1
Chapter 1
An overview of the recent trends in the
coordination chemistry of the functionalized
polycarboxylic acid ligands.
2
INTRODUCTION
In recent years metal complexes with functionalized polycarboxylic acid
ligands viz. 2,6-dipicolinic acid (H2PDA), iminodiacetic acid (H2IDA), oxydiacetic
acid (H2ODA), nitrilotriacetic acid (H3NTA), ethylenetriaminetetraacetic acid
(H4EDTA) or diethylenetriaminepentaacetic acid (H5DTPA) (Fig. 1) have attracted
attention of inorganic and bioinorganic chemists [1-6]. The ligating characteristics of
these flexible functionalized polycarboxylic acid ligands constitute an important class
of polydentate [N,O,O] or [O,O,O] chelators resulting in metal complexes differing in
coordination numbers, conformations and 3-D architectures. Furthermore, in mixed
ligand metal carboxylate complexes, the polycarboxylate moieties in ternary
complexes containing α-diimine spacers also play an important role to construct
supramolecular networks via. extensive H-bonding and interactions of aromatic rings
[7-9]. The carboxylate moiety may adopt a range of different coordination modes
whether it binds the metal ion as anionic (COO)¯ or neutral unionized (COOH) form.
[10-19] (Fig. 2).
A large numbers of polymeric metal organic frameworks (MOFs) containing
carboxylate ligands have been prepared and well characterized [7-9]. The
dicarboxylic ligands with additional donor sites as for example pyridyl ‘N’ in
pyridine-2, 6-dicarboxylic acid and iminic ‘N’ in iminodiacetic acid and ethereal ‘O’
in oxydiacetic acid and their derivatives are of great interest to medicinal chemists.
H2PDA and its derivatives are present in many natural products, such as alkaloids,
vitamins and coenzymes and are found to display a wide variety of physiological
properties. Metal complexes of these ligands therefore, have been exploited as
interesting model systems [20-22].
3
HN
O
OHO
OH
NHO OH
OO
OO O
OH OHN
O
OH
O OH
N N
HO
OH
OH
HO
O
O
O
O NN
N
O
O
HO
OH
O
O
OH
OH
HO O
O
HO
(a) (b) (c) (d)
(e) (f)
Fig. 1. (a) H2IDA, (b) H2PDA, (c) H2ODA, (d) H3NTA, (e) H4EDTA, (f) H5DTPA.
CO O
M
CO O
M
CHO O
M
CHO O
M
CO O
M
O O
M M
O O
M
O OMMM
i ii iii iv
v vi vii viii
Fig. 2. Various coordination modes in metal–carboxylate complexes.
4
Pyridine-2,6-dicarboxylic acid (H2PDA), typically called as dipicolinic acid,
has an elaborately attractive coordination chemistry. It has a rigid 120° angle between
the central pyridine ring and two carboxylate groups, henceforth could potentially
provide various coordination motifs to form both discrete and consecutive metal
complexes under appropriate synthetic conditions [23]. The systematic studies
undertaken by several research groups for transition [24] as well as rare earth [25]
metal complexes based on pyridine dicarboxylic acids indicate versatile coordination
motifs for pyridine-2,6-dicarboxylic acid (Fig. 3). The presence of additional donor
centers in the pyridine-2,6-dicarboxylic acid frame as for example OH group in 4-
hydroxypyridine-2,6-dicarboxylic acid generates a unified coordination geometry of
H2PDA and hydroxyl group providing more coordination motifs than H2PDA [26]
itself.
Many transition metal ions with dicarboxylate ligands have been exploited as
possible drugs for many diseases [24,27]. The recent successes in the development of
metal based drugs incorporating suitable organic moieties suggest that modifications
of the metal ion chemistry by the organic ligands not only enhance the efficacy but
also decrease toxicity. Many vanadium complexes containing pyridine dicarboxylic
acid and its derivatives have been reported in literature [28]. Most of them are of
particular interest because a range of these compounds are comparable with insulin
like effects. These compounds include simple salts (sodium ortho− and meta−
vanadate, vanadyl sulphate [29] and a few organic vanadium compounds. The
complexes bis(pyrrolidine−N−carbodithioate)oxovanadium(IV) and bis(cystein
methyl ester)oxovanadium(IV) [30], have been propagated as an effective insulin
mimics (antidiabetic drugs). The most successful vanadium complexes in this regard
5
are considered to have reasonably good solublity in both organic and aqueous media
and compatible with the human metabolism. Recent efforts are focused on identifying
vanadium compounds with increased potency and decreased toxicity [31-35]. Most of
the compounds reported contain bidentate ligands and have a 1:2 metal-to-ligand
stoichiometry. The functionalized dicarboxylate ligands are desirable because of low
toxicity and amphophilic nature. There are reports that different classes of Vanadium
compounds [36,37] exhibit the insulin-enhancing properties. Many V-dipicolinate
complexes viz. NH4[VVO2(PDA)] (1), [VIVO(PDA)(H2O)2]·2H2O (2),
[VIII(PDA)(HPDA)H2O]·3H2O (3) or [VIII(PDA)(HPDA)]·3H2O (4) (Fig. 4) and even
the H2PDA ligand alone, have been shown to be effective in diabetes.
