The effect of protonation on [Mn(IV)(μ2-O)]2 complexes

The series of complexes [Mn(IV)(X-SALPN)(₂-O)]₂, 1: X=5-OCH₃; 2: X=H; 3: X=5-Cl; 4: X=3,5-diCl; 5: X=5-NO₂, contain [Mn₂O₂]⁴⁺ cores with Mn-Mn separations of 2.7 . These molecules can be protonated to form [Mn(IV)(X-SALPN)(₂-O,OH)]₂ ⁺ in which a bridging oxide is protonated. The pK_a values for the series of [Mn(IV)(X-SALPN)(₂-O,OH)]₂ ⁺ track linearly versus the shift in redox potential with a slope of 84 mV/pKa. This observation suggests that the [Mn₂O₂]⁴⁺ core can be considered as a unit in which the free energy of protonation is directly related to the ability to reduce the Mn(IV) ion. The marked sensitivity of the reduction potential to the presence of protons presents a mechanism in which an enzyme can control the oxidizing capacity of an oxo manganese cluster by the degree and timing of oxo bridge protonation.Abbreviations AnH⁺CF₃SO₃ ^- anilinium triflate - DMA N,N-dimethyl acetamide - H₂SALPN 1,3-bis(salicylideneiminato)propane - H₂(5-Cl-SALPN) 1,3-bis(5-chlorosalicylideneiminato)propane; - H₂(3,5-diCl-SALPN) 1,3-bis(3,5-dichlorosalicylideneiminato)propane - H₂(5-NO₂-SALPN) 1,3-bis(5-nitorosalicylideneiminato)propane - H₂(5-OMe-SALPN) 1,3-bis(5-methoxysalicylideneiminato)propane - LuH⁺CF₃SO₃ ^- 2,4-lutidinium triflate - ME₃NH⁺Ph₄B^- trimethylammonium tetraphenylborate - OEC oxygen evolving complex - PyH⁺ClO₄ ^- pyridinium perchlorate - SCE saturated calomel electrode     

The tungsten(IV) aqua ion; reduction studies and detection of aquachloro complexes of [W3(μ3-O)(μ2-O)3]4+ by oxygen-17 and tungsten-183 NMR spectroscopy

Oxygen-17 and tungsten-183 NMR studies on solutions of orange tungsten(IV), prepared in 2 M p-toluenesulphonic acid (Hpts) solution via acid catalyzed hydrolysis of potassium hexachlorotungstate(IV) in 2 M Hpts followed by treatment with a single DOWEX 50W X2 cation-exchange column, confirm the formation of the species; [W₃(μ₃-O)(μ₂-O)₃(OH₂)₈Cl]³⁺ as major product in addition to [W₃(μ₃-O)(μ₂-O)₃(OH₂)₉]⁴⁺ under these conditions in a manner not possible by spectrophotometric means. Further characterization of the green W(III,III,IV) mixed-valence reduction product has also been carried out with in addition an estimation of the formal reduction potential for the W(IV)₃/W(III,III,IV) redox couple.     

Specific features of the kinetics of H2-O2-O2(a 1Δ g ) mixtures: I. formation and quenching of electronically and vibrationally excited (A′) molecules in H2-O2-O2(A 1Δ g ) mixtures at a temperature of 300 K

