Facile generation and morphology of Pd nanoparticles from palladacycles and carbon monoxide

Article abstract of DOI:10.1016/j.jorganchem.2003.09.049

Treatment of PdCys or palladium salts with carbon monoxide (1 atm) in DMF or toluene at room temperature results in a solution of palladium nanoparticles whose morphology depends on the PdCy or palladium salt. Unusual triangular shaped 2-12 nm nanoparticles result from PdCys 1 and 3.

Full text of DOI:10.1016/j.jorganchem.2003.09.049

Journal of Organometallic Chemistry 689 (2004) 170–173
www.elsevier.com/locate/jorganchem
Facile generation and morphology of Pd nanoparticles
from palladacycles and carbon monoxide
a,
Ronald Grigg , Lixin Zhang , Simon Collard ,
a
b
*
c
Peter Ellis , Ann Keep
b
a
Molecular Innovation, Diversity and Automated Synthesis (MIDAS) Centre, School of Chemistry,
Leeds University, Leeds LS2 9JT, UK
b
Johnson Matthey, Orchard Road, Royston, Herts SG8 5HE, UK
Johnsson Matthey Technology Centre, BlountÕs Court, Sonning Common, Reading RG4 9NH, UK
c
Received 4 July 2003; accepted 30 September 2003
Abstract
Treatment of PdCys or palladium salts with carbon monoxide (1 atm) in DMF or toluene at room temperature results in a
solution of palladium nanoparticles whose morphology depends on the PdCy or palladium salt. Unusual triangular shaped 2–12 nm
nanoparticles result from PdCys 1 and 3.
Ó 2003 Published by Elsevier B.V.
Keywords: Palladacycle; Nanoparticles; Carbon monoxide
1
A great variety of, largely reductive, methods have
been reported for the preparation of metal nanoparticles
croscopy (TEM) revealed interesting morphological
differences in addition to establishing the size range of
the nanoparticles.
[
[
1]. These include reduction of metal salts with hydrogen
2], alcohols [3] and hydride [4]. Carbon monoxide has
First, we examined the reduction of acetate bridged
PdCys 1–5 by stirring their solutions in DMF and tol-
uene under carbon monoxide (1 atm) at room temper-
ature. Reactions in DMF (Table 1, entries 1, 3, 5, 7, 9)
resulted in rapid formation of nanoparticles immedi-
ately after the introduction of CO. The generation of
nanoparticles is signalled by the formation of black so-
lutions which do not sediment. The reductions in tolu-
ene were slightly slower (entries 2, 6, 8, 10). HerrmannÕs
catalyst 2, the only P-donor, was particularly slow in
toluene (entry 4), whilst chloride bridged imine PdCys 6
and 7 either gave very little nanoparticle formation
(entry 11) or none (entries 12–14).
been used to reduce HAuCl4 [5] and PtO2 [6] and
Et P) PdCl was reported to give giant palladium
(
3
2
2
clusters containing both Et3P and CO when contacted
with CO in DMF [7].
During the course of our research we observed that
when PdCys were used as precatalysts in carbonylation
reactions the solution rapidly turned black suggesting
formation of palladium black. Further study of this ef-
fect led to a convenient and rapid synthesis of palladium
nanoparticles from PdCys, palladium acetate or palla-
dium trifluoroacetate and carbon monoxide (1 atm) at
room temperature. Structural studies of several of the
nanoparticle preparations by transmission electron mi-
1
Preparation and examination of TEM images: The samples were
examined to assess the size, morphology and particle distribution. A
small portion of sample was taken and dried on a holey carbon coated
copper TEM grid. The samples were examined in the FEI Tecnai F20
electron miscroscope using the following experimental conditions:
*
Corresponding author. Tel.: +44-113-343-6501/6500; fax: +44-113-
3
0
43-6530.
E-mail address: R.Grigg@chem.leeds.ac.uk (R. Grigg).
voltage (kV): 200; C
2
aperture (lm): 50; analytical take-off angle: +15°
and mode: microprobe.
022-328X/$ - see front matter Ó 2003 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2003.09.049

