From 2D to 3D: A single-crystal-to-single-crystal photochemical framework transformation and phenylmethanol oxidation catalytic activity

Article abstract of DOI:10.1002/chem.201101321

Lattice salad: A neutral 2D lattice was converted into a 3D porous network (see figure) through a photochemical [2+2] cycloaddition in a single-crystal-to-single-crystal manner. Both systems present interesting catalytic activity accompanied by a 2D 3D st

Full text of DOI:10.1002/chem.201101321

DOI: 10.1002/chem.201101321
From 2D to 3D: A Single-Crystal-to-Single-Crystal Photochemical
Framework Transformation and Phenylmethanol Oxidation Catalytic
Activity
Ming-Hua Xie, Xiu-Li Yang, and Chuan-De Wu*[a]
The solid-state structural transformation of coordination
networks is a fascinating research topic because some result-
ing compounds can not be easily obtained by traditional
synthetic routes.[1] Of particularly interest are solid-state
photochemical [2+2] cycloadditions, which can create new
covalent bonds with high regio- and stereoselectivity.[2] Over
the past decade, solid-state photochemical [2+2] cycloaddi-
tions have been observed in many supramolecular com-
Scheme 1.
plexes[3] and metal–organic compounds.[4–7] However, there
are only a few photochemical [2+2] cycloadditions of the
C=C bonds that occur in the solid state and accompany
single-crystal-to-single-crystal (SCSC) transformations due
to positional and geometrical constraints and the significant
rearrangement of the molecular components, which can de-
stroy the single-crystalline character.[5–7]
Brown crystals of 1 were synthesized by the reaction of
H2L and MnCl2·4H2O in a H2O/DMF mixture at 808C for
10 d. Single-crystal X-ray diffraction analysis revealed that
¯
compound 1 crystallizes in the triclinic P1 space group with
one MnII atom, one L, one aqua ligand, and 1.5 lattice water
molecules in the asymmetric unit.[9] Each MnII atom is octa-
hedrally coordinated to four carboxyl oxygen atoms of three
L, one L pyridine, and one aqua ligand (Figures 1a and 2a).
Two Mn centers are doubly coupled in a syn–anti fashion by
two carboxylate groups of two L ligands, and are further
linked by the second L carboxylate group in chelate fashion
to propagate into a linear network. The 1D chains are fur-
ther linked by the L pyridine groups to extend into a 2D
grid framework with cavity dimensions of 7.80(2)ꢀ
8.94(4) ꢁ2.
Metal–organic frameworks (MOFs) are a type of material
constructed from organic ligands that link metal nodes.[8]
Some photoactive olefin units in MOFs are positioned
within the required distance for [2+2] photodimerization.
To date, most of the SCSC transformations in metal–organic
compounds have been focused on linear bipyridine-type li-
gands.[6] Although many carboxylate ligands containing pho-
toactive groups have been used for the construction of coor-
dination networks, only one example have been used for
photochemical [2+2] cycloaddition in a SCSC manner.[7]
To avoid additional anionic effects in the synthesis of pho-
toreactive MOFs, we have synthesized an E-5-(2-(pyridin-4-
yl)vinyl)isophthalic acid ligand (H2L; Scheme 1). The pyri-
dine and carboxylate groups in H2L can bridge metal nodes
to form neutral MOFs, and the C=C bond can be photoacti-
vated for the topochemical reaction. Herein, we report a 2D
The 2D layered lattices are further packed into a 3D
supramolecular network in an ···AA···-stacked fashion to
give open 1D channels. PLATON calculations indicate that
1 contains 24.3% void space that accommodates lattice
water molecules.[10] Thermogravimetric analysis (TGA) of 1
indicates that a weight loss of 12.4% occurs between 30 and
1118C, which corresponds to the loss of water molecules
(predicted: 12.3%). After a sample of 1 was evacuated at
908C for 6 h under vacuum, powder X-ray diffraction
(PXRD) demonstrated that the framework structure of 1
was maintained upon the removal of all solvent molecules
(Figure S11 in the Supporting Information). The N2 sorption
isotherm of 1 at 77 K has confirmed its permanent porosity,
with a Brunauer–Emmett–Teller (BET) surface area of
7.54 m2 gꢀ1 (Figure 3).
coordination polymer [Mn2L2ACTHUNTRGNEUNG(H2O)2]·3H2O (1) that under-
goes quantitative topochemical [2+2] cycloaddition in a
SCSC manner in a 2D to a 3D porous network structure
transformation with interesting phenylmethanol oxidation
catalytic activity.
