Oxidative addition of Se-X bonds and reductive elimination of Se-C bonds in platinum compounds containing hydrocarbyl ligands

Article abstract of DOI:10.1016/S1387-7003(03)00251-X

The oxidative addition of PhSe-X molecules (X = Cl or Br) to Pt(II) complexes of formula [PtR(R′)(N,N-chelate)] [R, R′ = Me, CH(CO ₂Me)₂, CH(CO₂Me)(CN); N,N-chelate = 4,4′-(tert-butyl)₂-2,2′-bipyridine] is descr

Full text of DOI:10.1016/S1387-7003(03)00251-X

Inorganic Chemistry Communications 6 (2003) 1282–1286
www.elsevier.com/locate/inoche
Oxidative addition of Se–X bonds and reductive elimination of
Se–C bonds in platinum compounds containing hydrocarbyl ligands
*
Achille Panunzi, Giuseppina Roviello, Francesco Ruffo
Dipartimento di Chimica, Universitaꢀ di Napoli ‘‘Federico II’’, Complesso Universitario di Monte S. Angelo, via Cintia, Napoli I 80126, Italy
Received 22 July 2003; accepted 26 July 2003
Published online: 25 August 2003
Abstract
The oxidative addition of PhSe–X molecules (X ¼ Cl or Br) to Pt(II) complexes of formula [PtR(R0)(N,N-chelate)] [R, R0 ¼ Me,
CH(CO2Me)2, CH(CO2Me)(CN); N,N-chelate ¼ 4,40-(tert-butyl)2-2,20-bipyridine] is described. The reaction quantitatively affords
octahedral products [PtR(R0)(SePh)(X)(N,N-chelate)], and is highly selective, since only the isomers with -X and -SePh in trans
position are obtained. When R ¼ R0 ¼ CH(CO2Me)2 or R ¼ R0 ¼ CH(CO2Me)2(CN) an irreversible reductive elimination takes
place in solution, giving rise to Pt(II) compounds [PtR(X)(N,N-chelate)] and organoselenium molecules PhSeR0.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Platinum; Selenium; Oxidative addition; Reductive elimination; Octahedral complexes
1. Introduction
the opportunity to gain useful insight into the thermo-
dynamics of the reaction.
In the last few years, coordination chemistry of
Group 16 ligands has attracted increasing attention
[1,2]. Within this growing area, we explored the oxida-
tive addition of Se–Se bonds to platinum(0) [3] and
platinum(II) [4] complexes, respectively, of formula
[Pt(N,N-chelate)(olefin)] (1) and [PtR(R0)(N,N-chelate)]
(2), where N,N-chelate is a diimine, and R, R0 hydro-
carbyl groups. New five-coordinate Pt(II) (3) and octa-
hedral Pt(IV) (4) products could be isolated, according
to the Eqs. (1) and (2)
Stimulated by these results, we addressed our interest
to the addition of Se–X bonds (X ¼ halide) to Pt com-
pounds, which, as far as we know, has not yet been in-
vestigated [5]. Within this communication we wish to
present preliminary results obtained by using plati-
num(II) compounds of type 2 as precursors and phe-
nylselenyl halides PhSeX (X ¼ Cl, Br) as oxidants.
Isolable octahedral products (5) were obtained and
characterised through NMR spectroscopy
½PtRðR0ÞðN; N-chelateÞꢀð2Þ þ PhSe–X
½PtðN; N-chelateÞðolefinÞꢀð1Þ þ R00Se–SeR00
¼ ½PtRðR0ÞðSePhÞðXÞðN; N-chelateÞꢀð5Þ
ð3Þ
¼ ½PtðSeR00Þ ðN; N-chelateÞðolefinÞꢀð3Þ
ð1Þ
ð2Þ
2
Depending on the nature of the hydrocarbyl ligands,
in solution the complexes may undergo reductive elim-
ination of a Se–C bond with formation of new Pt(II)
complexes and organoselenium compounds.
