Solvent-free ball-milling subcomponent synthesis of metallosupramolecular complexes

Article abstract of DOI:10.1002/chem.201500734

Subcomponent self-assembly from components A, B, C, D, and Fe²⁺ under solvent-free conditions by self-sorting leads to the construction of three structurally different metallosupramolecular iron(II) complexes. Under carefully selected ball-milling conditions, tetranuclear [Fe₄(AD₂)₆]^4- 22-component cage 1, dinuclear [Fe₂(BD₂)₃]^2- 11-component helicate 2, and 5-component mononuclear [Fe(CD₃)]²⁺ complex 3 were prepared simultaneously in a one-pot reaction from 38 components. Through subcomponent substitution reaction by adding subcomponent B, the [Fe₄(AD₂)₆]^4- cage converts quantitatively to the [Fe₂(BD₂)₃]^2- helicate, which, in turn, upon addition of subcomponent C, transforms to [Fe(CD₃)]²⁺, following the hierarchical preference based on the thermodynamic stability of the complexes.

Full text of DOI:10.1002/chem.201500734

DOI: 10.1002/chem.201500734
Communication
&
Supramolecular Complexes
Solvent-Free Ball-Milling Subcomponent Synthesis of
Metallosupramolecular Complexes
Chandan Giri,[a, b] Prasit Kumar Sahoo,[a] Rakesh Puttreddy,[b] Kari Rissanen,*[b] and
Prasenjit Mal*[a]
aminal-(bis)imine FeII-complex.[7] In all the above-mentioned
Abstract: Subcomponent self-assembly from components
A, B, C, D, and Fe2+ under solvent-free conditions by self-
sorting leads to the construction of three structurally dif-
ferent metallosupramolecular iron(II) complexes. Under
carefully selected ball-milling conditions, tetranuclear
[Fe4(AD2)6]4ꢀ 22-component cage 1, dinuclear [Fe2(BD2)3]2ꢀ
11-component helicate 2, and 5-component mononuclear
[Fe(CD3)]2+ complex 3 were prepared simultaneously in
a one-pot reaction from 38 components. Through sub-
component substitution reaction by adding subcompo-
nent B, the [Fe4(AD2)6]4ꢀ cage converts quantitatively to
the [Fe2(BD2)3]2ꢀ helicate, which, in turn, upon addition of
subcomponent C, transforms to [Fe(CD3)]2+, following the
hierarchical preference based on the thermodynamic sta-
bility of the complexes.
cases the dynamic behavior of the imine bond is the key factor
to control the self-assembly process.[8]
In the last few years, mechanochemistry,[9] as a solvent-free
synthesis method, has drawn great interest due to its advan-
tages over traditional solution-based methods,[10a] including
some self-assembling systems.[10b,c] The core benefit of mecha-
nochemistry is to avoid traditional work-up.[11] This advantage
has huge implications to green processes, shown to be eco-
nomical, time efficient, and environmentally friendly. Quantita-
tive conversion, less by-products, and no purification bring
extra benefits to this method. The mechanochemical synthesis
of small organic molecules is well explored,[12] also including
multistep synthesis[11c] and the synthesis of small mononuclear
ZnII, CuII, and NiII salen/salophen complexes has been recently
demonstrated by James and co-workers.[13]
However, unlike in solution, subcomponent synthesis of
complex supramolecular assemblies in absence of any solvent
is still rare. Herein we demonstrate that mechanochemistry,
under one-pot solvent-free ball-milling conditions offers a very
efficient route to prepare three distinct, structurally different,
water-soluble metallosupramolecular iron complexes from
their respective subcomponents by self-sorting from 38 com-
ponents. These metallosupramolecular complexes are a tetrahe-
dral [Fe4(AD2)6] cage 1 (22 components), a dinuclear [Fe2(BD2)3]
triple helicate 2 (11 components), and a mononuclear [Fe(CD3)]
complex 3 (5 components) (Figure 1), in which AD2, BD2, and
CD3 are the bis-, bis-, and tris-Schiff base ligands formed in situ
during the one-pot reaction.
