Selective metal substitution for the preparation of heterobimetallic microporous coordination polymers

Article abstract of DOI:10.1021/ic8007427

The designed synthesis of heterobimetallic microporous coordination polymers (MCPs) is reported by a strategy employing the selective replacement of a single metal in homometallic MCPs with two unique metal coordination environments: octahedral and tetrahedral. This strategy is successful in the preparation of six mixed-metal MCPs, where Co/Zn and Ni/Zn versions of MOF-4, MOF-39, and a Zn-BTEC MCP are reported.

Full text of DOI:10.1021/ic8007427

Inorg. Chem. 2008, 47, 7942-7944
Selective Metal Substitution for the Preparation of Heterobimetallic
Microporous Coordination Polymers
Stephen R. Caskey and Adam J. Matzger*
Department of Chemistry and Macromolecular Science and Engineering Program, The UniVersity
of Michigan, 930 North UniVersity AVenue, Ann Arbor, Michigan 48109-1055
Received April 24, 2008
The designed synthesis of heterobimetallic microporous coordina-
tion polymers (MCPs) is reported by a strategy employing the
selective replacement of a single metal in homometallic MCPs
with two unique metal coordination environments: octahedral and
tetrahedral. This strategy is successful in the preparation of six
mixed-metal MCPs, where Co/Zn and Ni/Zn versions of MOF-4,
MOF-39, and a Zn-BTEC MCP are reported.
multifunctional carboxylated porphyrins.4 Other examples
of mixed-transition-metal MCPs have been arrived at by
empirical means.3a,5 The selective, direct replacement of one
transition metal within a homometallic coordination polymer
by using controlled stoichiometry has not generally led to
maintained structural fidelity.6 Postsynthetic incorporation
or sequestration of a second metal into an MCP has also
been examined extensively, but incomplete metal inclusion
is often observed.7 This leaves an important gap in the
chemistry of MCPs; if the selective replacement of a
transition metal within a given framework can be ac-
complished, it would provide the opportunity to investigate
the effect of a single metal site on the bulk sorbent properties
Microporous coordination polymers (MCPs) are a rela-
tively new class of materials formed by the reaction of a
metal ion and a multifunctional ligand that acts as a linker.
Reports on the synthesis of new MCPs have increased greatly
in recent years. Although new linker synthesis accounts for
some of this activity, variation of the metal cluster is the
most common strategy to make new materials. Beyond the
simple paradigm of changing the linker or the metal,1 novel
strategies are arising that yield new structure types from a
limited number of starting materials. For example, we have
recently demonstrated that copolymerization of two linkers
bearing the same functionality can generate a high-
performance microporous/mesoporous material, UMCM-1.2
Alternatively, the incorporation of two metals can increase
the diversity of MCPs. Heterobimetallic MCPs are now being
increasingly investigated for opportunities to incorporate
unusual metal coordination environments to enhance cata-
lytic, photoluminescent, or other properties. Rare-earth metals
have been widely incorporated into MCPs in conjunction
with transition metals, but these metals occupy noninter-
changeable coordination environments.3
(3) (a) Ren, P.; Shi, W.; Cheng, P. Cryst. Growth Des. 2008, 8, 1097. (b)
Cheng, J. W.; Zhang, J.; Zheng, S. T.; Yang, G. Y. Chem.sEur. J.
2008, 14, 88. (c) Cheng, J. W.; Zheng, S. T.; Ma, E.; Yang, G. Y.
Inorg. Chem. 2007, 46, 10534. (d) Cheng, J. W.; Zheng, S. T.; Yang,
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N.; Xu, G. F.; Liao, D. Z.; Yoshimura, K.; Yan, S. P.; Cheng, P. Inorg.
Chem. 2007, 46, 8454. (f) Zhao, X. Q.; Zhao, B.; Ma, Y.; Shi, W.;
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Wan, Y. H.; Sun, C. Y.; Wang, Z. M.; Wang, K. Z.; Jin, L. P. Inorg.
