A new “bicycle helmet”-like copper(ii),sodiumphenylsilsesquioxane. Synthesis, structure and catalytic activity

Article abstract of DOI:10.1039/c8dt03209b

A new “bicycle helmet”-like copper(ii),sodiumphenylsilsesquioxane Ph₁₂Si₁₂O₁₂(OH)(O^?)₁₁Cu₅Na(bipy)₃(H₂O) exhibited high catalytic efficiency in two homogeneous reactions: (i) functionalization of C-H compounds; (ii) formation of benzamides from alcohols.

Full text of DOI:10.1039/c8dt03209b

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A new “bicycle helmet”-like copper(II),
sodiumphenylsilsesquioxane. Synthesis, structure
and catalytic activity†
Cite this: Dalton Trans., 2018, 47,
15666
Received 6th August 2018,
Accepted 15th October 2018
Alena N. Kulakova,a,b Alexey N. Bilyachenko,
*
a,b Alexander A. Korlyukov,a,c
a
Lidia S. Shul’pina,a Xavier Bantreil, d Frédéric Lamaty, *d Elena S. Shubina,
Mikhail M. Levitsky,a Nikolay S. Ikonnikova and Georgiy B. Shul’pin*e,f
DOI: 10.1039/c8dt03209b
rsc.li/dalton
A new “bicycle helmet”-like copper(II),sodiumphenylsilsesquioxane organic) can promote the stability of catalytic species during
Ph12Si12O12(OH)(O−)11Cu5Na(bipy)3(H2O) exhibited high catalytic the reaction. Here, we present our recent observations regard-
efficiency in two homogeneous reactions: (i) functionalization of ing: self-assembly of Cu(II),Na-silsesquioxane in the presence
C–H compounds; (ii) formation of benzamides from alcohols.
of a widely used 2,2′-bipyridine; determination of the unusual
molecular topology of the product; testing of the product as a
precatalyst for homogeneous reactions of organic synthesis.
Target compound 1 was isolated (in 24% yield) through an
interaction between in situ-generated Cu,Na-silsesquioxane
species and 2,2′-bipyridine (Scheme 1) by varying the ratio
between reactants and the nature of solvents. Carrying out the
reaction/crystallization in THF favored formation of an earlier,
unknown type of cage metallasilsesquioxane molecular topo-
logy (Fig. 1, top).
The tremendous popularity of self-assembly reactions aimed at
the creation of different types of polyhedral metallacomplexes
is explicable by the great efficacy of the underlying method-
ology.1 A diverse family of the resultant compounds has been
studied widely due to potential material,2 biological appli-
cations3 and numerous catalytic tasks.4 Intriguing opportu-
nities for isolation of cage metallacomplexes could be provided
by using non-trivial ligands such as a silicon-based sesquiox-
ane (branched compound of general formula RSiO1.5). Using
silsesquioxanes has allowed a family of metalladerivatives
(including several examples of heterometallic ones) to be
obtained.5 In turn, some scholars have reported a “heteroli-
gand” approach6 to metallacomplexes of this type, whereby
organic ligands (e.g., tris(2-(dimethylamino)ethyl)amine,6a
cyclooctenyl,6b 2,2′-bipyridine6c,d) cooperate with the silses-
quioxane ligands in the assembling of the cage product. This
novel (L2) type of cage can be especially prospective for their
catalytic studies because both ligands (silsesquioxane and
X-ray diffraction indicated that 1 was a mixed metal
(Cu5Na) complex, which crystallized in the P21/c space group.
