-dimethylpyrazole generally failed to give the
addition of excess Me3SiCl as a hydride scavenger creates free desired B–N triptycenes, likely due to prohibitively high steric
coordination sites at the boron centres; this leads to the for- congestion. The use of 4-bromo- or 4-iodopyrazole led to very
mation of two intramolecular B–N adduct bonds and thereby poorly soluble products, which were hard to purify and charac-
closes the molecular cage. The synthesis protocol tolerates terise and are consequently not considered further.
various degrees of steric bulk at the 3,5-positions of the pyrazo-
The 11B{1H} NMR shifts of all the B–N triptycenes under
lyl rings: for example, derivatives carrying hydrogen atoms (3a, consideration here fall in the range between 0.2 ppm and
3b; Fig. 2), methyl groups (3c) or tert-butyl groups (3d) are all −7.0 ppm, thereby testifying to the presence of tetracoordi-
formed in good yields. It is also possible to employ both elec- nated boron nuclei.29 Compound [4a]Br, which possesses two
tron-richer (cf. the H3C-derivative 3c) and electron-poorer pyra- magnetically inequivalent boron centres, gives rise to two
zoles (cf. the CF3-derivative 3e) with similar success; 3e is, signals in the 11B{1H} NMR spectrum (−0.5 ppm (BC);
however, significantly more prone to hydrolysis than 3c. We −5.4 ppm (BH)); all other molecules show only one resonance
also found that the o-phenylene ring may be replaced by other each. B,H-coupling is resolved only in the cases of 3c (d, 1JB,H
=
1
1
(metal)organic fragments, because the synthesis sequence can 98 Hz) and [4a]Br (d, JB,H = 128 Hz). In the H NMR spectra,
also be applied to the 1,1′-ferrocenylene-bridged bisborate the boron-bonded hydrogen atoms give rise to very broad reso-
Li2[1,1′-fc(BH3)2]24,25 to give compounds like 3f (Fig. 2). 3f can nances in the interval between 5.5 ppm and 4.0 ppm; only for
not only be viewed as an Fe-containing B–N triptycene, but the signal of 3c the expected quartet multiplicity is visible.
also as an ansa-ferrocene with pyrazabole26 bridge.14
All the proton spectra of 3a–3e contain two characteristic
multiplets for the o-phenylene rings; the organometallic
derivative 3f shows two virtual triplets at 4.12 ppm and
3.22 ppm for the 1,1′-ferrocenylene backbone. For each of the
molecules, the overall integral ratio between the o-phenylene/
1,1′-ferrocenylene resonances on the one hand and the pyrazo-
lyl signals on the other confirms the proposed 1 : 2 ratio of the
two moieties in 3-type compounds. Moreover, the number of
1
signals in the H and 13C{1H} NMR spectra is in line with the
suggested symmetry of the molecular frameworks.
1
In the H NMR spectrum of the unsymmetrically B-substi-
tuted cation [4a]+, the pyrazolyl rings lead to two doublets with
3
characteristic low-field shifts and small JH,H coupling con-
stants (8.24 ppm, 8.02 ppm; 2.5 Hz) and to one high-field
shifted virtual triplet (6.51 ppm). The R substituent gives the
two approximate doublet resonances that are expected for
AA′BB′ spin systems. On going from [4a]Br to the symmetrically
B-substituted species like [4b]Br, the two pyrazolyl doublets
become one signal of double intensity and the pyrazolyl :
phenylene integral ratios change from 9 H : 4 H to 9 H : 8 H.
X-ray crystal structure analyses of B–N triptycenes
Selected crystallographic data for the structure analyses dis-
cussed in the following paragraphs are compiled in Tables
1–3. (Note: In a crystallographic context, we are, for reasons of
simplicity, treating compounds containing deuterium atoms
as if they were containing exclusively hydrogen atoms.)
The reaction providing compound 3f is less selective than
in the case of the o-phenylene congener 3a and the ferrocene
derivative is more sensitive to air and moisture. Thus, even
though we succeeded in an NMR spectroscopic and X-ray
Fig. 2 Compilation of the B–N triptycenes of types [2]−, 3 and [4]+ dis-
cussed in this paper.
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Table 1 Crystallographic data for 3f*·3C6H6, 3a and [4a]Br·CH2Cl2
3f*·3C6H6
3a
[4a]Br·CH2Cl2
Formula
Mr
C35H31B4Fe2I5N10·3 C6H6
1615.46
C12H10B2I2N4
485.66
C15H14B2Br2N6·CH2Cl2
544.69
Colour, shape
T [K]
Orange, block
173(2)
Colourless, block
173(2)
Colourless, block
173(2)
Radiation, λ [Å]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
MoKα, 0.71073
Monoclinic
P21/c
21.7729(16)
11.9678(12)
24.4436(18)
90
MoKα, 0.71073
MoKα, 0.71073
Orthorhombic
Pnma
12.0607(5)
9.6544(6)
18.1880(8)
90
Trigonal
ˉ
R3
18.7020(7)
18.7020(7)
23.5495(9)
90
α [°]
β [°]
115.688(5)
90
5739.9(9)
4
1.869
3096
3.240
0.10 × 0.10 × 0.10
41 538
90
120
7133.3(6)
18
2.035
4068
3.961
0.20 × 0.20 × 0.10
35 425
90
90
γ [°]
V [Å3]
Z
2117.79(18)
4
1.708
1072
4.095
0.20 × 0.10 × 0.05
37 320
Dcalcd [g cm−3
F(000)
]
μ [mm−1
]
Crystal size [mm3]
Rflns collected
Independent rflns (Rint
)
10 130 (0.1528)
10 130/390/667
0.914
3057 (0.0689)
3057/0/190
1.082
2170 (0.1048)
2170/0/147
1.180
Data/restraints/parameters
GOF on F2
R1, wR2 [I > 2σ(I)]
0.0678, 0.0973
0.1459, 0.1167
1.494, −0.848
0.0219, 0.0531
0.0221, 0.0533
1.246, −0.649
0.0353, 0.0780
0.0383, 0.0794
0.624, −0.367
R1, wR2 (all data)
Largest diff. peak and hole [e Å−3
]
Table 2 Crystallographic data for [4b]Br, 14a·3 C7H8 and 14b·2 CH2Cl2
[4b]Br
14a·3 C7H8
14b·2 CH2Cl2
Formula
Mr
C21H17B2Br3N6
614.76
C26H10B4F24N8·3 C7H8
1210.06
C30H50B4N8Si4·2 CH2Cl2
848.23
Colour, shape
T [K]
Colourless, block
173(2)
Light brown, block
173(2)
Colourless, block
173(2)
Radiation, λ [Å]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
γ [°]
MoKα, 0.71073
MoKα, 0.71073
MoKα, 0.71073
Triclinic
Triclinic
Triclinic
ˉ
P1
ˉ
P1
ˉ
P1
9.7625(8)
11.1593(8)
11.5455(9)
90.305(6)
107.707(6)
106.691(6)
1141.79(15)
2
1.788
600
5.322
0.20 × 0.15 × 0.10
10 831
8.8267(8)
11.3525(11)
14.2556(14)
99.583(8)
106.299(7)
