Visual chiral recognition through enantioselective metallogel collapsing: Synthesis, characterization, and application of platinum-steroid low-molecular-mass gelators

Article abstract of DOI:10.1002/anie.201100620

Seeing is believing: The visual chiral recognition of (R)- and (S)-binap has been realized through an enantioselective breakdown of metallogels prepared from novel aromatic-linker-steroidal-type pincer platinum gelators. Van der Waals interactions, π stacking, and metal-metal bonding are responsible for the aggregation and chiral recognition. Copyright

Full text of DOI:10.1002/anie.201100620

DOI: 10.1002/anie.201100620
Enantiomer Discrimination
Visual Chiral Recognition through Enantioselective Metallogel
Collapsing: Synthesis, Characterization, and Application of
Platinum–Steroid Low-Molecular-Mass Gelators**
Tao Tu,* Weiwei Fang, Xiaoling Bao, Xinbao Li, and Karl Heinz Dꢀtz
A simple protocol to distinguish between left- and right-
handed enantiomers is extremely intriguing and useful in
pharmacology, biology, and even for daily life.[1,2] Recently,
significant progress in chiral recognition has been achieved
based on further development of analytical methodology such
as NMR, UV/Vis, CD, and fluorescence spectroscopy.[3] The
straightforward and convenient visual discrimination of
enantiomers, however, still remains a major challenge in the
design of molecular chiral sensors. Very few examples of
visual chiral recognition by discernible changes in color and
precipitation have been realized.[4,5] In particular, the appli-
cation of molecular gels[6,7] in visual enantioselective discrim-
ination as highly sensitive nanostructured soft matter suscep-
tible to various stimuli has been widely neglected to date.[8]
Only very recently Pu and co-workers reported a chiral
molecular gel prepared from an (R)-binol-terpyridine based
CuII complex as a potential visual chiral sensor for the
enantiomers of amino alcohols.[8a]
As one type of the most studied and efficient low-
molecular-mass gelators (LMMGs),[6] aromatic–linker–ste-
roidal (ALS) molecular gels[6,7] have exhibited potential for
application in light and pH sensors.[6d] To date, however, to
our knowledge no report on their potential application in
visual enantioselective discrimination is available, although
cholesteryl crown ethers have been employed in the chiral
recognition of optically active ammonium ions.[4f] Despite the
major role that organometallic compounds play in synthesis,
catalysis, and material sciences, there are only very few
examples of organometallic compounds that are able to act as
metallogelators,[9] among them only two organometallic ALS
compounds (cholesterol ferrocene[10] and titanocene deriva-
tives),[11] which have been developed as efficient LMMGs.
Recently, we have developed pyridine-bridged palladium
pincer complexes as efficient metallogelators for a variety of
organic solvents and ionic liquids in extremely low gelator
concentration and demonstrated potential applications in
catalysis and solar cells.[12] p stacking and van der Waals
interactions as well as metal–metal bonding are assumed to be
responsible for the assembly and aggregation. Following our
recent interest in developing novel pincer metallacycles and
exploring their potential in soft materials and catalysis,[12,13]
we incorporated a steroidal skeleton to platinum pincer
metallacycles to generate a novel type of ALS metallogelators
and explored their potential in chiral recognition.
Gelation tests of ALS complexes 1 and 2 (Scheme 1) at a
1 wt% gelator concentration (wt/v; ca. 1.1 ꢀ 10À2 m) are
summarized in Table 1. Upon heating to reflux in a variety
of selected organic solvents and cooling to room temperature,
these robust metallogelators entrapped the preferred solvents
to form a thermoreversible jelly type of soft material within a
few hours, which could be stored under ambient condition
without any visible collapse. In contrast to complex 2,
compound 1 turned out to be a less powerful gelator
(entries 1–12, Table 1), which may result from the shorter
linker of gelator 1 inducing a lower flexibility and lower
solubility in most of the solvents applied. ALS metallogela-
tors 1 and 2 efficiently encage polar aprotic solvents such as
chloroform, dichloroethane (DCE), dimethylacetamide
(DMA), and benzonitrile (PhCN, entries 1–10, Table 1).
