Journal of Alloys and Compounds 392 (2005) 112–119

Binuclear lanthanide complexes of 2,3-dihydroxy-1,4-naphthoquinone

Prasad Khandagalea, Rajeev Chikateb,∗, S.B. Joshia, B.A. Kulkarnia

a Department of Chemistry, University of Pune, Pune 411007, Indiab Department of Chemistry, Post-graduate and Research Center, MES Abasaheb Garware College, Karve Road, Pune 411004, India

Received 5 August 2004; received in revised form 20 September 2004; accepted 21 September 2004Available online 8 December 2004

Abstract

Binuclear octa-coordinated lanthanide(III) complexes of 2,3-dihydroxy-1,4-naphthoquinone (isnphth) possessing [M2L3(H2O)4] composi-tion are synthesized and characterized by infrared spectroscopy,1H and13C NMR spectroscopy and thermal studies. The thermal decompositionrevealed two stage patterns, 1st step corresponding to loss of four coordinated water molecules while the bondedisnphthmolecules are decom-posed during 2nd step. During the 2nd stage, the structure of these complexes is reorganized forming two distinct monomeric species implyingthat the ligand bridges as well as intermolecular associations are essential for thermal stability. Energy of activation (Ea) for 2nd stage exhibitsl 1 13 ngh carbonylsa ated watera as revealedf©

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inear increase with decrease in the size of lanthanide ion.H and C NMR spectrum ofisnphthand its lanthanide chelate signifies stroydroxyl oxygen and quinone binding. IR absorption spectral data indicate bonding through both hydroxyl oxygen with quinonecting as bridge between lanthanide ions and there exists intensive network of intermolecular hydrogen bonding involving coordinnd quinone carbonyls. The absorption spectrum of Nd(III) complex suggests a weak covalent character in metal–ligand bonding

rom nephelauxetic ratio (β = 0.973), covalent factor (b1/2 = 0.0641) and Sinha’s factor (δ = 1.705).2004 Elsevier B.V. All rights reserved.

eywords:Lanthanides complexes; Dihydroxy-naphthoquinone; Binuclear complexes; Thermal decomposition; Energy of activation; NMR spectr

. Introduction

Quinone, as a class of ligand, constitutes important familyf biologically relevant molecules in many biological pro-esses such as electron transfer and redox reactions. Theyave been found to possess variety of structural peculiari-

ies forming mononuclear and polynuclear metal complexesith transition and lanthanide ions incorporating fully oxi-ized quinone; one electron reduced semiquinone and fullyeduced catechol forms[1]. Even though, the coordinationhemistry of transition metal–quinone complexes has beenhe subject of exploration for variety of purposes[2,3]; it isather surprising that there are relatively few reports on thetructural propensities of lanthanides–quinone chelates. For

∗ Corresponding author. Present address: Brain-Pool International Fellow,icro-Chemical Technology Laboratory, Korea Research Institute of Chem-

cal Technology, P.O. Box 107, Yuseong, Daejeon 305 600, South Korea.el.: +91 20 2543 7855; fax: +91 20 2544 0728.E-mail address:rajuchikate@rediffmail.com (R. Chikate).

example, there are reports on the polymeric complexesanthraquinone[4] and embelin[5], amino-naphthoquinon[6] and halolawsone-monoximes[7].

Lanthanide complexes possess a variety of applicain the field such as NMR shift reagents, catalysts fordrogenation and asymmetric synthesis, liquid crystalssurfactants, lasers and amplifiers, electroluminescentrial and biosensors for molecular recognition and chiralitbiological substrates[8].

In continuation of our earlier efforts in understandthe coordination properties of quinone towards lanthanwe have reported the thermal decomposition patternlanthanide chelates of naphthoquinones[6,7] wherein itwas observed that the complex molecules are stronglythrough the extensive network of intermolecular hydrobonding contacts involving coordinated water moleculesbound naphthoquinone ligands. The present communicdescribes the thermal and spectral properties of lanthchelates of 2,3-dihydroxy-1,4-naphthoquinone (Fig. 1,isnphth), a derivative of 2-hydroxy-1,4-naphthoquino

925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.

oi:10.1016/j.jallcom.2004.09.050

P. Khandagale et al. / Journal of Alloys and Compounds 392 (2005) 112–119 113

Fig. 1. (a) 2,3-Dihydroxy-1,4-naphthoquinone and (b) computer simulated structure ofisnphthusing energy minimization program.

