A Triangular Iron(III) Complex Potentially Relevant to Iron(III)-Binding Sites in Ferreascidin

Experimental and DFT Study of Monensinate and Salinomycinate Complexes Containing {Fe3(µ3-O)}7+ Core

Two trinuclear oxo-centred iron(III) coordination compounds of monensic and salinomycinic acids (HL) were synthesized and their spectral properties were studied using physicochemical/thermal methods (FT-IR, TG-DTA, TG-MS, EPR, Mössbauer spectroscopy, powder XRD) and elemental analysis. The data suggested the formation of [Fe3(µ3-O)L3(OH)4] and the probable complex structures were modelled using the DFT method. The computed spectral parameters of the optimized constructs were compared to the experimentally measured ones. In each complex, three metal centres were joined together at the axial position by a μ3-O unit to form a {Fe3O}7+ core. The antibiotics monoanions served as bidentate ligands through the carboxylate and hydroxyl groups located at the termini. The carboxylate moieties played a dual role bridging each two metal centres. Hydroxide anions secured the overall neutral character of the coordination species. Mössbauer spectra displayed asymmetric quadrupole doublets that were consistent with the existence of two types of high-spin iron(III) sites with different environments-two Fe[O5] and one Fe[O6] centres. The solid-state EPR studies confirmed the +3 oxidation state of iron with a total spin St = 5/2 per trinuclear cluster. The studied complexes are the first iron(III) coordination compounds of monensin and salinomycin reported so far.

A Trinuclear High-Spin Iron(III) Complex with a Geometrically Frustrated Spin Ground State Featuring Negligible Magnetic Anisotropy and Antisymmetric Exchange

The trinuclear high-spin iron(III) complex [Fe₃Cl₃(saltag^Br)(py)₆]ClO₄ {H₅saltag^Br = 1,2,3-tris[(5-bromo-salicylidene)amino]guanidine} was synthesized and characterized by several experimental and theoretical methods. The iron(III) complex exhibits molecular 3-fold symmetry imposed by the rigid ligand backbone and crystallizes in trigonal space group P3̅ with the complex cation lying on a crystallographic C₃ axis. The high-spin states (S = ⁵/₂) of the individual iron(III) ions were determined by Mößbauer spectroscopy and confirmed by CASSCF/CASPT2 ab initio calculations. Magnetic measurements show an antiferromagnetic exchange between the iron(III) ions leading to a geometrically spin-frustrated ground state. This was complemented by high-field magnetization experiments up to 60 T, which confirm the isotropic nature of the magnetic exchange and negligible single-ion anisotropy for the iron(III) ions. Muon-spin relaxation experiments were performed and further prove the isotropic nature of the coupled spin ground state and the presence of isolated paramagnetic molecular systems with negligible intermolecular interactions down to 20 mK. Broken-symmetry density functional theory calculations are consistent with the antiferromagnetic exchange between the iron(III) ions within the presented trinuclear high-spin iron(III) complex. Ab initio calculations further support the absence of appreciable magnetic anisotropy (D = 0.086, and E = 0.010 cm^-1) and the absence of significant contributions from antisymmetric exchange, as the two Kramers doublets are virtually degenerate (ΔE = 0.005 cm^-1). Therefore, this trinuclear high-spin iron(III) complex should be an ideal candidate for further investigations of spin-electric effects arising exclusively from the spin chirality of a geometrically frustrated S = ¹/₂ spin ground state of the molecular system.

Synthesis and structural, magnetic and spectroscopic characterization of iron(III) complexes with in situ formed ligands from methyl-2-pyridyl ketone transformations

