Preparation and properties of high-valent manganese carboxylate clusters: Single-molecule magnetism properties and models for water oxidation

A lot of effort has been devoted in the past to the development of preparative methods to polynuclear, oxide-bridged Mn carboxylate clusters with relevance to areas as diverse as magnetic materials and bioinorganic chemistry. The former area involves the ability of single molecules to retain, below a critical temperature, the orientation of their individual magnetic moments, resulting in the observation of bulk magnetization in the absence of a magnetic field without long range interaction of the spin centers. Such molecules are termed single-molecule magnets (SMMs). In the latter area, a tetranuclear Mn-oxo aggregate referred to as the Water Oxidation Complex (WOC) is known to catalyze the photosynthetic transformation of water to dioxygen near the Photosystem II reaction site.; In this work, different synthetic methods have been developed to prepare a variety of tetranuclear clusters that provide new members of the SMM family of complexes. The products have been characterized by X-ray crystallography, 1H-NMR spectroscopy, magnetic susceptibility studies, EPR spectroscopy and electrochemistry. Addition of mineral acids (HX; X = Cl− , Br−, F−, NO₃ −) to [Mn₄O₂(O₂CEt)₆(dbm) ₂] results in disproportionation leading to high yields of [Mn ₄O₃X(O₂CEt)₃(dbm)₃] complexes containing the [Mn₄(μ₃-O)3(μ₃-X)] core (dbm− is the anion of dibenzoylmethane). Chemical oxidation of [Mn₄O₂(O₂CEt)₆(dbm)₂] with permanganate leads to [Mn₄O₃(O₂CEt ₎₄(dbm)₃]. The latter crystallizes as a 1:1 mixture of two isomers comprising the [μ3:η1] and [μ 3:η2] forms of the triply bridging propionate group. The reaction of [Mn₄O₃(O₂CR) ₄(dbm)₃] (R = Me, Et) with trimethylsilyl acetamide results in the formation of [Mn₄O₃(OSiMe₃)(O ₂CR)₃(dbm)₃] (R = Me, Et). These complexes possess crystallographic C₃ symmetry, and thus represent the first axially symmetric cubane complexes with dbm− ligands. The crystal structure of these complexes indicates the presence of solvent molecules of crystallization.; In the area of bioinorganic chemistry, a series of complexes possessing the previously unknown [Mn₃O₄]4+ core have been synthesized and isolated for the first time. These complexes are [Mn₃O₄(O₂CMe)₄(bpy)₂] (bpy = 2,2′-bipyridine) and [Mn₃O₄(O ₂CR)₂Cl₂(bpy)₂] (R = Me, Et). The incorporation of Cl− in the latter series of complexes is a significant development as this ion is a cofactor of the WOC. (Abstract shortened by UMI.)...