Sulfoxide-Induced Homochiral Folding of ortho-Phenylene Ethynylenes (o-OPEs) by Silver(I) Templating: Structure and Chiroptical Properties

Article abstract of DOI:10.1002/chem.201704897

A new family of homochiral silver complexes based on carbophilic interactions with ortho-phenylene ethynylene (o-OPE) scaffolds containing up to two silver atoms are described. These compounds represent a unique class of complexes with chirality at the me

Full text of DOI:10.1002/chem.201704897

A Journal of
Accepted Article
Title: Sulfoxide-Induced Homochiral Folding of o-OPEs by Ag(I)
Templating: Structure and Chiroptical Properties
Authors: Juan M. Cuerva, Sandra Resa, Delia Miguel, Santiago
Guisán-Ceinos, Giuseppe Mazzeo, Duane Choquesillo-
Lazarte, Sergio Abbate, Luis Crovetto, Diego J. Cárdenas,
M. Carmen Carreño, María Ribagorda, Giovanna Longhi,
Antonio J. Mota, and Luis Álvarez de Cenfuegos
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To be cited as: Chem. Eur. J. 10.1002/chem.201704897
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Sulfoxide-Induced Homochiral Folding of o-OPEs by AgI
Templating: Structure and Chiroptical Properties
Sandra Resa,[a] Delia Miguel,[b] Santiago Guisán-Ceinos,[c] Giuseppe Mazzeo,[d] Duane Choquesillo-
Lazarte,[e] Sergio Abbate,[d] Luis Crovetto,[b] Diego J. Cárdenas,[c] M. Carmen Carreño,[c] María
Ribagorda,[c] Giovanna Longhi,[d] Antonio J. Mota,[f] Luis Álvarez de Cienfuegos,*[a] and Juan M.
Cuerva,*[a]
Dedication ((optional))
Abstract: A new family of homochiral silver complexes based on
carbophilic interactions with ortho-phenyleneethynylene (o-OPE)
scaffolds containing up to two silver atoms is described. These
compounds represent a unique class of complexes with chirality at
the metal. Chiral induction is based on the inclusion of chiral
sulfoxides which allow an efficient transfer of chirality to the helically
folded o-OPE, leading to CPL and VCD active compounds. In the
presence of AgI cation carbophilic interactions dominate, promoting
helical structures with a defined helicity. This is one of the very
scarce examples of the use of such interactions in the attractive field
of abiotic foldamers. The switching event has been extensively
studied using different chiroptical techniques including CD, CPL and
VCD, also representing one of the few CPL switches described in
literature.
shape as potential nanopores is a challenging task although
they could lead to important applications. Structures able to
include in their core very different species depending on the
specific functionalisation of the cavity are known.[2] Notable
examples correspond to the inclusion of 1-D water chains,[3]
different anions[4] and cations,[5] and chiral organic molecules.[6]
Moreover, the corresponding chirality is a useful feature in
different fields, such as chiral materials and asymmetric
catalysis. Among the corresponding chiroptical properties, the
ability of organic monomolecular emitters to produce efficient
chiral emission (circularly polarized luminescence, CPL) [7-9] has
been less explored, despite its huge interest for advanced
optoelectronic applications.[10] CPL switches[11] capable of
modulating their emission in the presence of external stimuli, are
especially appealing. To this end, such systems must be flexible
enough to accommodate different P/M helicities in a reversible
manner. Moreover, from a structural point of view, helical
Introduction
structures presenting
a conductive backbone have been
suggested as spin filters based on chiral-induced spin selectivity
(CISS) effect. [12]
Synthesis of new enantiopure abiotic helical nanostructures is
an intense research field owing to the expected properties
derived from their intrinsic dissymmetry.[1] For example,
designing homochiral helical systems with a precise size and
Within this context, the study of oligomeric structures able to
adopt well-defined secondary conformations is a research field
of growing interest. In particular, all-conjugated aryl-
ethynyloligomers have been extensively used as hosts of
