Surface Modification of Two-Dimensional Metal–Organic Layers Creates Biomimetic Catalytic Microenvironments for Selective Oxidation

Article abstract of DOI:10.1002/anie.201703675

Microenvironments in enzymes play crucial roles in controlling the activities and selectivities of reaction centers. Herein we report the tuning of the catalytic microenvironments of metal–organic layers (MOLs), a two-dimensional version of metal–organic frameworks (MOFs) with thickness down to a monolayer, to control product selectivities. By modifying the secondary building units (SBUs) of MOLs with monocarboxylic acids, such as gluconic acid, we changed the hydrophobicity/hydrophilicity around the active sites and fine-tuned the selectivity in photocatalytic oxidation of tetrahydrofuran (THF) to exclusively afford butyrolactone (BTL), likely a result of prolonging the residence time of reaction intermediates in the hydrophilic microenvironment of catalytic centers. Our work highlights new opportunities in using functional MOLs as highly tunable and selective two-dimensional catalytic materials.

Full text of DOI:10.1002/anie.201703675

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Surface Modification of Two-Dimensional Metal-Organic Layers
Creates Biomimetic Catalytic Microenvironments for Selective
Oxidation
Authors: Wenjie Shi, Lingyun Cao, Hua Zhang, Xin Zhou, Bing An,
Zekai Lin, Ruihan Dai, Jianfeng Li, Cheng Wang, and Wenbin
Lin
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201703675
Angew. Chem. 10.1002/ange.201703675
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http://dx.doi.org/10.1002/ange.201703675

10.1002/anie.201703675
Angewandte Chemie International Edition
COMMUNICATION
Surface Modification of Two-Dimensional Metal-Organic Layers
Creates Biomimetic Catalytic Microenvironments for Selective
Oxidation
Wenjie Shi[a], Lingyun Cao[a], Hua Zhang[a], Xin Zhou[a,b], Bing An[a], Zekai Lin[c], Ruihan Dai[a], Jianfeng
Li[a], Cheng Wang[a]*, and Wenbin Lin[a,c]
*
Abstract: Microenvironments in enzymes play crucial roles in
controlling the activities and selectivities of reaction centers. Herein
we report the tuning of the catalytic microenvironments of metal-
organic layers (MOLs), a two-dimensional version of metal-organic
frameworks (MOFs) with thickness down to a monolayer, to control
product selectivities. By modifying the secondary building units
(SBUs) of MOLs with monocarboxylic acids such as gluconic acid,
we successfully changed the hydrophobicity/hydrophilicity around
the active sites and fine-tuned the selectivity in photocatalytic
oxidation of tetrahydrofuran (THF) to exclusively afford butyrolactone
(BTL), likely via prolonging residence time of reaction intermediates
in the hydrophilic microenvironment of catalytic centers. Our work
highlights new opportunities in using functional MOLs as highly
tunable and selective two-dimensional catalytic materials.
molecules with carboxylate anchoring groups, offering a
convenient means to modify SBUs and alter the
hydrophilic/hydrophobic environments of catalytic sites. By
engineering FeII-tpy sites on the MOLs and the hydrophilic
environment around them, we have designed a heterogeneous
oxidase-mimicking catalyst for aerobic transformation of THF to
butyrolactone (BTL) with 100% selectivity,
a reaction of
significant commercial interest.[9] The selectivity can be tuned to
afford different products by simply modifying the SBUs with
various monocarboxylic acids (Scheme 1).
Enzymes achieve high catalytic activities and selectivities
under mild conditions by creating favorable microenvironments
such as hydrophobic pockets around the reaction centers.[1] For
example,
a number of oxidases achieve highly selective
oxidative C-H functionalization in highly tailored reaction pockets
using oxygen as the electron acceptor.[2] Various artificial
systems have been developed to mimic Nature’s designs,
ranging from cyclodextrin inclusion compounds[3] and
supramolecular cages[4] to surface-decorated nanoparticles[5]
and porous Metal-Organic Frameworks (MOFs).[6] However,
reactions like selective oxidation of C-H bonds still present
formidable challenges to synthetic chemists, in part due to the
inability to precisely control the microenvironments in artificial
catalysts.
