Kinetic Control of Interpenetration in Fe–Biphenyl-4,4′-dicarboxylate Metal–Organic Frameworks by Coordination and Oxidation Modulation

Phase control in
the self-assembly of metal–organic frameworks
(MOFs) is often a case of trial and error; judicious control over
a number of synthetic variables is required to select the desired
topology and control features such as interpenetration and defectivity.
Herein, we present a comprehensive investigation of self-assembly
in the Fe–biphenyl-4,4′-dicarboxylate system, demonstrating
that coordination modulation can reliably tune between the kinetic
product, noninterpenetrated MIL-88D­(Fe), and the thermodynamic product,
two-fold interpenetrated MIL-126­(Fe). Density functional theory simulations
reveal that correlated disorder of the terminal anions on the metal
clusters results in hydrogen bonding between adjacent nets in the
interpenetrated phase and this is the thermodynamic driving force
for its formation. Coordination modulation slows self-assembly and
therefore selects the thermodynamic product MIL-126­(Fe), while offering
fine control over defectivity, inducing mesoporosity, but electron
microscopy shows MIL-88D­(Fe) persists in many samples despite not
being evident by diffraction. Interpenetration control is also demonstrated
using the 2,2′-bipyridine-5,5′-dicarboxylate linker;
it is energetically prohibitive for it to adopt the twisted conformation
required to form the interpenetrated phase, although multiple alternative
phases are identified due to additional coordination of Fe cations
to its N donors. Finally, we introduce oxidation modulationthe
use of metal precursors in different oxidation states from that found
in the final MOFto kinetically control self-assembly. Combining
coordination and oxidation modulation allows the synthesis of pristine
MIL-126­(Fe) with BET surface areas close to the predicted maximum
for the first time, suggesting that combining the two may be a powerful
methodology for the controlled self-assembly of high-valent MOFs.