Unusual reactivity at a platinum center determined by the ligands and the solvent environment

Article abstract of DOI:10.1002/anie.200454111

The use of ancillary ligands resulted in the formation of (trans-[PtCl ₃(OH₂){N=C(OCH₃)CH₂CH ₂CH₂}₂]⁺ (see structure), the first intermediate aqua species in the oxidation of platinum(II) to platinum(IV) to be isolated and characterized. In chloroform this complex, but not the corresponding chloro species, reacts with excess Cl^- with O-demethylation of the ligands, indicating the importance of the coordinated water molecule.

Full text of DOI:10.1002/anie.200454111

Angewandte
Chemie
Oxidation
Unusual Reactivity at a Platinum Center
Determined by the Ligands and the Solvent
Environment**
Gabriella Tamasi, Renzo Cini, Francesco P. Intini,
Maria F. Sivo, and Giovanni Natile*
The oxidation of platinum(ii) to platinum(iv) by dihalides
(X2) has been widely investigated in the past years.[1] In this
process, two consecutive reactions occur in aqueous solution:
1) Rapid oxidation, assisted by solvent participation, that
leads to the formation of a trans-haloaquaplatinum(iv)
intermediate and a free halide ion, and 2) attack by the
halide ion upon the platinum(iv) intermediate to yield the
trans-dihaloplatinum(iv) end product.[2–4]
The intermediate aqua species formed by this procedure
have generally been characterized in solution, but, to the best
of our knowledge, they were never isolated as pure solids. We
have found that ancillary ligands can stabilize the aqua
species to such an extent that it becomes the stable end
product. Here we report, for the first time, the X-ray
=
structure of such
a
species: (trans-[PtCl3(OH2){NK
C(OCH3)CH2CH2CL H2}2]+ (2, see Scheme 1). Furthermore,
in chlorinated solvents and in the presence of excess
chloride, 2 undergoes O-demethylation of the ancillary
ligands with release of methyl chloride. The X-raystructure
of
the
demethylation
product
(trans-[PtCl3(OH)-
ꢀ
=
=
{NK C(OH)CH2CH2CL H2}{NK C(O)CH2CH2CL H2}]
(4, see
Scheme 1) is also presented.
=
The platinum(ii) complex trans-[PtCl2{NK C(OCH3)-
CH2CH2CL H2}2] (1) reacts with Cl2 in water to form trans-
=
[PtCl3(OH2){NK C(OCH3)CH2CH2CL H2}2]Cl (2Cl, Scheme 1).
This species is stable for days in water in the presence of 5 ”
10ꢀ2 m HCl and 10ꢀ3 m 1 (platinum(ii) catalyzes the anation
reaction, that is, replacement of the coordinated solvent
molecule by an entering anion, in platinum(iv) complexes).
Other aqua species, under similar reaction conditions,
undergo complete anation within minutes.[3] Because of the
low solubility of 1 in water, the reaction was repeated by
[*] Dr. F. P. Intini, Dr. M. F. Sivo, Prof. G. Natile
Dipartimento Farmaco-Chimico
Universitàdegli Studi di Bari
Via E. Orabona 4, 70125 Bari (Italy)
Fax: (+39)080-544-2230
E-mail: natile@farmchim.uniba.it
Dr. G. Tamasi, Prof. R. Cini
Dipartimento di Scienze e Tecnologie Chimiche e dei Biosistemi
Universitàdegli Studi di Siena
Via A. Moro 2, 53100 Siena (Italy)
[**] The authors are grateful to the University of Bari (Contribution ex
60%), the Ministero per Istruzione Universitàe Ricerca (MIUR,
Cofin. N8 2001053898), and the EC (COST Chemistry Action D20)
for support. Dr. Francesco Cannito is gratefully acknowledged for
his assistance in the preparation of the manuscript.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 5081 –5084
DOI: 10.1002/anie.200454111
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5081

Communications
methyl group has taken the place of the
methoxy substituent) reacts with Cl2 to give
the tetrachloro platinum(iv) species trans-
=
[PtCl4{NK C(CH3)OCH2CL H2}2] (see the Sup-
porting Information, Figure S2).[11] There-
fore, the presence of two methoxy oxygen
atoms is essential for the stabilization of the
aqua species 2.
