Inclusion compound based approach to arrays of artificial dipolar molecular rotors: Bulk inclusions

Article abstract of DOI:10.1021/jo3009897

We examine the insertion of two dipolar molecular rotors as guests into a host, tris(o-phenylenedioxy)cyclotriphosphazine (TPP, 1), using differential scanning calorimetry, solid-state NMR, powder X-ray diffraction, and dielectric spectroscopy. The rotors are 1-(4′-n-pentylbiphenyl-4-yl)-12-(2,3- dichlorophenyl)-p-dicarba-closo-dodecaborane and 1,12-bis(2,3-dichlorophenyl)-p- dicarba-closo-dodecaborane. Both enter the bulk even though their nominal diameter exceeds the nominal channel diameter and although a closely related rotor, 1-n-hexadecyl-12-(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane, is known to produce a surface inclusion compound. Rotational barriers of 5.4-9.3 kcal/mol were found for the dichlorophenyl rotator contained within the TPP channel. Clearly, van der Waals diameters in themselves do not suffice to predict TPP channel entry. It is suggested that the efficacy of the p-carborane stopper is reduced by the presence of the two relatively bulky adjacent benzene rings, which help to stretch the channel, and by the axial direction of its axis, which prevents the attached rotator from contributing to the stopping action.

Full text of DOI:10.1021/jo3009897

Article
pubs.acs.org/joc
Inclusion Compound Based Approach to Arrays of Artificial Dipolar
Molecular Rotors: Bulk Inclusions
Lukas Kobr,† Ke Zhao,‡ Yongqiang Shen,‡ Katerina Polívkova,§ Richard K. Shoemaker,† Noel A. Clark,‡
́
̌
̌
́
John C. Price,‡ Charles T. Rogers,‡ and Josef Michl*,†,§
†Department of Chemistry and Biochemistry and ‡Department of Physics, University of Colorado, Boulder, Colorado 80309, United
States
§Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 16610 Prague 6, Czech Republic
S
* Supporting Information
ABSTRACT: We examine the insertion of two dipolar molecular rotors as guests into a host,
tris(o-phenylenedioxy)cyclotriphosphazine (TPP, 1), using differential scanning calorimetry,
solid-state NMR, powder X-ray diffraction, and dielectric spectroscopy. The rotors are 1-(4′-
n-pentylbiphenyl-4-yl)-12-(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane and 1,12-bis-
(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane. Both enter the bulk even though their
nominal diameter exceeds the nominal channel diameter and although a closely related rotor,
1-n-hexadecyl-12-(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane, is known to produce a
surface inclusion compound. Rotational barriers of 5.4−9.3 kcal/mol were found for the
dichlorophenyl rotator contained within the TPP channel. Clearly, van der Waals diameters
in themselves do not suffice to predict TPP channel entry. It is suggested that the efficacy of
the p-carborane stopper is reduced by the presence of the two relatively bulky adjacent
benzene rings, which help to stretch the channel, and by the axial direction of its axis, which prevents the attached rotator from
contributing to the stopping action.
∼5 Å thick layers penetrated in the perpendicular direction by a
trigonal array of parallel hollow channels of 4.5−5 Å diameter,
separated by ∼11.5 Å.16 Their internal shape is locally
triangular, with the triangles rotated by 60° from layer to
layer (Figure 1). They readily include small molecules as guests,
INTRODUCTION
■
Molecular rotors have received much attention,1−7 and
purposely designed regular arrays of interacting dipolar rotors
would be expected to show interesting collective properties.
They might also be of practical utility, e.g., as ultrafast dielectric
materials in electronics, especially if they had a ferroelectric
ground state. Rotor arrays with designed properties such as
inter-rotor interactions are not easy to produce by growing
crystals of neat molecular rotor compounds,8−12 since many
features need to be controlled simultaneously: the highly polar
rotors must have very low rotational barriers, and the
orientation of their axes as well as their geometrical
arrangement and spacing need to be optimized. The issue has
been recently discussed,13 and it was proposed that the desired
arrays might be produced by inserting the rotors as guests into
channels in suitable hosts, allowing a separate tuning of the
properties of the guest and the host. A related idea is to use
metallo-organic frameworks (MOFs).14,15 If the guests in a
guest−host inclusion compound were constrained to the
surface of the host, a two-dimensional (2-D) rotor array
would result, and if they penetrated into the bulk, a three-
dimensional (3-D) array would be produced. In a 3-D array,
proper registry of rotors located in different channels might be
difficult to achieve, but the total density of rotors in a thin layer
of material would be much larger and the dielectric response
stronger. However, even in a 2-D array the response would still
be readily detectable.3
Figure 1. Crystal structure of hexagonal TPP.16 Above, top view, and
below, side view of two layers of the solid.
Special Issue: Howard Zimmerman Memorial Issue
Received: June 15, 2012
We have chosen as host hexagonal tris(o-phenylenedioxy)-
cyclotriphosphazene 1 (TPP), whose crystals are composed of
© XXXX American Chemical Society
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as shown by X-ray diffraction and solid-state NMR spectros-
copy.16−22 The inclusion of guests is easy to detect, since the
solid-state NMR chemical shifts of their nuclei are displaced by
several ppm by the ring currents in the TPP host. An even
easier way is to use TPP-d12 as host and look for cross-
polarization of TPP carbons by magnetized protons of the
guest.13 TPP also forms monoclinic crystal modification that
does not form inclusion compounds and whose presence is
readily recognized in NMR spectra and X-ray diffraction
patterns.17,23,24 Below about 150 °C, the monoclinic form is the
more stable crystal modification in the absence of a guest.
Having chosen hexagonal TPP as the host, we need to design
the molecular rotor guest to produce regular arrays. First, we
need to ensure the formation of an inclusion compound, which
reflects a competition between the cohesive forces in the crystal
of a neat dipolar rotor guest and the attractive forces between
the guest and the TPP channel walls. A tight fit of the rotor into
the TPP channel is desirable to maximize this attraction and
also to anchor the rotator axle firmly along the channel axis, but
excessively thick guests may encounter steric hindrance to
inclusion and fail to enter. TPP channels are able to expand to
some degree in response to the presence of a guest,25 but little
is known presently about the maximum lateral dimensions of an
acceptable guest.26
order to reduce the intrachannel electrostatic coupling, which
should not exceed interchannel coupling if the ground state is
to be ferroelectric.
In our first publication on the subject,13 we reported the
formation of a surface inclusion compound of type A from a
shaft−stopper−rotator guest but found that in samples with
high channel occupancy, the guests were not all inserted to the
same depth, suggesting imperfect registry of the rotators at the
surface. Barriers to rotation were relatively high (1.2−9 kcal/
mol), presumably because of a deviation of the rotator axle
from the surface normal and a resulting close contact of the
rotator with the surface at certain rotation angles. Presently, we
report results for an inclusion of type C, with 1-(4′-n-
pentylbiphenyl-4-yl)-12-(2,3-dichlorophenyl)-p-dicarba-closo-
dodecaborane (2) and 1,12-bis(2,3-dichlorophenyl)-p-dicarba-
closo-dodecaborane (3), shown in Chart 1, as guests in TPP.
