A new class of linear tetrapyrroles: acetylenic 10,10a-didehydro-10a-homobilirubins

Novel bilirubin analogues with dipyrrinones conjoined to an acetylene rather than a methylene group were synthesized and examined spectroscopically. Despite the increased separation of the dipyrrinones forced by replacing a -CH(2)- by a -C(triple bond)C- unit, molecular dynamics calculations show that, like bilirubin, they may still engage in intramolecular hydrogen bonding to carboxylic acid groups when the propionic acid chains are slightly lengthened, e.g., butanoic acids. Unlike bilirubin, however, which is bent in the middle and has a ridge-tile shape, the acetylene orients the attached dipyrrinones along a linear path, and intramolecular hydrogen bonding preserves a twisted linear molecular shape. The extended planes of the dipyrrinones intersect along the -C(triple bond)C- axis at an angle of 136 degrees for the conformation stabilized by intramolecular hydrogen bonding in the bis-butyric acid rubin (1b). With shorter acid chains (propionic), only one CO(2)H can engage an opposing dipyrrinone in intramolecular hydrogen bonding, and in this energy-minimum conformation of the linear pigment 1a, the intersection of the extended planes of the dipyrrinones has an angle of 171 degrees. Spectroscopic evidence for such linearized and twisted structures was found in the pigments' NMR spectral data and their exciton UV-vis and induced circular dichroism spectra.     

Synthesis,characterisation and solid state thermal behaviour of nickel(II) diamine complexes

Summary [NiL₂X₂] (L =N,N-dimethyl-1,2-ethanediamine; X = Cl^–, CF₃CO ₂ ^– , CC1₃CO ₂ ^– and CBr₃CO ₂ ^– ), [NiL₂C₂O₄] · H₂O and [NiL₂X₂] · 2 H₂O (X = Br^–, 0.5 SO ₄ ^2– and 0.5 SeO ₄ ^– ) have been synthesised and their thermal studies carried out. Thermally induced phase transition phenomena are noticed in [NiL₂X₂] (X = CF₃CO ₂ ^– and CCl₃CO ₂ ^– ) and their probable mechanisms are described. [NiL₂X₂] (X = Br^–, 0.5 SO ₄ ^2– and 0.5 SeO ₄ ^2– ) and [NiLX₂] (X = Cl^–, 0.5 C₂O ₄ ^2– and 0.5 SO ₄ ^2– ) have been prepared by solid state pyrolysis from the respective parent diamine complexes. [NiL₂X₂] have been made in solid state by temperature arrest technique from [NiL₂(CX₃CO₂)₂] (X = Cl^– and Br^–).     

Donor–acceptor complexes in copolymerization. XLIX. Cationic polymerization of donor monomers in presence of polar solvents and acrylic monomers. A revised mechanism for polymerization of comonomer complexes

α-Methylstyrene (MS) and isobutyl vinyl ether (VE) readily polymerize, styrene (S) polymerizes to a small extent, and isobutylene (IB), butadiene (BD), and isoprene (IP) fail to polymerize in the presence of catalytic amounts of AlCl₃ when propionitrile, ethyl propionate, and methyl isobutyrate are used as reaction media. MS polymerizes readily and S polymerizes with difficulty in the presence of AlCl₃ to yield homopolymers when acrylonitrile (AN) is present and copolymers with ethyl acrylate (EA) and methyl methacrylate (MMA). VE readily homopolymerizes, while IB, BD, and IP fail to polymerize in the presence of AlCl₃ and the acrylic monomers. VE readily homopolymerizes, S and MS polymerize to a very small extent, and IB, BD, and IP do not polymerize in the presence of ethylaluminum sesquichloride (EASC) in polar solvents. VE readily homopolymerizes in the presence of EASC and the acrylic monomers. MS polymerizes to a small extent in the presence of EASC and the acrylic monomers to yield equimolar copolymers with EA and MMA and a mixture of cationic homopolymer and equimolar copolymer with AN. S yields equimolar copolymers in low yield in the presence of EASC and the acrylic monomers. IB, BD, and IP in the presence of EASC do not polymerize to any significant extent when EA is present, form AN-rich copolymers and yield poly(methyl methacrylate) in the presence of MMA. A revised mechanism is presented for the formation of cationic, radical, random, and alternating copolymers as well as alternating copolymer graft copolymers in the copolymerization of donor and acceptor monomers.     

