REGULAR PAPERS

Spectroscopy and thermal studies of the reaction of iodine with cobalt (II) and copper (II) acetylacetonate

Sadeek, S.A.

Chemistry Department, Faculty of Science, Zagazig University, Zagazig, Egypt, Fax: +20 66 3341303, E-Mail:msrefat@yahoo.com

Received February 2^nd, 2005. In final form June 29, 2005

Abstract
A spectrophotometric study of 1:1 charge-transfer complexes derived from two metal acetylacetonates ( Co (II) and Cu (II) acetylacetonates) donor and iodine ( s-acceptor ) has been carried out. The equilibrium constants ( K ) and absortivity ( e ) for the formation of the chelate-iodine complexes have been evaluated. The suggested structure of the two solid triiodide charge-transfer complexes reported in this study is further supported by thermal analysis and far and mid infrared measurements.

Resumen
En este trabajo se realizó un estudio espectrofotométrico de complejos 1:1 de transferencia de carga derivados de dos acetilacetonatos metálicos ( acetilacetonatos de Co (II) y Cu (II) ) donores, y de yodo (aceptor s ). Se evaluaron la constante de equilibrio ( K ) y la absortividad ( e ) para la formación de los correspondientes complejos quelados con yodo. Se sugieren estructuras para dos complejos sólidos de transferencia de carga del triyoduro, las cuales fueron verificadas mediante análisis térmico y espectroscópía en el infrarrojo medio y cercano.

Introduction
The formation of stable charge-transfer complexes between iodine and several types of electron donors like cyclic polyamines, cyclic polysulphur and mixed oxygen-nitrogen cyclic bases are well known [1-7], in view of their good electrical conductivity. The formation of CT- complexes depends strongly on the type and nature of the donor bases as well as the electron acceptors.
Studies on the nature of the reaction of iodine with metal acetylacetonates [M(acac)_n] are rare in the literature and the available investigations [8-13] show that these complexes are similar to those formed by aromatic hydrocarbons with I₂ and that [M(acac)_n] compounds behave as p-electron donors. The interaction of iodine with metal acetylacetonate compounds were demonstrated by measurements of the electronic spectra, dielectric constants and refractive indices of solutions containing mixtures of the components.
The aim of present work is to investigate the new solid CT- complexes formed on the reaction of cobalt (II) and copper (II) acetylacetonates shown in Scheme (I) with iodine in chloroform using electronic absorption spectroscopy, infrared spectra as well as thermogravimetric (TG) and differential thermogravimetric analysis (DTG).

Scheme ( I ), M = Co( II ) or Cu ( II )

Experimental
All chemicals used were of high grade and used without further purification. The cobalt (II) acetylacetonate, Co(acac)₂ was obtained from Merck Chemical Co. and R. G. quality iodine ( Hayashi Pure Chemical Industries Ltd.) was used and its concentration in solution was checked spectrophotometrically. The copper (II) acetylacetonate was prepared [14] by using the general method that involves adding an ammoniacal solution of copper (II) nitrate or copper (II) acetate to an alcoholic solution of a 1,3-diketone. The precipitated chelates were filtered and recrystallized.
The [Cu(acac)₂]-I₂ CT-complex with the general formula [(Cu(acac)₂ ]₂ I⁺.I₃^- , was isolated as a greenish brown solid by the addition of an excess of saturated iodine solution (50 ml) to a saturated solution (10 ml) of the [Cu(acac )₂] in chloroform with constant stirring for about 25 min. The solid precipitate formed was filtered off immediately and washed several times with a little amount of CHCl₃ and dried under vacuum. The solid brown complex with the general formula [(Co(acac)₂ ]₂I⁺. I₃^-. 2H₂O, was prepared in a similar way to that described above by the reaction of [Co(acac)₂] with iodine using chloroform as a medium. The iodine solid reaction products were characterized by elemental analysis, infrared spectra, electronic absorption bands and thermal analysis data. The elemental analysis data of the solid reaction products were summarized and given as follow: [Cu(acac)₂]₂ I⁺.I₃^- : C, 22.97% (23.28%); H, 2.82% (2.71%); Cu, 12.53% (12.32%); I, 50.13% (49.24%), [(Co(acac)₂]₂I⁺.I₃^- . 2H₂O: C, 22.41% (22.69%); H, 2.96% (3.02%); Co, 11.75% (11.14%); I, 49.03% (48.00%) (calculated values are shown in parentheses).
The electronic absorption spectra of the donor [ M ( acac )₂ ] ( M = Co(II) or Cu(II)), acceptors (iodine) and the formed CT-complexes in chloroform were recorded in the region of 700-200 nm using a Shimadzu UV-spectrophotometer model 1601 PC with a quartz cell of 1 cm path length. The mid infrared spectra of the reactants and the formed CT-complexes were recorded with KBr discs using a Genesis II FT-IR, while the far infrared spectra for the donor [M(acac)₂] and the iodine complexes were recorded from Nujol mulls dispersed on polyethylene windows in the region 50-300 cm^-1 using a Mattson Infinity series FT-IR spectrometer. Thermogravimetric (TG) and differential thermo- gravimetric analysis (DTG) were carried out under a N₂-atmosphere using a detector model Shimadzu TG-50 H. Photometric titrations were performed [15] at 25°C for the reactions of M (II) acetylacetonate with the acceptor in chloroform as follows. The concentrations of the donor [M(acac)₂] in the reaction mixtures was kept fixed at ( 0.5x10^-4 M ), while the concentrations of the acceptor were changed over the range from 0.125x10^-4 to 1.5x10^-4 and these produced solutions with donor: acceptor ratios varying from 1 : 0.25 to 1 : 3, as shown in Table 1.