Chem. 2007, 46, 2956. (l) Gu, X. J.; Xue, D. F. Cryst. Growth Des.
2006, 6, 2551. (m) Zhai, B.; Yi, L.; Wang, H. S.; Zhao, B.; Cheng,
P.; Liao, D. Z.; Yan, S. P. Inorg. Chem. 2006, 45, 8471. (n) Shi, W.;
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The incorporation of two or more transition metals in a
coordination polymer is a current challenge for designed
synthesis. The most widely utilized approach has been to
incorporate a second metal as a generally innocent (coordi-
natively saturated) part of a linear linker as in the case of
* To whom correspondence should be addressed. E-mail: matzger@
umich.edu.
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7942 Inorganic Chemistry, Vol. 47, No. 18, 2008
10.1021/ic8007427 CCC: $40.75  2008 American Chemical Society
Published on Web 08/21/2008

COMMUNICATION
of the coordination polymer while maintaining the same
overall pore structure. Toward this goal, we investigate the
hypothesis that, with the appropriate choice of metal, it is
possible to selectively replace a single site in a homometallic
coordination polymer consisting of both octahedral and
tetrahedral metal sites in the same metal cluster. The rationale
for this hypothesis can be traced to crystal-field stabilization
energy and the relative stability of octahedral and tetrahedral
geometries for complexes of CoII, NiII, and ZnII, where CoII
and especially NiII each favor octahedral coordination while
ZnII favors tetrahedral coordination.8
MOF-4 is a three periodic framework derived from
trimesic acid (H3BTC) and zinc(II) nitrate.9 The secondary
building unit (SBU) of MOF-4 consists of two inequivalent
ZnII ions in the asymmetric unit of the single-crystal X-ray
structure: one tetrahedral and one octahedral metal ion. It
was reasoned that, upon addition of a second metal to the
reaction solution during the formation of this MCP, the
octahedral ion of the original structure could be systemati-
cally replaced by the second appropriately chosen metal. ZnII
ions are well-known to exist in either tetrahedral or octahedral
coordination modes. CoII can also exist in either octahedral
or tetrahedral geometries; however, octahedral coordination
is generally preferred in the presence of weak-field ligands
such as carboxylate and solvent molecules. Two distinct
colors of CoII-containing compounds are generally derived
from the different coordination modes where pink indicates
six coordination as in CoCl2 · 6H2O and blue indicates four
coordination as in anhydrous CoCl2 due to changes in the
electronic configuration. Therefore, to test our hypothesis as
to the selective replacement of octahedral ZnII ions with
another metal, CoII was chosen as an ideal color-specific
indicator for the coordination environment about the metal
center should it be incorporated into an MCP.
Figure 1. Views of (a) the SBU for Co/Zn-MOF-4 [C atoms (gray), O
atoms (red), Co atoms (pink), and Zn atoms (blue-gray)] and (b) the
framework for Co/Zn-MOF-4 (H atoms, solvent molecules, and nitrate ions
are omitted for clarity).
(DMF)/ethanol in the presence of pyridine at 85 °C, pink
cubic crystals were formed. The pink color of these Co/Zn-
MOF-4 crystals suggests octahedral coordination about the
CoII ions. The chemical composition of Co/Zn-MOF-4 was
determined by energy-dispersive X-ray (EDX) spectroscopy
where both Co and Zn were found to be incorporated.
Powder X-ray diffraction (PXRD) showed that Co/Zn-
MOF-4 and MOF-4 were indeed isostructural. Ultimately,
single-crystal XRD analysis of a pink cubic crystal confirmed
the identity of Co/Zn-MOF-4. The crystal structure of Co/
Zn-MOF-4, as shown in Figure 1, reveals two metal
coordination sites: an octahedral site occupied by the CoII
ion and a tetrahedral site occupied by the ZnII ion. Three
BTC linkers are bound through a carboxylate in a bidentate
fashion to each metal center (Figure 1a). The coordination
environment for the octahedral CoII ion is filled by three
solvent molecules, while that for the tetrahedral ZnII ion is
filled by a nitrate to maintain charge neutrality.
