Triple-Stranded Cluster Helicates for the Selective Catalytic Oxidation of C-H Bonds

Article abstract of DOI:10.1021/acs.inorgchem.6b01828

Triple-stranded cluster helicates with heptametallic dicubane cores are synthesized by entrapping metals in the cavities of linear triple helicates based on a C₂-symmetrical hexadentate Schiff-base ligand of ortho-substitued biphenol. The helicates are stable in both the solution and solid states, and the copper species could selectively catalyze the oxidation of C-H bonds of alkanes to ketones.

Full text of DOI:10.1021/acs.inorgchem.6b01828

Communication
pubs.acs.org/IC
Triple-Stranded Cluster Helicates for the Selective Catalytic
Oxidation of C−H Bonds
†
,‡
†,‡
†
,†
,†,§
Yu Fang, Wei Gong, Lujia Liu, Yan Liu,* and Yong Cui*
†School of Chemistry and Chemical Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong
University, Shanghai 200240, China
§
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China
*
S Supporting Information
Scheme 1. Synthesis of Helicates 1 and 2
ABSTRACT: Triple-stranded cluster helicates with hep-
tametallic dicubane cores are synthesized by entrapping
metals in the cavities of linear triple helicates based on a
C2-symmetrical hexadentate Schiff-base ligand of ortho-
substitued biphenol. The helicates are stable in both the
solution and solid states, and the copper species could
selectively catalyze the oxidation of C−H bonds of alkanes
to ketones.
elical structures are ubiquitous in nature, particularly in
H
biomacromolecules such as peptides and DNA, and are
1
integral to various biological functions. Their prominent
a
Only one ligand is shown in the product.
functions have motivated chemists to make artificial helical
2
,3
architectures. Of particular interest is the design and assembly
3
of multistranded helicates. Helicates are efficient units for
constructing novel materials, devices, and machines with tunable
functions and properties such as chirality, luminescence,
magnetic exchange, and DNA binding. Because the intrinsically
yield. Both of the air-stable products are soluble in common
organic solvents such as CH Cl and tetrahydrofuran. They were
2
2
4
formulated based on elemental analysis, IR, and mass
spectrometry (MS). The stability of the helicates in solution is
demonstrated by electrospray ionization mass spectrometry
(ESI-MS), which gave prominent peaks for [Cu (OH) L +
important physical, chemical, and biological properties of clusters
were transferred into helicates, all of these potential applications
5
could undergo remarkable development. Helicates are generally
7
2 3
+
+
constructed from flexible oligodentate strands and transition
4H] at m/z 2579.9 and [Zn
(OH)
7
2
L
3
+ K − 5H] at m/z
3
metals. However, complications associated with the simulta-
2616.8.1
13
neous coordination of more than three metal ions with organic
strands have limited most helicates to di- or trinuclear species.
There are no efficient synthetic routes to cluster helicates, and so
The H and C NMR spectra of 2 show one clear set of proton
signals as the free ligand, most of which are shifted upfield
slightly. The number of signals indicated that the C symmetry of
2
6
only a very few examples of such helicates have been made.
the biphenyl ligand is maintained in the helicate complex. The
disappearance of the signals around 2.86−4.88 ppm of MOM
groups suggests that the MOM group was completely removed
during the crystal growth process, whereas the disappearance of
the −OH protons supported deprotonation of the phenolic
hydroxyl group and complexation of the metal and ligand. The
Biphenol and its derivatives exhibit intrinsic C -symmetric
2
twisted conformations and are ideal platforms for the synthesis of
7
helical species. We have developed pyridyl- or carboxylic-
functionalized 1,1′-biphenol-based ligands through which a
variety of helical coordination polymers or chiral metal−organic
7a,8
UV−vis spectrum of L-(2MOM-2H) in CHCl shows three large
frameworks constituted by helicates can be assembled, In this
work, we report the synthesis and characterization of two triple-
stranded helicates consisting of corner-sharing heptanuclear
dicubane clusters and demonstrate the capability of the copper
species to selectively catalyze oxidation of C−H bonds.
