Semirigid aromatic sulfone-carboxylate molecule for dynamic coordination networks: Multiple substitutions of the ancillary ligands

Article abstract of DOI:10.1021/ic2007292

We report dynamic, multiple single-crystal to single-crystal transformations of a coordination network system based on a semirigid molecule, TCPSB = 1,3,5-tri(4′-carboxyphenylsulphonyl)benzene, which nicely balances shape persistence and flexibility to br

Full text of DOI:10.1021/ic2007292

ARTICLE
pubs.acs.org/IC
Semirigid Aromatic SulfoneꢀCarboxylate Molecule for Dynamic
Coordination Networks: Multiple Substitutions of the Ancillary
Ligands
Xiao-Ping Zhou,† Zhengtao Xu,*,† Matthias Zeller,‡ Allen D. Hunter,‡ Stephen Sin-Yin Chui,§ and
Chi-Ming Che§
†Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China
‡Department of Chemistry, Youngstown State University, One University Plaza, Youngstown, Ohio 44555, United States
§Department of Chemistry and HKU-CAS Joint Laboratory on New Materials, The University of Hong Kong, Pokfulam Road,
Hong Kong, China
S Supporting Information
b
ABSTRACT: We report dynamic, multiple single-crystal to single-crystal
transformations of a coordination network system based on a semirigid
molecule, TCPSB = 1,3,5-tri(40-carboxyphenylsulphonyl)benzene, which
nicely balances shape persistence and flexibility to bring about the frame-
work dynamics in the solid state. The networks here generally consist of
(1) the persistent core component (denoted as CoTCPSB) of linear CoII
aqua clusters (CoꢀOꢀCoꢀO-Co) integrated into 2D grids by 4,40-
bipyridine and TCPSB and (2) ancillary ligands (AL) on the two terminal
CoII ions—these include DMF (N,N0-dimethylformamide), DMA (N,N0-
dimethylacetamide), CH3CN, and water. Most notably, the ancillary
ligand sites are highly variable and undergo multiple substitution se-
quences while maintaining the solid reactants/products as single-crystals amenable to X-ray structure determinations. For example,
when immersed in CH3CN, the AL of an as-made single crystal of CoTCPSBꢀDMF (i.e., DMF being the AL) is replaced to form
CoTCPSBꢀCH3CN, which, in air, readily loses CH3CN to form CoTCPSBꢀH2O; the CoTCPSBꢀH2O single crystals, when
placed in DMF, give back CoTCPSBꢀDMF in single-crystal form. Other selective, dynamic exchanges include the following:
CoTCPSBꢀDMF reacts with CH3CN (to form CoTCPSBꢀCH3CN) but NOT with water, methanol, ethanol, DMA, or pyridine;
CoTCPSBꢀH2O specifically pick outs DMF from a mixture of DMF, DMA, and DEF; an amorphous, dehydrated solid from
CoTCPSBꢀH2O regains crystalline order simply by immersion in DMF (to form CoTCPSBꢀDMF). Further exploration with
functional, semirigid ligands like TCPSB shall continue to uncover a wider array of advanced dynamic behaviors in solid state
materials.
’ INTRODUCTION
can be monitored by X-ray crystal diffraction to unveil the relevant
structural and mechanistic details.5g
Porous coordination networks (PCNs, or metalꢀorganic frame-
works, MOF),1 constructed by using metal ions and organic linkers
through coordination bonds, are attracting much attention for their
remarkable porous features and potential application in sorption/
separation,2 catalysis,2h,3 and sensing.2e,4 Compared with the more
rigid inorganic zeolites or metal oxides, the inherent suppleness of
the organic building blocks tends to impart to the resultant
coordination networks significant structural flexibility in the solid
state, giving rise to dynamic behaviors in guest absorption and
transport (e.g., guest molecules traversing a host net without open
channels,1f,5 sorption hysteresis), as well as in the conversions
between different framework structures via external stimulus (e.g.,
heat, guest molecules)5m,6 or solution/solvent-mediated processes.7
On the other hand, the study of these dynamic behaviors can be
greatly facilitated by the availability of crystallographical data, for
example, single-crystal to single-crystal (SCSC) transformations
Among the increasing number of reported SCꢀSC transfor-
mations of coordination networks,5g,6aꢀ6h,8 most are based on
guest-dependent dynamic behaviors. To achieve reactions on the
coordination spheres of the metal centers, heating is often needed
to displace the volatile ancillary ligands (e.g., H2O) by other
donor species (e.g., carboxyl, pyridyl/MeOH, or NO3ꢀ).6aꢀe In
these cases, the substituting donors often originate from the host
network and they are usually preassembled around the metal
center. By contrast, it remains a challenge to engage externally
introduced guest ligands to effect substitution processes at the
metal center, even though progress along this direction has
recently been made by the groups of Bharadwaj6h and Zeng.6g
Received: April 8, 2011
Published: July 07, 2011
r
2011 American Chemical Society
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Scheme 1
Such substitution reactions proceed with both starting materials
and products persisting in the form of single crystals, despite the
lack of large open channel features in the single-crystal structures.
