Reversible Proton-Coupled Reduction of an Iron Nitrosyl Porphyrin within [DBU-H]+-Based Protic Ionic Liquid Nanodomains.

The reduction of [Fe(OEP)(NO)] has been studied in the presence of aprotic room-temperature ionic liquids (RTIL) and protic (PIL) ionic liquids dissolved within a molecular solvent (MS). The cyclic voltammetric results showed the formation of RTIL nanodomains at low concentrations of the RTIL/PIL solutions. The pKa values of the two PILs studied (i.e., trialkylammonium and [DBU-H]+-based ionic liquids) differed by four units in THF. While voltammetry in solutions containing all three RTILs showed similar potential shifts of the first reduction of [Fe(OEP)(NO)] to [Fe(OEP)(NO)]- at low concentrations, significant differences were observed at higher concentrations for the ammonium PIL. The trialkylammonium cation had previously been shown to protonate the {FeNO}8 species at room temperature. Visible and infrared spectroelectrochemistry revealed that the [DBU-H]+-based PIL formed hydrogen bonds with [Fe(OEP)(NO)]- rather than formally protonating it. Despite these differences, both PILs were able to efficiently reduce the nitrosyl species to the hydroxylamine complex, which could be further reduced to ammonia. On the voltammetric time scale and when the switching potential was positive of the Fe(II)/Fe(I) potential, the hydroxylamine complex was re-oxidized back to the NO complex via direct oxidation of the coordinated hydroxylamine at low scan rates or initial oxidation of the ferrous porphyrin at high scan rates. The results of this work show that, while [DBU-H]+ does not protonate electrochemically generated [Fe(OEP)(NO)]-, it still plays an important role in efficiently reducing the nitroxyl ligand via a series of proton-coupled electron transfer steps to generate hydroxylamine and eventually ammonia. The overall reaction rates were independent of the PIL concentration, consistent with the nanodomain formation being important to the reduction process.

Ion Pairing versus Solvation of Dinitrobenzene Anions in Room-Temperature Ionic Liquids (RTILs): Vibrational Signatures of RTIL-Substrate Interactions.

The mechanism of solvation of ions by ionic liquids is more complex than solvation in most molecular solvents as the ionic liquid itself provides the counter ion. Solvation and ion pairing of anionic substrates in room-temperature ionic liquids (RTILs) were investigated using resonance Raman spectroscopy and DFT calculations. The purpose of this study was to differentiate between the formation of discrete cation/anion structures and a double-layer cloud of counter ions without specific atomic interactions between the ionic species. In acetonitrile/RTIL mixtures, the radical anion and dianion of dinitrobenzene (DNB) are stabilized by RTILs through solvation and ion pairing. The formation of the lowest-energy ion pair led to the largest shifts in the Raman band in DNB-·, while significantly smaller shifts were predicted for general solvation. The effect of general solvation and ion pair formation was studied using DFT with the implicit solvation model. Identification of the bands most sensitive to tight ion pairing allowed for the interpretation of the observed vibrational changes. The formation of tight ion pairs between the anionic solutes depends on both cation-solute and RTIL cation-anion interactions. Tight ion pairs were observed in RTILs, but general solvation was also important. This work establishes the advantageous use of vibrational spectroscopy to provide detailed structural information not accessible from voltammetry alone.

Infrared Spectroelectrochemistry of Iron-Nitrosyl Triarylcorroles. Implications for Ligand Noninnocence.