Reactions of methyl-2-pyridyl ketone, pyCOMe, with FeCl₃·6H₂O in various solvents gave complexes [Fe₄Cl₆(OMe)₂(L1)₂]·0.7MeCN·0.4MeOH (1·0.7MeCN·0.4MeOH) and [Fe₃Cl₄(bicine)(L2)]·Me₂CO·0.2H₂O (2·Me₂CO·0.2H₂O). The ligands (L1)^2- = pyCO(Me)CHCOpy (in 1) and (L2)^2- = pyCO(Me)CH₂CO(OMe)py (in 2) are formed in situ, through an aldol reaction-type mechanism between the carbanion pyC(O)CH₂^- (formed by the nucleophilic attack of the MeO^- in pyCOMe) and pyCOMe which results in the formation of a new C-C bond. The intermediate compound undergoes attack in the -CH₂- or -CO- group by a MeO^- group, and the new ligands (L1)^2- and (L2)^2-, respectively, are formed. The molecular structure of 1 consists of three corner-sharing [Fe₂O₂] rhombic units in cis-arrangement. The two terminal Fe^III ions display distorted square pyramidal geometry and the two central Fe^III ions are distorted octahedral. The molecular structure of 2 consists of two corner-sharing [Fe₂O₂] rhombic units, with the two terminal Fe^III ions in distorted square pyramidal geometry and the central Fe^III in distorted octahedral. The differentiation in the coordination environment of the Fe^III ions in 1-2 is reflected in the values of the Mössbauer hyperfine parameters. In agreement with theoretical calculations, the square pyramidal sites exhibit a smaller isomer shift value in comparison to the octahedral sites. Magnetic studies indicate antiferromagnetic interactions leading to an S = 0 ground state in 1 and to an S = 5/2 ground state in 2, consistent with Electron Paramagnetic Resonance spectroscopy. Mössbauer spectra of 2 indicate the onset of relaxation effects below 80 K. At 1.5 K the spectrum of 2 consists of magnetic sextets. The determined hyperfine magnetic fields are consistent with the exchange coupling scheme imposed by the crystal structure of 2. Theoretical calculations shed light on the differences in the electronic structure between the square pyramidal and the octahedral sites.

Synthesis and Characterization of Symmetrically versus Unsymmetrically Proton-Bridged Hexa-Iron Clusters

Syntheses and magnetic and structural characterization of hexa-iron complexes of derivatized salicylaldoximes are discussed. Complexation of Fe(BF₄)₂·6H₂O with each ligand (H₂ L1 and H₄ L2) in a methanolic-pyridine solution resulted in hexa-iron compounds (C1 and C2, respectively), which each contain two near-parallel metal triangles of [Fe₃-μ₃-O], linked by six fluoride bridges and stabilized by a hydrogen-bonded proton between the μ₃-O groups. Within each metal triangle of C2, Fe(III) ions are connected via the amine "straps" of (H₄ L2-2H). Variable-temperature magnetic susceptibility and Mössbauer data of C1 and C2 indicate the presence of dominant antiferromagnetic interactions between the high-spin (S = 5/2) Fe(III) centers. For C1, two quadrupole doublets are observed at room temperature and 5 K, consistent with structural data from which discrete but disordered [Fe₃-μ₃-O] and [Fe₃-μ₃-OH] species were inferred. For C2, a single sharp quadrupole doublet with splitting intermediate between those determined for C1 was observed, consistent with the symmetric [Fe₃-μ₃-O···H···μ₃-O-Fe₃] species inferred crystallographically from the very short μ₃-O···μ₃-O separation. The differences in the physical properties of the complexes, as seen in the Mössbauer, X-ray, and magnetic data, are attributed to the conformational flexibility imparted by the nature of the linkages between the closely related ligands.

Half-Integer Spin Triangles: Old Dogs, New Tricks

Spin triangles, that is, triangular complexes of half-integer spins, are the oldest molecular nanomagnets (MNMs). Their magnetic properties have been studied long before molecular magnetism was delineated as a research field. This Review presents the history of their study, with references to the parallel development of new experimental investigations and new theoretical ideas used for their interpretation. It then presents an indicative list of spin-triangle families to illustrate their chemical diversity. Finally, it makes reference to recent developments in terms of theoretical ideas and new phenomena, as well as to the relevance of spin triangles to spintronic devices and new physics.

