New multidentate pyrazolyl-pyridine ligands - Synthesis and structures

Article abstract of DOI:10.1071/CH02251

Five new multidentate ligands have been prepared containing N,N-bidentate pyrazolyl-pyridine units linked to a central aromatic spacer unit. The ligands 3,3′-bis(3-{pyridin-2-yl}-pyrazol-1-yl-methyl)-p-terphenyl (I), 4,4′-bis(3-{pyridin-2-yl}-pyrazol-1-yl

Full text of DOI:10.1071/CH02251

CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2003, 56, 665–670
New Multidentate Pyrazolyl–Pyridine Ligands—Synthesis and Structures
Zöe R. Bell,A Jon A. McClevertyA and Michael D. WardA,B
A School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, UK.
B Author to whom correspondence should be addressed (e-mail: mike.ward@bristol.ac.uk).
Five new multidentate ligands have been prepared containing N,N -bidentate pyrazolyl–pyridine units linked to a
central aromatic spacer unit.The ligands 3,3ꢀ-bis(3-{pyridin-2-yl}-pyrazol-1-yl-methyl)-p-terphenyl (1), 4,4ꢀ-bis(3-
{pyridin-2-yl}-pyrazol-1-yl)biphenyl (2), and 1,8-bis(3-{pyridin-2-yl}-pyrazol-1-yl-methyl)naphthalene (3) have
two bidentate arms and are therefore potentially tetradentate; 2,6-bis(3-{pyridin-2-yl}-pyrazol-1-yl-methyl)pyridine
(4) has two bidentate arms with an additional nitrogen-donor in the aromatic spacer unit (a pyridyl group) and is
therefore potentially pentadentate; and 1,3,5-trimethyl-2,4,6-tris(3-{pyridin-2-yl}-pyrazol-1-yl-methyl)benzene (5)
has three bidentate arms and is therefore potentially hexadentate. The X-ray crystal structures of all of these ligands
have been determined.
Manuscript received: 4 December 2002.
Final version: 21 January 2003.
Introduction
and X-ray crystal structures of five new ligands based on
pyrazolyl–pyridine donor units.
Theself-assemblyofelaboratemetallo-supramoleculararchi-
tectures such as cages, helicates, grids, boxes, ladders, and
rings from labile metal ions and simple multidentate hetero-
nuclear ligands is becoming one of the most studied areas in
supramolecular chemistry.[1–6] It has been shown that such
architectures can be formed through the control of metal–
ligand interactions, and that depending upon the preference
of the metal ion for a specific coordination geometry, and the
arrangement of donor sites and the flexibility of the ligand,
it is possible to obtain—with a substantial degree of control
in some cases—a variety of one-, two-, or three-dimensional
architectures.[7] In the particular case of complexes which
contain two- or three-dimensional cavities, there is also the
possibility of templating effects associated with a central
guest species, usually a counter-ion.[4,8]
The ligands most commonly used in the above examples
of self-assembly are based on N-donor heterocycles. Our
work has focussed in particular on ligands which contain two
bidentate pyrazolyl–pyridine units separated by an aromatic
spacer; these are exemplified by ligands LA–LC in Scheme 1
which have all been used for assembly of adamantoid M4L6
tetrahedral cage complexes which incorporate anions in their
central cavities.[4,9,10] Related ligands, in which two or more
pyrazolyl-based donors are pendant from a central spacer,
have also been used in the self-assembly of some elaborate
cage-like complexes by numerous groups,[11–17] in particular
by Steel and coworkers.[13–17]
Results and Discussion
The new ligands, (1)–(5) in Scheme 1, are prepared by a
previously described method, involving the reaction of 3-(2-
pyridyl)pyrazolewithahalogenatedaromaticcoreinthepres-
ence of hydroxide ion under phase-transfer conditions.[4,18]
This is a simple and relatively general method that is only
limited by the availability of suitable halogenated aromatic
units to use as the central linker. Ligands (1)–(4) contain two
bidentatepyrazolyl–pyridineunits;withinthisgroupthereare
variations in the flexibility of the ligand, ligand (2); the length
of the ligand due to the variation in the length of the spacer
unit, ligand (1); and in the addition of a donor ligand within
the central core, ligand (4). Ligand (5), in contrast, is a hexa-
dentate tripodal ligand with three bidentate arms linked to
the central aromatic spacer through methylene groups.
