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Emerging Supramolecular Chemistry of Gases.

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Reviews
D. M. Rudkevich
Gas?Receptor Complexes
Emerging Supramolecular Chemistry of Gases
Dmitry M. Rudkevich*
Keywords:
host?guest systems и hydrogen bonds и
molecular recognition и
noncovalent interactions и
supramolecular chemistry
Angewandte
Chemie
558
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200300606
Angew. Chem. Int. Ed. 2004, 43, 558 ? 571
Angewandte
Chemie
Supramolecular Chemistry of Gases
Molecular recognition of gases is an emerging area of chemistry.
From the Contents
Supramolecular chemistry helps us to understand how gases interact
with biological molecules and offers delicate insights into the mechanisms of their physiological activity. Principles of molecular recognition have been used for gas sensing, and have provided fundamental
knowledge about the structure and dynamics of receptor?analyte
complexes, and novel materials for gas sensing and storage have been
developed. Supramolecular chemistry is also enabling us to learn how
to transform gases into synthetically useful reagents. The rational
design of novel catalysts for gas conversion and, more recently,
encapsulation complexes with gases open novel directions in preparative synthetic chemistry.
1. Introduction
Molecular chemistry is based on the covalent bond, while
weaker, noncovalent forces define the field of molecular
recognition and supramolecular chemistry. Accordingly,
supramolecular chemistry relies on intermolecular interactions and assembly of molecules.[1] Within the last three
decades revolutionary advances in supramolecular chemistry
have resulted in the successful complexation and sensing of
ions and neutral organic molecules.[2] Somewhat surprising,
however, is that the molecular recognition of gases is still at an
early stage. Gases comprise the atmosphere of the Earth and
also occupy a central position in biomedicine, science,
technology, and agriculture.
Oxygen (O2) is of great importance in such diverse areas
as medicine and steel making, while nitrogen (N2) is utilized in
space technology and in the production of ammonia (NH3).
Chemical, biomedical, and food industries widely use O2,
carbon dioxide (CO2), N2, NH3, chlorine (Cl2), and ethylene
(C2H4). CO2 and nitrous oxide (N2O) are major greenhouse
gases and huge amounts of CO2 are released into the air upon
the burning of fuels, such as oil, coal, wood, and natural gas.
As a result, CO2 is accumulating faster in the atmosphere than
the Earth's natural processes?plants and aqueous resources?can absorb it.[3] The excessive emission of N2O into the
atmosphere is caused by the widespread use of nitrogenous
fertilizers and the industrial manufacturing of nylon.[4] CO2
and N2O are also involved in a number of biochemical
processes and are called blood gases. N2O is heavily used in
anesthesia. Another crucial group of gases is NOx, which
comprises nitric oxide (NO), nitrogen dioxide (NO2), N2O3,
dinitrogen tetroxide (N2O4), and N2O5. NO has multiple roles
in the human organism:[5] It serves as an important messenger
in signal-transduction processes in smooth muscle cells and
neurons and it is also the key intermediate in global
denitrification. Other NOx gases are extremely toxic pollutants derived from fossil-fuel combustion, power plants, and
large-scale industrial processes. NOx gases participate in the
formation of ground-level ozone, global warming, and they
also form toxic chemicals, nitrate particles, and acid rain/
aerosols. NOx gases are aggressively involved in damaging
nitrosation processes in biological tissues.[6]The so-called SOx
Angew. Chem. Int. Ed. 2004, 43, 558 ? 571
1. Introduction
559
2. Lessons from Nature
560
3. Hydrogen Bonding of Gases
561
4. Lewis Acid?Base Interactions:
From Receptors to Materials
562
5. Encapsulation of Gases
565
6. Summary and Outlook
569
gases SO2 and SO3 as well as hydrogen sulfide (H2S)[7] also
contribute significantly to the production of smog and acid
rain.
Such an extensive circulation of gases necessitates the
development of novel methods for their sensing under a
variety of conditions. Another, still unresolved, issue is their
chemical fixation in environmentally benign processes. Surprisingly, the application of principles and techniques of
supramolecular chemistry and molecular recognition for
these purposes is only at the very beginning. Rules governing
reversible gas?receptor interactions, which are responsible for
binding selectivity and biochemical action, and usually
precede the covalent fixation, are still poorly understood.
The gases that surround us, however, are not ?ideal gases?
and they interact with other molecules, and thus their
recognition/complexation remains a great challenge. Furthermore, gases are technically more difficult to handle than
liquids and solids. In addition, their molecules are neutral, and
electrostatic binding interactions are much less effective than
they would be for similar-sized cations and anions, although a
number of gas molecules have dipole or quadruple moments.
Furthermore, gases are small molecules with a size of 2?4 >
and very close contacts with receptor sites are needed to
achieve their binding. The limited polarizability, dimensions,
and geometries of the gas molecules necessitates a high
degree of design in the receptor to achieve complementarity,
and often a combination of different binding forces is
necessary. Solvation may well compete with the recognition
event, and while some gases are soluble in water some are
lipophilic.
