Enantiopure vs. racemic metalloligands: Impact on metal-organic framework structure and synthesis

Article abstract of DOI:10.1039/b712118k

Carboxylate-decorated tris(dipyrrinato) cobalt(iii) complexes have been used to construct 2-D and 3-D metal-organic frameworks with infinite and trinuclear zinc secondary-building units. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712118k

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Enantiopure vs. racemic metalloligands: impact on metal–organic
framework structure and synthesis{
Sergio J. Garibay,a Jay R. Stork,a Zhenqiang Wang,a Seth M. Cohen*a and Shane G. Telferb
Received (in Austin, TX, USA) 9th August 2007, Accepted 3rd September 2007
First published as an Advance Article on the web 25th September 2007
DOI: 10.1039/b712118k
After screening a number of solvothermal conditions, dichroic
red-green crystals of MOF-Co/Zn-5a were obtained by the
following method: rac-[Co(4-cpdpm)3] (2.1 mg, 2.5 6 1026 mol)
and Zn(NO3)2?4H2O (6.5 mg, 2.5 6 1025 mol) were dissolved in
DEF–EtOH–H2O (1 : 0.25 : 0.25 mL) in a 4 mL screw top vial,
and the mixture was heated to 90 uC at a rate of 2.0 uC min21 for
Carboxylate-decorated tris(dipyrrinato) cobalt(III) complexes
have been used to construct 2-D and 3-D metal–organic frame-
works with infinite and trinuclear zinc secondary-building units.
Metal–organic frameworks (MOFs) constitute an exciting class of
materials for a wide variety of potential applications including gas
storage, heterogeneous catalysis, and separations.1–3 In an attempt
to introduce enhanced properties to MOFs, the use of metalloli-
gands, coordination compounds with peripheral metal-binding
functionalities, has been explored in order to add a second,
‘functional’ metal center into the lattice structure.4–9 Such metallo-
ligands offer the opportunity to enhance guest binding, introduce
catalytic sites, and confer useful opto-electronic properties to the
resulting MOFs. With these goals in mind, our laboratory has used
tris(dipyrrinato) metal complexes10,11 as metalloligands to success-
fully create a variety of heterobi- and heterotrimetallic MOFs.12,13
One of the most successful approaches to preparing MOFs with
high thermal stability and chemical robustness is the combination
of a polycarboxylate ligand with divalent and trivalent metal ions
such as copper(II), zinc(II), indium(III), and others under
solvothermal synthesis conditions.1,2,14,15 We sought to develop
dipyrrinato metalloligands that could mimic the MOF structures
obtained by these polycarboxylate ligands. Due to their range of
interesting topologies and high porosity, structures based on 3-fold
symmetric tricarboxylates were specifically targeted,16–18 Of
particular interest was MOF-177,16 which is comprised of 1,3,5-
benzenetribenzoate (BTB, Scheme 1) and Zn4O secondary-
building units (SBUs). MOF-177 has enormous pores capable of
absorbing guest molecules as large as Nile Red and C60. To this
end the metalloligand [Co(4-cpdpm)3] (4-cpdpm = 5-(4-carboxyl-
phenyl)-4,6-dipyrrinato) has been synthesized, characterized, and
resolved into its D and L enantiomers in an attempt to make chiral
analogues of structures such as MOF-177. Solvothermal reactions
with Zn(NO3)2?4H2O generated two novel MOFs, one of which
utilizes an infinite secondary-building unit (iSBU). Furthermore,
we show that the products of these solvothermal reactions depend
on whether racemic or enantiopure [Co(4-cpdpm)3] is utilized.
The synthesis of racemic [Co(4-cpdpm)3] was straightforward
and performed as shown in Scheme 1; spectroscopic data on this
compound were consistent with literature results.10,19
23 h, followed by cooling to room temperature at 0.2 uC min21
.
