Kinetics and mechanism of the reduction of chromium(VI) and chromium(V) by D-glucitol and D-mannitol

Article abstract of DOI:10.1002/1522-2675(20001220)83:12<3211::AID-HLCA3211>3.0.CO;2-4

The oxidation of D-glucitol and D-mannitol by Cr^VI yields the aldonic acid (and/or the aldonolactone) and Cr^III as final products when an excess of alditol over Cr^VI is used. The redox reaction occurs through a Cr^VI → Cr^V → Cr^III path, the Cr^VI → Cr^V reduction being the slow redox step. The complete rate laws for the redox reactions are expressed by: a) - d[Cr^VI]/dt = {k_M2H [H^-]² +k_MH [H⁺]}[mannitol][Cr^VI], where k_M2H = (6.7 ± 0.3) · 10^-2 M^-3 S^-1 and k_MH = (9±2) · 10^-3 M^-2 S^-1: b) - d [Cr^VI] /dt = {k_G2H[H⁺]² + k^GH [H⁺]}[GLUCITOL][Cr^VI]. where k_G2H = (8.5 ± 0.2) · 10^-2 M^-3 S^-1 and k_GH = (1.8 ± 0.1) · 10^-2 M^-2S^-1 at 33⁰. The slow redox steps are preceded by the formation of a Cr^VI oxy ester with λ_MAX 371 nm. at pH 4.5. In acid medium, intermediate Cr^V reacts with the substrate faster than Cr^VI does. The EPR spectra show that five- and six-coordinate oxo-Cr^V intermediates are formed, with the alditol or the aldonic acid acting as bidentate ligands. Pentacoordinate oxo-Cr^V species are present at any [H⁺], whereas hexacoordinate ones are observed only at pH < 2 and become the dominant species under stronger acidic conditions where rapid decomposition to the redox products occurs. At higher pH, where hexacoordinate oxo-Cr^V species are not observed, Cr^V complexes are stable enough to remain in solution for several days to months.

Full text of DOI:10.1002/1522-2675(20001220)83:12<3211::AID-HLCA3211>3.0.CO;2-4

Helvetica Chimica Acta ± Vol. 83 (2000)
3211
Kinetics and Mechanism of the Reduction of Chromium(VI) and
Chromium(V) by d-Glucitol and d-Mannitol
a
b
c
b
by Viviana P. Rold a n ), Ver o nica A. Daier ), Bernard Goodman ), Mabel I. Santoro ), Juan Carlos
b
b
b
b
Gonz a lez ), Nancy Calisto ), Sandra R. Signorella )*, Luis F. Sala )*
a) Departamento de Ciencias B a sicas, Facultad de Ciencias Veterinarias, UNL, R.P. Kreder 735,
3080 Esperanza, Argentina
b) Departamento de Química, Facultad de Ciencias Bioquímicas y Farmac e uticas, UNR, Suipacha 531,
2
000 Rosario, Argentina
c) Scottish Crop Research Institute, Invergowrie, Dundee, Scotland DD2 5DA
The oxidation of d-glucitol and d-mannitol by CrVI yields the aldonic acid (and/or the aldonolactone) and
III
VI
VI
Cr as final products when an excess of alditol over Cr is used. The redox reaction occurs through a Cr
!
V
III
VI
V
Cr ! Cr path, the Cr ! Cr reduction being the slow redox step. The complete rate laws for the redox
VI
‡
2
‡
VI
reactions are expressed by: a) � d[Cr ]/dt ˆ {kM2 H [H ] ‡ kMH [H ]}[mannitol][Cr ], where kM2 H ˆ (6.7 Æ
�
2
� 3 � 1
� 3
� 2 � 1
VI
‡
2
‡
VI
0
.3) ´ 10
m
s
and kMH ˆ (9 Æ 2) ´ 10
m
s
; b) � d[Cr ]/dt ˆ {kG2 H [H ] ‡ kGH [H ]}[glucitol][Cr ],
� 2 � 2 � 1
�
2
� 3 � 1
where kG2 H ˆ (8.5 Æ 0.2) ´ 10
m
s
and kGH ˆ (1.8 Æ 0.1) ´ 10
m
s
, at 338. The slow redox steps are
VI
V
preceded by the formation of a Cr oxy ester with lmax 371 nm, at pH 4.5. In acid medium, intermediate Cr
reacts with the substrate faster than CrVI does. The EPR spectra show that five- and six-coordinate oxo-CrV
intermediates are formed, with the alditol or the aldonic acid acting as bidentate ligands. Pentacoordinate oxo-
V
‡
Cr species are present at any [H ], whereas hexacoordinate ones are observed only at pH < 2 and become the
dominant species under stronger acidic conditions where rapid decomposition to the redox products occurs. At
higher pH, where hexacoordinate oxo-Cr species are not observed, CrV complexes are stable enough to remain
in solution for several days to months.
V
VI
1
. Introduction. ± Chromium(VI) (Cr ) is a potential hazard both in a biological
V
IV
and an ecological context [1]. The observation of Cr and Cr intermediates in the
selective oxidation of organic substrates by Cr and their implication in the mechanism
VI
of Cr-induced cancers [2 ± 3] has generated a considerable amount of interest in their
chemistry and biochemistry [4 ± 7]. The biological reduction of CrVI to lower states has
been observed with a wide variety of naturally occurring cellular reductants [8 ± 12].
Ligands that possess two O-atoms able to form five-membered rings about the metal
ion, such as 1,2-diols and a-hydroxy acids, are effective as nonenzymatic reductants and
complexation agents towards hypervalent chromium and can stabilize the labile
oxidation states of chromium [13 ± 19]. For this reason, it is interesting to look at the
VI
III
ability of polyhydroxy compounds to reduce Cr to Cr , in order to know the role they
VI
may play in the chemistry of Cr .
VI
V
We are studying the possible fate of Cr and Cr in biological systems by examining
VI
reactions of Cr with low-molecular molecules [20 ± 34]. Our studies on the reduc-
VI
V
tion of Cr and intermediate Cr by aldoses [20][30][31][33], deoxyaldoses
20][23][25][30][31], sugar acids [22][27][29], and methyl glycosides [34] showed
[
that the relative redox reactivities of these saccharides toward chromate is based on the
relative rates of the oxidation vs. complexation processes [31][32]. Under conditions of

