Self-assembly of mixed-valence and heterometallic metallocycles: efficient catalysts for the oxidation of alcohols to aldehydes in ambient air

Article abstract of DOI:10.1039/d0dt01340d

Two mixed-valence Cu^II/Cu^Iand two heterometallic Cu^II/Ag^Imetallocycles have been synthesized by the assembly of designed metalloligands and Cu^I/Ag^Iions, respectively. The Cu^II/Cu^Imetallocycle can catalyze the oxidation of alcohols to aldehydes mediated by a co-catalyst, TEMPO (2,2,6,6-tetramethylpiperdine-1-oxyl), with ambient air as an oxidant, while the Cu^II/Ag^Imetallocycle has no catalytic effect.

Full text of DOI:10.1039/d0dt01340d

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14 March 2017
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Self-assembly
of
mixed-valent
and
heterometallic
Received 00th XXXX 2019,
Accepted 00th XXXX 2019
metallocycles: efficient catalyst for oxidation of alcohols to
aldehydes with ambient air
DOI: 10.1039/c9ccxxxxxx
Ya-Liang Lai, Xue-Zhi Wang, Rui-Rong Dai, Yong-Liang Huang, Xian-
Chao Zhou, Xiao-Ping Zhou* and Dan Li
n
Two mixed-valent CuII/CuI and two heterometallic CuII/AgI
metallocycles have been synthesized by the assembly of designed
metalloligands and CuI/AgI ions, respectively. The CuII/CuI
metallocycle can catalyze oxidation of alcohols to aldehydes
mediated by co-catalyst TEMPO (2,2,6,6-tetramethylpiperdine-
1-oxyl) with ambient air as oxidant, while CuII/AgI metallocycle
has no catalytic effect.
M
N
N
N
N
N
N
n
n
Cu
N
Cu
n
N
Cu+/ Ag+
N
N
N
N
N
N
N
Cu2+
Cu
+
N
N
N
N
N
HN
NH
HN
NH
M
M
n = 2 M = Cu+ metallocycle 1
n = 1 M = Cu+ metallocycle 2
n = 2 M = Ag+ metallocycle 3
n = 1 M = Ag+ metallocycle 4
N
N
N
N
Cu
N
N
n
The design and construction of discrete supramolecular coordination
complexes (SCCs) including metallocycles and coordination cages
through coordination driven self-assembly have attracted gigantic
attentions for their aesthetically pleasing structures and advanced
Scheme 1. Schematic representation of stepwise synthesis of metallocycles
1-4
A complex that has appended functional binding sites to
coordinate to additional metal ions, is regarded as a metalloligand.
Metalloligands have been employed in the self-assembly of
metallocycles via a stepwise synthesis,12, 13 which is documented as
a useful approach, especially for heterometallic metallocycles.3 In
our previous studies, we found that a mononuclear metal complex
based on bis-imidazole ligand with Schiff base groups can act as a
metalloligand (Scheme 1 and Fig. S1 ESI†), which have been
employed as the building unit to construct coordination cages and
coordination polymers.14, 15 We hypothesized this metalloligand can
be probably used as a building unit for metallocycles: 1) The
mononuclear metal complex contains two convergent unbinding sites
(N-Cu-N  98 , Fig. S1, ESI†); 2) the soft nitrogen in imidazole
ligand can bind with soft linear coordinated coin metal ions (e.g. CuI
and AgI); 3) previous reports have shown that the imidazole ligand
can form metallocycles with coin metal ions.10 Herein, we report the
stepwise construction of two mixed-valent CuII/CuI and two
heterometallic CuII/AgI metallocycles by using two copper(II)-
imidazolate complexes as a metalloligand (Scheme 1). Both the
mixed-valent and metallocycles have identical triangular structures.
Interestingly, we found that the CuII/CuI metallocycle shows
excellent catalytic activity on the oxidation of alcohol mediated by
TEMPO, whereas there is no catalytic effect for heterometallic
CuII/AgI metallocycle. Furthermore, the CuII/CuI metallocycle can be
further used to catalyze the Knoevenagel condensation of
benzaldehyde and malononitrile to achieve a sequential catalysis.
1,
2
3, 4
functions (e.g. host-guest chemistry,
catalysis,
optical
materials,5-7 bioengineering 8). Metallocycles are large metal-organic
rings in which metal ions bind with organic ligands through dative
bonds. Due to the highly directional and relatively strong dative
bonds, the metallocycles can be designed and prepared with
predictable shapes and sizes. A huge number of metallocycles have
been successfully synthesized with varied geometries (e.g. triangles,
squares or rectangles, pentagons, hexagons, heptagon, and
octagon).9-11 In these metallocycles, the electronic state and
coordination geometry of metal ions may play critical roles in both
structures and functions. The metallocycle with mixed-valent or
heterometallic metal ions is important, 3 as the different metal centers
can probably act either as structural nodes or functional sites.
