Subhra Samanta

Direct observation of intermediates formed during steady-state electrocatalytic O2 reduction by iron porphyrins

Heme/porphyrin-based electrocatalysts (both synthetic and natural) have been known to catalyze electrochemical O2, H+, and CO2 reduction for more than five decades. So far, no direct spectroscopic investigations of intermediates formed on the electrodes during these processes have been reported; and this has limited detailed understanding of the mechanism of these catalysts, which is key to their development. Rotating disk electrochemistry coupled to resonance Raman spectroscopy is reported for iron porphyrin electrocatalysts that reduce O2 in buffered aqueous solutions. Unlike conventional single-turnover intermediate trapping experiments, these experiments probe the system while it is under steady state. A combination of oxidation and spin-state marker bands and metal ligand vibrations (identified using isotopically enriched substrates) allow in situ identification of O2-derived intermediates formed on the electrode surface. This approach, combining dynamic electrochemistry with resonance Raman spectroscopy, may be routinely used to investigate a plethora of metalloporphyrin complexes and heme enzymes used as electrocatalysts for small-molecule activation.

O2 reduction reaction by biologically relevant anionic ligand bound iron porphyrin complexes.

Iron porphyrin complex with a covalently attached thiolate ligand and another with a covalently attached phenolate ligand has been synthesized. The thiolate bound complex shows spectroscopic features characteristic of P450, including the hallmark absorption spectrum of the CO adduct. Electrocatalytic O2 reduction by this complex, which bears a terminal alkyne group, is investigated by both physiabsorbing on graphite surfaces (fast electron transfer rates) and covalent attachment to azide terminated self-assembled monolayer (physiologically relevant electron transfer rates) using the terminal alkyne group. Analysis of the steady state electrochemical kinetics reveals that this catalyst can selectively reduce O2 to H2O with a second-order k(cat.) ~10(7) M(-1 )s(-1) at pH 7. The analogous phenolate bound iron porphyrin complex reduces O2 with a second-order rate constant of 10(5) M(-1) s(-1) under the same conditions. The anionic ligand bound iron porphyrin complexes catalyze oxygen reduction reactions faster than any known synthetic heme porphyrin analogues. The kinetic parameters of O2 reduction of the synthetic thiolate bound complex, which is devoid of any second sphere effects present in protein active sites, provide fundamental insight into the role of the protein environment in tuning the reactivity of thiolate bound iron porphyrin containing metalloenzymes.

Electrocatalytic O2 reduction reaction by synthetic analogues of cytochrome P450 and myoglobin: in-situ resonance Raman and dynamic electrochemistry investigations.

Bioinspired electrodes have been constructed by physiabsorption of two air stable iron porphyrin complexes, one bearing an imidazole coordination and the other bearing a thiolate coordination. To control the electron transfer (ET) rate to these O2 reducing electrocatalysts, the complexes were immobilized on edge plane graphite electrode and alkyl thiol self-assembled monolayer (SAM) modified Au electrodes with varying chain lengths of the thiols. Catalyst immobilized SAM modified surfaces were characterized using surface enhanced resonance Raman spectroscopy (SERRS), and their electrocatalytic O2 reduction properties were investigated using rotating ring disc electrochemistry (RRDE). While the imidazole bound complex showed increase in partially reduced oxygen species (PROS) on decreasing ET rate, the thiolate bound complex showed the opposite trend, that is, the value of PROS reduced on decreasing the ET rate. SERRS coupled to rotating disc electrochemistry (SERRS-RDE) technique helps gain insight into the O2 reduction mechanism. The results obtained indicate that while the imidazole bound iron porphyrin complex reduces O2 through an inner sphere mechanism using a high-spin (HS) Fe(II) species, the thiolate ligated complex shows an inner sphere as well as outer sphere mechanism using a HS Fe(II) and low-spin (LS) Fe(II) species, respectively. The PROS formation by a HS Fe(II) species of this thiolate bound complex increases with decreasing ET rates while that of a LS Fe(II) species decreases with decreasing ET rates.

Selective 4e-/4H+ O2 reduction by an iron(tetraferrocenyl)porphyrin complex: from proton transfer followed by electron transfer in organic solvent to proton coupled electron transfer in aqueous medium.

