Porous metal-organic alloys based on soluble coordination cages

Article abstract of DOI:10.1039/d0sc04941g

Diverse strategies for the preparation of mixed-metal three-dimensional porous solids abound, although many of them lend themselves only moderate levels of tunability. Herein, we report the design and synthesis of surface functionalized permanently microporous coordination cages and their use in the isolation of mixed metal solids. Judicious alkoxide-based ligand functionalization was utilized to tune the solubility of starting copper(ii)-based cages and their resulting compatibility with the mixed-cage approach described here. We further prepared a family of isostructural molybdenum(ii) cages for a subset of the ligands. The preparation of mixed-metal cage solids proceeds under facile conditions where solutions of parent cages are mixed and product phases isolated. A suite of spectroscopic and characterization tools confirm the starting cages are intact in the amorphous product. Finally, we show that utilization of precise ligand functional groups can be used to prepare mixed cage solids that can be easily and cleanly separated into their constituent components through simple solvent washing or solvent extraction techniques.

Full text of DOI:10.1039/d0sc04941g

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Porous metal-organic alloys based on soluble coordination cages
Alexandra M. Antonio,†a Kyle J. Korman,†a Glenn P. A. Yapa and Eric D. Bloch*a
Received 00th January 20xx,
Accepted 00th January 20xx
Diverse strategies for the preparation of mixed-metal three-dimensional porous solids abound, although many of them
lend themselves only moderate levels of tunability. Herein, we report the design and synthesis of a suite of surface
functionalized permanently microporous coordination cages and their use in the synthesis of mixed metal solids. Judicious
alkoxide-based ligand functionalization was utilized to tune the solubility of starting copper(II)-based cages and their
resulting compatibility with the mixed-cage approach described here. We further prepared a family of isostructural
molybdenum(II) cages for a subset of the ligands. The preparation of mixed-metal cage solids proceeds under facile
conditions where solutions of parent cages are mixed and product phases isolated. A suite of spectroscopic and
characterization tools confirm the starting cages are intact in the amorphous product. Finally, we show that utilization of
precise ligand functional groups can be used to prepare mixed cage solids that can be easily and cleanly separated into
their constituent components through simple solvent washing or solvent extraction techniques.
DOI: 10.1039/x0xx00000x
guest chemistry can be studied,27 and homogeneous post-
synthetic functionalization can be employed.28 A number of
researchers have used this latter strategy in the post-synthetic
Introduction
Permanently microporous coordination cages have seen a
marked increase in their development over the past five
years.1,2,3 These molecules, which are also commonly referred
to as metal-organic polyhedra (MOPs) or porous coordination
cages (PCCs),4,5 are distinct from the broader class of
supramolecular containers6,7,8,9,10,11 in that they are stable to
solvent evacuation and display permanent porosity to gases in
the solid state. In this regard, they are similar to metal-organic
frameworks (MOFs), the well-known and widely studied class
of porous three-dimensional materials.12,13 For many porous
coordination cages, they can be thought of as molecular
analogs of metal-organic framework pores.14 Given their often-
limited solubility,15 many of them are actually more MOF-like
than molecular in nature.16 Their surface areas often pale in
comparison to the record values reported for MOFs.17
However, even for insoluble porous cages, their molecular
nature does endow them with significant advantages as
compared to MOFs.18 Chiefly among them is their modularity
and compatibility with molecular level design principles.19–23
These can be leveraged, for example, in synthetic routes
where precise ligand functionalization can be used to tune the
crystal packing and thus solid-state properties of a given cage
structure.24,25 Soluble cages have considerable added benefits
as compared to both MOFs and insoluble PCCs. Solution-based
synthesis and characterization methods can be used,26 host-
modification of cuboctahedral coordination cages (Figure 1).29
Fig 1. An example of an alkoxide-functionalized cuboctahedral copper cage used
for the synthesis of porous coordination cage alloys. The structure, Cu24(5-
hexoxy-bdc)24 features ligand functional groups on its surface that can be tuned
to optimize solubility.
Isophthalic acid, paddlewheel-based cuboctahedral
structures have been under investigation for nearly two
decades although they were initially limited to copper- and
molybdenum-based structures with simple ligand functional
groups.30,31 Relatively recently, however, there has been an
influx of publications regarding these types of materials,
whose related structures have been expanded to Cr, Ni, Ru,
and Rh.32-41 In addition to this, an impressive body of work has
been established regarding the incorporation of a variety of
functional groups on their surface.42 These are typically
installed prior to cage assembly, where precisely placed ligand
a. Department of Chemistry & Biochemistry, University of Delaware, Newark, DE
19716, USA. E-mail: edb@udel.edu
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
†These authors are joint first authors and contributed equally.
