Enhancement of CO2 Adsorption and Catalytic Properties by Fe-Doping of [Ga2(OH)2(L)] (H4L = Biphenyl-3,3′,5,5′-tetracarboxylic Acid), MFM-300(Ga2)

Article abstract of DOI:10.1021/acs.inorgchem.5b02108

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) ? and c = 11.9111(11) ? 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 [Ga_1.87Fe_0.13(OH)₂(L)], MFM-300(Ga_1.87Fe_0.13), 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 [Fe₃O_1.5(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) ?, b = 19.451(8) ?, and c = 11.773(5) ? 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(Ga_1.87Fe_0.13) 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(Ga_1.87Fe_0.13)-solv, MFM-300(Ga_1.87Fe_0.13), and MFM-300(Ga_1.87Fe_0.13)·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(Ga_1.87Fe_0.13) 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 structural integrity of the parent material.

Full text of DOI:10.1021/acs.inorgchem.5b02108

Article
pubs.acs.org/IC
Enhancement of CO Adsorption and Catalytic Properties by
2
Fe-Doping of [Ga (OH) (L)] (H L = Biphenyl-3,3′,5,5′-tetracarboxylic
2
2
4
Acid), MFM-300(Ga )
2
†
†
§
§
Cristina P. Krap, Ruth Newby, Amarajothi Dhakshinamoorthy, Hermenegildo García,
∥
,⊥
†,#
†
†
†
Izabela Cebula, Timothy L. Easun, Mathew Savage, Jennifer E. Eyley, Shan Gao,
†
†
∇
§
∇
∇
Alexander J. Blake, William Lewis, Peter H. Beton, Mark R. Warren, David R. Allan,
Mark D. Frogley, Chiu C. Tang, Gianfelice Cinque, Sihai Yang,* and Martin Schro
̈
der*,‡
∇
∇
,‡
†
‡
§
∥
School of Chemistry, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K.
School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.
Instituto de Technologia Quimica (UPV-CSIC), Universidad Politecnica de Valencia, Avenida de los Naranjos s/n, Valencia, 46022, Spain
School of Physics, University of Nottingham, University Park, Nottingham, NG7 2RD, U.K.
⊥
Department of Chemical and Process Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street,
Glasgow G1 1XJ, U.K.
#
School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, U.K.
∇
Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, U.K.
S Supporting Information
*
ABSTRACT: 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 [Ga (OH) (L)] (H L =
2
2
4
biphenyl-3,3′,5,5′-tetracarboxylic acid), designated as MFM-
00(Ga ), (MFM = Manchester Framework Material replacing
3
2
NOTT designation), by solvothermal reaction of Ga(NO ) and
3
3
H L in a mixture of DMF, THF, and water containing HCl for 3
4
days. MFM-300(Ga ) crystallizes in the tetragonal space group
2
I4 22, a = b = 15.0174(7) Å and c = 11.9111(11) Å and is isostructural with the Al(III) analogue MFM-300(Al ) with pores decorated
1
2
with −OH groups bridging Ga(III) centers. The isostructural Fe-doped material [Ga1.87Fe0.13(OH) (L)], MFM-300(Ga1.87Fe0.13), can
2
be prepared under similar conditions to MFM-300(Ga ) via reaction of a homogeneous mixture of Fe(NO ) and Ga(NO ) with
2
3 3
3 3
biphenyl-3,3′,5,5′-tetracarboxylic acid. An Fe(III)-based material [Fe O (OH)(HL)(L) (H O) ], MFM-310(Fe), was synthesized
3
1.5
0.5
2
3.5
with Fe(NO ) and the same ligand via hydrothermal methods. [MFM-310(Fe)] crystallizes in the orthorhombic space group Pmn2
3
3
1
with a = 10.560(4) Å, b = 19.451(8) Å, and c = 11.773(5) Å and incorporates μ -oxo-centered trinuclear iron cluster nodes connected
3
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 [Fe (OH) (L)], MFM-300(Fe ), which we have thus far been unable to prepare. Fe-doping of MFM-
2
2
2
3
00(Ga ) affords positive effects on gas adsorption capacities, particularly for CO adsorption, whereby MFM-300(Ga1.87Fe0.13) shows
2
2
a 49% enhancement of CO adsorption capacity in comparison to the homometallic parent material. We thus report herein the highest
2
−
1
CO uptake (2.86 mmol g at 273 K at 1 bar) for a Ga-based MOF. The single-crystal X-ray structures of MFM-300(Ga )-solv,
2
2
MFM-300(Ga ), MFM-300(Ga )·2.35CO , MFM-300(Ga1.87Fe0.13)-solv, MFM-300(Ga1.87Fe0.13), and MFM-300(Ga1.87Fe0.13)·
2
2
2
2
.0CO have been determined. Most notably, in situ single-crystal diffraction studies of gas-loaded materials have revealed that
2
Fe-doping has a significant impact on the molecular details for CO binding in the pore, with the bridging M−OH hydroxyl groups
being preferred binding sites for CO within these framework materials. In situ synchrotron IR spectroscopic measurements on CO
2
2
2
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(Ga ), Fe-doped MFM-300(Ga Fe ) shows improved catalytic properties for the ring-opening reaction of
2
1.87 0.13
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 structural integrity of the parent material.
Received: September 16, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b02108
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
INTRODUCTION
Porous metal−organic frameworks (MOFs) have attracted a
great deal of interest because of their potential applications in gas
confirmed a significant enhancement (49%) of CO adsorption
2
■
upon controlled Fe-doping. Single-crystal X-ray diffraction studies
on both solvated and desolvated materials confirm the structural
integrity of these materials, and notably, in situ single-crystal
1
−3
adsorption and separation, catalysis, and drug delivery.
The
diffraction studies under a CO atmosphere have been used to
assembly of MOF materials from various metal ions and organic
linkers, usually via solvothermal reactions, allows the fine-tuning
of their crystal structures and the incorporation of designed
functional groups for specific applications. Indeed, the proper-
ties, size, and functionality of the cavity of these porous materials
can be optimized by using different metal centers or organic
2
determine the preferred binding sites of CO within these MOFs
2
and to explain the changes in the molecular guest binding
mechanism upon Fe-doping. In situ synchrotron IR spectroscopy
has also been used to study the details of CO binding with respect
2
to the −OH groups in the pore. Furthermore, catalytic experi-
4
,5
ments have also confirmed that MFM-300(Ga Fe ) shows
ligands. Amine (−NH ) groups can bind selectively to CO
1.87 0.13
2
2
higher conversion rates than MFM-300(Ga ) for the ring-opening
due to the formation of strong electrostatic interactions between
2
reaction of styrene oxide.
the electronegative N center of the −NH group and the electro-
positive C center of the CO molecule. For this reason, a
number of amine-functionalized MOFs have been designed and
synthesized to capture CO from flue gases. However, the
effect of the different metal ions in MOFs on CO adsorption
properties has been rarely studied. While frameworks based on
divalent transition metal centers have been extensively explored
2
6
2
MATERIALS AND METHODS
■
(
7
,8
Chemicals. Gallium nitrate (Ga(NO ) , 99%, Aldrich), iron nitrate
3 3
2
Fe(NO ) ·9H O, 99%, Sigma), hydrochloric acid (HCl, >99%, Fisher
3
3
2
2
Scientific), nitric acid (HNO , >99%, Fisher Scientific), piperazine
3
(
(
99%, Sigma-Aldrich), DMF (>99%, Fisher Scientific), and THF
99.9%, Fisher Scientific) were used as purchased. Biphenyl-3,3′,5,5′-
9
and are now well-developed, MOFs with trivalent metal centers
tetracarboxylic acid (H L) (Scheme 1) was synthesized by a literature
method.
4
10
2
5
have generally attracted less attention. Furthermore, few
examples of gallium-carboxylate MOFs are described in the
1
1
literature. MOFs are usually constructed from a single type of
metal cation and organic linker, but there are increasing examples
of MOFs which contain two different types of ligand linkers or
metal cations with a homogeneous distribution, as found in solid
Scheme 1. ViewofBiphenyl-3,3′,5,5′-tetracarboxylic Acid (H L)
4
1
2−23
17
18
solutions.
