Charge-transfer complexes of 1-(2-aminoethyl) piperazine with σ- And π-acceptors

Article abstract of DOI:10.1016/j.molstruc.2010.08.045

The solid charge-transfer (CT) molecular complexes formed in the reaction of 1-(2-aminoethyl) piperazine (AEPIP) with the σ-acceptor iodine and π-acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), 7,7,8,8-tetracyanoquinodi-methane (TCNQ), 2,4,4,6-

Full text of DOI:10.1016/j.molstruc.2010.08.045

Journal of Molecular Structure 983 (2010) 153–161
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Charge-transfer complexes of 1-(2-aminoethyl) piperazine with
r- and p-acceptors
⇑
Adel Mostafa, Hassan S. Bazzi
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
a b s t r a c t
Article history:
Received 26 April 2010
Received in revised form 24 August 2010
Accepted 24 August 2010
Available online 31 August 2010
The solid charge-transfer (CT) molecular complexes formed in the reaction of 1-(2-aminoethyl) pipera-
zine (AEPIP) with the -acceptor iodine and -acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
r
p
(DDQ), 7,7,8,8-tetracyanoquinodi-methane (TCNQ), 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD)
and 2,3,5,6-tetrachloro-1,4-benzoquinone (CHL) were studied in chloroform at 25 °C. The products were
investigated through electronic and infrared spectra as well as elemental analysis. The obtained results
showed that the formed solid CT-complexes have the formulas [(AEPIP) I]+ I5ꢀ, [(AEPIP)(DDQ)2],
[(AEPIP)(TCNQ)2], [(AEPIP)2(TBCHD)3] and [(AEPIP)(CHL)] which are in full agreement with the known
reaction stoichiometries in solution as well as the elemental analysis measurements. The formation con-
Keywords:
1-(2-Aminoethyl) piperazine
Iodine
DDQ
TCNQ
TBCHD
CHL
stant KCT, molar extinction coefficient
for the CT-complexes [(AEPIP)(DDQ)2], [(AEPIP)(TCNQ)2] and [(AEPIP)(CHL)] as well.
Ó 2010 Elsevier B.V. All rights reserved.
eCT, free energy change DG
0 and CT energy ECT have been calculated
1. Introduction
electrical conductivity, and others [16]. In addition, these interactions
play an important role in biological systems [17].
The charge-transfer (CT) complexes formed in the reactions of
and p-electron acceptors with different donors like amines, crown
r
-
In the paper herein, we report the formation of five new CT-com-
plexes produced from the reaction of 1-(2-aminoethyl) piperazine
ethers, polysulfur bases and oxygen–nitrogen mixed bases have
been the subjects of many studies both in solution and in solid state
show interesting chemical and physical properties. These properties
have been the subjects of many studies both in solution and in solid
state [1–10]. The photometric methods based on these interactions
are usually simple and convenient because of the rapid formation of
the complexes.
with the p-acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ), 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,4,4,6-tetrab-
romo-2,5-cyclohexadienone (TBCHD) and 2,3,5,6-tetrachloro-1,4-
benzoquinone (CHL) and
r-acceptor iodine in CHCl3 as the solvent.
The purpose of this work is to make an assessment of the correct nat-
ure and stoichiometry of each of the resulting new CT-complexes
formed with each acceptor.
The electron donor we use in this study, 1-(2-aminoethyl) piper-
azine (AEPIP), is a derivative of piperazine and contains primary, sec-
ondary and tertiary nitrogen atoms (electron donating atom).
Polyiodide anions such as Iꢀ3 , I5ꢀ, I7ꢀ, or Iꢀ9 could be formed in charge-
transfer interaction between iodine and electron donors. The forma-
tion of a particular polyiodide species depends strongly on the nature
of the donor base and in some cases on the method of preparation
[11–15]. The
p-electron acceptors 7,7,8,8-tetracyanoquinodimeth-
ane (TCNQ) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
are known to form stable colored CT-complexes with many donor
bases. However, the reaction stoichiometries depend on several fac-
tors such as the nature of donor and acceptor and in some cases on
the solvent used. The increased interest in the study of charge-trans-
fer interactions stems from the important applications that CT-com-
plexes can have. These include electronics, solar cells, optical devices,
⇑
Corresponding author. Tel.: +974 4423 0018; fax: +974 4423 0060.
