Measurement of supramolecular effective molarities for intramolecular H-bonds in zinc porphyrin-imidazole complexes

Article abstract of DOI:10.1039/c3ob42246a

The association constants for formation of 1:1 complexes between five different imidazole ligands and eight different porphyrins have been measured by UV/vis titration experiments in two different solvents, toluene and 1,1,2,2-tetrachloroethane (TCE). Ligands equipped with H-bond acceptors (ester or amide) and porphyrins equipped with H-bond donors (phenol) can make H-bonds in addition to the zinc-nitrogen coordination interaction. The free energy contributions of these H-bonds to the overall stabilities of the complexes were determined using chemical double mutant cycles. Amide-phenol H-bonds contribute up to 5 kJ mol^-1 to the free energy change on complexation, and ester-phenol H-bonds contribute up to 3 kJ mol^-1. Porphyrin-ligand combinations with poor geometric complementarity do not make detectable H-bonding interactions. Effective molarities (EM) for the formation of H-bonds in the complexes were estimated by comparing the equilibrium constants for formation of the intramolecular interaction with the corresponding intermolecular interaction: the values are between 3 mM and 200 mM, which is comparable to previous results obtained for porphyrin-pyridine complexes. The values of EM measured for flexible and rigid ligand systems are comparable. This suggests that there is a trade off between restriction of conformational mobility in the flexible ligands and geometric strain in the rigid ligands, which results in similar binding affinities.

Full text of DOI:10.1039/c3ob42246a

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Measurement of supramolecular effective
molarities for intramolecular H-bonds in zinc
porphyrin–imidazole complexes†
Cite this: Org. Biomol. Chem., 2014,
12, 1440
Michael A. Jinks, Hongmei Sun and Christopher A. Hunter*
The association constants for formation of 1 : 1 complexes between five different imidazole ligands and
eight different porphyrins have been measured by UV/vis titration experiments in two different solvents,
toluene and 1,1,2,2-tetrachloroethane (TCE). Ligands equipped with H-bond acceptors (ester or amide)
and porphyrins equipped with H-bond donors (phenol) can make H-bonds in addition to the zinc–
nitrogen coordination interaction. The free energy contributions of these H-bonds to the overall stabilities
of the complexes were determined using chemical double mutant cycles. Amide-phenol H-bonds con-
tribute up to 5 kJ mol−1 to the free energy change on complexation, and ester-phenol H-bonds contrib-
ute up to 3 kJ mol−1. Porphyrin–ligand combinations with poor geometric complementarity do not make
detectable H-bonding interactions. Effective molarities (EM) for the formation of H-bonds in the com-
plexes were estimated by comparing the equilibrium constants for formation of the intramolecular inter-
action with the corresponding intermolecular interaction: the values are between 3 mM and 200 mM,
which is comparable to previous results obtained for porphyrin–pyridine complexes. The values of EM
measured for flexible and rigid ligand systems are comparable. This suggests that there is a trade off
between restriction of conformational mobility in the flexible ligands and geometric strain in the rigid
ligands, which results in similar binding affinities.
Received 12th November 2013,
Accepted 13th January 2014
DOI: 10.1039/c3ob42246a
www.rsc.org/obc
example, short complementary DNA oligomers do not form
stable duplexes in water, but when there is a sufficient number
Introduction
Supramolecular chemistry exploits non-covalent interactions to of base pairs, a highly stable duplex is formed.5 This type of
build multicomponent assemblies with well-defined three- cooperative binding is known as chelate cooperativity and
dimensional structure and reactivity.1 Biological systems have is central to most biological and supramolecular assembly
evolved to exploit non-covalent interactions in very sophisti- processes.6
cated ways, but synthetic equivalents are still a challenge to
Chelate cooperativity is maximised when molecules have
design using non-biological components.2 New supramolecu- good geometric complementarity and low conformational flexi-
lar systems are usually built on a small number of well-estab- bility.7 However, it is difficult to predict the magnitude of the
lished successful binding motifs, because it is difficult to effect of deviating from this ideal on the thermodynamic pro-
develop new ones.3 The development of quantitative supra- perties of the system. In practice, there will often be a trade off
molecular chemistry will allow prediction of the structural, between conformational flexibility, geometric complementarity
thermodynamic and kinetic properties of supramolecular com- and synthetic accessibility. We have therefore begun a program
plexes, allowing rational supramolecular design and exploita- to quantify the relationship between chemical structure and
tion of the full potential of the field.
chelate cooperativity.8 The parameter that is used to quantify
Non-covalent interactions are relatively weak, and stable chelate cooperativity is the effective molarity (EM).9 A supra-
supramolecular complexes are only formed when multiple molecular EM is defined as the ratio of the equilibrium con-
intermolecular interactions cooperate in binding.4 For stant for formation of an intramolecular non-covalent
interaction relative to the equilibrium constant for formation
of the corresponding intermolecular interaction. The value of
EM defines the concentration at which intermolecular inter-
actions start to compete with intramolecular interactions, i.e.
Department of Chemistry, University of Sheffield, S3 7HF, UK.
