Metal-Organic Layers for Synergistic Lewis Acid and Photoredox Catalysis

Article abstract of DOI:10.1021/jacs.9b12593

We report the design of a new multifunctional metal-organic layer (MOL), Hf₁₂-Ir-OTf, comprising triflate (OTf)-capped Hf₁₂ secondary building units (SBUs) and photosensitizing Ir(DBB)[dF(CF₃)ppy]₂⁺ [DBB-Ir-F, DBB = 4,4′-di(4-benzoato)-2,2′-bipyridine; dF(CF₃)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine] bridging ligands. Hf₁₂-Ir-OTf effectively catalyzed dehydrogenative cross-couplings of heteroarenes with ethers, amines, and unactivated alkanes with turnover numbers of 930, 790, and 950, respectively. Hf₁₂-Ir-OTf also competently catalyzed late-stage functionalization of bioactive and drug molecules such as caffeine, Fasudil, and Metyrapone. The superior catalytic performance of Hf₁₂-Ir-OTf over a mixture of photoredox catalyst and stoichiometric amounts of Br?nsted acids or substoichiometric amounts (20 mol %) of Lewis acids is attributed to the close proximity (1.2 nm) between photoredox and Lewis acid catalysts in Hf₁₂-Ir-OTf, which not only facilitates the reaction between the carbon radical and the activated heteroarene but also accelerates the electron transfer from the nitrogen radical intermediate to the Ir(IV) species in the catalytic cycle.

Full text of DOI:10.1021/jacs.9b12593

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Communication
Metal-Organic Layers for Synergistic Lewis Acid and Photoredox Catalysis
Yangjian Quan, Guangxu Lan, Yingjie Fan, Wenjie Shi, Eric You, and Wenbin Lin
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Metal-Organic Layers for Synergistic Lewis Acid and Photoredox
Catalysis
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Yangjian Quan, Guangxu Lan, Yingjie Fan, Wenjie Shi, Eric You, and Wenbin Lin*
Department of Chemistry, The University of Chicago, Chicago, IL 60637, USA
Supporting Information Placeholder
ABSTRACT: We report the design of a new multifunctional
a) MOFs with internal Lewis acid sites
metal-organic layer (MOL), Hf12-Ir-OTf, comprising triflate (OTf)-
capped Hf12 secondary building units (SBUs) and photosensitizing
+
Ir(DBB)[dF(CF
,2'-bipyridine;
trifluoromethyl)pyridine] bridging ligands. Hf12-Ir-OTf effec-
3
)ppy]
2
[DBB-Ir-F, DBB = 4,4'-di(4-benzoato)-
2
(
dF(CF
3
)ppy 2-(2,4-difluorophenyl)-5-
=
tively catalyzed dehydrogenative cross-couplings of heteroarenes
with ethers, amines, and unactivated alkanes with turnover num-
bers of 930, 790, and 950, respectively. Hf12-Ir-OTf also compe-
tently catalyzed late-stage functionalization of bioactive and drug
molecules such as caffeine, Fasudil, and Metyrapone. The superior
catalytic performance of Hf12-Ir-OTf over a mixture of photoredox
catalyst and stoichiometric amounts of Brønsted acids or sub-stoi-
chiometric amounts (20 mol%) of Lewis acids is attributed to the
close proximity (1.2 nm) between photoredox and Lewis acid cat-
alysts in Hf12-Ir-OTf, which not only facilitates the reaction be-
tween the carbon radical and the activated heteroarene but also ac-
celerates the electron transfer from the nitrogen radical intermedi-
ate to the Ir(IV) species in the catalytic cycle.
b) MOLs with external Lewis acid sites
Metal-organic frameworks (MOFs) have provided a tunable po-
rous material platform for the design of heterogeneous catalysts
with crystalline structures and uniform, isolated, and well-defined
c) Synergistic Lewis acid and photoredox catalysis
1
-10
active sites. In particular, MOFs with coordinatively unsaturated
metal sites in their secondary building units (SBUs) have long been
11-16
recognized and utilized as solid Lewis acid catalysts.
However,
Synergistic
modest Lewis acidity of and unfavorable substrate access to SBUs
have limited the applications of MOFs as solid Lewis acid catalysts.
