Indian Journal of ChemistryVol. 18A, August 1979, pp. 157-160

Complexes of 's-Methylpyrazole-3-carboxylic Acid with Cu(II) ,Ni(II) & Co(II)S. N. pODDAI & J. HALDAR

Indian Association for the Cultivation of Science, Calcutta 32

Received 5 April 1978; revised 26 December 1978; accepted 3 January 1979

5-Methylpyratole-3-carboxylic acid (Hmpe) acts as a bidentate chelating ligand to form stable complexes ofthe type [M(mpc)2(HaO)2]' wbere M = Cu(II), Ni(II) and Co(II). These compounds possess distorted octahedralstereochemistry with the two mpc molecules acting as chelating ligands in a plane and the two water molecules occupy-ing the trans axial positions. When these complexes are debydrated thermally, the anhydrous compounds [M(mpc)2]result, in which the metal is again hexa- coordinated, with mpc molecules acting as tridentate Iigands and the carbonyloxygen of the -COOH groups entering into coordination through nridging. Cu(II) forms, under different experimentalcondltions, two more compounds of the formulae [Cu(mpc)aJand [Cu(mllc).JHaO, which possess different planar structures.Probable structures have been assigned to the complexes on the basis of magnetic susceptibility, electronic and infraredspectra, DTA, DTG and TG data.

ALTHOUGH considerable has recently beenreported on the complexes of pyrazole and its

. substituted products', little attention has beenpaid to pyrazole-carboxylicacid as a ligand. In fact,the complexation reactions of the acid do not appearto have been studied at all. 5-Methylpyrazole-3-carboxylic acid+ generally behaves as a bidentateligand and forms inner complexes with five-memberedchelate rings.

In the present paper, Cu(IJ), Ni(ID and Co(IDcomplexes of the ligand are reported. These are ofthe following general types:

(i) [Munpcj.], where M = CuH, designated asbis-complex.

(ii) [M(mpc)z].HzO, where M = Cu2+. designatedas monohydrated bis-complex.

(Ui) [M(mpc)z(H20)2]' where M = Cu-r, Ni2+ &C02+, designed as bis-aquo complex.

(iv) [M(mpc)2]d, where M =,Cu2+, Ni2+ & C02+,designed as dehydrated bis-complex.

Materials and Methods5-Methylpyrazole-3-carboxylic acid was prepared

according to the method described by Knorr andMcdonald- bycondensing sodium salt of ethylaceto-pyruvate with hydrazine sulphate. The product wasrecrystallised from hot water using activated charcoal,when glistening white needles melting at 236° wereobtained. The substance is freely soluble in methanol,hot water and hot ethanol.

The Cu(IJ) complex of type (i), [Cuunpcj.], wasprepared by adding an aqueous solution of a Cu(IJ)salt (sulphate, chloride, nitrate or acetate) (0.001mole) to a hot aqueous solution of the ligand (0.002moles). A sky-blue silky micro-crystalline precipitatewas formed, which was filtered by suction, washedfirst with hot water and then with cold aqueous

tAn alternative nomenclature is 3-methylpyrazole-5-car-boxylic acid.

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ethanol and finally dried over fused calcium chloride.The Cu(IJ) complex of type (ii), [Cu (mpc)2]' H20,

was prepared by mixing an aqueous solution of aCu(IJ) salt (0.001 mole) with an aqueous solution ofthe sodium salt of the ligand (0.002 mole) at pH,...., 5.0(adjusted with acetic acid). The bluish-green granu-lar precipitate was collected on a filter, washed withwater and dried over fused calcium chloride.

The complex [Cu(mpcMH20)2] (type iii) wasobtained by dissolving either of the above two com-plexes in boiling aqueous methanol and allowingthe solution to cool, when deep-green needles wereobtained. These were filtered, washed with water andether and dried over fused calcium chloride.

The Co(IJ) and Ni(IJ) complexes of the type (iii),[M(mpc)2' (HzO)z], were obtained by mixing aqueoussolution of the corresponding metal salts (0.001mole) and the sodium salt of the ligand (0.002 moles)at pH 5.0 and heating the mixtures on steam-bath for1 hr. Deep-red crystals of the Co(IJ) complex anddeep-green crystals of the Ni(IJ) complex wereobtained. These were collected and dried as above.i The dehydrated complexes of type (iv) of all the

three metal ions were obtaind by heating the comple-xes obtained above at 120°-220° for 6-8 hr.