Ligands (1)–(5) have been fully characterized by standard
techniques (see Experimental section) and the X-ray crystal
structures have been determined. The molecular structures
are shown in Figures 1–5, respectively; hydrogen atoms are
omitted for clarity. In each case the pyrazolyl–pyridine units
are arranged in an approximately transoid manner such that
the adjacent lone pairs on the pyridyl and pyrazolyl rings
avoid each other.
The packing diagram of (1) reveals that there are two
distinct types of herringbone-like packing, a result of there
being two independent half molecules in the asymmetric unit.
Figure 6 shows a packing diagram for one of the independent
molecules, withdottedlinesindicatinga‘T-stacking’arrange-
ment of the molecules with a separation of 3.39 Å between
Given the popularity of ligands of this nature for stud-
ies in self-assembly, we have been interested in synthesizing
further examples to study their coordination behaviour. We
accordingly describe herein the synthesis, characterization,
© CSIRO 2003
10.1071/CH02251
0004-9425/03/070665

666
Z. R. Bell, J. A. McCleverty and M. D. Ward
X
N
N
N
N
N
N
LA X = 3,3"-p-biphenyl
LB X = 1,2-phenyl
LC X = 2,3-naphthyl
N
N
N
N
Fig. 1. Molecular structure of (1). Only one component of the dis-
ordered central phenyl ring is shown. There are two independent
half-molecules in the asymmetric unit of which only one (grown to
generate the whole molecule) is shown.
(1)
N
N
N
N
N
N
N
N
(2)
(3)
N
N
N
N
Fig. 2. Molecular structure of (2). The molecule lies on an inversion
centre with only a half molecule in each asymmetric unit.
N
N
N
N
N
N
N
N
N
(4)
N
N
N
N
N
Fig. 3. Molecular structure of (3).
N
N
N
N
(5)
Scheme 1. Ligands (1)–(5).
H5 of a pyrazole ring and the centre of the face of a phenyl
ring from the terphenyl unit on the adjacent molecule. The
terphenyl unit is not coplanar, with the central phenyl ring
being twisted by 29.4 or 36.7◦ (depending on which disorder
component is considered) from the plane of the two adja-
cent rings. The bidentate pyrazolyl–pyridine unit is also not
coplanar, with a twist between the two rings of 4.1◦. Figure 7
shows a packing diagram for the second independent mole-
cule in the structure. Although it seems very similar to that
illustrated in Figure 6, there are subtle differences that are
in part due to the degree of disorder of the central phenyl
ring in this molecule. The dotted lines indicate an inter-
action similar to those seen in the first molecule, but the
H· · · π separation here is slightly longer at 3.52 Å. The twist
of the central phenyl ring of the terphenyl spacer with respect
Fig. 4. Molecular structure of (4).
Fig. 5. Molecular structure of (5).

New Multidentate Pyrazolyl-Pyridine Ligands
667
to the other two is either 27.8 or 59.2◦, again depending
on which disordered component is considered. The larger
twist in the molecule generates the slightly different pack-
ing styles; adjacent molecules in the stack are slightly further
apart, hence the longer C–H· · · π interaction. The bidentate
pyrazolyl–pyridine units of this molecule have a twist angle of
7.5◦ between the rings. The chelating pyrazolyl–pyridine
units of both molecules are nearly perpendicular to the exter-
nalphenylringsofthecentralspacer, theanglesbetweenthem
being 86.1 and 88.0◦ for the two independent molecules (1)
and (2), respectively.
between the pyrazolyl and pyridine rings being 3.4◦.The cen-
tral biphenyl unit is approximately planar; the angle between
the mean planes of the pyrazolyl–pyridine and biphenyl units
is 17.9◦.
Ligand (3) (Fig. 9) crystallizes in the chiral space group
P21. The molecule itself has a chiral, twisted conformation,
with the angle between both chelating units and the plane
of the naphthalene spacer unit being approximately 86◦. The
pyrazolyl–pyridine units themselves are not coplanar, with
The packing of ligand (2) revealed extensive π–π stack-
ing interactions (Fig. 8) between adjacent layers of ligands.
The distance of this interaction is approximately 3.68 Å, and
involves overlapping pyridine and pyrazole rings. There is
also a weaker interaction (3.87 Å) between overlapping pyra-
zole rings of adjacent molecules (not shown in the Figure).
The chelating units are again almost coplanar, with the angle
Fig. 7. Packing diagram showing the herringbone motif for the second
independent molecule of (1) (both disorder components of the central
phenyl ring are shown).
