Integrative self-sorting synthesis of a Fe8Pt6L 24 cubic cage

Article abstract of DOI:10.1002/anie.201202050

96 bonds were formed when 62 building blocks (heterotopic ligands with Fe^II and Pt^II ions) self-assembled in a one-pot reaction into a heterometallic Fe₈Pt₆L₂₄ cubic cage (see model, Fe purple, Pt orange, N blue). The dynamic nature of this method also allowed an efficient cage-to-cage conversion: a tetrahedral Fe₄L ₆ cage was converted into the Fe₈Pt₆L ₂₄ cube, which was subsequently converted into a second tetrahedral Fe₄L'₆ cage. Copyright

Full text of DOI:10.1002/anie.201202050

Angewandte
Chemie
DOI: 10.1002/anie.201202050
Self-Sorting
Integrative Self-Sorting Synthesis of a Fe Pt L Cubic Cage**
8
6
24
Maarten M. J. Smulders, Azucena Jimꢀnez, and Jonathan R. Nitschke*
The many different chemical processes associated with life
run in parallel within the cytoplasm of a cell. These processes
are not spatially separated by membranes, yet they do not
interfere with each other. The biomolecular agents of these
[
1]
processes can be said to undergo self-sorting, which is
defined as “the high-fidelity recognition of self from nonself”,
into the locally organized systems that achieve individual
[2]
functions. The goal of creating synthetic self-sorting systems
is thus inspired in part by the prospect of synthesizing
functional supramolecular systems that carry out different
processes together in solution.
[
3]
Self-assembled metal–organic capsules have been shown
to be useful for a variety of applications, including guest
[
4]
[5]
binding and separation, cavity-controlled catalysis, gener-
[
6]
ation of unusual reaction products, and stabilization of
[
7]
reactive intermediates. New strategies have been explored
to prepare more complex, metal–organic capsules so as to
achieve new functions. The potential of self-sorting has only
recently been acknowledged by researchers as a method for
[
8]
preparing such capsules.
One strategy pursued by researchers to create progres-
sively more-complex metal–organic architectures has been
the preparation of heteroleptic species. This method relies on
the combination of at least two types of homotopic ligands
and a single metal ion for the construction of a metal–organic
[
9]
complex. An alternative method to create complexity,
employed herein, entails the use of a single heterotopic
ligand and more than one metal ion, wherein the fidelity of
II
Scheme 1. The two-step synthesis of cube 3 via the square-planar Pt
intermediate 2 and the one-pot synthesis of 3 starting from free
[
10]
the self-assembly relies on integrative self-sorting.
Our
subcomponents and metal ions. For clarity, the chemical structure of
only one of the six faces of the cube is shown.
construction method relies on two different metal ions and
coordination environments: in addition to the tris(pyridyli-
mine)iron(II) moiety, widely employed in the subcomponent
[
11]
[12]
self-assembly of complex structures, we have introduced
second coordination motif, the tetrakis-
pyridine)platinum(II) moiety. To this end, ditopic
a
(
Based on the fourfold symmetry of the tetrakis-
(pyridine)platinum(II) moiety and the threefold symmetry
of the tris(pyridylimine)iron(II) moiety, we envisioned the
[
9d,13]
ligand 1 (Scheme 1), was selected as a ligand that can
II
[4c,14]
coordinate to both Fe (as a tris(pyridylimine) complex)
possibility of preparing a heterometallic cubic capsule:
II
II
and Pt ions (through its terminal pyridine group).
after synthesis of the square-planar Pt complex 2, this
intermediate could be incorporated into heterometallic cube
II
3
by formation of Fe -stabilized pyridylimine bonds
(
Scheme 1, right). Self-sorting could be validated within
[
*] Dr. M. M. J. Smulders, Dr. A. Jimꢀnez, Dr. J. R. Nitschke
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge CB2 1EW (UK)
E-mail: jrn34@cam.ac.uk
such a system by starting with the free subcomponents and
metal ions, and allowing each to find its proper place within
the final product structure (Scheme 1, left). For this self-
Homepage: http://www-jrn.ch.cam.ac.uk/
2
assembly process to work, the two different kinds of sp -
[
**] Dr. J. K. Clegg is thanked for performing single-crystal X-ray analysis
on compounds 2 and 4. This work was supported by the EPSRC.
