Crystallisation of H3BTC, H3TPO or H2SDA with MII (M = Co, Mn or Zn) and 2,2′-bipyridyl: Design and control of co-ordination architecture, and magnetic properties (H3BTC = benzene-1,3,5-tricarboxylic acid, H3TPO = tris(4-carboxylphenyl)phosphine oxide, H2SDA = cis-stilbene-4,4′-dicarboxylic acid)

Article abstract of DOI:10.1039/a905332h

The hydrothermal reaction of benzene-1,3,5-tricarboxylic acid (H₃BTC) with M^II (M = Mn, Co or Zn), tris(4-carboxyphenyl)phosphine oxide (H₃TPO) or cis-stilbene-4,4′-dicarboxylic acid (H₂SDA) with Co^II and 2,2′-bipyridyl (BIPY) gave 1-D co-ordination networks formulated as: [M(HBTC)(BIPY)(H₂O)] (M = Mn 1, Co 2, or Zn 3; [Co₃(BTC)₂-(BIPY)₂(H₂O) ₆]·4H₂O 4, [Co₃(TPO)₂(BIPY)₂(H₂O) ₆]·xH₂O 5 and [Co(SDA)(BIPY)(H₂O)] 6. Structures 1 and 2 consist of double stranded chains of alternating HBTC dianions and dimeric units M^II-M^II linked by two μ-(1,1) bridging carboxylates. Magnetic properties of 1 and 2 indicate the presence of ferromagnetic exchange interactions within the dimers. Structures 4 and 5 consist of chains with a molecular ladder motif which are stacked creating large channels lined by hydrated cobalt(II) ions. The H₂O/D₂O solvent exchange in structure 5, studied by infrared spectroscopy and thermal gravimetric analysis-mass spectrometry, provides evidence for the porosity and zeolitic nature of the material. The Royal Society of Chemistry 1999.

Full text of DOI:10.1039/a905332h

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Crystallisation of H3BTC, H3TPO or H2SDA with MII (M ؍ Co,
Mn or Zn) and 2,2Ј-bipyridyl: design and control of co-ordination
architecture, and magnetic properties (H3BTC ؍ benzene-1,3,5-
tricarboxylic acid, H3TPO ؍ tris(4-carboxylphenyl)phosphine
oxide, H2SDA ؍ cis-stilbene-4,4Ј-dicarboxylic acid)†
M. John Plater,*a Mark R. St. J. Foreman,a Eugenio Coronado,*b Carlos J. Gómez-Garcíab
and Alexandra M. Z. Slawinc
a Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen, UK AB24 3UE.
E-mail: m.j.plater@abdn.ac.uk; Fax:Int. code ϩ44 1224 272921
b Department Quimica Inorganica, Universidad de Valencia, 46100 Burjassot, Spain.
E-mail: eugenio.coronado@uv.es; Fax: Int. code ϩ34 96 3864859
c Molecular Structure Laboratory, Department of Chemistry, University of St. Andrews,
St. Andrews, Fife, UK KY16 9ST. E-mail: amzs@st-andrews.ac.uk
Received 2nd July 1999, Accepted 5th October 1999
The hydrothermal reaction of benzene-1,3,5-tricarboxylic acid (H3BTC) with MII (M = Mn, Co or Zn), tris(4-carboxy-
phenyl)phosphine oxide (H3TPO) or cis-stilbene-4,4Ј-dicarboxylic acid (H2SDA) with CoII and 2,2Ј-bipyridyl (BIPY)
gave 1-D co-ordination networks formulated as: [M(HBTC)(BIPY)(H2O)] (M = Mn 1, Co 2, or Zn 3; [Co3(BTC)2-
(BIPY)2(H2O)6]ؒ4H2O 4, [Co3(TPO)2(BIPY)2(H2O)6]ؒxH2O 5 and [Co(SDA)(BIPY)(H2O)] 6. Structures 1 and 2
consist of double stranded chains of alternating HBTC dianions and dimeric units MII–MII linked by two µ-(1,1)
bridging carboxylates. Magnetic properties of 1 and 2 indicate the presence of ferromagnetic exchange inter-
actions within the dimers. Structures 4 and 5 consist of chains with a molecular ladder motif which are stacked
creating large channels lined by hydrated cobalt() ions. The H2O/D2O solvent exchange in structure 5, studied by
infrared spectroscopy and thermal gravimetric analysis–mass spectrometry, provides evidence for the porosity
and zeolitic nature of the material.
Introduction
The solid state assembly of open framework metal–organic co-
ordination polymers is attracting increasing attention owing to
the potential discovery of novel functional materials which are
expected to find applications in catalysis, non-linear optics,
magnetism, sensors and molecular recognition.1 Understanding
and controlling the factors which govern the assembly process
and lattice stability is critical to technological developement.
Such factors include the formation of 1-D, 2-D or 3-D co-
ordinative networks, the formation of interpenetrating lattices
versus the inclusion of solvent or guest molecules, lattice stabil-
ity to guest or solvent exchange or removal and re-absorp-
tion, counter ion mobility for ion exchange, pore volume and
accessibility of co-ordinatively unsaturated metal centres and
Fig. 1 Drawing of a possible 1-D infinite co-ordinative chain with the
assemblies which propagate magnetic interactions between
metal cations along the edges capped by a bis-donor ligand such as 2,2Ј-
paramagnetic metal ions.2 One approach to novel porous net-
bipyridyl.
