Molecular assembly directed by metal-aromatic interactions: Control of the aggregation and photophysical properties of Zn-salen complexes by aromatic mercuration

Article abstract of DOI:10.1002/chem.201103332

There is widespread interest in non-covalent bonding and weak interactions, such as electrostatic interactions, hydrogen bonding, solvophobic/hydrophobic interactions, metal-metal interactions, and π-π stacking, to tune the molecular assembly of planar π-

Full text of DOI:10.1002/chem.201103332

DOI: 10.1002/chem.201103332
Molecular Assembly Directed by Metal–Aromatic Interactions: Control of
the Aggregation and Photophysical Properties of Zn–Salen Complexes by
Aromatic Mercuration
Yuan-Bo Cai, Jinhui Zhan, Yang Hai, and Jun-Long Zhang*[a]
Abstract: There is widespread interest
in non-covalent bonding and weak in-
teractions, such as electrostatic interac-
tions, hydrogen bonding, solvophobic/
hydrophobic interactions, metal–metal
interactions, and p–p stacking, to tune
the molecular assembly of planar p-
conjugated organic and inorganic mole-
cules. Inspired by the roles of metal–ar-
omatic interaction in biological sys-
tems, such as in ion channels and met-
alloproteins, herein, we report the first
example of the use of Hg2+–aromatic
interactions to selectively control the
assembly and disassembly of zinc–salen
complexes in aqueous media; more-
over, this process exhibited significant
“turn on” fluorescent properties. UV/
Vis and fluorescence spectroscopic
analysis of the titration of Hg2+ ions
versus complex ZnL1 revealed that the
higher binding affinity of Hg2+ ions
(compared to 13 other metal ions) was
ascribed to specific interactions be-
tween the Hg2+ ions and the phenyl
rings of ZnL1; this result was also con-
firmed by 1H NMR spectroscopy and
HRMS (ESI). Further evidence for this
type of interaction was obtained from
the reaction of small-molecule ana-
logue L1 with Hg2+ ions, which demon-
strates the proximity of the N-alkyl
group to the aromatic protons during
Hg2+-ion binding, which led to the con-
sequential H/D exchange reaction with
D2O. DFT modeling of such interac-
tions between the Hg2+ ions and the
phenyl rings afforded calculated distan-
ces between the C and Hg atoms
À
(2.29 ꢀ) that were indicative of C Hg
bond-formation, under the direction of
the N atom of the morpholine ring.
The unusual coordination of Hg2+ ions
to the phenyl ring of the metallosalen
complexes not only strengthened the
binding ability but also increased the
steric effect to promote the disassem-
bly of ZnL1 in aqueous media.
Keywords: fluorescence · lumines-
cence · mercury · supramolecular
chemistry · zinc
Introduction
fine-tuning molecular assemblies may allow the assembly of
new materials with interesting new properties. To this end,
metal–aromatic interactions may offer exciting new opportu-
nities as are known to play important roles in biological sys-
tems, such as in ion channels and metalloproteins.[3] Despite
the widespread use of such metal–aromatic interactions in
biological systems, the use of such interactions to control as-
sembly or disassembly has been much-less explored. More-
over, model systems may also offer opportunities to help un-
derstand the nature of such interactions in biological sys-
tems. Herein, we report the first example of Hg2+–aromatic
interactions and their use in selectively controlling the as-
sembly and disassembly of zinc–salen (ZnL1) complexes in
aqueous media; such processes exhibited significant “turn
on” fluorescent properties, thereby allowing for the selective
sensing of Hg2+ ions (Scheme 1a). More importantly,
1H NMR and 2D NOSEY NMR spectroscopy and HRMS
(ESI) analysis of ZnL1 and its small-molecule analogue L1
Molecular assembly of planar p-conjugated organic or inor-
ganic molecules through non-covalent bonds or weak inter-
actions provides an entry into new classes of supramolecular
materials with tunable structures and functionalities.[1] Ex-
cellent examples of such non-covalent interactions include
electrostatic interactions, hydrogen bonding, solvophobic/hy-
drophobic interactions, metal–metal interactions, and p–p
stacking; these interactions have been used to control as-
sembly and disassembly processes in response to selective
stimuli, such as anions, metal ions, and biological cofactors,
to modulate photophysical and electronic properties.[2] Ex-
panding the types of non-covalent interactions available for
[a] Y.-B. Cai, Dr. J. Zhan, Y. Hai, Prof. Dr. J.-L. Zhang
Beijing National Laboratory for Molecular Sciences
State Key Laboratory of Rare-Earth Materials Chemistry
and Applications
(3-(N-methyl-N-(3-morpholinopropyl))aminoanisole)
al-
lowed the high binding affinity of Hg2+ ions to ZnL1 to be
ascribed to strong interactions between the Hg2+ ions and
the aromatic rings of ZnL1, owing to the p-electrophilicity
of the mercury ions.[4]
College of Chemistry and Molecular Engineering
Peking University
Beijing 100871 (P.R. China)
Fax : (+86)1062767034
E-mail: zhangjunlong@pku.edu.cn
We chose zinc–salen complexes as a model system to
demonstrate the use of metal–aromatic interactions in mo-
lecular-assembly processes because zinc–salen or zinc–salo-
Homepage: www.chem.pku.edu.cn/zhangjl/
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103332.
