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Journal of Molecular Structure 1106 (2016) 10e18

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Journal of Molecular Structure

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Charge-transfer complexes formed in the reaction of 2-amino-4-ethylpyridine with p-electron acceptors

Siham Y. AlQaradawi a, *, Adel Mostafa b, A.A. Bengali b

a Department of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar University, P.O Box 2713, Doha, Qatarb Department of Chemistry, Texas A&M University at Qatar, P.O Box 23874, Doha, Qatar

a r t i c l e i n f o

Article history:Received 3 July 2015Received in revised form30 August 2015Accepted 29 October 2015Available online xxx

Keywords:TCNEDDQTBCHDSpectraThermal

* Corresponding author.E-mail addresses: siham@qu.edu.qa (S.Y. AlQarada

http://dx.doi.org/10.1016/j.molstruc.2015.10.1000022-2860/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

Molecular charge-transfer complexes (CT) of electron donor 2-amino-4-ethylpyridine (2A4EPy) with p-acceptors tetracyanoethylene (TCNE), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD) have been studied spectrophotometrically in chloroform at25 �C. These were investigated through electronic, infrared, mass spectra and thermal measurements aswell as elemental analysis. All formed complexes exhibit well resolved charge-transfer bands in theregions where neither donor nor acceptors have any absorption. The obtained results show that theformed solid CT-complexes have the structures [(2A4EPy)(TCNE)2], [(2A4EPy)2(DDQ)] and[(2A4EPy)2(TBCHD)] for 2-amino-4-ethylpyridine in full agreement with the known reaction stoichi-ometries in solution as well as the elemental measurements. The formation constant KCT, molarextinction coefficient εC.T, free energy change DG0, CT energy ECT, ionization potential Ip and oscillatorstrength G have been calculated for these three CT-complexes.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

The study of charge-transfer complexes formed in the reac-tion of p-acceptors with many electron donors are of interest dueto their interesting physical and chemical properties. Charge-transfer complexes are known to take part in many chemicalreactions such as addition, substitution and condensation. Themolecular interactions between electron donors and acceptorsare generally identified by the intense color of these CT com-plexes which absorb radiation in the visible region [1e8]. Thephotometric methods used to study these interactions are usu-ally simple and convenient because of the rapid formation of thecomplexes. The chemical and physical properties of charge-transfer (CT) complexes formed by the reactions of p- and s-electron acceptors with different donors like amines, crownethers, polysulfur bases and oxygenenitrogen mixed bases havebeen the subject of many studies both in solution and in the solidstate [9e13]. It was found that the reaction stoichiometries as

wi), adel.saeid@qatar.tamu.edu (A.

well as the structure of these CT-complexes depend strongly onthe number of nitrogen donor atoms as well as on their terminalattached groups, hydrogen or donating groups like alkyl orwithdrawing atoms like halogens. Electrons donating alkylgroups were found to enhance the acceptor: donor stoichiom-etry. Interestingly, most of the CT-complexes have many appli-cations in chemical analysis like quantitative drug estimation andsome complexes have interesting physical properties like elec-trical conductivities [14e17].

In this paper, we report the formation of three new CT-complexes formed by the reaction of 2-amino-4-ethylpyridinewith different types of p-electron acceptors. The p-acceptors aretetracyanoethylene (TCNE), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD). All reactions were carried out in CHCl3 as a sol-vent. The obtained results enabled us to investigate the stoichi-ometries and structure of these new CT-complexes.

Mostafa).

S.Y. AlQaradawi et al. / Journal of Molecular Structure 1106 (2016) 10e18 11

2. Experimental

2.1. Materials

All chemicals used were of analytical grade and obtained fromSigmaeAldrich, USA, and used without further purification.

2.2. Instrumentation

The UV/Vis electronic absorption spectra of the CHCl3 solutionsof the solid CT-complexes formed in the reactions of the donor 2-amino-4-ethylpyridine and the acceptors tetracyanoethylene(TCNE), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and)2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD) as well as thereaction products were checked in the region 320e1500 nm using alambda 950 Perkin Elmer UVeViseNIR spectrometer with quartzcell of 1.0 cm path length. Elemental analysis was done using aPerkin Elmer CHNSO Elemental Analyzer model 2400 series II. Theobtained CT e complexes have been checked using Agilent TripleQuad LC MS/MS model 6420. The infrared spectra of the reactant,(2A4EPy), TCNE, DDQ, and TBCHD and the obtained CT-complexes(KBr pellets) were recorded on a Spectrum One Perkin Elmer FTIRspectrometer.

