Synthesis of a strained acetylenic macrocycle incorporating a para-oligo[2]cruciform bridge bent over nanoscopic dimensions: Structural, electronic, spectroscopic, and ion-sensing properties

Article abstract of DOI:10.1002/chem.201204569

An Eglinton-Galbraith diethyne cyclization preferentially yielded a structurally unusual macrocycle, comprising a strained conjugated oligo[2]cruciform wire, forced into a 2.2 nm bow-shape by a terpyridine rein or tether, and stabilized towards light and heat by four insulating triisopropylsilylacetylene (TIPSA) substituents. Spectroscopic ion-binding studies revealed the macrocycle to exhibit a particularly high UV/Vis selectivity for Pd^II in dilute solution, and one of its precursors to afford a variety of luminescence quenching and color responses to particular metals, suggestive of promising ion-sensor applications. Under more concentrated conditions, the new macrocycle is able to bind specific metals (e.g., Au ^I) within its cavity despite the steric constraints. Intriguingly, variable-temperature (VT) UV/Vis/¹H NMR investigations showed the TIPSA substituents to undergo restricted intramolecular motions along with reversible changes in the spectroscopic bandgap of the compound with temperature. In line with the theoretical calculations, the VT UV/Vis observations are consistent with a thermal modulation of the electronic conjugation through the strained oligo[2]cruciform bridge, which is coupled with redistributions within a mixture of conformational isomers of the macrocycle with differing relative twisting between the TIPSA-substituted phenyl rings. Overall, the generation of a para-oligo[2]cruciform, bent and flexed over nanoscopic dimensions through conformational tethering within the macrocyclic ring is noteworthy, and suggests a general approach to nanosized, curved, and strained, yet heat- and light-stable, para-phenyleneethynylene oligomers with unique physicochemical properties and challenging theoretical possibilities. Reining in the strain: An Eglinton-Galbraith diethyne cyclization yielded an unusual bow-shaped cycle (see scheme), incorporating a strained yet stable conjugated oligo[2]cruciform wire, bent over nanoscopic dimensions by a terpyridine tether. As well as spectroscopically sensing metal ions, it undergoes thermo-controllable bandgap changes in solution, consistent with hindered relative twisting of the substituted phenyl rings, thus functioning as an environmentally adaptive conjugation switch. Copyright

Full text of DOI:10.1002/chem.201204569

DOI: 10.1002/chem.201204569
Synthesis of a Strained Acetylenic Macrocycle Incorporating a para-
Oligo[2]cruciform Bridge Bent over Nanoscopic Dimensions: Structural,
Electronic, Spectroscopic, and Ion-Sensing Properties
Paul N. W. Baxter,*[a] Jean-Paul Gisselbrecht,[b] Lydia Karmazin-Brelot,[b]
Alexandre Varnek,[b] and Lionel Allouche[b]
Abstract: An Eglinton–Galbraith di-
ethyne cyclization preferentially yield-
ed a structurally unusual macrocycle,
comprising a strained conjugated oli-
the new macrocycle is able to bind spe-
cific metals (e.g., AuI) within its cavity
despite the steric constraints. Intrigu-
ingly, variable-temperature (VT) UV/
Vis/1H NMR investigations showed the
TIPSA substituents to undergo restrict-
ed intramolecular motions along with
reversible changes in the spectroscopic
bandgap of the compound with temper-
ature. In line with the theoretical calcu-
lations, the VT UV/Vis observations
are consistent with a thermal modula-
tion of the electronic conjugation
through the strained oligo[2]cruciform
bridge, which is coupled with redistrib-
utions within a mixture of conforma-
tional isomers of the macrocycle with
differing relative twisting between the
TIPSA-substituted phenyl rings. Over-
all, the generation of a para-oligo-
go[2]cruciform wire, forced into
a
2.2 nm bow-shape by a terpyridine rein
or tether, and stabilized towards light
and heat by four insulating triisopro-
pylsilylacetylene (TIPSA) substituents.
Spectroscopic ion-binding studies re-
vealed the macrocycle to exhibit a par-
ticularly high UV/Vis selectivity for
PdII in dilute solution, and one of its
precursors to afford a variety of lumi-
nescence quenching and color respons-
es to particular metals, suggestive of
AHCTUNGTRENGN[UN 2]cruciform, bent and flexed over
nanoscopic dimensions through confor-
mational tethering within the macrocy-
clic ring is noteworthy, and suggests a
general approach to nanosized, curved,
and strained, yet heat- and light-stable,
para-phenyleneethynylene
oligomers
with unique physicochemical properties
and challenging theoretical possibili-
ties.
Keywords: conjugation · macrocy-
clic ligands · N ligands · sensors ·
strained molecules
promising
ion-sensor
applications.
Under more concentrated conditions,
Introduction
Strained acetylenic macrocycles (SAMs) are synthetically
challenging materials that have been the subject of consider-
able interest over the past four decades due to their intrigu-
ing structural morphologies, their unusual chemical reactivi-
ty, and their high theoretical interest.[1–6] They currently en-
compass a diverse range of macrocyclic structures that in-
clude cyclophynes,[7–12] cyclic dendralenes and radia-
lenes,[13–15] fullerene cycloalkyne adducts,[5] [n]rotanes,[16] and
particular types of dehydro[n]annulenes and dehydroaryl-
[a] Dr. P. N. W. Baxter
Institut Charles Sadron, UPR 22-CNRS
23 rue du Loess, 67034 Strasbourg (France)
Fax : (+33)388-414-099
E-mail: paul.baxter@ics-cnrs.unistra.fr
[b] Dr. J.-P. Gisselbrecht, Dr. L. Karmazin-Brelot, Prof. A. Varnek,
Dr. L. Allouche
Facultꢀ de chimie, Universitꢀ de Strasbourg
1 rue Blaise Pascal, 67008 Strasbourg (France)
AHCTUNGTRENNUNG
[n]annulenes.[1,15,17,19–23] Enediyne SAMs also occur in
nature, functioning as antibiotics.[24,25] Some examples of the
more structurally elaborate SAMs prepared to date, such as
compounds 2,[1] 3,[12] 4,[11] 5,[20] and 6,[23] are shown in
Scheme 1.
In some cases, SAMs are very unstable due to extreme
ring strain, and/or the presence of contiguous series of the
ethyne groups, which further enhance inter- or intramolecu-
lar reactions.[10,18,26,27] However, the majority of the reported
SAMs are stable and isolable solids at room temperature, an
encouraging feature, which suggests viable materials and
chemical applications for these compounds. Accordingly,
they have been shown to function as new types of donor–ac-
Supporting information for this article (containing general experi-
mental details, that is, materials and methods, UV/Vis and lumines-
cence spectroscopic experimental details, luminescence emission spec-
tra of thin films of 1 and 7, 10, 11, 13, and 14, VT UV/Vis spectra of
13, UV/Vis and luminescence emission spectra of the binding of 13 to
Mn+
ACHTUNGTRENNUNG(CF3SO3)n, luminescence quantum yield estimations, electro-
chemistry, and general X-ray experimental details, crystallographic
data, and ORTEP representations for 1, peak assignment of the
1H NMR spectrum of 1, H and 13C NMR solution spectra of 1, 10,
1
11, 13, and 14, and VT H NMR spectra of 1 and 13, 1H NMR spectra
1
of metal complexation to 1, calculated structure Cartesian coordi-
nates of 1b, 1g, and 1i, TGA curves for 1, 8, 10, 11, 13, and 14, and
references (continued)) is available on the WWW under http://
dx.doi.org/10.1002/chem.201204569.
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Chem. Eur. J. 2013, 19, 12336 – 12349