Table 1. The electronic absorption spectral data for ( A ): [Co(acac)₂]₂I⁺. I₃^- .2H₂O and ( B ): [Cu(acac)₂]₂I⁺. I₃^- respectively in CHCl₃. 1ml donor ( 5x10^-4M ) + X ml of ( I₂ ) ( 5x10^-4M ) + Y ml solvent = 10 ml

 

Results and Discussion
The electronic absorption spectra of the reactants (cobalt (II), [Co(acac)2] or copper (II) acetylacetonates, [Cu(acac)₂]) (1x10^-4M) with iodine as s-acceptor ( 1x10^-4 M) in CHCl₃ along with those of the obtained 1:1 CT-complexes are shown in Figure 1 (A and B respectively).

Figure 1: Electronic absorption spectra of ( A ) [Co(acac)₂]- iodine reaction in CHCl₃ ( B ) [Cu(acac)₂]- iodine reaction in CHCl₃, ( a ) = acceptor (1x10^-4M), ( b ) = donor(1x10^-4M), ( c ) = donor-acceptor CT-complex

The spectra show that the formed CT-complexes have a real strong absorption band around 360 nm for the two new triiodide CT- complexes [Co ( acac )₂ ]₂ I⁺. I₃^- 2H₂O and [Cu(acac)₂]₂ I⁺.I₃^- . The stoichiometry of the [M(acac)₂(iodine)] (where M=Co(II) or Cu(II)) reactions were shown in all cases to be of 1:1 ratio. This was proposed on the base of the obtained elemental analysis data of the isolated solid CT-complexes as well as from the complexes infrared spectra, Figure 2 and Table 2, which indicate the existence of shifts in bands characteristic for both the [Co(acac)₂] and [Cu(acac)₂] chelates.

Figure 2: Infrared spectra of : ( A ) Cu(acac)₂ ( B ) [Cu(acac)₂]₂I⁺.I₃^- complex ( C ) Co(acac)₂, ( D ) [Co(acac)₂]₂I⁺.I₃^- 2H₂O complex

The stoichiometry of 1:1 is also strongly supported by photometric titration measurements. These measurements were based on the CT absorption bands exhibited by the spectra for each of the [M (acac)₂]-(iodine) [M=Co or Cu] systems (indicated above) and are given in Figure 3. The M(acac)₂-iodine equivalence points indicate that the donor: acceptor ratio in all cases is 1:1 and this result agrees quite well with the elemental analysis and infrared spectra of the solid CT-complexes.

Table 2. Infrared frequencies^{( a )} (cm ^-1 ) and tentative assignments ( A ): Co(acac)₂ , ( B ): Cu(acac) ₂, ( C ): [Co(acac)₂]₂I⁺.I₃^- 2H₂O and ( D ): [Cu(acac)₂]₂I⁺.I₃^-

(a): s = strong, w = weak, m = medium, sh = shoulder, v = very, br = broad. ( b ): n, stretching; d, bending.

Figure 3: Photometric titration curves for ( A ) : [Co(acac)₂]-I₂ and ( B ): [Co(acac)₂]-I₂ systems in chloroform at 364 and 361nm respectively.

Figure 4: The modified Benesi-Hildebrand plots for ( A ): Iodine- [Co(acac)₂ ] and ( B ): Iodine-[Cu(acac)₂] reactions; C_a, C_d, A and l are the acceptor concentration, donor concentration, absorbance and the pathlength, respectively.

Figure 5: TGA & DTG diagrams of (A): [Co(acac)₂]₂I⁺. I₃^- .2H₂O . I₃^- and (B): [Cu(acac)₂]₂I⁺.I₃^- complexes respectively.