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functional groups have been used to tune charge, solubility, from a mixture of judiciously-chosen porous organic cages.64
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DOI: 10.1039/D0SC04941G
phase, surface area, and stability.43,44,45 More recently, the This approach had the advantage in that any ratio of cages
post-synthetic modification of a subset of these structures has could be used to form a continuum of porous organic solid
been demonstrated where solution-stable cages can be solutions. The work presented here lies at the intersection of
modified with ester, ether, or amide groups.28,44,46,47,48 mixed-metal MOFs and mixed-cage POCs. We report the
Cuboctahedral cages have also shown utility as building blocks design, synthesis, characterization, and utilization of porous
for three-dimensional porous solids. It has been shown that coordination cage alloys (Figure 2). These materials are
the addition of an appropriate pillaring ligand can be used to comprised of alkoxide-functionalized copper and molybdenum
isolate cuboctahedra-based MOFs.49,50,51 Given this, it is not coordination cages where the synthesis, modification, and
surprising that these M24L24 paddlewheel structures are also separation of the materials is controllable based on the level of
present as prevalent pore types in
carboxylate-based MOFs.
a wide variety of alkoxide-functionalization.
As the organic and coordination chemistry of
cuboctahedral paddlewheel-based cages has been developed,
so too have new applications that have leveraged the inherent
solubility of these MOF-like structures. As reported by Li et al,
an organic-soluble, water-insoluble charged cuboctahedron
was solution processed to form
a uniform honeycomb
interface for potential biological applications.52 Other unique
properties can be realized through the solvation and
dispersion of these cages, exemplified by the marked increase
in gas adsorption and thermal stability when dispersed inside
mesoporous silica.53 We have recently shown that solutions of
oppositely charged, permanently porous cages based on Fig. 2 Modular approach for the synthesis of mixed-metal porous solids. Here, porous
cages are synthesized and isolated prior to combining to give a mixed-metal alloy.
differing metal cations can be combined to afford insoluble,
extended salt structures made solely of coordination cages.54
The synthesis of these porous salts illustrates a benefit of the
Results and Discussion
use of coordination cages for the synthesis of mixed
functionality materials. In targeting, for example, a mixed-
metal framework, one could simply combine oppositely-
charged porous cages that contain the metal cations of
interest. This is in stark contrast to the synthetic methods that
are required to afford mixed-metal and/or mixed-ligand
MOFs.55,56 Often, serendipity plays a role and a mixture of
metal cations in the MOF reaction mixture affords a mixed
metal product. More complex design strategies have also been
employed.57 For these, mixed-functionality ligands bind one
type of metal cation during framework synthesis, leaving the
other metal binding site accessible for post-synthetic
modification.58 Metal-chelating sites can also be added to
framework ligands, post-synthetically. Some MOF structures
are amenable to selective post-synthetic metal exchange.59
Here, for example, one of the four metal cations that make up
the secondary building unit of MOF-5 can selectively be
exchanged for other first-row metal cations.60 Here too,
however, the process is time consuming and lacks a level of
synthetic control as precise metal:metal ratios cannot
necessarily be easily tuned.
The higher fidelity synthesis of mixed-functionality
products has been reported for porous organic cages
(POCs).61,62,63 In a subset of these, scrambled organic cages
were synthesized through [4+6] cycloimination, with random
incorporation of vicinal diamines for each molecular cage. This
disrupted their ability to efficiently pack in the solid state,
leading to increased gas storage capabilities. Cooper and
coworkers also reported the synthesis of “porous organic
alloys” where a ternary crystal was designed and prepared
Although permanently porous coordination cages have been
widely investigated as a result of their potential compatibility
with molecular synthesis and characterization methods, the
vast majority of cuboctahedral cages are in fact insoluble in
most common organic solvents. We have recently shown that
judicious ligand functionalization can be used to improve the
solubility of these types of cage while still endowing them with
N2 or CO2 accessible surface area in the solid state.28 In
addition to this, ligand functional groups can be used to
control the phase of isophthalic acid-based cages. An
important starting point for the work outlined here is the
development of a family of coordination cages that have high
solubility, can be isolated for multiple metals, and maintain
porosity in the solid state upon solvent removal. With the
exception of Cu24(OPent-bdc)24, which is soluble in a limited
number of organic solvents, the alkoxide-functionalized cages
we previously reported contain insufficient alkyl chain length
to impart solubility.43 Although the 5-dodecoxyisophthalic
acid-based cage was reported some time ago by Yaghi, it has
limited surface area to CO2 and is essentially nonporous to
N2.65 To more systematically understand the relationship
between alkoxide chain length, solubility, and porosity, we
initiated a study wherein copper cages based on functionalized
ligands from hexoxy through dodecoxy were prepared.
