Journal of Molecular Structure 1081 (2015) 136–145

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Preparation, spectroscopic and thermal characterization ofcharge-transfer molecular complexes formed in the reactionof 4-dimethylaminopyridine with p-electron acceptors

http://dx.doi.org/10.1016/j.molstruc.2014.09.0740022-2860/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +974 4423 0018; fax: +974 4423 0060.E-mail address: bazzi@tamu.edu (H.S. Bazzi).

Adel Mostafa, G. Benjamin Cieslinski, Hassan S. Bazzi ⇑Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar

h i g h l i g h t s

� Three CT-complexes of donor 4DMAPwith the acceptors TCNE, TCNQ andTBCHD are obtained.� The CT-complexes are characterized

through FTIR, UV–Vis, TGA, elementalanalysis and LC MS/MS.� The values of KCT, eCT, ECT and DG0 are

calculated.

g r a p h i c a l a b s t r a c t

TCNE: Tetracyanoethylene

TCNQ: 7,7,8,8 – Tetracyanoquinodimethane

TBCHD: 2,4,4,6 - Tetrabromo-2,5- cyclohexadienone

a r t i c l e i n f o

Article history:Received 30 June 2014Received in revised form 23 September 2014Accepted 25 September 2014Available online 5 October 2014

Keywords:4-DimethylaminopyridineTCNQTBCHDTCNESpectraThermal

a b s t r a c t

The interactions of the electron donor 4-dimethylaminopyridine (4DMAP) with the p-acceptors tetracy-anoethylene (TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,4,4,6-tetrabromo-2,5-cyclohexadi-enone (TBCHD) were studied spectrophotometrically in chloroform at room temperature. The electronicand infrared spectra of the formed molecular charge-transfer (CT) complexes were recorded. Photometrictitration showed that the stoichiometries of the reactions were not fixed and depended on the nature ofboth the donor and the acceptor. The molecular structures of the CT-complexes were, however, affectedby the amino group in 4-dimethylaminopyridine and the two methyl groups and were formulated as[(4DMAP)(TCNE)2], [(4DMAP)(TCNQ)2] and [(4DMAP)(TBCHD)]. The formation constant (KCT), chargetransfer energy (ECT), molar extinction coefficients (eCT) and free energy change of the formed CT-com-plexes were obtained.

� 2014 Elsevier B.V. All rights reserved.

Introduction

Charge-transfer complexes are known to take part in manychemical reactions like addition, substitution and condensation[1–3]. The molecular interactions between electron donors andacceptors are generally associated in the formation of intensity

colored charge-transfer (CT) complexes which absorb radiation inthe visible region. We have been studying the synthesis andspectroscopic characterization of a variety of molecular donorsand acceptors in order to fully understand the nature of theseinteractions [4–10]. It was found that the reaction stoichiometriesas 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 or with-drawing atoms like halogens. Electrons donating like alkyl groups

A. Mostafa et al. / Journal of Molecular Structure 1081 (2015) 136–145 137

were found to enhance the acceptor:donor stoichiometry and theirinteraction is also important in the field of drug–receptor bindingmechanism [11], in solar energy storage [12] and in surface chem-istry [13] as well as in many biological fields [14]. Aminopyridinesconstitute an important group of electron donors and the study oftheir CT interactions can help elucidate many chemical and biolog-ical phenomena they take part in. Aminopyridines have beenshown to have various medical and pharmacological applications,in addition to their use in analytical chemistry [15–16]. 4-Dimeth-ylaminopyridine (4DMAP) is a derivative of pyridine with thechemical formula (CH3)2NC5H4N. This colorless solid is a usefulnucleophilic catalyst for a variety of reactions such as esterifica-tions with anhydrides and hydrosilylations.

In the paper herein, we report the formation of three new CT-complexes produced from the reaction of 4-dimethylaminopyri-dine (4DMAP) with the p-acceptors tetracyanoethylene (TCNE),7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,4,4,6-tetrab-romo-2,5-cyclohexadienone (TBCHD) in CHCl3 as the solvent. Theaim of this work is to make an assessment of the correct natureand stoichiometry of each of the resulting new CT-complexesformed with each acceptor. These acceptors are known to formstable colored CT-complexes with many donor bases.

