A porous coordination polymer exhibiting reversible single-crystal to single-crystal substitution reactions at Mn(II) centers by nitrile guest molecules

Article abstract of DOI:10.1021/ja9006035

The porous coordination polymer {[Mn(L)(H₂O)](H ₂O)_1.5(DMF)}_n (1) containing a water molecule coordinated at the apical position of each distorted octahedral Mn(II) center has been synthesized using the solvothe

Full text of DOI:10.1021/ja9006035

Published on Web 07/21/2009
A Porous Coordination Polymer Exhibiting Reversible
Single-Crystal to Single-Crystal Substitution Reactions at
Mn(II) Centers by Nitrile Guest Molecules
Madhab C. Das and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
Received January 26, 2009; E-mail: pkb@iitk.ac.in
Abstract: The porous coordination polymer {[Mn(L)(H2O)](H2O)1.5(DMF)}n (1) containing a water molecule
coordinated at the apical position of each distorted octahedral Mn(II) center has been synthesized using
the solvothermal technique by reacting Mn(NO3)2 ·4H2O with a new flexible ligand (LH2) having isophthalic
fragment and pyridine donors at the two ends. The coordinated water molecule could be substituted by
nitrile guest molecules such as acetonitrile, acrylonitrile, allylnitrile, and crotononitrile (affording compounds
2-5, respectively) without loss of crystallinity. Interestingly, compound 1 selectively captures cis-crotononitrile
into its cavity from a mixture of cis and trans isomers. Hence, the cis isomer can be separated from the
trans isomer. In each case, 1.5 lattice water molecules and a dimethylformamide (DMF) molecule are also
simultaneously replaced by certain numbers of these guest molecules. When these first-generation
compounds 2-5 are dipped in DMF at room temperature with the lid of the vial open to the atmosphere,
the mother crystal 1 is regenerated in each case. Thus, all of these substitution reactions are completely
reversible. Also, the first-generation compounds 2-5 can be interconverted among one another by dipping
them in appropriate nitrile guests. All of these phenomena could be observed in single-crystal to single-
crystal fashion.
Introduction
are known, examples of single-crystal to single-crystal (SC-SC)
phase transformation6-8 are fewer in number. Most of the
Recent years have witnessed considerable interest in porous
coordination polymers (PCPs) or metal-organic frameworks
(MOFs) because of their potential applications as functional
materials.1-3 One of the most important discoveries in the area
of PCPs involves their flexible and dynamic properties, which
are characteristic of the cooperative action of organic and
inorganic moieties.4,5 Guest-responsive changes between the
solid phases are particularly intriguing. Although various cases
of structural transformation (single-crystal to amorphous-phase)
reported cases involve the dimerization or polymerization of
unsaturated molecules6 or guest exchange of porous materials.7,8
Particularly rare are reactions within the coordination spheres
of transition metals. Such reactions are known in solids,9,10 but
they have often been accompanied by catastrophic failure of
the crystal, thus preventing the identification of the products.
As has been pointed out,4d many applications (e.g., catalysis
and storage) do not require single-crystal transformations, but
retention of single crystallinity would be essential if a crystal
were to be incorporated into a device such as a substrate-
triggered sensor,4d and could also find use in directing multiple
small molecules cooperatively in chemical reactivity. Besides,
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10.1021/ja9006035 CCC: $40.75  2009 American Chemical Society

Substitution Reactions in a Porous Coordination Polymer
A R T I C L E S
direct observation of the orientation the guest molecules adopt
in the voids will offer ways to modify the coordination space
in order to carry out chemical reactions. Our attempt here is
focused on SC-SC transformations through simultaneous M-D
(M ) metal atom; D ) ligand containing a donor atom such as
O or N) bond breaking/new bond formation within PCPs. The
formation of new bonds by photoinduced cross-linking reactions
of organic and coordination polymers in solids is known.6
Metal-ligand bond cleavage, exemplified by the photolytic
cleavage of Mn-CO bonds within a host-guest structure, has
also been reported.11 Bond breaking and new bond formation
by reversible bipy/MeOH substitution in the lattice of a Co(II)
complex,12 reversible cleavage and formation of two bonds
around a Ce(III) center leading to a 3D structure from a 2D
coordination polymer,13 and a reversible exchange of apical aqua
and nitrate ligands coordinated to a Co(II) center14 observed
through an SC-SC transformation by an external stimulus such
as temperature have very recently been reported. It should be
noted that in all three of the above cases, intramolecular
rearrangements of the ligands take place around the metal ions
in the crystal lattices. However, to our knowledge, there has
been just a single report to date of SC-SC transformation
through substitution reactions at a metal center by simultaneous
bond breaking and bond formation by an external guest molecule
(MeOH) in a nonporous coordination polymer.15 Most recently,
reversible loss of a weakly bound pyridine ligand from a square-
pyramidal Cu(II) complex16a and replacement of coordinated
dimethylformamide (DMF) molecules by pyridine ligands of a
Zn(II) coordination polymer16b (though not in SC-SC fashion)
have been reported. Herein, we report the crystallographic
observation of a substitution reaction at a Mn(II) center by a
series of external guest molecules containing a -CN group,
including acetonitrile (MeCN), acrylonitrile (ACN), and allylni-
trile (AN), in which the guest molecule replaces the apical aqua
ligand. The uptake of a particular nitrile in the presence of others
was also probed. The size selectivity of guest inclusion is
exemplified by the uptake of cis-crotononitrile (cis-CTN) from
a mixture of cis- and trans-CTN. A DMF molecule and 1.5
lattice water molecules are also simultaneously replaced by
certain numbers of these external guest molecules. The metal
ion is incorporated into a 3D coordination network that provides
a relatively fluid cavity within the solid. Although there are
reports of retention of single-crystallinity upon exchange of
lattice solvent molecules,7,8 substitution of a coordinated solvent
molecule by an external guest molecule has scarcely been
observed.15 In this regard, the present work is the first report of
SC-SC reaction in which all of the solvent molecules in a
crystal lattice (both coordinated and lattice) are simultaneously
replaced by external guest molecules. It is therefore of consider-
able importance to study the processes that govern cooperative
fluidity in dynamic crystals with a view to gaining better insight
into the fascinating phenomenon of SC transformations.
