Reversible gas uptake by a nonporous crystalline solid involving multiple changes in covalent bonding

Article abstract of DOI:10.1021/ja075265t

Hydrogen chloride gas (HCl) is absorbed (and reversibly released) by a nonporous crystalline solid, [CuCl₂(3-Clpy)₂] (3-Clpy = 3-chloropyridine), under ambient conditions leading to conversion from the blue coordination compound to the yellow salt (3-ClPyH)₂[CuCl ₄]. These reactions require substantial motions within the crystalline solid including a change in the copper coordination environment from square planar to tetrahedral. This process also involves cleavage of the covalent bond of the gaseous molecules (H-Cl) and of coordination bonds of the molecular solid compound (Cu-N) and formation of N-H and Cu-Cl bonds. These reactions are not a single-crystal-to-single-crystal transformation; thus, the crystal structure determinations have been performed using X-ray powder diffraction. Importantly, we demonstrate that these reactions proceed in the absence of solvent or water vapor, ruling out the possibility of a water-assisted (microscopic recrystallization) mechanism, which is remarkable given all the structural changes needed for the process to take place. Gas-phase FTIR spectroscopy has permitted us to establish that this process is actually a solid-gas equilibrium, and time-resolved X-ray powder diffraction (both in situ and ex situ) has been used for the study of possible intermediates as well as the kinetics of the reaction.

Full text of DOI:10.1021/ja075265t

Published on Web 11/23/2007
Reversible Gas Uptake by a Nonporous Crystalline Solid
Involving Multiple Changes in Covalent Bonding
Guillermo M´ınguez Espallargas,† Michael Hippler,† Alastair J. Florence,‡
Philippe Fernandes,‡ Jacco van de Streek,§ Michela Brunelli,| William I. F. David,
Kenneth Shankland, and Lee Brammer*,†
Contribution from the Department of Chemistry, UniVersity of Sheffield, Sheffield S3 7HF,
United Kingdom, Strathclyde Institute of Pharmacy and Biomedical Sciences, UniVersity of
Strathclyde, 27 Taylor Street, Glasgow G4 0NR, Scotland, Institute for Inorganic and Analytical
Chemistry, Frankfurt UniVersity, 60438 Frankfurt am Main, Germany, European Synchrotron
Radiation Facility, 38042 Grenoble, France, and ISIS Facility, Rutherford Appleton Laboratory,
Chilton, Didcot, Oxon OX11 0QX, United Kingdom
Received July 14, 2007; E-mail: lee.brammer@sheffield.ac.uk
Abstract: Hydrogen chloride gas (HCl) is absorbed (and reversibly released) by a nonporous crystalline
solid, [CuCl2(3-Clpy)2] (3-Clpy ) 3-chloropyridine), under ambient conditions leading to conversion from
the blue coordination compound to the yellow salt (3-ClpyH)2[CuCl4]. These reactions require substantial
motions within the crystalline solid including a change in the copper coordination environment from square
planar to tetrahedral. This process also involves cleavage of the covalent bond of the gaseous molecules
(H-Cl) and of coordination bonds of the molecular solid compound (Cu-N) and formation of N-H and
Cu-Cl bonds. These reactions are not a single-crystal-to-single-crystal transformation; thus, the crystal
structure determinations have been performed using X-ray powder diffraction. Importantly, we demonstrate
that these reactions proceed in the absence of solvent or water vapor, ruling out the possibility of a water-
assisted (microscopic recrystallization) mechanism, which is remarkable given all the structural changes
needed for the process to take place. Gas-phase FTIR spectroscopy has permitted us to establish that
this process is actually a solid-gas equilibrium, and time-resolved X-ray powder diffraction (both in situ
and ex situ) has been used for the study of possible intermediates as well as the kinetics of the reaction.
Introduction
Of particular relevance to the present study are solid-gas
reactions involving crystalline metal-organic compounds.8
Although chemical reactions in molecular crystals that
proceed without destruction of crystallinity have been known
for many years, their study has been largely confined to crystals
of organic compounds. Such reactions are typically induced
photochemically or thermally1-3 and require motions on a length
scale of a few Ångstroms by neighboring molecules within the
crystals either in order that covalent bonds can be formed or
subsequent to bond breaking during the reaction. However,
many such reactions are irreversible. An illustrative example is
carbon-carbon bond formation in cycloaddition reactions
between alkenes.1,2,4,5 Examples of metal-organic compounds
that permit irreversible reactions such as polymerizations in the
solid state have been known for some time.6 Reactions of
organometallic compounds as solids (surfaces, amorphous, and
crystalline) have also been reviewed.7
Reactions of this type, defined rather broadly, can be divided
into three general classes: (i) sorption of gases by crystalline
porous materials where the gas molecules are incorporated in
the interior of the pores; (ii) reactions between nonporous
crystals and aqueous vapors of volatile acids and bases leading
to the incorporation of these molecules into hydrogen-bonded
networks; and (iii) absorption of gas molecules by nonporous
molecular crystals with formation of covalent bonds to the
substrate, where retention of crystallinity is unexpected and
accordingly extremely rare.