In chloroform 2Cl slowly reacts to
=
the tetrachloro species trans-[PtCl4{NK C-
(OCH3)CH2CH2CL H2}2] (3; t1/2 ꢁ 12 h, see
Scheme 1 and the Supporting Information,
Figure S3). The reaction appears to be
catalyzed by a small amount of the platinu-
m(ii) compound 1, which is formed by
disproportionation of 2Cl into 1 and Cl2.[2]
Addition of excess Clꢀ does not help in the
conversion of 2 into 3, but instead promotes the demethyla-
Scheme 1. Synthesis of 2Cl and further reaction to form 3 or 4. See text for details.
treating a solution of 1 in chloroform with a solution of Cl2 in
CCl4 (undried solvents). Again, the only reaction product was
the aqua species 2 (see the Supporting Information, Fig-
ure S1).
=
tion of 2 with formation of trans-[PtCl3(OH){NK C(OH)-
CH2CH2CL H2}{NK C(O)CH2CH2CL H2}]
ꢀ
=
(4) according to
Equation (1) (see Scheme 1 and the Supporting Information,
Figure S4).
2 Cl þ ðPPh4ÞCl ! ðPPh4Þ 4 þ 2 CH3Cl
ð1Þ
Compound 4 is stable in chloroform, even in the presence
of excess chloride ion, and represents the first mononuclear
platinum complex with 5-hydroxy-3,4-dihydro-2H-pyrrole
ligands to be isolated and fully characterized. In contrast, a
large number of platinum complexes with bridging 5-oxy-3,4-
dihydro-2H-pyrrole ligands (a-pyrrolidonato) have been
reported.[12]
Figure 1. Drawing of complex 2 in the crystalline state. The ellipsoids
enclose 50% probability. Selected bond distances [Š] and angles [8]:
The X-ray structure of 4 is shown in Figure 2. The
ꢀ
coordination sphere of 4 is similar to that of 2 (Pt N
ꢀ
ꢀ
ꢀ
ꢀ
Pt1 Cl1 2.283(3), Pt1 Cl2 2.308(3), Pt1 Cl3 2.310(3), Pt1 O1
ꢀ
ꢀ
2.050(5), Pt O 2.025(4), Pt Cl 2.318(5) Š for mutually trans
chloride atoms and 2.312(1) Š for chloride trans to oxygen;
the bond angles between the cis ligands are in the range 87.5–
ꢀ
ꢀ
ꢀ
2.035(7), Pt1 N11 2.021(9), Pt1 N12 2.023(9), N11 C51 1.281(15),
ꢀ
ꢀ
ꢀ
C51 O11 1.333(14), N12 C52 1.331(15), C52 O12 1.311(15); O1-Pt1-
Cl2 94.6(2), O1-Pt1-Cl3 85.1(2), Cl1-Pt1-Cl2 88.8(1), Cl1-Pt1-Cl3
91.5(1), Pt1-N11-C51 129.6(8), Pt1-N12-C52 129.8(8), N11-C51-O11
120(1), N12-C52-O12 118(1).
ꢀ
91.68). The Pt O bond distance is 0.01 Š shorter in 4 than in 2,
The X-ray structure of 2 is shown in Figure 1.[5–7] The bond
distances agree well with those previously reported for PtIV
ꢀ
ꢀ
ꢀ
complexes (Pt N 2.022(9), Pt O 2.035(7), Pt Cl 2.309(3) Š
for mutually trans chloride ligands and 2.283(3) Š for chloride
trans to oxygen).[8] The bond angles between the cis ligands
are in the range of 88.7–91.58, except for H2O-Pt-Cl2
(94.6(2)8) and H2O-Pt-Cl3 (85.1(2)8). In the organicligands
there is extensive electron delocalization over the oxygen
=
atom and the C N p system, as already observed in related
systems (av C O 1.32(1), av C N 1.30(3) Š).[9,10] The methyl
groups of the 5-methoxy substituents are directed away from
the metal center, thus allowing the methoxy oxygen atoms to
be close to the coordinated water molecule (O1···O11 2.68(1),
O1···O12 2.87(1) Š).
