Polycationic Redox-Active Cyclophanes with Integrated Electron-Rich Diboron Units

Article abstract of DOI:10.1002/chem.202102656

Cationic cyclophanes are widely used in a variety of applications in supramolecular chemistry and materials science. In this work the authors systematically study the integration of electron-rich diboron units with B^II atoms into polycationic cyclophanes with viologen-like electron-acceptor units. They also report a first hexacationic cage-compound in which three diboron units connect two tris(4-pyridyl)triazine acceptor units. Moreover, di- and tetracationic open-structure compounds, in which one diboron unit connects two bispyridyl groups, were synthesized and the properties compared to those of the corresponding closed structures (cyclophanes). The combination of diboron electron-donor units and bi- or oligopyridyl electron-acceptor units leads to intriguing optical and redox properties.

Full text of DOI:10.1002/chem.202102656

Accepted Article
Accepted Article
Title: Polycationic Redox-Active Cyclophanes with Integrated Electron-
Rich Diboron Units
Authors: Erik Filbeck, Anna Widera, Elisabeth Kaifer, and Hans-Jörg
Himmel
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.202102656
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1/2020

Chemistry - A European Journal
10.1002/chem.202102656
Polycationic Redox-Active Cyclophanes with Integrated
Electron-Rich Diboron Units
Erik Filbeck, Anna Widera, Elisabeth Kaifer, Hans-Jörg Himmel *
Abstract. Cationic cyclophanes are widely used in a variety of applications in
supramolecular chemistry and materials science. In this work we systematically study
the integration of electron-rich diboron units with BII atoms into polycationic
cyclophanes with viologen-like electron-acceptor units. We also report a first
hexacationic cage-compound in which three diboron units connect two tris(4-
pyridyl)triazine acceptor units. Moreover, di- and tetracationic open-structure
compounds, in which one diboron unit connects two bispyridyl groups, were
synthesized and the properties compared to those of the corresponding closed
structures (cyclophanes). The combination of diboron electron-donor units and bi- or
oligopyridyl electron-acceptor units leads to intriguing optical and redox properties.
*
E. Filbeck, Dr. A. Widera, Dr. E. Kaifer, Prof. H.-J. Himmel,
Inorganic Chemistry, Ruprecht-Karls University of Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
E-mail: hans-jorg.himmel@aci.uni-heidelberg.de
1
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10.1002/chem.202102656
Introduction
Organic macrocycles and rings are intensively studied, since they are interesting for a
variety of applications in supramolecular chemistry and materials science. Especially,
[1]
viologen-like linkers are used in a number of polycationic cyclophanes. As example,
Figure displays the Lewis structure of the archeotypical tetracation
1
4+
cyclobis(paraquat-p-phenylene) (CBPQT ), synthesized already in 1988
independently by Stoddart and Hünig,[2,3] and used for host-guest chemistry and as
key component in molecular shuttles, switches and machines.[4] The electron-poor
viologen units are versatile electron-acceptors, enabling the incorporation of small
electron-donor molecules such as hydroquinone,[5] and producing intriguing redox
properties. A variety of further polycationic cyclophanes with viologen-like units were
synthesized,[4] with variable diameters (see, for example, the boxlike
4+
cyclobis(paraquat-p-biphenylene) tetraction (CBPQTBP ) in Figure 1 comprising two
,4’-phenylene-linked “extended” bipyridinium units,[6] and/or integrated functional
groups, as for example the tetrazine units in the recently reported “TetrazineBox“).[7]
4
[
8]
Very recently, the first cyclotris(paraquat-p-phenylenes) were synthesized. Jäkle et
al. synthesized several boracyclophanes with interesting optical and redox
properties.[9-14] One example is sketched in Figure 1. In these compounds, the BIII
atoms are Lewis-acidic sites and act as electron-acceptors.[10]
Figure 1. Lewis structures of two typical tetracationic viologen cyclophanes, illustrating
the possibility to tune the ring size, and of a boracyclophane.
2
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The aim of our research in this field is the integration of diboron electron-donor units
into polycationic cyclophanes with viologen-like electron acceptor units.[15] A suitable
compound for the synthesis of electron-rich diboron building blocks is the ditriflato-
diborane B
2 2
(hpp) (OTf)
2
(1) with hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-
]pyrimidinate (Scheme 1), exhibiting two strongly electron-donating bridging
guanidinate substituents and two triflato groups that could easily be substituted by
neutral Lewis bases.[16-19] The low oxidation state of the boron atoms (B ) and the
guanidinate substituents turn diborane 1 into an electron-donor (in difference to most
other known boron compounds). The hypothetical dicationic diboron building block
II
arising from abstraction of both triflato substituents by a Lewis acid (AlCl
precipitates from solution as insoluble, bright-red and fluorescent dimer with rhomboid
core and 4c,4e bonding (two closed B-B-B (3c,2e) bonds that share an edge).[16] Its
3
or GaCl
3
)
B
4
one-electron reduction produces a soluble radical trication (Scheme 1).[20,21,22]
Scheme 1. The ditriflato-diborane 1, an excellent precursor for dicationic diboron
building blocks for integration in polycationic cyclophanes with viologen-like electron
acceptor units. In the absence of a Lewis base, the abstraction of the two triflato groups
4+
with AlCl
3
or GaCl
3
induces dimerization to the tetraboron tetracation [B
4
(hpp) ] . The
4
3
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Chemistry - A European Journal
10.1002/chem.202102656
tetracation could be reduced to the radical trication, with retention of the rhomboid B
core. In the course of dimerization, the diboron unit changes from an electron-donor
highlighted in red) to an electron acceptor (highlighted in blue).
4
(
On the other hand, the addition of 4,4‘-bipyridine (L1) or 1,2-di(4-pyridyl)ethylene (L2)
to a solution of 1 leads to the formation of tetracationic cyclophanes [{B (hpp) (L1)
]4+
and [{B (hpp) (L2)
]4+ (Scheme 2).[15] The substitution of the triflato groups by the
neutral bipyridine is likely to occur via an S 1 reaction mechanism, initiated by triflate
2
}
2 2
2
2
}
2 2
2
N
dissociation (see Figure 2). The resulting two unprecedented novel polycationic
cyclophanes, combining for the first time bispyridyl electron-acceptor units with diboron
electron-donor units, were structurally characterized in the solid-state. The diboron
3
II
units with their tetracoordinate (sp -hybridized) B atoms act as ethylene-analogue
bridges, but in difference to ethylene linkers they are redox-active [23] by virtue of the
strong electron-donating character of the two bicyclic guanidinate bridges and the low
boron oxidation state (BII).
