Homochiral nickel coordination polymers based on salen(Ni) metalloligands: Synthesis, structure, and catalytic alkene epoxidation

Article abstract of DOI:10.1021/ic102181h

One-dimensional (1D) homochiral nickel coordination polymers [Ni ₃(bpdc)-(RR-L)₂· (DMF)]_n (2R RR-L = (R,R)-(-)-1,2-cyclohexanediamino-N,N'-bis(3-fert-butyl-5-(4-pyridyl) salicylidene), bpdc = 4,4'-biphenyldicarboxylic acid) and [Ni₃(bpdc) (SS-L)₂ · (DMF)]_n (2S, SS-L=(S,S)-(-)-1,2- cyclohexanediamino-N, N'-bis(3-tert-butyl-5-(4-pyridyl)salicylidene) based on enantiopure pyridyl-functionalized salen(Ni) metalloligand units NiL ((1,2-cyclohexanediamino-N,N'-bis(3-tert-butyl-5-(4-pyridyl)salicylidene)) Ni_II) have been synthesized and characterized by microanalysis, IR spectroscopy, solid-state UV-vis spectroscopy, thermogravimetric analysis (TGA), circular dichroism (CD) spectroscopy, cyclic voltammetric measurement, and powder and single crystal X-ray diffraction. Each NiL as unbridging pendant metalloligand uses one terminal pyridyl group to coordinate achiral unit (nickel and bpdc^2-) building a helical chain, while the other pyridyl group remains uncoordinated. Both 2R and 2S contain left-and right-handed helical chains made of the achiral building blocks, while the NiL as remote external chiral source is perpendicular to the backbone of the helices. The nickel coordination polymers 2R and 2S containing unsaturated active nickel center in metalloligand NiL can be used as self-supported heterogeneous catalysts. They show catalytic activity comparable with their homogeneous counterpart in alkene epoxidation and exhibit great potential as recyclable catalysts.

Full text of DOI:10.1021/ic102181h

ARTICLE
pubs.acs.org/IC
Homochiral Nickel Coordination Polymers Based on Salen(Ni)
Metalloligands: Synthesis, Structure, and Catalytic Alkene Epoxidation
Yuanbiao Huang, Tianfu Liu, Jingxiang Lin, Jian L€u, Zujin Lin, and Rong Cao*
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science,
Fuzhou 350002, China
S Supporting Information
b
ABSTRACT: One-dimensional (1D) homochiral nickel coordination polymers [Ni3(bpdc)-
(RR-L)2 (DMF)]n (2R, RR-L = (R,R)-(-)-1,2-cyclohexanediamino-N,N0-bis(3-tert-butyl-5-(4-
3
pyridyl)salicylidene), bpdc = 4,40-biphenyldicarboxylic acid) and [Ni3(bpdc)(SS-L)2 (DMF)]n
3
(2S, SS-L = (S,S)-(-)-1,2-cyclohexanediamino-N,N0-bis(3-tert-butyl-5-(4-pyridyl)salicylidene)
based on enantiopure pyridyl-functionalized salen(Ni) metalloligand units NiL ((1,2-
cyclohexanediamino-N,N0-bis(3-tert-butyl-5-(4-pyridyl)salicylidene))NiII) have been synthe-
sized and characterized by microanalysis, IR spectroscopy, solid-state UV-vis spectroscopy,
thermogravimetric analysis (TGA), circular dichroism (CD) spectroscopy, cyclic voltam-
metric measurement, and powder and single crystal X-ray diffraction. Each NiL as unbridging
pendant metalloligand uses one terminal pyridyl group to coordinate achiral unit (nickel and
bpdc2-) building a helical chain, while the other pyridyl group remains uncoordinated. Both
2R and 2S contain left- and right-handed helical chains made of the achiral building blocks, while the NiL as remote external chiral
source is perpendicular to the backbone of the helices. The nickel coordination polymers 2R and 2S containing unsaturated active
nickel center in metalloligand NiL can be used as self-supported heterogeneous catalysts. They show catalytic activity comparable with
their homogeneous counterpart in alkene epoxidation and exhibit great potential as recyclable catalysts.