As Lewis acidity of MOFs has recently been drastically enhanced
by introducing perfluorinated ligands and via SBU modifica-
Lewis
acid
1
5-16
tions,
access to highly Lewis acidic SBUs remains a key hurdle
to widespread applications of MOF-based Lewis acid catalysts.
Metal-organic layers (MOLs), a free-standing monolayered ver-
sion of MOFs, have been recently developed as a new type of 2D
Photosensitizer
17-18
molecular materials.
MOLs are highly dispersible and have
completely accessible active sites in addition to inheriting the ad-
vantages of MOFs including structural regularity, compositional
C
F
N
Ir Hf
O
S
1
9-22
diversity, and stability.
We surmised that MOLs could provide
Figure 1. (a) Schematic showing incompletely accessible internal
Lewis acid sites in MOFs (labeled by black arrows) exemplified by
coordinatively unsaturated metals on the SBUs of MOF-808 (left)
and defect UiO-MOFs with missing linkers (right). (b) Schematic
showing freely accessible external Lewis acid sites in the SBUs of
Hf12 MOLs (labeled by black arrows). (c) The proximity of Lewis
acidic metal sites and photosensitizers in a MOL promotes syner-
gistic Lewis acid and photoredox catalysis.
an excellent 2D material platform to engineer highly effective het-
erogeneous catalysts with freely accessible Lewis acidic and other
2
3-25
catalytic sites.
Herein we report the design of a multifunctional
MOL, Hf12-Ir-OTf, consisting of triflate (OTf)-capped Hf12 SBUs
+
as a strong Lewis acid and Ir(DBB)[dF(CF
DBB = 4,4'-di(4-benzoato)-2,2'-bipyridine; dF(CF
difluorophenyl)-5-(trifluoromethyl)pyridine] ligands as a photore-
3
)ppy]
2
[DBB-Ir-F,
3
)ppy = 2-(2,4-
2
6-34
dox catalyst
for synergistic catalytic dehydrogenative cross-
heteroarenes and ethers, protected amines, or unactivated alkanes.
As a result of the proximity (~1.2 nm) between the two catalytic
couplings between
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centers, 0.1 mol% Hf12-Ir-OTf catalyzed dehydrogenative cross-
NMR analysis of the reaction supernatant revealed the formation of
trimethylsilyl trifluoroacetate (TMS-TFA) and H-TFA (Figure 2b
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coupling reactions to afford functionalized heteroarenes with turn-
over numbers of up to 950, outperforming homogeneously cata-
lyzed reactions with the employment of stoichiometric amounts of
1
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and S10-15, SI). F NMR analysis of digested Hf12-Ir-OTf showed
the presence of DBB-Ir-F bridging ligand and triflic acid (HOTf)
in a 1:1 molar ratio but the absence of H-TFA (Figures 2d and S18,
3
5
Brønsted acids or sub-stoichiometric amounts of Lewis acids.
1
9
SI). F NMR analysis further showed that Hf12-Ir-OTf was free of
HOTf after washing with benzene twice (Figure S16, SI).
Hf12-Ir-OTf was synthesized in two steps. Free-standing Hf12-Ir-
TFA was first prepared through a solvothermal reaction between
HfCl
4
and H
2
(DBB-Ir-F) in N,N-dimethylformamide (DMF) at 80
TEM and AFM imaging showed that Hf12-Ir-OTf retained the
monolayer structure of Hf12-Ir-TFA (Figures 2h and S19, SI). Hf12-
⁰
C using trifluoroacetic acid (H-TFA) and water as modulators. In
Hf12-Ir-TFA, Hf12-SBUs are laterally bridged by DBB-Ir-F ligands
and vertically terminated by trifluoroacetate (TFA) capping agents
to generate an infinite 2D network with the formula of Hf12(µ
Ir-OTf exhibited a similar PXRD pattern to that simulated from the
1
structural model of the Hf12 MOL (Figure 2c). H NMR analysis of
3
-
digested Hf12-Ir-OTf showed only one group of signals assignable
to DBB-Ir-F (Figure S17, SI). UV-Vis and luminescence spectra
indicated that Hf12-Ir-OTf possessed the same characteristic ab-
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O) (µ -OH) (µ -OH) (DBB-Ir-F) (TFA) (Figure S5, SI). The
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19
DBB-Ir-F to TFA ratio of 1:1 was confirmed by F NMR analysis
of digested Hf12-Ir-TFA (Figure S7, SI). Transmission electron mi-
croscopy (TEM, Figures 2e and S8, SI) and atomic force micros-
copy (AFM, Figure 2f) imaging verified the monolayer morphol-
ogy of Hf12-Ir-TFA with a diameter of ~300 nm and a thickness of
~1.7 nm (Figure 2g).