The supramolecular chemistry and molecular recognition
of gases definitely deserve attention. Here, an overview of the
general principles towards the design and synthesis of
receptor molecules for gases is given. The possibilities of
hydrogen bonding and Lewis acid?base interactions, dipole?
[*] Prof. Dr. D. M. Rudkevich
Department of Chemistry & Biochemistry
University of Texas at Arlington
Arlington, TX 76019-0065 (USA)
Fax: (+ 1) 817-272-3808
E-mail: rudkevich@uta.edu
DOI: 10.1002/anie.200300606
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
559
Reviews
D. M. Rudkevich
dipole attractions, as well as encapsulation
will be discussed and experimental progress
in this direction will be analyzed. Furthermore, existing approaches towards studying
supramolecular complexes with gases will be
reviewed. Published thermodynamic and
kinetic parameters of these interactions will
be presented, as well as the data concerning
the geometry and stereochemistry of the
complexes. This knowledge should allow for
the development of more specific/selective
receptors and molecular containers for gases,
potentially useful chemical processes and
catalysts for their utilization, novel gas
sensors, and chemically modified materials
for detection and purification. It will be
shown that the application of principles of
molecular recognition for gas sensing, storage, and chemistry may lead to conceptually
new discoveries and inventions.
2. Lessons from Nature
Nature employs molecular recognition
for effectively discriminating between blood
gases, with differences in the O2/CO binding
by heme molecules being the most spectacular example.[8] In addition to the iron?gas
interaction, the histidine residue on the distal
Scheme 1. Molecular recognition of gases in nature.
porphyrin face of hemoglobin and myoglobin is involved in hydrogen bonding with O2
(1, Scheme 1), as evident from EPR, X-ray,
and neutron-diffraction studies. Such hydrogen-bonding
There is also a hydrogen bond between the tyrosine and
interactions not only affects oxygen affinity, but may also
glutamine fragments. Such a hydrogen-bonding network is
stabilize the oxy form and prevent autooxidation. The distal
believed to be responsible for the Kd values for O2 being four
cavity is also important, particularly the polarity of the walls,
orders of magnitude greater than that of human hemoglobin.
the functional composition, and the 3D arrangement.
The crystal structures of complexes of heme protein with
In oxygen-avid Ascaris hemoglobin, glutamine and tyroNO also suggest that the distal cavity is important in gas
sine residues participate in hydrogen bonding with O2 (2,
binding. Such cavities are rather hydrophobic and possibly
help to exclude the noncoordinated H2O molecule prior to
Scheme 1).[9] The crystal structure of complex 2 shows the
tyrosine hydroxy group to be perfectly positioned to make a
the complexation of NO. The X-ray structure of the ferrous?
strong hydrogen bond with the distal atom of the complexed
nitric oxide complex of native sperm whale myoglobin 3
O2, and the glutamine forms a somewhat weaker hydrogen
(Scheme 1) shows a Fe NO interaction as well as the
formation of a hydrogen bond between the gas molecule
bond to the oxygen atom coordinated to the iron center.
and the histidine 64 residue on the distal porphyrin face of the
myoglobin.[10] Overall, the NO binding event takes place in a
Dmitry M. Rudkevich studied chemistry at
the Institute of Organic Chemistry, National
tight cavity formed by the lipophilic leucine 69 and valine 68
Academy of Sciences of the Ukraine. He
fragments as well as histidine 64.
completed his PhD in the laboratory of Prof.
It is believed that upon atmospheric N2 fixation by
David N. Reinhoudt at the University of
enzymes, the NH nitrogen?metal intermediates 4 participate
Twente in the Netherlands in 1995. In 1996
in hydrogen-bonding interactions with the amino acid resihe joined the research group of Prof. Julius
dues of the enzyme (Scheme 1).[11]
Rebek, Jr. and was a Research Assistant ProThese examples offer great inspiration for designers of
fessor of the Skaggs Institute for Chemical
Biology at Scripps (1997?2001). He is curartificial receptors for gases. They directly point out the
rently an Assistant Professor of Chemistry at
binding forces to employ and explore: a) hydrogen bonding,
the University of Texas at Arlington. His
b) metal?gas interactions, and c) cavity effects.
research interests focus on molecular recognition.
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2004, 43, 558 ? 571
Angewandte
Chemie
Supramolecular Chemistry of Gases
3. Hydrogen Bonding of Gases
Hydrogen bonding is one of the most important interactions utilized by nature and is responsible for biotic selfassembly and the selectivity of enzymes. As seen in the
previous section, it readily works for both gases and heme
proteins. In addition to the natural examples, other information on hydrogen bonds with gases comes from gas-phase
adducts of acidic HF, HCl, HBr, and HCN with N2, CO, CO2,
and OCS.[12] The geometry of the BиииH X (B = N, O, S, etc.;
X = halogen) complexes was deduced from ab initio calculations and also from advanced spectroscopic measurements,
such as molecular beam FT microwave spectroscopy, vibration spectroscopy, and photoelectron spectroscopy. Zeroelectron kinetic energy (ZEKE) photoelectron spectroscopy
and resonance-enhanced multiphoton ionization (REMPI)
spectroscopy proved to be very useful for characterizing the
Aryl-OHиииgas (gas = Ar, N2, CO) molecular clusters.[13] Much
less is known about hydrogen bonding of gases in solution.