½PtRðR0ÞðN; N-chelateÞꢀð2Þ þ R00Se–SeR00
¼ ½PtRðR0ÞðSeR00Þ ðN; N-chelateÞꢀð4Þ
2
R00 ¼ Me or Ph
We also discovered that in some cases the addition is
a ‘‘tuneable’’ equilibrium [3b,4]. This rare event offered
2. Experimental
1H NMR and 13C NMR spectra were recorded on
Varian XL-200 or Varian Gemini spectrometers. 1H and
13C NMR chemical shifts are reported in d(ppm) relative
* Corresponding author. Tel.: +39-081-674460; fax: +39-081-674090.
E-mail address: ruffo@unina.it (F. Ruffo).
1387-7003/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S1387-7003(03)00251-X

A. Panunzi et al. / Inorganic Chemistry Communications 6 (2003) 1282–1286
1283
to the solvent (CHCl3, d ¼ 7:26; 13CDCl3, d ¼ 77:0).
The following abbreviations are used in describing
NMR multiplicities: s, singlet; d, doublet; t, triplet; m,
multiplet. Phenylselenenyl chloride and phenylselenenyl
bromide were commercially available. The platinum(II)
precursors [PtCl2(Bu2-bipy)] [6] and [Pt{CH(CO2Me)2}
(Me)(Bu2-bipy)] [4] (Bu2-bipy ¼ 4,40-(tert-butyl)2-2,20-
bipyridine) were obtained according to described pro-
cedures. Solvents and reagents were of AnalaR grade
and used without further purification.
Ph), 6.48 (t, 2H, 3,5-H-Ph), 4.07 (s, 1H, Pt–CH,
2JPt–H ¼ 89 Hz, [18.6 (1JPt–C ¼ 590 Hz)]), 3.78 (s, 3H,
2
OMe), 3.72 (s, 3H, OMe), 2.15 (s, 3H, Pt–Me, JPt–H
¼
66 Hz [1.3 (1JPt–C ¼ 524 Hz)]) ppm. Anal. Calcd for
C30H39ClN2O4PtSe (5aCl): C, 44.98; H, 4.91; N, 3.50.
Found: C, 45.26; H, 4.82; N, 3.63. Calcd for C30H39
BrN2O4PtSe (5aBr): C, 42.62; H, 4.65; N, 3.31. Found:
C, 42.57; H, 4.70; N, 3.44.
2.3. Synthesis of type 5 complexes [Pt{CH(CO2Me)2}2
(SePh) (X)(Bu2-bipy)] (X ¼ Cl, 5bCl; X ¼ Br, 5bBr) and
[Pt{CH(CO2Me)2(CN)}(SePh)(X)(Bu2-bipy)] (X¼ Cl, 5cCl;
X ¼ Br, 5cBr)
2.1. Synthesis of type 2 complexes [PtR(R0)(Bu2-bipy)]
R ¼ R0 ¼ CH(CO2Me)2 (2b), CH(CO2Me)(CN)(2c)
A stirred solution of [PtCl2(Bu2-bipy)] (0.20 g, 0.39
mmol) and KI (0.19 g, 1.2 mmol) in dimethylformamide
(2.5 mL) was heated at 373 K for 5 min. After the orange
solution was cooled at 313 K, the appropriate malonic
ester (0.97 mmol) and anhydrous K2CO3 (0.11 g, 0.78
mmol) were added, and the mixture was stirred for 24 h
at 313 K. The solvent and excess malonate were then