(lawsone).Fig. 1b depicts the computer-simulated structureof isbphthusing energy minimization program.

2. Experimental

All the chemicals used were of AR or equivalent grade.Solvents were purified by literature methods. The lanthanidechlorides were purchased from Aldrich, Central Drug House(P) Ltd. and Indian Rare Earth Company.

2.1. Synthesis of ligands

Isnphth is synthesized according to reported procedure[9].

2.1.1. Synthesis of lanthanide chelatesThe lanthanide chelates are prepared by a general proce-

dure given below.To a solution of 3 mmol ofisnphth (0.54 g) in 25 ml

of methanol, an aqueous solution of 1 mmol of metal(III) chloride hexa-hydrate was added with constant stir-ring. The mixture was allowed to reflux for 3 h in oilbath. After cooling the solution at room temperature, theproduct was obtained by adjusting the pH of the solu-tion (7.5–8.0) using 5% aqueous ammonia solution. Thes andm linep

2

landC d atr witha ectraw ec-tU car-r her-m do ingra spec-

trophotometer (300 MHz) in DMSO-d6 while chemical shiftswere measured against TMS.

3. Results and discussion

All the poly-chelates are amorphous in nature, bluecoloured products having [M2L3(H2O)4] stoichiometry(Table 1) and are insoluble in most of the organic solvents butare soluble in DMSO, DMF probably indicating presence ofbinuclear/polynuclear structures. The lowering in the roomtemperature magnetic moment values probably indicate thepresence of antiferromagnetic interactions between the lan-thanide pairs through ligand bridges in the complexes[4–7].

3.1. Thermal studies

The non-isothermal degradation profiles ofisnphthandits lanthanide complexes are examined for their decomposi-tion characteristics. The ligand is thermally stable (∼175◦C),close to the melting temperature and on further heating asharp decrease in weight loss is observed corresponding toan exothermic peak. It represents the 1st stage of decompo-sition probably involving phenolato and quinone functionswhile there is a gradual degradation of remaining part of lig-a uallya g thed

(III)cc grad-u harpd o-s a-t ts thes andb tactsb

reaki er -ia ules,i ed in

olid thus formed was filtered, washed with cold waterethanol and finally dried in vacuum giving semi-crystalroduct.

.2. Physical measurements

Elemental analyses were carried out using a Hosli-Hol, H Analyzer. The magnetic susceptibility was measure

oom temperature on a Faraday type magnetic balancepermanent magnetic field of 7000 G where as IR spere recorded in nujol mulls on a Perkin-Elmer FT-IR sp

rophotometer (Model 1600) in the range 4000–400 cm−1.V–visible spectral measurements (200–800 nm) were

ied out on Simatzu UV-160 spectrophotometer while togravimetric profiles in the range 0–800◦C were obtainen Mettler Toledo 851, USA analyzer in air with a heatate of 10◦C min−1. 1H and13C NMR spectrum ofisnphthnd its lanthanide complexes were recorded on Varian

nd during 2nd step. Such a behavior is found to be usssociated with quinone derivatives probably supportinisturbance in the quinonoidal structure[1].

Fig. 2 represents the thermal patterns of lanthanideomplexes in the temperature range 50–800◦C. Thermolyticurves of complexes reveal that they decompose in aal manner with two distinct stages, in contrast to the secomposition ofisnphth[10,11]. The 1st stage of decompition (190–225◦C) indicates the loss of four coordinated wer molecules. Higher temperature for this stage suggestronger associations of complex molecules through ligridges as well as intermolecular hydrogen bonding conetween coordinated water and bonded ligands[3,12].