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FULL PAPER
sidered the possible use of aro-
matic mercuration,
a typical
mode in metal–aromatic inter-
actions, in controlling the as-
sembly or disassembly of zinc–
salen and zinc–salophen com-
pounds. The interaction of mer-
cury(II) salts with aromatic
compounds
is
well-known,
owing to its p-electrophilicity.[4]
Such interactions have been ex-
ploited in the design of selec-
tive chemosensors and chemo-
dosimeters for mercury(II) ions
with aromatic rings, alkynes,[10]
and alkenes.[11]
Results and Discussion
Recently, we reported the use
of highly luminescent Zn–salen
compounds (salen=2,3-bis[(4-
dialkylamino-2-hydroxybenzyli-
Scheme 1. a) Schematic representation of the self-assembly of Zn–salen complexes in water and the disassem-
bly of Zn–salen aggregates by metal ions. b) Structures of Zn–salen complexes ZnL1–4
.
phen compounds, have been widely used in the construction
of functional, stimulus-responsive nano- and microsized
supramolecular devices and materials.[5] In addition to being
widely available and cost-effective, their p-conjugated pla-
narity was reported promote intermolecular p–p stacking
like in organic molecules.[6] More importantly, one unique
feature of these building blocks was the extended Zn···O in-
teractions between the zinc center and a phenolic oxygen
atom, which allowed the integration of multiple functional
groups for obtaining synergistic properties.[7] At the same
time, the disassembly of zinc–salen or zinc–salophen aggre-
gates was investigated by introducing anions or electron-do-
nating ligands to coordinate to the zinc center and cleave
the intermolecular Zn···O interactions. Such endeavors re-
sulted in simultaneously switching the photophysical proper-
ties of the aggregates to those of monomeric zinc–salen or
zinc–salophen, thereby allowing the detection of anions or
small molecules, such as pyridines and amines, at low con-
centrations.[5c,8] In contrast, few studies of the disassembly of
zinc–salen or zinc–salophen complexes triggered by metal
ions have been reported; this paucity may be because metal
ions have been shown by single-crystal X-ray-diffraction
analysis to bind to phenolic oxygen atoms in salicylalde-
hydes that were located inside the salen plane.[9] Therefore,
to expand the repertoire of zinc–salen and zinc–salophen
compounds that use metal ions as a stimulus to control their
self-assembly and disassembly processes, and to harness the
potential of such processes, it is crucial to understand their
types of interactions with metal ions, and at which sites such
interactions occur, which will affect the disassembly of zinc–
salen and zinc–salophen aggregates.
dene)amino]but-2-enedinitrile) for the non-invasive labeling
of organelles in living cells.[12] We found that the polarity of
the N-substituent at the 4-position of salicylaldehyde was
important not only for the specificity for intracellular locali-
zation but also for their photophysical properties in aqueous
media. The structures of ZnL1–4 are shown in Scheme 1b.
Among these compounds, ZnL1 exhibited significant fluores-
cence decrease along with the increasing content of water
increasing. UV/Vis and fluorescence spectroscopic studies
for ZnL1 in H2O/DMSO (0–99%, v/v; Figure 1) indicated
the formation of aggregates in aqueous solution, which
agreed with previously reports regarding the self-assembly
of Zn–salen and Zn–salophen compounds by MacLachlan
and co-workers and Kleij and co-workers.[6b,c,f,g,7g] In addi-
tion, electron spray ionization (ESI) MS of ZnL1 in aqueous
solution (see the Supporting Information, Figure S9) showed
defined signals that corresponded to the dimeric and trimer-
ic species, which indicated their propensity to aggregate in
water.
Hg2+-ion-induced disassembly of zinc–salen (ZnL1) aggre-
gates: As a preliminary screening of metal ions that could
fine-tune the assembly process and the emission properties,
absorption and emission titrations of ZnL1 against various
biologically and environmentally relevant metal ions, such
as K+, Na+, Ag+, Mg2+, Hg2+, Pb2+, Cu2+, Co2+, Zn2+, Mn2+,
Ni2+, Cd2+, Fe2+, and Ca2+ (20 equiv of each) were carried
out in aqueous media (1% DMSO, v/v). Interestingly, as
shown in Figure 2, only the addition of Hg2+ ions showed
a distinguishable fluorescence increase (Figure 2b,d) and
color change (Figure 2c). UV/Vis absorption showed a red-
shift of the absorbance centered at 352 nm (to 387 nm) and
a blue-shift of the absorbance centered at 440 nm (to
Inspired by the roles of metal–aromatic interactions in ion
channels with an accumulation of aromatic residues, we con-
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J.-L. Zhang et al.
metal ions such as Hg2+ ion may similarly induce the effi-
cient disassembly of ZnL1 aggregates.
