Mechanistic studies on oxidation of hydrazine by a mu-oxo diiron(III,III) complex in aqueous acidic media-proton coupled electron transfer

[Fe2(mu-O)(phen)4(H2O)2]4+ (1), one of the simplest mu-oxo diiron(III) complexes, quantitatively oxidises hydrazine to dinitrogen and itself is reduced to two moles of ferroin, [Fe(phen)3]2+ in presence of excess phenanthroline. The weak dibasic acid, 1 (pKa1= 3.71 +/- 0.05 and pKa2= 5.28 +/- 0.10 at 25.0 degrees C, I= 1.0 mol dm(-3)(NaNO3)) and its conjugate bases, [Fe2(mu-O)(phen)4(H2O)(OH)]3+ (2) and [Fe2(mu-O)(phen)4(OH)2]2+ (3) are involved in the redox process with the reactivity order 1 > 2 > 3 whereas N2H4 and not N2H5+ was found to be reactive in the pH interval studied 3.45-5.60. Cyclic voltammetric studies indicate poor oxidizing capacity of the title substitution-labile diiron complex, yet it oxidizes N2H4 with a moderate rate--a proton coupled electron transfer (1e, 1H+) drags the energetically unfavourable reaction to completion. The rate retardation in D2O media is substantially higher at higher pH due to the increasing basicity of the oxo-ligand in the order 3 > 2 > 1. Marcus calculations result an unacceptably high one-electron self-exchange rate for the iron center indicating an inner-sphere nature of the electron-transfer.     

Mechanistic studies on oxidation of nitrite by a {Mn3O4}4+ core in aqueous acidic media

[MnIV3(micro-O)4(phen)4(H2O)2]4+ (, phen=1,10-phenanthroline) equilibrates with its conjugate base [Mn3(micro-O)4(phen)4(H2O)(OH)]3+ in aqueous solution. Among the several synthetic multinuclear oxo- and/or carboxylato bridged manganese complexes known to date containing metal-bound water, to the best of our knowledge, only deprotonates (right harpoon over left harpoon+H+, pKa=4.00 (+/-0.15) at 25.0 degrees C, I=1.0 M, maintained with NaNO3) at physiological pH. An aqueous solution of quantitatively oxidises NIII (HNO2 and NO2-) to NO3- within pH 2.3-4.1, the end manganese state being MnII. Both and are reactive oxidants in the title redox. In contrast to a common observation that anions react quicker than their conjugate acids in reducing metal centred oxidants, HNO2 reacts faster than NO2- in reducing or . The observed rates of nitrite oxidation do not depend on the variation of 1,10-phenanthroline content of the solution indicating that the MnIV-bound phen ligands do not dissociate in solution under experimental conditions. Also, there was no kinetic evidence for any kind of pre-equilibrium replacement of MnIV-bound water by nitrite prior to electron transfer which indicates the substitution-inert nature of the MnIV-bound waters and the 1,10-phenanthroline ligands. The MnIV3 to MnII transition in the present observation proceeds through the intermediate generation of the spectrally characterised mixed-valent MnIIIMnIV dimer that quickly produces MnII. The reaction rates are substantially lowered when solvent H2O is replaced by D2O and a rate determining 1e, 1H+ electroprotic mechanism is proposed.     

Mechanistic studies on the oxidation of pyruvic acid by an oxo-bridged diiron(III,III) complex in aqueous acidic media

In aqueous solution [Fe₂(μ-O)(phen)₄(H₂O)₂]⁴⁺ (1, phen = 1,10-phenanthroline) equilibrates with its conjugate bases [Fe₂(μ-O)(phen)₄(H₂O)(OH)]³⁺ (2) and [Fe₂(μ-O)(phen)₄(OH)₂]²⁺ (3). In the presence of excess phen and in the pH range 2.5–5.5, the dimer quantitatively oxidizes pyruvic acid to acetic acid and carbon dioxide, the end iron species being ferroin, [Fe(phen)₃]²⁺. The observed reaction rate shows a bell-shaped curve as pH increases, but is independent of added phen. Kinetic analysis shows that (3) is non-reactive and (1) has much higher reactivity than (2) in oxidizing pyruvic acid. The basicity of the bridging oxygen increases with deprotonation of the aqua ligands. The reaction rate decreases significantly in media enriched with D₂O in comparison to that in H₂O, with a greater retardation at higher pH, suggesting the occurrence of proton coupled electron transfer (PCET; 1e, 1H⁺), which possibly drags the energetically unfavorable reaction to completion in presence of excess phen.     

