Charles E. Schulz

Identification of carbon-encapsulated iron nanoparticles as active species in non-precious metal oxygen reduction catalysts

The widespread use of fuel cells is currently limited by the lack of efficient and cost-effective catalysts for the oxygen reduction reaction. Iron-based non-precious metal catalysts exhibit promising activity and stability, as an alternative to state-of-the-art platinum catalysts. However, the identity of the active species in non-precious metal catalysts remains elusive, impeding the development of new catalysts. Here we demonstrate the reversible deactivation and reactivation of an iron-based non-precious metal oxygen reduction catalyst achieved using high-temperature gas-phase chlorine and hydrogen treatments. In addition, we observe a decrease in catalyst heterogeneity following treatment with chlorine and hydrogen, using Mössbauer and X-ray absorption spectroscopy. Our study reveals that protected sites adjacent to iron nanoparticles are responsible for the observed activity and stability of the catalyst. These findings may allow for the design and synthesis of enhanced non-precious metal oxygen reduction catalysts with a higher density of active sites.

Consequences of a linear two-coordinate geometry for the orbital magnetism and Jahn-Teller distortion behavior of the high spin iron(II) complex Fe[N(t-Bu)2]2.

Mossbauer, EPR, magnetic susceptibility, and DFT studies of the unusual two-coordinate iron(II) amide Fe[N(t-Bu)(2)](2) show that it retains a linear N-Fe-N framework due to the nonbonding delta nature of the (xy, x(2)-y(2)) orbitals. The resulting near-degenerate ground state gives rise to a large magnetic moment and a remarkably large internal hyperfine field. The results confirm that extraordinary orbital magnetic effects can arise in linear transition metal complexes in which orbital degeneracies are not broken by Jahn-Teller or Renner-Teller distortions.

Direct Spectroscopic Observation of Large Quenching of First Order Orbital Angular Momentum with Bending in Monomeric, Two-Coordinate Fe(II) Primary Amido Complexes and the Profound Magnetic Effects of the Absence of Jahn- and Renner-Teller Distortions in Rigorously Linear Coordination

The monomeric iron(II) amido derivatives Fe{N(H)Ar*}(2) (1), Ar* = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Pr(i)(3))(2), and Fe{N(H)Ar(#)}(2) (2), Ar(#) = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Me(3))(2), were synthesized and studied in order to determine the effects of geometric changes on their unusual magnetic properties. The compounds, which are the first stable homoleptic primary amides of iron(II), were obtained by the transamination of Fe{N(SiMe(3))(2)}(2), with HN(SiMe(3))(2) elimination, by the primary amines H(2)NAr* or H(2)NAr(#). X-ray crystallography showed that they have either strictly linear (1) or bent (2, N-Fe-N = 140.9(2) degrees ) iron coordination. Variable temperature magnetization and applied magnetic field Mossbauer spectroscopy studies revealed a very large dependence of the magnetic properties on the metal coordination geometry. At ambient temperature, the linear 1 displayed an effective magnetic moment in the range 7.0-7.50 mu(B), consistent with essentially free ion magnetism. There is a very high internal orbital field component, H(L) approximately 170 T which is only exceeded by a H(L) approximately 203 T of Fe{C(SiMe(3))(3)}(2). In contrast, the strongly bent 2 displayed a significantly lower mu(eff) value in the range 5.25-5.80 mu(B) at ambient temperature and a much lower orbital field H(L) value of 116 T. The data for the two amido complexes demonstrate a very large quenching of the orbital magnetic moment upon bending the linear geometry. In addition, a strong correlation of H(L) with overall formal symmetry is confirmed. ESR spectroscopy supports the existence of large orbital magnetic moments in 1 and 2, and DFT calculations provide good agreement with the physical data.

Hydrosulfide (HS-) coordination in iron porphyrinates.

