Rational Design of Pore Size and Functionality in a Series of Isoreticular Zwitterionic Metal–Organic Frameworks

The isoreticular
expansion and functionalization of charged-polarized
porosity has been systematically explored by the rational design of
11 isostructural zwitterionic metal–organic frameworks (ZW-MOFs).
This extended series of general structural composition {[M3F­(L1)3(L2)1.5]·guests}n was prepared by employing the solvothermal reaction of
Co and Ni tetrafluoroborates with a binary ligand system composed
of zwitterionic pyridinium derivatives and traditional functionalized
ditopic carboxylate auxiliary ligands (HL1·Cl = 1-(4-carboxyphenyl)-4,4′-bipyridinium
chloride, Hcpb·Cl; or 1-(4-carboxyphenyl-3-hydroxyphenyl)-4,4′-bipyridinium
chloride, Hchpb·Cl; and H2L2 = benzene-1,4-dicarboxylic
acid, H2bdc; 2-aminobenzene-1,4-dicarboxylic acid, H2abdc; 2,5-dihydroxy-1,4-benzenedicarboxylic acid, H2dhbdc; biphenyl-4,4′-dicarboxylic acid, H2bpdc;
or stilbene-4,4′-dicarboxylic acid, H2sdc). Single-crystal
structure analyses revealed cubic crystal symmetry (I-43m, a = 31–36 Å)
with a 3D pore system of significant void space (73–81%). The
pore system features three types of pores being systematically tunable
in size ranging from 17.4 to 18.8 Å (pore I), 8.2 to 12.8 Å
(pore II), and 4.8 to 10.4 Å (pore III) by the choice of auxiliary
ligands. All members of this series have noninterpenetrating structures
and exhibit robust architectures, as evidenced by their permanent
porosity and high thermal stability (up to 300 °C). The structural
integrity and specific surface areas could be systematically optimized
using supercritical CO2 exchange methods for framework
activation resulting in BET surface areas ranging from 1250 to 2250
m2/g. Most interestingly, as a structural landmark, we
found the pore surfaces lined with charge gradients employed by the
pyridinium ligands. This key feature results in significant adsorption
of carbon dioxide and methane which is attributed to polarization
effects. With this contribution we pioneer the reticulation of pyridinium
building blocks into extended zwitterionic networks in which specific
properties can be targeted.