The surprisingly high ligation energy of CO to Ruthenium porphyrins

International audienceA combined theoretical and experimental approach has been used to investigate the binding energy of a ruthenium metalloporphyrin ligated with CO, ruthenium tetraphenyl porphyrin [Ru II TPP] in the Ru II oxidation degree. Measurements made by VUV ionization with the DESIRS beamline at Synchrotron SOLEIL lead to adiabatic ionization energies of [Ru II TPP] and its complex with CO, [Ru II TPP-CO], to be 6.48±0.03 eV and 6.60±0.03 eV, respectively while the ion dissociation threshold of [Ru II TPP-CO] + is measured at 8.36±0.03 eV. These experimental data are used to derive binding energies of the CO ligand in the neutral and cationic complex (1.88±0.06 eV and 1.76±0.06 eV, respectively) using a Born-Haber cycle. Density Functional Theory calculations, in very satisfactory agreement with the experimental results, help to get insights into the metal-ligand bond. Notably, the high ligation energies can be rationalized in terms of the ruthenium orbital structure, which is singular from that of the iron atom. Thus, beyond indications of a strengthening of the Ru-CO bond from the decrease in the CO vibrational frequency in the complex as compared to the Fe-CO bond, high level calculations are essential to describe accurately the metal ligand (CO) bond and show that the Ru-CO bond energy is strongly affected by the splitting of triplet and singlet spin states in uncomplexed [Ru TPP]