A wide variety of heme models have been prepared which
are based on sterically hindered ?picket fence?, ?picnic
basket?, ?capped?, ?strapped?, ?hanging base?, and other
?superstructured? metalloporphyrins.[14] In some cases, the
increased affinity of these porphyrins towards O2 was
interpreted by favorable hydrogen-bonding interactions
between the complexed O2 molecule and the hydrogendonating fragment within the porphyrin structure.[14] In a most
recent impressive effort, dendritic iron porphyrins 5
(Scheme 2) have been introduced that possess highly protected gas-binding pockets. These also exhibited increased
affinity towards O2, for which hydrogen bonding between the
coordinated O2 and the C(O)NH function of the dendritic
arms has been proposed.[15]
Scheme 2. Dendritic porphyrins 5 prepared by Diederich and
co-workers.[15]
Angew. Chem. Int. Ed. 2004, 43, 558 ? 571
Spectroscopy offered direct proof of the presence of
hydrogen-bonding interactions. In the early 1H NMR studies
with the O2 complexes of amide-functionalized, ?baskethandle? porphyrins 6 (Scheme 3) a pronounced downfield
Scheme 3. Superstructured porphyrins 6 and 7 and their dioxygen
complexes.[16, 17] Piv = pivaloyl = trimethylacetyl.
shift of the signal for the amide C(O)NH proton was observed
in apolar solvents, thus indicating the presence of a
C(O)NHиииO2 hydrogen-bonding interaction.[16] This hydrogen bond, however, must be rather weak, since the estimated
nitrogen?O2 distance is approximately 4 >.
Recently, Naruta and co-workers presented Raman
spectroscopic evidence for the presence of hydrogen bonding
in the complex of ?twin-coronet? porphyrin 7 and O2
(Scheme 3).[17] Specifically, the hydrogen bond formed
between the bound dioxygen and the OH groups of the
binaphthyl moieties was studied through H/D exchange
experiments. Deuteration of the OH groups in 7 resulted in
the frequencies of the n(O O) bands being shifted by 2 cm 1
to a higher wavenumber. The estimated distances between the
binaphthyl oxygen atoms and the bound O2 are small, in the
range of 3 >.
Direct evidence of a hydrogen bond in oxyhemoglobin
was recently obtained by echo?anti-echo (1H, 15N) heteronuclear multiple quantum coherence (HMQC) NMR experiments.[18] Analysis of the pattern of cross-peaks for the distal
histidine residues in the HMQC spectrum showed that the
histidine NH hydrogen atom in the dioxygen complex is
stabilized against solvent exchange through formation of a
hydrogen bond with O2.
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
561
Reviews
D. M. Rudkevich
Studies with CO2 and N2O have also been reported. These
gases frequently circulate in biological fluids, however, the
possibility of their involvement in hydrogen-bonding interactions with proteins and enzymes had been routinely
ignored. Only recently, it was found that secondary cisamides, for example, e-caprolactam (8), interact with N2O and
CO2 in apolar solutions (Scheme 4).[19] Saturation of a
solution of 8 with N2O in CDCl3 and [D6]benzene at room
temperature resulted in the singlet corresponding to the NH
proton being shifted upfield by Dd 0.25 ppm. Ab initio
calculations of the complex formed between simple formamide (HC(O)NH2), which possesses a cis-amide arrangement,
and N2O confirm that a hydrogen bond can form between the
amide NH hydrogen atom and the partially negatively
charged oxygen atom of the gas molecule. A N H?O=N+=
ND distance of 2.33 > was obtained, which places the basic
amide C=O oxygen atom right in front of the central,
electron-deficient nitrogen atom of the N2O molecule.
There is an electrostatic attraction between the lone pair of
electrons and the latter partial positive charge, and a C=O?N+
distance of 2.94 > was calculated. These results taken
together indicate that a two-point noncovalent interaction
occurs, which is not possible for trans-amides. Semi-empirical
calculations with 8 and N2O yielded similar results. As
saturation with N2O disrupts the dimerization of cis-amides
(KD 5 m 1), this sets the lower limit for the described N2O
complexation to be DG295 0.9 kcal mol 1, which is typical for
weak interactions in apolar solution. Analogous conclusions
were obtained from molecular modeling studies, calculations,
and 1H NMR measurements with CO2, which is isoelectronic
to N2O.
Similar two-point interactions between a C(O)CH3 acetyl
group and CO2 were proposed for complexes of peracetylated
carbohydrates 9?11 and CO2.[20] This situation involves both
the acetate C=OиииCO2 electrostatic and C HиииO2C hydrogen-bonding interactions (Scheme 4). Derivatives 9?11 have
an enhanced solubility in supercritical CO2, and in general,
the Lewis basic C=O group is known as a CO2-phile. Ab initio
calculations on complexes formed between CO2 and simple
carbonyl compounds were performed, and gave binding
energy values of 2.69 kcal mol 1 per complex. The Raman
spectra of the complexes formed between acetaldehyde and
CO2 showed the maximum red-shift of the acetaldehyde
carbonyl band to be from 1746.0 to 1743.5 cm 1 and the
maximum blue-shift of the aldehyde CH proton band to be
from 2717.0 to 2718.3 cm 1. Both observations were consistent with the computer-predicted structures.
