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

Herein we report two pentanuclear MnII -substituted sandwich-type polyoxotungstate complexes, [{Mn(bpy)}2 Na(H2 O)2 (MnCl)2 {Mn(H2 O)}(AsW9 O33 )2 ]9- and [{Mn(bpy)}2 Na(H2 O)2 (MnCl){Mn(H2 O)}2 (SbW9 O33 )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-α-XW9 O33 ]9- subunits sandwiching a cyclic assembly of the hexagonal [{Mn(bpy)}2 Na(H2 O)2 (MnCl)2 {Mn(H2 O)}]9+ and [{Mn(bpy)}2 Na(H2 O)2 (MnCl){Mn(H2 O)}2 ]10+ moieties, respectively, and represent the first pentanuclear MnII -substituted sandwich-type polyoxometalates (POMs). Both compounds have been synthesized by reacting MnCl2 ·4H2 O with trilacunary Na9 [XW9 O33 ]·27H2 O (X = AsIII and SbIII ) 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 MnII ions and two six-coordinated MnII ions with an internal Na cation formed a coplanar six-membered ring that was sandwiched between two [B-α-XW9 O33 ]9- (X = AsIII and SbIII ) 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 Mnx Oy , as demonstrated by electrochemical quartz crystal microbalance experiments.