Available data on the kinetic processes in H₂-O₂-O₂(a ¹Δ _g ) mixtures are analyzed theoretically, and the ranges in which the rate constants of these processes can vary are determined. The processes of energy transformation in an O₂(a ¹Δ _g )-H₂-H-HO₂ system in the approximations of the fast and slow (in comparison with the vibrational relaxation time of the HO₂ radical) quenching of the electronically excited state are considered. The experiments on the quenching of singlet delta oxygen (SDO) molecules O₂(a ¹Δ _g ) excited in a microwave discharge at a temperature of 300 K and pressure of 6 Torr in a lean hydrogen-oxygen mixture are simulated (by a lean fuel mixture is meant a mixture in which the ratio of the fuel to the oxidizer mass fraction is less than the stoichiometric ratio, which is 2: 1 for a hydrogen-oxygen mixture). It is shown that, over the experimental observation times, the SDO quenching frequency depends on the concentration of molecular hydrogen, the residual odd oxygen fraction in the gas flow, and the ratio between the rate constants of kinetic processes involving HO₂ and HO₂* radicals. Simulations of the microwave discharge and the transport of excited oxygen along the drift tube make it possible to determine the residual odd oxygen concentration in the gas flow. Recommendations on the choice of the rate constants for the O₂(a ¹Δ _g ) + HO₂)A″, v₃″ = 0) ? O₂ + HO₂*(A′, v₃′ = 1), HO₂*(A′v₃′ ≤ 1) + O₂(a ¹Δ _g ) → HO₂*(A′,v₃′ ≥ 6) + O₂, and HO₂*(A′,v₃′ ≤ 1) + O₂(a ¹Δ _g ) → H + O₂ + O₂ processes are given. It is shown that, in the case of slow quenching in a H₂-O₂-O₂(a ¹Δ _g ) mixture at a low temperature, the intensity of hydrogen oxidation is enhanced due to the reaction + HO₂*(A′,v₃′ ≤ 1) + O₂(¹Δ) → H + O₂ + O₂.     

Binuclear copper(II) complexes of tetradentate (Ne) diazine ligands. Crystal structures of [μ-1,4-Bis(4,6-dimethyl-2-pyridylamino)phthalazine-N,μ-N3,μN3,N]-(μ-Dichloro)dichlorodicopper(II), [μ-3,6-Bis(2-pyridylthio)pyridazine-N,μ-N1,μ-N2,N] (μ-dichloro)dichlorodicopper(II) ethanol,and 3,6-Bis(2-pyridylthio)-pyridazine

The X-ray structures of two binuclear copper(II) chloride complexes of the tetradentate ligands 1,4- bis(4,6-dimethyl-2-pyridylamino)phthalazine (PAP46Me) and 3,6-bis(2-pyridylthio)pyridazine are reported. [Cu₂(PAP46Me)Cl₄] (1) and [Cu₂(PTP)Cl₄]· CH₃CH₂OH (2) contain triply bridged binuclear centres involving a diazine (NN) and two chlorobridges with copper-copper separations in the fange 3.19–3.25 Å and distorted square pyramidal copper stereochemistry. The reduced room temperature magnetic moments indicate antiferromagnetically coupled binuclear copper(II) centres.Complex 1 forms green crystals with a= 15.795(3), b=10.661(3), c=16.155(4) Å, β= 113.82(3)°, C2/c, Z = 4, R_f=0.031. Complex (2) forms green crystals with a=33.9022(8), b= 9.1626(5), c= 15.7885(5) Å,β= 114.853(2)°, C2/c, Z=8, R_f=0.047. The structure of the ligand PTP is also reported and compared with that of 2.     

Synthesis, structure and magnetic property of μ-phenoxo-μ-acetato-bis(η-acetato)dicobalt(II, II) complexes

Dinuclear cobalt(II) complexes Co₂(bpmp)(OAc)₃ (1) and Co₂(bpcp)(OAc)₃ (2) have been synthesized by using acyclic ligands 2,6-bis((4-(pyridin-2-yl)pyrimidin-2-ylthio)methyl)-4-methylphenol [H(bpmp)] and 2,6-bis((4-(pyridin-2-yl)pyrimidin-2-ylthio)methyl)-4-chlorophenol [H(bpcp)] with versatile coordination sites. X-ray analysis uncovered that complex 1 · 3H₂O contains a μ-phenoxo-μ-acetato-bis(η²-acetato) dicobalt(II, II) core. Magnetic susceptibility was measured for 1 over the temperature range 1.8-300 K, and the best theoretical fitting parameters were g = 2.12(6), J = −3.63(9) cm⁻¹ and D = −12(4) cm⁻¹.     