An iron porphyrin catalyst bearing four ferrocenes and a hydrogen bonding distal pocket is found to catalyze 4e(-)/4H(+) oxygen reduction reaction (ORR) in organic solvent under homogeneous conditions in the presence of 2-3 equiv of Trifluoromethanesulphonic acid. Absorption spectroscopy, electron paramagnetic resonance (EPR), and resonance Raman data along with H2O2 assay indicate that one out of the four electrons necessary to reduce O2 to H2O is donated by the ferrous porphyrin while three are donated by the distal ferrocene residues. The same catalyst shows 4e(-)/4H(+) reduction of O2 in an aqueous medium, under heterogeneous conditions, over a wide range of pH. Both the selectivity and the rate of ORR are found to be pH independent in an aqueous medium. The ORR proceeds via a proton transfer followed by electron transfer (PET) step in an organic medium and while a 2e(-)/1H(+) proton coupled electron transfer (PCET) step determines the electrochemical potential of ORR in an aqueous medium.

EPR, Resonance Raman, and DFT Calculations on Thiolate- and Imidazole-Bound Iron(III) Porphyrin Complexes: Role of the Axial Ligand in Tuning the Electronic Structure

Iron(III) porphyrin complexes bearing covalently attached imidazole and thiolate axial ligands are investigated using resonance Raman, electron paramagnetic resonance, and cyclic voltammetry. The thiolate ligand stabilizes a low-spin ground state in solvent-bound six-coordinate species, weakens the Fe-N(pyr) bonds, and shifts the Fe(III/II) potential more negative by ~500 mV relative to an imidazole-bound species. Density functional theory calculations reproduce the experimental observation and indicate that the covalent charge donation from thiolate to iron reduces the Z(eff) on the iron. This increases the Fe(3d) orbital energies, which changes the bonding interaction present in these complexes significantly. In particular, the increase of the Fe(3d) energies activates an iron-to-porphyrin π*-back-bonding interaction not present in the imidazole-bound complex.

Concerted Proton–Electron Transfer in Electrocatalytic O2 Reduction by Iron Porphyrin Complexes: Axial Ligands Tuning H/D Isotope Effect

The electrochemical O2 reduction by thiolate- and imidazole-bound iron porphyrin complexes and H/D isotope effects on 4e(-) (determined by rotating disc electrochemistry) and 2e(-) (determined by rotating ring disc electrochemistry) O2 reduction rates are investigated. The results indicate that a thiolate axial ligand shows an H/D isotope effect greater than 18 and 47 for the 4e(-) and 2e(-) O2 reductions, respectively. Alternatively, an imidazole axial ligand results in H/D isotope effects of 1.04 and 4.7 for the 4e(-) and 2e(-) O2 reduction, respectively. The catalytic O2 reduction mechanism is investigated in situ with resonance Raman coupled with rotating disc electrochemistry. The data indicate that the rate-determining step changes from O-O bond heterolysis of Fe(III)-OOH species for a thiolate axial ligand to an O-O bond heterolysis of an Fe(II)-OOH for an imidazole axial ligand.

A hydrogen bond scaffold supported synthetic heme FeIII–O2− adduct

A hydrogen bonded heme-Fe(III)-O(2)(-) adduct is stabilized and characterized using resonance Raman and EPR spectroscopy. The low O-O vibrations of this complex are quite different from those reported for other heme-Fe(III)-O(2)(-) adducts.

Effect of axial ligand, spin state, and hydrogen bonding on the inner-sphere reorganization energies of functional models of cytochrome P450.

