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PubMed2003

Synthesis, structure, and spectroscopic, photochemical, redox, and catalytic properties of ruthenium(II) isomeric complexes containing dimethyl sulfoxide, chloro, and the dinucleating bis(2-pyridyl)pyrazole ligands.

Two isomeric Ru(II) complexes containing the dinucleating Hbpp (3,5-bis(2-pyridyl)pyrazole) ligand together with Cl and dmso ligands have been prepared and their structural, spectroscopic, electrochemical, photochemical, and catalytic properties studied. The crystal structures of trans,cis-[Ru(II)Cl(2)(Hbpp)(dmso)(2)], 2a, and cis(out),cis-[Ru(II)Cl(2)(Hbpp)(dmso)(2)], 2b, have been solved by means of single-crystal X-ray diffraction analysis showing a distorted octahedral geometry for the metal center where the dmso ligands coordinate through their S atom. 1D and 2D NMR spectroscopy corroborates a similar structure in solution for both isomers. Exposure of either 2a or 2b in acetonitrile solution under UV light produces a substitution of one dmso ligand by a solvent molecule generating the same product namely, cis(out)-[Ru(II)Cl(2)(Hbpp)(MeCN)(dmso)], 4. While the 1 e(-) oxidation of 2b or cis(out),cis-[Ru(II)Cl(2)(bpp)(dmso)(2)](+), 3b, generates a stable product, the same process for 2a or trans,cis-[Ru(II)Cl(2)(bpp)(dmso)(2)](+), 3a, produces the interesting linkage isomerization phenomenon where the dmso ligand switches its bond from Ru-S to Ru-O (K(III)(O)(-->)(S) = 0.25 +/- 0.025, k(III)(O)(-->)(S) = 0.017 s(-1), and k(III)(S)(-->)(O) = 0.065 s(-1); K(II)(O)(-->)(S) = 6.45 x 10(9), k(II)(O)(-->)(S) = 0.132 s(-1), k(II)(S)(-->)(O) = 2.1 x 10(-11) s(-1)). Finally complex 3a presents a relatively high activity as hydrogen transfer catalyst, with regard to its ability to transform acetophenone into 2-phenylethyl alcohol using 2-propanol as the source of hydrogen atoms.

Journal Article↗
PubMed2003

Bis(pyrazol-1-yl)acetates as tripodal heteroscorpionate ligands in iron chemistry: syntheses and structures of iron(II) and iron(III) complexes with bpza, bdmpza, and bdtbpza ligands.

The molecular structure of the previously reported species "[Fe(bdtbpza)Cl]" has been revealed by X-ray structure determination to be a ferrous dimer [Fe(bdtbpza)Cl](2) (2c) [bdtbpza = bis(3,5-di-tert-butylpyrazol-1-yl)acetate]. The syntheses of ferrous 2:1 complexes [Fe(bpza)(2)] (3a) and [Fe(bdtbpza)(2)] (3c) as well as ferric 1:1 complexes [NEt(4)][Fe(bpza)Cl(3)] (4a) and [NEt(4)][Fe(bdmpza)Cl(3)] (4b) [bpza = bis(pyrazol-1-yl)acetate, bdmpza = bis(3,5-dimethylpyrazol-1-yl)acetate] are reported. Complexes 3a, previously reported [Fe(bdmpza)(2)] (3b), and 3c are high-spin. No spin crossover to the low-spin state was observed in the temperature range of 5-350 K. 4a and 4b are synthesized in one step and in high yield from [NEt(4)](2)[Cl(3)FeOFeCl(3)]. 4a and 4b are iron(III) high-spin complexes. Crystallographic information: 2c (C(24)H(39)ClFeN(4)O(2).CH(2)Cl(2).CH(3)CN) is triclinic, P1, a = 12.171(16) A, b = 12.851(14) A, c = 13.390(13) A, alpha = 98.61(9) degrees, beta = 113.51(11) degrees, gamma = 108.10(5) degrees, Z = 2; 3a (C(8)H(7)Fe(0.5)N(4)O(2)) is monoclinic, P2(1)/n, a = 7.4784(19) A, b = 7.604(3) A, c = 16.196(4) A, beta = 95.397(9) degrees, Z = 4; 3c (C(24)H(39)Fe(0.5)N(4)O(2)) is monoclinic, P2(1)/n, a = 9.939(6) A, b = 18.161(10) A, c = 13.722(8) A, beta = 97.67(7) degrees, Z = 4; 4b (C(20)H(35)Cl(3)FeN(5)O(2)) is monoclinic, C2/c, a = 30.45(6) A, b = 12.33(2) A, c = 16.17(3) A, beta = 118.47(5) degrees, Z = 8.

