High-Spin and Reactive Fe13 Cluster with Exposed Metal Sites

Atomically defined large metal clusters have applications in new reaction development and preparation of materials with tailored properties. Expanding the synthetic toolbox for reactive high nuclearity metal complexes, we report a new class of Fe clusters, Tp*₄W₄Fe₁₃S₁₂ , displaying a Fe₁₃ core with M−M bonds that has precedent only in main group and late metal chemistry. M₁₃ clusters with closed shell electron configurations can show significant stability and have been classified as superatoms. In contrast, Tp*₄W₄Fe₁₃S₁₂ displays a large spin ground state of S=13. This compound performs small molecule activations involving the transfer of up to 12 electrons resulting in significant cluster rearrangements.     

Metallorganische Verbindungen mit N-substituierten 3-Hydroxy-2-methyl-4-pyridon-Liganden: quadratisch-planare Rhodium(I)-, Iridium(I)- und Palladium(II)-Komplexe

Organometallic Compounds with N -substituted 3-Hydroxy-2-methyl-4-pyridone Ligands: square planar Rhodium(I), Iridium(I), and Palladium(II) Complexes Reactions of [(OC)₂MCl]₂ (M = Rh, Ir) or [(cod)RhCl]₂ with the anions of N-Aryl or N-Alkyl substituted 3-hydroxy-2-methyl-4-pyridones (O–O′) yield complexes of the general formula [L₂M(O–O′)]. Compounds of this type are also available from reactions of [(OC)₂Rh(acac)] with the corresponding neutral ligands. Substitution of one carbonyl-ligand of the N-phenyl complex [(OC)₂Rh(C₁₂H₁₀NO₂)] ( 2 ) with cyclooctene affords [(OC)(C₈H₁₄)Rh(C₁₂H₁₀NO₂)] ( 8 ). The palladium complexes [(R₃P)Pd(O–O′)Cl] (R = Et, Bu), [(C₆H₄CH₂NMe₂) · Pd(O–O′)] and [(Et₃P)₂Pd(O–O′)]BF₄ ( 9 – 12 ) were synthesized from [(R₃P)PdCl₂]₂, [(C₆H₄CH₂NMe₂)PdCl]₂ or [(Et₃P)PdCl₂]. The structures of the N-methyl compounds [(OC)₂Rh(C₇H₈NO₂)] ( 1 ) and [(Ph₃P)Pd(C₇H₈NO₂)Cl] ( 9 ) were determined by single crystal X-ray diffraction.     

Metallomacrocycles as ligands: synthesis and characterisation of aluminium-bridged bisglyoximato complexes of palladium and iron

Dialuminiummacrocycles based on bisglyoximato moieties were prepared and their coordination chemistry with Fe(II) and Pd(II) was investigated. The bridging aluminium centers were supported by several types of tetradentate diphenoxide diamine ligands. The nature of the ancillary ligands bound to aluminium was found to affect the overall geometry and symmetry of the metallomacrocycles. Enantiopure, chiral diphenoxide ligands based on the (R,R)-trans-1,2-diaminocyclohexane backbone afforded cleanly one metallomacrocycle isomer. The size and electronic properties of remote substituents on aluminium-bound ligands affected the binding mode and electronic properties of the central iron. A structurally characterized iron complex shows trigonal prismatic coordination mode, with phenoxide bridges between iron and aluminium. Increasing the size of the phenoxide substituents led to square bipyramidal coordination at iron. Employing p-NO(2)- instead of p-tBu-substituted phenoxide as supporting ligands for aluminium caused a 0.27 V positive shift of the Fe(III)/Fe(II) reduction potential. These results indicate that the present synthetic approach can be applied to a variety of metallomacrocycles based on bisglyoximato motifs to affect the chemistry at the central metal.     

