Tetraplatinum cluster complexes bearing hydrophilic anchors as precursors for γ-Al2O3-supported platinum nanoparticles

Article abstract of DOI:10.1039/c3dt50670c

Tetraplatinum cluster complexes bearing hydrophilic anchors, [Pt ₄(μ-OCOCH₃)₄(μ-OCOC₆H ₄OH-4)₄] (2a), [Pt₄(μ-OCOCH ₃)₄(μ-OCOC₆H₄B(OH) ₂-4)₄] (2b), and [Pt₄(μ-OCOCH ₃)₄(μ-OCOC₆H₄NH ₂-4)₄] (2c), were successfully prepared by a selective substitution reaction of four in-plane acetates of [Pt₄(μ- OCOCH₃)₈] (1) with the corresponding p-substituted benzoic acids. Solid-state structure determination of 2a revealed the 3D network structure through intermolecular hydrogen bonding between the hydroxy group of the p-hydroxybenzoate ligand and the oxygen atom of the carboxylate ligand of 2a. UV-vis analysis of 2a-c in CH₃CN or CH₃CN-H ₂O in the presence of γ-Al₂O₃ clearly indicated the adsorption efficiency of these platinum clusters on γ-Al₂O₃: 2a bearing a hydroxyl group and 2b bearing a B(OH)₂ group were effectively deposited onto γ-Al ₂O₃ from CH₃CN solution, whereas less than 40% of 1 and 2c were chemically adsorbed onto γ-Al₂O₃. Highly dispersed and very small platinum nanoparticles (less than 1 nm) on γ-Al₂O₃ were obtained by thermal treatment of Pt₄-deposited γ-Al₂O₃ at 500 °C.

Full text of DOI:10.1039/c3dt50670c

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Tetraplatinum cluster complexes bearing hydrophilic
anchors as precursors for γ-Al2O3-supported platinum
nanoparticles†
Cite this: Dalton Trans., 2013, 42, 12662
Shinji Tanaka,a Naoto Nagata,b Naoki Tagawa,a Hirohito Hirata,b
Shin-ichi Matsumoto,b Hayato Tsurugia and Kazushi Mashima*a
Tetraplatinum cluster complexes bearing hydrophilic anchors, [Pt4(μ-OCOCH3)4(μ-OCOC6H4OH-4)4] (2a),
[Pt4(μ-OCOCH3)4(μ-OCOC6H4B(OH)2-4)4] (2b), and [Pt4(μ-OCOCH3)4(μ-OCOC6H4NH2-4)4] (2c), were suc-
cessfully prepared by a selective substitution reaction of four in-plane acetates of [Pt4(μ-OCOCH3)8] (1)
with the corresponding p-substituted benzoic acids. Solid-state structure determination of 2a revealed
the 3D network structure through intermolecular hydrogen bonding between the hydroxy group of the
p-hydroxybenzoate ligand and the oxygen atom of the carboxylate ligand of 2a. UV-vis analysis of 2a–c
in CH3CN or CH3CN–H2O in the presence of γ-Al2O3 clearly indicated the adsorption efficiency of these
platinum clusters on γ-Al2O3: 2a bearing a hydroxyl group and 2b bearing a B(OH)2 group were effec-
tively deposited onto γ-Al2O3 from CH3CN solution, whereas less than 40% of 1 and 2c were chemically
adsorbed onto γ-Al2O3. Highly dispersed and very small platinum nanoparticles (less than 1 nm) on
γ-Al2O3 were obtained by thermal treatment of Pt4-deposited γ-Al2O3 at 500 °C.
Received 11th March 2013,
Accepted 16th April 2013
DOI: 10.1039/c3dt50670c
www.rsc.org/dalton
peripheral modification by ligand substituents allows for metal
clusters to be adsorbed onto solid surfaces.12a,16 We considered
Introduction
Platinum nanoparticles have attracted considerable attention the tetraplatinum cluster complex [Pt4(μ-OCOCH3)8] (1)17,18 to be
due to their versatile catalytic performance in exhaust gas con- a well-defined precursor for introducing platinum nanoparticles
verters
of
automobiles,
fuel
cells,
and
organic on surfaces,19 because four in-plane acetate ligands of 1 can be
transformations.1–4 To improve the catalytic efficiency of plati- selectively substituted by other carboxylic acids, amidinates, and
num nanoparticles, the size of the platinum particles must be so on.20 We previously demonstrated that such a labile property
reduced to nano-scale because minimization of the particle of the four acetate ligands of 1 was applicable for not only selec-
size to close to or less than 1 nm induces a drastic change in tive modification by redox active units21 but also rational con-
the activity and selectivity in catalysis,5,6 but precise size struction of supramolecular assemblies of Pt4 units.22 Herein,
control for extremely small particles is difficult. In fact, a finely we synthesized tetraplatinum cluster complexes bearing hydro-
designed macromolecular template led to the formation of philic functional groups at four in-plane sides of the square Pt4
platinum particles in a subnanometer scale,7–9 but removal of entity as precursors for introducing monodispersed Pt4 onto a
the template induced substantial platinum sintering.10,15
γ-Al2O3 surface, and characterized the calcined platinum nano-
Cluster complexes in which the number of metal atoms is particles by STEM and CO absorption studies.
