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

26

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

28

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.