A molybdenum crown cluster forms discrete inorganic-organic nanocomposites with metalloporphyrins

Article abstract of DOI:10.1002/anie.200460990

Filled doughnuts: Molybdenum crown cluster MC accommodates up to three molecules of aminophenyl-substituted metalloporphyrins through hydrogen-bonding interactions, to form discrete inorganic-organic nanocomposite materials (see picture; blue polyhedra = MC, space-filling model = metalloporphyrin). Ultrahigh-vacuum scanning tunneling microscopy, in conjunction with scanning tunneling spectroscopy, at 80 K confirms the formation of the inclusion complex. (Figure presented).

Full text of DOI:10.1002/anie.200460990

Angewandte
Chemie
Polyoxomolybdates
A Molybdenum Crown Cluster Forms Discrete
Inorganic–Organic Nanocomposites with
Metalloporphyrins**
Akihiko Tsuda,* Eri Hirahara, Yeong-Sang Kim,
Hiroyuki Tanaka, Tomoji Kawai,* and Takuzo Aida*
VI
Molybdenum blue (MB), formed by partial reduction of Mo
in an acidic aqueous solution, is a striking inorganic material
due to its vivid blue color. It is a mixture of polyoxomolybdate
V
VI
(
POM) clusters consisting of mixed-valent Mo and Mo
[
1–3]
centers.
Although the initial exploration was made more
[
4]
than 200 years ago, the first success in structural analysis of
POM clusters was only reported in 1995, when Müller and co-
workers isolated a crown-shaped POM cluster and obtained
its crystal structure. To date, they have also succeeded in
structural determination of large POM clusters with hollow
[
5]
[
6,7]
and spherical shapes. Despite their interesting potentials in
materials sciences, no examples have yet been reported of the
utilization of such inorganic nano-objects for the fabrication
of discrete inorganic/organic nanocomposite materials.
Herein we report that the crown-shaped POM (molybdenum
crown cluster; MC), upon mixing with metalloporphyrins
having meso-aminophenyl substituents, forms discrete inclu-
sion complexes, where the inorganic cavity of MC can
accommodate up to three molecules of the guest compounds,
to give spatially isolated metalloporphyrin molecules
(Scheme 1).
Nað32ÀnÞ½ðMoO3Þ176ðH OÞ ðMeOHÞ H ŠÁ
2
63
17
n
ꢀ
600 H OÁ ꢀ 30 MeOH MC
2
MC has a large cavity with a diameter of approximately
[
7]
2
.3 nm. We expected that this cavity can incorporate proton
acceptors through a hydrogen-bonding interaction, since a
[*] Dr. A. Tsuda, E. Hirahara, Dr. Y.-S. Kim, Prof. Dr. T. Aida
Department of Chemistry and Biotechnology
School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-7310
E-mail: tsuda@macro.t.u-tokyo.ac.jp
aida@macro.t.u-tokyo.ac.jp
[
+]
Dr. H. Tanaka, Prof. Dr. T. Kawai
The Institute of Scientific and Industrial Research
Osaka University
8-1 Mihogaoka, Ibaraki, Osaka 567-0047 (Japan)
E-mail: kawai@sanken.osaka-u.ac.jp
+
[
] Responsible for scanning tunneling microscopy.
[**] The present work was supported by a Grants-in-Aid for Scientific
Research (No. 15350128) and Encouragement of Young Scientists
(No. 15750028) from the Ministry of Education, Science, Sports,
and Culture, Japan. A.T. thanks the Kurata Memorial Hitachi Science
and Technology Foundation, and the Tokuyama Science Founda-
tion.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200460990
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tetrakis(aminophenyl)porphyrins, such as
(p-NH ) , 1(m-NH ) , and 1(o-NH ) ,
1
2
Zn
2
Zn
2 Zn
and 1(p-NH ) , a copper analogue of 1(p-
2
Cu
NH ) . Compounds 2 having 4-aminobi-
2
Zn
Zn
phenyl groups and 3Zn bearing 3,5-dihydroxy-
phenyl groups at the meso-positions were
used as references. According to molecular
models, 1(p-NH ) , 1(p-NH ) , 1(m-
2
Zn
2 Cu
NH ) , 1(o-NH ) , and 3 are 2.0, 2.0, 1.8,
2
Zn
2
Zn
Zn
1
.6, and 1.8 nm in diameter (longer molecular
[
9]
axis), respectively, and smaller than the
cavity of MC, whereas compound 2Zn is much
larger (2.8 nm). Compound 3Zn is different
from these complexes, in that it has Brønsted
acidic OH functionalities in place of the
amino functionalities.