precisely controlled at the molecular level as characterized by
X-ray analysis as well as spectroscopic measurements have been
utilized
as
precursors
for
size-controlled
metal
nanoparticles.11–15 The merit of cluster precursors is that
aDepartment of Chemistry, Graduate School of Engineering Science, Osaka
University, and CREST, JST, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531,
Results and discussion
Synthesis and characterization of Pt4 clusters bearing
hydrophilic groups
Japan. E-mail: mashima@chem.osaka-u.ac.jp
bToyota Motor Corporation, 1 Toyota-cho, Toyota, Aichi 471-8572, Japan
†Electronic supplementary information (ESI) available: X-ray crystallographic
data in CIF format for complex 2a, tables of crystal data and refinement para-
meters for 2a. CCDC 924685. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3dt50670c
We previously reported a substitution reaction of in-plane
acetate ligands of 1 by other carboxylate ligands:22b reaction of
12662 | Dalton Trans., 2013, 42, 12662–12666
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1 with an excess of p-hydroxybenzoic acid in a mixture of
CH2Cl2 and MeOH followed by treatment under reduced
pressure to remove the liberated acetic acid afforded [Pt4(μ-O-
COCH3)4(μ-OCOC6H4OH-4)4] (2a) in quantitative yield (eqn
(1)). The 1H NMR spectrum of 2a in DMSO-d6 showed one
singlet resonance assignable to the methyl group of out-plane
acetate ligands, and AB type doublet signals at δ 6.88 and 8.13
(3JHH = 8.8 Hz) assignable to ortho- and meta-aromatic protons.
The overall molecular structure was further determined by
single crystal X-ray analysis (vide infra). In a similar manner,
[Pt4(μ-OCOCH3)4(μ-OCOC6H4B(OH)2-4)4] (2b) and [Pt4(μ-O-
COCH3)4(μ-OCOC6H4NH2-4)4] (2c) were prepared by treating 1
with an excess of p-boronobenzoic acid and p-aminobenzoic
acid, respectively.
Fig. 2 Schematic drawing of the intermolecular hydrogen bonding network in
complex 2a (a), and that of stack structure (b). Out-plane acetate ligands were
omitted.
ð1Þ
Fig. 1 shows the molecular structure of complex 2a, where
four p-hydroxybenzoate ligands coordinate to in-plane sides of
a Pt4 square in a bridging manner with Pt–O bond distances of
2.110–2.176 Å. The Pt–Pt bond distances of 2a, 2.484–2.494 Å,
are comparable to those of 1 and reported tetracarboxylate
complexes20–22 and thus two adjacent PtII ions are bound to
each other with single bonds. Four aromatic rings of the
p-hydroxybenzoate ligands are parallel to the Pt4 square plane:
angles between the best plane of the Pt4 core and the four aro-
matic rings lie in the range of 1.52–15.16°, being much
smaller than those of the 2,6-diphenylbenzoate derivative.22b A
notable feature of 2a is the one-dimensional hydrogen
bonding network, in which solvated water bridges the hydroxy
group of p-hydroxyphenyl ligands and the oxygen atom of an
in-plane carboxylate ligand attached to another Pt4 unit to
make a one-dimensional flat network where a dihedral angle
between two consecutive Pt4 squares is 0° with distances of O
(phenoxy)–O(water) (2.732 Å) and O(water)–O(carboxylate)
(2.992 Å) (Fig. 2a). The other intermolecular hydrogen bonding
network is between the hydroxy group of p-hydroxyphenyl
ligands and the oxygen atom of the in-plane carboxylate
ligand, making a zig-zag one-dimensional network where the
dihedral angles between two Pt4 squares are 75.96° with the
distance of O(phenoxy)–O(carboxylate) (2.889 Å) (Fig. 2a and b).
