Drastic enhancement of catalytic activity via post-oxidation of a porous MnII triazolate framework

Article abstract of DOI:10.1002/chem.201403123

Mn^III is a powerful active site for catalytic oxidation of alkyl aromatics, but it can be only stabilized by macrocyclic chelating ligands such porphyrinates. Herein, by using benzobistriazolate as a rigid bridging ligand, a porous Mn^II azolate framework with a nitrogen-rich coordinated environment similar to that of metalloporphyrins was synthesized, in which the Mn^II ions can be post-oxidized to Mn^III to achieve drastic increase of catalytic (aerobic) oxidation performance. Later could be better: A robust and redox-active, porous coordination framework has been designed and constructed, in which the Mn^II ions on the pore surface can be post-oxidized to form unstable Mn^III sites with drastically enhanced activity for catalytic oxidation of alkylaromatics (see figure; TBHP=tert-butyl hydroperoxide).

Full text of DOI:10.1002/chem.201403123

DOI: 10.1002/chem.201403123
Communication
&
Heterogeneous Catalysis
Drastic Enhancement of Catalytic Activity via Post-oxidation of
a Porous MnII Triazolate Framework
Pei-Qin Liao, Xu-Yu Li, Jie Bai, Chun-Ting He, Dong-Dong Zhou, Wei-Xiong Zhang, Jie-
Peng Zhang,* and Xiao-Ming Chen[a]
ilar PCP structures easily decompose during activation and/or
Abstract: MnIII is a powerful active site for catalytic oxida-
tion of alkyl aromatics, but it can be only stabilized by
oxidation.[8]
Solvothermal reaction of MnCl2 and 1H,5H-benzo(1,2-d:4,5-
macrocyclic chelating ligands such porphyrinates. Herein,
d’)bistriazole (H2bbta) at 2108C gave single crystals of [Mn2Cl2-
(bbta)(H2O)2]·guest (MAF-X25, 1g). The bulk microcrystalline
sample of 1g can be obtained at 758C. Single-crystal X-ray dif-
fraction analysis revealed that 1g possesses a three-dimension-
by using benzobistriazolate as a rigid bridging ligand,
a porous MnII azolate framework with a nitrogen-rich coor-
dinated environment similar to that of metalloporphyrins
was synthesized, in which the MnII ions can be post-oxi-
al (3D) coordination framework with a honeycomb-like struc-
dized to MnIII to achieve drastic increase of catalytic (aero-
¯
ture (Figure 1a). The asymmetric unit of 1g (space group R3m)
bic) oxidation performance.
1
1
1
contains = MnII atom, = deprotonated bbta2À ligand, = ClÀ
2
4
2
1
ligand, and = coordinated H2O molecule (Figure 1b). Each MnII
2
is coordinated in a distorted octahedral fashion by three nitro-
gen atoms from three bbta2À ligands, two m-ClÀ anions and
a terminal H2O molecule, and each bbta2À ligand coordinates
to six MnII ions. The MnII ions are bridged by m3-triazolate ring
and m-ClÀ (Mn···Mn 3.6885(7) ꢀ) to form a 31 helical chain (Fig-
ure S1 in the Supporting Information), which are connected by
the phenyl backbone of bbta2À to generate a pillared-rod
structure with large 1D channels (dꢀ11 ꢀ if all guest and the
coordinating water are removed) parallel to the helical chains.
Similar structures have been observed in [M2(dobdc)(H2O)2]
(MOF-74/CPO-27; H4dobdc = 2,5-dihydroxyl-1,4-benzenedicar-
boxylic acid; M = Mg, Mn, Fe, Co, Ni, Zn)[8a–d] and [Mn2Cl2(bdt)-
(DMF)2] (H2bdt=1,4-benzeneditetrazole, DMF=N,N-dimethyl-
formamide).[8e] Because of the lower symmetries of dobdc4À
and bdt2À, these compounds crystallize in a slightly lower-sym-
Because pore size, shape and functionality can be tailored over
a wide range, porous coordination polymers (PCPs) are very at-
tractive as heterogeneous catalysts.[1] The MnIII ion is an effi-
cient active site for catalytic oxidation of alkyl aromatics to cor-
responding ketones.[2] Catalytic active MnIII centers are general-
ly chelated by macrocyclic (such as porphyrinates) or quasi-
macrocyclic (such as Salen) ligands.[3] Without protection of the
macrocyclic or polydentate chelating ligand, MnIII is usually un-
stable in solution and easily disproportionate to MnII and
MnIV,[4] which makes it difficult to directly synthesize a PCP with
MnIII active sites.
