# Reaction of Chromium(III) with 3,4-Dihydroxybenzoic Acid: Kinetics and Mechanism in Weak Acidic Aqueous Solutions

Authors: Kimon Zavitsanos; Konstantinos Tampouris; Athinoula L. Petrou
Journal: Bioinorganic Chemistry and Applications
 (2008)

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## Abstract

The interactions between chromium(III) and 3,4-dihydroxybenzoic acid (3,4-DHBA) were studied resulting in the formation of oxygen-bonded complexes upon substitution of water molecules in the chromium(III) coordination sphere. The experimental results show that the reaction takes place in at least three stages, involving various intermediates. The first stage was found to be linearly dependent on ligand concentrationk1(obs)_=k0+k1(obs)[3,4-DHBA], and the corresponding activation parameters were calculated as follows: ΔH1(obs)≠=51.2±11.5 kJ mol−1, ΔS1(obs)≠=−97.3±28.9 J mol−1K−1 (composite activation parameters) . The second and third stages, which are kinetically indistinguishable, do not depend on the concentrations of ligand and chromium(III), accounting for isomerization and chelation processes, respectively. The corresponding activation parameters are ΔH2(obs)≠=44.5±5.0kJ mol−1, ΔS2(obs)≠=−175.8±70.3 J mol−1K−1. The observed stages are proposed to proceed via interchange dissociative (Id, first stage) and associative (second and third stages) mechanisms. The reactions are accompanied by proton release, as is shown by the pH decrease.


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## Body

## 1. Introduction

The  ligand 3,4-dihydroxybenzoic
acid (3,4-DHBA) (Figure1), 1, is
known to be produced in the reaction of radicals, which are formed in
pathophysiological cases (e.g., ischemic stroke [1, 2], traumatic brain injury
[3], and Huntington’s disease [4]), the 4-hydroxybenzoic acid or salicylic acid
acting as radical-trapping agent [5]. It is used for assisting the Fenton
reaction in effluent treatment [6] and in dechlorination of polychlorinated
dioxins [7].Figure 1
3,4-DHBA.3,4-DHBA is employed in the
preparation of resins having antioxidant properties, acting as radical
scavenger [8] as well as in the preparation of composite polymer modified
electrodes [9]. It was also found to interfere
in the nucleation and crystal growth of radial alumina trihydrate particles
[10].Phenolic acids, in general, are
present in fruits and plants and are participating in the chemical structure of
humic substances, which can coordinate with nutrient ions, especially metal
ions, increasing thus their bioavailability [11–14]. 3,4-dihydroxybenzoic acid, in particular, bearing
both catecholic and carboxylic sites, shows special complexing properties.Cr(III), although
present in traces in biological systems, is considered to play a role in the
activation of some enzymes [15]. It has also been identified as a
reactive
component of an oligopeptide known as low-molecular-weight chromium binding
substance or chromoduline [16, 17]. Yet, the biological role of Cr(III) remains
mostly unclarified, one of the main reasons being the lack of intense
characteristics like charge-transfer bands in the spectra with the only
exception being the organochromium complexes [18, 19].In the present study,
the reaction of Cr(III) and 3,4-DHBA in weak acidic aqueous solutions is
investigated. The mechanism of the complex formation and the stability of the
complexes formed are studied and presented, in order to contribute in the
clarification of the role of complexation in the uptake of metals in various
biological systems (e.g., plants).


## 2. Experimental Results

### 2.1. Reagents and Materials

All reagents employed were of
analytical grade and were used as received. Aqueous solutions containing
3,4-DHBA (Alfa Aesar) in concentrations ranging from7.45×10−3 to 1.62×10−2M were prepared
using dilute (0.1 M) KOH solutions for pH adjustment in
order for the ligand to be dissolved. Stock solutions (0.2 M) of Cr(III) were prepared from Cr(NO3)2⋅9H2O.
The ionic strength was adjusted using KNO3. All solutions used in
the present study were freshly prepared in order to avoid side reactions
(transformation and decomposition). The addition of the Cr(III) solution kept
the pH below 4 due to its acidic hydrolysis.