Substitution of trans ligands in μ-oxo-bis(μ-acetato)diruthenium(III) complexes: Synthesis and kinetic studies

[Ru₂O(L)₆(acetate)₂](PF₆)₂ {L = pyridine 1; 4-picoline 2} undergo aquation in acetone-water (60:40 v/v) mixed solvent to form diaquo complexes in solution as shown by proton NMR studies. Ligands trans to the μ-oxo group are substituted. These diaquo complexes react with substituted pyridines and imidazoles to form respective disubstituted complexes. Rate constants for aquation and complexation under pseudo first order conditions of ligand are reported. Rate constants increase with increase in the basicity of incoming ligand. Disubstituted complexes proposed to be formed in solution have been isolated and characterized by elemental analyses, visible spectra, proton NMR. Single crystal X-ray structures of 4-picoline and 4-methylimidazole disubstituted complexes are reported. All the isolated complexes exhibit a strong peak between 570 and 585 nm in their visible absorption spectra. λ_max varies linearly with ∑pk_a of terminal ligands. In disubstituted complexes of 1 with 2-methyl and 4-methyl imidazole deprotonation of N(1)H of methylimidazoles takes place in solution.     

Human HE2 (μB) and μA motifs show the same function as whole IgH intronic enhancer in transgenic mice

In the murine IgH gene intronic enhancer (ENH_iH), two major functional domains were reported. One is the E4/octomer region and another includes the A and B motifs. In the human ENH_iH, it was reported that the HE2, which corresponds to the murine B, and E6 motifs play an important role in an enhancer activity and a tissue-specificity at cellular level. Here we examined thein vivo function of the E6, A and HE2 motifs within the human ENH_iH by using the transgenic mice technique. The A and HE2 motifs together revealed almost the same enhancer function as the whole human ENH_iH, but the E6 motif had lesser enhancer acitivty and tissue-specificity.     

Magnetostructural correlations in μ-pyrazolato-μ-acetato dinickel(II) complexes: Subtle effects of acetate tilting

A series of dinickel acetato complexes [LNi₂(OAc)(solvent)_x](ClO₄)₂ (1-3, 5) as well as related complexes [LNi₂(OAc)₂](OAc) (4) and [LNi₂(OAc)(NO₃)₂] (6), all derived from three pyrazolate-based binucleating ligands, have been prepared and characterized by X-ray crystallography. The solid state structures reveal different acetate binding modes (μ_1,3-bridging and bidentate chelating) plus severe twisting and tilting of the μ_1,3-acetate bridges with respect to their bimetallic scaffolds, reflecting the great flexibility of carboxylate coordination. Magnetic properties of all six complexes have been investigated, and the strength of antiferromagnetic coupling is discussed in the light of the structural differences, suggesting a magnetostructural correlation for acetato-bridged complexes.     

Synthesis of [{Os3(CO)10(μ2-H)}2{μ2,μ2-NC6H4C6H4N}] and [{Os3(CO)9(μ2-H)PPh3}2{μ2,μ2-NC6H4C6H4N}]: Carbon–carbon bond formation promoted by organorhodium species

Synthesis and characterization of linked cluster [{Os₃(CO)₁₀(μ₂-H)}₂{μ₂,μ₂-NC₆H₄C₆H₄N}] (1) from the reaction of [Os₃Rh(μ-H)₃(CO)₁₂] with aniline in the presence of an excess amount of 4-vinyl phenol in refluxing heptane is reported. A similar reaction with [Os₃(CO)₁₀(NCMe)₂] as starting material gave a known compound, [Os₃(CO)₁₀(μ₂-H)(μ₂-HNC₆H₅)] (2). The treatment of complexes 1 and 2 with Wilkinson’s catalyst in refluxing heptane respectively, yielded [{Os₃(CO)₉(μ₂-H)PPh₃}₂{μ₂,μ₂-NC₆H₄C₆H₄N}] (3). An interesting and unexpected C–C coupling of phenyl-amido ligands was observed in complexes 1 and 3, which is believed to be catalysed by the organometallic rhodium species. The newly synthesized compounds 1 and 3 were fully characterized by IR, ¹H NMR spectroscopy, mass spectroscopy, elemental analysis, and X-ray crystallography. Both structures 1 and 3 comprise two triangles of osmium atoms. The two triangular osmium metal cores are linked by a bi-amido ligand via the two nitrogen atoms N(1) and N(1)* and N(1) and N(2), at their equatorial sites. The electronic absorption spectra of complexes 1, 2, and 3 display both low energy absorption, dπ (Os) → π* (amido) metal-to-ligand charge-transfer (MLCT) transition, and π → π* intra-ligand electronic transitions of the amido and bi-amido ligands.     

