Excited-state distortions determined from structured luminescence of nitridorhenium(V) complexes

The nitridorhenium(V) complexes ReNCl(2)(PCy3)(2) (1), ReNBr(2)(PCy3)(2) (2), ReNCl(2)(PPh3)(2) (3), and ReNBr(2)(PPh3)(2) (4) produce structured emission spectra upon excitation at low temperature. The origin, E(00), occurs at 15 775, 16 375, 15 875, and 16 300 cm(-1), respectively. The vibronic peaks are regularly spaced with an average energy separation corresponding to the Re triple bond N stretching frequency. The nitridorhenium stretching frequency ranges from 1095 to 1101 cm(-1), as determined by Raman and IR spectroscopy. The excited-state distortions are calculated by fitting the emission spectra. The excited state arises primarily from a d(xy) (ReN nonbonding) to d(yz) (ReN pi antibonding) transition. The rhenium-nitrogen bond length in the excited state is 0.08 A longer than in the ground electronic state, which is consistent with the difference in bond lengths of ReN bonds of bond order 3 and bond order 2.5 as determined from molecular structures.     

Deuterium NMR spectroscopy is a versatile and economical tool for monitoring reaction kinetics in ionic liquids

Time-resolved 2H NMR spectroscopy is used to monitor the progress of and gain kinetic information for a variety of reactions in different ionic media.     

Multielectron atom transfer reactions of perchlorate and other substrates catalyzed by rhenium oxazoline and thiazoline complexes: reaction kinetics, mechanisms, and density functional theory calculations

The title complexes, the Re(O)L(2)(Solv)(+) complexes (L = hoz, 2-(2'-hydroxyphenyl)-2-oxazoline(-) or thoz, 2-(2'-hydroxyphenyl)-2-thiazoline(-); Solv = H(2)O or CH(3)CN), are effective catalysts for the following fundamental oxo transfer reaction between closed shell molecules: XO + Y --> X + YO. Among suitable oxygen acceptors (Y's) are organic thioethers and phosphines, and among suitable oxo donors (XO's) are pyridine N-oxide (PyO), t-BuOOH, and inorganic oxyanions. One of the remarkable features of these catalysts is their high kinetic competency in effecting perchlorate reduction by pure atom transfer. Oxo transfer to rhenium(V) proceeds cleanly to afford the cationic dioxorhenium(VII) complex Re(O)(2)L(2)(+) in a two-step mechanism, rapid substrate (XO) coordination to give the precursor adduct cis-Re(V)(O)(OX)L(2)(+) followed by oxygen atom transfer (OAT) as the rate determining step. Electronic variations with PyO derivatives demonstrated that electron-withdrawing substituents accelerate the rate of Re(VII)(O)(2)L(2)(+) formation from the precursor adduct cis-Re(V)(O)(OX)L(2)(+). The activation parameters for OAT with picoline N-oxide and chlorate have been measured; the entropic barrier to oxo transfer is essentially zero. The potential energy surface for the reaction of Re(O)(hoz)(2)(OH(2))(+) with PyO was defined, and all pertinent intermediates and transition states along the reaction pathway were located by density functional theory (DFT) calculations (B3LYP/6-31G). In the second half of the catalytic cycle, Re(O)(2)L(2)(+) reacts with oxygen acceptors (Y's) in second-order reactions with associative transition states. The rate of OAT to substrates spans a remarkable range of 0.1-10(6) L mol(-)(1) s(-)(1), and the substrate reactivity order is Ph(3)P > dialkyl sulfides > alkyl aryl sulfides > Ph(2)S approximately DMSO, which demonstrates electrophilic oxo transfer. Competing deactivation and inhibitory pathways as well as their relevant kinetics are also reported.     

