Hydrolytic and redox transformations of chromium(III) bis-oxalato complexes with glutaminic acid and glutamine: A kinetic, UV-Vis and EPR, study

Article abstract of DOI:10.1007/s11243-016-0039-2

The title complexes have been synthesized, chromatographically isolated and characterized by their ligands to metal ratio determinations and spectroscopic analyses. The kinetics of the first aquation stage, i.e., the amino acid chelate ring opening via the Cr-N bond cleavage, has been studied spectrophotometrically in acidic and alkaline media. Hydrogen peroxide oxidizes the complexes in alkaline media to CrO ₄^2- anion and a relatively stable Cr(V) complex. Consecutive biphasic kinetics through two first-order steps were observed for the base hydrolysis and the oxidation process, whereas the acid-catalyzed aquation obeys a simple first-order pattern. Based on the kinetic and spectroscopic data, mechanisms of the coordinated amino acid liberation and chromium(III) oxidation are discussed.

Full text of DOI:10.1007/s11243-016-0039-2

Transition Met Chem (2016) 41:435–445
DOI 10.1007/s11243-016-0039-2
Hydrolytic and redox transformations of chromium(III) bis-
oxalato complexes with glutaminic acid and glutamine: a kinetic,
UV–Vis and EPR, study
Emilia Kiersikowska1 Ewa Kita1
Przemysław Kita1 Grzegorz Wrzeszcz1
•
•
•
Received: 4 February 2016 / Accepted: 17 February 2016 / Published online: 1 March 2016
Ó The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract The title complexes have been synthesized,
chromatographically isolated and characterized by their
ligands to metal ratio determinations and spectroscopic
analyses. The kinetics of the first aquation stage, i.e., the
amino acid chelate ring opening via the Cr–N bond
cleavage, has been studied spectrophotometrically in acidic
and alkaline media. Hydrogen peroxide oxidizes the com-
plexes in alkaline media to CrO24- anion and a relatively
stable Cr(V) complex. Consecutive biphasic kinetics
through two first-order steps were observed for the base
hydrolysis and the oxidation process, whereas the acid-
catalyzed aquation obeys a simple first-order pattern. Based
on the kinetic and spectroscopic data, mechanisms of the
coordinated amino acid liberation and chromium(III) oxi-
dation are discussed.
states are toxic, carcinogenic and mutagenic. Particularly
dangerous are chromium complexes at the intermediate
oxidation states (IV) and (V). In the case of oxidative
stress, Cr(III)-DNA adducts are oxidized to Cr(V) and
2-
CrO4 anion. An overview of chromium redox transfor-
mations in biological systems is presented in [8]. In spite of
this health risk, some recent reports point at possible
application of selected chromium(III) complexes in medi-
cal therapy and diagnosis [9–12]. Very recent works on
biological and medicinal aspects of chromium coordination
chemistry include: (1) separation and speciation of chro-
mium in chromium yeast [13], (2) syntheses, characteris-
tics and biological activity of metforming drug for diabetes
patients with chromium(III) [14] and (3) kinetic and
mechanistic studies on Cr(V) ? Cr(III) transformations
[15–17]. As it was established, chromium(III) complexes
being to the left of the ‘‘oxo wall’’, vide [18], can be
involved not only in four-electron reduction of the oxygen,
but also can act as selective catalysts in two-electron
Introduction
For the last two decades, numerous dietary supplements for
humans containing so-called biochromium have been
offered. In fact, these pharmaceuticals contain only a few
different chromium(III) complexes as active substances
and all of them are recommended against obesity, arte-
riosclerosis and type II diabetes, e.g., [1–4]. Commercial
success of the mentioned preparations seems to be inde-
pendent of controversial discussion on the biological role
of the chromium(III), e.g., [5–8]. On the other hand, there
is no doubt that chromium species in higher oxidation
transformation
of
O2 ? H2O2.
In
that
case,
chromium(V) can be the catalyst precursor. The latest
findings [16] demonstrate the complexity of redox cycles
of chromium in living organisms.
Very low bioavailability of commercial chromium(III)
pharmaceuticals and their unknown mechanism in human
administration still inspire chemists to look for new chro-
mium(III) complexes and examine their chemical trans-
formations. We studied chemical and biological activity of
several chromium(III) complexes with amino acids, e.g.,
[19–21]. The present work deals with kinetics and mech-
anism of ligand substitution and redox transformations of
chromium(III)
bis-oxalato-glutaminic acid/glutamine
& Ewa Kita
complexes. While much work has been done on
Cr(VI) ? Cr(III) reduction, only a few articles have been
published on mechanism of Cr(III) ? Cr(VI) oxidation by
ewakita@chem.umk.pl
1
´
Nicolaus Copernicus University, Torun, Poland
123

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Transition Met Chem (2016) 41:435–445
Spectroscopic characteristics of [Cr(ox)2(Gln)]22
and [Cr(ox)2(Glu)]22 complexes
hydrogen peroxide. Glutamine (Gln), the ligand chosen in
this work, is the most abundant free amino acid in the
human body, 60 % of all free amino acids, and is involved
in many metabolic processes. Production of Gln decreases
with age, and external supplementation is recommended in
such cases. It plays an anti-aging role, acts as a neuro-
transmitter, is used in medical treatments and also as a
flavor enhancer (in the form of monosodium glutaminate,
MSG), nutrient, plant growth preparation, etc. [22]. Spe-
ciation of the proteolytic forms of glutaminic acid and
glutamine is shown in Scheme 1.