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Helvetica Chimica Acta ± Vol. 83 (2000)
VI
excess reductant over Cr , aldoses, deoxyaldoses, and sugar acids are selectively
oxidized at OH� C(1), and in methyl glycosides, where the anomeric position is
blocked, the oxidation takes place at OH� C(6). These results reflect the inertness of
VI
the secondary OH groups of saccharides to be oxidized by Cr , as compared to the
hemiacetal and primary OH groups. We decided to evaluate the oxidation of alditols
(
ˆsugar alcohols), in order to determine whether the selectivity of the oxidation of
VI
cyclic polyhydroxy compounds by Cr (inertness of the secondary OH group) is
retained in open-chain polyols. Alditols constitute a group of compounds closely
related to sugars and widely distributed in microorganisms, fungi, animals, and plants
[
35]. Among the physiological functions attributed to alditols are structural functions,
energy storage, coenzyme regulation, and sugar interconversion [36]. Hexitols are the
most common alditols, with mannitol being the most abundant sugar found in over 100
different species of higher plants [37]. d-Glucitol is a normal metabolite in mammals
and is used frequently in the industrial manufacture of food ingredients [38][39].
In this work, we study the CrVI oxidation of d-glucitol (Glc-ol) and d-mannitol
(
Man-ol), and provide information on the mechanism as well as on the structure of the
V
intermediate Cr species.
2
. Results and Discussion. ± 2.1. Rate Studies: CrVI Oxidation of Glc-ol and Man-ol.
Over the whole range of perchloric acid concentrations used in the kinetic measure-
VI
ments, UV/VIS studies show that the reaction of alditols with Cr result in an
absorbance band with lmax 350 nm, and the rate constants are calculated from the
absorbance changes at this wavelength. The kinetic traces show an initial deviation
V
from first-order decay over very short time periods, probably due to Cr species that
VI
also absorb at 350 nm and may superimpose the Cr absorbance [40]. However, it is
V
VI
known that when Cr reacts faster than Cr and exists in solution in a sufficiently small
VI
concentration, changes in absorbance at 350 nm essentially reflect changes in Cr
concentration ± except for the short initial period ± and the rate constants can be
calculated from the linear part of the In(Abs) vs. time curves. To avoid a wrong
interpretation of spectrophotometric absorbance decay values due to superimposition
V
VI
V
of Cr absorbance, we determined the relative values of the Cr vs. Cr reduction rate
constants (k vs. k ) from spectrophotometric data at 750 nm (see below), and we found
6
5
‡
that k ꢀ k , in the range of [H ] used in the kinetic measurements. Besides, we
5
6
III
followed the formation of Cr at 570 nm in the HClO concentration range 0.1 ± 0.8m to
4
III
VI
determine the rate of Cr formation relative to that of the Cr consumption. The
pseudo-first-order rate constants obtained at this wavelength are coincident with those
VI
‡
obtained from the Cr decay at 350 nm, with the same dependence on [H ] (Table 1).
III
VI
This means that Cr forms as fast as Cr is consumed, and that the slow redox path
effectively implies the reduction of Cr , which should react slower than Cr does. Thus,
VI
V

Helvetica Chimica Acta ± Vol. 83 (2000)
3213
pseudo-first-order rate constants (k ) calculated from the slopes of the linear part of the
6
VI
In(Abs) vs. time curves correspond to the Cr consumption. Table 1 summarizes values
of k for various concentrations of Glc-ol and Man-ol at different concentrations of
6
HClO . For both alditols, plots of k vs. [Ald-ol] (Figs. 1 and 2) give good straight lines
4
6
from which values of k and k can be determined (Table 1). Insets in Fig. 1 and 2 show
G
M
the dependence of k and k on [HClO ], which may be expressed by Eqn. 1, where
G
M
4
�
2
� 3 � 1
� 2
� 2 � 1
kG2 H ˆ (8.5 Æ 0.2) ´ 10
m
s
and kGH ˆ (1.8 Æ 0.1) ´ 10
m
s
for Glc-ol, and kM2 H
�
2
� 3 � 1
� 3
� 2 � 1
VI
ˆ
(6.7 Æ 0.3) ´ 10
m
s
and kMH (9 Æ 2) ´ 10
m
s
for Man-ol. The complete rate
law for the reaction of Glc-ol and Man-ol with Cr is then given by Eqn. 2.
‡
2
‡
kG(M) ˆ kG2 H(M2 H) [H ] ‡ k
[H ]
(1)
(2)
GH(MH)
VI
VI
‡
2
‡
VI
�
d[Cr ]/dt ˆ k [Cr ] ˆ {k
[H ] ‡ k [H ]}[Ald-ol] [Cr ]
6
T
A2 H
AH
VI
2
.2. Detection of the Intermediate Cr Ester. Differential UV/Vis spectra of
VI
mixtures of Cr and Glc-ol or Man-ol exhibit an absorption band at l
371 nm
max
VI
(
Fig. 3) consistent with that ascribed to Cr oxy esters [41][42]. At pH 4.5, the redox
VI
reaction of Cr with the alditols studied proceeds very slowly, with negligible reduction
of Cr within the first hour. Thus, at this pH, the ester-formation step can be
VI
distinguished clearly from the electron-transfer reaction. Spectra obtained within 20 s
Table 1. Observed Pseudo-First-Order Rate Constants (k
6
)a) for Different Concentrations of HClO
4
and Alditol. T 338;
VI
ˆ 6 ´ 10� 4 m; I ˆ 1m.
[
Cr ]
0
4
� 1
3
� 1 � 1
[
HClO
4
]/m
10 k
6
/s for [Glc-ol]
10 k
G
/m
s
0
.06m
0.12m
0.18m
0.21m
0.24m
0.3m
l 350 nm
l 570 nmb)
0
0
0
0
0
0
0
.10
.20
.30
.40
.50
.60
.75
1.3(1)
2.7(1)
8.2(1)
2.25(5)
7.75(5)
15.6(1)
25.5(2)
40.0(2)
54.1(9)
76(2)
4.0(1)
4.9(1)
5.8(1)
18.2(1)
31.6(7)
50.4(6)
75.9(8)
103(1)
146(1)
7.7(1)
19.4(1)
36.4(1)
57.6(5)
87.5(5)
120(2)
180(2)
7.2(2)
20.3(6)
34.6(5)
51(2)
82(2)
±
2.4(1)
6.8(2)
12.9(3)
20.3(4)
31.0(7)
41.6(9)
61(1)
12.7(1)
24.6(7)
37.0(3)
60.0(8)
71(2)
13.9(1)
28.1(5)
44.3(7)
64.8(7)
90(2)
14.1(1)
19.0(1)
27.3(4)
39.1(5)
4
107(1)
133(3)
±
� 1
3
� 1 � 1
[
HClO
4
]/m
10 k
6
/s for [Man-ol]
10 k
M
/m
s
0
.06m
0.12m
0.18m
0.24m
0.30m
l 350 nm
l 570 nmb)
l 350 nm
l 570 nmb)
0
0
0
0
0
0
0
0
.10
.20
.30
.40
.50
.60
.75
.80
1.55(1)
3.30(1)
6.01(2)
9.50(1)
13.1(1)
20.0(2)
25.2(3)
33.1(2)
1.95(5)
5.55(5)
11.7(1)
17.9(2)
26.3(4)
38.1(3)
55.3(3)
66.2(4)
2.65(5)
6.95(5)
17.0(1)
25.0(6)
38.0(9)
58.6(5)
79(1)
4.65(5)
11.0(2)
20.1(1)
34.8(9)
50(1)
77(2)
106(2)
121(2)
5.1(8)
10.8(9)
20.1(9)
33.4(2)
50.3(3)
71.0(2)
110(2)
124(1)
6.7(1)
14.4(3)
24.3(2)
45(1)
61(2)
86(2)
6.9(3)
13.7(1)
24.6(9)
40.5(9)
60.4(6)
85(2)
2.0(1)
4.6(2)
8.6(3)
14.7(2)
20.8(4)
30.7(9)
44(1)
128(2)
151(4)
129(3)
149(5)
93(2)
51(1)
a) Mean values from multiple determinations. b) [CrVI] ˆ 6 ´ 10� 3 m.

