Decomposition of ascorbic acid in the presence of cadmium ions leads to formation of a polymeric cadmium oxalate species with peculiar structural features.

Article abstract of DOI:10.1021/ic025598l

Slow decomposition of L-ascorbic acid, carried out under aerobic conditions and in the presence of cadmium ions, results in formation of a crystalline product that is highly insoluble in water. This compound has been identified as a cadmium oxalate polymeric species with formula Cd(C(2)O(4)).3H(2)O. The crystal structure of this compound is described. Relevant crystal data are the following: C(4)H(12)O(14)Cd(2), fw = 508.94; triclinic; space group P1 (No. 1); a = 6.010(1) A, b = 6.668(1) A, c = 8.498(1) A; alpha = 74.64(1) degrees, beta = 74.25(1) degrees, gamma = 80.91(1) degrees; V = 314.7(5) A(3); Z = 1.

Full text of DOI:10.1021/ic025598l

Inorg. Chem. 2002, 41, 4312−4314
Decomposition of Ascorbic Acid in the Presence of Cadmium Ions
Leads to Formation of a Polymeric Cadmium Oxalate Species with
Peculiar Structural Features
Pierluigi Orioli,* Bruno Bruni, Massimo Di Vaira, Luigi Messori, and Francesca Piccioli
Department of Chemistry, UniVersity of Florence, Via della Lastruccia 3,
50019 Sesto Fiorentino (FI), Italy
Received March 15, 2002
Slow decomposition of L-ascorbic acid, carried out under aerobic
conditions and in the presence of cadmium ions, results in formation
of a crystalline product that is highly insoluble in water. This
compound has been identified as a cadmium oxalate polymeric
degradation.8 Decomposition of DHA, in alkaline solutions,
produces oxalic acid and trihydroxybutyric acid in both
aerobic and anaerobic conditions.9
It is remarkable that oxalate represents one of the main
end-products of the degradation pathways, under both
oxidative and nonoxidative conditions. Indeed, Simpson et
al. demonstrated that 2,3-L-diketogulonate (2,3-DKG) is a
key intermediate of ascorbic acid catabolism, while oxalate
and L-erythrulose are the final degradation products regard-
less of which compound was used as the starting material.10
A recent work by U¨ naleroglu et al. specifically addressed
the effects of the presence of cobalt(II) and gadolinium(III)
ions on the degradation process of L-ascorbic acid.11 Interest-
ingly these authors found that decomposition of L-ascorbic
acid, carried out under various solution conditions, leads to
slow formation of highly insoluble cobalt(II) and gadolinium-
(III) oxalates. The crystal structure of the gadolinium oxalate
complex Gd2(C2O4)3‚6H2O was solved by X-ray diffraction.11
Notably, in this structure, symmetry-related gadolinium
atoms are linked by planar bridging oxalate ligands, forming
a polymeric network. Each gadolinium atom is coordinated
to six oxygens belonging to three different oxalate anions
and to three oxygens of water molecules.
We report here on the reaction of L-ascorbic acid with
cadmium ions, at equimolar ratios, performed under mildly
acidic conditions (pH 5). Although three different cadmium
salts were employed, namely, Cd(NO3)2, CdSO4, and CdCl2,
similar results were obtained in all cases, regardless of the
nature of the anion. The course of the reaction is peculiar.
At room temperature, a water-soluble yellow compound
slowly forms with time, probably associated with formation
of the six-membered δ lactone R-345, which is characteristic
of L-ascorbic acid degradation.12 Heating the solution to 60
species with formula Cd(C2O4)‚3H O. The crystal structure of this
2
compound is described. Relevant crystal data are the following:
C4H O14Cd2, fw) 508.94; triclinic; space group P1 (No. 1); a )
12
6.010(1) Å, b ) 6.668(1) Å, c ) 8.498(1) Å; R ) 74.64(1)°, â )
3
74.25(1)°, γ ) 80.91(1)°; V ) 314.7(5) Å ; Z ) 1.
The interactions of L-ascorbic acid with metals have been
extensively reviewed by Davies.1 However, despite intense
investigations, very few metal ascorbate complexes have
been isolated until now in the solid state; regrettably, most
of them are reluctant to form crystals suitable for X-ray
analysis.2
This issue is further complicated by the propensity of
L-ascorbic acid to undergo oxidative degradation. The
complex pathways of L-ascorbic acid transformations, both
in acidic and alkaline milieu, have been studied by several
authors and partially elucidated. It was shown that the rate
of oxidation of L-ascorbic acid to dehydroascorbic acid
(DHA) in aqueous solution and the tendency toward com-
plexation depend both on pH and on the presence (and
nature) of metal ions.3-7 When an ascorbic acid solution is
heated in acidic medium, carbon dioxide forms by anaerobic
* Corresponding author. E-mail: orioli@cerm.unifi.it. Fax: +39
0552757555.
(1) Davies, M. B. Polyhedron 1992, 11 (3), 285.
(2) Arendse, M.; Anderson, G. K.; Rath, N. P. Inorg. Chem. 1999, 38,
5864.
(3) Dixon, D. A.; Sadler, N. P.; Dasgupta, T. P. J. Chem. Soc., Dalton
Trans. 1993, 3489.
(4) Taqui Khan, M. M.; Shukla, R. S. Polyhedron 1991, 10, 2711.
(5) Taqui Khan, M. M.; Martell, A. E. J. Am. Chem. Soc. 1968, 90 (22),
(9) Herbert, R. W.; Hirst, E. L.; Percival, E. G. V.; Reynolds, R. J. W.;
Smith, F. J. Chem. Soc. 1933, 1270.
6011.
(6) Taqui Khan, M. M.; Martell, A. E. J. Am. Chem. Soc. 1969, 91 (17),
(10) Simpson, G. L. W.; Ortwerth, B. J. Biochim. Biophys. Acta 2000, 1501,
12.
4468.
(7) Taqui Khan, M. M.; Martell, A. E. J. Am. Chem. Soc. 1967, 89 (16),
(11) U¨ naleroglu, C.; Zu¨mreoglu-Karan, B.; Zencir, Y.; Ho¨kelek, T.
4176.
Polyhedron 1997, 16, 2155.
(8) Finholt, P.; Paulssen, R. B.; Alsos, I.; Higuchi, T. J. Pharm. Sci. 1965,
54, 124.
(12) Kimoto, E.; Tanaka, H.; Ohmoto, T.; Choami, M. Anal. Biochem. 1993,
214, 38.
4312 Inorganic Chemistry, Vol. 41, No. 17, 2002
10.1021/ic025598l CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/31/2002

