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Inorganic Chemistry Communications 9 (2006) 437–440

Evidence for the involvement of CrII and free radicalsas intermediates in the reduction of HCrO�4 by saccharides, alcohols

and hydroxyacids

Juan Carlos Gonzalez, Silvia Garcıa, Nadia Mamana, Luis F. Sala, Sandra Signorella *

Departamento de Quımica, Facultad de Ciencias Bioquımicas y Farmaceuticas, UNR, Suipacha 531, S2002LRK Rosario, Argentina

Received 11 November 2005; accepted 3 February 2006Available online 20 March 2006

Abstract

Intermediacy of free radicals and CrII in the reaction of HCrO�4 with a variety of neutral and acid saccharides, hydroxyacids and aly-phatic alcohols was demonstrated by the observation of free radical induced polymerization of acrylonitrile and acrylamide, and detec-tion of CrO2þ

2 when working in O2-saturated solutions and low ½HCrO�4 �. The results are interpreted in terms of a mechanism combiningthe CrVI! CrIV! CrII and CrVI! CrIV! CrIII pathways.� 2006 Elsevier B.V. All rights reserved.

Keywords: CrVI; Solution redox chemistry; Reaction intermediates; Spectrophotometry

The oxidation of aliphatic alcohols, aldehydes and car-boxylic acids by HCrO�4 has been widely studied [1–4].Based on the observation of CrO2þ

2 in the reactions carriedout in O2-saturated solutions, and the fact that this speciescan be exclusively formed by reaction of CrII with O2, inter-mediacy of CrIV and CrII has been established and a mech-anism in which the organic substrate exclusively acts as atwo-equivalent reducing agent toward CrVI and CrIV hasbeen proposed (Scheme 1) [5–8].

However, this mechanism does not explain the polymer-ization induced by free radicals observed in the oxidationof a number of substrates by CrVI [9–20], which impliesthat a pathway involving a one-electron step must alsooccur. Recently, based on the observation of CrO2þ

2 andthe successful trap of organic radicals using acrylonitrile,a combination of the CrVI! CrIV! CrII and CrVI!CrIV! CrIII pathways was proposed for the oxidation ofD-lactobionic [21] and D-galacturonic [22] acids byHCrO�4 . In view of these results, we decided to check thepossibility that the combination of one- and two-electron

1387-7003/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.inoche.2006.02.002

* Corresponding author. Tel./fax: +54 341 4350214.E-mail address: signorel@infovia.com.ar (S. Signorella).

pathways for the reduction of intermediate CrIV by theorganic substrate could be generalized. Therefore, weexplored the presence of free radicals and CrO2þ

2 in the oxi-dation of a number of neutral and acid saccharides, diolsand hydroxycarboxilic compounds by CrVI.

The presence of free radicals was tested by addition ofacrylonitrile or acrylamide to mixtures of the organic sub-strate and HCrO�4 in acid solution [23]. Polymer formationwas observed within a few minutes in the reaction ofHCrO�4 with D-ribose, D-glucose, D-galactose, D-fructose,D-galactono-1,4-lactone, D-glucuronic acid, 2-hydroxy-3-methylbutanoic acid, 3-hydroxybutanoic acid, quinic acid,D-mannitol, 2,3-butanediol and ethyleneglycol, in 0.5 MHClO4.

The formation of CrO2þ2 was examined by periodic spec-

trophotometric scanning of the O2-saturated solution(1.26 mM O2) of HCrO�4 + substrate reaction mixtures in1.0 M HClO4 [24]. In these reactions, [CrVI]0 was kept6 0.05 mM so that the reaction of CrII + O2 can competesuccessfully with that of CrII + CrVI and the autocatalyticconsumption of CrO2þ

2 by CrII [6–8,25]. The sequentialspectra showed appearance of two absorption bands atkmax = 247 and 290 nm, with Abs290/Abs247 = 0.443,

Table 1Yield of CrO2þ

2 from the reaction of saccharides hydroxyacids and polyolsby CrVI

Substrate % yielda of CrO2þ2

D-Ribose 29D-Glucose 26D-Galactose 36D-Fructose 30D-Galactono-1,4-lactone, 17D-Glucuronic acid 152-Hydroxy-3-methylbutanoic acid 323-Hydroxybutiric acid 32Quinic acidb 46D-Mannitol 322,3-Butanediol 5Ethyleneglycol 15

Conditions: [substrate] = 0.13 M, [CrVI]0 = 0.047 mM; [H+] = 1.0 M,[O2] = 1.26 mM, I = 1.0 M.

a Mean values from multiple determinations, with ±5% incertainty.b [H+] = 0.4 M.

