992 research outputs found
Geometric complementarity in assembly and guest recognition of a bent heteroleptic cis-[Pd₂LA₂LB₂] coordination cage
No full textDue to the inherent difficulties in achieving a defined and exclusive formation of multicomponent assemblies against entropic predisposition, we present the rational assembly of a heteroleptic [Pd2LA2LB2]4+ coordination cage achieved through the geometric complementarity of two carefully designed ligands, LA and LB. With Pd(II) cations as rigid nodes, the pure distinctly angular components readily form homoleptic cages, a [Pd2LA4]4+ strained helical assembly and a [Pd4LB8]8+ box-like structure, both of which were characterized by X-ray analysis. Combined, however, the two ligands could be used to cleanly assemble a cis-[Pd2LA2LB2]4+ cage with a bent architecture. The same self-sorted product was also obtained by a quantitative cage-to-cage transformation upon mixing of the two homoleptic cages revealing the [Pd2LA2LB2]4+ assembly as the thermodynamic minimum. The structure of the heteroleptic cage was examined by ESI-MS, COSY, DOSY, and NOESY methods, the latter of which pointed toward a cis-conformation of ligands in the assembly. Indeed, DFT calculations revealed that the angular ligands and strict Pd(II) geometry strongly favor the cis-[Pd2LA2LB2]4+ species. The robust nature of the cis-[Pd2LA2LB2]4+ cage allowed us to probe the accessibility of its cavity, which could be utilized for shape recognition toward stereoisomeric guests. The ability to directly combine two different backbones in a controlled manner provides a powerful strategy for increasing complexity in the family of [Pd2L4] cages and opens up possibilities of introducing multiple functionalities into a single self-assembled architecture.Witold M. Bloch, Yoko Abe, Julian J. Holstein, Claudia M. Wandtke, Birger Dittrich and Guido H. Cleve
Guest-induced crystal-to-crystal expansion and contraction of a 3-D porous coordination polymer
No full textFirst published on the web 12 Jan 2012A silver(I) 3-D porous coordination polymer (PCP, 1) with a sodalite topology undergoes a remarkable reversible crystal-to-crystal expansion and contraction upon exposure to different solvents. Notably, complete desolvation of 1 also facilitates a transformation to a non-porous 2-D coordination polymer (2).Witold M. Bloch and Christopher J. Sumb
Maximized axial helicity in a PdL cage: inverse guest size-dependent compression and mesocate isomerism
No full textHelicity is an archetypal structural motif of many biological systems and provides a basis for molecular recognition in DNA. Whilst artificial supramolecular hosts are often helical, the relationship between helicity and guest encapsulation is not well understood. We report a detailed study on a significantly coiled-up PdL metallohelicate with an unusually wide azimuthal angle (∼176°). Through a combination of NMR spectroscopy, single-crystal X-ray diffraction, trapped ion mobility mass spectrometry and isothermal titration calorimetry we show that the coiled-up cage exhibits extremely tight anion binding (K of up to 10 M) by virtue of a pronounced oblate/prolate cavity expansion, whereby the Pd–Pd separation decreases for mono-anionic guests of increasing size. Electronic structure calculations point toward strong dispersion forces contributing to these host–guest interactions. In the absence of a suitable guest, the helical cage exists in equilibrium with a well-defined mesocate isomer that possesses a distinct cavity environment afforded by a doubled Pd–Pd separation distance
Biomimetic total synthesis of (+/-)-garcibracteatone
No full textThe polycyclic polyprenylated acylphloroglucinol natural product garcibracteatone has been synthesized in four steps from phloroglucinol, using a strategy based on biosynthetic speculation. The key biomimetic transformation is a cascade of 7-endo-trig and 5-exo-trig radical cyclizations followed by a terminating aromatic substitution reaction.Henry P. Pepper, Hiu C. Lam, Witold M. Bloch, and Jonathan H. Georg
Total synthesis of (+)-aureol
No full textA total synthesis of the marine sponge meroterpenoid (+)-aureol has been achieved in 12 steps (6% overall yield) from (+)-sclareolide. Key steps of the synthesis include a biosynthetically inspired sequence of 1,2-hydride and methyl shifts, and a biomimetic cycloetherification reaction.Kevin K. W. Kuan, Henry P. Pepper, Witold M. Bloch, and Jonathan H. Georg
