Porphyrin-Metalation-Mediated Tuning of Photoredox Catalytic Properties in Metal-Organic Frameworks

Article abstract of DOI:10.1021/acscatal.5b00941

Photoredox catalytic activation of organic molecules via single-electron transfer processes has proven to be a mild and efficient synthetic methodology. However, the heavy reliance on expensive ruthenium and iridium complexes limits their applications for scale-up synthesis. To this end, photoactive metal-organic frameworks (MOFs) exhibit unique advantages as novel heterogeneous photocatalytic systems, yet their utilization toward organic transformations has been limited. Here we describe the preparation and synthetic applications of four isostructural porphyrinic MOFs, namely, UNLPF-10a, -10b, -11, and -12, which are composed of free base, In^III-, Sn^IVCl₂-, and Sn^IV-porphyrin building blocks, respectively. We demonstrate that the metalation with high valent metal cations (In^III and Sn^IV) significantly modifies the electronic structure of porphyrin macrocycle and provides a highly oxidative photoexcited state that can undergo efficient reductive quenching processes to facilitate organic reactions. In particular, UNLPF-12 exhibits both outstanding photostability and efficient photocatalytic activities toward a range of important organic transformations including aerobic hydroxylation of arylboronic acids, amine coupling, and the Mannich reaction.

Full text of DOI:10.1021/acscatal.5b00941

Subscriber access provided by UNIV OF CAMBRIDGE
Article
Porphyrin-Metalation-Mediated Tuning of Photoredox
Catalytic Properties in Metal-Organic Frameworks
Jacob A. Johnson, Jian Luo, Xu Zhang, Yu-Sheng Chen, Martha
Morton, Elena Echeverría, Fernand Echeverria Torres, and Jian Zhang
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 03 Aug 2015
Downloaded from http://pubs.acs.org on August 4, 2015
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
PorphyrinꢀMetalationꢀMediated Tuning of
Photoredox Catalytic Properties in MetalꢀOrganic
Frameworks
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
†
†
†
‡
†
Jacob A. Johnson, Jian Luo, Xu Zhang, YuꢀSheng Chen, Martha D. Morton, Elena
#
#
,†
Echeverría, Fernand E. Torres, and Jian Zhang*
†
Department of Chemistry, University of NebraskaꢀLincoln, Lincoln, Nebraska, 68588ꢀ0304,
United States
‡
ChemMatCARS, Center for Advanced Radiation Sources, The University of Chicago, 9700 S.
Cass Avenue, Argonne, Illinois, 60439, United States
#
Department of Physics and Astronomy, Theodore Jorgensen Hall, 855 N 16th St., University of
Nebraska, Lincoln, Nebraska 68588ꢀ0299, United States
KEYWORDS: metalꢀorganic frameworks, photocatalysis, microporous materials, porphyrins,
metalation
ACS Paragon Plus Environment
1

ACS Catalysis
Page 2 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ABSTRACT: Photoredox catalytic activation of organic molecules via single electron transfer
processes has proven to be a mild and efficient synthetic methodology. However, the heavy
reliance on expensive ruthenium and iridium complexes limits their applications for scale up
synthesis. To this end, photoactive metalꢀorganic frameworks (MOFs) exhibit unique advantages
as novel heterogeneous photocatalytic systems, yet their utilization toward organic
transformations has been limited. Here we describe the preparation and synthetic applications of
four isostructural porphyrinic MOFs, namely, UNLPFꢀ10a, ꢀ10b, ꢀ11, and ꢀ12, which are
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
III
IV
IV
composed of free base, In ꢀ, Sn Cl ꢀ, and Sn ꢀporphyrin building blocks, respectively. We
2
III
IV
demonstrate that the metalation with high valent metal cations (In and Sn ) significantly
modifies the electronic structure of porphyrin macrocycle and provides a highly oxidizing
photoexcited state that can undergo efficient reductive quenching processes to facilitate organic
reactions. In particular, UNLPFꢀ12 exhibits both outstanding photostability and efficient
photocatalytic activities toward a range of important organic transformations including aerobic
hydroxylation of arylboronic acids, amine coupling, and the Mannich reaction.
ACS Paragon Plus Environment
2

Page 3 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
Introduction
Visibleꢀlight photoredox catalysis allows for the use of solarꢀenergyꢀgenerated photochemical
1
potential to produce valueꢀadded organic compounds in a sustainable fashion. This process is
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
based upon the single electron transfer (SET) events between organic substrates and photoredox
2
+
catalysts that are typically transition metal complexes (e.g. [Ru(bpy) ] (bpy = 2,2’ꢀbipyridine)
3
and Ir(ppy) (ppy = 2ꢀphenylpyridine)), and it has recently realized many complicated organic
3
2
transformations. However, Ruꢀ and Irꢀ complexes are expensive, potentially toxic, and difficult
to recycle. To this end, metalꢀorganic frameworks (MOFs) represent an ideal heterogeneous
3
photocatalytic platform to reduce the cost especially for scaleꢀup synthesis. MOFs are a class of
porous crystalline solids that are constructed from the selfꢀassembly of inorganic metal ions or
4
clusters with organic ligands. The remarkably tunable pore sizes and large surface areas of
5
6
MOFs have rendered their wide array of applications in gas storage, small molecule separation,
7
8
9
optics, chemical sensing and imaging, drug delivery and therapy, among others. In particular,
MOFs comprising large open channels can facilitate the diffusion of substrates and products and
10
offer a unique advantage for their use as singleꢀsite solid catalysts and photocatalysts. Indeed,
over the past several years, a few photoactive MOFs have been prepared for inorganic reactions
1
1
12
such as visibleꢀlight driven hydrogen evolution and CO reduction.
2
Using MOFs for visibleꢀlight promoted organic transformations, however, has made much less
progress. This is probably because most of the photoactive MOFs reported to date are based on
the LMCT (ligandꢀtoꢀmetal charge transfer) transitions, which normally have limited, often not
1
3
readily tunable visible light absorption. More importantly, the large thermodynamic driving
force and small kinetic barrier associated with LMCT usually lead to poor photon energy
14
utilization. Thus, this type of photoactive MOF is mostly used for oxidative dye degradation
ACS Paragon Plus Environment
3

ACS Catalysis
Page 4 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
15
and generally shows inferior activities towards useful organic transformations. A more robust
and efficient strategy to construct photoactive MOFs for sophisticated organic transformations is
to employ linkers that are functionalized with organic and metal–organic chromophores with
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
16
wellꢀknown photocatalytic activities. For instance, using linkers based on Irꢀ and Ruꢀ
complexes, Lin et al. constructed a series of UiOꢀ67 MOFs that exhibit efficient activities
1
7
towards azaꢀHenry reactions, aerobic amine coupling, and photoꢀoxidation of thioanisole.
Recently, Duan et al. successfully utilized MOFs consisting of triphenylamine photoredox
1
8
moieties for αꢀalkylation of aldehydes with good enantioselectivity.
Porphyrins and metalloporphyrins are common chromophores that can efficiently engage
1
9
20
energy and/or electron transfer processes. Thus, photoactive porphyrinic MOFs can then be
used for heterogeneous photocatalysis. Indeed, their utility in photocatalytic oxidation of
21
22
thioanisole and hydrogen evolution reaction has been recently demonstrated. One unique yet
underexplored feature of porphyrinic MOFs is the high dependence of their photophysical and
photochemical properties on the coordinated metal ions, which can be easily modulated via the
metalation of porphyrin macrocycle. We recently reported an indium porphyrinic MOF, UNLPFꢀ
10 (UNLPF: University of Nebraska−Lincoln porous framework), with adjustable photocatalytic
2
1b
III
activity via simple inꢀsitu porphyrin metalation.
By changing the In /ligand ratio in the
III
solvothermal synthesis, we systematically increased the extent of In –porphyrin moieties in
UNLPFꢀ10. Correspondingly, the photocatalytic activity of UNLPFꢀ10 toward aerobic
III
oxygenation of organic sulfides was enhanced since In –porphyrin is a more efficient singlet
2
1b
oxygen sensitizer compared to the freeꢀbase porphyrin.
Additionally, it is known that
coordinated metal ions can also significantly affect the photoredox properties of the porphyrin
macrocycles. Herein, we report the use of metalation to fineꢀtune the photoredox catalytic
ACS Paragon Plus Environment
4