works involves the reaction of benzene-1,3,5-tricarboxylic acid
(H3BTC) with alkaline earths (M = Ca, Sr or Ba)3 or transition
bipyridyl upon co-ordination architecture during the assembly
metal ions (M = Co, Zn or Ni) under hydrothermal conditions.4
of aromatic acids with metal ions (M = Co, Mn or Zn). It was
This gives co-ordination polymers containing the HBTC2Ϫ
predicted that this ligand would favour the formation of 1-D
dianion or the BTC trianion respectively and the hydrated
metal ion. Anion H2BTCϪ has not yet formed under these con-
ditions. In search of a further class of materials of this type we
have investigated the influence of the bidentate ligand 2,2Ј-
co-ordinative chains, such as the possible chain shown in Fig. 1,
by reducing the available metal-ion binding sites that might
have led to polymer growth in other directions. The chains
shown are desirable because they possess pockets that might
give the compound porous properties. The edge bipyridyl lig-
ands were expected to create pockets allowing adjacent chains
to interweave and associate by van der Waals forces or π
stacking.5
† Supplementary data available: rotatable 3-D crystal structure diagram
in CHIME format. See http://www.rsc.org/suppdata/dt/1999/4209/
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This journal is © The Royal Society of Chemistry 1999
4209

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The N–Mn–N angle of bound bipy is 71.9Њ. The monodentate
carboxylate oxygen (O5) is cis to both BIPY nitrogen atoms,
while the water oxygen is trans to one nitrogen. The undeproto-
nated carboxylic acid groups C20 point in opposite directions
approximately along the a axis. The chains are stabilised by one
intrachain and two interchain hydrogen bonds. The same
hydrogen bonding motif occurs in structures 1–3 and is shown in
Fig. 5. Each carboxylic acid acts as a hydrogen bond donor and
acceptor. Atom H3O of carboxylic acid group C20 forms an
interchain hydrogen bond (1.72 Å) to O2ٞ of the monodentate
carboxylate group C19; H10B of the water molecule O10 forms
an interchain hydrogen bond (1.82 Å) to O4Љ of carboxylic
group C20 completing a 10-membered ring. The remaining pro-
ton H10A of the water molecule O10 forms an intrachain
hydrogen bond (1.75 Å) to O6Ј of the bifurcated carboxylate
anion C21 completing a 6-membered ring. The interchain and
intrachain hydrogen bonding is clearly visible in the packing
diagram shown for compound 2 in Fig. 4 (bottom) and in
Fig. 5. The hydrogen bonds present are all quite strong since
they are shorter than the average length for a hydrogen bond
which is typically in the range 1.9–2.1 Å. Hydrogen bonding is
Results and discussion
In this paper we report the hydrothermal synthesis and
X-ray structural characterisation6 of crystalline solids pre-
pared from (i) H3BTC and acetates of MnII, CoII and ZnII
with 2,2Ј-bipyridyl, (ii) tris(4-carboxylphenyl)phosphine oxide
(H3TPO) and cis-stilbene-4,4Ј-dicarboxylic acid (H2SDA)
with cobalt() acetate and 2,2Ј-bipyridyl. Table 1 gives a
summary of the crystallographic data and Table 2 summarises
key hydrogen bond lengths and angles. Hydrothermal synthesis
of H3BTC with the acetate of MnII, CoII, or ZnII and 2,2Ј-
bipyridyl in the ratio of 1:1:1 gives solids 1, 2 and 3 respec-
tively which possess a similar asymmetric unit shown in
Fig. 2. The same numbering scheme has been used for each. All
3 compounds have the same molecular formula but each has
a different structure presumably as a consequence of the pre-
ferred metal ion co-ordination environment. As expected
each structure has an infinite co-ordinative network in one
dimension only. Networks 1 and 2 formulated as [M(HBTC)-
(BIPY)(H2O)] where M = Mn or Co respectively consist of
similar double stranded chains of alternating HBTC dianions
and dimeric units MII–MII linked by two bridging oxygens
coming from two carboxylate ligands (Figs. 3 and 4). The metal
ions each possess a distorted octahedral geometry and are co-
ordinated by one bipyridyl ligand, one monodentate carb-
oxylate ion, two bifurcated carboxylate anions and one water
molecule. For each HBTC dianion one carboxylate anion
oxygen is monodentate and the other carboxylate anion forms
a bifurcated bridge between the two metal ions. The chains of
M2O2 squares are bridged by two HBTC dianions forming 16
membered metallocyclic rings.
For structure 1 the Mn ؒ ؒ ؒ Mn internuclear distance is 3.5 Å
and the chains run parallel to the b axis (Fig. 3). The HBTC
anions are stacked in pairs about 3.35 Å apart which corre-
sponds to the van der Waals distance between them. The angle
O–Mn–O between the 2 bridging oxygen atoms in the Mn2O2
ring is 74.9Њ while the Mn–O–Mn angle in the ring is 105.1Њ.
Fig. 4 Top: ball and stick drawing of the double stranded chains in
compound 2. Bottom: Packing diagram illustrating the interchain
hydrogen bonding network. Selected bond lengths [Å] and angles [Њ]:
Co1 ؒ ؒ ؒ Co1 3.37, Co1–O10 2.062(2), Co1–N12 2.106(2), Co1–N1
2.140(2), Co1–O1 2.074(2), Co1–O5Ј 2.130(2) and Co1–O5Љ 2.171(2);
Co1ٞ–O5–Co1Љ 102.94(6), O1–Co1–O5Ј 164.53(6) and O1–Co1–O5Љ
87.93(6).
Fig. 2 Drawing of the asymmetric unit for structures 1–3.
Fig. 3 Ball and stick drawing of the double stranded chains in com-
pound 1. Selected bond lengths [Å] and angles [Њ]: Mn1 ؒ ؒ ؒ Mn1 3.5,
Mn1–O10 2.133(2), Mn1–N12 2.302(2), Mn1–N1 2.229(2), Mn1–O1
2.2172(12), Mn1–O1Љ 2.2003(12) and Mn1–O5Ј 2.1524(12); Mn1Љ–O1–
Mn1 105.11(5) and O1Љ–Mn1–O1 74.88(5).
Fig. 5 Drawing showing the interchain hydrogen bonding present
between co-ordinative chains in structures 1–3.
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Fig. 6 Packing diagram illustrating the single stranded chains and
interchain hydrogen bonding network in compound 3. Selected bond
lengths [Å]: Zn1–O1 1.962(2), Zn1–O10 2.112(2), Zn1–N1 2.164(3),
Zn1–N12 2.118(2) and Zn1–O5Ј 2.016(2).
probably an important factor in the supramolecular assembly
and stabilisation of the lattice.
For structure 2 the Co ؒ ؒ ؒ Co internuclear distance is 3.37 Å
and the chains run parallel to the c axis. The obvious variation
from octahedral geometry are angles N–Co–N (76.7Њ) and O5–
Co–O5 (77.1Њ) of the 4-membered ring. The Co–O–Co angle is
102.9Њ. The water oxygen (O10) is trans to one of the BIPY
nitrogen atoms while O2 is cis to both nitrogens. The chains are
stabilised by one intrachain and two interchain hydrogen bonds
analogous to the structure of 1 although the hydrogen bonds
exist between different atoms. Atom H3O of the carboxylic
acid group C20 forms an interchain hydrogen bond of length
1.62 Å to O6Ј of the bifurcated carboxylate anion C21; H10A
of the water molecule O10 forms an interchain hydrogen bond
(1.80 Å) to O4Љ of carboxylate group C20. The remaining
proton H10B of the water molecule O10 forms an intrachain
hydrogen bond (1.71 Å) to O2 of monodentate carboxylate
anion C19 completing a 6-membered ring (see Fig. 6).
