Synthesis, Structural Studies, and Magnetic Exchange Interactions in Low-Valent Manganese Alkoxide Cubes

In synthetic manganese chemistry, the {Mn4O4}4+ unit has been characterized in several tetranuclear Mn(II) complexes with a single Schiff-base macrocycle encapsulating the metals to support the cube-type core.2,3 Highly distorted Mn cubes supported by catecholate ligands have been prepared,4 as has a cube with a {Mn4Te4}4+ core in which all the metals are tetrahedral.5 The {Mn4O4}4+ cube is also encountered as a building block about which higher nuclearity structures are constructed, for example, [Mn8Fe4O12(O2CMe)16(H2O)4]6,7 and [Mn12O12(O2CPh)16(H2O)4].8 In both cases, the cube is assembled during cluster synthesis. In the present note, we describe the synthesis and properties of two new examples of manganese alkoxide cubes, [Mn(DPM)- (OMe)(MeOH)]4, 1, and [Mn(DBM)(OMe)(MeOH)]4, 3, where HDPM is dipivaloylmethane and HDBM is dibenzoylmethane, as well as additional characterization of [Mn4(DPM)4(OEt)4- (EtOH)2], 2. The synthesis and structure of compound 2 were reported previously.9 Both methoxide cubes contain four octahedrally coordinated manganese atoms, in contrast to the ethoxide complex, where two of the four metal atoms are only pentacoordinate. A detailed analysis of the magnetic properties of 1-3 was undertaken, and a general scheme was obtained which fits the ø vs T data for all three compounds, despite the structural differences among them. The previously noted instability of the cubes in solution9 was further investigated by spectroscopic methods and fast atom bombardment (FAB) mass spectrometry.