Highly Variable π-Bonding in the Interaction of Iron(II) Porphyrinates with Nitrite

Electronic Transition and Magnetic Coupling Regulation in Trimetallic Complexes Featuring a New Bridging Ligand Obtained by Oxidative Addition

A series of trimetallic complexes [Fe^III(μ-L)(py)]₂M^II(py)_n (n = 2, M^II = Mn^II, 1; Fe^II, 2; Co^II, 3; Zn^II, 4; n = 3, M^II = Cd^II, 5) with a new bridging ligand L^4- (deprotonated 1,2-N¹,N²-bis(2-mercaptoanil) oxalimidic acid) were synthesized and fully characterized by elemental analysis, single-crystal X-ray crystallography, IR, and Mössbauer spectra. Interestingly, the bridging ligand was obtained by oxidative addition of the (gma^•)^3- ligand from the mononuclear precursor Fe(gma)py (gma = glyoxal-bis(2-mercaptoanil)). In the obtained complexes, the bridging ligand L^4- coordinates to the terminal Fe^III ions (intermediate-spin with S_Fe = 3/2) by the N, S atoms, and coordinate to the central metal M^II ion by the four O atoms. The resonance structure of the bridging ligand can be described as the two 4π-electron delocalized systems connected by one single-bond (C₁-C₂), which is different from the electronic structure of the precursor Fe(gma)py. Remarkably, the magnetic coupling interaction can be regulated through the central metal. The ferromagnetic coupling constant J gradually decreases as M^II changes from Fe^II to Co^II and Mn^II, while the paramagnetic behaviors are presented when M^II = Zn^II and Cd^II, confirmed by the magnetic susceptibility measurements and further supported by using the PHI program. Furthermore, the bridging ligand to the terminal Fe^III charge transfer (LMCT) transitions emerged in all complexes but the central Fe^II to terminal Fe^III charge transfer (MMCT) only presented in complex 2, strongly supported by the UV/vis-NIR electronic spectra and TDDFT calculations.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Selective Convergence to Atropisomers of a Porphyrin Derivative Having Bulky Substituents at the Periphery

Four kinds of possible atropisomers of a porphyrin derivative (1), having mesityl groups at one of the o-positions of each meso-aryl group, can be selectively converged to targeted atropisomers among the four isomers (αααα, αααβ, αβαβ, and ααββ) under appropriate conditions for each atropisomer. For example, protonation and subsequent neutralization of a free base porphyrin (H₂-1) induces a convergence reaction to the αβαβ atropisomer, H₂-1-αβαβ, from an atropisomeric mixture. The αααα isomer, H₂-1-αααα, was also obtained by heating a solution of H₂-1 in CHCl₃ in 60% isolated yield, probably owing to a template effect of the solvent molecule. Remarkably, when an atropisomeric mixture of its zinc complex, Zn-1, was heated at 70 °C in a ClCH₂CH₂Cl/MeOH mixed solvent, crystals composed of only Zn-1-αααα were formed. The hydrophobic space formed by the four mesityl groups in the αααα isomer can be used for repeatable molecular encapsulation of benzene, and the encapsulation structure was elucidated by powder X-ray diffraction analysis. Heating the solid of an atropisomeric mixture of Zn-1 to 400 °C afforded the ααββ isomer almost quantitatively. On the other hand, the solid of H₂-1-αααα can be converted by heating, successively to H₂-1-αααβ at 286 °C and then to H₂-1-ααββ at 350 °C.

Computational analysis of non-heme iron-oxo formation by direct NO release in nitrite reduction

A direct NO-releasing reaction of nitrite catalyzed by [N(afa^Cy)₃Fe(OTf)]⁺ (afa (azafulvene-amine); OTf (trifluoromethanesulfonate); Cy (cyclohexyl)) was investigated using density functional theory (DFT) with D3 dispersion correction. The complex featured a secondary coordination sphere that facilitated the formation of the iron-oxo product [N(afa^Cy)₃FeO]⁺ with three (Fe)OH-N hydrogen bonds. As a high-spin iron(ii), the O-binding initial intermediate Fe(O)-nitrito was thermodynamically favorable in the S = 2 state. The cleavage of the (Fe)O-NO bond was performed by a β-electron shift to produce Fe(iii)-O by electron rearrangement in the S = 5/2 state. The different electron configurations are responsible for the structural properties, the valence of iron in the complexes, and the pathways of the reactions. Moreover, the two different H-bonds, (Fe)OH-N and (Fe)O-HN (by O-protonation), in the product complexes played a role in determining the reaction channels by impacting the N-H bond rotation. Thus, an exothermic sequence of conversions Fe(ii) → Fe(iii)-O → Fe(iii)-OH → Fe(iii)-O was established for the targeted product formation. This process provided a clue to build two key intermediates, iron-oxo and iron-hydroxo, in a variety of biological and synthetic systems. The results of this study are in agreement with experimental observations and describe the roles of H-bonding in nitrite reduction catalyzed by the non-heme iron complex.

