Nanoporous three-dimensional networks topologically related to Cooperite from the self-assembly of copper(I) centres and the 'square-planar' building block 1,2,4,5-tetracyanobenzene

Article abstract of DOI:10.1039/a809144g

The deliberate assembly of a Cu¹ 3D coordination network with the Cooperite (PtS) topology, namely [Cu(μ4-TCNB)][PF₆] 1, has been accomplished by using the pseudo-square-planar building block 1,2,4,5- tetracyanobenzene (TCNB). It has been obtained by reacting [Cu(NCMe)₄][PF₆] with TCNB (in molar ratio 1: 1) in solvent mixtures of CH₂Cl₂ and of a poorly basic solvent, like ethanol, isopropanol, benzyl alcohol or acetone. The crystals are orthorhombic, space group Cccm, with a = 8.157(1), b = 18.634(2), c = 11.613(1) A, Z = 4. The framework is based on tetrahedral Cu¹ centres and tetradentate μ₄-TCNB ligands and presents cavities containing guest solvent molecules. On performing the same reaction in CH₂Cl₂-THF a 3D three-connected polymeric species, [Cu(μ₃-TCNB)(THF)][PF₆] 2, is obtained. Compound 2 gives monoclinic crystals, space group P2₁/c, with a = 21.617(2), b = 11.127(3), c = 16.294(2) A, β = 111.61(2)°, Z = 8. In 2 the TCNB ligands are tridentate and the copper centres achieve a tetrahedral coordination on forming a bond with a THF molecule. The network presents an unprecedented topology, (8²10)-b, and the crystals undergo, by thermal activation, an unusual crystal-to-crystal transformation, losing the THF molecules to give the same frame as 1.

Full text of DOI:10.1039/a809144g

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Nanoporous three-dimensional networks topologically related to
Cooperite from the self-assembly of copper(I) centres and the
““square-planarÏÏ building block 1,2,4,5-tetracyanobenzene
Lucia Carlucci, Gianfranco Ciani,* Dorothea W.v. Gudenberg and Davide M. Proserpio
Dipartimento di Chimica Strutturale e Stereochimica Inorganica and Centro CNR, V ia G.
V enezian 21, 20133 Milano, Italy.
Received (in Strasbourg, France) 23rd November 1998, Accepted 16th February 1999
The deliberate assembly of a CuI 3D coordination network with the Cooperite (PtS) topology, namely [Cu(l -
4
TCNB)][PF ] 1, has been accomplished by using the pseudo-square-planar building block 1,2,4,5-
6
tetracyanobenzene (TCNB). It has been obtained by reacting [Cu(NCMe) ][PF ] with TCNB (in molar ratio 1 : 1)
4
6
in solvent mixtures of CH Cl and of a poorly basic solvent, like ethanol, isopropanol, benzyl alcohol or acetone.
2
2
The crystals are orthorhombic, space group Cccm, with a \ 8.157(1), b \ 18.634(2), c \ 11.613(1) A, Z \ 4. The
framework is based on tetrahedral CuI centres and tetradentate l -TCNB ligands and presents cavities containing
4
2
guest solvent molecules. On performing the same reaction in CH Cl ÈTHF a 3D three-connected polymeric
2
species, [Cu(l -TCNB)(THF)][PF ] 2, is obtained. Compound 2 gives monoclinic crystals, space group P2 /c, with
3
6
1
a \ 21.617(2), b \ 11.127(3), c \ 16.294(2) A, b \ 111.61(2)¡, Z \ 8. In 2 the TCNB ligands are tridentate and the
copper centres achieve a tetrahedral coordination on forming a bond with a THF molecule. The network presents
an unprecedented topology, (8210)-b, and the crystals undergo, by thermal activation, an unusual crystal-to-crystal
transformation, losing the THF molecules to give the same frame as 1.
Many e†orts in the crystal engineering of networked materials
based on polymeric coordination compounds1,2 have been
devoted to the use of novel suitably tailored polydentate
ligands. Polycyano ligands, neutral and anionic, are attracting
a growing interest,3 both for their electronic properties and
also for their di†erent bonding modes and variable hapticity.
