Palladium(II)-selenoether complexes as new single source precursors: First synthesis of Pd4Se and Pd7Se4 nanoparticles

Article abstract of DOI:10.1039/c2dt12113a

Pd₄Se and Pd₇Se₄ nanoparticles (size 38-104 nm), protected by TOP, have been obtained for the first time using Pd(ii) ligated with selenated primary and secondary amines (see 1 and 2) as single source precursors respective

Full text of DOI:10.1039/c2dt12113a

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Palladium(II)–selenoether complexes as new single source precursors: First
synthesis of Pd4Se and Pd7Se4 nanoparticles†
Ved Vati Singh, Gyandshwar Kumar Rao, Arun Kumar and Ajai K. Singh*
Received 4th November 2011, Accepted 22nd November 2011
DOI: 10.1039/c2dt12113a
Pd4Se and Pd7Se4 nanoparticles (size 38–104 nm), protected
by TOP, have been obtained for the first time using Pd(II)
ligated with selenated primary and secondary amines (see 1
and 2) as single source precursors respectively. TEM, SEM,
powder XRD and photoluminescence have been used to
characterize them. 1 and 2 are also the first Pd(II)–selenoether
complexes used for the synthesis of nanoparticles containing
palladium and selenium.
ethyl selenolates, dimeric bridged species having Pd(II) or allylpal-
ladium(II) generally give the Pd17Se15 phase. One of the exceptions
is [Pd2(m-SePh)2(h -C4H7)2] which leads to the formation of a
3
Pd4Se phase.
Nanoparticles of PdSe2 and Pd17Se15 have been recently grown
using bis(N,N-diethyl-N¢-naphthoylselenoureato)palladium(II) as
a single source precursor.9 Phase-pure Pd17Se15 nanocrystals have
also been synthesized by heating a solid sample of [Pd(SeC6H5)2]•
under solvothermal conditions.10 The in situ formation of Pd17Se15
nanoparticles have been reported during the course of Suzuki–
Miyaura C–C coupling reactions catalyzed by a selenium ligated
palladacycle.7g These NPs have been proposed as true catalytic
species for the coupling reaction.7g To our knowledge there is no
report in the literature on the growth of nanoparticles of Pd4Se
and Pd7Se4. In fact nanoparticles of none of the remaining other
palladium selenide phases mentioned above have been reported.
In this communication we present the growth of Pd4Se and Pd7Se4
nanoparticles using the Pd(II) complexes 1 and 2. So far no
complex of selenated primary or secondary amines has been
used for this purpose. Furthermore no other Pd(II)–selenoether
complex has ever been used as a single source precursor for the
growth of nanoparticles containing palladium and selenium.
The [PdCl2(PhSe-CH2CH2CH2-NH2)] (1), Pd(II) complex of
a primary amine has been synthesized by a slight modification
in the method reported in the literature.11‡ However, from a
secondary amine the palladacycle (2), [PdCl{C6H4CH(C6H4-2-
OH)-NHCH2CH2CH2-SePh}] has been synthesized by following
a method reported earlier.7g
Platinum group metal chalcogenides find extensive applications
in catalysis,1–3 low resistance ohmic contacts of semiconducting
electronic devices,3 light image receiving materials with silver
halides,4 and recording films in optical discs and lithographic
films.3 The search of suitable precursors for these chalcogenides,
which may replace presently used volatile and/or toxic Se/Te
(
E) containing species like H2Se or R2E2, continues to be
a challenge of great importance. The complexes of platinum
group metals with chalcogenated ligands can be envisaged as
a good single source precursor. The chalcogenated Schiff bases,
pincer and other ligands form metal complexes which have rich
structural chemistry5–7 and relevance in catalysis7 have also shown,
in some cases, potential as molecular precursors for the synthesis
of metal chalcogenides.8 For low temperature preparation of
chalcogenide materials1–4 metal chalcogenolates1–3 constitute the
class of compounds which have mainly been explored as single-
source precursors.
Palladium, an important platinum group metal forms selenides,
the compositions of which have been identified as PdSe, Pd17Se15,
Pd7Se4, Pd2.5Se, Pd3Se, Pd7Se, Pd4Se, Pd4.5Se, Pd8Se and PdSe2.
The known routes for the preparation of these phases typically
involve heating of the two constituent elements in an evacuated
sealed tube followed by annealing at high temperatures for
several days. The formation of some of these phases has also
been reported on pyrolysis of several palladium(II) complexes.
