Convenient high-pressure syntheses of [PtX3(C2H 4)]- (X = Cl, Br) salts with a variety of organic cations from PtX2

Article abstract of DOI:10.1016/j.jorganchem.2012.12.017

Reaction between PtX₂ (X = Cl or Br) with an equimolar amount of the appropriate [Cat]⁺X^- in toluene in an autoclave under 25 bars of C₂H₄ yields the corresponding [Cat][PtX ₃(C₂H₄)] in moderate to excellent isolated yields and high purity (X = Cl, Cat = nBu₄P; X = Br, Cat = nBu ₄P, Ph₄P, nBu₄N). The same reaction does not yield any olefin complex if X = I and no reaction also occurs for the PtBr ₂ + KBr system in water, THF, acetone or toluene.

Full text of DOI:10.1016/j.jorganchem.2012.12.017

Journal of Organometallic Chemistry 730 (2013) 165e167
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Note
Convenient high-pressure syntheses of [PtX3(C2H4)]ꢀ (X ¼ Cl, Br) salts
with a variety of organic cations from PtX2
,
,b,c,
Aurélien Béthegnies a b, Rinaldo Poli a
*
a CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France
b Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
c Institut Universitaire de France, 103, bd Saint-Michel, 75005 Paris, France
a r t i c l e i n f o
a b s t r a c t
Reaction between PtX2 (X ¼ Cl or Br) with an equimolar amount of the appropriate [Cat]þXꢀ in toluene in
an autoclave under 25 bars of C2H4 yields the corresponding [Cat][PtX3(C2H4)] in moderate to excellent
isolated yields and high purity (X ¼ Cl, Cat ¼ nBu4P; X ¼ Br, Cat ¼ nBu4P, Ph4P, nBu4N). The same reaction
does not yield any olefin complex if X ¼ I and no reaction also occurs for the PtBr2 þ KBr system in water,
THF, acetone or toluene.
Article history:
Received 24 October 2012
Accepted 12 December 2012
Keywords:
Platinum
Ethylene ligand
Halide complexes
Zeise anion
Ó 2012 Elsevier B.V. All rights reserved.
Chojnacki’s anion
Synthesis
1. Introduction
by aniline derivatives, we have isolated the [PtX3(C2H4)]ꢀ salts
(X ¼ Cl, Br) in combination with the nBu4Pþ cation. This has allowed
The ethylenetrichloroplatinate(II) complex [PtCl3(C2H4)]ꢀ,
represents a milestone in organometallic chemistry, being the first
organometallic complex to be synthesized for a transition metal
and isolated as the hydrated potassium salt in 1825 by the Danish
chemist Zeise [1e3]. The tribromo analogue [PtBr3(C2H4)]ꢀ, was
introduced by Chojnacki in 1870, and also isolated as the potassium
salt [4]. Until recently, the chemistry of these and other analogous
(olefin)trihaloplatinate complexes has been investigated, specifi-
cally in terms of ligand exchange kinetics and mechanism [5,6],
only in combination with potassium or other alkali metals, mostly
in water or aqueous methanol. Although a Liþ salt was shown to be
soluble in THF [7,8], salts with bulky organic cations are expected to
provide solubility in a broader range of organic solvents and would
therefore allow the development of the coordination chemistry of
the Zeise, Chojnacki and related anions in these different media.
A compound of stoichiometry [nBu4N][PtBr3(C2H4)], character-
ized by IR spectroscopy, was mentioned [9], but no detailed
synthetic procedure has appeared in the literature to the best of our
knowledge. Only recently, motivated by the observation of
a remarkable catalytic activity of PtBr2 in the presence of nBu4PBr
towards the hydroamination of ethylene [10e12] and 1-hexene [13]
us to investigate ligand exchange equilibria in dichloromethane
[14] that, in combination with theoretical calculations [15], have led
to the elucidation of the full catalytic cycle for the ethylene
hydroamination reaction [16,17].
The synthesis of [nBu4P][PtX3(C2H4)] (X ¼ Cl, Br) made use of
a salt metathesis from Zeise’s salt or Chojnacki’s salt (the latter
being made in situ by halide exchange from Zeise’s salt and KBr),
respectively. The pure products were obtained in 88 and 61% yield,
respectively [14]. Although simple, this synthesis suffers from
a relatively high market price of Zeise’s salt and the need to use
a large amount of KBr (to insure complete halide exchange) for the
synthesis of the tribromo analogue, since Chojnacki’s salt is not
commercially available. In this contribution, we disclose a much
simpler and widely applicable synthetic method making use of the
less expensive PtCl2 and PtBr2, leading to the isolation in good to
excellent yields of salts with a few different organic cations. The
method should be easily adaptable to salts with many other cations
and different olefins for both halide series.