The synthesis of alkoxide-functionalized ligands is relatively
straightforward where the reaction of the appropriate alkyl
halide with the methyl-protected ester of 5-hydroxy
isophthalate in acetone in the presence of base affords the
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targeted ligands in high yield. A straightforward base-catalyzed stretch and the persistence of –OH stretch in the IR spectra.
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deprotection gives the free carboxylic acids. The synthesis of Finally, the –OH stretch became absenDt OfoI:l1l0o.w10i3n9g/Da0cStCiv0a4t9i4o1nG,
functionalized cuboctahedral cages is also simple where the indicating optimal activation conditions for each cage. We also
reaction of ligand with copper salts in amide solvents typically screened desolvation temperatures, with N2 and CO2
gives cage in high yield. However, in order to isolate diffraction isotherms recorded after successive evacuation steps. Optimal
quality single crystals (Figure 3), extensive screening with degas conditions were screened for the materials, with no
temperatures and solvent mixtures had to take place. Initially, discernible correlation to the length of the alkoxide tail. As
equimolar metal-ligand solutions were placed in several dry could be expected the shortest chain length, Cu24(5-hexoxy-
baths to screen rate of cage formation as a function of bdc)24 afforded the highest BET (Langmuir) surface area, 197
temperature. Due to the disordered nature of the alkoxide (523) m2/g, and was the only cage that had N2 accessible
chains decorating the periphery of the cages, several days of surface area. The CO2 accessible surface areas of the remaining
heating were necessary to afford diffraction quality single materials were generally slightly lower with BET (Langmuir)
crystals. Typically, solutions with too high of an alcohol content surface areas ranging from 102 to 168 (354-771) m2/g.
produced crystalline powders. A simple solubility test helped
Although as-synthesized cages showed considerable
indicate whether resultant products were cages or higher- solubility in a wide range of organic solvents, activated
dimensional solids, with the latter being insoluble. In instances material had decreased solubility profiles. We have generally
where microcrystalline powder had formed, solvent ratios found that solvation of the metal cation sites in porous
were then modified to give single crystals.
coordination cages with amides (e.g. DMF, DMA, etc.)
increases their solubility in less-polar solvents. As expected, we
saw a marked increase in solubility with increasing chain
length. Cu24(5-hexoxy-bdc)24 was only soluble in DMF and DMA
while Cu24(5-dodecoxy-bdc)24 was highly soluble in DMF, DMA,
THF, CHCl3, CH2Cl2, and benzene. The intermediate-length
alkoxide cages had intermediate solubility, for example,
Cu24(5-nonoxy-bdc)24 was highly soluble in DMF and DMA,
partially soluble in THF, CH3Cl, CH2Cl2, and insoluble in
benzene.
Given the differing solubilities of the hexoxy-, nonoxy-, and
dodecoxy-bdc based cages, we targeted the synthesis of
molybdenum-based cuboctahedra with these ligands. The
synthesis of Mo24(R-bdc)24 cages proceeds via the reaction of
Mo2(OAc)4 and the functionalized ligand in amide/alcohol
solvent mixtures at various ratios. For instance, Mo24(5-
hexoxy-bdc)24 was synthesized in an 80:20 DMA:MeOH ratio
after heating for 2 days at 100 °C in an N2 glovebox. In
contrast, Mo24(5-nonoxy-bdc)24 required a 90:10 DMF:EtOH
solvent ratio in order to precipitate crystalline orange solid.
Lastly, for the most soluble Mo(II) cage of the three described,
the synthesis of Mo24(5-dodecoxy-bdc)24 necessitated an 80:20
DMF:EtOH mixture to give crystalline solid.
