Vinylic deprotonation in the coordination sphere of platinum(II): Regioselective formation of alkenyl complexes

Article abstract of DOI:10.1002/ejic.200400512

Reaction of [PtCl(η²-CH₂=CHAr)(tmeda)](ClO ₄) (1) [Ar = C₆H₅ (1a), 4-CH₃OC ₆H₄ (1b), 3-NO₂C₆H₄ (1c); tmeda = N,N,N′,N′-tetramethylethylenediamine) with triethylamine, or even with inorganic carbonate, leads to the abstraction of a vinylic proton and formation of the (alkenyl)platinum species [PtCl{(E)-CH=CHAr}(tmeda)] (2) in high yield. The initially formed (E) isomer, in solution of chlorinated solvents, partly isomerizes into the (Z) form [equilibrium ratio (E)/(Z) ≈ 4:1]; the isomerization reaction is rather slow and is catalyzed by visible light. The nature of the phenyl substituent (electron donor OMe or electron-withdrawing NO₂ group) does not appear to affect the rate of deprotonation. The nature and stereochemistry of the formed (alkenyl)platinum species has been elucidated through 1D and 2D ¹H and ¹³C NMR spectroscopy. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400512

SHORT COMMUNICATION
Vinylic Deprotonation in the Coordination Sphere of Platinum(II):
Regioselective Formation of Alkenyl Complexes
Giuseppe Lorusso,[a] Giovanni Boccaletti,[a] Nicola G. Di Masi,[a] Francesco P. Fanizzi,[b,c]
Luciana Maresca,*[a] and Giovanni Natile[a]
Keywords: Platinum / Alkenes / Brönsted acidity / Deprotonation reaction / (E)/(Z) isomerization
Reaction of [PtCl(η2-CH2=CHAr)(tmeda)](ClO4) (1) [Ar =
C6H5 (1a), 4-CH3OC6H4 (1b), 3-NO2C6H4 (1c); tmeda =
N,N,NЈ,NЈ-tetramethylethylenediamine) with triethylamine,
or even with inorganic carbonate, leads to the abstraction of
a vinylic proton and formation of the (alkenyl)platinum spe-
cies [PtCl{(E)-CH=CHAr}(tmeda)] (2) in high yield. The ini-
tially formed (E) isomer, in solution of chlorinated solvents,
partly isomerizes into the (Z) form [equilibrium ratio (E)/(Z) ഠ
4:1]; the isomerization reaction is rather slow and is catalyzed
by visible light. The nature of the phenyl substituent (elec-
tron donor OMe or electron-withdrawing NO2 group) does
not appear to affect the rate of deprotonation. The nature and
stereochemistry of the formed (alkenyl)platinum species has
been elucidated through 1D and 2D 1H and 13C NMR spec-
troscopy.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
of triethylamine (3 h, 25 °C, 1a/NEt3 ϭ 1:2.5), but slower
in the presence of anhydrous sodium carbonate in hetero-
geneous conditions (ca. 80% yield at 25 °C after 3 d). More-
over, it is completely regioselective and leads to the exclus-
ive formation of the (E) isomer [PtCl{(E)-CHϭ
CHPh}(tmeda)] [(E)-2a] (Scheme 1).
The stereochemistry of 2a was unequivocally determined
on the basis of NMR spectroscopic data. The signals of the
vinylic protons appear at δ ϭ 6.46 and 7.31 ppm with
3JH,H ϭ 16.3 Hz which is typical for protons that are mutu-
ally trans with respect to a carbonϪcarbon double bond.[8]
13C and 1H/13C INVERSE HETCOR spectra set the signals
of the vinylic carbon atoms at δ ϭ 121.3 and 135.8 ppm
with JPt,C ϭ 1009 Hz for the former and Ͻ 60 Hz for the
latter (Figure 1 and Table 1). The more shielded and
strongly Pt-coupled carbon atom is the one directly bound
to the metal center. It is worth noting that this carbon atom
carries the less shielded proton (δ ϭ 7.31 ppm).
In a recent paper we described the allylic deprotonation
of the alkene in cationic [PtCl(η2-alkene)(tmeda)]ϩ com-
plexes (tmeda ϭ N,N,NЈ,NЈ-tetramethylethylenediamine).