2. Experimental section
2.1. General
* Corresponding author. CNRS, LCC (Laboratoire de Chimie de Coordination), 205
route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France.
E-mail address: rinaldo.poli@lcc-toulouse.fr (R. Poli).
All synthetic procedures were carried out under exclusion of air
and using solvents that had been dehydrated by conventional
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.12.017

166
A. Béthegnies, R. Poli / Journal of Organometallic Chemistry 730 (2013) 165e167
methods. The 1H, 31P and 195Pt NMR spectra were measured in
CD2Cl2 or CDCl3 with Brucker (DPX300, AV400) instruments. The
spectra were referenced against the residual solvent resonance for
1H and against external 85% H3PO4 for 31P and Na2[PtCl6] for 195Pt.
Elemental analyses were carried out with a PerkineElmer 2400
series II analyzer by the Microanalytical Service of the Laboratoire
de Chimie de Coordination. Tetra(n-butyl)phosphonium iodide was
prepared from nBu3P and nBuI according to the literature [11] and
stored protected from light and under argon in a freezer. All other
chemicals were purchased from commercial suppliers and were
used as received: PtCl2 (99.9%, Aldrich), PtBr2 (99.9%, Aldrich), PtI2
(Alfa Product), C2H4 (ꢁ99.5%, Air Liquid), nBu4PBr (98%, Aldrich),
nBu4PCl (96%, Aldrich), Ph4PBr (98þ%, Avocado), nBu4NBr (ꢁ98%,
Aldrich).
with the addition of 10 mL of dry toluene. The final mixture
consisted of a yelloweorange solution and a small amount of
black residue. The autoclave was washed with toluene (10 mL).
The combined liquids were evaporated to dryness and the
residue was dissolved in CH2Cl2 (3 mL) and filtered, leaving
behind a black material. The product was obtained as a yellow-
ochre precipitate after partial solvent evaporation to less than
1 mL and addition of diethyl ether (ca. 5 mL). After separation
from the mother liquor, the solid was washed twice with Et2O
(2 ꢃ 5 mL) and dried. Yield 62.8 mg (31%). 1H NMR (CD2Cl2):
2
d
4.59 (s þ d, 4H, JPteH ¼ 64 Hz, C2H4), 3.25 (s broad, 8H, CH2),
1.72 (s broad, 8H, CH2), 1.53 (s broad, 8H, CH2), 1.07 (t, 12H, 2JH
:
8 Hz, CH3). 195Pt NMR (CD2Cl2):
d
ꢀ3430 (quint, JPteH ¼ 65 Hz).
2
Anal. % Calcd. for C18H40Br3NPt (M ¼ 705.316): C, 30.65; H, 5.73;
N, 1.99. Found: 31.0; H, 5.6; N, 2.2.
2.2. Preparation of [nBu4P][PtBr3(C2H4)]
2.5. Preparation of [nBu4P][PtCl3(C2H4)]
PtBr2 (100 mg, 0.28 mmol) and one equivalent of nBu4PBr
(95.5 mg, 0.28 mmol) were introduced in a stainless steel autoclave,
which was then put under an argon atmosphere after several
purges. Toluene (10 mL) was then syringed into the autoclave,
followed by pressurization with ethylene (25 bars). After one night
(15 h) at 150 ꢂC under magnetic stirring, the autoclave was allowed
to cool down, depressurized and opened in air. The yellow solution
was transferred into a Schlenk flask and the autoclave was washed
with toluene, the washings being collected and added to the
reaction mixture. A small amount of black residue, probably
metallic Pt, remained in the vessel. The collected solution was
filtered and evaporated under reduced pressure until appearance of
a precipitate (ca. 8 mL). The product precipitation was completed
by addition of dry Et2O (5 mL). The product was recovered by
filtration in air and dried under vacuum. Yield 190 mg (94%). 1H
The same procedure described above for the preparation of
[nBu4P][PtBr3(C2H4)] was adopted, except for the use of PtCl2
(100.2 mg, 0.38 mmol) and nBu4PCl (102.4 mg, 0.38 mmol), with
the addition of 10 mL of dry toluene. Yield: 148.4 mg, 66%. 1H NMR
(CDCl3):
d
4.48 (s þ d, JPteH ¼ 64.3 Hz, 4H, C2H4), 2.30 (m, 8H, br,
PCH2), 1.60 (m, 4, br, CH2CH2), 1.05 (m, 3, br, CH3). 31P{1H} NMR:
d
33.14. 195Pt NMR:
d
ꢀ2743(quint 2JPteH ¼ 64 Hz). Anal. % Calcd. for
C18H40Cl3PPt (M ¼ 588.930): C, 36.71; H, 6.85. Found: 37.2; H, 6.9.
