NMR investigation of platinum-diphosphine complexes in [BMIM][PF 6] ionic liquid

Article abstract of DOI:10.1016/S0020-1693(03)00291-3

The formation of platinum-diphosphine complexes of catalytic importance was investigated in [BMIM][PF₆] (BMIM=butyl-methyl-imidazolium cation) ionic liquid by NMR for the first time. Both the PtCl(SnCl ₃)(bdpp) and Pt(SnCl₃)₂(bdpp) complexes (where bdpp=2,4-bis(diphenylphosphino)pentane) were identified in the insertion reaction of tin(II)chloride into the Pt-Cl bond of PtCl₂(bdpp) complex. The addition of PPh₃ to PtCl₂(bdpp) resulted in the formation of [Pt(bdpp)(PPh₃)Cl]⁺ complex cation as a minor component, which turned to be a major component in the presence of tin(II)chloride while it forms trichlorostannate counterion. Both types of reactions show high similarity with those obtained in conventional organic solvents. As a serious disadvantage of the use of [BMIM][PF₆] as ionic liquid, the partial decomposition of the hexafluorophosphate counterion in the presence of the smallest amount of water (and tin(II)chloride) is also discussed.

Full text of DOI:10.1016/S0020-1693(03)00291-3

Inorganica Chimica Acta 353 (2003) 301ꢀ305
/
www.elsevier.com/locate/ica
Note
NMR investigation of platinumꢀ
/
diphosphine complexes in
[BMIM][PF6] ionic liquid
Ga´bor Rangits a, Gyorgy Peto˝cz a, Zolta´n Berente b,c, La´szlo´ Kolla´r a,c,
¨
*
a Department of Inorganic Chemistry, University of Pe´cs, Ifju´sa´g u. 6, P.O. Box 266, H-7624 Pecs, Hungary
b Institute of Biochemistry, University of Pe´cs, P.O. Box 99, H-7643 Pecs, Hungary
c Research Group for Chemical Sensors of the Hungarian Academy of Sciences, P.O. Box 266, H-7624 Pecs, Hungary
Received 29 November 2002; accepted 10 March 2003
Abstract
The formation of platinumꢀ
methyl-imidazolium cation) ionic liquid by NMR for the first time. Both the PtCl(SnCl3)(bdpp) and Pt(SnCl3)2(bdpp) complexes
Cl bond of
/
diphosphine complexes of catalytic importance was investigated in [BMIM][PF6] (BMIMꢁbutyl-
/
(where bdppꢁ
/
2,4-bis(diphenylphosphino)pentane) were identified in the insertion reaction of tin(II)chloride into the PtÃ
/
PtCl2(bdpp) complex. The addition of PPh3 to PtCl2(bdpp) resulted in the formation of [Pt(bdpp)(PPh3)Cl]ꢂ complex cation as a
minor component, which turned to be a major component in the presence of tin(II)chloride while it forms trichlorostannate
counterion. Both types of reactions show high similarity with those obtained in conventional organic solvents. As a serious
disadvantage of the use of [BMIM][PF6] as ionic liquid, the partial decomposition of the hexafluorophosphate counterion in the
presence of the smallest amount of water (and tin(II)chloride) is also discussed.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Platinum; Phosphine; Ionic liquid; Nuclear magnetic resonance; Ligand exchange
1. Introduction
cessfully used as solvents for the nickel catalysed
dimerization of propene [6], for the dimerization of
butenes [7] and ethene [8] and for the rhodium catalysed
hydrogenation of cyclohexene [9].
Due to the high practical importance of hydroformy-
lation, the ‘ionic liquid version’ has been investigated as
early as 1972 by Parshall who described the platinum-
catalysed hydroformylation of ethene in [NEt4][SnCl3]
melts [10].
Fused salts are liquids containing only ions and are
often called ionic liquids. Especially those ones, that are
liquid at room temperature, provide a useful extension
to the range of solvents that are used by synthetic
chemists. Although these ionic liquids possess a number
of physical properties that make them interesting for a
wide audience of chemists, it is only in the past few years
that significant literature has become available [1].
