Stabilisation of water-soluble platinum nanoparticles by phosphonic acid derivatives

Article abstract of DOI:10.1039/c2dt12071b

Sodium 2-(diphenylphosphino)ethyl phosphonate (1) was investigated as a stabilising agent for platinum nanoparticles (Pt-NPs) in aqueous solution. This phosphino phosphonate is known to stabilise rhodium nanoparticles (NPs) in water. Here we report that i

Full text of DOI:10.1039/c2dt12071b

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PAPER
Stabilisation of water-soluble platinum nanoparticles by phosphonic acid
derivatives†
Mareike Richter,a Arndt Karschin,a Bernhard Spingler,b Peter C. Kunz,*a Wolfgang Meyer-Zaikac
and Wolfgang Kläuia
Received 1st November 2011, Accepted 19th December 2011
DOI: 10.1039/c2dt12071b
Sodium 2-(diphenylphosphino)ethyl phosphonate (1) was investigated as a stabilising agent for platinum
nanoparticles (Pt-NPs) in aqueous solution. This phosphino phosphonate is known to stabilise rhodium
nanoparticles (NPs) in water. Here we report that in the case of Pt-NPs this ligand is indirectly involved in
the stabilisation mechanism and the actual stabilisation agent is the platinum complex Na2[Pt(1)2] (2).
The reduction of platinum(II) salts in the presence of the phosphonates 1, 2, sodium
2-(diphenylphosphoryl)ethyl phosphonate (3) and 3,3,3-triphenylpropyl phosphonate (4) leads to stable
platinum NPs with a remarkably narrow particle size distribution. These platinum NPs show high
catalytic activity in the hydrogenation of 1-hexene and 1-chloro-3-nitrobenzene under biphasic as well as
heterogeneous (supported on charcoal) conditions. The activity of the supported NPs was 30 times higher
than the commercially available catalyst Pt(0) EnCat®. Furthermore, the single-crystal X-ray structures of
(1)(MeOH)2(H2O)2, (3)(H2O)4, and (4)2(H2O)17 have been determined.
prevent such agglomeration processes, stabilisation of such par-
ticles is of major interest, where usually two main concepts of
Introduction
In the field of nanoparticle (NP) research, synthesis and stabilis-
ation of well-characterized, small and monodisperse metal NPs
is of significant interest for catalytic applications,1–3 mainly due
to their very high relative surface area causing high mass activity.
Metallic systems at the nanoscale tend to have different proper-
ties in relation to bulk material due to their size.4,5 Today the
chemical “bottom up” method for preparation of nanosized par-
ticles is the most common technique used due to fairly simple
procedures and the advantage that a very narrow size distribution
can be obtained.6 Unfortunately, a general synthetic route to
monodisperse metal particles is not available, as the mechanisms
of formation of such NPs are poorly understood.7–11 Still a
variety of metal precursors, stabilisers (dendrimers, surfactants,
ionic liquids, polymers, cyclodextrines or ligands), solvents or
reducing agents is used.6,12–14 Due to their high surface energy,
NPs tend to agglomerate at low interparticle distances forming
bulk material which precipitates from the solution.15,16 To
stabilisation are discussed: (i) electrostatic repulsion and (ii)
steric shielding.6,13 Recently we reported on the synthesis
of highly water soluble rhodium NPs stabilised by sodium
2-(diphenylphosphino)ethyl phosphonate(1).17 The excellent
water solubility and the high stability of the NPs were attributed
to electrostatic repulsion of the negatively charged phosphonate
groups of this ligand covering the particle surface. Within this
concept we proposed that the phosphine phosphorus atom binds
to the metal surface and the phosphonate group is directed
towards the solvent causing water solubility and repulsion of the
NPs. Therefore 1 should be suitable to stabilise NPs of other
metals that form stable bonds to phosphine ligands as well. Here
we present platinum NPs synthesised using 1 and evaluated
further ligands that stabilise metal NPs without having “classi-
cal” soft donor atoms (Scheme 1). We discuss a different
concept of stabilisation for these water-soluble NPs.
aInstitut für Anorganische Chemie und Strukturchemie, Lehrstuhl I:
Bioanorganische Chemie und Katalyse, Heinrich Heine Universität
Düsseldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany.
