A review on electronic spectral studies of charge transfer complexes

Journal of Molecular Structure 1034 (2013) 393–403

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A review on electronic spectral studies of charge transfer complexes Usama M. Rabie ⇑ Chemistry Department, Faculty of Science at Qena, South Valley University, Qena 83523, Egypt

h i g h l i g h t s " Some of the CT interactions involving I2 or DDQ acceptors are time-dependently. " Effect of time on CT complexations has been reported and discussed. " Different reactions mechanisms reported for time-dependently CT interactions have been manifested and categorized. " Donor/acceptor interactions involving only CT complexations are time-independently.

a r t i c l e

i n f o

Article history: Available online 25 August 2012 Keywords: Charge transfer complexation Effect of time r- and p-acceptors UV/Visible spectral measurements Survey on time-dependently CT interactions

a b s t r a c t The electronic absorption spectra of several charge transfer (CT) interactions have showed appearance and/or disappearance of spectral bands in correlation with the lapse of time. Careful investigations of the structural formulae of the different species involved of such interactions including their intermediates and final products have led to provide adequate illustrations of the observed time-dependency. The different reported propositions which have explained this time-dependency observed in the UV/Visible spectral measurements of the interactions involving CT complexations have been surveyed, manifested and categorized. This review is not a literature survey for all CT interactions showed timedependency but we believe that it reports all the different propositions (reaction pathways/mechanisms) which have been previously presented to reveal this time-dependency, however. Iodine (I2) as a typical r-acceptor and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as a typical p-acceptor have been selected for this study. Other conventional r- and p-acceptors might follow, to some extent, one or more of the categories presented in this study. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction For last few decades, molecular interactions between electron donors and acceptors have gained a great attention as one of the important topics of research in many fields of chemistry. These molecular interactions are often associated with formation of intensely colored charge-transfer (CT) complexes accompanying with absorption of radiation in the visible or in the UV region [1]. Consequently, among the most widely used physical methods for investigation of these CT interactions is that of spectroscopy, particularly involving the UV and visible regions. Indeed, the various features of the spectra and other properties of the molecular complexes could be fully understood only after Mulliken [1–7] propounded the charge transfer theory. The essential feature of Mulliken’s theory is that a complex is formed between an electron donor (D) and an electron acceptor (A), whereas the quantum mechanical resonance structure of the ground state WN of such ⇑ Tel.: +20 106755160; fax: +20 965213383. E-mail address: [email protected] 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.08.032

complexes is described in the term of no-bond structure W0 (D, A), in which the binding results from ‘‘physical’’ forces as a dipole–dipole interaction, a dipole-induced dipole interaction, or dispersion forces (i.e. no charge transfer occurs there) and a wave function of the dative structure W1 (D+–A), in which one electron is completely transferred from the donor molecule to the acceptor molecule. The molecular orbital treatment [8–10] posited that the electron involved in such donor–acceptor interaction is transferred from the highest occupied molecular orbital (HOMO) of the donor to the lowest unoccupied molecular orbital (LUMO) of the acceptor. On the other hand, it has been reported [11–13] that the typical timescales of CT range between picoseconds and attoseconds. Consequently, an instance appearance of the CT spectral band(s) is characteristic for pure CT interactions. In turn, the time-dependency recognized for some interactions involving CT complexations have to be utilized and explained adequately. However, according to our knowledge, yet to date no previous work has been reported to categorize the different propositions that previously presented to ascertain the observed time-dependency of

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some interactions involving CT complexation. Accordingly this study has been devoted. Owing to their very wide usage, iodine (I2) as a typical r-acceptor and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as a typical p-acceptors have been selected for this study. In addition, these acceptors are easily recognized and tracked in their different states; as molecules, anions, and even if they are involved in chemical reactions. For instance, in 1966, Foster and Horman [14] reported the different species resulted from the stepwise reduction of the DDQ. Whereas each of these different anion radicals, semi hydroquinone and hydroquinone are well-defined by their characterized spectral bands form the peaks positions and shapes [14–21].

ner one was explained [23] on the basis of the following equilibrium:

C5 H5 N  I2 ðouter complexÞ C5 H5 Nþ I I ðinner complexÞ In 1966, Bhaskar et al. [24] reported that iodine forms a CT complex with the triphenylarsine (U3As) whereas the resulted CT band is located at 320 nm. When the concentration of triphenylarsine (U3As) is sufficiently high compared to that of iodine, the CT band intensity diminishes with time. The authors [24] showed that the variation of the CT band intensity with time is probably due to the transformation of the ‘‘outer complex’’ to the ‘‘inner complex’’ of the type proposed by Mulliken [1]:

U3 As þ I2 ½U3 As  I2  \outer complex" O

O CN

Cl

Cl

Cl

CN

O CN

Cl

Cl

CN

CN

Cl

CN

O_

O

O

OH

_

Cl

Cl O_

_

OH CN

Cl

CN

Cl

CN

CN OH

Now, it is important to mention that two points are being considered in this study. First – The study involves only the CT interactions in which their electronic spectral measurements have shown time dependency of the observed spectral bands. Second – Even though we may have our own opinion but we are not in a position to assist one proposition over another, meanwhile, we have enrolled the different propositions (reaction pathways/mechanisms) as they have been published and, in turn, we have categorized them, hereafter. In consequence, the other conventional r- and p-acceptors can be manifested and categorized. 2. CT complexations involving iodine as a typical r-acceptor 2.1. Transformation form outer CT complex into inner CT complex Mulliken [1,22] pointed out that a donor–acceptor pair can form either an associative outer complex or a dissociative inner complex depending on the distance of approach between the donor and the acceptor and the relative magnitude of the no-bond and the dative wave functions. It was also suggested [1,22] that the formation of the inner complex from the outer complex should be strongly dependent on the environmental conditions. If the environmental influence is sufficiently great, the inner complex may become the stable form of the donor–acceptor pair, and with small or no environmental influence the outer complex may represent the stable form. An early evidence for the transformation of the outer complex to the inner variety was illustrated [23] by the electrical conductance measurements of the pyridine–iodine CT interaction, whereas such transformation from the outer CT complex to the in-