Journal Article↗
PubMed2003

Blue phosphors of dinuclear and mononuclear copper(I) and silver(I) complexes of 3,5-bis(trifluoromethyl)pyrazolate and the related bis(pyrazolyl)borate.

The synthesis, structure, and photoluminescence properties are described for the three-coordinate mononuclear and dinuclear complexes [H(2)B(3,5-(CF(3))(2)Pz)(2)]M(2,4,6-collidine), M(1)(), and [[3,5-(CF(3))(2)Pz]M(2,4,6-collidine)](2), M(2)(), respectively (M = Cu; Ag). The solids exhibit bright blue phosphorescence, at room temperature for the copper compounds and at 77 K for all compounds. Ag(1)(), Cu(1)(), and Cu(2)() exhibit blue pyrazole-based structured emissions with short phosphorescence lifetimes, 10(1)-10(2) micros, due to an internal heavy-metal effect. Meanwhile, Ag(2)() exhibits curious multiple excitation-dependent emissions.

Journal Article↗
PubMed2004

Nonexponential relaxation of the metastable state of the spin-crossover system [Fe(L)2](ClO4)2.H2O [L = 2,6-Bis(pyrazol-1-ylmethyl)pyridine].

The relaxation of the metastable state of the spin-crossover compound [Fe(L)(2)](ClO(4))(2).H(2)O, with L = 2,6-bis(pyrazol-1-ylmethyl)pyridine, populated by the LIESST (light induced excited spin state trapping) effect, has been investigated by magnetic measurements. The time dependence of the relaxation curve at several temperatures, starting from different initial states, is in the shape of stretched exponentials, and the thermal variation of the photostationary state under constant photoexcitation is progressive and reversible. These features are satisfactorily modeled by considering noninteracting two-level systems with a distribution of activation energies. A suggested origin for the distribution is the conformational flexibility of the nonplanar heterocyclic ligands. The effect of the intensity distribution during the LIESST process is also accounted for in a simple way.

Journal Article↗
PubMed2006

Syntheses, crystal structures, and magneto-structural correlations of novel Cu(II) complexes containing a planar [Cu(mu-L1)]2 (HL1 = 3-(2-pyridyl)pyrazole) unit: from dinuclear to tetranuclear and then to one-dimensional compounds.

Seven new Cu(II) complexes based on a binuclear planar unit [Cu(mu-L(1))](2), [[Cu(mu-L(1))(NO(3))(H(2)O)](2) (1), [Cu(mu-L(1))(HL(1))(ClO(4))](2) (2), [Cu(4)(mu-L(1))(6)(NO(3))(2)] (3), [Cu(4)(mu-L(1))(6)(L(1))(2)] (4), [Cu(4)(mu-L(1))(6)(mu-L(2))](n) (5), [Cu(4)(mu-L(1))(6)(mu-L(3))](n) (6), [[Cu(4)(mu-L(1))(4)(mu-L(4))(2)](H(2)O)(3)](n) (7) (HL(1) = 3-(2-pyridyl)pyrazole, L(2) = 1,8-naphthalenedicarboxylate, L(3) = terephthalate, L(4) = 2,6-pyridinedicarboxylate)}, have been synthesized and characterized by elemental analysis, IR, and X-ray diffraction. In 1 and 2, the Cu(II) centers are linked by deprotonated pyrazolyl groups to form dinuclear structures. 3 and 4 have similar gridlike tetranuclear structures in which two additional deprotonated L(1) ligands bridge two [Cu(mu-L(1))](2) units perpendicularly. 5 and 6 consist of similar one-dimensional (1-D) chains in which gridlike tetranuclear copper(II) units similar to that of 3 are further linked by L(2) or L(3) ligands, respectively. And, in 7, L(4) ligands link [Cu(mu-L(1))](2) binuclear units to form a tetranuclear gridlike structure in chelating/bridging mode and simultaneously bridge the tetranuclear units to form a 1-D chain. The magnetic properties of all complexes were studied by variable-temperature magnetic susceptibility and magnetization measurements. The obtained parameters of J range from -33.1 to -211 cm(-1), indicating very strong antiferromagnetic coupling between Cu(II) ions. The main factor that affects the |J| parameter is the geometry of the Cu(N(2))(2)Cu entity. From the magnetic point of view, 1 and 2 feature "pure" dinuclear, 3 and 5 tetranuclear, and 4, 6, and 7 pseudodinuclear moieties.