To test the generality of this approach, NiII, which also
prefers octahedral coordination, was employed. Replacement
of Co(NO3)2 ·6H2O with Ni(NO3)2 ·6H2O in the mixed-metal
MOF-4 synthesis described above leads to the formation of
green cubic crystals of Ni/Zn-MOF-4, where NiII occupies
the octahedral coordination site and ZnII remains in the
tetrahedral site. EDX confirms the incorporation of both Ni
and Zn in Ni/Zn-MOF-4, while the PXRD confirms the
isostructural nature of Ni/Zn-MOF-4, Co/Zn-MOF-4, and
MOF-4.
This strategy for the preparation of heterobimetallic MCPs
was next applied to MOF-39.10 The SBU of MOF-39 consists
of three ZnII ions: two in a tetrahedral environment and one
in an octahedral environment. Here the synthetic conditions
were altered to add a coordinating ligand (pyridine) to replace
the bound water molecule found in the original MOF-39
structure,10 and it was observed that the addition of pyridine
limits the formation of any undesired byproduct. Upon
reaction of an equimolar quantity of Zn(NO3)2 ·4H2O and
M(NO3)2 · 6H2O (where M ) CoII or NiII) with 1,3,5-
(triscarboxyphenyl)benzene (H3BTB)11 in a 1:2 (v/v) mixture
of DMF/ethanol in the presence of pyridine, pink (Co/Zn-
Mixed-metal variants of MOF-4 were synthesized under
solvothermal conditions. Upon reaction of an equimolar
quantity of Zn(NO3)2 · 4H2O and Co(NO3)2 · 6H2O with
H3BTC in a 1:2 (v/v) mixture of N,N-dimethylformamide
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2006, 128, 15255. (h) Szeto, K. C.; Lillerud, K. P.; Tilset, M.; Bjørgen,
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A. K.; Fe´rey, G. Angew. Chem., Int. Ed. 2007, 46, 5877. (b) Wang,
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Tissler, A.; Fischer, R. W.; Fischer, R. A. Angew. Chem., Int. Ed.
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COMMUNICATION
Figure 3. Views of (a) the SBU for Co/Zn-BTEC [C atoms (gray), N
atoms (blue), O atoms (red), S atoms (yellow), Co atoms (pink), and Zn
atoms (blue-gray)] and (b) the framework for Co/Zn-BTEC (H atoms are
omitted for clarity).
octahedral metal sites, whereas ZnII ions occupy the tetrahedral
metal sites. On the basis of the crystal structure, this compound
was formulated as [CoZn(BTEC)(DMF)(DMSO)]n, where
DMF and DMSO are bound to CoII along with four O atoms
of three different BTEC linkers, as shown in Figure 3. The ZnII
ions are coordinated by four O atoms from four different BTEC
molecules; three of which bridge between the CoII and ZnII ions.
Further supporting the generality of this approach for the
design of mixed-metal MCPs, replacement of Co(NO3)2 ·
6H2O with Ni(NO3)2 · 6H2O results in the formation of
[NiZn(BTEC)(DMF)(DMSO)]n (Ni/Zn-BTEC), as con-
firmed by SEM/EDX and PXRD analysis.
In conclusion, a simple strategy for the direct substitution
of a single metal into homometallic MCPs with two distinct
metal coordination environments for the preparation of
heterobimetallic MCPs was developed. This strategy proved
to be successful where Co or Ni could be substituted for Zn
in three previously reported Zn-only MCPs. Direct metal
substitutions to generate series of isostructural homometallic
coordination polymers are quite rare. It is therefore significant
that metal substitution can be achieved for the synthesis of
mixed-metal MCPs that remain isostructural to the homo-
metallic counterparts. This strategy was used for the prepara-
tion of heterobimetallic derivatives of MOF-4, MOF-39, and
a Zn-BTEC MCP. This method broadens the field of MCPs
by introducing a systematic approach to designed heterobi-
metallic materials with unique metal coordination environ-
ments, which could potentially provide improved properties
toward catalysis, photoluminescence, or magnetism.