E-mail address: bazzi@tamu.edu (H.S. Bazzi).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.08.045

154
A. Mostafa, H.S. Bazzi / Journal of Molecular Structure 983 (2010) 153–161
3. Results and discussion
2. Experimental
3.1. Electronic absorption spectra
2.1. Materials and measurements
Instant and strong change in color was observed upon mixing
chloroform solutions of the donor AEPIP with each of the acceptor
iodine, DDQ, TCNQ, TBCHD and chloranil. The colors were dark
brown for AEPIP-I2, AEPIP–TBCHD and AEPIP–CHL, dark red for AE-
PIP–DDQ, and brown for AEPIP–TCNQ reaction mixtures. These
changes in colors clearly indicate the occurrence of the charge-
transfer interactions between the donor and each of the acceptors.
The electronic absorption spectra of the reactants along with those
of the CT-complexes formed between the donor AEPIP and I2, DDQ,
TCNQ, TBCHD, and CHL are shown in Figs. 1–5 respectively. Strong
absorption bands appeared at 365 and 290 nm for AEPIP-I2, 731
and 531 nm for AEPIP–DDQ, 852 and 657 nm for AEPIP–TCNQ,
527 nm for AEPIP-TBCHD and 553 nm for AEPIP–CHL products.
Photometric titration measurements for the five reactions in
CHCl3 were performed and shown in Figs. 6–10. Interestingly, the
measurements show that the donor–acceptor molar ratio is vari-
able depending on the type of acceptor. These molar ratios were
found to be 1:1 in the case of AEPIP–CHL and 1:2 in the case of AE-
PIP–DDQ and AEPIP–TCNQ, 1:3 in the case of AEPIP-I2 system, and
1:1½ in case of AEPIP–TBCHD. The structures of the five new
formed CT-complexes were thus formulated to be [(AEPIP)(I2)3]
The chemicals were purchased from Sigma–Aldrich, USA, and
used as received. The electronic absorption spectra of the CHCl3
solutions of the solid CT-complexes were checked in the region
1000–250 nm using a lambda 950 Perkin Elmer UV–Vis–NIR spec-
trometer with quartz cell of 1.0 cm path length. Elemental analysis
was done using a Perkin Elmer CHNSO elemental analyzer model
2400 series II.
Thermogravimetric (TG) and differential thermogravimetric
(DTG) analysis were carried out for all reactants and CT-com-
plexes; using a Perkin Elmer model pyris 6 TGA computerized ther-
mal analysis system. The rate of heating of the sample was kept at
10 °C minꢀ1 under nitrogen flow at 20 ml minꢀ1. Copper sulfate
pentahydrate was used as a calibration standard.
The infrared spectra of the reactants, AEPIP, DDQ, TCNQ, TBCHD
and CHL (iodine has no infrared activity) and the obtained CT-com-
plexes (KBr pellets) were recorded on a Perkin Elmer FTIR spec-
trometer model spectrum one.
Photometric titration measurements were performed for the
reactions between the donor AEPIP and each of the acceptors iodine,
DDQ, TCNQ, TBCHD and CHL in CHCl3 at 25 °C in order to determine
the reaction stoichiometries according to literature method [3,18].
The measurements were conducted under the conditions of fixed
donor AEPIP concentrations while those of the acceptors I2, DDQ,
TCNQ, CHL or TBCHD were changed over a wide range, to produce
in each case reaction solutions where the molar ratio of donor:
acceptor varies from 1:0.25 to 1:4. The peak absorbancies of the
formed CT-complexes were measured for all solutions in each case
and plotted as a function of the acceptor to donor molar ratio. The
infrared spectra of the reactants and the formed CT-complexes
(KBr pellets) were recorded on a perkin-Elmer Spectrum One FTIR
spectro-photometer.