[a] M.-H. Xie, X.-L. Yang, Prof. C.-D. Wu
Department of Chemistry, Zhejiang University
Hangzhou 310027 (P.R. China)
Fax : (+86)571-87951895
Note that the L olefinic bonds between the adjacent
layers are positioned parallel in a head-to-tail fashion, and
the distance between the olefinic centers (3.627 ꢁ) meets
Schmidt’s geometric criteria for photochemical [2+2] cyclo-
addition.[11] After crystals of 1 were subjected to Hg light ir-
E-mail: cdwu@zju.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101321.
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COMMUNICATION
Figure 2. Packing diagrams of a) 1 and b) 2 as viewed slightly off the c
axis, showing the structural differences before and after UV irradiation.
Figure 1. a) The packing diagram of the ···AA··· stacked-layer network in
1 viewed down the a axis. b) A view of the 3D framework of 2 down the
a axis.
radiation for 24 h, a photochemical [2+2] cycloaddition
compound [Mn2L’ACHTUNGTRENNUNG(H2O)2]·3H2O (2; H4L’=5,5’-(3,4-diphe-
nylcyclobutane-1,2-diyl)diisophthalic acid; Scheme 1) was
produced.[9]
Single-crystal X-ray diffraction study of photodimerized
product 2 revealed 100% photodimerization accompanied
by an SCSC transformation. The quantitative photoreactivi-
1
ty is evident from the H NMR spectrum (Figure S9 in the
Supporting Information). The 1H NMR spectrum shows
complete disappearance of the signals from olefinic protons
of L (d=7.50 and 7.79 ppm) and appearance of the signal of
the L’ cyclobutane protons (d=4.93 ppm). The purity of the
bulk sample was further confirmed by elementary analysis
and by comparing the simulated and experimental PXRD
patterns (Figure S10 in the Supporting Information). The
SCSC transformation is accompanied by an increase in the a
and b dimensions (1.27 and 1.96%) and a reduction in the c
dimension (3.28%), with a slightly increased cell volume of
1.36%. In compound 2, each MnII center has a distorted oc-
tahedral coordination geometry as described in 1 (Fig-
ure 1b). Most strikingly, the newly formed 2 presents a 3D
porous framework because the four carbon atoms of the cy-
clobutane ring are from two original neighboring layers in 1
Figure 3. N2 adsorption/desorption isotherms for the evacuated samples
~
~
*
of 1 and 2 at 77 K; : adsorption for 1; : desorption for 1; : adsorption
*
for 2; : desorption for 2.
(see Figures 1b and 2b). The resulting Mn-L’-Mn distance
(14.02(7) ꢁ) is slightly longer than the Mn-L-Mn distance
(12.90(7) ꢁ) in the precursor, which subsequently results in
an increased cell volume and enlarged 1D channels running
along the a axis (9.23(2)ꢀ9.35(4) ꢁ2). Calculations using
PLATON showed that the effective pore volume for the in-
clusion of compound 2 is about 238.6 ꢁ3 per unit cell
(882.7 ꢁ3) with slightly increased pore volume of 27.0%.
Topological analysis of 2 reveals that the 3D network is an
unusual (3,3,4,4)-connected tetranodal topology with
a
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C.-D. Wu et al.
Schlꢃfli symbol of (4.6.8)
(4.62.83)2(62.82.102)
E
creased to 97% (Table 1, entry 2). No trace of benzaldehyde
was detected in the absence of catalyst under otherwise
identical reaction conditions (Table 1, entry 3). Catalyst 2
can be easily separated by filtration and was reused in a suc-
cessive run without a decrease in the catalytic activity
(Table 1, entry 4). The PXRD of recovered solid 2 showed
that the structural integrity was maintained during the cata-
lytic reaction (Figure S13 in the Supporting Information).
