Cage-to-Cage Cascade Transformations

Article abstract of DOI:10.1002/chem.201602039

A series of Pd₂L₄-type binuclear self-assembled coordination cages (1–4), where L stands for a nonchelating bidentate ligand, were prepared. The strategies adopted for the synthesis of the cages were: combination of Pd^IIwith 1) a selected ligand or 2) subcomponents of the ligand. Highly efficient cage-to-cage transformation reactions are demonstrated by suitable covalent modification (from 1 to 2 or 3 or 4) or ligand-exchange reactions (from 1 to 2 or 3 or 4; from 2 to 3 or 4). Thus, new cascade transformations (from 1 to 2 to 3; from 1 to 2 to 4) are achieved beautifully.

Full text of DOI:10.1002/chem.201602039

DOI: 10.1002/chem.201602039
Communication
&
Reorganization Reactions |Hot Paper|
Cage-to-Cage Cascade Transformations
Sreenivasulu Bandi and Dillip Kumar Chand*[a]
Dedicated to Prof. P. K. Bharadwaj on the occasion of his retirement
bone; these can be derivatized with a chosen organic frag-
ment.[4] However, alterations/exchange of the metal, ligand,
counter anion or guest molecule are more commonly used for
post-modification.[4–9]
Abstract: A series of Pd2L4-type binuclear self-assembled
coordination cages (1–4), where L stands for a nonchelat-
ing bidentate ligand, were prepared. The strategies adopt-
ed for the synthesis of the cages were: combination of
PdII with 1) a selected ligand or 2) subcomponents of the
ligand. Highly efficient cage-to-cage transformation reac-
tions are demonstrated by suitable covalent modification
(from 1 to 2 or 3 or 4) or ligand-exchange reactions (from
1 to 2 or 3 or 4; from 2 to 3 or 4). Thus, new cascade
transformations (from 1 to 2 to 3; from 1 to 2 to 4) are
achieved beautifully.
In some cases of ligand-exchange reactions the bound
ligand units of a coordination complex are replaced with a re-
quired number of a suitable incoming ligand[5] or a coordinat-
ing fragment of the already coordinated ligand is covalently re-
placed by an incoming fragment of similar functionality in a dy-
namic covalent space (defined as subcomponent self-assembly
by Nitschke).[2–3] Transmetallation is another dynamic process
that could be considered a post-modification.[6] Transmetalla-
tion with or without structural change of a chosen self-assem-
bled compound is well known[6d] and included under disrup-
tive or direct exchange. Exchange of the counter anion of
a self-assembled cage is known to either retain the core archi-
tecture or induce structural change, forming yet another cage
of higher or lower nuclearity.[7] Post-modification induced by
a suitable guest[8] or stimuli, such as light,[9] has also been
shown successfully. The dynamic nature of the metal–ligand in-
teractions helps only when there is a requirement of metal–
ligand bond breaking and making. However, pure covalent
modifications or rearrangement without touching the metal–
ligand bonds does not really require the dynamic nature of the
metal–ligand bonds. Most of the post-modification phenom-
ena described above could be defined as cage-to-cage trans-
formations.
Coordination-driven self-assembly has been well recognized as
an efficient strategy for the construction of a vast range of
metallo-supramolecular architectures. The well-established
classical self-assembly[1] and relatively new subcomponent self-
assembly[2,3] routes are typically adopted for preparation of
such architectures. Thermodynamically favored self-assembled
coordination cages are often the final products of these com-
plexation reactions, because self-healing of incorrectly formed
bonds is facilitated by the dynamic nature of metal–ligand in-
teractions.[1] Post-self-assembly modification[4–9] of already self-
assembled coordination cages has been a recent trend of con-
siderable significance in the realm of supramolecular coordina-
tion chemistry.
The classical self-assembly route involves combination of
a chosen ligand with a suitable metal component under ap-
propriate reaction conditions to afford the targeted assembly.[1]
In the subcomponent self-assembly route, in situ synthesis of
the ligand is carried out in presence of the metal component
so that the self-assembly phenomenon could take place in
one-pot. Thus, the steps involved in the synthesis and isolation
of the ligand is avoided.[2,3] In the post-self-assembly modifica-
tion route, a suitably fabricated pre-prepared self-assembled
compound is subjected to alteration. The most studied phe-
nomenon among the post-modification strategies has been co-
valent modification of the back-bone of a self-assembled coor-
dination complex. For such modifications, it is necessary to
have suitable functional groups anchored at the ligand back-
Complexation of PdII with suitably designed bidentate non-
chelating ligands is known to afford binuclear Pd2L4-type self-
assembled coordination-cage molecules.[1,10] The ligands with
coordination vectors almost parallel to each other and point-
ing in the same direction are ideal designs for the construction
of these binuclear architectures.