Fig. 6. Packing diagram showing the herringbone motif for one of the
independent molecules of (1) (both disorder components of the central
phenyl ring are shown).
Fig. 8. Packing diagram of (2).

668
Z. R. Bell, J. A. McCleverty and M. D. Ward
Fig. 9. Packing diagram of (3).
internal twist angles of 5.6 and 5.8◦. There is an exten-
sive array of π–π interactions between the pyrazole ring of
one molecule and the pyridine ring of an adjacent molecule
(Fig. 9). The average distance between the stacked units is
3.42 Å, the interactions being indicated with a dotted line in
Figure 9.
The packing structure of (4) (Fig. 10) shows an array
of weak C–H· · · N hydrogen-bonding interactions between
adjacent molecules (shown in Figure 10 with dotted lines).
There are three distinct types of interaction: (a) between
the H5 of a pyrazole ring and the nitrogen atom of the
adjacent pyridine ring from the chelating unit (N· · · C dis-
tance 3.52 Å); (b) between the nitrogen of the pyridine ring
and a hydrogen atom from a methylene unit on an adjacent
molecule (N· · · C distance 3.68 Å); and (c) between a hydro-
gen atom from a methylene unit and the nitrogen atom of
a pyrazole ring on an adjacent molecule (N· · · C distance
3.56 Å). As is clearly indicated in Figure 10, this results in a
quadruplearrayofweakhydrogen-bondsconnectingadjacent
molecules.
Thepyrazolyl–pyridineunitsoftheligandarenotcoplanar,
with angles between pyrazolyl and pyridine rings in the two
‘arms’ being 9.0 and 11.5◦. The angles between the mean
planes of these ‘arms’and the pyridyl ring are 85.1 and 59.8◦,
respectively.
The hexadentate ligand (5) shows no significant π–π, C–
H· · · π, or C–H· · · N interactions in the packing arrangement,
although, as for the other structures, the bidentate units are
not exactly internally coplanar and are substantially twisted
with respect to the plane of the central phenyl ring due to the
effects of the methylene groups.
Fig. 10. Packing diagram of (4).
(where X = perchlorate or tetrafluoroborate), in which each
of the 18 bridging ligands spans an edge of a truncated-
tetrahedral M12 array. We expect the remaining new ligands
to have comparably fruitful coordination behaviour, which
will be discussed in due course.
Conclusions
This paper reports the syntheses and structures of five new
multidentate ligands (1)–(5). The phase-transfer reaction is
high yielding and has proved to be a simple and efficient
way of synthesizing multidentate nitrogen-donor ligands,
and appears to be limited only by the availability of the
halogenated aromatic spacers.
Experimental
General Details
Proton NMR spectra were recorded on Jeol GX-270 or Lambda 300
spectrometers, assignments are shown in Scheme 2. Electron impact
(EI) mass spectra were measured on aVG-Autospec instrument. Organic
reagents were of the highest commercially available grade and were used
as received.
Ligand (1)
1,4-Dibromo-2,5-diiodobenzene[19] was converted into 3,3ꢀꢀ-dimethyl-
p-terphenyl,[20] which was subsequently brominated to yield 3,3ꢀ-
bis(bromomethyl)-[1,1ꢀ,4ꢀ,1ꢀꢀ]terphenyl, as described below.
3,3ꢀ-Bis(bromomethyl)-[1,1ꢀ,4ꢀ,1ꢀꢀ]terphenyl.
3,3ꢀꢀ-Dimethyl-p-
The coordination chemistry of ligand (3) has been dis-
cussed recently;[18] this ligand reacts with Coii and Znii to
generate dodecanuclear cage complexes [M12(µ-(3))18]X24
terphenyl (1.00 g, 3.9 mmol) and N-bromosuccinimide (1.73 g,
9.7 mmol) were added under an atmosphere of nitrogen to 20 mL of
vigorously stirred carbon tetrachloride. To the resulting suspension

New Multidentate Pyrazolyl-Pyridine Ligands
669
H1
H
H5
CH2Cl2/hexane) to yield 1,8-bis(bromomethyl)naphthalene as a crys-
talline solid after the removal of solvent (1.55 g, 71%). δH (270 MHz;
CDCl3) 5.08 (4 H, s, CH2), 7.74 (2 H, d, J 8.0 Hz, H2), 7.43 (2 H, t,
J 7.0 Hz, H3), 7.18 (2 H,d, J 6.8 Hz, H4). EI mass spectrums m/z 314
(M+). (Found: C, 45.6; H, 3.2%. Br2C12H10 requires C, 45.9; H, 3.2%.)
