Chiral metal-organic frameworks with tunable open channels as single-site asymmetric cyclopropanation catalysts

Article abstract of DOI:10.1039/c2cc32232c

A pair of interpenetrated and non-interpenetrated chiral metal-organic frameworks with the same catalytic sites but different open channel sizes catalysed asymmetric cyclopropanation of substituted terminal alkenes with excellent diastereoselectivities (up to 9.6) and enantioselectivities (up to >99%).

Full text of DOI:10.1039/c2cc32232c

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COMMUNICATION
Chiral metal–organic frameworks with tunable open channels as
single-site asymmetric cyclopropanation catalystswz
Joseph M. Falkowski, Sophie Liu, Cheng Wang and Wenbin Lin*
Received 5th January 2012, Accepted 8th May 2012
DOI: 10.1039/c2cc32232c
A pair of interpenetrated and non-interpenetrated chiral metal–
organic frameworks with the same catalytic sites but different
open channel sizes catalysed asymmetric cyclopropanation of
substituted terminal alkenes with excellent diastereoselectivities
(up to 9.6) and enantioselectivities (up to >99%).
[L is deprotonated (R,R)-(ꢀ)-N,N0-bis(3-acrylate-5-tert-butyl-
salicylidene)-1,2-cyclohexanediamine, Fig. 1a], into the
Zn-carboxylate MOF of the primitive cubic unit (pcu) topology.
The different open channel sizes of this pair of chiral MOFs
exert a significant influence on the conversions and stereo-
selectivities of asymmetric cyclopropanation reactions.
Metal–organic frameworks (MOFs) represent an interesting
class of hybrid materials composed of organic bridging ligands
and metal ion or metal cluster connecting points.1 In the
past 15 years, MOFs have been explored for a wide range of
applications, including nonlinear optics,2 gas storage,3
chemical separations,4 molecular sensing,5 and drug delivery.6
Since MOFs are typically constructed from molecular building
blocks under mild conditions, they provide a great opportunity
to immobilize homogeneous catalysts7 in a modular and
tunable fashion leading to truly single-site solid catalysts,
which offer several advantages over their solution counter-
parts, including recyclability and reusability as well as facile
removal of the toxic catalyst components from the organic
products. Additionally, the highly ordered nature of MOFs
allows for the precise characterization of their catalytic sites
through X-ray diffraction studies, which is not possible in
other heterogenized catalysts.8 The detailed structural char-
acterization of MOF catalytic sites can in turn allow for the
elucidation of the structure–function relationships in this
unique class of single-site solid catalysts.
Although many MOFs have been examined as potential
heterogeneous catalysts,7,9 examples of stereoselective MOF
catalysts are still scarce.10 Highly efficient asymmetric MOF
catalysts have been generated either via a post-synthesis
modification strategy11 or by direct incorporation of catalyti-
cally competent bridging ligands into MOFs.12 In this
work, we report the synthesis of a pair of interpenetrated
and non-interpenetrated chiral MOFs with the same catalytic
sites but different open channel sizes by direct incorpora-
tion of an elongated dicarboxylate ligand, [Ru(L)(Py)2]Cl
Department of Chemistry, CB#3290, University of North Carolina,
Chapel Hill, NC 27599, USA. E-mail: wlin@unc.edu;
Fig. 1 (a) Synthesis of CMOF-1 and 2 from the [Ru(L-Me2)(Py)2]Cl
ligand. Stick and polyhedron structure model of 2-fold interpenetrated
1 (b) and non-interpenetrated 2 (c): blue tetrahedron = ZnO4, grey
node = C, red node = O, blue node = N, dark green node = Ru,
green network = the second set of the network. Space-filling model of
1 (d) and 2 (e) as viewed along the [10ꢀ2] direction.
Web: http://www.chem.unc.edu/people/faculty/linw/wlindex.html
w This article is part of the ChemComm ‘Chirality’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
details and characterization data including TGA, NMR, GC, PXRD,
and X-ray structures. CCDC 860688. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2cc32232c
c
6508 Chem. Commun., 2012, 48, 6508–6510
This journal is The Royal Society of Chemistry 2012

