Immobilization of “Capping Arene” Cobalt(II) Complexes on Ordered Mesoporous Carbon for Electrocatalytic Water Oxidation

We
report the synthesis, characterization, and electrocatalytic
water oxidation activity of two cobalt complexes, (6-FP)­Co­(NO3)2 (1) (6-FP = 8,8′-(1,2-phenylene)­diquinoline)
and (5-FP)­Co­(NO3)2 (2) (5-FP =
1,2-bis­(N-7-azaindolyl)­benzene), containing “capping
arene” bidentate ligands with nitrogen atom donors. The cobalt
complexes 1 and 2 were supported on ordered
mesoporous carbon (OMC) by π–π stacking, resulting
in heterogenized cobalt materials 6-FP-Co-OMC-1 and 5-FP-Co-OMC-2, respectively, and studied for electrocatalytic
water oxidation. We find that 6-FP-Co-OMC-1 exhibits
an overpotential of 355 mV for a current density of 10 mA cm–2 and a turnover frequency (TOF) of ∼0.53 s–1 at an overpotential of 400 mV at pH 14. 6-FP-Co-OMC-1 exhibits activity that is ∼1.6 times that of 5-FP-Co-OMC-2, which gives a TOF of 0.32 s–1 at 400 mV overpotential.
The structural stability of the single-atom Co site was demonstrated
for 6-FP-Co-OMC-1 using X-ray absorption spectroscopy
for the molecular complex supported on OMC, but slow degradation in
catalyst activity can be attributed to eventual formation of Co oxide
clusters. DFT computations of electrocatalytic water oxidation using
the molecular complexes as models provide a description of the catalytic
mechanism. These studies reveal that the mechanism for O–O
bond formation involves an intermediate CoIV oxo complex
that undergoes an intramolecular reductive O–O coupling to
form a CoII–OOH species. Further, the calculations
predict that the molecular 6-FP-Co structure is more active for electrocatalytic
water oxidation than 5-FP-Co, which is consistent with experimental
studies of 6-FP-Co-OMC-1 and 5-FP-Co-OMC-2, highlighting the possibility that the ligand structure influences
the catalytic activity of the supported molecular catalysts.