This type of nuclearity has not been observed for reported
metallasilsesquioxanes.7 All copper(II) sites (Fig. S1†)
aNesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
Vavilov Str., 28, 119991 Moscow, Russia. E-mail: bilyachenko@ineos.ac.ru
bPeoples’ Friendship University of Russia (RUDN University), Miklukho-Maklay Str.,
6, 117198 Moscow, Russia
cPirogov Russian National Research Medical University, Ostrovitianov str. 1, 117997
Moscow, Russia
dInstitut des Biomolécules Max Mousseron (IBMM), UMR 5247, CNRS, Université de
Montpellier ENSCM, Site Triolet, Place Eugène Bataillon, 34095 Montpellier cedex 5,
France. E-mail: frederic.lamaty@umontpellier.fr
eSemenov Institute of Chemical Physics, Russian Academy of Sciences, ulitsa
Kosygina, dom 4, Moscow 119991, Russia. E-mail: shulpin@chph.ras.ru
fPlekhanov Russian University of Economics, Stremyannyi pereulok, dom 36, Moscow
117997, Russia
†Electronic supplementary information (ESI) available: Details of synthesis and
catalysis, structural discussions. CCDC 1852742. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c8dt03209b
Scheme 1 General scheme of the self-assembling synthesis of 1.
15666 | Dalton Trans., 2018, 47, 15666–15669
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“globular” geometry, observed many times for Cu(II)-silses-
quioxanes.10 Detailed structural comparison of two of these
ligands is given in the ESI (Fig. S4 and Table S1†). Here, 11
positive charges from five Cu(II) ions and one Na(I) ion in 1
could not compensate for the 12 negative charges expected
from the dodecasilsesquioxane skeleton. Indeed, charge
balance became possible due to the presence of hydrogen,
which protonated one of the oxygen atoms of the silsesquiox-
ane framework. It elicited a very specific final composition of
the ligand: Ph12Si12(O)12(OH)(O−)11. The hydroxyl group and
encapsulated water molecules formed a system of hydrogen
bonds (Table 2, ESI†) within the metallasilsesquioxane cage
that has rarely (if ever) been observed in compounds of this
type. In view of its mono-hydroxyl functionality, only OH-
bearing silsesquioxanes belonging to the cubane structural
family11 and based-metallacomplexes11b,12 have been reported.
Logically, these compounds are quite different from 1 in terms
of composition and structure.
Being interested in the application of 1 in catalysis, we
undertook the first evaluation in homogeneous oxidative reac-
tions. First, oxidations of hydrocarbons with donors of an
oxygen atom (e.g., hydrogen peroxide or peroxy acids) consti-
tute an important field of contemporary catalytic chemistry.
Complex 1 exhibited high activity in the cyclohexane oxidation
with hydrogen peroxide in the absence of any added acid.
Thus, the reaction afforded after 2 h at 50 °C oxygenated in
20% yield and turnover number (TON) = 190 (Fig. 2). A com-
parison of chromatograms obtained before and after addition
of PPh3 (for this method developed by one of our research
team, see ref. 13) to the sample indicated that the main
product of the reaction, cyclohexyl hydroperoxide, was formed
in high concentration because the alcohol/ketone ratio rose
dramatically after the reduction, whereas this ratio was ∼1
before the reduction. Indeed, cyclohexyl hydroperoxide decom-
posed in the chromatograph (in the injector and columns) to
generate cyclohexanol and cyclohexanone in approximately
equal amounts. Unlike many oxidations under different con-
ditions, this reaction gave a much lower product concentration
Fig. 1 Top: Molecular structure of 1. Solvating ligands (THF) and encap-
sulated species (water) have been removed for clarity. Bottom. Structure
of the silsesquioxane ligand in 1.