The enantiopure RuII complex Ru(L-Me2)(Py)2 was
synthesized by a metathesis reaction between the potassium
salt of L-Me2 and [RuCl2(p-cymene)]2, which was hydrolyzed
by saponification followed by acidification with dilute hydro-
chloric acid. The resulting mixture was further oxidized with
nitric acid in air followed by ion exchange with chloride
to afford the pure RuIII-Salen derived dicarboxylic acid,
[Ru(L-H2)(Py)2]Cl. The diamagnetic complex [Ru(L-Me2)(Py)2]
was characterized by 1H and 13C{1H} NMR spectroscopy
whereas the paramagnetic [Ru(L-H2)(Py)2]Cl was characterized
by electrospray ionization-mass spectrometry.
Solvothermal reactions of Zn(NO3)2ꢁ6H2O with [Ru(L-H2)-
(Py)2]Cl in N,N-dibutylformamide (DBF)/dimethylformamide
(DMF) or diethylformamide (DEF) with added ethanol
resulted in single crystals of [Zn4(m4-O){[RuL(Py)2]Cl}3]2ꢁ
10DBFꢁ7DMF (CMOF-1) and [Zn4(m4-O){[RuL(Py)2]Cl}3]ꢁ
51DEF (CMOF-2), respectively.y Both 1 and 2 were constructed
from the octahedral Zn4(m4-O)(carboxylate)6 secondary building
blocks (SBUs), which are interconnected by the linear dicarboxy-
late ligand [RuL(Py)2]Cl to give 3D frameworks of the pcu
topology. 1 and 2 adopt two-fold interpenetrated and non-
interpenetrated structures, respectively (Fig. 1b and c),
presumably as a result of the different solubility of L-H2 in
different formamides. 1 crystallizes in the R32 space group,
with the asymmetric unit containing two [RuL(Py)2]Cl ligands
and two-thirds of the Zn4(m4-O) cluster. The two sets of
interpenetrated nets in 1 are related to each other by a two-
fold axis. CMOF-1 possesses 3-D inter-connected zigzag
channels (Fig. 1d), with the largest channel dimensions of
0.7 ꢂ 0.7 nm2 and a PLATON-calculated void space of 74.0%.
Although large crystals of 2 could be synthesized, their crystal-
linity is quite poor. Repeated attempts led to one single-crystal
diffraction dataset of 2.1 A resolution. 2 has the same asymmetric
unit as 1 but crystallizes in the R3 space group. The absence of
the 2-fold axis in the R3 space group (as compared to the R32
space group for 1) leads to the non-interpenetrated structure for
2 (ESIz). Based on the structural model, 2 possesses 3-D inter-
connected channels of 1.9 ꢂ 1.9 nm2 dimensions (Fig. 1e) with an
87.5% void space as calculated by PLATON.
Fig. 2 (a) TGA traces for 1 and 2. The solvent weight loss for 1 and 2
is 45 and 75%, respectively. 1 and 2 were soaked in DMF prior to
TGA analyses. (b) UV-Vis spectra of BBR-250 released from 1 and 2.
(c) PXRD patterns of 1, 1R, 1R after catalysis, and 2. (d) Vis-NIR
spectra of the oxidized (1) and reduced (1R) forms of CMOF-1 after
dissolution in MeOH/Na2EDTA.
(39 wt%) had much higher BBR-250 uptake capacity than the
2-fold interpenetrated 1 (14 wt%).
The new pair of CMOFs was screened for their catalytic
activities in the asymmetric cyclopropanation of substituted
olefins. Like the previously reported Ru-salen derived
CMOFs,13 1 and 2, in their RuIII oxidation states, are not
active catalysts for the cyclopropanation reactions (Table 1,
entry 3). However, 1 and 2 can be readily reduced to their RuII
counterparts 1R and 2R, respectively, upon treatment with
LiBEt3H or NaB(OMe)3H, leading to active catalysts for
asymmetric cyclopropanation of substituted olefins. The
reduction of 1 and 2 to afford 1R and 2R was indicated
by their colour change from dark green to dark red. The
characteristic RuIII-salen LMCT bands at 867 nm disappeared
in the solution Vis-NIR spectra of 1R and 2R upon dissolution
in MeOH/Na2EDTA (Fig. 2d). This result was further
The catenation assignments of 1 and 2 were supported by
several additional evidences. First, the powder X-ray diffrac-
tion (PXRD) pattern of 2 is very similar to that of a 2-fold
interpenetrated CMOF based on the [MnL(OAc)] bridging
ligands and Zn4(m4-O)(carboxylate)6 SBUs (ESIz),12b indicating
their isostructural nature. Although 1 and 2 have the same
systematic absence, the relative peak intensities are quite differ-
ent, consistent with their different structures (Fig. 2c). Second, 1
and 2 exhibit very different solvent weight loss in the 25 to 250 1C
temperature range of 45% and 75%, respectively (Fig. 2a).
Third, 1 and 2 exhibit very different Brilliant Blue R-250
(BBR-250) dye uptake of 14% and 39%, respectively (Fig. 2b).
N2 adsorption studies do not necessarily provide meaningful
information on the porosity of CMOFs with large open
channels, presumably due to framework distortion upon
removal of the solvent molecules. Instead, the uptake of bulky
dye molecules by MOFs has recently been used by us to quali-
tatively assess their substrate-accessible pore volumes.14 Both 1
and 2 allow the inclusion of BBR-250, indicating the presence of
large open channels in them, but the non-interpenetrated 2
Table 1 Cyclopropanation of terminal olefins catalysed by 1R and
2Ra
trans cis
Yield (%) ee (%) ee (%) dr
Entry Cat
R
1
2
3
4
5
6
7
8
9
10
1R
2R
2
Ph
Ph
Ph
39
55
93
94
0
92
79
76
80
40
19
80
80
92
35
93
95
>99
89
49
21
85
7.1
9.6
2.3
11.7
2.6
2.6
20
1.7
1.8
1.5
o2
Ru(L-Me2)(py)2 Ph
1R
2R
Ru(L-Me2)(py)2 OEt
1R
2R
56
22
OEt
OEt
22
18
CH3(CH)2 11
CH3(CH)2 19
Ru(L-Me2)(py)2 CH3(CH)2
8
a
Reactions were carried out at room temperature with 2 mol%
catalyst loading (based on L) and 15 equiv. of olefin. The yields and
selectivities were determined by GC on a b-Dex 120 or a 225 chiral
column.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 6508–6510 6509

