Transmetallierung von Zinn(II) in [Sn(μ3‐PSitBu3)]4 durch Barium – von Sn4P4‐Heterocuban‐Strukturen zu heterobinuklearen Käfigverbindungen mit einem zentralen BanSn4−nP4‐Heterocuban‐Polyeder (n = 1, 2 und 3)

Transmetallation of Tin(II) in [Sn(μ₃‐PSitBu₃)]₄ by Barium – from Sn₄P₄ Heterocubane Structures to Heterobinuclear Cage Compounds with a Central Ba_nSn_4?nP₄ Heterocubane Polyhedron (n = 1, 2 and 3) For the preparation of compounds of the type [Ba_nSn_4?n(PSitBu₃)₄] (n = 1 ( 2 ), 2 ( 3 ) and 3 ( 4 )) two synthetic routes are applicable: in the transmetallation reaction homometallic [Sn₄(PSitBu₃)₄] ( 1 ) reacts with barium metal and in a deprotonation reaction (metallation) tri(tert‐butyl)silylphosphane reacts simultaneously with (thf)₂Ba[N(SiMe₃)₂]₂ and Sn[N(SiMe₃)₂]₂. During the transmetallation reaction mixtures of the heterobimetallic cage compounds 2 to 4 are obtained, however, analytically pure compounds 2 and 3 are accessible by the metallation reaction. Compound 4 is formed as a minor product together with 3 . Due to the larger Ba‐P bond lengths compared to the Sn‐P values the substitution of tin by barium leads to strong distortions of the heterocubane moiety. With NMR‐spectroscopic experiments one could show that all the above mentioned compounds form Ba_nSn_4?nP₄ heterocubane cage structures.     

UV/Vis Spectroscopic Properties of N -(2′-Hydroxy-4′- N , N -dimethyl-aminobenzylidene)-4-nitroaniline in Various Solvents and Solid Environments

 N-(2′-Hydroxy-4′-N,N-dimethylaminobenzylidene)-4-nitroaniline [HDBN] has been used as a model for investigating intra- and intermolecular D–A (donor–acceptor) interactions in various environments by means of UV/Vis spectroscopy. UV/Vis spectra of HDBN have been measured in various solvents, ethanolic solutions of different pH, adsorbed on silica, and in the solid state. A bathochromic shift of ν_max is observed with increasing the dipolarity/polarizability and HBD (hydrogen bond donor) capacity of the solvent, which is described by means of a multiple LSE (linear solvation energy) relationship in terms of the empirical Kamlet-Taft solvent polarity parameters. The adsorption of HDBN on Aerosil^? 300-silica particles in non-HBA (hydrogen bond acceptor) solvents is explained in the same sense. Mobile protons and sol–gel entrapping cause a hypsochromic shift due to protonation of the lone electron pair of the 4′-N,N-dimethylamino group. Hydroxide ions attack the 2′-hydroxy group which causes a bathochromic shift. A strong intramolecular hydrogen bond between the 2′-hydroxyl hydrogen and the imine nitrogen atom is present in the solid-state structure causing an unprecedented bathochromic shift.     

Structural studies of transition metal complexes with 4,5-bis(2-pyridylmethylsulfanyl)-4',5'-ethylenedithiotetrathiafulvalene: probing their potential for the construction of multifunctional molecular assemblies

Three divalent transition metal complexes of 4,5-bis(2-pyridylmethylsulfanyl)-4',5'-ethylenedithiotetrathiafulvalene have been prepared and crystallographically characterized. The isostructural Co(II) and the Ni(II) complexes show octahedral geometries around the metal ions with the coordination sites occupied by the pyridyl nitrogen atoms and the thioether sulfur atoms of the ligand and cis coordination of the halide ions. Cyclic voltammetry reveals that the complexation leads to a small anodic shift in the first oxidation potential of the TTF system.     

The use of ferrocene in organometallic synthesis: A two-step preparation of cyclopentadienyliron acetonitrile and phosphine cations via photolysis of cyclopentadienyliron tricarbonyl or arene cations