Adsorption of Pt4 clusters 1 and 2a–c on γ-Al2O3
We performed an adsorption test of these Pt4 complexes 1 and
2a–c onto the γ-Al2O3 surface by monitoring the absorption
spectra of 1 and 2a–c in CH3CN or CH3CN–H2O and those for
which γ-Al2O3 was added in 0.5 wt% Pt–γ-Al2O3. Notably,
absorption peaks at 262 and 277 nm for 2a and 253 nm for 2b
with large ε values (>4 × 104) decreased upon the addition of
γ-Al2O3, indicating that Pt4 complexes in solution were efficien-
tly adsorbed onto the γ-Al2O3 surface with a grafting efficiency
of 99% for 2a and 80% for 2b (Fig. 3b and 3c). In sharp con-
trast, the addition of γ-Al2O3 to complexes 1 and 2c in CH3CN
resulted in a partial decrease of their absorbance (Fig. 3a and
3c) and grafting efficiency, estimated to be 39% for 1 and 36%
for 2c (Table 1). Accordingly, the hydroxy groups attached to
the Pt4 core acted as anchors suitable for interacting with the
Fig. 1 Molecular structure of complex 2a with thermal ellipsoids at 50% prob-
ability. All H atoms and solvent molecules (H2O and Et2O) were omitted for the
sake of clarity. Selected bond distances [Å]: Pt(1)–Pt(2) 2.4943(8), Pt(2)–Pt(3)
2.4947(8), Pt(3)–Pt(4) 2.4906(8), Pt(1)–Pt(4) 2.4984(8), Pt(1)–O(2) 2.154(12),
Pt(1)–O(3) 2.110(15), Pt(2)–O(4) 2.176(17), Pt(2)–O(5) 2.173(11), Pt(3)–O(6)
2.165(12), Pt(3)–O(7) 2.120(15), Pt(4)–O(8) 2.171(15), Pt(4)–O(1) 2.167(13).
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Fig. 3 UV-vis spectra of complexes 1 (a), 2a (b), 2b (c), and 2c (d) in CH3CN
(13 μM) for 1, 2a, 2c and CH3CN–H2O (15/1) (12 μM) for 2b, and those of the
supernatant in a mixture with γ-Al2O3 (0.5 wt% on Pt) in CH3CN.
Table 1 λmax values, absorbance at λmax, grafting efficiency based on UV-vis
measurement of complexes 1 and 2a–c
1
2a
2b
2c
λmax (nm)
Absorbance at λmax
246
0.60
0.37
39
262
1.28
0.02
99
253
1.35
0.27
80
301
1.61
1.04
36
Without γ-Al2O3
With γ-Al2O3
Grafting efficiency (%)
Fig. 4 STEM images of Pt nanoparticles on γ-Al2O3 prepared from 2a (a) and
2b (b).
γ-Al2O3 surface through hydrogen bonding interactions, and
promoted the spontaneous adsorption of Pt4 clusters onto
γ-Al2O3 surfaces.
were further analyzed by STEM (Fig. 4a and b) and were in a
highly dispersed form. The sizes of the particles were calcu-
lated to be 0.71 nm for complex 2a and 0.80 nm for complex
2b based on CO absorption studies (Table 2), suggesting that
platinum particles of a size less than 1 nm were efficiently gene-
rated by highly dispersed grafting of Pt4 cluster complexes on
γ-Al2O3 via multiple hydrogen bonds between hydrophilic
functional groups and the surface.
Characterization of Pt/γ-Al2O3
Based on the grafting efficiency of these Pt4 complexes on
γ-Al2O3, we prepared several Pt/γ-Al2O3 samples (0.5 wt% on
Pt) up to the maximum adsorption under operations by fil-
tration from CH3CN, drying under vacuum at 120 °C, and cal-
cination at 500 °C for 2 h under air. ICP analysis of the
resulting solid samples derived from complexes 2a–c indicated
that the platinum content of each sample was 0.49 wt% for 2a,
0.45 wt% for 2b, and less than 0.10 wt% for 2c (Table 2). Plati-
num particles on γ-Al2O3 derived from complexes 2a and 2b
Conclusions
We successfully prepared Pt4 cluster complexes bearing OH
group 2a, B(OH)2 group 2b, and NH2 group 2c by selective sub-
stitution reactions of 1 with benzoic acid derivatives. The
solid-state structure of 2a was determined by a single crystal
X-ray analysis, revealing a unique hydrogen bond network via
OH and carboxylate groups. Complexes 2a and 2b in CH3CN
were efficiently adsorbed onto γ-Al2O3. The sizes of platinum
particles derived from thermal treatment of grafted materials
were well controlled at less than 1 nm. This methodology for
Table 2 Pt content and size of Pt particles in Pt/γ-Al2O3 prepared from 2a–c
2a
2b
2c
Pt content (wt%)a
0.49
0.71
0.45
0.80
<0.10
—
Size of Pt particles (nm)b
a Determined by ICP analysis. b Determined by CO absorption
measurement.
