Redox Properties of Ruthenium Nitrosyl Porphyrin Complexes with Different Axial Ligation: Structural, Spectroelectrochemical (IR, UV-Visible, and EPR), and Theoretical Studies

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Inorganic Chemistry, 47 (16): 7106--7113 (2008)
DOI: 10.1021/ic702371t


Exptl. and computational results for different Ru nitrosyl porphyrin complexes (Por)Ru(NO)(X)n+ (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 species with an almost-linear RuNO arrangement at 178.1(3)°. (Por)Ru(NO)(H2O)BF4 undergoes 2 reversible 1-electron oxidn. processes. Spectroelectrochem. measurements (IR, UV-visible-NIR, and EPR) indicate that the 1st oxidn. occurs on the porphyrin ring, as evident from the appearance of diagnostic porphyrin radical-anion vibrational bands (1530 cm-1 for OEP.bul.- and 1290 cm-1 for TPP.bul.-), from the small shift of $\sim$20 cm-1 for nNO and from the EPR signal at giso $\approx$ 2.00. The 2nd oxidn., which is electrochem. reversible for the OEP compd., shows a 55. cm-1 shift in nNO, suggesting a partially metal-centered process. (Por)Ru(NO)(X)BF4, where X = pyridines, undergo a reversible 1-electron redn. The site of the redn. was detd. by spectroelectrochem. studies to be NO-centered with a ca. -300 cm-1 shift in nNO. The EPR response of the NO.bul. complexes was essentially unaffected by the variation in the substituted pyridines X. DFT calcns. support the interpretation of the exptl. results because the HOMO of (TPP)Ru(NO)(X)+, where X = H2O or pyridines, is centered at the porphyrin p system, whereas the LUMO of (TPP)Ru(NO)(X)+ has $\sim$50\% p*(NO) character. This confirms that the (1st) oxidn. of (Por)Ru(NO)(H2O)+ occurs on the porphyrin ring whereas the redn. of (Por)Ru(NO)(X)+ is largely NO-centered with the metal remaining in the low-spin Ru(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. on SciFinder(R)



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