Together with the significant changes in the lattice parameters,
packing motifs of the layers, and local bonding features that
accompanied the transformations, these observations point to
the substantial cooperative motion of the atoms and molecules
across the host lattice to accommodate the guest transport and
the substitution processes.
’ EXPERIMENTAL DETAILS
Starting materials, reagents, and solvents were purchased from
commercial sources (Aldrich and Acros) and used without further
purification. Elemental analysis was performed by a Vario EL III CHN
elemental analyzer. FT-IR spectra were measured using a Nicolet Avatar
360 FT-IR spectrophotometer. NMR spectra were measured using a
Varian YH300 300 MHz superconducting-magnet NMR spectrometer
using tetramethylsilane as the internal reference. Thermal analysis (TG)
was carried out in a nitrogen stream using PerkinElmer STA6000
thermal analysis equipment with a heating rate of 10 °C minꢀ1. X-ray
powder diffraction patterns of the bulk samples (the powder samples
were spread onto glass slides for data collection) were collected at room
temperature on a Siemens D500 powder diffractometer or Bruker D8
Advance diffractometer (Cu KR, λ = 1.5418 Å).
Scheme 2. Conversions among the Individual Phases of
CoTCPSBa
Synthesis of 1,3,5-Tris(p-tolylthio)benzene (TTTB). In a
nitrogen-filled glovebox, sodium 4-methylbenzenethiolate (3.285 g,
22.50 mmol) was loaded into a 100 mL two-neck round-bottom flask
equipped with a septum. After the flask was taken out of the glovebox,
DMEU (dimethylethyleneurea, 10.0 mL) was injected via cannula under
nitrogen. When the sodium 4-methylbenzenethiolate was completely
dissolved in DMEU, 1,3,5-trifluorobenzene (anhydrous, 0.660 g, 5.0
mmol) was added via cannula from a nitrogen-protected vial. The
reaction mixture was then stirred and heated at 90 °C under nitrogen
protection. After about 3 days, the orange-colored solution was poured
into 100 mL of water and extracted with toluene (3 ꢁ 50 mL). The
organic layers were then combined and washed with water (3 ꢁ 200 mL)
and dried with anhydrous magnesium sulfate. The solvent was removed
in vacuo, and the crude product was purified by flash chromatography
(silica gel, hexanes) to provide a white solid (1.582 g, 71% based on
1,3,5-trifluorobenzene). 1H NMR (CDCl3, 300 MHz): δ 2.37 (s, 9 H),
6.75 (s, 3H), 7.08ꢀ7.11 (m, 6H), 7.24ꢀ7.27 (m, 6H). 13C NMR(CDCl3,
75 MHz): δ 21.48, 125.44, 129.43, 130.38, 133.64, 138.54, 140.04.
Synthesis of 1,3,5-Tris(tosyl)benzene. To a solution of 1,3,5-
tris(p-tolylthio)benzene (0.250 g, 0.56 mmol) in dichloromethane
(20.0 mL) was added m-chloroperbenzoic acid (85ꢀ95%, 0.614 g).
The resultant solution was allowed to stir overnight at room tempera-
ture, after which it was diluted with dichloromethane (30.0 mL) and
washed with a sodium hydroxide solution (10%, 2 ꢁ 50 mL), water (2 ꢁ
100 mL), and brine (100 mL). The organic layer was separated, dried
with anhydrous magnesium sulfate, and concentrated in a vacuum
to furnish 1,3,5-tris(tosyl)benzene (0.275 g, 91% based on 1,3,5-tris-
(p-tolylthio)benzene). 1H NMR (CDCl3, 300 MHz): δ 2.43 (s, 9 H),
7.33ꢀ7.35(m, 6H), 7.78ꢀ7.81 (m, 6H), 8.51 (s, 3H). 13C NMR(CDCl3,
75 MHz): δ 17.95, 124.46, 126.28, 126.78, 132.63, 141.83, 142.05.
Synthesis of 1,3,5-Tri(40-carboxyphenylsulphonyl)benzene
(TCPSB). Dilute HNO3 (5.80 g of 65% HNO3 in 13.0 mL of water) was
mixed with 1,3,5-tris(tosyl)benzene (100 mg, 0.185 mmol) and heated at
220 °C in a Teflon autoclave for 12 h. After cooling to room temperature
and filtration, 87.5 mg of a white solid was obtained (75%, based on 1,3,5-
tris(tosyl)benzene). 1H NMR (300 MHz, DMSO-d6): δ 8.08ꢀ8.11 (m,
6 H), 8.22ꢀ8.25 (m, 6 H), 8.71 (s, 3H). The solubility of TCPSB is
very low in DMSO-d6, and the 13C NMR spectrum is not measured.