½U3 As  I2  ! ðU3 AsÞþ I \inner complex" They [24] pointed out that this inner complex is apparently energetically stable in this case. The rate constant K for the transformation from the outer complex to the inner complex was found to be 7.7  102 h1 at 31 °C in chloroform, indicating that the transformation is of the first order. Salman et al. [25] synthesized and characterized CT complexes of 4-amino-2,6-dimethylpyrimidine (1P), 2-amino-4,6-dimethylpyrimidine (2P), 2-amino-4-chloro-6-methylpyrimidine (3P) and 5-bromopyrimidine (4P) with the iodine (I2). Electronic absorption spectra of the mixture solution of each of the studied pyrimidines (1P–4P) with I2 in CH2Cl2 are characterized by the appearance of a new band ranged between 390 and 428 nm beside the presence of the iodine band at around 500 nm. The authors [25] reported that the observed CT band is time independent for all pyrimidines–iodine systems except for 1P–I2 in C2H4Cl2, CH2Cl2, or CHCl3, as a solvent. The band reaches its higher intensity after about 60 min. They [25] stated that compound 1P, of pKa = 6.99, has at least 2.4 pKa units more than the other studied pyrimidines. In turn, 1P is one of the moderately strength donors that can do the transformation from the outer CT complex (DI2) to the inner CT complex (D+II). Thus, the time consumed for CT band stability of the 1P–I2 system was discussed [25] in the scope of the transformation from the outer sphere CT complexes to the inner sphere ones for such pyrimidine donor–iodine system. 2.2. Formation of poly-iodide anions In many donor–acceptor systems involving halogen acceptors formation of the trihalide ions is often noticed, which can result only through the formation of inner complexes from the initial outer ones. Thus, in the CT interactions involving iodine, especially those which are time-dependent, it is very common to notice the   corresponding poly-iodide anions (I 3 ; I5 ; I7 , etc). The tri-iodide ðI Þ anions are well characterized spectrophotometrically [1– 3 3,26–28] by the appearance of two spectral bands located around 292 and 363 nm (depending on the solvent used). As shown hereafter, several publications have presented the following, general, reaction pathway, in which the triiodide anions ðI 3 Þ is resulted from the interaction between an electron donor (D) with iodine (I2) via an initial formation of the outer-sphere CT complex followed by a transformation into the inner-sphere CT complex.

D þ I2 ! þ



DI2 outer complex þ

D I  I þ I2 ! D 

! D þ I  I

inner complex

I3

ðtriiodide complexÞ

In 1965, Augdahl et al. [29] reported that the system triphenylarsine (Ph3As)–I2 gives a CT band at 321 nm and 316 nm in CCl4 and CH2Cl2, respectively. However, in solutions with low ([Ph3As]/[I2]) ratios, small amounts of the I 3 ions are formed which are indicated

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by the appearance of two spectral bands located at 295 and 365 nm. A few hours later this tendency is much more pronounced and an increase in the absorbance at 295 and 365 nm is observed, accompanied by a decrease in the absorption of the CT band. After 3 h most of the complex is destroyed, resulting in the two distinct I 3 bands. The authors [29] stated that a slow ionization of the complex followed by dissociation might be the reason for the formation of the triiodide ion. This explanation is contradicted by the fact that the reaction is almost as rapid in the non-polar solvent CCl4 as in the polar solvent CH2Cl2. In acetonitrile no CT is detected. Instead, bands of I 3 are detected. This has led to the suggestion that the CT complex, initially formed, very quickly undergoes an ionization  followed by the formation of I 3 from reaction between I and iodine. The net result would be:

Ph3 As þ I2 ! Ph3 As  I2 Ph3 As  I2 þ I2 ! Ph3 AsIþ  I3 After a year, in 1966, Bhaskar et al. [24] reported that iodine forms a CT complex with each of the donors; triphenylamine, triphenylarsine, triphenylphosphine and triphenylstibine. In case of triphenylphosphine (U3P), I 3 bands appear at 290 and 360 nm, whereas the intensities of these bands decrease with time, particularly when the donor concentration is sufficiently high. This variation with time is less at higher iodine concentrations. Thus these results can be interpreted in terms of the following scheme:

U3 P þ I2 ½U3 P  I2  \outer complex" very

½U3 P—I2  ! ðU3 PIÞþ I \inner complex" fast

ðU3 PIÞþ I þ I2 ðU3 PIÞþ I3 The authors [24] stated that the inner complex is apparently so much more stable in the U3P–I2 system that the outer complex is not even observed in the spectrum. The disappearance of the I 3 absorption bands with time and the appearance of free iodine are interpreted in terms of the equilibrium nature of the reaction 3 (for mation of the I 3 ). For some reason, the I3 is formed immediately after the formation of the inner complex which slowly goes back to I + I2. In the presence of excess of iodine, the formation of the triiodide ion will be favored and the time dependence of the I 3 band is also less. Tertiary phosphines are very strong bases. So that, the high basicity of U3P is responsible for making the inner complex more stable compared to the outer complex. Bhat and Rao [30] completed the work reported by Bhaskar et al. [24] for the triphenylarsine–iodine and triphenylstibine–iodine systems. They [30] found that kinetics of transformation of outer CT complexes to inner complexes is found to follow the first-order rate law and the rate constants are quite large. Thus, the intensity of the CT bands in these two systems decreases markedly with time. Accompanying the decrease in the intensity of the CT bands, progressive intensification of the triiodide ion absorption at 297 and 363 nm is noticed. It was felt [30] that the time dependence of the CT bands and the formation of I 3 are both due to the transformation of the initially formed 1:1 outer complex into the inner complex followed by the fast reaction of the inner complex with iodine to form the triiodide ion. slow

ðC6 H5 Þ3 M þ I2 ðC6 H5 Þ3 M  I2 ! ðC6 H5 Þ3 Mþ I I M¼AS or Sb

outer complex

inner complex

ðC6 H5 Þ3 Mþ I I þ I2 ðC6 H5 Þ3 Mþ I I3 The quaternary salts (inner complexes) have already been reported in both arsine and stibine systems [24]. However, in polar solvent like CH2Cl2 the transformation is very fast and the triiodide ion ap-

pears almost immediately after mixing (C6H5)3As with iodine. The transformation of the outer complex (C6H5)3SbI2 to the inner complex is much slower and is associated with a higher activation energy (Ea) than the transformation of (C6H5)3AsI2. The authors [30] stated that the formation of triiodide is a function of time, concentration of the donor, and nature of the solvent. Since the formation of I 3 ion from the inner complex is likely to be a fast step, the ratedetermining step in the formation of the I 3 ion is probably the transformation of the outer complex. Dwivedi and Banga [31] examined the interactions of iodine with 2-aminopyrimidine, 2-methylimidazole and benzimidazole. In the system 2-methylimidazole-I2, the CT as well as the blueshifted iodine bands could not be observed due to the formation of I 3 as evidenced by the appearance of the absorption bands at 295 nm and 365 nm even in CHCl3 solution. Progressive intensification of the I 3 absorption is noticed. They [31] felt that the formation of I 3 is due to the transformation of the initially formed 1:1 EDA complex (outer complex) into the inner complex followed by the fast reaction of the inner complex with iodine to form I 3 similar to that reported by Rao et al. [30].

D þ I2

D  I2 outer complex

slow

ƒ!

