Cyclic Bis-porphyrin-Based Flexible Molecular Containers: Controlling Guest Arrangements and Supramolecular Catalysis by Tuning Cavity Size

Article abstract of DOI:10.1002/chem.201700577

Three cyclic zinc(II) bis-porphyrins (CB) with highly flexible linkers are employed as artificial molecular containers that efficiently encapsulate/coordinate various aromatic aldehydes within their cavities. Interestingly, the arrangements of guests and their reactivity inside the molecular clefts are significantly influenced by the cavity size of the cyclic containers. In the presence of polycyclic aromatic aldehydes, such as 3-formylperylene, as a guest, the cyclic bis-porphyrin host with a smaller cavity (CB1) forms a 1:1 sandwich complex. Upon slightly increasing the spacer length and thereby the cavity size, the cyclic host (CB2) encapsulates two molecules of 3-formylperylene that are also stacked together due to strong π–π interactions between them and CH–π interactions with the porphyrin rings. However, in the cyclic host (CB3) with an even larger cavity, two metal centers of the bis-porphyrin axially coordinate two molecules of 3-formylperylene within its cavity. Different arrangements of guest inside the cyclic bis-porphyrin hosts are investigated by using UV/Vis, ESI-MS, and ¹H NMR spectroscopy, along with X-ray structure determination of the host–guest complexes. Moreover, strong binding of guests within the cyclic bis-porphyrin hosts support the robust nature of the host–guest assemblies in solution. Such preferential binding of the bis-porphyrinic cavity towards aromatic aldehydes through encapsulation/coordination has been employed successfully to catalyze the Knoevenagel condensation of a series of polycyclic aldehydes with active methylene compounds (such as Meldrum's acid and 1, 3-dimethylbarbituric acid) under ambient conditions. Interestingly, the yields of the condensed products significantly increase upon increasing spacer lengths of the cyclic bis-porphyrins because more substrates can then be encapsulated within the cavity. Such controllable cavity size of the cyclic containers has profound implications for constructing highly functional and modular enzyme mimics.

Full text of DOI:10.1002/chem.201700577

A Journal of
Accepted Article
Title: Cyclic Bisporphyrin Based Flexible Molecular Containers:
Controlling Guest Arrangements and Supramolecular Catalysis
by Tuning Cavity Size
Authors: Sankar Prasad Rath, Pritam Mondal, and Sabyasachi Sarkar
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Cyclic Bisporphyrin Based Flexible Molecular Containers:
Controlling Guest Arrangements and Supramolecular Catalysis
by Tuning Cavity Size
Pritam Mondal, Sabyasachi Sarkar and Sankar Prasad Rath*[a]
various guests within their cavities and the shape of the space
Abstract: Three cyclic zinc(II) bisporphyrins (CB) with highly flexible
linkers are employed as artificial molecular containers that efficiently
encapsulate/coordinate various aromatic aldehydes within their
cavities. Interestingly, the arrangements of the guests and their
reactivity inside the molecular clefts are significantly influenced by
the cavity size of the cyclic containers. In presence of polycyclic
aromatic aldehydes such as 3-formylperylene as a guest, the cyclic
bisporphyrin host having smaller cavity (CB1) forms 1:1 sandwich
complex. Upon slight increase of the spacer length and thereby
cavity size, the cyclic host (CB2) encapsulates two molecules of 3-
formylperylene that are also stacked together due to strong π-π
interactions between them and CH-π interactions with the porphyrin
rings. However, in the cyclic host (CB3) with even large cavity, two
metal centers of the bisporphyrin axially coordinate two molecules of
3-formylperylene within its cavity. Different arrangements of guest
inside the cyclic bisporphyrin hosts are investigated using UV-vis,
ESI-MS and 1H NMR spectroscopy along with X-ray structure
determination of the host-guest complexes. Moreover, strong binding
of guests within the cyclic bisporphyrin hosts support the robust
nature of the host-guest assemblies in solution. Such preferential
binding of the bisporphyrinic cavity towards aromatic aldehydes
through encapsulation/coordination has been employed successfully
to catalyze the Knoevenagel condensation of a series of polycyclic
aldehydes with active methylene compounds (such as Meldrum’s
acid and 1, 3-dimethylbarbituric acid) under ambient conditions.
Interestingly, the yields of the condensed products significantly
increase upon increasing spacer lengths of the cyclic bisporphyrins,
since more substrates can then be encapsulated within the cavity.
Such controllable cavity size of the cyclic containers has profound
implications for constructing highly functional and modular enzyme
mimics.
determines the arrangement of molecules inside.[5] Such
molecular entrapment completely isolates the encapsulated
molecules from the bulk solution, which eventually influence and
facilitate various chemical reactions within the cavities of the
containers. Various molecular containers[6] have been previously
reported which facilitate unusual chemical reactivity or promoting
specific reactions, often with the goal of mimicking enzymatic
catalysis.[7] In this regard, the design of molecular container
incorporating covalently linked bisporphyrins has enticed a great
deal of attention recently, because of the cofacial arrangement
through rigid/flexible linkers, which can act as molecular clefts
for the binding and activation of a variety of substrates.[8-10] Of
particular interest to our group is the design of bisporphyrins
appropriate for the applications involving efficient molecular
recognition and catalysis.[11] Bisporphyrin cavities of different
shapes and sizes can recognize the substrates for various
practical applications, while the presence of metal centers would
facilitate the scopes further.[12]
In the present investigation, we have employed three tunable
dizinc(II) cyclic bisporphyrins[13] (CB) varying spacer lengths as
flexible molecular containers that are capable of encapsulating
aromatic guests to modulate their reactivity and alter the product
distributions efficiently. We observe that the cyclic bisporphyrins
efficiently accelerate the Knoevenagel condensation of
Meldrum’s acid (MA)/1,3-dimethylbarbituric acid (DBA) with a
series of polycyclic aldehydes which are otherwise less reactive
under ambient conditions in the absence of any catalyst. The
cyclic bisporphyrins can preferentially encapsulate/coordinate
the polycyclic aldehydes inside its cavity according to the spacer
length and after the reaction the condensed products are
spontaneously released from the bisporphyrinic cavity due to
host-guest size discrepancy. Most interestingly, the yields of the
condensation products are regulated by the cavity size of the
cyclic bisporphyrins, more specifically the spacer length of the
bisporphyrins. As the spacer length of the cyclic host increases,
the yield of the condensed product also increases. So far,
various metal-organic frameworks, zeolites, and covalent
Introduction
Over the past several decades, supramolecular chemists have
given considerable efforts for the design of artificial molecular
entities that can behave as container-like systems.[1] The
growing interest of such systems is driven in part by their
potential applications and prospects in areas as diverse as
recognition,[2] catalysis,[3] and transport.[4] The molecular
containers generally offer a confined space to encapsulate
organic frameworks are used to act as
a size-selective
catalyst,[14] which means the selective conversion of one
substrate over others based on their size. However, influencing
the product distribution by tuning the container’s binding pocket
is rarely known in literature[15] and detailed understanding will
most certainly result in many promising developments and
applications in the field of molecular container and catalysis.
