Phase dependent η2-alkene versus η1-alkyl transformation in platinum(II) complexes

Article abstract of DOI:10.1016/S0020-1693(98)00345-4

Addition products of some nucleophiles to the coordinated olefin in [Pt(η²-olefin)Cl(tmen)]⁺ (1) (tmen = N,N,N′,N′-tetramethylethylenediamine) have shown to undergo a chemical transformation in the solid phase. The acetylacetonate (a

Full text of DOI:10.1016/S0020-1693(98)00345-4

Inorganica Chimica Acta 285 (1999) 301±304
Note
Phase dependent h2-alkene versus h1-alkyl transformation
in platinum(II) complexes
Luciana Maresca*, Giovanni Natile
Á
Dipartimento Farmaco-Chimico, Universita di Bari, via E. Orabona 4, I-70125 Bari, Italy
Received 20 March 1998; accepted 7 August 1998
Abstract
‡
Addition products of some nucleophiles to the coordinated ole®n in [Pt(h2-ole®n)Cl(tmen)] (1) (tmen ˆ N,N,N0,N0-tetramethylethy-
lenediamine) have shown to undergo a chemical transformation in the solid phase. The acetylacetonate (acac) addition product
[Pt{CH(H,Ph)CH(Ph,H)(acac)}Cl(tmen)] (2) is formed as a mixture of Marcovnikov and anti-Marcovnikov isomers; in the solid state
slow isomerization to the pure Marcovnikov isomer takes place. The complex [Pt{CH2CH2OC(O)CH3}Cl(tmen)] (3), obtained by addition
of acetate anion to 1, in the solid state slowly dissociates into its constituent species. Heterometallacycles [Pt{CH(H,Me)-
‡
CHMeONO}(tmen)] (4) formed by endo nucleophilic attack of an oxygen of the nitro ligand on the cis alkene ligand can be detected
in solution; however, they lose stability in the solid state where they revert to the acyclic species. The hydroxyl compound
[Pt(CH2CH2OH)Cl(tmen)] (5), formed by addition of OH to 1, undergoes, in the solid state, a condensation reaction to give the dimeric
ether species [(tmen)ClPtCH2CH2OCH2CH2PtCl(tmen)] (6) and a water molecule. Moreover, the 5 to 6 transformation is catalyzed by
carbon dioxide. All data are in accord with the solid state increasing the stability of the h2-ole®n species and free nucleophile over the h1-
alkyl addition product, a role for enthropic factors is envisaged. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Platinum complexes; Alkene complexes; Alkyl complexes
1. Introduction
was purchased from Aldrich and puri®ed before use by
distillation from potassium hydroxide.
The addition of nucleophiles to coordinated h2-ole®ns
leads to h1-alkyl species, the stability of which depends
upon several factors such as basicity of the added nucleo-
philes, charge of the complex, and substituents on the ole®n
(Scheme 1) [1±4].
2.2. Preparation of compounds
2.2.1. ꢀ2-Olefin complexes
The complexes [Pt(h2-ethene)X(tmen)](ClO4) (X ˆ Cl,
NO2; tmen ˆ N,N,N0,N0-tetramethylethylenediamine) were
prepared according to Refs. [6] and [7]. All other cationic
species were prepared by ole®n exchange. In a typical
experiment 1 mmol of the ethene complex was suspended
in dichloromethane (5 ml) and treated with an excess of the
exchanging ole®n under stirring for 2 days at room tem-
perature. Owing to their low solubility, the reaction products
were recovered quantitatively by ®ltration of the mother
liquor. In the case of E- and Z-2-butene the ole®n exchange
afforded a crude reaction product which contained some
non-ole®n containing species. The raw material was then
treated with a base (KOH in methanol) to form the methox-
ide addition product [Pt(CHMeCHMeOMe)(NO2)(tmen)]
which is very soluble in methanol. The solution was ®ltered
and treated with aqueous HClO4 to restore the h2-ole®n
complex which precipitates as a white solid. It is necessary
to use an E-2-butene which contains less than 1% of the Z-
isomer, otherwise a signi®cant amount of complex contain-
In our current studies on the chemistry of the cationic
‡
complexes [Pt(h2-ole®n)Cl(tmen)] (1) (tmen ˆ N,N,N0,N0-
tetramethylethylenediamine) [5±10], and in particular of
their reactivity towards nucleophiles, we have come across
a number of cases in which there was a reaction taking place
in the solid state. The description of these ®ndings, which
are indicative of a phase-dependent conformational prefer-
ence, are hereafter described.
2. Experimental
2.1. Starting materials
Commercial reagent-grade chemicals were used without
further puri®cation. N,N,N0,N0-tetramethylethylenediamine
*Corresponding author: Tel.: +39-080-544-2759; fax: +39-080-544-
2724.
0020-1693/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00345-4