The two intact B-B bonds enforce quite close proximity of the two viologen-like organic
4+

-systems. In difference to CBPQT and related polycationic organic cyclophanes, the
4
+
(L2)
]4+ are too small for introduction of
cavities in [{B
2
(hpp)
2
}
2
(L1)
2
] and [{B
(hpp) }
2 2 2
2
an electron-rich guest molecule, but the compounds show intriguing optical and redox
properties. They share a typical overall electron-acceptor property with Stoddart’s
cyclophanes, but are in addition amenable to intramolecular electron (charge) transfer.
This is demonstrated by the outcome of the reaction of 1 with the stronger electron-
acceptor pyrazine (Scheme 2).[15] Here, the formation of a tetracationic ring is
accompanied with electron-transfer from the diboron to the pyrazine units. In the
course of this electron transfer, the two B-B bonds are cleaved and a macrocyclic ring
III
compound with four B atoms is formed. Addition of two triflato substituents ensures
the preservation of two four-coordinate boron atoms. To highlight the presence of
electron-donor and electron-acceptor units, we use throughout this paper blue colour
for the electron-accepting viologen-like section of the cyclophanes, and red colour for
the electron-donating diboron units.
4
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10.1002/chem.202102656
Scheme 2. Reactions of the ditriflato-diborane 1 with L1/L2, leading to the first
polycationic cyclophanes in which bispyridyl acceptor units are combined with diboron
electron-donor units. In the case of reaction between 1 and pyrazine, ring formation is
accompanied by electron transfer from the diboron to the pyrazine units.
In this work, we report a first systematic study of this novel compound class. A variety
of cyclophanes with different rigid or flexible bis- and trispyridyl groups (see Figure 2)
was synthesized to address a number of important issues. First, it is analysed how the
optical and redox properties alter with the size of the viologen-like units. Moreover,
open-structure compounds with two bispyridyl units connected by only one diboron unit
were synthesized to compare their optical and redox properties with those of the
corresponding closed forms (cyclophanes). Finally, we report the first hexacationic
cage compound with three diboron units, paving the way to larger redox-active
macrocycles with integrated diboron electron-donor units and oligopyridyl electron-
acceptor units.
5
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Figure 2. Equilibrium showing the formation of free diboron cations by triflate
dissociation from diborane 1, as well as Lewis structures and labels (L1 – L8) of the
linker units for the construction of cyclophanes and related structures used in this work,
sorted by their specific features.
Results and Discussion
Rigid bispyridines. The optical and redox properties of [{B
2
(hpp)
}
2 2
(L1)
]4+ and
2
[
{B (hpp) (L2)
2
2
}
2
2
]4+ are very different to those of free L1 and L2, respectively, and
should respond specifically to an extension of the viologen-like units (see bispyridines
in the red box). Therefore, we aimed at the preparation of new extended cyclophanes
by using the “extended” bispyridines L3 and L4 (Figure 2). Indeed, reaction of 1 with
one equivalent of either L3 or L4 in CH
2
Cl at room temperature yielded the new
2
6
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10.1002/chem.202102656
tetracationic tetraboron cyclophanes [{B
2
(hpp)
was obtained in the form of yellow
O, and the salt [{B (hpp) (L4) ](OTf)
precipitated as a pale-yellow solid from the CH Cl reaction mixture. Unfortunately,
only for [{B (hpp) (L1) ](OTf) and [{B (hpp) (L3) ](OTf) crystals suitable for a
2
}
2
(L3)
2
]4+ and [{B
2
(hpp)
2
}
2
(L4)
]4+
2
(Scheme 3). The salt [{B
2 2 2 2
(hpp) } (L3) ](OTf)
4
3
crystals by layering a solution in CH CN with Et
2
2
}
2 2
2
4
2
2
2
}
2 2
2
4
2
2
}
2
2
4
structural analysis were obtained,[15,24] but all analytic data clearly shows that the
+
compound with L4 adopts a similar cyclophane structure. In the ESI spectra,
measured for solutions of the compounds in CH
tetracationic cyclophane unit and a varying number of triflate counter ions (e.g.
m/z = 679.2764 and 1507.5053 for [{B (hpp) (L3) (OTf) and
]2+
]+ respectively; m/z = 755.3075 and 1659.5750 for
]2+ and [{B ]+, respectively) indicate that the
(hpp) (L4) (OTf)
3
CN, peaks associated with the
2
}
2 2
2
2
[
[
{B
{B
2
(hpp)
2
}
2
(L3)
2
(OTf)
3
2
2
(hpp)
2
}
2
(L4)
2
(OTf)
2
}
2 2
2
3
cyclophanes are stable in solution. Also, the NMR spectra indicate the presence of
1
intact cyclophane units. In Figure 3, the H NMR (only aromatic region from  = 9.00
11
19
to 7.00 ppm), B and F NMR spectra of [{B
2
(hpp)
2 2
}
(L3)
2
](OTf) are compared to the
4
11
spectra obtained for the reactants. The B NMR spectrum contains a single signal for
all boron atoms at  = 4.19 ppm, being slightly shifted with respect to the diborane
reactant B
2
(hpp)
2
(OTf)
2
( = 4.53 ppm). Three signals, two doublets and a singlet,
1
(L3)
]4+, in line
appear in the aromatic region of the H NMR spectrum of [{B
(hpp) }
2 2 2
2
with the presence of two chemically-equivalent bispyridine units L3 (Figure 3). Their
chemical shifts are significantly different from those of the free bispyridine L3.
19
Moreover, the F NMR spectrum shows the sole presence of free triflate ( = -79.21
ppm). Hence, the triflate groups are completely substituted by bispyridine (L3). Similar
results were obtained for [{B
2
(hpp)
2
}
2
(L4)
2
](OTf) (see Supporting Information).
4
In the case of reaction between 1 and L1, we observed an immediate color change
from colorless to green; an EPR spectrum recorded for the solution at this stage
indicated the presence of radical intermediates.[15] Hence, the reaction seems to be
initiated by electron-transfer from the electron-rich diborane 1 to the electron-acceptor
L1. After some time, the reaction mixture turned red and formation of the diamagnetic
cyclophane product [{B
(hpp) }
2 2 2
(L1)
]4+ was completed. On the other hand, no
2
comparable observations were made for the reaction between 1 and L3 or L4; radical
intermediates arising from electron transfer are not formed in significant amount. This
7
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10.1002/chem.202102656
result is in line with the lower electron-acceptor character of L3 and L4 in comparison
with L1.
