Facile control of the charge density and photocatalytic activity of an anionic indium porphyrin framework via in situ metalation

Article abstract of DOI:10.1021/ja5092672

An anionic indium porphyrin framework (UNLPF-10) consisting of rare Williams β-tetrakaidecahedral cages was constructed using an octatopic ligand linked with 4-connected [In(COO)₄]^- SBUs. Remarkably, the extent of indium metalation of porphyrin macrocycles in UNLPF-10 can be facilely tuned in situ depending on the M/L ratio during synthesis, resulting in a controllable framework charge density and photocatalytic activity toward the selective oxygenation of sulfides.

Full text of DOI:10.1021/ja5092672

Communication
pubs.acs.org/JACS
Facile Control of the Charge Density and Photocatalytic Activity of
an Anionic Indium Porphyrin Framework via in Situ Metalation
†
†
†
‡
,†
Jacob A. Johnson, Xu Zhang, Tyler C. Reeson, Yu-Sheng Chen, and Jian Zhang*
†
Department of Chemistry, University of Nebraska−Lincoln, Lincoln, Nebraska 68588, United States
‡
ChemMatCARS, Center for Advanced Radiation Sources, The University of Chicago, 9700 South Cass Avenue, Argonne, Illinois
0439, United States
6
*
S Supporting Information
metalation allows for the remarkably precise control of the
charge density and photocatalytic activity of UNLPF-10.
UNLPF-10 was prepared using a solvothermal reaction of an
octatopic ligand, H tbcppp (tetrakis 3,5-bis[(4-carboxy)-
ABSTRACT: An anionic indium porphyrin framework
UNLPF-10) consisting of rare Williams β-tetrakaidecahe-
(
dral cages was constructed using an octatopic ligand linked
1
0
−
6
with 4-connected [In(COO)4] SBUs. Remarkably, the
phenyl]phenylporphine ), and In(NO ) ·H O in N,N-dime-
3 3 2
extent of indium metalation of porphyrin macrocycles in
UNLPF-10 can be facilely tuned in situ depending on the
M/L ratio during synthesis, resulting in a controllable
framework charge density and photocatalytic activity
toward the selective oxygenation of sulfides.
thylformamide (DMF). Single crystal X-ray crystallographic
studies revealed that each 8-connected tbcppp ligand links eight
4
-connected [In(COO)4]− SBUs. Interestingly, six tbcppp
−
ligands connect eight [In(COO)4] SBUs and form a rare
Williams β-tetrakaidecahedral cage, which consists of 14 faces,
7
2
4 vertices, and 36 edges (Figure 1a). Within each cage, two
orphyrinic metal−organic frameworks (porph-MOFs) are
P
a special class of porous coordination polymers that are
composed of (metallo)porphyrin-based linkers interconnected
by metal ions or metal carboxylate cluster secondary building
1
units (SBUs). In the past decade, porph-MOFs have received
resurgent attention due to the persistent interests to assemble
the tetrapyrrolic macrocycles that are quintessential in many
fundamental biochemical, enzymatic, and photochemical
2
functions. Indeed, new porph-MOFs with excellent catalytic
activity, light-harvesting capability, or ion conductivity have
now emerged, demonstrating a promising potential for their
3
future development and practical applications.
In principle, metalation of porphyrin macrocycles provides a
unique handle to control the chemical, optical, or catalytic
properties of porph-MOFs. Both premetalation (i.e., free base
3a−c
porphyrin linkers are metalated before MOF synthesis)
and
in situ metalation (i.e., free base porphyrin linkers are metalated
3d−f
during MOF synthesis)
are commonly used to tune the
Figure 1. (a) Illustration of the octatopic ligand tbcppp and
identity of the metalloporphyrin core. Recently, postsynthetic
solvent-assisted linker exchange (SALE) and transmetalation
−
[
In(COO)4] SBU forming the Williams β-tetrakaidecahedral cage.
4
5
(b) Close packing of tetrakaidecahedral cages. (c,d) Controlling the
were also employed to obtain porph-MOFs with specific
metalloporphyrin cores that otherwise cannot be prepared by
de novo synthesis. However, the extent of metalation, a
parameter that also controls the properties of porph-MOFs, has
not been well studied and utilized. To date, methods that can
lead to the controllable metalation have not been reported.