Electronic absorption spectroscopy in
MeCN with the five zinc porphyrin com-
plexes indicated that 1(p-NH2)Zn and 1(m-
NH2)Zn interact with MC. For example, upon
Scheme 1. Schematic representations of the complexation of molybdenum crown cluster (MC; blue
polyhedra) with zinc 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (1(p-NH2)Zn; space-filling model:
pink Zn, blue N, gray C, white H). Molecular models of MC and 1(p-NH2)Zn are based on X-ray crys-
tallography (see ref. [7]) and MM2 calculation, respectively.
great number of proton-donating m -O···H species are con-
3
centrated in its inner surface.
[
7]
MC was prepared according to a literature method, and
characterized by means of MALDI-TOF mass spectrometry
and scanning tunneling microscopy (STM), along with UV/
[
8]
Vis and IR spectroscopy. The MC, thus obtained, was highly
soluble in MeCN to give a clear blue solution, which showed
characteristic broad absorption bands at 730 and 1058 nm
(
Figure 1a). For the complexation studies with MC, we chose
as potential guest molecules zinc complexes of 5,10,15,20-
titration of 1(p-NH2)Zn which has 4-aminophenyl groups with
MC, the Soret-band at 430 nm decreased in intensity with a
small blue shift to 427 nm, while a red-shifted shoulder
appeared at 440 nm (Figure 1b). Compound 1(m-NH2)Zn
which has 3-aminophenyl groups also showed a spectral
change upon titration with MC, a considerable broadening of
[
8]
the Soret band in the longer wavelength region took place.
In contrast, 1(o-NH2)Zn bearing amino groups directed
towards the porphyrin unit (Figure 1c) and the larger sized
[
8]
2Zn hardly showed spectral changes under similar titration
conditions. Likewise, the absorption spectrum of 3Zn having
phenolic OH functionalities was virtually unchanged upon
addition of MC. These contrasting observations indicate that
the complexation of the zinc porphyrins with MC is chemo-,
À6
[10]
Figure 1. Absorption spectra (350–850 nm) of a) MC (2.2”10 m)
(
(
inset: 350–1400 nm), and zinc porphyrins, b) 1(p-NH )
2 Zn
À6
À6
2.4”10 m), and c) 1(o-NH2)Zn (2.5”10 m) in the absence and pres-
ence of MC (0.5 equiv) in MeCN at 208C.
6
328
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Chemie
regio-, and size-selective. Namely, the peripheral amino
groups are crucial for the complexation with MC, but they
should be oriented away from the porphyrin ring, and such
guest molecules should be smaller than the MC cavity.
Therefore, MC accommodates 1(p-NH2)Zn and 1(m-NH2)Zn
inside the cavity through the formation of multiple hydrogen
bonds (e.g., m -O···H···N), to give inclusion complexes
3
[
MCꢁ1(p-NH ) ], and [MCꢁ1(m-NH ) ], respectively.
2
Zn
2 Zn
Copper complex 1(p-NH ) , upon titration with MC, also
2
Cu
showed an essentially identical spectral change profile to that
[
8]
of 1(p-NH ) .
2
Zn
To determine the stoichiometry of the complexation,
spectroscopic titration of MC with 1(p-NH2)Zn was conducted
in MeCN (Figure 2a), where MC showed stepwise spectral
Figure 3. Left: ultrahigh-vacuum scanning tunneling microscopy (UHV-
STM) images and right: scanning tunneling spectroscopy (STS) data
on Cu(111) surfaces at 80 K. UHV-STM of a) intact MC (imaging con-
ditions: I=2.0 pA, V =1.0 V), b) a 1:3 mixture of MC and 1(p-NH )
s
2 Zn
(
I=2.0 pA, V =3.0 V), and c) [MCꢁ1(p-NH ) ] (I=2.0 pA, V =1.0 V).
s
2
Zn
s
STS differential conductance (dI/dV)–sample bias voltage (V ) correla-
s
À6
[10]
tions of area (1) an intact MC observed in (a), and areas (2) the shell
Figure 2. Spectroscopic titration of MC (2.5”10 m) with 1(p-
À5
and (3) the core of [MCꢁ1(p-NH ) ] observed in (c).