E-mail: Peter.Kunz@uni-duesseldorf.de; Fax: +49 211 12287; Tel: +49
211 12283
Results and discussion
We synthesised platinum NPs by reaction of K2[PtCl4] and 1 in
water under an atmosphere of hydrogen gas as described for the
preparation of rhodium NPs.17 Instead of hydrogen gas methanol
can be used as reducing agent. The reduction takes place only as
long as the ligand/platinum ratio (L/Pt) < 2. The course of the
reaction was followed by 31P NMR spectroscopy. Initially
complex 2 is formed by coordination of two 1 ligands to
platinum(II). The reaction is completed within a few minutes. At
all L/Pt ≤ 2 complex 2 was the only platinum species observed
bInstitut für Anorganische Chemie, Universität Zürich,
Winterthurerstrasse 190, 8057 Zürich, Switzerland
cInstitut für Anorganische Chemie, Universität Duisburg-Essen,
Universitätsstraße 5-7, 45117 Essen, Germany
†Electronic supplementary information (ESI) available: Details on
crystal data and structure refinement for 1, 3, and 4. CCDC reference
numbers 782185–782187. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2dt12071b
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Fig. 1 HRTEM micrograph of platinum NPs stabilised with 2 (left)
and size distribution thereof (right). The NPs were prepared using a
hydrogen atmosphere of p(H2) = 75 bar.
in solution. At L/Pt > 2 the only additional signal in the 31P
NMR spectra was that of non-coordinated ligand 1. Pt-NPs
formed from solutions of K2[PtCl4] and 1 with L/Pt < 2 within
5 days under a hydrogen atmosphere. These homogeneously
dispersed NP solutions are stable for months. The so-obtained
Pt-NPs can be isolated as black solid by removing the solvent,
and are easily redispersed in water. HRTEM micrographs of the
obtained NPs show an average diameter of 2.2 nm with a rela-
tively narrow size distribution (see Fig. 1).
Scheme 1 Water-soluble (amphiphilic) phosphonates.
The 31P NMR spectra of the so-prepared particle solutions
show only signals of complex 2. The reduction of aqueous sol-
utions of independently prepared complex 2 in the presence of
K2[PtCl4] proceeds in the same way as the reduction of solutions
prepared from ligand 1 and K2[PtCl4] with L/Pt < 2 in situ.
Furthermore we observed that once complex 2 had formed,
this compound is not reduced by methanol or hydrogen gas,
even under a hydrogen atmosphere up to a pressure of 80 bar;
complex 2 alone is inert to reduction. Thus, only free platinum(II)
(not coordinated by ligand 1) is reduced (Scheme 2). In the
absence of a stabilising agent (e.g. complex 2) reduction of
K2[PtCl4] in aqueous solution results in precipitation of platinum
black.
Scheme 2 Preparation of Pt-NPs using complex 2 as stabilizing agent.
In order to rule out the possibility that in an equilibrium reac-
tion some 1 is released by 2 and then acts as a stabilising agent,
we synthesised phosphonate 3, the oxidised form of compound
1. The phosphonate 3 has no phosphine donor centre and does
not form complexes with platinum(II). Here again, purging of
hydrogen gas through aqueous solutions of K2[PtCl4] and 3 or
addition of methanol to these solutions leads to deep brown NP
solutions. Therefore both complex 2 and the phosphine oxide 3
are able to stabilise Pt-NPs.
To disprove that under the reducing reaction conditions used
some phosphine oxide 3 might be reduced to phosphine 1 as
stabilisation agent we investigated sodium (3,3,3-triphenylpropyl)
phosphonate (4) as stabilizing agent. Compound 4 has steric and
electrostatic properties similar to those of 1 and 3 and does not
bear any phosphine or phosphine oxide functionalities. This
excludes coordination of a potential phosphine species to plati-
num. The synthesis of 4 starting from triphenylmethane and
diethyl-2-bromoethylphosphonate is done analogously to the
preparation of 1 (Scheme 3).17,18
Scheme 3 Synthesis of 4.
