(Invited) Climbing over the Volcano Correlation for O2 Reduction By the Effect of a Pyridine Axial Ligand Bound to Co Phthalocyanine Compared to Vitamin B12. 4 Versus 2 Electron Reduction of O2
Authors: Zagal Jose H , Tasca Federico , Neira Karinna , Riquelme Jorge , Hermosilla Patricio , Venegas Diego , Orellana Walter 
Published: February 2020

CoN4 macrocyclic complexes like Co phthalocyanines (CoPc) have been extensively studied as electrocatalysts for the ORR since the discovery by Jasinski [1] but they only promote the 2-e reduction of O2 to give peroxide, this, delivering less energy when used in a fuel cell. Further, they lack the long-term stability required for fuel cell performance. However, they can serve as excellent models for establishing reactivity descriptors and activity correlations that can be very useful for design of more active and stable catalysts such as the Pyrolyzed MNx materials. On the other hand, vitamin B12, a naturally occurring CoN4 macrocyclic molecule has attracted the attention of the scientific community because instead of catalysing the 2 electron reduction of O2 to H2O2 like the other Co macrocyclics in alkaline media it promotes the 4-electron reduction to H2O. In alkaline media vitamin B12 possesses an axial back ligand and this seems to be the reason for its higher activity and selectivity for the 4-e reduction of O2. This is also true for another natural occurring catalyst: FeN4 cytochrome c [2]. To test this hypothesis, we synthesized a CoPc axially coordinated to pyridine anchored to carbon nanotubes (Co-Py-CNT). The Co center becomes then coordinated to 5 nitrogen atoms as in vitamin B12. The modified CoPcPy containing catalytic material was characterized by EPR and XPS spectroscopy. DFT calculations showed that the binding energy of Co to O2 increases dramatically by the presence of the axial ligand (see volcano correlation down below [3]). Koutecky– Levich extrapolation and Tafel plots show the similarities between the 2 complexes (Tafel slopes ca. -RT/F) and reveal insights into the mechanism of action of Co penta-coordinated complexes. According to our results the pyridine back ligand increases the Co-O2 binding energy, making it more similar to VitB12, favouring the splitting of the O-O bond. The back ligand then plays a crucial role in modifying Co-O2 binding energy which is a well know reactivity descriptor [3]. A further evidence eof this dramatic change in reactivity of CoPc by the presence of the "Py" back ligand is that it moves from the weak binding leg of a volcano correlation to the strong leg of the volcano, as illustrated in the Figure down below. This might explain why nature uses a back ligand in cytochrome -c to catalyze the ORR.

ACKOWLEDGEMENTS: F.T. thanks for financial support the Fondecyt Iniciación Project 11130167, and Proyecto Basal. J.H.Z thanks the financial support of Millenium Project RC120001, Project Anillo ACT 1412 and Dicyt-USACH. Fondecyt 1140199.

REFERENCES:

1. R. Jasinski, Nature, 201 (1964) 1212-1213

2. R. Boulatov, N4 Macrocyclic Metal Complexes (J.H. Zagal, F. Bedioui, J.P. Dodelet Eds.) Springer, Berlin,2006, pp.1-60

3. J.H. Zagal, M.T.M. Koper, Angewandte Chemie Int. Ed.,
55 (2016) 14510-14521.

Figure 1