We also studied the catalytic reusability of 1 under the
ambient light source. Most surprisingly, the benzaldehyde
yield increased to 70% in the second run (Table 1, entry 5)
and reached 98% in the third run (Table 1, entry 6), which
is comparable to that of 2. The PXRD pattern of the recov-
ered solid is similar to that of 2, which is quite different to
ure S14 in the Supporting Information).[12]
The TGA plot confirmed that compound 2 retained the
solvents during the irradiation. The PXRD pattern showed
that the framework structure of 2 was maintained upon the
removal of the solvent molecules but the diffraction is weak-
ened (Figure S12 in the Supporting Information). The N2
sorption measurements showed that the evacuated sample
of 2 possessed permanent porosity with a BET surface area
of 59.06 m2 gꢀ1.
Of particular interest are the frameworks of 1 and 2,
which are comprised of the exposed MnII metal centers ori-
ented towards the 1D channels. Because one labile water
molecule occupies a coordination site of the MnII atom, we
speculated that the Mn centers can be activated as a source
of catalytic active sites. The development of efficient catalyst
systems for the selective oxidation of phenylmethanol to
form benzaldehyde has recently attracted considerable at-
tention.[13,14] To evaluate the catalytic properties of both
compounds, we have employed our system for the selective
oxidation of phenylmethanol.
1
that of freshly prepared solid 1. H NMR spectroscopy sug-
gests that L is completely transformed into L’. To gain in-
sight into the possible transformation mechanism, the cata-
lytic reaction was performed in the absence of light but
under otherwise identical conditions. The benzaldehyde
yield is only 28% for the fist run, 34% for the second run
1
and 33% for the third run (Table 1, entries 7–9). H NMR
After the samples of 1 and 2 were treated under vacuum
at 908C for 3 h, the dried samples were subsequently im-
mersed in phenylmethanol for 12 h. The solid samples were
separated by filtration and thoroughly washed with diethyl
ether to remove the surface-adsorbed solvent molecules.
The solid samples were digested with 10% aqueous NH3 so-
lution, which were subsequently extracted with diethyl
ether. GC analysis suggests that about 0.3 or 0.5 of a phenyl-
methanol molecule per formula unit is found in compound 1
or 2. The catalytic experiment was performed in acetonitrile
with NaIO4 as the oxidant and in the presence of solid 1
under ambient light; the reaction was monitored by using
GC-MS. When the reaction was performed at 608C for 18 h,
the oxidation product was benzaldehyde in 64% yield
(Table 1, entry 1). However, if 2 was used instead of 1 to
prompt the catalytic reaction, the benzaldehyde yield in-
spectroscopy suggests that L remained intact during these
catalytic experiments, which indicates that the transforma-
tion from 1 to 2 was induced by the ambient light source.
The distinct catalytic activities of 1 and 2 can be attributed
to the different cavity sizes for the diffusion of the substrate,
as mentioned above. For comparison, MnCl2·4H2O was used
as a catalyst under identical conditions. However, only an
80% yield of benzaldehyde was obtained, which is inferior
to that of 2 (Table 1, entry 10). No trace of benzaldehyde
was detected when phenylmethanol was treated with the hot
filtrate from a mixture of 1 or 2, NaIO4, and acetonitrile
under otherwise identical conditions, which proves the
nature of the heterogeneous oxidation process.
In summary, a novel approach to generate a neutral 2D
MOF that has C=C centers at a reasonable distance for pho-
tochemical [2+2] cycloaddition has been demonstrated. The
2D!3D topochemical structural transformation represents
the first example of 2D!3D SCSC photochemical transfor-
mation. Finally, the two MOFs present interesting catalytic
activity for phenylmethanol oxidation and an associated
2D!3D structural transformation induced by ambient light
under the catalytic conditions.