Electronic Vis spectra of the complexes were recorded in
aqueous solutions at pH 2. Values of molar absorption
coefficients were calculated based on chromium concen-
tration, which was determined spectrophotometrically as
2-
CrO4 at 372 nm, after decomposition of the complexes
(in NaOH ? H2O2). Spectroscopic characteristics are
given in Table 1.
Products of acid-catalyzed aquation
Two polynuclear chromium(III) complexes with glu-
taminic acid and glutamine or cysteine were isolated and
physicochemically characterized before [23].
A
solution of [Cr(ox)2(Gln)]2- or [Cr(ox)2(Glu)]2-
(0.2 mmol) in 0.2 M HClO4 (15 cm3) was kept at 313 K
for 30 min, i.e., time corresponding to 3 t1/2, determined
from the kinetic studies. Then, it was cooled, neutralized
with cold 2.5 M KOH and kept in the refrigerator for
30 min. Next, the deposit of KClO4 was filtered off, the
solution was diluted to 100 cm3 and introduced onto a
column of anionic exchanger. Two species were eluted and
Experimental
Materials
Glutamine (Aldrich), glutaminic acid (Aldrich), NaClO4-
H2O (Aldrich), 70 % HClO4 (Aldrich) and other chemicals
were used as supplied. Sephadex DEAE A-25 (ClO4-) and
Sephadex C-25 (H?) were used for chromatographic
separations.
identified as [Cr(ox)2(H2O)2]- and [Cr(ox)2(Aa)]2-
,
respectively. Some chromium species were also found in
the solution collected during sorption. As they did not
exhibit affinity to the cationic exchanger, they were
assumed to be [Cr(ox)(Aa)(H2O)2]0. The composition of
reaction mixtures obtained from acid-catalyzed aquation is
given in Table 2.
Preparation of [Cr(ox)2(Gln)]22
and [Cr(ox)2(Glu)]22 complexes
Products of base hydrolysis
These complexes were prepared from trans-K[Cr(ox)2(H2-
O)2]Á3H2O (2 mmol) and glutamine/glutaminic acid
(3 mmol) according to the description given in [19]. The
desired products were separated chromatographically using
DEAE ion exchanger (ClO4-) with 0.2 M NaClO4. The
yield was ca. 40 %.
A
solution of [Cr(ox)2(Gln)]2- or [Cr(ox)2(Glu)]2-
(0.2 mmol) in 0.2 M KOH (15 cm3) was kept at 303 K for
250 s. The reaction was quenched by addition of equimolar
2 M HClO4 (using an ice bath), the precipitate was filtered
(a)
NH2
O
O
O
NH2
O
O
NH2
O
−H+
+H+
−H+
+H+
-O
-O
HO
+
+
NH3
NH3
NH2
pKa2 = 9.00
pKa1 = 2.18
(b)
O-
O-
O-
O
OH
O
O
O
−H+
+H+
−H+
+H+
−H+
+H+
O-
O-
O
O
OH
O
O
OH
+
+
+
NH2
NH3
NH3
NH3
pKa2 = 4.45
pKa3 = 10.1
pKa1 = 2.19
Scheme 1 Protolytic forms of glutamine (a) and glutaminic acid (b) in aqueous solution; the pKa values at 298 K, I = 0 [8]
123

Transition Met Chem (2016) 41:435–445
437
Table 1 Spectroscopic characteristics of [Cr(ox)2(Aa)]2- in 0.01
HClO4
Table 3 Position of lower-energy d–d transition band in the spec-
trum of reaction mixture of [Cr(ox)2(Aa)]2- in 0.4 M HClO4 and after
pH increase
Complex
kmax (e), nm (M-1cm-1
)
pH
[Cr(ox)2(Gln)]2-
[Cr(ox)2(Glu)]2-
[Cr(ox)2Gln]2-
[Cr(ox)2Glu]2-
406 (95) 549 (78)
406 (96) 549 (81)
t (min)
kmax (nm)
t (min)
kmax (nm)
1
1
7
0
14
14
549
566
556
0
15
15
549
564
554
Table 2 Composition of the reaction mixture obtained from acid-
catalyzed aquation of [Cr(ox)2(Aa)]2- at 313 K
Complex
t (min) Composition of the reaction
mixture (%)
controller) by monitoring the absorbance in the visible
spectrum region, within the lower-energy d–d transition
band. The concentration of HClO4 and NaOH was varied
between 0.2 and 1.0 M, and the ionic strength was 1.0 M
(NaClO4). Spectroscopic changes for [Cr(ox)2(Gln)]2- are
shown in Figs. 1 and 2 (spectroscopic changes for
[Cr(ox)2(Glu)]2- were practically the same).