’ INTRODUCTION
include the use of (salen)Ni complex immobilization in zeolites
X and Y, as heterogeneous catalysts for the oxidation of phenol by
Over the past decade, the use of transition metal complexes as
catalysts for the olefin epoxidation has attracted considerable
attention in academic and industrial fields.1 Most of the publica-
tions concerned the use of highly enantioselective catalysts of
(salen)Mn complexes developed by Jacobsen and Katsuki in
the epoxidation of alkenes.1-3 (Salen)Ni complexes, which have
been used in homogeneous catalytic transformations such as the
tetralin oxidation,4 asymmetric aldol reaction,5 Mannich-Type
and Michael reactions,6 also showed to be highly active for alkene
epoxidation.7-11 Although homogeneous catalysts usually exhi-
bit high activity and selectivity in most of organic reactions, their
practical applications remain limited because of catalyst instabil-
ity and difficulty in catalyst/product separation. Immobilization
of homogeneous catalysts can facilitate its recovery and reuse and
therefore is of considerable interest to academia and industry.12
Recently, a number of approaches have been developed for this
purpose, typically including using inert inorganic materials or
organic polymers as supports or conducting the reactions in
some unconventional media such as ionic liquids or supercritical
CO2 fluid.12,13 Although extremely successful, classical immobi-
lization with various prefabricated supports is often plagued by
negative effects such as reduction of catalytic activity and/or
selectivity as a result of the poor accessibility, random anchoring,
or disturbed geometry of the active sites in the solid matrix.12,13
The immobilization of (salen)Ni complexes in heterogeneous
catalysis has not been extensively explored. Some examples
14
H2O2 and for the epoxidation of cyclohexene15 and trans-β-
methylstyrene10 by NaOCl. Corma used chiral (salen)Ni com-
plexes immobilizing in ordered mesoporous silica supports
(MCM-41), delaminated ITQ-2 and ITQ-6 zeolites, and amor-
phous silica for hydrogenation of imines.16 Very recently, a
sulfonato-salen-nickel(II) complex has been immobilized on a
Zn(II)-Al(III) layered double hydroxide (LDH) host for tetra-
lin oxidation.4
Infinite coordination polymers, especially metal-organic fra-
meworks (MOFs) with infinite network structures built from
organic bridging ligands and inorganic connecting nodes have
been emerging as very promising materials for gas storage,
separation, heterogeneous catalysis, sensing, and drug delivery.17-33
Two different approaches have been utilized to synthesize cataly-
tically active coordination polymers. The first method is the metal-
connecting points with unsaturated coordination environments
being utilized as catalytically active sites.12,24-30 In the second
one, catalytic sites are incorporated directly into the bridging ligands
used to construct coordination polymers.12,24-28,31-33 Although
more synthetically demanding, the second approach is much
more versatile and allows for the incorporation of a wide variety
of catalysts, especially asymmetry catalysts.25 Until now, a few
Received: August 31, 2010
Published: February 18, 2011
r
2011 American Chemical Society
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|

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Figure 1. Perspective view showing the chirality of cyclohexyl groups in 2R (a) and 2S (b) (one of the two repeating units). Ligands bpdc, solvent
molecules, and hydrogen atoms are omitted for clarity.
coordination polymers based on salen ligands with an additional
functional group such as carboxylates,34-42 p-benzoic acid groups,43
and p-pyridyl groups32,44,45 in the para or meta position to the OH
group have been reported. There were only dipyridyl32 and
dicarboxylate43b functionalized (salen)MnCl metalloligands incor-
porating into the framework of structures of Zn-MOFs, and
coordination polymers33 formed by the reaction of [bis(catechol)-
salen]MnIII with several di- and trivalent metal ions using for highly
effective olefin epoxidation. Herein, we report chiral dipyridyl
functionalized nickel salen complexes NiL (1R and 1S) as unbrid-
ging pendant metalloligands incorporating into infinite nickel
coordination polymers as self-supported catalysts for alkene
epoxidation.
single crystal X-ray diffraction, elemental analysis, and thermo-
gravimetric analysis (TGA). Interestingly, 2R and 2S can also be
prepared directly using chiral ligand (RR- or SS-)H2L, Ni-
(NO3)2 6H2O, and bpdc (2:1:1) under the same conditions
3
with high yields.