The sorption of gases by crystalline porous materials such
as metal-organic frameworks is well established9,10 and of
widespread interest for applications including gas storage,
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(8) Meijer, M. D.; Klein Gebbink, R. J. M.; van Koten, G. In PerspectiVes in
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A. J.; Thomas, K. M.; Bradshaw, D.; Rosseinsky, M. J. Science 2004, 306,
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Cheetham, A. K. J. Am. Chem. Soc. 2003, 125, 1309. (d) Lin, X.; Blake,
A. J.; Wilson, C.; Sun, X. Z.; Champness, N. R.; George, M. W.;
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† University of Sheffield.
‡ University of Strathclyde.
§ Frankfurt University.
| European Synchrotron Radiation Facility.
ISIS Facility.
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J. AM. CHEM. SOC. 2007, 129, 15606-15614
10.1021/ja075265t CCC: $37.00 © 2007 American Chemical Society

Reversible Gas Uptake
A R T I C L E S
chemical separations, and molecular sensing.9,11-13 Typically
physisorption results in gas molecules that are rather weakly
bound to the interior surfaces of the pores,14 but although
crystallinity of the solid is retained in some cases after sorption
or desorption,15 often this is not the case.16 Recent developments
have also shown that binding of gas molecules is feasible
through specific interactions, for example, hydrogen bonding
interactions17 or coordination bond formation,18,19 within pores.
Reactions between crystalline powders of the organometallic
zwitterion [Co(η5-C5H4CO2H)(η5-C5H4CO2)] and aqueous va-
pors of volatile acids and bases has been reported by Braga,
Grepioni, and co-workers.20-22 These reactions lead to the
formation of salts [Co(η5-C5H4CO2H)(η5-C5H4CO2H)]X‚nH2O
Very recent reports by us28 and by Orpen and co-workers29
have shown that microcrystalline samples of coordination
complexes with pyridine-derived ligands can undergo reaction
with hydrated vapors of HCl leading to formation of crystalline
hydrogen-bonded salts. These reactions require cleavage of the
M-N (M ) Cu,28 Co,29 Zn,29) and H-Cl bonds and formation
of N-H and M-Cl bonds, thereby inserting HCl into the M-N
coordination bond. However, although water molecules are not
included in the crystals of the product salts, given the presence
of excess water vapor in the reactions, a highly plausible
mechanism for these reactions has been thought to involve a
microscopic recrystallization front that migrates across the
crystals, analogous to that demonstrated for some anion
exchange reactions involving crystalline solids.30
(X ) Cl-, BF4- 21 CF3COO-,21 CHF2COO-,22 CH2ClCOO-22
, )
if exposed to acids or [Co(η5-C5H4CO2)2](HY)‚nH2O (Y )
NH3,20 NH2Me,20 NMe320) if exposed to bases (n g 0).
Formation and cleavage of hydrogen bonds as well as proto-
nation/deprotonation of the carboxylate/carboxyl groups is
required to accommodate the gas molecules in the solid.23 In
some cases, water molecules are also incorporated in the
products upon acid or base uptake (i.e., n > 0). These reactions
are all reversible upon thermal treatment of the resultant salts.
The third class of solid-gas reactions involves not only
absorption of gas molecules by nonporous molecular crystals
but also the resultant formation of metal-ligand covalent bonds.
Such reactions converting crystalline reactant into crystalline
product are rare. An early example reported by van Koten and
co-workers involves the reaction an organoplatinum complex
with SO2 gas.24 In this reversible reaction, the coordination
geometry at the platinum center is converted from square planar
to square pyramidal but nevertheless requires the formation of
only an axial Pt-S bond enabling SO2 to be bound upon its
uptake by these crystals. In two very recent publications,
reactions involving methanol coordination25 and pyridine co-
ordination26 have also been identified. In parallel with the present
paper, we have also reported reversible ethanol insertion into
and elimination from the Ag-O bond of a nonporous crystalline
coordination polymer.27
In this study, we substantially extend our earlier report and
are able to demonstrate that the molecular coordination com-
pound trans-[CuCl2(3-Clpy)2] 1 (3-Clpy ) 3-chloropyridine)
prepared as a microcrystalline powder can react directly with
gaseous HCl in the absence of water yielding the crystalline
salt (3-ClpyH)2[CuCl4] 2. This definitively rules out the pos-
sibility of a water-assisted (microscopic recrystallization) mech-
anism and requires instead a quite remarkable process that
involves transport of HCl through nonporous crystals,31 coupled
with reaction within these crystals that involves multiple changes
in covalent bonding and a major change in coordination
geometry at the metal center. This reaction has been examined
in detail using X-ray powder diffraction and gas-phase IR
spectroscopy, including establishing the operation of a solid-
gas equilibrium process and investigating the kinetics of the
reverse (HCl elimination) reaction.
Experimental Section
General. All reagents were purchased from Aldrich, Lancaster, or
Avocado and used as received. HCl gas was purchased from BOC
(grade N2.6, 99.6% HCl; H2O content <10 ppm).