2.3. Photometric titration

Photometric titration measurements were performed for thereactions between the donor (2A4EPy) and each of the acceptorsTCNE DDQ, and TBCHD in CHCl3 at 25 �C in order to determine thereaction stoichiometries according to a literature method [18]. Themeasurements were conducted under the conditions of fixed donor(2A4EPy) concentration while those of the acceptors TCNE, DDQ orTBCHD were changed over a wide range, to produce in each casereaction solutions where the molar ratio of donor: acceptor variesfrom 1:0.25 to 1:4. The peak absorbancies of the formed CT

complexes were measured for all solutions in each case and plottedas a function of the acceptor to donor molar ratio.

2.4. Preparation of the solid CT-complexes

The three solid CT-complexes formed in the reaction of 2A4EPywith each of TCNE, DDQ and TBCHD were prepared in CHCl3 by thedrop wise addition of a saturated solution (75 ml) of each of thedonors to a saturated solution (95 ml) of each of the acceptors. Ineach case the mixing of reactants was associated with a strongchange in color. The resulting precipitate in each case was filteredoff, washed with minimum amounts of CHCl3 and dried in vacuumover P2O5. The complexes were characterized using spectroscopictechniques (FTIR and UVevis) and by elemental analysis shown inTable 1.

3. Results and discussion

3.1. Electronic absorption spectra

Fig. 1 shows the electronic absorption spectra of the reactions oftetracyanoethylene (TCNE) with the donor 2A4EPy. While none ofthe reactants spectra display any measurable absorption in theregion 400e650 nm, the resulting CT-complexes show strong ab-sorptions centered on 397, 387 and 372 nm for 2A4EPy-TCNE re-actions. These absorptions are associated with a strong change incolor observed upon mixing of reactants (dark green from colorlesssolution) for 2A4EPy-TCNE, and reflect the electronic transitions inthe formed CT-complexes.

Photometric titrationmeasurements based on these absorptionswere performed in order to determine the reactions stoichiome-tries in CHCl3 (Fig. 2). The donor: TCNE molar ratio was found to be1:2. This is in good agreement with the elemental analysis of thethree solid CT-complexes (Table 1). On the basis of these experi-mental data, the complex obtained can be formulated as

Table 1Elemental analysis values of the CT-complexes [(2A4EPy)(TCNE)2], [(2A4EPy)2(DDQ)] and [(2A4EPy)2(TBCHD)] with theoretical values in parentheses.

Complex MW (g/mol) C% (Theo %) H% (Theo %) N% (Theo %)

[(2A4EPy)(TCNE)2] 378.35 60.24 (60.27) 2.58 (2.64) 37.07 (37.01)[(2A4EPy)2(DDQ)] 471.35 56.07 (56.02) 4.27 (4.24) 17.79 (17.82)[(2A4EPy)2(TBCHD)] 654.04 36.66 (36.70) 3.38 (3.36) 8.53 (8.58)

Fig. 1. Electronic absorption spectra of the 2-amino-4-ethylpyridine-TCNE reaction inCHCl3. (A) [2A4EPy] ¼ 1 � 10�3 M; (B) [TCNE] ¼ 1 � 10�3 M; 1:2 2A4EPy-TCNEmixture, [2A4EPy] ¼ [TCNE] ¼ 1 � 10�3 M.

Fig. 3. Electronic absorption spectra of the 2-amino-4-ethylpyridine-DDQ reaction inCHCl3. (A) [2A4EPy] ¼ 1 � 10�3 M; (B) [DDQ] ¼ 5 � 10�3 M; 2:1 2A4EPy-DDQ mixture,[2A4EPy] ¼ 1 � 10�3 M and [DDQ] ¼ 5 � 10�3 M.

S.Y. AlQaradawi et al. / Journal of Molecular Structure 1106 (2016) 10e1812

[(2A4EPy)(TCNE)2].Interestingly, the reaction stoichiometry using TCNE as a p-

acceptor is 1:2 donor and the alkyl group (ethyl group at position 4)is enhancing the electron donation in comparisonwith the reactionstoichiometry of 2-aminopyridine with TCNE which was 1:1 in ourprevious study [19].