E-mail: C.Hunter@shef.ac.uk; Fax: +44 (0)114 2229346; Tel: +44 (0)114 2229476
†Electronic supplementary information (ESI) available: Details of the partially
the concentration at which the formation of open polymeric
bound state populations and values of KrefEM. See DOI: 10.1039/c3ob42246a
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Fig. 1 (a) Formation of an intermolecular H-bond. (b) Stepwise equilibria in the formation of a porphyrin–ligand complex containing an intramole-
cular H-bond. Kref is the equilibrium constant for formation of the intermolecular H-bond. K0 is the intermolecular association constant for for-
mation of the zinc–nitrogen interaction. KrefEM is the equilibrium constant for formation of the intramolecular H-bond, and EM is the effective
molarity for the intramolecular interaction.
assemblies takes over from the folding of closed assemblies.
EM is therefore a key quantitative parameter that determines
the behaviour of supramolecular systems.10
Approach
We have previously used the binding of pyridine ligands
equipped with H-bond acceptors to zinc porphyrins equipped
with H-bond donors to measure values of EM for a wide variety
of different intramolecular H-bond interactions.8 In this
paper, we extend the approach to the interaction of imidazole
ligands with zinc porphyrins (Fig. 1). Imidazole binds more
strongly than pyridine to zinc porphyrins,11 so these exper-
iments will establish whether the strength of the coordination
bond has a significant effect on the EM of peripheral H-bonds.
Fig. 2 Chemical double mutant cycle (DMC) used to measure the free
energy contribution of the phenol–carbonyl H-bond to the overall
stability of complex A.
Fig. 1b shows stepwise equilibria involved in the formation
of an intramolecular H-bond in these systems. Coordination of
the nitrogen of the ligand to the zinc leads to formation of an
intermolecular complex. If the ligand is equipped with a
H-bond acceptor and the porphyrin is equipped with a H-bond
containing multiple non-covalent interactions.8,12 Fig. 2 illus-
donor, these two groups can subsequently make an intramole-
trates the approach. Complex A makes both the zinc–nitrogen
interaction and the H-bond. The mutation to give complex B
removes the H-bond acceptor from the ligand. However, this
mutation can also affect the strength of the zinc–nitrogen
interaction. The magnitude of this secondary effect can be
quantified by comparing the stabilities of complexes C and D,
where the same mutation is made, but the porphyrin does not
have a H-bond donor. Thus by measuring the stabilities of
all four complexes in Fig. 2, eqn (1) can be used to dissect
the contribution of the intramolecular H-bond to the overall
stability of complex A.
cular H-bond. Strictly, this is an intermolecular H-bond, but
we will refer to it as an intramolecular interaction, because it
takes place within the coordinated zinc porphyrin complex.
Comparison of the equilibrium constant for this intramolecu-
lar interaction with the corresponding intermolecular H-bond
measured using reference compounds containing the same
functional groups (Fig. 1a) allows experimental determination
of EM for the intramolecular process. By varying the solvent
and the structures of the ligand and porphyrin, it is possible
to quantify the effects of H-bond strength, geometric comple-
mentarity and conformational flexibility on the magnitude of
supramolecular EMs.
However, it is not possible to directly measure the equili-
brium constant for formation of the intramolecular H-bond in
Fig. 1b, because host–guest titration experiments measure the
overall stability of the porphyrin–ligand complex. We have
therefore developed the chemical double mutant cycle (DMC)
ΔΔG° ¼ ΔG°A ꢀ ΔGB° ꢀ ΔGC° þ ΔG°D
ð1Þ
The increase in the stability of complex
A
due to
to dissect the contributions made by individual functional H-bonding, which is measured by ΔΔG°, can be used to deter-
group interactions to the overall stability of a complex mine the EM for the intramolecular interaction.8
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Results and discussion
Fig. 3 and 4 show the structures of the porphyrins and ligands
used in this study. Porphyrins P1a–P4a each have four arms
bearing a H-bond donor group (phenol). P1b–P4b are the
corresponding controls, where the phenol hydroxyls are
replaced by methoxy groups, which cannot act as H-bond
donors. Ligands 1–4 contain an imidazole moiety for coordi-
nation to the zinc porphyrins and a H-bond acceptor group
(amide or ester). N-Methyl imidazole (5) is used as the control
ligand that cannot form H-bonds.
Synthesis
Scheme 1
The synthesis of the porphyrins has been described previous-
ly.8a Compound 5 is commercially available. Scheme 1 shows
the synthetic route used to prepare the other imidazole
ligands. Wittig chemistry was used to prepare ligands 1 and 2,
which have unsaturated linkers.13 Hydrogenation of these
compounds gave ligands 3 and 4, which have saturated linkers
and are more flexible.
Binding studies
Binding interactions between the ligands and the porphyrins
were studied using UV/vis absorption spectroscopy in toluene
and in 1,1,2,2-tetrachloroethane (TCE). The unbound zinc por-
phyrins have a strong UV/vis absorption band at 422–426 nm,
and addition of an imidazole ligand gives rise to a new band
that is shifted by about 4 nm. Monitoring the decrease in the
absorption due to the unbound porphyrin and the increase in
the absorption due to the bound porphyrin allowed determi-
nation of the association constants from UV/vis titrations.
