Recognition between V- and dumbbell-shaped molecules

Article abstract of DOI:10.1039/c3ra43470b

A series of 2,6-bis(imino)pyridyl-based V-shaped compounds bearing various para-substituents on the terminal aromatic rings [C₅H ₃N(CHN-C₆H₄R)₂; R = OMe, ⁱPr, Me, H, Cl, F, and CF₃] have been prepared and investigated for their reversible binding with the dumbbell-shaped cations NH₂⁺-{CH₂-C₆H₃(OMe-3,5) ₂}₂ and 9-anthryl-CH₂-NH₂ ⁺-CH₂-C₆H₃(OMe-3,5)₂. Three crystalline V-shaped compounds and a dumbbell hexafluorophosphate were characterised in the solid state by X-ray structural analysis. The binding mode of the 1 : 1 V-shaped molecule·dumbbell complexes was evaluated by ¹H NMR spectroscopy. The binding constants (90-400 M^-1 in dichloromethane) and stoichiometries of the complexes were determined using the Method of Continuous Variations and the Rose-Drago Method based on ¹H NMR spectroscopic data. In a series of V-shaped compounds, the binding strength with both dumbbell cations diminishes with the decreasing electron-donating ability of the R substituents. Specifically, one of the diimine V-shaped compounds shows a stronger binding with the symmetrical dumbbell than with the unsymmetrical anthracene-containing dumbbell. Fluorescence measurements of equimolar mixtures of the V-shaped compounds and the unsymmetrical dumbbell have revealed a reduced anthracene emission which is approximately 50% that of the original intensity. Rapid and complete dissociation (<5 min) of the V-shaped compounds from the dumbbells was realised using an excess of acid or base, whereas only partial dissociation of the complexes was achieved with a large excess of water (<1 h).

Full text of DOI:10.1039/c3ra43470b

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Recognition between V- and dumbbell-shaped
molecules†
Cite this: RSC Adv., 2013, 3, 26382
Wing-Yan Wong,a Siu-Fung Lee,a Hoi-Shan Chan,a Thomas C. W. Mak,a
Chi-Hin Wong, Lau-Shan Huang, J. Fraser Stoddart and Ken Cham-Fai Leung
b
b
c
bd
*
*
A series of 2,6-bis(imino)pyridyl-based V-shaped compounds bearing various para-substituents on the
i
terminal aromatic rings [C5H3N(CH]N–C6H4R)2; R ¼ OMe, Pr, Me, H, Cl, F, and CF3] have been prepared
and investigated for their reversible binding with the dumbbell-shaped cations NH2+–{CH2–C6H3(OMe-
3,5)2}2 and 9-anthryl–CH2–NH2+–CH2–C6H3(OMe-3,5)2. Three crystalline V-shaped compounds and a
dumbbell hexafluorophosphate were characterised in the solid state by X-ray structural analysis. The
binding mode of the 1 : 1 V-shaped molecule$dumbbell complexes was evaluated by 1H NMR
spectroscopy. The binding constants (90–400 Mꢀ1 in dichloromethane) and stoichiometries of the
complexes were determined using the Method of Continuous Variations and the Rose-Drago Method
based on 1H NMR spectroscopic data. In a series of V-shaped compounds, the binding strength with
both dumbbell cations diminishes with the decreasing electron-donating ability of the R substituents.
Specifically, one of the diimine V-shaped compounds shows a stronger binding with the symmetrical
dumbbell than with the unsymmetrical anthracene-containing dumbbell. Fluorescence measurements of
equimolar mixtures of the V-shaped compounds and the unsymmetrical dumbbell have revealed a
reduced anthracene emission which is approximately 50% that of the original intensity. Rapid and
complete dissociation (<5 min) of the V-shaped compounds from the dumbbells was realised using an
excess of acid or base, whereas only partial dissociation of the complexes was achieved with a large
excess of water (<1 h).
Received 9th July 2013
Accepted 18th October 2013
DOI: 10.1039/c3ra43470b
www.rsc.org/advances
cavities and topologies which account for their wide application
for binding neutral and cationic aromatic substrates.2–4
1. Introduction
The term molecular clip or tweezer was coined by Whitlock1 to
describe a class of synthetic receptors which contain two pre-
organised aromatic chromophores covalently linked by a single
spacer. Apropriate spacers allow molecular clips to form sand-
wich complexes with aromatic substrates employing p–p
stacking interactions. More sophisticated molecular clips carry
additional functional groups oriented towards their cles, so
that hydrogen bonding and electrostatic interactions can
provide some synergy when binding appropriate substrates.
These acyclic receptors possess distinct advantages over their
cyclic relatives—e.g., cyclophanes—owing to their pre-organised
About a decade ago, Park et al.5 reported a Schiff-base
molecular clip which forms a 1 : 1 complex with salicylaldehyde
with an association constant (log Ka) of 1.10 in acetonitrile. The
1
complex formation was monitored by H NMR titration of the
Schiff-base clip with salicylaldehyde. The driving forces for
complex formation are hydrogen bonding and p–p stacking
interactions between the aromatic faces. Other types of clips,
which display pH-dependent reversible binding of complexes,6
exhibit guest-exchange behaviour,7 enable recognition of
macro-molecular sequences,8 and serve as drug carriers,9 have
appeared in the recent literature.