Fe(TpivPP)(NO2)2]−: What is the nitrite orientation?

We have prepared a new crystalline derivative of [Fe(TpivPP)(NO2)2][Formula: see text] with the [K(222)][Formula: see text] cation. The newly determined structure of the anion resolves an ambiguity about the relative orientation of the two axial nitrite ligands which were found in a previous structure determination with a different cation. The two NO[Formula: see text] ligands are found to have a relative perpendicular orientation with Fe[Formula: see text]–N(NO2) distances of 1.974 (6) Åand 1.994 (6) Å. The average Fe[Formula: see text]–N[Formula: see text] distance in the low-spin six-coordinate iron(III) porphyrinate derivative is 1.987 (3) Å.

A mechanistic study of nitrite reduction on iron(ii) complexes of methylated N-confused porphyrins

Proton delivery to the prosthetic group is a crucial step to sustain the activity of nitrite reductase. An iron N-confused porphyrin (NCP) complex, which is capable of relaying protons from the outer pyrrolic nitrogen (N_out-H) of the inverted pyrrole ring to the axial coordinated ligand, has been demonstrated to facilitate facile nitrite reduction. Time-dependent FTIR studies on the reaction between [Fe^II(HCTPPMe)Br] (1) and a nitrite anion revealed a two-step process involving conversion of the starting complex 1 to an {Fe(NO)}⁷ intermediate, [Fe(CTPPMe)(NO)] (5), before the detection of [Fe(CTPPCH₂)(NO)] (3), an {Fe(NO)}⁶ end product. Moreover, spectroscopic data confirm that N_out-H on the NCP core is indispensable to the proceeding of the nitrite reduction reaction. Mass spectra have detected the coordination of a nitrite to the iron center while DFT theoretical calculations suggest that subsequent intramolecular proton transfer to a nitro group to form [Fe(CTPPMe)(HNO₂)] (6a) evokes a homolytic N-OH bond fission on axial nitrous acid due to an enhanced π-back-bonding to produce an {Fe(NO)}⁷ intermediate and to release a hydroxyl radical. The subsequent oxidation of an {Fe(NO)}⁷ intermediate by the hydroxyl radical gave the final product, {Fe(NO)}⁶ [Fe(CTPPCH₂)(NO)] (3). This study illustrates a proton assisted small molecule activation on the iron N-confused porphyrin coordination sphere and provides complemental insights into the mechanism of enzymatic nitrite reduction reactions.

The functional role of the structure of the dioxo-isobacteriochlorin in the catalytic site of cytochrome cd1 for the reduction of nitrite

Cytochrome cd1 is a key enzyme in bacterial denitrification and catalyzes one-electron reduction of nitrite (NO2-) to nitric oxide (NO) at the heme d1 center under anaerobic conditions. The heme d1 has a unique dioxo-isobacteriochlorin structure and is present only in cytochrome cd1. To reveal the functional role of the unique heme d1 in the catalytic nitrite reduction, we studied effect of the porphyrin macrocycle on each reaction step of the catalytic cycle of cytochrome cd1 using synthetic model complexes. The complexes investigated are iron complexes of dioxo-octaethylisobacteriochlorin (1), mono-oxo-octaethylchlorin (2) and octaethylporphyrin (3). We show here that the reduction potential for the transition from the ferric state to the ferrous state and the binding constant for binding of NO2- to the ferrous complex increases with a trend of 3 < 2 < 1. However, the reactivity of the ferrous nitrite complex with protons increases in the reversed order, 1 < 2 < 3. We also show that the iron bound NO of the ferric NO complex is readily replaced by addition of 1 equiv. of p-nitrophenolate. These results indicate that the dioxo-isobacteriochlorin structure is superior to porphyrin and mono-oxo-chlorin structures in the first iron reduction step, the second nitrite binding step, and the NO dissociation step, but inferior in the third nitrite reduction step. These results suggest that the heme d1 has evolved as the catalytic site of cytochrome cd1 to catalyze the nitrite reduction at the highest possible redox potential while maintaining its catalytic activity.

