Modulation of properties in analogues of Zeise's anion on changing the ligand trans to ethene. X-Ray crystal structures of trans-[PtCl 2(OH)(η2-C2H4)]- and trans-[PtCl2(η1-CH2NO2) (η2-C2H4)]-

Article abstract of DOI:10.1039/c2dt11934j

To get further insight in the reaction of nucleophilic substitution upon changing the ligand trans to a η²-olefin, the reactivity of some monoanionic platinum(ii) complexes (trans-[PtCl₂X(η²- C₂H₄)]^-, X = Cl^-, 1, OH^-, 2, and CH₂NO₂^-, 3) towards pyridines with different steric hindrance (py, 4-Mepy, and 2,6-Me₂py) has been tested. All crystallographic (2 and 3 reported for the first time) and spectroscopic data are in accord with a platinum-olefin interaction decreasing in the order 2 > 1 > 3, paralleling the decreasing electronegativity of the donor atom (O > Cl > C). Not only the platinum-olefin bond but also the bond between platinum and the ligand trans to the olefin appear to be strongest in 2 (Pt-O distance at the lower limit for this type of bond). In the reaction with py, the ligand trans to the olefin is displaced in 1 and 2. Moreover the reaction is in equilibrium in the case of sterically hindered 2,6-Me₂py, the equilibrium being shifted moderately or prevalently toward the reagents in the case of 1 and 2, respectively. In the case of 3, the reaction with pyridines leads to substitution of the olefin instead of the carbanion. This is in accord with the observation that carbanions strongly weaken the trans Pt-olefin bond.

Full text of DOI:10.1039/c2dt11934j

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PAPER
Modulation of properties in analogues of Zeise’s anion on changing the ligand
trans to ethene. X-Ray crystal structures of trans-[PtCl2(OH)(η2-C2H4)]− and
trans-[PtCl2(η1-CH2NO2)(η2-C2H4)]−†
Michele Benedetti,*a Carmen R. Barone,b Daniela Antonucci,a Vita M. Vecchio,a Andrea Ienco,c
Luciana Maresca,*b Giovanni Natileb and Francesco P. Fanizzia
Received 13th October 2011, Accepted 22nd November 2011
DOI: 10.1039/c2dt11934j
To get further insight in the reaction of nucleophilic substitution upon changing the ligand trans to a η2-
olefin, the reactivity of some monoanionic platinum(II) complexes (trans-[PtCl2X(η2-C2H4)]−, X = Cl−, 1,
OH−, 2, and CH2NO2−, 3) towards pyridines with different steric hindrance (py, 4-Mepy, and 2,6-Me2py)
has been tested. All crystallographic (2 and 3 reported for the first time) and spectroscopic data are in
accord with a platinum–olefin interaction decreasing in the order 2 > 1 > 3, paralleling the decreasing
electronegativity of the donor atom (O > Cl > C). Not only the platinum-olefin bond but also the bond
between platinum and the ligand trans to the olefin appear to be strongest in 2 (Pt–O distance at the lower
limit for this type of bond). In the reaction with py, the ligand trans to the olefin is displaced in 1 and 2.
Moreover the reaction is in equilibrium in the case of sterically hindered 2,6-Me2py, the equilibrium being
shifted moderately or prevalently toward the reagents in the case of 1 and 2, respectively. In the case of 3,
the reaction with pyridines leads to substitution of the olefin instead of the carbanion. This is in accord
with the observation that carbanions strongly weaken the trans Pt–olefin bond.
Such high reactivity towards nucleophiles depends upon the
π-acid character of the ethene molecule which stabilizes the tri-
gonal bipyramidal transition state, where the entering and the
Introduction
Zeise’s salt, K[PtCl3(η2-C2H4)], serendipitously prepared by
William Zeise in 1825,1 saw its full characterization only 150
leaving ligand and the trans-directing olefin occupy the equator-
ial sites.8 In the trigonal plane, in fact, apart from the lone pairs
years later, when a neutron diffraction study could precisely
locate also the hydrogen atoms.2 Apart from the historical inter-
of the three donors, there is accumulation of extra electron
est, it was in fact the first organometallic compound to be
reported, Zeise’s anion is a paradigmatic example of olefin to
density due to two filled non bonding d orbitals of platinum. The
π-back-donation from the metal to the unsaturated ligand can
remove part of this electron density and stabilize the transition
state.9 Moreover, in the five-coordinate transition state the repul-
sion between bonding and non bonding electron pairs is reduced
if the angle between entering and leaving ligands is rather small
(< 90°, against a theoretical value of 120°).9 This last aspect has
received full experimental support by the isolation of several
five-coordinate complexes of formula [PtCl2(η2-C2H4)(N–N)],
where the bite angle of the dinitrogen ligand is close to 70°.10
In water solution Zeise’s anion, 1, readily undergoes substi-
tution of the chloride trans to ethene by a solvent molecule
which, in basic conditions, can be deprotonated yielding the
mono-anionic hydroxo species trans-[PtCl2(OH)(η2-C2H4)]−, 2.