In general, conformationally induced self-assembly10
between
a structurally post-organised compound and a
aDepartment of Chemistry, The Chinese University of Hong Kong, Shatin, NT, Hong
Kong SAR, P.R. China
bDepartment of Chemistry and Institute of Creativity, The Hong Kong Baptist
University, Kowloon Tong, Kowloon, Hong Kong SAR, P.R. China
cDepartment of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208, USA
substrate will render a rapid change of physical properties
(solubility, organic functionality, etc.) of the substrate, and the
association process can, in principle, be reversible. There exist
only a few reports, however, on the dissociation of self-assem-
bled complexes induced by various stimuli. A recent dissocia-
tion study of a dynamic [2]rotaxane showed that an acyclic
diimine exhibits relative stability in the presence of an ammo-
nium dumbbell in a competitive mixture.11 This phenomenon
has prompted us to nd out how substituents on relatively rigid
dInstitute of Molecular Functional Materials, University Grants Committee, Hong Kong
SAR, P.R. China. E-mail: stoddart@northwestern.edu; ceung@hkbu.edu.hk
† Electronic supplementary information (ESI) available. CCDC 798464,
784761–784763. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3ra43470b
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diimine V-shaped compounds affect their binding affinities and
Dumbbell compounds 1-H$PF6 and 2-H$PF6 were prepared
photophysical properties.11 In the present investigation, self- according to literature procedures.11,13b The V-shaped
assembly10,12 and effective dissociation13 between selected compounds 3–9 were synthesised in high yields by condensa-
2,6-bis(imino)pyridyl (diimine) V-shaped compounds and tion of 2,6-diformylpyridine with the corresponding
dumbbell substrates have been demonstrated. Systematic R-substituted aniline (1 : 2 molar ratio) in the presence of
˚
exploration of these readily cleavable molecular recognition molecular sieves (4 A). Diimine V-shaped compounds 4, 7, 8,
processes may point the way to rapid functionalisation and and 9 are all new compounds. The V$dumbbell complexes were
disfunctionalisation of polyelectrolyte materials, such as poly- formed by mixing stochiometric amounts of the V-shaped
cationic polymers and nanoparticles. V-shaped compounds may compound and the dumbbell in a solvent mixture containing
be useful for designing 2,6-bis(imino)pyridyl transition metal- CH2Cl2 and MeCN at concentrations of approximately 20–120
1
based polymerization catalysts.14 Furthermore, they have been mM. It was conrmed by H NMR spectroscopy that equilibra-
designed and engineered to form Borromean rings15 and metal– tion to form complexes is complete within 10 minutes.
organic frameworks (MOFs).16
2.2 Characterisation of components
2.2.1 1H NMR spectroscopy. All the diimine V-shaped
2. Results and discussion
2.1. Design and synthesis
compounds were characterised by 1H NMR spectroscopy in
CD2Cl2. The V-shaped compounds, dumbbells, and their
complexes are well solvated in CH2Cl2, which is neither suffi-
ciently acidic to catalyse the hydrolysis of the imino groups, nor
inuential enough to affect electron donor–acceptor interac-
tions. Even although a very small amount of water is inevitably
present in the 1H NMR samples, all the diimine V-shaped
compounds showed no residual aldehyde signal at d ¼ 10.15
ppm in their NMR spectra, indicating that they are thermody-
namically stable. The imine proton resonances of the V-shaped
compounds show gradual changes in the chemical shis
according to the nature of electron-withdrawing and electron-
donating substituents. Compared to the reference compound 6
(R ¼ H), the imine protons of the more electron-withdrawing
group-substituted V-shaped compounds 7–9 (R ¼ Cl, F, CF3)
reveal upeld shis, while the more electron-donating
substituted V-shaped compounds 3–5 (R ¼ OMe, iPr, Me) show
downeld shis (Table 1).
Two dumbbell-like diimine salts,11,13b 1-H$PF6 and 2-H$PF6
(Fig. 1), were selected for investigation. The use of 1-H$PF6
allows the formation of V$dumbbell complexes for convenient
characterisation and comparison of the stereoelectronic effects
of dumbbell upon the binding event. On the other hand, the
anthracene-containing dumbbell 2-H$PF6 permits the forma-
tion of complexes with various V-shaped compounds with
enhanced aromatic interactions that can be characterised
conveniently by uorescence spectroscopy. A series (Fig. 1) of
2,6-bis(imino)pyridyl diimine V-shaped compounds 3–9 with
various para-substituents on the terminal aromatic rings were
synthesised. The stoichiometries and binding constants of the
V-shaped compounds with two dumbbells were determined by
monitoring the chemical shi changes in their 1H NMR spectra.
The uorescence properties of the complexes formed between
the V-shaped compounds and the dumbbell 2-H$PF6 were also
monitored. It is reasonable to expect that the self-assembly
between V-shaped compounds and the dumbbells is facilitated
by (i) signicant electrostatic interactions between the dumb-
bell's secondary dialkylammonium centre (–+NH2–) and the
V-shaped molecules' N atoms, (ii) hydrogen bonding between
2.2.2 Mass spectrometry. The new V-shaped compounds 4,
7, 8, and 9 were analysed by high-resolution matrix-assisted
laser desorption ionization-time-of-ight-mass spectrometry
(MALDI-TOF-MS). All mass spectra show the presence of the
expected molecular plus sodium ion peaks without any frag-
mentation of the hydrolysed imine V-shaped compounds or
2,6-diformylpyridine. It follows that these compounds possess
remarkable stabilities in the gas phase.