Recent reports of potential physiological roles of hydrogen sulfide have prompted interest in heme-sulfide interactions. Heme-H(2)S and/or heme-HS(-) interactions could potentially occur during endogenous production, transport, signaling events, and catabolism of H(2)S. We have investigated the interaction of the hydrosulfide ion (HS(-)) with iron porphyrinates. UV-vis spectral studies show the formation of [Fe(Por)(SH)](-), [Fe(Por)(SH)(2)](2-), and the mixed-ligand species [Fe(Por)(Im)(SH)](-). UV-vis binding studies of [Fe(OEP)] and [Fe(T-p-OMePP)] (OEP = octaethylporphyrinate; T-p-OMePP = tetra-p-methoxyphenylporphyrinate) with HS(-) allowed for calculation of the formation constants and extinction coefficients of mono- and bis-HS(-) complexes. We report the synthesis of the first HS(-)-bound iron(II) porphyrin compounds, [Na(222)][Fe(OEP)(SH)].0.5C(6)H(6) and [Na(222)][Fe(T-p-OMePP)(SH)].C(6)H(5)Cl (222 = Kryptofix-222). Characterization by single-crystal X-ray analysis, mass spectrometry, and Mossbauer and IR spectroscopy is all consistent with that of known sulfur-bound high-spin iron(II) compounds. The Fe-S distances of 2.3929(5) and 2.3887(13) A are longer than all reported values of [Fe(II)(Por)(SR)](-) species. An analysis of the porphyrin nonplanarity for these derivatives and for all five-coordinate high-spin iron(II) porphyrinate derivatives with an axial anion ligand is presented. In our hands, attempts to synthesize iron(III) HS(-) derivatives led to iron(II) species.

Discovery of acetylene hydratase activity of the iron–sulphur protein IspH

The final step of the methylerythritol phosphate isoprenoid biosynthesis pathway is catalysed by the iron-sulphur enzyme IspH, producing the universal precursors of terpenes: isopentenyl diphosphate and dimethylallyl diphosphate. Here we report an unforeseen reaction discovered during the investigation of the interaction of IspH with acetylene inhibitors by X-ray crystallography, Mößbauer, and nuclear magnetic resonance spectroscopy. In addition to its role as a 2H(+)/2e(-) reductase, IspH can hydrate acetylenes to aldehydes and ketones via anti-Markovnikov/Markovnikov addition. The reactions only occur with the oxidised protein and proceed via η(1)-O-enolate intermediates. One of these is characterized crystallographically and contains a C4 ligand oxygen bound to the unique, fourth iron in the 4Fe-4S cluster: this intermediate subsequently hydrolyzes to produce an aldehyde product. This unexpected side to IspH reactivity is of interest in the context of the mechanism of action of other acetylene hydratases, as well as in the design of antiinfectives targeting IspH.

Just a proton: distinguishing the two electronic states of five-coordinate high-spin iron(II) porphyrinates with imidazole/ate coordination.

We report detailed studies on two S = 2 electronic states of high-spin iron(II) porphyrinates. These two states are exemplified by the five-coordinate derivatives with either neutral imidazole or anionic imidazolate as the axial ligand. The application of several physical methods all demonstrate distinctive differences between the two states. These include characteristic molecular structure differences, Mossbauer spectra, magnetic circular dichroism spectroscopy, and integer-spin EPR spectral distinctions. These distinctions are supported by DFT calculations. The two states are characterized by very different spatial properties of the doubly occupied orbital of the high-spin that are consonant with the physical properties.

Relative axial ligand orientation in bis(imidazole)iron(II) porphyrinates: are "picket fence" derivatives different?

The synthesis of three new bis(imidazole)-ligated iron(II) picket fence porphyrin derivatives, [Fe(TpivPP)(1-RIm) 2] 1-RIm = 1-methyl-, 1-ethyl-, or 1-vinylimidazole) are reported. X-ray structure determinations reveal that the steric requirements of the four alpha,alpha,alpha,alpha-o-pivalamidophenyl groups lead to very restricted rotation of the imidazole ligand on the picket side of the porphyrin plane; the crowding leads to an imidazole plane orientation eclipsing an iron-porphyrin nitrogen bond. An unusual feature for these diamagnetic iron(II) species is that all three derivatives have the two axial ligands with a relative perpendicular orientation; the dihedral angles between the two imidazole planes are 77.2 degrees , 62.4 degrees , and 78.5 degrees . All three derivatives have nearly planar porphyrin cores. Mössbauer spectroscopic characterization shows that all three derivatives have quadrupole splitting constants around 1.00 mm/s at 100K.