Single crystal X-ray diffraction study shows that both com-
pounds 2R and 2S crystallize in the orthorhombic chiral space
group P2221 with absolute structure parameters of 0.028(1) and
0.055(2), respectively. X-ray crystallography shows that 2R and
2S are one-dimensional (1D) coordination polymers which have
the same formula and are therefore isomorphous (Figure 1 and
Supporting Information, Figure S1). Circular Dichroism (CD)
spectra (Figure 4) of bulk materials 2R and 2S made from (R, R)
and (S, S) enantiomers of the H2L ligand are mirror images of
each other, which indicates their enantiomeric nature.41 The
structures of the 2R and 2S are identical with the exception of
being opposite hands of each other (Figure 1), and only that of
2R is discussed here. There exist two different coordination
models of the nickel(II) centers in 2R (Figure 2). The coordina-
tion model of Ni2 is similar to that of Ni1, while Ni3 and Ni4 are
similar to each other. One unsaturated nickel center Ni1 (Ni2) as
active catalytic site from metalloligand NiL adopts nearly square
geometry (Ni-Oavg = 1.851 Å, Ni-Navg = 1.849 Å, Supporting
Information, Table S1). The other nickel center ion Ni3 (Ni4) as
metal node exhibits distorted octahedral coordination geometry,
which is bridged by four oxygen atoms of the two different
chelating bpdc2- (Ni-Oavg = 2.121 Å) and two nitrogen atoms
of the pyridyl groups from two different cis-NiL metalloligands
(Ni-Navg = 2.051 Å). The coordination geometry around the
metal node Ni3 (Ni4) is similar to that observed for the model
complex dibenzoatodipyridinnenickel(II)47a (5) and polycate-
’ RESULTS AND DISCUSSION
In a typical synthesis, the reaction of Ni(OAc)2 4H2O and
3
chiral ligand (R,R or S,S)-(-)-1,2-cyclohexanediamino-N,N0-
bis(3-tert-butyl-5-(4-pyridyl)salicylidene) (RR- or SS-)H2L)46
(1:1) in a MeOH solution at 60 °C resulted in yellow solids of
RR-NiL (1R) and the enantiomer SS-NiL (1S). The orange
crystals of 1S suitable for single crystal X-ray diffraction were
grown from the mixture of the solvent DMF/CH2Cl2/MeOH
(1:1:1). The NiII ion is coordinated in nearly square-plane
geometry with two nitrogen atoms and two oxygen atoms from
the chelating L ligand (Supporting Information, Figure S1). The
pyridyl groups of the L ligand are not involved in the coordina-
tion to the NiII ion, which can be used as metalloligands to
assemble coordination polymers.
The chiral metalloligand 1R or the enantiomer 1S reacts with
Ni(NO3)2 6H2O and 4,40-biphenyldicarboxylic acid (bpdc)
3
(1:1:1) in a mixture of DMF and EtOH (10:1) at 80 °C after
24 h afforded the red brown needle crystals of 2R or 2S with high
yields. The products 2R and 2S as nickel coordination polymers
are stable in air and insoluble in water and common organic
nated array of 1D nanotubes [Ni2(oba)2(bpy)2(H2O)2] bpy
3
(6), which was obtained from the reaction of the rigid 4,40-
bipyridine (bpy) and the long V-shaped 4,40-oxybis-(benzoate)
(oba) ligands.47b However, interestingly, each metalloligand unit
RR-NiL uses only one terminal pyridyl group to coordinate to a
solvents formulated as [Ni3(bpdc)L2 DMF]n on the basis of
3
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Figure 2. Top, a view of Ball-and-Stick and representation of the 1D chain in 2R. Bottom, a Space-filling model of the 1D chain in 2R. Color code: dark-
gray, carbon; blue, nitrogen; red, oxygen; blue-green, nickel.
nickel center Ni3 (Ni4), while another one remains uncoordi-
nated, which is similar to those of Zn(salen) polymers45 and
metallomacrocycles,48,49 and homochiral porous material POST-1
([Zn3O(L)6] 2H3O 12H2O)n (L = 4-aminopyridine amide
In general, when using enantiopure chiral molecules as build-
ing blocks, the helical compounds are usually chiral, with the
right- or left-handed feature. While right- and left-handed helices
are formed in equal amounts within a single crystal to produce a
meso compound, achiral or racemic molecules are used as
building blocks.52 Surprisingly, it is of particular interest to note
that left- and right-handed helix structures based on the enantio-
pure metalloligands NiL and achiral molecules (bpdc2- and
nickel) are observed in both the homochiral coordination poly-
mers 2R and 2S (Figure 3, Supporting Information, Figure S3).
The chiral molecules NiL (RR-NiL) in 2R coordinated to the
nickel node via the terminal nitrogen of the pyridyl groups are
almost perpendicular to the helical chains, while the helical
backbone are actually constructed from achiral units (nickel
and bpdc ligand). Therefore, in this case, the molecular chirality
of RR-NiL serves as an external chiral source to interact with the
helix made of achiral building blocks. The phenomenon is
different from the internal induction in which the chirality of
the molecular building block forms an inherent part of the helical
backbone (e.g., the helicity of DNA based on D-sugars).52 And
also, the chiral sources (cyclohexyl) in 2R are remote from the
helical chains (e.g., C13 Ni3 = 12.9969 Å, C18 Ni3 =
3
3
derivative of tartaric acid) being first used as coordination poly-
mer for asymmetric catalysis reaction,50 but different from the
coordination polymer [Ni2(oba)2(bpy)2(H2O)2] bpy (6) using
3
bpy as cross-linkers.47b Changes of the different nickel pre-
cursors (Ni(OAc)2 4H2O and NiCl2 6H2O) or the stoichio-
3
3
metric ratio of the Ni(NO3)2, H2L, and bpdc (3:1:1, 4:1:1)
still gave the same product 2R under the same conditions,
confirmed by the powder X-ray diffraction (PXRD) (Supporting
Information, Figure S7). The two independent NiN2O2 planes
have dihedral angles of 24.3° and 25.9° with the coordinated
pyridine rings and of 34.2° and 27.5° with the uncoordinated
pyridine rings, respectively. The two NiL units coordinated to a
same nickel node are nearly perpendicular (with a dihedral angle
of 88.3°). The distance of the two types of nickel centers (e.g., Ni1
and Ni3) is 11.798 Å, while the distance of the two unsaturated
active nickel centers (e.g., Ni1 and Ni1B (symmetry operation:
-x þ 1, y, -z þ 1/2)) from the NiL metalloligands anchoring on
the same nickel node is 15.877 Å. The resultant 1D coordination
polymeric chain exhibits a turning angle (defined by three adjacent
metal centers) of 126.33° and a repeating period of 26.229 Å
(Figure 2).
