Redox Properties Of Ruthenium Nitrosyl Porphyrin Complexes With Different Axial Ligation: Structural, Spectroelectrochemical (Ir, Uv-Visible, And Epr), And Theoretical Studies

Experimental and computational results for different ruthenium nitrosyl porphyrin complexes [(Por)Ru(NO)(X)](n+)(where Por(2-) = tetraphenylporphyrin dianion (TPP2-) or octaethylporphyrin dianion (OEP2-) and X = H2O (n = 1, 2, 3) or pyridine, 4-cyanopyridine, or 4-NN-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)degrees. 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 center dot- and 1290 cm(-1) for TPP center dot-), from the small shift of similar to 20 cm(-1) for v(NO) and from the EPR signal at g(iso) approximate to 2.00. The second oxidation, which was found to be electrochemically reversible for the OEP compound, shows a 55 cm(-1) shift in v(NO), 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 v(NO). The EPR response of the NO center dot 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 = H2O or pyridines, was calculated to be centered at the porphyrin pi system, whereas the LUMO of [(TPP)Ru(NO)(X)](+) has about 50% pi*(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.