Self-Assembly of a "cationic-cage": Via the formation of Ag-carbene bonds followed by imine condensation

Article abstract of DOI:10.1039/c9cc02341k

A new strategy for the synthesis of a "cationic-cage" (CC-Ag) has been developed via metal-carbene (M-C_NHC) bond formation followed by imine bond condensation. Reaction of a trigonal trisimidazolium salt H₃L(PF₆)₃ functionalized with three flexible N-phenyl-Aldehyde pendants with silver oxide yielded a trinuclear tricationic organometallic cage (OC-Ag). Subsequent treatment of the organometallic cage (OC-Ag) with 1,4-diaminobutane links the two tris-NHC ligands via imine bond condensation, which thus generates a 3D 'cationic-cage' (CC-Ag). Furthermore, post-synthetic replacement of the Ag(i) with Au(i) leading to the formation of CC-Au was achieved via trans-metalation, with the retention of the molecular architecture.

Full text of DOI:10.1039/c9cc02341k

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DOI: 10.1039/C9CC02341K
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Self-assembly of a “Cationic-Cage” via formation of Ag-carbene
bonds followed by imine condensation
a
a
a
a
Ritwik Modak, Bijnaneswar Mondal, Prodip Howlader and Partha Sarathi Mukherjee *
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A new strategy for the synthesis of a “Cationic-Cage” (CC-Ag) has their metal complexes, Ag(I)– and Au(I)–NHC complexes are of
been developed via metal-carbene (M−CNHC) bond formation specific interest due to their fascinating structural properties
followed by imine bond condensation. Reaction of a trigonal and various applications in homogeneous catalysis, medicine
1
1
trisimidazolium salt H
3 6 3
L(PF ) functionalized with three flexible N- and materials chemistry. Moreover, Ag(I)–NHC complexes are
phenyl-aldehyde pendants with silver oxide yielded a trinuclear frequently used as effective trans-metalating agents for several
1
1,12
tricationic organometallic cage (OC-Ag). Subsequent treatment of late-transition metals.
To date, several Ag(I)– and Au(I)–NHC
the organometallic cage (OC-Ag) with 1,4-diaminobutane links the complexes with variation of NHC ligands have been reported.
two tris-NHC ligands via imine bond condensation, which thereby However, only a handful of Ag(I)-NHC and Au(I)-NHC bond
1
2b
generates a 3D ‘cationic-cage’ (CC-Ag). Furthermore, post-synthetic driven macrocycles and cages are known.
replacement of the Ag(I) by Au(I) leading to the formation of CC-Au Parallel to the rapid growth of NHCs, dynamic covalent
1
3,14
was achieved via trans-metalation with the retention of the chemistry (DCC)
has emerged as an efficient method for the
molecular architecture.
easy construction of architectures having different shapes and
sizes. This advanced synthetic protocol relying on reversible
formation/cleavage of dynamic linkages has great importance
in the preparation of molecular architectures containing purely
organic building blocks. Moreover, the topological diversity of
the precursors/ligands has allowed the rapid preparation of a
variety of organic architectures that include molecular squares,
Metallo-supramolecular architectures1 have drawn special
interest over the past two decades due to their extensive range
of applications in molecular recognition, supramolecular
catalysis, drug delivery, metallo-pharmaceuticals and light-
emitting materials. For these purposes, several metallacycles
and metallacages having diverse sizes and shapes have been
prepared and well investigated. Such metallo-supramolecular
architectures are synthesized using ligands with two or more
binding domains separated by organic spacers in combination
with suitable metal fragments. Most of the known multitopic
ligands are based on pyridyl/carboxylate/imidazole/tetrazole
donors which have been employed with appropriate transition
metal-based acceptors.