Here we report Metal-Organic Layers (MOLs) as a new
platform to rationally construct catalytic centers having rich
secondary interactions with reaction intermediates for highly
selective oxidations. MOLs are a two-dimensional version of
Metal-Organic Frameworks (MOFs)[7] with one dimension
reduced to a monolayer in thickness.[8] By replacing BTB in our
previously reported [Hf6(µ3-O)4(µ3-OH)4(HCO2)6(BTB)2] MOLs
(where BTB is benzenetribenzoate) with (4’-(4-benzoate)-
(2,2’,2’’-terpyridine)-5,5’’-dicarboxylate) (TPY), we obtained a
new MOL of the composition [Hf6(µ3-O)4(µ3-OH)4(HCO2)6(TPY)2]
with the TPY groups available for postsynthetic coordination of
transition metal centers. Importantly, the six formate groups on
the [Hf6(µ3-O)4(µ3-OH)4(HCO2)6] SBUs are pointing away from
the nano-sheet and can be readily exchanged with functional
Scheme 1. Modifying the SBUs of FeII-MOLs with gluconic acid for the
selective oxidation of tetrahydrofuran.
[Hf6(µ3-O)4(µ3-OH)4(HCO2)6(TPY)2] was prepared following
the
published
procedure
for
[Hf6(µ3-O)4(µ3-
OH)4(HCO2)6(BTB)2].[8h] The MOLs appeared as wrinkled
ultrathin films of ~0.2 μm in dimensions by Transmission
Electron Microscopy (TEM) (Figure S1a & S2), and their Powder
X-Ray Diffraction (PXRD) patterns were consistent with the 3,6-
connected 2-D net of kgd topology (Figure 1a). The as-
synthesized MOLs were measured to be 1.4 ± 0.2 nm in
thickness by Atomic Force Microscopy (AFM), corresponding to
the van der Waals diameter of a single layer of Hf6 SBUs
(Figure S1b&c). FeII centers were coordinated to the tpy sites by
treating the MOLs with FeBr2, FeCl2, or Fe(OTf)2 (OTf = triflate)
at room temperature for 24 hours. Inductively coupled plasma-
optical emission spectroscopy (ICP-OES) gave an Fe loading of
10.35 wt%, indicating complete functionalization of the tpy sites.
FeII-MOLs were light yellow with broad absorptions at <350 nm
and a tail extending to 420 nm, while the corresponding FeIII-
MOLs were purple in color with broad absorptions before 350
[a]
W. Shi, L. Cao, H. Zhang, B. An, R. Dai, Prof. J. Li, Prof. C. Wang,
Prof. W. Lin
College of Chemistry and Chemical Engineering, iCHEM, PCOSS
Xiamen University, Xiamen 361005 (China)
E-mail: wangchengxmu@xmu.edu.cn
X. Zhou
School of Chemistry and Chemical Engineering, South China
University of Technology, Guangzhou 510640 (China)
Z. Lin, Prof. W. Lin
[b]
[c]
Department of Chemistry, University of Chicago
Chicago, IL 60637 (USA)
E-Mail: wenbinlin@uchicago.edu
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10.1002/anie.201703675
Angewandte Chemie International Edition
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nm and a pronounced absorption band centered at ~370 nm
(Figure 1b).
SBU modifications (Figure 1a&d). AFM images of GA-FeII-MOLs
gave a thickness of 1.4 ± 0.2 nm, similar to that of FeII-MOLs
within experimental error (1.4 ± 0.2 nm; Figure 1c and Figure
S1b&c).
The SBU modification of 2D-MOLs is more than a simple
extension of postsynthetic functionalization of SBUs in 3D MOFs
[10]. First, there is no limitation on the size of monocarboxylate
functional molecules due to the open space around a 2D net.
Second, the position/geometry of modifying groups around
catalytic sites can be easily controlled due to lower connectivity
and complexity in 2D MOLs. Third, a high degree of SBU
modification on MOLs can be achieved due to the absence of
diffusion barriers. Furthermore, SBU functionalization of MOLs
affords very different structures from surface modification of bulk
MOFs: polymer-treated MOFs exhibited pre-concentration/gating
effects in catalysis,[6a,6c] while SBU modification in this work
directly alters the microenvironment of reaction centers, allowing
for significant enhancement of product selectivity.
