Guest responsivity of a two-dimensional coordination polymer incorporating a cholesterol-based co-ligand

Article abstract of DOI:10.1039/c3dt51465j

To implement specific guest responsivity, a hydrophobic cholesterol-based co-ligand, cholest-5-en-3-yl-4-isonicotinate (Cholpy), was incorporated into a two-dimensional Hofmann-type Co(ii)Ni(ii) coordination polymer. The chemically programmed structure successfully demonstrated the unique guest response with remarkable chromatic changes.

Full text of DOI:10.1039/c3dt51465j

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Guest responsivity of a two-dimensional coordination
polymer incorporating a cholesterol-based co-ligand†
Cite this: Dalton Trans., 2013, 42, 15893
Received 3rd June 2013,
Accepted 3rd July 2013
Kazuki Kajitani,a Tomomi Koshiyama,a Akihiro Hori,b Ryo Ohtani,c Akio Mishima,a
Kohei Torikai,a Makoto Ebine,a Tohru Oishi,a Masaki Takata,b Susumu Kitagawab,c,d
and Masaaki Ohba*a,b
DOI: 10.1039/c3dt51465j
www.rsc.org/dalton
To implement specific guest responsivity, a hydrophobic chole- properties). To achieve such specific functions, a well-designed
sterol-based co-ligand, cholest-5-en-3-yl-4-isonicotinate (Cholpy), coordination programme of frameworks that closely links
was incorporated into a two-dimensional Hofmann-type Co(II)Ni(II) host–guest interactions with their properties is essential.
coordination polymer. The chemically programmed structure
Hofmann-type CPs {M(L)2[M′(CN)4]G} (M = Co, Mn, Fe, Ni
successfully demonstrated the unique guest response with etc.; M′II = Ni, Pd and Pt; L = axial co-ligand; G = guest mole-
remarkable chromatic changes.
cule) are some of the most attractive compounds to provide a
multifunctional platform.4 The Hofmann-type CPs have a two-
dimensional (2-D) layer structure extended by M–CN–M′
linkages of six coordinated octahedral MII ions and accommodate
guest molecules between the layers. Our group has investigated
The design strategy to produce functional coordination poly-
mers (CPs, or metal–organic frameworks (MOFs)), formed by
alternate arrays of mono- or polynuclear metal building units
and organic ligands as linkers, has made remarkable progress
in recent years.1 The structural diversity of CPs exceeds that of
the parental coordination compounds, and CPs can inherit
and merge the original characteristics of the constituent
coordination compounds depending on an appropriate mole-
cular design. CPs provide one promising platform to pro-
gramme the desired functions and properties. In the past
decade, CPs providing uniform pores within their structure
have been developed and are called porous coordination poly-
mers (PCPs), which have emerged as a new class of porous
materials that represent an evolved form of CPs.1 PCPs provide
a flexible and designable framework with highly regular pores.
It is expected that they will find applications as general use
porous solids2 (e.g., for gas storage, separation materials, ion
exchange and heterogeneous catalysts) and as new multifunc-
tional materials that combine porous properties with physical
properties3 (e.g., magnetic, proton conductive and optical
3-D Hofmann-type PCPs {Fe(pz)[M(CN)4]} (pz = pyrazine; MII
=
Ni, Pd and Pt).4c–j The compounds have pillared layer struc-
tures linking 2-D layers of {Fe[M(CN)4]}∞ through Fe–pz–Fe
bridges and show spin transition around room temperature.
The framework provides important interactive sites on the pz
pillar ligands and the coordinatively unsaturated MII centres.
The spin states of the FeII ion switch between the high-spin
(HS) and low-spin (LS) states, depending on the guest mole-
cule. By contrast, the 2-D Hofmann-type CPs {Fe(L)2[MII(CN)4]}
(L = pyridine, etc.) respond to no guest molecules because of
their closely packed interlayer structure, which results from
the stronger interaction between the axial co-ligands.4b,k To
use the 2-D structure for delivering specific guest responsivity,
we contrived to incorporate hydrophobic co-ligands between
the layers as additional guest-responsive sites. From the per-
spective of the hydrophobic area, we focused on an amphiphi-
lic cholesterol molecule having a steroid ring, an alkyl group
and a reactive hydrophilic hydroxyl group.5 We found that the
cholesterol derivatives could act as an axial co-ligand for the
2-D Hofmann-type structure (Scheme 1). Here, we report on a
new 2-D Hofmann-type CP, {Co(Cholpy)2[Ni(CN)4]} (Cholpy =
cholest-5-en-3-yl isonicotinate; 1), which incorporated a hydro-
phobic area, and its guest responsivity, which was associated
with chromatic changes.