With the evolving interest in the coordination chemistry of N-
heterocyclic carbenes (NHCs), discrete supramolecular
complexes held together by M−CNHC bonds have gained special
interest in recent time. The construction of such metallo-
supramolecular assemblies requires the synthesis of suitable
poly-NHC ligands to form labile bonds with metal centres for the
formation of thermodynamically most stable product. Among
13
tetrahedron, octahedron and others. Additionally, dynamic
2
-9
imine bonds are exploited in ‘error checking’ and ‘self-
14c
correction’ while seeking for the thermodynamic minimum.
Herein, we report a new class of compounds (CC-Ag and CC-Au)
obtained employing both the above-mentioned synthetic
protocols in stepwise fashion (Scheme 1). To the best of our
knowledge, our present systems (CC-Ag and CC-Au) represent
the first examples of three-dimensional “cationic-cage”
obtained by applying M−CNHC bonds formation followed by
dynamic imine bond chemistry. This new method to the
preparation of complex synthetic molecular architectures under
relatively benign conditions is relatively simple and certainly
unprecedented.
1
0
a. Department of Inorganic and Physical Chemistry, Indian Institute of Science,
Bangalore 560012, India. Email: psm@iisc.ac.in
†
Electronic supplementary information (ESI) available: Experimental procedures,
NMR, ESI-HRMS, computational details, CCDC 1901315. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/x0xx00000x
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Journal Name
aldehyde H
3
L(PF
6
)
3
with silver oxide (Ag
2
O) in acetonitrile led to
View Article Online
DOI: 10.1039/C9CC02341K
the formation of a semi-flexible tri-cationic organometallic cage
1
7
(
[
OC-Ag). The 3D-cage having a cylindrical shape of type
Ag L ] where those three N3, N3’, N3’’ substituted pendant
3 2
3
+
aldehyde groups were arranged nearly in gauche fashion.
Furthermore, imine condensation between OC-Ag and 1,4-
diaminobutane resulted exclusively one single product
“cationic-cage” (CC-Ag) (Fig. 2). This complex CC-Ag has two
different sized cages in the same structure. The inner cage is
formed via metal-carbene (M−CNHC) bond formation where the
outer cage was formed through dynamic imine bond chemistry.
The reaction of 2 equivalents of H
3
L(PF
6
)
3
with 3 equivalents of
o
2
Ag O in acetonitrile at 60 C for 24h under exclusion of light
resulted in the formation of the trinuclear Ag(I) hexacarbene
complex (OC-Ag) in excellent yields of 70 % as off-white solid
powder (Scheme 1). The formation of the Ag-NHC bond directed
molecular cage (OC-Ag) was confirmed by NMR spectroscopy
Scheme 1 Template synthetic routes for the synthesis of organometallic cage (OC-
Ag) and “cationic-cage” (CC-Ag).
1
13
1
(
H, C, 2D NMR) and HR-ESI mass spectrometry. The H NMR
Most of the imine-based cages are neutral in nature and thus
spectra of the complex OC-Ag no longer features resonance
peak at  = 9.09 ppm for the N-CH-N proton of the parent
1
3-15
their solubility is mainly restricted to organic solvents.
Design of charged organic cages is always a challenging task. For
this purpose, use of charged aldehyde is expected to be a
reliable approach. Recently, Hann and co-workers developed a
photochemical [2+2] cycloaddition reaction utilizing N-olefin-
substituted N-heterocyclic carbenes (NHCs) coordinated to
selected metal templates to prepare cationic metallacycles and
metallacages.16 However, our template approach is entirely
different from known approaches incorporating imine
condensation in combination with M-NHC bonding for the first
time to obtain a charged organic cationic-cage compound. The
†
13
1
imidazolium precursor (Fig. S6, ESI ). The C{ H} spectra
†
showed CNHC resonance peak at  = 180.7 ppm (Fig. S7, ESI ) for
†
OC-Ag in CD
in the parent H
range that previously observed for [Ag(I)--CNHC] complexes.