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Helvetica Chimica Acta ± Vol. 83 (2000)
‡
Fig. 1. Effect of [Glc-ol] on k
6
at 338, I ˆ 1.00m, and [H ]/m: a) 0.10, b) 0.20, c) 0.30, d) 0.40, e) 0.50, f) 0.60, and
G
g) 0.75. Inset: Effect of acidity on k .
after mixing show a distinctive absorption band at 371 nm. Continued scanning for
0 min reveals no further change in the spectra. Varying the excess concentration of
3
Glc-ol or Man-ol at pH4.5 shows that the absorbance at 371 nm increases with
increasing concentration of alditol, probably as a result of a shift toward the ester in the
esterification equilibrium.
V
VI
2
.3. Intermediacy of Cr . The EPR spectra of mixtures of Man-ol or Glc-ol and Cr
V
‡
in 0.1 ± 0.8m HClO show the formation of several intermediate Cr species. In this [H ]
4
range, the EPR spectra consist of a minor signal at giso ˆ 1.9793 ± which is a composite of

Helvetica Chimica Acta ± Vol. 83 (2000)
3215
‡
Fig. 2. Effect of [Man-ol] on k
6
at 338, I ˆ 1.00m and [H ]/m: a) 0.10, b) 0.20, c) 0.30, d) 0.40, e) 0.50, f) 0.60, g)
0
M
.75, and h) 0.80. Inset: Effect of acidity on k .
V
several Cr species, as discussed below ± and a major signal at g ˆ 1.9718 (or 1.9719,
iso
resp.). The signal at lower giso is not observed at pH > 1 where the electron-transfer
V
reaction becomes extremely slow and the Cr species with g ˆ 1.9793 remain a long
iso
‡
time in solution. By contrast, in the [H ] range used in the kinetics measurements, the
V
Cr EPR signal intensities grow and decay rapidly.
We investigated the kinetics of the formation and disappearance of CrV
V
spectrophotometrically, at 750 nm, in 0.1 ± 0.5m HClO . At this wavelength, only Cr
absorbs, and the experimentally observed intermediate Cr growth and decay curves
can be fitted to Eqn. 3, where e refers to the Cr molar absorptivity at this wavelength
4
V
V
V

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Helvetica Chimica Acta ± Vol. 83 (2000)
VI
Fig. 3. UV/VIS Difference spectra of Cr /Glc-ol solutions at pH 4.5, showing the increasing band at 371 nm with
VI
� 4
increasing [Glc-ol]: a) 0.12, b) 0.18, c) 0.24, and d) 0.30m. [Cr ] ˆ 6 ´ 10 m, T 338, I ˆ 1m. Spectra taken after
0 min.
2
V
�
� 1
� 1
and is assumed to be the same as for the complex [Cr (O)(ehba) ] (e ˆ 38 m cm ;
2
ehba ˆ 2-ethyl-2-hydroxybutanoato(2� )) [43]. Fig. 4 shows typical curves for runs at
7
50 nm and the curve fit according to Eqn. 3. Values of k and k determined from these
5 6
VI
‡
fits for a 50 :1 alditol/Cr ratio and different [H ] are listed in Table 2. The k values
6
obtained agree perfectly with values calculated from data at 350 and 570 nm.
Abs750 ˆ k e [Cr ] (e � e� k6t)/(k � k )
V
VI
� k5t
(3)
6
0
6
5

Helvetica Chimica Acta ± Vol. 83 (2000)
3217
Fig. 4. Absorbance vs. time curves for the chromic oxidation of a) Glc-ol and b) Man-ol. [CrVI] ˆ 1.8 ´ 10� 2 m,
‡
[
Ald-ol] ˆ 0.9m, [H ] ˆ 0.30m, I ˆ 1m, Tˆ 338, l 750 nm.
a
Table 2. Observed Pseudo-First-Order Rate Constants ) k
5
and k
6
for Different [HClO
4
] Obtained at l 750 nm.
VI
[
Cr ] ˆ 0.018m; [alditol] ˆ 0.9m; I ˆ 1m; T 338.
[
HClO
4
]/m
Man-ol
03 k6(calc.)b)/s� 1
10 k
3
/s
� 1
2
� 1
1
6
5
10 k /s
0
0
0
0
0
.1
.2
.3
.4
.5
1.3
4.0
8.0
13
20
1.0(2)
3.8(1)
6.1(1)
12(1)
4.4(1)
6.6(1)
9.9(1)
11(1)
18(2)
13(1)
[
HClO
4
]/m
Glc-ol
03 k6(calc.)b)/s� 1
10 k
3
/s
� 1
10 k
2
/s
� 1
1
6
5
0
0
.2
.3
6.3
12
6.2(2)
11(1)
7.4(1)
8.1(2)
a) Mean values from multiple determinations. b) Values calculated with Eqn. 2.

