MONODENTATE CHROMIUM(III) ASCORBATE COMPLEXES

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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY Vol. 32, No. 6, pp. 1071±1084, 2002

MONODENTATE CHROMIUM(III) ASCORBATE COMPLEXES PREPARED VIA CHROMATE REDUCTION IN THF BirguÈl ZuÈmreogÆlu-Karan,* Ahmet Nedim Ay, and Canan UÈnalerogÆlu Department of Chemistry, Hacettepe University, Beytepe Campus, 06532 Ankara, Turkey

ABSTRACT Two new Cr(III)-ascorbate complexes, K[Cr(C6H7O6)2(OH)2(H2O2)2]3H2O (1) and K2[Cr(C6H7O6)3(OH)2(H2O)]2H2O (2), have been prepared by reducing chromate with L-ascorbic acid in aqueous THF. The complexes have been characterized by various analytical and spectroscopic methods (UV=Vis, FTIR, 13C NMR and EI MS) and by thermal and magnetic measurements. Experimental results indicated a distorted octahedral environment around the Cr(III) centers and the coordination mode of ascorbic acid has been shown to be monodentate through its O(3) atom by NMR studies. The analytical and spectroscopic data available allowed the proposal of structural formulae for (1) and (2).

*Corresponding author. E-mail: [email protected] 1071 DOI: 10.1081=SIM-120013021 Copyright # 2002 by Marcel Dekker, Inc.

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ZUÈMREOGÆLU-KARAN, AY, AND UÈNALEROGÆLU

INTRODUCTION Chromium is known to exert serious toxic eects on humans and animals in its VI oxidation state. It is commonly believed that Cr(VI) compounds are metabolized by intracellular reduction to form powerful carcinogens and Vitamin C (L-ascorbic acid) is one of the cellular components for this reduction.[1] Ascorbic acid (AH2) (Fig. 1) is known to form both mono- and dianionic species, depending on the pH of the aqueous solution.[2] The monoanion (AH ) forms at pH 475 with deprotonation of the O(3)-H, while the dianion (A2 ) forms at pH 11712 with deprotonation of the O(2)H. The acid exhibits both one-electron and two-electron reducing ability.[3] The reduction of chromate by ascorbic acid is sketched in the scheme of Fig. 2 and occurs via the formation of a chromate ester intermediate, which undergoes rapid electron transfer reactions by an inner sphere mechanism, to produce chromium(III) and dehydroascorbate (DAH) as ®nal products.[4,5] With excess of ascorbate, the reduction mechanism for the overall reaction is the three-electron reduction of Cr(VI) by the two-electron reductant ascorbate, as shown[6,7] in Eq. (1). With excess of Cr(VI) the reactivity is more complicated.[8] 3 AH 2 CrVI ! 3 DAH 2 CrIII 3H

1

The oxidized form, DAH, is a much less eective ligand than ascorbate, AH , itself.

Figure 1.

Structure of L-ascorbic acid.

CHROMIUM(III) ASCORBATE COMPLEXES

Figure 2.

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A summary of the redox processes involving Cr(VI) and ascorbic acid.

In vitro chromate reduction by saccharides has been investigated by Rao et al.[9711] Studies on reduction by ascorbic acid are, however, less comprehensive and a family of complexes with the general formula shown below has been described by Cieslak-Golonka et al.[12715] Kn2mz n 0

3 Cr

III

AHn Am OHz yH2 O

3; m 0

2; x 0

3; y 0

7

Due to the complexity of the redox chemistry of Cr(VI) species and the chemical unstability of AH2 in aqueous solutions, the isolation and the characterisation of the ®nal products is troublesome. In addition, it is dif®cult to decide from the analytical results which form of ascorbic acid, AH or A2 , coordinates to the metal. Although the hydrolysis of the metalascorbate complexes might be expected to form the dianionic ligand (A2 ) in alkaline solutions according to Eq. (2), where z oxidation state of metal(M)-n, the ®rst coordination sphere involves the mono-anionic ligand at neutral pH for most transition metals used.[16]

z z MAHn H2 Om H2 O! MAHn 1 AH2 Om

1

H3 O

2

Structural studies in metal ascorbate chemistry suer from the reluctance of the complexes toward crystallization due to the formation of strong hydrogen bonds with the solvents commonly applied. Apart from the few mixed ligand platinum complexes characterized by crystallography and NMR,[17720] the structural descriptions derived from limited spectroscopic data are speculative and need further research. We attempted to perform the chromate reduction by AH2 in THF to avoid rapid redox reactions and de®ne the products with a variety of methods. Here we report the isolation and characterization of two new ascorbato complexes.

