Thermodynamics, kinetics, and mechanism of the stepwise dissociation and formation of Tris(L-lysinehydroxamato)iron(III) in aqueous acid

pK(a) values for the hydroxamic acid, alpha-NH(3)(+), and epsilon-NH(3)(+) groups of L-lysinehydroxamic acid (LyHA, H(3)L(2+)) were found to be 6.87, 8.89, and 10.76, respectively, in aqueous solution (I = 0.1 M, NaClO(4)) at 25 degrees C. O,O coordination to Fe(III) by LyHA is supported by H(+) stoichiometry, UV-vis spectral shifts, and a shift in nu(CO) from 1648 to 1592 cm(-1) upon formation of mono(L-lysinehydroxamato)tetra(aquo)iron(III) (Fe(H(2)L)(H(2)O)(4)(4+)). The stepwise formation of tris(L-lysinehydroxamato)iron(III) from Fe(H(2)O)(6)(3+) and H(3)L(2+) was characterized by spectrophotometric titration, and the values for log beta(1), log beta(2), and log beta(3) are 6.80(9), 12.4(2), and 16.1(2), respectively, at 25 degrees C and I = 2.0 M (NaClO(4)). Stopped-flow spectrophotometry was used to study the proton-driven stepwise ligand dissociation kinetics of tris(L-lysinehydroxamato)iron(III) at 25 degrees C and I = 2.0 M (HClO(4)/NaClO(4)). Defining k(n) and k(-n) as the stepwise ligand dissociation and association rate constants and n as the number of bound LyHA ligands, k(3), k(-3), k(2), k(-2), k(1), and k(-1) are 3.0 x 10(4), 2.4 x 10(1), 3.9 x 10(2), 1.9 x 10(1), 1.4 x 10(-1), and 1.2 x 10(-1) M(-1) s(-1), respectively. These rate and equilibrium constants are compared with corresponding constants for Fe(III) complexes of acetohydroxamic acid (AHA) and N-methylacetohydroxamic acid (NMAHA) in the form of a linear free energy relationship. The role of electrostatics in these complexation reactions to form the highly charged Fe(LyHA)(3)(6+) species is discussed, and an interchange mechanism mediated by charge repulsion is presented. The reduction potential for tris(L-lysinehydroxamato)iron(III) is -214 mV (vs. NHE), and a comparison to other hydroxamic acid complexes of Fe(III) is made through a correlation between E(1/2) and pFe.     

Mechanistic studies of nitric oxide reactions with water soluble iron(II), cobalt(II), and iron(III) porphyrin complexes in aqueous solutions: implications for biological activity.

The reactions of nitric oxide and carbon monoxide with water soluble iron and cobalt porphyrin complexes were investigated over the temperature range 298-318 K and the hydrostatic pressure range 0.1-250 MPa [porphyrin ligands: TPPS = tetra-meso-(4-sulfonatophenyl)porphinate and TMPS = tetra-meso-(sulfonatomesityl)porphinate]. Large and positive DeltaS(double dagger) and DeltaV(double dagger) values were observed for NO binding to and release from iron(III) complexes Fe(III)(TPPS) and Fe(III)(TMPS) consistent with a dissociative ligand exchange mechanism where the lability of coordinated water dominates the reactivity with NO. Small positive values for Delta and Delta for the fast reactions of NO with the iron(II) and cobalt(II) analogues (k(on) = 1.5 x 10(9) and 1.9 x 10(9) M(-1) s(-1) for Fe(II)(TPPS) and Co(II)(TPPS), respectively) indicate a mechanism dominated by diffusion processes in these cases. However, reaction of CO with the Fe(II) complexes (k(on) = 3.6 x 10(7) M(-1) s(-1) for Fe(II)(TPPS)) displays negative Delta and Delta values, consistent with a mechanism dominated by activation rather than diffusion terms. Measurements of NO dissociation rates from Fe(II)(TPPS)(NO) and Co(II)(TPPS)(NO) by trapping free NO gave k(off) values of 6.3 x 10(-4) s(-1) and 1.5 x 10(-4) s(-1). The respective M(II)(TPPS)(NO) formation constants calculated from k(on)/k(off) ratios were 2.4 x 10(12) and 1.3 x 10(13) M(-1), many orders of magnitude larger than that (1.1 x 10(3) M(-1)) for the reaction of Fe(III)(TPPS) with NO.     

Fe(bipy)(CN)(4)](-) as a versatile building block for the design of heterometallic systems: synthesis, crystal structure, and magnetic properties of PPh(4)[Fe(III)(bipy)(CN)(4)] x H(2)O, [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] x 4H(2)O, and [[Fe(III)(bipy)(CN)(4)](2)Zn(II)] x 2H(2)O [bipy = 2,2'-Bipyridine; M = Mn and Zn

