Palladiumd(II) complexes of N-{2-(aryltelluro)ethyl}morpholine/piperidine: Synthesis, structure, application in Heck coupling and unprecedented conversion into nano-sized PdTe

Article abstract of DOI:10.1016/j.inoche.2011.10.015

The complexes, [PdCl₂(L)] (1-2) (L = N-{2-(aryltelluro)ethyl} morpholine/piperidine) have been synthesized and characterized by multi-nuclei NMR and single crystal X-ray crystallography. They on reaction with aryl chloride/bromide and morpholine/piperidine give ~ 5 nm size nano-particles of PdTe. The 0.005 mol% of 2 is suitable for Heck coupling (conversion up to 93%).

Full text of DOI:10.1016/j.inoche.2011.10.015

Inorganic Chemistry Communications 15 (2012) 163–166
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Inorganic Chemistry Communications
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Palladiumd(II) complexes of N-{2-(aryltelluro)ethyl}morpholine/piperidine:
Synthesis, structure, application in Heck coupling and unprecedented conversion
into nano-sized PdTe
⁎
Pradhumn Singh, Dipanwita Das, Arun Kumar, Ajai K. Singh
Department of Chemistry, Indian Institute of Technology, New Delhi 110016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The complexes, [PdCl2(L)] (1–2) (L=N-{2-(aryltelluro)ethyl}morpholine/piperidine) have been synthesized
and characterized by multi-nuclei NMR and single crystal X-ray crystallography. They on reaction with aryl
chloride/bromide and morpholine/piperidine give ~5 nm size nano-particles of PdTe. The 0.005 mol% of 2
is suitable for Heck coupling (conversion up to 93%).
Received 10 September 2011
Accepted 17 October 2011
Available online 25 October 2011
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Palladium
Tellurium ligand
Synthesis
Structure
Nano-particle
Heck coupling
Palladium(II) complexes of organotellurium ligands have been in-
vestigated more as academic curiosities[1], while palladium(II) com-
plexes of many other ligands are known as catalysts or pre-catalysts
for various carbon–carbon [2] and carbon–heteroatom coupling reac-
tions (C\S [3], C\P [4], C\O [5] and C\N [6]). Among these cou-
plings, C\N bond formation is of special interest because the
resulting products are the rudiments in various compounds of rele-
vance to biological, pharmaceutical and material sciences [7]. Conse-
quently several new efficient palladium catalytic systems for C\N
cross-coupling reactions between aryl and heteroaryl halides and
amines have been reported recently [6f–h]. The complexes of palladi-
um with some organochalcogen ligands have been found promising
for C\C coupling reactions [8–10] such as Heck coupling and
Suzuki–Miyaura coupling. However, they need to be extensively ex-
plored for catalytic reactions as their potential is scantly known, par-
ticularly of organotellurium ligands about which virtually nothing is
reported. Further we are unaware of any report in which palladium com-
plex of any organochalcogen ligand is explored for carbon–heteroatom
coupling reaction, particularly C\N one. It was therefore thought worth-
while to design palladium complexes (1–2) of N-{2-(aryltelluro)ethyl}
morpholine/piperidine/ligands (Scheme 1) and explore their potential
for C\C and C\N cross-coupling reactions. Bidentate organotellurium li-
gands have been envisaged suitable for this purpose in view of desired
stability as well as accessibility of Pd in complexes. Thus ligands given in
Scheme 1 and their Pd(II) complexes have been synthesized, character-
ized and explored for their potential for C\N coupling and Heck
C\C Coupling. The conversions were found up to 92% when
0.005 mol% of 2 was used for Heck coupling. However, unprece-
dented formation of nano-size (~5 nm) particles of composition
PdTe occurs while attempting C\N reactions. The resulting
Pd\Te quantum dots have been characterized. These results are
described in this communication.
⁎
Corresponding author. Tel.: +91 11 26591379; fax: +91 11 26581102.
Scheme 1. Synthesis of L1–L2 and their Pd-complexes 1–2.
E-mail addresses: ajai57@hotmail.com, aksingh@chemistry.iitd.ac.in (A.K. Singh).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.10.015

