Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(II)]: structural and spin-state phase transitions and low-temperature magnetic ordering in a unique 2-dimensional material

Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(II)] was synthesized by the reaction of ferrocene with imidazole in an excess of 2,2'- bipyridine in a Carius tube at 130 degrees C. Dc magnetic susceptibility studies at an applied field of 1000 G reveal that on cooling from room temperature, the material undergoes two structural phase transitions designated alpha --> beta and beta --> gamma at 151 and 133 K, respectively. On warming, the gamma --> beta and beta --> alpha transitions are observed at 137 and 151 K, respectively; a 4 K thermal hysteresis clearly detectable in the lower temperature beta <--> gamma transition. These structural phase transitions have also been studied by detailed, variable-temperature, ac susceptibility and M?ssbauer spectroscopy techniques. Single-crystal X-ray diffraction studies done at 294, 143, and 113 K reveal 2, 12, and 6 unique iron centers in the alpha-, beta-, and gamma-forms, respectively. All three forms have the same basic structure involving 2D extended double layer sheets (bilayers) of alternating tetrahedral and octahedral irons singly bridged by imidazolate ligands, with the octahedral centers additionally coordinated by 2,2'-bipyridine ligands that occupy positions between the sheets. Magnetic susceptibility and bond length data reveal that in the gamma-phase one in three six-coordinate sites corresponds to spin singlet ground-state iron(II); i.e., the overall alpha --> gamma process involves a spin-crossover transition such that at least 1/6 of the iron sites in the gamma-phase correspond to S = 0. This is supported by the low-temperature M?ssbauer spectra of the gamma-phase, which reveal the simultaneous presence of both S = 2 and S = 0 iron(II) centers. The compound magnetically orders, with a resultant small remnant magnetization, at low temperatures. The magnetic phase transition, studied by a combination of dc and ac susceptibility and M?ssbauer techniques, occurs at T(c) approximately 6.5 K.     

On the use of two-dimensional potential surface cross sections in the discussion of vibronic spectra of transition metal complexes

For purposes of interpreting broad band electronic spectra of complexes, the 3N-5 dimensional potential energy hypersurface may be reduced substantially. Consideration of the geometry of the orbital transition involved allows selection of an appropriate cross section, depending on which features should be represented. In addition, the orbital transition geometry predicts which vibrational modes will contribute significantly to the width of the spectrum. In the particular case of the first and second spin-allowed bands in d³ and d⁶ complexes, α_1g and ?_g modes are responsible for the band width.     

Scanning electrochemical microscopy as a probe of Ag+ binding kinetics at Langmuir phospholipid monolayers

A new method has been developed for measuring local adsorption rates of metal ions at interfaces based on scanning electrochemical microscopy (SECM). The technique is illustrated with the example of Ag+ binding at Langmuir phospholipid monolayers formed at the water/air interface. Specifically, an inverted 25 microm diameter silver disc ultramicroelectrode (UME) was positioned in the subphase of a Langmuir trough, close to a dipalmitoyl phosphatidic acid (DPPA) monolayer, and used to generate Ag+ via Ag electro-oxidation. The method involved measuring the transient current-time response at the UME when the electrode was switched to a potential to electrogenerate Ag+. Since the Ag+/Ag couple is reversible, the response is highly sensitive to local mass transfer of Ag+ away from the electrode, which, in turn, is governed by the interaction of Ag+ with the monolayer. The methodology has been used to determine the influence of surface pressure on the adsorption of Ag+ ions at a phospholipid (dipalmitoyl phosphatidic acid) Langmuir monolayer. It is shown that the capacity for metal ion adsorption at the monolayer increased as the density of surface adsorption sites increased (by increasing the surface pressure). A model for mass transport and adsorption in this geometry has been developed to explain and characterise the adsorption process.     

