Computational Modeling of the Mechanism of Urease

In order to elucidate aspects of the mechanism of the hydrolytic enzyme urease, theoretical calculations were undertaken on a model of the active site, using density functional theory. The bridging oxygen donor that has been found in the crystal structures was determined to be a hydroxide ion. The initial coordination of urea at the active site occurs most likely through the urea oxygen to the nickel ion with the lowest coordination number. This coordination can be made without much gain in energy. The calculations also showed that weak coordination of one of the urea amine nitrogen atoms to the second nickel atom is energetically feasible. Furthermore, a proposed mechanism including a tetrahedral intermediate generated by hydrolytic attack on the urea carbon by the bridging hydroxide was modeled, and the tetrahedral intermediate was found to be energetically unfavorable relative to terminal coordination of the substrate (urea)

Activation of Tri(2-Furyl)Phosphine at a Dirhenium Centre: Formation of Phosphido-Bridged Dirhenium Complexes

Reaction of tri(2-furyl)phosphine (PFu3) with [Re2(CO)10−n(NCMe)n] (n = 1, 2) at 40 °C gave the substituted complexes [Re2(CO)10−n(PFu3)n] (1 and 2), the phosphines occupying axial position in all cases. Heating [Re2(CO)10] and PFu3 in refluxing xylene also gives 1 and 2 together with four phosphido-bridged complexes; [Re2(CO)8−n(PFu3)n(μ-PFu2)(μ-H)] (n = 0, 1, 2) (3–5) and [Re2(CO)6(PFu3)2(μ-PFu2)(μ-Cl)] (6) resulting from phosphorus–carbon bond cleavage. A series of separate thermolysis experiments has allowed a detailed reaction pathway to be unambiguously established. A similar reaction between [Re2(CO)10] and PFu3 in refluxing chlorobenzene furnishes four complexes which include 1, 2, 6 and the new binuclear complex [Re2(CO)6(η1-C4H3O)2(μ-PFu2)2] (7). All new complexes have been characterized by a combination of spectroscopic data and single crystal X-ray diffraction studies

A di-iron(III) mu-oxido complex as catalyst precursor in the oxidation of alkanes and alkenes

The oxido-bridged diiron(III) complex [Fe-2(mu-O)(mu-OAc)(DPEAMP)(2)](OCH3) (1), based on a new unsymmetrical ligand with an N4O donor set, viz. [2-((bis(pyridin-2-ylmethyl)amino)methyl)-6-((ethylamino)methyl)-4-meth-ylphenol (HDPEAMP)], has been prepared and characterized by spectroscopic methods and X-ray crystallog-raphy. The crystal structure of the complex reveals that each Fe(III) ion is coordinated by three nitrogen and three oxygen donors, two of which are the bridging oxido and acetate ligands. Employing H2O2 as a terminal oxidant, 1 is capable of oxidizing a number of alkanes and alkenes with high activity. The catalytic oxidation of 1,2-dimethylcyclohexane results in excellent retention of configuration. Monitoring of the reaction of 1 with H2O2 and acetic acid in the absence of substrate, using low-temperature UV-Vis spectroscopy, suggests the in situ formation of a transient Fe(III)(2)-peroxido species. While the selectivity and nature of oxidation products implicate a high-valent iron-oxido complex as a key intermediate, the low alcohol/ketone ratios suggest a simultaneous radical-based process.Peer reviewe

A quantum-mechanical study of the reaction mechanism of sulfite oxidase.

The oxidation of sulfite to sulfate by two different models of the active site of sulfite oxidase has been studied. Both protonated and deprotonated substrates were tested. Geometries were optimized with density functional theory (TPSS/def2-SV(P)) and energies were calculated either with hybrid functionals and large basis sets (B3LYP/def2-TZVPD) including corrections for dispersion, solvation, and entropy, or with coupled-cluster theory (LCCSD(T0)) extrapolated toward a complete basis set. Three suggested reaction mechanisms have been compared and the results show that the lowest barriers are obtained for a mechanism where the substrate attacks a Mo-bound oxo ligand, directly forming a Mo-bound sulfate complex, which then dissociates into the products. Such a mechanism is more favorable than mechanisms involving a Mo-sulfite complex with the substrate coordinating either by the S or O atom. The activation energy is dominated by the Coulomb repulsion between the Mo complex and the substrate, which both have a negative charge of -1 or -2

Carbon−Phosphorus Bond Activation of Tri(2-thienyl)phosphine at Dirhenium and Dimanganese Centers

