chapter ii review metal complexes of polydentate schiff...
TRANSCRIPT
Chapter II
REVIEW
Metal Complexes of Polydentate Schiff Bases Containing Thiophene
Ring System
Metal complexes derived from Schiff bases have been known
for over a hundred and fifty years. Schiff's elegant synthesis, in 1869, of
azomethine complexes of copper(II) from preformed metal salicylaldehyde
complex and primary amine, started a new era in coordination chemistry.
Although the immense potential of the new synthetic route was
immediately apparent, systematic studies were carried out only from 1931
onwards by Pfeiffer and coworkers who, in a classical series of papers
reviewed75
in 1940, brought out the details of a variety of complexes of
Schiff bases derived from salicylaldehyde.
Metal complexes of polydentate Schiff bases have occupied a central
role in the development of coordination chemistry. Their relevance is
manifested in the huge number of publications ranging from the purely
synthetic to modern physiochemical to biochemically relevant studies of
these complexes. A tremendous variety of stable chemical species has been
synthesized, containing both transition and non-transition metals and
24
multifarious ligand systems. A review of recent literature reveals that work
on lanthanide complexes of Schiff bases is scantly reported as compared to
transition metal complexes owing to the scarce availability of 4f-orbitals
for bond formation. It has also been noted that very little work has been
done on metal chelates of heterocyclic Schiff bases containing thiophene
ring system. 12 The scarcity of metal complexes of polydentate Schiff bases
containing thiophene ring system can be attributed to a certain extent to the
lack of suitable preparative method for this class of complexes.76 This
review is confined to metal complexe� of polydentate Schiff bases
containing thiophene ring system. While providing an overview of the
contents of earlier reviews, it lays greater stress on the synthesis, structural
aspects, stability, reactivity, biological activity, thermal behaviour, X-ray
diffraction studies, electrochemical studies, luminescence properties and
analytical applications of such polydentate heterocyclic Schiff bases. Most
of the studies reported in this review are of recent origin and many of them
are contemporaneous with this investigation.
Synthesis and characterization
A deep survey of literature on Schiff base complexes reveals that a
variety of preparative methods are available. It has also been observed that
25
the synthetic route adopted depends largely on the stability and solubility
factors of ligands as well as the metal complexes formed. These methods of
synthesis have been widely reviewed by Dayagi and Degani.77 Some
important synthetic methods so far employed successfully are briefly given
below.
a) Addition of metal salt in aqueous methanol to a methanolic solution of
the ligand. 78
b) Reaction between the anhydrous metal salt and the ligand in a non-
aqueous medium in the presence of a base. 79
c) Reaction of the ligand with a suspension of the metal hydroxide in a non-
aqueous solvent. 80
d) Displacement of a P-diketone from its metal complex by a Schiff
base.81,82
e) Template reaction involving metal ammine complex and the appropriate
carbonyl compound. 83
f) Refluxing a suspension of metal alkoxide with the ligand in a non-
d. 84-86aqueous me mm.
g) The classical method of Schiff. 87
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Since stability and solubility vary from ligand to ligand and from
one metal complex to another, none of the synthetic methods can be said to
be of general applicability.
Synthesis of metal complexes of Schiff bases containing thiophene
ring system started only recently. Most of the work reported so far have
been devoted to metal complexes of Schiff bases derived from thiophene-2
aldehyde and related compounds.
Bala and Sinha88-9o examined a senes of complexes with
heterocyclic Schiff bases containing thiophene ring system. They have
reported the ligational behaviour of the Schiff base formed from
thiophene-2-aldehyde and 2-amino-6-ethoxybenzothiazole towards
copper(II). The complex formed has been formulated as
[Cu(TAAEB)2lCh 2H20. In this complex the ligand is bonded to the metal
ion in a bidentate fashion through the azomethine nitrogen and ring sulphur
atom as evidenced by infrared spectral data. The non-ligand bands in the far
infrared region indicated the formation of (M-N) and (M-S) bonds. This is
a rare instance of coordination by the ring sulphur atom. But in many cases
the ring sulphur is not involved in coordination.
Singh91 et at. reported the· synthesis and structural studies of
N-(thiophene-2-carboxamido)salicylaldimine complexes with
27
aluminium(III), chromium(III) and iron(III) ions. The complexes were
characterized by elemental analyses, molar conductance, magnetic moment,
electronic, IR and MB spectral studies. The different modes of chelation of
the ligand and the stereochemistry of the complexes were discussed.
Delgado92 and coworkers synthesised the complexes
[IrH2(11 1S-Th(PPh3)2]PF6 (T=Thiophene, benzo[b]thiophene, dibenzo[b]
thiophene and tetrahydrothiophene) in high yields by hydrogenation of
[Ir(COD)(PPh3)2] PF6(COD = 1, 5-cyclooctadiene) in the presence of the
appropriate thiophene.