2.6. Attempted preparation of [nBu4P][PtI3(C2H4)]
The same procedure described above for the preparation of
[nBu4P][PtBr3(C2H4)] was adopted, except for the use of PtI2 and
nBu4PI. A pale yellow solution was obtained, together with a red
oil and abundant brown precipitate at the bottom of the autoclave.
The 1H NMR spectrum of both fractions did not reveal any reso-
nance indicative of coordinated ethylene. The 195Pt NMR spectrum
NMR (CD2Cl2):
d
4.60 (s þ d, JPteH ¼ 64.3 Hz, 4H, C2H4), 2.30 (m, 8H,
PCH2), 1.60 (m, 16H, CH2CH2), 1.05 (m, 12H, CH3). 31P{1H} NMR
(CD2Cl2):
d
33.11. 195Pt NMR (CD2Cl2): ꢀ3429 (quint, JPte
¼ 64.3 Hz). Anal. % Calcd. for C18H40Br3PPt (M ¼ 722.283): C,
H
29.93; H, 5.58. Found: 30.4; H, 5.6.
exhibits a single resonance at
coupling.
d
ꢀ3691 with no observable
2.3. Preparation of [Ph4P][PtBr3(C2H4)]
The same procedure described above for the preparation of
[nBu4P][PtBr3(C2H4)] was adopted, except for the use of PtBr2
(49.6 mg, 0.140 mmol) and Ph4PBr (59.0 mg, 0.141 mmol), with the
addition of 10 mL of dry toluene. At the end of the reaction, the
recovered mixture contained a large amount of yellowebrown
precipitate. The autoclave was washed with 5 mL of toluene and
the resulting suspension added to the reaction mixture. Then, the
autoclave was further washed with 5 mL of CH2Cl2, which
completely dissolved the residual precipitate. The combined liquids
were evaporated to dryness. The residue was redissolved in 3 mL of
CH2Cl2 and filtered, leaving behind a small amount of black solid.
The product was obtained as a yellow-ochre precipitate after partial
solvent evaporation to ca. 0.5 mL and addition of diethyl ether
(2 mL). After separation from the mother liquor, the solid was
washed twice with Et2O (2 ꢃ 5 mL) and dried. Yield 93 mg (82%). 1H
3. Results and discussion
During our recent investigation of the PtBr2/Brꢀ catalysed
addition of aniline to ethylene, we have been puzzled by the
systematic catalyst degradation with generation of Pt0 [18,19]. As
part of our mechanistic study of this decomposition process, we
have carried out tests using the same temperature (150 ꢂC) and
reaction time (10 h) as the catalytic process, but in the presence of
only one or the other of the two reactants. The optimized catalytic
conditions for the reaction involved aniline and 25 bar of ethylene
(C2H2/aniline ¼ ca. 2) in the absence of solvents in a stainless steel
autoclave, using 0.3% catalyst and 10 equivalents (3%) of nBu4PBr
relative to the aniline. The test carried out in the absence of
aniline, therefore, required the replacement of this component
with an inert solvent in order to favour the contact between the
two solids (PtBr2 and [nBu4P]Br) and the gas (C2H4). We chose
toluene in order to maintain a structural analogy with aniline. This
test resulted in the complete dissolution of PtBr2 to yield a yellow
solution and only trace amounts of a black precipitate that may
correspond to metallic Pt. While proving the principle that
ethylene alone was not the major cause of catalyst decomposition
during the hydroamination process, we also discovered, from the
NMR spectra recorded on the crude solution, that a selective
reaction resulting in the clean formation of the [PtBr3(C2H4)]ꢀ ion
had taken place according to the stoichiometry of Equation (1).
NMR (CD2Cl2): d 8.0 (m, 4H, Ph), 7.8 (m, 8H, Ph), 7.7 (m, 8H, Ph), 4.52
2
(s þ d, 4H, JPteH ¼ 65 Hz, C2H4). 195Pt NMR (CD2Cl2):
d
ꢀ3428
2
(quint, JPteH
(M ¼ 802.242): C, 38.93; H, 3.02. Found: C, 38.8; H, 2.6.
¼
65 Hz). Anal. % Calcd. for C26H24Br3PPt
2.4. Preparation of [nBu4N][PtBr3(C2H4)]
The same procedure described above for the preparation of
[nBu4P][PtBr3(C2H4)] was adopted, except for the use of PtBr2
(100.7 mg, 0.283 mmol) and nBu4NBr (90.8 mg, 0.282 mmol),

A. Béthegnies, R. Poli / Journal of Organometallic Chemistry 730 (2013) 165e167
167
PtX2 þ ½CatꢄþXꢀþC2H4/½Catꢄþ½PtX3ðC2H4Þꢄꢀ ðX ¼ Cl; BrÞ
Finally, the success of the simple ethylene and halide addition to
PtX2 in the presence of various cations made us wonder whether
this would also be a simple method for the synthesis of Chojnacki’s
salt. However, the reaction of PtBr2 and KBr using water as solvent
and carried out under the same conditions (150 ꢂC, 15 h, 25 bars of
C2H4) did not lead to any dissolution of PtBr2. Using THF, acetone or
toluene instead of water as solvent led to some dissolution as evi-
denced by the colour of the final liquid but the NMR analysis of the
crude solution gave no evidence of the formation of the desired
[PtB3(C2H4)]ꢀ anion.