In particular, some tetracyano species like TCNE
tions of [Cu(NCMe) ][PF ] with 1,2,4,5-tetracyanobenzene
4
6
(TCNB), leading to a Cooperite-like framework, [Cu(l -
4
TCNB)][PF ] 1, in solution mixtures of CH Cl with di†er-
6
2 2
ent poorly basic solvents (like acetone, ethanol, isopropanol,
benzyl alcohol), and to a three-connected net of uncommon
topology, [Cu(l -TCNB)(THF)][PF ] 2, in CH Cl ÈTHF
3
6
2 2
(tetracyanoethene)
and
TCNQ
(7,7,8,8-tetracyano-
quinodimethane) can potentially act as planar l -bridging
4
groups, as, indeed, found in some previously reported poly-
meric species.4,5
The self-assembly of ligands like these with tetrahedral
metal centres can produce open nanoporous frameworks
related to the prototypical structure of Cooperite (PtS, Fig. 1,
top), containing a 1 : 1 ratio of tetrahedral and square-planar
nodes. Networked materials with this topology (4284),6 have
been deliberately assembled by using tetrahedral CuI ions
together with square-planar metal centres, like [Pt(CN) ]2~ 7
4
or metalloporphyrins,8 but also a suitable organic group can
play the same role of a square-planar coordination centre, as
found in the case of the two-fold interpenetrated PtS-like
network of [Ag(TCNQ)],9 schematically shown in Fig. 1
(bottom).
Our previous attempts to build a PtS-like net using TCNE
together with AgI or CuI ions were unsuccessful, resulting in
frames of di†erent topology because of the transformation of
the ligand, under the reaction conditions, into a non-planar
carbanionic acetonyl derivative.10 We report here on the reac-
Fig. 1 The prototypal network of Cooperite (PtS) viewed down the
[1 0 0] direction (top) and a schematic view of the two-fold interpene-
trated PtS-like net of [Ag(TCNQ)] (bottom, black balls \ baricentres
of the TCNQ ligands).
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(THF \ tetrahydrofuran). Compounds 1 and 2 can be inter-
converted and the noteworthy crystal-to-crystal transform-
ation of
2 into 1 under thermal activation is also
demonstrated. During the preparation of this manuscript
another study on the reactivity of [Cu(NCMe) ][PF ] with
4
6
TCNB has been reported by Munakata et al.,11 leading to
quite di†erent polymeric products, of composition
[Cu (TCNB) ][PF ] , due to the use of a large stoichiomet-
2
3
6 2
ric excess of the TCNB ligand.
Results and discussion
Reactivity of [Cu(NCMe) ] [PF ] with TCNB
4
6
The reactions of [Cu(NCMe) ][PF ] with TCNB were per-
4
6
formed in di†erent solvents using a 1 : 1 molar ratio of the
reagents. The metal complex was dissolved in CH Cl and
2
2
treated with solutions of TCNB in ethanol, isopropanol,
benzyl alcohol, acetone or THF. A yellow microcrystalline
product is rapidly formed under stirring, which gives, after Ðl-
tration and drying, rather variable analyses corresponding
Fig. 2 A SCHAKAL perspective view of the framework of 1 down
[1 1 0], oriented to show the PtS topology (cf. Fig. 1 top). Hatched
spheres represent the P atoms of the PF ~ anions.
6
approximately to the 1 : 1 adduct [Cu(TCNB)][PF ] 1. Crys-
6
tals were grown in few days by slow di†usion, upon carefully
and 2.119(7) A] in a distorted tetrahedral geometry. In this
case, in contrast to 1, the solvent molecules are able to enter
into the metal coordination sphere, replacing a bound CN
group. The coordination is illustrated in Fig. 3, which shows
the shortest circuit in the net, consisting of alternated four CuI
ions and four TCNB ligands.
layering the solution of the ligand on that of the metal
complex. Similar elongated yellow crystals were obtained from
the acetone, ethanol, benzyl alcohol or isopropanol solutions,
containing the same 3D polymeric network, as conÐrmed by
X-ray di†raction analyses performed on crystals of all the
samples.
From a topological point of view, the 3D frame is com-
prised of three-connected metallic and organic centres in equal
ratio, which form a system of alternating 4 and 4 intercon-
From the CH Cl ÈTHF system, on the other hand, by
2
2
slow di†usion beautiful Ñattened hexagonal shaped orange-
1
3
yellow crystals separated, corresponding to [Cu(l -
nected parallel helices, extending in the b direction. It was
classiÐed by Wells6 as the Archimedian 3D net (8210)-b (see
Fig. 4) and, to the best of our knowledge, no example of this
type has been previously reported.