1,8-Bis(bromomethyl)naphthalene and 3-(2-pyridyl)pyrazole (2.1
equivalents) were reacted in the same manner used in the synthe-
sis of (1). The crude solid was purified by recrystallization from
dichloromethane/hexane to yield the desired product as an off-white
crystalline solid (0.64 g, 44%). X-ray quality crystals were grown by the
slow diffusion of hexane in to a solution of (3) in CH2Cl2. δH (300 MHz;
CDCl3) 5.87 (4 H, s, CH2), 8.56 (2 H, d, J 5.6 Hz, py6), 7.18 (2 H, t, J
5.6 Hz, py5), 7.45 (2 H, t, J 8.6 Hz, py4), 7.23 (2 H, d, J 7.5 Hz, py3),
7.07 (2 H, d, J 3.8 Hz, pz5), 6.80 (2 H, d, J 3.8 Hz, pz4), 7.90 (2 H, d, J
7.5 Hz, H7), 7.70 (2 H, td, J 1.9, 7.3 Hz, H6), 7.99 (2 H, d, J 7.5 Hz, H5).
EI mass spectrum: m/z 442 (M+). (Found: C, 76.3; H, 4.9; N, 19.3%.
C28H22N6 requires C, 76.0; H, 5.0; N, 19.0%.)
H6
H2
for (1)
pz H5
N
H3
H2
H pz4
H py3
H py4
H py5
N
N
for (2)
H py6
all ligands
H5
H4
N
H6
H7
H3
Ligand (4)
for (4)
for (3)
2,6-Bis(bromomethyl)pyridine and 3-(2-pyridyl)pyrazole (2.1 equiva-
lents) were reacted in the same manner used for the synthesis of (1).
The crude product was purified by column chromatography (alumina
III, 33 : 1 CH2Cl2/methanol), giving the desired product as a pale yellow
crystalline solid (0.51 g, 69%) following the removal of solvent. X-ray
quality crystals were grown by the slow diffusion of hexane into a solu-
tion of (4) in CH2Cl2. δH (300 MHz; CDCl3) 5.51 (4 H, s, CH2), 8.64
(2 H, d, J 8.3 Hz, py6), 7.38 (2 H, ddd, J 1.4, 5.2, 7.5 Hz, py5), 7.92
(2 H, td, J 2.1, 8.3 Hz, py4), 7.54 (2 H, d, J 7.5 Hz, py3), 7.58 (2 H, d,
J 2.8 Hz, pz5), 7.88 (2 H, d, J 8.3 Hz, central pyridine H3), 6.90–6.98
(3 H, m, pz4, central pyridine H4). EI mass spectrum m/z 393 (M+).
(Found: C, 70.2; H, 4.4; N, 24.8%. C23H19N7 requires C, 70.2; H, 4.9;
N, 24.9%.)
Scheme 2. Proton NMR spectroscopic assignments for ligands
(1)–(5).
2,2ꢀ-azobis-(2-methylpropionitrile) (0.05 g) was added and the reaction
mixture was heated to reflux with stirring for 4 h, after which time the
reaction mixture was allowed to cool and was filtered.The filter cake was
washed with carbon tetrachloride and the solvent was removed under
vacuum to yield a yellow oil, which was recrystallized from diethyl ether
to form a white crystalline solid (1.24 g, 76%).
Ligand (1) was then synthesized by further reaction with 3-(2-
pyridyl)pyrazole.[13] A mixture of 3,3ꢀ-bis(bromomethyl)-[1,1ꢀ,4ꢀ,1ꢀꢀ]-
terphenyl (0.87 g, 2.1 mmol), 3-(2-pyridyl)pyrazole (0.64 g, 4.4 mmol),
toluene (20 mL), [Bun4N][OH] (0.10 mL of a 40% aqueous solution),
and aqueous NaOH (1.49 g dissolved in 3.50 mL of water) were heated
to 85◦C for 24 h with vigorous stirring. After cooling, the organic phase
was separated, washed with water, and dried (MgSO4). Removal of the
solvent afforded a pale yellow oil, which, after recrystallization from
dichloromethane/hexane (1 : 2), yielded the desired product as an off-
white crystalline solid (0.74 g, 65%). X-ray quality crystals were grown
by slow diffusion of acetone into a solution of (1) in chloroform. δH
(300 MHz; CDCl3) 5.41 (4 H, s, CH2), 8.61 (2 H, d, J 5.8 Hz, py6),
6.49 (2 H, dt, J 2.3, 9.2 Hz, py4), 6.14 (2 H, td, J 2.3, 9.2 Hz, py3), 7.29
(2 H, d, J 1.3 Hz, pz5), 6.85 (2 H, d, J 1.3 Hz, pz4), 6.15 (4 H, s, phenyl
H1), 7.02 (4 H, ddd, J 1.2, 5.8, 9.2, Hz, phenyl H6), 7.39–7.55 (6 H, m,
py5, phenyl H2, H4, H5). EI mass spectrums m/z 544 (M+). (Found: C,
79.2; H, 5.3; N, 15.2%. C36H28N6 requires: C, 79.4; H, 5.2; N, 15.4%.)