Although we were unable to obtain diffraction-quality
single crystals of the Mo-based materials, the diamagnetic
nature of molybdenum(II) paddlewheels confers the added
advantage in that they are compatible with solution NMR
studies. After synthesizing the desired Mo(II) cages, the
mother liquor was discarded and the crystalline powders were
thrice washed with the respective alcohols used during
synthesis. After Mo24(5-hexoxy-bdc)24 was fully exchanged
with MeOH, it was dried under vacuum and dissolved in DMF-
d7 for NMR studies (Figure S15). Similarly, after Mo24(5-
nonoxy-bdc)24 was fully exchanged with EtOH, it was vacuum
dried and dissolved in DMF-d7 (Figure S18). Finally, after
Mo24(5-dodecoxy-bdc)24 was fully exchanged with EtOH, it was
vacuum dried and dissolved in CDCl3 for 1H NMR (Figure S21).
Thermogravimetric analysis measurements suggest the
Mo(II) cages have high thermal stability with minimal mass
losses up to 400 °C, while the Cu(II) analogues decompose at
Fig 3. Solid-state structure of Cu24(5-nonoxy-bdc)24 as viewed down the c-axis.
Subtle ligand functionalization changes tune the packing, and thus porosity, of
these types of materials.
Analogous to the synthesis of copper cages where a wide
variety of conditions can afford the target cage, general
desolvation can be achieved by simply heating the samples to
moderate temperatures under dynamic vacuum. However, to
optimize the surface areas for these cages, a number of
additional factors must be considered. Here we typically
screen a wide variety of solvent exchange conditions where
washes with volatile solvents are employed either directly or
after multiple amide washes. For the seven copper cages
reported here, optimal surface areas were achieved after
thorough methanol washes. We were able to monitor the
completion of washes and activation through infrared
spectroscopy. The IR spectra for all cages initially showed a
strong stretch at ~1660 cm-1 and a broad peak at ~3300 cm-1,
which were expected based on the solvent mixture used in the
synthesis of the cage, N,N-dimethylformamide (carbonyl
stretch) and an alcohol (OH stretch), respectively. The copper-
bound DMF molecules were replaced with methanol through
subsequent washes, indicated by the loss of the carbonyl
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much lower temperatures. As a result of this high thermal were dissolved an appropriate solvent (hexoxy = DMF, nonoxy
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DOI: 10.1039/D0SC04941G
stability, fully activated Mo(II) cages can be realized with no = THF, dodecoxy = benzene) and in the case of [(Cu24(5-hexoxy-
residual amide or alcohol solvent bound to the metal bdc)24/Mo24(5-hexoxy-bdc)24]
and
[Cu24(5-nonoxy-
paddlewheels and no structure collapse after proper washing bdc)24/Mo24(5-nonoxy-bdc)24] precipitated with anhydrous
methods. Although the Mo(II) cages retain porosity until ~350 methanol. [Cu24(5-dodecoxy-bdc)24/Mo24(5-dodecoxy-bdc)24]
°C, with surface areas fluctuating minimally, their Cu(II) was isolated by simply subliming frozen benzene from a
analogues collapse and lose porosity at higher temperatures, homogeneous solution to afford a porous solid. For all three
with the Cu24(5-hexoxy-bdc)24 displaying significant surface materials, the spectroscopic features displayed by the
area decrease at just 75 °C (Figure S98). Similar to its Cu(II) resultant solids are a weighted average of those of the parent
counterpart, Mo24(5-hexoxy-bdc)24 afforded the highest BET cages (Figures 5, S53-S66) while the thermal gravimetric
surface area of the molybdenum-based cages at 290 m2/g and analysis and differential scanning calorimetry results are
was the only Mo(II) cage that demonstrated pores accessible significantly different than either parent cage (Figure 5, S67-
to N2. The Mo(II) cages followed a trend where increasing the S79). As expected, the survey XPS scan indicates the presence
length of alkoxy chain decreased the BET surface area, with of both copper and molybdenum in the isolated amorphous
Mo24(5-nonoxy-bdc)24 and Mo24(5-dodecoxy-bdc)24 exhibiting sample. In a similar manner, the vibrational and electronic
surface areas of 172 m2/g and 104 m2/g, respectively.
spectra for the products indicate the parent cages are intact
when isolated as mixed materials. IR spectra clearly indicate
vibrational features that are specific to either Cu24(5-R-bdc)24
or Mo24(5-R-bdc)24 are present in the product. Finally,
elemental mapping experiments (Figure 4, S86-97) confirm a
truly homogenous dispersion of copper and molybdenum in
product particles.