COMMUNICATION
atoms in the equatorial positions of the bipyramids. These
anions form chains in the structure linking pairs of cadmium
atoms at a distance of 6.10 Å, as well as pairs at 3.87 Å via
bridging oxygens from different oxalate groups (Figure 1).
The other oxalates in the structure chelate cadmium atoms
belonging to different chains, through an apical and an
equatorial site, yielding a layered structure. The present
structure appears to be definitely different from that of Gd2-
(C2O4)3‚6H2O11 and related compounds,13 although these also
present polymeric networks. In fact, all of the metal atoms
in the latter structures are symmetry-related and form
comparable distances, all above 6 Å.
Attempts were made to prepare crystalline Cd2(C2O4)2‚
6H2O starting directly from cadmium sulfate and oxalic acid;
yet, these attempts failed due to rapid precipitation of
cadmium oxalate. Thus, it is probable that under the present
solution conditions decomposition of ascorbate and con-
comitant slow release of oxalate specifically favor the
formation of the peculiar cadmium oxalate polymeric
compound. We argue that a similar mechanism is operative
in the formation of the gadolinium oxalate complex described
by U¨ naleroglu. It is likely that the metal directs both the
L-ascorbic acid decomposition and the formation of the
polymeric crystalline lattice.
Figure 1. ORTEP drawing of part of the layered structure of Cd2(C2O4)2‚
6H2O with 50% probability ellipsoids. Symmetry operations, denoted by
the trailing letter in atomic labels, are (a) x + 1, y, z; (b) x - 1, y, z; (c) x
- 1, y, z + 1.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Cd2(C2O4)2‚6H2Oa
Bond Distances
Cd(1)-O(1)III
Cd(1)-O(9)
Cd(1)-O(3)III
Cd(1)-O(6)
Cd(1)-O(8)
Cd(1)-O(5)II
Cd(1)-O(10)
2.303(11)
2.220(8)
2.376(7)
2.341(7)
2.346(6)
2.432(6)
2.320(9)
Cd(2)-O(4)
Cd(2)-O(12)
Cd(2)-O(2)
Cd(2)-O(7)
Cd(2)-O(5)
Cd(2)-O(8)I
Cd(2)-O(11)
2.299(8)
2.396(12)
2.337(7)
2.393(7)
2.381(7)
2.331(6)
2.382(7)
We believe that such mechanisms may present some
degree of generality and might be relevant for the formation
of insoluble oxalate complexes also within biological systems
(e.g., calcium oxalate). Studies are under way to assess the
role of various metal ions on L-ascorbic acid decomposition
and on the formation of polymeric metal oxalate compounds.
Bond Angles
172.8(4)
O(1)III-Cd(1)-O(9)
O(1)III-Cd(1)-O(3)III
O(6)-Cd(1)-O(8)
O(8)-Cd(1)-O(5)II
O(5)II-Cd(1)-O(10)
O(10)-Cd(1)-O(3)III
O(3)III-Cd(1)-O(6)
O(4)-Cd(2)-O(12)
O(4)-Cd(2)-O(2)
O(7)-Cd(2)-O(5)
O(5)-Cd(2)-O(8)I
O(8)I-Cd(2)-O(11)
O(2)-Cd(2)-O(11)
O(2)-Cd(2)-O(7)
174.9(4)
73.2(2)
67.6(2)
71.3(2)
81.5(3)
78.1(3)
72.2(2)
69.2(3)
70.8(2)
70.1(2)
81.1(3)
74.8(3)
75.0(3)
L-Ascorbic acid (5 mmol) was reacted with equimolar
amounts of CdCl2, Cd(NO3)2, or CdSO4 in 20 mL of
deionized water. The reaction was carried out either at room
temperature or at 60 °C. In all cases, independently of the
nature of the anion, a characteristic yellow color appeared
within a few hours at RT; crystals formed in a few days.
Anal. Calcd for C2H6CdO7: C, 9.43; H, 2.36. Found: C,
9.46; H, 2.25.
a Symmetry transformations used to generate equivalent atoms: I, x+1,
y, z; II, x-1, y, z; III, x-1, y, z+1.
°C results in a faster appearance of this yellow product. On
aging, after a few days, large colorless crystals, suitable for