different species.[13] For instance, meta[14] and ortho-
phenyleneethynylenes (o-OPEs) [15] are able to reversibly fold
into helical arrangements[16-18] even in a chiral way when chiral
side chains[19] or chiral subunits exist in the backbone,[20] or in
the presence of chiral guests.[21]
We recently showed that folded o-OPE-based nanopores can
be filled with AgI cations through carbophilic interactions with
the alkynes,[22] to give a new class of metallofoldamers (Figure 1,
acyclic non-chiral organic emitters).[23] We also synthesized
helical stapled o-OPEs[24] with an enantiopure 2,3-
dihydroxybutanediether moiety, which showed excellent CPL
[a]
S. Resa, L. Álvarez de Cienfuegos, J. M. Cuerva
Department of Organic Chemistry
University of Granada, UGR
C. U. Fuentenueva, 18071 Granada, Spain
E-mail: jmcuerva@ugr.es, lac@ugr.es
D. Miguel, L. Crovetto,[b]
Department of Physical Chemistry
University of Granada, UGR
Cartuja Campus, 18071 Granada, Spain
S. Guisán-Ceinos, D. J. Cárdenas, M. C. Carreño, M. Ribagorda,c
Department of Organic Chemistry t
Universidad Autónoma de Madrid
Cantoblanco, 28049 Madrid, Spain
G. Mazzeo, S. Abbate, G. Longhi
Dipartimento di Medicina e Traslazionale
Università di Brescia
[b]
[c]
[d]
[e]
[f]
responses (Figure 1, cyclic chiral enantiopure organic emitters).
[25]
These homochiral helically folded structures suffer a change in
the CPL emission in the presence of AgI, exemplifying the first
case of CPL active small organic molecular emitters whose
response can be modulated by carbophilic interactions in a
reversible manner. These results showed the potential of OPEs
as efficient and versatile CPL emitters. Nevertheless,
enantiopure OPE foldamers having CPL emission have not
been reported up to date. To this end a chiral inducer must be
placed in the OPE. In this sense, sulfoxides are very efficient
chiral inducers and have been widely used in asymmetric
Viale Europa 11, 25123 Brescia, Italy
D. Choquesillo-Lazarte
Laboratorio de Estudios Cristalográficos
Instituto Andaluz de Ciencias de la Tierra, CSIC-UGR
Armilla, Granada, Spain.
A. J. Mota
Department of Inorganic Chemistry
University of Granada, UGR
C. U. Fuentenueva, 18071 Granada, Spain
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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synthesis. [26] This group has already been used to control, in a
very efficient manner, the conformation and configuration of
sulfinyl containing azobenzene photoswitches.[27] In that case, a
p-tolylsulfoxide situated in an aromatic ring adopted a fixed
conformation both in E and Z isomers of azobenzenes being
able to transfer the central chirality to the azobenzene core.[27]
also observed in the vibrational wavelengths range,[29] using
vibrational circular dichroism (VCD).[30]
Results and Discussion
The known compound (S)-1-bromo-2,5-dimethoxyphenyl-4-(p-
tolylsulfinyl)benzene(S)-A,[27-28] was chosen as the homochiral
starting material due to its availability and its known ability to
freeze a sulfoxide conformation thanks to the vicinal disposition
of methoxy and sulfinyl groups at the aryl ring. Sonogashira
reaction between (S)-A and trimethylsilylacetylene, using
catalytic amounts of Pd(CH3CN)2Cl2, tBu3P•HBF4 and CuI (3
mol %) in i-Pr2NH/THF (1:1) as solvent, was followed by a
desilylation to give p-tolylsulfinyl aryl substituted acetylene
derivative (S)-I in 75 % isolated yield (Scheme 1). This
intermediate was then used in the synthesis of acetylene
derivative (S,S)-1 and series of o-OPEs (S,S)-2 to (S,S)-8, with
up to eight o-phenyleneethynylene units (Scheme 1).[31-32]
Figure 1. Working hypothesis.
On this basis, we expected an effective transfer of chirality from
the stereogenic sulfur of chiral sulfoxides to a helically folded o-
OPE. Herein, we present the synthesis of enantiopure sulfinyl
substituted o-OPE foldamers and metallofoldamers and a study
of the structural and chiroptical properties of the resulting
systems (Figure 1, acyclic chiral enantiopure organic emitters).