To further characterize the Hg2+-ion-induced morphology
changes for nano- or microsized aggregates of ZnL1, we car-
ried out dynamic light scattering (DLS) experiments. As ex-
pected, ZnL1 formed nanosized species, with a mean radius
of about 170 nm (see the Supporting Information, Fig-
ure S8). Upon addition of Hg2+ ions, the scattered-light in-
tensity did not change with time (see the Supporting Infor-
mation, Figure S7), thereby indicating that there was no for-
mation of nanosized species. Control experiments with Pb2+
or Fe2+ ions showed that the distributed radii of ZnL1 (175
and 180 nm, respectively) were comparable to that in ab-
sence of metal ions, thereby suggesting that 20 equivalents
of Pb2+ or Fe2+ ions only slightly affected the aggregation of
ZnL1. TEM and SEM analysis (Figure 3a,b) revealed that
the suspension of ZnL1 contained 2D sheet-like nanostruc-
tures in aqueous media. The selected-area electron-diffrac-
tion (SAED) pattern of ZnL1 showed sharp and ordered
spots (Figure 3c), which confirmed its single-crystalline
nature. Compared with the previously reported formation of
Zn–salen and Zn–salophen nanofibers, we found that the N
substituent at salicylaldehyde was important for tuning the
morphology of the Zn–salen complexes.[6f,13–15] After the ad-
dition of 20 equivalents of HgCl2, TEM of ZnL1 showed
a change from quasi-2D nanosheets to nanoparticles, with
diameters of 2–5 nm (Figure 3d). Thus, DLS, TEM, and
SEM studies demonstrated that Hg2+ ions disassembled
ZnL1 aggregates in aqueous media, which was consistent
with the results of the titration experiments.
Mechanism for the Hg2+-ion-induced disassembly of ZnL1
aggregates: To understand the mechanism of the Hg2+-ion-
induced disassembly of ZnL1 aggregates in aqueous media,
we first investigated the effect of ion strength on the absorp-
tion and emission spectra. Progressive addition of NaCl (0–
Figure 1. a) UV/Vis spectra and b) emission spectra (excitation: 377 nm)
of ZnL1 (1.0ꢂ10À5 molLÀ1) in various mixtures of H2O/DMSO (v/v).
430 nm) after titration of Hg2+ ions against ZnL1 (Fig-
ure 2a). The charge-transfer band centered at 571 nm re-
mained unchanged but the tailing absorption at 600 nm de-
creased, whilst a new tailing absorption at 533 nm appeared,
which exhibited similar absorption to that in DMSO. The
lifetime of ZnL1 in aqueous solution was 0.66 ns, which in-
creased to 0.79 ns after the addition of Hg2+ ions (20 equiv),
thus indicating the presence of weak interactions between
Hg2+ ions and ZnL1. Emission spectra for the titration of 14
metal ions to aqueous solutions of ZnL1 showed that the
fluorescence intensity centered at 620 nm increased 38 fold
after the addition of Hg2+ (20 equiv), whereas other metal
ions showed little effect on either the fluorescence or ab-
sorption spectra. The binding constant was KD =
0.053 mmolLÀ1 (see the Supporting Information, Figure S6).
Control experiments using ZnL2–4 showed little change in
the UV/Vis and emission spectra after titration with Hg2+
ions under identical conditions (see the Supporting Informa-
tion, Figures S2–S4). Similar changes in the UV/Vis and
emission spectra have been observed previously in the
anion- and pyridine-derivative-induced disassemblies of Zn–
salen or Zn–salophen aggregates,[8] thus suggesting that
100 mmolLÀ1) to
a
solution containing ZnL1 (6.5ꢂ
10À6 molLÀ1) and 20 equivalents of HgCl2 caused the emis-
sion intensity to be dramatically quenched and the charac-
teristic absorption features to change to those of the aggre-
gated state, which displayed three absorption bands at 352,
440, and 571 nm, and a tailing peak at 610 nm (Figure 4).
These results indicated that the disassembly of ZnL1 upon
addition of Hg2+ ions may arise from the electrostatic effects
of coordinating Hg2+ ions to the morpholine group of salicy-
laldehyde.
If Hg2+ only binds to the N atom of the morpholine group
as a soft Lewis acid, the binding affinity should not be much
higher than that of Pb2+ and Fe2+ ions. Therefore, we hy-
pothesized that other factors may lead to the stronger bind-
ing between Hg2+ ions and ZnL1. Thus, the interaction of
1
ZnL1 with Hg2+ ions was investigated by H NMR spectros-
copy (Figure 5). The 1H NMR spectrum of ZnL1 in
[D6]DMSO showed sharp signals, with chemical shifts and
splitting patterns that were consistent with its chemical for-
mulation. When D2O (10%) was added, the sharp signals
for the aromatic resonances broadened and several protons
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Molecular Assembly Directed by Metal–Aromatic Interactions
FULL PAPER
Figure 2. a) Absorption spectra of ZnL1 (9.0ꢂ10À6 molLÀ1) in H2O/DMSO (99:1, v/v) treated with various metal ions (20 equiv); b) emission spectra of
ZnL1 (2.6ꢂ10À5 molLÀ1) in H2O/DMSO (99:1, v/v) treated with various metal ions (20 equiv, excited at 364 nm); c) the colors of the solutions under day-
light; and d) fluorescence (under 365 nm light) of ZnL1 aqueous solutions treated with various metal ions.