Lanthanum(lII), cerium(III), praseodymium(lII), neodymium(III),
samarium(III) and europium(III) formed complexes in 1:1, 1:2 and 1:3
stoichiometric ratios with thiophene-2-aldehydesalicyloyldydrazone. These
complexes have been characterized on the basis of elemental analyses,
spectral, magnetic and thermal data. Coordination number up to twelve has
been reported. The spectral parameters indicated that partial covalent
bonding existed between the lanthanide ion and the ligand.93
Spinu94 and coworkers prepared N-(2-thienylmethylidene)-2
aminothiophenol complexes of cobalt(II), nickel(II) and copper(II). The
complexes were characterized by elemental analyses, magnetic and
spectroscopic studies. IR and NMR spectra show that the nitrogen of the
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azomethine group and sulphur of the thiophene nng take part in
coordination in the [ML2CI2] complexes while in the [M~] complexes the
ligands act as tridentate coordinating through azomethine nitrogen and
both sulphur atoms. Magnetic, ESR and electronic spectral studies show a
distorted octahedral structure for the [ML2] and [CoL2CI2] complexes and
square-planar geometry for [Ni(TNATPh)2CI2] and [Cu(TNATPhhCI2]
complexes.
Thomas95 and coworkers studied the reactions of a senes of
5-alkyl-2-thiophenedithiocarboxylates with nickel(II) chloride afforded two
types of complexes, blue nickel(II) complexes with two terminal
dithiocarboxylate ligand and violet nickel(II) complexes with perthio and
dithiocarboxylate ligands.
Shailendra96 et al. carried out the synthesis of complexes of the type
[Ru(r(CsH12)(TSC)CI2] where TSC =Thiosemicarbazone from thiophene
2-Carboxaldehyde and cycloalkylaminothiocarbonylhydrazine with
[RU(T\4_CsH12)(CH3CN)2CI2]. All the compounds have been characterized
by elemental analyses, JR, IH NMR, electronic spectra and
thermogravimetric analysis. It is concluded that the thionic sulphur and the
azomethine nitrogen atom of the ligands are bonded to the metal ion.
29
Kisun 'ko97 and coworkers synthesised cyclopenta(b)thiophene 115
complexes with metal (manganese, ruthenium and chromium) coordination
at the thiophene ring.
Febre98 et al. synthesised [PdL2]C12 . 4H20, [PdLC12] and [PdLBr2]
complexes where L = 3, 4-diaminothiophene.
Rare earth complexes99 with Schiff base ligand derived from
ethylenediamine and 3-methyl-I-phenyl-4-(2-thenoyl)-2-pyrazol-5-one
were synthesized. On the basis of various physicochemical studies, the
complexes have been formulated as [Ln(SB)(N03h]N03.
Singh100 synthesised a new ligand 2,6-diacetylpyridine-2
thenoylpicolynoyldihydrazone (H2dapthph) and complexes of the type
[Ln(H2dapthph)Ch]Cl. (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb and Dy). The
complexes were characterized by analytical, molar conductance, IR, NMR,
electronic and emission spectral data. The bonding sites of the hydrazone
are deduced from IR and NMR spectra and the ligand is found to bond to
the metal ion in a hexadentate fashion.
Dias lOl reported the synthesis of lanthanide(III) complexes with
isophthalic acid or thiophenyl isophthalic acid. Rio 102 et al. synthesised
and characterized ruthenium 'pincer' complexes containing thiophene
moiety. The reactions of dialkyl sulphides or sulphur dioxide with
30
[RU2C4(113-NN'Nh(Il-N2)] (NN'N = 2,6-(M~NCH2)2C5H3N) leads to the
formation of mononuclear complexes of the general formula
[RuCl2(1l3-NN'N) (1l1-SR2)] (SR2= SMe2, SEt2, C4HgS, S02)'
Jeong103 and coworkers prepared lead ion selective PVC
membranes that were based on N, N' -bisthiophene-2-ylmethylene-ethane
1,2-diammine as a membrane carrier.
Chaviara104 et al. prepared two novel mononuclear copper(II)
coordination compounds of the type [Cu(dptas)CI2] and [Cu(dptas)Br2]
(dptas = 1,3-propanediamine-N(I)-[3-aminopropyl]-N(3)-[2-thienyl-
methylidene] as Schiff base of dipropylene triamine with thiophene-2
carboxaldehyde. The X-ray determined structure of the compound
[Cu(dptas)Ch] was confirmed by spectroscopic methods, magnetic and
molar conductivity measurements.
Sharaby105 synthesised and characterized the Schiff base thiophene
2-carboxaldehyde sulphametrole (HL) and its tri-positive and di-positive
metal complexes. The low molar conductance values suggested the non
electrolytic nature of these complexes. IR spectra showed that m.... is
coordinated to the metal ions in a tetradentate manner through hetero
five-membered ring-S, azomethine-N, enolic sulphonamide -OH and
thiadiazole -N, respectively.
31
Mohanan106 et al. reported the condensation of 2-amino,.3-
carboxyethyl-4, 5, 6, 7-tetrahydrobenzo[b]thiophene with salicylaldehyde
and prepared a series of copper(II) complexes of the type, [Cu(SAT)X] in
the presence of different coordinating anions (X) such as chloride, bromide,
nitrate, perchlorate, acetate or sulphate. These complexes were
characterized by elemental analyses, molar conductance, magnetic
susceptibility, UV-Visible, IR and EPR spectral data. The spectral studies
revealed that the ligand is bonded to the metal ion through the deprotonated
phenolate oxygen, azomethine nitrogen and ester carbonyl group.
Howell 107 and coworkers synthesized and characterized a series of
new thiophene amides containing a pyridine ring and their copper(II)
complexes.
Dissouky108 et al. synthesized and characterized new complexes
having the formulae [L2CoX2], [LCuCh], [LCuCl] and [LCu(Cl04)2].
