A convenient route to dinuclear chloro-bridged platinum(II) derivatives via nitrile complexes

Article abstract of DOI:10.1039/c1dt11709b

The syntheses of the complexes [PtCl₂(NCR)L] [R = Me, Et; L = PPh₃; R = Et, L = Py, CO] and [PtCl{(κ²-P,C) P(OC₆H₄)(OPh)₂}(NCEt)] are described starting from the easily available [PtCl₂(NCR)₂]. The stability of the products under different experimental conditions is discussed as well as their use as precursors to dinuclear complexes [Pt(μ-Cl)ClL]₂. The crystal and molecular structures of cis-[PtCl₂(NCEt)(PPh ₃)], [SP-4-2]-[PtCl{(κ²-P,C)P(OC₆H ₄)(OPh)₂}(NCEt)] and trans-[Pt(μ-Cl) {(κ²-P,C)P(OC₆H₄)(OPh)₂}] ₂ are reported.

Full text of DOI:10.1039/c1dt11709b

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PAPER
A convenient route to dinuclear chloro-bridged platinum(II) derivatives via
nitrile complexes†
Daniela Belli Dell’Amico,* Luca Labella, Fabio Marchetti and Simona Samaritani
Received 8th September 2011, Accepted 14th October 2011
DOI: 10.1039/c1dt11709b
The syntheses of the complexes [PtCl2(NCR)L] [R = Me, Et; L = PPh3; R = Et, L = Py, CO] and
2
[PtCl{(k -P,C)P(OC6H4)(OPh)2}(NCEt)] are described starting from the easily available
[PtCl2(NCR)2]. The stability of the products under different experimental conditions is discussed as well
as their use as precursors to dinuclear complexes [Pt(m-Cl)ClL]2. The crystal and molecular structures
2
of cis-[PtCl2(NCEt)(PPh3)], [SP-4-2]-[PtCl{(k -P,C)P(OC6H4)(OPh)2}(NCEt)] and
2
trans-[Pt(m-Cl){(k -P,C)P(OC6H4)(OPh)2}]2 are reported.
K
Introduction
4
⎯⎯→
cis,trans-[PtCl (CO)(alkene)] + CO
←⎯⎯
2
(4)
cis-[PtCl2(CO)2 ] + alkene
Organonitrile ligands are moderate s-donors and weak p-
acceptors towards metal centers. Consequently, their easy re-
placement by other ligands makes organonitrile complexes useful
intermediates in coordination chemistry.1
Platinum(II) organonitrile complexes appear to be suitable
precursors not only of a variety of derivatives through substitution
reactions, but also of dinuclear complexes, by releasing of the
coordinated nitrile. For instance, the equilibrium constant◦ of
reaction 5 has been determined8 [K5 = 3601 215 mol-1dm3, 25 C,
solvent = CH2Cl2] and its value suggests that the reverse reaction
can be easily forced.
As for the platinum(II) species [PtCl2(NCR)2], some substitution
reactions lead to displacement of both nitrile ligands, affording
2,3
4
mixtures of cis- and trans-[PtCl2L2] (L = PR3 or AsR3 ). In
other cases equilibrium reactions are encountered: for instance,
the study of the system [PtCl2(NCEt)2]/CO in CDCl3 has revealed
that reaction 1 is completely displaced to the right,◦while reaction
2 shows an equilibrium constant K2 = 48 6 (23.4 C).5
K
5
trans-[Pt(m-Cl)Cl(h2-C H )] + 2 MeCN
⎯⎯→
←⎯⎯
2
4
2
(5)
2 trans-[PtCl2(h2-C2H4 )(NCMe)]
[PtCl2(NCEt)2] + CO → [PtCl2(CO)(NCEt)] + EtCN
(1)
It has been also observed9,10 that cis-[PtX2(R2SO)(NCMe)]10
(X = Cl, Br; R = Me, Et, Ph) and cis-[PtCl2(Py)(NCMe)]9 lose
the coordinated nitrile upon heating in the solid phase (120–
145 ◦C), affording the corresponding dinuclear derivatives [Pt(m-
X)XL]2. Moreover, cis-[PtCl2(NCMe)2] reacts with Et2SO in hot
nitromethane producing [Pt(m-Cl)Cl(Et2SO)]2:10 this last outcome
suggests the intermediate formation of [PtCl2(Et2SO)(NCMe)],
followed by nitrile elimination.
K
2
⎯⎯→
(2)
[PtCl (CO)(NCEt)] + CO
cis-[PtCl2(CO)2 ] + EtCN
←⎯⎯
2
Moreover, for the competition between nitrile and alkenes
approximate values of the equilibrium constant for reaction 3
(RCN = EtCN; alkene = cyclohexene K3 ª 1 ¥ 10-1, alkene =
1-octene K3 ª 1) can be indirectly obtained by the combination
of the equilibrium constants of eqn (2) and (4) [K4 = (4.1 0.5)
¥ 102 and 36 5 for cyclohexene and 1-octene, respectively, in
1,2-dichloroethane (1,2-DCE), 21 ◦C].6,7
It is then reasonable to suppose that similar reactions (see eqn
(6)) can be exploited for the preparation of a series of dinuclear
derivatives.