Engineered Fe3 triangle for the rapid and selective removal of aromatic cationic pollutants: complexity is not a necessity

In this study, a low-cost oxo-bridged {Fe3} triangular cluster was constructed based on a benzoate ligand via slow evaporation. The cluster was thoroughly characterized by FTIR and UV-visible spectroscopy, TGA, and PXRD, and the exact structure was elucidated by single-crystal XRD. The formation of C-H⋯π and π-π interactions is responsible for the extra stability of {Fe3} clusters, which further enhances the dye adsorption property. The dye adsorption experiments performed on cationic [methylene blue (MB) and rhodamine-B (Rh-B)] as well as anionic [methyl orange (MO) and congo red (CR)] dyes revealed the ultimate selectivity of the present cluster towards the cationic ones. The {Fe3} cluster exclusively adsorbs the cationic dyes, i.e., MB and Rh-B even in the presence of anionic dyes, i.e., CR and MO. The extra stability, reusability and high efficiency of the {Fe3} molecular ensemble make it an attractive and fascinating material of importance. The kinetics analysis was evaluated employing different kinetics models. Furthermore, the plausible adsorption mechanism was also proposed, which suggests the interplay of cation-π and π-π interactions consolidating the efficient adsorption. Thus, the present work opens new doors for coordination chemists to further tune the structural features to modulate the adsorption/separation capacities of simple low-cost clusters for environmental protection for future efforts.

Selective Formation, Reactivity, Redox and Magnetic Properties of MnIII and FeIII Dinuclear Complexes with Shortened Salen-Type Schiff Base Ligands

The reactivity of the shortened salen-type ligands H₃salmp, H₂salmen and H₂sal(p-X)ben with variable para-substituent on the central aromatic ring (X = tBu, Me, H, F, Cl, CF₃, NO₂) towards the trivalent metal ions manganese(III) and iron(III) is presented. The selective formation of the dinuclear complexes [M₂(μ-salmp)₂], M = Mn (1a), Fe (2a), [M₂(μ-salmen)₂(μ-OR)₂)], R = Et, Me, H and M = Mn (3a–c) or Fe (4a–c), and (M₂(μ-sal[p-X]ben)₂(μ-OMe)₂), X = tBu, Me, H, F, Cl, CF₃, NO₂ and M = Mn (5a–g) or Fe (6a–g), could be identified by reaction of the Schiff bases with metal salts and the base NEt₃, and their characterization through elemental analysis, infrared spectroscopy, mass spectrometry and single-crystal X-ray diffraction of 2a·2AcOEt, 2a·2CH₃CN and 3c·2DMF was performed. In the case of iron(III) and H₃salmp, when using NaOH as a base instead of NEt₃, the dinuclear complexes [Fe₂(μ-salmp)(μ-OR)(salim)₂], R = Me, H (2b,c) could be isolated and spectroscopically characterized, including the crystal structure of 2b·1.5H₂O, which showed that rupture of one salmp³⁻ to two coordinated salim⁻ ligands and release of one salH molecule occurred. The same hydrolytic tendency could be identified with sal(p-X)ben ligands in the case of iron(III) also by using NEt₃ or upon standing in solution, while manganese(III) did not promote such a C–N bond breakage. Cyclic voltammetry studies were performed for 3b, 4b, 5a and 6a, revealing that the iron(III) complexes can be irreversibly reduced to the mixed-valence Fe^IIFe^III and Fe^II₂ dinuclear species, while the manganese(III) derivatives can be reversibly oxidized to either the mixed-valence Mn^IIIMn^IV or to the Mn^IV₂ dinuclear species. The super-exchange interaction between the metal centers, mediated by the bridging ligands, resulted in being antiferromagnetic (AFM) for the selected dinuclear compounds 3b, 4b, 5a, 5e,5f, 6a and 6e. The coupling constants J (–2JŜ₁·Ŝ₂ formalism) had values around −13 cm⁻¹ for manganese(III) compounds, among the largest AFM coupling constants reported so far for dinuclear Mn^III₂ derivatives, while values between −3 and −10 cm⁻¹ were obtained for iron(III) compounds.

New salicylaldoximato-borate ligands resulting from anion hydrolysis and their respective copper and iron complexes

Anion hydrolysis reactions between salicylaldoximato ligands (L'-L''') and copper and iron BF₄^- metal salts, have resulted in the formation of new salicylaldoximato borate containing transition metal complexes: [Fe₂(L' + 2H)₂](BF₄)₂(MeOH)₄ (C1), [Fe₃(L'' + 4H)(OH)₂(Py)₂](BF₄)₂(H₂O)₂(Py)₂ (C2), and [Cu₂(L''' + H)₂Cl₂] (C3). Each of the complexes have been structurally characterised, revealing the indirect role boron plays in the formation of these complexes. For complexes C1 and C2, Mössbauer spectroscopy confirmed the existence of Fe(iii) oxidation states. SQUID magnetometry measurements were performed on complexes C2 and C3, revealing the presence of two competing exchange pathways between the three Fe(iii) centres in C2, with antiferromagnetic exchange dominating. For C3 weak antiferromagnetic exchange dominated between the two Cu(ii) centres.

