R. Sadasivan, A. Patel and A. Patel
Polyhedron 193 (2021) 114896
of the same [22]. The 31P NMR of the mother solution of the sodium
salt indicated that the tri-palladium, though a major product,
always came with a mixture of unidentified by-products, and even
precipitation with KCl did not yield pure compound. Preliminary
single crystal studies revealed presence of an asymmetric unit
composed of three independent thirds of an anion along with 11
potassium cations. Further determination of the crystal structure
seemed not viable due to poor crystal quality and presence of
unidentified mixtures.
In the present paper, we have synthesized tri-palladium substi-
tuted sandwich type phosphotungstate using individual salts, i.e.,
sodium tungstate and potassium hydrogen phosphate. For the first
time, we were able to isolate the pure crystal and systematically
deduce its crystal structure. Further characterizations by various
spectral techniques were also carried out to support the data
obtained from single crystal XRD, such as FT-IR, UV–Visible, Cyclic
Voltammetry and NMR spectroscopy. A preliminary study on the
catalytic activity was carried out for industrially important organic
transformations such as hydrogenation of nitrobenzene, cyclohex-
ene and crotonaldehyde.
well as no other isomeric product formation indicating the stability
of the synthesized materials.
2.3. Characterization
The synthesized material was characterized by Single crystal
XRD, elemental analysis, FT–IR Spectroscopy, solution phase 31P
NMR Spectroscopy, UV–Visible spectroscopy and Cyclic
Voltammetry.
Single Crystal X-Ray diffraction studies were carried out in a
Bruker D8 Venture SC-X-Ray diffractometer using Cu(a) source
(k = 1.5406 Å), KAPPA goniometer and a PHOTON 100 CMOS detec-
tor. Data collection was carried out with APEX III software [26].
Reflections were merged and corrected for Lorenz and polarization
effects, scan speed, and background using SAINT [2]. Absorption
corrections, including odd and even ordered spherical harmonics
were performed using SADABS [27]. Space group assignments were
based upon systematic absences, E-statistics, and successful refine-
ment of the structures. Structures were solved by direct methods
with the aid of successive difference Fourier maps, and were
refined against all data using the APEX III software [26,28] in con-
junction with SHELXL-2014 [29]. H atoms of the phenyl rings were
placed in calculated positions and refined using a riding model.
Non-hydrogen atoms were refined with anisotropic displacement
parameters. Refinements were conducted by full-matrix least
squares against |F| using all data. Images of the crystal structures
were generated by Diamond, version 3.2 (software copyright, Crys-
tal Impact GbR) and PLATON [30]. Some of the disagreeable reflec-
tions were removed by the SQUEEZE command in PLATON [30].
However, it is well known that due to presence of heavy tungsten
atoms, artefacts may appear close to the lighter oxygen atoms. Fur-
ther, in the presence of heavy elements, such as W’s, it is almost
impossible to locate or fix hydrogens corresponding to water mole-
cules. Hence, the crystal data shows high Rint value as well as dis-
crepancy in the formula weight. The data is provided free of charge
by The Cambridge Crystallographic Data Centre (CSD 1979233).
2. Experimental
2.1. Materials
All chemicals used were of A.R. grade. Na2WO4Á2H2O, K2HPO4-
Á7H2O, PdCl2, CsCl and diethyl ether, were obtained from Merck
and used as received.
2.2. Synthesis
Tri-palladium substituted phosphotungstate, [Pd3(PW9O34)2]11-
À was synthesized by following method. PdCl2 (0.355 g, 2 mmol)
was dissolved in minimum amount of water by heating (addition
of 2–3 drops of conc. HCl is required for dissolution). To this, satu-
rated aqueous solution of K2HPO4Á7H2O (0.071 g, 1 mmol) was
added drop-wise, followed by addition of 1 mL conc. H2SO4 with
continuous stirring. Finally, saturated solution of Na2WO4 (1.65 g,
10 mmol) prepared in water was added to above resultant mixture,
followed by careful drop-wise addition of 5 mL conc. H2SO4 with
stirring. The reaction mixture was allowed to cool at room temper-
ature (due to exothermic nature of the resultant mixture), the
obtained product was then extracted with diethyl ether (twice,
using 25 mL). The etherate layer (lower layer) was bubbled out
thoroughly with molecular oxygen to break the POM-etherate
complex, finally the traces of ether were removed by gentle heat-
ing. To this, about 5 mL of distilled water was added, heated at
80 °C, filtered and the filtrate was kept aside for crystallization.
After 10 days, long needle shaped dark brown crystals are obtained
and designated Cs-K-Pd3(PW9)2. (The crystals growth after 10 days
and after 17 days are shown in Scheme 1) The visual appearance of
crystals after 17 days showed no aggregates formation of Pd as
2.4. Catalytic study
The synthesized material was used as a catalyst for the hydro-
genation of nitrobenzene, cyclohexene and crotonaldehyde. The
reaction was carried out in a parr reactor instrument with the fol-
lowing components: SS-316 batch reactor (100 mL), H2 reservoir
and electric temperature and pressure controller. Known amount
of the catalyst was taken along with a mixture of 20 mL acetonitrile
and 30 mL water. To this 1.23 mL (1 mmol) of nitrobenzene was
added. Preliminary reaction was carried out at 8 bar H2 pressure,
60 °C for 5 h. The reaction mixture was then extracted multiple
times using dichloromethane and analyzed in Shimadzu GC–
2014 gas chromatograph instrument with RTx-5 capillary column.
Hydrogenation reaction of cyclohexene and crotonaldehyde were
also carried out under similar conditions.
Scheme 1. Synthesis of Cs-K-Pd3(PW9)2 and crystal growth after (a) 10 days and (b) 17 days.
2