Cage-like Fe,Na-Germsesquioxanes: Structure, Magnetism, and Catalytic Activity

Article abstract of DOI:10.1002/anie.201607189

A series of four unprecedented heterometallic metallagermsesquioxanes were synthesized. Their cage-like architectures have a unique type of molecular topology consisting of the hexairon oxo {Fe₆O₁₉} core surrounded in a triangular manner by three cyclic germoxanolates [PhGe(O)O]₅. This structural organization induces antiferromagnetic interactions between the Fe^IIIions through the oxygen atoms. Evaluated for this first time in catalysis, these compounds showed a high catalytic activity in the oxidation of alkanes and the oxidative formation of benzamides from alcohols.

Full text of DOI:10.1002/anie.201607189

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201607189
German Edition: DOI: 10.1002/ange.201607189
Homogeneous Catalysis
Cage-like Fe,Na-Germsesquioxanes: Structure, Magnetism, and
Catalytic Activity
Alexey N. Bilyachenko,* Mikhail M. Levitsky, Alexey I. Yalymov, Alexander A. Korlyukov,
Viktor N. Khrustalev, Anna V. Vologzhanina, Lidia S. Shul’pina, Nikolay S. Ikonnikov,
Alexander E. Trigub, Pavel V. Dorovatovskii, Xavier Bantreil, Frꢀdꢀric Lamaty,* Jꢀrꢁme Long,
Joulia Larionova, Igor E. Golub, Elena S. Shubina, and Georgiy B. Shul’pin*
Abstract: A series of four unprecedented heterometallic
metallagermsesquioxanes were synthesized. Their cage-like
architectures have a unique type of molecular topology
consisting of the hexairon oxo {Fe6O19} core surrounded in
a triangular manner by three cyclic germoxanolates [PhGe-
(O)O]5. This structural organization induces antiferromag-
netic interactions between the FeIII ions through the oxygen
atoms. Evaluated for this first time in catalysis, these com-
pounds showed a high catalytic activity in the oxidation of
alkanes and the oxidative formation of benzamides from
alcohols.
silsesquioxane [RSiO1.5]n ligands. Surprisingly, examples of
cage-like architectures involving another metalloid close to
silicon, namely germanium, are scarce despite the great
potential of this element in organometallic chemistry. To our
knowledge there is only a series of compounds obtained from
germasesquioxane [RGeO1.5]2 (R = HOOCCH2CH2) and var-
ious cations.[4] Their hydrothermal synthesis requires rela-
tively hard conditions consisting of several days of reactions at
1108C. It is worth mentioning that to date focus has only been
on the synthesis, thermogravimetric analysis, luminescence,
and sorption properties of these architectures and no inves-
tigation of the catalytic activity of metallagermanates has
been reported.
Herein, we report an alternative and simplified synthetic
approach to design a series of unprecedented individual cage-
like metallagermsesquioxanes under mild conditions. In
addition, the catalytic activity and magnetic properties of
the metallagermsesquioxanes will be discussed.
The single crystals of Fe,Na-phenylgermsesquioxane
[(Ph5Ge5O10)3Fe6(OH)3(O)Na2]·(EtOH)2·H2O 1 (10% yield)
were obtained by a reaction between phenyltrimethoxyger-
mane PhGe(OMe)3, NaOH and anhydrous FeCl3 in ethanol
with further crystallization. The addition of N-containing
ligands, such as phenanthroline, bipyridine, or neocuproine
to this reaction allowed us to isolate complexes
T
he design of metal-based polycyclic cage-like compounds
has attracted a great deal of attention from researchers
worldwide over several decades owing to their structural
diversity and their potential technological applications espe-
cially in catalysis.[1] Numerous metal-oxygen clusters,[1b,c,j]
metal–organic frameworks[1d,l,m] or metal–organic polyhe-
drons[1f,n] have been designed by using a variety of self-
assembling or templated reactions. Among those, a wide
family of polynuclear cage-like metallasilsesquioxanes
(CLMSs) architectures with interesting structural topolo-
gies[2] and important catalytic activity (e.g. in epoxidation,