Results for a different inclusion type C are being reported
elsewhere.26
Chart 1. Molecular Structures of the Host (1) and Guests (2,
3)
Three insertion modes are of interest (Figure 2): (mode A)
surface inclusion, with the dipolar rotating part (rotator)
RESULTS
■
Figure 2. Three modes of guest inclusion. (A) Surface inclusion, with
the dipolar rotating part (rotator) located outside the host; (B) surface
inclusion, with the dipolar rotator inside the host; and (C) bulk
inclusion, with the whole molecular rotor including the rotator inside
the host.
The ease with which benzene enters TPP channels to form
benzene@TPP16 suggested that biphenyl will form an inclusion
compound biphenyl@TPP as well, and indeed it does
(Supporting Information, Figure S1). This result suggested
that oligophenyls might make good rotor shafts. Unlike the
long alkyl chain used in our first paper,13 they are expected to
direct the rotator axle perpendicular to the surface as desired. In
the following, we report the synthesis of 2 and 3, assignment of
their liquid NMR spectra, formation of inclusion compounds
with microcrystalline TPP and their characterization by
differential scanning calorimetry (DSC), solid-state NMR
spectra, X-ray powder diffraction, and dielectric spectroscopy.
Synthesis. To make rotor 2 (Scheme 1), an organozinc
reagent was prepared from p-(n-pentyl)bromobenzene and
Negishi coupled with p-iodonitrobenzene to 4 in 84% yield.
The product was reduced to 5 in 94% yield and converted to 6
84% yield by the Sandmayer reaction.
Ulmann coupling with the cuprous salt of p-carborane to
yield 7, analogous to previous work,27 proceeded in 92% yield.
A small amount of the disubstituted product was also formed.
When the reaction was tried under catalysis with various
palladium(0) catalysts, much p-(n-pentyl)biphenyl was formed,
and the yield of 7 was low. Addition of triethylamine to the
reaction mixture increased the yield, but the Ulmann conditions
remained superior. Ulmann-type reaction of 7 with 1,2-
dichloro-3-iodobenzene provided 2 in 87% yield. This
procedure was again better than using palladium as catalysts.
located outside the host; (mode B) surface inclusion, with the
dipolar rotator inside the host; and (mode C) bulk inclusion,
with the whole molecular rotor including the rotator inside the
host.
In a triangular lattice, good registry of the dipolar rotators in
a surface inclusion will be most likely opposed by their
electrostatic interactions and will require the guest molecules to
have high affinity for the channel walls. For a 2-D surface
inclusion, we therefore attach the dipolar rotator to a relatively
long shaft, and we prevent the entry of the whole rotator
molecule inside the channel by terminating the shaft with a
sufficiently bulky stopper. For insertion mode A, we search for
suitable guests of the type shaft−stopper−rotator and for mode
B, for guests of the type shaft−rotator−stopper. Mode A has
the advantage that electrostatic inter-rotator interactions are
not attenuated by the dielectric constant of the host. For mode
C, guests of the type shaft−rotator, shaft−rotator−shaft
(shown in Figure 2), or even rotator−shaft−rotator would all
appear possible, with the diameter of the shaft in excess of that
of the rotator. In this instance, the shafts would not only fix the
direction of the rotational axis, but also help control the
distance between dipolar rotators within the same channel in
B
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Scheme 1. Synthesis of 2
The rotor 3 was prepared by Ulmann coupling in 76% yield
(Scheme 2), and the preparation of TPP-d12 (isotopic purity
>99.8%) has been described.13
Formation and DSC of Inclusion Compounds 2@TPP
and 3@TPP. Two procedures for the formation of inclusion
compounds from solids were tested. Grinding yielded the TPP
inclusion of 5 mol % of 2 (5%2@TPP). Ball milling followed by
annealing was used to obtain 15%2@TPP-d12 and 15%3@TPP-
d12.
Scheme 2. Synthesis of 3
The DSC results are shown in the Supporting Information
(Figures S4 and S5). The trace of hexagonal TPP-d12 consists
of a broad exotherm centered around 148 °C and endotherms
at 222 and 252 °C.13 The neat 2 yields a single endotherm at
152 °C, and 15%2@TPP-d12 yields two very broad exotherms
at 100 and 210 °C and two endotherms at 251 and 282 °C.
Neat 3 gives a single endotherm at 265 °C, and 15%3@TPP-
d12, three endotherms at 232, 240, and 246 °C.
Solid-State NMR. The solid-state 13C CP MAS NMR
spectrum of neat 2 (Figure 3) shows chemical shifts and a
pattern that are similar to the 13C solution spectrum, except
that some signals are split into two peaks, suggesting that two
inequivalent crystallographic orientations are present in the unit
cell.
Solution NMR (Chart 1 and Figure 3). The assignment of
carbons and protons in 2 (Table 1) was deduced from their 1H
1
NMR, 13C NMR, H−1H gCOSY (Supporting Information,
1
Figure S2, top), H−13C gHSQC (Supporting Information,
1
Figure S2, bottom), H−13C gHMBC, (Figure S3, top), and
1
boron decoupled H−13C gHMBC (Supporting Information,
The solid-state 13C CP MAS NMR spectrum of 5%2@TPP
(Figure 3) displays three large resonances in the aromatic
region that are due to TPP. It also shows a pattern for the
resolved guest peaks that is similar to the solution 13C spectrum
of 2, but most are shifted upfield with respect to both the
solution and the solid 13C NMR spectra. Annealing sharpened
the signals. The carbons of TPP now exhibit multiple signals
since the empty portion of TPP became monoclinic.
Figure S3, bottom) spectra as described in the Supporting
Information.
A detailed assignment of the spectrum of 3 was not needed.
Of the six resolved aromatic signals in its 13C NMR spectrum,
Ca, Cb and Cc belong to the quaternary carbons of the 2,3-
dichlorophenyl rotators, and Cd, Ce and Cf to the protonated
carbons, on the basis of DEPT135. The signal Cg belongs to the
p-carborane carbons.
