The microwave synthesis of platinum(II) phosphine complexes

Article abstract of DOI:10.1016/j.ica.2011.06.028

The microwave synthesis of a series of platinum(II) phosphine complexes is reported. The complexes dppePtCl₂ (dppe = bis(diphenylphosphino) ethane), dpppPtCl₂ (dppp = bis(diphenylphosphino)propane), dppmPtCl₂ (dppm = bis(diphenylphosphino)methane) and cis-(Ph ₃P)₂PtCl₂ are synthesized from the reaction of potassium tetrachloroplatinate(II) and the phosphine. The isolated yields are 65% or better.

Full text of DOI:10.1016/j.ica.2011.06.028

Inorganica Chimica Acta 376 (2011) 638–640
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Note
The microwave synthesis of platinum(II) phosphine complexes
⇑
Nancy Carter Dopke , Holly E. Oemke
Department of Chemistry, Alma College, Alma, MI 48801, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The microwave synthesis of a series of platinum(II) phosphine complexes is reported. The complexes
dppePtCl2 (dppe = bis(diphenylphosphino)ethane), dpppPtCl2 (dppp = bis(diphenylphosphino)propane),
dppmPtCl2 (dppm = bis(diphenylphosphino)methane) and cis-(Ph3P)2PtCl2 are synthesized from the reac-
tion of potassium tetrachloroplatinate(II) and the phosphine. The isolated yields are 65% or better.
Ó 2011 Elsevier B.V. All rights reserved.
Received 13 October 2010
Accepted 7 June 2011
Available online 23 June 2011
Keywords:
Microwave synthesis
Platinum complexes
Phosphine complexes
1. Introduction
methylene chloride, and absolute ethanol were used as purchased.
Diethyl ether was either used as received or purified on a Glass
Metal complexes containing the bidentate phosphine ligands,
dppm, dppe and dppp, continue to be of interest. The complex
NiCl2(dppp) has recently been reported as part of a catalytic
mixed-ligand system for the borylation of catalysts [1]. A series
of platinum(II), palladium(II), nickel, cobalt, and tin mixed-ligand
systems containing bidentate phosphine ligands have been investi-
gated for use as antitumor agents [2]. The ligand dppm was used to
assemble cyclometalated organometallic platinum(II) complexes
[3].
Square planar bis(phosphine)platinum(II) complexes are useful
starting materials for the synthesis of multinuclear platinum com-
plexes and heterometallic complexes and clusters [4–7]. The con-
ventional syntheses of these compounds can be time consuming
and laborious [8–12]. We report the microwave synthesis of
dppe PtCl2 (dppe = bis(diphenylphosphino)ethane), dpppPtCl2 (dppp
= bis(diphenylphosphino)propane), dppmPtCl2 (dppm = bis(diphen-
ylphosphino)methane) and cis-(PPh3)2PtCl2. The microwave syn-
theses are relatively quick and straightforward with good yields.
Contour Seca Solvent System by SG Water USA, LLC. Phosphorus-
31 NMR was collected on a Bruker Avance 300 MHz NMR and
was used to confirm the identity of all products. The optimized
microwave syntheses are reported below. A 30-s prestir was in-
cluded in all microwave syntheses.
2.1. Conventional synthesis of dppePtCl2 [8]
To a slurry of K2PtCl4 (0.7140 g, 1.720 mmol) in 18 ml of chloro-
form was added a solution of dppe (0.6860 g, 1.722 mmol) in 16 ml
of hot ethanol. This combined solution was heated to reflux for
over 12 h under nitrogen. The resulting solid was filtered, rinsed
in ether and dissolved in very hot DMF. The cream colored solution
was layered with diethyl ether to result in a white powder (0.538 g,
47.1%) which was collected by filtration. 31P NMR (CDCl3):
41.6 ppm (JP–Pt = 3620 Hz).
2.2. Microwave synthesis of dppePtCl2
To a mixture of K2PtCl4 (0.0740 g, 0.178 mmol) and dppe
(0.0748 g, 0.188 mmol) was added 2 ml chloroform and 2 ml abso-
lute ethanol in a 2.0–5.0 ml microwave vial with a stir bar. The vial
was sealed and irradiated in the microwave system with a pro-
grammed temperature of 145 °C for 1 h. The resulting white pow-
der was isolated and loaded into a clean vial with 2 ml DMF and a
stir bar. The vial was sealed and heated in the microwave at 145 °C
for 3 min. The resulting solution was layered with diethyl ether
and cooled in a refrigerator. A white powder (0.0847 g, 71.6%)
was collected by filtration and dried. 31P NMR (CDCl3): 41.9 ppm
(JP–Pt = 3620 Hz).
2. Experimental
Microwave-assisted syntheses were done using
a Biotage
Microwave Initiator System. The potassium tetrachloroplatinate(II)
was purchased from Strem and ground using a mortar and pestle
prior to use. The phosphine ligands were purchased from Acros
Organics or Strem and were recrystallized prior to use. Chloroform,
⇑
Corresponding author.
E-mail address: ndopke@alma.edu (N.C. Dopke).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.06.028

N.C. Dopke, H.E. Oemke / Inorganica Chimica Acta 376 (2011) 638–640
639
2.3. Microwave synthesis of dppmPtCl2
yield for two trials using the large scale was 62.7%. Having an ex-
cess of dppe was not necessary for a good yield.
To a mixture of K2PtCl4 (0.0733 g, 0.177 mmol) and dppm
(0.0737 g, 0.192 mmol) was added 2 ml absolute ethanol, 1.5 ml
chloroform and 0.5 ml distilled water in a 2.0–5.0 ml microwave
vial with a stir bar. The vial was sealed and irradiated in the micro-
wave system with a programmed temperature of 145 °C for
30 min. The resulting white powder was isolated, washed with
ethanol and water, then loaded into a clean vial with 2 ml DMF
and a stir bar. The vial was sealed and heated in the microwave
at 145 °C for 3 min. The resulting yellow solution was layered with
diethyl ether and cooled. A white powder (0.0742 g, 64.6%) was
collected by filtration and dried. 31P NMR (CDCl3): À63.3 ppm
(JP–Pt = 3090 Hz).
The yield of the reaction decreased, down to 41.9%, when only
0.10 mmol of K2PtCl4 and excess dppe were used in the reaction.
A factor in this decline is that the percentage lost in the filtration
process is greater for the smaller scale reaction.
Given the success of the synthesis of dppePtCl2, the syntheses of
other platinum(II) phosphine complexes were explored. The com-
plexes dppmPtCl2 and dpppPtCl2 were successively synthesized
as clean products using a microwave reactor. Using the same sol-
vent system and conditions as employed successfully for dppePtCl2
for the reaction between K2PtCl4 and dppm to synthesize
dppmPtCl2, resulted in a yield of only 12.5%. Using inspiration from
the conventional synthesis [10] which includes water in the sol-
vent system increased the yield to 64.6% with a 30-min microwave
cycle time.
2.4. Microwave synthesis of dpppPtCl2
Using pure methylene chloride as the solvent for the reaction
between K2PtCl4 and dppp under the same microwave conditions
resulted in the product dpppPtCl2 from the crystallization of the
reaction filtrate by diethyl ether in yields up to 68.4%. Product
was also isolated in the reaction using a chloroform/ethanol
solvent system but in lower yields. The addition of water to the
solvent system resulted in a black reaction mixture that was not
analyzed.
The synthesis of the monodentate triphenylphosphine complex,
(Ph3P)2PtCl2, provides the opportunity for cis/trans mixtures. Reac-
tion times of 20 or more minutes result in a cis product with less
than 3% contamination of the trans product based upon 31P NMR
analysis [13]. The highest yield (93.2%) was obtained using meth-
ylene chloride as the solvent, but yields over 70% were achieved
using a mixed solvent system of water/ethanol and the pure sol-
vent water.
To a mixture of K2PtCl4 (0.4096 g, 0.9868 mmol) and dppp
(0.4076 g, 0.9882 mmol) was added 12 ml methylene chloride in
a 10.0–20.0 ml microwave vial with a stir bar. The vial was sealed
and irradiated in the microwave system with a programmed tem-
perature of 145 °C for 1 h. The resulting yellow solution was dec-
anted and the filtrate was layered with diethyl ether and cooled.
A white powder (0.4576 g, 68.4%) was collected by filtration and
dried. 31P NMR (CDCl3): À4.0 ppm (JP–Pt = 3410 Hz).
2.5. Microwave synthesis of cis-(Ph3P)2PtCl2
To a mixture of K2PtCl4 (0.0721 g, 0.174 mmol) and triphenyl-
phosphine (0.0921 g, 0.351 mmol) was added 4 ml methylene
chloride in a 2.0–5.0 ml microwave vial with a stir bar. The vial
was sealed and irradiated in the microwave system with a pro-
grammed temperature of 145 °C for 50 min. The resulting yellow
solution was layered with diethyl ether and cooled. The resulting
cream powder was recrystallized in chloroform layered with hep-
tane. A white powder (0.1280 g, 93.2%) was collected by filtration
and dried. 31P NMR (CDCl3): 15.5 ppm (JP–Pt = 3670 Hz).
A mixture of the trans and cis products was produced when the
microwave vial was heated for 5 min using an ethanol/water sol-
vent system. The pure trans product was not obtained by a de-
crease in time or temperature.
4. Conclusion
3. Results and discussion
A series of platinum(II) phosphines have been synthesized using
the microwave. The reactions are easy, use commercially available
materials, and are relatively fast. Clean products are obtained in
good isolated yields.
The use of a microwave greatly reduces the time of reaction for
the synthesis of dppePtCl2 and provides an easier methodology for
recrystallization of this bis(phosphine)platinum(II) complex. The
reaction time was reduced from over 8 h to 1 h using the micro-
wave instead of conventional methods starting with commercially
available starting materials. With isolated yields up to 71.5% using
a mixed solvent system of chloroform and ethanol, the yield is bet-
ter than those obtained by conventional methods [9]. The mixed
solvent system is composed of the same solvents as those found
in a published literature procedure [8]. Using pure absolute etha-
nol or pure chloroform as the solvent for the reaction decreased
the yield. The product yield is also sensitive to time. A reduction
in the microwave cycle time from 1 h to 45 min greatly reduced
the yield. Product purity was established by 31P NMR and the ab-
sence of pink K2PtCl4 starting material.
Using the microwave for the dissolution of the crude product
dppePtCl2 greatly decreases not only the time necessary to get
the product dissolved in DMF, but also decreases the risk of ther-
mal decomposition of DMF.
Increasing the scale of the reaction from 0.18 mmol of K2PtCl4 to
1.4 mmol necessitated a larger microwave vial, an increase in the
volume of solvent and the addition of 30 min to the microwave
time. An increase in the volume of DMF, from 2 to 8 ml, used for
recrystallization and an increase in time, from 3 to 6 min, was also
required for dissolution of the crude product. The average percent
Acknowledgments
We thank the American Chemical Society Petroleum Research
Fund for support of this research through ACS PRF Grant No.
48504-B3. We thank Nicholas A. Maciulis (graduate of Alma
College), Julia C. Riggs (graduate of Mercer University) and Prayash
Patel (graduate of Mercer University) for their previous work
on the conventional synthesis of platinum complexes. The NMR
was purchased through funds provided by the National Science
Foundation Grant USE-8951401. We thank the Alma College
Department of Chemistry, Provost Office and the Office of the Vice
President of Finance and Administration for partial support.
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