Compound 3 formulated as [Zn(HBTC)(BIPY)(H2O)] con-
sists of single stranded chains of alternating HBTC dianions
and zinc() cations running parallel to the a axis (Fig. 6). The
cations are co-ordinated by two monodentate carboxylate
anions, one bipyridyl group and one water molecule, and adopt
a distorted square based pyramidal geometry. Owing to the
fact that ZnII has a d10 electron configuration and lacks crystal
field stabilisation energy, the co-ordination of ligands around
Zn is likely to be controlled by steric effects. The chains are
stabilised by three interchain hydrogen bonds. Atom H30 of
carboxylic acid group C20 forms a hydrogen bond (1.63 Å) to
O6ٞ of carboxylate anion C21. Proton H10C of the water
molecule O10 forms a hydrogen bond (1.84 Å) to O4Љ of
carboxylate anion C20 completing a 10-membered ring. This
can be seen in the packing diagram in Fig. 6. The other proton
of the water molecule H10B forms a hydrogen bond (1.99 Å)
to O5Ј of carboxylate anion C21 in an adjacent layer. This
hydrogen bond is longer and weaker than the others owing to
the co-ordination of O5Ј to a zinc() cation.
The phase shown in Fig. 1 would have a different stoichio-
metry to the above structures of composition M:HBTC:BIPY
(1:1:1). The synthesis was repeated using Co:H3BTC:BIPY
in a 1.5:1:1 ratio. Two different phases could be identified by
visual examination of the red crystalline material. By carefully
selecting similar looking red crystals a material of approximate
composition [Co3(BTC)2(BIPY)2(H2O)6]ؒxH2O 4 (x ≈ 2) was
identified by microanalysis. The single crystal structure was
determined which showed that the lattice motif described in the
introduction had been formed. Fig. 7 shows a drawing of the
asymmetric unit and the infinite double stranded chains which
run along the a axis in the ac plane. The lattice contains two
different cobalt() cations. Those down the centre connecting
the chains together are octahedral and are co-ordinated by
two trans monodentate carboxylate anions and four water
Fig. 7 Top: ball and stick drawing of the asymmetric unit in com-
pound 4. Bottom: drawing of a double stranded chain. Selected bond
lengths [Å]: Co1–O1 2.068(3), Co1–O10 2.096(3), Co1–N1 2.101(3),
Co1–N12 2.119(3), Co1–O5Ј 2.116(3), Co2–O3ٞ 2.052(3), Co2–O30ٞ
2.093(3) and Co2-O20ٞ 2.146(3).
molecules. Those along the edges are five-co-ordinated by two
monodentate carboxylate anions, one bipyridyl group and one
water molecule. The bipy nitrogens, Co and water oxygens
are approximately coplanar possessing a N12–N1–Co1–O10
torsion angle of Ϫ178.6Њ. The carboxylate oxygens are out of
this plane with torsion angles N12–N1–Co1–O1 of Ϫ76.5Њ and
N12–N1–Co1–O5 of 76.1Њ. While the common co-ordination
geometries for CoII are octahedral and tetrahedral, examples of
trigonal bipyramidal complexes are known. The structure
resembles a molecular ladder made up of edge sharing 32-
membered rhombohedra. The hydrogen atoms of water
molecules O10 and O30 could not be located. The hydro-
gen atoms H20 of water molecule O20 which is co-ordinated
to Co2 were located: H20A forms a hydrogen bond (1.75 Å)
to O2Ј of an adjacent double strand as does H20B but on
the opposite side. Two non-co-ordinated water molecules
per asymmetric unit were found in four different locations
indicative of the free volume created in the lattice by the co-
ordinative network. This amounts to the inclusion of four
disordered water molecules for each rhombohedra (x = 4 in 4).
The double strands are stacked in a slightly staggered arrange-
ment along the b axis forming an open framework lattice
with molecular channels. However, owing to the difficulty of
obtaining this material as a pure phase further studies on its
porosity were not carried out.
Owing to the success of crystallising networks from H3BTC,
divalent metal ions and 2,2Ј-bipyridyl it was of interest to in-
vestigate the crystallisation of networks from different aromatic
carboxylic acids. Two further acids were studied namely
H3TPO7 and H2SDA. Heating H3TPO with Co(OAc)2 and 2,2Ј-
bipyridyl in water gave a red crystalline material of formula
J. Chem. Soc., Dalton Trans., 1999, 4209–4216
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Fig. 9 Ball and stick drawing of the asymmetric unit in compound 6.
Selected bond lengths [Å]: Co1–O12 2.039(2), Co1–O1 2.085(2), Co1–
O29Ј 2.103(2), Co1–O28Ј 2.504(3), Co1–N1 2.147(2) and Co1–N12
2.137(2).
and 170 ЊC three overlapping endothermic mass losses totalling
9.6% occurred. The conversion of the octahydrate into an
anhydrous solid requires a mass loss of 10% which indicates the
presence of at least two non-co-ordinating water molecules.
The structural integrity of the porous framework is maintained
after room temperature desorption of the non-co-ordinating
water molecules, because powder diffraction patterns of the as
synthesized and evacuated material are sharp and identical. The
framework is probably able to maintain its shape or crystallinity
and support internal void volume because water molecules
have not been removed from the metal-ion co-ordination sphere
which can therefore maintain its geometry. Full crystallo-
graphic characterisation of the porous solvated and desolvated
(ϪEtOH) molecular framework co-ordination polymer
[Ni(bipy)1.5(NO3)2]ؒEtOH was also reported recently.8 In this
structure the EtOH molecules are not bound in the metal ion
co-ordination sphere. Standing a sample of undried material in
heavy water for 3 d showed that solvent exchange had occurred
as evidenced by the appearance of a broad O–D absorption at
2493 cmϪ1. The infrared spectrum was identical to that of a
sample prepared in D2O. The exchange of H2O for D2O was
also studied by TG-MS. This showed the presence of some
D2O, DHO and H2O indicating that complete exchange of all
the included H2O for D2O had not occurred. Non-co-ordinated
H2O would presumably exchange the fastest by diffusion
whereas co-ordinated H2O molecules would probably exchange
more slowly. The H2O/D2O exchange illustrates the porous
nature of the framework. Attempted H2O/D2O exchange on
unevacuated samples of the previously reported porous solids
[Ca(HBTC)(H2O)]ؒH2O3 and [Co3(BTC)2(H2O)12]4a was un-
successful which illustrates the improved porosity of the new
TPO solid.