Fig. 3. Coordinating modes in PDA2–
6
Fig. 4.
Ternary complexes of oxygen-donor ligands (as primary ligands) and
heteroaromatic N-bases (as auxiliary ligands) have attracted much interest as they can
display exceptionally high stability [38]. The use of transition metal complexes of
iminodiacetic acid (H2IDA) and nitrilotriacetic acid (H3NTA) have been widely
adopted in biology, and are gaining uses specifically in biotechnology for the protein
purification. Organic ligands possessing multifunctional coordination sites such as N
and S have been shown to play crucial roles in the design and architecture of metal-
organized frameworks having interesting topologies [39, 40]. Furthermore ligands
with polyfunctional donor sites have been exploited for this purpose. N-
carbamoylmethyl-iminodiacetic acid [H2ADA = H2NCOCH2N(CH2COOH)2], which
is an amido (–CONH2) analogue of H2IDA, possesses three potential coordinating
groups, –N, –COOH and –CONH2 and has been exploited for this purpose. This
7
ligand can be partly or fully deprotonated (HADA– and ADA2–) and can perform a
wide range of coordination modes. However, the easily chelating effect of this
derivative (ADA2–) made it a limited ligand in coordination chemistry. Several papers
have appeared in recent years concerning the binary and ternary (mixed-ligand)
complexes of a variety of metal ions and ADA2– as primary ligand [41]. Most of the
complexes adopt mononuclear structures where ADA2– acts as a tridentate chelating
agent (Fig. 5). Carboxylates have been used extensively in the synthesis of di and
polynuclear metal complex [41]. Iminodiacetic acid (H2IDA) is one such ligand,
finding widespread synthetic applications. For example, using iminodiacetato–
copper(II) as a building block and polypyridine- type molecules as common cross-
linkers, several oligomeric complexes with aesthetically pleasing structural forms
have been reported in recent times [41]. The X-ray crystal structure analysis of a
dinuclear iminodiacetato–copper(II) complex,
[(H2O)2(IDA)Cu(PMZ)Cu(IDA)(OH2)2], (PMZ = N,N’-bis(pyridin-4-
ylmethylene)hydrazine ), where PMZ acts as a linear spacer was reported (Fig. 6).
Aromatic ring stacking interactions have also proved to be relevant intermolecular
forces for molecular recognition processes in chemistry and biology [42]. In this
broad context, a large variety of mixed-ligand metal complexes involving various
aromatic amino acetate ions or closely related derivatives and N-heterocycles as co-
ligands were investigated during the past 2-3 decades [41].
8
Fig. 5. Structure of [Ni(ADA)2]2–
Fig. 6.
9
Structural studies on many mixed−ligand compounds have revealed promising
informations not only on the metal ion surrounding but also on molecular recognition
phenomena involved in the crystal packing. copper(II) complexes having one IDA-
like chelating ligand and stoichiometric modes of N heterocycles such as imidazoles,
bipyridines, phenanthrolines and adenine have been reported [43-46]. Such complexes
have bio-inorganic interest because the copper(II)-IDA like chelate could stimulate a
protein-metal centre which would be able to display different recognition modes with
various N-heterocycles acting as “substrates” [43].
The crystal structure of the mixed-ligand complex had infinite multi-staked
chains with alternating benzyl-phen intra- and inter-molecular π–π interactions. The
salt also exhibited a large variety of inter-ligand π–π interactions [47]. The ternary
systems [Cu(II)/L/phen] where L is the substituted analogues of
nitrobenzyliminodiacetic acid (IDANBz) i.e. N-phenethyliminodiacetate(2-)(pheIDA)
or N-benzylaminoacetato-2-propionate(2-) (BAAP) instead of IDANBz as chelating
ligand are also reported [47].