3
TCNB)(THF)][PF ] 2, containing a di†erent 3D polymeric
6
network, characterized by X-ray analysis.
It is interesting to note that the reactions of
[Cu(NCMe) ][PF ] with a large excess (Ðve-fold) of TCNB
In spite of the di†erent topology, the structure of 2 displays
a great resemblance to that of 1, as can be seen on comparing
the projection of the frame of 1 down c with that of 2 down b
(Fig. 5). This structural similarity can account for the crystal-
to-crystal transformation of the two species, that is described
below.
4
6
give two di†erent polymeric species on varying the solvent, i.e.
[Cu (l -TCNB)(l -TCNB) ][PF ] Æ 4Me CO in acetone and
2
2
4
2
6 2
2
[Cu (l -TCNB)(l -TCNB) ][PF ] in methylethyl ketone.11
2
2
4
2
6 2
The former contains 2D undulated layers while the latter con-
sists of a two-fold interpenetrated 3D net topologically related
to the SiO polymorph moganite.12
2
The nanoporous network of compound 1
The three-dimensional networks of 1 and 2
When compound 1 is rapidly precipitated under stirring it
gives polycrystalline samples whose X-ray powder di†raction
patterns are almost identical whichever solvent system is used;
this is also the case for the THFÈCH Cl mixture, if the pre-
The crystal structure of 1 consists of a 3D net based on an
equal number of planar tetradentate organic centres and dis-
torted tetrahedral CuI ions [CuÈN 1.969(4) A, NÈCuÈN
95.2(3)È117.2(2)¡], with the desired PtS-like topology (see Fig.
2). The TCNB ligands (as well as TCNE and TCNQ) can be
better described as ““rectangularÏÏ rather than square centres
(with intraligand NÉ É ÉN long and short rectangular edges
having a ratio of ca. 1.8 : 1). The channels of the network are
2
2
cipitate is immediately removed from the mother-liquor.
Single crystals are grown within a few days by slow mixing of
the solution of the metal complex in CH Cl and the solution
2
2
of TCNB in the second solvent. All these single crystals give
very similar cell parameters, with the exception of those
obtained from THFÈCH Cl (compound 2). Structure
occupied by the PF ~ anions and by highly disordered
6
2
2
solvent molecules, which are difficult to rationalize (see
below). In contrast to what was observed for the two-fold
interpenetrated [Ag(TCNQ)] (Fig. 1, bottom), compound 1
contains a single net, very probably due both to the smaller
dimensions of the TCNB ligand and to the presence of the
counter ions.
The deliberate contruction of a PtS-like framework, using
proper building blocks, has thus been successfully carried out.
The same topology has also been obtained by the self-
assembly of TCNB and AgSbF .13 SigniÐcantly, these net-
6
works (both in the CuI and in the AgI species) are not
interpenetrated.
The structure of compound 2, [Cu(l -TCNB)(THF)][PF ],
3
6
reveals a di†erent 3D frame. The TCNB ligands use only three
of the four nitrile groups for bonding to CuI ions, which are
connected to three such groups [CuÈN 1.915(7)È1.999(7) A]
and to the oxygen atom of a THF molecule [CuÈO 2.117(7)
Fig. 3 The shortest circuit in the network of 2, showing the tri-
connected TCNB ligands and the distorted tetrahedral coordination
of the CuI ions.
398
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Fig. 5 A comparison of the networks in the structures of 1, down c
(top) and of 2, down b (bottom). The dashed lines in the case of 2
represent the directions of CuÈN bond formation in the course of the
2 ] 1 transformation (see text).
Fig. 4 The ideal triconnected (8210)-b network (top) and the topo-
logically equivalent schematized net of 2 (bottom), in which the black
balls represent the baricenters of the TCNB ligands.
mogravimetric analyses of 1 show a loss of solvent (not
constant), not exceeding ca. 4% of the mass, in the range 80È
100 ¡C. The network is stable up to ca. 280 ¡C and above this
temperature it loses the TCNB ligands. Almost constant ele-
mental analyses for 1, corresponding to the solvent-free
network, can be obtained after heating the samples in an oven
at 80 ¡C for some hours. The X-ray powder di†raction spectra
of these samples show patterns quite similar to the original
one, conÐrming that the polymeric network is not destroyed
on removing the solvents.
