Charge-Transfer Complexes of Some Metal 2,4-Pentanedionates with Picric Acid as π-Acceptor

INTRODUCTION

One interesting aspect of the chemistry of metal acetylacetonates [M(acac)n] concerns the pseudo aromatic π-electron delocalization in the [M(acac)] rings.1 The ability of [M(acac)n] compounds to form molecular complexes with σ-acceptor I2,2-8 is one property that has been taken as evidence for such delocalization.1 It was proposed that these complexes are similar to those formed by aromatic hydrocarbons with I2 and that [M(acac)n] compounds behave as π-electron donors.

Recently, we have reported the formation of new CT-complexes formed from the reaction of ferric(III) acetylacetonate, Fe(acac)3, with different types of σ(iodine) and π-electron acceptors (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) (DDQ), tetrachloro-p-benzoquinone (p-chloranil) and 7,7',8,8'-tetracyanoquinodimethane (TCNQ)).8

In the last few years, chemical and physical properties of some charge transfer complexes formed by the reaction of π-electron acceptors with some heterocyclic amines have been the subject of many investigations,9-11 some of these charge-transfer complexes show very interesting applications in the analysis of some drug in pure form or in pharmaceutical preparations12,13 and some of the CT-complexes show very interesting physical properties such as electrical conductivity.14-16

To continue these reports in this area,7,8 present investigation deals with the formation of the new CT-complexes obtained in the reaction of different metal acetylacetonates [Cu(II), Ni(II), Mn(II), Fe(III), Co(III), Cr(III), Al(III) and Zr(IV)] with picric acid (PA). All reactions were carried out in chloroform as a solvent. The obtained results lead to investigate the rapture of bonding and structure inherent in these new complexes.

 

EXPERIMENTAL

All chemicals used in this study were of high pure grade and used without further purification. Different metal acetylacetonates (Cu(II), Ni(II), Mn(II), Fe(III), Co(III), Cr(III), Al(III) and Zr(IV)) were obtained from Merck Chemical Co., while picric acid (PA) was purchased from BDH.

The solid donor-acceptor complexes were isolated as follows. Excess saturated solution of the acceptor (picric acid, PA) in chloroform (40 ml) was added to a saturated solution for each of the donors (10 ml) in chloroform. The mixture in each case was stirred for about 10-15 min. The CT-solid complexes formed were filtered immediately and washed several times with minimum amounts of chloroform (3-5 ml) and dried under vacuum.

The formed complexes were characterized by their elemental analysis, vibrational and electronic absorption spectroscopy. The analysis data were shown in Table 1. Copper(II), nickel(II), manganese(II), ferric(III), cobalt(III), chromium(III), aluminum(III) and zirconium(IV) contents in all charge-transfer complexes were determined gravimetrically as a stable metal oxide.

Table 1.Elemental analysis data and gravimetric measurements for [M(acac)n(PA)]

Absorption spectra of the donors [Cu(acac)2], [Ni(acac)2], [Mn(acac)2], [Fe(acac)3], [Co(acac)3], [Cr(acac)3], [Al(acac)3] or [Zr(acac)4]; acceptor (picric acid (PA)) and the formed CT-complexes in chloroform were scanned in the region of 700-200 nm using a Shimadzu UV-spectrophotometer model 1601 PC using 1 cm matched quartz cell. The mid infrared spectra of the reactants and the formed CTcomplexes were recorded from KBr discs using a Genesis II FT-IR. Photometric titration were performed17 at 25 ℃ for the reactions of different acetylacetonates with the acceptor (PA) in chloroform as follow. The concentrations of all donors [Cu(acac)2], [Ni(acac)2], [Mn(acac)2], [Fe(acac)3], [Co(acac)3], [Cr(acac)3], Al(acac)3] and [Zr(acac)4] in the reaction mixtures were kept fixed at (1.0×10−5 M), while the concentration of acceptor picric acid was changed over the range from 0.25×10−5 to 3.00×10−5 and these produced solutions with donor : acceptor ratios varying from 1 : 0.25 to 1 : 3, as shown in Table 2.