Fig 4. SEM image (top left) and corresponding EDX mapping (top right) of a
[Cu24(5-dodecoxy-bdc)24/Mo24(5-dodecoxy-bdc)24
] particle. In the EDX map,
orange and cyan represent Mo (bottom left) and Cu (bottom right), respectively,
clearly showing the homogeneous distribution of metals in the particle.
Fig 5. Characterization of Cu24(5-hexoxy-bdc)24 (blue), Mo24(5-hexoxy-bdc)24
(orange) and [Cu24(5-hexoxy-bdc)24/Mo24(5-hexoxy-bdc)24] green. The top left,
top right, bottom left, and bottom right plots show XPS, TGA, IR, and UV-vis data,
respectively. Importantly, the Cu/Mo alloy displays spectroscopic signatures of
both parent cage yet bulk thermal properties (TGA) that are dissimilar to either
cage.
Our previously reported syntheses of porous salts based on
coordination cages afford mixed-metal cages wherein product
phase is MOF-like in that it is completely insoluble in all
organic solvents as a result of the high lattice energy of the
product phase. Utilization of the surface-functionalized cages
presented here offers an additional level of tunability as the
solid phase based on alkoxide functionalized cages is still
expected to be soluble in certain solvents. To prepare mixed-
metal materials, we focused on M24(5-hexoxy-bdc)24, M24(5-
nonoxy-bdc)24, and M24(5-dodecoxy-bdc)24 (M = Cu, Mo) as
they all display appreciable solubility in solvents with varying
polarity. The syntheses of [Cu24(5-hexoxy-bdc)24/Mo24(5-
hexoxy-bdc)24], [Cu24(5-nonoxy-bdc)24/Mo24(5-nonoxy-bdc)24],
Although the product phases are amorphous, they display
CO2 accessible surface areas that are on par with the values of
the pure-metal parent cages and range from 286-382 m2/g
(Langmuir). In order to probe the accessibility of the Mo2+
cations in these materials, we turned to 273 K oxygen
isotherms as the reactivity of molybdenum(II)-based
paddlewheels toward O2 is well known. As expected, Cu24(R-
bdc)24 (R = hexoxy, nonoxy, dodecoxy) adsorbs minimal O2 at
273 K and 1.0 bar. In contrast, isotherms measured under the
and
[Cu24(5-dodecoxy-bdc)24/Mo24(5-dodecoxy-bdc)24]
proceeded similarly where equimolar quantities of each cage
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same conditions for Mo24(R-bdc)24 (R = hexoxy, nonoxy, washed with methanol and dried under dynamVieiwcArvtiaclceuOunlmine.
dodecoxy) indicate a strong interaction with O2 as the Addition of THF to the dried powderDOseI:l1e0c.1t0iv3e9l/yD0dSCis0s4o9lv4e1Gd
adsorption isotherms are incredibly steep before turning over Mo24(5-nonoxy-bdc)24, leaving Cu24(5-hexoxy-bdc)24 powder.
1
at approximately 0.5 mmol/g (Figure S111). The porous alloy The purity of Mo24(5-nonoxy-bdc)24 was confirmed via H NMR
materials based on mixtures of these cages display saturation spectroscopy. (Figure S25). However, the Mo(II) cage could not
O2 capacities that fall between those of the metal-pure cages be fully separated from the Cu24(5-hexoxy-bdc)24 cage as seen
(Figure S112). The structural stability of the Mo(II) cages allows via UV-Vis spectroscopy (Figure S26). Since the porous alloy
for recovery of the material even after chemisorption of O2, was precipitated from DMF and washed with methanol, there
where the cage can be reactivated, releasing bound O2 and was some residual DMF coordinated to the metal
retaining CO2 accessible surface area (Table S3).
paddlewheels that was not fully removed. Consequently, the
Previous work has shown that these types of cages are usually THF-insoluble Cu24(5-hexoxy-bdc)24 began to dissolve in
isolable with mixed-metal paddlewheel units.40 We turned to a THF upon subsequent washing intended to remove Mo24(5-
combination of experiments to rule out metal cation exchange nonoxy-bdc)24, rendering this separation technique
in these materials and to confirm the products are best unsatisfactory. Therefore, in order to fully separate the porous
described as [Cu24(R-bdc)24]/[Mo24(R-bdc)24] as compared to cage alloy, we utilized an extraction technique to promote
[Cu12Mo12(R-bdc)24] or (CuMo)12(R-bdc)24]. Importantly, phase transfer and subsequent recovery of parent cages.
