Synthesis, crystal structure and magnetism of a new mixed germanium–polyoxovanadate cluster

A new germanium–polyoxovanadate, (H3aep)4[V14Ge8O50]·2(aep)·13H2O (1), has been synthesized under solvothermal conditions applying GeO2, NH4VO3, Cu(NO3)2·3H2O and an aqueous solution of 1-(2-aminoethyl)-piperazine (aep, C6H18N3) in the temperature range from 110 to 150 °C. The compound crystallizes in the non-centrosymmetric tetragonal space group P-421c with a = 17.193(1) Å, c = 16.501(1) Å, V = 4877.9(5) Å3 and Z = 2. The structure consists of isolated spherical [VIV14GeIV8O50]12− cluster anions and protonated amine molecules as counterions. The cluster anion can be viewed as a derivative of the [V18O42] archetype by replacing four VO5 pyramids by four Ge2O7 units. The latter are formed by corner-sharing of two [GeO4]4− tetrahedra. At temperatures above 150 °C the compound (H2pip)4(Hpip)4[VIV14GeIV8O50(H2O)] (2) (pip = piperazine, C4N2H10) is formed and during the reaction Cu2+ is reduced to elemental copper. This redox reaction is essential for the formation of 2. The crystal water molecules in the structure of 1 are emitted at low temperatures. The magnetic properties are dominated by strong intra-cluster antiferromagnetic coupling and the strongest exchange between edge- and corner-sharing VO5 square pyramids results in an eight-membered spin ring to which two three-membered spin bridges are joined. The magnetic susceptibility data suggest that even at the low temperature of 2 K several multiplet states are still significantly populated.