Large-bite diboranes for the μ(1,2) complexation of hydrazine and cyanide

Article abstract of DOI:10.1039/c8sc01877d

As part of our interest in the chemistry of polydentate Lewis acids as hosts for diatomic molecules, we have investigated the synthesis and coordination chemistry of bidentate boranes that feature a large boron-boron separation. In this paper, we describe the synthesis of a new example of such a diborane, namely 1,8-bis(dimesitylboryl)triptycene (2) and compare its properties to those of the recently reported 1,8-bis(dimesitylboryl)biphenylene (1). These comparative studies reveal that these two diboranes feature some important differences. As indicated by cyclic voltammetry, 1 is more electron deficient than 2; it also adopts a more compact and rigid structure with a boron-boron separation (4.566(5) ?) shorter by ～1 ? than that in 2 (5.559(4) ?). These differences appear to dictate the coordination behaviour of these two compounds. While 2 remains inert toward hydrazine, we observed that 1 forms a very stable μ(1,2) hydrazine complex which can also be obtained by phase transfer upon layering a solution of 1 with a dilute aqueous hydrazine solution. The stability of this complex is further reflected by its lack of reaction with benzaldehyde at room temperature. We have also investigated the behaviour of 1 and 2 toward anions. In MeOH/CHCl₃ (1/1 vol) both compounds selectively bind cyanide to form the corresponding μ(1,2) chelate complexes with a B-CN-B bridge at their cores. Competition experiments in protic media show that the anionic cyanide complex formed by 1 is the most stable, with no evidence of decomplexation even in the presence of (C₆F₅)₃B.

Full text of DOI:10.1039/c8sc01877d

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Large-bite diboranes for the μ(1,2) complexation of hydrazine and
cyanide
Received 00th January 20xx,
Accepted 00th January 20xx
Chang-Hong Chen and François P. Gabbaï*
DOI: 10.1039/x0xx00000x
As part of our interest in the chemistry of polydentate Lewis acids as hosts for diatomic molecules, we have investigated
the synthesis and coordination chemistry of bidentate boranes that feature a large boron-boron separation. In this paper,
we describe the synthesis of a new example of such a diborane, namely 1,8-bis(dimesitylboryl)triptycene (2) and compare
its properties to those of the recently reported 1,8-bis(dimesitylboryl)biphenylene (1). These comparative studies reveal
that these two diboranes feature some important differences. As indicated by cyclic voltammetry, 1 is more electron
deficient than 2; it also adopts a more compact and rigid structure with a boron-boron separation (4.566(5) Å) shorter by
~1 Å than that in 2 (5.559(4) Å). These differences appear to dictate the coordination behaviour of these two compounds.
While 2 remains inert toward hydrazine, we observed that 1 forms a very stable μ(1,2) hydrazine complex which can also
be obtained by phase transfer upon layering a solution of 1 with a dilute aqueous hydrazine solution. The stability of this
complex is further reflected by its lack of reaction with benzaldehyde at room temperature. We have also investigated the
behaviour of 1 and 2 toward anions. In MeOH/CHCl3 (1/1 vol.) both compounds selectively bind cyanide to form the
corresponding μ(1,2) chelate complexes with a B-C≡N-B bridge at their core. Competition experiments in protic media
show that the anionic cyanide complex formed by 1 is the most stable, with no evidence of decomplexation even in the
presence of (C6F5)3B.
www.rsc.org/
pyridazine.9
Introduction
R'
The chemistry of main group-based polydentate Lewis acids1
has drawn considerable attention over the past decades,
leading to application in anion sensing,2 anion transport,3 small
molecule activation and catalysis.4 Diboranes featuring the
rigid 1,8-naphthalenediyl5 or ortho-phenylene6 backbones are
the most studied examples of such systems. Owing to the
short spacing of the two boron centres, these diboranes are
well-suited for the chelation of monoatomic anions such as
X−
X−
R'
N
R
R
R
R
R
R
R
R
N
B
B
B
B
B
Me
B
H4 or F4
Me
A
X = H, F, Cl, N3
B
X = F, Cl, N3
C
5b, 6a
5e, 5g, 6a
6b
hydride,5a,
fluoride5a,
and chloride5c,
or
polyatomic anions amenable to μ(1,1) ligation such as azide.5g,
6c
The strength of the diborane host-anionic guest interaction
Bearing in mind that the selectivity of these diboranes for their
respective guests is dictated by the size match that exists
between the host and the guest, we have recently become
interested in diboranes with an increased separation between
the Lewis acidic centres.10 It occurred to us that such systems
may display a different selectivity and may become well
in these system has led to the development of application in
selective anion sensing5a-c, 5e, 5g, 6a as well as catalysis.4g, 5e, 6b, 6c
Although no structural evidence has been obtained for the
ditopic complexation of neutral molecules by diboranes of
type
A and B, it has been demonstrated recently that 9,10-
dihydroanthracenes of type
C
are able to engage 1,2-diazines
adapted to the ꢀ
(1,2) chelation of diatomic molecules.11 We
in complex formation7 and catalyse their Diels Alder
also speculated that the rigidity of the targeted diboranes may
inform the molecular recognition properties of these bidentate
Lewis acidic hosts.