Synthesis of trans-[CuCl2(3-Clpy)2] (1). CuCl2 (143.6 mg, 1.068
mmol) was dissolved in MeOH (ca. 5 mL) (solution A), resulting in a
green solution. 3-Chloropyridine (222.3 mg, 1.958 mmol) was dissolved
separately in MeOH (ca. 5 mL) (solution B) to give a colorless solution.
Solution A was added to solution B with the immediate formation of
a blue precipitate. The sample was placed in an oven for 5 days at 50
°C for drying.
Synthesis of (3-ClpyH)2[CuCl4] (2). About 200 mg of dry blue
crystalline powder 1 was placed in three vials in the presence of vapors
of concentrated aqueous HCl (32%) for 2 days.28 Completion of the
reaction was observed after 2 days (complete color change from blue
to yellow).
(11) Kesanli, B.; Lin, W. Coord. Chem. ReV. 2003, 246, 305.
(12) James, S. L. Chem. Soc. ReV. 2003, 32, 276.
(13) Halder, G. J.; Kepert, C. J.; Moubraki, B.; Murray, K. S.; Cashion, J. D.
Science 2002, 298, 1762.
(14) Rowsell, J. L. C.; Spencer, E. C.; Eckert, J.; Howard, J. A. K.; Yaghi, O.
M. Science 2005, 309, 1350.
(15) (a) Serre, C.; Millange, F.; Thouvenot, C.; Nogue´s, M.; Marsolier, G.; Louer,
D. Fere´y, G. J. Am. Chem. Soc. 2002, 124, 13519. (b) Kitaura, R.; Seki,
K.; Akiyama, G.; Kitagawa, S. Angew. Chem., Int. Ed. 2003, 42, 428.
(16) (a) Rosi, N.; Eddaoudi, M.; Kim, J.; O’Keefe, M.; Yaghi, O. M. Angew.
Chem., Int. Ed. 2002, 41, 284. (b) Sun, J.; Weng, L.; Zhou, Y.; Chen, J.;
Chen, Z.; Liu, Z.; Zhao, D. Angew. Chem., Int. Ed. 2002, 41, 4471.
(17) Matsuda, R.; Kitaura, R.; Kitagawa, S.; Kubota, Y.; Belosludov, R. V.;
Kobayashi, T. C.; Sakamoto, H.; Chiba, T.; Takata, M.; Kawazoe, Y.; Mita,
Y. Nature 2005, 436, 238.
Formation of 1 and 2 and phase purity were confirmed by X-ray
powder diffraction.
Preparation of Samples for ex situ Powder Diffraction Study of
the Release of HCl Gas by 2. About 20 mg of 2 was placed in each
of 10 open vials. Under such conditions, conversion to 1 via HCl release
is known to occur over a period of 2 days.28 The vials were sealed
(18) Beauvais, L. G.; Shores, M. P.; Long, J. R. J. Am. Chem. Soc. 2000, 122,
2763.
(19) Bradshaw, D.; Warren, J. E.; Rosseinsky, M. J. Science 2007, 315, 977.
(20) Braga, D.; Cojazzi, G.; Emiliani, D.; Maini, L.; Grepioni, F. Chem.
Commun. 2001, 2272.
(21) Braga, D.; Cojazzi, G.; Emiliani, D.; Maini, L.; Grepioni, F. Organometallics
2002, 21, 1315.
(22) Braga, D.; Maini, L.; Mazzotti, M.; Rubini, K.; Grepioni, F. CrystEngComm
2003, 5, 154.
(23) Related gas-solid acid base reactions are also established for organic
compounds, see: Paul, I. C; Curtin, D. Y. Acc. Chem. Res. 1973, 6, 217.
(24) Albrecht, M.; Lutz, M.; Spek, A. L.; van Koten, G. Nature 2000, 406,
970.
(25) Supriya, S.; Das, S. K. J. Am. Chem. Soc. 2007, 129, 3464.
(26) Lennartson, A.; Håkansson, M.; Jagner, S. New J. Chem. 2007, 31, 344.
(27) Libri, S.; Mahler, M.; M´ınguez Espallargas, G.; Singh, D. C. N. G.;
Soleimannejad, J.; Adams, H.; Burgard, M. D.; Rath, N. P.; Brunelli, M.;
Brammer, L. Angew. Chem., Int. Ed. 2007, 46, in press.
(28) M´ınguez Espallargas, G.; Brammer, L.; van de Streek, J.; Shankland, K.;
Florence, A. J.; Adams, H. J. Am. Chem. Soc. 2006, 128, 9584.
(29) Adams, C. J.; Colquhoun, H. M.; Crawford, P. C.; Lusi, M.; Orpen, A. G.
Angew. Chem., Int. Ed. 2007, 46, 1124.
(30) (a) Khlobystov, A. N.; Champness, N. R.; Roberts, C. J.; Tendler, S. J. B.;
Thomson, C.; Schro¨der. CrystEngComm 2002, 4, 426. (b) Thompson, C.;
Champness, N. R.; Khlobystov, A. N.; Roberts, C. J.; Schro¨der, M.; Tendler,
S. J. B.; Wilkinson, M. J. J. Microsc. (Oxford) 2004, 214, 261.
(31) (a) Examination by the program PLATON31b indicates that the crystal
structure of the molecular coordination compound [CuCl2(3-Clpy)2] 1
contains no solvent accessible voids and that 76.4% of space is filled (i.e.,
close packing is achieved). (b) Spek, A. L. J. Appl. Cryst. 2003, 36, 7.