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Helvetica Chimica Acta ± Vol. 83 (2000)
VI
For alditol and Cr concentrations used in the kinetic measurements, the calculated
maximum CrV concentrations ([Cr ]max) represent up to 17% of the total Cr
V
concentration, and the tmax values are very short compared to the total redox-reaction
time, thus confirming that absorbance at 350 nm should essentially reflect changes in
VI
[
Cr ].
VI
2
.4. Mechanism of the Oxidation of Alditols by Cr . In the range of substrate and
VI
acid concentration used in this work, the oxidation of Glc-ol and Man-ol by Cr is a
complex multistep reaction yielding [Cr(H O] ] and the aldonic acid (AldA) ± in
3‡
2
6
equilibrium with its lactone ± as the final redox products. In the Scheme, we propose a
mechanism that takes into account a) the kinetic results, b) the polymerization of
VI
acrylonitrile added to the reaction mixture, c) the detection of an intermediate Cr
ester, and d) the detection of intermediate oxochromate(V) species.
Scheme
a) R,R',R'' ˆ polyhydroxy-substituted residues; w ˆ water.
‡
VI
�
In the [H ] range under study, Cr may exist as HCrO4 [44], and this species is
VI
proposed as the reactive form of Cr , in agreement with the first-order dependence of
the reaction rate on [Cr ]. It is known that the chromic oxidations of alcohols and
VI
glycols are preceded by the formation of a chromate ester [41][42]. The observation of
the absorbance band characteristic of chromate esters around 371 nm a few seconds
VI
after mixing alditols and Cr , under conditions where the redox reaction is extremely
slow, reveals that such an intermediate CrVI complex is rapidly formed prior to the
redox steps. Thus, the first step of the mechanism proposed in the Scheme may be
VI
interpreted as the formation of several linkage isomers I and II of the Cr -Ald-ol
VI
monochelate [18] with the alditol acting as a bidentate ligand bound to Cr via any pair
of properly disposed OH groups. Taking into account that in the redox reaction only the

Helvetica Chimica Acta ± Vol. 83 (2000)
3219
primary alcohol is oxidized, it seems reasonable to think that the complex II with the
VI
primary OH group bound to Cr should be the precursor of the slow redox steps.
The slow redox step in the mechanism might take place by either a one-electron or a
two-electron transfer [45]. The fit of absorbance data at 750 nm reveals that all of the
VI
III
V
Cr reaches Cr through the intermediate Cr following the consecutive first-order
VI
V
III
Cr ! Cr ! Cr redox pathway, indicating that the slow redox step in these reactions
VI
V
corresponds to a one-electron-transfer process (Cr ! Cr ). This result differs from
that observed for the oxidation of aldoses and methyl glycosides, in which the rate-
VI
determining step involves a two-electron-transfer process and only one-half of the Cr
III
V
reaches Cr through the intermediate Cr [20][23][25][30 ± 34]. The two terms in the
rate law (Eqn. 2) indicate that there are at least two transition states, differing in the
protonation degree but similar in energy, through which the reduction of CrVI can
proceed. Thus, in the slow redox step of the mechanism in the Scheme, complex II yields
the Ald-ol radical and Cr through two parallel acid-catalysed paths. Polymerization
after addition of acrylonitrile supports the formation of the Ald-ol radical. This Ald-ol
.
V
.
.
VI
radical formed in the slow redox steps may react with Cr to afford the final oxidized
III
V
product (AldA) and Cr . For either Glc-ol or Man-ol, Cr formed in the slow step
reacts with excess Ald-ol to yield the oxochromate(V) complex precursor of the
electron-transfer steps, which then yields the redox products.
In contrast to monohydric alcohols [28][46][47], the alditols under study are not
oxidized to the aldehyde, but directly to the carboxylic acid. For monohydric alcohols, it
has been shown that the aldehyde oxidation rates are of the same order of magnitude
than those of the corresponding alcohols [48][49]; so when a large excess of alcohol
VI
over Cr is used, the [aldehyde] is not high enough to compete with the alcohol to
VI
VI
reduce Cr . In the case of the polyols under study, the rate of Cr oxidation of aldoses,
i.e. of the hemiacetal form of the aldehyde formed by oxidation of the corresponding
alditol ± is slower than that of alditols: kGlc-ol/kGlc ˆ 9.4; kMan-ol/kMan ˆ 2.2 [20][26][31].
VI
Thus, if formed, the aldose should not be oxidized by Cr to the aldonic acid when a
large excess of alditol over Cr is used. The non-observation of the aldose in the
VI
reaction mixture means that the aldose is not formed in these redox reactions, and that
the alditols are directly oxidized to the corresponding aldonic acid. Consequently, the
redox path shown in the Scheme takes into account the direct oxidation of the primary
OH group to the carboxylate group with no formation of the aldehyde.
2
.5. Characterization of Intermediate CrV Species by EPR Spectroscopy. 2.5.1.
Preamble. The EPR spectral parameters, i.e. the giso and Aiso values, together with the
proton superhyperfine (shf) coupling, have been shown to be useful in determining the
V
binding modes of polyhydroxy compounds to the Cr center [20][25][30][31][50].
Polyols usually form weak complexes with metal cations. However, to compete with
H O molecules for the occupancy of the first coordination sphere of a metal cation,
2
these acyclic ligands must possess some pre-organization and contain particular spatial
arrangements of the ligating groups [51]. The threo arrangement of two vicinal OH
groups is particularly suitable for the complexation of hypervalent metal cations, which
3
‡
V
reinforces the CrO preference to form five-membered Cr chelate rings [52 ± 54].
V
V
Since the giso and Aiso values of the EPR signal of Cr complexes depend on the Cr
coordination number and the nature of the donor groups bound to Cr [55 ± 57], an
estimation of the Cr species in solution may be made according to the shf pattern
V
V