absorption features in solution-based UV-vis spectra of the
An important and practically useful embodiment of this
product phase correspond to those in the pure-cage solutions example involves the preparation of porous cage alloys based
(Figure S50), which is consistent with a lack of metal cation on both different metals and different ligands. Here one could,
exchange in a given paddlewheel. This does not, however, rule for example, leverage the differing solubility of each cage in
out the presence of mixed-paddlewheel cages. To investigate the preparation of porous cage alloys to recover phase-pure
this more thoroughly, we undertook a variety of ligand and cage after synthesis of the product phase. In an analogous
metal/ligand exchange experiments.
manner to the synthesis of ligand-pure mixed metal cages, we
It has been previously shown that copper-based prepared DMF solutions of [Cu24(R-bdc)24]/[Mo24(R-bdc)24]
cuboctahedral cages are amenable to ligand exchange where R = 12/6; 12/9; 6/12; or 9/12 carbon chain alkoxides. In
chemistry.15 To determine the propensity for Mo24(R-bdc)24 to all cases, and analogous to the pure ligand mixtures, the
engage in ligand exchange reactions a number of control resulting homogeneous solutions are dark brown. Substituent
experiments were performed. Here, Mo24(5-hexoxy-bdc)24 was cages can be separated, however, by simply layering a linear
dissolved in DMF, followed by addition of H25-nonoxy-bdc, alkane solvent on the DMF, shaking the vial, and pipette
heating to 100 °C, and allowing the sample to cool to room separating products where the longer chain alkoxide cages are
temperature before precipitating a product with methanol. soluble in the alkane while the 5-nonoxy-bdc and 5-hexoxy-bdc
The isolated product was only soluble in DMF, suggesting the cages remain in DMF (Figure 6). NMR characterization of the
1
absence of more solubilizing functional groups. H NMR of the dissolved or digested (Mo and Cu, respectively) cages rule out
product phase confirms this interpretation as pure Mo24(5- metal or ligand exchange reactions and confirm cage purity
hexoxy-bdc)24 was isolated (Figure S22). Similarly, we found (Figures S27-S34). Given the broad tunability of the solubility
that Mo24(5-nonoxy-bdc)24/H2dodecoxy-bdc and Mo24(5- of these types of cages, and by extension those of cages based
dodecoxy-bdc)24/H2hexoxy-bdc mixtures did not participate in on different divalent metal cations, we expect this approach to
ligand exchange reactions (Figures S23, S24).
be highly tunable for novel syntheses of mixed-metal porous
solids.
Conclusions
Mixed-metal, permanently porous metal-organic materials are
of high interest for a wide variety of applications. However,
there remain challenges for the straightforward and tuneable
synthesis of these materials. Here, we have shown that soluble
permanently microporous coordination cages can be used for
the preparation of mixed-metal products. Precise surface
functionalization of alkoxide-based cages allows for tuning of
cage solubility which, in turn, can be used for the construction
Fig 6. The separation of coordination cage-based porous alloys where solubility
differing solubility of functionalized cages can be used for straightforward
solvent extractions. Here a DMF mixture of Cu24(5-dodecoxy-bdc)24 and Mo24(5-
nonoxy-bdc)24 is layered with hexane and shaken to afford cleanly separated
cage solutions.
or deconstruction of porous material.
A
variety of
spectroscopic tools were used to confirm that the parent cages
persist after incorporation into amorphous alloys. Importantly,
this approach gives the added benefit that product phase
formation is reversible and starting cages are isolable by
simple solvent washing or extraction procedures. We expect
that the mixed-cage design principles outlined here can be
To rule out ligand exchange in the presence of cages based
on both metals, we combined Cu24(5-hexoxy-bdc)24 and
Mo24(5-nonoxy-bdc)24 in DMF, heated to 100 °C until both
cages dissolved then added excess methanol upon cooling to
precipitate product. The resulting brown solid was thoroughly
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used to prepare a nearly unlimited number of mixed-metal
porous solids with complete control over metal ratios in
product phase. Importantly, this approach can be utilized for
families of cages that display increased thermal, hydrolytic,
and chemical stabilities as compared to paddlewheel-based
cages.
21 C. A. Rowland, G. R. Lorzing, R. Bhattacharjee, S. Caratzoulas,
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This material is based upon work supported by the U.S.
Department of Energy’s Office of Energy Efficiency and
Renewable Energy under the Hydrogen and Fuel Cell
Technologies and Vehicle Technologies Offices under Award
Number DE-EE0008813.
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