FULL PAPER
ambient temperature, in part due to the presence of the ster-
ically shielding triisopropylsilylacetylene (TIPSA) substitu-
ents. The structure and formation of 1 is noteworthy, as
copper-catalyzed ethyne couplings are normally applied for
the generation of small SAMs starting from precursors of
higher rigidity, and with terminal ethynes in closer overall
proximity.[1–6] The formation of 1 suggests that the classical
Eglinton–Galbraith cyclization may be applicable for SAMs
incorporating large and normally linear conjugated oligom-
ers and molecular wires curved over nanoscopic or macro-
molecular dimensions. Such materials should be of funda-
mental interest within the domain of molecular electronics
for the study of NLO properties and electronic communica-
tion along curved conjugation pathways distributed over
nanoscopic distances.
We therefore detail below the synthesis and characteriza-
tion of 1 along with its electrochemical, spectroscopic,
metal-ion-binding, and thermal properties, compared to its
acyclic precursors. The physical studies collectively support
the macrocyclic formulation of 1, and a theoretical investiga-
Scheme 1. Examples of SAMs comprising strongly curved ethynylphenyl
units.
ꢀ
ceptor
and
nonlinear
optical
(NLO)
chromo-
tion of the unsubstituted and C CSiH3 substituted ana-
phores[15,17,19,28,29] and structurally novel photochromic[21,22]
and luminescent materials with ion-sensor properties,[30–32] as
well as undergoing chemical rearrangements to planar poly-
aromatic hydrocarbons,[33,34] fullerene/carbon clusters, and
nanotubes,[10,26,27,35] cycloadditions to afford novel congested
and strained phenyl-cyclo-
logues of 1 offered an explanation for the unexpected elec-
tro-reductive instability of 1 and its more selective metal-
ion-binding properties compared to its acyclic precursor 13.
phanes[36,37] and topotactic re-
actions to give carbon poly-
mers.[38] The preparation and
physicochemical properties of
particular strained ortho-dehy-
droaryl[n]annulenes have also
been the focus of attention as
they represent the elementary
monomer units of hypothetical
all-carbon
network
poly-
mers.[39]
As part of a program direct-
ed towards the syntheses of
pyridine-incorporated
acety-
lenic macrocycles for the pur-
pose of metal ion sensing and
the construction of metallosu-
pramolecular polymers and
nanostructures,[30,40] we report
the isolation of the unusual
strained terpyridine macrocy-
cle 1 (Scheme 2). The SAM 1
results from an intramolecular
Eglinton–Galbraith coupling
and incorporates
a para-oli-
go[2]cruciform bridge spanning
2 nm, which is bent into a
bow-shaped conformation by a
terpyridine tether. The macro-
cycle was found to be stable at
Scheme 2. Synthesis of the precursor terpyridines 8, 10, and 11, the terpyridine cruciforms 13 and 14, and the
strained macrocycle 1. i) THF/distilled H2O, [(nBu)4N]F/THF, 20–258C, 6 h–2.5 days (93% 8), (91% 11);
ii) [PdCl
(61% 10); iii) [PdCl
258C, 3 days (84% 14); v) excess Cu CTHNUGTRENNUNG
(dppf)]·CH2Cl2 (dppf=1,1’-bis(diphenylphosphino)ferrocene), CuI cat., toluene/Et3N, 55 8C, 5 days
2A(PPh3)2], CuI cat., toluene/Et3N, 55 8C, 8 days (41% 13); iv) K2CO3, 1:2 Et2O/MeOH, 20–
2A(OAc)4, 3:1 pyridine/toluene, 20–258C, 7 days, (40% 1).
CTHUNGTRENNUNG
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12337

P. N. W. Baxter et al.
Results and Discussion
The structural assignment of the strained macrocyclic ar-
chitecture 1 in Schemes 2 and 3, to the product isolated
Synthesis and characterization of macrocycle 1: Macrocycle
1 was constructed by using the six-step synthesis shown in
Scheme 2, starting from 7.[41] Thus, the Sonogashira reac-
tion[42] between a 1:2 stoichiometric ratio of 8[41] and 9[40] in
from the Cu2ACHTNGUTERNNU(G OAc)4-mediated coupling of 14 was made on
1
the basis of mass spectrometric as well as infrared, H and
13C NMR spectroscopic studies including COSY, NOESY,
ROESY, and HSQC correlation spectroscopy, and in partic-
ular X-ray crystallography (see also the Supporting Informa-
the presence of [PdCl2ACHTNUTGRNEUNG(dppf)]·CH2Cl2 and CuI as catalysts,
1
afforded terpyridine 10 in 61% yield after purification. Sub-
sequent de-silylation of 10 with [(nBu)4N]F in THF/H2O
yielded 11, which was isolated in 91% yield after workup. A
further Sonogashira reaction of 11 and two equivalents of
tion for H NMR spectra, their assignments, and crystallo-
1
graphic numbering). Interestingly, the H NMR spectrum of
the coupling product revealed that the protons of the central
phenyls of the oligo[2]cruciform bridge (H3/3’ and H6/6’),
and the protons H4/4’’ of the outer pyridines, which point
towards the interior of the macrocyclic cavity, exhibited
temperature-dependent changes in the chemical shifts (see
below and the Supporting Information), indicative of hin-
dered rotations of the 2,5-bis-[(triisopropylsilanyl)ethynyl]-
benzene units, and consistent with the strained cyclic struc-
ture of 1.
12[43] mediated by [PdCl
2ACHTNUGTRNEUNG(PPh3)2] and CuI as catalysts af-
forded 13 in 41% yield. Longer reaction times facilitated
higher yields of 13, although the overall moderate yields of
the latter terpyridine were ascribed to extensive byproduct
contamination. Treatment of 13 with a stoichiometric excess
of powdered K2CO3 in 1:2 Et2O/MeOH resulted in clean tri-
methylsilyl (TMS) deprotection to afford 14 in 84% yield
after purification.
CPK models of 14 reveal that the ethyne hydrogen atoms
can move in close enough proximity to touch each other
X-ray-crystal-structural characterization of 1: The product
obtained from the reaction of 14 with Cu2OAc4 crystallized
from pentane to yield crystals of uniformly small size and
extreme acicularity. However, it was possible to obtain an
X-ray crystal structure solution (Figure 1), which confirmed
the macrocyclic architecture proposed for 1 in Scheme 2.
Macrocycle 1 therefore clearly results from the intramo-
lecular oxidative ring closure of 14, and is significantly
(shown most clearly in
a
wireframe representation,
Scheme 3). Therefore, the generation of a reaction inter-
strained as anticipated, with a bow-shaped oligocruciform
[45]
ꢁ
bridge spanning 22 ꢂ (C3 C29 distance, Scheme 3) lying
to one side of the mean plane through the terpyridine unit.
The 5,5’’-bis-(phenylethynyl)-2,2’:6’,2’’-terpyridine moiety is
also bent into a shallow bow, whose direction of curvature
follows that of the oligocruciform bridge. The terpyridine
rests in a trans conformation that is significantly distorted,
due to rotational twisting of the outer pyridines in opposite
directions to each other, and which may originate from crys-
tal packing distortions.
ꢁ
When viewed along the para-H16 N2 axis of the central
pyridine ring the two 2,5-bis-[(triisopropylsilanyl)ethynyl]-
benzene moieties are seen to lie parallel to each other in a
trans-type conformation. However, they are further twisted
in opposite directions with respect to each other by approxi-
mately 608 when viewed along the C4–C28 trajectory of the
oligocruciform bridge mean plane. This twisting causes the
inner pointing TIPS substituent of each of the two 2,5-bis-
[(triisopropylsilanyl)ethynyl]benzene units to completely
cover above and below the cheese slice-shaped macrocyclic
cavity of the size 13ꢃ5.6 ꢂ (distances between van der
Waals surfaces).
In the crystal lattice, the molecules of 1 are packed hori-
zontally in layers that are separated by disordered TIPS sub-
stituents, in the ab plane. Although no p–p contacts are ob-
servable, some stabilization within the layers is achieved
through outer pyridine N1–outer phenyl H30 (2.409 ꢂ), and
alkyne C33–outer pyridine H10 (2.861 ꢂ) hydrogen bond-
ing.
Scheme 3. Top) Conformation of 14 with the terminal ethynes in close
proximity. Bottom) 1H NMR numbering scheme and crystallographic
numbering (in brackets) referred to in text.
mediate in which both ethynes of 14 form bonds with the
same pre-cyclization metal catalyst should be conformation-
ally accessible, suggesting the feasibility of intramolecular
coupling in this system. Thus, subjection of 14 to the Eglin-
ton–Galbraith ethyne-coupling protocol under medium/high
dilution conditions[44] did indeed afford a product, isolated
in a yield of 40% after purification, whose structural identi-
ty was consistent with that of the strained macrocycle 1
(Schemes 2 and 3).
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Synthesis of a Strained Acetylenic Macrocycle
FULL PAPER
the absorption spectra obeyed
the Beer and Lambert law over
the concentration range of 7ꢃ
10ꢁ7–1ꢃ10ꢁ5 molLꢁ1,
demon-
strating that aggregation was
absent in dilute solution.
Accordingly,
1
exhibits
a
unique absorption at l=
402 nm, the most red-shifted
absorption band within the
series investigated, characteris-
tic of a chromophoric unit pos-
sessing extended conjugation,
consistent with the presence of
the oligo[2]cruciform bridge
and therefore a cyclic structure.
The lowest-energy UV/Vis tran-
sitions, the highest-energy lumi-
nescence maxima, and the opti-
cal energy gaps (Eogpt) deter-
mined from the solution and
thin film absorption edges,[46,47]
follow the same trend in order
of decreasing energy, that is,
7>10ffi11>13ffi14>1, consis-
tent with an overall increase in
Figure 1. X-ray structure of 1. Plan view, cylindrical bond representation (upper left) and space-filling repre-
sentation (lower left). Cylindrical bond representation of the side view (upper right) with the curved oligocru-
ciform bridge in the foreground, and edge view (lower right) showing the opposing orientations of the two 2,5-
bis-[(triisopropylsilanyl)ethynyl]benzene units. (ORTEP representations of 1 with complete atom numbering
are included in the Supporting Information).
Spectroscopic properties of the macrocycle 1 and the precur-
sors 7, 10, 11, 13, and 14: With the expectation that 1 may
possess a spectroscopic fingerprint characteristic of its un-
usual strained architecture, we investigated its spectroscopic
properties along with the acyclic precursors 7, 10, 11, 13,
and 14 for comparison, the results of which are summarized
in Table 1 and Figures 2 and 3. In all compounds studied,
conjugation and decrease in the HOMO–LUMO energy dif-
ference (Table 1).
The highest-intensity emission maxima for all compounds
studied are of lower energy in the solid state (Figure 9 in
the Supporting Information) than in solution, which is in ac-
cordance with an expected increased radiationless energy
loss in the solid state and a consequent re-emission of the
Table 1. Luminescence emission and selected UV/Vis spectroscopic data for macrocycle 1, the precursor terpyridines 7, 10, and 11, and the terpyridine
cruciforms 13 and 14.
Compound
labssoln. [nm]
lemsoln. [nm]
lemsolid [nm]
Stokes
shift
Optical
bandgap
Quantum
yield F
[nm][a]
ACHTUNGTRENUN(NG solution)
Eogpt [eV][b]
7
319[c]
335[c]
350
372[c]
387
375 sh
431[c]
434[c]
4
4
6
2
3.44 (3.60)
0.20
0.81
0.85
0.47
331 sh[d]
336[c]
10
11
13
355[c]
373
3.22 (3.43)
3.16 (3.41)
3.12 (3.25)
351[d]
336[c]
357[c]
376
351[d]
273[c]
374[c]
394
395[c]
411[c]
372 sh[d]
406 sh
372[c]
391
404 sh
414[c]
443
14
1
277[c]
394 sh
426[c]
450 sh
430[c]
461
1
3.15 (3.26)
2.86 (2.95)
0.40
371 sh[d]
289[c]
402[d]
12
0.23,
[f]
0.24[e]
490 sh
[a] Highest-energy lemsoln.ꢁlowest-energy labssoln.. [b] Determined from the lowest-energy absorption edge of the thin film (and solution) UV/Vis spec-
trum. [c] Highest-intensity maximum. [d] Lowest-energy absorption in spectrum (sh=shoulder). [e] Quantum yield in argon-degassed n-heptane.
[f] Varies with temperature (see main text). The UV/Vis, luminescence solution and quantum yield measurements of 11 were performed in CH2Cl2 due
to its poor solubility in n-heptane. The solution spectroscopic measurements of all other compounds were performed in air-equilibrated n-heptane at
208C.
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12339

