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S. Madrahimov et al. / Journal of Molecular Structure 1199 (2020) 127021
2. Experimental
2.1. Materials and instrumentation
All the chemicals used in this study were of high grade and
purchased from Sigma-Aldrich, USA, and used without further
purification.
The UV/Vis electronic absorption spectra of the CHCl3 solutions
of the solid CT-complexes formed in the reactions of the donor 2-
amino-6-methylpyridine 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 the
reaction products were checked in the region 1200-320 nm using
PerkinElmer UVeViseNIR spectrometer model lambda 950 with
quartz cell of 1.0 cm path length. Elemental analysis was done using
a PerkinElmer CHNS/O Elemental Analyzer model 2400 series II.
The infrared spectra of the reactants, (2A6MPy), TCNE, DDQ, and
TBCHD and the obtained CT-complexes (KBr pellets) were recorded
using PerkinElmer FTIR spectrometer model spectrum one.
Fig. 1. Electronic absorption spectra of 2- amino-6-methylpyridine-TCNE reaction in
CHCl3. (A) [2A6MPy] ¼ 1 ꢁ 10ꢂ3 M; (B) [TCNE] ¼ 1 ꢁ 10ꢂ3 M; 1:2 2A6MPy-TCNE
mixture, [2A6MPy] ¼ [TCNE] ¼ 1 ꢁ 10ꢂ3 M.
2.2. Photometric titration
absorptions centered on 595, 553 and 523 nm for 2A6MPy-TCNE
reactions. These absorptions are associated with the strong change
in color observed upon mixing of reactants (dark green from
colorless solution for 2A6MPy-TCNE and reflect the electronic
transitions in the formed CT-complexes.
Photometric titration measurements based on these absorptions
were performed in order to determine the reaction stoichiometry
in CHCl3 (Fig. 2). The results showed that the donor: TCNE molar
ratio was found to be 1:2 for the donor. This is in good agreement
with the obtained elemental analysis of the three solid CT-
complexes. On the basis of these experimental data, the complex
obtained can be formulated as [(2A6MPy)(TCNE)2].
Photometric titration measurements were performed for the
reactions between the donor (2A6MPy) and each of the acceptors
TCNE DDQ, and TBCHD in CHCl3 at 25 ꢀC in order to determine the
reaction stoichiometries according to a literature method [3,18].
The measurements were conducted under the conditions of fixed
donor (2A6MPy) concentration while those of the acceptors TCNE,
DDQ 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.
Interestingly, the reaction stoichiometry using TCNE as a
p -
The infrared spectra of the reactants and the formed CT-
complexes (KBr pellets) were recorded on a spectrum one Perki-
nElmer FTIR spectrophotometer.
acceptor is 1: 2 and the alkyl group (methyl group at positions 6) is
enhancing the electron donation in comparison with the reaction
stoichiometry of 2-aminopyridine with TCNE which was 1:1 in our
previous study [19].
Fig. 3 shows the electronic spectra recorded in the region
400e1500 nm of the reaction of 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) with 2A6MPy. Similar to the reaction with
the previous acceptor, strong change in color is observed upon
mixing. Dark brown color indicates the formation of the 2A6MPy-
DDQ CT-complex and is associated with the electronic transitions at
863 and 486 nm for 2-amino-6-methylpyridine.
2.3. Preparation of CT-complexes
The three solid CT-complexes formed in the reaction of
(2A6MPy) with each of TCNE, DDQ and TBCHD were prepared in
CHCl3 by the dropwise addition of a saturated solution (85 ml) of
the donor to a saturated solution (95 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 CHCl3 and dried in vacuum
over P2O5. The complexes were characterized using spectroscopic
techniques (FTIR and UVeVis) and by elemental analysis: (theo-
retical values are shown in brackets):
2-amino-6-methylpyridine
CT-complexes:
[(2A6MPy)(TCNE)2] dark blue complex (M/W: 364.32g); C, 59.31%
(59.29%) H, 2.22% (2.20%); N, 38.38% (38.43%); [(2A6MPy)2(DDQ)]
dark brown complex (M/W:443.29g); C, 54.10% (54.14%); H, 3.64%
(3.61%); N, 18.90% (18.95%); [(2A6MPy)4(TBCHD)] dark brown
complex (M/W: 842.3g); C, 42.69% (42.74%); H, 3.07% (3.09%); N,
13.27% (13.30%).
3. Results and discussion
3.1. Electronic absorptions spectra
Fig. 1 shows the electronic absorption spectra of the reaction of
tetracyanoethylene (TCNE) with the donor 2A6MPy. While none of
the reactants’ spectra display any measurable absorption in the
region 400e650 nm, the resulting CT-complexes show strong
Fig. 2. Photometric titration curves for 2A6MPy e TCNE reaction in CHCl3 measured at
the 595 nm, 553 nm and 523 nm absorptions.