3
peaks centered near 290, 341, and 361 nm due to π−π*
transitions of aromatic rings and azomethine chromophore
groups. Upon complexation with metal ions, the peaks assigned
to the same transitions were observed at 295, 439, and 461 nm for
1
and 315, 437, and 457 nm for 2, respectively. There is one new
The ligand L-(2MOM-2H) (MOM = methoxymethyl) could
be readily synthesized by condensation of 3,3′-diamino-5,5′,6,6′-
tetramethyl-2,2′-bis(methoxymethoxy)-1,1′-biphenyl and 3,5-
di-tert-butyl salicylaldehyde. Heating metal(II) salts and the
ligand L-(2MOM-2H) (2:1 molar ratio) in aqueous organic
solvents afforded single crystals of [Cu (OH) L ]·2DMF·2H O
band around 646 nm for 1, which was assigned to the d−d
transition of Cu ions. The ligand is weakly luminescent in
II
9
CHCl with a signal at 412 nm under 290 nm excitation. The
3
Received: July 28, 2016
7
2
3
2
(
1; Scheme 1) and [Zn (OH) L ]·6DMF·2H O (2) in good
7 2 3 2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b01828
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
luminescence is significantly enhanced in 2 with a signal at 511
nm but is completely quenched in 1.
Single-crystal X-ray diffraction of 1 and 2 unambiguously
revealed the formation of triple-stranded cluster helicates (Figure
holds potential to mimic the active site of a metalloenzyme; the
metal site can bind the substrates, and the hydrophobic pocket
10
can recognize the substrates. There is, however, only a handful
11
of such complexes that have been reported.
1
). 1 crystallizes in the triclinic space group P1
̅
The highly stable framework and intriguing cluster structure
presented by 1 have prompted us to explore its catalytic oxidation
chemistry. 1 was found to exhibit impressive catalytic activity in
the peroxidative oxidation of alkanes to the corresponding
alcohols and ketones. In our model reaction, we converted
cyclopentane into cyclopentanone and cyclopentanol in
acetonitrile using an excess of 3 M aqueous H O as the
2
2
oxidizing agent at 55 °C. The gas chromatography (GC) trace of
the reaction mixture showed the formation of cyclopentanone in
a yield of 12.15% along with a trace amount of cyclopentanol,
when the molar ratio of catalyst to substrate was 1:125.
Under otherwise identical conditions, we could also convert
cyclohexane, cyclooctane, toluene, and ethylbenzene to their
corresponding ketones and alcohols (Table 1 and Scheme 2).
Table 1. Peroxidative Oxidation of Alkanes Catalyzed by 1
b
yield (%) of product
n(H O )/
2
2
c
entry substrate T/°C n(catalyst) time/h
4
5
total
3.55
1
3a
25
3360
72
3.55
trace
(
n = 1)
n = 2)
n = 4)
2
3
4
3a
3a
3b
55
55
25
670
4
6
10.43
12.15
3.12
trace
trace
3.04
10.43
12.15
6.16
3360
3360
72
(
Figure 1. (a) View of the triple-stranded heptametallic helicate in 1 and
. (b) Space-filling mode of the helicate with an included DMF
5
6
7
3b
3b
3c
55
55
25
670
4
6
8.24
10.42
4.51
7.98
16.22
20.75
4.51
2
3360
3360
10.33
trace
molecule.
72
(
8
9
1
3c
3c
3b
55
55
55
670
4
6
8.86
15.23
N.D
trace
trace
N.D
8.86
formula unit in the asymmetric unit, while 2 crystallizes in the
monoclinic space group C2/c, with half of the formula unit in the
3360
15.23
d
e
0
N/A
72
N.D
asymmetric unit and a crystallographic C axis passing through
2
a
Reaction conditions: catalyst (15.3−22.1 mg), H O (3.97−28.76
mmol), and substrate (0.1 mL). Moles of product/100 mol of alkane.