[Cr(ox)2Gln]2- 28
[Cr(ox)2(Glu)]2- 28
[Cr(ox)2Gln]2-
42
39
9
[Cr(ox)2(H2O)2]-
[Cr(ox)(Gln)(H2O)2]0
[Cr(ox)2(Glu)]2-
[Cr(ox)2(H2O)2]-
[Cr(ox)(Glu)(H2O)2]0
45
42
9
The reaction was performed in 1-cm tandem cell, where
equal volumes (0.85 cm3) of the stock complex solution
and the medium were mixed. Each kinetic run was per-
formed for at least 3 t1/2 of the substrate conversion and
was repeated trice. Absorbance-time data obeyed a pseudo-
first-order dependence for acid-catalyzed reactions. On the
other hand, a consecutive reaction model, A ? B ? C,
was applied for base hydrolysis. Values of pseudo-first-
order rate constants, kobs,H, were calculated from absor-
bance-time data collected at 550 nm using the Enzfitter
program, and kobs1,OH and kobs2,OH were calculated by
multifunctional analyses of scans recorded within
500–600 nm using SPECFIT software. The calculated kobs
were practically independent of the initial complex
concentration.
off, and the collected solution (diluted to 150 cm3) was
introduced onto a column of anionic exchanger. Only one
colored complex (ca. 50 % of the total chromium content)
eluted with 0.1 HClO4 ? 0.1 M NaClO4 was obtained and
identified as the substrate, [Cr(ox)2(Aa)]2-. The solution
(practically colorless) collected during sorption (containing
ca. 15 % of the total chromium content) was introduced
onto the cationic exchanger, where only polynuclear l-
hydroxo-aquachromium(III) species were found. A quite
large amount of chromium(III), ca. 40 % of the total
chromium content, was found in the collected precipitate,
2-
which was analyzed spectrophotometrically as CrO4
,
after dissolution of chromium species present in the deposit
in 0.4 M NaOH and oxidation by H2O2.
Kinetics of oxidation of [Cr(ox)2(Gln)]2- and
[Cr(ox)2(Glu)]2- were followed by the absorbance increase
within the 350–400 nm spectral range (Fig. 3a). The con-
centration of oxidant, H2O2, was changed between 0.2 and
Spectroscopic evidence for formation
of the metastable intermediate
Equal volumes (0.85 cm3) of the complex solution and
0.4 M HClO4 were placed in a tandem cell, respectively.
The reaction was followed spectrophotometrically at 313 K
for 3 t1/2. In parallel, the same experiment was performed
in a thermostated beaker and was quenched by the addition
of an appropriate amount of 2.5 M KOH (pH 4–6). Posi-
tions of lower-energy d–d bands in the spectra of the
mixtures at time 0 and 3 t1/2 in acidic and neutralized
reaction mixtures were compared (Table 3).
Kinetic measurements
Kinetics of the acid- and base-catalyzed aquation of the
[Cr(ox)2(Gln)]2- and [Cr(ox)2(Glu)]2- complexes were
studied spectrophotometrically (Spectrophotometer HP
8453 equipped with HP89090 Peltier temperature
Fig. 1 Spectroscopic changes during aquation of [Cr(ox)2(Gln)]2- in
0.6 M HClO4; [Cr(III)] = 8 9 10-3 M, I = 1 M, T = 303 K, scans
every 120 s
123

438
Transition Met Chem (2016) 41:435–445
Fig. 2 Spectroscopic changes at the initial period (a) and after it
(b) during base hydrolysis of [Cr(ox)2(Gln)]2- in 0.6 M NaOH;
[Cr(III)] = 8 9 10-3 M, I = 1 M, T = 303 K, scans every 40 s
Fig. 3 Spectroscopic changes in the UV (a) and Vis (b) spectral
range during oxidation of [Cr(ox)2(Gln)]2- by H2O2 in 0.6 M NaOH;
[Cr(III)] = 1.5 9 10-4
[H2O2] = 0.2 M, T = 298 K, scans every 120 s (a) and 60 s (b)
M
(a) and 6.2 9 10-3
M
(b),
1.0 M, the concentration of NaOH was kept constant at
0.6 M (taking into consideration neutralization of NaOH
by H2O2), the concentration of chromium(III) was
1.5 9 10-4 M, and the ionic strength was 2.0 M (NaClO4).
The reaction was initiated by injection of 0.1 cm3 of the
complex solution into 1.9 cm3 of the thermostated medium
solution, prepared directly before the measurement from
the appropriate amounts of NaOH and H2O2 stock solu-
tions. Additionally some kinetic runs were performed in the
Vis spectral range, using a 1-cm tandem cell (Fig. 3b).
Standard error of single pseudo-first-order rate con-
stants, kobs, was usually ca. 1 %, and that of the mean kobs
values was usually 1–2 %.
reagents. A flat quartz cell was used. The solution concen-
trations were: [Cr(III)] = 1.5 9 10-4 M, [H2O2] = 0.2 M,
[NaOH] = 0.6 M. The initial intensity of EPR signal was
relatively weak, but quickly increased during the reaction
course, reaching a maximum within ca. 30 min (Fig. 4). At
that point, the signal was stable for at least a few hours and
EPR measurements
EPR spectra of reaction mixtures, containing Aa-chromiu-
m(III) complexes and hydrogen peroxide in NaOH medium,
were recorded with a Radiopan EPR SE/x2541 M spec-
trometer in X band (ca. 9.33 GHz) with 100-kHz modula-
tion. The microwave frequency was monitored with an
automatic NMR-type magnetometer. EPR spectra were
recorded at room temperature, and the measurements were
carried out from ca. 180 s after the initial mixing of
Fig. 4 Changes of intensity of EPR signal generated during oxidation
of [Cr(ox)2(Gln)]2- by 0.2 M H2O2 in 0.6 M NaOH at 298 K;
[Cr(III)] = 1.5 9 10-4
M
123

Transition Met Chem (2016) 41:435–445
439
alkaline media (Fig. 2). In neutral solutions, both the
complexes are very inert.