1
Table 1. Summary of the H and 13C Assignments in Rotors 2 and 3 (cf. Chart 1)
2
3
atom
a
1H δ/ppm
7.41*
13C δ/ppm
atom
h
1H δ/ppm
13C δ/ppm
atom
o
1H δ/ppm
13C δ/ppm
atom
1H δ/ppm
13C δ/ppm
131.02
86.60
7.24
129.02
a−c
131.40
134.02
b
7.08
7.59
126.66
i
134.96
p
142.75
135.95
c
d
e
130.69
134.19
131.46
j
7.30
127.49
126.73
141.55
q
r
2.64
35.73
31.70
31.29
k
l
7.41*
1.65
d−f
7.41
7.07
7.56
126.71
130.53
131.15
s
1.36*
f
135.90
81.70
m
n
137.32
126.98
t
1.36*
0.91
22.71
14.20
g
7.44
u
g
77.36
C
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The long contact time spectrum shows three intense singlet
aromatic resonances that belong to TPP-d12. These three
signals and the differences from the spectrum of neat 2 provide
additional evidence for the formation of the inclusion
compound. In the short contact time spectrum the signals of
protonated carbons are enhanced relative to those of
quaternary carbons. A comparison of the two spectra shows
that the signals Cp, Cl, Cm, Cf, Ci, Cd, Ce, Ch, and Cg belong to
quaternary carbons, while the signals X, Y, Z, Cq, Cr, Cs, Ct, and
Cu belong to protonated carbons. The signals Cp, Cl, Cm, Cf, Ci,
Cd, Ce, Ch, Cq, Cr, Cs, Ct and Cu, in the solid-state 13C NMR
spectrum of 15%2@TPP-d12 were tentatively assigned by
comparison with the solution 13C NMR spectrum of 2. The
chemical shifts in the solid-state 13C NMR spectrum of 15%2@
TPP-d12 are displaced upfield from their positions in the
solution 13C spectrum of 2. The chemical shift changes for the
resolved carbons are summarized in Table 2. The protonated
aromatic carbons are not fully resolved, but they all display an
upfield change in the chemical shift.
Table 2. Differences in the Chemical Shifts of 15%2@TPP-
d12 and of the Solution Spectrum of 2
Δδ
Δδ
Δδ
Δδ
carbon
ppm
carbon
ppm
carbon
ppm
carbon
ppm
d
e
f
−2.1
−1.1
−1.9
−1.7
h
i
−1.7
−1.5
−1.4
−1.7
p
q
r
−2.0
−1.8
−0.1
−0.5
t
−0.7
−0.9
u
l
g
m
s
The solid-state 13C CP MAS NMR spectra of neat 3 and
15%3@TPP-d12 are shown in Figure 5.
Figure 3. Solid-state 13C CP MAS (5 ms contact time) and solution
13C NMR spectra of 2. (A) Solution of 2 in CDCl3; (B) neat solid 2;
(C) solid 5%2@TPP; (D) annealed solid 5%2@TPP. *: a spinning
sideband.
The solid-state 13C CP MAS NMR spectrum of 15%3@TPP
with dipolar dephasing only shows signals of nonprotonated
carbons, while that of 3 with a short (0.2 ms) contact time
displays protonated carbon signals enhanced relative to
nonprotonated ones (Figure 5). A comparison of the two
The solid-state 31P SPE MAS NMR spectrum of 15%2@
TPP-d12 (Figure 4) shows a singlet at 34.1 ppm, due to TPP-d12
in hexagonal form, and a few small signals around 0 ppm. The
solid-state 13C CP MAS NMR spectra of 15%2@TPP-d12 are
shown in Figure 4.
Figure 5. NMR spectra. (A) 13C CP MAS of 15%3@TPP-d12; (B) 13
C
CP MAS of neat 3; (C) 13C of CDCl3 solution of 3; (D) 13C CP MAS
with DPD of 15%3@TPP-d12; (E) 13C CP MAS of 15%3@TPP-d12
with 0.2 ms contact time; (F) 31P SPE MAS of 15%3@TPP-d12; (G)
31P CP MAS of 15%3@TPP-d12.
Figure 4. NMR spectra. (A) 13C CP MAS (5 ms) of 15%2@TPP-d12;
(B) 13C CP MAS (0.2 ms) of 15%2@TPP-d12; (C) 31P SPE MAS
NMR of 15%2@TPP-d12.
D
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spectra reveals that the signals A and B belong to quaternary
aromatic carbons, and signal C contains protonated and
nonprotonated carbon signals. The short contact time experi-
ment shows an additional protonated carbon, D. A comparison
of the solid-state 13C NMR spectrum and the solution spectrum
of 3 suggests that carbon Ca gives rise to the signal A in the
inclusion compound solid-state spectrum, carbon Cb is
responsible for signal B, and a part of the signal C belongs to
carbon Cc. The remaining intensity of signal C and signal D
belong to the protonated carbons Cd, Ce and Cf. Signal E
belongs to the p-carborane carbons Cg. The solid-state 31P SPE
MAS NMR and 31P CP MAS NMR both show a single
resonance at 34.1 and 34.0 ppm, respectively (Figure 5). All the
aromatic signals of 3 in 15%3@TPP-d12 are shifted upfield
relative to the solution 13C NMR spectrum of 3, and the p-
carborane carbons are shifted upfield by 1.2 ppm.
by 0.6%, respectively, relative to empty TPP. From peak widths,
the particle size for the hexagonal 15%3@TPP-d12 present in
the mixture is 100 20 nm.
Dielectric Spectroscopy. The results of dielectric loss
measurements on the same samples of 15%2@TPP-d12 and
15%3@TPP-d12 that were used for NMR and X-ray diffraction
studies are shown in Figure 7. 15%2@TPP-d12 displays
X-ray Powder Diffraction. The same solid-state NMR
samples 15%2@TPP-d12 and 15%3@TPP-d12 were also used
for copper Kα1 wavelength X-ray powder diffraction measure-
ments. Figure 6 shows the scattered X-ray photon counts versus
the magnitude of the scattering wave vector q, related to
scattering angle θ and X-ray wavelength λ by q = 4π sinθ/λ.
Figure 7. Dielectric loss at 120 Hz (black), 1200 Hz (red), and 12 kHz
(blue). (A) 15%2@TPP-d12; (B) 15%3@TPP-d12.
primarily a single barrier to rotation, with a slight shoulder
on the low temperature side. The barrier has a height Eb of 8.90
0.02 kcal/mol, the frequency factor A is (2.2 0.2) × 1013
Hz, and the asymmetry parameter s is 0.52 0.01 kcal/mol.
The shoulder can be fitted to Eb = 4.70 0.03 kcal/mol, A =
(5.1 0.6) × 1012 Hz, and s = 0.92 0.02 kcal/mol. 15%3@
TPP-d12 has a more complicated set of three loss peaks. Listing
from the lowest temperature peak to the highest, their Eb values
are 5.37 0.01, 6.73 0.03, and 9.25 0.04 kcal/mol, their A
values are (1.5 0.1) × 1013, (2.0 0.2) × 1013, and (2.5
0.3) × 1014 Hz, and their s values are 0.34 0.01, 0.51 0.01,
and 0.68 0.01 kcal/mol, respectively.
Figure 6. X-ray powder patterns from inclusion compounds. (A)
15%2@TPP-d12 (solid black line, diffraction data; red line, fit assuming
hexagonal peak positions). (B) 15%3@TPP-d12 with assignments (H,
empty hexagonal TPP; M, monoclinic TPP; I, a hexagonal inclusion
compound).
DISCUSSION
■
Choice of Guest. The rationale for choosing TPP as the
host system has been described elsewhere,13 and here we only
need to note that (i) the nominal internal diameter of the
channels available for the guest species is about 4.5−5 Å, that
(ii) TPP is capable of expanding its lattice to accommodate
even guests with a larger diameter, and that (iii) p-carborane
(∼7.6 Å across29) refused to enter TPP when it was attached to
a long alkyl chain and carried a rotator in the antipodal
position.