Scheme 3. Synthesis of polycationic tetraboron cyclophanes with different sizes of the
bispyridyl units.
1H NMR
9
,0
8,8
8,6
8,4
8,2
8,0
7,8
7,6
7,4
7,2
7,0
-10
11B NMR
2
0
15
10
5
0
-5
19F NMR
-70
-72
-74
-76
-78
-80
-82
-84
1
Figure 3. H NMR spectra of [{B
2
(hpp)
2
}
2
(L3)
2
](OTf)
4
(black) and L3 (red) in CH
4
3
CN;
CN.
11
19
B NMR and F NMR spectra of [{B (hpp)
2
2 2
} (L3)
2
](OTf) (black) and 1 (red) in CH
3
8
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The extension of the viologen-like units is accompanied by a marked decrease of the
solubility of the resulting cyclophanes. While [{B (hpp) (L1) ](OTf) still displays a
decent solubility in CH Cl and CH CN, [{B (hpp) (L4) ](OTf) is barely soluble in
CH Cl and only moderately soluble in CH CN. The UV-vis spectroscopic and cyclic
voltammetry measurements of the new cyclophanes were therefore carried out in
CH CN solutions.
In Figure 4, the UV-vis spectra are compared, and the embedded photo compares the
colours of the different cyclophanes. [{B (hpp) (L1) ](OTf) shows a broad absorption
2
}
2 2
2
4
2
2
3
2
}
2 2
2
4
2
2
3
3
2
}
2 2
2
4
centered at 397 nm, being responsible for its intense orange colour, and a sharper one
at 251 nm. For comparison, the UV-vis spectrum of free L1 contains a single strong
band in the UV region at 238 nm, the visible region is free of absorptions. The low-
energy band around 397 nm in the spectrum of [{B
2
(hpp)
2
}
2
(L1)
2
](OTf) could be
4
assigned to a charge-transfer transition from the electron-rich diboron units to the
pyridinium acceptor units (see below). Hence, this band is typical for this novel family
of donor-acceptor cyclophanes. The UV-vis spectrum of [{B
2 2 2
(hpp) } (L3)
2
](OTf) also
4
displays two absorptions, with maxima at 299 and 364 nm. The low-energy charge-
transfer absorption is slightly blue-shifted by 34 nm with respect to the cyclophane with
L1, in line with the higher LUMO energy of L3 compared with L1. On the other hand,
the band in the UV region is slightly red-shifted by 48 nm. For free L3, a band at 278
nm is observed, being red-shifted compared to free L1 by 40 nm. This band could be
assigned to an electronic excitation localized on the bispyridyl -system. As a result,
the energy separation between the two bands decreases. In [{B
2
(hpp)
2 2
}
(L4)
2
](OTf) ,
4
the energy difference between the two bands further decreases; only one absorption
maximum at 333 nm is visible, and the charge-transfer transition appears as a shoulder
at about 360 nm. The UV-vis spectra thus show a trend of the charge-transfer band
towards higher energy with increasing lengths of the bispyridyl units, being in line with
the trend of increasing LUMO energy of the free bispyridines.
9
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Figure
{B (hpp)
The embedded photo compares the colour of CH
{B (hpp) (L1) ](OTf) , [{B (hpp) (L3) ](OTf) and [{B (hpp) (L4)
to right).
4.
UV-vis
spectra
(red curve) and [{B
of
[{B
2
(hpp)
2
}
2
(L1)
2
](OTf)
(blue curve) in CH
CN solutions of
](OTf) (from left
4
(black
curve)
[
2
2
2
} (L3)
2
](OTf)
4
2
(hpp)
2 2
} (L4)
2
](OTf)
4
3
CN.
3
[
2
2
}
2
2
4
2
2
}
2
2
4
2
2
}
2
2
4
To obtain additional information, the cyclophanes [{B
{B (hpp) (L3)
]4+ were calculated by DFT methods (B3LYP+D3/def2-TZVP). The
structure calculated for [{B
the experimental solid-state structure.[15] The calculated B-B bond length amounts to
.765 Å (exp. 1.728(8)/1.724(8) Å) and the distance between the two dipyridyl units (C
atoms in position 4 of the pyridyl rings) to 3.927 Å (exp. 4.0266(1)/4.0335(1) Å). The
calculated structure for [{B (hpp) (L3)
]4+ is also in good agreement with the
2
(hpp)
2 2
}
(L1)
2
]4+ and
[
2
}
2 2
2
4+
2
(hpp)
2
}
2
(L1) ] was found to be in pleasing agreement with
2
1
2
}
2 2
2
experimentally determined structure from XRD (see Supporting Information),[24] with a
B-B bond length of 1.747 Å (exp. 1.7248(2) Å) and a distance between the C atoms in
position 4 of the pyridyl rings of 4.010 Å (exp. 3.8069(4) Å). Figure 5 displays isodensity
plots for the HOMO and LUMO of the two tetracations [{B
2
(hpp)
}
2 2
(L3)
]4+ and
2
4+
[
{B
2
(hpp)
2
}
2
(L1)
2
] . As expected, the HOMO is located on the diboron units and the
1
0
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LUMO on the bispyridyl units. The HOMO-LUMO gap increases from 2.6 eV in
{B (hpp) (L1) (hpp) (L3)
]4+ to 3.1 eV in [{B ]4+, in line with the blue shift of the
charge-transfer band in the UV-vis spectra.
[
2
}
2 2
2
2
}
2 2
2
Figure 5. Illustration of the isodensity surfaces (isovalue = 0.02) for the HOMO und
4+
4+
LUMO of a) [{B
2 2 2 2
(hpp) } (L3) ]
and b) [{B
2 2 2 2
(hpp) } (L1) ]
according to
B3LYP+D3/def2-TZVP calculations.