We tested the catalytic activity of as-synthesized FeII-MOLs
towards oxidative C-H activation of THF. Under the irradiation of
a blue LED (~462 nm), 2 mg of FeII-MOLs was placed in a flask
with 3 mL of aerated THF together with a catalytic amount (2 μl)
of acetic acid or formic acid. After 24 hours, the reaction mixture
was analyzed by gas chromatography (GC) and GC-mass
spectrometry (GC-MS). BTL and 2-hydroxyl-tetrahydrofuran (2-
OH-THF) were two major products, with a BTL selectivity of 53.5%
and a total turnover number (1/2 O2-TON) of (8.2 ± 0.3)×102
(Table 1, entry 1; Figure S8). Control experiments in the dark
with or without heating at 50 °C showed negligible amount of
products, confirming the photocatalytic nature of the reaction
(Table 1, entries 2&3). The MOLs without FeII centers were
inactive, indicating that tpy-FeII complex is the reaction center
(Table S2, entry 1). We also found that O2 and catalytic amount
of acid (2 μl) were essential for the oxidation of THF (Table 1,
entry 4; Table S2, entries 2&3).
Figure 1. (a) PXRD patterns of GA-FeII-MOLs and FeII-MOLs. (b) UV-Vis
spectra of MOLs (blue), FeII-MOLs (black), and FeIII-MOLs (red). (c) Atomic
force microscopy (AFM) image of GA-FeII-MOLs. (d) HRTEM and FFT images
of GA-FeII-MOLs after catalysis.
The SBUs of Fe-MOLs were modified by treatment with
different monocarboxylic acids in DMF at 60 oC for 24 hours
(Figure S4&5). Eight monocarboxylic acids of varying
hydrophilicity/hydrophobicity, including gluconic acid (GA), oleic
acid (OA), caprylic acid (CA), propionic acid (PA), 5-aminovaleric
acid (5-AA), 7-aminoheptanoic acid (7-AA), O-[2-(2-
Methoxyethoxy)ethyl]glycolic acid (O-GA), and protoporphyrin IX
(PPIX), were employed to tune the microenvironments around
the tpy-Fe sites.
Thermogravimetric analysis (TGA) exhibited additional weight
losses for MOLs treated with monocarboxylic acids, suggesting
successful modification of the SBUs (Figure S3 and Table S1).
For example, GA-MOLs exhibited an additional 8.4% weight loss
as compared to the as-synthesized MOLs, consistent with the
replacement of 16.7% of the capping formates by gluconates (i.e.
one gluconate per SBU).
The heterogeneous nature of the MOL catalyst was proved by
the lack of activity for the supernatant after removal of the MOL.
TEM (Figure 1d) and PXRD studies confirmed that the GA-FeII-
MOLs catalyst retained the same nanosheet structure after the
catalytic reaction (Figure S7).
A
homogeneous control
experiment using the tpy-FeII complex gave a low TON of 45 ± 2,
suggesting inter-molecular deactivation of the homogeneous
catalysts in solution and highlighting the advantage of site-
isolated MOL for efficient catalysis.
SBU modification greatly impacted the product selectivity of
THF oxidation (Table 1, entries 5-12; Figure 2d). OA-FeII-MOLs
with a hydrophobic surface gave a lower BTL selectivity of
56.7%, while the BTL selectivity increased to 78.7% for O-GA-
FeII-MOLs with a less hydrophobic surface (Figure S6). Notably,
the more hydrophilic GA-FeII-MOLs showed 100% selectivity for
BTL with a TON of (5.3 ± 0.3) × 102. The relationship between
BTL selectivities and MOL contact angles is shown in Figure 2d.
In reactions with hydrophobic MOLs, we also observed the
hydrolysis of some BTL (20%-40%) to the ring-opened product
4-hydroxyl-butanoic acid (Figures S8&S9).
We also explored the substrate scope of the MOL catalyst in
oxidative activation of other C-H bonds. For example,
tetrahydropyran was also oxidized to oxotetrahydropyran with a
TON of 9.0 ± 0.3 under the photocatalytic conditions (Figure
S10a). The MOLs functionalized with Fe(OTf)2 oxidized
cyclohexane with oxygen as the oxidant to form cyclohexanol
and cyclohexanone in a 1:1 ratio with a TON of 4.0 ± 0.2 (Figure
S10b).