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EXPERIMENTAL Materials K2CrO4 (Carlo Erba) was used as received. The purity of L-ascorbic acid (Merck) was checked by HPLC.[21] No signi®cant amount of DAH was detected and, therefore, AH2 was used without further puri®cation. THF and methanol (MeOH) were puri®ed by the common methods, distilled and kept under nitrogen to remove traces of oxygen. Deionized water was deoxygenated by re¯uxing at the boiling point for 8 h under nitrogen. Synthesis The experiments were done under anaerobic conditions under nitrogen atmosphere. Solid AH2 (3.575.3 g, 20.0730.0 mmol) and solid K2CrO4 (0.97 g, 5.0 mmol) were added to 100 mL THF in a molar ratio of AH2:Cr(VI) 4 and 6 for the complexes (1) and (2), respectively. A minor amount of water (172 mL) was added since no signi®cant reaction occurred in pure THF. The mixture was stirred for 30 min at 30 C while a green coloration was observed. After 30 min of more stirring at 40 C, a viscous dark-green phase started to separate. The ¯ask was kept in a refrigerator after the addition of 100 mL of MeOH. A few days later, the solution phase was decantated, the sticky green precipitate was broken up and dried in vacuo. The yield of (1) was ca. 70% (1.9 g) and for (2) was also ca. 70% (2.6 g), based on chromium. Anal.: Found for (1): C, 25.60; H, 4.85; Cr, 9.08; K, 6.40. Anal. Calc. for KCr(C6H7O6)2(OH)2(H2O)5 (FW 565 g mol 1): C, 25.50; H, 4.64; Cr, 9.19; K, 6.90%. Found for (2): C, 29.42; H, 3.57; Cr, 7.21; K, 10.90. Anal. Calc. for K2Cr(C6H7O6)3(OH)2(H2O)3 (FW 743 g mol 1): C: 29.37, H: 3.90, Cr: 7.00, K, 10.50%. The green complexes are soluble in water and DMSO, insoluble in acetone and other common organic solvents. Caution Chromium(VI) compounds and the intermediates generated during the reduction of Cr(VI) are toxic and mutagenic, therefore, care should be taken to prevent inhalation and contact with skin. Instruments and Methods Carbon and hydrogen contents were determined by a LECO CHNS-932 elemental analyzer. Chromium and potassium were determined spectro-


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photometrically on a Shimadzu AA-660 instrument. IR spectra were recorded on a System 2000 Perkin Elmer FTIR spectrophotometer as KBr disks. The electronic spectra were recorded in H2O on a Unicam UV2-l00 UV=VIS spectrophotometer. The EI mass spectrum of (1) was recorded on an Intectra M40 spectrometer and of (2) on a Micromass UK LCMS Platform II spectrometer. 13C NMR spectra were recorded on a Bruker GMbH DPX-400 spectrometer using the double tube technique. D2O in the outer tube was used as the NMR lock solvent and Me4Si as the reference. Magnetic susceptibility measurements were carried out at room temperature by a PAR Co. Model 150 A Parallel Field Vibrating Sample Magnetometer. Diamagnetic corrections were made using Pascal's constants and the value 73.29610 6 emu mol 1 was applied as the diamagnetic correction factor for ascorbic acid.[22] Thermogravimetric analyses of compounds (1) and (2) were performed under dynamic nitrogen atmosphere on a Dupont 951 thermal analyzer at a heating rate of 10 C min 1. RESULTS AND DISCUSSION Reduction of Cr(VI) ions by AH2 in THF resulted in the formation of stable Cr(III) ascorbate complexes. The complexes are formulated as KCr(C6H7O6)2(OH)2(H2O)5 (1) and K2Cr(C6H7O6)3(OH)2(H2O)3 (2) from the microanalytical results considering the monoanionic form of ascorbic acid, six-fold coordination of Cr(III) and the charge balance. OH ions and H2O molecules are provided by the esteri®cation reaction as outlined in the scheme of Fig. 2 or by the small amount of water added to the system. As a consequence of the applied molar ratios and the redox reaction, two and three molecules of the reduced form of ascorbate coordinate to Cr(III), respectively for (1) and (2). The vacant coordination sites are occupied by the OH and H2O ligands (Fig. 3).

Figure 3.

Suggested structures of the complexes (1) and (2).

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Electronic Spectra Aqueous solutions of (1) and (2) gave broad bands at 582 nm and 556 nm corresponding to the d-d (4T2g / 4A2g) transitions. The spectra exhibited another broad d-d band in the region of 380±420 nm (4T1g / 4A2g) as a shoulder overlapped with the charge transfer bands of higher intensity. The comparison of the data with those of the complexes reported previously is given in Table I. The values indicate a pseudo-octahedral environment around Cr(III) in (1) and (2). The six-fold coordination in the other complexes could be attained by the entrance of the water molecules in the coordination sphere, however, there exists no reasonable explanation for those proposed structures in the literature as to how AH2 behaves as a dianion[12] nor does it behaves as a monoanion and dianion simultaneously[14], under mild acidic to neutral conditions.