Crystal structure of bis-(benzyl-amine-κN)[5,10,15,20-tetra-kis-(4-chloro-phen-yl)porphyrinato-κ(4) N]iron(II) n-hexane monosolvate

In the title compound, [Fe(II)(C44H24Cl4N4)(C6H5CH2NH2)2]·C6H14 or [Fe(II)(TPP-Cl)(BzNH2)2]·n-hexane [where TPP-Cl and BzNH2 are 5,10,15,20-tetra-kis-(4-chloro-phen-yl)porphyrinate and benzyl-amine ligands, respectively], the Fe(II) cation lies on an inversion centre and is octa-hedrally coordinated by the four pyrrole N atoms of the porphyrin ligand in the equatorial plane and by two amine N atoms of the benzyl-amine ligand in the axial sites. The crystal structure also contains one inversion-symmetric n-hexane solvent mol-ecule per complex mol-ecule. The average Fe-Npyrrole bond length [1.994 (3) Å] indicates a low-spin complex. The crystal packing is sustained by N-H⋯Cl and C-H⋯Cl hydrogen-bonding inter-actions and by C-H⋯π inter-molecular inter-actions, leading to a three-dimensional network structure.

Facile nitrite reduction in a non-heme iron system: formation of an iron(III)-oxo

Reaction of tetrabutylammonium nitrite with [N(afa(Cy))3Fe(OTf)](OTf) cleanly resulted in the formation of an iron(III)-oxo species, [N(afa(Cy))3Fe(O)](OTf), and NO(g). Formation of NO(g) as a byproduct was confirmed by reaction of the iron(II) starting material with half an equivalent of nitrite, resulting in a mixture of two products, the iron-oxo and an iron-NO species, [N(afa(Cy))3Fe(NO)](OTf)2. Formation of the latter was confirmed through independent synthesis. The results of this study provide insight into the role of hydrogen bonding in the mechanism of nitrite reduction and the binding mode of nitrite in biological heme systems.

Multiple, disparate redox pathways exhibited by a tris(pyrrolido)ethane iron complex

Iron(III) complexes of the tris(pyrrolide)ethane trianion have been synthesized by reaction of one- and two-electron oxidants with [(tpe)Fe(THF)][Li(THF)4] (tpe = tris(5-mesitylpyrrolyl)ethane). X-ray crystallography, (57)Fe Mössbauer, (1)H NMR and EPR spectroscopy, SQUID magnetometry, and density functional theory calculations were employed to rigorously establish the iron 3+ oxidation state. All oxidants employed are proposed to operate via an inner-sphere electron transfer mechanism. Dialkyl peroxides and dibenzyldisulfide served to oxidize iron by one electron, and group transfer of an aryl nitrene unit to the Fe(2+) starting material resulted in formation of Fe(3+) amido species following H-atom abstraction by a presumed nitrenoid intermediate. Single electron transfer to and from diphenyldiazoalkane was also observed to yield a diphenyldiazomethanyl radical anion antiferromagnetically coupled to the S = 5/2 Fe(3+). Isolation of Fe(3+) complexes of tpe, in comparison with previous results wherein the tpe ligand was the redox active moiety, presents an unusual juxtaposition of two noncommunicating redox reservoirs, each accessible via different reaction pathways (namely, inner- and outer-sphere electron transfer).

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.

Structural insights into ligand dynamics: correlated oxygen and picket motion in oxycobalt picket fence porphyrins

Two different oxygen-ligated cobalt porphyrins have been synthesized and the solid-state structures have been determined at several temperatures. The solid-state structures provide insight into the dynamics of Co-O(2) rotation and correlation with protecting group disorder. [Co(TpivPP)(1-EtIm)(O(2))] (TpivPP = picket fence porphyrin) is prepared by oxygenation of [Co(TpivPP)(1-EtIm)(2)] in benzene solution. The structure at room temperature has the oxygen ligand within the ligand binding pocket and disordered over four sites and the trans imidazole is disordered over two sites. The structure at 100 K, after the crystal has been carefully annealed to yield a reversible phase change, is almost completely ordered. The phase change is reversed upon warming the crystal to 200 K, whereupon the oxygen ligand is again disordered but with quite unequal populations. Further warming to 300 K leads to greater disorder of the oxygen ligands with nearly equal O(2) occupancies at all four positions. The disorder of the tert-butyl groups of the protecting pickets is correlated with rotation of the O(2) around the Co-O(O(2)) bond. [Co(TpivPP)(2-MeHIm)(O(2))] is synthesized by a solid-state oxygenation reaction from the five-coordinate precursor [Co(TpivPP)(2-MeHIm)]. Exposure to 1 atm of O(2) leads to incomplete oxygenation, however, exposure at 5 atm yields complete oxygenation. Complete oxygenation leads to picket disorder whereas partial (40%) oxygenation does not. Crystallinity is retained on complete degassing of oxygen in the solid, and complete ordering of the pickets is restored. The results should provide basic information needed to better model M-O(2) dynamics in protein environments.