Titrations were carried out for the 80 different ligand–por-
phyrin–solvent combinations using an automated UV/vis plate
reader. In all cases, the titration data fit well to a 1 : 1 binding
isotherm, and a clearly defined isosbestic point was observed.
The association constants are listed in Tables 1 and 2. The
association constants are generally one to two orders of magni-
tude higher in toluene than in TCE, reflecting the relative
polarity of the two solvents. TCE competes better than toluene
for both the zinc–nitrogen interaction and any H-bonding
interactions.
DMC analysis
The association constants are illustrated graphically in Fig. 5
with the complexes organised and colored according to their
role in the DMC. In general, the complexes where intramolecu-
lar H-bonding is possible (blue data) are more stable than the
reference complexes where these H-bonds are absent (red,
green and yellow data).
Fig. 3 Porphyrin receptors: P1a–P4a have phenol groups that can form
intramolecular H-bonds, and P1b–P4b do not.
The free energy contributions due to intramolecular
H-bonding interactions, ΔΔG°, were determined using eqn (1),
and the results are presented in Tables 3 and 4. Many of the
complexes do not make detectable H-bonding interactions,
and the values of ΔΔG° are generally small compared with pre-
vious experiments using pyridine ligands, where we have
measured free energy contributions of up to 10 kJ mol−1 for
amide-phenol H-bonds.8g The results in Tables 3 and 4 do not
depend strongly on solvent, with similar values of ΔΔG° in
toluene and TCE. However, there is some variations in ΔΔG°
Fig. 4 Imidazole ligands: 1–4 have carbonyl groups that can form
H-bonds, and 5 is the control that does not.
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Table 1 Association constants (M−1) for the formation of 1 : 1 complexes at 298 K in toluene (errors in brackets)a
Ligand
Porphyrin
1
2
3
4
5
P1a
P2a
P3a
P4a
P1b
P2b
P3b
P4b
1.50 × 104 (30%)
6.87 × 104 (4%)
6.04 × 104 (8%)
2.23 × 104 (9%)
1.85 × 104 (3%)
1.34 × 104 (7%)
2.55 × 104 (4%)
2.36 × 104 (5%)
7.32 × 104 (7%)
9.99 × 104 (9%)
4.10 × 105 (7%)
1.00 × 105 (18%)
3.49 × 104 (5%)
2.91 × 104 (4%)
7.21 × 104 (5%)
5.01 × 104 (15%)
1.48 × 105 (10%)
3.86 × 105 (6%)
7.50 × 105 (9%)
3.41 × 105 (7%)
1.58 × 105 (10%)
1.06 × 105 (6%)
2.66 × 105 (10%)
1.74 × 105 (6%)
7.25 × 105 (20%)
6.37 × 105 (10%)
1.59 × 106 (20%)
5.13 × 105 (10%)
1.58 × 105 (10%)
1.36 × 105 (8%)
2.69 × 105 (6%)
2.10 × 105 (9%)
4.40 × 104 (10%)
1.23 × 105 (5%)
1.83 × 105 (7%)
1.31 × 105 (9%)
7.49 × 104 (22%)
5.66 × 104 (7%)
1.13 × 105 (6%)
6.99 × 104 (4%)
a Errors are quoted as twice the standard deviation of the values from 6–12 repeats of the same titration.
Table 2 Association constants (M−1) for the formation of 1 : 1 complexes at 298 K in TCE (errors in brackets)a
Ligand
Porphyrin
1
2
3
4
5
P1a
P2a
P3a
P4a
P1b
P2b
P3b
P4b
2.78 × 103 (8%)
6.79 × 103 (7%)
4.75 × 103 (5%)
2.49 × 103 (3%)
2.15 × 103 (7%)
1.57 × 103 (6%)
2.65 × 103 (9%)
1.95 × 103 (4%)
2.62 × 103 (5%)
1.09 × 104 (6%)
8.91 × 103 (3%)
1.50 × 103 (3%)
1.60 × 103 (8%)
1.17 × 103 (7%)
1.73 × 103 (10%)
1.41 × 103 (9%)
1.17 × 104 (7%)
1.45 × 104 (5%)
1.33 × 104 (3%)
3.79 × 103 (4%)
5.19 × 103 (6%)
3.94 × 103 (4%)
5.80 × 103 (4%)
5.43 × 103 (4%)
2.31 × 104 (22%)
2.77 × 104 (17%)
4.65 × 104 (5%)
5.46 × 103 (5%)
7.72 × 103 (6%)
5.92 × 103 (10%)
8.81 × 103 (7%)
7.75 × 103 (9%)
4.89 × 103 (15%)
1.06 × 104 (4%)
7.93 × 103 (14%)
6.30 × 103 (7%)
6.20 × 103 (3%)
3.99 × 103 (5%)
7.02 × 103 (3%)
6.64 × 103 (2%)
a Errors are quoted as twice the standard deviation of the values from 6–12 repeats of the same titration.