1
1
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The curves obtained from cyclic voltammetry measurements are reproduced in Figure
, and the measured potentials are compiled in Table 1. Please note that the CV curve
of [{B (hpp) (L1) ](OTf) in CH Cl solution, showing “better-resolved” redox waves,
was published previously.[15] However, since the solubility in CH
Cl is too low for the
cyclophane with L4 units, we show here the CV curve in CH CN. As expected from the
6
2
}
2 2
2
4
2
2
2
2
3
trend in the electron-acceptor strength of the bispyridines, the redox potentials of the
three cyclophanes vary. Since both bispyridyl units could be reduced, two-electron
redox processes are expected to show in the cyclic voltammograms if the two units are
not electronically coupled. On the other hand, the presence of electronic coupling
between the two units leads to one-electron redox processes at different potential. A
two-electron redox process at E1/2 = -1.72 V, assigned to the redox couple
4
+
]2+, is observed for the cyclophane with L4
[
{B
2
(hpp)
2
}
2
(L4)
2
] /[{B
2
(hpp)
2
}
2
(L4)
2
bispyridyl groups. Hence, the cyclophane [{B
2
(hpp) (L4) ](OTf) exhibits the lowest
2
}
2
2
4
reduction potential and therefore is the weakest electron acceptor. The redox potential
clearly decreases with increasing size of the viologen-like linker. This result is in full
agreement with the UV-vis results and also with the trend of the irreversible reduction
potentials of the free bispyridines. Two one-electron redox events at different potential
occur for the cyclophanes with L1 and L3, [{B
2 2 2
(hpp) } (L1)
2
](OTf)
4
and
[
[
[
{B
{B
{B
2
2
2
(hpp)
(hpp)
(hpp)
2
2
2
}
}
}
2
2
2
(L3)
(L1)
(L3)
2
2
2
](OTf)
4
;
E
1/2 values
of -1.14 V
for
the
redox
couple
4
+
]3+ and E1/2 = -1.51 V for the redox couple
] /[{B
2
(hpp)
2
} (L1)
2
2
4
+
]3+ were measured. (Please note that the
] /[{B
2
(hpp)
2
} (L3)
2
2
assignment of the waves to one- or two-electron redox events is motivated by the
relative intensities of the waves in the same CV curve.). Hence, the two bispyridyl
groups are electronically coupled in the cyclophanes with L1 and L3 units, but not in
the cyclophane with L4 units.
1
2
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0
-
10
20
[
{B (hpp) } (L1) ](OTf)
2 2 2 2 4
-
-
-
-
2
2
2
-1
1
0
0
-
-
-
10
20
30
[{B (hpp) } (L3) ](OTf)
4
2
2 2
2
-1
20
0
-
20
40
[
{B (hpp) } (L4) ](OTf)
2
2 2
2
4
-
-1
E / V
Figure 6. Cyclic voltammograms of [{B
2 2 2 2 4
(hpp) } (L1) ](OTf) , [{B
2
(hpp)
2 2
}
(L3)
2
](OTf)
4
and [{B
2
(hpp)
2
}
2
(L4)
2
](OTf)
4
(all measured in CH
3
CN at 298 K with a scan rate of 100
mV s−
1
(glassy carbon working-electrode, Pt counter-electrode, Ag/AgCl
NPF as supporting electrolyte). Potentials are
reference electrode) with 0.1 M nBu
given vs. the ferrocenium/ferrocene (Fc /Fc) redox couple.
4
6
+
+
Table 1. Potentials (in V, given vs. Fc /Fc) measured in CH
3
CN solution.
Compound
E1/2(1)/Ered(1) E1/2(2)/Ered(2) E1/2(3)/Ered(3) E1/2(4)/Ered(4)
a
[
[
[
{B
{B
{B
2
2
2
(hpp)
(hpp)
(hpp)
}
2 2
}
2 2
}
2 2
(L1)
(L3)
(L4)
2
](OTf)
2
](OTf)
2
](OTf)
4
4
4
-1.14/-1.18
-1.51/-1.53
-1.72/-1.78
-1.21/-1.38
-1.64/-1.68
-1.72/-1.78
-1.80/-1.87
-1.84/-1.88
-1.89/-1.95
-2.06/-2.10
-1.84/-1.88
-1.89/-1.95
a
Broad waves.
We also tested the possibility to construct cyclophanes with bispyridines in which the
two pyridyl units are connected by the rigid butadiyne unit (linker di(4-pyridyl)butadiyne,
L5) as well as a more flexible ethane unit (linker 1,2-di(4-pyridyl)ethane, L6). Reaction
between 1 and L5 in a CH
a few crystals of the salt [{B
crystals were suitable for a structural analysis by SCXRD. The resulting solid-state
Cl
2 2
solution resulted in a dark reaction mixture, from which
2
(hpp) (L5) ](OTf) precipitated. Fortunately, these
2
}
2
2
4
4+
structure of the tetracationic tetraboron cyclophane [{B
2
(hpp)
2
}
2
(L5) ] is visualized in
2
1
3
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Figure 7a.[24] The B-B bond length of 1.7390(3) Å indicates the presence of intact B-B
bonds. The B-N bonds connecting the L5 units with the diboron units measure
1
.5807(3) and 1.5804(3) Å. The separation between the two L5 units in the new
polycationic cyclophane is 3.8496(6) Å for the C atoms in 4-position of the pyridyl
groups (see Figure 7a). The compound turned out to be extremely light-sensitive, and
had to be kept in the dark to avoid light-induced decomposition, leading to so far
unidentified products. It was not possible to obtain a larger amount of this compound
in pure form, prohibiting further analysis.
Reaction between diborane
1
and 1,2-di(4-pyridyl)ethane (L6) gave
[
{B (hpp) (L6) ](OTf) . The flexible structure leads to a slight increase in the
2
2
}
2
2
4
separation of the two bispyridine units (Figure 7b).[24] Hence, the distances between
the carbon atoms in 4-position of the pyridyl groups is 4.5952(5) Å in
4+
[
{B
2
(hpp)
2
}
2
(L6) ] . The B-B bonds are 1.7155(2) Å long, and the B-N bond distances
2
connecting the diboron units with the L6 units measure 1.5844(1) and 1.5912(2) Å. As
expected, charge-transfer bands are absent in the UV-vis spectrum of
[
{B
2
(hpp)
2
}
2
(L6)
2
](OTf) (see Supporting Information); the visible region is free of
4
absorptions. Additional experiments with 1,2-di(4-pyridyl)propane produced product
mixtures from which it was not possible to isolate a pure compound in larger yield.
Nevertheless, a small amount of crystals was obtained; a preliminary structural
2+
analysis with XRD revealed the presence of a cyclic compound with a [B
to which one 1,2-di(4-pyridyl)propane is bound by two B-N bonds to both pyridyl groups
see Supporting Information). This result indicates the limits for the synthesis of
cyclophane boranes with flexible linkers.
2
(hpp) ] unit,
2
(
1
4
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Figure 7. Illustration of the solid-state structure of the tetracationic cyclophane
4+
4+
[
{B
counterions and hydrogen atoms are omitted. Displacement ellipsoids drawn at the
0% probability level. Colour code: B purple, C grey, N blue.