Synthesis, structures and reactivity of triosmium clusters containing terminal pyrazines, bridging hydroxy and methoxycarbonyl ligands

Four triosmium carbonyl clusters bearing terminal pyrazines, bridging hydroxy and methoxycarbonyl ligands of general formula [Os₃(CO)₉(μ-OH)(μ-OMeCO)L] (1, L = pyrazine; 2, L = 2-methylpyrazine; 3, L = 2,3-dimethylpyrazine; 4, L = 2,3,5-trimethylpyrazine) were synthesized by the reactions of [Os₃(CO)₁₂] with the corresponding pyrazine derivatives and water in the presence of a methanolic solution of Me₃NO in moderate yields. Compounds [Os₃(CO)₉(μ-OH)(μ-OMeCO)L] react with a series of two electron donor ligands, L′ at ambient temperature to give [Os₃(CO)₉(μ-OH)(μ-OMeCO)L′] (5, L′ = PPh₃; 6, L′ = P(OMe)₃; 7, L′ = ^tBuNC; 8, L′ = C₅H₅N) in good yields by the displacement of the pyrazine ligands. This implies that the pyrazine ligands in 1–4 are relatively labile. Compounds 2, 3, 4, and 8 were characterized by single crystal X-ray diffraction analyses. All the four compounds possess two metal–metal bonds and a non-bonded separation of two osmium atoms defined by Os(1)Os(3), which are simultaneously bridged by OH and MeOCO ligands and a heterocyclic ligand is terminally coordinated to one of the two non-bonded osmium atoms.     

DNA-binding studies of mixed ligand cobalt(III) complexes

The DNA binding characteristics of mixed ligand complexes of the type [Co(en)₂(L)]Br₃ where en = N,N′-ethylenediamine and L = 1,10-phenanthroline (phen), 2,2′-bipyridine (bpy), 1,10-phenanthroline-5,6-dione (phendione), dipyrido[3,2-a:2′,3′-c]phenazine (dppz) have been investigated by absorption titration, competitive binding fluorescence spectroscopy and viscosity measurements. The order of intercalative ability of the coordinated ligands is dppz > phen > phendione > bpy in this series of complexes.     

Oxovanadium(IV) and -(V) complexes of dithiocarbazate-based tridentate Schiff base ligands: syntheses,structure, and photochemical reactivity of compounds involving imidazole derivatives as coligands

The tridentate dithiocarbazate-based Schiff base ligands H(2)L (S-methyl-3-((5-R-2-hydroxyphenyl)methyl)dithiocarbazate, R = NO(2), L = L(2); R = Br, L = L(3)) react with [VO(acac)(2)] in the presence of imidazole derivatives as coligands to form oxovanadium(IV) and cis-dioxovanadium(V) complexes. With benzimidazole and N-methylimidazole, the products are oxovanadium(IV) complexes, viz. [VOL(3)(BzIm)].0.5CH(3)CN (1a) and [VOL(N-MeIm)(2)] (L = L(3), 1b; L = L(2), 1c), respectively. In both 1a,b, the O and S donor atoms of the tridentate ligand are cis to the terminal oxo group (in the "equatorial" plane) and mutually trans, but the N donor atom is respectively cis and trans to the oxo atom, as revealed from X-ray crystallography. When imidazole or 4-methylimidazole is used as the ancillary ligand, the products obtained are water-soluble cis-dioxovanadium(V) complexes [VO(2)L(R'-ImH)] (L = L(3) and L(2), R' = H and Me, 2a-d). These compounds have zigzag chain structures in the solid state as confirmed by X-ray crystallographic investigations of 2a,d, involving an alternating array of LVO(2)(-) species and the imidazolium counterions held together by Coulombic interactions and strong hydrogen bonding. Complexes 2a-d are stable in water or methanol. In aprotic solvents, viz. CH(3)CN, DMF, or DMSO, however, they undergo photochemical transformation when exposed to visible light. The putative product is a mixed-oxidation divanadium(IV/V) species obtained by photoinduced reduction as established by EPR, electronic spectroscopy, and dynamic (1)H NMR experiments.     

