N,N′-Dialkylimidazolium Chloroplatinate(II), Chloroplatinate(IV), and Chloroiridate(IV)
Salts and an N-Heterocyclic Carbene Complex of Platinum(II): Synthesis in Ionic Liquids
and Crystal Structures
Masihul Hasan, Ivan V. Kozhevnikov,* M. Rafiq H. Siddiqui, Cristina Femoni,
²
Alexander Steiner,
²
and Neil Winterton
Leverhulme Centre for Innovative Catalysis, Department of Chemistry, University of Liverpool,
Liverpool L69 7ZD, U.K.
ReceiVed June 6, 2000
The first imidazole-type carbene complex of platinum(II), cis-(C
2
H
4
)(1-ethyl-3-methylimidazol-2-ylidene)PtCl
2
,
has been obtained by reacting PtCl
2
and PtCl
4
with ethylene in the basic [EMIM]Cl/AlCl
3
(1.3:1) ionic liquid
(where [EMIM]
+
) 1-ethyl-3-methylimidazolium) at 200 °C and structurally characterized (monoclinic P2
1
/c
space group, a ) 10.416(2) Å, b ) 7.3421(9) Å, c ) 15.613(2) Å, ) 101.53(2)
o
, Z ) 4). This complex can be
regarded as a stable analogue of the π-alkene-Pd(II)-carbene intermediate in the Heck reaction. In addition, a
series of new N,N′-dialkylimidazolium salts of platinum group metals of the type [RMIM]
2
[MCl
n
], where [RMIM
+
]
) 1-alkyl-3-methylimidazolium and M ) Pt(II), Pt(IV), or Ir(IV), have been prepared and characterized. The
salts [EMIM]
2
[PtCl
6
](1) and [EMIM]
2
[PtCl
4
](2) were prepared in the ionic liquid [EMIM]Cl/AlCl
3
and the salts
[BMIM]
2
[PtCl
4
](3) and [BMIM]
2
[PtCl
6
](4) (where [BMIM]
+
) 1-n-butyl-3-methylimidazolium) and [EMIM]
2
-
[IrCl
6
](5) in aqueous or acetonitrile media. From TGA measurements, salts 1-5 decompose in air in several
steps eventually to form the corresponding metal, the onset of decomposition being observed at (°C) 260 (1), 220
(2), 200 (3), 215 (4), and 210 (5). The structures of 1, 2, and 5 were determined by single-crystal X-ray analysis.
The three salts crystallize in the monoclinic P2
1
/n space group (1, a ) 7.6433(9) Å, b ) 16.353(2) Å, c )
9.213(1) Å, ) 113.56(1)
o
, Z ) 2; 2, a ) 8.601(1) Å, b ) 8.095(2) Å, c ) 13.977(2) Å, ) 91.75(2)
o
, Z )
2; 5, a ) 10.353(2) Å, b ) 9.759(2) Å, c ) 10.371(2) Å, ) 92.98(3)
o
, Z ) 2).
Introduction
Ambient temperature molten salts (ionic liquids) with very
low vapor pressure, especially those comprising mixtures of
N,N′-dialkylimidazolium (e.g., 1-ethyl-3-methylimidazolium,
[EMIM]
+
, and 1-n-butyl-3-methylimidazolium, [BMIM]
+
) chlo-
ride and aluminum chloride, are increasingly finding a range
of laboratory, developmental, and technical applications, for
example, as media for organic and inorganic chemical synthesis,
in materials production, in electrochemical and separation
processes, and as prototype novel materials.
1-3
Much current
research is focused upon the possible use of ionic liquids as
media for cleaner organic synthesis and processing
4
and upon
better understanding of liquid structure and solvation phenomena
of ionic liquids and its relevance to reactivity.
2,5,6
Ionic liquids containing transition-metal complexes, especially
platinum group metal complexes, are of great interest regarding
their potential catalytic applications.
4
However, relatively few
studies have explored the use of ionic liquids for the purposeful
synthesis and characterization of coordination complexes since
Hussey reviewed the chemistry of transition-metal halide
complexes in chloroaluminate ionic liquids over 10 years ago.
2c
Several reports have apppeared describing spectrochemical and
electrochemical investigations of transition-metal complexes in
solution in these media
5
and structural characterization in the
solid state.
6,7
Recently ionic liquids have been used as media
for the preparation of some halometalate complexes and
clusters.
8
We ourselves have described the synthesis, thermo-
stability, and single-crystal X-ray structure of two salts, [EMIM]-
[AuCl
4
] and [BMIM][AuCl
4
].
8j
No attempt, however, has been
made so far to study Pt(II) and Pt(IV) complexes in ionic liquids.
²
Department of Chemistry, University of Liverpool, Crown St., Liverpool
L69 7ZD, U.K.
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10.1021/ic000606o CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/13/2001