Accordingly, the formed CT-complexes upon the reaction of [ M ( acac )₂ ] as a donor with the p-acceptor under investigation in chloroform have the general formula [(M(acac)₂ ]₂ I⁺.I₃^- . Benesi-Hildebrand plots [16] and also the modified Benesi-Hildebrand equation (1) was used to calculate the values of the equilibrium constant, K (Lmol ^-1 ) and the extinction coefficient, e (Lmol ^-1 cm^-1 ) which give a straight line by plotting the values of C_a^o.C_d^o.l/A against C_a^o + C_d^o values in each system showing 1:1 donor-acceptor composition in the charge transfer complexes (Figure 4).

Here C⁰_a and C⁰_d are the initial concentrations of the iodine and the donor [M(acac)₂ ] (M = Co or Cu), respectively, while A is the absorbance of the strong bands at 364 nm for [Co(acac)₂ ]₂I⁺.I₃^- 2H₂O and 361 nm for [Cu(acac)₂ ]₂ I⁺.I₃^- complexes. The data obtained throughout these calculations are given in Table 3.

Table 3. The values C_d^o, C_a^o, C_d^o+C_a^o and C_d^o.C_a^o.l /A, for ( A ): [Co(acac)₂]₂ I⁺.I₃^- .2H₂O and ( B ): [Cu(acac)₂ ]₂ I⁺.I₃^-complexes respectively in CHCl₃.

A straight line is obtained with a slope of 1/e and intercept of 1/Ke as shown in Figure 4. The values of both K and e associated with these complexes are given in Table 4.

Table 4. Spectrophotometric results of CT-complexes of [ Co ( acac )₂ ]₂ I⁺.I₃^-. 2H₂O and [ Cu ( acac )₂ ]₂ I⁺.I₃^- in CHCl₃.

These complexes show large values of both the equilibrium constants ( K ) and the extinction coefficients ( e ) indicate the stability of the chelate rings under the conditions employed, and, when considered along with the accompanying facts, support the present interpretation of the 364 or 361 nm absorption bands.
The appearance of the absorption band around 360 nm is well known [17-19] as characteristic for the formation of the triiodide ion (I₃^-). This was also supported by the far infrared spectra of the iodine complexes, Table 5.

Table 5. Fundamental vibrations for some triiodide compounds.

n₁^*, n_s(I-I); n₂, d(I₃^- ); n₃, n_as (I-I).

 

This spectra show the characteristic bands of the triiodide ion for the two CT-complexes [Co(acac)₂ ]₂ I⁺.I₃^-. 2H₂O and [Cu(acac)₂]₂ I⁺.I₃^- at around 140, 106 and 75 cm^-1, which are assigned to n_as(I-I), n_s(I-I) and n( I₃^- ), respectively. These three absorptions do not exist in the spectrum of the donor. However, the I₃^- ion may be linear ( D_infinityh) or non linear ( C_2v ). Group theoretical analysis indicates that the I₃^- with C_2v symmetry displays three vibrations n_s (I-I); A_l, n_as (I-I); B₂ and n (I₃^-); A_l, all are infrared active in good agreement [13, 17, 20-22] with the observed three infrared bands for [M(acac)₂ ]₂I⁺.I₃^- as shown in Table 5.
According to the foregoing discussion a general mechanism is proposed for the formation of [M(acac)₂]₂ I⁺.I₃^- complexes as follows:

The formation of the iodine intermediate [M(acac)₂ ]₂ I⁺.I₃^- is well known between iodine and cyclic polyamines [20,23]. To confirm the proposed formula and structure for the two new [Co(acac)₂ ]₂I⁺.I₃^- .2H₂O and [Cu(acac)₂]₂ I⁺.I₃^- complexes, thermogravimetric (TG) and differential thermogravimetric analysis (DTG) were carried out for these complexes under N₂ flow. TG curves are shown in Figure 5 (A and B). Table (6) gives the maximum temperature values, T_max/°C, together with the corresponding weight loss for each step of the decomposition reactions of these complexes.

Table 6. The maximum temperature, T_max/°C, and weight loss values of the decomposition stages for the [ Co ( acac )₂ ]₂ I⁺.I₃^-.2H₂O and [Cu(acac)₂ ]₂ I⁺.I₃^- complexes.