Recent DFT calculations have suggested that iron nitrosyl triarylcorrole complexes have substantial {FeNO}7-corrole•2- character. With this formulation, reduction of Fe(C)(NO) complexes, where C = triarylcorrole, should be centered on the corrole macrocycle rather than on the {FeNO}7 moiety. To verify this proposition, visible and infrared spectroelectrochemical studies of Fe(C)(NO) were carried out and the results were interpreted using DFT (B3LYP/STO-TZP) calculations. The first reduction of Fe(C)(NO) led to significant changes in the Soret and Q-band regions of the visible spectrum as well as to a significant downshift in the νNO and changes in the corrole vibrational frequencies. DFT calculations, which showed that the electron was mostly added to the corrole ligand (85%), were also able to predict the observed shifts in the νNO and corrole bands upon reduction. These results underscore the importance of monitoring both the corrole and nitrosyl vibrations in ascertaining the site of reduction. By contrast, the visible spectroelectrochemistry of the second reduction revealed only minor changes in the Soret band upon reduction, consistent with the reduction of the FeNO moiety.

Voltammetry and Spectroelectrochemistry of TCNQ in Acetonitrile/RTIL Mixtures.

Understanding the solvation and ion-pairing interactions of anionic substrates in room-temperature ionic liquids (RTIL) is key for the electrochemical applications of these new classes of solvents. In this work, cyclic voltammetry and visible and infrared spectroelectrochemistry of tetracyanoquinodimethane (TCNQ) was examined in molecular (acetonitrile) and RTIL solvents, as well as mixtures of these solvents. The overall results were consistent with the formation of RTIL/acetonitrile nanodomains. The voltammetry indicated that the first electrogenerated product, TCNQ-, was not incorporated into the RTIL nanodomain, while the second electrogenerated product, TCNQ2-, was strongly attracted to the RTIL nanodomain. The visible spectroelectrochemistry was also consistent with these observations. Infrared spectroelectrochemistry showed no discrete ion pairing between the cation and TCNQ- in either the acetonitrile or RTIL solutions. Discrete ion pairing was, however, observed in the acetonitrile domain between the tetrabutylammonium ion and TCNQ2-. On the other hand, no discrete ion pairing was observed in BMImPF6 or BMImBF4 solutions with TCNQ2-. In BMImNTf2, however, discrete ion pairs were formed with BMIm+ and TCNQ2-. Density function theory (DFT) calculations showed that the cations paired above and below the aromatic ring. The results of this work support the understanding of the redox chemistry in RTIL solutions.

Proton Transfer versus Hydrogen Bonding in a Reduced Iron Porphyrin Nitrosyl Complex.

The 1H NMR spectra of Fe(OEP)(HNO), which was formed from Fe(OEP)(NO)- in the presence of 3,5-dichlorophenol, were studied as a function of temperature. The chemical shift of the HNO proton showed a unique behavior which could be explained based on the equilibrium between the protonated complex, Fe(OEP)(HNO), and the hydrogen-bonded complex, Fe(OEP)(NO)-···HOPh. This equilibrium was consistent with UV/visible spectroscopy and the voltammetric data. UV/visible stopped-flow experiments showed that the hydrogen-bonded complex, which was formed when weak acids such as phenol were added, and the Fe(OEP)(HNO) complex were quite similar. In addition to the HNO proton resonance, the meso-resonances were consistent with the proposed equilibrium. Density functional theory calculations of various Fe(OEP)(NO)-/Fe(OEP)(HNO) species were calculated, and the results were consistent with experimental data.

Altering the Coordination of Iron Porphyrins by Ionic Liquid Nanodomains in Mixed Solvent Systems.

The solvent environment around iron porphyrin complexes was examined using mixed molecular/RTIL (room temperature ionic liquid) solutions. The formation of nanodomains in these solutions provides different solvation environments for substrates that could have significant impact on their chemical reactivity. Iron porphyrins (Fe(P)), whose properties are sensitive to solvent and ligation changes, were used to probe the molecular/RTIL environment. The addition of RTILs to molecular solvents shifted the redox potentials to more positive values. When there was no ligation change upon reduction, the shift in the E° values were correlated to the Gutmann acceptor number, as was observed for other porphyrins with similar charge changes. As %RTIL approached 100 %, there was insufficient THF to maintain coordination and the E° values were much more dependent upon the %RTIL. In the case of FeIII (P)(Cl), the shifts in the E° values were driven by the release of the chloride ion and its strong attraction to the ionic liquid environment. The spectroscopic properties and distribution of the FeII and FeI species into the RTIL nanodomains were monitored with visible spectroelectrochemistry, 19 F NMR and EPR spectroscopy. This investigation shows that coordination and charge delocalization (metal versus ligand) in the metalloporphyrins redox products can be altered by the RTIL fraction in the solvent system, allowing an easy tuning of their chemical reactivity.