2.2. Preparation of the solid CT-complexes
The five solid CT-complexes formed in the reaction of AEPIP
with each of I2, DDQ, TCNQ, TBCHD and CHL were isolated in CHCl3
by drop wise addition of a saturated solution (60 ml) of the donor
to a saturated solution (85 ml) of the acceptor. The resulting mix-
ture was stirred for about 10–15 min. The mixing of reactants was
associated with a strong change in color. The resulting precipitate
was filtered immediately and washed several times with minimum
amounts of CHCl3, then dried under vacuum.
Fig. 1. Electronic absorption spectra of 1-(2-aminoethyl) piperazine (AEPIP)-I2 in
CHCl3. (A) [AEPIP] = 5 ꢁ 10ꢀ3 M; (B) [I2] = 5 ꢁ 10ꢀ3M; (C) 1: 3 AEPIP-I2 mixture,
[AEPIP] = [I2] = 5 ꢁ 10ꢀ3M.
The yields of the obtained CT-complexes were 1.98 g (2.2 mmo-
les, 69.7%) for [(AEPIP)I]+I5ꢀ 2.19 g (3.8 mmoles, 63.7%) for [(AE-
,
PIP)(DDQ)2], 2.5 g (4.6 mmoles, 77.4%) for [(AEPIP)(TCNQ)2],
2.32 g (1.6 mmoles, 75.3%) for [(AEPIP)2(TBCHD)3], and 2.13 g
(5.7 mmoles, 74.7%) for [(AEPIP)(CHL)].
The complexes were characterized using spectroscopic tech-
niques (FTIR and UV–Vis) and by elemental analysis (theoretical
values are shown in brackets):
[(AEPIP)I]+Iꢀ5 dark brown complex (M/W: 890.63 g); C, 8.11%
(8.08%) H, 1.70% (1.68%); N, 4.68% (4.72%); I, 85.43% (85.52%).
[(AEPIP)(DDQ)2] dark red complex (M/W: 583.22 g); C, 45.22%
(45.27%); H, 2.59% (2.57%); N, 16.77% (16.80%).
[(AEPIP)(TCNQ)2] brown complex (M/W: 537.58 g); C, 66.91%
(66.97%); H, 4.24% (4.28%); N, 28.61% (28.65%).
[(AEPIP)2(TBCHD)3] dark brown complex (M/W: 1487.5 g); C,
24.18% (24.20%); H, 2.44% (2.42%); N, 5.62% (5.65%).
[(AEPIP)(CHL)] dark brown complex (M/W: 375.08 g); C, 38.37%
(38.39%); H, 4.02% (4.0%); N, 11.95% (11.98%).
Fig. 2. Electronic absorption spectra of 1-(2-aminoethyl) piperazine (AEPIP) – DDQ
in CHCl3. (A) [AEPIP] = 5 ꢁ 10ꢀ3M; (B) [DDQ] = 1 ꢁ 10ꢀ3M; (C) 1: 2 AEPIP–DDQ
mixture, [AEPIP] = 5 ꢁ 10ꢀ3M and [DDQ] = 1 ꢁ 10ꢀ3M.

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155
Fig. 6. Photometric titration curve for 1-(2-aminoethyl) piperazine (AEPIP) – iodine
reaction in CHCl3 measured at 365 nm.
Fig. 3. Electronic absorption spectra of 1-(2-aminoethyl) piperazine (AEPIP)–TCNQ
in CHCl3. (A) [AEPIP] = 5 ꢁ 10ꢀ3M; (B) [TCNQ] = 5 ꢁ 10ꢀ3M; (C) 1: 2 AEPIP–TCNQ
mixture, [AEPIP] = [TCNQ] = 5 ꢁ 10ꢀ3M.
Fig. 7. Photometric titration curves for 1-(2-aminoethyl) piperazine (AEPIP)–DDQ
reaction in CHCl3 measured at 731 and 531 nm.
Fig. 4. Electronic absorption spectra of 1-(2-aminoethyl) piperazine (AEPIP)–
TBCHD in CHCl3. (A) [AEPIP] = 5 ꢁ 10ꢀ3M; (B) [TBCHD] = 1 ꢁ 10ꢀ3M; (C) 1:1½
AEPIP–TBCHD mixture, [AEPIP] = 5 ꢁ 10ꢀ3 M and [TBCHD] = 1 ꢁ 10ꢀ3M.