The subcomponent self-assembly[1] is an integral part of self-
sorting reactions[2] in which ligands of the metallo-supramolec-
ular complexes are formed in situ from their subcomponents.
This concept has proved to offer a promising method for the
synthesis of high-purity products from complex mixtures of
starting materials.[3] The metal-ion-assisted subcomponent self-
assembly of a rigid aromatic linear bisamine, pyridine-2-carbox-
aldehyde, and iron(II) resulting in tetrahedral M4L6 cage in
water reported by the group of Nitschke and us opened a new
page on supramolecular tetrahedral complexes.[4] Recently, by
exploiting the same dynamic imine chemistry,[5] we have used
this concept to prepare the smallest possible tetrahedral M4L6
cage complex[6] and for the anion-controlled formation of an
The one-pot solvent-free ball-milling reaction emulates the
similar solution reaction[14] in which six equivalents of 4,4’-dia-
minobiphenyl-2,2’-disulfonic acid A, three equivalents of 6,6’-
oxybis(3-ammoniobenzenesulfonate) B, one equivalent of
tris(2-aminoethyl)amine C, 21 equivalents of 2-formyl pyridine
D, 18 equivalents of sodium bicarbonate, together with seven
equivalents of iron(II) sulfate heptahydrate are mixed together;
however, here they were mechanically milled without any sol-
vent in a ball mill (21 Hz) resulting in the same three metallo-
supramolecular complexes 1, 2, and 3 as in solution. This one-
pot solvent-free reaction cannot be considered a true solid-
state reaction as two of the components (C and D) are liquids.
Individually, under the same ball-milling conditions, the supra-
molecular complexes 1, 2, and 3 were also produced from
their respective subcomponents, A+D for 1, B+D for 2 and
C+D for 3.
[a] Dr. C. Giri,+ P. K. Sahoo,+ Dr. P. Mal
School of Chemical Sciences
National Institute of Science Education and Research (NISER) Bhubaneswar
Institute of Physics Campus
P.O. Sainik School, Bhubaneswar, Odisha 751005 (India)
E-mail: pmal@niser.ac.in
[b] Dr. C. Giri,+ Dr. R. Puttreddy, Prof. Dr. K. Rissanen
University of Jyvaskyla, Department of Chemistry
Nanoscience Center
P.O. Box. 35, 40014 University of Jyvaskyla (Finland)
E-mail: kari.t.rissanen@jyu.fi
[+] These authors contributed equally.
Supporting information for this article (including details of the experimen-
tal methods, spectroscopic investigations, synthetic procedure, characteriza-
tion data, and spectra of the complexes) is available on the WWW under
http://dx.doi.org/10.1002/chem.201500734.
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tone to remove the excess pyridine-2-aldehyde (1.1 equiv). The
complexes were characterized and their purity verified in com-
parison to the 1H NMR spectra of a standard sample of the
same complexes synthesized in water. The cage 1 was original-
ly synthesized in water in 20 h at 508C;[14] however, in here the
solvent-free ball-milling reaction times are 2 h for 1, 1 h for 2,
and 0.5 h for 3, respectively. The one-pot synthesis of 1, 2, and
3 took 2 h.
The X-ray structures of the tetraanionic tetrahedral cage 1 as
tetra-TMA (tetramethylammonium)+ and di-Fe(H2O)2+ salts[4,16]
and the mononuclear complex 3 as perchlorate,[17] tetrafluoro-
borate,[18] and hexafluorophosphate[19] salts have been report-
ed previously. But the X-ray structure of the triple-helicate 2
has been unknown prior to this study. The negatively (overall
2ꢀ) charged triple-stranded helicate 2 is structurally very simi-
lar to the corresponding nonsulfonated diphenyl ether Ni- and
Co-triple-helicates.[20] Whereas the Fe…Fe separation in 2 is
11.25 ꢁ, the Ni- and Co-helicates have Ni…Ni and Co…Co distan-
ces of 11.41 and 11.37 ꢁ, respectively (Figure 3). The triple-heli-
cate 2 has tighter helix structure, with a CꢀH…centroid dis-
tance of the adjacent phenyl ring of 2.80 ꢁ, when in the Ni-
and Co-diphenyl ether triple-helicates the same distance is
3.00 ꢁ.
