Spectrophotometric and thermal studies on the charge - Transfer complexes of 4-(aminomethyl) piperidine as donor with σ- And π-electron acceptors

Article abstract of DOI:10.1016/j.saa.2013.09.091

The spectroscopic characteristics of the solid charge-transfer molecular complexes (CT) formed in the reaction of the electron donor 4-(aminomethyl) piperidine (4AMP) with the σ-acceptor iodine and the π-acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), 2,4,4,6-tetrabromo-2,5- cyclohexadienone (TBCHD) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) have been studied in chloroform at 25 C. These were investigated through electronic, infrared spectra and thermal analysis as well as elemental analysis. The results show that the formed solid CT-complexes have the formulas [ ^(4AMP)I]+I3-, [(4AMP)(DDQ)₂] and [(4AMP)(TBCHD)] while in the case of 4AMP-TCNQ reaction, a short-lived CT complex is formed followed by rapid N-substitution by TCNQ forming the final reaction product 7,7,8-tricyano-8-aminomethylpiperidinylquinodimethane [TCAMPQDM] in full agreement with the known reaction stoichiometries in solution as well as the elemental measurements and the thermal analysis confirmed the structure of the obtained compounds. The formation constant k_CT, molar extinction coefficient ε_CT, free energy change ΔG⁰ and CT energy E_CT have been calculated for the CT-complexes [ ^(4AMP)I]+I3-, [(4AMP)(DDQ)₂] and [(4AMP)(TBCHD)].

Full text of DOI:10.1016/j.saa.2013.09.091

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1012–1019
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectrophotometric and thermal studies on the charge – Transfer
complexes of 4-(aminomethyl) piperidine as donor
with
r
- and
p
-electron acceptors
a
a
b,
⇑
Adel Mostafa , Nada El-Ghossein , Siham Y. AlQaradawi
a
Science Program, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar
Department of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar University, P.O. Box 2713, Doha, Qatar
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
Three CT-complexes of 4-
aminomethyl) piperidine with the
(
acceptors DDQ, TBCHD and iodine are
obtained.
ꢀ
ꢀ
ꢀ
The adduct 7,7,8-tricyano-8-amino-
methylpiperidinylquinodimethane
[
TCAMPQDM] is yielded with TCNQ.
These CT-complexes are
characterized through FTIR, UV–vis,
elemental and thermal analysis.
C
The values of K , eCT, ECT and D
G0 are
calculated.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 June 2013
Received in revised form 6 September 2013
Accepted 26 September 2013
Available online 9 October 2013
The spectroscopic characteristics of the solid charge-transfer molecular complexes (CT) formed in the
reaction of the electron donor 4-(aminomethyl) piperidine (4AMP) with the
-acceptors 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), 2,4,4,6-tetrabromo-2,5-cyclohexadie-
none (TBCHD) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) have been studied in chloroform at
5 °C. These were investigated through electronic, infrared spectra and thermal analysis as well as
elemental analysis.
r-acceptor iodine and the
p
2
Keywords:
þ
ꢂ
The results show that the formed solid CT-complexes have the formulas ½ð4AMPÞ Iꢁ I , [(4AMP)(DDQ)
2
]
3
4
-(Aminomethyl) piperidine
and [(4AMP)(TBCHD)] while in the case of 4AMP-TCNQ reaction, a short-lived CT complex is formed
followed by rapid N-substitution by TCNQ forming the final reaction product 7,7,8-tricyano-8-amin-
omethylpiperidinylquinodimethane [TCAMPQDM] in full agreement with the known reaction stoichiom-
etries in solution as well as the elemental measurements and the thermal analysis confirmed the
DDQ
TBCHD
TCNQ
Spectra
structure of the obtained compounds. The formation constant kCT, molar extinction coefficient eCT, free
0
þ
ꢂ
energy change
D
G
and CT energy ECT have been calculated for the CT-complexes ½ð4AMPÞ Iꢁ I ,
3
[
(4AMP)(DDQ) ] and [(4AMP)(TBCHD)].
2
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
charge-transfer (CT) complexes which absorb radiation in the
visible region [1–9]. The photometric methods based on these
interactions are usually simple and convenient because of the
rapid formation of the complexes. The chemical and physical prop-
erties of charge-transfer (CT) complexes formed in the reactions of
The molecular interactions between electron donors and accep-
tors are generally associated in the formation of intensity colored
p
- and
r-electron acceptors with different donors like amines,
⇑
Corresponding author. Tel.: +974 55507904; fax: +974 44835750.
