Similar, yet different: long-range metal-metal coupling and electron-transfer processes in metal-free 5,10,15,20-tetra(ruthenocenyl)porphyrin

The electronic structure, redox properties, and long-range metal-metal coupling in metal-free 5,10,15,20-tetra(ruthenocenyl)porphyrin (H(2)TRcP) were probed by spectroscopic (NMR, UV-vis, magnetic circular dichroism (MCD), and atmospheric pressure chemical ionization (APCI)), electrochemical (cyclic voltammetry, CV, and differential pulse voltammetry, DPV), spectroelectrochemical, and chemical oxidation methods, as well as theoretical (density functional theory, DFT, and time-dependent DFT, TDDFT) approaches. It was demonstrated that the spectroscopic properties of H(2)TRcP are significantly different from those in H(2)TFcP (metal-free 5,10,15,20-tetra(ferrocenyl)porphyrin). Ruthenocenyl fragments in H(2)TRcP have higher oxidation potentials than the ferrocene groups in the H(2)TFcP complex. Similar to H(2)TFcP, we were able to access and spectroscopically characterize the one- and two-electron oxidized mixed-valence states in the H(2)TRcP system. DFT predicts that the porphyrin Jr-system stabilizes the [H(2)TRcP](+) mixed-valence cation and prevents its dimerization, which is characteristic for ruthenocenyl systems. However, formation of the mixed-valence [H(2)TRcP](+) is significantly less reproducible than the formation of [H(2)TRcP](+). DFT and TDDFT calculations suggest the ruthenocenyl fragment dominance in the highest occupied molecular orbital (HOMO) energy region and the presence of the low-energy MLCT (Rc -> porphyrin (pi*)) transitions in the visible region with energies higher than the predominantly porphyrin-centered Q-bands.