with the polarity of the H-bond acceptor. The H-bonds formed the zinc–nitrogen interaction is made, and the fully bound
with the amide ligands (2 and 4) are generally more favourable state, where both the zinc–nitrogen interaction and the intra-
than the corresponding ester ligands (1 and 3). This result is molecular H-bond are made, is given by KrefEM, where Kref is
consistent with the H-bond acceptor parameters that quantify the equilibrium constant for formation of the corresponding
the polarity of these functional groups: β = 5.4 for esters, and intermolecular H-bond. Thus in order to determine EM, we
β = 8.3 for amides.14 Comparison of the results obtained for have to establish the distribution of partially bound states in
the more flexible ligands with the corresponding rigid ligands the porphyrin–ligand complex and estimate the value of Kref
reveals no significant differences in the ΔΔG° values (compare using an appropriate model system (Fig. 1a). For a porphyrin–
1 with 3 or 2 with 4).
ligand complex that can make one intramolecular H-bond
There are no detectable H-bonding interactions in any of (complex A in the DMC in Fig. 2), the association constant
the P4a complexes, because the phenol H-bond donor sites are measured by UV/vis titration, KA, is the sum of the association
located on the periphery of the porphyrin and are too far away constants for the partially and fully bound states (eqn (2)).8h
to contact the ligand H-bond acceptor sites. The porphyrins
that are most complementary to the ligands are P1a and P3a.
KA ¼ ð1 þ 4KrefEMÞK0
ð2Þ
In these porphyrins, the H-bond donors can be directed
towards the centre of the ligand binding site. However, Fig. 6
shows a representation of the structures of the P1a and P3a
complexes, which suggests that the geometric complementar-
ity is quite different in these two systems. Clearly, the com-
plexes are sufficiently flexible to allow P1a to make just as
strong a H-bond as P3a, despite the implications of Fig. 6.
where K0 is the intermolecular association constant for for-
mation of only the zinc–nitrogen interaction, and 4 is a statisti-
cal factor that accounts for the fact that the porphyrins have 4
H-bond donor sites.
The other three complexes in the DMC cannot make intra-
molecular H-bonds, so the values of KB, KC and KD are all equi-
valent to K0. The strength of the zinc–nitrogen interaction (K0)
Effective molarity
The free energy contributions in Tables 3 and 4 can be used to is not the same in all four complexes, but these differences
determine the effective molarity (EM) for the intramolecular cancel out in a pairwise manner in the DMC. Thus EM can
H-bonds in these complexes. Fig. 1b shows that the equili- be related to the value of ΔΔG° measured in the DMC using
brium constant between a partially bound state, where only eqn (3).
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Table 4 Free energy contributions from carbonyl–phenol H-bonds
(ΔΔG°/kJ mol−1) determined using the DMC in Fig. 2 at 298 K in TCEa
Ligand
Porphyrin
1
2
3
4
P1a
P2a
P3a
P4a
−1
−1
−1
−1
−2
−3
−4
0
−3
−1
−2
+1
−3
−1
−4
+1
a Errors 1 kJ mol−1
.
Fig. 6 Structures of the H-bonded complexes formed between the imi-
dazole ligands and porphyrins P1a and P3a.
Table 5 Effective molarities (EM/mM) for formation of intramolecular
carbonyl–phenol H-bonds at 298 K in toluenea,b
Ligand
Porphyrin
1
2
3
4
P1a
P2a
P3a
P4a
nb
100
nb
nb
8
nb
7
nb
nb
nb
nb
20
3
8
nb
nb
a Errors are
detectable H-bonds.
50%. b nb indicates complexes that do not make
Fig. 5 Association constants (log K/M−1) for formation of porphyrin–
ligand complexes at 298 K in toluene (a) and in TCE (b). The data are
colour coded according to their role in the DMC (c). L = imidazole
ligand, P = zinc porphyrin, A = H-bond acceptor, D = H-bond donor.
between p-cresol and N,N-diethyl acetamide. The values of Kref
for the ester-phenol H-bond are 3 and 2 M−1 in toluene and
TCE respectively, and values of Kref for the amide-phenol
H-bond are 86 and 22 M−1 in toluene and TCE respectively.8g
We assume that there is no difference between the H-bond
properties of the saturated and unsaturated ligands. By
analogy, the experimentally determined H-bond acceptor para-
meter for ethyl cinnamate, an unsaturated ester, is comparable
to the value for saturated esters (β = 5.6 and 5.4 respectively).15
The values of Kref were used in eqn (3) to determine EM for the
intramolecular H-bonds in the porphyrin–ligand complexes,
and the results are shown in Tables 5 and 6.
The values of EM range from 3 mM to 200 mM, which is
similar to the range we have found previously for intramole-
cular H-bonds in zinc porphyrin–pyridine complexes (1 mM to
1 M).7g There is insufficient data for a detailed comparison of
ligand families. However, EM was determined for all combi-
nations of ligands 2 and 4, porphyrins P1a and P3a, and
solvent. The values of EM are similar for all eight complexes.
Table 3 Free energy contributions from carbonyl–phenol H-bonds
(ΔΔG°/kJ mol−1) determined using the DMC in Fig. 2 at 298 K in
toluenea
Ligand
Porphyrin
1
2
3
4
P1a
P2a
P3a
P4a
−1
−2
−1
+2
−3
−1
−3
0
−1
−1
−1
0
−5
−2
−3
−1
a Errors 1 kJ mol−1
.