NH2)Zn in MeCN at 208C. [1(p-NH ) ]=a) 0–2.1”10 m (overall spec-
tral change), b) 0–0.5”10 m, c) 0.6”10 –1.0”10 m, and
2 Zn
2
Zn
À5
À5
À5
À5
À5
d) 1.2”10 –2.1”10 m.
[
10]
of a mixture of MC and 1(p-NH2)Zn (1:3) was sprayed by
the pulse injection technique onto a clear flat Cu(111) surface.
The UHV-STM image showed the presence of numerous
doughnut-like discrete nano-objects with a diameter of 4–
5 nm (Figure 3b), which is consistent with the reported
changes in response to [1(p-NH ) ]/[MC] at 0–2, 2–4 , and 4–
2
Zn
[
10]
1
1
0 (Figure 2b–d), with isosbestic points at 1121, 1138, and
143 nm, respectively. This observation indicates the stepwise
formation of 1:1, 1:2, and 1:3 complexes between MC and 1(p-
[
11]
[7]
NH ) .
Slow evaporation of a concentrated MeCN
dimension of crystallographically defined MC. Some of
2
Zn
solution of a 1:3 mixture of MC and 1(p-NH2)Zn allowed the
isolation of [MCꢁ1(p-NH ) ] as a green precipitate, which
these nano-objects in the STM image (Figure 3c) appear to
[
13]
have a filled cavity.
spectroscopy (STS) on one of these filled objects, correla-
By means of scanning tunneling
2
Zn
[
14]
was subjected to inductively coupled plasma atomic emission
spectroscopy (ICP-AES) to give a Mo/Zn ratio of 61.9. This
value is in good agreement with that calculated for the 1:3
tions between differential conductance (dI/dV) and sample
bias voltage (V ) were measured separately for the shell and
s
[
8]
inclusion complex between MC and 1(p-NH )
(58.7).
core parts. As shown in Figure 3, these two parts exhibit
2
Zn
[
7]
According to the crystal structure, the cavity of MC is
.3 nm deep and large enough to accommodate up to three
molecules of 1(p-NH2)Zn when they are stacked face to face
different dI/dV–V profiles. Although the shell part, as with
s
1
intact MC, displays a gradual increase in dI/dV with either
decreasing or increasing V , the core part, in contrast, shows
s
(
Scheme 1). Thus, the spectral changes observed for the
two bands at À0.7 and + 1.6 V, which are assignable to
[
8]
complexation of 1(p-NH2)Zn (Figure 1b) and 1(m-NH2)Zn
resonant tunneling currents through the HOMO and LUMO
[
15]
with MC are considered to reflect such a stacking association
of these guest molecules in the MC cavity, as well as the
proton-donation from their peripheral amino groups.
Ultrahigh-vacuum scanning tunneling microscopy (UHV-
of the guest compound. Since the energy gap of 2.3 eV, thus
observed, is consistent with the optical HOMO–LUMO
energy gap of 1(p-NH2)Zn (2.1 eV), it is clear that the object
with a filled cavity (Figure 3c) is [MCꢁ1(p-NH ) ].
2
Zn
[
12]
STM)
(Figure 3) demonstrated that the product upon
We also conducted an electron paramagnetic resonance
(EPR) study on the inclusion complex of MC with copper
porphyrin 1(p-NH2)Cu ([MCꢁ1(p-NH ) ]). MC is diamag-
complexation of MC with 1(p-NH2)Zn is indeed an inclusion
complex [MCꢁ1(p-NH ) ]. Thus, a dilute aqueous solution
2
Zn
2 Cu
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netic at the ground state, while copper porphyrins are
paramagnetic (S = 1/2) and generally show anisotropic EPR
spectra with two distinct g and g values and hyperfine
controller. IR spectra were recorded on a JASCOmodel FT/IR-610
spectrometer. MALDI-TOF mass spectrometry was performed on a
Perceptive Biosystems model Voyager-DE spectrometer with 9-
nitroanthracene as matrix. ICP-AES was performed on a Seiko
Instruments Inc. model SPS4000 inductively coupled plasma atomic
emission analyzer. Ultrahigh-vacuum scanning tunneling microscopy
k
?
splittings induced by copper (I = 3/2) and nitrogen (I = 1)
[
16]
nuclei. In contrast, 1(p-NH2)Cu alone in MeCN at 103 K
displayed an isotropic EPR pattern possibly arising from an
irregular aggregation caused by hydrogen-bonding interac-
tions at the peripheral amino groups (Figure 4). On the other
(UHV-STM) was performed on a Unisoku Japan model USM-1200.