2
(hpp)
2
}
2
(L5)
2
] (a) and [{B
2
(hpp)
2
}
2
(L6) ] in crystals of their triflate salts. Triflate
2
5
Open-structure compounds. Compounds with an open structure, in which two
viologen-like units are connected only by one diboron unit, are of interest for a variety
of reasons. First, the comparison of their structures as well as redox and optical
properties with those of the corresponding closed structures (cyclophanes) should be
informative. Then, they eventually might provide access to polycationic cyclophanes
with one diboron unit and a different (organic) unit of variable size and properties at
the other end of the macrocyclic ring. Herein, we concentrate on the first point, the
comparison of the redox and optical properties.
Indeed, it was possible to synthesize the open-structure compound
[
B
2
(hpp)
2
(L1)
2
](OTf)
2
by reacting the ditriflato-diborane 1 with an excess (five
+
equivalents) of L1 in CH
Cl
2 2
at room temperature (Scheme 4a). The ESI mass
1
5
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spectrometric analysis found a peak of maximal intensity at m/z = 759.3140 due to
+
[
B
2
(hpp)
2
(L1)
2
(OTf)] (see Supporting Information), indicating the presence of the
open-structure dication in solution. In the 11B NMR spectrum (see Supporting
Information), a signal appeared at  = 3.57 ppm, shifted slightly from the value of  =
1
4
.53 ppm in 1. The H NMR spectrum indicates that both bispyridyl (L1) groups are
chemically equivalent (four signals in the aromatic region). Interestingly, the reaction
mixture first turned from colourless to deep-green. At the end of the reaction period of
1
8 h, the solution adopted an orange colour. Red crystals of the final product
(hpp) (L1) ](OTf) suitable for structural analysis by SCXRD were grown from a
CH Cl solution layered with n-pentane. The analogy to the observations made for the
reaction in a 1:1 ratio leading to the closed structure [{B (hpp) (L1) ](OTf) (see
discussion above and ref. 15) indicates that the same radical intermediate is formed in
both reactions. This intermediate might be the 1:1 complex [B (hpp) (L1)(OTf)]OTf in
[
B
2
2
2
2
2
2
2
}
2 2
2
4
2
2
which one of the triflato groups is substituted by L1, and an electron transferred from
the electron-rich diboron unit to L1. Scheme 4b illustrates possible pathways to this
intermediate. Obviously, substitution of the first triflate from the neutral diborane
B
2
(hpp)
2
(OTf)
2
by a neutral bipyridine in an S 1 mechanism should be faster than
N
substitution of the second one, since elimination of the second triflate ion from the
+
[
B
2
(hpp)
2
(L1)(OTf)] intermediate stage is hindered by the opposite charges. The
electron transfer enables addition of a second bispyridine prior to triflate elimination.
Another possibility is that the produced [B (hpp) (L1)
]2+ is first reduced to a radical
2
2
2
monocation by unconsumed diborane molecules 1, before it is re-oxidized in the course
of the reaction to allow further product formation. However, further addition of diborane
1
to a solution of the product did not lead to a colour change. Therefore, this alternative
is not plausible.
We were also able to prepare the open-structure compound [B
2
(hpp)
2
(L2)
2
](OTf) by
2
reacting diborane 1 with five equivalents of L2 in CH
2
Cl
signal is observed in the B NMR spectrum, located at  = 3.28 ppm. The compound
was isolated in 28% yield in the form of yellow crystals by layering a solution of CH Cl
with n-pentane. An additional experiment, in which the possibility for a photoinduced
2+2] cycloaddition reaction between the two double bonds in [B (hpp) (L2)
]2+ was
2
(Scheme 4a). Again, only one
11
2
2
[
2
2
2
tested by irradiating a solution of the compound at different wavelengths, gave no clear
results.[25-27]
1
6
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2
+
Scheme 4. a) Synthesis of the open-structure compounds [B
2
(hpp)
2
(L1) ] ,
2
4+
2+
[
B
2
(hpp)
2
(L1+Me)
2
]
and [B
2
(hpp)
2
(L2)
2
] . Photo of a CH
2
Cl solution of 1 with two
2
equivalents of (L1+Me)I after 24 h reaction time showing the precipitated salt
1
7
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10.1002/chem.202102656
[
B
2
(hpp)
2
(L1+Me)
2
](OTf)
2
I
2
and the remaining green solution. b) Possible mechanisms
2+
for the reaction of 1 with two equivalents of L1 leading to [B
2
(hpp)
2
(L1) ] (IET denotes
2
intramolecular electron transfer).
Finally, the salt [B
2
(hpp)
2
(L1+Me)
2
](OTf)
2
I was obtained in 76% isolated yield by
2
11
reacting diborane 1 with two equivalents of (L1+Me)I in CH
2
Cl solution. The B NMR
2
1
spectrum shows a single signal at  = 3.70 ppm. The H NMR spectrum contains four
signals in the aromatic region (see Supporting Information), indicating the equivalence
of the two units of (L1+Me). The 19F NMR spectrum confirms the sole presence of
−
solvated triflate (OTf ) ions in solution; all boron-bound triflate groups were substituted.
During the reaction, an immediate color change to cyan-green was observed. In the
EPR spectrum recorded of the reaction mixture at this stage, a signal at g = 2.0035
appeared, indicating the formation of radical intermediates (see Supporting Information
for details). On the basis of the considerations and experiments made for related
reactions (see discussion above), we postulate this intermediate to be
2+
[
B
2
(hpp)
2
(L1+Me)(OTf)] . The final product [B
2
(hpp)
2
(L1+Me)
2
](OTf)
2
I
2
precipitated as
pale-yellow solid from the green reaction mixture (see photo imbedded in Scheme 4a).
2+
The solid-state structure of the dication [B
visualized in Figure 8.[24] The compound exhibits an intact B-B bond with a length of
.7005(1) Å. The two B-N bonds connecting the two units of L1 with the diboron unit
2
(hpp)
2 2
(L1) ] from an SCXRD analysis is
1
measure 1.5864(1) and 1.5816(1) Å. The distance between the two equivalent units of
L1 widens with increasing distance to the diboron unit. Hence, the two N atoms directly
bound to B atoms are separated by only 3.1450(3) Å, but the two N atoms at the other
end of the bipyridine unit are already 8.4507(7) Å apart. The distance between the
carbon atoms in 4-position of the pyridyl groups measures 5.1278(4) Å (Figure 8). The
two B-B-N angles (N being one of the pyridine-nitrogen atoms directly bound to boron)
measure 115.6° and 118.6°.
