Properties and reactivity of model manganese complexes for the oxygen-evolving complex of photosystem II.

Abstract

This dissertation explores the reactivities, properties and structures of manganese complexes as models for manganese redox enzymes, with an emphasis on model complexes related to the Oxygen Evolving Complex, OEC, and manganese superoxide dismutase, MnSOD. The OEC modeling chemistry is focused on a series of complexes with formula [Mn<super>IV</super>(L)(mu-O)]₂, where L is a tetradentate Schiff base ligand for example H₂salpn (N,N<super>'</super>-bis(salicylideneimino)-1,3-diaminopropane). One significant result from this work has been the establishment of a magneto-structural correlation for [Mn<super>IV</super>((L)(mu-O)]₂ complexes, between the magnetic susceptibility coupling constant J and the average, crystallographically determined, Mn-O-Mn angle of the [Mn<super>IV</super>(mu-O)]₂ core. This is the first established magneto-structural correlation for [Mn<super> IV</super>(mu-O)]₂ dimers, and it may provide insight into the structure of the OEC. Another significant result arises from the reaction of [Mn<super>IV</super>(L)(mu-O)]₂ complexes with protons. The addition of protons to a solution of [Mn<super>IV</super>(salpn)(mu-O)]₂, initiates a redox process that is designated the proton-triggered reaction. An earlier literature report had suggested that a related [Mn<super> IV</super>(L)(mu-O)]₂ compound produced hydrogen peroxide in an analogous reaction. Hydrogen peroxide production by the proton-triggered reaction was not observed when this reaction was reexamined, nor did experiments with other [Mn<super>IV</super>(L)(mu-O)]₂ compounds provide positive results. This implies that dioxygen may not be evolved from the oxo bridges of the manganese cluster of the OEC. Reaction of [Mn<super>IV</super>(salpn)(mu-O)]₂ with hydrochloric acid generates the complex Mn<super>IV</super>(salpn)Cl₂, which <italic> trans</italic> chlorinates alkenes, in good yields with some regiospecificity. This is the first Mn-based halogenating reagent for which both the initial reagent and final manganese product are well-defined and crystallographically characterized. Insights from Mn<super>IV</super>(salpn)Cl₂ may assist the understanding of less well-characterized Mn-based halogenation reagents. Progress toward the generation of structurally corroborative and functional MnSOD model compounds is presented. Pentadentate ligands, H₃salshedr and derivatives, formed dimeric Fe<super>III</super> and Mn<super>III</super> mu-alkoxo bridged complexes. The manganese dimers exhibited weak ferromagnetism, a rare trait among Mn<super>III</super>₂ compounds. structurally corroborative MnSOD models using tetradentate tripodal amine ligands formed chains in the solid state and monomers or dimers in ethanol or glycerol/water mixtures.