In this work we have used Pd2L4-type coordination-cage
molecules to study the cage-to-cage transformation phenom-
enon (Figure 1 and Scheme 1). We have conceptualized the
phenomenon of covalent modification of a coordination cage
by a direct condensation reaction of the coordinating atoms of
the cage with a chosen incoming organic functionality in such
a manner that the ligation loyalty is either retained (Figure 1a
and Scheme 1a) or transferred (Figure 1c,e and Scheme 1c,e).
In consequence, the size of the cavity is either retained or ex-
panded. Ligand-exchange reactions were also performed by
choosing suitable incoming ligands so that the cavity size is
either retained (Figure 1b and Scheme 1b) or expanded (Fig-
ure 1d,f and Scheme 1d,f). The cage-to-cage transformations
in the steps (a) and (b) could be continued further in a cascade
[a] S. Bandi, Prof. Dr. D. K. Chand
Department of Chemistry, Indian Institute of Technology Madras
Chennai 600036 (India)
E-mail: dillip@iitm.ac.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201602039.
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Figure 3. Energy-minimized structures of the ligands L1–L4 (from left to
right) showing the bent conformations and structure of L4 in the compara-
ble conformation.
porting Information (see Section S1). The energy-minimized
structures of ligands L1–L4 are shown in the Figure 3; these
were calculated with the Gaussian 09 software package.[12] The
ligands adopt an overall bent shape with one of the arms in
an extended conformation and the other somewhat retracted
as shown in the chemical drawing of L4 in Figure 3. The con-
formations of the ligands in the corresponding Pd2L4 com-
plexes are worthy of comparison and discussed in a later sec-
tion.
Figure 1. Cartoon diagram showing the cage-to-cage transformations in
a one-step manner through covalent modifications (a)/(c)/(e), or ligand-ex-
change reactions (b)/(d)/(f)/(g)/(h). The two-step cascade transformations are
represented as step (a) followed by step (g) or (h), and step (b) followed by
step (g) or (h). Steps (a)–(h) are comparable to the same in Scheme 1.
Although L1 is commercially available and the crystal struc-
ture of L3 has been reported recently,[13] both of these com-
pounds have not been explored for complexation with metal
ions. Ligand L2 displays CÀH activation when reacted with
Fe2(CO)9 to form organometallic complexes,[14] whereas ligand
L4 has been used for complexation with Cu(NO3)2 for prepara-
tion of 1D coordination polymers.[15] The smaller ligands, L5
and L6, have not been explored for complexation with metal
ions. Compounds L1, L2, and L4 are typical bidentate nonche-
lating ligands for which the coordinating atoms are a pair of
well-separated amine, imine, and pyridine nitrogen centers, re-
spectively. Ligand L3 has four binding sites, that is, two imine
and two terminal pyridine nitrogen centers. Thus L3 could act
as a bidentate nonchelating ligand by utilizing either the imine
pair or the pyridine pair; actually the pyridine set is found to
coordinate with PdII.
fashion, for example, through ligand-exchange reactions as
shown in the step (g) or (h).
Ligands L1–L4 considered for complexation reactions with
PdII, to prepare Pd2L4-type cages, are shown in Figure 2. Anoth-
er relevant compound L2a (not a ligand) and two other mono-
dentate ligands L5 and L6 are also shown in Figure 2. Ligands
L2–L4 are crafted with a pair of arms that are shown extended
and wide open (Figure 2). Ligands L2–L4 and compound L2a
were prepared from commercially available bis(4-aminophe-
nyl)methane, L1, and other appropriate reagents either by
using literature methods,[11] sometimes with slight modifica-
tions. Thus, condensation of L1 with benzaldehyde, benzoyl
chloride, nicotinaldehyde, or nicotinoyl chloride resulted in L2,
L2a, L3, and L4, respectively. Nicotinoyl azide can be used in-
stead of nicotinoyl chloride for the preparation of L4. The ex-
perimental details of the synthesis are provided in the Sup-
Complexation of Pd(NO3)2 with ligands L1–L4 at a ratio of
2:4 resulted in the quantitative formation of the Pd2L4-type
cages 1–4, respectively. The general formula of these com-
plexes is [Pd2(L)4](NO3)4, where L stands for the ligand. Similarly,
complexation of Pd(NO3)2 with the monodentate ligands L5
and L6 at a ratio of 1:4 resulted in the Pd1L4-type complexes
5–6, with the general formula [Pd(L)4](NO3)2. The chemical
structures of complexes 1–6 can be seen in the Scheme 1.