2
sorption, excitation, and emission peaks as Me DBB-Ir-F and Hf12-
Ir-TFA (Figure S20, SI).
We determined Lewis acidity of Hf12-Ir-OTf, Hf12-Ir-TFA, and
Sc(OTf) using the reported N-methylacridone (NMA) fluores-
cence method.
3
15-16
Upon excitation at 413.0 nm, NMA shows an
emission maximum (λmax) at 433.0 nm (Figure 3a, black). The λmax
of NMA red-shifts upon binding to a Lewis acid and the extent of
the red-shift correlates to the Lewis acidity. After binding to Hf12
Ir-OTf and Hf12-Ir-TFA, the λmax of NMA shifted to 472.8 and
67.7 nm (Figure 3a, blue and red), respectively. As a control, ho-
mogeneous Sc(OTf) displayed a NMA emission λmax at 472.6 nm
Figure 3a, green). These results indicate that Hf12-Ir-OTf pos-
-
4
3
(
sesses a similar Lewis acidity to Sc(OTf)
acidity than Hf12-Ir-TFA.
3
and much higher Lewis
The identical photophysical property of Hf12-Ir-OTf to
Me DBB-Ir-F suggests its photosensitizing ability. With the short-
2
est distance of 1.2 nm between the Lewis acid and photoredox sites
in Hf12-Ir-OTf (Figure S21, SI), we were intrigued if the proximity
of the two active sites could promote synergistic catalysis. We used
Minisci-type cross-couplings of heteroarenes as model reactions to
evaluate the synergistic catalytic performance of Lewis acid and
3
5
photoredox sites in Hf12-Ir-OTf. Functionalization of het-
eroarenes is of great significance owing to their prevalence in phar-
3
6-37
maceutical compounds and bioactive molecules.
Upon blue
LED irradiation at room temperature, 0.1 mol% Hf12-Ir-OTf effi-
ciently catalyzed dehydrogenative cross-couplings of heteroarenes
38
39
40-41
with ethers, amines, and unactivated alkanes
with turnover
numbers of up to 930, 790, and 950, respectively. Cyclic and acy-
clic ethers underwent coupling reactions with isoquinoline
smoothly to afford target products 1c-4c in 85-93% isolated yields
and with excellent regioselectivity. The addition of ethers at the C1
position of isoquinoline is likely due to the polar effects of Lewis
4
2
acid-bound isoquinoline. 2-Substituted quinoline afforded the
C4-alkylated product while 4-substituted quinoline resulted in the
functionalization at the C2 position (5c & 11c in Table 1). Pyridine
and phenanthridine also underwent coupling reactions with cyclic
ethers to afford 6c and 7c in high isolated yields.
Figure 2. Preparation and characterization of Hf12-Ir-OTf. (a) Re-
placement of TFA by OTF on Hf12-SBU. (b) F NMR spectrum of
the reaction supernatant and assignment of F peaks. (c) PXRD
patterns of Hf12-Ir-TFA and Hf12-Ir-OTf (freshly prepared or after
catalytic reaction) in comparison to the simulated pattern for the
Hf12 MOL. (d) F NMR spectrum of digested Hf12-Ir-OTf. The
molar ratio of DBB-Ir-F to HOTf is calculated as ~1:1. (e-g) TEM
image (e), AFM topography (f), and height profile (g) of Hf12-Ir-
TFA. (h) TEM image of Hf12-Ir-OTf shows its monolayer morphol-
ogy.
19
19
In the presence of 0.1 mol% Hf12-Ir-OTf, alkylamide and carba-
mate coupled with isoquinoline to give 9c and 10c in 79% and 74%
isolated yields, respectively. No coupling product was observed
when an electron-rich trialkylamine was used as the amine partner,
due probably to the overoxidation of trialkylamine to the iminium
cation which cannot react with Lewis acid-activated electron defi-
cient heteroarenes. Quinoline and benzothiazole also coupled with
carbamate to afford the corresponding products 11c and 12c in
good isolated yields.
