Room temperature solution studies of complexation between o-chloranil and a series of anilines by spectrophotometric method

Spectrochimica Acta Part A 57 (2001) 2409– 2416 www.elsevier.com/locate/saa

Room temperature solution studies of complexation between o-chloranil and a series of anilines by spectrophotometric method Sumanta Bhattacharya, Manas Banerjee, Asok K. Mukherjee * Department of Chemistry, The Uni6ersity of Burdwan, Burdwan, West Bengal 713104, India Received 19 January 2001; accepted 4 February 2001

Abstract Electron donor–acceptor (EDA) complex formation between o-chloranil and a series of anilines has been studied in CCl4 medium. In all the cases, EDA complexes are formed instantaneously on mixing the donor and acceptor solutions. N,N-dimethylaniline and N,N-dimethyl-p-toluidine form stable EDA complexes with o-chloranil while the other complexes decay slowly into secondary products. The kinetics of all these reactions has been studied by UV–VIS absorption spectrophotometric method and the rate constants of the reactions and formation constants of the EDA complexes have been determined. The charge transfer (CT) transition energies of the complexes are found to change systematically with change in the number and position of the methyl groups in the donor molecules (methylanilines). From an analysis of this variation, the electron affinity of o-chloranil has been found to be 2.54 eV. A perturbational inductive effect Hu¨ckel parameter hMe has been found from this trend and the value obtained ( −0.27) is very close to that ( −0.3) obtained by Lepley (J. Am. Chem. Soc., 86 (1964) 2545) from a study of tetracyano ethylene (TCNE)–methylbenzene complexes. © 2001 Elsevier Science B.V. All rights reserved. Keywords: o-chloranil; EDA complex; Electron affinity; Formation constant; Kinetic study

1. Introduction Electron donor –acceptor (EDA) complexes are currently of great importance since these materials can be utilised as organic semiconductors [1], photocatalysts [2] and dendrimers [3]. They are also important in studying redox processes [4], second order non-linear optical activity [5], and * Corresponding author. Tel.: + 91-342-60810; fax: +91342-64452. E-mail address: [email protected] (A.K. Mukherjee).

micro-emulsion [6]. There exists a vast literature on theoretical [7,8] and experimental studies [9– 18] in EDA complexes. Although p-chloranil as an acceptor has been studied extensively, the ortho isomer has received much less attention. Recently some EDA complexes involving o-chloranil as acceptor have been studied [19 –21]. The present paper reports studies on EDA complexes of o-chloranil with anilines and a series of methylanilines as donors in CCl4 medium by electronic absorption spectroscopy. For N,Ndimethylaniline and N,N-dimethyl-p-toluidine

1386-1425/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 1 ) 0 0 4 2 7 - 9

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(NNDP), the EDA complexes are found to be stable with 1:1 (donor:acceptor) stoichiometry, while the other anilines in the series form labile complexes and their decay rate and formation constants have been determined. The charge transfer (CT) bands have been located in each case. The CT transition energy varies systematically with change in the number and location of methyl groups in the donor moiety. Some important molecular parameters have been obtained from such variation.

2. Experimental o-chloranil was collected from Sigma and was purified by sublimation just before use. The anilines were distilled with Zn dust just before the preparation of the experimental solution. The solvent CCl4 was dried over fused CaCl2 and doubly distilled before use. All optical measurements were done on a UV 2101 PC model spectrophotometer fitted with a TB 85 thermo bath.

band could not be observed. However, the intensity at 460 nm which is the absorption maximum of o-chloranil decreases from the very beginning.

3.1. Determination of 6ertical electron affinity of o-chloranil and an inducti6e effect Hu¨ckel parameter for the methyl group from CT absorption bands The wavelength at the broad absorption maxima and the corresponding transition energies (hw) are summarised in Table 1, together with those (as collected from literature [22,23]) of the charge transfer complexes of the same anilines with tetracyanoethylene (TCNE) as an acceptor. An excellent linear correlation exists between the present hw values with those (hw) of the TCNE –aniline complexes:

3. Results and discussion Fig. 1 shows the electronic absorption spectra of a series of mixtures of o-chloranil and NNDP in CCl4 medium against the pristine o-chloranil solution as reference. NNDP does not absorb in the 400– 900 nm wavelength range. At 821 nm, where o-chloranil and NNDP have no absorption, the mixture shows a CT type broad absorption band whose intensity increases systematically with increase in the concentration of NNDP. No variation of intensity with time was observed in this case. Similar spectral features were obtained with N,N-dimethyl aniline. In case of aniline, o-toluidine and 2,4-dimethyl aniline, the intensities of the broad CT absorption bands were found to remain approximately constant with respect to time but a new absorption band appeared at 510 nm, whose intensity increased with time from the very beginning. This new band may be attributed to product formation, as will be evident from the subsequent discussions. A typical case is shown in Fig. 2. In case of N-methyl aniline, this 510 nm

Fig. 1. Spectra of mixtures containing o-chloranil and NNDP, the concentrations of the components are as in Table 3.

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potentials (ID) of the first four donors namely, aniline, N-methylaniline, N,N-dimethylaniline and o-toluidine (which were available from [24]) is approximately linear with a correlation coefficient of 0.7. These confirm that the observed new absorption bands of the o-chloranil–aniline complexes are of CT nature and are summarised in Table 1. In the subsequent discussions, the umax of the broad absorption bands has been written as uCT and the corresponding transition energy, hwCT. To estimate the electron affinity of o-chloranil, Mulliken’s theory [25] has been used, according to which the CT transition energies are related to the vertical ionisation potentials (I wD) of the donors by the relation, hwCT = I wD − C1 +

C2 I − C1

(2)

w D

Here, C1 = E wA + G1 + G0

Fig. 2. Variation of absorption spectrum of o-chloranil –2,4dimethylaniline mixture with time. [o-chloranil] = 1.2488× 10 − 5 mol dm3. [2,4-dimethylaniline] = 2.5×10 − 3 mol dm3.

hwCT(TCNEaniline complexes) = 0.391hw(present series)91.182

(1)

with a correlation coefficient of 0.83. Moreover, a plot of the present hn values against the ionisation

(3)

where E wA is the vertical electron affinity of the acceptor, G0 is the sum of several energy terms (like dipole–dipole, van der Waal’s interaction, etc.) in the ‘no-bond’ state and G1 is the sum of a number of energy terms in the dative state. In most cases, G0 is small and can be neglected while G1 is largely the electrostatic energy of attraction between D+ and A−. The term C2 in Eq. (2) is related to the resonance energy of interaction between the ground and excited state and for a given acceptor it may be supposed constant. A rearrangement of Eq. (2) yields

Table 1 CT absorption maxima and transition energies of o-chloranil–aniline and TCNE–aniline complexes; ionisation potentials of the donors; and inductive effect perturbational quantity SrC 2rj of the donors; temperature 303 K Donor

Aniline o-toluidine N-methylaniline N,N-dimethylaniline N,N-dimethyl-p-toluidine 2,4-dimethylaniline

uCT (nm)

hwCT (eV)

o-chloranil complex

TCNE complex

o-chloranil complex

TCNE complex

607 677 570 784 821 754

574 – 619 655 688 674

2.044 1.830 2.177 1.582 1.511 1.646

2.097 – 1.946 1.839 1.749 1.786

ID (eV)

SrC 2rj

7.95 7.75 7.60 7.30 7.055 7.215

0.0 0.1467 0.2430 0.4860 0.7260 0.3867

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2I wD −hwCT = (1/C1)I wD(I wD −hwCT) + (C1 +C2/C1) (4) With the observed transition energies we have obtained the correlation 2I wD −hwCT = (0.1498)I wD(I wD −hwCT) + (6.807) (5) with a correlation coefficient of 0.99. This also confirms the CT nature of the transitions observed and yields C1 =6.675 eV. Neglecting G0 and taking the typical D– A distance in p-type EDA complexes to be 3.5 A, , the major part of G1 is estimated to be e2/4po0r =4.13 eV. Now using Eq. (3), E wA of o-chloranil in solution is found to be 2.54 eV, which is in fair agreement with the value of 2.87 eV obtained by Briegleb [26,27]. The ID values of NNDP 2,4-dimethylaniline were not available in the literature; they were calculated by using the measured hwCT values and the present correlation (Eq. (5)) and are reported in Table 1. The CT transition energies of the present series of complexes have also been utilised to get the inductive effect Hu¨ ckel parameter (hMe) of the methyl group. According to Coulson– Longuet – Higgins perturbation theory [28] in HMO formalism the energy of the highest occupied molecular orbital (HOMO) of a methylsubstituted aniline is given by Ej = E +hMeiSr C 0 j