8
chemistry.6g, The ability of
C to complex 1,2-diazines is
reminiscent of the adduct formed by 2,2′-diborabiphenyl and
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communication between the boron atoms, an effect that we
assign to the absence of extended conjugation in the
triptycene backbone. The importance of electronic
3.00 − 3.38 Å
R2B BR2
3.11 − 3.30 Å
R2B BR2
> 4.5 Å
R2B BR2
communication in
1
is further supported by the fact that the
(0.51 V) is comparable to the largest
ΔE1/2 measured for
1
This work
value measured for naphthalene-based diboranes (ΔE1/2 range
= 0.3 V - 0.52 V), a class of compounds in which the two boron
atoms are separated by only 3.002 – 3.385 Å.5e, 14
In this article, we report a series of results concerning the
synthesis and properties of bidentate diboranes in which the
two boron atoms are separated by more than 4.5 Å. We also
show that this increased separation allows for the selective
(1,2) complexation of hydrazine and cyanide, two diatomic
molecules that are known for their high toxicity.
Computational studies show that the lowest unoccupied
molecular orbital (LUMO) of
1 (Figure 1a) is dominated by the
ꢀ
two boron pπ orbitals which are in conjugation with the
biphenylene π system. The makeup of this LUMO underscores
the effective electronic communication that exists between
the two boron centres. In the case of 2, which lacks mirror
Results and discussion
symmetry because of steric repulsions between the mesityl
substituents, the LUMO bears a larger contribution from the pπ
orbital of one boron atom. The LUMO+1, which lies only 0.14
ev above the LUMO, shows a large contribution from the
opposite boron atom (see SI). More importantly, π-conjugation
Synthesis, characterization and properties of the diboranes
We have recently described the synthesis
of
1,8-bis(dimesitylboryl)biphenylene
),12 a diborane in which the two boron
(
1
between the two boron atoms in
triptycene bridgehead carbon atoms. These features
corroborate the conclusion that the two boron atoms of are
, as confirmed
2
is interrupted by the sp3
atoms are separated 4.566(5) Å. To assess
the influence of the backbone over the
1
properties of such large-bite diboranes,
in closer electronic communication than in
from the CV measurements.
2
we have now decided to prepare its 1,8- triptycenediyl
analogue (
2). By analogy with the approach we employed to
access 1, diborane
2
was obtained via the dilithiation of 1,8-
dibromotriptycene, followed by metathesis with Mes2BF
(Scheme 1).
Br
Br
Mes2B
BMes2
a
2
Scheme 1. Synthesis of diborane 2. a) 2.3 equiv nBuLi, 2.2 equiv Mes2BF, THF, -78 oC.
Compound
2 is a colourless air-stable solid which was isolated
in moderate yield. The 1H NMR signals of the triptycene
backbone are consistent with a symmetrical structure. The
detection of six sharp signals arising from the methyl groups
and the four meta-H signals of the mesityl groups suggest that
the molecule retains mirror symmetry in solution. The 11B
NMR spectrum shows a broad peak at 73.0 ppm which falls
within the typical range expected for triarylboranes. The cyclic
voltammogram (CV) of compound
2 shows two quasi-
reversible reduction waves at -2.62 and –3.00 V vs Fc/Fc+,
suggesting that the molecule can be reduced by two electrons
Figure 1. a) LUMO of 1 (isovalue = 0.03). b) LUMO of 2 (isovalue = 0.05). c) Cyclic
voltammograms of 1 (dash line, E1/2 = -2.23 and –2.74 V) and 2 (solid line, E1/2 = -2.62
and –3.00 V) in THF. Scan rate = 100 mVs-1
.
(Figure 1c). Compared to the biphenylene derivative
which the two reduction waves are observed at E1/2 = -2.23
and -2.74 V (Figure 1c),12 the first reduction of
is shifted
1 for
Single crystals of compound
solution of in CH2Cl2 with MeOH. The solid-state structure of
shows that the boron atoms adopt a trigonal planar
geometry as indicated by the sum of the Caryl-B-Caryl angles
(∑∠
= 359.7o, ∑∠ = 359.4o) (Figure 2).15 The B1-B2
2 could be obtained by layering a
2
2
toward cathodic potentials by 390 mV. This observation
indicates that the biphenylene backbone is substantially more
electron withdrawing. This conclusion is in agreement with
two previous studies dealing with related biphenylene boron
derivatives.12-13 Also, the shorter separation observed between
2
C-B1-C
C-B2-C
separation of 5.559(4) Å in 2 is notably larger than that in 1
(4.566(5) Å)12 and the 1,8-bis(diphenylboryl)naphthalene
(3.002(2) Å).14a In CHCl3/MeOH (1/1 vol.), the UV-Vis spectra of
the first and second reduction wave in 2 (ΔE1/2 = 0.51 V for
1 vs
ΔE1/2 = 0.38 V for signals a decreased electronic
2)
1
and 2 feature a low-energy absorption band at 424 nm for 1
2 | J. Name., 2012, 00, 1-3
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(Figure 3) and 316 nm for 2 (Figure 5). Time-dependent density fluorescence (Figure 3). In stark contrast, diborane 2 does not
functional theory (TD-DFT) calculations and Natural Transition complex hydrazine under the same condition, even with a
Orbitals (NTO) analysis show the dominant HOMO-LUMO large excess of N2H4 H2O. The lack of reactivity of toward
character of these transitions as observed for most triaryl hydrazine may be correlated to its lower Lewis acidity as
boranes (see SI).16 Both
and are weakly fluorescent (QY = suggested by the CV measurements. It is also possible that the
0.05 for and 0.04 for ) at a wavelength of 528 nm and 383 increased spacing between the two boron atoms, the lower
nm, respectively. Given that the boron orbitals are involved in rigidity of the backbone22 and the presence of a bridgehead
‧
2
1
2
1
2
the LUMO of both derivatives, it can be anticipated that the methine group interfere with the ability of
Ha
2
to bind hydrazine.