At high pH values, substitution of another Cl− by OH− can take
place.11 In basic alcoholic medium RO− takes the place of OH−,
2′.4a,c
3
metal bond and a still important starting material for the syn-
thesis of platinum-based organometallic compounds.4 Moreover,
Zeise’s salt can be used to detect the presence of the hepatitis
associated antigen (HAA) in humans.5
The great reactivity of Zeise’s anion, essentially related to the
great lability of the chloride trans to the olefin,6 allows to bring
in the metal coordination shell, and in very mild reaction con-
ditions, any donor having a reasonable affinity to platinum.7
aDipartimento di Scienze e Tecnologie Biologiche ed Ambientali,
Università del Salento, Via Monteroni, I-73100 Lecce, Italy.
E-mail: michele.benedetti@unisalento.it; Fax: +39 0832 298626;
Tel: +39 0832 298973
bDipartimento Farmaco-Chimico, Università degli Studi di Bari, Via
E. Orabona 4, I-70125 Bari, Italy. E-mail: maresca@farmchim.uniba.it;
Fax: +39 080 5442230; Tel: +39 080 5442759
cCNR, I-50019 Sesto Fiorentino, Firenze, Italy. Istituto di Chimica dei
Composti Organometallici (ICCOM), Consiglio Nazionale delle
Ricerche, Via Madonna del Piano 10, I-50019 Sesto Fiorentino, Firenze,
Italy
Substitution of OH− (2) or RO− (2′) for Cl− (1) should not
alter the pattern of nucleophilic substitution at the platinum
center, however we noticed different reactivity towards a steri-
cally hindered ligand such as 2,9-dimethyl-1,10-phenanthroline,
Me2phen. While 1 reacts readily with Me2phen forming
†CCDC reference numbers 848868 and 848869. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2dt11934j
3014 | Dalton Trans., 2012, 41, 3014–3021
This journal is © The Royal Society of Chemistry 2012

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Table 2 Comparison of selected bond lengths [Å] for: [PtCl3(η2-
C2H4)]−, 1 (potassium salt, ref. 2); trans-[PtCl2(OH)(η2-C2H4)]−, 2
(PPh4 salt, this work); trans-[PtCl2(η1-CH2NO2)(η2-C2H4)]−, 3 (PPh4
salt, this work)
[PtCl2(η2-C2H4)(Me2phen)], in contrast species of type 2 do not
react at all.4a Moreover, if trans-[PtCl2(OR)(η2-C2H4)]−, 2′, is
transformed into trans-[PtCl2(OR)(η1-C2H4–OR)]2− by a nucleo-
philic attack of RO− onto the coordinated ethene in basic metha-
nol, the dianion, differently from 2′, reacts readily with Me2phen
to give the stable [PtCl(η1-C2H4–OR)(Me2phen)] end product.4a
To get further insights in the reaction of nucleophilic substi-
tution upon changing the ligand trans to the η2-olefin, we have
Bond lengths
[Å]
1
2
3
Pt–O
Pt–Col
—
1.991(10)
2.096(15) and
2.071(19)
—
—
2.135(3) and
2.128(3)
—
2.340(2)
2.303(2) and
2.302(2)
1.375(4)
2.200(6) and
2.217(7)
2.078(7)
—
2.293(5) and
2.301(5)
1.354(11)
tested the reactivity of some monoanionic platinum(II) complexes
Pt–Cσ
Pt–Cltrans
Pt–Clcis
−
(trans-[PtCl2X(η2-C2H4)]−, X = Cl−, 1, OH−, 2, and CH2NO2
,
—
3) towards aromatic imines with different steric requirements
(pyridine, py; 4-methylpyridine, 4-Mepy; and 2,6-dimethylpyri-
dine, 2,6-Me2py). We also report the X-ray crystal structures of
2 and 3 (the X-ray structure of 1 has been resolved long time
ago 2).
2.287(5) and
2.272(5)
1.41(2)
CvC
Results and discussion
Synthesis and structural characterization of species 2 and 3
Complexes 1, 2, and 3 were isolated as tetraphenylphosphonium
(PPh4) salts in order to increase their solubility in organic sol-
vents. Therefore (PPh4)1 was obtained from (PPh4)Cl and
Zeise’s salt in water at neutral pH, while the same reaction, per-
formed at pH ≈14, gave (PPh4)2. (PPh4)3 was obtained by a
reaction of (PPh4)2 with a moderate excess of nitromethane in
organic solvent.12
Fig. 1 Single crystal X-ray diffraction molecular structure of (PPh4)
{trans-[PtCl2(OH)(η2-C2H4)]}, (PPh4)2. ORTEP drawing, 50% prob-
ability ellipsoids, of the trans-[PtCl2(OH)(η2-C2H4)]− anion. Selected
bond lenghts [Å] and angles [°]: Pt1–O1 1.991(10); Pt1–C1 2.096(15);
Pt1–C2 2.071(19); Pt1–Cl1 2.287(5); Pt1–Cl2 2.272(5); C1–C2 1.41(2);
O1–Pt1–C1 159.3(6); O1–Pt1–C2 161.2(6); C1–Pt1–C2 39.5(7); O1–
Pt1–Cl2 90.5(4); O1–Pt1–Cl1 88.9(4); C1–Pt1–Cl1 90.6(6); C1–Pt1–
Cl2 90.5(6); C2–Pt1–Cl2 89.7(7); C2–Pt1–Cl1 90.2(7); Cl1–Pt1–Cl2
178.20(19).
