Insights into Second-Sphere Effects on Redox Potentials, Spectroscopic Properties, and Superoxide Dismutase Activity of Manganese Complexes with Schiff-Base Ligands.

Six Mn-Schiff base complexes, [Mn(X-salpn)]0/+ (salpn = 1,3-bis(sal-ic-ylidenamino)propane, X = H [1], 5-Cl [2], 2,5-F2 [3], 3,5-Cl2 [4], 5-NO2 [5], 3,5-(NO2)2 [6]), were synthesized and characterized in solution, and second-sphere effects on their electrochemical and spectroscopic properties were analyzed. The six complexes catalyze the dismutation of superoxide with catalytic rate constants in the range 0.65 to 1.54 × 106 M-1 s-1 obtained through the nitro blue tetrazolium photoreduction inhibition superoxide dismutases assay, in aqueous medium of pH 7.8. In solution, these compounds possess two labile solvent molecules in the axial positions favoring coordination of the highly nucleophilic O2 •- to the metal center. Even complex 5, [Mn(5-(NO2)salpn) (OAc) (H2O)], with an axial acetate in the solid state, behaves as a 1:1 electrolyte in methanolic solution. Electron paramagnetic resonance and UV-vis monitoring of the reaction of [Mn(X-salpn)]0/+ with KO2 demonstrates that in diluted solutions these complexes behave as catalysts supporting several additions of excess O2 •-, but at high complex concentrations (≥0.75 mM) catalyst self-inhibition occurs by the formation of a catalytically inactive dimer. The correlation of spectroscopic, electrochemical, and kinetics data suggest that second-sphere effects control the oxidation states of Mn involved in the O2 •- dismutation cycle catalyzed by complexes 1-6 and modulate the strength of the Mn-substrate adduct for electron-transfer through an inner-sphere mechanism.

Synthesis and structural characterisation of unprecedented primary N-nitrosamines coordinated to iridium(iv).

The redox chemistry of the N-nitrosamine complexes [IrCl5(RN(H)N[double bond, length as m-dash]O)]2- (R = benzyl or n-butyl) was studied in detail. One-electron oxidations at around 200 mV vs. ferrocene/ferrocenium were reversible in cyclic voltammograms. UV-vis spectroelectrochemistry reveals spectra characteristic of IrIV species but also partial decomposition of the oxidised species [IrIVCl5(RN(H)N[double bond, length as m-dash]O)]˙- on this timescale (minutes). Detailed studies on chemically oxidised solutions of the parent IrIII complexes gave evidence for paramagnetic IrIV from NMR spectra. Final products of the decomposition were the corresponding alcohols and presumably [IrIIICl5(L)]2- (L = N2, solvent, amine) complexes. Similar decomposition reactions of acidic DMSO solutions of [IrCl5(RN(H)N[double bond, length as m-dash]O)]2-revealed that this combination produces the so-called "activated" DMSO (Me2S+-O- or Me2S+-OE, with "E" being an electrophile) which oxidises the parent IrIII complexes. Finally, with the very reactive purple IrIV compound (PPh4)[IrCl5(BnN(H)N[double bond, length as m-dash]O)], the first primary N-nitrosamine coordinated to [IrIVCl5]- was isolated and characterised by UV-vis absorption, FTIR, NMR spectroscopy, ultra-high resolution electrospray mass spectrometry (UHR-ESI-MS) and iridium L3 X-ray absorption near-edge spectroscopy (XANES).

Dimerization, redox properties and antioxidant activity of two manganese(III) complexes of difluoro- and dichloro-substituted Schiff-base ligands.