After the initial loss during the 1st stage, very sharp bn the temperature range 310–500◦C has occurred with thapid change in the weight for the 2nd stage[13], correspondng to the loss of coordinated ligand molecules[14,15]. As

consequence of the loss of coordinated water molect seems that the structural reorganizations have occurr

114 P. Khandagale et al. / Journal of Alloys and Compounds 392 (2005) 112–119

Table 1Analytical and magnetic moment data of lanthanide chelates ofisnphth

Complex Yield (%) Elemental analysisa (%) µeff (B.M.)

C H M

La2 C30H20O16 [M2(L)3 (H2O)4] 81.5 39.08 (39.41) 2.60 (2.20) 30.56 (30.38) DiamagneticCe2 C30H20O16 [M2(L)3 (H2O)4] 81.3 39.17 (39.30) 2.81 (2.19) 30.14 (30.56) DiamagneticPr2 C30H20O16 [M2(L)3 (H2O)4] 84.5 39.14 (39.23) 2.34 (2.19) 30.43 (30.68) 3.99Nd2 C30H20O16 [M2(L)3 (H2O)4] 85.5 38.53 (38.95) 2.58 (2.18) 31.24 (31.18) 4.11Sm2 C30H20O16 [M2(L)3 (H2O)4] 81.5 37.82 (38.44) 2.87 (2.15) 32.24 (32.08) DiamagneticEu2 C30H20O16 [M2(L)3 (H2O)4] 83.5 37.44 (38.31) 2.88 (2.14) 32.72 (32.31) 4.7Gd2 C30H20O16 [M2(L)3 (H2O)4] 81.5 37.34 (37.89) 2.86 (2.12) 33.15 (33.07) 9.9Tb2 C30H20O16 [M2(L)3 (H2O)4] 83.5 37.36 (37.75) 3.02 (2.11) 33.10 (33.30) 11.0Dy2C30H20O16 [M2(L)3 (H2O)4] 82.5 37.91 (37.47) 2.53 (2.09) 33.10 (33.80) 10.61Ho2 C30H20O16 [M2(L)3 (H2O)4] 84.5 37.72 (37.28) 2.17 (2.08) 33.92 (34.13) 2.74Er2 C30H20O16 [M2(L)3 (H2O)4] 80.5 37.21 (37.11) 2.87 (2.07) 37.14 (34.46) 3.10Yb2 C30H20O16 [M2(L)3 (H2O)4] 84.5 36.08 (36.67) 2.80 (2.05) 35.30 (35.22) 5.43

a Values in parentheses indicate calculated ones.

Fig. 2. TG/DTA curves for (a) Nd(III), (b) Sm(III) and (c) Eu(III) complexes ofisnphth.

P. Khandagale et al. / Journal of Alloys and Compounds 392 (2005) 112–119 115

Fig. 3. Coats and Redfern plots of (a) Sm(III), (b) Nd(III) and (c) Dy(III) complexes ofisnphth.

the complex thus resulting in the formation of thermally sta-ble structure. Moreover, higher temperature of decompositionsuggests that the binuclear species has become structurallymore stable leading to increased thermal stability. Finally,the total destruction of the complex occurs after∼500◦Cresulting in the formation of stable oxide. The DTA curvesalso reveal that there is only one sharp endothermic peak inthe range 390–420◦C corroborating earlier speculation thatsome structural modifications have occurred within the bin-uclear structure of the complex after the loss of coordinatedwater molecules[16,17].

From thermogravimetric profiles, it is evident that the 2ndstage of decomposition is the predominant for all chelates.Significantly, such a behavior has also been observed for ma-jority of lanthanide complexes with quinone derivatives[6].Therefore, this step was selected for evaluating the kineticparameters using Coats and Redfern[18] non-isothermal in-tegral equations (Fig. 3). The DTG peaks relating to this stageare separated out and corrected for area using Anderson andFreeman methodology[19]. The energy of activation (Ea) forall the chelates was computed from their DTG curves. The

order of the thermal reaction is found close to unity implyingthat only one thermo-chemical species is formed after the lossof four coordinated water molecules, which on further heatingundergoes the distinct decomposition without further disso-ciation. It suggests that binuclear structure is decomposedwith the formation of two monomeric species.