Acid-catalyzed aquation of [Cr(ox)2(Glu)]22
and [Cr(ox)2(Gln)]22
The rate of aquation in acidic media increases with the
concentration increase of Hþaq and leads to liberation of the
amino acid. The parallel process, dissociation of oxalate,
takes place in much smaller extent; the molar ratio of the
bis-oxalato to mono-oxalato products is ca. 10 (Table 2).
Moreover, spectral changes accompanying oxalato ligand
dissociation are much smaller than those accompanying the
amino acid liberation, and for this reason absorbance-time
data can be attributed only to reaction (1):
Fig. 5 EPR spectrum recorded for reaction mixture of 1.5 9 10-4
M
[Cr(ox)2(Gln)]2- and 0.2 M H2O2 in 0.6 M NaOH after 30 min from
the initiation of the reaction; microwave frequency 9.33235 GHz,
room temp
Â
Ã
½Cr(ox)2ðGlnފ2Àþ2H2OþHþ ! Cr(ox) ðH2OÞ ÀþGlnH
2
2
ð1Þ
then slowly disappeared (within a few days or even weeks)
due to decomposition of the hydrogen peroxide.
Formation of the diaqua product (Eq. 1) results in absor-
bance decrease in the whole visible region and a redshift of
The chromium(V) EPR spectrum (Fig. 5) was identical
with that recorded during oxidation of other chromiu-
m(III)-amino acid complexes by hydrogen peroxide [20,
21] and was assigned to [CrV(O2)4]3- species based on
comparison of the obtained spectral parameters with the
literature data [24, 25]: a strong isotropic signal due to
chromium isotopes with I = 0 (90.5 %) at giso = 1.9722
0.0003 and peak-to-peak width, DBpp 9 0.2 mT. A small
isotropic satellite signal, split into a quartet due to hyper-
fine coupling by the less abundant (9.5 %) 53Cr isotope
(I = 3.2), is also observed with a (53Cr) = 18.59 9 10-4
cm-1 [24, 25].
4
4
the A2g ? T2g transition (pseudo-Oh symmetry approx-
imation) (Fig. 1). The presence of isosbestic points at 439,
467 and 610 nm (for both the Gln/Glu-complexes) during
the reaction course (up to 0.9 of the conversion degree)
indicates that only one step from the two steps of amino
acid dissociation is observed spectrophotometrically.
Additionally performed experiments (vide Experimental)
revealed cumulation of the intermediate complex,
[Cr(ox)2(AaH)(H2O)]- (where AaH is coordinated through
the carboxylate oxygen atom), in the reaction mixture
(Table 2). This intermediate is metastable in acidic media,
but undergoes fast conversion into the substrate upon
neutralization, manifested by a blueshift of the lower-en-
ergy d–d transition band in the spectrum of the reaction
mixture after 3 t1/2 if pH increases from 1 to 7. Thus, the
first step of the amino acid dissociation, the chelate ring
opening at the Cr–N bond, is faster than liberation of the
one-end bonded amino acid.
Results and discussion
Two new complexes, [Cr(ox)2(Gln)]2- and [Cr(ox)2(-
Glu)]2-, were obtained via anation of cis-[Cr(ox)2(H2O)2]-
by glutamine and glutaminic acid followed by chromato-
graphic separation. Electronic spectra of the complexes
within d–d transition range (Table 1) are practically the
same as those for analogous complexes with other amino
acids [19–21] and are consistent with bidentate N,O-coor-
dination of the amino acid. Substitution of H2O ligands by
Gln/Glu in the starting complex results in ca. 10-nm
blueshift of the lower-energy d–d band and an intensity
increase. The absence of H2O ligands in the coordination
sphere of the anation products was confirmed by inde-
pendence of their spectra of pH within the 1–13 range,
recorded immediately after preparation of the complexes
solution.
Absorbance-time data obey a first-order kinetics at
constant [H?]. Values of the pseudo-first-order rate con-
stants are shown in Table 4, and their acid dependence is
presented in Fig. 6.
The chelate ring-opening process is catalyzed by H?,
and the kobs increases linearly with [H?] increase (Eq. 2):
kobs ¼ a þ b½HþŠ
ð2Þ
Values of the a and b parameters and the activation
parameters, calculated from the b parameter, are given in
Table 5.
Based on the obtained results (identified reaction inter-
mediates and products, spectroscopic and kinetic data), the
following reaction mechanism can be proposed (Scheme 2).
