Indian Journal of ChemistryVol. 31A, February 1992, pp, 110-115

Syntheses of polystyrene supported chelating resin containing the schiff basederived from 3-formylsalicylic acid and o-hydroxybenzylamine and its

copper(II), nickekll), iron(III), zinc(II), cadmium(II), zirconium(IV),molybdenum(V and VI) and uranium(VI) complexes

A Syamal* & M M SinghDepartment of Chemistry. Regional Engineering College, Kurukshetra 132 119

Received II February 1991: revised and accepted 10 October 1991

A series of new polystyrene-anchored coordination complexes has been synthesized by the reactionof metal complex/metal salt with the chelating resin containing the schiff base derived from 0-

hydroxybenzylamine and 3-formylsalicylic acid. The pofymer supported complexes have the formulae:PS-LCu.DMF, PS-LNi.3DMF, PS-LFeCI.2DMF, PS-LZn.DMF, PS-LCd.DMF, PS-LMoOCI.DMF, PS-LMoO~.DMF, PS-LUOc.DMF and PS-LZr(OHh.2DMF (where PS-L= deprotonated polystyrenebound schiff base). The polymer supported complexes have been characterized by elemental analysis, in-frared, electronic and EPR spectra and magnetic susceptibility measurements. The polymer-anchoredcoppertll), nickel(II), iron(lII) and molybdenum(V) complexes are paramagnetic while the zinc(II), cad-mium(Il), molybdenum(VI), uranium(VI) and zirconium(IV) complexes are diamagnetic. The magneticand EPR data indicate the magnetically dilute nature of the complexes. The shifts of the v(C = N) (azome-thine) and v(C - 0) (phenolic) modes have been followed to find out the donor sites of the ligand. Thepolymer-anchored copper(ll) complex is square-planar, zinc(ll) and cadmium(Il) complexes are te-trahedral, nickel(II). iron(IIl). molybdenum(V and VIi and uranium(VI) complexes are octahedral andzirconium(IV) complex is pentagonal-bipyrarnidal. The structures of the complexes are comparable withthe corresponding metal complexes of the non-anchored ligand.

In recent years there has been considerable researchinterest in the immobilisation of transition metalcomplexes on insoluble polymer supports I - 4. Theseinsoluble polymer supported complexes have theadvantages over their soluble counter-parts of easyseparation from the reaction mixture leading to op-erational flexibility, of their facile regenerability andof higher stability' - 7. Several polystyrene supportedligands like dipyridyldiimine", dithiocarbamate",porphyrin 10, acetyl acetone II, 8-hydroxyquinoline I~

and their metal complexes have been reported. Al-though the. schiff bases are the most versatile andthoroughly studied ligands 1.1.14, only a few schiffbases have been immobilised to polystyrenematrix 15 - II!. In this paper the syntheses and charac-terization of coordination complexes of the polymerbound schiff base(I) with copper(II), nickel(lI), ir-on(III), zinc(II), cadmium(II), molybdenum(V andVI). uranium(VI) and zirconium(IV) are described.

Materials and MethodsChloromethylated polystyrene containing 0.0012

mol of chlorine per gram of resin and 1% cross-linked witl! divinylbenzene was the product of Po-Iysciences Inc. (U.S.A.). 3-Formylsalicylic acid 1'1, zir-

conium(IV) diacetate-", ammonium oxopentachlor-omolybdatefv)" and bis( acetylacetonato )dioxom-olybdenumrVlj" were synthesized by following theliterature procedures. Copper(II) acetate mono-hydrate, acetyl acetone and ammonium molybdatetetrahydrate were purchased from Glaxo Laborato-ries. Dimethylformarnide, salicylic acid, zinc(II)acetate dihydrate and nickel(II) acetate tetrahydratewere the products of S.D.'s Fine Chemicals. The po-lymer-anchored schiff base (I, PS-lR2) was pre-pared by following the reported procedure-". Sol-vents were dried over molecular sieves.

