Electrochemistry of Niobium(V) in Sulfuric and Methanesulfonic Acids: Formation of the Nb3O2(SO4)6(H2O)35- Cluster and Designed Electrochemical Generation of "Nb3O2" Core Clusters by Double Potential Pulse Electrolysis

The electrochemical and spectroelectrochemical properties of niobium(V) and the Nb3O2(SO4)6(H2O)35- 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 "Nb3O2" core trinuclear clusters. In 9 M H2SO4 the cluster Nb3O2(SO4)6 (H2O)35- exhibited a reversible 1-electron reduction peak at Epc = -1.30 V vs Hg/Hg2SO4 electrode, as well as a 4-electron irreversible oxidation peak at Epa = -0.45 V. Controlled potential reduction at E = -1.40 V produced the green Nb(3.33+) cluster anion Nb3O2(SO4)6(H2O)36-. In 12 M H2SO4 Nb(V) displayed two reduction peaks at Epc = -1.15 V and Epc = -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 Nb3O2(SO4)6(H2O)35- cluster (ECC process). The second reduction peak at Epc = -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 H2SO4, 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 Nb3O2(SO4)6 (H2O)35- 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 Nb3O2(SO4)6(H2O)35-. A double potential pulse electrolysis waveform was developed to direct the reduction of Nb(V) toward selective formation of the Nb3O2(SO4)6(H2O)35- cluster. Proper application of dcvoltage pulses alternating between E1 = -1.45 V and E2 = -0.86 V yields only the target trinuclear cluster. Analogous double potential pulse electrolysis of Nb(V) in methanesulfonic acid generates the "Nb3O2" core cluster Nb3O2(CH3SO3)6(H2O)3+.