Spectroelectrochemical Evidence for the Nitrosyl Redox Siblings NO+, NO?, and NO− Coordinated to a Strongly Electron-Accepting FeII Porphyrin: DFT Calculations Suggest the Presence of High-Spin States after Reduction of the FeII?NO− Complex

JUAN PELLEGRINO; RALPH HUBNER; FABIO DOCTOROVICH; WOLFGANG KAIM

CHEMISTRY-A EUROPEAN JOURNAL

WILEY-V C H VERLAG GMBH

Año: 2011 vol. 17 p. 7868 - 7874

0947-6539

Abstract: Experimental and computational results for the electron-deficient porphyrin complex, [Fe(NO)(TFPPBr8)] = 1 (TFPPBr8 = 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis-(pentafluorophenyl)porphyrin) are reported with respect to its electron transfer behavior. The complex 1 undergoes three one-electron processes: two reversible reductions and one irreversible oxidation. Spectroelectrochemical measurements (IR and UV-Vis-NIR) of 14NO and 15NO containing material indicate that the first reduction to 1? occurs largely on the NO ligand to produce nitroxyl anion (NO?) character as evident from the considerable change in íNO from 1715 to ~1550 cm-1. The second reduction to 12? does not result in a further shift of íNO to lower frequencies but to a surprising high-energy shift to 1590 cm-1. This and the notable changes of the characteristic porphyrin vibrations as well as significant changes of the UV-Vis absorptions indicate a porphyrin-centered process. The oxidation of 1 is irreversible on the voltammetry time scale but chemically reversible in the spectroelectrochemical experiment, suggesting that the cationic form dissociates to the corresponding ferric porphyrin and NO. DFT calculations support the interpretation of the experimental results. The SOMO of 1 has 34 % Fe and 26 % NO character, which confirms that the first oxidation and the first reduction involve the NO ligand to a significant extent, thus yielding species formulated as [{FeNO}6(TFPPBr82?)]+ and [{FeNO}8(TFPPBr82?)]?, respectively. The second reduction product, 12?, might possibly be described as a nitroxyl ferrous porphyrin radical trianion complex, [{FeNO}8(TFPPBr83?)]2?, which, however, should result in an EPR signal at g » 2.00 and a lower nNO value at 243 K. The high-energy shift of nNO, the EPR silence at 243 K, the UV-Vis-NIR spectroelectrochemical results, and the DFT calculations with a porphyrin-centered LUMO suggest the significance of higher spin states, best described as [{FeNO}7(TFPPBr84?)]2?, caused by the extraordinary acceptor properties of the perhalogenated TFPPBr8n- porpyhrin ligand.