Catalase-peroxidase activity of iron(III)-TAML activators of hydrogen peroxide

Exceptionally high peroxidase-like and catalase-like activities of iron(III)-TAML activators of H 2O 2 ( 1: Tetra-Amidato-Macrocyclic-Ligand Fe (III) complexes [ F e{1,2-X 2C 6H 2-4,5-( NCOCMe 2 NCO) 2CR 2}(OH 2)] (-)) are reported from pH 6-12.4 and 25-45 degrees C. Oxidation of the cyclometalated 2-phenylpyridine organometallic complex, [Ru (II)( o-C 6H 4py)(phen) 2]PF 6 ( 2) or "ruthenium dye", occurs via the equation [ Ru II ] + 1/2 H 2 O 2 + H +-->(Fe III - TAML) [ Ru III ] + H 2 O, following a simple rate law rate = k obs (per)[ 1][H 2O 2], that is, the rate is independent of the concentration of 2 at all pHs and temperatures studied. The kinetics of the catalase-like activity (H 2 O 2 -->(Fe III - TAML) H 2 O + 1/2 O 2) obeys a similar rate law: rate = k obs (cat)[ 1][H 2O 2]). The rate constants, k obs (per) and k obs (cat), are strongly and similarly pH dependent, with a maximum around pH 10. Both bell-shaped pH profiles are quantitatively accounted for in terms of a common mechanism based on the known speciation of 1 and H 2O 2 in this pH range. Complexes 1 exist as axial diaqua species [FeL(H 2O) 2] (-) ( 1 aqua) which are deprotonated to afford [FeL(OH)(H 2O)] (2-) ( 1 OH) at pH 9-10. The pathways 1 aqua + H 2O 2 ( k 1), 1 OH + H 2O 2 ( k 2), and 1 OH + HO 2 (-) ( k 4) afford one or more oxidized Fe-TAML species that further rapidly oxidize the dye (peroxidase-like activity) or a second H 2O 2 molecule (catalase-like activity). This mechanism is supported by the observations that (i) the catalase-like activity of 1 is controllably retarded by addition of reducing agents into solution and (ii) second order kinetics in H 2O 2 has been observed when the rate of O 2 evolution was monitored in the presence of added reducing agents. The performances of the 1 complexes in catalyzing H 2O 2 oxidations are shown to compare favorably with the peroxidases further establishing Fe (III)-TAML activators as miniaturized enzyme replicas with the potential to greatly expand the technological utility of hydrogen peroxide.     

Iron(III) activation hits a [4 + 4] macrocycle

The tetranuclear complex [Fe(III)2(L')(OH)(CH3O)]2, 1, has been synthesised from the reaction of either ferrous [in excess as 4:1 or stoichiometric 2:1 iron(II) : H4L] or ferric ions [4:1 iron(III) : H4L] with the large macrocycle, H4L, using aerobic conditions in methanol in the presence of triethylamine. The structure of 1 was determined by single-crystal X-ray diffraction. These reaction conditions lead to the modification of the original macrocycle through the incorporation of a methylene group between two amine groups to give an imidazolidine ring in (L')4-. The controlled addition of formaldehyde into the reaction system results in a significantly improved yield of 1, suggesting that it is involved in the reaction mechanism. The (L')4- macrocycle binds to two, well-separated, iron(III) centres [Fe(1)...Fe(1a) > 8 A]. Each iron(III) centre is further linked via hydroxy and methoxy bridges to equivalent iron(iii) centres contained in a second macrocycle. Overall this gives a structure containing two {Fe(OH)(CH(3)O)Fe} dimers [Fe(1)...Fe(2)ca. 3.2 A] sandwiched by two (L')4- macrocycles. The complex was further characterised by SQUID magnetic measurements and can be interpreted in terms of two isolated antiferromagnetically coupled Fe(III) dimers (J=-23.75 K).     

mer-[Fe(pcq)(CN)3]-: a novel cyanide-containing building block and its application to assembling cyanide-bridged trinuclear FeIII2MnII complexes [pcq- = 8-(pyridine-2-carboxamido)quinoline anion

A new cyanide-containing building block K[Fe(pcq)(CN)(3)] [1; pcq(-) = 8-(pyridine-2-carboxamido)quinoline anion] containing a low-spin Fe(III) center with three cyanide groups in a meridional arrangement has been successfully designed and synthesized. Three cyanide-bridged trinuclear Fe(III)(2)Mn(II) complexes, [Fe(pcq)(CN)(3)](2)[Mn(CH(3)OH)(2)(H(2)O)(2)].2H(2)O (2), [Fe(pcq)(CN)(3)](2)[Mn(bipy)(2)].CH(3)OH.2H(2)O (3), and [Fe(pcq)(CN)(3)](2)[Mn(phen)(2)].CH(3)OH.2H(2)O (4), have been synthesized and structurally characterized. The magnetic susceptibilities of the three heterometallic complexes have been investigated.     

Organic-inorganic hybrids based on novel bimolecular [Si2W22Cu2O78(H2O)]12- polyoxometalates and the polynuclear complex cations [Cu(ac)(phen)(H2O)]n n+ (n=2, 3)

The reaction of a monosubstituted Keggin polyoxometalate (POM) generated in situ with copper-phenanthroline complexes in excess ammonium or rubidium acetate led to the formation of the hybrid metal organic-inorganic compounds A7[Cu2(ac)2(phen)2(H2O)2][Cu3(ac)3(phen)3(H2O)3][Si2W22Cu2O78(H2O)].approximately 18 H2O (A=NH4+ (1), Rb+ (2); ac=acetate; phen=1,10-phenanthroline). These compounds are constructed from inorganic and metalorganic interpenetrated sublattices containing the novel bimolecular Keggin POM, [Si2W22Cu2O78(H2O)]12-, and Cu-ac-phen complexes, [Cu(ac)(phen)(H2O)]n n+ (n=2, 3). The packing of compound 1 can be viewed as a stacking of open-framework layers parallel to the xy plane built of hydrogen-bonded POMs, and zigzag columns of pi-stacked Cu-ac-phen complex cations running along the [111] direction. Magnetic and EPR results are discussed with respect to the crystal structure of the compounds. DFT calculations on [Cu(ac)(phen)(H2O)]n n+ cationic complexes have been performed, to check the influence of packing in the complex geometry and determine the magnetic exchange pathways.     