[a]
P. Mondal, Prof. Dr. S. P. Rath
Department of Chemistry, Indian Institute of Technology Kanpur,
Kanpur-208016, India.
E-mail: sprath@iitk.ac.in
Homepage: http://home.iitk.ac.in/~sprath/
Supporting information for this article is given via a link at the end of
the document.
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Results and Discussion
Scheme 1. Cyclic bisporphyrins and their complexes with aldehydes.
Dizinc(II) cyclic bisporphyrin hosts, CB1 and CB2 have been
Similarly, upon gradual addition of peryald to a dichloromethane
solution of CB2, the intensity of the Soret band decreases and
red shifted (from 410 to 411 nm) (Figure 1B) owing to the
formation of CB2·(peryald)2, which has also been isolated and
structurally characterized. However, in case of CB3, a modest
bathochromic shift (410 to 413 nm) with a concomitant decrease
in absorption in the Soret band intensity is observed upon
addition of peryald. During such transformation, several
isosbestic points are also observed at 386, 421, 515, 590 nm
(Figure 1C) and the corresponding host-guest complex is
isolated and structurally characterized as CB3·(peryald)2.
Similarly, the interactions of the cyclic bisporphyrins (CB) with
other aromatic aldehydes, e.g. pyrene-1-carbaldehyde (pyald)
and 9-anthraldehyde (antald) are also monitored using UV-
visible spectroscopy and displayed in Figures S1-S2. Scheme 1
portrays the synthetic outline of all the complexes along with the
abbreviations used.
d]
prepared using the procedures reported earlier.[13c, CB3 has
been synthesized by alkaline-mediated coupling of the
corresponding bromoalkylated mono porphyrin with another
mono porphyrin having phenol functionalities. Further stirring at
room temperature with an excess of zinc acetate in methanol
followed by chromatographic purification yielded dark red CB3
and unambiguously characterized by 1H NMR, ESI-MS, and X-
ray crystallography. The synthetic outline is displayed in Scheme
S1.
The interactions of the cyclic hosts with polycyclic guests are
probed using UV-visible spectroscopy (Figure 1). The absorption
spectrum of CB1 (in dichloromethane) shows an intense Soret
band at 409 nm and two Q bands at 541 and 575 nm. Upon
incremental addition of 3-formylperylene (peryald) to the
dichloromethane solution of CB1, the Soret band intensity
decreases and slightly red shifted (from 409 to 410 nm) (Figure
1A) due to the formation of the 1:1 sandwich complex which has
been isolated in solid as CB1·peryald and spectroscopically
characterized.
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X-ray structures of the host-guest complexes CB2·(peryald)2 and
CB3·(peryald)2 are reported here, which allow us to make a
direct structural and spectroscopic comparison upon guest
binding. Although the single crystals of CB1·peryald, suitable for
X- ray structure determination, could not be obtained, but the
complex is geometrically optimized using density functional
theory (DFT) (vide infra). Additionally, X-ray structures of the
host-guest complexes between CB1 and 6-methoxy 2-
naphthaldehyde (CB1·Me-napald), CB2 and 9-anthraldehyde
[CB2·(antald)2], CB3 and pyrene-1-carbaldehyde [CB3·(pyald)2]
are also reported here, which further offer substantial insight into
the rational control over guest encapsulation and also the
reactivity therein. Detailed synthetic procedures of all the host-
guest complexes and their spectral characterizations are
provided in the experimental section.
Crystallographic characterization
Dark red crystals of CB1·Me-napald are obtained through slow
diffusion of n-hexane into a solution of the respective complex in
dichloromethane. The complex crystallizes in the monoclinic
crystal system with C2/c space group. Perspective view of
CB1·Me-napald is displayed in Figure 2, while the molecular
packing is shown in Figure S5. In CB1·Me-napald, the Zn
centers have four-coordinate square-planar geometry and the
average Zn–Npor distance is 2.04 Å, which is a typical distance
observed in the X-ray structure of Zn(II) porphyrins known in the
literature.[9a, f] The intramolecular Zn···Zn nonbonding distance is
found to be 6.31 Å, while the Cmeso···Cmeso distance is 7.047 Å.
Figure 1. UV-visible spectral changes (at 298K in dichloromethane) of (A)
CB1 (3×10-6 M) upon addition of peryald as the CB1:peryald molar ratio
changes from 1:0 to 1:30, (B) CB2 (3×10-6 M) upon addition of peryald as the
CB2:peryald molar ratio changes from 1:0 to 1:65, (C) CB3 (3×10-6 M) upon
addition of peryald as the CB3:peryald molar ratio changes from 1:0 to 1:80,
and (D) UV-visible spectra of polycrystalline samples of CB1·peryald (red),
CB2·(peryald)2 (blue), CB3·(peryald)2 (green). Arrows indicate the increasing
or decreasing trend in intensity.
Figure 2. A perspective view of X-ray crystal structure (at 100 K) of CB1·Me-
napald (H atoms and solvent molecules have been omitted for clarity).
Distances shown are the separation between two mean planes.
The formation of the host-guest complexes are further
substantiated by ESI-MS spectroscopy. ESI mass spectra of
polycrystalline sample of CB1·peryald reveals the desired signal
at m/z 1953.8616, which is assigned to [CB1·peryald+H]+
(Figure S3), and thus confirms the formation of 1:1 inclusion
complex between the host (CB1) and the guest (peryald).
However, in case of CB2·(peryald)2 and CB3·(peryald)2, the
desired signals at 1144.3190 and 2344.1565 are observed for
[CB2·(peryald)2+2H]2+ and [CB3·(peryald)2+H]+, respectively,
which manifest the formation of 1:2 complexes between the
hosts (CB2 and CB3) and the guest (peryald) (Figure S4).