106.005(7)
1271.1(2)
1
1.581
608
0.156
0.26 × 0.25 × 0.22
15 695
10.610(2)
11.003(3)
12.115(3)
63.855(17)
70.231(17)
80.803(17)
1194.7(5)
1
1.179
446
0.380
0.21 × 0.14 × 0.09
10 352
V [Å3]
Z
Dcalcd [g cm−3
F(000)
]
μ [mm−1
]
Crystal size [mm3]
Rflns collected
Independent rflns (Rint
)
4185 (0.0570)
4185/0/289
1.015
4864 (0.0645)
4864/0/397
1.046
4202 (0.1236)
4202/0/243
0.856
Data/restraints/parameters
GOF on F2
R1, wR2 [I > 2σ(I)]
0.0363, 0.0872
0.0474, 0.0911
0.596, −0.721
0.0451, 0.1127
0.0580, 0.1189
0.262, −0.265
0.0774, 0.1452
0.1890, 0.1809
0.681, −0.406
R1, wR2 (all data)
Largest diff. peak and hole [e Å−3
]
crystallographic characterisation of 3f, we have not been able storage of an NMR tube containing a solution of crude 3f in
to isolate the compound in an analytically pure form (cf. the C6D6. X-ray crystallography revealed a neutral dinuclear ferro-
ESI† for a plot of the molecular structure, which revealed no cene species (3f*; Fig. 3) containing 1 pyrazole ligand, 4 pyra-
significant differences compared to other pyrazabole-bridged zolyl moieties and 4 B–H bonds. As a remarkable feature of
ansa-ferrocenes14). In two instances, single crystals of a side this side product, we note that the interannular bridge of each
product/decomposition product of 3f grew upon prolonged of the two ferrocene fragments consists of one pyrazolyl ring
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Compounds 3a, [4a]Br and [4b]Br have also been character-
ised by X-ray crystallography (Fig. 4 and 5). Compounds 3a and
[4b]+ possess no symmetry element in the crystal lattice; the
solid-state structure of the cation [4a]+ features a mirror plane
containing the pyrazolyl ring pz(N(1)). All three molecules
exhibit the aimed-for paddle-wheel structure with angles
between the three core planes [B(1)N(1)N(2)B(2), B(1)N(11)-
N(12)B(2) and B(1)C(21)C(22)B(2) (3a) or B(1)N(11A)N(12A)B(2)
([4a]Br) or B(1)N(21)N(22)B(2) ([4b]Br)] close to 120°.30 The
same is true for the angles between the corresponding planes
in Li[2a] (Fig. 2).19 In the cases of the two cationic B–N tripty-
cenes, we observe kinks along certain N–N bonds resulting in
dihedral angles between the pyrazolyl blades that can be as
small as pz(N(11))//pz(N(11A)) = 100.6(1)° ([4a]Br) or as large as
pz(N(1))//pz(N(21)) = 138.7(2)° ([4b]Br).
The average B–N bond lengths decrease continuously upon
going from Li[2a] (1.605 Å) to 3a (1.579 Å) to [4b]Br (1.562 Å).
For [4a]Br, a comparison of the average B–N bond lengths
about B(1) (1.570 Å) and B(2) (1.546 Å) indicates considerable
steric repulsion between the p-BrC6H4 substituent and the
bridging pyrazolide moieties. A related effect is obvious in the
case of 9-phenyltriptycene, which shows longer endocyclic C–C
bonds about the phenylated bridgehead carbon atom (average
value = 1.554 Å) than about the unsubstituted bridgehead
atom (average value = 1.513 Å).31
Table 3 Crystallographic data for 11a and 11b
11a
11b
Formula
Mr
C
20H20B2Br2N4O2
C20H20B2Br2N4O2
529.84
Colourless, plate
173(2)
MoKα, 0.71073
Monoclinic
P21/n
7.6702(5)
12.4132(7)
11.2406(7)
90
102.351(5)
90
1045.47(11)
2
1.683
528
3.902
0.24 × 0.19 × 0.13
14 759
2137 (0.0915)
2137/0/136
0.965
0.0293, 0.0660
0.0415, 0.0696
0.347, −0.570
529.84
Colour, shape
T [K]
Colourless, block
173(2)
Radiation, λ [Å]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
MoKα, 0.71073
Monoclinic
P21/c
15.7852(7)
22.9613(10)
12.9046(5)
90
α [°]
β [°]
113.391(3)
90
4292.9(3)
8
1.640
2112
3.801
0.32 × 0.31 × 0.31
92 971
γ [°]
V [Å3]
Z
Dcalcd [g cm−3
F(000)
]
μ [mm−1
]
Crystal size [mm3]
Rflns collected
Independent rflns (Rint
)
9294 (0.0782)
9294/0/541
1.025
Data/restraints/parameters
GOF on F2
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0598, 0.1343
0.1048, 0.1562
0.553, −1.016
Largest diff. peak and
hole [e Å−3
]
Similar to the B–N bonds, the intramolecular B⋯B dis-
tances contract along the series Li[2a] (2.734(6) Å) → 3a
(2.707(5) Å) → [4a]Br (2.676(6) Å)/[4b]Br (2.702(5) Å). Even
shorter through-space distances are found between the two
bridgehead carbon atoms in 9-phenyltriptycene (2.630 Å)31
and in pristine triptycene (2.604 Å).32
Fig. 3 Molecular structure of 3f*·3C6H6 in the solid state. Hydrogen
atoms (except on boron and N(42)) and the C6H6 molecules have been
omitted for clarity; displacement ellipsoids are drawn at the 50% prob-
ability level. The bond lengths and bond angles are not given due to
poor crystallographic data which lead to large error margins. The H
atom at N(42) was found in the difference Fourier map, but it could not
be isotropically refined.
Fig. 4 Molecular structure of 3a in the solid state. Hydrogen atoms
except on boron have been omitted for clarity; displacement ellipsoids
are drawn at the 50% probability level. Selected bond lengths (Å),
atom⋯atom distance (Å), bond angles (°) and dihedral angles (°): B(1)–N(1)
1.575(4), B(1)–N(11) 1.588(4), B(1)–C(21) 1.611(5), B(2)–N(2) 1.574(4),
B(2)–N(12) 1.579(4), B(2)–C(22) 1.604(5), B(1)⋯B(2) 2.707(5); N(1)–B(1)–
N(11) 101.5(2), N(1)–B(1)–C(21) 105.7(3), N(11)–B(1)–C(21) 104.0(2), N(2)–
B(2)–N(12) 100.9(2), N(2)–B(2)–C(22) 105.8(3), N(12)–B(2)–C(22)
104.4(3); N(1)B(1)N(11)//N(2)B(2)N(12) 84.1(3), N(1)B(1)N(11)//N(1)N(2)-
N(11)N(12) 47.7(2), N(2)B(2)N(12)//N(1)N(2)N(11)N(12) 48.2(3), B(1)N(1)-
N(2)B(2)//B(1)N(11)N(12)B(2) 117.3(1), B(1)N(1)N(2)B(2)//B(1)C(21)C(22)B(2)
122.4(2), B(1)N(11)N(12)B(2)//B(1)C(21)C(22)B(2) 120.3(2), pz(N(1))//pz-
(N(11)) 123.0(1), pz(N(1))//C6H4 122.5(1), pz(N(11))//C6H4 114.4(1); pz(N(X))
= pyrazolyl ring containing N(X).
only. A third ring acts as a linker between the organometallic
units, whereas the remaining two heterocyclic rings are term-
inal substituents. A proton residing on the nitrogen atom
N(42) forms a hydrogen bond with the nitrogen atom N(2) to
give a HB4N10 macrocycle.