(Hf,e,g,j) exhibited upfield shifts (Figure 5b), owing to a de-
crease in the T2 relaxation time when aggregates of ZnL1
were formed and/or owing to the hydrophobic effect in
aqueous media. The addition of Hg2+ ions (20 equiv) to a so-
lution of [D6]DMSO/D2O (90:10, v/v) containing ZnL1 re-
sulted in the disappearance of the Hc,d signals, whereas the
Hb doublet changed into a singlet. For the resonances corre-
sponding to the N substituents, protons He,f were shifted
downfield by as much as 0.47 ppm. These results showed
that Hg2+-ion binding to ZnL1 involved specific interaction
between the Hg2+ ions and the phenyl ring (Hc,d) and the N
atom of the morpholine group of ZnL1. Indeed, the addition
of 40 equivalents of Hg2+ ions to a [D8]THF/D2O (90:10, v/
v) solution containing ZnL1 also showed downfield shifts of
the aromatic carbons in the 13C NMR spectra (see the Sup-
porting Information, Figure S15), as a result of the interac-
tions between the Hg2+ ions and the phenyl rings.[4a] Control
experiments were performed in which 20 equivalents of Pb2+
ions were added to the same solution of ZnL1: the aromatic
and N-alkyl protons did not change but a small upfield shift
Figure 3. a) TEM and b) SEM images of nanosheet assemblies of ZnL1 in
for the Ha proton and a downfield shift for the Hd proton
aqueous media (1% DMSO, v/v); c) SAED pattern of ZnL1, which cor-
were observed (Figure 5c). Even successive addition of up
responds to the morphology shown in the Supporting Information, Fig-
to 110 equivalents of Pb2+ ions did not cause the disappear-
ance of the Hc,d protons (see the Supporting Information,
ure S8; d) TEM image of the disassembled ZnL1 after the addition of
Hg2+ ions (20 equiv).
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J.-L. Zhang et al.
Figure S12) and protons Hd,f,e only exhibited gradual down-
field shifts of about 0.2 ppm. MS (ESI) studies of the titra-
tion of 20 equivalents of Hg2+ ions against ZnL1 in D2O/
[D6]DMSO were performed (see the Supporting Informa-
tion, Figure S10). We observed two molecular ion peaks (m/
z 693.2819, 929.2097), which corresponded to the ZnL1 com-
plex with two deuterium atoms ([ZnL1(D)2]) and the ZnL1
complex with two deuterium atoms and one HgCl molecule
([ZnL1(D)2+HgCl]), respectively. The MS (ESI) study
strongly suggested that the disappearance of the Hc,d protons
1
in the aromatic region in H NMR spectrum was due to H/
D exchange in the presence of Hg2+ ions.
To further elucidate the possible interactions between the
Hg2+ ions and the phenyl rings of ZnL1, we chose small mol-
ecule L1, which contained an appending morpholine group,
as a model compound (Figure 6 and S13 in the Supporting
Figure 4. a) Absorption spectra and b) steady-state emission spectra (ex-
citation: 377 nm) of ZnL1 (6.5ꢂ10À6 molLÀ1) and HgCl2 (20 equiv) treat-
ed with various amounts of NaCl in aqueous solution (H2O/DMSO, 99:1,
v/v).
Figure 5. 1H NMR spectra of ZnL1 ([ZnL1]=10 mmolLÀ1
)
in
Figure 6. a) Stacked 1H NMR spectra of compound L1 with increasing
amounts of HgCl2: i–v) 0–40 equiv HgCl2 in 10 equiv increments; vi) 40 e-
quiv HgCl2, D2O/[D8]THF (1:9, v/v). b) Changes in the chemical shift
upon addition of HgCl2.
a) [D6]DMSO; b) D2O/[D6]DMSO (1:9, v/v); c) in the presence of Pb2+
ions (20 equiv) in D2O/[D6]DMSO (1:9, v/v) and d) in the presence of
Hg2+ ions (20 equiv) in D2O/[D6]DMSO (1:9, v/v).
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Molecular Assembly Directed by Metal–Aromatic Interactions
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Information) because it was similar to the salicyaldehyde
moiety in ZnL1 but with higher solubility in aqueous solu-
tion. We used anhydrous [D8]THF to avoid the residual
water peak in [D6]DMSO. Figure 6a showed the changes in
1
the H NMR spectrum for the aromatic resonances after the
addition of Hg2+ ions. In pure [D8]THF, the addition of 0–
40 equivalents of HgCl2 resulted in a decrease in the intensi-
ty of the triplet peak of Ha along with the appearance of
a new doublet peak (Haꢃ) at d=7.12 ppm. Similar changes
were observed for Hc, with a new peak (Hcꢃ) appearing at
d=6.30 ppm. The chemical shifts of protons Hb,d did not ex-
hibit obvious changes but the signals became broadened.