On account of the tridentate chelating property of the iminodiacetate (IDA2–)
ligand and the flexibility of the Cu(II) coordination stereochemistry, extensive
research has been devoted to the structural investigations of a variety of the mixed-
ligand complexes of Cu(II), with IDA-like groups as primary ligands and N-
heterocyclic donors as auxillary ligands [44-47]. Such ternary complexes are useful
bioinorganic model compounds for mono- and dinuclear copper proteins. In this
context, the Cu(II) complex serves as a metal centre, while the tridentate H2IDA
ligand or its derivative acts as a protein-like moiety and the N-heterocyclic donors
play the role of substrate or inhibitor. All mixed-ligand complexes having 1:1:2 ratio
10
of Cu/IDA-like/N-heterocyclic donor results in complexes in which the primary
ligand adopts a fac-chelating conformation. In contrast, the complexes with an
equimolar Cu/IDA-like/N-heterocyclic donor ratio (1:1:1) adopt a distorted square-
pyramidal coordination (4+1 type), or more rarely in an elongated octahedral
coordination (4+2 or 4+1+1 type). In these complexes, the IDA-like ligand itself
adopts a mer-conformation binding as tridentate donor. Furthermore, the
stoichiometry and binding nature of the auxiliary ligand influence the coordination
geometry of IDA. It has been shown that 1,10-phenanthroline (phen) possesses a
stronger coordination ability (Figs. 7–9). In the mixed-ligand homo-bimetallic
complex [Cu(phen)2Cu(IDA)(phen)], IDA2– serves as a bridging ligand to link the two
different [Cu(phen)2]2+ and [Cu(phen)]2+ moieties via a carboxylate group. However,
the two Cu(II) centers exhibit totally different coordination geometries.
Fig. 7. The molecular structure of [Cu(phen)2Cu(IDA)(phen)] showing the atomic
labelling scheme. Thermal ellipsoids are drawn at 50% probability. H atoms, solvent molecules and counter ions have been omitted for clarity.
11
Fig. 8. The H-bonding network and octahedral coordination geometry around metal
atom in [Cu(phen)2Cu(IDA)(phen)].
Fig. 9. The π–π stacking interactions in [Cu(phen)2Cu(IDA)(phen)].
12
H-bonding and π–π interaction in mixed ligand complexes generally result in
the formation of a supramolecular framework. The supramolecular chemistry was first
introduced in the year 1978 [48,49] as the chemistry of molecular assemblies.
Supramolecular chemistry is the chemistry of intermolecular bonds comprising of
strong electrostatic interactions and often weak interactions such as hydrogen bonding
and Vander Waals forces, by mutual recognition. Molecules recognize each other
through a complex combination of geometrical and chemical factors and the
complementary relationship between interacting molecules is characteristic of
recognition process [50]. The characteristic properties of supramolecules are distinct
from the aggregate properties of their molecular constituents. The construction of a
supramolecular network from molecular components implies an accurate knowledge
of the chemical and geometrical properties of intermolecular interactions [51].
Although supramolecular chemistry is still novel, it has drawn a great deal of attention
for further investigations. Since Supramolecular chemistry is a highly
interdisciplinary field of science covering the chemical, physical, and biological
features of the chemical species of greater complexity than molecules themselves
[52]. Coordination chemistry of metal-carboxylate species is an attractive subject
from the bioinorganic standpoint because the carboxylate group of glutamate and
aspartate works as a supporting ligand for the metal centers in various metallo-
proteins. The carboxylate group is also suggested to play an important role for
structural holding and proton transfer via a hydrogen bonding interaction in proteins.
Reports on metallo-proteins reveal the variety of the coordination mode of the
carboxylate ligands and the formation of the hydrogen-bonding interaction with the
other functions [53]. Dinuclear carboxylate-bridged complex is the most attractive
synthetic target because the carboxylate bridged bimetallic core is the common
13
structural motif of the various proteins [54]. The pH of the reaction medium plays an
important role [55] in deciding the formation of the carboxylate bridged complexes as
mononuclear or polynuclear unit. By adjusting a suitable pH, one can obtain desired
product under certain conditions [56,57].
