402
M.I. Rodríguez Cordero et al. / Journal of Molecular Liquids 211 (2015) 401–405
Brønsted acids are classified in general as weak, mild and strong
2. Experimental part
according to their ability to donate a proton to the redox species. The
CV curve will have a particular trend/response, and the donor ability
will be affected by the environment [1]
2.1. Synthesis of the complex
The presence of hydrogen bonding agents with weak Brønsted acids
such as ethanol upon the reaction of 2,5-dichloro-1,4-benzoquinone at
The synthesis of Chromium (III) picolinate has been previously
reported [11,12]; some modifications were made in this work to
ensure efficient percentage yield. Chemical reagents used for experi-
mental development are of analytical grade (Sigma-Aldrich). The
metal salts (E. Merck), and all solvents were of spectroscopic or biolog-
ical grade.
Crpic was synthesized by first dissolving 3.76 g CrCl3·6H2O in 25 mL
warm (50 °C) ethanol. Thereafter, 4.79 g of picolinic acid in 25.0 mL eth-
anol was added, the resulting solution was mixed in a round-bottom
three necked saturated Ar vessel, and maintained under continuous stir-
ring for 24 h at room temperature. The mixture was then refluxed with
stirring for 8 h at 60 °C until reddish crystals began to form. The solution
was then left standing at 15 °C for 2 weeks and the supernatant was
discarded, with t vacuum filtered removed excess of the unreacted
ligand. The crystals were washed 5 times with 3 mL of cold ethanol,
then washed 3 times with 3 mL of ether and dried under vacuum for
1
00 mV/s, tends to change the couple redox potential to more positive
values [1–3,7]. These changes increase with the concentration of the
agent, without change in the electrochemical reversibility. Potential
changes are due to the fast equilibrium of the hydrogen bond and the
mono and di-anion, where the constant of the hydrogen bond is
negligible.
For benzoquinone (BQ) in the presence of an agent of mild strength,
such as trifluoroethanol, there would be changes on the positive side of
the potential, and an increase in the height of the first redox signal will
occur with the irreversibility of the second redox couple. As in the pre-
vious case, the change in the behavior of the potential is related to the
basicity of the BQ. The increase in the current is related to the dispropor-
tionate benzoquinone [2,3,7]
2
4 h. Analysis of the crystals indicated that the complex consisted of
:
− →
←
2−
3+
2
BQ
BQ þ BQ
:
ð1Þ
3 mol picolinate per 1 mol Cr . Analytical calculations for the
Cr(pic) ]·H O Cr (C18 ) complex; correspond to C: 49.55%;
[
3
2
14 3 7
H N O
H: 3.23%; and N: 9.63%, and the elemental analysis gave C: 49.78%,
N: 9.57% and H: 2.36%. IR measurements were taken using a Thermo
Scientific Nicolet IS10 model, DTGS detector with 32 scans and resolu-
tion of 2. For Crpic stretching (ν) of the COOH groups was observed,
and the deformations in the plane (γ) and out of plane (δ) were absent,
suggesting deprotonation of the ligand. The ν(C=O) and ν(CO) ligand
bands became ν as (COO–) and νs (COO–) [13], which implies forma-
tion of the complex. The difference between the vibrational stretching
ν as (COO–) and νs (COO–) is greater than the corresponding to the
When a quinone has a strong basicity, a new shoulder appears
before the first original redox couple, even at low concentrations of
the binding agent and the formation of hydrogen bridges take place.
The new signal is attributed to the reduction of the hydrogen bonded
to the quinone, which is confirmed by a slight change in the UV–visible
spectrum to the red on the quinine [3].
Chloranil is a weak basic quinine. In the presence of a strong acid
such as trifluoroacetic acid, a cathodic peak is observed before the first
redox couple. This peak grows in height at the expense of the original
signal. This suggests the presence of two species in reducible equilibri-
um [3]. Protonation of the quinone is discarded and a hydrogen-
bonded complex is favored.
−
1
3
free ion CH COO–(~164 cm ) [13], the connection of the metal with
the group carboxylate is monodentate, and deformation out of the
plane although both are present in the complex are shifted to higher fre-
quencies [13]. Other bands appearing in the IR spectrum of the complex
−
1
include the Cr–O around 474 cm stretching. The UV–visible spectrum
in HClO /CH COOH 70% (Agilent 8453 diode array) showed two bands
4
3
1
1
.1. Method to evaluate a new peak caused by Brønsted acid upon
,4-benzoquinone
in the visible region, at 539 nm, and a third one in the UV region is oc-
cult, but it was calculated semi-empirically using equations reported
3
by Lever for complex d , 250 nm [14]. Another experiment on the com-
When ΔEp is plotted versus pKa for the appropriate aprotic medium,
a linear behavior is observed with a correlation coefficient in the range
of 0.99 to 0.97 for DMSO and acetonitrile, assuming ΔEp is less than
plex used was electron EPR (Bruker EMX spectrometer). A g = 1.98
value was measured, which is characteristic of chromium (III)
complexes [14].
5
0 mV between two adjacent signals. Based on the ECE mechanism,
the electrochemical and chemical processes can be represented by
Scheme 1 such as [6]:
2.2. Electrochemical measurements
0
1
þ
0
2
E
H
E
:
Cyclic voltammogram experiments were done using Autolab PGSTS
BQ
BQ−:
BQH
BQH−:
Scheme1
→
→
→
12 potenciostat/galvanostat controlled with a computer. The standard
three electrode cell was used with a Pt counter electrode, a GC work
←
←
←
2
electrode (area = 0.283 cm ) and Ag/AgCl (BAS, MF-2052) as reference
electrode. The electrode was calibrated against the [Fe(C
5
H
5
)
2
]/
redox couple, +0.43 V vs Ag/AgCl. The CV of acetic acid
15], benzoic acid [16], and ascorbic acid [17] without complexes had
After combination of the Nernst equation the acid–base equilibrium
expression is [6]:
+
[
[
5 5 2
Fe(C H ) ]
been already published by other authors, founding, as this work also
with picolinic acid, that no peaks were observed even in working poten-
tial range. The stretch of acid is important to study different electro-
chemical behaviors. The acetic acid had been used to study catalyst
process by electroreduction of proton with complexes under anaerobic
conditions [16,18–21]; benzoic acid [16,19,22–24] and ascorbic acid
ꢀ
ꢁ
0
0
Q
0
0
1
2:3RT pK −pKa
0
0
0
E −E
a
ΔE0 ¼ E −E
0
0
¼
2
þ
ð2Þ
new
original
2
2F
assumingΔE00 ≅ ΔEp, where ΔE = Enew − Eoriginal; considered Eoriginal
0`
=
[5,17,25] had been assessed as strong acid in DMF medium.
0
`
0`
E
E
1
as the peak potential of BQ due to the first reduction, and E
¼
New
For the experiments DMF (Sigma, biotech. grade), 0.1 M of
0
`
0`
Q
a
2F
þE
2:3RTðpK −pKaÞ
2
1
þ
as the peak potential of BQ in the presence of
acid, when [BQ] = [BQH ]. The initial concentrations of HA and BQ
are similar, discarding the transfer of a hydrogen atom [6].
tetrabutylammonium hexafluorophosphate and 1 mM of Crpic and
Brønsted acid solutions were used. All measurements were recorded
without oxygen by purging the solution with Ar.
2
−
Rodríguez Cordero, Mildred I.