+
the –CH2–NH2 –CH2– segment of the dumbbells and the
V-shaped molecules' N atoms, and (iii) any p–p interactions
involving the aromatic rings.17
2.2.3 X-Ray crystallography. The crystal structures of the
dumbbell 2-H$PF6, and the V-shaped compounds 3, 5 and 8
Table 1 Comparison of chemical shift of imine signal in the 1H NMR spectra of V-
shaped compounds 3–9 at 296 K in CD2Cl2
Chemical shi of
V-shaped compound
R group substitution
imine proton d (ppm)
3
4
5
6
7
8
9
OMe
iPr
Me
H
Cl
F
8.68
8.68
8.66
8.66
8.63
8.64
8.64
Fig.
1
Structural formulae of the 2,6-bis(imino)pyridyl (diimine) V-shaped
compounds 3–9, the symmetrical dumbbell 1-H$PF6, the unsymmetrical dumb-
bell 2-H$PF6, and the self-assembly of V$Dumbbell complex.
CF3
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were obtained following X-ray diffraction analysis (Fig. 2 and 3).
Single crystals of 2-H$PF6 were obtained by slow vapour diffu-
t
sion of BuOMe into a solution of 2-H$PF6 in ClCH2CH2Cl. In
the solid state structure (Fig. 2) of 2-H$PF6, two adjacent
dumbbell molecules are in contact with each other with their
anthracene planes lying nearly perpendicular to one another.
The shortest distance between a hydrogen atom of one
anthracene moiety and the plane of the other anthracene
˚
moiety is 2.99 A, providing evidence for a moderate C–H/p
interaction.18 Single-crystal X-ray analyses (Fig. 3) of the free V-
shaped molecules reveal that they have relaxed linear shapes,
with the pyridyl and imine nitrogen atoms orientated on
opposite sides of the molecules.
2.3 Characterisation of V$Dumbbell complexes
Fig. 3 Different views of the conformations of the V-shaped compounds 3 (R ¼
OMe), 5 (R ¼ Me), and 8 (R ¼ F) from their respective crystal structures. Atom
colour key—red: oxygen; violet: nitrogen; yellow: fluorine; grey: carbon; and
green: hydrogen.
All attempts to grow single crystals of V$dumbbell complexes by
slow vapour diffusion of an equimolar mixture of the compo-
nents in CH2Cl2 or MeCN or ClCH2CH2Cl with BuOMe were
t
unsuccessful. Consequently, the V$dumbbell complexes were
characterised by 1H NMR spectroscopy, mass spectrometry, and
UV/visible absorption and uorescence spectroscopies.
spectra of dumbbells aer addition of the V-shaped
compounds. By way of an example, for an equimolar mixture of
the V-shaped compound 3 and the dumbbell 2-H$PF6 (Fig. 4),
signicant shis in the resonances for protons (Hd, He, Hi, Hl,
Hm, and Hn) signify the emergence of intermolecular N+–H/N
and N+C–H/N hydrogen bonding, p–p stacking, and electro-
static interactions on complexation.19 Noticeably, an aldehyde
proton signal at d ¼ 10.09 ppm is present since a small amount
of 3 inevitably underwent hydrolysis in the presence of the
acidic dumbbell 2-H$PF6.
2.3.1 1H NMR spectroscopy. The V-shaped compounds 3–9
and the two dumbbells 1-H$PF6 and 2-H$PF6 were mixed and
matched (1 : 1 ratio, 20 mM), and le for 6 h to reach equilib-
1
1
rium before recording their H NMR spectra. From all the H
NMR spectra of the V$dumbbell complexes, proton signals of
free and complexed V-shaped molecules appeared as weighted-
averages, suggesting that all complexation/decomplexation is
fast on the 1HNMR timescale. Complex formation can be
detected by the pronounced shis of signals in the 1H NMR
Upon complexation, the imine nitrogen atoms and pyridine
unit on the V-shaped compound donate electrons to the
ammonium centre of the dumbbells and, in return, disperse the
electron density around the imine. Therefore, by monitoring
the extent of upeld shi of the resonance for the imine proton,
the binding strength between the V-shaped compound and the
dumbbell can be estimated. From Table 2, the imine proton
upeld shis decrease down the series of complexes, suggesting
a gradual decline in the binding strength. Furthermore,
Table 2 Comparison of upfield shift of the imine 1H NMR signal in the V-shaped
compounds after complexation with equimolar quantities of the dumbbells 1-
H$PF6 and 2-H$PF6
Upeld shi of imine proton Dd (ppm)a
V-shaped compound
R
Dumbbell 1-H$PF6 Dumbbell 2-H$PF6
3
4
5
6
7
8
9
OMe 0.06
0.13
0.13
0.09
0.10
0.06
0.06
0.03
iPr
Me
H
0.06
0.05
0.03
0.01
0.01
0.01
Cl
F
CF3
a
Fig. 2 Molecular packing in the crystal structure of the dumbbell 2-H$PF6. Atom
colour key—red: oxygen; violet; nitrogen; yellow: fluorine; orange: phosphorus;
grey: carbon; and green: hydrogen.