However, the alkyl protons He–k on the N-substituents of
compound L1 exhibited downfield shifts upon the addition
of Hg2+ ions; the differences in chemical shifts are summar-
ized in Figure 6b. Protons He,f exhibited the largest down-
field shift (about d=0.9 ppm, whilst protons Hg,h showed the
smallest downfield shift (about d=0.4 ppm), and the other
protons (Hi,j,k) only changed a little (d<0.1 ppm). HRMS
(ESI) of the reaction mixture of 20 equivalents of Hg2+ with
L1 showed a molecular ion peak at m/z 501.1225 (see the
Supporting Information, Figure S11a), which was assigned
to [L1+HgCl]+ and showed the characteristic isotopic peak
pattern of Hg2+ ions, thus indicating there was no H/D ex-
change in [D8]THF. H/D exchange took place when 10%
1
D2O (v/v) was added. In the H NMR spectra, the proton
signals of Hb,d disappeared, and protons Ha,c showed singlet
peaks and downfield shifts of d=0.15 and 0.02 ppm, respec-
tively. A molecular ion peak at m/z 503.1350 was observed
by HRMS (ESI; see the Supporting Information, Fig-
ure S11b), which indicated that two protons had been deu-
terated, and was consistent with the disappearance of the
1
Hb,d signals in the H NMR spectrum. Control experiments
in which Pb2+ ions replaced Hg2+ ions did not show the dis-
appearance of the Hb,d protons, although the protons corre-
sponding to the N-substituents of compound L1 showed
downfield shifts in aqueous solution (90% [D8]THF, v/v).
This result indicated that H/D exchange only took place in
the presence of D2O, which was strong evidence for aromat-
ic mercuration.
Figure 7. 2D NOESY spectrum of compound L1 versus HgCl2 in:
a) 0 equiv HgCl2 b) 6 equiv HgCl2 (D2O/[D8]THF (1:9, v/v) ([L1]=90 mm
for all of the spectra).
2D NOESY spectra of compound L1 showed interactions
between the phenyl protons and the alkyl protons on the N-
substituents in the presence or absence of Hg2+ ions in aque-
ous solution ([D8]THF/D2O, 90:10, v/v). In pure [D8]THF,
cross-peaks were mainly seen for neighboring protons, such
as between protons Hb,c and Hj,j, and between protons Hc,d
and Hk (see the Supporting Information, Figure S17). When
10% D2O was added, the appearance of cross-peaks be-
tween the tail CH2 protons (Hg, d=3.6 ppm) of the morpho-
line ring and aromatic protons Hc,d indicated that the mor-
pholine ring had moved closer to the aromatic ring in aque-
ous media, owing to hydrophobic effects (Figure 7a).[11] Ti-
tration of HgCl2 (6 equiv) against compound L1 in [D8]THF/
D2O (9:1, v/v) showed at least two species in the solution:
one was the starting compound (L1) and the other was
a new product, which was similar to that in Figure 6a. A
of the molpholine conjugate and the new protons (Ha’,c’) of
the aromatic ring were observed (Figure 7b), thus suggest-
ing that the alkyl protons were proximal to the aromatic
protons, with a distance parameter that was governed by the
NOE after Hg2+ binding.
Finally, to gain a greater insight into the interaction prop-
erties between the Hg2+ ions and compound L1, two coordi-
nation complexes were modeled by DFT calculations.[16] The
N atom of the morpholine ring could coordinate to Hg2+
ions and the calculated distances between the N atoms and
the Hg2+ ions for the two structures were 2.31 and 2.33 ꢀ,
respectively (Figure 8). These computational results suggest-
À
ed that the C Hg bonds could be formed through two
À
À
modes, C(Hb) Hg and C(Hd) Hg, both of which gave the
2+
À
same C Hg distance (2.29 ꢀ). Therefore, the Hg ions
could be stabilized by anchoring the aromatic C atom (Hb
strong NOE effect (cross-peaks) between the protons (Hh,e,f
)
or Hd) through direction by the morpholine N atom of L1.
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J.-L. Zhang et al.
concentrations ꢀ1ꢂ10À5 molLÀ1. Single-photon luminescence spectra of
ZnL1 titrated against HgCl2 in aqueous solution were recorded by using
a Hitachi FL4500 spectrometer. Other single-photon luminescence spec-
tra were recorded on an Edinburgh Instrument FLS920 Combined Fluo-
rescence-Lifetime and Steady-State spectrophotometer. Fluorescence
lifetimes were measured by an Edinburgh Instrument LifeSpec Red
Spectrometer (laser: 372 nm) All measurements were performed at RT.
Dynamic light scattering experiments were performed on an ALV/DLS/
SLS-5022F Laser Light Scattering Spectrometer. TEM was conducted on
a JEOL JEM-2011F TEM (acceleration voltage: 200 kV). The sample for
DLS experiments was centrifuged at 5000 rpm for 30 min.
Compounds ZnL1–4 were synthesized according to literature proce-
dures.[12]
À
Synthesis of (3-(N-methyl-N-(3-morpholinopropyl))aminoanisole) (L1):
N-(3-Bromopropyl)-3-methoxy-N-methylaniline was synthesized accord-
ing to a literature procedure.[12] A mixture of N-(3-bromopropyl)-3-me-
thoxy-N-methylaniline (1 g, 3.9 mmol), morpholine (1.5 g, 17.2 mmol),
and KHCO3 (0.4 g, 3.9 mmol) were dissolved in CH3CN (50 mL), and the
solution was heated to reflux under a nitrogen atmosphere overnight.
