Mild and Regioselective Hydroxylation of Methyl Group in Neocuproine: Approach to an N,O-Ligated Cu6 Cage Phenylsilsesquioxane

Article abstract of DOI:10.1021/acs.organomet.7b00845

The self-Assembly synthesis of Cu(II)-silsesquioxane involving 2,9-dimethyl-1,10-phenanthroline (neocuproine) as an additional N ligand at copper atoms was performed. The reaction revealed an unprecedented aerobic hydroxylation of only one of the two methyl groups in neocuproine to afford the corresponding geminal diol. The produced derivative of oxidized neocuproine acts as a two-centered N,O ligand in the assembly of the hexacopper cage product [Cu₆(Ph₅Si₅O₁₀)₂·(C₁₄H₁₁N₂O₂)₂] (1), coordinating two of the six copper centers in the product. Two siloxanolate ligands [PhSi(O)O]₅ in the cis configuration coordinate to the rest of the copper(II) ions. Compound 1 is a highly efficient homogeneous precatalyst in the oxidation of alkanes and alcohols with peroxides.

Full text of DOI:10.1021/acs.organomet.7b00845

Communication
Cite This: Organometallics XXXX, XXX, XXX−XXX
Mild and Regioselective Hydroxylation of Methyl Group in
Neocuproine: Approach to an N,O-Ligated Cu6 Cage
Phenylsilsesquioxane
Alexey N. Bilyachenko,*,†,‡ Mikhail M. Levitsky,† Victor N. Khrustalev,‡ Yan V. Zubavichus,§
Lidia S. Shul’pina,† Elena S. Shubina,† and Georgiy B. Shul’pin*,∥,⊥
†Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov Str., 28, Moscow, Russia
‡Peoples’ Friendship University of Russia (RUDN University), Miklukho-Maklay Str., 6, Moscow, Russia
§National Research Center “Kurchatov Institute”, Akademika Kurchatova pl., 1, Moscow, Russia
∥Semenov Institute of Chemical Physics, Russian Academy of Sciences, ulitsa Kosygina, dom 4, Moscow, Russia
⊥Plekhanov Russian University of Economics, Stremyannyi pereulok, dom 36, Moscow, Russia
S
* Supporting Information
ABSTRACT: The self-assembly synthesis of Cu(II)-silsesquioxane
involving 2,9-dimethyl-1,10-phenanthroline (neocuproine) as an addi-
tional N ligand at copper atoms was performed. The reaction revealed an
unprecedented aerobic hydroxylation of only one of the two methyl
groups in neocuproine to afford the corresponding geminal diol. The
produced derivative of oxidized neocuproine acts as a two-centered N,O
ligand in the assembly of the hexacopper cage product
[Cu6(Ph5Si5O10)2·(C14H11N2O2)2] (1), coordinating two of the six
copper centers in the product. Two siloxanolate ligands [PhSi(O)O]5 in
the cis configuration coordinate to the rest of the copper(II) ions.
Compound 1 is a highly efficient homogeneous precatalyst in the
oxidation of alkanes and alcohols with peroxides.
n the ever-expanding field of C−H functionalization,
significant attention has been paid to the use of complexes
dppe5). Herewith, we present an unexpected reactivity of 2,9-
dimethyl-1,10-phenanthroline (neocuproine) exhibited by this
heterocyclic ligand in the course of such a reaction.
I
of inexpensive metals. The most intriguing feature is that the
catalytic activity of metal complexes can be dramatically
enhanced by selecting an appropriate ligand environment of
the metal site in such systems. In the context of these works,
some of us presented the first results concerning an application
of polynuclear complexes of non-noble metals in sesquioxane
(REO1.5, E stands for silicon (Si) or germanium (Ge))
matrixes.1−3 Very recently, it has been shown that both the
structural and catalytic features of such metalla compounds may
be tailored due to the involvement of additional organic
(nitrogen-4 or phosphorus-containing5) ligands. Thus, further
exploration of the chemistry of sesquioxane metallacomplexes
seemed to be of significant profit. According to known
literature data, there are three strategies for the “ligand assisted”
synthesis of metallasilsesquioxanes. First, the ligand could enter
into the composition of the metallasilsesquioxane along with a
metal ion in the case of using a metallacomplex as a reactant for
the synthesis (e.g., Cp*TaCl4,6 (dcpe)FeCl2,7 [(PPh3)2PtI-
Interaction of PhSi(OMe)3 (1 g; 5.04 mmol) in 20 mL of an
ethanol/methanol (1/1) mixture with solid NaOH (0.20 g; 5
mmol) was carried out by heating under reflux for 1.5 h.