ꢀ
ꢀ
Figure 2. Drawing of complex 4 in the crystalline state. The ellipsoids
enclose 50% probability. Selected bond distances [Š] and angles [8]:
ꢀ
ꢀ
ꢀ
ꢀ
Pt1 Cl1 2.312(1), Pt1 Cl2 2.313(1), Pt1 Cl3 2.322(1), Pt1 O1
ꢀ
ꢀ
ꢀ
2.025(4), Pt1 N11 2.055(5), Pt1 N12 2.045(4), N11 C51 1.310(7),
ꢀ
ꢀ
ꢀ
C51 O11 1.273(7), N12 C52 1.311(7), C52 O12 1.258(7); O1-Pt1-Cl2
87.5(1), O1-Pt1-Cl3 91.1(1), Cl1-Pt1-Cl2 91.1(1), Cl1-Pt1-Cl3 90.4(1),
Pt1-N11-C51 124.1(4), Pt1-N12-C52 124.5(4), N11-C51-O11 125.8(5),
N12-C52-O12 126.3(5).
Under analogous conditions, the related dihydro oxazole
complex trans-[PtCl2{NK C(CH3)OCH2CL H2}2] (in which a
=
5082
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Angew. Chem. Int. Ed. 2004, 43, 5081 –5084

Angewandte
Chemie
ꢀ
ꢀ
formation of 2Cl. A solution of 2 in chloroform, left standing at
while the Pt Cl distance trans to Pt O is 0.03 Š longer in 4
than in 2; this reflects the different nature of the O donor
(OHꢀ and OH2, respectively). The hydroxo ligand, while
donating a hydrogen bond to the 5-oxy-dihydropyrrole ligand,
accepts a hydrogen bond from the 5-hydroxy-dihydropyrrole
ligand (O1···O12 2.551(7) Š, O1-H···O12 165(1)8; O1···O11
2.426(7) Š, O1···H-O11 167(1)8). Furthermore, there is
extensive electron delocalization over the oxygen atom and
room temperature, undergoes slow transformation with formation of
3 (t1/2 ꢁ 12 h at 258C). Treatment of a solution of 2Cl in chloroform
with excess (PPh4)Cl leads to ligand demethylation with formation of
(PPh4)4 and release of CH3Cl (t1/2 ꢁ 5 h in 2 ” 10ꢀ3 m (PPh4)Cl at
258C).
Further details of the synthesis and elemental analysis of 1–4 are
given in the Supporting Information. The 1H NMR spectra of 1–4
were recorded at 258C on a Bruker DMX 300 spectrometer. The
spectral changes for the oxidation of 1 to 2Cl by Cl2, for the
spontaneous transformation of 2Cl into 3, and for the reaction of 2Cl
with excess (PPh4)Cl to give (PPh4)4 and CH3Cl are given in the
Supporting Information (Figures S1, S3, and S4, respectively).
Because of fast proton exchange between residual water in the
solvent and coordinated water molecule (2) or hydroxyl groups (4),
only a broad signal is observed below 2 ppm. This is clearly shown in
Figure S3 of the Supporting Information, where the very broad signal
at ca. 1.6 ppm in the initial spectrum becomes sharp and shifts to
1.5 ppm as the coordinated water molecule of 2 is replaced by Clꢀ.
=
ꢀ
the C N p system of the dihydropyrrole ligands (av C O
ꢀ
1.27(1), av C N 1.31(1) Š).
The demethylation reaction does not take place in the
case of compound 3. Therefore, the coordinated water
molecule in 2 appears to play an important role in the
nucleophilic substitution of the nearby methoxy group by an
external chloride ion. The coordinated water molecule can
help in capturing this chloride ion. Furthermore, 2 is
positively charged while 3 is neutral, and therefore in
chloroform 2 can form an ionic couple with Clꢀ. The inertness
Received: February 26, 2004
Revised: July 5, 2004 [Z54111]
of 2 in water, even in the presence of 5 ” 10ꢀ2 m HCl and 10ꢀ3
platinum(ii) catalyst—as compared to its slow reactivity in
chloroform, where it affords 3 or, in the presence of excess
Cl , 4—must be connected with the greater ability of water to
solvate ionicspecies.
m
Keywords: demethylation · O ligands · oxidation · platinum ·
ꢀ
.
solvent effects
The O-demethylation is an important reaction in bio-
logical systems[13] as well as in organicchemistry.
