A novel type of tetranuclear copper(II) cluster, containing deprotonated 3-(pyridin-2-yl)-5-(pyrazin-2-yl)-1,2,4-triazole (Hppt) as a diatomic bridging ligand. Preparation, magnetism, and X-ray crystal structure of [Cu(µ-ppt)(H2O)]4(NO3)4(H2O)12