K
3
2 [PtCl2L(NCR)] ꢀ [Pt(m-Cl)ClL]2 + 2 RCN
(6)
⎯⎯→
cis,trans-[PtCl (CO)(NCEt)] + alkene
←⎯⎯
2
(3)
cis-[PtCl2(CO)(alkene)] + EtCN
Halo-bridged derivatives [Pt(m-X)X(L)]2 are key intermediates
in platinum coordination chemistry,11 allowing the easy prepara-
tion of complexes containing two different types of neutral ligands
of general formula [PtX2(L)(L¢)]. Among the many examples
described, the following can be mentioned: L = PR3 and L¢ = CO,12
C2H4,12c R2S,13 R2SO,14 amines,12b,15,16 PhCN;17 L = amine and L¢ =
alkenes, nitrogen and sulphur nucleophiles, nitrogen unsaturated
nucleophiles;18 L = P(OR)3 and L¢ = amine,16 CO, P(OMe)3.19
The preparation of dinuclear complexes of phosphines and
Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35, 56126,
Pisa, Italy. E-mail: belli@dcci.unipi.it; Fax: +39 050 2219246; Tel: +39 050
2219210
† CCDC reference numbers 833973–833975 for cis-[PtCl2(NCEt)(PPh3)],
2
[SP-4-2]-[PtCl{(k -P,C)P(OC6H4)(OPh)2}(NCEt)]
and
trans-[Pt(m-
2
Cl){(k -P,C)P(OC6H4)(OPh)2}]2, respectively. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1dt11709b
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phosphites is usually carried out by reacting at high temperature
the suitable [PtCl2(L)2] systems with equimolar amounts of PtCl2
in high boiling solvents,12,20,21 although for some PR3 derivatives
syntheses starting from the Zeise’s salt13 or the Zeise’s dimer22
are also described. Some preparations of dinuclear complexes of
amines, [Pt(m-Cl)Cl(amine)]2, are carried out by photochemical
1JP-Pt = 3560 Hz). Their relative intensities slowly varied with
the progress of the reaction in favour of the signal at 6.0 ppm.
Since conversion was scarce after a relatively long reaction time
(28 h), it was supposed that more drastic conditions were
necessary: the reaction mixture was thus heated at 145◦ C in a
sealed tube (Scheme 2).
2
decomposition of [PtCl2(h -C2H4)(amine)]23 or by addition of
HClO4 to K[PtCl3(amine)].24
Although the lability of nitrile complexes is exploited in several
syntheses, a systematic study of RCN substitution on plat-
inum(II) is not present. Since dichlorobisorganonitrile complexes
[PtCl2(NCR)2] are easily prepared,25 we have considered that they
could be good precursors of [PtCl2(NCR)L] derivatives by reaction
with various type of ligands L in suitable molar ratios. Moreover,
the above described lability of coordinated nitrile suggested that
[PtCl2(NCR)L] complexes could generate families of mononuclear
compounds of the type [PtCl2LL¢] or dinuclear derivatives [Pt(m-
Cl)ClL]2 (Scheme 1).
Scheme 2
After 12 h a yellow, homogeneous solution was obtained
and its 31P NMR spectrum showed the absence of the cis-
[PtCl2(PPh3)2] signal. The solution, upon cooling, separated out
pale yellow crystals of cis-[PtCl2(PPh3)(NCEt)] (cis-1a) in 68%
1
yield. The presence of coordinated nitrile was confirmed by H
and 13C NMR analyses on CDCl3 solutions of the recovered
solid and by IR spectroscopy. Single crystal X-ray diffraction
analysis confirmed the cis stereochemistry of the complex. The
31P NMR spectrum of the liquid phase of the reaction mixture
showed, besides the signal of cis-(1a) (6.0 ppm, 1JP-Pt = 3560 Hz),
1
a less intense signal (3.4 ppm, JP-Pt = 4100 Hz) ascribed to
trans-(1a).27 The prevalence of the cis-isomer in propionitrile
and its low solubility allows its clean recovery by crystallization.
In platinum(II) complexes, trans–cis isomerization in solution is
commonly observed and examples where the cis isomer appears
to be more stable, independently of the solvent, are for example
[PtCl2(PPh3)(CO)]28 and [PtCl2(CO)2].29
Scheme 1
In order to define the right conditions for the synthesis
of [PtCl2(NCR)L], we have reacted [PtCl2(NCR)2] with PPh3,
P(OPh)3, py, CO and checked the effects of solvent, temperature
and reagent molar ratio on the outcome of the reactions. The
results obtained for the different incoming ligands is described
here, together with the dimerization of the obtained complexes to
chloro-bridged derivatives via nitrile release.
The molecular structure of cis-(1a) is reported (Fig. 1), together
with a selection of bond lengths and angles (Table 1).
Crystallographic data are in good agreement with the structure
of the analogous cis-[PtCl2(PPh3)(NCMe)]26 (cis-1b) and show
terminal coordination of propionitrile, with a linear geometry
involving platinum, coordinated nitrogen, nitrile carbon and
methylene carbon, [Pt–N–C(19)–C(20)]. Of the two Pt–Cl bond
distances, Pt–Cl(2), trans to PPh3, is significantly longer than Pt–
Cl(1), trans to NCEt.
The synthesis of cis-(1b) was carried out by the same procedure,
starting from the mixture of the two geometrical isomers of
[PtCl2(NCMe)2].25 The reaction was slightly slower and the
complete conversion was reached after 24 h at 150 ◦C. The product
crystallized out of the reaction mixture and was obtained in 77%
yield as a single isomer. Although cis-(1b) has been described,26
its spectroscopic data are not reported. We assigned the cis
configuration to our product on the basis of its 31P NMR spectrum,
which showed strong analogies with the 31P NMR spectrum of the
propionitrile derivative cis-(1a) (5.4 ppm, 1JP-Pt = 3530 Hz). Also in
this case the 31P NMR spectrum of the liquid phase of the reaction
Results and discussion
The PPh3/[PtCl2(NCR)2] system
The formation of [PtCl2(PR3)(NCR)] complexes has been ob-
served, beyond the already mentioned17 bridge-splitting reaction
between [Pt(m-Cl)Cl(PR3)]2 and benzonitrile, as a result of the
accidental substitution of ethylene by acetonitrile during the
2
attempted purification of [PtCl2(PPh3)(h -C2H4)] (eqn (7)),26 but
the procedure is not described in detail.