Cyanide — a Strong Field Ligand for Ferrohemes and Hemoproteins?

Cyanide ion, a versatile diatomic ligand, has been extensively investigated as both a classic inhibitor and as a ligand for exploring properties of hemes and hemoproteins. Unlike CO and O2 which bind only to iron(II) species, CN− can bind to both iron(II) and -(III) hemo-proteins. Stable low-spin (LS) iron(III) proteins can be straightforwardly prepared.[1–3] In contrast, (cyano)iron(II) hemoproteins are usually indirectly formed by reduction of (cyano)iron(III) proteins. Cyanide bound, iron(II) forms of myoglobin,[4] hemoglobin,[5] horseradish peroxidase[6] and a number of cytochrome oxidase derivatives[7] are known. Many, but not all, of the iron(II) species, have lower binding constants than the iron(III) analogues. The equilibrium constant for cyanide binding for iron(III) hemoproteins is often ≥105 M−1 compared to ≤102 M−1 for iron(II) species.[3] Since the first reported isolation of a (cyano)heme was reported by us in 1980,[8] a number of electronic and geometric structure issues have been brought forward.[9] All of the known species are LS iron(III) derivatives, either bis(cyano) [FeIII(Por)(CN)2]− or mixed-ligand [FeIII(Por)(CN)(L)] complexes.[9] However, there are currently no (cyano)iron(II) porphyrinate derivatives reported, presumably because of its known lower stability/affinity compared to iron(III). It might be thought that (cyano)iron(II) species would be preferred since a filled d6 shell should strongly π-bond to the π-accepting cyanide ligand. We now report the first (cyano)iron(II) porphyrinate species, five-coordinate [K(222)][Fe(TPP)(CN)] (Figure 1). The average equatorial Fe–Np bond distance (1.986 (7) A) and the axial Fe–C distance (1.8783 (10) A) are consistent with a LS state.[10] However, T-dependent Mossbauer spectra reveal a more complicated picture of the iron spin state. A single quadrupole doublet is observed, whose value decreases from 1.827 mm/s at 25 K to 0.85 mm/s at 300 K; the isomer shift varies between 0.37 to 0.47 mm/s. The most probable explanation is that a thermally induced spin crossover is occurring, whose interconversion is rapid on the Mossbauer time scale (< 10−8 s).[11]a This interpretation has been confirmed by both DFT calculations and magnetic susceptibility measurements. Figure 1 100 K ORTEP diagram of [Fe(TPP)(CN)]−. Thermal ellipsoids are contoured at the 50% probability level. Hydrogens omitted for clarity. The magnetic susceptibility of [K(222)][Fe(TPP)(CN)] was investigated over the temperature range of 2–400 K. Figure 2 shows the product of the molar susceptibility (χm) (corrected for paramagnetism (TIP)) and temperature (T) in an external magnetic field of 2 T, which provides direct evidence for an S = 0 (LS) ↔ S = 2 (HS) spin crossover. AT 400 K, the value of χmT (2.96 cm3 K mol−1) is close to that expected for the HS state, but the lack of significant plateau suggests that the transition is not quite complete at this temperature. The spin-state transition occurs over a large temperature range (~175–400 K) and is reversible; both ascending and descending temperature measurements are shown in Figure 2 and no hysteresis was observed. The transition temperature T1/2 (defined as temperature at which complexes shows a population of 50% in the HS state) of this gradually proceeding spin transition is about 265 K. Figure 2 also plots the observed time-averaged quadrupole splitting value against temperature; the strong correlation between the quadrupole splitting and the susceptibility is clear. Figure 2 χmT versus T plot for [K(222)][Fe(TPP)(CN)] at 2T applied field. The Mossbauer quadrupole splitting values are also presented for comparison. To gain a better understanding of the thermodynamics regarding the spin-states, density functional theory was employed (see Supporting Information).[12] At low temperature only the low-spin S = 0 state was thermodynamically accessible. With increasing temperature the S = 2 state became significant, and a spin-crossover event is predicted to occur near 325 K (Figure S1) in good agreement with the value of 265 K from experiment. The intermediate-spin S = 1 state was disfavored over the entire temperature range explored. We have also investigated the T-dependent structures of the iron complex, since changes in metal–donor atom distances, along with changes in magnetic properties, are the two hallmarks of spin-state transitions. Structures have been determined at 100 K (two crystals) and 296, 325 and 400 K.