Table 2.The electronic absorption spectral data for [M(acac)n(PA)] (where n=2 for M=Cu(II), Ni(II), Mn(II); n=3 for M=Fe(III), Co(III), Cr(III), Al(III); n=4 for M=Zr(IV)) complexes in CHCl3

 

RESULTS AND DISCUSSION

The ultraviolet-visible absorption spectra of the reactants, metal acetlacetonates (M=Cu(II), Ni(II), Mn(II), Fe(III), Co(III), Cr(III), Al(III) and Zr(IV)), (0.2×10−4 M) and π-acceptor (PA=picric acid) (0.2×10−4 M) in CHCl3 along with those of the obtained 1:1 CT-complexes are shown in Fig. 1 (AH, respectively). The spectra demonstrate that the formed CT-complexes have strong absorption bands around 321 and 420 nm for [Cu(acac)2(PA)]; 387, 411 and 495 nm for [Ni(acac)2(PA)]; 324, 376 and 407 nm for [Mn(acac)2(PA)]; 345, 405 and 483 nm for [Fe(acac)3(PA)]; 319, 369 and 450 nm for [Co(acac)3(PA)]; 361 and 424 nm for [Co(acac)3(PA)]; 358 and 423 nm for [Al(acac)3(PA)]; 360 and 440 nm for [Zr(acac)4(PA)], complexes. These bands do not exist in the spectra of the reactants. The stoichiometry of the [M(acac)n]-PA (n=2 for M=Cu, Ni and Mn; n=3 for M=Fe, Co, Al and Cr; n=4 for M=Zr(IV)) reactions was shown in all cases to be of ratio 1 : 1. This was interpreted on the bases of the obtained elemental analysis data of the isolated solid CTcomplexes as indicated in the experimental section, gravimetric measurements by calculated the weight loss and the final thermal products as metal oxides for all CT-complexes, as well as from the complexes infrared spectra, which indicate the existence of the bands characteristic for both the [M(acac)n] and the picric acid as π-acceptor. The stoichiometry of 1 : 1 is also strongly supported by photometric titration measurements. These measurements were based on strong absorption bands at 321 and 420 nm for [Cu(acac)2]-PA; at 387, 411 and 495 nm for [Ni(acac)2]-PA; at 324, 376 and 407 nm for [Mn(acac)2]-PA; at 345, 405 nm and 483 nm for [Fe(acac)3]-PA; at 319, 369 and 450 nm for [Co(acac)3]-PA; at 361 and 424 nm for [Cr(acac)3]-PA; at 358 and 423 nm for [Al(acac)3]-PA and at 360 and 440 nm for [Zr(acac)4]-PA, see Table 2.

Fig. 1.Electronic absorption spectra of (A): [Cu(acac)2]-PA reaction in CHCl3; (B): [Ni(acac)2]-PA reaction in CHCl3. (C): [Mn(acac)2]-PA reaction in CHCl3; (D): [Fe(acac)3] PA reaction in CHCl3. (E): [Co(acac)3]-PA reaction in CHCl3; (F): [Cr(acac)3]-PA reaction in CHCl3. (G): [Al(acac)3]PA reaction in CHCl3; (H): [Zr(acac)4] PA reaction in CHCl3. (a) =acceptor (0.2×10−4 M), (d)=donor (0.2×10−4 M) and (c)= donor-acceptor CT-complex.

Fig. 2.Photometric titration curves for the [M(acac)n]-PA reactions in CHCl3: (A): [Cu(acac)2]-PA, (B):[Ni(acac)2]-PA, (C): [Mn(acac)2]-PA, (D): [Fe(acac)3]-PA, (E): [Co(acac)3]-PA, (F): [Cr(acac)3]-PA, (G): [Al(acac)3]-PA, (H): [Zr(acac)4]-PA.

In these measurements, concentration of [M(acac)n] was kept fixed, while the concentration of the acceptor (PA) was varied over the range of 0.25×10−5 M to 3.00×10−5 M as described in the experimental section. Photometric titration curves based on these measurements are shown in Fig. 2(A-H). The [M(acac)n]-acceptor equivalence points indicate that the [M(acac)n]: 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. Accordingly, the formed CT-complexes upon the reaction of [M(acac)n] as a donor with the π-acceptor picric acid (PA) under investigation in chloroform have the general formula [M(acac)n(PA)]. The 1 : 1 modified Benesi-Hildebrand equation18 was used in calculating the values of the equilibrium constant K (l mol-1) and the extinction coefficient, ε (l mol−1 cm−1).

