Oxidation of nucleic acid bases by hexavalent manganese in aqueous alkaline medium: a kinetic study

Oxidation of uracil or thymine by Mn^VI in aqueous alkali shows first order dependence on [MnO ₄ ²⁻ ] and fractional order dependence on [uracil] or [thymine]. The effect of ionic strength is negligible on the rate of oxidation. The product of oxidation is the 5,6-dihydroxy compound. A mechanism is proposed involving the formation of a 5-membered cyclic complex between the substrate and manganate in a fast step, which subsequently disproportionates to give the final products. The large negative values of the entropy of activation suggest the formation of an intermediate complex. The formation constants of the latter and the rate of its disproportionation have been calculated from the derived rate law. The rate of oxidation of thymine is faster than that of uracil.     

Bipyricorrole: A Corrole Homologue with a Monoanionic Core as a Fluorescence ZnII Sensor

In the corrole homologue, 6,11,16‐triarylbipyricorrole, the bipyrrole unit is replaced by a 2,2′‐bipyridine unit. This modification effectively alters the corrole N4 coordination sphere from the trianionic [(NH)₃N] to the monoanionic [N₃NH] state. The newly formed monoanionic core stabilizes Zn^II ions with enhanced emission properties. The enhanced emission was further utilized for metal ion sensing studies and exploited for the selective detection of Zn^II ions.     

Synthesis and X-ray structural characterization of mono(aminophosphine) derivatives of molybdenum hexacarbonyl, Mo(CO)5L {L?=?P(NC5H10)3, P(Ph)(NC4H8O)2 or P(Ph){N(i-C3H7)2}(NC4H8O)}

Equimolar reactions of molybdenum hexacarbonyl with tris(piperidino)phosphine (L1), bis(morpholino)(phenyl)phosphine (L2), and (Di-isopropylamino)(morpholino)(phenyl) phosphine (L3), which are examples of symmetrically and unsymmetrically substituted tertiary(amino)phosphines, afford the corresponding mono derivatives, Mo[P(NC₅H₁₀)₃)](CO)₅ (1), Mo[P(Ph)(NC₄H₈O)₂](CO)₅ (2), and Mo[P(Ph){N(i-C₃H₇)₂}(NC₄H₈O)](CO)₅ (3) in moderate yields as air stable crystalline solids. In the case of L1, some amount of trans-bis derivative, Mo[P(NC₅H₁₀)₃)]₂(CO)₄ (4), was also isolated. X-ray structures of 1, 2, and 3 have been determined. Compounds 1 and 2 crystallize in the monoclinic system with space group P21/c, while 3 crystallizes in the triclinic system with space group Pī. While the Mo-P and Mo-C_ax bond distances in these complexes are comparable with those of other P-C bonded phosphines, the presence of chiral phosphine seems to induce a relatively weaker interaction with the molybdenum center. Intermolecular hydrogen bonding is seen in compound 1.     

Kinetics of oxidation of adenosine by t-butoxyl radical: Protection and repair by caffeic acid

The kinetics of oxidation of adenosine and caffeic acid by t-BuO^• has been studied by the photolysis of t-BuOOH in the presence of t-BuOH. The rates and the quantum yields (φ) of oxidation of caffeic acid by t-BuO^• radicals have been determined in the absence and presence of varying concentrations of adenosine. An increase in the concentration of adenosine has been found to decrease the rate of oxidation of caffeic acid suggesting that adenosine and caffeic acid compete for t-BuO^• radicals. From competition kinetics, the rate constant of t-BuO^•–caffeic acid reaction has been calculated to be 8.15 × 10⁸ dm³ mol⁻¹ s⁻¹. The results of experimentally determined quantum yield (φ_exptl) values of oxidation of caffeic acid and the quantum yield values calculated (φ_cal) by assuming that caffeic acid reacts only with t-BuO^• radicals suggest that caffeic acid not only protects adenosine from t-BuO^• radicals but also repairs adenosine radicals formed by the reaction of t-BuO^• radicals. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 515–521, 2005     

Stable (BIII-Subporphyrin-5-yl)dicyanomethyl Radicals

Stable B^III-subporphyrin-substituted dicyanomethyl radicals were synthesized by S_NAr reaction of meso-bromo- or meso-chlorosubporphyrins with malononitrile followed by oxidation with PbO₂. Different from previously reported dicyanomethyl radicals that underwent σ- or π-dimer formation both in the solid state and in solutions, subporphyrin-stabilized dicyanomethyl radicals exist as monomers in solutions even at low temperature. DFT calculations revealed efficient spin delocalization over the entire subporphyrin. In the solid state, these radicals form weak π-dimers with antiferromagnetic interactions depending on the crystal packing structures.     