Relevant NMR data for the three anionic complexes are
reported in Table 1. It has to be noted that on going from 1 to 2
(substitution of Cl− trans to ethene by OH−), the proton and
carbon atoms of the unsaturated ligand undergo a significant
upfield shift. In contrast, when the Cl− of 1 is replaced by the
CH2NO2− carbanion, to give 3, the resonances of ethene protons
2
and carbons shift to a lower field. The JPt,H value, similar for 1
and 2, is significantly smaller in the case of 3. As far as the 195Pt
resonance is concerned, 1 and 3 exhibit an almost coincident
value (−2747 and −2749 ppm, respectively) while in 2 the
signal is slightly shifted to a higher field (−2772 ppm).
It was possible to obtain crystals of 2 and 3 suitable for X-ray
investigation; their structures are depicted in Fig. 1 and 2. Sig-
nificant bond distances are reported in Table 2 together with the
Fig. 2 Single crystal X-ray diffraction molecular structure of (PPh4)
{trans-[PtCl2(η1-CH2NO2)(η2-C2H4)]}, (PPh4)3. ORTEP drawing, 50%
probability ellipsoids, of the trans-[PtCl2(η1-CH2NO2)(η2-C2H4)]−
anion. Selected bond lenghts [Å] and angles [°]: Pt1–C1 2.217(7); Pt1–
C2 2.200(6); Pt1–C3 2.078(7); Pt1–Cl1 2.293(5); Pt1–Cl2 2.301(5);
C1–C2 1.354(11); N1–O1 1.180(9); N1–O2 1.286(10); N1–C3 1.356
(10); C1–Pt1–C2 35.7(3); C1–Pt1–C3 160.7(3); C1–Pt1–Cl1 90.0(3);
C1–Pt1–Cl2 90.5(3); C2–Pt1–C3 163.5(3); C2–Pt1–Cl1 90.2(3); C2–
Pt1–Cl2 90.7(3); C3–Pt1–Cl1 91.0(3); C3–Pt1–Cl2 88.3(3); Cl1–Pt1–
Cl2 179.06(11); O1–N1–O2 116.3(8); O1–N1–C3 129.1(10); O2–N1–
C3 114.4(8).
Table 1 Selected 1H, 13C and 195Pt chemical shifts for: [PtCl3(η2-
C2H4)]−, 1 (tetrabutylammonium salt, ref. 13); trans-[PtCl2(OH)(η2-
C2H4)]−, 2 (PPh4 salt, this work and ref. 14); trans-[PtCl2(η1-CH2NO2)
(η2-C2H4)]−, 3 (PPh4 salt, this work)a
Complex
1
2
3
1H (η2-C2H4) 4.33 (2JPt,H = 60) 3.72 (2JPt,H = 55) 4.46 (2JPt,H = 44)
1H (Pt–OH) 3.82 (2JPt,H = 49) —
—
13C (η2-C2H4) 68 (1JPt,C = 93) 56 (1JPt,C = 68) 83 (1JPt,C = 83)
1H (η1-CH2-)
13C (η1-CH2-) —
195Pt
−2747
—
—
—
−2772
5.26 (2JPt,H = 104)
71 (1JPt,C = 722)
−2749
corresponding values for Zeise’s salt, K1. The coordination shell
of platinum has, in both cases, the usual square planar arrange-
ment, when the centroid of the olefin bond is considered. The
displacement of the platinum atom from the coordination plane
is 0.022 and 0.019 Å for 2 and 3, respectively. The two mutually
trans Pt–Cl distances remain practically unaltered in all
compounds.
a Spectra collected in CDCl3; 1H and 13C NMR chemical shifts are
given in ppm by using the residual solvent signal as reference; 195Pt
NMR chemical shift is given in ppm by using the standard reference
K2PtCl6; J values are given in Hz.
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The Pt–OH bond in 2 (1.991(10) Å) is shorter than in any
other reported Pt(II) complex where the trans ligand is other than
an olefin (e.g. hydride (2.16 Å),15 phenyl (2.094 Å),16 phosphine
(2.042 Å, average),17 or nitrogen donor (2.028 Å).18 On the
other hand, the Pt–Cσ bond in 3 [2.081(7) Å] lies close to the
mean value for platinum–carbon σ-bond distances which may
range from 1.903 19 to 2.432 Å.20
hydroxo complex. Conversely, the back-bonding of the ethene
ligand will reduce the repulsion between the Pt atom and the
hydroxo group so that the Pt–OH distance becomes particularly
short.