Complex 2 also immobilized nonpolar aromatic solvents
such as benzene and toluene (entries 11,12, Table 1). The
gelation scope of ALS pincer complexes 1 and 2 is further
extended by addition of a cosolvent. In the presence of
chlorinated solvents, the gelators 1 and 2 also gelate alkanes,
alcohols, and acetic acid (see the Supporting Information).
[*] Dr. T. Tu, W. Fang, X. Bao, X. Li
Shanghai Key Laboratory of Molecular Catalysts and Innovative
Materials, Department of Chemistry, Fudan University
220 Handan Road, 200433, Shanghai (China)
Fax: (+86)21-65102412
E-mail: taotu@fudan.edu.cn
Prof. K. H. Dꢀtz
Kekulꢁ-Institute of Organic Chemistry and Biochemistry
University of Bonn (Germany)
[**] Financial support from the National Natural Science Foundation of
China (No. 20902011), the Shanghai Municipal Science and
Technology Commission (Qimingxing Program No. 10A1400500),
the Shanghai Leading Academic Discipline Project (B108), and
Shanghai Key Laboratory of Molecular Catalysts and Innovative
Materials (2010MCIMKF04), Department of Chemistry, Fudan
University is gratefully acknowledged. We thank Prof. Tao Yi and
Prof. Qi-Lin Zhou for technical support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100620.
Scheme 1. ALS-type PtII pincer complexes 1 and 2.
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Table 1: Gelation ability of ALS platinum pincer complexes 1 and 2 in
various organic solvents.[a]
Entry
Solvent
1
2
Phase
Tg [8C][b]
Phase
Tg [8C][b]
1
2
3
4
5
6
7
8
CHCl3
CH2Cl2
DCE
DMA
DMF
DMSO
PhCN
dioxane
THF
pyridine
benzene
toluene
G
105
/
/
96
/
/
106
/
/
/
S[c]
P[c]
G
PG
G
/
/
65
/
/
124
63
84
65
[d]
G
P[c]
G
[d]
I[c]
I[c]
G[e]
I[c]
I[c]
I[c]
I[c]
I[c]
[d]
G
G[e]
G
9
G
G
G
G
[d]
10
11
12
/
85
91
Figure 1. Selected SEM images of gels formed by ALS platinum pincer
complexes 1 and 2: a) Gel 1/PhCN (2 wt%); b) gel 2/PhCN (2 wt%);
c) gel 2/DCE (1 wt%); d) gel 2/(DCE+EtOH) (1 wt%).
/
/
[a] Gelator concentration: 1 wt%; G gel formed at RT, PG partial gel,
S solution, P precipitate, I insoluble. [b] Determined by the “dropping
ball method”. [c] Gel formed with additional cosolvents (see the
Supporting Information). [d] Tg is near RT, hardly detected by the
“dropping ball method”. [e] Gel formed with 2 wt% gelator concen-
tration at RT.
EtOH and MeOH), a morphology of short and thick helices
was observed (Figure 1d). Similar images were also found
with the gels 2/(CHCl3+EtOH) and 2/(CHCl3+MeOH).
Long, thin helical wool-type fibers were encountered with
gels prepared from 2 with ACN, EtOAc, hexane, acetone, and
HOAc when DCE has been added. These examples clearly
demonstrate that the addition of a cosolvent is suited to
control the morphologies of xerogels 2. Gels prepared with
metallogelator 1 and a mixture of solvents consisted of long,
straight thick fibers. Moreover, straight rods (100–200 nm
wide and 3–5 mm long) characterize the morphologies of the
gels 2/DMSO and 2/DMF (see the Supporting Information).