Polyoxometalate-supported transition metal complexes and their charge complementarity: synthesis and characterization of [M(OH)6Mo6O18[Cu(Phen)(H2O)2]2][M(OH)6Mo6O18[Cu(Phen)(H2O)Cl]2].5H2O (M = Al(+, Cr3+)

Two Anderson-type heteropolyanion-supported copper phenanthroline complexes, [Al(OH)6Mo6O18[Cu(phen)(H2O)2]2]1+ (1c) and [Al(OH)6Mo6O18[Cu(phen)(H2O)Cl]2]1- (1a) complement their charges in one of the title compounds [Al(OH)6Mo6O18[Cu(phen)(H2O)2]2][Al(OH)6Mo6O18[Cu(phen)(H2O)Cl]2].5H2O [1c][1a].5 H2O 1. Similar charge complementarity exists in the chromium analogue, [Cr(OH)6Mo6O18[Cu(phen)(H2O)2]2][Cr(OH)6Mo6O18[Cu(phen)(H2O)Cl]2].5 H2O [2c][2a].5 H2O 2. The chloride coordination to copper centers of 1a and 2a makes the charge difference. In both compounds, the geometries around copper centers are distorted square pyramidal and those around aluminum/chromium centers are distorted octahedral. Three lattice waters, from the formation of intermolecular O-H.....O hydrogen bonds, have been shown to self-assemble into an "acyclic water trimer" in the crystals of both 1 and 2. The title compounds have been synthesized in a simple one pot aqueous wet-synthesis consisting of aluminum/chromium chloride, sodium molybdate, copper nitrate, phenanthroline, and hydrochloric acid, and characterized by elemental analyses, EDAX, IR, diffuse reflectance, EPR, TGA, and single-crystal X-ray diffraction. Both compounds crystallize in the triclinic space group P. Crystal data for 1: a = 10.7618(6), b = 15.0238(8), c = 15.6648(8) angstroms, alpha = 65.4570(10), beta = 83.4420(10), gamma = 71.3230(10), V = 2182.1(2) angstroms3. Crystal data for 2: a = 10.8867(5), b = 15.2504(7), c = 15.7022(7) angstroms, alpha = 64.9850(10), beta = 83.0430(10), gamma = 71.1570(10), V = 2235.47(18) angstroms3. In the electronic reflectance spectra, compounds 1 and 2 exhibit a broad d-d band at approximately 700 nm, which is a considerable shift with respect to the value of 650-660 nm for a square-pyramidal [Cu(phen)2L] complex, indicating the coordination of [M(OH)6Mo6O18]3- POM anions (as a ligand) to the monophenanthroline copper complexes to form POM-supported copper complexes 1c, 1a, 2c, and 2a. The ESR spectrum of compound 1 shows a typical axial signal for a Cu2+ (d9) system, and that of compound 2, containing both chromium(III) and copper(II) ions, may reveal a zero-field-splitting of the central Cr3+ ion of the Anderson anion, [Cr(OH)6Mo6O18]3-, with an intense peak for the Cu2+ ion.     

Synthesis, reactivities, and magnetostructural properties of FeIII, FeIII-O-FeIII, and ZnIIFeIII-O-FeIIIZnII complexes of a tetraiminodiphenolate macrocycle

The mononuclear iron(III) complexes [Fe(LH2)(H2O)Cl](ClO4)2.2H2O (1) and [Fe(LH2)(H2O)2](ClO4)3.H2O (2) have been prepared by reacting [Pb(LH(2))](ClO4)2 with FeCl3.6H2O and Fe(ClO(4))(3).6H(2)O, respectively. Complex 2 upon treatment with 1 equiv of alkali produces the oxo-bridged dimer [{Fe(LH2)(H2O)}2(mu-O)](ClO4)4.2H2O (3). In these compounds, LH2 refers to the tetraiminodiphenol macrocycle in the zwitterionic form whose two uncoordinated imine nitrogens are protonated and hydrogen-bonded to the metal-bound phenolate oxygens. The aqua ligands of complexes 1-3 get exchanged in acetonitrile. Reaction equilibria involving binding and exchange of the terminal ligands (Cl-/H2O/CH3CN) in these complexes have been studied spectrophotometrically. The equilibrium constant for the aquation reaction (K(aq)) [1]2+ + H2O <==> [2]3+ + Cl- in acetonitrile is 8.65(5) M, and the binding constant (K(Cl)-) for the reaction [1]2+ + Cl- [1Cl]+ + CH3CN is 4.75(5) M. The pK(D) value for the dimerization reaction 2[2]3+ + 2OH- <==> [3]4+ + 3H(2)O in 1:1 acetonitrile-water is 9.38(10). Complexes 1-3 upon reaction with Zn(ClO4)(2).6H(2)O and sodium acetate (OAc), pivalate (OPiv), or bis(4-nitrophenyl)phosphate (BNPP) produce the heterobimetallic complexes [{FeLZn(mu-X)}2(mu-O)](ClO4)2, where X = OAc (4), OPiv (5), and BNPP (6). The pseudo-first-order rate constant (k(obs)) for the formation of 4 at 25 degrees C from either 1 or 3 with an excess of Zn(OAc)2.2H2O in 1:1 acetonitrile-water at pH 6.6 is found to be the same with k(obs) = 1.6(2) x 10(-4) s(-1). The X-ray crystal structures of 3, 4, and 6 have been determined, although the structure determination of 3 was severely affected because of heavy disordering. In 3, the Fe-O-Fe angle is 168.6(6) degrees, while it is exactly 180.0 degrees in 4 and 6. Cyclic and square-wave voltammetric (CV and SWV) measurements have been carried out for complexes 1-4 in acetonitrile. The variation of the solvent composition (acetonitrile-water) has a profound effect on the E(1/2) and DeltaE(p) values. The binding of an additional chloride ion to an iron(III) center in 1-3 is accompanied by a remarkable shift of E(1/2) to more negative values. The observation of quasi-reversible CV for complexes containing a Fe(III)-O-Fe(III) unit (3 and 4) indicates that in the electrochemical time scale unusual Fe(III)-O-Fe(II) is produced. The 1H NMR spectra of complexes 3-6 exhibit hyperfine-shifted signals in the range 0-90 ppm with similar features. The metal-hydrogen distances obtained from T(1) measurements are in good agreement with the crystallographic data. Variable-temperature (2-300 K) magnetic susceptibility measurements carried out for 3 and 4 indicate strong antiferromagnetic exchange interaction (H = -2JS1.S2) between the high-spin iron(III) centers in the Fe-O-Fe unit with J = -114 cm(-1) (3) and -107 cm(-1) (4).     

