Hexacoppergermsesquioxanes as complexes with N-ligands: Synthesis, structure and catalytic properties

Article abstract of DOI:10.1016/j.jorganchem.2019.01.004

A simple and versatile strategy of synthesis of cage metallagermaniumsesquioxanes is demonstrated for isolation of Cu₆-based phenylgermsesquioxanes 1–6. Structure of newly synthesized coppergermsesquioxanes was established by single-crystal X-ray diffraction analysis. General principle of these cages’ topology implies the presence of two linear Cu₃ fragments, coordinated by three pairs of ligands. These are: (i) cyclic germsesquioxanes [PhGeO_1,5]₅, products 1–6, (ii) 1,10-phenanthrolines, (1, 3–5) or 2,2’-bipyridines, (2, 6), (or and (iii) OX species (X = H, 1–2, CH₃, 3, H and C₂H₅O, 4, HCO, 5, CH₃CO, 6). Appearance of non-expected species (formiate for 5, acetate for 6), resulted from corresponding alcohols used as reaction media, points at easyness of oxidation processes in the conditions of such self-assembling reactions. Catalytic tests showed high activity of complexes 1 and 2 as precatalysts in homogeneous oxidations of alkanes (cyclohexane, methylcyclohexane, n-heptane, cis-1,2-dimethylcyclohexane with hydrogen peroxide in acetonitrile solution. Hydroxyl radicals take part in the reaction. The same complexes catalyze oxidation of alcohols (cyclohexanol, 2-heptanol, 1-phenylethanol) to corresponding ketones with tert-butyl hydroperoxide in almost 100% yield. With the addition of various alkyl ammonium salts, complex 2 could also furnish corresponding amides in high yield.

Full text of DOI:10.1016/j.jorganchem.2019.01.004

Accepted Manuscript
Hexacoppergermsesquioxanes as complexes with N-ligands: Synthesis, structure and
catalytic properties
Alena N. Kulakova, Alexander A. Korlyukov, Yan V. Zubavichus, Victor N. Khrustalev,
Xavier Bantreil, Lidia S. Shul'pina, Mikhail M. Levitsky, Nikolay S. Ikonnikov, Elena S.
Shubina, Frédéric Lamaty, Alexey N. Bilyachenko, Georgiy B. Shul'pin
PII:
S0022-328X(19)30012-9
DOI:
https://doi.org/10.1016/j.jorganchem.2019.01.004
JOM 20682
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 9 November 2018
Revised Date: 7 January 2019
Accepted Date: 8 January 2019
Please cite this article as: A.N. Kulakova, A.A. Korlyukov, Y.V. Zubavichus, V.N. Khrustalev, X.
Bantreil, L.S. Shul'pina, M.M. Levitsky, N.S. Ikonnikov, E.S. Shubina, Fréé. Lamaty, A.N. Bilyachenko,
G.B. Shul'pin, Hexacoppergermsesquioxanes as complexes with N-ligands: Synthesis, structure
and catalytic properties, Journal of Organometallic Chemistry (2019), doi: https://doi.org/10.1016/
j.jorganchem.2019.01.004.
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ACCEPTED MANUSCRIPT
REVISION No 3
Manuscript 18-00326 for Journal of Organometallic Chemistry, Special Issue “Synthesis and Applications of
Organometallic Compounds" dedicated to Professor Armando Pombeiro
Journal of Organometallic Chemistry XXX (2019) xx-xx
Research Article
Hexacoppergermsesquioxanes as complexes with N-ligands:
synthesis, structure and catalytic properties
Alena N. Kulakova,a,b Alexander A. Korlyukov,a,c Yan V. Zubavichus,d,e Victor N. Khrustalev,b,d
Xavier Bantreil,f Lidia S. Shul’pina,a Mikhail M. Levitsky,a Nikolay S. Ikonnikov,a Elena S. Shubina,a
Frédéric Lamaty,*f Alexey N. Bilyachenko,*a,b Georgiy B. Shul’pin *b,g,h
a.
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul.
orcid: org/0000-0002-1448-9228 (Shul’pina), orcid:
Vavilova, dom 28, Moscow 119991, Russia
org/
0000-0002-5825-6581 (Ikonnikov), orcid.org/0000-0001-8057-3703 (Shubina)
orcid.org/0000-0003-3136-3675 (Bilyachenko)
Peoples’ Friendship University of Russia (RUDN University), ul. Miklukho-Maklaya, dom
6, Moscow 117198, Russia orcid.org/0000-0001-8806-2975 (Khrustalev)
Pirogov Russian National Research Medical University, ul. Ostrovitianova, dom 1, Moscow
117997, Russia
b.
c.
d.
National Research Center “Kurchatov Institute”, pl. Akad. Kurchatova, dom 1, Moscow
Boreskov Institute of Catalysis SB RAS, prosp. Akad. Lavrentieva, dom 5, Novosibirsk
Institut des Biomolécules Max Mousseron, UMR 5247, CNRS, Université de Montpellier,
123182, Russia
e.