Figure 2. (a-c) Micrographs of water droplets on films of as-synthesized MOLs
(a), OA-modified MOLs (OA-MOLs) (b), and GA-MOLs (c). d) Relationship
between BTL selectivity and MOL contact angles in aerobic oxidation of
tetrahydrofuran. (Red: BTL selectivity; Black: 2-hydroxyl-tetrahydrofuran
selectivity).
The unmodified MOLs were hydrophobic with a contact angle
o
o
of 131.1 ± 0.5 in water wettability test (Figure 2a). Upon
o
modification with OA, the contact angle increased to 134.9
±
0.5 o, suggesting the creation of a more hydrophobic surface
(Figure 2b). In contrast, after treating the MOLs with GA, the
surface became hydrophilic with a contact angle of 86.8 o ± 0.5 o
(Figure 2c). TEM images and PXRD patterns of the modified
samples indicated the preservation of the MOL structure after
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10.1002/anie.201703675
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adducts but an FeIII center for the tpy-Fe-OOH complex (see SI
section 10). The v(O-O) vibration in FeIII-▪OOH was observed at
1021 cm-1 in the Raman spectrum (Figure 3b), as expected for a
protonated superoxide.[13]
Table 1. Photocatalytic Oxidation of THF
Entry
Catalyst
Selectivity%
TON
2-OH-THF
43 ± 4
-
-
-
BTL
57 ± 4
-
-
-
1
2
3
FeII-MOLs
(8.2 ± 0.3) ×102
2.0 ± 0.1
5.0 ± 0.2
4.0 ± 0.2
FeII-MOLs[a]
FeII-MOLs[b]
FeII-MOLs[c]
4
5
6
7
8
9
10
11
12
GA-FeII-MOLs[d]
O-GA-FeII-MOLs
PPIX-FeII-MOLs
OA-FeII-MOLs
CA-FeII-MOLs
PA-FeII-MOLs
5-AA-FeII-MOLs
7-AA-FeII-MOLs
0
100
(5.3 ± 0.3) ×102
(6.8 ± 0.3) ×102
(5.9 ± 0.3) ×102
(7.8 ± 0.3) ×102
(9.3 ± 0.3) ×102
(8.8 ± 0.3) ×102
(7.2 ± 0.3) ×102
(7.8 ± 0.3) ×102
21 ± 4
31 ± 4
43 ± 4
44 ± 4
42 ± 4
44 ± 4
44 ± 4
79 ± 4
69 ± 4
57 ± 4
56 ± 4
58 ± 4
56 ± 4
56 ± 4
Figure 3. (a) ESR spectra of FeII-MOLs in THF at 90K (black), with acetic acid
but without oxygen (red), with acetic acid and oxygen in dark (blue) and after
irradiation with blue light (cyan). (b) Raman spectra of FeII-MOLs (black) with
acetic acid and FeII-MOLs (red) in THF.
[a] Without light. [b] Heating at 50 oC without light. [c] Without oxygen.
The second step in the cycle involves photochemical oxidation
of THF by tpy-FeIII-▪OOH. In the MLCT excited state of tpy-FeIII-
▪OOH, i.e. tpy-▪-FeIV-▪OOH, the tpy-▪ radical anion transfers one
electron to FeIV-▪OOH to generate tpy-FeIV--OOH (3). The FeIV
center oxidizes coordinated THF via a two-electron transfer to
form FeII-coordinated oxonium cation (4), which is attacked by
water to generate 2-OH-THF. The oxonium as an intermediate
was supported by the exclusive formation of 2-methoxyl-
tetrahydrofuran when methanol was added to the reaction
mixture. H2O2 can also be produced in the process, as
corroborated by electrochemical detection of H2O2 with a
Faraday efficiency of 97% on a rotating ring-disk electrode using
FeII-MOLs as the oxygen reduction electrocatalyst (Figure S12).