Dd ¼ dobs – d; where dobs is the observed imine proton signal in the fast
equilibrating mixture and d is the imine proton signal of the free V-
shaped compounds.
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Fig. 4 Stacked partial 1H NMR spectra of the dumbbell 2-H$PF6, the V-shaped compound 3 and the equimolar mixture of 2-H$PF6 and 3 in CD2Cl2. Asterisk denotes
the solvent residual signal.
complexes formed with the dumbbell 2-H$PF6 show (Fig. S3†)
greater shis than the corresponding complexes with 1-H$PF6.
Table 3 Comparison of the 1H NMR signal shift of the V$dumbbell complex 3$2-
H$PF6. Significant signal shifts were observed for protons Hd, He, Hl, Hm, and Hn
The greater binding strength may be attributed to additional
p–p stacking interactions provided by the anthracene moiety of
2 H$PF6.
In order to investigate the geometry of the V$dumbbell
complexes, two-dimensional NOESY 1H NMR spectroscopy was
employed. From an equimolar mixture of the V-shaped
compound 3 and the dumbbell 2-H$PF6 in dry CD2Cl2, the Hb (D)
resonances for protons Hd and He, which are located next to the
ammonium centre of the dumbbell, shi signicantly (Fig. S1†
and Table 3). Moreover, the resonances for the protons Hl, Hn,
and Hm, which constitute the aryl protons and the imine proton
of the V-shaped compound, shi signicantly as well. Hence, Hh (D)
the electronic environments of these moieties are drastically
altered upon complexation.
Furthermore, strong nuclear Overhauser effect (NOE) signals
appear (Fig. S1† and Table 3) among the dumbbell protons Hc
(with Hp and Hn), Hd (with Ho), and Hj (with Hk and Hl). Protons Hn (V)
on the dumbbell Ha, Hb, and Heꢀi do not show any correlations
(Italic).
D denotes the dumbbell 2-H$PF6 while V denotes the V-shaped
compound 3
Uncomplex
Complexed
Proton
Ha (D)
d (ppm)
d (ppm)
Dd (ppm)
Correlation
3.70
6.42
6.51
3.61
4.76
7.91
7.49
7.44
7.89
8.39
7.93
8.25
8.68
7.36
6.97
3.84
3.64
6.38
6.56
3.87
4.89
7.95
7.44
7.42
7.91
8.37
7.98
8.02
8.53
7.16
6.92
3.84
0.06
0.04
0.05
0.26
0.13
0.04
0.05
0.02
0.02
0.02
0.05
0.23
0.15
0.20
0.05
0.00
—
—
Hc (D)
Hd (D)
He (D)
Hf (D)
Hp (V), Hn (V)
Ho (V)
—
—
—
Hg (D)
—
—
Hi (D)
Hj (D)
Hk (V)
Hl (V)
Hk (V), Hl (V)
Hj (D)
Hj (D)
—
Hc (D)
Hd (D)
Hc (D)
Hm (V)
Ho (V)
H
p (V)
with the protons of the V-shaped compound, while the imine
proton Hm also shows no correlation with any dumbbell
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Table 4 Comparison of the 1H NMR signal shift of the V$dumbbell complex
Attempts to determine binding constants (Ka), as well as
stoichiometries of the complexes formed between the V-shaped
compounds 3–9 and the dumbbells 1-H$PF6 and 2-H$PF6, have
been made by 1H NMR titration experiments. The total
concentration of the solution was kept constant while the ratio
between the V-shaped compound and the dumbbell was varied.
The dependence of complexation-induced 1H NMR shis of
methylene protons adjacent to the ammonium centres on the
dumbbells was monitored as the mole fraction of dumbbell was
varied. The stoichiometries were determined from the x-coor-
dinate at the maximum in a Job plot.20 Complexation between
V-shaped compound 5 and the dumbbell 1-H$PF6 is taken as an
example (Fig. S8†). Extrapolation of data points near the mole
fractions of 0 and 1 gave an interception point at a mole fraction
of 0.51. Therefore, V-shaped compound 5 and the dumbbell
1-H$PF6 form a 1 : 1 complex. Since the chemical shi of the
methylene protons adjacent to the ammonium centre of the
complexed dumbbell dc cannot be obtained directly, the Rose-
Drago method21 was applied to evaluate the complex concen-
tration and hence the binding constant (Ka). By tting the data
to a 1 : 1 complexation model, dc and Ka were determined
(Table S1†) to be 4.10 ppm and 360 Mꢀ1, respectively. For
instance, the complexation between 4 and 1-H$PF6 revealed a
binding constant of 400 Mꢀ1, a value that is comparable with
that of a pre-organised crown ether-based complexation.17b
Some of the complexes, however, especially those with the
dumbbell 2-H$PF6, are unstable and can exist in several co-
conformations as a result of varying the mole fractions of the
complexes. Thus, large errors arose during the calculation and
their Job plots could not be obtained.