Kinetic studies on the acid catalyzed reaction of hydroxamic acids in β-cyclodextrin/surfactant mixed systems

The acid catalyzed hydrolysis of two N-substituted hydroxamic acids (C₆H₅CON(OH)R, R = C₆H₅ (PBHA), R = C₆H₅CH₂(BBHA)) in mixed systems containing -cyclodextrin (—CD) and a surfactant (sodium dodecyl sulfate, SDS) has been studied. The reactions are inhibited by —CD. The inhibition is attributed to the formation of inclusion complex and competition between the micellization and complexation processes.This revised version was published online in December 2005 with corrections to the Cover Date.     

Electron transfer. 138. Potentials involving chelated chromium(V)

The bis(chelated) complex of Cr^V(0) derived from the dianion (L^{2 −}) of 2-ethyl-2-hydroxybutanoic acid is readily reduced to a bis(chelate of Cr^III, featuring the monoanion (LH⁻) [Cr ^V(0)(L²⁻)₂]⁻+4H⁺+H₂O+2e⁻→[Cr^III(OH₂)₂(LH⁻ ₂]⁺ of this acid. Potentials estimated by Ghosh in 1993 for this 2e⁻ change, E⁰ (pH 0) 1.32 V, E^eff (pH 3.3) 0.93 V, are in accord with the nearly irreversible reductions of the Cr(V) species (in 1∶1 ligand buffer) by Fe²⁺, V0²⁺, IrCl₆ ^{3 −} and I⁻, whereas lower values reported by Bose in 1996, E⁰ (pH 0) 0.84 V, E^eff (pH 3.3) 0.45 V, are potentiometrically inconsistent with these conversions. A similar discrepancy is noted for potentials for Cr(V,IV) estimated in 1996, E⁰ (pH 0) 0.84 V, E^eff (pH 3.3) 0.46 V, which, wholly contrary to observation, predict that the reductions of excess Cr(V) to CR(IV) by Fe²⁺, V0²⁺, and I⁻ are thermodynamically disfavored.     

Cupric sulfate pentahydrate (CuSO4·5H2O): a mild and efficient catalyst for tetrahydropyranylation/depyranylation of alcohols and phenols

Various alcohols and phenols can be smoothly converted to the corresponding THP ethers using 20 mol % CuSO₄·5H₂O under mild reaction conditions at room temperature. Some of the major advantages of this procedure are nonaqueous work-up, very good yields, less expensive catalyst and compatibility with other protecting groups.     

Kinetics and mechanism of the interaction of DL-methionine with cis-diaquaethylenediamine platinum(II) perchlorate in aqueous medium

The kinetics of the interaction of DL-methionine with [Pt(en)(H2O)2]2+ have been studied spectrophotometrically as a function of [Pt(en)(H2O)22+], [DL-methionine], PH and temperature. The reaction proceeds via a rapid outer sphere association followed by two slow consecutive steps having first order dependence on the aqua ion and methionine concentrations. Activation parameters have been evaluated using the Eyring equation. The low H1 (15.6 kJ mol–1) and large negative value of S1 (–230 J K–1 mol–1) as well as H2 (19.5 kJ mol–1) and S2 (–226 J K–1 mol–1) indicate an associative mode of activation for both the aqua ligand substitution processes in the two consecutive steps.