P. N. W. Baxter et al.
luminescence at lower energy. The thin-film lmax were also
sensitive to solvent inclusion, shifting to lower energy upon
air drying (see Supporting Information for thin-film deposi-
tion conditions). However, the lmax of the thin-film emis-
sions of 10, 11, 14, and 1 are strikingly similar suggesting
that the 2,2’:6’,2’’-terpyridine-5,5’’-diylbis-[1-(2,1-ethynediyl)-
2-ethynyl-phenylene] (11) chromophoric unit may be pri-
marily responsible for the emission of these compounds in
the solid state.
Although solutions of 1, 10, 11, 13, and 14 are strongly lu-
minescent under UV illumination, the precursors 10 and 11,
which lack the 1,4-diethynyl-2,5-bis[(triisopropylsilanyl)ethy-
nyl]benzene units, are found to exhibit the highest quantum
yields within the series studied, thereby offering interesting
potential for luminescence metal-ion-sensing applications. In
contrast, the quantum yield of 1 was comparatively moder-
ate and unaffected by the presence or absence of air, dem-
onstrating that the reduction in its luminescence was not
due to oxygen quenching but rather due to the presence of
the oligo[2]cruciform bridge.
Figure 2. UV/Vis spectra of 1, 7, 10, 13, and 14 in n-heptane solution. The
UV/Vis spectrum of terpyridine 11 was recorded in CH2Cl2.
Thermally controlled conjugation modulation of 1, 13, and
14: The UV/Vis spectra of solutions of 1, 13, and 14 in n-
heptane undergo small but significant reversible changes
upon heating and cooling, as shown for 1 in Figure 4 and for
13 in Figure 10 of the Supporting Information. Upon heating
from 10–708C, the strongest absorptions shift 1 nm to higher
energies, and the lowest-energy absorption edges also move
to higher energies, corresponding to an increase of Eogpt in
solution of 0.004 eV for the acyclic precursors. Of particular
note however, is the observation that the weak lowest-
energy absorption at l=402 nm of 1, attributable to the oli-
go[2]cruciform bridge, merges into the l=337 nm absorp-
tion upon heating. This is accompanied by a more clearly
defined retreat of the lowest-energy absorption edge corre-
sponding to an increase of Eogpt for 1 of 0.010 eV in solution.
Figure 3. Normalized luminescence emission spectra of the precursor ter-
pyridines 7 and 10, the terpyridine cruciforms 13 and 14, and macrocycle
1 in n-heptane solution. The luminescence emission spectrum of the pre-
cursor terpyridine 11 was recorded in CH2Cl2.
Figure 4. Reversible temperature-dependent UV/Vis spectral changes of 1 (5.0ꢃ10ꢁ6 molLꢁ1) in n-heptane. The arrows indicate the direction of the
change upon heating from 10–708C in 108C increments. Complete spectrum (left) and magnification of the lowest-energy maximum (right), showing the
shift of the absorption edge to lower energies with cooling. The spectra have been corrected for concentration deviations arising from solvent volume
changes.
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Chem. Eur. J. 2013, 19, 12336 – 12349

Synthesis of a Strained Acetylenic Macrocycle
FULL PAPER
All these spectral changes are completely reversible upon
successive heating/cooling cycles.
Thus for 1, 13, and 14, heating causes an increase in the
solution Eogpt, which in the case of 1 corresponds to a de-
crease in conjugation over the oligo[2]cruciform bridge that
is consistent with an overall shift in the distribution of ex-
changing conformers in favor of those with greater twist
angles between the 2,5-bis[(triisopropylsilanyl)ethynyl]ben-
zene units (see theoretical investigations below). The tem-
perature dependence of the UV/Vis spectra of 13 and 14
suggests a slight energetic preference for all-planar confor-
mations with optimal conjugation at lower temperatures,
whereas the lower magnitude of the Eogpt changes is consis-
tent with reduced conformer conjugation differences expect-
ed for the acyclic precursors.[48] The conclusion that relative
TIPSA-substituted phenyl rotations lie at the heart of the
variable temperature (VT)-UV/Vis phenomena in 1, 13, and
14 is also consistent with the finding that increasing the
inter-ring dihedral angle of sterically constrained bridged
1,2-diphenylethynes causes an increase in the solution
Figure 5. UV/Vis spectra of the binding of 1 to [PdCl2ACTHNUGTRNE(UNG MeCN)2] in 10%
MeCN/CH2Cl2, compared to pure 1 in 10% MeOH/CH2Cl2. The spec-
trum of the solution with a 1/PdII ratio of 1:1 was recorded after ageing
for 48 h, and those of a 1/PdII ratio of 1:10 were obtained after 4 and 24 h
ageing.
Eogpt [49]
.
changes in the presence of FeII, CoII, NiII, CuII, ZnII, CdII,
and PdII, indicative of metal–terpyridine coordination with
these latter transition metals (Figure 11 in the Supporting
Information). When the 13/Mn+ ratio was changed to 1:10,
UV/Vis binding responses to InIII, AlIII, PbII, ScIII and the
lanthanide HfIV were also observed, (Figure 12 in the Sup-
porting Information).
Spectroscopic metal-ion-binding properties of 1 and 13: The
highly sterically congested environment about the terpyri-
dine binding site of 1 would be expected to block the forma-
tion of [M(1)2]n+ complexes, and even enforce ion-size re-
strictions resulting in selectivity for particular solvent-ligated
metal ions. We therefore carried out spectroscopic investiga-
tions of the metal-ion-binding behavior of 1 as well as the
acyclic precursor 13 for comparison, in order to obtain some
qualitative insight into the metal complexation properties of
this structurally unusual ligand.
In contrast, a 1/Mn+ ratio of 1:1 in 10% MeOH/CH2Cl2
afforded a UV/Vis binding response only for PdII (Figure 5).
However, when the quantity of the metal is increased to a 1/
Mn+ ratio of 1:10, the UV/Vis spectrum of 1 does exhibit
clear metal–ligand binding to FeII, CoII, NiII, CuII, ZnII, CdII,
InIII, and HfIV (Figure 6). A more detailed UV/Vis examina-
tion of the binding of 1 and 13 to ZnII revealed that the
onset of binding occurred at L/Mn+ ratios of 1:>3 for 1 and
1:>0 for 13, confirming that 1 is a weaker ligand than 13.
Interestingly, the UV/Vis spectra of the binding of 1 to CoII,
ZnII, CdII, and InIII and 13 to FeII, CoII, ZnII, and CdII show
Initially, the UV/Vis spectra of 1:1 combinations of 13/M-
ACHTUNGTRENNUNG
(CF3SO3)n, where Mn+ =LiI, NaI, KI, MgII, CaII, BaII, MnII,
FeII, CoII, NiII, CuII, AgI, ZnII, CdII, HgII, PbII, TlI, AlIII, InIII,
HfIV, ScIII, YIII, LaIII, EuIII, TbIII, YbIII in 10% MeOH/
CH2Cl2, and PdII (as [PdCl
2ACHTNUTRGNE(NUG MeCN)2]) in 10% MeCN/
CH2Cl2 were scanned for evidence of metal-ion-binding. As
expected, the UV/Vis spectrum of 13 exhibited significant
Figure 6. UV/Vis spectra of the binding of 1 to Mn+
(CF3SO3)n in 10% MeOH/CH2Cl2 (with a 1/Mn+ ratio of 1:10), compared to pure 1 in 10% MeOH/
ACHTUNGTRENNUNG
CH2Cl2. The 1+Mn+ spectra were recorded 4–48 h after solution preparation.
Chem. Eur. J. 2013, 19, 12336 – 12349
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12341