analyses of crystals of 1 from di†erent solvents, both at room
temperature and at [30 ¡C, gave similar results, showing the
presence of some variable residual electron density peaks due
to highly disordered solvent molecules, that are difficult to
rationalize on the basis of the X-ray data only, located within
the cavities (of approximate volume 239 AŽ 3)14 illustrated in
Fig. 6 by big spheres. These solvent regions are disposed down
[0 0 1], along rhombic channels topologically equivalent to
the square ones down the tetragonal axis of PtS. Di†erent
experiments have been carried out in order to characterize the
guest solvent molecules. IR microscopy on the crystals shows
bands due to the presence of both CH Cl and the second
Crystal-to-crystal 2 Ç 1 conversion
Thermal analyses (DSC and TGA) have shown that 2 loses
the coordinated THF molecule at 110È130 ¡C and starts to
decompose above 280 ¡C. Examination under the microscope
of the samples previously heated to ca. 140 ¡C and cooled at
room temperature revealed that the macroscopic crystal shape
and colour seem almost unchanged in the process. These crys-
tals, submitted to single crystal X-ray di†raction, showed
marked broadening of the peaks and lack of di†raction above
2
2
solvent, and, in the case of the crystals obtained from
CH Cl Èacetone, monitoring of the acetone l(CO) stretching
2
2
reveals that this band rapidly decreases and disappears on
heating the sample to 80 ¡C. 1H NMR spectra of di†erent
samples from CH Cl Èethanol, collected after drying the crys-
2
2
tals under N Ñux and dissolution in acetone-d , show signals
2
6
at d 8.93 (2H, TCNB), 5.63 (2H, CH Cl ) and 1.5 (t, 3H, CH
ethanol), of variable relative intensities (TCNBÈCH Cl from
1 : 1 to 1 : 033; TCNBÈethanol from 1 : 0.33 to 1 : 0.20). Ther-
h
ca. 10¡, but gave an orthorhombic cell [a \ 8.30(1),
b \ 18.66(5), c \ 11.60(4) A, V \ 1797(8) A3] similar to that of
1. The same conversion has been conÐrmed on bulk samples
2
2
3
Ž
2
2
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399

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of 2 by monitoring the heating process by X-ray powder dif-
fraction methods (Fig. 7). A crystal-to-crystal tranformation
takes therefore place, leading from 2 to 1, with the cavities and
channels along the c axis (see Fig. 5, top and Fig. 6) left empty.
The process is made possible by the limited extent of the
required rearrangements: the displacement of the THF mol-
ecules of 2, which can travel through the crystal channels, is
accompanied by the formation of the fourth CuÈN bond (the
relevant CuÉ É ÉN contact varying from ca. 3.8 A in 2 to ca 2.0
A in 1, along the directions illustrated by the dashed lines in
Fig. 5, bottom). So Fig. 5 gives the extreme situations of the
solid-state displacement reaction path CNÉ É ÉCuÉ É ÉTHF.
Crystal-to-crystal reactions are in general rare,15 particularly
in coordination polymer chemistry.16,17 Some evidence of the
reverse 1 ] 2 solid state transformation has also been
achieved: for instance, samples of 1 left in THF at room tem-
perature for a week gave X-ray powder di†raction patterns
which can be ascribed to a ca. 50% mixture of 1 and 2.
Experimental
All manipulations were carried out under a nitrogen atmo-
sphere with use of standard Schlenk techniques. Solvents were
distilled under nitrogen from NaÈPh CO (THF) or P O
2
2 5
(CH Cl ). [Cu(NCMe) ][PF ] was prepared and puriÐed
2
2
4
6
according to the literature method.18 All the other reagents
were used as purchased from Aldrich. IR spectra in Nujol and
IR microscopy data were collected on a PerkinÈElmer FT-IR
Paragon 1000 spectrometer. 1H NMR spectra were recorded
on a Bruker AC200 instrument. Thermal analyses were per-
formed on DSC 7 and TGA 7 PerkinÈElmer instruments with
a heating rate of 10 ¡C per min. X-Ray powder di†raction
spectra were collected on a Rigaku D/Max horizontal-scan
di†ractometer. Elemental analyses were carried out at the
Microanalysis Laboratory of this University.
Fig. 6 A view of the frame of 1 that illustrates the rhombic channels
down c containing the regions (big spheres) occupied by the disor-
dered solvent molecules. The PF ~ anions are also shown with van
6
der Waals radii.
