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+(NbIII,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 NbV. 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–, [NbIII,IV,IV3(µ3-S)(µ-O)3(NCS)9]6–(S2– 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 BO45– 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.