3 3 3
3 3 3
11.7099 Å), which result in almost no chiral induction interac-
tion to the helical backbone chains constructed from achiral
building blocks.52b So, it is not a surprise that there exist left- and
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Table 1. Effect of Solvent on the Oxidation of Styrene with
NaClO Catalyzed by 2Ra
entry solvent (5 mL) conversionb(%) epoxide (%) selectivityc (%)
1
2
3
4
5
6
CH2Cl2
34
19
13
11
10
56
46
48
45
CH3CN
28
CH3COCH3
CH3OH
23
22
DMF
trace
36
CH3CN (60 °C)
9
25
a Reaction conditions: styrene (0.5 mmol); NaClO (2 mmol); Catalyst
(0.01 mmol); T = 25 °C. b Based on substrate taken. c Epoxide yield/%
Conversion.
Figure 3. View of left-handed (a) and right-handed (b) helix polymeric
chains of 2R.
coordinated salen ligand, H2L.9,46 The absence of the expected
characteristic band at 1689 cm-1 for the protonated carboxylate
groups (appears at ca. 1679 cm-1) indicates the complete
deprotonation of bpdc ligand and coordination to the nickel
center47 for 2R and 2S. The solid state UV-vis spectrum of
complex 1R (Supporting Information, Figure S8) with salen
ligand presents high intense bands occurring at λ < 500 nm,
because of metal-to-ligand and ligand-to-metal charge transfer
and intraligand transitions, while one broad band in the visible
region at λ = 500-650 nm is attributed to the nonresolved d-d
transitions from the four low-lying d orbitals (dxz, dyz, dz2, dxy)
to the empty orbital.9,10 The solid state UV-vis spectrum of 2R
(Supporting Information, Figure S8) shows similar features to
the free complex 1R, indicating that no change at the Ni(II)
coordination center took place during self-assmebly.
The cyclic voltammogram (Supporting Information, Figure
S12) ofcomplex1R showsthe anodic peak potential Epa= -1.18 V
and the cathodic peak potential Epc= -1.31 V, which are
indicative of a quasi-reversible electrode process, corresponding
to the Ni3þ/Ni2þ couple of the NiL.9,51 The values of Epa and
Epc of 2R are -0.98, -1.11 V, respectively. These peak
potentials are more positive than that of complex 1R. Interest-
ingly, both of the ΔEp values of 1R and 2R are about 0.13 V,
which indicates that the coordination polymer 2R shows the
salen nickel electrochemistry property, while the metal node Ni3
shows no electrochemistry property.53
The chiral coordination polymers 2R and 2S containing
unsaturated active centers in NiL can be viewed as self-supported
heterogeneous catalysts and prompt us to explore their applica-
tions in alkene epoxidation. Compounds 2R and 2S are stable in
air even after the loss of solvent molecules, different from most of
3D MOFs containing large open channels which are mechani-
cally unstable.43b Compounds 2R and 2S are insoluble in water
and common organic solvents such as DMF, DMSO, CH2Cl2,
and EtOH. The catalytic activity results from the surface catalytic
sites, similar to that of the coordination polymers formed by the
reaction of [bis(catechol)salen]MnIII with several di- and triva-
lent metal ions.33 Therefore, the samples of 2R were pestled to
powder and dried under vacuum to remove the solvent and then
were subjected to ultrasonication for 30 min to increase the
surface active sites prior to use. The choice of solvent is crucial for
the catalytic epoxidation of alkenes (Table 1). Among dichlor-
omethane, acetonitrile, methanol, acetone, and DMF, the weak
donor solvent dichloromethane was chosen as reaction medium
because the higher yield epoxide was observed for the epoxida-
tion of styrene. Lower catalytic activity in the presence of a
stronger coordinating solvent was observed because the axially
Figure 4. CD spectra of 2R (solid line) and 2S (dashed line).
right-handed helices in the homochiral coordination polymers
2R and 2S, but the metalloligands NiL coordinated to the
backbone of the left- and right-handed helices in 2R keep the
same absolute configuration (Supporting Information, Figure S1
and S2), which is the enantiomer of that in 2S (Supporting
Information, Figure S1 and S3).