Infrared spectra were recorded in KBr on a Beck-man IR-12 spectrophotometer calibrated with po-lystyrene. EPR spectra were recorded at 77 Kin po-Iycrystalline solids on a Varian V4502-12 X-bandEPR spectrometer with 100 kHz modulation usingdiphenylpicrylhydrazide as the g-marker. Frequen-cy was monitored with the help of a frequency me-ter. The g values were corrected using a second ordercorrection. 7A~/4H~., for coppertll ) and 17NI4H'.4for molybdenumtv )". Electronic spectra were re-corded on a Cary model 2390 spectrophotometerattached with a reflectance arrangement. The mag-netic susceptibilities were determined at room tern-

SYAMAL et al.: SYNTHESES OF POLYSTYRENE SUPPORTED CHElATING RESIN & ITS COMPLEXES 111

perature by the Gouy method using Hg [CO(NCS)4]as the standard. A double-ended one-side sealedglass tube with zero diamagnetic susceptibility wasused for better accuracy. The diamagnetic correc-tion of the metal-ligand system was computed usingthe Pascal's constants following a procedure special-ly designed for polymer-supported complexes ".The magnetic susceptibilities were corrected for di-amagnetism and temperature independent param-agnetism term [copper(II), 60 x 10-6 cgs units; mo-lybdenum(V), 55 x 10-6 cgs units; nickel(ll),200 x 10-6 cgs units; iron(III), zero].

To a weighed amount ( - 0.15 g) of polymer-an-chored complex of copper(II), nickel(II), zinc(II) orcadmium(II), acetic acid (4N, 20 mI) was added andthe mixture was heated on a water bath for !h forcopper(II) and cadmium(II), 1h for zinc(II) and 2hfor nickel(II) complex. The resin was filtered andwashed with acetic acid (2N) followed by distilledwater. Nickel was estimated in the filtrate gravimet-rically as bis(dimethylglyoximato )nickel(II). Cadmi-um and zinc were determined complexometricallyby titrating with EDTA using xylenol orange as theindicator. Copper was analysed iodometrically by ti-trating with a standard solution of sodium thio-sulphate. The iron(III) complex was decomposed byheating the complex with HCI (6N, 25 mI) on a wa-ter bath for 1h. The resin was filtered and washedwith 2N HCI followed by distilled water. Iron wasdetermined in the filtrate by reducing it with a solu-tion of tin(ll) chloride and then titrating with astandard solution of potassium dichromate.' Mo-lybdenum(V/VI) complex was converted to Mo03

by decomposing the complex with a few drops ofconc. HN03 and cone. H2S04 and then igniting in amuffle furnace at 500°e. Mo03 was dissolved in adilute solution of NaOH (6N) and molybdenum wasanalysed gravimetrically as bis( 8-hydroxyquinolina-to}dioxomolybdenum(VI). Uranium and zirconiumwere determined gravimetrically as U30g and Zr02

respectively after decomposing the complexes witha few drops of cone. HN03 and then igniting. Chlo-rinc analysis was done by fusing the complex withNaOH pellets and Na202 in a nickel crucible andthen precipitating as AgCl. Dimethylformamide wasanalysed by heating the complex at 160°C in an airoven for 3h. In Ni(II) complex the temperature was170°C and in Fe(IIl) and Zn(II) complexes the tem-perature was 180°C.

The per cent reaction conversion(P) of the polym-er-anchored complex was calculated by use of theformula:

observed metal ion percentageP= x 100

calculated metal ion percentage

The calculated metal ion percentage was computedon the basis of 100% conversion from polymer-an-chored ligand to polymer-anchored complex.

Synthesis of PS-LM.xDMF, where M = copper; Il),nicke~11), zind,Jl), cadmium Ib, dioxouranium; VI)andx= 1or3

The polymer-anchored schiff base (0.5 g) wasswelled in dimethylformamide (20 ml) for 45 min.To this a dimethylformamide solution (30-50 mI) ofthe appropriate metal acetate (0.001 mol) in dime-thylformamide was added and the mixture was ref-luxed on a suspended type heating mantle for 8hwhile stirring magnetically and then cooled to roomtemperature. The solid precipitated was fil-tered under reduced pressure, washed with dime-thylformamide, ethanol, methanol and acetone andwas dried in vacuo at room temperature.

Synthesis of PS-LMo02·DMFThe polymer-anchored schiff base (0.5 g) was

swelled in dry dimethylformamide (20 mI) for 45rnin in a three-necked flask fitted with a condenser,and N2 gas was passed through the mixture. To this afiltered dimethylformamide solution (40 rnl) of(NH4h [MoOCls] (0.325 g, 0.001 mol) was addedand the mixture was refluxed under N2 atmosphereon a heating mantle for 5h while stirring magnetical-ly. The mixture was cooled to room temperature, fil-tered under reduced pressure, washed with dime-thylformamide, absolute ethanol and petroleum

.. ethertboiling range 60-80°C). The compound wasdried in vacuo at room temperature.