630090, Russia
f.
ENSCM, Site Triolet, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France orcid.org/0000-
0002-2676-6851 (Bantreil), orcid.org/0000-0003-2213-9276 (Lamaty)
g.
Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina, dom
orcid: org/0000-0002-7053-4812 (Shul’pin)
4, Moscow 119991, Russia
h.
Plekhanov Russian University of Economics, Stremyannyi pereulok, dom 36, Moscow
117997, Russia
_____________________________________________________
ARTICLE INFO
We dedicate this paper to our dear friend and colleague Prof. Armando José Latourrette de Oliveira Pombeiro.
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_____________________________________________________
Keywords:
Alkanes
Catalysis
Complexes of copper
Functionalization of CH bonds
Metallasilsesquioxanes
Oxidation with hydrogen peroxide
Hydroxyl radicals
* Corresponding authors. Tel.: +7 495 7118443; fax: +7495 6512191 (GBS).
E-mails: frederic.lamaty@umontpellier.fr (for FL), bilyachenko@ineos.ac.ru (for ANB),
shulpin@chph.ras.ru (for GBS)
Electronic Supplementary Information (ESI) available: details of X-ray diffraction study. See
DOI: 10.1039/x0xx00000x
Highlights
- New hexanuclear coppersilsesquioxanes have been synthesized
- The structures of complexes in crystals have been determined
- The complexes are good catalysts in oxidation of alkanes and alcohols with H2O2 or tert-butyl
hydroperoxide
- Hydroxyl radicals take part in the reaction
________________________________________________________
ABSTRACT
A simple and versatile strategy of synthesis of cage metallagermaniumsesquioxanes is demonstrated for
isolation of Cu6-based phenylgermsesquioxanes 1-6. Structure of newly synthesized
coppergermsesquioxanes was established by single-crystal X-ray diffraction analysis. General principle
of these cages’ topology implies the presence of two linear Cu3 fragments, coordinated by three pairs of
ligands. These are: (i) cyclic germsesquioxanes [PhGeO1,5]5, products 1-6, (ii) 1,10-phenanthrolines, (1,
3-5) or 2,2’-bipyridines, (2, 6), (or and (iii) OX species (X = H, 1-2, CH3, 3, H and C2H5O, 4, HCO, 5,
CH3CO, 6). Appearance of non-expected species (formiate for 5, acetate for 6), resulted from
corresponding alcohols used as reaction media, points at easyness of oxidation processes in the
conditions of such self-assembling reactions. Catalytic tests showed high activity of complexes 1 and 2
as precatalysts in homogeneous oxidations of alkanes (cyclohexane, methylcyclohexane, n-heptane, cis-
1,2-dimethylcyclohexane with hydrogen peroxide in acetonitrile solution. Hydroxyl radicals take part in
the reaction. The same complexes catalyze oxidation of alcohols (cyclohexanol, 2-heptanol, 1-
phenylethanol) to corresponding ketones with tert-butyl hydroperoxide in almost 100% yield. With the
addition of various alkyl ammonium salts, complex 2 could also furnish corresponding amides in high
yield.
_____________________________________________________________________________
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Graphical Abstract:
1. Introduction
Multinuclear complexes are among the most efficient catalysts for chemical processes of
high demand [1-6]. Multinuclearity’s main advantages are well known: ( ) opportunity of the
i
realization of multi-electron processes [7-9]; (ii) selectivity [10-12]; (iii) higher activity than for
mononuclear catalysts [13-16]; (iv) cooperative interactions [17-19], especially prominent in
case of involvement of metal ions with different Lewis acidity [20-22]. Significant benefits in
the field are brought by the use of cage-like compounds, combining metal centres and functional
ligands in the frameworks of multiple topologies. Regarding the latter, it is worth to emphasize
the specific organisation of cage’ components, giving synergistic effects for impressive catalytic
applications [23-31]. Among other types of metallacages, significant attention has been paid to
those constructed from branched organoelement, sesquioxane, blocks (RXO1.5).
Metallasilsesquioxane (X = Si) compounds, firstly discovered in 1986 [32-33], are thoroughly
described in several book chapters and reviews [34-44], including numerous examples of their
catalytic activity [41, 45-49]. In turn, metallasilsesquioxanes’ close analogues,
metallagermsesqioxanes (X = Ge), still remain in the shadow. Hydrothermal method was
reported for isolation of MOF-like objects starting from germanium sesquioxide [50-59]. Some
of these products were isolated in the presence of additional ligands, namely 2-picolinic acid
[50, 56-57], 2-pyrazinecarboxylic acid [56-57], 2,6-pyridinedicarboxylic acid [57] or 1,10-
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phenanthroline [58-59]. These unusual products demonstrated luminescent [50, 51, 56-57, 59]
sorption (of Ar gas [50]) properties and high thermal stability [50-59].