The formation of co-ordination polymers from the ligand
H2SDA was of interest owing to its existence as cis and trans
isomers. Either isomer or a mixture of both might give
networks. Heating a commercially available sample, which
consisted of a mixture of cis and trans isomers, with Co(OAc)2
and 2,2Ј-bipyridyl in water gave a red crystalline material of
composition [Co(SDA)(BIPY)(H2O)] 6 which was admixed
with some unchanged ligand. This suggested that only one
isomer had formed a co-ordination polymer. The unchanged
ligand was removed by washing with ethanol. The crystal
structure consists of infinite single stranded chains of cis-SDA
running along the b axis (Fig. 9). The cobalt() cations are
co-ordinated by one monodentate carboxylate anion, one bi-
dentate carboxylate anion, one bipyridyl ligand and one water
molecule. One of the cis-SDA aromatic rings is conjugated to
the alkene group with a torsion angle of 3.6Њ (C17–C20–C21–
C22) while the other is twisted out of the plane of the alkene
π system at an angle of 90Њ (C20–C21–C22–C23). Atom
H1B of the water molecule O1 forms an intrachain bifurcated
hydrogen bond between O13Ј (2.10 Å) and O28Ј (2.22 Å); H1A
of the water molecule O1 forms an interchain hydrogen bond to
Fig. 8 Top: ball and stick drawing of the asymmetric unit in com-
pound 5. Bottom: drawing of a double stranded chain. Selected bond
lengths [Å]: Co1–O20 2.056(4), Co1–O1 2.095(5), Co1–O26Ј 2.094(5),
Co1–N1 2.114(5), Co1–N12 2.103(5), Co1–O27Ј 2.317(5), Co2–O34ٞ
2.054(6), Co2–O40ٞ 2.101(6) and Co2–O50ٞ 2.149(6).
[Co3(TPO)2(BIPY)2(H2O)6] 5 determined by microanalysis after
drying under vacuum. The single crystal structure consists of
infinite double stranded chains in one dimension which
resemble a molecular ladder (Fig. 8). In this respect it is similar
to the previously predicted BTC derived lattice found in com-
pound 4. There are two different cobalt cations in the network.
Those along the edges adopt a distorted octahedral environ-
ment and are co-ordinated by one monodentate carboxylate
anion, one bidentate carboxylate anion, one bipyridyl ligand
and one water molecule. In common with structures 1 and 2
the water (O1) is trans to one of the bipy nitrogens; O27 is
2.32 Å from the cobalt while the other carboxylate oxygens
are all within 2.1 Å. Angle O26–Co1–O27 is 59.9Њ and N1–
Co1–N12 is 77.6Њ. The physical constraints of assembling
the lattice probably force O27 to be removed from the ideal
octahedral position. The cobalt cations connecting the strands
are octahedrally co-ordinated by 4 water molecules and 2 trans
carboxylate anions. The hydrogen atoms of two of these waters
(O40) could not be located. Hydrogen H1B of the water O1
forms an intrachain hydrogen bond (1.68 Å) to O19 completing
a 6-membered ring; H1A of the water O1 forms an interchain
hydrogen bond (1.77 Å) to the oxygen O2Ј of the phosphine
oxide. Non-co-ordinating, disordered water molecules were
observed which were absent according to microanalytical
data. The sample used for microanalysis had been dried under
vacuum whereas that for crystallography was taken from water.
This suggests that the water is loosely bound and can escape
from the porous structure very easily. This is supported by
thermal gravimetric analysis of an as synthesized sample which
showed ready loss in mass immediately at 25 ЊC. Between 40
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Table 1 Summary of crystal data for compounds 1–6
1
2
3
4
5
6
Formula
Mr
T/K
C19H14MnN2O7
437.26
293
C19H14CoN2O7
441.25
293
C19H14N2O7Zn
447.69
293
C19H25Co1.5N2O11
545.80
293
C62H54Co3N4O21P2 C26H20CoN2O5
1429.82
293
499.37
293
Crystal system
Space group
Monoclinic
P21/n
Triclinic
P1
Triclinic
P1
Triclinic
P1
Triclinic
P1
Monoclinic
P21/a
¯
¯
¯
¯
a/Å
b/Å
c/Å
α/Њ
10.364(3)
10.504(10)
17.083(4)
9.560(5)
10.114(5)
10.836(5)
98.55(10)
111.59 (10)
104.92(10)
906
9.462(10)
9.482(2)
11.462(3)
111.95
93.091(10)
101.878(10)
923.58(3)
2
10.044(7)
10.341(7)
12.109(9)
74.719(2)
74.947(2)
79.909(2)
1163.98(14)
2
10.910(10)
11.437(11)
13.716(13)
110.791(2)
93.6520(10)
99.355(2)
1565.1(3)
1
11.238(10)
13.552(2)
14.579(3)
β/Њ
103.3090(10)
94.4970(10)
γ/Њ
V/Å3
1810
4
2213.45(6)
4
Z
2
Reflections collected
Independent reflections 4074
10478
4373
2596
4015
2627
7104
5190
9933
7016
9617
3190
Reflections observed
R(int)
Final R [I > 2σ(I)]
Final wR2
4024
2546
2577
5140
0.0950
0.0482
0.0938
7016
0.0358
0.0736
0.1997
3135
0.0306
0.0314
0.0670
0.0345
0.0284
0.0632
0.0399
0.0295
0.0767
0.0288
0.0327
0.0781
Table 2 Summary of hydrogen bond lengths and angles
Compound
O
H
X
H ؒ ؒ ؒ X/Å
O ؒ ؒ ؒ X/Å
O–H ؒ ؒ ؒ X/Њ
1 [Mn(HBTC)(BIPY)(H2O)]
O3
H30
O2ٞ
O4Љ
O6Ј
O6Ј
O4Љ
O2
O6ٞ
O4Љ
O5Ј
O2Ј
O50
O2Ј
O19
O12
O13Ј
O28Ј
1.72
1.82
1.75
1.62
1.80
1.71
1.63
1.84
1.99
1.75
1.92
1.77
1.68
1.90
2.10
2.22
2.66
2.72
2.61
2.59
2.78
2.63
2.58
2.77
2.84
2.71
2.75
2.74
2.64
1.90
2.10
2.22
173
173
164
169
172
156
164
156
144
165
165
176
169
175
120
130
O10
O10
O3
O10
O10
O3
O10
O10
O20
O20
O1
H10B
H10A
H30
H10A
H10B
H30
H10C
H10B
H20A
H20B
H1A
H1B
2 [Co(HBTC)(BIPY)(H2O)]
3 [Zn(HBTC)(BIPY)(H2O)]
4 [Co3(BTC)2(BIPY)2(H2O)6]ؒ4H2O
5 [Co3(TPO)2(BIPY)2(H2O)6]ؒxH2O
6 [Co(SDA)(BIPY)(H2O)]
O1
O1
O1
O1
H1A
H1B
H1B
to the square of the magnetic moment) vs. temperature. At
room temperature the χT value is ca. 4.7 emu K per manganese,
which is close to the value expected for magnetically isolated
manganese() centres possessing a spin sextuplet S = 5/2. Upon