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In a second series of experiments, we investigated the sensi-
tivity of B–N triptycenes toward air and moisture. While the
B–H bonds of free 9,10-dihydro-9,10-diboraanthracene are
extremely prone to hydrolysis,34 it takes several hours until the
1H NMR spectrum of compound Li[2a] in non-dried d8-THF
shows significant signs of decomposition.19 In the cases of 3b,
[4a]Br and [4b]Br, even quick aqueous workup is possible; 3b
can also be purified by column chromatography under
ambient conditions (silica gel, non-dried solvents). Further
detailed protonolysis reactions were carried out in non-dried
CD3OD at room temperature and were monitored by NMR
spectroscopy (plots of the spectra are included in the ESI,†
together with the spectra of selected authentic samples of pro-
1
posed decomposition products). H NMR spectra recorded on
solutions of compound 3b approximately 10 min after sample
preparation already contained resonances assignable to degra-
dation products. After 2 h, about 10% of the initial amount of
3b was left and after 2 d, the sample consisted exclusively
of DpzTMS and o-C6H4(B(OCD3)2)2 (9) in a stoichiometric ratio
of 2 : 1 (Scheme 2; for simplicity reasons, we have drawn the
Fig. 5 Molecular structures of [4a]Br·CH2Cl2 (top) and [4b]Br (bottom)
in the solid state. Hydrogen atoms except on boron, the bromide coun-
terions and the solvent molecule have been omitted for clarity; displace-
ment ellipsoids are drawn at the 50% probability level. Selected bond
lengths (Å), atom⋯atom distances (Å), bond angles (°) and dihedral
angles (°): [4a]Br: B(1)–N(1) 1.561(5), B(1)–N(11) 1.574(3), B(1)–C(21)
1.582(5), B(2)–N(2) 1.541(6), B(2)–N(12) 1.549(3), B(1)⋯B(2) 2.676(6);
N(1)–B(1)–N(11) 104.4(2), N(11)–B(1)–N(11A) 101.3(3), N(2)–B(2)–N(12)
104.8(2), N(12)–B(2)–N(12A) 102.7(3); B(1)N(1)N(2)B(2)//B(1)N(11)N(12)B(2)
121.3(1), B(1)N(11)N(12)B(2)//B(1)N(11A)N(12A)B(2) 117.5(1), pz(N(1))//pz-
(N(11)) 129.7(1), pz(N(11))//pz(N(11A)) 100.6(1); symmetry transformation
used to generate equivalent atoms: A: x, −y + 3/2, z. [4b]Br: B(1)–N(1)
1.561(5), B(1)–N(11) 1.558(5), B(1)–N(21) 1.563(4), B(1)–C(41) 1.589(5),
B(2)–N(2) 1.561(5), B(2)–N(12) 1.575(5), B(2)–N(22) 1.556(5), B(2)–C(31)
1.590(5), B(1)⋯B(2) 2.702(5); N(1)–B(1)–N(11) 102.8(3), N(1)–B(1)–N(21)
103.9(3), N(11)–B(1)–N(21) 103.2(3), N(2)–B(2)–N(12) 102.6(3), N(2)–
B(2)–N(22) 104.5(3), N(12)–B(2)–N(22) 102.5(3); B(1)N(1)N(2)B(2)//B(1)-
N(11)N(12)B(2) 119.0(2), B(1)N(1)N(2)B(2)//B(1)N(21)N(22)B(2) 121.6(2), B(1)-
N(11)N(12)B(2)//B(1)N(21)N(22)B(2) 119.4(2), pz(N(1))//pz(N(11)) 105.6(1),
pz(N(1))//pz(N(21)) 138.7(2), pz(N(11))//pz(N(21)) 115.5(1); pz(N(X)) = pyra-
zolyl ring containing N(X).
Investigations into the relative cage stabilities of B–N
triptycenes
Hydroboration reactions of alkenes and alkynes require three-
coordinate hydroborane species.33 If borane adducts are
employed (e.g., BH3·SMe2), the donor has to come off first in a
preceding association–dissociation equilibrium step, before
the B–H bond is able to add across a CvC-double or a CuC-
triple bond.
A comparison of the reactivities of Li[2a]
(Fig. 2), 3b and [4a]Br toward the hydroboration of tBuCuCH
therefore provides useful first insight into the relative
stabilities of the individual B–N cage bonds. Upon treatment
with excess tBuCuCH in THF at 60 °C, Li[2a] adds two equi-
valents of the alkyne to give Li[tBuC(H)v(H)CB(μ-pzTMS)-
(μ-o-C6H4)2BC(H)v(H)CtBu] (HpzTMS = 4-(trimethylsilyl)pyra-
zole).19 Under the same conditions, neither 3b nor [4a]Br
undergoes a hydroboration reaction, but can be recovered in
essentially quantitative yield; this result remains valid if the
temperature is increased to 80 °C.
Scheme 2 Degradation reactions of 3b, [4a]Br and [4b]Br in non-dried
CD3OD.
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decomposition products as CD3OD esters, even though the
corresponding boronic acids or mixed acid/ester species may
well be present, too). Under the same measurement con-
ditions, about 55% of [4a]Br was still present in the sample
after 2 h. Two days later, however, the proton NMR spectrum
showed only the resonances of free pyrazole, together with a
broad multiplet at 7.48–7.52 ppm. The 11B{1H} NMR spectrum
contained resonances at 28.3 ppm and 18.6 ppm with an inte-
gral ratio of 1 : 1.4. The first value is characteristic of boron
nuclei surrounded by one carbon and two oxygen atoms
(cf. C6H5B(OCH3)2: δ(11B) = 28.629), whereas the second signal
corresponds to B(OCD3)3 (cf. B(OCH3)3: δ(11B) = 18.3; in
CH3OH29,35). These data thus indicate that the cage of [4a]Br
has completely been broken down to pyrazole, the p-bromo-
phenylboronic acid methyl ester 10 (resulting from the substi-
tuted bridgehead boron atom) and B(OCD3)3 (resulting from
the unsubstituted bridgehead boron atom; Scheme 2).
The diphenylated derivative [4b]Br turned out to be more
1
stable towards CD3OD than 3b and [4a]Br. After 76 d, H NMR
spectroscopy indicated the formation of four major products:
(i) free pyrazole, (ii) 10 and (iii) two more species still consist-
ing of both pyrazolide and p-BrC6H4 units. From the NMR
tube, we were able to isolate the syn and anti pyrazaboles 11a36
and 11b in a single-crystal form (Fig. 6). Since the molecular
structures of 11a and 11b are consistent with the proton reso-
nance patterns of the two incompletely degraded species, we
conclude that pyrazaboles are comparatively long-lived inter-
mediates of the protonolysis pathway of [4b]Br.36 In line with
that, the 11B NMR spectrum of [4b]Br in CD3OD after 76 d
exhibited a resonance not only at 28.6 ppm (10), but also at
3.7 ppm (cf. (C6H5)(CH3O)B(μ-pz)2B(OCH3)(C6H5): δ(11B) =
3.5;37 in CDCl3).