Weak interchain π π interactions exist between extended
3 3 3
π conjugated NiL units and pyridyl groups (plane-to-plane
separation ca. 3.90 Å, respectively, Supporting Information,
Figure S4). There exist C-H π interactions between the
3 3 3
cyclohexyl groups and coordinated pyridyl groups (the distance
is ca. 3.59 and 3.49 Å, respectively Supporting Information,
Figure S5). The void space between the adjacent polymeric
chains is occupied by dimethylformamide (DMF) molecules
(Supporting Information, Figure S6). Interestingly, the super-
molecular structure (Supporting Information, Figure S6) is
further stabilized by the weak hydrogen-bonding interactions
(Supporting Information, Figure S5) formed by the DMF
molecules with uncoordinated pyridyl groups, as well as the
phenol hydroxyl groups with tert-butyl groups. TGA (Supporting
Information, Figure S9) and PXRD (Supporting Information,
Figure S10) measurements indicate that the framework of 2R
remains intact upon complete removal of solvent DMF mole-
cules and remains stable up to about 305 °C.
IR spectra of complexes 1R, 1S, 2R, and 2S (Supporting
Information, Figure S11) show that the CdN stretching vibra-
tions shift to about 1597 cm-1 expected for the nickel
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ARTICLE
coordinated solvent prevents the oxidant OCl- from formation
of catalytically active intermediate nickel(IV)-oxo species and the
assessment of substrates to active centers.9
unsubstituted salen(Ni) complex 3 (entry 3, Table 3).10,11 The
reason may be that the bulky steric hindrance tert-butyl group is
unfavorable for alkene to access the active nickel center. How-
ever, the moderate epoxide selectivity (for styrene oxidation,
benzaldehyde and other products were also obtained) was quite
similar to that of the unsubstituted salen nickel complex (entry 3
and 4, Table 3).8-11 Interestingly, no significant changes about
conversion and selectivity were observed when the pyridyl
functional group was introduced to the para position of OH
group, as in complex 1R (entries 1 and 4, Table 3). The conversion
and epoxide selectivity of the styrene oxidation is higher than the
cyclohexene reaction catalyzed by both 1R (entries 1 and 7, Table 3)
and 2R (entries 2 and 8, Table 3), indicating that the electron-rich
olefins are more reactive for oxidation than electron poor ones.8
Interestingly, the conversion and epoxide selectivity of the styrene
and cyclohexene oxidation catalyzed by 2R as self-supported
heterogeneous catalyst are close to that of 1R (entries 1 and 2,
Table 3). The reason may be that the self-supported catalyst 2R
contains much more active centers than those of other hetero-
geneous supported catalysts such as zeolites.12 And the arranged
regular active centers coordinated to the helical chains may therefore
enforce a cooperative reaction pathway resulting in enhanced
reaction rates and higher selectivities.54 The unsaturated active
nickel center is far away from the coordination polymeric chain
(e.g., the distance of Ni1 and Ni3 is 11.798 Å), and only one
terminal pyridyl group is coordinated to the metal node which
makes the substrate access easily to get the comparative conversion
and selectivity of complex 1.25-28 While complexes 5 and 6, in
which the coordination model of nickel ion is similar to those of
metal nodes (e.g., Ni3) in 2R and 2S, showed no activity for styrene
epoxidation (entries 5 and 6, Table 3) under the same conditions.
The results further prove that the catalytic activity of 2R comes from
the salen(Ni) metalloligands, but not from Ni3 metal node that is
positioned in a distorted octahedral six-coordination geometry
without space for substrate insertion.
The effect of different oxidants on the catalytic activity of 2R in
the oxidation of styrene was studied (Table 2). When NaOCl,
tert-BuOOH, H2O2, and O2 were used as the oxygen source in
dichloromethane for the oxidation of styrene, the economic
domestic bleach NaClO as the best oxidant gave higher conver-
sion and selectivity. Although the oxidant tert-BuOOH gave the
comparative conversion with that of NaOCl, the epoxide selec-
tivity reduced to only 27%. A series of the above controlled
experiments shows that each component is essential for an
effective catalytic epoxidation. Neither 2R (entry 6, Table 2)
nor NaClO (entry 5, Table 2) alone is able to catalyze styrene
epoxidation. The effect of the temperature on the epoxidation of
styrene was also evaluated. Although the conversion of the
styrene slightly increased to 36% when the reaction temperature
increased to 60 °C in CH3CN, the epoxide product decreased to
only 9% (entry 6, Table 1).