Kitagawa et al. and Cheetham et al. have
reported the syntheses and properties of some binary and ternary
MOF solid solutions which contain a mixture of similar organic
linkers. In addition, Serre et al. have reported mixed-metal
MIL-53(FeCr), which exhibits a new type of framework
breathing behavior that is distinct from that in either pure
Synthesis of MFM-300(Ga )-solv. Biphenyl-3,3′,5,5′-tetracarbox-
2
ylic acid (H L, 21.8 mg, 0.067 mmol) and Ga(NO ) (38.0 mg,
4
3 3
0
.148 mmol) in a 2:5:1 mixture of DMF, THF, and water (8 mL) slightly
acidified with 37% HCl (2 drops) were reacted in a pressure tube at
348 K for 3 days. The white crystalline product was separated by
filtration as single crystals (∼0.05 mm cubes), washed several times
with warm DMF, and dried in air. Yield: 85%. Elemental analysis
(% calc/found) for [Ga (OH) (L)]·5H O·1DMF (C H Ga NO ):
19
MIL-53(Fe) or MIL-53(Cr). Hill et al. reported that CO uptake
2
2
0
could be increased through metal ion exchange in UiO-66. Thus,
postsynthetic modification of UiO-66 by stirring UiO-66 in a
solution of TiCl (THF) for 1, 5, or 15 days gave UiO-66 (Ti ),
4
2
32
2
2
2
19 25
2
16
UiO-66 (Ti ), and UiO-66 (Ti ), respectively. The BET surface
area of these materials was found to increase as the proportion
of Ti within the framework increased, and an increase in Ti
C, 34.43/34.21; H, 3.80/3.14; N, 2.11/1.70. Rapid loss of free solvent
molecules and hydration during the sample transfer are responsible for
the discrepancy and the variability observed in the elemental analytical
data for this material. Selected IR: ν/cm : 3648(m), 3236(w),
1667(vs), 1541(vs), 1458(s), 1425(s), 1376(m), 1318(m), 1095(m),
44
56
−1
content from 0−56% gave an increase in CO uptake by up to 81%
2
20
compared to UiO-66 (Zr ). Similar replacement of heavier
100
9
15(w), 775(m), 664(m).
Synthesis of MFM-310(Fe). Fe(NO ) ·9H O (290 mg, 0.72 mmol),
metal ions with lighter ones has also been shown to increase
2
1,22
23
3
3
2
surface area.
Long et al. have recently described
a
H L (60 mg, 0.18 mmol), and piperazine (0.10 g, 1.26 mmol) were mixed
4
4−
magnesium-diluted [Fe Mg (dobdc)] [dobdc = 2,5-dioxido-
and dispersed in water (10.0 mL) containing nitric acid (2.0 mL, 2.7M) in
a Teflon autoclave and heated at 483 K for 3 days. The yellow crystalline
product was separated by filtration, washed several times with water, and
dried in air. Yield: 10%. These hydrothermal conditions afforded two
phases which were light yellow and dark purple in color. Single-crystal
X-ray diffraction identified the light yellow phase as MFM-310(Fe), while
the other phase (Fe O ) was isolated only as a fine powder. Since we have
0.1
0.9
1
,4-benzenedicarboxylate] analogue of [Fe (dobdc)], also known
2
as Fe-MOF-74. [Fe Mg (dobdc)] was synthesized by mixing
0
.1
1.9
MgCl , FeCl , and H (dobdc) in a mixture of DMF (N,N′-
2
2
4
dimethylformamide) and MeOH in a Schlenk flask at 120 °C.
Fe Mg (dobdc)] converts ethane into EtOH and acetaldehyde
[
0.1
1.9
3
4
using N O as the terminal oxidant, in contrast to the complex
2
found no method as yet for their separation as pure phases, no further
characterization has been undertaken.
Synthesis of MFM-300(Ga1.87Fe0.13)-solv. A mixture of
23
mixture of products formed by its analogue Fe-MOF-74.
We report herein the synthesis and structural characterization
of a Ga(III)-based metal−organic material [Ga (OH) (L)]·
Fe(NO ) ·9H O (3.16 mg, 0.00782 mmol), Ga(NO ) (38 mg,
2
2
3
3
2
3 3
1
.6DMF·4H O [denoted as MFM-300(Ga )-solv; MFM =
0.1486 mmol), and H L (22.0 mg, 0.067 mmol) in a 2:5:1 mixture of
2
2
4
Manchester Framework Material replacing NOTT designation],
DMF, THF, and water (8 mL) slightly acidified with 37% HCl (5 drops)
was reacted at 348 K for 3 days. The microcrystalline pale orange
precipitate was separated by filtration, washed several times with warm
DMF, and dried in air. Yield: 84%. Elemental analysis (% calc/found)
which is found to be isostructural with the previously reported
2
4
MFM-300(Al2), and, to the best of our knowledge, shows the
highest CO uptake (20.8 wt % at 195 K, 1 bar) for Ga-based
2
1
1
for [Ga1.87Fe0.13(OH)
3
3
2
(L)]·1.5H
6.61/36.80; H, 2.11/2.11; N, 0/0. Selected IR: ν/cm : 3648(m),
232(w), 1662(vs), 1539(vs), 1455(s), 1418(s), 1374(m), 1318(m),
2
O (C16Fe0.13Ga1.87
H
11
−1
O
11.5): C,
MOFs. In order to investigate the effect of metal doping on CO
2
adsorption and catalytic properties, we synthesized the isostruc-
tural mixed-metal solid solution material, [Ga1.87Fe0.13(OH) (L)]
2
1096(m), 913(w), 803(w), 775(m), 659(m). The presence and quantity
of Fe within the single crystals was determined by SEM and EDX, which
also confirmed the even distribution of Fe throughout the material
[
denoted as MFM-300(Ga1.87Fe0.13)], by including Fe(NO3)3
in the synthesis of MFM-300(Ga ). Gas adsorption studies have
2
B
DOI: 10.1021/acs.inorgchem.5b02108
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
8
(
Figure S16), and ICP analysis confirmed the overall metal content of
by subsequent difference Fourier analysis using TOPAS. The final
structure refinement was carried out using the Rietveld method with
isotropic displacement parameters for all atoms. The highly disordered
DMF molecules in the pores could not be located and modeled and,
therefore, were treated as discrete water molecules in the refinement.
A total of 40 disordered water molecules per unit cell were found
within the channels and included in the final structure refinement for
the bulk product. The Ga:Fe stoichiometry in the final product was
consistent over a range of syntheses using the above conditions.
Growth of MOF Crystals on SiO Surfaces. Silicon (100) wafers
2
with a 300 nm SiO thin oxide layer were cut mechanically and then
washed in methanol, acetone, and isopropanol for 10 min each. The
wafers were dried under a stream of N and placed into the above reac-
2
2
tion solutions, which were then heated in a pressure tube. Single crystals
of MOF products grew on these surfaces over 3 days under the same
solvothermal conditions. We found this methodology to be highly
effective in growing good quality single crystals for subsequent structural
characterization.
SEM, EDX, and ICP Measurements. MOF crystals grown on SiO2
substrates were coated with carbon using an Agar turbo carbon coater to
improve conductivity. Scanning electron microscopic (SEM) analysis
was performed on a FEI Quanta 3D 200 dual beam focused ion beam
scanning electron microscope (FIB-SEM). The images were acquired
using secondary electron imaging at an accelerating voltage of 5 kV.
An Oxford Instruments integrated INCA Energy 250 Microanalysis
System was used for EDX elemental spectra and mapping, and ICP-OES
measurements were made on a PerkinElmer Optima 2000.
MFM-300(Ga2−xFe )-solv (x = 0, 0.13). The final stage of the Rietveld
refinement involved soft restraints to the C−C bond lengths within the
benzene rings.
x
Structure Determination and Refinement for MFM-300(Ga )-
2
solv, MFM-300(Ga ), MFM-300(Ga )·2.35CO , MFM-300-
2 2 2
(Ga1.87Fe0.13)-solv, MFM-300(Ga1.87Fe0.13), MFM-300-
(
3
Ga1.87Fe )·2.0CO . Single-crystal diffraction data of MFM-
0.13 2
00(Ga ) and MFM-300(Ga1.87Fe0.13) were collected on Beamline I19
2
at Diamond Light Source within a gas cell system. A single crystal of
MFM-300(Ga )-solv or MFM-300(Ga1.87Fe0.13)-solv was loaded into
2
the gas cell, which was then flushed with dry N . The diffraction data
2
were collected at room temperature. The gas cell was then evacuated at
−
4
1
0
mbar and heated at 393 K using an Oxford Cryosystems open-flow
nitrogen cryostat for 4 h to generate the desolvated material MFM-
00(Ga ) or MFM-300(Ga1.87Fe0.13). Diffraction data were collected
3
Growth of Single Crystals of MFM-300(Ga1.87Fe0.13)-solv.