Synthesis of the compounds
Compound 1. In a typical preparation a solution of TCNB
(0.021 g, 0.12 mmol) in ethanol (5È6 mL) was added to a solu-
tion of [Cu(NCMe) ][PF ] (0.044 g, 0.12 mmol) in CH Cl (8
4
6
2 2
mL). The solution was left to stir for 1 h. A yellow precipitate
of 1 was obtained, which was Ðltered o†, washed with ethanol
and then with hexane and dried under vacuum (ca. 0.025 g,
yield 55%). Crystals were grown by slow di†usion of a solu-
tion of TCNB in ethanol into a solution of the copper
complex in CH Cl . The crystals are air stable for few days.
2
2
Constant elemental analyses, corresponding to the solvent-free
product, were obtained after heating at 80 ¡C for some hours
(Calc. for C H CuF N P: C 31.06, H 0.52, N 14.49. Found:
10
2
6 4
C 30.94, H 0.75, N 14.10%. IR (Nujol, cm~1): CH 3117, 3044,
CN 2269, PF ~ 832.
6
Compound 2. A solution of TCNB (0.028 g, 0.16 mmol) in
THF (5 mL) was added to a solution of [Cu(NCMe) ][PF ]
4
6
(0.058 g, 0.16 mmol) in CH Cl (7È8 mL). After stirring for
2
2
one night the orange-yellow precipitate of 2 was Ðltered o†,
washed with ethanol and then with hexane and dried under
vacuum (ca. 0.052 g, yield 71%) (Calc. for C
C 36.65, H 2.20, N 12.21. Found: C 36.12, H 2.06, N 11.96%.
H
CuF N OP:
14 10
6 4
IR (Nujol, cm~1): CH 3128, 3059; CN 2262, 2252; THF 1043,
PF ~ 833.
6
X-Ray crystallography
Single crystals of 1 and 2 were mounted under a coating of
cyanoacrylic glue, to prevent decomposition, on a Siemens
SMART CCD area-detector di†ractometer. Crystal data for
1: C H CuF N P, M \ 386.67, orthorhombic, space group
Fig. 7 X-Ray powder di†raction spectra showing the transformation
of a polycrystalline sample of 2 (see top) left at 110 ¡C for one night to
give 1 (see bottom). Each XRPD trace is in the 5 \ 2h \ 30¡ range
(Cu-Ka); observed (top) and calculated (bottom) patterns. Starred
peaks arise from mutual sample contaminations.
10
2
6 4
Cccm (no. 66), a \ 8.157(1), b \ 18.634(2), c \ 11.613(1),
V \ 1765.1(3) A
Ž
3, Z \ 4, D \ 1.455 Mg m~3, Mo-Ka radi-
c
400
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4 F. A. Cotton and Y. Kim, J. Am. Chem. Soc., 1993, 113, 8511.
ation (j \ 0.71073 A), k \ 1.381 mm~1. Crystal data for 2:
5 C. Campana, K. R. Dunbar and X. Ouyang, Chem. Commun.,
1996, 2427.
C
H
CuF N OP, M \ 458.77, monoclinic, space group
14 10
6 4
P2 /c (no. 14), a \ 21.617(2), b \ 11.127(3), c \ 16.294(2) A,
6 A. F. Wells, T hree-dimensional Nets and Polyhedra, Wiley, New
York, 1977.
7 R. W. Gable, B. F. Hoskins and R. Robson, J. Chem. Soc., Chem.
Commun., 1990, 762.
8 B. F. Abrahams, B. F. Hoskins, D. M. Michail and R. Robson,
Nature (L ondon), 1994, 369, 727.
9 L. Shields, J. Chem. Soc., Faraday T rans., 1985, 81, 1. A rational-
ization of the topology of this species is given in ref. 1(c).
10 L. Carlucci, G. Ciani, D. M. Proserpio and A. Sironi, Angew.
Chem., Int. Ed. Engl., 1996, 35, 1088.
11 M. Munakata, G. L. Ning, T. Kuroda-Sowa, M. Maekawa, Y.
Suenaga and T. Horino, Inorg. Chem., 1998, 37, 5651.