Redox and Spectroscopic Properties of Iron Porphyrin Nitroxyl in the Presence of Weak Acids.

The spectroelectrochemistry and voltammetry of Fe(OEP) (NO) in the presence of substituted phenols was studied. Cyclic voltammetry showed that two closely spaced waves were observed for the reduction of Fe(OEP) (NO) in the presence of substituted phenols. The first wave was a single electron reduction under voltammetric conditions. The second wave was kinetically controlled, multielectron process. Visible spectroelectrochemistry of Fe(OEP) (NO) in the presence of substituted phenols showed that three species were present during the electrolysis. Additional spectroscopic studies indicated that the two reduction species were Fe(OEP) (HNO) and Fe(OEP)(H2NOH). The Fe(OEP) (HNO) species, which can be generated chemically, was stable over a period of hours. Additional acid did not lead to further protonation. Proton NMR spectroscopy confirmed the Fe(OEP) (HNO) species could be deprotonated under basic conditions. The third species, Fe(OEP)(H2NOH), was generated by the further reduction of the chemically generated Fe(OEP) (HNO) complex. Both the Fe(OEP) (HNO) and Fe(OEP)(H2NOH) complexes could be slowly oxidized back to Fe(OEP) (NO). At millimolar concentrations of Fe(OEP) (HNO), there was no evidence for the disproportionation of Fe(OEP) (HNO) to Fe(OEP) (NO) and H2 in the presence of substituted phenols. Nor was there evidence for the generation of N2O. The FTIR spectroelectrochemistry showed changes in the infrared spectra in the presence of substituted phenols, but no isotopic sensitive bands were observed for the reduced species between 1450 and 1200 cm-1. This may be because the νNO band shifted into a region (1500-1450 cm-1) where it would be difficult to observe.

X-ray Structure and Properties of the Ferrous Octaethylporphyrin Nitroxyl Complex.

The preparation and characterization of the iron octaethylporphyrin nitroxyl ion, [Fe(OEP)(NO)(-)], is reported. The complex was synthesized by the one-electron reduction of Fe(OEP)(NO) using anthracenide as the reducing agent. The compound was isolated as the potassium (2.2.2)cryptand salt. The anion was characterized using X-ray analysis with visible and infrared spectroscopy. The spectral features of the iron nitroxyl complex were consistent with previous literature reports. The important structural changes upon reduction were a significant decrease in the Fe-N-O bond angle from 142° to 127° and an increase in the N-O bond length from that in the starting nitrosyl moiety. The porphyrin ring became significantly less planar upon reduction, but the displacement of the iron atom from the 24-atom plane was essentially unchanged. In spite of the attempt to encapsulate the potassium ion with the (2.2.2)cryptand, significant interaction between K(+) and the oxygen of the nitroxyl were observed, indicating a contact ion pair in the crystal structure. Comparison between the experimental structure and the DFT-calculated parameters were reported. The results are consistent with the Fe-N-O moiety being the site of the reduction, with little evidence for the reduction of the iron itself or the porphyrin ring. The proton NMR spectrum was also obtained, and the chemical shifts were significantly different from other S = 0 metalloporphyrin complexes. These shifts, though, were consistent with the DFT calculations.

Spectroscopic Evidence of Nanodomains in THF/RTIL Mixtures: Spectroelectrochemical and Voltammetric Study of Nickel Porphyrins.