2
under vacuum after cooling to room temperature. The gas cell was
then cooled to 195 K and exposed to dry CO for 2 h. The diffraction
Single crystals of MFM-300(Ga )-solv were obtained under solvother-
2
mal conditions as detailed above, and we also attempted to grow crystals
2
data were then collected at 195 K under 1.0 bar pressure of CO . The
of both MFM-300(Ga )-solv and MFM-300(Ga1.87Fe0.13)-solv on a
2
2
structures were solved by direct methods and developed by difference
SiO surface. Immersion of the prepared SiO substrate into a reaction
2
2
29
Fourier techniques using the SHELXTL software package. A total of
.4 DMF and 16 water molecules, and 10.8 water molecules per unit cell
were found within the pores, and these were included in the refinement
of MFM-300(Ga )·1.6DMF·4H O and MFM-300(Ga1.87Fe0.13)·
solution of MFM-300(Ga1.87Fe0.13) leads to the formation of uniform
6
cubic single crystals of 30−50 μm in length. The growth of MOF
26
crystallites on surfaces has been observed previously but has been
used rarely for the growth of single crystals. We were unable to grow
single crystals of MFM-300(Ga1.87Fe0.13)-solv in the absence of the SiO2
substrate (Figure 1).
2
2
2
.7H O, respectively. Importantly, no residual electron density was
2
apparent within the pores of desolvated MFM-300(Ga ) and desolvated
2
MFM-300(Ga1.87Fe0.13), confirming the complete removal of guest
solvent molecules. A total of 9.4 and 8.0 CO molecules per unit cell
2
were found to occupy two independent sites within the pore and were
included in the refinement of MFM-300(Ga )·2.35CO and MFM-
2
2
3
00(Ga1.87Fe013.)·2.0CO , respectively. The hydrogen atoms on the
2
ligands and DMF molecules were placed geometrically and refined using
a riding model. The hydrogen atoms of free water molecules could not
be located but are included in the molecular formula and in values
derived from it. CCDC 951538−951543 and 1009553 contain the
supplementary crystallographic data for this paper.
In Situ Infrared Spectroscopy. IR spectroscopic measurements
were carried out on the B22 MIRIAM beamline at the Diamond Light
Source using a Hyperion 3000 infrared microscope with a 15× objective
and a liquid N cooled MCT detector. The sample was mounted in a
2
Linkam FTIR600 variable-temperature gas-tight stage fitted with ZnSe
windows, and ZnSe-based linear IR polarizers were used to obtain
polarized IR spectra. Spectra were collected (256 scans) in the range
−
1
−1
6
50−4000 cm with 4 cm resolution and an infrared spot size at the
sample of approximately 25 by 25 μm. The sample was activated in the
stage prior to measurements by heating at 150 °C under a slow flow
of N for 10 h before being transferred to the microscope and purged
2
under He flow at room temperature. CO and He were delivered to the
2
Linkam stage using separate mass-flow controllers, the partial pressure
Figure 1. SEM images for (a, b) [MFM-300(Ga1.87Fe0.13)]-solv and
of CO being controlled by varying the volumetric flow of the two gases.
2
(
c, d) [MFM-300(Ga )]-solv grown on SiO surfaces.
Both gases were predried through zeolite filters placed between the
cylinders and the mass-flow controllers before being flowed through the
Linkam stage.
2
2
Structure Determination and Refinement for MFM-300-
Ga2−xFe )-solv (x = 0, 0.13). High-resolution powder diffraction
(
Gas Sorption. Sorption isotherms for CO
, CH , N2 and H were
2 4 2
x
data for the as-synthesized MOFs were collected on Beamline I11 at
Diamond Light Source using multi-analyzing-crystal (MAC) detec-
tors. The powder pattern was first indexed on a body-centered
recorded at 77 K (liquid N ), 87 K (liquid Ar), or 273−303 K
2
(temperature-programmed water bath from Hiden Company) on an
IGA-003 system under ultrahigh vacuum from a diaphragm and turbo
pumping system. All gases used were ultrapure research grade
(99.999%) purchased from BOC or AIRLIQUIDE. The density of the
27
tetragonal lattice, and the independent unit cell parameters were
28
refined using TOPAS. The body centring and the reflection condition
0l: l = 4n indicates that the space group is one of the enantiomeric pair
I4 22 and I4 22. In the absence of any component capable of directing
−3
0
desolvated samples used in buoyancy corrections (1.80 g cm ) was
estimated from the crystallographic density of the desolvated sample by
1
3
3
0
chirality, the product is expected to be a 1:1 racemic mixture. The
PLATON/SOLV. In a typical gas adsorption experiment, ca. 50 mg of
structure solution was initially established in space group I4 22 starting
as-synthesized material was loaded into the IGA and degassed at 100 °C
1
−10
from the structural model of MFM-300(Ga ) and further developed
and high vacuum (10 bar) for 1 day to give fully desolvated samples.
2
C
DOI: 10.1021/acs.inorgchem.5b02108
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. Views of the crystal structure of MFM-300(Ga ). (a) View of the [Ga(OH) O ] chain; (b) projection of the structure along the a-axis;
2
2
4 ∞
4−
(
c) view of the channel running through the framework parallel to the c-axis. (Ga: green; O: red; H: white; ligand L is represented by gray lines).
Catalytic Reactions. All reagents and starting materials were
framework structure constructed from one-dimensional helical
obtained commercially from Aldrich and used without any further
purification. The percentage conversion, purity, and relative yields of the
final products were determined by using a Hewlett-Packard 5890 series
II gas chromatograph with an FID detector and high-purity helium
as the carrier gas. The products were identified by GC−MS by using
a Hewlett-Packard 6890 series spectrometer. The regioselectivity of
the ring-opening was determined by mass spectrometry, which shows
different fragmentation patterns depending on the relative position of
the nucleophilic group. In the case of 2-methoxy-2-phenylethanol, the
structure was confirmed by comparison with an authentic sample.
In a typical experiment, the activated MOF catalyst (20 mg) was
suspended in MeOH (5 mL), followed by the addition of 1a or
benzaldehyde. The reaction was performed at the required temperature,
the evolution of products was monitored by GC, and the products were
analyzed by GC−MS. After the required time, the catalyst was removed
by filtration, washed with MeOH, and dried at 80 °C. This dried catalyst
was activated again and used for the second run with fresh 1a and
methanol.
[
(
Ga(OH) O ] chains bridged by tetracarboxylate ligands
Figure 2). The Ga(III) ion is octahedrally coordinated by six
2 4 ∞
O-donors: four from carboxylate groups [Ga−O = 2.004(3) and
1
μ -OH [Ga−O = 1.928(3) Å]. This coordination also affords a
square-shaped, one-dimensional channel running through the
framework parallel to the c-axis. In the analogous complex
MFM-300(Al )-solv, the four bonds from the carboxylates are
slightly shorter [Al−O = 1.935(1) and 1.929(2) Å] than those in
MFM-300(Ga )-solv, but the two bonds from the μ -OH groups
are very similar [Al−O = 1.930(1) Å]. The approximate
diameter of the channel in MFM-300(Ga )-solv, taking into
account the van der Waals radii of surface atoms, is 6.7 × 6.7 Å.
The channel contains uncoordinated water (2.0 per Ga) and
DMF (0.8 per Ga) molecules, giving a formula of [Ga (OH) -
.999(4) Å] and two from mutually cis bridging hydroxyl groups
2
2
2
2
24
2
2
2
(
C H O )]·(DMF) ·(H O) for the as-synthesized material.
16 6 8 1.6 2 4
The DMF molecule interacts with the hydroxyl group to form a
moderate hydrogen bond [O···O = 2.694(6) Å; ∠O−H···O = 180°],
and two disordered water molecules form weak intermolecular
hydrogen bonds [O···O = 2.94(3) Å]. The hydrogen atoms on
the water molecules were not located. The total pore voids
occupied by the free solvents were estimated by PLATON/
RESULTS AND DISCUSSION
■
Synthesis and Structural Description. Biphenyl-3,3′5,5′-
tetracarboxylic acid (Scheme 1), H L, was reacted with
4
Ga(NO ) in a 2:5:1 mixture of DMF, THF, and water (8 mL)
3
3
slightly acidified with HCl in a pressure tube at 348 K for 3 days
30
SOLV to be 50%.
The solvated framework complex MFM-300(Ga1.87Fe0.13)-solv
to give MFM-300(Ga )-solv. MFM-300(Ga1.87Fe0.13)-solv was
2
prepared in a similar manner but using an intimate mixture of
was prepared via solvothermal reaction of H L and Ga(NO ) /
Fe(NO ) ·9H O and Ga(NO ) as the metal source. The struc-
4
3 3
3
3
2
3 3
Fe(NO ) ·9H O in DMF/THF/water containing HCl. Whereas
tures of MFM-300(Ga )-solv and MFM-300(Ga1.87Fe0.13)-solv
3 3
2
2
were solved by single-crystal and high-resolution synchrotron
powder diffraction data, respectively. Indexation of powder
diffraction patterns (PXRD) of these complexes confirms that
these conditions yielded cube-shaped single crystals of MFM-
300(Ga)-solv with an average size of 20−50 μm, the solvothermal
synthesis of MFM-300(Ga1.87Fe0.13)-solv yielded microcrystalline
solids with an approximate particle size of 5 μm rather than
crystals large enough for single-crystal structure determination.
