Preparation of a chiral Pt12 tetrahedral cage and its use in catalytic Michael addition reaction

Article abstract of DOI:10.1039/c8cc01487f

The reaction of chiral cis-[(1S,2S)-dch]Pt(NO₃)₂ (M) [where (1S,2S)-dch = (1S,2S)-1,2-diaminocyclohexane] with a hexadentate ligand (L) in 3:1 stoichiometric ratio yielded a [12+4] self-assembled chiral M₁₂L₄ mo

Full text of DOI:10.1039/c8cc01487f

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A chiral Pt12 tetrahedral cage and its use in catalytic Michael
addition reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Imtiyaz Ahmad Bhata, Anthonisamy Devaraj a, Prodip howladera, Ki‐Whan Chib and Partha Sarathi
Mukherjeea*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Reaction of chiral cis‐[(1S, 2S)‐dch]Pt(NO3)2 (M) with
a
cages.1a,9 Tetrahedral clusters, as the basic platonic polydrons
hexadentate ligand (L) in 3:1 stoichiometric ratio yielded a [12+4] feature many interesting host‐guest chemistry owing to their
self‐assembled chiral M12L4 molecular tetrahedron (T) [where (1S, intrinsic structural beauty.10 Several tetrahedral molecular
2S)‐dch =(1S, 2S)‐1,2‐diaminocyclohexane]. The cage T features cages are reported in recent past mostly employing octahedral
internal 3D nanocavity with large open windows, enables it to metal ions or lanthanides with higher coordination number.
catalyze the Michael addition reactions of a series of water‐ Solubility of such tetrahedral cages is largely restricted to
insoluble nitrostyrene derivatives with indole in (9:1) water : organic solvents only.11 However, due to geometric restriction,
methanol mixture.
a very few tetrahedral cages are reported with square planar
metal ions like Pd(II) acceptors through coordination driven
self‐assembly.12 Relatively more kinetic inertness of the Pt‐N
bonds makes amine blocked Pt(II) acceptors even more
difficult to achieve large architectures; in many cases self‐
assembly of amine blocked Pt(II) acceptors lead to mixture of
products. However, such Pt(II) architectures are more
appealing as molecular vessels for diverse chemical reactions
due to better stability compared to analogous Pd(II)
counterparts.
The general concept of “cavity‐directed synthesis” which deals
with molecular containers having confined space of size
comparable to that of reactants to alter the profile of the
chemical reactions is well established.1 Enzymes, nature’s
molecular containers, having molecular pockets capable of
binding substrates through non‐covalent interactions and
catalyze many important enzymatic reactions.2 Initially,
covalent hosts such as cyclodextrins, carcerands and
hemicarcerands as molecular containers attracted much focus
with diverse applications.3 Over the last two decades, with the
advent of coordination‐driven self‐assembly, the focus has
greatly shifted to exploit weak metal‐ligand coordination for
the self‐assembly of molecular containers from individual
components. The simple and dynamic nature of coordination
driven self‐assembly has led to construct various capsules and
cages with nanometer‐size cavities.4 The shape and size of
inner cavity of the coordination cages even without possessing
definite covalent interactions between the catalyst and
substrate play a paramount role in altering the reactivity and
properties of the contained molecules.3b,8 Large rate
enhancement, modulation in selectivity, or shifts in the
thermodynamic properties of encapsulated guests have been
achieved by the appropriate choice of substrates and the host
Herein we report the synthesis of an unusual chiral M12L4
tetrahedral cage
(pyrimidin‐5‐yl)phenyl)amine (
dch]Pt(NO3)2 ) in 1:3 stoichiometric ratio in 1 :1 water :
T
by treating exo‐hexadentate ligand tris(4‐
L
) with chiral cis‐[(1S, 2S)‐
(
M
methanol mixture (Scheme 1). The formation of a tetrahedral
cage of Pt(II) in the present case as the single product is unique
because an analogous hexadentate tripyrimidine ligand with
cis‐blocked Pd(NO3)2 yielded a hexahedron cage. Due to the
fact that Pt(II)‐pym bond is more stronger than Pd(II)‐pym
bond,13 the tetrahedral cage
T attains the thermodynamic
stability through the positive cooperative effect of only 24
Pt(II)‐pym coordination bonds compared to the 36 Pd(II)‐pym
weak coordination bonds of hexahedron reported earlier.14
The tetrahedral cage
T features interior hydrophobic cavity of
size ~ 860 Å3 surrounded by four exo‐hexadentate ligands
paneling four faces of tetrahedron. The confined cavity of the
cage was successfully utilized to promote the Michael addition
reactions of a series of aromatic nitro‐styrenes with indole
through supramolecular catalysis in water : methanol mixture
(9:1).