E-mail address: siham@qu.edu.qa (S.Y. AlQaradawi).
crown ethers, polysulfur bases and oxygen–nitrogen mixed bases
1
386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.09.091

A. Mostafa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1012–1019
1013
have been the subjects of many studies both in solution and in
solid state [10–14]. The donor 4-(aminomethyl) piperidine
II. The infrared spectra of the reactants, (4AMP), DDQ, TBCHD and
TCNQ and the obtained CT-complexes (KBr pellets) were recorded
on a spectrum one Perkin Elmer FTIR spectrometer.
(4AMP) is evaluated as small molecule inhibitor of CD4 – gp 120
binding and involved in solvothermal preparation of germanium
oxide hydroxide 3D net zeotype, zeolite synthesis and coupling agent
for adhesive systems. It was found that the reaction stoichiometries
as well as the structure of these CT-complexes depend strongly on
the number of nitrogen donor atoms as well as on their terminal
attached groups, hydrogen or donating groups like alkyl or withdraw-
ing atoms like halogens. Electrons donating like alkyl groups were
found to enhance the acceptor:donor stoichiometry [15].
Interestingly, most of the CT-complexes have many applications
in chemical analysis like quantitative drug estimation and some
complexes have interesting physical properties like electrical con-
ductivities [16–19]. In this paper, we report the formation of three
new CT-complexes formed on the reaction of 4-(aminomethyl)
Photometric titration
The measurements of Photometric titration were obtained for
the reactions between the donor (4AMP) and each of the acceptors
DDQ, TBCHD and iodine in CHCl at 25 °C in order to determine the
3
reaction stoichiometries according to a literature method [5,20].
The measurements were conducted under the conditions of fixed
donor (4AMP) concentration while those of the acceptors DDQ,
TBCHD and iodine 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 com-
plexes (KBr pellets) were recorded on a spectrum one Perkin–El-
mer FTIR spectrophotometer.
piperidine (4AMP) with
-acceptors like 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
DDQ) and 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD) but
r-acceptor iodine and different types of
p
(
with the acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) an
adduct 7,7,8-tricyano-8-aminomethylpiperidinylquinodimethane
The three solid CT-complexes formed in the reaction of (4AMP)
with each of DDQ, TBCHD and iodine were prepared in CHCl by the
3
[
TCAMPQDM] has been formed.
drop wise addition of a saturated solution (75 ml) of each of the do-
nors to a saturated solution (90 ml) of each of the acceptors. In each
case the mixing of reactants was associated with a strong change in
color. The resulting precipitate in each case was filtered off, washed
with minimum amounts of CHCl
The complexes were characterized using spectroscopic techniques
FTIR and UV–vis) and by elemental analysis: (theoretical values
3 2 5
and dried in vacuum over P O .
(
are shown in brackets): [TCAMPQDM] light yellow complex (M/
W: 291.38 g); C, 70.11% (70.01%) H, 5.79% (5.83%); N, 24.08%
þ
ꢂ
(
(
24.02%); ½ð4AMPÞ Iꢁ
I
brown complex (M/W: 621.81C, 11.53%
3
2
11.58%); H, 2.23% (2.25%); N, 4.48% (4.50%); [(4AMP)(DDQ) ] dark
brown complex (M/W: 568.19 g); C, 46.39% (46.46%); H, 2.43%
2.46%); N, 14.71% (14.78%) and [(4AMP)(TBCHD)] dark brown
(
complex (M/W: 523.89 g); C, 27.44% (27.49%); H, 3.06% (3.05%);
N, 12.44% (5.34%).
Results and discussion
Electronic absorptions spectra
3
The UV–vis spectra of the CHCl solution of the formed solid
þ
ꢂ
complexes, ½ð4AMPÞ Iꢁ I , [(4AMP)(DDQ)
2
] and [(4AMP)(TBCHD)]
3
are given in Table 1. The elemental analysis of these solid products
strongly supports the proposed complex structures and in good
agreement with stoichiometrics reaction based on photometric
titration method as indicated below.
Experimental
Materials and instrumentation
The chemicals used in this study were of high grade and
obtained from Sigma–Aldrich, USA, and used without further
purification.
