Can Ni Complexes Behave as Molecular Water Oxidation Catalysts?

The present report
uncovers the borderline between homogeneous
and heterogeneous water oxidation catalysis using a family of Ni complexes
containing oxamidate anionic type of ligands. In particular, the Ni
complex [(L1)­NiII]2– (12–; L1 = o-phenylenebis­(oxamidate)) and its modified analogues [(L2)­NiII]2– (22–; L2 = 4,5-dimethyl-1,2-phenylenebis­(oxamidate))
and [(L3)­NiII]2– (32–; L3 = 4-methoxy-1,2-phenylenebis­(oxamidate)) have been prepared and evaluated as molecular
water oxidation catalysts at basic pH. Their redox features have been
analyzed by means of electrochemical measurements revealing a crucial
involvement of the ligand in the electron transfer processes. Moreover,
the stability of those complexes has been assessed both in solution
and immobilized on graphene-based electrodes at different potentials
and pHs. The degradation of the molecular species generates a NiOx
(Niquel oxides of general formula NixOyHz) layer, whose stability and activity as water oxidation catalyst
have also been established. Electrochemical methods, together with
surface characterization techniques, have shown the complex mechanistic
scenario in water oxidation catalyzed by this family of Ni complexes,
which consists of the coexistence of two catalytic mechanisms: a homogeneous
pathway driven by the molecular complex and a heterogeneous pathway
based on NiOx. The electronic perturbations exerted through the ligand
framework have manifested a strong influence over the stability of
the molecular species under turnover conditions. Finally, 12– has been used as a molecular precursor
for the formation of NiFeOx (Niquel/Iron oxides of general formula
NixFe1-xOyHz) anodes that
behave as extremely powerful water oxidation anodes.