Journal Article↗
PubMed1997

High Superoxide Dismutase Activity of a Novel, Intramolecularly Imidazolato-Bridged Asymmetric Dicopper(II) Species. Design, Synthesis, Structure, and Magnetism of Copper(II) Complexes with a Mixed Pyrazole-Imidazole Donor Set.

A new dinucleating ligand, 1,5-bis(1-pyrazolyl)-3-[bis(2-imidazolyl)methyl] azapentane (Hbpzbiap), containing pyrazoles and imidazoles has been designed and synthesized. The synthesis and characterization of the copper complexes with the ligand Hbpzbiap and its dehydronated form are described. This study is aimed at modeling the active site of copper-zinc superoxide dismutase (SOD). Single crystals of the imidazolato-bridged complex [Cu(2)(bpzbiap)Cl(3)] (1) and non-imidazolato-bridged complex [Cu(2)(Hbpzbiap)Cl(4)] (2) were obtained and their structures determined by X-ray diffraction. Both structures show two copper centers in two different coordination environments: a distorted square pyramid and a distorted tetrahedron. The Cu-nitrogen bond lengths range from 1.919(4) to 2.039(3) Å and are as expected. The copper-copper distances from 5.566(1) to 6.104(1) Å being only slightly shorter than that found in bovine erythrocyte SOD. Temperature-dependent magnetic susceptibility study of 1 shows antiferromagnetic behavior with -2J = 96 cm(-)(1). From pH-dependent electron paramagnetic resonance and electronic spectra, [Cu(2)(bpzbiap)Cl(3)] has been demonstrated to be stable over a quite wide pH range including the physiological pH values. A low concentration of this complex (1) catalyzes the dismutation of superoxide at biological pH. Voltammetric studies indicate a quasi-reversible redox behavior in aqueous solution at pH 7. These results clearly indicate that complex 1 is a good model for superoxide dismutase.

Journal Article↗
PubMed1999

Mononuclear and Binuclear Ruthenium(II) Complexes Containing 2,2'-Bipyridine or 1,10-Phenanthroline and Pyrazole-3,5-Bis(benzimidazole). Synthesis, Structure, Isomerism, Spectroscopy, and Proton-Coupled Redox Activity.

