Indian Journal of Chemistry Vol. 41A, October 2002, pp. 2092-2095

Synthesis, characterization, thermal decomposition and kinetic parameters of

Ni(II) and eu(II) terephthalate-8Hq complexes

A P Mishra, V K Tiwari & R Singhai*

Department of Chemistry, Dr. H.S. Gour University, Sagar, 470003 (India)

E-mail: rashmitiwari @eptra.com

Received 22 October 2001; revised 2 May 2002

Ni(1l)-8Hq terephthalate and Cu(II)-8Hq terephthalate complexes have been synthesised and characterized by elemental analysi s, magnetic measurements, IR spectra and UV - Vis­spectra. The complexes have also been analysed thermogravimetrically for the evaluation of their decomposition kinetic parameters. The thermal decomposi tion of the compounds occurs mainly in two steps. Kinetic parameters, (£*,Z and /',. S*) have been determined by using three different methods.

Terephthalates are known in industry for their polymeric, resistant and tolerant nature. Besides industrial and biological significance, mixed ligand chelates have their theoretical and chemical importance where we interpret the phenomena of ligand -ligand and metal-metal interaction in solution as well as in isolated solid state l ,2. The preparation and thermal decomposition of metal terephthalates have been studied earlier3

. Sharma et al. 4 have prepared some complexes of metal terephthalates and their thermal studies have also been studied but very limited work on high temperature solid state behaviour of metal terephthalate complexes have been done. The present work describes the kinetic parameters (E*, Z, and L1S*) of Ni(II) and Cu(II) chelates with 8-hydroxyquinoline. Such values are determined through TG and DT A curves using Coats­Redfems, Piloyan-Novikova6 and Horowitz-Metzger7

methods.

Experimental Metal terephthalate complexes have been prepared

by the reaction of metal terephthalate of Ni(II) and Cu(II) with 8-hydroxyquinoline in 1:2 molar ratio in ethanol, slight amount of O.IN HCI was added and the pH of the reaction mixture was adjusted to 4.5-5.5. The reaction mixture was then refluxed for about 7 h on a water bath. The refluxate was allowed to cool

and stand for about 2h. The coloured precipitate thus obtained was filtered and washed three or four times with ethanol followed by ether. The product was dried first in a desicator over anhydrous CaCh and after that in an electric oven at 70°C. The reagents used for the synthesis of the complexes were of analytical grade BDH Merck, Sisco and Loba.

Structural characterization was carried out by elemental analysis, infrared spectra, UV -Vis (drs.) spectra and magnetic measurements. The compounds were thermogravimetric ally analysed for obtaining simultaneous DT A-TG curves Coats-Redfems, Piloyan-Novikova6 and Horowitz-Metzger7 methods were employed for finding the kinetic parameters and mechanism (Fig. 1).

Elemental analysis, infrared spectra and UV -Vis (diffuse reflectance) spectra were recorded at CDRI Lucknow. The magnetic measurements were made by Gouy method. The thermograms were recorded at RSIC Nagpur, at a heating rate of 20K min- ' In aIr atmosphere.

Results and discussion The elemental analysis data (Table 1) reveal that

compounds have the composition [Ni Lz(HzO)2 T] and H2 [CuL2 T]. A feeble solubility and slight solvolysis in common organic solvents precluded the measurement of conductance of these complexes4

.8

.l).

Rather high decomposition temperature, practically insoluble behaviour towards solvents and to certain extents lowering magnetic moments values also suggest about the slight polymeric tendency of the complexes4•1O,11,

For observing the phenomenon of complexation, the IR band positions of 8-hydroxyquinoline have been taken from the literature as a reference

,z-'4. The strong band as a result of the vibration of pyridine ring nitrogen (C=N-) has been found to shift from 1580 cm- I (in 8-Hq) to 1560±10 cm- I (in complexes), suggesting its involvement in coordination. An intense band at around 1400 cm· 1 and another at around 1100 cm- I in 8-Hq have been assigned to phenolic-OH and C-O, respectively ,14. '6 In the IR spectrum of Ni(II) complex, practically negligible change occurs in the band position of both the groups i.e. phenolic-OH and C-O. But in case of Cu(JI) complex the phenolic -OH band disappears and a

NOTES 2093

positive shift of about 50 cm-I appear also in vC-O. This indicates that the ligand 8-Hq behaves as a monodentate neutral coordinating species with Ni(lI); while in Cu(ll) complex, 8-Hq acts as an anionic bidentate ligand chelating through nitrogen and d d h 1· 12-16 Th eprotonate p eno IC group oxygen. e characteristic carboxylate bands due to terephthalate