Reaction of 4AMP with TCNQ
ꢂ3
When adding the donor 4-aminomethylpiperidine (1 ꢃ 10 M)
ꢂ3
The UV–vis electronic absorption spectra of the CHCl
3
solutions
to a solution of TCNQ (3 ꢃ 10 M) in CHCl
3
, a brown color is
of the solid CT-complexes formed in the reactions of the donor
formed which changes very fast forming a stable yellow yield.
Fig. 1 shows the electronic spectrum recorded at region
430–1100 nm of the reactants 4AMP and 7,7,8,8-tetracyanoquino-
dimethane (TCNQ) along that of the final yellow product.
Interestingly, both reactants do not show any absorption in the
region of study while the reaction product has a number of three
absorptions at 852, 754 and 654 nm. The formation of the short
– lived brown product which gives the stable yellow reaction
product is understood on the basis that 4AMP acts as an electron
4
2
-(aminomethyl) piperidine (4AMP) and the acceptors iodine,
,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 2,4,4,6-tet-
rabromo-2,5-cyclohexadienone (TBCHD) as well as the reaction
products were checked in the region 1100–250 nm using a lambda
9
50 Perkin Elmer UV–vis spectrometer with quartz cell of 1.0 cm
path length.
The adduct 7,7,8-tricyano-8-benzylpiperidinylquinodimethane
TCAMPQDM] formed in the reaction of the donor 4-(amino-
methyl) piperidine (4AMP) and 7,7,8,8-tetracyanoquinodimethane
TCNQ) has been checked using Agilent GC/MS model 7890A with
[
donor reacts with
p-acceptor TCNQ forming 4AMP-TCNQ
(
CT-complex. This short-lived product (brown in color) undergoes
elimination reaction forming the yellow final product which
is identified by elemental analysis to be 7,7,8-tricyano-8-amin-
the thermal separation probe (TSP). Elemental analysis was done
using a Perkin Elmer CHNSO elemental analyzer model 2400 series

1
014
A. Mostafa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1012–1019
Table 1
3
Spectroscopic data for the CHCl solutions of solid CT-complexes of 4AMP with the acceptors iodine, DDQ, TCNQ and TBCHD.
Complex/adduct
Color
Absorptiona (nm)
Stoichiometry (donor:acceptor)
1:2
þ
ꢂ
½
ð4AMPÞ Iꢁ
I
Brown
382 s, 294 s
3
[
(4AMP)(DDQ)
2
]
Dark brown
Dark brown
830 s, 483 s
555 m
1:2
1:1
[(4AMP)(TBCHD)]
a
The reactants 4AMP, I2, DDQ and TBCHD have no measurable absorptions in the region of study with used concentrations; m, medium; s, strong; sh,
shoulder.
NH2
N
CN
Fig. 1. Electronic absorption spectra of 4-(aminomethyl) piperidine (4AMP) – TCNQ
ꢂ3
ꢂ3
reaction in CHCl
3
. (A) [4AMP] = 1 ꢃ 10 M; (B) [TCNQ] = 3 ꢃ 10 M; (C) 4AMP-
ꢂ3 ꢂ3
TCNQ mixture, [4AMP] = 1 ꢃ 10 M and [TCNQ] = 3 ꢃ 10 M.
NC
CN
Formula 1. 7,7,8-Tricyano-8-aminomethylpiperidinylquinodimethane(TCAMQDM).
omethylpiperidinylquino-dimethane (TCAMPQDM), Formula 1. A
general mechanism for the reaction is proposed as follows:
(
i) Formation of CT-complex:
TCNQ þ 4AMPꢂ! ½ ð 4AMPÞðTCNQÞꢁðbrown short ꢂ livedÞ
(
ii) N-substitution by TCNQ:
½
ð4AMPÞðTCNQÞꢁꢂ!7; 7; 8 ꢂ tricyano ꢂ 8
ꢂ aminomethylpiperidinylquino
ꢂ dimethaneðTCAMPQDMÞ þ HCNðyellow; stableÞ
Our results agree quite well with that reported by Frey et al.