coordination of Lewis bases to the tricoordinate boron centres
will dramatically affect these spectral features.16e, 17
Hb
Ha'
N
1
+
BMes2
Mes2B
BMes2
N
Hb'
b
a
H
Ph
N
NH2
1-ꢀ2-N2H4
Scheme 2. Synthesis of 1-μ2-N2H4 and the reaction of 1-μ2-N2H4 with benzaldehyde. a)
2.5 equiv N2H4·H2O, THF, rt. b) 1 equiv benzaldehyde, CDCl3, 60 oC.
Figure 2. Solid-state structure of 2. Thermal ellipsoids are drawn at the 50% probability
level. The hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles
[o]: B1-C1 1.564(4), B1-C21 1.518(3), B1-C30, 1.571(4), B2-C8 1.572(4), B2-C39 1.577(4),
B2-C48 1.570(4); C1-B1-C21 117.5(2), C1-B1-C30 119.1(2), C21-B1-C30 123.1(2), C8-B2-
C39 117.6(2), C8-B2-C48 121.1(2), C39-B2-C48 120.7(2).
Figure 3. UV-Vis absorption (left) and emission (right) spectra of 1 (solid line) and 1-μ2-
N2H4 (dash line) (4.52 × 10-5 M, λex = 370 nm) in THF. The pictures of the solutions were
taken at the same concentration and the fluorescent images were illuminated with a
hand-held UV lamp.
The 1H NMR spectrum recorded for the resulting hydrazine
Reactions of the diboranes with neutral diatomic molecules
adduct 1-μ2-N2H4 in CD2Cl2 features six methyl-proton and four
With
1 and 2 at our disposal, we became eager to investigate
meta-proton resonances, indicating that the complex adopts
C2 symmetry in solution. The most conspicuous evidence for
the formation of a B-NH2-NH2-B bridge is the detection of two
broad multiplets at 5.15 and 6.29 ppm, indicating that the B-
NH2-NH2-B unit gives rise to an AA’BB’ spin system. These
spectroscopic features also indicate that the conformation of
the hydrazine molecule in the diboron pocket is rigidly locked
the behaviour of these two compounds in the presence of
simple diatomic molecules including hydrogen peroxide,
hydroxylamine and hydrazine. While complexes containing
doubly coordinated peroxide11a, 11b and hydrazine18 molecules
have generated much interest in the context of energy related
research, we also note that coordinated hydrazine has been
identified as a possible intermediate in nitrogen fixation
reactions.10c, 19 Because of its acute toxicity, there are also
numerous ongoing efforts aimed at developing selective
receptors for molecular recognition and sensing applications.20
in solution. The solid-state structure of
1-μ2-N2H4 offers a
consistent picture (Figure 4). This neutral complex crystalizes
in the monoclinic C2/c space group with a half molecule in the
asymmetric unit.15 The hydrazine nitrogen atoms are
coordinated to the boron atoms via a B-N bond length of
1.688(2) Å which is longer than that in H3B-NH2-NH2-BH3 (1.609
Å)23 but comparable to that recently reported by Szymczak for
Inspired by the recent discovery that perfluorinated boranes
can doubly complex the peroxide and superoxide anion,21 we
first tested the reaction of
1 and 2 with 30% H2O2(aq).
complex
D
-μ2-N2H4 (1.697(2) Å and 1.698(2) Å) (Scheme 3).10c
Unfortunately, we observed decomposition, illustrating the
vulnerability of these non-perfluorinated diboranes under
oxidative conditions. We also tested the reaction of
with 50% NH2OH(aq), which also resulted in decomposition of
the hosts. By contrast, reaction of diborane with hydrazine
monohydrate (N2H4 H2O) in THF proceeded smoothly to afford
the corresponding hydrazine complex -μ2-N2H4 as an air- and
1 and 2
1
‧
1
moisture-stable off-white solid (Scheme 2). This reaction
results in a distinct loss of the yellow colour indicating
coordination of the hydrazine to each boron atoms. The same
effect is responsible for the observed quenching of the green
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formation of the free diborane
1
and benzaldehyde hydrazone
H
after 20 h as confirmed by 1H NMR spectroscopy (Scheme 2).25
H
N
B
B
N
B
B
N
Reaction of diboranes with cyanide anion
H
H
a
Next in our survey of the properties of these diboranes, we
decided to focus on the case of anions and especially the
cyanide anion which is known for its acute toxicity and which
has often been considered as a target in molecular recognition
assays,26 including using boron-based hosts.17b, 27
Br
Br
Br
Br
Fe
N
Fe
N
N
N
N
N
N
N
N
D
D ꢀ
- 2-N2H4
Both diboranes quickly react with KCN in a CH2Cl2/MeOH (1/1
vol.) mixture containing dibenzo-18-crown-6. As expected, this
Scheme 3. Reaction of Iron complex (D) with hydrazine to afford D-μ2-N2H4 according
to Szymczak et al. a) 1 equiv N2H4, THF, rt.