302
L. Maresca, G. Natile / Inorganica Chimica Acta 285 (1999) 301±304
windows) on a Perkin-Elmer FT 1600 spectrophotometer.
1H NMR spectra were obtained with a Bruker AM 300
spectrometer.
Scheme 1.
3. Results and discussion
ing the Z-2-butene is also formed. The yield of the isolated
product was 65% referred to platinum in both cases. Anal.
Calc. for C10H24ClN3O6Pt: C, 23.4; H, 4.7; N, 8.2. Found
(E-2-butene): C, 23.2; H, 4.7; N, 8.1; (Z-2-butene): C, 23.3;
H, 4.7; N, 8.1%.
Addition of a nucleophile to a metal coordinated ole®n
takes place with a high degree of regioselectivity according
to the Markovnikov rule (preferred attack on the more
substituted carbon). When also the incoming nucleophile
is encumbered, the anti-Markovnikov addition product
gains stability [11].
2.3. Addition products
Therefore, the styrene complex [Pt(h2-CH2CHPh)Cl-
‡
(tmen)] reacts with acetylacetonate to give 2 as a mixture
[Pt{CH(H,Ph)CH(Ph,H)(acac)}Cl(tmen)] (2). The catio-
nic complex, [Pt(h2-styrene)Cl(tmen)](ClO4) (1 mmol) was
treated with acetylacetone (1 mmol) in a few ml of anhy-
drous dichloromethane in the presence of sodium carbonate
(2 mmol). After 24 h stirring at room temperature, the
solution was ®ltered and the solvent evaporated. The oily
residue, upon trituration with pentane and diethyl ether,
gave a pale yellow solid which proved to be a mixture of
Markovnikov and anti-Markovnikov isomers of 2. Anal.
Calc. for C19H31ClN2O2Pt: C, 41.5; H, 5.7; N, 5.1. Found:
C, 41.3; H, 5.7; N, 5.0%.
of Markovnikov (ꢀ75%) and anti-Markovnikov isomers
(ꢀ25%) (Scheme 2) [10]. 1H NMR spectra of aged samples
of 2, dissolved in CDCl3 just before the registration of the
spectrum, are reported in Fig. 1 as a function of aged time1.
The most signi®cant part of the spectrum (in the region
between 3.5 and 4.5 ppm down®eld from TMS) is related to
the signals of the residual proton on the attacking carbon of
the acetylacetonate anion (in g position with respect to
platinum). This particular proton gives rise to a neat doublet
in the case of the Markovnikov addition product (coupling
with the single proton on the b carbon) and a doublet of
doublets in the anti-Markovnikov addition product (coup-
ling with the two protons on the b carbon which are
unequivalent because of the diastereotopic splitting induced
by the asymmetric carbon in a position). The g proton in the
two isomers resonates at suf®ciently different ®elds to
allow, on the basis of the integration ratio, a quantitative
measure of their relative concentrations. The spectra show a
progressive lowering with time of the intensity of the signals
assignable to the anti-Markovnikov addition product and
their complete disappearance after storage of the solid
sample at 208C for 14 weeks (Fig. 1).
[Pt{CH2CH2OC(O)CH3}Cl(tmen] (3), was prepared
according to Ref. [6].
[Pt(CH2CH2OH)Cl(tmen)] (5). [Pt(h2-ethene)Cl(tmen)]-
(ClO4) (0.5 mmol) was reacted with KOH (2 mmol) in 5 ml
of water; after 0.5 h stirring, the mother liquor was ®ltered
and extracted with dichloromethane. The CH2Cl2 fractions
were allowed to stand (1 h) over anhydrous Na2SO4; the
solution was ®ltered and the solvent evaporated under
reduced pressure to leave a white solid of 5. To obtain 5
free from 6 (see below), all reported manipulations were
carried out in argon atmosphere and the used solvent were
¯ushed with the same gas (to exclude any carbon dioxide).
The yield, referred to platinum, was 80%. Anal. Calc. for
C8H21ClN2OPt: C, 24.5; H, 5.4; N, 7.1. Found: C, 24.7; H,
5.5; N, 7.1%.
The observed chemical transformation taking place in the
solid state requires detachment of the added carbanion and
its subsequent readdition in the thermodynamically more
favored site. This rearrangement cannot be detected in
solution where irreversible decomposition to other species
takes place in a few days. In contrast, dissociation of the
carbanion in the solid phase leaves the nucleophile in close
[(tmen)ClPtCH2CH2OCH2CH2PtCl(tmen)] (6). [Pt(h2-
ethene)Cl(tmen)](ClO4) (0.5 mmol) was reacted with ®nely
ground KOH (2 mmol) in a chlorinated solvent (either
dichloromethane or 1,2-dichloroethane, 5 ml). After 24 h
stirring, the mother liquor was ®ltered and the solvent
evaporated under reduced pressure to leave 6 as a white
solid. The yield, referred to platinum, was 80%. Anal. Calc.
for C16H40Cl2N4OPt: C, 25.1; H, 5.3; N, 7.3. Found: C, 24.9;
H, 5.4; N, 7.1%.
2.4. Physical measurements
Scheme 2.
IR spectra in the range 4000±600 cm 1 were recorded as
KBr pellets, Nujol mulls or solution spectra (in chlorinated
solvents, using a cell of 0.1 mm thickness with NaCl
1The complete solubilization of all the samples excludes the presence of
decomposition products.