3
CN which was earlier at  = 136.6 ppm (Fig. S4, ESI )
3
6 3
L(PF ) . These experimental values fall in the
1
6,17
The HR-ESI mass spectra (positive ions +1 and +2) feature the
†
strong signals (Fig. S10 and S11, ESI ) at m/z = 1875.1267 (calcd
1
875.1277) and m/z = 865.0804 (calcd 865.0784) respectively,
confirming the formation of the 3D-organometallic cage (OC-
Ag).
trisimidazolium salt H
N-phenyl-aldehyde pendants was used as a charged aldehyde
Scheme 1). Treatment of this trialdehyde with 1,4-
3 6 3
L(PF ) functionalized with three flexible
(
butanediamine yielded an insoluble product, presumably due to
the formation of polymer instead of expected discrete cationic
[
2+3] imine condensed cage.
3 6 3
The precursor tri-cationic trialdehyde H L(PF ) was synthesized
by the reaction between 1,3,5-tri(1H-imidazol-1-yl)benzene
with 4-(bromomethyl) benzaldehyde in dry DMF for 72h under
†
nitrogen atmosphere (Scheme S1, ESI ). Subsequently, counter
anion exchange by NH
triimidazolium trialdehyde H
4
PF
6
in aqueous medium yields the
L(PF as white solid in 93% yield.
3
6 3
)
Fig. 1 (a) X-ray crystal structure of organometallic cage (OC-Ag) and (b) DFT
optimized structure of “cationic-cage” (CC-Ag).
Moreover, the triimidazolium ligand precursor was
characterized by NMR spectroscopy ( H, 13C) and high-
1
resolution electrospray ionization (HR-ESI) mass spectrometry
Single crystals for an X-ray diffraction study were obtained by
slow vapor diffusion of diethyl ether into a saturated solution of
OC-Ag in acetonitrile at room temperature. X-ray diffraction
†
(
Fig. S3, S4 and S5, ESI ). Treatment of the aldehyde H
L(PF )
3 6 3
with 1.5 equivalents of 1,4-diaminobutane at room
temperature in CD CN forms some insoluble oligomers and
polymers (Scheme 1). The outcomes were solely due to the
flexible nature of -CH groups linked with the N-imidazolium
3
3
+
analysis confirmed that the tri-cationic complex [Ag
consists of three Ag ions sandwiched in between two trigonal
3
(I)L
2
]
+
2
tri-carbene ligands (Fig. 1). The CNHC-Ag-CNHC angles [173.52–
centre which leads to form some imine based polymeric
products.
To overcome this problem of polymer formation, we employed
o
1
76.02 ] and Ag-CNHC [2.051–2.093 Å] distances fall in the range
reported previously for related silver polycarbene assemblies.
1
6,17
The two central benzene rings are separated by 4.9 Å, and
a new template method where both metal-carbene (M−CNHC
)
the three Ag--Ag separations are almost equidistant and
bond formation and dynamic imine bond chemistry were used
in a stepwise fashion (Scheme 1). Treatment of the flexible tri-
†
measured to be 5.9 Å (Fig. S1, ESI ).
2
| J. Name., 2012, 00, 1-3
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I
Moreover, the trinuclear OC-Ag was tested for post-synthetic point energy it was found that structure CC-Ag is more stable
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II
DOI: 10.1039/C9CC02341K
modification via imine condensation to form a “cationic-cage” than CC-Ag by ~21 Kcal/mol (Table S3). These computational
CC-Ag). Upon treating 1 equivalent of OC-Ag with 3.1 results were supported by the 2D-NMR result. For example, if
equivalents of flexible 1,4-diaminobutane in acetonitrile-D or we compare the distance between the protons of imidazole and
DMSO-D at room temperature for 24h yielded the imine the 4-formylphenyl moieties (protons d and f) we can see that
bonded trinuclear Ag(I) hexacarbene complex CC-Ag
(
3
6
II
.