Table 2Effect of [CrVI]0 on the yield of CrO2þ

2 from the oxidation of D-galactose,D-glucuronic acid and 2,3-butanediol by CrVI

Substrate [CrVI]0/mM % yield of CrO2þ2

D-Galactose 0.047 360.01 590.00048a 60

D-Glucuronic acid 0.047 150.025 300.02 420.01 520.008 750.005 74

2,3-Butanediol 0.047 50.01 30

CrVI + S CrIV + Sox

CrIII + CrV

CrII

CrVI

O2 CrO22+

CrIII + SoxCrV + S

CrIII

fast

fast

k6

k4

k5

CrII + SoxCrIV + S

(1)

(2)

(3)

(4)

(5)

Scheme 1.

438 J.C. Gonzalez et al. / Inorganic Chemistry Communications 9 (2006) 437–440

characteristic of CrO2þ2 [5], as shown in Fig. 1 for the reac-

tion of D-galactose + CrVI in 1.0 M HClO4 [26]. Table 1summarizes the yield of CrO2þ

2 obtained from the reactionof 0.13 M substrate and 4.7 · 10�5 M HCrO�4 in 1.0 MHClO4. As it can be observed, CrO2þ

2 forms in all the testedredox reactions, under similar reaction conditions althoughwith different yield of CrO2þ

2 .For D-galactose, D-glucuronic acid and 2,3-butanediol,

we evaluated the percent yield of CrO2þ2 as the ½HCrO�4 �0

was lowered. The data summarized in Table 2 clearly showthat the yield of CrO2þ

2 increases as the [CrVI]0 decreases,and, as observed for D-galactose and D-glucuronic acid, itreaches a limiting value of �62% for D-galactose and75% for D-glucuronic acid. This result suggests that partof the HCrO�4 is reacting through a pathway that doesnot involve CrII. The yield of CrO2þ

2 is expected toapproach 100% if the reaction should take place exclusivelythrough the CrVI! CrIV! CrII pathway [6].

200 250 300 350 400 450 5000.00

0.05

0.10

0.15

0.20

0.25

0 10 20 30 400.00

0.01

0.02

0.03

0.04

0.05

Abs

: (nm)

[CrVI]

[CrO2

2+]

[Cr]

(m

M)

t (min)

λ

Fig. 1. Formation of CrO2þ2 from the reaction of 0.13 M D-galactose,

0.047 mM HCrO�4 and 1.26 mM O2, in 1.0 M HClO4. Inset: Evolution of½CrO2þ

2 � and [CrVI] with time.

0.005 36

Conditions: [substrate] = 0.13 M, [H+] = 1.0 M, [O2] = 1.26 · 10�3 M,I = 1.0 M.

a 10-cm cell.

The possible influence of the [substrate] on the yield ofCrO2þ

2 was also evaluated, and the results obtained frommixtures of D-glucuronic acid + CrVI and D-galac-tose + CrVI are shown in Table 3. For the reaction ofD-glucuronic acid + CrVI performed in 1.0 M HClO4, theyield of CrO2þ

2 varies inversely with the [substrate].

Table 3Effect of the substrate concentration on the yield of CrO2þ

2 from theoxidation of D-galactose and D-glucuronic acid by CrVI

D-Glucuronic acid/M [CrVI]0/mM [H+]/M % yield of CrO2þ2

0.032 0.01 1.0 690.064 0.01 1.0 590.13 0.01 1.0 520.064–0.26 0.005 0.1 50

D-Galactose/M0.064–0.13 0.01 0.1 49

[O2] = 1.26 mM, I = 1.0 M.

(14)

(15)

CrII + Sox

rIV + S CrIV- S K42, SK41

k4c

S2-CrIV S-Cr III + Sk4d

Scheme 3.