Rietveld refinement confirms the phase purity of the bulk material
24
they are isomorphous to MFM-300(Al )-solv.
2
MFM-300(Ga )-solv crystallizes in the chiral tetragonal
2
space group I4 22 and shows an open three-dimensional
1
D
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Inorganic Chemistry
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3
3
Figure 3. Views of the crystal structure of MFM-310(Fe): (a) Comparison of the common trinuclear Fe O(O CR) cluster (left) and the cluster
3
2
6
present in MFM-310(Fe) (right); (b) view of the structure along the a-axis; (c) view of the structure along the c-axis. (FeO octahedron: green;
6
Fe: yellow; O: red; C: gray/black).
and confirms that the Fe-doped material MFM-300(Ga1.87Fe0.13
)
PXRD data for MFM-300(Ga1.87Fe0.13)-solv confirm the absence
of the MFM-310(Fe) phase, suggesting that a solid solution
has been formed, rather than a mechanical binary mixture of
MFM-300(Ga )-solv, MFM-300(Fe )-solv and MFM-310(Fe).
Furthermore, no splitting or broadening of PXRD peaks
attributable to the presence of a binary mixture of phases was
observed, even when using a synchrotron radiation source
(Δ2θ < 0.005°), supporting the formation of a fully mixed doped
material (Figure S1).
The synthesis of MFM-300(Ga1.87Fe0.13)-solv emanated
from a preparation using metal salt starting materials in an
Fe:Ga molar ratio of 5:95. Characterization of the Fe content
by ICP for successive isolated samples from the above synthetic
procedure consistently gave metal content ratios of MFM-
300(Ga1.87Fe0.13)-solv (x = 0.13) (equivalent to 6.5% doping).
SEM/EDX confirmed the even spread of Fe throughout the
sample.
has the same framework structure as MFM-300(Ga ), with very
2
small differences in unit cell parameters (Table S1). This is
consistent with the similar ionic radii of Ga(III) and Fe(III) and the
low level of doping. A similar modulation in unit cell parameters has
2
2
2
+
been observed in the Fe -exchanged material Mn [(Mn Cl) -
3
4
3
3
−
(
BTT) ] (BTT = 1,3,5-benzenetristetrazolate) following
8 2
31
postsynthetic cation methathesis. Subsequently, single crystals
of MFM-300(Ga1.87Fe0.13)-solv were successfully grown on SiO2
substrates as described above, and their single-crystal structure
determination is entirely consistent with the structure derived
from PXRD.
Attempts to form the isostructural MFM-300(Fe )-solv
2
have thus far failed. A nonisostructural complex was synthesized
under different hydrothermal conditions using only an Fe(III)
metal source to give the trinuclear Fe -cluster-based material
3
[
Fe O (OH)(HL)(L) (H O) ] [denoted as MFM-310(Fe)].
3 1.5 0.5 2 3.5
MFM-310(Fe) crystallizes in the orthorhombic space group
We also targeted materials MFM-300(Ga2−xFe )-solv with
x
Pmn2 with a = 10.560(4) Å, b = 19.451(8) Å, and c = 11.773(5) Å
and shows an entirely different framework structure to the
higher Fe doping levels of 10, 15 and 20%. With increasing Fe
content, the PXRD patterns of the isolated powders indicated
the presence of more amorphous materials, and more critically,
we found that the reproducibility of these higher Fe-doped
materials was very poor, with significant variations in Fe content
1
MFM-300 series. In MFM-310(Fe), μ -oxo-centered trinuclear
3
4
−
iron clusters connected by L ligands generate a three-
dimensional nonporous framework (Figure 3). This trinuclear
iron cluster is connected through an oxygen positioned at
the center of an isosceles triangle of 3.57(3) and 3.27(2) Å
from batch to batch. Thus, only MFM-300(Ga ) doped with
2
Fe at the 5% level in the synthesis (giving 6.5% in the product)
is presented here. The partial replacement of one metal
cation by another is most likely to be successful in the case of
(Figure 3a). Interestingly, this trinuclear iron cluster incorporates
different carboxylate connectivity compared to the more usual
32,33
23
Fe O(CO ) -type cluster reported previously in the literature.
a series of isostructural compounds. In this case, however, the
3
2 6
E
DOI: 10.1021/acs.inorgchem.5b02108
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Inorganic Chemistry
Article
Figure 4. CO adsorption isotherms for MFM-300(Ga2−xFe ) (x = 0, 0.13) (a) at 195 K and (b) at 293 K up to 20 bar; (c) variation of isosteric heat of
2
x
CO adsorption.
2
isostructural MFM-300(Fe ) cannot be isolated using the
synthetic conditions reported here.
molecules remain in the pores, leading to blocking effects limiting
gas entry and uptake. The phase purity of the as-synthesized bulk
samples has been confirmed by high-resolution synchrotron
X-ray powder diffraction data, which do not show the presence
of bulk impurities (Figure S4). The total micropore volume
for MFM-300(Ga ) was estimated from CO isotherms to be
2
Gas Adsorption Properties. MFM-300(Ga2−xFe )-solv
x = 0, 0.13) are thermally stable up to 573 K before decomposi-
x
(
tion, as confirmed by TGA (Figure S7). The desolvated materials
were prepared by soaking the as-synthesized sample in acetone
for 5 days, followed by degassing under dynamic vacuum
2
2
0.201 cc g− increasing to 0.299 cc g upon Fe-doping (Table 1).
Thus, such doping has a positive effect on the micropore volume,
increasing it by 49%; we ascribe this effect to increased structural
integrity of the Fe-doped material giving a more ordered, and
thus more porous, material with more accessible pores.
1
−1
−
10
(
10 bar) at 100 °C for 20 h, to obtain the fully desolvated
samples. The CO isotherms at 273 K show type-I behavior with
2
reversible uptake (Figure 4). The pore size distributions (PSDs)
for both MFM-300(Ga ) and MFM-300(Ga1.87Fe0.13) show one
2
broad peak at approximately at 5.5 Å, close to the approximate
diameter of the channel window (6.7 Å). The surface areas from
Gravimetric CO sorption isotherms were also measured
at 195 K (1 bar) and 303 K (up to 20 bar) for desolvated
2
2
−1
the CO isotherms were calculated to be 392 and 491 m g for
MFM-300(Ga2−xFe ) (x = 0, 0.13) to evaluate the effect of
2
x
MFM-300(Ga ) and MFM-300(Ga1.87Fe0.13), respectively. The
values for BET surface area and pore size obtained from the N2
isotherms are in good agreement with those calculated from CO2
mixing metal cations on CO uptake capacity. At 195 K and 1 bar,
2
2
−1
MFM-300(Ga ) shows a maximum uptake of 4.72 mmol g ,
2
whereas MFM-300(Ga1.87Fe0.13) shows a maximum uptake of
−
1
(
Table 1). These surface areas are considerably lower than that
7.02 mmol g , corresponding to a 49% increase in CO
2
2
−1
of the isostructural MFM-300(Al ) (1370 m g ), taking into
adsorption (Figure 3). This result further suggests that MFM-
300(Ga1.87Fe0.13) is a homogeneous, single-phase material: if it
were simply a mechanical binary mixture of two pure M(III)
2
account the increased formula weight by ∼21% on going from Al
to Ga. A similar difference in BET surface area between Al- and
Ga-containing isostructural materials has been also observed in
(M = Ga, Fe), in order to achieve 7.02 mmol g− CO uptake for
1
2
−
34
[
Al(BTB)] and [Ga(BTB)] (BTB3 = benzenetribenzoate),
the mixture, MFM-300(Fe ) would need to have a CO uptake of
2
2
2
−1
−1
where the surface areas are reported as 1045 and 62 m g ,
35.17 mol g , which is 3 times higher than the maximum uptake
calculated based on the single-crystal structure.
respectively. The BET surface areas for MFM-300(Ga ) are
2
reproducible over many batches of material. This discrepancy
could be due to a number of factors: not all the void space within
a porous material is necessarily accessible by the gaseous
substrate in the BET and gas adsorption experiments, while the
bulk powder sample may not correspond precisely to the
idealized structure determined by single-crystal diffraction due to
partial collapse of the framework and/or blocking of pores
by phase changes and different domains within the material.