Besides the functionalized dicarboxylic acid ligands, the
diethylenetriaminepantaacetic acid (H5DTPA), a higher homologue of polycarboxylic
acids is also an important polydentate ligand. The ligand appears to have eight
coordination sites comprising of three amino nitrogen donors and five carboxyl
groups, capable to bind metal ions. However, the dianion, [H3DTPA]2– forms stable
solid complexes with first row transition metal ions acquires flexible coordination
geometry where the ligand invariably involves lower than the available octa-
coordination sites [57]. This behaviour is comparable to that the lower homologous
like H4EDTA. Albeit, it has been reported that the resulting complexes have a less
number of water molecules binding (including two that present in the hydration
sphere) compared to that observed for the complexes of lower polycarboxylic acids
analogues H4EDTA [58]. This is consistent with higher stabilities (or formation
constant) of the H5DTPA compared to corresponding ethylenediamine tetraacetic
acetate (H4EDTA) complexes. The most marked reduction in water binding on
replacing EDTA by DTPA occurs with Fe3+ and Mn2+. Furthermore, there is a change
in overall stereochemistry from a seven- to a predominantly six- co-ordinate structure
for Fe3+. Other H5DTPA complexes adopt octahedral symmetry as do all the
derivatives of H5DTPA especially the diethylenetriaminepentamethyl phosphonate
(DTPMP) analogues [58]. Complexes of DTPMP with Co2+, Ni2+ and a lesser extent,
Cu2+ have lower hydration numbers than the corresponding ethylenediaminetetram-
ethylphosphonate (EDTMP) counterparts, due to possibility of simultaneous binding
14
of three nitrogen donors in DTPMP. The [Fe3+−(DTPMP)] complex has one bound
water similar to [Fe3+−(EDTMP)], but the manganese (II) complex of DTPMP is
more hydrated than the corresponding EDTMP complex [58].
Radioactive labeling has been an indispensable tool in the investigation of
biological activities that requires high sensitivity and specificity. In recent years, the
use of fluorescence of lanthanide chelates (e.g., europium and terbium) has been
gaining favour as a viable alternative to the radioactivity. This is based on the facts
that lanthanide fluorescence has high sensitivity and its fluorescence is more specific
than that of conventional fluorescent probes [59,60]. The higher specificity and
sensitivity result from the fact that the fluorescence emission of these lanthanide ions
is narrow banded with a long Stokes shift and a very long fluorescence decay time
[61]. The latter two characteristics allow detection of lanthanide fluorescence in the
presence of background fluorescence derived from biological samples and common
plastic ware, especially if the time-resolved fluorescence measurement is made.
The obvious advantage of lanthanide is that there is no need for special
handling, including the waste disposal, such as that required for radioactive materials.
However, the major disadvantage of lanthanide labeling is the requirement for
incorporating a chelating agent, a structure much larger than radioisotopes, which
may affect the property of the compound labeled. For this reason it is prudent to aim
for minimum level of incorporation of a chelater (ideally <1 mol/mol on the average)
into biological macromolecules, such as proteins.
Diethylenetriaminepentaacetic acid (H5DTPA) and related
polyaminocarboxylic acids and their derivatives are proven chelaters of lanthanide
ions. Simple polyaminocarboxylate-lanthanide chelates are not fluorescent. To
15
potentiate the fluorescence of lanthanide for measurement, two strategies are
generally used. One way is to incorporate a suitable aromatic group close to the
chelater which will deliver the captured light to lanthanide so as to make it
fluorescent. The second way is to use dissociation-enhanced lanthanide fluorescence
immunoassay (DELFIA) technique, in which at the final stage of fluorescence
measurement the lanthanide is displaced from the polyaminocarboxylate chelate and
transferred into a micellar chelating environment, which will make it highly
fluorescent. The former method is required when a lanthanide is used for certain
experiments, e.g., fluorescence resonance energy transfer (FRET), for which a
number of synthetic compounds based on H5DTPA and related
polyaminocarboxylates have been reported [62-65 ]. The second method requires an
extra enhancement step, but the incorporation of H5DTPA group into proteins is as
simple as reacting them directly with commercially available HDTPA dianhydride
[66]. The shortcomings of reacting directly with the dianhydride are (1) difficulty in
controlling the extent of H5DTPA incorporation; (2) possibility of conjugation via
both anhydride groups, which will cause undesirable cross-linking and incorporation
of a lanthanide chelate that is unstable in the acidic pHs; and (3) possible change in
protein stability due to neutralization of the amino group utilized in conjugation. A
monofunctional reagent, p-isothiocyanatobenzyl DTPA, is available commercially
(Wallac, Gaithersburg, MD). However, this reagent introduces an aromatic group as
well as neutralizes the positive charge of the amino group when it reacts with
proteins. Both of these facts may introduce undesirable characteristics to the protein
modified [67].