reaction is accompanied by
a quenching of both the
absorption and emission band (Figure 5). In the case of
1
,
The N-N bond length of 1.469(2) Å is similar to the N-N bond
length determined previously in the literature.24 This
these changes results in a distinct loss of both the yellow
colour and the green fluorescence of the starting diborane
(Figure 5a). Workup of these reactions afforded the
-μ2-CN]− and [ -μ2-CN]− as
corresponding cyanide complexes [1 2
air-stable [K(dibenzo-18-crown-6)]+ salts (Scheme 4). The 11B
observation clearly indicates that diborane
the chelation of hydrazine molecule. We also note that
bind two hydrazine molecules, one at each boron atom. This is
not the case of diborane which only forms a 1:1 complex,
1
is well suited for
D
can
1
NMR signals detected for these anionic complexes (-15.8 ppm
even in the presence of an excess of hydrazine. This difference
in behaviour illustrates the selectivity imparted by the rigid
for [1 2
-μ2-CN]− and -11.7 ppm for [ -μ2-CN]−) are consistent
preorganization of the two boron atoms in 1 10c
.
with the existence of four-coordinate boron atoms. These
signals are somewhat broad, possibly indicating the presence
of two closely spaced and overlapping resonances
corresponding to the NCN-bound and CCN-bound boron atoms,
respectively. The presence of the cyanide anion in these
complexes is confirmed by IR spectroscopy which shows that,
in both cases, the cyanide stretching frequency (
ν
CN = 2229 cm-
) is higher than that of free cyanide
νCN = 2158 cm-1). The higher energy of these vibrations is
1
for
1 2
and 2184 cm-1 for
(
typical of cyanoborates. This effect can be rationalized by
invoking a stabilization of the π and π* molecular orbitals of
the cyanide anion.28 The absence of π-backbonding can also be
invoked as a cause for this effect. The greater
-μ2-CN]- can be corroborated to the CV results and again
νCN observed for
[
1
suggest that the biphenylene backbone is more electron
withdrawing than the triptycene backbone. The methyl region
1
Figure 4. The solid-state structure of 1-μ2-N2H4. Thermal ellipsoids are drawn at the
50% probability level. The hydrogen atoms, except that bound to the nitrogen atoms,
are omitted for clarity. Selected bond lengths [Å] and angles [o]: B1-N1 1.688(2), N1-
N1‘ 1.469(2), B1-C1 1.627(2), B1-C7 1.651(2), B1-C16 1.647(2); B1-N1-N1‘ 113.7(1), C7-
B1-C16 118.7(1), C1-B1-C7 119.7(1), C1-B1-C16 106.6(1).
of the H NMR spectrum recorded for both [
1
-μ2-CN]- and [
2
-
μ2-CN]- as [K(dibenzo-18-crown-6)]+ salts in CDCl3 shows a level
of complexity that is not consistent with equivalence of the
two boryl moieties. We interpret these features as an evidence
that the two boryl moieties are differentiated by their ligation
to the C or N terminus of the cyanide anion.
To further establish the high affinity that
hydrazine, we have studied the complexation under dilute
conditions. We observed that diborane is capable of
capturing N2H4 from dilute aqueous solutions under biphasic
conditions. Quantitative formation of -μ2-N2H4 was observed
by 1H NMR spectrometry when a solution of
in CH2Cl2 (1 mL,
] = 7.7 mM) was stirred for 24 hours with an aqueous
solution (2.5 mL) containing 1 weight% of N2H4. Complex -μ2-
1 displays for
1
1
1
[1
1
N2H4 is remarkably stable. It can be stored in air and shows no
evidence of decomposition for months. It is also resistant to
reactions with aldehydes. For example,
1-μ2-N2H4 failed to
react with benzaldehyde in CDCl3 at room temperature over
o
the course of 20 h. However, upon heating to 60 C, a clean,
yet slow reaction is observed leading to the quantitative
4 | J. Name., 2012, 00, 1-3
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Figure 5. UV-Vis absorption (left) and emission (right) spectra of (a) 1 (solid line) and [1-
μ2-CN]− (dash line) (6.91 × 10-5 M, λex = 370 nm) and (b) 2 (solid line) and [2-μ2-CN]−
(dash line) (6.22 × 10-5 M, λex = 280 nm) in CHCl3/MeOH (1/1 vol.). The pictures of the
solutions were taken at the same concentration and the fluorescent images were
illuminated with a hand-held UV lamp.