Notwithstanding the full structural analysis of these materials, it
remains possible that residual free solvent molecules or reactant
At 273 K and up to 1 bar, MFM-300(Ga2−xFe ) (x = 0, 0.13)
x
−1
show a maximum CO uptake of 2.87 and 3.51 mmol g for x = 0
2
and x = 0.13, respectively. Compared with MFM-300(Ga ), the
2
CO uptake in the Fe-doped MOF thus shows an increase of
2
22%. This value is consistent with those observed at 195 K and
1 bar, confirming the enhancement of CO adsorption in
2
the doped material. Interestingly, although Fe-doping has a
positive effect on CO uptake capacities, the heat of CO adsorp-
2
2
tion counterintuitively decreases upon Fe-doping [34.7 and
30.9 kJ mol− for MFM-300(Ga ) and MFM-300(Ga1.87Fe0.13),
1
2
F
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Inorganic Chemistry
Article
respectively], indicating that the Fe-doped framework actually
has a reduced interaction with regard to binding of CO2
molecules, consistent with an increased pore volume. This
same effect was also observed for hydrogen adsorption, as
described below, and may reflect the presence of more open and
available pores within MFM-300(Ga1.87Fe0.13) rather than any
specific enhanced chemical effects of doping with Fe.
MFM-300(Ga1.87Fe0.13) adsorbs more H at 77 K and 20 bar
2
(
1.61 wt %) than MFM-300(Ga ) (1.45 wt %). In addition, both
2
materials display substantially higher H2 uptake capacities
compared to MFM-300(Al ) (<0.2 wt %), even though the
2
35
latter has the higher internal surface area. A slight decrease in
heat of adsorption (Q ) is observed for MFM-300(Ga1.87Fe
)
st
0.13
−1
−1
(
9.7 kJ mol ) in comparison to MFM-300(Ga ) (10.3 kJ mol ),
2
indicating slightly weaker H -framework interactions as a result
2
of Fe-doping (Figure S10). The high heat of adsorption for
H uptake in MFM-300(Ga Fe ) (x = 0, 0.13) is due to strong
2
2−x
x
overlapping potential between the adsorbed H molecules and
2
the narrow pore size (5.5 Å) of these materials.
In Situ Single-Crystal Synchrotron Infrared Spectros-
copy. To investigate the nature of the CO interactions with the
2
framework, we have studied the CO sorption in MFM-300(Ga )
2
2
by in situ IR spectroscopy on a single crystal using a high
brightness synchrotron IR source coupled to an IR microscope.
In the related complex MFM-300(Al )·3.2CO studied by in situ
2
2
PXRD at 273 K, the adsorbed CO molecules interact via
2
24
hydrogen bonding with the framework Al−OH groups.
We therefore monitored in MFM-300(Ga ) both the ν(OH)
2
stretch of the MOF host and the combination bands of CO in the
2
−1
36
region 3500−3800 cm . The fundamental antisymmetric CO2
stretch at 2348 cm− has too great an absorbance at pressures
above 0.2 bar in our experiment to be used as an effective probe,
1
but the higher energy combination bands, centered at 3714 and
612 cm− for gaseous CO , are much less intense and hence
1
3
2
more readily studied at higher uptake capacities of CO . The
2
activated MFM-300(Ga ) crystals show in the absence of CO
2
2
−1
−1
the ν(OH) stretch at 3660 cm , which shifts ∼7 cm to lower
energy to 3653 cm− on increasing the partial pressure of CO
1
2
from 0 to 1 bar (Figure 5). The difference spectra (Figure S17)
clearly show a loss of the original band and the formation of a
new, lower energy band, consistent with the formation of a
hydrogen bond between −OH and adsorbed CO molecules in
2
Figure 5. In situ FTIR spectra of a single crystal of MFM-300(Ga ) in
2
the ν(OH) and CO combination band region on increasing the partial
2
pressure of CO gas from 0 to 1 bar. The contribution from gaseous CO
2
2
has been subtracted with reference to the blank cell as a function of CO2
loading.
G
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MFM-300(Ga ). Unsurprisingly, the combination bands of
2
gaseous CO increase in intensity as the CO pressure increases
2
2
and, on subtracting these bands from the spectra, two new
−1
lower-energy bands are revealed at 3696 and 3592 cm . These
new bands grow concurrently with the change in the −OH
absorption and are consistent with the formation of an adsorbed
CO species in MFM-300(Ga ). We have also examined the
2
2
adsorbed CO concentration in MFM-300(Ga ) and the changes
2
2
in the −OH vibration by fitting the peaks in this region to
Lorentzian line shapes. The plot of peak area vs partial pressure
of CO (Figure S18), normalized such that the final absorption of
2
each peak is set to 1, is entirely consistent with the isotherms
obtained by gas sorption experiments and confirms that the
formation of the lower energy ν(OH) band is concurrent with
the rise in the adsorbed CO combination bands. A new absorp-
2
−1
tion band was also observed at 2275 cm (data not shown here),
which we attribute as the fundamental vibration of adsorbed
−1
CO , but the rest of that region (2280−2400 cm ) is obscured
2
by the absorbance for free CO gas.
2
To probe further the Ga−OH···OCO interaction within
the framework, we studied a single crystal of MFM-300(Ga )
2
with well-defined faces using polarized IR spectroscopy. IR spectra
measured using a synchrotron radiation source, which gives
approximately 100 times higher photon flux density over typical
laboratory sources, have much improved signal-to-noise ratios.
This allows the accurate measurement of IR spectra with a
polarized beam to study the orientation of the vibration modes
with respect to the functional groups. The microscope was used
to locate a single crystal that appeared to be lying flat on the stage
window, and spectra were recorded of the sample after activation
(
under He) and under 1 bar of CO . Two orthogonal directions
2
of polarized light were employed, both parallel to a crystal edge
Figure 6c), and the spectra are shown in Figure 6a. In the
(
absence of CO , the IR spectrum recorded with polarization
2
along the ab plane of the crystal (P1 as shown in Figure 6c)
−1
shows a single intense ν(OH) stretch at 3660 cm , as observed
in the unpolarized spectra. The spectrum recorded with polariza-
tion P2 (oriented 90° to P1, along c-axis) is markedly different,
with an almost complete absence of the ν(OH) absorption.
In the crystal structure, all the −OH groups in the framework
are found pointing into the channels (in the ab plane), with no
component along the c-axis (along the channels). Therefore,
the polarized IR results are consistent with the polarization in P1
being aligned with the ab plane containing the −OH groups and
P2 being aligned along the c-axis of the structure, orthogonal to
the −OH groups and hence hardly interacting with the −OH
dipole. Having identified the directionality of the crystal, the
same orientations were investigated on loading the sample with
Figure 6. In situ polarized FTIR spectra of a single crystal of MFM-
300(Ga ) in the ν(OH) and CO combination band region, (a) under
2
2
1 bar He and (b) under 1 bar of CO
CO has been subtracted), recorded at two orthogonal polarizations P1
(the contribution from gaseous
2
2
(
red) and P2 (blue), each aligned with a crystal edge. Arrows indicate the
direction of changes on rotating polarization from P1 to P2. The face
index of the single crystal which was used for this study is shown in (c).
P2 is along the c-axis, and P1 is along the ab plane.
1
bar of CO (Figure 6b). With polarization P1, the −OH
2
vibration is still intense and, as with the unpolarized spectra,
is shifted to lower energy (accompanied by a degree of peak
broadening), indicative of a hydrogen bonding interaction with
adsorbed CO . The CO combination bands are again both
A3696:A(OH)3660 = 0.28:1 with unpolarized IR light], suggesting
2
2
−1
present at 3696 and 3591 cm ; the higher energy CO band is
that there must be additional adsorbed CO present in other
2
2
a combination of the ν and ν symmetric and antisymmetric
positions in the pore, not aligned with this polarization. Indeed,
1
3
3
6
vibrations, both of which are oriented along the OCO
bond axis, leading to the potential assignment of the 3696 cm−
band in the P1 polarized IR spectrum to the presence of a CO2
molecule hydrogen-bonded in an “end-on” fashion to and
aligned along the same axis as the −OH groups. This combina-
tion band has a lower relative absorbance as a fraction of the
ν(OH) band when compared with the unpolarized spectra
peaks [A3696:A(OH)3660 = 0.14:1 with polarization P1 vs
on rotating the polarization to orientation P2, the CO -loaded
2
1
spectrum changes significantly (Figure 6b). As in the activated
sample, the ν(OH) stretch at 3660 cm− is almost completely
removed, although this time a very weak, but well-defined,
band at 3657 cm− remains (with approximately 10% of the
absorbance of the corresponding peak observed for P1).