Page 5 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
activities of porphyrinic MOFs. Specifically, using UNLPFꢀ10 as the prototypic structural motif,
we synthesized and systematically characterized the photophysical and electrochemical
properties of four isostructural porphyrinic MOFs, namely, UNLPFꢀ10a, ꢀ10b, ꢀ11, and ꢀ12,
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
III
IV
IV
which are composed of free base, In ꢀ, Sn Cl ꢀ, and Sn ꢀporphyrin building blocks,
2
respectively. We found that UNLPFꢀ12, which is composed of coordinatively unsaturated highꢀ
IV
valent metal centers (Sn ), exhibits the strongest oxidizing capability and the highest efficiency
in promoting the hydroxylation of arylboronic acids, aerobic amine coupling, and the Mannich
reaction.
Results and Discussion
Synthesis and characterization.
Porphyrin ligands. The octatopic porphyrin ligand, tetrakis 3,5ꢀbis((4ꢀhydroxycarbonyl)ꢀ
phenyl)phenylporphine (H L, 1b) was synthesized using acidꢀmediated deprotection from its
2
2
0g
corresponding methyl ester (1a) according to our previously reported procedure (Scheme 1).
IV
The reaction of 1a with SnCl in pyridine under air yielded the Sn Cl ꢀmetalated ester 2a. To
2
2
–
23
remove the two coordinated Cl , we employed an anion metathesis procedure by reacting 2a
with two equivalents of AgBF in THF under argon which generated ester 3a. The coordination
4
IV
119
119
environment of Sn in 2a and 3a was confirmed by Sn NMR: the chemical shift of Sn in 2a
(
δ = –588.4 ppm) appears higher than that in 3a (δ = –620.1 ppm), which matches well with their
IV
2–
structural analogues such as Sn Cl TPP (TPP = mesoꢀtetraphenylporphyrinate) (δ = –588.5
2
IV
ppm) and [Sn TPP](BF ) (δ = –619.9 ppm) (see Supporting Information for detailed synthetic
4
2
1
19
procedure and Figure S1 for Sn NMR spectra). In general, stronger axial ligands such as
1
19
IV
chloride can cause a downꢀfield shift of Sn in Sn ꢀporphyrins, possibly due to the effect of
24
deshielding. Further deprotection of 2a by HCl and 3a by HBF yielded the octatopic ligand
4
ACS Paragon Plus Environment
5

ACS Catalysis
Page 6 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
IV
IV
–
Sn Cl L (2b) and [Sn L](BF ) (3b), respectively. The presence of the two BF in 3b was
2
4 2
4
1
9
confirmed by F NMR using AgBF as an internal standard (Figure S2).
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 1. Synthesis of porphyrinic MOFs.
UNLPFꢀ10a and ꢀ10b. Two UNLPFꢀ10 samples, UNLPFꢀ10a and ꢀ10b, which contain ~0%
III
and ~100% In ꢀporphyrin moieties, respectively, were prepared via a solvothermal reaction of
III
In(NO ) ꢁH O and 1b in a N,Nꢀdimethylformamide (DMF)/acetic acid mixture using In /ligand
3
3
2
ratios of 2:1 and 50:1, respectively (see Supporting Information for detailed procedures). Note
III
III
that a high In /ligand ratio (50:1) is essential here to achieve a complete In ꢀmetalation in
UNLPFꢀ10b.
UNLPFꢀ11 and ꢀ12. UNLPFꢀ11 and ꢀ12 were synthesized as dark red octahedral crystals
IV
IV
(
Figure S3) by a similar reaction of In(NO ) ꢁH O with Sn Cl L (2b) and [Sn L](BF ) (3b),
3 3 2 2 4 2
respectively (see Supporting Information for detailed procedures). Notably, the synthesis of
III
UNLPFꢀ11 and ꢀ12 is highly sensitive to the In /ligand ratio: two equivalents of In(NO ) ꢁH O
3
3
2
1
are required to achieve a good product yield and crystallinity. H NMR spectra of the acid
ACS Paragon Plus Environment
6

Page 7 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
digested samples of UNLPFꢀ11 and ꢀ12 confirm that all porphyrin moieties are coordinated with
IV
Sn and the possible demetalation or metal ion exchange during the solvothermal synthesis does
–
not occur (Supporting Information, Figure S4). It should be noted that BF is not present within
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
19
19
the framework of UNLPFꢀ12: no signal of F was observed in the F NMR spectrum of an
–
acidꢀdigested sample of UNLPFꢀ12 (Figure S5). Presumably, BF4 remains in the original
1
9
mother liquor in the solvothermal synthesis: the F NMR spectrum indeed revealed a strong
signal that can be attributed to [Me NH ]BF (Figure S5).
2
2
4
Single crystal Xꢀray structural analysis reveals UNLPFꢀ11 crystalizes in the orthorhombic
21b
space group Pnnm and is isostructural to UNLPFꢀ10. Overall, the framework can be viewed as
2
1b,25
the close packing of 14ꢀfaced Williams βꢀtetrakaidecahedral cages.
Each cage is comprised
IV
–
of six Sn Cl L ligands and eight [In(COO) ] SBUs (secondary building units) (Figure 1a), and
2
4
is enclosed by two square faces (two porphyrin planes), eight pentagonal faces (formed by
IV
perpendicularly oriented Sn Cl L ligands), and four hexagonal faces (formed by parallelly
2
IV
oriented Sn Cl L ligands). Notably, pentagonal shaped oneꢀdimensional channels (1.1 Å × 1.1
2
Å) along the cꢀaxis are beneficial for efficient mass transport, a good attribute for catalysis
(Figure 1b). The phase purity of UNLPFꢀ11 is confirmed by the powder Xꢀray diffraction pattern
of a bulk sample, which matches well with the simulated pattern (Figure 1c). Likewise, the
powder Xꢀray diffraction pattern of UNLPFꢀ12 is also in good accordance to the simulated
pattern (Figure 1c), indicating the isostructural nature. Additionally, Xꢀray photoemission
IV
spectroscopy (XPS) confirmed the presence of Sn in UNLPFꢀ12 with binding energies of 485.6
eV (3d ) and 494.2 eV (3d ) (Figure S7a).
5/2
3/2
ACS Paragon Plus Environment
7