After the reaction had been completed, the solvent was removed under
reduced pressure. The brown residue was extracted with CH2Cl2 and pu-
rified by column chromatography on silica gel to give a yellow oil
Figure 8. DFT-optimized structures of the C Hg complexes. Left:
À
À
C(Hb) Hg; right: C(Hd) Hg.
Conclusion
We have demonstrated the first example of the disassembly
of Zn–salen aggregates by metal–aromatic interactions,
which was accompanied by “switch-on” fluorescence in
aqueous media. Upon addition of Hg2+ ions, DLS, TEM,
and SEM analysis confirmed the disassembly of ZnL1 aggre-
gates in aqueous media, which was consistent with the re-
sults from spectrophotometric and fluorimetric titrations.
(540 mg, yield: 52.6%). 1H NMR (300 MHz, D
8ACTHNUTRGNEUNG[THF], 258C, TMS): d=
1.707 (m, 2H), 2.308 (t, J=6.9 Hz, 2H), 2.354 (m, 4H), 2.895 (s, 3H),
3.388 (t, J=7.2 Hz, 2H), 3.620 (t, J=4.8 Hz, 4H), 3.710 (s, 3H), 6.178
(dd, J=2.1, 7.8 Hz, 1H), 6.253 (t, 1H, J=2.4 Hz), 6.322 (dd, J=2.4,
8.4 Hz, 1H), 7.004 ppm (t, J=8.4 Hz, 1H). MS (ESI): m/z calcd for
C15H25N2O2: 265.2 [M+H]+; found: 265.2.
1
More interestingly, H NMR spectroscopy and HRMS (ESI)
analysis of the titration of Hg2+ ions versus ZnL1 ascribed
the higher binding affinity of Hg2+ ions versus 13 other
metal ions to specific interactions between Hg2+ ions and
the phenyl rings of ZnL1. Further evidence for this type of
interaction was obtained from the reaction of small-mole-
cule analogue L1 with Hg2+ ions, which demonstrates the
proximity of the N-alkyl group to the aromatic protons
during Hg2+-ion binding, which led to the consequential H/
D exchange reaction with D2O. DFT modeling of such inter-
actions between the Hg2+ ions and the phenyl rings afforded
calculated distances between the C and Hg atoms (2.29 ꢀ)
Acknowledgements
This project was supported by National Key Basic Research Support
Foundation of China (NKBRSFC; 2010CB912302) and by the National
Scientific Foundation of China (grant no. 20971007). Y.-B.C. and Y.H.
thank the National Funding for Fostering Talents of Basic Sciences
(J0630421). We thank Prof. Y. Tao for helpful discussions.
[1] a) F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer, A. Schenning, Chem.
Rev. 2005, 105, 1491–1546; b) T. Baumgartner, R. Reau, Chem. Rev.
2006, 106, 4681–4727; c) K. Ariga, T. Nakanishi, J. P. Hill, Curr.
Opin. Colloid Interface Sci. 2007, 12, 106–120; d) S. Yagai, A. Kita-
mura, Chem. Soc. Rev. 2008, 37, 1520–1529; e) M. Hasegawa, M.
Iyoda, Chem. Soc. Rev. 2010, 39, 2420–2427; f) J. K. H. Hui, M. J.
MacLachlan, Coord. Chem. Rev. 2010, 254, 2363–2390; g) D.
Grosso, F. Ribot, C. Boissiere, C. Sanchez, Chem. Soc. Rev. 2011, 40,
829–848.
[2] For selected reviews, see: a) A. P. de Silva, H. Q. N. Gunaratne, T.
Gunnlaugsson, A. J. M. Huxley, C. P. McCoy, J. T. Rademacher,
T. E. Rice, Chem. Rev. 1997, 97, 1515–1566; b) F. P. Schmidtchen,
M. Berger, Chem. Rev. 1997, 97, 1609–1646; c) F. Wꢄrthner, Chem.
Commun. 2004, 1564–1579; d) T. W. Bell, N. M. Hext, Chem. Soc.
Rev. 2004, 33, 589–598; e) J. L. Sessler, P. A. Gale, W.-S. Cho, Anion
Receptor Chemistry, Royal Society of Chemistry, Cambridge, UK,
2006; f) D. A. Uhlenheuer, K. Petkau, L. Brunsveld, Chem. Soc.
Rev. 2010, 39, 2817–2826; g) H.-J. Schneider, Angew. Chem. 2009,
121, 3982–4036; Angew. Chem. Int. Ed. 2009, 48, 3924–3977;
h) S. K. S. Karuppannan, J. C. Chambron, Chem. Asian J. 2011, 6,
964–984; i) A. Wild, A. Winter, F. Schlutter, U. S. Schubert, Chem.
Soc. Rev. 2011, 40, 1459–1511; j) J. S. Wu, W. M. Liu, J. C. Ge, H. Y.
Zhang, P. F. Wang, Chem. Soc. Rev. 2011, 40, 3483–3495; k) G.