2
[PtCl2(PPh3)(h -C2H4)] + MeCN (excess) →
(7)
[PtCl2(PPh3)(NCMe)] + C2H4
In order to find more convenient conditions for the preparation
of [PtCl2(PPh3)(NCR)] (1) derivatives, we have treated a CH2Cl2
solution of the mixture of the two geometrical isomers of
[PtCl2(NCEt)2] (cis/trans molar ratio ª 6)25 with a solution of PPh3
in the same solvent at room temperature (Pt/PPh3 molar ratio =
1). A pale yellow solid immediately formed, identified as trans-
[PtCl2(PPh3)2],2b while the analysis of the liquid phase showed the
presence of unreacted [PtCl2(NCEt)2].25 In a further experiment,
propionitrile was used as solvent and the reaction mixture was
refluxed (97 ◦C). Under these conditions a yellow suspension was
obtained, the liquid phase of which was analysed by 31P NMR
spectroscopy. Two signals were observed due to cis-[PtCl2(PPh3)2]
(16.9 ppm, 1JP-Pt = 3660 Hz)2b and to a new compound (6.0 ppm,
◦
˚
Table 1 Bond lengths [A] and angles [ ] around Pt atoms in cis-(1a)
Pt–N
Pt–P
1.976(6)
2.2355(19)
2.2832(19)
2.342(2)
92.9(2)
N–C(19)
C(19)–C(20)
C(20)–C(21)
1.120(10)
1.464(11)
1.451(13)
Pt–Cl(1)
Pt–Cl(2)
N–Pt–P
N–Pt–Cl(1)
P–Pt–Cl(1)
N–Pt–Cl(2)
P–Pt–Cl(2)
Cl(1)–Pt–Cl(2)
C(19)–N–Pt
178.70(8)
90.10(8)
174.3(6)
177.6(9)
176.7(2)
89.24(7)
87.8(2)
N–C(19)–C(20)
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The P(OPh)3/[PtCl2(NCEt)2] system
[PtCl2(NCEt)2] was reacted with P(OPh)3 under experimental
conditions analogous to those used for the preparation of cis-
(1a) (EtCN as solvent, sealed tube, T = 125 ◦C, Pt/P(OPh)3 molar
ratio = 1). As analogously observed in the case of triphenylphos-
phine, addition of triphenylphosphite caused the immediate con-
version of half the precursor into cis-[PtCl2(P(OPh)3)2],30 identified
via 31P NMR spectroscopy on a sample of the reaction mixture.
The reaction of this intermediate with the residual unreacted bis-
nitrile precursor proceeded quite slowly: the maximum conversion
(85%, checked by 31P NMR) was reached after 48 h at 150 ◦C.
The 31P NMR spectrum of the reaction mixture showed two
signals (relative intensity 85/15) due to the products, the main
one at 99.6 ppm (1JP-Pt = 6728 Hz) and the other at 92.7
ppm (1JP-Pt = 7550 Hz). White crystals separated out of the
mixture upon cooling and X-ray diffraction analysis showed
2
we had obtained the ortho-metalated product [SP-4-2]-[PtCl{(k -
P,C)P(OC6H4)(OPh)2}(NCEt)] ([SP-4-2]-(3)), 40% yield). Activa-
tion of C–H bonds leading to ortho-metalation products of aryl-
phosphite halo-complexes of palladium and platinum has been
already described19,31 under rather drastic experimental conditions
(boiling decalin). Yields of ortho-metalation products starting
from halo-complexes are generally enhanced by working in the
presence of flushing dinitrogen to help hydrogen halide removal.19
In our case, the choice of a sealed tube as reactor prevents HCl
from being removed and explains why the conversion is not com-
plete. The two 31P NMR signals observed in the reaction mixture
could be reasonably ascribed to the [SP-4-2] and [SP-4-4] isomers
of the ortho-metalation product 3, in equilibrium in propionitrile
solution (Scheme 4). It is reasonable that [PtCl2(P(OPh)3)(NCEt] is
an intermediate of the reaction and that, once formed, it is rapidly
converted to the products; nevertheless, at the temperatures we
have used, there was no evidence of its presence in the reaction
mixture.
Fig. 1 View of the molecular structure of cis-[PtCl2(PPh3)(NCEt)]
(cis-(1a)). Thermal ellipsoids are at 30% probability.
mixture showed, besides the already mentioned signal of the main
product, another signal (3.2 ppm, 1JP-Pt = 4110 Hz), ascribable to
minor amounts of trans-(1b).
Solutions of pure cis-(1a) in EtCN and pure cis-(1b) in MeCN
were prepared and analysed by 31P NMR spectroscopy at room
temperature. In both cases signals of the trans isomer were
observed after about 48 h and equilibrium between the two
isomers was reached after about 7 days. In both cases the cis
isomer appeared to be more stable than the trans one, being the
main component of the equilibrium mixtures (about 90% for both
derivatives, on the basis of the relative intensities of the 31P NMR
signals).
When cis-(1a) was dissolved in CDCl3 and analysed by 31P NMR
at room temperature, the appearance of the resonance of trans
isomer was followed by the precipitation of an orange solid, which
was identified as trans-[Pt(m-Cl)Cl(PPh3)]2 (trans-2).21,22,26 In the 1H
NMR spectrum, besides signals due to coordinated EtCN of the
two geometrical isomers of 1a, signals of free EtCN were observed.
These data show that, in the absence of excess EtCN, coordinated
nitrile is promptly released from trans-(1a) with formation of the
corresponding dinuclear derivative 2 (Scheme 3).
Scheme 4
The molecular structure of [SP-4-2]-(3) is reported in Fig. 2,
together with a selection of bond lengths and angles (Table 2).
As can be seen in Fig. 2, the metal, P, O(1) and Cl atoms, phenyl
ring C(1P) > C(6P) and the propyl group C(1) > C(3) make an
Scheme 3
˚
The synthesis of trans-(2) was optimized in toluene as solvent at
the reflux temperature. Isomerization from cis- to trans-(1a) was
much faster and, due to the low solubility of the dinuclear complex
in toluene, equilibrium reactions (Scheme 3) were completely
displaced to the right. Under these conditions pure trans-(2) was
prepared in 80% yield. Analogous results were obtained when cis-
(1b) was used as precursor. By 31P NMR spectroscopy it is observed
that the partial isomerisation of cis-1b to trans-1b in CDCl3 is
followed by precipitation of trans-2. The difference in the nitrile
alkyl group (Me or Et) does not differentiate the reactivity of the
complexes.
