Slow Magnetic Relaxation in a Co2Dy Trimer and a Co2Dy2 Tetramer

The combination of Co(III) and Dy(III) with a compartmental Schiff base ligand (H₃L=3-[(2-Hydroxy-3-methoxy-benzylidene)-amino]-propane-1,2-diol), presenting three different coordinating pockets, has allowed the synthesis of two novel Co(III)−Dy(III) complexes: [Co₂Dy(HL)₄]NO₃ ⋅ 2CH₃CN ( 1 ), a rare example of trinuclear linear Co^III₂Dy^III complex (and the first with slow relaxation of magnetization in absence of a DC field) and [Co₂Dy₂(μ₃−OH)₂(HL)₂(OAc)₆] ⋅ 4.6H₂O ( 2 ), the first tetranuclear Co^III₂Dy^III₂ cluster with a rhomb-like structure where the Co(III) ions are connected along the short diagonal of the rhomb. 1 presents two different relaxation processes: a fast relaxation dominated by Quantum tunnelling (QT) and a slow relaxation with an energy barrier of 40 K. 2 shows two close relaxation processes without applied DC fields that follow QT and Orbach mechanisms whereas for H_DC=500 Oe, the QT is cancelled and a direct term appears. Here we present the synthesis, X-ray structure and magnetic characterization of these two Co(III)−Dy(III) single-ion/molecule magnets.     

2 + 2]Photocyclization in a single-crystal-to-single-crystal transformation of a TTF-amido-pyridine

The redox active ligand EDT-TTF-CONH-3-pyridine undergoes a [2 + 2] cycloaddition upon irradiation with polychromatic light, an unprecedented single-crystal-to-single-crystal transformation for a TTF derivative.     

2-Benzoyloxynitroethylene as a cis-2-aminoethenol equivalent

Nitroester 1 reacts readily with dienes to afford trans-nitroesters which are reduced to cis-aminoalcohols.     

Reaction Mechanisms in Both a CCl2F2/O2/Ar and a CCl2F2/H2/Ar RF Plasma Environment

Decomposition of dichlorodifluoromethane (CCl₂F₂ or CFC-12) in aradiofrequency (RF) plasma system is demonstrated. The CCl₂F₂decomposition fractions _{CCl 2 F 2} and mole fractionsof detected products in the effluent gas stream of CCl₂F₂/O₂/Ar andCCl₂F₂/H₂/Ar plasma, respectively, have been determined. The experimentalparameters including input power wattage, O₂/CCl₂F₂ or H₂/CCl₂F₂ ratio,operational pressure, and CCl₂F₂ feeding concentration wereinvestigated. The main carbonaceous product in the CCl₂F₂/O₂/Arplasma system was CO₂, while that in the CCl₂F₂/H₂/Ar plasma systemwas CH₄ and C₂H₂. Furthermore, the possible reaction pathways werebuilt-up and elucidated in this study. The results of the experimentsshowed that the highly electronegative chlorine and fluorine wouldeasily separate from the CCl₂F₂ molecule and combine with the addedreaction gas. This led to the reactions terminated with the CO₂,CH₄, and C₂H₂ formation, because of their high bonding strength. Theaddition of hydrogen would form a preferential pathway for the HCland HF formations, which were thermodynamically stable diatomicspecies that would limit the production of CCl₃F, CClF₃, CF₄, andCCl₄. In addition, the HCl and HF could be removed by neutral orscrubber method. Hence, a hydrogen-based RF plasma system provideda better alternative to decompose CCl₂F₂.     

Synthese und Kristallstruktur von [(PhCH2)2GaF(tBuNH2)]2 · 2 THF

Synthesis and Crystal Structure of [(PhCH₂)₂GaF(^tBuNH₂)]₂ · 2 THF (PhCH₂)₂GaF reacts with ^tBuNH₂ to the adduct [(PhCH₂)₂GaF(^tBuNH₂)] ( 1 ). 1 was characterized by NMR, IR and MS techniques. 1 can be recrystallized from THF forming crystals of [ 1 ]₂ · 2 THF. According to an X-ray structure analysis [ 1 ]₂ · 2 THF consists of dimers of 1 formed by hydrogen bridges. The THF molecules are coordinated to [ 1 ]₂ by hydrogen bridges, too.     