oxidation and polymerization)[3] has been synthesized
through a reaction involving transition-metal ions and
[*] Dr. A. N. Bilyachenko, Dr. M. M. Levitsky, A. I. Yalymov,
Prof. A. A. Korlyukov, Prof. V. N. Khrustalev, Dr. A. V. Vologzhanina,
L. S. Shul’pina, Dr. N. S. Ikonnikov, Dr. I. E. Golub, Prof. E. S. Shubina
A. N. Nesmeyanov Institute of Organoelement Compounds
Russian Academy of Sciences
Dr. X. Bantreil, Dr. F. Lamaty
Institut des Biomolꢀcules Max Mousseron (IBMM), UMR 5247
CNRS
Universitꢀ Montpellier, ENSCM CNRS, Universitꢀ de Montpellier,
ENSCM should be on the same line
Vavilov str., 28, Moscow (Russia)
E-mail: bilyachenko@ineos.ac.ru
Universitꢀ de Montpellier Campus Triolet, Place Eugꢁne Bataillon,
34095 Montpellier cedex 5 (France)
E-mail: frederic.lamaty@umontpellier.fr
Dr. A. N. Bilyachenko, Prof. V. N. Khrustalev
People’s Friendship University of Russia
Prof. G. B. Shul’pin
Miklukho-Maklay Str., 6, 117198, Moscow (Russia)
Semenov Institute of Chemical Physics
Russian Academy of Sciences
Prof. A. A. Korlyukov
Ulitsa Kosygina, dom 4, Moscow 119991 (Russia)
E-mail: shulpin@chph.ras.ru
Pirogov Russian National Research Medical University
Ostrovitianov str., 1, Moscow (Russia)
Prof. G. B. Shul’pin
Dr. A. E. Trigub, P. V. Dorovatovskii
Plekhanov Russian University of Economics
Stremyannyi pereulok, dom 36, Moscow 117997 (Russia)
National Research Center “Kurchatov Institute”
Akademika Kurchatova pl., 1, Moscow (Russia)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201607189.
Dr. J. Long, Prof. J. Larionova
Institut Charles Gerhardt de Montpellier (ICGM), UMR5253, Equipe
IMNO, Universitꢀ de Montpellier
Site Triolet, Place Eugꢁne Bataillon, 34095 Montpellier cedex 5
(France)
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[(Ph5Ge5O10)3Fe6(OH)3(O)Na2(phen)]· (EtOH)4· (H2O)0.5
(2), [(Ph5Ge5O10)6Fe6(OH)3OFe6(OH)2(O)2Na3(bipy)]·
(EtOH)14.5· (C5H5N)0.5· (H2O) (3), and [(Ph5Ge5O10)3Fe6-
(OH)3(O)Na2(neocuproine)]· (EtOH)2· (H2O)1.5 (4;
Scheme S1 in the Supporting Information) in higher yields
(up to 26%). Single-crystal X-ray diffraction analyses of 1–4
revealed similar structural organization with the cage-like
molecular architecture. The complexes are formed by a cen-
tral hexairon(III) oxo {Fe6O19} core, reminiscent of the
“Lindqvist polyanion”, coordinated by three pentameric
germoxanolate [PhGe(O)OÀ]5 ligands, forming an unprece-
dented three-pointed star geometry.
Pentameric germoxanolate fragment of 1 adopts a crown
configuration (Figure S1), with the main structural parame-
À
À À
ters: Ge O(Ge) 1.740–1.791 ꢀ, ]Ge O Ge 117.6–129.08.
Note that the only example of cyclic germanoxane fragments
in metallagermsesquioxane structures describe hexameric
units.[4a] The structural organization of the related compounds
2–4 is similar to 1. The difference consists in additional
coordination of the respective bidentate ligands (1,10-phe-
nanthroline, 2,2’-bipyridine, neocuproine) to the Na ions
(Figure S2). Noteworthy is that sodium ions are located in
external positions to the cage and, thus, are responsible for
the supramolecular packing (Figure S3). According to the
higher yields of 2–4 in comparison to 1, bidentate ligands
might give additional stabilization of the metallagermses-
quioxane cage and facilitate its crystallization. These results
show that germsesquioxanes indeed provide great potential
for creation of cage metalladerivatives, even when using
simple alkoxygermane as an initial reagent. Drawing an
analogy between Fe-based sesquioxanes of germanium and
silicon, we would like to emphasize that known types of cage
ironsilsesquioxanes (Refs. [2a,c,j]) have quite different struc-
tural features in comparison to germanium-based compounds
1–4. This suggests a unique role of germanium centers in cage-
compound formation. Results of EXAFS study on iron and
germanium K-edges for 1–4 confirmed that the oxidation
states of iron and germanium are + 3 and + 4, respectively
(Figures S4–S5, Tables S3–S4).
