Buffered Coordination Modulation as a Means of Controlling Crystal Morphology and Molecular Diffusion in an Anisotropic Metal–Organic Framework

Significant
advances have been made in the synthesis of chemically
selective environments within metal–organic frameworks, yet
materials development and industrial implementation have been hindered
by the inability to predictively control crystallite size and shape.
One common strategy to control crystal growth is the inclusion of
coordination modulators, which are molecular species designed to compete
with the linker for metal coordination during synthesis. However,
these modulators can simultaneously alter the pH of the reaction solution,
an effect that can also significantly influence crystal morphology.
Herein, noncoordinating buffers are used to independently control
reaction pH during metal–organic framework synthesis, enabling
direct interrogation of the role of the coordinating species on crystal
growth. We demonstrate the efficacy of this strategy in the synthesis
of low-dispersity single-crystals of the framework Co2(dobdc)
(dobdc4–= 2,5-dioxido-1,4-benzenedicarboxylate)
in a pH 7-buffered solution using cobalt­(II) acetate as the metal
source. Density functional theory calculations reveal that acetate
competitively binds to Co during crystallization, and by using a series
of cobalt­(II) salts with carboxylate anions of varying coordination
strength, it is possible to control crystal growth along the c-direction. Finally, we use zero length column chromatography
to show that crystal morphology has a direct impact on guest diffusional
path length for the industrially important hydrocarbon m-xylene. Together, these results provide molecular-level insight
into the use of modulators in governing crystallite morphology and
a powerful strategy for the control of molecular diffusion rates within
metal–organic frameworks.