Formation in case of CC-Ag the distance is considerably shorter (~2.7 Ǻ)
I
1
1
of the [1+3] imine condensed charged CC-Ag was established by than that in case of CC-Ag (~4.2 Ǻ). Therefore, in the H- H
NMR spectroscopy ( H, 2D NMR) and HR-ESI mass NOESY we expect to observe a NOE between these two protons.
spectrometry. The H NMR spectrum of CC-Ag no longer But, experimental spectra (Fig. S15) did not display such NOE
features peak at  = 9.93 ppm corresponding to pendant interaction, which indirectly confirms the formation of CC-Ag in
1
1
I
aldehyde proton of the OC-Ag. Moreover, appearance of new the solution.
peak at  = 8.23 ppm (Fig. 2) clearly attributed to the imine bond
1
3
1
formation in CD
3
CN. C{ H} spectrum showed Cimine resonance
†
peak at  = 153.3 ppm (Fig. S18, ESI ) for CC-Ag in DMSO-D
which was earlier at  = 193 ppm (Fig. S7, ESI ) in the parent OC-
Ag for aldehyde group in CD
6
†
1
3
CN. Furthermore, H DOSY NMR of
CC-Ag shows single diffusion coefficient value -10.16 with
hydrodynamic radius of 1.44 nm whereas the parent OC-Ag
shows single diffusion coefficient value of -9.19 with
†
hydrodynamic radius of 0.86 nm (Fig. S8 and S17, ESI ). This
increase in hydrodynamic radius is due to tethering of flexible
aldehyde groups by imine condensation. Finally, the HR-ESI
mass spectrum (positive ions +2 and +3) of complex CC-Ag
†
features the strong signals (Fig. S21 and S22, ESI ) at m/z =
9
43.1987 (calcd 943.1985) and m/z = 580.4722 (calcd 580.4784)
respectively, which clearly confirm the formation of the
tricationic trinuclear Ag(I)-imine cage (CC-Ag).
Scheme 2 Synthetic routes for the transmetalation of tricarbene-derived CC-Ag.
Transmetalation is a common procedure for the transfer of NHC
bound silver (I) complexes to other higher transition metal
1
8
congeners. The gold complexes were prepared as they are less
light-sensitive than the silver complexes. The reaction of 1 equiv
of CC-Ag with
tetrahydrothiophene) in DMSO-D
4
equivalents of [Au(THT)Cl] (THT=
at ambient temperature for
6
1
24h exclusively yielded the homotrinuclear Au(I) hexacarbene
3 6 3 3
Fig. 2 H NMR spectra of H L(PF ) (1), OC-Ag (2), and CC-Ag (3) in CD CN.
complex (CC-Au). The formation of the gold complex (CC-Au)
1
was confirmed by NMR spectroscopy ( H, 2D NMR) and HR-ESI
Several attempts to grow single crystals of the newly
synthesized CC-Ag remained unsuccessful. Therefore, the
structural details of CC-Ag were obtained through various 2D
1
mass spectrometry. The H NMR spectrum of the complex (CC-
Au) exhibited -CH
ESI ) in DMSO-D
2
broad peak at  = 5.47-5.55 ppm (Fig. S23,
, which are shifted slightly downfield
compared to the -CH peaks in parent CC-Ag. Also, C{ H}
2
spectrum showed CNHC resonance peak at  = 182.6 ppm (Fig.