In very stable/robust mesoporous organosilicas and zeolites,
unstable metal ions such as MnIII, CuI, and ZnI with high cata-
lytic activities are usually obtained by redox treatment of the
more stable precursors MnII,[5] CuII,[5] and Zn0,[6] respectively.
However, this strategy can be hardly applied in PCPs because
they are less robust than the covalent adsorbents.[7] In this
work, we report a porous metal azolate framework (MAF)
based on MnII and a rigid bridging bis-triazolate ligand, in
which the MnII ions can be post-oxidized to MnIII to drastically
enhance the catalytic activity for (aerobic) oxidation of alkyl ar-
omatics. The triazolate donors are beneficial for stabilization of
not only the MnIII ions but also the porous coordination net-
work during the harsh oxidation reaction conditions, while sim-
¯
metry space group R3. According to the bridging lengths of
the ligands, the pore size of [Mn2Cl2(bbta)] is similar to those
of [M2(dobdc)] but much smaller than that of [Mn2Cl2(bdt)].
Preliminary synthetic experiments showed that, FeII, CoII, NiII,
CuII analogues of 1g can be also synthesized under similar re-
action conditions (Figure S2 in the Supporting Information), al-
though their purity still needs to be improved. On the other
hand, ZnII always led to a known cubic structure [Zn5Cl4-
(bbta)3].[9]
Thermogravimetry (TG) and powder X-ray diffraction (PXRD)
analysis showed that 1g releases all guest and coordinated
water molecules below 2008C and retains high crystallinity up
to at least 2508C (Figure S3 in the Supporting Information),
which is similar to [M2(dobdc)(H2O)2].[10] Additionally, 1g is
stable in hot acidic/basic (808C, 4ꢁpHꢁ12) solution for at
least 48 h (Figure S3). On the other hand, [Mn2Cl2(bdt)(DMF)2]
transforms to a less porous, amorphous structure after guest
removal,[8e] which might be ascribed to the high acidity and
flexibility of the long bistetrazolate ligand. Single-crystal struc-
ture of guest-free 1 was measured to confirm the retention of
[a] P.-Q. Liao, X.-Y. Li, J. Bai, C.-T. He, D.-D. Zhou, W.-X. Zhang,
Prof. Dr. J.-P. Zhang, Prof. Dr. X.-M. Chen
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry
State Key Laboratory of Optoelectronic Materials and Technologies
School of Chemistry and Chemical Engineering
Sun Yat-Sen University, Guangzhou 510275 (P.R. China)
E-mail: zhangjp7@mail.sysu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403123.