Moreover, the experimental peaks are isotopically resolved and
are also in good agreement with their theoretical distributions.
As can be seen in the structure, a Me-napald molecule is
sandwiched between the two porphyrin rings with an average
Me-napald···porphyrin distance of ~3.32 Å. Moreover, the guest
(Me-napald) and porphyrin rings are also nearly coplanar, with a
small offset that manifests strong π-π interactions to form a
robust host–guest assembly.
The dark-red crystals of CB2·(peryald)2 are grown via slow
diffusion of acetonitrile into the dichloromethane solution of the
compound. The compound crystallizes in the triclinic crystal
system with P-1 space group. Perspective view of
CB2·(peryald)2 is displayed in Figure 3A, while the molecular
packing is depicted in Figure S6. In CB2·(peryald)2, two Zn
centers have four-coordinate square-planar geometry with the
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metal center close to the least squares plane of the C20N4
porphyrinato core. The average Zn–Npor and the intramolecular
CB2 cavity in CB2·(peryald)2. The selected bond distances and
angles are listed in Table 1, while crystal data and data
Zn···Zn nonbonding distance is found to be 2.039 Å and 11.50 Å, collection parameters are given in Table S1.
respectively, whereas, the Cmeso···Cmeso distance is 11.67 Å. As
can be seen in the structure, two peryald molecules are
cofacially stacked together in the bisporphyrinic cleft with an
angle of 60.10° with the C20N4 porphyrinato plane. The average
distance between two encapsulated peryald molecules is 3.40 Å,
which shows an appreciable π-π interaction between them
(Figure 3B). Also, the average distance from the nearest peryald
carbon atom to C20N4 porphyrinato plane is found to be 3.32 Å,
leading to a strong CH-π interaction with the porphyrin ring
(Figure S7) which eventually facilitates the unusual stacking of
the guests that are observed.
Dark-red crystals of CB2·(antald)2 are obtained similarly as
described above and the complex crystallizes in the monoclinic
crystal system with P21/n space group. Perspective view of
CB2·(antald)2 is displayed in Figure 3C and Figure S8 illustrates
the molecular packing. In CB2·(antald)2, each Zn center has
five-coordinate square-pyramidal geometry in which the metal
centers are displaced by approximately 0.28 Å from the mean
porphyrin plane. The average Zn-Npor and intramolecular Zn···Zn
nonbonding separation are found to be 2.060 and 11.077 Å.
The dark red crystals of CB3·(peryald)2 and CB3·(pyald)2 are
grown via slow diffusion of n-hexane into the dichloromethane
solutions of the respective complexes and both the complexes
crystallize in the triclinic crystal system with P-1 space group.
Perspective views of CB3·(peryald)2 and CB3·(pyald)2 are
displayed in Figure 4. In both the complexes, each Zn centre
has five-coordinate square-pyramidal geometry in which the
metal ions are displaced by approximately 0.25
Å
[CB3·(peryald)2] and 0.33 Å [CB3·(pyald)2] from the C20N4
porphyrinato core. The Zn–Npor distances are found to be 2.051
and 2.075 Å, whereas, Zn-Oax distances are 2.196 and 2.189 Å
for CB3·(peryald)2 and CB3·(pyald)2, respectively. The
intramolecular Zn···Zn nonbonding distances are found to be
15.493 Å [CB3·(peryald)2] and 13.069 Å [CB3·(pyald)2].
As can be seen from the structure of CB3·(peryald)2, the two
peryald molecules are axially coordinated to two Zn centers of
the CB3 with the aldehyde ‘O’ atom, as also observed in the
case of CB2·(antald)2 and the separation between two cofacially
stacked peryald guests inside the cleft is 3.33 Å (Figure 4B).
Such axial coordination of peryald is possible due to the larger
cavity size of CB3, which is not attainable with CB1 and CB2.
However, two more peryald guests are stacked between two
molecules of CB3·(peryald)2 with an average distance of 3.43 Å
between them. The average non bonding separation between
the coordinated
Figure 3. (A) Perspective view of X-ray crystal structure (at 100 K) of
CB2·(peryald)2, (B) diagram illustrating the interactions between two peryald
molecules inside CB2, (C) perspective view of X-ray crystal structure (at 100
K) of CB2·(antald)2, and (D) diagram illustrating the interactions between two
antald molecules inside CB2 (H atoms and solvent molecules have been
omitted for clarity). Distances shown are the separation between two mean
planes.
Figure 4. (A) Perspective view of X-ray crystal structure (at 100 K) of
CB3·(peryald)2, (B) diagram illustrating the interactions between peryald
molecules inside CB3, (C) perspective view of X- ray crystal structure (at 100
K) of CB3·(pyald)2, and (D) diagram illustrating the interactions between pyald
molecules inside CB3 (H atoms and solvent molecules have been omitted for
clarity). Distances shown are the separation between two mean planes.
As can be seen in the crystal structure, two antald molecules are
axially coordinated to the Zn centers of the porphyrin units with
the aldehyde ‘O’ atom (Zn‒Oax: 2.153 Å), and the intermolecular
separation between two cofacially stacked antald molecules
inside the cleft is 3.45 Å (Figure 3D). Due to the smaller size of
the antald (7.29 Å×4.25 Å), CB2 can easily accomodate and
axially coordinates within its cavity to form CB2·(antald)2,
whereas, peryald, having lager size (9.84×4.85 Å) could not
coordinate with the Zn atoms and stacked unusually inside the
and the non-coordinated peryald molecules are found to be 3.35
Å
and thus form strong π-π interaction between them.
Furthermore, the stacking arrangement of the non-coordinated
peryald molecules are stabilized by strong CH-π interactions
with the nearby porphyrin ring, as also observed in
CB2·(peryald)2. Similar stacking pattern is also observed in case
of CB3·(pyald)2, where the interplanar distance between
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Table 1. Selected bond lengths (Å) and bond angles (°).