Metalloproteins in the Biology of Heterocysts

Cyanobacteria are photoautotrophic microorganisms present in almost all ecologically niches on Earth. They exist as single-cell or filamentous forms and the latter often contain specialized cells for N₂ fixation known as heterocysts. Heterocysts arise from photosynthetic active vegetative cells by multiple morphological and physiological rearrangements including the absence of O₂ evolution and CO₂ fixation. The key function of this cell type is carried out by the metalloprotein complex known as nitrogenase. Additionally, many other important processes in heterocysts also depend on metalloproteins. This leads to a high metal demand exceeding the one of other bacteria in content and concentration during heterocyst development and in mature heterocysts. This review provides an overview on the current knowledge of the transition metals and metalloproteins required by heterocysts in heterocyst-forming cyanobacteria. It discusses the molecular, physiological, and physicochemical properties of metalloproteins involved in N₂ fixation, H₂ metabolism, electron transport chains, oxidative stress management, storage, energy metabolism, and metabolic networks in the diazotrophic filament. This provides a detailed and comprehensive picture on the heterocyst demands for Fe, Cu, Mo, Ni, Mn, V, and Zn as cofactors for metalloproteins and highlights the importance of such metalloproteins for the biology of cyanobacterial heterocysts.

Hexa- and octanuclear iron(III) salicylaldoxime clusters

The syntheses, structures and magnetic properties of six iron complexes stabilised with the derivatised salicylaldoxime ligands Me-saoH(2) (2-hydroxyethanone oxime) and Et-saoH(2) (2-hydroxypropiophenone oxime) are discussed. The four hexanuclear and two octanuclear complexes of formulae [Fe(8)O(2)(OMe)(4)(Me-sao)(6)Br(4)(py)(4)]·2Et(2)O·MeOH (1·2Et(2)O·MeOH), [Fe(8)O(2)(OMe)(3.85)(N(3))(4.15)(Me-sao)(6)(py)(2)] (2), [Fe(6)O(2)(O(2)CPh-4-NO(2))(4)(Me-sao)(2)(OMe)(4)Cl(2)(py)(2)] (3), [Fe(6)O(2)(O(2)CPh-4-NO(2))(4)(Et-sao)(2)(OMe)(4)Cl(2)(py)(2)]·2Et(2)O·MeOH (4·2Et(2)O·MeOH), [HNEt(3)](2)[Fe(6)O(2)(Me-sao)(4)(SO(4))(2)(OMe)(4)(MeOH)(2)] (5) and [HNEt(3)](2)[Fe(6)O(2)(Et-sao)(4)(SO(4))(2)(OMe)(4)(MeOH)(2)] (6) all are built from a series of edge-sharing [Fe(4)(μ(4)-O)](10+) tetrahedra. Complexes 1 and 2 display a new μ(4)-coordination mode of the oxime ligand and join a small group of Fe-phenolic oxime complexes with nuclearity greater than six.