Synthesis of PS-LFeCI.2DMFThe polymer-anchored schiff base (0.5 g) was al-

lowed to swell in dimethylformamide (20 mI) for 45min. To this suspension, a dimethylformamide solu-tion (40 mI) of Mof), (acetylacetone), (0.33 g, 0.001mol) was added and .the mixture was refluxed on aheating mantle for 5h while stirring magnetically.The mixture was cooled to room temperature. Thecompound was filtered under reduced pressure,washed with dimethylformamide, ethanol and ace-tone and dried in vacuo at room temperature.

Synthesis of PS-LZr{ OHh.2DMFThe polymer-anchored schiff base (1.0 g) was

swelled in dimethylformamide (30 mI) for 45 min.To this a dimethylformamide solution (50 ml) of an-hydrous ferric chloride (0.24 g, 0.0015 mol) wasadded and the mixture was refluxed for 6h whilestirring magnetically, the condensor being fittedwith a calcium chloride guard tube. The mixture wascooled to room temperature and the resin was fil-

112 INDIANJCHEM,SEC.A,FEBRUARY 1992

tered under reduced pressure, washed with dime-thylformarnide, absolute ethanol and petroleumether (boiling range 60-80°C) and dried in vacuo atroom temperature.

Synthesis of PS- LZ~ OH)2' 2DMFThe polymer-anchored schiff base (0.5 g) was

swelled in dimethylformarnide (25 ml) for 45 min.To this a freshly prepared dimethylformarnide solu-tion (40 ml) of zirconium(IV) acetate (0.001 mol,prepared in situ) was added and the mixture was ref-luxed on a heating mantle for 5h while stirring mag-netically. The mixture was cooled to room tempera-ture and the compound was filtered under reducedpressure, washed with dimethylformamide, ethanoland petroleum ether (boiling range 60-80°C). Thecompound was dried in vacuo at room temperature.

Results and DiscussionThe polymer-bound chelating agent(I) is insoluble

in water and common coordinating and non-coordi-

oII

ps-oc

@(OH HO)§]C = N - C H2

H/I

nating solvents. Dimethylformamide is a convenientsolvent for studying the metal loading property of Idue to its high dielectric constant and ability to swellthe polymer support and to dissolve a large numberof metal salts and metal complexes. The results ofthe synthetic studies (Table 1) demonstrate that I iscapable of chelating a variety of transition metalions. The synthetic reactions were carried out in 1:2molar ratio [1:1.5 in case of iron(II1) complex] of po-lymer-bound ligand :metal salt/metal complex. 'Theanalytical data show that the polymer bound com-plexes have the compositions: PS-LCu.DMF, PS-LiNi.3DMF, PS-LZn.DMF, PS-LCd.DMF, PS-FeCI.2DMF, PS-LMo02.DMF, PS-LMoOCI.DMFand PS-LZr(OHh.2DMF.

The metal ions are coordinated by tridentate che-lating moieties anchored on the polymer and therest of the coordination sphere is completed byDMF, 02 - , CI- or OH - . Assuming that each metalion is coordinated to one ligand, the metal ions werefound to occupy 39 to - 100% (Table 1) of theavailable sites in I. It is evident from the per centreaction conversion of the complexes that there isno apparent correlation between per cent reactionconversion and size of the metal ion. The polymerbound chelating ligand(I) is yellow in colour. As thereacton of polymer bound ligand with metal salt/metal complex proceeds, the colour of the oolymerchanges from yellow to cream, green or yellowish

Table I-Analytical and magnetic susceptibility data of polymer supponed complexes

Complex Found(calc.)% Binding capa. of resina Conversion Magnetic(mmol metal per (%) moment~.BM

M CI DMF gram ofresin x 10- !) Temp.(K)

PS-LNi.3DMF 4.3 16.5 73.3 98 3.02(4.2R) (15.97) (302)

PS-LCu.DMF 2.4 2.95 37.8 46 1.87(2.42) (2.79) (289)

PS-LFeCl.2DMF 2.2 1.5 6.0 39.4 51 6.01(2.19) (l.3R) (5.71 ) (298)