Magnetic Studies Room temperature magnetic susceptibility measurements on (1) and (2) gave me values comparable with those calculated for the related complexes in the literature (Table I). The values are slightly lower than the expected one (me 3.87 B.M.) for a d3 (Oh) con®guration, indicating distorted octahedral symmetry. A slight contamination of the products with Cr(V)

Table I. d-d (4T2g/4A2g) Band Positions and Eective Magnetic Moments of Some Anionic-type Cr(III)-Ascorbate Complexesa Compound

l (nm)

K[Cr(C6H6O6)2(OH)2]0.5KOH K2Cr(C6H6O6)2(OH)]6H2O K[Cr(C6H6O6)2]7H2O K[Cr(C6H7O6)2(OH)2]4H2O K2[Cr(C6H706)2(C6H6O6)9H2O K[Cr(C6H7O6)2(OH)2(H2O)2]3H2O K2[Cr(C6H7O6)3(OH)2(H2O)]2H2O

590 581 581 602 602 582 556

a

(38) (48) (92) (57) (35)

meffb (B.M.)

Reference

7 3.27 3.76 3.77 3.76 3.51c 3.30c

5 12 12 14 14 this work this work

From water solution. The molar absorption coef®cients, e (dm3 mol given in parentheses. b Measured in the temperature range 807300 K. c Measured at 293 K.

1

cm 1), are


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(me 1.8 B.M.) and Cr(IV) (me 2.8 B.M.) species generated during the reduction process is also possible. FTIR Spectra Almost all characteristic bands of AH2 broaden upon complexation with metals. It is, therefore, dicult to assign the binding mode of the acid as monodentate [through O(3)] or bidentate [through O(3) and O(2)] from the IR spectra. Compounds (1) and (2) gave almost the same spectral patterns. An asymmetric band at ca. 3410 cm 1 resulted from the overlapping O-H stretching vibrations of the ligands and water. AH2 carbonyl C(1)O stretching at 1754 cm 1 bathochromically shifted to 1727 cm 1. Similarly, the broad and very strong band at 1670 cm 1 in the free acid, attributed to the combination band due to n(CO) n(CC), was shifted to 1622 cm 1. A broad band with strong intensity at 1372 cm 1 is assigned to the C(3)-O stretching vibration. The appearance of this band as a broad and strong peak suggests the coordination of the O(3) atom of the ascorbate anion. There was a reduction of the number of bands in the ring C-O and C-C vibrations (11007900 cm 1) of ascorbic acid and accompanying shifts were observed through interactions with chromium. NMR Studies The assignments of the 13C NMR chemical shifts of AH2 and some of its metal salts have been reported in the literature. The results in D2O are summarized in Table II. Upon acid ionization, drastic changes are observed for the chemical shifts of C(3) (20 ppm down®eld), C(2) (down®eld or up®eld), C(1) (down®eld) and C(4) (down®eld).[23,24] The positions of C(5) and C(6) are not aected signi®cantly. The observed change for C(3) is due to the related variations in bond lengths and delocalization of the electron distribution throughout the ene-diol and carbonyl groups as a result of ionization of the acid at this position. Consequently, the C(2) carbon shows a considerable up®eld shift if the binding to the metal is through the O(3) atom only, as for alkaline and alkaline earth salts. On the other hand, a down®eld shift of C(2) is indicative of chelation through the O(2) and O(3) atoms.[25727] The structures of zinc- and lead ascorbates are assumed to be of the chelate-type on the basis of related spectroscopic data. Since the transition metal ascorbate complexes have low solubilities in common organic solvents and the metals in the complexes are mostly paramagnetic, diculties are faced in obtaining and evaluating their NMR


1078 Table II.

13

C NMR Dataa of L-Ascorbic Acid and Some Metal Compounds

Compound L-Ascorbic acid Na(C6H7O6) Mg(C6H7O6)24H2O Ca(C6H7O6)22H2O Sr(C6H7O6)22H2O Zn(C6H7O6)22H2O Pb(C6H7O6)22H2O (1) (2) a

C(1)

C(3)

C(2)

C (4)

C(5)

C(6)

Reference

173.74 178.09 178.39 178.19 178.19 177.80 176.27 177.88 176.98

156.07 176.34 175.85 175.85 176.20 174.37 175.12 175.97 172.28

118.51 113.85 114.64 114.45 113.93 122.45 120.00 113.59 114.34

76.87 79.28 79.90 79.30 79.21 78.77 78.73 78.87 78.52

69.66 70.46 70.52 70.52 70.46 70.60 70.26 70.07 69.89

62.80 63.46 63.49 63.30 63.38 63.24 62.89 63.07 62.99

24 24 24 24 24 26 27 this work this work

Values are in ppm.