164
P. Singh et al. / Inorganic Chemistry Communications 15 (2012) 163–166
The 1 and 2 have been characterized by X-ray crystallography and
their ORTEP diagrams are given in Figs. 1 and 2 with some bond
lengths and angles (more values are given in Table S1 and S2 of
Supplementary Material) respectively. The geometry around Pd in 1
and 2 is nearly square planar and the ligands are coordinated with
Pd in a bidentate (Te, N) mode forming five membered ring. This is
also supported by NMR spectroscopy. The Pd–Te bond lengths of
´
1, 2.5054(6) and 2, 2.5145(6) Å are similar but shorter than the sum
´
of their covalent radii 2.64 Å. They are consistent with the reported
values [1,13] 2.5007(6)-2.5158(5) Å for [PdCl(O−, N, Te)ligand]
´
and [PdCl2(N, Te)ligand]. They are also not significantly longer than
the value 2.4781(3) Å reported for [PdCl2(N-{2-(4-methoxyphenyl-
telluro)ethyl}pyrrolidine)] [14] but in comparison to values 2.5865
´
Fig. 1. ORTEP diagram of 1 with 30% probability ellipsoids; bond length(Å): Pd(1)–
Te(1) 2.5054(6), Pd(1)–N(1) 2.119(5), Pd(1)–Cl(1) 2.3574(15), Pd(1)–Cl(2) 2.2880
(16); bond angle (°): Cl(1)–Pd(1)–Te(1)172.44(4), Cl(2)–Pd(1)–Te(1) 84.55(4),
N(1)–Pd(1)–Te(1) 90.26(13), N(1)–Pd(1)–Cl(1) 95.17(14), N(1)–Pd(1)–Cl(2) 174.59
(14), Cl(2)–Pd(1)–Cl(1) 90.14(6).
´
(2)-2.6052(2) Å reported for [PdCl2(N-{2-(4-methoxyphenyltelluro)
ethyl}morpholine)2] [15] they appear to be somewhat shorter. This
difference is easily understandable in terms of minor steric factor in
first case and monodentate nature of (Te, N) ligand in the other com-
´
plex. The Pd\N bond lengths of 1–2 [2.119(5) to 2.126(5) Å] are con-
´
sistent with the sum of their covalent radii 2.03 Å. The Pd\Cl bond
´
lengths in 1 and 2 are normal (2.2880(16) to 2.3609(16) Å). The
bond angles at Te and N atoms are as expected for nearly trigonal-
pyramidal and tetrahedral geometries, respectively.
The reactions between morpholine or piperidine and aryl chloride
or bromide (having substituent: \MeO, \CN, \CHO, \NO2) and
complexes 1 and 2 in the presence of KOH as a base, at 100 °C carried
out with a motive of C\N coupling (Scheme 2) resulted in PdTe
nano-particles (for detailed procedure see, Supplementary Material)
due to decomposition of 1 and 2.
The resulting nano-sized product was separated (See S4 in Sup-
plementary material). To characterize it was converted from amor-
phous phase to crystalline phase by annealing at 550 °C in argon
atmosphere for 5 h. Thereafter SEM-EDX (Supplementary Material:
Figure S5.2–S5.3) powder X-ray diffraction (see S5 and Figure S5.1
in Supplementary Material), HR-TEM (Fig. 3 and Figure S5.4) and
TEM-EDX (Figure S5.5 in Supplementary Material) were used for its
characterization. These studies revealed that spherical shaped nano-
particles of average size ~5 nm formulated as PdTe (composition
established on the basis of matching of its powder XRD pattern with
that of a known standard phase of the same composition; S5 in
Supplementary Material) were formed (Fig. S5.1 in Supplementary
Material).
These nano-particles were explored for C\N coupling but found
catalytically inactive even immediately after formation. This appears
to be contributed by steric effects of large size Te and protective liga-
tion effect of morpholine or piperidine present in the reaction mix-
ture [16–17], which probably coordinates strongly with palladium
present on the surface of these nano-particles. The filtrates from
which nano-particles were separated were extremely complex mix-
ture of organic compounds and therefore presence of C\N coupled
products could not be unequivocally established. However, the pre-
sent results provide a novel low temperature (100 °C) synthetic
route for preparations of PdTe quantum dots. The mechanism of for-
mation of these quantum dots appears to be highly complicated, in
´
Fig. 2. ORTEP diagram of 2 with 50% probability ellipsoids; bond length(Å): Pd(1)–
Te(1) 2.5145(6), Pd(1)–N(1) 2.126(5), Pd(1)–Cl(1) 2.3609(16), Pd(1)–Cl(2) 2.3099(17);
bond angle (°): Cl(1)–Pd(1)–Te(1)175.96(5), Cl(2)–Pd(1)–Te(1) 85.42(5), N(1)–Pd(1)–
Te(1) 90.20(14), N(1)–Pd(1)–Cl(1) 93.79(14), N(1)–Pd(1)–Cl(2) 175.56(14), Cl(2)–
Pd(1)–Cl(1) 90.66(6).
The L1 has been reported earlier [11] while L2 prepared for the
first time. The 1 and 2 were synthesized by reaction of Na2PdCl4
with L1 and L2 respectively. The detailed procedures are given in
Supplementary Material. The signals in 125Te{1H} NMR spectra [12]
of 1 and 2 appear at higher frequency (222 and 244 ppm respectively)
than those of free L1 and L2 (Supplementary Material), as Te donor
sites are coordinated to the Pd centre. In 1H and 13C{1H} NMR
spectra of 1 and 2 signals of all protons and carbon atoms respectively
appear at higher frequency[11] relative to those of free ligands (See
Supplementary Material) which coordinate with Pd in a bidentate
mode. The magnitude of shifting to higher frequency is up to ~5 ppm
for C5 and ~46 ppm for C7 in 13C{1H} NMR spectra. In 1H NMR spectra
also protons attached to these carbon atoms appear shifted (up to
1 ppm) to higher frequency.
Scheme 2. C–N coupling (unsuccessful).