The monoselenocarboxylates, benzylselenolates, b-functionalised
The complexes 1 and 2 give characteristic proton and carbon-
13 NMR (Fig. S5.5, S5.7–S5.9, ESI†). The assignments of signals
1
in 13C{ H} NMR of the ligand of 1 were made using HMQC
experiments (Fig. S5.4, ESI†). In 77Se NMR spectrum of 1 (Fig.
S5.6, ESI†) the signal is deshielded (by 21 ppm) with respect to
that of the free ligand (Fig. S5.3, ESI†) but in the case of 2 (Fig.
S5.12, ESI†) it is shielded by 27 ppm in comparison to that of its
free ligand (Fig. S5.10, ESI†). The molecular complexes 1 and 2
were used as single source precursors to synthesize nanoparticles
of Pd4Se and Pd7Se4 in a one-step process in moisture and oxygen-
free atmosphere.
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi,
110016, India. E-mail: aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com;
Fax: +91-11-26581102; Tel: +91-11-26591379
The process (Scheme 1) involves the thermolysis of an 11 : 1
mixture of tri-n-octylphosphine (TOP) and 1 or 2 at 280–300 ◦C.
The composition of resulting products analyzed using powder
XRD was found to be precursor dependent. The Pd4Se was
selectively obtained from precursor 1 and Pd7Se4 from 2 even when
† Electronic supplementary information (ESI) available: Synthesis and
full characterization of 1 and 2; DLS, powder XRD and SEM-EDX
1
1
studies of Pd4Se and Pd7Se4 nanoparticles; 1H, 13C{ H} and 77Se{ H}
NMR spectra of known compounds (PhSe-CH2CH2CH2-NH2, PhSe-
CH2CH2CH2-NH-CH(Ph)-C6H4-2-OH, and Pd(II) complexes 1 and 2. See
DOI: 10.1039/c2dt12113a
1142 | Dalton Trans., 2012, 41, 1142–1145
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The morphology of these nanoparticles as revealed by SEM
is shown in Fig. 4. The results of SEM-EDX show that they are
capped with TOP (Fig. S4.1–S4.4, ESI†). HRTEM images of TOP
capped nanoparticles obtained from 1 show their particle size in
the range 38–43 nm, while in case of those obtained from 2 the
size was found in the range 62–104 nm. Nanoparticles of Pd4Se are
cubic (Fig. 3) and those of Pd7Se4 are spherical (Fig. 3) in shape.
In the absorption spectra of both Pd4Se and Pd7Se4 nanoparticles
in visible region there is no excitonic peak corresponding to band
edges at 385 nm (3.22 eV) and 384 nm (3.22 eV) respectively (Fig.
5). Both Pd4Se and Pd7Se4 show broad peaks in their luminescence
spectra. The emission maxima of Pd4Se (489 nm) as well as Pd7Se4
(522 nm) are red shifted in relation to these band edges (Fig. 6
and 7). In these nanoparticles the broad shape of the emission
spectra is not characteristic of band edge luminescence previously
observed in the case of CdSe.12 This could be due to a broad
size distribution of the particles or inefficient passivation of the
surface traps by TOP. However the particle size distribution as
determined by DLS analysis (Fig. S3.1, S3.2, ESI†) is narrow
in the case of Pd7Se4 nanoparticles relative to that of Pd4Se
nanoparticles. Therefore the broad emission spectrum could be
due to recombination from shallow traps, present on the surface,
which have not been passivated by TOP.
Scheme 1 Methodology for syntheses of palladium selenide NPs.
similar thermolytic conditions were used. Each of the powder
XRD patterns of both types of nanoparticles was found matching
with that of the corresponding known standard phase of the same
composition (Fig. S4, ESI†). The sharp nature of the powder XRD
patterns (Fig. 1 and Fig. 2) indicates high crystallinity and the
possibility of the presence of somewhat large sized nanoparticles.
Powder XRD patterns also suggest that the Pd4Se phase has a
tetragonal structure whereas Pd7Se4 is orthorhombic.
Fig. 3 HTEM images of NPs of Pd7Se4 and Pd4Se respectively.
Fig. 1 Powder XRD pattern of Pd4Se.
Fig. 4 SEM images of NPs of Pd7Se4 and Pd4Se respectively.
In conclusion, good quality TOP capped nanoparticles of
compositions Pd4Se (38–43 nm) and Pd7Se4 (62–104 nm) in good
yield have been prepared from one pot synthesis for the first
time by thermolysis in TOP of [PdCl2(PhSe-CH2CH2CH2-NH2)]
Fig. 2 Powder XRD pattern of Pd7Se4 nanoparticles.