For comparison of the effectiveness of different substituent
groups on the aromatic rings of the ligands in nickel complexes,
we also synthesized (R,R)-(-)-1,2-cyclohexanediamino-N,N0-bis-
(salicylidene)(Ni) (3) and (R,R)-(-)-1,2-cyclohexanediamino-
N,N0-bis(3-tert-butyl-salicylidene)(Ni) (4) by a similar proce-
dure to that for complex 1R. The results of the epoxidation of
alkenes by complexes 1R, 2R, 3, 4, 5, and 6 are provided in
Table 3. Introduction of the tert-butyl group (bulky steric
hindrance and electron-donating group) in the third position
of the aldehyde fragment (entry 4, Table 3) could be responsible
for the lower reaction conversion compared with that of the
Table 2. Effect of Various Oxidants on the Oxidation of
Styrene Catalyzed by 2Ra
entry oxidant (2 mmol) conversionb (%) epoxide (%) selectivityc(%)
The supernatant of catalysis system of 2R did not result in
further oxidation of the substrate under identical experimental
conditions, and no metal species leaching into the organic phases
were detected (<1 ppm) by inductively coupled plasma (ICP)
spectroscopic analysis, confirming the heterogeneous nature of
the present catalytic system.31b,c The FTIR spectra (Supporting
Information, Figure S11) and PXRD patterns (Supporting
Information, Figure S13) of the catalyst 2R before and after
the reaction were compared. The results show that no changes
are observed in the FTIR and PXRD before and after reaction,
1
NaClO
H2O2
34
19
56
27
2
12
trace
9
3
tert-BuOOH
O2
33
4
trace
trace
trace
5d
6e
NaClO
a Reaction conditions: styrene (0.5 mmol); catalyst (0.01 mmol);
CH2Cl2 (5 mL); T = 25 °C. b Based on substrate taken. c Epoxide
yield/% conversion. d No nickel catalyst. e No oxidant.
Table 3. Oxidation of Alkenes Catalyzed by Complex 1 and 2Ra
entry
substrate (0.5 mmol)
catalyst (0.01 mmol)
conversionb(%)
epoxide (%)
selectivityc (%)
1
styrene
1R
2R
3
39
21
19
38
27
54
56
61
68
2
styrene
34
3
styrene
62
4
styrene
4
40
5
styrene
5
Trace
Trace
29
6
styrene
6
7
cyclohexene
cyclohexene
styrene
1R
2R
2Rd
2Re
8
27
32
58
56
8
22
7
9
29
17
10
styrene
22
13
a Reaction conditions: NaClO (2 mmol); Catalyst (0.01 mmol); CH2Cl2 (5 mL); T = 25 °C; t = 24 h. b Based on substrate taken. c Epoxide yield/%
conversion. d Second reuse. e Third reuse.
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Table 4. Crystallographic Data for Compounds 1S, 2R, and 2S
1S
2R
2S
empirical formula
formula weight
temperature (K)
cryst syst
space group
a (Å)
C
82H98N10O6Ni2
C93H99N9O9Ni3
1662.94
C93H99N9O9Ni3
1662.94
1437.12
298(2)
173(2)
298(2)
monoclinic
P21
orthorhombic
P2221
orthorhombic
P2221
9.789(3)
24.528(8)
16.262(5)
106.721(5)
3739(2)
2
27.486(4)
11.0171(18)
26.229(4)
90
27.514(3)
11.0404(15)
26.236(4)
90
b (Å)
c (Å)
β (deg)
V (Å3)
7942(2)
4
7969.6(18)
4
Z
Dcalc (Mg m-3
)
1.276
1.391
1.386
crystal size (mm)
0.12 ꢀ 0.10 ꢀ 0.08
0.563
0.15 ꢀ 0.12 ꢀ 0.08
0.771
0.15 ꢀ 0.12 ꢀ 0.08
0.768
μ (mm-1
F(000)
)
1528
3504
3504
θmax, θmin (deg)
27.46, 2.17
-12f12
-31f30
-15f21
0.0477
27.49, 2.00
-32f35
-14f14
-34f34
0.0519
27.47, 2.00
-35f32
-13f14
-34f34
0.0519
index range h
k
l
Rint
absolute structure parameter
no.of indep refls
no. of obsd refls
no. of variables
0.002(1)
9468
0.028(1)
15332
0.055(2)
14510
15131
18192
18237
917
1071
1081
a
b
final R indices [I > 2σ(I)]: R1 , wR2
0.0524, 0.0989
0.0890, 0.1164
0.992
0.0592, 0.1686
0.0710, 0.1821
1.211
0.0655, 0.1936
0.0854, 0.2230
1.043
a
b
final R indices [all data]: R1 , wR2
GOF on F2
largest diff.Peak*(hole)(e Å-3
a R1 = ∑||Fo| - |Fc||/∑|Fo|, wR2 = [∑(|Fo|2 - |Fc|2)2/∑(Fo )]1/2
)
0.239(-0.376)
1.017 (-0. 850)
1.183 (-0. 850)
2
.
which indicate that the self-supported catalyst remains stable,
even though in the basic reaction medium. Encouraged by the
persistence of catalytic activity for 2R, we examined its recycl-
ability. Samples of 2R were easily recovered by centrifugation
after catalytic reaction and rinsed by dichloromethane before
being used again for styrene epoxidation. Remarkably, after three
cycles no loss of epoxide selectivity and only a slight loss of
conversion were observed (Table 3). The loss of conversion may
be caused by the loss of the catalyst during the recovery process.