P. Singh et al. / Inorganic Chemistry Communications 15 (2012) 163–166
165
synthesized and their structures are solved. Attempts with 1 and 2
of catalyzing C\N cross-coupling reactions of aryl bromides and chlo-
rides result in the formation of PdTe nano-particles of size ~5 nm un-
expectedly. These nanoparticles are inactive for C\N coupling but the
procedure in which they are formed is a novel low temperature route
to prepare quantum dots of PdTe. The 2 has potential for Heck cou-
pling as its 0.005 mol% gives conversions more than 90% in some
cases.
Acknowledgement
Authors thank Council of Scientific and Industrial Research (India)
award of project 01(2421)10/EMR-II and University Grants Commis-
sion (India) for JRF/SRF. Authors thank Professor A. K. Ganguli of IIT
Delhi for HRTEM facility.
Appendix A. Supplementary material
Fig. 3. HR-TEM image of PdTe obtained from 1and 2.
Experimental details, crystal data and refinements table, bond
lengths and angles for complex 1–2. 125Te{1H} NMR spectra of L1,
L2, 1 and 2. Powder XRD data, SEM, TEM and EDX of Pd\Te NPs.
CCDC nos. 778778 (1) and 709838 (2) have full crystallographic data.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2011.10.015.
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3.36–3.70 (4 H, m, C7-H), 3.84 (3 H, s, OCH3), 3.89–4.38 (2 H, m, C5-H),
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2993, 2860 (s; νC–H
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