21] on their reaction between the -acceptor tetracyanoethylene
[
p
and electron donor like aniline where the later undergoes rapid
N-substitution by TCNE:
C
6
H
5
NH
2
þ C
2
ðCNÞ ! C
6
H
5
NHðC
2
ðCNÞ Þ þ HCN
4
3
and with the work on the reaction of TCNE and o-phenylenediamine
21,22]
[
Reaction of 4AMP with I
Fig. 3 shows the electronic absorption spectra of the reaction of
the -acceptor iodine with the donor 4-(aminomethyl) piperidine
2
, DDQ and TBCHD
C
6
H
8
N
2
þ C
2
ðCNÞ ! C
6
H
6
N
2
ðC
2
ðCNÞ Þ þ 2HCN
4
2
Mass spectral measurements were performed only for 4AMP –
r
TCNQ adduct TCAMPQDM. Fig. 2 shows the mass spectrum in the
region m/z = 30–300. The molecular ion M is observed as a short
peak at m/z = 292 in good agreement with the calculated value
for the molecular weight of the reaction product (TCAMPQDM) of
(4AMP). While none of the spectra of the reactants display any
measurable absorption in the region 250–650 nm, the resulting
CT-complex shows strong absorptions centered on 382 and
294 nm. These absorptions are associated with the strong change
in color to dark brown observed upon mixing of reactants and
reflect the electronic transitions in the formed CT-complexes.
+
2
91.38 g. The found difference of about 0.62 between the observed
and calculated molecular weight value is acceptable within the al-
lowed experimental errors. The mass spectrum (Fig. 2) also shows
a number of other peaks. The peaks at m/z 179, 97 and 83 are re-
lated to the formation of fragments shown in (A), (B) and (C)
respectively.
3
Photometric titration measurements for the reaction in CHCl were
performed. The molar ratio was found to be 1:2, as shown in Fig. 4.
This reaction stoichiometry was supported by the obtained
elemental analysis of the formed solid product. However, the two

A. Mostafa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1012–1019
1015
Fig. 2. Mass spectrum for 7,7,8-tricyano-8-aminomethylpiperidinylquinodime-
thane (TCAMPQDM).
Fig. 5. Electronic absorption spectra of 4-(aminomethyl) piperidine (4AMP) – DDQ
ꢂ3
ꢂ3
reaction in CHCl
3
. (A) [4AMP] = 1 ꢃ 10 M; (B) [DDQ] = 1 ꢃ 10 M; (C) 1:2 4AMP-
ꢂ3
DDQ mixture, [4AMP] = [DDQ] = 1 ꢃ 10 M.
Fig. 3. Electronic absorption spectra of 4-(aminomethyl) piperidine (4AMP) –
ꢂ3
ꢂ3
iodine reaction in CHCl
AMP-I
of diluted solution of 4AMP/I
3
. (A) [4AMP] = 5 ꢃ 10 M; (B) [I
2
] = 1 ꢃ 10 M; (C) 1:2
ꢂ3
ꢂ3
3
Fig. 6. Photometric titration curve for 4AMP–DDQ reaction in CHCl measured at
4
2
mixture, [4AMP] = 5 ꢃ 10 M and [I ] = 1 ꢃ 10 M. The band at 294 nm
2
483 nm absorptions.
2
CT-complex (inset).
ð4AMPÞ Iꢁþ
I
ꢂ
could be understood considering the three reaction
½
3
steps, the first involves the formation of the outer complex:
(
i) ð4AMPÞ þ I
2
ꢂ!½ð4AMPÞꢁ ꢄ I
2
ꢀ
Formation of the inner complex,
(
ii) ½ð4AMPÞꢁ:I
2
ꢂ!½ð4AMPÞ Iꢁþ ꢄ Iꢂ
ꢀ
Formation of the triiodide CT-complex
(
iii) ½ð4AMPÞ IꢁþIꢂ þ I
2
½ð4AMPÞ IꢁþIꢂ
3
Fig. 5 shows the electronic spectrum recorded in the region
4
4
00–1100 nm of the reaction of 2,3-dichloro-5,6-dicyano-1,
-benzoquinone (DDQ) with 4AMP. Similar to the reaction with the
previous acceptor, strong change in color is observed upon mixing.
Reddish brown color indicates the formation of the
4
AMP – DDQ charge-transfer complex and is associated with the
electronic transitions at 787 and 483 nm. Photometric titration
measurements were performed for 4AMP – DDQ reaction in CHCl
Fig. 4. Photometric titration curves for 4AMP–iodine reaction in CHCl
82 nm absorptions.