Fig. 3 shows the electronic spectra recorded in the region400e1500 nm of the reaction of DDQ with 2A4EPy. Similar to thereaction with the previous acceptor, a strong change in color isobserved upon mixing and a dark brown color indicated the for-mation of the 2A4EPy-DDQ charge-transfer complex and is asso-ciated with an electronic transitions at 1108, 744 and 498 nm.Photometric titration measurement was performed for 2A4EPy-DDQ reaction in CHCl3 as shown in Fig. 4. The results showed thatthe 2A4EPy-DDQ molar ratio was 2:1. This is in good agreementwith the obtained elemental analysis of the solid CT-complex

Fig. 2. Photometric titration curves for 2A4EPyeTCNE reaction in CHCl3 measured atthe 397 nm, 387 nm and 372 nm absorptions.

which accordingly can be formulated as [(2A4EPy)2(DDQ)].The reaction stoichiometry using DDQ as a p-acceptor is 2:1

with the presence of alkyl group (ethyl group at position 4) but forthe steric hindrance, the donation became less (2: 1) than that incase of 2-aminopyridine with DDQ which was 1:1 in our previousstudy [19].

It is observed in the reactions of 2A4EPy-TCNE and 2A4EPy-DDQthe appearance of multiple absorption bands; this can be explainedin terms of multiple ion e pair states participating in chargetransfer interactions.

Fig. 5 shows the electronic spectra recorded in the region320e450 nm of the reactions of 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD) with 2A4EPy. A strong change in coloris observed upon mixing and a dark brown color indicated theformation of the 2A4EPy-TBCHD charge-transfer complex and isassociated with the electronic transitions at 370 nm.

Fig. 4. Photometric titration curves for the 2-amino-4-ethylpyridine - DDQ reaction inCHCl3 measured at the 1108 nm, 744 nm and 498 nm absorptions.

Fig. 5. Electronic absorption spectra of the 2-amino-4-ethylpyridine-TBCHD reactionin CHCl3. (A) [2A4EPy] ¼ 1 � 10�3 M; (B) [TBCHD] ¼ 5 � 10�3 M; 2:1 2A4EPy-TBCHDmixture, [2A4EPy] ¼ 1 � 10�3 M and [TBCHD] ¼ 5 � 10�3 M.

Fig. 6. Photometric titration curve for the 2A4EPyeTBCHD reaction in CHCl3 measuredat the 370 nm absorption.

Table 2Spectroscopic data for the CHCl3 solutions of the solid CT-complexes of 2A4EPy withthe acceptors TCNE, DDQ and TBCHD.

Complex Color Absorptiona (nm) Stoichiometry(Donor: acceptor)

[(2A4EPy)(TCNE)2] Dark green 397sh, 387s, 372sh 1:2[(2A4EPy)2(DDQ)] Dark brown 1108s, 744sh, 498sh 2:1[(2A4EPy)2(TBCHD)] Dark brown 370 m 2:1

a The reactants 2A4EPy, TCNE, DDQ and TBCHD have no measurable absorptionsin the region of study at the concentrations used; m, medium; s, strong; sh,shoulder.

Fig. 7. Spectral determination of the formation constant and molar extinction co-efficients of the CT-complex [(2A4EPy)(TCNE)2] at 397 nm.

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A photometric titration measurement was performed for2A4EPy-TBCHD reaction in CHCl3 as shown in Fig. 6. Straight linehas been obtained for the donor 2A4EPy but the obtainedelemental analysis of the solid CT-complex has matched accuratelythe molar ratio 2:1 for 2A4EPy-TBCHD, which can be formulated as[(2A4EPy)2(TBCHD)].

Table 2 shows the spectroscopic data of the resulting CT-complexes.

Table 3Spectrophotometric and free energy results of the CT-complexes of [(2A4EPy)(TCNE)2], [

Complex KCT/l mol�1 �DG0/cal mol�1

[(2A4EPy)(TCNE)2] 4.10 � 105 7.65 � 103

[(2A4EPy)2(DDQ)] 1.48 � 105 7.05 � 103

[(2A4EPy)2(TBCHD)] 0.16 � 105 5.74 � 103

These obtained UV/Vis spectra of the CT-complexes[(2A4EPy)(TCNE)2], [(2A4EPy)2(DDQ)] and [(2A4EPy)2(TBCHD)]have clarified that the variation of the CT-absorption of TCNE, DDQand TBCHD should be related to the electron affinity of eachacceptor with the donor 2A4EPy.