Where L = (2-thiophene)-(5,6-diphenyl-[1,2,4]-triazin-3-yl)hydrazone,
TDPTH; X = Cl, OAc or Cl04. The IR spectra indicate that TDPTH is a
neutral bidentate ligand, coordinating via a triazin nitrogen and azomethine
nitrogen in [L2CoX2] and [LCuC12] with the thiophene sulphur · not
coordinated but is tridentate in [LCuCl] and [LCu(Cl04h] through the same
two nitrogen atoms and thiophene sulphur.
32
Maurya 109 and coworkers reported the synthesis, magnetic and
spectral studies of some mono- and binuclear dioxomolybdenum(VI)
complexes with chelating hydrazones derived from salicylic acid I benzoic
acid I isonicotinic acid hydrazide with thiophene-2-aldehyde or furfural
with general formulae [Mo02(L1) (OH)] and [Mo02(L2)(0H)h-
Loire110 and coworkers synthesised oligothiophene
substituted arene tricarbonyl chromium complexes using efficient
palladium and nickel catalysed coupling reactions. a-coupled
polythiophenes have unusual properties and have attracted substantial
interest.
Singh and Srivastav111
prepared the complexes ML2 (HL=2-
(salicylideneamino )benzophenone-2-thenoylhydrazone; M=VO, Mn, Co,
Ni, Cu, Zn), M(HL)2Ch (M=Mn, Co, Ni, Cu, Zn) and [VO(HL)i]S04 and
studied them by elemental analyses, molar conductance, magnetic
susceptibility, electronic, ESR, IR and NMR (1H and 13C).
Singh and Tiwari 112 reported the reaction between N-(thiophene-2
carboxamido )salicylaldimine (H2 TCS) and lanthanide complexes in the
presence of KOH to give K[Ln(TCS)2] (Ln = La, Pr, Nd, Sm, Eu, Gd
and Dy). A tentative structure of these six coordinate complexes is
suggested on the basis of molar conductance, TGA and DTA, magnetic
33
susceptibility, electronic, IR and IH and 13C-NMR data. The nephelauxetic
ratio (~), covalency (b) and bonding parameter (bl/2) were calculated from
the electronic spectrum of the Nd(III) complex.
Mishral13 et ai. studied the reaction of Schiff bases derived
from thiophene-2-aldehyde and ethylenediamine, o-phenylenediamine
or 4-methyl-o-phenylenediamine with bis(cyclopentadienyl)titanium(IV)
and zirconium(IV) dichloride in THF, in the presence and absence of base.
Two types of derivatives [Cp2MChL] and [(Cp2MCI)2L]Ch have been
isolated and characterized by elemental analyses, conductance
measurements, magnetic moment and spectral (electronic, IR,tH and 13C_
NMR) studies.
Xu114 et al. prepared twelve lanthanide(III) complexes,
Ln(TBGhnH20 [Ln = La, Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, n=3;
Ln = ·Nd, n = 4; HTBG = 4-(21~thiophenaldiminobenzoinglycin)]. The
complexes were characterized by elemental analyses, magnetic moment
and molar conductance measurements, IR, UV and IH NMR spectra as
well as TGA and DSC methods.
Alsfasser and Vahrenkamp 115 carried out reactions of
bis(pyrazol-l-yl)(thien-2-yl) methane (L) with zinc halides and .obtained
complexes [LZnX2] (X = CI, Br, I). A structure determination of the Br
34
complex showed it to be tetrahedral with 2Br and 2pyrazole N ligand
atoms. The thiophene sulphur is non-coordinating.
Mishra and Srivastava116 reported the synthesis and characterization
of ML2S04.nH20 (M = Co, Ni, and Cu, L = Schiff base formed from
thiophene-2-aldehyde and nicotinamide).
Agrawal and SaxenaII7 synthesised and characterized ML(H20)2
(M = Mn, Co, Ni, eu; H2L = 2-thienylglyoxal-4-iminobenzoic acid)
and ~ I (HLI = p-tolyl-2-thienylglyoxalimine-2-thienylglyoxal-2I
iminopyridine) on the basis of elemental analyses, magnetic moments, IR
and electronic spectral studies.
Balaloet al. reviewed Schiff bases derived from heterocyclic
compounds such as pyrrole, di-and triazoles, pyridine, furan and thiophene
and their transition metal complexes.
Habibi and Movahhed118 prepared and characterized a new
macrocyclic Schiff base ligand(L) by the 1:2 condensation of 2, 5
bis(aminomethyl)thiophene with 2-formyl pyridine and the relevant Ni(II)
and copper(II) complexes.
Zahid and Farooq119 reported some acylhydrazine derived furanyl
and thienyl Schiff bases and their cobalt(II), copper(II), nickel(II) and
zinc(II) complexes. The Schiff base ligands functioned as tetradentates
35
forming octahedral complexes with cobalt(II), nickel(II) and zinc(II) ions
and a square planar complex with the copper(II) ion.