The presence of this residual band indicates a degree of bending
of the −OH group out of the ab plane, as a direct result of the
1
1
H
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−4
Figure 7. Views of single-crystal structures of (a, d) MFM-300(Ga )-solv; (b, e) MFM-300(Ga ) activated at 393 K and vacuum 10 mbar for 4 h; (c, f)
2
2
MFM-300(Ga )·2.35CO , loaded with CO at 1 bar at 195 K for 2 h. The guest molecules (DMF, water, CO ) in the channels are highlighted using a
2
2
2
2
II
ball-and-stick style (Ga: green; C: gray; O: red; H: white; N: blue). The carbon atom of CO2 is highlighted in orange. The hydrogen bonding
interaction between the guest molecule and free −OH group is highlighted in purple. The electrostatic dipole interactions between CO molecules are
2
highlighted in cyan (O ···C ) and green (O ···C ).
3s
1s
2s
2s
Figure 8. Views of single-crystal structures of (a, d) MFM-300(Ga1.87Fe0.13)-solv; (b, e) MFM-300(Ga1.87Fe0.13), activated at 393 K and vacuum
−
4
10
mbar for 4 h; (c, f) MFM-300(Ga1.87Fe0.13)·2.0CO , loaded with CO at 1 bar at 195 K for 2 h. The guest molecules (DMF, water, CO ) in the
2 2 2
II
channels are highlighted as ball-and-stick models (Ga: green; C: gray; O: red; H: white; N: blue). The carbon atom of CO2 is highlighted in orange. The
hydrogen bonding interaction between the guest molecule and free −OH group is highlighted in purple. The electrostatic dipole interactions between
CO molecules are highlighted in cyan (O ···C ) and green (O ···C ).
2
3s
1s
2s
2s
interactions with adsorbed CO adjacent to the −OH group.
volume contractions ΔV/V of 2.1% and 0.65%, respectively.
2
Most notably, the CO combination bands increase in absorbance
The relatively small contraction for MFM-300(Ga Fe ) is
2
1.87 0.13
by a factor of ∼1.6 compared with the same bands observed at P1,
due to the synthesized crystals prepared on a SiO surface
2
commensurate with the presence of CO molecules in an
substrate being partially dried during the sample transfer before
the experiment. No electron density was found in the pore for
these desolvated samples, confirming the effectiveness of the
activation procedure and the retention of the framework
structure which incorporates uncoordinated −OH groups within
the pore (Figures 7 and 8). The activated crystals were then
2
orientation much better aligned with the P2 polarization than P1.
This suggests the presence of a second (or more) binding site
within the framework and that, at 1 bar, the CO molecules at
2
the second site have higher occupancy than the end-on CO2
molecules hydrogen-bonded to the −OH groups. We thus sought
to investigate this further by gaining more insight into the adsorp-
tion sites within the porous host by crystallographic methods.
In Situ Single-Crystal Diffraction Studies. To determine
exposed to 1 bar of CO at 195 K, and the diffraction data were
2
collected for the resulting CO -loaded crystal. Significant residual
2
electron density was now found in the pore by difference Fourier
map analysis and was sequentially assigned as two independent
CO molecules populating at sites I (O C O ) and II
the preferred binding sites for adsorbed CO molecules in
2
MFM-300(Ga ) and MFM-300(Ga1.87Fe0.13), single crystals
2
2
1s
1s
2s
of as-synthesized MFM-300(Ga )-solv and MFM-300-
(O C O ).
In MFM-300(Ga ), the occupancies for these CO molecules
2 2
2
3s
2s
4s
(
Ga1.87Fe0.13)-solv were studied using an in situ gas cell system
on a synchrotron radiation source. Upon degassing at 393 K
under high vacuum (10 mbar), the desolvated materials
at sites I and II refined to values of 0.43(3) and 0.74(3), re-
spectively, yielding the overall formula [Ga (OH) (C H O )]·
−4
2
2
16
6
8
MFM-300(Ga ) and MFM-300(Ga1.87Fe0.13) show cell
2.35CO for the CO -loaded material. This result is in excellent
2 2
2
I
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Inorganic Chemistry
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agreement with the experimental uptake from the CO isotherm
hydrogen bonding is a direct result of Fe-doping into the
2
at 195 K (1.18 CO per Ga). The CO at site I is ordered and
framework structure. In addition to this hydrogen bond, O also
2
2
1s
binds to the −OH group in an end-on fashion via a moderate
forms weak supramolecular contacts with aromatic hydrogen
hydrogen bonding interaction [H···O = 1.883(10) Å; ∠O−H···
atoms from the phenyl rings [O ···H = 3.055(4), 3.163(3) Å,
1
s
1s
O = 180°], entirely consistent with the interaction observed by
each occurring twice]. The CO at site II is disordered over two
1s
2
II
the IR spectroscopy. Interestingly, this hydrogen bond distance is
equally-occupied positions. However, CO2 adopts a different
much shorter than that observed in the MFM-300(Al )·3.2CO2
orientation to that observed in MFM-300(Ga )·2.35CO , forming
2
2
2
I
system [H···O = 2.376(13) Å; ∠O−H···O = 180°] studied by
a typical “T-shaped” dipole interaction with CO between the
2
28
in situ PXRD at 273 K, indicating the formation of a stronger
hydrogen bonding interaction in MFM-300(Ga)·2.35CO2.
Given that MFM-300(Ga ) and MFM-300(Al ) have the same
electropositive C center and electronegative O center [C ···O =
1
s
3s
3.207(6) Å, occurring twice]. Thus, the small percentage of
Fe-doping into this solid solution has a significant effect on the
2
2
framework structure and pore surface chemistry, this difference
in hydrogen bond length is most likely due to the different metal
center (Al or Ga) affecting the relative acidity of the M−OH
CO binding details, including the formation of both adsorbate−
2
adsorbent hydrogen bonds and adsorbate−adsorbate inter-
molecular dipole interactions. We could not of course distinguish
between Ga and Fe centers in the above experiments, reflecting
also the random distribution of Fe within the Ga-based host.
(
M = Al, Ga) group. In addition to this hydrogen bond, O also
1s
forms weak supramolecular contacts with aromatic hydrogen
atoms from the phenyl rings [O ···H = 2.98(4), 3.14(3) Å,
We have thus determined the detailed binding of CO in
1
s
2
each occurring twice]. The CO at site II is disordered over two
2
two isostructural MOFs and have confirmed the formation of
hydrogen bonding, C−H and O−H supramolecular contacts,
and the complementary dipole interactions and cooperative
I
2
II
2
equally occupied positions. In contrast to CO , CO does not
interact directly with framework atoms, instead forming two
I
2
weak electrostatic dipole interactions with CO , between the
binding of CO molecules within the pores to form one-
2
electropositive C and electronegative O centers [C ···O
=
1s
3s
dimensional chains of CO molecules. In both CO -loaded
2
2
3
.85(4) Å; C ···O = 4.39(4) Å]. Significantly, this pattern
2s 2s
samples, the CO distances lie within the range 1.176(8)−
.207(5) Å, consistent with typical CO distances [1.155(1) Å]
observed in solid CO . The OCO angles in CO /CO are
2 2 2
of intermolecular interactions is distinct from the traditional
1
I
II
“
[
T-shaped” dipole interaction observed in solid CO , in
2
−
Zn (atz) (ox)]·1.3CO (atz = 3-amino-1,2,4-triazolate;
2
2
2
180°/160(4)° and 180°/165(1)° for MFM-300(Ga )·2.35CO
2 2
2
−
6
24
ox = oxalate) and in MFM-300(Al )·3.2CO , and is, for
2
2
and MFM-300(Ga1 Fe0.13)·2.0CO , respectively. The slight
.87
II
2
the first time, determined here. Furthermore, both the position
and the relative occupancy of these sites are in excellent
agreement with the experimentally observed polarized IR
bending of the CO2 molecules is probably an artifact of the
disorder rather than any true distortion in bonding and is
6
comparable to that observed in [Zn (Atz) (ox)]·1.3CO .
2
2
2
absorbances for adsorbed CO aligned with and perpendicular
2
Catalytic Reactions. Two general reactions requiring acid
catalysis, namely epoxide ring-opening and the acetylation of
benzaldehyde, were selected to determine the influence of the
to the −OH axis at room temperature. CO molecules at site I are
2
aligned with the direction of P1, and CO molecules at site II,
2
while not perfectly orthogonal to site I, have a significantly better
alignment with P2 than P1. The increase in IR absorbance of
Fe-doping on the catalytic activity of MFM-300(Ga ). In the first
2
example, the catalytic activity of as-synthesized MFM-300(Ga2)
and MFM-300(Ga1.87Fe0.13) was studied in the ring-opening
of styrene oxide (1a) with methanol to yield 2-methoxy-2-
phenylethanol. In the absence of catalyst, we observed 4%
conversion of 1a to 2-methoxy-2-phenylethanol after 30 h at
the higher energy CO combination band (factor of 1.6) in P2
2
compared with P1 is remarkably close to the crystallographically
observed ratio of site occupancies (II:I = 1.7:1), both experi-
ments having been carried out at 1 bar CO pressure.