J.C. Gonzalez et al. / Inorganic Chemistry Communications 9 (2006) 437–440 439

However, at pH 1 and [CrVI]0 low enough to avoid theCrVI + CrII competitive reaction, for either D-glucuronicacid or D-galactose, the [substrate] does not affect the yieldof CrO2þ

2 . Moreover, the two substrates yield 50% ofCrO2þ

2 .Based on the above results, we propose the mechanism

shown in Scheme 2, in which the oxidation of the organicsubstrate by HCrO�4 involves CrIV, CrII and organicradicals.

In this scheme we have included the two competitiveone- and two-electron reductions of CrIV by the substrate.In O2-saturated solutions (1.26 mM) and [CrVI]0 6 0.01mM, reactions (9) and (11) can be neglected and CrII andS� intermediates formed in reactions (7) and (8) should berapidly trapped by O2 [27]. The proposed mechanism isin accordance with the previous observation that O2 hasnot kinetic effect on this reaction [1–4], because when[CrVI]0 P 0.1 mM (as usually employed in the kinetic mea-surements), both CrII and S� react with HCrO�4 faster thanthey do with O2 [6,27,28].

This model also explains the 50% yield of CrO2þ2 from

the reaction of either D-glucuronic acid or D-galactose withCrVI, at pH 1. It is known that the two-electron reductionof CrVI to CrIV by D-galactose [17] and D-glucuronic acid[29] has a second order dependence on [H+], and that atpH 1 this step is the rate controlling one. Therefore, at thispH, reactions (7) and (8) are much faster than reaction (6),so that CrII and S� are produced at the same rate to afford½CrII�=½S�� ¼ ½CrO2þ

2 �=ð½CrVI�0 � ½CrO2þ2 �Þ ¼ 1 (here ½CrO2þ

2 �is taken as a measure of the [CrII]). On the other hand, in1.0 M HClO4, the CrVI! CrIV step is not rate limiting

CrIV + Sox (6)S + CrVI k6

fastCrV + CrIII

CrII + CrVI (9)

(7)

CrIII + S

CrII + Sox

CrIV + Sk4b

k4a

(8)

(10)CrII + O2 CrO22+

fastCrIII

S + CrV CrIII + Soxk5

(13)

fastCrV + SoxCrVI + S (11)

fastS'oxO2 + S (12)

Scheme 2.

C

and the ½CrO2þ2 �=ð½CrVI�0 � ½CrO2þ

2 �Þ ratio depends on therelative rates of the two competitive reactions of CrIV

(Eqs. (7) and (8)).The inverse dependence of the yield of CrO2þ

2 with the[D-glucuronic acid] suggests that, for this substrate, reac-tion (8) involves one additional substrate molecule to yieldCrIII–S and S�. The formation of mono- and bis-chelates ofCrIV with hydroxycarboxylato ligands is well documented[30–32]. Therefore, it seems reasonable to propose thatCrIV might form S–CrIV and S2–CrIV chelates, so thatEqs. (7) and (8) can be rewritten as in Eqs. (14) and (15)(Scheme 3). According to Eqs. (14) and (15), in 1.0 MHClO4 and [CrVI]0 low enough so that Eqs. (9) and (11)can be neglected, the ½CrO2þ

2 �=ð½CrVI�0 � ½CrO2þ2 �Þ ratio

can be expressed by k4c/K42k4d [S]. At pH 1, reaction (6)is rate controlling, and at very low [CrVI]0,½CrO2þ

2 �=ð½CrVI�0 �½CrO2þ2 �Þ ¼ 1, such as observed.

The mechanistic model shown in Scheme 2 also explainsthe results obtained in the reaction of iPrOH with HCrO�4in acidic media. In 2.0 M HClO4 and [CrVI]0 = 0.006 mM([O2] = 1.26 mM, [iPrOH] = 0.23 M), this reaction yields�100% of CrO2þ

2 and no polymer is observed upon addi-tion of acrylonitrile [33], suggesting that k4a� k4b. Thus,under these conditions, the reaction should take placemainly through the CrVI! CrIV! CrII pathway, such aspreviously proposed [6]. However, when [H+] = 0.1 Mand [CrVI]0 = 0.006 mM ([O2] = 1.26 mM, [iPrOH] = 0.23M, I = 1.0 M), the yield of CrO2þ

2 diminishes to 70% andpolymerization of acrylonitrile is then observed [33],suggesting that the CrVI! CrIV! CrIII pathway becomesevident as the acidity decreases.