### 2.2. Kinetic Experiments

Electronic spectra were recorded on
a Varian Cary 3E, UV-vis
spectrophotometer. The kinetic experiments were also followed at the above
instrument. All kinetic experiments were performed at pH values below 4 in the
presence of air.Pseudo-first-order conditions were
employed for most of the kinetic experiments. For the first stage of the
reactions which was studied at 279–295 K, the
duration of the step was ~16 000–5000 seconds,
the concentrations were of the order of10−2M for
the ligand and 10−3M for Cr(III), and plots of ln(At−A∞) against time (the absorbance is decreasing)
were set up, where At and A∞ are absorbances at time t and at infinite time
(after the completion of this step). The plots were found to be linear (Figure 2) for at least three half lives. The rate constants were calculated from the
constant slope of the line. At the higher temperatures where the second and
third stages were studied, the first stage being faster is not observed. For
the second and third (consecutive) stages which were studied at 303–323K, the
duration of the steps was ~20 000–8000 seconds,
the concentrations were of the order of 10−2M for the ligand and 10−3M for Cr(III), and plots of ln(A∞−At) against time (the absorbance is increasing)
were also found to be linear. This is the case of a polyfunctional compound
acting sequentially; a linear
plot of ln(A∞−At)=f(t) could be obtained when a biphasic reaction
takes place resembling to a single-stage reaction [20]. This could also happen
when k3≫k2 or k2≫k3.
In the case of our system, k2 could be a lot smaller than k3.
The k1 and k2(k2≪k3) values
at various temperatures are given in Table 1.Table 1
Values ofk1(obs) and k2(k2≪k3) at various temperatures.
Temperature (K)Rate constantsk1(obs)×102(M−1s−1)k2×104(s−1)2791.10—2862.52—2903.15—2953.74—303—0.91309—1.35313—1.54318—2.11323—3.11Figure 2
Typical
linear plot ofln(At−A∞) versus time at 279 K and [3,4-DHBA]=1.50×10−2M,
[Cr(III)]=2.50×10−3M, I
= 0.02 M.The
kinetics was followed
at various wavelengths yielding identical results though changes in absorbance
were in all cases small. Uncomplexed Cr(III) species does not interfere in the
(absorbance) measurements since it is included in bothAt and A∞ and is thus eliminated.The activation parameters (ΔH≠ and ΔS≠) were
calculated from the linear Eyring plots according to activated complex theory.
The activation parameters ΔH1(obs)≠ and ΔS1(obs)≠ corresponding to k1(obs) and ΔH2(obs)≠, ΔS2(obs)≠ corresponding to k2(k2≪k3)were thus estimated and presented in Table 2.Table 2
Activation parameters for
steps1(k1(obs)) and 2(k2≪k3).
ΔH1(obs)≠(kJmol−1)ΔS1(obs)≠(Jmol−1K−1)ΔH2(obs)≠(kJmol−1)ΔS2(obs)≠(Jmol−1K−1)51.2±11.5−97.3±28.944.5±5.0−175.8±70.3TheA∞ values were obtained from the kinetic
measurements and from the A=f(t)
plots (at the certain wavelengths) making it possible to check if the reaction
was run to completion.


## 2.1. Reagents and Materials

All reagents employed were of
analytical grade and were used as received. Aqueous solutions containing
3,4-DHBA (Alfa Aesar) in concentrations ranging from7.45×10−3 to 1.62×10−2M were prepared
using dilute (0.1 M) KOH solutions for pH adjustment in
order for the ligand to be dissolved. Stock solutions (0.2 M) of Cr(III) were prepared from Cr(NO3)2⋅9H2O.
The ionic strength was adjusted using KNO3. All solutions used in
the present study were freshly prepared in order to avoid side reactions
(transformation and decomposition). The addition of the Cr(III) solution kept
the pH below 4 due to its acidic hydrolysis.


## 2.2. Kinetic Experiments

Electronic spectra were recorded on
a Varian Cary 3E, UV-vis
spectrophotometer. The kinetic experiments were also followed at the above
instrument. All kinetic experiments were performed at pH values below 4 in the
presence of air.Pseudo-first-order conditions were
employed for most of the kinetic experiments. For the first stage of the
reactions which was studied at 279–295 K, the
duration of the step was ~16 000–5000 seconds,
the concentrations were of the order of10−2M for
the ligand and 10−3M for Cr(III), and plots of ln(At−A∞) against time (the absorbance is decreasing)
were set up, where At and A∞ are absorbances at time t and at infinite time
(after the completion of this step). The plots were found to be linear (Figure 2) for at least three half lives. The rate constants were calculated from the
constant slope of the line. At the higher temperatures where the second and
third stages were studied, the first stage being faster is not observed. For
the second and third (consecutive) stages which were studied at 303–323K, the
duration of the steps was ~20 000–8000 seconds,
the concentrations were of the order of 10−2M for the ligand and 10−3M for Cr(III), and plots of ln(A∞−At) against time (the absorbance is increasing)
were also found to be linear. This is the case of a polyfunctional compound
acting sequentially; a linear
plot of ln(A∞−At)=f(t) could be obtained when a biphasic reaction
takes place resembling to a single-stage reaction [20]. This could also happen
when k3≫k2 or k2≫k3.
In the case of our system, k2 could be a lot smaller than k3.
The k1 and k2(k2≪k3) values
at various temperatures are given in Table 1.Table 1
Values ofk1(obs) and k2(k2≪k3) at various temperatures.
Temperature (K)Rate constantsk1(obs)×102(M−1s−1)k2×104(s−1)2791.10—2862.52—2903.15—2953.74—303—0.91309—1.35313—1.54318—2.11323—3.11Figure 2
Typical
linear plot ofln(At−A∞) versus time at 279 K and [3,4-DHBA]=1.50×10−2M,
[Cr(III)]=2.50×10−3M, I
= 0.02 M.The
kinetics was followed
at various wavelengths yielding identical results though changes in absorbance
were in all cases small. Uncomplexed Cr(III) species does not interfere in the
(absorbance) measurements since it is included in bothAt and A∞ and is thus eliminated.The activation parameters (ΔH≠ and ΔS≠) were
calculated from the linear Eyring plots according to activated complex theory.
The activation parameters ΔH1(obs)≠ and ΔS1(obs)≠ corresponding to k1(obs) and ΔH2(obs)≠, ΔS2(obs)≠ corresponding to k2(k2≪k3)were thus estimated and presented in Table 2.Table 2
Activation parameters for
steps1(k1(obs)) and 2(k2≪k3).
ΔH1(obs)≠(kJmol−1)ΔS1(obs)≠(Jmol−1K−1)ΔH2(obs)≠(kJmol−1)ΔS2(obs)≠(Jmol−1K−1)51.2±11.5−97.3±28.944.5±5.0−175.8±70.3TheA∞ values were obtained from the kinetic
measurements and from the A=f(t)
plots (at the certain wavelengths) making it possible to check if the reaction
was run to completion.