P. N. W. Baxter et al.
clear isosbestics, indicating that spectroscopically similar
species were generated in each case.
Evidence of slow binding kinetics were particularly no-
ticeable for a 1:10 ratio of 1/PdII (Figure 5), and a 1:1 ratio
of 13/NiII. The former requiring at least 24 h and the latter,
7 days to reach equilibrium.
the central pyridine H4’ triplet in the former three cases,
and its complete disappearance in the case of the NiII salt,[52]
indicating intra-cyclic coordination. Similar terpyridine
chemical shifts were found for a 1:1 combination between
[AuCl
which gave no further movements upon changing the
1/[AuCl(tht)] ratio to 1:10, verifying the formation of an
ACHTUNGTRENUN(NG tht)] (tht=tetrahydrothiofuran) and 1 in pure CDCl3,
The luminescence responses of 1 and 13 to metal ion
binding were also briefly investigated. Thus, the lumines-
cence spectrum of acyclic 13 was found to exhibit varying
degrees of luminescence quenching in the presence of FeII,
CoII, NiII, CuII, and PdII with 13/Mn+ ratios of 1:1. Increasing
the relative quantities of metal ion to L/Mn+ ratios of 1:10–
1:100 resulted in luminescence color changes characteristic
of the emission from excited-state aggregates in the case of
CdII, InIII, HfIV, and PbII (See Figure 13 in the Supporting In-
formation). In contrast, 1 showed only varying degrees of lu-
minescence quenching with 1/Mn+ ratios of 1:ꢂ10 with CuII,
InIII, and HfIV.
Thus, the spectroscopic ion-binding investigations re-
vealed that the macrocycle 1 exhibits significantly weaker
metal coordination than the acyclic compound 13 under
dilute conditions, and to a more restricted range of metal
ions, which is consistent with the sterically constrained ion-
binding pocket of the former as well as the specific electron-
ic properties of its frontier molecular orbitals (FMOs; see
below). However, the marked selectivity of 1 for PdII sug-
gests that potential ion-sensing applications exist for this
metal/ligand system.
A
ACHTUNGTRENNUNG
intra-cyclic 1:1 [AuCl1] complex.
1
The H NMR spectra of the HfIV, FeII, CoII, and AuI com-
plexes exhibited no additional line broadening compared to
that of pure 1, and remained simplified over 1/Mn+ ratios
ranging between 1:0.5 to 1:10 (the experimental limit) indi-
cative of fast metal–ligand exchange on the NMR time-
scale.[53]
On the other hand, 1:1 combinations of 1/Mn+ of the com-
plexes [Pd
CD3CN/CDCl3 mixture, and [Pt
(MeCN)4]
E
(MeCN)2] in a 8%
(CF3SO3)2 in pure
N
ACHTUNGTRENNUNG
CDCl3, all exhibited complex spectra with significant pertur-
bations of the central pyridine H4’ triplet suggesting forma-
tion of multiple products as well as metal binding within the
macrocycle.[54]
Thus, despite the steric restrictions, 1 exhibits intra-cyclic
coordination to a range of metal salts, worthy of further
study with respect to potential catalytic applications.
Cyclic voltammetric studies: The cyclic conjugation of 1 sug-
gested that it may form a radical anion of enhanced stability
upon electrochemical reduction, compared to those generat-
ed from the non-cyclic precursors. We therefore performed
electrochemical investigations on 1 and the non-cyclic pre-
cursors 7, 10, and 13, the results of which are summarized in
Table 2 and Figure 7. The cyclic voltammograms of 7, 10,
1H NMR investigation of the metal-ion complexation of 1:
The uniquely hindered terpyridine coordination pocket of 1
raises interesting issues concerning the nature, strength, and
extent of the metal–ligand interactions that are possible
with this macrocycle. Metal ions may be expected to prefer-
entially bind to the sterically less encumbered outer pyri-
dines of the terpyridine subunit to give [M21]n+ complexes,
whereas those that are able to bind within the macrocyclic
cavity may possess incomplete coordination geometries, or
weakly solvated sterically protected sites with possible cata-
lytic activity.[50] With these considerations in mind, we under-
Table 2. Electrochemical data of 1, 7, 10, and 13 observed by cyclic vol-
tammetry at a scan rate of v=0.1 Vsꢁ1 in CH2Cl2 with 0.1 molLꢁ1
[(nBu4)N]PF6.[a]
Compound
E8 versus Fc+/Fc [V][b]
DEp [mV][c]
Epc [V][d]
7
ꢁ2.30[e]
10
13
1
ꢁ2.24
ꢁ2.14
90
70
ꢁ2.38
1
ꢁ2.07
took initial exploratory H NMR investigations of the ion-
[a] All potentials are given versus ferrocene/ferrocenium (Fc/Fc+), used
as internal standard. [b] E8=(Epa+Epc)/2, where Epa and Epc are the
respective anodic and cathodic peak potentials. [c] DEp=EpaꢁEpc.
[d] Irreversible peak potential. [e] Reversible electron transfer at a scan
binding of 1 at a concentration of 1 of 1.2ꢃ10ꢁ3 molLꢁ1 in a
8% CD3OD/CDCl3 mixture (see the Supporting Informa-
tion). Possible interference by ligand aggregation during the
studies was found to be negligible as the chemical shifts of 1
were found to vary by less than 0.02 ppm in 8% CD3OD/
rate >1 Vsꢁ1
.
CDCl3 over
a
concentration range of 5ꢃ10ꢁ3–5ꢃ
10ꢁ4 molLꢁ1.
The NMR studies revealed that in the case of 1:1 stoichio-
metric combinations of 1/Mn+ with [Fe
(dmso)6](CF3SO3)2
and Sc(CF3SO3)3, the coordination-induced shifts (Dd) were
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
greatest for the outer pyridine protons and small to negligi-
ble for the triplet corresponding to the central pyridine H4’,
indicating weak binding mainly to the outer pyridines. On
the other hand, 1:1 combinations of 1/Mn+ with Pb-
Figure 7. Cyclic voltammetry of 1 recorded in CH2Cl2 with 0.1 molLꢁ1
[(nBu4)N]PF6 and with ferrocene as internal standard.
G
ACHTUNGTRENNUNG ACHTUNRTEGN(NGUN dmso)6]ACHTNUGTRNE(UNGN CF3SO3)2, and [Ni-
(CF3SO3)4,[51] [Co
G
ACHTUNGTRENNUNG
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Chem. Eur. J. 2013, 19, 12336 – 12349

Synthesis of a Strained Acetylenic Macrocycle
FULL PAPER
and 13 are shown in Figure 14 of the Supporting Informa-
tion.
hindered rotational motions of the substituted phenyl rings
of the oligo[2]cruciform bridge and flipping motions of the
bridge across the terpyridine mean plane, which may be an-
ticipated to result in a series of conformational oscillations
depicted in part in Scheme 4.
Surprisingly, 1 only gave an irreversible electron transfer
at any scan rates ꢂ0.1 Vsꢁ1 (Figure 7). However, 1 is indeed
easier to reduce than the precursors, as shown in Table 2,
where the first reduction is seen to shift to less negative po-
tentials upon going from 7 (Epc=ꢁ2.30 V) through 10
(E8=ꢁ2.24 V) and 13 (E8=ꢁ2.14 V) to 1 (Epc=ꢁ2.07 V),
which is in agreement with the progressive increase in con-
jugation extension.
The instability of 1 towards reduction is supported by the-
oretical calculations (see later), which show that the electron
can only enter the LUMOs of the strained butadiyne-linked
oligo[2]cruciform bridge, thereby affording an unstable spe-
cies, which may then undergo rapid follow-up chemical reac-
tions.
The highest-energy interconversions would be expected to
involve passage of substituents through the macrocyclic
cavity (“through cavity” motions, Scheme 4); a process,
which would place the inner-pointing substituents on either
side (conformers 1a–1d), or together on the same side (con-
formers 1e–1h) of the terpyridine mean plane. The passage
of the TIPSA groups through the cycle cavity, by analogy to
a “molecular turnstile”,[60] would be particularly sterically
restricted, requiring considerable flexing and breathing of
the macrocyclic frame. However, the variable-temperature
1H NMR and UV/Vis spectra of 1 did indeed show changes
continuing to elevated temperatures, which may possibly in-
volve such a process.[61]
VT NMR investigations of the conformational properties of
1 and 13: Due to the extraordinary flexibility of the triple
bond,[55–57] rotational isomerism along the ethyne axis is pos-
sible within significantly hindered systems.[58,59] Macrocycle 1
would therefore be expected to be a flexible structure, with
1
Thus, the VT H NMR studies of 1 in CDCl3 and [D16]n-
heptane revealed the existence of hindered motions con-
cerning the TIPS substituents and the inner phenyl protons
(H3/3’ and H6/6’) of the oligo[2]cruciform bridge, as well as
ꢀ
Scheme 4. Schematic representation of the conformers of 1 which may be generated through stepwise inter-phenyl rotations, where R=C CSi
(iPr)3 or
ꢀ
C CSiH3 in the case of the DFT models of 1b and 1g. A complementary set of enantiomers can also exist below the horizontal line, generated through
continued substituent rotations of the meso conformers. The conformations symmetrically disposed either side of the vertical line are related through
flipping of the curved oligo[2]cruciform bridge across the mean plane through the terpyridine.
Chem. Eur. J. 2013, 19, 12336 – 12349
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12343

