Isolation of a Dodecanuclear Polyoxo Cluster {Nb₁₂O₂₁} Showing a Rare Case of Five-fold Coordinated Niobium(V) Centers with a Square Pyramidal Geometry

The reactivity of the complexing anthracene-9-carboxylate ligand has been investigated with a niobium(IV) tetrachloride precursor (NbCl4 center dot 2THF) in isopropanol solvent. This resulted in the crystallization of a molecular assembly containing two distinct {Nb12O21} cores surrounded by multiple isopropanolate and anthracenoate ligands. The compound is formulated [Nb-12((mu(3)-O)(3)(mu-O)(18)(C15H9O2)(8)((OPr)-Pr-i)(10)] center dot [Nb-12(mu(3)-O)(2)(mu-O)(19)(C15H9O2)(8)((OPr)-Pr-i)(10)] illustrating the two different dodecameric oxo-clusters, for which the niobium(IV) precursor was oxidized in the niobium(V) state during the reactional process. The two distinct {Nb12O21} units mainly differs by the environment of the niobium centers, which exhibits unexpected five-fold coordination (square pyramid) for some of them, together with the classical six-fold coordination (octahedron) as usually found for niobium(V). In the crystallization process, the. IR spectroscopy was used to analyze the esterification reaction occurring between the anthracene acid an isopropanolate ligands responsible of the production of water used in the oxo-condensation of the niobium centers. Nb-93 Solid state NMR was tentatively used to assess the occurrence of the different niobium environments.