removed in vacuo, and the resulting solid was extracted
with chloroform. The yellow product was purified on a
silica gel column, eluting first with dichloromethane and
then with dichloromethane/methanol (methanol 5% v/v,
To a magnetically stirred solution of the type 2 pre-
cursor (0.15 mmol) in toluene (1.5 ml) the phenylselenyl
halide compound (0.15 mmol) was added. The dark red
solution was stirred for 5 min at room temperature. The
addition of hexane afforded the product, which was
washed with hexane and dried (yield: 70–80%). Selected
1H NMR data (d, 295 K): 5bCl 10.50 (d, 2H, NCH,
3JPt–H ¼ 22 Hz), 6.97 (t, 1H, 4H-Ph), 6.58 (d, 2H, 2,6-H-
Ph), 6.40 (t, 2H, 3,5-H-Ph), 4.80 (s, 2H, Pt–CH,
2JPt–H ¼ 91 Hz), 3.80 (s, 6H, OMe), 3.70 (s, 6H, OMe)
3
ppm; 5bBr 10.52 (d, 2H, NCH, JPt–H ¼ 29 Hz), 6.92 (t,
1
yield: 60–70%). Selected H [13C]NMR data (d, 295 K):
1H, 4H-Ph), 6.58 (d, 2H, 2,6-H-Ph), 6.42 (t, 2H, 3,5-H-
2
2b 10.00 (d, 2H, NCH,3JPt–H ¼ 32 Hz), 4.66 (s, 2H,
Ph), 4.80 (s, 2H, Pt–CH, JPt–H ¼ 92 Hz), 3.80 (s, 6H,
2
Pt–CH, JPt–H ¼ 128 Hz, [19.1 (1JPt–C ¼ 452 Hz)]), 3.61
OMe), 3.72 (s, 6H, OMe) ppm; 5cCl 9.70 (d, 2H, NCH,
3JPt–H ¼ 26 Hz), 7.00 (t, 1H, 4H-Ph), 6.74 (d, 2H, 2,6-H-
Ph), 6.47 (t, 2H, 3,5-H-Ph), 4.76 (s, 1H, Pt–CH,
(s, 12H, OMe) ppm; 2c (major diastereomer) 9.71 (d, 2H,
3
2
NCH, JPt–H ¼ 23 Hz), 4.04 (s, 2H, Pt–CH, JPt–H ¼ 123
Hz, [11.0 (1JPt–C ¼ 302 Hz)]), 3.70 (s, 6H, OMe) ppm; 2c
(minor diastereomer) 9.50 (d, 2H, NCH, 3JPt–H ¼ 20 Hz),
2
2JPt–H ¼ 104 Hz), 4.69 (s, 1H, Pt–CH, JPt–H ¼ 96 Hz),
3.97 (s, 3H, OMe), 3.84 (s, 3H, OMe) ppm; 5cBr 9.67 (d,
2H, NCH, 3JPt–H ¼ 25 Hz), 7.07 (t, 1H, 4H-Ph), 6.72 (d,
2H, 2,6-H-Ph), 6.45 (t, 2H, 3,5-H-Ph), 4.80 (s, 1H, Pt–
2
4.00 (s, 2H, Pt–CH, JPt–H ¼ 97 Hz, [12.0]), 3.45 (s, 6H,
OMe) ppm. Anal. Calcd for C28H38N2O8Pt (2b) C,
46.35; H, 5.28; N, 3.86. Found: C, 46.51; H, 5.19; N,
3.77; C26H32N4O4Pt (2c) C, 47.35; H, 4.89; N, 8.49.
Found: C, 47.34; H, 4.86; N, 8.88.
2
2
CH, JPt–H ¼ 108 Hz), 4.70 (s, 1H, Pt–CH, JPt–H ¼ 100
Hz), 3.98 (s, 3H, OMe), 3.88 (s, 3H, OMe) ppm. Anal.
Calcd for C34H43ClN2O8PtSe (5bCl): C, 44.53; H, 4.73;
N, 3.05. Found: C, 44.79; H, 4.75; N, 2.99. Calcd for
C34H43BrN2O8PtSe (5bBr): C, 42.47; H, 4.51; N, 2.91.