Bond lengths, (Å)
Zn(1)-N(1)
CB1·Me-napald
2.041(5)
2.039(5)
2.040(6)
2.040(5)
-
CB2·(peryald)2
2.039(2)
2.043(2)
2.036(2)
2.039(2)
-
CB2·(antald)2
2.063(2)
CB3·(peryald)2
2.034(6)
CB3·(pyald)2
2.086(5)
2.073(6)
2.060(6)
2.083(6)
2.189(5)
Zn(1)-N(2)
2.061(2)
2.050(6)
Zn(1)-N(3)
2.052(2)
2.061(6)
Zn(1)-N(4)
2.067(2)
2.061(6)
Zn(1)-O(1L)
2.153(2)
2.196(5)
Bond angles, (°)
N(1)- Zn(1)-N(2)
N(1)- Zn(1)-N(3)
N(1)- Zn(1)-N(4)
N(2)- Zn(1)-N(3)
N(2)- Zn(1)-N(4)
N(3)- Zn(1)-N(4)
N(1)-Zn(1)-O(1L)
N(2)-Zn(1)-O(1L)
N(3)-Zn(1)-O(1L)
N(4)-Zn(1)-O(1L)
92.4(2)
92.71(9)
92.26(9)
170.34(9)
86.04(9)
86.72(9)
162.83(9)
92.10(9)
97.16(8)
95.94(8)
92.50(8)
101.22(8)
92.6(2)
168.1(2)
86.2(2)
86.3(2)
167.8(2)
92.3(2)
100.1(2)
93.6(2)
91.7(2)
98.6(2)
91.6(2)
165.8(2)
85.7(2)
86.9(2)
165.6(2)
92.3(2)
97.4(2)
100.2(2)
96.7(2)
94.2(2)
176.6(2)
179.31(9)
87.0(2)
87.10(9)
87.3(2)
87.51(9)
170.4(2)
176.50(10)
92.7(2)
92.71(9)
-
-
-
-
-
-
-
-
highest in case of CB3·(peryald)2 (15.53 Å), while, lowest in
case of CB1·Me-napald (6.65 Å). Similarly, the intramolecular
Zn···Zn and Cmeso···Cmeso nonbonding distances are also found
to be maximum in case of CB3·(peryald)2, which are 15.493 Å
and 15.384 Å, respectively, whereas, lowest in case of CB1·Me-
napald. Also, due to axial coordination, the displacement (in Å)
of metal atom from the C20N4 porphyrinato core is higher in case
of CB2·(antald)2, CB3·(peryald)2, and CB3·(pyald)2 than the
other three complexes viz. CB2, CB1·Me-napald, and
CB2·(peryald)2. Thus, the comparative structural analyses
between the complexes clearly manifest the exceptional ability
of the cyclic bisporphyrin platforms to flip its cavity by a large
vertical displacement. Such a unique feature is due to the
presence of flexible linkers that can easily be folded to adjust the
various conformations according to the shape and size of the
guests.
coordinated pyald molecules is 3.29 Å and the non-coordinated
pyald molecules is 3.39 Å (Figure 4D). Figures S9 and S10
depict the molecular packing of CB3·(peryald)2 and
CB3·(pyald)2, respectively, whereas, the selected bond
distances and angles are provided in Table 1.
The salient structural features of all the complexes, reported
herein, are compared in Table 2 which further enable us to
analyze the structural and conformational changes upon guest
entrapment within the bisporphyrin hosts. X-ray structure of CB2
has been reported by us previously[9a], where two porphyrin rings
are aligned in a perpendicular fashion to each other with an
angle of 84.97°. However, upon intercalation of two peryald
molecules, the two perpendicular rings immediately switches to
parallel orientation with
a
zero interplanar angle in
CB2·(peryald)2. Similar observations are also obtained for other
complexes reported here. Axial coordination has increased Zn-
Npor distance and metal displacement from the mean porphyrin
plane (ΔZn24). The average mean plane separation is found to be
NMR spectroscopy
1H NMR titration experiments are performed at 298 K in CDCl3 to
gain further insight into the host–guest complexation process in
solution. Usually, cyclic bisporphyrins, CB exist in solution as a
mixture of conformational isomers due to its flexible linkers, thus
showing a complicated 1H NMR spectrum.[11b] However, upon
addition of guest ligands, the spectra are substantially simplified,
indicating that the encapsulation of the guest is accompanied by
an induced-fit conformational change in the host molecules.
Upon addition of 1 equivalent of peryald to a CDCl3 solution of
CB1, upfield shift of the guest protons are observed because of
close proximity and strong ring current effect of two porphyrin
units (trace C, Figure S11). Also, no peaks are observed in the
free guest ligand region, which manifest the robust nature of the
host-guest assemblies in solution. However, addition of one
more equivalent of peryald, no further shifting of the peryald
protons are observed (trace D, Figure S11). Figure 5 shows the
relevant spectra obtained upon gradual addition of peryald to a
Table 2. Selected structural parameters of the complexes.
a
b
24
c
e
Complex
Zn-Npor ΔZn
Δ24 MPSd Cm···Cm Zn···Zna
θf Ref
9a
tw
CB2
2.035 0.09 0.18
-
9.114
9.018 84.97
6.312 0.00
11.500 0.00
CB1·Me-
napald
2.040 0.18 0.26 6.65 7.047
tw
CB2·(peryald)2 2.039 0.03 0.14 10.96 11.672
CB2·(antald)2 2.060 0.28 0.24 11.13 11.440
CB3·(peryald)2 2.051 0.25 0.19 15.53 15.384
CB3·(pyald)2 2.075 0.33 0.08 12.96 13.644
11.077 0.00 tw
15.493 0.00 tw
13.069 0.00 tw
[a] Average value in Å. [b] Displacement (in Å) of Zn from the least-square
plane of C20N4 porphyrinato core. [c] Average displacement (in Å) of atoms
from the least-square plane of C20N4 porphyrinato core. [d] Average distance
(in Å) of two least-squares plane of C20N4 porphyrinato core. [e] Non-bonding
distance (in Å) between two meso carbons that are covalently connected. [f]
Angle between two least-square planes of C20N4 porphyrinato core
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CDCl3 solution of CB2. Traces A and B show the spectra of CB2
and peryald alone, respectively, whereas, traces C and D
illustrate the spectra after additions of one and two equivalents
of peryald, respectively. As can be seen from the spectra, the
largest upfield shifts of the peryald protons take place when two
equivalents of peryald are added. Even after the addition of
more than two equivalents of peryald, no further shifting of the
peryald protons are observed, which confirms the 1:2
complexation between CB2 and peryald (trace E, Figure 5)
of CB3 (Figure 6). However, addition of one more equivalent of
peryald to CB3, another set of peak is generated in the upfield
region (trace E, Figure 6). The newly generated peaks are
shifted further upfield region upon addition of another equivalent
of peryald (trace F, Figure 6). Such shifting of the peryald
protons clearly suggests the presence of two different types of
peryald in the CB3·(peryald)2 assembly, as also observed in the
X-ray crystal structure of the complex (vide supra). In sharp
contrast, slight upfield shifts of the peryald protons are observed
when monomeric unit of the cyclic host [5,15-bis(3′-
hydroxyphenyl)-octaethylporphyrin, mono] is used in the titration,
as shown in Figure S12.