ACS Catalysis
Page 8 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
IV
−
Figure 1. (a) Illustration of the octatopic ligand Sn Cl L and [In(COO) ] SBU forming the
2
4
Williams βꢀtetrakaidecahedral cage in UNLPFꢀ11. (b) Crystal structure of UNLPFꢀ11 showing
the oneꢀdimensional channel along crystallographic c axis. (c) Simulated and experimental
powder Xꢀray diffraction patterns. (d) Controlling the charge density of UNLPFꢀ10a, ꢀ10b, ꢀ11
and ꢀ12 via metalation of porphyrins (charges are shown with respect to cage occupancy in the
overall framework).
–
We have previously shown that the presence of [In(COO) ] SBUs renders UNLPFꢀ10 an
4
+
26
anionically charged framework with encapsulated mobile counter cations (Me NH ).
2
2
III
Moreover, the overall framework charge density is tunable by controlling the extent of In ꢀ
III
metalation. Since the In ꢀporphyrin moiety exhibits a “+1” charge, the effective charge density
per cage in UNLPFꢀ10a and UNLPFꢀ10b are “–2” and “–1”, respectively (Figure 1d). Similarly,
IV
in UNLPFꢀ11, the porphyrin metal center Sn Cl gives way to a chargeꢀneutral macrocycle and
2
thus the effective charge density per cage is “–2” (Figure 1d). Accordingly, the “+2” charged
IV
Sn ꢀporphyrin centers in UNLPFꢀ12 results in a net neutral framework (Figure 1d). Indeed, dye
ACS Paragon Plus Environment
8

Page 9 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
adsorption experiments confirmed the predicted charge densities. UNLPFꢀ10a, 10b, and ꢀ11
adsorb approximately two, one, and two equivalents of cationic methylene blue within 24 h,
respectively, and do not adsorb the anionic dye, acid orange 7 (Supporting Information, Figure
S8aꢀc). Conversely, the chargeꢀneutral UNLPFꢀ12 shows minimal adsorption for either the
cationic or anionic dye (Figure S8d).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Spectroscopic and electrochemical characterization.
The UVꢀvis absorption and fluorescence emission spectra of UNLPFꢀ10a, ꢀ10b, ꢀ11, and ꢀ12
were measured and depicted in Figure 2. In order to make proper peak assignments, we also
synthesized and measured the absorption and fluorescence emission spectra of H TPP (mesoꢀ
2
2
7
28
29
tetraphenylporphyrin) and its metal complexes including InClTPP, In(OAc)TPP, SnCl TPP,
2
23a
and [SnTPP](BF4)2 as the model compounds (Supporting Information, Figures S9 and S10).
The absorption spectrum of UNLPFꢀ10a (Figure 2a and Table 1) shows similar features as that
of H TPP (Figure S9), including a Soret band (B(0,0), 416 nm) and four Q bands in the visible
2
region corresponding to πꢀπ* transitions: Q (1,0), 519 nm; Q (0,0), 562 nm; Q (1,0), 596 nm;
y
y
x
and Q (0,0), 641 nm. The electronic absorption spectra of UNLPFꢀ10b, ꢀ11, and ꢀ12 (Figure 2bꢀd
x
and Table 1) display broad Soret bands (B(0,0)) with peak maxima around 430 nm and two
weaker, less wellꢀdefined Q bands near 560 and 600 nm, respectively. Based on the absorption
spectra of their model compounds (Figure S9), we assign the higher energy Q band as the Q(1,0)
transition (Table 1): the peak positions of Q(1,0) band are at 560, 565, and 556 nm when
III
IV
IV
porphyrin macrocycle is coordinated to In , [Sn Cl ], and Sn , respectively. It is noted that the
2
–
IV
–1
coordination of Cl in [Sn Cl ] causes a small red shift (~270 cm ) of Q(1,0) band in both
2
model compounds (SnCl TPP and [SnTPP](BF ) ) and MOFs (UNLPFꢀ11 and UNLPFꢀ12)
2
4 2
30
(
Figure 2c,d and Table 1), consistent with previous observations. Similarly, according to the
ACS Paragon Plus Environment
9

ACS Catalysis
Page 10 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
fluorescence emission spectra obtained from model compounds, the emission peak around 650
3
1
nm is assigned as Q(0,0) for UNLPFꢀ10a and Q(0,1) for UNLPFꢀ10b, ꢀ11, and ꢀ12. Based on
these spectroscopic analysis, we were able to estimate the relevant zeroꢀzero vibrational state
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0ꢀ0
excitation energy (E ) for UNLPFꢀ10a (1.91 V, intercept of the normalized absorption and
emission of Q(0,0) state) and for UNLPFꢀ10b, ꢀ11, and ꢀ12 (2.05, 2.04, and 2.07 V, respectively,
medium wavelengths between corresponding absorbance Q(1,0) and emission Q(0,1) state)
(Table 1).
Figure 2. Steady state electronic UVꢀvis absorption (blue) and fluorescence emission (green)
spectra for UNLPFꢀ10a (a), ꢀ10b (b), ꢀ11 (c), and ꢀ12 (d).
ACS Paragon Plus Environment
10

Page 11 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
Cyclicꢀvoltammetry was used to determine the ground state oxidation and reduction potentials
for model compounds as well as four porphyrinic MOFs. The measured reduction and oxidation
3
2
33
34
35
couples of H TPP, InClTPP, In(OAc)TPP, and SnCl TPP were consistent with previously
2
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
reported values (Table 1 and Supporting Information, Figure S11). Briefly, all model compounds
undergo two reversible singleꢀelectron reductions and two reversible singleꢀelectron oxidations,
–
−
which are porphyrinꢀcentered. Notably, the E (M/M ) (describing the halfꢀreaction M + e →
1
/2
–
M ) for all four metalated porphyrins are less negative compared to that of H TPP (Table 1).
2
–
Moreover, as the valence of the coordinated metal ion increases from 3+ to 4+, the E (M/M )
1
/2
III
III
anodically shifts from –1.06/–1.07 V (In ClTPP/In (OAc)TPP) to –0.80/–0.76 V
IV
IV
+
(
Sn Cl TPP/[Sn TPP](BF ) ). Likewise, the E (M /M) anodically shifts from +1.21/+1.16 V
2 4 2 1/2
III
III
IV
IV
(In ClTPP/In (OAc)TPP) to +1.43/+1.44 V (Sn Cl TPP/[Sn TPP](BF ) ). Combining the
2
4 2
+
spectroscopic and electrochemical data, the excitedꢀstate reduction potentials, E (M /*M)
1
/2
+
−
−
(
(
(
describing the halfꢀreaction M + e → *M, “*” denotes the excited state) and E (*M/M )
1/2
−
–
describing the halfꢀreaction *M + e → M ) of the model compounds, have been estimated
Table 1). It is clear that the strong electron withdrawing effect induced by high valent metal
cations on the porphyrin ring stabilizes both HOMO (highest occupied molecular orbital) and
LUMO (the lowest unoccupied molecular orbital) energy levels, leading to the reduction of
porphyrin macrocycle occurring at a less negative potential. Accordingly, the photoexcited
–
porphyrin is transformed from the weakly oxidizing H TPP (E (M*/M ) = +0.68 V) to
2
1/2
–
moderately oxidizing InClTPP/In(OAc)TPP (E (*M/M ) = +0.98/+0.97 V), and to strongly
1
/2
–
oxidizing SnCl TPP/[SnTPP](BF ) (E (*M/M ) = +1.23/+1.30 V).
2
4 2
1/2
ACS Paragon Plus Environment
11