(1)
Under the conditions of the above mentioned test, isolation of
pure [nBu4P][PtBr3(C2H4)] was hampered by the large excess of
nBu4PBr. However, repetition of the same reaction in the presence
of the stoichiometric amount of nBu4PBr still resulted in the
selective formation of Chojnacki’s anion and in the isolation of the
pure nBu4Pþ salt in nearly quantitative yield.
Encouraged by this result, we have carried out the same reaction
using Ph4PBr and nBu4NBr, obtaining the corresponding [Ph4P]
[PtBr3(C2H4)] and [nBu4N][PtBr3(C2H4)] salts in 93 and 31% yield,
respectively, also in spectroscopically and analytically pure form.
The lower yield obtained for the nBu4Nþ salt may be related to an
insufficient time allowed for the solubilization and reaction of the
reagents and/or to a more extensive decomposition to Pt0. The 1H
NMR analysis of the residual mother liquor confirms the efficiency
of the precipitation step. It may be possible to further optimize this
synthesis.
Since all the platinum complexes indicated in this contribution
are know as air stable, we also wondered whether the synthesis
could be carried out without special precautions to avoid contact
with air. We have tested the synthesis of nBu4Pþ salt under these
conditions but only obtained a low yield (ca. 15%) of pure product,
the main product being an uncharacterized red oily material. We
therefore recommend the use of an inert atmosphere.
4. Conclusions
A simple observation of the behaviour of PtBr2 in toluene in the
presence of [nBu4P]Br and a moderate pressure (25 bars) of C2H4
has led us to develop a simple and general method for the synthesis
of a variety of [Cat][PtX3(C2H4)] salts for X ¼ Cl and Br. Extension to
the preparation of the yet unknown triiodido analogue does not
seem possible, at least at the relatively low ethylene pressure used
in our study. It is possible that these syntheses may be further
optimized by using different solvents, the optimum solvent
possibly being cation dependent. It is also possible that the same
strategy could lead to a wide variety of similar salts with other
coordinated alkenes.
Acknowledgements
Extension of this procedure to the chloride system was tested
only for the tetra-n-butylphosphonium salt. Compound [nBu4P]
[PtCl3(C2H4)] could be obtained in pure form and in 66% yield from
PtCl2 and nBu4PCl. The same procedure was also attempted for the
iodide system, reacting PtI2 and nBu4PI. However, the solution
recovered from the autoclave after depressurization did not show
any NMR resonance attributable to coordinated C2H4. Analysis of
the literature reveals that no stable compound containing the
[PtI3(C2H4)]ꢀ anion exists. On the other hand, complex [Pt2I6]2ꢀ is
known, obtained by anion metathesis from the reaction of
[PtCl4]2ꢀ and excess KI and isolated as the [nPr4Nþ] salt [20]. It
seems that ligand addition reactions to [Pt2X6]2ꢀ to yield [PtX3L]ꢀ
(neutral L) or [PtX3Y]2ꢀ (anionic Yꢀ) adducts become less and less
favourable in the order X ¼ Cl > Br > I. The [PtI4]2ꢀ species has
been reported but is described as highly unstable [21,22], existing
in solution only in the presence of a large excess of Iꢀ salts [23],
and apparently no stable salt with this dianion has ever been
isolated and characterized. Thus, the equilibrium shown in
Equation (2) is probably highly shifted to the left hand side for the
iodide system and the final product of the reaction between PtI2
and nBu4PI in the presence of C2H4 could indeed be the
[nBu4P]2[Pt2I6] salt. Although the reaction may well represent
a convenient synthetic method for this salt, we have not pursued
the isolation and characterization of this product. It is interesting
to note that, whereas complex [PtCl3(C2H4)]ꢀ is stable towards
loss of ethylene, complex [PtBr3(C2H4)]ꢀ tends to lose ethylene,
because its synthesis by anion exchange from [PtCl3(C2H4)]ꢀ was
shown to require a protecting C2H4 atmosphere to avoid the
formation of [Pt2Br6]2ꢀ [24].
We are grateful to the CNRS (Centre National de la Recherche
Scientifique), the ANR (Agence Nationale de la Recherche, Pro-
gramme Blanc HYDROAM, Grant No. NT09_442499) and the IUF
(Institut Universitaire de France) for financial support.
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½Pt2X6ꢄ2ꢀþ2 C2H4%2 ½PtX3ðC2H4Þꢄꢀ
(2)