Schemes outlining the syntheses, through the above-men-
tioned classical self-assembly protocol, are provided in the
Supporting Information (Supporting Information Schemes S1–
S6).
All six complexes (1–6) were characterized by recording
1
their H NMR, 13C NMR, H-H COSY, C-H COSY, DOSY, and ESI-MS
Figure 2. Ligands L1–L6 and compound L2a.
data (see the Supporting Information Figures S1–S24, and S44–
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Scheme 1. Transformation of cage 1 by covalent modification/ligand-exchange reactions to form: (a)/(b) cage 2, (c)/(d) cage 3 and (e)/(f) cage 4. Transforma-
tion of cage 2 by ligand-exchange reactions to form: (g) cage 3 and (h) cage 4. Covalent modification of 5 and 6 to form: (i) cage 3 and (j) cage 4, respective-
ly. The cascade transformation reactions are represented by the conversion of cage 1 to 2 to 3 (or 4).
1
S48). The H NMR spectra of ligands L1–L4 and complexes 1–4
are provided in Figure 4. The energy-minimized structures of
complexes 1–4 are shown in Figure 5; these were obtained by
DFT methods by using the Gaussian 09 software package.[12]
The conformations of ligands L2–L4 in the free and bound
states (i.e., in complexes 2–4) are worth noting. Whereas L2
moieties adopted a fully extended conformation in the struc-
ture of complex 2, albeit with a slim cavity, the arms of L3 and
L4 are fully retracted, resulting in wider cavities of 3 and 4.
The single crystal X-ray structure of complex 4 unequivocally
confirmed the Pd2L4 architecture of the molecule with retract-
ed arms (Figure 6) in line with the energy-minimized structure
of 4.
1
The H NMR spectra of complexes 1–6 are compared with
the corresponding free ligands L1–L6 (Figure 4, and Figures S1,
S5, S9, S13, S17, and S21 in the Supporting Information). Com-
plexation-induced changes in the chemical shifts of the deci-
sive signals are summarized here. The downfield and upfield
shifts are designated with plus (+) and minus (À) signs, re-
spectively. The NH2 protons in 1 are shifted by 2.1 ppm, indi-
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complexation of the pyridine units, that is, 0.8 ppm shift for Ha
protons. Complexes 5 and 6 showed similar complexation-in-
duced shifts of the pyridine protons.
The DOSY spectra of complexes 1, 2, 3, and 4 showed single
bands in each case, corresponding to single species. The diffu-
sion coefficient, D, for compounds 1–4 are 1.115ꢁ10À10
,
1.752ꢁ10À10, 9.723ꢁ10À11, and 8.280ꢁ10À11 m2 sÀ1, respectively.
The radius of the solvated cages (often referred to as hydrody-
namic radius) 1, 2, 3, and 4 were calculated. The shapes of 1,
3, and 4 were considered spherical and that of 2 as spheroidal.
Accordingly, suitable equations[16] were used for the calculation
of the radius of the molecules. The calculated radiuses of 1–4
are 9.84, 9.47, 11.28, and 13.25 ꢂ, respectively. Thus, the sizes
of cages 1 and 2 are comparable, and so are those of 3 and 4
as well. However, the sizes of 1 and 2 are smaller than those of
3 and 4. The DOSY spectra and detailed calculations are pro-
vided in the Supporting Information (Figures S44–S48 and Sec-
tion S3).
ESI-MS data of complexes 1–4, support the Pd2L4 formula-
tions of the architectures (see the Supporting Information, Fig-
ures S4, S8, S12, S16), whereas the same data for 5 and 6 indi-
cate mononuclear Pd1L4-type complexes (see the Supporting
Information, Figures S20, S24). The peak patterns at m/z=
564.59 for the fragment [1À2NO3]2+; m/z=917.06 for the frag-
ment [2À2NO3]2+; m/z=593.49 and 429.62 corresponding to
1
Figure 4. 400 MHz H NMR spectra in [D6]DMSO for (i) L1; (ii) [Pd2(L1)4](NO3)4,
1; (iii) L2; (iv) [Pd2(L2)4](NO3)4, 2; (v) L3; (vi) [Pd2(L3)4](NO3)4, 3; (vii) L4; and
(viii) [Pd2(L4)4](NO3)4, 4.
the fragments [3À3NO3]3+and [3À4NO3]4+
; m/z=985.31,
636.26 and 461.68 corresponding to the fragments [4À2NO3]2+
, [4À3NO3]3+ and [4À4NO3]4+; m/z=267.08 and 349.03 corre-
sponding to the fragments [5À2NO3]2+ and [6À2NO3]2+ are
found to be comparable to the theoretically calculated isotopic
peak patterns (see the Supporting Information, Figures S4a,
S8a, S12a, S16a).