Aragay, J. Pons, A. MerkoÅi, Chem. Rev. 2011, 111, 3433–3458.
[3] a) D. A. Dougherty, Science 1996, 271, 163–168; b) J. C. Ma, D. A.
Dougherty, Chem. Rev. 1997, 97, 1303–1324; c) J. T. Kittleson, I. R.
Loftin, A. C. Hausrath, K. P. Engelhardt, C. Rensing, M. M.
McEvoy, Biochemistry 2006, 45, 11096–11102; d) Y. Xue, A. V.
À
that were indicative of C Hg bond-formation, under the di-
rection of the N atom of the morpholine ring. The unusual
coordination of Hg2+ ions to the phenyl ring of the metallo-
salen complexes not only strengthened the binding ability
but also increased the steric effect to promote the disassem-
bly of ZnL1 in aqueous media. This result is important for
the further development of metallosalen-based, metal-ion-
responsive devices and materials.
Experimental Section
General: All reactions were performed under a nitrogen atmosphere.
Commercially available reagents were used without further purification
unless otherwise stated. All reactions were monitored by thin layer chro-
matography (TLC). 1H NMR and 13C NMR spectroscopy was performed
on a Varian (300 MHz) NMR spectrometer. 2D NOESY NMR experi-
ments were recorded on a Bruker Advance (600 MHz) spectrometer.
Electrospray ionization (ESI) mass spectra were recorded on a Fourier
Transform Ion Cyclotron Resonance Mass Spectrometer (Bruker, USA).
Steady-state absorption spectra were recorded on an Agilent 8453 UV/
Vis spectrophotometer in quartz cuvettes (path-length: 10 mm) with dye
4248
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Chem. Eur. J. 2012, 18, 4242 – 4249

Molecular Assembly Directed by Metal–Aromatic Interactions
FULL PAPER
Davis, G. Balakrishnan, J. P. Stasser, B. M. Staehlin, P. Focia, T. G.
Spiro, J. E. Penner-Hahn, T. V. OꢃHalloran, Nat. Chem. Biol. 2008,
4, 107–109.
C. Pasquini, K. Rissanen, L. Russo, L. Schiaffino, New J. Chem.
2007, 31, 1633–1638; c) P. Chen, R. Lu, P. C. Xue, T. H. Xu, G. J.
Chen, Y. Y. Zhao, Langmuir 2009, 25, 8395–8399; d) A. Dalla Cort,
P. De Bernardin, L. Schiaffino, Chirality 2009, 21, 104–109; e) S. J.
Wezenberg, E. C. Escudero-Adan, J. Benet-Buchholz, A. W. Kleij,
Chem. Eur. J. 2009, 15, 5695–5700; f) E. A. Katayev, M. B. Schmid,
Dalton Trans. 2011, 40, 2778–2786; g) G. Salassa, A. M. Castilla,
A. W. Kleij, Dalton Trans. 2011, 40, 5236–5243; h) A. M. J. Devoille,
P. Richardson, N. L. Bill, J. L. Sesser, J. B. Love, Inorg. Chem. 2011,
50, 3116–3126.
[4] a) G. A. Olah, S. H. Yu, D. G. Parker, J. Org. Chem. 1976, 41, 1983–
1986; b) S. Fukuzumi, J. K. Kochi, J. Org. Chem. 1981, 46, 4116–
4126; c) W. Lau, J. C. Huffman, J. K. Kochi, J. Am. Chem. Soc. 1982,
104, 5515–5517; d) W. Lau, J. K. Kochi, J. Am. Chem. Soc. 1986,
108, 6720–6732; e) L. A. Fowley, J. C. Less, Jr. R. H. Crabtree,
P. E. M. Siegbahn, J. Organomet. Chem. 1995, 504, 57–67; f) A. S.
Borovik, S. G. Bott, A. R. Barron, Angew. Chem. 2000, 112, 4283–
4284; Angew. Chem. Int. Ed. 2000, 39, 4117–4118; g) A. S. Borovik,
S. G. Bott, A. R. Barron, J. Am. Chem. Soc. 2001, 123, 11219–
11228; h) M. R. Haneline, M. Tsunoda, F. P. Gabbai, J. Am. Chem.
Soc. 2002, 124, 3737–3742; i) T. J. Taylor, C. N. Burress, F. P. Gabbai,
Organometallics 2007, 26, 5252–5263.
[5] a) P. A. Gale, Coord. Chem. Rev. 2001, 213, 79–128; b) S. J. Wezen-
berg, A. W. Kleij, Angew. Chem. 2008, 120, 2388–2399; Angew.
Chem. Int. Ed. 2008, 47, 2354–2364; c) A. W. Kleij, Dalton Trans.
2009, 4635–4639; d) A. Dalla Cort, P. De Bernardin, G. Forte, F. Y.
Mihan, Chem. Soc. Rev. 2010, 39, 3863–3874; e) M. E. Germain,
M. J. Knapp, J. Am. Chem. Soc. 2008, 130, 5422–5423; f) M. E. Ger-
main, T. R. Vargo, P. G. Khalifah, M. J. Knapp, Inorg. Chem. 2007,
46, 4422–4429.