Fig. 8. Photometric titration curves for 1-(2-aminoethyl) piperazine (AEPIP)–TCNQ
reaction in CHCl3 measured at 852 and 657 nm.
be indicated here that the absorption of the iodine complex shows
two strong absorptions at 365 and 290 nm. Neither free iodine nor
AEPIP show these absorptions (Fig. 1 and Table 1). These absorp-
tions are well known to be characteristic of the polyiodide ion of
the type Iꢀ5 [19]. Accordinglꢀy, the structure of the formed CT-com-
plex should be [(AEPIP)I]+ꢂI5 and the formation of iodide interme-
diate [(AEPIP) I]+ꢂIꢀ is characterized by itsꢀknown absorption at
245 nm [20]. The formation of [(AEPIP) I]+ I5 could be understood
considering the four reaction steps, the first involves the formation
of the outer complex:
Fig. 5. Electronic absorption spectra of 1-(2-aminoethyl) piperazine) (AEPIP)–CHL
in CHCl3. (A) [AEPIP] = 5 ꢁ 10ꢀ3M; (B) [CHL] = 1 ꢁ 10ꢀ3M; (C) 1:1 AEPIP–CHL
mixture, [AEPIP] = 5 ꢁ 10ꢀ3M and [CHL] = 1 ꢁ 10ꢀ3M.
[(AEPIP)(DDQ)2], [(AEPIP)(TCNQ)2] [(AEPIP)2(TBCHD)3] and
[(AEPIP)(CHL)].
These structures and stoichiometries agree quite well with the
elemental analysis of the formed solid CT-complexes. It should
(i) (AEPIP) + I2 ? [(AEPIP)]ꢂI2

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A. Mostafa, H.S. Bazzi / Journal of Molecular Structure 983 (2010) 153–161
Fig. 9. Photometric titration curve for 1-(2-aminoethyl) piperazine (AEPIP)–TBCHD
reaction in CHCl3 measured at 527 nm.
Fig. 11. Spectral determination of formation constant and molar extinction
coefficient of CT-complex [(AEPIP)(DDQ)2] at 731 nm.
(iii) [(AEPIP) I]+ꢂIꢀ + I2 ? [(AEPIP) I]+ꢂIꢀ3
(iv) [(AEPIP) I]+ꢂI3ꢀ + I2 ? [(AEPIP) I]+ꢂIꢀ5
It is of interest to see that the reaction stoichiometries of AEPIP
with the acceptors vary from 1:1 to 1:3 depending on the type of
acceptor (Table 1). For the
p-acceptors, the stoichiometric ratio
(AEPIP:acceptor) values are: 1:1 in the case of chloranil, 1:2 for
DDQ and TCNQ, and 1:1½ for TBCHD.
These pronounced variations of CT interaction stoichiometries
are definitely connected to several factors such as the donor molec-
ular symmetry, the type of electron withdrawing groups or atoms
(Cl, Br, C@O, or CN) as well as the steric hindrance between reac-
tants. All of these factors are expected to play an important role
in the electron donation process from the nitrogen electron pairs
of the donor AEPIP and the aromatic ring of each acceptor.
The formation constant (KCT) and molar extinction coefficient
Fig. 10. Photometric titration curve for 1-(2-aminoethyl) piperazine (AEPIP)–CHL
reaction in CHCl3 measured at 553 nm.
(eCT) values for the formed CT-complexes of AEPIP with DDQ, TCNQ
and CHL in CHCl3 at 25 °C were calculated. The 1:1 modified Bene-
si–Hildebrand Eq. (1) [21] was used to calculate the values of the
formation constant, KCT (L molꢀ1), and the molar extinction coeffi-
cient eCT (L molꢀ1cmꢀ1), for the complex [(AEPIP)(CHL)].
Table 1
Spectroscopic data for the CHCl3 solutions of CT-complexes of AEPIP with the
acceptors I2, DDQ, TCNQ, TBCHD and CHL.