UV photolysis of [CpFe^II(CO)₃]⁺ PF6₆^? (I) or [CpFe^II(η⁶-toluene)]⁺ PF₆^?? (II) in CH₃CN in the presence of 1 mole of a ligand L gives the new air sensitive, red complexes [CpFe^II(NCCH₃)₂L]⁺PF₆^? (III, L = PPh₃; IV; L = CO; VIII, L = cyclohexene; IX, L = dimethylthiophene) and the known air stable complex [CpFe^II(PMe₃)₂(NCMe)]⁺ PF₆^? (V). The last product is also obtained by photolysis in the presence of 2 or 3 moles of PMe₃. In the presence of dppe, the known complex [CpFe^II (dppe)(NCCH₃)]⁺ (XI) is obtained. Complex III reacts with CO under mild conditions to give the known complex [CpFe(NCCH₃)(PPh₃)CO]⁺ PF₆^? (X). UV photolysis of I in CH₃CN in the presence of 1-phenyl-3,4-dimethylphosphole (P) gives [CpFe^IIP₃]⁺ PF₆^? (XII); UV photolysis of II in CH₂Cl₂ in the presence of 3 moles of PMe₃ or I mole of tripod (CH₃C(CH₂Ph₂)₃) provides an easy synthesis of the known complexes [CpFe^II(PMe₃)₃]⁺ PF₆^? (VII) or [CpFe^IIη³-tripod]⁺ PF₆^t- (XIII). Since I and II are easily accessible from ferrocene, these photolytic syntheses provide access to a wide range of piano-stool cyclopentadienyliron(II) cations in a 2-step process from ferrocene.     

Relaxation processes in semidilute solutions of polymers in liquid crystal solvents

We investigate the relaxation phenomena in a polymer (polystyrene)/liquid crystal (4-cyano-4'-n-octyl-biphenyl) system, in its homogeneous isotropic phase near the isotropic-isotropic, isotropic-nematic, and isotropic-smectic coexistence curve, using both polarized and depolarized photon correlation spectroscopy (PCS). We study this system for different polystyrene molecular weights (4750, 12 500, and 65 000 g/mol), different compositions (50, 40, 30, and 10% polystyrene (PS) by weight), and different temperatures close to phase boundaries. First of all, we determine the phase diagrams of this system for the different molecular weights. The shape of the phase diagrams strongly depends on the molecular weight. However, in all cases, at low temperatures, these systems separate into an almost pure liquid crystalline (LC) phase and polystyrene-rich phase. PCS measurements show that the relaxation processes in the homogeneous phase are not affected by the proximity of the nematic, or smectic, boundaries (even at a temperature of 0.1 degrees C above the phase separation in two phases). In polarized PCS experiments, we always see three relaxation processes well separated in time: one, very fast, with a relaxation time of the order of 10(-5) s; a second one with a relaxation time within the range 10(-2)-10(-3) s; and a last one, very slow, with a relaxation time of the order of 1 s. Both the fast and slow modes are independent of the wave vector magnitude, while the intermediate relaxation process is diffusive. In depolarized PCS experiments, the intermediate mode disappears and only the fast and slow relaxation processes remain, and they are independent of the magnitude of the wave vector. The diffusive mode is the classical diffusive mode, which is associated with the diffusion of polymer chains in all polymer solutions. The fast mode is due to the rotational diffusion of 4-cyano-4'-n-octyl-biphenyl (8CB) molecules close to polystyrene chains (transient network). Finally, we assign the slowest mode to reorientational processes of small aggregates of PS chains that are not dissolved in 8CB.     

The intermediacy of metathiophosphonate PhPSO in the reaction of N-tert-butyl-P-phenylphosphonamidothioic acid with alcohols

Kinetic studies of the reaction of N-tert-butyl-P-phenylphosphonamidothioic acid (1) with alcohols were carried out in CH2Cl2 by means of 31P NMR spectrometry. The reaction is of the first order with respect to thio acid 1. The first-order rate constant at 30 degrees C increases with increasing methanol concentration below 0.25 M, but otherwise the rate constants are either independent of alcohol concentration (MeOH above 0.25 M, BuOH) or decrease with increasing alcohol concentration (i-PrOH, t-BuOH). The effect of alcohols on the order of the reaction and parameters of activation, as well as results of competition experiments, lead us to the conclusion that reaction of 1 with alcohols occurs by an elimination-addition mechanism involving the association of the thio acid 1 and the alcohol and then formation in the rate-determining step of an encounter complex 2' ' involving metathiophosphonate 4, amine, and alcohol. Metathiophosphonate 4 reacts preferentially with the alcohol as the encounter complex (primary alcohols) or after diffusion apart as a "free" intermediate (hindered alcohols).     