In turn, an alternative approach to individual cagelike metallagermsesquioxanes was
developed by some of us recently [60-62]. This method is based on use of reactive germane
PhGe(OMe)3 species and represents several important features such as mild conditions of
synthesis and opportunity of isolation of bimetallic (Fe/Na [60], Fe/Cu [61], Ni/Na [62])
products. First representatives of this new class of metallagermsesquioxanes exhibit unusual
(spin glass) magnetic behaviour [62] and possess high catalytic activity [63].
Taking in mind positive results, presented recently by some of us for catalytic properties
of Cu-phenylsilsesquioxanes bearing additional N-ligands, namely 2,2’-bipyridine and 1,10-
phenanthroline [64], we were interested in synthesis of Cu-based compounds including at the
same time two types of metal’ surrounding, (i) phenylgermsesquioxane and (ii) ligand of 2,2’-
bipy family. Known prominent metallic character of germanium, according to our expectations,
could provide higher activity of such compounds in comparison to silicon-based predecessors.
For the catalysed reactions we chose two processes of highest interest, namely: (
i) amidation and
(
ii) oxidation of C-H compounds. The first reaction is known as a key reaction for contemporary
green chemistry, requiring solutions for minimization of waste production [65]. Several high
efficient catalytic systems (on base of Ru [66-68] or Rh [69]) were suggested for the amides
formation from alcohols and amines [66-67] or via reductive amidation of aldehydes without an
external hydrogen source [68-69]. Recently, an alternative approach, an oxidative amidation of
alcohols, was developed. This method implies an use of cheap metal-based complexes (Cu [70-
72], Fe [73-76], Zn [77]). The catalyst loading in these reactions, though, could not be decreased
below 1%. In turn, as some of us showed previously [78], an application of copper
silsesquioxanes allow to work effectively even at 100 ppm of Cu loading.
Regarding the second process of interest, C-H compounds functionalization, the role of
metallacomplexes for its catalysis could not be overestimated [79-82]. Along with numerous
examples of expansive metal-based (e.g. Au [83], Ir [84], Os [85-86], Pd [87-88], Rh [89-90])
catalysts used for such transformations, several successful efforts were reported for an
application of the less expansive metals’ complexes, e.g. Co [91], Cu [92], Fe [93], Ni [94].
Important to emphasize that significant amount of publications witness higher catalytic activity
of multinuclear complexes in comparison with mononuclear ones [6, 95-96].
In the present study, we have developed the self-assembly approach to Cu(II)-cage
complexes, bearing two types of ligands, namely, O-based anionic phenylgermsesquioxanes
[PhGe(O)O-]5 and N-based bidentate 2,2’-bipyridines (or 1,10-phenanthrolines):
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[(PhGeO1.5)10(CuO)6(HO0.5)2(C12H8N2)2][H2O]2 (
[(PhGeO1.5)10(CuO)6(HO0.5)2(C10H8N2)2][EtOH]5 (
[(PhGeO1.5)10(CuO)6(CH3O0.5)2(C12H8N2)2][MeOH]2 (
[(PhGeO1.5)10(CuO)6(HO0.5)1.6)(C2H5O0.5)0.4(C12H8N2)2] [(DMF)0.75]2 [(H2O)0.2]2 [EtOH]2 (
[(PhGeO1.5)10(CuO)6(HCOO0.5)2(C12H8N2)2][MeOH] ( ),
[(PhGeO1.5)10(CuO)6(CH3COO0.5)2(C10H8N2)2][MeOH]0.5
An application of the isolated Cu6-phenylgermsesquioxanes
1
),
2),
3
),
4
),
5
(6).
1
and
2
in homogeneous
catalysis of oxidative amidation and C-H functionalization turned to be very prospective both in
terms of catalyst’ activity and minimization of loading.
2. Results and discussion
Synthesis and structure of Cu6-phenylgermsesquioxanes containing N-ligands
For the method of synthesis of target mixed-ligand Cu-complexes, we chose an
interaction of CuCl2 at room temperature with a product of alkaline hydrolysis of PhGe(OMe)3
in methanol/ethanol solution, followed by complexation with corresponding bidentate N-ligand
(2,2’-bipyridine or 1,10-phenanthroline). This approach allowed us to get a family of complexes
of the same, Cu6, nuclearity and composition of Ge-based ligands, [PhGe(O)O-]5
Fig. 1). The general type of structure remained the same in conditions of crystallization of
product from DMF media (complex ) instead of typical (alcohol-containing) system used for
isolation of and
1-6 (Scheme 1,
4
1-
3
5-6.
5

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Scheme 1. General scheme of synthesis of hexacoppergermsesquioxanes
additional N-ligands (1,10-phenanthroline or 2,2′-bipyridine)
1−6 containing
6

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Figure 1. Molecular structure, formula and yields of compounds 1‒6.