In compound 3 there is a carbanion (NO2CH2−) trans to the
olefin. Carbanions are known to be very strong σ-donors. The
carbanion trans to the ethene molecule has a dramatic effect on
the platinum–olefin bond system and causes a remarkable
lengthening of the Pt–Col bonds (0.10 Å longer than in 1 and
0.14 Å longer than in 2). Most probably, olefin-to-platinum σ-
donation and platinum-to-olefin π-back-donation operate syner-
gistically, therefore a reduction in the olefin-to-platinum σ-bond
(due to the strong σ-donor capacity of the trans carbanion) can
have a strong (weakening) effect on the platinum-to-olefin
π-back-donation. Comparing the three trans ligands, their σ-
donor ability can be confidently rated in the order carbanion >
chloride > hydroxyl. The weaker σ-donor ability of chloride, as
compared to a carbanion, is also witnessed by the X-ray structure
of [PtCl(η1-CHRCH2–L)(tmen)]+, tmen = N,N,N′,N′-tetramethyl-
ethane-1,2-diamine, where the tmen nitrogen trans to the η1-
carbon is 0.09 Å longer than that trans to chloride.28 On the
other hand the much weaker σ-donor capacity of a hydroxyl, as
compared to a carbanion, can be easily deduced by the much
greater electronegativity of oxygen with respect to carbon.
Therefore, the much smaller σ-donor and π-acceptor capacity of
the olefin in 3 can fully account for the remarkable elongation
(as compared to 1 and 2) of the Pt–Col distances and for the shift
to a lower field of the carbon and hydrogen NMR frequencies
(the electronic situation more shifted towards that of a regular
olefin than towards that of a cyclopropane).29,30 In summary, all
crystallographic and spectroscopic data are in accord with a plati-
num–olefin interaction decreasing in the order: 2 > 1 > 3, paral-
leling the decreasing electronegativity of the donor atoms (O >
Cl > C). In 2, not only the platinum–olefin bond but also the
bond between platinum and the ligand trans to the olefin appear
to be strongest (Pt–O distance at the lower limit for this type of
bond).
The ethene ligand is perpendicular to the coordination plane,
the angle between the CvC bond axis and the orthogonal to the
coordination plane being only 0.1(9)° in 2 and 0.6(4)° in 3.21
The preferred upright orientation of a coordinated olefin in
square planar complexes has essentially a steric origin. In fact,
an in-plane orientation of the olefin would cause short repulsive
contacts with the two adjacent chlorine atoms, as revealed by the
significant energy barrier (at least 20 kcal mol−1) for olefin
rotation detected in similar species.22 This interpretation (steric
effect favoring the orthogonal disposition of the olefin with
respect to the coordination plane in square planar complexes) is
fully supported also by the observation that when there is less
steric hindrance in the coordination plane, such as in the case of
the platinum(II) complex [Pt{η3-CH2C(Me)CH2}(PPh3)(η2-
23
CH2CHPh)]+
and, more recently, in the rhodium(I) species
[Rh(PCP)(η2-C2H4)] (PCP is a three-coordinate pincer dipho-
sphite ligand), a coplanar arrangement of the coordinated olefin
is observed. In the latter complexes the structure of the allyl
group (platinum case) and that of the pincer ligand (rhodium
case) is such as to leave more room in the coordination plane
allowing though the in-plane orientation of the olefin. Accord-
ingly, a very low energy barrier for olefin rotation has been
found in the quoted platinum complex.23
24
From the data of Table 2 it appears that on going from 1 to 2,
that is, by substitution of a hydroxyl group for the chloride trans
to ethene, there is a significant shrinking of the Pt–Col distances.
In contrast, the substitution of a carbanion for the chloride trans
to ethene, to form 3, produces considerable elongation of the Pt–
Col bond distances.
Effect of the trans ligand upon the platinum–olefin bond
The extent of π-back donation from the metal to a coordinated
olefin can be sized by the degree of elongation of the CvC
bond of the unsaturated ligand.25 However this single parameter
could not be sufficient for ranking the strength of the π-bond
since also pure σ donation from the olefin to the metal can cause
a similar effect.26 Another parameter which could be exploited
for sizing the extent of π-back donation is the Pt–Col bond
length. Compared to 1, complex 2 reveals a significant shorten-
ing of the two Pt–Col bond lengths; therefore it is possible to
conclude that, as far as the bonding between the metal and the
ethene ligand is concerned, the π-back bonding is greater in 2
than in 1. This feature is also supported by the NMR data (see
above) revealing how the olefin proton and carbon atoms are sig-
nificantly more shielded in 2 than in 1 (see Table 1).
The lone pairs of anionic oxygen and nitrogen ligands have
long been known to engage in repulsion interactions with filled
late transition metal d-orbitals.27 As a consequence the filled d-
orbitals are pushed up in energy and these are exactly the orbitals
engaged in the back-bonding to the ethene ligand. Thus, it is not
surprising that the ethene ligand is strongly bonded with the
Reactivity towards an entering nucleophile
The second aim of this work was to investigate how the different
ground state stability of the three substrates could influence their
reactivity towards a monodentate nucleophile. It has already
been mentioned in the Introduction that, while compound 1
readily reacts with chelating, but sterically hindered, neocuproine
(Me2phen) to yield [PtCl2(η2-C2H4)(Me2phen)], compound 2
does not react at all.