N

NH3C CH3

4-Dimethylaminopyridine (4DMAP) 3D structure of 4DMAP

www.chemtube3d.com

CC

CN

CN

NC

CN

C C

CC

NN

NN

O

Br

BrBr

Br

TCNE TCNQ TBCHD

Experimental

Materials and measurements

The chemicals used were purchased from Sigma–Aldrich, USA,and used as received. The electronic absorption spectra of the reac-tants, 4-dimethylaminopyridine (4DMAP), tetracyanoethylene(TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,4,4,6-tet-rabromo-2,5-cyclohexadienone (TBCHD) as well as the reactionproducts were recorded in the region 300–1000 nm using a lambda

950 Perkin Elmer UV–Vis spectrophotometer with a quartz cell of1.0 cm path length and elemental analysis was done using model2400 Perkin Elmer C,H,N,S/O elemental analyzer.

Photometric titration measurements were performed (Table 1)for the reactions between the donor 4DMAP and each of the accep-tors TCNE, TCNQ and TBCHD in CHCl3 at 25 �C in order to determinethe reaction stoichiometries according to a literature method [17].The measurements were conducted under the conditions of fixeddonor 4DMAP concentration while those of the acceptors TCNE,TCNQ and TBCHD were changed over a wide range, to produce ineach case solutions where the molar ratio of donor:acceptor variesfrom 1:0.25 to 1:4. The peak absorbencies of the formed CT-com-plexes were measured for all solutions in each case and plotted asa function of the acceptor to donor molar ratio. The obtained CT-complexes have been checked using Agilent Triple Quad LC MS/MS model 6420. The infrared spectra of the reactants and theformed CT-complexes (KBr pellets) were recorded on a Perkin ElmerFTIR spectrophotometer model Spectrum one.

Preparation of the solid CT-complexes

The three solid CT-complexes formed in the reaction of 4DMAPwith each of TCNE, TCNQ and TBCHD were prepared by the

addition of a saturated solution of donor 4DMAP (75 ml) to asaturated solution of each acceptor TCNE, TCNQ and TBCHD(85 ml) in CHCl3.

The resulting precipitate in each case was filtered off, washedwith minimum amounts of CHCl3 and dried in vacuum over P2O5.The complexes were characterized spectrophotometrically (FTIRand UV–Vis), and elemental analysis: (Theoretical values areshown in brackets):

4-Dimethylaminopyridine complexes: [(4DMAP)(TCNE)2] yellow(MW: 378.35 g): C, 60.23% (60.26%); H, 2.61% (2.64%); N, 36.97%

Table 1Spectroscopic data for the formed 4-dimethylaminopyridine (4DMAP)–acceptor complexes.

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

[(4DMAP)(TCNE)2] Yellow 400s, 417s 1:2[(4DMAP)(TCNQ)2] Dark green 750 m, 849s 1:2[(4DMAP)(TBCHD)] Brown 375 h 1:1

a The reactants, 4DMAP, TCNE, TCNQ TBCHD have no measurable absorptions in the region of study with used concentrations; m, medium, s, strong, h, hump.

Fig. 1. Electronic absorption spectra of 4-dimethylaminopyridine (4DMAP)–TCNEreaction in CHCl3. (A) [4DMAP] = 1 � 10�3 M; (B) [TCNE] = 1 � 10�3 M; (C) 1:2DMAP–TCNE mixture, [4DMAP] = [TCNE] = 1 � 10�3 M.

Fig. 2. Photometric titration curves for 4-dimethylaminopyridine (4DMAP)–TCNEreaction in CHCl3 based on 400 nm and 417 nm absorptions.