Two mononuclear MnIII complexes [Mn(3,5-F2salpn)(H2O)2][B(C6H5)4]·2H2O (1·2H2O) and [Mn(3,5-Cl2salpn)(OAc)(H2O)]·H2O (2·H2O), where H2salpn=1,3-bis(salicylidenamino)propane, have been prepared and characterized. The crystal structure of 1·H2O shows that this complex forms µ-aqua dimers with a short Mn⋯Mn distance of 4.93Å. Under anaerobic conditions, the two complexes are stable in solution and possess trans-diaxial symmetry with the tetradentate Schiff base ligand symmetrically arranged in the equatorial plane. When left in air, these complexes slowly dimerize to yield high-valent [MnIV2(3,5-X2-salpn)2(µ-O)2] in which each X2-salpn ligand wraps the two Mn ions. This process is favored in basic medium where the deprotonation of the bound water molecule is concomitant with air oxidation. The two complexes catalyze the dismutation of superoxide (superoxide dismutase (SOD) activity) and peroxide (catalase (CAT) activity) in basic medium. The phenyl-ring substituents play an important role on the CAT reaction but have little effect on SOD activity. Kinetics and spectroscopic results indicate that 1 and 2 catalyze H2O2 disproportionation through a cycle involving MnIII2 and MnIV2 dimers, unlike related complexes with a more rigid and smaller chelate ring, which employ MnIII/MnVO monomers.

Halide coordinated homoleptic [Fe4S4X4](2-) and heteroleptic [Fe4S4X2Y2](2-) clusters (X, Y = Cl, Br, I)--alternative preparations, structural analogies and spectroscopic properties in solution and solid state.

New facile methods to prepare iron sulphur halide clusters [Fe4S4X4](2-) from [Fe(CO)5] and elemental sulphur were elaborated. Reactions of ferrous precursors like tetrahalidoferrates(ii) or simple ferrous halides with [Fe(CO)5] and sulphur turned out to be efficient methods to prepare homoleptic [Fe4S4X4](2-) (X = Cl, Br) and heteroleptic clusters [Fe4S4X4-nYn](2-) (X = Cl, Br; Y = Br, I). Solid materials were obtained as salts of BTMA(+) (= benzyltrimethylammonium); the new compounds containing [Fe4S4Br4](2-) and [Fe4S4X2Y2](2-) (X, Y = Cl, Br, I) were all isostructural to (BTMA)2[Fe4S4I4] (monoclinic, Cc) as inferred from synchrotron X-ray powder diffraction. While the solid materials contain defined heteroleptic clusters with a halide X : Y ratio of 2 : 2, dissolving these compounds leads to rapid scrambling of the halide ligands forming mixtures of all five possible [Fe4S4X4-nYn](2-) clusters as could be shown by UHR-ESI MS. The variation of X and Y allowed assignment of the absorption bands in the visible and NIR; the long-wavelength bands around 1100 nm were tentatively assigned to intervalence charge transfer (IVCT) transitions.

Chloro-bis-(naphthalen-1-yl)phosphane.

In the title compound, C(20)H(14)ClP, the dihedral angle between the naphthyl rings is 81.77 (6)°. The crystal packing suggests weak π-π stacking inter-actions between the naphthyl rings in adjacent units [minimum ring centroid separation 3.7625 (13) Å].

5-Hy-droxy-3-methyl-5-phenyl-4,5-di-hydro-1H-pyrazole-1-carbothio-amide.

In the title compound C(11)H(13)N(3)OS, the aromatic ring and the dihydro-pyrazole ring are oriented orthogonally with respect to each other, making a dihedral angle of 89.92 (9)°. An intra-molecular O-H⋯S hydrogen bond occurs. In the crystal, weak N-H⋯N and N-H⋯S hydrogen bonds link the mol-ecules into a columnar stack propagating along the b axis.

N-butylpyridinium undecachlorocarbadodecaborate and comparison with similar compounds.

The title compound, C(9)H(14)N(+)·CHB(11)Cl(11)(-), was obtained in the course of our continuing studies of the low-melting salts of closo- and nido-carborane cage anions with alkylpyridinium and dialkylimidazolium cations. The title compound is the first example of a pyridinium salt of a perchlorinated carborane anion. The structure consists of one N-butylpyridinium cation counterbalanced by one perchlorinated carborane cage anion per asymmetric unit. By changing the counter-ion, different packings are observed, and to try to understand this the new structure is compared with five similar compounds.