PS-LZn.DMF 4.7 5.2 71.9 86 Diamag.(4.67) (5.45)

PS-LCd.DMF 4.1 2.8 36.5 46 Diamag.(4.13) (2.68)

PS-LMoOCl.DMF 3.2 1.2 2.6 33.3 43 1.65(3.20) (UR) (2.44) (300)

PS-LMoO,.DMF 6.0 4.7 62.5 79 Diamag.(5.99) (4.55)

PS-LZr( OH ),.2 DMF 6.75 10.9 74.6 99 Diamag.(6.82) (10.81)

PS-LUO,.DMF 6.6 2.3 27.3 39 Diamag.(6.59) (2.02)

(a) Calculated from the observed value of metal percentage in the polymer-anchored complex.(b) Calculated using the relation: lirif = 2.83(xM'rrx T)' , B.M.

SYAMAL et al.: SYNTHESES OF POLYSTYRENE SUPPORTED CHELATING RESIN & ITS COMPLEXES 113

brown depending on the metal ion. The colour ofthe polymer-anchored complexes does not changeeven after prolonged washing with dimethylforma-rnide, methanol, ethanol and petroleum ether. Themetal ions can be easily stripped from the polymer-anchored complexes by dilute acids with little effecton the ability to regenerate the polymer-anchoredcomplexes. Regeneration studies indicated that aftersix complete cycles, about 90% of the ligand siteswere still available for complexation. The polymer-anchored complexes lose the coordinated dimethyl-formamide completely on heating in air.

The infrared spectrum of the schiff base derivedfrom salicylaldehyde and o-hydroxybenzylarnineexhibits a band at 1655 cm-I characteristic ofv(C = N) (azornethiner". The polymer bound schiffbase(I) exhibits the v(C = N) (azomethine) at 1645em - 1 (Table 2). This band shifts to lower energy by5-20 cm - I in the infrared spectra of the polymer-an-chored complexes indicating the nitrogen coordina-tion of the schiff basetl}". The v(C - 0) (phenolic)occurs at i550 em - I in the schiff base(l) and thisband moves to higher energy by 5-10 em - I in thepolymer-anchored complexes. This is indicative ofthe phenolic oxygen coordination of the schiffbase(l).ln the non-anchored schiff base the v(C - 0)(phenolic) occurs at 1540 em - I. The IR data pre-clude the presence of a dimetallic structure and in-dicate a monometallic structure as in the event of adimetallic structure the v(C - 0) (phenolic) is ex-pected to shift to higher energy by > 10 em -I (ref.28). The absence of v(OH) in the complexes indi-cates the deprotonation of both the phenolic hy-droxyl groups. Thus, the IR data indicate the ONOdonor tridentate behaviour of I.

Dimethylformarnide exhibits the v(C = 0) at 1680

Table 2-Infrared spectral data of polymer supported ligand andcomplexes"

Complex/ligand . v(C=N) v(C-O) v(C=O)(phenolic) (DMF)

PS-LH2 1645 1550PS-LNi.3DMF 1035 1555 1635

PS-LCu.DMF 1625 1500 1055PS-LFeCI.2DMF 1635 1560 1050PS-LCd.DMF 1630 1500 1045

PS-LZn.DMF 1035 1560 1045PS-LMoOCI.DMF 1640 1560 1650PS-LMo02.DMF 1030 1555 1650PS-LZr(OH),.2DMF 1630 1560 1655

PS-LUO!.DMf 1625 1560 IMo

"AIIIR bands are in ern I

em - 1 which shifts to lower energy by 20-45 em - 1 inthe complexes indicating the oxygen coordination ofDMF!<).