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Regarding the composition of products 1-6, it is clear that observed feature (six Cu2+ cations vs.
two Ge-ligands, bearing 10 negative charges) is not adequate from the positions of formal charge
balance. Indeed, two additional negative charge’ carriers are presented in each product structure. These
additional anionic groups are as follows: HO- (compounds 1-2), CH3O- (3), HCOO- (5), CH3COO- (6).
In case of compound 4 we observed superposition of several (HO- and CH3CH2O-) groups (1.6/0.4).
Fig. 2. Presumable scheme of oxidation of alcohols (on the example of ethanol) to the carboxylic acid in
the presence of Cu centers of germsesquioxane.
The presence of hydroxyl fragments as well as methoxy or ethoxy groups could be easily
explained by the water/alcohol presence in the reaction media. In turn, an appearance of carboxylic units
is much more unusual. We suggest that formiate (in 5) and acetate (in 6) groups were formed as
products of oxidation of corresponding, MeOH or EtOH, alcohols (Figure 2). A possible mechanism is
presented in Scheme 2. That is quite possible due to the presence of air during synthesis/crystallization
of 5-6 (complexes 1-4 were obtained under inert atmosphere).
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Scheme 2. A proposed mechanism for the oxidation of ethanol to afford acetate ligand.
This “oxidative” effect has never been observed for metallagermsesquioxanes, while
some reports concerning oxidation properties of metallasilsesquioxanes are available. These
works reported, e.g. CeIII (to CeIV) [97], UIV (to UVI) [98], CrII (to CrIV) [99] and CuI (to CuII)
[100] oxidations observed in the course of synthesis of corresponding silsesquioxanes.
Intriguing mild-conditioned, “ferment-like”, oxidation of methyl group of neocuproine (into diol
fragment) was reported for the self-assembly reaction of Cu-silsesquioxane [101]. In close
parallel to presenting work, several examples of solvent’ oxidation were reported as well:
tetrahydrofuran to γ-butyrolactone (in the course of Fe-silsesquioxane synthesis) [102], ethanol
to acetate (in the course of Cu-silsesquioxane synthesis) [64, 103-104].
In the context of other features of 1-6‘ molecular structure, some points are to be emphasized.
First of all, differences in the structure of 1-6 provoked some deviations in copper ions locations (Fig.
3), e.g. the longest Cu…Cu distance varies from 8.01 Å (in 2) to 8.20 Å (in 6), while Cu…Cu…Cu
angle in Cu-trimers varies from 173.6 deg (in 6) to 175.7 deg (in 2).
Secondly, complexes 1-6 include germoxanolate fragments of the same composition
̶
[PhGe(O)O]5. These fragments consist from two types of oxygen atoms: (i) ones belonging to cyclic -
Ge-O-Ge- “germoxane” fragments, (ii) ones belonging to -Ge-O-Cu- “germanolate” fragments (Fig. 4).
Difference in geometries of products could be also concluded from the deviation of longest Ge…O
9

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distance in cyclic germoxane fragments (Fig. 3), varying from 5.471 Å (in 2) to 5.650 Å (in 3). It was of
interest to compare an influence of additional structural factors on the cage geometry.
Fig. 3. Location of Cu centers in 1-6.
This could be done through discussion of the connectivity of two neighbouring copper centers
(Figs. 5-7). It was found that distance between peripheral copper ion (coordinated by N-ligand) and near
copper ion inside the cage is quite sensitive to nature of oxygen atom bridging them. These oxygen
atoms, in turn, are fragments of “charge balancing” groups OX, mentioned above [CuOX, Х = H, Me,
Et, CR(O), R= H, Me]. Symptomatically that in case of OH fragment (in 4), possessing highly basic
oxygen atom, a maximum shortening of Cu-Cu distance occurred (Fig. 5). Fragment ОMе fragment (in
3) provokes the lengthening of this distance (Fig. 6). Finally, Cu-Cu contact reached its maximum value
in case of the ОСR(O) fragment’ presence (in 6) (Fig. 7)
Fig. 4. Structure of Ge-ligands in 1-6.
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Fig. 5. Structural unit in structure of 4, describing distortion of molecular architecture.
Fig. 6. Structural unit in structure of 3, describing distortion of molecular architecture.
Fig. 7. Structural unit in structure of 6, describing distortion of molecular architecture.
Crystallographic data for 1‒6 have been deposited with the Cambridge Crystallographic Data
Center, CCDC 1888654-1888659. Copies of this information may be obtained free of charge from the
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: þ44 1223 336033; e-mail:
deposit@ccdc.cam.ac.uk or www.ccdc.cam.ac.uk).
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We used catalysts 1 and 2 for the oxidation of alkanes and alcohols under mild conditions.
Hydrogen peroxide and tert-butyl hydroperoxide were employed as oxidants acetonitrile was a solvent.
Hydroperoxidation of alkanes with H2O2
We have found that cyclohexane can be converted into cyclohexyl hydroperoxide in the presence
of catalyst 2 which is gradually transformed in a mixture of cyclohexanol and cyclohexanone in the
course of the oxidation (Fig. 8).