Metalation of porphyrins: Typically, a saturated MeOH solution
of Zn(OAc)
base porphyrin, and the resulting mixture was heated under reflux for
–2 h. After the complete metalation was confirmed by thin layer
or Cu(OAc) was added to a CHCl solution of a free-
2
2
3
1
chromatography (TLC) or MALDI-TOF MS, the mixture was poured
into water and extracted with CHCl . The combined organic extract
3
was washed with water and brine, dried over anhydrous Na SO , and
2
4
then evaporated to dryness. Recrystallization of the residue from
THF/hexane gave the corresponding metalloporphyrin in an analyti-
cally pure form.
2Zn
:
(1,1’-Biphenyl)-4-nitro-4’-carboxaldehyde
(786 mg,
3.5 mmol) and pyrrole (232 mg, 3.5 mmol) were heated in refluxing
propionic acid (25 mL) for 3 h. A crystalline precipitate, obtained
from the reaction mixture on cooling, was isolated by filtration and
washed with water and MeOH. The precipitate (70 mg) and
anhydrous SnCl (250 mg, 1.1 mmol) were dissolved in concentrated
2
aq.HCl/THF (2:1; 15 mL), and the resulting solution was stirred for
24 h at 508C. Aqueous KOH was added to the mixture until it turned
basic, and the mixture was extracted with CH Cl (4 ” 50 mL). The
2
2
combined organic extract was dried over anhydrous Na SO4 and
2
evaporated to dryness. The residue was purified by chromatography
on silica gel with CHCl :MeOH (95:5) as eluent, and a reddish purple
3
fraction isolated was stirred with excess Zn(OAc) in CHCl for 1 h.
2
3
À5
The reaction mixture was evaporated to dryness under reduced
Figure 4. EPR spectra at 103 K in MeCN. a) MC (3.0”10 m), b) 1(p-
À5
À5
pressure, and the residue was extracted with CHCl /water. The
3
NH2)Cu (3.0”10 m), c) a mixture of 1(p-NH2)Cu (3.0”10 m) and MC
À5
[10]
combined organic extract was dried over anhydrous Na SO4 and
2
(
1.0”10 m).
evaporated to dryness. Recrystallization of the residue from THF/
hexane gave 2Zn (5 mg, 4.7 mmol) as purple powdery substance.
MALDI-TOF MS m/z 1040, calcd for C H N Zn 1040; UV/Vis
hand, when 1(p-NH ) was mixed with MC at a 1:3 molar
2
Cu
6
8
48
1
8
[
10]
ratio,
the resulting inclusion complex [MCꢁ1(p-NH ) ]
2
Cu
(CHCl ): l = 427, 522, 559, and 600 nm; H NMR (500 MHz,
3 max
showed in its EPR spectrum an anisotropic pattern with g
[D ]THF, 258C): d = 4.61 (s, 8H, NH ), 6.72 (d, J = 10.0 Hz, 8H,
k
8
2
and g values of 2.187 and 2.085, respectively, along with
bipheny), 7.62 (d, J = 10.0 Hz, 8H, bipheny), 7.87 (d, J = 10.0 Hz, 8H,
bipheny), 8.14 (d, J = 10.0 Hz, 8H, bipheny), and 8.88 ppm (s, 8H,
pyrrole-b).
?
sharp hyperfine arising from the copper (a = 19.3 mT) and
k
nitrogen nuclei. Thus, 1(p-NH2)Cu is freed from its hydrogen-
bonded irregular assembly and incorporated into the MC
cavity to form a uniform assembly in the confined nanospace.
In conclusion, we have demonstrated the first example of
organic functionalization of doughnut-like molybdenum
crown cluster (MC). In its nanocavity MC can accommodate
metal complexes of aminophenyl-substituted porphyrins, such
as 1(p-NH2)M (M = Zn, Cu) and 1(m-NH2)Zn as a result of
hydrogen-bonding interactions, to form discrete inorganic/
organic nanocomposite materials. The results indicate a new
potential for MC as an inorganic host in supramolecular
chemistry and also as an building block for nanoscopic
materials science.
Received: June 16, 2004
Keywords: inorganic/organic nanocomposites ·
.
polyoxomolybdates · porphyrinoids · scanning probe
microscopy · supramolecular chemistry
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2 Zn
[
7,17]
literature methods,
while 5,10,15,20-tetrakis(4-aminophenyl)-
2
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6
330
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Angewandte
Chemie
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[
[
[
[
[
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Angew. Chem. Int. Ed. 2004, 43, 6327 –6331
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