Dimeric allylpalladium(II) complexes with pyrazolate bridges: Synthesis, characterization, structure and thermal behaviour

Article abstract of DOI:10.1016/j.jorganchem.2005.12.037

The synthesis, characterization and thermal behaviour of some new dimeric allylpalladium (II) complexes bridged by pyrazolate ligands are reported. The complexes [Pd(μ-3,5-R2′pz)(η3-CH2C(R)CH2)]2[R=H,R′=CH(CH3) 2(1a); R = H, R′ = C(CH₃)₃ (1b), R = H, R′ = CF₃ (1c); R = CH₃, R′ = CH(CH₃) ₂ (2a); R = CH₃, R′ = C(CH₃)₃ (2b); and R = CH₃, R′ = CF₃ (2c)] have been prepared by the room temperature reaction of [Pd(η³-CH ₂C(R)CH₂)(acac)](acac = acetylacetonate) with 3,5-disubstituted pyrazoles in acetonitrile solution. The complexes have been characterized by NMR (¹H, ¹³C{¹H}), FT-IR, and elemental analyses. The structure of a representative complex, viz. 2c, has been established by single-crystal X-ray diffraction. The dinuclear molecule features two formally square planar palladium centres which are bridged by two pyrazole ligands and the coordination of each metal centre is completed by allyl substituents. The molecule has non-crystallographic mirror symmetry. Thermogravimetric studies have been carried out to evaluate the thermal stability of these complexes. Most of the complexes thermally decompose in argon atmosphere to give nanocrystals of palladium, which have been characterized by XRD, SEM and TEM. However, complex 2c can be sublimed in vacuo at 2 mbar without decomposition. The equilibrium vapour pressure of 2c has been measured by the Knudsen effusion technique. The vapour pressure of the complex 2c could be expressed by the relation: ln (p/Pa)(±0.06) = -18047.3/T + 46.85. The enthalpy and entropy of vapourization are found to be 150.0 ± 3 kJ mol^-1 and 389.5 ± 8 J K^-1 mol^-1, respectively.

Full text of DOI:10.1016/j.jorganchem.2005.12.037

Journal of Organometallic Chemistry 691 (2006) 1402–1410
www.elsevier.com/locate/jorganchem
Dimeric allylpalladium(II) complexes with pyrazolate bridges:
Synthesis, characterization, structure and thermal behaviour
a
a
a,
b
c
A. Singhal , R. Mishra , S.K. Kulshreshtha *, Paul V. Bernhardt , Edward R.T. Tiekink
a
Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra 400 085, India
School of Molecular and Microbial Sciences, University of Queensland, Brisbane 4072, Qld, Australia
Department of Chemistry, The University of Texas at San Antonio, San Antonio, TX 78249-0698, USA
b
c
Received 9 November 2005; received in revised form 6 December 2005; accepted 13 December 2005
Available online 20 January 2006
Abstract
The synthesis, characterization and thermal behaviour of some new dimeric allylpalladium (II) complexes bridged by pyrazolate
ligands are reported. The complexes ½Pdðl-3; 5-R02pzÞðg3-CH2CðRÞCH2Þꢀ2 ½R ¼ H; R0 ¼ CHðCH3Þ2 ð1aÞ; R = H, R0 = C(CH3)3 (1b),
R = H, R0 = CF3 (1c); R = CH3, R0 = CH(CH3)2 (2a); R = CH3, R0 = C(CH3)3 (2b); and R = CH3, R0 = CF3 (2c)] have been prepared
by the room temperature reaction of [Pd(g3-CH2C(R)CH2)(acac)](acac = acetylacetonate) with 3,5-disubstituted pyrazoles in acetoni-
trile solution. The complexes have been characterized by NMR (1H, 13C{1H}), FT-IR, and elemental analyses. The structure of a rep-
resentative complex, viz. 2c, has been established by single-crystal X-ray diffraction. The dinuclear molecule features two formally square
planar palladium centres which are bridged by two pyrazole ligands and the coordination of each metal centre is completed by allyl sub-
stituents. The molecule has non-crystallographic mirror symmetry. Thermogravimetric studies have been carried out to evaluate the ther-
mal stability of these complexes. Most of the complexes thermally decompose in argon atmosphere to give nanocrystals of palladium,
which have been characterized by XRD, SEM and TEM. However, complex 2c can be sublimed in vacuo at 2 mbar without decompo-
sition. The equilibrium vapour pressure of 2c has been measured by the Knudsen effusion technique. The vapour pressure of the complex
2c could be expressed by the relation: ln (p/Pa)( 0.06) = ꢁ18047.3/T + 46.85. The enthalpy and entropy of vapourization are found to
be 150.0 3 kJ molꢁ1 and 389.5 8 J Kꢁ1 molꢁ1, respectively.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: g3-Allylpalladium complexes; Pyrazolate ligands; NMR spectroscopy; X-ray crystal structure; Transmission electron microscopy
1. Introduction
has been significant interest in developing suitable precur-
sor molecules for the chemical vapour deposition (CVD)
of palladium thin films and nanoparticles. The lability of
the allyl groups in allylpalladium compounds is well docu-
mented and complexes such as Pd(g3-allyl)2, [Pd(g3-
CH2CHCHMe)2] and [CpPd(g3-allyl)] have proven to be
excellent precursors for the deposition of high-purity palla-
dium thin films by thermal CVD [5]. However, these com-
plexes suffer from low thermal- and air-stability. Recently,
complexes of the type (g3-allyl)(b-diketonato)palladium(II)
[6] and (g3-allyl)(b-diketoiminato)palladium(II) [7] which
exhibit excellent thermal stability and volatility have been
reported.