0
3$1-H$PF6. Significant signal shifts were observed for protons Hc , Hd0, Hl, and Hn
(Italic). D denotes the dumbbell 1-H$PF6 while V denotes the V-shaped compound 3
Uncomplex
Complexed
Proton
d (ppm)
d (ppm)
Dd (ppm)
Correlation
0
Ha (D)
3.74
6.40
6.66
3.88
7.93
8.25
8.68
7.36
6.97
3.84
3.65
6.35
6.54
4.06
8.02
8.02
8.62
7.22
6.97
3.85
0.09
0.05
0.12
0.18
0.09
0.23
0.06
0.14
0.00
0.01
—
—
Hn (V)
—
—
—
—
Hc (D)
—
—
0
Hb (D)
0
Hc (D)
0
Hd (D)
H
k (V)
Hl (V)
m (V)
Hn (V)
Ho (V)
Hp (V)
H
0
protons. Therefore, it is reasonable to conclude that the V-
shaped compound's three N atoms are in close proximity to the
dumbbell's ammonium centre, thereby forming the V-shaped
dumbbell complexes, stabilised by electrostatic interactions
and hydrogen bonding.
Two-dimensional NOESY 1H NMR spectroscopy was employed
to investigate an equimolar mixture of V-shaped compound 3
0
and the dumbbell 1-H$PF6 in dry CD2Cl2. The aryl protons Hc
0
and methylene protons Hd of the dumbbell, as well as the aryl
protons Hl and Hn of the V-shaped compound, shi signicantly
(Fig. S2,† Table 4). Again, the electronic environments of these
moieties are altered considerably upon complexation. Strong
0
NOE signals appear only for Hc on the dumbbell and Hn of the V-
shaped compound. Other protons show no correlation. This
observation provides further evidence that the V-shaped com-
pound's three N atoms are in close proximity to the dumbbell's
ammonium centre. Based on the Dd values, the V$dumbbell
complex 3$2-H$PF6 possesses enhanced binding interactions
compared with that of the V$dumbbell complex 3$1-H$PF6.
2.3.2 Mass spectrometry. Mixtures of V-shaped compounds
3–9 with the dumbbells 1-H$PF6 and 2-H$PF6 in dry CH2Cl2
(total conc. ¼ 0.02 mM) were prepared according to the stoi-
chiometries obtained from the Job plots and analysed
(Table S2†) with high resolution ESI-MS. It transpires that the
Fig. 5 (A): UV/visible absorption spectra of the V-shaped compounds 3–9 and the dumbbell 2-H$PF6 (0.02 mM in CH2Cl2). (B): Fluorescence emission spectra
(excitation wavelength ¼ 290 nm) of the dumbbell 2-H$PF6 and its equimolar mixture with the V-shaped compounds 3–9 (0.02 mM in CH2Cl2).
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1 : 1 molecular V-shaped compound/dumbbell ratio is the most which contains both the averaged 1H NMR signals between the
stable co-conformation in the gas phase. All the detected 1 : 1 V$dumbbell complex and the free dumbbell 1-H$PF6
V-shaped molecule/dumbbell molecular ions agree well with (Fig. S6B†). The addition of base (Et3N, 10 equiv.) to both
their calculated values.
V$dumbbell complexes aer 5 min leads to subsequent depro-
2.3.3 Absorption and uorescence spectroscopies. The tontation of the ammonium dumbbells, while 4 and the
UV/visible absorption spectra (Fig. 5A) of all the V-shaped deprotonated dumbbells 1/2 become completely dissociated.
compounds show a broad imino residual band, while the spec- Noticeably, diimine 4 is stable under such basic conditions
trum of the dumbbell 2-H$PF6 shows the characteristic anthra- (Fig. S5 and S7†). On the other hand, the mixture of the
cene absorption band at 300–400 nm. Since the dumbbell 1-H$PF6 V$dumbbell complexes with a large excess of water (D2O, 200
does not have signicant uorescent properties, attention has equiv.) requires almost an hour to reach equilibrium, whereas
been focused on the anthracene-containing dumbbell 2-H$PF6. the complexes and the V-shaped compounds only undergo
The uorescence spectra (excitation at 290 nm, xed dumbbell partial dissociation (Fig. S4C and S6C†).
concentration to 0.02 mM) of equimolar mixtures of the V-shaped
Fluorescent properties of the anthracene-containing dumb-
compounds 3–9 and the dumbbell 2-H$PF6 show (Fig. 5B) the bell compound 2-H$PF6 in the presence of acid, base, and water
characteristic anthracene emission (lmax ¼ 418 nm), yet with have been reported as a comparison in the present study.11
lower intensities than that of the free 2-H$PF6. This result Mixtures of the anthracene-containing V$dumbbell complex
suggests that, although the binding strengths of V-shaped 4$2-H$PF6 treated separately with acid (3 M HCl, 10 equiv.), base
compounds are not strong enough to quench 2-H$PF6 completely, (Et3N, 10 equiv.) and water (200 equiv.), were analysed (Fig. 6,
they are able to lower the original intensity of the anthracene Table 5) by uorescence spectroscopy with an excitation wave-
emission by approximately 50%. This result agrees well with the length of 290 nm and a xed dumbbell concentration at
previous NMR spectroscopic investigation wherein it was shown 0.02 mM. By monitoring the anthracene uorescence intensity
that the binding strength decreases down the series.