The various gel network morphologies formed by gelators
1 and 2 inspired us to investigate the principles of the
aggregation of the steroid–ligand–metal complex hybrid in
The gel–sol phase-transition temperatures (Tg) of the
metallogels were determined by the “dropping ball method”
(Table 1).[14] In comparison to its analogue 2, the gel prepared
from the less soluble pincer complex 1 revealed a higher
thermostability, as demonstrated by higher Tg values
observed for the immobilization of PhCN (1068C vs. 638C,
entry 7, Table 1). Moreover, most gels prepared from mix-
tures of solvents exhibited higher Tg values than those found
for the gels resulting from the corresponding single-compo-
nent solvents. This observation suggests a practical way to
increase the thermal stability of gels. The stability of the gel
networks increases with the gelator concentration, as estab-
lished for the series of 1/PhCN and 2/PhCN gels formed with a
gelator concentration varying from 1.8–3.2 wt% (Figure S1 in
the Supporting Information). Again, the gels obtained from 1
revealed higher Tg values than those resulting from 2,
independent of the applied gelator concentration (see the
Supporting Information).
The morphologies of three-dimensional xerogel networks
obtained from metallogelators 1 and 2 with the respective
solvents were characterized by scanning electron microscopy
(SEM). Characteristic images are presented in Figure 1. A
2 wt% gel 1/PhCN revealed ribbon-like Chinese chives leaves
that are approximately 10–50 mm long and up to 3 mm wide
(Figure 1a). In contrast, the image of 2 wt% xerogel 2/PhCN
was quite different and revealed very long (up to 20 mm) and
thin (ca. 10 nm wide) fibers (Figure 1b). Similar character-
istics were found for the gels 1/DMA and 2/DMA. The
different morphologies between the gels obtained from 1 and
2 suggest that a longer linker between the cholesterol group
and the pincer ligand results in a better gelator. Even a low
gelator concentration of 1 wt% is sufficient to create a very
dense three-dimensional network, as demonstrated by gel 2/
DCE (Figure 1c). Similar xerogel networks were also
observed with the gels 2/dioxane and 2/benzene (see the
Supporting Information). When DCE was used as a cosolvent
to assist the gelation of “poorly gelling” solvents (such as
1
the gelation process. Temperature-dependent H NMR spec-
troscopy studies of gel 2/[D6]DMSO (5 wt%) revealed broad
signals of low intensity, indicating strong aggregation as
characteristic for the gel state at ambient temperature
(Figure S38 in the Supporting Information). Upon warming
by 108C steps, the signals of complex 2 steadily sharpened.
When the temperature was raised to 958C, the broad signals
still existed, but their intensity had increased, which supports
a strong aggregation even at elevated temperature. With
1
increasing temperature the H NMR spectral signals of the
steroidal part were shifted upfield (e.g., the signal of the
cholesterol-3H was shifted from d = 4.09 to 3.88 ppm over a
temperature window of 708C), whereas the signals of the
heteroaromatic hydrogen atoms were shifted downfield upon
heating (e.g. the signal at d = 8.94 was shifted to 9.00 ppm).
These phenomena were thermoreversible and thus reflect
both reduced intermolecular van der Waals interactions
between the steroidal part and decreasing p–p interactions
between coplanar heteroaromatic rings upon warming. These
interactions may rationalize the gelator molecular assembly
and further coordination of solvents to form solvates within a
modified network that is capable of trapping additional
solvent, leading to the gel formation (Scheme 2).
To gain further insight into the assembly process, the
structure and dimensions of the xerogel networks of 2/toluene
(5 wt%) were studied by X-ray diffraction (Figure S39 in the
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tion of a chloro ligand in complexes 1 and 2 for a bulky
phosphine was expected to block the assembly process
between the heteroaromatic rings and, thus, to induce a
collapse of the gel network owing to the overcrowded ligand
sphere (Scheme 2).
Binaphthyl derivatives, such as 2,2’-bis(diphenylphos-
phino)-1,1’-binaphthyl (binap) and 1,1’-binaphthalene-2,2’-
diol (binol), are well-established as powerful ligands in
asymmetric transformations[17] and supramolecular chemis-
try[18] owing to their elegant chiral steric hindrance and strong
donor properties. Aiming at an unprecedented approach to
visual discrimination of enantiomers, we studied the effi-
ciency of metallogelators 1 and 2 in the presence of chiral
binap. The addition of one equivalent of (R)- or (S)-binap to
the stable turbid gel 1/CHCl3 (1 wt%, Figure 2a) or to gel 2/
toluene with subsequent heating to reflux and cooling to room
temperature resulted in a collapse of the metallogels. These
results may be rationalized in terms of a substitution of the
chloro ligand in complexes 1 and 2 for binap blocking the
intermolecular p stacking and metal–metal interactions
between the heteroaromatic rings, which finally led to a
collapse of the gel network. To determine the minimum
amount of chiral binap required for gel collapsing, gradually
reduced amounts of (R)- or (S)-binap were added to samples
of gel 1/CHCl3. Surprisingly, when the amount of chiral
phosphine was reduced to 0.1 equiv (0.36 mg), we observed a
striking difference in the behavior of the respective gels:
Whereas the gel sample containing (S)-binap survived the
heating and cooling sequence as a robust gel (Figure 2b), the
gel sample containing the (R)-binap enantiomer collapsed.
