The reaction of the polyoxomolybdate anion $$[\hbox{Mo}_{32}^{\rm VI}\hbox{(MoNO)}_{4}\hbox{O}_{108}(\hbox{H}_{2}\hbox{O})_{16}]^{12-} =\{\hbox{Mo}_{36}\hbox{(NO)}_{4}\}$$ with FeII salts and hydrazine sulfate yields the cluster anion $$[\hbox{Mo}^{\rm V/VI}_{51}(\hbox{Mo}^{\rm VI}\hbox{O})_{2}\hbox{Fe}^{\rm III}_{6}\hbox{(MoNO)}_{6}\hbox{O}_{176}\hbox{(OH)}_{3} (\hbox{H}_{2}\hbox{O})_{22}]^{15-}$$ (1a), isolated as its corresponding ammonium salt (NH4)15 1a · 36H2O (1) (space group P63/mmc, a=b=23.607(5) A, c=26.767(6) A, 4076 unique reflections, 293 parameters). Despite the absence of oxygen and the reducing conditions the product comprises only FeIII centers as determined by a combination of spectroscopic, magnetic and crystallographic analysis. Reduction of peripheral Mo centers causes the aggregation of two additional [MoVIO]4+ groups onto the archetypal {Mo57M6} structure. The single-crystal X-ray determined structure of 1 is reported.
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