The first order thermal decomposition kinetics of thesechelates has overall energy of activation (Ea) in the range of43–113 kJ mol−1 (Table 2). Inspection of this table reflectsthat the energy of activation is inversely proportional to thesize of the metal ion. Such a variation seems to be originat-ing from the strength ofisnpthligand bridges resulting in acloser association of two metal ions in a dimeric structure.It is plausible that intermolecular hydrogen bonding associ-ations through coordinated water molecules may also be adecisive factor for the stability and strength of metal–ligandbonding. The energies of activation for present series of com-plexes are significantly lower than those reported earlier forlanthanide complexes of quinone derivatives[6,7,20]imply-ing that the thermally stable binuclear nature is destroyedduring the 2nd step. These parameters also suggest that the

116 P. Khandagale et al. / Journal of Alloys and Compounds 392 (2005) 112–119

Table 2Thermal and kinetic data of lanthanide chelates ofisnphth

Complex Step I Step II Energy of activationb, Ea (kJ/mol)

Temperature (◦C) Loss (wt.%)a Temperature (◦C) Loss (wt.%)a

La(III)–isnphth 200–225 7.81 (7.88) 325–475 60.64 (61.72) 42.43Ce(III)–isnphth 210–225 7.53 (7.85) 335–480 60.92 (61.54) 47.58Pr(III)–isnphth 200–225 7.62 (7.84) 320–490 60.37 (61.43) 53.58Nd(III)–isnphth 210–225 7.54 (7.78) 345–435 59.64 (60.97) 54.59Sm(III)–isnphth 200–220 7.71 (7.68) 355–480 59.16 (60.20) 88.58Eu(III)–isnphth 200–215 7.55 (7.66) 340–450 58.76 (59.97) 100.58Gd(III)–isnphth 190–210 7.48 (7.57) 342–484 58.36 (59.43) 103.58Tb(III)–isnphth 200–225 7.38 (7.54) 350–500 58.77 (59.11) 107.58Dy(III)– isnphth 190–215 7.31 (7.49) 300–467 57.94 (58.67) 108.34Ho(III)–isnphth 195–220 7.18 (7.45) 310–460 57.66 (58.38) 110.58Er(III)–isnphth 200–225 7.06 (7.41) 310–450 57.31 (58.10) 112.54Yb(III)– isnphth 200–215 6.98 (7.33) 380–502 56.87 (57.42) 113.50

a Values in parentheses indicate calculated ones.b For 2nd step.

order of thermal decomposition reaction is unity indicatingthat decomposition of binuclear species is the crucial step inthermal degradation.

3.2. 1H and13C NMR spectra

It has been reported earlier[21] that hydroxyl resonance of2-hydroxy-1,4-naphthoquinone (Lawsone) in CDCl3 is ob-served as a broad signal at 7.42 ppm due to formation ofdimeric structure. However, when the spectrum is recordedin DMSO-d6, the dimeric association is destroyed and thisband undergoes down field shift[22] of 4.2 ppm. Such a fea-ture is characteristics of intramolecularly hydrogen bondedhydroxyl absorption which is also observed for juglone (5-hydorxy-1,4-naphthoquinone) and plumbagin (2-methyl-5-hydorxy-1,4-naphthoquinone)[22]. Isonaphthazarin, on theother hand, exhibits a sharp hydroxyl resonance at 9.61 ppmin DMSO-d6 (Fig. 4a) probably indicating the presence ofboth strong intermolecular hydrogen bonding as well asdimeric associations. The aromatic protons (H6, H7 and H5,H8) appear as doublets at 7.75 and 8.04 ppm.

Upon complexation with lanthanide ions, C2- and C3-hydroxyl band disappears with appearance of a broad sig-

nal around 4.8 ppm (Table 3). Such an observation is sup-porting the presence of coordinated water molecules aswell as their involvement in strong intermolecular hy-drogen bonding with coordinated ligand molecules. Itseems that coordinated water molecules have strongly in-fluenced the extent of intermolecular H-bonding thus re-sulting in a better structural stability of lanthanide com-plexes, an observation in concurrence with thermal data.The other signals originating from the benzonoide pro-tons are, however, not affected due to the presence of lan-thanide ion supporting the diamagnetic behavior of La(III)chelate.

