An Isolated Nitridyl Radical‐Bridged {Rh(N.)Rh} Complex

Photochemical activation of [(PNNH)Rh(N₃)] (PNNH=6‐di‐(tert‐butyl)phosphinomethyl‐2,2′‐bipyridine) complex 2 produced the paramagnetic (S=1/2), [(PNN)Rh?N^.‐Rh(PNN)] complex 3 (PNN^?=methylene‐deprotonated PNNH), which could be crystallographically characterized. Spectroscopic investigation of 3 indicates a predominant nitridyl radical (^.N^2?) character, which was confirmed computationally. Complex 3 reacts selectively with CO, producing two equivalents of [(PNN)Rh^I(CO)] complex 4 , presumably by nitridyl radical N,N‐coupling.     

On the Effect of Secondary Nucleation on Deracemization through Temperature Cycles

Herein, the pivotal role of secondary nucleation in a crystallization-enhanced deracemization process is reported. During this process, complete and rapid deracemization of chiral conglomerate crystals of an isoindolinone is attained through fast microwave-assisted temperature cycling. A parametric study of the main factors that affect the occurrence of secondary nucleation in this process, namely agitation rate, suspension density, and solute supersaturation, confirms that an enhanced stereoselective secondary nucleation rate maximizes the deracemization rate. Analysis of the system during a single temperature cycle showed that, although stereoselective particle production during the crystallization stage leads to enantiomeric enrichment, undesired kinetic dissolution of smaller particles of the preferred enantiomer occurs during the dissolution step. Therefore, secondary nucleation is crucial for the enhancement of deracemization through temperature cycles and as such should be considered in further design and optimization of this process, as well as in other temperature cycling processes commonly applied in particle engineering.     

Catalytic Synthesis of 8-Membered Ring Compounds via Cobalt(III)-Carbene Radicals

The metalloradical activation of o-aryl aldehydes with tosylhydrazide and a cobalt(II) porphyrin catalyst produces cobalt(III)-carbene radical intermediates, providing a new and powerful strategy for the synthesis of medium-sized ring structures. Herein we make use of the intrinsic radical-type reactivity of cobalt(III)-carbene radical intermediates in the [Co^II(TPP)]-catalyzed (TPP=tetraphenylporphyrin) synthesis of two types of 8-membered ring compounds; novel dibenzocyclooctenes and unprecedented monobenzocyclooctadienes. The method was successfully applied to afford a variety of 8-membered ring compounds in good yields and with excellent substituent tolerance. Density functional theory (DFT) calculations and experimental results suggest that the reactions proceed via hydrogen atom transfer from the bis-allylic/benzallylic C−H bond to the carbene radical, followed by two divergent processes for ring-closure to the two different types of 8-membered ring products. While the dibenzocyclooctenes are most likely formed by dissociation of o-quinodimethanes (o-QDMs) which undergo a non-catalyzed 8π-cyclization, DFT calculations suggest that ring-closure to the monobenzocyclooctadienes involves a radical-rebound step in the coordination sphere of cobalt. The latter mechanism implies that unprecedented enantioselective ring-closure reactions to chiral monobenzocyclooctadienes should be possible, as was confirmed for reactions mediated by a chiral cobalt-porphyrin catalyst.     

Redox-Active Supramolecular Heteroleptic M4L2L′2 Assemblies with Tunable Interior Binding Site

Three Pt₄L₂L′₂ heteroleptic rectangles ( 1 – 3 ), containing ditopic redox-active bis-pyridine functionalized perylene bisimide (PBI) ligands PBI-pyr₂ ( L ) are reported. Co-ligand L′ is a dicarboxylate spacer of varying length, leading to modified overall size of the assemblies. ¹H NMR spectroscopy reveals a trend in the splitting and upfield chemical shift of the PBI-hydrogens in the rectangles with respect to free PBI, most pronounced with the largest strut length ( 3 ) and least with the smallest strut length ( 1 ). This is attributed to increased rotational freedom of the PBI-pyr ₂ ligand over its longitudinal axis (N_py-N_py), due to increased distance between the PBI-surfaces, which is corroborated by VT-NMR measurements and DFT calculations. The intramolecular motion entails desymmetrization of the two PBI-ligands, in line with cyclic voltammetry (CV) data. The first (overall two-electron) reduction event and re-oxidation for 1 display a subtle peak-to-peak splitting of 60 mV, whilst increased splitting of this event is observed for 2 and 3 . The binding of pyrene in 1 is probed to establish proof of concept of host-guest chemistry enabled by the two PBI-motifs. Fitting the binding curve obtained by ¹H NMR titration with a 1:1 complex formation model led to a binding constant of 964±55 m ⁻¹. Pyrene binding is shown to directly influence the redox-chemistry of 1 , resulting in a cathodic and anodic shift of approximately 46 mV on the first and second reduction event, respectively.     