which was earlier at  = 180.1
ppm (Fig. S18, ESI ) in the parent CC-Ag. These experimental
values fall in the range that previously observed for [Au(I)--CNHC
The HR-ESI mass spectrum (positive ions +2 and
3) features the strongest signals (Fig. S26 and S27, ESI ) at m/z
= 1076.8012 (calcd 1076.7975) and m/z = 669.5369 (calcd
669.5381) respectively which clearly confirmed the complete
transformation of “Ag cationic-cage” to “Au cationic-cage” (CC-
Ag →CC-Au). H DOSY NMR of CC-Ag and CC-Au showed that
the diffusion coefficient values were -10.16 and -10.18 which
†
†
1
6
NMR spectroscopy (Fig. S13-S15, S17-S20, ESI ) such as H DOSY,
1
3
1
1
1
1
1
H- H COSY and H- H NOESY along with DFT calculations. From
the crystal structure of the OC-Ag it can be clearly seen that the
phenyl moieties containing the aldehyde group are extremely
flexible in nature. Therefore, this conformational flexibility may
allow the aldehyde groups to react with the flexible diamine in
various ways: 1) adjacent aldehyde moieties from the same
aldehyde ligand, 2) aldehyde moieties connected to the same
†
S25, ESI ) for CC-Au in DMSO-D
6
†
]
1
6,17
complexes.
+
†
I
Ag(I) centre (CC-Ag ), and 3) aldehyde moieties that are
II
connected to two different Ag(I) centres (CC-Ag ). All the
possible modes of aldehyde-aldehyde linking were attempted
1
†
to model (Fig. S29, ESI ); although, the structure 1) could not be
optimized. Structures 2) and 3) were optimized. From the single
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J. Name., 2013, 00, 1-3 | 3
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2
073; (c) T. Tateishi, S. Kai, Y. Sasaki, T. Kojima,ViSew. TAratikclaehOanslinhei
provide their hydrodynamic radii of 1.44 and 1.50 nm
†
and S. Hiraoka, Chem. Commun., 2018,
54, 7758-7761.
DOI: 10.1039/C9CC02341K
respectively, in DMSO-D
6
(Fig. S17 and S24, ESI ). These results
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E. C. Constable, Chem. Soc. Rev., 2007, 36, 246-253; (c) S. Y.
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indicate that both the structures were comparable which
advocates the successful transmetallation with the retention of
core 3D-architecture.
2
018, 24, 9274-9284.
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In summary, we have designed and synthesized a shape-
obstinate 3D “cationic-cage” compound (CC-Ag) by employing
metal-carbene (M−CNHC) bond formation followed by dynamic
imine bond chemistry. We have demonstrated that two trigonal
tricarbene ligands in Ag-NHC complex decorated with terminal
N, N’, N’’-aldehyde pendants can be linked via imine
condensation to form a 3D cationic cage (CC-Ag). This reaction
requires the preorganization of the two tricarbene ligands
2
Soc. Rev., 2015, 44, 815-832; (c) L. R. Holloway, P. M. Bogie, Y.
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aldehyde substituents. Also, a successful transmetallation was
observed via “CC-Ag →CC-Au” transformation with the
retention of the molecular architecture. Surprisingly, the
cationic trialdehyde alone didn’t yield expected [2+3]
condensed imine cage, which was only formed upon formation
of intermediate organometallic complex OC-Ag. Our results
demonstrate the crucial role of metal templates in the
realization of the imine condensation reaction for exclusive
product formation. This facile two-step synthetic strategy
1
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cationic polyimidazolium architectures by varying the nature of
aldehyde and amine.
P. S. M. is grateful to CSIR-New Delhi (India) for financial
support. R.M. and B. M. acknowledge the SERB-New Delhi
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5
54-557.
Conflicts of interest
There are no conflicts to declare.
1
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Page 5 of 5
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Journal Name
COMMUNICATION
Self-assembly of a “Cationic-Cage” via
formation of Ag-carbene bonds followed
by imine condensation
View Article Online
DOI: 10.1039/C9CC02341K
a
a
a
Ritwik Modak, Bijnaneswar Mondal, Prodip Howlader
a
and Partha Sarathi Mukherjee *
We develop a new strategy for the synthesis of a “cationic-
cage” (CC-Ag) via metal-carbene (M−CNHC) bond formation
followed by imine condensation. While the aldehyde alone
doesn’t yield the desired cage with the amine, Ag-NHC bond
formation allows such condensation leading to the formation
of a “cationic-cage”.
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