The band positions and their assignments of13C-NMRspectra forisnphthand its La(III) chelate are made on thebasis of 13C-spectra of lawsone and some of its deriva-tives[22–24]. Isnphthexhibits three sets of signals (Table 3)in the range 125–181 ppm (Fig. 4b). The first set com-prising of quinonoidal carbon skeleton (C1 and C4) isstrongly deshielded by carbonyl oxygen atoms and ap-pears at 181 ppm while the second set of carbon atoms(C2 and C3) resonate fairly up field at 140 ppm indicat-ing the endocyclic character of these carbon atoms. Thethird set of signals in the range 125–135 ppm is indica-

Table 31

N1

N

4

1 181 31184 31184 31

H and13C NMR dataa for isnphthand its lanthanide complex

MR Compound

H IsnphthLa–isnphthLawsoneb

MR Compound Data

C1 C2 C3 C3C Isnphth 181 140 140

La–isnphth 184 161 161Lawsoneb 181 159 111

a Shifts in ppm relative to TMS.b Ref. [22].

Data

7.75 (d; H6,7) 8.04 (d; H5,8), 9.61 (b; O H)7.6 (d; H6,7), 7.82 (d; H5,8), 4.18 (vb; coordinated water)6.15 (s; H3), 7.79 (q; H6,7), 7.90 (d; H5), 7.95 (d; H8) 11.63 (b; O H)

C5 C6 C7 C8 C9 C10

126 135 134 126 130 1125 134 134 125 129 1125 134 133 125 130 1

P. Khandagale et al. / Journal of Alloys and Compounds 392 (2005) 112–119 117

Fig. 4. (a)1H NMR and (b)13C NMR spectra ofisnphthin DMSO-d6.

tive of benzonoidal (C5–C10) carbon ring structure. In the13C-NMR spectrum of La(III) complex, the first set ofsignal due to carbonyl carbon (C1 and C4) is shifted to-wards lower energy side (by 3 ppm) suggesting the binu-clear association of complex through ligand bridges. Sub-stantial downfield shift of∼21 ppm is observed for C2and C3 signal implying that the complexation with lan-thanide ion has exerted appreciable effect on this setof signal due to the oxygen donor atoms of hydroxylgroups. Such features are also observed for complexes ofcatecholes[25], catecholamine[26], tropolone [27] andmaltol [28]. The third set of signal from aromatic ben-zonoide skeleton, however, does not undergo any significantchange.

3.2.1. Infrared spectraThe infrared spectra of freeisnphthligand exhibit a sharp

and strong signal at around 3290 cm−1 (Table 4) attributedto intermolcularly hydrogen-bonded phenolato (–OH) group[6]. For binuclear metal complexes, this absorption is disap-peared indicating participation of phenolato oxygen atomsas coordinating centers. However, a new broad signal in therange 3379–3422 cm−1 is observed which is probably origi-nating from hydroxyl stretch of coordinated water molecules.The substantial increase in this frequency reflects the inter-molecular hydrogen bonding between quinone carbonyl andcoordinated water molecules.

In the double bond region, the band at 1640 cm−1 as-signed to C1 and C4 carbonyl-stretching[3] modes is shifted

118 P. Khandagale et al. / Journal of Alloys and Compounds 392 (2005) 112–119

Table 4Significant peaks (cm−1) in IR spectra of lanthanide chelates ofisnphth

Complex ν (O H) ν (C O) ν (C C) ν (C O) ν (M O)