A number of mixed-ligand mononuclear and binuclear ruthenium(II) complexes of composition [(bpy)(2)Ru(H(3)pzbzim)](ClO(4))(2).2H(2)O (1), [(phen)(2)Ru(H(3)pzbzim)](ClO(4))(2).3H(2)O (2), [(bpy)(2)Ru(H(2)pzbzim)Ru(bpy)(2)](ClO(4))(3).5H(2)O (3), [(phen)(2)Ru(H(2)pzbzim)Ru(phen)(2)](ClO(4))(3).4H(2)O (4), [(bpy)(2)Ru(H(2)pzbzim)Ru(phen)(2)](ClO(4))(3).4H(2)O (5), [(bpy)(2)Ru(pzbzim)Ru(bpy)(2)](ClO(4)).3H(2)O (6), and [(phen)(2)Ru(pzbzim)Ru(phen)(2)](ClO(4)).2H(2)O (7), where H(3)pzbzim = pyrazole-3,5-bis(benzimidazole), bpy = 2,2'-bipyridine, and phen = 1,10-phenanthroline, have been prepared and characterized. Complexes 3-5 isolated as mixtures of diastereoisomers have been separated by fractional recrystallization. In the cases of 3 and 4, the meso (LambdaDelta) and racemate (rac) (LambdaLambda, DeltaDelta) forms, and for 5, two enantiomeric pairs [(LambdaDelta, DeltaLambda) and (LambdaLambda, DeltaDelta)] have been obtained. These, as well as the meso and rac diastereoisomers of 6, have been characterized by (1)H and (13)C NMR spectroscopy. The crystal structure of the meso (LambdaDelta) form of 3 (C(57)H(53)N(14)Cl(3)O(17)Ru(2)) has been determined, which crystallizes in the monoclinic space group P2(1)/c with a = 11.672(2) Å, b = 41.696(9) Å, c = 12.871(2) Å, beta = 90.03(2)(o), and Z = 4. The acid-base and redox chemistry of the binuclear complexes has been studied over the pH range 1-12 in acetonitrile-water (3:2) medium. The equilibrium constants of the species involving protonation and deprotonation of the benzimidazole NH protons and the metal oxidation states covering +2 and +3 have been evaluated by spectrophotometric and cyclic voltammetric measurements. During spectrophotometric titrations of the complexes with cerium(IV), the metal-to-ligand charge transfer transitions are replaced by the newly generated ligand-to-metal charge transfer transition. The luminescence spectra of the complexes in solution (at 298 K) and in frozen glass (at 77 K) and their quantum yields have been reported. In contrast to the luminescent behavior of 1-5, corresponding deprotonated compounds do not fluoresce even at 77 K.

Journal Article↗
PubMed2000

Pd(II) complexes with polydentate nitrogen ligands. Molecular recognition and dynamic behavior involving Pd-N bond rupture. X-ray molecular structures of [[Pd(C6HF4)2](bpzpm)] and [[Pd(eta 3-C4H7)]2(bpzpm)] (CF3SO3)2 [bpzpm = 4,6-bis(pyrazol-1-yl)pyrimidine].

The ligands 4,6-bis(pyrazol-1-yl)pyrimidine (bpzpm) and 4,6-bis(4-methylpyrazol-1-yl)pyrimidine (Me-bpzpm) were synthesized and their reactions with some palladium derivatives explored. Mononuclear or dinuclear neutral or cationic complexes were obtained by reaction of the ligands with 1 or 2 equiv of Pd(C6XF4)2(cod) (cod = 1,5-cyclooctadiene; X = F, H) or the palladium fragment [Pd(eta 3-2-Me-C3H4)(S)2]+ (S = acetone). The reaction of the dinuclear derivatives with 1 equiv of the respective free ligand immediately led to the regeneration of the mononuclear complexes. Except in the case of the synthesis of [[Pd(C6HF4)2][Pd(C6F5)2](bpzpm)], where two similar metallic groups are present, all attempts to obtain dinuclear asymmetric complexes with two different palladium fragments failed. Instead, the dinuclear symmetric complexes were formed. This result could be considered as an example of molecular recognition with the ligand acting as a ditopic receptor. This behavior is comparable to chemical symbiosis but in this case applied to the ligand rather than to the metal center as occurs normally. The polyfluorophenyl rings are situated on average in a perpendicular orientation with respect to the coordination plane. Their restricted rotation results in several atropoisomers for the complexes with m-C6HF4. Different cross-reaction experiments were carried out, and these showed the mobility of the metallic fragments, with the more difficult process being that involving the more strongly bonded polyfluorophenyl palladium groups. By means of 1H NMR variable temperature studies, the interconversion of the two isomers of [[Pd(eta 3-C4H7)]2-(bpzpm)]Tf2 (Tf = CF3SO3) was analyzed. In the case of [[Pd(eta 3-C4H7)](bpzpm)]Tf the existence of two processes, an intramolecular apparent allyl rotation and an intermolecular exchange of the allylpalladium fragments, has been demonstrated. Different delta Gc++ values at the coalescence temperatures have also been determined. An X-ray single-crystal analysis was carried out on [[Pd(eta 3-C4H7)]2(bpzpm)]Tf2, which crystallizes in the monoclinic system, space group I2/m, with a = 9.368(2), b = 16.191(3), c = 20.228(6) A, beta = 101.26(3), and Z = 4. Compound [[Pd(C6HF4)2](bpzpm)] crystallized in the triclinic system, space group P1, with a = 8.845(6), b = 12.6609(9), c = 12.826(3) A, alpha = 88.45(2), beta = 74.36(3), gamma = 89.32(2), and Z = 2.

