An unprecedented CoII cuboctahedron as the secondary building unit in a Co-based metal-organic framework

Article abstract of DOI:10.1039/c3cc46396f

A cubic metal-organic framework with an unprecedented octanuclear secondary building unit (SBU) was isolated. The obtained SBU is composed of 8 Co ^II ions at each vertex, 6 μ₄-OH groups at each face, and 12 cpt^- ligands framing the metal core. The cuboctahedra arrange in a ubt framework topology, eliciting a highly symmetrical MOF structure. Magnetic measurements as well as DFT calculations on this crystalline MOF reveal intramolecular antiferromagnetic coupling between Co^II ions in the octanuclear SBU. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc46396f

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II
An unprecedented Co cuboctahedron as the
secondary building unit in a Co-based metal–organic
framework†
Cite this: Chem. Commun., 2014,
0, 5333
5
Received 21st August 2013,
Accepted 19th October 2013
Rebecca J. Holmberg, Marika Kay, Ilia Korobkov, Eugene Kadantsev, Peter G. Boyd,
Tomoko Aharen, Serge Desgreniers, Tom K. Woo and Muralee Murugesu*
DOI: 10.1039/c3cc46396f
www.rsc.org/chemcomm
A cubic metal–organic framework with an unprecedented octa-
nuclear secondary building unit (SBU) was isolated. The obtained
II
SBU is composed of 8 Co ions at each vertex, 6 l
4
-OH groups at
ꢀ
each face, and 12 cpt ligands framing the metal core. The cub-
octahedra arrange in a ubt framework topology, eliciting a highly
symmetrical MOF structure. Magnetic measurements as well as DFT
calculations on this crystalline MOF reveal intramolecular anti-
II
ferromagnetic coupling between Co ions in the octanuclear SBU.
Metal–organic frameworks (MOFs) continue to fascinate the
scientific community with their ubiquitous potential application
1
2
3
within: catalysis, gas storage, chemical sensing, separation
4
methods and drug delivery. Due to the inherent demand for
the development of such sought-after materials, new MOFs are
continuously synthesised and reported with a plethora of different
metal ions and organic linkers. Often rigid organic linkers are
utilized in synthetic tuning of the structural topology and pore
sizes to suit particular applications. However, even with attempted
structural tuning, metal secondary building units (SBUs) are often
5
serendipitously isolated with low metal nuclearities. Higher
nuclearity SBUs are rare, this is mainly due to the difficulty in
Scheme 1 Synthetic scheme highlighting the subtle difference in synthetic
controlling large cluster formation as well as the challenge of conditions, leading to a significant change in the SBU.
6
nucleating single crystals of suitable quality for X-ray diffraction.
We recently reported two unique cobalt-based MOFs: Co-MOF1
7
0
and Co-MOF2, through the use of an asymmetric ligand, 4-(4 - tweaking the reaction conditions to promote structural changes
carboxyphenyl)-1,2,4-triazole (Hcpt). We observed a 2D structure while retaining the same rigid Hcpt ligand and metal source.
II
as well as a distinct 3D structure, with the latter containing a
triangular SBU (Scheme 1, Co-MOF1) as often seen in well- [Co
established MIL series MOFs.
We present a novel cubic octanuclear Co MOF structure,
II
8
(OH)
6
(cpt)
6
][CoCl
4
]
2
ꢁ2H
2
O, hereafter termed Co-MOF3. This
With the aim of isolating MOFs compound was obtained through subtle modification of solvo-
5a,8
with novel, high nuclearity SBUs we have focused our attention on thermal synthetic methods, predicating the use of a Teflon bomb
as the reaction takes place at 150 1C. This is followed by a gradual
cooling procedure found to be essential for the isolation of X-ray
quality single crystals. The crystallization was further optimized by
varying the ratio of DMF, ethanol and water within the solvent
Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, ON,
Canada K1N 6N5. E-mail: m.murugesu@uottawa.ca;
Tel: +1 (613) 562 5800 ext. 2733
†
Electronic supplementary information (ESI) available: Crystallographic infor-
7
mixture, as was previously reported. The crystalline quality was
mation for Co-MOF3, experimental details, IR and Raman, TGA, SQUID magnetic
measurements, details of DFT calculations, and supplementary figures. CCDC
particularly dependent upon the amount of water present within the
solvent mixture, as well as a low pH environment to induce crystal
growth. This low pH mother liquor was achieved through the
9
44829. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3cc46396f
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Fig. 2 The octanuclear complex connected to twelve neighboring com-
plexes to form a cuboctahedron.