The reaction requires the use of a noncoordinating base
(NEt3) and, under suitable conditions, it can lead to the
removal of two allylic protons and formation of a dimeric
species with an η1,η3-allyl bridging ligand.[1] We have ex-
tended the investigation to styrene analogues of the pre-
vious compounds, [PtCl(η2-styrene)(tmeda)](ClO4), for
which only vinylic protons can be removed. The Brönsted
acidity of vinylic protons has already been reported for styr-
enes coordinated to a metal center and in a cationic
substrate.[2Ϫ6] There is, however, only one report concerning
platinum complexes, which involves dicationic species that
are known to be rather reactive towards deprotonation.[7]
Complex (E)-2a in chloroform solution partially iso-
merizes to the (Z) form. The transformation is photocata-
lyzed since at ambient temperature (ca. 25 °C) the equilib-
rium is attained in the dark after ca. 100 d and if the sample
is exposed to visible light after ca. 35 h. At equilibrium the
(E)/(Z) ratio is ca. 4:1. In the (Z) isomer the two vinylic
Results and Discussion
[PtCl(η2-CH2ϭCHPh)(tmeda)](ClO4) (1a) is readily de-
protonated by a base in chlorinated solvents. The reaction
is fast and quantitative in the presence of a moderate excess
3
[a]
protons have JH,H ϭ 10.7 Hz consistent with their mutual
`
Dipartimento Farmaco-Chimico, Universita di Bari,
cis position.[9] All other proton resonances change slightly
on going from the (E) to the (Z) form, with the only excep-
tion being the phenyl ortho-proton signals which are shifted
by more than 1 ppm at lower field [δ ϭ 7.27 and 8.50 ppm
for the (E) and (Z) isomer, respectively]. In the (Z) isomer
the phenyl ring is cis to the platinum atom with respect to
Via E. Orabona 4, 70125 Bari, Italia
Fax: (internat.) ϩ 39-0805442230
E-mail: maresca@farmchim.uniba.it
[b]
[c]
Dipartimento di Scienze
e
Tecnologie Biologiche ed
`
Ambientali, Universita di Lecce,
Via Monteroni, 73100 Lecce, Italia
Consortium C.A.R.S.O., Cancer Research Center,
70010 Valenzano-Bari, Italia
Eur. J. Inorg. Chem. 2004, 4751Ϫ4754
DOI: 10.1002/ejic.200400512
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4751

G. Lorusso, G. Boccaletti, N. G. Di Masi, F. P. Fanizzi, L. Maresca, G. Natile
SHORT COMMUNICATION
Scheme 1. Deprotonation and isomerization reactions
the olefin double bond, the ortho protons are close to and
An interesting aspect of the present reaction is its com-
plete stereoselectivity leading to the exclusive formation of
the (E) isomer. The (E) isomer is also thermodynamically
preferred because of smaller intramolecular steric interac-
tions. However, the kinetic preference for the (E) isomer
is greater than the thermodynamic preference for the same
isomer, therefore, starting from the pure (E) isomer (kine-
tically determined composition), a significant amount of
the (Z) form (ca. 20%) is also found at equilibrium.
deshielded by the metal center.[10]
The behavior of [PtCl(η2-4-MeO-styrene)(tmeda)](ClO4)
(1b) in which the phenyl ring carries an electron-donor sub-
stituent, and that of [PtCl(η2-3-NO2-styrene)(tmeda)](ClO4)
(1c) carrying an electron-withdrawing substituent, is practi-
cally the same as that of 1a. Therefore, the presence of sub-
stituents of different types on the styrene phenyl ring does
not appear to significantly affect the rate of deprotonation.
Treatment of the alkenyl complexes with mineral acids
(HClO4, 0.02  in water) reverses the reaction, reforming
the parent η2-alkene complexes.