In contrast to experiments using 1, the L/Pt ratio was not crucial
as 4 has no soft donor centre which coordinates strongly to
platinum(II) and therefore does not influence the concentration of
free Pt(II) in solution. The suspension turned deep brown within
a few hours and the pH value decreases from 8 to 6. This
decrease of the pH value is caused by the oxidation of methanol
to formaldehyde.19–22 NP formation was completed within 24 h.
Water and sodium hydroxide was added to a final pH of 8–9 in
order to ensure that the phosphonate group of 4 is fully deproto-
nated. The deep brown NP solutions synthesised this way are
stable for months and the NPs can be redissolved after isolation.
HRTEM micrographs confirmed formation of Pt-NPs with a rela-
tively narrow size distribution of 2.1 nm (Fig. 2). The 31P NMR
spectra of the air-stable solutions show only the signal of 4.
These observations can be rationalized by applying the
concept of double layer stabilisation.13 In this model, compounds
2, 3 and 4 respectively interact with the surface of the NPs by
their phosphonate groups. The phenyl rings of this first layer
Stabilisation of platinum NPs using sodium
(3,3,3-triphenylpropyl) phosphonate (4)
In order to explore 4 as a stabilising agent for NPs, one equival-
ent of PtCl2 and two equivalents of 4 were reacted in methanol.
3408 | Dalton Trans., 2012, 41, 3407–3413
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Catalytic activity in biphasic systems
The formation of dark-brown coloured solutions is an indication
of the existence of platinum NPs. The actual formation of NPs
was proven by TEM and HRTEM micrographs, although these
methods show only an infinitesimally small part of the sample.
Platinum(II) compounds do not catalyse hydrogenation reac-
tions,26 which is in accordance with the observation that 2 shows
no catalytic activity in the hydrogenation of 1-hexene. Activity
in the catalytic hydrogenation of 1-hexene is a simple, though
indirect, test for the formation of Pt-NPs from platinum(II)
compounds.
Fig. 2 HRTEM micrograph of platinum NPs stabilised with 4 (left)
and corresponding size distribution (right).
Catalytic activities of NP solutions stabilised by phosphonates
1–4 (solutions 5–8) in the hydrogenation of 1-hexene under
biphasic conditions are presented in Table 1. The NP solutions
did not show loss of activity or coagulation in up to five sub-
sequent catalytic runs. The turnover frequencies (TOFs)
observed are within the range reported for hydrogenation reac-
tions using Pt NPs in ionic liquids.11,27
The catalytic activities determined under biphasic conditions
are limited by mass transport across the hydrophilic/lipophilic
interface. This was shown by variation of the stirring rate where
we could observe an almost linear dependence of the conversion
with the stirring rate. Even with the highest stirring rates
(∼1000 rpm) the mass transport limitations could not be
avoided. To circumvent these limitations we exposed the NPs
directly to the substrate using a carbon support and used them in
heterogeneous catalysis where no diffusion processes through a
biphasic interface are taking place. To achieve this, an aqueous
solution of Pt NPs 8 (2.1 nm) was added to activated charcoal
and stirred for five hours.6,28–30
interact with those of ligands of a second layer whose phos-
phonate groups facilitate solubility in the aqueous phase.
Monflier and Roucoux proposed an analogous stabilisation of
ruthenium(0) and rhodium(0) NPs by polyhydroxylated
ammonium chloride salts.23–25 Such double layer structures are
also dominant characteristics in the packing found in the solid-
state structures of 1, 3 and 4 (see Fig. 3). All three compounds
show an alternating stacked hydrophilic-hydrophobic layered
structure in the solid state. The hydrophilic double layer consists
of a two-dimensional net of phosphonate and oxygen atoms
coordinating the sodium ions (for further discussion of the solid-
state structures see the supporting information†).
Using this concept we can also understand why the addition
of water is inevitable after formation of NPs using 4 as stabilis-
ing agent in methanol as solvent. Compound 4 is amphiphilic
and shows high solubility in methanol, whereas the correspond-
ing NPs bear phosphonate groups at the surface making them
less soluble in methanol but highly soluble in water. The
addition of water is therefore vital to keep the NPs in solution.