possessed a similar distorted square pyramidal pentacoordi- before PPh3 than after reduction. This finding can be
nated geometry even though three of them were heteroligated explained by the assumption that cyclohexyl hydroperoxide
(O from silsesquioxane and N from 2,2′-bipyridine). The pres- was not completely reduced with PPh3 and was not directly
ence of 2,2′-bipyridine ligands provoked specific packing in detected by our chromatograph. Regioselectivity parameters in
the crystal, with two of the three 2,2′-bipyridines participating the oxidation of n-heptane catalyzed by complex 1 [C(1) : C
in π–π stacking interactions of aromatic systems with phenyl (2) : C(3) : C(4) = 1.0 : 5.5 : 5.0 : 4.5] and the oxidation of cis-1,2-
groups (Fig. S2†). Cu–N bond lengths ranged from 1.996 to dimethylcyclohexane without retention of the configuration
2.214 Å, and Cu–O bond lengths from 1.919 to 2.782 Å. The (trans/cis = 0.9 taking into account the mutual position of the
longest (2.523 Å and 2.782 Å) Cu–O contacts implied the two methyl groups) indicated that this reaction occurred with
coordination of copper centers to the oxygen atom of encapsu- the participation of free hydroxyl radicals.13b
lated water (Fig. S3†). Such water “capture”, despite some
Complex 1 was also evaluated in the oxidative amidation of
known examples reported for other types of complexes,8 is not benzylic alcohol in the presence of alkylammonium chlorides,
a widespread feature among the encapsulation abilities of cage calcium carbonate and TBHP (Scheme 2). This reaction
silsesquioxanes.9
allows to circumvent the classical coupling of acids and
The silsesquioxane skeleton of 1 comprised 12 silicon amines to furnish the corresponding amides (which are
centers to form one cyclic ligand (Fig. 1, bottom). The topology present in many organic molecules, including pharmaceutical
of the ligand was unprecedented. First, it was important to drugs), but which require at least a stoichiometric amount of
differentiate this Si12-based ligand from the classical one, of coupling agent, which could be toxic and generate a stoichio-
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83% and 88% yields, respectively. N-Butylbenzamide and N,N-
dibenzylbenzamide could also be isolated in good yields of
70% and 68%, respectively. TON and turnover frequency (TOF)
were calculated, giving values up to 220 and 9.2 h−1 (see ESI†
for details on TON and TOF), respectively. If the value of TOF
was similar to the one calculated in our seminal report
because of the prolonged reaction times with use of 1 com-
pared with CuO (24 h vs. 4 h), TON was increased fivefold,
thereby confirming the efficiency of complex 1 in this reaction.
Complex 1 was soluble under the reaction conditions, and
seemed to be involved as
a
homogeneous catalyst.
Consequently, because of the difficulty of recovery, recycling
experiments were not considered at this stage of the project.
Conclusions
We presented a brand-new type of bimetallic (Cu,Na) oxaclus-
ter in a heteroligand (phenylsilsesquioxane and 2,2′-bipyri-
dine) surrounding to form
a uniquely shaped product
Ph12Si12O12(OH)(O−)11Cu5Na(bipy)3(H2O). Catalytic tests of
this complex showed its high activity in oxidations. Further
investigation of this prospective “ligand plus ligand” approach
to self-assembly of complicated cage metallastructures and
their evaluation in catalysis is currently underway in our
research team.
Fig. 2 Accumulation of cyclohexanol and cyclohexanone with time in
the cyclohexane (0.46 M) oxidation in acetonitrile with H2O2 (50%,
aqueous, 1.5 M) catalyzed at 50 °C by compound 1 (5 × 10−4 M).
Concentrations measured before (Graph A) and after (Graph B) reduction
of the samples with PPh3 are shown.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the RUDN University Program “5-
100”, the Russian Foundation for Basic Research (17-03-00993,
16-03-00254, 16-53-150008, 16-29-05180, 16-29-05180), French
Embassy in Moscow, and University of Montpellier, CNRS
(Programme de Recherche Conjoint).
References
Scheme 2 Catalytic activity of 1 in oxidative amidation. Reaction con-
ditions: Ammonium chloride (0.5 mmol), benzyl alcohol (1.0 mmol),
CaCO3 (0.25 mmol), TBHP (5.5 M, 2.5 mmol), 1 (0.4 mol% of Cu),
CH3CN (1 mL), 80 °C, 24 h.
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