The trace for 15%2@TPP-d12 shows a sequence of diffraction
peaks that fit a single hexagonal structure with lattice
parameters expanded in the in-plane direction to 12.067
0.001 Å and contracted in layer spacing to 9.979 0.001 Å.
These values represent a 5.4% expansion and a 1.8%
contraction, respectively, compared to the 11.454
0.005 Å
Guest rotors that form a 2-D surface inclusion compound,
with a transverse dipole carrying rotator at the surface, either
just outside (Figure 2A) or just inside (Figure 2B), would be
particularly novel and attractive. Our first attempt to attain this
goal with a rotor molecule consisting of a hexadecyl shaft, a p-
carborane stopper, and a 2,3-dichlorophenyl rotator with a ∼2.5
D dipole moment30 was successful,13 but the 2-D rotor array
suffered from two shortcomings. First, the most concentrated
samples, in which the TPP channels were close to saturated
with the rotor guests, were heterogeneous. In these samples,
different rotors were apparently inserted to different depths and
and 10.160
0.005 Å values reported for empty hexagonal
TPP.28 Peak widths suggest a typical powder particle size of 40
nm. These results demonstrate that 15%2@TPP-d12 adopts a
global hexagonal structure, consistent with the local hexagonal
structure shown by the solid state NMR data.
The trace for 15%3@TPP-d12 displays a complicated peak
structure that is consistent with a mixture of empty TPP,
monoclinic TPP, and a single hexagonal structure with in-plane
lattice parameter of 12.56 0.01 Å and a layer spacing of 10.10
0.01 Å. These values are expanded by by 9.7% and contracted
E
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had different rotational barriers. Second, most of the barriers to
rotation were too high to be useful, 1.2−9 kcal/mol, although
in the model for the isolated molecule a barrier of only 1.56
kcal/mol was calculated.13 The higher values were tentatively
attributed to rotators that rubbed the surface because they were
close to it and their axles did not lie along the surface normal.
Presently, we tried to solve the second difficulty by assuring
that the rotator axle, whose direction is fixed by the C−C axis
of the p-carborane stopper, lies perpendicular to the surface.
Unlike the terminal bond of an alkyl chain, which does not lie
along the long axis, that of a polyphenylene chain does, and for
2, one can expect the C−C axis of the attached p-carborane to
lie normal to the crystal surface. If the p-carborane stopper
performed its intended function similarly as when it was
attached to an alkyl chain, prevented entry of the rotator inside
the guest channel, and forced it to remain just outside the TPP
surface, an inclusion compound with a 2-D array of rotators
with small barriers to rotation would result. The large (∼7.8
Å31) diameter of the o-dichlorobenzene rotator makes it
unlikely that the rotor guest would enter the TPP channels
rotator first; such orientation was not observed for the
previously studied rotor molecule.13 By analogy, we did not
expect the double rotor 3 to form any inclusion compound with
TPP. Below, we discuss the data obtained for 2@TPP and 3@
TPP, both of which form bulk inclusions, illustrating how little
it takes to thwart the best laid plans of mice and men.
Synthesis. The Ulmann conditions for the coupling of p-
carborane to an iodo arene27 produced good results in all the
reactions necessary for the syntheses of 2 and 3. In contrast, the
more popular Pd-catalyzed coupling of halides with cuprous p-
carborane derivatives was troublesome, most likely because of
slow transmetalation of cuprous p-carborane derivatives with
arylpalladium halides. The latter can apparently also undergo a
competing reduction to p-(n-pentyl)biphenyl by the solvent or
trace impurities in the solvent. This hypothesis is supported by
the fact that when reducing solvents such as THF are avoided,
the reduction is partially suppressed. The addition of ligands
that are able to activate the copper in p-carborane for
transmetalation, such as triethylamine, also suppresses the
formation of p-n-pentylbiphenyl, but the classic Ulmann
conditions remain superior.
the differences in chemical shifts in the spectra of neat biphenyl
and biphenyl@TPP (Figure S1, Supporting Information), and
sharpening of the lines in the inclusion compound biphenyl@
TPP, demonstrated that biphenyl enters as well. Biphenyl
carbons of the inclusion compound exhibit both upfield and
downfield changes in the chemical shifts relative to the 13C
NMR of neat solid biphenyl.
The detailed assignment of peaks in the solution 13C NMR
spectrum of 2 is unambiguous. In the spectrum of 3, we have
only distinguished the protonated from the unprotonated
carbons of the benzene rings, and the carborane carbons. The
solid-state 13C NMR spectra of 2@TPP and 3@TPP show the
same signal pattern as the solution spectra of the free guests.
The differences in their chemical shifts relative to the solution
spectra of 2 and 3 are much more consistent than the
differences relative to the spectra of the neat solids 2 and 3. The
resolved aromatic signals in the 13C solid-state spectra of the
inclusion compounds were therefore assigned by comparison
with the solution spectra. The differences in the chemical shifts
are assumed to be due to the local magnetic anisotropy of the
TPP environment, i.e., by its shielding and the deshielding
effects. The assignments were verified by solid-state short-
contact time and dipolar dephasing experiments.
Inclusion Compounds of Rotor 2 (Figures 3−5). The
aromatic carbon assignments in the solid-state NMR spectrum
of neat 2 cannot be reliably made by comparison with the
solution spectrum, because the chemical shifts of the various
aromatic carbons in the rotor 2 do not differ much. The
solution spectra are averaged over all accessible conformations,
while there is little conformational freedom in the solid. The
effect of conformation on the chemical shift can be very strong
and larger than the signal separation in the aromatic region of
the 13C NMR spectra of 2.
5%2@TPP. Most signals in the solid-state 13C CP MAS
NMR spectrum of 5%2@TPP overlap with the middle TPP
signal, and for interpretation it was necessary to reduce the TPP
signal. This was achieved by using TPP-d12 instead of TPP.
This also permits a quick detection of inclusion compound
formation, since a mechanical mixture of a protonated
compound with TPP-d12 containing no guests inside the
channels will not give rise to any 13C or 31P NMR signal in the
CP MAS experiment unless the contact times are very long. If
the guest molecule is located within the channels, the 13C or
31P atoms of TPP-d12 become cross-polarized by the guest
protons, demonstrating that an inclusion compound formed,
and providing information on the local crystallographic
structure of the host. By this criterion, 2 clearly makes an
inclusion compound with TPP.
Inclusion Compounds. The DSC traces in Figures S4 and
S5 (Supporting Information) are compatible with the formation
of inclusion compounds of TPP with both 2 and 3. The nearly
13
identical DSC traces of TPP17,32 and TPP-d12 are well
understood. The single endotherm of neat 2 corresponds to
melting. The two very broad exotherms in the DSC of 15%2@
TPP-d12 are attributed to the first order transition of hexagonal
TPP-d12 into its monoclinic form (100 °C) and further
insertion of the guest rotors into TPP-d12 to form a hexagonal
inclusion compound (210 °C). The endotherms are assigned to
the melting of TPP-d12 (251 °C) and of the inclusion
compound (282 °C). The origin of the small endotherm at
298 °C is unknown.