We could evidence that they behave as a new class of very
scarce CPL active switches, based in this case on AgI
carbophilic interactions. Moreover, the synthetized complexes
were able to include more than one silver atom in their structure
depending on the length of the oligomer. This fact would
generate simple molecules that, in principle, could have two
conductive pathways: an axial one due to the presence of AgI
cations and a helical one due to the organic foldamer. The
switching event could be followed by NMR, ECD, fluorescence,
less common CPL spectroscopy and also by X-ray
crystallography. Remarkably, the switching process could be
Scheme 1. Synthesis of (S,S)-1 to (S,S)-8. Reaction conditions: i)
Pd(CH3CN)2Cl2 (0.03 equiv), tBu3P•HBF4 (0.06 eq), CuI (0.03 equiv),
trimethylsylil acetylene (2 equiv) in i-Pr2NH/THF, RT. ii) Bu4NF, THF. iii)
Pd(CH3CN)2Cl2 (0.03 equiv), tBu3P•HBF4 (0.06 equiv), CuI (0.03 equiv), III-VII
(1 equiv), (S)-I (2 equiv) in i-Pr2NH/THF, 60 ºC. iv) Pd(PPh3)2Cl2 (0.05 equiv),
CuI (0.1 equiv), diiodobenzene (1 equiv),(S)-I (2 equiv) in Et3N/THF.
Photophysical characterization of (S,S)-1 to (S,S)-8. To
evaluate the changes associated with folding in the OPEs
containing the enantiopure (S)-sulfoxides, we recorded the
electronic spectra of compounds (S,S)-1 to (S,S)-8 in CH3CN
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(common helicogenic solvent, Figure S1) and CH2Cl2 (non-
helicogenic solvent, Figure S2). Such solvents were selected
syn conformation shown in Figure 3, may predominate in
solution.
based on the previous behavior described for o- and m-OPEs.
[33]
Compound (S,S)-1 was studied as a model, owing to the
presence of two sulfoxides as the sole chiral elements without
any potential interference of emerging helical structures. CD
spectra of (S,S)-1 to (S,S)-8 in CH2Cl2 are shown in Figure 2.
Two main absorption bands appeared in the UV-vis, one in the
region around 260-280 nm, where the sulfoxide absorbs, and a
band at 330-380 nm ( transition) owing to the aromatic
(phenylene-ethynylene) chromophore. The CD spectra of (S,S)-
1-8 also showed a common positive band at = 260-270 nm
corresponding to the (S)-configured sulfoxides slightly red
shifted in the more conjugated (S,S)-3-8. Slight differences could
also be observed in the other bands for compounds (S,S)-1 and
(S,S)-2 when compared with (S,S)-3-8. Derivatives(S,S)-1 and
(S,S)-2 present a peak with a negative Cotton effect, which was
assigned to the aromatic groups, at  = 330 nm and 370 nm
respectively. The appearance of this band suggested that the
sulfoxide is inducing a chiral perturbation in the aromatic system.
Higher oligomers, (S,S)-3 to (S,S)-8, where folded
conformations are possible, showed two negative bands at ca.
320 nm and ca. 360 nm, also corresponding to the aromatic
absorptions. The intensity of these chiral peaks increases with
the number of phenyleneethynylene units (Δε = -25 to -55),
being the largest for compound (S,S)-8 (Δε = -55). Thus, the
increasing number of phenyleneethynylene units seemed to
produce
a significant folding of these systems giving a
chiroptical response that could be due to the presence of
helically chiral foldamers, even in a non-helicogenic solvent. We
used dimensionless dissymmetry factor gabs for comparison
between the different structures, showing that their values do not
increase linearly with the number of alkynes (See Figure S3).
