STEPWISE SUBSTITUTING MAGNETIC CENTERS IN A GIANT CLUSTER HOST SYSTEM

In the polyoxomolybdate shell of the cluster compound O)(2)(mu-OH)Mo)( 3)(Mo-15(MoNO)(2)O-58(H2O)(2))(3)] ([V-6]), {Fe-III(H2O)(2)} and (Fe-I I(H2O)(2)) moieties can be incorporated at the positions of the (VO(H2 O)) groups. On adding FeCl3 . 6H(2)O in excess to a solution from whic h pure [V-6] can be Isolated, the cluster compound O)(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 seri es of compounds can be isolated with different ratios of oxovanadium( IV), iron(III), and iron(II) centres. The experimental conditions allo w 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 for mulae correspond to those of [V-6] and [Fe-6] with the relevant interm ediate NH4+ content, The synthesized compounds were characterized by e lemental analyses, different spectroscopic methods, and X-ray crystal structure analyses as well as Fe-57-Mossbauer spectroscopy which allow ed determination of the iron(III)/iron(II) ratio. The magnetic propert ies of all cluster compounds were carefully investigated. For one of t he mixed systems an interesting ferrimagnetic behaviour is observed an d 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.