Compound 11a crystallises from CD3OD with two crystallo-
graphically independent molecules in the asymmetric unit
(11aA, 11aB). Since all key structural parameters of 11aA and
11aB are the same within the error margins, only the data for
11aA are compiled in Fig. 6. The pyrazabole core of 11aA
adopts a boat conformation such that the two phenyl rings
become almost coplanar. The two boron atoms are 3.115(7) Å
apart from each other and the distance between the phenyl-
ring centroids amounts to 3.808 Å. The Ci-symmetric molecule
11b features a central B2N4 ring in a shallow chair confor-
mation and a crystallographically imposed dihedral angle
between the two pyrazolide rings of 0°.
Fig. 6 Molecular structures of 11aA (top) and 11b (bottom) in the solid
state. Hydrogen atoms have been omitted for clarity; displacement ellip-
soids are drawn at the 50% probability level. Selected bond lengths (Å),
atom⋯atom distances (Å), bond angles (°) and dihedral angles (°): 11aA:
B(1)–O(1) 1.415(6), B(1)–N(11) 1.577(6), B(1)–N(21) 1.587(6), B(1)–C(31)
1.602(7), B(2)–O(2) 1.442(6), B(2)–N(12) 1.600(6), B(2)–N(22) 1.578(6),
B(2)–C(41) 1.589(7), B(1)⋯B(2) 3.115(7), COG(Ar(C(31)))⋯COG(Ar(C(41)))
3.808; N(11)–B(1)–N(21) 103.7(4), N(12)–B(2)–N(22) 103.0(3); N(11)B(1)-
N(21)//N(12)B(2)N(22)
54.6(3),
N(11)B(1)N(21)//N(11)N(12)N(21)N(22)
27.5(3), N(12)B(2)N(22)//N(11)N(12)N(21)N(22) 27.1(3), B(1)N(11)N(12)B(2)//
B(1)N(21)N(22)B(2) 140.2(2), pz(N(11))//pz(N(21)) 154.1(1), Ar(C(31))//Ar(C-
(41)) 9.3(1); Ar(C(X)) = aryl ring containing C(X); COG(Ar(C(X))) = centroid
of the aryl ring containing C(X). 11b: B(1)–O(1) 1.428(3), B(1)–N(11)
1.568(3), B(1)–N(12A) 1.597(3), B(1)–C(21) 1.615(3), B(1)⋯B(1A) 3.266(5);
N(11)–B(1)–N(12A) 104.9(2); N(11)B(1)N(12A)//N(11)N(12)N(11A)N(12A)
14.3(3), pz(N(11))//pz(N(11A)) 0; pz(N(X)) = pyrazolyl ring containing N(X).
Symmetry transformation used to generate equivalent atoms: A: −x, −y
+ 1, −z + 1.
C–C coupling reactions on a B–N triptycene platform
Using compound [4c]Br, we compared the relative perform-
ance of Sonogashira-38 versus Stille-type39,40 C–C-coupling pro-
tocols for the introduction of alkynyl substituents into B–N
triptycene scaffolds (Scheme 3). The room-temperature reac-
tion between [4c]Br and tBuCuCH in dry NEt3 using
PdCl2(dppf)·CH2Cl2/CuI as the catalyst system afforded [4e]Br
in approximately 45% yield after 7 d. (Note: It took 7 d until
[4e]X (X = Cl, Br or I)), even though some of the bromide counter-
ions will likely be exchanged against chloride and/or iodide
ions under the conditions applied. The ill-defined nature of
the counterion in the reaction product is one shortcoming
of the Sonogashira protocol, because it affects the calculation of
precise stoichiometries as well as the determination of accurate
yields. Another setback arises from the waste salt, [HNEt3]X,
1
the last traces of the starting material had vanished in the H
NMR spectrum of the reaction mixture.) For reasons of simpli-
city, we still refer to the reaction product as [4e]Br (instead of
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analytically pure, single-crystalline form; however, the crystals
did not re-dissolve in any common inert solvent. Compound
14a was therefore characterised only by elemental analysis and
X-ray crystallography (Fig. 7, top). The introduction of SiMe3
Scheme 3 Derivatisation of [4c]Br through Sonogashira-type and
Stille-type coupling reactions. Reaction conditions: (i) +2 tBuCuCH,
+PdCl2(dppf)·CH2Cl2/CuI (10 mol%), NEt3, rt, 7 d. (ii) +2 nBu3SnCuCtBu,
+Pd(PtBu3)2 (15 mol %), CH3CN–toluene, rt, 24 h.
which is difficult to separate completely from the target com-
pound. These problems did not occur upon application of the
Stille protocol (nBu3SnCuCtBu, CH3CN–toluene, 15 mol%
Pd(PtBu3)2), which gave comparable yields of [4e]Br as the
Sonogashira reaction and required a reaction time of only 24 h
at room temperature.
The B–N cage of [4e]Br revealed similar NMR characteristics
to the other symmetrically substituted [4]+-type compounds. A
1
singlet at 1.38 ppm in the H NMR spectrum is to be assigned
to the tert-butyl substituents. The 13C NMR resonances of the
alkynyl–carbon atoms appear at 101.7 ppm and 78.4 ppm.
Synthesis and characterisation of B–N-analogues of
pentiptycenes
The key to success for the synthesis of 3-type B–N triptycenes
was the availability of the lithium 1,2-bisborate Li2[7]
(Scheme 1). Recently, we have also prepared the lithium
1,2,4,5-tetrakisborate Li4[13],41 which can consequently be
employed for the synthesis of B–N pentiptycenes 14a–14d
(Scheme 4). The general synthesis protocol for 14a–14d was
essentially the same as that previously described in the cases
of 3a–3e. We note, however, a markedly reduced solubility of
some B–N pentiptycenes as compared to the B–N triptycenes
(the same is true for the all-carbon frameworks).4 For example,
the CF3 derivative 14a was obtained from toluene in an
Fig. 7 Molecular structures of 14a·3C7H8 (top) and 14b·2CH2Cl2
(bottom) in the solid state. Hydrogen atoms except on boron and the
solvent molecules have been omitted for clarity; displacement ellipsoids
are drawn at the 50% probability level. Selected bond lengths (Å),
atom⋯atom distances (Å), bond angles (°) and dihedral angles (°): 14a:
B(1)–N(1) 1.601(2), B(1)–N(11) 1.595(3), B(1)–C(21) 1.597(2), B(2)–N(2)
1.601(2), B(2)–N(12) 1.601(2), B(2)–C(22) 1.595(3), B(1)⋯B(2) 2.712(3);
N(1)–B(1)–N(11) 99.7(1), N(1)–B(1)–C(21) 105.1(1), N(11)–B(1)–C(21)
106.1(1), N(2)–B(2)–N(12) 99.5(1), N(2)–B(2)–C(22) 105.5(1), N(12)–B(2)–
C(22) 106.0(1); N(1)B(1)N(11)//N(2)B(2)N(12) 82.4(1), N(1)B(1)N(11)//N(1)-
N(2)N(11)N(12) 48.6(1), N(2)B(2)N(12)//N(1)N(2)N(11)N(12) 49.0(1), B(1)-
N(1)N(2)B(2)//B(1)N(11)N(12)B(2) 115.1(1), B(1)N(1)N(2)B(2)//B(1)C(21)C(22)-
B(2) 121.92(7), B(1)N(11)N(12)B(2)//B(1)C(21)C(22)B(2) 123.00(9), pz(N(1))//
pz(N(11)) 117.77(8), pz(N(1))//C6H2 117.15(6), pz(N(11))//C6H2 125.07(8).
