Ethanol oxidation by chromium(III) complexes of chloranilic acid. Spectroscopic, electrochemical, magnetic, and chromatographic studies of chloranilate semiquinone-bridged polynuclear chromium ions
The oxidation of ethanol by chloroanilic acid (H/sub 2/CA/sub ox/; 2,5-dihydroxy-3,6-dichlorobenzoquinone), catalyzed by Cr(III), has been reexamined. Rate and spectrophotometric studies showed that a binuclear complex of the form Cr/sub 2/(CA/sub ox/)/sup 4 +/ is the most reactive catalyst species;...
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Veröffentlicht in: | Inorg. Chem.; (United States) 1985-01, Vol.24 (2), p.153-159 |
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Zusammenfassung: | The oxidation of ethanol by chloroanilic acid (H/sub 2/CA/sub ox/; 2,5-dihydroxy-3,6-dichlorobenzoquinone), catalyzed by Cr(III), has been reexamined. Rate and spectrophotometric studies showed that a binuclear complex of the form Cr/sub 2/(CA/sub ox/)/sup 4 +/ is the most reactive catalyst species; reduction of Cr/sub 2/(CA/sub ox/)/sup 2 +/ is first order with respect to ethanol (95-99%), with a rate constant of 1.5 X 10/sup -3/ M/sup -1/s/sup -1/ (60/sup 0/C). Spectrophotometric, chromatographic, magnetic susceptibility, electron paramagnetic resonance, and cyclic voltammetric measurements on the reduced chloroanilate- (CA/sub r/-) containing catalyst reduction product are also reported. Both 2:1 (I) and 5:2 Cr-Ca/sub r/ complexes, with intense absorption maxima at 300, 355, 428, 458, 494, 600, and 654 nm, are formed in ethanol/water solutions. Binuclear complex I in 99% ethanol exhibits an effective magnetic moment per Cr atom of 3.81 ..mu../sub B/ (298.2 K), near the spin-only value for three unpaired electrons, an EPR resonance at g = 1.97 (100K), and a quasi-reversible cyclic voltammogram (0.1 M (n-C/sub 4/H/sub 9/)/sub 4/NClO/sub 4/, 25.0/sup 0/C) indicative of a two-electron oxidation with E/sub 1/2/ = +0.46 V vs. SCE. Cation-exchange separations on SP-Sephadex C-25 resin confirmed the stoichiometry of I and demonstrated that its immediate oxidation product rapidly decays to give Cr(H/sub 2/O)/sub 6//sup 3 +/ and a 1:1 complex (Cr(CA/sub ox/)/sup +/) in aqueous acidic solution. The 400-500 nm spectra of I and the chloranilate semiquinone trianion radical (CASQ/sup 3 -/) are remarkably similar, suggesting that the predominant resonance form of I is Cr(II)-bridging CASQ/sup 3 -/-Cr(III) rather than Cr(III)-bridging chloranilate hydroquinone-Cr(III). Qualitative molecular orbital arguments are presented to reconcile the magnetic results with this hypothesis and account for the electronic spectra of CASQ/sup 3 -/ and I. 45 references, 3 figures, 2 tables. |
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ISSN: | 0020-1669 1520-510X |
DOI: | 10.1021/ic00196a007 |