The resulting red-green, plate-like crystals were filtered, washed
with DEF (diethylformamide), and briefly dried in air.
The structure of MOF-Co/Zn-5a was determined by single-
crystal X-ray diffraction,{ and found to be a 3-D MOF with
iSBUs (Fig. S1).{ The asymmetric unit consists of a single [Co(4-
cpdpm)3] complex, a portion of an infinite, zigzag chain of zinc-
hydroxo clusters, a guest DEF solvent molecule, and a region that
appeared to contain partially occupied and severely disordered
solvent. The [Co(4-cpdpm)3] complex has an octahedral geometry
similar to other dipyrrinato complexes reported in the litera-
ture;10,19,20 however, the complex is slightly distorted such that the
disposition of the peripheral carboxylate ligands is of considerably
lower symmetry than idealized D3 symmetry would predict.
A portion of the zinc-hydroxo chain is shown in Fig. 1. This
iSBU is an extended cluster that can be considered as an
alternating arrangement of two types of corner-sharing
Zn3(CO2)3OH triangles (designated type I and II). The type I
triangles are close to equilateral and contain m3-hydroxy groups
…
with Zn Zn separations of 3.332, 3.380, and 3.547 A. Type II
˚
…
triangles are more nearly isosceles with one long Zn Zn
˚
separation of 5.250 A and two shorter distances of 3.497 and
˚
3.741 A. The type II triangles incorporate m2-hydroxy groups that
are statistically distributed between two edges of the triangles. The
two types of triangular units share an edge motif that involves one
edge that is bridged by two carboxylate moieties from [Co(4-
cpdpm)3], one edge comprised of a single bridging carboxylate
aDepartment of Chemistry and Biochemistry, University of California,
San Diego, La Jolla, California 92093-0358, USA.
E-mail: scohen@ucsd.edu; Fax: +1 858-822-5598; Tel: +1 858-822-5596
bMacDiarmid Institute for Advanced Materials and Nanotechnology,
Institute of Fundamental Sciences, Massey University, Private Bag 11
222, Palmerston North, New Zealand. E-mail: s.telfer@massey.ac.nz
{ Electronic supplementary information (ESI) available: Synthetic and
crystallographic details, Figures S1–S6. See DOI: 10.1039/b712118k
Scheme 1 Synthesis of racemic [Co(4-cpdpm)3]. In box: the 3-fold
symmetric ligand BTB that was used to prepare MOF-177.
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Chem. Commun., 2007, 4881–4883 | 4881

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Fig. 3 Structure of MOF-Co/Zn-5b (left). Trinuclear SBU in MOF-Co/
Zn-5b (right). Hydrogen atoms and solvent molecules have been omitted
(or partially omitted) for clarity. Cobalt (green), tetrahedral zinc (pink),
octahedral zinc (cyan), oxygen (red, highlighted only on the right).
Fig. 1 A portion of the extended zinc cluster in MOF-Co/Zn-5a. Detail
of the type I and type II triangles (upper right).
moiety, and one open edge that is not bridged by a carboxylate
group.