cooling χT reveals a continuous increase with a tendency of
levelling below 5 K, reaching a value of 7.25 emu K molϪ1 at
1.8 K. Such a behaviour is characteristic of a ferromagnetic
coupling between two spin sextuplets, whereby the plateau at
lower temperature corresponds to the exclusive thermal popul-
ation of the S = 5 ground spin state. Therefore, the magnetic
data were conveniently analysed with the appropriate van Vleck
equation for the magnetic susceptibility of a high spin man-
ganese() dimer with the isotropic Heisenberg hamiltonian
Hex = Ϫ2JSAؒSB, where J is the magnetic coupling parameter. A
fitting of the data to this model leads to J = 0.90 cmϪ1 and g =
2.0; 9% of monomeric impurity had to be introduced. The
agreement is excellent in the whole temperature range (Fig. 10).
These results indicate that although the structure of this
compound consists of a double chain of dimers connected
through two HBTC bridging ligands, the exchange interaction
is restricted to the MnII–MnII pairs which are magnetically
coupled through two bridging oxygens from two carboxylate
groups. In this case the spin coupling through the HBTC
bridging ligands is quite negligible, in agreement with the fact
that these anions keep the manganese() ions at a distance as
large as 9 Å. An additional proof of this picture comes from
the low temperature magnetisation data. Thus, the isothermal
variation of the magnetisation at 2 and 5 K vs. the applied
magnetic field closely follows the behaviour expected for a total
Fig. 10 Thermal dependence of the product susceptibility times the
temperature (χT) for complex 1 (᭺) and the theoretical behaviour of
—
a ferromagnetic manganese() dimer ( ). The inset shows the plateau
of χT in the low temperature region.
O12 of a monodentate carboxylate anion in an adjacent layer.
The separation of the stilbene isomers illustrates an example
of hydrothermal synthesis in which a single diastereomer is
recognised and selectively crystallised from a mixture into a
co-ordination network. This might have practical applications
for some separations.
Magnetic properties
The magnetic properties of the manganese() compound 1 are
depicted in Fig. 10 with a plot of the product χT (proportional
J. Chem. Soc., Dalton Trans., 1999, 4209–4216
4213

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Fig. 11 Plot of the isothermal magnetisation M (per pair of manga-
nese() ions) vs. H/T at 2 and 5 K for complex 1. The Brillouin
function for a spin S = 5 with g = 2.0 is plotted as a solid line and
compared to the behaviour calculated for two non-interacting local spins
Fig. 13 Plot of the isothermal magnetisation M (per pair of cobalt()
ions) vs. H/T at 2 and 5 K for complex 2. The theoretical behaviour
calculated from the parameters obtained in the fit of the magnetic
susceptibility data is plotted as solid (2 K magnetisation) and dashed
(5 K magnetisation) lines.
—
SA = SB = 5/2 ( ).
has provided a satisfactory fitting of the magnetic behaviour
in the low temperature region (below 30 K). The following set
of parameters has been obtained: Jz = 2.1 cmϪ1, Jz/Jxy = 1 to 2,
gz = 7.9, gxy = 2.5 (inset in Fig. 12). As for other cobalt()
systems, the fitting procedure has not been an easy task due to
the large number of adjustable parameters. In particular, it has
shown to be largely insensitive to the amount of exchange
anisotropy as we can see in the figure (dashed and solid lines).
Still, this magnetic analysis has provided reliable information
on the sign and magnitude of the CoII–CoII exchange inter-
action. Other complementary techniques, for example specific
heat measurements, need to be employed in order precisely to
determine the amount of exchange anisotropy. Furthermore,
this analysis has indicated that these exchange interactions are
restricted, as in the manganese derivative, to the dimeric units
present in the chain. Interdimer antiferromagnetic interactions
may be also present, but they remain negligible since they only
affect the magnetic behaviour below 4 K. In fact, the com-
parison of the low temperature magnetisation data indicates
that at 2 K the magnetisation curve is only slightly below that at
5 K, suggesting that quite weak antiferromagnetic interdimer
interactions are operative at the lower temperature (Fig. 13).
The unusual presence of ferromagnetic coupling in these two
compounds deserves explanation. In the cobalt case the result is
not too surprising as the M–O–M angles within the dimeric
unit are equal to 103Њ, which is in the range in which the Co–Co
ferromagnetic pathways are dominant as a consequence of
an accidental orthogonality of the magnetic orbitals that are
pointing towards the bridging oxygen atoms. In fact, ferro-
magnetic couplings of the order of Jz = 10 cmϪ1 have also been
observed in tetranuclear Co4O16 clusters having M–O–M angles
between 90 and 100Њ.10 However, in the manganese derivative
this kind of interaction is extremely rare as for this and for
the other d5 ions (like high-spin FeIII) the five d orbitals are mag-
netically active and therefore some of the orbital pathways are
bound to be antiferromagnetic. For example, the manganese()
cluster which is isomorphous to the tetranuclear cobalt()
cluster exhibits weak antiferromagnetic pairwise interactions.11
The first precedent in which the net interaction is ferromagnetic
has been provided by complexes having as bridging ligands
two azido ions double bonded in an end-on (µ-(1,1)) fashion.