Does functionalisation enhance CO2 uptake in interpenetrated MOFs? An examination of the IRMOF-9 series
No full textThe effect of pore functionalisation (-I, -OH, -OCH3) on a series of topologically equivalent, interpenetrated metal-organic frameworks (MOFs) was assessed by both simulation and experiment. Counter-intuitively, a decreased affinity for CO2 was observed in the functionalised materials, compared to the non-functionalised material. This result highlights the importance of considering the combined effects of network topology and chemical functionality in the design of MOFs for enhanced CO2 adsorptionRavichandar Babarao, Campbell J. Coghlan, Damien Rankine, Witold M. Bloch, Gemma K. Gransbury, Hiroshi Sato, Susumu Kitagawa, Christopher J. Sumby, Matthew R. Hill and Christian J. Doona
Assembly of a Heterometallic Cu(II)-Pd(II) Cage by Post-assembly Metal Insertion
No full textPorous structures based on multi-metallic motifs are receiving growing interest, but their general preparation still remains a challenge. Here, we report the self-assembly and structure of a CuII metal−organic cage (MOC) that is functionalized with free bis(pyrazolyl)methane sites. The homometallic Cu₄L₄ cage is isolated as a water-stable crystalline solid, and its formation is dependent on metal−ligand stoichiometry and the pre-organization of the Cu₂ paddlewheel. We show by X-ray diffraction and SEM−EDX that PdII chloride can be quantitatively inserted into the free chelating sites of the MOC to yield a [Cu₄(L(PdCl₂))₄] structure. Moreover, the solvent employed in the metalation dictates the solid-state isomerism of the heterometallic cage - a further handle to control the MOC’s structural diversity and permanent porosity.Mei Tieng Yong, Oliver M. Linder-Patton, and Witold M. Bloc
VEGF coordinates interaction of pericytes and endothelial cells during vasculogenesis and experimental angiogenesis
No full textBiological activities of vascular enclothelial growth factor (VEGF) have been studied extensively in endothelial cells (ECs), but few data are available regarding its effects on pericytes. In murine embryoid body cultures, VEGF-induced expression of desmin and a-smooth muscle actin (alpha-SMA) in CD-31(+) cells. The number of CD-31(+)/desmin(+) vascular chords increased with VEGF treatment time and peaked during a differentiation window between 6 and 9 days after plating. In vivo, VEGF-induced elongation and migration of desmin-positive pericytes and coverage of angiogenic capillaries, as revealed by analysis of Sambucus nigra lectin-stained vascular beds of the chick chorioallantoic membrane, VEGF also caused significant decrease of intercapillary spaces, an indicator for intussusceptive vascular growth. These VEGF-mediated effects point at a more intricate interaction between ECs and pericytes cells than previously demonstrated and suggest that pericytes may be derived from EC progenitors in vitro and not only stabilize capillaries but also participate in vascular remodeling in vivo. (C) 2004 Wiley-Liss, Inc
Dynamics of Network Formation Processes in the Co-Author Model
No full textThis article studies the dynamics in the formation processes of a mutual consent network in game theory setting: the Co-Author Model. In this article, a limited observation is applied and analytical results are derived. Then, 2 parameters are varied: the number of individuals in the network and the initial probability of the links in the network in its initial state. A simulation result shows a finding that is consistent with an analytical result for a state of equilibrium while it also shows different possible equilibria.Dynamics, Network, Game Theory, Model,Simulation, Equilibrium, Complexity
A covalent deprotection strategy for assembling supramolecular coordination polymers from metal-organic cages
No full textA Cu4L4 metal-organic cage (MOC) composed of amine-protected ligands forms supramolecular coordination polymers (SCPs) upon covalent post-assembly deprotection. The amorphous SCPs form by virtue of aniline-copper coordination and possess a tunable porosity based on the rate of deprotection.Matthew L. Schneider, Oliver M. Linder-Patton and Witold M. Bloc
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