almost ideal plane, the maximum deviation being 0.06 A for C(6)
˚
atom. The Pt–C(2P) bond distance, 2.010 A, is within the range
reported for similar compounds,32 whereas the Pt–P distance,
˚
˚
2.156 A, is shorter than the range 2.26–2.19 A normally found
in the mononuclear ortho-metalated Pt–phosphito derivatives.32
The Pt–Cl distance is slightly longer than that we have found
for chlorine trans to phosphine in cis-(1a), but is however in
the range normally found in similar compounds.33 The nitrile
coordination is very similar to that found in cis-(1a). The Pt–N
˚
bond distance in [SP-4-2]-(3) is remarkably long (2.08 A), being
well above the average value (1.98 A) of the Pt–N bond lengths in
˚
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◦
˚
Table 2 Bond lengths [A] and angles [ ] around Pt atoms in [SP-4-2]-(3)
2 [PtCl{(k 2-P,C)P(OC6H4 )(OPh)2}(NCEt)] ꢀ
(3)
(8)
Pt–C(2P)
Pt–N
Pt–P
Pt–Cl
N–C(1)
C(1)–C(2)
O(3)–C(13P)
C(2P)–Pt–N
C(2P)–Pt–P
N–Pt–P
C(2P)–Pt–Cl
N–Pt–Cl
P–Pt–Cl
2.010(7)
2.077(8)
2.1557(17)
2.3506(18)
1.126(11)
1.468(13)
1.408(9)
178.8(3)
82.18(18)
96.6(2)
P–O(1)
P–O(3)
P–O(2)
O(1)–C(1P)
C(2P)–C(3P)
O(2)–C(7P)
1.577(6)
1.587(5)
1.593(5)
1.422(7)
1.410(9)
1.398(8)
[Pt(m-Cl){(k 2-P,C)P(OC6H4 )(OPh)2}]2 + 2 EtCN
4
The existence of equilibrium (8) in CDCl3 solution was con-
firmed by 31P NMR spectroscopy, where signals ascribable to
2
19
cis- and trans-[Pt(m-Cl){(k -P,C)P(OC6H4)(OC6H5)2}]2 (4) were
C(1)–N–Pt
N–C(1)–C(2)
O(1)–P–Pt
O(3)–P–Pt
O(2)–P–Pt
175.2(8)
178 (1)
108.1(2)
121.0(2)
121.8(2)
114.5(4)
observed (85.1 ppm [1JP-Pt = 7864 Hz] and 87.7 ppm [1JP-Pt
=
7770 Hz], respectively), while the signals of the two geometrical
isomers of the mononuclear precursor showed a relatively weak
intensity. The same behaviour was observed in toluene. In this
case, slow evaporation of the solvent afforded the pure dinuclear
derivative in 70% yield. The overall preparation procedure of
4 could be optimized by carrying out the reaction between
[PtCl2(NCEt)2] and P(OPh)3 at 160 ◦C in benzonitrile (b.p. 191 ◦C).
In this case the use of a sealed vessel could be avoided, thus making
the elimination of HCl(g) easier. After 48 h, a 31P NMR spectrum
on a sample of the reaction mixture revealed the complete
disappearance of the intermediate [PtCl2{P(OPh)3}2] and the
formation of two main products, with resonances at 98.8 ppm
(1JP-Pt = 6709 Hz) and 96.4 ppm (1JP-Pt = 7223 Hz), with 3/1 molar
ratio, according to the relative intensities of the observed signals.
94.32(19)
86.9(2)
176.47(9)
C(1P)–O(1)–P
2
We suggest the formation of [SP-4-2]- and [SP-4-4]-[PtCl{(k -
P,C)P(OC6H4)(OPh)2}(NCPh)] in view of the strong analogies
with the previously described spectrum of the propionitrile
derivatives 3, which in this case were observed in traces (signals at
98.7 and 91.9 ppm). The dinuclear derivative 4 was finally obtained
in 65% overall yield upon slow evaporation of excess benzonitrile.
A sample of the mixture of the two geometrical isomers of 4 was
recrystallized from 1,2-dichloroethane/pentane and the molecular
structure of the trans isomer could be determined by single crystal
X-ray diffraction methods (Fig. 3).
Fig. 2 View of the molecular structure of [SP-4-2]-(3). Thermal ellipsoids
are at 30% probability.
square planar platinum(II) nitrile complexes.34 In effect there is
a bimodal distribution of the Pt–NCR bond distances: a rather
large group of compounds exhibits distances centered around
˚
˚
1.98 A, while a restricted group shows distances of about 2.07 A.
The complexes of the former group contain several types of
˚
ligands trans to the nitrile: examples are cis-(1a) (1.98 A, see Table
35
˚
1); cis-[PtCl2(NCEt)2] (1.96 and 1.99 A); cis-[PtCl2(CO)(NCEt)]
(1.99 A); trans-[PtCl2Py(NCMe)] (1.98 A) and, on the
5
36
˚
˚
2
8
˚
borderline, trans-[PtCl2(h -C2H4)(NCMe)] (2.02 A). The latter
group is formed by a few complexes that have strong s-donor
ligands (alkyl, aryl, phosphide), trans to the nitrile. Examples of
this type are: [1,4-{Pt(MeCN)}2{C6(CH2NMe2)4-2,3,5,6}](BPh4)2
Fig. 3 View of the molecular structure of trans- 4. Thermal ellipsoids are
at 30% probability. ¢ = 1 - x, 1 - y, 1 - z. For sake of clarity only one of the
two conformations of the disordered rings has been represented.
37
38
˚
˚
˚
(2.09 A); [(C6F5)2Pt(m-PPh2)2Pt(NCMe)2] (2.07 and 2.08 A);
39
[PtMe-{(R)-C10H7CHMeNCHC4H2S}(NCMe)]
(2.05
A);
[Pt{C(E) C(E)–C(E) C(E)}(N,N¢-dimethylbenzimidazol-2-
40
˚
ylidene)(NCMe)] (E = CO2Me) (2.06 A).
Table 3 reports some geometrical details of the coordination
around the metal.
While the presence of coordinated nitrile in [SP-4-2]-(3) was
˜
observed by IR spectroscopy (ATR on solid sample: nCN at
As shown in Fig. 3, the molecule is centrosymmetric and,
as the moiety Pt(1), Cl(1), P(1), O(1) and the ring C(1P)
> C(6P) are almost perfectly planar, the symmetry related
atoms lie on the same plane. Such a feature is not present
1
2295 cm-1), it was not possible to obtain the direct H NMR
characterization of the derivative. As a matter of fact, pure [SP-
4-2]-(3), when dissolved in CDCl3, promptly released coordinated
nitrile (eqn (8)), and hydrogen resonances due to the free ligand
were observed.