Ba2Ti2Si2O9F2, a new titanium silicate

Dibarium dititanium difluoride dioxide heptaoxidodisilicate, Ba₂Ti₂Si₂O₉F₂, is a new edge‐sharing titanate with a unique titanium silicate framework. All atoms in the structure are in general positions. Titanium oxyfluoride octahedra combine with silicon tetrahedra to form a double stacked chain, which is the base unit of the layered framework. The Ba atoms lie in channels that extend along the a axis.     

Formation of a PNA2-DNA2 hybrid quadruplex

DNA guanine (G) quadruplexes are stabilized by an interesting variation of the hydrogen-bonding schemes encountered in nucleic acid duplexes and triplexes. In an attempt to use this mode of molecular recognition, we target a dimeric G-quadruplex formed by the Oxytricha nova telomeric sequence d(G(4)T(4)G(4)) with a peptide nucleic acid (PNA) probe having a homologous rather than complementary sequence. UV-vis and CD spectroscopy reveal that a stable hybrid possessing G-quartets is formed between the PNA and DNA. The four-stranded character of the hybrid and the relative orientation of the strands is determined by fluorescence resonance energy transfer (FRET) experiments. FRET results indicate that (i) the two PNA strands are parallel to each other, (ii) the two DNA strands are parallel to each other, and (iii) the 5'-termini of the DNA strands align with the N-termini of the PNA strands. The resulting PNA(2)-DNA(2) quadruplex shows a preference of Na(+) over Li(+) and displays thermodynamic behavior consistent with alternating PNA and DNA strands in the hybrid. The formation of this novel supramolecular structure demonstrates a new high-affinity DNA recognition mechanism and expands the scope of molecular recognition by PNA.     

Oxidation-induced ring expansion of a 2-phosphino-2H-azirine

Oxidation of the 2-phosphino-2H-azirine 1 by preparative scale electrolysis quantitatively leads to the four-membered phosphonium heterocycle 3, while addition of tetrachloro-o-benzoquinone to 1 produces an eight-membered heterocycle 4 with 60 % yield. In both cases, the first step is the formation of a transient phosphorus centered radical cation 1^•+, which induces the cleavage of the CN bond and the ring expansion reaction of the azirine moiety affording the four-membered ring intermediate 2^•+.     

Direct Synthesis of H2O2 by a H2/O2 Fuel Cell

Direct synthesis of H₂O₂ solutions by a fuel cell method was reviewed. The fuel cell reactor of [O₂, gas-diffusion cathode electrolyte solutions Nafion membrane electrolyte solutions gas-diffusion anode, H₂] is very effective for formation of H₂O₂. The three-phase boundary (O₂(g)–electrode(s)–electrolyte(l)) in the gas-diffusion cathode is essential for efficient formation of H₂O₂. Fast diffusion processes of O₂ to the active surface and of H₂O₂ to the bulk electrolyte solutions are essential for H₂O₂ accumulation. The maxima H₂O₂ concentrations of 1.2 M (3.5 wt%) and 2.4 M (7.0 wt%) were accomplished by the heat-treated Mn-OEP/AC electrocatalyst with H₂SO₄ electrolyte and by the VGCF electrocatalyst with NaOH electrolyte, respectively, under short circuit conditions.     

Oxidation by a H2O2-vanadium complex-2-pyrazinecarboxylic acid reagent

Evidence for the formation of hydroxyl radicals and their participation in hydrocarbon oxidation by a H₂O₂-vanadium complex-2-pyrazinecarboxylic acid reagent has been obtained by the spin trap method and kinetically, by competitive oxidation of a benzenealiphatic alcohol mixture.For part 2, see Ref. l.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1212–1214, July, 1994.This work was financially supported by the Russian Foundation for Basic Research (project No. 93-03-5226) and the International Science Foundation (Grant No. MMS000). The authors are grateful to D. Attanasio and L. Suber (Italy) for help in the work and to S. N. Dobryakov and Yu. N. Kozlov for their participation in the discussions.     