[9] For selected examples, see: a) J.-P. Costes, F. Dahan, A. Dupuis, J.-P.
Laurent, Inorg. Chem. 1996, 35, 2400; b) M. L. Colon, S. Y. Qian, D.
Vanderveer, X. R. Bu, Inorg. Chim. Acta 2004, 357, 83–88; c) T.
Wei, S. S. Zhao, W. Y. Bi, X. Q. Lu, Y. I. Hui, J. R. Song, W. K.
Wong, R. A. Jones, Inorg. Chem. Commun. 2009, 12, 1216–1219;
d) C. E. Burrow, T. J. Burchell, P. H. Lin, F. Habib, W. Wernsdorfer,
R. Clerac, M. Murugesu, Inorg. Chem. 2009, 48, 8051–8053; e) E. C.
Constable, G. Q. Zhang, C. E. Housecroft, J. A. Zampese, Dalton
Trans. 2010, 39, 1941–1947.
[10] S. Ando, K. Koide, J. Am. Chem. Soc. 2011, 133, 2556–2566.
[11] M. Santra, B. Roy, K. H. Ahn, Org. Lett. 2011, 13, 3422–3425.
[12] Y. Hai, J. J. Chen, P. Zhao, H. B. Lv, Y. Yu, P. Y. Xu, J. L. Zhang,
Chem. Commun. 2011, 47, 2435–2437.
[6] a) S. Mizukami, H. Houjou, Y. Nagawa, M. Kanesato, Chem.
Commun. 2003, 1148–1149; b) A. W. Kleij, M. Kuil, D. M. Tooke,
M. Lutz, A. L. Spek, J. N. H. Reek, Chem. Eur. J. 2005, 11, 4743–
4750; c) J. K. H. Hui, Z. Yu, M. J. MacLachlan, Angew. Chem. 2007,
119, 8126–8129; Angew. Chem. Int. Ed. 2007, 46, 7980–7983;
d) A. W. Kleij, M. Kuil, D. M. Tooke, A. L. Spek, J. N. H. Reek,
Inorg. Chem. 2007, 46, 5829–5831; e) L. Chen, J. Kim, T. Ishizuka,
Y. Honsho, A. Saeki, S. Seki, H. Ihee, D. L. Jiang, J. Am. Chem.
Soc. 2009, 131, 7287–7292; f) J. K.-H. Hui, Z. Yu, T. Mirfakhrai,
M. J. MacLachlan, Chem. Eur. J. 2009, 15, 13456–13465; g) X. Roy,
M. J. MacLachlan, Chem. Eur. J. 2009, 15, 6552–6559; h) G. Consi-
glio, S. Failla, P. Finocchiaro, I. P. Oliveri, R. Purrello, S. Di Bella,
Inorg. Chem. 2010, 49, 5134–5142.
[7] a) C. T. L. Ma, M. J. MacLachlan, Angew. Chem. 2005, 117, 4250–
4254; Angew. Chem. Int. Ed. 2005, 44, 4178–4182; b) A. L. Singer,
D. A. Atwood, Inorg. Chim. Acta 1998, 277, 157–162; c) J. Reglin-
ski, S. Morris, D. E. Stevenson, Polyhedron 2002, 21, 2175–2182;
d) S. Akine, T. Taniguchi, T. Nabeshima, Inorg. Chem. 2004, 43,
6142–6144; e) L. San Felices, E. C. Escudero-Ad an, J. Benet-Buch-
holz, A. W. Kleij, Inorg. Chem. 2009, 48, 846; f) K. G. Vladimirova,
A. Y. Freidzon, O. V. Kotova, A. A. Vaschenko, L. S. Lepnev, A. A.
Bagatur’yants, A. G. Vitukhnovskiy, N. F. Stepanov, M. V. Alfimov,
Inorg. Chem. 2009, 48, 11123–11130; g) M. M. Belmonte, S. J. We-
zenberg, R. M. Haak, D. Anselmo, E. C. Escudero-Adan, J. Benet-
Buchholz, A. W. Kleij, Dalton Trans. 2010, 39, 4541–4550.
[13] J. K. H. Hui, M. J. MacLachlan, Dalton Trans. 2010, 39, 7310–7319.
[14] M. Kuil, I. M. Puijk, A. W. Kleij, D. M. Tooke, A. L. Spek, J. N. H.
Reek, Chem. Asian J. 2009, 4, 50–57.
[15] G. Consiglio, S. Failla, I. P. Oliveri, R. Purrello, S. Di Bella, Dalton
Trans. 2009, 10426–10428.
[16] Gaussian 09, Revision A.02, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A.
Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, ꢅ.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian,
Inc. Wallingford CT, 2009. The structural optimizations were carried
out by using the B3LYP hybrid functional with Stuttgart RSC ECP
and associated basis sets on mercury and 6–31G* basis sets on all
other atoms.
[8] For recent examples, see: a) F. H. Zelder, R. Salvio, J. Rebek,
Chem. Commun. 2006, 1280–1282; b) A. Dalla Cort, L. Mandolini,
Received: October 24, 2011
Published online: February 29, 2012
Chem. Eur. J. 2012, 18, 4242 – 4249
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