These pronounced variations of CT-interaction stoichiometriesare relatively complicated issue to be sorted out. It is definitelyconnected to many factors such as the donor molecular symmetry,the type of electron withdrawing groups or atoms Cl, Br or C^N aswell as the sterric hinderance between reactants. All of these fac-tors are expected to play an important role on the electron donationprocess from the nitrogen electron pairs of the donor 2A4EPy andthe aromatic ring of DDQ and TBCHD acceptors. The aromatic ringin TBCHD has lower electron accepting ability compared with thatin DDQ, related to the lower electron withdrawing ability of thesubstituent Br in TBCHD compared with C^N in DDQ. Thiscertainly, allows stronger electron donation from 2A4EPy base toDDQ compared to TBCHD.

3.2. Formation constant and molar extinction coefficient

The formation constant (KCT) and molar extinction coefficient(εCT) values for the three CT-complexes in CHCl3 at 25 �C werecalculated.

The formation constant, KCT (lmol�1), and the molar extinctioncoefficient εCT (lmol�1 cm�1) havebeen calculated for the complexes[(2A4EPy)(TCNE)2], [(2A4EPy)2(DDQ)] and [(2A4EPy)2(TBCHD)]using the known [20] Equation (1) of 1:2 complexes:

ðA0Þ2D0lA

¼ 1kε

þ A0ðA0 þ 4D0Þε

(1)

where A0 and D0 are the initial concentrations of the acceptors anddonors, respectively, while A is the absorbance at the mentioned CTbands and [ is the light path length (1 cm). The data obtainedthroughout this calculation are given in Table 3. Plotting the values(A0)2D0 [/A versus A0(A0þ 4D0) values of Equation (1), straight lineswere obtained with a slop of 1/εCT and intercept of 1/KCT εCT asshown in Figs. 7e9.

(2A4EPy)2(DDQ)] and [(2A4EPy)2(TBCHD)] in CHCl3.

ECT/eV lmax/nm εCT/l mol�1cm�1

3.13 397 0.466 � 103

2.50 498 0.417 � 103

3.36 370 0.428 � 103

Fig. 8. Spectral determination of the formation constant and molar extinction co-efficients of the CT-complex [(2A4EPy)2(DDQ)] at 498 nm.

Fig. 9. Spectral determination of formation constant and molar extinction coefficientof CT-complex [(2A4EPy)2(TBCHD)] at 370 nm.

S.Y. AlQaradawi et al. / Journal of Molecular Structure 1106 (2016) 10e1814

These complexes show high values of both the formation con-stant (KCT) and the molar extinction coefficient (εCT). These highvalues of KCT confirm the expected high stabilities of the formed CT-complexes as a result of the expected strong donor ability of2A4EPy. The formation constants are strongly dependent on thenature of the used acceptors including the type of electron with-drawing substituent on it such as cyanide group in DDQ and TCNE.

3.3. Measurement of DG0, ECT, Ip and G

The free energy change DG0 (cal mol�1) values of the complexes[(2A4EPy)(TCNE)2], [(2A4EPy)2(DDQ)] and [(2A4EPy)2(TBCHD)]were calculated from Gibbs free energy of formation according tothe Equation (2) [21,22]:

DG0 ¼ � RT ln KCT (2)

where DG0 is the free energy of the charge transfer complexes; Rthe gas constant (1.987 cal mol�1 0C); T the temperature inKelvin; KCT the formation constant of donoreacceptor complexes(l mol�1). The DG0 values of the complexes are given in Table 3.The obtained results of DG0 reveal that the CT-complexes for-mation process is spontaneous. The results of DG0 are generallymore negative as the formation constants of the CT-complexesincrease.

The charge transfer energy ECT of the formed solid CT-complexesis calculated using the following Equation (3) [23,24]:

ECT ðnmÞ ¼ 1243:667lCT

(3)

where lCT is the wavelength of the band of the studied CT-complexes [(2A4EPy)(TCNE)2], [(2A4EPy)2(DDQ)] and[(2A4EPy)2(TBCHD)]. The ECT values calculated from Equation (3)are listed in Table 3.

