Design and synthesis of capped-paddlewheel-based porous coordination cages

Article abstract of DOI:10.1039/c9cc05002g

To leverage the structural diversity of metal-organic frameworks, the ability to controllably terminate them for the isolation of porous coordination cages is advantageous. However, the strategy has largely been limited to ligand termination methods, part

Full text of DOI:10.1039/c9cc05002g

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Gosselin, B. S. Lindner, R. Bhattacharjee, G. Yap, S. Caratzoulas and E. D. Bloch, Chem. Commun., 2019,
DOI: 10.1039/C9CC05002G.
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DOI: 10.1039/C9CC05002G
COMMUNICATION
Design and Synthesis of Capped-Paddlewheel-Based Porous
Coordination Cages
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a
a
c
Gregory R. Lorzing, Eric J. Gosselin, Brian S. Lindner, Rameswar Bhattacharjee, Glenn P. A.
a
c
a,b,c
Yap, Stavros Caratzoulas, and Eric D. Bloch*
DOI: 10.1039/x0xx00000x
To leverage the structural diversity of metal-organic frameworks,
the ability to controllably terminate them for the isolation of
porous coordination cages is advantageous. However, the strategy
has largely been limited to ligand termination methods,
particularly for paddlewheel-based materials. Here, we show a
paddlewheel-capping strategy can be employed to afford
previously unattainable coordination cage structures that are
mimetic of metal-organic framework pores.
Coordination cages have been widely investigated for a variety
1
2
of applications including uses in catalysis, biomedicine,
separations,3 and sensing. The discovery of permanent
4
porosity in
a subset of these materials has spurred
5
6
investigations into applications in gas storage, separation,
and small-molecule activation. Analogous to record surface
area metal-organic frameworks, the highest surface area
7
8
porous coordination cages are typically based on carboxylic
9
acid ligands and divalent or trivalent metal cations. In fact, Figure 1. The structure of HKUST-1 (upper left) contains three types of
many porous cages are essentially molecular analogues of pores. To afford molecular cages the large pore is terminated at the ligand
portions of well-known metal-organic frameworks. For these, while the other two rely on metal capping units.
discrimination between cage and extended structure is than ligands. A cluster capping strategy has previously been
achieved by simply terminating a multitopic carboxylate ligand successfully employed for a variety of systems to afford, for
1
4
15
16
to a lower nuclearity. For example, the reaction of copper(II) example, MIL-101, UiO-66, and PCN-9 cage analogues by
5
17
7,18
with trimesic acid affords a three-dimensional framework capping with sulfate, cyclopentadienyl, and calixarene,
whereas reaction with isophthalic acid affords a discrete respectively. This strategy is particularly challenging for
molecular species.1
0,11
paddlewheel-based cages where cis-divacant coordination
This approach, however, necessarily limits the number of sites in starting materials are necessary. Herein we report
pores within a metal-organic framework that can be isolated synthetic routes where either 2,2’-bipyridine or formamidinate
as molecular species. This is illustrated for HKUST-1 in Figure 1. ligands are utilized in combination with copper(II) or
The large pore in the material is the isophthalic acid-based molybdenum(II) paddlewheels to terminate cluster growth and
Cu24(bdc)24 (bdc2–
=
1,3-benzenedicarboxylate) cage.
10
afford discrete porous cages of trimesic acid.
Coordination cage analogues of the other pores in the
For this strategy, we initially targeted Mo-based materials
material, which have been implicated as key adsorption sites given the well-developed coordination chemistry of multiply-
1
2,13
4+
for gas storage at high pressure,
are not accessible via this bonded Mo
2
paddlewheel units. Importantly, Cotton et al.
route. These pores are terminated at paddlewheels rather demonstrated the isolation of an octahedral cage analogous to
the small pore in HKUST-1 via the reaction of trimesic acid with
[
Mo
2
(DAniF)
2
(MeCN)
2
][BF
4
]
2
(DAniF
=
N,N’-di-p-
anisylformamidinate), a Mo-based paddlewheel unit featuring
two capping ligands on cis coordination sites. The cage,
Mo12(btc)
a. Department of Chemistry and Biochemistry, University of Delaware, Newark,
Delaware, 19716, USA. E-mail: edb@udel.edu
19
3–
4
(DAniF)12 (btc
= 1,3,5-benzenetricarboxylate),
b.
Center for Neutron Science, Department of Chemical and Biomolecular
features six dimolybdenum paddlewheel units coordinated to
four trimesic acid ligands and capped with 12 formamidinate
ligands and is directly analogous to the small cage in HKUST-1.
Although this molecule is isolable in high yield and features
potential porosity, efforts to achieve a porous product via a
Engineering, University of Delaware, Newark, Delaware, 19716, USA.