2
in the analogous derivative [Pt(m-Cl){(k -P,C)P(OC6H2-2,4-t-
Bu2)}((R)-binolate)]2,41 where the Pt coordination planes are tilted
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◦
˚
Table 3 Bond lengths [A] and angles [ ] around Pt atoms in trans-4
[PtCl2(NCEt)(CO)] (cis-6) was obtained in 70% yield (see Experi-
mental section).
Pt(1)–C(2P)
Pt(1)–P(1)
Pt(1)–Cl(1¢)
Pt(1)–Cl(1)
2.018(8)
2.1449(18)
2.4054(18)
2.439(2)
81.8(2)
97.0(2)
177.42(7)
179.2(2)
P(1)–O(2)
P(1)–O(1)
P(1)–O(3)
1.576(5)
1.583(6)
1.589(5)
With the aim of obtaining [Pt(m-Cl)Cl(CO)]2 (7), a sample of
cis-6 was dissolved in dry mesitylene and the solution was heated
(75 ◦C) and slowly evaporated under vacuum. The outcome of
the reaction was followed by 195Pt NMR. The analysis of the
residue, dissolved in CDCl3, showed a signal due to trans-(6),5
and two more signals due to trans-(7).29 The presence of two 195Pt
resonances for this dinuclear complex has been already observed
and discussed.29a The heating/evaporation cycle was repeated and
afforded an orange residue, whose 195Pt NMR analysis in CDCl3
showed the complete conversion of the precursor into 7. The
formation of the dinuclear carbonyl derivative was confirmed by
IR spectroscopy.29 The observed data suggest that isomerization
from cis- to trans-(6) takes place easily in mesitylene solution upon
heating. This observation is in good agreement with the known
behaviour of cis-(6), which is completely isomerized to trans isomer
in chlorinated solvents at room temperature in 24 h.5 On the basis
of our results, it seems reasonable that trans-(6) is the isomer which
releases coordinated nitrile in an equilibrium reaction leading to
the formation of the chloro-bridged dimer (eqn (9)).
C(2P)–Pt(1)–P(1)
C(2P)–Pt(1)–Cl(1¢)
P(1)–Pt(1)–Cl(1¢)
C(2P)–Pt(1)–Cl(1)
P(1)–Pt(1)–Cl(1)
Cl(1¢)–Pt(1)–Cl(1)
Pt(1¢)–Cl(1)–Pt(1)
O(2)–P(1)–O(3)
97.69(7)
83.53(6)
96.47(6)
94.4(3)
around the Cl(1) ◊ ◊ ◊ Cl(1¢) axis making a Pt(1)–Cl(1) ◊ ◊ ◊ Cl(1¢)–
Pt(1¢) torsion angle of 163◦. The analogies concern, instead, the
rather short Pt–P bond: 2.12 vs. 2.14 A, and the different Pt–Cl
distances relative to the Cl(1) and Cl(1¢) bridging chlorine atoms.
In both cases the Pt–Cl bond trans to the phenyl group are longer
than those trans to phosphorus. The M–Cl bonds show instead the
same length in the analogous cis derivative known for palladium,42
disregarding the position of bridging chlorine trans with respect
to C or P.
˚
The Py/[PtCl2(NCEt)2] system
2 trans-[PtCl2(NCEt)(CO)] ꢀ [Pt(m-Cl)Cl(CO)]2 + 2EtCN
(9)
trans-6
7
The reaction between pyridine and a mixture of cis- and trans-
[PtCl2(NCEt)2] (Pt/Py = 1/1 molar ratio) was initially carried out
in EtCN at 130 ◦C in a sealed tube. A unique product was obtained
in 63% yield, identified as PtCl2(NCEt)(Py) (5). The presence of
At room temperature, equilibrium (9) is completely displaced to
the left and the removal of nitrile under vacuum at relatively high
temperature is necessary to obtain the dinuclear derivative.
1
coordinated propionitrile was observed by both H NMR and
The trans-[PtCl2(g2-C2H4)(NCEt)]/[Pt(l-Cl)Cl(g2-C2H4)]2
IR spectroscopy. Since in this case no traces of the disubstituted
product [PtCl2Py2] were observed, in a further experiment the
reaction was carried out at room temperature. The outcome of
the reaction was essentially the same. A single isomer was always
observed in solution (1H and 195Pt NMR) and we propose that we
are dealing with the trans-isomer. Isomerization from cis to trans
isomers is described for analogous [PtCl2(NCMe)(Am)] complexes
(Am = amine) in organic solvents,36,43 where free MeCN catalyzes
the process. The complex is stable in solution at room temperature
in chlorinated or aromatic solvents, nevertheless coordinated
nitrile was released when a toluene solution of trans-(5) was heated
(75 ◦C) and slowly evaporated under vacuum. The conversion
to a brown, scarcely soluble material was complete only after
several evaporation cycles. Unfortunately, the low solubility of
the product prevented its 195Pt NMR characterization, but the
absence of coordinated nitrile could be verified by 1H NMR.
The IR spectrum, which shows the presence of bands due to
the coordinated pyridine and the absence of bands attributable
to EtCN, is in agreement with that reported in the literature
for Pt2Cl4py2.44 Moreover, addition of propionitrile to a CDCl3
suspension of the product caused its slow conversion to the original
trans-(5), thus supporting the hypothesis of the chloro-bridged
nature of the product.
system
As already mentioned, for L = C2H4 the reverse of reaction 6 has
been reported (see eqn (5)).8 We have observed that the treatment
◦
2
of trans-[PtCl2(h -C2H4)(NCEt)] (trans-8) under vacuum at 25 C
afforded the corresponding dinuclear derivative [Pt(m-Cl)Cl(h -
C2H4)]2 (9) in essentially quantitative yield.
