Characterisation of a cationic triniobium aqua ion cluster and related studies

Abstract

A green air-sensitive niobium cation has been characterised in acidic aqueous solutions following Dowex cation-exchange column chromatography of hydrolysed solutions of [NbCl₃(dme)](dme = 1,2-dimethoxyethane) in aqueous HCl. Redox titrations and oxygen-17 NMR studies on the green ion suggest the presence of the incomplete cuboidal structure [Nb₃(µ₃-Cl)(µ-O)₃(OH₂)₉]⁴⁺(Nb^III,IV,IV). Air-free solutions in HCl, CF₃SO₃H and p-MeC₆H₄SO₃H are stable for weeks frozen at –18 °C but the use of HClO₄ results in fairly rapid oxidation to colourless Nb^V. Water-exchange kinetic studies (use of ¹⁷O NMR) have identified two distinct types of water ligand consistent with the incomplete cuboidal triangular structure. Those water ligands trans to the capping (Cl) atom (one per Nb)(k²⁹⁸= 8.3 × 10^–3 s^–1) are significantly more inert than those trans to the bridging oxo groups (two per Nb)(k²⁹⁸= 352 s^–1). The reaction products obtained by treatment of the green ion with aqueous methanolic NCS^–, [Nb^III,IV,IV₃(µ₃-S)(µ-O)₃(NCS)₉]^6–(S^2– deriving from NCS^–), and with aqueous ethanolic hydridotris(pyrazol-1-yl)borate, HB(pz)₃^–, [Nb₃(µ₃-BO₄){µ-(O,OH)}₃{HB(pz)₃}₃][the BO₄^5– ligand deriving from the hydrolysis of HB(pz)₃^–], suggest the ready formation of a range of triangular Nb₃ cluster cores with different complexing ligands.