IR (cmꢀ1): 3081w, 2986w, 2846w, 2666w, 2543w, 1698s, 1601w, 1576w,
a CoTCPSBꢀDMA1 and CoTCPSBꢀDMF were first prepared sol-
vothermally to initiate the conversion studies. We collected X-ray datasets
for CoTCPSBꢀDMF (and CoTCPSBꢀCH3CN) from two different
batches of samples, e.g., labeled as CoTCPSBꢀDMF1 and CoTCPSBꢀ
DMF2. Except CoTCPSBꢀDMA1, which contains two types of 2D
grids, all other single-crystal structures herein are isostructural and
contain only one type of 2D grid.
The major novelty of this work lies in the development of
molecular building block systems for tackling this challenge of
exploring ancillary ligand substitution in coordination networks.
As a general consideration, dynamic properties in solid state
networks call for a balance between the degree of shape
persistence and the conformational flexibility in the molecular
building blocks, as the former generally imparts the strength and
structural integrity of the host net while the latter allows for the
concerted motion throughout the network to create the dynamic
solid state properties. We thus conceived the molecule 1,3,5-
tri(40-carboxyphenylsulphonyl)benzene (TCPSB, Scheme 1) as
a semirigid, tritopic building block equipped with two distinct
sets of functions: (1) the commonly used carboxylate groups for
building up the coordination scaffold and (2) the sulfone groups
around the core of the molecule to bring about moderate
flexibility and to play a functional role under certain circum-
stances (e.g., hydrogen-bond formation with the guest molecules
or additional ligands).
The coordination networks reported here are based on a
trinuclear Co(II) aggregate integrated by the linkers of TCPSB
and 4,40-bipyridine into 2D grids (denoted as CoTCPSB).
Besides being bonded to the donor atoms from the linker
molecules, the Co(II) centers can also bond to water and other
small solvent molecules of DMA, DMF, and CH3CN as ancillary
ligands. The dynamic nature of the networks is mostly reflected
in the multiple substitutions of the ancillary ligands (see
Scheme 2 for an overview meant to help navigate the main text).
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1489w, 1353m, 1343 m, 1286s, 1155s, 1123w, 1096w, 1079w, 1015w,
931w, 861w, 824 w, 806w, 763m, 713m, 687w, 665w, 621w, 601m, 583m,
567m, 555m, 446w.
Crystallization of CoTCPSBꢀDMF. A mixture of Co(NO3)2
3
6H2O (3.0 mg), TCPSB (3.1 mg), 4,40-bipyridine (3.6 mg), and
DMFꢀwater (1.0 mL, 4:1, v/v) was sealed in a glass tube, heated in
an oven at 100 °C for 50 h, and slowly cooled to room temperature over
a period of 12 h. Red block-like crystals were obtained (3.4 mg, 70%,
based on TCPSB). Experiments on a larger scale (e.g., with 15.0 mg of
TCPSB) in a large glass tube provided the same product (as checked
by X-ray powder diffraction). Chemical analysis of the product
C77.5H77.5Co3N6.5O32.5S6 (corresponding to [Co3(H2O)4(TCPSB)2-
(bipy)(DMF)2] 2.5DMF) yields the following: Anal. Calcd C, 46.80;
3
H, 3.93; N, 4.58. Found: C, 46.82; H, 3.64; N, 4.74.
Crystallization of CoTCPSBꢀDMA1. The procedure here is
similar to that of CoTCPSBꢀDMF, except for the solvent (DMAꢀ
water, 4:1, v/v). Red block-like crystals were obtained (61%, based
on TCPSB). Chemical analysis of the product (corresponding to
Figure 1. Co2+ coordination environments in CoTCPSBꢀDMF (a)
[Co3(H2O)4(TCPSB)2(bipy)(DMA)2]2 [Co3(H2O)4(TCPSB)2-
3
and the resultant 2D grid (b).
(bipy)] 8DMA 9H2O)) yields the following: Anal. Calcd C, 46.58; H,
3
3
4.30; N, 4.07. Found: C, 46.29; H, 4.19; N, 4.33.