The new cyano complexes of formulas PPh(4)[Fe(III)(bipy)(CN)(4)] x H(2)O (1), [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] x 4H(2)O with M = Mn (2) and Zn (3), and [[Fe(III)(bipy)(CN)(4)](2)Zn(II)] x 2H(2)O (4) [bipy = 2,2'-bipyridine and PPh(4) = tetraphenylphosphonium cation] have been synthesized and structurally characterized. The structure of complex 1 is made up of mononuclear [Fe(bipy)(CN)(4)](-) anions, tetraphenyphosphonium cations, and water molecules of crystallization. The iron(III) is hexacoordinated with two nitrogen atoms of a chelating bipy and four carbon atoms of four terminal cyanide groups, building a distorted octahedron around the metal atom. The structure of complexes 2 and 3 consists of neutral centrosymmetric [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] heterotrinuclear units and crystallization water molecules. The [Fe(bipy)(CN)(4)](-) entity of 1 is present in 2 and 3 acting as a monodentate ligand toward M(H(2)O)(4) units [M = Mn(II) (2) and Zn(II) (3)] through one cyanide group, the other three cyanides remaining terminal. Four water molecules and two cyanide nitrogen atoms from two [Fe(bipy)(CN)(4)](-) units in trans positions build a distorted octahedron surrounding Mn(II) (2) and Zn(II) (3). The structure of the [Fe(phen)(CN)(4)](-) complex ligand in 2 and 3 is close to that of the one in 1. The intramolecular Fe-M distances are 5.126(1) and 5.018(1) A in 2 and 3, respectively. 4 exhibits a neutral one-dimensional polymeric structure containing two types of [Fe(bipy)(CN)(4)](-) units acting as bismonodentate (Fe(1)) and trismonodentate (Fe(2)) ligands versus the divalent zinc cations through two cis-cyanide (Fe(1)) and three fac-cyanide (Fe(2)) groups. The environment of the iron atoms in 4 is distorted octahedral as in 1-3, whereas the zinc atom is pentacoordinated with five cyanide nitrogen atoms, describing a very distorted square pyramid. The iron-zinc separations across the single bridging cyanides are 5.013(1) and 5.142(1) A at Fe(1) and 5.028(1), 5.076(1), and 5.176(1) A at Fe(2). The magnetic properties of 1-3 have been investigated in the temperature range 2.0-300 K. 1 is a low-spin iron(III) complex with an important orbital contribution. The magnetic properties of 3 correspond to the sum of two magnetically isolated spin triplets, the antiferromagnetic coupling between the low-spin iron(III) centers through the -CN-Zn-NC- bridging skeleton (iron-iron separation larger than 10 A) being very weak. More interestingly, 2 exhibits a significant intramolecular antiferromagnetic interaction between the central spin sextet and peripheral spin doublets, leading to a low-lying spin quartet.     

Kinetics and mechanism of iron(III)-nitrilotriacetate complex reactions with phosphate and acetohydroxamic acid

The kinetics and mechanism of the substitution of coordinated water in nitrilotriacetate complexes of iron(III) (Fe(NTA)(OH(2))(2) and Fe(NTA)(OH(2))(OH)(-)) by phosphate (H(2)PO(4)(-) and HPO(4)(2)(-)) and acetohydroxamic acid (CH(3)C(O)N(OH)H) were investigated. The phosphate reactions were found to be pH dependent in the range of 4-8. Phosphate substitution rates are independent of the degree of phosphate protonation, and pH dependence is due to the difference in reactivity of Fe(NTA)(OH(2))(2) (k = 3.6 x 10(5) M(-)(1) s(-)(1)) and Fe(NTA)(OH(2))(OH)(-) (k = 2.4 x 10(4) M(-)(1) s(-)(1)). Substitution by acetohydroxamic acid is insensitive to pH in the range of 4-5.2, and Fe(NTA)(OH(2))(2) and Fe(NTA)(OH(2))(OH)(-) react at equivalent rates (k = 4.2 x 10(4) and 3.8 x 10(4) M(-)(1) s(-)(1), respectively). Evidence for acid-dependent and acid-independent back-reactions was obtained for both the phosphate and acetohydroxamate complexes. Reactivity patterns were analyzed in the context of NTA labilization of coordinated water, and outer-sphere electrostatic and H-bonding influences were analyzed in the precursor complex (K(os)).     

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.     

A supramolecular receptor of diatomic molecules (O2, CO, NO) in aqueous solution

A per-O-methylated beta-cyclodextrin dimer, Py2CD, was conveniently prepared via two steps: the Williamson reaction of 3,5-bis(bromomethyl)pyridine and beta-cyclodextrin (beta-CD) yielding 2A,2'A-O-[3,5-pyridinediylbis(methylene)bis-beta-cyclodextrin (bisCD) followed by the O-methylation of all the hydroxy groups of the bisCD. Py2CD formed a very stable 1:1 complex (Fe(III)PCD) with [5,10,15,20-tetrakis(p-sulfonatophenyl)porphinato]iron(III) (Fe(III)TPPS) in aqueous solution. Fe(III)PCD was reduced with Na2S2O4 to afford the Fe (II)TPPS/Py2CD complex (Fe(II)PCD). Dioxygen was bound to Fe(II)PCD, the P(1/2)(O2) values being 42.4 +/- 1.6 and 176 +/- 3 Torr at 3 and 25 degrees C, respectively. The k(on)(O2) and k(off)(O2) values for the dioxygen binding were determined to be 1.3 x 10(7) M(-1) s(-1) and 3.8 x 10(3) s(-1), respectively, at 25 degrees C. Although the dioxygen adduct was not very stable (K(O2) = k(on)(O2)/k(off)(O2) = 3.4 x 10(3) M(-1)), no autoxidation of the dioxygen adduct of Fe(II)PCD to Fe(III)PCD was observed. These results suggest that the encapsulation of Fe (II)TPPS by Py2CD strictly inhibits not only the extrusion of dioxygen from the cyclodextrin cage but also the penetration of a water molecule into the cage. The carbon monoxide affinity of Fe(II)PCD was much higher than the dioxygen affinity; the P(1/2)(CO), k(on)(CO), k(off)(CO), and K(CO) values being (1.6 +/- 0.2) x 10(-2) Torr, 2.4 x 10(6) M(-1) s(-1), 4.8 x 10(-2) s(-1), and 5.0 x 10(7) M(-1), respectively, at 25 degrees C. Fe(II)PCD also bound nitric oxide. The rate of the dissociation of NO from (NO)Fe(II)PCD ((5.58 +/- 0.42) x 10(-5) s(-1)) was in good agreement with the maximum rate ((5.12 +/- 0.18) x 10(-5) s(-1)) of the oxidation of (NO)Fe(II)PCD to Fe(III)PCD and NO3(-), suggesting that the autoxidation of (NO)Fe(II)PCD proceeds through the ligand exchange between NO and O2 followed by the rapid reaction of (O2)Fe(II)PCD with released NO, affording Fe(II)PCD and the NO3(-) anion inside the cyclodextrin cage.     