Therefore the three platinum complexes ([PtCl3(η2-C2H4)]−,
1; trans-[PtCl2(OH)(η2-C2H4)]−, 2; and trans-[PtCl2(η1-
CH2NO2)(η2-C2H4)]−, 3) were reacted with py, 4-Mepy, and 2,6-
Me2py. The reaction course was monitored via NMR in organic
solvents where all reactants and products are soluble, the results
are summarized in Table 3. The expected reaction products,
trans-[PtCl2(L)(η2-C2H4)] (L = py, 4a; 4-Mepy, 4b; 2,6-Me2py,
4c), were independently prepared by a reaction of Zeise’s salt, K
[PtCl3(η2-C2H4)], with the relevant base (py, 4-Mepy, or 2,6-
Me2py) in water, where complexes of type 4 are practically inso-
luble and precipitate from solution in almost quantitative yield.31
3016 | Dalton Trans., 2012, 41, 3014–3021
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Table 3 Percentage of the product in the reaction of 1, 2 and 3 with an
entering pyridine
ethanide moiety. These species have been investigated in the past
and in the case of 4-MePy even isolated in the solid state;
however, they do not survive in solution at temperatures >
243 K.32 To confirm the nature of species 5, py or 4-Mepy was
added to a solution of independently prepared 4a or 4b in
CDCl3 at 294 K, no reaction whatsoever was observed (sharp
py
4-Mepy
2,6-Me2py
1
2
3
100% 4
100% 5a
100% 6
100% 4
100% 5a
100% 6
≈ 40% 4
≈ 0% 5a
100% 6
1
signals of 4 and of free aromatic base were present in the H
a Denotes that 5 is the major reaction product but it is in equilibrium
with the precursor 4 (5 derives from 4 by nucleophylic addition of the
released hydroxyl upon the π-ethene).
NMR spectra). When the same solution of species 4 was treated
with a few drops of KOH in D2O, signals due to 5 immediately
appeared in the organic phase (1H NMR). Species 5 of the
present investigation are not indefinitely stable in solution and
with time (a few hours at room temperature) they decompose
releasing the aromatic base (1H NMR) and forming platinum oli-
gomers (ESI-MS data). This decomposition, however, was not
further investigated.
The reaction of (PPh4)1 with L, performed in organic solvent
(CDCl3, 294 K), was complete within the mixing time of the
1
reagents (monitored by H NMR). However, a quantitative con-
Unlike py and 4-Mepy, 2,6-Me2py, treated with 2, does not
give the substitution product, neither immediately nor after
several hours from the mixing of the reagents (eqn (IV)),
showing a strict analogy with Me2phen previously found not to
give reaction products by treatment with the anionic complex 2
in MeOH solution.4a,c
version into 4 was achieved only in the case of py and 4-Mepy
(eqn (I)), while in the case of 2,6-Me2py
½PtCl3ðη2-C2H4Þꢀꢁ þ L ! trans-½PtCl2ðLÞðη2-C2H4Þꢀ þ Clꢁ
1
4
L ¼ py ðaÞ; 4-Mepy ðbÞ
In order to show that the lack of formation of reaction pro-
ducts in the reaction of 2 with sterically hindered aromatic bases
(2,6-Me2py and Me2phen) is not due to kinetic inertness, but to
the shift of the equilibrium in favor of the reactants (eqn (IV)),
the complex trans-[PtCl2(2,6-Me2py)(η2-C2H4)], 4c, indepen-
dently prepared as previously described, was reacted with KOH
(made soluble in CDCl3 by the addition of a crown ether). The
reverse reaction, with formation of 2 and free 2,6-Me2py, did
occur quantitatively in the mixing time of the reagents.
Finally we tested the reactivity towards the same set of pyri-
dines of (PPh4)3. In all cases the reaction produced, within the
mixing time, 100% conversion of the starting substrate into the
anionic complex trans-[PtCl2(η1-CH2NO2)(L)]−, 6, (L = py, 6a;
4-Mepy, 6b; or 2,6-Me2py, 6c) formed by substitution of the aro-
matic base for ethene (eqn (V)).
ðIÞ
an equilibrium between reagents and products was established
and the estimated K value at the working temperature was ca.
0.4 (eqn (II)).
ꢁ
2
2
ꢁ
!
½PtCl ðη -C H Þꢀ þ L trans-½PtCl ðLÞðη -C H Þꢀ þ Cl
3
2
4
2
2
4
1
4
L ¼2;6-Me2py ðcÞ
ðIIÞ
The reaction of (PPh4)2 (CDCl3, 294 K) with py and 4-Mepy
also went to completion within the mixing time of the reagents
and produced 4 together with trans-[PtCl2(L)(η1-CαH2CβH2-
OH)]−, 5, (L = py, 5a, and 4-Mepy, 5b. eqn (III)).
Complex 5, which formally derives from addition of the
released OH− to the coordinated ethene in complex 4, was in
both cases the major component ([5]/[4] = 5/1). The presence of
5 was recognized by the diagnostic pattern of the ethanide
moiety [1H NMR (CDCl3, 294 K): CαH2 at ca. 2.1 ppm (2JPt,H
ca. 85 Hz) and CβH2 at ca. 3.2 ppm (3JPt,H ca. 30 Hz)].