centers and one TCPSB ligand, one-half of a 4,40-bipy ligand, two
H2O units (one bridging Co1 and Co2, and the other bonded
to Co1), and two DMF molecules (one bonded to Co1 and the
other being noncoordinated). Both Co2+ atoms adopt an octa-
hedral geometry. Co1 is bound by 5 oxygen atoms (i.e., O7 and
O12 from TCPSB, O13 and O14 from water, and O15 from
DMF; CoꢀO distances 2.022ꢀ2.214 Å; see Figure 1a) and one
N atom of 4,40-bipy (CoꢀN1, 2.116 Å); Co2 is bound by six
oxygen atoms (e.g., O1 and O8 from TCPSB and O13 from
water; CoꢀO distances 1.920ꢀ2.227 Å, Figure 1a). The brid-
ging/bonding patterns of the carboxylate and aqua groups on the
trinuclear Co(II) cluster are similar to that of a reported tri-
nuclear Co2+ cluster.9 Topologically, the Co(II) cluster can be
considered a 4-connected node integrated by two types of linear
struts into a 2D parallelogram net. One type of strut is provided
by a single bipy molecule, while the other by a pair of TCPSB
molecules is aligned in a parallel, centrosymmetric fashion. Such
a complex linear link is partly made possible by the flexible
conformation of the individual TCPSB molecules, which allows
two of its branches to converge onto one metal cluster (the other
branch being bound to another cluster). The 2D coordination
grid contains significant cavities (Figure 1b) in which the non-
coordinated DMF guests are located. On switching the solvent
from DMF to DMA, one obtains, under similar conditions,
CoTCPSBꢀDMA1, which, like CoTCPSBꢀDMF, also crystal-
lizes in the space group Pꢀ1 and features linear trinuclear cobalt
clusters integrated into 2D open networks by the TCPSB and
bipy linkers (Figure 2). However, CoTCPSBꢀDMA1 manifestly
differs from CoTCPSBꢀDMF (and other structures reported
herein) in that it consists of not one but two types of 2D
coordination grids, each with different coordination environ-
ments around the trinuclear cobalt clusters, as shown in Figure 2c
and2d. The network composition for type I is Co3(H2O)4-
(TCPSB)2(bipy)(DMA)2 and for type II Co3(H2O)4(TCPSB)2-
(bipy). The trinuclear cobalt cluster in type I is similar to the
cluster in CoTCPSBꢀDMF, except that DMA now takes the
place of the DMF molecules (Figure 2a). Type II, by comparison,
is not associated with any DMA ligands, with a carboxylate oxygen
atom from one of the TCPSB molecules filling in to maintain
the octahedral geometry. As a result, the adjacent Co atoms here
(i.e., Co4 and Co5 of Figure 2b) are bridged by two carboxylate
groups (c.f., one single bridging carboxylate in type I), with a
Preparations of CoTCPSBꢀCH3CN. The crystals of CoTCPSBꢀ
DMF (10.0 mg) and CoTCPSBꢀDMA1 (10.0 mg) were immersed in
acetonitrile (2.0 mL) for 2 days, and CoTCPSBꢀCH3CN was obtained
after filtration. After being exposed to air for 1 h, chemical analysis of
the product found (C, 41.88; H, 3.67; N, 1.62), which closely matches
CoTCPSBꢀH2O (C64H66Co3N2O38S6, corresponding to [Co3(H2O)6-
(TCPSB)2(bipy)] 8H2O) (calcd C, 41.77; H, 3.6; N, 1.52) andindicates
3
that the CH3CN in the solid sample has already been replaced by water.
Single-Crystal XRD Analyses. For CoTCPSBꢀDMF1, CoTCPSBꢀ
DMA1, CoTCPSBꢀCH3CN2, and CoTCPSBꢀDMA2 data collec-
tions were conducted on a Bruker AXS SMART APEX CCD system
using Mo KR (λ = 0.71073 Å) radiation at 100(2) K. All absorption
corrections were performed using the SADABS program. Crystals
of CoTCPSBꢀCH3CN1, CoTCPSBꢀH2O, and CoTCPSBꢀDMF2
were studied on an Oxford Gemini S Ultra diffractometer. The crystals
of CoTCPSBꢀCH3CN1 and CoTCPSBꢀCH3CN2 were immediately
placed into grease after removal from the mother liquors. The crystals
of CoTCPSBꢀH2O are obtained by exposing CoTCPSBꢀCH3CN
crystals to air for about 1 h. The structures of CoTCPSBꢀDMF1 and
CoTCPSBꢀDMA1 were solved by direct methods using SHELXTL
6.14 (Bruker AXS Inc., Madison, WI, 2003). All other structures were
solved by isomorphous replacement starting with the structure of
CoTCPSBꢀDMF1 under omission of the auxiliary ligands. All struc-
tures were refined by full-matrix least-squares on Fo2 using SHELXTL
6.14. For summaries of the X-ray crystallographic data, see Tables
S1ꢀS4, Supporting Information. CCDC 812036ꢀ812042 (in the same
order as listed in Tables S1ꢀS4, Supporting Information) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/datarequest/cif.