crystallographic measurements does not allow for
a
comparison of the B-CCN and B-NCN bond distances which fall in
the 1.54(4) Å-1.62(3) Å for [
for [
-μ2-CN]− (4.187 (4) Å) and
of the molecule. Compared to
B2 separation on going from 2 (B1-B2 5.559(4) Å) to [
1
-μ2-CN]− and 1.63(5) Å-1.67(8) Å
2
-μ2-CN]−. The small difference of the B1-B2 separations in
[
1
1 (4.566(5) Å) reflects the rigidity
1
, the large variation of the B1-
Interestingly, diborane
1 and 2 appear to be highly selective
−
2
-μ2-CN]−
−
for cyanide since no interaction with HCO3−, HSO4 , H2PO4 ,
CH3COO−, Cl−, Br−, I−, and N3 is observed in CHCl3/MeOH (1/1
−
(B1-B2 4.321(5) Å) speaks to the flexibility of the triptycene
backbone and its pincer ability toward the cyanide anion. The
vol.) by UV-Vis spectrometry. Boranes are known to also
display a large affinity for the fluoride anion. However, under
behaviour of
ends of the cyanide anion is reminiscent of the behaviour of
dicopper(II) complexes ( ) which also form μ(1,2) cyanide
complexes as described by Krämer (Scheme 5).29 This bonding
mode differs from that observed for stibonium ( ) and
sulfonium boranes ( ) in which the cyanide anion interact
1 and 2 and their ability to coordinate to both
these conditions, neither
1 nor 2 show any evidence of binding
E
with fluoride. We propose that this selectivity is assisted by the
specific architecture of the compounds that are well adapted
F
G
C
N
BMes2
Mes2B
Mes2B
BMes2
BMes2
Mes2B
primarily with the boron atom, with a weak side on contact to
the adjacent sulfonium or stibonium cation (Scheme 5).27d, 30
Finally, it is important to point out that while [
-μ2-CN]− are the first bimolecular μ(1,2) complexes formed
1
-μ2-CN]− and
[1-ꢀ2-CN]−
1
2
[
2
a
by a diborane and cyanide, their core is reminiscent of that
found in termolecular complexes of general formula [Ar3B-CN-
BAr3]− (Ar = Ph or C6F5).31
C
N
BMes2
Mes2B
−
2
[ -ꢀ2-CN]
Scheme 4. Synthesis of [1-μ2-CN]− and [2-μ2-CN]− as [K(dibenzo-18-crown-6)]+ salts. a) 1
equiv KCN, 1 equiv dibenzo-18-crown-6, CH2Cl2/MeOH (1/1 vol.), rt.
to cyanide complexation (vide infra). However, considering
that this selectivity could be biased by the protic nature of the
medium which solvates the fluoride anion more effectively
than the cyanide anion, we also tested the reaction with
fluoride in dry CDCl3. When
1 and 2 were combined with
tetrabutylammonium difluorotriphenylsilicate (TBAT), no
changes were observed in the 1H NMR spectra of the
diboranes indicating the absence of any interaction. When
tetrabutylammonium fluoride trihydrate (TBAF
used in dry CDCl3, underwent hydrolysis as previously
described while
remained unperturbed.12 Under the same
conditions, both and quickly react with
‧3H2O) was
1
2
1
2
tetrabutylammonium cyanide (TBACN) to give the
corresponding cyanide complexes. These results show that
these diboranes are also selective for cyanide in organic
solvents. Such selectivity is not unprecedented in the
chemistry of boron-based Lewis acids.17b,
27
Lastly, these
diboranes do not react with LiHBEt3 in dry CDCl3.
The structures of the cyanide complexes as [K(dibenzo-18-
crown-6)]+ salts have been investigated by single crystal X-ray
diffraction which confirms the μ(1,2) chelation of the cyanide
anions (Figure 6).15 Both structures were solved on the basis of
a model in which the cyanide anion is disordered over two
overlapping head-to-tail positions, which as indicated by the
refinements, contribute almost equally to the observed
structures. In both structures, the boron atoms are distinctly
pyramidal as indicated by the sum of the Caryl-B-Caryl angles
which fall in the 341.7(4)o-343.2(4)o range for [
1
-μ2-CN]− and
Figure 6. Solid-state structure of cyanide complexes [K(dibenzo-18-crown-6)][1-μ2-CN]-
(CH2Cl2) (a) and [K(dibenzo-18-crown-6)][2-μ2-CN]-(CH2Cl2)2 (b). Thermal ellipsoids are
342.0(5)o-344.6(5)o for -μ2-CN]−. The accuracy of the
[
2
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drawn at the 50% probability level. The [K(dibenzo-18-crown-6)]+, the solvate
molecules and the hydrogen atoms are omitted for clarity. The cyanide anions shown
correspond to that with the higher positional occupancy. Selected bond lengths [Å] and
angles [o] for [1-μ2-CN]−: B1-C1 1.635(4), B1-C13 1.649(4), B1-C22 1.639(4), B1-N2
1.58(2), B2-C8 1.641(4), B2-C31 1.643(4), B2-C40 1.657(3), B2-C72 1.54(4) C72-N2
1.21(4); C1-B1-C13 121.1(2), C13-B1-C22 113.8(2), C1-B1-C22 108.3(2), C1-B1-N2
100.9(7), C13-B1-N2 98.7(8), C22-B1-N2 112.9(7), B1-N2-C72 157.0(19), C8-B2-C31
108.4(2), C8-B2-C40 118.0(2), C31-B2-C40 115.3(2), C8-B2-C72 99.8(8), C31-B2-C72
115.8(9), C40-B2-C72 98.5(9), B2-C72-N2 163(2); for [2-μ2-CN]−: C1-B1 1.657(5), C21-B1
1.644(5), C30-B1 1.665(5), C81-B1 1.66(4), C81-N1 1.16(8), C8-B2 1.632(5), C39-B2
1.646(5), C48-B2 1.649(5), N1-B2 1.64(5); C1-B1-C21 109.1(3), C1-B1-C30 120.2(3), C21-
B1-C30 112.7(3), C1-B1-C81 102.6(11), C21-B1-C81 111.4(16), C30-B1-C81 99.8(15), B1-
C81-N1 158(6), C8-B2-C39 113.5(3), C8-B2-C48 113.1(3), C39-B2-C48 118.0(3), C8-B2-
N1 99.8(15), C39-B2-N1 110.6(18), C48-B2-N1 99.1(13), B2-N1-C81 168(6).
Scheme 6. The competition reaction of [nBu4N][2-μ2-CN] and 1 (top) and [nBu4N][3-CN]
and 1 (bottom). The [nBu4N]+ cations are not shown for clarity.