2
Conclusions
Starting from the easily available [PtCl2(NCEt)2], the syntheses of
[PtCl2(NCEt)(L)] (L = PPh3 (1a), Py (5)), [PtCl2(NCMe)(PPh3)]
(1b) and [SP-4-2]-[PtCl{(k -P,C)P(OC6H4)(OPh)2}(NCEt)] ([SP-
2
4-2]-3) have been carried out in moderate to good yields and the
preparation of [PtCl2(NCEt)(CO)]5 (6) has been optimized. While
for L = Py the stoichiometric substitution reaction (L/Pt molar
ratio = 1, eqn (10)) can be carried out at room temperature in
propionitrile and affords the trans-isomer of the expected 5, in
the case of PPh3 and P(OPh)3 the analogous reactions carried
out at room temperature afford mixtures containing the starting
platinum complex and the double substitution product, [PtCl2L2],
probably because the strong trans-effect of these ligands makes
the second substitution reaction (eqn (11)) faster than the former
(eqn (10)).
The CO/[PtCl2(NCEt)2] system
[PtCl2(NCR)2] + L → [PtCl2L(NCR)] + RCN
(10)
(11)
As already mentioned, the reaction between [PtCl2(NCEt)2] and
CO affording a mixture of monosubstitution and disubstitution
products had been already studied.5 The preparation of the
monosubstitution product was optimized in the course of this
work by a modification of the reported5 procedure and cis-
[PtCl2L(NCR)] + L → [PtCl2L2] + RCN
Suitable conditions for the conversion of the obtained mixtures
into the desired products have been found. In the case of L =
P(OPh)3 the procedure afforded the ortho-metalated product 3.
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Although 1a, 1b and 3 are stable in RCN solution, in other
solvents the formation of dinuclear species upon release of
coordinated nitrile (Scheme 3 and eqn (8)) is observed. In CDCl3
3 is rapidly and extensively converted into the soluble dinuclear
for C21H20Cl2NPPt: C 43.2; H 3.5; N 2.4%. Found: C 43.1; H 3.6;
N 3.3%.47 A single crystal was selected for X-ray analysis, which
confirmed the cis stereochemistry. 1H NMR: d 7.78–7.45 (m, 15H,
2
aromatic hydrogens); 2.14 (q, 2H, JH-H = 7.5 Hz, CH2); 0.87 (t,
2
2
[Pt(m-Cl){(k -P,C)P(OC6H4)(OPh)2}]2 (4) at room temperature.
3H, JH-H = 7.5 Hz, CH3). 13C NMR: d 134.6 (d, JC-P = 10 Hz),
In the case of 1a the dinuclear derivative 2 produced according to
Scheme 3 is scarcely soluble either in toluene or CDCl3 and slowly
separates out of the solution, thus displacing equilibrium to the
right. On the other hand, trans-(6) and trans-(8), in spite of the
strong trans effect of CO and C2H4, need relatively more severe
conditions to release the coordinated nitrile.
131.5, 128.4 (d, JC-P = 11 Hz), 126.3 (d, JC-P = 67 Hz), 118.0, 12.3,
8.8. 31P NMR: d 5.8 (1JP-Pt = 3560 Hz) 195Pt NMR: d -3595 (d,
1
JPt-P = 3560 Hz). FTIR (ATR): (nCN) 2312 cm-1.
˜
Synthesis of cis-[PtCl2(NCMe)(PPh3)] (1b)
Release of nitrile appears particularly easy for 3 and we suppose
that the process involves the [SP-4-2]-isomer, where the ligand
is trans to the phenyl group. The presence of strong s-donors
in platinum(II) square planar complexes has been associated to
dissociative paths according to kinetic data regarding substitution
reactions.45 We can reasonably suppose that a dissociative route
is followed by this complex to form the dinuclear species, this
hypothesis being indirectly corroborated by the rather long Pt–
NCR bond distance observed in its structure.
A mixture of [PtCl2(NCMe)2] (1.41 g, 4.06 mmol), PPh3 (1.06
g, 4.05 mmol) and acetonitrile (8.0 mL) was heated (150 ◦C) in
a sealed tube (total volume ª 100 mL), until a yellow solution
was obtained (18 h). Heating was stopped and the solution was
slowly cooled to room temperature (25 ◦C). Light yellow crystals
were filtered and dried under vacuum (1.82 g, 77% yield). Anal.
Calcd. for C20H18Cl2NPPt: C, 42.2; H, 3.2; N, 2.5%. Found: C,
42.5; H, 3.3; N, 3.3%.47 1H NMR: d 7.84–7.46 (m, 15H, aromatic
hydrogens); 1.80 (s, 3H, CH3). 13C NMR: d 134.7 (d, JC-P = 11 Hz),
131.6, 128.6 (d, JC-P = 12 Hz), 127.6 (d, JC-P = 65 Hz), 114.4, 3.5.