’ RESULTS AND DISCUSSION
The first compound, CoTCPSBꢀDMF, with a network
composition of Co3(H2O)4(TCPSB)2(bipy)(DMF)2 was ob-
tained in a solvothermal synthesis from Co(NO3)2 6H2O,
3
4,40-bipyridine, and TCPSB with a mixture of DMF (N,N0-
dimethylformamide) and water as the solvent. This compound
crystallizes in the space group Pꢀ1 and features centrosym-
metric, linear trinuclear CoII aqua clusters (Co1ꢀOꢀCo2ꢀ
OꢀCo1) integrated into a 2D open network by the linkers
TCPSB and 4,40-bipy, as shown in Figure 1. The asymmetric
portion of the unit cell of CoTCPSBꢀDMF contains 1.5 Co2+
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Figure 2. Coordination environments of the Co2+ ions in CoTCPSBꢀDMA1 (a, b) and the corresponding 2D grids ((a and c) type I and (b and d) type II).
shorter CoꢀCo distance of 3.585 Å (c.f., 3.700 and 3.706 Å in
type I; see also Figure 2b). Networks of type I (the ones with
DMA ligands) and type II (without DMA) do not occur in equal
numbers—the ratio is 2:1. In other words, type I nets aggregate
in pairs that alternate with the single nets of type II along
the stacking direction (the c axis). In the cavities of the crystal
structure are found the guests of DMA and water molecules.
When immersed in acetonitrile, single crystals of both
CoTCPSBꢀDMF and CoTCPSBꢀDMA1 transformed into
the new compound of CoTCPSBꢀCH3CN in the single-crystal
form: the crystal derived from CoTCPSBꢀDMF is labeled as
CoTCPSBꢀCH3CN1 and the crystals from CoTCPSBꢀDMA1
CoTCPSBꢀCH3CN2. X-ray diffraction studies on both batches
of CoTCPSBꢀCH3CN crystals revealed that they are of the
same structure, containing one single type of coordination grid
that is isostructural to the above CoTCPSBꢀDMF but with
the DMF molecules replaced by CH3CN (Figure 3a and 3c) to
give a network composition of Co3(H2O)4(TCPSB)2(bipy)-
(CH3CN)2. In fact, CoTCPSBꢀCH3CN (and the other struc-
tures in the rest of the paper) was refined with the framework
of the DMF-containing precursor as the starting point. Taken
together, the three types of precursor networks—the one in
CoTCPSBꢀDMF, type I and type II in CoTCPSBꢀDMA1—
can all transform into this new net in CoTCPSBꢀCH3CN.
Notably, the transformation of the type II network in
CoTCPSBꢀDMA1, which does not contain coordinated DMA
molecules, involves more drastic motions in bond breaking/
formation (e.g., displacing one carboxylate O atom by CH3CN
on Co5 and rearranging its coordination sphere) as well as in the
conformational change of the TCPSB molecules.
evidence to support a single-crystal to single-crystal nature of
the above transformations. Also, the formation/crystallization of
the main framework from solution seems to be highly dependent
on the templating molecules, with CH3CN not being a suitable
template in contrast to DMF or DMA. The transition from the
liquid/solution state to the solid state network often involves
exceptionally complicated processes, and a mechanistic account
for the different templating effects of CH3CN vs DMF and DMA
is beyond this work. However, from the crystal structures one can
infer to some degree the potential roles played by the steric
and other local interactions on the ancillary ligand substitution
processes. For example, the coordinated water molecules adjacent
to DMF in CoTCPSBꢀDMF (O14) or DMA in CoTCPSBꢀ
DMA1 (O28, O29) on Co2+ centers were very stable; they
were not substituted by acetonitrile. This stability appears to be
consistent with the steric situation (the area around this aqua
ligand is crowded by the neighboring groups, whereas the site of
the CH3CN ligand is less obstructed, see also Figures 1a and 2a)
and the strong hydrogen bonding between the water and
carboxylate group (e.g., the shortest hydrogen-bond distances
for the water O14ꢀO1 in CoTCPSBꢀDMF and O28ꢀO20,
O29ꢀO5 in CoTCPSBꢀDMA1 are all under 2.74 Å).
When we performed IR measurements of the newly formed
CoTCPSBꢀCH3CN crystals (ground in air), we were surprised
by the absence of the CtN stretching peak (from CH3CN) in
the spectrum (Figure S1, Supporting Information), even though
the disappearance of the characteristic peak of the CdO
stretching of DMF at 1652 cmꢀ1 (the peak of CdO vibration
of DMA is overlapped with the peak of the network) and the
methyl CꢀH stretching at 2930 cmꢀ1 is within expectations.