Thin films of palladium find many applications in elec-
tronic industry, such as electrical contacts (replacing gold),
multi-layer magneto-optical data storage materials, hydro-
gen sensors or infrared-sensors, multilayer chip capacitors,
and electrode coating materials [1]. They are also useful as
catalysts for a variety of gas-phase reactions [2] and mem-
brane materials for gas separation [3]. Nano-sized palla-
dium particles deposited on suitable substrates find wide
applications in heterogeneous catalysis [4]. Thus, there
*
Corresponding author. Tel.: +91 22 25593829; fax: +91 22 25505151.
Pyrazolato ligands are an interesting group of ligands
[8] whose coordination chemistry with transition and
E-mail addresses: kulshres@magnum.barc.ernet.in (S.K. Kulsh-
reshtha), Edward.Tiekink@utsa.edu (E.R.T. Tiekink).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.12.037

A. Singhal et al. / Journal of Organometallic Chemistry 691 (2006) 1402–1410
1403
non-transition metals has been thoroughly reviewed [9].
Cl
Cl
R
R
Pd
Pd
These ligands can be easily prepared and their properties
can be suitably modified by appropriate choice of the
carbon-bound substituents. In particular, 3,5-disubstituted
derivatives such as 3,5-di-tert-butyl- [10,11] and 3,5-bis(tri-
fluoromethyl)-pyrazolate [12] are known to impart
increased volatility to their metal complexes. Further, pyr-
azolates, being nitrogen donor ligands, are more basic than
common oxygen donor ligands, such as b-diketonate and
can therefore bind strongly to metal centres resulting in
source compounds with enhanced stability. With these
considerations, we have investigated the chemistry and ther-
mal behaviour of a series of pyrazolato-bridged dimeric
allylpalladium complexes, ½Pdðl-3; 5-R02pzÞðg3-CH2CðRÞ-
CH2Þꢀ2 ðR¼H ð1ÞÞ or CH3 (2) R0 = (CH(CH3)2, C(C-H3)3,
CF3). The complex [Pd(l-3,5-(CF3)2pz)(g3-C3H5)]2 (1c)
has recently been reported by Jones et al. [12] and was
obtained by reacting [Pd(l-Cl)(g3-C3H5)]2 with 3,5-
(CF3)2pzLi at room temperature for 12 h. In this work,
we report a faster and convenient method to synthesize
pyrazolato-bridged dimeric allylpalladium(II) complexes,
their characterization and thermal stability, and in the case
of 2c, equilibrium vapour pressure measurements.
Tlacac
Me
Me
O
O
R
Pd
3,5-R'2PzH -acacH
H
R'
R'
N
N
R
Pd
R
Pd
N
N
R'
R'
H
2. Results and discussion
1a 1b 1c 2a
2b
2c
2.1. Synthesis and characterization of the complexes
R
R’
H
H
H CH3 CH3 CH3
Pri But CF3 Pri But CF3
The complexes, Pdðl-3; 5-R02pzÞðg3-CH2CðRÞCH2Þꢀ2
R = H or CH3; R0 = (CH(CH3)2, C(CH3)3, CF3) are pre-
pared in high-yields from the reaction of the appropriate
pyrazole with the complex, [Pd(acac)(g3-CH2C(R)CH2)
(generated in situ by the reaction between the chloro-
bridged allylpalladium dimer and thallium(I)acetylaceto-
nate) (Scheme 1). The acac method reported by us represents
a general and convenient synthetic approach to synthesize
several new dimeric allylpalladium(II) complexes with
different bridging ligands [13]. Most of the complexes are
air-stable solids and can be stored at room temperature
for long periods. However, complexes 1b and 2b tend to
decompose if kept in air at room temperature for long peri-
ods and have to be stored under argon below 0 ꢁC. All
complexes are white to off-white solids and have been char-
acterized by 1H, 13C{1H} NMR, FTIR, and C, H, N
analyses.
Scheme 1.
5.8 and 6.8 ppm are attributed to the single proton bound
to C-4 of the pyrazolate ring. For complexes, 2a and 2c
two sets of resonances each for anti, syn and methyl
groups were observed. These data suggest that for these
complexes in solution a mixture of isomers are present.
Of relevance here is the proton NMR data for the original
series of dinuclear palladium(II) allyl derivatives of dim-
ethylpyrazolate or of the parent pyrazolate reported by
Trofimenko [14]. For these complexes also, the 1H
NMR spectra were found to be more complex than those
expected for a molecule of simple C2v symmetry. It was
proposed that the NMR data were compatible with a
mixture of conformational isomers resulting from different
orientations of the allyl groups which may arise due to the
formal rotation of allyl groups, inversion of the entire
metallocycle or the presence of isomer I (Fig. 1). Of the
different conformational isomers possible, structure I has
each set of syn and anti protons in a different environment
and the left- and right-side R0 groups are also non-equiv-
alent. Both structures II and III have C2v symmetry. The
room temperature spectra of 1a and 1c would be compat-
ible with either I or a 1:1 mixture of II and III. The latter
possibility looks less attractive as the putative steric inter-
actions between the R groups would disfavour structure
2.2. NMR of allylpalladium complexes
1
The room temperature H NMR spectra of the com-
plexes 1 and 2 were complex but showed the correct pro-
ton ratios for both ligands. For example, in complexes 1a
and 1c four sets of resonances each for anti and syn pro-
tons with both showing up as doublets (anti JHH
=
ꢂ12 Hz, syn JHH = ꢂ7 Hz) are observed. Two/three mul-
tiplets appeared for the proton bound to central carbon
atom of the allyl ligand. The three resonances between

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A. Singhal et al. / Journal of Organometallic Chemistry 691 (2006) 1402–1410
Fig. 1. Conformational isomers possible for ½Pdðl-3; 5-R02pzÞðg3-CH2-
CðRÞCH2Þꢀ2. (Reprinted from Ref. [12] with permission from Elsevier and
Ref. [14] with permission from American Chemical Society.)
