Cu(I)-Based Metal-Organic Frameworks as Efficient and Recyclable Heterogeneous Catalysts for Aqueous-Medium C-H Oxidation

Article abstract of DOI:10.1021/acs.cgd.8b01527

The enantioselective transformation of ubiquitous C-H bonds into valuable C-O bonds offers an efficient synthetic approach to construct carbonyl functionalized molecules. However, the grand obstacles in the reaction are the selectivity issues and side reactions under the harsh reaction conditions. In order to overcome the limits, two Cu(I)-based MOFs {(NEt₄)_0.5[Cu₃(TTPB)_0.75(CN)_0.5(H₂O)]·H₂O}_n (1) and {[Cu₂(TTPB)_0.5]·DMF·2H₂O}_n (2) were synthesized (H₄TTPB = 5,5′-(4′,5′-bis(4-(1H-tetrazol-5-yl)phenyl)-[1,1′:2′,1′′-terphenyl]-4,4′′-diyl) bis(1H-tetrazole)) under hydrothermal conditions with (triethylamine (TEA) and ethyldiisopropylamine (DIPEA) as structure-directing agents, respectively. Of these, 1 shows an anionic three-dimensional (3D) framework composed of two kinds of cagelike micropores with 7 × 17 ? and 10 × 17 ?, respectively. In comparison, 2 exhibits a 3D framework with open channels (14 × 8 ?). The stability studies showed that the crystallinity of 1 and 2 could remain in a series of organic solvents (ethanol, N,N-dimethylformamide, chloroform, dioxane, toluene) and acid and alkali aqueous solutions (pH = 1-13) at room temperature for 48 h. 1 and 2 with coordinatively unsaturated Cu(I) sites were applied as heterogeneous catalysts for the oxidation of arylacycloalkanes in aqueous medium and exhibited excellent catalytic activities, selectivities, and recyclabilities. Moreover, free-radical reaction mechanism and reversible valence-tautomeric conversions of central copper were confirmed during the process by control experiment.