Hydroxyl radical is the active species in photochemical DNA strand scission by bis(peroxo)vanadium(V) phenanthroline

Bis(peroxo)vanadium(V) complexes are widely investigated as anticancer agents. They exert their antitumor and cyctotoxic effects through inhibition of tyrosine phosphatases and DNA cleavage, respectively. The latter process remains poorly understood. The mechanism of DNA cleavage by NH(4)[(phen)V(O)(eta(2)-O(2))(2)] (phen = 1,10-phenanthroline) was investigated. Kinetic studies on DNA cleavage revealed that the complex is a single-strand nicking agent with no specificity. EPR experiments using 2,2,6,6-tetramethyl-4-piperidone (TMP) and 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) as spin-traps for singlet oxygen and hydroxyl radical, respectively, implicated hydroxyl radical production upon photodecomposition of bis(peroxo)vanadium(V). This was corroborated by benzoate inhibition of DNA strand scission and stoichiometric oxidation of 2-propanol to acetone upon irradiation of bis(peroxo)vanadium(V) phenanthroline. High-resolution polyacrylamide gel analysis of the vanadium cleavage reaction and [Fe(II)EDTA](2)(-)/H(2)O(2) resulted in comigration of "ladder" pattern bands, which superimposed when both reactions were run on the same lane. These findings identify hydroxyl radical produced from the photooxidation of the peroxo ligand on vanadium as the active species in DNA cleavage.     

Observation of inductive effects that cause a change in the rate-determining step for the conversion of rhenium azides to imido complexes

The cationic oxorhenium(V) complex [Re(O)(hoz)(2)(CH(3)CN)][B(C(6)F(5))(4)] [1; Hhoz = 2-(2'-hydroxyphenyl)-2-oxazoline] reacts with aryl azides (N(3)Ar) to give cationic cis-rhenium(VII) oxoimido complexes of the general formula [Re(O)(NAr)(hoz)(2)][B(C(6)F(5))(4)] [2a-2f; Ar = 4-methoxyphenyl, 4-methylphenyl, phenyl, 3-methoxyphenyl, 4-chlorophenyl, and 4-(trifluoromethyl)phenyl]. The kinetics of formation of 2 in CH(3)CN are first-order in both azide (N(3)Ar) and oxorhenium(V) complex 1, with second-order rate constants ranging from 3.5 × 10(-2) to 1.7 × 10(-1) M(-1) s(-1). A strong inductive effect is observed for electron-withdrawing substituents, leading to a negative Hammett reaction constant ρ = -1.3. However, electron-donating substituents on phenyl azide deviate significantly from this trend. Enthalpic barriers (ΔH(?)) determined by the Eyring-Polanyi equation are in the range 14-19 kcal mol(-1) for all aryl azides studied. However, electron-donating 4-methoxyphenyl azide exhibits a large negative entropy of activation, ΔS(?) = -21 cal mol(-1) K(-1), which is in sharp contrast to the near zero ΔS(?) observed for phenyl azide and 4-(trifluoromethyl)phenyl azide. The Hammett linear free-energy relationship and the activation parameters support a change in the mechanism between electron-withdrawing and electron-donating aryl azides. Density functional theory predicts that the aryl azides coordinate via N(α) and extrude N(2) directly. For the electron-withdrawing substituents, N(2) extrusion is rate-determining, while for the electron-donating substituents, the rate-determining step becomes the initial attack of the azide. The barriers for these two steps are inverted in their order with respect to the Hammett σ values; thus, the Hammett plot appears with a break in its slope.     

Synthèse dc la pyrido[2,3-f] phtalazine et des diformyl-6,7; -4,6; -4,7; -2,4 quinoleines

This article describes the synthesis of a new heterocycle, pyrido[2,3,f]phtalazine and three new diformylquinolincs.     

Synthèse des pyrido[2,3-d] el [3,2-d] -s-triazolo[3,4-f] pyrimidines el de (pyrazolyl-1')-4 pyrido[2,3-d] et [3,2-d] pyrimidines

The synthesis of two new heterocycles is described: pyrido-[2,3-d]-.s-triazolo[ 3,4-f] pyrimidine and pyrido[3,2-d]-.s-triayzolo-[3,4-f] pyrimidine. 4-[I'-Pyrazolyl]pyrido[2,3-d]pyrimidines and 4-[1′-pyrazoly1] pyrido[ 3,2-d] pyrimidine are obtained by the action of 4-hydrazinopyrido[2,3-d]pyrimidine and 4-hydrazinopyrido-[3,2-d]pyrimidine with several β-diketones.