High-Cluster (Cu9) Cage Silsesquioxanes: Synthesis, Structure, and Catalytic Activity

Article abstract of DOI:10.1021/acs.inorgchem.8b01496

Unusual high-cluster (Cu₉) cage phenylsilsesquioxanes were obtained via complexation of in situ Cu^II,Na-silsesquioxane species formed with phenanthroline and neocuproine. In the first case, phenanthroline, acting as "a silent ligand" (not participating in the composition of the final product), favors the formation of an unprecedented cagelike phenylsilsesquioxane of Cu₉Na₆ nuclearity, 1. In the second case, neocuproine ligands withdraws two Cu ions from the metallasilsesquioxane matrix, producing two cationic fragments Cu⁺(neocuproine)₂. The remaining metallasilsesquioxane is rearranged into an anionic cage of Cu₉Na₄ nuclearity, finalizing the formation of a specific ionic complex, 2. The impressive molecular architecture of both types of complexes, e.g., the presence of different (cyclic/acyclic) types of silsesquioxane ligands, was established by single-crystal X-ray diffraction studies. Compound 1 was revealed to be highly active in the oxidative amidation of benzylic alcohol and the catalyst loading could be reduced down to 100 ppm of Cu. Catalytic studies of compound 1 demonstrated its high activity in hydroperoxidation of alkanes with H₂O₂ and oxidation of alcohols to ketones with tert-BuOOH.

Full text of DOI:10.1021/acs.inorgchem.8b01496

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
High-Cluster (Cu ) Cage Silsesquioxanes: Synthesis, Structure, and
9
Catalytic Activity
†
,‡
,†,‡
Alexander A. Korlyukov, Mikhail M. Levitsky,†
†,§
Grigorii S. Astakhov, Alexey N. Bilyachenko,*
Lidia S. Shul’pina, Xavier Bantreil, Fred
́ ́
ic Lamaty,* Anna V. Vologzhanina, Elena S. Shubina,†
†
∥
er
Pavel V. Dorovatovskii, Dmytro S. Nesterov, Armando J. L. Pombeiro,
,∥
†
⊥
#
#
,
¶,∇
and Georgiy B. Shul’pin*
†
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov Strasse 28, Moscow, Russia
Peoples’ Friendship University of Russia (RUDN University), Miklukho-Maklay Strasse 6, Moscow, Russia
Pirogov Russian National Research Medical University, Ostrovitianov Strasse 1, Moscow, Russia
‡
§
∥
Institut des Biomolec
́
ules Max Mousseron, UMR 5247, CNRS, Universite
́
de Montpellier, ENSCM, Site Triolet, Place Eugen
̀
e
Bataillon, 34095 Montpellier Cedex 5, France
⊥
National Research Center “Kurchatov Institute”, Akademika Kurchatova Place 1, Moscow, Russia
#
Centro de Química Estrutural, Instituto Superior Tec
́
nico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa,
Portugal
¶
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Ulitsa Kosygina, dom 4, 119991 Moscow, Russia
∇
Plekhanov Russian University of Economics, Stremyannyi Pereulok, dom 36, 117997Moscow, Russia
S Supporting Information
*
ABSTRACT: Unusual high-cluster (Cu ) cage phenylsilses-
9
II
quioxanes were obtained via complexation of in situ Cu ,Na-
silsesquioxane species formed with phenanthroline and neo-
cuproine. In the first case, phenanthroline, acting as “a silent
ligand” (not participating in the composition of the final
product), favors the formation of an unprecedented cagelike
phenylsilsesquioxane of Cu Na nuclearity, 1. In the second
9
6
case, neocuproine ligands withdraws two Cu ions from the
metallasilsesquioxane matrix, producing two cationic fragments
+
Cu (neocuproine) . The remaining metallasilsesquioxane is
2
rearranged into an anionic cage of Cu Na nuclearity, finalizing
9
4
the formation of a specific ionic complex, 2. The impressive
molecular architecture of both types of complexes, e.g., the
presence of different (cyclic/acyclic) types of silsesquioxane ligands, was established by single-crystal X-ray diffraction studies.
Compound 1 was revealed to be highly active in the oxidative amidation of benzylic alcohol and the catalyst loading could be
reduced down to 100 ppm of Cu. Catalytic studies of compound 1 demonstrated its high activity in hydroperoxidation of
alkanes with H O and oxidation of alcohols to ketones with tert-BuOOH.
