The field of multimetallic catalysis is rapidly developing and some multimetallic complexes catalyze organic transformations to yield the desired products in more efficient ways owing to the combined ...action of different metals in a cooperative fashion. This Concept article describes the recent advances of cooperative catalysis playing in multimetallic systems such as homo‐multimetallic complexes with short metal‐metal distances, homo‐multimetallic complexes with long metal‐metal distances, hetero‐multimetallic complexes and metallocene‐based multimetallic complexes with special attention towards redox‐switchable catalysis. Examples are illustrated in which the use of multimetallic complexes show clear enhancement of catalytic outcomes when compared with the sum of their corresponding mononuclear counterparts. Furthermore, several examples are discussed showing the effects of electronic communication in cooperative systems.
Teamwork: In this Concept article the authors discuss selected examples of cooperativity in multimetallic systems. Homobimetallic systems with short and long metal‐metal distances are considered, and the interplay of the different metals in several heterometallic complexes is reviewed. Cooperativity in metallocene‐based systems and their utilization in redox‐switchable catalysis is also discussed.
For the first time, free base and N-methylated porphyrins have been utilized as bifunctional organocatalysts in Michael additions and it was found that distortion of the macrocycle is a vital ...prerequisite for their catalytic activity. Conformational design has been used to tailor the properties of nonplanar porphyrins with regards to availability of the N-H units for hydrogen bonding (distortion-dependent hydrogen bonding) and the basicity of the heterocyclic groups. NMR spectroscopic- and catalyst screening studies provided insight into the likely mode of catalyst action. This unprecedented use of free base and N-substituted porphyrins as organocatalysts opens a new functional role for porphyrins.
Nonconjugated hydrocarbons, like bicyclo1.1.1pentane, bicyclo2.2.2octane, triptycene, and cubane are a unique class of rigid linkers. Due to their similarity in size and shape they are useful mimics ...of classic benzene moieties in drugs, so‐called bioisosteres. Moreover, they also fulfill an important role in material sciences as linear linkers, in order to arrange various functionalities in a defined spatial manner. In this Review article, recent developments and usages of these special, rectilinear systems are discussed. Furthermore, we focus on covalently linked, nonconjugated linear arrangements and discuss the physical and chemical properties and differences of individual linkers, as well as their application in material and medicinal sciences.
Rigid, linear, versatile, and increasingly in use are attributes of nonconjugating hydrocarbons, such as bicyclo1.1.1pentane, bicyclo2.2.2octane, triptycene, and cubane. This Review article details their fundamental properties, synthetic chemistry, and use as linker systems in materials sciences and drug development (see figure).
Mucin-related carbohydrates are overexpressed on the surface of cancer cells, providing a disease-specific target for cancer immunotherapy. Here, we describe the design and construction of ...peptide-free multivalent glycosylated nanoscale constructs as potential synthetic cancer vaccines that generate significant titers of antibodies selective for aberrant mucin glycans. A polymerizable version of the Tn-antigen glycan was prepared and converted into well-defined glycopolymers by Reversible Addition–Fragmentation chain Transfer (RAFT) polymerization. The polymers were then conjugated to gold nanoparticles, yielding ‘multicopy-multivalent’ nanoscale glycoconjugates. Immunological studies indicated that these nanomaterials generated strong and long-lasting production of antibodies that are selective to the Tn-antigen glycan and cross-reactive toward mucin proteins displaying Tn. The results demonstrate proof-of-concept of a simple and modular approach toward synthetic anticancer vaccines based on multivalent glycosylated nanomaterials without the need for a typical vaccine protein component.