## 3. Discussion

### 3.1. Kinetics and Mechanisms

The UV-vis spectra of
Cr(III) and 3,4-DHBA solutions of concentrations5.5×10−3M are
presented in Figure 3. At pH<4, where all the kinetic experiments were
conducted, Cr(III) exists mainly in the hexaaqua monomeric form. However,
reaction with Cr(H2O)5(OH)2+ should be considered
at the pH range 3 to 4, since a small amount of Cr(H2O)5(OH)2+ is present due to the equilibrium which is characterized by a pKa value (Cr3+/Cr(OH)2+) of about 4 [21].  The visible
spectrum of Cr(H2O)63+ exhibits two maxima in
the region of 410 and 575 nm (Figure 3) accounting for the A42g→T41g and the A42u→T42g Cr(III)
transitions, respectively.Figure 3
UV-vis spectra of
Cr(III) (spectrum 1) and 3,4-DHBA (spectrum 2) solutions of5.5×10−3M
concentrations and spectrophotometric cell path d=1cm.Under the applied experimental
conditions, the ligand exists as neutral molecule and monoanion [22, 23]
abbreviated as DHBA and DHBA−, respectively.
In the ligand molecules, intramolecular hydrogen bonding between the two
hydroxyl groups occurs,
favored by the formation of a five-membered ring.The acid ligand and theCr(H2O)63+ complex dissociation equilibria established are as follows: (1)DHBA⇌K1DHBA−+H+DHBA−⇌K2DHBA2−+H+DHBA2−⇌K3DHBA3−+H+Cr(H2O)63+⇌KaCr(H2O)5(OH)2++H+ΔHa,ΔSaThe corresponding pK
values for the above acid dissociation constants, at25°C, are given  pK1=4.5,pK2=8.7,pK3=12.8, and pKa=4.0.Upon mixing of the reactants,
violet Cr(III) and light-brown ligand solutions, formation of a light green
complex,2, assigned to be oxygen-bound Cr(III) compound takes place.
The substitution of water molecules in the Cr(III) coordination sphere by
3,4-DHBA molecule results in a change of the ligand field. Decrease in
absorbance at 575 nm is at first observed. The formation of 2 is
consistent with the formation and subsequent transformation (substitution)
kinetics. Experiments conducted at various temperatures (279, 286, 290, and 295 K) yielded identical kinetic behavior.However, the attacks by Cr(III)
can take place only by releasing protons because the hydroxyls and the
carboxylic groups are efficiently blocked (are protonated). This results in the
pH decrease of the solution (Figure4). Thus, the k1 pathway occurs
by attack of Cr(H2O)5OH2+ at the carboxylic
ligand group leading to a complex which, in acidic solutions, is protonated, 2.Figure 4
Typical pH
versus time plot of 3,4-DHBA/Cr(III) mixtures at 313 K. Conditions: [3,4-DHBA]0 = 1.62×10−2M,
[Cr(III)]0 = 2.50×10−3M, [KOH]=4.00×10−3M, I = 0.02 M.The kinetics of the first stage of
the reaction (k1(obs)′) followed a first-order rate law, k1(obs)′=k0+k1(obs)[3,4-DHBA].
The rate constant k0 corresponds to a reaction of Cr(III) when
[3,4-DHBA]=0.0, that is, to a reaction which does not involve the
ligand. This may be any reaction of Cr(III) with KOH or with other Cr(III)
ions, that is, an oligomerization reaction which always takes place in Cr(III)
aqueous solutions. This does not affect the reactions and the mechanism of the
reaction with 3,4-DHBA.That is, in order to obtain
information on how many ligands react in the first step, the dependence ofk1(obs)′ on the ligand concentration was studied (Figure 5). In previous studies [24, 25], where  the reactions of
Cr(III) with 3,4-dihydroxyphenylpropionic acid (dihydrocaffeic acid) and
3,4-dihydroxyphenylpropenoic acid (caffeic acid) were investigated kinetically,
the experimental data led to the
conclusion that the reactive form of the metal ion is Cr(OH)2+ and the reactive form of the ligand is the neutral species.Figure 5
Dependence
ofk1(obs)′ on ligand concentration at 279 K
and [Cr(III)]  =2.50×10−3M, I =
0.02 M.A dissociative  mechanism,Id is expected for step 1 since, in the case of the conjugate base Cr(H2O)5OH2+,
the dissociative mechanism Id is favored [26]. This is because of
the strong labilizing effect, which is induced by the coordinated OH−,
presumably, on the trans H2O molecule. This leads to a 102-103 fold enhanced reaction rate for the hydroxy-aqua over
the hexaaqua ion.In conclusion, for the first attack, a reaction
ofCr(OH)2+ can be proposed (Scheme 1). This would suggest an
inverse dependence on [H+] since [Cr(OH)2+]=Ka[Cr(aq)3+]/[H+],
which is not observed, implying that a protonation of the hydrogen-bonded
ligand molecule exists; a breakout of the hydrogen bonding must take place
before the attack by Cr(OH)2+. We suggest
attack on the –OH of the protonated
–COOH group (–HOC=OH+) since there the proton is loosely attached to
oxygen due to the fact that withdrawing of negative charge by the H+ of the C=OH+ is taking place. The protonation of the –COOH group
though in small extent makes the H of the OH more labile and so the attack by
Cr(H2O)5(OH)2+ is easier. The small extent of the protonation
of the –COOH group is the reason for the small yield of the reaction which is
expressed by the small absorbance change ΔA.Scheme 1Scheme1 presents the mechanism proposed, according to
the above experimental results.Therefore, it holds that Rate =k1[Cr(OH)2+][3,4-DHBAH+]=k1K0[3,4-DHBA][H+](Ka[Cr(aq)3+]/[H+])=k1K0Ka[3,4-DHBA][Cr(aq)3+]=k1(obs)[3,4-DHBA][Cr(aq)3+],
where k1(obs)=k1K0Ka.In Figure6, spectra of the
reaction mixture at 288 K are recorded at various times after mixing. The
decrease in absorbance at short reaction times and the increase in longer
reaction times suggest that the reaction is a multistep process. Spectra are shown only for the decrease in absorbance since at
longer reaction time the increase in absorbance that follows would cause
