posted on 2016-03-08, 00:00authored byChristopher
M. Kane, Arash Banisafar, Timothy P. Dougherty, Leonard J. Barbour, K. Travis Holman
The stable, guest-free
crystal form of the simple molecular cavitand, Me,H,SiMe2, is shown to
be intrinsically porous, possessing discrete, zero-dimensional
(0D) pores/microcavities of about 28 Å3. The incollapsible
0D pores of Me,H,SiMe2 have been exploited for the enclathration and room
temperature (and higher) confinement of a wide range of small gases.
Over 20 isostructural x(gas/guest)@Me,H,SiMe2 (x ≤ 1) clathrates
(guest = H2O, N2, Ar, CH4, Kr, Xe,
C2H4, C2H6, CH3F, CO2, H2S, CH3Cl, CH3OCH3, CH3Br, CH3SH, CH3CH2Cl, CH2Cl2, CH3I,
CH3OH, BrCH2Cl, CH3CH2OH, CH3CN, CH3NO2, I2), and a propyne clathrate (CH3CCH@Me,H,SiMe2·2CHCl3), have been prepared
and characterized, and their single crystal structures determined.
Gas enclathration is found to be highly selective for gases that can
be accommodated by the predefined, though slightly flexible 0D pore.
The structure determinations provide valuable insight, at subangstrom
resolution, into the factors that govern inclusion selectivity, gas
accommodation, and the kinetic stability of the clathrates, which
has been probed by thermal gravimetric analysis. The activation (emptying)
of several clathrates (guest = H2O, N2, CO2, Kr, CH3F) is shown to occur in a single-crystal-to-single-crystal
(SC → SC) fashion, often requiring elevated temperatures. Akin
to open pore materials, water vapor and CO2 gas are shown
to be taken up by single crystals of empty Me,H,SiMe2 at room temperature,
but sorption rates are slow, occurring over weeks to months. Thus, Me,H,SiMe2 exhibits very low, but measurable, gas permeability, despite there
being no obvious dynamic mechanism to facilitate gas uptake. The unusually
slow exchange kinetics has allowed the rates of gas (water vapor and
CO2) sorption to be quantified by single crystal X-ray
diffraction. The data are well fit to a simple three-dimensional diffusion
model.