Found: C, 42.12; H, 4.38; N, 3.01. Calcd for
C32H37ClN4O4PtSe (5cCl): C, 45.16; H, 4.38; N, 6.58.
Found: C, 45.30; H, 4.47; N, 6.89. Calcd for
C32H37BrN4O4PtSe (5cBr): C, 42.92; H, 4.16; N, 6.26.
Found: C, 43.11; H, 4.25; N, 6.27.
2.2. Synthesis of type 5 complexes [Pt{CH(CO2Me)2}
(Me)(SePh)(X)(Bu2-bipy)] (X ¼ Cl, 5aCl; X ¼ Br, 5aBr)
The phenylselenyl halide (0.11 mmol) was added to a
solution of the precursor 2a (0.070 g, 0.11 mmol) in
dichloromethane (1.5 ml). After stirring for 5 min at
room temperature, addition of diethyl ether afforded the
deep rose-violet product, which was washed with hexane
2.4. Behaviour in solution of type 5b and 5c complexes
1
and dried (yield: 80%). Selected H [13C]NMR data (d;
3
Compound 5bCl (0.10 g, 0.11 mmol) was dissolved in
toluene (2 ml), and change of the colour of solution
from orange to yellow was observed. After 3 h, the
products of the reductive elimination [PtCl{CH(CO2
Me)2}(Bu2-bipy)] and PhSeCH(CO2Me)2 were isolated
through filtration on a silica gel column, eluting first
with dichloromethane and then with dichloromethane/
methanol (methanol 5% v/v, yield: 70%). An analogous
295 K): 5aCl 10.58 (d, 1H, NCH, JPt–H ¼ 13 Hz), 8.84
(d, 1H, NCH, 3JPt–H ¼ 26 Hz), 6.94 (t, 1H, 4H-Ph), 6.68
(d, 2H, 2,6-H-Ph), 6.48 (t, 2H, 3,5-H-Ph), 4.18 (s, 1H,
Pt–CH, 2JPt–H ¼ 75 Hz, [19.5 (1JPt–C ¼ 600 Hz)]), 3.82 (s,
3H, OMe), 3.78 (s, 3H, OMe), 2.18 (s, 3H, Pt–Me,
2JPt–H ¼ 62 Hz [2.9 (1JPt–C ¼ 540 Hz)]) ppm; 5aBr 10.48
3
(d, 1H, NCH, JPt–H ¼ 14 Hz), 8.80 (d, 1H, NCH,
3JPt–H ¼ 17 Hz), 6.94 (t, 1H, 4H-Ph), 6.62 (d, 2H, 2,6-H-

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A. Panunzi et al. / Inorganic Chemistry Communications 6 (2003) 1282–1286
behaviour was observed by aging NMR samples of 5bBr
1
½PtCl2ðBu2-bipyÞꢀ þ 2KCHðCO2MeÞðCNÞ
and 5cCl. Selected H NMR [13C] data (d; 295 K): [Pt
¼ ½PtfCHðCO2MeÞðCNÞg2ðBu2-bipyÞꢀð2cÞ þ 2KCl
Cl{CH(CO2Me)2}(Bu2-bipy)]: 10.18 (d, 1H, NCH, 3JPt–H
¼
ð4Þ
3
39 Hz), 9.60 (d, 1H, NCH, JPt–H ¼ 26 Hz), 5.18 (s, 1H,
2
Pt–CH, JPt–H ¼ 122 Hz, [13.7 (1JPt–C ¼ 574 Hz)]), 3.59
The yellow complexes fairly dissolve in chlorinated
solvents, also thanks to the presence of Bu2-bipy as
chelate, which is known [6] to improve the solubility of
the complexes in the common organic solvents.
