Two New Sandwich-Type Manganese {Mn5}-Substituted Polyoxotungstates: Syntheses, Crystal Structures, Electrochemistry, and Magnetic Properties

Herein we report two pentanuclear Mn-II-substituted sandwich-type polyoxotungstate complexes, [{Mn(bpy)}(2)Na(H2O)(2)(MnCl)(2){Mn(H2O)}(AsW9O33)(2)](9-) and [{Mn(bpy)}(2)Na(H2O)(2)(MnCI){Mn(H2O)}(2)(SbW9O33)(2)](8-) (bpy = 2,2'-bipyridine), whose structures have been obtained by single-crystal X-ray diffraction (SCXRD), complemented by results obtained from elemental analysis, electrospray ionization mass spectrometry, Fourier transform infrared spectroscopy, and thermogravimetric analysis. They consist of two [B-alpha-XW9O33](9-) subunits sandwiching a cyclic assembly of the hexagonal [{Mn(bpy)}(2)Na(H2O)(2)(MnCl)(2){Mn(H2O)}](9+) and [{Mn(bpy)}(2)Na(H2O)(2)(MnCl){Mn(H2O)}(2)](10+) moieties, respectively, and represent the first,pentanuclear Mn-II-substituted sandwich -type polyoxometalates (POMs). Both compounds have been synthesized by reacting MnCl2 center dot 4H(2)O with trilacunary Na-9[XW9O33]center dot 27H(2)O (X = As-III and Sb-III) POM precursors in the presence of bpy in a 1 M aqueous sodium chloride solution under mild reaction conditions. SCXRD showed that the alternate arrangement of three five-coordinated Mn-II ions and two six-coordinated Mn-II ions with an internal Na. cation formed a coplanar six-membered ring that was sandwiched between two [B-alpha-XW9O33]9- (X = As-III and Sb-III) subunits. The results of temperature dependent direct-current (dc) magnetic susceptibility data indicated ferromagnetic interactions between Mn ions in the cluster. Moreover, alternating-current magnetic susceptibility measurements with a dc-biased magnetic field showed the existence of a ferromagnetic order for both samples. Electrochemistry studies revealed the presence of redox processes assigned to the Mn centers. They are associated with the deposition of material on the working electrode surface, possibly MnxOy as demonstrated by electrochemical quartz crystal microbalance experiments.