Experimental Section
Materials. 5-Aminoisophthalic acid, 4-pyridinecarboxaldehyde,
metal salts, and the nitriles were acquired from Aldrich and used
as received. Prior to use, solvents were purified following standard
procedures.
Physical Measurements. Spectroscopic data were collected as
follows: IR spectra (KBr disk, 400-4000 cm-1) were recorded on
a PerkinElmer Model 1320 spectrometer. X-ray powder patterns
(Cu KR radiation, 3 deg/min scan rate, 293 K) were acquired using
a Siefert ISODEBYEFLEX-2002 X-ray generator or a Philips
PW100 diffractometer. Thermogravimetric analysis (TGA) (5 °C/
min heating rate under a nitrogen atmosphere) was performed with
1
a Mettler Toledo Star System. H NMR spectra were recorded on
a JEOL JNM-LA500 FT instrument (500 MHz) in CDCl3 with TMS
as the internal standard. Microanalysis data for the compounds were
obtained from CDRI, Lucknow. Adsorption isotherms were mea-
sured in a Micromeritics Tristar 3000 volumetric instrument under
continuous adsorption conditions. Prior to measurement, samples
were heated at 250 °C for 4 h and outgassed to 10-3 Torr using a
Micromeritics FlowPrep degasser. The specific surface areas of the
compounds were obtained by applying Brunauer-Emmett-Teller
(BET) (N2 isotherm) and Langmuir (CO2 isotherm) analyses. The
temperatures of the measurements for CO2 and N2 were 273 and
77 K, respectively. The cis- and trans-CTN were separated
following a literature method.17
X-ray Structural Studies. Single-crystal X-ray data were
collected at 100 K on a Bruker SMART APEX CCD diffractometer
using graphite-monochromatized Mo KR radiation (λ ) 0.71069
Å). The linear absorption coefficients, scattering factors for the
atoms, and anomalous dispersion corrections were taken from
International Tables for X-ray Crystallography. The data integration
and reduction were processed with SAINT18 software. An empirical
absorption correction was applied to the collected reflections with
SADABS19 using XPREP.20 The structure was solved by the direct
methods using SHELXTL21 and refined on F2 by full-matrix least-
squares techniques using the SHELXL-9722 program package. The
non-hydrogen atoms were refined anisotropically (except as noted).
The H atoms of the coordinated water molecule of 1 were located
by difference Fourier synthesis. The H atoms of the noncoordinated
water molecules in 1 and all of the water molecules of the
regenerated crystals could not be found in the difference Fourier
map. All of the other H atoms were placed in calculated positions
using idealized geometries (riding model) and assigned fixed
isotropic displacement parameters. Several DFIX commands were
used to fix the bond distances of solvent molecules.
Synthesis of LH2. Triethylamine (2.8 mL, 20 mmol) was added
to a stirred mixture of 5-aminoisophthalic acid (0.90 g, 5 mmol),
4-pyridinecarboxaldehyde (0.49 mL, 5.2 mmol), and dry methanol
(50 mL). After 1 h, the mixture became limpid, and an excess of
NaBH4 was slowly added at 4 °C. After 2 h at 4 °C, the solvent
was concentrated in vacuo. The residue was dissolved in water (50
mL) and acidified with AcOH to pH 5-6. After filtration, the
product was obtained as a pale-yellowish solid (1.2 g, 88%; mp
1
162 °C). H NMR (DMSO-d6): δ 4.57 (s, 2H, -CH2-), 7.42 (s,
2H, C-CH), 7.64 (d, J ) 8.6 Hz, 2H, N-CH-CH), 8.05 (s, 1H,
CH-), 8.85 (d, J ) 8.4 Hz, 2H, N-CH-). Anal. Calcd for
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2006, 128, 6558.
