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Query: UMLS:C0847097 (acidity)
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The decarboxylation rate of the tetramethylguanidinium salt of 3-carboxy-6-nitrobenzisoxazole in 24 pure solvents and 36 dimethyl sulfoxide binary mixtures with diglyme, acetonitrile, benzene, dichloromethane, chloroform, and methanol was analyzed in the light of the SPP, SA, and SB pure solvent scales. The results allow one to rationalize the high sensitivity of this kinetics to the reaction medium and to assess the potential use of this compound as a probe in biochemical environments. The natural environment for comparison of this kinetics was found to be the gas phase rather than the aqueous medium. In the latter, the process is much faster owing to such high polarity, which, however, is strongly diminished by the high acidity of the medium. Based on our calculations, the rate constant for the decarboxylation kinetics in the gas phase must be in the region of 2 x 10(-10) s(-1) (i.e., 3 orders of magnitude smaller than in water).
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PMID:Effects of medium on decarboxylation kinetics: 3-carboxybenzisoxazoles and their potential use as environmental probes in biochemistry. 1084 24

The concerned azo ligands are 2-(phenylazo)pyridine (HL) and 2-((p-chlorophenyl)azo)pyridine (ClL). The reaction of KReO4 with HL in hot concentrated HCl is attended with metal reduction and ligand chlorination affording the oxo complex ReVOCl3(ClL), 2, which furnishes ReIII(OPPh3)Cl3(ClL), 3, upon treatment with PPh3. Aromatic amines, ArNH2, convert 2 to the imido complex ReV(NAr)Cl3(ClL), 5, and the unusual oxo-imido dimer (ClL)-Cl2(O)ReVOReV(NAr)Cl2(ClL), 7. The complex ReIII(OPPh3)Cl3(HL), 4, has been generated from ReVOCl3(PPh3)2 and HL. Reaction of 4 with HL has yielded ReV(NPh)Cl3(HL), 6, via azo splitting. The complexes have been characterized with the help spectral, magnetic, and X-ray structural data (2, 3, 5c (Ar = pClC6H4) and 7.CH2Cl2 (Ar = pMeC6H4)). In 2, 3, and 5c the ReCl3 fragment is meridionally disposed, and in 7 the ReCl2 fragments have a trans configuration. The Re-O(oxo) bond, 1.663(6) A, in 2 and Re-N(imido) bond, 1.719(5) A, in 5c are triple bonds. The corresponding bonds are slightly longer in 7 wherein the (O)Re(1)-O(2)-Re(2)(NAr) bridge is angular (151.0(5) degrees) and unsymmetrical, the Re(1)-O(2) bond, 1.849(7) A, having a large double-bond character (Re(2)-O(2), 1.954(7) A). In effect, cis-ReVO2 acts as a monodentate oxygen ligand toward ReVNAr in 7. In all cases the pyridine nitrogen binds trans to the oxo, OPPh3, or NAr donor. Bond length data are consistent with the presence of substantial d(Re)-pi*(azo) back-bonding. In acetonitrile solution the complexes display electrochemical one-electron metal (ReVI/ReV or ReIV/ReIII) and azo redox. The imido ligand in 5 stabilizes the ReVI state (E1/2 approximately 1.4 V) better than the oxo ligand in 2 (approximately 1.9 V). Parallely it is more difficult to reduce the azo group in 5 (approximately -0.4 V) than in 2 (approximately 0.0 V). In 7 the metal (approximately 1.0 V) and azo (approximately -0.4 V) couples correspond to the imido and oxo halves, respectively. The significantly higher (by 0.2-0.6 V) metal reduction potentials of the azopyridine compared to pyridine-2-aldimine complexes is ascribed to the superior pi-acidity and electron-withdrawing character of the azo function relative to the aldimine function. This also makes the transfer of the ReVO oxygen function much more facile under azopyridine chelation as in 2. For the same reason, ReOCl3(PPh3)2 reacts with HL affording only 4 while it reacts with pyridine-2-aldimines furnishing oxo species. Crystal data for the complexes are as follows: 2, empirical formula C11H8Cl4N3ORe, crystal system triclinic, space group P1, a = 7.118(4) A, b = 8.537(4) A, c = 13.231(9) A, alpha = 79.16(5) degrees, beta = 78.03(5) degrees, gamma = 70.96(4) degrees, V = 737.2(7) A3, Z = 2; 3, empirical formula C29H23Cl4N3OPRe, crystal system monoclinic, space group P2(1)/n, a = 11.264(2) A, b = 15.221(3) A, c = 17.628(4) A, beta = 94.21(3) degrees, V = 3014(1) A3, Z = 4; 5c, empirical formula C17H12Cl5N4Re, crystal system triclinic, space group P1, a = 9.683(3) A, b = 10.898(3) A, c = 11.522(3) A, alpha = 63.67(2) degrees, beta = 71.24(2) degrees, gamma = 86.79(2) degrees, V = 1026(1) A3, Z = 2; 7.CH2Cl2, empirical formula C30H25Cl8N7O2Re2, crystal system triclinic, space group P1, a = 12.522(6) A, b = 12.857(8) A, c = 13.182(7) A, alpha = 67.75(4) degrees, beta = 88.30(4) degrees, gamma = 82.09(4) degrees, V = 1945(2) A3, Z = 2.
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PMID:Chemistry of the rhenium-azopyridine family: an oxo parent and derivatives thereof including a novel oxo-imido dimer. 1122 35