12664 | Dalton Trans., 2013, 42, 12662–12666
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generating subnanometer platinum particles is thus simple for C36H36B4O24Pt4: C 25.79, H 2.16. Found: C 25.83, H 2.30, N
and controllable.
0.07.
Preparation of [Pt4(μ-OCOCH3)4(μ-OCOC6H4NH2-4)4] (2c)
Experimental section
General procedures
This compound was prepared in a similar manner as for com-
pound 2a using p-aminobenzoic acid instead of p-hydroxyben-
zoic acid. Orange powders (98% yield), mp 204–206 °C
All manipulations involving air- and moisture-sensitive com-
pounds were carried out under argon using standard Schlenk
techniques or in an argon-filled glovebox. [Pt4(μ-OCOCH3)8] (1)
was prepared according to literature procedures.20 p-Hydroxy-
benzoic acid, p-aminobenzoic acid, and p-boronobenzoic acid
were purchased and used as received. γ-Al2O3 powder (57 m2
g−1) was purchased from Nanotek. Dehydrated hexane, Et2O,
and CH2Cl2 were purchased from Kanto Chemical and further
purified by passage through activated alumina under positive
argon pressure as described by Grubbs et al.23 Dehydrated
MeOH and DMSO-d6 were degassed and stored under an
argon over activated 3 Å (MeOH) and 4 Å (DMSO-d6) molecular
1
(decomp.). H NMR (400 MHz, DMSO-d6, 30 °C, δ/ppm): 1.96
3
(s, 12H, axCH3CO2), 5.91 (s, 8H, –NH2), 6.61 (d, JHH = 8.8 Hz,
3
8H, Ar H), 7.95 (d, JHH = 8.8 Hz, 8H, Ar H). MS (ESI positive,
CH3CN, m/z): 1583 ([M + Na]+), 1562 ([M + H]+), 1424 ([M −
O2CC6H4NH2]+). Anal. Calcd for C36H36N4O16Pt4: C 27.70,
H 2.32, N 3.59. Found: 28.01, H 1.95, N 3.37.
UV-vis spectroscopy
UV-vis spectra were recorded on an Agilent 8453 spectrometer.
Complexes (9.7 μmol) were dissolved in CH3CN (30 mL) for 1,
2a, and 2c and CH3CN/H2O (30 mL + 2 mL) for 2b, and each
solution was diluted by 4% (1.3 × 10−5 M) before to measure
UV-vis spectra. For grafted samples, 1.5 g of γ-Al2O3 was added
to the CH3CN solution, and the suspension was stirred for
30 min. After filtration, the filtered solution diluted by 4% was
used for UV-vis spectroscopy.
1
sieves. H NMR (400 MHz) and 13C{1H} NMR (100 MHz) were
measured on Bruker AVANCEIII-400 spectrometers, and all
spectra were recorded at 30 °C unless mentioned otherwise
and referenced to an internal solvent. Elemental analyses were
recorded on a Perkin-Elmer 2400II microanalyzer in the
Department of Chemistry, Faculty of Engineering Science,
Osaka University. Mass spectrometric data were obtained
using ESI techniques on a BRUKER micrOTOF spectrometer.
Melting points were measured in sealed tubes and were not
corrected.