Aquation of the complexes is conveniently monitored by
spectral changes in strongly acidic (Fig. 1) or strongly
123

440
Transition Met Chem (2016) 41:435–445
Table 4 Dependence of kobs on [H?] for acid-catalyzed aquation of
+ H+, Kp1
- H+
(Aa)]2-
-
[Cr(ox)2(Gln)]2- (A) and [Cr(ox)2(Glu)]2- (B)
[C(ox)
[Cr(ox) (AaH)
2
2
(S)
[H?], M
103kobs (s-1
303 K
)
103kobs (s-1
313 K
)
103kobs (s-1
)
323 K
(SH)
+ H2O k0
+ H2O k1
+ H+
rapid
-
(A)
0.2
0.4
0.6
0.8
1.0
(B)
0.1
0.2
0.4
0.6
0.8
1.0
[Cr(ox) (O-Aa)(H O)]2-
[Cr(ox) (O-AaH)(H O)]
2
2
2
2
–
1.35 0.04
2.34 0.07
3.52 0.05
4.72 0.13
5.25 0.19
3.64 0.17
6.12 0.19
9.18 0.30
12.5 0.32
–
(P
)
0.852 0.037
1.35 0.09
1.86 0.19
2.24 0.15
(P
)'
1,Aa
1,Aa
Scheme 2 Mechanism for the first stage of amino acid dissociation
from [Cr(ox)2(Aa)]2-
kobs1 ¼ ðk0 þ k1Kp1½HþŠÞ=ð1 þ Kp1½HþŠÞ
If the protolytic equilibrium is shifted left, i.e.,
Kp1[H?] ꢀ 1, Eq. 3 simplifies to the observed linear
dependence kobs1 on [H?]:
ð3Þ
–
–
1.80 0.05
3.29 0.12
5.68 0.16
8.26 0.12
11.2 0.3
–
0.472 0.038
0.803 0.003
1.19 0.06
1.61 0.01
2.04 0.13
1.28 0.18
2.25 0.05
3.29 0.05
4.34 0.05
5.49 0.23
kobs1 ¼ k0 þ kH½HþŠ
where kH = k1Kp1.
Thus, comparison of Eq. 2 with Eq. 4 gives mechanistic
ð4Þ
interpretation of the experimental parameters a and b:
a ¼ k0 and b ¼ kH ¼ k1Kp1:
ð5Þ
The apparent activation parameters, shown in Table 5,
are the sums of activation enthalpy (entropy) of the chelate
ring-opening process and enthalpy (entropy) of the chelate
ring protonation. An analogous mechanism operates for
H?-catalyzed aquation of other chromium(III) bis-oxalato
complexes with amino acids [19]. Comparison of the rate
data (the pseudo-second-order rate constants, b parameter)
leads to the conclusion that the side chain of a-amino acids
plays a minor role in determination of the complex reac-
Fig. 6 Dependence of kobs on [H?] for acid-catalyzed aquation of
tivity. Values of the 103b [M-1 -1
s ] at 298 K, I = 1.0 M
[Cr(ox)2(Gln)]2-
are as follows: 0.84 (Val), 0.96 (Asp), 0.99 (Cys), 1.08
(Glu), 1.20 (Gln), 1.21 (Ala), 1.58 (His), 1.64 (Ser) [19].
Values of the apparent activation parameters for these
complexes are quite similar; DH= is usually
74–80 kJ mol-1 and DS= is near -50 eu, which clearly
indicates an analogous mechanism for all [Cr(ox)2(Aa)]2-
complexes.
The acid-catalyzed reaction path predominates in
strongly acidic media. This process is initiated by proto-
nation of the coordinated carboxylic group (SH) leading to
destabilization of the chelate ring, which results in the Cr–
N bond cleavage and formation of the monoaqua inter-
mediate, P1,Aa. The rate law derived from the presented
reaction model is of the form:
Hydrolytic Cr–N bond cleavage in H?-catalyzed reac-
tion paths for [Cr(Gly)3]0 and [Cr(Asp)2]- complexes is
Table 5 Values of the a and
b parameters (Eq. 2) determined
for the acid-catalyzed aquation
of [Cr(ox)2(Aa)]2- and
[Cr(ox)2(Gln)]2-
[Cr(ox)2(Glu)]2-
103 a (s-1
103 a (s-1
)
103 b (s-1 M-1
)
)
103 b (s-1 M-1
)
activation parameters at 298 K
for the Cr–Aa ring opening
calculated from the
303 K
0.036 0.089
0.160 0.031
0.427 0.273
0.028
2.13 0.11
5.54 0.05
14.6 0.49
1.20
0.063 0.016
0.241 0.011
0.476 0.089
0.059
1.88 0.02
5.00 0.02
13.0 0.2
1.08
313 K
323 K
b parameters
298 K
DH= (kJ/mol)
DS= (J/mol K)
85
8
78
1
65 12
77.1 0.6
-47 25
-41
4
-11 38
-43
2
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Transition Met Chem (2016) 41:435–445
441
faster than that for the bis-oxalato complexes; values of the
103 b [M-1 -1
s ] at 298 K are 4.5 and 8.4, respectively [26,
relatively small redshift (ca. 10 nm) of the lower-energy d–
d band which is much smaller than that expected for sub-
stitution of only the N-donor atom by the OH- ligand.
These intermediates are assumed to be in comparable
amounts. Thus, the determined kobs1 rate constants are
sums of the rate constants:
27]. Thus, oxalato ligands slightly retard the amino acid
chelate ring-opening process.
Base hydrolysis of [Cr(ox)2(Glu)]22
and [Cr(ox)2(Gln)]22
kobs1 ¼ k10 þ k100
ð6Þ
As it was mentioned, neutralization of the reaction mixture
at the first reaction stage reverses the hydrolysis and the
initial complex is restored. In the second stage, monoden-
tate bonded ligands (Aa’ and ox’) are liberated and then
decomposition of the complexes leading to chromates(III)
takes place. Thus, the kobs2 values are also sums of the rate
constants for spontaneous one-end bonded ligand (Aa’ and
ox’) liberation.
In strongly alkaline media (pH [ 13), both the complexes
undergo relatively slow ligand dissociation to give chro-
mates(III). Chromatographic separations of the neutralized
reaction mixture obtained after t1/2 of the substrate con-
version revealed the starting complex and greenish
polynuclear l-hydroxo-aquachromium(III). Spectroscopic
changes during the reaction course (Fig. 2) are character-
istic for a two-step process. At the first step (Fig. 2a), ca.