Fig. 3. Electronic absorption spectra of 4-dimethylaminopyridine (4DMAP)–TCNQin CHCl3. (A) [4DMAP] = 5 � 10�3 M; (B) [TCNQ] = 3 � 10�3 M and (C) 1:2 4DMAP–TCNQ mixture, [4DMAP] = 5 � 10�3 M and [TCNQ] = 3 � 10�3 M.

Fig. 4. Photometric titration curves for 4-dimethylaminopyridine (4DMAP)–TCNQreactions in CHCl3 based on 849 nm and 750 nm absorptions.

138 A. Mostafa et al. / Journal of Molecular Structure 1081 (2015) 136–145

(37.0%); [(4DMAP)(TCNQ)2] dark green complex (MW: 530.55 g):C, 70.14% (70.12%); H, 3.41% (3.39%); N, 26.34% (26.39%);[(4DMAP)(TBCHD)] brown complex (M/W: 531.87 g): C, 29.28%(29.33%); H, 2.28% (2.26%); N, 5.23% (5.26%).

Results and discussion

Electronic spectra

Reaction of TCNE with 4DMAPFig. 1 shows the electronic absorption spectra of the reactions of

tetracyanoethylene (TCNE) with the donor 4DMAP. While none ofthe reactants’ spectra display any measurable absorption in theregion 300–500 nm, the resulting CT-complexes show strongabsorptions centered on 417 and 400 nm for 4DMAP–TCNE reac-tions. These absorptions are associated with the strong change incolor observed upon mixing of reactants (yellow from colorless

solution), and reflect the electronic transitions in the formed CT-complexes.

Photometric titration measurements based on these absorp-tions were performed in order to determine the reactions stoichi-ometries in CHCl3 (Fig. 2). The results showed that thedonor:TCNE molar ratio was found to be 1:2 in the reaction. Thisis in good agreement with the obtained elemental analysis of thesolid CT-complex. On the basis of these experimental data, thecomplex obtained can be formulated as [(4DMAP)(TCNE)2].

Reaction of TCNQ with 4DMAPFig. 3 shows the electronic spectra recorded in the region 450–

950 nm of the reactions of 7,7,8,8-tetracyanoquinodimethane(TCNQ) with 4DMAP. Strong changes in color are observed uponmixing. The formed dark green color indicates the formation ofthe 4DMAP–TCNQ charge-transfer complex and is associated withthe electronic transitions at 849 and 750 nm. Photometric titration

Fig. 5. Electronic absorption spectra of 4DMAP–TBCHD in CHCl3. (A)[4DMAP] = 1 � 10�3 M; (B) [TBCHD] = 1 � 10�3 M and (C) 1:1 4DMAP–TBCHDmixture, [4DMAP] = [TBCHD] = 1 � 10�3 M.

Fig. 6. Photometric titration curve for 4-dimethylaminopyridine (4DMAP)–TBCHDreaction in CHCl3 based on 376 nm absorption.

A. Mostafa et al. / Journal of Molecular Structure 1081 (2015) 136–145 139

measurements were performed for the 4DMAP–TCNQ reaction inCHCl3 as shown in Fig. 4. Photometric titration based on theabsorption at 849 and 750 nm shows a reaction stoichiometry of1:2 (4DMAP:TCNQ) and indicate that these absorptions belong tothe CT-complex formulated as [(4DMAP)(TCNQ)2]. The 4DMAP–TCNQ complex shows absorption at 849 and 750 nm (Fig. 4) andthe photometric titration based on these two absorptions showsa reaction stoichiometry of 1:2 (4DMAP:TCNQ). Elemental analysisshowed that the 4DMAP–TCNQ CT-complex can be formulated as[(4DMAP)(TCNQ)2]. This can be related to the presence of twomethyl groups in 4DMAP which are enhancing the electron dona-tion and also the TCNQ is a strong electron acceptor being containsfour cyano groups which are strong electron withdrawing groups.

It is observed in the reactions of 4DMAP–TCNQ and 4DMAP–TCNQ the appearance of multiple absorption bands; this can beexplained in terms of multiple ion-pair states participating incharge transfer interactions. The relative positions of these bandscan be predicted based on gas-phase ionization potentials of thedonor and the electron affinity of the acceptors.