3
measured at
3
3
as shown in Fig. 6. The results showed that the donor – DDQ molar
ratio was found to be 1:2. This is in good agreement with the
obtained elemental analysis of the solid CT-complex which
accordingly can be formulated as [(4AMP)(DDQ)2].
Fig. 7 shows the electronic spectrum obtained in the region
350–750 nm of the reaction of 2,4,4,6-tetrabromo-2,5-cyclohexa-
dienone (TBCHD) with 4AMP. Strong change in color is observed
absorption bands of the formed iodine complex around 382 and
2
94 nm in Fig. 3, are known to be characteristics of the formation
ꢂ
of the polyiodide ion of the type I3 [23]. Based on this, the structure
of the formed CT-complex should be ½ð4AMPÞ Iꢁ
þ
ꢂ
I
and the forma-
3
+
ꢂ
tion of iodide intermediate [(4AMP) I] . I is characterized by its
known absorption [24] at 245 nm and the formation of

1
016
A. Mostafa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1012–1019
Fig. 7. Electronic absorption spectra of 4-(aminomethyl) piperidine (4AMP) –
ꢂ3
ꢂ3
TBCHD reaction in CHCl
1
3
. (A) [4AMP] = 5 ꢃ 10 M; (B) [TBCHD] = 1 ꢃ 10 M; (C)
:1 4AMP-TBCHD mixture, [4AMP] = 5 ꢃ 10ꢂ3 M and [TBCHD] = 1 ꢃ 10 M.
ꢂ3
Fig. 9. Spectral determination of formation constant and molar extinction Coeffi-
þ
ꢂ
cients of CT-complexes: ½ð4AMPÞ Iꢁ I3 at 360 nm and (4AMP)(DDQ)
2
] at 830 nm.
Fig. 8. Photometric titration curve for 4AMP–TBCHD reaction in CHCl
55 nm absorptions.
3
measured at
5
upon mixing giving a dark brown color indicating the formation of
the 4AMP-TBCHD charge-transfer complex and is associated with
the electronic transitions at 555 nm. Photometric titration mea-
3
surements were performed for 4AMP-TBCHD reaction in CHCl as
shown in Fig. 8. The results showed that the donor-TBCHD molar
ratio was found to be 1:1. This is in good agreement with the ob-
tained elemental analysis of the solid CT-complex which accord-
ingly can be formulated as [(4AMP)(TBCHD)].
These obtained UV–vis spectra of the CT-complexes
Fig. 10. Benesi–Hildebrand plots for the CT-complexes: (A) [(4AMP)(TBCHD)] at
555 nm.
þ
ꢂ
½
ð4AMPÞ Iꢁ I , [(4AMP)(DDQ)
2
], and [(4AMP)(TBCHD)] have clari-
3
fied that the variation of the CT-absorption of iodine, DDQ and
TBCHD should be related to the electron affinity of each acceptor
with the donor 4AMP.
well as the sterric hinderance between reactants. All of these fac-
tors are expected to play an important role on the electron dona-
tion process from the nitrogen electron pairs of the donor 4AMP
and the aromatic ring of DDQ, TCNQ and TBCHD acceptors. The
aromatic ring in TBCHD has lower electron accepting ability com-
These pronounced variations of CT-interaction stoichiometries
are relatively complicated issue to be sorted out. It is definitely
connected to many factors such as the donor molecular symmetry,
the type of electron withdrawing groups or atoms Cl, Br or C„N as
Table 2
þ
ꢂ
Spectrophotometric and free energy results of CT-complexes ½ð4AMPÞ Iꢁ I , [(4AMP)(DDQ)
2
] and [(4AMP)(TBCHD)] in CHCl
3
.
3
ꢂ1
G0 (cal molꢂ1
ꢂ1
ꢂ1
Complex
K
CT (l mol
)
ꢂ
D
)
E
CT (eV)
k
max (nm)
e
CT (l mol cm
)
þ
ꢂ
5
3
3
½
ð4AMPÞ Iꢁ
I
13.801 ꢃ 10
8.38 ꢃ 10
6.58 ꢃ 10
4.41 ꢃ 10
3.26
382
0.980 ꢃ 10
3
5
3
3
3
3
[
[
(4AMP)(DDQ)
2
]
0.670 ꢃ 10
1.50
2.24
830
555
1.192 ꢃ 10
0.097 ꢃ 10
5
(4AMP)(TBCHD)]
0.017 ꢃ 10

A. Mostafa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1012–1019
1017
ꢂ1
(
A)
constant, KCT (L mol ) and the molar extinction coefficient
(
e
CT
ꢂ1
ꢂ1
L mol cm ), for the complex [(4AMP)(TBCHD)].