With the azido ligand a dinuclear manganese() complex12 as
well as two iron() dimers13,14 exhibiting weak ferromagnetic
couplings have been reported. In these complexes the angles at
the azido bridge are similar to that observed in our compound
(105 to 107Њ). Recently an isophthalate-bridged manganese()
chain complex which contains ferromagnetically coupled
manganese() dimers with double carboxylates in syn-syn con-
formation has also been reported.15 Nevertheless, to the best
of our knowledge, the manganese() complex reported in the
present work constitutes the first manganese() complex in
Fig. 12 Thermal dependence of the product susceptibility times the
temperature (χT) for complex 2 (᭺). In the inset the best fits of the
data in the low temperature region by an anisotropic exchange dimeric
model are plotted as solid (Jxy/Jz = 1) and dashed (Jxy/Jz = 2) lines.
spin S = 5 with g = 2.0, indicating that the ground spin state
of the system is the ferromagnetic one (Fig. 11). The curve lies
well above that for two uncoupled local spins S = 5/2 (dashed
line in Fig. 11). The fact that at two different temperatures
(2 and 5 K) the curves of M vs. H/T are coincidental, within the
experimental error, is conclusive evidence of the good magnetic
insulation of these ferromagnetic dimers.
The magnetic properties of the analogous cobalt() deriva-
tive 2 are depicted in Fig. 12. Upon cooling the χT product
shows a continuous decrease from 3 emu K molϪ1 at 300 K
to 2.5 emu K molϪ1 at 25 K where a rounded minimum is
observed. Below this temperature χT increases and reaches a
sharp maximum at ca. 4 K. This behaviour can be qualitatively
explained if we bear in mind that high-spin octahedral CoII
has an orbitally degenerate ground state 4T1. Thus, the decrease
in χT down to 25 K basically corresponds to a single-ion
4
behaviour. It accounts for the splitting of the T1 term into six
Kramers doublets as a consequence of the combined effect of
spin–orbit coupling and distortion from octahedral symmetry.9
However, the observed increase in χT below this temperature is
no longer coming from the single-ion behaviour but rather from
a ferromagnetic CoII–CoII exchange interaction. More precisely,
it reflects a ferromagnetic coupling between two low-lying
Kramers doublets since at this temperature this is the only sig-
nificantly populated cobalt() level. Therefore, the quantitative
analysis of these interactions can be conveniently described
by an anisotropic exchange Hamiltonian of the type (1). In
Hex = Ϫ2JzSz1Sz2 Ϫ 2Jxy(Sx1Sx2 ϩ Sy1Sy2)
(1)
this expression Si (i = 1 or 2) represents an effective spin 1/2
associated to the lowest Kramers doublet of the octahedral
CoII. The exchange anisotropy assumed by this model (with
Jz = Jxy) comes from the large anisotropy of this low-lying spin
doublet, as evidenced by EPR spectra. This anisotropic model
4214
J. Chem. Soc., Dalton Trans., 1999, 4209–4216

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which the ferromagnetic coupling is transmitted through a µ-
(1,1) bridging carboxylate. In this respect this compound
adopts the same co-ordination mode as the end-on azide
ligand, a mode that is unusual for the carboxylate ligand.
[{Co(HBTC)(BIPY)(H2O)}n] 2. The compounds H3BTC (100
mg, 476 µmol), Co(OAc)2ؒ4H2O (120 mg, 482 µmol), 2,2Ј-
bipyridyl (75 mg, 480 µmol) and water (10 ml) were placed in
a bomb then heated at 100 ЊC hϪ1 to 220 ЊC. After holding
at this temperature for 2 h the bomb was cooled at 5 ЊC hϪ1
to 180 ЊC. After 6 h at 180 ЊC the bomb was cooled at 5 ЊC hϪ1
to 100 ЊC and then at 6 ЊC hϪ1 to 20 ЊC. Compound 2 was
collected by filtration, washed with water and air dried to
give red crystals (186 mg, 89%). Found: C, 51.8(51.6); H,
3.2(3.0); N, 6.2(6.3). [Co(HBTC)(BIPY)(H2O)] = C19H14Co-
N2O7 requires C, 51.7; H, 3.2; N, 6.4%. IR: 3397s, 3103m,
3092m, 3031w, 1695vs, 1655w, 1615s, 1606s, 1579s, 1564s,
1493w, 1475m, 1442m, 1411m, 1371s, 1337s, 1317m, 1246m,
1231s, 1179m, 1158m, 1100m, 1063w, 1046w, 1025w, 1014w,
1006(sh), 962w, 944w, 894m, 882w, 817w, 802m, 762s, 736m,
Conclusion
Six different co-ordination polymers have been prepared by
hydrothermal synthesis of a polyaromatic acid with a divalent
metal ion (M = Co, Mn or Zn) and 2,2Ј-bipyridyl. In each case
a co-ordination polymer with infinite chains in one dimension
only was formed as anticipated owing to the tight binding of
2,2Ј-bipyridyl to some or all of the metal cations. In all cases the
chains ‘slot together’ and associate by van der Waals forces
between the 2,2Ј-bipyridyl rings. The acids H3TPO and H2SDA
failed to give co-ordination polymers with metal cations in
hot water in the absence of 2,2Ј-bipyridyl which exemplifies
the easier formation of 1-D versus 2-D or 3-D infinite lattices.
This is attributed to the less directional nature of van der
Waals forces and hydrogen bonding compared to the geo-
metrical requirements of metal ion–ligand co-ordination
which propagates through the infinite network. Lattices 4 and
5 incorporating BTC and TPO respectively have an open
framework structure with large molecular channels occupied
by disordered water molecules. Retention of the crystallinity
and porosity of [Co3(TPO)2(BIPY)2(H2O)6]ؒxH2O 5, after
mild desorption of the non-co-ordinating water molecules,
is attributed to the still fully co-ordinated metal ions. They
undergo H2O/D2O solvent exchange characteristic of a zeolite.
Further studies are in progress of the scope for rational design
of acid and bidentate ligand for solid state assembly.
692m, 674s, 652m, 606w, 530m, 466w, 439w and 416m cmϪ1
.
TGA: when heated in nitrogen at 2 ЊC minϪ1, between 190
and 260 ЊC a mass loss of 4.3% occurred corresponding to
dehydration of the monohydrate (4.1% theoretical).
