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An Anionic N‑Donor Ligand Promotes Manganese-Catalyzed Water Oxidation

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posted on 01.07.2013, 00:00 by Karin J. Young, Michael K. Takase, Gary W. Brudvig
Four manganese complexes of pentadentate ligands have been studied for their ability to act as oxygen evolution catalysts in the presence of Oxone or hydrogen peroxide. The complexes [Mn­(PaPy3)­(NO3)]­(ClO4) (1) (PaPy3H = N,N-bis­(2-pyridylmethyl)-amine-N-ethyl-2-pyridine-2-carboxamide) and [Mn­(PaPy3)­(μ-O)­(PaPy3)­Mn]­(ClO4)2 (2) feature an anionic carboxamido ligand trans to the labile sixth coordination site, while [Mn­(N4Py)­OTf]­(OTf) (3) (N4Py = N,N-bis­(2-pyridylmethyl)-N-bis­(2-pyridyl)­methylamine) and [Mn­(PY5)­(OH2)]­(ClO4)2 (4) (PY5 = 2,6-bis­(bis­(2-pyridyl)­methoxymethane)-pyridine) have neutral ligands of varying flexibility. 1 and 2 are shown to evolve oxygen in the presence of either Oxone or hydrogen peroxide, but 3 evolves oxygen only in the presence of hydrogen peroxide. 4 is inactive. The activity of 1 and 2 with Oxone suggests that the presence of an anionic N-donor ligand plays a role in stabilizing putative high-valent intermediates. Anionic N-donor ligands may be viewed as alternatives to μ-oxo ligands that are prone to protonation in low-valent Mn species formed during a catalytic cycle, resulting in loss of catalyst structure.