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Helvetica Chimica Acta ± Vol. 83 (2000)
together with the isotropic EPR parameters (giso and Aiso values) based on the empirical
method developed by Lay and co-workers [55]. In the discussion that follows, we will
V
use this approach to assign the coordination sphere of the intermediate Cr species
VI
formed in the reaction of Cr with Glc-ol, Man-ol, and meso-galactitol (Gal-ol).
2
.5.2. (d-Glucitol)chromium(V). In the reaction of CrVI with Glc-ol, at pH 4 ± 5 and
V
ligand-to-metal ratios < 20 (Fig. 5,a, Table 3), the EPR spectra of the intermediate Cr
species show three quint. at g ˆ 1.9800, g ˆ 1.9798, and g ˆ 1.9797 and A ˆ 16.5(3) ´
1
2
3
iso
�
4
� 1
V
10
cm . The g and A values correspond to those calculated for five-coordinate
iso iso
oxo-Cr complexes with four alcoholato donors [55], and the shf pattern is that
expected for four equivalent CH� O protons ± belonging to the diolate donor site of
V
V
two bidentate Glc-ol bound to Cr ± coupled to the Cr electronic spin. The
V
independence of the spectra on [Glc-ol] confirms that all the species have a Cr
coordination sphere saturated with respect to the alditol (Cr /Glc-ol ). The different
shf coupling constants indicate that the different species are linkage isomers of the Cr
V
2
V
2
(3)
3(4)
�
2(3)
3(4)
bis-chelate [Cr(O)(O ,O -Glc-ol) ] (type A), with O ,O -Glc-ol correspond-
2
V
2
3
3
4
ing to the ligand bound to Cr at any threo-diolate group (O ,O or O ,O ). The
2
3
4
2
3
4
formation of [Cr(O)(O ,O ,O -Glc-ol)(OH )] (g ˆ 1.9783) or [Cr(O)(O ,O ,O -Glc-
2
calc
2
3
4
2
3
ol)(OH ) ] (g ˆ 1.9735) monochelates and [Cr(O)(O ,O ,O -Glc-ol)(O ,O -Glc-
2
2
calc
2�
ol)] (gcalc ˆ 1.9759) bis-chelate with one Glc-ol acting as a tridentate ligand via the
2
,3,4-xylo-triolate (threo-threo) moiety is disregarded because no signal with the shf
pattern and spectral parameters (giso and Aiso values) corresponding to such species are
observed.
At pH 2 and ligand-to-metal ratios up to 10 :1, and at pH 4 ± 5 and ligand-to-metal
ratios ꢁ20, the spectra show the three signals attributed to the linkage isomers
2
(3)
3(4)
�
2
[
Cr(O)(O ,O -Glc-ol) ] , besides an additional signal at g ˆ 1.9787 (Fig. 5,b). The
4
V
giso value of the new signal corresponds to that calculated for a five-coordinate oxo-Cr
complex, and the formation of this additional species is opposed to the formation of the
a) R,R',R'' ˆ polyhydroxy-substituted residues; w ˆ water.

Helvetica Chimica Acta ± Vol. 83 (2000)
3221
Table 3. EPR-Spectral Parameters
CrV species
g
A
iso/10 cm
4
� 1
a
H
/10 cm (N)a)
4
� 1
iso
�
[
Cr(O)(O2(3),O3(4)-Glc-ol)
2
]
1.9800
.9798
.9797
1.9787
1.9719
1.9799
16.5 Æ 0.3
0.78 (4)
0.68 (4)
0.60 (4)
0.68 (2)
1
1
1
2
�
b)
[
[
[
Cr(O)(O ,O -GlcA)
2
]
1
(2,3)
2(3,4)
‡
b
Cr(O)(O
,O
-Glc-ol)(H
2
O)
3
]
20 Æ 1
)
3
4
�
Cr(O)(O ,O -Man-ol)
2
]
16.5 Æ 0.3
0.90 (4)
0.88 (4)
1.9798
1
2
�
b
b
[
[
[
Cr(O)(O ,O -ManA)
2
]
1.9787
1.9718
1.9799
)
)
)
)
1
(3)
2(4)
‡
b
b
Cr(O)(O ,O -Man-ol)(H
2
O)
3
]
2
3
�
Cr(O)(O ,O -Gal-ol)
2
]
16.3 Æ 0.2
0.88 (4)
0.80 (4)
0.85 (4)
1
.9798
.9795
1.9720
1
Cr(O)(O1(2),O2(3)-Gal-ol)(H
O)
]
‡
20 Æ 1
b
)
[
2
3
a) N ˆ number of equivalent protons. b) Not determined.
VI
VI
Fig. 5. Experimental and simulated X-band EPR spectra of solutions of Glc-ol and Cr : a) Glc-ol/Cr 10 :1,
VI
VI
‡
pH 5 (mod. ampl. 0.4 G); b) Glc-ol/Cr 20 :1, pH 5 (mod. ampl. 0.4 g); c) Glc-ol/Cr 10 :1, [H ] ˆ 0.1m (mod.
ampl. 1.01 G). T 258, frequency 9.7815 GHz. [CrVI] ˆ 2.5 ´ 10� 3 m.

