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May M, Gantt M, Hoadley C, Batten T, Sayers W, Katovic V: Electrochemistry of niobium (V) in sulfuric and methanesulfonic acids: formation of the Nb3O2 (SO4) 6 (H2O)(3)(5-) cluster and designed electrochemical generation of "Nb3O2" core clusters by double potential pulse electrolysis. Inorg Chem. 2003 Nov 3;42(22):7137-47.
The electrochemical and spectroelectrochemical properties of niobium (V) and the Nb (3) O (2)(SO (4))(6)(H (2) O)(3)(5-) cluster in sulfuric acid and methanesulfonic acid were investigated using cyclic voltammetry, constant potential electrolysis, and spectroelectrochemistry. These chemical systems were suitable to probe the formation of "Nb (3) O (2)" core trinuclear clusters. In 9 M H (2) SO (4) the cluster Nb (3) O (2)(SO (4))(6)(H (2) O)(3)(5-) exhibited a reversible 1-electron reduction peak at E (pc) = -1.30 V vs Hg/Hg (2) SO (4) electrode, as well as a 4-electron irreversible oxidation peak at E (pa) = -0.45 V. Controlled potential reduction at E = -1.40 V produced the green Nb (3.33+) cluster anion Nb (3) O (2)(SO (4))(6)(H (2) O)(3)(6-). In 12 M H (2) SO (4) Nb (V) displayed two reduction peaks at E (pc) = -1.15 V and E (pc) = -1.30 V. It was determined that the first process involves a quasi-reversible 2-electron reduction. After reduction of Nb (V) to Nb (III) the following chemical step involves formation of [Nb (III)](2) dimer, which further reacts with Nb (V) to produce the Nb (3) O (2)(SO (4))(6 (H (2) O)(3)(5-) cluster (ECC process). The second reduction peak at E (pc) = -1.30 V corresponds to further 2-electron reduction of Nb (III) to Nb (I). The electrogenerated Nb (I) species also chemically reacts with starting material Nb (V) to produce additional [Nb (III)](2). In 5 M H (2) SO (4), the rate of the second chemical step in the ECC process is relatively slower and reduction of Nb (V) at E = -1.45 V/-1.2 V produces a mixture of Nb (3) O (2)(SO (4))(6)(H (2) O)(3)(5-) and [Nb (III)](2) dimer. [Nb (III)](2) can be selectively oxidized by two 2-electron steps at E = -0.65 V to Nb (V). However, if the oxidation is performed at E = -0.86 V, the product is Nb (3) O (2)(SO (4))(6)(H (2) O)(3)(5-). A double potential pulse electrolysis waveform was developed to direct the reduction of Nb (V) toward selective formation of the Nb (3) O (2)(SO (4))(6)(H (2) O)(3)(5-) cluster. Proper application of dc-voltage pulses alternating between E (1) = -1.45 V and E (2) = -0.86 V yields only the target trinuclear cluster. Analogous double potential pulse electrolysis of Nb (V) in methanesulfonic acid generates the "Nb (3) O (2)" core cluster Nb (3) O (2)(CH (3) SO (3))(6)(H (2) O)(3)(+). |
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