2
2
INTRODUCTION
matrixes for the production of cagelike metal-containing
derivatives. Important data concerning the reactivity and
■
5
Hybrid inorganic−organic molecular materials are the focus of
both academia and industry chemists because of their unusual
reactivity, structure formation principles, and numerous
structure formation were reported for the cases when6
additional ligands (e.g., 1,2,3,4,5-pentamethylcyclopentadiene,
7
8
1
silylamido, and triethylamine ) were used along with the
silsesquioxane one in the assembly of polyhedral metal-
lacomplexes. In all of these works, the “ligands-synergy”
principle was reported when two types of ligands (silsesquiox-
ane and organic ones) coordinated metal ions of the cagelike
product.
potential applications. Among the best models for such
hybrid materials are derivatives of siloxanes and molecular
combinations of inorganic (Si−O−Si) skeletons and organic
2
(
R−Si) surroundings. An exclusive feature of monosubsti-
tuted siloxanes (silsesquioxanes of the general formula
3
[
RSiO ] ) is an ability to form different polyhedral clusters.
1.5 n
These well-defined polyhedral structures are the objects of
4
utmost interest as precursors of materials in high demand. In
Received: June 1, 2018
turn, it should be mentioned that silsesquioxanes are suitable
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01496
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Inorganic Chemistry
Article
RESULTS AND DISCUSSION
■
Here we discuss a different scenario of additional ligand
involvement in the cage metallasilsesquioxane synthesis. First,
the self-assembly reaction of the in situ formed sodium
phenylsiloxanolate [(PhSiO )(NaO )] and copper(II)
1
.5
0.5
n
chloride in the presence of 1,10-phenanthroline (ethanol
media) gave unprecedented cagelike Cu ,Na -silsesquioxane 1
9
6
(Scheme 1 and Figure 1, top).
Scheme 1. Reaction Scheme of the Synthesis of 1 and 2
Figure 1. Molecular structures of 1 (top) and 2 (bottom). Solvating
ethanol and water molecules of 1 are omitted for clarity. Color code:
Si, yellow; O, red; Cu, green; N, blue; C, black.
runs of this reaction gave an analogue of complex 1, with small
differences from 1; see complex 1a; Figures S1 and S2).
In the second stage of work, a similar synthetic procedure
but in the presence of another type of N-based ligand,
neocuproine, was studied. To our surprise, this reaction
provoked the formation of a principally different type of
molecular architecture, compound 2 (Scheme 1 and Figure 1,
bottom). To the best of our knowledge, compound 2
represented a novel principle of metal-containing cage
compound design in conditions of the presence of O and N
ligands in the reaction mixture. The only example of similar
reaction in the presence of P-ligand was presented by some of
5g
us in recent work. The product 2 composition could be
described as a consequence of unusual Cu ion redistribution
between two types of ligands. Namely, neocuproine ligands
+
form two cationic fragments Cu (neocuproine) , while
2
Cu Na -silsesquioxane plays the role of a negatively (2−)
9
4
charged component of compound 2. Such a difference of
behavior between phenanthroline and neocuproine could be
explained by the better σ donation of the latter ligand, as
demonstrated in the formation of copper ethylene com-
10
plexes.
The compositions of the silsesquioxane fragments of
complexes 1 and 2 are quite different from each other.