Influence of thiolate ligands on reductive N-O bond activation. Probing the O2(-) binding site of a biomimetic superoxide reductase analogue and examining the proton-dependent reduction of nitrite

Nitric oxide (NO) is frequently used to probe the substrate-binding site of "spectroscopically silent" non-heme Fe(2+) sites of metalloenzymes, such as superoxide reductase (SOR). Herein we use NO to probe the superoxide binding site of our thiolate-ligated biomimetic SOR model [Fe(II)(S(Me(2))N(4)(tren))](+) (1). Like NO-bound trans-cysteinate-ligated SOR (SOR-NO), the rhombic S = 3/2 EPR signal of NO-bound cis-thiolate-ligated [Fe(S(Me(2))N(4)(tren)(NO)](+) (2; g = 4.44, 3.54, 1.97), the isotopically sensitive ν(NO)(ν((15)NO)) stretching frequency (1685(1640) cm(-1)), and the 0.05 Å decrease in Fe-S bond length are shown to be consistent with the oxidative addition of NO to Fe(II) to afford an Fe(III)-NO(-) {FeNO}(7) species containing high-spin (S = 5/2) Fe(III) antiferromagnetically coupled to NO(-) (S = 1). The cis versus trans positioning of the thiolate does not appear to influence these properties. Although it has yet to be crystallographically characterized, SOR-NO is presumed to possess a bent Fe-NO similar to that of 2 (Fe-N-O = 151.7(4)°). The N-O bond is shown to be more activated in 2 relative to N- and O-ligated {FeNO}(7) complexes, and this is attributed to the electron-donating properties of the thiolate ligand. Hydrogen-bonding to the cysteinate sulfur attenuates N-O bond activation in SOR, as shown by its higher ν(NO) frequency (1721 cm(-1)). In contrast, the ν(O-O) frequency of the SOR peroxo intermediate and its analogues is not affected by H-bonds to the cysteinate sulfur or other factors influencing the Fe-SR bond strength; these only influence the ν(Fe-O) frequency. Reactions between 1 and NO(2)(-) are shown to result in the proton-dependent heterolytic cleavage of an N-O bond. The mechanism of this reaction is proposed to involve both Fe(II)-NO(2)(-) and {FeNO}(6) intermediates similar to those implicated in the mechanism of NiR-promoted NO(2)(-) reduction.

(Cryptand-222)potassium(+) (hydrogensulfido)[5,10,15,20-tetra-kis(2-pival-amido-phen-yl)porphyrinato]ferrate(II)

As part of a systematic investigation for a number of Fe(II) porphyrin complexes used as biomimetic models for cytochrome P450, crystals of the title compound, [K(C(18)H(36)N(2)O(6))][Fe(II)(C(64)H(64)N(8)O(4))(HS)], were prepared. The compound exhibits a non-planar conformation with major ruffling and saddling distortions. The average equatorial iron-pyrrole N atom [Fe-N(p) = 2.102 (2) Å] bond length and the distance between the Fe(II) atom and the 24-atom core of the porphyrin ring (Fe-P(C)= 0.558 Å) are typical for high-spin iron(II) penta-coordinate porphyrinates. One of the tert-butyl groups in the structure is disordered over two sets with occupancies of 0.84 and 0.16.

Hydrogen bonding influence of 1,10-phenanthroline on five-coordinate high-spin imidazole-ligated iron(II) porphyrinates

The influence of a hydrogen bond to the coordinated imidazole on the geometric and electronic structure of iron has been further studied in new complexes of five-coordinate high-spin imidazole-ligated iron(II) porphyrinates. With 1,10-phenanthroline (1,10-phen) as the hydrogen-bond acceptor, several new octaethylporphyrin dianion (OEP) and meso-tetraphenylporphyrin dianion (TPP) derivatives have been synthesized and characterized by X-ray crystallography and Mössbauer spectroscopy. In all three new structures, the porphyrin molecules and 1,10-phenanthroline molecules have been found with a ratio of 1:1. All the porphyrin derivatives are five-coordinate 2-methylimidazole-ligated iron(II) species. 1,10-Phenanthroline is hydrogen bonded to the coordinated imidazole to form two unequal hydrogen bonds. The Fe-N p and Fe-N Im bond lengths and displacement of the iron atom out of the porphyrin plane are similar to those in imidazole-ligated species. Mössbauer measurements showed remarkable temperature dependence; the analysis of the data obtained in applied magnetic field for [Fe(OEP)(2-MeHIm)].(1,10-phen) gave a negative quadrupole splitting value and large asymmetry parameters. All the structural and Mössbauer properties suggest that these new hydrogen-bonded species have the same electronic configuration as imidazole-ligated species.

Sulfoxide as a ligand in iron(II) porphyrinates: S- or O-bound?

