Stopped-flow injection analysis. The influence of diffusion on dispersion of normal and reverse flow injection

A relationship is derived to enable the comparison of the dispersion heights of normal and reverse flow injection analysis (FIA). A single channel flow system is employed in the absence of a chemical reaction. The stopped-flow injection method is used to probe the influence of molecular diffusion on the overall dispersion of normal and reverse FIA, which appeared to demonstrate fundamentally different diffusion behaviors. Small discrepancies are observed between the dispersion heights, which are enhanced by the stopped-flow period, especially when unmatched matrix ionic compositions of the indicator and counter solutions were involved. For these conditions, the diffusion flux rate is enhanced considerably, displaying a peak, in addition to the transient, for both methods. The influence of diffusion on the dispersion characteristics of normal and reverse FIA is discussed theoretically. Diffusion in the proposed model is postulated to oppose dispersion by convection. The latter initiates concentration gradients in the injection zone and propagates it with flow time over the dispersion zone profile. The diffusion flux then reacts in order to confine the indicator dispersion for normal FIA and to enhance it for reverse FIA. This model is consistent with the experimental results and accounts for most of the phenomena encountered. Probably owing to the influence of secondary flow phenomena, the use of coiled tubes has suppressed the effects of diffusion on the overall dispersion behavior.Part of the experimental work was performed at IMI Institute for Research and Development, Haifa, Israel.     

Determination of the chemical nature of active surface sites present on bulk mixed metal oxide catalysts

CH3OH temperature programmed surface reaction (TPSR) spectroscopy was employed to determine the chemical nature of active surface sites for bulk mixed metal oxide catalysts. The CH3OH-TPSR spectra peak temperature, Tp, for model supported metal oxides and bulk, pure metal oxides was found to be sensitive to the specific surface metal oxide as well as its oxidation state. The catalytic activity of the surface metal oxide sites was found to decrease upon reduction of these sites and the most active surface sites were the fully oxidized surface cations. The surface V5+ sites were found to be more active than the surface Mo6+ sites, which in turn were significantly more active than the surface Nb5+ and Te4+ sites. Furthermore, the reaction products formed also reflected the chemical nature of surface active sites. Surface redox sites are able to liberate oxygen and yield H2CO, while surface acidic sites are not able to liberate oxygen, contain either H+ or oxygen vacancies, and produce CH3OCH3. Surface V5+, Mo6+, and Te4+ sites behave as redox sites, and surface Nb5+ sites are Lewis acid sites. This experimental information was used to determine the chemical nature of the different surface cations in bulk Mo-V-Te-Nb-Ox mixed oxide catalysts (Mo(0.6)V(1.5)Ox, Mo(1.0)V(0.5)Te(0.16)Ox, Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox). The bulk Mo(0.6)V(1.5)Ox and Mo(1.0)V(0.5)Te(0.16)Ox mixed oxide catalytic characteristics were dominated by the catalytic properties of the surface V5+ redox sites. The surface enrichment of these bulk mixed oxide by surface V5+ is related to its high mobility, V5+ possesses the lowest Tammann temperature among the different oxide cations, and the lower surface free energy associated with the surface termination of V=O bonds. The quaternary bulk Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox mixed oxide possessed both surface redox and acidic sites. The surface redox sites reflect the characteristics of surface V5+ and the surface acidic sites reflect the properties normally associated with supported Mo6+. The major roles of Nb5+ and Te4+ appear to be that of ligand promoters for the more active surface V and Mo sites. These reactivity trends for CH3OH ODH parallel the reactivity trends of propane ODH because of their similar rate-determining step involving cleavage of a C-H bond. This novel CH3OH-TPSR spectroscopic method is a universal method that has also been successfully applied to other bulk mixed metal oxide systems to determine the chemical nature of the active surface sites.     

Direct preparation and structure determination of tertiary and secondary amine boranes from primary or secondary amine boranes

New secondary and tertiary amine borane derivatives were prepared in a one-pot reaction starting from primary amine boranes. The reaction involves treatment of an amine borane with 2 equivalents of s-BuLi at −78 °C. In general, mixtures of mono and di metallated products were obtained. Alkyl iodides and benzyl chloride reacted with the lithiated amine, but aldehydes and ketones were reduced. Conversion was high as determined by NMR, but moderate to low yields were obtained after chromatography, possibly due to decomposition on silica. Crystal structures were obtained for the compounds 3a, 3b and 3c.     

57Fe-labeled octamethylferrocenium tetrafluoroborate. X-ray crystal structures of conformational isomers, hyperfine interactions, and spin-lattice relaxation by Moessbauer spectroscopy

X-ray structure determinations of two different single crystals of octamethylferrocenium tetrafluoroborate (OMFc(+)BF(4)(-)) revealed conformational polymorphism with ligand twist angles of 180 degrees and 108 degrees , respectively. Their concomitant occurrence could be explained by the small lattice energy difference of 3.2 kJ mol(-1). Temperature-dependent Moessbauer spectroscopy of (57)Fe-labeled OMFc(+)BF(4)(-) over the range 90 < T < 370 K did not show the anomalous sudden increase in the motion of the metal atom as observed in neutral OMFc. Broadened absorption curves characteristic of relaxation spectra were obtained with an isomer shift of 0.466(6) mm s(-1) at 90 K. The temperature dependence of the isomer shift corresponded to an effective vibrating mass of 79 +/- 10 Da and, in conjunction with the temperature dependence of the recoil-free fraction, to a Moessbauer lattice temperature of 89 K. The spin relaxation rate could be better described by an Orbach rather than a Raman process. At 400 K, a reversible solid-solid transition to a plastic crystalline mesophase was noted.     