’ EXPERIMENTAL SECTION
Materials and Instruments. Ligands (R,R)-(-)-1,2-cyclohexane-
diamino-N,N0-bis(3-tert-butyl-5-(4-pyridyl)salicylidene46 (RR-H2L),
(S,S)-(-)-1,2-cyclohexanediamino-N,N0-bis(3-tert-butyl-5-(4-pyridyl)
salicylidene)46 (SS-H2L) (Supporting Information, Scheme S1) and
complexes dibenzoatodipyridinnenickel(II) (5), [Ni2(oba)2(bpy)2(H2
O)2] bpy (6) were synthesized according to literature procedures.
3
Unless otherwise stated all other chemicals are commercial available,
and used without further purification. Fourier-transform infrared (FT-
IR) spectra were taken on a Magna 750 FTIR spectrometer with samples
as KBr pellets in the range of 450-4000 cm-1. PXRD patterns were
recorded on a Rigaku-Dmax2500 diffractometer using Cu KR radiation
(λ = 0.154 nm). GC-MS measurements were performed on a Varian
450-GC/240-MS. Elemental analyses of C, H, and N were determined
using a Vario MICRO E III elemental analyzer. Thermogravimetric
analyses were performed on an NETZSCH STA 449C unit at a heating
rate of 10 °C/min under nitrogen atmosphere. Solid-state circular
dichroism (CD) spectra of compounds 2R and 2S were recorded using
a JASCO J-810 spectrometer. For each CD measurement about 0.5 mg
of crystalline sample was taken to be mixed with 40 mg of dried and well
ground KCl powder. This mixture was then pressed into a disk by a
literature method.41 The UV-vis diffuse reflectance spectra were
measured on a Perkin Elmer Lambda 900 UV/vis spectrometer
equipped with an integrating sphere over the 200-2000 nm wavelength
range at room temperature. A BaSO4 plate was used as reference
material (100% reflectance). Cyclic voltammogram (CV) was recorded
’ CONCLUSIONS
In summary, we have successfully synthesized 1D homochiral
nickel coordination polymers based on pyridyl substituted
metalloligands salen(Ni) units via two self-assembly methods,
in which only one pyridyl group of the NiL unit as unbridging
pendant metalolligands coordinated to polymeric chains. It is
very interesting that both of the homochiral coordination poly-
mers 2R and 2S contain left- and right-handed helical chains
made of the achiral building blocks, while the NiL as remote
external chiral source is perpendicular to the backbone of the
helices. The compounds show catalytic activity comparable to
their homogeneous counterpart in alkene epoxidation reaction
and exhibit great potential as recyclable catalysts. This assembly
approach provides a facile and efficient strategy to synthesize self-
supported catalysts.
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Inorganic Chemistry
ARTICLE
on a 384B polarographic analyzer. An epsilon Electrochemical Work-
station connected to a Digital-586 personal computer was used for
control of the electrochemical measurements and for data collection. A
conventional three-electrode system was used. The working electrode
was a modified carbon paste electrode (CPE). An Ag/AgCl (saturated
KCl) electrode was used as a reference electrode and Pt gauze as a
counter electrode. All potentials were measured and reported versus the
Fcþ/Fc couple. The measurements were carried out under N2, in
degassed DMF (distilled from CaH2 under N2), using 0.1 M N(n-
Bu)4ClO4 as the supporting electrolyte. The CPE was fabricated as
follows:55 25 mg graphite powder and 3 mg compound (1R and 2R)
were mixed and ground together by agate mortar and pestle to achieve
an even mixture, then 0.10 mL of paraffin oil was added to the above
mixture and stirred with a glass rod, then the mixture was used to paste
on a 5 mm diameter carbon bar, and the surface was pressed tightly by a
clean knife. Electrical contact was established through a carbon bar
electrode.
Crystal structure determinations of compounds 1S, 2R, and 2S were
collected with a Saturn 70 single-crystal diffractometer equipped with
graphite-monochromated Mo KR radiation (λ = 0.71073 Å). The
Crytal-Clear software was used for data reduction and empirical
absorption correction.56a The structures of compounds 1S, 2R, and
2S were solved by direct methods and successive Fourier difference
syntheses, and refined by the fullmatrix least-squares on F2.56b Details of
the crystal parameters, data collection, and refinement are summarized
in Table 4. Selected bond lengths and bond angles are listed in
Supporting Information, Table S1.
