Nitric Oxide at a Surface-Bound Nickel Porphyrinoid

A stable NO2 species forms at the Ni sites at room temperature upon exposure to NO of a Nickel-containing tetraphenyl porphyrins monolayer grown on Cu(100). The NO uptake was characterized by a quantitative analysis of the NO2 formation rates as a function of the NO exposure conditions, and we conclude that NO2 origins through a disproportionation mechanism. Together with the support of ab initio simulations, we propose an atomistic model of the reaction that is compatible with our observations, paving the way towards further investigations on NO disproportionation at biomimetic single-atom sites within the framework of surface science.

M. Stredansky et al. DOI:10.1002/anie.202201916
       Disproportionation of Nitric Oxide at a Surface-Bound Nickel Porphyrinoid

In a biomimetic picture at surfaces, novel model systems for heterogeneous catalysis may be built by exploiting  a rich variety of 2-dimensional self-assembled structures formed by tetrapyrrolic compounds such as metal-containing tetraphenyl porphyrins. These supramolecular assemblies act as a network, stabilizing ordered arrays of single metal atom active sites and can be considered single-atom catalyst offering axial coordination for the anchoring of ligands and for the chemical conversion of small molecules. In the specific case of NO adsorption at 2D arrays of surface-supported porphyrins, it is commonly accepted that only a single NO molecule coordinates to the metal center under UHV conditions. We present the results obtained upon exposure of a Ni tetraphenyl porphyrin (NiTPP) film deposited on the Cu(100) surface to NO at room temperature, providing a first evidence of NO disproportionation observed at tetrapyrroles in UHV, yielding the NO2-NiTPP complex. By means of vibronic and vibrational spectroscopies and scanning tunneling microscopy, we unequivocally identify the ligand coordinated with the Ni atom of the porphyrin to be the NO2 molecule. Independent photoemission-based experiments allow to draw the same conclusions. Density Functional Theory calculations indicate that the disproportionation reaction path (3 NO→NO2+N2O) is extremely exothermic so that the formation of the NO2-NiTPP complex is energetically favorable, although kinetically hindered, being a high order reaction. We report in detail the results obtained by means of each of the many techniques, both experimental and theoretical, that we exploited to investigate this complex system. We also discuss our observations in light of the well-known, unavoidable, and non-trivial issues associated with the role of contaminants in NO experiments, critically considering the role of pathways including: i) NO2 contamination of the NO bottles, ii) NO2 formation at the UHV chamber walls, iii) NO2 formation at the Cu substrate, iv) NO2 formation at the single-atom Ni sites.

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Disproportionation of Nitric Oxide at a Surface-Bound Nickel Porphyrinoid M. Stredansky, S. Moro, M. Corva, H. Sturmeit, V. Mischke, D. Janas, I. Cojocariu, M. Jugovac, A. Cossaro, A. Verdini, L. Floreano, Z. Feng, A. Sala, G. Comelli, A. Windischbacher, P. Puschnig, C. Hohner, M. Kettner, J. Libuda, M. Cinchetti, C. M. Schneider, V. Feyer, E. Vesselli, G. Zamborlini Anghew. Chem. Int. Ed. 60 25988-25993 (2022) 
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