We have devised domino reaction conditions by utilizing the
concept of the dynamic subcomponent self-assembly process
to prepare complexes 2–4. Thus, the ligands (L2–L4) were syn-
thesized in the presence of Pd(NO3)2 to afford the targeted
complexes (see the Supporting Information, Schemes S7–S9).
For instance, a mixture of Pd(NO3)2, L1, and benzaldehyde at
a ratio of 2:4:8 resulted in complex 2. The use of nicotinalde-
hyde or nicotinoyl azide instead of benzaldehyde resulted in
complexes 3 or 4, respectively. The progress of the domino re-
1
actions were monitored by H NMR spectroscopy. No major dif-
ferences can be seen between the 1H NMR spectra of com-
plexes 2–4 prepared by the domino methods (see the Sup-
porting Information, Figures S25–S27) and the same complexes
prepared by usual self-assembly methods.
Figure 5. Energy-minimized structures of the complexes 1–4.
After characterization of the complexes, the cage-to-cage
transformations through the covalent modifications and
ligand-exchange reaction routes were probed (Scheme 1). In
the covalent-modification method, the pre-prepared cage 1,
with the amine nitrogen atoms coordinated to the PdII centers,
was allowed to react with eight equivalents of benzaldehyde,
nicotinaldehyde, or nicotinoyl azide to exclusively prepare the
corresponding complexes 2–4 (Figure 1 and Scheme 1). The
schematic representation and 1H NMR spectra of the com-
plexes are given in the Supporting Information (Schemes S10–
S12, Figures S28–S30). The amine functionality of 1 was cova-
cating the coordination of an amine nitrogen atom to the
metal center. The CH=N protons in 2 are shifted by 1.4 ppm,
indicating the coordination of a Schiff base nitrogen atom to
the metal center. The CH=N protons in 3 are marginally shifted
by À0.1 ppm, indicating that there is no interaction between
the Schiff base nitrogen atom and the metal center. However,
the pyridine-ring protons of 3 showed a very common com-
plexation-induced shift, that is, 1.0 ppm for Ha protons. Similar-
ly, the shift of the pyridine-ring protons in 4 also supports
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lently modified to form a Schiff base or an amide group by re-
action with suitable functional groups. Simultaneous reorgani-
zation also happened during the covalent modifications. The
nitrogen center of L1, which is responsible for coordination in
1, retained the ligation loyalty even after complex 1 was react-
ed with benzaldehyde, because no other coordination sites are
available in the covalently modified ligand moiety. Thus, the
imine functionality was involved in coordination and formation
of complex 2. However, upon reaction of 1 with nicotinalde-
hyde, the coordination site got shifted in favor of the pyridine
nitrogen atoms (and not the imine nitrogens) to form 3. When
complex 1 was reacted with nicotinoyl azide, the coordination
site was shifted to the only possible pyridine nitrogen atoms
to form 4, because the amide groups formed were inefficient
for complexation with PdII. Thus, the ligation loyalty was either
retained or transferred, depending on the nature of the func-
tional group generated via covalent modification. Reaction of
1 with benzoyl azide, however eliminated the ensuing com-
pound L2a due to concomitant decomplexation (see the Sup-
porting Information, Scheme S13, Figure S31).
by simply combining L1 with nicotinoyl azide, where upon
ligand L4 was formed (see the Supporting Information,
Scheme S25, Figure S43).
The geometries of the reactant and product molecules were
optimized and the frequencies were calculated at the B3LYP/6-
31G* level of theory (see the Supporting Information, Fig-
ure S53). The overall Gibbs free energies (DG) and the enthal-
pies (DH) for the formation were also calculated and support
the formation of all experimental products (see the Supporting
Information, Table S3), except for the formation of 2. A brief
discussion is provided in the Supporting Information (Sec-
tion S5). The energy-minimized structures of the complexes
can be seen in Figure 5.
Single crystals of compound 4 suitable for X-ray diffraction
data collection were grown by slow diffusion of ethyl acetate
into a dimethyl sulfoxide solution of 4. The crystal structure of
4 reveals that four units of the ligand is coordinated with two
units of PdII ions in a square planar geometry. The carbonyl
groups point towards the inner side of the cavity of 4. The
PdÀN bond lengths in the complex span the range of 2.009–
2.035 ꢂ and the cis-N-PdÀN bond angles span the range of
88.03–91.248 (Figure 6a). Further details on the crystal struc-
ture are briefly discussed in the Supporting Information (Sec-
tion S6).