(lmax ¼ 418 nm) in the equilibrium mixture of the reactants and
products, we can estimate the V$dumbbell complex concen-
tration. The uorescence spectrum of the V$dumbbell complex
4$2-H$PF6 treated with acid reveals (Fig. 6A) the characteristic
anthracene emission of pure [2-H]+. The uorescence spectrum
2.4 Dissociation of V$Dumbbell complexes
In a further set of experiments, complexes 4$1-H$PF6 and 4$2-
H$PF6 were exposed to an excess of acid, base, and water, in
order to dissociate the V-shaped compounds from the dumbbell
substrates.11,13b All resulting V$dumbbell mixtures were ana-
1
lysed by H NMR spectroscopy from which the signals of the
methylene protons adjacent to the dumbbell ammonium group
0
(Hd, He, and Hd ) as well as the aldehyde proton are compared
(Fig. S4–S7†). The outcome for both the V$dumbbell complexes
4$1-H$PF6 and 4$2-H$PF6 aer the same treatment, is some-
what similar (Table 5). Aer the addition of 10 equiv. of 3 M HCl
to the V$dumbbell complex 4$1-H$PF6, the complex dissociates
into its separate components 4 and 1-H$PF6 within 5 min,
whereas the diimine 4 is only partially dissociated into the
aldehyde and amine moieties (Fig. S4B†). Somewhat unexpect-
edly, the acid-promoted dissociation of the V$dumbbell
complex 4$2-H$PF6 yields a fast exchange equilibrium mixture
Fig. 6 Fluorescence emission spectra (excitation wavelength ¼ 290 nm) of the
V$dumbbell mixture 4$2-H$PF6 after addition of (A) acid, (B) water, and (C) base
(fixed dumbbell concentration at 0.02 mM in 9 : 1 CH2Cl2–MeCN).
Table 5 The outcome of 4$1-H$PF6 (left) and 4$2-H$PF6 (right) and the V$dumbbell complexes (20 mM, CD2Cl2/CD3CN 9 : 1) upon addition of excess of acid, base,
and water, characterised by 1H NMR spectroscopy. “1” represents the presence of the output substrate while “0” represents the absence of the output substrate
Input
Acida
Input
Acida
Output
Baseb
Waterc
Output
Baseb
Waterc
4$1-H$PF6
1-H$PF6
4
p-Isopropyl aniline
2,6-Diformyl pyridine
Fluorescencee
0
1
1
1
1
0
0
1d
1
0
0
1
1
1
1
1
0
4$2-H$PF6
2-H$PF6
4
p-Isopropyl aniline
2,6-Diformyl pyridine
Fluorescencee
1
1
1
1
1
1
0
1d
1
0
0
1
1
1
1
1
1
0
0
a
HCl (3 M, 10 equiv.), NMR spectrum obtained aer 5 min of acid addition, nal conc. ¼ 18.7 mM. b Et3N (10 equiv.), NMR spectrum 5 min aer
c
d
base addition, nal conc. ¼ 19.5 mM. D2O (200 equiv.), NMR spectrum obtained aer 1 h of water addition, nal conc. ¼ 18.6 mM. In
e
deprotonated form. Fluorescence intensity > 50 a.u. with a lmax of 418 nm.
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of 4$2-H$PF6 treated with water demonstrates (Fig. 6B) the procedures.11,13b,17 Compounds 4, 7, 8, and 9 have not been
characteristic anthracene emission, yet with an intensity of reported in the literature before. All non-aqueous reactions were
ꢁ
15%. Therefore, the dissociation of V$dumbbell complex carried out under dry, high-purity N2 with oven-dried (115 C)
4$2-H$PF6 in the aqueous solution is incomplete. In contrast, glassware. Unless otherwise specied, all solvents and reagents
the uorescence spectra of 4$2-H$PF6, aer treatment with base were purchased commercially and used without further puri-
reveals (Fig. 6C) complete uorescence quenching of the cation. Dichloromethane (CH2Cl2) was freshly distilled from
anthracene emission. This result reects the fact that the CaH2. Acetonitrile (MeCN) was pre-dried by stirring with CaH2
covalently linked secondary amine of the deprotonated dumb- for 24 h and distillation under N2, and stored in the presence of
˚
bell 2 deactivates the lower-lying anthracene uorescence.
molecular sieves (4 A). Thin layer chromatography (TLC) was
As a result, the V$dumbbell complexes possess relative performed on silica gel 60 F254 (Merck). Melting points were
stabilities toward hydrolysis in the presence of a large excess of measured on an Electrothermal 9100 digital melting point
water. Generally, rapid and complete dissociation of the apparatus and are uncorrected. UV/visible absorption spectra
V$dumbbell complexes can be achieved on addition of acid were obtained using a Cary 5 G UV/visible/NIR spectropho-
(complex 4$2-H$PF6 requires large excess of acid) or base. The tometer. Excitation and uorescence spectra were recorded
resulting substrates could be isolated by column chromatog- using a Hitachi F-4500 Fluorescence spectrophotometer. 1H,
raphy. When a cationic polymer substrate is employed, even- 13C, and 1H NOESY NMR spectra for structural characterization
tually, the polymer substrate can be puried by a simple were recorded on Bruker Avance 400 (1H: 400 MHz; 13C:
precipitation in a specic solvent system wherein the V-shaped 101 MHz) spectrometer at 296 K. NMR samples were dissolved
molecular components are soluble.