Interfacial layers from the protein HFBII hydrophobin: dynamic surface tension, dilatational elasticity and relaxation times

The pendant-drop method (with drop-shape analysis) and Langmuir trough are applied to investigate the characteristic relaxation times and elasticity of interfacial layers from the protein HFBII hydrophobin. Such layers undergo a transition from fluid to elastic solid films. The transition is detected as an increase in the error of the fit of the pendant-drop profile by means of the Laplace equation of capillarity. The relaxation of surface tension after interfacial expansion follows an exponential-decay law, which indicates adsorption kinetics under barrier control. The experimental data for the relaxation time suggest that the adsorption rate is determined by the balance of two opposing factors: (i) the barrier to detachment of protein molecules from bulk aggregates and (ii) the attraction of the detached molecules by the adsorption layer due to the hydrophobic surface force. The hydrophobic attraction can explain why a greater surface coverage leads to a faster adsorption. The relaxation of surface tension after interfacial compression follows a different, square-root law. Such behavior can be attributed to surface diffusion of adsorbed protein molecules that are condensing at the periphery of interfacial protein aggregates. The surface dilatational elasticity, E, is determined in experiments on quick expansion or compression of the interfacial protein layers. At lower surface pressures (<11 mN/m) the experiments on expansion, compression and oscillations give close values of E that are increasing with the rise of surface pressure. At higher surface pressures, E exhibits the opposite tendency and the data are scattered. The latter behavior can be explained with a two-dimensional condensation of adsorbed protein molecules at the higher surface pressures. The results could be important for the understanding and control of dynamic processes in foams and emulsions stabilized by hydrophobins, as well as for the modification of solid surfaces by adsorption of such proteins.     

Specific Features of Strain Dependences of Loss Modulus in Highly Stretched Elastomers

The loss modulus E″ in elastomer strips stretched by a factor of λ is studied theoretically. A small oscillating deformation is applied to these strips in the transition frequency zone between the glassy and rubbery states. The range of λ under study ranges up to ultimate extensions close to fracture. A model of E″/λ dependence in the transition frequency zone is suggested that considers the possible distortion in the local chain structure in the ultimate extension range. It is found that E″ in the range of moderate and high λ increases owing to the finite extensibility of the chains. However, upon further extension up to ultimate values, E″ decreases and a maximum appears on the E″/λ curves. These features are also confirmed experimentally.

     

Calorimetric Studies of Biopolymers and Aggregates of Phospholipids

The structural transformations that biopolymers undergo in aqueous solution are complex processes, whose mechanisms can only be explained by the coupling of various partial processes. A spectroscopic analysis of the states of the system often requires considerable effort. In many cases, calorimetric studies have proven to be sufficient for a general characterization of the behavior of the system. These measurements provide information on the stability of the initial state and on the cooperativity of the total process. From the thermodynamic parameters of the system, knowledge on the structure-determining influence of the various types of inter- and intramolecular interactions is obtained. This not only applies to solutions of biopolymers and analogous model substances, but also to solutions of biopolymer complexes with low-molecular-weight ligands and to aqueous suspensions of self-aggregating phospholipids. Possibilities and limitations of the calorimetric methods of measurement are demonstrated for typical examples from the numerous polypeptide and polynucleotide systems and the phospholipid bilayer systems that have been studied. In addition, the special problems involved in carrying out measurements on dilute solutions are pointed out. Here, the latest advances made in measuring techniques are readily apparent.     

Methionin als Vorläufer zur enantioselektiven Synthese α-verzweigter Vinylglycine und anderer Aminosäuren

Methionine as Precursor for the Enantioselective Synthesis of α-Branched Vinylglycines and of Other Amino Acids Methionine is converted by previously published methods into the diastereoisomerically pure 3-thiabutyl-substituted oxazolidinone ( 7 ) and imidazolidinones 5 and 6 . An X-ray crystal structure determination of cis-3-benzoyl-2-(tert-butyl)-4-(3-thiabutyl)oxazolidin-5-one ( 7 ) confirms the configurational assignments made by NOE-NMR measurements. Oxidation to sulfoxides and pyrolytic elimination produce vinyl-substituted heterocycles (see 19, 21 ). Diastereoselective alkylations of the enolate 14 from the imidazolidinone 5 and of the dienolate 23 from the vinyl derivative 19 give geminally alkyl- and/or vinyl-substituted heterocycles. Some of these products were hydrolyzed to free amino acids, such as (R)-2-methyl- ( 25a ) and (R)-2-ethyl-2-vinylglycine ( 25b ) (R)-2-methylhomoserine ( 27 ). Raney-Ni desulfurization of 5 and oxidative degradation of 19 lead to enantiomerically pure derivatives of α-aminobutyric acid (see 28 ) and of glycine (see 31 ), respectively.