Excellent reviews and papers appeared quite recently
showing the potential of ionic liquids both in classical
Room temperature ionic liquid, [BMIM][PF6]
(BMIMꢁbutyl-methyl-imidazolium cation) (Fig. 1),
/
whose application could bring a real breakthrough in
this field, proved to be a good solvent in rhodium-
catalysed hydroformylation of 1-pentene (catalyst:
[Rh(CO)2(acac)]/PPh3) [11] and long-chain a-olefins
[12]. In the latter case a cationic cobaltocenium ligand
has been used. Wasserscheid and Waffenschmidt did a
seminal work in the field of platinum-catalysed hydro-
formylation in [BMIM][SnCl3] [13].
organic reactions like Friedelꢀ
homogeneous catalysis [2ꢀ4].
The application of ionic liquids in transition metal
catalysed reactions turned up in the middle of the
nineties, and reviewed recently [5]. The chloroaluminate
and ethylchloroaluminate ionic liquids have been suc-
/Crafts alkylation and
/
Although several types of catalytic reactions have
been studied, sporadic results were published concerning
even the simplest reactions of complexes in ionic liquid.
* Corresponding author. Tel.: ꢂ
50 1527.
E-mail address: kollar@ttk.pte.hu (L. Kolla´r).
/
36-72-32 7622/4153; fax: ꢂ36-72-
/
0020-1693/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0020-1693(03)00291-3

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G. Rangits et al. / Inorganica Chimica Acta 353 (2003) 301ꢀ305
/
sertion takes place nearly quantitatively resulting in the
formation of the corresponding trichlorostannato com-
plexes.
However, in [BMIM][PF6] ionic liquid the
PtCl2(bdpp) complex (1) shows a limited reactivity
towards SnCl2 (anhydrous) even at higher temperature
(70 8C) and elevated reaction time (50 h). Up to 15% of
the starting complexes have been converted to the
corresponding trichlorostannato complex (1a). How-
Fig. 1. The structure of the [BMIM][PF6] ionic liquid.
Therefore, our results on kinetically relatively inert
platinumꢀphosphineꢀtin(II)chloride systems, also of
/
/
potential catalytic importance, will be presented.
Furthermore, the preliminary results on potential side
reactions of the [BMIM][PF6] ionic liquid will also be
discussed.
ever, when PtCl(SnCl3)(bdpp) (bdppꢁ2,4-bis(diphenyl-
/
phosphino)pentane) 1a was prepared in crystalline form
and a twofold excess of SnCl2 was added, a mixture of
PtCl2(bdpp) 1, Pt(SnCl3)2(bdpp) 1b, and the starting
complex, 1a has been obtained (Scheme 1). Following
the reaction by 31P NMR, the composition of the
mixture was found temperature dependent. By increas-
ing the temperature, the amount of 1b was slightly
increased (Fig. 2), so the equilibrium was shifted
towards the formation of trichlorostannato species, 1a
and 1b. Although the determination of the extremely
weak 117,119Sn satellites could have been done with high
2. Results and discussion
2.1. NMR investigations on platinumꢀ
/
diphosphineꢀ
/
tin(II)chloride complexes in ionic liquids
The solution structure of well-known hydroformyla-
tion catalysts, platinum(II)-bdpp-tin(II)chloride in situ
catalytic systems, as well as that of the
PtCl(SnCl3)(bdpp) precursors were investigated in
[BMIM][PF6] ionic liquid. The NMR data are sum-
marised in Table 1.
One of the key-points of the platinum catalysed
hydroformylation is the role of tin(II)chloride. It has
been proved by detailed high pressure NMR investiga-
tion that it acts as a counterion in the elementary steps
of the hydroformylation reaction [14]. Its insertion into
uncertainty, the prior information on PtꢀSnCl3 com-
/
plexes (e.g. the relative downfield shifts and the magni-
tude of 1J(Pt,P) coupling constants assigned to
phosphorus trans to chloro or trichlorostannato ligand)
enables the identification of these species. However,
even by using the threefold excess of tin(II)chloride,
unlike the reaction in CDCl3, neither the formation of
1a nor that of 1b are complete.