Afterward, the solution was cooled to room temperature and
0.226 g (1.68 mmol) of CuCl2 was added at once. The solution
was stirred for 3 h, and 0.175 g (0.840 mmol) of neocuproine
(solution in 35 mL of a 1/1 1,4-dioxane/DMF mixture) was
added (Figure 1). The reaction solution was intensely stirred
for 2 h and then filtered to remove the insoluble precipitate.
The filtrate was stored in a flask in air to admit a slow
evaporation of the solvents for crystal growth. After roughly 2
weeks the formation of crystalline material 1 of the composition
[Cu6(Ph5Si5O10)2·(C14H11N2O2)2]·0.5(dioxane)·DMF oc-
curred; a few selected single crystals were used for the X-ray
diffraction analysis. Anal. Calcd for [Cu6(Ph5Si5O10)2.
(C14H11N2O2)2]: Cu, 17.08; N, 2.51; Si, 12.59. Found (for
vacuum-dried sample): Cu, 16.89; N, 2.42; Si, 12.40 (elemental
analysis was carried out with an XRF VRA-30 spectrometer).
Yield: 0.27 g (43%).
9
(Ph),8 Fe[N(SiMe3)2]2 ). In the second method, a postsyn-
thesis treatment of a metallasilsesquioxane with the ligand was
suggested (e.g., by tmeda,10 PMe3,11 NO,12 NEt313). Finally,
the simplest and most convenient approach, developed by our
team, involves self-assembly of a metallasilsesquioxane in the
presence of an additional ligand (e.g., phenanthroline4 or
Received: November 24, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00845
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aforementioned synthetic protocols earlier gave rise to
alcohol,20 aldehyde, or carboxylic21 fragments. We could only
compare our results to works discussing “methyl to gem-diol”
transformation as an intermediate step in enzyme-assisted
oxidations.22
We assume that, in our neocuproine case, the molecular
oxygen is coordinated to two copper ions to give species B
(Figure 3), and then a peroxo dicopper intermediate is
generated. Such complexes containing one or two copper
ions surrounded with N-donor ligands have been reported
earlier.23−26 These compounds are capable of hydroxylating C−
H bonds to afford phenols for the intramolecular reaction
involving a benzene ring or THF oxygenated products in the
intermolecular case.25 A copper(II)−OOH complex has been
isolated and characterized as a structural/functional model of
dopamine β-hydroxylases hydroxylating sp3-CH bonds.23
Hydroxyl radicals have been produced from H2O2 under the
action of Cu(II)-bridged peroxo species.26 The oxygen atom
from coordinated O2 in species B abstracts a hydrogen atom to
produce in species C a hydroperoxy ligand and alkyl radical
fragment. A recombination of the two radicals −CuO• and
•CH(OH)− in D and then H atom abstraction in E gives
finally structure G (isolated complex 1) via a rebound
mechanism in structure F.
Multicopper complexes are known to catalyze functionaliza-
tion of alkanes and other C−H compounds under mild
conditions.27−31 These complexes are models of the reaction
sites of certain enzymes.32−34 It is worth mentioning that earlier
work of Waymouth and colleagues35 noted that oxidation of a
methyl group at the neocuproine (to a carboxylate fragment)
strongly inhibited the corresponding palladium complex toward
the aerobic alcohol oxidation. In contrast to this, we have found
that compound 1 is a highly efficient homogeneous catalyst in
the oxidation of alkanes and alcohols with peroxides. As we
indicated previously, the samples obtained in the alkane
oxidation were typically analyzed twice (before and after their
treatment with PPh3) by GC. This method (the comparison of
chromatograms of the same sample obtained before and after
addition of PPh3) which was proposed by Shul’pin earlier1,36−42
allows us to estimate the real concentrations of alkyl
hydroperoxide, ketone (aldehyde), and alcohol present in the
reaction solution. The experiments on the methylcyclohexane
oxidations catalyzed by 1 are described in the Supporting
Information. All of the peculiarities of alkane oxidation indicate
that the oxidation with H2O2 occurs with the participation of
free hydroxyl radicals.43−53 Complex 1 also efficiently catalyzes
the oxidation of alcohols to ketones with TBHP in acetonitrile.