The reaction of [Os(H)(Br)(CO)(PPh3)3], 5, with 2-(phenylazo)pyridine (pap) in boiling dry heptane has afforded the azo anion radical complex [Os(pap.-)(Br)(CO)(PPh3)2], 6a, as the major product and [Os(pap)(H)(CO)(PPh3)2]Br, 7, as a minor byproduct. Upon replacing pap by the better pi-acceptor azo-2,2'-bipyridine (abp) in the above synthesis, the radical complex [Os(abp.-)(Br)(CO)(PPh3)2], 6b, becomes the sole product. It is proposed that 6 is formed via homolytic cleavage of the Os-H bond in 5; in the formation of 7, the Os-Br bond of 5 is heterolytically cleaved. The X-ray structures of 6b and 7.CH2Cl2 have been determined. In 6b, the N-N length is 1.35(2) A, consistent with the anion radical description; in 7.CH2Cl2 the length is 1.27(1) A. The spin-bearing extended Huckel HOMO in a model of 6 is found to be approximately 70% azo-pi* in character associated with a small metal contribution. An electronic band observed in the range 600-700 nm in solutions of 6 is assigned to the HOMO --> LUMO transition, the LUMO being 95% pyridine-pi* in character. One-electron paramagnetic 6 displays well-defined anisotropic EPR features near g = 2.00. The anisotropy arises from the metal character of HOMO and is magnified by the large spin-orbit coupling in osmium. In a moisture-free environment 6 is indefinitely stable in the solid state, but in CH2Cl2-MeCN solution 6a is rapidly oxidized by air, affording [Os(pap)(Br)(CO)(PPh3)2]+, 6a+, which has been isolated as the diamagnetic PF6- salt; 6b+PF6- has been similarly prepared. The voltammetric reduction potentials of the 6+/6 couple follow the order 6a+/6a < 6b+/6b, and the carbon monoxide stretching frequencies follow the order 6a < 6b and 6a+ < 6b+. These trends are consistent with the pi-acidity order pap < abp. Crystal data are as follows: (6b, C47H38BrN4OOsP2) monoclinic, space group P21/c (no. 14), a = 10.215(4) A, b = 17.634(7) A, c = 22.473(8) A, beta = 97.67(3) degrees , Z = 4; (7.CH2Cl2, C49H42BrCl2N3OOsP2) monoclinic, space group P2(1/n) (no. 14), a = 15.323(7) A, b = 15.201(6) A, c = 19.542(7) A, beta = 92.51(3) degrees, Z = 4.
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PMID:Azo anion radical complexes of osmium and related nonradical species. 1127 25

A straightforward synthesis of the zwitterionic benzoquinonemonoimine 8 is reported. This molecule is a rare example of a zwitterion being more stable than its canonical forms. It is shown that 8 is best described as constituted of two chemically connected but electronically not conjugated 6 pi electron subunits. Its reactivity with electrophiles such as H(+), CH(3)(+), and metal salts leads to the synthesis of new 12 pi electron molecules 12 (H(+)), 14 (CH(3)(+)), and 20 (Pd(2+)), respectively, in which one or both 6 pi electron subsystems localize into an alternation of single and double bonds, as established by X-ray diffraction. The acidity of the N[bond]H protons of 8 can be modulated by an external reagent. Dependent on the electrophile used, the control of the pi system delocalization becomes possible. When the electrophile simply adds to the zwitterion as in 12, 14, or 15, there is no more negative charge to be delocalized and only the positive charge remains delocalized between the nitrogen atoms. Furthermore, when a reaction with the electrophilic reagent results in deprotonation, as in 17-21, there remains no charge in the system to be delocalized. DFT calculations were performed on models of 8, 12, 14, 20, and on other related zwitterions 9 and 10 in order to examine the influence of the fused cycles on the charge separation and on the singlet-triplet energy gap. An effect of the nitrogen substituents in 8 is to significantly stabilize the singlet state. The dipole moment of 8 was measured to be 9.7 D in dichloromethane, in agreement with calculated values. The new ligands and complexes described in this article constitute new classes of compounds relevant to many areas of chemistry.
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PMID:A 6 pi + 6 pi potentially antiaromatic zwitterion preferred to a quinoidal structure: its reactivity toward organic and inorganic reagents. 1451 10