resulted in red block-like crystals of a new compound, MOF-Co/
Zn-5b. Racemic [Co(4-cpdpm)3] (2.1 mg, 2.5 6 1026 mol) and
Zn(NO3)2?4H2O (1.3 mg, 4.9 6 1026 mol) were dissolved in
DEF–EtOH (1 : 0.50 mL) in a 4 mL screw top vial. This mixture
was subjected to an identical gradient of heating to 90 uC at a rate
of 2.0 uC min21 for 23 h, followed by cooling back to room
Two zinc ions in the iSBU do not link adjacent triangles, Zn2
and Zn4 (Fig. 1), and bear one or two coordinated water
molecules to attain distorted square pyramidal or octahedral
coordination spheres, respectively. Each of the [Co(4-cpdpm)3]
metalloligands bridges three distinct iSBUs so that the chains are
arranged in a somewhat distorted hexagonal network (Fig. 2). The
rod-shaped chains present in MOF-Co/Zn-5a extend along the
crystallographic b-axis, and MOF-Co/Zn-5a thus represents an
example of a small family of MOFs that are derived from
iSBUs.5,14,21–24 Because of the complicated connectivity of iSBU-
based MOFs, as opposed to the simple node-and-spacer type
MOFs, different topological perspectives are necessary in order to
facilitate their structural illustration. Yaghi, O’Keeffe et al. have
recently enumerated 14 different ways in which rod-shaped iSBUs
can be arranged in three-dimensional structures.14 In this context,
MOF-Co/Zn-5a exemplifies, if only iSBUs are considered, the
prototypical hexagonal rod-packing motif (hex); however, if the
[Co(4-cpdpm)3] metalloligands are also considered and treated as
trigonal nodes, a new structural type, which is associated with the
(3,8)-connected net tfz-d (RCSR database, http://rcsr.anu.edu.au/)
can be formulated (Fig. 2).§ It should be noted that most existing
iSBU-based MOFs are constructed from ditopic ligands and
structures incorporating higher connectivity ligand systems remain
largely unexplored.14
temperature at 0.2 uC min21
.
The structure of MOF-Co/Zn-5b was determined by single-
crystal X-ray diffraction,{ which revealed a 2-D lattice (Fig. 3,
Fig. S2).{ The asymmetric unit consists of one molecule of [Co(4-
cpdpm)3], two disordered DEF molecules, and 1.5 Zn2+ ions, with
one located at an inversion center. A trinuclear Zn2+ SBU was
found, which is composed of a central zinc atom that is
octahedrally coordinated by six carboxyl groups (Fig. 3). This
central, 6-coordinate Zn2+ ion is flanked by two tetrahedral Zn2+
ions that are each connected to the central zinc atom by three
bridging carboxylate groups. Disordered DEF molecules bind the
apical sites of the flanking 4-coordinate Zn2+ ions. Each SBU is
connected by six carboxylate groups each from a distinct [Co(4-
cpdpm)3] metalloligand. This creates a hexagonal, pinwheel-like
geometry and an overall S6 symmetry at the SBU (Fig. 3, Fig. S3).{
The topology underlying MOF-Co/Zn-5b is related to the
hexagonal kgd (Kagome´ dual) net, which is a 2-D (3,6)-connected
lattice containing 4-membered rings. This topology is exceedingly
rare among observed MOF structures (RCSR database). It should
be noted that, structurally, MOF-Co/Zn-5b and MOF-Co/Zn-5a
are closely related to each other in that a schematic representation
of the 2-D MOF resembles the individual layers of the 3-D MOF
Upon obtaining MOF-Co/Zn-5a, additional reaction conditions
were explored, in a continuing effort to obtain more novel MOF
structures. One slight modification of the reaction conditions
Fig. 2 TopologyofMOF-Co/Zn-5a. OverallstructureofMOF-Co/Zn-5a(left);hydrogenatomsandsolventmoleculesomittedforclaritywithcobalt(III)and
zinc(II)centersshownasgreenandpinkpolyhedra,respectively.SchematicrepresentationsofMOF-Co/Zn-5a(centerandright)fromarod-packingperspective.
The resulting model is related to the tfz-d net. Green spheres represent cobalt(III) centers and magenta spheres/rods represent the zinc(II)-based iSBUs.
4882 | Chem. Commun., 2007, 4881–4883
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Notes and references
{ Crystal data for MOF-Co/Zn-5a: C53H41.5CoN7O9.5Zn2, M = 1118.10,
˚
monoclinic, space group P21/n, a = 15.229(4) A, b = 8.796(2) A, c =
˚
3
˚
˚
38.650(11) A, a = 90u, b = 89.977(5)u, c = 90u, V = 5177(2) A , T =
100(2) K, Z = 4, 43650 reflections measured, 10531 unique which were used
in all data, (Rint = 0.0841), R1 = 0.0787 (I . 2s(I)), GOF = 1.042.
