An anionic phosphenium complex as an ambident nucleophile

A unique anionic phosphenium complex was prepared from reaction of an N-heterocyclic chlorophosphine with Collman's reagent or K[HFe(CO)(4)]/NaH and characterized by spectral and XRD data. The complex behaves as an ambident nucleophile. Reactions with acetic acid, ClSnPh3, and a further equivalent of an N-heterocyclic chlorophosphine proceed via electrophilic functionalization at the metal site to yield appropriate mono- or bis-phosphenium complexes. Reaction with MeI at -70 degrees C produces a P-alkylation product as the first spectroscopically detectable intermediate, which decays at a higher temperature to give a mixture of free P-methylated N-heterocyclic phosphine and its Fe(CO)(4) complex. The different reaction products were characterized by spectral and XRD data. Computational studies indicate that the NHP units in all complexes display p-acceptor behaviour but show no disposition to adopt phosphide-like character or formally oxidize the metal centre.Peer reviewe

STUDIES ON ANTIBODY-PRODUCING CELLS : II. APPEARANCE OF3H-THYMIDINE-LABELED ROSETTE-FORMING CELLS

A study of the kinetics of antibody-producing cells has been carried out by the use of rosette formation for detection of individual antibody-producing cells, and labeling with tritiated thymidine, in cells obtained from mouse spleens at intervals after injection of SRBC. Following a primary injection of the antigen, the number of RFC per million cells was found to increase to a peak at 5 days, then, after a decrease, to a second peak at about the 10th day. The curve of tritium labeling of RFC was also biphasic, with peaks on the 3rd and 7th day. The second increase in rosette-forming cells could be shown to involve, especially between the 7th and 9th day, a second increase in lymphoid cell RFC and, among these, 7S antibody-producing cells. When the population examined was restricted to large lymphocytes, two peaks of RFC per million cells and two peaks of labeling were again found. In this case, however, the peaks of RFC and of labeling were reached on the same day in each instance, rather than with the 2 day difference found in the entire spleen cell suspension or the entire lymphoid cell population. Electron microscopic examination of labeled rosette-forming cells showed these to be largely lymphocytes, but to include rather well differentiated plasmablasts as well. No macrophages were found among labeled RFC in the primary response. A substantial number of labeled lymphocytes were found in close contiguity with rosette-forming macrophages. The percentage of labeling in such lymphocytes was as high, on the respective days, as the percentage of labeled cells among the RFC of the entire suspension

On the energetics of P-P bond dissociation of sterically strained tetraamino-diphosphanes

The homolytic P-P bond fission in a series of sterically congested tetraaminodiphosphanes (R2N)(2)P-P-(NR2)(2) ({4}(2)-{9}(2), two of which were newly synthesized and fully characterized) into diaminophosphanyl radicals (R2N)(2)P-center dot (4-9) was monitored by VT EPR spectroscopy. Determination of the radical concentration from the EPR spectra permitted to calculate free dissociation energies Delta G(Diss)(295) as well as dissociation enthalpies Delta H-Diss and entropies Delta S-Diss, respectively. Large positive values of Delta G(Diss)(295) indicate that the degree of dissociation is in most cases low, and the concentration of persistent radicals - even if they are spectroscopically observable at ambient temperature - remains small. Appreciable dissociation was established only for the sterically highly congested acyclic derivative {9}(2). Analysis of the trends in experimental data in connection with DFT studies indicate that radical formation is favoured by large entropy contributions and the energetic effect of structural relaxation (geometrical distortions and conformational changes in acyclic derivatives) in the radicals, and disfavoured by attractive dispersion forces. Comparison of the energetics of formation for CC-saturated N-heterocyclic diphosphanes and the 7 pi-radical 3c indicates that the effect of energetic stabilization by pi-electron delocalization in the latter is visible, but stands back behind those of steric and entropic contributions. Evaluation of spectroscopic and computational data indicates that diaminophosphanyl radicals exhibit, in contrast to aminophosphenium cations, no strong energetic preference for a planar arrangement of the (R2N)(2)P unit.Peer reviewe

Synthesis and Ambiphilic Reactivity of Metalated Diorgano-Phosphonite Boranes

Unprecedented metalated phosphonite boranes were prepared from PH-substituted precursors and silyl amides. Although potassium derivatives were thermally stable and could even be isolated and structurally characterised, lithiated analogues proved to be unstable towards self-condensation under cleavage of LiOR at ambient temperature. Reaction studies revealed that the metalated phosphonite boranes exhibit ambiphilic character. Their synthetic potential as nucleophilic building blocks was demonstrated in the synthesis of the first stannylated phosphonite representing a new structural motif in phosphine chemistry.Peer reviewe

Bildung und Eigenschaften anorganischer Kieselsäure-Schadstoffverbindungen im Boden