Oxidative nuclease activity of ferromagnetically coupled μ-hydroxo-μ-propionato copper(II) complexes [Cu3(L)2(μ-OH)2(μ-propionato)2] (L = N-(pyrid-2-ylmethyl)R-sulfonamidato, R = benzene, toluene, naphthalene)

Three doubly-bridged, trinuclear copper(II) compounds with hydroxo and carboxylato bridges, ^∞₁[Cu₃(L1)₂(μ-OH)₂(μ-propionato)₂](1), [Cu₃(L2)₂(μ-OH)₂(μ-propionato)₂(DMF)₂] (2) and ^∞₁{[Cu₃(L3)₂(μ-OH)₂(μ-propionato)₂]} [Cu₃(L3)₂(μ-OH)₂(μ-propionato)₂(DMF)₂]} (3) [HL1 = N-(pyrid-2-ylmethyl)benzenesulfonylamide, HL2 = N-(pyrid-2-ylmethyl)toluenesulfonylamide, HL3 = N-(pyrid-2-ylmethyl)naphthalenesulfonylamide], have been synthesized and characterized. 1 is built from [Cu₃(L1)₂(μ-OH)₂(μ-propionato)₂] clusters. Each unit contains three copper(II) with two different coordination environments: the terminal centers are square-base pyramidal whereas the central copper is square planar. 2 presents a similar square-base pyramidal geometry in the terminal centers, but the central copper is six-coordinate. 3 shows an unusual 1D coordination polymer comprised of two distinct building blocks: one similar to that found in 1 and the other similar to that found in 2. The magnetic susceptibility measurements (2-300 K) reveal a ferromagnetic interaction between the Cu(II) ions with J values of 76.0, 55.0, and 48.0 cm⁻¹ for 1, 2, and 3, respectively. Emission spectroscopy, thermal denaturation, viscosimetry and cyclic voltammetry show an interaction of the complexes with DNA through the sugar-phosphate backbone. All three Cu(II) complexes were found to be very efficient agents of plasmid DNA cleavage in the presence of ascorbato or mercaptopropionic acid. Both the kinetics and the mechanism of the cleavage reaction have also been examined.     

Structure and characterization of a μ-phenoxo-bis μ-acetato dinuclear Mn(II,II) complex [Mn2(LO)(μ-OAc)2](ClO4) (LOH = 2,6-bis{bis(2-(2-pyridyl)ethyl)aminomethyl}-4-methylphenol): Substituent effects