Synthesis and Structures of RhI and IrI Complexes Supported by N‐Heterocyclic Carbene‐Phosphinidene Adducts

N‐Heterocyclic carbene‐phosphinidene adducts of the type (IDipp)PR [R = Ph ( 5 ), SiMe₃ ( 6 ); IDipp = 1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene] were used as ligands for the preparation of rhodium(I) and iridium(I) complexes. Treatment of (IDipp)PPh ( 5 ) with the dimeric complexes [M(μ‐Cl)(COD)]₂ (M = Rh, Ir; COD = 1,5‐cyclcooctadiene) afforded the corresponding metal(I) complexes [M(COD)Cl{(IDipp)PPh}] [M = Rh ( 7 ) or Ir ( 8 )] in moderate to good yields. The reaction of (IDipp)PSiMe₃ ( 6 ) with [Ir(μ‐Cl)(COD)]₂ did not yield trimethylsilyl chloride elimination product, but furnished the 1:1 complex, [Ir(COD)Cl{(IDipp)PSiMe₃}] ( 9 ). Additionally, the rhodium‐COD complex 7 was converted into the corresponding rhodium‐carbonyl complex [Rh(CO)₂Cl{(IDipp)PPh}] ( 10 ) by reaction with an excess of carbon monoxide gas. All complexes were fully characterized by NMR spectroscopy, microanalyses, and single‐crystal X‐ray diffraction studies.     

N‐Heterocyclic Carbene–Phosphinidene Complexes of the Coinage Metals

Coinage metal complexes of the N‐heterocyclic carbene–phosphinidene adduct IPr ? PPh (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) were prepared by its reaction with CuCl, AgCl, and [(Me₂S)AuCl], which afforded the monometallic complexes [(IPr ? PPh)MCl] (M=Cu, Ag, Au). The reaction with two equivalents of the metal halides gave bimetallic [(IPr ? PPh)(MCl)₂] (M=Cu, Au); the corresponding disilver complex could not be isolated. [(IPr ? PPh)(CuOTf)₂] was prepared by reaction with copper(I) trifluoromethanesulfonate. Treatment of [(IPr ? PPh)(MCl)₂] (M=Cu, Au) with Na(BAr^F) or AgSbF₆ afforded the tetranuclear complexes [(IPr ? PPh)₂M₄Cl₂]X₂ (X=BAr^F or SbF₆), which contain unusual eight‐membered M₄Cl₂P₂ rings with short cuprophilic or aurophilic contacts along the chlorine‐bridged M???M axes. Complete chloride abstraction from [(IPr ? PPh)(AuCl)₂] was achieved with two equivalents of AgSbF₆ in the presence of tetrahydrothiophene (THT) to form [(IPr ? PPh){Au(THT)}₂][SbF₆]₂. The cationic tetra‐ and dinuclear complexes were used as catalysts for enyne cyclization and carbene transfer reactions.     

A 6,11,16‐Triarylbiphenylcorrole with an adj‐CCNN Core: Stabilization of an Organocopper(III) Complex

An adj‐dicarbacorrole with CCNN in the core is achieved by replacing the bipyrrole moiety by a simple polycyclic aromatic hydrocarbon, such as biphenyl unit. Spectroscopic studies and structural analyzes confirm the absence of macrocyclic aromatization, thus leading to overall nonaromatic character. The trianionic core is effectively utilized to stabilize a copper(III) ion to form an organocopper complex.     

Cyclophane-Type Chlorin Dimers from Dynamic Covalent Chemistry of 2,18-Porphyrinyl Dicyanomethyl Diradicals

2,18-Bis(dicyanomethyl)-substituted Ni^II porphyrin 8 and Zn^II porphyrin 11 were prepared and subjected to oxidation with PbO₂ in CH₂Cl₂ at 298 K to give cyclophane-type chlorin dimers ( 9 )₂ and ( 12 )₂ as a consequence of double recombination of biradicals 9 and 12 , respectively. Dimer ( 9 )₂ takes a syn-conformation of two distorted Ni^II chlorins but ( 12 )₂ takes an anti-conformation of relatively planar Zn^II chlorins. At 298 K, dimer ( 9 )₂ is stable and its ¹H NMR spectrum is sharp but becomes broad at high temperature, while the ¹H NMR spectrum of ( 12 )₂ is considerably broad even at 298 K but becomes sharper at low temperature. These results indicate that the chlorin dimers dissociate to radical species, but the activation barrier of the dissociation of ( 12 )₂ is much less than that of ( 9 )₂. The involvement of diradicals in dynamic covalent chemistry has been suggested by thermal scrambling of hetero dimer ( 16 )₂ to give homo dimers ( 9 )₂ and ( 15 )₂.