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977.
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(18) SAINT, version 6.02; Bruker AXS: Madison, WI, 1999.
(19) Sheldrick, G. M. SADABS: Empirical Absorption Correction Program;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(20) XPREP, version 5.1; Siemens Industrial Automation Inc.: Madison,
WI, 1995.
(21) Sheldrick, G. M. SHELXTL Reference Manual, version 5.1; Bruker
AXS: Madison, WI, 1997.
(22) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(13) Ghosh, S. K.; Zhang, J. P.; Kitagawa, S. Angew. Chem., Int. Ed. 2007,
46, 7965.
(14) Takaoka, K.; Kawano, M.; Tominaga, M.; Fujita, M. Angew. Chem.,
Int. Ed. 2005, 44, 2151.
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31, 344. (b) Farha, O. K.; Mulfort, K. L.; Hupp, J. T. Inorg. Chem.
2008, 47, 10223.
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A R T I C L E S
Das and Bharadwaj
Scheme 1. Ligand LH2
C14H12N2O4 (272.08): C, 61.76; H, 4.44; N, 10.29. Found: C, 61.71;
H, 4.48; N, 10.33. ESI-MS (methanol) m/z: calcd for C14H12N2O4,
272.0797; found, 272.0805.
Synthesis of {[Mn(L)(H2O)](H2O)1.5(DMF)}n (1). This com-
pound was synthesized by mixing 1 mmol of LH2 and 1 mmol of
MnCl2 ·4H2O in 3 mL of DMF and 1 mL of water in a Teflon-
lined autoclave, which was heated under autogenous pressure to
130 °C for 36 h. Cooling to room temperature afforded the desired
product as pale-pink cubic crystals in ∼60% yield. The same
crystalline product was obtained when the acetate or sulfate salt of
Mn(II) was used in place of the chloride salt. Anal. Calcd for
C17H23N3O7.5Mn: C, 45.95; H, 5.22; N, 9.46. Found: C, 45.93; H,
5.26; N, 9.50%. IR (KBr) ν (cm-1): 3356, 2927, 1664, 1578, 1547,
1431, 1384.
Figure 1. Dimeric unit in the crystal structure of 1 (H atoms of the ligand
moieties have been omitted for clarity).
These channels are hydrophilic, since the coordinated water
molecules are directed inside the channels. The channel dimen-
sions are ∼7.36 × 4.37 Å2 (as measured by considering the
van der Waals radii of the constituent atoms), providing a void
space composed of 45.2% of the total crystal volume as
estimated by PLATON.24 C-H· · ·O interactions among the
lattice DMF molecules, O · · ·O interactions among the coordi-
nated and free water molecules (forming a discrete pentameric
water cluster), C-H· · ·π interactions between the C-H groups
of DMF and the phenyl rings of the ligands, and intermolecular
N-H· · ·O(carboxylate) interactions (see the Supporting Infor-
mation) are responsible for stabilizing the overall 3D architec-
ture.
TGA of 1 indicates that all of the solvent molecules can be
removed up to 250 °C and that no decomposition takes place
up to 375 °C. It is a well-known phenomenon that guest solvent
molecules located in the channels can often be removed from
a framework structure without causing framework collapse, and
furthermore, they can sometimes be reinserted. To explore this
possibility, a single crystal of 1 was heated at 120 °C under
vacuum for 2 h to yield a partially desolvated crystal, 1a. The
X-ray structure of 1a showed that the space group had changed
from C2/c to P21/n and that the asymmetric unit consists of
two Mn(II) ions, two ligand moieties, two coordinated water
molecules, one lattice water molecule, and two DMF molecules.
Partial expulsion of lattice water takes place, retaining the
original 3D framework with the formation of two new H-
bonding networks (the coordinated water molecule and the
lattice water molecule together with their symmetry-related
molecules form a discrete tetrameric water cluster, instead of
the discrete pentameric water cluster found in 1, the H-bonding
network among the coordinated water molecules and the O
atoms of the DMF molecules; see the Supporting Information).