4. Lewis Acid?Base Interactions: From Receptors to
Materials
Inorganic and organometallic compounds are widely
known to react with N2, NO, N2O, CO, CO2, O2, and other
gases through formation of a metal?gas bond. Some of these
processes are reversible and can be described as supramolecular in nature. Irreversible chemistry between metals and
gases has been effectively employed in catalysis and also in
photo and redox processes which are used in industrial,
environmental, and biomedical gas sensing and monitoring.
This has been thoroughly described in numerous reviews and
original papers.[21] Here, we discuss the most characteristic
advances related to molecular recognition of gases. Molecular
recognition is usually defined by the delicate balance of
energy and chemical information involved in the binding and
selection processes.[1]
Scheme 4. Involvement of N2O and CO2 in hydrogen-bonding and electrostatic interactions during their complexation with cis-amides and
acetates.[19, 20]
562
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Angew. Chem. Int. Ed. 2004, 43, 558 ? 571
Angewandte
Chemie
Supramolecular Chemistry of Gases
Scheme 5. Exchange experiments with gases in ?picnic-basket? porphyrin 12 as well as the structure of 13.[22] Py = pyridine.
Collman et al. identified controlled gas exchange within
the cavity of ruthenium ?picnic-basket? porphyrins 12
(Scheme 5).[22] The RuII?gas interactions were the driving
force for these processes, but the cavity also played a crucial
role, especially in the kinetics and regioselectivity. First, the
so-called bis-solvent complex [12(thf)2] was prepared by
photolysis of the corresponding carbonyl derivative
[12(CO)(thf)]. Treatment of this complex with bulky 1,5dicyclohexylimidazole (1,5-DCI) resulted in formation of
[12(thf)in(1,5-dci)out], with the thf molecule coordinated inside
the cavity. Dry O2 quantitatively replaced the internally
coordinated thf molecule in toluene solution, and this
processes was shown to be reversible. Furthermore, the
introduction of N2 enabled reversible exchange between the
corresponding dioxygen [12(O2)in(1,5-dci)out] and dinitrogen
[12(N2)in(1,5-dci)out] complexes to be achieved. Dinitrogen
complex [12(N2)in(1,5-dci)out] was also converted into the
carbon monoxide analogue [12(CO)in(1,5-dci)out] simply by
exposure to CO gas, and also to the gas-free derivative
[12(1,5-dci)out]. This latter complex was readily converted into
[12(O2)in(1,5-dci)out] by exposure to O2. The gas-exchange
processes were effectively monitored by visible, IR, and
1
H NMR spectroscopies.
The combination of a metal center and hydrogen-bonding
sites was also employed for the recognition of other gases. For
example, preorganized and rigid UO2-containing macrocycles
13 were shown to selectively bind NH3 over primary alkyl
amines (Scheme 5).[23] Kass values of 3 M 102 to 1 M 103 m 1 were
obtained for NH3 in CDCl3 solution, which is more than an
order of magnitude higher than for amines RNH2 (R = Me,
Et, nPr, Bz). In addition to the binding of the lone pair of
Angew. Chem. Int. Ed. 2004, 43, 558 ? 571
electrons on the NH3 nitrogen atom to the Lewis acidic
UO2 cation, several N HиииO hydrogen bonds may be formed
between the gas molecule and the phenolic oxygen atoms on
the receptor. Furthermore, the preorganized macrocycle apparently introduced steric restrictions in binding larger guests.
In a series of thorough papers, van Koten and co-workers
reported on square-planar N,C,N-terdentate pincer PtII complexes 14 for the reversible sensing of sulfur dioxide (SO2)[24]
in which SO2 is directly bound to the metal center. In the
presence of SO2, complexes 14 spontaneously adsorbed the
gas and formed pentacoordinate, square-pyramidal adducts
[14(SO2)] (Scheme 6). The process is fast and reversible, both
in apolar solution (CH2Cl2, CHCl3, C2F4Br2, and toluene) and
in the solid state. Importantly, a characteristic color change of
the complex from colorless to bright orange occurred upon
entrapment of SO2. The exchange of SO2 is fast at room
temperature, but can be slowed down and studied by conventional 1H NMR spectroscopy at 167?188 K (in C2F4Br2). For
example, the rate constants for exchange are k = (1.5 0.5) M
108 s 1 at 298 K and k = (2.34 0.08) M 103 s 1 at 174 K.
Variable-temperature NMR analysis of the equilibrium constant between complex 14 and adduct [14(SO2)] (X = I) gave
DH8 = 36.6 0.8 kJ mol 1, DS8 = 104 3 J K 1 mol 1, and
K298 = 9 4 m 1.
Reactions of 14 and SO2 in the crystalline-state produced
orange-colored, reversible adducts, which may be used for gas
storage and switchable optoelectronic devices. Compound 14
was also covalently linked to the amino acid valine (Val)
through the N-terminus to form a compound that may act as a
diagnostic peptide-labeling tool for biomedical applications.