Darstellung,Schwingungsspektren und Kristallstrukturen von [(Ph3P)2N]2[(W6Cl)I] · 2 Et2O · 2 CH2Cl2 und [(Ph3P)2N]2[(W6Cl)(NCS)] · 2 CH2Cl2

Synthesis, Crystal Structures, and Vibrational Spectra of [(Ph₃P)₂N]₂[(W₆Cl )I ] · 2 Et₂O · 2 CH₂Cl₂ and [(Ph₃P)₂N]₂[(W₆Cl )(NCS) ] · 2 CH₂Cl₂ By treatment of [(W₆Cl)I]^2– with (SCN)₂ in dichloromethane at –20 °C the hexaisothiocyanato cluster anion [(W₆Cl)(NCS)]^2– is formed. X‐ray structure determinations have been performed on single crystals of [(Ph₃P)₂N]₂[(W₆Cl)I] · 2 CH₂Cl₂ · 2 Et₂O ( 1 ) (triclinic, space group P1, a = 10.324(5), b = 14.908(3), c = 17.734(8) Å, α = 112.78(2)°, β = 99.13(3)°, γ = 92.02(3)°, Z = 1) and [(Ph₃P)₂N]₂[(W₆Cl)(NCS)] · 2 CH₂Cl₂ ( 2 ) (triclinic, space group P1, a = 11.115(2), b = 14.839(2), c = 17.036(3) Å, α = 104.46(1)°, β = 105.75(2)°, γ = 110.59(1)°, Z = 1). The thiocyanate ligands of 2 are bound exclusively via N atoms with W–N bond lengths of 2.091–2.107 Å, W–N–C angles of 173.1–176.9° and N–C–S angles of 178.1–179.3°. The vibrational spectra exhibit characteristic innerligand vibrations at 2067–2045 (ν_CN), 879–867 (ν_CS) and 490–482 (δ_NCS). Based on the molekular parameters of the X‐ray determination of 1 the vibrational spectra of the corresponding (n‐Bu₄N) salt of 1 are assigned by normal coordinate analysis. The valence force constants are f_d(WW) = 1.61, f_d(WI) = 1.23 and f_d(WCl) = 1.10 mdyn/Å.     

Ni(Et2PCH2NMeCH2PEt2)2]2+ as a functional model for hydrogenases

The reaction of Et(2)PCH(2)N(Me)CH(2)PEt(2) (PNP) with [Ni(CH(3)CN)(6)](BF(4))(2) results in the formation of [Ni(PNP)(2)](BF(4))(2), which possesses both hydride- and proton-acceptor sites. This complex is an electrocatalyst for the oxidation of hydrogen to protons, and stoichiometric reaction with hydrogen forms [HNi(PNP)(PNHP)](BF(4))(2), in which a hydride ligand is bound to Ni and a proton is bound to a pendant N atom of one PNP ligand. The free energy associated with this reaction has been calculated to be -5 kcal/mol using a thermodynamic cycle. The hydride ligand and the NH proton undergo rapid intramolecular exchange with each other and intermolecular exchange with protons in solution. [HNi(PNP)(PNHP)](BF(4))(2) undergoes reversible deprotonation to form [HNi(PNP)(2)](BF(4)) in acetonitrile solutions (pK(a) = 10.6). A convenient synthetic route to the PF(6)(-) salt of this hydride involves the reaction of PNP with Ni(COD)(2) to form Ni(PNP)(2), followed by protonation with NH(4)PF(6). A pK(a) of value of 22.2 was measured for this hydride. This value, together with the half-wave potentials of [Ni(PNP)(2)](BF(4))(2), was used to calculate homolytic and heterolytic Ni-H bond dissociation free energies of 55 and 66 kcal/mol, respectively, for [HNi(PNP)(2)](PF(6)). Oxidation of [HNi(PNP)(2)](PF(6)) has been studied by cyclic voltammetry, and the results are consistent with a rapid migration of the proton from the Ni atom of the resulting [HNi(PNP)(2)](2+) cation to the N atom to form [Ni(PNP)(PNHP)](2+). Estimates of the pK(a) values of the NiH and NH protons of these two isomers indicate that proton migration from Ni to N should be favorable by 1-2 pK(a) units. Cyclic voltammetry and proton exchange studies of [HNi(depp)(2)](PF(6)) (where depp is Et(2)PCH(2)CH(2)CH(2)PEt(2)) are also presented as control experiments that support the important role of the bridging N atom of the PNP ligand in the proton exchange reactions observed for the various Ni complexes containing the PNP ligand. Similarly, structural studies of [Ni(PNBuP)(2)](BF(4))(2) and [Ni(PNP)(dmpm)](BF(4))(2) (where PNBuP is Et(2)PCH(2)N(Bu)CH(2)PEt(2) and dmpm is Me(2)PCH(2)PMe(2)) illustrate the importance of tetrahedral distortions about Ni in determining the hydride acceptor ability of Ni(II) complexes.     