a. Inorganic and Physical Chemistry Department, Indian Institute of Science
Bangalore, Karnataka, 560012 (India).
b. Department of Chemistry, University of Ulsan, Ulsan 680‐749, Republic of Korea
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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‐ 4+
‐ 5+
‐ 6+
4NO3 ] ,
[
T
‐5NO3 ]
and
[
T
‐6NO3 ]
charge fragments,
respectively (Fig. S4‐S5). Finally, X‐ray crystallographic analysis
provided a clear evidence for the formation of Pt12L4
tetrahedral complex
shown in Fig. 3. Vapor diffusion of acetone into the water–
methanol (1:1) solution of for seven days afforded single
crystals of . A suitable single crystal was diffracted using the
T. A view of the X‐ray structure of T is
T
T
synchrotron radiation source. The complex crystalized in
orthorhombic system with P222 space group. Twelve square
planar Pt(II) centers are bridged by four exo‐hexadentate
ligands, occupying the four faces of the tetrahedron. The four
corners of the tetrahedron are made up of chiral triangular
[Pt(II)‐pym]3 secondary building unit, imparting the overall
Scheme 1. Schematic representation of the synthesis of tetrahedral cage T.
The exo‐hexadentate ligand (L) was synthesized by Suzuki
coupling of tris(4‐bromophenyl)amine and pyrimidine‐5‐
boronic acid at 85 °C following the reported procedure.15 Self ‐
chirality to the cage
T. The enantiopurity of the cage T was
confirmed by CD spectroscopy as shown in Fig. 2.
assembly reaction was carried out by treating ligand (
clear colorless solution of cis‐[(1S, 2S)‐dch]Pt(NO3)2 (
L
M
) to the
) in 1:3
stoichiometric ratio in water: methanol mixture (1:1). The
white suspension thus obtained was stirred for 36 h at 45 °C
temperature. A sharp visual color change from colorless to
clear yellow solution indicated the progress of the reaction.
Pure product as an off‐yellow precipitate was isolated by
adding excess acetone to the concentrated reaction mixture.
The product is poorly soluble in water but soluble in water:
methanol mixture (9:1) at room temperature. The 1H NMR
spectrum of the product (T) showed three singlets and two
doublets with integration ratio of 1:1:1:2:2, which clearly
advocates the symmetrical nature of the assembly (Fig. S2). In
1
comparison to H NMR of free ligands which shows only four
1
peaks, the H NMR of the isolated complex (
T
) showed five
peaks with significant downfield shift due to metal‐ligand
coordination (Fig. 1b).
Fig. 2 Circular dichroism spectra of (S, S)‐T, (R, R)‐T and (S, R)‐T in water : methanol
(1:1).
The non‐coplanar nature of ligands paneling the four faces of
the tetrahedron engenders large open windows of dimension
(5.67 × 5.36 Å2) along the edges of the tetrahedron
T
. The
presence of large open windows together with free space
inside the cage prompted us to explore the cage as
supramolecular catalyst. To authenticate our observation,
host‐guest study was first performed by treating the cage
T
T
T
with (E)‐(2‐nitrovinyl)benzene (1a) and (E)‐1‐methoxy‐4‐(2‐
nitrovinyl)benzene (1f) guests in H2O : methanol mixture (9:1)
separately at room temperature. As expected, both the
aromatic guests 1a and 1f were encapsulated in cage
T as
evidenced by 1H NMR and UV‐Visible spectroscopy (Figs. S6, S9
1
and S13). The H NMR of the host‐guest complex showed the
1
substantial shift and peak broadening for host peaks. The H
Fig. 1 Partial 1H NMR of ligand (L) in CDCl3 and cage T in D2O: CD3OD (1:1).