31P NMR: d 4.8 (1JP-Pt = 3530 Hz). 195Pt NMR: d -3606 (d, 1JPt-P
=
Experimental section
3530 Hz). FTIR (ATR): (nCN) 2327 cm-1.
˜
General procedures
Synthesis of cis-[PtCl2(CO)(NCEt)]5 (6)
All manipulations were performed under a dinitrogen atmosphere,
unless otherwise stated. Solvents and liquid reagents were dried
according to reported procedures.46 Triphenylphosphine (Aldrich,
Steinheim-Westfalia, DE) was recrystallized from ethanol before
use. [PtCl2(NCEt)2] and [PtCl2(NCMe)2] were prepared (both
as the mixture of the two geometrical isomers) according to
A solution of 0.474 g of [PtCl2(NCEt)2] (1.26 mmol) in 9.0 mL
of 1,2-DCE was treated with 1.0 mL of propionitrile (14.0 mmol,
EtCN/Pt molar ratio = 11) in a 80 mL Carius tube under CO. The
mixture was heated (65 ◦C) under stirring until the 195Pt NMR
signals of precursors (-2253 and -2342 ppm of cis-and trans-
[PtCl2(NCEt)2], respectively) disappeared and signals at -3280,
-3430 and -3910 ppm (cis-(6), trans-(6) and cis-[PtCl2(CO)2],
respectively) were observed (48 h). The mixture was then cooled
to room temperature, CO was replaced with Ar and solvents were
eliminated under vacuum. Residual solid was dissolved in 1,2-
DCE (8.0 mL), 1.0 mL of propionitrile were added and solvents
were eliminated under vacuum. Light-orange crystals of cis-(6)
2
reported procedures.25 trans-[PtCl2(h -C2H4)(NCEt)] (trans-8) was
2
prepared from [Pt(m-Cl)Cl(h -C2H4)]2 (9) according to the proce-
2
dure described for trans-[PtCl2(h -C2H4)(NCMe)].8
1H, 13C, 31P and 195Pt NMR spectra were recorded with a
Bruker Avance DRX 400 spectrometer (1H 401.36 MHz, 13C
100.93 MHz, 31P 162.49 MHz and 195Pt 86.02 MHz operating
frequencies). Solvent was CDCl3 if not otherwise stated. Chemical
shifts were measured in ppm (d) from TMS by residual solvent
peaks for 1H and 13C, from aqueous (D2O) H3PO4 (85%) for
31P and from aqueous (D2O) hexachloroplatinic acid for 195Pt. A
sealed capillary containing C6D6 was introduced into the NMR
tube to lock the spectrometer to the deuterium signal when non-
deuterated solvents were used. FTIR spectra in the solid phase
were recorded with a Perkin–Elmer Spectrum One spectrometer,
with ATR technique. FTIR spectra in solution were recorded with
a Perkin–Elmer Paragon 500 or a Perkin–Elmer Spectrum 100
spectrometer. A 0.1 mm cell supplied with CaF2 windows was used.
Elemental analyses (C, H, N) were performed by Dipartimento di
Scienze e Tecnologie Chimiche, Universita` di Udine.
were obtained (0.346 g, 79% yield). 1H NMR: d 2.95 (q, 2J
=
H-H
2
7.7 Hz, CH2) and 1.50 (t, JH-H = 7.7 Hz, CH3). 195Pt NMR: d
-3285. FTIR (ATR): (nCN) 2325; (nCO) 2125 cm-1.
˜
˜
Synthesis of [PtCl{(j2-P,C)P(OC6H4)(OC6H5)2}(NCEt)] (3)
A solution of 0.620 g (1.65 mmol) of [PtCl2(NCEt)2] in 4.0 mL
of EtCN was treated with 0.500 g (1.63 mmol) of P(OPh)3 and
heated (150 ◦C) under stirring (44 h) in a sealed tube (total volume
ª 100 mL). The hot solution was filtered and then cooled (-30 ◦C).
Crystals were filtered and dried under vacuum. A single crystal was
selected for X-ray diffraction studies. A second crop of crystals
was obtained from the mother liquor. The overall yield (0.330 g)
Synthesis of cis-[PtCl2(PPh3)(NCEt)] (1a)
1
of (3) was 40%. H NMR: d 7.29 (m), 7.10 (m), 6.88 (m), 2.38
A mixture of [PtCl2(NCEt)2] (1.18 g, 3.13 mmol), PPh3 (◦0.819 g,
3.12 mmol) and propionitrile (8.0 mL) was heated (145 C) in a
sealed tube (total volume ª 100 mL) until a yellow solution was
obtained (12 h). Heating was stopped and the solution was slowly
cooled to room temperature (25 ◦C). Light yellow crystals were
filtered and dried under vacuum (1.24 g, 68% yield). Anal. Calcd.
(q) and 1.33 (t) (coordinated and free nitrile). 31P NMR: d 97.2
(1JP-Pt = 6853 Hz) [SP-4-2]-(3); 89.7 (1JP-Pt = 7575 Hz) [SP-4-4]-(3);
87.7(1JP-Pt = 7770 Hz) (trans-4)); 85.1 (1JP-Pt = 7864 Hz) (cis-(4)).
195Pt: d -4038 d (1JPt-P = 7864 Hz) (cis-(4)), -4142 d (1JPt-P = 7770 Hz)
(trans-(4)), -4272 d (JPt-P = 6853 Hz) ([SP-4-2] -(3)), -4329 d (JPt-P
=
7575 Hz) ([SP-4-4]-(3)). FTIR (ATR): (nCN) 2295 cm-1.
˜
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Synthesis of trans-[PtCl2(NCEt)(Py)] (5)
in 10.0 mL of benzonitrile was treated with a solution of 0.864
g of triphenylphosphite in 4.0 mL of the same solv◦ent (Pt/P
molar ratio = 1). The yellow solution was heated (160 C) under
stirring (48 h) and turned reddish. The mixture was cooled, filtered
and excess benzonitrile was eliminated under vacuum. The solid
residue was washed with diethylether (15.0 mL) and filtered. The
light-brown product was obtained as the mixture of the two
geometrical isomers (0.958 g, 63% yield). 31P NMR:19 d 87.7
(1JP-Pt = 7770 Hz), 85.1 (1JP-Pt = 7864 Hz). 195Pt NMR: d -4144
A solution of 0.540 g (1.44 mmol) of [PtCl2(NCEt)2] in 4.0 mL
of EtCN was treated with 0.117 mL (1.44 mmol) of pyridine and
stirred in a sealed tube (total volume ª 100 mL) initially at room
temperature (3 h) and then at 130 ◦C (5 h). The yellow solution was
◦
then cooled (25 C) to afford yellow crystals, which were filtered
and dried under vacuum. A second crop of crystals was obtained
from the cooled (–30 ◦C) mother liquor (0.370 g, 63.3% overall
yield). An analogous experiment carried out at room temperature
afforded similar results. 1H NMR: d 9.06 (m, 2H); 7.89 (m, 1H);
1
1
(d, JPt-P = 7770 Hz); -4038 (d, JPt-P = 7864 Hz). Molar ratio
between the two isomers, according to the relative intensity of 31
P
7.46 (m, 2H); 2.89 (q, 2H); 1.39 (t, 3H). 13C NMR: d 153.6, 138.7,
NMR signals, was about 4/1. A small sample of the mixture was
recrystallized (1,2-DCE/pentane) and a single crystal was selected
for X-ray analysis, which showed the trans stereochemistry of the
selected product.