14b: B(1)–N(1) 1.574(9), B(1)–N(11) 1.590(8), B(1)–C(21) 1.597(9), B(2)–
N(2) 1.593(7), B(2)–N(12) 1.578(8), B(2)–C(22) 1.597(9), B(1)⋯B(2) 2.701(1);
N(1)–B(1)–N(11) 101.7(5), N(1)–B(1)–C(21) 105.9(5), N(11)–B(1)–C(21)
105.3(4), N(2)–B(2)–N(12) 100.9(5), N(2)–B(2)–C(22) 105.3(4), N(12)–
B(2)–C(22) 104.1(5); N(1)B(1)N(11)//N(2)B(2)N(12) 84.0(6), N(1)B(1)N(11)//
N(1)N(2)N(11)N(12) 48.7(5), N(2)B(2)N(12)//N(1)N(2)N(11)N(12) 47.3(4), B(1)-
N(1)N(2)B(2)//B(1)N(11)N(12)B(2) 117.3(3), B(1)N(1)N(2)B(2)//B(1)C(21)C(22)-
B(2) 122.0(3), B(1)N(11)N(12)B(2)//B(1)C(21)C(22)B(2) 120.7(3), pz(N(1))//-
pz(N(11)) 127.2(3), pz(N(1))//C6H2 120.4(3), pz(N(11))//C6H2 112.4(3);
pz(N(X)) = pyrazolyl ring containing N(X).
Scheme 4 Synthesis of the B–N pentiptycenes 14a–14d. General reac-
tion conditions: (i) THF or toluene, rt or 80 °C, between 1 d and 7 d.
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substituents in the 4-position of the pyrazolyl rings leads to rt with Bruker AM 250, DPX 250, Avance II 300, Avance 400
good solubility of 14b in C6H6, toluene, CHCl3, CH2Cl2 or and Avance III HD 500 spectrometers. Chemical shifts are
1
THF; NMR spectra were recorded in C6D6. The H NMR spec- referenced to (residual) solvent signals (1H/13C{1H}; C6D6:
trum of 14b reveals a singlet at 8.66 ppm for the two protons 7.16/128.06; d8-THF: 1.72/25.31; CD3CN: 1.94/1.32; CDCl3:
on the central six-membered ring, another singlet at 7.35 ppm 7.26/77.16; CD3OD: 3.31/49.00) or external BF3·OEt2 (11B,11B-
for the eight magnetically equivalent hydrogen atoms on the {1H}) and Si(CH3)4 (29Si INEPT). Abbreviations: s = singlet, d =
pyrazolyl fragments and one resonance at −0.08 ppm for the doublet, tr = triplet, vtr = virtual triplet, q = quartet, m = multi-
SiMe3 groups (36 H). The 11B{1H} NMR spectrum features one plet, br = broad, n.o. = signal not observed; n.r. = multiplet
1
broad signal at −2.7 ppm (cf. 3b: −3.2 ppm). The molecular expected in the H NMR spectrum but not resolved. Combus-
structure of 14b was finally confirmed also by X-ray crystallo- tion analyses were performed by the Microanalytical Labora-
graphy (Fig. 7, bottom). The NMR spectra of the derivatives tory of the Goethe University Frankfurt; HRMS measurements
14c and 14d (CDCl3) show no peculiarities compared to those were performed on a MALDI LTQ Orbitrap spectrometer
of 14b; all corresponding data are compiled in the ESI.†
(Thermo Scientific). The compounds Li2[7]·OEt2,23 Li2[1,1′-fc-
25
Both compounds, 14a and 14b, are Ci-symmetric in the (BH3)2]·(OEt2)0.25
,
K[HBpz3],42
K[p-BrC6H4Bpz3],43
solid state. All key metrical parameters of pentiptycene 14a are p-BrC6H4BBr2,44 4-iodopyrazole,45 nBu3SnCuCtBu,46 4-(tri-
similar to those of its triptycene analogue 3e and the same methylsilyl)pyrazole,47 and 3,5-di(tert-butyl)pyrazole48 are lit-
applies to the bond lengths and angles of 14a compared to erature known; the amount of OEt2 present in the samples of
14b. In the crystal lattice of 14a·3C7H8, the void spaces Li2[1,1′-fc(BH3)2]·(OEt2)0.25 after prolonged storage in a screw-
between the pyrazolyl rings pz(N(1)) and pz(N(11)) as well as capped vial in the glove-box was determined by 1H NMR
pz(N(1)) and pz(N(11A)) are occupied by toluene molecules. In spectroscopy.
the case of 14b·2CH2Cl2, only the compartments above and
below the central six-membered ring are hosting solvent
Synthesis of 3a
molecules.
A Schlenk flask was charged with Li2[7]·OEt2 (50 mg,
0.26 mmol) and 4-iodopyrazole (101 mg, 0.521 mmol). Et2O
(10 mL) was added at rt with stirring, whereupon vigorous gas
evolution (H2) was observed, which lasted for approximately
2 min. After 30 min, neat Me3SiCl (0.1 mL, 0.08 g, 0.8 mmol)
Conclusion
The concept of B–N/C–C isosterism was applied for the devel- was added to the light yellow solution to give a white suspen-
opment of a complete series of B–N analogues of triptycene, sion. Stirring was continued for 12 h, the formed LiCl was
i.e. [HB(μ-pzR)(μ-o-C6H4)2BH]− ([2]−), HB(μ-pzR)2(μ-o-C6H4)BH removed by filtration and the filtrate was evaporated under
(3) and [RB(μ-pzR)3BR′]+ ([4]+). Instead of employing highly vacuum to yield a colourless solid. Single crystals were grown
reactive benzyne intermediates, as they are required to prepare by slow evaporation of a solution of 3a in Et2O. Yield: 89 mg,
all-carbon triptycenes, the synthesis of the B–N congeners 70%. 1H NMR (300.0 MHz, C6D6): δ = 7.85–7.79 (2 H, m,
relies on the facile formation of B–N adduct bonds. We have H-3,6), 7.30–7.24 (2 H, m, H-4,5), 6.84 (4 H, s, pzH-3,5), 4.25*
also shown that the general design strategy can be extended to (2 H, br, BH); 13C{1H} NMR (75.4 MHz, C6D6): δ = 138.5
the preparation of B–N analogues of pentiptycene. Most of the (pzC-3,5), 130.3 (C-3,6), 126.6 (C-4,5), 55.7 (pzC-4), n.o. (CB);
compounds are sufficiently stable towards air and moisture to 11B{1H} NMR (96.3 MHz, C6D6): δ = −3.7 (h1/2 = 240 Hz); HRMS
allow for a quick aqueous workup. As a proof-of-principle, we (MALDI-TOF): m/z = 486.92435 ([M + H]+, calcd 486.92537).
have also shown that the molecular framework of 4-type com- *This signal sharpens upon 11B decoupling.
pounds is compatible with further derivatisation through
Stille-type C–C-coupling protocols. Thus, B–N iptycenes are
Synthesis of 3b
highly promising building blocks for the facile assembly of A Schlenk flask was charged with Li2[7]·OEt2 (0.1 g, 0.5 mmol)
rigid supramolecular frameworks with selectable overall and 4-(trimethylsilyl)pyrazole (0.15 g, 1.1 mmol). Et2O (20 mL)
charges.
was added at rt with stirring, whereupon vigorous gas evol-
ution (H2) was observed, which lasted for approximately 2 min.