Similar phenomena were also observed with other chiral
phosphine ligands (see Figures S43 and S44 in the Supporting
Information). Further decrease of the amount of chiral
phosphine resulted in the conservation of the stable gel
network, independent of the binap enantiomer used. This
Scheme 2. Suggested gel assembly and gel collapsing process through
interactions hampered by coordination of bulky phosphine ligands.
Supporting Information).[15] The major features in the XRD
pattern are three broad reflections at 2q = 7.88 (L001), 15.68
(L002) and 25.68 (d001). L001 and L002 are assigned to a lamellar
structure with a mean distance of 11.38 ꢁ, which may reflect
the distance between stacked layers of the gelator molecule.
The peak of d001 indicates another vital unit with a typical
distance of 3.47 ꢁ, which is assumed to result from the
corresponding p stacking and metal–metal interactions
between the coplanar heteroaromatic rings of the pincer
ligands in complex 2.
Since p stacking and metal–metal interactions are con-
sidered as key contributions to the gel formation, structural
modifications of the metallogelator suited to hamper these
interactions are expected to allow a control over formation
and collapse of the gel. According to Che and co-workers,
chloride ligands coordinated to platinum or gold pincer
complexes readily undergo ligand exchange, for example, by
phosphines, alkenes, or other ligands, to give mono-, di- or
trinuclear cyclometalated complexes revealing potential in
sensor and imaging development.[16] Therefore, the substitu-
Figure 2. Visual chiral recognition of (R)- and (S)-binap through ALS platinum gel 1/CHCl3 (1 wt%) enantioselective collapsing: SEM images of
a) gel 1/CHCl3; b) gel (1+0.1 equiv (S)-binap)/CHCl3; c) collapsing sol of (1+0.1 equiv (R)-binap)/CHCl3; d) 31P NMR spectra of the solutions
prepared with gel 1/CDCl3 (1 wt%) and (S)-binap or (R)-binap; and CD spectra of e) (R)- or (S)-binap; f) Sol: 1/CHCl3 (0.125 wt%); +R: Solution
prepared with 1/CHCl3 (0.125 wt%) and 1 equiv (R)-binap; and +S: Solution prepared with 1/CHCl3 (0.125 wt%) and 1 equiv (S)-binap.
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Communications
exciting phenomenon demonstrates the potential of the
blocked, leading to the collapse of the gel. Application of (S)-
binap, however, represents a chiral unmatched case. This
enantiomer is difficult to coordinate to the metal center and
consequently has no major impact on the assembly process.
The chiral recognition phenomenon of the metallogels
established for the binap enantiomers was also characterized
by circular dichroism (CD) spectroscopy (Figure 2e,f). Sol 1/
CHCl3 (0.125 wt%) revealed at room temperature only a
weak CD effect between 450 and 500 nm with a split at [q] = 0
line. In contrast, the orange solution prepared from 1/CHCl3
(0.125 wt%) with 1 equiv (R)-binap afforded an intensive
split CD band crossing the [q] = 0 line at 415 nm, revealing a
slightly negative first and a strong positive second Cotton
effect, which significantly differs from the CD pattern
observed for (R)-binap (Figure 2e). Whereas gelator 1 and
(R)-binap represent the chiral matched case, the combination
of sol 1/CHCl3 (0.125 wt%) and (S)-binap results in a non-
mirror-image CD pattern characterized by a first positive and
second negative allosterism.
