DOI: 10.1002/anie.201106429
Functional MOFs
Metal–Organic Framework Regioisomers Based on Bifunctional
Ligands**
Min Kim, Jake A. Boissonnault, Phuong V. Dau, and Seth M. Cohen*
Metal–organic frameworks (MOFs) are crystalline, hybrid
materials that consist of inorganic connecting nodes and
organic linker molecules. MOFs are attractive materials for
applications in gas adsorption,[1] separations,[2] catalysis,[3] and
other technologies[4] because of their high porosity, thermal
stability, and chemical tunability. The ability to utilize differ-
ent organic ligands in MOFs is particularly advantageous, as it
allows for the introduction of a wider variety of functional
groups into the pores of the MOF when compared to other
porous, crystalline solids. The use of postsynthetic modifica-
tion (PSM) has provided broader access to functional groups
within MOFs.[5,6] Both solvothermal and PSM routes have
demonstrated that multifunctional or “multivariate” MOFs
can be prepared, with more than one functional group
displayed within the MOF pores.[7–13] In these multifunctional
MOF materials, the relative abundance of different ligands
(and hence different functional groups) can be controlled, but
not the distribution nor spatial orientation of the functional
groups with respect to each other. To truly achieve the next
level of tailored, multi-purpose materials,[14] control over the
relative position of different functional groups would be
required. Herein, we describe the first class of bifunctional
MOF “ligand regioisomers” and show that even these subtle
changes can result in materials with dramatically different
physical properties.
described and it is found that these regioisomers manifest
themselves as distinct conformational isomers with notably
different physical properties. Furthermore, these studies are
the first to control the position of targeted functional groups
in a porous, crystalline material.
We chose a previously unreported class of bifunctional
amino-halo benzene dicarboxylates (NH2X-BDC, where X =
Cl, Br, or I) as the building blocks for MOF regioisomers.
Independently, amino and halide groups are well-known in
MOFs,[5,16] and PSM routes for both amino and halide groups
have been reported,[13] leaving open the possibility of PSM on
MOF regioisomers. The target ligands were synthesized by
halogenation of dimethyl-2-amino terephthalate (1) using N-
halosuccinimides (NCS, N-chlorosuccinimide; NBS, N-bro-
mosuccinimide; NIS, N-iodosuccinimide; Table 1 and
Scheme S1 in the Supporting Information).[17] Depending on
the N-halosuccinimide used, it was possible to obtain two
different regioisomers that could be isolated by column
chromatography. Electronic effects dictate that the ortho- and
para-positions, relative to the amino group, will be preferen-
tially halogenated over the meta-position. In addition, steric
considerations would suggest that the para-position might be
more accessible than the ortho-position. Indeed, chlorination
with NCS gave a nearly equal mixture of the ortho- (2a) and
Table 1: Preparation of bifunctional amino-halo BDC ligands.[a]
Recently, framework isomers of MOFs have been classi-
fied into three major groups: interpenetrated, conforma-
tional, and orientation isomers—which all describe different
structures comprised of the same ligand and metal ion
composition.[15] These isomers tend to have different proper-
ties from each other, albeit sometimes minor. MOFs derived
from different ligands are referred to as “ligand-originated
isomers”. Although many different ligands have been inves-
tigated for MOF formation, we are unaware of any systematic
studies of ligand-originated isomers that arise from differ-
ences is regiochemical isomerism in a multifunctional ligand.
In the studies presented here, the first MOF regioisomers are
[*] Dr. M. Kim, J. A. Boissonnault,[+] P. V. Dau,[+] Prof. Dr. S. M. Cohen
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093 (USA)
E-mail: scohen@ucsd.edu
[+] These authors contributed equally to this work.
X
Yield of
Yield of
ortho-halogenation[b]
para-halogenation[b]
Cl
Br
I
2a, 34%
3a, 16%
0%
2b, 44%
3b, 57%
4, 47%
[**] We thank Dr. Y. Su (UCSD) for assistance with mass spectrometry
experiments and D. Martin (UCSD) for assistance with crystallog-
raphy. This work was supported by a grant from the National
Science Foundation (CHE-0952370).
[a] Reaction conditions for halogenation: 1 (5 mmol) and NCS
(5.5 mmol) in isopropyl alcohol (100 mL) under reflux; 1 (10 mmol) and
NBS (11 mmol) in chloroform (150 mL) at room temperature; 1
(1 mmol) and NIS (1.1 mmol) in acetic acid (20 mL) at room temper-
ature. [b] Yields of isolated products.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2011, 50, 12193 –12196
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12193