Namely, compound 1 comprises two acyclic Ph Si O
10 21
1
0
ligands, unprecedented for cage metallasilsesquioxanes (Figure
, top). In turn, the cage component of complex 2 includes two
pairs of ligands: cyclic (PhSiO ) and acyclic (PhSiO )
2
1
.5 6
1.5 4
(
Figure 2, bottom). While (PhSiO ) ligands are widespread
1.5 6
5a,b
It is worth mentioning that the phenanthroline ligand is not
participating in the composition of the final product;
nevertheless, its influence on the reaction course could not
be denied: early reported “phenanthroline-less” synthesis of
Cu,Na-phenylsilsesquioxane in ethanol media gave a Cu Na
complex of principally different molecular topology. Thus, it
may be concluded that phenanthroline acts as a “silent witness”
ligand, and this effect was found to be reproducible (additional
among metallasilsesquioxane structures,
only work-described (PhSiO ) ligands in the composition of
cage Ti-silsesquioxanes. This publication, unlike compound
2, reported cyclic (PhSiO ) rings, sandwiched between Ti-
based planes. Detailed topological analysis of the structures of
compounds 1 and 2 is presented in Figure S4. Finally, we
could mention Mn-based phenylsilsesquioxane possessing
the same [(PhSiO ) and (PhSiO ) ] combination of ligands
we could cite the
1
.5 4
11
1
.5 4
2
2
9
12
1
.5 6
1.5 4
B
DOI: 10.1021/acs.inorgchem.8b01496
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Inorganic Chemistry
Article
quick optimization of the reaction conditions, for the
16
formation of amides directly from alcohol, revealed that
compound 1 was as efficient as the previously reported CLMSs
because only 100 ppm of Cu was required to catalyze the
reaction. When benzyl alcohol was reacted with different
amines, protected as their ammonium salt, in the presence of
calcium carbonate, tert-butyl hydroperoxide (TBHP) as the
terminal oxidant, and 1 as the catalyst, the corresponding
amides could be isolated in good yield (Scheme 2). To ensure
Scheme 2. Catalytic Activity of Compound 1 in Oxidative
a
Amidation
a
Reaction conditions: ammonium chloride (0.5 mmol), benzyl
alcohol (1.0 mmol), CaCO3 (0.25 mmol), TBHP (5.5 M, 2.5
mmol), 1 (0.01 mol % of Cu), CH CN (1 mL), 80 °C, 24 h.
3
Figure 2. Silsesquioxane ligand structures in 1 (top) and 2 (bottom).
that compound 1, and not traces of metals, was indeed
responsible for the catalytic activity, high-purity calcium
carbonate (>99.999% purity) was used. Because low catalyst
loading was used, the turnover number and frequency (TON
as compound 2. It is worth noting that the resulting structure
of the compound from ref 12 is quite unlike complex 2.
The structure and composition of Si−O−Si skeletons in
both products 1 and 2 are worth mentioning as new evidence
of the role of additional factors during Si−O−Si fragment
17
and TOF) were calculated. The reaction was found to be
efficient with primary and secondary amines, with TON and
−1
TOF values reaching 7700 and 325 h , respectively. These
values are slightly lower than those obtained with a Cu -CLMS
4
assembly. It is known that the basic reaction [PhSi(OR′) with
featuring a globulelike structure (TON and TOF of 9400 and
3
−
1
15
NaOH] leads to the cyclic trimer or tetramer product
392 h , respectively).
However, compound 1 was
5a,13
(
[PhSi(O)ONa)] , n = 3 or 4; Figure S3).
In turn,
undoubtedly more efficient than the CuO that was used in
n
numerous further syntheses, using these trimers or tetramers
for interaction with transition-metal salts, showed the
structural rearrangement of these cycles into ligands of other
our seminal publication on this reaction, with TON and TOF
values of 44 and 11 h , respectively.
Impressive results were obtained in the catalyzed alkane
−
1
16e
5
a,14
forms.
Keeping in mind that the procedures presented here
oxidations with peroxides (H O and m-chloroperoxybenzoic
2
2
that allowed the isolation of compounds 1and 2 are even more
complicated because of the involvement of additional N
ligands, it is not surprising that the silsesquioxane ligand
topology in 1 and 2 is far from the classical one.
Regarding the properties of complexes 1 and 2, we could
state that both compounds are air-stable, nonhygroscopic, and
well soluble in polar organic solvents (e.g., N,N-dimethylfor-
mide, diemthyl sulfoxide, or benzonitrile) or in their mixtures
with aromatic solvents (benzene and toluene).