50 % of absorbance decrease, sharp isosbestic points (at
440, 478 and 607 nm for the Gln/Glu-complex) are held
very well, then (Fig. 2b) they disappear, and further
absorbance decrease and a redshift are observed. In spite of
substantial spectroscopic changes during the first reaction
step (kmax 550 ? 557 nm, Amax 0.6 ? 0.4), neutralization
of the reaction mixture causes practically complete reverse
of the spectrum to that at t = 0 s. These results are con-
sistent with parallel chelate rings opening via the Cr–N and
Cr–ox bond cleavage and formation of the [Cr(ox)2
(Gln’)(OH)]3- and [Cr(ox)(ox’)(Gln)(OH)]3- or [Cr(ox)2
(Glu’)(OH)]4- and [Cr(ox)(ox’)(Glu)(OH)]4- (Glu is
dianionic because of its two carboxylate groups) interme-
diates. Then ligands substitution takes place. The back
reaction is insignificant at pH [ 13, because the monoaqua
intermediate is deprotonated and the hydroxido ligand
blocks the chelate ring closure. Lack of [Cr(ox)2(H2O)2]-
and Gln/Glu-tetraaquachromium(III) complex in the acid-
ified reaction mixture indicates that both ligands, oxalato
and Aa, are liberated with similar rates.
kobs2 ¼ k20 þ k200
Values of apparent activation parameters are given in
ð7Þ
Table 7.
Comparison of the kobs1,H in 1 M HClO4 (ca. 2 9 10-3
s-1) and kobs1,OH (ca. 8 9 10-3 s-1) determined at 303 K
shows that the rates of the first reaction stage markedly
depend on medium; in acidic solution, the process is slower
than in alkaline medium. Rationalization of this kinetic
effect can be based on hindrance of the reverse process,
i.e., the chelate ring closure by hydroxido ligand in
monohydroxido intermediates.
It seems to be remarkable that the kobs1,OH values are
higher than those of the kobs2,OH in spite of the presence of
OH- ligand in the coordination sphere which labilizes the
complex. Determined values of (DS=)1 and (DS=)2 are
apparent activation parameters because they are the sums
of activation entropies for two parallel processes. High
negative values of the activation entropies for both the
reaction stages suggest an associative mode of the transi-
tion state formation classified as IA-type mechanism. For
dissociative mode of activation, expected increase of
hydration leading to the entropy decrease would be coun-
terbalanced by entropy increase caused by Cr–L bond
breaking, resulting in close to zero entropy value.
Kinetics of the base hydrolysis were examined using
SPECFIT software for scans registered within the
500–600 nm region for the consecutive first-order
A ? B ? C reaction model. The calculated values of the
kobs1 and kobs2 are collected in Table 6.
As it is seen, values of the first-order rate constants for
both the stages are only slightly dependent on [OH-] and
therefore we calculated only one average value for kobs1
and kobs2 at each temperature studied (Table 7). It is also
seen that the kobs1 values are ca. twice higher than those for
Oxidation of [Cr(ox)2(Aa)]22 by H2O2
Oxidation of the bis-oxalato complexes with Gln and Glu,
and additionally also [Cr(ox)2(H2O)2]- and [Cr(ox)(H2O)4]?,
carried out under a large excess of H2O2 and OH- was
monitored spectrophotometrically within the UV–Vis
range and additionally using EPR technique. This process
is quite slow with the exception of [Cr(ox)(H2O)4]? oxi-
dation; spectroscopic changes take place for ca. 30 min at
298 K. Two chromium species at higher oxidation states,
i.e., CrO42- anion and Cr(V) identified as [Cr(O2)4]3-, are
k
obs2. The presented data are consistent with the reaction
mechanism shown in Scheme 3.