KAKD
KBKC
eꢀΔΔG°=RT
¼
¼ 1 þ 4Kref EM
ð3Þ
The values for Kref were estimated using the 1 : 1 H-bonded
complexes formed between p-cresol and ethyl acetate and
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1, then the intramolecular interaction does not form, and this
determines the lower limit on the values of EM that can be
measured.6 The highest supramolecular EM that we have
measured in porphyrin–ligand systems 1 M, which is many
orders of magnitude lower than the values of EM that are com-
monly observed for intramolecular covalent reactions.9
Table 6 Effective molarities (EM/mM) for formation of intramolecular
carbonyl–phenol H-bonds at 298 K in TCEa,b
Ligand
Porphyrin
1
2
3
4
P1a
P2a
P3a
P4a
nb
nb
nb
nb
10
30
40
nb
200
nb
100
30
nb
40
nb
nb
Experimental
UV/vis titrations
a Errors are 50%. b nb indicates complexes that do not make
detectable H-bonds.
A 5 ml sample of porphyrin solution was prepared at a concen-
tration of 2–5 μM in spectroscopic grade solvent (TCE was
dried over calcium chloride and neutral alumina). A 10–100 ml
sample of ligand solution was prepared at a concentration of
10–2500 μM in spectroscopic grade solvent. 150 μl of porphyrin
solution was added to each of 12 wells of a 96 well Hellma
quartz plate, and the UV/vis absorbance was recorded at three
wavelengths using a BMG FLUOstar Omega plate reader. The
plate reader was thermostatted at 298 K for all measurements.
Aliquots of ligand solution were added successively to each
well containing porphyrin solution, and the UV/vis absorbance
was recorded after each addition. The absorbance at three
different wavelengths was fit to a 1 : 1 binding isotherm using
Microsoft Excel to obtain the association constant. Each titra-
tion was repeated at least six times, and the experimental error
is quoted as twice the standard deviation at a precision of one
significant figure.
This implies that there are no significant solvent effects on EM
in these systems. In addition, the values of EM measured for
the flexible ligands are similar to the values measured for the
more rigid ligands. The implication is that any entropic
benefit associated with decreased conformational flexibility in
the rigid ligand is offset by the ability of the flexible ligand to
optimise the geometry of the H-bond interaction.
Conclusions
The complexation of five different imidazole ligands with
eight different zinc porphyrins has been studied in two
different solvents. Ligands equipped with H-bond acceptor
groups (amide or ester) and zinc porphyrins equipped with
H-bond donor groups (phenol) can make intramolecular
H-bonds in addition to the zinc–nitrogen coordination inter-
action. The matrix of 80 association constants was used to con-
struct 32 double mutant cycles to quantify the free energy
contributions of intramolecular H-bonding to the stabilities of
these complexes. In most systems, no H-bond interactions
were detected. For the systems that make detectable H-bonds,
these interactions contribute up to 5 kJ mol−1 to the stability
of the complexes. Ligands equipped with amide H-bond accep-
tors make stronger H-bonds than the corresponding ligands
equipped with ester H-bond acceptors. This reflects the relative
polarity of these two functional groups: the H-bond acceptor
parameter for esters is β = 5.4, and β = 8.3 for amides. Two
different types of linker were used to connect the H-bond
acceptor to the imidazole core: a flexible CH2CH2 chain, and
the corresponding alkene, which is more rigid. The results
obtained for pairs of ligands that differ only in the nature of
this linker are comparable. This suggests that any entropic
penalty associated with restricting the conformational mobility
of the flexible linker are comparable to conformational strain
in H-bond geometry encountered for the rigid linker.
Synthesis
Ligand 1. 1-Methylimidazole carboxaldehyde (1.0019 g,
9.10 mmol) dissolved in DCM (10 ml) was added to (carbethoxy-
methylene)triphenylphosphorane (4.22 g, 12.1 mmol) dis-
solved in DCM (10 ml) and protected by
a nitrogen
atmosphere. The mixture was stirred at room temperature for
24 hours, quenched with water (20 ml) and washed with DCM
(2 × 20 ml). The combined organic extracts were washed with
aqueous Na2CO3 (2 × 10 ml), brine (2 × 10 ml), dried with
Na2SO4, and the solvent was removed on a rotary evaporator.
The solid residue was purified on silica, eluting with EtOAc–
MeOH (1 : 0 to 9 : 1). The product was isolated as a white solid
(0.95 g, 58%); FT-IR (ATR) vmax/cm−1 3088, 2986, 1702, 1627,
1491, 1478, 1306, 1259, 1171; 1H NMR (400 MHz, CDCl3)
δ 7.63 (s, 1H), 7.53 (d, 1H, J = 16 Hz), 7.49 (s, 1H), 6.29 (d, 1H,
J = 16 Hz), 4.28 (q, 2H, J = 7 Hz), 3.76 (s, 3H), 1.35 (t, 3H, J =
7 Hz); 13C NMR (101 MHz, CDCl3) δC = 116.1, 140.8, 131.8,
129.3, 116.7, 60.7, 32.5, 14.3; MS (ES+) m/z (%) = 181.1 [M+H+]
(100); HRMS (ES+) calc. C9H13N2O3 181.2137, found 181.0872.