The NPs were adsorbed on charcoal as seen by the discolour-
ation of the solution. The supported NPs (8/C) were washed
Fig. 3 Except for the packing of ligands 1, 3 and 4 in the solid state (1 and 3 view along the b-axis, 4 view along the a-axis), the inorganic bi-layer
structure with the coordinated sodium atoms is shown in a ball and stick representation whereas the hydrophobic organic parts are presented as wire-
frames. Hydrogen atoms are omitted for clarity.
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Table 1 Catalytic activity of Pt NPs stabilised by phosphonates 1–4 in
the hydrogenation of 1-hexene under biphasic conditions.
Hydrogenation trials were performed at 25 °C using a hydrogen gas
pressure of 20 bar
(3,3,3-triphenylpropyl) phosphonate (4), can also act as stabilis-
ing agents for platinum NPs in aqueous solution. Therefore, the
term “ligand-stabilised” is not appropriate for these NPs which
was also shown by utilisation of the phosphine ligand 1, which
forms complex 2 with a Pt(II) precursor species, themselves
being the true stabilisers. An applicable stabilisation model in
this context is the concept of double layer stabilisation, where a
micelle-like structure coats the particle surface. All of the syn-
thesised stabilisers show amphiphilic behaviour in their solid-
state structures which supports this model. The water-soluble
NPs show activity in the hydrogenation of 1-hexene under bipha-
sic conditions with a TOF of up to 2100 h−1. The activity is
increased tremendously when the NPs are supported on active
charcoal. The supported NPs furthermore show high activity in
the hydrogenation of 1-chloro-3-nitrobenzene without formation
of any dehalogenation products.
Pt NP solution
Stabiliser
NP size/nm
TOF/h−1
5
6
7
8
1
2
3
4
2.2
n.d.
n.d.
2.1
1600
1400
1000
2100
Table 2 Turnover frequencies (TOF) for the hydrogenation of
1-chloro-3-nitrobenzene
Catalyst
Solvent
System
TOF/h−1
5
8
Water/hexane
Water/hexane
THF
Hexane
Thf
Biphasic
Biphasic
Heterogeneous
Heterogeneous
Heterogeneous
Heterogeneous
25
50
25
300
800
750
Pt(0) EnCat®
Pt/C
8/C
8/C
Hexane
Experimental section
Activated charcoal (Darco® KB, −100 mesh, wet powder)
(Aldrich), platinum on activated charcoal (Fluka), and 3-chloro-
nitrobenzene (Merck, > 99%) were used as received. Pt(0)
EnCat® (Aldrich) was washed with methanol and dried before
use. Solvents were dried and distilled under nitrogen before use.
Hydrogen gas, purity 5.0 (Air Liquide), was used for hydrogen-
ation reactions. 1H NMR and 31P{1H} NMR spectra were
recorded with a Bruker Avance DRX200, and 13C NMR spectra
repeatedly with water until the 31P NMR spectrum of the filtrate
showed no more signals. 8/C was used as heterogeneous catalyst
in the hydrogenation of 1-hexene and showed a TOF of about
200 000 h−1 under mild conditions (20 bar H2 and ambient
temperature) thus being as active as the industrially used hydro-
genation catalyst Pt/C. We assume that this higher activity of
supported NPs compared to non-supported particles under bipha-
sic conditions is most likely due to three effects: (i) no mass
transport limitation of the substrate over the liquid/liquid inter-
face, (ii) the appearance of “naked” NPs on the support which
are not covered by ligands, (iii) catalytic electron transfer effects
of the carbon surface. Whether or not the support plays a major
role in the hydrogenation mechanism, influencing the catalytic
activity, is currently under investigation.
We also examined the potential of the NPs in the hydrogen-
ation of 1-chloro-3-nitrobenzene (Table 2). The selective
reduction of halogenated nitrobenzene to chloroaniline is of
great industrial interest as the corresponding amines are impor-
tant intermediates for pharmaceutical and agrochemical sub-
stances.31 The dehalogenation of the aryl compound is the most
common side reaction and should be minimized.