Figure 2. Views of (a) the SBU for Co/Zn-MOF-39 [C atoms (gray), N
atoms (blue), O atoms (red), Co atoms (pink), and Zn atoms (blue-gray)]
and (b) the framework for Co/Zn-MOF-39 (H atoms and solvent molecules
are omitted for clarity).
MOF-39) or green (Ni/Zn-MOF-39) plate crystals were
formed. Scanning electron microscopy (SEM)/EDX again
confirms the incorporation of two metals in both Co/Zn-
MOF-39 and Ni/Zn-MOF-39. PXRD confirms the isostruc-
tural nature of Co/Zn-MOF-39, Ni/Zn-MOF-39, and MOF-
39, while single-crystal XRD analysis of a pink plate
confirms the identity and coordination environment of each
of the metals as predicted for Co/Zn-MOF-39, as shown in
Figure 2. A similar analysis of the single-crystal XRD data
of Ni/Zn-MOF-39 also confirms the incorporation of an
octahedral NiII and two tetrahedral ZnII ions (see the
Supporting Information).
To further emphasize the generality of this approach to the
synthesis of mixed-metal MCPs, another homometallic MCP
with both octahedral and tetrahedral metal sites was chosen in
[Zn2(BTEC)(DMF)2]n (Zn-BTEC),12 where BTEC represents
the 4-connector 1,2,4,5-benzenetetracarboxylate. The crystal
structure of this material was shown to have two coordination
environments for the ZnII ions; again one Zn is tetrahedral, and
the other is octahedral. However, there is an added complication
in the case of BTEC because of the possibility of the preferential
formation of one of an extensive variety of known homometallic
polymers for both Co and Zn. A search of the Cambridge
Structure Database resulted in 28 hits for Zn/BTEC crystal
structures and 55 hits for Co/BTEC crystal structures. Therefore,
the application of this strategy to BTEC provides a particular
challenge because of the potential formation of numerous
homometallic polymers.
Acknowledgment. This work is supported by Grant 454
of the 21st Century Jobs Trust Fund received through the
SEIC Board from the State of Michigan. The authors thank
Dr. Antek Wong-Foy and Dr. William W. Porter III for
assistance with X-ray data analysis, Dr. Kyoungmoo Koh
for discussion of nets, and Carl Henderson at the University
of Michigan’s Electron Microbeam Analysis Laboratory for
assistance with SEM equipment (NSF Grant EAR-9628196).
This strategy for selective metal substitution at the octahedral
metal site was applied to Zn-BTEC. Upon reaction of an
equimolar quantity of Zn(NO3)2 ·4H2O and Co(NO3)2 ·6H2O in
the presence of pyromellitic dianhydride (BTEC-anhydride) in
a 2:1 (v/v) mixture of DMF/dimethyl sulfoxide (DMSO) at 100
°C, pink crystals were formed. Analysis of the single-crystal
XRD data for this new Co/Zn-BTEC phase revealed the
isostructural nature with the Zn-only MCP, where both fall into
the same P42bc space group. Again the CoII ions occupy the
Supporting Information Available: Synthetic procedures, ther-
mogravimetric analysis data, crystallographic data (in the CIF
format), PXRD data, SEM/EDX data, ORTEP diagrams, and
photomicrographs of crystals. This material is available free of
charge via the Internet at http://pubs.acs.org.
(12) Yang, S. Y.; Long, L. S.; Huang, R. B.; Zheng, L. S.; Ng, S. W.
Appl. Organomet. Chem. 2004, 18, 91.
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