9
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A R T I C L E S
M´ınguez Espallargas et al.
sequentially at different stages of this reaction (0, 3, 4, 6, 14, 16, 17,
19, 22, and 49 h), stopping the reaction at these points.
underwent 18 scans. Indexing of the pattern indicated that two phases
were present, namely compound 2 and a minor impurity phase, but no
traces of compound 1 were observed. Nevertheless, compound 1 was
included in the refinement to obtain the composition of the sample
more accurately. Atomic positions and displacement parameters were
refined for compound 2. A spherical harmonic correction of the
intensities for preferred orientation was also applied for phase 2 in the
final refinement. The ratio of 1:2 in the yellow material was determined
as 0:100 from the refinement, and the final value of Rwp obtained was
0.0963.
Uptake of Dry HCl by 1 (75% Conversion). Crystalline powder
1 (78.9 mg) (0.22 mmol) was placed in a gas cell of volume ca. 200
cm3 at room temperature and evacuated using a vacuum line connected
to a rotary vacuum pump (∼5 × 10-3 Torr) for 30 min prior to the
introduction of 0.69 mmol (pressure 65 Torr) of dry gaseous HCl. A
change of color of the powder from blue to green occurred within an
hour. After 24 h, the powder had become yellow and the sample was
removed.
Uptake of Dry HCl by 1 (100% Conversion). Crystalline powder
1 (10.0 mg) (0.03 mmol) was placed in a gas cell of volume ca. 625
cm3 at room temperature (ca. 295 K) and evacuated using a vacuum
line connected to a rotary vacuum pump (∼5 × 10-3 Torr) for
approximately 30 min prior to the introduction of dry gaseous HCl.
About 11 mmol of dry HCl gas was introduced into the gas cell
(pressure 335 Torr). A change of color of the powder from blue to
green occurred within minutes, but the sample was allowed to react
for 7 days, during which time it became yellow in color.
Ex situ Monitoring of HCl Release Reaction. For each of the 10
samples for which the conversion of 2 to 1 by HCl release was stopped
at different stages, X-ray powder diffraction data were collected in the
2θ range 0-40° at 100 K at beam line ID31 at ESRF. The sample
temperature was maintained at 100 K by the coaxial nitrogen gas flow
of an Oxford Cryosystems Cryostream. A continuous scan mode (20°
min-1 for 48 min) was employed in which each sample underwent 24
scans. The capillaries were translated after each scan to a sequence of
8 different positions to avoid degradation of the sample in the X-ray
beam, resulting in 3 scans being made at each translation position. The
data for each sample were rebinned in the range 0-40° to a step size
of 0.001°. Inspection of the patterns clearly indicated that complete
conversion from 2 to 1 had occurred in the sample for which the vial
had been sealed after 49 h. Structure solution and Rietveld refinements
were first conducted for the start and end points of the reaction (t ) 0
h and t ) 49 h, respectively). Two-phase Rietveld refinements were
then performed for each of the 10 samples using the refined structure
models obtained for the start and end points.
X-Ray Powder Diffraction. All of the polycrystalline samples were
lightly ground in an agate mortar and pestle and filled into 0.5 mm
glass capillaries (dry HCl experiment: 75% conversion), 0.7 mm glass
capillaries (dry HCl experiment: 100% conversion), or 0.7 mm
borosilicate capillaries (in situ and ex situ experiments) prior to being
mounted and aligned on Station 9.1 at the Synchrotron Radiation Source
(SRS), Daresbury Laboratory, UK, using λ ) 0.998622 Å (dry HCl
experiment: 75% conversion) or on beam line ID3132 at the European
Synchrotron Radiation Facility (ESRF), Grenoble, France using λ )
0.91912(1) Å (dry HCl experiment: 100% conversion) or λ ) 0.80103-
(2) Å (ex situ and in situ experiments). Two-phase Rietveld33 refine-
ments were undertaken using the program TOPAS34 using as the starting
point the crystal structures determined at room temperature for
compounds 1 and 228 or the structure solutions from the simulated
annealing (SA) global optimization procedure implemented in the
DASH computer program35,36 (see Supporting Information for details
of the structure solution). A small number of minor peaks not
attributable to 1 or 2, nor to ice formation, remain constant throughout
the series of patterns measured in both the in situ and ex situ
experiments, as is evident from the residual profile. These peaks are
attributed to the presence of a very small amount of an impurity phase.
Being constant throughout the reaction, these cannot be taken as an
indication of a reaction intermediate.
Product of Reaction of 1 with Dry HCl (75% Conversion). For
the yellow product of the exposure of 1 to gaseous dry HCl, X-ray
powder diffraction data were collected at SRS station 9.1. Measurements
were made at room temperature in the 2θ range 3-40° employing a
variable count time (VCT) scheme (3.0-20.0° ) 2 s; 20.0-30.0° ) 4
s; 30.0-35.0° ) 6 s; 35.0-40.0° ) 8 s). Indexing of the pattern
indicated that two phases were present, namely compounds 1 and 2,
with the latter being the majority phase. Atomic positions and
displacement parameters were refined. A spherical harmonic correction
of the intensities for preferred orientation was applied for both phases
in the final refinement. The ratio of 1:2 in the yellow material was
determined as 24.1:75.9 from the refinement, and the final value of
Rwp obtained was 0.0433.