Complex
Color
Absorption
(nm)a
Stoichiometry
(donor: acceptor)
A0D0‘
A
1
A0 þ D0
[(AEPIP)I] I5
Dark brown
Dark red
Brown
dark brown
dark brown
365s, 290s
731 h, 531s
852 h, 657 m
527sh
1:3
1:2
1:2
1:1½
1:1
¼
þ
ð1Þ
ke
e
[(AEPIP)(DDQ)2]
[(AEPIP)(TCNQ)2]
[(AEPIP)2(TBCHD)3
[(AEPIP)(CHL)]
The corresponding spectral parameters for the complexes [(AE-
PIP)(DDQ)2] and [(AEPIP)(TCNQ)2] were calculated using the
known [22] Eq. (2) of 1:2 complexes:
553s
a
The reactants AEPIP, Iodine, TCNQ, DDQ, TBCHD and CHL have no measurable
absorptions in the wavelength region of study with used concentrations; m, med-
ium; s, strong; sh, shoulder; h, hump.
2
ðA0Þ D0‘
1
A0ðA0 þ 4D0Þ
¼
þ
ð2Þ
A
ke
e
where A0 and D0 are the initial concentrations of the acceptor and
donor, respectively, while A is the absorbancy of the mentioned
CT band(s) and ‘ is the cell path length (1 cm). The data obtained
via this calculation are given in Table 2. Plotting the values of
(A0D0 ‘)/A against (A0 + D0) values of Eq. (1) and plotting values of
(A02D0 ‘)/A versus A0(A0 + 4D0) values of Eq. (2), generated straight
lines with a slope of 1/eCT and intercept of 1/KCT eCT as shown in
Figs. 11–13.
Followed by the formation of the inner complex,
(ii) [(AEPIP)]ꢂI2 ? [(AEPIP) I]+ꢂIꢀ
The product of (ii) combines with one iodine molecule to form
Iꢀ3 which reacts with the third iodine molecule to give the final
pentaiodide, Iꢀ5 , complex.
Table 2
Values of KCT, eCT, ECT and D
G0 for the measured CT-complexes.
Complex
KCT (L molꢀ1
)
kmax (nm)
ꢀ
D
G0 (cal molꢀ1
)
ECT (eV)
e )
CT/(L molꢀ1 cmꢀ1
[(AEPIP)(DDQ)2]
[(AEPIP)(TCNQ)2]
[(AEPIP)(CHL)]
55.45 ꢁ 103
102.5 ꢁ 103
0.369 ꢁ 103
731
852
553
6.468 ꢁ 103
6.832 ꢁ 103
3.499 ꢁ 103
1.70
1.46
2.25
0.243 ꢁ 103
0.041 ꢁ 103
0.865 ꢁ 103

A. Mostafa, H.S. Bazzi / Journal of Molecular Structure 983 (2010) 153–161
157
Fig. 13. Spectral determination of formation constant and molar extinction
coefficient of CT-complex [(AEPIP)(CHL)] at 553 nm.
Fig. 12. Spectral determination of formation constant and molar extinction
coefficient of CT-complex [(AEPIP)(TCNQ)2] at 852 nm.
nitrogen atoms. The formation constants are strongly dependent
on the nature of the used acceptors that include strong electron
withdrawing groups.
The obtained data show that the CT-complex [(AEPIP)(CHL)] has
much lower KCT value compared with that of [(AEPIP)(DDQ)2]
and [(AEPIP)(TCNQ)2]. The value of formation constant of
In general these complexes show high values of both the forma-
tion constant (KCT) and the molar extinction coefficient (eCT). The
values of KCT confirm the expected high stabilities of the formed
CT-complexes as a result of the expected powerful electron dona-
tion of 1-(2-aminoethyl) piperazine which contains three donor
Fig. 14. Infrared absorption spectra of: (A) 1-(2-Aminoethyl) piperazine (AEPIP), (B) [(AEPIP)(TCNQ)2],(C) [(AEPIP)(DDQ)2], (D) [(AEPIP)2(TBCHD)3], (E) [(AEPIP)(CHL)] and (F)
[(AEPIP) I]+ Iꢀ5
.