-6.1

t -6.0

N t-

~ -5 .8 L.......1

Ol ,g

-5.6

P-N

•

- 5. 4 '--........ ......J---Io._"-:--'--~-....J..-,J~ 1.71 1-77 1.83 1.89 1.95

t -6.0

r--, N

t-

:::- -5.8

~ Ol

~-5.6

1000jT ~

C-R

,g •

-5.4

- 5.3 ~""""--L::---L-"---'---'---,----,,--1.71 1.77 1.83 1.89 1.95

-0.5

t -0.4

1000/T_

H-M

eJK~

Fig. I- Piloyan-Novikova (P-N). Coats-Redfern (C-R) and Horowitz - Metzger (H-M) Plots of Ni(II) Complex I First decomposition stagel

anion appears at 1640± I 0 and 1340± I 0 em-I In complex spectra with respect to their positions in ligand at 1680 and 1360 cm- I (ref. 14-16). This observation works as an evidence for terephthalate's chelation with metal ion. A broad band at 3450 cm-I

and a medium sharp band at 770 cm- I have respectively been assigned to stretching and rocking modes of coordinated water in Ni(II) complex. New bands at 470 and 390 cm- I in nickel (II) complex and 400 and 500 cm- I in copper (II) complex have respectively been assigned to vM-O and vM-N vibrations 15.1 6.

In the reflectance spectrum of Ni (II) complex, three bands have been observed. A shoulder occurring at 11235 cm- I may be assigned to the ]A2g ---1 ]T2g(VI)

transition. The middle band V2 has splitted into two, perhaps due to spin-orbit coupling and di stortion in regular symmetryI7.1 8. Its band position around 16393 cm- I to 17543 cm- I has been attributed to ]A2g---1 ]TIg(F) transition. Relatively, a higher energy band at 24390 cm-I has been ascribed to 3A2g(F) ---1 3TIg(p) (v]) transition. The ligand field parameters viz., 100q, B (Racah parameter), ~(Nephelauxetic ratio) , ~o(covalent character) and LFSE have been evaluated l7.18 and their values are as 11235 em-I, 606 cm- I, 0.56,43.3 and 161.65 kJ/mot. respectively . Values of ligand field parameters l7.18 reflects that the M-L bond is sufficiently strong bearing 43.8% covalency; which in turn suggest enough overlapping of metal orbitals with the ligand orbitals. Ligand behaves as moderately strong field ligand. The value of observed magnetic moment of this Ni(ll) complex is 2.92 B.M. The reflectance spectrum of Cu(II) complex exhibits a broad band which appears rather bifurcated into two. The two band positions at 12345 and 15384 cm- I may be because of the Jahn-Teller distortion, spin-orbit coupling and lowering of symmetry and hence have been considered belonging to transition 2Eg ---1 2T2g (splitted) . The value of 10 Oq and LFSE have been found l7.18 to be 14371 cm- I and

Table I-Analytical and physical data of the complexes

Complex Molecular Colour Found Calc (0/0 ) Magnetic Formula/wI. m.p. / (dec.) C H N M measurements

Ni(ll ) [Ni L2 (H2OhT] Yellowish 56.10 4.05 5.49 10.39 303 Temp (K) L = C9 H7NO, green (56.85) (4.00) (5. 10) (10.65) 3513.5 XM( 10.6)

T = CgH404 >340°C 2.92 ~lcll(B.M) 548.8

Cu(ll) H2 [Cu L2 T] Greenish 59.11 3.38 5.20 11.31 303 Temp (K) L=C9H6NO, cream (60.48) (3.10) (5.42) (12.31 ) 1291.01 XM(10-6)

T = CSH404 >310°C 1.77 J..lcll(B. M) 515.8

2094 INDIAN J CHEM, SEC A, OCTOBER 2002

103.38 kllmo\. The observed magnetic moment IS

1.77 B.M. Electroni.c spectral assignments '7.' 8, ligand field

data and magnetic moments favour the distorted octahedral stereoarrangement for both of the above complexes. '7.ls

Thermal decomposition (a) Ni(lI) 8-Hq terephthalate complex - A careful

analysis of thermogram of Ni(II) 8-Hq terephthalate complex indicates that complex

compound is stable upto IOO°C, an elimination of two coordinated molecules of water has been

observed on progression upto temperature 150°C (Remaining weight% obs./ca\.92/92.66). Almost negligible weight loss occurs between the

temperature range ISO-250°C indicating the complexes having two coordinated water molecules which in turn reflects as a horizontal

from the pyrolysis curve. Above 250°C, a gradual

weight loss continues upto 310°C indicating the decomposition of the ligand moiety; however a very small fraction of the chelated ligand i.e. two nitrogens, two oxygens and four carbon atoms remain attached with metal ion at this temperature (Remaining weight%, obs./ca\. 58.0/60.2). No change III weight appears between the

temperature 130°C to 430°C, but after 430°C, a quick loss in weight occurs, which is reflected as an inflection in the curve between the temperature

region 430°C to 450°C. A horizontal zone beyond 450°C suggest about the ultimate product which in oxygenated atmosphere, is an oxide of nickel (Remaining weight %, obs./ca\. 14.00/13.58).