In situ Monitoring of HCl Release Reaction. The conversion of 2
to 1 through HCl release was also followed in situ using the
polycrystalline sample of 2 contained in a sealed 0.7 mm borosilicate
capillary that was used for the ex situ powder pattern for t ) 0 h. The
end of the capillary was removed to permit HCl loss during the
subsequent reaction, and the capillary was immediately placed on the
diffractometer at beam line ID31, ESRF in the coaxial nitrogen gas
flow of an Oxford Cryosystems Cryostream already set at 370 K to
accelerate the reaction. A series of powder patterns were then measured
over the 2θ range 3-23° in continuous scanning mode at a scan rate
of 20° min-1. Successive measurements were made after translation of
the capillary to a sequence of 4 positions, a strategy adopted to minimize
radiation-induced degradation of the sample in the intense X-ray beam.
Each sequence of four measurements and the corresponding translations
of the capillary and resetting of the detector angles took 5 min. Eighteen
such sequences of measurements were made over a total of 90 min,
during which time complete conversion of 2 into 1 could be seen to
have taken place due the absence of peaks for 2 and the lack of further
growth of the peaks for 1. The translation position furthest from the
base of the capillary was later determined to place a much smaller
volume of sample in the beam than other capillary positions and so
was excluded from subsequent calculations. The data for each sequence
of measurements were summed and rebinned in the range 3-23° to a
step size of 0.001°. Two-phase Rietveld refinements were performed
for the start and end point of the reaction, using as starting models the
crystal structures determined at room temperature.28 The structures
established at 370 K for 1 and 2 from Rietveld refinements of the start
and end points of the reaction were then used for two-phase Rietveld
refinements from the patterns obtained at the remaining 16 time intervals
sampled during the reaction.
Product of Reaction of 1 with Dry HCl (100% Conversion). For
the yellow product of exposure of 1 to gaseous dry HCl, X-ray powder
diffraction data were collected at beam line ID31 at ESRF. Measure-
ments were made at room temperature in the 2θ range 3-40° employing
a continuous scan mode (20° min-1 for 36 min) in which the sample
Infrared Spectroscopy. Fourier transform infrared (FTIR) spectro-
scopic experiments were conducted using a double-walled 10 cm glass
IR absorption cell fitted with NaCl windows and a suspended sample
container. Temperature control was achieved via a flow of silicone oil
within the double-wall jacket of the cell. The temperature of the cell
was maintained within a range of 0.2 K (1 K at room temperature)
using a Haake DC30-K20 circulator bath. To obtain time-dependent
concentrations of HCl vapor, gas-phase IR spectra in the region of
(32) Fitch, A. N. Natl. Inst. Stand. Technol. 2004, 109, 133.
(33) Rietveld, H. M. J. Appl. Cryst. 1969, 2, 65.
(34) Coehlo, A. A. TOPAS-Academic; 2004. See http://members.optusnet.com.au/
∼alancoehlo.
(35) David, W. I. F.; Shankland, K.; Shankland, N. Chem. Commun. 1998, 931.
(36) David, W. I. F.; Shankland, K.; van de Streek, J.; Pidcock, E.; Motherwell,
S.; Cole, J. C. J. Appl. Cryst. 2006, 39, 910.
9
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Reversible Gas Uptake
A R T I C L E S
Figure 1. Reaction of crystalline 1 (blue) with dry gaseous HCl yields crystalline 2 (yellow). Shown after (a) 0 h, (b) 1 h, and (c) 24 h (alongside original
vial of unreacted 1).
Figure 2. Observed (blue) profile, profile calculated from two-phase fit (red) and difference plot [(Iobs - Icalcd)] (gray) for the two-phase Rietveld refinement
of the product of exposure for (a) 24 h and (b) 7 days of 1 to dry HCl gas (2θ range 3.0-40.0°; maximum resolution (a) 1.46 Å and (b) 1.34 Å).
2500-6500 cm-1 were acquired using a FTIR spectrometer (Perkin-
Elmer Paragon 1000, resolution 1 cm-1, no apodization). The spec-
trometer was operated in the single-beam mode, that is, sample and
background (empty cell) spectra were recorded separately. The area
under the HCl absorption spectrum was integrated from 2600 to 3100
cm-1 after background subtraction and baseline correction to obtain a
measure for the HCl concentration (which was previously calibrated,
see Supporting Information). The absorption cell was loaded with ca.
50 mg of polycrystalline compound 2 and then connected to a vacuum
line to remove all the H2O and other gases present. Sixteen scans were
accumulated over a period of 2 min for each IR spectrum to provide a
satisfactory signal-to-noise ratio. Additional scans to improve accuracy
further were not made since continuous extrusion of HCl was expected.
Scans were continued until no increase in the HCl signal was observed.