SUMMARY
The mantle transition zone (TZ) is expected to influence vertical mass flow between upper and lower mantle as it hosts a complex set of mineral phase transitions and an increase in viscosity ...with depth. Still, neither its seismic structure nor its dynamic effects have conclusively been constrained. The seismic discontinuities at around 410 and 660 km depth (‘410’ and ‘660’) are classically associated with phase transitions between olivine polymorphs, the pressure of which is modulated by lateral temperature variations. Resulting discontinuity topography is seismically visible and can thus potentially provide insight on temperature and phase composition at depth. Besides the olivine phase changes, the disassociation of garnet may additionally impact the 660 at higher temperatures. However, the volume of material affected by this garnet transition and its dynamic implications have not yet been quantified. This study presents hypothetical realizations of TZ seismic structure and major discontinuities based on the temperature field of a published 3-D mantle circulation model for a range of relevant mineralogies, including pyrolite and mechanical mixtures (MM). Systematic analysis of these models provides a framework for dynamically informed interpretations of seismic observations and gives insights into the potential dynamic behaviour of the TZ. Using our geodynamic-mineralogical approach we can identify which phase transitions induce specific topographic features of 410 and 660 and quantify their relative impact. Areal proportions of the garnet transition at the 660 are ∼3 and ∼1 per cent for pyrolite and MM, respectively. This proportion could be significantly higher (up to ∼39 per cent) in a hotter mantle for pyrolite, but remains low (<2 per cent) for MM. In pyrolite, both slabs and plumes are found to depress the 660—with average deflections of 14 and 6 km, respectively—due to the influence of garnet at high temperatures indicating its complex dynamic effects on mantle upwellings. Pronounced differences in model characteristics for pyrolite and MM, particularly their relative garnet proportions and associated topography features, could serve to discriminate between the two scenarios in Earth.
SUMMARY A major challenge in seismic tomography consists in quantifying and representing model resolution and uncertainty, particularly at global scales. This information is crucial for ...interpretations of tomographic images and their technical application in geodynamics. However, due to large computational costs, there have been only few attempts so far to coherently analyse the spatially varying resolving power for a complete set of model parameters. Here, we present a concept for an effective evaluation and global representation of the 3-D resolution information contained in a full set of averaging kernels. In our case, these kernels are constructed using the ‘Subtractive Optimally Localized Averages’ (SOLA) method, a variant of classic Backus-Gilbert inversion suitable for global tomography. Our assessment strategy incorporates the following steps: (1) a 3-D Gaussian function is fitted to each averaging kernel to measure resolution lengths in different directions and (2) we define a classification scheme for the quality of the averaging kernels based on their focus with respect to the estimated 3-D Gaussian, allowing us to reliably identify whether the inferred resolution lengths are robust. This strategy is not restricted to SOLA inversions, but can, for example, be applied in all cases where point-spread functions are computed in other tomographic frameworks. Together with model uncertainty estimates that are derived from error propagation in the SOLA method, our concept reveals at which locations, resolution lengths and interpretations of model values are actually meaningful. We finally illustrate how the complete information from our analysis can be used to calibrate the SOLA inversion parameters—locally tunable target resolution kernels and trade-off parameters—without the need for visual inspection of the individual resulting averaging kernels. Instead, our global representations provide a tool for designing tomographic models with specific resolution-uncertainty properties that are useful in geodynamic applications, especially for linking seismic inversions to models of mantle flow.
The synthesis of heterobimetallic AuI/RuII complexes of the general formula syn‐ and anti‐{AuCl}(L1∩L2){Ru(bpy)2}PF62 is reported. The ditopic bridging ligand L1∩L2 refers to a P,N hybrid ligand ...composed of phosphine and bipyridine substructures, which was obtained via a post‐functionalization strategy based on Diels‐Alder reaction between a phosphole and a maleimide moiety. It was found that the stereochemistry at the phosphorus atom of the resulting 7‐phosphanorbornene backbone can be controlled by executing the metal coordination and the cycloaddition reaction in a different order. All precursors, as well as the mono‐ and multimetallic complexes, were isolated and fully characterized by various spectroscopic methods such as NMR, IR, and UV‐vis spectroscopy as well as cyclic voltammetry. Photophysical measurements show efficient phosphorescence for the investigated monometallic complex anti‐(L1∩L2){Ru(bpy)2}PF62 and the bimetallic analogue syn‐{AuCl}(L1∩L2){Ru(bpy)2}PF62, thus indicating a small influence of the {AuCl} fragment on the photoluminescence properties. The heterobimetallic AuI/RuII complexes syn‐ and anti‐{AuCl}(L1∩L2){Ru(bpy)2}PF62 are both active catalysts in the P‐arylation of aryldiazonium salts promoted by visible light with H‐phosphonate affording arylphosphonates in yields of up to 91 %. Both dinuclear complexes outperform their monometallic counterparts.