Although the coordination environment around the two metal
centers remains identical to that in the mother crystal, the
corresponding bond lengths and angles are slightly changed (see
the Supporting Information). Most noteworthy is the increment
in the length of the Mn· · ·Mn separation by 0.166 Å in the
dimeric {Mn(II)}2 unit along with appreciable changes in the
conformation of the two ligand moieties [the C-C-N-C
dihedral angles are -75.02(7)° and 57.05(8)°, as opposed to
-75.71(8)° found in 1]. Because of this, the channels are much
Results and Discussion
Over the past few years, we have been exploring23 multi-
dentate ligands incorporating aromatic carboxylate and pyridine
donors. Here we have designed the new ligand LH2 in which
these donors are separated by a flexible spacer (Scheme 1). The
compound {[Mn(L)(H2O)](H2O)1.5(DMF)}n (1) was prepared
using the solvothermal technique by reacting the ligand LH2
with Mn(NO3)2 ·4H2O in aqueous DMF. Once formed, the
crystalline product is insoluble in most organic solvents, making
it possible to study the substitution reactions. Complex 1
(hereafter called the mother crystal) crystallizes in the mono-
clinic space group C2/c, and the asymmetric unit contains one
Mn(II), one L2-, and two types of solvent molecules: one bound
water molecule along with 1.5 free water molecules as well as
a free DMF molecule in the channel. The Mn(II) ion is
surrounded by five oxygen atoms and one nitrogen atom (Figure
1) in a distorted octahedral geometry: one water molecule and
one pyridyl group occupy axial positions, and one bidentate and
two bridging carboxylate groups from three different ligand
moieties are at equatorial positions. The degree of distortion
from the ideal octahedral geometry is reflected in the cisoid
andtransoidangles[58.14(9)-106.72(8)and155.43(5)-177.4(2)°,
respectively]. The Mn-N bond distance is 2.268(6) Å, and the
Mn-O distances are in the range 2.099(4)-2.307(4) Å. The
ligand is not planar; rather, the pyridyl group is highly twisted
with respect to the phenyl ring [C-C-N-C dihedral angle )
-75.71(8)°]. Two metal ions with two bridging carboxylate
groups form a dimeric {Mn(II)}2 unit with a Mn · · ·Mn
separation of 4.480(8) Å. Each of these units connects six
different ligand moieties, and three such units share one ligand
through one bidentate carboxylate, one chelating carboxylate,
and a pyridyl N atom. These dimeric units fashion wavelike
arrangements along the crystallographic c axis and the twisted
pyridyl rings act as pillars, giving rise to an overall complicated
3D porous framework structure (Figure 2). It is worth noting
that no interpenetration is observed in the structure, which
possesses channels occupied by water and DMF molecules.
(23) (a) Ghosh, S. K.; Savitha, G.; Bharadwaj, P. K. Inorg. Chem. 2004,
43, 5495. (b) Neogi, S.; Bharadwaj, P. K. Inorg. Chem. 2005, 44,
816.
(24) Spek, A. L. PLATON; The University of Utrecht: Utrecht, The
Netherlands, 1999.
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Substitution Reactions in a Porous Coordination Polymer
A R T I C L E S
Figure 2. (a) 3D framework of 1 viewed down the crystallographic c axis (channels are occupied by DMF and water molecules). (b) Space-filling representation
showing the pores in complex 1 (all of the solvent molecules have been omitted).
Figure 3. X-ray structure of the MeCN-substituted crystal 2 (H atoms of
the ligand moieties have been omitted for clarity).
Figure 4. View of the ACN-substituted crystal 3 (for clarity, only the H
atoms of the ACN molecules have been shown).
more rectangular in shape than previously. The partial loss of
lattice water molecules is accompanied by a decrease of 12.03%
in the cell volume (from 4188 to 3684 Å3) and a decrease in
the volume of the channel (from 45.2 to 37.9% of the total
crystal volume) as well as a noteworthy shrinkage of 3.36 Å in
the cell parameter along the a axis and an opening of the pores
along the b axis. Exposing a crystal of 1a to the ambient
atmosphere for a day results in complete rehydration of the
original structure to afford 1b, which includes the H-bonding
network observed in the original structure 1 (see the Supporting
Information). In 1b, the channel constitutes 45.4% of the total
crystal volume, which is almost the same in 1. Thus, removal
and reinsertion of the guest water molecules is completely
reversible. However, complete dehydration was unsuccessful,
as the crystal became opaque upon heating at high temperature.
Interestingly, the crystals of 1 show a remarkable ability to
substitute the coordinated water as well as lattice solvent
molecules for acetonitrile. To probe SC-SC transformation with
different nitriles, a mother crystal of suitable size was chosen
and used throughout at room temperature (RT). When the
mother crystal was immersed in acetonitrile for 4 h, it was
transformed into a crystal of {[Mn(L)(MeCN)](MeCN)1.5}n (2)
(Figure 3). The structural determination of 2 revealed that the
crystal system and the space group remain the same as that of
the mother crystal, but the metal-bound water and lattice solvent
molecules (H2O + DMF) of 1 are completely replaced by MeCN
molecules. The TGA of 2 showed that a 24% weight loss occurs
at 210 °C, corresponding to 2.5 MeCN molecules in the crystal
lattice, and no decomposition was observed up to 370 °C.