The presence and effect of RTIL nanodomains in molecular solvent/RTIL mixture were investigated by studying the spectroelectrochemistry and voltammetry of nickel octaethylporphyrin (Ni(OEP)) and nickel octaethylporphinone (Ni(OEPone)). Two oxidation and 2-3 reduction redox couples were observed, and the UV-visible spectra of all stable products in THF and RTIL mixtures were obtained. The E° values for the reduction couples that were studied were linearly correlated with the Gutmann acceptor number, as well as the difference in the E° values between the first two waves (ΔE12° = |E1° - E2°|). The ΔE12° for the reduction was much more sensitive to the %RTIL in the mixture than the oxidation, indicating a strong interaction between the RTIL and the anion or dianion. The shifts in the E° values were significantly different between Ni(OEP) and Ni(OEPone). For Ni(OEP), the E1° values were less sensitive to the %RTIL than were observed for Ni(OEPone). Variations in the diffusion coefficients of Ni(OEP) and Ni(OEPone) as a function of %RTIL were also investigated, and the results were interpreted in terms of RTIL nanodomains. To observe the effect of solvation on the metalloporphyrin, Ni(OEPone) was chosen because it contains a carbonyl group that can be easily observed in infrared spectroelectrochemistry. It was found that the νCO band was very sensitive to the solvent environment, and two carbonyl bands were observed for Ni(OEPone)(-) in mixed THF/RTIL solutions. The higher energy band was attributed to the reduced product in THF, and the lower energy band attributed to the reduced product in the RTIL nanophase. The second band could be observed with as little as 5% of the RTIL. No partitioning of Ni(OEPone)(+) into the RTIL nanodomain was observed. DFT calculations were carried out to characterize the product of the first reduction. These results provide strong direct evidence of the presence of nanodomains in molecular solvent/RTIL mixtures.

Electrochemistry and spectroelectrochemistry of 1,4-dinitrobenzene in acetonitrile and room-temperature ionic liquids: ion-pairing effects in mixed solvents.

Room-temperature ionic liquids (RTILs) have been shown to have a significant effect on the redox potentials of compounds such as 1,4-dinitrobenzene (DNB), which can be reduced in two one-electron steps. The most noticeable effect is that the two one-electron waves in acetonitrile collapsed to a single two-electron wave in a RTIL such as butylmethyl imidazolium-BF4 (BMImBF4). In order to probe this effect over a wider range of mixed-molecular-solvent/RTIL solutions, the reduction process was studied using UV-vis spectroelectrochemistry. With the use of spectroelectrochemistry, it was possible to calculate readily the difference in E°'s between the first and second electron transfer (ΔE12° = E1° - E2°) even when the two one-electron waves collapsed into a single two-electron wave. The spectra of the radical anion and dianion in BMImPF6 were obtained using evolving factor analysis (EFA). Using these spectra, the concentrations of DNB, DNB(-•), and DNB(2-) were calculated, and from these concentrations, the ΔE12° values were calculated. Significant differences were observed when the bis(trifluoromethylsulfonyl)imide (NTf2) anion replaced the PF6(-) anion, leading to an irreversible reduction of DNB in BMImNTf2. The results were consistent with the protonation of DNB(2-), most likely by an ion pair between DNB(2-) and BMIm(+), which has been proposed by Minami and Fry. The differences in reactivity between the PF6(-) and NTf2(-) ionic liquids were interpreted in terms of the tight versus loose ion pairing in RTILs. The results indicated that nanostructural domains of RTILs were present in a mixed-solvent system.

In situ study of the photodegradation of carbofuran deposited on TiO2 film under UV light, using ATR-FTIR coupled to HS-MCR-ALS.

