An EPR Spectroscopic Study of Chromium(V) Oxalato Complexes in Aqueous Solutions. Mechanism of the Chromium(VI) Oxidation of Oxalic Acid

Inorganic Chemistry

Volumn 37, Issue 13, 1998, Pages 3159-3166

  Farrell, Rodney P ^a   Lay, Peter A ^a   Levina, Aviva ^a   Maxwell, Ian A ^a   Bramley, Richard ^b   Brumby, Steven ^b   Ji, Ji Ying ^b  

^a UNIVERSITY OF SYDNEY   (Australia)

^b AUSTRALIAN NATIONAL UNIVERSITY   (Australia)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0000704824     PISSN: 00201669     EISSN: None     Source Type: Journal    
DOI: 10.1021/ic971070i     Document Type: Article

References (39)

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19. 11 Therefore, the relative areas of EPR signals, estimated by digital simulations, are directly proportional to the concentrations of corresponding EPR-active species.
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23. 2 system.
24. 11
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31. 3
32. The formation of Cr(V) adducts with different forms of Cr(III) present in the reaction mixture (for example, Cr(III)a-c in Scheme 2) is expected. These adducts can include both oxalato- and oxo-bridged Cr(III)-Cr(V) complexes. However, most of these mixed-valence complexes will not result in such sharp EPR signals at room temperature due to interactions of the paramagnetic Cr(III) and Cr(V) centers. It is unclear as to the nature of the species giving rise to signal 6, but the most likely species would be Cr(III)-Cr(III)-oxalato-Cr(V) trimers. Rapidly relaxing Cr(III) dimers with hydroxo and other bridges are common in the aqueous chemistry of Cr(III), and such dimeric centers bound to Cr(V) via an oxalato bridge would minimize broadening of the Cr(V) signal. Such a species appears to be the most viable structure to account for signal 6.
33. Niemann, A.; Bossek, U.; Haselhorst, G.; Wieghardt, K.; Nuber, B. Inorg. Chem. 1996, 35, 906-915.
34. Farrell, R. P.; Maxwell, I. A.; Lay, P. A. To be submitted for publication.
35. The presence of two μ-O atoms in the structure of the Cr(V)-Cr(VI) dimer, 7, is consistent with the structure of the crystallographically characterized Cr(V)-Cr(V) dimer. di-μ-oxobis[oxo(perfluoropinacolato)chromate(V)] (Nishino, H.; Kochi, J. K. Inorg. Chim. Acta, 1990, 174, 93-102). However, alternative structures of the complex 7, including oxalate as a bridging ligand, cannot be ruled out.
36. 0 (Levina, A. Unpublished results).
37. 0 (Levina, A. Unpublished results).
38. 2 system, the formation of bis-oxalato complexes is also expected to be thermodynamically favorable. However, the experimental evidence (refs 11 and 31 and the current work) points to Cr(VI) existing mainly in the monooxalato form. This suggests that the Cr(VI) bis-oxalato intermediates are rapidly consumed in the following redox reactions. Therefore, the steady-state approximation for the complex Cr(VI)d (Scheme 4) is appropriate.
39. 2 was not taken into account).