X-ray analysis, separate from the yellow solution. These
crystals are insoluble in water and in common organic
solvents. Elemental analysis revealed that the crystalline
material corresponds to cadmium oxalate with the formula
Cd(C2O4)‚3H2O. Overall, these observations suggest that
decomposition of L-ascorbic acid in the presence of cadmium
ions resembles the decomposition process described by
U¨ naleroglu et al. in the presence of cobalt(II) or gadolinium-
(III) ions.
Crystals appropriate for X-ray diffraction were obtained
in the form of colorless elongated plates from solutions
containing cadmium sulfate and sodium ascorbate. Accurate
lattice constants were measured from the angular settings of
60 reflections with 15° < θ < 40°, collected with a crystal
mounted on a P4 Bruker-AXS diffractometer, using Cu KR
radiation (λ ) 1.54184 Å). Intensity data in the 3-27° θ
range were measured on a Enraf-Nonius CAD4 diffracto-
meter, with graphite-monochromatized Mo KR radiation (λ
) 0.71069 Å). Data were corrected for Lorentz and polariza-
tion effects. An empirical absorption correction was applied
(ψ scans, Tmin/max ) 0.596/0.998). The structure was solved
by direct methods and heavy atom techniques, with SIR14
and SHELXL 93,15 and was refined by full-matrix least
The crystal structure, solved by X-ray diffraction methods,
consists of two-dimensional arrays formed by the Cd2-
(C2O4)2‚6H2O repeat unit, where both of the symmetry-
independent cadmium atoms are in a seven-coordinate
environment of five oxalate and two water oxygens (Figure
1). Two water molecules of the repeat unit are uncoordinated
and are involved in the hydrogen bond network present in
the structure. Values of selected bond lengths and angles
are reported in Table 1. The coordination geometry about
each metal atom is approximately that of a pentagonal
bipyramid with a water and an oxalate oxygen in the apical
positions. Half of the planar oxalate anions provide donor
(13) Trollet, D.; Rome´ro, S.; Mosset, A.; Trombe, J. C. Polyhedron 1998,
17, 3977.
(14) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo,
C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27, 435.
(15) Sheldrick, G. M. SHELXL 93, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Germany, 1993.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4313

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squares on F2. In the final cycles all non-hydrogen atoms
were refined anisotropically and the hydrogen atoms were
given an overall isotropic temperature factor. The H atom
positions were refined, with restraints on the O-H distances
and the H-O-H angles. A correction for the extinction
effect was applied, and the absolute structure was assigned,
based on Flack’s test.16 Crystal data for 1 are as follows:
C4H12O14Cd2, fw ) 508.94; colorless plates; triclinic;
space group P1 (No. 1); a ) 6.010(1) Å, b ) 6.668(1)
Å, c ) 8.498(1) Å; R ) 74.64(1)°, â ) 74.25(1)°, γ )
80.91(1)°; V ) 314.7(5) Å3; Z ) 1; Dcalc ) 2.685 g cm-3;
µ(Mo KR) ) 3.45 mm-1; R1) 0.024, wR2 ) 0.061.
Supporting Information Available: Crystallographic tables and
CIF file. These materials are available free of charge via the Internet
at http://pubs.acs.org.
(16) Flack, H. D. Acta Crystallogr., Sect. A 1983, 39, 876.
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4314 Inorganic Chemistry, Vol. 41, No. 17, 2002