Anion control in the lonothermal synthesis of coordination polymers

Article abstract of DOI:10.1021/ja0737671

Two anionic 3D coordination frameworks (EMIm)₂[Co₃(TMA)₂(OAc)₂] 1 and (EMIm)[Co(TMA)] 2, one condensed hydroxide network [Co₅(OH)₂(OAc)₈](H₂O)_x 3, and one 2D layer compound (EMIm)[Co₂(TMA)₄H₇(22bpy)₂] 4 were prepared from three relevant ionic liquids of 1-ethyl 3-methyl imidazolium bromide (EMIm-Br), EMIm bistriflamide (EMIm-Tf2N), and mixtures of the two. The use of anions in ionic liquids plays a significant role in solvent polarity/hydrophibicity, which in turn controls the structures of materials formed in the ionothermal synthesis. The use of mixed anion ionic liquids opens up many possibilities to further tailor the properties of ionic liquids for ionothermal synthesis applications. The ionic liquid anions are not incorporated into the resultant materials. Copyright

Full text of DOI:10.1021/ja0737671

Published on Web 08/04/2007
Anion Control in the Ionothermal Synthesis of Coordination Polymers
Zhuojia Lin, David S. Wragg, John E. Warren, and Russell E. Morris*
School of Chemistry, UniVersity of St. Andrews, St Andrews, Fife, Scotland, U.K. KY16 9ST
Received May 25, 2007; E-mail: rem1@st-and.ac.uk
The most common method of controlling the structure of a
product in a solvothermal synthesis uses structure directing agents
(SDAs, sometimes also called templates). In the synthesis of porous
materials such as zeolites and some types of coordination polymers
these are often organic cations of varying sizes, which are
incorporated into the structure of the product. Templating is much
less common in the preparation of coordination polymer materials
but can still produce interesting solids. In this Communication we
report an alternative method of controlling the structure of a product
in a solvothermal reaction of coordination polymers. In this method
we use the anions present in ionic liquids to control the structure
of materials formed in ionothermal synthesis. The cations of the
ionic liquid, which in this study are always the same, are
incorporated into the resultant materials, but the anions are not. Of
particular interest is that mixed anion ionic liquids show different
product selectivity to single anion liquids, indicating the level of
structure direction available in this synthetic methodology.
Ionothermal synthesis has been used recently to produce zeo-
types,1,2 supramolecular architectures,3 and coordination polymers/
metal organic frameworks (MOFs).4,5 To date, much of the work
on ionothermal synthesis has concentrated on the use of 1-ethyl-
3-methyl imidazolium bromide (EMIm-Br) as the solvent. Ionic
liquids have been referred to as “designer solvents” as their
properties, including melting point, density, viscosity, and miscibil-
ity with water, can be easily altered by judicious choice of cations
and anions.6 The influence of the anion deserves particular attention;
for example, replacing the Br- in EMIm-Br with bis((trifluorom-
ethyl)sulfonyl)amide (Tf2N-) dramatically decreases the melting
point (from 81 to -3 °C) and the water solubility (from highly
water-soluble to 1.4 wt % water content saturated at 20 °C).7 EMIm-
Tf2N displays exceptional properties, combining high hydrophobic-
ity, low polarity, good thermal stability, low viscosity, and a very
low melting point and has been used in synthetic and electrochemi-
cal applications.7b Nevertheless, very few investigations have been
carried out on how the choice of anion in ILs affects the reaction
outcome in materials synthesis, although our recent illustration of
chiral induction by a chiral IL anion is one example.5
Figure 1. The structures of 1 (a) synthesized from EMIm-Br and Co2-
(COO)6 SBU and 2 (b) synthesized using mixed EMIm-Br/EMIm-Tf2N:
Co, purple octahedra (1) or tetrahedra (2); C, gray; O, red; H, white. The
EMIm+ cations in 1 are severely disordered. EMIm+ (blue line) outside
the unit cells are omitted for clarity.
When the reaction was carried out in a less polar, more
hydrophobic solvent, that is, using an EMIm-Br/EMIm-Tf2N (1:1
molar ratio) mixture a different 3D anionic framework 2 was
formed. The overall structure is built from dinuclear Co2(syn,anti-
COO)2(mono-COO)4 secondary building units (SBUs) where each
cobalt is tetrahedrally coordinated (Figure 1). Four SBUs are
arranged in rectangular grids with dimensions of ∼5 Å × 11 Å,
and which are further arranged in a herringbone packing mode in
the ab plane to form an infinite layer. Different layers are connected
into a 3D network through stacking in an ABAB sequence. The
staggered layers result in rectangular void space with small windows
of ∼4 Å × 4 Å along the c-axis. Each rectangular void encapsulates
two EMIm+ ions to compensate the negative-charge of the
framework.
Here we report four cobalt compounds (EMIm)2[Co3(TMA)2-
(OAc)2] 1, (EMIm)[Co(TMA)] 2, [Co5(OH)2(OAc)8](H2O)x 3, and
(EMIm)[Co2(TMA)4H7(22bpy)2] 4 (TMA ) trimesate and 22bpy
) 2, 2′-bipyridine).8,9 The controlling feature of the synthesis is
the IL anion chosen. Compound 1 is formed using EMIm-Br as
the solvent, 3 and 4 when EMIm-Tf2N is the solvent, and 2 when
a mixture of EMIm-Br and EMIm-Tf2N (1:1 molar ratio) is used.