The electronic spectra in solution (,....,10-2M) in thevisible region were recorded on a Hilger Uvispeckspectrophotometer and those in nujol mull on aHungarian MOM 201 spectrophotometer. The IRspectra were recorded on a Perkin-Elmer 21 spectro-photometer. Magnetic susceptibility measurementswere carried out using a Guoy type balance calibratedwith Co[Hg(SCN)4]' The electrolytic conductancesof the solutions of complexes were measured using aPhilips PR 9500 conductivity bridge. A Hungarianderivatograph (MOM) was used for thermal analysis.

Results and DiscussionThe analytical data of the complexes (Table 1)

support the formulae assigned to them.

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INDIAN J. CHEM., VOL. 18A, AUGUST 19751

TABLE1 - ANALYTICALANDMAGNETICMOMENTDATAFORTHECOMPLEXES

Found (Calc.) (%)Complexes Colour !leU· (B.M.)

Metal N C H Water at 27°

[Co(mpc).(H.O).] Bright-red 16.98 16.32 34.60 4.00 10.74(17.09) (16.23) (34.79) (4.06) (10.43) 4.90

[Co(mpc)2] Violet 19.15 18.35 38.82 3.14(19.08) (18.126) (38.85) (3.24) 4.80

[Ni(mpc),.(HzO)s] Bright-green 16.96 16.36 34.75 4.10 10.18(17.03) (16.245) (34.81) (4.06) (10.44) 3.10

[Ni (mpc),] Pale-yellowish green 18.92 18.40 38.80 3.18(19.02) (18.14) (38.87) (3.24) 3.20

[Cu(mpc).] Sky-blue 19.9 18.02 38.15 3.08(20.26) (17.86) (38.27) (3.19) 1.84

[Cu(mpc)s] H2O Bluish-green 18.98 17.10 36.00 3.56 6.60(19.16) (16.89) (36.19) (3.62) (5.43) 1.79

[Cu(mpc>s(HsO).l2H.O Deep-green 16.20 14.83 31.10 3.58 17.8(16.48) (14.52) (31.13) (3.63) (18.67) 1.98

[Cu(mpc)21 Dirty-green 19.94 17.92 38.15 3.05(20.26) (17.86) (38.27) (3.19) 2.06

The molar conductance values in methanol of10-3 M solutions of the Cu (Il) complexes lie in therange 17-20 ohrrr ' ern" mole-', indicating theirnon-electrolytic nature. The bis-aquo complexes ofNi (Il) and Co (Il) and all the dehydrated metalcomplexes are insoluble incommon solvents and hencetheir conductance values could not be measured.

Compounds of the type (iv) have the same empiricalformulae as those of type (i) and are obtained bycontrolled thermal dehydration of the bis-aquocomplexes (iii). These compounds are different inboth structure and properties from the simple bis-complexes (i). The dehydrated complexes (iv) havebeen shown to possess octahedral stereochemistry,where the ligand exhibits tridentate function byestablishing intermolecular bridging through thecarboxyl oxygen atoms of the carboxylic groups,and thus forming polymeric chelates .. This struc-tural change, which was observed and studied only ina very few other cases+' has been investigated byvarious physico-chemical methods. The Cu (II)complexes of type (i) and (ii) are assigned planarstructure. On controlled thermal treatment, bothof them give products identical in colour, appear-ance and properties with those of the type (iv). Thistype of structural change is known for metallic com-plexes of picolinic acid and some other aminoacids,whose structures are definitely known from X-raycrystallographic data=".

The results of DT A, DTG and TG of the bis-aquocomplexes of Cu(Il), Ni/Il) and Coffl) (type)are almost identical. They are stable upto 280°,after which they lose two molecules of water and giverise to the anhydrous complexes'[ Mtmpc), (type iv),which are stable upto 340°. This species then decom-poses with faster rates. The DTG as well as DT Apeaks indicate all the intermediate stages of decom-position which are not often shown in the TG curves.

Decarboxylation of the ligand takes place between340° and 380°, and the complex decomposes through

tAlthough the TG curve shows that complete conversion tothe anhydrous state occurs at 280°-300°, the same compoundswere also formed when the bis-aquo complexes were heated for6-8 hr in an air oven at about 200°.