III and even if it did exist, it is unlikely that it would exist
in an exactly 1:1 ratio with II.
2.3. Crystallography
The molecular structure of 2c is shown in Fig. 2 from
which it can be seen that the dimeric complex features
two bidentate bridging pyrazolate ligands. These form
experimentally equivalent Pd–N bond distances in the
˚
range 2.100(2)–2.108(2) A. The formally square planar
coordination geometry for each palladium centre is com-
pleted by an allyl group. Key bond lengths and bond angles
are given in Table 1. The Pd–C separations are equal at the
˚
5r level, falling in the range 2.103(3)–.129(3) A, with the
longer separation in each case being formed by the palla-
dium atom and the central carbon atom of each allyl
group. In terms of the possible isomers represented in
Fig. 2. The molecular structure of 2c showing (a) the atomic numbering
scheme and (b) a side view highlighting the disposition of the pyrazolate
ligands.
Table 1
˚
Selected bond lengths (A) and angles (ꢁ) for 2c
Fig. 1, the molecular structure of 2c adopts the conforma-
tion in which the allyl groups have an opposite orientation
with respect to the Pd2N4 core, i.e., isomer I, as emphasised
in the lower view of Fig. 2b. The molecule adopts a boat
conformation as seen in the dihedral angles between the
two allyl groups (ca. 75ꢁ) and the two five-membered rings
(ca. 83ꢁ). This conformation found for 2c has also been
reported for the previously described structure of 1c [12].
Indeed, the geometric parameters reported for 1c are in
good agreement to those reported above, i.e., the ranges
Bond lengths
Pd(1)–N(1a)
Pd(1)–N(1b)
Pd(1)–C(1)
Pd(1)–C(2)
Pd(1)–C(3)
2.100(2)
2.108(2)
2.103(3)
2.120(3)
2.113(3)
Pd(2)–N(2a)
Pd(2)–N(2b)
Pd(2)–C(5)
Pd(2)–C(6)
Pd(2)–C(7)
2.100(2)
2.105(2)
2.114(3)
2.129(3)
2.115(3)
Bond angles
N(1a)–Pd(1)–N(1b)
N(1a)–Pd(1)–C(1)
N(1a)–Pd(1)–C(2)
N(1a)–Pd(1)–C(3)
N(1b)–Pd(1)–C(1)
N(1b)–Pd(1)–C(3)
N(1b)–Pd(1)–C(2)
C(1)–Pd(1)–C(2)
C(1)–Pd(1)–C(3)
C(2)–Pd(1)–C(3)
89.17(9)
101.42(12)
132.96(12)
169.20(12)
169.35(12)
101.52(12)
132.46(12)
38.71 (14)
67.87(14)
38.33(13)
N(2a)–Pd(2)–N(2b)
N(2a)–Pd(2)–C(5)
N(2a)–Pd(2)–C(6)
N(2a)–Pd(2)–C(7)
N(2b)–Pd(2)–C(5)
N(2b)–Pd(2)–C(6)
N(2b)–Pd(2)–C(7)
C(5)–Pd(2)–C(6)
C(5)–Pd(2)–C(7)
C(7)–Pd(2)–C(6)
88.42(9)
101.31(12)
133.75(11)
168.82(12)
169.18(12)
134.43(11)
101.90(11)
38.58(13)
68.06(14)
38.48(13)
˚
of Pd–N and Pd–C are 2.101(3)–2.111(3) A and 2.099(4)–
˚
2.133(4) A, respectively. In a closely related structure, i.e.,
[Pd(l-3,5-Me2pz)(g3-CH2C(H)CH2)]2 [15] the molecule
has a configuration comparable to that shown as isomer
II in Fig. 1. A partial crystallographic analysis was also
conducted on complex 1b but problems in modelling the

A. Singhal et al. / Journal of Organometallic Chemistry 691 (2006) 1402–1410
1405
Table 2
steps to leave a residue of metallic palladium, with the mass
loss in the first step corresponding to the loss of the allyl
moieties and the mass loss in the second step corresponding
to the loss of pyrazolate groups. The mass loss obtained
from TGA agrees well within experimental error with the
calculated mass loss for each step for all the above com-
plexes (cf. Table 2). However, complex 1c, containing the
3,5-bis(trifluromethyl) pyrazolate ligand, partly evaporated
and partly decomposed (ꢂ90% evaporation was observed)
leaving a very small amount of metallic Pd residue. The
TGA of 2c is of particular interest as it showed a simple
one-step loss of nearly 100% within the temperature range
(166–280 ꢁC) (Fig. 3b). This observation is consistent with
highly effective transportation of palladium metal into the
gas phase without decomposition. The enthalpy and
entropy of evaporation for 2c has been determined. From
the data collected in Table 2, the stabilities of the com-
plexes can be arranged in the following order:
TGA data for ½Pdðl-3; 5-R02pzÞðg3-CH2CðRÞCH2Þꢀ2
Compound
Step I
Step II
Temp. range
% wt. lossa
Temp. range
% wt. loss
(ꢁC)
(ꢁC)
1a
1b
1c
2a
2b
2c
a
175–269
97–169
160–260
200–265
160–204
166–284
13.5 (13.7)
11.7 (12.5)
89.9
19.7 (17.6)
17.4 (16.2)
100
269–406
171–260
–
280–380
206–227
–
51.5 (50.6)
55.8 (54.9)
–
49.5 (49.6)
51.2 (52.6)
–
The values in parentheses correspond to expected weight loss.
significant disorder precluded a full report (see Section 4).