It has to be noted, that tin(II)chloride insertion takes
place to much smaller extent with PtCl2(chiraphos) 2
and PtCl2(prophos) 3 (where chiraphos and prophos
stands for 2,3-bis(diphenylphosphino)butane and 1,2-
bis(diphenylphosphino)propane, respectively). Similar
situation has been observed with PtCl2(cis-1,2-bis(di-
the PtÃ
/
Cl bond of various cis- and trans-Pt(tertiary
phosphine)2Cl2 complexes was investigated in detail as
early as 1978 by Pregosin and Sze [15]. The extent of the
formation of the corresponding PtCl(SnCl3)(dipho-
sphine) complexes is highly solvent dependent. In
chloroform or dichloromethane the tin(II)chloride in-
Table 1
NMR data of Pt-diphosphine complexes in [BMIM][PF6] ionic liquid
Complexes
d(31P) (ppm) 1J(31P,195Pt) (Hz)
2J(31P,31P) (Hz)
d(13C) a (ppm) 1J(13C,195Pt) (Hz)
2J(13C,13C) (Hz)
PtCl2(bdpp)
PtCl(SnCl3)(bdpp)
1
8.5
1a 9.1
3431
3370
3121
ꢀ
24.4
24.4
/
5.0
Pt(SnCl3)2(bdpp)
[Pt(bdpp)(PPh3)Cl]ꢂ
1b 10.6
1c 27.2
18.4
33509
24039
23009
35009
/
8 b
8 b
8 b
8 b
ꢀ
/
/
390; 15.0
390; 28.0
15.0; 28.0
/
15.8
/
[Pt(bdpp)(PPh3)Cl]ꢂ c
1c 22.1
14.6
12.1
2368
2309
3454
2233
2380
2384
392; 20.0
392; 25.4
20.0; 25.4
[Pt(bdpp)2]2ꢂ
Pt(13CN)2(bdpp)
1d 9.0
1e 3.4
2.6
28.2
126.4
127.7
1022
1025
25.0
Solvent: [BMIM][PF6]; 31P NMR and 13C NMR spectra were recorded at 161.9 and 100.6 MHz, respectively (temperature: 70 8C).
Chemical shift of the CN carbon.
a
b
Due to the Pt-satellites of very low intensity, the 1J(Pt,P) coupling constants could be determined with a given deviation, only.
31P NMR in CDCl3 (see Ref. [16]).
c

G. Rangits et al. / Inorganica Chimica Acta 353 (2003) 301ꢀ
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303
Fig. 2. The 31P NMR of the complexes formed in the reaction of PtCl2(bdpp) and SnCl2 in [BMIM][PF6] at 303, 323 and 343 K (see also Scheme 1).
phenylphosphino)ethene), where the five-membered
platinumꢀphosphine chelate-ring is even more rigid
than in 2 and 3.
satellites are inherently broader than the ‘central line’,
and are extremely broad and weak in ionic liquid. In
certain cases their assignment cannot be done with usual
accuracy (e.g. for CDCl3 solutions) due to the low
resolution (high viscosity even at 70 8C) and low
solubility of the complex in ionic liquid (Fig. 3, Table
1). However, perfect resolution of the signals due to
both the 2Jcis(P,P) and 2Jtrans(P,P) coupling constants of
diagnostic value can be achieved at 70 8C.
Upon addition of 1 M equiv. of bdpp and 2 M equiv.
of SnCl2 to 1, the quantitative formation of
[Pt(bdpp)2](SnCl3)2 was observed (Scheme 3, Table 1).
It has to be mentioned, that the reaction resulting in the
same complex cation 1d is complete upon addition of 1
equiv. of SnCl2 only. This result shows clearly, that in
case of a favourable six-membered chelate ring even the
formation of a good leaving ligand, SnCl3ꢃ is not a
precondition of the formation of ‘PtP4’ cationic species.