Association constant determination
The association constants for all the host-guest complexes are
determined by the UV-visible spectroscopic titration method,
which clearly rationalize the efficient binding and stoichiometry
of the host-guest complexes. The association constants are
calculated using the HypSpec computer program (Protonic
Software, U.K.), while the species distribution plots of the
complexes are calculated using the program HySS2009
(Protonic Software, U.K.).[16] During the experiment, the
concentrations of the cyclic hosts CB are kept constant at 3×10-6
M while the concentration of peryald is varied within the range of
10-6-10-4 M. The association constants between the host and
guest are calculated by measuring the changes in intensity of
the peak at 450 nm in the UV-visible spectra. Best-fits are
obtained for the 1:1 binding between the CB1 and peryald with
an association constant of 6.3±0.2×105 M-1. However, best-fits
are obtained by applying two step binding model, 1:1 (K1) and
1:2 (K2) complex, between the hosts (CB2 and CB3) and
peryald as guest. For the complexation of peryald, the K1 and K2
are found to be 1.3±0.2×104 M-1 and 3.8±0.3×104 M-1,
respectively, with CB2and 1.6±0.3 ×104 M-1 and 2.5±0.3 ×105 M-
1, with CB3. Larger value of K2 suggests the cooperative binding
of the guest ligand in the 1:2 host-guest complexes. Figures
S13-S15 depict the relevant plots for the binding of peryald
within the cyclic bisporphyrin hosts.
Figure 5. Partial 1H NMR spectra (in CDCl3 at 298 K) of (A) CB2, (B) peryald,
(C) after addition of 1 eqv. of peryald, (D) after addition of 2 eqv. of peryald,
and (E) after addition of 3 eqv. of peryald. Asterisked signals originate from the
excess peryald.
CB catalyzed Knoevenagel condensation reactions
Generally, in the biological systems, the reactivity of a substrate
is tuned by isolating it from the bulk to achieve high selectivity
and catalytic activity.[7b,e] In the present investigation, the
efficient binding between the polycyclic aromatic aldehyde
guests and the cyclic hosts (CB) prompted us to investigate the
potential application of CB as a catalyst for Knoevenagel
condensation. Initially, the inclusion complex CB2·(peryald)2 is
treated with Meldrum’s acid (MA, 4 eqv.) in an aqueous THF
(1:1) medium at room temperature. After 3 hours of stirring at
room temperature, the solution turns dark red and turbid owing
to the precipitation of the condensed product (peryald·MA) as a
yellowish powder, which is further isolated and characterized by
NMR and ESI-MS spectroscopy. 1H NMR spectra show the
formation of the condensation product peryald·MA in 50% yield
(Figure 7). Without the cyclic host as catalyst, peryald gives only
Figure 6. Partial 1H NMR spectra (in CDCl3 at 298 K) of (A) CB3, (B) peryald,
(C) after addition of 1 eqv. of peryald, (D) after addition of 2 eqv. of peryald,
(E) after addition of 3 eqv. of peryald, and (F) after addition of 4 eqv. of peryald.
in solution. A similar observation is obtained upon gradual
addition of peryald upto two equivalents into the CDCl3 solution
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Similarly, 1-naphthaldehyde gives 1-napald·MA in 48% with CB2,
whereas, the yield is found to be 12% in the absence of CB2
(entry 5). In case of other polycyclic aldehydes (i.e., phenald, 2-
napald, Me-napald, indald), the condensation reactions are also
found much faster in the presence of CB2, which, however,
proceed poorly with very low yield without CB2 (Table 3).
Similarly, in presence of more-reactive DBA, the yield of the
condensation products of all polycyclic aldehydes are
significantly increased, which is tabulated in Table S2. In
contrast, smaller aldehyde, such as benzaldehyde, which poorly
fits into the bisporphyrinic cavity, gives almost similar yield of the
a trace amount of the condensation product (∼3%) under similar
experimental condition. 1H NMR spectral measurements also
confirm that the condensation product peryald·MA is not
properly fitted within CB2 cavity (Figure S16), since the guest
size is too large to be accommodated inside the cavity. The
spontaneous release of the condensed product from the
bisporphyrinic cavity encourage us to explore further the
catalytic efficiency of CB2 for Knoevenagel condensation of a
number of polycyclic aromatic aldehydes with Meldrum’s acid
(MA) and 1,3-dimethylbarbituric acid (DBA) under ambient
conditions. Scheme 2 portrays the synthetic outline of the
Knoevenagel condensation of aromatic aldehydes reported here,
whereas, detailed synthetic procedure and spectroscopic
characterizations of all the condensed products are given in the
supporting information (Figures S17-S32).
Table 3. Yields of the Knoevenagel condensations of aromatic aldehydes and
Meldrum’s acid. [a]
Entry
Aldehyde
Reaction
time, h
Yield of product,
Ar·MA (%)
Without
CB2
3
With
CB2
42
1
peryald
pyald
10
10
2
6
50
3
4
5
6
7
8
9
antald
72
10
24
10
10
10
10
2
35
55
48
72
75
54
40
phenald
8
1-napald
2-napald
Me-napald
indald
12
15
16
8
Benzaldehdye
35
[a] Reaction conditions: aldehyde (0.10 mmol), MA (0.10 mmol), and
bisporphyrin (CB2) (1 mol %) in THF:H2O (5.0 mL) at room temperature.
Yields were determined by 1H NMR analysis.
condensed product with MA (35%) corresponding to the reaction
without CB2 (40%) (entry 9, Table 3). Further, we have
investigated the condensation reaction of aliphatic aldehyde like
cyclohexanecarboxaldehyde and isobutyraldehyde with MA for
10 h in presence of CB2, however, no significant enhancement
of the yield is observed. Such observation clearly manifests the
selectivity of the bisporphyrinic cavity towards the polycyclic
aldehydes, which are otherwise less reactive under ambient
conditions without any catalyst.