Cheri120 et at. synthesised three copper(II) complexes with
thiophene-2,5-dicarboxylic acid (H2Tda) and 1, 10-phenanthroline(phen)
ligands and structurally characterized: [Cu(Tda)(phen)(H20h].3H20(l),
[Cu(Tda)(phen)2] Jh H 2Tda.2H20 (2) and [{ Cu(Tda)(phen) H20 }n] (3). The
copper atom in all of the three complexes is in five coordinate, distorted
square pyramidal environments. The thiophene-2,5-dicarboxylate molecule
is monodentate in (1) and (2) in which the molecular structure is stabilized
by intermolecular aromatic ring stacking interaction between the thiophene
ring and the aromatic ring in the 1,10-phenanthroline molecule and
hydrogen bonding. In (3) the thiophene-2,5-dicarboxylate IOn IS
bis-monodentate and bridges the [Cu(phen) (H20)] units to form a
one -dimensional polymer.
Teoh121 and coworkers prepared thiophene-2-carboxaldehyde
thiosemicarbazone, C4H3S-CH N-NH-C(S)NH2 (tctscH) by the
condensation reaction of thiophene-2-carboxaldehyde with
thiosemicarbazide and synthesized [Snph2CI(tctsc)]. They found that
36
(tctscH) deprotonated and functioned as an anlOnlC bidentate ligand,
coordinating to the tin atom through its azomethine-nitrogen and thiol
sulphur atoms.
The complexes [Zn2(S2CTR)4] (T =2, 5-disubstituted thiophene,
R = C4H9 (1), C6H13 (2), C5H17(3), C12H25(4) and C16H33(5) have been
recently synthesized and their structural features investigated by
Thomas122 et al.
Meyer123 et al. synthesised titanocene complexes [TiCp2(CI)R](l),
[TiCp2(CI)SR](2), [TiCp2(SR)2](3) with R =benzothienyl, (BT)A and
dibenzothienyl ,(DBT)B.
Sengupta124 and coworkers synthesised a series of ruthenium(II)
complexes of potentially NNS tridentate but functionally NS bidentate
chelating ligands in the form of 4-substitnted-4-phenyl and 4-cyclohexyl
thiosemicarbazones of ~yridine-2-aldehyde and thiophene-2-aldehyde
(LH) using Ru (PPh3)Ch as the starting material. All the complexes were
characterized by elemental analyses, measurements of conductance in
solution, magnetic susceptibility at room temperature and by spectroscopic
techniques.
Shailendra125 et al. prepared a senes of ~-substituted
thiosernicarbazones palladium complexes derived from thiophene-2-
37
carboxaldehyde. These complexes were characterized by analytical
methods and spectral studies.
Chohan 126 and coworkers described the preparation and ligational
properties of some pyrazinedicarboxamide derived furanyl, thionyl and
pyrrolyl compounds with cobalt(II), copper(II), nickel(II) and zinc(II)
metals. Magnetic moments, electronic, IR, NMR spectra and elemental
analyses data indicate that coordination of the ligands with the metal ions
takes place through the pyrazine ring nitrogen, azomethine nitrogen and
heteroatom of heterocyclic ring system.
Liu 127 et al. synthesised europium and terbium complexes of 2, 5-
(2-thiophene )pyridine (TPY) and 5, 5 1-bis( 5-(2,2 1 -bithiophene) )-2,21-
bipyridine (B2TBPY).
Chen128 and coworkers synthesized five copper(II) complexes with
thiophene-2,5-dicarboxylic acid (H2Tda) : [Cu(Tda}(H20)].(H20h (I),
[Cu(Tda)(im)2].H20 (2), [Cu(Tda)(im)4] (3), [Cu(Tda)(Py)2Jn(4) and
[Cu(Tda)(bipy)(H20)Jn.n[Cu(Tda)(bipy)(H20)2].2nH20(5) (im = imidazole,
Py = pyridine and bipy = 2, 21 -bipyridine) and then spectroscopic and
thermal properties investigated. Three of them (3, 4 and 5) are structurally
characterized and the copper atom is in five coordinate, distorted square
pyramidal environments.
38
Gupta129 et al. reported palladium(II) compl~xes of Schiff bases
derived from thiophene-2-aldehyde and hydrazine sulphate. The complexes
were characterized through elemental analyses, magnetic moment
measurement, molecular weight determination, molar conductance and
spectral studies. A neutral diamagnetic complex with square planar
geometry has been isolated.
Complexes of some 3d transition metals and uranyl ions with
thiophene-2-aldehydethiosemicarbazone have been reported.130
Palladium(II) complex of thiophene-2-aldehydethiosemicarbazone has
been studied by Mukkanti131 and coworkers. The complexes have been
formulated as [Pd(TATS)2X2]. The ligand acted in a bidentate fashion
coordinating through the azomethine nitrogen and hydrazine group. Later
Guan132 et al. also reported several complexes of the same kind. Complexes
of the type [SnPh2CI(TATS)2] and [SnPhCh (TATS)2] respectively were
formed with SnPh2Ch and SnPhCh. In both the complexes, TATS was
deprotonated and it functioned as an anionic bidentate ligand coordinating
to the Sn(IV) through the azomethine nitrogen and thiol sulphur atom.
Copper(II) complexes of this ligand were also reported. 133
39
Kumar and Radhakrishnan134 synthesized lanthanide(III) complexes
with 4-[N-(SI-nitro-21-thienylidene)aminoJantipyrine. The ligand acted as
neutral tridentate; yielding complex of composition [Ln(NTAAPh(CI04hJ
CI04 in which the ligand bonded through carbonyl oxygen, azomethine
nitrogen and sulphur of the thiophene ring. A coordination number of eight
was assigned.
