Catalysis in a porous molecular capsule: Activation by regulated access to sixty metal centers spanning a truncated icosahedron

Sivil Kopilevich, Adrià Gil, Miquel Garcia-Ratés, Josep Bonet-Ávalos, Carles Bo, Achim Müller, Ira A. Weinstock

Research output: Contribution to journalArticlepeer-review

69 Scopus citations

Abstract

The 30 cationic {Mo V 2O 4(acetate)} + units linking 12 negatively charged pentagonal "ligands," {(Mo VI)Mo VI 5O 21(H 2O) 6} 6- of the porous metal-oxide capsule, [{Mo VI 6O 21(H 2O) 6} 12{Mo V 2O 4(acetate)} 30] 42- provide active sites for catalytic transformations of organic "guests". This is demonstrated using a well-behaved model reaction, the fully reversible cleavage and formation of methyl tert-butyl ether (MTBE) under mild conditions in water. Five independent lines of evidence demonstrate that reactions of the MTBE guests occur in the ca. 6 × 10 3 Å 3 interior of the spherical capsule. The Mo atoms of the {Mo V 2O 4(acetate)} + linkers - spanning an ca. 3-nm truncated icosahedron - are sterically accessible to substrate, and controlled removal of their internally bound acetate ligands generates catalytically active {Mo V 2O 4(H 2O) 2} 2+ units with labile water ligands, and Lewis- and Brønsted-acid properties. The activity of these units is demonstrating by kinetic data that reveal a first-order dependence of MTBE cleavage rates on the number of acetate-free {Mo V 2O 4(H 2O) 2} 2+ linkers. DFT calculations point to a pathway involving both Mo(V) centers, and the intermediacy of isobutene in both forward and reverse reactions. A plausible catalytic cycle - satisfying microscopic reversibility - is supported by activation parameters for MTBE cleavage, deuterium and oxygen-18 labeling studies, and by reactions of deliberately added isobutene and of a water-soluble isobutene analog. More generally, pore-restricted encapsulation, ligand-regulated access to multiple structurally integral metal-centers, and options for modifying the microenvironment within this new type of nanoreactor, suggest numerous additional transformations of organic substrates by this and related molybdenum-oxide based capsules.

Original languageEnglish
Pages (from-to)13082-13088
Number of pages7
JournalJournal of the American Chemical Society
Volume134
Issue number31
DOIs
StatePublished - 8 Aug 2012

ASJC Scopus subject areas

  • Catalysis
  • Chemistry (all)
  • Biochemistry
  • Colloid and Surface Chemistry

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