1
8
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10.1002/chem.202102656
2+
Figure 8. Illustration of the solid-state structure of [B
2
(hpp)
2
(L1) ] . Triflate counterions
2
and hydrogen atoms omitted. Displacement ellipsoids drawn at the 50% probability
level. Colour code: B purple, C grey, N blue.
To complement our analysis, we calculated the tetracation [B
2
(hpp)
2
(L1+Me)
]4+ by
2
DFT methods (B3LYP/def2-TZVP). Figure 9a sketches the resulting energy minimum
+
structure. Since the organic units (L1+Me) carry an additional charge, the two equally-
2
+
charged (L1+Me) units sense a larger columbic repulsion than in [B
2
(hpp)
2
(L1) ] .
2
While the two nitrogen atoms directly bound to the B atoms are separated by 3.631 Å,
the methyl-bound nitrogen atoms at the other side of the organic linker units are already
separated by 13.569 Å. With 7.346 Å, the (calculated) distance between the carbon
atoms in 4-position of the pyridyl groups is significantly larger than in [B
exp. 5.1278(4) Å in the solid-state). The B-B-N angles (N being one of the pyridine-
nitrogen atoms directly bound to boron) measure 125.2° and 125.3°, being significantly
2
(hpp)
2
(L1)
2
]2+
(
2+
larger than the corresponding angles in [B
2
(hpp)
2
(L1) ] . In addition, the two pyridines
2
in the (L1+Me) units are twisted, with an angle of 36.21° and 36.14° between the two
pyridyl planes, whereas this angle measures only 15.53° and 13.77° in
2+
[
B
2
(hpp)
2
(L1)
2
] . With 1.765 Å, the B-B bond length is calculated to be larger in
]4+ than in [B ]2+ (exp. 1.7005(1) Å). Similar to the
(hpp) (L1)
(hpp) (L1) (hpp) (L3)
]4+ and [{B ]4+, the HOMO is located
exclusively on the diboron unit and the LUMO on the (L1+Me) units (Figure 9b). The
[
B
2
(hpp)
2
(L1+Me)
2
2
2
2
cyclophanes [{B
2
}
2 2
2
2
}
2 2
2
4
+
HOMO-LUMO gap of 2.6 eV is similar to that in [{B
2
(hpp)
2
}
2
(L1) ] .
2
1
9
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Figure 9. a) Visualization of the structure of [B
2
(hpp)
2
(L1+Me)
2
]4+ (hydrogen atoms
omitted) from a B3LYP+D3/def2-TZVP calculation. Colour code: B purple, C grey, N
blue. b) Illustration of the isodensity surfaces (isovalue = 0.02) for the HOMO und
4
+
LUMO of [B
2
(hpp)
2
(L1+Me) ] .
2
In Figure 10, the UV-vis spectra of the two open-structure compounds [B
2
(hpp)
2
(L1)
]2+
2
and [B (hpp) (L1+Me)
2
2
2
]4+ are compared with that of the cyclophane
2
0
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4
+
]2+, a shoulder at 316 nm can be assigned to a
[
{B
2
(hpp)
2
}
2
(L1)
2
] . For [B
2
(hpp)
2
(L1)
2
charge-transfer excitation from the diboron unit to the bipyridyl units (Figure 10). The
2+
energy of the charge-transfer band decreases from 316 nm for [B
2
(hpp)
] . Obviously, (L1+Me) is a better electron acceptor than
L1. The charge-transfer band of [{B (hpp) (L1)
]4+ shows again at lower energy
397 nm). The larger extinction coefficient of the charge-transfer transition of
2
(L1) ] to 361
2
4+
+
nm for [B
2
(hpp)
2
(L1+Me)
2
2
}
2 2
2
(
4+
[
{B
2
(hpp)
2
}
2
(L1) ] is in line with the presence of two instead of only one diboron unit.
2
60000
50000
40000
30000
20000
10000
0
[
[
[
{B (hpp) } (L1) ](OTf)
2 2 2 2 4
B (hpp) (L1) ](OTf)
2
2
2
2
B (hpp) (L1+Me) ](OTf) I
2
2
2
2 2
300
400
500
600
700
800
l / nm
Figure 10. UV-vis spectra of [{B
2
(hpp)
2
}
2
(L1)
2
](OTf)
](OTf)
4
(black curve),
(blue curve) in
[
B
2
(hpp) (L1) ](OTf) (red curve) and [B (hpp)
2
2
2
2
2
(L1+Me)
2
2 2
I
CH CN.
3
The cyclic voltammograms of the two open-structure compounds containing L1 or
(L1+Me) are shown in Figure 11. In Table 2 the potentials are compared to those of
4+
the cyclophane [{B
2
(hpp)
2 2 2 4
} (L1) ](OTf) . The tetracation [B
2
(hpp)
2
(L1+Me) ] , with
2
E
1/2 = -0.58 V for the first redox process assigned to the redox couple
4
+
]2+, is the strongest electron acceptor of the
[
B
2
(hpp)
2
(L1+Me)
2
] /[B
2
(hpp)
2
(L1+Me)
2
4+
three compounds. In contrast to [{B
2
(hpp)
2
}
2
(L1) ] , this two-electron reduction step is
2
not split into two one-electron reduction steps, indicating the absence of significant
electronic coupling between the two dicationic organic units in the tetracationic
compound. This result could be explained by the large spatial separation of the two
units. Further reduction is split into two one-electron events, with E1/2 values of -1.06 V
2
1
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2
+
]+ and -1.15 V for
for the redox couple [B
2
(hpp)
2
(L1+Me)
2
] /[B
2
(hpp)
2
(L1+Me)
2
[
B
2
(hpp)
2
(L1+Me)
2
]+/[B
2
(hpp)
2
(L1+Me)
]0. Thus, electronic coupling is switched on
2
after two-electron reduction, that decreases the electronic repulsion between the two
organic units and presumably decreases their spatial separation. Please note that
irreversible oxidation of the iodide counter-ions is observed at quite high potential (Eox
=
+0.67 V), in line with strong interactions between the iodide ions and the tetracations
in CH CN solution. Another irreversible oxidation at Eox = +0.86 V is tentatively
assigned to oxidation of the diboron unit (B → B ) accompanied with decomposition.