Very recently Mohanan and Devi135 synthesized two series of new
lanthanide(III) complexes of the type [Ln(HSAT)z(HzO)3CI3J and
[Ln(HSAT)z(N03)3J, where Ln =La, Pr, Nd, Sm, Eu, Gd, Dy, Tm, Yb
or Lu and HSAT = 2-(N-salicylideneamino)-3-carboxyethyl-4,5,6,7
tetrahydrobenzo(b)thiophene. The complexes were characterized by
elemental analyses, magnetic moment values, molar conductivity, IR,
UV-Visible and 1HNMR spectral data.
Anderson136 and coworkers conducted the reaction
of [pt2Me4(/-t-SMezhJ with substituted urumc thiophenes and
2-phenylpyridine gives platinum(II) [C,NJ cyclometallated complexes
which contain a labile ligand (SMez or CH3CN). Several platinum(Il)
complexes have been synthesized by substitution reactions with phosphine
or sulfoxide ligands to introduce a second chiral center. The reactions of the
new complexes with methyl iodide were subsequently studied and they
40
showed results that are dependent on the steric and electronic effects of
both the cyclometallated ligand and the ancillary phesphine or sulfoxide
ligand. The structure of [PtMe(R)-CIOH7CHMeNCHC4H2S)(CH3CN)], a
synthetic precursor is also reported.
Munakata137 et al. synthesized three novel silver(1) complexes
with 1, 2-bis[2-methyl-5'-(2"-pyridyl)-3' -thienyl]perfluorocyclopentene
(BM-2-PTP) by the reaction of Ag(CF3S03) or Ag(CF3COO) with
(BM-2-PTP) in benzene at different temperatures. The difference in
structures of the three complexes shows the interesting anionic effect on
coordination and the subtleness of crystal engineering.
Vetter138 and coworkers conducted the reaction of
[PtMe3(OAC)(bpy)] with N, S and S, S containing heterocycles and
thiophene-2-thiol, resulting in the formation of monomeric complexes. The
complexes were characterized by microanalysis, IH and l3C NMR
spectroscopy.
SU l39 et al. prepared two new 14-membered hexaaza macrocyclic
complexes with the formulae [NiL](CI04)2 CH3COCH3 (I) and
[CuL](CI04)2.CH3COCH3 (2), where L =3, IO-bis(2-thiophene methyl)
1, 3, 5, 8, 10, 12-hexaazacyclotetradecane and characterized by elemental
analyses, electronic spectra, 1R and TG-DTA.
41
Sharaby140 synthesized [M2X(HL)(H20)4J where M =manganese(ll),
cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II); X =; CI and
HL = 2-thiophene carboxaldehyde sulfametrole, [F~CI6(HL)(H20)2J,
[Fe(S04h(HL)(H20)4J and [(U02)2(HL)(N03)4].H20. They are
characterized by elemental analyses, IR, IH NMR, solid reflectance,
magnetic moment, molar conductance, mass spectra and UV-Visible
methods.
Lobana141 and coworkers conducted the reaction of copper(D
chloride with thiophenecarbaldehydethiosemicarbazone (HttSC) in
acetonitrile in the presence of Ph3P yielding a sulphur bridged
similar
reaction with isatin-3-thiosemicarbazone (H2itSC) formed a monomer,
[CuCl(H2itSC)(ph3P)2].2CH3CN (2). The complexes have been
characterized using elemental analyses, IR, I H and 31p NMR
spectroscopy and single crystal X-ray crystallography.
Howell142 et al. prepared and characterized complexes of zinc(II),
copper(II) and cobalt(ll) with either N-(2-methylpyridyl)-3-thienyl-alkyl
carboxamide or N-(2-pyridyl)-3-thienylalkylcarboxamide groups. With all
ligands, bidentate complexation is through the carbonyl oxygen and
42
pyridine nitrogen atoms and the amide nitrogen atom remain
protonated.
Biological activity
Schiff bases and their metal complexes are well known for their
biological activities.143-145 The Schiff bases derived from heterocyclic
aldehydes such as furan-2-aldehyde, thiophene-2-aldehyde and pyridine-
2-aldehyde have been found to act as potential ligands and they show
'd bl 'b 'al .. 146-158consl era e antI acten actIvIty.
Mohanan and Devi135 conducted the screemng of the
ligand,2-(N-salicylideneamino)-3-carboxyethyl-4,5,6,7-tetrahydrobenzo[b]-
thiophene and its lanthanide complexes for their antimicrobial properties
and found that antimicrobial activities of the ligand increased III
coordination with the metal ions.
Teoh 121 et al. studied the anticancer activity of [SnPh2CI(tctsc)](I)
and [SnCh(tctsC)2] (2), (tctscH) =C4H3S-CH =N-NH-C(S)NH2 and found
that (I) is an effective complex against the cell tested: remarkable activities
were found against human breast adenocarcinoma and human acute
lymphoblastic leukemia.
Biological properties of some pyrazinedicarboxaimide derived
furanyl, thienyl and pyrrolyl compounds with cobalt(II), copper(II),
43
nickel(II) and zinc(II) metals were studied by. Chohan126 et al. The ligands
and metal complexes were screened against bacterial species Escherichia
coli, Pseudomanas aeruginosa, Staphylococcus aureus and Klebsiella
pneumonae. The results showed that ligands and all their metal complexes
were biologically active against one or more bacterial species and the metal
complexes have been shown to be more antibacterial than the simple
uncomplexed parent ligands.