This fact may reflect a conformational disorder in solution, which
increases with the number of alkynes. Moreover, gabs values are
relatively low (0.5-1 x 10-3) in comparison to the usually high
dissymmetric factor found in helical structures, thus supporting
such conformational disorder in solution. In CH3CN, the
recorded CD and UV-spectra were very similar to those obtained
in CH2Cl2 (See Figures S1-S4), also suggesting that for these
structures either the solvophobic promoted folding is minimum or
that they present an intrinsic tendency to fold in solution
independently of the nature of the solvent.
Figure 2. (a) Experimental CD spectra of compounds (S,S)-1 to (S,S)-8. (b)
Calculated CD spectra of compounds of (S,S)-1anti, (S,S)-1syn, (S,S)-3 with
two CH2Cl2 molecules, (S,S)-4 with two CH2Cl2 molecules, (S,S)-5 with two
CH2Cl2 molecules, (S,S)-6, and (S,S)-8. Spectra of compounds (S,S)-3-(S,S)-8
have been offset 70 nm.
Calculations of compound (S,S)-2 gave a series of different
conformations, which are very close in energy and easily
interconvertible.
Consequently
many
relative
minima
conformations must coexist in solution. This conformational
flexibility was experimentally confirmed in the corresponding
Time Resolved Emission Spectroscopy (TRES) experiments
(details can be found below). Moreover, the calculated CD signal
of (S,S)-2 was highly dependent on the particular conformation,
with no apparent relationship with the emerging helicity.
Therefore, the structure in solution for (S,S)-2 is uncertain.
To obtain more information on these highly flexible systems we
carried out (TD)-DFT-based theoretical calculations.[32] Despite
the increasingly complex conformational space of these
structures, we could obtain some representative minima, which
could explain the experimental data. For the simplest compound
For higher oligomers the study is much more complex owing to
the presence of unfolded, partially folded and folded conformers
with different P or M helicities. Moreover, the p-tolyl groups of
the chiral sulfoxides can adopt different spatial dispositions: (a)
open conformation: the p-tolyl groups pointing outside the helical
structure: or (b) compact conformation: the p-tolyl groups placing
along the axis of the helix. In any case, experimental CD
showed a peak with a negative Cotton effect at the longest
wavelength. Theoretical calculations, setting CH2Cl2 as solvent,
(S,S)-1,
calculations
gave
two
minimum
energy
conformations,[34] corresponding to the anti and syn disposition
of the tolyl groups:(S,S)-1anti and (S,S)-1syn, shown in Figure
3. The syn conformer is slightly favored (−0.6 Kcal·mol−1) over
the anti. They also present opposite theoretical CD spectra
(Figure 2b). Comparison of experimental and calculated CDs of
(S,S)-1 at the longest wavelength suggests that the most stable
[32]
revealed small differences in energy between the different
conformations, thus suggesting that both P/M helical epimers
can coexist in solution. Nevertheless, (S,S,M) configuration with
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a compact conformation of the p-tolylsulfinyl group is slightly
[39] Recently, Itami et al. reported a similar behavior for related
cyclophenyleneethynylenes. [40]
favored, with the exception of (S,S,M)-6, in which open
[32]
conformation is the only possible one (See Figure 3).
This
Time Resolved Emission Spectroscopy (TRES) is a useful
technique to gather information about the fluorescent species
involved in the emission process as long as their interconversion
occurs in a time scale similar or higher than the emission lifetime.
[32] Within this limitation, we observed only one lifetime (1 = 1.37
ns, CH2Cl2) for (S,S)-1, which suggested that all the
conformations are equilibrated at room temperature. For (S,S)-2
and (S,S)-3 the emission spectra in CH2Cl2 and CH3CN
presented three different time resolved emissions (1, 2 and 3),
preference for the (S,S,M) folded conformations qualitatively
matches with the experimental negative bands appearing at
340-370 nm.[35-36]
which are very similar in shape, suggesting
a dynamic
interconversion between them (Figures S9 y S10). These results
are in agreement with the expected complex conformational
behavior predicted by calculations. Compound (S,S)-3 is able to
fold in solution creating helical P/M epimers and also unfolded
conformations thus justifying the three different time-resolved
emissions with different shapes (Figure S10), which could be
related with three structurally different excited states (roughly
unfolded, P-folded and M-folded states). These states also
represent the average emission of fast conformational equilibria.