DOSY NMR of the host‐guest complexes showed an identical
diffusion coefficients value for host and guest peaks (Figs. S7
DOSY NMR confirmed the formation of a single product with a
clear single band at D = 1.2 × 10‐10 m2/s (log D = ‐9.3 m2/s) (Fig.
and S10). The 1H NMR spectrum of
T⊃1f revealed the
stoichiometry of cage vs guest as 1: 2 (Fig. S9). Unfortunately
various attempts to crystallize host‐guest complexes remained
unsuccessful. Semi‐empirical method with PM6 basis set was
modelled to optimize the T⊃1f host‐guest (1: 2) complex (Fig.
3b). The optimized structure showed that the phenyl moieties
S3). An ESI‐MS spectral analysis of the
T in water : methanol
mixture (1:1) revealed the formation of Pt12L4 assembly with
the appearance of peaks at m/z =1719.0012, 1363.0014 and
1125.0033 matching with isotopic distribution patterns of [T‐
2 | J. Name., 2012, 00, 1‐3
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of guest 1f are stabilized through π–π stacking interactions and of the cage. To further explore the catalytic role of cage
its nitro‐groups are aligned towards [Pt(II)‐pym]3 cyclic several reactions were investigated by taking various
framework of host and are stabilized by electrostatic derivatives of nitro‐styrene treated with Indole in water :
interactions (Fig. 3b).1H NOESY NMR of
1f showed the methanol (9 : 1) mixture at room temperature (Table 1).
correlation cross peak between the methoxy protons of guest
1f and the phenyl protons of host which further support in
T,
T
T
⊃
Table 1. Michael addition of 1a‐1f with indole promoted by cage T
T
favor of energy optimized orientation of guest 1f in T⊃1f host‐
8.03 ppm
NH
H
guest complex (Fig. S12).
H
N
NO2
(10 mol % T)
H2O : MeOH
H
5.23 ppm
H
R
(90:10) , RT, 5-24 h
H
H
R
NO2
1a-1g
2a-2g
R
Conv. %
Entry
1.
Conv. % Time (h)
with (T)xx
without
xx
(T)
H
91
22
5
2,3‐CH=CH‐
CH=CH‐
2.
3.
60
87
12
28
24
Fig. 3 (a) Capped‐sticks presentation of the crystal structure of the tetrahedron T (all
the hydrogen atoms are omitted for clarity). Color codes: cyan‐green: carbon, blue:
nitrogen and red: platinum. (b) PM6 energy optimized structure of the guest
encapsulated cage T⊃1f. Color codes: green: carbon, blue: nitrogen, red: platinum and
yellow : oxygen.
2‐chloro
4.5
4‐chloro
4.
5.
68
50
20
27
6
6
In organic synthesis, Michael addition of indole as
nucleophile to electron deficient alkenes has been well studied
and documented in the literature using different acid catalysts
2‐4 dichloro
in organic solvents.16 Moreover, the nitro‐styrenes not only 6.
being good Michael acceptors but also feature the advantage
of forming a wide range of different functionalized Michael
88
25
24
4‐OMe
4‐Me
7.
70
25
24
adducts with indoles.17 However, most of such reported
protocols suffer from the disadvantage of forming other
products due to the possible polymerization of electron‐rich
heteroaromatic rings in presence of acid catalyst.18 Hence,
there are continuous efforts for the development of new and
effective protocol for the synthesis of Michael adducts with
indoles and nitro styrenes. After the successful encapsulation
XX
Percentage conversion is based on 1H NMR of crude sample.
The reaction times and the efficiency of the reaction were
both affected by the nature of the substituent on the β‐
nitrostyrene. The β‐nitrostyrenes having strong electron‐
withdrawing groups (entries 3‐5) gave Michael adducts in
relatively short time with good to excellent yields while the β‐
nitrostyrenes bearing strong electron‐donating groups (entries
2,6–7), required longer reaction time. The Michael adducts 2a‐
2g were formed through nucleophilic addition of electron‐rich
indole to β‐nitrostyrene derivatives. The substrates after
of the aromatic nitro‐alkenes inside the cage
carry out supramolecular catalysis utilizing the cage
T
, we planned to
as
T
supramolecular catalyst in water : methanol (9:1). Initially,
nitrostyrene 1a was treated with indole in presence of 10
mol% of cage
T in 1 mL of water : methanol (9:1) solvent and
the mixture was stirred for 5 h at room temperature. The
reaction mixture containing Michael adduct (2a) was extracted
with CDCl3 and the 1H NMR spectral analysis of the crude
sample revealed the formation of 91% of product 2a (Fig. S14).