4,4′-Bipyridine complexes of molybdenum(II) and tungsten(II)

Treatment of [MI₂(CO)₃(NCMe)₂] with two equivalents of 4,4-bipyridine (4,4-bipy) in CH₂Cl₂ at room temperature gave the MeCN displaced products, [MI₂(CO)₃(4,4-bipy-N)₂] (1) and (2). Equimolar amounts of [MI₂(CO)₃(NCMe)₂] and L (L = PPh₃, AsPh₃ or SbPh₃) react to give [MI₂(CO)₃(NCMe)L], which when reacted in situ with 4,4-bipy yield the new complexes, [MI₂(CO)₃(4,4-bipy-N)L] (3)–(8). Reaction of equimolar quantities of [WI₂(CO)(NCMe)( ²-RC₂R)₂] (R = Me or Ph) and 4,4-bipy gave the new bis(alkyne) complexes, [WI₂(CO)(4,4-bipy-N)( ²-RC₂R)₂] (9) and (10). Treatment of [MI₂(CO)₃(NCMe)₂] with two equivalents of (9) or (10) in CH₂Cl₂ at room temperature affords the bimetallic complexes, [MI₂(CO)₃{WI₂(CO)(4,4-bipy-N,N)( ²-RC₂R)₂}₂] (11)–(14). Equimolar quantities of [MI₂(CO)₃(NCMe)(PPh₃)] (prepared in situ) and (9) or (10), react to give the 4,4-bipy-bridged complexes, [MI₂(CO)₃{WI₂(CO)(4,4-bipy-N,N)( ²-RC₂R)₂}(PPh₃)] (15)–(18). All the new complexes, (1)–(18) were characterised by elemental analysis (C, H and N), i.r. and ¹H-n.m.r. spectroscopy.     

The role of proficiency testing in the detection and resolution of calibration bias in the LeadCare® blood lead analyzer; limitations of peer-group assessment

Following implementation of the CLIA ‘88 laboratory regulations, the primary role of proficiency testing (external quality assessment, PT) in the U.S. has been widely viewed as one of assuring regulatory compliance. PT can also be an effective tool for detecting widespread analytical problems, subject to limitations based on the method of PT assigned value determination. A recent case study describes the role of two PT programs in detecting and resolving a calibration bias in the LeadCare blood lead analyzer, and illustrates the limitations of peer-group target determination in fulfilling that PT role.     

Catalytic asymmetric addition of alkylzinc and functionalized alkylzinc reagents to ketones

We describe our full report of the catalytic asymmetric addition of simple and functionalized dialkylzinc reagents to a broad range of saturated ketones and enones. The functionalized organozinc reagents contain esters, silyl ethers, alkyl chlorides, and alkyl bromides. In general, the resulting tertiary alcohol products are isolated with high ee's. With some substrates, yields are low as a result of the formation of aldol byproducts. Most substrates undergo additions with good yields reaching as high as 91%.     

Rapid intramolecular interactions between organic isocyanates and hexafluorobut-2-yne on a dirhodium centre; X-ray crystal structure of (η-C5H5)2Rh2 {μ2(η3-C(CF3)C(CF3)C(O)N(Ph)}

Complexes of formula (η-C₅H₅₂Rh₂{CF₃C₂CF₃ · RNCO} have been prepared by three methods, from reactions between organic isocyanates and (η-C₅H₅)₂Rh₂(CO)(CF₃C₂CF₃) or (η-C₅H₅)₂Rh₂(CO)₂(CF₃C₂CF₃); by treatment of (η-C₅H₅)₂Rh₂(CO)(CF₃C₂CF₃) with organic azides; and by oxidation with Me₃NO of the organic isocyanide in (η-C₅H₅)₂Rh₂(CO)(CNR)(CF₃C₂CF₃). The crystal and molecular structure of the complex (η-C₅H₅)₂Rh₂{CF₃C₂CF₃ · RNCO} with R = Ph has been determined from single crystal X-ray diffraction data. This reveals that the isocyanate has condensed with the hexafluorobut-2-yne to form an amide ligand of the form C(CF₃)C(CF₃)C(=O)N(R); this bridges the two rhodium atoms in a μ₂η³-manner.     