[{Zn(HBTC)(BIPY)(H2O)}n] 3. The compounds H3BTC
(101 mg, 481 µmol), Zn(OAc)2ؒ2H2O (105 mg, 480 µmol), BIPY
(75 mg, 483 µmol), and water (20 ml) were placed in a 45 ml
bomb. This was heated at 20 ЊC hϪ1 to 210 ЊC, maintained for
2 h, then cooled at 5 ЊC hϪ1 to 180 ЊC. After 6 h at 180 ЊC the
bomb was cooled at 5 ЊC hϪ1 to 50 ЊC. From 50 to 20 ЊC the rate
of cooling was 10 ЊC hϪ1. The bomb was opened and colourless
crystals of 3 were collected by filtration, washed with water and
dried in air (190 mg, 88%). Found: C, 51.1(50.9); H, 2.9(2.9); N,
6.2(6.2). [Zn(HBTC)(BIPY)(H2O)] = C19H14N2O7Zn requires
C, 51.0; H, 3.2; N, 6.3%. IR: 3410(br) s, 3088m, 1701s, 1626s,
1600m, 1584m, 1568s, 1509w, 1493w, 1477w, 1439s, 1399w,
1356vs, 1249m, 1232m, 1184m, 1103w, 1058w, 1023w, 1013w,
940w, 860w and 795w cmϪ1. TGA: an endothermic mass loss
of 3.8% was observed between 110 and 160 ЊC corresponding
to dehydration of the monohydrate (4% theoretical); no further
mass loss occurred below 290 ЊC after which the material
decomposed.
Experimental
All hydrothermal synthesis experiments were performed in
Teflon lined 23 or 45 ml bombs (Parr 4749 and 4744) heated
using a Heraeus Instruments oven (type 6030) fitted with a
Eurotherm 902PX programmer. All infrared spectra were
recorded on KBr discs using a ATI Mattson Genesis series
FTIR spectrometer. Thermal gravimetric analysis was per-
formed in an atmosphere of either flowing nitrogen gas or air,
using a Stanton Redcroft STA-780 instrument. The C,H,N
microanalyses were performed by Butterworth Laboratories
Ltd. Cobalt microanalysis was by means of atomic absorp-
tion spectroscopy using a Varian SpectrAA-10 using an air/
acetylene flame. In some cases microanalyses were performed in
duplicate on separate batches to verify the sample homogeneity.
Variable temperature susceptibility measurements were carried
out in the temperature range 2–300 K at a magnetic field of
0.1 T on polycrystalline samples with a magnetometer (Quan-
tum Design MPMS-XL-5) equipped with a SQUID sensor.
The susceptibility data were corrected from the diamagnetic
contributions as deduced by using Pascal’s constant tables.
[Co3(BTC)2(BIPY)2(H2O)]ؒ4H2O 4. The compounds H3BTC
(100 mg, 476 µmol), Co(OAc)2ؒ4H2O (178 mg, 714 µmol),
BIPY (113 mg, 723 µmol) and water (16 ml) were placed in
a 45 ml bomb. This was heated at 100 ЊC hϪ1 to 180 ЊC, main-
tained for 2 h, then cooled at 5 ЊC hϪ1 to 20 ЊC. A mixture of
a pink solid and blood red crystals of 4 (48 mg, 19%) was
obtained. The red crystals were separated mechanically. Found:
C, 44.4(44.2); H, 3.4(3.6); N, 5.2(5.1). [Co3(BTC)2(BIPY)2-
(H2O)6]ؒ2H2O = C38H38Co3N4O20 requires C, 43.6; H, 3.7; N,
5.4 (the solid has partially dehydrated loosing some of the non-
co-ordinating H2O molecules that were definitely observed by
crystallography). IR: 3315vs(br), 1615vs, 1560s, 1492w, 1474m,
1443s, 1373vs, 1316w, 1250w, 1175w, 1158w, 1105m, 1059w,
1043w, 1024m, 930w, 816w, 762s, 732s, 653w, and 633w cmϪ1
.
TGA: an endothermic mass loss of 18% was observed up to
100 ЊC; conversion of an octahydrate into an anhydrous solid
requires a mass loss of 14%.
Syntheses
[{Mn(HBTC)(BIPY)(H2O)}n] 1. The compounds H3BTC
(102 mg, 486 µmol), Mn(OAc)2ؒ4H2O (119 mg, 486 µmol), 2,2Ј-
bipyridyl (77 mg, 493 µmol), and water (10 ml) were placed in
a 23 ml bomb. The bomb was heated at 10 ЊC hϪ1 to 210 ЊC;
after 2 h it was cooled to 180 ЊC at a rate of 5 ЊC hϪ1, before
being cooled at 1 ЊC hϪ1 to 20 ЊC. Compound 1 (199 mg, 94%)
was collected as large yellow crystals. Found: C, 51.7; H, 3.1;
N, 6.2 [Mn(HBTC)(BIPY)(H2O)] = C19H14MnN2O7 requires C,
52.2; H, 3.2; N, 6.4%. IR 3416s, 1686s, 1655w, 1613s, 1531m,
1560m, 1496w, 1479m, 1444m, 1404w, 1371s, 1336s, 1321s,
1245m, 1185m, 1162w, 1098w, 1064w, 1019w, 1003w, 941w,
893w, 801w, 760m, 737m, 673m, 649w and 534w cmϪ1. TGA: no
significant mass loss up to 230 ЊC after which decomposition
occurred.
[Co3(TPO)2(BIPY)2(H2O)6]ؒxH2O 5. The compounds
H3TPO (207 mg, 505 µmol), Co(OAc)2ؒ4H2O (200 mg, 803
µmol), BIPY (80 mg, 512 µmol) and water (20 ml) were placed
in a 45 ml bomb. This was heated at 100 ЊC hϪ1 to 210 ЊC,
maintained for 2 h, then cooled at 3 ЊC hϪ1 to 20 ЊC. Compound
5 was obtained as a homogeneous red solid (324 mg, 91%) by
filtration and drying in air. Found: C, 52.5; H, 3.9; N, 3.9; P, 4.2.