In order to study the bonding of sulfoxides to iron(II) porphyrinates, an equilibrium study of Fe(TPP) with tetramethylenesulfoxide (TMSO) has been performed. UV-vis spectra at different concentrations of TMSO have shown distinct character belonging to three species: four-coordinate Fe(II)(TPP), five-coordinate [Fe(II)(TPP)(TMSO)], and six-coordinate [Fe(II)(TPP)(TMSO)2]. The isosbestic points for the low TMSO concentrations suggest that the equilibrium constant K1 is much larger than K2. Analysis of spectral data by the nonlinear least-squares program SQUAD gives K1 = 267 and K2 approximately 1. Even though the five-coordinate species is the dominant species under the synthetic conditions, only the six-coordinate species was crystallized and characterized by an X-ray diffraction study. [Fe(TPP)(TMSO)2] (C52H44Fe-N4O2S2): monoclinic, P2(1)/c, a = 11.2580(3) A, b = 15.9262(5) A, c = 12.3930(4) A, beta = 116.246(1) degrees , V = 1992.95(10) A3, Z = 2. X-ray crystallography demonstrates the complex is a low-spin bis-TMSO ligated species. The average Fe-Np distance is 1.999(4) A. The most important feature is that TMSO is coordinated to iron(II) by the sulfur donors, not oxygen. The Fe-S distance is 2.2220(3) A.

Molecular and electronic structure of five-coordinate complexes of iron(II/III) containing o-diiminobenzosemiquinonate(1-) pi radical ligands

The reaction of the ligand N-phenyl-1,2-benzenediamine (N-phenyl-o-phenylenediamine), H2[L(PDI)], in dry acetonitrile with [FeIII(dmf)6](ClO4)3 (dmf = N,N-dimethylformamide) affords the dimer (mu-NH,NH)[FeIII(L(ISQ))(L(PDI))]2 (1), where (L(ISQ))*- represents the pi radical monoanion N-phenyl-o-diiminobenzosemiquinonate and (L(PDI))2- is its one-electron-reduced, closed-shell form. Complex 1 possesses a diamagnetic ground-state St = 0. Addition reactions of tri-n-butylphosphane, tert-butyl isocyanide, cyclohexyl isocyanide, 4,5-diphenylimidazole, and 4-(1-phenylpentyl)pyridine with 1 in acetonitrile or toluene yields [FeII(L(ISQ))2(PBu3)] (2), [Fe(II)(L(ISQ))2(CN-tBu)] (4), [FeII(L(ISQ))2(CNCy)] (5), [FeIII(L(ISQ))2(Ph2Im)] (6), and [FeIII(L(ISQ))(L(PDI))(BuPhCH-py)].BuPhCH-py (7). Oxidation of 1 with iodine affords [FeIII(L(ISQ))2I] (3), and oxidation of 2 with ferrocenium hexafluorophosphate yields [FeIII(L(ISQ))2(PBu3)](PF6) (2ox). The structures of complexes 2, 2ox, 3, 5, 6, and 7 have been determined by X-ray crystallography at 100(2) K. Magnetic susceptibility measurements and EPR, UV-vis, and Mössbauer spectroscopy have established that mononuclear complexes containing the [FeII(L(ISQ))2X] chromophore (2, 4, 5) are diamagnetic (St = 0) whereas those with an [FeIII(L(ISQ))2X]n chromophore (3, 2(ox), 6) are paramagnetic (St = 1/2) and those with an [FeIII(L(ISQ))(L(PDI))X] chromophore (7) possess an St = 1 ground state. It is established that all ferric species have an intrinsic intermediate spin (SFe = 3/2) which is intramolecularly antiferromagnetically coupled to one or two (L(ISQ))*- ligand radicals yielding an St = 1 (7) or St = 1/2 (2ox, 3, 6) ground state, respectively. In the ferrous complexes 2, 4, and 5 the intrinsic spin at the iron ion is either low spin (SFe = 0) or intermediate spin (SFe = 1). Antiferromagnetic coupling between two radicals (L(ISQ))*- or, alternatively, between the intermediate spin ferrous ion and two radicals yields then the observed diamagnetic ground state. In 1 two [FeIII(L(ISQ))(L(PDI))] halves with S = 1 couple antiferromagnetically affording an St = 0 ground state.