The in situ study of the photodegradation of carbofuran deposited on a TiO2 catalyst film under UV light was carried out using the ATR-FTIR technique. The data were analyzed using a Hard-Soft Multivariate Curve Resolution-Alternating Least Squares (HS-MCR-ALS) methodology. Using S-MCR-ALS, four factors were deduced from the evolving factor analysis of the data, and their concentrations and spectra were determined. These results were used to draw qualitative and quantitative analyses of the major products of carbofuran photodegradation. The results of this analysis were in good agreement with GC-MS results and with reported mechanisms. Hard-MCR-ALS was then used to refine the spectra and concentrations, using a multistep kinetic model. The rate constant for the first step in the photodegradation of carbofuran was found to be 2.9 × 10(-3) min(-1). The higher magnitude of the correlation (96.87%), the explained variance (99.87%) and LOF (3.01), are good indicators of the reliability of the outcome of this approach. This method has been shown to be an efficient approach to study in situ photodegradation of pesticides on a solid surface.

Infrared spectroelectrochemical reduction of iron porphyrin complexes.

The spectroelectrochemistry of iron porphinones and their nitrosyl complexes were examined by infrared spectroscopy, as well as ferrous octaethylporphyrin nitrosyl. With the use of d(8)-THF, the solvent was transparent down to 1200 cm(-1). For the porphinones, the reduction of the macrocycle ring could be observed by the changes in the nu(CO) band and, for the nitrosyl complex, the changes in the nitrosyl ligand were directly observable from the nu(NO) band. Formation of the ferrous complexes led to a small downshift in the nu(CO) band. Further reduction to the formal Fe("I") complex led to more complex spectra which were interpreted with the help of density functional theory (DFT) calculations. The reduction of Fe(OEP)(NO) and its porphinone analogues was also examined. The reduction of the iron porphyrin and porphinone nitrosyl complexes lead to substantial decreases in the nu(NO) band from 1665 to 1670 cm(-1) to 1442-3 cm(-1). The energy of the nu(NO) band in the reduced complex was unaffected by the presence of carbonyl groups on the porphinone ring, indicating little additional delocalization of the electron density of the Fe-NO moiety because of the carbonyl groups. The identity of the nu(NO) bands was confirmed with (15)N substitution of the Fe(OEP)(NO) complex. The nu(CO) band on the porphinone ring was found to be sensitive to the degree than electron density was delocalized to the ring.

The redox couple of the cytochrome c cyanide complex: the contribution of heme iron ligation to the structural stability, chemical reactivity, and physiological behavior of horse cytochrome c.

Contrary to most heme proteins, ferrous cytochrome c does not bind ligands such as cyanide and CO. In order to quantify this observation, the redox potential of the ferric/ferrous cytochrome c-cyanide redox couple was determined for the first time by cyclic voltammetry. Its E0' was -240 mV versus SHE, equivalent to -23.2 kJ/mol. The entropy of reaction for the reduction of the cyanide complex was also determined. From a thermodynamic cycle that included this new value for the cyt c cyanide complex E0', the binding constant of cyanide to the reduced protein was estimated to be 4.7 x 10(-3) L M(-1) or 13.4 kJ/mol (3.2 kcal/mol), which is 48.1 kJ/mol (11.5 kcal/mol) less favorable than the binding of cyanide to ferricytochrome c. For coordination of cyanide to ferrocytochrome c, the entropy change was earlier experimentally evaluated as 92.4 J mol(-1) K(-1) (22.1 e.u.) at 25 K, and the enthalpy change for the same net reaction was calculated to be 41.0 kJ/mol (9.8 kcal/mol). By taking these results into account, it was discovered that the major obstacle to cyanide coordination to ferrocytochrome c is enthalpic, due to the greater compactness of the reduced molecule or, alternatively, to a lower rate of conformational fluctuation caused by solvation, electrostatic, and structural factors. The biophysical consequences of the large difference in the stabilities of the closed crevice structures are discussed.

Chemical and Electrochemical Studies of Cl(2)FeS(2)MS(2)FeCl(2)(n)()(-) Clusters [M = Mo (n = 2), W (n = 2), V (n = 3)].