[13] A change from a LS to a HS state in the five-coordinate complex is expected to lead to increases in the axial Fe–C distance, the equatorial Fe–Np bond distances, and the displacement of the iron atom from the mean porphinato plane. The results are summarized in the ORTEP drawings given in Figure 3, for simplicity only the cyanide group and FeN4 porphyrin core are shown. The Fe–C distance elongates by 0.23 A (Figure 3), which is amongst the largest changes in bond lengths that have been observed for iron(II) spin crossover compounds.[11b]b This is in part because the axial and equatorial bond distance increases must be asymmetric owing to the macrocyclic constraints of the porphyrin ring; note that Fe–Np has increased by 0.103 A over the same temperature range. The 100 K Fe–Np average bond length of 1.986 (7) A is that for a pure LS state whereas the 400 K value of 2.089 (8) A is slightly less than expected for anionic HS iron(II) complex, consistent with the idea that the spin state transition is not quite complete. Also completely consonant with expectation are the increases in the displacement of the iron from the mean plane of the four nitrogen plane. Figure 3 Four ORTEP diagrams of [K(222)][Fe(TPP)(CN)] displaying the cyanide groups and the core atoms of porphyrin (Fe and four pyrrole N atoms). Values of axial ligand and average equatorial bond distances are given as well as the iron displacement from the ... The anisotropic thermal parameters also show evidence of the spin crossover. As expected, the magnitude of all atomic anisotropic displacement parameters increase upon increasing temperature. However, the cyanide carbon atom shows different behavior over the temperature range. The thermal parameters at 100 and 400 K are close to isotropic, consistent with a single carbon atom site, whereas at intermediate temperatures with substantial populations of two spin states and differing carbon sites, the thermal parameters are much more prolate with elongation along the Fe–C bond direction. Importantly, the C–N bond distance in all structures remains nearly constant, as expected if only CN− atoms occupy two sites. Additional evidence for the spin crossover comes from T-dependent infrared spectra, which has the advantage of a shorter time scale (10−13 s) and thus can detect both spin isomers. Measurements at 296 K, as either Nujol mulls or KBr pellets, show two distinct ν(C–N) frequencies at 2070 and 2105 cm−1, with the first being the stronger. (S.I.) On cooling, the 2105 cm−1 peak gradually decreases while the 2070 cm−1 peak increases. At 150 to 160 K, the stretch at 2105 cm−1 disappears and thus corresponds to the HS stretch. A similar pattern of T-dependent azide stretches was observed in a 5/2, 3/2 spin crossover complex.[14] In coordination chemistry, cyanide and CO have been deeply entrenched as strong field ligands.[15, 16] Recently, Miller et al. showed that [(NEt4)3][Cr(II)(CN)5][17] is a distorted trigonal bipyramidal complex that was not low spin. Two different theoretical calculations[18] have suggested that the HS state results from the buildup of electrostatic (ligand–ligand) repulsions and not the ligand field of cyanide per se; the cyanide ligand is behaving as a strong field ligand in this Cr complex. However, [K(222)][Fe(TPP)(CN)] represents a case where the CN− should unequivocally lead to LS species. That it does not, strongly demonstrates the weaker field nature of cyanide, even in a case where π-back bonding should be maximized. In summary, the synthesis and characterization of the first cyanoiron(II) porphyrinate, [K(222)][Fe(TPP)(CN)], is presented. It forms a LS to HS crossover complex; coordination of a single axial cyanide ligand does not generate a sufficiently strong ligand field to ensure a low-spin complex under all conditions.[19] This is in distinct contrast to the five-coordinate CO complex, that is low spin under all known conditions.