In the representative crystal structure of 1, the octahedral
iron ions are located in vertexes of an octahedron (Figure 1).
They are connected through the central and peripheral
À
oxygen atoms. Fe O(Fe) distances are in the range of 1.99–
À
À
2.05 ꢀ, those of Fe O(Ge) and Fe Fe are in the range of
1.87–1.90 ꢀ and 3.16–3.27 ꢀ, respectively. In turn, the values
À À
of the Fe O Fe angle depend on the position of the oxygen
atom. It is close to 908 when the oxygen occupies the central
position and significantly exceeds 908 in the other cases
À
À
À À
(]Fe O1 Fe: 88.1–91.78; ]Fe O Fe: 101.9–108.28). The
The magnetic properties of these compounds, investigated
by using a SQUID magnetometer working in the temperature
range 1.8–300 K up to 7 T, indicate that they are paramagnetic
behavior in this temperature range. The temperature depend-
ence of the magnetic susceptibility for 1 is given as an example
(Figure S7, ESI). The room temperature value of cT, equal to
14.29 cm3 KmolÀ1, is considerably lower than the theoretical
one of 26.25 cm3 KmolÀ1, expected for six non-interacting
FeIII ions (S = 5/2 and g = 2.00). Such a discrepancy most
likely originates from the occurrence of strong antiferromag-
netic interactions between the spin carriers that are still
operative at room temperature. This is indeed confirmed by
the temperature dependence of cT, which reveals a progres-
sive decrease upon cooling to reach 3.80 cm3 KmolÀ1 at 1.8 K.
Antiferromagnetic interactions between the FeIII ions can be
mediated through both the central and bridging oxygen
atoms, as found in similar hexanuclear architectures.[5] For
temperatures lower than 10 K, a steep decrease of cT occurs
and can be assigned to the zero-field splitting and/or
intermolecular interactions between the polynuclear com-
plexes. In addition, the non-null value of cT at low temper-
ature indicates the presence of a non-diamagnetic ground
state that should be expected for a pure S = 0 species. This
could reflect either the presence of thermally populated
excited states or antagonist magnetic interactions mediated
by the different bridging oxygen atoms. The field dependence
of the magnetization at 1.8 K for 1 further confirms the non-
diamagnetic state with a magnetization value of 2.72 mB
without any evidence of the saturation indicating the presence
of a magnetic anisotropy (Figure S8). Alternating current
(AC) measurements performed as the temperature depend-
ence of the in-phase (c’) and out-of-phase (c’’) susceptibilities
Figure 1. Top: Molecular structure of 1. Bottom: The structure of
Fe6O19 core of 1. Full color version is given in ESI (Figures S1,S2).
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measured in a zero DC magnetic field show an increase upon
cooling but without the presence of a maximum, precluding
an in-depth characterization of the relaxation dynamics
(Figure S9).
We also present herein the first applications of cage
metallagermsesquioxanes in catalysis. As complexes 1–4 have
similar molecular architectures, we concentrated on the study
of one of them as an example, namely, compound 2. It is well
established that iron-based compounds[6] are capable of
catalyzing the oxidation of hydrocarbons with peroxides. We
have found that cyclohexane and other alkanes as well as
benzene can be oxidized to the alkyl hydroperoxides and
phenol, respectively, by hydrogen peroxide in the presence of
catalytic amounts of 2 and CF3COOH (some examples are
shown in Figure 2, the experimental details are described in
Figure 3. Dependence of the initial rate of oxygenate (the sum cyclo-
hexanol + cyclohexanone) formation W0 on the initial concentration of
2 in the oxidation of cyclohexane (0.46m, curve 1) and benzene
(0.45 M, curve 2) with hydrogen peroxide (50% aqueous) in the
presence of CF3COOH (0.05m) in MeCN at 408C. Concentrations of
cyclohexanone and cyclohexanol were determined after reduction of
the aliquots with solid PPh3.
ure S11) indicating a competitive interaction of the oxidizing
species with the alkane and acetonitrile.[8] The rate depend-
ence on the initial concentration of hydrogen peroxide is
linear at [H2O2]0 < 1.5 m (Figure S12).