acid). Oxidation of cyclohexane (CyH) with H O , in the
2 2
presence of 1 as a catalyst and HNO as a promoter either in
3
1
8
air (see Figures S10−S15) or under an atmosphere of O2
(see Figures 3 and 4 and S16−S24), afforded cyclohexyl
hydroperoxide as the main reaction product, which is
selectively reduced with PPh (refs 18) to produce cyclo-
3
1
8
hexanol. The highest incorporation of O into cyclohexyl
hydroperoxide was observed at the beginning of the reaction
(Figure S16). The chromatogram recorded before the addition
We previously reported the activity of cagelike metal-
lasilsesquioxanes (CLMSs) as precatalysts in the homogeneous
catalysis of highly relevant organic reactions: amidation and
hydrocarbons functionalization. Importantly, the use of
CLMSs allowed minimization of the catalyst loading (down
to 100 ppm of Cu) with retention of the catalytic activity. A
of PPh reveals a strong peak of cyclohexyl hydroperoxide,
3
disappearing upon treatment. The pattern of byproducts
(Figure 3) is typical for reaction systems oxidizing with
hydroxyl radicals, showing isomers of cyclohexanediols and
hydroxycyclohexanones. Cyclohexanediols are mainly single
15
1
8
O-labeled. Hydroxycyclohexanones are probably only labeled
C
DOI: 10.1021/acs.inorgchem.8b01496
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Inorganic Chemistry
Article
reaction centers available for approaching both H O and the
2
2
substrate. As in the case of oxidations catalyzed by
polyoxometallates, in our oxidation, a few species with active
reaction centers work. Figures S10−S14 demonstrate pecu-
liarities of CyH oxidation with H O in acetonitrile. The
2
2
maximum yield of the products was 21%. The products of
methylcyclohexane oxidation by the same system are shown in
Figure S15. Oxidations of n-heptane and n-octane allowed us
to measure regioselectivity parameters: 1:5.4:5.4:4.9 and
1
:5.4:5.3:4.8, respectively. The reaction of cis-1,2-dimethylcy-
clohexane with H O catalyzed by compound 1 gave a trans/
2
2
cis ratio of 1.0 for tert-alcohols. The interaction of tert-butyl
hydroperoxide (1.5 M) with 1-phenylethanol or cyclohexanol
(
0.5 M) in the absence of HNO afforded after 10 or 5 h at 60
3
Figure 3. Fragments of the chromatograms showing the main reaction
products and byproducts (inset) in the course of CyH oxidation with
H O , in the presence of HNO , catalyzed by 1 (for conditions, see
°C cyclohexanone or acetophenone in 63% or 98% yield,
respectively.
2
2
3
The system 1/HNO /H O shows low stereoselectivity with
4% retention of the stereoconfiguration (RC), corresponding
3
2
2
Figure S16), before (top, green) and after (bottom, red) the addition
of solid PPh3 to the reaction sample. Percentages indicate the
incorporations of 18O into the respective oxygenates measured after
1
to a trans/cis ratio of 1.34 of tertiary alcohols. Such a value of
the RC index indicates the involvement of free alkyl radical
intermediates. The normalized bond selectivity for cis-1,2-
dimethylcyclohexane is 3°:2°:1° = 90:21:1. Although the ratio
1
8
the addition of PPh . The incorporation of O into the main
3
products is shown in Figure S16, right.
19
3
°:2° = 4.3:1 is expected for the hydroxyl radical range, the
amounts of primary oxygenates are abnormally low. This may
be explained by a high reactivity of closely located tertiary H
atoms of cis-1,2-dimethylcyclohexane.
Oxidation of cis-1,2-dimethylcyclohexane with m-CPBA,
catalyzed by 1, in the presence of a HNO promoter and
3
1
8
under an atmosphere of O , afforded much higher stereo-
2
selectivity with an RC index of up to 51% (trans/cis ratio of up
1
8
to 3.1 of tertiary alcohols). The incorporation of O into the
tertiary alcohols is also higher than those in the case of H O ,
2
2
reaching 70% after 3 h (Figure 4, bottom). Remarkably, while
1
8
cis-alcohol accumulated ca. 2 times more O than trans-
1
8
alcohol (in percent), the absolute amounts of both O-labeled
alcohols are nearly equal (Figure 4, top). The initial reaction
1
8
−7
rates of O-labeled tertiary alcohols (1.4 × 10 and 1.0 ×
−
7
M s−1 for trans- and cis-alcohols, respectively) are ca. 2
10
−
7
times lower than the overall initial rates (5.0 × 10 and 1.7 ×
−
7
−1
10
M s for trans- and cis-alcohols, respectively).
Recently, we found that a hexacopper complex with
neocuproine (an analogue of complex 2) is a highly efficient
homogeneous catalyst in the oxidation of alkanes and alcohols
20
with peroxides. All of the peculiarities of alkane oxidation
indicate that the oxidation with H O occurs with the
2
2
participation of free hydroxyl radicals. The hexacopper
complex with neocuproine also efficiently catalyzes the
oxidation of alcohols to ketones with TBHP in acetonitrile.
The oxidation of alcohols apparently involves the generation of
hydroxyl radicals.
