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

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.

本研究针对四种五齿配体（pentadentate ligands）锰配合物，考察其在Oxone或过氧化氢存在下作为析氧催化剂的性能。配合物1为[Mn(PaPy3)(NO3)](ClO4)（其中PaPy3H = N,N-双(2-吡啶甲基)-胺-N-乙基-2-吡啶-2-甲酰胺），配合物2为[Mn(PaPy3)(μ-O)(PaPy3)Mn](ClO4)2，二者第六配位易解离位点的反位均带有阴离子型酰胺配体；配合物3为[Mn(N4Py)OTf](OTf)（其中N4Py = N,N-双(2-吡啶甲基)-N-双(2-吡啶基)甲胺），配合物4为[Mn(PY5)(OH2)](ClO4)2（其中PY5 = 2,6-双(双(2-吡啶基)甲氧基甲基)-吡啶），后两者的配体均为中性配体且具有不同柔性。实验表明，配合物1和2在Oxone或过氧化氢存在下均可催化析氧，而配合物3仅在过氧化氢存在下才能实现析氧，配合物4则无催化活性。配合物1与2在Oxone体系中的催化活性提示，阴离子型氮供体配体的存在对稳定推测的高价锰中间体具有关键作用。阴离子型氮供体配体可作为桥氧配体的替代选择：桥氧配体在催化循环生成的低价锰物种中易发生质子化，进而导致催化剂结构失活。