The present results show that the reduction of CrVI byalcohols, saccharides and hydroxyacids can be welldescribed by a mechanism combining the CrVI! CrIV!CrII and CrVI! CrIV! CrIII pathways and that depend-ing on the experimental conditions and the relative valuesof k6, k4a and k4b, one or the other may prevail.

Acknowledgements

We thank the National Research Council of Argentina(CONICET), the National Agency for Sciences Promotion(ANPCyT) and the National University of Rosario(UNR), for financial support.

References

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[23] Polymerization of acrylonitrile. In a typical experiment, 0.5 mL ofacrylonitrile was added to a solution of HCrO�4 (8.3 · 10�3 mmol)and organic substrate (0.51 mmol) in 3 mL of 0.64 M HClO4 at 30 �C.Within a few minutes, a white precipitate appeared. Controlexperiments (without HCrO�4 or reductant present) did not producethe precipitate. No precipitation occurred on mixing the CrV or CrIV

complexes with acrylonitrile under the conditions used in the CrVI+organic substrate reaction. Polymerization of acrylamide. Typically,an aqueous solution of HCrO�4 (0.05 mmol) was added to an aqueoussolution containing the organic substrate (2.16 mmol) and acrylamide

(3.7 mmol), in a total volume of 1.5 mL. When the colour of CrVI

disappeared the mixture was diluted with 5 mL of methanol at 0 �Cand a white polymer precipitated. Control experiments showed thatno polymerization of acrylamide takes place under the experimentalconditions with either HCrO�4 or the organic substrate alone. Withsome substrates, such as D-glucuronic acid, a CrIII–substrate complexco-precipitated with the polymer. In these cases, the precipitate wascentrifuged, the coloured complex was redissolved with water, and thepolymer remained as a white suspension. In all cases, the final CrIII

was characterized by its two typical d–d bands in the electronicspectra.

[24] The spectrophotometric monitoring of CrO2þ2 formed in the redox

reactions was carried out in O2-saturated solutions. The ionicstrength, I, (HClO4 + NaClO4) was 1.0 M in all the experiments.The saturated solutions of O2 (1.26 mM at 25 �C) were prepared bybubbling O2 through the solutions for about 1 h. In a typicalexperiment, 47 lL of an O2-saturated solution of 2.4 mMHCrO�4 ð½CrVI�final ¼ 4:7� 10�5 MÞ was injected into a 1-cm spectro-photometric cell filled with 2.4 mL of an O2-saturated solution of0.133 M substrate. The UV–Vis absorption spectra were recorded ona Jasco V-530 spectrophotometer with fully thermostated cellcompartments. Reactant solutions were thermostated prior to theexperiment. All measurements were made at 25 �C. O2-saturatedsolution of the substrate were used as reference.

[25] A. Bakac, T.J. Won, J.H. Espenson, Inorg. Chem. 35 (1996)2171.

[26] The [CrO2þ2 ] was calculated from the electronic spectra using

e290 = 3100 M�1 cm�1 and e247 = 7000 M�1 cm�1. In the concentra-tion vs time profiles, the CrO2þ

2 absorbance was estimated from thetotal Abs by subtracting the CrVI absorbance. The [CrVI] wascalculated from Abs350.

[27] P.A. Lay, A. Levina, J. Am. Chem. Soc. 120 (1998) 6704.[28] O. Pestovsky, A. Bakac, J.H. Espenson, J. Am. Chem. Soc. 120 (1998)

13422.[29] J.C. Gonzalez, PhD Thesis, Universidad Nacional de Rosario, 2006.[30] E.S. Gould, Coord. Chem. Rev. 135/136 (1994) 651.[31] A. Levina, R. Codd, G.J. Foran, T.W. Hambley, T. Maschmeyer,

A.F. Masters, P.A. Lay, Inorg. Chem. 43 (2004) 1046.[32] A. Levina, P.A. Lay, Coord. Chem. Rev. 249 (2005) 281.[33] The attempts to trap organic radicals by using acrylonitrile failed for

the reaction of iPrOHþHCrO�4 in 2.0 M HClO4. However, thepolymer was observed after a few minutes when acrylonitrile (0.5 mL)was added to the reaction of iPrOH (0.51 mmol) with CrVI

(8.3 · 10�3 mmol) in 0.5 M HClO4 or 0.1 M HClO4 + 0.9 M NaClO4

(3 mL).