Fe2 and Fe4 clusters encapsulated in vacant polyoxotungstates: hydrothermal synthesis, magnetic and electrochemical properties, and DFT calculations

While the reaction of [PW(11)O(39)](7-) with first row transition-metal ions M(n+) under usual bench conditions only leads to monosubstituted {PW(11)O(39)M(H(2)O)} anions, we have shown that the use of this precursor under hydrothermal conditions allows the isolation of a family of novel polynuclear discrete magnetic polyoxometalates (POMs). The hybrid asymmetric [Fe(II)(bpy)(3)][PW(11)O(39)Fe(2) (III)(OH)(bpy)(2)]12 H(2)O (bpy=bipyridine) complex (1) contains the dinuclear {Fe(micro-O(W))(micro-OH)Fe} core in which one iron atom is coordinated to a monovacant POM, while the other is coordinated to two bipyridine ligands. Magnetic measurements indicate that the Fe(III) centers in complex 1 are weakly antiferromagnetically coupled (J=-11.2 cm(-1), H=-JS(1)S(2)) compared to other {Fe(micro-O)(micro-OH)Fe} systems. This is due to the long distances between the iron center embedded in the POM and the oxygen atom of the POM bridging the two magnetic centers, but also, as shown by DFT calculations, to the important mixing of bridging oxygen orbitals with orbitals of the POM tungsten atoms. The complexes [Hdmbpy](2)[Fe(II)(dmbpy)(3)](2)[(PW(11)O(39))(2)Fe(4) (III)O(2)(dmbpy)(4)]14 H(2)O (2) (dmbpy=5,5'-dimethyl-2,2'-bipyridine) and H(2)[Fe(II)(dmbpy)(3)](2)[(PW(11)O(39))(2)Fe(4) (III)O(2)(dmbpy)(4)]10 H(2)O (3) represent the first butterfly-like POM complexes. In these species, a tetranuclear Fe(III) complex is sandwiched between two lacunary polyoxotungstates that are pentacoordinated to two Fe(III) cations, the remaining paramagnetic centers each being coordinated to two dmbpy ligands. The best fit of the chi(M)T=f(T) curve leads to J(wb)=-59.6 cm(-1) and J(bb)=-10.2 cm(-1) (H=-J(wb)(S(1)S(2)+S(1)S(2*)+S(1*)S(2)+S(1*)S(2*))-J(bb)(S(2)S(2*))). While the J(bb) value is within the range of related exchange parameters previously reported for non-POM butterfly systems, the J(wb) constant is significantly lower. As for complex 1, this can be justified considering Fe(w)--O distances. Finally, in the absence of a coordinating ligand, the dimeric complex [N(CH(3))(4)](10)[(PW(11)O(39)Fe(III))(2)O]12 H(2)O (4) has been isolated. In this complex, the two single oxo-bridged Fe(III) centers are very strongly antiferromagnetically coupled (J=-211.7 cm(-1), H=-JS(1)S(2)). The electrochemical behavior of compound 1 both in dimethyl sulfoxide (DMSO) and in the solid state is also presented, while the electrochemical properties of complex 2, which is insoluble in common solvents, have been studied in the solid state.     