Chlorine dioxide reduction by aqueous iron(II) through outer-sphere and inner-sphere electron-transfer pathways

The reduction of ClO(2) to ClO(2)(-) by aqueous iron(II) in 0.5 M HClO(4) proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M(-1) s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO(2)(2+) that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M(-1) s(-1)) is controlled by ClO(2) substitution of a coordinated water to give an inner-sphere complex between ClO(2) and Fe(II) that very rapidly transfers an electron to give (Fe(III)(ClO(2)(-))(H(2)O)(5)(2+))(IS). The composite activation parameters for the ClO(2)/Fe(aq)(2+) reaction (inner-sphere + outer-sphere) are the following: DeltaH(r)++ = 40 kJ mol(-1); DeltaS(r)++ = 1.7 J mol(-1) K(-1). The Fe(III)ClO(2)(2+) inner-sphere complex dissociates to give Fe(aq)(3+) and ClO(2)(-) (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)++= 76 kJ mol(-1); DeltaS(d)++= 32 J K(-1) mol(-1). The reaction of Fe(aq)(2+) with ClO(2)(-) is first order in each species with a second-order rate constant of k(ClO2)- = 2.0 x 10(3) M(-1) s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO(2) in H(2)SO(4) medium ([H(+)] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H(2)SO(4) are DeltaH(Cl(III))++ = 41 kJ mol(-1) and DeltaS(Cl(III))++ = 48 J mol(-1) K(-1).     

Mononuclear cyano- and hydroxo-complexes of iron(III)

A detailed investigation of the iron(III)-cyanide and iron(III)-hydroxide systems has been made in NaClO(4) media at 25 degrees C, using combined UV-vis spectrophotometric and pH-potentiometric titrations. For the Fe(III)/OH- system, use of low total Fe(III) concentrations (< or =10 microM) and a wide pH range (0 < or = pH < or = 12.7) enabled detection of six mononuclear complexes, corresponding to the following equilibria: Fe3+(aq)+rH2O<=>Fe(OH)r(3-r)+(aq) + rH(+)(aq), where r = 1-6 with stability constants (log *beta 1r) of -2.66, -7.0, -12.5, -20.7, -30.8, and -43.4, respectively, at I = 1 M (NaClO(4)). It was also found to be possible to measure, for the first time, stability constants for most of the following equilibria: Fe3+(aq)+qCN-(aq)<=>Fe(CN)q(3-q)+(aq), despite a plethora of complicating factors. Values of log beta(1q) = 8.5, 15.8, 23.1, and 38.8 were obtained at I = 1.0 M (NaClO(4)) for q = 1-3 and 6, respectively. No reliable evidence could be obtained for the intermediate (q = 4 or 5) complexes. Similar results were obtained for both systems at I = 0.5 M(NaClO(4)). Spectra for the individual mononuclear complexes detected for Fe(III) with OH- and CN- are reported. Attempted measurements on the Fe(II)/CN- system were unsuccessful, but values of log beta(16)(Fe(CN)(6)(4-)) = 31.8 and log beta(15)(Fe(CN)(5)(3-) approximately 24 were estimated from well established electrode potential and other data.     

