(Lecture) Organic Chemistry In A Porous Metal-Oxide Capsule

Topic: Organic Chemistry In A Porous Metal-Oxide Capsule

Speaker: Prof. Ira A. Weinstock (Ben-Gurion University of the Negev)

Time: Friday, July 27, 2018, 10:00

Venue: Room 324, Building No.2, Science Park, Wushan Campus


Abstract

In recent years, we have used porous icosahedral-symmetry metal-oxide capsules, [{MoVI6O21(H2O)6}12{MoV2O4(L)}30]42– {Mo132} (L = an endohedrally coordinated h2-bound carboxylate anion), to investigate organic reactions within nano-confined domains in water. Topics investigated include: 1) diffusion through flexible pores, 2) catalysis, and 3) self-assembly.

Flexible pores. Using the capsule as a soluble analogue of porous solid-state (rigid) oxides, we initially showed that branched-alkane carboxylate “guests” could enter the capsule’s interior by negotiating passage through flexible subnanometer Mo9O9 apertures whose geometrical dimensions were smaller than the entering species themselves .

Catalysis. Nano-containers in which structurally integral metal centers serve as catalytic sites for encapsulated substrates are relatively rare, and the {Mo132} capsules provide a unique opportunity for exploring this class of reactions. These include the cleavage of methyl tert-butyl ether under mild conditions in water, and for theMichaelis-Menten compliant hydrolysis of epoxides, an enzyme-like rate acceleration (kcat / kuncat) of 182,800, the largest yet reported for a cage or container at room temperature in water .

Self-assembly. Here, a {Mo132} capsule was used to reveal the energetics of individual steps in the formation of a “micelle”-like organic aggregate of n-butyrate ions (see also ref). In other work, two distinct host domains within the propionate-ligand form of the capsule are preferentially populated as a function of alkane size, with n-butane guests between the propionate ligands, and ethane molecules rotating rapidly in the 11-Å diameter (700 Å3) hydrophobic cavity at the capsule’s center. Finally, H-bonding between the hydroxyl groups of polyols does not give rise to self-assembly in bulk water. This is because aggregation in energetically prohibited by the strongly favorable enthalpies of solvation of alcohols in water. As the same time, alcohol solvation is accompanied by large (unfavorable) decreases in entropy, closely paralleling the entropic signature of hydrocarbon dissolution in water. Recognizing this, the unexpectedly favorable sequestration of L-glycerate ligands by a {Mo132} capsule is shown to be entropically driven, revealing the inherently “hydrophobic” nature of poly-alcohols, whose self-assembly from water bears an energetic signature closely analogous to that of “classical” hydrophobes.

Biography

Prof. Ira A. Weinstock is currently the Irene Evans Professor of Inorganic Chemistry in Ben-Gurion University of the Negev. He obtained his Ph.D. in 1990 from the Massachusetts Institute of Technology (MIT), where he worked on alkyne metathesis with Richard R. Schrock. After one year at the Sandia National Laboratory, Albuquerque, New Mexico, he served as Team Leader at the U.S. Department of Agriculture, Madison, Wisconsin, where he initiated the use of polyoxometalates as green catalysts for aerobic oxidations of biomass in water. In 2006, he joined the Ben-Gurion University of the Negev, where his research concerns the use of polyoxometalates in molecular and supramolecular chemistry and nanoscience and more recently as redox-active ligands for metal-oxide nanocrystals.


Announced by South China advanced Institute for Soft Matter Science and Technology


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