Reaction of [Re2(CO)9(NCMe)] with tri(2-thienyl)phosphine (PTh3) in refluxing cyclohexane affords three substituted dirhenium complexes: [Re2(CO)9(PTh3)] (1), [Re2(CO)8(NCMe)(PTh3)] (2), and [Re2(CO)8(PTh3)2] (3). Complex 2 was also obtained from the room-temperature reaction of [Re2(CO)8(NCMe)2] with PTh3 and is an unusual example in which the acetonitrile and phosphine ligands are coordinated to the same rhenium atom. Thermolysis of 1 and 3 in refluxing xylene affords [Re2(CO)8(μ-PTh2)(μ-η1:κ1-C4H3S)] (4) and [Re2(CO)7(PTh3)(μ-PTh2)(μ-H)] (5), respectively, both resulting from carbon−phosphorus bond cleavage of a coordinated PTh3 ligand. Reaction of [Re2(CO)10] and PTh3 in refluxing xylene gives a complex mixture of products. These products include 3−5, two further binuclear products, [Re2(CO)7(PTh3)(μ-PTh2)(μ-η1:κ1-C4H3S)] (6) and [Re2(CO)7(μ-κ1:κ2-Th2PC4H2SPTh)(μ-η1:κ1-C4H3S)] (7), and the mononuclear hydrides [ReH(CO)4(PTh3)] (8) and trans-[ReH(CO)3(PTh3)2] (9). Binuclear 6 is structurally similar to 4 and can be obtained from reaction of the latter with 1 equiv of PTh3. Formation of 7 involves a series of rearrangements resulting in the formation of a unique new diphosphine ligand, Th2PC4H2SPTh. Reaction of [Mn2(CO)10] with PTh3 in refluxing toluene affords the phosphine-substituted product [Mn2(CO)9(PTh3)] (10) and two carbon−phosphorus bond cleavage products, [Mn2(CO)6(μ-PTh2)(μ-η1:η5-C4H3S)] (11) and [Mn2(CO)5(PTh3)(μ-PTh2)(μ-η1:η5-C4H3S)] (12). Both 11 and 12 contain a bridging thienyl ligand that is bonded to one manganese atom in a η5-fashion. The molecular structures of eight of these new complexes were established by single-crystal X-ray diffraction studies, allowing a detailed analysis of the disposition of the coordinated ligands

Hexakis(tetra­aqua­sodium) deca­vanadate(V) dihydrate

The title compound, {[Na(H2O)4]6[V10O28]·2H2O}n, crystallized from a H2O/THF/CH3CN solution (pH ca 6) containing equimolar amounts of NaVO3 and N-(2-hydroxy­benz­yl)-N-(2-picol­yl)glycine. In the crystal structure, the deca­vanadate [V10O28]6− anion ( symmetry) is coordinated, via four terminal oxide ligands of V centres, to two dinuclear [{Na(H2O)3}2(μ-H2O)2]2+ units. Inter­connection of these aquasodium-ion-sandwiched deca­vanadates to chains parallel to [001] is effected by μ-[{Na(H2O)3}2(μ-H2O)2]2+ units, bridging adjacent deca­vanadates via O=V. The structure is consolidated by an extensive network of O—H⋯O hydrogen bonds

A pyrazine amide – 4-aminoquinoline hybrid and its rhodium and iridium pentamethylcyclopentadienyl complexes; evaluation of anti-mycobacterial and anti-plasmodial activities

The synthesis and characterization of N-(2-((7-chloroquinolin-4-yl)amino)ethyl)pyrazine-2-carboxamide (L), an aminoquinoline – pyrazinamide hybrid, and the complexes (N-(2-((7-chloroquinolin-4-yl)amino)ethyl)pyrazine-2-carboxamide)(cyclopentadienyl) chlorido-rhodium or iridium hexafluorophosphate ([M(L)(Cp*)Cl] PF6; M = Rh, Ir) and the corresponding chlorido salts ([M(L)(Cp*) Cl]Cl; M = Rh, Ir) are described. The ligand and the hexafluorophosphate salts of the metal complexes have been evaluated for anti-plasmodial and anti-mycobacterial activity. The rhodium and the iridium complexes were significantly more active against M. tuberculosis than the free ligand. The crystallographically determined molecular structures of complexes (N-(2-((7-chloroquinolin-4-yl)amino)ethyl)pyrazine-2-carboxamide)(cyclopentadienyl)chlororhodium hexafluoro-phosphate and (N-(2-((7-chloroquinolin-4-yl)amino)ethyl)pyrazine-2-carboxamide)(cyclopentadienyl)chloro-iridium chloride are presented