[Co3(TPO)2(BIPY)2(H2O)6] = C62H52Co3N4O20P2 requires C,
52.8; H, 3.7; N, 4.0; P, 4.4%. IR: 3397(br) s, 3111w, 3093w,
3045w, 1693w, 1598m, 1541m, 1499w, 1474w, 1443w, 1425m,
1385s, 1314w, 1251w, 1164w, 1136w, 1111m, 1087w, 1080w,
1019m, 862w, 828w, 768m, 739m, 700(sh), 653w, 635w, 585w,
561w, 493w and 425w cmϪ1. TGA: between 40 and 170 ЊC three
J. Chem. Soc., Dalton Trans., 1999, 4209–4216
4215

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1995, 378, 703; (d) R. H. Groeneman, L. R. MacGillivray and
overlapping endothermic mass losses totalling 9.6% occurred;
conversion of the octahydrate into an anhydrous solid requires
a mass loss of 10%.
J. L. Atwood, Chem. Commun., 1998, 2735; (e) S. Subramanian and
M. J. Zaworotko, Angew. Chem., 1995, 107, 2295; Angew. Chem., Int.
Ed. Engl., 1995, 34, 2127; ( f ) C. J. Kepert, D. Hesek, P. D. Beer and
M. J. Rosseinsky, Angew. Chem., 1998, 110, 3335; Angew. Chem.,
Int. Ed., 1998, 37, 3158; (g) C. J. Kepert and M. J. Rosseinsky,
Chem. Commun., 1998, 31; (h) M. Kondo, T. Yoshitomi, K. Seki,
H. Matsuzaka and S. Kitagawa, Angew. Chem., 1997, 109, 1844;
Angew. Chem., Int. Ed. Engl., 1997, 36, 1725; (i) H. Li, C. E. Davis,
T. L. Groy, D. G. Kelley and O. M. Yaghi, J. Am. Chem. Soc., 1998,
120, 2186; (j) O. M. Yaghi and H. Li, J. Am. Chem. Soc., 1995, 117,
10401; (k) M. J. Plater, M. R. St. J. Foreman and A. M. Z. Slawin,
J. Chem. Res., 1999, (S) 74; (M) 0728.
[Co(SDA)(BIPY)(H2O)] 6. The compounds cis-H2SDA
(103 mg, 384 µmol), Co(OAc)2ؒ4H2O (96 mg, 385 µmol), BIPY
(60 mg, 384 µmol) and water (10 ml) were placed in a 23 ml
bomb. This was heated at 100 ЊC hϪ1 to 220 ЊC, held for 2 h,
then cooled at 5 ЊC hϪ1 to 180 ЊC. After 6 h at 180 ЊC the bomb
was cooled at 5 ЊC hϪ1 to 100 ЊC and then at 6 ЊC hϪ1 to 20 ЊC.
Large orange needle-shaped crystals were collected by filtration,
washed with saturated aqueous NaHCO3, then with water
followed by drying in air (171 mg, 89%). Found: C, 62.6; H, 4.0;
N, 5.6. [Co(SDA)(BIPY)(H2O)] = C26H20CoN2O5 requires C,
62.5; H, 4.0; N, 5.6. IR: 3404(br), 3105m, 3077w, 3055m, 3043w,
3034w, 3008m, 1602s, 1577s, 1550s, 1520s, 1490m, 1473m,
1440s, 1427s, 1412vs, 1358vs, 1314m, 1277w, 1263w, 1248m,
1187w, 1175w, 1154w, 1137w, 1118w, 1105w, 1057w, 1020m,
982w, 908(sh), 897m, 868(sh), 859m, 850(sh), 836w, 800m,
781w, 764s, 732s, 699m, 652w, 629w, 598w, 584w, 563w, 552w,
529w, 483w, 430w, 417m and 403m cmϪ1. TGA: an endothermic
mass loss of 3.4% was observed between 140 and 190 ЊC
corresponding to dehydration of the monohydrate (theoretical
3.6%). An attempt at using a mixture of cis- and trans-H2SDA
in the reaction gave a mixture of 6 and some recovered starting
material.
3 (a) M. J. Plater, A. J. Roberts and R. A. Howie, Chem. Commun.,
1997, 893; (b) M. J. Plater, A. J. Roberts, J. Marr, E. E. Lachowski
and R. A. Howie, J. Chem. Soc., Dalton Trans., 1998, 797.
4 (a) O. M. Yaghi, H. Li and T. L. Groy, J. Am. Chem. Soc., 1996, 118,
9096; (b) O. M. Yaghi, C. E. Davis, G. Li and H. Li, J. Am. Chem.
Soc., 1997, 119, 2861; (c) T. M. Reineke, M. Eddaoudi, M. Fehr,
D. Kelley and O. M. Yaghi, J. Am. Chem. Soc., 1999, 121, 1651.
5 (a) N. Yoshida, H. Oshio and T. Ito, Chem. Commun., 1998, 63;
(b) E. Ishow, A. Gourdon and J.-P. Launay, Chem. Commun., 1998,
1909; (c) H.-P. Wu, C. Janiak, L. Uehlin, P. Klufers and P. Mayer,
Chem. Commun., 1998, 2637; (d) I. M. Muller, T. Rottgers and
W. S. Sheldrick, Chem. Commun., 1998, 823; (e) H.-P. Wu, C. Janiak,
G. Rheinwald and H. Lang, J. Chem. Soc., Dalton Trans., 1999, 183;
( f ) B.-H. Ye, X.-M. Chen, G.-Q. Xue and L.-N. Ji, J. Chem. Soc.,
Dalton Trans., 1998, 2827.
6 SMART system and hemisphere data collection. All structures were
solved by direct methods and refined against F2 using SHELXTL,
Version 5.01, Bruker AXS, Madison, WI, 1995.
CCDC reference number 186/1671.
See http://wwtw.rsc.org/suppdata/dt/1999/4209/ for crystallo-
graphic files in .cif format.
Acknowledgements
7 W. Chou and M. Pomerantz, J. Org. Chem., 1991, 56, 2762.
8 C. J. Kepert and M. J. Rosseinsky, Chem. Commun., 1999, 375.
9 R. L. Carlin, Magnetochemistry, Springer, New York, 1983.
10 C. J. Gómez-García, E. Coronado and J. J. Borrás-Almenar, Inorg.
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11 C. J. Gómez García, E. Coronado, P. Gómez-Romero and N. Casañ-
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12 R. Cortés, J. L. Pizarro, L. Lezama, M. L. Arriortua and T. Rojo,
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13 G. De Munno, T. Poerio, G. Viau, M. Julve and F. Lloret, Angew.
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We are grateful to the Leverhulme Trust for financial support.
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J. Chem. Soc., Dalton Trans., 1999, 4209–4216