The Pt-NPs show remarkable catalytic activities in the
reduction of 1-chloro-3-nitrobenzene under biphasic—but
especially under heterogeneous—conditions. We never observed
dehalogenation of either the reactant nor the product. For com-
parison, catalytic activities of two commercially available hetero-
geneous hydrogenation catalysts (platinum on activated
charcoal, Pt/C and Pt(0) EnCat®) were tested. The TOF of both
commercial catalysts are significantly lower than those of the
supported NPs 8/C; the supported NPs are about two times as
active as Pt/C and about 30 times as active as Pt(0) EnCat®.
1
with a Bruker Avance DRX500 spectrometer. The H and 13C
NMR chemical shifts were referenced to the solvent residual
signals. Measured chemical shifts for the residual solvent
CDCl3: 7.30 ppm (1H), 77.6 ppm (13C); CD3OD: 3.3 ppm (1H),
49.4 ppm (13C); D2O: 4.7 ppm (1H). The 13C chemical shifts
were assigned by HMQC spectroscopy. The assignment was
confirmed by simulation using the program CS ChemDraw Ultra
12.0. For HRTEM imaging a Philips microscope TEM CM200-
FEG was available. An acceleration voltage of 200 kV was used.
The ESI mass spectrum was recorded with a Finnigan LCQ
Deca Ion-Trap-API mass spectrometer. GC/MS(EI) spectra were
recorded on a FINNIGAN Trace GC-Ultra equipped with a
DB5MS column (15 m, ∅ 0.25 mm). The injection temperature
was set to 220 °C, whereas the column temperature started at
50 °C and was increased linearly by 20 °C min−1 up to 250 °C.
For FT-IR analysis of reaction mixtures from hydrogenation
experiments of 1-hexene to determine quickly the conversion a
BRUKER IFS 66 was used. Conversion was determined by
comparing the intensity of the CvC valence vibration peak of
residual 1-hexene to the one of pure 1-hexene. This procedure
was verified to be reliable by GC analysis. Turnover frequencies
(as presented in Table 1) were calculated as mass activities with
the assumption that all precursor platinum species were reduced
and catalytically active. Due to the fact that in the case of the Pt
NP solution 5, 25% of the platinum(II) precursor species are con-
verted into the catalytically inactive complex 2, this amount of
platinum species was subtracted for the calculation of TOF.
TOFs were calculated as n(hexane) × n(Pt)−1 × time−1. Each
measurement was repeated at least three times. Values are given
as averaged.
Conclusions
We have shown that for the stabilisation of platinum NPs, a
phosphorus(III) atom as soft donor is not essential. Amphiphilic
phosphonates, such as the herein reported new sodium
3410 | Dalton Trans., 2012, 41, 3407–3413
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Crystallography
Sodium (2-(diphenylphosphoryl)ethyl)phosphonate (3)
Crystals of the solvates 1, 3, and 4 were obtained by slow evap-
oration of their solutions. 1 was crystallized from methanol/
acetonitrile, 3 from ethanol/water, and 4 was crystallized from
water.
1 (17.3 mg, 0.0480 mmol) was dissolved in water (5 mL).
Hydrogen peroxide (30% H2O2 in water, 0.1 mL, 0.9 mmol) was
added and the solution and stirred for 10 min. After removing
the solvent in vacuo, the phosphine oxide (3) was obtained quan-
titatively as a white solid. 1H NMR (200 MHz, D2O): δ =
7.2–7.6 (10H, m, C6H5), 2.1–2.3 (2H, m, C2H2), 1.2–1.5 (2H,
m, C1H2). 31P{1H} NMR (81 MHz, D2O, 25 °C): 44.3 (1P, d,
Crystallographic data were collected at 183(2) K on an Oxford
Diffraction Xcalibur system with a Ruby detector using Mo Kα
radiation (λ = 0.7107 Å) that was graphite monochromated. Suit-
able crystals were covered with oil (Infineum V8512, formerly
known as Paratone N), mounted on top of a glass fibre and
immediately transferred to the diffractometer. The program suite
CrysAlisPro was used for data collection, semi-empirical absorp-
tion correction and data reduction.32 Structures were solved with
direct methods using SIR9733 and were refined by full-matrix
least-squares methods on F2 with SHELXL-97.34 The crystal of
(3)(H2O)4 is a non-merohedral twin. The two twin domains are
related by a 180° rotation around (100); their ratio is 50.5 : 49.5.