overlapping of the spectra.Figure 6
Typical UV-vis spectra of
3,4-DHBA/Cr(III) mixture at various times after mixing at 288 K. Conditions:
[3,4-DHBA]=6.80×10−3M,
[Cr(III)]=3.75×10−2M.The calculated negative value ofΔS1(obs)≠ would suggest an associative mechanism for the
first stage of the reaction. Cr(H2O)5OH2+,
however, as stated above, bears a dissociative, (Id), mechanism.
This suggests, as presented above, that a complex reaction is taking place and
composite activation parameters exist, that is, ΔH1(obs)≠=ΔH0+ΔHa+ΔH1≠ and ΔS1(obs)≠=ΔS0+ΔSa+ΔS1≠,
where ΔH0 and ΔS0 correspond to an equilibrium established prior
to step 1. ΔH1≠ and ΔS1≠ correspond to the first step (k1).
Therefore, the resulting negative value of ΔS≠ as well as the resulting value of ΔH≠ do not correspond only to step 1 reaction
which is actually taking place by an Id mechanism due to the
reactive species Cr(H2O)5OH2+.Dependence on ligand and Cr(III)
concentrations was studied in order to find if a second or third ligand
molecule or Cr(III) species enters the coordination sphere of the already
formed complex2. Therefore, the possibility of two consecutive steps
involving transformations of 2 to 3 and of 3 to 4(2→k23→k34) is investigated.The above scheme2→k23→k34 should give a biphasic reaction resembling a
single-stage reaction when a polyfunctional compound reacts sequentially [20].
Thus, in our case, the polyfunctional ligand 3,4-DHBA reacting with
Cr(III) gives the polyfunctional
compound 2 which reacts sequentially and
the biphasic reaction resembles a single-stage reaction giving a linear plot of ln(A∞−At)=f(t).  This could also happen when k3≫k2.The experimental results (Figure7)
clearly demonstrate that the concentrations of 3,4-DHBA and Cr(III) have no
effect on the observed rate constants k2(obs)(≪k3(obs))
in the concentration range studied. However, study at higher concentrations was
restricted due to the low solubility of 3,4-DHBA in weak acidic aqueous
solutions. Values of k2(obs)(≪k3(obs)) are
presented in Table 1 and the
corresponding ΔH2(obs)≠,ΔS2(obs)≠ values are listed in Table 2.Dependence ofk2(obs) upon (a) ligand concentration at [Cr(III)]=2.50×10−3M, I =
0.02 M, 313 K and (b) Cr(III) concentration at [3,4-DHBA]=7.45×10−3M, I = 0.5 M, 318 K.
(a)(b)The
fact that the second and third steps(k2,k3) were found
to be independent both on ligand and Cr(III) concentrations, that is, the rate exhibits a first-order dependence on the product of step 1, suggests
that transformations are taking place within the already formed complex, 2.The
activation parameters deduced from the temperature-dependence experiments may
be used for proposing structures of the activated complexes (Scheme2), and the
taking place mechanism.Scheme 2
Activated complexes2≠ and 3≠.The negative value ofΔS2≠,
the independence of k2(obs)(k2≪k3) on both ligand and Cr(III) concentrations,
and the increase in absorbance (i.e., of the extinction coefficients) led to
the assignment of the observed transformations as associatively activated
substitution of water molecules from the Cr(III) coordination sphere by
3,4-DHBA through isomerization and chelation in two consecutive steps (2→k23→k34). The pH decrease suggests a concomitant
proton release as proposed in the mechanism (Scheme 1). Scheme 1 shows that 2, the first formed
species, undergoes isomerization to give another oxygen-bonded species, 3.
Complex 3 chelates in a k3 step, (k2≪k3)
chelation, to produce 4, the final chelated complex [Cr(3,4-DHBA−3H)(H2O)4].
Isolation in the solid form of the final product gives a compound the elemental
analyses of which (15.4% C and 3.7% H) correspond to the formula [Cr(3,4-DHBA−3H)(H2O)4]⋅2KNO3⋅4H2O,
for which the calculated percentages for C and H are 15.3% and 3.5%,
respectively.Alternatively, the first attack
could occur at the hydroxyl group (stepk1) and a following
chelation could account for a second step (k2) only. We prefer
though the already suggested mechanism which is in accord with the mechanisms
of the reactions of Cr(III) with 3,4-dihydroxyphenylpropionic acid and
3,4-dihydroxyphenylpropenoic acid where the three stages (i.e., complexation, isomerization,
and chelation) were kinetically distinguishable [24, 25]. In our present case,
three steps are also suggested because of the necessity for isomerization prior
to chelation. Chelation between the groups carboxylic and phenolic is
unfavorable due to the formation of seven-membered ring. We exclude the
possibility of a reversible reaction, where the final absorbance could
correspond to Cr(III) and complex (in equilibrium) resulting in kobs=kf+kr based on the inertness of the various species
(small values of reaction rates) and the existence of further reactions which,
then, would shift an equilibrium well to the right.Another possibility is attack at
the carboxylic group and subsequent chelation at the same group through the
oxygens. We are discarding this alternative because a four-membered ring would
result if chelation at the carboxylic group took place whereas a more
preferable situation, that is, a five-membered ring is formed according to the
suggested mechanism.Since Cr,
Mo, and W are in the same group of the periodic table, the following comparison
can be made:[Cr(H2O)5(OH)]2+ in our system
reacts with a rate constant two orders of magnitude slower than that of [W3O4(H2O)9]4+ and three orders of magnitude slower than that of [Mo3O4(H2O)9]4+ indicating an extremely less labile species [27]. The comparison is made for
substitution reactions of the conjugate base forms of the above aqua ions.