2
In contrast, the occupancies of the CO molecules at sites I and
2
4
0 °C. In contrast, the presence of heterogeneous MOF catalyst
II in MFM-300(Ga1.87Fe0.13) both refined to values of 0.50 (1),
yielding a formula of [Ga1.87Fe0.13(OH) (C H O )]·2.0CO for
resulted in significant conversions: MFM-300(Ga ) showed 40%
2
2
16
6
8
2
conversion of 1a to 2-methoxy-2-phenylethanol in 30 h while
MFM-300(Ga1.87Fe0.13) resulted almost in complete conversion
of 1a in the same time (Figure 9). Both materials showed no
leaching of active sites, as evidenced from the hot filtration test,
in which the solid catalyst was removed from the reaction
mixture at 40 °C at conversions of about 20% and the clear
the CO -loaded material. However, this result is lower than
2
the experimental uptake from the CO isotherm at 195 K, which
2
implies the formation of MFM-300(Ga1.87Fe0.13)·2.9CO . The
2
discrepancy could be due to two factors: (i) the kinetic effect for
the population of CO molecules within the whole single crystal;
2
(
ii) some of the faces of the single crystals could have been
blocked or damaged when it was removed from the surface
substrate, thereby reducing the diffusion rate for CO molecules.
2
Nevertheless, this discrepancy does not prevent us from
determining the binding mechanisms. The CO at site I is
2
ordered and binds to the −OH group in an end-on fashion via
a weak hydrogen bonding interaction [H···O = 2.259(12) Å,
1
s
∠
O−H···O = 180°]. Interestingly, this hydrogen bond distance
1
s
is longer than that observed in the MFM-300(Ga )·2.35CO
2
2
system [H···O1s = 1.883(10) Å], indicating the formation
of a weaker hydrogen bonding interaction in MFM-300-
(
Ga1.87Fe0.13)·2.0CO . This is consistent with the observed heat
2
of adsorption which also decreases, indicating a weaker interaction
Figure 9. Time conversion plots for the ring-opening of 1a with MeOH
between the mixed-metal material and the CO molecule. Given
the identical framework structures and pore surface chemistry
2
using MFM-300(Ga ) (black squares) and MFM-300(Ga1.87Fe0.13) (red
2
circles) as catalysts, and ring-opening of 1a with EtOH using MFM-
of MFM-300(Ga ) and MFM-300(Ga1.87Fe0.13), this difference in
300(Ga1.87Fe0.13) (green triangles) as catalyst.
2
J
DOI: 10.1021/acs.inorgchem.5b02108
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
solution in the absence of solid was then allowed to react further;
no further conversion was then observed after 30 h. The stability
of MFM-300(Ga1.87Fe0.13) as a solid catalyst was studied by
cycling it under identical conditions at 40 °C. The percentage
conversion of 1a in the first, second, and third cycles was 98, 93
and 83, respectively, in 30 h (Table 2) reflecting some gradual
deactivation of the catalyst with time.
Table 3. Ring-Opening of Epoxides with Various Substrates
Catalyzed by MFM-300(Ga1.87Fe0.13) Using Different
a
Nucleophiles
Table 2. Ring-Opening of 1a to 2-Methoxy-2-phenylethanol
Catalyzed by MFM-300(Ga ) and MFM-300(Ga1.87Fe0.13
Catalysts
)
2
a
b
b
run
catalyst
time (h)
conversion (%)
selectivity (%)
1
2
3
4
5
6
7
no catalyst
Ga2
30
4
98
97
97
98
97
97
97
30
46
c
Ga2
6−30
30
10−12
98
(Ga1.87Fe0.13
(Ga1.87Fe0.13
(Ga1.87Fe0.13
(Ga1.87Fe0.13
)
)
)
)
d
e
f
3−30
30
11−15
93
30
83
a
Reaction conditions: 1a (2.08 mmol), catalyst (20 mg) activated at
b
1
00 °C for 2 h under vacuum, methanol (5 mL), 40 °C. Determined
c
by GC using nitrobenzene as internal standard. Catalyst filtered at 6 h
and the reaction continued to 30 h. Catalyst filtered at 3 h and the
reaction continued to 30 h. Second reuse. Third reuse.
d
e
f
We have also studied the reaction of styrene oxide with other
alcohols such as ethanol and tert-butanol. As shown in Table 3,
MeOH reacts more quickly than EtOH, whereas tert-BuOH
showed no reaction even after 30 h. This lack of reactivity with
tert-BuOH may be due to the impeded diffusion at low tem-
perature of this larger molecule through the pore system of the
MOF, and similar results have been reported for other MOFs and
a
37
Reaction conditions: epoxide (0.250 mL), MFM-300(Ga1.87Fe0.13)
with graphene oxide as heterogeneous catalysts. This result
suggests that the catalytic sites reside in the pore rather than
on the surface. The porous nature of the MOFs used in this
study also plays a role in determining the reactivity of other
substrates and nucleophiles, as shown in Table 3, where the scope
of the catalytic activity of MFM-300(Ga1.87Fe0.13) was screened.
In most of the cases using MFM-300(Ga1.87Fe0.13) as catalyst,
the reaction exhibits high regioselectivity toward a single
regioisomer. The alcoholysis of 2,3-epoxypropylbenzene (1b)
with MFM-300(Ga1.87Fe0.13) resulted in 7% conversion with 88%
selectivity for 3b, the observed regioselectivity probably
reflecting the steric hindrance caused by the phenyl ring to the
direction of methanol attack. Similar reactivity was observed for
activated at 100 °C for 2 h under vacuum (20 mg), methanol
5 mL), 30 h, 40 °C. Determined by GC. Selectivity refers to
b
c
(
2
-methoxycyclohexanol.
the major product over the more-substituted regioisomer 3g
(10%) clearly indicates that the reaction proceeds predominantly
via an S 1 mechanism, with a transition state in which a con-
N
siderable positive charge density has developed on the epoxide
carbon atom as a consequence of the interaction of the epoxide
oxygen atom with the catalyst acid sites.
Having demonstrated the catalytic activity of MFM-300(Ga2)
and MFM-300(Ga1.87Fe0.13) in the ring-opening of epoxides,
we further tested them in the acetylation of benzaldehyde by
MeOH. In the absence of solid catalyst, acetylation leads to
24% conversion to 1,1-dimethoxytoluene after 30 h at room
1
,2-epoxy-3-phenoxypropane (1c), giving 3c as the major isomer
with respect to 2c. In the case of 1-hexene oxide (1d), ring-
opening of epoxide using methanol as solvent favored isomer 2d
over 3d. Epichlorohydrin (1e) showed 13% conversion with 3e
as the major isomer, suggesting a directing inductive effect
of the chloride group on the position of the attack by MeOH.
The methanolysis of cyclohexene oxide (1f) resulted in 12%
conversion within 30 h to 2-methoxycyclohexanol, 2f with
temperature (Figure 10). The presence of MFM-300(Ga ) and
2
MFM-300(Ga1.87Fe0.13) significantly increased the conversion to
1,1-dimethoxytoluene. The two catalysts show almost identical
activity, as shown by almost identical conversions of 71% and
72%, respectively, at 30 h. MFM-300(Ga1.87Fe0.13) was also
tested in a leaching experiment. The reaction was started in
the presence of MFM-300(Ga1.87Fe0.13), and the solid catalyst
was filtered after 4 h with 11% conversion of benzaldehyde. The
reaction mixture was then allowed to react further in the absence
of catalyst. After 30 h, 39% (instead of 72%) of benzaldehyde
was converted, consistent with the expected total conversion for
the uncatalyzed reaction based on the data from blank controls in
the absence of catalyst. The similar reactivity exhibited by these
two MOFs could be followed by in situ IR spectroscopic analysis
9
7% selectivity. Finally, norbornene oxide (1h) exhibited only
trace conversion even after a prolonged reaction time. Overall,
the unoptimized catalytic data (Table 3) show that MFM-
3
00(Ga1.87Fe0.13) exhibits general activity as a solid Lewis acid
catalyst.