In conclusion, a method for the visual chiral recognition of
(R)- and (S)-binap has been established through an enantio-
controlled breakdown of gels prepared from novel ALS-type
pincer metallogelators 1 and 2, which may be relevant for the
design of chiral sensors. Van der Waals interactions between
the steroid part, p stacking of the heteroarene moiety, and
metal–metal bonding are responsible for the aggregation.
Based on SEM and XRD studies as well as CD and 31P NMR
spectroscopy, an assembly model has been proposed that is
suited to rationalize the potential and scope of these novel
ALS pincer platinum metallogels.
metallogel 1/CHCl3 in a simple protocol of visual chiral
recognition. Further increase or decrease of the amount of
(R)- and (S)-binap added to gel 2/toluene (0.2 wt%) resulted
in a less pronounced differentiation effect: Higher binap
concentrations led to the formation of clear orange solutions,
while the gel network was preserved when lower binap
concentrations were applied, irrespective of the binap enan-
tiomer used.
The influence of the binap additives was further con-
firmed by an SEM study. In contrast to the SEM morphology
of xerogel 1/CHCl3 (1 wt%, Figure 2a), the images of (1 +
(S)-binap)/CHCl3 and (1 + (R)-binap)/CHCl3 reveal addi-
tional thick crystalline rods in the fiber networks (Fig-
ure 2b,c). In particular, the presence of (R)-binap leads to
the formation of a dominating agglomerate of crystalline rods
(Figure 2c) that are bulkier, denser, and longer than those
observed in (1 + (S)-binap)/CHCl3 (Figure 2b). This differ-
ence in superstructure may be responsible for the enantiose-
lective collapse of the gel network. A similar but less
pronounced trend of labilization is observed for gel 2/toluene
(0.2 wt%). Thick but less bulky rods which are not suited for
an efficient collapse of the gel network are observed in the gel
with 0.1 equiv (R)-binap (Figure S42 in the Supporting
Information). The image of (2 + (S)-binap)/toluene does not
reveal any clearly defined crystalline rods, which makes the
gel 2/toluene a less promising candidate for visual chiral
recognition. The different gel-collapsing behavior of metal-
logels obtained from pincer complexes 1 and 2 may be
ascribed to the increased molecular flexibility of the extended
linker in ALS platinum complex 2.
31P NMR spectroscopy study of metallogel 1/CDCl3
Received: January 25, 2011
Revised: March 22, 2011
A
(1 wt%) in the presence of one equivalent (R)- or (S)-binap
confirmed the chiral recognition results of enantioselective
gel collapse and the difference in the SEM morphologies.
After one equivalent (R)-binap was added to metallogel 1/
CDCl3 (1 wt%) and the sample was stirred for 48 h at 608C to
achieve (almost) complete coordination, the signal of (R)-
binap (d = À14.9 ppm) had almost vanished; instead, five new
intense downfield signals (d = 10.2, 11.9, 16.7, 17.2, and
29.1 ppm)—indicative of phosphorus coordination—had
appeared (Figure 2d). In an analogous experiment under
identical conditions, however, the signal of (S)-binap was
preserved and turned out to be much more intense than the
downfield 31P signals. These experiments indicate that the
chirality of (R)-binap matches the chiral environment of the
cholesterol fragment directly attached to the pincer hetero-
aromatic rings, and the biphosphine readily coordinates to the
Pt center in various coordination modes. Further support was
provided by high-resolution mass spectrometry (HR-MS)
experiments: The substitution of the chloro ligand in complex
1 for (R)-binap results in an intense adduct signal of [M(1+binap-
Cl) + 1]+ (m/z found: 1461.5805; calculated: 1461.5969),
whereas its intensity in the analogous experiment applying
(S)-binap is distinctly weaker (m/z found: 1461.6013, Figur-
es S62 and S63 in the Supporting Information). Accordingly,
owing to the hindrance of the bulky binaphthalene skeleton
and the PPh2 group, the p stacking and metal–metal bonding
and, thus, the assembly of the metallogelator molecules, are
Keywords: chiral recognition · enantioselectivity · gels ·
metallacycles · platinum
.
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