Electron spectroscopy relies on the quantized nature of energystates. Given enough energy, an electron can be excited from itsinitial ground state or initial excited state and briefly exist in ahigher energy excited state. Electron transitions involve excitingan electron from one principle quantum state to another. Withoutincentive, an electron will not transition to a higher level. Only byabsorbing an energy, can an electron be excited. Once it is in theexcited state, it will relax back to its original more energeticallystable state.

Reaction of TBCHD with 4DMAPFig. 5 shows the electronic spectra recorded in the region 320–

500 nm of the reactions of 2,4,4,6-tetrabromo-2,5-cyclohexadie-none (TBCHD) with 4DMAP. Similar to the reactions with the previ-ous two acceptors, strong changes in color are observed uponmixing. Brown color indicates the formation of the 4DMAP–TBCHDcharge-transfer complex and is associated with the electronic tran-sitions at 458 and 376 nm. Photometric titration measurementswere performed for the 4DMAP–TBCHD reactions in CHCl3 as shownin Fig. 6. Photometric titration based on the absorptions at 376 nmfor 4DMAP–TBCHD showed a reaction stoichiometry 1:1. Accord-ingly CT-complex can be formulated as [(4DMAP)(TBCHD)]. This isin good agreement with the obtained elemental analysis of the solidCT-complex.

This reaction stoichiometry could be related to main factors,firstly the steric hindrance which is relatively high in the case of

TBCHD due to the large substituent (Br) atomic size and the pres-ence of two methyl groups in 4DMAP, secondly, the aromatic ringin TBCHD has lower electron accepting ability related to the lowerelectron withdrawing process of the substituent bromine atoms(Br) in TBCHD. This certainly, allows stronger electron donationfrom 4DMAP base to TBCHD.

Formation constant and charge transfer energy of CT-complexesThe spectrophotometric data were used to calculate the values

of the formation constant, K (L mol�1), charge transfer energy ECT

(eV) and molar extinction coefficient, e (L mol�1 cm�1), of the CT-complexes in CHCl3. The values of K (L mol�1) and e (L mol�1 cm�1)for the CT-complex [(4DMAP)(TBCHD)] based on the 1:1 were cal-culated using the modified Benesi–Hildebrand equation [18].

A0D0‘

A¼ 1

keþ A0 þ D0

eð1Þ

The corresponding spectral parameters for the complexes[(4DMAP)(TCNE)2] and [(4DMAP)(TCNQ)2] were calculated usingthe known [19] Eq. (2) of 1:2 complexes:

ðA0Þ2D0‘

A¼ 1

keþ A0ðA0 þ 4D0Þ

eð2Þ

where A0 and D0 are the initial concentrations of the acceptor anddonor, respectively, while A is the absorbance at the mentionedCT bands and ‘ is the cell path length (1 cm). The KCT value of theCT-complex [(4DMAP)(TCNE)2] is 11.37 � 106 L mol�1 calculatedas an average of 11.44 � 106 and 11.30 � 106 L mol�1 (at 417 and400 nm), the KCT of [(4DMAP)(TCNQ)2] is 0.024 � 106 L mol�1 asan average of 0.025 � 106 and 0.023 � 106 L mol�1 at wavelengths849 and 750 nm respectively and the value of KCT of[(4DMAP)(TBCHD)] is 0.0002 � 106 L mol�1 is calculated at wave-length 375 nm. The data obtained throughout this calculation aregiven in Table 2. Plotting the values of (A0D0l/A) against (A0 + D0)values of Eq. (1), and plotting values of (A0)2D0‘/A versusA0(A0 + 4D0) values of Eq. (2), straight lines were obtained with aslop of 1/eCT and intercept of 1/KCT eCT as shown in Figs. 7–9 for[(4DMAP)(TCNE)2], [(4DMAP)(TCNQ)2] and [(4DMAP)(TBCHD)]respectively.

The CT energy of the formed solid CT-complexes is calculatedusing the following equation [20,21].