### 3.2. Structure of the Complexes—Mode of Binding

In the
UV-vis spectra, the
changes are caused only by the change of the ligand field. Since oxidation does
not take place under the experimental conditions, only a shift of the maximum
and disappearance of the shoulder due to the high absorbtivities in the UV
region of the produced complexes take place.A 1:1 stoichiometry for the
reaction of Cr(III) with 3,4-DHBA is proposed because of the observedk1 dependence on ligand concentration. The elemental analyses of the isolated
final product also suggest a 1:1 stoichiometry. The consecutive isomerization and
chelation reactions taking place in the Cr(III) center do not cause any change
in stoichiometry.The proposed structures of the
activated complexes2≠ and 3≠ are given in Scheme 2. Associative mechanism has been supported to operate in
reactions of Cr(III) [18, 28, 29]. In the above suggested mechanism, the
phenolic groups act as internal attacking groups to the H2O
molecules of the Cr(III) coordination sphere supplying thus a proton, which is
then released as H3O+ (Scheme 1).The driving force for the
production of the final chelated product4 must be the most stabilized chelated
form compared to forms 2 or 3.The suggested catecholic mode of
binding was also found to operate in the coordination complexes of
3,4-dihydroxyphenylpropionic acid (dihydrocaffeic acid) and
3,4-dihydroxyphenylpropenoic acid (caffeic acid) with Cr(III) [24, 25] as already mentioned. This type of binding
was also reported for complexes of dihydrocaffeic, caffeic, and ferulic acids
with Co(II), Ni(II), Cu(II), Fe(III), Mn(II), Mn(III), V(V), V(IV,V), and Zn(II)
[22, 23, 30–32]. Catecholic
type of coordination was also suggested for the Fe(III)-2,3-DHBA complex
[33, 34].