Anion-pi-pi interactions in a dinuclear M2L2 metallocycle

The combination of the square-planar cis-protected [Pt(dppp)(O 3SCF 3) 2] (M) with the ditopic N, N'-bis(3,5-dimethyl-4-pyridinyl)-4-ethoxy-2,6-pyridinedicarboxamide ligand (L) leads both in solution and in the solid state to the quantitative self-assembly of a M 2L 2 metallocycle. We also report crystallographic evidence derived from the solid-state structure of the dinuclear metallocycle in support of the existence of anion-pi-pi interactions.     

Microperoxidase-11/NH2-FSM16 as a H2O2-resistant heterogeneous nanobiocatalyst: a suicide-inactivation study

The catalytic activity of heme peptides is an area of intense investigation. They are utilized for exploring the fine details of structural and functional properties of an active site, and to create minimized and industrial catalysts. The peroxidase activity and kinetics of suicide-inactivation of microperoxidase-11/FSM16 as a heterogeneous nanobiocatalyst in oxidation reaction of guaiacol were studied in the presence of high concentration of hydrogen peroxide (2?mM), as its natural suicide-substrate. The substrate concentration was first-order in relation to aromatic substrate (AH), and the ratio of suicide-substrate (H₂O₂) was kept much higher than the benign substrate (guaiacol). The results of kinetic analysis confirmed a similar mechanism for suicide-peroxide inactivation of horseradish peroxidase (HRP), microperoxidase (MP-11) and MP-11/NH₂-FSM16. Inactivation kinetic parameters, including intact activity of MP-11/NH₂-FSM16, ??_i, and the apparent inactivation rate constant (k _i) were obtained as 0.229?±?0.009?min^?1 and 0.651?±?0.041?min^?1 at [H₂O₂]?=?2.0?mM, respectively, in 5.0?mM phosphate buffer solution (PBS; pH 7.0) at 27?°C. Our results indicated that covalent immobilization of microperoxidase onto NH₂-FSM16 protected the heme group against peroxide inactivation resulting in generation of an efficient peroxide-resistant heterogeneous nanobiocatalyst.     

Thermal [2 + 2] cycloreversion of a cyclobutane moiety via a biradical reaction

The nature of the Woodward-Hoffmann-forbidden, thermal activated cycloreversion mechanism of cyclobutane has long been the subject of speculation and intense research. We were now able to prove the theoretically postulated biradicalic mechanism directly from radical scavenging reactions and electron paramagnetic resonance (EPR) experiments on [2 + 2] heterodimers of 5-fluoro-1-heptanoyluracil and 7-methoxy-1,1-dimethylnaphthalenon. The dimers show both the "allowed" photochemically as well as the "forbidden" thermally triggered [2 + 2] cycloreversion of the cyclobutane ring. The quantum efficiency of the photochemical cleavage is about 1%. The thermal cycloreversion reaction is independent from solvent and occurs at low activation energies of about 13 kcal/mol, even in the solid state. The radical scavenger and EPR results are further supported by the finding, that the reaction products are solely the educts for the anti-head-to-tail heterodimer. But for the syn-head-to-head heterodimer two additional products are observed, which require a sufficiently stable biradical intermediate to facilitate the required intramolecular rearrangements. Because of the surprisingly high lifetime of the radical species of these heterodimers it was possible to prove the long-discussed biradical mechanism experimentally.