In a thorough work by Gladysz on CϪH bond activation
of coordinated olefins in cationic complexes of rhenium()
promoted by K(tBuO), the attention was focused on the
two different deprotonation pathways involving either al-
lylic or vinylic protons.[3d] It was shown that both types of
deprotonation can take place depending upon the exper-
imental conditions. Vinylic deprotonation was preferred in
THF or DMSO, while allylic deprotonation was preferred
in tBuOH. A greater thermodynamic stability of the alkenyl
species (sp2 carbon atom bound to the metal atom), as com-
pared to the η1-allyl complex (sp3 carbon atom bound to
the metal atom), was given as a rationale for the observed
preference in THF and DMSO. On the contrary, the forma-
tion of large hydrogen-bonded aggregates of solvent mol-
ecules, around the tBuOϪ anion, was proposed to be re-
sponsible for the enhanced selectivity for less hindered al-
lylic protons in tert-butyl alcohol. The relevance of steric
Figure 1. Aromatic portion of the 1H/13C HETCOR spectrum
(500 MHz, CDCl3, 25 °C) for an equilibrated solution of (E)- and
(Z)-2a; some two-bond carbon-to-hydrogen correlation peaks are
also visible
1
Table 1. H NMR spectroscopic data (300 MHz, CDCl3, 25 °C) for [PtCl(η1-CαHϭCβHAr)(tmeda)] complexes 2; coupling constants in
Hz are reported in parentheses (3JH,H), brackets [JPt,H] and braces {JPt,C}; ortho, meta, and para protons of the aryl substituents are
indicated by o, m and p, respectively
PtϪCαHαϭCβHβϪAr
Ar
tmeda
NCH3
Compound Hα
Hβ
Cα
Cβ
o
m
p
CH2
(E)-2a
(Z)-2a
(E)-2b
(Z)-2b
(E)-2c
7.31 (16.3) [48] 6.46 (16.3) [50] 121.3 {1009} 135.8 {Ͻ 60} 7.27
7.18 7.01
7.24 7.10
2.62, 2.83 2.77, 2.91
2.60, 2.71 2.82, 2.97
7.35 (10.7)
6.79 (10.7)
126.8
134
8.50
7.18
8.43
7.06 (16.2) [46] 6.38 (16.2) [50] 117.6 {1009} 134.0 {79}
7.14 (10.7)
6.74 3.74 (OCH3) 2.60, 2.81 2.76, 2.90
6.80 3.78 (OCH3) 2.60, 2.68 2.81, 2.95
6.71 (10.7)
122.5
133.0
134.2
7.61 (16.4) [48] 6.57 (16.4) [53] 125.9
7.53, 8.11 7.31 7.84
2.65, 2.85 2.79, 2.94
4752
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Eur. J. Inorg. Chem. 2004, 4751Ϫ4754

Regioselective Formation of (Alkenyl)platinum(II) Complexes
SHORT COMMUNICATION
factors was also supported by the observation that vinylic base. Moreover, in the case of the dicationic alkene com-
deprotonation in tBuOH increased as the number of sub- plex, the formed alkenyl species, being insoluble in meth-
stituents on the allylic carbon increased.
anol, precipitated from the solution and this could have fav-
In our investigations (ref.[1] and present work), we always ored its formation.
use a very bulky base (NEt3) since the use of less bulky
nucleophiles such as OHϪ, OMeϪ or even NHEt2 leads to
stable addition products {[PtCl{η1-CH2CHR(X)}(tmeda)];
Conclusion
R ϭ H, alkyl or aryl; X ϭ OH, OMe or NHEt2} which
have no tendency, whatsoever, to lose HX and form allyl-
This study, besides providing a quick and clean route to
or alkenylplatinum species.[11] Also triethylamine can add the synthesis of (E)-alkenyl complexes, has also given valu-
to the coordinated olefin, leading to the formation of an able information on the possible reaction mechanism. Ab-
alkylplatinum derivative, but only in the case of unsubsti- straction of the proton trans to the phenyl substituent leads
tuted ethylene.[11] Having used a bulky base it is obvious to the exclusive formation of the (E) isomer. The (E) isomer
that, in the presence of allylic protons (more exposed to is also thermodynamically favored but not to the extent of
the solvent than vinylic protons), these are preferentially precluding the formation of a small amount of (Z) isomer
removed leading to formation of allylplatinum species. Only (ca. 20%) at equilibrium. The equilibration reaction is
in the absence of allylic protons (such as in the case of styr- rather slow (at room temperature in the dark it requires
ene), will vinylic deprotonation take place. Moreover, it is some weeks) and can be accelerated by visible light. There
very likely that both allylic and vinylic deprotonations take appears to be a direct correlation between overall charge of
place through a direct attack of the base (NEt3) on the leav- the complexes and deprotonation propensity of the coordi-
ing proton. The understanding of the reaction on the basis nated alkene.
of a simple Brönsted acid-base model is also supported by
the observation that mineral acids can protonate the alkenyl
species leading back to the π-olefin complex.
