Redox Metal–Ligand Cooperativity Enables Robust and Efficient Water Oxidation Catalysis at Neutral pH with Macrocyclic Copper Complexes

Water
oxidation catalysis stands out as one of the most important
reactions to design practical devices for artificial photosynthesis.
Use of late first-row transition metal (TM) complexes provides an
excellent platform for the development of inexpensive catalysts with
exquisite control on their electronic and structural features via
ligand design. However, the difficult access to their high oxidation
states and the general labile character of their metal–ligand
bonds pose important challenges. Herein, we explore a copper complex
(12–) featuring an extended, π-delocalized,
tetra-amidate macrocyclic ligand (TAML) as water oxidation catalyst
and compare its activity to analogous systems with lower π-delocalization
(22– and 32–). Their characterization evidences a special metal–ligand
cooperativity in accommodating the required oxidative equivalents
using 12– that is absent in 22– and 32–. This
consists of charge delocalization promoted by easy access to different
electronic states at a narrow energy range, corresponding to either
metal-centered or ligand-centered oxidations, which we identify as
an essential factor to stabilize the accumulated oxidative charges.
This translates into a significant improvement in the catalytic performance
of 12– compared to 22– and 32– and leads
to one of the most active and robust molecular complexes for water
oxidation at neutral pH with a kobs of
140 s–1 at an overpotential of only 200 mV. In contrast, 22– degrades under oxidative conditions,
which we associate to the impossibility of efficiently stabilizing
several oxidative equivalents via charge delocalization, resulting
in a highly reactive oxidized ligand. Finally, the acyclic structure
of 32– prevents its use at neutral
pH due to acidic demetalation, highlighting the importance of the
macrocyclic stabilization.