Ketone + alcohol. No catalyst but excess H O was added. Not
detected by GC−MS.
the central metal. Both 1 and 2 can be viewed as a linear
hexanuclear triple-stranded helicate enclosing one metal ion in
the cavity. The metal ions are engaged in two M O distorted
2
2
b
c
d
e
2
2
4
4
cubanes sharing one metal ion. The six outer metal ions each
adopt a distorted square-pyramidal geometry with the equatorial
Scheme 2. Peroxidative Oxidation of Toluene and
Ethylbenzene Catalyzed by 1
plane occupied by the NO donors of one L ligand and one OH
3
group and the apical position by one O atom of another L ligand,
whereas the central metal ion adopts a distorted octahedral
geometry by coordinating to three N and three O atoms from
three L ligands. In the cubane unit, the adjacent M−M distances
range from 3.1116(1) to 3.3036(1) Å for 1 and from 3.1089(1)
to 3.2472(1) Å for 2. All three ligands possess the same R
conformation, and each ligand coordinates to four metal ions by
using its two tridentate NO donors and two biphenolate O
2
atoms. The bulk of the biphenol moieties are pointing outward
from the cluster core, and the dihedral angles between the two
twisted phenyl rings of the L ligands are 74.1−81.5° for 1 and
The oxidation of cyclohexane led to a mixture product of
cyclohexanol (10.33%) and cyclohexanone (10.42%) (entry 6 in
Table 1), whereas cyclooctane gave its corresponding ketone as
the predominant product (4.51−15.23% yield) with a trace
amount of alcohol (entries 7−9 in Table 1). Besides, the
oxidation of toluene and ethylbenzene gave their corresponding
ketones as the main products (11−15% yield) with a trace
amount of alcohol. Although yields of up to 20.75% for 1 are
moderate in the oxidation of inert alkanes, the selectivities for the
oxidation of cyclopentane and cyclooctane are fairly comparable
with those of documented high-performing systems including
7
6.8−79.1° for 2.
This arrangement of the dicubane cluster and hexadentate L
ligands thus results in a P-configured triple-stranded helicate with
overall lengths of 24.1 and 24.5 Å and maximum widths of 16.1
and 15.9 Å for 1 and 2, respectively. The three pairs of tert-butyl
groups of the ligand L are organized such that they cover two
terminal sides of the helicate, thereby generating two hydro-
phobic pockets, each of which is occupied by one N,N-
dimethylformamide (DMF) molecule. A metal cluster with a
coordinatively unsaturated metal center in a hydrophobic pocket
12
polynuclear copper complexes. A further study on the catalytic
performance of this helicate and related analogues and
B
DOI: 10.1021/acs.inorgchem.6b01828
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
understanding the origin of the catalytic selectivity is still
underway.
ACKNOWLEDGMENTS
This work was supported by the Nationl Science Foundation of
China (Grants 21371119, 21431004, 21401128, 21522104, and
1620102001), the National Key Basic Research Program of
China (Grants 2014CB932102, 2012CB8217, and
016YFA0203400), and the Shanghai “Eastern Scholar”
Program.
■
In experiments in which the concentration of H O was varied,
2
2
comparable yields were obtained, whereas further increasing the
amount of the oxidant led to a decrease in the yield. Decreasing
the reaction temperature to 25 °C also led to a decrease in the
yield. However, the molar ratio of products ketone to alcohol
with 1 did not change when lower amounts of oxidant were
applied to the substrate and/or the reaction time was decreased
2
2
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■
*
*
Corresponding Authors
E-mail: liuy@sjtu.edu.cn.
E-mail: yongcui@sjtu.edu.cn.
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Author Contributions
These authors contributed equally.
‡
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.inorgchem.6b01828
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DOI: 10.1021/acs.inorgchem.6b01828
Inorg. Chem. XXXX, XXX, XXX−XXX