Indian Journal of ChemistryVol. 19A, Marcb 1980, pp. 229-232

Nickel(II) Complexes of I-Nitroguanyl-5-methylpyrazole-3-carboxylic Acid

"

NITY ANANDA SAHA * & N. C. GA YENDepartment of Chemistry, University College of Science, Calcutta 700 009

Received 31 March 1979; revised 23 May 1979; accepted I June 1979

New mixed ligand complexes of Ni(II) with l-nitroguanyl-5-methylpyrazole-3-carboxylic acid (NMPCH;)as the primary ligand and heterocyclic amines as the secondary ligands have been prepared and characterised. Themagnetic moment and electronic spectral data of the complexes are consistent with an average octahedral environment.The IR spectra of the complexes indicate that the primary ligand (NMPCH.) is attached to the metal ion through thepyrazolyl (tertiary) nitrogen, the carboxylic oxygen and the nitrogen of the nitroguanyl group, thus showing triden-tate (ONN) behaviour of the ligand. The derivatographic studies have been carried out to study the thermal stabi-lities and the mode of cleavage of these mixed-ligand complexes.

PYRAZOLE derivatives are well known fortheir medicinal value and recently certainpyrazole-pyrimidines have been studied as

anti-cancer reagents'. The coordination chemistryof pyrazole derived ligands has received muchattention" in recent years, primarily because of thebiological implications. However, most of thestudies including recent work from our laboratory>"have centered around syntheses and structural studieson the metal complexes of pyrazole-containingbidentate ligands . The only report on a pyrazole-derived tridentate ligand to our knowledge, is about1-(8-hydroxy - 2 - quinolyl) - 3,S - dirnethylpyrazole"which has been proposed as an analytical reagentfor the determination of Cd. As a part of ourprogramme on a systematic study of the chelatingtendency of new pyrazole-based tridentate ligandswe report here the synthesis and structural charac-terisation of nickel(Il) complexes with l-nitroguanyl-S-methyl pyrazole-3-carboxylic acid [I, NMPCH2],

a potential tridentate ligand, synthesised for thefirst time in our laboratory

HCOOH

MC N/• IC=NH

IHN -N01

(1\

Materials and Methods

All the reagents used were of AR grade andthe solvents were purified before use.

Preparation of the ligand (NMPCHJ - l-Nitro-guanyl- S-methylpyrazole - 3 - carboxylic acid wasprepared by condensation of the sodium saltof ethylacetopyruvate with nitroaminoguanidine'"following a method very similar to that adoptedby Knorr and Macdonald'! for the synthesis of S(3)-

methylpyrazole-3(S)-carboxylic acid. The producton recrystallisation from hot ethanol produced awhite crystalline compound, m. p. 189° [Found :C, 33.80; H, 3.28; N, 32.86. C6H7Ns04 requiresC, 33.70; H, 3.24; N, 32.82%].

Preparation of the mixed ligand complexes : (a)Ni (NMPC).B.2H20 (B = H20, NHa) - Ethanolicsolutions of hydrated nickel(II) chloride (0.005mole) and ligand (O.OOS mole) were mixed. Theresulting solution was treated with ammonia whenthe water adduct (at pH = S 6) and the ammoniaadduct (at pH 8-9) separated out. In each case,the adduct was filtered off, washed with water, alcoholand then dried over fused calcium chlo ride.

(b) Ni (NMPC). 3B (B=py, ~-pico and y-pico)-Asolution of the hydrated nickel(II) chloride «O.OOSmole) ill ethanol was mixed with a solution of theligand (O.OOS mole) in the same solvent followedby the addition of the respective base to adjust thepH of the reaction mixture around S-6. The precipi-tated compounds were filtered off, washed well withwater, alcohol and finally with acetone and thendried over fused calcium chloride.

(c) Ni (NMPC). B. SH20 (B = bipy, o-pheny-s-The solution obtained by mixing ethanolic solutionsof hydrated nickel(II) chloride (O.OOS mole) andthe ligand (O.OOSmole) was treated with the respec-tive base (bipy or o-phen) (O.OOSmole) dissolved inmethanol. To this was added ammonia solutiondropwise to adjust the pH ~ S-6 and then thesolution was left at room temperature ( 2S0) forabout an hour. The coloured microcrystallinecompound separated in each case, was collected asbefore.