La–snphth 3398 1610 1400 1220 667Ce–isnphth 3398 1612 1405 1221 668Pr–isnphth 3405 1612 1400 1219 667Nd–isnphth 3422 1611 1400 1220 667Sm–isnphth 3395 1610 1349 1220 669Eu–isnphth 3395 1614 1349 1220 669Gd–isnphth 3415 1615 1419 1221 667Tb–isnphth 3379 1611 1405 1221 668Dy–isnphth 3404 1610 1407 1222 668Ho–isnphth 3399 1613 1402 1219 667Er–isnphth 3399 1611 1404 1220 667Yb–isnphth 3392 1610 1406 1221 669

towards lower energy side (∼30 cm−1) upon complexation.From the nature of this band, it seems that carbonyl oxy-gen atoms are not directly bonded to metal but are involvedin strong intermolecular hydrogen bonding network result-ing a dimeric structure. The CC skeleton of quinonoidalstructure has a vibrational mode at∼1410 cm−1 in free lig-and while for chelates, this vibration is slightly affected(∼30–40 cm−1) indicating the influence of presence of metalion in the close proximity as well as hydrogen bonding asso-ciations of quinone carbonyls.

In the single bond region, a medium intensity band is ob-served at 1240 cm−1 assigned to CO stretching frequency.On coordination with metal ions, this band shifted to higherfrequency by 15 cm−1. This observation is in agreement withthe participation of both hydroxyl groups in the complex for-mation.

As the isnphthmolecule is planer molecule, bonding ofall four oxygen to one lanthanide atom is not possible. Ashoulder at 475 and 668 cm−1 was attributed to metal–ligand(M O) vibrations.

3.3. Electronic spectra

The absorption spectrum ofisnphth in DMSO exhibitsthree distinct bands at 274, 302 and 454 nm. The firstt ben-z ans-fc

andu en-e er ata byw nm.N itiona h9 b-s withs c pa-rv er

(δ = 1.705)[31] clearly signifies weak covalent character inlanthanide–isnphthbonding.

4. Conclusions

Lanthanide complexes of 2,3-dihydroxy-1,4-naph-thoquinone having [M2L3(H2O)2] composition exhibit ther-mal degradation pattern involving loss of four coordinatedwater molecules during 1st step of decomposition at highertemperatures. As a consequence, the binuclear structureundergoes some structural reorganization probably throughintermolecular molecular stacking as evidenced from the lossof ligand molecules at fairly high temperatures. It, therefore,indicates that these complexes achieve thermal stabilitythrough intermolecular associations involving binuclearstructure and coordinated quinone carbonyls. However, afterthis stage, the complex structure is collapsed as evidencedfrom the lowering of energy of activation for this step. Forsome of the chelates, there seems to be more than two stepsof decomposition suggesting the formation of intermediatespecies thus probably revealing the polymeric nature throughmolecular associations. It is further corroborated with1Hand13C NMR studies, where in, it is observed that resonancepeaks due to hydrogen and carbon atoms that are in closeproximity of lanthanide ion undergo substantial down-fields ob-s genb allyf exes.P fori ndb g fors

A

andP in-f

R

45., S.B.

Loth,

ye,

arni,

rein.

wo intense bands are originating from characteristiconoide electron transfer (BET) and quinonoid electron trer (QET) while the last absorption peak arises due to�–�*

harge transfer transition (CT)[20].Upon complexation with lanthanide ions, the first b

ndergoes negligible shift of about 2 nm on lowerrgy side while second band is appears as shouldbout 310 nm. However, the third band is replacedeak f–f transitions with a red shift of about 125–150eodymium compound exhibits a characteristic transt 582 nm assigned to9/2I4 → 5/2G4 transition, althoug/2I4 → 7/2G4 and9/2I4 → 1/2P4 transitions could not be oerved possibly due to intermixing of these transitionstrong ligand–metal charge transfer. The spectroscopiameters such as nephelauxetic ratio (β = 0.973) [29], co-alency factor (b1/2 = 0.0641) [30] and Sinha’s paramet

hift. Moreover, stronger molecular associations areerved for binuclear unit through intermolecular hydroonding resulting in a polymeric network, a feature usu

ound to be associated with lanthanide–quinone complhenolato oxygen atoms are the coordinating sitessnphthwith carbonyl oxygen bridges. The metal–ligaonding has weak covalent character thus augmentintronger interactions between binuclear units.

cknowledgements

RC is thankful to Head, Department of Chemistryrincipal MES Abasaheb Garware College for providing

rastructure and constant encouragement.

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