Kinetics of Degradation of Biapenem

The kinetic and thermodynamic analysis of biapenem degradation was conducted during acid–base hydrolysis, under the influence of buffer components and in the solid state at increased temperature (dry air) and relative air humidity (RH > 50%). The effects of the initial concentration of biapenem and the formation of degradation products on its stability were investigated.     

Tetranuclear iron(III) complexes of an octadentate pyridine-carboxylate ligand and their catalytic activity in alkane oxidation by hydrogen peroxide

Reaction of the octadentate ligand 2,6-bis{3-[N,N-di(2-pyridylmethyl)amino]propoxy}benzoic acid (LH) with Fe(ClO4)3 leads to the formation of the tetranuclear complexes [Fe4(mu-O)2(LH)2(ClCH2-CO2)4](ClO4)4 (1), [{Fe2(mu-O)L(R-CO2)}2](ClO4)4 (2 R = C6H5-, 3 R = CH3-, 4, R = ClCH2-). The crystal structures of complexes 1 and 2 reveal that they consist of two Fe(III)2(mu-O)(mu-RCO2)2 cores that are linked via the two LH/L ligands to give a "dimer of dimers" structure. Complex assumes a helical shape, with protonated carboxylic acid moieties of the two ligands forming a hydrogen-bonded pair at the center of the cation. In complexes 2, 3 and 4, central carboxylates of the two ligands bridge the iron ions in each of the two Fe2O units, with an interdimer iron-iron separation of approximately 10 A and an intradimer separation of approximately 3.1 A. The second carboxylate bridge within the Fe2O units is defined by exogenous benzoate (2), acetate (3) or chloroacetate (4) ligands. The aqua complex [{Fe2(mu-O)L(H2O)2}2](ClO4)6 (5) is proposed to have a similar structure, but with the exogenous bridging carboxylates replaced by two terminal water ligands. These complexes exhibit electronic and M?ssbauer spectral features that are similar to those of (mu-oxo)diiron(III) proteins as well as other related (mu-oxo)bis(mu-carboxylato)diiron(III) complexes. This similarity shows that these properties are not significantly affected by the nature of the bridging exogenous carboxylate, and that the octadentate framework ligand is essential in stabilizing the "dimer of dimers" structure. This structural feature remains in highly diluted solution (10(-5) M) as evidenced by electrospray ionization mass-spectroscopy (ES MS). Cyclic voltammetric studies of complexes 2 and 5 showed two irreversible two-electron reductions, indicating that the two Fe2O units of the tetranuclear complexes behave as distinct redox entities. Complexes 2, 3 and, especially, the aqua complex 5 are active alkane oxidation catalysts. Catalytic reactions carried out with alkane substrate molecules and hydrogen peroxide predominantly gave alcohols. High stereospecificity in the oxidation of cis-1,2-dimethylcyclohexane supports the metal-based molecular mechanism of O-insertion into C-H bonds postulated for non-heme iron enzymes such as methane monooxygenase.     

Iron-Semiquinone, Semiquinone-Semiquinone, and Iron-Iron Magnetic Exchange in Monomeric and Dimeric Ferric Complexes Containing the 3,6-Di-tert-butyl-1,2-semiquinonate Ligand

Fe(3,6-DBSQ)(3) has been prepared by reacting 3,6-di-tert-butyl-1,2-benzoquinone with Fe(CO)(5). The complex has been characterized structurally [orthorhombic, Pbca, a = 18.277(5) ?, b = 11.634(3) ?, c = 39.903(10) ?, V = 8485(4) ?(3), Z = 8, R = 0.063], electrochemically, and magnetically. Ligand-based redox couples are observed in electrochemical experiments that consist of reversible or quasireversible Cat/SQ steps at negative potentials and irreversible SQ/BQ oxidations at positive potentials. Magnetic measurements show temperature dependence that arises from antiferromagnetic exchange. Data have been fit to an expression that includes the effects of both Fe-SQ and SQ-SQ exchange with the result that J(SQ-SQ) is larger in magnitude than J(Fe-SQ). In methanol, the complex undergoes solvolysis to form [Fe(3,6-DBSQ)(2)(&mgr;-OMe)](2). Structural characterization [triclinic, P&onemacr;, a = 11.441(2) ?, b = 11.514(2) ?, c = 14.552(2) ?, alpha = 67.86(1) degrees, beta = 70.51(1) degrees, gamma = 72.79(1) degrees, V = 1641.8(5) ?(3), Z = 1, R = 0.048] has shown that the molecule is a centrosymmetric dimer with no close intermolecular contacts. The temperature dependence of magnetic measurements has been fit to a model consisting of two interacting S = (3)/(2) centers that arise from strong Fe-SQ exchange with J(Fe-Fe) = -22.4 cm(-1).     

Zur Geschichte der Gleichung des V. Grades (bis 1858)

Ohne Zusammenfassung     

Synthesis of a quinone-functionalized macrocyclic ligand and the intense fluorescence of its zinc complex

The redox-active quinone-functionalized macrocyclic ligand 1,4,14,17-tetrahydroxyhemiporphyrazine, H2hp(OH)4, has been synthesized and its zinc complex, [Zn(hp(OH)4)(py)], found to exhibit intense fluorescence.     

Evidence for strong tantalum-to-boron dative interactions in (silox)3Ta(BH3) and (silox)3Ta(eta2-B,Cl-BCl2Ph) (silox = tBu3SiO)1

Treatment of (silox)3Ta (1, silox = tBu3SiO) with BH3.THF and BCl2Ph afforded (silox)3Ta(BH3) (2) and (silox)3Ta(eta2-B,Cl-BCl2Ph) (3), which are both remarkably stable Ta(III) compounds. NMe3 and ethylene failed to remove BH3 from 2, and no indication of BH3 exchange with BH3.THF-d8 was noted via variable-temperature 1H NMR studies. Addition of BH3.THF to (silox)3TaH2 provided the borohydride-hydride (silox)3HTa(eta3-BH4) (5), and its thermolysis released H2 to generate 2. Exposure of 2 to D2 enabled the preparation of isotopologues (silox)3Ta(BH3-nDn) (n = 0, 2; 1, 2-D; 2, 2-D2; 3, 2-D3) for isotopic perturbation of chemical shift studies, but these failed to distinguish between "inverse adduct" (i.e., (silox)3Ta-->BH3) or (silox)3Ta(eta2-B,H-BH3) forms of 2. Computational models (RO)3Ta(BH3) (R = H, 2'; SiH3, 2SiH SiMe3, 2SiMe, and SitBu3, 2SiBu) were investigated to assess the relative importance of steric and electronic effects on structure and bonding. With small R, eta2-B,H structures were favored, but for 2SiMe and 2SiBu, the dative structure proved to be similar in energy. The electonic and vibrational features of both structure types were probed. The IR spectrum of 2 was best matched by the eta2-B,H conformer of 2SiBu. In related computations pertaining to 3, small R models favored the oxidative addition of a BCl bond, while with R = SitBu3 (3SiBu), an excellent match with its X-ray crystal structure revealed the critical steric influence of the silox ligands.