Having established that the reaction is a gas-solid equilibrium (Vide
infra), the equilibrium position was further studied as a function of
temperature. The study was performed using a cylindrical glass IR
absorption cell of length 10 cm fitted with NaCl windows and a
suspended sample container, but without a double-walled jacket, as
seen in Figure 4a (Vide infra). The cell was loaded with ca. 50 mg of
a fresh sample of polycrystalline compound 2 and the reaction was
allowed to reach equilibrium at room temperature over a period of 2
months (although only hours are needed). After this time, IR spectra
were acquired by accumulating 64 scans (total 10 min). The cell was
then placed in an oven at 40 °C for 24 h to permit equilibrium to be
reached at the elevated temperature, after which another spectrum was
acquired (accumulating only 16 scans to avoid significant cooling of
the unjacketed cell). The cell was raised to 50 °C for a period of 72 h
and subsequently to 60 °C for a period of 24 h. After each temperature
rise, a spectrum was obtained (again accumulating 16 scans). Finally,
the cell was then left at room temperature for 120 h to permit
re-establishment of equilibrium at the reduced temperature, and another
spectrum was collected.
Results and Discussion
HCl Gas Uptake by 1 in the Absence of Water. Blue
coordination compound 1 was placed in the sample holder of a
gas cell equipped with NaCl windows and a valve suitable for
introduction and removal of gases, as shown in Figure 1a. To
eliminate the presence of water, the cell was evacuated using a
vacuum line (ca. 5 × 10-3 Torr) for ca. 30 min. Upon
introduction of dry gaseous HCl, a color change was visible
within minutes, and over a 24-hour period, the crystalline solid
changed color from blue to yellow (Figure 1), indicating the
formation of tetrahedral [CuCl4]2- anions.
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M´ınguez Espallargas et al.
polymorph of 2 is formed as when 1 reacts with aqueous HCl
vapor.28 This process involves multiple changes in covalent
bonding. Specifically, HCl molecules are inserted into all Cu-N
bonds requiring rupture of the covalent bond of the gaseous
molecules (H-Cl) and of coordination bonds of molecular solid
compound (Cu-N) in order that N-H and Cu-Cl bonds can
be formed. The crystalline product, the salt (3-ClpyH)2[CuCl4]
2 (3-ClpyH ) 3-chloropyridinium), now contains four Cu-Cl
bonds in
a
distorted tetrahedral arrangement rather
than two Cu-Cl and two Cu-N bonds in the square planar
geometry of 1, consistent with the change in color. The resultant
ionic compound forms 1D chains where the 3-chloropyridinium
cations and [CuCl4]2- anions are linked via bifurcated N-
H‚‚‚Cl2Cu hydrogen bonds37 and intermolecular Cu-Cl‚‚‚Cl-C
halogen bonds,38 the latter being also present in the crystal
structure of 1, which forms 2D layers (Figure 3).
Figure 3. Crystal structures showing (a) 2D network formed in 1,
propagated via Cu-Cl‚‚‚Cl-C halogen bonds represented as dotted lines
and (b) 1D network formed by 2, propagated via N-H‚‚‚Cl2Cu hydrogen
bonds and Cu-Cl‚‚‚Cl-C halogen bonds, indicated as dotted lines. Copper
atoms and chloride ligands shown in red, carbon-bound chlorine atoms in
green, and all other atoms in blue.
Establishment of a Solid-Gas Equilibrium. The reverse
of the HCl uptake process, that is, the extrusion of HCl in the
absence of water to convert salt 2 into neutral coordination
compound 1, was monitored by time-resolved Fourier transform
infrared (FTIR) spectroscopy to establish the variation in
concentration of HCl gas with time. The possible formation of
a solid-gas equilibrium in this reaction had been previously
The crystallinity and identity of the yellow product as the
ionic material 2 was confirmed by X-ray powder diffraction.
Rietveld refinement of the final product shows that the reaction
had not reached completion in 24 h (75% conversion; see Figure
2a), although 100% conversion can be obtained by extending
the period of reaction with the HCl gas (Figure 2b). The same
Figure 4. (a) Gas cell used for the IR spectroscopy measurements containing a sample of 2; (b) variation with time of the IR absorbance for HCl, after 8
min (green), 2 h (blue), 7 h (red), and 144 h (black); (c) variation of HCl pressure with time at 35 °C (exponential fit); (d) variation with temperature of the
IR absorbance for HCl at equilibrium.
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Reversible Gas Uptake
A R T I C L E S
Figure 5. Samples from the ex situ reaction study showing the change in color over time as 1 (yellow) is converted to 2 (blue) through extrusion of gaseous
HCl (left) together with the corresponding synchrotron X-ray powder patterns (T ) 100 K).
Figure 6. Observed (blue) and calculated (red) profiles and difference plot [(Iobs - Icalcd)] (gray) of the two-phase Rietveld refinement after having stopped
the reaction of 2 f 1 after (a) 0 h; (b) 6 h; and (c) 49 h (2θ range 2.0-40.0°, maximum resolution of 1.17 Å). A full set of patterns is provided in Supporting
Information.
anticipated but not confirmed. Specifically, no change of color
a sealed vessel. However, release of HCl by 2 can be promoted
is observed over many months if crystalline 2 is placed inside
within a closed system by trapping the eliminated HCl gas with
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A R T I C L E S
M´ınguez Espallargas et al.