3222
Helvetica Chimica Acta ± Vol. 83 (2000)
bis-chelate [Cr(O)(O2(3),O3(4)-Glc-ol) ] , which is independent of pH and [ligand]. The
�
2
dependence of the signal at g ˆ 1.9787 on both the pH and the [ligand] is consistent
4
V
with an oxo-Cr species formed with the oxidation product GlcA, coordination
occurring via the 1-carboxylate and 2-hydroxy donor groups. The calculated g
iso
1
2
�
(
1.9783) for [Cr(O)(O ,O -GlcA) ] (type B) is in reasonable agreement with the
2
observed giso value. Besides, coordination to the oxidized ligand is consistent with the
VI
fact that Cr is a stronger oxidant in more acidic medium.
At pH 1 and ligand-to-metal ratio 10 :1, one additional signal at g ˆ 1.9719 appears,
5
together with signals at g ˆ 1.9800, g ˆ 1.9798, g ˆ 1.9797 and g ˆ 1.9787. The shf
1
2
3
4
pattern of this new signal is not resolved. However, its pH dependency and the g value
iso
reveal some information on its nature. The formation of the species at g is favored at
5
higher acid concentrations, where it is the dominant one (Fig. 5,c). The low g value of
iso
V
this signal suggests the presence of six-coordinate oxo-Cr species, possibly
1
(2,3)
2(3,4)
‡
[
Cr(O)(O
,O
-Glc-ol)(H O) ] (type C). The calculated giso value for a six-
2
3
V
coordinate oxo-Cr species with two alcoholato donor sites and three H O molecules
2
(
1.9724) is in agreement with the observed giso value. The positive charge of this species
‡
V
is also consistent with its appearance at high [H ]. This Cr mono-chelate is possibly
responsible for the higher rates observed for the Cr -Ald-ol redox reactions at pH < 1.
At higher pH, where this Cr mono-chelate is not observed, Cr complexes are stable
V
V
V
enough to remain in solution for several days to months.
V
2
.5.3. (d-Mannitol)chromium(V). Intermediate Cr -Man-ol species formed in the
VI
reaction of Cr with Man-ol, at pH 4 ± 5 and ligand-to-metal ratios up to 25, have EPR
spectra (Fig. 6,a, Table 3) consisting of two quint. at g ˆ 1.9799 and g ˆ 1.9798 with
1
2
�
4
� 1
A
ˆ 16.5(3) ´ 10 cm . The g and A values and the shf pattern indicate these
iso
iso
iso
V
signals may be attributed to two geometric isomers of the five-coordinate oxo-Cr
3
4
�
V
complex [Cr(O)(O ,O -Man-ol) ] (type A), with Man-ol bound to Cr via the 3,4-
2
threo-diolate moiety.
At pH 2 and ligand-to-metal ratios up to 10 :1, the spectra show the two signals
3
4
�
attributed to the geometric isomers [Cr(O)(O ,O -Man-ol) ] , together with an
2
additional non-resolved signal at g ˆ 1.9787 (Fig. 6,b). As in the case of Glc-ol, this
3
V
1
2
�
signal may be attributed to the five-coordinate oxo-Cr [Cr(O)(O ,O -ManA) ] (type
2
B), with ManA acting as a bidentate ligand via the 2-hydroxy acid moiety.
At pH 1 and ligand-to-metal ratio 10 :1, Man-ol behaves similarly to Glc-ol. The
EPR spectrum is now dominated by a non-resolved signal at g ˆ 1.9718 (85%), besides
4
minor signals at g ˆ 1.9799, g ˆ 1.9798, and g ˆ 1.9787 (Fig. 6,c). The g value of the
1
2
3
iso
V
signal at g corresponds to a six-coordinate oxo-Cr species and may be assigned to
4
1
(3)
2(4)
‡
[
Cr(O)(O ,O -Man-ol)(H O) ] (type C).
2
3
2
.5.4. (meso-Galactitol)chromium(V). To evaluate whether the pattern observed
VI
for Glc-ol and Man-ol may be generalized to other alditols, the reaction of Cr with
Gal-ol was also studied. At pH 4 ± 5 and ligand-to-metal ratios up to 25 (Fig. 7,a,
V
Table 3), the EPR spectra of intermediate Cr consist of three quint. at g ˆ 1.9799, g ˆ
1
2
V
1
.9798, and g ˆ 1.9795, which may be attributed to three geometric isomers of the Cr
3
2 3 � 2 3
bis-chelate [Cr(O)(O ,O -Gal-ol) ] (type A), with O ,O -Gal-ol corresponding to the
2
V
2
3
4
5
ligand bound to Cr at any threo-diolate group (coordination to O ,O and O ,O are
equivalent). Here again, the independence of the signals on [Gal-ol] indicates that all
V
three species have a Cr coordination sphere saturated with respect to the alditol.

Helvetica Chimica Acta ± Vol. 83 (2000)
3223
Fig. 6. Experimental and simulated X-band EPR spectra of solutions of Man-ol and CrVI: a) pH 5 (mod. ampl.
‡
0
.4 G, frequency 9.7819 GHz); b) pH 2 (mod. ampl. 0.4 G, frequency 9.7808 GHz); c) [H ] ˆ 0.1m (mod. ampl.
VI
� 3
VI
1
.01 G, frequency 9.7788 GHz). T 258, [Cr ] ˆ 2.5 ´ 10 m, Man-ol/Cr 10 :1.
In 0.5m HClO and ligand-to-metal ratio 10 :1, the EPR spectrum is dominated
4
(
84%) by a new signal at g ˆ 1.9720, with non-resolved shf pattern (Fig. 7,b). The
4
species at g , g , and g are still present under these conditions, but the signal is now
1
2
3
badly resolved, probably due to their very low concentration in the reaction mixture.
V
Also in this case, the signal at g may be assigned to the six-coordinate oxo-Cr complex
4
1
(2)
2(3)
‡
[
Cr(O)(O ,O -Gal-ol)(H O) ] (type C).
2
3
The ultimate fate of the Cr-atom in all of these reactions (i.e. also in 2.5.2 and 2.5.3)
III
III
is a Cr species, and a typical broad Cr EPR signal centered at g ꢂ 1.98 is always
observed at a later time.
3
. Conclusions. ± Alditols are selectively oxidized by CrVI at the primary OH group
VI
to yield the aldonic acid as the only oxidation product. The reaction involves a Cr !
V
III
VI
V
Cr ! Cr reduction path, with the Cr ! Cr step being the rate-determining one.