Membrane dynamics of the amphiphilic siderophore, acinetoferrin

Acinetobacter haemolyticus is an antibiotic resistant, pathogenic bacterium responsible for an increasing number of hospital infections. Acinetoferrin (Af), the amphiphilic siderophore isolated from this organism, contains two unusual trans-2-octenoyl hydrocarbon chains reminiscent of a phospholipid structural motif. Here, we have investigated the membrane affinity of Af and its iron complex, Fe-Af, using small and large unilamellar phospholipid vesicles (SUV and LUV) as model membranes. Af shows a high membrane affinity with a partition coefficient, K(x)= 6.8 x 10(5). Membrane partitioning and trans-membrane flip-flop of Fe-Af have also been studied via fluorescence quenching of specifically labeled vesicle leaflets and (1)H NMR line-broadening techniques. Fe-Af is found to rapidly redistribute between lipid and aqueous phases with dissociation/partitioning rates of k(off) = 29 s(-1) and k(on) = 2.4 x 10(4) M(-1) s(-1), respectively. Upon binding iron, the membrane affinity of Af is reduced 30-fold to K'(x) = 2.2 x 10(4) for Fe-Af. In addition, trans-membrane flip-flop of Fe-Af occurs with a rate constant, k(p) = 1.2 x 10(-3) s(-1), with egg-PC LUV and a half-life time around 10 min with DMPC SUV. These properties are due to the phospholipid-like conformation of Af and the more extended conformation of Fe-Af that is enforced by iron binding. Remarkable similarities and differences between Af and another amphiphilic siderophore, marinobactin E, are discussed. The potential biological implications of Af and Fe-Af are also addressed. Our approaches using inner- and outer-leaflet-labeled fluorescent vesicles and (1)H NMR line-broadening techniques to discern Af-mediated membrane partitioning and trans-membrane diffusion are amenable to similar studies for other paramagnetic amphiphiles.     

Iron porphyrin-cyclodextrin supramolecular complex as a functional model of myoglobin in aqueous solution

The 1:1 inclusion complex of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato iron(II) (Fe(II)TPPS) and an O-methylated beta-cyclodextrin dimer having a pyridine linker (1) binds dioxygen reversibly in aqueous solution. The O2 adduct was very stable (t(1/2) = 30.1 h) at pH 7.0 and 25 degrees C. ESI-MS and NMR spectroscopic measurements and molecular mechanics (MM) calculations indicated the inclusion of the sulfonatophenyl groups at the 5- and 15-positions of Fe(III)TPPS or Fe(II)TPPS into two cyclodextrin moieties of 1 to form a supramolecular 1:1 complex (hemoCD1 for the Fe(II)TPPS complex), whose iron center is completely covered by two cyclodextrin moieties. Equilibrium measurements and laser flash photolysis provided the affinities ( and ) and rate constants for O2 and CO binding of hemoCD1 (k(O2)(on), k(O2)(off), k(CO)(on), and k(CO)(off)). The CO affinity relative to the O2 affinity of hemoCD1 was abnormally high. Although resonance Raman spectra suggested weak back-bonding of d(pi)(Fe) --> pi(CO) and hence a weak CO-Fe bond, the CO adduct of hemoCD1 was very stable. The hydrophobic CO molecule dissociated from CO-hemoCD1 hardly breaks free from a shallow cleft in hemoCD1 surrounded by an aqueous bulk phase leading to fast rebinding of CO to hemoCD1. Isothermal titration calorimetry furnished the association constant (K(O2)), DeltaH degrees , and DeltaS degrees for O2 association to be (2.71 +/- 0.51) x 10(4) M(-1), -65.2 +/- 4.4 kJ mol(-1), and -133.9 +/- 16.1 J mol(-1) K(-1), respectively. The autoxidation of oxy-hemoCD1 was accelerated by H+ and OH-. The inorganic anions also accelerated the autoxidation of oxy-hemoCD1. The O2-Fe(II) bond is equivalent to the O2.--Fe(III) bond, which is attacked by the inorganic anions or the water molecule to produce met-hemoCD1 and a superoxide anion.     

Efficient energy transfer via the cyanide bridge in dinuclear complexes containing Ru(II) polypyridine moieties

We report the synthesis, structure and properties of the cyanide-bridged dinuclear complex ions [Ru(L)(bpy)(μ-NC)M(CN)(5)](2-/-) (L = tpy, 2,2';6',2'-terpyridine, or tpm, tris(1-pyrazolyl)methane, bpy = 2,2'-bipyridine, M = Fe(II), Fe(III), Cr(III)) and the related monomers [Ru(L)(bpy)X](2+) (X = CN(-) and NCS(-)). All the monomeric compounds are weak MLCT emitters (λ = 650-715 nm, ? ≈ 10(-4)). In the Fe(II) and Cr(III) dinuclear systems, the cyanide bridge promotes efficient energy transfer between the Ru-centered MLCT state and a Fe(II)- or Cr(III)-centered d-d state, which results either in a complete quenching of luminescence or in a narrow red emission (λ ≈ 820 nm, ? ≈ 10(-3)) respectively. In the case of Fe(III) dinuclear systems, an electron transfer quenching process is also likely to occur.     

Electron and hydrogen-atom self-exchange reactions of iron and cobalt coordination complexes

