Abstract
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.
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