supported by the diffuse reflectance Vis-NIR spectra of 1R and
2R solids (ESIz).
graduate fellowship under the DOE contract number
DE-AC05-06OR23100. C.W. acknowledges the UNC
Department of Chemistry for an Ernest L. Eliel Fellowship.
S.L. was supported by a UNC William W. and Ida W. Taylor
Fellowship.
As shown in Table 1, both 1R and 2R are competent
cyclopropanation catalysts for substituted alkenes. In the case
of styrene, the non-interpenetrated 2R gave higher isolated
yields of cyclopropanation products than the 2-fold inter-
penetrated 1R, presumably as a result of the larger open
channels in 2R that facilitate the substrate and product diffu-
sion through the CMOFs. This trend was not observed for
ethyl vinyl ether, possibly due to the limited steric demand for
this substrate. The isolated yields of cyclopropanation
products afforded by 2R rival those of the homogeneous
control catalyst Ru(L-Me2)(py)2. For styrene and ethyl
vinyl ether substrates, 1R and 2R gave diastereoselectivities
(dr’s) and enantioselectivities (ee’s) comparable to those of
Ru(L-Me2)(py)2. Comparison of the present results to our
earlier asymmetric cyclopropanation reactions catalysed by
CMOFs built from shorter Ru-salen-derived dicarboxylic acid
[Ru(L0)(py)2] (L0 is the deprotonated form of (R,R)-(ꢀ)-N,N0-
(3-carboxyl-5-tert-butylsalicylidene)-1,2-cyclohexanediamine;
the 2-fold interpenetrated and non-interpenetrated MOFs are
herein denoted 3 and 4, respectively) reveals several interesting
insights. First, while 1R is catalytically competent, the 2-fold
interpenetrated 3 is totally inactive. This result supports the
heterogeneous nature of all these CMOF catalysts (3 would
have been active if it had dissolved under catalytic conditions)
and reinforces the importance of open channels in MOF
catalysis. Second, while 2R only provided slight enhancements
in isolated yields for the cyclopropanation products compared
to 4, significant increases in both dr’s and ee’s were observed
for 2R. For the cyclopropanation of styrene, 2R gave an 8%
increase in the cis ee and a 3% increase in the trans ee
compared to those of 4. For the cyclopropanation of ethyl
vinyl ether, 2R gave a 16% increase in the cis ee and a 2%
increase in trans ee compared to those of 4.
Notes and references
y Crystal data for 1: trigonal, R32, a = b = 35.140(2) A, c = 92.240(8) A,
V = 98640(11) A3, rcalc = 0.530 g cmꢀ3. R1 = 0.132, wR2 = 0.318.
CCDC 860688.
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We have also carried out experiments to demonstrate that the
present MOF catalysts are heterogeneous and recyclable. First,
the supernatant of the MOF catalyst showed no cyclopropana-
tion activity. Second, the MOF recovered from the catalytic
reactions remained active for cyclopropanation reactions, albeit
with lower yields and selectivities (Fig. S30, ESIz). The deteriora-
tion in the catalytic performance is likely a result of catalyst
deactivation caused by the loss of the axial pyridine ligands.15
In summary, we have synthesized a new pair of porous
chiral MOFs that were constructed from the same catalytically
active bridging ligand but possessed different open channel
sizes as a result of the different catenation modes. Upon
reduction, this pair of chiral MOFs became active catalysts
for highly diastereo- and enantio-selective cyclopropanation
reactions of substituted alkenes. Both the yields and selectivities
of the cyclopropanation reactions are markedly dependent on
the MOF open channel sizes. Chiral MOFs thus provide
a tunable platform for the design of highly efficient and
stereoselective single-site solid catalysts.
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We thank the NSF (CHE-1111490) for financial support
and Kathryn deKrafft and Rachel Huxford-Phillips for experi-
mental help. J.M.F. is supported by a DOE Office of Science
c
6510 Chem. Commun., 2012, 48, 6508–6510
This journal is The Royal Society of Chemistry 2012