Metal–organic
frameworks (MOFs) are usually synthesized
using a single type of metal ion, and MOFs containing mixtures of
different metal ions are of great interest and represent a methodology
to enhance and tune materials properties. We report the synthesis
of [Ga2(OH)2(L)] (H4L = biphenyl-3,3′,5,5′-tetracarboxylic
acid), designated as MFM-300(Ga2), (MFM = Manchester Framework
Material replacing NOTT designation), by solvothermal reaction of
Ga(NO3)3 and H4L in a mixture of
DMF, THF, and water containing HCl for 3 days. MFM-300(Ga2) crystallizes in the tetragonal space group I4122, a = b = 15.0174(7) Å
and c = 11.9111(11) Å and is isostructural with
the Al(III) analogue MFM-300(Al2) with pores decorated
with −OH groups bridging Ga(III) centers. The isostructural
Fe-doped material [Ga1.87Fe0.13(OH)2(L)], MFM-300(Ga1.87Fe0.13), can be prepared
under similar conditions to MFM-300(Ga2) via reaction of
a homogeneous mixture of Fe(NO3)3 and Ga(NO3)3 with biphenyl-3,3′,5,5′-tetracarboxylic
acid. An Fe(III)-based material [Fe3O1.5(OH)(HL)(L)0.5(H2O)3.5], MFM-310(Fe), was synthesized
with Fe(NO3)3 and the same ligand via hydrothermal
methods. [MFM-310(Fe)] crystallizes in the orthorhombic space group Pmn21 with a = 10.560(4) Å, b = 19.451(8) Å, and c = 11.773(5)
Å and incorporates μ3-oxo-centered trinuclear
iron cluster nodes connected by ligands to give a 3D nonporous framework
that has a different structure to the MFM-300 series. Thus, Fe-doping
can be used to monitor the effects of the heteroatom center within
a parent Ga(III) framework without the requirement of synthesizing
the isostructural Fe(III) analogue [Fe2(OH)2(L)], MFM-300(Fe2), which we have thus far been unable
to prepare. Fe-doping of MFM-300(Ga2) affords positive
effects on gas adsorption capacities, particularly for CO2 adsorption, whereby MFM-300(Ga1.87Fe0.13)
shows a 49% enhancement of CO2 adsorption capacity in comparison
to the homometallic parent material. We thus report herein the highest
CO2 uptake (2.86 mmol g–1 at 273 K at
1 bar) for a Ga-based MOF. The single-crystal X-ray structures of
MFM-300(Ga2)-solv, MFM-300(Ga2), MFM-300(Ga2)·2.35CO2, MFM-300(Ga1.87Fe0.13)-solv, MFM-300(Ga1.87Fe0.13), and
MFM-300(Ga1.87Fe0.13)·2.0CO2 have been determined. Most notably, in situ single-crystal
diffraction studies of gas-loaded materials have revealed that Fe-doping
has a significant impact on the molecular details for CO2 binding in the pore, with the bridging M–OH hydroxyl groups
being preferred binding sites for CO2 within these framework
materials. In situ synchrotron IR spectroscopic measurements
on CO2 binding with respect to the −OH groups in
the pore are consistent with the above structural analyses. In addition,
we found that, compared to MFM-300(Ga2), Fe-doped MFM-300(Ga1.87Fe0.13) shows improved catalytic properties
for the ring-opening reaction of styrene oxide, but similar activity
for the room-temperature acetylation of benzaldehyde by methanol.
The role of Fe-doping in these systems is discussed as a mechanism
for enhancing porosity and the s...