The synthesis, structure and characterization of the dinuclear Mn(II) complex [Mn₂(LO)(μ-OAc)₂](ClO₄) (1) where LOH = 2,6-bis{bis(2-(2-pyridyl)ethyl)aminomethyl)}-4-methylphenol are reported. The reaction of Mn(ClO₄)₂ · 6H₂O with the dinucleating ligand LOH and H₃CCOONa in the presence of NEt₃ in dry, degassed methanol and under an argon atmosphere, yields 1 as a colorless powder. The crystal structure of 1, determined by X-ray diffraction methods, shows a dinuclear Mn(II) complex in which two Mn(II) ions, each in six-coordinate approximate octahedral coordination, are bridged by the phenolate oxygen of LO⁻ and by two acetate ions in a syn,syn-1,3-bridging mode. The Mn-Mn distance is 3.557(1) Å and Mn-O_phenolate-Mn angle is 112.50(9)°. Cyclic voltammetry of 1 in acetonitrile solution shows a quasi-reversible wave at E_1/2 = 0.65 V, for the Mn₂(II,II)/Mn₂(II,III) redox process, and an irreversible oxidation peak at E_p,c = 1.22 V versus Ag/AgCl for the Mn₂(II,III) to Mn₂(III,III) oxidation process. Controlled potential electrolysis of 1 in acetonitrile solution at 0.85 V (versus Ag/AgCl) takes up 1 F of charge per mole of 1 to yield a brown solution of the Mn₂(II,III) state of the complex, which, however, is unstable and reverts back to the Mn₂(II,II) state in solution at room temperature. Least square fitting of the variable temperature magnetic susceptibility measurements on powdered sample of 1 is obtained with g = 1.888, J = −2.75 cm⁻¹, Par = 0.008, TIP = 0. The low −J value and the room temperature calculated magnetic moment of the complex (5.30 BM per Mn(II)), which is less than the spin-only moment of Mn(II), show that the two Mn(II) ions are weakly antiferromagnetically coupled.     

Microsecond single-molecule tracking (μsSMT)

Here we report on a method to track individual molecules on nanometer length and microsecond timescales using an optical microscope. Our method is based on double-labeling of a molecule with two spectrally distinct fluorophores and illuminating it with laser pulses of different wavelengths that partially overlap temporally. We demonstrate our method by using it to resolve the motion of short DNA oligomers in solution down to a timescale of 100 μs.     

Magnetostructural correlations and catecholase-like activities of μ-alkoxo-μ-carboxylato double bridged dinuclear and tetranuclear copper(II) complexes

Two μ-alkoxo-μ-carboxylato bridged dinuclear copper(II) complexes, [Cu₂(L¹)(μ-HCO₂)] (1) ((H₃L¹ = 1,3-bis(5-bromosalicylideneamino)-2-propanol)), [Cu₂(L²)(μ-HCO₂)] · dmf (2) (H₃L² = 1,3-bis(3,5-chlorosalicylideneamino-2-propanol)), and two μ-alkoxo-μ-dicarboxylato doubly bridged tetranuclear copper(II) complexes, [{Cu₂(L³)}₂(μ-O₂C–C(CH₃)₂–CO₂)] · 5H₂O · 3CH₃OH (3) ((H₃L³ = 1,3-bis(salicylid-deneamino)-2-propanol)) and [{Cu₂(L³)}₂(μ- O₂CCH₂–C₆H₄–CH₂CO₂)] · 2H₂O (4) have been prepared and characterized. The single crystal X-ray analysis shows that the structures of complexes 1 and 2 are dimeric with two adjacent copper(II) atoms bridged by μ-alkoxo-μ-carboxylato ligands with the Cu⋯Cu distances and Cu–O(alkoxo)–Cu angles are 3.511 Å and 132.85° for 1, 3.517 Å and 131.7° for 2, respectively. Complexes 3 and 4 consist of μ-alkoxo-μ-dicarboxylato doubly bridged tetranuclear Cu(II) complexes with mean Cu–Cu distances and Cu–O–Cu angles of 3.158 Å and 108.05° for 3 and 3.081 Å and 104.76° for 4, respectively. Magnetic measurements reveal that 1 and 2 are strong antiferromagnetically coupled with 2J = −156 and −152 cm⁻¹, respectively, while 3 and 4 exhibit ferromagnetic coupling with 2J = 86 and 155.2 cm⁻¹, respectively. The 2J values of 1–4 are linearly correlated to the Cu–O–Cu angles. Dependence of the pH at 25 °C on the reaction rate of oxidation of 3,4-di-tert-butylcatechol (3,5-dtbc) to the corresponding quinone catalyzed by 1–4 was studied. Complexes 1–4 exhibit high catecholase-like activity at pH 9.0 and 25 °C for oxidation of 3,5-di-tert-butylcatechol.