ꢀ
Each edge of the cubic SBU is capped by a disordered cpt
ligand, thus linking the core to 12 neighbouring cubic SBUs and
forming a cuboctahedron, as is shown in Fig. 2. The observed 3-D
network is reminiscent of a ubt framework topology, which is a
II
8
2ꢀ
secondary
Fig. 1 (a) Molecular core structure of [Co
(OH)
6
(cpt)12
]
building unit. (b) An isosurface plot of the spin density difference of the face-centered cubic (fcc) style packing of cuboctahedra within 3D
5
b,9
calculated S = 0 spin ground state.
space.
Thus, we can elucidate that this is a high surface area
9a,10
MOF structure with a highly symmetric arrangement.
The Co ions from each cube, and those which are arranged
addition of concentrated HCl, the chloride ions from which led to within the lattice framework, are well separated (Fig. 2), with
II
2ꢀ
the formation of [CoCl
4
]
counter ions. These molecules not only the nearest distances being: Co
8
–Co
8
(Co1–Co1) 12.275(1) Å,
–CoCl (Co3–Co4)
provide the necessary charges, but also serve to stabilize the lattice Co
8
–CoCl (Co1–Co3) 7.5169(7) Å, and CoCl
4
4
4
framework. Octahedron-shaped dark purple-blue crystals with tri- 11.6783(3) Å. Thus, these intermolecular distances preclude any
angular faces (Fig. S1 and S2, ESI†) were isolated and studied using intercluster interactions (vide infra).
single crystal X-ray.‡
Raman and Infrared spectroscopic analyses of Co-MOF3 serve as
The Co-MOF3 crystallizes with the formation of a MOF structure in fingerprints of the structure, confirming its stability upon filtration
11
the cubic space group, F23, with cell constants a, b, c = 23.3565(6) Å, and drying (Fig. S3 and S4, ESI†). In order to ascertain the bulk
and a, b, g = 901. The cubic system of the MOF structure elicits a sample purity X-ray Powder Diffraction (XRPD) experiments were
highly symmetrical framework. Further crystallographic informa- also performed, and were found to be in good agreement with the
tion can be found in Tables S1 and S2 (ESI†). Most notably within generated pattern from single crystal XRD (Fig. S5, ESI†). According
the molecular structure of the Co-MOF3, shown in Fig. 1, is the to thermo-gravimetric analysis (TGA) measurements (Fig. S6, ESI†)
II
core: an octanuclear Co cube. This high nuclearity constituent the Co-MOF3 structure is stable up to 200 1C. Significant weight
represents a new SBU in the rapidly expanding field of MOFs.
changes in the material were observed around 200 and 275 1C,
The unprecedented octanuclear cubane SBU consists of Co–Co where presumably the ligand is undergoing decomposition. Slight
bonds of 2.999(1) Å and CoꢁꢁꢁCoꢁꢁꢁCo corner angles of 90.00(6)1. losses in weight were also observed prior to 200 1C, which can be
4 2
The faces are capped by m -hydroxide groups, with Co–OH–Co attributed to solvent losses (2H O molecules, 6.4 wt%).
II
angles of 86.19(8)1. Each Co ion displays an octahedral coordina-
Direct current (dc) susceptibility measurements were performed
tion environment: one O atom (O2/O3) from the carboxylate group in order to elucidate the magnetic behaviour of the Co-MOF3,
ꢀ
of a cpt ligand, two N atoms (N2/N3) from the triazole rings of the specifically the Co
8
SBU. The Co
8
core was determined to be well
ꢀ
other cpt ligands, and three m
In the crystal lattice, two tetrahedral [CoCl
4
-O (O1) atoms of the cubane core. isolated according to the crystal structure, with the shortest non-core
2ꢀ
4
]
guest molecules, per Co–Co distance being (Co1–Co3) 7.5169(7) Å. Thus, any magnetic
II
cubane SBU, can be seen. They serve to stabilize the lattice, and interactions observed between Co ions most-likely originate from
prevent the presence of disordered solvents, while simultaneously within the octanuclear SBU. The temperature dependence of the dc
acting as counter anions to balance the overall charges. The magnetic susceptibility (1.8–300 K) was measured at 1000 Oe. Plots
structure contains voids which are accessible to solvent molecules, of the temperature dependence of w and w (Fig. S7 and S8, ESI†)
made evident through several partially occupied water molecules were fit in order to obtain the Weiss constant, y, as well as the
ꢀ
1
which were localized and modelled during refinement.