D þ I I

inner complex

Dþ I I þ I2 Dþ I I3 The transformation is very fast and I 3 appears almost immediately after mixing in all the solvents studied. However, the choice of the solvents was very limited because in CCl4, the yellow color of I 3 fades with time (possibly due to the interaction of I with CCl4). In polar solvents like acetonitrile [30], iodine alone yields I 3. The interaction between iodine and 1,4,8,11-tetraazacyclotetradecane (TACTD) was studied spectrometrically in various solvents like CCl4, CHCl3, CH2Cl2 and 1,2-dichloroethane [32]. The results reveal that in each solvent the (TACTD):I2 ratio is 1:2 and the iodine complex is formulated as (TACTD) Iþ  I 3 . The existence of the triiodide ions is supported by Raman spectra of the formed solid complex. The iodine complex is shown to be strongly dependent on the polarity of the solvent. It has been reported [33] that the spectra of the nicotine–I2 system show two strong absorptions at 292 and 362 nm which are characteristic to the triiodide anion. This indicates that the nicotine–I2 complex formed exists as (nicotine) Iþ  I 3 . As it has been indicated [33] that a strong convincing evidence for this assignment is obtained from the behavior that lowering temperature, in the range of 10–25 °C, does not affect the intensity ratio of the observed two bands. Therefore, it is deduced that the two CT bands should have the same enthalpies of formation and therefore they are of the same nature (i.e., due to absorption of the I 3 ion formed). Thus, the formation of the (nicotine) Iþ  I 3 complex could be represented by the following equations:

nicotine þ I2 ! nicotine  I2 ! ðnicotineÞþ I  I outer complex þ



inner complex þ

ðnicotineÞ I  I þ I2 ! ðnicotineÞ I:I3 Hamed et al. [21] reported that the CT molecular complex formed of atropine with iodine is of strong kind (n–r) and it exists in the ionic structure, ðatropineÞIþ  I 3 , whereas, the solvent polarity plays an important role in determining the stability of such kind of CT complexes. It has been shown [21] that the electronic absorption spectra of the CH2Cl2 solution mixture of atropine with I2 clearly reveal that the complex formed has two strong absorptions with maxima at 288 and 396 nm which are characteristic to the triiodide ions. Furthermore, the recorded spectrum of the solution of the synthesized CT complex in CH2Cl2 displays these two strong absorption bands at 295 and 363 nm. However, the observed red shift in the longer wavelength band of the atropine–I2 mixture solution

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(396 nm) on comparison with that reported for I 3 (365 nm) could likely be attributed to its overlap with the broad absorption band of the uncomplexed I2 (kmax = 510 nm). The authors [21] represented the following mechanism for the formation of this CT complex:

Atropine þ I2 ! Atropine  I2 ! ðAtropineÞIþ  I outer complex þ

inner complex

ðAtropineÞI  I þ I2 ! ðAtropineÞIþ  I3

D þ I2 ! DI2 ! Dþ I  I outer complex

inner complex

þ

þ



D I  I þ I2 ! D I  I3 ðtriiodide complexÞ This pathway requires a lone pair on the donor, not present in the ferrocene case. Thus, the authors [37] believed that the following pathway can better explain the interaction of ferrocene donors with iodine:



Meanwhile, according to this mechanism, they [21] stated that the initially formed 1:1 outer complex is transformed into the inner one followed by its reaction with I2. Whereas, the expected high electron donating power of atropine as a result of its high basicity is considered to play an important role in the formation of the triiodide ion. However, in terms of this mechanism the observed increase in absorption intensities of the two bands of the triiodide with time to the increase in [I 3 ] is ascribed. The host–guest molecular complexation reaction between crypt and C222 and iodine were studied spectrophotometrically in C2H4Cl2 solution [34]. The resulting 1:1 and 2:1 (I2 to C222) complexes are formulated as (C222  I+)I and (C222  I+) I 3 , respectively. The time dependence of the triiodide absorption bands and the large negative DS value obtained indicate the transformation of an initially formed 1:1 C222–I2 outer complex into an inner complex (C222  I+)I followed by the fast reaction of the inner complex with iodine to form I 3 ion [35]. Hasani and Shamsipur [36] investigated spectrophotometrically in CHCl3 solutions the interaction of Hexaaza-18-crown-6 (HA18C6) and Tetraaza-14-crown-4 (TA14C4) with iodine. The observed time dependence of the CT band and subsequent formation of I 3 in solution are related [36] to the slow transformation of the initially formed 1:1 macrocycleI2 outer complex to an inner electron donor–acceptor (EDA) complex, followed by fast reaction of the inner complex with iodine to form a triiodide ion. The pseudo-first-order rate constants at various temperatures for the transformation process were evaluated form the absorbance-time data [36]. Decamethylferrocene (Cp2Fe), ferrocene (Cp2Fe), 1,1-dimethylferrocene ((MeCp)2Fe) and 1,2-diferrocenylethane (Fc2C2H4) have been studied [37] as donors for CT complexations with I2. Oxidation of the ferrocenes to the corresponding ferricinium slats is proposed [37] to occur via initial formation of outer sphere CT complexes. UV–Vis spectra for the mixtures solutions of any of the ferrocene donors with I2 showed the characteristic bands of the I 3 anion radical, meanwhile, elemental analyses of the isolated solid complexes indicated the formation of [ferrocene]I3 for ferrocenes and I2, except for (Fc2C2H4) which gave [(Fc2C2H4)]I5. The authors [37] indicated that the reaction of ferrocenes with I2 in CH2Cl2 always led to the formation of salts via electron transfer. All attempts to identify CT band(s) for mixtures of ferrocenes and I2 in CH2Cl2 failed. Instead, the characteristic bands of the I 3 at 292 and 363 nm grew in, while the yellow color of the ferrocenes turned to the green color of the ferricinium salts. The electronic spectra of the MeOH solutions of the solid complexes indicate the presence of ferricinium [38] and I 3 ions [26–28]. For Cp2Fe/I2 in CH2Cl2 the triiodide bands appeared immediately on mixing indicating rapid interaction, but for Cp2Fe/I2, (MeCp)2Fe/I2 and Fc2C2H4/I2, this oxidation took about 5– 10 min. The stoichiometric ratios of these complexations in CH2Cl2 were ascertained by applying the C.V. method [39] which gave a symmetrical curve with a maximum at a mole fraction 0.40 of the ferrocene donors with I2, indicating that the complexes are of the form ferrocene–I3. The authors [37] stated that the only reaction pathway previously reported [21,30,40] for triiodide formation via the interaction of iodine as r-acceptor with strong donors is:

ðferroceneÞ þ I2 ðferroceneÞ  I2 ðouter CT complexÞ ðferroceneÞ  I2 ! ðferriciniumÞþ I þ 1=2I2 ðferriciniumÞþ  I þ I2 ! ðferriciniumÞþ I3 Thus, a fast formation of the CT complex followed by a slow step to give the ferricinium ion, followed by formation of triiodide has been proposed [37]. Although the system Fc2C2H4/I2 gave both the triiodide and the ferricinium bands in CH2Cl2 solution, indicating the formulation (Fc2C2H4)I3, the elemental analyses of the isolated solid complex indicated the formulation (Fc2C2H4)I5; pentaiodide compounds are well known [41,42]. This formulation implies the presence of one Fe (III) and one Fe(II) in the complex. Accordingly, the UV–Vis spectra of (Fc2C2H4)I3 solution has a shoulder corresponding to the Fe(II) center. Rabie et al. [43] reported that CT complexes of 1,2,4-triazole (1T), 3-amino-1,2,4-triazole (2T), 4-amino-4H-1,2,4-triazole (3T), and guanazole (3,5-diamino-1,2,4-triazole) (4T) with iodine have been synthesized and characterized. The system triazoles/I2 is characterized by the formation of triiodide ions (I 3 ), which is proposed to occur via the initial formation of outer-sphere CT complexes according to the previously reported [20,30,40] general mechanism for triiodide formation via an interaction of the racceptor iodine with strong donors. Thus, a lone pair of electrons present on each of the nitrogen atoms of the triazoles emphasizes this interaction pathway. It has been reported [44] that addition of antipyrine to iodine solution in CHCl3 results in two absorption bands presumably due to the formation of antipyrine–iodine complex. The spectra of the mixture show bands characteristic of free iodine and complexed iodine. The latter has two absorption bands, the hypsochromically shifted iodine band (in the region of 360 nm) and a CT band (<300 nm) that overlaps the intense band of the free donor. It has been observed [44] that the spectra recorded for the complexes between antipyrine and iodine are time dependent. The intensity of the 360 nm band increased markedly with the lapse of time. The observed time dependence of the CT band and the subsequent formation of the I 3 ion in solution are most probably due to the transformation of the initially formed outer complex into an inner EDA complex followed by a fast reaction of the resulting inner complex with iodine to form a triiodide ion [36,40,45]. By analogy with other n–r complexes [23,30,46,47] the authors [44] concluded that the following reaction occurs between antipyrine (D) and iodine in solution.