Figure 1. The one-pot solvent-free synthesis of 22-component [Fe4(AD2)6]4ꢀ
1, 11-component [Fe2(BD2)3]2ꢀ 2, and 5-component [Fe(CD3)]2+ 3. A slight
excess of 2-formylpyridine (D) was used (1.1 equivalent).
The one-pot and individual reactions were performed under
ambient conditions and monitored during the initial synthesis
1
tests using H NMR spectroscopy (Figure 2). During monitoring,
the ball-milling apparatus was stopped, a small portion of the
sample was extracted from the reaction chamber, and the
1H NMR spectrum was recorded in D2O. The final synthesis was
done in a fixed time and the products were removed, placed
on a paper filter, and washed with a minimum quantity of ace-
Figure 3. The X-ray structure of the Fe-triple-helicate 2.
Previous studies in solution[14] have established that com-
plexes 1, 2, and 3 can undergo transformation to a thermody-
namically more stable structure upon an appropriate subcom-
ponent substitution reaction. The role of the solvent has been
considered to be crucial for the transformation from the ther-
modynamically less stable complex to the thermodynamically
more stable complex, like 1!2. However, under solvent-free
conditions, due to restrictions of the molecular movements,
the concept of dynamic chemistry has not been explored or
defined. However, it has been anticipated that under ball-mill-
ing conditions the products formed are thermodynamically
controlled.
The well-defined differences in the thermodynamical stabili-
ty of 1, 2, and 3 enable post-assembly transformation (substi-
tution) chemistry to progress in a predictable, hierarchical path
also under solvent-free ball-milling conditions. The addition of
Figure 2. 1H NMR spectra of the complexes obtained under solvent-free ball
milling condition; a) 1, 2, and 3 from the one-pot reaction; individual reac-
tions b) 1, c) 2, d) 3.
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the tris-amine subcomponent C to either 1 or 2 under ball-
milling conditions yielded 3, with ejection of diamine subcom-
ponent A or B, respectively. Similarly adding B into 1 resulted
in 2, which could then subsequentially be transformed into 3
upon addition of C (Figure 4).
molecular networks which ultimately assist us to acquire new
systems[25] with properties and functions unlike any conven-
tional materials.[26] In other words, the dynamic self-sorting ap-
proach can enable easy access to certain materials of interests
by changing inputs in either sub- or multicomponent sys-
tems.[27] To the best of our knowledge, this is the first example
of a subcomponent self-assembly under solvent-free condition
in which both cationic (1 and 2) and anionic (3) complexes
were generated together.
In conclusion, an unprecedented example of self-sorting of
three distinct iron(II) complexes under solvent-free mechano-
milling condition has been demonstrated. The obtained three
distinct structures were obtained from 38 subcomponents in
one-pot reaction at room temperature. However, these com-
plexes were transformed to their smaller and more stable
counterparts upon subcomponent substitution under solvent-
free conditions. Currently, in our laboratory, we are focusing on
developing more complex supramolecular architectures whose
molecular recognition can be reversibly controlled to facilitate
control over product distribution of a particular system of in-
terest.
Experimental Section
Detailed experimental procedures are given in the Supporting In-
formation. All the complexes are known from literature and were
characterized using NMR spectroscopy.
During the transformation reaction from 2 to 3, silica gel (column
chromatography grade, 100–200 mesh) was used to avoid sticki-
ness at the milling ball. In absence of silica gel the reaction of 2 to
3 was unsuccessful.
