Solid state coordination chemistry: secondary metal–ligand influences on the structures of vanadium oxides. Hydrothermal syntheses and structures of [Zn(tpytrz)2V2O6], [Zn3(tpytrz)2(H2O)2V6O18]·6H2O and [Cu3(tpytrz)2(H2O)2V8O23]·3H2O (tpytrz = 2,4,6-tri(4-pyridyl)-1,3,5-triazine)