Various C-H· · ·N and C-H· · ·O interactions among the MeCN
molecules and the host framework appear to impart the
enhancement of thermal stability (see the Supporting Informa-
tion). The IR spectrum of compound 2 remained similar overall
to that of compound 1 except for the appearance of a new peak
at 2255 cm-1, which corresponds to the Ct N stretching
vibration. A packing diagram is shown in Figure S1 in the
Supporting Information.
When this crystal 2 was immersed in DMF in a sample vial
open to air at RT for 8 h, 2′ was obtained; X-ray structural
analysis of 2′ revealed that the original mother crystal (complex
1) was regenerated, including the H-bonding network observed
in 1 (Table S1 in the Supporting Information). Thus, this
substitution reaction is completely reversible.
Similar results were obtained when the mother crystal was
immersed in acrylonitrile and allylnitrile to obtain {[Mn(L)-
(ACN)](ACN)}n (3) (Figure 4) and {[Mn(L)(AN)](AN)}n (4)
(Figure 5), respectively; the crystal system and space group
remained unaltered during these substitution processes. There
are several C-H · · · O, C-H · · · N, and C-H · · · π H-bonding
interactions among the solvent molecules and the host frame-
work (see the Supporting Information). The appearance of peaks
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A R T I C L E S
Das and Bharadwaj
Scheme 2. Nitrile Molecules Used in This Study
strong C-H· · ·O contacts with the carboxylate O atoms from
the nearest wall. Within the cavities, the coordinated AN
molecules lie at a dihedral angle of 78.91° with respect to the
noncoordinated ones. Thus, these H bonds play an important
role in accommodating these nitriles in certain fashions within
the channels and stabilizing the overall 3D structure with
retention of single-crystallinity. A 3D packing diagram of
complex 4 is depicted in Figure S2 in the Supporting Informa-
tion.
Figure 5. Representation of the AN-substituted crystal 4 (ligand H atoms
have been omitted for clarity).
at 2227 and 2275 cm-1 in the IR spectra are attributed to the
Ct N stretching vibrations for 3 and 4, respectively. TGA of
these two complexes showed that they lose all the nitrile
molecules cleanly up to ∼290 °C, after which no weight loss
occurs until ∼360 °C. Thus, the cavity dimensions of the mother
crystal are large enough to accommodate acrylonitrile and its
larger analogue, allylnitrile. The 3D array of complex 3 is
displayed in Figure 6, which shows that the ACN molecules
are packed in two different ways within the channels. The
coordinated ones are directed toward the channel from the two
wavelike walls of Mn(II) dimeric units in a face-to-face fashion
with a distance of 3.541(6) Å between C17 and the centroid of
the benzene moieties of alternative walls, forming C-H· · ·π
contacts. The noncoordinated ones are almost perpendicular in
direction with respect to the coordinated ones within the
channels (the dihedral angle between the mean planes constituted
by these two types of ACN molecules is 86.66°). The point to
note is that noncoordinated ACNs lie very close to the walls,
with the carbon atoms (C19, C20) forming strong C-H· · ·O
contacts with two different carboxylate O atoms of a single wall.
Similar interactions are observed for complex 4: the coordinated
AN molecules are in C-H· · ·π contact, with C17 and the
benzene π moieties of alternative walls at a distance of 3.449(4)
Å, whereas the noncoordinated ANs are in intermolecular
C-H· · ·N interactions with the coordinated ones and also form
Since we observed that planar ACN and AN molecules
occupy special orientations within the channels, we chose to
examine the reaction with crotononitrile; CTN has two geo-
metrical isomers, with the cis isomer having an orientation very
similar to ACN and AN (Scheme 2). Thus, to investigate the
substitution phenomenon by CTN, the mother crystal 1 was
immersed in a 40:60 (v/v) cis-CTN/trans-CTN mixture for a
period of 8 h. The crystallographic determination showed that
the system remains the same, yielding {[Mn(L)(cis-CTN)](cis-
CTN)}n (5). Only the cis form replaces the coordinated water
molecule. The other lattice solvent molecules (1.5H2O + DMF)
are also simultaneously replaced by another cis-CTN molecule
(Figure 7). The cis-CTN bound to the metal is disordered, with
carbon atoms C17 and C18 showing positional disorder. This
was satisfactorily resolved by applying 0.75 occupancy to C17
and C18 and 0.25 occupancy to C17a and C18a. We collected
the data at least five times on different single crystals, and the
same results were obtained in each case. Data collected on other
crystals at a slower speed indicate a structure without this
disordering problem. There is a very short C-H· · ·π interaction
[3.343(6) Å] between the cis-oriented hydrogen (H17) of the
coordinated cis-CTN and the benzene moiety, as can be seen
for its above counterparts. The cis hydrogen atoms (H20 and
H21) of the lattice CTN molecules are strongly H-bonded with
Figure 6. 3D packing diagram for 3. The highlighted portion represents
one of the channels occupied by the ACN molecules (both coordinated and
noncoordinated), which are shown as a space-filling model.