D þ I2

DI2 outer complex

DI2 ðD  IÞþ I

Fast Slow

inner complex

ðD  IÞþ I þ I2 ðD  IÞþ þ I3

Fast

The kinetics of transformation of the outer complex was followed by the time dependence of I 3 absorption band at 364 nm [44]. The transformation of the outer complex into the corresponding inner complex was found to follow pseudo-first order kinetics. Pandeeswaran and Elango [48] reported that the interaction between the drug atenolol and iodine proceeds through the initial formation of a CT complex as an intermediate species. The stoichiometry of the complex was found to be 1:2. The rate of their reaction has been measured as a function of time and solvent. It is

U.M. Rabie / Journal of Molecular Structure 1034 (2013) 393–403

evident [48] from the absorption spectra of the atenolol/I2 mixture solution in tert-butyl alcohol that the absorption due to I 3 progressively intensifies with time. Such a time dependence of the absorption spectrum suggests that the outer complex initially formed at 230 nm is transformed into an inner complex, followed by its fast reaction with iodine to form I 3 ion according to the following scheme:

Drug þ Iodine Drug  Iodine ðouter complexÞ Drug  Iodine ½Drug  Iþ I ðinner complexÞ ½Drug  Iþ I þ I2 ½Drug  Iþ I3 2.3. Formation of a new acceptor resulting in situ The CT complex of 4-(dimethylamino)pyridine (DMAP) with iodine acceptor has been synthesized and characterized [49]. The system DMAP/I2 is characterized by formation of triiodide ions (I 3) which is proposed to occur via initial formation of outer-sphere CT complex. In this case, DMAP/I2, two new bands at 292 and 363 nm which are characteristic to the triiodide ion (I 3 ) appeared and reached almost maximum intensities after 15 min. The author [49] reported that formation of the triiodide complexes as a result of the interactions of n-donors (D) with the r-acceptor (I2) has been reported [21,30,36,40] to follow the following reaction pathway: (1) formation of the associative outer-sphere CT complex DI2, (2) transformation to the dissociative inner-sphere complex (DII), (3) association with another iodine molecule to form the triiodide complex (Dþ I  I 3 ). Thus, in this case, DMAP/I2, the 15 min might be required for completion of this transformation. Meanwhile, the lone pair of electrons on each of the ring nitrogen and/or amino nitrogen atom of the DMPA donor emphasizes this interaction pathway although the pKa values of the aminopyridines, in general, have been shown to correspond to protonation of the ring nitrogen [50–52]. However, the electronic spectrum of the CH2Cl2 solution of the solid complex DMAP/I2 indicates the presence of I 3 ions. Meanwhile, the elemental analysis of the formed solid CT complex of DMAP with I2 indicates that the stoichiometric ratio D: A is either 1:1, as previously reported by Daisey and Sonnessa [52], or 2:2. The author [49] stated that formation of a compound having triiodide ions means that this compound (complex) contains, definitely, at least 1½ iodine atoms. In turn, the 2 + 2 assembly in which two molecules of both donor and acceptor come to bonding each other might be truly describe the case meaning that another molecule of the donor DMAP is somehow bonded with the triiodide complex (DMAPþ I  I 3 ). Thus, it has been suggested by the author [49] that the first formed triiodide complex (DMAPþ I  I 3 ) still has the ability to accept charges. It serves as a new acceptor resulting in situ and this undergoes CT interaction with another molecule of the donor to form the solid CT complex (DMAP)(DMAPþ I  I 3 ) having the ratio 1:1, (DMAP): (DMAPþ I  I 3 ). Also, and accordingly, the author [49] suggested that the previously reported [21,25,30,37,40,43] reaction pathway for the formation of the triiodide complexes may be extended by the interaction with another molecule of the donor (DMAP) according to the following scheme:

DMAP þ I2 !

DMAP  I2 ðoutersphere CT complexÞ

!

DMAPþ I  I

ðinnersphere complexÞ

DMAPþ I  I þ I2 ! DMAPþ I  I3 ðfirst formed complexÞ

DMAP þ DMAPþ I  I3 ! ðDMAPÞ  ðDMAPþ I  I3 Þ ðdonorÞ

ðnew acceptorÞ

ðCT complexÞ

In 2007, Rabie et al. [53] reported that electronic adsorption spectra have been recoded for the mixtures solutions of some pyrimidine

397

derivatives, 4-amino-2,6-dimethylpyrimidine, Kyanmethin, (4AP), 2-amino-4,6-dimethylpyrimidine (2AP), 2-aminopyrimidine (AP), 2-amino-4-methylpyrimidine (AMP), 2-amino-4-methoxy-6-methylpyrimidine (AMMP), and 4-amino-5-chloro-2,6-dimethylpyrimidine (ACDP) as electron donors, with the r-acceptor iodine in several organic solvents. The electronic spectra of each of these mixtures solutions are characterized by the instance appearance of a new band ranged between 395 and 417 nm, which does not belong to each donor and acceptor, beside the presence of the iodine band [1–3] at around 500 nm. The new band is located at 400, 410, 395, 403, 402, and 417 nm for mixtures solutions 4AP/I2, 2AP/I2, AP/I2, AMP/I2, AMMP/I2 and ACDP/I2, respectively. However, electronic absorption spectra of the solutions of the synthesized pyrimidines–iodine, P–I2, CT complexes have shown the characteristic bands of the triiodide ion, I 3 located at 292 and 363 nm. The authors [53] reported that UV/Visible spectral tracking of each of the P–I2 mixtures solutions in CH2Cl2 has showed that in all cases, except 2AP–I2 and AMP–I2 mixtures, the primarily formed CT band, located in the range 395–417 nm, is dramatically affected by time, as it gradually shifts toward lower wavelengths, hypschromic shift, through lapse of time. Thus, in all that cases the destination of the gradual shift of the first formed CT bands is directed toward the wavelength region near to that characterizing to the triiodide ions (I 3 ), ca. at 364 nm in CH2Cl2, meanwhile a parallel gradual diminishment of the primary formed CT band is observed through that time, which is due to a decrease in the concentration of the CT complex. This means [53] that iodine presents in each of the solid products with at least 1½ atom. But, the elemental analyses of these synthesized solid compounds indicated that the stoichiometric ratio, pyrimidine donor (P): acceptor (I2), is either 1:1, as previously reported by Daisey and Sonnessa [52], or 2:2. Thus, in agreement with the idea previously presented by Rabie [49] the authors [53] suggested that the first formed triiodide complex ðPþ I  I 3 Þ still has the ability to accept charges. It serves as a new acceptor resulting in situ and this undergoes CT interaction with another molecule of the donor to form the solid CT complex ðPÞ  ðPþ I  I 3 Þ having the ratio 1:1, (P): ðPþ I  I Þ. 3 Also, it has been found [54] that interaction of 2-aminopyrimidine (AP) with iodine has followed the same trend as in the case of pyrimidines/iodine system [53] in which a new acceptor having the formula ðAPþ I  I 3 Þ was resulted in situ which led to form a CT complex with another molecule of the donor, ðAPÞ  ðAPþ I  I 3 Þ.