However, the self-assembly of metallocycles featuring identical
structure with mixed-valent and heterometallic metal ions is unusual,
in which precisely designed building unit is required for binding with
different types of metal ions.
College of Chemistry and Materials Science, Jinan University,
Guangzhou, Guangdong 510632, P. R. China. E-mail:
zhouxp@jnu.edu.cn
†
Electronic supplementary information (ESI) available:
Experimental section, characterization and physical measurements.
CCDC Nos. 1963369-1963370, 1974814-1974815. See DOI:
10.1039/xxxxxxx
Ligands H2L1 (N,N'-(butane-1,4-diyl)bis(1-(1H-imidazol-4-
yl)methanimine) and H2L2
(
N,N'-(propane-1,3-diyl)bis(1-(1H-
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imidazol-4-yl)methanimine)
)
were synthesized from the longer (2.452 – 2.524 Å) due to a Jahn–Teller effect for copper
condensation of 4-formylimidazole with corresponding diamine centers. The CuII-N bond distances are longer than that of CuI-N bond
molecules (1,4-diaminobutane for H2L and 1,3-diaminopropane for distances (1.857 -1.868 Å), which is in good agreement with the
14
H2L2).
Then, the mononuclear complexes Cu(H2L1) and reported mixed-valent CuII/CuI complexes. 16, 17 L1 coordinates with
Cu(H2L2) were obtained by reacting the H2L1 and H2L2 with one CuII ions and two CuI ions in 1, bridging the metal centers to
Cu(ClO4)2·6H2O, respectively. Finally, the reacting metalloligands construct metallocycle 1. The CuIICuII distances in metallocycle 1
(Cu(H2L1) and Cu(H2L2)) with Cu2O/AgNO3 in mixed solvents (N, were 11.594, 11.678, and 11.724 Å, respectively, while that of
N’-dimethylacetamide (DMA)/ ethanol, 2:1, v/v) at 100 C for 72 h CuICuI were 7.687, 8.021, 8.181 Å, respectively. The cavity of
-
afforded four metallocycles (see the ESI† for details), formulated as metallocycle 1 is occupied by a ClO4 anion (Fig. 2a), as the guest.
CuI3(CuIIL1)33ClO4
(1),
CuI3(CuIIL2)33ClO4
(2), Due to the ClO4 anion binding on the CuII, an inter-threading
-
AgI3(CuIIL1)33NO3 (3), and AgI3(CuIIL2)33NO3 (4), respectively. supramolecular dimer is formed, as shown in Fig. 2b. Although the
Single crystals of metallocycles 1-4 were obtained in the solvothemal metallocycles with triangular structure have been reported previously,
processes. Further studies found that metallocycles 1-4 can also be to our best knowledge, the assembly of two metallocycles by
synthesized in one step by mixing the ligands and metal salts under threading to form a supramolecular dimer is firstly observed.
similar conditions (Scheme S1, see ESI† for details).
Fig. 2 Crystal structure of metallocycle 1 filled with ClO4- anion (a) and the
structure of inter-threading supramolecular dimer (b). Color codes: CuII,
Green; CuI, orange; C, light gray; N, blue; O, red; Cl, turquoise; H, omitted.
The structure of metallocycle 2 is similar to 1, in which the L1 is
replaced with L2. The CuIICuII distances (11.519-11.605Å) in
metallocycle 2 were lightly shorter than that of 1 (11.594 - 11.724
Å), while its CuICuI distances (8.167 – 8.230 Å) were lightly
longer than that of 1 (7.687 - 8.181 Å). By replacing the CuI with
AgI, similar metallocycles 3 and 4 were obtained (Figs. 1c and d),
respectively. However, both the CuIICuII distances in 3 (11.961 –
12.177 Å) and 4 (11.870 – 12.068 Å) were longer than that of 1 and
2, which was probably due to their longer Ag-N coordination bond
distances (2.060 – 2.116 Å). Interestingly, both the metallocycles 3
Fig. 1 Crystal structures of metallocycles 1 (a), 2 (b), 3(c), and 4 (d). Color
codes: CuII, Green; CuI, orange; AgI, purple; C, light gray; N, blue. All
and 4 selectively host the NO3- anions as the guests (Fig. S3 ESI†),
-
although the same amount of ClO4 anion also presented in the
hydrogen atoms and anions are omitted for clarity.
reaction mixture.
Powder X-ray diffraction (PXRD) analysis showed that the
PXRD patterns of metallocycles 1-4 closely matched with their
simulated ones from SCXRD data, indicating that the pure phase of
these metallacycles can be obtained (Figs. 3a and b, Fig. S4 ESI†).