Herein, we report the first example of controllable, in situ
metalation of an anionic indium porph-MOF, UNLPF-10
charge density of UNLPF-10 via indium metalation of porphyrins
(charges are shown with respect to cage occupancy in the overall
framework).
planar porphyrin macrocycles serve as the two square faces, and
the pentagonal and hexagonal faces are formed with two tbcppp
−
ligands linked by two [In(COO)4] SBUs in orientations
where two porphyrin macrocycles are perpendicular and
parallel to each other, respectively (Figure S1, Supporting
Information). The overall structure of UNLPF-10 can be
viewed as the close packing of the tetrakaidecahedral cages
(
UNLPF: University of Nebraska−Lincoln porous framework).
The extent of metalation of porphyrin macrocycles in UNLPF-
0 is tunable by simply varying the In/L (indium to ligand)
1
ratio during MOF synthesis. Since the indium-metalated
porphyrin macrocycle (In-porph) exhibits a net +1 charge
and outstanding photosensitizing capability, such in situ
Received: September 8, 2014
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ja5092672 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Figure 1b). UNLPF-10 is highly porous: the internal
dimensions of the tetrakaidecahedral cage are 33 × 23 × 23
Å, and the hexagonal and pentagonal window sizes are 19 × 16
Å and 17 × 13 Å, respectively. Notably, UNLPF-10 also
exhibits channel-like pores along all three crystallographic axes
Communication
(
premetalation of tbcppp linkers with In3+ was unsuccessful
(Figure S5, Supporting Information); thus, the observed in situ
metalation is likely due to the tendency of minimizing the
overall framework charge density (vide infra). Further,
inductively coupled plasma mass spectrometry (ICP−MS)
analysis shows a good correlation between the M/L ratio and
the mass content of indium in UNLPF-10 (Figure S7,
Supporting Information), suggesting that the M/L ratio indeed
controls the extent of metalation in UNLPF-10.
(
Figure S2, Supporting Information), an appealing feature for
catalysis (vide infra). β-Tetrakaidecahedral cage-like topology is
seldom observed in crystals structures. To the best of our
8
knowledge, UNLPF-10 represents the first MOF that is
composed of Williams β-tetrakaidecahedral cages.
1
0
Similar to many In-MOFs, UNLPF-10 forms as an anionic
−
Interestingly, highly crystalline samples of UNLPF-10 with
comparable PXRD patterns can be obtained using a wide range
of M/L ratios (5:1, 10:1, 20:1, 30:1, 40:1, and 50:1; Figure S3,
Supporting Information). This is noteworthy since it is well-
known that the high yield synthesis of a specific crystalline
phase of MOFs typically requires a narrow range of M/L
framework due to the presence of [In(COO) ] SBUs.
4
+
Me NH2 is presumably the counter cation (Figure S8,
Supporting Information) that is loosely bound within the
framework. The anionic nature was also confirmed by ion
2
1
1
exchange with organic dyes. Specifically, cationic dyes such as
+
+
methylene blue (MB ), methylene green (MG ), methyl violet
9
+
+
ratios. Thus, the assembly of UNLPF-10 is robust and not
(MV ), and rhodamine B (RB ) completely exchanged with
Me NH2 within 24 h (Figures 3a and S10, Supporting
+
significantly affected by the M/L ratio. More importantly, single
crystal X-ray structure analysis reveals partial occupancy of
indium atoms at the porphyrin centers for two UNLPF-10
2
samples prepared at different M/L ratios: U = 1.045 for M/L
eq
=
5:1 and Ueq = 0.445 for M/L = 30:1 (U : equivalent
eq
isotropic displacement parameter). In crystallography, a
significantly high U value as compared to that of neighboring
eq
atoms usually indicates a low atomic occupancy. As −a
comparison, the occupancies of In atoms at the [In(COO)4]
SBUs in these two samples were found to be similar (Ueq
=
0
.117 and U = 0.121, respectively). Therefore, a higher M/L
eq
ratio indeed results in a higher extent of metalation of
porphyrins in UNLPF-10.