Assembly of azido- or cyano-bridged binuclear complexes containing the bulky [Mn(phen)2]2+ building block: syntheses, crystal structures, and magnetic properties

Two new cyano-bridged heterobinuclear complexes, [Mn(II)(phen)2Cl][Fe(III)(bpb)(CN)2] x 0.5CH3CH2OH x 1.5H2O (1) and [Mn(II)(phen)2Cl][Cr(III)(bpb)(CN)2] x 2H2O (2) [phen = 1,10-phenanthroline; bpb(2-) = 1,2-bis(pyridine-2-carboxamido)benzenate], and four novel azido-bridged Mn(II) dimeric complexes, [Mn2(phen)4(mu(1,1)-N3)2][M(III)(bpb)(CN)2]2 x H2O [M = Fe (3), Cr (4), Co (5)] and [Mn2(phen)4(mu(1,3)-N3)(N3)2]BPh4 x 0.5H2O (6), have been synthesized and characterized by single-crystal X-ray diffraction analysis and magnetic studies. Complexes 1 and 2 comprise [Mn(phen)2Cl]+ and [M(bpb)(CN)2]- units connected by one cyano ligand of [M(bpb)(CN)2]-. Complexes 3-5 are doubly end-on (EO) azido-bridged Mn(II) binuclear complexes with two [M(bpb)(CN)2]- molecules acting as charge-compensating anions. However, the Mn(II) ions in complex 6 are linked by a single end-to-end (EE) azido bridging ligand with one large free BPh4(-) group as the charge-balancing anion. The magnetic coupling between Mn(II) and Fe(III) or Cr(III) in complexes 1 and 2 was found to be antiferromagnetic with J(MnFe) = -2.68(3) cm(-1) and J(MnCr) = -4.55(1) cm(-1) on the basis of the Hamiltonian H = -JS(Mn)S(M) (M = Fe or Cr). The magnetic interactions between two Mn(II) ions in 3-5 are ferromagnetic in nature with the magnetic coupling constants of 1.15(3), 1.05(2), and 1.27(2) cm(-1) (H = -JS(Mn1)S(Mn2)), respectively. The single EE azido-bridged dimeric complex 6 manifests antiferromagnetic interaction with J = -2.29(4) cm(-1) (H = -JS(Mn1)S(Mn2)). Magneto-structural correlationship on the EO azido-bridged Mn(II) dimers has been investigated.     

Unusual structural types in polynuclear iron chemistry from the use of N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine (edteH4): Fe5, Fe6, and Fe12 clusters

The syntheses, crystal structures, and magnetochemical characterization of five new iron clusters [Fe5O2(O2CPh)7(edte)(H2O)] (1), [Fe6O2(O2CBut)8(edteH)2] (2), [Fe12O4(OH)2(O2CMe)6(edte)4(H2O)2](ClO4)4 (3), [Fe12O4(OH)8(edte)4(H2O)2](ClO4)4 (4), and [Fe12O4(OH)8(edte)4(H2O)2](NO3)4 (5) (edteH4= N,N,N',N'-tetrakis(2-hydroxyethyl) ethylenediamine) are reported. The reaction of edteH4 with [Fe3O(O2CPh)6(H2O)3](NO3) and [Fe3O(O2CBut)6(H2O)3](OH) gave 1 and 2, respectively. Complex 3 was obtained from the reaction of edteH4 and NaO2CMe with Fe(ClO4)3, whereas 4 and 5 were obtained from the reaction of edteH4 with Fe(ClO4)3 and Fe(NO3)3, respectively. The core of 1 consists of a [Fe4(mu3-O)2]8+ butterfly unit to which is attached a fifth Fe atom by four bridging O atoms. The core of 2 consists of two triangular [Fe3(mu3-O)]7+ units linked together by six bridging O atoms. Finally, the cores of 3-5 consist of an [Fe12(mu4-O)4(mu-OH)2]26+ unit. Variable-temperature (T) and -field (H) solid-state direct and alternating current magnetization (M) studies were carried out on complexes 1-3 in the 1.8-300 K range. Analysis of the obtained data revealed that 1, 2, and 3-5 possess an S = 5/2, 5, and 0 ground-state spin, respectively. The fitting of the obtained M/N(muB) vs H/T data was carried out by matrix diagonalization, and this gave values for the axial zero-field splitting (ZFS) parameter D of -0.50 cm-1 for 1 and -0.28 cm-1 for 2.     

Organically directed iron sulfate chains: structural diversity based on hydrogen bonding interactions

Six organically directed 1-D iron sulfates hydrated and hydrolyzed to different extents have been prepared hydrothermally. [C2H10N2]1.5[Fe(SO4)(3)].2H2O (I), [C2H10N2][Fe(SO4)2(OH)].H2O (II), [C6H18N2]0.5[Fe(SO4)2(H2O)2] (III), and [C6H18N2]0.5[Fe2(SO4)(H2O)4(OH)].H2O (V) possess the linear topological structures observed in ferrinatrite, sideronatrite, kr?hnkite, and copiapite minerals, respectively. [C4H12N2][Fe2(SO4)3(OH)2(H2O)2].H2O (IV) shows a novel linear structure that can be regarded as a hybrid of the tancoite and butlerite types. [C6N4H22]0.5[Fe(SO4)2(OH)].2H2O (VI) adopts a cis configuration, compared with II, to give a rare inorganic helical iron sulfate chain which is a new member of the organically directed transitional metal sulfates. The results reveal that the starting molar proportion of the reactants and the type of amines are critical for the structural motif. There is an obvious relationship between the constitution of chains and the type of amino groups, involving the amount of N-H...O hydrogen bonds.     

