Preparation, Properties, and Reactivities of Unprecedented Oxo-Sulfido Nb(IV) Aqua Ions and Crystal Structure of (Me<sub>2</sub>NH<sub>2</sub>)<sub>6</sub>[Nb<sub>5</sub>(μ<sub>3</sub>-S)<sub>2</sub>(μ<sub>3</sub>-O)<sub>2</sub>(μ<sub>2</sub>-O)<sub>2</sub>(NCS)<sub>14</sub>]·3.5H<sub>2</sub>O
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By treatment of Zn-reduced ethanolic solutions of NbCl5 with HCl in the presence of sulfide followed by cation-exchange chromatography, two oxo-sulfido niobium aqua ions, the red [Nb4(μ4-S)(μ2-O)5(H2O)10]4+ and the yellow-brown [Nb5(μ3-S)2(μ3-O)2(μ2-O)2(H2O)14]8+, were isolated. Both readily form their respective thiocyanate complexes, for which the structure for the former has been previously reported. Brown crystals of (Me2NH2)6[Nb5S2O4(NCS)14]·3.5H2O (1) were isolated in the case of the latter, and the structure was determined by X-ray crystallography (space group: a = 15.4018(5) Å, b = 21.1932(8) Å, c = 22.0487(8) Å, α = γ = 90°, β = 103.4590(10)°, and R1 = 0.0659). An unprecedented pentanuclear Nb5S2O48+ core is revealed in which short Nb−Nb distances (2.7995(8)−2.9111(8) Å) are consistent with metal−metal bonding. A stopped-flow kinetic study of the 1:1 equilibration of NCS- with [Nb4(μ4-S)(μ2-O)5(H2O)10]4+ has been carried out. Equilibration rate constants are independent of [H+] in the range investigated (0.5−2.0 M) and at 25 °C; kf = 9.5 M-1 s-1, kaq = 2.6 × 10-2 s-1, and K = 365 M-1. Conditions with first NCS- and then [Nb4(μ4-S)(μ2-O)5(H2O)10]4+ in excess revealed a statistical factor of 4, suggesting the presence of four kinetically equivalent Nb atoms. Attempts to study the 1:1 substitution of NCS- with [Nb5(μ3-S)2(μ3-O)2(μ2-O)2(H2O)14]8+ showed signs of saturation kinetics. Quantum chemical calculations using the density functional theory (DFT) approach were performed on both the Nb4O5S4+ and Nb5O4S28+ naked clusters. The highest occupied and lowest unoccupied molecular orbitals have dominant Nb(4d) character. The HOMO for Nb4O5S4+ is a nondegenerate fully filled MO, whereas for Nb5O4S28+, it is a nondegenerate partially filled MO with one unpaired electron. EPR spectroscopy on [Nb5(μ3-S)2(μ3-O)2(μ2-O)2(H2O)14]8+ shows that the molecule has total anisotropy (C2v), with all three tensors, gx = 2.399, gy = 1.975, and gz = 1.531, resolved. No hyperfine interaction expected from the nuclear moment of I = 9/2 for 93Nb was observed.