1
9
The TFA groups in Hf12-Ir-TFA were completely replaced with
OTf groups via a metathesis reaction with trimethylsilyl triflate
(TMS-OTf) to afford Hf12-Ir-OTf of the formula Hf12(µ
Table 1. Hf12-Ir-OTf catalyzed dehydrogenative cross-coupling re-
a
-O)
8
(µ
3
-
actions.
3
(Figures 2a and S9, SI). 9F
1
OH) (µ -OH) (DBB-Ir-F) (OTf)
8
2
6
6
6
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sites on the SBUs. As shown in Figure 3b (black), Hf12-Ir-TFA cat-
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alyzed reactions under identical conditions afforded the coupling
product 1c in only 8% yield. Although possessing similar Lewis
acidity as Hf12-Ir-OTf, Sc(OTf) in combination with Hf12-Ir-TFA
3
produced 1c in 23% yield only (Figure 3b, red). These results and
additional control experiments (Table S1, SI) support the synergis-
tic catalytic performance of the photoredox and Lewis acidic sites
in Hf12-Ir-OTf as a result of their proximity, highlighting the ad-
vantage of hierarchically installing cooperative catalytic sites in
MOLs for synergistic catalysis.
Hf12-Ir-OTf
a
1.2
NMA only
Hf12-Ir-OTf
Hf12-Ir-TFA
Sc(OTf)3
472.6
b 100
80
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Hf12-Ir-TFA + Sc(OTf)3
Hf12-Ir-TFA
1.0
0.8
0.6
0.4
0.2
0.0
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72.8
467.7
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Time (h)
Wavelength (nm)
c
aReactions at 0.1 mmol scale with 0.1 mol% Hf-Ir-OTf in dichloro-
methane at room temperature.
d
Hf12-Ir-OTf catalyzed dehydrocoupling of isoquinoline with un-
activated cyclopentane, cyclohexane, and cycloheptane to afford
target products 13c-15c in 92%, 95%, and 76% isolated yields, re-
spectively. Such alkylated heteroarenes are difficult to prepare by
conventional methods. Quinoline and pyrimidine derivatives also
coupled with cyclohexane to afford 16c and 17c in 71% and 46%
isolated yields, respectively. It is noteworthy that some of the prod-
43
ucts such as 3c and 9c are key scaffolds in drug molecules. More
importantly, Hf12-Ir-OTf competently catalyzed late-stage func-
tionalization of bioactive and drug molecules. We successfully
functionalized caffeine (the most consumed neurostimulant),
Fasudil (a potent Rho-kinase inhibitor and vasodilator), and
Metyrapone (a diagnostic agent for adrenal insufficiency) with tet-
rahydrofuran in the presence of 0.1 mol% Hf12-Ir-OTf to afford
C
F
N
Ir Hf
O
S
Figure 3. (a) Fluorescence spectra of free NMA (black), NMA
binding to Hf12-Ir-OTf (blue), Hf12-Ir-TFA (red), and Sc(OTf)
3
1
8c-20c in high to moderate isolated yields. Freely accessible
(green) in MeCN at 298 K. (b) Time-dependent yields of 1c with
different catalysts. (c) Control experiments under different condi-
tions. (d) Proposed reaction mechanism.
Lewis acidic and other catalytic sites in MOLs can thus be lever-
36
aged for synergistic catalysis and drug discovery.
The low loading (0.1 mol%) of Hf12-Ir-OTf in these coupling re-
actions led to very high turnover numbers of 460 to 950 calculated
on the basis of both the photosensitizer and the Lewis acid. This
catalytic performance is vastly superior to homogeneously cata-
lyzed reactions which require stoichiometric amounts of Brønsted
Hf12-Ir-OTf was stable under photocatalytic conditions as evi-
denced by the retention of the PXRD pattern for the recovered Hf12
-
Ir-OTf (Figure 2c) and the leaching of <1% Hf and <2% Ir into the
supernatant by inductively coupled plasma mass spectrometry
(ICP-MS). In addition, Hf12-Ir-OTf was recovered and used in five
runs of dehydrogenative cross-coupling between isoquinoline and
tetrahydrofuran without significant decrease in catalytic activity
(Figure S22, SI).