N,N-Diethyl-2-(triphenylphosphoranylidene)acetamide13
The association constants for formation of the corres-
ponding intermolecular H-bonding interactions in reference Triphenylphosphine (13.18 g, 50.2 mmol) was added to N,N-
compounds were used to determine values of EM for the intra- diethylchloroacetamide (6.9 ml, 50 mmol) dissolved in aceto-
molecular H-bonds in the porphyrin–ligand complexes. The nitrile (100 ml). The mixture was refluxed for 26 hours and
EMs range from 3 mM to 200 mM, which is comparable to the then allowed to cool to room temperature. The solvent was
results we have obtained previously using complexes formed removed under reduced pressure, and the residue was dis-
between zinc porphyrins and pyridine ligands.8b,g If KrefEM ≪ solved in water (200 ml). After filtering off any solid, NaOH
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(10 ml of a 10 M aqueous solution) was added with cooling.
The mixture was stirred for 20 minutes, and then DCM
(300 ml) was added. The aqueous layer was separated and
washed with DCM (2 × 100 ml). The combined organic extracts
were dried over Na2SO4, and the solvent was removed on a
rotary evaporator. The product was isolated as a yellow solid,
which was used without further purification (15.84 g, 84%);
Notes and references
1 (a) D. Philp and J. F. Stoddart, Angew. Chem., Int. Ed. Engl.,
1996, 35, 1154–1196; (b) J. M. Lehn, Angew. Chem., Int. Ed.
Engl., 1990, 29, 1304–1319; (c) J. M. Lehn, Supramolecular
Chemistry: Concepts and Perspectives, Wiley, Weinheim,
1995; (d) F. Vögtle, Supramolecular Chemistry: An Introduc-
tion, Wiley, Chichester, 1991.
1
FTIR (nujol) vmax/cm−1 3490, 1640; H NMR (400 MHz, CDCl3)
δH 7.90–7.36 (m, 15H), 3.40–3.24 (m, 4H), 2.09 (s, 1H), 1.13 (tt,
6H, J = 7 Hz, J = 11 Hz); 31P NMR (100 MHz, CDCl3) δP = 23.3
(d, 1P, J = 1121 Hz); 13C NMR (100 MHz, CDCl3) δC = 133.1,
133.0, 132.1, 131.1, 131.9, 131.3, 128.6, 128.4, 42.8, 40.4, 14.2,
13.1.
2 (a) S. Shinkai and O. Manabe, Top. Curr. Chem., 1984, 121,
67–104; (b) N. Koumura, R. W. J. Zijlstra, R. A. van Delden,
N. Harada and B. L. Feringa, Nature, 1999, 401, 152–155;
(c) V. Balzani, A. Credi, F. M. Raymo and J. F. Stoddart,
Angew. Chem., Int. Ed., 2000, 39, 3349–3391; (d) E. R. Kay,
D. A. Leigh and F. Zerbetto, Angew. Chem., Int. Ed., 2007,
46, 72–191; (e) B. Lewandowski, G. D. Bo, J. W. Ward,
Ligand 2. 1-Methylimidazole carboxaldehyde (1.10 g,
9.98 mmol) dissolved in DCM (10 ml) was added to N,N-
diethyl-2-(triphenylphosphoranylidene)acetamide
(4.83
g,
M.
Papmeyer,
S.
Kuschel,
M.
J.
Aldegunde,
12.86 mmol) dissolved in DCM (10 ml) and protected by a
nitrogen atmosphere. The mixture was stirred at room temp-
erature for 24 hours, quenched with water (20 ml) and washed
with DCM (2 × 20 ml). The combined organic extracts were
washed with aqueous Na2CO3 (2 × 10 ml), brine (2 × 10 ml),
dried over Na2SO4, and the solvent was removed on a rotary
evaporator. The solid residue was purified on silica, eluting
with EtOAc–MeOH (1 : 0 to 9 : 1). The product was isolated as a
yellow solid (yield 0.79 g, 38%); FT-IR (thin film) vmax/cm−1
P. M. E. Gramlich, D. Heckmann, S. M. Goldup,
D. M. D’Souza, A. E. Fernandes and D. A. Leigh, Science,
2013, 339, 189–193.
3 (a) P. R. Ashton, T. T. Goodnow, A. E. Kaifer,
M. V. Reddington, A. M. Z. Slawin, N. Spencer,
J. F. Stoddart, C. Vicent and D. J. Williams, Angew. Chem.,
Int. Ed. Engl., 1989, 28, 1396–1399; (b) M. Fujita, J. Yazaki
and K. Ogura, J. Am. Chem. Soc., 1990, 112, 5645–5647;
(c) S. K. Chang, D. V. Engen, E. Fan and A. D. Hamilton,
J. Am. Chem. Soc., 1991, 113, 7640–7645; (d) A. G. Johnston,
D. A. Leigh, R. J. Pritchard and M. D. Deegan, Angew.
1
3400, 3100, 1639, 1585, 1485, 1461, 1436, 1278, 1238, 1128; H
NMR (400 MHz, CDCl3) δ 7.58 (d, 1H, J = 15 Hz), 7.50 (s, 1H),
7.40 (s, 1H), 6.69 (d, 1H, J = 15 Hz), 3.67 (s, 3H), 3.48 (q, 2H,
J = 7 Hz), 3.41 (q, 2H, J = 7 Hz), 1.25 (t, 3H, J = 7 Hz), 1.18 (t,
3H, J = 7 Hz); 13C NMR (100 MHz, CDCl3) δC = 165.3, 140.0,
129.9, 129.7, 127.3, 116.4, 42.3, 41.2, 31.9, 15.1, 13.2; MS (ES+)
m/z (%) = 208.1 [M+H+] (100); HRMS (ES+) calc. C11H18N3O
208.2832, found 208.1315.