Figure 7. Partial 1H NMR spectra (in CDCl3 at 298 K) of (A) CB2·(peryald)2,
(B) after the condensation of peryald with MA, (C) condensation product
peryald·MA obtained after extraction and purification
Encouraged by these results, we have further investigated the
catalytic activity of the other cyclic hosts (CB1 and CB3) towards
the Knoevenagel condensation reaction. Interestingly, we notice
that under identical condition, the yields of the condensation
products are varied according to the cavity size of the
bisporphyrins, more specifically the spacer length of the
bisporphyrins. In case of cyclic host with smallest cavity size,
CB1, the product peryald·MA is obtained in only 17% yield.
However, using CB2, an impressive progress of the
condensation reaction is observed and the product peryald·MA
is obtained in 42% yield. Further increasing the cavity size in
CB3, yield of the product increases significantly upto 50%.
Similar observation has also been obtained in case of the
condensation reaction of other aldehydes (i.e., pyald, antald and
2-napald) with MA (Table 4). Such observation can be explained
in the light of the guest arrangement inside the bisporphyrinic
cavity. The sequestering of the aldehydes from the bulk and
strong substrate binding within the bisporphyrinic cavity is
possibly the primary reason for enhancing the catalytic ability of
the CB. In case of CB1, only one molecule of aldehyde gets
encapsulated within the cleft which drives the reaction to the
condensed product resulting only a slight increase of the yields.
Scheme 2. Knoevenagel condensation of aromatic aldehydes in aqueous THF.
In presence of CB2 (1 mol %), the condensation between
peryald and MA significantly increases to 42% yield within 10 h
(with respect to aldehyde). Similarly, the condensation of pyald
with MA efficiently promote to 50% within 10 h, whereas, the
reaction proceed hardly without CB2, 6% (entry 2, Table 3).
Moreover, the condensation of sterically hindered antald with
MA is efficiently elevated to 35% yield, while the condensation of
antald scarcely occur (<2%) without CB2 (entry 3, Table 3).
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In case of CB2·(peryald)2, however, two aldehyde molecules are
held together with a strong π-π stacking interactions which are
further stabilized by the CH-π interactions with the porphyrin
rings. After the reaction, the condensed products become too
bulky for enclathration and readily come out of the cleft and thus
easily replaced by the incoming reactants, which eventually
increases the yield of the products. However, in case of
CB3·(peryald)2, the Zn(II) centers of the bisporphyrin
coordinatively interacts with the aldehydic ‘O’ atom to selectively
accommodate them inside the macrocyclic cavity and activate
the substrates leading to better yield. The accommodations of
other aldehydes (such as pyald, antald and 2-napald) inside
CB2 and CB3 cleft are very similar, although the yields of the
condensed products are slightly higher with CB3. This is
because more number of aldehyde substrates can then be
encapsulated in CB3 due to larger cavity size, as observed in
the X-ray crystal structure of the host-guest complexes.
Moreover, to justify the cavity effect, a control experiment is
performed in presence of monomeric unit of the cyclic host
[5,15-bis(3′-hydroxyphenyl)-octaethylporphyrin, mono] which,
however, does not regulate the yield of the condensed products
(Table 4). Furthermore, the condensation reaction between
peryald and MA is performed in different solvent medium, and
found that the reaction is significantly promoted in aqueous THF
(1:1) medium in a homogeneous fashion. The low solubility of
MA in organic solvents (such as chloroform and
dichloromethane), and insolubility of CB in polar protic solvents
(such as methanol, ethanol and water) are responsible for the
poor catalytic transformation (Table S3). To check the reusability
of the cyclic host, it is separated and collected from the reaction
mixture by simple column chromatography. No significant
changes have been observed in the isolated cyclic host (Figure
S33) and can be easily employed for further catalytic processes.
Figure 8. Proposed mechanism for the catalytic Knoevenagel condensation of
peryald with MA in the presence of cyclic host CB3. For simplicity, only one
guest molecule is shown in this catalytic cycle.
Theoretical calculation
Computational studies are carried out by using density functional
method (DFT) at the B3LYP/6-31G** level[17] to get further insight
of the host-guest assemblies. All the coordinates are taken
directly from the X-ray crystal structure of the complexes and all
Table 4. Yields of the products in presence of CB.[a]
Entry
Aldehyde
Yield of product, Ar·MA (%)
CB1
CB2
CB3
mono
1
2
3
peryald
pyald
antald
17
22
11
42
50
35
50
61
42
6
8
2
4
2-napald
30
72
82
15
[a] Reaction conditions: aldehyde (0.10 mmol), active methylene compound
(0.10 mmol), and bisporphyrin (1 mol %) in THF:H2O (5.0 mL) at room
temperature. Yields are determined by 1H NMR analysis. Reaction time: 10 h
(in case of antald, 72 h).
The proposed mechanism for the catalytic Knoevenagel
condensation within cyclic host CB3 is displayed in Figure 8.
Generally, water soluble MA is not a suitable guest for CB3, and
it exists in equilibrium with its enolate form because of its low
pKa value.[5a] We assume that, after accommodating the
aldehyde inside the bisporphyrin cleft, the enolate form of MA
attacks the encapsulated aldehyde. After that, the loss of water
readily takes place (due to the hydrophobicity of the cavity) to
generate the dehydrated product peryald·MA. Since, the product
peryald·MA is too bulky to be fitted within the cavity, it
spontaneously comes out of the bisporphyrin cavity while a new
aldehyde substrate comes in.
Figure 9. Optimized molecular structure of (A) CB1·peryald, (B)
CB2·(peryald)2, and (C) CB3·(peryald)2 calculated by DFT method at the
B3LYP/6-31G** level. Values show the Mulliken charges on the formyl carbon
of peryald and mean plane separation between two porphyrin rings.
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the molecules are successfully optimized. The DFT optimized
structures of CB1·peryald is displayed in Figure 9 along with the
optimized structures of CB2·(peryald)2 and CB3·(peryald)2,
whereas the optimized structure of CB1·pyald is displayed in
Figure S34. The computed bond distances and angles of
CB1·peryald and CB1·pyald are shown in Table S4. The
Mulliken charge density on the formyl carbon atom of the
encapsulated aldehydes within CB cleft are also calculated and
displayed in Figure 9. Due to the axial coordination, the +ve
charge on the formyl carbon is highest in case of CB3·(peryald)2,
resulting a feasible nucleophilic attack of the enolate form of MA.