Curie constant, C, which were calculated to be ꢀ32.07 K and
5
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3
ꢀ1
3
ꢀ1
2
ꢀ1
8
.34 cm K mol , respectively. The negative value of y indicates the (0.18 cm g ), and surface area (1201 m g ) calculations were
presence of non-negligible antiferromagnetic interaction between also performed on Co-MOF3.
II
Co ions. The wT vs. T plot (Fig. S9, ESI†) suggests the presence of a
In summary, we have successfully synthesized a unique, highly
II
II
predominant antiferromagnetic interaction between Co ions, symmetrical cubic MOF structure with an octanuclear Co cubane
which is developed at higher temperatures than room temperature core as SBU. The cuboctahedral arrangement of the Co SBU elicits
8
since the plot does not show saturation of wT in the measured a ubt framework topology. This structure has been shown to be
range. The room temperature wT value decreases monotonically thermally stable up to 200 1C, and displays antiferromagnetic
3
ꢀ1
3
ꢀ1
8
from 14.76 cm K mol to 2.88 cm K mol at 1.8 K. This interactions within the Co core leading to an S = 0 ground state.
behaviour further indicates the presence of strong antiferro- This unprecedented high nuclearity SBU provides a highly symme-
II
magnetic interaction between Co ions even at higher temperature, trical building block on which up to 12 rigid pillars can link to
II
as the theoretical spin-only value for a single non-interacting Co
form highly symmetrical MOF structures. Subsequently, these
3
ꢀ1
4
ion is 1.88 cm K mol ( F9/2, S = 3/2, L = 3, g = 2), and is thus MOFs can be further altered through systematic spacer length
8.8 cm K mol per molecular formula. The decrease to a wT (ligand) tuning to create MOFs with large surface area. Overall, this
product of 2.88 cm K mol at 1.8 K indicates the presence of an new MOF with cuboctahedral arrangement has the potential to be
S = 0 spin ground state for the Co cluster, attributed to strong a customizable, highly functional material based on a unique SBU.
antiferromagnetic interactions, and 2 isolated Co counter ions
3
ꢀ1
1
3
ꢀ1
8
II
3
ꢀ1
(
3.76 cm K mol ). This type of antiferromagnetic coupling leading
Notes and references
to an S = 0 spin ground state was observed in a similar Ni
8
II
ꢀ1
12
II
‡ Data for [Co
8
(OH)
6
(cpt)
6
][CoCl
4
]
2
ꢁ2H
2
O (Co-MOF3), FW: 2146.06 g mol
:
molecular species, as well as in other cluster-based Co organic
3
cubic, a, b, c = 23.3565(6) Å, a, b, g = 90.00(6)1, V = 12 741.58(57) Å , s.u.mean
=
5c
frameworks.
0.000, s.u.max = 0.002, T = 200(2) K, F23, Z = 16, a total of 55 588 reflections
collected in the range 2.471 o y o 30.491, of which 3203 were unique, Rint
.0294, R = 0.0512 [I > 2s(I)], wR = 0.1182 (for all data), largest diff. peak
=
The isotherm magnetisation (M) measurements (Fig. S10, ESI†)
show no saturation, suggesting the presence of non-negligible
anisotropy within the system as well as population of excited states GOF = 1.046, F(000) = 4488.
even at 1.8 K. This behaviour is most likely arising from Co counter
0
1
2
ꢀ3
ꢀ1
and hole = 0.489 and ꢀ0.693 e Å . Absorption coefficient = 1.487 mm
,
II
1
2
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3
4
2ꢀ
magnetism is primarily due to two isolated [CoCl
4
]
counter ions.
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of the SBU in Co-MOF3, periodic density functional theory calcula-
tions were performed (the full details of which are provided in the
ESI†). For calculation purposes, a simulation cell was used that
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2
2
1
1
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2
(298 K) is almost negligible. The uptake
ꢀ
1
2
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2
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 5333--5335 | 5335