In the stage I, two parallel reactions—the chelate ring
opening at Cr–ox and Cr–Aa—take place and monohy-
droxido intermediates, [Cr(ox)2(Aa’)(OH)]3- (P1,OH) and
[Cr(ox)(ox’)(Aa)(OH)]3- (P2,OH)’, are formed. The pres-
ence of these two intermediates is consistent with a
123

442
Transition Met Chem (2016) 41:435–445
Table 6 Dependence of kobs1
and kobs2 on [OH-] for base
hydrolysis of [Cr(ox)2(Gln)]2-
(A) and [Cr(ox)2(Glu)]2- (B)
[OH-] M
103kobs1 (s-1
)
103kobs2 (s-1
)
293 K
303 K
313 K
293 K
303 K
313 K
A
0.2
0.4
0.6
0.8
0.9
B
2.76 0.15
2.80 0.07
3.02 0.11
2.92 0.19
2.97 0.25
7.91 0.14
8.16 0.70
8.08 0.30
8.10 0.10
8.19 0.33
18.2 1.3
18.8 1.2
19.6 1.3
19.5 0.4
20.8 0.9
1.45 0.004
1.56 0.06
1.58 0.06
1.64 0.44
1.67 0.25
3.94 0.08
3.93 0.28
4.21 0.07
4.46 0.11
4.56 0.07
9.75 0.15
10.4 0.44
10.5 0.4
11.0 0.4
10.9 0.1
0.2
0.4
0.6
0.8
0.9
–
6.88 0.38
7.33 0.60
7.81 0.55
8.70 0.64
8.10 0.72
17.2 1.2
18.4 0.3
19.5 1.4
18.2 0.5
17.8 1.0
–
3.76 0.04
3.97 0.12
4.06 0.03
4.14 0.08
4.29 0.02
9.38 0.13
10.0 0.2
10.3 0.4
10.5 0.4
11.2 0.3
2.60 0.13
2.98 0.12
2.71 0.47
2.88 0.25
1.48 0.03
1.48 0.01
1.62 0.13
1.59 0.01
Table 7 Values of the kobs1 and
kobs2 and activation parameters
at 298 K determined for base
hydrolysis of [Cr(ox)2(Aa)]2-
[Cr(ox)2(Gln)]2-
[Cr(ox)2(Glu)]2-
103 kobs1 (s-1
103 kobs1 (s-1
)
103 kobs2 (s-1
)
)
103 kobs2 (s-1
)
293 K
2.89 0.13
8.08 0.14
19.4 1.32
4.98
1.58 0.11
4.22 0.32
10.5 0.63
2.61
2.79 0.27
7.76 0.87
18.2 1.6
4.81
1.54 0.10
4.04 0.27
0.3 0.92
2.50
303 K
313 K
298 K
DH= (kJ/mol)
DS= (J/mol K)
68.1 1.9
-61.1 6.1
69.6 0.2
-60.7 0.6
66.6 2.2
-65.7 7.2
70.9 0.7
-56.9 2.1
Scheme 3 Mechanism of base
hydrolysis of
[Cr(ox)2(Aa)(H2O)2]2-; Aa0 and
ox0 denote one-end bonded
ligand through oxygen atom
K '
a
k2'
[Cr(ox) (Aa')(H O)]2-
[Cr(ox) (Aa')(OH)]3-
2
[Cr(ox) (OH) ]3-
-
2
+ Aa
2
2
2
(P )
1
(P
)
(P
)
1,OH
2,OH
k1' k-1
'
[Cr(ox) (Aa)]2-
2
(S)
k
-1''
k1''
K ''
k ''
[Cr(ox)(ox')(Aa)(H O)]2-
2
a
[Cr(ox)(ox')(Aa)(OH)]3-
(P )'
[Cr(ox)(Aa)(OH) ]2-
2
2
+ ox2-
(P )'
1
1,OH
(P
)'
2,OH
observed practically from beginning of the reaction
(Figs. 3, 4). Distribution of Cr(VI) and Cr(V) species can
be evaluated from the spectra shown in Fig. 3. As it is seen,
at the ‘‘end’’ of the reaction ca. 35 % of chromium is in the
form of the metastable chromium(V), which practically
does not absorb within the monitored spectral range at 10-4
M total concentration of chromium.
because it requires two orders of magnitude higher sub-
strate concentration, causing additional complication due
to oligomerization of Cr(III) species [28]. For this reason,
kinetic measurements were done at low Cr(III) concentra-
tion, in the UV region, and only in a few cases, the
obtained results were compared with those obtained from
visible spectra and from EPR data. Multiwavelength
analysis of the absorbance-time data shows that reaction is
biphasic and proceeds through two consecutive first-order
steps. Similar results including values of the rate constants
were obtained from visible range data and EPR signal
intensity changes with time. Thus, the rates of CrO42- and
The rate parameters can be determined from the absor-
bance increase at 372 nm (the kmax for CrO42-), from EPR
signal intensity increase (formation of CrV) or from
absorbance decrease within the d–d transition region (CrIII
disappearance). The later method is less convenient
123

Transition Met Chem (2016) 41:435–445
443
Cr(V) formation and the rate of Cr(III) disappearance are
practically equal. The data were processed using a con-
secutive first-order A ? B ? C reaction pattern with the
help of SPECFIT software. Additionally, absorbance-time
data collected after 300 s for oxidation of the Gln- and Glu-
complexes and after 100 s for [Cr(ox)2(H2O)2]- were fitted
to a simple first-order A ? B reaction model. Calculated
values of the pseudo-first-order rate constants are shown in
Table 8.
Cr(III) d orbital into the LUMO r* hydroperoxido orbital.
All individual rate constants for electron transfer are
approximated by a common ket rate constant (Scheme 4).
The formed CrVO is further transformed in two fast parallel
reaction paths: (1) It decays by disproportionation, and (2)
its coordination sphere is rearranged by ligands exchange
giving long-lived [CrV(O2)4]3-. The ratio of rate constants
for these two fast reaction paths determines the relative
content of CrO4 and [CrV(O2)4]3- in the reaction mix-
2-
Analogous consecutive biphasic kinetics for base
hydrolysis and the oxidation process and very close values
of the rate constants for two stages (especially those for the
second one) for both the reactions (Tables 7, 8) firmly
confirm the importance of ligand substitution during oxi-
dation. Electron transfer from Cr(III) leading to Cr(V) and
ture. EPR measurements at low hydrogen peroxide con-
centration revealed that chromium(V) not stabilized by a
2-
large excess of [H2O2] decays fast, forming finally CrO4
anion.