aDepartment of Chemistry, Faculty of Science, Kyushu University, 6-10-1 Hakozaki,
Higashi-ku, Fukuoka, Japan. E-mail: ohba@chem.kyushu-univ.jp;
Fax: +81-92-642-2570; Tel: +81-92-642-2570
bQuantum Order Research Group, RIKEN SPring-8 Center, Koto 1-1-1, Sayo-cho,
Sayo, Hyogo, Japan
cDepartment of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto, Japan
dInstitute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida,
The cholesterol-based co-ligand Cholpy‡ was prepared
by modification of a method reported in the literature.6
The crystal structure of Cholpy is shown in Fig. S1.†
{Co(Cholpy)2[Ni(CN)4]H2O} (1·H2O)‡ was obtained as orange
Sakyo-ku, Kyoto, Japan
†Electronic supplementary information (ESI) available: Details of physical
measurements and additional experimental data. See DOI: 10.1039/c3dt51465j
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Scheme 1
microcrystals by the reaction of a tetrahydrofuran (THF) solu-
tion of Co(NO3)2·6H2O and Cholpy, and an aqueous MeOH
solution of K2[Ni(CN)4]·H2O.
Infrared (IR) spectra of 1·H2O and Cholpy are shown in
Fig. S2.† Compound 1·H2O showed typical bands attributed to
Scheme 2
Cholpy and
a strong single CuN stretching band at
2165 cm−1. The formation of Ni–CuN–Co bridges in 1·H2O
was confirmed by the upshift of the νCN band from that of
K2[Ni(CN)4] (2122 cm−1).
estimated by thermogravimetric analysis (Fig. S4†). Magnetic
measurements showed the CoII ion in the HS state at room
temperature in all clathrates.
Compound 1·H2O showed unique guest responses with
chromatic changes through somewhat complicated processes
(Scheme 2). The guest responsivity was detected by solid-state
reflectance spectra measured under guest vapour (Fig. S5†).
The initial state 1·H2O was changed to guest-free 1 by vacuum
drying at 120 °C for 12 h (process [1] in Scheme 2). During
dehydration, the sample colour changed to purple and new
adsorption bands appeared around 540 and 580 nm. In
process [2], the colour of guest-free 1 was returned to orange
(1·H2O) by rehydration. The colour of 1·H2O was changed to
yellow by absorption of MeOH (1·H2O·MeOH) with decreasing
intensity of an absorption band around 550 nm (process [3]).
Compound 1·H2O·MeOH was changed to 1 by desolvation
(process [4]) and was returned to the initial state 1·H2O by
rehydration (process [2]). The three-step colour change
between orange, purple and yellow was reversible through
guest adsorption and desorption processes [3] → [4] → [2]. By
contrast, the colour of 1·H2O was changed to green by adsorp-
tion of CHCl3 (1·H2O·CHCl3; process [5]), and new absorption
bands appeared around 600 and 650 nm. Vacuum drying
(process [6]) did not change the colour of 1·H2O·CHCl3 to
purple but to dark blue (1′) and produced a new absorption
band around 550 and 580 nm (process [7]). The desolvated 1′
was not returned to the initial state 1·H2O by rehydration
(process [8]) but changed to a light-yellow solid (1′·H2O). Com-
pound 1′·H2O showed a reflectance spectrum similar to that of
1·H2O·MeOH with additional weak bands around 650 and
680 nm. The three-step colour change between orange, green
and light yellow through processes [5] → [7] → [9] was irrevers-
ible, and the guest-free 1 exhibited different guest responsivi-
ties under a N2 atmosphere. In processes [10] and [12], 1
adsorbed MeOH (1·MeOH) and CHCl3 (1·CHCl3), respectively,
Microcrystals of 1·H2O showed a broad X-ray powder diffrac-
tion (XRPD) pattern indicating low crystallinity of 1·H2O
(Fig. S3†). The low-angle region XRPD pattern of 1·H2O
resembled the simulation pattern of {Fe(pep)2[Pt(CN)4]}
(2; pep = 4-(2-phenylethyl) pyridine).4k Compound 2 had an
interdigitated 3-D network, linking the 2-D Hofmann-type
layers by weak π–π interactions between the relatively large co-
ligands pep. Several peaks were indexed based on the structure
of 2, in which in-plane peaks upshifted because the atomic
radius of Ni is shorter than that of Pt, and because the out-of-
plane (interlayer) peaks downshifted. These results and the
single νCN band in the IR spectrum suggested that 1·H2O
formed a 2-D Hofmann-type framework with an elongated
interlayer structure by the large co-ligand Cholpy. The large
interlayer separation should explain the low crystallinity.
