In Situ Fe K-edge X-ray Absorption Spectroscopy of a Nitrosyl Iron(II) Porphyrin Adduct Adsorbed on a High-Area Carbon Electrode in Aqueous Electrolytes

Abstract

The first in situ Fe K-edge X-ray absorption fine structure study is reported on the electronic and structural properties of a nitrosyl [meso-tetrakis(p-methoxyphenyl) porphyrinato]iron(II) adduct, [Fe(NO)(TMPP)], adsorbed onto a high-area carbon electrode. This adsorbed complex displayed a remarkable stability over a wide potential range, and multiple-scattering XAFS analysis yielded bond lengths of Fe−Np = 1.99, Fe−NNO = 1.74, and Fe−OH2 = 2.02 Å, and an Fe−N−O bond angle of 155°, characteristic of FeIINO porphyrin adducts.

In Situ Surface-Enhanced Raman Scattering Studies of the Nitrosyl Adduct of Hemin Adsorbed on Roughened Silver Surfaces in Aqueous Electrolytes

“…As clearly evident from these data, the formation of the NO adduct is accompanied by the disappearance of the redox peaks of Hm. This behavior is in harmony with that found for Hm adsorbed on glassy carbon (see light curve, Right Panel, Figure ) and also with FeTMPP and FePc adsorbed on high-area carbon electrodes, which is reported in earlier work . The much less-defined oxidation feature in the scan in the positive direction has also been noted by Rywkin et al.…”

“…This behavior is in harmony with that found for Hm adsorbed on glassy carbon (see light curve, Right Panel, Figure 1) and also with FeTMPP and FePc adsorbed on high-area carbon electrodes, which is reported in earlier work. 3 The much less-defined oxidation feature in the scan in the positive direction has also been noted by Rywkin et al for FeTMPyP adsorbed on roughened Ag electrode in acidic solutions. 14 It must be stressed that both the voltammetric curves and the in situ SERS spectrum obtained for bare roughened Ag (without adsorbed Hm) in the same neat electrolyte were found to be featureless over the same potential range both before and after exposure to NO using the same precise protocol (Experimental Section).…”

“…A number of iron macrocycles of the porphyrins and phthalocyanine (Pc) type have been shown to display electrocatalytic properties for the reduction of nitrite in aqueous electrolytes. , Much of the experimental evidence points to the formation of a nitrosyl adduct as a key intermediate in the reaction pathway. , Efforts in our laboratory have focused on the in situ structural and electronic characterization of two such adduct materials adsorbed on high-area carbon surfaces using synchrotron-based X-ray absorption fine structure (XAFS), , [ meso -tetrakis( p -methoxyphenyl)porphyrinato]iron(II), Fe(TMPP), and iron phthalocyanine, FePc. As shown therein, statistical best fits to the extended X-ray absorption fine structure (EXAFS) data placed the Fe center in a pentacoordinated square-pyramidal configuration shifted away from the macrocyclic plane toward the axially bound NO bent at an angle of ca.…”

XAS spectroelectrochemistry: reliable measurement of X-ray absorption spectra from redox manipulated solutions at room temperature

“…Several approaches have been reported that allow in situ XAS measurements from samples in controlled redox states and these use methodologies that range from bulk electrosynthesis cells (typically of volume 1-10 ml) with largesurface-area electrodes (Dewald et al, 1986;Bae et al, 2001;Levina et al, 2004;Hennig et al, 2005;Milsmann et al, 2006;Takao et al, 2010) to channel-flow cells (Wiltshire et al, 2009) and cells optimized for the study of electrode transformations (McBreen et al, 1987;Nakanishi et al, 2014). While bulk electrosynthesis cells have been used to obtain XAS spectra, the long electrosynthesis period (typically > 10 min) makes them impracticable for the study of short-lived reactive species, or for samples, such as metalloproteins, that are only available in small quantities.…”

Excited state molecular structure determination in disordered media using laser pump/X-ray probe time-domain X-ray absorption spectroscopy

“…4p z transition is smeared out, resulting in a smooth transition edge. Such changes in the transition edge due to ligation have been well documented for metalloporphyrins without and with one or two axial ligands, [55][56][57][58][59][60] as well as other transition metal complexes. [61][62][63][64] Therefore, the smooth rising edge in the XANES spectrum of the MLCT state in Fig.…”