2 rj

(6)

where j is the index for the HOMO, r denotes the location of the methyl group and the superscript 0 stands for the unperturbed system, viz. aniline. Taking Ej as the negative of the ionisation potential of the methyl aniline donor (according to Koopman’s theorem) and using the empirical McConnel – Ham –Plat [29] equation, hwCT =ID −EA +D

(7)

one obtains hwCT = − hMeiSr C 2rj +constant

(8)

In Eq. (7) EA is the electron affinity of the acceptor (o-chloranil) and D is an energy term coming from solvation etc. The values of hwCT, ID and the perturbational quantity SrC 2rj for the systems under study are given in Table 1. The values of SrC 2rj

for all the methylated anilines were calculated after diagonalising the Hu¨ ckel matrix for aniline (taking hN = 1.5 and kCN = 1 as recommended by Streitweisser [30]). A least square calculation yields the following correlation hwCT = (−0.8253)SrC 2rj + (2.0718)

(9)

with a correlation coefficient of −0.80. Comparing Eqs. (7) and (8) and taking i= −3.1 eV as obtained from first four singlet–singlet transitions of benzene [31] the inductive effect Hu¨ ckel parameter for the methyl group, hMe, is found to be − 0.279 0.1, which is very close to the value obtained by Lepley [31] by a similar study on complexes of a series of methylbenzenes with tetracyanoethylene as an acceptor.

3.2. Determination of formation constant (K) Stoichiometry and formation constants of the complexes are determined by using Benesi– Hildebrand [32] equation for cells with 1 cm optical path length: 1 [A]0[D]0 [D]0 = + d% m% Km%

(10a)

with d%= d−d 0A − d 0d

(10b)

Here [A]0 and [D]0 are the initial concentrations of the acceptor and donor, respectively, d% is the absorbance of the donor–acceptor mixture at uCT measured against the solvent as reference, d 0A and d 0d are the absorbances of the acceptor and donor solutions with same molar concentrations as in the mixture at the same wavelength (i.e. uCT). The quantity m%= mc − mA − mD means the corrected molar absorptivity of the complex, mA and mD being those of the acceptor and the donor, respectively, at uCT. K is the formation constant of the complex. Eq. (10a) is valid [32] under the condition [D]0  [A]0 for 1:1 donor–acceptor complex. Experimental data are shown in Table 2. In all the cases very good linear plots according to Eq. (10a) are obtained, one typical case being shown in Fig. 3. The correlation coefficients of all such plots were about 0.9. Values of K and m% of the complexes obtained from such plots are shown in Table 2.

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Table 2 Formation constants and molar absorptivities of EDA complexes from absorbance data of donor–acceptor mixtures measured against acceptor blank; temperature 303 K Donor (D)

105[o-chloranil]0 (mol dm−3)

Aniline

5.366

o-toluidine

5.365

N-methylaniline

N,N-dimethylanili ne

13.960

1.248

N,N-dimethyl-p-to 0.286 luidine

2,4-dimethylanilin e

1.248

m% (dm3 mol−1 cm−1)

103[D]0 (mol dm−3)

Absorbances at uCT

Formation constant (dm3 mol−1)

2.5 7.5 10.0 12.5 13.5 2.5 5.0 7.5 10.0 12.0 2.5 5.0 7.5 10.0 12.5 3.062

0.003 0.012 0.024 0.035 0.042 0.037 0.044 0.048 0.046 0.049 0.035 0.023 0.024 0.070 0.051 0.027

3365.2

1880.7

1042.5

971

94.7

653

6.12 9.2 12 15 7.5

0.028 0.028 0.031 0.029 0.015

53.5

21 786

10.0 12.5 17.5 20.0 2.5

0.028 0.025 0.029 0.032 0.027

95.1

9823

5.0 7.5 10.0 13.0

0.032 0.058 0.059 0.067

3.3. Results of kinetic studies (a) Complexes of N-methylaniline (D) with ochloranil (A) were formed instantaneously on mixing the solutions of D and A in CCl4 and their decay started from very beginning. This was indicated by the decrease with time in intensity at the absorption maximum (umax) of o-chloranil, namely, 460 nm while that at uCT absorption band remained approximately constant. The following reaction scheme was tried: A +DUAD (fast); AD“ Products (rate con-

2204

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stant k1). Here the donor D is N-methylaniline. The rate equation is, −d[complex] = k1[complex] dt

(11)

which yields [complex]= [complex]0exp(− k1t)

(12)

where the square bracket denotes concentration and the subscript zero corresponds to t=0.