Ligand (5)
2,4,6-Tris(bromomethyl)mesitylene and 3-(2-pyridyl)pyrazole (3.2
equivalents) were reacted in the same manner used for the synthesis
of (1). The crude product was purified by washing with diethyl ether
to leave a white crystalline solid (0.56 g, 76%). X-ray quality crystals
were grown by the slow diffusion of pentane into a solution of (5) in
CH2Cl2. δH (300 MHz; CDCl3) 2.45 (9 H, s, CH3), 5.55 (6 H, s, CH2),
8.63 (3 H, d, J 6.0 Hz, py6), 7.20 (3 H, t, J 5.3 Hz, py5), 7.72 (3 H, td,
J 2.1, 8.6 Hz, py4), 7.92 (3 H, d, J 8.6 Hz, py3), 7.08 (3 H, d, J 2.1 Hz,
pz5), 6.83 (3 H, d, J 2.1 Hz, pz4). EI mass spectrum m/z 591 (M+).
(Found: C, 73.3; H, 5.5; N, 21.1%. C36H33N9 requires C, 73.1; H, 5.6;
N, 21.3%.)
Crystal Structure Determinations
Ligand (2)
Suitable crystals were quickly transferred from the mother liquor to a
stream of cold N2 (173 K) on a Bruker SMART-CCD diffractometer. In
all cases a hemisphere of data was collected at 173 K using graphite-
monochromatized MoKα radiation. All of the structures studied were
solved by direct methods. Ligand (1) crystallizes with two independent
half molecules in the asymmetric unit. Ligands (1) and (2) have a crys-
tallographic centre of inversion. The structural determination of (1) was
complicated by disorder of the central phenyl ring.The disorder has been
successfully modelled in two parts with 50% site occupancy for each
part. The structural determinations for (2)–(5) were straightforward.
All hydrogen atoms were constrained to ideal geometries and
assigned isotropic displacement parameters 1.2 times that of their adja-
cent carbon atom. Data collection: SMART; cell refinement: SAINT;
data reduction, program(s) used to solve structure and program(s) used
to refine structure: SHELXTL.[21] Crystal data and details of the data
collection and processing for each crystal are given in Table 1. CIF
files have been deposited in the Cambridge Crystallographic Data
Centre (CCDC), nos. 186825–186829 for (1)–(5), respectively. Copies
of the data can be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44-(0)1223-336033 or
email:deposit@ccdc.cam.ac.uk.
Preparation of (2) was based on a published method.[17] 4,4ꢀ-
Dibromobiphenyl (0.50 g, 1.6 mmol), 3-(2-pyridyl)pyrazole (2.32 g,
16 mmol), CuI (0.790 g, 4.1 mmol), and K2CO3 (1.766 g, 12.8 mmol)
in nitrobenzene (10 mL) were heated to reflux with stirring for 3 h.
The solvent was removed under vacuum to yield a dark green residue
which was purified by column chromatography (AluminaV, 99 : 0.5 : 0.5
CH2Cl2/hexane/pyridine). The desired product was obtained as a pale
yellow crystalline solid after the removal of solvent (0.41 g, 58%). X-ray
quality crystals were grown from slow evaporation of a saturated solu-
tion of (2) in dimethylformamide (DMF). δH (270 MHz; CDCl3) 8.57
(2 H, d, J 6.4 Hz, py6), 7.25 (2 H, t, J 4.6 Hz, py5), 8.11 (2 H, dd, J 2.2,
7.6 Hz, py4), 7.76 (2 H, d, J 8.6 Hz, py3), 8.01 (2 H, d, J1.6 Hz, pz5),
7.10 (2 H, d, J 1.7 Hz, pz4), 7.69 (4 H, d, J 2.2 Hz, phenyl H3), 7.83
(4 H, d, J 2.2, phenyl H2). EI mass spectrums m/z 440 (M+). (Found: C,
76.2; H, 4.6; N, 19.0%. C28H20N6 requires C, 76.4; H, 4.6; N, 19.1%.)
