Imidazole and imidazolate iron complexes: on the way for tuning 3D-structural characteristics and reactivity. Redox interconversions controlled by protonation state

X-ray structures for six Fe(II) and Fe(III) complexes from two closely heptadentate N-tripodal ligands, L1H(3) = tris[(imidazol-4-yl)-3-aza-3-butenyl]amine and L2H(3) = tris[(imidazol-2-yl)-3-aza-3-butenyl]amine, are described: three complexes in the L1 series (namely, [Fe(II)(L1H(3))](2+) and [Fe(III)(L1H(3))](3+) at low pH and [Fe(III)(L1)](0) at high pH) and three complexes in the L2 series (namely, [Fe(II)(L2H(3))](2+) at low pH and [Fe(II)(L2H)](0) and [Fe(III)(L2)](0) at high pH). Most of these complexes are stable in both Fe(II) and Fe(III) redox states and with the ligand in various protonation states. In the solid state, hydrogen bonds networks were obtained. Structural differences induced by 2- or 4-imidazole substitution are described and discussed. In solution, interconversions between different forms, with regard to oxidation and protonation states, were investigated by UV-visible spectroscopy, cyclic voltammetry, and potentiometry. The deprotonation pattern of these polyimidazole iron(II) and iron(III) complexes is described in detail. pK(a)s of the imidazolate/imidazole moieties in MeOH/H(2)O are reported. Two new species, namely, [Fe(II)(L1)](-) and [Fe(II)(L2)](-), were shown to be obtained in DMSO upon strong base addition and characterized by UV-vis spectroscopy and cyclic voltammetry. Half-wave potentials of Fe(III)/Fe(II) complexes with ligand moieties in several protonation states are reported, both in DMSO and in MeOH/H(2)O. Because of the presence of free imidazole groups coordinated to the iron, the potential of the iron(III)/iron(II) couples can be tuned by pH. A shift of DeltaE = E(deprot) - E(prot) ranging from -270 to -320 mV per exchanged proton in DMSO was measured. This study shows moreover that interconversions (with regard to both redox and protonation states) can be reversed several times. As the complexes have been isolated in order to be tested as superoxide dismutase mimics, preliminary reactions with dioxygen and with superoxide, considered as oxidant and reducer of biological importance, are reported. In these two series, O(2)(-) behaves either as a base or as a reducer and no adducts have been observed.     

Multi-iron tungstodiarsenates. Synthesis,characterization, and electrocatalytic studies of alphabetabetaalpha-(Fe(III)OH(2))(2)Fe(III)(2)(As(2)W(15)O(56))(2)(12)(-)

Reaction of the trivacant lacunary complex, alpha-Na(12)[As(2)W(15)O(56)], with an aqueous solution of Fe(NO(3))(3).9H(2)O yields the sandwich-type polyoxometalate, alphabetabetaalpha-Na(12)(Fe(III)OH(2))(2)Fe(III)(2)(As(2)W(15)O(56))(2) (Na1). The structure of this complex, determined by single-crystal X-ray crystallography (a = 13.434(1) A, b = 13.763(1) A, c = 22.999(2) A, alpha = 90.246(2) degrees, beta = 102.887(2) degrees, gamma = 116.972(1) degrees, triclinic, Ponemacr;, R1 = 5.5%, based on 25342 independent reflections), consists of an Fe(III)(4) unit sandwiched between two trivacant alpha-As(2)W(15)O(56)(12)(-) moieties. UV-vis, infrared, cyclic voltammetry, and elemental analysis data are all consistent with the structure determined from X-ray analysis. Magnetization studies confirm that the four Fe(III) centers are antiferromagnetically coupled. A cyclic voltammogram of Na1 reveals that a three-wave W(VI) system replaces the two-wave W(VI) system found in the precursor alpha-As(2)W(15)O(56)(12)(-) complex. The observed modifications in the CV patterns of Na1 and alpha-As(2)W(15)O(56)(12)(-) are most likely due to subsequent changes in the acid-base properties of two reduced POMs that occur as a result of Fe(III) incorporation. Na1 is shown to be more efficient than the monosubstituted complex alpha(2)-As(2)(Fe(III)OH(2))W(17)O(61)(7)(-) in the electrocatalytic reduction of dioxygen. This is attributed to cooperativity effects among the adjacent Fe(III) centers in Na1.     

Cleavage of hydrazine N-N bonds by RuMo3S4 cubane-type clusters

The mixed-metal cubane-type clusters [(Cp*Mo)3(mu3-S)4RuH2(PR3)][PF(6)] [Cp* = eta5-C5Me5; R = Ph (2), Cy (5)] were effective for the N-N bond cleavage of hydrazine and phenylhydrazine via a disproportionation reaction. The ammonia cluster [(C*Mo)3(mu3-S)4Ru(NH3)(PPh3)][PF6] (3) and/or the unprecedented double-cubane-type cluster with bridging nitrogenous ligands [{(Cp*Mo)3(mu3-S)4Ru}2(mu2-NH2)(mu2-NHNH2)][PF6]2 (4) were isolated from the reaction mixtures, and their structures were determined by X-ray diffraction studies.     