Binding properties of (S,S)-1 to (S,S)-8 with AgI cation. As
we described previously, AgI cation is able to selectively bind to
the alkynes presented in o-OPE systems, thus promoting a
folding process through carbophilic interactions. In structures
(S,S)-1-8, the enantiopure sulfoxides and methoxy groups of the
o-OPE structures offer additional coordination sites. Taking this
variable into consideration, we studied the effect of the presence
of AgI cation in the structure and the chiroptical properties of o-
OPEs (S,S)-1-8. The products were dissolved in a 9:1 mixture of
CH2Cl2:acetone in the presence of AgBF4, having a non-
coordinating counteranion. The resulting absorbance spectra are
shown in Figure S5 and the CD spectra are depicted in Figure 4.
For (S,S)-1 no significant changes in the CD spectrum could be
observed after AgI addition. This result points out that the
chiroptical response of (S,S)-1 is not affected upon AgI addition.
Nevertheless, the change in the chemical shift of the 13C NMR
alkyne signal suggested that the metal binds to the alkyne.[32] On
the other hand, dramatic changes can be clearly observed in the
CD spectra (both in shape and sign) upon AgI addition to
compounds (S,S)-2 to (S,S)-8 (Figure 4a). The appearance of a
positive Cotton effect at ca. 390 nm indicated a change in the
helicity after coordination with AgI cation. Taking into account
this CD band at the highest  we could deduce a preferred
opposite P helicity after coordination with AgI cation. Molar
circular dichroism values () at 390 nm monotonically increased
from (S,S)-2 (= +26) to a maximum for (S,S)-6 (= +144),
which could be related with an increase in the magnetic dipole
transition moment owing to the created helicity.
Figure 3. Side and top views of representative more stable calculated
conformers of compounds (S,S)-1, (S,S)-3 to (S,S)-8. Solvent molecules
(CH2Cl2) have been removed for clarity (see SI).
The fluorescent properties of (S,S)-1 to (S,S)-8 in different
organic solvents, including CH2Cl2 and CH3CN, were also
evaluated (see Figure S8). All of them were fluorescent with
quantum yields highly dependent on the solvent but ranging
from 0.56 for (S,S)-1 to 0.20 for (S,S)-8 in CH2Cl2. Besides, the
photochemical stability of these compounds was highly
dependent on the number of alkynes. o-OPEs with up to three
Titration experiments using CD variations revealed an initial
binding event with increasing binding constants: K(S S = 79.5 ±
alkynes,(S,S)-1 to (S,S)-3, were stable. However, beyond that
,
)-2
[32]
number they become unstable after intense irradiation.
In
4% M-1, K(S S = 4.91x103 ± 3% M-1, K(S S = 9.56x104 ± 4%M-1,
,
)-3
, )-4
those cases, the proximity of two alkynes in the folded structure
could facilitate a photochemically induced [2+2] cycloaddition, [37]
being this the origin of this lack of stability. The resulting
cyclobutadienes[38] could not be isolated due to their instability.
K(S S)-5 = 2.80x105 ± 10.6% M-1 and K(S S)-6 = 1.19x106 ± 25% M-1.
,
,
[41]
Accordingly with this finding, a trend of the binding constant
with the number of alkynes can be observed (See Figure S19).
Despite the minor variations in the CD signals of model
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compound (S,S)-1 a binding constant value of K(S,S)-1 = 4.09x104
±
40% M-1 could be estimated, which is unusually high
compared to those obtained in the other related systems. On the
other hand, compound (S,S)-3 showed a very intense chiroptical
response in the presence of an excess of AgI cation, (= +125
at 390 nm after addition of 215 equiv of AgI). In this case the CD
titration experiments also showed the presence of two additional
different binding constants, 2.06x102 ± 15% M-1, and 1.26x103 ±
6% M-1, assigned to (S,S)-3:Ag3 complex.