Both domains were integrated and the structure refined with the
hklf5 command. Because of the twinning, not all the hydrogen
atoms of the water molecules could be found. In (1)
(MeOH)2(H2O)2, all 6 hydrogen atoms of O–H groups, and in
(4)2(H2O)17, all hydrogen atoms of the 17 water molecules,
could be localized and refined (partially with restraints). The
structures were checked for higher symmetry with help of
the program Platon.35 CCDC entries 782185–782187 contain the
supplementary crystallographic data for this paper.†
3
3JPP = 62.8 Hz, P-Oxide), 21.0 (1P, d, JPP = 62.8 Hz, PV). 13C
4
{1H} NMR (126 MHz, D2O): 133.2 (2C, d, JC,P = 2.5 Hz,
2
1
C
para), 131.1 (4C, d, JC,P = 9.9 Hz, Cortho), 129.9 (2C, d, JC,P
= 99.9 Hz, Cipso), 129.5 (4C, d, JC,P = 11.9 Hz, Cmeta), 23.9
3
(1C, dd, 1JC,P = 70.6 Hz, 2JC,P = 4.0 Hz, C2), 20.5 (1C, dd, 1JC,P
= 129.6 Hz, JC,P = 5.7 Hz, C1). C14H14O4P2Na2·5H2O
2
(444.27): calc. C 37.84, H 5.44; found C 37.57, H 5.14.
Sodium 3,3,3-triphenylpropyl phosphonate (4)
Diethyl(3,3,3-triphenylpropyl)phosphonate. Triphenylmethane
(11.2 g, 46.0 mmol) was placed in a 250 mL three-necked flask,
charged with a dropping funnel and a nitrogen inlet. After dissol-
ution of triphenylmethane in dry tetrahydrofuran (50 mL), the
mixture was cooled to 0 °C in an ice bath. n-Butyllithium
(29.0 mL, 46.4 mmol) was added dropwise over a period of
20 min. The red suspension was stirred for 2 h at 0 °C. After
cooling to −78 °C diethyl (2-bromoethyl)phosphonate (8.30 mL,
46.0 mmol) was added and the red suspension turned into a
yellow solution while warming slowly to ambient temperature.
After stirring for 2 h the solvent was removed in vacuo. The
yellow viscous residue was dissolved in dichloromethane
(30 mL) and washed 3 times with water (10 mL). Dichloro-
methane was removed in vacuo and the product was purified by
Kugelrohr distillation (bp 240 °C; 0.01 mbar). Yield 6.9 g (37%)
of diethyl (3,3,3-triphenylpropyl) phosphonate as yellowish
Sodium (2-(diphenylphosphino)ethyl)phosphonate (1)
The compound was synthesised according to a literature pro-
cedure.18 To obtain the ligand free of the corresponding oxide
(3) and further organic impurities, the product was recrystallized
from ethanol : water (9 : 1). Small amounts of water are essential
to start the crystallization process. Yield 79%.1H NMR
(200 MHz, D2O): δ = 7.3–7.8 (10H, m, C6H5), 2.4–2.7 (2H, m,
C2H2), 1.3–1.6 (2H, m, C1H2); 31P{1H} NMR (81 MHz, D2O):
1
viscous oil. H NMR (200 MHz, CDCl3): δ = 7.4–7.1 (15H, m,
C6H5), 4.2–3.9 (4H, m, OCH2), 3.0–2.8 (2H, m, C2H2), 1.6–1.4
3
(2H, m, C1H2), 1.3 (6H, t, JH,H = 7 Hz, CH3). 31P{1H}
3
3
δ = 22.4 (1P, d, JPP = 63 Hz, PV), −11.5 (1P, d, JPP = 63 Hz,
(81 MHz, CDCl3): δ = 33.9 (1P, s). 13C{1H} (126 MHz,
CDCl3): δ = 145.3 (3C, s, Cipso), 128.0 (6C, s, Cmeta), 127.0
PIII); 13C{1H} NMR (126 MHz, D2O): δ = 137.8 (2C, d, 1JC,P
=
7.7 Hz, Cipso), 133.0 (4C, d, JC,P = 17.5 Hz, Cortho), 129.5 (2C,
2
2
(6C, s, Cortho), 125.0 (3C, s, Cpara), 60.5 (2C, d, JC,P = 6.5 Hz,
s, Cpara), 129.2 (4C, d, JC,P = 6.8 Hz, Cmeta), 25.5 (1C, d, JC,P
3
1
2
OCH2), 55.5 (1C, s, C3), 31.6 (1C, d, JC,P = 2.5 Hz, C2), 21.3
= 128.7 Hz, JC,P = 11.7 Hz, C1), 21.6 (1C, dd, JC,P = 6.9 Hz,
2JC,P = 4.3 Hz, C2). ESI MS (H2O): m/z = 309 (M − 2Na+ + O
+ H+), 293 (M − 2Na+ + H+).