ECT ðnmÞ ¼ 1243:667kCT

ð3Þ

where kCT is the wavelength of the CT band of the threeCT-complexes of 4DMAP with TCNE, TCNQ and TBCHD. The ECT

values calculated from equation (3) are listed in Table 2.

Table 2Spectrophotometric and free energy results of CT-complexes of [(4DMAP)(TCNE)2], [(4DMAP)(TCNQ)2] and [(4DMAP)(TBCHD)] in CHCl3.

Complex K (L mol�1) kmax (nm) ECT (eV) �DG0 (cal mol�1) emax (L mol�1 cm�1)

[(4DMAP)(TCNE)2] 11.44 � 106 417 2.98 9.63 � 103 3.123 � 103

[(4DMAP)(TCNQ)2] 0.025 � 106 849 1.46 5.99 � 103 0.253 � 103

[(4DMAP)(TBCHD)] 0.0002 � 106 375 3.32 3.13 � 103 0.557 � 103

Fig. 7. Spectral determination of formation constant and molar extinction coeffi-cients of CT-complex [(4DMAP)(TCNE)2] at 417 nm.

Fig. 8. Spectral determination of formation constant and molar extinction coeffi-cients of CT-complex [(4DMAP)(TCNQ)2] at 849 nm.

Fig. 9. Spectral determination of formation constant and molar extinction coeffi-cients of CT-complex [(4DMAP)(TBCHD)] at 375 nm.

140 A. Mostafa et al. / Journal of Molecular Structure 1081 (2015) 136–145

The free energy change DG0 (cal mol�1) values of the complexes[(4DMAP)(TCNE)2], [(4DMAP)(TCNQ)2] and [(4DMAP)(TBCHD)]were calculated from Gibbs free energy of formation according toEq. (4) [22,23]:

DG0 ¼ �RT ln KCT ð4Þ

where DG0 is the free energy of the charge transfer complexes; Rthe gas constant (1.987 cal mol�1 K); T the temperature in Kelvindegrees; KCT the formation constant of donor–acceptor complexes(L mol�1). The DG0 values of the complexes are given in Table 2.

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. As the bond between the components becomes strongerand thus the components are subjected to more physical strainor loss of freedom, the values of DG0 become more negative. Themore negative the value for DG0, the farther to the right the reac-tion will proceed in order to achieve equilibrium.

These results in Table 2 (KCT, eCT, ECT and DG0) are showing thatthe obtained solid CT-complexes formed with the donor 4DMAPwith p-acceptors TCNE, TCNQ and TBCHD have high CT energyECT and formation constants KCT. These high values of KCT confirmthe expected high stabilities of the formed CT-complexes as aresult of the expected high donation of 4DMAP. The formation con-stant is strongly dependent on the nature of the used acceptorincluding the type of electron withdrawing substituents on it suchas cyano groups. This could be related to main factors, firstly thesteric hindrance which is relatively high in the case of TBCHDdue to the larger substituent Br atomic size and the presence oftwo methyl groups in the donor 4DMAP. This will weaken theinteraction between 4DMAP and TBCHD. Secondly, the aromaticring in TBCHD has lower electron accepting ability, related to thelower electron withdrawing process of the substituent Br inTBCHD.

Mass spectra

Mass spectral measurements were performed for the CT-com-plexes 4DMAP–TCNE, 4DMAP–TCNQ and 4DMAP–TBCHD.Fig. 10(A) shows the mass spectrum of [(4DMAP)(TCNE)2] in theregion m/z = 100–390. The molecular ion M+ is observed as a strongpeak at m/z = 380.0 in good agreement with the calculated valuefor the molecular weight of the CT-complex of 378.35 g. The founddifference of about 1.65 between the observed and calculatedmolecular weight value is acceptable within the allowed experi-mental errors. The mass spectrum (Fig. 10A) also shows a numberof other peaks. The peak at m/z 177 is for the mass of the fragmentC10HN4 (C19H10N10/complex formula), the m/z 191.1 is the mass offragment C10HN5 and the m/z 123.1 is the mass of the donor4DAMP (m/z 122.17 calculated) with difference 0.93.

