Simple and quantitative mechanochemical preparation of a porous crystalline material based on a 1D coordination network for uptake of small molecules

Article abstract of DOI:10.1002/anie.200502597

Kneading them in: A versatile porous material based on the 1D coordination network [CuCl₂(dace)]∞ (dace = trans-1,4-diaminocyclohexane) can be inexpensively prepared by a mechanochemical reaction followed by mild thermal treatment. The system is able to reversibly absorb molecules from solution or by simple kneading, while guest desorption from the product invariably leads back to the unsolvated form. (Figure Presented).

Full text of DOI:10.1002/anie.200502597

Communications
Host–Guest Systems
DOI: 10.1002/anie.200502597
Simple and Quantitative Mechanochemical
Preparation of a Porous Crystalline Material
Based on a 1D Coordination Network for Uptake
of Small Molecules**
Dario Braga,* Marco Curzi, Anna Johansson,
Marco Polito, Katia Rubini, and Fabrizia Grepioni*
Many research groups worldwide are engaged in the quest for
nanoporous solids that are able to absorb and release small
[*] Prof. D. Braga, M. Curzi, Dr. A. Johansson, Dr. M. Polito, K. Rubini,
Prof. F. Grepioni
Department of Chemistry, G. Ciamician
Via F. Selmi 2, 40126 Bologna (Italy)
Fax: (+39)051-209-9456
E-mail: dario.braga@unibo.it
fabrizia.grepioni@unibo.it
[**] This work was supported by MIUR (PRIN2004, FIRB2001), the
University of Bologna, and the Bertil Lundqvist Foundation.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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molecules in a controllable and selective fashion.[1] In many
cases, coordination networks have been shown to allow
formation of structures with channels and cavities (zeotypes),
which can often be emptied and refilled without disruption of
the crystalline edifice.[2] The preparation of zeotype com-
pounds by hydrothermal methods or by reactions in solution,
followed by crystallization, is often difficult because of the
tendency of coordination networks to form highly insoluble
materials. Recently we have begun to explore the use of solid-
state, solvent-free methods for the preparation of molecular
crystals[3] and more recently of coordination network com-
pounds.[4,5]
It has long been known that co-grinding and co-milling of
solid reactants are viable routes to molecular compounds or
molecular crystals.[6] Early work dates back to the pioneering
investigations of Etter and co-workers,[7] Rastogi and co-
workers,[8] and Curtin, Paul, and co-workers.[9] Recently,
mechanochemical methods have begun to be successfully
applied also in the field of molecular crystal engineering[10] for
solvent-free preparation of supramolecular aggregates,[11]
cocrystals, and coordination networks.[12] Importantly, crystals
formed by co-grinding of crystals in the absence of liquid can
be different from those obtained from solutions or melts.[13]
Examples of the utilization of mechanochemical methods
in coordination chemistry are not numerous, but these
procedures are increasingly attractive because of environ-
mental and sustainability issues. For example, cis-platinum
complexes such as cis-(Ph3P)2PtCl2 and cis-(Ph3P)2PtCO3
have been prepared mechanochemically from solid reac-
tants,[14] and the supramolecular self assembly of a number of
two- or three-dimensional helicates has also been recently
reported.[15]
Herein we describe the simple mechanochemical prepa-
ration of an inexpensive and versatile porous material based
on the 1D coordination network [CuCl2(dace)]1 (1; dace =
trans-1,4-diaminocyclohexane), which is able to absorb and
release small molecules. Compound 1 can be obtained by mild
thermal treatment of the hydrated compound [CuCl2
(dace)]1·nH2O (3) (see below), which is prepared as a
polycrystalline material by manual kneading of solid CuCl2
and dace in the presence of a small quantity of water (see
Scheme 1). The system 1 reversibly absorbs small molecules
either from solution or by simple kneading, whereas guest
desorption invariably results in reversion to the unsolvated
form 1.
The structure of compound 1 is not known in crystallo-
graphic detail because the insolubility of the product does not
permit the growth of X-ray crystallographic-quality single
crystals. However, insight into the structure of 1 has been
obtained from the knowledge of the structure of the dimethyl
sulfoxide (DMSO) adduct [CuCl2(dace)(dmso)]1 (2), which
has been fully characterized by single-crystal X-ray crystallo-
graphic diffraction (see the Experimental Section). As in the
case of 3, compound 2 can be easily and quantitatively
prepared by kneading solid CuCl2 and dace with a few drops
of DMSO. Comparison of the X-ray powder diffractogram
measured on the kneaded product with that calculated on the
basis of the single crystal structure allowed unambiguous
formulation of the former product as 2.