Metal-
[14]
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mediated O-demethylation of coordinated ligands is also
described in the literature.[15,16] However, in the case of
platinum substrates there are only very few reports. These
include Arbuzov-like dealkylation of coordinated phosphites
promoted by Clꢀ,[17] demethylation of a coordinated O6-
methyldeoxyguanosine with thiophenol,[18] and dealkylation
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In conclusion, it appears that concepts that usually apply
to complex natural substrates—such as intramolecular hydro-
phobic–hydrophilic interactions and organization of a reac-
tive site—can also apply to simple coordination compounds
with interesting and unexpected changes in their behavior.
Therefore, compound 2, in which the coordinated water
molecule is surrounded by the oxygen atoms of two 5-
methoxy-dihydropyrrole ligands, is favored over the fully
chlorinated species 3 and represents the end product in
aqueous solution. Moreover, the hydrophilicsite of the
complex, with a coordinated water molecule and nearby
methoxy oxygen atoms, is suited to host the Clꢀ anion, which
promotes demethylation in a hydrophobicsolvent.
[4] M. Crespo, R. Puddephat, Organometallics 1987, 6, 2548 – 2550.
[5] Crystal data for 2Cl·2H2O (C10H24Cl4N2O5Pt): Mw = 589.2,
colorless parallelepiped crystal, monoclinic space group P21/c,
a = 16.246(2), b = 7.865(1), c = 15.055(1) Š, b = 94.46(1)8, V=
1917.8(4) Š3, Z = 4, 1calcd = 2.041 gcmꢀ3, Siemens P4 diffractom-
eter, 2qmax = 508, radiation MoKa, l = 0.71073 Š, scan mode w,
T= 293 ꢂ 2 K; of 4351 measured reflections, 3350 were inde-
pendent and 2859 were included in the refinement (I > 2s(I)).
Lorentz and polarization corrections were performed, absorp-
tion corrections were made through the y-scan technique, m =
7.893 mmꢀ1. The structure was solved by direct methods and
refined with Fourier methods (SHELX97-WINGX);[6,7] no. of
parameters 223, hydrogen atoms were included in calculated
positions (HFIX-SHELX97),[6,7] R1 = 0.0450 and wR2 = 0.1494
for the observed reflections, R1 = 0.0592 and wR2 = 0.1785 for
ꢀ3
all reflections, refinement residual electron density 1.813 eŠ
.
Crystal data for (PPh4)4 (C32H34Cl3N2O3PPt): Mw = 827.06, pale
yellow prism, monoclinic space group P21/n, a = 11.596(2), b =
11.040(2), c = 25.058(5) d Š, b = 99.55(1)8, V= 3163.5(10) Š3,
Z = 4, 1calcd = 1.736 gcmꢀ3
, 2qmax = 508, radiation MoKa, l =
0.71073 Š, scan mode w, T= 293 ꢂ 2 K; of 7186 measured
reflections, 5574 were independent and 5574 were included in
the refinement (I > 2s(I)). Lorentz and polarization corrections
were performed, absorption corrections were made through the
y-scan technique, m = 4.776 mmꢀ1. The structure was solved by
direct methods and refined with Fourier methods; no. of
parameters 383, R1 = 0.0323 and wR2 = 0.0699 for the observed
reflections, R1 = 0.0463 and wR2 = 0.0749 for all reflections,
refinement residual electron density 0.602 eŠꢀ3. Methods for
treatment of hydrogen atoms, instruments, computers and
software as for 2. CCDC-231767 (2Cl·2H2O) and CCDC-
231768 ((PPh4)4) contain the supplementary crystallographic
Experimental Section
All reagent-grade chemicals were purchased from Aldrich and used
without further purification.
Compound 1 was synthesized by ligand cyclization of trans-
=
[PtCl2{HN C(OCH3)CH2CH2CH2Cl}], which was prepared accord-
ing to the procedure described in reference [9] for analogous
compounds. The cyclization reaction was performed in acetone with
powdered KOH and is complete in 1 h at 258C, leading to the
quantitative formation of 1.
The treatment of 1 in chloroform with a solution of Cl2 in CCl4
(undried solvents) leads to the instantaneous and quantitative
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ccdc.cam.ac.uk).
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=
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Angew. Chem. Int. Ed. 2004, 43, 5081 –5084