The mass spectrum (Fig. 10B) for the complex [(4DMAP)(TCNQ)2] in the region m/z = 120–540 is showing the molecularion M+ as a short peak at m/z = 530.0 in a good agreement withthe calculated value for the molecular weight of that complex of530.55 g. The peak at m/z = 123.1 is for the donor 4DMAP (122.17calculated) with difference 0.93, the peak at 151.2 is for one mole-cule of TCNQ with 2CN groups lost (A), the peak at m/z 204.2 is for

Fig. 10. Mass spectra of (A) [(4DMAP)(TCNE)2], (B) [(4DMAP)(TCNQ)2] and (C) [(4DMAP)(TBCHD)].

A. Mostafa et al. / Journal of Molecular Structure 1081 (2015) 136–145 141

the acceptor TCNQ (204.19 calculated), the peak at m/z 223.2 is forthe fragment C11H7N6 (C31H18N10/complex formula), the peak at m/z 297.1 is for the fragment C12H13N10 (C31H18N10/complex formula)and the peak at m/z 495.4 is for the fragment C30H9N9 (C31H18N10/complex formula).

The mass spectrum (Fig. 10C) for the complex [(4DMAP)(TBCHD)] in the region m/z = 120–540 is showing the molecularion M+ as a short peak at m/z = 531.1 in a good agreement withthe calculated value for the molecular weight of that complex of531.87 g. The peak at m/z 411.0 is for the acceptor TBCHD (409.7

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 450.0

cm-1

%T

(A)

(B)

(C)

(D)

Fig. 11. Infrared absorption spectra of (A) 4-dimethylaminopyridine (4DMAP), (B) [(4DMAP)(TCNQ)2], (C) [(4DMAP)(TCNE)2] and (D) [(4DMAP)(TBCHD)].

142 A. Mostafa et al. / Journal of Molecular Structure 1081 (2015) 136–145

calculated), the peak at m/z 391.2 is for the acceptor TBCHD withan oxygen atom lost (B), the peak at m/z 381.3 is for the sameacceptor with carbonyl group lost (381.7 calculated) (C) and thepeak at m/z 123.1 is for the donor 4DMAP (122.17 calculated).

CNNC

C

(A) m/z 152

Br

Br

C

O

BrBr

BrBr

- CO

(C) m/z

Infrared spectra

The infrared absorption spectra of the complexes of 4DMAP,[(4DMAP)(TCNE)2], [(4DMAP)(TCNQ)2] and [(4DMAP)(TBCHD)]are shown in Fig. 11.

BrBr

Br Br

(B) m/z 391

Br

Br

C

BrBr

BrBr

381

A. Mostafa et al. / Journal of Molecular Structure 1081 (2015) 136–145 143

The infrared band assignments are given in Table 3. Theseassignments are based on the comparison of the spectra of theformed products with the spectra of the free reactants, the donor4-dimethylaminopyridine and the acceptors TCNE, TCNQ andTBCHD. Interestingly, the spectra of the reaction products contain

Table 4Thermal decomposition dataa for the [(4DMAP)(TCNE)2], [(4DMAP)(TBCHD)] and [4DMAP

Complex Reaction stoichiometry DTG max. (�C

[(4DMAP)(TCNE)2] 1:2 212370

[(4DMAP)(TBCHD)] 1:1 125, 234, 315

432[(4DMAP)(TCNQ)2] 1:2

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

Table 3Infrared wavenumbers (cm�1) and tentative band assignments for 4-dimethylaminopyrid

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

the main infrared bands for both the reactants in each case. Thisstrongly supports the formation of the CT-complexes. However,the absorptions of 4DMAP and acceptors in the formed productsshow some changes in band intensities and in some cases smallshifts in the frequency wavenumber values. These changes could

)(TCNQ)2] CT-complexes.