Electronic, magnetic, and structural characterization of the five-coordinate, high-spin iron(II) nitrato complex [Fe(TpivPP)(NO3)]-

The preparation and characterization of the five-coordinate iron(II) porphyrinate derivative [Fe(TpivPP)(NO3)]- (TpivPP = picket-fence porphyrin) is described. Structural and magnetic susceptibility data support a high-spin state (S = 2) assignment for this species. The anionic axial nitrate ligand is O-bound, through a single O atom, with an Fe-O bond length of 2.069(4) A. The planar nitrate ligand bisects a N(p)-Fe-N(p) angle. The average Fe-N(p) bond length is 2.070(16) A. The Fe atom is located 0.49 A out of the 24-atom mean porphyrin plane toward the nitrate ligand. From solid-state Mössbauer data, the isomer shift of 0.98 mm/s at 77 K is entirely consistent with high-spin iron(II). However the quadrupole splitting of 3.59 mm/s at 77 K is unusually high for iron(II), S = 2 systems but within the range of other five-coordinate high-spin ferrous complexes with a single anionic axial ligand. Crystal data for [K(222)][Fe(TpivPP)(NO3)] x C6H5Cl: a = 17.888 (5) A, b = 21.500 (10) A, c = 22.514 (11) A, beta = 100.32 (3) degrees, monoclinic, space group P2(1)/n, V = 8519 A3, Z = 4.

Iron(III)−Nitro Porphyrins: Theoretical Exploration of a Unique Class of Reactive Molecules

DFT(PW91/TZP) calculations, including full geometry optimizations, have been carried on [FeII(P)(NO2)]-, Fe(III)(P)(NO2), [Fe(II)(P)(NO2)(py)]-, Fe(III)(P)(NO2)(py), [Fe(III)(P)(NO2)2]-, and Fe(III)(P)(NO2)(NO), where P is the unsubstituted porphine dianion, as well as on certain picket fence porphyrin (TPivPP) analogues. The bonding in [Fe(II)(P)(NO2)]- and Fe(III)(P)(NO2), as well as in their pyridine adducts, reveals a sigma-donor interaction of the nitrite HOMO and the Fe dz2 orbital, where the Fe-Nnitro axis is defined as the z direction and the nitrite plane is identified as xz. Both molecules also feature a pi-acceptor interaction of the nitrite LUMO and the Fe dyz orbital, whereas the SOMO of the Fe(III)-nitro complexes may be identified as dxz. The Fe(III)-nitro porphyrins studied all exhibit extremely high adiabatic electron affinities, ranging from about 2.5 eV for Fe(III)(P)(NO2) and Fe(III)(P)(NO2)(py) to about 3.4 eV for their TPivPP analogues. Transition-state optimizations for oxygen-atom transfer from Fe(III)(P)(NO2) and Fe(III)(P)(NO2)(py) to dimethyl sulfide yielded activation energies of 0.45 and 0.77 eV, respectively, which is qualitatively consistent with the observed far greater stability of Fe(III)(TPivPP)(NO2)(py) relative to Fe(III)(TPivPP)(NO2). Addition of NO to yield {FeNO}6 nitro-nitrosyl adducts such as Fe(P)(NO2)(NO) provides another mechanism whereby Fe(III)-nitro porphyrins can relieve their extreme electron affinities. In Fe(P)(NO2)(NO), the bonding involves substantial Fe-NO pi-bonding, but the nitrite acts essentially as a simple sigma-donor, which accounts for the relatively long Fe-Nnitro distance in this molecule.

Experimental and Density Functional Theoretical Investigations of Linkage Isomerism in Six-Coordinate {FeNO}6 Iron Porphyrins with Axial Nitrosyl and Nitro Ligands

A critical component of the biological activity of NO and nitrite involves their coordination to the iron center in heme proteins. Irradiation (330 < lambda < 500 nm) of the nitrosyl-nitro compound (TPP)Fe(NO)(NO(2)) (TPP = tetraphenylporphyrinato dianion) at 11 K results in changes in the IR spectrum associated with both nitro-to-nitrito and nitrosyl-to-isonitrosyl linkage isomerism. Only the nitro-to-nitrito linkage isomer is obtained at 200 K, indicating that the isonitrosyl linkage isomer is less stable than the nitrito linkage isomer. DFT calculations reveal two ground-state conformations of (porphine)Fe(NO)(NO(2)) that differ in the relative axial ligand orientations (i.e., GS parallel and GS perpendicular). In both conformations, the FeNO group is bent (156.4 degrees for GS parallel, 159.8 degrees for GS perpendicular) for this formally {FeNO}(6) compound. Three conformations of the nitrosyl-nitrito isomer (porphine)Fe(NO)(ONO) (MSa parallel, MSa perpendicular, and MSa(L)) and two conformations of the isonitrosyl-nitro isomer (porphine)Fe(ON)(NO(2)) (MSb parallel and MSb perpendicular) are identified, as are three conformations of the double-linkage isomer (porphine)Fe(ON)(ONO) (MSc parallel, MSc perpendicular, MSc(L)). Only 2 of the 10 optimized geometries contain near-linear FeNO (MSa(L)) and FeON (MSc(L)) bonds. The energies of the ground-state and isomeric structures increase in the order GS < MSa < MSb < MSc. Vibrational frequencies for all of the linkage isomers have been calculated, and the theoretical gas-phase absorption spectrum of (porphine)Fe(NO)(NO(2)) has been analyzed to obtain information on the electronic transitions responsible for the linkage isomerization. Comparison of the experimental and theoretical IR spectra does not provide evidence for the existence of a double linkage isomer of (TPP)Fe(NO)(NO(2)).