The physicochemical measurements like diffusereflectance spectra, infrared spectra and magneticsusceptibilities in the solid state were carried out asdescribed earlier", The electronic solution spectrawere taken on a Hilger-Watts UVISPEK spectro-photometer using DMF as solvent in a cell of I cm

229

INDIAN J. CHEM., VOL. 19A, MARCH 1980

Found (Calc.), %

TABLE 1 - ANALYTICAL, MAGNETIC AND ELECTRONIC SPECTRAL DATA fOR THE COMPLEXES

Complexes !-leI! Absorption maxima (cm-1)(E)N Ni (B.M.)

at 304°K

Ni(NMPC).3H.O 21.70 18.20 22000sh; 16000; 11400; 10400·(21 63) (18.13) 3.03 27030(83.1); 15380(7.9); 13330sh(4.6)t

Ni(NMPC).NH3·2H20 26.21 18.30 17000; 13JOOsh; 11000; 10500·(26.03) (18.19) 3.14

27030(53.2); Hi130(IO.l)t

Ni(NMPC).3py 22.19 11.60 17200; 13200sh; 10500·(22.10) (J 1.58) 3.06

15620 (12.2); 10100 (7.0)t

Ni(NMPC.3 j3-pico. 20.16 10.75 19000brsh; 16500; 11000·(20.41) (10.69) 3.16

27030(175.9) ; 16130(10.8)t

Ni(NMPC).3 y-pico. 20.47 10.65 18800brsh; 16600; 10600·(20.41) (10.69) 3.28

26670(138.1); I 5870(9.7)t

Ni(NMPC).bipy.5H.O J8.93 11.49 23000sh; t 7700; 13200sh; llOOO·(19.00) (J 1.40) 3.30

Ni(NMPC).o-phen.5H.O 17.90 10.88 17400; 11000·(18.10) (10.86) 3.05

27030(241.9) ; 17240(9.6) ; 16130(8.6) ; 14080(3.3);10530(9.9)t

·Reflectance spectratElecronic spectra in DMF

light path. Derivatographic analyses were donewith the help of a Hungarian derivatograph [DERI-VATOGRAPH; system : Paulik, F., Paulik,J. and Erdey, L., MOM (Budapest)].

The complexes were analysed for nickel gravime-trically as nickel dimethyl glyoximate after decom-posing the complex species with a mixture of cone,sulphuric acid and nitric acid. C, Hand N analyseswere performed microanalytically.

Results and DiscussionThe condensation product of ethyl acetopyruvate

and nitro amino guanidine is characterised as 1-nitroguanyl-5-methylpyrazole-3-carboxylic acid (1)on the basis of elemental analyses and from aknowledge of the mode of formation of pyrazolederivatives'". The acid dissociation constants ofthe ligand, NMPCH2, determined potentiome-trically by Bjerrum's method had the values, pKPf= 3.92 and pK! = 8.96, which correspond tothe deprotonation of the carboxylic and the nitro-guanyl groups respectively. This indicates that theligand has the potentiality of showing a diacidic(tridentate) character in forming metallic complexesat controlled pH values.

Analytical data show (Table 1) that the presentmixed ligand complexes of nickel(fl) conform tothe general compositions Ni(NMPC). B.nH20(B = H20/NHa/bipy/o-phen) and Ni (NMPC).3B (B = py, ~-pico, y-pico). The complexes arenormally insoluble in water and in all the commonorganic solvents; but except Ni(NMPC). bipy. 5HzO

230

others are fairly soluble in coordinating solventslike DMF, DMSO etc.

The room-temperature magnetic moments (Table I)for the present nickel(II) complexes lie between3.0 and 3.2 B. M., quite typical of 6-coordinatecomplexes with the singly degenerate 3A2/ all) groundstate involving distorted octahedral structures'>.

The diffuse reflectance spectral data of the complexes(Table I) reveal that they probably belong to 0,.or D411 (or a lower) symmetry due to the presenceof mixed ligand fields. Two major absorptionbands in the regions 10000-11000 and 16000-7000cm-1 are attributed to the spin-allowed d-d tran-sitions, 3AI1 -+ 3Tzg (F) (Vl) and 3A2g -+ 3T11f(F)(v 2) respectively in 0" syrnrnetry'". The twosplit components of the v 1 bands in Ni(NMPC).3H20 and Ni(NMPC). NH3.2H20 suggest loweringof the symmetry from A,. to D 4"; it results in asplitting of orbital triplets 3T2, into 3Eg and 3H211

components", The onset of charge-transfer peaksapparently masks the expected third or highestenergy band for an octahedral Ni(Il) species. Thevery weak bands at 13000 and 18000-23000 em'?in a few cases (Table I) might well be due to thespin-forbidden transitions, 3A2g -+ lEg (D) 16 and3A2, -+ ITzg(D)17 respectively. The values of va/vIratio (•...• 1.50-1.64) are in good agreement withan octahedral environment (0,.) of the complexes.suggesting a strong interaction between the high-spin TII(P) TII(F) excited states's.