To confirm the mixed valence state of copper centers in
metallocycles 1 and 2, X-ray photoelectron spectra (XPS) of 1 and 2
were measured. As shown in Figs. 3c and d, intense asymmetrical Cu
2p3/2 photoelectron peaks with satellite peaks are presented in both
Single-crystal X-ray diffraction (SCXRD) analysis revealed that
all the crystals of metallocycles 1-4 crystalize in a P space group,
featuring similar hexanuclear triangular structures (Fig. 1). The
structure of metallocycle 1 will be described in detail as an example.
Due to the low symmetry, the asymmetric unit in 1 contains one
triangular metallocycle composed of 6 copper centres and 3 ligands
-
and three ClO4 anions to balance the positive charges (Fig S2, ESI†).
As shown in Fig. 1a, there are two types of copper centers in 1. The
first one probably has an oxidation state of +2, where CuII was
chelated by ligand L1 to form a copper(II)-imidazolate metalloligand.
The second one should have an oxidation state of +1, which CuI links
the metalloligands to form a triangular metallocycle. Two CuII
centers adopt five-coordinated (four nirtrogen from the ligand, one
spectra, indicating the mixed-valent feature of copper centers in 1
19
and 2.18,
The Cu 2p3/2 peaks can be deconvoluted into two
contributions located at 933.04 and 934.89 eV for 1 (Fig. 2c) and
932.87 and 934.59 eV for 2 (Fig. 2d), corresponding to the CuI and
CuII species in peak area ratios of approximately 3 : 2 and 2 : 3,
respectively. The peak area ratio of CuI and CuII of XPS results is not
in agreement with the crystal structure. The possible reason is that
the XPS is surface-sensitive quantitative technology. On the other
hand, bond valence sum (BVS) has been used to determine the
oxidation state of metal ions based on their metal-ligand bond
-
oxygen from ClO4 ) square pyramidal geometry and one CuII adopts
a four-coordinated square planar geometry (four nirtrogen from
ligand), while the CuI adopts a linear coordination geometry (N-Cu-
N, 171.788 – 175.313 ). The CuII-N bond distances were in the range
of 1.976 - 2.026 Å, while the CuII -O bond distances were obviously
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distances in SCXRD structure or extended X-ray absorption fine complexes have been employed for catalysing the oxidation of
structure (EXAFS).20 The calculation of BVS values for the three alcohols to aldehydes by combining with a stable nitroxyl radical
CuII centers in 1 gives 2.037, 2.043, and 2.031, respectively, while such as TEMPO and molecular oxygen.23-25 We hypothesize that the
that of three CuI centers gives 0.981, 0.964, and 0.958, respectively. CuII/CuI mixed-valent metallocycles can probably act as an efficient
Similarly, the BVS of three CuII centers in 2 are 2.068, 2.105, and catalyst for the aerobic oxidation of alcohols to aldehyde by using
2.088, respectively, while that of three CuI centers are 0.978, 0.976, TEMPO as co-catalyst.
and 0.990, respectively. All these BVS values are in good agreement
with the results of SCXRD and XPS, proving the mixed-valent
property of both metallocycles 1 and 2
The solubility of metallocycle 1 is better than 2 in acetonitrile.
Thus, the catalytic activity of 1 for the oxidization of alcohol was
assessed, and the oxidation of benzyl alcohol was selected as a model
Mass spectrometry was usually employed for checking the reaction. Gas chromatography (GC) results demonstrated that the
solution states of cages and metallocycles. The high-resolution mass oxidation of benzyl alcohol into benzyl aldehyde is successful by
spectra of metallocycles 1-4 were measured by an electrospray using metallocycle 1, TEMPO, N-methylimidazole (NMI), and
ionization time-of-flight mass spectrometer (ESI-TOF-MS) in acetonitrile at room temperature with ambient air, in which the
acetonitrile. As shown in Fig. S5 (ESI†), the main cationic peaks TEMPO and NMI acts co-catalyst and base, respectively. After
arise mainly from the broken species of metallocycles (e.g. [CuL1- optimization of the reaction parameters, the catalyst system
H]+, [Cu3L2-4H]+ in spectrum of 1), indicating that metallocycles 1- employed here consists of 5 mol% 1, 10 mol % TEMPO, and 50 mol
4 are probably not stable under the ESI-TOF-MS measurement. % NMI in acetonitrile. As shown in Fig. S6 (ESI†), although the
However, we can find some weak cationic peaks arising from the loss reaction rate is slow in the first 40 min, metallocycle 1 could catalyse
of anions and hydrogen from the metallocycles (such as m/z 603.395, the oxidation of benzyl alcohol with 99.99 % conversion in 2 hours.