Moreover, we have optimized the geometry of CB2·(peryald)2 by
stacking the two peryald molecule parallel with the porphyrin
planes. However, the original geometry of CB2·(peryald)2 is
found to be energetically stable by 7.64 kcal/mol than the
forcefully optimized structure (Figure 10). Further, the geometry
of the condensed product (peryald·MA) within the CB2 cleft is
successfully optimized and it is found that the product is ejecting
out of the cleft. Figure S35 displays the optimized geometry of
the peryald·MA within CB2, while the coordinates of optimized
geometries are provided in supporting information.
Experimental Section
Materials: All the reagents and solvents are purchased from commercial
sources and purified by standard procedures before use. 3-
formylperylene was synthesized according to a reported procedure.[18]
Zn(II) cyclic bisporphyrin hosts, CB1 and CB2 have been prepared using
the procedures reported earlier.[13c,d] CB3 have been synthesized
according to the synthetic strategy displayed in Scheme S1. 5, 15-bis (3′-
hydroxyphenyl)-octaethylporphyrin, A was synthesized according to a
reported procedure.[13b] The other synthetic steps and the preparation
host-guest complexes, reported in the present work, are described below.
Preparation
of
5,l5-bis(3′-(1-bromooctanyloxy)
phenyl)-
octaethylporphyrin, B: Into a 100 mL round-bottom flask equipped with
a magnetic stir bar and a water-cooled reflux condenser were combined
with A (100 mg, 0.14 mmol), K2CO3 (100 mg, 0.72 mmol), 18-crown-6
(20 mg, 0.075 mmol), dry acetone (50 mL), and 1,8-dibromooctane (3.0 g,
11.02 mmol) and the resulting mixture was refluxed under N2 atmosphere
for 12 h. The reaction mixture was then cooled to room temperature,
filtered, and further washed with 50 mL acetone. The combined solution
was evaporated to dryness and it was further purified by silica gel column
chromatography (hexane/CH2Cl2 2:1 v/v) to yield the desired product as a
red solid. Yield: 141 mg (92%). 1H NMR (CDCl3, 298 K): 10.29 (s, 2H;
meso-H), 7.92 (d, 2H; Ar-H), 7.86 (s, 2H; Ar-H), 7.62 (t, 2H; Ar-H), 7.40
(d, 2H; Ar-H), 4.19 (t, 4H; -OCH2), 3.56 (t, 4H; -CH2Br), 3.43 (m, 8H; -
CH2CH3), 3.28 (m, 8H; -CH2CH3), 2.92 (m, 12H; -OCH2C6H12CH2Br),
1.96 (m, 12H; -OCH2C6H12CH2Br), 1.82 (t, 12H; -CH2CH3), 1.35 (t, 12H; -
CH2CH3), -2.18 (br, 2H, -NH); UV-vis (CH2Cl2) [max, nm (ε, M-1 cm-1)]:
411 (5.0 x 105), 509 (1.6 x 104), 541 (1.2 x 104), 577 (5.3 x 103), 631(3.0
x 103); ESI-MS: m/z 1100.51[M]+.
Preparation of free base cyclic bisporphyrin, C: To a magnetically
stirred DMF suspension (150 mL) of K2CO3 (600 mg, 4.32 mmol) in a
250 mL round-bottom flask was added dropwise a DMF solution (25 mL)
of A (200 mg, 0.278 mmol) and B (306 mg, 0.278 mmol) over a period of
12 h. After the addition was completed, the reaction mixture was allowed
to stir for an additional 60 h. Then the reaction mixture was poured into
toluene (300 mL), washed with water (2×200 mL), dried over Na2SO4,
and evaporated to dryness. The cyclic dimer was purified by silica gel
column chromatography (hexane/CH2Cl2 1:1 v/v), where the second band
was collected and evaporated to dryness to afford C as reddish solid.
Yield: 110 mg (24%).1H NMR (CDCl3, 298 K): 10.14 (s, 4H; meso-H),
7.78-7.28 (m, 16H; Ar-H), 3.98 (m, 20H; -OCH2, -CH2CH3), 3.24 (m, 12H;
-CH2CH3), 2.72 (m, 16H; -OCH2C6H12CH2Br, -CH2CH3), 2.12-0.92 (m,
64H; -CH2CH3, -OCH2C6H12CH2Br, -CH2CH3), -2.22 (br, 4H, -NH); UV-vis
(CH2Cl2) [max, nm (ε, M-1 cm-1)]: 412 (5.0 x 105), 509 (4.5 x 104), 540 (3.1
x 104), 578 (1.1 x 104), 631(6.5 x 103); ESI-MS: m/z 1659.11 [M+H]+.
Figure 10. Relative energies of the optimized molecular structure of two
differently oriented CB2·(peryald)2.
Conclusion
Preparation of Zn(II) cyclic bisporphyrin, CB3: To a magnetically
stirred solution of C (100 mg, 0.06 mmol) in chloroform (50 mL) in a 250
mL round-bottom flask was added a solution of Zn(OAc)2 (100 mg, 0.54
mmol) in methanol (10 mL). The resulting mixture was allowed to stir at
room temperature for 5-6 h. The reaction mixture was then evaporated
and the residue was purified by silica gel column chromatography
(hexanes/CH2Cl2 1:1 v/v) to yield the desired product CB3 as a bright red
solid. Yield: 100 mg (93%). 1H NMR (CDCl3, 298 K): 10.05 (s, 4H; meso-
H), 7.84-7.30 (m, 16H; Ar-H), 4.02 (m, 20H; -OCH2, -CH2CH3), 3.30 (m,
12H; -CH2CH3), 2.80 (m, 16H; -OCH2C6H12CH2Br, -CH2CH3), 1.87-0.88
In summary, we have synthesized three cyclic bisporphyrins
varying chain lengths (and hence cavity size) that can
preferentially encapsulate/coordinate various polycyclic aromatic
aldehydes and thereby influence the reactivity in a controlled
way. Different stacking arrangements are observed within the
bisporphyrin cavity based on the host-guest size compatibility,
which is substantiated by using UV-vis, 1H NMR, and X-ray
crystal structure determination. The preferential and efficient
binding of the cyclic bisporphyrin hosts towards aromatic
aldehyde as guest has been successfully utilized to catalyze the
Knoevenagel condensation of a series of aromatic aldehydes
with Meldrum’s acid/1,3-dimethylbarbituric acid in aqueous THF
medium. It is found that the condensation reactions are quite
selective towards the polycyclic aromatic aldehydes here. Upon
increasing the chain length and thereby cavity, the yield of the
condensed product also increases. Thus, we have utilized here
(m, 64H; -CH2CH3, -OCH2C6H12CH2Br,-CH2CH3); UV-vis (CH2Cl2) [max
,
nm (ε, M-1 cm-1)]: 410 (6.7 x 105), 540 (4.7 x 104), 575 (3.0 x 104); ESI-
MS: m/z 1785.90 [M+H]+.