Figure 7. Dimeric unit of cis-CTN-substituted crystal 5 (for clarity, only
the H atoms of CTN molecules have been shown).
9
10946 J. AM. CHEM. SOC. VOL. 131, NO. 31, 2009

Substitution Reactions in a Porous Coordination Polymer
A R T I C L E S
Scheme 3. Schematic Representation of the Reversible
Substitution Reactions at Mn(II) Centers within the Pores of
Complex 1
Table 1. Preference for Nitriles as Guests in 1 with Variation of
Molar Ratio
MeCN/ACN/AN ratio
complex formed
1:1:0
1:0:1
1:1:1
0:1:1
1:g5:0
1:0:g15
0:1:g10
2
2
2
3
3
4
4
respectively) showed almost the same cell dimensions as that
of mother crystal 1. In each case, the detailed crystallographic
investigation showed that the original structure, including
the H-bonding network, was restored (see the Supporting
Information). Thus, these substitution reactions are com-
pletely reversible.
the carboxylate oxygen atoms from its nearby wall. These
H-bonding interactions among the solvent molecules and
the host framework sustain the porous framework structure
(Table S1 in the Supporting Information). If trans isomer would
have been incorporated, then possibly there would have been
steric hindrance between the terminal methyl moieties and the
opposite walls. To the best of our knowledge to date, there is
no example of separation of geometrical isomers either by PCPs
or inorganic zeolites. Such selective inclusion of geometrical
isomers within the PCP is unprecedented. However, shape-
selective separation of positional isomers of C8 alkylaromatic
compounds has been reported recently.25 When the mother
crystal 1 was immersed in pure trans-CTN for several days, no
substitution reaction took place according to the single-crystal
X-ray determination. The detailed crystallographic investigation
showed that the original structure of 1, including the H-bonding
network, was preserved. In complex 5, the dihedral angle between
the planes of coordinated cis-CTN molecules and noncoordinated
cis-CTN molecules is 58.3°. TGA of 5 revealed that it lost all the
solvent molecules at 300 °C, with a mass loss of ∼29.4%
(calculated loss of 29.2%), and no further loss occurred up to 350
°C. The band appearing at 2218 cm-1 in the IR spectrum is
attributed to the cyanide stretching frequency of complex 5.
To probe the selectivity further, 50, 40, 30, and 20 µL aliquots
of CTN (60:40 trans/cis) were added sequentially to four sample
vials containing 1 (50 mg each). After 8 h, CDCl3 was added,
and the liquids were transferred to NMR tubes. The gradual
increase of the trans/cis ratio from 65:35 to 87:13 indicates that
the cis isomer was adsorbed by complex 1, causing the trans/
cis ratio to increase (Figure S15 in the Supporting Information).
The peaks corresponding to water and DMF molecules released
from compound 1 were also observed in the NMR spectra.
To investigate whether these substitutions are reversible,
as in the case of MeCN, the transformed crystal (3, 4, or 5)
was dipped in DMF at RT with the lid of the vial open to
the atmosphere. In each case, the mother crystal 1 was formed
in 10-15 h (Scheme 3). The resulting crystal (3′, 4′, or 5′,
In an attempt to understand the preferential substitution of
one guest molecule over others, a crystal of compound 1 was
immersed in mixtures of nitriles (MeCN, ACN, AN) with
different proportions. When a crystal of 1 was dipped into an
equimolar mixture of MeCN, ACN, and AN, only MeCN was
taken up, forming a crystal of 2. In other situations, the relative
concentration of a nitrile in the mixture was found to be the
deciding factor. Thus, when present in equimolar amounts in
binary mixtures, MeCN is the preferred guest for 1 over either
ACN or AN. Between ACN and AN, ACN is preferred. The
results are collected in Table 1.
It should be noted here that none of the first-generation
compounds (2-5) could be obtained by direct solvothermal
synthesis. During the substitution reactions, transparency of the
single-crystal was retained. The possibility of dissolution of 1
in the successive liquids followed by crystallization or renucle-
ation at the surface and growth of the new phase was excluded
by photographs of the mother crystal and its transformations to
2, 3, 4, and 5 (Figure 8), which show no change in size,
morphology, color, or transparency. In addition, insolubility in
different nitriles was checked by NMR spectra measured with
commercially available deuterated CH3CN and CH2dCH-CN
solvents after immersion of the crystals of the original compound
for 10 h; these showed no peak corresponding to framework 1.
It is also notable that the formation of first-generation com-
pounds is accompanied by the preservation of intermolecular
N-H· · ·O H-bonding interactions. Selected crystallographic
parameters are summarized in Table 2.
