Redox properties of ruthenium nitrosyl porphyrin complexes with different axial ligation: Structural, spectroelectrochemical (IR, UV-Visible, and EPR), and theoretical studies

Abstract

Experimental and computational results for different ruthenium nitrosyl porphyrin complexes [(Por)Ru(NO)(X)]n+ (where Por2- = tetraphenylporphyrin dianion (TPP2-) or octaethylporphyrin dianion (OEP2-) and X = H2O (n = 1,2,3) or pyridine, 4-cyanopyridine, or 4-N,N-dimethylaminopyridine (n = 1,0)) are reported with respect to their electron-transfer behavior. The structure of [(TPP)Ru(NO)(H2O)]BF4 is established as an {MNO} 6 species with an almost-linear RuNO arrangement at 178.1(3)°. The compound [(Por)Ru(NO)(H2O)]BF4 undergoes two reversible one-electron oxidation processes. Spectroelectrochemical measurements (IR, UV-vis-NIR, and EPR) indicate that the first oxidation occurs on the porphyrin ring, as evident from the appearance of diagnostic porphyrin radical-anion vibrational bands (1530 cm-1 for OEP•- and 1290 cm-1 for TPP•-), from the small shift of ∼20 cm-1 for νNO and from the EPR signal at g iso ≈ 2.00. The second oxidation, which was found to be electrochemically reversible for the OEP compound, shows a 55 cm-1 shift in νNO, suggesting a partially metal-centered process. The compounds [(Por)Ru(NO)(X)]BF4, where X = pyridines, undergo a reversible one-electron reduction. The site of the reduction was determined by spectroelectrochemical studies to be NO-centered with a ca. -300 cm-1 shift in νNO. The EPR response of the NO• complexes was essentially unaffected by the variation in the substituted pyridines X. DFT calculations support the interpretation of the experimental results because the HOMO of [(TPP)Ru(NO)(X)]+, where X = H 2O or pyridines, was calculated to be centered at the porphyrin π system, whereas the LUMO of [(TPP)Ru(NO)(X)]+ has about 50% π*(NO) character. This confirms that the (first) oxidation of [(Por)Ru(NO)(H2O)]+ occurs on the porphyrin ring wheras the reduction of [(Por)Ru(NO)(X)]+ is largely NO-centered with the metal remaining in the low-spin ruthenium(II) state throughout. The 4% pyridine contribution to the LUMO of [(TPP)Ru(NO)(py)]+ is correlated with the stability of the reduced form as opposed to that of the aqua complex.