Crystal data for MOF-Co/Zn-5b: C55.5H46CoN7.5O7.5Zn1.5, M =
¯
˚
˚
1094.98, triclinic, space group P1, a = 13.626(3) A, b = 14.766(3) A,
˚
c = 17.357(4) A, a = 81.062(4)u, b = 81.660(4)u, c = 75.534(4)u, V =
3
3319.8(12) A , T = 100(2) K, Z = 2, 36922 reflections measured, 13380
˚
unique which were used in all data, (Rint = 0.0800), R1 = 0.0605 (I . 2s(I)),
GOF = 0.906.
CCDC 656116–656117. For crystallographic data in CIF format see
DOI: 10.1039/b712118k
§ Alternative analysis of MOF-Co/Zn-5a topology. iSBU-based MOF
classification involves recognition of simple patterns (e.g., ladders, helices)
of coordinating carboxylate groups in the iSBUs. Although this approach
is not always applicable due to the complex nature of some iSBUs, those in
MOF-Co/Zn-5a can be simplified as 6-fold helices which in turn are cross-
linked by the metalloligands (Fig. S6).{ This gives rise to a new binodal
3-connected net that has not, to the best of our knowledge, been observed
in other MOFs but is recognized as net sqc946 (vertex symbols,
(6?6?122)(6?122?122); coordination sequences, 3, 6, 10, 18, 32, 52, 73, 98,
125, 156 and 3, 6, 11, 18, 31, 52, 73, 98, 125, 156) in the EPINET database
(http://epinet.anu.edu.au/).
Fig. 4 Simulated (red) and experimental (black) XRD pattern for MOF-
Co/Zn-5a. The pattern obtained from the reaction product using L-[Co(4-
cpdpm)3] is shown in blue, indicating an amorphous material.
when viewed along the b-axis (Fig. 2, right). The resemblance of
the MOFs is due to the similar connectivity found in the SBUs of
these MOFs.
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With both a 2-D and 3-D MOF in hand, we sought to generate
homochiral structures by exploiting the chirality of the octahedral,
tris(chelate) metal center of the [Co(4-cpdpm)3] metalloligand.
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cinchonidine using a published procedure.19 Enantiopurity of the
resolved metalloligand was confirmed by chiral HPLC.19
Subjecting enantiopure D-[Co(4-cpdpm)3] or L-[Co(4-cpdpm)3] to
identical reaction conditions as those used for MOF-Co/Zn-5a
or MOF-Co/Zn-5b produced amorphous, red precipitates.
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showed that they had no significant crystallinity (Fig. 4, Fig. S4).{
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was obtained by thermal gravimetric analysis (TGA). While both
MOF-Co/Zn-5a and MOF-Co/Zn-5b are stable up to y400 uC,
the amorphous solids obtained from L-[Co(4-cpdpm)3] showed
significant weight losses at temperatures y100 uC (Fig. S5).{
The latter finding is important in the context of using
metalloligands in the preparation of chiral MOFs and the capacity
of such systems to produce predictable structure types. When
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homochiral metalloligand; indeed similar metalloligands have been
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as differences in solubility of the enantiopure versus racemic
metalloligand, may prevent the direct substitution of one for the
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In summary, two new MOFs based on the metalloligand rac-
[Co(4-cpdpm)3] have been prepared and characterized. Both
MOFs contain unusual structural motifs, one involving an iSBU
and the other a trinuclear zinc(II) SBU. When preparing these
MOFs, we find that the products are obtained only when the
racemate is used. Under identical reaction conditions enantiopure
metalloligands lead only to amorphous solids. These findings are
significant within the context of designing and synthesizing new
MOFs and their homochiral analogues.
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Chem. Commun., 2007, 4881–4883 | 4883