In principle, also a pyridine molecule (L) could add to the
coordinated ethene of 4, and form the zwitterionic species trans-
[PtCl2(L)(η1-CαH2CβH2-L)], with a similar pattern for the
Selectivity in the substitution reaction
In the nucleophilic substitution at a platinum(II) center, the reac-
tion course is deeply influenced by the nature of the entering
ligand, which generally replaces its closest similar (π-acid
ligands replace π-acid coordinated groups and σ-donors replace
ligands having prevalently a σ-donor character). The reason for
such selectivity is that, in the trigonal bipyramidal transition
(III)
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trans-½PtCl2ðOHÞðη2-C2H4Þꢀꢁ þ L trans-½PtCl2ðLÞðη2-C2H4Þꢀ þ OHꢁ
(IV)
2
4
L ¼2;6-Me2py ðcÞ
trans-½PtCl2ðη1-CH2NO2Þðη2-C2H4Þꢀꢁ þ L ! trans-½PtCl2ðη1-CH2NO2ÞðLÞꢀꢁ þ C2H4
(V)
3
6
L ¼ py ðaÞ; 4-Mepy ðbÞ; 2;6-Me2py ðcÞ:
state, entering, leaving, and trans groups share the equatorial
plane and ligands with a similar character (σ-donor or π-accep-
tor) are in stronger competition to one another with one finally
losing and leaving the metal core. In accord with this expec-
tation, Vicente & Co.33 observed that the anionic complex cis-
[PtCl2(CH2COCH3)(η2-C2H4)]− (where a σ-carbon and an
olefin, having both trans labilizing properties, are mutually cis)
reacts with ligands having a π-acidic character replacing the
olefin. In contrast, ligands which are essentially σ-donors replace
capacity of the ligands can be ranked in the order: OH− < Cl− ≪
Cσ−. A carbanion exerts an exceptionally high weakening effect
on the bond between platinum and a trans-olefin. As a conse-
quence the olefin is displaced in preference to the carbanion not
only by another olefin but also by σ-donors such as pyridines.
The remarkable effect of substituents at the pyridine ring upon
the stability of platinum complexes has also been highlighted.
Experimental
the chloride lying trans to the σ-carbon. In accord with the same
34
view, 1
and 2 (having only one trans-labilizing ligand, the
Reagents and methods
olefin) and also 3 (having two trans-labilizing groups trans to
one another) react with π-acid ligands (another olefin) displacing
ethene.35 Always in accord with the same rule, pyridines (which
are essentially σ-donors) react with 1 and 2 replacing the σ-
donor ligand trans to the trans-labilizing olefin. Of course, the
reaction can be of equilibrium and the equilibrium composition
reflects the relative stability of the starting and of the end pro-
ducts. It is interesting to note the deep influence exerted by steric
effects upon the stability of pyridine’s end products. In the case
of 2,6-Me2py, the steric hindrance exerted by the two methyl
substituents in ortho position to the N-donor causes destabiliza-
tion of compound 4c to such an extent that the starting substrate
is still present (partially, 1, or nearly exclusively, 2) at the end of
the reaction (eqn (II) and (IV)).32,36 The different extent of for-
mation of 4c in eqn (II) and (IV) confirms the greater stability of
2 over 1, already deduced from crystallographic and spectro-
scopic parameters. The stability of 2 is such that even a biden-
tate, but sterically hindered, diimine such as neocuproine fails to
give reaction products.
It is noteworthy to observe that complex 3 reacts with pyri-
dines (mostly σ-donor ligands) giving an olefin rather than car-
banion substitution. Therefore olefin substitution is observed not
only in the case of reaction with π-acceptor ligands but also in
the case of reaction with σ-donor ligands like pyridines. In the
above discussion it was pointed out that a carbanion weakens the
interaction between platinum and a trans olefin. We can now
conclude that the weakening is such that the olefin becomes the
most labile ligand and is preferentially displaced not only by
π-acceptor but also by σ-donor ligands. Moreover, also an ortho
disubstituted pyridine, such as 2,6-Me2py, can give complete
displacement of the olefin, further supporting the great destabili-
zation of the platinum-to-olefin bond in 3.
Reagents and solvents were commercially available and used as
received without further purification. Zeise’s salt was prepared
4c
by a modified procedure
of the method reported by Chock
et al.37 The complexes trans-[PtCl2(η2-olefin)(L)], 4, (L = py,
4a; 4-Mepy, 4b; 2,6-Me2py, 4c) were prepared in water from
Zeise’s salt and the appropriate imine according to a reported
procedure.29 Elemental analyses were performed with a CHN
Eurovector EA 3011. H, and 13C NMR spectra were recorded
1
with Avance Bruker DPX-WB 300 instrument equipped with
probes for inverse detection and with z gradient for gradient-
1
accelerated spectroscopy. H and 13C NMR spectra were refer-
enced to TMS; the residual proton and carbon signals of the
solvent were used as internal standard. For 195Pt NMR spectra,
1
K2PtCl4 (−1643.00 ppm) was used as external standard. H/13C
inversely detected gradient-sensitivity enhanced heterocorrelated
2D NMR spectra for normal coupling (INVIEAGSSI) were
acquired using standard Bruker automation programs and pulse
sequences. Each block of data was preceeded by eight dummy
scans. The data were processed in the phase-sensitive mode. The
ESI-MS spectra were recorded with an Agilent 1100 Series
LC-MSD Trap System VL.