The first endotherm in the DSC trace of 15%3@TPP-d12
corresponds to the first order transition of monoclinic to
hexagonal TPP-d12 and represents evidence for expulsion of the
guest compound 3. The two phase transitions at high
temperatures are associated with the melting of TPP-d12 and
of 3 at temperatures lower than the melting points of the pure
components, due to the mixture effect.
The annealing process greatly improves the homogeneity of
the sample. The signals of TPP in the annealed sample split
into several. Some belong to filled hexagonal TPP and the rest
to empty TPP that collapsed into the monoclinic form during
the annealing process.
15%2@TPP-d12. The solid-state 31P SPE MAS NMR
(Figure 4) shows a singlet peak for the phosphorus atom of
TPP-d12, demonstrating that hexagonal TPP-d12 is present. The
small signals around 0 ppm, a value of chemical shift typical for
phosphates,33 were assigned to impurities in TPP-d12 formed by
oxidation during the annealing process.
The upfield changes in the 13C chemical shifts of the shaft,
the stopper and the rotator in the 15%2@TPP-d12 solid-state
13C spectrum relative to the solution spectrum of 2 indicate
that all the rotor parts are fully inserted inside the channel.
NMR Spectra. The benzene ring was already known to
enter TPP channels with ease,16 and it was not a surprise when
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Some incompletely annealed samples were heterogeneous and
contained more than one NMR peak for each carborane
carbon, presumably due to the presence of a fraction of
incompletely inserted guest molecules. The upfield changes in
the chemical shifts of the aliphatic carbons in 15%2@TPP-d12
are smaller than those of remaining carbons. This is attributed
to the presence of other chain conformations than anti in the
solution and the averaging of their chemical shift into the
observed value.34 The aliphatic chains inside TPP are largely
constrained to the anti conformation.35,36
The X-ray powder diffraction data indicate that the local
structural information from solid-state NMR is also reflected in
the longer range coherent structure. In the case of 15%2@TPP-
d12, the sample displays a single phase of hexagonal TPP
structure in both the NMR and in the X-ray powder patterns.
The expanded in-plane lattice parameter and slightly contracted
layer spacing indicate how much the TPP channels have
changed to allow for the insertion of a guest molecule. The
nominal internal van der Waals diameter of TPP channels now
is ∼5.15 instead of ∼4.95 Å, somewhat closer to the ∼6.7 Å
value for the benzene ring, but still much smaller than the 7.8 Å
values for p-carborane and 1,2-dichlorobenzene, which are both
included nevertheless. This is apparently possible because of
the “alternating triangle” nature of channels in TPP, which
permits an inclusion of molecules with a larger than nominal
diameter, as long as they fit into a single layer.
guest protons and the phosphorus atoms of the TPP-d12 lattice.
In absence of a guest with protons, TPP-d12 shows no 31P
signal. Therefore the single phosphorus signal in 31P CP MAS
NMR is an indication of inclusion compound of a hexagonal
structure filled with guest molecules. The 31P SPE MAS solid-
state NMR reveals the structure of the entire sample, and the
single peak in the spectrum of the inclusion compound shows
hexagonal lattice throughout. The upfield shift of p-carborane
carbons and all the aromatic carbons signals of 3 in 15%3@
TPP-d12 means that the whole rotor 3 is inserted inside the
TPP-d12 channels. While the NMR and X-ray both show a
hexagonal inclusion compound, the X-ray clearly shows the
presence of both empty hexagonal TPP and monoclinic TPP.
The absence of monoclinic TPP signals in the 31P SPE MAS
solid-state NMR spectrum demonstrates that at the time when
the inclusion 15%3@TPP-d12 was prepared, monoclinic TPP-
d
12 was absent. However, over time before the X-ray diffraction
was measured, some of the free hexagonal TPP-d12 collapsed
into the more stable monoclinic form. This was subsequently
confirmed by a second 31P SPE MAS solid-state NMR
experiment.
Independent structural information was obtained from
results of X-ray diffraction, which reflect long-range order and
are entirely compatible with the information gleaned from
solid-state NMR spectra.
The multiple peaks observed in dielectric spectroscopy for
15%3@TPP-d12 are consistent with a situation where some of
the rotators are near the surface, some are internal to the TPP,
and some may interact with neighboring rotators from other
bulk-included rotor molecules. The lowest rotational barrier in
the inclusion compound 15%3@TPP-d12 is thought to be due
to rotators at the channel end, but fully included in the channel,
probably in the first TPP layer. The two higher energy barriers
would then be due to rotator rotations in the bulk that have
different immediate environments. For instance, one of the
barriers could correspond to the rotation of a dipolar rotator
located next to a void, and another to a rotator located next to
the dipolar rotator end of its neighbor.
Using procedures described earlier,3,13,14 dielectric spectros-
copy yielded rotational barriers Eb, attempt frequencies A, and
asymmetry parameters s. The results demonstrate that dipolar
rotation in 15%2@TPP-d12 takes place with a single significant
rotational barrier. This would be expected if all the rotors are in
the same near-surface location, but it could be accounted for in
other ways, too. Perhaps the TPP platelets are thin enough that
all guest molecules are necessarily near the surface. The 15 mol
% loading was selected as it is the rough molar percentage that
one expects for a TPP platelet that is entirely populated on top
and bottom by 2, such that the rotator end is just barely
immersed in the first TPP layer at the surface, and the ends of
the alkane tails touch along the platelet midplane. This
geometry offers the maximum surface to volume ratio for
TPP crystallites, but additional investigations would be
necessary to confirm that it is indeed present.
The frequency factors again have the usual values and the
asymmetry factors are small compared to the barrier heights,
indicating largely symmetrical rotational potentials.
Surface versus Bulk Inclusion. It is instructive to compare
the inclusion compound formation between TPP and the first
rotor guest studied, n-C16H33−C2B10H10−C6H3Cl2,13 and of
the rotor 2 that has been examined presently, n-C5H11−C6H4−
C6H4−C2B10H10−C6H3Cl2, which differs only in the choice of
shaft. In both cases, the shaft enters the TPP channel readily, as
desired. The inclusion of the rotor 2 is most likely of the bulk
type (Figure 2C), although surface inclusion with the rotator
inserted in the top TPP layer cannot be ruled out. The
observation that the same p-carborane stopper prevents the
entry of the rest of the rotor in the former case, forcing the
formation of a surface inclusion compound, while in the latter
case, it permits the rotator to insert into the channel, is quite
remarkable. It suggests the existence of a fine balance between
the energy needed to stretch the diameter of the TPP channels,
the lattice energy of the guest, and the energy gained from the
guest−host interaction. Considerable additional experimenta-
tion will be required before the balance is understood in detail
and rotors specifically designed for surface inclusion or bulk
inclusion can be designed confidently. Apparently, objects
whose diameter exceeds the nominal channel width may be
The considerable barrier height of 8.9 kcal/mol is consistent
with the finding that the dipolar rotator is included within the
host, whose channel is too narrow to accommodate it
comfortably. The frequency factor is unexceptional and
characteristic of molecular vibrational frequencies. The
asymmetry factor of 0.5 kcal/mol is small compared to the
barrier height and indicates that the rotator is moving in a
largely symmetrical rotational potential as expected.
