I. Autoxidation of manganese(II) B-diketonates. II. Synthesis of dipivaloylmethane chelates of manganese(II)

Abstract

NOTE: Text and symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Part I. The oxidation of several manganese(II) [beta]-diketonates by molecular oxygen has been studied in benzene and ethanol with the goals of elucidating the mechanisms and understanding the influence of ligand structure on the reactivities of the chelates. Autoxidation of bis(acetylacetonato)diaquomanganese(II) in benzene follows a two-term rate law which can be interpreted in terms of an uncatalyzed bimolecular reaction and an acetylacetone-catalyzed reaction of manganese(II) complex and oxygen. Excess acetylacetone inhibits autoxidation of the manganese(II) chelates in ethanol, suggesting that a key step is formation of a bond between metal and molecular oxygen. Kinetic studies in ethanol are complicated by autoxidation of [beta]-diketone coordinated to the manganese(III) product and competition between manganese(II) and manganese(III) for available ligand. The ligand bound to manganese(II) strongly influences the reactivity of the compound towards oxygen. A factor of 10[superscript 3] was observed between the rates of autoxidation of bis(ethylacetoacetato)-manganese(II) and bis(3methylacetylacetonato)manganese(II). Results have been interpreted in terms of steric, resonance, and inductive effects. Manganese [beta]-diketonates catalyze the autoxidation of several organic cosubstrates. Catalytic behavior falls into three classes in which manganese is predominantly divalent, trivalent, or in between the two extremes. Formation of a ternary complex of substrate, manganese, and oxygen is consistent with the limited data available. Part II. Synthesis of bis(dipivaloylmethanato)manganese(II), Mn(DPM)[subscript 2], was attempted in aqueous acetone and methanol solution. A bis compound, Mn(DPM)[subscript 2](H[subscript 2]O)[subscript 2], was prepared in aqueous acetone. The compound is octahedral, unlike the DPM chelates of other first-row transition metals, which suggests that steric factors alone do not control the geometry of these compounds. The product obtained from methanol solution contains bridging methoxy ligands and is probably a dimer, [Mn(DPM)(OCH)[subscript 3])][subscript 2].