Dissecting Porosity in Molecular Crystals: Influence of Geometry, Hydrogen Bonding, and [π···π] Stacking on the Solid-State Packing of Fluorinated Aromatics

Porous molecular
crystals are an emerging class of porous materials
that is unique in being built from discrete molecules rather than
being polymeric in nature. In this study, we examined the effects
of molecular structure of the precursors on the formation of porous
solid-state structures with a series of 16 rigid aromatics. The majority
of these precursors possess pyrazole groups capable of hydrogen bonding,
as well as electron-rich aromatics and electron-poor tetrafluorobenzene
rings. These precursors were prepared using a combination of Pd- and
Cu-catalyzed cross-couplings, careful manipulations of protecting
groups on the nitrogen atoms, and solvothermal syntheses. Our study
varied the geometry and dimensions of precursors, as well as the presence
of groups capable of hydrogen bonding and [π···π]
stacking. Thirteen derivatives were crystallographically characterized,
and four of them were found to be porous with surface areas between
283 and 1821 m2 g–1. Common to these
four porous structures were (a) rigid trigonal geometry, (b) [π···π]
stacking of electron-poor tetrafluorobenzenes with electron-rich pyrazoles
or tetrazoles, and (c) hydrogen bonding between the terminal heteroaromatic
rings.