This highlight summarizes new developments made in group 5 polyoxometalate science of high nuclearity clusters with focus on synthetic approaches.

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.

Systematic variation of the titanium source and the reaction temperature applied during hydrothermal synthesis led to crystallization of four new titanoniobates: {[Ni(cyclam)]4[Ti2Nb8O28]}n·∼28nH2O (I), K[Ni(cyclam)]3[TiNb9O28]·xH2O; x = 18 (II), x = 14 (III) and x ∼ 10(IV).