Interestingly, the first-generation compounds (2-5) can be
interconverted in SC-SC fashion by dipping them in appropriate
solvents (Scheme 3). For example, dipping 2 in ACN affords
3, and when this crystal of 3 is dipped in MeCN, 2 is
regenerated. This type of interconversion among the first-
generation compounds through SC-SC transformations is
unprecedented.
It must be mentioned that no substitution reactions take place
when a crystal of 1 is dipped in different ethers, thioethers, and
Figure 8. Photographs of single crystals of (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5.
9
J. AM. CHEM. SOC. VOL. 131, NO. 31, 2009 10947

A R T I C L E S
Das and Bharadwaj
Table 2. Crystal Data and Structure Refinement Parameters
1
C34H46Mn2N6O15
888.65
1a
C34H40Mn2N6O13
850.60
1b
C34H46Mn2N6O15
888.65
2
C38H35Mn2N9O8
855.63
3
C20H16MnN4O4
431.31
4
C22H20MnN4O4
459.36
empirical formula
formula weight
T (K)
100(1)
100(1)
100(1)
100(1)
100(1)
100(1)
radiation
λ (Å)
cryst syst
space group
a (Å)
Mo KR
0.71069
monoclinic
C2/c
18.278(6)
15.032(4)
16.444(5)
90.00
112.032(5)
90.00
4188(2)
4
Mo KR
Mo KR
Mo KR
Mo KR
0.71069
monoclinic
C2/c
18.237(6)
15.078(3)
16.285(5)
90.00
114.835(5)
90.00
4064(2)
8
Mo KR
0.71069
monoclinic
P21/n
0.71069
monoclinic
C2/c
0.71069
monoclinic
C2/c
0.71069
monoclinic
C2/c
14.918(5)
15.596(5)
16.442(5)
90.00
105.606(5)
90.00
3684(2)
4
1.533
18.406(7)
15.041(5)
16.344(4)
90.00
111.540(4)
90.00
4209(2)
4
1.402
17.777(7)
15.303(5)
16.267(6)
90.00
113.510(5)
90.00
4058(3)
4
1.401
18.826(7)
14.890(2)
16.288(4)
90.00
112.937(6)
90.00
4205(2)
8
1.451
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
Fcalc (Mg/m3)
1.409
0.675
1848
1.410
0.683
1768
µ (mm-1
F(000)
)
0.760
1760
0.671
1848
0.684
1760
0.665
1896
independent reflns
reflns used [I > 2σ(I)]
Rint
3101
5204
0.0578
4817
9102
0.0617
3122
5197
0.0573
3670
4918
0.0586
3132
4968
0.0686
2925
5172
0.0677
refinement method
full-matrix least-
squares on F2
1.065
full-matrix least-
squares on F2
1.044
full-matrix least-
squares on F2
1.053
full-matrix least-
squares on F2
1.051
full-matrix least-
squares on F2
1.075
full-matrix least-
squares on F2
1.047
GOF
R1, wR2 [I > 2σ(I)]
0.0615, 0.1497
0.1024, 0.2141
5
C22H20MnN4O4
459.36
0.0731, 0.1410
0.1016, 0.2045
2′
0.0649, 0.1511
0.1011, 0.2207
3′
0.0649, 0.1537
0.0932, 0.2191
4′
0.0720, 0.1844
0.1080, 0.2440
5′
0.0701, 0.1849
0.1072, 0.2352
R1, wR2 (all data)
empirical formula
formula weight
T (K)
C34H46Mn2N6O15
888.65
C34H46Mn2N6O15
888.65
C34H46Mn2N6O15
888.65
C34H46Mn2N6O15
888.65
100(1)
100(1)
100(1)
100(1)
100(1)
radiation
λ (Å)
cryst syst
space group
a (Å)
Mo KR
0.71069
monoclinic
C2/c
18.874(6)
14.891(5)
16.299(4)
90.00
114.015(2)
90.00
4184(2)
8
Mo KR
Mo KR
Mo KR
Mo KR
0.71069
monoclinic
C2/c
18.343(6)
15.054(4)
16.343(5)
90.00
111.808(4)
90.00
4190(2)
4
0.71069
monoclinic
C2/c
0.71069
monoclinic
C2/c
0.71069
monoclinic
C2/c
18.409(7)
15.009(6)
16.341(6)
90.00
111.682(7)
90.00
4196(3)
4
1.407
18.408(6)
15.037(5)
16.357(6)
90.00
111.575(5)
90.00
4210(3)
4
1.402
18.421(5)
15.029(4)
16.355(5)
90.00
111.668(6)
90.00
4208(3)
4
1.403
b (Å)
c (Å)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
Fcalc (Mg/m3)
1.458
0.668
1896
1.409
0.674
1848
µ (mm-1
F(000)
)
0.674
1848
0.671
1848
0.672
1848
independent reflns
reflns used [I > 2σ(I)]
Rint
2675
5233
0.0710
3083
5169
0.0581
3740
5185
0.0606
3105
5168
0.0584
2970
5155
0.0556
refinement method
full-matrix least-
squares on F2
1.044
full-matrix least-
squares on F2
1.078
full-matrix least-
squares on F2
1.064
full-matrix least-
squares on F2
1.052
full-matrix least-
squares on F2
1.065
GOF
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0733, 0.1870
0.1181, 0.2406
0.0675, 0.1549
0.1079, 0.2241
0.0645, 0.1524
0.1064, 0.2114
0.0637, 0.1586
0.1024, 0.2210
0.0633, 0.1515
0.1019, 0.2186
alcohols, or aromatic solvents such as toluene, benzene, xylene,
etc. For ethers, thioethers, and alcohols, the crystal becomes
opaque and does not diffract to the extent that the crystal
structure can be determined. For the case of aromatic guests,
the substitution reaction does not take place as well, although
the crystallinity is retained, as evidened by single-crystal X-ray
diffraction studies.