The chloro ligand could also fully dissociate and form a
chloride counterion.
/
2.2. NMR investigations of ligand exchange reactions
diphosphine complexes in [BMIM][PF6]
ionic liquid
with platinumꢀ
/
In order to investigate some ligand-exchange reac-
tions in ionic liquid, 1 equiv. of PPh3 was added to 1a.
Although some ‘P2-complexes’ 1, 1a, 1b, featuring the
approximately 20ꢀ
plexes, remained
/
28% of the total amount of com-
in the reaction mixture,
[Pt(bdpp)(PPh3)Cl][SnCl3] (1c) containing both biden-
tate and monodentate phosphines coordinated in a
square-planar arrangement, has been formed as a major
product (Scheme 2). Both the chemical shifts and
coupling constants correspond to those obtained in
chloroform [16]. It has to be noticed that platinum
Scheme 2. Formation of Pt-bdpp-monophosphine cation.
Scheme 1. Insertion of SnCl2 into PtÃCl bond.
/

304
G. Rangits et al. / Inorganica Chimica Acta 353 (2003) 301ꢀ305
/
Fig. 3. The 31P NMR of the complexes formed in the reaction of PtCl2(bdpp), SnCl2 and PPh3 in [BMIM][PF6] at 303 and 343 K (see also Scheme 2).
Similarly, complete ligand exchange reaction has been
observed with 2 M equiv. of Na13CN. The resulting
[Pt(bdpp)(13CN)2] complex (1e) shows an AA?XX?
second order coupling system.
Our preliminary NMR experiments clearly show, that
in addition to the potential formation of transition
ing to that of apical fluoro ligands. (The detailed
investigation of the formation of species containing
PF2 fragments is in progress.)
Therefore, it has to be noted, that the application of
ionic liquids of this type has to be done with care and ionic
liquids cannot be considered as innocent solvents.
As a conclusion it can be stated that both tin(II)chlor-
ide insertion and ligand exchange reactions take place in
[BMIM][PF6] ionic liquid analogously to those observed
in conventional organic solvents like chlorinated hydro-
carbons resulting in well-defined species. However, the
tin(II)halide insertion depends even stronger on the
platinum-ligand chelate size. (To the best of our knowl-
edge, in spite of the widespread application of ionic
liquids, this is the first NMR investigation of the
elementary reactions of transition metal complexes in
this novel reaction media.)
As one of the most important advantage of the above
systems, the extreme stability towards oxidation has to
be mentioned. (Probably one of the most important
features of this system is that no phosphine dioxides and
hemioxides were obtained even after exposing the
samples to air for a week. From practical reasons this
feature cannot be over-emphasized.) However, the
decomposition of the [PF6]ꢃ counterion in the presence
metalꢀimidazolium carbene complexes [17], the partial
/
decomposition of the hexafluorophosphate counterion
in the presence of the smallest amount of water and
tin(II)chloride should be considered. (The usual work-
up of the catalytic mixtures occurs very often by adding
water to the ionic liquid system.)
When the [BMIM][PF6] is exposed to moisture, two
types of PF2Xn species showing a triplet are formed (31P
NMR: d1ꢁ
/
ꢃ
/
18.5 ppm (br t, 1J(31P,19F)ꢁ
/
989 Hz);
970 Hz). The formation
1
d2ꢁ
/
ꢃ
/
24 ppm (t, J(31P,19F)ꢁ
/
of one of these species (d1) is supported by tin(II)chlor-
ide addition and shows species in fast equilibrium. A
probable explanation of the formation of PF2 moieties is
the relatively easy substitution of equatorial fluoro
ligands of PF5 of trigonal bipiramidal structure compar-
Scheme 3. Formation of Pt-bis(bdpp) cation.

G. Rangits et al. / Inorganica Chimica Acta 353 (2003) 301ꢀ
/
305
305
of water (and tin(II)chloride) should also be considered.