Reported here are self-exchange reactions between iron 2,2'-bi(tetrahydro)pyrimidine (H(2)bip) complexes and between cobalt 2,2'-biimidazoline (H(2)bim) complexes. The (1)H NMR resonances of [Fe(II)(H(2)bip)(3)](2+) are broadened upon addition of [Fe(III)(H(2)bip)(3)](3+), indicating that electron self-exchange occurs with k(Fe,e)(-) = (1.1 +/- 0.2) x 10(5) M(-1) s(-1) at 298 K in CD(3)CN. Similar studies of [Fe(II)(H(2)bip)(3)](2+) plus [Fe(III)(Hbip)(H(2)bip)(2)](2+) indicate that hydrogen-atom self-exchange (proton-coupled electron transfer) occurs with k(Fe,H.) = (1.1 +/- 0.2) x 10(4) M(-1) s(-1) under the same conditions. Both self-exchange reactions are faster at lower temperatures, showing small negative enthalpies of activation: DeltaH++(e(-)) = -2.1 +/- 0.5 kcal mol(-1) (288-320 K) and DeltaH++(H.) = -1.5 +/- 0.5 kcal mol(-1) (260-300 K). This behavior is concluded to be due to the faster reaction of the low-spin states of the iron complexes, which are depopulated as the temperature is raised. Below about 290 K, rate constants for electron self-exchange show the more normal decrease with temperature. There is a modest kinetic isotope effect on H-atom self-exchange of 1.6 +/- 0.5 at 298 K that is close to that seen previously for the fully high-spin iron biimidazoline complexes.(12) The difference in the measured activation parameters, E(a)(D) - E(a)(H), is -1.2 +/- 0.8 kcal mol(-1), appears to be inconsistent with a semiclassical view of the isotope effect, and suggests extensive tunneling. Reactions of [Co(H(2)bim)(3)](2+)-d(24) with [Co(H(2)bim)(3)](3+) or [Co(Hbim)(H(2)bim)(2)](2+) occur with scrambling of ligands indicating inner-sphere processes. The self-exchange rate constant for outer-sphere electron transfer between [Co(H(2)bim)(3)](2+) and [Co(H(2)bim)(3)](3+) is estimated to be 10(-)(6) M(-1) s(-1) by application of the Marcus cross relation. Similar application of the cross relation to H-atom transfer reactions indicates that self-exchange between [Co(H(2)bim)(3)](2+) and [Co(Hbim)(H(2)bim)(2)](2+) is also slow, < or =10(-3) M(-1) s(-1). The slow self-exchange rates for the cobalt complexes are apparently due to their interconverting high-spin [Co(II)(H(2)bim)(3)](2+) with low-spin Co(III) derivatives.     

Chiral, Three-Dimensional Supramolecular Compounds: Homo- and Bimetallic Oxalate- and 1,2-Dithiooxalate-Bridged Networks. A Structural and Photophysical Study

In analogy to the [M(II)(bpy)(3)](2+) cations, where M(II) is a divalent transition-metal and bpy is 2,2'-bipyridine, the tris-chelated [M(III)(bpy)(3)](3+) cations, where M(III) is Cr(III) or Co(III), induce the crystallization of chiral, anionic three-dimensional (3D) coordination polymers of oxalate-bridged (&mgr;-ox) metal complexes with stoichiometries [M(II)(2)(ox)(3)](n)()(2)(n)()(-) or [M(I)M(III)(ox)(3)](n)()(2)(n)()(-). The tripositive charge is partially compensated by inclusion of additional complex anions like ClO(4)(-), BF(4)(-), or PF(6)(-) which are encapsulated in cubic shaped cavities formed by the bipyridine ligands of the cations. Thus, an elaborate structure of cationic and anionic species within a polymeric anionic network is realized. The compounds isolated and structurally characterized include [Cr(III)(bpy)(3)][ClO(4)] [NaCr(III)(ox)(3)] (1), [Cr(III)(bpy)(3)][ClO(4)][Mn(II)(2)(ox)(3)] (2), [Cr(III)(bpy)(3)][BF(4)] [Mn(II)(2)(ox)(3)] (3), [Co(III)(bpy)(3)][PF(6)][NaCr(III)(ox)(3)] (4). Crystal data: 1, cubic, P2(1)3, a = 15.523(4) ?, Z = 4; 2, cubic, P4(1)32, a = 15.564(3) ?, Z = 4; 3, cubic, P4(1)32, a = 15.553(3) ?, Z = 4; 4, cubic, P2(1)3, a = 15.515(3) ?, Z = 4. Furthermore, it seemed likely that 1,2-dithiooxalate (dto) could act as an alternative to the oxalate bridging ligand, and as a result the compound [Ni(II)(phen)(3)][NaCo(III)(dto)(3)].C(3)H(6)O (5) has successfully been isolated and structurally characterized. Crystal data: 5, orthorhombic, P2(1)2(1)2(1), a = 16.238(4) ?, b = 16.225(4) ?, c = 18.371(5) ?, Z = 4. In addition, the photophysical properties of compound 1 have been investigated in detail. In single crystal absorption spectra of [Cr(III)(bpy)(3)][ClO(4)][NaCr(III)(ox)(3)] (1), the spin-flip transitions of both the [Cr(bpy)(3)](3+) and the [Cr(ox)(3)](3)(-) chromophores are observed and can be clearly distinguished. Irradiating into the spin-allowed (4)A(2) --> (4)T(2) absorption band of [Cr(ox)(3)](3)(-) results in intense luminescence from the (2)E state of [Cr(bpy)(3)](3+) as a result of rapid energy transfer processes.     