2
1
(1C, d, 1JC,P = 139.7 Hz; C1), 15.4 (2C, s, CH3) ppm.
(3,3,3-Triphenylpropyl)phosphonic acid. Diethyl(3,3,3-triphe-
nylpropyl)phosphonate (5.10 g, 12.5 mmol) was placed in a
100 mL Schlenk flask and dissolved in dry dichloromethane
(25 mL). The solution was cooled to −78 °C and bromotri-
methylsilane (7.65 g, 50.0 mmol) was added. The solution was
stirred for 2 h while warming to ambient temperature. Water
(15 mL) was added to hydrolyse the silyl ester. The mixture was
stirred for 1 h and the solvent was removed in vacuum. The
residue was dissolved in ethanol and the phosphonic acid preci-
pitated upon addition of water. The suspension was placed in the
refrigerator overnight and the precipitate isolated by filtration.
The white product is dried in vacuo at 80 °C for several hours.
cis-Na2[Pt{Ph2P(CH2)2(PO3)}2] (2)
1 (173 mg, 0.481 mmol) and PtCl2 (63.8 mg, 0.240 mmol) were
stirred in water/methanol. After 1 h the solution was filtered
through a syringe filter and all volatiles were removed in vacuo
to yield an off-white solid. Yield 153 mg (82%).1H NMR
(200 MHz, CD3OD): δ = 7.4–7.0 (20H, m, C6H5), 2.7–2.5 (4H,
m, C2H2), 1.5–1.3 (4H, m, C1H2); 31P{1H} NMR (81 MHz,
1
CD3OD): δ = 19.9 (2P, m, PV), 0.6 (2P, m, JPt,P = 3840 Hz,
PIII). ESI MS (CHOOH): m/z = 780 (M − 2Na+ + H+), 802
(M − Na+). C28H28O6P4PtNa2Cl2·2H2O·3CH3OH (1028.54):
calc. C 36.20, H 4.31; found C 36.38, H 4.44.
Yield 3.5 g (79%). H NMR (200 MHz, CD3OD): δ = 7.4–7.1
1
(15H, m, C6H5), 3.0–2.8 (2H, m, C2H2), 1.5–1.3 (2H, m, C1H2).
31P{1H} (81 MHz, CD3OD): δ = 31.2 (1P, s). 13C{1H}
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(126 MHz, CD3OD): δ = 148.3 (3C, s, Cipso), 130.7 (6C, s,
Cmeta), 129.5 (6C, s, Cpara), 127.6 (3C, s, Cortho), 58.2 (1C, s,
C3), 34.7 (1C, s, C2), 25.5 (1C, d, 1JC,P = 137.0 Hz; C1).
autoclave with glass inlet and a magnetic stirring bar. 1-Hexene
(2.0 mL, 16 mmol) was added and after flushing the autoclave
with nitrogen, a pressure of 20 bar of hydrogen gas was applied.
The biphasic mixture was stirred vigorously at ambient tempera-
ture to ensure very good mixing. The reaction was stopped after
20 min and the organic phase was separated and analysed by IR
spectroscopy.
Sodium 3,3,3-triphenylpropyl phosphonate. In a 100 mL
flask 3,3,3-triphenylpropyl phosphonic acid (2.00 g, 5.68 mmol)
was dissolved in dichloromethane (20 mL) and the solution
cooled using an ice bath. Sodium hydroxide (1 M solution in
water; 11.4 mL) was added whereupon a white solid precipitated.