L. Maresca, G. Natile / Inorganica Chimica Acta 285 (1999) 301±304
303
free acetate anion at 1570 and 1450 cm 1 gradually appear
and grow. These aged samples, when redissolved in chlori-
nated solvents, did show complete restoring of the ester
function. Thus, the stability of the compound is phase
dependent: in solution the addition product is the preferred
one, in the solid state the detached form gains stability. Both
events could be favored by enthropic factors: desolvation of
the ions (cationic complex and anionic nucleophile) in
solution and dissociation into two separate units in the solid.
A further example of phase dependent conformational
preference came from the different behavior in the solid
and in solution of the cationic species [Pt(h2-
‡
ole®n)(NO2)(tmen)] (4) having NO2 , instead of Cl ,
as anionic ligand. When the ole®n is propene or E-2-butene,
1
the H NMR spectra in CD2Cl2 were consistent with the
presence of a heterometallacycle formed by endo-nucleo-
philic attack of an oxygen of the nitro ligand on the cis
alkene ligand [9]. The heterometallacycle is the only species
present in solution in the case of propene. It represents ca.
50% of the total (the remaining 50% is the h2-ole®n species)
in the case of E-2-butene, and it is not detectable when the
ole®n is either ethene or Z-2-butene. The formation of the
heterometallacycle was also supported by FTIR spectra of
the same solutions used for 1H NMR, which showed a band
at 1582 cm 1 (propene), or 1588 cm 1 (E-2-butene) assign-
able to the N=O stretching of a ±N(O)OR group (this band
was missing in the case of ethene and Z-2-butene). However,
the bands at 1400 and 1340 cm 1, characteristic of a
terminal nitro ligand, missing in the case of propene, were
of moderate intensity in the case of E-2-butene, and were the
strongest of the spectrum in the case of ethene and Z-2-
butene. When the IR spectra were recorded in the solid
phase (Nujol mulls) all the series of cationic complexes did
not show any absorption in the region of 1600 cm 1, but
only those assignable to a terminal NO2 group. Therefore,
the endo-addition with the formation of the heterometalla-
cycle is favored in solution, while only the h2-alkene
complex is present in the solid state.
Fig. 1. 1H NMR spectra (CDCl3) of samples of [Pt{CH(H,Ph)CH(Ph,H)-
(acac)}Cl(tmen)], 2, registered immediately after dissolution: (1) soon
after preparation; (2) after two days; (3) after 55 days; (4) after 89 days.
((A) g proton of the Markovnikov isomer; (B) g proton of the anti-
Markovnikov isomer; (C) b proton of the Markovnikov isomer which is
coupled to three different nuclei (the g and the two diastereotopic a
protons)).
proximity to the reactive cationic complex allowing its
prompt readdition before decomposition patterns become
effective.
It has been reported in the literature that the photode-
carboxylation of 3-indolacetic acid using phenanthridine as
sensitizer occurs with high yield and selectivity if the
reaction is performed in the solid state and at low tempera-
ture ( 708C). Higher temperature, or performing the reac-
tion in solution, severely lowers the selectivity [12]. The
pictured situation is understood on the basis of the presence
of an intermediate radical species; the lower the tempera-
ture, the less mobile is the radical though the more straight-
forward and cleaner the observed reaction. The analogy
with the system we have described is quite evident.
We have also found that the complex [Pt{CH2CH2O-
C(O)CH3}Cl(tmen)] (3) [6], obtained by addition of acetate
anion to 1, in the solid state slowly dissociates into its
We have also observed the spontaneous condensation of
two molecules of [Pt(CH2CH2OH)Cl(tmen)] (5) (formed by
addition of OH to 1), [6] to give the dimeric species
[(tmen)ClPtCH2CH2OCH2CH2PtCl(tmen)] (6) and a water
molecule. The reaction is reversible and the equilibrium is
shifted towards the monomer as the water content of the
medium increases. Pure 5 can be prepared only in water,
while in aprotic solvents or in the solid phase the 5 to 6
condensation reaction takes place. Furthermore, this trans-
formation is catalyzed by carbon dioxide. For instance, a
solid sample of complex 5 kept under argon atmosphere
remains unaltered for several days. The same sample in
open air undergoes 50% conversion to 6 after one week at
208C, a much faster dimerization reaction (80% transforma-
tion after 5 h at 208C) is observed under a pressure of CO2
(2.5 atm). No signi®cant catalytic effect is observed in
solution (CDCl3 saturated with CO2 at 1 atm) where a
sample of 5 shows only 15% conversion after three days
constituent species, as it can be monitored by IR spectra in
1
the solid [6]. The absorption bands at 1730 and 1245 cm
,
present in the freshly prepared sample and assignable to the
ester group, lose intensity with time while absorptions of