Do S,S'-coordinated o-dithiobenzosemiquinonate(1-) radicals exist in coordination compounds? the [AuIII(1,2-C6H4S2)2]1 -/0 couple

The square planar, light-green, diamagnetic complex [N(n-Bu)(4)][Au(III)(L(t)()(-)(Bu))(2)] (1) reacts with iodine in acetone affording the neutral paramagnetic species [Au(L(t)()(-)(Bu))(2)] (1a) (S = (1)/(2)) where H(2)[L(t)()(-)(Bu)] represents the ligand 3,5-di-tert-butyl-1,2-benzenedithiol. The corresponding complexes containing the unsubstituted ligand H(2)[L], 1,2-benzenedithiol, namely [N(n-Bu)(3)H][Au(L)(2)] (2) and [Au(L)(2)] (2a), have also been prepared and characterized by X-ray crystallography; the structure of the latter has been reported in ref 10. (197)Au M?ssbauer spectra of 1 and 1a clearly show that the one-electron oxidation is ligand-centered and does not involve the formation of Au(IV) (d(7)). The spectroscopic features of the ligand mixed-valent species 1a were determined by UV-vis, EPR, and IR spectroscopy which allows the detection of S,S-coordinated 1,2-dithiobenzosemiquinonate(1-) radicals in coordination compounds.     

Iron(III) complex of a crown ether-porphyrin conjugate and reversible binding of superoxide to its Iron(II) form

The synthesis and characterization of the Fe(III) complex of a novel crown ether-porphyrin conjugate, 52-N-(4-aza-18-crown-6)methyl-54,104,154,204-tetra-tert-butyl-56-methyl-5,10,15,20-tetraphenylporphyrin (H2Porph), as well as the corresponding hydroxo, dimeric, Fe(II), and peroxo species are reported. The crystal structure of [FeIII(Porph)Cl].H3O+.FeCl4-.C6H6.EtOH is also reported. [FeIII(Porph)(DMSO)2]+ and K[FeIII(Porph)(O22-)] are high-spin species (M?ssbauer data: delta = 0.38 mm s(-1), DeltaEq = 0.83 mm s(-1) and delta = 0.41 mm s(-1), DeltaEq = 0.51 mm s(-1), respectively), whereas in a solution of reduced [FeIII(Porph)(DMSO)2]+ complex the low-spin [FeII(Porph)(DMSO)2] (delta = 0.44 mm s(-1), DeltaEq = 1.32 mm s(-1)) and high-spin [FeII(Porph)(DMSO)] (delta = 1.27 mm s(-1), DeltaEq = 3.13 mm s(-1)) iron(II) species are observed. The reaction of [FeIII(Porph)(DMSO)2]+ with KO2 in DMSO has been investigated. The first reaction step, involving reduction to [FeII(Porph)(DMSO)2], was not investigated in detail because of parallel formation of an Fe(III)-hydroxo species. The kinetics and thermodynamics of the second reaction step, reversible binding of superoxide to the Fe(II) complex and formation of an Fe(III)-peroxo species, were studied in detail (by stopped-flow time-resolved UV/vis measurements in DMSO at 25 degrees C), resulting in kon = 36 500 +/- 500 M(-1) s(-1), koff = 0.21 +/- 0.01 s(-1) (direct measurements using an acid as a superoxide scavenger), and KO2- = (1.7 +/- 0.2) x 10(5) (superoxide binding constant kinetically obtained as kon/koff), (1.4 +/- 0.1) x 10(5), and (9.0 +/- 0.1) x 10(4) M(-1) (thermodynamically obtained in the absence and in the presence of 0.1 M NBu4PF6, respectively). Temperature-dependent kinetic measurements for kon (-40 to 25 degrees C in 3:7 DMSO/CH3CN mixture) yielded the activation parameters DeltaH = 61.2 +/- 0.9 kJ mol(-1) and DeltaS = +48 +/- 3 J K(-1) mol(-1). The observed reversible binding of superoxide to the metal center and the obtained kinetic and thermodynamic parameters are unique. The finding that fine-tuning of the proton concentration can cause the Fe(III)-peroxo species to release O2- and form an Fe(II) species is of biological interest, since this process might occur under very specific physiological conditions.     

Synthesis and characterization of iron(III)-substituted, dimeric polyoxotungstates, [Fe(4)(H(2)O)(10)(beta-XW(9)O(33))(2)](n-) (n = 6, X = As(III), Sb(III); n = 4, X = Se(IV), Te(IV))