spectra. The presence of unpaired electrons produces additional relaxations which cause signals to broaden and shift. However, fast relaxation processes may give rise to narrow signals[28] and in recent studies NMR has been applied as a valuable tool for the characterization of the structures and reaction mechanisms of paramagnetic complexes.[29731] The complexes gave narrow NMR signals despite their paramagnetism. The clear-cut de®nition of six carbon atoms for both (1) and (2) suggests that the ascorbate anions present in the complexes are in similar chemical environments. The C(3) chemical shifts are 20 ppm down®eld [175.97 and 172.28 ppm for (1) and (2)] as expected. The C(2) shifts for (1) and (2) are, however, close to those of metal ascorbates where unidentate binding [via O(3)] prevails. The up®eld chemical shifts of the C(2) carbons (113.59 and 114.34 ppm, respectively) indicate that the coordination mode of ascorbic acid in the complexes is monodentate through the O(3) atom. Accordingly, the observed results may allow the proposal of the following complex structures for (1) and (2) as shown in Fig. 3. Two additional peaks were observed at 68.37 and 25.52 ppm corresponding to THF carbons. Apparently, some THF is captivated during the preparation process. The facts that the chemical shifts match with those of pure THF and the agreement of the microanalytical results with the calculated values removes the possibility of THF acting as a ligand as well. Thermal Analyses Thermogravimetric decomposition curves of (1) and (2) exhibited similar patterns. The decompositions up to 120 C correspond to dehydration of


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non-coordinated water. The mass losses at this step are 9.6% for (1) (calc. 9.5% from K[Cr(C6H7O6)2(OH)2(H2O)]3H2O) and 5.2 for (2) (calc. 4.8% from K2[Cr(C6H7O6)3(OH)2(H2O)2H2O). Above 200 C, removal of ligand water and decomposition of the complex proceed simultaneously. The results are in accord with the thermal data of related complexes[15] and support the proposed chemical and structural formulae of the complexes. Mass Spectral Studies EI mass spectra, given in Figs. 4 and 5, show fragments of the complexes and the degradation of ascorbate groups. Molecular ion peaks were not observed. The proposed fragments for (1) are presented in the scheme of Fig. 6. On the other hand, two fragmentation pathways are possible for (2). A regular fragmentation from the Cr(AH)3 structure (m=z 577) proceeds through the removal of m=z 73 and m=z 61 moieties from the ascorbate ligands (scheme of Fig. 7). Successive removal of m=z 73 gives rise to the following peaks: m=z 504 [m=z 577 7 m=z 73], m=z 503 [m=z 504 7 H], m=z 430 [m=z 503 7 m=z 73], m=z 429 [m=z 430 7 H], m=z 356 [m=z 429 7 m=z 73], m=z 355 [m=z 577 7 H]. Removal of m=z 61 produces the following peaks: m=z 283 [m=z 577 7 m=z 175 (AH) 7 2 m=z 613H], m=z 253 [m=z 283 7 m=z 30 (HCOH) (AH)]

Figure 4.

EI mass spectrum of complex (1).


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Figure 5.

EI mass spectrum of complex (2).

Figure 6.

Suggested fragments of (1).


Figure 7.

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Suggested fragmentations of the ascorbate group in (2).

or [m=z 4297m=z 176 (AH2)], m=z 238 [m=z 2837m=z 45(COOH)], m=z 223 [m=z 223 7 m=z 30 (HCOH)], m=z 207 [m=z 253 7 m=z 46 (HCOOH)]. CONCLUSION Cr(III) complexes can be isolated from the reduction of chromate ion with ascorbic acid in THF. The described procedure appears to be eective in avoiding the formation of undesired products and allows the characterization of the Cr(III) products that do not suer from the rapid and complex redox chemistry as in aqueous solutions. AH2 not only reduces Cr(VI) to Cr(III) but also coordinates to the metal. UV=Vis spectra and magnetic measurements on the complexes indicated that the Cr(III) centers have pseudo-octahedral geometries. Structural information obtained by 13C NMR spectroscopy demonstrates the monodentate coordination of the ascorbate ligands through their O(3) atoms while vacant sites are occupied by hydroxo and aqua ligands. The combination of the observed analytical and spectroscopic results leads to the conclusion that the complexes are anionic with potassium ions acting as counter ions. Mass spectra of the complexes include typical peaks expected from the proposed structures. In addition to the known complexes of platinum involving monodentate ascorbate ligands,[18,20] the information reported here provides a new insight into the vagueness of the binding modes of ascorbic acid in the complexes with ®rst-raw transition metals. ACKNOWLEDGMENTS This work constitutes a part of a research project (no. 1692) granted by Turkish Scienti®c and Technological Research Council. The authors are

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Received September 17, 2001 Accepted April 5, 2002

Referee I: P. Augustin Referee II: M. Veith