Building Fe(III) clusters with derivatised salicylaldoximes

The syntheses, structures and magnetic properties of nine new iron complexes containing salicylaldoxime (saoH(2)) or derivatised salicylaldoximes (R-saoH(2)), [Fe(3)O(OMe)(Ph-sao)(2)Cl(2)(py)(3)].2MeOH (1.2MeOH), [Fe(3)O(OMe)(Ph-sao)(2)Br(2)(py)(3)].Et(2)O (2.Et(2)O), [Fe(4)(Ph-sao)(4)F(4)(py)(4)].1.5MeOH (3.1.5MeOH), [Fe(6)O(2)(OH)(2)(Et-sao)(2)(Et-saoH)(2)(O(2)CPh)(6)] (4), [HNEt(3)](2)[Fe(6)O(2)(OH)(2)(Et-sao)(4)(O(2)CPh(Me)(2))(6)].2MeCN (5.2MeCN), [Fe(6)O(2)(O(2)CPh)(10)(3-(t)But-5-NO(2)-sao)(2)(H(2)O)(2)].2MeCN (6.2MeCN), [Fe(6)O(2)(O(2)CCH(2)Ph)(10)(3-(t)But-sao)(2)(H(2)O)(2)].5MeCN (7.5MeCN), {[Fe(6)Na(3)O(OH)(4)(Me-sao)(6)(OMe)(3)(H(2)O)(3)(MeOH)(6)].MeOH}n (8.MeOH) and [HNEt(3)](2)[Fe(12)Na(4)O(2)(OH)(8)(sao)(12)(OMe)(6)(MeOH)(10)] (9) are discussed. The predominant building block appears to be the triangular [Fe(3)O(R-sao)(3)](+) species which can self-assemble into more elaborate arrays depending on reaction conditions. An interesting observation is that the R-saoH(-)/R-sao(2-) ligand system tends to adopt coordination modes similar to carboxylates. The most unusual molecule is the [Fe(4)F(4)] molecular square, 3. While Cl(-) and Br(-) appear to act only as terminal ligands, the F(-) ions bridge making a telling impact on molecular structure and topology.

Polymetallic clusters of iron(III) with derivatised salicylaldoximes

The synthesis and magnetic properties of the compounds [HNEt(3)][Fe(2)(OMe)(Ph-sao)(2) (Ph-saoH)(2)].5MeOH (1.5MeOH), [Fe(3)O(Et-sao)(O(2)CPh)(5)(MeOH)(2)].3MeOH (2.3MeOH), [Fe(4)(Me-sao)(4)(Me-saoH)(4)] (3), [HNEt(3)](2)[Fe(6)O(2)(Me-sao)(4)(SO(4))(2)(OMe)(4)(MeOH)(2)] (4), [Fe(8)O(3)(Me-sao)(3)(tea)(teaH)(3)(O(2)CMe)(3)] (5), [Fe(8)O(3)(Et-sao)(3)(tea)(teaH)(3)(O(2)CMe)(3)] (6), and [Fe(8)O(3)(Ph-sao)(3)(tea)(teaH)(3)(O(2)CMe)(3)] (7) are reported (Me-saoH(2) is 2'-hydroxyacetophenone oxime, Et-saoH(2) is 2'-hydroxypropiophenone oxime and Ph-saoH(2) is 2-hydroxybenzophenone oxime). 1-7 are the first Fe(III) compounds synthesised using the derivatised salicylaldoxime ligands, R-saoH(2). 1 is prepared by treatment of Fe(2)(SO(4))(3).6H(2)O with Ph-saoH(2) in the presence of NEt(3) in MeOH; 2 prepared by treatment of Fe(ClO(4))(2).6H(2)O with Et-saoH(2) and NaO(2)CPh in the presence of NEt(4)OH in MeOH; 3 prepared by treatment of Fe(ClO(4))(2).6H(2)O with Me-saoH(2) and NaO(2)CCMe(3) in the presence of NEt(4)OH in MeOH; and 4 prepared by treatment of Fe(2)(SO(4))(3).6H(2)O with Me-saoH(2) in the presence of NEt(3) in MeOH. 4 is a rare example of a polynuclear iron complex containing a coordinated SO(4)(2-) ion. Compounds 5-7 are prepared by treatment of Fe(O(2)CMe)(2) with Me-saoH(2) (5), Et-saoH(2) (6), Ph-saoH(2) (7) in the presence of H(3)tea (triethanolamine) in MeOH, and represent the largest nuclearity Fe(III) clusters containing salicyladoxime-based ligands, joining a surprisingly small family of characterised octanuclear Fe complexes. Variable temperature magnetic susceptibilty measurements of 1, 3 and 5-7 reveal all five complexes possess S = 0 spin ground states; 2 possesses an S = 1/2 spin ground state, while 4 has an S = 4 +/- 1 spin ground state.