The ionization potential of the free donor was determined fromthe CT energies of the CT band of its complexes. In case of the ac-ceptors TCNE and DDQ the relationship becomes the followingEquation (4) [25]:

ECT ¼ Ip� 5:2þ 1:5Ip� 5:2

(4)

where Ip is the ionization potential and ECT is the charge transferenergy of the formed solid CT-complexes. The obtained values of Ipare 6.79 eV and 6.45 eV and 6.89 eV for the CT-complexes[(2A4EPy)(TCNE)2], [(2A4EPy)2(DDQ)] and [(2A4EPy)2(TBCHD)]respectively. It has been reported that the ionization potential ofthe electron donor may be correlated with the charge transfertransition energy of the complex [25].

The oscillator strength G has a dimensionless quantity that isused to express the transition probability of the CT band and can becalculated from the following equation:

f ¼ (4.319 � 10�9) εmax n1/2 (5)

where n1/2 is the band ewidth for half intensity in cm�1 and εmax isthe extinction coefficient. The obtained values of G are 0.1899,0.0346 and 0.2844 for the complexes [(2A4EPy)(TCNE)2],[(2A4EPy)2(DDQ)] and [(2A4EPy)2(TBCHD)] respectively.

The results in Table 3 (KCT, εCT, DG0 and ECT) suggest that the solidCT-complexes formed in the reaction of the donor 2A4EPy with thep-acceptors TCNE, DDQ and TBCHD have high CT energy and for-mation constants KCT. These high values confirm the expected highstabilities of the formed CT-complexes.

S.Y. AlQaradawi et al. / Journal of Molecular Structure 1106 (2016) 10e18 15

3.4. Mass spectral studies

The mass spectra for the obtained three CT-complexes and themolecular ion were consistent with the proposed formula. Massspectral measurements were performed for the CT e complexes[(2A4EPy)(TCNE)2], [(2A4EPy)2(DDQ)] and [(2A4EPy)2(TBCHD)].Fig. 10 (A)shows the mass spectrum of [(2A6EPy)(TCNE)2] in theregionm/z¼ 100e380. The molecular ion Mþ is observed as a shortpeak atm/z¼ 377.2 in good agreement with the calculated value forthe molecular weight of the CT-complex of 378.3 g. The founddifference of about 1.1 between the observed and calculated mo-lecular weight value is due to a loss of one proton. The massspectrum (Fig. 10 A) also shows a number of other peaks; the peakat m/z ¼ 106.1 is for the donor 2A6EPy losing a methyl group (A),

Fig. 10. Mass spectra of CT-complexes: (A) [(2A4EPy)(TCNE)

the peak at m/z 123.1 is for the mass of the donor (122.17 calcu-lated) which is probably a protonated 2A4EPy. The peak at m/z187.2 is for the fragment C10H10N4 (C19H10N10/complex formula),them/z 224.2 is the mass of fragment C12H12N5 and them/z 256.2 isthe mass equivalent to two molecules of the acceptor TCNE (m/z256.18 calculated) and m/z 315.2 is for the fragment C16H10N8(C19H10N10/complex formula).

Themass spectrum (Fig.10 B) for the complex [(2A4EPy)2(DDQ)]in the regionm/z ¼ 100e480 is showing the molecular ion Mþ as ashort peak atm/z¼ 470.2.0 very close to the calculated value for themolecular weight of that complex of 471.29 g. The peak at m/z ¼ 106.1 is for the donor 2A4EPy losing a methyl group (A), thepeak at m/z 123.1 is for one molecule of donor (122.17 calculated),the peak at m/z 138.1 is for fragment C8H13N2 (C22H20N6Cl2O2/

2], (B) [(2A4EPy)2(DDQ)] And (C) [(2A4EPy)2(TBCHD))].

S.Y. AlQaradawi et al. / Journal of Molecular Structure 1106 (2016) 10e1816

complex formula), at m/z 200.1 is for the cation of 2,3-dichloro-6-cyano-1,4-benzoquinone (200 calculated) (B) and the peak at m/z270.2 is for the fragment (C15H20N5).

The mass spectrum (Fig. 10C) for the complex[(2A4EPy)2(TBCHD)] in the region m/z ¼ 100e900 is showing themolecular ion Mþ as a medium peak at m/z ¼ 655.1 in a goodagreement with the calculated value for the molecular weight ofthat complex of 654.04 g. The peak at m/z 106.1 is for the donor(2A4EPy) losing a methyl group (A), the peak atm/z 123.1 is for onemolecule of donor (122.17 calculated) (A), the peak at m/z 408.1 isfor the acceptor cation [TBCHD]þ (C).

