Reactions of Superoxide with Iron Porphyrins in the Bulk and the Near-Surface Region of Ionic Liquids

Abstract

The redox reaction of superoxide (KO2) with highly charged iron porphyrins (Fe(P4+), Fe(P8+), and Fe(P8-)) has been investigated in the ionic liquids (IL) [EMIM][Tf2N] (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and [EMIM][B(CN)4] (1-ethyl-3-methylimidazolium tetracyanoborate) by using time-resolved UV/vis stopped-flow, electrochemistry, cryospray mass spectrometry, EPR, and XPS measurements. Stable KO2 solutions in [EMIM][Tf2N] can be prepared up to a 15 mM concentration and are characterized by a signal in EPR spectrum at g = 2.0039 and by the 1215 cm(-1) stretching vibration in the resonance Raman spectrum. While the negatively charged iron porphyrin Fe(P8-) does not react with superoxide in IL, Fe(P4+) and Fe(P8+) do react in a two-step process (first a reduction of the Fe(III) to the Fe(II) form, followed by the binding of superoxide to Fe(II)). In the reaction with KO2, Fe(P4+) and Fe(P8+) show similar rate constants (e.g., in the case of Fe(P4+): k1 = 18.6 ± 0.5 M(-1) s(-1) for the first reaction step, and k2 = 2.8 ± 0.1 M(-1) s(-1) for the second reaction step). Notably, these rate constants are four to five orders of magnitude lower in [EMIM][Tf2N] than in conventional solvents such as DMSO. The influence of the ionic liquid is also apparent during electrochemical experiments, where the redox potentials for the corresponding Fe(III)/Fe(II) couples are much more negative in [EMIM][Tf2N] than in DMSO. This modified redox and kinetic behavior of the positively charged iron porphyrins results from their interactions with the anions of the ionic liquid, while the nucleophilicity of the superoxide is reduced by its interactions with the cations of the ionic liquid. A negligible vapor pressure of [EMIM][B(CN)4] and a sufficient enrichment of Fe(P8+) in a close proximity to the surface enabled XPS measurements as a case study for monitoring direct changes in the electronic structure of the metal centers during redox processes in solution and at liquid/solid interfaces.

Ultrathin ionic liquid films on metal surfaces: adsorption, growth, stability and exchange phenomena

“…These investigations highlight the importance of the strength of the organic-metal interaction and its correlation to the stability and arrangement in stacked multilayer architectures [156,161,162,[164][165][166][167][168][169][170]. Although combinations of porphyrins and ILs were already realized for catalysis [171] and dye-sensitized solar cell technologies [40,41], dedicated interface studies of porphyrin/IL hybrid systems are still missing. Porphyrins are a diverse and fascinating class of materials.…”

“…They offer great functional diversity through a vast degree of freedom concerning the variation of the substituents at the periphery of their tetrapyrrole core [83]. In the mentioned applications, the function, performance and stability of the composite thin films are strongly determined by the properties of the interface to the support [11,41,57,[61][62][63][64][65]171]. To obtain some insights, we address temperature-dependent changes in the bilayer film structure of the IL [C 8 C 1 Im][PF 6 ] and the 5,10,15,20-tetraphenylporphyrin (2H-TPP, see Table 1) on Ag (111) and Au(111) in the following; see Figure 12 [83].…”

“…One example could be the poisoning of a SCILL system, where the adsorption of large molecular reactants might block the reactive sites on the surface, irreversibly lowering the catalytic activity over time, similar to poisoning of Pt catalysts by sulfur or other nonreactive species [60,[195][196][197][198]. Another example would concern SILP systems based on large molecules (like metalloporphyrin derivates) as catalytic centers [171]. It would cause major challenges if the catalyst complexes preferentially adsorb to the support surface and thus deplete from the liquid phase.…”

Stability and Exchange Processes in Ionic Liquid/Porphyrin Composite Films on Metal Surfaces

“…Knowledge and control of the formation and structure of self-assembled porphyrin adlayers on the surface of solid supports are essential for the synthesis of custom-tailored environments and catalysts. 5,8,9,13,14,18 The preparation of catalytically active porphyrin-functionalized metal surfaces can be achieved by physical vapor deposition (PVD) in ultrahigh vacuum (UHV). 5,9,19 One particularly popular and well-studied representative is 5,10,15,20-tetraphenylporphyrin (2H-TPP, see Figure 1a).…”

Perspective: Chemical reactions in ionic liquids monitored through the gas (vacuum)/liquid interface

“…[67][68][69][70][71][72] We also exclude studies where the reaction was not carried out in situ, that is, spectra were not recorded while the reaction proceeded, or the sample was moved to a different site in the XPS setup or was even removed from the XPS setup in order to carry out the reaction. [73][74][75][76][77] We also do not address studies that incorporate electrochemical sample manipulation, 66,67,[69][70][71][78][79][80][81][82][83][84][85][86] electrodeposition of metal or organic layers from ILs, [87][88][89][90][91] and IL decomposition 84,[92][93][94][95][96][97][98][99][100][101][102] or nanoparticle formation in ILs. 62,75,103 Each of these topics was studied extensively and would justify a separate review.…”