3
II
III
2+
In the case of [B
2
(hpp)
2
(L1) ] , the reduced electron-acceptor character of the bipyridyl
2
units leads to a significant shift of the redox potentials to lower values. Two one-
electron processes with only slightly different potentials are visible, at -1.68 V for the
2
+
+
redox
(hpp)
couple
(L1)
]+/[B
[B
2
(hpp)
2
(L1)
2
] /[B
2
(hpp)
2
(L1)
2
] ,
and
at
-1.77
V
for
[
B
2
2
2
2
(hpp)
2
(L1)
2
]. The reduced electronic repulsion allows a smaller
spatial separation between the bipyridyl units, initiating weak electronic coupling
already before reduction. An irreversible oxidation at Eox = +0.89 V is likely to belong
II
III
to oxidation of the diboron unit (B → B ) accompanied with decomposition.
50
0
[
B (hpp) (L1+Me) ](OTf) I
2 2 2 2 2
-50
-
2
0
2
30
0
[
B (hpp) (L1) ](OTf)
2
2
2
2
-30
-
2
0
2
E / V
Figure 11.
Comparison
between
the
(L1) ](OTf)
cyclic
in CH
voltammograms
CN at 298 K, measured at
for
[
B
2
(hpp) (L1+Me)
2
2
](OTf)
2
I
2
and [B (hpp)
1
2
2
2
2
3
−
a scan rate of 100 mV s (glassy carbon working-electrode, Pt counter-electrode,
2
2
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Ag/AgCl reference-electrode, 0.1 M nBu
are given vs. the ferrocenium/ferrocene (Fc /Fc) redox couple.
4
NPF
6
as supporting electrolyte). Potentials
+
+
Table 2. Potentials (in V, given vs. Fc /Fc) measured in CH
3
CN solution.
Compound
E
1/2(1)/Ered(1)
E
1/2(2)/Ered(2)
E
1/2(3)/Ered(3)
E1/2(4)/Ered(4)
[
[
[
{B
2
(hpp)
2
}
2
(L1)
(L1+Me)
(L1)
2
](OTf)
4
-1.14/-1.18
-0.58/-0.64
-1.68/-1.74
-1.21/-1.38
-0.58/-0.64
-1.77/-1.81
-1.80/-1.87
-2.06/-2.10
B
B
2
(hpp)
(hpp)
2
2
](OTf)
2
I
2
-1.06/-1.10
-1.15/-1.19
a
a
2
2
2
](OTf)
2
a
Outside the available potential window in the solvent.
In addition to the dicationic open-structure compounds [B
2
(hpp)
2
(L1)
2
]2+ and
2
+
2+
[
B
2
(hpp)
L7 also containing two uncoordinated nitrogen atoms. Reaction of 1 with 2,2`-
bipyrimidine (L7) in CH Cl solution at room temperature yielded the salt
(hpp) (L7)](OTf) , that precipitated in 38% isolated yield in the form of orange
crystals suitable for SCXRD from a CH Cl solution layered with n-pentane (Figure 12).
It was not possible to coordinate a second [B
excess of diborane 1. Moreover, attempts to coordinate a transition metal (reactions
with Mn(CO) Br, Ru(bipy) Cl , [Rh(cod)Cl] , and W(CO) ) were fruitless. Obviously, the
2
(L2)
2
] , we synthesized the dicationic diborane [B
2
(hpp)
2
(L7)] , with a unit
2
2
[
B
2
2
2
2
2
2+
2
(hpp) ] unit to L7, even not with a large
2
5
2
2
2
6
Lewis basicity of the two remaining uncoordinated N atoms is significantly reduced by
the coordination of L7 to one diboron unit. Figure 12 visualizes the solid-state structure
2+ [24]
of the dication [B
B-N bonds connecting the diboron unit with the bipyrimidine unit measure 1.600 and
.606 Å.
2
(hpp) (L7)] . The B-B bond length measures 1.670 Å, and the two
2
1
2+
Figure 12. Synthesis of [B
2
(hpp)
(L7)] . Triflate counterions and hydrogen atoms omitted. Displacement
ellipsoids drawn at the 50% probability level. Colour code: B purple, C grey, N blue.
2
(L7)] and illustration of the solid-state structure of
2+
[
B
2
(hpp)
2
2
3
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10.1002/chem.202102656
The cyclic voltammogram of [B
2
(hpp)
2
(L7)](OTf)
2
shows two (one-electron) redox
processes at E1/2(1) = −0.96 V and E1/2(2) = −1.52 V (Figure 13). The Lewis structures
on the right side sketch the overall, 2,2’-bipyrimidine-centered two-electron redox
process. However, the reduction waves are more intense than the oxidation waves,
arguing for partial loss of compound after the second reduction. Attempts to chemically
reduce the compound with CoCp*
product could not be isolated.
2
resulted in product mixtures from which a clean
Figure 13. Cyclic voltammogram of a 1 mM solution of [B
at 298 K, measured at a scan rate of 100 mV s−1 (glassy carbon working electrode,
platinum counter electrode, Ag/AgCl reference electrode, 0.1 M nBu NPF as
2 2
(hpp) (L7)](OTf)
2
in CH CN
3
4
6
+
supporting electrolyte). Potentials given vs. the ferrocenium/ferrocene (Fc /Fc) redox
couple.
Construction of the first hexaboracyclophane cage. An important aim of this work
was the construction of a first polycationic macrocycle with cage-like structure and
integrated electron-rich diboron units. To demonstrate the possibility to synthesize
such compounds with a charge higher than +4, we used the trispyridine compound
tris(4-pyridyl)triazine (L8). To our delight, reaction of L8 with the diborane 1 yielded the
6+
unprecedented hexacationic hexaboron cage compound [{B
2
(hpp)
2
}
3
(L8) ] (Scheme
2
5
), that crystallized together with six triflato counterions in the form of dark-red crystals
2
4
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Chemistry - A European Journal
10.1002/chem.202102656
in 38% isolated yield. Several experiments showed that the use of an excess of L8 has
a positive impact on the yield. The red salt was barely soluble in CH Cl , but moderately
2
2
11
soluble in CH
3
CN. The B NMR spectrum contained a single signal at  = 4.08 ppm,
1
showing the equivalence of all six boron atoms. Also, in the H NMR spectrum, only
two doublets appeared in the aromatic region (at  = 8.58 and 8.30 ppm), in line with
the chemical equivalence of all six pyridyl groups. Interestingly, here again the reaction
mixture turned deep-green instantly upon dissolving the two reactants in CH
Cl
2 2
(Scheme 5). After some time it changed colour to red, signaling formation of the final
product. Following the conclusions drawn for other reactions (see discussion above),
we postulate an intermediate in which only one of the triflates of the diborane reagent
is substituted by L8 to be responsible for the dark-green colour.