To differentiate between S 1 and S 2 reaction mechanisms,
N
N
1,2-epoxy-2-methylpropane (1g) was selected as a substrate and
the nucleophilic ring-opening of this epoxide was carried out with
MeOH. Formation of the less-substituted alcohol 2g (90%) as
K
DOI: 10.1021/acs.inorgchem.5b02108
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
CONCLUSIONS
■
MFM-300(Ga ) and the isostructural mixed-metal solid solution
2
MFM-300(Ga0.87Fe0.13) were prepared from homogeneous
solvothermal reactions. Significant enhancement of CO2
adsorption capacity by up to 49% was observed by doping
with Fe(III), reflecting the increased structural integrity of the
Fe-doped material giving a more ordered material with more
accessible pores. Thus, Fe-doping can be used to improve the
materials properties of the host while also monitoring the effects
of the heteroatom center within a parent framework without the
requirement of synthesizing the isostructural MFM-300(Fe2).
Figure 10. Time conversion plots for the acetylation of benzaldehyde
In situ single-crystal X-ray diffraction studies of CO -loaded
with MeOH using MFM-300(Ga ) (black squares) and MFM-
2
2
3
00(Ga1.87Fe0.13) (red circles) as catalysts. Reaction conditions:
materials revealed, on a molecular level, key details into the
preferred binding sites within the pores of these materials that
were in excellent agreement with the results of the in situ
benzaldehyde (0.94 mmol), catalyst activated at 100 °C, 2 h (20 mg),
MeOH (3 mL), room temperature.
polarized IR spectroscopic study of CO -loaded MFM-
2
3
00(Ga ). The adsorbed CO molecules in MFM-300(Ga )
2 2 2
hydrogen bond with the free −OH groups on the surface of
the pores, and a previously unobserved pattern of intermolecular
dipole interactions was found to stabilize the two CO molecules
2
within the pores. The CO molecule at site I binds to the −OH
2
group in an end-on fashion [H···O = 1.883(10) Å] and also
1
s
interacts with the CO molecule at site II. In contrast, the adsorbed
2
CO molecules in MFM-300(Ga1.83Fe0.13) form only very weak
2
hydrogen bonds with free −OH groups [H···O = 2.259(12) Å],
1s
and a traditional “T-shaped” CO2 intermolecular dipole
interaction was responsible for the stability of the CO molecular
2
chain. The reduction on the strength of the hydrogen bond is also
reflected in the isosteric heat of adsorption for CO uptake.
2
In addition to the enhancement of the gas adsorption properties,
Fe-doping has also shown a positive effect on specific catalytic
reactions such as the ring-opening reactions of styrene oxide.
We anticipate that the strategy developed here, based upon
mixing transition and main group metal nodes in MOFs, can lead
to the design and discovery of new materials with improved
properties and functions.
)
2
Figure 11. IR spectra of MFM-300(Ga ) and MFM-300(Ga1.87Fe0.13
after CO adsorption.
using CO as probe molecule (Figure 11), which showed that
ASSOCIATED CONTENT
■
both MOFs contained a similar number of active sites. The IR
band at 2165 cm−1 could be due to Lewis acid sites, whereas the
*
S
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.5b02108.
−
1
IR band at 2145 cm is attributed to hydroxyl groups. As
confirmed by Figure 11, the majority of the centers interacting
with CO are Bro
̈
nsted acid sites, and since MFM-300(Ga ) and
Experimental details, additional figures, graphs, and tables
(PDF)
Crytallographic data (CIF)
2
MFM-300(Ga1 Fe0.13) have similar populations of acid sites and
.87
the demand of acid strength on the benzaldehyde acetalization is
low, both solids should exhibit very similar catalytic activity, as is
experimentally observed. Differences in catalytic activity between
AUTHOR INFORMATION
MFM-300(Ga ) and MFM-300(Ga1.87Fe0.13) are expected for
■
*
*
2
other reactions such as S 1 epoxide ring-opening, which require
Corresponding Authors
E-mail: Sihai.Yang@manchester.ac.uk (S.Y.).
E-mail: M.Schroder@manchester.ac.uk (M.S.).
N
higher acid strength. In this way, the combination of spectro-
scopic titration of acid sites using CO as a probe with the catalytic
results for acetylation and epoxide ring-opening indicates that
Notes
MFM-300(Ga ) and MFM-300(Ga1.87Fe0.13) have the same
2
The authors declare no competing financial interest.
population of acid sites, but the presence of Fe-doping increases
the acid strength, enabling the use of MFM-300(Ga1.87Fe0.13
)
ACKNOWLEDGMENTS
as catalyst for those reactions requiring sites with higher acid
strength. This effect of Fe-doping increasing the strength of
the acid sites contrasts with the weaker hydrogen bonding
■
We thank the Universities of Nottingham and Manchester for
support. M.S. acknowledges receipt of an ERC Advanced Grant
and EPSRC Programme Grant. We also thank EPSRC for funding
of X-ray equipment. We acknowledge the use of instrumentation
within the Nottingham Nanotechnology and Nanoscience Centre,
and thank Dr. Christopher Parmenter for assistance. We thank
Conacyt, Mexico, for funding to C.P.K. We are especially grateful to
interaction between the framework and CO in single crystals
2
of MFM-300(Ga1.87Fe0.13)·2.0CO . It is, therefore, likely that the
2
most active sites in catalysis for MFM-300(Ga1.87Fe0.13) are those
present in defects, and these will be absent in highly crystalline
samples of MFM-300(Ga2).
L
DOI: 10.1021/acs.inorgchem.5b02108
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Diamond Light Source for access to Beamlines I11 (EE8943), I19
(12) Furukawa, H.; Ko, N.; Go, Y. B.; Aratani, N.; Choi, S. B.; Choi, E.;
̈
Yazaydin, A. O.; Snurr, R. Q.; O'Keeffe, M.; Kim, J.; Yaghi, O. M. Science
010, 329, 424−428.
13) Kettner, F.; Worch, C.; Moellmer, J.; Glas
Krautscheid, H. Inorg. Chem. 2013, 52, 8738−8742.
14) Deng, H.; Doonan, C. J.; Furukawa, H.; Ferreira, R. B.; Towne, J.;
Knobler, C. B.; Wang, B.; Yaghi, O. M. Science 2010, 327, 846−850.
15) Wang, L. J.; Deng, H.; Furukawa, H.; Gandara, F.; Cordova, K. E.;
Peri, D.; Yaghi, O. M. Inorg. Chem. 2014, 53, 5881−5883.
16) Breeze, M. I.; Clet, G.; Campo, B. C.; Vimont, A.; Daturi, M.;
(MT7548, MT8448, MT8937), and B22 (SM11279).
2
(
̈
er, R.; Staudt, R.;
LIST OF MATERIALS
■
4
(
MFM-300(Ga )-solv: [Ga (OH) (C H O )]·1.6DMF·
2
2
2
16
6
8
H O
2
(
́
MFM-300(Ga ): [Ga (OH) (C H O )]
2
2
2
16
6
8
MFM-300(Ga )·2.35CO : [Ga (OH) (C H O )]·2.35CO
2
2
2
2
2
16
6
8
(
MFM-300(Ga1.87Fe0.13)-solv: [Ga1.87Fe0.13(OH)2-
Greneche, J.-M.; Dent, A. J.; Millange, F.; Walton, R. I. Inorg. Chem.
(
C H O )]·2.7H O
16 6 8 2
2013, 52, 8171−8182.
MFM-300(Ga1.87Fe0.13): [Ga1.87Fe0.13(OH) (C H O )]
(17) Fukushima, T.; Horike, S.; Inubushi, Y.; Nakagawa, K.; Kubota,
Y.; Takata, M.; Kitagawa, S. Angew. Chem., Int. Ed. 2010, 49, 4820−4824.
2
16
6
8
MFM-300(Ga1.87Fe0.13)·2.0CO : [Ga1.87Fe0.13(OH)2-
2
(
C H O )]·2.0CO
̈
(18) Yeung, H. H. M.; Li, W.; Saines, P. J.; Koster, T. K. J.; Grey, C. P.;
1
6
6
8
2
Cheetham, A. K. Angew. Chem., Int. Ed. 2013, 52, 5544−5547.
19) Nouar, F.; Devic, T.; Chevreau, H.; Guillou, N.; Gibson, E.; Clet,
MFM-310(Fe): [Fe O (OH)(HL)(L) (H O) ]
3
1.5
0.5
2
3.5
(
G.; Daturi, M.; Vimont, A.; Greneche, J. M.; Breeze, M. I.; Walton, R. I.;
Llewellyn, P. L.; Serre, C. Chem. Commun. 2012, 48, 10237−10239.
(20) Lau, C. H.; Babarao, R.; Hill, M. R. Chem. Commun. 2013, 49,
3634−3636.
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DOI: 10.1021/acs.inorgchem.5b02108
Inorg. Chem. XXXX, XXX, XXX−XXX