One-pot ball-milling synthesis of 1, 2, and 3: Tris(2-ethylamino)-
amine A (1.46 mg, 10.0 mmol), 4,4’-diaminobiphenyl ether-2,2’-di-
sulfonic acid B (4.5 mg, 80%, balance water, 6.6 mmol), 4,4’-diami-
nobiphenyl-2,2’-disulfonic acid C (4.6 mg, 75%, balance water,
6.6 mmol), sodium bicarbonate (5.0 mg, 60 mmol), FeSO4·7H2O
(6.5 mg, 23.3 mmol), and 2-formyl pyridine (6.7 mL, 70 mmol) were
added in a 10 mL Teflon reaction chamber with a one 5 mm diame-
ter steel ball. After reaction the mixture was washed with acetone
Figure 4. The solvent-free post-assembly transformation (substitution) reac-
tion[15] of 1 and 2 leading to 3.
1
and dissolved in 0.5 mL D2O and H NMR spectra recorded.
Crystal data for 2: M=2160.60, violet block, 0.32ꢂ0.26ꢂ0.18 mm3,
¯
triclinic, space group P1, a=15.8610(8) ꢁ, b=15.9946(7) ꢁ, c=
Our focus on the competition among the iron complexes in-
cludes the flexibility/rigidity of their backbones and the entro-
py associated with degrees of freedom of mono-, di-, and
tetra-iron(II) complexes. The geometries of 1, 2, and 3 derive
from the flexibilities of their respective amine subcomponents:
To maximize entropy, each imine ligand will generate the
smallest self-assembled structure possible in which all iron(II)
ions are hexacoordinate and all imine nitrogen atoms are coor-
dinated to iron(II). As can be seen from the structural analysis
of the iron complexes, the number of building blocks presents
in the systems are 22, 11, and 5 for 1, 2, and 3, respectively.
22.5064(8) ꢁ, a=97.075(3)8, b=102.500(4)8, g=97.641(4)8, V=
5456.2(4) ꢁ3,
Z=2,
1calcd =1.315 gcmꢀ3
,
F000=2195,
m=
0.594 mmꢀ1, T=120 K, 2qmax =50.68, 19722 reflections used, 10748
with Io > 2s(Io), Rint =0.0965, 1502 parameters, 175 restraints, GoF=
1.076, R=0.1361 [Io > 2s(Io)], wR=0.3811 (all reflections), 1.122 <
D1 <ꢀ1.097 eꢁ3. CCDC-1043159 contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Therefore the relative thermodynamic stability of the com- Acknowledgements
plexes could be as follows, 3>2>1. Thus the experimental
outcome shown in Figure 4 may be justified.
The Department of Science and Technology, New Delhi, India
Many self-sorting methodologies[2c,3b,21] are known in ’sys-
tems biology’.[22] However, the study of ’systems chemistry’[3c,23]
provides primary insights into the self-sorting principles[24] of
(PM, grant no. INT/FINLAND/P-06 and SR/S1/IC-59/2010) and
the Academy of Finland (KR, grant no. 263256 and 265328) are
acknowledged. P.K.S. thanks UGC India for a fellowship.
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Communication
tesberger, G. W. V. Cave, M. C. Ferrarelli, M. W. Heaven, C. L. Raston, J. L.
Atwood, Chem. Commun. 2005, 892–894.
Keywords: ball mill
·
dynamic imine chemistry
·
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COMMUNICATION
&
Supramolecular Complexes
C. Giri, P. K. Sahoo, R. Puttreddy,
K. Rissanen,* P. Mal*
&& – &&
Solvent-Free Ball-Milling
Subcomponent Synthesis of
Metallosupramolecular Complexes
Four different subcomponents self-
sorted around iron(II) templates to
create Fe4, Fe2, and Fe1 complexes
under ball milling. Input-controlled clear
hierarchical preferences were observed:
the Fe4 could be converted to Fe2 or
Fe1. Similarly Fe2 was switchable to Fe1
depending on the input used.
Chem. Eur. J. 2015, 21, 1 – 5
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These are not the final page numbers! ÞÞ