Conclusions
In summary, we describe two “large-bite” diboranes that are
Scheme 5. Dicopper(II) complexes (E), stibonium (F) and sulfonium boranes (G).
ideally suited for the selective
ꢀ(1,2) complexation of
-μ2-CN]− and
, we
hydrazine and cyanide, two diatomic molecules that are
known for their high toxicity. The high selectivity displayed for
these two specific molecules highlights the defining role
played by the backbone. The most potent binder is the
biphenylene-based diborane which complexes both hydrazine
and cyanide while the triptycene derivative only binds cyanide.
A similar picture emerges from competition experiments
Since the CV data of
-μ2-CN]− suggest that
became eager to experimentally confirm that
outcompete with regard to cyanide binding. In accordance
with this prediction, we observed, using 1H NMR spectroscopy,
the quantitative transfer of the cyanide anion from to
when [nBu4N][
-μ2-CN] was mixed with an equimolar quantity
of
in CDCl3/CD3OD (1/1 vol.) at 60 oC over the course of 12 h
(Scheme 6). This observation confirms the higher cyanide ion
affinity of in protic solution, which is consistent with the
1
and
is a stronger Lewis acid than
would indeed
2 and the IR data of [1
[2
1
2
1
2
2
1
2
which show that
cyanide than . Our results and analyses indicate that the
enhanced properties of arise from the electron withdrawing
1 is a better molecular recognition unit for
1
2
1
1
nature of the biphenylene backbone. We also propose that the
rigidity of this diborane is a favourable factor that helps
sequester hydrazine and the cyanide anion in the diboron
pocket. This binding appears to be irreversible in the case of
the cyanide anion. In the case of the hydrazine complex, we
observe that slow release can triggered upon elevation of the
temperature and in the presence of a reaction partner such as
benzaldehyde. This last feature shows that such bidentate
hosts could be used for the slow release of reactive
compounds.
electrochemical and IR measurements described above.
Furthermore, a competition experiment was also performed
with the monofunctional model borane Mes2BPh (
3
). In these
-CN]− to
quantitatively in CDCl3/CD3OD (1/1 vol.) at 60 C (Scheme 6).
However, no transfer was observed between [
-CN]− and
The absence of a reaction points to the weaker cyanide affinity
of when compared to . The reversibility of the cyanide
binding was also investigated by allowing [nBu4N][
-μ2-CN] and
[nBu4N][
-μ2-CN] to react with one equivalent of (C6F5)3B in
CDCl3/CD3OD (1/1 vol.).32 Under these conditions, [ -μ2-CN]-
was readily converted into while [
-μ2-CN]- remained
tests, the cyanide anion was transferred from [
3
1
o
3
2.
2
1
1
2
2
2
1
Experimental
untouched, again supporting the superior Lewis acidity of 1.
Synthesis of 2. nBuLi (2.65 M, 1 mL, 2.65 mmol) was slowly
added to a solution of 1,8-dibromotriptycene (500 mg, 1.2
o
mmol) in dry THF (8 mL) under N2 at -78 C. After 30 min of
stirring at low temperature, a solution of Mes2BF (715 mg, 2.6
mmol) in THF (5 mL) was slowly transferred into the reaction
flask using a cannula. The resulting solution was stirred for an
additional 12 h at room temperature. The solution was then
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treated with saturated NH4Cl(aq) (1 mL) and the solvent was under vacuum and the resulting solid was washed by Et2O (10
removed under vacuum. The resulting white solid was then mL) twice to afford [K(dibenzo-18-crown-6)][1-μ2-CN] as a
dissolved in CH2Cl2 (5 mL). The resulting solution was filtered white powder in 90 % yield. 1H NMR (499.53 MHz, 25 oC,
and brought to dryness under vacuum. The off-white solid was CDCl3): δ 7.00-6.97 (m, 4 H, -C6H4-), 6.84-6.79 (m, 4 H, -C6H4-),
3
then washed with MeOH (15 mL) twice and dried under 6.66 (t, 2 H, J = 8.78 Hz, biphenylene-CH), 6.50 (br, 8 H, mes-
3
as a white powder in 60 % yield. CH), 6.20-6.16 (m, 2 H, biphenylene-CH), 6.10 (d, J = 6.34 Hz,
vacuum to afford diborane
o
2
3