) TG% mass loss found (calc.) Lost species

88.38/88.37 2TCNE + C5H4N11.62/11.63 (C2H6N); part of donor29.76/28.24 4DMAP + CO55.33/71.76 C5H2Br4

14.91% ResidueBroad peak

ine (4DMAP), [(4DMAP) (TCNE)2], [(4DMAP)(TCNQ)2] and [(4DMAP)(TBCHD)].

144 A. Mostafa et al. / Journal of Molecular Structure 1081 (2015) 136–145

be attributed to the expected symmetry and electronic structuremodifications in both donor and acceptor units in the formed prod-ucts relative to the free molecules.

In addition, the m(C„N) vibrations of the acceptors TCNE, TCNQshow some changes particularly in terms of band wavenumber val-ues upon complexation. The m(C„N) vibrations for free TCNE areobserved a triplet at 2262, 2228 and 2213 cm�1 and for free TCNQat 2224 cm�1. These vibrations occur at 2195 and 2167 cm�1 for

0

10

20

30

40

50

60

70

80

90

100

105

30 50 100 150 200 250 300 350 400 450 500

TG

DTG

TG

DTG

Wei

ght (

%)

Wei

ght (

%)

Temperature (oC)

Temperature (oC)

0

10

20

30

40

50

60

70

80

90

100

105

30 100 200 300 400 500 600 700 800 900 950

Fig. 12. TG–DTG curves of (A) [(4DMAP)(TCNE)2 and (B) [(4DMAP)(TBCHD)].

[(4DMAP)(TCNE)2] and at 2187 and 2167 cm�1 for [(4DMAP)(TCNQ)2].

Thermal analysis measurement

The thermal analysis (TG and DTG) were carried out under anitrogen gas flow (20 ml min�1) within a temperature range 30–950 �C and heating rate 10 �C min�1 for the CT-complexes[(4DMAP)(TCNE)2], [(4DMAP)(TBCHD)] and [(4DMAP)(TCNQ)2] toconfirm the proposed formula and structure for the obtained prod-ucts as shown in, Fig. 12(A) and (B) for [(4DMAP)(TCNE)2],[(4DMAP)(TBCHD)] and a broad peak has been obtained for[(4DMAP)(TCNQ)2] and was difficult to interpret it accurately.

The thermogravimetric data for these complexes are shown inTable 4. The obtained data support the calculated formulas andstructures of the formed CT-complexes. The degradations stepsand their associated temperatures vary from one complex toanother depending on the type of constituents as well as on thestoichiometry in each case. Obviously, these two factors have pro-nounced effects on the type of bonding, relative complex stabilitiesand geometries. It is of interest to see that the complex[(4DMAP)(TCNE)2] shown in Fig. 12(A) decomposes in two degra-dation steps; at temperature 212 �C corresponds to the decomposi-tion of [2TCNE] plus a part of donor C5H4N (donor formula/C7H10N2) with a total mass loss of 88.38% very close to the calcu-lated value of 88.37%. This step is followed by another degradationstep at 370 �C corresponds to the loss of the remaining two methylgroups linked with nitrogen atom with a mass loss of 5.52%; thedifference 6.10 is equivalent to the remaining residue (total masslost 11.62%) as shown in the in Fig. 12(A). This is an evident thatthe donor 4DMAP represents 32.29% of the complex, Table 4.Accordingly, a proposed mechanism for the thermal decompositionof [(4DMAP)(TCNE)2] as follows:

(i) [(4DMAP)(TCNE)2] ���!212 �C (2TCNE + C5H4N) + [C2H6N].

(ii) [C2H6N] ���!370 �C Decomposition.

For the CT-complex [(4DMAP)(TBCHD)] shown in Fig. 12 (B); thedecomposition temperatures at 125, 234 and 315 �C are associatedwith a total mass loss of 29.76% corresponds to the loss of the donor4DMAP plus C@O (from TBCHD) very close to the calculated value28.24% with difference 1.53%. At 432 �C the acceptor TBCHD hasbeen decomposed with mass loss 55.33% with a remaining residueof mass 14.91%; the total decomposed mass (70.24) is very closeto the calculated value 71.77% with difference 1.5%. Accordingly, aproposed mechanism for the thermal decomposition of[(4DMAP)(TBCHD)] is as follows:

(i) [(4DMAP)(TBCHD)]���������!125;234;315;�C

(4DMAP + CO) + [TBCHD].