Ca° and Cd° are the initial concentrations of the π-acceptor (PA) and the donor [M(acac)n] (where n=2 for Cu(II), Ni(II) and Mn(II); n=3 for Fe(III), Co(III), Cr(III) and Al(III); n=4 for Zr(IV)), respectively, while A is the absorption of the strong bands around 321 and 420 nm for [Cu(acac)2(PA)]; 387, 411 and 495 nm for [Ni(acac)2(PA)]; 324, 376 and 407 nm for [Mn(acac)2(PA)]; 345, 405 and 483 nm for [Fe(acac)3(PA)]; 319, 369 and 450 nm for [Co(acac)3(PA)]; 361 and 424 nm for [Cr(acac)3(PA)]; 358 and 423 nm for [Al(acac)3(PA)] and 360 and 440 nm for [Zr(acac)4(PA)] complexes. The data obtained throughout these calculations are given in Table 3(A-H). Plotting the values of the Ca°· Cd°/A against Ca°+Cd° values for each donor, a straight line is obtained with a slope of 1/ε and intercept of 1/Kε as shown in Fig. 3(A-H), for the reactions of various [M(acac)n] with PA respectively in CHCl3. The values of both K and ε associated with these complexes [Cu(acac)2(PA)], [Ni(acac)2(PA)], [Mn(acac)2(PA)], [Fe(acac)3(PA)], [Co(acac)3(PA)], [Cr(acac)3(PA)], [Al(acac)3(PA)] and [Zr(acac)4(PA)] are given in Table 4. These complexes show high values of both the formation constant K and the extinction coefficients ε. These high values of K confirm the expected high stabilities of the formed CT-complexes as a result of the expected high donation of the metal acetylacetonates [M(acac)n].

Table 3.The values Ca°, Cd°, Ca°+Cd° and Ca°·Cd° for [M(acac)n(PA)] (where n=2 for M=Cu(II), Ni(II), Mn(II); n=3 for M=Fe(III), Co(III), Cr(III), Al(III); n=4 for M=Zr(IV)) complexes in CHCl3

Fig. 3.The plot of (CA° .CD° )/A values against (CA°+CD°) values for the reaction of: (A): [Cu(acac)2]-PA, (B): [Ni(acac)2]-PA, (C): [Mn(acac)2]-PA, (D): [Fe(acac)3]-PA, (E): [Co(acac)3]-PA, (F): [Cr(acac)3]-PA, (G): [Al(acac)3]-PA, (H): [Zr(acac)4]-PA.

Table 4.Spectrophotometric results of CT-complexes of [M(acac)n(PA)] (where n=2 for M=Cu(II), Ni(II), Mn(II); n=3 for M=Fe(III), Co(III), Cr(III), Al(III); n=4 for M=Zr (IV)] in CHCl3

Fig. 4(A-H) shows the infrared spectra of the formed CT-complexes, [Cu(acac)2(PA)], [Ni(acac)2(PA)], [Mn(acac)2(PA)], [Fe(acac)3(PA)], [Co(acac)3(PA)], [Cr(acac)3(PA)], [Al(acac)3(PA)] and [Zr(acac)4(PA)], respectively. The spectral bands of the formed CTcomplexes and their band assignments are reported in Table 5. The formation of the [M(acac)n]-PA, CT-complexes are strongly supported by the observation of the main infrared bands for both reactants, [M(acac)n] and acceptor (PA) in the product spectra. However, the bands of the [M(acac)n] and acceptor in the spectra of the new CT-complexes show small shifts in the frequency values as well as some changes in their intensities compared with those of the free [M(acac)n] base and acceptor. This could be attributed to the expected symmetry and electronic structure changes upon the formation of CT-complexes.

Fig. 4.Infrared spectra of: (A): [Cu(acac)2(PA)] (E): [Co(acac)3(PA)], (B): [Ni(acac)2(PA)] (F): [Cr(acac)3(PA)], (C): [Mn(acac)2(PA)], (G): [Al(acac)3(PA)], (D): [Fe(acac)3(PA)], (H): [Zr(acac)4(PA)].

Table 5.as=strong; w=weak, m=medium; sh=shoulder; v=very and br=broad bν=stretching; and δ=bending.

Moreover, in general, the IR spectra of the molecular complexes [M(acac)n](M=Cu(II), Ni(II), Mn(II), Fe(III), Co(III), Cr(III), Al(III) and Zr(IV)) with picric acid indicate that the single ν(NO2) band of PA shifted to lower wavenumber values on complexation.