Figure 5. Side and top views of X-Ray structure of (S,S)-8:Ag2. Solvent
molecules have been removed for clarity.
o-OPE (S, S)-8, with ten potential binding sites (eight alkynes
and two of the four OMe groups) becomes insoluble in the
presence of an excess AgI. We were able to roughly obtain the
corresponding CD spectra and an estimation of constant for the
first binding event (K(S S)-8= 8.6x105± 41%M-1). Fortunately, a
,
suitable crystal for X-ray diffraction analysis was obtained for
(S,S)-8:Ag2 complex. As it can be seen in Figure 5, o-OPE
(S,S)-8 is able to accomodate two AgI cations in a chiral helical
environment with a AgI-AgI distance about 5.98 Å. In this
situation, the number of binding sites seems to be optimal.
Moreover, in the X-ray structure (Figure 5)
a compact
conformation, with the p-tolylsulfinyl groups situated along the
axis of the helix is clearly observed. In such disposition, the
electrostatic repulsions between the SO and the vicinal OMe
groups are minimized. Interestingly, the configuration of the helix
in the solid state is M, which is the opposite to that suggested by
CD in solution. This finding, together with the decreasing
intensity of the CD signal during the second binding event for
(S,S)-6, strongly suggested that the helicity for mono and
bimetallic structures is not the same.
Figure 4. Experimental CD spectra of compounds (S,S)-1 to (S,S)-8 in
presence (a) of an excess of AgI and (b) calculated CDs of compounds of
(S,S)-1anti:Ag, (S,S)-3:Ag, (S,S)-3:Ag3, (S,S)-4:Ag, (S,S)-5:Ag, (S,S)-6:Ag
and (S,S)-6:Ag2.
Accordingly with CD spectra, significant changes were also
evident in 1H and 13C NMR chemical shifts. A representative
example is shown in Figure 6 for compound (S,S)-5. As it can be
seen, the shielding of the1H-NMR signals corresponding to the
aromatic hydrogens in both 1,4-dimethoxyaryl groups (see
arrows in Figure 6) as well as the shielding of one of the OMe
signals strongly suggested the existence of folded structures.
13C NMR spectra showed variations in the chemical shifts of the
acetylenic carbons due to the coordination to AgI. Accordingly,
the NMR signals of ligands (S,S)-3 and (S,S)-6 are quite
complex in the presence of less than 3 equiv of AgI. Probably, at
such concentrations (0.01 M) the signals were due to a mixture
of mono-and polymetallic complexes.
Compounds (S,S)-5, and (S,S)-6 present seven and eight
potential binding sites (five/six alkynes and two of the four OMe
groups), respectively. In principle, under this situation just one
AgI cation is unable to simultaneously bind to all of them, and a
second AgI cation may take part in the interaction. However,
electrostatic repulsions could result in an unfavorable bimetallic
coordination. The titration experiments of such compounds only
showed a second binding event for (S,S)-6 with a modest
associated binding constant (K2(
= 60.3 ±
16% M-1),
S S
¸ )-6
suggesting that the second coordination event was not very
favorable despite the number of available binding sites. It is
worth noting that this second binding event is related with a
decrease in the CD signal. [42]
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Compound (S,S)-3 was also a CPL active derivative with a
better glum value around 1x10-3. This value is similar to the gabs
previously obtained for that compound (see SI), showing that
chirality is retained in the excited state.[43] Remarkably, in the
presence of an excess of AgI cation[44] the CPL signal of (S,S)-3
is inverted (Figure 7b), with a glum value of 5x10-3. This is a
significant value for a small organic-based monomolecular
emitter, even though the fluorescence is quenched by Ag.
Moreover, the initial signal can be recovered by addition of
CH3CN and therefore this homochiral o-OPE can be considered
a new class of CPL switch based on a folding process with
interesting glum values.
Figure 6. Representative 1H-NMR spectra of compounds (S,S,M)-5 and
(S,S,P)-5:Ag.
CPL and VCD studies. Owing to the problems encountered in
the photochemical stability of compounds (S,S)-4 to (S,S)-8,
reliable CPL studies were not satisfactory, so we limited our
study to compounds (S,S)-1 to (S,S)-3. Before complexation
with AgI cation, o-OPEs(S,S)-1 and (S,S)-2 were shown to be
modest CPL active compounds. After AgI addition, their
corresponding glum factor increased from roughly 3x10-4 to 6x10-4.