Inclusion Compounds of Rotor 3 (Figure 5). The solid-
state 13C NMR spectrum of 15%3@TPP-d12 is different from
that of neat 3. It shows three intense aromatic resonances that
belong to TPP-d12 and signals A, B, C and D that belong to the
guest compound 3. The three TPP-d12 signals provide strong
evidence for the formation of an inclusion compound. The
splitting of the TPP-d12 signal located the farthest upfield into
two peaks is attributed to TPP-d12 molecules of the filled
channel and TPP-d12 molecules of unfilled or only partially
filled channels. The latter are still in the proximity of the guest
molecules 3, since their carbons are cross-polarized.
The single resonance in the solid-state 31P CP MAS NMR of
15%3@TPP-d12 is a result of cross-polarization between the
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4-Nitro-4′-n-pentylbiphenyl (4). A dried high-pressure Schlenk
flask (500 mL) with triply sealed Teflon valve was equipped with a
magnetic stir bar and a septum. It was charged with p-
pentylbromobenzene (6.00 g, 26.42 mmol, 1.2 equiv), evacuated,
and filled with argon (three times). Anhydrous THF (80 mL) was
added, and the solution was cooled in an acetone/dry ice bath. After 5
min, 1.6 M solution of n-BuLi in hexanes (16.6 mL, 26.42 mmol, 1.2
equiv) was added to the stirred solution. After 10 min, 1.0 M solution
of zinc chloride (52.8 mL, 26.42 mmol, 1.2 equiv) was added, and the
resulting mixture was allowed to warm to room temperature. A
suspension of p-iodonitrobenzene (5.48 g, 22.01 mmol, 1.0 equiv) and
tetrakis(triphenylphosphino)palladium (763 mg, 0.66 mmol, 3 mol %)
in anhydrous THF (20 mL) was added to the mixture. The valve was
then closed, and the reaction mixture was stirred at room temperature
for 48 h. The solvent was removed under reduced pressure, and the
residue was dissolved in dichloromethane (300 mL). The solution was
washed with 10% hydrochloric acid (2 × 100 mL) and water (2 × 100
mL). The organic layer was dried over anhydrous sodium sulfate,
filtered, and the solvents were removed under reduced pressure. The
crude product was purified by flash chromatography in hexanes and
toluene (3/1) to yield 4.969 g (84%) of 4 as yellow crystals: mp
(CH2Cl2) 46 °C; 1H NMR (400 MHz, CDCl3) δ 8.31−8.25 (m, 2H),
7.76−7.69 (m, 2H), 7.59−7.52 (m, 2H), 7.32 (d, J = 8.3 Hz, 2H),
2.76−2.60 (m, 2H), 1.67 (ddd, J = 15.1, 8.3, 4.8 Hz, 2H), 1.43−1.30
(m, 4H), 0.97−0.87 (m, 3H); 13C{1H} NMR (101 MHz, CDCl3) δ
147.7 (s), 146.9 (s), 144.2 (s), 136.1 (s), 129.3 (s), 127.6 (s), 127.3
(s), 124.2 (s), 35.7 (s), 31.6 (s), 31.2 (s), 22.7 (s), 14.2 (s); IR (KBr,
cm−1) 398, 450, 488, 509, 529, 543, 693, 731, 746, 755, 812, 847, 854,
866, 895, 950, 962, 969, 1004, 1023, 1108, 1127, 1176, 1186, 1234,
1250, 1288, 1308, 1339, 1398, 1429, 1454, 1466, 1483, 1513, 1562,
1808, 1917, 1932, 2343, 2858, 2869, 2926, 2954, 2966, 3028, 3077;
HRMS (ESI-TOF) 270.1488 (calcd. for MH+: 270.1491); UV−vis
(CH2Cl2, nm) λmax (εmax) 324 (23700). Elemental analysis calcd. for
C17H19NO2 C: 75.81%, H: 7.11%, N: 5.20%. Found C: 75.71%, H:
7.23%, N: 5.25%.
accommodated quite readily as long as they fit into a single
layer.
Initial tentative conclusions are (i) the moieties adjacent to
the stopper should not make it easier to stretch the channel
diameter, i.e., should not be bulky, and (ii) when the direction
of the p-carborane symmetry axis is strictly along the channel,
the attached rotator does not contribute to the stopping action,
whereas when the p-carborane axis is tilted, the direction of
attachment of the rotator is skewed, permitting it to cooperate
as a part of the stopper. The formation of a surface inclusion of
type A or B in Figure 2, with a rotator axle normal to the
surface, requires the use of a stopper larger than the parent p-
carborane.
SUMMARY
■
Inclusion compounds of the rotors 2 and 3 with TPP were
prepared and characterized by solid-state NMR, differential
scanning calorimetry, X-ray powder diffraction, and dielectric
spectroscopy. Annealing was found to be important for their
detailed structure. It was critical to use TPP-d12 in place of TPP
for detailed NMR spectroscopic analysis.
Against expectations, in all inclusion compounds of rotors 2
and 3 were inserted completely into TPP channels, and the p-
carborane moiety did not act as a stopper.
Longer range structural information from X-ray powder
patterns, along with the observed dielectric loss peaks, suggest
that in 2@TPP all rotators, while included into the TPP
channel, are near the crystal surface, whereas 3@TPP is more
likely a true fully bulk inclusion compound, with an associated
range of local structural variations around the rotator. The
rotational barriers observed for both inclusion systems are high
compared with expectations for the isolated rotor molecule,
indicating the presence of substantial steric hindrance to
rotation of the dichlorophenyl rotator included in a TPP
channel.