3. CT complexations involving DDQ as a typical -acceptor 3.1. Formation of a dative structure It has been reported by many authors that the observed timedependency of the electronic absorption spectral measurements of the donor (D)–acceptor (A) interaction involving the p-acceptor DDQ is referred to the transformation of the formed CT complexes from the outer sphere type D–A (non-bonding structure) to the inner sphere type D+ + A (dative structure). Mahmoud et al. [55] reported the interactions of the donors 2amino-5-X-1,3,4-thiadiazole (X = H, I; = CH3, II; = phenyl, III) with the p-type electron acceptor DDQ. Generally the spectra recorded for the methanolic CT complex solutions of the compounds I–III (D) with DDQ (A) are time dependent where some bands disappear and new bands appear. This can be interpreted on the principle that the formed CT complexes are strong, i.e. the ground CT state of each complex has predominately a dative structure D+–A. In addition, the authors [55] indicated that since methanol is also quite polar and the 2-amino-1,3,4-thiadiazoles exist mainly in the amino form, one should expect that the very polar ground CT state would be highly stabilized by such solvent molecules through

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dipole–dipole or dipole–induced dipole and H-bonding interactions. Accordingly, dissociation of the CT complex to D+ and A radicals can be found to take place in the ground state. The authors [55] also reported that the appearance of isosbestic points in the spectra recorded at later times from mixing the reactants (>15 min) suggests that the reaction has to proceed first to form D+–A, then an equilibrium dissociation to the radicals D+ and A is established. This behavior can be represented by the following equation:

D þ A ! Dþ  A Dþ þ A The results of chemical analyses of the synthesized solid CT complexes indicate the formation of 1:1 CT complexes. In 1989, it has been reported [56] that the electronic spectra of the CT complexes of some different substituted pyridines (2-hydroxy-, 2-methoxy-, 2-methyl-, 4-methyl-, 2-aminopyridine) with the p-acceptor (DDQ) in CH2Cl2 show two CT absorption bands. Generally, these two bands appeared are broad and complex. The absorption intensities of the observed bands increase with time and reach maximum constant values after 2 h, 1/2 h, 1 h, 10 min and 5 min for 2-hydroxy-, 2-methoxy-, 2-methyl-, 4-methyl- and 2-aminopyridine, respectively. In all cases the general features of the spectra do not change by time denoting no side reaction. Only in the case of the donors 4-methyl- and 2-amino-pyridine the absorption intensities of the observed bands start to decrease after a period of time >20 min, owing to the precipitation of the solid complexes. The authors [56] stated that the observed time dependence of the intensities of the CT bands can probably be ascribed to the transformation of the complex from the outer-sphere type D-A (py-DDQ), non-bonding structure, to the inner-sphere complex D+–A (py+-DDQ), dative structure, i.e. the extent of contribution of the dative structure to the ground state is increased. Accordingly, the observed broad or structural features of the absorption bands of the CT complexes between pyridines and DDQ can be explained on the principle of the possible dissociated state of the CT complexes in the ground state (D+–A M D+ + A) under the effect of the relatively high polarity of the CH2Cl2 (D = 9.08). Mahmoud and co-workers [57] reported that molecular complexes of different substituted thiazoles with the p-electron acceptor (DDQ) have been investigated. It is deduced that the complexes formed are of the p–p type and the solid complexes have 1:1 stoichiometry. The absorption spectra of the studied thiazoles–DDQ CT complexes in CH2Cl2 (except in case of 2-aminothiazole where methanol was employed) display two absorption bands in case of the donors thiazole, 4-methylthiazole, 2,4-dimethylthiazole, 2aminothiazole. On the other hand, one broad absorption band is observed in case of the phenylthiazoles. Except in case of the complexes of 4-phenyl-and-2-methyl-4-phenylthiazole, the absorption intensities of the observed CT bands of the other studied thiazoles compounds increase with time where the general features of the recorded spectra do not vary indicating no formation of reaction products, i.e. no side chemical reaction. Since the chemical analysis data of the isolated solid thiazoles–DDQ complexes are in accordance with the formation of 1:1 complexes, the authors [57] deduced that the two CT bands appeared in the spectra of the studied CT complexes are two CT transitions for one molecular complex. However, the structural character of the two CT bands in case of thiazole and 4-methyl-, 2,4-dimethyl-, 2-aminothiazole-DDQ complexes can be likely ascribed to the existence of these complexes in the dissociated states (D+–A M D+ + A). Thus, the above described time-dependence of the absorptions intensities of the CT bands can be probably due to [57] the transformation of the formed CT complexes from the outer sphere type D–A (nonbonding structure) to the inner sphere type D+ + A (dative structure).

One year later, El-Gyar et al. [58] studied the CT molecular complexations of 2,20 -bipyridyl (I), 1,10-phenanthroline (II) and 4,20 ,60 ,400 -terpyridyl (III) with the p-electron acceptor DDQ. It has been found [58] that, generally, the electronic spectra of the CT complexes of the donors I–III with DDQ in CH2Cl2 show two main CT absorptions and these two bands appear broad and complex. The absorption intensities of the observed bands increase with time and reach maximum constant values after a period of time ranging from 15 to 20 min. In all cases the general features of the spectra do not change with time, denoting no side reactions. The authors [58] stated that the two CT bands are double CT transitions for one molecular CT complex. Since it is known that pyridines have predominately n-donor properties and, in addition, have partial p-donor properties, the authors [58] assigned the donor orbital involved in the CT transition responsible for the shorter CT band as likely to be of p-type. They [58] reported that the observed time dependence of the intensities of the CT bands can probably be ascribed to the transformation of the complex from an outer sphere type D–A, non-bonding structure, to an inner sphere complex D+–A, dative structure i.e., the extent of contribution of the dative structure to the ground state is increased. Accordingly the observed broad structural features of the absorption bands of the CT complexes with DDQ can be interpreted on the principle that their ground states are dissociated (D+–A M D+ + A) under the influence of the relatively high polarity of the solvent used. Boraei [59] reported that upon mixing CH2Cl2 solutions of some pyrazole donors with DDQ, colored solutions are formed immediately indicating the formation of CT complexes. The absorption spectra recorded for the CT solutions of each of the pyrazole donors–DDQ systems have displayed a new absorption band in the visible region. This new band is [59] typical of the electron donor–electron acceptor CT transition between the pacceptor and pyrazoles. The absorption intensities of the recorded CT bands increase with lapse of time and reach maximum constant values after a period of time (10–15 min) depending on the nature of the donor and acceptor used. The general features of CT spectra do not vary with time indicating no side chemical reactions. The author [59] pointed out that the observed time dependence of intensities of CT bands can be ascribed to the possible transformation of the formed complexes from the nonbonding structure D–A (outer-sphere type) to the dative structure D+–A (inner-sphere type). That is, the extent of the contribution of the dative structure to the ground state wave function is increased. 3.2. Formation of anion radicals It has been reported in several publications that the observed time-dependency of the electronic absorption spectra of the donor (D)–acceptor (A) interaction involving the p-acceptor DDQ is due to the stepwise reduction of the acceptor (DDQ) to the corresponding anion radicals. Whereas these anion radicals are well-defined by their characterized spectral bands form the peaks positions and shapes [15–21]. Abdellatef [60] described simple, rapid, accurate and sensitive spectrophotometric methods for determination of perinfpril drug. The methods are based on the reaction of this drug as n-electron donor with DDQ as p-acceptor to give highly colored complex species. The author [60] noticed that the optimum reaction time has been determined by following the color developed at ambient temperature (20–25 °C), whereas, complete color development is attained after 20 min; formation of the anion radicals (DDQ). Meanwhile, the observed spectral bands are characteristic for these anion radicals (DDQ) from peaks positions and shapes.