in CD2Cl2 unless otherwise stated. Chemical shis were repor-
ted as parts per million (ppm) on the d scale and calibrated by
using the solvent residual peak as internal standard. Coupling
constants (J) are reported in Hertz. Electrospray ionization (ESI)
3. Conclusions
A series of diimine V-shaped compounds 3–9 with different para- mass spectra were obtained on a Thermo Finnigan MAT 95XL
substituents (R ¼ OMe, iPr, Me, H, Cl, F, and CF3) on the terminal mass spectrometer using 1 : 1 CH2Cl2–MeOH as the mobile
aromatic rings have been prepared for complexation with two phase. The reported m/z values correspond to the most abun-
dumbbell substrates. Single crystals of the V-shaped compounds dant monoisotopic masses. X-ray diffraction data of crystalline
3, 5, and 8, as well as the dumbbell 2-H$PF6 have been obtained compounds were collected with a Bruker APEX II diffractometer
and subjected to X-ray structure determination. The self-assembly or a Bruker ASX CCD X-ray system.
binding mode of the V$dumbbell complexes was evaluated by 1H
NMR spectroscopy. The binding constants and stoichiometries General procedure for synthesising diimine V-Shaped
between the V-shaped compounds and the dumbbells were compounds
determined using the Method of Continuous Variations and the
A solution of the p-substituted aniline (2.0 equiv.) and 2,6-
Rose-Drago Method based on 1H NMR spectroscopic data. Within
diformylpyridine (1.0 equiv.) in anhydrous CH2Cl2 (1 mL
the series of V-shaped compounds, the binding strengths with
both dumbbells weaken with the decreasing electron-donating
ability of the R substituents. Moreover, the V-shaped compound 4
shows a stronger binding with the symmetrical dumbbell 1-H$PF6
than with the unsymmetrical anthracene-containing dumbbell 2-
H$PF6. Fluorescence measurement of equimolar mixtures of 3–9
and the dumbbell 2-H$PF6 caused reduced anthracene emission
by approximately 50%, showing that the complexes formed could
not completely quench the anthracene uorescence. Rapid
(<5 min) and complete dissociation of the V-shaped compounds
from the dumbbells could be realised by using an excess of acid or
base, whereas only partial dissociation of the complexes was
observed (<1 h) with a large excess of water. Further exploration of
these complexes with uorinated dumbbells22 may give enhanced
bindings. Such modications may potentially lead to rapid
and controlled functionalisation/disfunctionalisation of poly-
electrolyte materials,23 and their acting as a gate-switchable part at
the surface of nanoparticles.24
mmolꢀ1) was stirred in the presence of molecular sieves (4 A) for
˚
4 h. The molecular sieves were ltered off and washed with
CH2Cl2. The excess of solvent in the ltrate was removed under
vacuum to yield the product.
3 (R ¼ OMe)
Slow vapour diffusion of tBuOMe into a solution of 3 in
ClCH2CH2Cl yielded single crystals (colourless prisms, 0.40 ꢂ
0.30 ꢂ 0.20 mm) suitable for X-ray crystallography. Crystal data:
[C21H19N3O2], M ¼ 345.39, orthorhombic, Aba2, a ¼ 7.1660(6),
3
ꢁ
˚
˚
b ¼ 40.204(3), c ¼ 6.3427(5) A, a ¼ b ¼ g ¼ 90 , V ¼ 1827.4(3) A ,
Z ¼ 4, Dcalcd ¼ 1.255 mg mꢀ3, m(Mo-Ka) ¼ 0.083 mmꢀ1, F(000) ¼
728, T ¼ 296 K, 2019 independent measured reections, full-
matrix least-squares on F2 renement, R1 ¼ 0.0439, wR2
¼
0.0920, 1226, independent observed reections, [|Fo| > 4s(|Fo|),
2qmax ¼ 55.9ꢁ], 119 parameters. CCDC code: 784762.
4 (R ¼ iPr)
Following the general procedure, a mixture of 4-isopropylani-
line (0.49 g, 3.60 mmol) and 2,6-diformylpyridine (0.24 g,
1.80 mmol) was stirred in anhydrous CH2Cl2 (5 mL) with
4. Experimental section
General methods
˚
1-H$PF6, 2-H$PF6, 3, 5, and 6 are known compounds which were molecular sieves (4 A) for 4 h. On ltration and removal of the
synthesised and characterised according to literature solvent, the diimine 4 was obtained as a bright yellow solid
26388 | RSC Adv., 2013, 3, 26382–26390
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(0.55 g, 83%). Rf: 0.81 (hexane–EtOAc ¼ 1 : 1). M.p. 126.8–127.2
ꢁC. 1H NMR (CD2Cl2): 1.28 (d, J ¼ 6.9 Hz, 12H, CH(CH3)2), 2.96
(quin, J ¼ 6.9 Hz, 2H, CH(CH3)2), 7.26–7.34 (m, 8H, ArH), 7.94 (t,
J ¼ 7.8 Hz, 1H, ArH), 8.27 (d, J ¼ 7.8 Hz, 2H, ArH), 8.68 (s, 2H,
HC ¼ N). 13C NMR (CD2Cl2): 24.4, 34.3, 121.7, 123.2, 127.8,
137.8, 148.6, 149.1, 155.4, 159.9. HRMS (MALDI-TOF): C32H27N3
[M + Na]+: calcd 392.2097; found 392.2100.