Synthesis of complexes
(PPh4)[PtCl3(η2-C2H4)], (PPh4)1. Zeise’s salt, K[PtCl3(η2-
C2H4)]·H2O, (193 mg, 0.50 mmol) was dissolved in 4 mL of
water and treated at 273 K with a water solution of (PPh4)Cl
(206 mg, 0.55 mmol, in ∼3 mL of solvent). A cream-colored
solid instantaneously precipitated. The formed solid was recov-
ered by filtration on a sintered glass filter, washed twice with
water and dried in vacuo. All manipulations were carried out at
room temperature. The yield of (PPh4)1, with respect to plati-
num, was nearly quantitative. Anal. Calcd (%) for C26H24Cl3PPt
(668.88): C, 46.69; H 3.62. Found: C 46.49; H 3.57. Peaks of
greatest intensity in ESI-MS: m/z 328 [M − PPh4]−, and 339
[M − PtCl3(η2-C2H4)]+. NMR (CDCl3, 300 MHz, 294 K): δH
Conclusions
Our data have highlighted the nature of the Pt–Cl, Pt–OH, and
−
Pt–Cσ bonds trans to an olefin (Cσ = carbanion). The σ-donor
3018 | Dalton Trans., 2012, 41, 3014–3021
This journal is © The Royal Society of Chemistry 2012

View Article Online
2
4.32 (pseudo triplet, 4H, JPt,H = 60 Hz, C2H4), 7.62–7.93 (m,
20H, P(C6H5)4) ppm.
(L)], 4. The transformation was complete in the case of py and
4-Mepy and partial (≈ 40%) in the case of 2,6-Me2py.
(PPh4){trans-[PtCl2(OH)(η2-C2H4)]}·H2O, (PPh4)2·H2O. The
preparation was similar to that of (PPh4)1, but in this case
Zeise’s salt was dissolved in basic water (KOH) at a pH close to
14, while the working temperature was always 273 K. The yield
of (PPh4)2 with respect to platinum was nearly quantitative.
Anal. Calcd (%) for C26H25Cl2OPPt·H2O (668.45): C, 46.72; H,
4.07; Cl, 10.61. Found: C 46.89; H 3.95; Cl, 10.95. NMR
trans-[PtCl2(η2-C2H4)(py)], 4a. NMR (CDCl3, 300 MHz,
294 K): δH 4.80 (pseudo triplet, JPt,H = 61 Hz, C2H4), 7.13 (t,
2
2H, Hmeta), 7.71 (t, 1H, Hpara), 9.12 (d, 2H, Hortho) ppm.
trans-[PtCl2(η2-C2H4)(4-Mepy)],
4b. NMR
(CDCl3,
300 MHz, 294 K): δH 2.45 (s, 3H, 4-Me-py), 4.77 (pseudo
2
triplet, 4H, JPt,H = 58 Hz, C2H4), 7.29 (d, 2H, Hmeta), 8.73 (d,
(CDCl3, 300 MHz, 294 K): δH 3.71 (pseudo triplet, 4H, 2JPt,H
=
2H, Hortho) ppm.
2
55 Hz, C2H4), 3.85 (pseudo triplet, 1H, JPt,H = 49 Hz, OH),
7.62–7.93 (m, 20H, P(C6H5)4) ppm.
trans-[PtCl2(η2-C2H4)(2,6-Me2py)],
4c. NMR
(CDCl3,
300 MHz, 294 K): δH 3.2 (s, 6H, Me2-py), 4.81 (pseudo triplet,
Crystals of (PPh4)2 were obtained from a solution prepared by
dissolving Zeise’s salt, K[PtCl3(η2-C2H4)]·H2O, and (PPh4)Cl in
basic methanol (NaOMe). The basic methanol was obtained by
reaction of sodium metal with methanol dried over molecular
sieves of Type 4A (activated by heating at 393 K for 48 h), the
solution was kept at 253 K (NaCl/ice bath) until the evolution of
hydrogen ceased and was then let to warm up to 273 K. After
removal by filtration of the precipitated KCl, a sample of the
resulting solution was placed in a vial, layered under diethyl
ether and placed in a deep freezer (253 K). After two weeks,
pale yellow needles formed of what turned to be anhydrous
(PPh4)2.
2JPt,H = 57.0 Hz, C2H4), 7.19 (d, 2H, Hmeta), 7.63 (t, 1H, Hpara
)
ppm.