Mass spectra were provided by the EPSRC Mass Spectrometry
Service at Swansea. M.M.J.S. acknowledges the Netherlands
Organization for Scientific Research (NWO Rubicon Fellowship).
A.J. acknowledges a Basque Country postdoctoral fellowship.
hybridized nitrogen donors must bind to the correct metal
ions, without scrambling.
Herein we employ integrative self-sorting in the synthesis
of the heterometallic Fe Pt L cube 3 (Scheme 1). A one-pot
8
6
24
synthesis of cube 3 from four different components, in which
a total of 62 building blocks are brought together, was found
to be possible, thus demonstrating both the potential of self-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202050.
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6681

.
Angewandte
Communications
sorting in the fabrication of larger and more complex
structures, as well as the dynamic nature of the employed
self-assembly method. The dynamic nature of the linkages
that knit the subcomponents of 3 together also enabled
efficient cage-to-cage conversions: in a first transformation
step, a tetrahedral Fe L cage was converted into the Fe Pt L
4
6
8
6
24
cube, which in a subsequent step was converted into a second
tetrahedral Fe L’ cage.
4
6
The square-planar tetrakis(pyridine)platinum(II) precur-
sor 2 was prepared according to a modified literature
[15]
procedure (Scheme 1):
Pt(C H CN) Cl (1 equiv) was
6 5 2 2
treated with ligand 1 (4 equiv) and AgBF4 (2 equiv) in
acetonitrile. After removal of the co-product AgCl by
filtration, compound 2 was recrystallized by diffusion of
diethyl ether vapor into an acetonitrile solution and was
isolated in 71% yield. The formation and purity of 2 were
1
13
confirmed by H and C NMR spectroscopy, electrospray
mass spectrometry (ESI-MS), elemental analysis, and single-
crystal X-ray diffraction (see the Supporting Information).
[4c]
In contrast to a previously studied tetrakis(4-amino-
phenyl)porphyrin subcomponent, which was found only to
undergo self-assembly in dimethylformamide because of its
Figure 2. Energy-minimized molecular model of cube 3. Color scheme:
dark gray: C, light gray: H, blue: N, purple: Fe, orange: Pt (for details
of the optimization, see the Supporting Information).
II
insolubility in other solvents, square-planar Pt ligand 2 was
found to be sufficiently soluble in acetonitrile to allow for 3 to
self-assemble in this solvent. The reaction of metallo-sub-
component 2 (6 equiv), iron(II) trifluoromethanesulfonate
of cube 3 (Figure 2; see the Supporting Information). To
1
(
Fe(OTf) , 8 equiv), and 2-formylpyridine (24 equiv) in
reproduce the O symmetry deduced from H NMR spectros-
2
acetonitrile resulted in the formation of cubic Fe Pt L cage
copy, we assigned the three bidentate pyridylimine ligands
around each Fe center to have facial geometry, and the eight
8
6
24
II
3
(Scheme 1) as the unique product, as evidenced by NMR
spectroscopy (78% yield of isolated product). In contrast to
previously reported heterometallic cubes, cage 3 displayed
tris(pyridylimine)iron(II) corners to be either all D or all L.