Hydrolysis of phosphodiesters by diiron complexes: design of nonequivalent iron sites in purple acid phosphatase models

New mu-oxo-diferric complexes have been designed for hydrolysis of phosphodiesters. To mimic the diiron active site of purple acid phosphatase, a combinatorial method has been used to select complexes containing two distinct iron coordination spheres. The introduction of a bidentate ligand, a substituted phenanthroline (L) into complex 1, [Fe2O(bipy)4(OH2)2](NO3)4, generates in solution the complex [Fe2O(bipy)3(L)(OH2)2](NO3)4 as shown by ESI/MS and 1H NMR studies. The latter complex was found to be 20-fold more active than complex 1. On the basis of kinetic studies, we demonstrated that the complex [Fe2O(bipy)3(L)(OH)(OH2)](NO3)3 was the active species and the reaction proceeded through the formation of a ternary complex in which one iron binds a hydroxide and the second, the substrate. At nonsaturating concentrations of the substrate, the increased activity with increased methyl substituents in L was due to an increased affinity of the complex for the substrate. The activity of [Fe2O(bipy)3(33'44'Me2-Phen)(OH2)2](NO3)4 [33'44'Me2Phen = 3,3',4,4'-dimethyl-1,10-phenanthroline] was found to be comparable to that reported for Co(III) or Ce(IV) complexes.     

Nitrosation of N-methylhydroxylamine by nitroprusside. A kinetic and mechanistic study

The kinetics of the reaction between aqueous solutions of Na(2)[Fe(CN)(5)NO].2H(2)O (sodium pentacyanonitrosylferrate(ii), nitroprusside, SNP) and MeN(H)OH (N-methylhydroxylamine, MeHA) has been studied by means of UV-vis spectroscopy, using complementary solution techniques: FTIR/ATR, EPR, mass spectrometry and isotopic labeling ((15)NO), in the pH range 7.1-9.3, I = 1 M (NaCl). The main products were N-methyl-N-nitrosohydroxylamine (MeN(NO)OH) and [Fe(CN)(5)H(2)O](3-), characterized as the [Fe(CN)(5)(pyCONH(2))](3-) complex (pyCONH(2) = isonicotinamide). No reaction occurred with Me(2)NOH (N,N-dimethylhydroxylamine, Me(2)HA) as nucleophile. The rate law was: R = k(exp) [Fe(CN)(5)NO(2-)] x [MeN(H)OH] x [OH(-)], with k(exp) = 1.6 +/- 0.2 x 10(5) M(-2) s(-1), at 25.0 degrees C, and DeltaH(#) = 34 +/- 3 kJ mol(-1), DeltaS(#) = -32 +/- 11 J K(-1) mol(-1), at pH 8.0. The proposed mechanism involves the formation of a precursor associative complex between SNP and MeHA, followed by an OH(-)-assisted reversible formation of a deprotonated adduct, [Fe(CN)(5)(N(O)NMeOH)](3-), and rapid dissociation of MeN(NO)OH. In excess SNP, the precursor complex reacts through a competitive one-electron-transfer path, forming the [Fe(CN)(5)NO](3-) ion with slow production of small quantities of N(2)O. The stoichiometry and mechanism of the main adduct-formation path are similar to those previously reported for hydroxylamine (HA) and related nucleophiles. The nitrosated product, MeN(NO)OH, decomposes thermally at physiological temperatures, slowly yielding NO.     

Synthesis, crystal structures and magnetic properties of single and double cyanide-bridged bimetallic Fe2(III)Cu(II) zigzag chains

The bimetallic complexes [[Fe(III)(phen)(CN)4]2Cu(II)(H2O)2].4H2O (1), [[Fe(III)(phen)(CN)4]2Cu(II)].H2O (2) and [[Fe(III)(bipy)(CN)4]2Cu(II)].2H2O (3) and [[Fe(III)(bipy)(CN)4]2Cu(II)(H2O)2].4H2O (4) (phen = 1,10-phenanthroline and bipy = 2,2'-bipyridine) have been prepared and the structures of 1-3 determined by X-ray diffraction. The structure of 1 is made up of neutral cyanide-bridged Fe(III)-Cu(II) zigzag chains of formula [[Fe(III)(phen)(CN)4]2Cu(II)(H2O)2] and uncoordinated water molecules with the [Fe(phen)(CN)4]- entity acting as a bis-monodentate bridging ligand toward two trans-diaquacopper(II) units through two of its four cyanide groups in cis positions. The structure of 2 can be viewed as the condensation of two chains of 1 connected through single cyanide-bridged Fe(III)-Cu(II) pairs after removal of the two axially coordinated water molecules of the copper atom. The structure of 3 is like that of 2, the main differences being the occurrence of bipy (phen in 2) and two (one in 2) crystallization water molecules. The crystals of 4 diffract poorly but the analysis of the limited set of diffraction data shows a chain structure like that of 1 the most important difference being the fact that elongation axis at the copper atom is defined by the two trans coordinated water molecules. 1 behaves as a ferromagnetic Fe(III)2Cu(II) trinuclear system. A metamagnetic-like behavior is observed for 2 and 3, the value of the critical field (Hc) being ca. 1100 (2) and 900 Oe (3). For H > Hc the ferromagnetic Fe(III)2Cu(II) chains exhibit frequency dependence of the out-of-phase ac susceptibility signal at T < 4.0 K. The magnetic behavior of 4 corresponds to that of a ferromagnetically coupled chain of low spin iron(III) and copper(II) ions with frequency dependence of the out-of-phase susceptibility at T < 3.0 K. Theoretical calculations using methods based on density functional theory (DFT) have been employed to analyze and substantiate the exchange pathways in this family of complexes.     