In the case of complex 2, py and 4-Mepy gave an instan-
taneous and complete substitution product with the released
OH− taken up by the olefin (trans-[PtCl2(η1-CH2CH2–OH)(L)]−,
5). No reaction was observed with 2,6-Me2py. In our opinion the
lack of reactivity was caused by a shift of the equilibrium
Table 4 Crystallographic data for (PPh4)2 and (PPh4)3
(PPh4)2
(PPh4)3
(PPh4){trans-[PtCl2(η1-CH2NO2)(η2-C2H4)]}, (PPh4)3. A sol-
ution of (PPh4)2·H2O (267 mg, 0.4 mmol) in CH2Cl2 (5 mL) at
room temperature, was reacted with CH3NO2 (49 mg, 0.8 mmol,
43 μL). A rapid reaction took place and the color of the solution
changed from pale to bright yellow. The solution was evaporated
under reduced pressure, leaving a yellow oil from which, after
trituration with diethyl ether, a yellow solid was obtained. The
yield of (PPh4)3 with respect to platinum was nearly quantitat-
ive. Anal. Calcd (%) for C27H26Cl2NO2PPt (693.46): C, 46.76;
H 3.78; N, 2.02. Found: C 46.85; H 3.77; N, 2.12. NMR
Empirical formula C26H25Cl2OPPt
C27H26Cl2NO2PPt
693.45
150(2)
0.71069
Monoclinic
Cc
Formula weight
T/K
Wavelength [Å]
Crystal system
Space group
Unit cell dimensions
a/Å
b/Å
c/Å
α (°)
β (°)
650.42
293(2)
0.71069
Triclinic
ˉ
P1
7.956(2)
11.887(2)
13.184(2)
91.152(16)
90.206(18)
100.160(16)
1227.0(4)
2
17.8100(4)
7.41700(10)
19.2250(6)
90.000
91.915(2)
90.000
2538.14(10)
4
1.815
γ (°)
Volume [Å3]
Z
(CDCl3, 300 MHz, 294 K): δH 4.45 (pseudo triplet, 4H, 2JPt,H
=
2
40 Hz, C2H4), 5.27 (pseudo triplet, 2H, JPt,H = 104 Hz,
-CH2NO2) 7.62–7.93 (m, 20H, P(C6H5)4) ppm.
Density
1.76
(calculated)
[Mg m−3
]
Crystals of (PPh4)3 were obtained from a CHCl3 solution con-
taining (PPh4)2·H2O and a slight excess of CH3NO2. After
10 min at room temperature (298 K), the solution was layered
under pentane and placed in a deep freezer (253 K). After
several days bright yellow crystals of (PPh4)3 formed.
Absorption
coefficient [mm−1
F(000)
6.017
632
5.827
]
1352
0.15 × 0.13 × 0.1
4.33 to 29.22°.
Crystal size [mm3] 0.25 × 0.225 × 0.15
Theta range for
data collection
Index ranges
1.74 to 22.97°.
−8 ≦ h ≦ 8, −13 ≦ k
≦ 13, 0 ≦ l ≦ 14
3575
−23 ≦ h ≦ 23, −9 ≦ k ≦
9, −24 ≦ l ≦ 26
9451
Reactions of platinum complexes with pyridines
Reflections
collected
All reactions were performed in NMR tubes at 294 K using
CDCl3 as solvent. 1 mL of a 0.03 M solution of the aromatic
imine [L = py (a); 4-Mepy (b); 2,6-Me2py (c)] was placed in an
NMR tube and, after recording the 1H NMR spectrum, was
treated with a stoichiometric amount of the appropriate platinum
complex. The resulting mixture was shaken until homogeneous
solution was obtained and then the 1H NMR spectrum was
recorded.
Independent
reflections
Data/restraints/
parameters
Goodness-of-fit on 1.52
F2
Final R indices
[I > 2σ(I)]
R indices (all data) R1 = 0.0781,
wR2 = 0.1335
Absolute structure
parameter
3400 [R(int) = 0.0175] 4760 [R(int) = 0.0345]
3400/0/281
4760/26/308
1.023
R1 = 0.059,
wR2 = 0.1287
R1 = 0.0339,
wR2 = 0.0735
R1 = 0.0382,
wR2 = 0.077
0.569(9)
The reaction products obtained in the different cases are
reported in Table 3.
Largest diff. peak
1.613 and −2.003
1.401 and −1.209
In the case of complex 1 all imines (L) gave an instantaneous
and hole/e Å−3
substitution reaction with formation of trans-[PtCl2(η2-C2H4)
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 3014–3021 | 3019

View Article Online
towards the reagents, see Results and discussion section; to
prove this complex 4c (≈ 2 mg), independently prepared, was
reacted with KOH (≈ 5 mg) made partially soluble in CDCl3
(600 μL, NMR tube) by addition of 18-Crown-6 ether (≈ 5 mg).
Formation of
quantitatively.
Dr Carlo Mealli (ICCOM, CNR, Firenze, Italy) is gratefully
acknowledged for active discussion.
2 and free 2,6-Me2py, did occur almost
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X-Ray structure determinations. Data collection for (PPh4)2
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room temperature. The intensities were corrected for Lorentz-
polarization and empirical correction for absorption, using Ψ
scan, was applied.38 For complex (PPh4)3 the data were collected
on an Oxford Diffraction Excalibur 3 diffractometer equipped
with CCD area detector at 150 K. The program CrysAlis CCD39
was used. Data reductions (including absorption corrections)
were carried out with the program CrysAlis RED.40 Data collec-
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The structures were solved by direct methods using SIR9741
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3020 | Dalton Trans., 2012, 41, 3014–3021
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This journal is © The Royal Society of Chemistry 2012
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