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(MAF-X25ox or 1’). Scanning electron microscopy (SEM) images
of 1 and 1’ showed that the crystals crashed but remained
well crystalline after treatment with H2O2 (Figure S4 in the Sup-
porting Information). The oxidation states of the metal ions in
1 and 1’ were characterized by X-ray photoelectron spectros-
copy (XPS) and electron paramagnetic resonance (EPR) spec-
troscopy (Figure S5 in the Supporting Information). The bind-
ing energies of Mn 2P2/3 (642.2 eV) and Mn 2P1/2 (654.2 eV) as
well as the shake-up satellite peaks in the XPS of 1 confirm the
oxidation state of +2 for manganese in the synthesized sam-
ples. The MnII species in 1 can be also characterized by the
strong broad EPR signal with g=1.997. Because the MnII coor-
dination sphere consists of three kinds of coordination bonds,
a well resolved signal with a hyperfine structure of six principal
lines cannot be observed. The shake-up peak is invisible in the
XPS of 1’, indicating that some MnII ions have been oxidized.[12]
On the other hand, the EPR spectrum of 1’ was silent,[13] also
suggesting the existence of MnIII in 1’. It is worth pointing out
that, without protection of a macrocyclic or quasi-macrocyclic
ligand, MnIII will be easily oxidized to MnIV (MnO2) under alka-
line conditions. Even chelated by a Salen ligand in the PCP,
H2O2 can also break the coordination bond between MnIII and
the Salen ligand.[14] Fourier transformed infrared (IR) spectros-
copy of 1’ showed the characteristic stretching vibration at
3600 cmÀ1 (strong and narrow) and in-plane bending vibration
at 1500–1300 cmÀ1 (strong and broad) for the OHÀ ions[15] and
also the characteristic stretching vibration at 562 cmÀ1 for the
MnIIIÀOH coordination bond (Figure 2b).[16] N2 sorption iso-
therm of 1’ measured at 77 K (Figure 2a) showed a type-I char-
acteristic with saturated uptake of 293 cm3 gÀ1, corresponding
to an apparent Langmuir surface area of 1286 m2 gÀ1 and
a pore volume of 0.46 cm3 gÀ1. Assuming complete oxidation,
the pore volume can be calculated as 0.38 cm3 gÀ1 (void=
48.4%, 1calcd =1.285 gcmÀ3). Therefore, about 50% MnII ions
were oxidized to be MnIIIÀOH. Although the crystal easily
cracked in the oxidation process, we successfully solved the
crystal structure of 1’ by Rietveld refinement of its PXRD pat-
tern (Figure 1d, and Figure S6 and Table S1 in the Supporting
Information). The unit-cell volume of guest-free 1’ is about 7%
smaller than that of 1g, because all coordination bonds in 1’
are significantly shortened by about 0.05 ꢀ, being consistent
with the smaller ionic radius of MnIII. The refinement also sug-
gests that 1’ is composed of about 50:50 MnII/MnIII-OH. These
data further confirm the retention of framework integrity and
reduction of pore size mainly due the introduction of coordi-
nated hydroxide groups. Under the oxidation condition for 1,
[Mn2(dobdc)(H2O)2] becomes amorphous (Figure S7 in the Sup-
porting Information), which might be ascribed to its poor sta-
Figure 1. a) The 3D network of 1, and coordination environments of: b) 1g,
c) 1, and d) 1’ (thermal ellipsoids are drawn with probability 30%; symmetric
codes: A: 4/3Ày, À1/3+xÀy, À1/3+z; B: 1/3+xÀy, 2/3Ày, 2/3Àz; C: 5/
3Àx+y, 4/3Àx, 1/3+z; D: 2/3+y, À2/3+x, 4/3Àz; E: 5/3Àx, 1/3Ày, 2/3Àz;
F: 1Ày, 1Àx, z).
host framework and complete removal of guest and coordinat-
ed molecules (Table S1 in the Supporting Information). After re-
moving the coordinated H2O molecules, the coordination fash-
ion of MnII was changed from octahedral geometry to tetrago-
nal-pyramid geometry with an open coordination site (Fig-
ure 1c). To confirm the porosity of activated 1, N2 sorption iso-
therm was measured at 77 K (Figure 2a), which showed
a type-I characteristic with saturated uptake of 358 cm3 gÀ1,
giving an apparent Langmuir surface area of 1566 m2 gÀ1 and
a pore volume of 0.56 cm3 gÀ1. For comparison, the crystallog-
raphy pore volume of 1 can be calculated as 0.55 cm3 gÀ1
(void=59.2%, 1calcd =1.074 gcmÀ3).
Considering the coordination environment of 1 is similar to
that of metalloporphyrin and hydrogen peroxide can oxidize
MnII to MnIII in small molecular complexes,[11] we treated 1 with
H2O2 under alkaline condition to obtain the oxidation product
bility under aqueous/basic solution.[17] Recently, Dinca et al.
˘
demonstrated that oxidation of [Mn2(dobdc)(H2O)2] by iodo-
benzene dichloride in dichloromethane at À358C give rise to
the oxidized quinoid ligand (2,5-dicarboxyl-p-benzoquinone)
rather than MnIII ions. When more aggressive oxidants such as
O3 and Cl2 were used, [Mn2(dobdc)(H2O)2] decomposed to give
Mn3O4.[8f] In contrast, 1’ exhibited high chemical stability in hot
acidic/basic solutions similar to 1 (Figure S8 in the Supporting
Information), so that it can endure the more severe oxidation
Figure 2. a) N2 isotherms, and b) IR spectra of 1 and 1’.