Redox‐Active‐Ligand‐Mediated Formation of an Acyclic Trinuclear Ruthenium Complex with Bridging Nitrido Ligands

Coordination of a redox‐active pyridine aminophenol ligand to Ru^II followed by aerobic oxidation generates two diamagnetic Ru^III species [ 1 a (cis) and 1 b (trans)] with ligand‐centered radicals. The reaction of 1 a / 1 b with excess NaN₃ under inert atmosphere resulted in the formation of a rare bis(nitrido)‐bridged trinuclear ruthenium complex with two nonlinear asymmetrical Ru‐N‐Ru fragments. The spontaneous reduction of the ligand centered radical in the parent 1 a / 1 b supports the oxidation of a nitride (N^3?) to half an equivalent of N₂. The trinuclear omplex is reactive toward TEMPO‐H, tin hydrides, thiols, and dihydrogen.     

Cholesterol Hydroperoxides as Substrates for Cholesterol‐Metabolizing Cytochrome P450 Enzymes and Alternative Sources of 25‐Hydroxycholesterol and other Oxysterols

The interaction of the primary autoxidation products of cholesterol, namely 25‐ and 20ξ‐hydroperoxides, with the four principal cholesterol‐metabolizing cytochrome P450 enzymes is reported. Addition of cholesterol 25‐hydroperoxide to the enzymes CYP27A1 and CYP11A1 induced well‐defined spectral changes while generating 25‐hydroxycholesterol as the major product. The 20ξ‐hydroperoxides induced spectral shifts in CYP27A1 and CYP11A1 but glycol metabolites were detected only with CYP11A1. CYP7A1 and CYP46A1 failed to give metabolites with any of the hydroperoxides. A P450 hydroperoxide‐shunt reaction is proposed, where the hydroperoxides serve as both donor for reduced oxygen and substrate. CYP27A1 was shown to mediate the reduction of cholesterol 25‐hydroperoxide to 25‐hydroxycholesterol, a role of potential significance for cholesterol‐rich tissues with high oxidative stress. CYP27A1 may participate in the removal of harmful autoxidation products in these tissues, while providing a complementary source of 25‐hydroxycholesterol, a modulator of immune cell function and mediator of viral cell entry.     

On the propagation of elastic waves through composite media II

Summary The propagation of elastic waves (both longitudinal and transverse) through polyurethane rubbers filled with different amounts of sodium chloride particles was studied at 0.8 MHz and 5 MHz. At a constant filler concentration (∼10% by volume), the velocity of these waves appeared to be independent of filler size. On the other hand, both velocities were found to increase with filler content. From the wave velocities, the effective modulus for longitudinal waves, L, bulk modulus, K, and shear modulus, G, were calculated according to the relations for a homogeneous isotropic material. All three moduli appear to be monotonically increasing functions of filler content, c, over the whole experimentally accessible temperature range (−80°C to +80°C for L and K; −80°C to about −30°C for G) and they, moreover, reflect the glass-rubber transition of the binder. Poissons ratio, μ, was found to decrease with increasing filler content and shows a rise at about −30°C as a result of the approach of the glass-rubber transition. The attenuation of the elastic waves was also measured in the temperature ranges mentioned. For filler particles beyond a critical size both tan δ_L and tan δ_G in the hard region are independent of the filler content within the accuracy of the measurements. The critical size depends on the type of wave and on its frequency. In the rubbery region, however, tan δ_L increases with particle size (at a constant content of 10% by volume) and even shows an enhancement with the smallest particles (1–5 μ) at 0.8 MHz. Moreover, it is found that for the same filler size tan δ_L increases with filler content. In some cases an anomalous damping behaviour was found, such that in the rubbery region the attenuation rises indefinitely with temperature. For filler particles larger than the above-mentioned critical size, tan δ_G and tan δ_L increase in the hard region as well. Finally, the experimental results are compared with existing theories on the elastic properties of and wave propagation through composite media.     

Die Bestimmung und die Trennung seltenerer Metalle von anderen Metallen

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