Self-metalation of porphyrins at the solid-gas interface.

Self-metalation is one of the most promising synthesis routes to include a single metal atom in a tetrapyrrolic macrocycle in the case of organic frameworks supported by metal surfaces. The molecule-surface interaction may provide the charge transfer and the geometric distortion of the molecular plane that are necessary for metal inclusion, thus mimicking enzymatic metalation mechanisms of porphyrins in biology. However, while the latter process proceeds at room temperature, at a metal surface, instead, the presence of an activation barrier can represent a technical obstacle that cannot be compensated by a higher substrate temperature without affecting the layer integrity. The formation of the intermediate state, the sitting-atop complex that weakens the two N-H bonds in the tetrapyrrole preceding the metal insertion step, can be facilitated in some cases by oxygen pre-adsorption at the supporting metal surface, like in the case of the 2H-TPP/Pd(100) system. In such cases, the activation barrier can be overcome by mild annealing, yielding the formation of desorbing products (hydrogen, water) and of the metalated tetrapyrrole. We show here that the self-metalation of 2H-TPP molecules at the Pd(100) surface can be promoted already at room temperature by the presence of an oxygen gas phase at close-to-ambient conditions via an Eley-Rideal mechanism, resembling the asymmetric interactions available in enzymatic pockets, thus opening the way to novel routes to metalate organic frameworks at surfaces.