195
˜
125.2, 12.6, 9.6. Pt NMR: d -2139. FTIR (ATR): (nCN) 2310
cm-1.
Synthesis of trans-[Pt(l-Cl)Cl(g2-C2H4)]2 (9)
29
Synthesis of [Pt(l-Cl)Cl(CO)]2 (7)
A solution of 1.13 mmol of trans-(8) in toluene (50.0 mL) was
slowly evaporated under vacuum at 25 C and the solid residue
◦
A solution of 45.0 mg (0.13 mmol) of cis-(6) in 2.0 mL of mesitylene
was heated (75 ◦C) and slowly evaporated to dryness. The
dissolution–evaporation cycle was repeated twice. The product
was dried under vacuum (9 h). trans-(9) was obtained in essentially
quantitative yield. 195Pt NMR (CH2Cl2): d -2498.48
was recovered in essentially quantitative yield. 195Pt NMR:29
d
-3019; -3034. FTIR (CDCl3): (nCO) 2136 cm-1.
˜
Synthesis of trans-[Pt(l-Cl)Cl(PPh3)]2 (2)
A suspension of 0.292 g of cis-(1a) (0.501 mmol) in 20.0 mL of
toluene was refluxed (110 ◦C) until the complete precipitation of an
orange solid was achieved and the 31P NMR signal of the precursor
disappeared (2 h). The orange crystals were filtered, washed with
toluene (3.0 mL) and dried under vacuum (0.217 g, 82% yield).
Anal. Calcd. for C36H30Cl4P2Pt2: C, 40.9; H, 2.9%. Found: C, 40.6;
H 2.8%. 31P NMR: d 5.1 (1JP-Pt = 4100 Hz). 195Pt NMR: d -3322
(d, 1JPt-P = 4100 Hz).
X-ray structure determinations
The X-ray diffraction experiments were carried out at room
temperature (T = 293 K) by means of a Bruker P4 diffractometer
operating with graphite-monochromated Mo-Ka radiation. The
samples were sealed in a glass capillary under a dinitrogen atmo-
sphere. The intensity data collection was carried out with the w/2q
scan mode, collecting a redundant set of data. Three standard
reflections were measured every 97 measurements to check the
sample decay. The intensities were corrected for Lorentz and
polarisation effects and for absorption by means of a integration
method based on the crystal faces49 in the case of the well shaped
crystal of [SP-4-2]-(3) and by a semiempirical method in the case
Synthesis of [Pt(l-Cl){(j2-P,C)P(OC6H4)2(OPh)2}]2 (4)
19
In a round-bottomed flask, equipped with magnetic stirrer and
condenser, a solution of 1.06 g (2.81 mmol) of [PtCl2(NCEt)2]
Table 4 Crystal data and structure refinements
cis-(1a)
[SP-4-2]-(3)
trans-(4)
Empirical formula
Formula weight
Crystal system
Space group
C21H20Cl2NPPt
583.34
C21H19ClNO3PPt
594.88
C36H28Cl2O6P2Pt2
1079.60
Monoclinic
P21/c (No. 14)
14.8678(19)
8.9329(9)
17.2269(18)
—
112.879(8)
—
2108.0(4)
4
1.838
6.991
5508
4351 [0.0368]
236
Triclinic
Triclinic
¯
¯
P1 (No. 2)
P1 (No. 2)
˚
a/A
9.1034(10)
9.8626(16)
12.6092(12)
80.004(9)
75.284(7)
79.865(10)
1068.0(2)
2
8.910(2)
9.699(1)
10.987(2)
96.04(1)
106.00(1)
105.80(1)
861.4(3)
1
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
U/A
Z
D
calc/g cm-3
1.850
6.790
5294
4427 [0.0279]
254
0.0389, 0.0822
0.0554, 0.0885
1.033
2.081
8.406
4244
3527 [0.0318]
182
0.0380, 0.0898
0.0459, 0.0934
1.062
m/mm-1
No. measured
No. unique [Rint
No. parameters
]
R1, wR2 [I>2s(I)]a
R1, wR2 [all data]a
Goodness of fitaon F2
0.0390, 0.0853
0.0593, 0.0935
1.029
ꢀ
ꢀ
ꢀ
ꢀ
1
2
2
2
2
2
a R(Fo) = ꢀFo| - |Fcꢀ/ |Fo|; Rw(Fo2) = [ [w(Fo - Fc2)2]/ [w(Fo ) ]] ; w = 1/[s (Fo2) + (AQ)2 + BQ] where Q = [MAX(Fo2,0) + 2Fc2]/3; GOF =
ꢀ
1
2
2
2 2
[
[w(Fo - Fc ) ]/(N - P)] , where N, P are the numbers of observations and parameters, respectively.
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of the other two samples. The structure solutions were obtained
by means of the automatic direct methods contained in SIR-9250
for [SP-4-2]-(3) and by means of the direct methods contained
in SHELXS9751 for the other two compounds. The refinement,
based on full-matrix least-squares on F2, was done by means of
the SHELXL9751 programme. Some other utilities contained in
the WINGX suite52 were also used. The more relevant crystal
parameters for the three compounds are listed in Table 4.
In the refinement of the centrosymmetric structure of trans-
(4) the thermal ellipsoids of carbon atoms of one of independent
phenyl rings of the phosphite ligand appeared abnormally prolate.
The orientation of their main axes suggested the presence of a
conformational disorder of the phosphite, characterized by two
slight different P–O–phenyl angles for the same phenyl ring. Thus a
model with two different orientations for that ring was introduced
in the calculations, the total occupancy of each corresponding
carbon atoms being fixed to 1. The following refinement cycles
showed some reduction in the reliability factor calculating the
occupancy of each position at about 50%.
19 A. Albinati, S. Affolter and P. S. Pregosin, Organometallics, 1990, 9,
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effect of triphenylphosphine.
=
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We gratefully acknowledge the Ministero dell’Universita` e della
Ricerca Universitaria (MIUR), Progetti di Ricerca di Rilevante
Interesse Nazionale (PRIN 2007) for financial support. We thank
Chimet S.p.A., I–52041 Badia al Pino (Arezzo) for a loan of
platinum.
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