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Salman et al. [25] studied CT complexations of 4-amino-2,6dimethylpyrimidine (1P), 2-amino-4,6-dimethylpyrimidine (2P), 2-amino-4-chloro-6-methylpyrimidine (3P) and 5-bromopyrimidine (4P) with the p-acceptor DDQ. The authors [25] discussed the time dependency of the observed spectral bands for the pyrimidine donors–DDQ systems in the scope of stepwise reduction of the DDQ to its anion radicals. Whereas, electronic absorption spectra obtained of (1P–3P)–DDQ complex solutions in CH2Cl2 display six main absorption bands located around 348, 412, 436, 460, 520 and 568 nm which could be ascribed to the anion radicals (DDQ). The recorded spectra [49] for CH2Cl2 mixture solutions of 4(dimethylamino)pyridine (DMAP) and DDQ have displayed a series of absorption bands appearing at kmax = 340, 441, 460, 562, and 605 nm. These bands are time-dependent reaching stability after about 80 min on mixing the reactant species. Undoubtedly, these bands are ascribed to the anion radicals (DDQ) based on the peaks positions and shapes. This means that the DMAP/DDQ complex exists in solution predominantly in the dissociated state radical cations and anions, DMAP+ and DDQ, respectively. The author [49] stated that appearance of the structural bands characterizing the DDQ anion radicals seems to follow the previously reported [37,55,58,61] interaction pathway. Thus, it can be probably ascribed to the transformation of the CT complex from an outersphere type of the DMAP–DDQ non-bonding structure to an inner-sphere complex DMAP+–DDQ of dative structure. Then equilibrium dissociation to the radicals DMAP+ and DDQ is established in the ground state. The structure of the formed CT complex (DMAP+–DDQ) has been confirmed by IR and electronic spectra of the synthesized solid complex in addition to the elemental analysis.

reaction pathway that the only rout to obtain the anion radicals (DDQ) is the first formation of the CT complex between 4T and DDQ. Meanwhile, this complex then dissociates to the corresponding dative structure, consisting of the cation (4T+) and the anion (DDQ): (4T + DDQ ? 4TDDQ outer sphere CT complex ? 4T+. DDQ inner sphere (dative structure)), i.e. the first recognized structural bands. It follows that the formed DDQ has only two routs: either a chemical reaction with the 4T or to be reduced to the corresponding hydroquinone (DDQH2). Consequently, it has been concluded [43] that the time dependency observed for spectra of the system 4T/DDQ is accounted for by the stepwise reduction of DDQ to its anion radicals, and then to the corresponding hydroquinone (2,3-dichloro-5,6-dicyanohydroquinone), DDQH2. However, because the resulting in situ (DDQH2) is an unconventional new acceptor, it forms a CT complex with the donor (4T). On the other hand, Rabie et al. [63] reported that interaction of thiazolidine-2-thione (T2T) as an electron donor with DDQ as an electron p-acceptor has led to a redox reaction in which T2T has been oxidized to the corresponding dehydrogenated T2T (T2T2H), meanwhile DDQ has been fully reduced to the corresponding hydroquinone (DDQH2). However, the two new species, resulting in situ, have been interacted, whereas a CT complex having the formula (T2T-2HDDQH2) has been occurred. IR, 1H NMR and Mass spectra were used for ascertaining the structural formula of the synthesized CT complex.

S

NH

S

S

SH

(I) Thione

Thiole

( II )

3.3. Formation of a new donor and/or a new acceptor resulting in situ As aforementioned that formation of the anion radicals (DDQ) is responsible for the elapsing of time observed in the absorption spectra of the CT complexations between some electron donors and DDQ. However, careful examination of the synthesized solid CT complexes has led (sometimes) to indicate formation of a new donor and/or a new acceptor resulting in situ. In turn, these new species can undergo a second CT complexation. In 1997, for the first time, it has been reported [62] that the reduced hydroquinone (CHLH2) resulting during the interaction of p-chloranil (CHL) with the organomettalic donor, pentahydridotris(triphenylphosphine)rhenium, ReH5(PPh3)3, can also interact as an electron p-acceptor. However, the slow formation and stabilization of the UV–Visible CHL CT band could be explained by the first reduction of CHL to CHLH2. Seven years later, Rabie et al. [43] reported the same trend for DDQ in its interaction with guanazole (3,5-diamino-1,2,4-triazole) (4T). The electronic absorption spectra of the methanolic CT complex solutions of 4T with DDQ have displayed the structural characteristic bands to the anion radicals, DDQ. Surprisingly, these bands are time dependent. Within a period of 45 min, these bands decrease until they completely vanish, meanwhile, a new band at 400 nm increases. The new band (at kmax = 400 nm) reaches its maximum intensity within 45 min. This band could be assigned [43] as a CT band arising from charge transfer from the highest occupied molecular orbital (HOMO) of the 4T donor to the lowest unoccupied molecular orbital (LUMO) of the resulting new acceptor, DDQH2. Thus, in this system, 4T/DDQ, it has been assumed [43] that DDQ follows CHL in forming the corresponding hydroquinone. Much evidence has been presented [43] to support the suggestion that the reduction of DDQ to DDQH2 can account for the time dependency (45 min). The resulting, in situ, hydroquinone (DDQH2) reacts as a CT p-acceptor with the donor 4T. The authors [43] agreed with the previously reported [55,61]

N

Thiazolidine-2-thione (T2T)

Structures (I) and (II): Thione – Thiole Tautomeres

N H

S

H N

S

S S

Structure (III): Dimeric Hydrogen-Bonded Thioamide Complex OH

S

NH NC

Cl

NC

Cl

.