The model shows how ligand 2 forms the faces of the cube,
[
14]
II
high solubility (in acetonitrile), which facilitated its character-
with the square-planar Pt ion residing in the middle of each
1
13
II
ization. The H and C NMR spectra of cage 3 displayed only
one set of ligand resonances in solution, consistent with
a chiral octahedral, O-symmetric structure (Figure 1 and
Figure S3 in the Supporting Information). Two-dimensional
NMR spectra, DOSY, ESI-MS, and elemental analysis also
confirmed the formation of cube 3 (see the Supporting
Information).
face. Moreover, the model shows how the octahedral Fe ions
define the vertices of the cube, and how heterotopic ligand
1 connects the two metal ions, thereby yielding the hetero-
metallic Fe Pt L cubic cage. No features of the model, or
8
6
24
spectroscopic features of 3, suggested apparent strain; nor
were indicators of the strain observed in a previously reported
[
4c]
Fe L6 porphyrin-faced cube.
8
These observations suggest
Numerous attempts to grow crystals of 3 of sufficient
quality for single-crystal X-ray analysis were unsuccessful. We
therefore constructed a MM2-optimized molecular model
that the ability to form a Fe L assembly may be a general
8
6
feature of fourfold-symmetric tetrakis(anilines).
[
16]
II
The eight Fe ions define a cube with a volume that could
II
II
allow encapsulation of a large guest. The shortest Fe ···Fe
distance in 3 is calculated to be approximately 17.6 ꢀ along an
edge, whereas the tetrakis(4-aminophenyl)porphyrin-based
[
4c]
cubic cage reported previously is characterized by a metal-
[
17]
to-metal distance of 14.8 ꢀ.
A range of molecules was
investigated as potential guests (see Table S2 in the Support-
ing Information), but there was no evidence of the encapsu-
1
lation of any of them within cage 3 by H NMR spectroscopy
and ESI-MS. From inspection of the structure of 3, we
inferred the absence of guest binding to be due to the open
structure of the cage; the large pores along the cubeꢁs edges
reduce the amount of solvent-accessible surface area that
would be covered upon guest binding, a parameter which has
1
Figure 1. H NMR spectrum of cube 3 in CD CN (1.1 mm). Only one
3
[18]
been observed to correlate with binding strength.
set of ligand resonances is evident in solution, consistent with an O-
1
1
In addition to the two-step preparation of 3 involving the
symmetric structure. The signals were assigned with the aid of H- H
COSY and NOESY spectra (see Figures S5 and S6 in the Supporting
Information).
II
synthesis and isolation of square-planar Pt metallo-subcom-
ponent 2 (Scheme 1), we also effected the one-pot synthesis of
6
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Chemie
3
(Scheme 1). To this end, ligand
1
(24 equiv), Pt-
omethanesulfonimidate) (Fe(NTf ) , 4 equiv), and 2-formyl-
2 2
(
(
C H CN) Cl (6 equiv), AgBF4 (12 equiv), Fe(OTf)2
pyridine (12 equiv; see the Supporting Information). The
addition of tetraamine 2 (4.1 equiv) to tetrahedral cage 5
(1 equiv) led to the formation of cubic cage 3 and diamine 4,
6
5
2
2
8 equiv), and 2-formylpyridine (24 equiv) were found to
1
react overnight in acetonitrile at 338 K. The H NMR
spectrum (see Figure S8 in the Supporting Information)
revealed that, apart from excess 2-formylpyridine, the major
product in the reaction mixture was structure 3; ESI-MS also
confirmed the formation of cube 3 (see Figure S9 in the
Supporting Information).
1
as evidenced by H NMR spectroscopy (Figure 3b and Fig-
In this one-pot synthesis of the cube, 62 building blocks
were brought together, ultimately forming a single self-
assembled complex through the formation of 96 new bonds.
[
19]
The dynamic nature of the imine and coordination bonds—
in particular, the iron(II)–pyridylimine bonds—means that
the synthesis of 3 is carried out under thermodynamic control,
thus allowing for error-checking during the cageꢁs assembly
and ultimately leading to the formation of the energetically
[
20]
most favored structure, that is, cube 3.
