Spectroelectrochemical evidence for the nitrosyl redox siblings NO+, NO*, and NO- coordinated to a strongly electron-accepting Fe(II) porphyrin: DFT calculations suggest the presence of high-spin states after reduction of the Fe(II)-NO- complex

Experimental and computational results for the electron-deficient porphyrin complex [Fe(NO)(TFPPBr(8))] (1; TFPPBr(8)=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. Complex 1 undergoes three one-electron processes: two reversible reductions and one irreversible oxidation. Spectroelectrochemical measurements (IR and UV/Vis/NIR spectroscopy) of (14)NO- and (15)NO-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 ν(NO) from 1715 to around 1550 cm(-1). The second reduction to 1(2)(-) does not result in a further shift of ν(NO) 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; DFT calculations predict the shift of ν(NO) to higher frequencies for the intermediate- and high-spin states of 1(2-). The oxidation of 1 is irreversible on the voltammetry timescale, but chemically reversible in spectroelectrochemical experiments, suggesting that the cationic form dissociates to the corresponding ferric porphyrin and NO. DFT calculations support the interpretation of the experimental results.