The data obtained strongly support the structure proposed for the two iodine complexes and indicate that, the thermal decomposition of these complexes in inert atmosphere proceeds approximately with three or two main degradation steps ( Figure 5 ). The thermal decomposition reactions of the complexes can be summarized as follows:

The infrared spectra of the formed [Co(acac)₂]₂ I⁺.I₃^- and [Cu(acac)₂]₂I⁺.I_3- CT-complexes, are shown in Figure (2), and their band assignments are given in Table (2). These assignments are based on a comparison of the spectra of CT-complexes with those of the reactants which detected some changes in intensities and in some cases show small shifts in the frequency values compared with those of the free reactants. This can be explained on the basis of symmetry and electronic structure changes in both acceptor and [M(acac)₂] in the formed CT-complexes compared with those of the free molecules.
Whereas all the metal acetylacetonates used in the present study absorb strongly only in the region 270-300 nm due to p₃–> p₄ transitions, a new absorption band between 360 and 366 nm has been observed in the case of each metal acetylacetonate interacting with iodine. This new absorption band is interpreted as a charge-transfer band of the M(acac)-I₂ molecular complex. Since this absorption band falls remarkably close to the region where one of the two characteristic bands (ca. 290 and 360 nm) for the triiodide ion is to be found [24,25], an alternative possibility, in which an electrophilic attack by iodine on metal acetylacetonates may produce corresponding iodo chelates and the triiodide ion (resulting from a combination of iodine ion with iodine), must be considered.

References

[1] Robin, M.B., J. Chem. Phys., 1964, 40, 3369.         [ Links ]

[2] Gabes, W.; Stufkens, D., J. Spectrochim. Acta, 1974, 30A, 1835.         [ Links ]

[3] Trotter, P.J.; White, P.A., Appl. Spectrosc., 1978, 32, 232.         [ Links ]

[4] Refat, M.S.; Teleb, S.M.; Grabchev, I., Spectrochim. Acta., 2005, 61(1-2)A, 205.         [ Links ]

[5] Nour, E.M.; Chen, L.H.; Laane, J., J. Raman, Spectrosc., 1986, 17, 467.         [ Links ]

[6] Zhang, F.; Li, Y.Z.; Gao, X., Chem. Lett. 2004, 33,556.         [ Links ]

[7] Nour, E.M.; Chen, L.H.; Laane, J., J. Phys. Chem., 1986, 90, 2841.         [ Links ]

[8] Thomson, D.W., Structure and Bonding, 1971, 9, 27.         [ Links ]

[9] Singh, P.R.; Sahai, R., Aust. J. Chem., 1970, 23, 269.         [ Links ]

[10] Sahai, R.; Badoni, V.N., Ind. J. Chem., 1978, 16A, 1060.         [ Links ]

[11] Kulevsky, N.; Butamina, K.N., Spectrochim. Acta., 1990, 46A, 79.         [ Links ]

[12] Sahai, R.; Singh, V.; Verma, R.J., J. Ind. Chem. Soc., 1980, 52, 493.         [ Links ]

[13] Nour, E.M.; Teleb, S.M.; El-Mosallamy, M.A.F.; Refat, M.S., South Afr. J. Chem., 2003, 56, 10.         [ Links ]

[14] Holtzclaw, H.F.; Carlson, A.H.; Collman, J.P., J. Am. Chem. Soc., 1956, 78, 1838.         [ Links ]

[15] Skoog, D.A., “Principles of Instrumental Analysis”, 3rd edn., Saunders College Publishing, New York, USA, Ch. 7, 1985.         [ Links ]

[16] Abu-Eittah, R.; Al-Sugeir, F., Can. J. Chem., 1976, 54, 3705.         [ Links ]

[17] Kiefer, W.; Bernstein, H.J., Chem. Phys. Lett., 1972, 16, 5.         [ Links ]

[18] Andrews, L.; Prochaska, E.S.; Loewenschuss, A., Inorg. Chem., 1980, 19, 463.         [ Links ]

[19] Kaya, K.; Nikami, N.; Udagawa, Y.; Ito, Y., Chem. Phys. Lett., 1972, 16, 151.         [ Links ]

[20] Nour, E.M.; Shahada, L.A., Spectrochim. Acta., 1989, 45A, 1033.         [ Links ]

[21] Maki, A.G.; Forneris, R., Spectrochim. Acta., 1967, 23A, 867.         [ Links ]

[22] Parrett , F.W.; Taylor, N.J., J. Inorg. Nucl. Chem., 1970, 32, 2458.         [ Links ]

[23] Nour, E.M.; Shahada, L.A., Spectrochim. Acta., 1988, 44A, 1277.         [ Links ]

[24] Lang, R.P., J. Am. Chem. Soc., 1962, 84, 1185.         [ Links ]

[25] Singh, P.R.; Sahai, R., Aust. J. Chem., 1970, 23, 269.         [ Links ]