Also, the X-ray powder pattern of the CoTCPSBꢀCH3CN
crystals that were ground in air differs substantially from the
one simulated from the single-crystal structure of CoTCPSBꢀ
CH3CN (Figure 4). These observations suggest that the
CoTCPSBꢀCH3CN crystals very probably have transformed
into a new phase after being exposed to air. Fortunately, the new
It should be noted here that attempts to directly synthesize
CoTCPSBꢀCH3CN from Co(NO3)2 6H2O and TCPSB
3
(using CH3CN as the solvent) were not successful, suggesting
that a dissolution/recrystallization process was unlikely to be
involved in going from CoTCPSBꢀDMF (or CoTCPSBꢀ
DMA1) to CoTCPSBꢀCH3CN, and thus, lending further
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Figure 3. Coordination environment of Co2+ (a) and grid network in CoTCPSBꢀCH3CN (c); coordination environment of Co2+ (b) and grid-like
network in CoTCPSBꢀH2O (d); packing of the 2D coordination nets in CoTCPSBꢀCH3CN (e) and CoTCPSBꢀH2O (f). Color code for atoms:
green, Co; red, O; blue, N; gray, C; yellow, S.
phase can be determined by single-crystal X-ray diffraction
(i.e., the single crystallinity of the mother CoTCPSBꢀCH3CN
crystals was preserved in this transformation), which reveals
that the CH3CN ligands in CoTCPSBꢀCH3CN have been
substituted by water molecules to form the new compound
CoTCPSBꢀH2O (Figure 3b and 3d), with a network composi-
tion of Co3(H2O)6(TCPSB)2(bipy). The single-crystal structure
of CoTCPSBꢀH2O thus accounts for the absence of the
CH3CN peak in the IR spectrum, and the simulated X-ray powder
pattern matches nicely with the observed pattern as shown in
Figure 4. Furthermore, elemental analysis data also closely matched
the CH3CN-free structure from X-ray diffraction data, thus further
indicating the expulsion of the CH3CN molecules from the crystals
of CoTCPSBꢀCH3CN after being exposed to air.
The coordination network of CoTCPSBꢀH2O is topologi-
cally similar to that of CoTCPSBꢀCH3CN, with water mol-
ecules taking the place of CH3CN as an ancillary ligand.
Compared with the three progenitor systems, the “parallelogram
window” of the 2D network in CoTCPSBꢀH2O (Figure 5) is
significantly “squashed”. Specifically, while the parallelogram in
Figure 4. X-ray powder patterns (Cu KR, λ = 1.5418 Å) (a) of the
CoTCPSBꢀH2O features an acute angle of 66.31°, the figure
in the other phases rises by about 10° to a range between 74° and
78°. Also, the 2D coordination networks of CoTCPSBꢀH2O
appear to be stacked in a more compact fashion, with significant
calculated pattern from the single-crystal structure of CoTCPSBꢀ
CH3CN (data collected at 100 K), (b) observed from an as-made sample
of ‘CoTCPSBꢀCH3CN’ at 298 K, (c) of calculated pattern from the
single-crystal structure of CoTCPSBꢀH2O (data collected at 173 K).
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it could also facilitate the departure of the CH3CN, as illustrated
in its rapid replacement by H2O in open air (to form
CoTCPSBꢀH2O). Replacement of CH3CN by H2O is, evi-
dently, also facilitated by its volatility as well as the plenitude of
H2O in open air, which serves to push forward the transforma-
tion into CoTCPSBꢀH2O.
Can CoTCPSBꢀH2O be reverted to CoTCPBSꢀDMF or
CoTCPBSꢀDMA1? For this, crystals of CoTCPSBꢀH2O were
immersed in DMF or DMA at room temperature for 1 day (for
DMF) and 3 days (for DMA), respectively, and the resultant
3
Figure 5. Parallelogram window in the network of CoTCPSBꢀH2O
crystals (labeled as CoTCPBSꢀDMF2 and CoTCPBSꢀDMA2,
respectively) were subjected to single-crystal X-ray diffraction
studies. It turns out that CoTCPBSꢀDMF2 is of the same
structure as the originally characterized CoTCPBSꢀDMF, with
the aqua ligand (Figure 3b, O15, cis to the pyridine ring) in
CoTCPSBꢀH2O changed back to DMF. The reversion of
CoTCPSBꢀH2O to CoTCPSBꢀDMF was also supported by
the IR spectra, which exhibit almost identical features between
CoTCPBSꢀDMF2 and the originally prepared CoTCPBSꢀ
DMF (Figure S6, Supporting Information). In short, one
achieves a full cycle from CoTCPSBꢀDMF to CoTCPSBꢀ
CH3CN, then to CoTCPSBꢀH2O, and finally back to
CoTCPSBꢀDMF (Scheme 2).
