Iron(II) Bis(imidazole) Derivatives of a Binuclear Porphyrin Model: Crystal Structures and Mössbauer Properties

A new sterically hindered “picket fence-like” porphyrin with chelates for the second metal atom, H₂TImPP (TImPP = <i>meso</i>-tetrakis­[α,α,α,α-<i>o</i>-(5-imidazolecarboxylaminophenyl)]­porphyrinato), is developed and used in the synthesis of four iron­(II) bis­(imidazole) derivatives, which are characterized by single crystal X-ray and other spectroscopies. The comprehensive studies on the crystal structures revealed noteworthy features including new axial ligand arrangements, deformed porphyrin planes, and strongly tilted pickets which can be rationalized by analysis of the intra- and intermolecular interactions. Solid-state Mössbauer experiments on [Fe­(TImPP)­(1-MeIm)₂] were conducted at several temperatures from 295 to 25 K. The quadrupole splitting (Δ<i>E</i>_Q) in the range of 1.01–1.03 mm/s confirmed the low-spin state of the iron

Hydrogen-Bonding Effects in Five-Coordinate High-Spin Imidazole-Ligated Iron(II) Porphyrinates

The influence of hydrogen binding to the N–H group of coordinated imidazole in high-spin iron­(II) porphyrinates has been studied. The preparation and characterization of new complexes based on [Fe­(TPP)­(2-MeHIm)] (TPP is the dianion of tetraphenylporphyrin) are reported. The hydrogen bond acceptors are ethanol, tetramethylene sulfoxide, and 2-methylimidazole. The last acceptor, 2-MeHIm, was found in a crystalline complex with two [Fe­(TPP)­(2-MeHIm)] sites, only one of which has the 2-methylimidazole hydrogen bond acceptor. This latter complex has been studied by temperature-dependent Mössbauer spectroscopy. All new complexes have also been characterized by X-ray structure determinations. The Fe–N_P and Fe–N_Im bond lengths, and displacement of the Fe atom out of the porphyrin plane are similar to, but marginally different than, those in imidazole-ligated species with no hydrogen bond. All the structural and Mössbauer properties suggest that these new hydrogen-bonded species have the same electronic configuration as imidazole-ligated species with no hydrogen bond. These new studies continue to show that the effects of hydrogen bonding in five-coordinate high-spin iron­(II) systems are subtle and challenging to understand

Hydrogen-Bonding Effects in Five-Coordinate High-Spin Imidazole-Ligated Iron(II) Porphyrinates

The influence of hydrogen binding to the N–H group of coordinated imidazole in high-spin iron­(II) porphyrinates has been studied. The preparation and characterization of new complexes based on [Fe­(TPP)­(2-MeHIm)] (TPP is the dianion of tetraphenylporphyrin) are reported. The hydrogen bond acceptors are ethanol, tetramethylene sulfoxide, and 2-methylimidazole. The last acceptor, 2-MeHIm, was found in a crystalline complex with two [Fe­(TPP)­(2-MeHIm)] sites, only one of which has the 2-methylimidazole hydrogen bond acceptor. This latter complex has been studied by temperature-dependent Mössbauer spectroscopy. All new complexes have also been characterized by X-ray structure determinations. The Fe–N_P and Fe–N_Im bond lengths, and displacement of the Fe atom out of the porphyrin plane are similar to, but marginally different than, those in imidazole-ligated species with no hydrogen bond. All the structural and Mössbauer properties suggest that these new hydrogen-bonded species have the same electronic configuration as imidazole-ligated species with no hydrogen bond. These new studies continue to show that the effects of hydrogen bonding in five-coordinate high-spin iron­(II) systems are subtle and challenging to understand

Bis(cyano) Iron(III) Porphyrinates: What Is the Ground State?

