Siloxyaluminate and Siloxygallate Complexes as Models for Framework and Partially Hydrolyzed Framework Sites in Zeolites and Zeotypes

Abstract

Anionic molecularm odels for nonhydrolyzed and partially hydrolyzeda luminum and gallium framework sites on silica, M[OSi(OtBu) 3 ] 4 À and HOM[OSi(OtBu) 3 ] 3 À (whereM= Al or Ga), were synthesized from anionic chlorides Li{M[O-Si(OtBu) 3 ] 3 Cl} in salt metathesisr eactions. Sequestration of lithium cationsw ith[ 12]crown-4a fforded charge-separated ion pairs composed of monomeric anionsM [OSi(OtBu) 3 ] 4 À with outer-sphere [([12]crown-4) 2 Li] + cations, and hydroxides {HOM[OSi(OtBu) 3 ] 3 }w ith pendant [([12]crown-4)Li] + cations. These molecular modelsw ere characterized by single-crystal X-ray diffraction, vibrational spectroscopy,m ass spectrometry and NMR spectroscopy.U pon treatment of monomeric [([12]crown-4)Li]{HOM[OSi(OtBu) 3 ] 3 }c omplexes with benzyl alcohol, benzyloxide complexes were formed, modeling a possible pathway for the formation of actives ites forM eerwin-Ponndorf-Verley (MPV) transfer hydrogenations with Al/ Ga-doped silica catalysts.

A Molecular Analogue of the C−H Activation Intermediate of the Silica‐Supported Ga(III) Single‐Site Propane Dehydrogenation Catalyst: Structure and XANES Signature

“…Since molecular metal siloxide complexes have been shown to be a successful approach to model surface species in particular for silica‐supported heterogeneous catalysts, [28–30] we therefore present the synthesis of a series of tailored molecular Ga(III) complexes, containing a varying number of siloxide and alkyl ligands to mimic possible reaction intermediates, and structural characterization through single crystal X‐Ray Diffraction (XRD) as well as XAS. We report in particular the isolation of a molecular analogue of the proposed mono‐propyl Ga(III) intermediate ( Scheme 1 ,b ) and provide its XANES signature.…”

Acidity of Al–O(H)–Al Sites in Molecular Aluminosilicate Models Enables Alcohol Dehydration Reactions

“…To our knowledge, there is just one other example, very recently published by Tilley and coworkers, where comparable exchange reactions have been achieved with alumino siloxides in simple organic solvents as reaction media . Another example from 2019 shows the exchange of bridging hydroxide with xylitol at γ-alumina under autoclave conditions .…”

“…To our knowledge, there is just one other example, very recently published by Tilley and coworkers, where comparable exchange reactions have been achieved with alumino siloxides in simple organic solvents as reaction media. 10 Another example from 2019 shows the exchange of bridging hydroxide with xylitol at γ-alumina under autoclave conditions. 11 Apart from this, closest to our observations are 16 OH/ 17 OHexchange studies between water and an Al 13 cluster, O, followed by stirring of the product in a THF solution containing excessive H 2 18 O, led to the formation of 1 labeled with, on average, 2.55 18 O atom equivalents per molecule according to the ESI-MS signal (see Supporting Information, Figure S4) and the subsequent treatment with 18 O label, respectively (see Supporting Information, Figures S5 and S7); this indicates that the O atom of the released water comes from CH 3 16 OH.…”