Catalysis Center for Energy Innovation (CCEI) University of Delaware, Newark,
Delaware, 19716, USA
Electronic Supplementary Information (ESI) available: Detailed experimental
procedures, crystallographic information files, and computational methods. See
DOI: 10.1039/x0xx00000x
c.
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Journal Name
with paddlewheel-paddlewheel distances of ~10 VÅie.wTAhrteiclecOanglienes
pack in the solid state with cage cente is
D
r
O
–
I
c
:
e
10
n
.1
t
0
e
3
r
9/
d
C
9
t
C
a
C
n
0
c
5
e
0
s of
02
G
approximately 20 Å. Close inspection of the crystal structure of
this compound reveals significant solvent-accessible voids.
Accordingly, thermogravimetric analysis (TGA) of
a
recrystallized sample shows a mass loss of 20 % by 150 °C. It
noteworthy that the medium pore in HKUST-1 contains the
identical 3:1:3 metal:ligand:cap ratio as this small octahedral
cage. However, our attempts to isolate this species,
8
Mo24(btc) (DTolF)24, as a molecular cage were unsuccessful
regardless of reaction conditions. Tuning the donating strength
of the capping formamidinate ligand may be a viable strategy
for isolating the higher nunclearity cage, which is the
thermodynamically less favoured product.
To achieve appreciable surface areas for porous cages,
great care must be taken with solvent exchange and removal.
Coordination cages are potentially soluble in many organic
solvents, limiting the number of these accessible for exchange.
Similarly, as porous cages lack three-dimensional connectivity,
lower activation temperatures are often required. For
4 2 2
Mo12(btc) (DTolF)12, recrystallization from CH Cl via MeCN
layering, followed by thorough MeCN exchanges and room
temperature activation resulted in an optimal surface area.
The cage displays a BET (Langmuir) surface area of 446 (547)
2
m /g as determined from N
2
adsorption at 77 K. Pore-size
distribution calculations indicate the octahedral cage in the
material is the most prevalent pore, with additional porosity
ascribed to pore-space extrinsic to the octahedral cage. The
coordination cage displays appreciable solubility in a variety of
organic solvents, including N,N-dimethylformamide (DMF),
chloroform, and methylene chloride. The utility of a porous
molecular material is illustrated by rapidly removing solvent
4
Figure 2. (Top) Octahedral Mo12(btc) (DTolF)12 cage where capping
formamidinate ligands coordinate to two metals in an analogous manner to
2 2
from a CH Cl -dissolved sample which affords a material with a
2
4+
the carboxylate groups. (Bottom) Cuboctahedral [Cu24(btc)
8
(bipy)24
]
cage
2
Langmuir surface area of 507 m /g.
capped with 2,2-bipyridine. Green, orange, red, blue, and grey spheres
represent copper, molybdenum, oxygen, nitrogen, and carbon atoms,
respectively. Hydrogen atoms, charge balancing perchlorate, and solvent
molecules have been omitted for clarity.
We expect this formamidinate-capping strategy to be
widely applicable to isolate porous molecular analogues of a
variety of metal-organic framework pores based on second-
row metals. For the isolation of cages based on first-row
metals, however, a significantly altered strategy must be
employed. This is a result of relative difficulty in preparing not
only formamidinate-based paddlewheel units, but mixed
carboxylate/formamidinate paddlewheels with first-row
metals. The few reported examples of these have been limited
variety of crystallization, solvent exchange, and activation
methods were unsuccessful. This is likely a result of the
interplay of intra- vs. inter-cage porosity.
We have previously shown that small degrees of
functionalization of ligands in porous cages can have a
2
0
profound effect on porosity and gas adsorption properties.
2
1
to chromium(II). Additionally, it is difficult to discriminate
between cis- and trans-formamidinate capping in these
systems. This is avoidable with the use of ligands where cis-
coordination is enforced by bidentate ligands such as
bipyridine or phenanthroline. A library of such coordination
complexes has been reported for copper(II) paddlewheels. For
these, two carboxylate ligands (typically acetate) bridge two
copper(II) cations while bipyridine ligands coordinate to the
Although the carboxylate-based ligands in Cotton’s cage lack
sites accessible to straightforward functionalization, the
capping formamidinate ligands are highly tunable. For these,
the reaction of triethylorthoformate with 4-functionalized
aniline derivatives affords capping ligands, coincident with
release of ethanol, in high yield. We adopted a slightly
modified procedure to produce M12btc L12 cages based on
4
tolyl-functionalized caps. Here, the reaction of molybdenum
hexacarbonyl with N,N’-di-p-tolylformamidinate (HDTolF) in
orthodichlorobenzene affords Mo (DTolF) in quantitative
2 4
2
2
two remaining paddlewheel coordination sites. Analogous to
the liberation of coordinated acetonitrile upon reaction of
2
+
2
Mo (DTolF)
2
(MeCN)
4
with trimesic acid, the acetate ligands
yield. Reaction of this paddlewheel with six equivalents of
trimethyloxonium tetrafluoroborate in acetonitrile (MeCN)
selectively oxidizes cis paddlewheel ligands to afford
of bipyridine or functionalized bipyridine capped copper
paddlewheels can be selectively removed upon reaction with a
multitopic carboxylic acid ligand.