Figure 7. (Top:) Powder sample of 2 (yellow) prior to reaction and as 1 (blue) after reaction. (Bottom) Powder patterns (T ) 370 K) of samples from in
situ study depicting the transformation of crystalline 2 (yellow pattern) to 1 (blue pattern).
an aqueous solution containing Ag+ ions (through formation
of insoluble AgCl).28
Synchrotron Radiation Facility in Grenoble, France. We rea-
soned that a plausible mechanism could involve stepwise loss
of the two equivalents of HCl, requiring the formation of a
transient intermediate phase resulting from the loss of one
molecule of HCl per formula unit of the ionic compound, as
has been suggested in studies of thermal treatment of a related
platinum system.40 The high intensity and narrow line width of
a third generation synchrotron source provides the optimum
chance of detecting such a phase even if present in very low
relative abundance. Both ex situ and in situ approaches to
monitoring the reaction have been undertaken.
Compound 2 (yellow salt) was placed in a gas cell under
vacuum (ca. 5 × 10-3 Torr), and the gas-phase IR spectrum
was measured at intervals over a period of 1 week (Figure 4).
The pressure of HCl increased rapidly with time at the outset
but reached a maximum pressure, indicating the establishment
of a gas-solid equilibrium (Figure 4c). At 35 °C, the pressure
reached ca. 2.5 Torr after 2 days39 and resulted in no perceptible
change in color of the solid; this corresponds to a 10.9(2)%
conversion of 2 to 1 at equilibrium for 50 mg of 2 used in a
cell of volume ca. 200 cm3. The equilibrium constant (K )
PHCl2) is calculated at 1.03(5) × 10-5 [∆GQm ) 29.5(1)
kJ.mol-1] and indicates good sensitivity of 1 to HCl gas in the
range 200-20 000 ppm. Futhermore, we have also shown that
the position of the equilibrium can be displaced by changing
the temperature (Figure 4d). Specifically, the equilibrium
pressure of HCl increases with increase in temperature, indicat-
ing that HCl extrusion is endothermic. Importantly, a reduction
in temperature returns the equilibrium pressure to its original
lower value, further confirming the ability of the blue coordina-
tion compound (1) to react directly with gaseous HCl in the
absence of water.
(a) Ex situ Monitoring of the HCl Release. A large sample
of 2 was prepared and divided into 9 vials. One was sealed
immediately to prevent conversion to 1 beyond the equilibrium
established at room temperature. Each of the others was sealed
in sequence at intervals of a few hours over a total period of 2
days during which full conversion to 1 occurs.28 Given that
equilibrium should be established upon sealing the vials
following irreversible loss of some HCl to the atmosphere, the
samples should represent a sequence of snapshots of the reaction
during its progress (Figure 5). In this way, the procedure is
analogous to the well-established protocol of sampling aliquots
of a solution-phase reaction and quenching the reaction prior
to (spectroscopic) analysis. X-ray powder patterns were mea-
sured at 100 K to a resolution of 1.17 Å for each sample in a
flame-sealed capillary (Figure 5). It can be observed that the
peaks corresponding to 2 diminish in intensity with time whereas
those corresponding to 1 increase. Rietveld analysis revealed
an excellent fit to a two-phase model for each pattern, indicating
the absence of a detectable intermediate crystalline phase in
the reaction (Figure 6). Furthermore, there is no evidence for
formation of an amorphous phase during the reaction. A small
Study of the Reaction Path and Reaction Kinetics. To
understand better the path of the reaction, we have followed
the extrusion of HCl from 2 by time-resolved synchrotron X-ray
powder diffraction on beam line ID3132 at the European
(37) Brammer, L.; Swearingen, J. K.; Bruton, E. A.; Sherwood, P. Proc. Natl.
Acad. Sci. U.S.A. 2002, 99, 4956.
(38) Zordan, F.; Brammer, L.; Sherwood, P. J. Am. Chem. Soc. 2005, 127, 5979.
(39) From the exponential curve fit to the data in Figure 4c, the pressure at
equilibrium is 2.41(4) Torr [321(5) Pa]. This compares with a corresponding
pressure of 1.37(4) Torr [183(5) Pa] for the measurements made at 25 °C.
(Full details in Supporting Information.)
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Reversible Gas Uptake
A R T I C L E S
Figure 8. Observed (blue) and calculated (red) profiles and difference plot [(Iobs - Icalcd)] (gray) of the two-phase Rietveld refinement after (a) 2.5 min; (b)
22.5 min; and (c) 87.5 min of the reaction of 2 f 1 at 370 K (2θ range 2.0-23.0°, maximum resolution 2.01 Å). A full set of patterns is provided in
Supporting Information.
number of minor peaks not attributable to 1 or 2, nor to ice
formation, remain constant throughout the series of patterns
measured, as is evident from the residual profile. These peaks
are attributed to the presence of a very small amount of an
impurity phase. Being constant throughout the reaction, these
cannot be taken as an indication of a reaction intermediate. It
is plausible that this impurity phase corresponds to a solvate of
the coordination compound, 1‚MeOH, since a similar compound
has been previously reported for the Pt analogous compound.38
However, it has not been possible to confirm this due to the
small number and low intensity of the peaks.