Journal Article↗
PubMed2004

Dynamic NMR study of the mechanisms of double, triple, and quadruple proton and deuteron transfer in cyclic hydrogen bonded solids of pyrazole derivatives.

Using dynamic solid state (15)N CPMAS NMR spectroscopy (CP = cross polarization, MAS = magic angle spinning), the kinetics of the degenerate intermolecular double and quadruple proton and deuteron transfers in the cyclic dimer of (15)N labeled polycrystalline 3,5-diphenyl-4-bromopyrazole (DPBrP) and in the cyclic tetramer of (15)N labeled polycrystalline 3,5-diphenylpyrazole (DPP) have been studied in a wide temperature range at different deuterium fractions in the mobile proton sites. Rate constants were measured on a millisecond time scale by line shape analysis of the doubly (15)N labeled compounds, and by magnetization transfer experiments on a second timescale of the singly (15)N labeled compounds in order to minimize the effects of proton-driven (15)N spin diffusion. For DPBrP the multiple kinetic HH/HD/DD isotope effects could be directly obtained. By contrast, four rate constants k(1) to k(4) were obtained for DPP at different deuterium fractions. Whereas k(1) corresponds to the rate constant k(HHHH) of the HHHH isotopolog, an appropriate kinetic reaction model was needed for the kinetic assignment of the other rate constants. Using the model described by Limbach, H. H.; Klein, O.; Lopez Del Amo, J. M.; Elguero, J. Z. Phys. Chem. 2004,218, 17, a concerted quadruple proton-transfer mechanism as well as a stepwise consecutive single transfer mechanism could be excluded. By contrast, using the kinetic assignment k(2) approximately k(3) approximately k(HHHD) approximately k(HDHD) and k(3) approximately k(HDDD) approximately k(DDDD), the results could be explained in terms of a two-step process involving a zwitterionic intermediate. In this mechanism, each reaction step involves the concerted transfer of two hydrons, giving rise to primary kinetic HH/HD/DD isotope effects, whereas the nontransferred hydrons only contribute small secondary effects, which are not resolved experimentally. By contrast, the multiple kinetic isotope effects of the double proton transfer in DPBrP and of the triple proton proton transfer in cyclic pyrazole trimers studied previously indicate concerted transfer processes. Thus, between n = 3 and 4 a switch of the reaction mechanism takes place. This switch is rationalized in terms of hydrogen bond compression effects associated with the multiple proton transfers. The Arrhenius curves of all processes are nonlinear and indicate tunneling processes at low temperatures. In a preliminary analysis, they are modeled in terms of the Bell-Limbach tunneling model.

Journal Article↗
PubMed1987

Cardiotonic agents. 6. Synthesis and inotropic activity of 2,4-dihydro-5-[4-(1H-imidazol-1-yl)phenyl]-3H-pyrazol-3-ones: ring-contracted analogues of imazodan (CI-914).

A series of 2,4-dihydro-5-[4-(1H-imidazol-1-yl)phenyl]-3H-pyrazol-3-ones was synthesized and evaluated for positive inotropic activity. Only compounds with two small alkyl groups at C-4 showed significant activity. The structure-activity relationships for optimal inotropic activity are presented and compared with those of the 4,5-dihydro-3(2H)-pyridazinone series. The phosphodiesterase inhibitory activity is also reported and correlated with the substitution pattern at C-4 in the pyrazolone ring.