(color code for atoms: green, Co; red, O; blue, N; gray, C; yellow, S).
hydrogen bonds between the sulfone groups and the aqua ligands
across the 2D sheets (OꢀO distance = 2.961 Å, see also
Figure 3f). Notice that the interlayer hydrogen bond entails
the newly introduced aqua ligand (i.e., the one taking the place of
the CH3CN ligand) and might contribute, in part, to the facile
transformation from CoTCPSBꢀCH3CN to CoTCPSBꢀH2O
by means of “locking in” the aqua ligand. This is contrary to a
recently reported 3D porous manganese coordination network
in which departure of the CH3CN ligands entailed heating at
about 210 °C.6h
In contrast to the sensitivity of CoTCPSBꢀCH3CN, the
mother crystals of CoTCPSBꢀDMF and CoTCPSBꢀDMA1
(reminder: they both convert to CoTCPSBꢀCH3CN in
CH3CN) are stable in air, and TGA studies (Figure S2, Support-
ing Information) showed that they start to lose the DMF or
DMA molecules at about 117 and 133 °C, respectively. After
immersing the crystals of CoTCPSBꢀDMF and CoTCPSBꢀ
DMA1 in water for 2 days, both IR and X-ray powder diffrac-
tion measurements indicated that the crystalline phase of
CoTCPSBꢀDMF remains unchanged (Figure S3, Supporting
Information), while the crystals of CoTCPSBꢀDMA1 become
amorphous (Figure S4, Supporting Information). These tests
indicate that direct transformation from CoTCPSBꢀDMF or
CoTCPSBꢀDMA1 to CoTCPSBꢀH2O is not as straightfor-
ward as from CoTCPSBꢀCH3CN, and acetonitrile is a useful
medium for preparing the phase of CoTCPSBꢀH2O.
The product of CoTCPBSꢀDMA2 features a structure
similar to CoTCPSBꢀDMF (with DMA taking the place of
DMF) but is structurally different from the originally prepared
CoTCPBSꢀDMA1 (recall that CoTCPBSꢀDMA1 contains
two types of networks, type I with DMA ligands and type II
without). In other words, CoTCPBSꢀDMA2 only contains the
type I network of CoTCPBSꢀDMA1. Thus, DMA is able to
replace the water molecules in CoTCPSBꢀH2O to generate
CoTCPBSꢀDMA2 but unable to recover the original structure
of CoTCPBSꢀDMA1 (Figure S7, Supporting Information).
The difficulty in converting CoTCPSBꢀH2O back to
CoTCPBSꢀDMA1 is likely due to the more complex structure
of the latter, which consists of two different types of networks
stacked in a highly ordered manner (i.e., pairs of type I alternating
with single nets of type II). To revert the simpler structure of
CoTCPSBꢀH2O to CoTCPBSꢀDMA1 would require the
DMA molecules to differentiate, in a correspondingly highly
ordered manner, the homologous layers of CoTCPSBꢀH2O.
Such an increase in orderliness seems very unlikely to occur
simply by immersion at room temperature.
Out of curiosity, we then immersed the crystals of CoTCPSBꢀ
H2O in a mixture of equal volumes of DMF, DMA, and DEF
(N,N-diethylformamide). After 3 days, the IR spectrum features
the peak of the characteristic CdO stretching of DMF at
1654 cmꢀ1 (Figure S8a, Supporting Information) and solution
1H NMR measurement (by dissolving the resultant crystals in a
DCl/D2O/DMSO-d6 mixture) indicates that only DMF mol-
ecules from the ternary mixture were taken up by the host net
(Figure S8b, Supporting Information). Thus, a strong selectivity
in favor of DMF (relative to DMA and DEF) was observed of
CoTCPSBꢀH2O, which could be due to its smallest size among
the three homologues. Further experiments also found that unlike
DMF and DMA the largest size DEF does not replace the water
molecules of CoTCPSBꢀH2O under similar conditions (see the
IR spectra in Figure S9, Supporting Information, and the PXRD
patterns in Figure S10, Supporting Information). Preferential
uptake of small-sized molecules observed of CoTCPSBꢀH2O is
quite common among porous coordination networks.2e,6h,10 For
example, a reported 3D porous manganese coordination network
We also tested the potential exchange reactions of
CoTCPSBꢀDMF toward other solvent molecules (e.g., metha-
nol, ethanol, DMA, and pyridine) in procedures similar to
the treatment with acetonitrile and water. The crystals of
CoTCPSBꢀDMF remain visually unchanged in these tests,
and the IR spectra do not exhibit significant new features, with
the characteristic peaks (e.g., 1652 cmꢀ1) of DMF persisting in
the spectra (Figure S5, Supporting Information). These tests
indicate a particularly strong binding of the DMF to the Co(II)
center, which can be ascribed to (1) the donor ability of its amide
O atom (due to abetment from the N lone pair; by comparison,
H2O, methanol, and ethanol are weaker donors) and (2) its
smaller size (e.g., relative to DMA) that reduces steric hindrance
with the neighboring groups.
On the other hand, these tests also serve to highlight acetoni-
trile in its particularly strong ability to dislodge DMF from the
host net. We suspect that such ability is associated with the linear,
slender shape of CH3CN and its open, unobstructed nitrile lone
pair (also a better donor than the O atoms in water and alcohols),
which serves to minimize the steric hindrance in the substitution
process. By comparison, the lone pair on pyridine is flanked by
two carbon groups, and it is therefore not as approachable.