Synthesis, characterization, and study of octanuclear iron-oxo clusters containing a redox-active Fe4O4-cubane core

A one-pot synthetic procedure yields the octanuclear Fe(III) complexes Fe(8)(micro(4-)O)(4)(micro-pz(*))(12)X(40, where X = Cl and pz(*) = pyrazolate anion (pz = C(3)H(3)N(2)-) (1), 4-Cl-pz (2), and 4-Me-pz (3) or X = Br and pz(*) = pz (4). The crystal structures of complexes 1-4, determined by X-ray diffraction, show an Fe(4)O(4)-cubane core encapsulated in a shell composed of four interwoven Fe(micro-pz(*))(3)X units. Complexes 1-4 have been characterized by 1H NMR, infrared, and Raman spectroscopies. Mössbauer spectroscopic analysis distinguishes the cubane and outer Fe(III) centers by their different isomer shift and quadrupole splitting values. Electrochemical analyses by cyclic voltammetry show four consecutive, closely spaced, reversible reduction processes for each of the four complexes. Magnetic susceptibility studies, corroborated by density functional theory calculations, reveal weak antiferromagnetic coupling among the four cubane Fe centers and strong antiferromagnetic coupling between cubane and outer Fe atoms of 1. The structural similarity between the antiferromagnetic Fe(8)(micro(4-)O)(4) core of 1-4 and the antiferromagnetic units contained in the minerals ferrihydrite and maghemite is demonstrated by X-ray and Mössbauer data.

A Cube in a Tetrahedron: Microwave-Assisted Synthesis of an Octametallic FeIII Cluster

The microwave-assisted reaction of Fe(O2CMe)2 with salicylaldoxime (saoH2) in pyridine produces an octametallic cluster in crystalline form in 2 min. The core describes a cube encapsulated in a tetrahedron, while sao2- exhibits a novel coordination mode.

An Octanuclear Complex Containing the {Fe3O}7+ Metal Core: Structural, Magnetic, Mössbauer, and Electron Paramagnetic Resonance Studies

A new asymmetrically coordinated bis-trinuclear iron(III) cluster containing a [Fe(3)O](7+) core has been synthesized and structurally, magnetically, and spectroscopically characterized. [Fe(6)Na(2)O(2)(O(2)CPh)(10)(pic)(4)(EtOH)(4)(H(2)O)(2)](ClO(4))(2).2EpsilontOH (1.2EpsilontOH) crystallizes in the P space group and consists of two symmetry-related {Fe(3)O](7+) subunits linked by two Na(+) cations. Inside each [Fe(3)O](7+) subunit, the iron(III) ions are antiferromagnetically coupled, and their magnetic exchange is best described by an isosceles triangle model with two equal (J) and one different (J ') coupling constants. On the basis of the H = -2SigmaJ(ij)S(i)S(j) spin Hamiltonian formalism, the two best fits to the data yield solutions J = -27.4 cm(-1), J ' = -20.9 cm(-1) and J = -22.7 cm(-1), J ' = -31.6 cm(-1). The ground state of the cluster is S = (1)/(2). X-band electron paramagnetic resonance (EPR) spectroscopy at liquid-helium temperature reveals a signal comprising a sharp peak at g approximately 2 and a broad tail at higher magnetic fields consistent with the S = (1)/(2) character of the ground state. Variable-temperature zero-field and magnetically perturbed Mössbauer spectra at liquid-helium temperatures are consistent with three antiferromagnetically coupled high-spin ferric ions in agreement with the magnetic susceptibility and EPR results. The EPR and Mössbauer spectra are interpreted by assuming the presence of an antisymmetric exchange interaction with |d| approximately 2-4 cm(-1) and a distribution of exchange constants J(ij).

A novel carboxylate-free ferromagnetic trinuclear mu(3)-oxo-manganese(III) complex with distorted pentagonal-bipyramidal metal centers

In methanol, the reaction of Mn(ClO(4))(2).6H(2)O and 1,2-bis(biacetylmonoximeimino)ethane (H(2)bamen) in the presence of triethylamine affords a trinuclear complex having the formula [Mn(3)(mu(3)-O)(mu(3)-bamen)(3)]ClO(4).2H(2)O. The structure of this complex shows a symmetric planar central [Mn(III)(3)(mu(3)-O)] unit coordinated to three hexadentate bridging (via oximate groups) ligands. The N(4)O(3) coordination sphere around each metal center is very close to pentagonal-bipyramidal. A cyclic voltammogram of the complex displays two reversible and an irreversible response due to Mn(III)(3) --> Mn(III)(2)Mn(IV), Mn(III)(2)Mn(IV) --> Mn(III)Mn(IV)(2), and Mn(III)Mn(IV)(2) --> Mn(IV)(3) oxidation processes, respectively. Cryomagnetic data reveal that the complex is ferromagnetic.