Ligand 3. A solution of 1 (0.22 g, 1.27 mmol) in methanol
(25 ml) was hydrogenated using a H-cube Pd/C CatCart®
(25 °C, 1 atmosphere, 1 ml min−1 flow rate). The solvent was
removed on a rotary evaporator to give the product as a clear
oil (0.23 g, 100%); vmax/cm−1 (ATR) 3088, 2986, 1702, 1627; δH
(250 MHz; CDCl3) 7.32 (1H, s), 6.71 (1H, d J = 1 Hz), 4.09 (2H,
q, J = 7.5 Hz), 3.52 (3H, s), 2.87–2.75 (2H, m), 2.64–2.55 (2H,
m), 1.20 (3H, t, J = 7.5 Hz); δC (100 MHz; CDCl3) 173.3, 137.7,
130.5, 126.0, 60.64, 32.8, 31.2, 19.2, 14.1; MS (ES+) m/z (%) =
183.1 (100) [M H+], 365.2 (23) [2M H+]. HRMS (ES+) m/z, calcd
for C9H15N2O2 183.1134, found 183.0947.
Ligand 4. A solution of 2 (0.21 g, 1.09 mmol) in methanol
(22 ml) was hydrogenated using a H-cube Pd/C CatCart®
(25 °C, 1 atmosphere, 1 ml min−1 flow rate). The solvent was
removed on a rotary evaporator to give the product as a clear
oil (0.22 g, 96%); vmax/cm−1 (ATR) 2976, 1711, 1631, 1163; δH
(250 MHz; CDCl3) 7.35 (1H, s), 6.74 (1H, d J = 1 Hz), 3.57 (3H,
s), 3.34 (2H, q, J = 7.0 Hz), 3.29 (2H, q, J = 7.0 Hz), 2.93–2.83
(2H, m), 2.67–2.57 (2H, m), 1.15 (3H, t, J = 7.0 Hz), 1.09 (3H, t,
J = 7.0 Hz); δC (100 MHz; CDCl3) 170.4, 137.5, 131.6, 125.8,
41.9, 40.3, 31.8, 19.5, 14.3, 13.1; MS (ES+) m/z (%) = 210.2 (100)
[M H+], 419.2 (23) [2M H+]; HRMS (ES+) m/z, calcd for
C11H20N3O210.2960, found 210.1582.
Chem.,
Int.
Ed.
Engl.,
1995,
34,
1209–1212;
(e) R. P. Sijbesma, F. H. Beijer, L. Brunsveld, B. J. B. Folmer,
J. H. K. K. Hirschberg, R. F. M. Lange, J. K. L. Lowe and
E. W. Meijer, Science, 1997, 278, 1601–1604.
4 (a) A. Mulder, J. Huskens and D. N. Reinhoudt, Org.
Biomol. Chem., 2004, 2, 3409–3424; (b) J. D. Badjić,
A. Nelson, S. J. Cantrill, W. B. Turnbull and J. F. Stoddart,
Acc. Chem. Res., 2005, 38, 723–732; (c) J. K. Sprafke,
B. Odell, T. D. W. Claridge and H. L. Anderson, Angew.
Chem., Int. Ed., 2011, 50, 5572–5575.
5 W. Saenger, Principles of nucleic acid structure, Springer-
Verlag, New York, 7th edn, 1984.
6 (a) C. A. Hunter and H. L. Anderson, Angew. Chem., Int. Ed.,
2009, 48, 7488–7499; (b) G. Ercolani and L. Schiaffino,
Angew. Chem., Int. Ed., 2011, 50, 1762–1768.
7 (a) E. T. Mack, P. W. Snyder, R. Perez-Castillejos,
B. Bilgicer, D. T. Moustakas, M. J. Butte and
G. M. Whitesides, J. Am. Chem. Soc., 2012, 134, 333–345;
(b) D. J. Cram, Angew. Chem., Int. Ed. Engl., 1986, 25, 1039–
1057; (c) J. M. Lehn, Rep. Prog. Phys., 2004, 67, 249–265;
(d) A. Metzger, V. M. Lynch and E. V. Anslyn, Angew. Chem.,
Int. Ed. Engl., 1997, 36, 862–865; (e) E. Ghidini, F. Ugozzoli,
R. Ungaro, S. Harkema, A. A. El-Fadl and D. N. Reinhoudt,
J. Am. Chem. Soc., 1990, 112, 6979–6985; (f) H. J. Hogben,
J. K. Sprafke, M. Hoffmann, M. Pawlicki and
H. L. Anderson, J. Am. Chem. Soc., 2011, 133, 20962–20969;
(g) A. T. ten Cate, H. Kooijman, A. L. Spek, R. P. Sijbesma
and E. W. Meijer, J. Am. Chem. Soc., 2004, 126, 3801–
3808.