Fig. 8. Accumulation of cyclohexanol and cyclohexanone with time in the oxidation of cyclohexane
(initial concentration was 0.46 M) with H2O2 (50% aqueous; initial concentration 2 M) catalyzed by
compound 2 (5 × 10‒4 M), in the presence of HNO3 (0.05M, Graph A) and in the absence HNO3 (Graph
B). Solvent was acetonitrile (total volume of the reaction solution was 5 mL); 50 °C. All samples were
reduced by PPh3 before GC.
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The oxidation of cis-1,2-dimethylcyclohexane with H2O2 occurs without retention of
configuration (the trans/cis ratio was 0.8 where the trans/cis is the ratio of isomers of tert-alcohols with
mutual trans- and cis-orientation of the methyl groups). Regioselectivity parameters in the oxidation of
n-heptane catalyzed by complex 2 are given in Table 1. Concentrations of products (isomeric alcohols)
were measured after reduction with PPh3. The parameter C(1):C(2):C(3):C(4) is given after normalization
which takes into account the number of hydrogen atoms at each carbon. The parameter of bond-
selectivity in oxidation of methylcyclohexane 1º:2º:3º = 1.0:5.4:16.0. All these parameters indicate that
the oxidation with H2O2 occurs with the participation of free hydroxyl radicals [105-107]. It is important
to note that catalyzed by 2 oxidation of cis-1,2-dimethylcyclohexane with meta-CPBA occurs
stereoselectively (the trans/cis ratio was 0.3; (we found previously analogous effect; see, for example,
ref. [108]).
Table 1
Oxidation of n-heptane (0.5 M) with H2O2 (50% aqueous, 2 M) catalyzed by complex 2 (5 × 10‒4 M) in
the presence of HNO3 (0.05 M) at 50 °C.
Entry Time (min)
C(1):C(2):C(3):C(4) <Total yield of oxygenates, %>
1.0:6.25:6.25:5.60 <12>
1
2
3
60
120
180
1.0:3.6:3.6:4.3 <12>
1.0:6.4:6.4:6.4 <12>
Complex 2 catalyses also the oxidation with H2O2 of benzene to phenol (Fig. 9) and with TBHP
of cyclohexanol to cyclohexanone (Fig. 10). Oxidation of 1-phenylethanol catalysed by 2 gave
acetophenone in 98% yield after 9 h at 50 ºC. Dependence of initial oxidation rate on initial
concentration of cyclohexane is shown in Fig. 11.
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OH
0.16
0.12
O
O
0.08
0.04
0
4
12
16
0
8
Time (h)
Fig. 9. Oxidation of benzene (0.45 M) to phenol and p-quinone with H2O2 (2 M; 50% aqueous)
catalyzed by 2 (5 × 10–4 M) in the presence of HNO3 (0.05M) in MeCN at 50 ºC. All samples were
reduced by PPh3 before GC.
Fig. 10. Oxidation of cyclohexanol (0.23 M) to cyclohexanone with TBHP (1.2 M; 70% aqueous)
catalyzed by 2 (5 × 10–4 M) in the presence of HNO3 (0.05M) in MeCN at 50 ºC. All samples were
reduced by PPh3 before GC.
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Fig. 11. Dependence of the initial rate W0 of oxygenate accumulation in the cyclohexane oxidation with
H2O2 catalyzed by 2 in MeCN on the initial concentration of cyclohexane. Conditions: H2O2 (50%
aqueous; initial concentration 2 M), compound 2 (5 × 10‒4 M), in the presence of HNO3 (0.05M).
Solvent was acetonitrile (total volume of the reaction solution was 5 mL); 50 °C. All samples were
reduced by PPh3 before GC.
Complex 1 turned also out to be an efficient catalyst. The data given in Fig. 12 indicate that the
oxidation of cyclohexane with H2O2 proceeds with the formation of cyclohexyl hydroperoxide which in
the course of the reaction decomposes in part to afford cyclohexanol and cycohexanone [109-118]. In
the presence of catalytic amounts of complex 1, tert-butyl hydroperoxide oxidizes 1-phenylethanol and
2-heptanol into acetophenone and 2-heptanone, respectively (Fig. 13).
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Fig. 12. Accumulation of cyclohexanol and cyclohexanone with time in the oxidation of cyclohexane
(initial concentration was 0.23 M) with H2O2 (50% aqueous; initial concentration 1.5 M) catalyzed by
compound 1 (5 × 10‒4 M), samples were either reduced by PPh3 before GC (Graph A) or were not treated
with PPh3 before GC (Graph B) (for this method see refs. [109-118]). Solvent was acetonitrile (total
volume of the reaction solution was 5 mL); 50 °C. Total yield of cyclohexanol and cyclohexanone after
120 min and reduction with PPh3 was 29%.