The direct extraction of alkylphenols, chlorophenols and bisphenol A from an acid-digested sediment suspension for GC-MS analysis was studied. The sediment was digested with acid while the hydrolyzed analytes were being extracted with dichloromethane. The conditions of the acid digestion and extraction were optimized in terms of time, acidity of digestion, and extracting solvent. It is possible to complete the extraction within 20 min with 5 ml of 0.1 M HCl digesting solution and three portions of 5 ml of dichloromethane. The recoveries of analytes were mostly around 90% with about 10% relative standard deviations. With this technique parallel treatment of large numbers of sediment samples is possible without any expensive special equipment or heating process. The analytical characteristics of this extraction technique were compared with Soxhlet extraction and the pressurized liquid extraction technique. The technique was examined and evaluated for real environmental sediment samples and certified reference material of natural matrix.
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PMID:Direct extraction of alkylphenols, chlorophenols and bisphenol A from acid-digested sediment suspension for simultaneous gas chromatographic-mass spectrometric analysis. 1452 16

Low-spin nickel(II) complexes containing bidentate ligands with modulated nitrogen donor ability, Py(Bz)2 or MePy(Bz)2 (Py(Bz)2 = N,N-bis(benzyl)-N-[(2-pyridyl)methyl]amine, MePy(Bz)2 = N,N-bis(benzyl)-N-[(6-methyl-2-pyridyl)methyl]amine), and a beta-diketonate derivative, tBuacacH (tBuacacH = 2,2,6,6-tetramethyl-3,5-heptanedione), represented as [Ni(Py(Bz)2)(tBuacac)](PF6) (1) and [Ni(MePy(Bz)2)(tBuacac)](PF6) (2) have been synthesized. In addition, the corresponding high-spin nickel(II) complexes having a nitrate ion, [Ni(Py(Bz)2)(tBuacac)(NO3)] (3) and [Ni(MePy(Bz)2)(tBuacac)(NO3)] (4), have also been synthesized for comparison. Complexes 1 and 2 have tetracoordinate low-spin square-planar structures, whereas the coordination environment of the nickel ion in 4 is a hexacoordinate high-spin octahedral geometry. The absorption spectra of low-spin complexes 1 and 2 in a noncoordinating solvent, dichloromethane (CH2Cl2), display the characteristic absorption bands at 500 and 540 nm, respectively. On the other hand, the spectra of a CH2Cl2 solution of high-spin complexes 3 and 4 exhibit the absorption bands centered at 610 and 620 nm, respectively. The absorption spectra of 1 and 2 in N,N-dimethylformamide (DMF), being a coordinating solvent, are quite different from those in CH2Cl2, which are nearly the same as those of 3 and 4 in CH2Cl2. This result indicates that the structures of 1 and 2 are converted from a low-spin square-planar to a high-spin octahedral configuration by the coordination of two DMF molecules to the nickel ion. Moreover, complex 1 shows thermochromic behavior resulting from the equilibrium between low-spin square-planar and high-spin octahedral structures in acetone, while complex 2 exists only as a high-spin octahedral configuration in acetone at any temperature. Such drastic differences in the binding constants and thermochromic properties can be ascribed to the enhancement of the acidity of the nickel ion of 2 by the steric effect of the o-methyl group in the MePy(Bz)2 ligand in 2, which weakens the Ni-N(pyridine) bond length compared with that of the nonsubstituted Py(Bz)2 ligand in 1.
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PMID:Equilibrium of low- and high-spin states of Ni(II) complexes controlled by the donor ability of the bidentate ligands. 1510 94

The aldol-Tishchenko reaction of ketone aldols as enol equivalents has been developed as an efficient strategy to furnish differentiated 1,3-anti-diol monoesters in one step. The thermodynamically unstable ketone aldols undergo a facile retro-aldolization to yield a presumed zirconium enolate in situ, which then undergoes the aldol-Tishchenko reaction in typically high yields and with complete 1,3-anti diastereocontrol. Evaluation of a broad range of metal alkoxides as catalysts and optimization of the reaction protocol led to a modified zirconium alkoxide catalyst with attenuated Lewis acidity and dichloromethane as solvent, which resulted in suppression of the undesired acyl migration to a large extent. Various ketone aldols have been prepared and subjected to the general process, giving rise to a broad range of differently substituted 1,3-anti-diol monoesters, which may be hydrolyzed to the corresponding 1,3-anti-diols.
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PMID:The zirconium alkoxide-catalyzed aldol-Tishchenko reaction of ketone aldols. 1577 Jul 13