12 We have rationalized the topology of this binodal network, that
has a symbol (426282)(4864). This is the same topology of the
““honeycombÏÏ frame described in ref. 16, and of a polymorph of
1
b \ 111.61(2)¡, V \ 3643.8(11) A
Ž
3, Z \ 8, D \ 1.673 Mg
c
m~3, Mo-Ka radiation (j \ 0.71073 A), k \ 1.356 mm~1. The
data collections were performed at 243 K, within the limits
2 \ h \ 28¡ for 1 and at 293 K within the limits 2 \ h \ 26¡
for 2. The structures were solved by direct methods (SIR97)19
and reÐned by full-matrix least-squares (SHELX-97)20 against
F2 . In both cases the anions were found disordered and a
o
suitable doubled model [F atoms with 83È17% occupancy (1)
and 50% (2)] was reÐned. Final residual peaks do not exceed
0.795 e A
reported for
CH Cl Èethanol. Three other single crystal analyses, based on
Ž
~3 for 1 and 0.440 e A
Ž
~3 for 2. All the results here
a sample obtained from
1
refer to
2
2
data collected at room temperature on samples obtained from
di†erent solvent systems, gave similar results with greater
residual peaks around 2/m special positions which were not
easily rationalized, indicative of disordered solvent molecules.
SiO called moganite (see M. OÏKee†e and B. G. Hyde, Crystal
2
structures. I. Patterns and symmetry, Mineralogical Society of
America, Washington, DC, 1996).
13 L. Carlucci, G. Ciani and D. M. Proserpio, unpublished work.
14 Obtained by using the PLATON program: A. L. Spek, PLATON,
A Multipurpose Crystallographic Tool, Utrecht University, 1998.
15 See for example, T. Wagner, U. Eigendorf, G. E. Herberich and U.
Englert, Struct. Chem., 1994, 5, 233 and refs. therein.
16 B. F. Abrahams, M. J. Hardie, B. F. Hoskins, R. Robson and G. A.
Williams, J. Am. Chem. Soc., 1992, 114, 10641.
17 A photo-induced solid state diacetylene polymerization has been
The Ðnal agreement indexes R were 0.0614 for 735 indepen-
1
dent signiÐcant [F [ 4p(F )] absorption corrected data for 1
o
o
and 0.0647 for 3365 independent signiÐcant [F [ 4p(F )]
o
o
absorption corrected data for 2. Anisotropic thermal param-
eters were assigned to all the non-hydrogen atoms, but to the
atoms of the minor component of the disordered anion in 1.
All the drawings were produced with SCHAKAL 97.21
CCDC reference number 440/103.
observed in the networked polymer [Cd(CN) L] [L \ 1,4-bis(4-
2
pyridyl)butadiyne)], but the photo-product has not been characc-
terized: B. F. Abrahams, M. J. Hardie, B. F. Hoskins, R. Robson
and E. E. Sutherland, J. Chem. Soc., Chem. Commun., 1994, 1049.
18 G. J. Kubas, B. Nozyk and A. L. Crumbliss, Inorg. Synth., 1979, 2,
90.
19 A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M. C.
Burla, G. Polidori and M. Camalli, J. Appl. Crystallogr., 1994, 27,
435.
20 G. M. Sheldrick, SHELX-97, Program for structure reÐnement,
University of Gottingen, 1997.
21 E. Keller, SCHAKAL 97, A computer program for the graphical
representation of crystallographic models, University of Freiburg,
1997.
Notes and references
1 (a) B. F. Hoskins and R. Robson, J. Am. Chem. Soc., 1990, 112,
1546; (b) R. Robson, B. F. Abrahams, S. R. Batten, R. W. Gable,
B. F. Hoskins and J. Liu, Supramolecular Architecture, ACS pub-
lications, Washington DC, 1992, ch. 19; (c) S. R. Batten and R.
Robson, Angew. Chem., Int. Ed., 1998, 37, 1461.
2 For some recent reviews see: M. J. Zaworotko, Chem. Soc. Rev.,
1994, 283; C. L. Bowes and G. A. Ozin, Adv. Mater., 1996, 8, 13;
O. M. Yaghi, H. Li, C. Davis, D. Richardson and T. L. Groy, Acc.
Chem. Res., 1998, 31, 474.
3 See for example, K. R. Dunbar, Angew. Chem., Int. Ed. Engl., 1996,
35, 1619 and refs. therein.
Paper 8/09144G
New J. Chem., 1999, 23, 397È401
401