Stepwise substituting magnetic centres in a giant cluster host system

In the polyoxomolybdate shell of the cluster compound (NH4)(21)[{(VO)-O-IV(H2O)}(6)(Mo(mu-H2O)(2)(mu-OH)Mo)(3)(Mo-15(MoNO)(2)O-58(H2O)(2))(3)] ([V-6]), {Fe-III(H2O)(2)} and (Fe-II(H2O)(2)) moieties can be incorporated at the positions of the (VO(H2O)) groups. On adding FeCl3 . 6H(2)O in excess to a solution from which pure [V-6] can be Isolated, the cluster compound (NH4)(15)[(Fe-III(H2O)(2))(6)(Mo(mu-H2O)(2)(mu-OH)Mo)(3)(Mo-15(MoNO)(2)O-58(H2O)(2))(3)] ([Fe-6]) is formed. By adding varying amounts of FeCl2 . 4H(2)O or FeCl3 . 6H(2)O to such a solution, a series of compounds can be isolated with different ratios of oxovanadium( IV), iron(III), and iron(II) centres. The experimental conditions allow tuning of these ratios in the prepared compounds, [V4Fe2], [V3Fe3], [V2Fe4] and [VFe5] with varying iron(III)/iron(II) ratio (depending on the degree of oxidation of iron(II) centres) and [VFe5III]. These formulae correspond to those of [V-6] and [Fe-6] with the relevant intermediate NH4+ content, The synthesized compounds were characterized by elemental analyses, different spectroscopic methods, and X-ray crystal structure analyses as well as Fe-57-Mossbauer spectroscopy which allowed determination of the iron(III)/iron(II) ratio. The magnetic properties of all cluster compounds were carefully investigated. For one of the mixed systems an interesting ferrimagnetic behaviour is observed and can only be explained if the different magnetic centres occur within a single cluster shell and are not separated in different clusters. (C) 1998 Elsevier Science S.A.