ACS Catalysis
Page 12 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Table 1. Photophysical and electrochemical parameters of porphyrinic MOFs and their model
compounds.
[
a]
[a]
Absorption (nm)
Emission (nm)
0ꢀ0
–
+
+
–
E
E
1/2(M/M )
(V)
E
1/2(M /M)
(V)
E
1/2(M /*M)
(V)
E
1/2(*M/M )
(V)
Photocatalyst
[c]
[c]
[c], [d]
[c], [e]
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(V)
B(0,0) Q(1,0)
Q(0,0)
Q(0,0) Q(0,1)
[b]
[b]
5
14
547
649
H
2
TPP
416
[b]
[b]
656
714
1.90
–1.22
+1.01
–0.89
+0.68
590
557
558
560
552
InClTPP
423
423
426
417
597
598
600
590
609
606
617
603
657
655
659
651
2.04
2.04
2.03
2.06
–1.06
–1.07
–0.80
–0.76
+1.21
+1.16
+1.43
+1.44
–0.83
–0.88
–0.60
–0.62
+0.98
+0.97
+1.23
+1.30
In(OAc)TPP
2
SnCl TPP
[
SnTPP](BF )
4 2
[b]
[b]
[b]
5
588
16
550
647
1
a
421
426
[b]
655
656
705
714
1.90
1.91
–1.23
–1.08
+1.02
ꢀ
–0.88
ꢀ
+0.67
+0.83
[
b]
[b]
5
5
19
96
562
641
UNLPFꢀ10a
[b]
[b]
UNLPFꢀ10b
UNLPFꢀ11
UNLPFꢀ12
429
430
431
560
565
556
600
598
591
606
607
593
648
648
642
2.05
2.04
2.07
–0.80
–0.71
–0.65
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
+1.25
+1.33
+1.42
[
a] In this notation, the numbers in parentheses are number of vibrational quanta in the excited and ground state states, respectively. [b] Split
+ − +
due to the reduced symmetry of free base porphyrins compared to their metalated counterparts. [c] E1/2(M /*M), E1/2(*M/M ), E1/2(M /M),
and E1/2(M/M ) describe halfꢀreactions M + e → *M, *M + e → M , M + e → M, and M + e → M , respectively. [d] E1/2(M /*M) = E
– E1/2(M /M). [e] E1/2(*M/M ) = E – E1/2(M/M ).
−
+
−
−
−
+
−
−
−
+
0ꢀ0
+
−
0ꢀ0
–
Cyclic voltammograms of the four porphyrinic MOFs exhibit weaker reduction current and
–
only the E (M/M ) were detectable and appeared less negative compared to those of their
1
/2
corresponding model compounds (Supporting Information, Figure S12): the anodic shifts are
0.09 V in UNLPFꢀ10a, ~0.26 V in UNLPFꢀ10b, and ~0.10 V in UNLPFꢀ11 and ꢀ12. We
tentatively attribute the observed anodic shift to the slight change in electronic structure of the
porphyrin ligand upon MOF formation, not the difference in substituents on porphyrin
macrocycle. Indeed, H TPP and its derivative 1a exhibit essentially the same spectroscopic and
2
−
electrochemical properties (Table 1 and Figure S9ꢀ11). The E1/2 (*M/M ) of four porphyrinic
MOFs were calculated to be +0.79 V, +1.25 V, +1.33 V, and +1.42 V for UNLPFꢀ10a, ꢀ10b, ꢀ11,
ACS Paragon Plus Environment
12

Page 13 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
and ꢀ12, respectively (Table 1). Compared to their model compounds, porphyrinic MOFs are
−
slightly more oxidative. In particular, with fairly positive E1/2 (*M/M ) values (+1.25 ꢀ 1.42 V),
UNLPFꢀ11b, ꢀ11, and ꢀ12 are expected to exhibit excellent activity in promoting organic
reactions via a reductive quenching pathway, where the excited state of the porphyrinic MOF
induces the oxidation of an electron donor. Here, we set out to test the activities of porphyrinic
MOFs in three representative photoredox catalytic organic transformations where an alkylamine
is used as either a sacrificial electron donor or an electronꢀrich substrate to initialize the
photoredox catalytic processes.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Oxidative hydroxylation of arylboronic acids.
3
6
We first used the aerobic oxidative hydroxylation of arylboronic acids to evaluate the
photocatalytic activities of porphyrinic MOFs. According to the previously proposed reaction
36
mechanism (Scheme 2), the excited photocatalyst first obtains an electron from a sacrificial
electron donor (e.g. N,Nꢀdiisopropylethylamine, DIPEA) and then reduces O in air to the
2
•–
36
•–
superoxide radical anion (O ). Further, O adds to the vacant pꢀorbital of boron leading to
2
2
the subsequent 1,2ꢀaryl shift and hydrolysis to produce the final phenolic product. Since the
+
37
–
38
E (M /M) of DIPEA is +0.90 V vs SCE and E (M/M ) of O is –0.86 V vs SCE (in DMF),
1
/2
1/2
2
–
–
photocatalysts with an E (*M/M ) higher than +0.90 V and E (M/M ) lower than –0.86 V are
1
/2
1/2
expected to oxidize the amine and lead to the subsequent O reduction.
2
ACS Paragon Plus Environment
13

ACS Catalysis
Page 14 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Scheme 2. Proposed mechanism for the photocatalytic oxidative hydroxylation of arylboronic
acids.
We systematically examined the photocatalytic activities of four porphyrinic MOFs as well as
their model compounds toward the hydroxylation of 4ꢀformylbenzeneboronic acid (4a), which
generates 4ꢀformylphenol (5a) as the product (Table 2). It is found that the photocatalyst’s
–
efficiency is largely dictated by a thermodynamically favorable E (*M/M ) for amine
1
/2
oxidation. For example, due to its weak oxidizing capability toward DIPEA, H TPP gives
2
–
essentially no conversion (E (*M/M ) = +0.68 V vs SCE) (entry 1, Table 2). Conversely, both
1
/2
InClTPP and In(OAc)TPP finish the reaction within ~24 hours (entries 2 and 3, Table 2),
−
consistent with their higher E1/2 (*M/M ) (~+0.97 V vs SCE, Table 1). SnCl TPP and
2
−
[
SnTPP](BF ) with even stronger oxidizing capabilities (E1/2 (*M/M ) = +1.23/+1.30 V vs SCE,
4 2
Table 1) exhibit the highest efficiency among all model compounds, finishing the reaction within
only 12 hours (entries 4 and 5, Table 2). Surprisingly, porphyrinic MOFs exhibit significantly
higher efficiencies than their model compounds. For example, compared to the inactive H TPP,
2
ACS Paragon Plus Environment
14