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[(AEPIP)(TCNQ)2] is the highest and this can be understood on the
basis of the differences in the electronic structure of the acceptors
TCNQ, DDQ and CHL. TCNQ has four strong withdrawing
cyano groups in conjugation with an aromatic ring which causes
high delocalization leading to an increase in the Lewis acidity of
the acceptor.
Fig. 15. Infrared absorption spectra of electron acceptors DDQ, TCNQ, TBCHD and TG.
Table 3
Infrared wavenumbersa (cmꢀ1
)
and tentative band assignments for 1-(2-aminoethyl) piperazine (AEPIP), and CT-complexes [(AEPIP)(TCNQ)2], [(AEPIP)(DDQ)2],
[(AEPIP)2(TBCHD)3] [(AEPIP)(CHL)]and [(AEPIP)I] I5.
AEPIP
[(AEPIP)(TCNQ)2]
[(AEPIP)(DDQ)2]
[(AEPIP)2(TBCHD)3]
[(AEPIP)(CHL)]
[(AEPIP) I] I5
Assignments
3436s
3355s
3270s
2938s
3403s,br
3341w
3039w
2953m, 2829sm
2269w, 2220w
3416s,br
3269w
2944s, 2840s
3436s,br
3248w
2954sm, 2830m
3417s,br
3269w
2962sm,
2839sm
3429s,br
3165w
2972s
m
m
m
(H2O); KBr
(NAH); AEPIP
(CAH); AEPIP
2824sm
2210sm
1623s
m
(CꢃN); TCNQ and DDQ
1635sm
1621s
m
(C@O); DDQ, TBCHD and CHL
1538sm
1496s, 1439w
1380w, 1358m
1563sm
1596w, 1557s
1491sm, 1456s
1374w, 1347w
1567sm
1491sm, 141465w
1344m, 1323sm
m
(C@C); TCNQ, DDQ TBCHD and CHL
1444s, 1456s
1361sm
1337s
1494s, 1458sm
1348sm, 1324s
1306s
1453sm
1361m, 1302sm
m(CH2); AEPIP
Free and complexed
AEPIP
1267S1182sm
1144s
1226w, 1188m
1134sm
1270sm, 1182m
1154sm
1241m, 1163w
1158w
1267m, 1152w
1137m
1281w, 1178m
1149m
m
m
(CAN); AEPIP
(CAC); AEPIP
1091sm
1097m
1095m
1049m
1084w
1101sm
759sm
760sm
m(CACl); CHL and DDQ
a
m, medium; s, strong; w, weak; br, broad; m, stretching.

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159
An unsuccessful attempt was made to calculate the correspond-
The charge transfer energy ECT of the formed solid CT-com-
ing values of KCT and
e
CT for both the 1:1½ and 1:3 complexes [(AE-
plexes was calculated using: [25,26]:
PIP)2 (TBCHD)3] and [(AEPIP)I]+ꢂI5ꢀ respectively.
1243:667
D
G0 (cal molꢀ1) values of the complexes
ECT ðnmÞ ¼
ð4Þ
The free energy change
kCT
[(AEPIP)(DDQ)2], [(AEPIP)(TCNQ)2] and [(AEPIP)(CHL)] were calcu-
lated from Gibbs free energy of formation according to Eq. (3)
[23,24] and are given in Table 2:
where kCT is the wavelength of the band of the studied CT-com-
plexes [(AEPIP)(DDQ)2], [(AEPIP)(TCNQ)2] and [(AEPIP)(CHL)]. The
ECT values calculated from Eq. (4) are listed in Table 2.
D
G0 ¼ ꢀRT ln KCT
ð3Þ
3.2. Infrared absorption spectra
The obtained results of
D
G0 reveal that the CT-complexes for-
D
mation process is spontaneous.
ative as the formation constants of the CT-complexes increase.