Complex 2 was also evaluated in the formation of amides
directly from alcohols and amines, in the presence of tert-butyl
hydroperoxide as the terminal oxidant. For this purpose, we
chose the newly developed metal-catalyzed microwave-acti-
vated amidation[9,2c] between benzylic alcohol and (Æ)-a-
methylbenzylammonium chloride (Scheme 1). The use of
Figure 2. Accumulation of oxygenates (cyclohexanol, curves 1, and
cyclohexanone, curves 2) in the oxidation of cyclohexane (0.46m) with
H2O2 (50%, aqueous, 1.5m) catalyzed by 2 (5ꢂ10À4 m) in the presence
of CF3COOH (0.05m). Solvent was acetonitrile (total volume of the
reaction solution was 5 mL) at 408C. Concentrations of cyclohexanone
and cyclohexanol were determined before (curves a) and after (cur-
ves b) reduction of the aliquots with solid PPh3 (for this method, see
Ref. [7]).
the Supporting Information). The oxidation with H2O2
catalyzed by complex 2 was very efficient as it gave oxygen-
ates from alkanes in a very high yield of 44%. This yield is one
of the highest values obtained so far in metal-catalyzed
oxidations of cyclohexane with H2O2 since usually yields are
not higher than 5–30%. For example, cyclohexane oxida-
tion[6c] with H2O2 catalyzed by a hexanuclear iron(III)
carboxylate gave maximum yield of oxygenates of 29%.
The S-shape mode of the initial rate W0 dependence on
the concentration of 2 attests that the reaction order with
respect to 2 is not constant and gradually changes from an
apparent second order to first order, when [2]0 changes in the
interval from 1.5 ꢁ 10À4 to 3 ꢁ 10À4 m (Figure 3).
On the basis of these data, a mechanism can be proposed
which includes a stage of the starting complex monomer-
ization.[2c] Further dimerization of these particles affords
species which are responsible for the catalytic activity of the
system.
Dependence of the initial oxidation rate on the concen-
tration of the CF3COOH co-catalyst is given in Figure S10.
The dependence of W0 on the initial concentration of
cyclohexane approaches a plateau at [CyH]0 > 0.4m (Fig-
Scheme 1. Microwave activation by 2 or FeCl2·4H2O.
complex 2 allowed amide 5 to be obtained in a yield of 71%.
This result was compared to that obtained with a more simple
iron salt, FeCl2·4H2O, which gave the product in a yield of
only 51%. This result clearly shows that the germsesquioxane
structure exerts a stabilizing effect around the metallic center,
resulting in an improved catalytic activity.
In conclusion, a family of unprecedented cage-like Fe,Na-
germsesquioxanes having a central hexairon(III) oxo {Fe6O19}
core coordinated by three pentameric germoxanolate ligands
has been obtained. Their structural arrangement induces
antiferromagnetic interactions between FeIII ions through the
oxygen bridges resulting in a paramagnetic behavior. This
class of compounds was evaluated for the first time for its
catalytic activity. Complex 2, displayed a high efficiency in
catalysis for alkane oxidation and amide formation from
benzylic alcohol and amine. A further investigation on the
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near future.
Acknowledgements
This work was supported by RFBR (projects 16-03-00609, 16-
03-00206, 16-03-00254, 16-29-05180, 16-53-150008). French
Embassy in Moscow, Balard Foundation, University of
Montpellier, CNRS (Programme de Recherche Conjoint),
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Balard), and “Science without Borders Program, Brazil–
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Received: July 25, 2016
Revised: September 28, 2016
Published online: && &&, &&&&
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Communications
Homogeneous Catalysis
GeOmetry: Unprecedented Fe,Na-germ-
sesquioxanes were synthesized. Their
unique molecular topology includes
a {Fe6O19} core surrounded by cyclic
germoxanolates [PhGe(O)O]5. This
structural organization induces antiferro-
magnetic interactions between FeIII ions
and high catalytic activity for alkane
oxidation and amide formation from
benzylic alcohol and amines.
A. N. Bilyachenko,* M. M. Levitsky,
A. I. Yalymov, A. A. Korlyukov,
V. N. Khrustalev, A. V. Vologzhanina,
L. S. Shul’pina, N. S. Ikonnikov,
A. E. Trigub, P. V. Dorovatovskii,
X. Bantreil, F. Lamaty,* J. Long,
J. Larionova, I. E. Golub, E. S. Shubina,
G. B. Shul’pin*
&&&& — &&&&
Cage-like Fe,Na-Germsesquioxanes:
Structure, Magnetism, and Catalytic
Activity
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www.angewandte.org
5
These are not the final page numbers!