Experimental Section
The kinetic preference for the (E) isomer is also worthy
of further discussion. The alkene complex [PtCl(η2-CH2ϭ
CHAr)(tmeda)]ϩ is present in solution in two enantiomeric
forms having opposite configurations at the ϭCHAr stere-
ocenter (Scheme 1). The proton abstraction by an achiral
base (such as NEt3) will occur at a similar rate for the two
enantiomers and the (E) or (Z) configuration of the formed
alkenyl complex will depend exclusively upon the trans or
cis position of the abstracted proton with respect to the
Ar substituent. The kinetic preference for the (E) isomer
General Methods: The reagents and solvents employed are commer-
cially available and were used as received without further purifi-
cation. The complex [PtCl(η2-propene)(tmeda)](ClO4) was pre-
pared as already reported.[11] All complexes described in this work
gave satisfactory elemental analysis. 1H and 13C NMR spectra were
recorded with DRX500 Avance and DPX300 Avance Bruker in-
struments equipped with probes for inverse detection and with z
gradient for gradient-accelerated spectroscopy. 1H and 13C NMR
spectra were referenced to TMS; the residual proton signal of the
solvent was used as internal standard. Standard Bruker automation
indicates that the proton is removed exclusively in the posi- programs and pulse sequences were used for 1H/13C inversely de-
tected gradient-sensitivity enhanced heterocorrelated 2D NMR for
normal coupling (INVIEAGSSI). 1H 2D NOESY experiments
were performed at room temperature by collecting a 512 ϫ 1024
matrix with 64 scans per t1, and a mixing time of 1200 ms. Each
block of data was preceded by eight dummy scans. The data were
processed in the phase-sensitive mode.
tion trans to Ar, which is more exposed to the solvent and
more susceptible to undergo attack by a bulky nucleophile.
In this context it could be worth mentioning that acidity
data for the free styrene indicate that the most acidic pro-
tons are not those on the unsubstituted atom, but on the
carbon atom bearing the phenyl substituent.[13,14]
Preparation of Alkene Complexes: [PtCl(η2-CH2ϭCHC6H4X)(tme-
da)](ClO4) [X ϭ H, (1a), 4-OCH3 (1b), 3-NO2 (1c)]. Complexes
1aϪc were prepared by propene exchange in the cationic complex
The formation of an alkenyl complex by deprotonation
of a coordinated alkene is unprecedented in platinum chem-
istry except for a recent report on dicationic platinum spec-
ies of formula [Pt(PNP)(alkene)]2ϩ [PNP ϭ 2,6-bis(diphen-
ylphosphanylmethyl)pyridine]. In methanol the styrene de-
rivative exhibited a spontaneous deprotonation leading to
formation of the alkenyl complex [Pt(PNP)(HCϭCHPh)]ϩ
which precipitated from solution.[7] The authors did not
specifically discuss the (E) or (Z) configuration of the ob-
tained alkenyl complex; however, their NMR spectroscopic
data are in full agreement with those found in our case for
the (E) isomer. Moreover, since the (E) isomer is usually
preferred, also on a thermodynamic basis, it is not clear if
the obtained isomer was also the one preferred on a kinetic
basis. Our alkene complexes, being monocationic, have a
smaller propensity to undergo deprotonation than the di-
[PtCl(η2-propene)(tmeda)](ClO4).[11] In
a
typical experiment
[PtCl(η2-propene)(tmeda)](ClO4) (250 mg, 0.51 mmol) was sus-
pended in 4 mL of dichloromethane and treated with a 10-fold ex-
cess of the incoming olefin (a ϭ styrene, b ϭ 4-MeO-styrene, c ϭ
3-NO2-styrene). After 24 h of stirring at room temperature, the
solid was recovered on a sintered glass filter, washed with diethyl
ether and dried. The yield was always above 90%.