After 30 min, neat Me3SiCl (0.20 mL, 0.17 g, 1.6 mmol) was
added to the pale yellow solution to give a white suspension.
Stirring was continued for 12 h, the formed LiCl was removed
Experimental methods
Unless otherwise specified, all reactions were carried out in by filtration and the filtrate was evaporated under vacuum to
carefully dried solvents under dry nitrogen or argon using yield a colourless solid. The solid was suspended in n-hexane
Schlenk or glove-box techniques. n-Pentane, n-hexane, cyclo- (20 mL) and quenched with saturated brine (10 mL). The two
hexane, C6H6, C6D6, toluene, THF, d8-THF and Et2O were dried liquid phases were separated from each other and the aqueous
over Na/benzophenone; NEt3, CH3CN, CD3CN, CHCl3, CDCl3, phase was extracted with n-hexane (20 mL). The combined
CH2Cl2 and Me3SiCl were dried over CaH2 and freshly distilled organic phases were quickly extracted with H2O (20 mL), dried
prior to use. Column chromatography was performed using over anhydrous MgSO4, filtered, and the filtrate was evaporated
silica gel 60 (Macherey-Nagel). NMR spectra were recorded at to dryness under vacuum. The crude product was further
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purified by short column chromatography (silica gel, ethyl (15 mL). Neat solid K[p-BrC6H4Bpz3] (50 mg, 0.12 mmol) was
acetate, Rf = 0.86). Yield: 103 mg (52%). (Found: C, 57.22; H, added, the tab was closed and the vessel was heated with stir-
7.76; N, 14.52. Calc. for C18H28B2N4Si2 [378.24]: C, 57.16; H, ring for 8 h at 120 °C. After cooling to rt, all insolubles were
7.46; N, 14.81%.) 1H NMR (400.1 MHz, C6D6): δ = 8.04–8.00 collected on a frit and washed with n-pentane (6 × 5 mL). The
(2 H, m, H-3,6), 7.35 (4 H, s, pzH-3,5), 7.32–7.28 (2 H, m, H-4,5), solid material was suspended in CHCl3 (20 mL) and quenched
4.84* (2 H, br, BH), −0.07 (18 H, s, SiMe3); 13C{1H} NMR with a saturated aqueous solution of NaHCO3 (10 mL). The
(100.6 MHz, C6D6): δ = 138.2 (pzC-3,5), 130.3 (C-3,6), 126.4 two liquid phases were separated from each other and the
(C-4,5), 114.5 (pz-C4), −0.6 (SiMe3), n.o. (CB); 11B{1H} NMR aqueous phase was extracted with CHCl3 (20 mL). The com-
(128.4 MHz, C6D6): δ = −3.2 (h1/2 = 300 Hz); 29Si INEPT NMR bined organic phases were washed with H2O (20 mL), dried
(79.5 MHz, C6D6): δ = −10.6. *This signal sharpens upon 11B over anhydrous MgSO4, filtered and the filtrate was evaporated
decoupling.
to dryness under vacuum. Single crystals were obtained as col-
ourless blocks by slow evaporation of a solution of [4b]Br in
CHCl3. Yield: 60 mg, 80%. (Found: C, 40.35; H, 2.87; N, 13.42.
Synthesis of 3f
A Schlenk flask was charged with Li2[1,1′-fc(BH3)2]·(OEt2)0.25 Calc. for C21H17B2Br3N6 [614.76]: C, 41.03; H, 2.79; N, 13.67%.)
3
(50 mg, 0.20 mmol) and 4-iodopyrazole (81 mg, 0.42 mmol). 1H NMR (500.2 MHz, CD3CN): δ = 8.08 (6 H, d, JH,H = 2.5 Hz,
Et2O (10 mL) was added at rt with stirring, whereupon vigor- pzH-3,5), 7.94 (4 H, d, 3JH,H = 8.3 Hz, ArH), 7.87 (4 H, d, 3JH,H
=
ous gas evolution (H2) was observed, which lasted for approxi- 8.3 Hz, ArH), 6.52 (3 H, tr, 3JH,H = 2.5 Hz, pzH-4); 13C{1H} NMR
mately 2 min. After 30 min, neat Me3SiCl (0.1 mL, 0.08 g, (75.4 MHz, CD3CN): δ = 139.8 (pzC-3,5), 136.6 (ArC), 133.2
0.8 mmol) was added to the yellow solution to give a white sus- (ArC), 108.6 (pzC-4), n.o. (CBr), n.o. (CB); 11B{1H} NMR
pension. Stirring was continued for 12 h, the formed LiCl was (96.3 MHz, CD3CN): δ = −0.5 (h1/2 = 160 Hz); HRMS (MALDI-
removed by filtration and the filtrate was evaporated under TOF): m/z = 535.00411 ([M − Br]+, calcd 535.00416).
vacuum. The remaining orange solid was dissolved in Et2O
Synthesis of [4c]Br
and the solution was stored at −30 °C to grow single crystals of
1
3f·OEt2. Yield: 104 mg, 76%. H NMR (300.0 MHz, C6D6): δ = In a Schlenk vessel equipped with a Teflon Young’s tab,
7.03 (4 H, s, pzH-3,5), 4.12 (4 H, vtr, JH,H = 1.7 Hz, C5H4), 3.22 p-IC6H4BBr2 (123 mg, 0.330 mmol) was dissolved in toluene
(4 H, vtr, JH,H = 1.7 Hz, C5H4), n.o. (BH); 13C{1H} NMR (15 mL). Neat solid K[p-IC6H4Bpz3] (150 mg, 0.330 mmol) was
(75.4 MHz, C6D6): δ = 140.5 (pzC-3,5), 70.9 (C5H4), 70.3 (C5H4), added, the tab was closed and the vessel was heated with stir-
56.3 (pzC-4), n.o. (CB); 11B{1H} NMR (96.3 MHz, C6D6): δ = ring for 6 h at 120 °C. After cooling to rt, all insolubles were
−3.5 (h1/2 = 240 Hz); MS (MALDI-TOF): m/z 593.8 ([M]+, 100%); collected on a frit, washed with H2O (20 mL), n-pentane (6 ×
1
HRMS (MALDI-TOF): m/z = 593.88323 ([M]+, calcd 593.88379).