1H NMR (399.50 MHz, 25 C, CDCl3): δ 7.49 (d, 2 H, JH-H = 6.6 biphenylene-CH), 4.03-4.01 (m, 8 H, -CH2-), 3.76-3.74 (m, 8 H, -
3
Hz, triptycene-CH), 7.23 (d, 1 H, JH-H = 7.0 Hz, triptycene-CH), CH2-), 2.13 (br, 12 H, -CH3), 1.84 (br, 24 H, -CH3) ppm. H NMR
1
3
6.97 (pseudo t, 2 H, JH-H = 7.4 Hz, triptycene-CH), 6.90 – 6.86 (399.46 MHz, -50 C, CDCl3): δ 6.95-6.94 (m, 4 H, -C6H4-), 6.77-
o
3
(m, 3 H, triptycene-CH), 6.82 (s, 2 H, mes-CH), 6.77 (s, 2 H, 6.74 (m, 4 H, -C6H4-), 6.67 (t, 2 H, J = 9.34 Hz, biphenylene-
3
mes-CH), 6.73 (s, 2 H, mes-CH), 6.61 (pseudo t, 1 H, JH-H = 7.4 CH), 6.58-6.52 (m, 6 H, mes-CH), 6.41 (s, 1 H, mes-CH), 6.39 (s,
Hz, triptycene-CH), 6.35 (s, 2 H, mes-CH), 5.40 (s, 1 H, 1 H, mes-CH), 6.19-6.12 (m, 4 H, biphenylene-CH), 3.94 (br, 8
3
triptycene-CH), 5.30 (d, 1 H, JH-H = 7.4 Hz, triptycene-CH), 5.16 H, -CH2-), 3.37 (br, 8 H, -CH2-), 2.18 (s, 6 H, -CH3), 2.14 (s, 3 H, -
(s, 1 H, triptycene-CH), 2.34 (s, 6 H, -CH3), 2.30 (s, 6 H, -CH3), CH3), 2.11 (s, 3 H, -CH3), 2.05 (s, 6 H, -CH3), 1.84-1.80 (m, 12 H,
2.17 (s, 6 H, -CH3), 2.13 (s, 6 H, -CH3), 1.79 (s, 6 H, -CH3), 0.60 -CH3), 1.70 (s, 6 H, -CH3) ppm. 13C NMR (125.62 MHz, 25 oC,
(s, 6 H, -CH3) ppm. 13C NMR (100.46 MHz, 25 oC, CDCl3): δ CDCl3): δ 158.33, 157.36, 150.05, 146.14, 137.01, 136.28,
150.86, 145.50, 145.23, 144.42, 144.27, 143.12, 141.52, 132.48, 132.37, 128.55, 124.36, 122.25, 112.02, 111.77, 80.29,
141.17, 140.90, 140.77, 139.89, 138.58, 138.57, 131.21, 68.97 (-CH2-), 66.59 (-CH2-), 24.44 (-CH3), 20.82 (-CH3) ppm. 11
B
129.92, 129.90, 128.47, 128.14, 127.90, 126.58, 126.17, (128.16 MHz, 25 C, CDCl3): δ -15.8 ppm. IR νCN = 2229 cm-1.
125.05, 124.16, 123.50, 121.60, 55.45 (triptycene-CH), 54.26 Elemental analysis calculated (%) for C69H74B2KO6 0.55 ×
(triptycene-CH), 25.20 (-CH3), 24.96 (-CH3), 22.37 (-CH3), 22.03 CH2Cl2: C, 75.48; H, 6.84; found C, 75.49; H, 6.78. These EA
o
o
(-CH3), 21.39 (-CH3), 21.25 (-CH3) ppm. 11B (128.16 MHz, 25 C, results indicate partial loss of the interstitial solvent molecules.
CDCl3): δ 73.0 (br) ppm. Elemental analysis calculated (%) for
C56H56B2: C, 89.60; H, 7.52; found C, 89.62; H, 7.47.
Synthesis of [K(dibenzo-18-crown-6)][2-μ2-CN]. A solution
of KCN (10 mg, 0.15 mmol) in MeOH (2 mL) was slowly added
Syntheis of 1-μ2-N2H4. A solution of N2H4 H2O (20 mg, 0.4 at room temperature into a CH2Cl2 solution (10 mL) containing
mmol) in THF (2 mL) was combined with a solution of diborane (116 mg, 0.15 mmol) and dibenzo-18-crown-6 (56 mg, 0.15
(100 mg, 0.154 mmol) in THF (5 mL) at room temperature. mmol). The resulting colourless solution was stirred for an
2
1
The resulting solution was stirred for an additional hour and additional 1 h at room temperature. The solvent was removed
brought to dryness under vacuum. The resulting solid was under vacuum and the resulting solid was washed by Et2O (10
washed twice with MeOH (5 mL) to afford 1-μ2-N2H4 as a off- mL) twice to afford [K(dibenzo-18-crown-6)][2-μ2-CN] as a
white powder in 90 % yield. 1H NMR (499.48 MHz, 25 oC, white powder in 88 % yield. 1H NMR (499.49 MHz, 25 oC,
3
CD2Cl2): δ 6.85 (s, 2 H, mes-CH), 6.80 (s, 2 H, mes-CH), 6.68 (s, 2 CD2Cl2): δ 7.32 (d, 1 H, JH-H = 7.3 Hz, triptycene-CH), 7.09 (d, 2
3
H, mes-CH), 6.57 (s, 2 H, mes-CH), 6.55 - 6.50 (m, 6 H, H, JH-H = 7.3 Hz, triptycene-CH), 7.04 – 6.98 (m, 5 H), 6.93 –
3
biphenylene-CH), 6.31 – 6.28 (m, 2 H, -NH2-), 5.17 – 5.13 (m, 2 6.88 (m, 6 H), 6.84 (d, 1 H. JH-H = 7.3 Hz, triptycene-CH), 6.66
3
H, -NH2-), 2.25 (s, 6 H, -CH3), 2.24 (s, 6 H, -CH3), 2.15 (s, 6 H, - (s, 2 H, mes-CH), 6.58 (pseudo t, 2 H, JH-H = 7.3 Hz, triptycene-
CH3), 1.80 (s, 6 H, -CH3), 1.74 (s, 6 H, -CH3), 1.67 (s, 6 H, -CH3) CH), 6.53 (s, 1 H, mes-CH), 6.52 (s, 2 H, mes-CH), 6.48 (s, 1 H,
o
3
ppm. 13C NMR (125.61 MHz, 25 C, CD2Cl2): δ 151.26, 149.80, mes-CH), 6.41 (d, 1 H, JH-H = 7.8 Hz, triptycene-CH), 6.37 (d, 1
143.94, 141.61, 139.50, 137.63, 137.42, 136.01, 135.58, H, 3JH-H = 7.8 Hz, triptycene-CH), 6.31 (s, 1 H, mes-CH), 6.27 (s, 1
132.50, 131.24, 130.65, 130.60, 128.00, 115.17, 26.77(-CH3), H, mes-CH), 5.74 (s, 1 H, triptycene-CH), 5.26 (s, 1 H,