The aqueous work-up of the reaction mixtures might
lead to serious damage both in products and catalyst.
Acknowledgements
L. K. thanks the Hungarian National Science Foun-
dation (T035047) for the financial support and Johnson
Matthey Technology Centre for the loan of platinum
precursors.
3. Experimental
3.1. Materials
References
The Pt(diphosphine)Cl2 catalytic precursors were
prepared according to a known procedure [18]. The
chiral bidentate ligands, bdpp, chiraphos and prophos
were purchased from Strem and Aldrich, respectively.
The ionic liquid [BMIM][PF6] was purchased from
Aldrich. Anhydrous tin(II)chloride was prepared from
[1] T. Welton, Chem. Rev. 99 (1999) 2071.
[2] R.T. Carlin, J.S. Wilkes, in: G. Mamantov, A. Popov (Eds.),
Advances of in Nonaqueous Chemistry, VCH Publishing, New
York, 1994.
[3] K.R. Seddon, Kinet. Catal. 37 (1996) 693.
[4] H. Olivier-Bourbigou, in: B. Cornils, W.A. Herrmann (Eds.),
Aqueous Phase Organometallic Catalysis: Concepts and Applica-
tions, Wiley-VCH, Weinheim, 1998.
SnCl2ꢄ2H2O (Aldrich) by treating with 2 M equiv. of
/
acetic anhydride and by washing with dry ether.
[5] P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39 (2000)
3772.
3.2. General
[6] Y. Chauvin, S. Einloft, H. Olivier, Ind. Eng. Chem. Res. 34 (1995)
1149.
The 31P and 13C NMR spectra were taken on a Varian
Inova 400 spectrometer operating at 161.9 and 100.6
MHz, respectively. Chemical shifts are downfield rela-
tive to 85% H3PO4. Due to the viscosity of the medium,
NMR signals are much broader than those in usual
deuterated solvents, introducing some uncertainty into
[7] Y. Chauvin, H. Olivier, C.N. Wyrvalski, L.C. Simon, R.F. de
Souza, J. Catal. 165 (1997) 275.
[8] S. Einloft, F.K. Dietrich, R.F. de Souza, J. Dupont, Polyhedron
15 (1996) 3257.
[9] P.A.Z. Suarez, J.E.L. Dullius, S. Einloft, R.F. de Souza, J.
Dupont, Polyhedron 15 (1996) 1217.
[10] G.W. Parshall, J. Am. Chem. Soc. 94 (1972) 8716.
[11] Y. Chauvin, H. Olivier, L. Mussmann, FR 95/14,147, (1995)
(Chem. Abstr. 127 (1977) P341298k).
the detection and quantification of satellite (195Pt and
117,119
especially
broader than the central lines.
Sn) signals which are inherently
[12] C.C. Brasse, U. Englert, A. Salzer, H. Waffenschmidt, P.
Wasserscheid, Organometallics 19 (2000) 3818.
[13] P. Wasserscheid, H. Waffenschmidt, J. Mol. Catal. A: Chemical
164 (2000) 61.
All operations (including preparation of complexes)
were done under argon atmosphere. The appropriate
complex was measured into an NMR tube, put under
argon and the sufficient amount of [BMIM][PF6] as well
as the further reagents (tin(II)chloride, triphenylpho-
sphine) were added. Locking and referencing was
performed using an insert tube (2-mm o.d.) filled with
D2O.
[14] I. To´th, T. Ke´gl, C.J. Elsevier, L. Kolla´r, Inorg. Chem. 33 (1994)
5708.
[15] P.S. Pregosin, S.N. Sze, Helv. Chim. Acta 61 (1978) 1848.
[16] L. Kolla´r, G. Szalontai, J. Organomet. Chem. 421 (1991) 341.
[17] L.J. Xu, W.P. Chen, J.L. Xiao, Organometallics 19 (2000) 1123.
[18] L. Kolla´r, J. Bakos, I. To´th, B. Heil, J. Organomet. Chem. 350
(1988) 277.