1446 | Org. Biomol. Chem., 2014, 12, 1440–1447
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Paper
8 (a) C. A. Hunter, M. C. Misuraca and S. M. Turega, J. Am.
Chem. Soc., 2011, 133, 582–594; (b) C. A. Hunter,
M. C. Misuraca and S. M. Turega, J. Am. Chem. Soc., 2011,
133, 20416–20425; (c) M. C. Misuraca, T. Grecu, Z. Freixa,
V. Garavini, C. A. Hunter, P. W. van Leeuwen,
M. D. Segarra-Maset and S. M. Turega, J. Org. Chem., 2011,
76, 2723–2732; (d) C. A. Hunter, M. C. Misuraca and
S. M. Turega, Chem. Sci., 2012, 3, 589–601;
(e) E. Chekmeneva, C. A. Hunter, M. C. Misuraca and
S. M. Turega, Org. Biomol. Chem., 2012, 10, 6022–6031;
(f) H. Adams, E. Chekmeneva, C. A. Hunter,
M. C. Misuraca, C. Navarro and S. M. Turega, J. Am. Chem.
Soc., 2013, 135, 1853–1863; (g) H. Sun, C. A. Hunter,
B. Bocquet and C. Piguet, Inorg. Chem., 2010, 49, 1252–
1265; (d) S. L. Tobey and E. V. Anslyn, J. Am. Chem. Soc.,
2003, 125, 10963–10970; (e) D. H. Williams, N. L. Davies,
R. Zerella and B. Bardsley, J. Am. Chem. Soc., 2004, 126,
2042–2049; (f) D. V. Kondratuk, L. M. A. Perdigao,
M. C. O’Sullivan, S. Svatek, G. Smith, J. N. O’Shea,
P. H. Beton and H. L. Anderson, Angew. Chem., Int. Ed.,
2012, 51, 6696–6699; (g) M. A. Hossain and H. J. Schneider,
Chem.–Eur. J., 1999, 5, 1284–1290; (h) W. Jiang,
K. Nowosinski, N. L. Loew, E. V. Dzyuba, F. Klautzsch,
A. Schaefer, J. Huuskonen, K. Rissanen and C. A. Schalley,
J. Am. Chem. Soc., 2012, 134, 1860–1868; (i) M. S. Searle and
D. H. Williams, J. Am. Chem. Soc., 1992, 114, 10690–10697.
C. Navarro and S. Turega, J. Am. Chem. Soc., 2013, 135, 11 (a) O. Middel, W. Verboom and D. N. Reinhoudt, J. Org.
13129–13141; (h) E. Chekmeneva, C. A. Hunter,
M. J. Packer and S. M. Turega, J. Am. Chem. Soc., 2008, 130,
17718–17725; (i) C. A. Hunter, N. Ihekwaba,
Chem., 2001, 66, 3998–4005; (b) R. P. Bonar-Law and
J. K. M. Sanders, J. Chem. Soc., Chem. Commun., 1991, 574–
577.
M. C. Misuraca, M. D. Segarra-Maset and S. M. Turega, 12 (a) S. L. Cockroft and C. A. Hunter, Chem. Soc. Rev., 2007,
Chem. Commun., 2009, 3964–3966.
9 (a) A. J. Kirby, Adv. Phys. Org. Chem., 1980, 17, 183–278;
(b) M. I. Page and W. P. Jencks, Proc. Natl. Acad.
36, 172–188; (b) A. Camara-Campos, D. Musumeci,
C. A. Hunter and S. Turega, J. Am. Chem. Soc., 2009, 131,
18518–18524.
Sci. U. S. A., 1971, 68, 1678–1683; (c) L. Mandolini, Adv. 13 (a) E. Cereda, E. Bellora and A. Donetti, Tetrahedron Lett.,
Phys. Org. Chem., 1987, 22, 1–111; (d) C. Galli and
L. Mandolini, Eur. J. Org. Chem., 2000, 3117–3125.
10 (a) V. M. Krishnamurthy, V. Semetey, P. J. Bracher, N. Shen
1980, 21, 4977–4980; (b) O. Garcia-Calvo, E. Coya, S. Lage,
I. Coldham, N. Sotomayor and E. Lete, Eur. J. Org. Chem.,
2013, 1460–1470.
and G. M. Whitesides, J. Am. Chem. Soc., 2007, 129, 1312– 14 C. A. Hunter, Angew. Chem., Int. Ed., 2004, 43, 5310–
1320; (b) M. Mammen, S. K. Choi and G. M. Whitesides, 5324.
Angew. Chem., Int. Ed., 1998, 37, 2755–2794; 15 F. Besseau and C. Laurence, J. Chem. Soc., Perkin Trans. 2,
(c) J. F. Lemonnier, L. Guenee, G. Bernardinelli, J. F. Vigier,
1994, 485–489.
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