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(India) and the Council of Scientific and Industrial Research
(India). The authors also thanks the NSIT (DST) funded HRTEM
central facility installed at IIT Delhi.
Notes and references
‡ All the reactions were performed under a nitrogen atmosphere by using
standard Schlenk techniques. All chemicals were bought from Sigma-
Aldrich. NMR spectra were obtained using CDCl3 as solvent on a Bruker
Spectrospin DPX300 NMR instrument. H2N-(CH2)3-SePh and C6H5-
CH(-C6H4-2-OH)-HN-(CH2)3-SePh: the preparation of the primary amine
was carried out by a slight modification of the method reported in the
literature.11 Instead of alkaline ethanolic solution of NaBH4 we used
an alkaline aqueous solution of NaBH4 for reduction of the diphenyl
diselenide. Yield, 70%; elemental analysis (found: C, 50.52; H, 6.09; N,
6.49%, calc: C, 50.47; H, 6.12; N, 6.54%) IR: 3256 n(N–H), 1208 n(C–N),
495 n(Se–C). 1HNMR (CDCl3, 300 MHz, TMS): 1.822 (quintet, 2H, –
Calkyl–CH2–Calkyl), 1.972 (broad s, 2H, –NH2), 2.772 (t, 2H, –NCH2), 2.932
Fig. 5 UV-VIS spectra of Pd4Se and Pd7Se4.
(t, 2H, –SeCH2), 7.223–7.28 (m, 3H, ArH), 7.467–7.492 (m, 2H, ArH);
1
13C{ H}NMR (CDCl3, 300 MHz, TMS), 24.81 (–SeCH2), 33.37 (–Calkyl
–
CH2–Calkyl), 41.55 (–NCH2), 126.58, 128.84, 130.04, 132.30; 77Se{ H}NMR
(CDCl3, 300 MHz, Me2Se): 289.49 ppm. [PdCl2(PhSe–CH2CH2CH2–
NH2)] (1). The preparation of the palladium complex of the primary
amine (1) was also carried out by a modification of the method reported in
the literature.11 A solution of compound H2N-(CH2)3-Se-C6H5 (1 mmol)
in acetone (10 cm3) was added dropwise with stirring to a solution of
sodium tetrachloropalladate (0.294 g, 1 mmol) in water (15 cm3). A yellow
precipitate separated after about 15 min but the mixture was stirred for
another 2 h and the solid then filtered off, washed with cold methanol,
and dried in vacuo. Repeated attempts to get good quality crystals of
the palladium complex of primary amine were unsuccessful. yield 70%,
elemental analysis (found: C, 27.61; H, 3.57; N, 3.63%), calculated: C
27.54, H 3.60, N 3.57%. IR: 3235 n(N–H), 1158 n(C–N), 479 n(Se–C),
1
330, 304 n(Pd–Cl) cm-1. HNMR (CDCl3, 300 MHz): 1.59–2.28 (m, 4H,
1
NH2 + Calkyl–CH2–Calkyl), 2.27–3.01 (m, 4H, SeCH2 + NCH2), 7.28–7.51
(m, 3H, meta to Se + para to Se), 8.21 (d, 2H, ortho to Se); 77Se{ H}
1
NMR (CDCl3, 300 MHz): 310.36 ppm. Synthesis of palladium selenide
nanoparticles: A mixture of 5.0 mL (11.0 mmol) of tri-n-octylphosphine
(TOP) and precursor (2: 0.537 g, 1.0 mmol; 1: 0.391 g, 1.0 mmol) was
heated to 280–300 ◦C under nitrogen atmosphere in a three neck flask
for 2 h with continuous stirring. The colour of the mixture changed from
yellowish to red–brown within 40 min and brown–black precipitate started
appearing after two hours. The reaction mixture was cooled to room
temperature and 20 mL of acetone was added into the flask to obtain
a brown–black precipitate which was separated by centrifugation. The
obtained precipitate was washed three times with methanol (20 mL) and
dried.
Fig. 6 Photoluminescence spectrum of Pd4Se.
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Fig. 7 Photoluminescence spectrum of Pd7Se4.
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molecule precursors of this type, which are stable, air/moisture
insensitive and are easy to synthesize by room temperature
reactions of selenoether type ligands with Na2PdCl4.
This work (SR/WOS–A/CS–22/2009) was supported by the
Department of Science and Technology, New Delhi-110016
1144 | Dalton Trans., 2012, 41, 1142–1145
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