Variation of oxo-transfer reactivity of (nitro)cobalt picket fence porphyrin with oxygen-donating ligands

Derivatives of (nitro)cobalt picket fence porphyrin with oxygen-donating ligands have been prepared in solution and in the solid state. Crystal structures of two of these derivatives, (H2O)CoTpivPP(NO2) and (CH3OH)CoTpivPP(NO2), have been determined. The ethanol complex (C2H5OH)Co(TPP)(NO2) has been obtained and spectrally characterized using sublimed layers methodology. The formation constant and the DeltaH degrees value of the association reaction with ethanol have been determined by FTIR measurements in CCl4 solution. Catalytic oxygen activation and oxo-transfer reactions of these derivatives have been assessed in solution. Correlations between the oxo-transfer reactivity, thermodynamics, and characteristics of the nitro ligand show that although calculated and observed ONO vibrational spectra and bond lengths suggest activation of the NO2 ligand and enhanced oxo-transfer reactions as seen in the analogous five-coordinate complexes, density functional theory calculations support that thermodynamics limits oxo-atom transfer reactions in these six-coordinate systems.

Electrochemical Generation of Free Nitric Oxide from Nitrite Catalyzed by Iron meso-Tetrakis(4-N-methylpyridiniumyl)porphyrin

Electrochemical generation of free nitric oxide (NO) from nitrite (NO(2)(-)) catalyzed by iron meso-tetrakis(4-N-methylpyridiniumyl)porphyrin, [Fe(III)(TMPyP)](5+), has been developed in this study. To obtain free NO, a cathodic electrolysis and an anodic electrolysis were performed in two connected flow electrolytic cells in sequence. The flow electrolytic cell upstream was used for cathodic electrolysis, where the solution of [Fe(III)(TMPyP)](5+) and NO(2)(-) was reduced at -0.25 V (vs Ag/AgCl) into [Fe(II)(NO(2)(-))(2)(TMPyP)](2+) and [Fe(II)(NO)(TMPyP)](4+) in sequence. The flow electrolytic cell downstream was utilized for anodic electrolysis, where [Fe(II)(NO)(TMPyP)](4+) formed from the upstream cell was oxidized at +0.40 V (vs Ag/AgCl) into [Fe(III)(TMPyP)](5+) and free NO. Finally, NO was bubbled out from anodic electrolyte by argon gas. The mechanism and the optimum conditions for electrochemical generation of NO from NO(2)(-) catalyzed by [Fe(III)(TMPyP)](5+) were studied in detail by voltammetric and spectroelectrochemical methods.

Variable pi-bonding in iron(II) porphyrinates with nitrite, CO, and tert-butyl isocyanide: characterization of [Fe(TpivPP)(NO2)(CO)]-

The addition of the strongly pi-bonding ligands CO or tert-butyl isocyanide to the low-spin five-coordinate iron(II) nitrite species [Fe(TpivPP)(NO2)]- (TpivPP = picket fence porphyrin) gives two new six-coordinate species [Fe(TpivPP)(NO2)(CO)]- and [Fe(TpivPP)(NO2)(t-BuNC)]-. These species have been characterized by single-crystal structure determinations and by UV-vis, IR, and Mössbauer spectroscopies. All evidence shows that in the mixed-ligand iron(II) porphyrin species, [Fe(TpivPP)(NO2)(CO)]-, the two trans, pi-accepting ligands CO and nitrite compete for pi density. The CO ligand however dominates the bonding. The Fe-N(NO2) bond lengths for the two independent anions in the unit cell at 2.006(4) and 2.009(4) A are lengthened compared to other nitrite species with either no trans ligands or non-pi-accepting trans ligands to nitrite. The Fe-C(CO) bond lengths are 1.782(4) A and 1.789(5) A for the two anions. The two Fe-C-O angles at 175.5(4) and 177.5(4) degrees are essentially linear in both anions. The quadrupole splitting for [Fe(TpivPP)(NO2)(CO)]- was determined to be 0.32 mm/s, and the isomer shift was 0.18 mm/s at room temperature in zero applied field. Both of the Mössbauer parameters are much smaller than those found for six-coordinate low-spin iron(II) porphyrinates with neutral nitrogen-donating ligands as well as iron(II) nitro complexes. However, the Mössbauer parameters are typical of other six-coordinate CO porphyrinates signifying that CO is the more dominant ligand. The CO stretching frequency of 1974 cm(-1) is shifted only slightly to higher energy compared to six-coordinate CO complexes with neutral nitrogen-donor ligands trans to CO. Crystal data for [K(222)][Fe(TpivPP)(NO2)(CO)].1/2C6H5Cl: monoclinic, space group P2(1)/c, Z = 8, a = 33.548(6) A, b = 18.8172(15) A, c = 27.187(2) A, beta = 95.240(7) degrees, V = 17091(4) A3.

