In situ observation of a reversible single-crystal-to-single-crystal apical-ligand-exchange reaction in a hydrogen-bonded 2D coordination network

Article abstract of DOI:10.1002/anie.200462214

(Chemical Equation Presented) Aqua and nitrate forms of a Co^II center are reversibly interconverted within a square-grid complex because of flexible side chains attached to a rodlike ligand (see scheme; Co yellow, N blue, O red). The ligand exc

Full text of DOI:10.1002/anie.200462214

Angewandte
Chemie
Coordination Networks
In Situ Observation of a Reversible Single-
Crystal-to-Single-Crystal Apical-Ligand-
Exchange Reaction in a Hydrogen-Bonded 2D
Coordination Network
Kanji Takaoka, Masaki Kawano,* Masahide Tominaga,
and Makoto Fujita*
Functionalization of coordination networks is a crucial step
for their application.[1] One of the salient features of
coordination networks is that they often contain substitu-
tion-active metal sites in their framework. Ligand exchange
will considerably affect their d-orbital configuration and lead
to the electronic and spin control of coordination networks.
Cobalt is a classical example that shows chromism upon
ligand exchange in the solid state, as commonly observed in
[*] K. Takaoka, Prof. Dr. M. Kawano, Dr. M. Tominaga, Prof. Dr. M. Fujita
Department of Applied Chemistry
School of Engineering
The University of Tokyo
and Genesis Research Institute, Inc.
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax : (+81)3-5841-7257
E-mail: mkawano@appchem.t.u-tokyo.ac.jp
mfujita@appchem.t.u-tokyo.ac.jp
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 2151 –2154
DOI: 10.1002/anie.200462214
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2151