2+.
Mo
2
(DTolF)
2
(MeCN)
4
This bimetallic unit featuring
3
–
Here, the reaction of 2,2’bipyridine capped copper acetate,
coordinating acetonitrile ligands further reacts with btc in
MeCN to afford the octahedral cage Mo12(btc) (DTolF)12 as
2
3
Cu
2
2
(OAc) (bipy)
2
[ClO
4 2
] ,
3
with H btc in methanol at room
4
temperature affords a teal-blue powder in high yield (Caution!-
transition metal perchlorate salts are potentially explosive,
particularly when anhydrous). Diffraction-quality single crystals
determined by single-crystal X-ray diffraction (Figure 2). The
quadruply bonded bimetallic units in this cage feature Mo–Mo
bonds of 2.09 Å and reside at the vertices of the octahedron
2
| J. Name., 2012, 00, 1-3
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that afforded the cuboctahedral structure, we weVrieew uArnticalebOlenlitnoe
isolate this material under any syntheti
D
c
O
c
I
o
:
n
10
d
.1it
0
i
3
o
9
n
/C
s
9
t
C
h
C
a
0
t
5
were
00
2
G
attempted. As ligand functionalization can be used to control
phase in both MOFs and coordination cages, we turned to
computational methods to give insights into design criteria
that would allow for the tunable control of cage phase. Similar
to the 4-position ligand functionalization to control crystal
2
+
packing in the Mo cage, we investigated 4- and 5-
functionalization of 2,2’-bipyridine. Here we utilized a 6-
metal/1-btc portion of the cage shown in Figure 3 that is
terminated with acetate ligands. Structure optimization of the
unfunctionalized cluster and a 4,4’-dimethoxy-functionalized
bipy ligand at the M06-L/6-31G(d,p) level show the methoxy
groups are sufficient to tune the spacing between acetate
carbon atoms in the cluster, with an average C–C distance of
7
.94 Å in the unfunctionalized structure and 7.53 Å in the
methoxy-functionalized structure (Figure 3). Further, these
calculations suggest no appreciable differences in the
electronic structure of the copper ions in each cluster.
Indeed, the reaction of a 4,4’-dimethoxy-functionalized
bipyridine-capped paddlewheel, Cu
2 2 2 4 2
(OAc) (OMe-bipy) [ClO ]
with H btc in methanol at room temperature affords a teal-
3
blue powder in high yield. Diffraction-quality single crystals of
this material were prepared via layering of diethyl ether on a
saturated acetonitrile solution at
determination reveals the cage indeed adopts a structure
analogous to Mo12(btc) (DTolF)12 This copper cage,
Cu12(btc) (OMe-bipy)12][ClO 12, similarly features six dicopper
8
°C. Structural
Figure
Cu (bipy)
3.
(OAc)
Optimized
(btc)] (top) and [Cu (OMe-bipy) (OAc)
broken-symmetry
structures
for
4
.
6
+
6+
[
6
6
3
6
6
3
(btc)] (bottom).
[
4
4
]
–
The steric effect of the –methoxy groups on the latter decreases the RCOO
paddlewheel units in an octahedral arrangement coordinated
–
btc–RCOO OAc angle and drives assembly toward the small cage. The average
3–
to four btc ligands on four of the faces of the cage (Figure 4).
C
acetate–Cacetate distance depicted in the top structure is 7.94 Å as compared
As expected based on the computational model, the Ccarboxylate
–
to 7.53 Å in the methoxy-functionalized cluster.
C
carboxylate distances analogous to those shown in Figure 3 are
of this material were prepared via slow evaporation of a
saturated acetonitrile solution at room temperature.