25.72(-CH3), 23.61(-CH3), 23.21(-CH3), 21.07(-CH3), 20.87(-CH3) triptycene-CH), 4.18 – 4.16 (m, 8 H, -CH2-), 3.93 – 3.91 (m, 8 H,
ppm. 11B (128.16 MHz, 25 C, CDCl3): not observed. Elemental -CH2-), 2.25 (s, 3 H, -CH3), 2.23 (s, 3 H, -CH3), 2.19 (s, 6 H, -CH3),
o
analysis calculated (%) for C48H54B2N2•0.93 × CHCl3: C, 74.24; 2.14 (s, 3 H, -CH3), 2.02 (s, 3 H, -CH3), 2.00 (s, 3 H, -CH3), 1.95
H, 6.99; found C, 74.22; H, 7.03. This EA result indicates the (s, 3 H, -CH3), 1.63 (s, 3 H, -CH3), 1.62 (s, 3 H, -CH3), 1.18 (s, 3 H,
crystal sample obtain from CHCl3/MeOH contains interstitial -CH3), 1.13 (s, 3 H, -CH3) ppm. 13C NMR (125.61 MHz, 25 oC,
solvent molecules. This view is consistent with result of the CD2Cl2): δ 151.30, 150.72, 147.52, 146.65, 146.59, 144.42,
crystallographic measurements which show the presence of 144.27, 143.19, 142.76, 142.53, 142.21, 141.89, 141.66,
disordered interstitial solvents equivalent to an electron count 140.88, 133.17, 130.01, 132.75, 132.63, 131.47, 131.34,
of 59 as indicated by application of the SQUEEZE protocol.33 129.29, 129.12, 129.02, 128.90, 128.73, 128.60, 128.15,
This electron count corresponds exactly to the number of 126.69, 124.49, 123.46, 122.85, 122.57, 122.48, 121.56,
electrons of a chloroform molecule (58). The EA results suggest 120.50, 120.40, 111.95, 69.85 (-CH2-), 67.21 (-CH2-), 56.15
partial loss of the interstitial chloroform molecule.
(triptycene-CH), 51.86 (triptycene-CH), 26.87 (-CH3), 25.97 (-
Synthesis of [K(dibenzo-18-crown-6)][1-μ2-CN]. A solution CH3), 25.91 (-CH3), 25.77 (-CH3), 25.55 (-CH3), 25.48 (-CH3),
of KCN (10 mg, 0.15 mmol) in MeOH (2 mL) was slowly added 24.79 (-CH3), 24.48 (-CH3), 21.24 (-CH3), 20.86 (-CH3) ppm. 11B
o
at room temperature into a CH2Cl2 solution (10 mL) containing (128.16 MHz, 25 C, CDCl3): δ -11.7 ppm. IR νCN = 2184 cm-1.
1
(100 mg, 0.15 mmol) and dibenzo-18-crown-6 (56 mg, 0.15 Elemental analysis calculated (%) for C77H80B2KNO6 0.43 ×
mmol). The resulting colourless solution was stirred for an CH2Cl2: C, 76.69; H, 6.72; found C, 76.70; H, 6.71. These EA
additional 1 h at room temperature. The solvent was removed results indicate partial loss of the interstitial solvent molecules.
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Emery, J. Mareda, S. K. Nayak, P. Metrangolo, G. Resnati,
N. Sakai and S. Matile, Nature Communications, 2012, 3,
905; (g) S. Benz, M. Macchione, Q. Verolet, J. Mareda, N.
Sakai and S. Matile, J. Am. Chem. Soc., 2016, 138, 9093-
9096.
Computational details.
Density functional theory (DFT) structural optimizations of
1
and
2 were carried with the Gaussian 09 program. In all cases,
the structures were optimized using the B3LYP functional and
the following mixed basis set: C/H, 6-31g; B, 6-31g+(d’). For all
optimized structures, frequency calculations were carried out
to confirm the absence of imaginary frequencies. The
molecular orbitals were visualized and plotted using the Jimp2
program. The TD-DFT and NTO calculations were carried out
using the MPW1PW91 functional and the above-mentioned
basis sets.
4.
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Bayne, I. Mallov, R. Dobrovetsky and D. W. Stephan, J.
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Conflicts of interest
The authors declare no conflict of interest.
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Katz, J. Am. Chem. Soc., 1985, 107, 1420-1421; (c) H. E.
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Acknowledgements
Acknowledgment is made to the donors of the American
Chemical Society Petroleum Research Fund for partial support
of this research (Grant 56871-ND3). We also acknowledge
support from the Welch Foundation (Grant A-1423), Texas
A&M University (Arthur E. Martell Chair of Chemistry), and the
Laboratory for Molecular Simulation at Texas A&M University
(software and computational resources) is gratefully
acknowledged.
Zhao and F. P. Gabbaı,
2335.
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