Cyano-bridged FeIII2CuII3 and FeIII4NiII4 complexes: syntheses, structures, and magnetic properties

Two tricyanometallate precursors, (Bu4N)[(Tp4Bo)Fe(CN)3].H2O.2MeCN (1) and (Bu4N)[(pzTp)Fe(CN)3] (2) [Bu4N+ = tetrabutylammonium cation; Tp4Bo = tris(indazolyl)hydroborate; pzTp = tetrakis(pyrazolyl)borate], with a low-spin FeIII center have been synthesized and characterized. The reactions of 1 or 2 with [Cu(Me3tacn)(H2O)2](ClO4)2 (Me3tacn = N,N',N' '-trimethyl-1,4,7-triazacyclononane) afford two pentanuclear cyano-bridged clusters, [(Tp4Bo)2(Me3tacn)3Cu3Fe2(CN)6](ClO4)4.5H2O (3) and [(pzTp)2(Me3tacn)3Cu3Fe2(CN)6](ClO4)4.4H2O (4), respectively. Assembly reactions between 2 and [Ni(phen)(CH3OH)4](ClO4)2 (phen = 1,10-phenanthroline) or Zn(OAc)2.2H2O afford a molecular box [(pzTp)4(phen)4Ni4Fe4(CH3OH)4(CN)12](ClO4)4.4H2O (5) and a rectangular cluster [(pzTp)2Zn2Fe2(OAc)2(H2O)2(CN)6] (6). Their molecular structures were determined by single-crystal X-ray diffraction. In complexes 1 and 2, the central FeIII ions are coordinated by three cyanide carbon atoms and three nitrogen atoms of Tp4Bo- or pzTp-. Both complexes 3 and 4 show a trigonal-bipyramidal geometry, in which [(L)Fe(CN)3]- units occupy the apical positions and are linked through cyanide to [Cu(Me3tacn)]2+ units situated in the equatorial plane. Complex 5 possesses a cubic arrangement of eight metal irons linked through edge-spanning cyanide bridges, while complex 6 shows Zn2Fe2(CN)4 rectangular structure, in which FeIII and ZnII ions are alternately bridged by the cyanide groups. Intramolecular ferromagnetic couplings are observed for complexes 3-5, and they have S = 5/2, 5/2, and 6 ground states and appreciable magnetic anisotropies with negative D values equal to -0.49, -2.39, and -0.39 cm-1, respectively.     

A comparative mechanistic study of the reversible binding of NO to a water-soluble octa-cationic Fe(III) porphyrin complex

The water-soluble, non-mu-oxo dimer-forming porphyrin, [5,10,15,20-tetrakis-4'-t-butylphenyl-2',6'-bis-(N-methylene-(4'-t-butylpyridinium))porphyrinato]iron(III) octabromide, (P(8+))Fe(III), with eight positively charged substituents in the ortho positions of the phenyl rings, was characterized by UV-vis and 1H NMR spectroscopy and 17O NMR water-exchange studies in aqueous solution. Spectrophotometric titrations of (P(8+))Fe(III) indicated a pKa1 value of 5.0 for coordinated water in (P(8+))Fe(III)(H2O)2. The monohydroxo-ligated (P(8+))Fe(III)(OH)(H2O) formed at 5 < pH < 12 has a weakly bound water molecule that undergoes an exchange reaction, k(ex) = 2.4 x 10(6) s(-1), significantly faster than water exchange on (P(8+))Fe(III)(H2O)2, viz. k(ex) = 5.5 x 10(4) s(-1) at 25 degrees C. The porphyrin complex reacts with nitric oxide to yield the nitrosyl adduct, (P(8+))Fe(II)(NO+)(L) (L = H2O or OH-). The diaqua-ligated (P(8+))Fe(III)(H2O)2 binds and releases NO according to a dissociatively activated mechanism, analogous to that reported earlier for other (P)Fe(III)(H2O)2 complexes. Coordination of NO to (P(8+))Fe(III)(OH)(H2O) at high pH follows an associative mode, as evidenced by negative deltaS(double dagger)(on) and deltaV(double dagger)(on) values measured for this reaction. The observed ca. 10-fold decrease in the NO binding rate on going from six-coordinate (P(8+))Fe(III)(H2O)2 (k(on) = 15.1 x 10(3) M(-1) s(-1)) to (P(8+))Fe(III)(OH)(H2O) (k(on) = 1.56 x 10(3) M(-1) s(-1) at 25 degrees C) is ascribed to the different nature of the rate-limiting step for NO binding at low and high pH, respectively. The results are compared with data reported for other water-soluble iron(III) porphyrins with positively and negatively charged meso substituents. Influence of the porphyrin periphery on the dynamics of reversible NO binding to these (P)Fe(III) complexes as a function of pH is discussed on the basis of available experimental data.     