Animals↗
PubMed2002

Design, synthesis, and antiproliferative activity of some new pyrazole-fused amino derivatives of the pyranoxanthenone, pyranothioxanthenone, and pyranoacridone ring systems: a new class of cytotoxic agents.

A series of novel pyranoxanthenones, pyranothioxanthenones, and pyranoacridones have been designed and synthesized as analogues of the acridone alkaloid acronycine, and their DNA binding and in vitro cytotoxicities have been investigated. The title compounds were derived by reaction of the corresponding 6-tosylates 5a-e with 2-hydroxyethylhydrazine, followed by conversion of the free hydroxyl of the substituted ethanols 6a-e to the corresponding mesylates, which were then treated with the suitably substituted secondary amines to provide the target derivatives 8-27. An alternative synthetic procedure for the preparation of these types of compounds is also developed, which resulted in an improvement of the overall yield. The new compounds exhibited interesting cytotoxic activity against the murine leukemia L1210 cell line, being more active than the parent compound, and a number of them possessed cytotoxicity against some human solid tumor cell lines. Especially in the case of a colon adenocarcinoma cell line, their IC(50) values were comparable to that of mitoxantrone. The results of this study indicate that the incorporation of an amino-substituted pyrazole ring into the acronycine chromophore, or into its isosteres, results in an improvement of the lead compound's activity, and therefore, it may be of use in the search of new anticancer agents derived from this natural product.

Acridines↗
PubMed2001

Flash vacuum pyrolysis over solid catalysts. 1. Pyrazoles over zeolites.

Flash vacuum pyrolysis (fvp) reactions of 1H-pyrazole (1), 3,5-dimethylpyrazole (2), and 3,5-diphenylpyrazole (3) were carried out over zeolites. Reactions were performed using ZCOY-7, NH(4)-Y, and Na-Y zeolites. Reaction temperatures of heterogeneous reactions were lower than the corresponding temperatures in the homogeneous system, showing a catalytic effect of the zeolites. Compounds 1-3 afforded nitrogen extrusion in homogeneous fvp reactions while in the heterogeneous ones different reactions were present. Compounds 1 and 2 also afforded nitrogen extrusion; products arising from ring fragmentation were found in reactions of 2 and 3 while an isomeric imidazole was isolated in reactions of 3. Isomerization of 3 is attributed to a transition-state selectivity by the catalyst due to the relation between the size of the molecule and the cavity of the zeolite. This isomerization reaction was present only when zeolites with active Brönsted sites were used.

Journal Article↗
PubMed2002

Flash vacuum pyrolysis over solid catalysts. 2. pyrazoles over hydrotalcites.

Flash vacuum pyrolysis (fvp) reactions of NH-pyrazole (1) and 3,5-diphenylpyrazole (2) were investigated in the presence of anionic clays having hydrotalcite structure (HT). Solid catalysts with Mg:Al ratio equal to 2:1 containing carbonate (HT-1), nitrate (HT-2), and silicate (HT-3) as interlayer anions were employed. Between 400 and 600 degrees C, compound 1 remained almost unchanged and only unidentified volatile products were detected in small amounts. In contrast, 2 afforded benzonitrile (3) and phenylacetonitrile (4) by a ring fragmentation reaction at 450 degrees C. At a higher temperature (660 degrees C), the same products obtained in homogeneous fvp reactions, i.e., 2-phenylindene (5) and 3-phenylindene (6), were obtained showing no catalysis by HT under these conditions. Results showed that the yield is strongly dependent on the nature of the interlayer anion in the hydrotalcite structure. In comparison with reactions of 2 over zeolites, HTs exhibit selectivity for ring fragmentation reaction.

Journal Article↗
PubMed2005

Palladium-catalyzed coupling of pyrazole triflates with arylboronic acids.

[reaction: see text] A general protocol for the palladium-mediated Suzuki coupling reaction of pyrazole triflates and aryl boronic acids has been developed. The use of additional dppf ligand was determined to increase product yields allowing for the use of a broad range of reaction substrates.