## 3.1. Kinetics and Mechanisms

The UV-vis spectra of
Cr(III) and 3,4-DHBA solutions of concentrations5.5×10−3M are
presented in Figure 3. At pH<4, where all the kinetic experiments were
conducted, Cr(III) exists mainly in the hexaaqua monomeric form. However,
reaction with Cr(H2O)5(OH)2+ should be considered
at the pH range 3 to 4, since a small amount of Cr(H2O)5(OH)2+ is present due to the equilibrium which is characterized by a pKa value (Cr3+/Cr(OH)2+) of about 4 [21].  The visible
spectrum of Cr(H2O)63+ exhibits two maxima in
the region of 410 and 575 nm (Figure 3) accounting for the A42g→T41g and the A42u→T42g Cr(III)
transitions, respectively.Figure 3
UV-vis spectra of
Cr(III) (spectrum 1) and 3,4-DHBA (spectrum 2) solutions of5.5×10−3M
concentrations and spectrophotometric cell path d=1cm.Under the applied experimental
conditions, the ligand exists as neutral molecule and monoanion [22, 23]
abbreviated as DHBA and DHBA−, respectively.
In the ligand molecules, intramolecular hydrogen bonding between the two
hydroxyl groups occurs,
favored by the formation of a five-membered ring.The acid ligand and theCr(H2O)63+ complex dissociation equilibria established are as follows: (1)DHBA⇌K1DHBA−+H+DHBA−⇌K2DHBA2−+H+DHBA2−⇌K3DHBA3−+H+Cr(H2O)63+⇌KaCr(H2O)5(OH)2++H+ΔHa,ΔSaThe corresponding pK
values for the above acid dissociation constants, at25°C, are given  pK1=4.5,pK2=8.7,pK3=12.8, and pKa=4.0.Upon mixing of the reactants,
violet Cr(III) and light-brown ligand solutions, formation of a light green
complex,2, assigned to be oxygen-bound Cr(III) compound takes place.
The substitution of water molecules in the Cr(III) coordination sphere by
3,4-DHBA molecule results in a change of the ligand field. Decrease in
absorbance at 575 nm is at first observed. The formation of 2 is
consistent with the formation and subsequent transformation (substitution)
kinetics. Experiments conducted at various temperatures (279, 286, 290, and 295 K) yielded identical kinetic behavior.However, the attacks by Cr(III)
can take place only by releasing protons because the hydroxyls and the
carboxylic groups are efficiently blocked (are protonated). This results in the
pH decrease of the solution (Figure4). Thus, the k1 pathway occurs
by attack of Cr(H2O)5OH2+ at the carboxylic
ligand group leading to a complex which, in acidic solutions, is protonated, 2.Figure 4
Typical pH
versus time plot of 3,4-DHBA/Cr(III) mixtures at 313 K. Conditions: [3,4-DHBA]0 = 1.62×10−2M,
[Cr(III)]0 = 2.50×10−3M, [KOH]=4.00×10−3M, I = 0.02 M.The kinetics of the first stage of
the reaction (k1(obs)′) followed a first-order rate law, k1(obs)′=k0+k1(obs)[3,4-DHBA].
The rate constant k0 corresponds to a reaction of Cr(III) when
[3,4-DHBA]=0.0, that is, to a reaction which does not involve the
ligand. This may be any reaction of Cr(III) with KOH or with other Cr(III)
ions, that is, an oligomerization reaction which always takes place in Cr(III)
aqueous solutions. This does not affect the reactions and the mechanism of the
reaction with 3,4-DHBA.That is, in order to obtain
information on how many ligands react in the first step, the dependence ofk1(obs)′ on the ligand concentration was studied (Figure 5). In previous studies [24, 25], where  the reactions of
Cr(III) with 3,4-dihydroxyphenylpropionic acid (dihydrocaffeic acid) and
3,4-dihydroxyphenylpropenoic acid (caffeic acid) were investigated kinetically,
the experimental data led to the
conclusion that the reactive form of the metal ion is Cr(OH)2+ and the reactive form of the ligand is the neutral species.Figure 5
Dependence
ofk1(obs)′ on ligand concentration at 279 K
and [Cr(III)]  =2.50×10−3M, I =
0.02 M.A dissociative  mechanism,Id is expected for step 1 since, in the case of the conjugate base Cr(H2O)5OH2+,
the dissociative mechanism Id is favored [26]. This is because of
the strong labilizing effect, which is induced by the coordinated OH−,
presumably, on the trans H2O molecule. This leads to a 102-103 fold enhanced reaction rate for the hydroxy-aqua over
the hexaaqua ion.In conclusion, for the first attack, a reaction
ofCr(OH)2+ can be proposed (Scheme 1). This would suggest an
inverse dependence on [H+] since [Cr(OH)2+]=Ka[Cr(aq)3+]/[H+],
which is not observed, implying that a protonation of the hydrogen-bonded
ligand molecule exists; a breakout of the hydrogen bonding must take place
before the attack by Cr(OH)2+. We suggest
attack on the –OH of the protonated
–COOH group (–HOC=OH+) since there the proton is loosely attached to
oxygen due to the fact that withdrawing of negative charge by the H+ of the C=OH+ is taking place. The protonation of the –COOH group
though in small extent makes the H of the OH more labile and so the attack by
Cr(H2O)5(OH)2+ is easier. The small extent of the protonation
of the –COOH group is the reason for the small yield of the reaction which is
expressed by the small absorbance change ΔA.Scheme 1Scheme1 presents the mechanism proposed, according to
the above experimental results.Therefore, it holds that Rate =k1[Cr(OH)2+][3,4-DHBAH+]=k1K0[3,4-DHBA][H+](Ka[Cr(aq)3+]/[H+])=k1K0Ka[3,4-DHBA][Cr(aq)3+]=k1(obs)[3,4-DHBA][Cr(aq)3+],
where k1(obs)=k1K0Ka.In Figure6, spectra of the
reaction mixture at 288 K are recorded at various times after mixing. The
decrease in absorbance at short reaction times and the increase in longer
reaction times suggest that the reaction is a multistep process. Spectra are shown only for the decrease in absorbance since at
longer reaction time the increase in absorbance that follows would cause
overlapping of the spectra.Figure 6