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conditions. In this context, the
nitrogen-rich and more stable
bistriazolate ligand should play
an important role in stabilizing
the MnIII ions in 1’.
Table 1. Oxidation of alkyl benzenes for the formation of phenyl ketones.
Entry
Substrate
Catalyst
Product
Conv.
[%]
Sel.
[%]
1
2
3
4
5
1
22
97
21
66
75
76
We compared the catalytic ac-
tivities of 1 and 1’ for oxidation
of ethylbenzene (EB). EB oxida-
tion was performed in a mixed
solvent of acetonitrile, acetic
acid, and water at 808C for 12 h
using tert-butyl hydroperoxide
(TBHP) as the oxidant. The oxi-
dized product was examined by
gas chromatography-mass spec-
trometry (GC-MS), which showed
that 1 can only convert 22%
with a selectivity of 76% for ace-
tophenone (entry 1 in Table 1
and Figure S9a in the Supporting
Information), which is consistent
with the poor catalytic activity of
MnII for oxidation of alkyl aroma-
tics.[3a] On the other hand, the
conversion and selectivity of 1’
are 97 and 99%, respectively
(entry 2 in Table 1 and Fig-
ure S9b), confirming that the
MnIII ions in 1’ are similar to the
well-known MnIII–porphyrinate
species.[2a,3a,18] It needs to be
noted that 1’ displayed higher
catalytic performance than ho-
mogeneous catalysts MnCl2 and
[Mn(TPP)Cl] (H2TPP=5,10,15,20-
tetraphenylporphyrin) for the ox-
idation of the small substrate EB
(entries 3 and 4 in Table 1 and
Figures S10 and 11 in the Sup-
porting Information).
1’
>99
68
MnCl2
Mn(TPP)Cl
1’
86
97
6
1’
23
98
7
8
9
1’
64
25
93
93
94
1’
Mn(TPP)Cl
>99
10
11
12
1’
1’
1
94[a]
37[b]
5[b]
96
79
63
Reaction conditions: alkyl benzene (0.10 mmol), TBHP (0.15 mmol), catalyst (0.01 mmol), acetonitrile (0.5 mL),
acetic acid (0.01 mL), water (2.5 mL), and m-dichlorobenzene (internal standard): 0.13 mmol were stirred at
808C for 10 h. Conversion [%] and selectivity [%] were determined by GC-MS on an HP-5 MS column. [a] The
fifth cycle. [b] Reaction conditions: catalyst (0.05 mmol), EB (20 mmol), water (8 mL), acetic acid (0.05 mL),
oxygen pressure (3.0 MPa), T (1208C) and time (14 h).
To demonstrate that the cata-
lytic reaction occurred in the
channel of 1’, we carried out the
oxidation reactions for larger substrates. As showed in en-
tries 5–8 in Table 1 and Figures S12–15 in the Supporting Infor-
mation, the conversions of propylbenzene and tetralin are ob-
viously lower than for EB, because the molecular sizes of these
three substrates are similar to the effective aperture diameter
of 1’ (6.8 ꢀ) and EB is the smallest one of them (Table S2 in the
Supporting Information). For much larger substrates diphenyl-
methane and 4-ethylbiphyl, the conversions are very low, indi-
cating that they cannot access the internal pores and the cata-
lytic reactions mainly occur on the outer crystal surfaces. On
the other hand, the homogeneous catalyst [Mn(TPP)Cl]
showed better catalytic activity for oxidation of diphenylme-
thane than for EB (entries 4 and 9 in Table 1 and Figure S16 in
the Supporting Information), because the methylene of diphe-
nylmethane is more activated.