9 (R ¼ CF3)
Following the general procedure, a mixture of 4-triuoromethyl
aniline (0.37 g, 2.28 mmol) and 2,6-diformylpyridine (0.15 g,
1.14 mmol) was stirred in anhydrous CH2Cl2 (3 mL) with
˚
molecular sieves (4 A) for 4 h. On ltration and removal of the
solvent, the diimine 9 was obtained as an off-white powder
(0.54 g, quantitative). Rf: 0.68 (hexane–EtOAc ¼ 1 : 1). M.p.
149.1–149.7 ꢁC. 1H NMR (CD2Cl2): 7.37 (d, J ¼ 8.3 Hz, 4H, ArH),
7.70 (d, J ¼ 8.3 Hz, 4H, ArH), 8.02 (t, J ¼ 7.8 Hz, 1H, ArH), 8.33 (d,
J ¼ 7.8 Hz, 2H, ArH), 8.64 (s, 2H, HC ¼ N). 13C NMR (CD2Cl2):
121.8, 124.2, 125.2 (q, J ¼ 215.5 Hz), 127.0 (m), 128.8 (q, J ¼ 32.7
Hz), 138.1, 154.7, 154.9, 162.7. HRMS (MALDI-TOF): C21H13F6N3
[M + H]+: calcd 422.1086; found 422.1095.
5. (R ¼ Me)
Slow vapour diffusion of MeCN into a solution of 5 in CH2Cl2
yielded single crystals (colourless prisms, 0.50 ꢂ 0.40 ꢂ 0.30
mm) suitable for X-ray crystallography. Crystal data: [C21H19N3],
M ¼ 313.39, monoclinic, P2/c, a ¼ 4.7249(7), b ¼ 6.2811(9), c ¼
3
˚
˚
28.965(4) A, a ¼ 90, b ¼ 94.639(3), g ¼ 90, V ¼ 856.8(2) A , Z ¼ 2,
Dcalcd ¼ 1.215 mg mꢀ3, m(Mo-Ka) ¼ 0.073 mmꢀ1, F(000) ¼ 332,
T ¼ 296 K, 1566 independent measured reections, full-matrix
Dumbbell 2-H$PF6
least-squares on F2 renement, R1 ¼ 0.1810, wR2 ¼ 0.6184, 1391, Slow vapour diffusion of BuOMe into a solution of 2-H$PF6 in
t
independent observed reections, [|Fo| > 4s(|Fo|), 2qmax
¼
CH2Cl2 yielded single crystals (yellow prisms, 0.50 ꢂ 0.40 ꢂ
50.5ꢁ], 111 parameters. CCDC code: 784763.
0.30 mm) of the dumbbell suitable for X-ray crystallography.
ꢀ
Crystal data: [C48H48N2O4]Cl(PF6), M ¼ 897.30, triclinic, P1, a ¼
˚
7 (R ¼ Cl)
12.0530(4), b ¼ 12.8073(4), c ¼ 16.0081(9) A, a ¼ 95.8310(10),
3
˚
b ¼ 99.9830(10), g ¼ 114.1490(10), V ¼ 2179.41(16) A , Z ¼ 2,
Following the general procedure, a mixture of 4-chloroaniline
(0.33 g, 2.60 mmol) and 2,6-diformylpyridine (0.18 g, 1.30 mmol)
was stirred in anhydrous CH2Cl2 (4 mL) with molecular sieves
Dcalcd ¼ 1.367 Mg mꢀ3, m(Mo-Ka) ¼ 0.197 mmꢀ1, F(000) ¼ 936,
T ¼ 296 K, 7855 independent measured reections, full-matrix
least-squares on F2 renement, R1 ¼ 0.0420, wR2 ¼ 0.1117, 6456,
˚
(4 A) for 4 h. On ltration and removal of the solvent, the diimine
independent observed reections, [|Fo| > 4s(|Fo|), 2qmax
50.5ꢁ], 562 parameters. CCDC code: 784761.
¼
7 was obtained as a yellow powder (0.66 g, quantitative). Rf: 0.59
(hexane–EtOAc ¼ 1 : 1). M.p. > 198.4 ꢁC (decomposed). 1H NMR
(CD2Cl2): 7.27 (d, J ¼ 8.7 Hz, 4H, ArH), 7.41 (d, J ¼ 8.7 Hz, 4H,
ArH), 7.97 (t, J ¼ 7.8 Hz, 1H, ArH), 8.29 (d, J ¼ 7.8 Hz, 2H, ArH),
8.63 (s, 2H, HC ¼ N). 13C NMR (CD2Cl2): 123.1, 123.7, 129.9,
Acknowledgements
132.8, 138.0, 150.0, 155.1, 161.2. HRMS (MALDI-TOF): This research was supported by a General Research Fund
C19H13Cl2N3 [M + Na]+: calcd 376.0376; found 376.0378.
(201412) awarded by the Hong Kong Research Grants Council,
and a grant from the University Grants Committee of Hong
Kong SAR (AoE/P-03/08).
8 (R ¼ F)
Following the general procedure, a mixture of 4-uoroaniline
(0.30 g, 2.72 mmol) and 2,6-diformylpyridine (0.18 g,
1.36 mmol) was stirred in anhydrous CH2Cl2 (4 mL) with
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This journal is ª The Royal Society of Chemistry 2013