P. N. W. Baxter et al.
protons H4/4’’ of the outer pyridines from ꢁ50 to 708C,
which is consistent with the coexistence of several dynamic
conformational processes, which were not directly mechanis-
tically distinguishable. In CDCl2CDCl2, which has a higher
boiling point than the former solvents, simplification of the
1H NMR spectra is almost complete only at 1108C (see the
Supporting Information).
rotate to bring the 2,2’-TIPS substituents in proximity with
the outer pyridine rings. Thus, as for 1, the primary steps
lying at the origin of the VT NMR and UV/Vis changes in
13 are consistent with conformational interchanges involving
rotation of the TIPSA-substituted phenyl rings.
Theoretical investigations of the conformational properties
of 1: To obtain greater insight into the conformational and
electronic properties of 1, we performed density functional
theory (DFT) calculations on the representative conforma-
tions 1b and 1g, where the TIPSA substituents were re-
Apart from intensity and chemical shift variations, the
spectral changes were broadly similar in both CDCl3 and
[D16]n-heptane, undergoing coalescence at 40–508C, suggest-
ing a related dynamic process in both solvents. As the X-ray
structure analysis unambiguously revealed that the macrocy-
cle can exist as conformer 1a, it follows that conformations
1a–1d must contribute to at least a part of the conforma-
tional species present in solution. Mechanistically, the
lowest-energy pathway for conformer interconversion in-
volves stepwise rotational twisting of the substituted inner
phenyl rings of the oligo[2]cruciform bridge, and it is there-
fore more probable that bridge-flipping motions such as the
interconversion between 1b and 1c occur in a stepwise
rather than a concerted process. Consistent with the afore-
mentioned considerations, the dynamic mechanisms respon-
sible for the observed VT NMR and UV/Vis changes must
fundamentally operate through stepwise rotational motions
of the inner phenyls of the oligo[2]cruciform bridge.[62]
In the case of 13 a VT NMR investigation in [D16]n-hep-
tane afforded simplified spectra from ꢁ50–708C, with an ab-
sence of coalescence phenomena. However, marked chemi-
cal shift movements were observed for the 5/5’ TIPS protons
(d=0.067 ppm), and the terminal Ph H6/6’ (d=ꢁ0.041 ppm)
protons, as well as the pyridine (Py) H3/3’’ (d=
ꢁ0.069 ppm), Py H3’,5’ (d=ꢁ0.066 ppm), and Py H4/4’’ (d=
ꢁ0.031 ppm) protons, upon heating from ꢁ50–708C (see
Scheme 5 and the Supporting Information). These results
ꢀ
placed by C CSiH3 (Scheme 4) in order to simplify the cal-
culations, and the unsubstituted compound 1i for compari-
son. The geometry minimizations, FMO surfaces, and ener-
gies were calculated by using the Spartan software[64] em-
ploying the B3LYP functional and the 6-31G* basis set.[65,66]
Views of the calculated FMO surfaces of 1b and 1g are
shown in Figure 8. The DFT calculations on 1b and 1g cor-
rectly reproduce the direction of the curvature of the oligoc-
ruciform bridge and its displacement to one side of the ter-
pyridine mean plane, as seen in the crystal structure of 1,
and revealed the following observations:
1) The LUMOs of 1b and 1g are located entirely on the
strained oligo[2]cruciform bridge and not on the normal-
ly electron-accepting terpyridine unit. The site of electro-
reduction would therefore be expected to be located ex-
clusively on the strained oligo[2]cruciform bridge, which
is consistent with the electrochemical instability of the
radical anion of 1.
2) Conformation 1g, with an increased twist angle between
the oligo[2]cruciform inner phenyl groups, undergoes
switching-off of part of the bridge HOMO, and an ex-
pansion of the terpyridine HOMOs.
3) The increased interruption in the FMO conjugation over
the oligo[2]cruciform bridge of conformer 1g causes an
increase in the calculated HOMO–LUMO bandgap com-
pared to 1b (Table 3). However, the presence of the
Table 3. Energy properties and dipole moments calculated (B3LYP/6-
ꢀ
31G*) for 1b and 1g, where R= C CSiH3, and 1i, where R=H, in the
gas phase.
Cycle
E
N
E
A
DE
m
[eV]
[eV]
(LUMOꢁ
HOMO)
[eV]
[D]
Scheme 5. 1H NMR numbering scheme for 13.
1b
1g
1i
ꢁ5.595
ꢁ5.674
ꢁ5.422
ꢁ2.693
ꢁ2.591
ꢁ2.315
2.902
3.083
3.107
1.22
0.91
0.58
are consistent with a temperature-controlled redistribution
of an equilibrating mixture of conformers that is fast com-
pared to the NMR timescale.[63] NOE investigations showed
cross peaks between the 5/5’ TIPS protons and the Py H3/
3’’, Py H3’,5’, and Py H4/4’’ protons, revealing that 13 exists
in part as the least hindered conformer with the 5/5’-TIPS
substituents pointing towards the interior of the molecule
(as depicted in Schemes 2 and 5). A cross peak assigned to
the Py H6,6’ is averaged between the 2,2’ TIPS and 5/5’
TIPS protons showing that the substituted phenyl rings also
TIPSA substituents causes some reduction in the calcu-
lated bandgap compared to the unsubstituted 1i, which
is in accord with an increase in FMO surface area.
As detailed above, the X-ray crystallographic and VT
1H NMR findings are consistent with 1 existing in solution
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Chem. Eur. J. 2013, 19, 12336 – 12349

Synthesis of a Strained Acetylenic Macrocycle
FULL PAPER
Figure 8. HOMO (middle) and LUMO (top) surfaces of 1b, 1g, and 1i calculated by using density functional B3LYP 6-31G* theory, with molecular sche-
matics shown at the bottom. The calculated model of 1b in the graphic, is the enantiomer of that shown in Scheme 4.
at least in part as a series of the conformations 1a–1d
(Scheme 4), which interconvert through rotational move-
ments of the TIPSA groups outside the macrocyclic cavity.
The VT UV/Vis findings are therefore consistent with cool-
ing causing an increase in the population of conformers 1b
and 1c, possessing an “in-plane” arrangement of the inner
phenyls of the oligo[2]cruciform bridge with improved con-
jugation and a lower HOMO–LUMO bandgap.[67]
An increase in the proportion of conformers 1e–1h pos-
sessing poorer conjugation, due to increased inner phenyl
twist angles within the oligo[2]cruciform bridges would also
be expected in the case of through-cavity substituent rota-
result in the direct formation of carbon fullerenes and nano-
tubes.[26,27,35,68,70]
With these considerations in mind, we thought it of inter-
est to compare the thermogravimetric analytical (TGA) be-
havior of 1, 8, 10, 11, 13, and 14 (Figure 15 in the Supporting
Information). These experiments revealed that the TGA
curves of 1, 10, 13, and 14 were essentially identical, with
greatest mass loss between 300–4508C, which is consistent
with the TIPS substituents significantly contributing to the
thermal stability of 1.
tions, resulting in an overall increase in HOMO–LUMO
Conclusion
opt
bandgap and thus increase in Eg
.
In any event, the thermally synchronized decrease in con-
jugation with heating and increase with cooling, mediated
through redistributions within a mixture of conformational
isomers of 1 is intriguing, as it forms the basis for a tempera-
ture-controllable conjugation switch.
The Eglinton–Galbraith cyclization of 14 proceeds intramo-
lecularly to yield the structurally unusual macrocycle 1. The
X-ray crystal structure shows that it comprises a strained
conjugated oligo[2]cruciform bridge forced into a curve of
nanoscopic dimensions by the terpyridine tether, and sur-
rounded by bulky TIPSA substituents, which stabilize it to-
wards heat and light.
Along with its precursors 7, 10, 11, 13, and 14, the macro-
cycle 1 is emissive both in solution and the solid state, with
10 and 11 in particular exhibiting high solution quantum
Thermochemical properties: Sterically unsubstituted SAMs
and many of their unstrained congeners are thermally unsta-
ble materials that are well documented to undergo explosive
decompositions upon heating,[27,35,68,69] and in some cases
Chem. Eur. J. 2013, 19, 12336 – 12349
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12345