3224
Helvetica Chimica Acta ± Vol. 83 (2000)
VI
VI
Fig. 7. Experimental and simulated X-band EPR spectra of solutions of Gal-ol and Cr . a) Gal-ol/Cr 20 :1,
VI
‡
pH 5 (mod. ampl. 0.4 G, frequency 9.7814 GHz); b) Gal-ol/Cr 10 :1, [H ] ˆ 0.5m (mod. ampl. 2.0 G, frequency
VI
� 3
9
.7827 GHz). T 258, [Cr ] ˆ 2.5 ´ 10 m.
The reaction requires protons to be present, and, at low acidity, the redox rate is
negligible. The same mechanism is found for the oxidation of the two alditols Glc-ol
and Man-ol, and the rate constants are very similar, meaning that the configuration at
C(2) affects only slightly the redox rate. Glc-ol and Man-ol are oxidized 10 and 20 times
faster than the respective methyl glycosides, for which the primary OH is also oxidized
to the carboxylate. This different reactivity is consistent with the ability of alditols to
V
form five-membered Cr chelate rings involving the primary OH, which is not possible
in methyl glycosides with the vicinal OH group involved in the acetal bond. As in the
case of sugars and their derivatives, the secondary OH groups are inert to oxidation
V
when an excess of reductant over oxidant is used. Alditols stabilize Cr , yielding penta-
‡
‡
and hexacoordinate oxo-chromate(V) complexes depending on the [H ]. At [H ]
ꢁ
V
0.1m, positively charged hexacoordinate Cr monochelates are formed, and are
‡
believed to be responsible for the fast electron-transfer process in the [H ] range 0.1 ±
V
0
.8m. At pH > 1, only pentacoordinate Cr species are present, intramolecular redox

Helvetica Chimica Acta ± Vol. 83 (2000)
3225
V
reactions are very slow, and the Cr complexes remain in solution for several days to
weeks. The EPR parameters together with the shf pattern indicate that the five-
V
coordinate oxo-Cr complexes formed with excess alditol involve metal binding to four
secondary alcoholato donors, belonging to the diolate donor site of two bidentate Ald-
V
V
V
ol bound to Cr . Thus, the more stable oxo-Cr complexes do not involve Cr
coordinated to the primary OH group, similar to what has been observed for sugars
and their derivatives [20][23][25][30][31][33][34].
Experimental Part
1
. Materials. d-Sorbitol (Sigma grade), d-mannitol (Sigma grade), meso-galactitol (Sigma grade), K
Cicarelli c.a.), acrylonitrile (Aldrich grade), perchloric acid (Merck p.a.), and sulfuric acid (Merck p.a.) were
used without further purification. H O was purified by deionization, followed by double distillation from a
KMnO soln. For experiments performed in the pH range 1 ± 5, the pH of the solns. was adjusted by addition of
2 2 7
Cr O
(
2
4
‡
0
.5m HClO
NaClO
concentration of stock solns. of HClO
4
. In experiments performed at constant ionic strength (I ˆ 1.0m) and different [H ], mixtures of
solns. and HClO solns. were used. NaClO solns. were prepared from NaOH and HClO solns. The
was determined by titration by standard anal. methods [58].
4
4
4
4
4
The stability of the org. substrate under conditions used in the kinetic studies was tested by paper
‡
chromatography (PC) at a given [H ] and by HPLC.
2
. Spectrophotometry. Spectrophotometric measurements were made by monitoring absorbance changes at
350, 570, and 750 nm on a Jasco-V-530-UV/VIS spectrophotometer with fully thermostated cell compartments.
Reactant solns. were previously equilibrated and transferred into a cell of 1-cm path length immediately after
mixing, and the reaction was followed until at least 80% conversion. Experiments were performed at 338 unless
otherwise mentioned.
VI
The Cr consumption was followed spectrophotometrically at 350 nm. In all experiments, the
VI
� 4
concentration of Cr was kept constant at 6 ´ 10 m, while the alditol concentration was varied from 0.06 to
.30m. The observed pseudo-first-order rate constants (k ), determined from the slopes of the linear part of plots
0
6
3
50
of In(Abs ) vs. time, were deduced from multiple determinations and were within Æ5% of each other. The
VI
first-order dependence of the rate upon [Cr ] was verified by calculating the pseudo-first-order rate constants
VI
‡
VI
at various [Cr
]
o
, but at constant temperature, [alditol]
o
, [H ], and I. As expected from a � d(In[Cr ])/dt ˆ k
6
‡
VI
rate law, where k
6
ˆ f([alditol][H ]), k
6 o
was found to be essentially constant with increasing [Cr ] .
III
VI
� 3
The formation of Cr was monitored at 570 nm ([Cr ] ˆ 6 ´ 10 m) in the presence of excess alditol and
VI
[
HClO
4
] in the 0.10 ± 0.80m range. Under these conditions, the first-order dependence of the rate upon [Cr ]
III
6
was verified, and k were obtained following the Cr growth at 570 nm. Initial reaction rates were obtained by
fitting the absorbance at 570 nm/time data to a polynomial expression [59] and calculating the slope of the
tangent at time zero (r ). First-order rate constants were obtained from these initial rates by means of the
equation � r /(A � A ), with average values of the initial rate and calculated estimates of (A � A ). The results
at 570 nm were in total agreement with those obtained at 350 nm (Table 1). At the end of the reaction, the two
i
i
o
f
o
f
III
3
� 1
� 1
d-d bands ascribed to Cr were observed at lmax 409 nm (e 16.2 dm mol cm ) and lmax 574 nm
3
� 1
� 1
(
e 13.6 dm mol cm ). Both VIS and UV spectral maxima and intensities were in close agreement with
3
‡
those observed for the [Cr(H
2
O)
6
]
ion.
V
VI
Cr was monitored by following the absorbance growth and decay at 750 nm of mixtures of Cr (0.018m)
and excess alditol (0.9m) at different [HClO
4
]. At this wavelength, kinetic traces were evaluated from a
VI
nonlinear iterative computer fit of the rate expression derived from two consecutive first-order reactions Cr
!
V
III
Cr ! Cr . Values of k
6
obtained at this wavelength were coincident with those calculated from data at 350 and
5
70 nm. In multiple measurements, the reproducibility of the two rate constants was better than 10%.
Chromate esters were investigated by UV/VIS spectrophotometry in the 350 ± 400 nm region in which these
esters show characteristic absorption bands. Reactions were performed at pH 4.5, a pH where the redox reaction
is slow enough to enable the observation of ester formation. The instrument was zeroed to an arrangement of
VI
2
the reference and sample beams passing through matched cuvettes, both containing Cr in H O at pH 4.5. The
�
4
VI
soln. in the sample cell was replaced with the reaction soln. containing 6 ´ 10 m Cr and 0.12 ± 0.30m alditol at
pH 4.5, I ˆ 1.0m and 338. Spectra obtained within 30 min after mixing revealed a distinctive absorption at
371 nm.