Interaction of the lacunary [alpha-XW(9)O(33)](9-) (X = As(III), Sb(III)) with Fe(3+) ions in acidic, aqueous medium leads to the formation of dimeric polyoxoanions, [Fe(4)(H(2)O)(10)(beta-XW(9)O(33))(2)](6-) (X = As(III), Sb(III)) in high yield. X-ray single-crystal analyses were carried out on Na(6)[Fe(4)(H(2)O)(10)(beta-AsW(9)O(33))(2)] x 32H(2)O, which crystallizes in the monoclinic system, space group C2/m, with a = 20.2493(18) A, b = 15.2678(13) A, c = 16.0689(14) A, beta = 95.766(2) degrees, and Z = 2; Na(6)[Fe(4)(H(2)O)(10)(beta-SbW(9)O(33))(2)] x 32H(2)O is isomorphous with a = 20.1542(18) A, b = 15.2204(13) A, c = 16.1469(14) A, and beta = 95.795(2) degrees. The selenium and tellurium analogues are also reported, [Fe(4)(H(2)O)(10)(beta-XW(9)O(33))(2)](4-) (X = Se(IV), Te(IV)). They are synthesized from sodium tungstate and a source of the heteroatom as precursors. X-ray single-crystal analysis was carried out on Cs(4)[Fe(4)(H(2)O)(10)(beta-SeW(9)O(33))(2)] x 21H(2)O, which crystallizes in the triclinic system, space group P macro 1, with a = 12.6648(10) A, b = 12.8247(10) A, c = 16.1588(13) A, alpha = 75.6540(10) degrees, beta = 87.9550(10) degrees, gamma = 64.3610(10) gamma, and Z = 1. All title polyanions consist of two (beta-XW(9)O(33)) units joined by a central pair and a peripheral pair of Fe(3+) ions leading to a structure with idealized C(2h) symmetry. It was also possible to synthesize the Cr(III) derivatives [Cr(4)(H(2)O)(10)(beta-XW(9)O(33))(2)](6-) (X = As(III), Sb(III)), the tungstoselenates(IV) [M(4)(H(2)O)(10)(beta-SeW(9)O(33))(2)]((16)(-)(4n)-) (M(n+) = Cr(3+), Mn(2+), Co(2+), Ni(2+), Zn(2+), Cd(2+), and Hg(2+)), and the tungstotellurates(IV) [M(4)(H(2)O)(10)(beta-TeW(9)O(33))(2)]((16-4n)-) (M(n+) = Cr(3+), Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), and Hg(2+)), as determined by FTIR. The electrochemical properties of the iron-containing species were also studied. Cyclic voltammetry and controlled potential coulometry aided in distinguishing between Fe(3+) and W(6+) waves. By variation of pH and scan rate, it was possible to observe the stepwise reduction of the Fe(3+) centers.     

Enterobactin protonation and iron release: structural characterization of the salicylate coordination shift in ferric enterobactin

The siderophore enterobactin (Ent) is produced by many species of enteric bacteria to mediate iron uptake. This iron scavenger can be reincorporated by the bacteria as the ferric complex [Fe(III)(Ent)](3)(-) and is subsequently hydrolyzed by an esterase to facilitate intracellular iron release. Recent literature reports on altered protein recognition and binding of modified enterobactin increase the significance of understanding the structural features and solution chemistry of ferric enterobactin. The structure of the neutral protonated ferric enterobactin complex [Fe(III)(H(3)Ent)](0) has been the source of some controversy and confusion in the literature. To demonstrate the proposed change of coordination from the tris-catecholate [Fe(III)(Ent)](3)(-) to the tris-salicylate [Fe(III)(H(3)Ent)](0) upon protonation, the coordination chemistry of two new model compounds N,N',N'-tris[2-(hydroxybenzoyl)carbonyl]cyclotriseryl trilactone (SERSAM) and N,N',N'-tris[2-hydroxy,3-methoxy(benzoyl)carbonyl]cyclotriseryl trilactone (SER(3M)SAM) was examined in solution and solid state. Both SERSAM and SER(3M)SAM form tris-salicylate ferric complexes with spectroscopic and solution thermodynamic properties (with log beta(110)() values of 39 and 38 respectively) similar to those of [Fe(III)(H(3)Ent)](0). The fits of EXAFS spectra of the model ferric complexes and the two forms of ferric enterobactin provided bond distances and disorder factors in the metal coordination sphere for both coordination modes. The protonated [Fe(III)(H(3)Ent)](0) complex (d(Fe)(-)(O) = 1.98 A, sigma(2)(stat)(O) = 0.00351(10) A(2)) exhibits a shorter average Fe-O bond length but a much higher static Debye-Waller factor for the first oxygen shell than the catecholate [Fe(III)(Ent)](3)(-) complex (d(Fe)(-)(O) = 2.00 A, sigma(2)(stat)(O) = 0.00067(14) A(2)). (1)H NMR spectroscopy was used to monitor the amide bond rotation between the catecholate and salicylate geometries using the gallic complexes of enterobactin: [Ga(III)(Ent)](3)(-) and [Ga(III)(H(3)Ent)](0). The ferric salicylate complexes display quasi-reversible reduction potentials from -89 to -551 mV (relative to the normal hydrogen electrode NHE) which supports the feasibility of a low pH iron release mechanism facilitated by biological reductants.     

Accessibility and Selective Stabilization of the Principal Spin States of Iron by Pyridyl versus Phenolic Ketimines: Modulation of the (6)A(1) ↔ (2)T(2) Ground-State Transformation of the [FeN(4)O(2)](+) Chromophore