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Fig. 13. Oxidation of 1-phenylethanol (0.16 M) and 2-heptanol (0.23 M) to acetophenone and
heptanone-2, respectively, with TBHP (1.2 M; 70% aqueous) catalyzed by 1 (5 × 10–4 M) in the
presence of CF3COOH (0.05M) in MeCN at 50 ºC. All samples were reduced by PPh3 before GC.
Having good results in hand for oxidation of alkanes and alcohols using peroxides,
complex
2 was evaluated in the formation of amides starting from alcohols and amines. Indeed,
this reaction was already described using copper [119], iron[120-123] and zinc[124] salts, but
also cage-like copper and iron compounds[60, 61, 64]. As already demonstrated in literature, the
formation of amides involves an oxidation of benzylic alcohol into corresponding aldehyde,
reaction with the amine to form a hemiaminal, which is further oxidized into amide. Of note, the
amines have to be protected as their hydrochloride salt, and deprotonated in situ, in order to
avoid side reaction with TBHP. In a view for comparison with our previous reports on oxidative
amidation using cage compounds, reaction conditions optimized previously were directly
adapted to the use of compound 2. Benzyl alcohol was reacted with the appropriate ammonium
chloride salts in the presence of TBHP as oxidant and calcium carbonate as a weak base
(Scheme 3). Interestingly, reactions could be performed using as low as 0.2 mol% of copper in
the reaction mixture, thanks to the good solubility of
yields of desired amides. Secondary amides, starting from the hydrochloride salts of
cyclohexyl, and ( )-α-methylbenzylamine, were isolated in 79 – 88 % yield. Formation of
tertiary amides was slightly less efficient and compounds featuring morpholine and
2
in acetonitrile, and still provide high
n-butyl,
±
dibenzylamine could be obtained in 72 and 79% yield, respectively. Turnover number (TON)
and frequency (TOF) were calculated to evaluate the efficiency of the catalyst in this reaction.
Even though TOF values were not as high as in previous reports, TON values were interesting in
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comparison with our seminal report on CuO-catalyzed. Indeed, in the presence of CuO, TON
values reach a maximum at 44 [119]. Thanks to the use of complex 2, catalyst loading could be
reduced to 0.2 mol% of Cu and TON values up to 220 could be obtained.
Scheme 3. Oxidative amidation of alcohols and amines using 2 as catalyst
3. Experimental Section
Materials and Methods
All the chemicals were of analytical grade and used as received. PhGe(OMe)3 was purchased from
ABCR. XRF analysis was performed using a spectrometer VRA-30. IR spectra were recorded on
Shimadzu IR Prestige21 FTIR spectrometer in KBr pellets (νGe-Ph 1100 cm-1;νas Ge-O 840-820 cm-1;
νs Ge-O 950 cm-1; σC–H of mono-substituted phenyl groups 680-670 cm-1 , δ Ge-O 459 cm-1).
Syntheses of 1-4 were performed under nitrogen atmosphere.
Syntheses of Cu6-phenylgermsesquioxanes containing N-ligands
[(PhGeO1.5)10(CuO)6(HO0.5)2(C12H8N2)2][H2O]2 (1)
PhGe(OMe)3 (1.20 g; 4.9 mmol) in 60 ml of an ethanol/methanol (1:1) solution was
mixed with solid NaOH (0.24 g; 5.9 mmol). Mixture was heated under reflux for 2 h.
Afterwards, the solution was cooled to room temperature and 0.40 g (3.0 mmol) of anhydrous
CuCl2 was added at once. The solution was left under stirring overnight and 0.18 g (0.9 mmol)
of 1,10-phenanthroline were added. Resulted mixture was left under stirring for 4 h and then
18

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filtered from the insoluble part. The filtrate was stored in a warm place in a flask equipped by
septum with two needles to admit a slow current of nitrogen and evaporation of the solvents.
After approximately 7 days the formation of crystalline material
1 was occured; a few selected
single crystals were used for the X-ray diffraction analysis (see ESI for details). Anal. Calcd for
[(PhGeO1,5)10(CuO)6(HO0,5)2(C12H8N2)2] Cu, 14.70; N, 2.16; Ge, 28.01. Found (for vacuum-
dried sample): Cu, 14.61; N, 2.13; Ge, 27.95%. Yield 0.42 g (21%).
[(PhGeO1.5)10(CuO)6(HO0.5)2(C10H8N2)2][EtOH]5 (2)
PhGe(OMe)3 (1.20 g; 4.9 mmol) in 60 ml of an ethanol/methanol (1:1) solution was
mixed with solid Na (0.14 g; 5.9 mmol). Mixture was heated under reflux for 2 h. Afterwards,
the solution was cooled to room temperature and 0.40 g (3.0 mmol) of anhydrous CuCl2 was
added at once. The solution was left under stirring overnight and 0.15 g (0.9 mmol) of 2,2’-
bipyridine were added. Resulted mixture was left under stirring for 4 h and then filtered from the
insoluble part. The filtrate was stored in a warm place in a flask equipped by septum with two
needles to admit a slow current of nitrogen and evaporation of the solvents. After approximately
7 days the formation of crystalline material
2 was occurred; a few selected single crystals were
used for the X-ray diffraction analysis (see ESI for details). Anal. Calcd for
[(PhGeO1,5)10(CuO)6(HO0,5)2(C10H8N2)2] Cu, 14.98; N, 2.20; Ge, 28.54. Found (for vacuum-
dried sample): Cu, 14.89; N, 2.14; Ge, 28.35%. Yield 0.78 g (39%).