(ii) [TBCHD] ����!432 �C Decomposition.

For the CT-complex [(4DMAP)(TCNQ)2] a broad peak has beenobtained and was difficult to interpret it accurately.

Conclusion

A comprehensive study of the charge transfer interactionsbetween the donor 4-dimethylaminopyridine with the p-acceptorsTCNE, TCNQ and TBCHD was conducted in CHCl3. We were able toshow that the reaction stoichiometry is not the same for acceptorsTCNE, TCNQ and TBCHD and the resulting CT-complexes wereshown to have the formulas: [(4DMAP)(TCNE)2], [(4DMAP)(TCNQ)2] and [(4DMAP)(TBCHD)].

Our obtained results indicate that the presence of two methylgroups in 4-dimethylaminopyridine (4DMAP) were enhancing

A. Mostafa et al. / Journal of Molecular Structure 1081 (2015) 136–145 145

the electron donation with TCNE and TCNQ (1:2) but interruptingthis donation through steric hindrance with TBCHD (1:1).

References

[1] E.M. Kosower, Prog. Phys. Org. Chem. 3 (1965) 81.[2] F.P. Fla, J. Palou, R. Valero, C.D. Hall, P. Speers, JCS Perkin Trans. 2 (1991) 1925.[3] T. Roy, K. Dutta, M.K. Nayek, A.K. Mukherjee, M. Banerjee, B.K. Seal, JCS Perkin

Trans. 2 (2000) 531.[4] A. Mostafa, H.S. Bazzi, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 74

(2009) 180.[5] A. Mostafa, H.S. Bazzi, J. Mol. Struct. 983 (2010) 126.[6] A. Mostafa, H.S. Bazzi, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 79

(2011) 1613.[7] H.S. Bazzi, A. Mostafa, S.Y. AlQaradawi, E.M. Nour, J. Mol. Struct. 842 (2007) 1.[8] H.S. Bazzi, S.Y. AlQaradawi, A. Mostafa, E.M. Nour, J. Mol. Struct. 879 (2008) 60.[9] S.Y. AlQaradawi, A. Mostafa, H.S. Bazzi, J. Mol. Struct. 1037 (2013) 209.

[10] A. Mostafa, Nada El-Ghossein, S.Y. AlQaradawi, Spectrochim. Acta Part A Mol.Biomol. Spectrosc. 118 (2014) 1012.

[11] A. Korolkovas, Essentials of Medical Chem, second ed., Wiley, New York, 1998.[12] K. Takahosi, K. Horino, T. Komura, K. Murata, Bull. Chem. Soc. Jpn. 66 (1993)

733.[13] S.M. Andrade, S.M.B. Costa, R. Pansu, J. Colloid Interf. Sci. 226 (2000) 260.[14] A.M. Slifkin, Charge-Transfer Interaction of Biomolecules, Academic Press, New

York, 1971.[15] Y. Büyükmurat, E. Akalin, A.E. }Ozel, S. Akyüz, J. Mol. Struct. 482–483 (1999)

579.[16] Y. Büyükmurat, S. Akyüz, J. Mol. Struct. 563–564 (2001) 545.[17] D.A. Skoog, Principle of Instrumental Analysis, Sunder College Publishing, New

York, 1985 (Chapter 7).[18] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703.[19] A. El-Kourashy, Spectrochim. Acta A 37 (1981) 399.[20] G. Briegleb, Z. Angew. Chem. 72 (1960) 401.[21] G. Briegleb, Z. Angew. Chem. 76 (1964) 326.[22] A.A.A. Boraei, Spectrochim. Acta A 58 (9) (2002) 1895.[23] M. Arslan, H. Duymus, Spectrochim. Acta A 67 (2007) 573–577.