Fig. 11. Infrared absorption spectra of: (A) 2-mino-4-ethylpyridine (2A4EPy); (

3.5. Infrared spectral studies

The most important infrared bands of the donor 2-amino-4-ethylpyridine (2A4EPy) and the CT e complexes[(2A4EPy)(TCNE)2], [(2A4EPy)2(DDQ)] and [(2A4EPy)2(TBCHD)] areshown in Fig.11. The infrared band assignments are given in Table 4.

These assignments are based on the comparison of the spectraof the formed products with the spectra of the free reactants, thedonor 2-amino-4-ethylpyridine and the acceptors TCNE, DDQ andTBCHD. Interestingly, the spectra of the reaction products containthe main infrared bands for both the reactants in each case.

This strongly supports the formation of the donoreacceptor CT-

B) [(2A4EPy)(TCNE)2]; (C) [(2A4EPy)2(DDQ)] and (D) [(2A4EPy)2(TBCHD)]

Table 4Infrared wavenumbers (cm�1) and tentative band assignments for 2-amino-4-ethylpyridine (2A4EPy), [(2A4EPy)(TCNE)2], [(2A4EPy) 2(DDQ)] and [(2A4EPy) 2(TBCHD].)

2A4EPy [(2A4EPy)(TCNE)2] [(2A4EPy)2 (DDQ)] [(2A4EPy)2 (TBCHD)] Assignments

3460 s 3439 m 3429 m 3412 ms n(H2O); KBr3307 ms,3 3367 ms 3328s 3296w n(NH); 2A4EPy153 ms 3299 ms, 3189 ms 3180 ms 3140 m2967 ms 2972 m 2969 ms 2967w n(CeH); 2A4EPy2931 m 2928w 2934 m 2928w

2214s 2201 ms n(C^N); DDQ2191w and TCNE

1638s 1668 s n(C]O); DDQ,TBCHD

1613 m1552 s

1567 s1544w

1624 s1557 ms

1628 s1563w

n(C]C); DDQ,TBCHD, 2A4EPy

1393w1312 m

1377 ms1309 ms

1394w1355w

1396w1372 m

Free and complexed 2A4EPy

1276 m 1284 ms 1233 m 1276 m1237 m 1219s 1233 m 1229 m n(CeN); 2A4EPy1181 m 1153 ms 1177 m 1179 m1132 m 1153 ms 1139w 1149w n(CeC); 2A4EPy1054 m 1056 m 1069 ms 1053 m997 m 985 m 996ms 995 m d(CH)deformation,867 m 849 m 890 m 858 m 2A4EPy802 ms 809w 812 ms 805 m d(CH); out of plan

Wag; 2A4EPy748 m n(CeCl); DDQ

471 m 481w 461w 474w d(CH); out of planebending; 2A4EPy

m, medium; s, strong; w, weak; br, broad; n,stretching; d,bending.

S.Y. AlQaradawi et al. / Journal of Molecular Structure 1106 (2016) 10e18 17

complexes. However, the absorptions of 2A4EPy and acceptors inthe formed products show same changes in band intensities and insome cases small shifts in the frequency wavenumber values. Thesechanges could be understood on the basis of the expected sym-metry and electronic structure modifications in both donor andacceptor units in the formed products compared with those of thefree molecules.

For example, the n(NeH) vibrations of the free 2-amino-4-ethylpyridine in [(2A4EPy) 2 (DDQ)] has two strong absorptionsat 3328and 3180 cm�1 while in the [(2A4EPy) (TCNE)2], threestrong absorptions are observed at 3367, 3299 and 3189 cm�1 andin [(2A4EPy)2(TBCHD)], twoweak absorptions are observed at 3296and 3140 cm�1.

The outlined changes in n(NeH) upon complexation clearlysupport the involvement of the nitrogen atom of the donor 2A4EPyin the CT e interaction process. It might also to indicate here thatn(C^N) vibrations of the acceptors TCNE and DDQ show somechanges particularly in terms of band wavenumber values uponcomplexation. The n(C^N) vibrations for free TCNE are observed asa doublet at 2196 and 2182 cm�1 and for free DDQ at 2203 cm�1.These vibrations occur at 2214s and 2191w cm�1 in the spectrum of[(2A4EPy)(TCNE)2] and at 2201 ms cm�1 in the spectrum of[(2A4EPy)2(DDQ)] complexes.