Typical UV-vis spectra of
3,4-DHBA/Cr(III) mixture at various times after mixing at 288 K. Conditions:
[3,4-DHBA]=6.80×10−3M,
[Cr(III)]=3.75×10−2M.The calculated negative value ofΔS1(obs)≠ would suggest an associative mechanism for the
first stage of the reaction. Cr(H2O)5OH2+,
however, as stated above, bears a dissociative, (Id), mechanism.
This suggests, as presented above, that a complex reaction is taking place and
composite activation parameters exist, that is, ΔH1(obs)≠=ΔH0+ΔHa+ΔH1≠ and ΔS1(obs)≠=ΔS0+ΔSa+ΔS1≠,
where ΔH0 and ΔS0 correspond to an equilibrium established prior
to step 1. ΔH1≠ and ΔS1≠ correspond to the first step (k1).
Therefore, the resulting negative value of ΔS≠ as well as the resulting value of ΔH≠ do not correspond only to step 1 reaction
which is actually taking place by an Id mechanism due to the
reactive species Cr(H2O)5OH2+.Dependence on ligand and Cr(III)
concentrations was studied in order to find if a second or third ligand
molecule or Cr(III) species enters the coordination sphere of the already
formed complex2. Therefore, the possibility of two consecutive steps
involving transformations of 2 to 3 and of 3 to 4(2→k23→k34) is investigated.The above scheme2→k23→k34 should give a biphasic reaction resembling a
single-stage reaction when a polyfunctional compound reacts sequentially [20].
Thus, in our case, the polyfunctional ligand 3,4-DHBA reacting with
Cr(III) gives the polyfunctional
compound 2 which reacts sequentially and
the biphasic reaction resembles a single-stage reaction giving a linear plot of ln(A∞−At)=f(t).  This could also happen when k3≫k2.The experimental results (Figure7)
clearly demonstrate that the concentrations of 3,4-DHBA and Cr(III) have no
effect on the observed rate constants k2(obs)(≪k3(obs))
in the concentration range studied. However, study at higher concentrations was
restricted due to the low solubility of 3,4-DHBA in weak acidic aqueous
solutions. Values of k2(obs)(≪k3(obs)) are
presented in Table 1 and the
corresponding ΔH2(obs)≠,ΔS2(obs)≠ values are listed in Table 2.Dependence ofk2(obs) upon (a) ligand concentration at [Cr(III)]=2.50×10−3M, I =
0.02 M, 313 K and (b) Cr(III) concentration at [3,4-DHBA]=7.45×10−3M, I = 0.5 M, 318 K.
(a)(b)The
fact that the second and third steps(k2,k3) were found
to be independent both on ligand and Cr(III) concentrations, that is, the rate exhibits a first-order dependence on the product of step 1, suggests
that transformations are taking place within the already formed complex, 2.The
activation parameters deduced from the temperature-dependence experiments may
be used for proposing structures of the activated complexes (Scheme2), and the
taking place mechanism.Scheme 2
Activated complexes2≠ and 3≠.The negative value ofΔS2≠,
the independence of k2(obs)(k2≪k3) on both ligand and Cr(III) concentrations,
and the increase in absorbance (i.e., of the extinction coefficients) led to
the assignment of the observed transformations as associatively activated
substitution of water molecules from the Cr(III) coordination sphere by
3,4-DHBA through isomerization and chelation in two consecutive steps (2→k23→k34). The pH decrease suggests a concomitant
proton release as proposed in the mechanism (Scheme 1). Scheme 1 shows that 2, the first formed
species, undergoes isomerization to give another oxygen-bonded species, 3.
Complex 3 chelates in a k3 step, (k2≪k3)
chelation, to produce 4, the final chelated complex [Cr(3,4-DHBA−3H)(H2O)4].
Isolation in the solid form of the final product gives a compound the elemental
analyses of which (15.4% C and 3.7% H) correspond to the formula [Cr(3,4-DHBA−3H)(H2O)4]⋅2KNO3⋅4H2O,
for which the calculated percentages for C and H are 15.3% and 3.5%,
respectively.Alternatively, the first attack
could occur at the hydroxyl group (stepk1) and a following
chelation could account for a second step (k2) only. We prefer
though the already suggested mechanism which is in accord with the mechanisms
of the reactions of Cr(III) with 3,4-dihydroxyphenylpropionic acid and
3,4-dihydroxyphenylpropenoic acid where the three stages (i.e., complexation, isomerization,
and chelation) were kinetically distinguishable [24, 25]. In our present case,
three steps are also suggested because of the necessity for isomerization prior
to chelation. Chelation between the groups carboxylic and phenolic is
unfavorable due to the formation of seven-membered ring. We exclude the
possibility of a reversible reaction, where the final absorbance could
correspond to Cr(III) and complex (in equilibrium) resulting in kobs=kf+kr based on the inertness of the various species
(small values of reaction rates) and the existence of further reactions which,
then, would shift an equilibrium well to the right.Another possibility is attack at
the carboxylic group and subsequent chelation at the same group through the
oxygens. We are discarding this alternative because a four-membered ring would
result if chelation at the carboxylic group took place whereas a more
preferable situation, that is, a five-membered ring is formed according to the
suggested mechanism.Since Cr,
Mo, and W are in the same group of the periodic table, the following comparison
can be made:[Cr(H2O)5(OH)]2+ in our system
reacts with a rate constant two orders of magnitude slower than that of [W3O4(H2O)9]4+ and three orders of magnitude slower than that of [Mo3O4(H2O)9]4+ indicating an extremely less labile species [27]. The comparison is made for
substitution reactions of the conjugate base forms of the above aqua ions.