(s, 6H, OMe) ppm; [PtBr{CH(CO2Me)2}(Bu2-bipy)]:
10.18 (d, 1H, NCH, 3JPt–H ¼ 42 Hz), 9.86 (d, 1H, NCH),
5.52 (s, 1H, Pt–CH, 2JPt–H ¼ 126 Hz), 3.72 (s, 6H, OMe)
ppm; [PtCl{CH(CO2Me)(CN)}(Bu2-bipy)]: 9.57 (d, 1H,
3
3
The NMR spectral features in CDCl3 are in full
agreement with the proposed structures. It should be
noted that two stereogenic carbon atoms are directly
bound to platinum in 2c. This means that the com-
pound can exist as racemate (RR/SS) and as meso
molecule (RS). Remarkably, the NMR spectrum
clearly shows the prevalence of one isomer (5:1 ratio),
thus indicating that formation of 2c according to Eq.
(4) occurs with a significant degree of stereoselectivity.
It is likely that the configuration of the first entered
hydrocarbyl group directs the geometry of the second
incoming carbanion. However, (at least at this stage,
see below) the NMR features are not sufficient for
identifying the geometry of the more abundant isomer.
In fact, due to the symmetry (C2 for the racemate and
Csfor the meso), in both isomers the two hydrocarbyl
groups are NMR equivalent, as well as the two halves
of the chelate.
NCH, JPt–H ¼ 42 Hz), 9.38 (d, 1H, NCH, JPt–H ¼ 31
2
Hz), 4.57 (s, 1H, Pt–CH, JPt–H ¼ 125 Hz, [)1.8
(1JPt–C ¼ 627 Hz)]), 3.62 (s, 6H, OMe) ppm. Anal. Calcd
for C23H31ClN2O4Pt ([PtCl{CH(CO2Me)2}(Bu2-bipy)]):
C, 43.85; H, 4.96; N, 4.45. Found: C, 43.68; H, 4.92; N,
4.57.
3. Results and discussion
The three platinum(II) complexes used as substrates
(2a–c) are shown in Scheme 1. The N,N-chelate is 4,40-
(tert-butyl)2-2,20-bipyridine (Bu2-bipy). Complex 2a is a
mixed bis-hydrocarbyl compound containing a Me and
a –CH(CO2Me)2 group. Instead, 2b and 2c display, re-
spectively, two –CH(CO2Me)2 and two –CH(CO2Me)
(CN) groups. The compounds can be obtained by
treating the parent halo-precursors with the appropriate
malonic derivative under basic conditions [7], as previ-
ously reported for 2a [4]. E.g.
Precursors of type 2 readily react with phenylsele-
nium halides PhSe–X (X ¼ Cl, Br) in chloroform or
toluene (Scheme 1), as revealed by the rapid change of
Scheme 1.

A. Panunzi et al. / Inorganic Chemistry Communications 6 (2003) 1282–1286
1285
colour of the reaction mixture from yellow to orange.
Addition of hexane affords the products (5) as orange
powders.
Within a couple of hours an irreversible reductive
elimination is complete, with formation of the new
square-planar Pt(II) compounds [Pt{CH(CO2Me)2}(X)
(Bu2-bipy)] [9] and the organo-selenium species PhS-
eCH(CO2Me)2 [10]. When X ¼ Cl, both compounds
have been isolated by filtration on a short column of
Silica gel. An analogous reaction involving 5cCl has
been monitored by NMR spectroscopy, although no
efforts have been made to separate the products of the
elimination.
The just described solution behaviour can be inter-
preted by assuming that the presence of two electron-
poor hydrocarbyl groups destabilises the oxidation state
IV in 5b and 5c species, and favours the reduction of the
metal centre to a lower oxidation state. We are not
aware of a similar coupling prompted by Pt(IV) species,
while C–Se bond formation has been previously ob-
served in the chemistry of octahedral Pd(IV) compounds
[1i,1j]. On the other hand, the co-presence of a donor
methyl group and an electron poor –CH(CO2Me)2 li-
gand apparently allows a stable electronic balance
within 5aCl and 5aBr.