Structural determination reveals the cage does not adopt a
structure analogous to Mo12(btc)
cage [Cu24(btc) (bipy)24][ClO 24 adopts
structure with eight triangular windows and six square
windows. The cage consists of twelve dicopper paddlewheel
units coordinated to eight btc ligands on every triangular
window (Figure 2). The square windows feature metal-bound
perchlorate or solvent molecules at the axial sites of the
copper paddlewheels. In this cage, the bipy capping ligands
remain on the paddlewheel with one 2,2’-bipyridine
coordinated to cis sites on each copper(II). This cage is
conceptually similar to the previously reported isophthalic
approximately 7.65 Å and the methoxy groups are indeed
incompatible with the close proximity enforced by the larger
cuboctahedral cage. The paddlewheel-paddlewheel distance of
4
(DTolF)12. Rather, this copper
cuboctahedral
~13.2 Å is slightly shorter than the distance of 13.5 Å in the
8
4
]
a
molybdenum cage. This is likely partially a result of the
chelating nature of bipy, which displays significantly decreased
N–Cu–N angles of ~82° as compared to N–Mo–N angles of
approximately 97° for the formamidinate-capped cage. This
causes the trimesic acid ligands of the copper species to
3
–
pucker significantly, decreasing the size of the cage. A CO
2
1
0
acid-based
cuboctahedral
cage
Cu24(bdc)24
.
The
paddlewheel-paddlewheel distances are slightly shorter in the
bipy-capped cage with distances of approximately 9.21 Å along
the edges of the square and triangular windows, as compared
to 9.35 Å in Cu24(bdc)24. As a result of this, the paddlewheel-
paddlewheel distance of 18.54 Å across the cage is slightly
shorter than the distance in the isopthalic acid cage (18.69 Å).
Although [Cu24(btc)
accessible surface area, fits to a CO
8
(bipy)24][ClO
4
]
24 displays negligible
N
2
2
isotherm collected at 195
K on a sample activated at 100 °C reveals a BET (Langmuir)
2
surface area of 104 (334) m /g. The inaccessibility of the
2
structure to N at 77 K is likely a result of pore blockage from
Figure 4. Octahedral [Cu12(btc)
4
(OMe-bipy)12]12+ where the bipyidine caps
copper-coordinated charge balancing perchlorate. However,
coordinate to cis sites on a single copper center and feature pi-pi stacking
with the bipy ligand on the adjacent copper. Green, red, blue, and grey
spheres represent copper, oxygen, nitrogen, and carbon atoms,
respectively. Hydrogen atoms, perchlorate ions, and solvent molecules
have been omitted for clarity.
2
the surface area is similar to the BET value of 248 m /g
2
4
reported for Cu24(bdc)24
.
Although it is expected that the lower nuclearity
octahedral cage should be isolable under similar conditions
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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isotherm collected at 195 K reveals a BET (Langmuir) surface
area of 151 (362) m /g for this material upon activation at 100
5
6
A. C. Sudik, A. R. Millward, N. W. Ockwig, A. P. C
M. Yaghi, J. Am. Chem. Soc., 2005, 127, 7110-7118.
V
o
ie
t
w
e,
A
J
rt
.
ic
K
leimOn
, O.
li
ne
2
DOI: 10.1039/C9CC05002G
°
C. Pore-size distribution calculations reveal a majority of the
E. V. Perez, K. J. Balkus, J. P. Ferraris, I. H. Musselman, J. Memb,
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nor cuboctahedral copper cages display measurable surface
areas upon rapid evacuation of solvent from acetonitrile
7
8
solutions. In this case, both [Cu24(btc) (bipy)24][ClO
8
4
]24 and
5
7, 5283-5287.
[
Cu12(btc) (OMe-bipy)12][ClO 12 have CO accessible surface
4
4
]
2
2
areas that are less than 10 m /g. However, we envision that his
property can be tuned by utilizing ligand functionalization to
tune density, and thus volumetric storage capacity, in designer
cages. Further, it is expected that utilization of porous counter
anions with these porous cationic cages will be a viable route
to tune porosity and gas uptake properties.
O. K. Farha, I. Eryazici, N. C. Jeong, B. G. Hauser, C. E. Wilmer,
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The aforementioned results illustrate how the judicious
utilization of capping ligands in paddlewheel systems can
afford porous coordination cages analogous to metal-
terminated pores in metal-organic frameworks. For metal
cations amenable to cis-divacant coordination sites, such as
molybdenum(II), formamidinate-type ligands can potentially
be incorporated into porous cages. Bipyridine-based capping
ligands can be used with first-row metals in order to isolate
novel paddlewheel-based cages. We envision the strategies
outlined here can be widely applied to a variety of first- and
second-row transition metals and numerous carboxylate-
based multitopic organic ligands. Future work in our lab along
these lines will focus on preparing copper(II) and chromium(II)
materials with a variety of anions for gas storage and
separation applications.
This material is based upon work supported as part of the
Catalysis Center for Energy Innovation, an Energy Frontier
Research Center funded by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences under Award
Number DE-SC0001004. We also thank the University of
Delaware for start-up funds that made this work possible. We
thank Casey Rowland for help with crystal structure
refinements.
8
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Conflicts of interest
There are no conflicts to declare.
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