The electrochemistry and spectroelectrochemistry of [Cl(2)FeS(2)MS(2)FeCl(2)](n)()(-) clusters (where n = 2 for M = Mo and W and n = 3 for M = V; Ia,Ib, and Ic, respectively) and the dimetal complex [Cl(2)FeS(2)MoS(2)](2)(-) (IIIa) were examined in order to characterize the structures and properties of the one-electron-reduced complexes. A stable reduction product for Ia was observed spectroelectrochemically at -1.05 V, which could be oxidized back to the starting complex. Reduction at more negative potentials caused complete bleaching of the spectrum, and the starting complex could not be obtained by reoxidation. Similar behavior was observed for the tungsten complex, Ib, but the dimetal complex [Cl(2)FeS(2)WS(2)](2)(-) was formed upon reoxidation. Chemical and electrochemical reduction of Ia and Ib both led to the same products (IIa and IIb), but by different mechanisms. Borohydride reduction of Ia and Ib led to the initial formation of the dimetal complex, while the electrochemical reduction of Ia proceeded by way of the formation of [Cl(2)FeS(2)MoS(2)FeCl(2)](3)(-). Spectral changes were observed in the reduction of Ic, but they were not reversible. Resonance Raman spectroscopy of the reduced complexes was carried out in order to characterize the reduction product. Two polarized bands in the sulfur bridging region were observed in the resonance Raman spectra of electrochemically and chemically generated IIa and IIb. The relative intensities of these bands were dependent upon the excitation frequency. Reduction of Ic led to the loss of all resonance Raman bands. Reduction of IIIa gave rise to a complex (IVa) that was spectrally quite similar to IIa. These results, along with the previously reported result that the reduction complex was diamagnetic, indicate that the complex IIa is a dimeric species. The most likely structure consistent with these data is a Mo(2)Fe(2)S(4) cubane structure.

Reactions of Hydroxylamine with Metal Porphyrins.

The reaction of hydroxylamine with a series of metal porphyrins was examined in methanol/chloroform media. The reductive nitrosylation reaction was observed for the manganese and iron porphyrins, leading to a nitrosyl complex that precipitated out of the solution in good isolatable yield (80-90%). This reaction could be used synthetically for the generation of iron and manganese porphyrin nitrosyl complexes and was particularly useful for making isotopically labeled nitrosyl complexes. On the other hand, Co(II)(TPP) and Cr(TPP)(Cl) did not react with hydroxylamine under anaerobic conditions. With trace amounts of oxygen, the reaction of Co(II)(TPP) with hydroxylamine led to the formation of a stable cobalt(III)-bis(hydroxylamine) complex. The infrared, resonance Raman, and proton NMR spectra were consistent with a cobalt(III)-bis(hydroxylamine) complex. The cyclic voltammetry and visible spectroelectrochemistry of this complex were examined. The one-electron reduction of Co(III)(TPP)(NH(2)OH)(2)(+) formed Co(II)(TPP), for which there was no evidence for the coordination of hydroxylamine. Further reduction led to Co(I)(TPP)(-), which reacted with the halogenated solvent to form a cobalt-alkyl complex. The difference in the reactivity of these four metal porphyrins with hydroxylamine correlated well with their E(1/2) values. Iron(III) and manganese(III) porphyrins were relatively easy to reduce and readily underwent the reductive nitrosylation reaction, while cobalt(II) and chromium(III) porphyrins are unreactive. The one-electron oxidation of the hydroxylamine complex with a M(III) porphyrin would be expected to oxidize the N-atom in the coordinated hydroxylamine. The oxidation of M(III)(NH(2)OH) with the loss of a proton would form M(II)(N(I)H(2)O)(+) by an internal electron transfer, which will eventually lead to M(NO). The relationship between the reductive nitrosyl reaction and the enzymatic interconversion of NO and hydroxylamine was discussed.