Figure 8. (a) Experimental VCD spectra of compounds (S,S)-3, (S,S)-5 and
(S,S)-6 in absence and presence of AgI salt. (b) Comparison of experimental
and calculated spectra of AgI complexes of compounds (S,S)-3, (S,S)-5 and
(S,S)-6. In all graphs an offset was employed in Y-axis scales.
On the other hand, VCD is very useful for conformational
analysis, due to the fact that vibrational chiroptical response can
reflect local differences and may be heavily dependent on
hindered torsions.[45] Therefore, VCD gives relevant structural
information about the sample in solution, which in many cases
cannot be inferred by other spectroscopic techniques. [46]
In the representative studied cases (S,S)-3, (S,S)-5, and (S,S)-6,
Figure 7. (a) Fluorescence and (b) CPL spectra of compound (S,S)-3 in
CH2Cl2 solutions in absence of AgI (blue line), in the presence of an excess of
AgI (burgundy line, signal has been multiplied by 10 to show it in the graph)
and after CH3CN addition (green line)
[47]
the corresponding VCD spectra should be sensitive to the
open and compact spatial disposition of the p-tolyl moieties of
the sulfoxides. This fact was relevant to understand the behavior
of the ligands in solution. It is also worth noting the observed
inversion of some signals in the presence of AgI, which could be
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explained considering
a
significant change in the spatial
agreement with the observed chiroptical response of (S,S)-3 in
the presence of an excess of AgI. Moreover, experimental and
theoretical VCD is in this case in agreement with such spatial
disposition of the sulfoxides, confirming the sign and shape of
the observed bands. As it can be also seen in Figure 8b,
mononuclear silver complex (navy blue line) does not invert the
sign of the signal at 1211 cm-1 as trinuclear complex does
(burgundy line). Bi- (pink line) and trimetallic (dark yellow line)
ones show the suitable set of signals with the right signs,
strongly supporting the existence of such complexes in solution.
Simulation of VCD of free (S,S)-3 is more difficult taking into
account its conformational freedom. Nevertheless simulated
VCD of representative conformations of (S,S)-3 showed a
reasonable fitting with the experimental one. [32]
disposition of such p-tolyl groups before and after AgI addition.
In particular, in the presence of AgI a shift and intensification of
the band at about 1490 cm-1 due to C=C ring stretching vibration
(shift from 1492 to 1485 cm-1) was observed in the absorption
spectra (for representative examples see Figure S7). In VCD
spectra, all samples without AgI present a weak doublet at 1494-
1487 cm-1(Figure 8a). Upon salt addition, all cases showed a
change in this region, presenting an opposite intense doublet
1489-1482 cm-1. However, the higher energy component at 1489
cm-1 loses intensity for longer oligomers. Especially relevant was
the case of compound (S,S)-3,which in the presence of Ag
presented additional VCD bands, quite intense and opposite in
sign to the bands observed without AgI (see bands at ca 1386
cm-1 and 1212 cm-1). Interestingly, the sign inversion of the latter
bands was not observed in the other cases. These results were
pointing out the different disposition of the p-tolyl groups in
(S,S)-3:Ag3 complex compared to the other ones. These
experimental results represent one of the very scarce examples
of VCD switches reported up to date. [48]
Theoretical studies of o-OPE: AgI complexes To better
understand all these observations, we carried out theoretical
calculations for the AgI complexes of o-OPE ligands (S,S)-1 to
(S,S)-8. For (S,S)-1, theoretical calculations revealed that the
more favorable situation corresponded to silver coordinated
between the alkyne and the OMe (Figure 9), where both -OMe
groups are oriented to the Ag+ ion, leading to a highly stabilizing
interaction. The system in such a disposition was adopting an
anti-conformation. The calculated CD for (S,S)-1anti:Ag is in
fact very similar in shape and sign to that obtained for
uncomplexed (S,S)-1syn. Therefore, theoretical calculations
supported the experimental observation that after AgI
coordination the CD sign and shape of (S,S)-1 is retained
despite the fact that the syn/anti conformation of the sulfoxides
is reversed.