However, despite this difficultly, the molecular connectivity
was established unambiguously. The essential molecular
structure is as described above with the notable exception
of the relative orientation of allyl groups which correspond
to the conformation represented as isomer II in Fig. 1. This
observation underscores the inherent flexibility in these sys-
tems with respect to the orientations of the allyl groups to
the Pd2N4 cores of the complexes.
1b < 1a < 1c and 2b < 2a < 2c.
2.5. Vapour pressure measurements
The thermogravimetric (TG) plot of 2c (Fig. 3b) showed
that the complex congruently vapourizes above 439 K in a
single step. No break in the TG plot for the decomposition
of the complex was seen. NMR (1H, 13C{1H}), FT-IR and
melting point data for the residue of the partially evapo-
rated sample obtained from the Knudsen effusion studies
showed the presence of the original complex. The NMR
spectra (1H, 13C{1H}) of the condensed phase deposited
on the cooler part of the reaction tube showed the presence
2.4. TG analysis
The thermogravimetric analyses (TGA) of ½Pdðl-3;
5-R02pzÞðg3-CH2CðRÞCH2Þꢀ2 were carried out in flowing
argon atmosphere and the results are summarized in Table
2. The typical TG plots for two representative complexes
(1a and 2c) are shown in Fig. 3. From TG analysis it was
found that most of the dimeric allyl complexes with pyraz-
olate bridges (i.e., 1a, 1b, 2a, and 2b) decomposed in two
0
-2
-4
-6
0
-1
-2
-3
100 150 200 250 300 350 400 450 500
100
150
200
250
300
350
400
Temperature (oC)
Temperature (oC)
a
b
Fig. 3. TGA traces for: (a) 1a (initial weight: 4.95 mg) which decomposed in two steps leaving a 35% residue; and (b) 2c (initial weight: 5.9 mg) which
showed 100% evaporation between 166 and 284 ꢁC.

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A. Singhal et al. / Journal of Organometallic Chemistry 691 (2006) 1402–1410
of pure 2c. This is indicative of the fact that the chemical
composition of the complex is retained after the reaction
and that the vapourization process is congruent.
lnðp=PaÞ ꢄ 0:06 ¼ ꢁ18047:3=T þ 46:85ð359 6 T=K 6 385Þ.
ð2Þ
The slope and intercept of the linear equation gave the val-
ues of mean molar enthalpy and entropy, respectively, for
the vapourization of the compound. The values of enthalpy
and entropy were found to be 150.0 3 kJ molꢁ1 and
389.5 8.0 J Kꢁ1 molꢁ1, respectively.
The isothermal rate of effusion of the vapours from the
orifice was obtained from the observed total mass loss over
a time, t, at steady state. The vapour pressure of 2c in the
Knudsen cell was derived from the mass loss/time data
using the relation of effusive flow obtainable from the
kinetic theory of gases applied to vapour fluxes. The fol-
lowing relation then expresses the vapour pressure of 2c
(g) in terms of the mass loss m:
2.6. Thermal decomposition of the complexes
Thermolysis of the complexes 1b and 2b in refluxing
xylene and of complexes 1a and 2a at 500 ꢁC at a rate
of 5 ꢁC minꢁ1 in a furnace under an argon atmosphere
were carried out and nanocrystals of palladium metal
were obtained in each case. X-ray diffraction patterns of
the nanocrystals revealed broad reflections which com-
pared well with the pattern reported for palladium metal
[16]. Fig. 5 shows the XRD spectra of the powders
obtained from thermolysis of 1b, 2a, and 2b. The average
crystallite size of 5.4 0.2 nm was estimated with the
Scherrer equation for the nanocrystals derived from com-
plexes. The surface morphology of the nanoparticles
obtained after thermolysis of complex 2b was determined
by high-resolution scanning electron micrographs (SEM)
and transmission electron microscopy (TEM). While
SEM analyses showed spherically shaped aggregates of
palladium with more or less uniform size of 0.5 lm with
very small distribution (Fig. 6), TEM analyses revealed
spherical nanoparticles having mean diameters in the
range 5–12 nm (Fig. 7). The method represents a facile
synthetic procedure to produce palladium nanoparticles
at moderately low temperature. Kim et al. [17] have ear-
lier obtained palladium nanoparticles with particle sizes
of 3.5, 5, and 7 nm from the thermal decomposition of
1=2
p ¼ ð1=AÞ ꢃ 1=Kc ꢃ ðm=tÞ ꢃ ð2pRT=MÞ
ð1Þ
In this equation, p is the vapour pressure of the complex, A
is the orifice area, Kc is the Clausing factor, T is the abso-
lute temperature in Kelvin, M is the molecular weight of
complex (729.15), and R is the universal gas constant. In
the present experiment for thin knife edge orifice, Kc was
taken to be 1.
The vapour pressure of 2c in the temperature range 359–
385 K as calculated from experimentally determined
parameters involved in the above equation are given in
Table 3 for all the experimental runs. The linear least-
squares fit of ln (p) versus 1/T is given in Fig. 4. The
least-square fitted equation is represented as:
Table 3
Vapour pressure data for 2c
Temp (K)
Time (s)
Mass loss (lg)
Pressure (Pa)
359
364
374
377.5
381
385
240
240
240
240
240
240
285
64
19
10
174
113
0.034
0.065
0.222
0.394
0.610
1.004
0.5
0.0
ln(p/pa) 0.06 = -18047.3/T + 46.85
dHvap = 150.3 3KJmole-1
2b
2a
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
1b
2.60
2.65
2.70
1/Tx103
2.75
2.80
20
30
40
50
60
70
80
2θ
Fig. 4. Graph showing the relationship between vapour pressure and
Fig. 5. X-ray diffraction spectra for the nanocrystals obtained after
temperature for the complex 2c.
thermolysis of complexes 1b, 2a and 2b.