Synthesis and characterization of a new family of bi-, tri-, tetra-, and pentanuclear ferric complexes

Nine members of a new family of polynuclear ferric complexes have been synthesized and characterized. The reaction of Fe(O(2)CMe)(2) with polydentate Schiff base proligands (H(2)L) derived from salicylidene-2-ethanolamine, followed in some cases by reaction with carboxylic acids, has afforded new complexes of general formulas [Fe(2)(pic)(2)(L)(2)] (where pic(-) is the anion of 2-picolinic acid), [Fe(3)(O(2)CMe)(3)(L)(3)], [Fe(4)(OR)(2)(O(2)CMe)(2)(L)(4)], and [Fe(5)O(OH)(O(2)CR)(4)(L)(4)]. The tri-, tetra-, and pentanuclear complexes all possess unusual structures and novel core topologies. M?ssbauer spectroscopy confirms the presence of high-spin ferric centers in the tri- and pentanuclear complexes. Variable-temperature magnetic measurements suggest spin ground states of S = 0, 1/2, 0, and 5/2 for the bi-, tri-, tetra-, and pentanuclear complexes, respectively. Fits of the magnetic susceptibility data have provided the magnitude of the exclusively antiferromagnetic exchange interactions. In addition, an easy-axis-type magnetic anisotropy has been observed for the pentanuclear complexes, with D values of approximately -0.4 cm(-)(1) determined from modeling the low-temperature magnetization data. A low-temperature micro-SQUID study of one of the pentanuclear complexes reveals magnetization hysteresis at nonzero field. This is attributed to an anisotropy-induced energy barrier to magnetization reversal that is of molecular origin. Finally, an inelastic neutron scattering study of one of the trinuclear complexes has revealed that the magnetic behavior arises from two distinct species.     

Alkyl-Diol Silica (ADS): restricted access precolumn packings for direct injection and coupled-column chromatography of biofluids

The direct and repetitive injection of untreated biological fluids (e.g., hemolyzed blood, plasma, serum, cell culture and tissue homogenates) onto an HPLC-system and the subsequent analysis of low-molecular weight compounds (e.g. drugs, xenobiotics, metabolites) is rendered possible by a coupled-column configuration and special precolumn packings. For this purpose a new family of chemically and enzymatically tailored reversed-phase packing materials have been prepared. The LC-integrated sample clean-up with these restricted access (bimodal) phases is based on the complete nonadsorptive size exclusion of macromolecules (e.g. proteins) and on the simultaneous dynamic partitioning of the target molecules. The bonded phase which exclusively covers the internal pore surface of a glyceryl-modified silica is a butyryl-(C-4), capryloyl-(C-8) or stearoyl-(C-18) moiety. These ligands allow a classical reversed-phase or ion-pair chromatography during the sample work-up step. The capacity of the n-alkyl phase is comparable with conventional silica based RP-materials. The broad hydrophobic retentive capability of these packings allows the extraction of a wide variety of compounds of biomedical interest. The electroneutral and hydrophilic particle exterior (glyceryl-residues) was generated using either soluble or immobilized enzymes (lipase, esterase) which cleave the fatty acid esters exclusively at the outer surface. Unwanted macromolecular components of a sample (e.g. proteins) are quantitatively eluted in the void volume due to the restricted access given by the pore size (6 nm) and the nonadsorptive external diol coverage. The lifetime of a precolumn (25 × 4 mm I.D.) packed with these novel bimodal, i.e. RP-SEC phases exceeds more than 200 injections of 500 l plasma. In addition to the synthesis, this paper describes an application of each of these Alkyl-Diol Silica (ADS) precolumn packings in fully automated coupled-column HPLC systems for the analysis of drugs and endogenous compounds in different biological matrices.Dedicated to Professor Dr. Dr. h.c. mult. J.F.K. Huber on the occasion of his 70th birthday     

Synthesis and spectroscopic characterization of a new family of Ni(4) spin clusters

A new family of tetranuclear Ni complexes [Ni(4)(ROH)(4)L(4)] (H(2)L = salicylidene-2-ethanolamine; R = Me (1) or Et (2)) has been synthesized and studied. Complexes 1 and 2 possess a [Ni(4)O(4)] core comprising a distorted cubane arrangement. Magnetic susceptibility and inelastic neutron scattering studies indicate a combination of ferromagnetic and antiferromagnetic pairwise exchange interactions between the four Ni(II) centers, resulting in an S = 4 spin ground state. Magnetization measurements reveal an easy-axis-type magnetic anisotropy with D approximately -0.93 cm(-)(1) for both complexes. Despite the large magnetic anisotropy, no slow relaxation of the magnetization is observed down to 40 mK. To determine the origin of the low-temperature magnetic behavior, the magnetic anisotropy of complex 1 was probed in detail using inelastic neutron scattering and frequency domain magnetic resonance spectroscopy. The spectroscopic studies confirm the easy-axis-type anisotropy and indicate strong transverse interactions. These lead to rapid quantum tunneling of the magnetization, explaining the unexpected absence of slow magnetization relaxation for complex 1.