[Cu3(CuL1)3(ClO4)-6H]2+ for 1, 582.695, [Cu3(Cu3L2)3(ClO4)-6H]2+ The further oxidation benzyl aldehyde to benzoic acid is not observed
for
2,
620.594
[Ag3(CuL1)3-7H]2+ for
3,
630.530, during the reaction even with longer reaction time, indicating the
[Ag3(CuL2)3(NO3)-6H]2+ for 4) in the spectra, suggesting that excellent selectivity of the catalyst. For comparison, the catalytic
metallocycles 1-4 should exist in acetonitrile solution.
performance of metallocycle 3 was also accessed under the same
conditions. Experimental results showed that trace benzyl aldehyde
was detected, indicating that 3 can not catalyse the oxidation of
benzyl alcohol to benzyl aldehyde. The structure difference between
1 and 3 is the linear coordination metal ions (CuI 1, AgI 3), suggesting
that the catalytic site for 1 is likely the CuI centre, than CuII. The
possible reason is that CuII in both 1 and 3 is chelated by L1, which
do not have accessible active site to catalyse the benzyl alcohol.
Therefore, as proposed by Stahl., et al,23 the plausible mechanism
may contain two important separate half-reactions (Fig. S7). First,
the O2 of air oxidize the CuI and TEMPOH. Second, the yielded
intermediated CuII species oxidize the alcohol to aldyhyde, which is
mediated by TEMPO.
Inspired by the good catalytic performance of metallocycle 1 for
the oxidation of benzyl alcohol into benzyl aldehyde under ambient
conditions, the scope of this oxidation reaction was further explored.
As shown in Fig. 4, 16 aromatic alcohol molecules with various
substitutional groups and heteroaromatic alcohols were tested. Both
electro-withdrawing and -donating substitutional groups on the
benzene ring of benzyl alcohols were well-tolerated (conversion >
99.9 %). On the other hand, the heteroaromatic alcohols including 2-
furylmethanol, 2-thienylmethanol, and 3-pyridinylmethanol also
afford excellent conversion (> 99.9%) in 2 h. However, for 1-
naphthalenemethanol, the conversion ( 70%) is obviously lower
than that of 2-naphthalenemethanol, which is probably due to its
steric hindrance.
Fig. 3 PXRD patterns of 1 (a) and 2 (b), and XPS spectra of 1 (a) and 2 (d)
with fitting peaks (blue CuI, green CuII).
Aldehydes act as important roles in organic reactions and high-
value components in the perfume industry. They can be produced
from the oxidation of alcohols by employing stoichiometric amounts
of inorganic oxidizing reagents, including chromium and manganese
oxides.21 Such oxidization reactions are poorly selective and generate
copious amounts of heavy-metal waste, which are not
environmentally friendly. Therefore, the development of green
catalysis for oxidation of alcohols by using green oxidizing reagents
(e.g. O2) seems highly desirable and is fundamentally important in
laboratory and industrial settings.22 The galactose oxidase (GO)
enzyme containing copper catalytic centres can selectively oxidize
primary alcohols to aldehydes. To mimic GO enzyme, copper(I/II)
Due to metallocycle 1 also contains weak base groups (e.g.
imidazole and Schiff base groups), it can probably catalyze the
Knoevenagel condensation reaction.26 Thus, the sequential alcohol
oxidation/Knoevenagel condensation reaction can probably be
reached by employing 1 as catalyst (Scheme S2, ESI†). Upon the
completion of oxidation of benzyl alcohol, malononitrile (2.0 equiv.)
was added to the reaction mixture. The product
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benzylidenemalononitrile was acquired in 90.0 % conversion after strategy for designing mixed-valent and heterometallic metallocycles.
1.5 h. The sequential reaction afforded an extremely low conversion It can be expected that these interesting metallocycles can be
(< 2%) without add 1 as catalyst. This preliminary result showed that employed as catalyst for more organic reaction.
metallocycle 1 can be used as a potential catalyst for sequential
reactions.
This work was supported by the National Natural Science
Foundation of China (Nos. 21731002, 21871172, 21801095), the
Major Program of Guangdong Basic and Applied Research (No.
2019B030302009), the Fundamental Research Funds for the Central
Universities (21619405), and Jinan University.
1
5 mol%
10 mol% TEMPO
50 mol% NMI
R
OH
R
O
Room Temperature
CH3CN, Air, 2h
Conflicts of interest
There are no conflicts to declare.
Notes and references
O
O
O
O
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>99.9%
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>99.9%
O
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>99.9%
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>99.9%
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In summary, we have successfully constructed two mixed-valent
CuII/CuI and two heterometallic CuII/AgI metallocycles by using the
metalloligand approach. These metallocycles feature similar
triangular structures. Interesting inter-threading supramolecular
dimers were observed between two adjacent metallocycles with their
binding anions. Metallocycle 1 is an efficient catalyst for oxidation
aromatic alcohols to aldehyde. Furthermore, metallocycle 1 can
catalyze the Knoevenagel condensation reaction, providing a
potential material for sequential catalysis. This work provides a good
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