The tpy-FeII species can also react with the generated H2O2 in
a Fenton-like process to generate tpy-FeIII-▪OH adduct (5, in
resonance to tpy-FeIV--OH) that oxidizes THF to oxonium cation
via two-electron transfer (Scheme 2b). The much higher redox
potential of OH/-OH (1.90 V vs. SHE) than that of OOH/-OOH
(0.75 V vs. SHE)[14] enables direct oxidation of FeIII to FeIV in the
absence of photo-excitation. We confirmed in control
experiments that H2O2 can oxidize THF in dark with FeII-MOLs
as the catalyst [TON = (8.8 ± 0.3) × 102 in 24 hours].
The 2-OH-THF from THF oxidation can also be adsorbed onto
the Fe centers and further oxidized by both the tpy-▪-FeIV--OOH
and tpy-FeIV--OH to form BTL (Scheme 2c), as confirmed by
using 2-OH-THF as the substrate in the aerobic photo-oxidation
with a TON of (6.3 ± 0.3) × 102 in 24 hours.
To identify the rate-determining step in this catalytic cycle, the
H/D kinetic isotope effect (KIE) for THF oxidation was measured
using C4H8O and C4D8O as the substrates in two separate
experiments. The resulting KIE values are 1.3 ± 0.2 for 2-OH-
THF and 1.2 ± 0.2 for BTL, suggesting only secondary isotopic
effect. In a related experiment using H2O2 as the oxidant in dark,
the KIE values are 3.0 ± 0.2 for 2-OH-THF and 2.4 ± 0.2 for BTL,
which are typical values for primary kinetic isotope effect. The
absence of the primary KIE in the photocatalysis and its
presence in the dark reaction suggest the step after light
activation as the rate-determining step (RDS) in the
photocatalysis, possibly the step of electron transfer from tpy-▪ to
▪OOH. This assignment of RDS is also consistent with the ESR
observation of FeIII species as the catalyst resting state, which is
the state before light activation. We have also added D2O and
H2O into the reaction mixtures without observing any rate
differences in THF oxidation, confirming that the protonation of
adsorbed oxygen on FeII or the addition of water to oxonium is
not the rate-determining step.
We studied the mechanism of the photocatalytic aerobic THF
oxidation.
Singlet oxygen was first ruled out as the oxidant, since
Ru(bpy)32+, which is known to generate singlet oxygen,[11] did not
oxidize THF under the photocatalytic conditions.
The reaction is also unlikely to proceed via hydroxyl or
hydroperoxyl free radicals from activated oxygen. The Fenton’s
reagent FeSO4/H2O2 that generated these radicals, oxidized
THF to a complex mixture of oxirane (4%), oxetane (3%), 2-OH-
THF (8%), 2-hydroperoxyl-tetrahydrofuran (11%), 1,4-butan-diol
(17%), BTL (43%), 4-hydroxyl-butanoic acid (3%) and other
unidentified species (11%) (Figure S9), in contrast to the high
selectivity towards BTL and 2-OH-THF by the MOLs (Figure S8).
Consistent with the absence of free radical involvement, adding
5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a radical trapping
agent (80 mM) in the reaction failed to produce DMPO adducts
with either hydroxyl or hydroperoxyl radicals as shown by
electron spin resonance (ESR) spectroscopy.
▪
▪
We proposed a catalytic cycle as shown in Scheme 2a. The
tpy-FeII (1) is oxidized to tpy-FeIII (2) by oxygen in the presence
of protons, followed by photochemical oxidation of THF by tpy-
FeIII-▪OOH.
The generation of FeIII species is confirmed by the X-band
ESR spectroscopy. The starting high-spin tpy-FeII complex was
ESR silent with S=2 (Figure 3a) and was confirmed by X-ray
photoelectron spectroscopy (Figure S7c). In the presence of
both acid and oxygen, a THF suspension of the Fe-MOLs in the
dark at 90 K showed a rhombic ESR spectrum with g=6.17,
g=4.23 and g=1.93, typical of high-spin FeIII centers with S=5/2
(Figure 3a; Figure S11).[12] In the absence of acid or air, no ESR
signal was detected with or without light irradiation. These
results also suggest that light is not needed in the generation of
FeIII, consistent with the observation of the reaction mixture
turning purple to form tpy-FeIII in the absence of light.