Intramolecular electron transfer in nitrite reductases.

The copper- and heme-containing nitrite reductases (NiRs) are key enzymes in denitrification. Their subunits contain two distinct redox-active metal centers, an electron-accepting site and a nitrite-reducing site, to carry out the single-electron reduction of nitrite to nitric oxide. Catalytic cycles of both enzyme families employ intramolecular electron transfer that can be rate-determining for their activity. Herein, we report results comparing these two enzyme families in order to resolve the different mechanisms controlling intramolecular electron transfer in these proteins.     

Diastereoisomerically selective enantiomerically pure titanium complexes of Salan ligands: synthesis, structure, and preliminary activity studies

The degree of diastereoselectivity in the wrapping of four new chiral Salan ligands to form chiral-at-metal titanium complexes ranged from mild to perfect as a function of the ligands' N substituents; the enantiomerically pure complexes catalyzed the addition of diethyl zinc to benzaldehyde in 73-76% enantiomeric excess.     

Vanadium(III) and vanadium(V) amine tris(phenolate) complexes

The coordination chemistry of amine tris(phenolate) ligands around V(III) and V(V) is described for the first time. Three amine tris(phenolate) ligands were employed featuring different steric and electronic influence exerted by the phenolate substituents in the ortho and para positions being either t-Bu, Me, or Cl. V(III) complexes of all ligands (1-3) were readily obtained by reaction between the ligand precursors and VCl3(THF)3 in the presence of triethylamine. The complexes obtained were pentacoordinate, a THF ligand completing the coordination sphere of the metal, which was found to be of almost perfect TBP geometry, as revealed by crystallography. V(V) oxo complexes of all the ligands (4-6) were readily obtained by a reaction between the ligand precursors and VO(OPr)3. The oxo complexes of the alkyl-bearing ligands (4 and 5) could also be synthesized by the air oxidation of the corresponding V(III) complexes (1 and 2); however, the attempted air oxidation of the V(III) complex bound to the electron-poor ligand (3) did not yield the corresponding oxo complex 6. 1H NMR and crystallographic analysis of complexes 4 and 5 supported their TBP structures. Complex 6, on the other hand, was found to be composed of a TBP complex (6a) and an octahedral complex (6b) in equilibrium, the octahedral complex being more stable at lower temperatures. An X-ray structure of 6b revealed a mononuclear oxo complex, the sixth coordination site being occupied by an aqua ligand to which two THF molecules are H-bonded. Complexes 4-6 catalyze the epoxidation of olefins by t-BuOOH, albeit slowly. These complexes may thus be considered as structural and functional models of vanadium-dependent haloperoxidase enzymes.     

Addition of palladium and platinum tri-tert-butylphosphine groups to Re-Sn and Re-Ge bonds

The reaction of Re2(CO)8[mu-eta2-C(H)=C(H)Bu(n)](mu-H) with Ph3SnH at 68 degrees C yielded the new compound Re2(CO)8(mu-SnPh2)2 (10) which contains two SnPh2 ligands bridging two Re(CO)(4) groups, joined by an unusually long Re-Re bond. Fenske-Hall molecular orbital calculations indicate that the bonding in the Re2Sn2 cluster is dominated by strong Re-Sn interactions and that the Re-Re interactions are weak. The 119Sn M?ssbauer spectrum of 10 exhibits a doublet with an isomer shift (IS) of 1.674(12) mm s(-1) and a quadrupole splitting (QS) of 2.080(12) mm s(-1) at 90 K,characteristic of Sn(IV) in a SnA2B2 environment. The IS is temperature dependent, -1.99(14) x 10(-4) mm s(-1) K(-1); the QS is temperature independent. The temperature-dependent properties are consistent with the known Gol'danskii-Kariagin effect. The germanium compound Re2(CO)8(mu-GePh2)2 (11) was obtained from the reaction of Re2(CO)8[mu-eta2-C(H)=C(H)Bu(n)](mu-H) with Ph3GeH. Compound 11 has a structure similar to that of 10. The reaction of 10 with Pd(PBu(t)3)2 at 25 degrees C yielded the bis-Pd(PBu(t)3) adduct, Re2(CO)8(mu-SnPh2)2[Pd(PBu(t)3)]2 (12); it has two Pd(PBu(t)3) groups bridging two of the four Re-Sn bonds in 10. Fenske-Hall molecular orbital calculations show that the Pd(PBu(t)3) groups form three-center two-electron bonds with the neighboring rhenium and tin atoms. The mono- and bis-Pt(PBu(t)3) adducts, Re2(CO)8(mu-SnPh2(2)[Pt(PBu(t)3)] (13) and Re2(CO)8(mu-SnPh2)2[Pt(PBu(t)3)]2 (14), were formed when 10 was treated with Pt(PBu(t)3)2. A mono adduct of 11, Re2(CO)8(mu-GePh2)2[Pt(PBu(t)3)] (15), was obtained similarly from the reaction of 11 with Pt(PBu(t)3)2.