Complex copper(II) fluorides. XV. The Ternary System BaF2?CuF2?ScF3

The liquid-solid phase diagram of the binary system BaF₂? ScF₃ is established by D.T.A. and radiocrystallography. Three fluorides are disclosed: Ba₃Sc₂F₁₂, Ba₅Sc₃F₁₉ and a cubic high temperature phase Ba_1?xSc_xF_2+x (x = 0.17), the structure of which derives from that of BaF₂. A solid solution between BaF₂ and ScF₃ is also evidenced at high temperature. The ternary system BaF₂? CuF₂? ScF₃ is investigated by radiocrystallography and an isothermal section at 670°C is given. It shows the existence of four phases: a complex quaternary fluoride Ba₁₀Cu₁₂ScF₄₇, two “polytypic” phases the structure of which derives from that of BaCuF₄ and a tetragonal solid solution Ba₅Sc_3?xCu_xF_19?x with 0 ≤ x ≤ 1.     

Proton induced P-H and Mo-H bond activation at the phosphide bridged dimolybdenum complexes [Mo2Cp2(mu-H)(mu-PHR)(CO)4](R = Cy, 2,4,6-C6H2R'3; R'= H, Me, tBu)

The new hydride complexes [Mo2Cp2(mu-H)(mu-PHR)(CO)4] having bulky substituents (R = 2,4,6-C(6)H2tBu3= Mes*, R = 2,4,6-C6H2Me3= Mes) have been prepared in good yield by addition of Li[PHR] to the triply bonded [Mo2Cp2(CO)4] and further protonation of the resulting anionic phosphide complex [Mo2Cp2(mu-PHR)(CO)4]-. Protonation of the Mes* compound with either [H(OEt2)2][B{3,5-C6H3(CF3)2}4] or HBF4.OEt2 gives the cationic phosphinidene complex [Mo2Cp2(mu-H)(mu-PMes*)(CO)4]+ in high yield. In contrast, protonation of the analogous hydride compounds with Mes or Cy substituents on phosphorus give the corresponding unsaturated tetracarbonyls [Mo2Cp2(mu-PHR)(CO)4]+, which are unstable at room temperature and display a cis geometry. Decomposition of the latter give the electron-precise pentacarbonyls [Mo2Cp2(mu-PHR)(mu-CO)(CO)4]+, also displaying a cis arrangement of the metal fragments. In the presence of BF4- as external anion, fluoride abstraction competes with carbonylation to yield the neutral fluorophosphide hydrides [Mo2Cp2(mu-H)(mu-PFR)(CO)4]. Similar results were obtained in the protonation reactions of the hydride compounds having a Ph substituent on phosphorus. In that case, using HCl as protonation reagent gave the chloro-complex [Mo2ClCp2(mu-PHPh)(CO)4] in good yield. The structures and dynamic behaviour of the new compounds are analyzed on the basis of solution IR and 1H, 31P, 19F and 13C NMR data as well as the X-ray studies carried out on [Mo2Cp2(mu-H)(mu-PHMes)(CO)4](cis isomer), [Mo2Cp2(mu-H)(mu-PFMes)(CO)4](trans isomer), [Mo2Cp2(mu-PHCy)(mu-CO)(CO)4](BF4) and [Mo2ClCp2(mu-PHPh)(CO)4].     

Photochemically modulated endothelial cell thrombogenicity via the thrombomodulin-tissue factor pathways

Photodynamic therapy (PDT) is based on a photochemical reaction using a photosensitizer and light to produce reactive oxygen species that have biological effects. Although its application in some fields is largely based on thrombosis, in the vascular setting thrombosis must be prevented. In this study we examined the effects of PDT on the changes in activity of thrombomodulin (TM) and tissue factor (TF) as important regulators of the coagulation process of endothelial cells. Human umbilical vein endothelial cells were treated with PDT (chloro-aluminum-sulfonated phthalocyanine, lambda = 630 nm) at different light-energy doses, and TM and TF levels were measured using fluorescence spectroscopy. Microparticles (MP) were analyzed using flow cytometry analysis. PDT alters the thrombogenic state of endothelial cells by causing decreased expression of TM and increased expression of functional TF in a light-energy dose-dependent way. PDT-treated endothelial cells shed large numbers of MP containing high levels of TF. TF functionality of PDT-treated cells, measured by a Factor Xa-generating assay, was high. TF was located mostly intracellularly and in MP. The disturbed anticoagulant balance described in this study may explain the occurrence of thrombosis induced by PDT and, if not contained, dispute the suitability of PDT as an adjuvant modality to treat vascular restenosis.