5 mL) and dried under vacuum. Yield: 196 mg, 84%. H NMR
(300.0 MHz, CD3CN): δ = 8.11–8.04 (10 H, m, ArH, pzH-3,5),
7.79 (4 H, d, JH,H = 8.1 Hz, ArH), 6.52 (3 H, tr, JH,H = 2.5 Hz,
Synthesis of [4a]Br
3
3
A solution of p-BrC6H4BBr2 (194 mg, 0.594 mmol) in toluene pzH-4); 13C{1H} NMR (125.8 MHz, CD3CN): δ = 139.7 (pzC-3,5),
(15 mL) was added dropwise with stirring at rt to a solution of 139.2 (ArC), 136.4 (ArC), 108.6 (pzC-4), 97.9* (CI), n.o. (CB);
K[HBpz3] (150 mg, 0.595 mmol) in CH3CN–toluene (1 : 1, 11B NMR (96.3 MHz, CD3CN): δ = −0.9 (h1/2 = 230 Hz); HRMS
20 mL). The resulting mixture was stirred for 24 h at rt. All (MALDI-TOF): m/z = 628.97883 ([M − Br]+, calcd 628.97847).
insolubles were collected on a frit and quickly washed with *The position of this signal was confirmed by an HMBC
H2O (3 mL) to remove KBr. The crude product was immedi- experiment.
ately after that washed with n-pentane (2 × 15 mL) and dried
Synthesis of [4d]Br
under vacuum. Single crystals were grown by slow evaporation
of a solution of [4a]Br in CH2Cl2. Yield: 255 mg, 93%. 1H NMR In a Schlenk vessel equipped with a Teflon Young’s tab,
3
(500.2 MHz, CD3CN): δ = 8.24 (3 H, d, JH,H = 2.5 Hz, pzH-3 or p-Me3SiC6H4BBr2 (172 mg, 0.538 mmol) was dissolved in
pzH-5), 8.02 (3 H, d, 3JH,H = 2.5 Hz, pzH-3 or pzH-5), 7.91 (2 H, toluene (15 mL). Neat solid K[p-Me3SiC6H4Bpz3] (215 mg,
3
3
d, JH,H = 8.5 Hz, ArH), 7.84 (2 H, d, JH,H = 8.5 Hz, ArH), 6.51 0.537 mmol) was added, the tab was closed and the vessel was
(3 H, vtr, pzH-4), 5.35–4.30 (1 H, br m, BH). 13C{1H} NMR heated with stirring for 6 h at 120 °C. After cooling to rt, all
(125.8 MHz, CD3CN): 139.7 (pzC-3 or pzC-5), 139.2 (pzC-3 or insolubles were collected on a frit, washed with n-pentane (6 ×
pzC-5), 136.5 (ArC), 133.1 (ArC), 125.6 (CBr), 108.6 (pzC-4), n.o. 5 mL), suspended in CHCl3 (20 mL) and quenched with a satu-
(CB); 11B{1H} NMR (160.5 MHz, CD3CN): δ = −0.5 (h1/2
=
rated aqueous solution of NaHCO3 (10 mL). The two liquid
110 Hz, BC), −5.4 (h1/2 = 100 Hz, BH); 11B NMR (160.5 MHz, phases were separated from each other and the aqueous phase
1
CD3CN): δ = −0.5 (s, BC), −5.4 (d, JB,H = 128 Hz, BH); MS was extracted with CHCl3 (20 mL). The combined organic
(ESI+): m/z 379.2 ([M − Br]+, 100%); HRMS (MALDI-TOF): m/z = phases were washed with H2O (20 mL), dried over anhydrous
379.06527 ([M − Br]+, calcd 379.06439).
MgSO4, filtered and the filtrate was evaporated to dryness
under vacuum. Single crystals were obtained as colourless
blocks by slow evaporation of a solution of [4d]Br in CHCl3.
Synthesis of [4b]Br
In a Schlenk vessel equipped with a Teflon Young’s tab, Yield: 259 mg, 80%. (Found: C, 54.29; H, 6.11; N, 13.86. Calc.
1
p-BrC6H4BBr2 (40 mg, 0.12 mmol) was dissolved in toluene for C27H35B2BrN6Si2 [601.30]: C, 53.93; H, 5.87; N, 13.98%.) H
8250 | Dalton Trans., 2014, 43, 8241–8253
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3
NMR (400.1 MHz, CDCl3): δ = 8.12 (6 H, d, JH,H = 2.5 Hz, 14a did not re-dissolve in any common inert solvent, NMR
pzH-3,5), 7.96 (4 H, d, 3JH,H = 8.0 Hz, ArH), 7.83 (4 H, d, 3JH,H
=
data cannot be provided.
8.0 Hz, ArH), 6.71 (3 H, tr, 3JH,H = 2.5 Hz, pzH-4), 0.36 (18 H, s,
SiMe3); 13C{1H} NMR (100.6 MHz, CDCl3): δ = 144.4 (CSi),
138.8 (pzC-3,5), 134.6 (ArC), 132.5 (ArC), 108.9 (pzC-4),
−1.1 (SiMe3), n.o. (CB); 11B NMR (128.4 MHz, CDCl3): δ =
0.2 (h1/2 = 400 Hz); 29Si INEPT NMR (99.4 MHz, CDCl3): δ =
−3.3; MS (ESI+): m/z 521.8 ([M − Br]+, 100%).
Synthesis of 14b
A
Schlenk flask was charged with Li4[13]·3thf (50 mg,
0.13 mmol) and 4-(trimethylsilyl)pyrazole (75 mg, 0.53 mmol).
THF (20 mL) was added at rt and the resulting solution was
stirred for 15 min; the reaction proceeds with evolution of H2.
Neat Me3SiCl (0.1 mL, 0.08 g, 0.8 mmol) was added to the pale
yellow solution and stirring was continued for 24 h. All vola-
Synthesis of [4e]Br
Method A: [4c]Br (50 mg, 0.071 mmol), PdCl2(dppf)·CH2Cl2 tiles were removed under vacuum. The yellow solid residue was
(6 mg, 0.007 mmol) and CuI (2 mg, 0.01 mmol) were sus- extracted with toluene (20 mL). The extract was evaporated to
pended in NEt3 (20 mL). tBuCuCH (18 μL, 12 mg, 0.15 mmol) dryness under vacuum to obtain 14b as a colourless solid.
was added via a syringe and the solution was stirred for 7 d at Yield: 66 mg, 73%. Single crystals of 14b·2CH2Cl2 were
rt. All volatiles were removed under reduced pressure. The obtained in the form of colourless blocks by slow evaporation
remaining dark brown solid was washed with H2O (20 mL), of a solution of 14b in CH2Cl2.
THF (20 mL), C6H6 (20 mL) and n-pentane (30 mL) to obtain a
1H NMR (300.0 MHz, C6D6): δ = 8.66 (2 H, s, H-3,6), 7.35
colourless solid, which was dried in vacuo. Yield: 21 mg, 48% (8 H, s, pzH-3,5), 4.96* (4 H, br, BH), −0.08 (36 H, s, SiMe3);
(note: the product was still contaminated with small quantities 13C{1H} NMR (75.4 MHz, C6D6): δ = 137.8 (pzC-3,5), 132.6
of [HNEt3]X (X = Cl, Br or I); NMR spectroscopic control).