P. N. W. Baxter et al.
small oblong plates; 1H NMR (CDCl3, 400.14 MHz, 258C): d=8.840 (dd,
4J(6,4/6’’,4’’)=2.0, 5J(6,3/6’’,3’’)=0.9 Hz, 2H; Py H6/6’’), 8.628 (dd, 3J(3,4/
yields, suggesting promising applications as new scaffolds for
luminescence metal-ion and biomolecule sensing.[71,72] Fur-
thermore, the very high selectivity of 1 for PdII under dilute
conditions, demonstrated its potential for the UV/Vis-spec-
troscopic detection of this metal. At higher concentrations,
1H NMR studies revealed 1 to bind a range of metal ions
within its cavity, which in view of the associated steric con-
straints, may provide potential catalytic properties.
VT UV/Vis/1H NMR investigations showed 1 to undergo
dynamic intramolecular motions involving restricted rota-
tions of the TIPSA substituents, in which heating causes an
increase in the spectroscopic bandgap, whereas cooling re-
sults in bandgap reduction. Theoretical calculations on the
ꢀ
5
3
3’’,4’’)=8.3, J(3,6/3’’,6’’)=0.7 Hz, 2H; Py H3/3’’), 8.512 (d, J(3’,4’;5’,4’)=
3
7.9 Hz, 2H; Py H3’,5’), 7.993 (t, J(4’,3’;4’,5’)=7.9 Hz, 1H; Py H4’), 7.980
(dd, 3J(4,3/4’’,3’’)=8.2, 4J(4,6/4’’,6’’)=2.0 Hz, 2H; Py H4/4’’), 7.610–7.576
(m, 2H; Ph H6/6’), 7.569–7.537 (m, 2H; Ph H3/3’), 7.356–7.293 (m, 4H;
Ph H4,5/4’5’), 1.143-1.139 ppm (m, 42H; Si(CH
(CDCl3, 100.62 MHz, 278C): d=154.8, 154.7, 151.7, 139.4, 138.0, 132.9,
ꢁ ꢀ
(CH3)2)3); 13C NMR
ACHTUNGTRENNUNG
ꢁ
132.3, 128.4, 128.1, 125.9, 125.1, 121.6, 120.5, 120.3, 105.1 ( C ), 95.5 (
ꢁ ꢀ ꢁ ꢀ
ꢀ
C ), 92.4 ( C ), 89.9 ( C ), 18.7 (CHACHTUNTRGENNU(G CH3)2), 11.3 ppm (CHACHTUNGTRENNUNG(CH3)2);
ꢀ
IR: n˜ =2941 (s), 2890 (m), 2863 (s), 2156 (m) (C C), 1586 (m), 1573 (m),
1542 (m), 1486 (s), 1464 (s), 1444 (vs), 1368 (m), 1253 (m), 1212 (m),
1101 (m), 1094 (m), 1075 (m), 1018 (s), 995 (s), 882 (s), 869 (m), 839 (m),
817 (vs), 800 (m), 755 (vs), 702 (s), 676 (vs), 662 (vs), 639 (s), 578 cmꢁ1
(m); UV/Vis (n-heptane): lmax (e)=227 (6.1ꢃ104), 237 (6.4ꢃ104), 257
(6.7ꢃ104), 263 (7.1ꢃ104), 336 (8.5ꢃ104), 351 nm (7.5ꢃ104 mꢁ1 cmꢁ1); HR
ESI-MS: m/z (%) calcd for C53H60N3Si2: 794.432; found: 794.434 [M+H+
] (100).
unsubstituted and C CSiH3-substituted analogues of
1
showed the LUMO to be located on the strained oligo[2]c-
ruciform bridge and not on the terpyridine unit, which is
consistent with the electrochemical instability of 1. The cal-
culations re-inforce the conclusion that the VT UV/Vis be-
havior may stem from reversible temperature-modulated re-
distributions within a library of conformers, in which the
electronic conjugation through the strained bridge varies ac-
2,2’:6’,2’’-Terpyridine-5,5’’-diylbis-[1-(2,1-ethynediyl)-2-ethynyl-phenylene]
(11): To a stirred solution of 10 (0.495 g, 6.23ꢃ10ꢁ4 mol) in THF (60 mL,
to which ten drops of H2O had been added), was added dropwise
[(nBu)4N]F (1.0m in THF, 1.5 mL, 1.50ꢃ10ꢁ3 mol) and the reaction was
stirred at ambient temperature for 2.5 days in the absence of light. The
solvent was then removed under reduced pressure at ambient tempera-
ture and water (60 mL) added to the residue. The suspended solid was
isolated by filtration under vacuum, washed with excess distilled water
followed by excess MeOH and dried in air. The solid was then re-sus-
pended in MeOH (20 mL), the mixture was homogenized by brief ultra-
sonication, and filtered under vacuum. The collected product was finally
washed with excess MeOH then re-crystallized from MeCN and finally
dried at 958C under a gentle stream of argon for 12 h, to afford 11
(0.273 g, 91%) as an amorphous-looking cream-colored, light-sensitive
solid. M.p. partial melting and re-solidification at 188.0–189.08C to a
dark brown solid; the proportion of solid melting increases with increas-
ing heating rate; 1H NMR (CDCl3, 500.13 MHz, 258C): d=8.901 (dd,
4J(6,4/6’’,4’’)=2.0, 5J(6,3/6’’,3’’)=1.0 Hz, 2H; Py H6/6’’), 8.662 (dd, 3J(3,4/
ꢀ
cording to the relative twisting of the C CSiR3-substituted
phenyl rings. The environmentally adaptive conjugation
switching of 1 points to intriguing possibilities for control-
ling the conjugation of molecular electronic materials.
The isolation of 1 therefore justifies the possibility of gen-
erating heat- and light-stable electronically conjugated oli-
gomeric rods and wires, with curved pathways for electronic
communication over nanoscopic distances. Furthermore, the
rich spectroscopic properties of conjugated cruciforms[73,74]
make them attractive candidates for the development of
sensors,[75,76] switches,[77] metallo-assembly units,[78] and nano-
mechanical structures.[79] Materials, such as 1, may therefore
be anticipated to serve as a new class of cruciform molecular
electronic platforms for the construction of molecular sole-
noids[80,81] and conjugation switches, while continuing to
pose intriguing theoretical possibilities.
5
3
3’’,4’’)=8.5, J(3,6/3’’,6’’)=1.0 Hz, 2H; Py H3/3’’), 8.530 (d, J(3’,4’;5’,4’)=
3
4
8.0 Hz, 2H; Py H3’,5’), 8.039 (dd, J(4,3/4’’,3’’)=8.5, J(4,6/4’’,6’’)=2.0 Hz,
2H; Py H4/4’’), 8.007 (t, 3J(4’,3’;4’,5’)=7.8 Hz, 1H; Py H4’), 7.608–7.571
(m, 4H; Ph H3,6/3’,6’), 7.399–7.331 (m, 4H; Ph H4,5/4’,5’), 3.419 ppm (s,
2H; C CH); 13C NMR (CDCl3, 100.61 MHz, 248C): d=154.9, 154.8,
ꢀ
151.8, 139.4, 138.0, 132.7, 131.9, 128.7, 128.5, 125.6, 124.9, 121.6, 120.4,
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
120.3, 92.0 ( C ), 90.5 ( C ), 82.0 ( C ), 81.5 ppm ( C ); IR: n˜ =3290
ꢀ
(m), 3254 (m), 3057 (w), 2104 (w) (C C), 1586 (m), 1571 (m), 1542 (s),
1487 (s), 1444 (s), 1368 (m), 1297 (m), 1254 (m), 1149 (w), 1128 (m), 1098
(m), 1092 (m), 1083 (m), 1021 (s), 992 (m), 950 (m), 933 (m), 860 (m),
820 (vs), 756 (vs), 739 (s), 689 (s), 656 (s), 651 (s), 601 cmꢁ1 (s); UV/Vis
(CH2Cl2): lmax (e)=252 (4.4ꢃ104), 336 (7.6ꢃ104), 351 nm (7.4ꢃ
104 mꢁ1 cmꢁ1); FAB MS: (1% CF3COOH/3-nitrobenzylalcohol (NBA)):
m/z (%): 482.1 [M+H+] (100); HR ESI-MS: m/z (%) calcd for C35H20N3:
482.165; found: 482.158 [M+H+] (100).
Experimental Section
General experimental details are described in the Supporting Informa-
tion.
2,2’:6’,2’’-Terpyridine-5,5’’-diylbis-[1-(2,1-ethynediyl)-2-(1-(4-trimethylsily-
lethynyl-2,5-bis-triisopropylsilylethynyl-phenyl)ethynyl)-phenylene] (13):
To a mixture of 11 (0.288 g, 5.98ꢃ10ꢁ4 mol), 12 (0.806 g, 1.22ꢃ10ꢁ3 mol)
Synthetic procedures and characterization data for 1, 10, 11, 13, and 14
2,2’:6ꢀ,2’’-Terpyridine-5,5’’-diylbis-[1-(2,1-ethynediyl)-2-(2-(1-triisopropylsi-
lyl)ethynyl)-phenylene] (10): To a mixture of 8 (0.320 g, 1.14ꢃ10ꢁ3 mol),
and [PdCl2ACTHNUTRGNEUNG
(PPh3)2] (0.067 g, 9.55ꢃ10ꢁ5 mol) under argon, was added tol-
9
(0.910 g, 2.37ꢃ10ꢁ3 mol) and [PdCl
2ACHTNGUETRNNU(G dppf)]·CH2Cl2 (0.061 g, 7.47ꢃ
uene (40 mL) through a syringe, and the stirred reaction mixture was
heated to 858C in order to dissolve the suspended 11. After allowing to
cool to 458C (the minimum temperature at which 11 remained dis-
solved), a solution of CuI (0.053 g, 2.78ꢃ10ꢁ4 mol) in Et3N (2 mL), previ-
ously prepared under argon, was added through a syringe, and the reac-
tion mixture was stirred in subdued light for 8 days at a bath temperature
of 558C. All solvent was then removed under reduced pressure on a
water bath, and the residue extracted with hexane (5ꢃ40 mL). The sol-
vent was removed from the combined hexane extracts under reduced
pressure on a water bath to yield a brown oil, which was chromatograph-
ed on a column of silica by eluting with CH2Cl2. The product thus ob-
tained was then re-chromatographed on silica by using 1:1 CH2Cl2/
hexane and boiled in absolute EtOH (600 mL), gravity filtered, and al-
lowed to stand for 24 h at ambient temperature. The resulting suspension
10ꢁ5 mol) under argon, was added toluene (30 mL) through a syringe,
and the reaction vessel was placed in a bath at 558C and the mixture was
stirred for 0.25 h, after which time all 8 had dissolved. A solution of CuI
(0.040 g, 2.10ꢃ10ꢁ4 mol) in Et3N (2 mL), previously prepared under
argon, was syringed into the reaction and the resulting mixture was stir-
red at 558C under subdued light for 5 days. All solvent was then removed
under reduced pressure on a water bath, and the residue was purified by
chromatography on a column of silica, consecutively eluting with CH2Cl2
and 1% MeOH/CH2Cl2. The crude product thus obtained was re-chro-
matographed on silica, with CH2Cl2 as eluent, and isolated as a honey-
colored glass. Final purification was achieved upon recrystallisation from
MeCN to afford 10 (0.547 g, 61%) as a white microcrystalline solid after
washing with MeCN and drying in air. M.p. 138.5–139.98C; crystal form:
12346
www.chemeurj.org
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 12336 – 12349

