Spectral and electrochemical studies of axial ligand binding reactions of carbonylruthenium(II) meso-tetramesitylporphyrin

The spectral and electrochemical properties of carbonylruthenium(II) meso-tetramesitylporphyrin, Ru(CO)(TMP) in the presence of imidazole, N-methylimidazole and hydroxide anion as axial ligands were investigated. Spectrophotometric titration of Ru(CO)(TMP) of the nitrogeneous bases caused the absorption spectrum to shift to longer wavelengths. Larger shifts in wavelength were observed in the titration using tetrabutylammonium hydroxide. The formation constants for these ligands coordinated to the ruthenium center were calculated. The effect of axial ligand ligation caused the decrease of vibrational frequency of CO as detected from FT-IR spectroscopy. The CO stretching frequency (νCO) of Ru(CO)TMP in CH2Cl2 is 1940 cm−1, which is lowered to 1936 and 1913 cm−1 upon coordination of nitrogenous bases and hydroxide anion, respectively. Cyclic voltammetry of Ru(CO)(TMP) in CH2Cl2 showed an irreversible reduction wave at −1.63 V and two reversible oxidations at E1/2 = 0.78 and 1.27 V vs. Ag|AgCl, respectively. Addition of imidazole and hydroxide into Ru(CO)(TMP) solution caused shifts in the redox potential and accordingly, the binding constants of the ligands to the one- and two-electron oxidized ruthenium porphyrins were estimated and compared. Spectroelectrochemical methods were used to characterize the above electron-transfer reactions.