Next, we used nuclear magnetic resonance (NMR) spec-
troscopy to evaluate the extent of metalation at bulk level. H
1
Figure 3. (a) Photographs of organic dye solutions before and after
treating with UNLPF-10. (b) MB exchange with UNLPF-10 with
NMR spectra of acid digested samples of UNLPF-10 showed
two sets of signals in the aromatic region assigned to free base
porphyrin (porph) and indium-metalated porphyrin (In-
porph), respectively (Figure 2). Specifically, as the M/L ratio
+
different percent of In-porph. The dye content was monitored by UV−
vis in DMF (λ = 665 nm).
max
Information). However, UNLPF-10 shows weak adsorption of
neutral Sudan I (Supporting Information) and no adsorption of
−
anionic acid orange (AO ) (Figure 3a). Clearly, the selective
adsorption of cationic dyes is due to the Coulombic interaction
of an anionic framework with cationic guests.
In-porph typically exhibits a +1 effective charge since the
axially coordinated anion (NO3− in this case) has a strong
12
tendency to dissociate in polar solution. Therefore, we
reasoned that the charge density of UNLPF-10 is dependent on
the extent of porphyrin metalation. When the entire framework
is composed of porph (0% metalation) or In-porph (100%
metalation), the calculated charge density is −2 or −1 per cage,
respectively (Figure 1c,d). We further quantified the charge
density of UNLPF-10 by calculating the average amount of
+
1
adsorbed MB in one tetrakaidecahedral cage using UV−vis
Figure 2. H NMR spectra in the aromatic region of tbcppp linkers
(
spectroscopy (see Supporting Information for detailed
procedure). Remarkably, as the extent of metalation in
UNLPF-10 increased, the equivalents of MB absorbed per
circle, In-porph; triangle, porph) obtained via acid digestion of
UNLPF-10 samples prepared at different M/L ratios.
+
cage decreased from ∼2 (8% metalation) to ∼1 (100%
increases, the peak intensities of porph at 8.25, 8.72, 8.85
metalation) (Figure 3b), corresponding to a charge density per
1
(
terphenyl), and 9.11 ppm (β-pyrrole) decrease. and the peak
intensities of In-porph at 8.15, 8.60, and 9.21 ppm increase
Figure 2). Complete metalation with indium was only possible
cage of −2 and −1, respectively. Further, H NMR analysis of
+
the MB @UNLPF-10 samples using acid digestion treatment
(
also confirmed the varying equivalents of extraframework MB+
(Figure S11, Supporting Information).
when the M/L ratio was 50:1, indicating slow reaction kinetics.
This is in sharp contrast to the metalation of other metals such
Because of their rich photophysical and photochemical
properties, porph-MOFs are of particular interest for artificial
3
e
3d
3f
as zinc, copper, and cobalt, for which a low M/L ratio can
lead to a complete metalation. Interestingly, attempted
3
c,g,13
photosynthesis and photocatalysis.
In-porph and their
B
dx.doi.org/10.1021/ja5092672 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
derivatives are excellent photosensitizers and have been used in
activity: the rate of photo-oxygenation of thioanisole increased
as the percent of In-porph sites increased (Figure 4). For
14
photodynamic therapy; however, their use in light-promoted
chemical reactions has not been documented. We evaluated the
photocatalytic activity of UNLPF-10 for the selective oxygen-
ation of sulfides, which is an important industrial process, and
the resulting sulfoxides are biologically active compounds
15
commonly used in pharmaceutics. The photocatalytic oxy-
genation of sulfides was investigated in the presence of O2
(open to air) and UNLPF-10 (0.1 mol % based on In-porph)
under irradiation with blue LED (135 mW, λmax = 465 nm;
Figure S12, Supporting Information), and it generally
proceeded in excellent yields (Table 1, entries 1−7). For
Table 1. Photo-Oxygenation of Sulfides
Figure 4. Photo-oxygenation of thioanisole-catalyzed UNLPF-10 with
1
00% In-porph (purple), 71% In-porph (green), and 25% In-porph
(red).
a
entry
substrate
conversion
time (h)
instance, UNLPF-10 with ∼25% In-porph sites required 40 h to
achieve a complete conversion, whereas ∼98% In-porph only
requires 8 h (Figure 4). It is noted that free base porphyrin is
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
>99%
96%
73%
98%
>99%
92%
97%
0%
8
18
24
18
18
24
24
48
48
48
48
8
20
also a common photosensitizer. However, our result clearly
shows that it is less photocatalytically active than In-porph for
oxygenation of sulfides. Indeed, the photo-oxygenation using
tetraphenylporphyrin (TPP) and its indium-metalated deriva-
tive (In-TPP) proceeded with conversions of 31% and 73%,
respectively (Table 1, entries 13 and 14). This result also
indicates the catalytic activity of heterogeneous UNLPF-10 is
1g
1h
1a
1a
1a
1a
1a
1a
b
0%
13a
c
superior to that of the homogeneous In-TPP.