A
0
D
A
0
‘
1
e
A
0
þ D
0
¼ k
þ
ð1Þ
e
(
(
B)
C)
ꢂ1
The formation constant, KCT (l mol ), and the molar extinction
ꢂ1
ꢂ1
coefficient
e
CT (l mol cm ) have been calculated for the com-
þ
ꢂ
plexes ½ð4AMPÞ Iꢁ
I
and [(4AMP)(DDQ)
2
], using the known [27]
3
Eq. (1) of 1:2 complexes:
2
ðA
0
Þ D
0
‘
1
e
A
0
ðA
0
þ 4D
0
Þ
¼ k
þ
ð2Þ
A
e
0 0
where A and D are the initial concentrations of the acceptors and
donors, respectively, while A is the absorbance at the mentioned CT
bands and ‘ is the light path length (1 cm).
(
D)
The data obtained throughout this calculation are given in
2
Table 2. Plotting the values (A
) D
0 0
‘/A versus A
0
(A
0
+ 4D
0
) values
CT and
of Eq. (1), straight lines were obtained with a slop of 1/
intercept of 1/KCT CT as shown in Figs. 9 and 10.
These complexes show high values of both the formation
constant (K ) and the molar extinction coefficient ( CT). These high
e
e
(
E)
C
e
values of KCT confirm the expected high stabilities of the formed
CT-complexes as a result of the expected high donation of 4-(ami-
nomethyl) piperidine. The formation constants are strongly depen-
dent on the nature of the used acceptors including the type of
electron withdrawing substituent on it such as cyanide group in
DDQ.
4
000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 450.0
-1
Wavenumber (cm )
Fig. 11. IR spectra of: (A) 4-(aminomethyl) piperidine (4AMP); (B) [TCAMPQDM];
C) [(4AMP)(DDQ)
þ
ꢂ
(
2
]; (D) ½ð4AMPÞ Iꢁ I and (E) [4AMP)(TBCHD)].
0
ꢂ1
3
The free energy change
D
G (cal mol ) values of the complexes
] and [(4AMP)(TBCHD)] were calcu-
þ
ꢂ
½
ð4AMPÞ Iꢁ I [(4AMP)(DDQ)
2
3
lated from Gibbs free energy of formation according to the follow-
ing equation [28,29]:
pared with that in DDQ, related to the lower electron withdrawing
process of the substituent Br in TBCHD compared with C„N in
DDQ. This certainly, allows stronger electron donation from
AMP base to DDQ compared with that with TBCHD. In case of
AMP-TBCHD the steric hindrance is high and the stoichiometric
0
D
G ¼ ꢂRT ln KCT
ð3Þ
4
4
where
D
G0 is the free energy of the charge transfer complexes; R
ꢂ1
ratio is lower (1:1) than that of the other two acceptors (1:2).
the gas constant (1.987 cal mol K); T the temperature in Kelvin
The formation constant (KCT) and molar extinction coefficient
degrees; K the formation constant of donor–acceptor complexes
CT
ꢂ1
0
(
e
CT) values for the formed CT-complexes of the donor 4-amino-
methylpiperidine (4AMP) with iodine, DDQ and TBCHD in CHCl
at 25 °C were calculated. The 1:1 modified Benesi–Hildebrand Eq.
1) [25,26] was used to calculate the values of the formation
(l mol ). The
The obtained results of
formation process is spontaneous. The results of
more negative as the formation constants of the CT-complexes
DG values of the complexes are given in Table 2.
0
3
D
G
reveal that the CT-complexes
0
D
G are generally
(
Table 3
ꢂ1
þ
ꢂ
Infrared wavenumbers ðcm Þ and tentative band assignments for 4-(aminomethyl) piperidine (4AMP), ½ð4AMPÞ Iꢁ I , [(4AMP) (DDQ)
2
], [(4AMP)(TBCHD)] and [TCAMPQDM].