The synthesis of six new bis­(cyano) iron­(III) porphyrinate derivatives is reported. The anionic porphyrin complexes utilized tetraphenylporphyrin, tetramesitylporphyrin, and tetratolylporphyrin as the porphyrin ligand. The potassium salts of Kryptofix-222 and 18-C-6 were used as the cations. These complexes have been characterized by X-ray structure analysis, solid-state Mössbauer spectroscopy, and EPR spectroscopy, both in frozen CH₂Cl₂ solution and in the microcrystalline state. These data show that these anionic complexes can exist in either the (d_<i>xz</i>,d_<i>yz</i>)⁴(d_<i>xy</i>)¹ or the (d_<i>xy</i>)²(d_<i>xz</i>,d_<i>yz</i>)³ electronic configuration and some can clearly readily interconvert. This is a reflection that these two states can be very close in energy. In addition to the effects of varying the porphyrin ligand, subtle effects of the cyanide ligand environment in the solid state and in solution are sufficient to shift the balance between the two electronic states

Electronic Configuration and Ligand Nature of Five-Coordinate Iron Porphyrin Carbene Complexes: An Experimental Study

The five-coordinate iron porphyrin carbene complexes [Fe­(TPP) (CCl₂)] (TPP = tetraphenylporphyrin), [Fe­(TTP) (CCl₂)] (TTP = tetratolylporphyrin) and [Fe­(TFPP) (CPh₂)] (TFPP = tetra­(penta­fluoro­phenyl)­porphyrin), utilizing two types of carbene ligands (CCl₂ and CPh₂), have been investigated by single crystal X-ray, XANES (X-ray absorption near edge spectroscopy), Mössbauer, NMR and UV–vis spectroscopies. The XANES suggested the iron­(II) oxidation state of the complexes. The multitemperature and high magnetic field Mössbauer experiments, which show very large quadrupole splittings (QS, Δ<i>E</i>_Q), determined the <i>S</i> = 0 electronic configuration. More importantly, combined structural and Mössbauer studies, especially the comparison with the low spin iron­(II) porphyrin complexes with strong diatomic ligands (CS, CO and CN^–) revealed the covalent bond nature of the carbene ligands. A correlation between the iron isomer shifts (IS, δ) and the axial bond distances is established for the first time for these donor carbon ligands (:C–R)

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 <i>tert</i>-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 <i>tert</i>-butyl pickets

Iron Nitrosyl “Natural” Porphyrinates: Does the Porphyrin Matter?

The synthesis and spectroscopic characterization of three five-coordinate nitrosyliron­(II) complexes, [Fe­(Porph)­(NO)], are reported. These three nitrosyl derivatives, where Porph represents protoporphyrin IX dimethyl ester, mesoporphyrin IX dimethyl ester, or deuteroporphyrin IX dimethyl ester, display notable differences in their properties relative to the symmetrical synthetic porphyrins such as OEP and TPP. The N–O stretching frequencies are in the range of 1651–1660 cm^–1, frequencies that are lower than those of synthetic porphyrin derivatives. Mössbauer spectra obtained in both zero and applied magnetic field show that the quadrupole splitting values are slightly larger than those of known synthetic porphyrins. The electronic structures of these naturally occurring porphyrin derivatives are thus seen to be consistently different from those of the synthetic derivatives, the presumed consequence of the asymmetric peripheral substituent pattern. The molecular structure of [Fe­(PPIX-DME)­(NO)] has been determined by X-ray crystallography. Although disorder of the axial nitrosyl ligand limits the structural quality, this derivative appears to show the same subtle structural features as previously characterized five-coordinate nitrosyls

Revealing the Role of the Metal in Non-Precious-Metal Catalysts for Oxygen Reduction via Selective Removal of Fe