Compound (S,S)-2 was able to coordinate to AgI through
different binding groups adopting different conformations of both
sulfinyl groups. Nevertheless, as in the case of this compound
without silver, none of all the calculated structures yielded any
simulated VCD and ECD able to fit reasonably well with the
experimental ones. This complex is probably too flexible and
different structures are equilibrated in the time scale of the
experimental measurement.
Theoretical calculations for ligand (S,S)-3 showed a preference
for the coordination of AgI cation with the three alkynes in a
helical environment, as it was previously observed.[22], [25] Such
arrangement forces the coordination with a OMe group, and the
configuration of the helix is inverted from (S,S,M) in the absence
of AgI to the (S,S,P)-3:Ag complex. This inversion is
energetically favored by 10.4 kcal mol-1, and is in agreement
with the observed inversion of the Cotton effect in the
experimental CD spectra (see Figure 4). Under this new
disposition, the sulfoxides offer a perfect environment for
accepting new AgI cations (up to two).This extra binding event
using the sulfoxides arms create a much more rigid structure.
The simulated CD for such bi and trimetallic complexes is also in
Figure 9. Side views of representative calculated structures for AgI complexes
(S,S)-1:Ag to (S,S)-8:Ag.
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The good matching between calculated and experimental VCD
for (S,S)-5:Ag confirmed the monometallic nature of this
complex and the spatial open disposition of the sulfoxide side
groups. A more complex situation resulted for o-OPE (S,S)-6,
which is able to accept two AgI cations. Theoretical calculations
showed that for the bimetallic complex, the most favorable
structure adopted a M helicity resembling the X-ray structure of
(S,S)-8:Ag2 (Figure 5). As a consequence of the M helicity, the
calculated CD sign of the last peak is negative, which is in
agreement with the observed decrease of the CD signal in the
presence of an excess of AgI. On the other hand, The simulation
of monometallic (S,S)-6:Ag was more complex. The best fit was
obtained for the structure in Figure 9, in which the AgI is placed
in the middle of the helix and the p-tolyl substituents are pointing
out in the opposite directions to thus calculated/observed for
(S,S)-6:Ag2 and (S,S)-8:Ag2. Calculated ECD and VCD for the
monometallic complex is in reasonable agreement with the
experimental ones, thus supporting such structure for (S,S)-
6:Ag. The small discrepancies observed in the VCD are
probably due to the presence of minor amounts of (S,S)-6:Ag2 in
the sample. Accordingly, the calculated VCD of (S,S)-6:Ag2 is
not in accordance with the experimental one but could explain
the distortions.
We thank MINECO (FEDER funded grant CTQ2014-53598-R).
Computing Center CINECA, Bologna, Italy, Regione Lombardia
for the LISA Grant No. “ChiPhyto: HPL13POZE1” and the
“Centro de Supercomputación de la UGR” (UGRGRID) are
acknowledged for access to computational facilities. Sandra
Resa thanks the MECD for a FPU fellowship.
Keywords: Supramolecular Chemistry • Chiroptical properties •
Helical structures • Host-guest systems • Nanostructures
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Figure S11). Nevertheless control experiments showed that the
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10.1002/chem.201704897
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A new family of homochiral silver
complexes based on carbophilic
interactions with ortho-
phenyleneethynylene (o-OPE)
scaffolds containing up to two silver
atoms is described
S. Resa, D. Miguel, S. Guisán-Ceinos, G.
Mazzeo, D. Choquesillo-Lazarte, S.
Abbate, L. Crovetto, D. J. Cárdenas, M.
C. Carreño, M. Ribagorda, G. Longhi, A.
J. Mota, L. Álvarez de Cienfuegos,* and
J. M. Cuerva,*
Page No. – Page No.
Sulfoxide-Induced Homochiral
Folding of o-OPEs by AgI
Templating: Structure and Chiroptical
Properties
This article is protected by copyright. All rights reserved.