4-Amino-4′-n-pentylbiphenyl (5). A round-bottom flask (500 mL)
was charged with 4 (4.80 g, 17.820 mmol, 1.0 equiv), powdered tin
(15.00 g, 126.24 mmol, 7 equiv), concentrated hydrochloric acid (80
mL), water (80 mL) and ethanol (50 mL). The resulting mixture was
refluxed for 6 h. The solvents were removed under reduced pressure,
the remaining solid was suspended in water (100 mL), and 10%
sodium carbonate was added to pH = 9. The solids were filtered,
washed with water, and dried. The product was extracted with boiling
ethanol, and the suspension was filtered hot. The solvent was removed
under reduced pressure to give 4.024 g (94%) of white crystalline 5:
EXPERIMENTAL SECTION
■
NMR Spectroscopy, Differential Scanning Calorimetry, X-ray
Diffraction, and Dielectric Spectroscopy. Measurements were
made and evaluated as described elsewhere.13
1
mp (CH2Cl2) 69 °C; H NMR (400 MHz, CDCl3) δ 7.47−7.43 (m,
Sample Preparation. The synthesis and storage of TPP, catechol-
d6, TPP-d12, and benzene-d6@TPP-d12 followed published proce-
dures.13 The amounts used in the preparation of inclusion compounds
were as follows. Biphenyl@TPP: biphenyl, 3.1 mg; TPP, 180.0 mg
(ground). 5%2@TPP: 2, 10.6 mg; TPP, 180.0 mg (ground). 15%2@
TPP-d12: 2, 26.8 mg; TPP-d12, 140.0 mg (ball milling for 90 min was
followed by annealing for 1 day, ball milling for 120 min, and
annealing for 3 days). 15%3@TPP-d12: 3, 24.3 mg; TPP-d12, 150.0 mg
(ball milling for 90 min was followed by annealing for 3 days).
Synthesis. General Methods. Air-sensitive reactions were carried
out under argon, using standard Schlenk techniques. Tetrahydrofuran
was distilled from Na/K under argon and dimethylformamide from
calcium hydride into a flask with freshly activated 3 Å molecular sieves
under reduced pressure. Degassing was performed by the standard
freeze−pump−thaw technique (three times). Pyridine, piperidine,
diisopropylamine, acetonitrile, and dichloromethane were distilled
from calcium hydride under argon. Triethylamine was distilled from
calcium hydride or from sodium/benzophenone under argon. Benzene
and toluene were distilled from sodium under argon. NMR solvents
were kept over molecular sieves (except for D2O). Molecular sieves (3
and 4 Å) were freshly activated at 500 °C before use, and they were
allowed to cool in a vacuum. Cuprous chloride was purchased in sealed
ampules and handled in a glovebox. Only pure white cuprous chloride
was used in reactions. All chemicals were used as received.
2H), 7.43−7.38 (m, 2H), 7.23−7.18 (m, 2H), 6.78−6.72 (m, 2H),
3.70 (s, 2H), 2.68−2.55 (m, 2H), 1.70−1.58 (m, 2H), 1.40−1.28 (m,
4H), 0.96−0.84 (m, 3H); 13C{1H} NMR (101 MHz, CDCl3) δ 145.7
(s), 141.2 (s), 138.6 (s), 131.8 (s), 128.9 (s), 128.0 (s), 126.4 (s),
115.5 (s), 35.7 (s), 31.7 (s), 31.4 (s), 22.7 (s), 14.2 (s); IR (KBr,
cm−1) 459, 479, 504, 547, 569, 673, 714, 727, 751, 804, 832, 843, 999,
1134, 1177, 1200, 1230, 1273, 1374, 1403, 1429, 1453, 1466, 1500,
1529, 1579, 1606, 1624, 1886, 1900, 2855, 2868, 2922, 2950, 3026,
3198, 3214, 3304, 3384; UV−vis (CH2Cl2, nm) λmax (εmax) 280
(41590). MS (ESI-TOF) 240.2 (calcd. for MH+: 240.2). Elemental
analysis calcd. for C17H21N C: 85.30%, H: 8.84%, N: 5.85%. Found C:
85.28%, H: 9.02%, N: 5.89%.
4-Iodo-4′-n-pentylbiphenyl (6). A round-bottom flask (500 mL)
was charged with 5 (3.80 g, 1.0 equiv), water (160 mL) and
concentrated sulfuric acid (30 mL), and the suspension was sonicated
for 1 h. The resulting fine suspension was cooled to 0 °C, and a
solution of sodium nitrite (2.19 g, 31.748 mmol, 2.0 equiv) in water
(30 mL) was added dropwise with vigorous stirring. The temperature
of the reaction mixture was kept below 5 °C during the addition. After
2 h at 0−5 °C a solution of urea (5 g) in water was added dropwise to
the reaction mixture while the temperature was kept below 5 °C.
Then, the reaction mixture was slowly poured into a hot (80−90 °C)
solution of potassium iodide (15 g) in water (100 mL), and the
mixture was stirred for 1 h. The product was extracted into diethyl
H
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ether (3 × 200 mL), the combined organic layers were washed with
10% sodium thiosulfate (2 × 100 mL), the organic layer was dried over
anhydrous sodium sulfate, filtered, and the solvents were removed
under reduced pressure. The crude product was purified by flash
chromatography in hexanes and toluene (3/1) to yield 4.652 g (84%)
1H NMR (500 MHz, CDCl3) δ 7.60 (dd, J = 8.4, 1.4 Hz, 1H), 7.48−
7.37 (m, 5H), 7.32−7.28 (m, 2H), 7.25 (t, J = 6.3 Hz, 2H), 7.09−7.04
(m, 1H), 2.67−2.60 (m, 2H), 3.59−2.05 (m, 10H), 1.71−1.59 (m,
2H), 1.42−1.29 (m, 4H), 0.91 (t, J = 7.0 Hz, 3H); 13C{1H} NMR
(101 MHz, CDCl3) δ 142.7 (s), 141.6 (s), 137.3 (s), 135.9 (s), 135.0
(s), 134.2 (s), 131.5 (s), 131.0 (s), 130.7 (s), 129.0 (s), 127.5 (s),
127.0 (s), 126.7 (s), 126.7 (s), 86.6 (s), 81.7 (s), 35.7 (s), 31.7 (s),
31.3 (s), 22.7 (s), 14.2 (s); 11B{1H} NMR (96 MHz, CDCl3) δ
−12.25 (s); IR (KBr, cm−1) 462, 488, 514, 548, 593, 607, 660, 698,
738, 758, 790, 805, 848, 861, 871, 901, 933, 1004, 1059, 1069, 1082,
1123, 1164, 1194, 1301, 1347,1375, 1398, 1421, 1450, 1464, 1493,
1501, 1561, 1578, 1866, 1904, 1931, 1956, 2613, 2852, 2867, 2927,
2951, 3030, 3051, 3088; HRMS (ESI-TOF) 535.2751 (calcd. for
MNa+: 535.2758); UV−vis (CH2Cl2, nm) λmax (εmax) 269 (33950).
Elemental analysis calcd. for C25H32B10Cl2: 58.70%, H: 6.31%. Found
C: 58.46%, H: 6.41%.
1
of 6 as white crystals: mp (CH2Cl2) 110 °C; H NMR (400 MHz,
CDCl3) δ 7.82−7.74 (m, 2H), 7.54−7.47 (m, 2H), 7.39−7.33 (m,
2H), 7.33−7.27 (m, 2H), 2.78−2.64 (m, 2H), 1.77−1.64 (m, 2H),
1.47−1.33 (m, 4H), 1.03−0.91 (m, 3H); 13C{1H} NMR (101 MHz,
CDCl3) δ 142.7 (s), 140.7 (s), 137.9 (s), 137.4 (s), 129.1 (s), 128.6
(s), 126.8 (s), 92.8 (s), 35.7 (s), 31.7 (s), 31.3 (s), 22.7 (s), 14.2 (s);
IR (KBr, cm−1) 478, 500, 550, 616, 642, 673, 698, 725, 737, 754, 802,
825, 852, 999, 1066, 1110, 1133, 1276, 1376, 1386, 1456, 1466, 1479,
1519, 1550, 1581, 1607, 1644, 1906, 2852, 2866, 2922, 2949, 3026,
3059; UV−vis (CH2Cl2, nm) λmax (εmax) 266 (24210); HRMS (EI-
TOF) 350.0529 (calcd. for C17H19I+: 350.0532).