[(PhGeO1.5)10(CuO)6(CH3O0.5)2(C12H8N2)2][MeOH]2 (3)
PhGe(OMe)3 (1.20 g; 4.9 mmol) in 60 ml of an ethanol/methanol (1:1) solution was
mixed with solid Na (0.14 g; 5.9 mmol). Mixture was heated under reflux for 2 h. Afterwards,
the solution was cooled to room temperature and 0.40 g (3.0 mmol) of anhydrous CuCl2 was
added at once. The solution was left under stirring overnight and 0.15 g (0.9 mmol) of 1,10-
phenanthroline were added. Resulted mixture was left under stirring for 4 h and then filtered
from the insoluble part. The filtrate was stored in a warm place in a flask equipped by septum
with two needles to admit a slow current of nitrogen and evaporation of the solvents. After
approximately 7 days the formation of crystalline material
3 was occured; a few selected single
crystals were used for the X-ray diffraction analysis (see ESI for details). Anal. Calcd for
[(PhGeO1,5)10(CuO)6(CH3O0,5)2(C12H8N2)2] Cu, 14.55; N, 2.14; Ge, 27.71. Found (for vacuum-
dried sample): Cu, 14.40; N, 2.09; Ge, 27.59%. Yield 0.30 g (15%).
[(PhGeO1.5)10(CuO)6(HO0.5)1.6)(C2H5O0.5)0.4(C12H8N2)2][(DMF)0.75]2[(H2O)0.2]2[EtOH]2 (
19
4)

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PhGe(OMe)3 (1.20 g; 4.9 mmol) in 14 ml of an ethanol/methanol (1:1) solution was
mixed with solid Na (0.14 g; 5.9 mmol). Mixture was heated under reflux for 2 h. Afterwards,
the solution was cooled to room temperature and 0.40 g (3.0 mmol) of anhydrous CuCl2,
dissolved in 55 ml of DMF, was added at once. The solution was left under stirring overnight
and 0.15 g (0.9 mmol) of 1,10-phenanthroline were added. Resulted mixture was left under
stirring for 4 h and then filtered from the insoluble part. The filtrate was stored in a warm place
in a flask equipped by septum with two needles to admit a slow current of nitrogen and
evaporation of the solvents. After approximately 7 days the formation of crystalline material
4
was occured; a few selected single crystals were used for the X-ray diffraction analysis (see ESI
for details). Anal. Calcd for [(PhGeO1,5)10(CuO)6(HO0,5)1.6(C2H5O0.5)0.4(C12H8N2)2] Cu, 14.64;
N, 2.15; Ge, 27.89. Found (for vacuum-dried sample): Cu, 14.61; N, 2.11; Ge, 27.81%. Yield
0.64 g (31%).
[(PhGeO1.5)10(CuO)6(HCOO0.5)2(C12H8N2)2][MeOH] (5)
PhGe(OMe)3 (1.20 g; 4.9 mmol) in 60 ml of an ethanol/methanol (1:1) solution was
mixed with solid Na (0.14 g; 5.9 mmol). Mixture was heated under reflux for 2 h. Afterwards,
the solution was cooled to room temperature and 0.40 g (3.0 mmol) of anhydrous CuCl2 was
added at once. The solution was left under stirring overnight and 0.15 g (0.9 mmol) of 1,10-
phenanthroline were added. Resulted mixture was left under stirring for 4 h and then filtered
from the insoluble part. The filtrate was stored in a warm place in a flask with contact to
atmosphere to let slow evaporation of the solvents. After approximately two weeks the
formation of crystalline material
5 was occured; a few selected single crystals were used for the
X-ray diffraction analysis (see ESI for details). Anal. Calcd for
[(PhGeO1,5)10(CuO)6(HCOO0,5)2(C12H8N2)2] Cu, 14.39; N, 2.12; Ge, 27.42. Found (for vacuum-
dried sample): Cu, 14.30; N, 2.08; Ge, 27.31%. Yield 0.29 g (14%).