Synthesis of a Strained Acetylenic Macrocycle
FULL PAPER
was filtered under vacuum, and the collected solid was washed with abso-
lute EtOH (4ꢃ3 mL), dried in air, and re-chromatographed on silica with
CH2Cl2 as eluent. Final purification was achieved upon treating the prod-
uct with boiling absolute EtOH (100 mL) as above to afford 13 (0.382 g,
41%) as a white solid after isolation by vacuum filtration, EtOH washing
and drying in air. M.p.: 208.9–211.58C; crystal form: microcrystalline rec-
tangular plates; 1H NMR (CDCl3, 400.14 MHz, 268C): d=8.804 (dd,
4J(6,4/6’’,4’’)=2.0, 5J(6,3/6’’,3’’)=0.6 Hz, 2H; Py H6/6’’), 8.577 (dd, 3J(3,4/
104 mꢁ1 cmꢁ1); HR ESI-MS: m/z (%) calcd for C95H108N3Si4: 1403.764;
found: 1403.760 [M+H+] (100); C95H107N3Si4Na: 1425.746; found:
1425.747 [M+Na+] (12); C190H215N6Si8: 2807.522; found: 2807.534
[2M+H+] (6); elemental analysis calcd for C95H107N3Si4: C 81.31, H 7.69,
N 2.99; found: C 80.98, H 7.80, N, 2.80.
Macrocycle 1: In a well-ventilated hood, anhydrous Cu2ACTHNUGTRNE(UNG OAc)4 (1.50 g,
4.13ꢃ10ꢁ3 mol) was added to 3:1 pyridine/toluene (400 mL), and the stir-
red mixture was bubbled with argon for 2 h, during which time the sus-
pended solid dissolved. A solution of 14 (0.207 g, 1.48ꢃ10ꢁ4 mol) in de-
gassed 3:1 pyridine/toluene (50 mL) was then added dropwise to the Cu2-
5
3
3’’,4’’)=8.2, J(3,6/3’’,6’’)=0.8 Hz, 2H; Py H3/3’’), 8.487 (d, J(3’,4’;5’,4’)=
3
7.9 Hz, 2H; Py H3’,5’), 7.947 (t, J(4’,3’;4’,5’)=8.0 Hz, 1H; Py H4’), 7.919
3
4
(dd, J(4,3/4’’,3’’)=8.3, J(4,6/4’’,6’’)=2.2 Hz, 2H; Py H4/4’’), 7.671 (s, 2H;
terminal Ph H6/6’), 7.631 (s, 2H; terminal Ph H3/3’), 7.631–7.607 (m, 2H;
corner Ph H6/6’), 7.579–7.557 (m, 2H; corner Ph H3/3’), 7.391–7.319 (m,
AHCTUNGRTEG(NNNU OAc)4 solution over 5 h with rapid stirring and continued argon bub-
bling. The argon flow was then terminated and the reaction mixture was
stirred for 7 days at ambient temperature in the absence of light. All sol-
vent was removed by distillation under reduced pressure at 35–408C, ice-
cold concentrated aqueous solution of KCN (10 mL) was added, and the
suspension was stirred for 0.25 h in order to allow complete cyanide com-
plexation of the copper salts. Subsequent extraction with CH2Cl2 yielded
the crude product as a glass, which was twice chromatographed on short
(3ꢃ5 cm diameter) columns of silica with CH2Cl2 as eluent. After each
column, the solvent was removed from the product under reduced pres-
sure at ambient temperature. The clear glassy solid thus obtained was dis-
solved in pentane and stored at ꢁ308C for 24 h, which induced crystalli-
zation. The isolated solid was again re-crystallized from pentane to
afford 1 (0.083 g, 40%) as pale caramel-colored needles, after washing
with ice-cold pentane and drying in air. The product was best kept below
08C for long term storage (several months to years). M.p.: crystals
change color from clear pale caramel yellow to opaque dark brown over
249–2528C with no melting; crystal form: elongated parallelogram plates;
1H NMR (CDCl2CDCl2, 500.13 MHz, 1108C): d=8.918 (dd, 4J(6,4/
4H; corner Ph H4,5/4’,5’), 1.081–1.075 (m, 42H; 2/2’-Si(CH
ACHTNUGTRNE(NUGN CH3)2)3),
1.043 ppm (s, 42H; 5/5’-Si(CH
AHCTUNGTRENNUNG
268C): d=154.8, 154.7, 151.6, 139.1, 137.8, 136.9, 136.3, 132.0, 131.9,
128.4, 128.3, 125.6, 125.5, 125.4, 125.31, 125.25, 125.15, 121.6, 120.4, 120.2,
ꢁ ꢀ ꢁ
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
(CH3)2), 18.65 (CH-
(CH3)3); IR:
104.0 ( C ), 102.3 ( C ), 100.6 ( C ), 97.8 ( C ), 97.6 ( C ), 93.4 (
ꢀ
ꢁ ꢀ ꢁ ꢀ ꢁ ꢀ
C ), 92.2 ( C ), 91.5 ( C ), 90.8 ( C ), 18.68 (CH
ACHTUNGTRENNUNG
G
N
N
ACHTUNGTRENNUNG
ꢀ
n˜ =2942 (s), 2891 (m), 2864 (s), 2155 (m) (C C), 1543 (m), 1489 (s), 1463
(m), 1445 (s), 1382 (m), 1367 (m), 1250 (s), 1187 (m), 1073 (m), 1019 (m),
996 (m), 902 (m), 879 (vs), 858 (vs), 842 (vs), 817 (s), 798 (m), 790 (s),
754 (vs), 677 (vs), 659 (s), 634 (s), 590 cmꢁ1 (s); UV/Vis (n-heptane): lmax
(e)=273 (1.9ꢃ105), 307 (9.3ꢃ104), 321 (1.0ꢃ105), 355 (7.3ꢃ104), 372 nm
(sh, 3.3ꢃ104 mꢁ1 cmꢁ1); FAB MS: (1% CF3COOH/NBA): m/z (%):
1547.6 [M+H+] (100); HR ESI-MS: m/z (%) calcd for C101H124N3Si6:
1547.843; found: 1547.873 [M+H+] (100); calcd for C202H246FeN6Si12
1575.303; found: 1575.317 [2M+Fe2+] (46).
:
5
3
6’’,4’’)=2.5, J(6,3/6’’,3’’)=0.8 Hz, 2H; Py H6/6’’), 8.643 (d, J(3’,4’;5’,4’)=
2,2’:6’,2’’-Terpyridine-5,5’’-diylbis-[1-(2,1-ethynediyl)-2-(1-(4-ethynyl-2,5-
3
5
8.0 Hz, 2H; Py H3’,5’), 8.469 (dd, J(3,4/3’’,4’’)=8.0, J(3,6/3’’,6’’)=0.8 Hz,
bis-triisopropylsilylethynyl-phenyl)ethynyl)-phenylene] (14): Powdered
K2CO3 (0.030 g, 2.17ꢃ10ꢁ4 mol) was added to a stirred solution of 13
(0.312 g, 2.02ꢃ10ꢁ4 mol) in 1:2 Et2O/MeOH (40 mL). After 24 h stirring
at ambient temperature in the absence of light, more powdered K2CO3
(0.028 g, 2.03ꢃ10ꢁ4 mol) was added and stirring in the dark was contin-
ued for further 48 h. All solvent was then removed under reduced pres-
sure on a water bath, water (60 mL) was added, and the suspended solid
was isolated by filtration under vacuum, the residue was washed with
excess water, followed by MeOH (4ꢃ5 mL) and dried in air. The product
was subsequently chromatographed on a column of silica by eluting with
CH2Cl2 and boiled in absolute EtOH (250 mL), the volume was reduced