In addition, the successful one-pot synthesis of 3 implies
an efficient self-sorting between two different coordination
motifs that both contain the pyridine moiety. In the synthesis
of 3, the terminal pyridine moiety present in subcomponent
II
1
was observed to preferentially coordinate to the Pt ion,
1
whereas the subcomponentꢁs amine moiety, after condensa-
tion with 2-formylpyridine, coordinated to the Fe ion. The
Figure 3. H NMR spectrum of: a) tetrahedral cage 5 in CD CN;
3
II
(b) cubic cage 3 (resonances marked with #), obtained after addition
of ligand 2 to a solution of 5 in CD CN; c) tetrahedral cage 7
(resonances marked with *), obtained after addition of ligand 6 to the
solution of 3 in CD CN. In (b) and (c) the resonances corresponding
to liberated ligands 2 and 4 can also be observed. The dotted lines
show how the chemical shifts of the protons on ligands 4 and 2
change as a result of the conversion from 5 into 3 and from 3 into 7,
respectively. Resonances corresponding to free 2 and 4 are marked by
and *, respectively. The full assignment of the H NMR spectra is
ability of this system to self-sort is further supported by the
3
II
observation that, in the absence of Fe ions and ligand 1, 2-
II
3
formylpyridine can coordinate to a Pt ion (see Figures S10
and S11 in the Supporting Information), while in the absence
II
II
of Pt ions, ligand 1 can also coordinate to an Fe ion through
the pyridine moiety (see Figure S12 in the Supporting
Information).
1
&
given in the Supporting Information (see Figures S21 and S24).
The two-step synthesis of cube 3 starts from a tetrafunc-
tional amine (i.e. compound 2), which stands in contrast to the
preparation of tetrahedral cages previously reported by our
research group that relied either on bifunctional amine
ure S21 in the Supporting Information), DOSY NMR spec-
troscopy (Figure S23 in the Supporting Information), and
ESI-MS (Figure S22 in the Supporting Information). We infer
that this cage-to-cage transformation is entropically favorable
because for each tetraamine 2 that is incorporated in cube 3,
two equivalents of diamine 4 are released; overall, 8 moles of
reagents are transformed into 13 moles of products in this
transformation. This increase in entropy appears to offset the
[
12d,21]
[22]
ligands
or trifunctional amine ligands.
Since in all
these metal–organic capsules the dynamic tris(pyridylimi-
ne)iron(II) moiety is present at the capsuleꢁs vertices, we set
out to investigate whether a cage-to-cage conversion through
[23]
amine residue exchange was possible (Scheme 2).
To this end, tetrahedral cage 5 was prepared by reaction of
the electron-poor diamine 4 (6 equiv), iron(II) bis(trifluor-
Scheme 2. Schematic representation of the conversion of tetrahedral cage 5 into cubic cage 3 upon addition of square-planar metallo-
subcomponent 2, followed by the conversion of cubic cage 3 into tetrahedral cage 7 upon addition of 6. For clarity, the chemical structure of only
one of the ligands is shown for cages 3, 5, and 7.
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.
Angewandte
Communications
enthalpic cost associated with the incorporation of the more
electron-poor tetraamine 2 compared to diamine 4 (Hammett
spara values for the ethynyl and 4-pyridyl groups are + 0.23 and
3: Iron(II) triflate (22.1 mg, 62.4 mmol), 2 (49.1 mg, 46.7 mmol), 2-
formylpyridine (17.8 mL, 187 mmol), and acetonitrile (8 mL) were
added to a 25 mL Schlenk flask. The solution was purified of dioxygen
by three vacuum/nitrogen fill cycles. The reaction was then stirred for
[
24]
+
0.44, respectively).