Page 15 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
UNLPFꢀ10a gives an 87% conversion after 24 hours (entry 6, Table 2), which can be attributed
–
to its slightly more positive E (*M/M ) of + 0.83 V compared to that of H TPP (Table 1).
1/2
2
UNLPFꢀ10b and ꢀ11 exhibit even faster reaction rates, finishing the hydroxylation within 4 and
.5 hours, respectively (entries 7 and 8, Table 2). More remarkably, UNLPFꢀ12 reaches a full
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3
conversion within 2.5 hours with a 92% isolated yield (entry 9, Table 2) despite its inadequate
+
E (M /M) (–0.65 V vs SCE) to reduce O . Therefore, the oxidation of the amine has a greater
1
/2
2
effect on the overall reaction rate compared to the reduction of O , which is reasonable
2
39
considering the amine’s higher concentration (~0.2 M) versus that of O (~1.31 mM in DMF).
2
•–
Indeed, the formation of O2 in the presence of DIPEA was confirmed via EPR (electron
paramagnetic resonance) spectroscopy by employing the superoxide radical trapping agent
DMPO (5,5ꢀdimethylꢀ1ꢀpyrroline Nꢀoxide) (Figure S17).
Table 2. Screening photocatalysts for oxidative hydroxylation of 4ꢀformylbenzeneboronic
[
a]
acid.
Photocatalyst
(0.5 mol%)
OH
OHC
OH
OHC
B
OH
DIPEA (2.0 eq)
DMF, air, hv
4a
5a
[b]
entry
Photocatalyst
TPP
t (h)
24
24
24
12
10
24
4
% yield
1
2
3
4
5
6
7
8
9
H
2
trace
97
InClTPP
In(OAc)TPP
94
SnCl
2
TPP
96
[SnTPP](BF
4
)
2
>99
87
UNLPFꢀ10a
UNLPFꢀ10b
UNLPFꢀ11
UNLPFꢀ12
UNLPFꢀ12
UNLPFꢀ12
UNLPFꢀ12
UNLPFꢀ12
>99
>99
>99
95
3.5
2.5
2.5
24
24
24
[
c]
1
1
1
0
1
2
[d]
n.r.
trace
trace
[
e]
f]
[
13
[
a] Reaction condition: photocatalyst (0.5 ꢂmol, 0.5 mol% based on porphyrin moiety), 4a (0.1
mmol), DIPEA (0.2 mmol), 1.0 mL dry DMF, 14 W CFL (distance app. 8 cm). [b] Determined
1
th
by H NMR. [c] After 4 recycle. [d] No photocatalyst. [e] No light. [f] Ar atmosphere.
ACS Paragon Plus Environment
15

ACS Catalysis
Page 16 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
However, a favorable photoredox potential alone cannot explain the dramatic difference in
reaction rates between model compounds with MOFs. In fact, the slower reaction rate of model
compounds is largely due to inferior photostability and severe decomposition under the reaction
conditions. We observed a significant color change following the hydroxylation of 4a. Indeed,
UVꢀvis absorption spectroscopy indicates a significant amount (>80%) of [SnTPP](BF4)2
decomposed after the reaction and new species that absorb at approximately 620 and 510 nm
formed (Figure 3a). Further reuse of [SnTPP](BF ) for a second time only gave a 12% yield
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
2
(Supporting Information, Figure S15). In contrast, UNLPFꢀ12 exhibits no sign of deactivation
even after five times of repeated reaction (entry 10, Table 2). The UVꢀvis spectrum (Figure 3b)
and powder Xꢀray diffraction pattern (Figure S16) of UNLPFꢀ12 after reuse exhibits no
significant structural change, also indicating its outstanding resistance to deactivation. It should
IV
be noted that the coordination environment and oxidation state of Sn porphyrin metal center are
preserved during the reaction as indicated by the XPS spectra (Figure S7c and Table S2) and
1
19
Sn NMR (Figure 3c) for a sample of UNLPFꢀ12 before and after photooxidative
hydroxylation of 4a.
ACS Paragon Plus Environment
16

Page 17 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. UVꢀvis absorption spectra of reaction mixtures of 4a, DIPEA, and [SnTPP](BF4)2
(
a) and UNLPFꢀ12 (b) in DMF before (black) and after (blue) photooxidative hydroxylation and
1
19
respective photographs (insets). (c)
Sn NMR spectra for acid (DCl:d ꢀDMSO, v/v=1:6)
6
digested samples of UNLPFꢀ12 before (black) and after (blue) photooxidative hydroxylation of
–
4
a. Note that Cl easily binds to the UNLPFꢀ12 Snꢀporphyrin centers in the presence of DCl, as
1
19
indicated by the similar Sn chemical shift of ligand 2b (red).
ACS Paragon Plus Environment
17

ACS Catalysis
Page 18 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Control experiments (entries 11ꢀ13, Table 2) confirmed the essential role of the photocatalyst,
light, and oxygen in this reaction. Due to its outstanding performance, we chose UNLPFꢀ12 as
the prototypic photocatalyst to study the reaction scope (Table 3). Arylboronic acids bearing
electronꢀwithdrawing (entries 1ꢀ7, Table 3) and electronꢀdonating (entries 8ꢀ11, Table 3)
substituents can all be smoothly converted into the corresponding aryl alcohols in good to
excellent yields (83~96%) within 2~10 hours. Generally, the reaction proceeds faster for
electronꢀdeficient arylboronic acids due to greater accessibility of the vacant boron pꢀorbital.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Notably, UNLPFꢀ12 is more efficient than common transition metal complex photocatalysts
2+
including [Ru(bpy) ] in promoting the hydroxylation of 4ꢀformylbenzeneboronic acid (entries
3
3
6a
13ꢀ15, Table 3).
[
a]
Table 3. Photocatalytic oxidative hydroxylation of arylboronic acids.
R2
R2
OH UNLPF-12, 0.5 mol%
R
1
B
OH
R
1
OH
DIPEA, 2.0 eq
DMF, Air, hv
R
3
R
3
4
a-l
5a-l
[
b]
entry
R
1
R
2
R
3
product
5a
5b
5c
t (h)
% yield
1
CHO
H
H
H
H
H
H
H
H
H
H
H
Me
H
H
H
H
H
2.5
3.5
2.0
2.5
6
>99 (92)
95 (94)
96 (89)
>99 (96)
95 (87)
98 (95)
97 (95)
94 (88)
95 (93)
88 (83)
93 (90)
96 (95)
21
2
CHO
H
3
CN
4
CO
2
Me
H
5d
5e
5
B(OH)
2
H
6
Cl
H
5f
4
7
H
Cl
H
5g
5h
5i
3.5
6
8
Me
9
H
Me
Me
H
6
10
H
5j
10
8
1
1
1
1
1
1
2
3
4
5
MeO
Ph
5k
5l
H
6
[
c]
d]
e]
CHO
CHO
CHO
H
5a
5a
5a
2.5
2.5
2.5
[
H
65
[
H
62
[
a] Reaction condition: UNLPFꢀ12 (0.5 ꢂmol, 0.5 mol% based on porphyrin moiety), 4 (0.1 mmol), DIPEA (0.2
2+
mmol), 2.5 mL dry DMF, 14 W CFL (distance app. 8 cm). [b] Isolated yields in parenthesis. [c] [Ru(bpy)
catalyst. [d] [Ir[dF(CF
trifluoromethylpyridine, dtbbpy = 4,4'ꢀdiꢀtertꢀbutylꢀ2,2'ꢀbipyridine). [e] [Ir[dF(CF
3
]
used as
+
3
)ppy]
2
(dtbbpy)]
used as catalyst (dF(CF
3
)ppy
=
2ꢀ(2,4ꢀdifluorophenyl)ꢀ5ꢀ
+
3
)ppy] (bpy)] used as catalyst.
2
ACS Paragon Plus Environment
18