## 3.2. Structure of the Complexes—Mode of Binding

In the
UV-vis spectra, the
changes are caused only by the change of the ligand field. Since oxidation does
not take place under the experimental conditions, only a shift of the maximum
and disappearance of the shoulder due to the high absorbtivities in the UV
region of the produced complexes take place.A 1:1 stoichiometry for the
reaction of Cr(III) with 3,4-DHBA is proposed because of the observedk1 dependence on ligand concentration. The elemental analyses of the isolated
final product also suggest a 1:1 stoichiometry. The consecutive isomerization and
chelation reactions taking place in the Cr(III) center do not cause any change
in stoichiometry.The proposed structures of the
activated complexes2≠ and 3≠ are given in Scheme 2. Associative mechanism has been supported to operate in
reactions of Cr(III) [18, 28, 29]. In the above suggested mechanism, the
phenolic groups act as internal attacking groups to the H2O
molecules of the Cr(III) coordination sphere supplying thus a proton, which is
then released as H3O+ (Scheme 1).The driving force for the
production of the final chelated product4 must be the most stabilized chelated
form compared to forms 2 or 3.The suggested catecholic mode of
binding was also found to operate in the coordination complexes of
3,4-dihydroxyphenylpropionic acid (dihydrocaffeic acid) and
3,4-dihydroxyphenylpropenoic acid (caffeic acid) with Cr(III) [24, 25] as already mentioned. This type of binding
was also reported for complexes of dihydrocaffeic, caffeic, and ferulic acids
with Co(II), Ni(II), Cu(II), Fe(III), Mn(II), Mn(III), V(V), V(IV,V), and Zn(II)
[22, 23, 30–32]. Catecholic
type of coordination was also suggested for the Fe(III)-2,3-DHBA complex
[33, 34].


## 4. Conclusions

In the present study, the reaction
between Cr(III) and 3,4-DHBA in weak acidic aqueous solutions was investigated.
The experimental results are consistent with a three-step mechanism in which an
initial attack (step 1) between the acid molecule (ligand) and theCr(H2O)5OH2+ complex giving a carboxylate bound Cr(III), 2, is followed by two
consecutive kinetically indistinguishable nonligand and non-Cr(III) dependent
steps. The two consecutive steps are assigned as isomerization and chelation
step (steps 2 and 3). The reactions are followed by a pH decrease because
proton release is taking place, according to an associative mode of activation
(steps 2 and 3).The negative value of the entropy
of activation of step 2, the independence on ligand and Cr(III) concentrations,
the increase of the extinction coefficients, and the pH decrease due to release
of protons upon complexation led to the proposed mechanism (Scheme1). The
observed transformations were assigned as substitution of water molecules from
the coordination sphere of Cr(III) by the ligand through complexation (step 1)
following an Id mechanism, isomerization, and chelation in two
consecutive kinetically indistinguishable steps supported to follow associative
mechanisms (step 2 and step 3).



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*Source: 212461-2009-02-04.xml*