PS-LMoO,DMF exhibits the v, (OMoO) and v,,,(OMoO) at Y30 and 862 ern - I respectively whichoccur in the usual range (v" 892-964 em I; va,'840-925 ern I) observed for the majority of Mo(YI)complexes:". The appearance of these bands ind~-cates a ci~~MoO, structure as only the va, (OMoO) IS

expected in case of trans-MoO! structure since thev; (OMoO) is Ik-inactive. The IR data rule out thepresence of a chain structure ...Mo = °...Mo = °...in which v(Mo = 0) is expected I~ to oecur at < 850cm-I. PS-LMoOCI.DMF shows v(Mo=O) at 945em - I which occurs in the usual range (900-1007em - I) observed for the majority of Mo(Y) com-plexes':'. PS-LUO!DMF exhibits v"JOUO) at 905cm - 1 which occurs in the usual range (870-950em - I) observed for the majority of trans-UO! com-plexes". The force constant (fu-o) was calculatedaccording to the method given by McGlynn et al:12

and the value obtained (6.S0 mdyne/ 'A) is in the ex-pected range. The U - 0 bond distance was calcu-lated using Jones' equation": Ru-o = l.Ost-ln+ 1.17 and the value was found to be 1.74 A whichis in the usual range (1.60-1.92 'A) reported for themajority of U(VI) complexes. PS-LZr(OH)2.2DMFexhibits a band at 1115em - 1 due to the ~(Zr-OH ).1~.

The complexes PS-LCu.DMF and PS-iMoOCI.DMF record magnetic moments of 1.S7and 1.65 BM respectively characteristic of magneti-cally dilute Cu(II) and Mo(Y) complexes+t". It is ofinterest to note that the Cu(II) and Mo(Y) com-plexes of the schiff base derived from salicylalde-hyde and o-hydroxybenzylamine are dimetallic andare involved in antiferromagnetic exchange 14.27. Thepresence of bulky diamagnetic polymer backbonehas prevented the M - M interaction in polymer-an-chored complexes and has resulted in the formation ofmagnetically dilute complexes. The ESR spectra ofthe complexes described later also support the mag-netically dilute nature of the complexes. PS-LFeCl.2DMF and PS-LNi,3DMF exhibit magne-tic moments of 6.01 and 3.l)2 B.M. respectivelywhich are in the normal range expected for magneti-cally dilute octahedral complexes of Fe(JIl) andNi(II)3f>. PS-LZn.DMF, PS-LCdDMF, PS-iMoO,DMF, PS-LZr(OH),.2DMF and PS-- -LUO!DMF are diamagnetic as expected for d", dill

and f' systems. A tetrahedral structure for Zn(Il)and Cd(lI) complexes, octahedral structure forMo(VI) and U(Vl) complexes and pentagonal-hi-pyramidal structure for the Zr(IY) complex arc sug-gested. The structures of the complexes are compar-

114 INDIAN J CHEM. SEe. A. FEBRUARY 1992

able with those of the corresponding complexes ofnon-anchored ligand 14J7.3H.

The polymer-anchored complexes are insolublein water and common organic solvents and this pre-cluded the recording of their solution electronicspectra. The complexes also do not form a goodmull with nujol and hence nujol mull spectra couldnot be recorded. Hence, the reflectance spectra ofthe complexes were recorded. The electronic spec-trum of PS-LCu. DMF exhibits a band at 17700em - I, characteristic of CuN03 coordinationsphere.17. The complex does not exhibit a band at8000-10000 cm - I which precludes the presence ofa tetrahedral structure'". PS-LNi. 3DMF exhibitsbands at 8300, 15100 and 25000 ern -I due to the\4eg-+~7;!i (VI)' ~Azg-+3I;g(F) (v2) and3Az.i(-+3I;'/r (P) (v3) transitions respectively inan octahedral field. The V 2: v I ratio of PS-LNi.3DMF is 1.82 which lies in the usual range(1.6-1.82) reported for the majority of octahedralNi(II) cornplexes'". The spectral parameters of theNi(II) complex calculated by following the methodof Lever" are as follows: Dq = 830 cm - I, B' = 884em - I, 13= B'I B= 0.83, 13° = 17%. The reduction ofthe Racah parameter from the free ion value of1056 em - I and 13° value of 17% indicate the pres-ence of strong covalent interaction of the metal ionwith the ligand". PS-LMoOCI. DMF shows twobands at 12250 and 17400cm-1 duetothe2Be-+2£

and e Be -+ eA, transitions respectively 30. The banddue to the ~B2 -+ 2 B\ transition, which occurs around19600 cm - I was not observed and it is probablycovered underneath the intense:' B2 ~ 2AI transition.