3
m, medium; s, strong; w, weak; br, broad; m, stretching; d, bending.

1
018
A. Mostafa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1012–1019
increase. As the bond between the components becomes stronger
The degradations steps and their associated temperatures vary
from one complex to another depending on the type of constitu-
ents as well as on the stoichiometry in each case. Obviously, these
two factors have pronounced effects on the type of bonding, rela-
tive complex stabilities and geometries.
and thus the components are subjected to more physical strain
0
or loss of freedom, the values of
more negative the value for
D
G
become more negative. The
0
DG , the farther to the right the reac-
tion will proceed in order to achieve equilibrium.
The charge transfer energy ECT of the formed solid CT-com-
plexes is calculated using the following equation [30,31]:
þ
ꢂ
It is of interest to see that the triiodide complex ½ð4AMPÞ Iꢁ I
3
shown in Fig. 12(A) decomposes in five degradation steps; at tem-
peratures 115 and 156 °C correspond to the decomposition of
[4AMP] with a mass loss of 15.54% very close to the calculated va-
lue of 18.36%. This step is followed by another degradation steps at
243:667
E
CTðnmÞ ¼ 1
ð4Þ
k
CT
where kCT is the wavelength of the band of the studied CT-com-
206, 268 and 353 °C corresponds to the loss of [2 I ] species with a
total mass loss of 79.09% with difference 2.55% from the calculated
2
þ
ꢂ
plexes ½ð4AMPÞ Iꢁ I [(4AMP)(DDQ)
2
] and [(4AMP)(TBCHD). The
CT values calculated from Eq. (4) are listed in Table 2. These results
3
E
value (81.64%). This is an evident that the donor 4AMP represents
18.36% of the complex, Table 4. Accordingly, a proposed mecha-
0
CT
in Table 2 (KCT, e , DG and ECT) are clarifying that the obtained so-
þ
ꢂ
lid CT-complexes formed in the reaction of the donor 4AMP with
the -acceptors DDQ and TBCHD have high CT energy and forma-
tion constants KCT as well as with the -acceptor iodine. These high
nism for the thermal decomposition of ½ð4AMPÞ Iꢁ I as follows:
3
p
ꢅ
ꢂ 115;156
C
(
i) ½ð4AMPÞ IꢁI3
ꢂ! 2½I ꢁ þ 4AMP
2
r
values confirm the expected high stabilities of the formed CT-com-
plexes as a result of the expected high donation of 4AMP.
ꢅ
2
06;268;353
ꢂ!
C
(ii) 2½I
2
ꢁ
2I
2
Infrared spectroscopy
The infrared absorption spectra of the donor 4-(aminomethyl)
(
A)
þ
ꢂ
piperidine (4AMP) and the formed complexes ½ð4AMPÞ Iꢁ I
DTG
3
100
80
[
[
2
(4AMP)(DDQ) ] and [(4AMP)(TBCHD)] in addition to the adduct
TCAMPQDM] are given in Fig. 11. The infrared band assignments
are given in Table 3. These assignments are based on the compar-
ison of the spectra of the formed products with the spectra of the
free reactants, the donor 4-aminomethylpiperidine, and the accep-
tors iodine, TCNQ, DDQ and TBCHD. Interestingly, 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 4AMP 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.
6
0
40
20
0
TG
o
Temperature ( C)
(B)
1
00
DTG
8
6
0
0
For example, the m(NAH) vibrations of the free 4-(aminomethyl)
piperidine in [(4AMP) (DDQ)
2
] has two strong absorption at 3327
ꢂ1
and 3181 cm
while in the [(4AMP) (TBCHD)], two medium
ꢂ1
absorptions are observed at 3288 and 3127 cm
The outlined changes in
support the involvement of the nitrogen atoms of the donor
4AMP in the CT-interaction process. It might also to indicate here
that (C„N) vibrations of the acceptors TCNQ and DDQ show some
changes particularly in terms of band wavenumber values upon
.
40
m
(NAH) upon complexation clearly
20
0
TG
2
m
o
Temperature ( C)
complexation. The
m
(C„N) vibrations for free TCNQ are observed
ꢂ1
as a doublet at 2196 and 2182 cm
and for free DDQ at
ꢂ1
ꢂ1
(
C)
2
203 cm . These vibrations occur at 2217, 2174 and 2127 cm
1
00
ꢂ1
in the spectrum of [TCAMPQDM)] and at 2200 cm in the spec-
DTG
2
trum of [(4AMP)(DDQ) ] complexes.