Scheme 5. Synthesis of the first hexacationic hexaboron cage [{B
2
(hpp)
2 3
}
(L8)
2
](OTf)
6
with three diboron units. The photos show (from left to right) the colorless mixture of
the two reactants 1 and L8, the dark-green reaction mixture directly after addition of
2 2
CH Cl , and a solution of the red final product [{B
2
(hpp)
2
}
3
(L8)
2
](OTf)
6
in CH CN.
3
The solid-state structure from SCXRD is shown in Figure 14 from two perspectives.[24]
The three B-B bond lengths are only slightly different in the solid state, measuring
1
.7259(1), 1.7213(1) and 1.7280(1) Å, in line with the presence of three intact B-B
2
5
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Chemistry - A European Journal
10.1002/chem.202102656
bonds. The B-N bonds, connecting the diboron units with the L8 units, are 1.5711(1) -
.5867(2) Å long. The shortest C···C distance between C atoms from different triazine
1
units amounts to 3.8278(3) Å.
Figure 14. Illustration of the solid-state structure of [{B
(hpp) }
2 2 3
(L8)
]6+ from two
2
perspectives. Triflate counterions and hydrogen atoms omitted. Displacement
ellipsoids drawn at the 50% probability level. Colour code: B purple, C grey, N blue.
The UV-vis spectrum of [{B
2
(hpp)
2
}
3
(L8)
2
](OTf)
6
in CH CN solution (Figure 15a) shows
3
a sharp band at 241 nm and a broad band centered at 391 nm, extending far into the
visible region. The broad band is again assigned to a charge-transfer transition from
an orbital localized on the electron-rich diboron units (HOMO) to an unoccupied orbital
at the trispyridyl (L8) electron-acceptor units (LUMO). For comparison, the UV-vis
spectrum of free L8 is dominated by a strong band at 245 nm; L8 does not absorb in
the visible region.
2
6
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Chemistry - A European Journal
10.1002/chem.202102656
Cyclic voltammetry measurements show that the hexacation [{B
2
(hpp)
}
2 3
(L8)
]6+ can
2
be reduced reversibly in five steps to the neutral compound (Figure 15b). On the basis
of their relative intensities, we assign the waves to four one-electron and one two-
electron redox events. The four reversible one-electron processes are observed at
E
1/2 = -0.88, -1.03, -1.51 and -1.68 V. Then a reversible two-electron redox process
follows at E1/2 = -2.07 V. For comparison, free L8 exhibits a reduction wave at -1.73 V.
The observation of one-electron redox processes indicates electronic communication
between the two tris(4-pyridyl)triazine units, enabled by their small spatial separation.
The possibility for reversible stepwise reduction with up to six electrons makes the new
cage-like hexacation interesting for applications (e.g. as energy-storage material). The
6
+
(L8)
]5+ is quite
reduction potential for the redox couple [{B
2
(hpp)
2 3
}
(L8)
2
] /[{B
(hpp) }
2 2 3
2
high, motivating attempts to chemically reduce the compounds. Unfortunately, up to
date all attempts to chemically reduce the hexacationic cyclophane and to isolate a
product of reduction failed, although some redox experiments look promising. For
example, reaction with two or four equivalents of CoCp
2
or CoCp*
2
in CH
2
Cl led to a
2
colour change from red to green, pointing to a change in redox state. However, the
solution darkened after some time indicating conversion to other, so far unidentified
products. Hence, we could not yet confirm the stability of the compounds in several
redox states.
2
7
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Chemistry - A European Journal
10.1002/chem.202102656
Figure 15. a) UV-vis spectrum of the new hexaboron cage [{B
2
(hpp)
in CH
measured at a scan rate of 100 mV s (glassy carbon working-electrode, Pt counter-
electrode, Ag/AgCl reference-electrode, 0.1 M nBu NPF as supporting electrolyte).
2
}
3
3
(L8)
CN at 298 K,
2
](OTf)
6
in
CH
3
CN. b) Cyclic voltammogram of [{B
2
(hpp)
2 3
}
(L8)
2
](OTf)
6
−
1
4
6
+
Potentials are given vs. the ferrocenium/ferrocene (Fc /Fc) redox couple.
Conclusions
This work is the first systematic study of polycationic cyclophanes and cage-like
compounds built of bi- or tris-pyridyl electron-acceptor units and electron-rich diboron
2
8
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Chemistry - A European Journal
10.1002/chem.202102656
units. The synthesis of these novel compounds relies on an easily accessible, electron-
rich ditriflato-diborane precursor with strongly electron-donating guanidinate groups,
that allows the integration of electron-rich diboron electron-donor units in a variety of
polycationic cyclophanes. An important result of this work is the synthesis of a first
hexacationic cage-like compound, in which two tris(4-pyridyl)triazine electron-acceptor
units are connected by three diboron units. Due to the presence of the diboron electron-
donor units in combination with the bi- or oligopyridyl electron-acceptor units, the
compounds display intriguing redox and optical properties, and the results demonstrate
that these properties could be tuned in a rational way by the size and other properties
of the viologen-like electron-acceptor units. Quantum-chemical calculations on
selected compounds show that the HOMO is localized on the diboron units and the
LUMO exclusively on the bispyridyl units, leading to characteristic charge-transfer
excitations in the visible region.
We demonstrate in addition the possibility to synthesize open-structure compounds, in
which two viologen-like units are connected by only one diboron unit. Such compounds
might provide access to asymmetric rings and cages with only one diboron unit and a
variable organic unit at the “other side” that binds to both bispyridyl units to close the
ring.
Research on this novel class of compounds is still at its infancy. An important aim of
ongoing research is the isolation and characterization of cyclophanes in different redox
states. Another aim is the increase of the ring or cavity size. The diboron units permit
only a small spatial separation between the aromatic systems in the cyclophanes with
bi- or oligopyridyl linkers, too small for the uptake of an organic donor molecule. On
the other hand, the increase of the electron-acceptor property by introduction of further
electron-withdrawing substituents, e.g. at the phenylene ring of the L3 units in
4+
[
{B
2
(hpp)
2
}
3
(L3) ] , might trigger an intramolecular electron transfer (IET) from the B-
2
B bond to the bispyridyl units. Similar IET processes were already observed for
cyclophanes with pyrazine units (see Introduction and Scheme 2). The cleavage of the
B-B bond in the course of this IET increases the size of the ring system, thereby
possibly initiating the uptake of an aromatic guest molecule. Moreover, rings with larger
diameter, able to receive organic guest molecules, could be realized in asymmetric
compounds in which the two bispyridyl groups are connected by one diboron unit and
an organic group of variable size at the other side. A suitable precursor to such
2
9
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