Ligand (3)
1,8-Dimethylnaphthalene was brominated using the method described
in the synthesis of 3,3ꢀ-bis(bromomethyl)-[1,1ꢀ,4ꢀ,1ꢀꢀ]terphenyl (above).
The crude solid was purified by column chromatography (silica, 1 : 1

670
Z. R. Bell, J. A. McCleverty and M. D. Ward
Table 1. X-ray crystal data collection and refinement details for (1)–(5)
Parameter
(1)
(2)
(3)
(4)
(5)
Formula
M
C36H28N6
544.64
C28H20N6
440.50
C28H22N6
442.52
C23H19N7
393.45
C36H33N9
591.71
Colour
colourless
monoclinic
P21/c
36.750 (18)
5.611 (3)
13.618 (7)
100.65 (4)
2760 (2)
4
colourless
monoclinic
P21/c
9.746 (2)
8.760 (2)
12.528 (3)
93.777 (4)
1067.2 (4)
2
colourless
monoclinic
P21
14.753 (4)
4.9545 (13)
14.853 (3)
98.00 (3)
1075.1 (5)
2
colourless
monoclinic
C2/c
28.01 (2)
6.702 (3)
21.686 (11)
107.28 (4)
3888 (4)
8
colourless
monoclinic
P21/c
11.700 (3)
23.730 (5)
10.977 (4)
95.61 (3)
3033.1 (14)
4
Crystal system
Space group
a (Å)
b (Å)
c (Å)
β (deg)
U (Å3)
Z
Dcalc (mg m−3
)
1.311
1.371
1.367
1.344
1.296
Crystal size (mm3)
Reflections collected:
total, independent, Rint
θ range for data (deg)
Data, restraints,
0.5 × 0.3 × 0.2
0.3 × 0.2 × 0.1
0.5 × 0.2 × 0.2
0.5 × 0.2 × 0.1
0.35 × 0.2 × 0.05
17872, 6311, 0.0431 4972, 1847, 0.0652 6948, 4667, 0.03189 12093, 4444, 0.0802 16176, 5337, 0.0806
1.52–27.54
6311, 0, 377
2.09–25.00
1847, 0, 154
1.39–24.47
4667, 1, 307
1.97–27.59
4444, 0, 271
1.72–25.00
5337, 0, 406
parameters
Final R1, wRA2
0.0523, 0.1239
0.0411, 0.0778
0.0458, 0.0850
0.0494, 0.0912
0.0627, 0.1499
A Structure was refined on Fo2 using all data; the value of R1 is given for comparison with older refinements based on Fo with a typical threshold
of F ≥ 4σ(F). The value of wR2 is based on all data.
Acknowledgment
[10] R. L. Paul, S. M. Couchman, J. C. Jeffery, J. A. McCleverty,
Z. R. Reeves, M. D. Ward, J. Chem. Soc., Dalton Trans. 2000,
845.
Financial support was provided by the EPSRC (UK).
[11] M. Loi, M. W. Hosseini, A. Jouaiti, A. De Cian, J. Fischer, Eur.
J. Inorg. Chem. 1999, 1981.
[12] W.-Y. Sun, J. Fan, T. Okamura, J. Xie, K.-B. Yu, N. Ueyama,
Chem. Eur. J. 2001, 40, 2557.
[13] D. A. McMorran, S. Pfadenhauer, P. J. Steel, Inorg. Chem.
Commun. 2002, 5, 449.
[14] C. M. Hartshorn, P. J. Steel, Inorg. Chem. Commun. 2000, 3, 476.
[15] C. M. Hartshorn, P. J. Steel, Chem. Commun. 1997, 541.
[16] D. A. McMorran, P. J. Steel, Chem. Commun. 2002, 2120.
[17] C. M. Hartshorn, P. J. Steel, Aust. J. Chem. 1995, 48, 1587.
[18] Z. R. Bell, J. C. Jeffery, J. A. McCleverty, M. D. Ward, Angew.
Chem. Int. Ed. 2002, 41, 2515.
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