Scheme 1. Preparation of compound 1 (middle) from its hydrated
precursor 3 (top), and its behavior as a porous material to reversibly
take up small guest molecules: in the example here (bottom) DMSO
is absorbed to yield compound 2.
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Communications
Figure 1 shows how compound 2 is formed out of 1D
coordination networks in which the CuCl2 units are bridged
by dace ligands located in chains. Parallel 1D CuCl2–dace
and the addition of a few drops of DMSO is sufficient to
revert 1 into 2. Compound 2 can also be obtained by mixing
stoichiometric amounts of CuCl2 and dace directly in DMSO
with the resultant formation of an insoluble purple micro-
crystalline powder. In all cases, the products were identified
unambiguously through comparison of X-ray powder diffrac-
tion patterns. From the structure of 2 and from the knowledge
of its thermal behavior, it is possible to infer that the structure
of 1 is based on a stacking sequence of layers as in 2, but
ꢀsqueezedꢁ at a shorter interlayer separation as a consequence
of DMSO removal (Scheme 2). When the guest molecules
enter between the layers, the spacing between the CuCl2–dace
chains is expanded and the layers are shifted back in position.
Figure 1. a) A schematic representation of the structure of 2. b) A
perspective view of the packing of 2, showing the layers formed by
parallel 1D networks of alternating CuCl2 and dace units. H atoms
omitted for clarity.
Scheme 2. Schematic representation of the stacking sequence of layers
in compounds containing water (3), DMSO (2), and a generic guest
molecule; in 1 a shorter interlayer separation is present, as a
consequence of the guest removal.
networks form layers, which host, in intercalation fashion, the
cocrystallized DMSO molecules (see view of the packing in
Figure 1b). The copper atoms in 2 adopt a square-planar
coordination and the cyclohexane rings of dace are in the
chair conformation. The earlier reported structure of cis-1,4-
diaminocyclohexane, PtCl4, and similar Pt compounds con-
tain dace in the boat conformation.[16] Recently, we reported
that solid-state and solution reactions between dace and silver
acetate yield three isomeric forms of the coordination net-
work [Ag(H2NC6H10NH2)+]1, all characterized by the pres-
ence of chevronlike Ag+···dace···Ag+···dace networks.[5]
The interest in compound 2 and its congeners stems not so
much from their structures, but from their behavior upon
thermal treatment. When polycrystalline 2, which can be
taken as a prototype of this family of compounds, is heated to
1308C in TGA and DSC experiments, conversion into
compound 1 is observed (Scheme 1). This was easily ascer-
tained through comparison of X-ray powder diffractograms
after and before DMSO absorption. The process is reversible,
Scheme 2 also shows that 1, produced by guest removal,
can in turn be used as a starting material in further uptake/
release reactions. Table 1 lists the behavior of compound 1
towards a series of small molecules and how the formation of
a host–guest compound depends on the preparation method,
that is, kneading, suspension in the liquid guest, and kneading
followed by suspension. This latter approach is the most
productive; when suspended in the desired liquid guest,
compound 1 only takes up relatively small molecules (DMSO,
methanol, acetone, etc.). In contrast, kneading results in the
uptake of other guest molecules as well, probably because the
kneading procedure breaks crystallites and reduces surface
clogging. Indeed, if polycrystalline 1 is first kneaded with a
small amount of the desired liquid and then left to stir in the
same liquid guest for 12 h, partial or complete filling of the
compound is observed, independent of the guest molecule.
Kneading is widely used in the pharmaceutical and food
industry and also in the preparation of host–guest compounds
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Chemie
A typical kneading experiment was followed by suspension in the
Table 1: Guest uptake by 1 as a function of the preparation route.
same solvent that was used for the kneading process: After kneading,
the powder mixture was left in suspension in the appropriate solvent
for 12 h. In all cases, treatment of the product under vacuum and at
1008C for 3 h quantitatively yields the solid reagent 1.
Three-phase crystallization of 2: In a sample tube, a saturated
solution of trans-1,4-diaminocyclohexane in DSMO (0.5 mL), meth-
anol/DMSO (1:1, 0.5 mL), and a saturated solution of CuCl2·2H2O in
methanol (0.5 mL) was layered in three phases. A light-purple
powder slowly formed between the DMSO layer and the methanol/
DMSO layer after 3 days and in the powder a few single crystals of 1
of X-ray crystallographic quality were recovered.