The product 2a was obtained as pure form by column
chromatography and was fully characterized by NMR (Fig. S16‐
S17) and ESI‐MS spectroscopy. Similar reaction when carried
out in absence of the cage under identical reaction condition
resulted in 22% of 2a formation (Fig. S15). The product
encapsulation inside the confined cavity of the cage
stabilized through π‐π interactions with the aromatic walls of
the cage . Based on the structure of the optimized host‐guest
complex (Fig. 3b), we presume that the large cationic charge of
[Pt(II)‐pym]3 cyclic framework present at the vertices of the
T get
T
cage
T activated the nitro groups of β‐nitrostyrene systems
through electrostatic interaction, which facilitates its
electrophilic substitution on indole to form final Micheal
adducts at room temperature in aqueous methanol medium.
The products formed being bulky and probably have less π‐π
enhancement from 22 to 91% in presence of cage
T reflected
the catalytic role of its confined cavity. However no
enantioselectivity of product 2a was found as determined by
the chiral HPLC. The racemic product formation is obvious as
the chiral amine groups are present at the peripheral positions
interactions with the cage
T and are thus easily released and
replaced by the new substrates to carry out the further
catalytic cycle. Due to the presence of robust Pt‐N bonds, the
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cage features an advantage to act as a molecular vessel for
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T
the reactions even with highly basic Michael donors like
indole. Such reactions in presence of basic indole were found
to break the Pd based tetrahedral cage (T1)
12a due to relatively
labile Pd‐N bonds .12a
In conclusion, we report here two‐component self‐assembly
of a 3D chiral tetrahedral Pt(II) cage synthesized by reaction
T
5
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acceptor cis‐[(1S, 2S)‐dch]Pt(NO3)2. Due to kinetic inertness of
amine blocked Pt(II) acceptor compared to analogous Pd(II)
acceptor, formation of an uncommon tetrahedral cage
employing square planar Pt(II) is quite remarkable. The
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tetrahedral cage
by four aromatic walls of the ligand (
successfully utilized to encapsulate water insoluble nitro‐
styrene derivatives in its molecular pocket. The presence of
large open windows along the four edges of tetrahedron
T
features inner hydrophobic void surrounded
). The cage was
L
T
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T
together with inner hydrophobic void made it suitable nano‐
vessel to catalyze the Micheal addition of a series of nitro‐
styrene derivatives with indole in catalytic fashion. To the best
of our knowledge, the present M12L4 assembly represents the
first example of Pt(II) based chiral tetrahedral cage reported so
far; and pays the way forward for further design and
exploration of Pt(II) based self‐assembled architectures
towards supramolecular catalysis.
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Acknowledgements
P.S.M. is grateful to SERB‐India for financial support (Grant No.
EMR/2015/002353). I.A.B. and D.A are grateful to UGC (New
Delhi) for SRF and D. S. Kothari postdoctoral fellowship,
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4 | J. Name., 2012, 00, 1‐3
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Graphical Abstract
A chiral Pt12 tetrahedral cage and its use in catalytic Michael addition reaction
a
Imtiyaz Ahmad Bhata, Anthonisamy Devaraj , Prodip howladera, Ki‐Whan Chib and Partha Sarathi Mukherjeea*
A chiral M12L4 molecular tetrahedron (
T
) was synthesized by self‐assembly of chiral cis‐[(1S, 2S)‐dch]Pt(NO3)2 (
M
)
with a
hexadentate ligand ( ) in 3:1 stoichiometric ratio. The cage
L
T
was found to catalyze the Michael addition reactions of series of
nitrostyrene derivatives with indole in (9:1) water‐methanol mixture.
This journal is © The Royal Society of Chemistry 20xx
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