[(PhGeO1.5)10(CuO)6(CH3COO0.5)2(C10H8N2)2][MeOH]0.5 (6)
PhGe(OMe)3 (1.20 g; 4.9 mmol) in 60 ml of an ethanol/methanol (1:1) solution was
mixed with solid Na (0.14 g; 5.9 mmol). Mixture was heated under reflux for 2 h. Afterwards,
the solution was cooled to room temperature and 0.40 g (3.0 mmol) of anhydrous CuCl2 was
added at once. The solution was left under stirring overnight and 0.18 g (0.9 mmol) of 2,2’-
bipyridine were added. Resulted mixture was left under stirring for 4 h and then filtered from the
insoluble part. The filtrate was stored in a warm place in a flask with contact to atmosphere to let
slow evaporation of the solvents. After approximately two weeks the formation of crystalline
20

ACCEPTED MANUSCRIPT
material
6
was occured; a few selected single crystals were used for the X-ray diffraction
analysis (see ESI for details). Anal. Calcd for [(PhGeO1,5)10(CuO)6(CH3OO0,5)2(C10H8N2)2] Cu,
14.50; N, 2.13; Ge, 27.63. Found (for vacuum-dried sample): Cu, 14.42; N, 2.09; Ge, 27.54%.
Yield 0.23 g (11%).
Hydroperoxidation of Alkanes by Hydrogen Peroxides
Catalyst 1 or 2 was introduced into the reaction mixture in the form of solid powder. The alkane
was then added and the reaction started when hydrogen peroxide was introduced in one portion.
(CAUTION. The combination of air or molecular oxygen and H2O2 with organic compounds at
elevated temperatures may be explosive!). The reactions after addition of nitromethane as a standard
compound were analyzed by GC. As we made previously, the samples obtained in the alkane oxidation
were typically analyzed twice (before and after their treatment with PPh3) by GC. The chromatograph-
3700 (fused silica capillary column FFAP/OV-101 20/80 w/w, 30 m × 0.2 mm × 0.3 µm; helium as a
carrier gas was employed. The method of comparison of chromatograms of the same sample obtained
before and after addition of PPh3 which was proposed by Shul’pin earlier [109-118] allowed us to
estimate real concentration of alkyl hydroperoxide, ketone (aldehyde) and alcohol present in the reaction
solution. Attribution of peaks was made by comparison with chromatograms of authentic samples and
by GC–MS. Blank experiments with cyclohexane showed that in the absence of a catalyst products were
formed in negligible concentrations.
Oxidation of Alcohols to Ketones by TBHP
These reactions were carried out analogously. Triphenylphosphine was added in order to reduce
remain TBHP to tert-butanol. The reactions with 1-phenyethanol were analyzed by 1H NMR method
(solutions in acetone-d6; “Bruker AMX-400” instrument, 400 MHz).
Oxidation of Alcohols and Amines into Amides
In a sealed tube were added successively amine hydrochloride (0.5 mmol), CaCO3 (50.1 mg, 0.5
mmol), CH3CN (1mL), 2 (0.9 mg, 0.002 mmol, 0.2 mol% in Cu), benzylic alcohol (105 µL, 1.0 mmol)
and TBHP (5.5M in nonane, 225 µL, 1.25mmol). The mixture was stirred at 80°C for 2h and TBHP
(5.5M in nonane, 225 µL, 1.25mmol) were again added to the mixture. After 22h at 80°C, the mixture
was cooled to room temperature and 1N HCl and EtOAc were added. The mixture was extracted twice
with EtOAc, and the combined organic phases were washed with a saturated solution of NaHCO3 and
brine, and concentrated under reduced pressure. To remove the excess of benzylic alcohol, 80 mL of
H2O were added and evaporated under reduced pressure. Crude product was then purified using silica
21

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gel chromatography using gradients of cyclohexane/EtOAc to yield the pure compounds. Purity of
amides was confirmed by 1H and 13C NMR analysis and comparison with literature data.
Conclusions
Here we presented that simple self-assembling reaction (starting from PhGe{OMe}3)
could be an effective instrument for the synthesis of cage-like Cu(II)-based
phenylgermsesquioxanes, containing N,N-ligands of 2,2’-bipyridine family. Similar composition
of all six isolated products, with the same ratio of “key elements”, namely Ge/Cu/N = 10/6/4,
pointed out to an increased stability of this type of molecular geometry. Additional ligation of
cage complexes 1-6 came from OX species (X = H, CH3, C2H5, HCO, CH3CO). Among these,
especial interest is attracted by formiate and acetate fragments, formed due to spontaneous
oxidation of corresponding alcohols (MeOH and EtOH), used as reaction media.
Proneness to oxidation activity of Cu-germsesquioxane was indeed confirmed by
catalytic tests, used as precatalysts. This investigation demonstrated that compounds
efficient catalysts in the oxidation of alkanes, benzene and alcohols with hydroperoxides.
Additionally, complex was also highly efficient in the formation of amides from alcohols and
1 and 2 are
2
amines. Further studies of cage metallacomplexes (based on germanium sesquioxanes) and their
catalytic properties are currently on their way in our teams.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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, 16-
03-00609, and 19-03-00142А and 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).
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Appendix A. Supplementary data
Supplementary data related to this article (details of X-ray experiments, Table S1) can be found
at https:// doi.org/…xxxx
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