A cobalt complex of 5,15-bis(pentafluorophenyl)-10-(4)-(1-pyrenyl)phenyl corrole that contains a triphenylphosphine axial ligand (1-PPh3 ) was synthesized and examined as an electrocatalyst for the hydrogen evolution reaction (HER). If supported on graphene (G), the resulting 1-PPh3 /G material can catalyze the HER in aqueous solutions over a wide pH range of 0-14 with a high efficiency and durability. The significantly enhanced activity of 1-PPh3 /G, compared with that of its analogues 1-py/G (the Co-bound axial ligand is pyridine instead of triphenylphosphine) and 2-py/G (Co complex of 5,10,15-tris(pentafluorophenyl)corrole), highlights the effects of the pyrenyl substituent and the triphenylphosphine axial ligand on the HER activity. On one hand, the pyrenyl moiety can increase the π-π interactions between 1 and graphene and thus lead to a fast electron transfer from the electrode to 1. On the other hand, the triphenylphosphine axial ligand can increase the electron density (basicity) of Co and thus make the metal center more reactive to protons at the trans position through a so-called "push effect". This study concerns a significant example that shows the trans effect of the axial ligand on the HER, which has been investigated rarely. The combination of various ligand-design strategies in one molecule has been realized in 1-PPh3 to achieve a high catalytic HER performance. These factors are valuable to be used in other molecular catalyst systems.
Keywords: cobalt; electrochemistry; graphene; hydrogen; macrocyclic ligands.
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