Crystallographic determination of 2a
Crystals of 2a (red, block) were obtained via diffusion of
diethyl ether into the acetonitrile solution of 2a. A crystal was
mounted on the CryoLoop (Hampton ReseArCh Corp.) with a
layer of mineral oil and placed in a nitrogen stream. Complex
Preparation of [Pt4(μ-OCOCH3)4(μ-OCOC6H4OH-4)4] (2a)
2a was measured with
a Rigaku RAXIS-RAPID Imaging
To a solution of [Pt4(μ-OCOCH3)8] (86.8 mg, 0.0693 mmol) in
CH2Cl2 (6 mL) was added a solution of p-hydroxybenzoic acid
(76.2 mg, 0.552 mmol, 8 equiv.) in MeOH (5 mL). The reaction
mixture was stirred for 20 h at ambient temperature under
reduced pressure. During this operation, a mixture of CH2Cl2
(6 mL) and MeOH (6 mL) was repeatedly added before all vola-
tiles were completely removed. Removal of all volatiles gave the
solid, which was washed with Et2O (8 mL, three times) to leave
complex 2a as orange powders (110 mg, quant.), mp
210–212 °C (decomp.). 1H NMR (400 MHz, DMSO-d6, 30 °C,
Plate equipped with a sealed tube X-ray generator (40 kV,
50 mA) with graphite monochromated Mo-Kα (0.71075 Å) radi-
ation in a nitrogen stream at 113(1) K. The unit cell parameters
and the orientation matrix for data collection were determined
by the least-squares refinement with the setting angles, which
are listed in Table S1.† The structure was solved by direct
methods on SIR97,24 after being refined on F2 by full-matrix
least-squares methods using SHELXL-97.25 Measured nonequi-
valent reflections with I > 2.0σ(I) were used for the structure
determination. The hydrogen atoms were included in the
refinement on calculated positions riding on their carrier
3
δ/ppm): 1.99 (s, 12H, axCH3CO2), 6.88 (d, JHH = 8.6 Hz, 8H, Ar
3
H), 8.13 (d, JHH = 8.6 Hz, 8H, Ar H). MS (ESI positive, CH3CN,
2
2
atoms. The function minimized was [Σw(Fo − Fc )] (w =
m/z): 1587 ([M + Na]+), 1427 ([M − O2CC6H4OH]+). Anal. Calcd
for C36H32O20Pt4: C 27.63, H 2.06. Found: 27.83, H 1.99,
N 0.08.
2
2
2
1/[σ2(Fo ) + (aP)2 + bP]), where P = (Max(Fo ,0) + 2Fc )/3 with
σ2(Fo ) from counting statistics. The functions R1 and wR2 were
2
(Σ||Fo| − |Fc||)/Σ|Fo| and [Σw(Fo − Fc ) /Σ(wFo )]1/2, respecti-
vely. In crystals of 2a, a disordered diethyl ether molecule with
its occupancy of 0.75 is involved. We refined those atoms in an
2
2 2
4
Preparation of [Pt4(μ-OCOCH3)4(μ-OCOC6H4B(OH)2-4)4] (2b)
This compound was prepared in a similar manner as for com- isotropic form because their separation into a disordered form
pound 2a using p-boronobenzoic acid instead of p-hydroxy- and anisotropic refinement were unsuccessful. ADP ratio
benzoic acid. Orange powders (87% yield), mp 212–215 °C values of atoms O7 and O10 were relatively high, presumably
1
(decomp.). H NMR (400 MHz, DMSO-d6/D2O, 30 °C, δ/ppm): due to the location of those atoms near the disordered diethyl
3
2.02 (s, 12H, axCH3CO2), 7.97 (d, JHH = 7.6 Hz, 8H, Ar H), 8.25 ether molecule. The ORTEP-3 program was used to draw mole-
3
(d, JHH = 7.6 Hz, 8H, Ar H). MS (ESI positive, CH3CN/H2O, cules.26 CCDC 924685 (2a) contains the supplementary crystal-
m/z): 1669 ([M + Na]+), 1511 ([M − O2CC6H4B(OH)2]+). Anal. Calcd lographic data for this paper.
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Preparation and characterization of Pt/γ-Al2O3
7 R. M. Crooks, M. Zhao, L. Sun, V. Chechik and L. K. Yeung,
Acc. Chem. Res., 2001, 34, 181–190.
γ-Al2O3 powder (1.5 g) was added to a solution of 2a (15 mg,
9.6 μmol) in CH3CN (30 mL). The mixture was kept under stir-
ring at room temperature for 2 h. After filtration, the residue
was washed with CH3CN, dried at 120 °C overnight, and cal-
cined at 500 °C in air for 2 h. Other catalysts were prepared in
a similar manner to that of the 2a/Al2O3. The Pt concentration
was determined by inductively coupled plasma atomic emis-
sion spectroscopy (ICP-AES) using a Shimadzu ICPV-8100 after
decomposing the catalyst with aqua regia. A scanning trans-
mission electron microscopy (STEM) image was measured on a
Hitachi HD2000 STEM operated at 200 kV in the z-contrast
mode. CO chemisorption for the estimation of Pt particle size
was measured using an Okura Riken R6015 apparatus
equipped with a thermal conductivity detector (TCD). A
sample of 150 mg was introduced into a glass tube and pre-
treated sequentially in O2 for 15 min, H2 for 15 min, He for
15 min at 400 °C and then cooled to 50 °C under a He flow.
Adsorbed CO was counted using a pulse injection method at
50 °C.
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Acknowledgements
This work was supported by CREST, JST. S.T. is thankful for
the Global-COE Program of Osaka University (2010–2012).
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