Compound 1 was synthesized from EMIm-Br in a sealed
autoclave and is isostructural to the nickel analogue (EMIm)2[Ni3-
(TMA)2(OAc)2].4a The liquid solubilizes all the starting materials
very well, and the mechanism of formation appears be solution
mediated. The whole network can be described as a (3,6)-connected
net constructed by octahedral Co3(µ2-η1:η2-CH3COO)2(µ2-RCOO)4
(chelating-RCOO)2 secondary building units (SBUs) and topologi-
cally 3-connected TMA ligands.
The solvent polarity has great influence on the coordination
behaviors of the metal atoms, and the partial replacement of EMIm-
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J. AM. CHEM. SOC. 2007, 129, 10334-10335
10.1021/ja0737671 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
In summary we synthesized two anionic 3D coordination
frameworks and one 2D layer compound from three relevant ionic
liquids with various solvent polarity/hydrophibicity, controlled by
changing the anion in the IL. Hydrophilic ionic liquids containing
small amounts of water tend to facilitate the production of
framework compounds, while the hydrophobic nature of the EMIm-
Tf2N solvent results in poor solubilization of metal ions and organic
ligands, which in turn inhibits the formation of polymeric structures.
We have also shown that adding a solubilizing agent such as 2,2′-
bipyridine, can be used to circumvent some of the problems
associated with the use of hydrophobic ionic liquids.
Acknowledgment. We thank the EPSRC (U.K.) for funding.
Supporting Information Available: Crystallographic information
(CIF) for 1, 2, 3, and 4. This material is available free of charge via
the Internet at http://pubs.acs.org.
References
(1) (a) Cooper, E. R.; Andrews, C. D.; Wheatley, P. S.; Webb, P. B.; Wormald,
P.; Morris, R. E. Nature 2004, 430, 1012. (b) Parnham, E. R.; Morris, R.
E. Acc. Chem. Res., published online, June 21, 2007, http://dx.doi.org/
10.1021/ar700025k.
(2) (a) Parnham, E. R.; Wheatley, P. S.; Morris, R. E. Chem. Commun. 2006,
380. (b) Parnham, E. R.; Morris, R. E. J. Am. Chem. Soc. 2006, 128,
2204. (c) Wang, L.; Xu, Y.; Wei, Y.; Duan, J.; Chen, A.; Wang, B.; Ma,
H.; Tian, Z.; Lin, L. J. Am. Chem. Soc. 2006, 128, 7432.
Figure 2. Inorganic framework of 3 (a) and supramolecular framework of
4 (b) from EMIm-Tf2N: Co, purple ball or polyhedron; C, gray; N, blue;
H, white. (EMIm)+ in 4 are omitted.
(3) Reichert W. M.; Holbrey, J. D.; Vigour K. B.; Morgan T. D.; Broker G.
A.; Rogers, R. D. Chem. Commun. 2006, 4767.
Br with EMIm-Tf2N leads to the change of coordination geometries
of cobalt from octahedral in 1 to tetrahedral in 2. Further reducing
the solvent polarity results in poor solubilization of the starting
materials, and only a mixture of trimesic acid and an inorganic
compound [Co5(OH)2(OAc)8](H2O)x 3 was obtained from EMIm-
Tf2N. In the structure of 3, three crystallographically distinct cobalt
atoms in octahedral geometry are linked by one µ3-OH and four
different acetates (one η1:η1, two η1:η2 and one η2:η2) into a 3D
condensed hydroxide network with a nearly hexagonal motif when
viewed along the b-axis (Figure 2). Compound 3 can be prepared
by solvothermal treatment of cobalt(III) acetylacetonate in a THF
solution containing 4 vol % H2O.11 This accords with the solva-
tochromatic studies that ILs have polarities similar to those of short-
chain alcohol and other polar solvents.7a
(4) (a) Lin, Z.; Wragg, D. S.; Morris, R. E. Chem. Commun. 2006, 2021. (b)
Dybtsev, D. N.; Chun H.; Kim K. Chem. Commun. 2004, 1594. (c) Jin,
K.; Huang, X.; Pang, L.; Li, J.; Appel, A.; Wherland, S. Chem. Commun.
2002, 2872.
(5) Lin, Z.; Slawin, A. M. Z.; Morris, R. E. J. Am. Chem. Soc. 2007, 129,
4880.
(6) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772.
(7) (a) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH:
Weinhein, Germany, 2003. (b) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou,
N.; Kalyanasundaram, K.; Gra¨tzel, M. Inorg. Chem. 1996, 35, 1168.