A. Singhal et al. / Journal of Organometallic Chemistry 691 (2006) 1402–1410
1407
3. Conclusions
Pyrazolat0e-bridged dimeric allylpalladium(II) complexes
½Pdðl-3; 5-R2pzÞðg3-CH2CðRÞCH2Þꢀ2 have been synthes-
ised following a facile synthetic approach and character-
ised. The X-ray structure of 2c, shows that the dinuclear
complex contains two square planar palladium centres
bridged by two pyrazole ligands with the coordination of
each metal centre completed by allyl substituents, and
adopts the conformation in which the allyl groups have
an opposite orientation with respect to the Pd2N4 core.
Their thermal behaviour has also been studied. The com-
plex, [Pd(l-3,5-(CF3)2pz)(g3-CH2C(CH3)CH2)]2 (2c) could
be a potential precursor for the deposition of palladium
thin films by chemical vapour deposition. The equilibrium
vapour pressure of this complex as determined by Knudsen
effusion method, could be expressed by the relation: ln
(p/Pa)( 0.06) = ꢁ18047.3/T + 46.85. Thermolysis of com-
plexes 1b and 2b in refluxing xylene (ꢂ140 ꢁC) affords pal-
ladium nanoparticles having mean sizes between 5 and
12 nm under mild conditions.
Fig. 6. Scanning electron micrograph of the nanocrystals derived from
thermolysis of complex 2b in refluxing xylene.
4. Experimental
4.1. General experimental details
All the reactions were carried out under an atmosphere
of dry argon using standard Schlenk techniques. Solvents
were distilled from the appropriate drying agents and
degassed before use. The chloro-bridged allylpalladium
dimers, [Pd2(l-Cl)2(g3-C3H4R)2] [18], 3,5-diisopropylpy-
razole [19], 3,5-di-tert-butyl pyrazole [20], 3,5- bis(trifluo-
romethyl)pyrazole [21] were prepared either by following
literature procedures or modifications of them. Thus, in
the preparation of [Pd2(l-Cl)(g3-CH2CHCH2)]2 from
K2PdCl4 and allyl halide, allyl bromide was used instead
of allyl chloride and the reaction mixture was refluxed for
3 h, thereby reducing the reaction time from 24 to 3 h.
Melting points were determined in sealed capillaries with
an electrothermal melting point apparatus and are uncor-
rected. C, H, N analyses were carried out with a Thermo
Finnigan Flash 1112 series elemental analyser. Infrared
spectra were recorded in the range 4000–200 cmꢁ1 on a
Bomem MB-series FT-IR spectrophotometer as Nujol
1
mulls between polythene sheets. H and 13C{1H} spectra
were recorded on a Bruker DPX 300 spectrometer. Chem-
ical shifts are referenced to the internal chloroform peak.
Thermogravimetric analyses (TGA) were carried out either
on a SETARAM 92-16-18 or NETZSCH STA 409 PC/PG
instrument. The TG curves were recorded at a heating rate
of 10 ꢁC/min under a flow of argon gas. X-ray powder dif-
fraction data were collected on a Philips X-ray diffractom-
Fig. 7. Transmission electron micrograph of nanoparticles derived from
thermolysis of 2b in refluxing xylene.
˚
eter (Model PW 1729) using Cu Ka radiation (k 1.54060 A)
at 30 kV and 20 mA. SEM micrographs of the palladium
nanocrystals were obtained using a Vega MV2300t/40
scanning electron microscope. Energy dispersive spectros-
copy (EDS) analyses were carried out using a Inca Energy
the [Pd(acac)2] complex in presence of trioctylphosphine
at 300 ꢁC. EDS results confirmed the presence of palla-
dium in the nanocrystals.

1408
A. Singhal et al. / Journal of Organometallic Chemistry 691 (2006) 1402–1410
250 instrument coupled to Vega MV2300t/40 scanning
electron microscope. Transmission electron microscopy
(TEM) was performed using a JEOL 2000 FX electron
microscope. The sample of metal nanocrystals was
prepared by treating 5 mg of finely ground powder with
methanol (15 ml), sonicating the mixture for 10 min, and
then applying one drop of the solution to carbon film
coated copper grid (300 mesh).
CH2CHCH2), 6.79 (s), 6.81 (s) [2H, H-4 pz ring].
13C{1H} NMR data: d 59.80 (s), 60.22 (s), 60.75 (s),
61.10 (s) (allyl CH2); 104.74 (s), 114.80 (s) (CH pz ring),
115.10 (s), 115.96 (s), 116.58 (s) (CH2CHCH2), 120.90 (q,
1
1
CF3, JC–F = 268 Hz), 121.11 (q, CF3, JC–F = 268 Hz),
2
2
142.95 (q, CCF3, JC–F = 38 Hz), 143.10 (q, CCF3, JC–F
= 38 Hz).
4.2.3. Complex 2a, [Pd(l-3; 5-Pri2pz)(g3-CH2C(CH3)CH2)]2
White solid. Yield: 79%. m.p.: 203–206 ꢁC. Anal. Calc.
for C26H44N4Pd2: C, 49.92; H, 7.09; N, 8.95. Found: C,
49.81; H, 7.23; N, 8.79%. IR (cmꢁ1): 1516, 1377, 1358,
1294, 1175, 1142, 1105, 1042, 1020, 921, 897, 835, 764,
4.1.1. Synthesis of complexes
All the complexes were prepared by using the acac
method reported by us earlier [13]. The characterization
data for the complexes 1b and 2b are also reported in
Ref. [13]. A typical preparation is described below.