The oxidation of alcohols apparently involves the generation of
hydroxyl radicals (see the Supporting Information).
Figure 1. General scheme of synthesis of hexacopper silsesquioxane 1
containing hydroxylated neocuproine ligands. Color code: Si, yellow;
O, red; Cu, green; N, blue; C, black; H, gray.
An X-ray diffraction study of 1 revealed a cage consisting of
two Cu3 linear trimers sandwiched between two cisoid
[PhSi(O)O]5 siloxanolate ligands. This is an unusual
combination of complex componentswhile hexacopper
compounds are in principle well-known in the chemistry of
metallasilsesquioxanes,14 the presence of five-membered
siloxanolate rings is not so common.15 Since five siloxanolate
oxygen atoms from each side of such a sandwich are obviously
insufficient to complete coordination of the six Cu(II) ions, the
involvement of two additional (i.e., neocuproine) ligands is
needed. The actual method of this involvement, to the best of
our knowledge, turned out to be absolutely unexpected and
unprecedented. Surprisingly, one methyl group of each
neocuproine moiety was oxidized (or, more strictly speaking,
hydroxylated) to produce a geminal diol fragment, −CH(OH)-
(OH). One of the −OH fragments of this diol is then
deprotonated to coordinate the Cu(II) ion, −CH(OH)(O···
CuII). Simultaneously, N atoms of the neocuproine ligand also
participate in the coordination with the same copper site
(Figure S1, Supporting Information). Thus, such a trans-
formation of neocuproine into the two-centered N,O ligand
provides better coordination conditions for copper sites and
therefore can be considered as a driving force of the whole
product 1 assembly.
In summary, an unprecedented “methyl to diol” hydrox-
ylation of one methyl group in a neocuproine ligand
coordinated to copper was observed. This unusual reaction
became possible, most likely, via peroxo dicopper intermediate
formation. Importantly, this intermediate was formed during
the metallasilsesquioxane assembly in the presence of ambient
oxygen, without the use of any added oxidant reagent. The
resulting neocuproine derivatives act as two-centered N,O
ligands, essential for the assembly of the hexacopper cage
product 1. The rest of the coordination sites of copper ions are
filled by oxygen centers of siloxanolate ligands [PhSi(O)O]5.
Compound 1 is a very efficient precatalyst in the homogeneous
oxidation of alkanes and alcohols with peroxides.
Several earlier works concerning metallasilsesquioxane syn-
thesis pointed at the “oxidation prospective” conditions of such
reaction systems. For example, CeIII to CeIV,16 UIV to UVI 17 and
18
CrII to CrIV oxidations were observed in the course of
synthesis of cerium-, uranium- and chromium-based silses-
quioxanes, respectively. Moreover, tetrahydrofuran to γ-
butyrolactone oxidation was reported for iron-silsesquioxane
synthesis performed in THF as a reaction medium.19 Thus, the
oxidation of neocuproine’s methyl groups under conditions of
the synthesis of 1′ is not inexplicable.20,21 The most attractive
feature in our report is the formation of a geminal diol as a
product of the neocuproine methyl group oxidation, while the
B
DOI: 10.1021/acs.organomet.7b00845
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Communication
Figure 3. Mechanism proposed for the hydroxylation of neocuproine’s methyl group to afford complex 1.
ORCID
ASSOCIATED CONTENT
■
Alexey N. Bilyachenko: 0000-0003-3136-3675
Victor N. Khrustalev: 0000-0001-8806-2975
Elena S. Shubina: 0000-0001-8057-3703
Notes
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00845.
The authors declare no competing financial interest.
Details of X-ray experiments and catalytic oxidations
(PDF)
ACKNOWLEDGMENTS
■
This publication was prepared with the support of the “RUDN
University Program 5-100”, the Russian Foundation for Basic
Research (Grants 16-03-00254, 17-03-00993, 16-29-05180).
Synchrotron single-crystal diffraction measurements were
performed at the unique scientific facility Kurchatov Synchro-
tron Radiation Source supported by the Ministry of Education
and Science of the Russian Federation (project code
RFMEFI61917X0007).
Accession Codes
CCDC 1565125 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
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■
Corresponding Authors
*E-mail for A.N.B.: bilyachenko@ineos.ac.ru.
*E-mail for G.B.S.: shulpin@chph.ras.ru.
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