Complexes 1–4 were probed for the ligand-exchange reac-
tions by combining a given complex with four equivalents of
ligands of another variety (Figure 1 and Scheme 1). The sche-
1
matic representation and H NMR spectra of the complexes are
given in the Supporting Information (Schemes S14–S18, Figur-
es S32–S36). Cage 1 was allowed to react with four equivalents
of L2, L3, or L4 where upon complexes 2–4 are formed, re-
spectively. This could happen smoothly as ligand L1 was re-
leased in each experiment in favor of the incoming ligands.
Likewise, combination of complex 2 with four equivalents of
L3 or L4 lead to the release of L2 in both cases and made way
for complexes 3 and 4, respectively. However, combination of
3 with L4 (see the Supporting Information, Scheme S19, Fig-
ure S37) or 4 with L3 resulted in a mixture of products. Thus,
the order of binding abilities of the ligands with PdII can be
considered as L1<L2<L3ꢀL4. A mixture of Pd(NO3)2, L1, and
L2 (or L3, or L4) at a ratio of 2:4:4 resulted complex 2 (or 3, or
4) and L1 remained free. Similarly, combination of Pd(NO3)2,
L2, and L3 (or L4) at a ratio of 2:4:4 resulted complex 3 (or 4)
and L2 remained free (see the Supporting Information,
Scheme S20, Figure S38). However, a combination of Pd(NO3)2,
L3, and L4 at a ratio of 2:4:4 resulted in a mixture of products
with no particular preference to any of the ligands (see the
Supporting Information, Scheme S21, Figure S39). An equimo-
lar mixture of 3 and 4 also resulted in a mixture of products
(see the Supporting Information, Scheme S22, Figure S40).
Thus, the cavity size of the cage before and after the ligand-ex-
change reactions are either unchanged or changed, depending
on the nature of incoming ligand.
Figure 6. (a) Crystal structure of 4 (the counter anions and solvent molecules
are omitted for clarity) and (b) energy-minimized structure of
[C60ꢁPd2(L4)4]4+
.
Cages 3 and 4 were considered suitable, for encapsulation
of C60. This was presumed on the basis of the size, shape, and
internal chemical environment of the cages. Preliminary inves-
1
tigation by H and 13C NMR techniques (see the Supporting In-
formation, Figures S48–S51) indicates the intended host–guest
interactions. The 1H NMR signals of the host molecules are
found to be shifted upfield. The sole 13C NMR signal of the
guest molecule was also found to be shifted upfield. These
changes are in line with the literature,[17] where binding of C60
in the cavity of 2D and 3D metallocages were successfully
demonstrated. The energy-minimized structure of the host–
guest complexes are shown in Figure 6b and in the Support-
ing Information, Figure S52.
Covalent modification of the mononuclear complexes 5 and
6 were probed by using ligand L1. Complexes 3 and 4 were
thus prepared by this alternative route by combining four
equivalents of L1 with two equivalents of 5 and 6 to afford
complexes 3 and 4, respectively (Scheme 1, and in the Sup-
porting Information, Schemes S23–S24, Figures S41–S42).
During the covalent-modification routes, in situ formations of
ligand moieties L3 and L4 occur. One such case was achieved
In summary, a series of Pd2L4-type, binuclear, self-assembled,
coordination cages were prepared by metal–ligand complexa-
tion through classical self-assembly or domino routes through
subcomponent self-assembly. Cage-to-cage transformations of
these cages were carried out by covalent modifications or
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D. K. C. thanks the SERB, Department of Science and Technolo-
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Keywords: cage-to-cage transformation
reorganization · self-assembly · subcomponents
·
palladium
·
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Received: May 1, 2016
Published online on && &&, 0000
&
&
Chem. Eur. J. 2016, 22, 1 – 7
www.chemeurj.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Communication
COMMUNICATION
Let’s cascade! Cage-to-cage transforma-
tion reactions of Pd2L4-type (L is a non-
chelating bidentate ligand) coordination
cages are achieved by covalent modifi-
cations or ligand-exchange reactions of
suitable pre-prepared complexes. The
palladium atoms retain their positions
or move away to increase the cavity
size depending on the nature of the
modification/exchange.
&
Reorganization Reactions
S. Bandi, D. K. Chand*
&& – &&
Cage-to-Cage Cascade
Transformations
Chem. Eur. J. 2016, 22, 1 – 7
www.chemeurj.org
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