The electronic absorption spectra of the com-plexes in DMF solution (Table 1) suggest that

SAHA & GA YEN : Ni(lJ) COMPLEXES WITH A POTENTIAL TRIDENTATE LIGAND

NiO _

n n

fIIlN +HCAw.-i3 I

H

n(mp) ~iO(mpi]

nScheme1

there is no gross change in the geometrical structureon dissolution in coordinating solvents. The lowmolar extinction coefficients (€-13 cm-1 mole+')of the major bands point out a pseudo-octahedralenvironment for these complexes in solution, whilethe high molar extinction coefficient for the bandaround 26300-27100 crrr" suggests it to be of charge-transfer origin.

IR spectra of all the metal complexes showabsorp-tions characteristic of the aromatic ligands; Vas COO(at 1600-1630 crrr? as compared to 1720 crrr+in the free ligand) and vsCOO (1450-1470 crrr-')for coordinated carboxylate group attached to thepyrazole ring. The IR spectrum of the ligand,NMPCH2 shows two strong bands at 3340 and 3170crrr ' which are assigned to vssN-H and VsN-H modes of vibration respectively-". Thesebands disappear with the appearance of a strongbroad band around 3280-3330 cm-I in the metalcomplexes thereby suggesting deprotonation of oneof the N-H groups on complexation formingnitrogen to metal bond 20. However, the diffusecharacter and broadness of this band indicate inter-or intra-molecular hydrogen bonding between amide(=NH) group and H20 molecules present in thecomplexes. The possibility of mixing of a bandexpected in this region (3,u) due to V OH of watermolecules with this broad band cannot also be ruledout. The characteristic positions of vibrationsdue to secondary co-ligands (py, pico, bipy, phenetc.) are in good agreement with those reportedearlier-" 22. The hydrated complexes show anadditional band around 880-920 crrr ", characteris-tic of the rocking mode of coordinated water-".X-ray crystallographic studies on several pyrazole-metal complexess+w show that pyrazole ring(tertiary) nitrogen is a common bonding site andthis most likely in the present complexes also.The IR spectrum of Ni(NMPC).3H20 (con-taining no secondary heterocyclic base) recordeddown to 250 cm-1 shows one strong absorption

in the region 310-320 cm-I which is absent in thefree ligand; therefore, it seems reasonable to assignthis band to vM-N modet". The IR spectralstudies, thus, suggest that I-nitroguanyl-5-methyl-pyrazole-3-carboxylic acid (NMPCH2) functionsas a binegative tridentate ligand having ONN donor.

The results of derivatographic analyses provideuseful information reagarding the thermal stabilityof the complexes and the mode of cleavage of certainside chains of the ligand. The DTG and DT Apeaks in the derivatogram of the ligand indicatethat it is stable up to 100°. It decomposes in twostages, the first weight-loss step at 190-210° canbe rationalised in terms of the loss of nitroguanylside chain forming methylpyrazole carboxylic acid(mpCH); the next step due to decarboxylation of theligand resulting in the formation of methylpyrazole(mp). These observations are in good agreementwith those reported for the decarboxylation of otherheterocyclic carboxylic acids above 200° (ref. 28,29)The derivatograms for the complexes indicate, ingeneral, the existence of a number of intermediatesformed primarily through the loss of water andsecondary base molecules; simultaneous decar-boxylation and loss of nitroguanyl group are thenindicated by the DTG and DT A peaks. Exceptthe anhydrous species, none of the intermediates,because of their short existence at such high tempera-tures, could be isolated. The water adduct, Ni(NMPC). 3HP is initially much more stable thanthe other complex species. In derivatogram ofNi(NMPC).3B (B = py, ~-pico or "(-pico), thesuccessive loss of the secondary base molecules (B)is quite consistent with their formulations. Ingeneral, the DTG data suggest the formation ofa apolyrneric intermediate species, [Ni(NMPC)]nabove 200°, which is rapidly decomposed leavinganother intermediate oxo-mono complex, [NiO(mp)] ••(mp=methylpyrazole). It may be speculated thatthis oxo-mono complex [NiO(mp)]n is formedby the loss of one methylpyrazole molecule froman oxo-bis complex, [NiO(mp)2]n which unfor-tunately could not be detected in the present derivato-graphic studies. The oxo-mono complex is ther-mally unstable and is finally converted to NiOaround 450° (Scheme 1).

Acknowledgement

The authors are thankful to Prof. S. E. Living-stone, University of New South Wales, Australia,for the diffuse reflectance spectra of the complexes.The authors also acknowledge their gratefulness toDr. S. N. Poddar of lACS, Calcutta for deriva-tographic analyses of the complexes and to Dr G. N.Mukherjee of their Department for determinationof the acid-dissociation constants of the ligand.

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