1
476. H NMR (CDCl3, 300 MHz): d 1.16–1.32 (m, 24 H,
HC(CH3)2); 2.02 (s, 3H, CH3 of 2-methylallyl), 2.16 (s,
3H, CH3 of 2-methyl allyl); 2.47 (s, 2H, anti-H, allyl
CH2), 2.72–2.79 (m, 4H, allyl CH2 + CH(CH3)2); 3.06 (h,
6.9 Hz, CH(CH3)2); 5.75 (s, 2H, H-4 pz ring). 13C{1H}
(CDCl3, 75.47 MHz): d 22.83 (s), 23.00 (s) (HC(CH3)2);
23.46 (s), 23.83 (s) [CH3 of 2-methyl allyl]; 24.51 (s),
24.74 (s) (HC(CH3)2); 29.21 (s) 29.27 (s) (CH(CH3)2),
54.21 (s) 54.66 (s), 54.94 (s), 55.32 (s), (allyl CH2), 92.78
(s), 94.80 (s) 95.02 (s) (CH pz ring), 127.00 (s), 127.94 (s),
128.28 (s) (CH2CHCH2) 158.60 (br, C3 + C5pz ring).
4.1.2. Synthesis of [Pd(l-3; 5-Pri2pz)(g3-CH2C(CH3)CH2]2)
Solid Tl(acac) (0.790 g, 2.60 mmol) was added to a
degassed acetonitrile (50 ml) solution of [Pd(l-Cl)(g3-
CH2)C(CH3)CH2]2 (0.5 g, 1.27 mmol) at 0 ꢁC. After it
was stirred for 30 min at 0 ꢁC, an acetonitrile (5 ml) solu-
tion of 3,5-PripzH (0.39 g, 2.56 mmol) was added to it.
The resulting mixture was stirred at room temperature
for 2 h and then filtered through a bed of Celite. The filtrate
was evaporated to dryness under reduced pressure and the
residue was recrystallised from acetonitrile–ether mixture
at ꢁ10 ꢁC to obtain complex 2a as white solid.
4.2.4. Complex 2c, [Pd(l-3,5-(CF3)2pz)(g3-CH2C(CH3)-
CH2)]2
4.2. Characterization data for the complexes
Colourless solid. Yield: 82%. m.p.: 195–197 ꢁC. Anal.
Calc. for C18H16F12N4Pd2 C, 29.65; H, 2.21; N, 7.68.
Found: C, 30.14; H, 1.94; N, 7.82%. IR (cmꢁ1): 1527,
4.2.1. Complex 1a, [Pd(l-3; 5-Pri2pz)(g3-CH2CHCH2)]2
White solid, Yield: 80%. m.p.: 182–184 (dec). Anal.
Calc. for C24H40N4Pd2: C, 48.25; H, 6.74; N, 9.37. Found:
C, 48.10; H, 6.93; N, 9.23%. IR(cmꢁ1): 1519, 1462, 1376,
1358, 1305, 1290, 1174, 1142, 1133, 1106, 1043, 941, 928,
768, 503. 1H NMR (CDCl3, 300 MHz) d 1.12–1.33 (m,
24H, HC(CH3)2); 2.65 (d, 12.4 Hz), 2.76 (d, 12.4 Hz),
2.80 (d, 12.4 Hz), 2.89 (d, 12.4 Hz) [4H, anti H, allyl
CH2]; 2.96–3.05 (m, 4H, HC-(CH3)2); 3.71 (d, 6.8 Hz),
3.79 (d, 6.8 Hz), 3.81 (d, 6.8 Hz), 3.90 (d, 6.8 Hz) [4H,
syn H, allyl CH2]; 5.21–5.76 (m, 2H, CH2CHCH2).
13C{1H} (CDCl3, 75.47 MHz): d 22.90 (s), 24.49 (s), 24.73
(s) (HCCH3)2,; 29.16 (s) [HC-(CH3)2], 55.54 (s), 55.17 (s),
54.86 (s) (allyl CH2); 95.41 (s) 95.16 (s), 94.77 (s) (CH pz
ring); 111.66 (s), 111.92 (s), 113.26 (s), 113.67 (s)
(CH2CHCH2); 158.65 (s), 159.07 (s), 159.19 (s) (C3 + C5
pz ring).
1
1501, 1261, 1125, 1021, 818, 810, 486. H NMR (CDCl3,
300 MHz): d 2.10 (s, 3H), 2.23 (s, 3H) [CH3 of 2-methylal-
lyl], 2.69 (s), 3.09 (s) ( anti-H, allyl CH2), 3.90 (s), 3.93 (s)
(syn-H, allyl CH2), 6.79 (s) (2H, H-4 pz ring). 13C NMR
(CDCl3, 75.47 MHz): d 22.36 (s) 23.74 (s) [CH3 of 2-meth-
ylallyl], 58.70 (s), 59.05 (s), 60.15 (s) (allyl CH2), 104.69 (s)
1
(C-4, pz ring), 121.00 (q, CF3, JC–F = 268 Hz), 121.18 (q,
1
CF3, JC–F = 268 Hz); 130.97 (s) [CH2C(CH3)CH2], 132.84
2
(s), 133.01(s), [CH2C(CH3)CH2], 142.94 ( q, CCF3, JC–F
38 Hz), 143.44 (q, CCF3 JC–F = 38 Hz).
=
2
4.3. Vapour pressure measurements
The vapour pressure of the 2c was determined by mea-
suring its partial pressure in the temperature range 359–
385 K, using Knudsen effusion mass loss technique. The
vapour pressure data for 2c were generated by monitoring
the rate of mass loss of the sample under isothermal condi-
tions with the help of a thermobalance (SETARAM, Model
B24). A detailed description of the apparatus and the proce-
dure for the data collection has been described elsewhere
[22] The mass calibration of the micro-balance was done
using standards weights at room temperature. The
temperature calibration for the sample was done by the
drop method [23], using high-purity indium metal as
standard.