R. Grigg et al. / Journal of Organometallic Chemistry 689 (2004) 170–173
171
Table 1
Synthesis of Pd nanoparticles by treatment of PdCys with CO (1 atm)
at room temperaturea
(entries 16 and 18), whilst sodium chloropalladate did
not give any nanoparticles in either DMF or toluene
(entries 19 and 20).
Entry
PdCy
Solvent
Nanoparticles
formation (min)
b
1
DMF
5
2
3
Toluene
DMF
10
10
We investigated the fate of the palladacycle ligands
from the reactions between PdCys 1 and 4 and CO. Work
up of the nanoparticle solutions afforded ca. 3:1 mixtures
of the ligand 8 and ketone 9 [8] derived from insertion of
4
5
Toluene
DMF
360
5
CO into the ligand [9]. The structure of the ketones 9a and
2
9
b were confirmed by X-ray crystallography, e.g., Fig. 2.
TEM technology was utilized to determine the mor-
6
7
Toluene
DMF
10
5
1
phology of the nanoparticles . The results show the
nanoparticles derived from 1 (Fig. 1(a)) and 3 (Fig. 1(b))
to consist of triangular palladium nanoparticles [10]
from 2 to 12 nm in size which tend to exist as small
clusters. In contrast the nanoparticles generated from
8
9
Toluene
DMF
10
5
Pd(OAc) (Fig. 1(c)), HerrmannÕs catalyst 2 (Fig. 1(d))
2
and 5 (Fig. 1(e)) exhibited conventional morphology
and were faceted palladium particles from 3 to 10 nm
which tend to exist as small clusters.
Studies of the growth and morphology of three-di-
mensional metal clusters of catalytically active transition
metals on inert crystal surfaces has underpinned ad-
vances in the mechanisms of heterogeneous catalysts
1
1
0
1
Toluene
DMF
10
c
[
10]. A recent paper [11] concerned with the high tem-
1
1
2
3
Toluene
DMF
d
c
perature formation of palladium nanoparticles on nat-
ural graphite single crystals reports palladium clusters
comprised of mixed morphology including some ap-
parent triangular shaped particles. In this work, TEM
experiments in the weak beam dark field mode showed
them to be regular tetrahedra. Thus it appears likely that
our triangular shaped particles are also tetrahedra.
The precise factors that control the shapes of the
palladium nanoparticles in our case are not clear.
However, triangular nanoparticles were derived from 1
1
1
1
1
1
1
2
4
5
6
7
8
9
0
Toluene
DMF
d
5
Pd(OAc)
2
Toluene
DMF
c
5
2 3 2
Pd(CO CF )
Toluene
DMF
Toluene
360
d
Na PdCl
2
4
2
and 3 both of which are sp carbon metallated PdCys
whilst faceted nanoparticles were derived from
d
a
Treating a solution of PdCys or Pd salts in toluene or DMF (0.1
mmol/20 ml) with CO (1 atm, via balloon) at r.t. for 24 h.
b
Diagnosed by formation of a non-sedimentry black solution.
c
Very little nanoparticle formation after 24 h.
2
Crystallographic data for 9a: audit creation method: SHELXL-97,
different ambient temperature: 150(2) K, different radiation wave-
d
No nanoparticle formation.
ꢀ
length: 0.71073 A [Mo K
a
], different radiation source: Ôfine-focus
, M ¼ 452.4, monoclinic, a ¼ 12.3417(3),
sealedtubeÕ, C27
14 2 2 3
H F N O
Next we examined the reduction of palladium salts
using carbon monoxide (1 atm) at room temperature in
DMF and toluene. The reduction of palladium acetate
and palladium trifluoroacetate with CO was fast in
DMF (entries 15 and 17) but was very slow in toluene
ꢀ
ꢀ
b ¼ 10.9267(3), c ¼ 15.8504(6) A, V ¼ 2049.14(11) A [3], T ¼ 150(2) K,
ꢀ1
space group P21=n, Z ¼ 4, absorption coefficient ¼ 0.109 mm , 17,536
reflections collected, 4018 independent (Rint ¼ 0:0841) which were used
1
in all calculations. The final R indices were 0.049 (R ) and 0.1118
2
(wR ).

172
R. Grigg et al. / Journal of Organometallic Chemistry 689 (2004) 170–173
Fig. 1. TEM of nanoparticles arising from treatment of PdCys 1–3, 5 and Pd(OAc)2 with (1 atm) in DMF: (a) nanoparticles from 1, (b) nanoparticles
from 3, (c) nanoparticles from Pd(OAc)2, (d) nanoparticles from 2 and (e) nanoparticles from 5.
in our cascade reaction chemistry [14] and have shown
that these nanoparticles also allow p-allylpalladium
species to exhibit nucleophilic chemistry rather than the
normal electrophilic chemistry [15]. Most recently we
have demonstrated the efficacy of nanoparticles gener-
ated by our protocol from PdCys in the [2 + 2 + 1]-cy-
cloaddition of 1,6-dienes and CO [16] and in cyclative
carbonylation/amination cascades [17].
In conclusion, a new protocol for generating palla-
dium nanoparticles has been established which involves
treating PdCys or selected palladium salts with carbon
monoxide (1 atm) in DMF or toluene at room temper-
ature. The possibility that the unusual morphology of
the nanoparticles derived from 1 and 3 is a general
2
Fig. 2. X-ray crystal structure of 9a.
feature for all sp carbon cyclopalladated complex pre-
cursors is under study as are the mechanistic and syn-
thetic implication of these observations.
3
HerrmannÕs catalyst 2 and from 5 both of which are sp
carbon metallated PdCys. It is also known that disso-
ciative adsorption can alter the shape/morphology of
nanoparticles [12] and that in the reductive generation of
nanoparticles in the presence of ‘‘capping’’ polymers the
morphology is dependent on the ‘‘capping’’ polymer.
Thus, a significant proportion (41%) of triangular Pt
nanoparticles are produced from K2PtCl4 in the pres-
ence of sodium poly(acrylate) [13].
When toluene was used as solvent, the Pd nanopar-
ticles aggregated and separated out, in our reductive
protocol, whilst in DMF the nanoparticles did not ag-
gregate which implies DMF is acting as a stabiliser.
We have previously reported excellent catalytic ac-
tivity of nanoparticles derived from precursors of type 1
Acknowledgements
One of us (L.Z.) thanks Leeds University (Tetley and
Lupton) and Johnson Matthey for scholarship. We also
thank Dr. Gregory Goodlet (Johnson Matthey) for the
TEM research.
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