The monohydrate 1·H2O adsorbed various guest molecules
in the gas phase or in solution and retained the lattice water
molecule. A change in colour of 1·H2O accompanied the guest
adsorption (Table 1). Guest molecules were grouped into two
classes according to their effect on the sample colour. Most of
the solvents gave yellow products, as observed for MeOH. By
contrast, with CHCl3 and CH2Cl2 the colour changed to green.
Here, MeOH and CHCl3 were selected as representative guest
molecules. The compositions of guest clathrates 1·H2O·G were
Table 1 Summary of the guest responsivity of 1·H2O
Colour Guest molecules (G)
Yellow MeOH, EtOH, 1-PrOH, 2-PrOH, MeCN, MeNO2, Et2O, THF,
hexane, benzene, toluene and CS2
Green CHCl3 and CH2Cl2
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Scheme 3
1′·H2O, a new peak appeared at 1672 cm−1. The spectrum
changes corresponded well to the structural changes in Cholpy
as indicated by the Raman spectra.
To confirm the structural change in Cholpy, the co-ligand
was extracted from 1·H2O·CHCl3 (see ESI†). After an isolation
treatment of 1·H2O·CHCl3 with a solution of disodium ethylene-
diaminetetraacetate, a white precipitate was obtained from
the organic phase. IR spectra of the extracts showed that a
peak appeared at 1672 cm−1 as shown for 1·H2O·CHCl3, 1′ and
1′·H2O. 1H- and 13C-nuclear magnetic resonance (NMR) and
MALDI-TOF-MS spectra of the extracts revealed that Cholpy
was changed to Cholest-5-en-7-one-3-yl isonicotinate (calcd for
C33H48NO3 ([M + H]+) m/z 506.3, found 507.4) during the
absorption process of CHCl3 (Scheme 3, Fig. S8 and S9†),
which corroborated the irreversible changes observed in IR
and Raman spectra. The allylic oxidation of steroid is
explained by a radical reaction involving Cl˙ and ˙OH
species.7
These guest responses emphasize the following two points:
(a) the chromatic changes through processes [3], [5], [11] and
[13] were assisted by lattice water, and (b) the origin of the
chromatic change induced by CHCl3 uptake (process [5]) was
different from that induced by MeOH (process [3]). Such chro-
matic changes were not observed in the analogous 2-D
Hofmann-type compounds {Co(L)2[Ni(CN)4]} (L = pep, 4-styryl-
pyridine and pyridine derivatives), which highlights the effect
of the co-ligand Cholpy on the guest responsivity (Fig. S10†).
In addition, another analogue {Ni(Cholpy)2[Ni(CN)4]·nH2O}
(3·H2O) did not exhibit a notable colour change when exposed
to MeOH and CHCl3 vapour (Fig. S11†). These results suggest
that both CoII and Cholpy are essential factors for the specific
guest responsivity.
Fig. 1 Raman spectra of 1·H2O, Cholpy and the resulting compounds through
the guest adsorption and desorption processes.
after exposure to the guest vapour under N2, but without chan-
ging colour. Compounds 1·MeOH and 1·CHCl3 were converted
to 1·H2O·MeOH and 1·H2O·CHCl3 by hydration processes [11]
and [13], respectively. The responses of 1 through the pro-
cesses [10] → [11] and [12] → [13] suggest that H2O plays an
important role in guest responsivity.
Raman spectra of Cholpy, 1·H2O, 1, 1·H2O·MeOH, 1·CHCl3,
1·H2O·CHCl3, 1′ and 1′·H2O are shown in Fig. 1 and S6.† Com-
pared with the Raman spectrum of 1·H2O, the guest-free 1 pre-
pared by process [1] showed a new band that could be
attributed to the coordination bond around CoII at 246 cm−1
.