(b) Cu(II) 8-Hq terephthalate complex - The thermogram of the complex shows no weight loss

upto 215°C indicating the absence of any lattice or coordinated water in the complex. A gradual increase in temperature above 215°C is

accompanied by loss in weight upto 300°C, which indicates the decomposition of coordinated ligand, but incomplete. Some directly chelated atoms (a ligand moiety) still remains associated with central metal (Remaining weight %,

obs./calc. 63.0/64.8). Above 340°C a deep inflection in the curve occurs which ranges upto 360°C. The loss in weight in the region corresponded mainly with the degradation of terephthalate unit. A further increase III

temperature above 360°C witnesses no loss in weight and this constant weight region is due to

copper oxide which appears as a final product (Remaining weight %, ) 6 .00/15.36).

Kinetics of decomposition

pyrolysis obs./ca\.

The fractional weight loss(a) and the

corresponding (1-a)" have been calculated from TG curves at different temperatures , where 'n ' depends

upon the reaction model and a =(wt-w,) I (wo-w,). The T vs (I-a) curves constructed on the basis of TG data for all the complexes, are more or less of the same pattern. The curves begin with an acceleratory period without any apparent induction period, thu s indicating that no surface nucleation on branching

f I ~ ryo occurs before the start 0 these steps '- . The decomposi tion reaction of the complexes was subjected to non-isothermal kinetic studies and the weighted least square method was used for obtaining best fit for linear plots by applying the data to various equations5

.7 and thus the kinetic parameters vi z.,

energy of activation (E*), frequency factor (2) and

entropy of activation (L~S*) were evaluated from plots . The value of frequency factor was evaluated using Eq. (1) for C-R5 and P_N6 methods, while the value of Z in case of H_M7 method was calculated us ing Eq. (2).

The entropy of activation (L~S* ) was calculated by employing Eq. (3).

Intercept = log ZR/~E

Z = E/RT m2 ~ exp (E/RTm)

or ~S*= 2.303 R log Zh/KT

. . . (I)

. .. (2)

... (3)

where K represents Boltzman constant, h the

Planck' s constant, ~ rate of heating, R mo lar gas constant and Till peak temperature. The values fo E\Z and ~S* are given in Table. The mechanism involved in decomposition is random nucleation and apparent order of reaction is one.

The value of thermal kinetic parameters (Table 2) obtained by C-R5 and P_N6 methods are quite consistent while values by H_M7 method differs noticeably (Fig. 1) . Therefore C-R and P-N methods may be considered more suitable for assessing the parameters. Generally, values of Z increases with decrease in E* and the higher value of activation energy suggests the higher stabil ity ,21 but there lies some more inherent physical and chemical factors which may cause a change or deviation in this trend 1'1.23. Higher values of E* (activation energy) and lower values of Z (frequency factor) favours the reaction to proceed slower than norma\. In present

NOTES 2095

Table 2-Thermal kinetic parameters of the metal complexes

Complex Equation Decomposi tion E* Z f;.S* due to stage/temp. (K) (kJ mol- I) (S- I) (JK- I mo l- I)

P-N 104.44 1341.63 - 190.01 Ni (8-Hq) terephtha late complex C-R I" /523 114.84 2047 .96 - 186.50

H-M 111.73 9.343xI0.4 - 307.90 P-N 325.80 8582.63 -176.85 C-R lInd 1703 382.80 1124.27 - 193.68 H-M 385.31 1.945x I 0-4 -246.68 P-N 153.12 5973.95 - 177.28

Cu (8-Hq) terephthalate complex C-R 1st /493 133. 18 4248 .83 -180. 12 H-M 112.50 1.0 153x 1 0.4

- 326.13 P-N 325.38 1360.56 -171.90 C-R II"d /523 306.24 1251.99 -172.69 H-M 319.43 2.105xI0·3

-302.37 P-N = Piloyan-Novikova, H-M = Horowitz-Metzger, C-R = Coats Redfern ,

studies, the numerical values of activation energy E*, frequency factor (2) and entropy of activation altogether indicates about the smoothness of the feasibility and reaction rate of the initial reactants and intermediate stage compounds. The negative values for entropy of activation indicates that the activated complex has a more ordered or more rigid structure than the reactants or intermediate (activated complex) and the reactions are slower than normaI 2o

.23

•

Quantitatively, slight change in the entropies S* of the compounds (than E*) can set a reaction governing factor 22-D.

Such kinetic parameters can necessarily be treated as reaction course governing under particu lar conditions, but their particular physical significance and intrinsic behaviour become generalised with the h . d' . 2223 Th d f b'l' c ange III con Itlons . .. e or er 0 sta t tty on

thermal decomposition temperature and the energy of activation is almost similar or different for two decomposition steps . The vari ation in the trend might be interpreted to be on account of some intermolecular interactions (structural as well as electronic) occurring therein, besides several experimental factors.

Acknowledgement The authors are grateful to the Head , RSIC Nagpur,

for providing thermal analysis data. The authors are al so gratefu l to Head, Department of Chemistry , Dr. H S Gour University, Sagar, for providing laboratory faci lity.

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