Effect of the sixth axial ligand in CS-ligated iron(II)octaethylporphyrinates: structural and Mössbauer studies

The effect of a sixth ligand in a series of low-spin thiocarbonyl-ligated iron(II)octaethylporphyrinates has been investigated. Six-coordinate complexes have been synthesized and characterized by Mössbauer and infrared spectroscopy and single-crystal X-ray structure determinations. The results are compared with the five-coordinate parent complex. The crystal structures of [Fe(OEP)(CS)(1-MeIm)] and [Fe(OEP)(CS)(Py)] are reported and discussed. The 1-methylimidazole and pyridine derivatives exhibit Fe-C(CS) bond distances of 1.703(4) and 1.706(2) A that are significantly longer than the 1.662(3) A reported for five-coordinate [Fe(OEP)(CS)] (Scheidt, W. R.; Geiger, D. K. Inorg. Chem. 1982, 21, 1208). The trans Fe-N(ligand) distances of 2.112(3) and 2.1550(15) A observed for the 1-methylimidazole and pyridine complex are approximately 0.13 A longer than those observed for analogous bis-ligated complexes and are consistent with a significant structural trans effect for the CS ligand. Mössbauer investigations carried out for five- and six-coordinate thiocarbonyl derivatives with several different sixth axial ligands reveal interesting features. All derivatives exhibit very small isomer shift values, consistent with a very strong interaction between iron and CS. The five-coordinate derivative has delta(Fe) = 0.08 mm/s, and the six-coordinate complexes exhibit delta(Fe) = 0.14 to 0.19 mm/s at 4.2 K. The five-coordinate complex shows a large quadrupole splitting (DeltaE(q) = 1.93 mm/s at 4.2 K) which is reduced on coordination of the sixth ligand (DeltaE(q) = 0.42-0.80 mm/s at 4.2 K). Addition of a sixth ligand also leads to a small decrease in the value of nu(CS). Correlations in structural, IR, and Mössbauer results suggest that the sixth ligand effect is primarily induced by changes in sigma-bonding. The structure of [Fe(OEP)(CS)(CH(3)OH)] is briefly reported. Crystal data: [Fe(OEP)(CS)(1-MeIm)] crystallizes in the monoclinic system, space group P2(1)/n, Z = 4, a = 9.5906(5) A, b = 16.704(4) A, c = 23.1417(6) A, beta = 100.453(7) degrees. [Fe(OEP)(CS)(Py)] crystallizes in the triclinic system, space group P1, Z = 5, a = 13.9073(6) A, b = 16.2624(7) A, c = 22.0709(9) A, alpha = 70.586(1) degrees, beta = 77.242(1) degrees, gamma = 77.959(1) degrees. [Fe(OEP)(CS)(CH(3)OH)] crystallizes in the triclinic system, space group P1, Z = 1, a = 9.0599(5) A, b = 9.4389(5) A, c = 11.0676(6) A, alpha = 90.261(1) degrees, beta = 100.362(1) degrees, gamma = 114.664(1) degrees.

Mechanism of the six-electron reduction of nitrite to ammonia by cytochrome c nitrite reductase

Cytochrome c nitrite reductase catalyzes the six-electron reduction of nitrite to ammonia without the release of potential reaction intermediates, such as NO or hydroxylamine. On the basis of the crystallographic observation of reaction intermediates and of density functional calculations, we present a working hypothesis for the reaction mechanism of this multiheme enzyme which carries a novel lysine-coordinated heme group (Fe-Lys). It is proposed that nitrite reduction starts with a heterolytic cleavage of the N-O bond which is facilitated by a pronounced back-bonding interaction of nitrite coordinated through nitrogen to the reduced (Fe(II)) but not the oxidized (Fe(III)) active site iron. This step leads to the formation of an [FeNO](6) species and a water molecule and is further facilitated by a hydrogen bonding network that induces an electronic asymmetry in the nitrite molecule that weakens one N-O bond and strengthens the other. Subsequently, two rapid one-electron reductions lead to an [FeNO](8) form and, by protonation, to an Fe(II)-HNO adduct. Hereafter, hydroxylamine will be formed by a consecutive two-electron two-proton step which is dehydrated in the final two-electron reduction step to give ammonia and an additional water molecule. A single electron reduction of the active site closes the catalytic cycle.