Correlated Ligand Dynamics in Oxyiron Picket Fence Porphyrins: Structural and Mössbauer Investigations

Disorder in the position of the dioxygen ligand is a well-known problem in dioxygen complexes and, in particular, those of picket fence porphyrin species. The dynamics of Fe-O2 rotation and tert-butyl motion in three different picket fence porphyrin derivatives has been studied by a combination of multitemperature X-ray structural studies and Mössbauer spectroscopy. Structural studies show that the motions of the dioxygen ligand also require motions of the protecting pickets of the ligand binding pocket. The two motions appear to be correlated, and the temperature-dependent change in the O2 occupancies cannot be governed by a simple Boltzmann distribution. The three [Fe(TpivPP)(RIm)(O2)] derivatives studied have RIm = 1-methyl-, 1-ethyl-, or 2-methylimidazole. In all three species there is a preferred orientation of the Fe-O2 moiety with respect to the trans imidazole ligand and the population of this orientation increases with decreasing temperature. In the 1-MeIm and 1-EtIm species the Fe-O2 unit is approximately perpendicular to the imidazole plane, whereas in the 2-MeHIm species the Fe-O2 unit is approximately parallel. This reflects the low energy required for rotation of the Fe-O2 unit and the small energy differences in populating the possible pocket quadrants. All dioxygen complexes have a crystallographically required 2-fold axis of symmetry that limits the accuracy of the determined Fe-O2 geometry. However, the 80 K structure of the 2-MeHIm derivative allowed for resolution of the two bonded oxygen atom positions and provided the best geometric description for the Fe-O2 unit. The values determined are Fe-O = 1.811(5) Å, Fe-O-O = 118.2(9)°, O-O = 1.281(12) Å, and an off-axis tilt of 6.2°. Demonstration of the off-axis tilt is a first. We present detailed temperature-dependent simulations of the Mössbauer spectra that model the changing value of the quadrupole splitting and line widths. Residuals to fits are poorer at higher temperature. We believe that this is consistent with the idea that population of the two conformers is related to the concomitant motions of both Fe-O2 rotations and motions of the protecting tert-butyl pickets.

Hydrogen Bonding Effects on the Electronic Configuration of Five-Coordinate High-Spin Iron(II) Porphyrinates

The characterization of a new five-coordinate derivative of (2-methylimidazole)(tetraphenylporphinato)iron(II) provides new and unique information about the effects of forming a hydrogen bond to the coordinated imidazole on the geometric and electronic structure of iron in these species. The complex studied has two crystallographically distinct iron sites; one site has an axial imidazole ligand modified by an external hydrogen bond, and the other site has an axial imidazole ligand with no external interactions. The iron atoms at the two sites have distinct geometric features, as revealed in their molecular structures, and distinct electronic structures, as shown by Mössbauer spectroscopy, although both are high spin (S = 2). The molecule with the external hydrogen bond has longer equatorial Fe-N(p) bonds, a larger displacement of the iron atom out of the porphyrin plane, and a shorter axial bond compared to its counterpart with no hydrogen bonding. The Mössbauer features are distinct for the two sites, with differing quadrupole splitting and isomer shift values and probably differing signs for the quadrupole splitting as shown by variable-temperature measurements in applied magnetic field. These features are consistent with a significant change in the nature of the doubly populated d orbital and are all in the direction of the dichotomy displayed by related imidazole and imidazolate species where deprotonation leads to major differences. The results points out the possible effects of strong hydrogen bonding in heme proteins.