Subsequent addition of 4,4’-diaminobiphenyl (6;
2.9 equiv per equiv of capsule 3) to cubic cage 3 (in the
À
À
24 h at 343 K before cooling to room temperature. The BF4 /TfO
salt of 3 was precipitated as a purple microcrystalline solid by the
1
1
addition of diethyl ether. Yield: 68 mg (78%). H NMR (400 MHz,
presence of the free ligand 4) resulted in the formation of
tetrahedral cage 7 and concomitant release of ligand 2
298 K, CD CN) d = 9.29 (d, 48H), 8.68 (s, 24H), 8.44 (d, 24H), 8.35 (t,
3
24H), 8.06 (d, 48H), 7.72 (t, 24H), 7.63 (d, 48H), 7.30 (d, 24H),
1
3
5
.23 ppm (brs, 48H); C NMR (126 MHz, 298 K, CD CN) d =
(
Figure 3c; see Figures S24–S27 in the Supporting Informa-
3
[
25]
176.46, 158.93, 156.99, 153.08, 152.60, 151.62, 140.75, 136.36, 132.46,
tion). Although entropically disfavored, this transforma-
tion appears to be driven by the incorporation of diamine 6,
which is more electron-rich than tetraamine 2 (the Hammett
1
31.01, 129.88, 126.34, 122.91 ppm; ESI-MS: the complex was
À
obtained as the 28 + complex with a varying number of BF4 and
OTf counterions. The assigned ESI mass spectrum is given in the
À
[
24]
19
value spara for the phenyl group is À0.01). Also, F NMR
Supporting Information (see Figure S4). Elemental analysis calcd for
À
spectroscopy revealed that a BF4 ion resides inside the cavity
C H N72Fe Pt (BF )12(F CSO )16: C 45.20, H 2.79, B 1.15, F 16.19,
408 312 8 6 4 3 3
Fe 3.97, N 8.95, O 6.82, Pt 10.39, S 4.55; found: C 45.07, H 2.81, N 9.14.
of cage 7 (see Figure S25 in the Supporting Information), thus
indicating that anion templation could provide a further
[
26]
Received: March 15, 2012
Published online: June 5, 2012
driving force to the transformation of 3 to 7. We will treat
the behavior of cage 7 in greater detail in a separate
[
27]
contribution.
Keywords: coordination modes · heterometallic structures ·
In conclusion, we have shown how, on the basis of
integrative self-sorting, the formation of a soluble hetero-
metallic cube starting from a heterotopic ligand and two
different metal ions is possible. Self-sorting allowed a one-pot
synthesis of the cubic complex, thus bringing together 62
building blocks through the formation of 96 new bonds. Even
though bis(pyridylimine)platinum(II) complexes are known
and stable, as are hexakis(pyridine)iron(II) complexes, effi-
cient self-sorting during the reaction of ligand 1 in the
.
metal–organic capsules · self-assembly ·
supramolecular chemistry
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Experimental Section
2
: Dichlorobis(benzonitrile)platinum(II) (242 mg, 0.512 mmol), 4-
(
(
pyridin-4-yl)aniline (350 mg, 2.06 mmol), silver tetrafluoroborate
213 mg, 1.09 mmol), and acetonitrile (100 mL) were added to
a 250 mL round bottom flask. The reaction was stirred for 24 h at
38 K, during which time a precipitate was observed to form (inferred
3
to be AgCl). After allowing the reaction mixture to cool to room
temperature, the precipitate was removed by filtration through celite.
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in vacuo. Slow diffusion of diethyl ether vapor into the remaining
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1
crystals of 2. Yield: 380 mg (71%). H NMR (400 MHz, 298 K,
CD CN): d = 8.52 (d, 2H), 7.65 (d, 2H), 7.57 (d, 2H) 6.72 (d, 2H)
3
1
3
4
.75 ppm (s, 2H); C NMR (126 MHz, 298 K, [D ]DMSO) d = 152.3,
6
1
51.0, 150.7, 128.7, 121.7, 119.8, 114.1 ppm; ESI-MS: 437.69 ([Pt-
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2
+
+
(C H N ) ] ) and 962.32 ([Pt(C H N ) BF ] ). Elemental analysis
11 10 2 4 11 10 2 4 4
calcd for C H B F N Pt·0.9CH CN: C 50.63, H 3.96, B 1.99, F 13.99,
N 11.47, Pt 17.96; found: C 50.74, H 4.09, N 11.34.
4
4
40
2
8
8
3
6
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