G0 values are generally more neg-
The infrared absorption spectra of 1-(2-aminoethyl) piperazine
(AEPIP) along with those of the formed complexes [(AEPIP)I]+ꢂI5ꢀ,
(A)
(B)
DTG
DTG
TG
TG
Temperature (oC)
Temperature (oC)
(C)
(D)
DTG
DTG
TG
TG
Temperature (oC)
Temperature (oC)
Fig. 16. Thermograms of (A) [(AEPIP) I] I5, (B) [(AEPIP)(DDQ)2], (C) [(AEPIP)2(TBCHD)3] and (D) [(AEPIP)(CHL)].

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A. Mostafa, H.S. Bazzi / Journal of Molecular Structure 983 (2010) 153–161
Table 4
*
Thermal data for the CT-complexes [(AEPIP) I]ꢂI5, [(AEPIP)(DDQ)2], [(AEPIP)(CHL)], [(AEPIP)2(TBCHD)3] and [(AEPIP)(TCNQ)2].
*
Thermal measurements were carried out under N2 flow at 20 ml minꢀ1
.
[(AEPIP)(DDQ)2], [(AEPIP)(TCNQ)2], [(AEPIP)2(TBCHD)3] and [(AE-
PIP)(CHL)] are shown in Fig. 14. Those of the electron acceptors
DDQ, TCNQ, TBCHD, and CHL (I2 has no infrared activity) are shown
in Fig. 15 (the infrared band assignments are given in Table 3).
These assignments are based on the comparison of the spectra
of the formed products with the spectra of the free reactants. Inter-
estingly, the spectra of the reaction products contain the main
infrared bands for both the reactants in each case. This strongly
supports the formation of the donor–acceptor CT-complexes.
However, the absorptions of AEPIP and acceptors in the formed
products show same changes in band intensities and in some cases
small shifts in the frequency wavenumber values. These changes
could be understood on the basis of the expected symmetry and
electronic structure modifications in both donor and acceptor units
in the formed products compared with those of the free molecules.
It is of interest to see that the pentaiodide complex
[(AEPIP)I]+ꢂIꢀ5 decomposes in three degradation steps, at 188 °C cor-
responding to the loss of AEPIP with a weight loss of 14.9% in an
agreement with the calculated value of 14.5%. This step is followed
by degradation at 273 °C corresponding to the loss of [I2ꢂIꢀ] species
with a weight loss of 38.2% with about 4.55% deviation from the
calculated value (42.75%). The third degradation step at 365 °C cor-
responds to the loss of another [I2ꢂI+] species with a weight loss of
41.5% with about 1.25% deviation from the calculated value. It is
clear then that the AEPIP represents 14.5% of the complex, Table 4.
The second complex [(AEPIP)(DDQ)2] decomposes at four tem-
peratures at 165 °C corresponding exactly to the loss of the donor
AEPIP with weight loss of 18.2% (22.16% calculated). The two DDQ
molecules are lost at three temperatures of 348, 474 and 706 °C
with total weight loss of 80.28% which is close to the calculated va-
lue of 77.84%. The deviation between the experimental and calcu-
lated values was 2.44%.
The third complex [(AEPIP)2(TBCHD)3] is shown in Fig. 16C. This
thermogram is relatively complicated. It decomposes at seven tem-
peratures at 166, 225, 279, 375, 540, 586 and 708 °C. The first at
166 °C shows weight loss of 16.3% corresponding to the loss of
the donor AEPIP, which is very close to the calculated value of
17.37%. The other decomposition temperatures at 225, 279, 375,
540, 586 and 708 °C are associated with a total weight loss of
79.2% corresponds to the loss of the acceptor TBCHD in agreement
with the calculated value of 82.63%.
For example, the
m(NAH) vibrations of the free 1-(2-amino-
ethyl) piperazine in [(AEPIP)(DDQ)2] appear at 3269 cmꢀ1, while
in the [(AEPIP)(TCNQ)2] two absorptions are observed at 3341
and 3039 cmꢀ1 and in the [(AEPIP)I]+ꢂIꢀ5 , only one absorption is ob-
served at 3165 cmꢀ1. The outlined changes in
m(NAH) upon com-
plexation clearly support the involvement of the nitrogen atom
of the amino group in the donor AEPIP through the CT-interaction
process.