Thiophene-2-carboxaldehyde thiosemicarbazones and their
palladium(II) complexes were tested125 for amoebiasis in vitro against
(HM-l : IMSS) strain of E histolitica by micro dilution method. 149 It was
found that most of the palladium(II) complexes showed significant
antiamoebic activity.
Antiproliferative activity of [Cu(dptas)Ch] and
[Cu(dptas)Br-(dptas)2](dptas) = 1,3-propanediamine N(I)-[3-aminopropyl]
N(3)-[2-thienylmethylidene] was examinedlO4 against a panel of different
normal and cancer cell lines and it showed that the copper(II) Schiff bases
exhibit increased activity as compared to the starting materials.
The ligand obtained by the condensation of 2-amino-3-carboxyethyl
4,5,6,7-tetrahydrobenzo[b]thiophene with salicylaldehyde and its copper
complexes of different counter anions have been screenedlO6 for
44
antibacterial activity against E.coli, S.aureus and S.caure with a view to
studying the effect of anion coordination on biological properties and found
that the ligand has been physiologically active and chelation enhanced its
activity. It was also found that the nitrate complex exhibited less inhibition
when compared to the chloro complex. It was explained in such a way that
the bonding capacity of nitrate ion towards the central metal ion was
greater than that of the chloride ion.
The biological activity of [NiL2J (I), where HL = thiophene-2
carbaldehyde thiosemicarbazone, is compared150 to that of the free ligand
and the cobalt(II) (2) and copper(II) (3) derivatives. The observed order of
cytotoxicity against melanoma B16FIO and Friend erythroleukemia cells is:
1.$ ligand < 2 < 3.
Sharabyl40 performed the antimicrobial activity of
[M2X4(HL)(H20)4J (where M = manganese(II), cobalt(II), nickel(II),
copper(II), zinc(II) and cadmium(II); X = CI and HL = 2-thiophene
carboxaldehydesulfametrol), [Fe2CI6(HL)(H20)2J, [Fe(S04)2(HL)(H20)4J
and [(U02)z(HL)(N03)4]. H20 using Chloramphenicol and Grisoflumine as
standards, indicating that in some cases metallation increases activity
more than the ligand.
45
Reactivity
Reactions involving coordinated ligands have evoked considerable
interest in recent years mainly because of their applications. Most of such
studies have been carried out on transition metal complexes with different
types of ligands. 106,151 There are several reports that metal complexes of
carboxylic esters undergo facile transesterification on refluxing with
alcohol. 152,153 The complex [La(HSAT)2(N03)3], where (HSAT) =2-(N
salicylideneamino)-3-carboxyethyl-4,5,6,7-tetrahydrobenzo[b]thiophene,
has been subjected135 to transesterification reaction in methanol medil;lm. It
was found that the crystallinity, appearance and solubility behaviour of the
product obtained after transesterification were distinctly different from
those of [La(HSAT)2(N03h].
Ruthenium(III) complex of 5-chloro-2-hydroxybenzophenone
thiophenecarbaldehyde-o-phenylenediamine is reported to be capable of
catalyzing the oxidation of styrene by sodium hypochlorite in the presence
154of a phase transfer agent.
Pal and Tarafder155 have investigated the kinetics and mechanism
of oxidation (using H20 2) of [Cr(1,3-DAP)2(NCS)2]NCSAH20 and
[CrLCh]C1.2H20 (L = Schiff base formed from N,N-bis-(2-aminoethyl)
1,3-propanediamine and thiophene-2-aldehyde), spectrophotometrically
46
at 30 °e. The reactions proceeded through a rapid pre-equilibrium
formation of a peroxide adduct which dissociate through oxygen-hydrogen,
metal-chlorine and oxygen-oxygen bonds to give a dioxochromium(IV)
product. This product on further oxidation by a second hydrogen peroxide
molecule gave a chromium(VI) oxoproduct. Different kinetic parameters
of this reaction have been calculated and the general mechanism of the
reaction has been postulated.
Sehlotho and Nyokong156 reported reduction of oxygen
electrocatalyzed by adsorbed films of manganese tetraethoxythiophene
phthalocyamine. The reaction was conducted in buffer solution of pH
range 1 - 12. Rotating disk electrode voltammetry revealed two electron
reduction in acidic and slightly alkaline media due to the formation of
hydrogen peroxide. In highly basic media, water is the major product
formed via four electron transfer. The reaction was found to be first order
in the diffusing analyte oxygen.
Albano 157 and coworkers conducted a combined catalytic and
crystallographic investigation· on the effectiveness of chiral diamino
lithiophene (2) and the corresponding diamino molecules (I) as chiral
ligands for Pd(II) catalysed transformations.
47
Thermal behaviour
Thermal studies on metal complexes of Schiff bases containing
thiophene moiety are rare, especially of lanthanide complexes.
Thermal stabilities of rhodium(IIl), palladium(lV) and platinum(IV)
complexes of thiophene-2-aldehyde-o-toluidine have been compared by
Athar and Ahmed. 158 The order of stabilities was found to be Rh>Pd>Pt.
Singh and Agarwala93 examined thermal stabilities of some
lanthanide(IIl) complexes of thiophene-2-aldehydesalicyloylhydrazone.