To further demonstrate the heterogeneous nature and stabil-
ity of 1’, it was filtrated out from the reaction mixture and ad-
ditional reactants were added into the filtrate to carry out the
reaction again. As expected, just a trace of product could be
generated using the filtrate as catalyst. Inductively coupled
plasma–atomic emission spectrometry showed that, just 0.17%
Mn ion was leached into the reaction solution after the reac-
tion. Compound 1’ can be easily separated from the reaction
mixture. After five consecutive catalytic batches, the PXRD pat-
tern and sample weight of recovered 1’ were almost un-
changed (Figure S7 in the Supporting Information), and the
product yield only marginally decreased (entry 10 in Table 1,
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Communication
and Figure S17 in the Supporting Information). As shown in
Figure S18 in the Supporting Information, we also studied the
effect of reaction time in the conversion of EB over 1’, which
achieved a turnover frequency (TOF) of 118 hÀ1, being higher
than for most heterogeneous catalysts such as metal oxides,
polyoxometalates and PCPs.[18a,19]
Acknowledgements
This work was supported by the “973 Project” (2012CB821706
and 2014CB845602), NSFC (21225105, 21121061, and
21371181),
and
NSF
of
Guangdong
Province
(S2012030006240).
It has been shown that aluminophosphate molecular sieves
containing CoIII or MnIII species can catalyze the oxy-functionali-
zation of alkanes using oxygen or air as a greener oxidant.[5,20]
Considering that PCPs have been rarely studied for catalytic
aerobic oxidation,[1a,b,21] we studied the catalytic activities of
1 and 1’ for aerobic oxidation of EB under a typical reaction
condition (in a mixed solvent of acetonitrile and acetic acid at
1208C for 14 h using oxygen as the oxidant). Compound 1’ ex-
hibited much higher activity and selectivity than 1, and the
blank reaction carried out without oxygen showed no oxida-
tive products (entries 11–12 in Table 1 and Figure S19 in the
Supporting Information). It should be noted that, aerobic oxi-
dation of EB using PCP as the catalysts has been rarely realized
in the absence of the co-catalyst N-hydroxyphthalimide
(NHPI).[21a]
Keywords: aerobic oxidation · alkyl aromatics · heterogeneous
catalysis metal–organic framework postsynthetic
modifications
·
·
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H2bbta (0.020 g, 0.126 mmol), and isopropanol (0.8 mL) was sealed
in a 2 mL glass tube and heated at 2108C for 3 days, then cooled
to room temperature. The resulting pink, rod-like crystals were col-
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ca. 43% based on H2bbta). Microcrystalline powder of 1g was ob-
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H2bbta (0.020 g, 0.126 mmol), N,N-dimethylformamide (DMF, 4 mL),
methanol (4 mL) and 12m HCl (0.2 mL) in a 15 mL Teflon reactor at
708C for 72 h (yield 72% based on H2bbta); anal. calcd (%) for
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[Mn2Cl2(bbta)(H2O)2]·0.55DMF·1.45H2O (C4.65H9.75N3.55ClMnO3):
22.00, H 3.87, N 19.59; found: C 22.16, H 3.76, N 19.54.
C
Synthesis of 1: The as-synthesized complex 1g was placed in the
quartz sample tube and dried under high vacuum at 1808C for
24 h.
Synthesis of 1’: A solution of H2O2 (0.5 mmol) in water (10 mL) was
slowly added in 12 h to a suspension of 1 (0.0169 g, 0.05 mmol) in
CH3CN (5.0 mL), water (5.0 mL), and triethylamine (0.2 mmol) at
08C under stirring, during which the color of the suspension
turned brown gradually. The mixture was further stirred for 2 days
at room temperature, and then filtered, washed with CH3CN and
dried in nitrogen flow to give the dark green oxidized sample 1’
(0.0152 g, yield 97%).
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Received: April 16, 2014
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
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ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
Later could be better: A robust and
redox-active, porous coordination
framework has been designed and con-
structed, in which the MnII ions on the
pore surface can be post-oxidized to
form unstable MnIII sites with drastically
enhanced activity for catalytic oxidation
of alkylaromatics (see figure).
&
Heterogeneous Catalysis
P.-Q. Liao, X.-Y. Li, J. Bai, C.-T. He,
D.-D. Zhou, W.-X. Zhang, J.-P. Zhang,*
X.-M. Chen
&& – &&
Drastic Enhancement of Catalytic
Activity via Post-oxidation of a Porous
MnII Triazolate Framework
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&
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
6
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!