Page 19 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
Oxidative amine coupling.
We next tested the activities of MOFꢀbased photocatalysts in the aerobic oxidation of primary
4
0
amines to yield imines, a widely studied model reaction. According to the typical electronꢀ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
41
transferꢀbased mechanism (Figure 4a), the excited photocatalyst first oxidizes the amine to
•–
generate a cationic amine radical and then reduces molecular oxygen to O . The subsequent
2
•–
reaction between the amine radical and O2 yields the imine intermediate, which undergoes a
nucleophilic addition with a second primary amine to give the coupled imine product (Figure
4a). Here, benzylamine (6a) was used to optimize the coupling reaction condition. First, the
photocatalyst, light irradiation, and air were found to be essential components for this
transformation (entries 3ꢀ5, Table 4). From a thermodynamic perspective, UNLPFꢀ10b, ꢀ11, and
ꢀ12 are expected to exhibit a good photocatalytic activity for this reaction since their E1/2
−
+
(
*M/M ) is more positive than the E1/2 (M /M) of benzylamine (~+1.23 V vs SCE, Figure S19).
As expected, UNLPFꢀ12 appears to be the most active photocatalyst among all porphyrinic
MOFs studied. The reaction of benzylamine in the presence of UNLPFꢀ12 (0.4 mol% based on
IV
Sn ꢀporphyrin) in CH CN exposed to air under irradiation with a 14W white CFL results in
3
imine 7a in 99% yield after 2 h (entry 1, Table 4), which is faster that than UNLPFꢀ11 (5.5 h)
and UNLPFꢀ10b (5 h) (entries 7 and 9, Table 4).
ACS Paragon Plus Environment
19

ACS Catalysis
Page 20 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 4. (a) Proposed mechanisms for the photocatalytic oxidative amine coupling reaction:
singlet oxygen (energy transfer) and superoxide radical anion (electron transfer) pathways. (b)
Plots of conversion of benzylamine versus time in oxidative amine coupling using porphyrinic
MOFs as photocatalysts in CH CN (open circle) and CD CN (solid circle).
3
3
ACS Paragon Plus Environment
20

Page 21 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
[a]
Table 4. Photocatalytic oxidative amine coupling.
UNLPF-12, 0.4 mol%
MeCN, Air, hv
Ar
NH2
a-h
Ar
N
7a-h
Ar
6
[
b]
[b]
entry
1
aryl
product
7a
7a
7a
7a
7a
7a
7a
7a
7a
7a
7a
7a
7b
7c
t (h)
2
% yield
>99
>99
trace
n.r.
% selectivity
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
>99
>99
ꢀ
[
[
[
c]
d]
e]
2
3
4
5
6
7
8
9
2
24
24
24
2
ꢀ
[f]
n.r.
ꢀ
[g]
98
97
>99
>99
99
>99
97
99
99
99
98
97
91
99
[
h]
h], [c]
i]
5.5
4.5
5
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
88
[
[
[
i], [c]
10
11
4.5
11.5
7.5
2
[j]
[j],[c]
1
1
1
1
1
1
2
3
4
5
6
7
4ꢀFꢀPh
4ꢀClꢀPh
4ꢀMeꢀPh
1.5
3
7d
7e
4ꢀMeOꢀPh
4ꢀPyridinyl
2ꢀFuranyl
4
7f
8
18
7g
10
[
a] Reaction condition: UNLPFꢀ12 (1.0 ꢂmol, 0.4 mol% based on porphyrin moiety), 6 (0.27
1
mmol), 1 mL dry CH
3
CN, 14W CFL (distance app. 8 cm). [b] Determined by H NMR, and
CN used as solvent. [d] No
the byproduct is the corresponding aryl aldehyde. [c] CD
3
th
photocatalyst. [e] No light. [f] Ar atmosphere. [g] After 4 recycle. [h] UNLPFꢀ11 used as
catalyst. [i] UNLPFꢀ10b used as catalyst. [j] UNLPFꢀ10a used as catalyst.
−
Interestingly, in spite of its inadequate E1/2 (*M/M ) (+0.79 V vs SCE), UNLPFꢀ10a also
exhibits a good catalytic activity albeit with a slower reaction rate: a 99% conversion with 97%
selectivity was achieved within 11.5 h (entry 11, Table 4). Therefore, the observed activity of
UNLPFꢀ10a should not be attributed, at least solely, to the electron transfer mechanism. In fact,
1
an alternative mechanism based on the singlet oxygen ( O ) induced amine oxidation is also
2
4
2
operable. In this mechanism, a photosensitizer that can efficiently generate highly oxidizing
1
singlet oxygen ( O ) via energy transfer is necessary to form the essential phenylmethanimine
2
intermediate (Figure 4a). Indeed, using EPR experiments where TEMP (2,2,6,6ꢀ
ACS Paragon Plus Environment
21

ACS Catalysis
Page 22 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
tetramethylpiperidine) was employed as the trapping agent, we found that UNLPFꢀ10a can
1
1
efficiently generate O (Supporting Information, Figure S21). Further, since the lifetime of O
2
2
4
3
is generally longer in deuterated solvents, an increase of reaction rate in deuterated solvents
compared to their protonated counterparts enhanced in the presence of UNLPFꢀ10a (99%, 7.5 h)
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
compared to in CH CN (99%, 11.5 h) (entries 11 and 12, Table 4 and Figure 4b). Conversely, no
3
significant solvent effect for UNLPFꢀ12, ꢀ11, and 10b was observed (entries 1, 2, 7ꢀ10, Table 4
and Figure 4b). This result indicates that for reactions catalyzed by UNLPFꢀ12, ꢀ11, and 10b, the
reaction primarily involves the photoinduced electron transfer pathway. It should be noted,
however, the electron transfer pathway cannot be completely ruled out for UNLPFꢀ10a. Indeed,
EPR spectra indicated that when DMPO was present in an aerated CH CN solution of
3
benzylamine and UNLPFꢀ10a, weak signals were observed upon light irradiation (Figure S22),
•–
which suggests UNLPFꢀ10a is weakly capable of generating O2 to initiate the amine coupling
via photoredox reaction pathway.
The reaction scope was studied using UNLPFꢀ12 as the photocatalyst. Photoꢀoxidative
coupling of benzylamine and its derivatives generally proceed smoothly with high conversion
(>88%) and high selectivity (>91%) (entries 13ꢀ16, Table 4). The substituent on the phenyl ring
has an insignificant effect on the reaction rate and selectivity. Additionally, this photocatalytic
system is tolerant to substrates containing heteroatoms (such as O, S, and N) on the aromatic ring
(entries 17–19, Table 4) with only a small decrease in conversion, consistent with previous
41a
results where TiO was used as the photocatalyst. Notably, the photocatalytic efficiency of
2
UNLPFꢀ12 for the coupling of primary amine 6a is comparable to those promoted by common
transition metal complexes (Supporting Information, Table S3). UNLPFꢀ12 also exhibits
ACS Paragon Plus Environment
22