Using a combination of self-assembly and synthesis, bioinspired electrodes having dilute iron porphyrin active sites bound to axial thiolate and imidazole axial ligands are created atop self-assembled monolayers (SAMs). Resonance Raman data indicate that a picket fence architecture results in a high-spin (HS) ground state (GS) in these complexes and a hydrogen-bonding triazole architecture results in a low-spin (LS) ground state. The reorganization energies (λ) of these thiolate- and imidazole-bound iron porphyrin sites for both HS and LS states are experimentally determined. The λ of 5C HS imidazole and thiolate-bound iron porphyrin active sites are 10-16 kJ/mol, which are lower than their 6C LS counterparts. Density functional theory (DFT) calculations reproduce these data and indicate that the presence of significant electronic relaxation from the ligand system lowers the geometric relaxation and results in very low λ in these 5C HS active sites. These calculations indicate that loss of one-half a π bond during redox in a LS thiolate bound active site is responsible for its higher λ relative to a σ-donor ligand-like imidazole. Hydrogen bonding to the axial ligand leads to a significant increase in λ irrespective of the spin state of the iron center. The results suggest that while the hydrogen bonding to the thiolate in the 5C HS thiolate bound active site of cytochrome P450 (cyp450) shifts the potential up, resulting in a negative ΔG, it also increases λ resulting in an overall low barrier for the electron transfer process.

Valence tautomerism in synthetic models of cytochrome P450

Significance Cytochrome P450 is a mammalian enzyme responsible for the catabolism of organic molecules (food, drug, etc.) as well as biosynthesis of hormones and cholesterol. Unfortunately, in stark contrast to the stability of the enzyme, its synthetic mimics rapidly degrade on being exposed to oxygen impeding erstwhile efforts of the scientific community to understand its chemistry in detail. Here the discovery of a valence tautomerism between a ferric thiolate and an arcane ferrous-thiyl species in synthetic models at room temperature is proposed to be at the root of this three-decade-old mystery. Hydrogen bonding to the thiolate sulfur, also present in the enzyme active site, restrains the system to the ferric thiolate form imparting its stability in the presence of oxygen. CytP450s have a cysteine-bound heme cofactor that, in its as-isolated resting (oxidized) form, can be conclusively described as a ferric thiolate species. Unlike the native enzyme, most synthetic thiolate-bound ferric porphyrins are unstable in air unless the axial thiolate ligand is sterically protected. Spectroscopic investigations on a series of synthetic mimics of cytP450 indicate that a thiolate-bound ferric porphyrin coexists in organic solutions at room temperature (RT) with a thiyl-radical bound ferrous porphyrin, i.e., its valence tautomer. The ferric thiolate state is favored by greater enthalpy and is air stable. The ferrous thiyl state is favored by entropy, populates at RT, and degrades in air. These ground states can be reversibly interchanged at RT by the addition or removal of water to the apolar medium. It is concluded that hydrogen bonding and local electrostatics protect the resting oxidized cytP450 active site from degradation in air by stabilizing the ferric thiolate ground state in contrast to its synthetic analogs.

Second sphere control of spin state: Differential tuning of axial ligand bonds in ferric porphyrin complexes by hydrogen bonding

An iron porphyrin with a pre-organized hydrogen bonding (H-Bonding) distal architecture is utilized to avoid the inherent loss of entropy associated with H-Bonding from solvent (water) and mimic the behavior of metallo-enzyme active sites attributed to H-Bonding interactions of active site with the 2nd sphere residues. Resonance Raman (rR) data on these iron porphyrin complexes indicate that H-Bonding to an axial ligand like hydroxide can result in both stronger or weaker Fe(III)-OH bond relative to iron porphyrin complexes. The 6-coordinate (6C) complexes bearing water derived axial ligands, trans to imidazole or thiolate axial ligand with H-Bonding stabilize a low spin (LS) ground state (GS) when a complex without H-Bonding stabilizes a high spin (HS) ground state. DFT calculations reproduce the trend in the experimental data and provide a mechanism of how H-Bonding can indeed lead to stronger metal ligand bonds when the axial ligand donates an H-Bond and lead to weaker metal ligand bonds when the axial ligand accepts an H-Bond. The experimental and computational results explain how a weak Fe(III)-OH bond (due to H-Bonding) can lead to the stabilization of low spin ground state in synthetic mimics and in enzymes containing iron porphyrin active sites. H-Bonding to a water ligand bound to a reduced ferrous active site can only strengthen the Fe(II)-OH2 bond and thus exclusion of water and hydrophilic residues from distal sites of O2 binding/activating heme proteins is necessary to avoid inhibition of O2 binding by water. These results help demonstrate the predominant role played by H-Bonding and subtle changes in its orientation in determining the geometric and electronic structure of iron porphyrin based active sites in nature.