In the polymer-anchored complexes the bulky di-amagnetic polymer backbone. forces the metal cen-tres to remain considerably separated and asa resultdipolar broadening is reduced to a minimum. Thecomplexes exhibit reasonably good EPR spectra inpolycrystalline solids in the absence of a host coor-dination complex diluent. PS-LCu. DMF shows twog values (gll= 2.23, gi = 2.08) indicating the pres-ence of a tetragonal type symmetry about the Cu(II)iorr". The EPR parameters are as follows:Afu = IgO x 10 - 4 em - I. Alu = 45 x IO - 4 ern - I. TheEPR data indicate that gi <&1 and Al < All whichare according to the expectations. gll is normally< 2.3 for covalent environments and is ~ 2.3 forionic environments ". The gll value of the Cu(II)complex identifies it as a covalent type complex. Gwas calculated using the relation ", G = (.&1- 2.(02)!(gl - 2.0(2). The Cu(lI) complex exhibits a G valueof 2.92 which identifies the ligand as a strong field li-gand; strong field ligands are known to exhibit the:Gvalue < 4.0. The in-plane covalence parameter. aluwas calculated using the relatiorr':'.

a~u =(~ - 2.002)+~(gl - 2.002)-~+0.047 0.036

The overlap integral (S) is related to a as follows:":

a2-2Saa'+(a'?= 1

S has a value of 0.076 for oxygen donor atoms and0.093 for nitrogen donor atoms. The Cu(II) com-plex contains NO.1 coordination sphere and, hence,an S value of 0.080 was calculated for NO) donorsfrom the above values and this was used for the cal-culation of a~u' The copper(II) complex exhibits a~uvalue of 0.80 which indicates the covalent nature ofthe complex. The bonding in a complex is regardedmore covalent if the al-u value is smaller; a~,u = 1.0indicates complete ionic bonding and a~·u= 0.5 sug-gests complete covalent bonding. The bonding in acomplex is considered more covalent if the (a')2 va-lue is larger; (a')2 = 0 indicates complete ionic bond-ing. The (a')2 value of 0.27 in the Cu(II) complexidentifies it as a covalent complex. The parameterKPd is the Fermi contact contribution to the couplingand Pd is the dipolar contribution. Pd was calculatedusing the relation": Pd =(AI - 1\.)/0.78. K was cal-culated using the relation: K= - (A/Pd) - 0.48. TheCu(II) complex exhibits the P, and K values of1.73 x 10-2 cm-I and 0.50 respectively. The reduc-tion of the Pd value from the free ion value of3.5 x 10- 2 cm - I is indicative of the presence of cov-alent bonding in the complex": The positive valueof K predicts I\. > Al and the same was observed.The CU(Il) complex exhibits the Fermi.contact con-tribution (A) value of~.65 x 10 -.1 ern -I.

. The Mo(Y) complex exhibits the following EPRparameters: &1= 1.92, gi = 1.96, g" = 1.95,A~o=83X 10-4 cm-I and A~"=24x lW~ ern I.

The EPR data indicate that gll< gland A 1 < 1\which are according to expectations. The spectralshape, g and A values of the Mo(Y) complex arecomparable to those of a monometallic complex[MoO( salicylaldehyde- o-aminophcnol)CI'CH30H] (&1= 1.923. e. = 1.947. &,,= 1.939.I\.Mo= 82.6 x 10-4 cm -I, A~I<l= 36.8 x lo-~em - 1)45. This similarity suggests that the' stereo-chemistry and electronic structure in these systemsare identical. The absence of ~Ms = 2 line around1500 gauss in the EPR spectra of the present Cut II)and Mo(Y) complexes rules out the presence ofM - M interaction. A rough calculation indicatesthat the metal ions are situated on the phenyl rings(of polystyrene) which are seven to eight styrene un-its apart when the per cent conversion is 100% andthe styrene units are more than eight when the percent conversion is < 100%. Thus a magnetically di-lute environment around the metal ions is main-

SYAMAL et al.: SYNTHESES OF POLYSTYRENE SUPPORTED CHELATING RESIN & ITS COMPLEXES 115

tained since the pathway for M - M interaction isreduced. But the polymer is 1% crosslinked with thepolymer chains overlapped and twisted and thismay bring some reactive groups closer leading toM - M interaction which was undetectable by theEPR method. The Ni(II) complex exhibits single lineEPR spectrum with gav value of 2.03 which is char-acteristic of octahedral Ni(II) complexes.".

AcknowledgementThe authors are grateful to the Department of

Atomic Energy (Govt. of India), Bombay for finan-cial support of the work.

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