We hypothesize that the FeIII was generated by proton-
coupled electron transfer (PCET) from the FeII center to a
coordinated O2. DFT calculations supported this PCET process,
as spin and orbital analysis showed an FeII center for tpy-Fe-O2
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Chem., Int. Ed. 2016, 55, 4962-4966; Angew. Chem. 2016, 128, 5046; i) Z. Hu,
E. M. Mahdi, Y. Peng, Y. Qian, B. Zhang, N. Yan, D. Yuan, J.-C. Tan, D. Zhao,
J. Mater. Chem. A 2017. doi: 10.1039/C7TA00413C.
With this catalytic cycle, we can rationalize the effects of
microenvironments on reactivity by retaining 2-OH-THF in
hydrophilic pockets, which leads to subsequent oxidation of 2-
OH-THF to BTL. We measured the uptake of 2-OH-THF by
MOLs with different modifications via equilibrating 23 mM of the
adsorbates with 3.4 mM of MOLs followed by determining the
adsorbate concentration in the supernatant by GC-MS.[6b] The 2-
OH-THF uptake was
0 for as-synthesized MOLs, 0 for
hydrophobic CA-MOLs and 1.18 mol/mol MOLs for hydrophilic
GA-MOLs, supporting the retention of 2-OH-THF in hydrophilic
microenvironment.
In summary, we developed a method to modify the SBUs of
catalytic metal-organic layers with monocarboxylate compounds,
which creates hydrophilic/hydrophobic microenvironments
around the reaction centers and controls the selectivity in the
photocatalytic aerobic oxidation of THF. This assembly mimics
oxidase in using secondary interactions to direct reaction
pathways, and highlights opportunities in using functionalized
metal-organic layers in biomimetic catalysis.
Acknowledgements
[9] a) P. Padmalatha, P. K. Khatri, S. L. Jain, Synlett 2013, 24, 1405-1409; b)
A. Ausavasukhi, T. Sooknoi, Green Chem. 2015, 17, 435-441.
We acknowledge funding support from the National Natural
Science Foundation and Ministry of Science and Technology of
[10] a) J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon, K. Kim,
Nature 2000, 404, 982-986; b) Z. Wang, S. M. Cohen, Chem. Soc. Rev. 2009,
38, 1315-1329.
the
P.R.China
(NSFC21671162,
2016YFA0200702,
NSFC21471126), the National Thousand Talents Program of the
P. R. China, Post-Doctoral Innovative Talents Project
(BX201600053) from China Postdoctoral Science Foundation
and the U.S. National Science Foundation (DMR-1308229).
[11] A. A. Abdel-Shafi, H. A. Hassanin, S. S. Al-Shihry, Photochem. Photobiol.
Sci. 2014, 13, 1330-1337.
[12] G. Villar-Acevedo, E. Nam, S. Fitch, J. Benedict, J. Freudenthal, W.
Kaminsky, J. A. Kovacs, J. Am. Chem. Soc. 2011, 133, 1419-1427.
[13] B. S. Yeo, S. L. Klaus, P. N. Ross, R. A. Mathies, A. T. Bell,
ChemPhysChem 2010, 11, 1854-1857.
Keywords: Metal-Organic Layers; Metal-Organic Frameworks;
[14] C. Adriaanse, J. Cheng, V. Chau, M. Sulpizi, J. VandeVondele, M. Sprik,
The J. of Phys. Chem. Lett. 2012, 3, 3411-3415.
microenvironment; biomimetic; photocatalysis
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10.1002/anie.201703675
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Biomimetic catalysis
Wenjie Shi, Lingyun Cao, Hua Zhang,
Xin Zhou, Bing An, Zekai Lin, Ruihan
Dai, Jianfeng Li, Cheng Wang*, and
Wenbin Lin*
Two dimensional metal-organic-layers
(MOLs) with FeII catalytic centers
mimic oxidase in aerobic photo-
oxidization of tetrahydrofuran. The
product selectivity is fine-tuned
Page No. – Page No.
through
modification of the MOLs, akin to the
creation of suitable
microenvironments in enzymes for
selective catalysis.
hydrophilic/hydrophobic
Surface
Dimensional Metal-Organic Layers
Creates Biomimetic Catalytic
Microenvironments for Selective
Oxidation
Modification
of
Two-
This article is protected by copyright. All rights reserved.