Sharaby105 reported the thermal study of the ligand HL derived
from [4 1_(4-methoxy-1 ,2,5-thiadiazol-yl)sulphanilamide] and thiophene
2-carboxaldehyde and its Zn(II), Co(Il), Ni(II), Fe(IIl), and DOz(II)
complexes. The thermogram of the Fe(III), Co(Il), Ni(II) and Zn(II)
complexes show three to five steps of decomposition while DOz(II)
complex has four steps of decomposition. The kinetic parameters were
evaluated by employing the Coats-Redfern equation. It was found that all
the complexes had negative entropy values indicating that activated
complexes had ordered.
Aravindakshan and Muraleedharan 159 carried out thermal analysis of
newly prepared MLz (M = Ni, Pd; HL = thiophene-2-carboxaldehyde
thiosemicarbazene) complexes by Coats-Redfern equation.
Chen 128 et al. conducted
48
thermogravimetric studies for
the complexes [Cu(Tda)(H20)](H20)2 (I), [Cu(Tda)(im)z].H20 (2),
[Cu(Tda)(Py)z]n(3) (H2Tda = thiophene-2,5-dicarboxylic acid, im =
imidazole and py = pyridine). They found that (1) first loses water of
crystallization between 100 °C and 170 °C with a total loss of two water
molecules and then losing the coordinated water molecule at 170 °C. In
complex(2) the water molecule of crystallization was lost in 120-160 °C
temperature range while complex(3) lost the first pyridine molecule
between 180 °C and 235 °C and the second molecule in the 235-275 °C
temperature range.
Thomas122 and coworkers reported the thermal properties of the
complexes [Zn2(S2CTR)4] (T = 2,5-disubstituted thiophene, R = C4H9(l),
C6H13(2), CsH17(3), C12H25(4) and C16H35(5) by DSC. Mesa phases and
liquid crystal phases can be inferred from DSC measurements by detecting
the enthalpy change that is associated with a phase transition. Complexes
(1), (2) and (5) showed sharp single endothermic peaks while (3) and (4)
showed two broad endothermic peaks.
Mohanan and Devi 135 conducted thermal studies on the complexes
[La(HSAT)2(H20)3Cl3] and [La(HSAT)i(N03)3], where (HSAT) =
2-(N-salicylideneamino)-3-carboxyethyl-4,5,6,7-tetrahydrobenzo[b]-
49
thiophene. The thermogram of the aquachlorocomplex indicated three
stage decomposition involving a loss of coordinated water molecules,
oxidation of the organic moieties while that of the nitrato complex showed
only a two stage decomposition pattern. The residue in both cases was
found to be La203.
Sharaby140 studied the thermal behaviour of [M2X4(HL)(H20)4J
(where M =Mn(H), Co(H), Ni(H), Cu(H), Zn(H) and Cd(H); X = Cl and
HL = 2-thiophenecarboxaldehydesulfametrole), [Fe2C16(HL)(H20)2J,
[Fe(S04h(HL)(H20)4J and [(U02)2(HL)(N03)4]' H20 and found that the
hydrated complexes lost water of hydration in the first step in the case of
uranium complex, followed by loss of coordinated water, followed
immediately by decomposition of anions and ligand molecules in the
subsequent steps. The activation thermodynamic parameters such as ~E*,
. ~H*, ~S* and ~G* are calculated from the DTG curves using Coats
Redfern method.
X-ray diffraction study
Singh and Srivatsavll1 conducted X-ray powder diffraction
studies for the complexes ML2 [HL = 2-(salicylidenearnino)
benzophenone-2-thenoylhydrazone: M = Vo, Mn, Co, Ni, Cu, ZnJ,
50
M(HLhCI2 (M = Pm, Co, Ni, Cu, Zn and [VO(HLh]S04' Lattice
parameters are reported for the orthorhombic copper complexes. Square
pyramidal geometry around oxovanadium(II) and octahedral geometry
around the rest of the metal ion are proposed.
Delgad092 et al. carried out X-ray diffraction studies for
[IrH2(111S-T)2(PPh3h]PF6 (T - thiophene,benzo[b]thiophene, dibenzo[b]-
thiophene and tetrahydrothiophene) and concluded that the coordination
geometry around the metal atom consisted of a distorted octahedron
with mutually cis S-bonded thiophenes, cis hydrides and trans
triphenylphosphines.
D· 101 d d .las reporte the X-ray diffraction studies of lanthani e Ion
complexes with isophthalic acid or thiophenyl isophthalic acid and
confirmed the cross linking structure of the complexes.
Mohanan106 and coworkers carried out X-ray diffraction studies on
[Cu(SAT)OAC], where HSAT = condensation product of 2-amino-3-
carboxyethyl-4,5 ,6,7-tetrahydrobenzo[b]thiophene with salicylaldehyde,
and found that the complex is crystalline. The complex was successfully
indexed to orthorhombic crystal system.
SU 139 et al. conducted single-crystal X-ray diffraction analysis of
two new 14-membered hexaaza macrocyclic complexes with the
51
formulae [NiL](CI04)2. CH3COCH3 (1) and [CuL](CI04)2. CH3COCH3
(2), where L =3, 10 - bis (2-thiophene methyl) - 1, 3, 5, 8, 10, 12 - hexaa
zacyclotetradecane and found that in (1), the nickel(II) ion is four
coordinated with four nitrogen atoms from the macrocycle and forms a
square planar coordination geometry and in (2), the copper(II) ion is six
coordinated with four nitrogen atoms from the macrocyclic ligand in the
equatorial plane and two oxygen atoms from the perchlorate anions in the
axial position exhibiting an elongated octahedron coordination geometry.