Guest
Kneading
Suspension[a]
Kneading followed
by suspension[a]
1-hexanol
1-pentanol
acetone
acetonitrile
cyclohexane
dichloromethane
dioxane
dimethylsulphoxide
dimethoxyethane
dimethylformamide
ethylacetate
ethanol
–
–
x
–
–
–
–
x
–
–
–
x
x
x
–
x
x
–
–
–
–
–
–
–
x
–
–
–
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
Thermogravimetric measurements: Thermogravimetric investi-
gations were carried out on dried samples by using a Perkin–Elmer
TGA-7. Heating was performed under a nitrogen flow (20 cm3 minꢀ1
)
by using a platinum crucible at a rate of 58Cminꢀ1 up to 4508C. The
weight of the samples was around 3 mg. Calorimetric measurements
were performed by using a Perkin–Elmer DSC-7 equipped with a
model PII intracooler. Temperature and enthalpy calibration were
performed by using high purity standards (n-decane, benzene and
indium). Heating was carried out at 58Cminꢀ1 in the temperature
range 45–1408C. Samples in the weight range from 5 to 10 mg were
analyzed in open aluminum pans.
glyme[b]
H2O
hexane
2-propanol
methanol
tert-butyl alcohol
tetrahydrofuran
TMEDA[c]
-
x
–
–
x
–
x
–
x
x
x
–
Crystal-structure determination: Data for 2 were collected at
room temperature on an Enraf–Nonius CAD4 diffractometer,
monochromator graphite. 2: C6H14Cl2CuN2 2(C2H6OS), Mr = 404.89,
light purple crystals, crystal dimensions: 0.2 ” 0.10 ” 0.10 mm3; tri-
toluene
–
[a] Suspension occurs in the liquid guest. [b] Glyme=ethylene glycol
dimethyl ether. [c] TMEDA=N,N,N’,N’-tetramethylethylenediamine
¯
clinic P1, a = 5.911(2), b = 8.515(2), c = 9.621(4) Š, a = 86.81(3), b =
78.09(4), g = 70.42(2)8, V= 446.4(3) Š3, Z = 1, 1calcd = 1.506 gcmꢀ3, R1
(wR2) 0.0410 (0.0985) for 1547 observed independent reflections;
MoKa radiation (l = 0.71073 Š), 2q range = 6.0–50.0. All non-hydro-
gen atoms were refined anisotropically. SHELXL97[18] was used for
structure solution and refinement on F2, and SCHAKAL[19] was used
for the molecular graphics. CCDC-279106 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. See Supporting Information
for crystallographic data in CIF format, X-ray powder patterns for
compounds 1–4, and TGA and DSC measurements for 2. Powder data
were collected on a Philips XꢁPert automated diffractometer with
CuKa radiation, graphite monochromator. The program Powder-
Cell 2.2 was used for calculation of X-ray crystallographic powder
patterns.[20]
of cyclodextrins.[17] All uptake/release processes were moni-
tored by powder diffraction (see Supporting Information).
In conclusion, we have found a simple way to prepare a
relatively inexpensive material that absorbs and releases a
variety of small molecules through an intercalation mecha-
nism. Similar behavior is shown by intercalation compounds,
such as metal calcogenides (TiS2, ZrS2, etc.) and metal oxides
(MO3, V2O5, MOPO4, etc.). However, to the best of our
knowledge, system 1 is the first example of a 1D coordination
network with reversible intercalation capacity. Furthermore,
guest uptake in 1 appears to be somewhat selective and
depends on the method used.
Received: July 25, 2005
Published online: November 21, 2005
Experimental Section
Keywords: host–guest systems · mechanochemistry ·
porous materials · solid-state reactions
All starting materials were purchased from Aldrich. Reagent grade
solvents and bidistilled water were used.
.
Kneading experiments; 1: trans-1,4-Diaminocyclohexane (1.14 g,
1.0 mol) and CuCl2·2H2O (1.70 g, 1.0 mol) was ground together in an
agate mortar to a fine powder, and water (0.5 mL) was then added.
The mixture was kneaded for 5–10 min, and the water was then
removed by thermal and vacuum treatment (1008C under vacuum for
5 h).2: See synthesis of 1, but DMSO was used instead of water.
3: In an agate mortar, 1 (0.2 g, 0.80 mmol) and water (0.5 mL)
were kneaded for 5 min and left to stand for 1 h before measuring the
powder diffractogram.
1·n(guest): see synthesis of 3, with the exception that all the
compounds listed in Table 1 were used instead of water.
Typical kneading experiment: See synthesis of 3, but different
solvents were used, and the product was left to stand for 1–6 h.
Synthesis of 2 in DMSO: In a round-bottomed flask CuCl2·2H2O
(1.70 g, 1 mol) was dissolved in DMSO (75 mL). A solution of trans-
1,4-diaminocyclohexane (1.14 g, 1 mol) in DMSO (100 mL) was
slowly added dropwise, and a blue-purple powder was formed in
92% (2.86 g) yield.
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