Sorption experiments of desolvated 1 showed that CO2
sorption (surface area ) 17.9 Å2, kinetic diameter ) 3.3 Å26)
occurs at 273 K without any hysteresis (Figure 9), which
accounts for a moderate adsorption capacity of ∼29 cm3/g
at P/P° ) 0.035 and a specific Langmuir surface area of ∼150
m2/g. To our surprise, no nitrogen diffusion (16.3 Å2, 3.64
Å) was observed at 77 K (BET surface area < 1 m2/g), despite
the stable framework and adequate effective pore size. The
CO2 selectivity may be due to the higher kinetic energy of
CO2 molecules and the higher thermal energy of the
coordination framework itself at 273 K, which permit
diffusion of CO2 molecules. In contrast, N2 adsorption
experiments performed at 77 K indicate that the kinetic
(26) Webster, C. E.; Drago, R. S.; Zerner, M. C. J. Am. Chem. Soc. 1998,
120, 5509.
(25) Alaerts, L.; Kirschhock, C. E. A.; Maes, M.; van der Veen, M. A.;
Finsy, V.; Depla, A.; Martens, J. A.; Baron, G. V.; Jacobs, P. A.;
Denayer, J. F. M.; Vos, D. E. D. Angew. Chem., Int. Ed. 2007, 46,
4293, and references therein.
(27) (a) Maji, T. K.; Matsuda, R.; Kitagawa, S. Nat. Mater. 2007, 6, 142.
(b) Navarro, J. A. R.; Barea, E.; Salas, J. M.; Masciocchi, N.; Galli,
S.; Sironi, A.; Ania, C. O.; Parra, J. B. J. Mater. Chem. 2007, 17,
4826.
9
10948 J. AM. CHEM. SOC. VOL. 131, NO. 31, 2009

Substitution Reactions in a Porous Coordination Polymer
A R T I C L E S
Conclusion
In summary, we have succeeded in the crystallographic
observation of substitution reactions of apical aqua ligands along
with exchange of all other crystallized solvent molecules by a
series of external nitrile guest molecules at Mn(II) centers within
a 3D porous coordination network in an SC-SC fashion.
Moreover, a certain shape selectivity of the porous network was
observed in the selective incorporation of cis-CTN as opposed
to trans-CTN. For the first time, we have observed such a
selectivity in an SC-SC manner, which is unprecedented. Since
ligand substitution at a metal center is a binuclear reaction, the
process can only rarely be observed by X-ray crystallography.
We have realized the crystallographic observation of these
dynamic processes. The in situ observation of various chemical
reactions in an SC-SC fashion, such as restricted cycloaddition,
selective Michael Addition, etc., in the fluid cavity of these first-
generation compounds is our next challenge, and the results will
be reported in due course.
Figure 9. CO2 adsorption isotherm of desolvated 1 at 273 K.
Acknowledgment. We gratefully acknowledge the financial
support from the DST, India. M.C.D. thanks CSIR, India for SRF.
We thank Prof. J. A. R. Navarro (Universidad de Granada, 18071
Granada, Spain) for gas adsorption measurements.
energy of the adsorbate and the thermal energy of the network
are not high enough to permit diffusion of the adsorbate
molecules through the pores.27 Also, there is the possibility
of strong interactions of N2 with the pore window, which
effectively block the pores from passing other molecules.27a
This could also occur as a consequence of the probable partial
collapse of the porous structure, as it has been found that
the partial dehydration gives rise to a noteworthy shrinkage
of the a cell parameter that along with b defines the opening
of the pores.
Supporting Information Available: Crystallographic data for
1-5, 1a, 1b, and 2′-5′ (CIF); IR and TGA data; additional
figures; and nonbonding distances and angles for all of the
complexes. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA9006035
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J. AM. CHEM. SOC. VOL. 131, NO. 31, 2009 10949