All the host-guest complexes reported in the present work were prepared
using the general procedure; details for one representative case are
described below.
Preparation of CB1•peryald: To a solution of CB1 (20 mg, 0.012 mmol)
in 50 mL dichloromethane, peryald (30 mg, 0.107 mmol) was added and
stirred at room temperature for 15 minutes. The resulting solution was
then evaporated to complete dryness. The solid thus obtained was
dissolved in a minimum volume of dichloromethane and carefully layered
with acetonitrile which was then kept for slow diffusion in air. On standing
for 6-7 days, dark red crystalline solid of the host-guest complex was
formed in excellent yields which was then collected and dried in vacuum.
tunable bisporphyrinic container molecules to manifest
a
trademark feature of enzymatic catalysis. Moreover, such
flexible molecular containers can be useful in developing
artificial molecular devices and molecular machines. Efforts
toward developing of these areas are currently under
investigation.
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10.1002/chem.201700577
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Yield: 17 mg (75%). 1H NMR (CDCl3, 298 K): 9.99 (s, 4H; meso-H), 9.88
(s, 1H; CHO), 8.78 (d, 1H; peryald-H), 7.85-7.16 (m, 26H; Ar-H, peryald-
H), 4.10-3.74 (m, 16H; OCH2C2H4CH2O, -CH2CH3), 2.72-2.60 (m, 32H; -
OCH2C2H4CH2O, -CH2CH3), 1.74 (t, 24H; -CH2CH3), 1.09 (t, 24H; -
CH2CH3); UV-vis (CH2Cl2): [max, nm (ε, M-1cm-1)]: 410 (3.5 x 105), 541
(1.8 x 104), 575 (6.2 x 103); ESI-MS: m/z 1953.86 [M+H]+.
determinations (Figures S13–S15), typical procedure for the
Knoevenagel condensation, 1H NMR spectra of the condensed products
(Figures S17–S32), tables of yields of the products (Table S2, S3),
optimized geometries of CB1•pyald and peryald•MA within CB2 (Figures
S34, S35), atom numbering schemes (Figure S36), computed bond
distances and angles (Table S4), cartesian coordinates of the optimized
geometry.
Preparation of CB2•(peryald)2: Yield: 21 mg (78%). 1H NMR (CDCl3,
298 K): 10.01 (s, 4H; meso-H), 9.67 (s, 2H; CHO), 8.52 (d, 2H; peryald-
H), 7.86-6.84 (m, 36H; Ar-H, peryald-H), 3.89-3.53 (m, 40H;
OCH2C4H8CH2O, -CH2CH3), 2.68 (m, 16H; -OCH2C4H8CH2O), 1.75 (t,
24H; -CH2CH3), 1.07 (t, 24H; -CH2CH3); UV-vis (CH2Cl2) [max, nm (ε, M-1
cm-1)]: 411 (2.8 x 105), 540 (1.2 x 104), 575 (3.8 x 103); ESI-MS: m/z
Acknowledgements
We are thankful to Science and Engineering Research Board
(SERB), India for financial support and PM thanks IIT Kanpur,
for fellowship.
1144.31 [M+2H]2+
.
Preparation of CB3•(peryald)2: Yield: 18 mg (70%). 1H NMR (CDCl3,
298 K): 10.05 (s, 4H; meso-H), 9.82 (br, 2H; CHO), 8.77 (br, 2H; peryald-
H), 7.83-7.18 (m, 36H; Ar-H, peryald-H), 4.03-3.87 (m, 20H;
OCH2C6H12CH2O, -CH2CH3), 2.81-2.70 (m, 36H; -OCH2C6H12CH2O, -
[1]
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CCDC 1531241 [CB3•(peryald)2], 1531242 [CB2•(peryald)2], 1531243
(CB1•Me-napald), 1531244 [CB2•(antald)2], and 1531245 [CB3•(pyald)2]
contain the supplementary crystallographic data for this paper. These
data are provided free of charge by The Cambridge Crystallographic
Data Centre.
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a
B3LYP hybrid functional and the Gaussian 03, revision B.04,
package.[17] The method used was Becke’s three-parameter hybrid-
exchange functional,[22] the non-local correlation provided by the Lee,
Yang, and Parr expression, and the Vosko, Wilk, and Nuair 1980
correlation functional (III) for local correction.[23] The basis set was 6–
31G** for C, N, O, and H atoms and LANL2DZ for Zn atom. Full
geometry optimizations were done in which all the coordinates were
taken from the single-crystal X-ray structure of the molecules. The
optimized geometry was confirmed to be the potential energy minima by
vibrational frequency calculations at the same level of theory as no
imaginary frequencies were found. The orbital surfaces were visualized
by Chemcraft software program. The molecular structures of all the
complexes were also generated and prepared graphically with this
software. The DFT optimized structures of CB1•peryald and CB1•pyald
are displayed in Figures 9A and S34, respectively, and the computed
bond distances and angles of CB1•peryald and CB1•pyald are shown in
Tables S3.
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Table of Contents
FULL PAPER
Three
cyclic
zinc(II)
Pritam Mondal, Sabyasachi Sarkar
and Sankar Prasad Rath*
bisporphyrins (CB) with flexible
linkers are employed as artificial
Page No. – Page No.
molecular
containers.
Interestingly, the arrangements
of the guests and the reactivity
inside the containers are
significantly influenced by the
cavity size of the cyclic
containers.
Cyclic Bisporphyrin Based
Flexible Molecular Containers:
Controlling Guest
Arrangements and
Supramolecular Catalysis by
Tuning Cavity Size
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