Porphyrins Reactions at Near Ambient Pressure

Porphyrins represent a class of molecules of relevant biological, chemical, physical, and industrial importance and are formed by an organic macrocycle that can bind a metal ion in the center. Both the organic part, that can be chemically functionalized, and the central single metal atom can be used to control the electronic properties of the molecule. Porphyrins have been extensively investigated in the framework of surface chemistry and physics, together with their synthetic counterpart, phthalocyanines.^1,2 In this talk I will present two reactions involving porphyrin macrocycles self-assembled at surfaces, both in ultra high vacuum (UHV) and at near ambient pressure (NAP) conditions. The first example is a self-metalation reaction, a process that takes place on a metal free porphyrin deposited on a metal substrate and leads to the formation of a complex between the molecule and a metal ion coming from the surface. It is a redox reaction that has been studied for many years. There are examples of this reaction conducted in UHV with different metals.³ We have explored the self-metalation of 5,10,15,20-tetraphenylporphyrin on the Pd(100) termination. In particular, we investigated the role of pre-adsorbed oxygen in UHV as a reaction promoter, allowing it to take place at temperatures below the porphyrin decomposition yield at the supporting metal surface.⁵ The role of oxygen in facilitating the reaction becomes even more relevant at NAP conditions, where a different mechanism allows the metalation reaction to occur at room temperature. As a second example, I will discuss a metalorganic framework formed by 5,10,15,20-tetra(4-pyridyl)21H,23H-porphyrines with two metal species. This system has already been reported for its good electrocatalytic activity in solution for oxygen evolution reaction.⁴ We studied the adsorption and stabilization of molecular oxygen at NAP conditions. We propose an activated adsorption mechanism and measure the associated energy barrier.⁷ All experimental results were obtained by means of X-ray photoelectron spectroscopy and IR-Vis sum-frequency generation spectroscopy, both in UHV and at NAP conditions.
References:

1. Auwärter, W., Écija, D., Klappenberger, F. & Barth, J. V. Porphyrins at interfaces. Nat. Chem. 7, 105–120 (2015).

2. Gottfried, J. M. Surface chemistry of porphyrins and phthalocyanines. Surf. Sci. Rep. 70, 259–379 (2015).

3. Marbach, H. Surface-Mediated in Situ Metalation of Porphyrins at the Solid–Vacuum Interface. Acc. Chem. Res. 48, 2649–2658 (2015).

4. Wurster, B., Grumelli, D., Hötger, D., Gutzler, R. & Kern, K. Driving the Oxygen Evolution Reaction by Nonlinear Cooperativity in Bimetallic Coordination Catalysts. J. Am. Chem. Soc. 138, 3623–3626 (2016).

5. A. Goldoni, P. Shinde, G.L. Montanari, G. Di Santo, M. Caputo, L. Floreano, E. D'Incecco, M. Corva, E. Vesselli, C.A. Pignedoli, D. Passerone, A. Verdini: "Water formation for self-metalation of tetraphenyl-porphyrin on an oxidized hitherto unreactive substrate as Pd(100)", in preparation.

6. F. Armillotta, E. D’Incecco, M. Corva, M. Stredansky, J.-J. Gallet, F. Bournel, A. Goldoni, A. Morgante, E. Vesselli, A. Verdini: "Room temperature self-metalation of porphyrins at the solid-gas interface", in preparation.

7. unpublished result

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gez. Prof. Seibold