After stirring the suspension for 30 min the precipitate was
filtered off. The solid was washed with several small portions of
dichloromethane and recrystallized from methanol. After drying
in vacuo 4 was obtained as a white, non-hygroscopic solid,
soluble in water, methanol, ethanol, acetone and tetrahydrofuran.
Hydrogenation of 1-hexene using 8/C and Pt/C
8/C (10 mg, 2.7 μmol) and Pt/C (10 mg, 10 wt% Pt/C,
5.1 μmol) respectively were used in an experimental setup as
described above. The reaction time was set to 2 min due to the
high activities.
1
Yield 1.5 g (67%). H NMR (200 MHz, CD3OD): δ = 7.5–7.1
(15H, m, C6H5), 3.1–2.9 (2H, m, 4CH2), 1.4–1.2 (2H, m, 3CH2).
31P{1H} (81 MHz, CD3OD): δ = 24.0 (1P, s). 13C{1H}
(126 MHz, CD3OD): δ = 149.7 (3C, s, Cipso), 131.2 (6C, s,
Cmeta), 129.0 (6C, s, Cortho), 126.9 (3C, s, Cpara), 58.8 (1C, s,
Hydrogenation of 1-chloro-3-nitrobenzene using 5 and 8
A solution of 5 (2.4 μmol, 1 mL) and 8 (3.8 μmol, 1 mL)
respectively was placed in a Schlenk tube equipped with a mag-
netic stirrer bar and a nitrogen gas inlet. A solution of 1-chloro-
3-nitrobenzene (0.5 mol L−1) in hexane (2 mL) was added. After
flushing with hydrogen the reaction mixture was stirred under a
pressure of 1.2 bar of hydrogen gas for 30 min at ambient temp-
erature. The mixture was analysed by GC-MS. The conversion
was determined by 1H NMR spectroscopy using the signal inten-
sities of the aromatic protons.
1
C3), 36.9 (1C, s, C2), 28.1 (1C, d, JC,P = 129.3 Hz, C1).
C21H19Na2PO3·5.5H2O: calc. C 50.9, H 6.1%; found C 51.0, H
6.1%.
Pt(0) NPs with phosphonates 1–3 (5–7)
K2PtCl4 (10.0 mg, 24,1 μmol) and the respective phosphonates
1 (4.32 mg, 12,0 μmol), 2 (19.9 mg, 24,1 μmol) or 3 (9.06 mg,
24.1 μmol) were placed in a 25 mL Schlenk tube and set under a
nitrogen atmosphere. Water (10.0 mL) was added and the solu-
tion stirred under an atmosphere of hydrogen for 5 days. The
solutions 5–7 were used for catalytic experiments without further
purification.
Hydrogenation of 1-chloro-3-nitrobenzene with 8/C, Pt/C or
Pt(0) EnCat®
8/C (5.0 mg, 1.3 μmol), Pt/C (6.7 mg, 3.4 μmol) and Pt(0)
EnCat® (10 mg, 1.2 μmol) respectively were used in an ex-
perimental setup as described above.
Pt(0) NPs with phosphonate 4 (8)
PtCl2 (10.0 mg, 37.6 μmol) and 4 (29.8 mg, 54.3 μmol) were
placed in a 25 mL Schlenk tube and set under a nitrogen atmos-
phere. Methanol (5.0 ml) was added and the suspension turned
from light brown to dark brown within an hour. For complete
formation of NPs the suspension was stirred for 5 days. Then
water (5.0 mL) was added and the solution was neutralized by
addition of a few drops of aqueous sodium hydroxide solution
(0.1 M). A black solid was obtained after evaporation of the
dark-brown solution. For the following experiments this solid (8)
was dissolved in 10.0 mL of water.
Acknowledgements
We thank Dr M. Meyer and Mr P. Stec from Oxford Diffraction
for helpful crystallographic discussions about the twin treatment
of 3(H2O)4.
Notes and references
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3412 | Dalton Trans., 2012, 41, 3407–3413
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