Preparation of Alkenyl Complexes: [PtCl(η1-CHϭCHC6H4X)-
(tmeda)] {X ϭ H [(E)-2a], 4-OCH3 [(E)-2b], 3-NO2 [(E)-2c]}, by
deprotonation with NEt3. In a typical experiment the styrene com-
plex 1 (0.4 mmol) was suspended in dichloromethane (8 mL) and
triethylamine was added whilst stirring at room temperature (2.5-
fold the stoichiometric amount). The mixture was left stirring
for several hours, while dissolution of the solid was observed. The
cationic complex mentioned above and require the use of a resulting solution was concentrated under vacuum; the viscous resi-
Eur. J. Inorg. Chem. 2004, 4751Ϫ4754
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G. Lorusso, G. Boccaletti, N. G. Di Masi, F. P. Fanizzi, L. Maresca, G. Natile
Madhavarao, S. Raghu, A. Rosan, M. Rosenblum, J. Am.
SHORT COMMUNICATION
due was triturated with diethyl ether and dried. The obtained solid
was then washed with water (to remove the alkylammonium salt)
and dried again under vacuum leaving a pale yellow compound
which was identified as the alkenyl complex 2. The isolated yield,
based on platinum, was ca. 85% for (E)-2a and (E)-2b and ca. 50%
for (E)-2c, which required purification of the crude reaction
product (Sefadex; eluent chloroform/acetone, 98:2, v/v). All ma-
nipulations were carried out by shielding the reaction vessel from
light. The alkenyl complexes (E)-2 were also prepared by depro-
tonation of the corresponding alkene complexes with Na2CO3. In
a typical experiment 1 (0.25 mmol) was suspended in chloroform
(5Ϫ6 mL), where it is sparingly soluble, treated with powdered
Na2CO3 (0.50 mmol), and the mixture stirred for 70 h at 25 °C.
After filtration, the mother liquor was concentrated under vacuum
and the viscous residue was triturated with diethyl ether and dried.
The obtained solid was the alkenyl species 2, the yield (always
based on platinum) was 80% for (E)-2a and 85% for (E)-2b.
[2c]
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). After 48 h of stirring, the solid was recovered by filtration of
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The coupling constants with the 195Pt nucleus are JPt,H ϭ 50
and 48 Hz for the more and the less shielded proton, respec-
tively.
[9]
In the (Z) isomer the vinyl hydrogen signals appear at δ ϭ 6.79
and 7.35 ppm, and the vinyl carbon signals at δ ϭ 126.8 and
134.0 ppm; also in this case the more shielded carbon atom is
bound to the less shielded proton.
[10]
(E)/(Z) Isomerization of the Alkenyl Ligand: NMR tubes containing
CDCl3 solutions (0.7 mL of a 6 m solution) of the (E)-alkenyl
species, were exposed to a neon lamp (75 W) and the 1H NMR
spectrum scanned every 4Ϫ6 h. The intensity ratio of two well-
separated signals, one belonging to the (E) and the other to the (Z)
isomer [the high-field vinylic signal of the (E) isomer and the ortho-
phenyl signals of the (Z) form], was plotted against time. A con-
stant ratio was reached at about 35 h for all compounds.
Large downfield shifts are typical for signals of protons at a
pseudoaxial position above the platinum center in a square-
planar complex: A. Albinati, P. J. Pregosin, F. Wombacher, In-
org. Chem. 1990, 29, 1812; Y. Liu, C. Pacifico, G. Natile, E.
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posed that a CϪH bond, close to the apical position of the Pt
atom, may undergo 3-center-4-electron interaction with a filled
orbital of the Pt atom. This interaction, unlike the 3-center-2-
electron agostic interactions, would cause a downfield shift in
the 1H NMR spectrum: W. Yao, O. Eisenstein, R. H. Crabtree,
Inorg. Chim. Acta 1997, 254, 105; L. Bremmer, J. M. Charnock,
P. L. Goggin, R. J. Goodfellow, A. G. Orpen. T. F. Koetzle, J.
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Acknowledgments
We thank the University of Bari, and the Italian Ministero dell’Is-
[11]
[12]
F. P. Fanizzi, L. Maresca, G. Natile, C. Pacifico, Gazz. Chim.
Ital. 1994, 124, 184.
`
truzione, Universita
e Ricerca (MIUR) (Cofin. 2003 no.
2003039774-005) for financial support.
L. Maresca, G. Natile, Comments Inorg. Chem. 1994, 16, 95.
[13] [13a]
When the vinylic CϪH bond is closely parallel to the π
[1] [1a]
electrons of the benzene ring, it is activated by σϪπ* orbital
interaction; similarly, the vinyl anion, generated by proton ab-
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Received June 14, 2004
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Eur. J. Inorg. Chem. 2004, 4751Ϫ4754