Table 1. Selective oxidation of phenylmethanol.[a]
Entry
Catalyst
Yield%[b]
1
1
64
2
2
97
Experimental Section
3
4
5
6
7
8
9
10
–
2
1
1
1
1
1
0
95[c]
70[d]
98[c]
28[e]
34[d,e]
33[c,e]
80
Synthesis of [Mn2L2ACHTUNGTRNEGN(U H2O)2]·3H2O (1): Ligand H2L (10 mg, 0.037 mmol)
and MnCl2·4H2O (20 mg, 0.10 mmol) were dissolved in a mixture of
DMF (10 mL) and H2O (16 mL). After the resulting yellow solution was
heated at 808C for ten days, brown crystals were filtered, washed with
DMF, H2O, and EtOH, and dried at RT (yield: 6.0 mg; 44.2%, based on
H2L). IR (KBr pellet): n˜ =1608 s, 1560 s, 1449 s, 1424 m, 1387 s, 1293 w,
1253 w, 1224 w, 1205 w, 1015 m, 964 m, 870 w, 814 m, 781 m, 731 s,
547 cmꢀ1 m; elemental analysis calcd. (%) for 1: C 49.06, H 3.84, N 3.81;
found: C 48.30, H 3.82, N 3.82.
MnCl2·4H2O
[a] Catalyst (0.005 mmol), phenylmethanol (0.1 mmol), and NaIO4
(0.15 mmol) in CH3CN (2 mL) were stirred at 608C for 18 h under ambi-
ent light. [b] Yields were determined by using GC on a SE-54 column.
[c] The third cycle. [d] The second cycle. [e] Performed in the absence of
light.
Synthesis of [Mn2L’
ACHTNUGERTN(NGNU H2O)2]·3H2O (2): A crytalline sample of 1 (15 mg)
was placed between two quartz glass slides at a distance of 10 cm from a
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Chem. Eur. J. 2011, 17, 11424 – 11427

A Photochemical Framework Transformation
COMMUNICATION
300 W Hg lamp, which was irradiated under the ambient atmosphere at
RT for 24 h to afford yellow crystals of 2 in quantitative yield. Because 2
is insoluble in common organic solvents and water, the crystals of 2 were
decomposed by using an aqueous 10% HCl solution. The resulting solid
was filtered, washed with H2O and EtOH, and dried in vacuum to obtain
H4L’. 1H NMR ([D6]DMSO, 500 MHz, TMS): d=13.30 (brs, 4H), 8.44–
8.48 (m, 4H), 8.19 (s, 2H), 8.02 (s, 4H), 7.59 (s, 4H), 4.93 ppm (s, 4H);
elemental analysis calcd. (%) for 2: C 49.06, H 3.84, N 3.81; found: C
48.46, H 3.79, N 3.92.
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This work was financially supported by the NNSF of China (grant nos.
21073158 and J0830413), Zhejiang Provincial Natural Science Foundation
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Keywords: crystal
heterogeneous catalysis
photochemistry
engineering
·
cycloaddition
·
·
·
metal–organic frameworks
[9] Crystal data for 1: C30H28Mn2N2O13; Mr =734.42; triclinic; space
¯
group P1; a=7.7506(7), b=9.7954(7), c=13.034(1) ꢁ; a=69.561(8),
b=86.021(8), g=70.154(8)8; V=870.81(13) ꢁ3; Z=1; T=293(2)K;
R
int =0.0319; 1calcd =1.400 gcmꢀ3
;
m=0.788 mmꢀ1
;
FACTHNGUTERNNU(G 000)=376;
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GOF=1.055; 6313 reflections measured; 3553 unique. The final
R1 =0.0773, wR2 =0.2001 for 2271 observed reflections with I>2s(I)
and 211 parameters. Crystal data for 2: C30H28Mn2N2O13; Mr =
¯
734.42; triclinic; space group P1; a=7.8494(4), b=9.9876(6), c=
12.6067(9) ꢁ; a=104.131(6), b=91.094(5), g=111.882(6)8; V=
882.69(9) ꢁ3; Z=1; T=293(2)K; Rint =0.0485; 1calcd =1.382 gcmꢀ3
;
m=0.788 mmꢀ1; F
ACTHNUGRTENUNG(000)=376; GOF=1.235; 6071 reflections mea-
sured; 3596 unique. The final R1 =0.0811, wR2 =0.2057 for 1615 ob-
served reflections with I>2s(I) and 211 parameters. CCDC-808588
(1) and 808589 (2) contain the supplementary crystallographic data
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Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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Received: April 30, 2011
Published online: September 9, 2011
Chem. Eur. J. 2011, 17, 11424 – 11427
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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