Fe(Phen)(CN)4]-: a versatile building block for the design of heterometallic systems. Crystal structures and magnetic properties of PPh4[Fe(Phen)(CN)4]*2H2O and [[Fe(Phen)(CN)4]2M(H2O)2]*4H2O [Phen = 1,10-phenanthroline; M = Mn(II) and Zn(II)

The mononuclear PPh4[Fe(phen)(CN)4]*2H2O (1) complex and the cyanide-bridged bimetallic [[Fe(phen)(CN)4]2M(H2O)2]*4H2O compounds [M = Mn(II) (2) and Zn(II) (3); phen = 1,10-phenanthroline; PPh4 = tetraphenylphosphonium cation] have been synthesized and structurally and magnetically characterized. Complex 1 crystallizes in the monoclinic system, space group P2(1)/c, with a = 9.364(4) A, b = 27.472(5) A, c = 14.301(3) A, beta = 97.68(2) degrees, and Z = 4. Complexes 2 and 3 are isostructural and they crystallize in the monoclinic system, space group P2(1)/n, with a = 7.5292(4) A, b = 15.6000(10) A, c = 15.4081(9) A, beta = 93.552(2) degrees, and Z = 2 for 2 and a = 7.440(1) A, b = 15.569(3) A, c = 15.344(6) A, beta = 93.63(2) degrees, and Z = 2 for 3. The structure of complex 1 is made up of mononuclear [Fe(phen)(CN)4]- anions, tetraphenyphosphonium cations, and water molecules of crystallization. The iron(III) is hexacoordinate with two nitrogen atoms of a chelating phen (2.018(6) and 2.021(6) A for Fe-N) and four carbon atoms of four terminal cyanide groups (Fe-C bond lengths varying in the range 1.906(8)-1.95(1) A) building a distorted octahedron around the metal atom. The structure of complexes 2 and 3 consists of neutral double zigzag chains of formula [[Fe(phen)(CN)4]2M(H2O)2] and crystallization water molecules. The [Fe(phen)(CN)4]- entity of 1 is present in 2 and 3 acting as a bridging ligand toward M(H2O)2 units [M = Mn(II) (2) and Zn(II) (3)] through two cyanide groups in cis positions, the other two cyanide remaining terminal. Two water molecules in trans positions and four cyanide-nitrogen atoms from four [Fe(phen)(CN)4]- units build a distorted octahedral surrounding Mn(II) (2) and Zn(II) (3). The M-O bond lengths are 2.185(3) (2) and 2.105(3) A (3), whereas the M-N bond distances vary in the ranges 2.210(3)-2.258(3) A (2) and 2.112(3)-2.186(3) A (3). The structure of the [Fe(phen)(CN)4]- complex ligand in 2 and 3 is as in 1. The shorter intrachain Fe-M distances through bridging cyano are 5.245(5) and 5.208(5) A in 2 and 5.187(1) and 5.132(1) A in 3. The magnetic properties of 1-3 have been investigated in the temperature range 2.0-300 K. Complex 1 is a low-spin iron(III) complex with an appreciable orbital contribution. The magnetic properties of 3 correspond to the sum of two magnetically isolated spin triplets, the magnetic coupling between the low-spin iron(III) centers through the -CN-Zn-NC- bridging skeleton (iron-iron separation larger than 10.2 A) being negligible. More interestingly, 2 exhibits one-dimensional ferrimagnetic behavior due to the noncompensation of the local interacting spins (S(Mn) = 5/2 and S(Fe) = 1/2) which interact antiferromagnetically through bridging cyano groups. A comparison between the magnetic properties of the isostructural compounds 2 and 3 allow us to check the antiferromagnetic coupling in 2.     

4,2-Ribbon like ferromagnetic cyano-bridged Fe(III)2Ni(II) chains: a magneto-structural study

The low-spin iron(III) complex AsPh(4)[Fe(III)(bpy)(CN)(4)].CH(3)CN (1) [AsPh(4) = tetraphenylarsonium cation] and the heterobimetallic chains [{Fe(III)(L)(CN)(4)}(2)Ni(II)(H(2)O)(2)].4H(2)O with L = bpy (2) and phen (3) [bpy = 2,2'-bipyridine and phen = 1,10-phenanthroline] have been prepared and their structures determined by X-ray diffraction methods. The structure of 1 consists of mononuclear [Fe(bpy)(CN)(4)](-) anions, tetraphenylarsonium cations and acetonitrile molecules of crystallization. The iron(III) is hexacoordinated with two nitrogen atoms of the bidentate bpy and four carbon atoms of four terminal cyanide groups building a distorted octahedral surrounding around the metal atom. 2 and 3 are isomorphous compounds whose structure is made up of neutral 4,2-ribbon like bimetallic chains of formula [{Fe(III)(L)(CN)(4)}(2)Ni(II)(H(2)O)(2)] where the [Fe(III)(L)(CN)(4)](-) unit acts as a bis-monodentate bridging ligand toward the trans-diaquanickel(II) units through two of its four cyanide groups in cis positions. The chains exhibit two orientations in the unit cell and they interact with each other through hydrogen bonds involving the coordination and crystallization water molecules together with the uncoordinated cyanide nitrogen atoms of the [Fe(L)(CN)(4)](-) units. Compounds 2 and 3 behave as ferromagnetic Fe(III)(2)Ni(II) chains which interact ferromagnetically at very low temperatures in the case of 2, whereas metamagnetic-like behaviour is observed for with a critical field (H(c)) around 200 G. For H > H(c) the ferromagnetic Fe(III)(2)Ni(II) chains of 3 exhibit a frequency dependence of the out-of-phase ac susceptibility signal at T < 3.5 K.