Page 23 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
excellent recyclability: no diminished conversion was observed after four times of reuse (entry 6,
Table 4) and the recovered catalyst retained the crystalline integrity (Figure S20).
Aerobic Photooxidative Mannich Reaction.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Lastly, to demonstrate their wider utility, we examined the catalytic activities of porphyrinic
4
4
MOFs in the wellꢀstudied photooxidative Mannich reactions. This dehydrogenation crossꢀ
coupling reaction involves the catalytic oxidation of αꢀamino CꢀH bonds to generate reactive
iminium ions and subsequent CꢀC bond formation between the iminium ions and a carbon
nucleophile. A typical Mannich reaction involves the irradiation of an aerated mixture containing
photocatalyst, tertiary amine, acetone and Lꢀproline (to form the enamine nucleophile). An
excellent conversion of 98% was achieved after 7 h when UNLPFꢀ12 and 8a was employed as
the photoredox catalyst and tertiary amine, respectively; the reaction rate is again faster than
those of the other three porphyrinic MOFs (entries 1ꢀ4, Table 5) and many common transitionꢀ
metal complex photocatalysts (Supporting Information, Table S4). The photocatalyst, light, and
air were confirmed to be integral to this transformation (entries 5ꢀ7, Table 5). UNLPFꢀ12 also
exhibits an excellent recyclability (entry 8, Table 5) without losing crystallinity (Figure S25). We
also examined the reaction scope using UNLPFꢀ12 as the photocatalyst under the optimized
condition (entries 9ꢀ13, Table 5). In general, the crossꢀdehydrogenative coupling products with
acetone were obtained in good to excellent yields (87ꢀ98%).
ACS Paragon Plus Environment
23

ACS Catalysis
Page 24 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
[a]
Table 5. Photocatalytic oxidative Mannich reaction.
R
R
R
R
O
UNLPF-12, 0.4 mol%
+
N
N
1
0 mol% L-proline
MeCN, Air, hv
Ar
Ar
O
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
8a-f
9a-f
[
b]
entry
R
H
H
H
H
H
H
H
H
H
H
Ar
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
product
9a
t (h)
48
15
12
7
% yield
83
[
c]
1
2
3
4
5
6
7
8
9
[d]
[e]
9a
92
9a
97
9a
98
[f]
9a
7
trace
trace
trace
96
[g]
9a
7
[h]
9a
7
[i]
9a
7
4ꢀBrꢀPh
4ꢀMeOꢀPh
Ph
9b
9c
7
98
10
8
87
1
1
1
1
2
3
MeO
MeO
MeO
9d
9e
7
96
4ꢀBrꢀPh
4ꢀMeOꢀPh
8
95
9f
9
89
[
a] Reaction condition: UNLPFꢀ12 (1 ꢂmol, 0.4 mol% based on porphyrin moiety), 8
(0.25 mmol), acetone (0.25 mmol), Lꢀproline (0.025 mmol, 10 mol%), 6.0 mL dry
1
CH
bromoacetophenone as an internal standard. [c] UNLPFꢀ10a used as catalyst. [d] UNLPFꢀ
0b used as catalyst. [e] UNLPFꢀ11 used as catalyst. [f] No photocatalyst. [g] No light.
3
CN, 14W CFL (distance app. 8 cm). [b] Determined by H NMR with 2ꢀ
1
th
[h] Ar atmosphere. [i] After 4 recycle.
According to the proposed mechanism (Supporting Information, Scheme S2), photoexcited
UNLPFꢀ12 oxidizes a tertiary amine (e.g. 8a) into a radical cation species and the reduced
•–
UNLPFꢀ12 promotes the reduction of O to O . The radical cation species is then deprotonated
2
2
•–
by O2 to form the highly reactive iminium ion, which enters the organocatalytic cycle and
reacts with the enamine nucleophile generated by Lꢀprolineꢀactivated acetone to give the desired
•–
crossꢀcoupling product. The formation of O2 was confirmed by EPR spectroscopy. UNLPFꢀ12
•–
alone was not able to generate O ; however, in the presence of 8a, DMPO, air, and UNLPFꢀ12,
2
•–
a strong radical signal was observed, confirming the formation of O2 under the reaction
condition (Figure S24).
ACS Paragon Plus Environment
24

Page 25 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
Conclusions
We have synthesized four isostructural porphyrinic MOFs and investigated their photoredox
catalytic activities toward three representative organic transformations including aerobic
hydroxylation of arylboronic acids, oxidative primary amine coupling, and the Mannich reaction.
Compared to their molecular model compounds, porphyrinic MOFꢀbased photocatalysts exhibit
a considerably enhanced photostability and excellent recyclability. Most importantly, metalation
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
III
IV
with highꢀvalent metal cations (In and Sn ) significantly modifies the electronic structure of
the porphyrin and provides a highly oxidizing photoexcited state that undergoes efficient
reductive quenching processes to facilitate subsequent organic transformations. Porphyrin
metalation indeed provides a convenient approach to fineꢀtune and optimize the photoredox
catalytic activities of MOFs.
Supporting Information. Materials, general procedure, synthesis of UNLPFꢀ11, ꢀ12,
crystallographic data, spectroscopic and electrochemical characterization, photocatalytic study.
This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
jzhang3@unl.edu
Notes
The authors declare no competing financial interest.
ACS Paragon Plus Environment
25

ACS Catalysis
Page 26 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ACKNOWLEDGMENT
The authors acknowledge the support from the University of NebraskaꢀLincoln. This work is
also partially supported by the donors of the American Chemical Society Petroleum Research
Fund (53678ꢀDNI10). Use of the Xꢀray photoemission spectrometer was supported by National
Science Foundation through the Nebraska MRSEC (DMRꢀ1420645). ChemMatCARS Sector 15
is principally supported by the Divisions of Chemistry (CHE) and Materials Research (DMR),
National Science Foundation, under grant number NSF/CHEꢀ1346572. Use of the Advanced
Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE
under Contract No. DEꢀAC02ꢀ06CH11357.
ACS Paragon Plus Environment
26

Page 27 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
REFERENCES
1) (a) Ciamician, G. Science 1912, 36, 385ꢀ394; (b) Fagnoni, M.; Dondi, D.; Ravelli, D.;
Albini, A. Chem. Rev. 2007, 107, 2725ꢀ2756; (c) Hoffmann, N. Chem. Rev. 2008, 108, 1052ꢀ
103; (d) Ravelli, D.; Fagnoni, M.; Albini, A. Chem. Soc. Rev. 2013, 42, 97ꢀ113.
(
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
(
2) (a) Yoon, T. P.; Ischay, M. A.; Du, J. Nat. Chem. 2010, 2, 527ꢀ532; (b) Narayanam, J.
M.; Stephenson, C. R. Chem. Soc. Rev. 2011, 40, 102ꢀ113; (c) Prier, C. K.; Rankic, D. A.;
MacMillan, D. W. Chem. Rev. 2013, 113, 5322ꢀ5363; (d) Schultz, D. M.; Yoon, T. P. Science
2014, 343, 1239176.
(3) Zhang, T.; Lin, W. Chem. Soc. Rev. 2014, 43, 5982ꢀ5993.
(4) (a) Long, J. R.; Yaghi, O. M. Chem. Soc. Rev. 2009, 38, 1213ꢀ1214; (b) Zhou, H. C.;
Long, J. R.; Yaghi, O. M. Chem. Rev. 2012, 112, 673ꢀ674; (c) Furukawa, H.; Cordova, K. E.;
O'Keeffe, M.; Yaghi, O. M. Science 2013, 341, 1230444; (d) Wang, C.; Liu, D.; Lin, W. J. Am.
Chem. Soc. 2013, 135, 13222ꢀ13234; (e) Foo, M. L.; Matsuda, R.; Kitagawa, S. Chem. Mater.
2
014, 26, 310ꢀ322.
(5) (a) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D. W. Chem. Rev. 2012, 112, 782ꢀ835; (b)
Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.
H.; Long, J. R. Chem. Rev. 2012, 112, 724ꢀ781; (c) He, Y.; Zhou, W.; Qian, G.; Chen, B. Chem.
Soc. Rev. 2014, 43, 5657ꢀ5678; (d) An, J.; Rosi, N. L. J. Am. Chem. Soc. 2010, 132, 5578ꢀ5579.
(
6) Li, J. R.; Sculley, J.; Zhou, H. C. Chem. Rev. 2012, 112, 869ꢀ932.
7) Wang, C.; Zhang, T.; Lin, W. Chem. Rev. 2012, 112, 1084ꢀ1104.
(
ACS Paragon Plus Environment
27