Indeed, such organoplatinum-labeled peptides may be traced
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Scheme 6. Reversible binding of SO2 by pincer PtII complexes 14 and 15 prepared by van Koten and co-workers.[24]
and located specifically in biological systems by ?highlighting? the platinum site with SO2 gas.
Dendritic organoplatinum(ii) complexes 15 were prepared
as reusable SO2-sensing and -storing materials (Scheme 6).
Such dendrimers offer monodisperse, globular nanostructures
with defined size and shape that satisfy the new challenges of
chemistry to build macromolecular entities composed of
many identical components, which are arranged to function as
receptors for given binding units or ligands. Multiple, multivalent interactions enable macromolecular receptors 15 to
display an increased affinity towards the gas substrate.
A dendritic model of hemocyanin was prepared which is
capable of storing oxygen.[25] Hemocyanin proteins are known
to bind O2 at a site containing two copper atoms directly
ligated by protein side chains. A multi-oxygen complex was
obtained from the synthesized CuI dendrimer with 10?
11 molecules of O2 per one dendritic molecule. It was
shown that 60?70 % of the CuI centers were involved in the
binding of oxygen. The high local concentration of oxygenating equivalents in such a dendrimer represents an exciting
promise for synthetic chemistry.
Borovik and co-workers developed porous organic materials for the reversible binding of CO, O2, and NO (Scheme 7).
Templated co-polymerization was employed for the immobilization of metal centers. For example, template complex 16,
along with cross-linking agent EGDMA, and solvent, which
served as the porogen during the radical polymerization
process, was used for NO gas.[26] The preformed kinetically
inert CoIII?4-dimethylaminopyridine (DMAP) complex 17
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
contains the styrene-modified salen ligand (salen = N,N?bis(salicylidene)ethylenediamine dianion), which is covalently attached to the porous methacrylate host. The CoIII ions
and DMAP were removed under acidic conditions to afford
18. The two immobilized salicylaldehyde moieties in 18
further reacted with ethylenediamine to reform the immobilized tetradentate salen ligand. The resulting polymer now
readily binds CoII ions to form 19 which contains immobilized
four-coordinate CoII centers, with the cobalt concentration
ranging from 180 to 230 mol g 1 and an average pore diameter
of 25 >. Polymer 19 binds NO in toluene solution and even at
the air?solid interphase, but it is relatively inert toward other
biologically important gases such as O2, CO2, and CO.
Moreover, NO is slowly released from 19 under ambient
conditions. It was shown that approximately 80 % of the NO
was lost from 19 after 30 days and heating the sample
accelerated the gas release. Such controlled, slow release of
NO is important for medical applications.[5]
The NO-binding properties of 19 differ from those of the
corresponding monomolecular analogues, and this is attributed to the presence of the porous polymethacrylate host in
these materials. The pronounced color change resulting from
the conversion of 19 into [19(NO)] suggests potential use in
the detection of NO. The polymer can be recycled, retains a
high binding affinity for NO, and works equally well in the
solid state or as a suspension in a liquid. Similar results have
been obtained for O2-binding polymers 20 which are made by
template copolymerization methods (Scheme 7).[27]
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Supramolecular Chemistry of Gases
Scheme 7. Formation of porous organic materials 19 and 20 for reversible binding of NO and O2.[26, 27] EGDMA = ethylene glycol dimethacrylate,
AIBN = azobisisobutyronitrile, EDTA = ethylenediaminetetraacetate.
5. Encapsulation of Gases
It has been known for several decades that low-molecularweight hydrocarbons, N2, O2, Cl2, CO2, N2O, H2S, SO2, and
PH3 can be encapsulated within self-assembled (H2O)n
lattices in the solid state.[28] Caged zeolites have recently
been shown to reversibly incarcerate NOx.[29] Early experiments on gas encapsulation within organic cavities were
performed with cyclodextrins in the 1950s.[30] a-Cyclodextrin,
for example, interacted with Cl2, Kr, Xe, O2, CO2, C2H4, CH4,
C2H6, C3H8, and n-C4H10 in water with the formation of stable
clathrates. The corresponding CO2cyclodextrin complex can
be isolated simply through precipitation, and this process is
used for the purification of the cyclodextrin itself.[31]
Over the years, the desirability of controlling the size,
shape, and dynamics of the cavity led to the synthesis of
various molecular containers, such as cavitands, carcerands,
and capsules. These days, chemists focus on nanoscale
cavities, which are capable of complexing two or even more
guest molecules or a single guest with a size of up to 15?
20 >.[32] However, much more modest dimensions are
Angew. Chem. Int. Ed. 2004, 43, 558 ? 571
required for gas complexation, and closer contacts with the
inner walls of the cavity are needed. Additional binding sites
may also be desirable.
Hemicarcerand 21?covalently sealed molecular containers with aromatic walls?prepared by Cram et al. encapsulated O2, N2, CO2, and Xe (Scheme 8).[33] The exchange
between the free and occupied hemicarcerand was slow on
the NMR time-scale, with association constants Kass of 180 m 1
(N2), 44 m 1 (O2), and 200 m 1 (Xe) being obtained in CDCl3 at
22 8C, assuming a 1:1 stoichiometry. The volume of the inner
cavity in 21 is relatively large (ca. 120 >3) compared to the gas
molecules (ca. 40 >3). Hemicarcerand 21 can probably
accommodate more than one gas molecule, but this has not
been investigated.