16
The H5DTPA complexes of copper prepared in acidic media apparently
decomposed in-vivo with the radiometal being trapped in the liver [68,69]. That was
the reason why they were not used for in-vivo studies by direct injection into blood
stream, while their in vitro use as cell radio labeling tracer were proposed as an
alternative option. Meanwhile, there have been reports in the literature concerning the
preparation and use of 64Cu-DTPA for cisternography in the early 1980’s [70].
However, at that time the positron emission tomography (PET) was not yet developed
for regular clinical studies. The idea of incorporation of a (PET) radioisotope into a
suitable chelate used in cisternographic and blood cell labeling, i.e. H5DTPA led to
emphasis on the use of one of the chelate complex [CuDTPA] (Fig. 10) as a
suitable, non−conventional PET tracers.[71]. This important utility of the transition
metal−DTPA complexes has attracted attention of the coordination chemist to design
and develop the synthesis of novel binary as well as ternary complexes incorporating
H5DTPA as main ligating moiety incorporating suitable π−donor as auxillary or
supporting chelator (or spacers).
Fig.10. Scheme of Cu−DTPA complex
17
Carboxylic acid anhydrides are common acylating agents for the modification
of proteins at the amino group of lysine residues and at the hydroxyl group of tyrosine
residues [72,73] .There are two reported anhydride methods for conjugation of
H5DTPA to proteins: (a) the H5DTPA carboxycarbonic mixed anhydride method [74-
80], and (b) the cyclic DTPA anhydride method [81-83]. The former, however,
requires extreme caution because the DTPA carboxycarbonic mixed anhydride is
thermally unstable and hydrolyzes very rapidly. On the other hand, cyclic DTPA
anhydride is sufficiently stable to be isolated for identification. Hnatowich and
coworkers. [81] reported a successful conjugation of DTPA to human serum albumin
(18.8 mg/ml, 2.81×10−4 M) using cDTPAA as an acylating agents.
Gadolinium complexes of linear poly(aminocarboxylate) ligands are of
considerable interest as contrast agents in magnetic resonance
imaging (MRI) [84,85]. The octachelating ligands,
diethylenetriaminepentaacetic acid (H5DTPA), di(methylcarbamoylmethyl)
diethylenetriaminetriacetate (H3DMPTTA), 10-(2-hydroxypropyl)-1,4,7,10-
tetraazacyclododecane-1,4,7-triacetic acid (H3HDPDOTRA) and 1,4,7,10-
tetraazacyclododecanetetraacetic acid (H4DOTA) are effective MRI contrast agents
when complexed with the trivalent gadolinium ion [86]. These gadolinium chelates
possess sufficient paramagnetism and high stability. In order to obtain high contrast
for lesion, administration of high doses of contrast agent is sometimes required,
especially for non-ionic gadolinium complexes which may be used as low-osmolality
contrast agents for MRI [87] The toxic effect of uncomplexed Gd3+ and free pro-
ligand arising from dissociation of the metal complex is one of the major concerns in
MRI [88-94]. The acute toxicity of gadolinium complexes of the
18
poly(aminocarboxylates) correlates well with the selectivity of the later for Gd3+. The
release of Gd3+ is related to the stability constants of the gadolinium complexes
[95,96]. Three important bis(amide) derivatives of H5DTPA, i.e. bis(isopropylamide),
bis(tert-butylamide) and the bis(benzylamide) are reported. Their protonation
constants, thermodynamic stability constants of complexes with Gd3+, Cu2+, Zn2+ and
Ca2+ and their selectivity for Gd3+ over endogenously available metal ions are already
discussed in literature [97] . The Gd(III) ion, with a 4f7 electronic configuration has a
S = 7/2 ground state, it is particularly attractive as an imaging agent. However, the
free gadolinium metal ion, which is similar in size to free Ca2+ binding capablility to
calcium binding sites [84,99], is toxic and so must be sequestered in a metal complex
[84,100]. The first such agent in wide use has been [Gd(DTPA)]2−[101], but the
negative charge of this complex adversely affects tissue distribution for many
applications. Additionality, the high osmolity of formations containing this complex
make administration by painful injection [84, 98].