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different intermediate steps with the formation of'oxo' species, as shown by the breaks in the DTAand DTG curves. But, owing to their short life atsuch high temperatures and due to the limitation ofthe instruments used, these intermediates could notbe isolated or characterised. Above this tempera-ture, total decomposition of the complex takes place,resulting in the formation of the correspondingoxides above 550°.

Similar is the observation with the copper(II)complex, [Cu(mpcMH20)2]' 2H20, in which thefour water molecules are continuously lost in therange 120°-220°, resulting in the formation of theanhydrous complex [Cu(mpc)2] (type iv). which isstable up to 340°. Although both the coordinatedwater and water of crystallization were eliminatedduring the heating process, the thermal analysiscurves did not indicate any sign to differentiatebetween the two types of water molecules, exceptthat the DTG and DTA peaks are unsymmetricalwith shoulders on the lower temperature side(,.....,160°) and that the rate of loss in weight as shownby the TG curve is more rapid above 160°-170°than that at lower temperatures. As in the casesof the Co(Il) and Ni(Il) analogues, here also, theformation of intermediate oxo-cornplexes was indi-cated by sharp DTG and DT A peaks occurringapproximately in the same temperature ranges. Butthey could not be isolated for reasons stated earlier.

Since the anhydrous (type iv) and the bis-aquo(type iii) complexes of Co(Il) and Ni(Il) are inso-luble in common solvents, their spectra were recordedin the solid state through mulling in nujol", Solidmull spectra of the Cu(II) complexes have been re-corded along with their solution spectra in methanol.

The spectra of the Co(Il) complexes consist of twomain bands in the region 800-10,000 cnr-' ('11) andat 20,000 crrr ' (V3)' These bands are split into twoor three components, the third component beingobserved sometime as a shoulder and sometimes it isobscured by other overlapping bands, resulting in thebroadening of the main band. The positions of thesplit components of the VI band are approximatelyaround 10,000 (sh), 9,000 and 8,000 cm-I and those

rPODDAR & HALDAR : COMPLEXES OF 5-METHYLPYRAZOLE-3-CARBOXYLIC ACID

of the V3 band around 20,000, 19,000 and 18,000 cm-I•

The V2 band, expected in the region 16,000 cm-',is not easily detectable in the spectra under study,but the unsymmetrical nature of the curves in thisregion indicates that it might be obscured by theoverlapping Va bands.

The spectra of the Ni(ll) complexes are of typicaloctahedral type, showing bands at 9,000-10,000 (VI)'16,000(v2) and a weak band or sometimes a shouldeat 13,000 crrr-'. Here also, the VI band is splitinto two components (at about 9,500 and 10,000crrr") and the energy separation between the twocomponents is of the order of 300 crrr-'. The V2 bandsare somewhat broad and are likely to contain morethan one overlapping bands.

The spectra of the Cu(ll) complexes give a broadenvelope as usual, centering around the region 14,000-16,000 crrr+. This may contain the two typicalclosely spaced octahedral transitions from dX2_y!

to dxy and dz2 levels. The band due to the thirdtransition (from dx' _y.dxz.dyz) being placed fartherin the ultra-violet region, could not be detected.

The positions of the band maxima in the spectraof the bis-aquo complexes of Co(ll), Ni(II) andCu(ll) (type iii) and the bis-complexes of Cu(II)(type i & ii) are shifted towards lower frequencysides in the spectra of the anhydrous complexes (typeiv), indicating thereby the lowering of the fieldstrength (due to the formation of polymeric bridgedstructure through the oxygen atoms of the carbonylgroup) in the latter compounds. In the case of thetwo varieties of the Cu(Il) complexes, type (i) sky-blue and type (ii) bluish-green respectively, theabsorption maxima appear at the same positions.

The magnetic moment values of the bis-aquo Ni(II)and Co(Il) complexes suggest their octahedral con-figuration. The observed high moments [4.9 B. M.for Co(Il) complexes and 3.2 B. M. for the Ni(JI)complexes] may be due to large orbital contri-bution9-I2• The corresponding dehydrated com-plexes also have magnetic moment values of the sameorder of magnitude, indicating thereby that theoctahedral configuration is also maintained in thesecases". The magnetic moment values of the Cu(II)complexes, although a little higher than the spin-onlyvalues, lie well within the range usually observed forCu(Il) complexes.