The ?tennis balls? 22 developed by Rebek and co-workers
self-assembled through the formation of hydrogen bonds
around a molecule of CH4, ethylene, or Xe in CDCl3 solution
(Scheme 8).[34] The volumes of the gas molecules are 28, 40,
and 42 >3, respectively, and the inner cavity of 22 is
approximately 50 >3. Similar to 21, the exchange between
the free and occupied capsule was slow on the 1H NMR time-
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Scheme 8. Encapsulation of gases within hemicarcerand 21[33] and self-assembling capsule
222.[34]
scale. The same was true for the guest species: uncomplexed
CH4 exhibited a singlet at d = 0.24 ppm, while the encapsulated species gave a signal at d = 0.91 ppm; both signals
could be seen simultaneously. For this gas, DH = 9 kcal mol 1, DS = 20 J K 1 mol 1, and K273 = 300 m 1 were estimated in CDCl3. Clearly, the inclusion of the gas involved a
large entropic cost, as was seen for complexes 14.
Cryptophane-A (23, Scheme 9) developed by Collet and
co-workers possesses an inner cavity of 95 >3 and was shown
to encapsulate CH4 with Kass = 130 m 1 and Xe with Kass 3000 m 1 in (CDCl2)2.[35] The gas molecules were clearly better
guests for receptors 21?23 than bulky (> 70 >3) solvent
molecules, which are too big to enter and/or occupy the
interiors. In general, this provides a strong driving force for
the entrapment of the gas.
Gas encapsulation within molecular containers is still in
its infancy, but may lead to important applications. For
example, studies with cyclodextrins showed that it is possible
to use 129Xe NMR spectroscopy to obtain quantitative
information about complexation processes. Xenon is highly
polarizable, but inert and hydrophobic. Two isotopes are
easily accessible to NMR spectroscopy: 129Xe (I = 1/2, natural
abundance of 26 %) and 131Xe (I = 3/2, abudance 21 %). The
NMR parameters of these isotopes are very sensitive to the
environment in which Xe is located, and this can be used to
probe structural and dynamic properties of host?guest complexes, both in the solid state and in solution.
Specifically, protein NMR studies have shown that weak
Xe?protein interactions influence the 129Xe chemical shift
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depending on the accessible protein surface, and
allow the protein void space and conformations
to be monitored. Recently, Pines and co-workers
attached cryptophane-A (23) to biotin and used
129
Xe NMR spectroscopy to detect biotin?avidin
binding (Scheme 9).[36]
Free Xe in water generates a signal at d =
193 ppm, while the signal for Xe24 is located at
d 70 ppm. Addition of avidin results in the
appearance of a third signal 2.3 ppm downfield of
the Xe24 signal, which was assigned to the
protein-bound Xe24 complex. The intensity of
this signal increased when the concentration of
avidin was increased. The proposed methodology
offers the capability of attaching different ligands
to different cages, thus forming Xe sensors that
show distinct chemical shifts upon encapsulation.
Recently, Kochi and co-workers demonstrated the encapsulation of NO by 1,3-alternate
calix[4]arene 25 (Scheme 10).[37] Initially, calixarene 25 was oxidized to its radical cation, which
then complexed NO gas with the formation of the
calix?nitrosonium species NO+25.
Strong charge-transfer interactions between
NO+ and the p surface of the calixarene placed
the gas molecule at a distance of 2.4 > from the
cofacial aromatic rings, which is much shorter
than the typical van der Waals contact (3.2 >).
An association constant of Kass > 5 M 108 m 1 was
estimated. However, NO+ was easily released
from the cavity upon addition of Cl ions, as a
Scheme 9. Cryptophane-A (23) and its complexes with Xe.[35] Biotinylated cryptophan-A (24) for binding to proteins.[36]
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Supramolecular Chemistry of Gases
NO+ ion within their cavities (Scheme 10). NO+ is
generated from N2O4, which is known to disproportionate to NO+NO3 upon exposure to aromatic compounds. Stable nitrosonium complexes NO+26 and
NO+27 were quantitatively isolated upon addition of
a stabilizer?Lewis acidic SnCl4. Only one NO+ ion was
found per cavity. These interactions are reversible. The
addition of H2O or alcohols resulted in the dissociation
of the complexes NO+26 and NO+27 and the
recovery of 26 and 27. The complex with 27 (R? = H)
decomposes instantly, but it takes several minutes for
complex 26 b (R? = tBu) to break up. Bulky tBu groups
at the upper rim of 26 b protect the encapsulated
NO+ ion. Such stability of the arene?NO+ complex is
without precedent.
Dramatic color changes result from the charge
transfer. Moreover, these interactions are specific. The
detection of NO2 in the presence of gases such as H2O,
O2, HCl, HBr, SOx, NH3, and neat NO can be fully
expected, since none of these vapors/gases undergoes
reactions with calixarenes. All this may be of interest for
sensing technology.