3.6. Thermal analysis

Thermal analysis (TG and DTG) were carried out under a ni-trogen gas flow (20 ml min�1) within a temperature range30e950 �C and heating rate 10 �C ml�1 to confirm the proposedformula and structure for the obtained CT-complexes, Fig. 12 (A)and (B) show the thermograms of [(2A4EPy)(TCNE)2],[(2A4EPy)2(DDQ)] and [(2A4EPy)2(TBCHD)] respectively. The ther-mogravimetric data for these complexes are shown in Table 5. Theobtained data support the calculated formulas and structures of theformed CT-complexes. The degradations steps and their associatedtemperatures vary from one complex to another depending on the

type of constituents as well as on the stoichiometry in each case.Obviously, these two factors have pronounced effects on the type ofbonding, relative complex stabilities and geometries.

For the CT-complex [(2A4EPy)(TCNE)2]; the obtained thermo-gram was including a broad peak and can't be interpreted.

The second product [(2A4EPy)2(DDQ)] is shown in Fig. 12 (A); at207 and 260 �C correspond to the loss of 2[2A4EPy] with total massloss of 48.5% (51.89% calculated). The acceptor DDQ lost a mass of26.0% at 291 and 329 �Cwith remaining carbon residue of mass lossof 25.5% as follows:

ðiÞ �ð2A4EPyÞ2ðDDQÞ� ����!207; 260 �C2ð2A4EPyÞ þ ½DDQ �

ðiiÞ ½DDQ � ����!291; 329 �CDecomposition with carbon residue

The third complex [(2A4EPy)2(TBCHD)] is shown in Fig.12 (B); at196 and 254 �C corresponds to the loss of the 2[2A4EPy] with amass loss of 36.8% very close to the calculated value of 37.36%. Apart the acceptor TBCHD is decomposed at 338, 368 and 396 �Cwith mass loss of 38.8% with carbon residue (24.4%); a proposedmechanism for the thermal decomposition of [(2A4EPy)2(TBCHD)]is as follows:

ðiÞ �ð2A4EPyÞ2ðTBCHDÞ�

����!196; 254 �C2ð2A4EPyÞ þ ½TBCHD�

ðiiÞ ½TBCHD� �������!338; 368; 396 �CDecomposition with carbon residue

4. Conclusion

The chargeetransfer interactions between the donor 2-amino-4-ethylpyridine with the p-acceptors TCNE, DDQ and TBCHD werestudied in CHCl3 at 25 �C. We were able to show that the reactionstoichiometry is the same (2:1) for p-acceptors, DDQ, TBCHD and1:2 for TCNE; the obtained CTe complexes were shown to have theformulas: [(2A4EPy)(TCNE)2], [(2A4EPy)2(DDQ)] and

Fig. 12. Thermograms of: (A) [(2A4EPy)2(DDQ)] and (B) [(2A4EPy)2(TBCHD)].

Table 5Thermal decomposition dataa for the [(2A4EPy)(TCNE)2], [(2A4EPy)2(DDQ)] and[(2A4EPy)2(TBCHD)] CT-complexes.

Complex Reactionstoichiometry

DTGmax.(�C)

TG% masslossfound/calc.

Lostspecies

[(2A4EPy)2(DDQ)] 2: 1 207, 260291, 329

48.5/51.8426.0/48.1625.5

Two[2A4EPy]Part of DDQCarbonresidue

[(2A4EPy)2(TBCHD)] 2: 1 196, 254338, 368,396

36.8/37.3638.8/62.6424.4

Two[2A4EPy]Part ofTBCHDCarbonresidue

[(2A4EPy)(TCNE)2] 1:2 Very broadpeak

a Thermal measurements were carried out under N2 flow rate at 20 ml min�1.

S.Y. AlQaradawi et al. / Journal of Molecular Structure 1106 (2016) 10e1818

[(2A4EPy)2(TBCHD)]. Our obtained results indicate that the nitro-gen atom (>NH) of the three donors is involved in the complexationwith acceptors. Next studies will focus on using different donorswith different substituent groups attached to the nitrogen atoms tofurther investigate the nature of such complexation.

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