Notice that such steric agility is like a double-edged sword in that
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Inorganic Chemistry
ARTICLE
selectively takes up acetonitrile and not the larger molecules of
acrylonitrile and allylnitrile.6h
structure probably retains certain structural information from the
crystalline mother compound of CoTCPSBꢀH2O: information
that serves to program the reestablishment of an isostructural
crystalline network upon immersion in DMF. We should caution
that it is generally difficult to completely rule out a dissolution/
recrystallization mechanism in a transformation as above, even
though in this particular case we can draw some assurance from
the fact that the crystals of CoTCPSBꢀDMF were obtained
under hydrothermal conditions and the direct mixing of the
starting materials at room temperature does not result in the
crystalline phase of CoTCPSBꢀDMF. The regaining of crystal-
line order from an amorphous solid sample has also been
reported in several other systems5d,10a,11,12 and in general reflects
the structural flexibility of the networks.
Thermogravimetric analysis (TGA, Figure S11, Supporting
Information) of the as-made sample of CoTCPSBꢀH2O reveals
a 11.8% weight loss at 146 °C, which corresponds to evacuation
of 8 guest and 4 coordinated water molecules per formula unit
of [Co3(H2O)6(TCPSB)2] [bipy]] 8H2O (calculated guest
3
3
and coordinated water molecules (bridge water is not included)
proportion in CoTCPSBꢀH2O 11.7%), and no significant
further weight loss was observed up to 381 °C. In fact, heating
the crystals of CoTCPSBꢀH2O under N2 atmosphere at 100 °C
for 2 h led to a color change from pink to purple, and the purple
changes back to pink after exposure to air overnight (see
Figure 6); this color change can be repeated multiple times
and is reminiscent of CoCl2 6H2O, which changes from pink to
3
deep blue upon dehydration. In addition, similar color changes
associated with dynamic guest exchanges have been earlier
observed in a Co(II)-based coordination polymer system.11
Powder X-ray diffraction study reveals that both the sample
heated at 100 °C and the subsequent pink sample recovered from
exposure in air lost crystallinity (see Figure 7b, for example).
In spite of the loss of crystallinity, the reversible color changes
(i.e., pink to purple, and purple to pink) persist in the amorphous
sample (as shown by repeating the heating/air exposure cycle
three times). More interestingly, like the crystalline mother
compound CoTCPSBꢀH2O, these noncrystalline powder sam-
ples change to the crystalline phase of CoTCPSBꢀDMF after
being immersed in DMF at room temperature for 3 days
(Figure 7c). Such a transformation suggests that the amorphous
’ CONCLUSION
Multiple ligand substitution reactions at the cobalt centers
can be carried out in the dynamic coordination network of
CoTCPSB. Such reactions take place with both the reactant
and the product networks in the form of single crystals and serve
to highlight the remarkable structural flexibility that is imparted
to the host network by the molecule TCPSB as a semirigid
building block. Other interesting properties of the versatile host
net of CoTCPSB uncovered in this exercise include the follow-
ing: CoTCPSBꢀDMF selectively absorbs and reacts with acet-
onitrile in preference to water, methanol, ethanol, DMA, and
pyridine; CoTCPSBꢀH2O specifically pick outs DMF from a
mixture of DMF, DMA, and DEF; the amorphous, dehydrated
solid from CoTCPSBꢀH2O regains crystalline order (and forms
CoTCPSBꢀDMF) simply by immersion in DMF. Further
exploration with functional, semirigid ligands like TCPSB shall
continue to uncover a wider array of advanced dynamic behaviors
in the resultant solid state framework materials.
Figure 6. Photograph of the crystals of CoTCPSBꢀH2O (a), the solids
after heating at 100 °C for 2 h (b), and the solids after being exposed in
air overnight (c).
’ ASSOCIATED CONTENT
S
Supporting Information. Full crystallographic data in
b
CIF format, additional powder X-ray diffraction patterns, TGA
plots, IR spectra, and solution NMR data. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: zhengtao@cityu.edu.hk.
’ ACKNOWLEDGMENT
This work was supported by City University of Hong Kong
(Project No. 7002471) and the Research Grants Council of
HKSAR [Project 9041212 (CityU 103407)]. The single-crystal
diffractometer at YSU was funded by NSF grant 0087210, the
Ohio Board of Regents grant CAP-491, and Youngstown State
University.
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Figure 7. X-ray powder patterns (Cu KR, λ = 1.5418 Å) (a) observed of
an as-made sample of CoTCPSBꢀH2O at 298 K, (b) observed of a solid
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observed of a sample of b immersed in DMF for 3 days at 298 K, and (d)
a calculated pattern from the single-crystal structure of CoTCPSBꢀ
DMF (dataset collected at 100 K).
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