Some other data presented in Table 8 can be considered
as helpful in elucidation of the oxidation mechanism. A
small but systematic increase of the kobs1 value with the
[H2O2] increase suggests that the equilibrium leading to an
2-
CrO4 is preceded by chelate ring opening and then one-
end bonded ligand liberation. The above-mentioned argu-
ments and determined equal rates of Cr(III) disappearance
and formation of [CrV(O2)4]3- and CrO4
I1,OOH-type an intermediate is not completely shifted to the
2-
can be
right and moreover, the value of the rate for the
intramolecular electron transfer (ket) is comparable with
k10 þ k100. However, the predominant reaction path is that
through I2,OH,OOH-type intermediate and for that reason
kinetics of the overall oxidation process can be satisfac-
torily described by a simple A ? B model, if the initial
part of the reaction is omitted. This conclusion is supported
by the very close values of kobs2 and kobs (Table 8). Prac-
tically equal values of the rate constants for the second step
of the base hydrolysis (Table 7) and the oxidation
(Table 8) support the assumption that ligand liberation is
the rate-limiting step for the second oxidation stage and
thus k20 þ k200 ꢀ ket. Interestingly, the value of kobs for
rationalized assuming the following simplified reaction
scheme for [Cr(ox)2(Aa)]2- oxidation (Scheme 4).
The rate controlling steps of the overall redox process
are substitution reactions, i.e., formation of the monohy-
droxido and dihydroxido derivatives: (P1,OH) (P2,OH
)
(P1,OH)0 and (P2,OH)0, expressed by the rate constants k10 þ
k100 for the first oxidation stage and by the sum k20 þ k200 for
the second stage. In the subsequent faster steps, hydrogen
peroxide is coordinated to the Cr(III) center as hydroper-
oxido anion forming two types of redox active intermedi-
ates, I1,OOH and I2,OH,OOH which generate CrVO via the
inner-sphere two-electron transfer from the HOMO p*
Table 8 Dependence of kobs on
Complex
A
[H2O2], M
103 kobs (s-1 a
)
103 kobs1 (s-1 b
)
103 kobs2 (s-1 b
)
[H2O2] during oxidation of
[Cr(ox)2(Gln)]2- (A) and
[Cr(ox)2(Glu)]2- (B),
[Cr(ox)2(H2O)2]- (C) and
[Cr(ox)(H2O)4]? (D) in 0.6 M
NaOH at 298 K
0.2
0.4
0.6
0.8
1.0
0.2
0.4
0.6
0.8
1.0
0.2
0.8
1.0
0.2
2.04 0.04
2.20 0.05
2.40 0.08
2.54 0.06
2.70 0.06
1.94 0.05
2.01 0.12
2.08 0.06
2.20 0.05
2.36 0.09
0.873 0.014
0.894 0.012
0.918 0.022
6.28 0.06
6.52 0.12
7.34 0.16
8.22 0.17
9.93 0.31
7.24 0.22
9.07 0.37
8.45 0.41
7.41 0.22
9.02 0.26
2.36 0.01
2.56 0.01
2.60 0.01
2.70 0.01
2.69 0.01
2.20 0.01
2.14 0.01
2.28 0.01
2.50 0.02
2.50 0.01
B
C
D
51
4
a
Data calculated for A ? B model from spectra recorded at 300–1500 s for A and B and at 100–2500 s
for C
b
Data calculated for A ? B ? C model from spectra recorded at 60–2000 s for A and B
123

444
Transition Met Chem (2016) 41:435–445
Scheme 4 Mechanism of
oxidation of [Cr(ox)2(Aa)]2- by
H2O2 in NaOH medium
-
HO2
fast
-
[Cr(ox) (OH)(OOH)]3
2
-
[Cr(ox) (OH) ]3
2
2
(P
k2' slow
)
2,OH
(I
)
2,OH,OOH
-
-
HO2
[Cr(ox) (Aa')(OH)]3
-
[Cr(ox) (Aa')(OOH)]3
2
2
fast
V
3-
[Cr (O ) ]
2 4
(P
)
1,OH
(I
)
-
1,OOH
HO2
slow
k1'
fast
-
[Cr(ox) (Aa)]2
ket
CrVO
2
k1''
slow
fast
-
HO2
fast
[Cr(ox)(ox')(Aa)(OH)]3
(P
[Cr(ox)(ox')(Aa)(OOH)]3
(I )'
-
-
2-
CrO
4
)'
k2''
slow
1,OH
1,OOH
-
-
HO2
fast
[Cr(ox)(Aa)(OH) ]2
-
[Cr(ox)(Aa)(OH)(OOH)]2
(I )'
2
(P
)'
2,OH
2,OH,OOH
[Cr(ox)2(H2O)2]- is twice lower than that for [Cr(ox)2
(Aa)]2- (Table 8) though in this case chromium(III)-ox-
alato ring opening and then the ligand liberation is not a
necessary prerequisite for formation of the redox active
intermediates of the Cr(III)–OOH type. In this case, the
rate constant, ket, controls the rate of the overall oxidation
process.
hydroxido ligands are much higher. Thus, chromium(III)
complexes with four unblocked coordination sites are
oxidized rapidly by hydrogen peroxide in alkaline media.
Open Access This article is distributed under the terms of the
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made.
Very close values of the analogous kobs for [Cr(ox)2
(Aa)]2- studied and lower kobs values for [Cr(ox)2(H2O)2]-
imply that the presence of an amino acid in the inner
coordination sphere accelerates the electron transfer. The
much higher rate constant for oxidation of [Cr(ox)(H2O)4]?
(Table 8) comparable to that for chromates(III) [29]
demonstrates redox lability of chromium complexes with
four or more unblocked coordination sites.
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