Separation and on-line concentration of bisphenol A and alkylphenols by micellar electrokinetic chromatography with anionic surfactant

Separation and on-line concentration of bisphenol A and three alkylphenols were investigated by micellar electrokinetic chromatography with the anionic surfactant, sodium dodecyl sulfate. The separation conditions were optimized by the simultaneous addition of the organic solvent and cyclodextrin to the running solution. The separation of hydrophobic analytes and 4-nonylphenol isomers was improved by the addition of 10% methanol and 5 mM beta-cyclodextrin to the running solution. When the sweeping with the running solution was used as the on-line concentration procedure, 69-, 48-, 55- and 41-fold increases in detection sensitivity were obtained for bisphenol A, 4-tert.-butylphenol and 4-(1,1,3,3-tetramethylbutyl)phenol, and the second peak of 4-nonylphenol isomers, respectively. The detection limits were 0.0071, 0.0065, 0.021 and 0.055 mg/l, respectively. These results were better than those with the cationic surfactant, tetradecyltrimethylammonium bromide.     

Construction of Robust Ruthenium(salen)(CO) Complexes and Asymmetric Aziridination with Nitrene Precursors in the Form of Azide Compounds That Bear Easily Removable N‐Sulfonyl Groups

We synthesized new Ru(salen)(CO) complexes of high durability and achieved aziridination with good to excellent enantioselectivity by using azide compounds that contain an easily removable N‐sulfonyl group, such as the 2‐(trimethylsilyl)ethanesulfonyl group, as a nitrene precursor. Aziridination of less‐reactive α,β‐unsaturated esters (and amides) proceeded with excellent enantioselectivities, from which it is inferred that an electrophilic species is the active species of this reaction. The present asymmetric aziridination provides a useful tool for introducing optically active nonprotected amine groups.     

Synthesis of a novel water-soluble polyamide dendrimer based on a facile convergent method

A novel, rapid, inexpensive, and highly efficient convergent approach has been developed for the synthesis of a 32-amine-terminated G3 polyamide dendrimer by the hydrolysis of the dendrimer with trifluoroacetamide groups. The resulting dendrimer could be successfully modified with oligo(ethylene glycol) chains at its periphery to afford a novel water-soluble polyamide dendrimer. The structural homogeneity of the dendrimers was confirmed by NMR and MALDI-TOF mass spectroscopies.     

Total synthesis of 4-epi-atpenin A5 as a potent nematode complex II inhibitor

It is clear that atpenins and their analogs are useful chemical tools for elucidation of complex II functionality and that they could act as lead compounds for the development of novel helminth complex II-specific inhibitors. Recently, we discovered 4-epi-atpenin A5 as a potent nematode complex II inhibitor during our SAR studies of atpenin A5. This result led us to embark on a concise total synthesis of 4-epi-atpenin A5. In this study, we describe the total synthesis of 4-epi-atpenin A5. Importantly, this was more concise and practical synthesis than our previous total synthesis of atpenin A5.     

Large-scale quantum mechanical simulations of carbon nanowires

The stability of quasi-one-dimensional structures of carbon is investigated using a generalized tight-binding molecular-dynamics scheme. Large-scale simulations are made possible by the parallel implementation of the diagonalization routines. Our results show that these structures can be stable provided that their geometries consist of a core of four-fold coordinated atoms, surrounded by a three-fold coordinated outer surface accommodating one of the most stable reconstructions of bulk diamond structure.     

Pentalenolactone biosynthesis. Molecular cloning and assignment of biochemical function to PtlI, a cytochrome P450 of Streptomyces avermitilis

A gene cluster encoding all of the enzymes for the biosynthesis of the antibiotic pentalenolactone (1) has recently been identified in Streptomyces avermitilis. The biosynthetic gene cluster contains the ptlI (SAV2999) gene which encodes a cytochrome P450 (CYP183A1). PtlI was cloned by PCR and expressed in Escherichia coli as a C-terminal His6-tag protein. Recombinant PtlI bound pentalenene (3) with high affinity (KD = 1.44 +/- 0.14 muM). Incubation of recombinant PtlI with (+/-)-3 in the presence of NADPH, E. coli flavodoxin and flavodoxin reductase, and O2 resulted in conversion to a single enantiomer of pentalen-13-al (7), by stepwise allylic oxidation via pentalen-13-ol (6). The steady-state kinetic parameters for the oxidation of pentalenene (3) to pentalen-13-ol (6) were kcat = 0.503 +/- 0.006 min-1 and Km = 3.33+/-0.62 muM for 3.