It is also noteworthy that
m(C„N) vibrations of the acceptors
TCNQ and DDQ show some changes particularly in terms of band
wavenumber values upon complexation. The m(C„N) vibration for
free TCNQ is observed at 2223 cmꢀ1 and for free DDQ at 2230
cmꢀ1 These vibrations occur at 2269 and 2220 cmꢀ1 in the spectrum
of [(AEPIP)(TCNQ)2] and at 2210 cmꢀ1 for [(AEPIP)(DDQ)2].
The fourth complex [(AEPIP)(CHL)] shows main degradation
steps, at 159, 248, 352, 646, 732 and 793 °C. The first two temper-
atures correspond to the loss of AEPIP and the degradations at 352,
646, 732 and 793 °C correspond to the loss of the acceptor CHL
molecule. The total weight loss of those steps is 67.2% very close
to the calculated value of 65.55%, Table 4. The deviation between
the found and calculated values was small (1.65%).
3.3. Thermal analysis measurements
Thermogravimetric (TG) and differential thermogravimetric
(DTG) analysis were carried out in order to confirm the decompo-
sitions and structures of the formed solid CT-complexes. Figs. 16A–
D show the thermograms of [(AEPIP)I]+ꢂI5ꢀ, [(AEPIP)(DDQ)2], [(AE-
PIP)2(TBCHD)3] and [(AEPIP)(CHL)], respectively.
The complex [(AEPIP)2(TCNQ)2] has been decomposed with
broad decomposition peak with one maxima for a complete loss
of the whole compound.
Table 4 shows the thermogravimetric data for all complexes.
The data support the calculated formulas and structures of the
formed CT-complexes. The degradations steps and their associated
temperatures vary from one complex to another depending on the
type of constituents as well as on the stoichiometries in each case.
Obviously, these two factors have pronounced effects on the type
of bonding, relative complex stabilities and geometries.
4. Conclusion
In conclusion, charge-transfer interactions between the donor
1-(2-aminoethyl) piperazine (AEPIP) and the
r-acceptor iodine
and the -acceptors DDQ, TCNQ, TBCHD and CHL were thoroughly
p
studied in CHCl3 at 25 °C. We were able to show that the reaction
stoichiometries for the acceptors iodine, DDQ, TCNQ, TBCHD and

A. Mostafa, H.S. Bazzi / Journal of Molecular Structure 983 (2010) 153–161
161
[3] H.S. Bazzi, S.Y. AlQaradawi, A. Mostafa, E.M. Nour, J. Mol. Struct. 879 (2008) 60.
[4] L. Shahada, A. Mostafa, E.M. Nour, H.S. Bazzi, J. Mol. Struct. 933 (2009) 1.
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[6] S.Y. AlQaradawi, H.S. Bazzi, A. Mostafa, E.M. Nour, Spectrochim. Acta Part A 71
(2008) 1594.
CHL are 1:3, 1:2, 1:2, 1:1½ and 1:1 respectively. The resulting CT-
complexes were shown to have the formulas: [(AEPIP)(DD-
Q)2],[(AEPIP)(TCNQ)2], [(AEPIP)2(TBCHD)3], [(AEPIP)(CHL)] and
[(AEPIP)I]+ꢂIꢀ5 . Our obtained results indicate that the nitrogen atom
of the amino group in the donor 1-(2-aminoethyl) piperazine (AE-
PIP) is involved in the complexation with the acceptors. The do-
nor–acceptor molar ratio is different with the acceptors 2,4,4,6-
tetrabromo-2,5-cyclohexadienone (TBCHD), 2,3,5,6-tetrachloro-
1,4-benzoquinone (CHL) and iodine reactions and were found to
be 1: 1½, 1:1 and 1:3 respectively. This could be related to many
factors, including steric hindrance which is relatively high in the
case of TBCHD and CHL. This will weaken the interaction between
AEPIP and TBCHD and CHL compared with that with the other
three acceptors. The aromatic rings in TBCHD and CHL have also
lower electron accepting ability compared with that in other
acceptors.
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