General Procedure for Ulmann Coupling with p-Carboranes. A
pressure Schlenk flask (25 to 50 mL), equipped with a triply sealed
pressure valve, a magnetic stir bar, a septum and a hose connector, was
charged with the p-carborane derivative (1 mmol, 1.0 equiv or 5 mmol,
5.0 equiv if p-carborane was the starting compound) and with cuprous
chloride (1.3 mmol, 1.3 equiv) in a glovebox. The flask was closed,
taken outside the glovebox and connected to the Schlenk line. THF
(10 mL) was added through the septum under argon, and the resulting
mixture was cooled in an acetone/dry ice bath. After 5 min of stirring,
n-BuLi (1.6 M solution in hexanes, 0.75 mL, 1.2 equiv) was added
dropwise under argon. The reaction mixture was stirred for 15 min at
−78 °C and then for 30 min at room temperature. Anhydrous pyridine
(1 mL) was added to the mixture, and it was stirred until all the
cuprous chloride dissolved. The flask was then closed, and a solvent
trap was connected to it. The trap was evacuated and filled with argon
three times. The trap was then evacuated and cooled with liquid
nitrogen. Special precaution was taken to seal all leaks. The reaction
mixture was stirred vigorously and slowly opened to the trap. The
solvents were removed under reduced pressure, and the flask was filled
with argon. After that, a suspension/solution of the iodo derivative
(1.5 mmol, 1.5 equiv) in anhydrous DMF (5 to 10 mL) was added
under argon. The resulting mixture was heated in a closed vessel at 120
°C for 24 h and then at 140−150 °C for 24−48 h. The reaction
progress was monitored by 1H NMR. After the reaction was complete,
the solvents were removed under reduced pressure, and the solids
were dissolved in dichloromethane (50−150 mL), washed with 5%
hydrochloric acid (50 mL), with 10% ammonium hydroxide (2 × 50
mL), and with water (50 mL). The organic layer was dried over
anhydrous sodium sulfate, filtered and concentrated under reduced
pressure. If p-carborane was used in the reaction, it was recovered by
sublimation under reduced pressure. The crude product was purified
by column chromatography.
1,12-Bis(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane (3).
The general Ulmann coupling procedure was followed, starting with
150 mg (1.0 equiv) of p-carborane, 1.95 mL (3.0 equiv) of 1.6 M n-
BuLi, and with 329.0 mg (4.0 equiv) of 1,2-dichloro-3-iodobenzene.
The product was purified by column chromatography in pentanes and
toluene (2/1), and the product was further purified by crystallization
from cyclohexane to yield 345.0 mg (76%) of white crystalline
1
product: mp (CH2Cl2) 265 °C; H NMR (500 MHz, CDCl3) δ 7.56
(dd, J = 8.4, 1.5 Hz, 2H), 7.41 (dd, J = 7.9, 1.5 Hz, 2H), 7.07 (t, J = 8.1
Hz, 2H), 2.10−3.70 (m, 10H); 13C{1H} NMR (75 MHz, CDCl3)
δ−136.0 (s), 134.0 (s), 131.4 (s), 131.2 (s), 130.5 (s), 126.7 (s), 77.4
(s); 11B{1H}NMR (96 MHz, CDCl3) δ −11.89 (s); IR (KBr, cm−1)
484, 685, 699, 738, 747, 762, 796, 814, 867, 947, 964, 1000, 1040,
1057, 1067, 1080, 1124, 1137, 1166, 1207, 1227, 1245, 1304, 1313,
1348, 1378, 1398, 1448, 1557, 1578, 1604, 1643, 1682, 1700, 1715,
1734, 1874, 1943, 2595, 2607, 2636, 2643, 3060, 3100; HRMS (ESI-
TOF) 469.0680 (calcd. for MCl−: 469.0695); UV−vis (CH2Cl2, nm)
λmax (εmax) 241 (15630), 230 (26490). Elemental analysis calcd. for
C14H16B10Cl4 C: 38.73%, H: 3.71%. Found C: 38.88%, H: 3.68%.
ASSOCIATED CONTENT
* Supporting Information
■
S
Results for biphenyl@TPP, Figures S1−S5, NMR assignments
for 2, and NMR spectra for compounds 2−7, Figures S6−12.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: michl@eefus.colorado.edu.
■
Notes
1-(4′-n-Pentylbiphenyl-4-yl)dicarba-closo-dodecaborane (7). The
general Ulmann coupling procedure was followed, starting with 800
mg (2.284 mmol) of 6. The product was purified by column
chromatography in hexanes and toluene (3/1) to yield 773.1 mg
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
1
■
(92%) of 7 as white crystals: mp (CH2Cl2) 79 °C; H NMR (300
The research leading to these results has received funding from
the European Research Council (FP7/2007-2013/ERC Grant
227756) and from the U.S. National Science Foundation (CHE
0848663 and Materials Research Science and Engineering
Centers Grant No. DMR-0820579). We thank Professors Piero
Sozzani and Angiolina Comotti (Milan, Italy) for useful
discussions.
MHz, CDCl3) δ 7.48−7.34 (m, 4H), 7.31−7.21 (m, 4H), 2.70−2.56
(m, 2H), 3.67−1.86 (m, 10H), 1.74−1.59 (m, 2H), 1.42−1.27 (m,
4H), 1.00−0.86 (m, 3H); 13C{1H} NMR (101 MHz, CDCl3) δ 142.7
(s), 141.3 (s), 137.4 (s), 135.6 (s), 129.0 (s), 127.5 (s), 127.0 (s),
126.6 (s), 86.4 (s), 59.9 (s), 35.7 (s), 31.7 (s), 31.3 (s), 22.7 (s), 14.2
(s); 11B{1H} NMR (96 MHz, CDCl3) δ −12.20 (s), −14.89 (s); IR
(KBr, cm−1) 505, 548, 582, 693, 721, 726, 737, 749, 768, 786, 801,
846, 896, 912, 939, 1004, 1016, 1022, 1040, 1090, 1124, 1140, 1147,
1193, 1374, 1397, 1429, 1451, 1464, 1501, 1529, 1577, 1906, 2609,
2856, 2866, 2926, 2951, 3027, 3060; UV−vis (CH2Cl2, nm) λmax
(εmax) 264 (27030). Elemental analysis calcd for C19H30B10 C: 62.26%,
H: 8.25%. Found C: 62.02%, H: 8.36%.
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