3
5
acids or sub-stoichiometric amounts (20 mol%) Lewis acids. We
attribute the excellent catalytic performance of Hf12-Ir-OTf to the
proximity between DBB-Ir-F photosensitizers and Lewis acidic Hf
3
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We conducted several experiments to probe the reaction mecha-
Corresponding Author
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nism (Figure 3c). The dehydrogenative cross-coupling between
isoquinoline and THF did not proceed in the absence of light or the
photosensitizer, indicating the photocatalytic nature of this syner-
gistic catalysis. The addition of a radical capture agent (2,2,6,6-tet-
ramethylpiperidin-1-yl)oxyl (TEMPO) completely inhibited the
dehydrogenative cross-coupling reaction, supporting the involve-
ment of a radical pathway in the reaction mechanism. Finally, the
addition of 1,1-diphenylethene as a radical capture agent afforded
wenbinlin@uchicago.edu
Author Contributions
‡
These authors contributed equally.
Notes
The authors declare no competing financial interests.
t
2-(2,2-diphenylvinyl)tetrahydrofuran 1d and 1,1-diphenyl- butox-
ACKNOWLEDGMENT
yethene 1e in 12% and 35% yields, respectively, but completely
shut down the formation of 1c. This result indicates the presence of
We thank Xiaomin Jiang, Xuanyu Feng, and Yang Song for exper-
imental help. We acknowledge the National Science Foundation
and the University of Chicago for founding support and the
MRSEC Shared User Facilities at the University of Chicago (NSF
DMR-1420709) for instrument access. XAS analysis was per-
formed at Beamline 10-BM, supported by the Materials Research
Collaborative Access Team (MRCAT). Use of the Advanced Pho-
ton Source, an Office of Science User Facility operated for the U.S.
DOE Office of Science by ANL, was supported by the U.S. DOE
under Contract No. DE-AC02-06CH11357.
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t
both butoxy radical and α-tetrahydrofuran radical in the reaction.
On the basis of these experimental results and literature prece-
3
5
dents, we propose a plausible reaction mechanism in Figure 3d.
t
Single electron reduction of BuOOBz by the photoexited DBB-Ir-
F ligand in Hf12-Ir-OTf generates the Ir(IV) species and electro-
t
philic butoxy radical A, which undergoes hydrogen atom transfer
3
3
(HAT) with sp C-H to give the nucleophilic sp carbon radical B.
In the meantime, upon coordinating to the strongly Lewis acidic
SBU, the heteroarene C becomes electron-deficient and susceptible
3
to attack by the nucleophilic sp carbon radical B generated by the
REFERENCES
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process produces the electron-rich nitrogen radical intermediate D,
which is oxidized by the Ir(IV) species to afford the secondary
amine cation E and regenerate the Ir(III) species. The Ir(III) photo-
catalyst is excited by blue LED to restart the catalytic cycle on the
left. Deprotonation of the amine cation E afforded Lewis acid-
bound functionalized heteroarene F. An incoming heteroarene sub-
strate releases the functionalized heteroarene P and restarts the cat-
alytic cycle on the right. In this mechanistic scenario, the close
proximity of photoredox and Lewis acidic centers in Hf12-Ir-OTf
not only facilitates the reaction between carbon radical B and acti-
vated heteroarene C, but also accelerates the electron transfer from
the nitrogen radical intermediate D to the Ir(IV) species.
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triflate to afford strongly Lewis acidic Hf sites. The resultant Hf12
-
Ir-OTf MOL effectively catalyzed dehydrogenative cross-cou-
plings of heteroarenes with ethers, amines, and unactivated alkanes
with turnover numbers of 930, 790, and 950, respectively. Hf12-Ir-
OTf also competently catalyzed late-stage functionalization of bio-
active and drug molecules such as caffeine, Fasudil, and
Metyrapone. We attribute the excellent catalytic performance of
Hf12-Ir-OTf to the close proximity (1.2 nm) between photoredox
and Lewis acid catalysts which not only facilitates the reaction be-
tween the carbon radical and the activated heteroarene but also ac-
celerates the electron transfer from the nitrogen radical intermedi-
ate to the Ir(IV) species in the catalytic cycle. MOLs thus provide
an excellent platform to design novel 2D materials with synergistic
catalysts for the sustainable synthesis of complex organic mole-
cules.
6.
Liu, J.; Chen, L.; Cui, H.; Zhang, J.; Zhang, L.; Su, C.-Y.,
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