Low valent managanese complexes as functional models for the manganese catalases and the alternate catalase reaction of the oxygen evolving complex.

Abstract

Synthetic dinuclear manganese complexes were prepared as structural, spectroscopic, and functional models for manganese containing enzymes. We synthesized a series of manganese dimers with the invariant ligand set of the pentadentate N,N'-bis(salicylimino)-1,3-diaminopropan-2-ol (2-OHsalpn). The dimers prepared in this work formed as 1:1 Mn:L complexes with each ligand chelating both Mn ions bridged by the alkoxide oxygen. These complexes were isolated in the four known oxidation states of the manganese catalases: $\rm Mn\sp{II}\sb2,\ Mn\sp{II/III},\ Mn\sp{III}\sb2,\ Mn\sp{III/IV}$ and each was characterized crystallographically. This represents the first series of Mn dimers for which four different oxidation state levels have been isolated using an invariant ligand set. The $\rm Mn\sp{II}\sb2$ and $\rm Mn\sp{III}\sb2$ dimers act as functional mimics for the Mn catalases in that they react catalytically as dimeric complexes to disproportionate hydrogen peroxide with rates that are first-order in both substrate and catalyst and exhibit substrate saturation kinetics at high H$\sb2$O$\sb2$ concentrations. The kinetics can be fit to the Michaelis-Menton equation to give the parameters $V\sb{\rm max}=21.9\pm0.2$ s$\sp{-1}$ and $K\sb{\rm M}=72\pm1$ mM. This is the first example of a functional catalase mimic which has been isolated and structurally characterized in both the oxidized and reduced forms, demonstrated to react as a dimer during the catalytic cycle, to exhibit saturation kinetics and which can be driven to an inactivated form as a dioxo-bridged Mn$\sp{\rm III/IV}$. These mechanistic features are consistent with those observed for the Mn catalases. We also examined the kinetics of H$\sb2$O$\sb2$ disproportionation by the high-valent dimer (Mn$\sp{\rm IV}$(salpn)(O)) $\sb2$. This dimer which has a different catalytic mechanism also exhibits both first-order catalyst and substrate dependence and saturation kinetics ($V\sb{\rm max}=250$ s$\sp{-1}$ and $K\sb{\rm M}=230$ mM). The reactivity of a tetrameric functional mimic, (Mn$\sp{\rm II}$(2-OHpicpn)) $\sb4$, was compared to these dimeric systems. This complex exhibits kinetics similar to the dimeric systems, with first-order peroxide dependence and saturation kinetics ($V\sb{\rm max}=140$ s$\sp{-1}$ and $K\sb{\rm M}=2.6$ M) as well as first-order catalyst dependence above 25 $\mu$M. This tetramer decomposes into less active species below 20 $\mu$M as has been determined by kinetic measurements. These systems provide insight into the reaction chemistry of multinuclear manganese complexes and the mechanistic features observed for the Mn catalases and the alternate catalase reaction of the OEC.