Vibronic Fingerprints of the Nickel Oxidation States in Surface-Supported Porphyrin Arrays

The 2D self-assembly of Ni-containing tetrapyrroles on Cu(100) allows control of the Ni atom oxidation state, yielding inactive Ni­(II) or active Ni­(I) upon modification of the molecule–substrate interaction, resembling the behavior of the biochemical counterpart. Ni­(I) is indeed the active site of methanogenic bacteria in the tetrahydrocorphin of the F₄₃₀ coenzyme of methyl-coenzyme reductase. Tuning of the electronic configuration of the Ni atom in the 2D system is accomplished by exploiting the surface trans effect, by analogy to the biologic enzymatic pocket, which is activated by a molecular trans effect. In this report, we identify the vibrational fingerprint of the molecular macrocycle that reflects the actual Ni oxidation state in the 2D system showing that, despite the apparent differences of the two cases, the fact that the Ni-porphin in the F₄₃₀ pocket is accessible to the reactants but not to the solvent makes the two situations more similar than expected.