In the IR spectra of the complexes, very broadand diffused bands occur around 3480, 3300, 3200and 3050 crrr+, whereas vN-H in the free ligandappears at 3580 and 3470 cm+. This shifting of thebands to the lower frequency side is indicative of theinvolvement of the nitrogen atom(s) of the pyrazolering in coordination. The diffused character andbroadness of the bands are obviously due to extensivehydrogen-bonding, both intra-molecular and inter-molecular, through the hydrogen atoms of N-H aswell as those of H20 molecules (wherever present,either as lattice water or as coordinated water).

Although too many bands appear in the region1650-1400 crn-', it is not very difficult to find outthe bands for the asymmetric and symmetric vibra-tional modes of OCO groups. Vas COO in thefree ligand appears at 1650 cm-', which is shifted to1610-1600 cm-! in the case of the complexes. This

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shows that OCO group is strongly bonded to themetal and the bond is more covalent than ionic.The energy separation between the bands due toasym. and sym. OCO vibrations (.6 v OCO) isgreater for the aquo-complexes (,.., 190 crrr") thanthat for the anhydrous complexes (__ 170 cm-'),This is due to the fact that as the covalent characterof the M-O bond increases, the OCO group becomesmore asymmetric as compared to the symmetry ofthe free ion, resulting in the increase in frequencyseparation of the two carbonyl stretching bands.The participation of carbonyl oxygen in coordi-nation restores the symmetry of the carboxyl group,resulting in the decrease in.6 v3'13.

The asymmetric stretching frequency of the OCOgroup in the bis-complex [Cufrnpcjj] (type i), appearsat 1615 crrr ' with a .6 v value of 195 crrr-'. In thecases of the monohydrated bis-complex( [Cufrnpcj.].H20 (type ii) and the diaquo-complexes of type(iii), there is further lowering of this frequency to,.., 1603 crrr-'. The asymmetric frequency of theunpreturbed carbonyl group [as in the type (i)complex] is obviously taken to be 1615 crrr-' and itslowering in the cases of the monohydrated and thediaquo-complexes is attributed to the hydrogen-bonding between the carbonyl oxygen and the hydro-gen atom of the lattice water (in the bluish grenecomplex) or the coordinated water (in the diaquo-complexesj-! .

The medium to medium-strong bands at ,.., 900crrr ' in the spectra of the bis-aquo complexes areattributed to the presence of coordinated watermolecules-v": In the spectra of the ligand itselfor of the simple bis-complex and the dehydratedbis-complexes, they do not appear at all.

From the above experimental data, the followingstructures are suggested for the complexes underinvestigation :

(a) The bis-aquo complexes of type (iii), [Mtmpc),(H20)2]' have an overall octahedral geometry, butwith much lower symmetry (C2). The two watermolecules are coordinated to the central metal ionoccupying two trans apical positions. The twopyrazole carboxylate ions occupy four positions intrans-planar form. The two water molecules arestrongly bonded not only through coordination butalso by hydrogen-bonding.

(b) The anhydrous complexes of type (iv),Mfrnpcj.; have distorted octahedral polymericstructure, the intermolecular bridging being establish-ed by oxygen atoms of the carbonyl group of thecarboxylate ions. The ligand exhibits tridentatefunction in these cases.

(c) The 'simple' bis-complexes of Cu(II) of type(i) and (ii), [Cufmpcjj] (sky-blue) and [Cufrnpcj.].H20 (bluish-green), are structural isomers as indi-cated by the following reasons : (i) On dehydration,the bluish-green complex does not give rise to thesky-blue form and hence the former is not merely ahydrated variety of the latter. (ii) The JR spectrumof the hydrated form does not contain any bandcharacteristic of coordinated water and hence it isprobably a tetracoordinated complex. (iii) Thewater molecule is held more or less strongly by hydro-gen-bonding as is evidenced by the IR spectral data

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INDIAN J. CHEM., VOL. 18A, AUGUST 1979 •

and derivatographic analysis. Both the forms are,therefore, supposed to be planar one havingtrans-trans (sky-blue) and the other cis-cis (bluish-green) configuration of the ligand",

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