The aqueous 1HNMR spectra of [Ln(DTPA)]2−(Ln = La, Pr, Eu, and Lu)
complexes have been studied in some detail, [102-105] although later reports came to
different assingements for the structures and exchange mechanisms. Neutral
complexes of Gd(III) with DTPA derivatives have been shown to have properties
somewhat different from the parent ionic complex. Preliminary studies of the
bis(amide) ligands, which have effective symmetry for the ligand, showed highly
complex spectra due to rapidly interconverting [106,107]. The constraint of tethering,
the two amide groups together substantially diminishes both the number of isomers
and the pathways for interconversion. Hence, macrocyclic bis(amide) complexes
[108] were prepared (Fig.11, 12) to demonstrate this behaviour (Scheme 1).
19
Scheme 1. Synthetic procedure for the preparation of DTPA–dien.
The crystal structures of the La(III) and Eu(III) complexes (Fig 11 and 12) are
reported in the literature [108].
Fig. 11. Crystal structure of [La(DTPA−dienH+)]2
20
Fig. 12. Crystal structure of [Eu(DTPA−dienH+)]4
Schiff base ligands have played an integral and important role in the
development of coordination chemistry since the late 19th century. The studies of the
metal complexes of these ligands have attracted attention due to their facile syntheses,
wide applications and the accessibility of diverse structural modifications [109,110].
The chemistry of Schiff base ligands and their metal complexes have been widely
studied including the various aspects of bioinorganic chemistry [110]. Schiff bases
have been reported to show a variety of biological actions by virtue of the azomethine
linkage, which is responsible for various antibacterial, antifungal, herbicidal, clinical
and analytical activities [111-114]. The chemistry of metal complexes containing
salen–type Schiff base ligand derived from condensation of aldehydes and amines is
of enduring significance, since they have common features with metalloporphyrin
with respect to their electronic structure and catalytic activities that mimic enzymetic
21
oxidation [115]. Manganese plays an important role in several biological redox–active
systems, e.g. manganese superoxide dismutase [115], manganese catalase [116],
manganese peroxidase [117] manganese ribonucleotide reductase [118] and the
oxygen–evolving complex (OEC) [119,120]. In general manganese (II) complexes
with Schiff–bases are air–sensitive and easily oxidized to manganese (III) complexes
by molecular oxygen. The crystal structure consists of diverse binuclear clustering.
The hexa–coordinate coordinate geometry around each Mn atom is attained from
coordination of additional water molecules [120,121] in axial direction. The transition
metal Schiff base complexes with an N2O2 donor set have been extensively studied
[122-129]. The manganese complexes containing N, N΄–propane–1, 3–bis
(salicylideneaminate) derivatives are best documented [130-133]. The uses of
hydroxyl–rich Schiff base ligands to form dimeric manganese (III) complexes to
mimic the substructure of oxygen evolution centre (OEC) [134-136] are also known.
The water alkoxo bridge dimeric manganese complexes like
[(Mn2(Salpa)2(H2O)2Cl2]·DMF and [Mn2(Salpa)2(H2O)2(N3)3]·EtOH with coordinated
H2O are recently reported [137] according to the scheme (Fig. 14).
22
Fig.13. Formation of dimeric manganese complexes from Schiff base ligand L [L=1–
(salicyaldeneamino)–3–hydroxypropane and the derivatives].
23
The crystal structure of the dimer has been certained from single crystal
structure as shown in Fig. 14.
Fig. 14. Perspective view of [Mn2(Salpa)2(H2O)2Cl2]∙2DMF
The novel polynuclear complexes of paramagnetic metals have attracted
considerable current interest due to the potential of these spin clusters to act as single-
molecule magnets [138,139]. The single-molecule magnets need to have relatively
high spin ground states with uniaxial magnetic anisotropy. It is, therefore, expected
that multi-nuclear nickel (II) complexes can be a good candidate for the single-
molecule magnets [140]. Utilization of polydentate Schiff base ligands, which can
function in both bridging and chelating capacities, represents a promising route to the
synthesis of new spin clusters and single-molecule magnets. Ligands derived from
salicylidene-2-ethanolamine incorporated into a number of mono- and dinuclear
24
transition metal complexes are known to have a tetranuclear Fe(II) and Cu(II) clusters
with cubane type structures [141].
The structure of the [Ni2(Hsae)2(CH3COO)2(H2O)2] has been certained from
single crystal structure as shown in Fig. 15.
Fig.15. Molecular structure of [Ni2(Hsae)2(CH3COO)2(H2O)2]
25
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