Angewandte
Communications
Chemie
[28]
internal reference tetramethylsilane (TMS). The changes
observed in relative volume, V /VTMS, upon variation of
1
b
concentration and temperature [36 Æ 2 (1 mm, 258C), 50 Æ 2
(
15 mm, 258C), 56 Æ 2 (30 mm, 258C), 112 Æ 9 (30 mm,
À108C)], are consistent with pCp self-association and the
trends discussed above.
A dramatic change in solution behavior is observed when
(
Æ)-1b is dissolved in a less-polar hydrocarbon solvent (e.g.,
cyclohexane or methylcyclohexane). Even at mm concentra-
tions, the solutions are considerably viscous (see the movie in
the Supporting Information), a hallmark macroscopic prop-
[
29]
erty of H-bonded supramolecular polymers , thus suggest-
ing a significant boost in association strength. While this is
expected from the standpoint of H-bonding, it must also mean
that the pCp p-stacking is, at the very least, accommodated
under these conditions. The solution spectroscopic data is
1
consistent with the macroscopic result. H NMR analysis in
[
D ]cyclohexane, for example, shows concentration invariant
12
chemical shifts for (Æ)-1b between 0.1–1.0 mm. The result
confirms a persistent aggregated state at even this level of
dilution, further evidenced by slow monomer exchange on the
NMR timescale which affords two broad and downfield
(dHb = 9.6 ppm; dHc = 8.8 ppm) NH resonances. Meanwhile,
the amide NH resonance of (Æ)-6b remains upfield (d =
Figure 4. Solution-phase self-assembly of (Æ)-1b versus non-assem-
bling pCp-4-monocarboxamide comparator (Æ)-6b. [a] IR spectrum of
À1
the NÀH stretch region (3550–3050 cm ) of (Æ)-1b (solid lines) and
À3
(
Æ)-6b (dashed lines) in the solid state (black lines), at 30ꢁ10 m in
À3
chloroform (red lines), and at 30ꢁ10 m in cyclohexane (abbreviated
cyhex; blue lines). NÀH stretch energies are provided in the text.
5
.2 ppm, 30 mm in [D ]cyclohexane; see Figures S24 and
12
S25). FT-IR data in cyclohexane is fully consistent with the
[
b] Graphical representation of DOSY data obtained for (Æ)-1b at
1
H NMR data, thus showing exclusively two H-bonded NÀH
variable concentrations and temperatures in CDCl , thus indicating
3
stretches for (Æ)-1b and one solvent-exposed NÀH stretch for
(Æ)-6b (Figure 4a). Worth noting, while the NÀH stretch of
a decrease in the diffusion coefficient and increase of hydrodynamic
radius relative to a TMS standard upon aggregation. c) Temperature-
dependent absorption spectra (corrected for temperature-dependent
(
(
(
Æ)-1b associated
with intramolecular H-bonding
3063 cm ) in cyclohexane is comparable to chloroform
À6
solvent density changes) of (Æ)-1b (40ꢁ10 m in MCH) collected
À1
with a constant cooling rate of 18C per minute. Arrows indicate the
À1
3068 cm ), the intermolecularly H-bonded NH shows
direction of decreasing temperature.
À1
a stretch at lower energy (3222 cm ) consistent with stronger
H-bonding in this solvent.
4
À1
Finally, the larger K value (> 10 m , estimated from the
el
1
ures S11–S15). In the solid state, a predominant and broad NÀ
H NMR dilution data) provides the opportunity to evaluate
À1
H stretch is observed for (Æ)-6b at 3276 cm (associated with
assembly by UV/Vis spectroscopy, which can report on both
inter- and intramolecular pCp electronic interactions. Quite
preliminary frontier MO analysis of [(R )-1a ] in the gas
intermolecular H-bonding), while two broad NÀH stretches
À1
[
ascribable to weaker, intermolecular (3215 cm ) and stron-
p
anti 2
À1
ger, intramolecular (3052 cm ) H-bonding] are found for
phase (DFT M06-2X/6-31 + G*) shows that through-space
(intermonomer) orbital delocalization could accompany
dimerization (see Figure S6). Absorption spectra of (Æ)-1b
(40 mm) recorded at various temperatures (15–908C) in
methylcyclohexane (MCH) show a hypsochromic shift of
the higher energy (lmax = 208 nm at 908C; 204 nm at 158C)
(
Æ)-1b. At 30 mm in CHCl , (Æ)-6b shows only a sharp,
3
À1
solvent-exposed NÀH stretch (3442 cm ), a resonance also
À1
found for (Æ)-1b (3437 cm ). Two additional broad NÀH
À1
stretches are observed for (Æ)-1b (3257 and 3068 cm ), thus
mirroring the solid-state behavior and consistent with H-bond
association at these concentrations.
transition and bathochromic shift of the lower energy (lmax
=
The equilibrium constant for (Æ)-1b is relatively small in
chloroform, but reasonable given the number of intermono-
mer H-bonding interactions, and a bit larger than some
280 nm at 908C; 287 nm at 158C) transition, with clean
isosbestic points, upon cooling (aggregation; Figure 4c). The
former absorption presumably originates from a p–p*
[
27]
À1
[30]
BTAs
(K = 15 Æ 5m ). Indeed, DOSY NMR measure-
transition involving H-aggregated benzene decks,
while
ments in CDCl (Figure 4b) could be used to verify supra-
the latter cyclophane band reports on the distance between
3
[
31]
molecular growth through translational diffusion coefficients
and the deformation of the pCp benzene rings, deformation
which occurs upon assembly. A bathochromic shift and
attenuated intensity of the cyclophane band has been
reported as paracyclophane decks are positioned closer in
(
D) obtained at various concentrations and temperatures. For
À10
2
À1
(
1
Æ)-1b at 258C, D (10 m s ) decreases from 6.7 Æ 0.1 at
mm, to 5.5 Æ 0.1 at 15 mm, to 5.1 Æ 0.1 at 30 mm. The latter
[
32]
value expectedly decreases further (to 2.7 Æ 0.1) upon low-
ering the temperature to À108C. The trends, consistent with
pCp self-assembly, are alternatively expressed through esti-
mated hydrodynamic volumes relative to a non-aggregating
space. A similar trend of the cyclophane band is observed
for (Æ)-1b (40 mm) when comparing chloroform (l
=
max
283 nm) and methylcyclohexane (l = 287 nm) spectra,
max
where the latter solvent is able to support stronger hydrogen
4
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Angew. Chem. Int. Ed. 2016, 55, 1 – 7
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