Non-precious-metal catalysts have been investigated as alternatives to Pt-based oxygen reduction reaction catalysts for more than 50 years. While the incorporation of a metal is known to be necessary to generate a catalyst with high activity, the exact role of the metal is still not well-understood. In this work, we prepare an active oxygen reduction reaction catalyst containing Fe and then selectively remove the Fe from the catalyst while preserving the carbon and nitrogen species. By comparing the oxygen reduction reaction activity of the catalyst before and after treatment, we show that in the absence of Fe the carbon and nitrogen sites in the catalyst exhibit a larger overpotential and lower selectivity for the 4<i>e</i>^– reduction of oxygen in both acidic and alkaline conditions. These findings reveal the direct involvement of the metal in the active site of non-precious-metal catalysts and provide important guidance for future catalyst improvements

Redesigning the Blue Copper Azurin into a Redox-Active Mononuclear Nonheme Iron Protein: Preparation and Study of Fe(II)-M121E Azurin

Much progress has been made in designing heme and dinuclear nonheme iron enzymes. In contrast, engineering mononuclear nonheme iron enzymes is lagging, even though these enzymes belong to a large class that catalyzes quite diverse reactions. Herein we report spectroscopic and X-ray crystallographic studies of Fe­(II)-M121E azurin (Az), by replacing the axial Met121 and Cu­(II) in wild-type azurin (wtAz) with Glu and Fe­(II), respectively. In contrast to the redox inactive Fe­(II)-wtAz, the Fe­(II)-M121EAz mutant can be readily oxidized by Na₂IrCl₆, and interestingly, the protein exhibits superoxide scavenging activity. Mössbauer and EPR spectroscopies, along with X-ray structural comparisons, revealed similarities and differences between Fe­(II)-M121EAz, Fe­(II)-wtAz, and superoxide reductase (SOR) and allowed design of the second generation mutant, Fe­(II)-M121EM44KAz, that exhibits increased superoxide scavenging activity by 2 orders of magnitude. This finding demonstrates the importance of noncovalent secondary coordination sphere interactions in fine-tuning enzymatic activity

Measurement of Extreme Hyperfine Fields in Two-Coordinate High-Spin Fe²⁺ Complexes by Mössbauer Spectroscopy: Essentially Free-Ion Magnetism in the Solid State

Mössbauer studies of three two-coordinate linear high-spin Fe²⁺ compounds, namely, Fe­{N­(SiMe₃)­(Dipp)}₂ (<b>1</b>) (Dipp = C₆H₃-2,6-^<i>i</i>Pr₂), Fe­(OAr′)₂ (<b>2</b>) [Ar′ = C₆H₃-2,6-(C₆H₃-2,6-^<i>i</i>Pr₂)₂], and Fe­{C­(SiMe₃)₃}₂ (<b>3</b>), are presented. The complexes were characterized by zero- and applied-field Mössbauer spectroscopy (<b>1</b>–<b>3</b>), as well as zero- and applied-field heat-capacity measurements (<b>3</b>). As <b>1</b>–<b>3</b> are rigorously linear, the distortion(s) that might normally be expected in view of the Jahn–Teller theorem need not necessarily apply. We find that the resulting very large unquenched orbital angular momentum leads to what we believe to be the largest observed internal magnetic field to date in a high-spin iron­(II) compound, specifically +162 T in <b>1</b>. The latter field is strongly polarized along the directions of the external field for both longitudinal and transverse field applications. For the longitudinal case, the applied field increases the overall hyperfine splitting consistent with a dominant orbital contribution to the effective internal field. By contrast, <b>2</b> has an internal field that is not as strongly polarized along a longitudinally applied field and is smaller in magnitude at ca. 116 T. Complex <b>3</b> behaves similarly to complex <b>1</b>. They are sufficiently self-dilute (e.g., Fe···Fe distances of ca. 9–10 Å) to exhibit varying degrees of slow paramagnetic relaxation in zero field for the neat solid form. In the absence of EPR signals for <b>1</b>–<b>3</b>, we show that heat-capacity measurements for one of the complexes (<b>3</b>) establish a <i>g</i>_eff value near 12, in agreement with the principal component of the ligand electric field gradient being coincident with the <i>z</i> axis