Several potentially tridentate pyridyl and phenolic Schiff bases (apRen and HhapRen, respectively) were derived from the condensation reactions of 2-acetylpyridine (ap) and 2'-hydroxyacetophenone (Hhap), respectively, with N-R-ethylenediamine (RNHCH(2)CH(2)NH(2), Ren; R = H, Me or Et) and complexed in situ with iron(II) or iron(III), as dictated by the nature of the ligand donor set, to generate the six-coordinate iron compounds [Fe(II)(apRen)(2)]X(2) (R = H, Me; X(-) = ClO(4)(-), BPh(4)(-), PF(6)(-)) and [Fe(III)(hapRen)(2)]X (R = Me, Et; X(-) = ClO(4)(-), BPh(4)(-)). Single-crystal X-ray analyses of [Fe(II)(apRen)(2)](ClO(4))(2) (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the Fe(II)-N bond distances (1.896-2.041 ?) pointing to the (1)A(1) (d(π)(6)) ground state; the existence of this spin state was corroborated by magnetic susceptibility measurements and M?ssbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex [Fe(III)(hapMen)(2)]ClO(4), determined at 100 K, demonstrated stabilization of the ferric state; the compression of the coordinate bonds at the metal center is in accord with the (2)T(2) (d(π)(5)) ground state. Magnetic susceptibility measurements along with EPR and M?ssbauer spectroscopic techniques have shown that the iron(III) complexes are spin-crossover (SCO) materials. The spin transition within the [Fe(III)N(4)O(2)](+) chromophore was modulated with alkyl substituents to afford two-step and one-step (6)A(1) ? (2)T(2) transformations in [Fe(III)(hapMen)(2)]ClO(4) and [Fe(III)(hapEen)(2)]ClO(4), respectively. Previously, none of the X-salRen- and X-sal(2)trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron(II) and SCO iron(III) complexes display intense d(π) → p(π)* and p(π) → d(π) CT visible absorptions, respectively, which account for the spectacular color differences. All the complexes are redox-active; as expected, the one-electron oxidative process in the divalent compounds occurs at higher redox potentials than does the reverse process in the trivalent compounds. The cyclic voltammograms of the latter compounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H(2)salen-type quadridentate ketimine H(2)hapen complexed with an equivalent amount of iron(III) to afford the μ-oxo-monobridged dinuclear complex [{Fe(III)(hapen)}(2)(μ-O)] exhibiting a distorted square-pyramidal geometry at the metal centers and considerable antiferromagnetic coupling of spins (J ≈ -99 cm(-1)).     

Kinetic and mechanistic studies on the reaction of nitric oxide with a water-soluble octa-anionic iron(III) porphyrin complex

The polyanionic water-soluble and non-mu-oxo-dimer-forming iron porphyrin iron(III) 5(4),10(4),15(4),20(4)-tetra-tert-butyl-5(2),5(6),15(2),15(6)-tetrakis[2,2-bis(carboxylato)ethyl]-5,10,15,20-tetraphenylporphyrin, (P(8-))Fe(III) (1), was synthesized as an octasodium salt by applying well-established porphyrin and organic chemistry procedures to bromomethylated precursor porphyrins and characterized by standard techniques such as UV-vis and (1)H NMR spectroscopy. A single pK(a1) value of 9.26 was determined for the deprotonation of coordinated water in (P(8-))Fe(III)(H(2)O)(2) (1-H(2)()O) present in aqueous solution at pH <9. The porphyrin complex reversibly binds NO in aqueous solution to give the mononitrosyl adduct, (P(8-))Fe(II)(NO(+))(L), where L = H(2)O or OH(-). The kinetics of the binding and release of NO was studied as a function of pH, temperature, and pressure by stopped-flow and laser flash photolysis techniques. The diaqua-ligated form of the porphyrin complex binds and releases NO according to a dissociative interchange mechanism based on the positive values of the activation parameters DeltaS() and DeltaV() for the "on" and "off" reactions. The rate constant k(on) = 6.2 x 10(4) M(-1) s(-1) (24 degrees C), determined for NO binding to the monohydroxo-ligated (P(8-))Fe(III)(OH) (1-OH) present in solution at pH >9, is markedly lower than the corresponding value measured for 1-H(2)O at lower pH (k(on) = 8.2 x 10(5) M(-1) s(-1), 24 degrees C, pH 7). The observed decrease in the reactivity is contradictory to that expected for the diaqua- and monohydroxo-ligated forms of the iron(III) complex and is accounted for in terms of a mechanistic changeover observed for 1-H(2)O and 1-OH in their reactions with NO. The mechanistic interpretation offered is further substantiated by the results of water-exchange studies performed on the polyanionic porphyrin complex as a function of pH, temperature, and pressure.     

An ionic liquid form of DNA: redox-active molten salts of nucleic acids.

Ionic liquids are described that contain duplex DNA as the anion and polyether-decorated transition metal complexes based on M(MePEG-bpy)(3)(2+) as the cation (M = Fe, Co; MePEG-bpy = 4,4'-(CH(3)(OCH(2)CH(2))(7)OCO)(2)-2,2'-bipyridine). When the undiluted liquid DNA-or molten salt-is interrogated electrochemically by a microelectrode, the molten salts exhibit cyclic voltammograms due to the physical diffusion (D(PHYS)) of the polyether-transition metal complex. When M = Co(II), the cyclic voltammogram of the melt shows an oxidative wave due to the Co(III/II) couple at E(1/2) = 0.40 V (versus Ag/AgCl) and a D(PHYS) of 6 x 10(-12) cm(2)/s, which is significantly lower than that for Co(MePEG-bpy)(3)(ClO(4))(2) (D(PHYS) = 2.6 x 10(-10) cm(2)/s) due to greater viscosity provoked by the DNA polymer. When a 1:1 mixture is made of the Co(MePEG-bpy)(3).DNA and Fe(MePEG-bpy)(3)(ClO(4))(2) melts, two redox waves are observed. The first is due to the Co(III/II) couple, and the second is a catalytic wave due to oxidation of guanine in DNA by electrogenerated Fe(III) in the undiluted melt. Independent experiments show that the Fe(III) form of the complex selectively oxidizes guanine in duplex DNA. These DNA molten salts constitute a new class of materials whose properties can be controlled by nucleic acid sequence and that can be interrogated in undiluted form on microelectrode arrays.