(8) Synthesis: for 1 Co(OAc)2‚4H2O (380 mg, 1.5 mmol, Fisons), Trimesic
acid (TMA-H3, 210 mg, 1 mmol, Avocado) and ∼10 mmol EMIm-Br
were sealed and heated in a 23 mL Teflon-lined stainless steel autoclave
at 150 °C for 3 days. Dark-purple block crystals of 1 were collected with
75% yield. The similar method was followed for other compounds. For 2
Co(OAc)2‚4H2O (1.5 mmol), TMA-H3 (1 mmol) and ∼5 mmol EMIm-
Br and ∼5 mmol EMIm-Tf2N were mixed and heated at 150 °C for 3
days. Purple plate crystals were collected with 65% yield. Some samples
of 2 contain a small amount of Li+ impurity that is present in the EMIM-
Tf2N starting material. For 3 Co(OAc)2‚4H2O (1.5 mmol), TMA-H3 (1
mmol) and ∼10 mmol EMIm-Tf2N were mixed and heated at 110 °C for
5 days. A mixture of orange bar crystals and white solid was collected
with a total 50% yield. For 4 Co(OAc)2‚4H2O (1.5 mmol), TMA-H3 (1
mmol), 22bpy (2 mmol) and ∼10 mmol EMIm-Tf2N were mixed and
heated at 150 °C for 3 days. Orange plate crystals were collected with
total 60% yield.
(9) Crystal data for 1: T ) 90(2) K, λ ) 0.67130 Å, orthorhombic, Pbca, a
) 14.2616(4) Å, b ) 16.1525(5) Å, c ) 16.4867(5) Å, V ) 3797.9(2)
Å3, Z ) 4, Dcalcd ) 1.629 Mg/m3, µ ) 1.373 mm-1, 2θ ) 2.98 to 26.99°,
data/restraints/parameters ) 4883/0/251, GOF ) 1.098, R1 ) 0.0454, wR2-
(all) ) 0.1328. Crystal data for 2: T ) 113(2) K, λ ) 0.71073 Å,
orthorhombic, Pbca, a ) 15.425(2) Å, b ) 12.184(2) Å, c ) 16.059(2)
Å, V ) 3018.1(7) Å3, Z ) 8, Dc ) 1.558 Mg/m3, µ ) 0.800 mm-1, 2θ
) 2.48 to 26.00°, data/restraints/parameters ) 2828/0/235, GOF ) 1.198,
R1 ) 0.0882, wR2(all) ) 0.2199. Crystal data for 3: T ) 113(2) K, λ )
0.71073 Å, tetragonal, I4(1)/a, a ) 23.705(3) Å, c ) 11.409(2) Å, V )
6410.6(18) Å3, Z ) 8, Dc ) 1.726 Mg/m3, µ ) 2.608 mm-1, 2θ ) 1.98
to 25.99°, data/restraints/parameters ) 3118/0/196, GOF ) 1.187, R1 )
0.0906, wR2(all) ) 0.2262. Crystal data for 4: T ) 113(2) K, λ ) 0.71073
Å, monoclinic, P2(1)/c, a ) 6.579(1) Å, b ) 16.775(3) Å, c ) 11.409(2)
Å, â ) 93.944(4)°, V ) 3002.0(8) Å3, Z ) 2, Dc ) 1.400 Mg/m3, µ )
0.636 mm-1, 2θ ) 1.93 to 26.00°, data/restraints/parameters ) 5684/0/
391, GOF ) 1.065, R1 ) 0.0767, wR2(all) ) 0.2014. The program
SQUEEZE was used to model the electron density in the pores from the
severely disordered EMIm+ cations in 1 and 4.
To help solubilize the starting materials in the EMIm-Tf2N ionic
liquid, 2,2′-bipyridine was added as a co-ligand, yielding (EMIm)-
[Co2(TMA)4H7(22bpy)2], 4, as the major product. Compound 4 has
a 2D layer structure connected by very strong hydrogen bonding
into a 3D supramolecular framework. While in 1 and 2 the TMA
units bind in several different modes to the cobalt, the TMAs in 4
connect two cobalt atoms only via monodentate linkages. As two
of the coordination sites are blocked by 2,2′-bipyridine, the metal
centers are linked in a seesaw shape into an infinite fluctuated (4,4)-
square net. Four carboxylate groups around the metal center form
1
two very short R1 (8) hydrogen bonds (O‚‚‚H‚‚‚O ) 2.435(5) and
2.489(5) Å), which helps cobalt to retain the octahedral coordination
geometry. Adjacent layers are held together by hydrogen bonds
among the uncoordinated carboxylate groups (O‚‚‚H‚‚‚O ) 2.429-
(8) and 2.651(5) Å). The EMIm+ cations are highly disordered and
situated between layers.
A well-known function of the anion is to control the amount of
water present in the IL. EMIm-Br is hygroscopic and contains a
significant amount of water even after a moderate drying process.11
Although this may be a problem for some applications, the trace
of water can act as a mineralizer and is essential for the cry-
stallization of the coordination polymers in these reactions.
(10) Kuhlman, R.; Schimek, G. L.; Kolis, J. W. Inorg. Chem. 1999, 38, 194.
(11) (a) Cammarata, L.; Kazarian, S. G.; Slater, P. A.; Welton, T. Phys. Chem.
Chem. Phys. 2001, 3, 5192.
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