Communications
the hydration-induced color change of silica gel. Such a
flexibility inside the cavity by allowing guest molecules to
behave as in solution. As regards the stabilization effect, we
assume that the crystal is stabilized by infinite 1D hydrogen-
ligand-exchange process is, however, seldom observable by
single-crystal X-ray diffraction studies because the drastic
structural changes during the process readily cause loss of
crystallinity. Here we report the crystallographic observation
of reversible apical-ligand exchange at a cobalt center in situ.
The metal center is incorporated in a square-grid coordina-
tion network that provides a relatively fluid cavity within the
solid. We show that aqua and nitrate ligands can be reversibly
coordinated to the cobalt center in a single-crystal-to-single-
crystal fashion.[2–4]
We designed a 4-pyridyl-terminated, rodlike ligand con-
taining ethylene glycol side chains (1).[5] Slow complexation of
Co(NO3)2·6H2O in MeOH over a solution of 1 in toluene
produced orange-yellow single crystals of {[Co(1)2(H2O)2]-
(NO3)2·1.5H2O}n (2) in 76% yield [Eq. (1)].
À
bond (O H···O) arrays that interpenetrate the layers (Fig-
ure 1c).[6] In fact, a similar square-grid coordination network
that does not contain an ethylene glycol chain on the ligand is
much less stable than 2:[7] a crystal of the coordination
network with no substituent gradually deteriorated above
room temperature due to the loss of guest molecules.
The stability and flexibility of 2 enabled us to directly
observe a reversible apical-ligand-exchange reaction at the
hinge metals by X-ray crystallography. Two water molecules
are coordinated to the cobalt(ii) center at the apical positions
(aqua form) with four pyridyl groups at the equatorial
positions (Figure 2a). Upon heating at 1508C for 24 h the
X-ray crystallographic analysis revealed the square-grid
network structure of 2 (Figure 1a). The grid sheet layers are
stacked above each other in an offset fashion on both edges
(Figure 1b). The ethylene glycol chains play two important
roles: they add considerable stabilization to the crystal due to
hydrogen-bond formation and they also introduce moderate
Figure 2. Photographs and crystal views around the cobalt ion:
a) original crystal 2, b) the crystal after heating at 1508C for 24 h (3),
and c) the crystal after exposure of 3 to air (2’). Two other disordered
nitrate ions have been omitted for clarity.
yellow crystal turned red, with no loss of crystallinity. The
crystallographic analysis revealed that the apical water
molecules had been substituted by two nitrate ions and
extruded from the crystal to give the nitrate complex
[CoII(1)2(NO3)2]n (3; Figure 2b). Upon exposure of 3 to air
at room temperature, it changed back to the initial aqua form.
The color of the crystal returned to yellow within a few
minutes and the crystallinity was again retained. Crystallo-
graphic and elemental analysis showed that the newly
obtained aqua form {[Co(1)2(H2O)2](NO3)2·1.5H2O}n (2’) is
almost superimposable on the original structure of 2 (Fig-
ure 2c).
The chromism of the crystal is principally ascribed to the
energy-level change of the d orbitals of cobalt induced by
apical-ligand exchange. The nitrate anion, which has a weaker
crystal-field-splitting ability than water, decreases the
HOMO–LUMO energy gap in the complex, which causes a
red shift of the absorption bands. The energy-level change
also influences the geometrical parameters of the cobalt
center.[8]
The void present after removal of the water molecules is
mainly offset by framework distortion. The grid framework is
slightly compressed, which causes a decrease of cell volume
from 4263.9(7) to 4036.0(16) ꢀ3. Interestingly, the void is also
Figure 1. a) The square-grid network of 2. Guest molecules (H2O) have
been omitted for clarity. b) The two-layer stack of 2. c) The infinite 1D
À
hydrogen bond (O H···O) array in 2. The oxygen atoms of the OH
groups are represented by a space-filling model.
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Angewandte
Chemie
offset by dynamic disordering of the flexible ethylene glycol
chains. The mobility of the carbon and oxygen atoms in the
side chains significantly increases in the absence of guest
molecules, as shown by the ORTEP drawing of the square-
grid cavity in Figure 3.
Experimental Section
Single crystals of 2 were grown by layering a methanol solution of
Co(NO3)2·6H2O (5.8 mg in 1 mL) onto a toluene solution of 1
(15.0 mg in 2 mL). After allowing the solution to stand for 4 d, the
crystals were isolated in 76% yield by filtration. Elemental analysis
(%) calculated: C 50.53, H 4.98, N 8.84; found: C 50.34, H 4.86, N
8.72. Crystal data for 2: orthorhombic, Pbcn, a = 20.580(6), b =
23.717(7), c = 8.820(3) ꢀ, V= 4263.9(7) ꢀ3, Z = 4, T= 80 K, 1calcd
=
1.434 gcmÀ3, m = 0.483 mmÀ1, final R1 (I > 2s(I)) = 0.0762, wR2 (all
data) = 0.2062, GOF = 1.044.
Crystal data for 3: orthorhombic, Pbcn, a = 20.572(5), b =
23.092(5), c = 8.496(2) ꢀ, V= 4036.0(16) ꢀ3, Z = 4, T= 423 K,
1calcd = 1.441 gcmÀ3 m = 0.506 mmÀ1
, , final R1 (I > 2s(I)) = 0.1039,
wR2 (all data) = 0.2384, GOF = 1.039.
2’: Elemental analysis (%) calculated: C 50.53, H 4.98, N 8.84;
found: C 50.60, H 4.89, N 8.73. Crystal data for 2’: orthorhombic,
Pbcn, a = 20.576(2), b = 23.677(3), c = 8.7155(9) ꢀ, V= 4246.0(8) ꢀ3,
Z = 4, T= 80 K, 1calcd = 1.304 gcmÀ3, m = 0.466 mmÀ1, final R1 (I >
2s(I)) = 0.0948, wR2 (all data) = 0.2206, GOF = 1.025.
All the diffraction data were measured on a Siemens SMART/
CCD diffractometer (MoKa radiation, l = 0.71073 ꢀ). Non-hydrogen
atoms were refined anisotropically and hydrogen atoms were fixed at
calculated positions and refined using a riding model. CCDC-250540
(2), -250541 (3), and -250542 (2’) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
Received: October 6, 2004
Published online: February 25, 2005
Keywords: cobalt · coordination polymers · host–guest systems ·
.
hydrogen bonds · ligand design
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[2] Single-crystal-to-single-crystal guest inclusion/removal in coordi-
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[3] For the dynamic behavior of coordination networks in a single-
crystal-to-single-crystal fashion, see: a) K. Biradha, M. Fujita,
Angew. Chem. 2002, 114, 3542 – 3545; Angew. Chem. Int. Ed.
2002, 41, 3392 – 3395; b) K. Biradha, Y. Hongo, M. Fujita, Angew.
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coordination networks, see: a) B. Rather, M. J. Zaworotko, Chem.
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Figure 3. ORTEP drawing (30% probability) of a square-grid unit of 2
(a) and 3 (b). The bridging ligand of 3 shows considerable distortion
compared to that of 2. The disordered guest water molecules in (a)
have been omitted for clarity.
Thermogravimetric analysis of the crystal of 2 showed loss
of all water molecules in the range 25–908C. The number of
water molecules removed (weight loss of 6.47%) agrees well
with that determined by X-ray analysis (6.6%). DSC analysis
of a crystal of 2 showed two endothermic peaks at 93.88C and
77.48C, which are attributed to the release of coordinated
water from the cobalt ion and noncoordinated water from the
cavity, respectively.
In summary, we have succeeded in the crystallographic
observation of an apical-ligand-exchange reaction at a cobalt
ion within a square-grid coordination network. Since ligand
exchange at a metal is a bimolecular reaction, the process can
only rarely be observed by X-ray crystallography. We have
realized the in situ crystallographic observation of this
dynamic process, which involves drastic structural changes
around the reaction center, by utilizing functionalized bridg-
ing ligands. The in situ observation of various chemical
processes in the fluid cavity is our next challenge, and the
results will be reported in due course.
Angew. Chem. Int. Ed. 2005, 44, 2151 –2154
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Communications
2801; c) S. Takamizawa, E. Nakata, H. Yokoyama, K. Mochizuki,
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[5] For the synthesis of 1, see the Supporting Information.
[6] The hydrogen-bond length (average O···O distance is 2.60 ꢀ) is
particularly short despite the absence of charged species: B. G.
Hong, J. Y. Lee, C.-W. Lee, J. C. Kim, S. C. Bae, K. S. Kim, J. Am.
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[7] For the square-grid coordination network that does not contain an
ethylene glycol chain on the ligand, see the Supporting Informa-
tion.
À
[8] The change of the geometrical parameters of cobalt: Co OH O
2
À
À
2.095(4) ꢀ to Co ONO 2.129(8) ꢀ; Co NPy1 from 2.160(3) to
3
À
2.144(5) ꢀ; Co NPy2 from 2.142(3) to 2.129(5) ꢀ. These values
are typical of CoII–pyridyl aqua complexes: a) M. Felloni, A. J.
Blake, N. R. Champness, P. Hubberstey, C. Wilson, M. Schrꢂder,
J. Supramol. Chem. 2002, 163 – 174; b) S. Banfi, L. Carlucci, E.
Caruso, G. Ciani, D. Proserpio, J. Chem. Soc. Dalton Trans. 2002,
2714 – 2721.
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