to 50 mL under reduced pressure on a water bath and the suspension was
left to stand at ambient temperature for 24 h. Product 14 (0.238 g, 84%)
was obtained as a cream powdery solid after isolation by filtration under
vacuum, washing with absolute EtOH (3ꢃ2 mL), drying in air, and fur-
ther drying under vacuum (0.01 mmHg/558C/24 h). M.p.: slow softening
from 70–1008C then slow melting to the isotropic at 1168C. The onset
temperature of softening increased with increasing heating rate; crystal
form: irregular glassy plates; 1H NMR (CDCl3, 500.13 MHz, 288C): d=
8.818 (dd, 4J(6,4/6’’,4’’)=2.1, 5J(6,3/6’’,3’’)=1.0 Hz, 2H; Py H6/6’’), 8.577
(dd, 3J(3,4/3’’,4’’)=8.3, 5J(3,6/3’’,6’’)=0.8 Hz, 2H; Py H3/3’’), 8.488 (d,
3J(3’,4’;5’,4’)=7.8 Hz, 2H; Py H3’,5’), 7.950 (t, 3J(4’,3’;4’,5’)=7.9 Hz, 1H;
Py H4’), 7.930 (dd, 3J(4,3/4’’,3’’)=8.2, 4J(4,6/4’’,6’’)=2.1 Hz, 2H; Py H4/
4’’), 7.690 (d, 5J(6,3/6’,3’)=0.7 Hz, 2H; terminal Ph H6/6’), 7.659 (s, 2H;
terminal Ph H3/3’), 7.632–7.612 (m, 2H; corner Ph H6/6’), 7.580–7.561
(m, 2H; corner Ph H3/3’), 7.391–7.330 (m, 4H; corner Ph H4,5/4’,5’),
3
2H; Py H3/3’’), 7.976 (t, J(4’,3’;4’,5’)=8.0 Hz, 1H; Py H4’), 7.862 (s, 2H;
inner Ph H6/6’), 7.744 (dd, 3J(4,3/4’’,3’’)=2.5, 4J(4,6/4’’,6’’)=8.0 Hz, 2H;
Py H4/4’’), 7.713–7.694 (m, 2H; corner Ph H6/6’), 7.660 (s, 2H; inner Ph
H3/3’), 7.578–7.559 (m, 2H; corner Ph H3/3’), 7.444–7.387 (m, 4H;
corner Ph H4,5/4’,5’), 1.167–1.165 (m, 42H; inner Ph 2/2’-Si(CH
ACHTUNGTRENNUNG(CH3)2)3),
1.123–1.112 ppm (m, 42H; inner Ph 5/5’-Si(CH
AHCTUNGTRENNUNG
(CDCl3, 100.61 MHz, 258C): d=155.1, 154.5, 152.4 (Py C6/6’’), 138.4 (Py
C4/4’’), 137.6 (Py C4’), 136.4 (inner Ph C6/6’), 134.9 (inner Ph C3/3’),
132.2 (corner Ph C6/6’), 130.5 (corner Ph C3/3’), {128.6, 128.4, (corner Ph
C4,5/4’,5’)}, 127.4, 126.8, 126.5, 125.1, 124.7, 124.6, 122.2 (Py C3’,5’), 120.3,
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
120.2, 103.6 ( C ), 103.3 ( C ), 99.2 ( C ), 98.4 ( C ), 95.0 ( C ), 92.3
ꢁ ꢀ ꢁ ꢀ ꢁ ꢀ ꢁ ꢀ ꢁ ꢀ
( C ), 91.1 ( C ), 86.4 ( C ), 81.6 ( C ), 77.7 ( C ), 18.6 (CH
ACHTUNGTRENNUNG
18.5 (CH(CH3)2), 11.3 (CH(CH3)2), 11.2 ppm (CH(CH3)2); IR: n˜ =2942
G
G
ACHTUNGTRENNUNG
ꢀ
(s), 2890 (m), 2864 (s), 2157 (m) (C C), 1583 (m), 1543 (m), 1485 (s),
1463 (s), 1442 (s), 1383 (m), 1367 (m), 1247 (m), 1183 (m), 1072 (m),
1019 (m), 996 (m), 919 (m), 900 (m), 882 (s), 870 (m), 861 (m), 851 (s),
820 (s), 789 (s), 754 (vs), 742 (m), 734 (m), 703 (m), 677 (vs), 659 (vs),
612 cmꢁ1 (m); UV/Vis (n-heptane): lmax (e)=289 (1.9ꢃ105), 337 (1.1ꢃ
105), 402 nm (1.2ꢃ104 mꢁ1 cmꢁ1); HR ESI-MS: m/z (%) calcd for
C95H106N3Si4: 1401.749; found: 1401.748 [M+H+] (99); C95H105N3NaSi4:
1423.730; found: 1423.729 [M+Na+] (100); C95H105KN3Si4: 1439.704;
found: 1439.704 [M+K+] (16); C190H211N6Si8: 2802.489; found: 2802.477
[2M+H+] (5); C190H210N6Si8Na: 2824.471: found: 2824.436 [2M+Na+]
(4).[82]
CCDC-913133 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
ꢀ
3.349 (s, 2H; C CH), 1.087–1.074 (m, 42H; terminal Ph 2/2’-Si(CH-
(CH3)2)3), 1.040–1.037 ppm (m, 42H; terminal Ph 5/5’-Si(CH
N
13C NMR (CDCl3, 100.61 MHz, 248C): d=154.8, 154.7, 151.6, 139.3,
137.8, 136.6, 135.8, 132.0, 131.9, 128.5, 128.3, 126.3, 125.8, 125.5, 125.4,
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
125.2, 124.6, 121.6, 120.4, 120.3, 103.9 ( C ), 103.6 ( C ), 98.07 ( C ),
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ ꢀ
Acknowledgements
98.05 ( C ), 93.5 ( C ), 92.3 ( C ), 91.4 ( C ), 90.8 ( C ), 82.9 ( C ),
ꢁ ꢀ
81.3 ( C ), 18.64 (CH
N
G
ACHTUGNRTNE(NUNG CH3)2),
11.2 ppm (CHACHTUNGTRENNUNG
The Centre National de la Recherche Scientifique and the Institut
Charles Sadron are acknowledged for financial support (P.N.W.B). Dr.
Raymonde Baltenweck-Guyot and Romain Carriere of the Service de
Spectrometrie de Masse, Inst. de Chimie, UdS, as well as Dr. Jean-Marc
Strub and Dr. Alain Van Dorsselaer of the Laboratoire de Spectromꢀtrie
de Masse BioOrganique, Dꢀpartement des Sciences Analytiques IPHC,
ꢀ
(s), 2155 (m) (C C), 1585 (m), 1573 (m), 1542 (m), 1489 (s), 1462 (s),
1445 (s), 1382 (m), 1367 (m), 1253 (m), 1236 (m), 1186 (m), 1073 (m),
1019 (m), 996 (m), 902 (m), 882 (s), 849 (s), 817 (s), 790 (s), 753 (vs), 738
(m), 676 (vs), 659 (vs), 618 (m), 586 cmꢁ1 (s); UV/Vis (n-heptane): lmax
(e)=266 (1.4ꢃ105), 277 (1.4ꢃ105), 316 (8.9ꢃ104), 354 nm (6.4ꢃ
Chem. Eur. J. 2013, 19, 12336 – 12349
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12347

P. N. W. Baxter et al.
ECPM, UdS, are thanked for the MALDI and ESI mass spectrometric
measurements. Dr. Benoꢄt Heinrich of the IPCMS, Strasbourg, is thanked
for the use of and assistance with the VT UV/Vis setup, and Y. Guilbert
of the ICS is thanked for the TGA measurements. Jacques Druz and
Marie-France Peguet of the Service de Microanalyse, ICS, are thanked
for the elemental analyses.
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ACTHNUTRGNEUNG
was found to dissociate to give a similar [M+H+]/[2M+H+] peak in-
tensity ratio as that of the initial MS. However, the [2M+H+] peak
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Received: December 22, 2012
Published online: July 23, 2013
Chem. Eur. J. 2013, 19, 12336 – 12349
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12349