Heterotrinuclear Oxo-Bridged Fe(III)ORu(IV)OFe(III) Complexes of Ruthenium Porphyrin and Borylated Low-Spin Iron Dioximes(1)

The synthesis, structure, spectra, and reactivity of diamagnetic linear chain heterotrinuclear oxo-bridged complexes L-Fe(III) N(4)-O-Ru(IV)(TPP')-O-Fe(III)N(4)-L are described where TPP' is tetrakis(4-methoxyphenylporphyrinate), N(4) is one of the borylated bis(dioximate) macrocycles, N(4) = ((DMG)BF(2))(2) in 1, ((DMG)BPh(2))(2) in 2, and ((DPG)BF(2))(2) in 3, and L is a monodentate terminal ligand, L = BuNH(2), CH(3)CN, Py, 1-MeIm, (BuO)(3)P, etc. Crystal data for 3-(BuNH(2))(2) = [(BuNH(2))Fe((DPG)BF(2))(2)-O](2)Ru(TPP') monoclinic space group P2(1)/n, a = 16.845 Å, b = 27.184 Å, c = 24.593 Å, beta = 90.41 degrees, Fe-O = 1.79(1) Å, Ru-O = 1.80(1) Å, Fe-O-Ru = 175 degrees. Large porphyrin and phenyl ring current shifts permit ready characterization of the materials by (1)H NMR. A red-shifted Soret band at 433 nm and two charge transfer transitions in the 650-850 nm region are observed in the visible spectra. Clean reduction of 1, 2, and 3-(CH(3)CN)(2) with 4-tert-butylcatechol is observed giving Fe(II) and Ru(II) products. The kinetics of reduction parallel those of low-spin (&mgr;-oxo)diiron FeN(4) systems showing a 1/[CH(3)CN] dependence, but the rates are 10(3)-10(5) times slower. The Ru(IV) is proposed to stabilize the &mgr;-oxo-Fe bonds toward reductive cleavage via delocalization of the oxo pi electrons onto the ruthenium. Electrochemical data for the heterotrinuclear Fe-O-Ru-O-Fe systems are compared with those for the binuclear Fe-O-Fe systems. The HOMO is stabilized by 120 mV over the corresponding Fe-O-Fe system.

Rapid and parallel formation of Fe3+ multimers, including a trimer, during H-type subunit ferritin mineralization

Conversion of Fe ions in solution to the solid phase in ferritin concentrates iron required for cell function. The rate of the Fe phase transition in ferritin is tissue specific and reflects the differential expression of two classes of ferritin subunits (H and L). Early stages of mineralization were probed by rapid freeze-quench Mossbauer, at strong fields (up to 8 T), and EPR spectroscopy in an H-type subunit, recombinant frog ferritin; small numbers of Fe (36 moles/mol of protein) were used to increase Fe3+ in mineral precursor forms. At 25 ms, four Fe3+-oxy species (three Fe dimers and one Fe trimer) were identified. These Fe3+-oxy species were found to form at similar rates and decay subsequently to a distinctive superparamagentic species designated the "young core." The rate of oxidation of Fe2+ (1026 s(-1)) corresponded well to the formation constant for the Fe3+-tyrosinate complex (920 s(-1)) observed previously [Waldo, G. S., & Theil, E. C. (1993) Biochemistry 32, 13261] and, coupled with EPR data, indicates that several or possibly all of the Fe3+-oxy species involve tyrosine. The results, combined with previous Mossbauer studies of Y30F human H-type ferritin which showed decreases in several Fe3+ intermediates and stabilization of Fe2+ [Bauminger, E. R., et al. (1993) Biochem. J. 296, 709], emphasize the involvement of tyrosyl residues in the mineralization of H-type ferritins. The subsequent decay of these multiple Fe3+-oxy species to the superparamagnetic mineral suggests that Fe3+ species in different environments may be translocated as intact units from the protein shell into the ferritin cavity where the conversion to a solid mineral occurs.