304
L. Maresca, G. Natile / Inorganica Chimica Acta 285 (1999) 301±304
Scheme 3.
at 208C (in absence of CO2 the conversion is slightly less
than 10%).
synthetic point of view either for their stereo- or chemo-
selectivity [13±15].
A possible mechanism for the catalysis is depicted in
Scheme 3 and implies an electrophilic attack of carbon
dioxide upon the hydroxyl group and formation of hydrogen
carbonate anion and parent cationic complex 1, the latter
can then react with the hydroxyl of 5 to give 6 releasing a
proton (the proton and the hydrogen carbonate can give back
carbon dioxide and water). This reaction pattern is very
similar to the classical mechanism of formation of an ether
from an alcohol in the presence of an electrophile. In our
case the electrophile is CO2. Stronger electrophiles, such as
Acknowledgements
Á
This work was supported by the Ministero dell'Universita
e della Ricerca Scienti®ca e Tecnologica (MURST) and the
Italian National Research Council (CNR).
References
‡
H , would have shifted the equilibrium towards the forma-
[1] P.D. Kaplan, P. Schmidt, M. Orchin, J. Am. Chem. Soc. 90 (1968)
4175.
tion of a single species (1), thus not allowing the condensa-
tion reaction; in contrast CO2 leads to an equilibrium
between 1 and 5, so allowing the interaction of these two
species to form the dimeric complex 6. The role played by
CO2 is quite evident and its higher ef®cacy in the solid state
is not surprising according to the examples we have just
described; the solid phase, in fact, appears to favor the ®rst
step reported in the scheme.
[2] D. Hollings, M. Green, D.V. Claridge, J. Organometal. Chem. 54
(1973) 399.
[3] G. Natile, J. Chem. Soc., Chem. Commun. (1978) 654.
[4] O. Eisenstein, R. Hoffmann, J. Am. Chem. Soc. 103 (1981) 4308.
[5] G. Gervasio, S.A. Mason, L. Maresca, G. Natile, Inorg. Chem. 25
(1986) 2207.
[6] F.P. Fanizzi, F.P. Intini, L. Maresca, G. Natile, F. Gasparrini, J.
Chem. Soc., Dalton Trans. (1990) 1019.
[7] F.P. Fanizzi, F.P. Intini, L. Maresca, G. Natile, J. Chem. Soc., Dalton
Trans. (1992) 309.
[8] L. Maresca, G. Natile, F.P. Fanizzi, J. Chem. Soc., Dalton Trans.
(1992) 1867.
4. Conclusions
[9] F.P. Fanizzi, L. Maresca, G. Natile, C. Pacifico, J. Chem. Soc.,
Dalton Trans. (1994) 949.
The several examples reported in this work indicate that
the solid phase stabilizes the h2-coordination mode, thus
favoring the shift of existing equilibria or allowing chemical
transformations promoted by the dihapto species. The
observed phase-dependent conformational preference also
indicates that crystal packing effects can act in opposite
direction than solvation effects.
The transformations here described (which take place
smoothly and require a very limited amount of energy)
should deserve some attention also in relation to the growing
interest, as witnessed by several recent reports, in reactions
occurring in the solid phase that can be useful from a
[10] F.P. Fanizzi, L. Maresca, G. Natile, C. Pacifico, Gazz. Chim. Ital. 124
(1994) 137.
[11] M.K. Cooper, P.J. Guerny, R.S. Nyholm, J. Organometal. Chem. 91
(1975) 3874.
[12] H. Koshima, K. Ding, T. Matsuura, J. Chem. Soc., Chem. Commun.
(1994) 2053.
[13] J.A. Hunt, W.R. Roush, J. Am. Chem. Soc. 118 (1996) 9998 and
references therein.
[14] M. Sakamoto, M. Takahashi, K. Kamiya, K. Yamagouchi, T. Fujita,
S. Watanabe, J. Am. Chem. Soc. 118 (1996) 10664.
Á
[15] M. Smrcina, S. Vyskocil, V. Hanus, M. Polasek, V. Langer, B.G.M.
Chew, D.B. Zax, H. Verrier, K. Harper, T. Claxton, P. Kocovsky, J.
Am. Chem. Soc. 118 (1996) 487.