4.2.2. Complex 1c, [Pd(l-3,5-(CF3)2pz)(g3-CH2CHCH2)]2
Colourless solid. Yield: 83%. m.p.: 224–226 ꢁC. Anal.
Calc. for C16H12F12N4Pd2: C, 27.41; H, 1.72; N, 7.99.
Found: C, 27.24; H, 1.64; N, 8.10%. IR (cmꢁ1): 1527,
1502, 1377, 1253, 1142, 1023, 822, 815, 753, 486. 1H
NMR: d 2.90 (d, 12.4 Hz), 3.03 (d, 12.4 Hz), 3.16 (d,
12.4 Hz), 3.26 (d, 12.4 Hz) (4H, anti-H, allyl CH2); 4.06
(d, 6.9 Hz), 4.12 (d, 6.9 Hz), 4.16 (d, 6.9 Hz), 4.24 (d,
6.9 Hz)
(
syn-H, allyl CH2); 5.44–5.84 (m, 2H,

A. Singhal et al. / Journal of Organometallic Chemistry 691 (2006) 1402–1410
1409
About 200 mg of powdered sample was put in a per-
fectly degassed graphite cup inside the Knudsen cell of
15 mm diameter and 15 mm height, having a circular knife
edged orifice of diameter 0.5 mm. In order to ensure the
quick equilibrium between sample and its vapour products,
the sample was well spread in the crucible so that ratio of
the projected surface of sample to the orifice is high (about
50). The vapour pressures were measured both in ascending
and descending mode of temperatures to ensure the
absence of kinetic impediments of the vapour generation
inside the cell.
ucts for both 1a and 2a were black shiny powders. The typ-
ical yields obtained were 84% (1a) and 88% (2a)
respectively. Bulk thermolyses of 1b and 2b were conducted
by refluxing a xylene solution of 1b and 2b (0.6 g in 40 ml of
xylene) for 1 h under an argon atmosphere. A shiny black
powder separated in both the cases. The resulting suspen-
sion was centrifuged to obtain the black powder which
was washed with diethyl ether (3 · 2 ml) and dried in air.
The typical yields obtained were 40% (1b) and 68% (2b).
5. Supplementary material
The rate of mass loss from the Knudsen cell was mea-
sured for the sample at different temperatures. The
observed reproducibility in the mass loss rate in each iso-
thermal run in ascending or descending orders of tempera-
ture fixation confirms the absence of kinetic hindrance in
the evaporative loss. Several measurements were carried
out in the range of temperature 359–385 K over the sample.
Crystallographic data for the structural analysis has
been deposited with the Cambridge Crystallographic Data
Centre, CCDC no. 287140 for compound 2c. Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (Fax: +44 1223 336033; e-mail: deposit@ccdc.ac.uk
or www: http://www.ccdc.cam.ac.uk).
4.4. Crystallography
Acknowledgements
Intensity data for 2c were measured at 223 K on a Enraf
Nonius FR590 diffractometer employing Mo Ka radiation
so that hmax = 25.0ꢁ. The structure was solved by heavy-
atom methods [24] and refinement anisotropic displace-
ment parameters, hydrogen atoms in the riding model
approximation and a weighting scheme of the form
The authors thank Drs. S.K. Gupta and Shovit Bhat-
tacharya for SEM and EDS analysis. The authors also
thank Dr. R.K Singhal and Dr. G.K. Dey for providing
microanalyses of the samples and TEM analysis,
respectively.
2
w ¼ 1=½r2ðF o2Þ þ ð0:023PÞ þ 2:086Pꢀ for P ¼ ðF o2 þ 2F 2cÞ=
3 was on F2 [25]. Positional disorder was noted in the struc-
ture so that two positions were resolved for the CF3 groups
bound to the C6b and C7b atoms. The major component
(anisotropic refinement) had a site occupancy = 0.934(6).
The molecular structure, shown in Fig. 2, was drawn with
ORTEP [26] at 25% probability ellipsoids. Data manipula-
tion were performed using TEXSAN [27]. The refinement con-
verged to final R [for 3712 reflections with I > 2(I)] = 0.021
and wR = 0.052 (all 4236 data)]. Crystal data: C18H16-
F12N4Pd2, M = 729.15, monoclinic, space group P21/n,
References
[1] P.D. Gurney, R.J. Seymour, in: F.R. Hartley (Ed.), Chemistry of
Platinum Group Metals, Elsevier, Oxford, UK, 1991.
[2] L. Sordelli, G. Martra, R. Psaro, C. Dossi, S. Coluccia, J. Chem.
Soc., Dalton Trans. (1996) 765.
[3] (a) G. Saracco, V. Specchia, Catal. Rev. Sci. Eng. 36 (1994) 305;
(b) J.N. Armor, Catal. Today 25 (1995) 199.
´
´
´
[4] (a) A. Beck, A. Horvath, A. Sarkany, L. Guczi, in: Z. Bing, S.
Hermans, G.A. Somorjai (Eds.), Nanotechnology in Catalysis, vol. 1,
Springer, Berlin, 2004, pp. 83–110 (Chapter 5);
(b) S.U. Son, Y. Yang, J. Park, H.B. Na, H.M. Park, H.J. Yun, J.
Lee, T. Hyeon, J. Am. Chem. Soc. 126 (2004) 5026.
[5] (a) A.A. Zinn, L. Brandt, H.D. Kaesz, R.F. Hicks, in: T.T. Kodas,
M.J. Hampden-Smith (Eds.), The Chemistry of Metal CVD, VCH,
Weinheim, Germany, 1994;
(b) J.E. Gozum, D.M. Pollina, J.A. Jensen, G.S. Girolami, J. Am.
Chem., Soc. 110 (1988) 268.
[6] Y. Zhang, Z. Yuan, R.J. Puddephatt, Chem. Mater. 10 (1998) 2293.