Catalytic epoxidation using dioxidomolybdenum(VI) complexes with tridentate aminoalcohol phenol ligands

Reaction of the tridentate aminoalcohol phenol ligands 2,4-di-tert-butyl-6-(((2 hydroxyethyl)(methyl)amino)methyl)phenol (H2L1) and 2,4-di-tert-butyl-6-(((1-hydroxybutan-2-yl)amino)methyl)phenol (H2L2) with [MoO2(acac)(2)] in methanol solutions resulted in the formation of [MoO2(L-1)(MeOH)] (1) and [MoO2(L-2)(MeOH)] (3), respectively. In contrast, the analogous reactions in acetonitrile afforded the dinuclear complexes [Mo2O2(mu-O)(2)(L-1)(2)] (2) and [Mo2O2(mu-O)(2)(L-2)(2)] (4). The corresponding reactions with the potentially tetradentate ligand 3-((3,5-di-tert-butyl-2-hydroxybenzyl)(methyl)amino)propane-1,2-diol (H3L3) led to the formation of the mononuclear complex [MoO2(L-3)(MeOH)] (5) in methanol while in acetonitrile solution a trinuclear structure [Mo3O3(mu-O)(3)(L-3)(3)] (6) was obtained. In both cases, the ligand moiety L-3 coordinated in a tridentate fashion. The catalytic activities of complexes 1-6 in epoxidation of five different olefins, S1-5, with tert-butyl hydroperoxide and hydrogen peroxide were studied. The catalytic activities were found to be moderate to good for the reaction of substrate cis-cyclooctene S1, while all complexes were less active in the epoxidation of the more challenging substrates S2-5. The molecular structures of 1, 2, 4 and 6 were determined by single crystal X-ray diffraction analyses

Di- and Tetrairon(III) mu-Oxido Complexes of an N3S-Donor Ligand : Catalyst Precursors for Alkene Oxidations

The new di- and tetranuclear Fe(III) mu-oxido complexes [Fe-4(mu-O)(4) (PTEBIA)(4)]CF3SO3)(4)(CH3CN)(2)] (1a) , [Fe-2(mu-O)Cl-2(PTEBIA)(2)(CF3SO3)(2) (1b), and [Fe-2(mu-O)(HCOO)(2)(PTEBIA)(2)](ClO4)(2) (MeOH) (2) were prepared from the sulfur-containing ligand (2-((2,4-dimethylphenyl)thiO)-N,N-bis ((1-methyl-benzimidazol-2-yl)methyl)ethanamine (PTEBIA). The tetrairon complex 1a features four mu-oxido bridges, while in dinuclear 1b, the sulfur moiety of the ligand occupies one of the six coordination sites of each Fe(III) ion with a long Fe-S distance of 2.814(6) angstrom . In 2, two Fe(III) centers are bridged by one oxido and two formate units, the latter likely formed by methanol oxidation. Complexes 1a and 1 b show broad sulfur-toiron charge transfer bands around 400-430 nm at room temperature, consistent with mononuclear structures featuring Fe-S interactions. In contrast, acetonitrile solutions of 2 display a sulfur-to-iron charge transfer band only at low temperature (228 K) upon addition of H2O2/CH3COOH, with an absorption maximum at 410 nm. Homogeneous oxidative catalytic activity was observed for 1a and 1b using H2O2 as oxidant, but with low product selectivity. High valent iron-oxo intermediates could not be detected by UV-vis spectroscopy or ESI mass spectrometry. Rather, evidence suggest preferential ligand oxidation, in line with the relatively low selectivity and catalytic activity observed in the reactions.Peer reviewe

An experimental and theoretical study of a heptacoordinated tungsten(VI) complex of a noninnocent phenylenediamine bis(phenolate) ligand

W(N2O2)(HN2O2)] (H4N2O2 = N,N'-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,2-phenylenediamine) with a noninnocent ligand was formed by reaction of the alkoxide precursor [W(eg)(3)] (eg = the 1,2-ethanediolate dianion) with two equivalents of ligand. The phenol groups on one of the ligands are completely deprotonated and the ligand coordinates in a tetradentate fashion, whereas the other ligand is tridentate with one phenol having an intact OH group. The molecular structure, magnetic measurements, EPR spectroscopy, and density functional theory calculations indicate that the complex is a stable radical with the odd electron situated on the tridentate amidophenoxide ligand. The formal oxidation state of the metal center is W(VI), with the paramagnetic properties being due to the unpaired electron on the ligand