Electrochemical studies
An indepth survey of literature reveals that studies of cyclic
voltammograms of transition metal complexes especially copper complexes
have been extensively reported. 120,124,160-163 Voltammetric studies of
lanthanide complexes have been very few.
Rabia164 et al. conducted studies on cyclic voltammetric behaviour
of [La(naph.phala)(N03) I-mlm].2H20, [Ce(naph-gly)(N03) 1-mlm].2H20
and [Ce(naph-gly)(N03)1,2-mlm]. 2H20 (mlm = methylimidazole) and
found that these complexes are characterized by three cathodic irreversible
waves lacking their anodic partners in the potential range of -1.4 - (-2.4) V.
The observed waves are attributed to the reduction of the complex species.
52
Electrical conductivity, thermo electric power and refractive index
dispersion properties of the Co(II) complex of tetrasubstituted thiophene
attached phthalocyanine have been investigated by Y akuphanogiu et al. 165
Electrical conductivity and thermoelectric power results suggest that the
2,9, 16,23-tetrakis-6-(thiophene-2-carboxylate )-hexylthiophthalocyaninato
cobalt(II) complex is a P-type organic semiconductor.
Luminescence properties
Luminescence microscopy with lanthanide ions is a relatively simple
technique that offers inherent advantages over conventional fluorescence
and transmission microscopy.166-168 Luminescence rmcroscopy 1s
increasingly common for europmm and terbium complexes.169-177
Attaching polymerizable thiophene groups to chelating ligands could tum
out to be a perfect strategy for developing highly luminescent lanthanide
complexes that can be processed into thin films for a variety of
optoelectronic applications. Ana de Buttencourt Dias described her group's
effort to design such complexes.178 They developed new ligand systems
capable of doing electropolymerised, and efficiently complexing
europium(III) and terbium(III) ions and sensitizing their luminescence.
These complexes emit red and green light, respectively, which are two of
53
the three primary colours, along with blue, necessary for the manufacture of
multicolour displays.
With an aim to developing novel luminescence materials, Liu
et al. 127 synthesised europium and terbium complexes of 2,5-(2-thiophene)-
pyridine (TPY) and 5,5' -bis(5-(2,2' -bithiophene))-2,2' -bipyridine
(B2TBPY) and studied their luminescence properties. The complexes
exhibited ligand sensitized emission, which is typical of europium(III) and
terbium(III) ions.
Analytical applications
The exponential growth of knowledge on the properties of functional
groups and nature of central atom led to the development of more
sensitive and selective polydentate Schiff bases as analytical reagent. 179
Among such promising reagents, those containing thiophene ring system
have been very few.
Complexes with Schiff bases180 derived from thiophene-2-aldehyde
find application in solvent extraction of some metal ions. In most of these
complexes the ligand coordinates to the metal ion through azomethine
nitrogen and sulphur atom.
54
Bis-(thiophene-2-aldehyde )thiocarbohydrazone obtained as
condensation product of thiocarbohydrazide with thiophene-2-aldehyde has
been recommended as a sensitive complexing agent181
for the
spectrophotometric determination of ruthenium(III) and iridium(III).
Thiophene-2-aldehyde-2-quinolylhydrazone has been an effective
reagent in the spectrophotometric determination of vanadium(V). Another
Schiff base viz., thiophene-2-aldehyde-2-benzothiazolylhydrazone finds
application in the spectrophotometric determination of copper(II) ions. 182• 183
Trans-2-thiophene trans-aldoxime can be used to determine palladium
gravimetrically. 184 2-thiophenecarbohydroxamic acid was used as an
effective gravimetric reagent for vanadium. 1 85
A new chelating agent 186
2-thiophene aldehyde -4-phenyl -3- thiosemicarbazone has been examined
for HPLC separations of cobalt(II), copper(II) and iron(II) or cobalt(II),
nickel(II), iron(II), copper(II) and mercury(II) metal chelates. These
chelates were eluted with methanol, acetonitrile, water containing sodium
acetate and tetra butyl ammonium bromide.
Conclusion
Review clearly indicates that although coordination chemistry of
polydentate hetrocyclic Schiff bases with transition metal ions has been
55
receiving increased attention, that of polydentate thiophene Schiff bases
with lanthanide ions has been scantly investigated, thus affording ample
scope for detailed and systematic further investigation. Researchers will
realise that chemistry of this class of complexes is rich and poses
interesting challenges to inorganic chemists.
Microanalytical, conductance, magnetic susceptibility, IR, UV-
Visible, 1H NMR and thermogravimetric data have been largely used in
these reports for elucidating composition and structure of the
complexes. 13C NMR and EPR spectral data have also been used sparingly.
There are rare cases where X-ray powder diffraction study was used to
establish the structure. There are also very rare cases where cyclic
voltammograms were used in studies of the complexes. Spectral parameters
were calculated in quite a few studies for assessing the covalent character
of the metal ligand bonding. Antibacterial and antifungal activities of some
ligands and complexes spread light on promising biological applications.