[7] Y.L. Tung, W.C. Tseng, C.Y. Lee, P.F. Hsu, Y. Chi, S.M. Peng,
G.H. Lee, Organometallics 18 (1999) 864.
˚
a = 14.485(3), b = 12.1112(8), c = 15.437(2) A, b =
3
3
˚
117.21(1)ꢁ, V = 2408.4(6) A , Z = 4, Dc = 2.011 g/cm ,
l = 1.600 mmꢁ1, F(000) = 1408.
A preliminary crystallographic examination was also
conducted on complex 1b, isolated as its 1:1 acetonitrile
adduct. The structure was found to be badly distorted, even
at ꢁ50 ꢁC, which precluded a complete determination and
so full details are not reported here (see Section 2). Crystal
data: C30H51N5Pd2, M = 694.56, orthorhombic, space
[8] S. Trofimenko, Chem. Rev. 72 (1972) 497;
S. Trofimenko, Prog. Inorg. Chem. 34 (1986) 115.
[9] (a) G. La Monica, G.A. Ardizzoia, Prog. Inorg. Chem. 46 (1997) 151;
(b) A.P. Sadimenko, S.S. Basson, Coord. Chem. Rev. 147 (1996) 247;
(c) J.E. Cosgriff, G.B. Deacon, Angew. Chem., Int. Ed. Engl. 37
(1998) 286.
˚
group Pbca, a = 10.343(3), b = 18.802(3), c = 33.927(6) A,
3
˚
V = 6598(2) A , Z = 8.
4.5. Thermolysis procedure
[10] (a) D. Pfeiffer, M.J. Heeg, C.H. Winter, Inorg. Chem. 39 (2000) 2377;
(b) C.H. Winter, P.J. Mckarns, J.T. Scheper, Mater. Res. Soc. Symp.
Proc. 495 (1998) 95;
Bulk thermolyses of 1a and 2a at 500 ꢁC were performed
by heating 0.5 g of 1a and 2a in a quartz boat at a rate of
5 ꢁC minꢁ1 from 25 to 500 ꢁC under high-purity argon.
Once at 500 ꢁC, an isotherm was maintained for 2 h before
allowing the sample to cool to room temperature. The prod-
(c) C.H. Winter, J.L. Sebestl, M.J. Heeg, Mater. Res. Soc. Symp.
Proc. 495 (1998) 147;
(d) D. Pfeiffer, M.J. Heeg, C.H. Winter, Angew. Chem., Int. Ed.
Engl. 37 (1998) 2517.

1410
A. Singhal et al. / Journal of Organometallic Chemistry 691 (2006) 1402–1410
[11] (a) J.G. Cederberg, T.D. Culp, B. Bieg, D. Pfeiffer, C.H. Winter,
K.L. Bray, T.F. Kuech, J. Appl. Phys. 85 (1999) 1825;
(b) D. Pfeiffer, B.J. Ximba, L.M. LiableSandes, A.L. Rheingold,
M.J. Heeg, D.M. Coleman, H.B. Schlegel, T.F. Kuech, C.H. Winter,
Inorg. Chem. 38 (1999) 4539;
[19] N. Kitajima, K. Fujisawa, C. Fujimoto, Y. Moro-oka, S. Hashimoto,
T. Kitagawa, K. Tatsumi, A. Nakamura, J. Am. Chem. Soc. 114
(1992) 1277.
´
[20] C. Fernandez-Castano, C. Foces-Foces, N. Jagerovic, J. Elguero, J.
˜
Mol. Struct. 355 (1995) 265.
(c) C.L. Dezelah IV, O.M. El-Kadri, M.J. Heeg, C.H. Winter, J.
Mater. Chem. 14 (2004) 3167.
[12] Z. Wang, C.D. Abernethy, A.H. Cowley, J.N. Jones, R.A. Jones,
C.L.B. Macdonald, L. Zhang, J. Organomet. Chem. 666 (2003)
35.
[21] (a) S. Trofimenko, J. Am. Chem. Soc. 89 (1967) 3165;
(b) O. Renn, L.M. Venanzi, A. Marteletti, V. Gramlich, Helv. Chim.
Acta 78 (1995) 993.
[22] R. Mishra, S.R. Bhardwaj, D. Das, J. Nucl. Mater. 321 (2003) 318.
[23] O. Kubaschewski, C.B. Alcock, P.J. Spencer, Metallugical Thermo-
chemistry, sixth ed., Pergamon Press, Oxford, 1993.
[13] A. Singhal, J. Chem. Res.(S) (2005) 702.
[14] S. Trofimenko, Inorg. Chem. 10 (1971) 1372.
[15] G.W. Henslee, J.D. Oliver, J. Cryst. Mol. Struct. 7 (1977) 137.
[16] Joint Committee on Powder Diffraction Standards (JCPDS) card No.
5-681.
[24] G.M. Sheldrick, SHELXS-97, Program for the Solution of Crystal
Structures, University of Go¨ttingen, Germany, 1997.
[25] G.M. Sheldrick, SHELXL-97. Program for Crystal Structure Refine-
ment, University of Go¨ttingen, Germany, 1997.
[17] S.-W. Kim, J. Park, Y. Jang, Y. Chung, S. Hwang, T. Hyeon, Y.W.
Kim, Nano Lett. 3 (2003) 1289.
[26] C.K. Johnson, ORTEPII. Report ORNL-5138, Oak Ridge National
Laboratory, TN, 1976.
[18] R.C. Palenik, G.J. Palenik, Synth. React. Inorg. Met-Org. 22 (1992)
1395.
[27] TEXSAN: Single Crystal Structure Analysis Software. Molecular
Structure Corporation, The Woodlands, TX, Version 1.7, 1997.