8
6
0
0
Thermal analysis
Thermal analysis (TG and DTG) were carried out for the CT-
40
20
þ
ꢂ
complexes ½ð4AMPÞ Iꢁ I , [(4AMP)(TBCHD)] and the adduct
3
ꢂ1
[
TCAMPQDM] under a nitrogen gas flow (20 ml min ) within a
ꢂ1
temperature range 30–950 °C and heating rate 10 °C ml to con-
firm the proposed formula and structure for the obtained products
as shown in Fig. 12(A)–(C) respectively. The thermogravimetric
data for these complexes and adduct are shown in Table 4. The ob-
tained data support the calculated formulas and structures of the
formed CT-complexes and the adduct.
TG
0
o
Temperature ( C)
Fig. 12. TG–DTG curves of (A) ½ð4AMPÞ IꢁþI
ꢂ
, (B) [TCAMPQDM] and (C)
3
[
4AMP)(TBCHD)].

A. Mostafa et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 1012–1019
1019
Table 4
Thermal decomposition dataa for the ½ð4AMPÞ Iꢁ I , [4AMP)(DDQ)
þ
ꢂ
2
], [(4AMP)(TBCHD)] CT-complexes and the adduct [TCAMPQDM].
3
a
ꢂ1
Thermal measurements were carried out under N
2
flow rate at 20 ml min
.
duct
7,7,8-tricyano-8-aminomethyl-piperidinylquinodimethane
For the adduct [TCAMPQDM] shown in Fig. 12(B); at119 °C cor-
responds to the loss of C (donor) with total mass loss of
0.66% (38.78% calculated). The remaining of the adduct has been
[
TCAMPQDM]. The resulting CT-complexes were shown to
6
13 2
H N
þ
ꢂ
have the formulas: ½ð4AMPÞ Iꢁ I , [(4AMP)(DDQ)
2
]
and
4
3
[
(4AMP)(TBCHD)]. Our obtained results indicate that the nitrogen
decomposed at 148 and 165 °C with mass loss of 53.10% (61.09%
atom (>NH) of the donor is involved in the complexation with
acceptors for the three CT-complexes. The measurements show
the donor–acceptor molar ratio is the same for the reactions of
the donor with iodine and DDQ and was found to be 1:2 and 1:1
with the acceptor TBCHD.
calculated) and 6.2% remaining residual carbon as clarified below:
ꢅ
1
19
C
(
i) ½TCAMPQDMꢁ ꢂ! ½C11
H
4
N
3
ꢁ þ 4AMP
ꢅ
1
48;165
ꢂ!
C
(
ii) ½C11
H
4
N
3
ꢁ
11 4 3
C H N
þ 6:2% residual carbon
This thermal decomposition mechanism is depending on that a
molecule of HCN has been removed during the formation of
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For the CT-complex [(4AMP)(TBCHD)] shown in Fig. 12(C); The
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loss of 13.49% corresponds to the loss of one bromine atom [Br]
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H
6 2
3
tures 166, 349, 362, 379 and 439 °C with total mass loss 65.16%
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ꢅ
1
25
C
(
i) ½ð4AMPÞðC
6
6
H
2
2
Br
4
3
OÞꢁ ꢂ! ½ð4AMPÞðC
6
H
2
Br
3
OÞꢁ þ Br
ꢅ
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141
C
(
ii) ½ð4AMPÞðC
H
Br
OÞꢁ ꢂ! ½C
6
H
2
Br
3
Oꢁ þ 4AMP
[
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1
66;349;362;379;439
C
(
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6
H
2
Br
3
Oꢁ
ꢂ!
C
6
H
2
Br
3
O
[
[
[
[
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Charge-transfer interactions between the donor 4-(amino-
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[
[
[
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p
r
3
able to show that the reaction stoichiometry is not the same for the
acceptors DDQ, TBCHD; the reaction of the donor 4AMP with the
acceptor TCNQ has been undergone N-substitution producing ad-
[29] M. Arslan, H. Duymus, Spectrochim. Acta A 67 (2007) 573–577.
[
[
30] G. Briegleb, Z. Angew. Chem. 72 (1960) 401.
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