The band was also observed in 1·CHCl3 prepared by process
[12] under N2. The appearance of the new band suggested that
the symmetry around CoII was reduced with elongation of the
axial Co–N bond by dehydration and that the reduced sym-
metry was not recovered without the lattice H2O. The reduced
symmetry allows the d–d transition with lifting of the Laporte-
forbidden transition, which explained the strong absorption
band around 580 nm in the solid-state reflectance spectra of 1
and 1·CHCl3. Compound 1·H2O and Cholpy showed three
characteristic Raman bands that could be attributed to a
steroid ring around 610, 665 and 703 cm−1. In the case of
1·H2O·CHCl3 compared with 1·H2O, two of the bands around
610 and 703 cm−1 disappeared and a new band appeared at
1633 cm−1 but without symmetry changes around CoII. The
latter band was attributed to a structural change in Cholpy.
A similar spectrum pattern was also observed in desolvated 1′
(process [7]) and rehydrated 1′·H2O (process [9]), whereas
1·CHCl3 (process [12]) showed no change around 610, 703 and
Conclusions
The guest-responsive 2-D Hofmann-type coordination polymer
{Co(Cholpy)2[Ni(CN)4]H2O} (1·H2O) was prepared successfully
by incorporating a hydrophobic cholesterol-based co-ligand,
Cholest-5-en-3-yl isonicotinate (Cholpy). Compound 1·H2O
showed a unique guest response with remarkable chromatic
changes assisted by lattice water molecules. This article dis-
cussed the complicated guest responsivity of 1·H2O as shown
by solid-state reflectance, Raman, IR and NMR spectroscopic
studies. The colour changes were classified into three patterns:
(i) a reversible change between orange and purple induced by
1633 cm−1
.
IR spectra of 1·H2O and the resulting compounds (1,
1·H2O·MeOH, 1·H2O·CHCl3, 1·MeOH, 1·CHCl3, 1′ and 1′·H2O)
are shown in Fig. S7.† The spectrum patterns of 1, 1·MeOH,
1·H2O·MeOH and 1·CHCl3 showed no remarkable changes
compared with 1·H2O. In the cases of 1·H2O·CHCl3, 1′ and
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dehydration and hydration (processes [1] and [2]), (ii) a rever-
sible change between orange and yellow induced by adsorp-
tion–desorption of MeOH and various other solvents
(processes [3], [4] and [2]), and (iii) an irreversible change from
orange to green induced by adsorption of CHCl3 and CH2Cl2
(process [5]). The origin of the deep colour observed for 1, 1′,
1·MeOH and 1·CHCl3 was attributed to the allowed d–d tran-
sition under low-symmetric CoII. The chromatic change in (ii)
would stem from slight structural changes caused by an
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Acknowledgements
This work was supported by a Grant-in-Aid for Science
Research for Innovative Areas “Coordination Programming
(Area 2107, no. 22108512)” and a Grant-in-Aid for Scientific
Research Program (A) (no. 23245014) from MEXT, Japan. T. K.
thanks the MEXT for a Grant-in-Aid for Young Scientists (B)
(no. 30467279). R. O. is grateful to JSPS Research Fellowships
for Young Scientists. The synchrotron radiation experiments
were performed at BL44B2 in SPring-8 with the approval of
RIKEN (proposal no. 20120035). This work was performed
under the Cooperative Research Program of “Network Joint
Research Center for Materials and Devices” and the MEXT
Project of Integrated Research on Chemical Synthesis.
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Notes and references
‡Cholest-5-en-3-yl 4-isonicotinate (Cholpy): isonicotinic acid (1.23 g, 10 mmol)
and 1,1′-carbonyl-bis(1H-imidazole) (1.62 g, 10 mmol) were dissolved in dry THF
(15 mL) and stirred for 10 min. The THF solution was evaporated to dryness,
and dry benzene (20 mL) and cholesterol (3.86 g, 10 mmol) were added to the
solid. The mixture was refluxed for 4 h, then cooled to room temperature,
washed with brine twice and dried by MgSO4. After filtration, the filtrate was
evaporated to dryness to give a white solid dried in vacuo. Yield: 4.23 g (86%).
Elemental analysis (%) calcd for Cholpy (C33H49NO2): C, 80.60; H, 10.04; N 2.85.
Found: C, 80.66; H, 10.14; N, 2.69. IR (cm−1): 2953–2868(νCH), 1724(νCO).
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