S

OH

Structure (IV): (T2T-2HDDQH2) CT complex The electronic absorption spectra [63] of the T2T–DDQ mixture solution in MeOH (or EtOH), a polar protic solvent, are characterized by developing of a new spectral band at 350 nm which does not instantaneously appeared on mixing the two interacting species. The intensity of this new band is gradually increasing by lapse of time and it reaches its maximum intensity after about 70 min from the time of mixing the two reactants but the position

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of this band (at 350 nm) does not change during this lapse of time. Meanwhile, the electronic absorption spectra of the T2T–DDQ solution mixtures in non-polar chlorinated solvents are characterized by the instant observation of that new spectral band in the range of 340–355 nm, and the intensity of this new band is also gradually increasing by lapse of time without a considerable change in the band positions too. Thus, the authors [63] believed that the observed lapse of time, which is necessary for the CT spectral band of the T2T–DDQ interaction to reach its maximum intensity, might be explained by: First – Dissociation of the dimeric thioamide hydrogen-bonded solid complex (St. III) in the solutions to its individual molecules, whereas the resulting individual molecules exist in the solutions in two tautomeric forms (thione (St. I) and thiole (St. II)). Second – Reduction of the DDQ to its corresponding hydroquinone; DDQH2. Third – Simultaneous oxidation of the T2T (either in thione or thiole form) to the corresponding dehydrogenated T2T; T2T-2H. In turn, the observed CT band is referred to a charge transfer arising from the highest occupied molecular orbital (HOMO) of the resulting dehydrogenated new donor (T2T-2H) to the lowest unoccupied molecular orbital (LUMO) of the resulting reduced new acceptor, DDQH2, whereas the two new interacting species (T2T-2H and DDQH2) are resulting in situ through a period of time depending on the polarity of the solvent used. In addition, the authors [60] reported that quinones, in general, and DDQ, in specific, are well known [19,64–66] as dehydrogenating agents since the pioneering work [67] published in 1954. Several mechanisms, have been suggested [19,65] for these kinds of redox reactions that caused by the action of the quinones as dehydrogenating agents. However, it has been reported [65,66] that in many of these redox reactions involving DDQ, CT complexations have been occurred at least initially. In turn, in the case of interaction between T2T and DDQ, time dependency of the established dehydrogenation of T2T with DDQ, to yield T2T-2H and DQQH2 via first formation of a CT interaction followed by a redox reaction, has found an adequate explanation. Thus, the following reaction pathway might be hypothesized [63] for the interaction between the T2T, as a donor, with the DDQ, as an acceptor, meanwhile the multiple and sequential CT complexations involved through this T2T–DDQ interaction has been suggested too:

T2T

Thioamide dimer complex

!

T2T

Individual free molecules

T2TðthioneÞ T2T ðthioleÞ

O

CH3

Cl

CN

Cl

CN

CH3 v. fast

+ (Me) n

v. fast (Me) n

O

O Cl

CN

Cl

CN O

slow

slow +

O

CH2

_

Cl

CN

+ Cl

CN OH

(Me) n

NC

CN OH

CH2 O Cl

(Me) n

NC

Cl

CN O_

CH2 O Cl

(Me) n

NC

CN O H2C

CH2 O (Me) n

Cl

Cl

Cl

(Me) n

ðtautomerismÞ

T2T þ DDQ ! T2T  DDQ ðfirst CT interactionÞ T2T  DDQ ! T2T-2H þ DDQH2 ðredox reactionÞ T2T-2H þ DDQH2 ! T2T-2H  DDQH2 ðsecond CT interactionÞ

3.4. Formation of a chemical reaction Foster and Horman [14] reported that when a solution of DDQ in CH2Cl2, C2H4Cl2 or CHCl3 is mixed with a methylbenzene, the solution becomes colored apparently instantaneously. The resulting absorption band is typical of a CT complex (p). With time the color fades because of further, slow, irreversible reaction. It has been suggested [14] that the reaction proceeds by a slow, hydride-ion transfer process [67,68], to produce the carbonium ion (II) and the quinol anion (III), which then react together to form the monoether (IV). Then, this may react through (V) with a second carbonium ion to form the bis-compound, 2,3-dichloro-5,6-dicyanoquinol-diether (VI).

The rate of decay [14] of the CT complexes formed between DDQ and various methylated benzenes is interpreted in terms of a monomolecular decomposition of the CT complex and/or a bimolecular reaction of free DDQ with free hydrocarbon. The association constants for initial complex formation have been evaluated from the kinetic data. Also, the authors [14] reported that several aromatic hydrocarbons have been observed to form CT complexes with DDQ which underwent subsequent irreversible reaction. Intermolecular CT bands of these complexes have been observed (in C2H4Cl2): hexamethybenzene, 621; pentamethylbenzene, 595; durene, 589; mesitylene, 516; p-xylene, 521; toluene, 446; 1-methylbaphthalene, 486; 2-methylbaphthalene, 494, 653; fluorene, 624; acenaphthalene, 482sh, 743; anthracene, 496sh, 824; indene, 586 nm. Although all show a fall of intensity with time, only hexamethylbenzene, pentamethylbenzene and durene have suitable kinetic rates for the present study and also yield identifiable products. Anthracene reacts very rapidly, to yield, apparently, a Diels-Alder product. The CT interaction of DDQ with various hydrocarbons (benzene, toluene, biphenyl, m-terphenyl, naphthalene, phenanthrene, fluorene, chrysene and pyrene) has been studied spectrophoto-

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metrically [69] in CHCl3. In all the systems CT bands characteristic of 1:1 complex have been observed. The spectra are quite sharp and well separate from the transition of either of the components. The optical density of most of the solutions has been found to decrease with time. In the case of acenaphthene, the optical density changes very rapidly with time and the green color of the system is almost discharged within an hour. Naphthalene, biphenyl, chrysene and benzene have showed two CT bands, meanwhile, the band for benzene at shorter wave length has been found to increase considerably in intensity with time. It shows a main band at 425 nm which is observed when benzene added to DDQ and yellow color of quinone intensifies. Slowly the intensity of this band diminishes with lapse of time and a new band appears at 355 nm. Seven years later, the same authors [64] reported spectrophotometric studies in CHCl3 of the reaction between DDQ with benzene, acenaphthene, diphenylamine and o-toluidine. The results indicated that in all cases CT bands appear initially (benzene at 427 nm, acenaphthene at 490 and 720 nm, o-toluidine at 690 nm, and diphenylamine at 700 nm) which vanish with simultaneous emergence of a band near 340 nm. When the colorless solutions of the different donors are mixed with the yellow solution of DDQ, a distinct change in the color occurs. The resultant initial color, however, fades with time and is replaced by another color. The color changes have been found to be faster when higher donor and acceptor concentrations are used. The authors [64] deduced that if the initial band is supposed to be a CT transition band, the decrease in its intensity in time is due to a decrease in the concentration of the CT complex. Thus, in the initial stage the complex has the characteristics of a CT complex, i.e. the ground state of the complex is an admixture of the ionic state (D+A) and the no bond state (DA), the contribution of the latter being predominates. On standing, however, a complete transfer of an electron from the donor to the acceptor occurs, and an ion pair is formed. The intensity increase of the band at 340 nm is not due to a CT complex but to the emergence of an ion pair (inner complex). In addition, the authors [64] indicated that Petruska [70] has previously reported that a sigma complex is formed as a final product for the interaction of DDQ with polyaromaticamines (Structure A). In this sigma complex configuration the quinone chromofore has been transformed into a benzene chromofore. Such substituted benzenes [70] are known to absorb near 340 nm. Thus this mechanism explains well the intensification of the band at 340 nm when DDQ reacts with hydrocarbons. In addition, the authors [64] stated that, on the other hand, Nogami et al. [71] suggested that when amines react with DDQ, beside the intensification of the 340 nm band, a shoulder near 370 nm is also observed. This indicates that quinone chromofore is probably not disturbed (not reduced to DDQH2) by the reaction of DDQ with amines (Structure B). Nogami et al. [71] suggested the following formula (Structure B) which maintains the quinone chromofore. In such case the intensification has to be explained on the basis of interaction between the transition moments of the donor and acceptor molecules. Charge resonance could also contribute to a rise in intensity.