Verschiedene Studien demonstrieren den positiven Effekt von Si-Dünger auf die Widerstandsfähigkeit einiger Pflanzen gegenüber anorganischen Schadstoffen, wie Pb, Cd und Cu. Als eine der Ursachen wird die Schadstoffimmobilisierung nach Si-Zugabe durch eine Komplexierungsreaktion angesehen, sodass die Schadstoffe mit der Kieselsäure in einer für die Pflanze nicht verfügbaren Form gebunden werden. Dieser Prozess wurde jedoch bisher nicht systematisch untersucht und aufgeklärt. Das Hauptziel dieser Arbeit ist es die Prozesse der Immobilisierung von Pb, Cd und Cu im Boden mittels Kieselsäure zu klären. Dafür sollen die Interaktionen zur Bildung von Kieselsäure-Schadstoffverbindungen zwischen der gelösten Kieselsäure und den Metallkationen in wässriger Lösung in einer Reihe von Fällungsexperimenten untersucht werden. Im ersten Schritt wird die Bildung der Kieselsäure-Schadstoffverbindungen in wässriger Lösung bei variierenden Synthesebedingungen (Si-Metall-Verhältnis und Zeit) und Raumtemperatur untersucht. Für die Synthese werden Tetraethylorthosilikat (TEOS) als Si-Quelle sowie die drei gelösten Metallsalze Pb(NO3)2, Cd(NO3)2 bzw. Cu(NO3)2 verwendet. Zur Untersuchung der Selektivität des Si gegenüber den genannten Metallkationen wird TEOS zu einer äquimolaren Lösung aller drei Metalle gegeben. Die Erfassung der Zeitabhängigkeit des Partikelwachstums erfolgt mittels Entnahme eines 50 ml Aliquots an 10 unterschiedlichen Terminen. Nach Dialyse in Reinstwasser werden die Proben mittels dynamischer Lichtstreuung und Elektrophorese analysiert, um die Größenverteilung und die Ladung der kolloidalen Partikel zu ermitteln. Die Bestimmung der relativen Partikelkonzentration in den Suspensionen erfolgt durch Messung der optischen Dichte mittels UV/VIS-Spektroskopie. Die chemische Zusammensetzung der dialysierten Suspension wird mittels ICP-OES ermittelt. Die synthetisierten Verbindungen werden mittels FTIR- und 29Si-NMR-Spektroskopie charakterisiert

Comparing the Ligand Behavior of N-Heterocyclic Phosphenium and Nitrosyl Units in Iron and Chromium Complexes

N-Heterocyclic phosphenium (NHP) and nitro-sonium (NO+) ligands are often viewed as isolobal analogues that share the capability to switch between different charge states and thus display redox "noninnocent" behavior. We report here on mixed complexes [(NHP)M(CO)(n)(NO)] (M = Fe, Cr; n = 2, 3), which permit evaluating the donor/acceptor properties of both types of ligands and their interplay in a single complex. The crystalline target compounds were obtained from reactions of N-heterocyclic phosphenium triflates with PPN[Fe(CO)(3)(NO)] or PPN[Cr(CO)(4)-(NO)], respectively, and fully characterized (PPN = nitride-bistriphenylphosphonium cation). The structural and spectroscopic (IR, UV-vis) data support the presence of carbene-analogue NHP ligands with an overall positive charge state and pi-acceptor character. Even if the structural features of the M-NO unit were in all but one product blurred by crystallographic CO/NO disorder, spectroscopic studies and the structural data of the remaining compound suggest that the NO units exhibit nitroxide (NO-) character. This assignment was validated by computational studies, which reveal also that the electronic structure of iron NHP/ NO complexes is closely akin to that of the Hieber anion, [Fe(CO)(3)(NO)](-). The electrophilic character of the NHP units is further reflected in the chemical behavior of the mixed complexes. Cyclic voltammetry and IR-SEC studies revealed that complex [(NHP)Fe(CO)(2) (NO)] (4) undergoes chemically reversible one-electron reduction. Computational studies indicate that the NHP unit in the resulting product carries significant radical character, and the reduction may thus be classified as predominantly ligand-centered. Reaction of 4 with sodium azide proceeded likewise under nucleophilic attack at phosphorus and decomplexation, while super hydride and methyl lithium reacted with all chromium and iron complexes via transfer of a hydride or methyl anion to the NHP unit to afford anionic phosphine complexes. Some of these species were isolated after cation exchange or trapped with electrophiles (H+, SnPh3(+)) to afford neutral complexes representing the products of a formal hydrogenation or hydrostannylation of the original M=P double bond.Peer reviewe

Spin quantum jumps in a singly charged quantum dot

Quantum Matter and Optic

Hydrochlorination of ruthenaphosphaalkenyls: unexpectedly facile access to alkylchlorohydrophosphane complexes

The novel ruthenaphosphaalkenyls [Ru{P═C(H)SiMe2R}Cl- (CO)(PPh3)2] (R = p-C6H4CF3, nBu) have been prepared for the first time, and studied alongside precedent analogues (R = Me, Ph, p-tol) for their reactions with HCl. In contrast to chemistry defined for the tert-butyl congener [Ru{P═C(H)tBu}Cl(CO)(PPh3)2], which initially adds a single equivalent of HCl across the Ru−P linkage, all five silyl derivatives undergo spontaneous addition of a second equivalent to afford [Ru{η1-PHCl−CH2SiMe2R}Cl(CO)(PPh3)2], extremely rare examples of coordinated “PHXR” type ligands. Where R = SiMe3, a distorted octahedral geometry with a conformationally restricted “PHXR” ligand is observed crystallographically; this structure is appreciably retained in solution, as determined from multinuclear NMR spectroscopic features, which include a Karplus-like PPh3−Ru−P−H spin−spin coupling dependence. Computational data suggest a silyl-induced increase in negative charge density at the phosphaalkenic carbon, rather than an intrinsic thermodynamic driver, as the likely origin of the disparate reactivity

Diazaphospholene and diazaarsolene derived homogeneous catalysis

The past 20 years has seen significant advances in main group chemistry and their use in catalysis. This minireview showcases the recent emergence of phosphorus and arsenic containing heterocycles as catalysts. With that, we discuss how the Group 15 compounds diazaphospholenes, diazaarsolenes, and their cationic counterparts have proven to be highly effective catalysts for a wide range of reduction transformations. This minireview highlights how the initial discovery by Gudat of the hydridic nature of the P–H bond in these systems led to these compounds being used as catalysts, and discusses the wide range of examples currently present in the literature