Complexes NO+26 and NO+27 may release
NO+, which acts as an encapsulated nitrosating species
(Scheme 11). Thus, secondary amides were quite reactive towards complex NO+27 b (R? = tBu) and showed
remarkable selectivity.[39] Amides with NCH3 substituents reacted smoothly with NO+ (298 K, CHCl3 ; 50?
95 %), however, bulkier amides did not react because of
steric hindrance: N substituents larger than CH3 do not
allow
the substrate to enter the cavity. Such stability and
Scheme 10. Encapsulation complexes of NOx gases with calix[4]arenes 25?27;
selectivity cannot be easily achieved for the existing
formation of NO+calixarene complexes.[37, 38]
nitrosating agents NO+ salts, NaNO2/H2SO4, NO/O2,
N2O3, and NO2/N2O4.
Simple solids held together by van der Waals forces may
result of the formation of nitrosyl chloride (NOCl). The
also be used for gas storage, processing, and controlled
complexation can also be controlled by redox and/or temperrelease. Recently, Atwood et al. elegantly demonstrated that
ature changes. The charge-transfer complex NO+25 is
CH4, CF4, C2F6, CF3Br, and other low-boiling halogenated C1deeply colored and thus can be used for colorimetric NO
sensing.
alkanes can be reversibly entrapped and retained within the
More recently calix[4]arenes were employed to visually
lattice voids of a crystalline calix[4]arene framework.[40] Such
[38]
detect and chemically transform NO2/N2O4 gases.
gas-storing crystals appeared to be extremely stable and
The
release their guests only at rather elevated temperatures?
tetrakis-O-alkylated calix[4]arenes 26 and 27 reversibly
hundreds of 8C above their boiling points.
interacted with NO2/N2O4 and entrapped the highly reactive
Scheme 11. Left: transformation of NO2/N2O4 into NO+calixarene complexes?encapsulated nitrosating reagents. Right: proposed mechanism
of amide nitrosation within the calixarene cavity.[39] The dimensions and shape of the R? substituent are crucial. The amide substrate approaches
the cavity facing the carbonyl oxygen atom, which places the NR? alkyl group in proximity to the bulky groups on the calixarene. This situation
could be sterically unfavorable for sizeable R? groups so that the substrate C=O and the encapsulated NO+ ion would not reach each other.
Angew. Chem. Int. Ed. 2004, 43, 558 ? 571
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Reviews
D. M. Rudkevich
An interesting approach to encapsulate gases, which is
based on the delicate interplay between hydrophilic and
lipophilic interactions, was demonstrated for decamethylenecucurbit[5]uril 28 (Scheme 12). Water-soluble molecular container 28?a smaller relative of cucurbituril?was found to
entrap a large variety of lipophilic gases (H2, He, Ne, Ar, N2,
O2, N2O, NO, CO, CO2, CH4, and acetylene) within its
interior.[41] The encapsulation of the gases was studied by
single-crystal X-ray analysis and also by NMR spectroscopy in
D2O. For example, the encapsulated CH4 singlet appeared at
d = 0.87 ppm (1H NMR), the weak resonance at d =
127.4 ppm (13C NMR) for the dissolved CO2 transformed
into a rather strong signal at d = 127.1 ppm (13C NMR) upon
encapsulation, and likewise, the typically insoluble acetylene
produced signals at d = 1.02 ppm (1H NMR) and d =
73.6 ppm (13C NMR) in the presence of 28. Most of the
gases were encapsulated upon bubbling them through the
solution of 28 in D2O at room temperature. Larger gases (for
example, CH4, Kr, and Xe), however, required heating to
80 8C. Most probably, competition with earlier encapsulated
water molecules was responsible for this behavior.
Circulating air through 28 powder resulted in a decrease
in the level of CO2 from 5 % to 0.01 %. Although 28 has a
lower capacity for CO2 than 5-> molecular sieves, it has a
lower regeneration temperature of 110 8C (versus 350 8C) and
a lower affinity to water.
Much attention is currently focused on fullerenes and
nanotubes for fabricating nanoscale structures for potential
biomedical and materials applications. Fullerenes are known
to permanently trap noble gases (for example, 29,
Scheme 12). Heating fullerenes at 650 8C and at 2000?
3000 atm pressures of noble gases leads to incorporation of
He, Ne, Ar, Kr, and Xe within the C60 and C70 interiors. Even
though the conditions are extreme, the fractional occupational levels are typically about 0.1 %.[42] It was proposed that
the noble gas enters through a ?window? formed by the
reversible breaking of one or more carbon?carbon bonds.
Theoretical barriers for He to penetrate the benzene rings of
fullerene were calculated to be 200 kcal mol 1. At the same
time, the encapsulated gases can be released by the breaking
of one or more carbon?carbon bonds upon heating under a
vacuum.
Fullerenes can also be ?opened through molecular
surgery?.[43] Thus, reaction of C60 with bis-azide 30 affords
open-neck bislactam 31, which encapsulates gases much more
easily (Scheme 12). Calculated insertion barriers for He and
H2 are 24.5 and 41.4 kcal mol 1, respectively, which corresponds to insertion temperatures of 124 and 397 8C, respectively. Experimentally, 3He gas was inserted into 31 at 475 atm
and approximately 300 8C over 7.5 h (1.5 % maximal yield),
while for H2, it took 100 atm, 400 8C, and 48 h (5 % yield).