According to the relative position of the substituents
in the coordination sphere, type 5 products may give
rise to several isomers (up to six in the case of 5aCl
and 5aBr). However, high stereoselectivity is observed:
if the equatorial plane is defined by the N,N-chelate,
only the isomers showing –X and –SePh in axial po-
sition are detectable in solution. This geometry can be
formulated on the grounds of the NMR features. We
note:
(i) In all cases, the phenyl protons of the –SePh ligand
undergo high-field shift with respect to the parent
X–SePh molecule (ca. 1 ppm), which has been typ-
ically observed when this group occupies one axial
position [1j,1r,3,4].
(ii) For 5aCl and 5aBr, the chemical shift of Pt–Me (d
2.18 and 2.15, respectively) clearly identifies its posi-
tion in the equatorial plane defined by an aromatic
N,N-chelate [8]. Furthermore, the –CH(CO2Me)2
resonances (d 4.12 and 4.07, respectively) are
very close to that observed in the related type 4
compound [Pt{CH(CO2Me)2}(Me)(SePh)2(Bu2-bipy)]
[4], where this substituent is in the equatorial plane.
(iii) Also complexes 5bCl and 5bBr clearly display the
hydrocarbyl groups in the equatorial plane, as
indicated by the NMR equivalence of the two
halves of Bu2-bipy.
(iv) Due to the geometrical isomerism in the parent
complex 2c (see above), 5cCl should exist as dia-
stereomers as well. Anyway, the NMR spectrum
reveals that only the more abundant one crystal-
lises from the reaction mixture. Both the two
halves of the chelating ligand and the two hydro-
carbyl –CH(CO2Me)(CN) groups are not NMR
equivalent. The methinic signals are very close (d
4.76 and 4.69), which suggests that they lie in
the equatorial plane (since one axial position is al-
ready assigned to –SePh, see point (i). The just
mentioned lack of equivalence indicates that the
stereogenic carbon atoms must display the same
configuration. In fact, if the configuration had
been RS, the complex would have shown a mirror
plane bisecting the chelating ligand. This means
that the major isomer in 2c (see above) is the ra-
cemic molecule.
4. Conclusion
This preliminary communication reports the synthe-
sis of new bis-hydrocarbyl square-planar Pt(II) com-
pounds [PtR(R0)(N,N-chelate)] and their reaction with
phenylselenyl halides, PhSe–X. The addition is in all
cases quantitative, by contrast with the behaviour of
related diselenides, e.g., MeSe–SeMe or PhSe–SePh [4],
which react with the analogous precursors establishing
an equilibrium in solution.
High stereoselectivity has been observed, and six oc-
tahedral Pt(IV) products bearing –X and –SePh ligands
in trans position have been obtained. The stability of the
compounds seems related to the electronic properties of
the hydrocarbyl ligands. When both are electron-poor,
the Pt(IV) species undergo an irreversible reductive
elimination of a Se–C bond, which affords organosele-
nim molecules and Pt(II) compounds.
The synthesis and the study of the properties of sev-
eral Pt(IV) compounds of type 5 will be pursued in the
next future by using different R and R0 substituents.
(v) Analogous considerations hold true for 5cBr, al-
though the signals of the major reaction product
are accompanied by those of a minor species
(<10%), conceivably the diastereomer arising from
the meso form of 2c.
Complexes of type 5a are stable in solution and do
not appreciably decompose after several hours. On the
other hand, species 5b undergo a slow and clean trans-
formation, as depicted by paths (iv) and (v) in Scheme 1.
Acknowledgements
The authors thank the MURST (Programmi di Ric-
erca Scientifica di Rilevante Interesse Nazionale, Cof-
inanziamento 2002–2003) for financial support, and the
Centro Interdipartimentale di Metodologie Chimico-
ꢀ
Fisiche, Universita di Napoli ‘‘Federico II’’ for NMR
facilities.

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A. Panunzi et al. / Inorganic Chemistry Communications 6 (2003) 1282–1286
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