1
1
1
1
1
0
1
2
3
4
0%
d
In summary, a new anionic indium porphyrin framework,
UNLPF-10, has been constructed as the first example of a MOF
composed of close-packed Williams β-tetrakaidecahedral cages.
The metalation of porphyrin macrocycles can be simply
controlled by adjusting the M/L ratio during synthesis,
resulting in fine-tuning of the charge density of the framework.
UNLPF-10 exhibits excellent photocatalytic activity toward the
selective oxygenation of sulfides, which is also tunable by
indium metalation. In light of its ionic feature, additional
functions can be easily incorporated to UNLPF-10 via ionic
exchange with extra-framework guests with interesting
2%
e
97%
f
31%
18
g
73%
18
a
Determined by 1NMR. No light. No catalyst. Under an Ar
atmosphere. After 5th recycle of UNLPF-10. TPP used as the
b
c
d
e
f
g
catalyst. In-TPP used as the catalyst.
instance, thioanisole 1a was completely and selectively
converted to the corresponding sulfoxide after 8 h (Table 1,
entry 1). Electron-withdrawing groups such as −Cl and −Br
were found to slightly decrease the reaction rate (Table 1,
entries 2 and 3), consistent with previous reports. UNLPF-
0 was not able to oxidize diphenyl sulfide 1h (Table 1, entry
21
22
23
chemical, catalytic, or optical properties. Thus, this
study paves a way for realizing new applications of porph-
MOFs, and such investigations are currently underway in our
laboratory.
1
6a
1
8
16b
) due to the less accessible p orbital on the sulfur atom.
z
Control experiments revealed the essential role of light,
ASSOCIATED CONTENT
photocatalyst, and O in this reaction (Table 1, entries 9−
2
■
11). UNLPF-10 exhibits excellent stability indicated by its well-
S
*
Supporting Information
preserved crystalline nature after reactions (Figure S3−4,
Supporting Information). Importantly, it can also be repeatedly
used without decrease of activity (Table 1, entry 12). The
possible overoxidized byproduct, sulfone, was not detected,
suggesting that the reaction involves the singlet oxygen
Materials, general procedure, synthesis of UNLPF-10, crystallo-
graphic data, dye uptake, and photocatalytic study. This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
7
pathway, which was further supported by electron para-
AUTHOR INFORMATION
18
■
*
magnetic resonance (EPR) spectroscopy and a control
19
Corresponding Author
jzhang3@unl.edu.
reaction performed in the deuterated solvent (d -MeOD)
4
(
see Supporting Information for details).
We next demonstrated that the controllable in situ
metalation of UNLPF-10 leads to its tunable photocatalytic
Notes
The authors declare no competing financial interest.
C
dx.doi.org/10.1021/ja5092672 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(
14) (a) Dolmans, D. E.; Kadambi, A.; Hill, J. S.; Waters, C. A.;
ACKNOWLEDGMENTS
■
Robinson, B. C.; Walker, J. P.; Fukumura, D.; Jain, R. K. Cancer Res.
002, 62, 2151. (b) Chen, Y.; Zheng, X.; Dobhal, M. P.; Gryshuk, A.;
The authors acknowledge the support from the University of
Nebraska−Lincoln and NSF through Nebraska MRSEC
DMR-0820521). ChemMatCARS Sector 15 is principally
2
Morgan, J.; Dougherty, T. J.; Oseroff, A.; Pandey, R. K. J. Med. Chem.
2005, 48, 3692.
(
supported by the Divisions of Chemistry (CHE) and Materials
Research (DMR), National Science Foundation, under grant
number NSF/CHE-1346572. Use of the Advanced Photon
Source, an Office of Science User Facility operated for the U.S.
Department of Energy (DOE) Office of Science by Argonne
National Laboratory, was supported by the U.S. DOE under
Contract No. DE-AC02-06CH11357.
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dx.doi.org/10.1021/ja5092672 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX