A fast and easy approach to the synthesis of Zeise's salt using microwave heating

Article abstract of DOI:10.1016/j.inoche.2009.02.015

A fast and easy approach to the synthesis of Zeise's salt, KPtCl₃(C₂H₄), is reported using microwave heating. The reaction is complete after 15 min at 130 °C using K₂PtCl₄ as starting material, a 1:1:

Full text of DOI:10.1016/j.inoche.2009.02.015

Inorganic Chemistry Communications 12 (2009) 341–342
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Inorganic Chemistry Communications
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A fast and easy approach to the synthesis of Zeise’s salt using microwave heating
*
Krista A. Shoemaker, Nicholas E. Leadbeater
Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, CT 06269-3060, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A fast and easy approach to the synthesis of Zeise’s salt, KPtCl3(C2H4), is reported using microwave heat-
ing. The reaction is complete after 15 min at 130 °C using K2PtCl4 as starting material, a 1:1:1 ratio of
water:ethanol:concentrated HCl as solvent, and a loading of 50 psi of ethene.
Ó 2009 Elsevier B.V. All rights reserved.
Received 10 February 2009
Accepted 11 February 2009
Available online 27 February 2009
Keywords:
Microwave
Platinum
Zeise’s salt
Alkene complex
Zeise’s salt, trichloro(ethylene)-platinate(II) 1, is credited as the
first organometallic compound to be isolated in essentially pure
form [1,2]. The synthesis and study of this, and the related dimeric
[13]. We believed that by extending this approach we could pre-
pare Zeise’s salt using ethene as a gaseous reagent.
compound trans-di-l–chlorodichloro-bis(ethylene)diplatinum(II)
2 played a considerable role in the development of bonding theory
in both inorganic and organic chemistry [3,4]. It is probable that
Zeise first prepared 1 by first boiling platinum (IV) chloride in eth-
anol and then treating this solution with potassium chloride [5].
For a long while after its discovery, modifications of the procedure
for making 1 all generally required prolonged reaction times
(7–14 days) or else the use of high pressures [6]. More recently a
method has been developed which utilizes tin (II) chloride as a cat-
alyst for the reaction between ethene and potassium hexachloro-
platinate [7]. The reaction can be performed in a few hours at
atmospheric pressure. The use of a tin catalyst, as well as being
somewhat undesirable, can lead to problems in product isolation.
The applications of Zeise’s salt in catalysis [8] and as a precursor
for functional materials [9] as well as it proving a versatile starting
material for synthesis of organometallics [10] sparked our interest
in developing a fast, easy route for its preparation using microwave
heating. Interestingly, while organic chemists have used micro-
wave heating extensively [11], there have been relatively few re-
ports of its use in preparative organometallic chemistry [12].
Using scientific microwave apparatus it is possible to perform reac-
tions either in sealed vessels or open reflux arrangements. Using a
custom-built gas-loading accessory for introduction of carbon
monoxide or hydrogen into a reaction vessel we have recently re-
ported the synthesis of Ru3(CO)12 from RuCl3 as well as
H4Ru4(CO)12 from Ru3(CO)12 and H2Os3(CO)10 from Os3(CO)12
As an initial set of reaction conditions we chose to use K2PtCl4
(3) as starting metal salt and dilute hydrochloric acid as the sol-
vent. Working on the 50 mg scale of 3 and loading with 50 psi
(3.45 bar) of ethene in a 10 mL capacity sealed tube, we heated
the reaction mixture to 130 °C and held it at this temperature for
15 min [14]. This yielded a slightly yellow solution but with signif-
icant plating of platinum metal on the inner surface of the reaction
tube. This is undesirable since, in the presence of a microwave
field, metal films heat very rapidly and can lead to the melting of
glass tubes which, if under pressure, can fail. We attributed the
deposition of platinum, at least in part, to the limited solubility
of 3 in dilute hydrochloric acid. Thus, we repeated the reaction
using more concentrated acid. A 1:1 mix of water to concentrated
HCl (v:v) proved to be more suitable, 3 being totally soluble. Upon
heating under a pressure of ethene, an orange solution was formed
but with significantly less metal deposition. Analysis of the solu-
tion showed that very little 3 had reacted. Ethene is poorly soluble
in water so we believed that by modifying the solvent we could
facilitate the reaction. Thus, we decide to trial ethanol as a co-sol-
vent since this would increase the solubility of ethene in the reac-
tion mixture. A 1:1:1 (v:v:v) ratio of water to concentrated
* Corresponding author.
E-mail address: nicholas.leadbeater@uconn.edu (N.E. Leadbeater).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.02.015

342
K.A. Shoemaker, N.E. Leadbeater / Inorganic Chemistry Communications 12 (2009) 341–342
[2] For a historical perspective on the discovery of Zeise’s salt, see D. Seyferth,
Organometallics 20 (2001) 2.
[3] (a) M. Dewar, Bull. Soc. Chim. Fr. 18 (1951) C79;
MW
H O / EtOH, H SO
4
KPtCl3(C2H4)
C2H4
K2PtCl4
+
2
2
(b) J. Chatt, L.A. Duncanson, J. Chem. Soc. (1953) 2939.
1
[4] M. Black, R.H.B. Mais, P.G. Owston, Acta Cryst. B. B25 (1969) 1753.
[5] (a) W.C. Zeise, Overs K. Dan, Vidensk. Selsk. Forh. (1825–26) 13.;
(b) W.C. Zeise, Poggendorf’s Ann. Phys. Chem. 21 (1831) 497.
[6] (a) J. Chatt, M.L. Searle, Inorg. Synth. 5 (1957) 210;
Scheme 1. Fifty psi ethene, heat to 130 °C and hold until a total time of 15 min has
elapsed.
(b) W. MacNevin, A. Giddings, A. Foris, Chem. Ind. (1958) 577.
[7] (a) P.B. Chock, J. Halpern, F.E. Paulik, Inorg. Synth. 28 (1990) 349;
(b) R. Pietropaolo, M. Graziani, U. Belluco, Inorg. Chem. 8 (1969) 1506.
[8] (a) A.R. Rajaram, L. Pu, Org. Lett. 8 (2006) 2019;
hydrochloric acid to ethanol proved to be successful and, after
15 min at 130 °C we obtained a clear yellow solution, analysis of
which showed that 1 was present in significant quantities (Scheme
1).
(b) Y. Chen, J.K. Snyder, J. Org. Chem. 66 (2001) 6943;
(c) J. Beyer, P.R. Skaanderup, R. Madsen, J. Am. Chem. Soc. 122 (2000) 9575;
(d) J. Beyer, R. Madsen, J. Am. Chem. Soc. 120 (1998) 12137;
(e) L. Giraud, T. Jenny, Organometallics 17 (1998) 426;
Our next objective was to find an easy way to obtain 1 in pure
form so we could reliably quantify our results. Filtration to remove
any metallic platinum formed during the course of the reaction fol-
lowed by removal of the ethanol/water solvent mixture under re-
duced pressure led to a yellow solid. Not unexpectedly, this
contained KCl, the byproduct formed during the reaction. To re-
move this we took advantage of the fact that while 1 is readily sol-
uble in acetone, KCl is not. Thus, simply adding acetone to the
crude solid product mixture allowed for separation of the two
components. Using this approach we were able to isolate 1 in pure
form by filtration to remove the KCl followed by evaporation of the
acetone under reduced pressure. Using this method we were able
to isolate 1 in 89% yield.
In many cases, reactions performed using microwave heating
can reach completion in very short times. To determine if 15 min
was the optimum time for the preparation of 1, we repeated the
reaction but held it at 130 °C for just 5 min. We obtained a 60%
yield of 1 showing that, while the reaction is well advanced within
5 min, to reach completion a longer reaction time is required.
Extending the time beyond 15 min had no significant impact on
product yield.
(f) K. Ikura, I. Ryu, N. Kanube, N. Sonoda, J. Am. Chem. Soc. 114 (1992) 1520;
(g) J.O. Hoberg, P.W. Jennings, Organometallics 15 (1996) 3902.
[9] (a) H.A. Andreas, S.K.Y. Kung, E.J. McLeod, J.L. Young, V.I. Birss, J. Phys. Chem. C
111 (2007) 13321;
(b) E.V. Shtykova, D.I. Svergun, D.M. Chernyshov, I.A. Khotina, P.M. Valetsky,
R.J. Spontak, L.M. Bronstein, J. Phys. Chem. B 108 (2004) 6175.
[10] (a) For selected recent examples, see C.R. Barone, M. Benedetti, V.M. Vecchio,
F.P. Fanizzi, L. Maresca, G. Natile, Dalton Trans. (2008) 5313;
(b) B.L. Stocker, J.O. Hoberg, Organometallics 25 (2006) 4537;
(c) M.L. Clarke, A.M.Z. Slawin, J.D. Woollins, Polyhedron 22 (2003) 19.
[11] For a recent overview of the field see A. Loupy (Ed.), Microwaves in Organic
Synthesis, Wiley, Weinheim, 2006.
[12] (a) For examples, see: B.J. Tardiff, A. Decken, G.S. McGrady, Inorg. Chem.
Commun. 11 (2008) 44;
(b) K.D. Johnson, G.L. Powell, J. Organomet. Chem. 693 (2008) 1712;
(c) Y.T. Lee, S.Y. Choi, S.I. Lee, Y.K. Chung, T.J. Kang, Tetrahedron Lett. 47 (2006)
6569;
(d) M. Ardon, G. Hogarth, D.T.W. Oscroft, J. Organomet. Chem. 689 (2004)
2429;
(e) S.L. VanAtta, B.A. Duclos, D.B. Green, Organometallics 19 (2000) 2397;
(f) D.R. Baghurst, S.R. Cooper, D.L. Greene, D.M.P. Mingos, S.M. Reynolds,
Polyhedron 9 (1990) 893.
[13] N.E. Leadbeater, K.A. Shoemaker, Organometallics 27 (2008) 1254.
[14] Reactions were performed using a CEM Discover microwave unit. This consists
of a continuous focused microwave power delivery system with operator
selectable power output from 0 to 300 W. Reactions were performed either in
10 mL or 80 mL capacity sealed tubes. The temperature of the contents of the
vessel was monitored using an IR sensor located underneath the reaction vessel
or a fiber-optic temperature probe inserted directly into the reaction mixture.
Pressure was controlled by a load cell connected directly to the vessel. The
contents of the reaction vessel are stirred by means of an electromagnet located
below the floor of the microwave cavity and a Teflon-coated magnetic stir bar
in the vessel. Temperature, pressure and power profiles were monitored using
commercially available software provided by the microwave manufacturer. For
loading reaction vessels with gas, either a commercially available gas-loading
interface or an in-house built interface was used.
Our final objective was to test the scalability of the protocol.
Moving first to using 100 mg of 3, we performed the reaction in
a larger 80 mL capacity sealed tube. We kept all conditions the
same (a 1:1:1 ratio of water:conc. HCl:ethanol as solvent, 50 psi
ethene, 130 °C for 15 min) and obtained an identical product yield
after purification [15]. Scaling the reaction to the 0.5 g level was
also successful, an 87% yield of 1 being obtained.
[15] Synthesis of [KPtCl3(C2H4)]: To a dry 80-mL glass vessel equipped with a
magnetic stirbar was added a solution of K2PtCl4 (100 mg, 0.241 mmol) in a
mixture of water (5 mL), ethanol (5 mL) and concentrated hydrochloric acid
(5 mL). The vessel was sealed in the microwave apparatus, with a septum
In summary, we have developed a fast and easy approach to the
synthesis of Zeise’s salt using microwave heating. The reaction is
complete after 15 min at 130 °C using K2PtCl4 as starting material,
a 1:1:1 ratio of water: concentrated HCl: ethanol as solvent and
50 psi of ethene.
containing ports for pressure and temperature measurement devices.
A
pressure of 50 psi ethene was introduced into the vessel, the pressure sensor
being kept closed. The line to the carbon monoxide regulator was then closed
and the pressure vented to the atmosphere through the pressure sensor. This
process was repeated two more times, then the vessel loaded to 50 psi with
ethene and sealed. With stirring, the reaction mixture was heated to 130 °C
using an initial microwave power of 300 W and held at this temperature until
a total time of 15 min had elapsed. The reaction mixture was then cooled to
50 °C, at which time the remaining pressure was carefully vented. The
contents of the reaction vessel were transferred into a round bottom flask and
the solvent removed on a rotary evaporator. Acetone (5 mL) was added to the
flask to extract KPtCl3(C2H4). This was repeated two times, and the combined
acetone washings cooled to approximately 0 °C and filtered. Removal of the
solvent left pure KPtCl3(C2H4) in 89% yield (79 mg). 1H NMR (400 MHz, d6-
acetone, d in ppm): 4.21 (1 J Pt = 64 Hz). 13C NMR (100 MHz, d6-acetone, d in
ppm): 66.0 (1 J Pt = 195 Hz).
Acknowledgement
We thank the University of Connecticut and the American
Chemical Society Petroleum Research Foundation (45433-AC1)
for funding.
References
[1] (a) For reviews on Zeise’s salt, see J.S. Thayer, J. Chem. Ed. 46 (1969) 442;
(b) J.S. Thayer, Adv. Organomet. Chem. 13 (1975) 1.