1537 (m), 1469 (m), 1455 (m), 1393 (w), 1349 (m), 1325 (m), 1224 (w),
1201 (w), 1147 (w), 1124 (w), 1045 (w), 1027 (w), 908 (w), 847 (w), 811
(w), 760 (w), 739 (w), 621 (w).
Synthesis of Complex (R,R)-(-)-1,2-cyclohexanediamino-
N,N0-bis(3-tert-butyl-salicylidene)(Ni) (4). The synthesis pro-
cedure was similar to that of 1R except using ligand (R,R)-(-)-1,2-
cyclohexanediamino-N,N0-bis(3-tert-butyl-salicylidene).46 Yield: 84%
(based on Ni). Anal. Calcd for 3, C28H36N2O2Ni: C, 68.45; H, 7.39;
N, 5.70. Found: C, 68.49; H, 7.42; N, 5.67%. IR (KBr, cm-1): 3056 (w),
3007 (w), 2944 (m), 2901 (w), 2865 (w), 1598 (s), 1539 (m), 1466 (m),
1420 (m), 1388 (m), 1339 (m), 1318 (m), 1244 (w), 1194 (w), 1144
(w), 1091 (w), 1045 (w), 871 (w), 808 (w), 744 (m), 665 (w).
Catalysis of Alkene Epoxidation. In a typical experiment, a
solution of the oxidant (for NaClO buffered to pH = 11 with a solution
of Na2HPO4) was stirred at designated temperature in a solution of
solvent enriched with substrate, the nickel catalyst (substrate/catalyst =1
:0.025, phase transfer catalyst benzyltributylammoniumchloride was
added to the CH2Cl2 solution for complexes 1R 3, 4 and 5; while for
2R and 6, no phase transfer reagent was employed), and internal
standard n-decane. Aliquots of organic layer were withdrawn at chosen
intervals of time and subjected to gas chromatographic analysis for
products. After being used for the catalytic reaction, the mixture was
centrifuged at 8000 rpm for 3 min. The supernatant solution was
decanted for analysis by GC. The remaining solid was washed with
dichloromethane before being dried in air for 0.5 h prior to being reused.
’ ASSOCIATED CONTENT
Synthesis of Complex RR-NiL (1R). Ligand RR-H2L (1 mmol)
S
was dissolved in MeOH (40 mL), and Ni(OAc)2 4H2O (1 mmol) was
Supporting Information. Procedures for syntheses of 1S
b
3
added to give to a yellow solution. The reaction mixture was stirred
under reflux for 24 h and then concentrated to about 10 mL. The yellow
products were filtered and washed with cold MeOH and diethyl ether,
collected, and dried under vacuum. Yield: 86%. Anal. Calcd for 1R,
C38H42N4O2Ni: C, 70.71; H, 6.56; N, 8.68. Found: C, 70.65; H, 6.52; N,
8.62%. IR (KBr, cm-1): 3391 (br), 2942 (m), 2864 (w), 1597 (s), 1552
(m), 1439 (m), 1405 (m), 1346 (m), 1280 (m), 1224 (m), 1173 (m),
1017 (w), 902 (w), 830 (m), 788 (w), 649 (m), 576 (w).
and 2S; CIF files, IR spectra, UV-vis spectra, CV, TGA, and
PXRD diagrams and supplementary figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Fax: þ86 59183796710. Phone: þ86 059183796710. E-mail:
rcao@fjirsm.ac.cn.
Synthesis of Coordination Polymer 2R. Method A: A mixture
of Ni(NO3)2 6H2O (0.02 mmol), RR-NiL (1R) (0.02 mmol), and bpdc
3
(0.02 mmol) was placed in Teflon-lined stainless autoclave containing
DMF (10 mL) and EtOH (1 mL). The mixture was heated at 80 °C for
24 h and then cooled to room temperature. The red brown crystals were
filtered and washed with DMF, EtOH, and diethyl ether, and collected
and dried in air. Yield: 64.2% (based on Ni). Anal. Calcd for 2R,
C93H99N9O9Ni3: C, 67.17; H, 6.00; N, 7.58. Found: C, 67.02; H, 5.94;
N, 7.49%. IR (KBr, cm-1): 3388 (b), 3069 (w), 3026 (w), 2949 (m),
2865 (w), 1679 (s), 1597 (s), 1551 (m), 1434 (s), 1406 (m), 1385 (m),
1347 (m), 1324 (m), 1281 (m), 1223 (m), 1174 (m), 1087 (m), 1051
(w), 1025 (w), 991 (w), 899 (w), 834 (w), 820 (w), 787 (w), 650 (m),
’ ACKNOWLEDGMENT
We acknowledge the financial support from the 973 Program
(2011CB932504, 2007CB815303), NSFC (20731005, 20821061,
21003128), CAS and FJIRSM (SZD07002). We also acknowledge
Prof. Jian Zhang, Hui Zhang, Xiaoying Huang, and Jiutong Chen for
the crystal structures determination.
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