Full text of DOI:10.1021/acs.cgd.8b01527

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Article
Cu(I)-based Metal-organic Frameworks as Efficient and Recyclable
Heterogeneous Catalysts for Aqueous-medium C-H Oxidation
Kuan Gao, Chao Huang, Yishen Yang, Hong Li, Jie Wu, and Hongwei Hou
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01527 • Publication Date (Web): 14 Dec 2018
Downloaded from http://pubs.acs.org on December 16, 2018
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Cu(I)-based Metal-organic Frameworks as Efficient and Recyclable
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Heterogeneous Catalysts for Aqueous-medium C-H Oxidation
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Kuan Gao,† Chao Huang,‡ Yishen Yang,† Hong Li,† Jie Wu,*, † and Hongwei Hou*, †
† College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, P. R.
China
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‡
Center for Advanced Materials Research, Zhongyuan University of Technology, Zhengzhou
450001, P. R. China
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
* To whom correspondence should be addressed. e-mail: houhongw@zzu.edu.cn (H. Hou),
wujie@zzu.edu.cn (J. Wu)
† Zhengzhou University
‡ Zhongyuan University of Technology
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ABSTRACT: The enantioselective transformation of ubiquitous C−H bonds into valuable C=O
bonds offers an efficient synthetic approach to construct carbonyl functionalized molecules.
However, the grand obstacles in the reaction are the selectivity issues and side reactions under the
harsh reaction conditions. In order to overcome the limits, two Cu(I)-based MOFs
{(NEt4)0.5[Cu3(TTPB)0.75(CN)0.5(H2O)]•H2O}n (1) and {[Cu2(TTPB)0.5]•DMF•2H2O}n (2) were
synthesized (H4TTPB =5,5'-(4',5'-bis(4-(1H-tetrazol-5-yl)phenyl)-[1,1':2',1''-terphenyl]-4,4''-diyl)
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bis(1H-tetrazole))
under
hydrothermal
condition
with
(triethylamine
(TEA)
and
ethyldiisopropylamine (DIPEA) as structure-directing agents, respectively. Of these, 1 shows an
anionic three-dimensional (3D) framework composed of two kinds of cage-like micropores with 7 ×
17 Å and 10 × 17 Å, respectively. In comparison, 2 exhibits 3D framework with open channels (14 ×
8 Å). The stability studies showed that the crystallinity of 1 and 2 could remain in a series of organic
solvents (ethanol, N,N-dimethylformamide, chloroform, dioxane, toluene), acid and alkali aqueous
solutions (pH=1-13) at room temperature for 48 h. 1 and 2 with coordinatively unsaturated Cu(I)
sites were applied as heterogeneous catalysts for the oxidation of arylacycloalkanes in aqueous
medium and exhibited excellent catalytic activities, selectivities and recyclabilities. Moreover,
free-radical reaction mechanism and reversible valence-tautomeric conversions of central copper
were confirmed during the process by control experiment.
Introduction
Metal-organic frameworks (MOFs), a class of crystalline materials, which demonstrate ample
structure types, tunable pores and intriguing functionalities, have attracted great research interests in
numerous fields including catalysis, gas storage and separation, luminescence, chemical sensors,
drug delivery1-10. In recent years, particular interesting has been paid to the application of MOFs as
heterogeneous molecular catalysts and catalyst supports. Especially, due to their highly tunable
platforms to construct molecule catalysts for plentiful chemical transformation, MOFs have been
given much promise as single site catalysts to modulate size-, shape-, chem- and enantioselective
reactions.11,12 Compared with homogeneous catalysts, MOFs materials are easy to separate and
recycle from the reaction mixture, offering particular environmental and sustainable advantages for
the chemical transformation.13,14 Besides, MOFs possess uniform pore sizes/environments and
unparalleled surface areas in contrast to traditional heterogeneous catalysts, including zeolite, SiO2,
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and activated carbon. More importantly, MOFs possess uniformly distributed catalytically active
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centers with identical microenvironments throughout the solid, which can achieve superior
selectivity and reactivity to other catalytic systems.15,16 In recently, our group has reported that some
Cu(I) coordination polymers can be as green heterogeneous catalysts for direction C-H bond
activation of arylalkanes to ketones and perform the reaction with high regioselectivity due to the
unique spatial confinement effect.17
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On the other hand, the oxidation of benzylic methylene groups usually aim to obtain products,
such as cyclic ketones, esters and amides, which have been widely used to build blocks of dyes and
other fine chemicals18-20. In the traditional methods, strong oxidizing agents (e.g., KMnO4) usually
require massive oxidants and multivalent transition metals (e.g., Cu, Fe, V, Ru) are irreversible
conversions of central metal valence, resulting in a large amount of waste.21-23 Therefore, to
ameliorate these defects, catalytically active metal-organic frameworks (MOFs) may represent a
feasible platform to as single-site heterogeneous catalysts, achieving excellent activity of
arylacycloalkanes oxidation with satisfied selectivity to ketone.
In this work, two Cu(I)-based MOFs {(NEt4)0.5[Cu3(TTPB)0.75(CN)0.5(H2O)]•H2O}n (1) and
{[Cu2(TTPB)0.5]•DMF•2H2O}n (2) (H4TTPB=5,5'-(4',5'-bis(4-(1H-tetrazol-5-yl) phenyl) -[1,1':2',1''-
terphenyl]-4,4''-diyl)bis(1H-tetrazole))
were
successfully
synthesized
with
different
structure-directing agents (TEA and DIPEA). The stability experiments showed that the frameworks
of 1 and 2 could remain in a series of organic solvents, acidic and basic solution. We also
demonstrated that 1 and 2 can serve as promising heterogeneous catalysts toward C=O bond
formation in the oxidation of arylacycloalkanes in aqueous medium.
Experimental section
Materials and Physical Measurements. All reagents and solvents were purchased from J&K
Chemicals and used without any purification. NMR spectroscopy was recorded on 400 MHz Bruker
Avance-400 spectrometer and the chemical shift was reported relative to the signals of an internal
standard of TMS (0.0 ppm). Powder X-ray diffraction (PXRD) data was recorded on a Bruker D8
Advance X-ray diffractometer using Cu Kα (λ = 1.5418 Å) radiation. The thermal decomposition
characteristic of the samples was analyzed by TGA ranging from 30 to 800 °C by using Netzsch
STA 449C thermal analyzer. An Elementar Vario MICRO Elemental analyzer was performed to the
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