NC

O

CN

HO

O Cl

Cl

D HN O

CH3

401

The electron donor–acceptor complexes of DDQ with a number of aromatic hydrocarbons have been studied [72]. All the aromatic hydrocarbons used on mixing each of them with DDQ in CHCl3 solution exhibit well-defined CT bands characteristic of the EDA complexes. The spectra quite sharp and well separate from the transitions of either component. However, in the case of biphenyl, naphthalene and a-bromo-naphthalene, two CT bands are observed. MO calculations on the CT spectra [73] have also indicated the existence of two CT bands in the aromatic hydrocarbon–DDQ systems. In the case of toluene and m-xylene as electron donors, the color of the initially formed electron donor–acceptor complex fades with time and the CT band shows a decrease in intensity with time and a new band of increasing intensity appears in the spectrum. The authors [72] referred this behavior to the occurrence of a further slow reaction. Müller and Joly [19] investigated the dehydrogenation of some hydroaromatic compounds (1,4-cyclohexadiene, 1,4-dihydronaphtalene and 9,10-dihydroanthracene) as electron donors with the DDQ as an electron acceptor. A mechanism consisting in fast formation followed by slow decomposition of an intermediate can be ruled out. It has been suggested [19] that this behavior could be due to contribution of CT-complexes or HOMO–LUMO interactions for determining the reactivity of the substrates. The authors [19] reported that oxidation of hydrocarbons by quinones have been studied extensively since the pioneering work of Braude [67,74], whereas several reaction mechanisms have been reported [14,75–84] for the oxidation of hydrocarbons by quinones. Müller and Joly [19] observed that the UV/Visible spectra of DDQ in dioxane show bands with maxima at 270 and 375 nm. Upon addition of 1,4-cyclohexadiene these bands disappear while a new band corresponding to the hydroquinone, DDQH2 arises at 350 nm. The authors [19] suggested that dehydrogenation of the hydrocarbons (AH) by the quinones (Q) follows the following reaction pathway involving a sequential electron–proton–electron transfer, which most likely occurs via a CT-complex:

AH þ Q $ ½AH    Q  AHþ Q  ! ðslowÞA  QH ! Aþ QH Kinetics and spectroscopic evidence shows that the disappearance of the substrate proceeds at the same rate as the product-forming step and the second-order rate constants for disappearance of DDQ and that for formation of DDQH2 have been determined [19]. Electron donor–acceptor interactions of toluene, m-xylene, pxylene, acenaphthalene and indene with DDQ have been examined [85] in CHCl3 and CH2Cl2. It has been found that the intensity of the CT band due to electron-acceptor (EDA) complex decreases with time and thereafter new bands appear, the intensities of which increase with time. These new bands are attributed to the products formed by reactions involving EDA complexes. The rate constants of the decomposition of EDA complexes and the formation of new products are measured for the first 40 min, assuming the absorbance of EDA complexes and the products are directly proportional to their respective concentrations. The authors [85] suggested that the formation of the product, in each case, from EDA complex proceeds via an intermediate. Pandeeswaran and Elango [86] studied spectroscopically the interaction of DDQ with ketoconazole and povidone drugs. The kinetics and mechanism of this interaction have been investigated. In the presence of a large excess of donor, the 1:1 CT complex is transformed into a final product. The rate of formation of the product has been measured as a function of time in different solvents at three temperatures. The authors [86] reported that review of the literature has revealed that a fairly extensive study has been carried out on the possible role of CT and inner (s) complexes as reaction intermediates in the reaction of organic and inorganic molecules

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with electron acceptors, particularly quinones [66,71,87–93]. So that, addition of the drugs to the DDQ solution results in some absorption bands, presumably due to the formation of a CT complex which causes the appearance of a deep red color in solution. Obviously, the spectra recorded for the CT complex between the drugs and DDQ are time dependent. With increase in time the deep color solution begins to disappear and the intensity of the absorption bands in the 400–600 nm region decreases whereas the intensity of the 298–350 nm bands (depending on solvent) continues to increase until the end of the reaction. It has been [86] showed that the absorption spectra of the povidone–DDQ system studied reveals that the spectra are characterized by maximum absorptions at the wavelengths 588, 542, 458 and 348 nm. Such spectral features are in agreement with those reported for the DDQ radical ion. Similar spectral features have been observed for the other system too [93,94]. The observed enhanced absorption band intensities, immediately after mixing D and A, supports the fact that the CT complex formed is of the dative-type structure which consequently converts to an ionic intermediate possessing the spectral characteristics of radical ion. However, the observed gradual decrease in the intensity of the CT bands in the 400–600 nm spectral regions could be [86] due to the consumption of the ionic intermediate through an irreversible chemical reaction, while the continuous increase of the 298–350 nm bands with elapse of time is indicative of the formation of the final reaction product. Further, in the case of ketoconazole– DDQ system [86], a shoulder appearing around 380 nm in the spectrum of the mixture of DDQ and the drug reveals that the quinone chromofore is probably not disturbed by the reaction of DDQ with the drug [93]. The pseudo-first-order rate constants have been evaluated [86] by employing the increase in absorbance of 290–350 nm bands at different temperatures and solvents. 4. Conclusions In general, the timescales of the CT interactions range between picoseconds to attoseconds and the CT spectral bands of pure CT complexations are, consequently, characteristic by their instance appearance and they are time-independently. Meanwhile, in such CT interactions which show time-dependency, several propositions have been provided to reveal this unusual behavior. In the case of electron donor/iodine system, yet only three of such propositions have been adequately explained the lapse of time accompanying some of these D/I2 interactions: (a) transformation form the outer CT complex into the inner CT complex, (b) formation of the poly-iodide anions, and (c) formation of a new acceptor resulting in situ. Appearance or disappearance of the characteristic bands of the triiodide anions (I 3 ) and the free iodine band, or, even, the change in their positions and intensities are indicative for ascertaining the interaction pathway of the D/I2 system. For interactions involving DDQ as an electron p-acceptor, when this system (Donor/DDQ) shows time-dependency it is then evidently characteristic by the stepwise reduction of the DDQ from its quinone chromofore to its corresponding dative structure, anion radical, semi hydroquinone, and hydroquinone of benzenoid chromofore to the end by the probable occurrence of a chemical reaction. However, all propositions which have been dedicated to ascertain the lapse of time observed in some of the donor/DDQ interactions have confirmed the initial formation of CT interactions. Knowing the structural formulae of the final products of such donor/DDQ interactions are truly indicative for ascertaining what is exactly happened in each of these interactions, whether it is a pure CT interaction or it is a CT complexation followed by anything else; formation of dative structures and the corresponding anion radicals, formation of the semi hydroquinone or the fully reduced hydroquinone, or even the occurrence of a chemical reaction.

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