3226
Helvetica Chimica Acta ± Vol. 83 (2000)
3
. Product Analysis. 3.1. General. HPLC was employed to detect the reaction products under the conditions
VI
used in the kinetic measurements (excess of alditol over Cr ): KNK-500A chromatograph, 7125 HPLC pump;
�
3
Aminex-HPX-87 H HPLC column (300 Â 7.8 mm, Bio-Rad Laboratories), 3 ´ 10 m H
2 4
SO as eluent, flow rate
0
.6 ml/min, at 358; refractive-index detector (ERC-7522, Erma Inc.). The pH of the standard and the reaction-
mixture samples was adjusted to 3 by addition of NaOH (to avoid precipitation inside the column), and then the
samples were filtered through a 0.2-mm membrane prior to the injection into the chromatographic system.
Standard solns. of the Glc-ol, Man-ol, d-glucose, d-mannose, d-glucono-1,5-lactone, d-gulono-1,4-lactone,
d-mannono-1,4-lactone, d-glucuronic acid, and d-mannofuranurono-6,3-lactone were prepared individually in
0
.5m HClO
Under all experimental conditions (large to moderate excess of alditol over CrVI), CO
as a reaction product, indicating that no degradation to the lower homologues occurred.
4 R
and chromatographed separately to determine the t of each sample.
2
was never observed
Alternatively, the possible formation of the aldose as the reaction product was tested by addition of (2,4-
VI
dinitrophenyl)hydrazine to the Cr /alditol reaction mixtures. The 2,4-dinitrophenylhydrazone derivative was
never observed.
.2. Glc-ol/Cr System. A reaction mixture obtained from 8 ´ 10 m CrVI and 0.04m Glc-ol in 0.5m HClO
VI
� 4
3
4
was submitted to HPLC: only 1 product at t
increased product peak; no d-glucuronic acid (t
8 min 20 s) and d-gulono-1,4-lactone (t 8 min 22 s) with t
R
8 min, 26 s. Co-HPLC with d-glucono-1,5-lactone resulted in an
R
7 min 25 s) was observed, however, the presence of d-glucose
(
t
R
R
R
s very close to that of d-glucono-1,5-lactone, could
VI
not be excluded. For this reason, and in the presence of an excess of alditol over Cr , the aldonolactone
2
produced was qualitatively identified by PC with BuOH/AcOH/H O 4 :1:5 as eluent and visualization by
aniline hydrogenphthalate [60], a reagent specific for the detection of aldoses and uronic acids, and by ferric
hydroxamate [61], a reagent specific for lactones. No colored spots appeared in the PC developed with aniline
hydrogenphthalate, whereas with ferric hydroxamate, a violet compound was detected. An additional PC with
BuOH/EtOH/H
,5-lactone in the reaction mixture [62]; however, the presence of a small amount of d-gulono-1,4-lactone can
not be excluded.
.3. Man-ol/Cr System. A reaction mixture obtained from 7.3 ´ 10 m CrVI and 0.36m Man-ol in 0.5m HClO
was chromatographed: only 1 product at t 10 min 13 s, corresponding to d-mannono-1,4-lactone. d-Mannose
8 min 55 s) and d-mannofuranurono-6,3-lactone (t 11 min 30 s) were not observed.
. EPR Measurements. All of the EPR experiments were carried out at r.t. with a Bruker ESP-300 E
2
O 5 :1:4 as eluent, a solvent separating g-lactones from d-lactones, revealed only d-glucono-
1
VI
� 3
3
4
R
(
t
R
R
4
spectrometer. The microwave frequency was generated with a Bruker 04-ER (9 ± 10 GHz) and measured with a
Racal-Dana frequency meter. The magnetic field was measured with a Bruker NMR-probe gaussmeter. EPR
Spectra were simulated by means of the program PEST WinSIM [63], with 100% Lorentzian lineshapes.
5
. Free-Radicals Test. The presence of free radicals was tested for the reaction of the alditols with CrVI. In a
�
3
2 2 7 4
typical experiment, to a soln. of K Cr O (2 ´ 10 mmol) and alditol (0.02 mmol) in 2 ml of 0.50m HClO ,
acrylonitrile (0.5 ml) was added at 338. After a few minutes, a white precipitate appeared. Control experiments
V
(
without K
2
Cr
2
O
7
or reductant present) did not show the formation of a precipitate. The possible reaction of Cr
V
with acrylonitrile was excluded based on the absence of a precipitate one day after mixing [Cr O(ehba)
acrylonitrile under the same reaction conditions than before.
2
]K and
We tried several spin traps with the intention of detecting by EPR the intermediate radical formed during
the redox reaction. The addition of 4,5-dihydro-5,5-dimethyl-1H-pyrrole 1-oxide (DMPO) to the reaction
�
4
� 1
mixture resulted in the appearance of an EPR signal at g ˆ 2.003 with splitting constants of 7.48 ´ 10 cm and
�
4
� 1
.
3
[
.65 ´ 10 cm (Â2), corresponding to the oxidized spin trap, and the signal of the DMPO-OH spin adduct
.
64]. The EPR signal of the DMPO-sorbitol spin adduct has been reported and has a distinctive well defined
.
hyperfine pattern [65]. However, in the present case, no signal attributable to the DMPO-alditol spin adduct
was detected. Similarly, 5-(diethoxyphosphoryl)-4,5-dihydro-5-methyl-1H-pyrrole 1-oxide (DEPMPO) and (4-
pyridyl-1-oxide)-N-(tert-butyl)nitrone (ˆ1,1-dimethyl-N-[(1-oxidopyridin-4-yl)methylidene]ethanamine N-ox-
.
ide; POBN) trapped only OH . Probably the intermediate radical could not be trapped because the spin trap
V
preferred the redox reaction with Cr , which yields the paramagnetic products observed by EPR [66]. In previous
works, the EPR m at g ˆ 2.003 had been erroneously attributed to a DMPO-sugar spin adduct [30][31].
.
We thank the National Research Council of Argentina (CONICET), the Scottish Office Rural Affairs
Department, the Third World Academy of Sciences (TWAS), the National University of Rosario (UNR), the
International Foundation for Science (IFS), the National Agency for Sciences Promotion, the ALFA Program
(CROCON Network, Project 7-0064-9), and ANTORCHAS Foundation for financial support.

Helvetica Chimica Acta ± Vol. 83 (2000)
3227
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[
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[
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Received July 3, 2000