(C-3,6), 113.8 (pz-C4), −0.6 (SiMe3), n.o. (CB); 11B{1H} NMR
Method B: [4c]Br (20 mg, 0.028 mmol) and Pd(PtBu3)2 (96.3 MHz, C6D6): δ = −2.7 (h1/2 = 900 Hz); 29Si INEPT NMR
(2 mg, 0.004 mmol) were dissolved in CH3CN (10 mL) and (59.6 MHz, C6D6): δ = −10.8; HRMS (MALDI-TOF): m/z =
toluene (10 mL). Neat nBu3SnCuCtBu (21 μL, 21 mg, 678.36164 ([M]+, calcd 678.36253 (C30H50B4N8Si4)). *This
0.058 mmol) was added via a syringe and the clear yellow solu- signal sharpens upon 11B decoupling.
tion was stirred for 24 h at rt. All volatiles were removed under
reduced pressure, the yellow solid residue was treated with
C6H6 (10 mL) to give a yellow suspension, and the insoluble
material was collected on a frit, washed with n-hexane (10 mL)
and dried in vacuo. Yield: 8 mg, 46%.
X-ray crystal structure determinations
Data for 11a, 11b and 12a were collected on a STOE IPDS II
two-circle diffractometer with graphite-monochromated MoKα
radiation (λ = 0.71073 Å) and corrected for absorption with an
empirical absorption correction using the program PLATON.49
Data for 3a, 3c, 3d, 3e, 3f·OEt2, 3f*·3C6H6, [4a]Br·CH2Cl2,
[4b]Br, 14a·3C7H8, and 14b·2CH2Cl2 were collected on a STOE
IPDS II two-circle diffractometer with a Genix Microfocus tube
with mirror optics using MoKα radiation (λ = 0.71073 Å) and
were scaled using the frame scaling procedure in the X-AREA
program system.50 The structures were solved by direct
methods using the program SHELXS51 and refined against
F2 with full-matrix least-squares techniques using the program
SHELXL-97.51
3
1H NMR (500.2 MHz, CDCl3): δ = 8.11 (6 H, d, JH,H = 2.5
3
Hz, pzH-3,5), 7.95 (4 H, d, JH,H = 8.0 Hz, ArH), 7.72 (4 H, d,
3
3JH,H = 8.0 Hz, ArH), 6.66 (3 H, tr, JH,H = 2.5 Hz, pzH-4), 1.38
(18 H, s, CH3); 13C{1H} NMR (125.8 MHz, CDCl3): δ = 138.8
(pzC-3,5), 133.2 (ArC), 132.8 (ArC), 127.3* (CCuC), 108.8
(pzC-4), 101.7* (tBuCuC), 78.4* (ArCuC), 31.1 (CH3), 28.3
(C(CH3)), n.o. (CB); 11B NMR (96.3 MHz, CD3CN): δ = 0.1 (h1/2
=
500 Hz); HRMS (MALDI-TOF): m/z = 537.31069 ([M − Br]+,
calcd 537.31038). *The position of this signal was confirmed
by an HMBC experiment.
The H atoms bonded to B in 3a, 3c, 3e and [4a]Br·CH2Cl2
were isotropically refined. The coordinates of the H atoms
Synthesis of 14a
A
Schlenk flask was charged with Li4[13]·3thf (50 mg, bonded to B in 3d were refined. The crystal of 3a was mero-
0.13 mmol) and 3,5-bis(trifluoromethyl)pyrazole (0.11 g, hedrally twinned (twin law: −1 −1 0/0 1 0/0 0 −1) with a frac-
0.54 mmol). Toluene (20 mL) was added at rt with stirring, tional contribution of 0.1971(7) for the minor domain. The
whereupon a gas (H2) evolved. After 10 min, neat Me3SiCl crystal of 3f·OEt2 was very weakly diffracting. The displacement
(0.1 mL, 0.08 g, 0.8 mmol) was added to the colourless suspen- parameters of all B, N and C atoms were restrained to an iso-
sion and stirring was continued for 4 d. All insolubles were col- tropic behaviour. In 3f*·3C6H6, the displacement parameters
lected on a frit (77 mg; 100% LiCl = 22 mg) and the filtrate was of B2, B4 and the benzene-C atoms C71 to C76 as well as C91
stored at 4 °C for 14 d to obtain block-shaped crystals of to C96 were restrained to an isotropic behaviour. Bond lengths
14a·3C7H8. Yield: 11 mg, 7%. The mother liquor was evapor- and angles in the benzene solvent molecules were restrained
ated to dryness to give 14a as a colourless solid; an elemental to be equal. One toluene molecule in 14a·3C7H8 was dis-
analysis revealed that, in contrast to the single crystals, the ordered about a centre of inversion over two equally occupied
microcrystalline product no longer contained toluene. Yield: positions; the H atoms bonded to B were isotropically refined.
34 mg, 27%. (Found: C, 33.55; H, 1.24; N, 12.01. Calc. for The crystal of 14b·2CH2Cl2 was just weakly diffracting; the H
C26H10B4F24N8 [933.62]: C, 33.45; H, 1.08; N, 12.00%.) Since atoms bonded to B were isotropically refined.
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Paper
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CCDC reference numbers: 985503 (3a), 985504 (3c), 985505
F. Fabrizi de Biani, P. Zanello and M. Wagner,
J. Organomet. Chem., 2001, 637–639, 390–397; (c) K. Ma,
F. Fabrizi de Biani, M. Bolte, P. Zanello and M. Wagner,
Organometallics, 2002, 21, 3979–3989; (d) L. Ding, K. Ma,
G. Dürner, M. Bolte, F. Fabrizi de Biani, P. Zanello and
M. Wagner, J. Chem. Soc., Dalton Trans., 2002, 1566–1573;
(e) C. Cui, J. Heilmann-Brohl, A. Sánchez Perucha,
M. D. Thomson, H. G. Roskos, M. Wagner and F. Jäkle,
Macromolecules, 2010, 43, 5256–5261.
(3d), 985506 (3e), 985507 (3f·OEt2), 985508 (3f*·3C6H6), 985509
([4a]Br·CH2Cl2), 985510 ([4b]Br), 985511 (11a), 985512 (11b),
985513 (12a), 985514 (14a·3C7H8) and 985515 (14b·2CH2Cl2).
Acknowledgements
This work has been supported by the Beilstein-Institut,
Frankfurt/Main, Germany, within the research collaboration 16 (a) F. Jäkle, T. Priermeier and M. Wagner, Chem. Ber., 1995,
NanoBiC through a Ph.D. grant for Ö.S.
128, 1163–1169; (b) M. Fontani, F. Peters, W. Scherer,
W. Wachter, M. Wagner and P. Zanello, Eur. J. Inorg. Chem.,
1998, 1453–1465; (c) M. Grosche, E. Herdtweck, F. Peters
and M. Wagner, Organometallics, 1999, 18, 4669–4672;
(d) L. Ding, F. Fabrizi de Biani, M. Bolte, P. Zanello and
M. Wagner, Organometallics, 2000, 19, 5763–5768;
(e) R. E. Dinnebier, M. Wagner, F. Peters, K. Shankland and
W. I. F. David, Z. Anorg. Allg. Chem., 2000, 626, 1400–
1405; (f) K. Ma, H.-W. Lerner, S. Scholz, J. W. Bats, M. Bolte
and M. Wagner, J. Organomet. Chem., 2002, 664, 94–
105.
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