Applying identical amounts of starting materials allowed the solvothermal preparation of two new polyoxoniobates by controlling the pH value of the reaction mixture. Stirring the slurries afforded crystallization of K5[Cu­(H2O)2(cyclam)]1.5{[Cu­(cyclam)]­[Cu­(H2O)­(cyclam)]2HSiNb18O54}­(NO3)·30H2O (I) and {[Cu­(cyclam)­(H2O)]2[Cu­(cyclam)]­[Nb10O28]} n ·9nH2O (II) within short reaction times and in high yields. While compound I crystallizes from the mother liquor at room temperature after hydrothermal treatment at pH values >10.3, compound II is isolated at pH < 10.3 by filtration, i.e., II is formed at the reaction conditions applied. Time-dependent experiments demonstrate that under stirring, pure samples of both compounds can be obtained within 30 min. Syntheses without stirring the educt mixtures leads to very low yields of I and crystallization of II in comparable yields afforded about 20 h reaction time. In the structure of I, the rare [SiNb18O54]14– anion is found, which is surrounded by [Cu­(cyclam)]2+ complexes and K+ cations. The water molecules form a very unusual hydrogen bonding pattern which may be classified as a L4(2)­4(4)­5(4)­10(4)­16(6)­42(14) water cluster. Compound II features the decaniobate anion [Nb10O28]6–, is obtained after short reaction time in high yields and exhibits a reversible release/uptake of crystal water molecules.

Diffusion-based room temperature syntheses using K8{Ta6O19}·16H2O, 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 K4{[Cu­(cyclam)]2Ta6O19}·18H2O (I), K4{[Zn­(cyclam)]2Ta6O19}·18H2O (II), and {[Cd­(cyclam)]4Ta6O19}·19H2O (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.

The new compound K2[Ni(dien)2]{[Ni(dien)]2Ta6O19}·11 H2O crystallized at room temperature applying a diffusion based reaction in a H2O/DMSO mixture using K8{Ta6O19}·16 H2O, Ni(NO3)2·6H2O and dien (diethylenetriamine). In the crystal structure, the Lindqvist-type anion [Ta6O19]8– is structurally expanded by two octahedrally Ni2+-centered complexes via three Ni–µ 2-O–Ta bonds thus generating the new {[Ni(dien)]2Ta6O19}4– anion. Two KO8 polyhedra share a common edge to form a K2O14 moiety, which connects the {[Ni(dien)]2Ta6O19}4– cluster shells into chains. The isolated [Ni(dien)2]2+ complexes are located in voids generated by the structural arrangement of the chains. An extended hydrogen bonding network between the different constituents generates a 3D network. The crystal water molecules can be thermally removed to form a highly crystalline dehydrated compound. Partial water uptake leads to the formation of a crystalline intermediate with a reduced unit cell volume compared to the fully hydrated sample. Water sorption experiments demonstrate that the fully dehydrated sample can be fully reconverted to the hydrated compound. The crystal field splitting parameters for the octahedrally coordinated Ni2+-centered complexes have been evaluated from an UV/Vis spectrum yielding D q = 1056 cm−1 and B = 887 cm−1.

Reaction of K8{Ta6O19}·16H2O with [Ni(tren)(H2O)Cl]Cl·H2O in different solvents led to the formation of single crystals of the title compound, [Ni4Ta6O19(C6H18N4)4]·19H2O or {[Ni2(κ4-tren)(μ-κ3-tren)]2Ta6O19}·19H2O (tren is N,N-bis(2-aminoethyl)-1,2-ethanediamine, C6H18N4). In its crystal structure, one Lindqvist-type anion {Ta6O19}8– (point group symmetry \overline{1}) is connected to two NiII cations, with both of them coordinated by one tren ligand into discrete units. Both NiII cations are sixfold coordinated by O atoms of the anion and N atoms of the organic ligand, resulting in slightly distorted [NiON5] octahedra for one and [NiO3N3] octahedra for the other cation. These clusters are linked by intermolecular O—H...O and N—H...O hydrogen bonding involving water molecules into layers parallel to the bc plane. Some of these water molecules are positionally disordered and were refined using a split model. Powder X-ray diffraction revealed that a pure crystalline phase was obtained but that on storage at room-temperature this compound decomposed because of the loss of crystal water molecules.