Both complexes were studied by NMR spectroscopy. Thus,
the helium complex showed a 3He NMR signal at d =
10.10 ppm, relative to the free 3He gas at d = 0 ppm. The
encapsulated H2 was seen at d = 5.43 ppm in the 1H NMR
spectrum.[44]
Recent studies have revealed that some industrially and
biologically important gases can be adsorbed and stored
inside single-walled carbon nanotubes (SWNTs). The initial
synthesis of SWNTs typically produces sealed structures,
which prevents adsorption of the gas within the interiors.
Chemical treatments, for example, oxidation, leads to open-
Scheme 12. Top: decamethylenecucurbit[5]uril (28) and fullerene 29 provides enclosed inner cavities for gases.[41, 42] Bottom: ?molecular surgery?
on C60 allows opening of the fullerene to form bislactam 31, which complexes with gases.[43]
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Supramolecular Chemistry of Gases
ended SWNTs. In addition to the interiors, gases may also be
adsorbed on the outside surfaces, in groove sites, and in the
interstitial sites of nanotubes. Calculations showed that the
interiors of SWNTs are energetically more favorable for
absorption than their surfaces.[45] Entrapping isotopes of
noble gases by SWNTs may improve their use in medical
imaging, where it would be desirable to confine the gas
physically before injection.[46] SWNTs also trap N2, O2, NO,
and CF4 within their interiors.[47] They have been shown to
effectively sense O2, NH3, and NO2 through a dramatic
change in the electrical resistance of the nanotubes.[48] Storage
of H2 in single-walled carbon nanotubes is extremely promising in the design of energy-rich fuel-cell electric devices. This
area is currently a subject of extensive study.[49]
6. Summary and Outlook
The chemistry of gases dates back to the 18th century,
when most of them were discovered and described in detail.
Experimental studies of gases by Joseph Black, Henry
Cavendish, Joseph Priestley, Carl Scheele, and others, coupled
with the revolutionary ideas of Antoine Laurent Lavoisier,
prepared the groundwork for modern chemistry.[50] Supramolecular chemistry of gases emerged almost 300 years later.
Chemists have reached a molecular-level understanding of
the forces that bind gases, trap them, fix them in time and in
space, and further chemically transform them. This understanding, of course, required inspiration and lessons from
nature, which indeed had offered a number of superb
examples of gas binding and storing by heme proteins. After
this, synthetic receptors, which mimicked the natural systems,
were guaranteed to succeed.
It is clear now, that Lewis acid?base, dipole?dipole
interactions, hydrogen bonding, van der Waals forces, constrictive encapsulation within enclosed spaces, and a smart
combination of all these are most likely to work for gas
recognition. Some important receptor/sensing systems, which
function through these intermolecular forces, have already
been constructed.
Despite the tiny dimensions and simple atomic composition, each gas is different in its physical and chemical
properties and requires an individual approach. However, it
is possible to rationalize the kinetics, thermodynamics, and
selectivity of molecular hosts for gases on the basis of
electronic and geometrical complementary between the
receptor and substrate. It is also clear that widely accepted
concepts of ion coordination and neutral host?guest chemistry, such as the preorganization, multiplication of binding
sites, and encapsulation, are applicable to gases. As for all
other areas of supramolecular chemistry, spectroscopic techniques and molecular modeling studies are crucial for
elucidating the specific contributions of various binding
forces, as well as solvent effects.
Supramolecular chemistry provides deeper understanding
in biomimetic areas of how gases interact with biological
molecules?peptides, enzymes, nucleic acids?and tissues.
This may offer some insight into the mechanisms of the
physiological activity of gases. The principles and techniques
Angew. Chem. Int. Ed. 2004, 43, 558 ? 571
of molecular recognition can be used for gas sensing. In
contrast to conventional, mostly electrochemical sensors, this
approach provides fundamental knowledge about the structure and dynamics of receptor?analyte complexes on a
molecular level, thus opening the way for desirable detection
selectivity and sensitivity through the development of novel
materials and membranes for gas sensing, storage, and
controlled release. Finally, supramolecular chemistry may
and will find ways to chemically transform gases into
synthetically useful reagents.[51] The rational design of novel
catalysts for gas conversion remains a mainstream area of
research. However, ?molecule-within-molecule? or encapsulation complexes with gases also offer a promising breakthrough as they can entrap and release guest species at will,
under subtle chemical or physical control. Most excitingly,
however, is that this is just the beginning of much bigger
developments.
Financial support is acknowledged from the University of
Texas at Arlington and the Donors of The Petroleum Research
Fund, administered by the American Chemical Society.
D.M.R. is an Alfred P. Sloan Research Fellow. Special thanks
are also due to the D.M.R. research group and Prof. R. A.
Paselk and the Robert A. Paselk Scientific Instrument Museum
(Humboldt State University).
Received: April 11, 2003 [A606]
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