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Fig. 3. Benesi – Hildebrand plot for complexes of o-chloranil with o-toluidine, the concentrations of the components being as in Table 2.

If D is taken in large excess the concentration of A at any time t is given by [A] =

[AD] [AD] [AD]0exp( − kt) : = K[D] K[D]0 K[D]0

(13)

which simplifies to

wavelength (510 nm) conforms to the following reaction scheme:A+ DUAD (fast)“ products (rate constant k2); In terms of concentration of the product, the rate equation is given by, d[P] = k2[AD]: k2K[A]0[D]0 dt

ln [A]= ln [AD]0 − kt −ln K + ln [D]0

(15)

or, ln d = −kt + Z

(14)

where Z is a constant and d is the absorbance of the mixture at the umax of A. Experimental data for each set of o-chloranil – N-methylaniline mixtures with initial concentrations mentioned in Table 2 fit Eq. (14) excellently, one typical plot being shown in Fig. 4. Rate constants obtained from the slopes of such plots are close to one another, the mean value being (3.307×10 − 2 9 1.98× 10 − 3) min − 1. In case of the EDA complexes of o-chloranil with aniline, o-toluidine and 2,4-dimethylaniline a new absorption band (other than the CT band) appeared at 510 nm. Intensity of this new band increased with time and this was indicative of product formation. Variation of absorbance of the D– A mixtures with time at this

Fig. 4. Plot of -ln d against time for o-chloranil– N-methylaniline complex. [o-chloranil]=1.396 ×10 − 4 mol dm − 3. [Nmethylaniline]=12.5 ×10 − 3 mol dm3.

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d = k2KmPt+ KmAD [A]0[D]0

2415

(19)

Experimentally observed absorbance values at 510 nm at different instants of time for the mixtures o-chloranil with aniline, o-toluidine and 2,4dimethylaniline having initial concentrations mentioned in Table 2 fit Eq. (19) excellently, the case of o-toluidine being shown in Fig. 5 as a typical illustration. This supports the proposed reaction scheme. An effective rate constant viz., k2mP/mAD can be obtained from slope/intercept of such plots. The calculated values are shown in Table 3.

Fig. 5. Plot of absorbance (d) against time for o-chloranil –otoluidine complex. [o-chloranil] =5.365 ×10 − 5 mol dm − 3. [o-toluidine]= 5 × 10 − 3 mol dm3.

Table 3 Effective rate constant of the decay viz., k2mP/mAD for the complexes of o-chloranil–aniline, o-chloranil–o-toluidine and o-chloranil–2,4-dimethylaniline, temperature 303 K Donor

k2mP/mAD (min−1)

o-toluidine 2,4-dimethylaniline Aniline

0.112 0.035 0.515

which on integration yields [P] = k2K[A]0[D]0t+Z1

(17)

Now the absorbance (d) at 510 nm, where the product and presumably also the complex absorb, is given by d=mP[P]l +mADK[A]0[D]0l

o-chloranil forms stable 1:1 EDA complexes with substituted anilines having no free NH2 group. In case of anilines with free NH2 and NHMe group, 2:1 and 1:1 (donor:acceptor) complexes, respectively, are formed and they decay into products by a reaction path, which is of the first order with respect to the complex. Charge — transfer bands could be located in all the cases and from an analysis of these bands, a good estimate of electron affinity of o-chloranil and an inductive effect parameter of the methyl group have been obtained. Vertical ionisation potentials of N,N-dimethyl-p-toluidine and 2,4-dimethylaniline have also been determined.

(16)

When t=0, no product is formed. Thus Z1 =0 and [P]= k2K[A]0[D]0t

4. Conclusion

Acknowledgements One of the authors, S. Bhattacharya, gratefully acknowledges CSIR, India for providing junior research fellowship. Financial assistance by the UGC, India extended through the DSA project in Chemistry, is also gratefully acknowledged.

(18)

where mP and mAD represent the molar extinction coefficients of the product and the complex AD, respectively, and l is the optical path length. Taking l =1 cm, rearrangement of (Eq. (18)) gives

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