The decreasing fractional composition of 2 and increasing
composition of 1 follow an exponential (and inverse exponential)
progression analogous to that observed by IR spectroscopy for
the evolution of HCl gas (Figure 4c), consistent with a first-
order reaction and indicating complete conversion in a period
of ca. 17 h (see Supporting Information).
aforementioned diffraction studies. Thus, we undertook an in
situ X-ray powder diffraction study of the conversion of 2 to 1,
again employing synchrotron radiation. An open capillary
containing a microcrystalline sample of 2 was placed on the
diffractometer under a flow of N2 gas at an elevated temperature
(370 K) to accelerate the reaction.
Powder patterns were measured continuously within 5 min
intervals to a resolution of 2.01 Å over a period of 90 min
(Figure 7 shows the powder patterns measured every 10 min).
Again, Rietveld analysis provided a very good two-phase fit
for each pattern, and the same impurity peaks as those found
in the ex situ experiments were present, remaining constant
during the reaction (Figure 8). The changes in fractional
composition of 1 and 2 obtained from the Rietveld fit follow a
very similar (but smoother) trajectory to those obtained from
the ex situ studies and indicate complete conversion of 2 to 1
after ca. 70 min at 370 K for the sample used (Figure 9). Again,
no evidence for an intermediate phase is found. A logarithmic
plot of the change in composition versus time shows a linear
relationship (Figure 9b) permitting the first-order rate constant
to be determined as 1.1 × 10-3 s-1 for the sample studied.
(b) In situ Monitoring of the HCl Release. Although
compelling, one might argue that establishment of equilibrium
upon sealing each sample vial in the ex situ study could in
principle lead to removal of an intermediate crystalline or
amorphous phase and thereby prevent its observation in the
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M´ınguez Espallargas et al.
HCl molecules are trapped by insertion into metal-ligand
coordination bonds is not a microscopic recrystallization process
but a true gas-solid reaction. IR spectroscopic studies of the
reverse reaction, the release of HCl gas, have established that
this reversible process is actually an equilibrium process, whose
equilibrium position varies with temperature. In situ and ex situ
powder diffraction studies of the transformation resulting from
release of HCl gas do not indicate the presence of an intermedi-
ate of the reaction. It has also been established that the release
of HCl follows first-order reaction kinetics. The change in color
between blue (no HCl) and yellow (HCl included) suggests
potential exploitation in sensing of HCl, and it has been
established that the sensitivity of coordination compound 1 to
HCl gas is good in the range 200-20 000 ppm. This is currently
being explored alongside studies that establish the generality
of the reaction to other metal coordination compounds and other
hydrogen halides. In this regard, preliminary diffraction results
for reactions involving HBr suggest similar behavior to that
found for HCl.
More generally, these results describe unprecedented flex-
ibility within molecular crystals and indicate that the molecular
solid state can be far more dynamic than it is generally perceived
to be. The observations are pertinent to the development of
responsive molecular crystalline materials for sensing, gas
sorption, and catalysis and to efforts to develop solvent-free or
solvent-minimized reactions for green chemistry.41,42
Acknowledgment. Support from the Cambridge Crystal-
lographic Data Centre and the Centre for Molecular Structure
and Dynamics of the Science and Technology Funding Council,
UK is gratefully acknowledged. We thank the Synchrotron
Radiation Source, Daresbury, UK and the European Synchrotron
Radiation Facility, Grenoble, France for access to experimental
facilities. We are grateful to Dr. Sam Hawxwell (New York
University) for assistance with some of the studies conducted
at ESRF.
Figure 9. (a) Exponential variation with time of the percentage composition
of 1 and 2 in the reaction products obtained during the in situ monitoring
of HCl release. (b) Logarithmic plot to determine the first-order rate constant
for conversion of 2 to 1 ([2] ) mole fraction of 2).
Conclusions
The reversible uptake of HCl molecules from the gas phase
by a nonporous crystalline solid has been established to proceed
in the absence of any solvent and with retention of crystallinity,
despite the many chemical and geometrical changes needed for
this process to take place. Specifically, cleavage and formation
of coordination bonds (cleavage of H-Cl and Cu-N and
formation of Cu-Cl and N-H bonds) and hydrogen bonds (N-
H‚‚‚Cl2Cu), as well as a change in the geometry of the metal
center from square planar to distorted tetrahedral, are required.
The results provide clear evidence that this process in which
Supporting Information Available: Full details of experi-
mental work including additional figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA075265T
(40) Adams, C. J.; Crawford, P. C.; Orpen, A. G.; Podesta, T. J.; Salt, B. Chem.
Commun. 2005, 2457.
(41) Braga, D.; Grepioni, F. Angew. Chem., Int. Ed. 2004, 43, 4002.
(42) Lazuen Garay, A.; Pichon, A.; James, S. L. Chem. Soc. ReV. 2007, 36,
846.
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