Figure 4. Top: Accumulations of all and 18O-labeled trans and cis
tertiary alcohols in the course of oxidation of cis-1,2-dimethylcyclo-
CONCLUSIONS
In summary, two unique cage Cu,Na-silsesquioxanes were
synthesized through self-assembly reactions of sodium
■
hexane (0.14 M) with m-CPBA (0.18 M), in the presence of HNO
3
0.05 M), catalyzed by 1 (5 × 10−4 M), in CH CN at 50 °C;
(
3
siloxanolate species with CuCl . The specific role of N-based
measured after the addition of solid PPh to the samples. Bottom:
2
3
Dependence of the 18O percentage in the tertiary alcohols.
ligands (phenanthroline and neocuproine) in this reaction was
demonstrated. Namely, the presence of phenanthroline
provokes the formation of compound 1, containing no
phenanthroline moieties in the composition (phenanthroline
at the hydroxyl O atom because of oxygen exchange of their
ketone groups with H216O.
21
as a “silent witness” ligand). In turn, neocuproine-assisted
We clearly realize that synthesized complexes are only
precatalysts in hydrocarbon oxidations. Under the action of
synthesis led to the formation of complex 2 with an unusual
redistribution of Cu centers between O (silsesquioxane) and N
(neocuproine) ligands. These observations point to the huge
HNO acid, the complex is modified to open fragments, with
3
D
DOI: 10.1021/acs.inorgchem.8b01496
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Inorganic Chemistry
Article
influence of various types of ligands, depending on their
binding affinity, on the mechanism of cage metallacomplex
formation. It is worth noting that small differences in the N-
ligand structures (phenanthroline vs neocuproine) cause the
formation of dramatically different cage products. This
indicates that the formation of cage metallasilsesquioxane is
complicated, especially in the presence of additional ligands,
which could provoke “ligand-to-metal” competitions and metal
redistribution. In turn, the isolation of compounds 1 and 2 was
performed for the conditions of different solvent media. No
doubt, the difference in the solvates’ surroundings may also be
crucial during the assembly/crystallization of the cage product.
In summary, we could state that at least three parameters could
influence/control the metallasilsesquioxane formation: (i) ratio
between “key elements” in reaction media, namely, Si:-
Cu:Na:N; (ii) nature of the ligand; (iii) ability of solvent
molecules to play a role of solvating ligands. All of these factors
could be of high influence for the process under discussion,
and in our future reports, we will try to shed some light on this
point. Compound 1 was found to be efficient in the alkane
oxidation and oxidative amidation of benzylic alcohols, with a
catalyst loading down to 100 ppm of Cu and TON and TOF
and 17-03-00993, French Embassy in Moscow, University of
Montpellier, CNRS (Programme de Recherche Conjoint).
Synchrotron single-crystal diffraction measurements were
performed at the unique scientific facility Kurchatov
Synchrotron Radiation Source supported by the Ministry of
Education and Science of the Russian Federation (Project code
RFMEFI61917X0007). This work was also supported by the
State within the frameworks of the state task for ICP RAS
0082-2014-0007. The number of state registration is AAAAA-
A18-118012390374-3. Support from the Fundaca
Ciencia e a Tecnologia, Portugal (Projects UID/QUI/00100/
̧ o para a
̃
̂
2013 and PTDC/QEQ-QIN/3967/2014 and Fellowship
SFRH/BPD/99533/2014 (to D.S.N.)], is also acknowledged.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01496.
■
*
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AUTHOR INFORMATION
Corresponding Authors
E-mail: bilyachenko@ineos.ac.ru.
E-mail: frederic.lamaty@umontpellier.fr.
E-mail: shulpin@chph.ras.ru.
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Alexey N. Bilyachenko: 0000-0003-3136-3675
Xavier Bantreil: 0000-0002-2676-6851
Anna V. Vologzhanina: 0000-0002-6228-303X
Elena S. Shubina: 0000-0001-8057-3703
Pavel V. Dorovatovskii: 0000-0002-2978-3614
Georgiy B. Shul’pin: 0000-0002-7053-4812
Author Contributions
The manuscript was written through contributions of all
authors.
(
(
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The publication was prepared with the support of RUDN
University Program 5-100 and funded by RFBR according to
Research Projects 16-03-00254, 16-53-150008, 16-29-05180,
E
DOI: 10.1021/acs.inorgchem.8b01496
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Inorganic Chemistry
Article
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DOI: 10.1021/acs.inorgchem.8b01496
Inorg. Chem. XXXX, XXX, XXX−XXX