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

Abstract

A new germanium–polyoxovanadate, (H 3aep) 4[V 14Ge 8O 50]·2(aep)·13H 2O ( 1), has been synthesized under solvothermal conditions applying GeO 2, NH 4VO 3, Cu(NO 3) 2·3H 2O and an aqueous solution of 1-(2-aminoethyl)-piperazine (aep, C 6H 18N 3) in the temperature range from 110 to 150 °C. The compound crystallizes in the non-centrosymmetric tetragonal space group P-42 1 c with a = 17.193(1) Å, c = 16.501(1) Å, V = 4877.9(5) Å 3 and Z = 2. The structure consists of isolated spherical [V IV 14Ge IV 8O 50] 12− cluster anions and protonated amine molecules as counterions. The cluster anion can be viewed as a derivative of the [V 18O 42] archetype by replacing four VO 5 pyramids by four Ge 2O 7 units. The latter are formed by corner-sharing of two [GeO 4] 4− tetrahedra. At temperatures above 150 °C the compound (H 2pip) 4(Hpip) 4[V IV 14Ge IV 8O 50(H 2O)] ( 2) (pip = piperazine, C 4N 2H 10) is formed and during the reaction Cu 2+ 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 VO 5 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.