Synthesis and Selected Properties of New Complex Cation Decorated Polyoxotantalates

Diffusion-based room temperature syntheses using K-8{Ta6O19}center dot 16H(2)O, d-block metal (M) salts (M = Cu, Zn, or Cd), and the macrocyclic ligand 1,4,8,11-tetraazacyclotetradecane (cyclam) led to crystallization of the three new polyoxotantalates with the composition K-4{[Cu(cyclam)](2)Ta6O19}center dot 18H(2)O (I), K-4{[Zn(cyclam)](2)Ta6O19}center dot 18H(2)O (II), and {[Cd-(cyclam)](4)Ta6O19}.19H(2)O (III). The latter two compounds represent the first polyoxotantalates (POTas) with Zn2+ and Cd2+ centered amine complexes and also the first POTas synthesized using a macrocyclic amine. Compounds I and II are isostructural with two complexes attached to the anionic cluster via Cu-O-Ta respectively Zn-O-Ta bonds. Despite that these two compounds are isostructural, the geometry of the M2+ complexes differs significantly with Cu2+ in a square-pyramidal and Zn2+ in a trigonal-bipyramidal environment. In the structure of III, the Cd2+ cations are in the rare hepta-coordinated monocapped trigonal prismatic geometry covalently linked to the anionic core via Cd-O-Ta bonds. While compounds I and II are quite stable under ambient conditions, III shows a fast loss of crystal water molecules. Temperature-dependent in situ XRD measurements were performed with I demonstrating stepwise structural rearrangements caused by water release.

Automated summary

This study synthesized three new polyoxotantalates using diffusion-based methods at room temperature, with two compounds containing zinc and cadmium-centered amine complexes for the first time, and being the first to use a macrocyclic amine for polyoxotantalate synthesis. Compounds I and II exhibit stability under ambient conditions, while compound III shows rapid loss of crystal water molecules. In situ XRD measurements revealed stepwise structural rearrangements in compound I due to water release.