ACS Catalysis
Page 28 of 34
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
8) (a) Della Rocca, J.; Lin, W. Eur. J. Inorg. Chem. 2010, 2010, 3725ꢀ3734; (b) Cui, Y.;
Yue, Y.; Qian, G.; Chen, B. Chem. Rev. 2012, 112, 1126ꢀ1162; (c) Kreno, L. E.; Leong, K.;
Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Chem. Rev. 2012, 112, 1105ꢀ1125;
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
d) Hu, Z.; Deibert, B. J.; Li, J. Chem. Soc. Rev. 2014, 43, 5815ꢀ5840.
(9) (a) Lin, W.; Rieter, W. J.; Taylor, K. M. Angew. Chem. Int. Ed. 2009, 48, 650ꢀ658; (b)
Horcajada, P.; Chalati, T.; Serre, C.; Gillet, B.; Sebrie, C.; Baati, T.; Eubank, J. F.; Heurtaux, D.;
Clayette, P.; Kreuz, C.; Chang, J. S.; Hwang, Y. K.; Marsaud, V.; Bories, P. N.; Cynober, L.;
Gil, S.; Ferey, G.; Couvreur, P.; Gref, R. Nat. Mater. 2010, 9, 172ꢀ178; (c) Lu, K.; He, C.; Lin,
W. J. Am. Chem. Soc. 2014, 136, 16712ꢀ16715.
(
10) (a) Ma, L.; Abney, C.; Lin, W. Chem. Soc. Rev. 2009, 38, 1248ꢀ1256; (b) Lee, J.; Farha,
O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Chem. Soc. Rev. 2009, 38, 1450ꢀ
459; (c) Yoon, M.; Srirambalaji, R.; Kim, K. Chem. Rev. 2012, 112, 1196ꢀ1231; (d) Liu, J.;
1
Chen, L.; Cui, H.; Zhang, J.; Zhang, L.; Su, C. Y. Chem. Soc. Rev. 2014, 43, 6011ꢀ6061; (e)
Dhakshinamoorthy, A.; Garcia, H. Chem. Soc. Rev. 2014, 43, 5750ꢀ5765.
(11) (a) Kataoka, Y.; Sato, K.; Miyazaki, Y.; Masuda, K.; Tanaka, H.; Naito, S.; Mori, W.
Energy Environ. Sci. 2009, 2, 397; (b) Gomes Silva, C.; Luz, I.; Llabres i Xamena, F. X.; Corma,
A.; Garcia, H. Chem.ꢀEur. J. 2010, 16, 11133ꢀ11138; (c) Horiuchi, Y.; Toyao, T.; Saito, M.;
Mochizuki, K.; Iwata, M.; Higashimura, H.; Anpo, M.; Matsuoka, M. J. Phys. Chem. C 2012,
1
16, 20848ꢀ20853; (d) He, J.; Yan, Z.; Wang, J.; Xie, J.; Jiang, L.; Shi, Y.; Yuan, F.; Yu, F.;
Sun, Y. Chem. Commun. 2013, 49, 6761ꢀ6763; (e) Pullen, S.; Fei, H.; Orthaber, A.; Cohen, S.
M.; Ott, S. J. Am. Chem. Soc. 2013, 135, 16997ꢀ17003; (f) He, J.; Wang, J.; Chen, Y.; Zhang, J.;
Duan, D.; Wang, Y.; Yan, Z. Chem. Commun. 2014, 50, 7063ꢀ7066; (g) Wen, M.; Mori, K.;
ACS Paragon Plus Environment
28

Page 29 of 34
ACS Catalysis
1
2
3
4
5
6
7
8
9
Kamegawa, T.; Yamashita, H. Chem. Commun. 2014, 50, 11645ꢀ11648; (h) Nasalevich, M. A.;
Becker, R.; RamosꢀFernandez, E. V.; Castellanos, S.; Veber, S. L.; Fedin, M. V.; Kapteijn, F.;
Reek, J. N. H.; van der Vlugt, J. I.; Gascon, J. Energy Environ. Sci. 2015, 8, 364ꢀ375.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(
12) (a) Li, L.; Zhang, S.; Xu, L.; Wang, J.; Shi, L.ꢀX.; Chen, Z.ꢀN.; Hong, M.; Luo, J. Chem.
Sci. 2014, 5, 3808; (b) Wang, D.; Huang, R.; Liu, W.; Sun, D.; Li, Z. ACS Catal. 2014, 4, 4254ꢀ
4
260.
(13) Nasalevich, M. A.; van der Veen, M.; Kapteijn, F.; Gascon, J. CrystEngComm 2014, 16,
4
919.
(14) (a) Laurier, K. G.; Vermoortele, F.; Ameloot, R.; De Vos, D. E.; Hofkens, J.; Roeffaers,
M. B. J. Am. Chem. Soc. 2013, 135, 14488ꢀ14491; (b) Wang, C.ꢀC.; Li, J.ꢀR.; Lv, X.ꢀL.; Zhang,
Y.ꢀQ.; Guo, G. Energy Environ. Sci. 2014, 7, 2831.
(15) (a) Long, J.; Wang, S.; Ding, Z.; Wang, S.; Zhou, Y.; Huang, L.; Wang, X. Chem.
Commun. 2012, 48, 11656ꢀ11658; (b) Nasalevich, M. A.; Goesten, M. G.; Savenije, T. J.;
Kapteijn, F.; Gascon, J. Chem. Commun. 2013, 49, 10575ꢀ10577; (c) Sun, D.; Ye, L.; Li, Z.
Appl. Catal., B 2015, 164, 428ꢀ432.
(16) (a) Wang, C.; deKrafft, K. E.; Lin, W. J. Am. Chem. Soc. 2012, 134, 7211ꢀ7214; (b)
Zhou, T.; Du, Y.; Borgna, A.; Hong, J.; Wang, Y.; Han, J.; Zhang, W.; Xu, R. Energy Environ.
Sci. 2013, 6, 3229; (c) Toyao, T.; Saito, M.; Dohshi, S.; Mochizuki, K.; Iwata, M.; Higashimura,
H.; Horiuchi, Y.; Matsuoka, M. Chem. Commun. 2014, 50, 6779ꢀ6781.
(17) Wang, C.; Xie, Z.; deKrafft, K. E.; Lin, W. J. Am. Chem. Soc. 2011, 133, 13445ꢀ13454.
ACS Paragon Plus Environment
29