Journal Article↗
PubMed1997

A Proton-Ionizable Ester Crown of 3,5-Disubstituted 1H-Pyrazole Able To Form Stable Dinuclear Complexes with Lipophilic Phenethylamines.

A convenient synthesis of the proton-ionizable crown 3 is reported that uses dibutyltin oxide. In acetonitrile, the reaction of 3 (LH(2)) with phenethylamine and homoveratrylamine (molar ratio 1:2) affords solid dinuclear complexes [LH(2)]2RNH(2) (4a,b), which spectroscopic (FAB-MS, IR, (1)H and (13)C NMR) data point toward a strong participation of the pyrazole nitrogens in the amine complexation. In DMSO-d(6) solution, a (13)C NMR study demonstrates the formation in situ of analogous neutral 4a-d[LH(2)]2RNH(2) or charged 5a-d[L(2)(-)]2RNH(3)(+) dinuclear complexes by reaction of 3 [LH(2)] or 3'[L(2)(-)]2Na(+) with RNH(2) (phenethylamine, homoveratrylamine, dopamine, and norepinephrine) or their RNH(3)(+)Cl(-) salts, respectively. Differences between the structure of complexes 4 and 5 have been evaluated by taking the homoveratrylamine derivatives 4b and 5bas models. An (1)H and (13)C NMR study (by raising the temperature) and measurements of intermolecular NOE effects (from NOESY and ROESY spectra) demonstrate that both complexes behave as prototropic isomers showing different conformations. By increasing the ionic strength, the 4b isomer structure becomes similar to that of 5b. The molecular modeling (GenMol software) of 4a-d and 5a-d shows that the assemblage in which both amine molecules are on the same side of the crown is the more stable. Lipophilic amines afford more stable complexes than hydrophilic ones and charged species are much more stable than the neutral ones.

Journal Article↗
PubMed1999

A Pyrazole to Furan Rearrangement. Thermolysis of 5-Azido-4-formylpyrazoles.

5-Azido-3-benzyl-4-formyl-1-phenylpyrazoles 1a-c extrude dinitrogen upon heating in toluene to give the corresponding nitrenes, which immediately rearrange via a new ring-opening ring-closure reaction to produce an equimolar mixture of 4-cyano-2-phenyl-3-phenylazofurans 2a-c and 3-benzyl-4-cyano-1-phenylpyrazoles 3a-c. The formation of the 4-cyano-2-phenyl-3-phenylazofurans 2a-c is the first example in the pyrazole series of a nitrene rearrangement, in which the parent heterocyclic system of the product differs from that of the starting material. The isolation of equimolar amounts of the two products points to the fact that their formation occurs by two mechanistically interconnected pathways, between which the exchange of a redox equivalent takes place. Evidence for the existence of two mechanistically interlinked pathways is presented, and the insight into the stoechiometry of the reaction is taken advantage of to optimize the reaction with respect to either of the two products 2 or 3. Thus, it is demonstrated how one can bias the two pathways using external reagents, thereby changing the product distribution ratio 2:3 from 1:1 in the unbiased case, to 1:4 in one direction, and to better than 20:1 in the other direction.

Journal Article↗
PubMed2001

Study on the co-luminescence system of Dy-Gd-1,6-bis(1'-phenyl-3'-methyl-5'-pyrazol-4'-one)hexanedionecetyltrimethylammonium bromide and its analytical application.

Dy-1,6-bis(1'-phenyl-3'-methyl-5'-pyrazol-4'-one)hexanedione-cetyltrimethylammonium bromide (Dy-BPMPHD-CTMAB) ion association system has strong fluorescence intensity. In this system, some rare earth ions such as Gd3+, Y3+ and La3+ can exert a fluorescence enhancement effect, leading to a newly found co-luminescence system. From this, a rapid, simple and sensitive method was developed for the determination of trace amounts of Dy3+. The results indicate that the fluorescence intensity of the system is linearly related to the concentration of Dy3+ in the range 1.0 x 10(-7)-1.2 x 10(-5) mol L(-1) and the detection limit (S/N = 3) is 3.0 x 10(-8) mol L(-1). The luminescence mechanism of the system is discussed.

Journal Article↗