Optimal conditions for the extraction of casearins from Casearia sylvestris were determined using response surface methodology. The maceration and sonication extraction techniques were performed using a 3 x 3 x 3 full factorial design including three acidity conditions, three solvents of different polarities and three extraction times. The yields and selectivities of the extraction of casearins were significantly influenced by acidity conditions. Taking into account all variables tested, the optimal conditions for maceration extraction were estimated to involve treatment with dichloromethane saturated with ammonium hydroxide for 26 h. Similar yields and selectivities for casearins were determined for sonication extraction using the same solvent but for the much shorter time of 1 h. The best results for stabilisation of the fresh plant material were obtained using leaves that had been oven dried at 40 degrees C for 48 h.
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PMID:Optimisation of conditions for the extraction of casearins from Casearia sylvestris using response surface methodology. 1674 24

A method combining liquid/liquid extraction and chromatographic fractionation has been developed for the preparation of pure monophenyltin (MPhT), diphenyltin (DPhT), and triphenyltin (TPhT), synthesized from isotope-enriched Sn metal using phenylation of SnI(4) in diethylether (DEE) followed by quenching with HBr and water. After two successive extractions of the aqueous HBr phase with DEE, >99% of both DPhT and TPhT was recovered in the combined DEE phase and 94% of the MPhT remained in the aqueous phase. The MPhT in the aqueous phase was extracted into dichloromethane. The organic phases were vaporized and the PhTs were redissolved in MeOH/water/acetic acid/sodium acetate (59/30/6/8, v/v/v/w), which was also used as storing solution. Aliquots of the two solutions containing either DPhT and TPhT or MPhT were injected into a silica-based C(18) column for isolating and purifying single species. The yields of pure MPhT, DPhT, and TPhT, each synthesized from isotope-enriched (118)Sn metal, (122)Sn metal, and (124)Sn metal, were better than 99%. After chromatographic separation, the single phenyltin compounds were mixed to prepare a spike for multiple-isotope species-specific isotope dilution (MI-SSID). MI-SSID was successfully used to determine phenyltin compounds in the certified reference material, mussel tissue BCR CRM-477. At -20 degrees C, all of the fractionated phenyltin species were stable in the storage solution for at least 197 days. When these standards were stored at 4 degrees C or 22 degrees C, 4-6% of the DPhT and TPhT degraded within 27 days. The degradation of DPhT and TPhT increased with the ionic strength and acidity of the storage solution.
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PMID:Preparation, preservation and application of pure isotope-enriched phenyltin species. 1693 90

Oxidation of Cp*Ir((rac-TsDPEN)H (DPEN = H2NCHPhCHPhNTs) with Cp2FePF6 or Ph3CPF6 in MeCN solution generates [Cp*Ir(TsDPEN)(NCMe)]PF6 ([1H(NCMe)]PF6) together with H2 and Ph3CH, respectively. Labeling studies revealed that the Ir-H was abstracted. The formation of a transient electrophilic species is implicated by the formation of a cyclometalated derivative. The labile species [1H(NCMe)]+ was also obtained by protonation of the diamido derivative Cp*Ir(TsDPEN-H) (1) in MeCN solution (BArF4- = B(C6H3-3,5-(CF3)2)4-). The unsaturated, "naked" cation [1H]BArF4 can be prepared by protonation of 1 with H(OEt2)2BArF4 in CH2Cl2 solution or by thermal elimination of MeCN from [1H(NCMe)]+. Crystallographic analysis confirms the structure of this 16e cation in [1H]BArF4. The formally unsaturated species 1 and [1H]BArF4 have strongly contrasting Lewis acidities, with the cation binding PPh3, CO, and NH3. 1 does not measurably bind these same ligands. [1H]BArF4 is reactive toward H2, at least in the absence of inhibiting donor ligands such as MeCN. [1H]BArF4 (CH2Cl2 solutions) catalyzes the addition of H2 to 1 by proton transfer from an apparent dihydrogen complex. This work demonstrates that the protonation activates the Lewis acidity of unsaturated Ir(III) amides, giving rise to novel organometallic Lewis acids.
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PMID:Proton-induced lewis acidity of unsaturated iridium amides. 1701 73


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