Acting in concert: Novel heterobimetallic AuI/RuII complexes synthesized in a Diels‐Alder reaction are reported. The applied post‐functionalization strategy allows which isomer is obtained to be controlled. Their application as catalysts in carbon‐phosphorus cross‐coupling showed that the dinuclear complexes can outperform their monometallic counterparts
SUMMARY
Tomographic-geodynamic model comparisons are a key component in studies of the present-day state and evolution of Earth’s mantle. To account for the limited seismic resolution, ‘tomographic ...filtering’ of the geodynamically predicted mantle structures is a standard processing step in this context. The filtered model provides valuable information on how heterogeneities are smeared and modified in amplitude given the available seismic data and underlying inversion strategy. An important aspect that has so far not been taken into account are the effects of data uncertainties. We present a new method for ‘tomographic filtering’ in which it is possible to include the effects of random and systematic errors in the seismic measurements and to analyse the associated uncertainties in the tomographic model space. The ‘imaged’ model is constructed by computing the generalized-inverse projection (GIP) of synthetic data calculated in an earth model of choice. An advantage of this approach is that a reparametrization onto the tomographic grid can be avoided, depending on how the synthetic data are calculated. To demonstrate the viability of the method, we compute traveltimes in an existing mantle circulation model (MCM), add specific realizations of random seismic ‘noise’ to the synthetic data and apply the generalized inverse operator of a recent Backus–Gilbert-type global S-wave tomography. GIP models based on different noise realizations show a significant variability of the shape and amplitude of seismic anomalies. This highlights the importance of interpreting tomographic images in a prudent and cautious manner. Systematic errors, such as event mislocation or imperfect crustal corrections, can be investigated by introducing an additional term to the noise component so that the resulting noise distributions are biased. In contrast to Gaussian zero-mean noise, this leads to a bias in model space; that is, the mean of all GIP realizations also is non-zero. Knowledge of the statistical properties of model uncertainties together with tomographic resolution is crucial for obtaining meaningful estimates of Earth’s present-day thermodynamic state. A practicable treatment of error propagation and uncertainty quantification will therefore be increasingly important, especially in view of geodynamic inversions that aim at ‘retrodicting’ past mantle evolution based on tomographic images.
Cubane Cross‐Coupling and Cubane–Porphyrin Arrays Bernhard, Stefan S. R.; Locke, Gemma M.; Plunkett, Shane ...
Chemistry : a European journal,
January 24, 2018, Letnik:
24, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Herein, an improved methodology for aryl‐cubane cross‐coupling is reported. The peculiarities of the cubane core and its behavior during cross‐coupling conditions were analyzed, while the versatility ...of this adapted Baran cross‐coupling methodology was demonstrated by the synthesis of various aryl‐cubane systems, including coupling products of cubanes and porphyrins. Furthermore, arm extension of alkynyl‐cubanes by Sonogashira reactions is demonstrated, showcasing the first proof of the stability of the cubane core in the presence of palladium catalysts.
Alea Iacta Est: The compatibility of cubanes and transition metals (Ni, Pd) was explored, and a direct arylation of cubanes via a Baran‐type nickel‐catalyzed radical cross‐coupling and arm‐extended Sonogashira coupling of alkynyl‐cubanes is reported. Furthermore, the first series of cubane–porphyrins was prepared (see figure).
Modeling past states of Earth's mantle and relating them to geologic observations such as continental‐scale uplift and subsidence is an effective method for testing mantle convection models. However, ...mantle convection is chaotic and two identical mantle models initialized with slightly different temperature fields diverge exponentially in time until they become uncorrelated, thus limiting retrodictions (i.e., reconstructions of past states of Earth's mantle obtained using present information) to the recent past. We show with 3‐D spherical mantle convection models that retrodictions of mantle flow can be extended significantly if knowledge of the surface velocity field is available. Assimilating surface velocities produces in some cases negative Lyapunov times (i.e., e‐folding times), implying that even a severely perturbed initial condition may evolve toward the reference state. A history of the surface velocity field for Earth can be obtained from past plate motion reconstructions for time periods of a mantle overturn, suggesting that mantle flow can be reconstructed over comparable times.
Key Points
Long‐term retrodictions of mantle convection are hindered by chaotic drift
Reconstructions of past plate motion provide fundamental information about mantle flow in the past
Assimilation of past plate motion data greatly improves long‐term retrodictions of mantle convection
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