[{Pt(CO)3}2]2+: The first homoleptic, dinuclear, cationic platinum(I) carbonyl complex formed in concentrated sulfuric acid

Article abstract of DOI:10.1002/(SICI)1521-3773(20000103)39:1<208::AID-ANIE208>3.0.CO;2-J

Prolonged evacuation of [{Pt(CO)₃}₂]²⁺ (1), the first homoleptic, dinuclear, cationic platinum(I) carbonyl complex, results in reversible disproportionation. Complex 1 was formed by dissolution of PtO₂ in concentrated H₂SO₄ under an atmosphere of CO [Eq. (a)], and completely characterized by NMR (¹³C, ¹⁹⁵Pt), IR, and Raman spectroscopy.

Full text of DOI:10.1002/(SICI)1521-3773(20000103)39:1<208::AID-ANIE208>3.0.CO;2-J

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2
‡
We now report on the preparation and spectroscopic
characterization of the first homoleptic, dinuclear, cationic
[
{Pt(CO) } ] : The First Homoleptic,
3
2
Dinuclear, Cationic Platinum(i) Carbonyl
Complex Formed in Concentrated Sulfuric
Acid**
2‡
platinum(i) carbonyl complex, [{Pt(CO) } ] (1), in which the
3
2
Pt atoms are bound by a direct Pt�Pt bond. This unusual
complex is formed as a result of the reductive carbonylation of
Qiang Xu,* Yoshie Souma, Brian T. Heaton,*
Chacko Jacob, and Kan Kanamori
PtO by CO in concentrated sulfuric acid at room temperature
and atmospheric pressure. The reaction takes about two
weeks to complete [Eq. (1)], producing a colorless solution of
2
In the last ten years there has been a rapid development in
the preparation and structural characterization of homoleptic
metal carbonyl cations of late transition metals, following the
discovery that they can be prepared in superacid media.
[
1]
1. Prolonged (ca. 1 d) evacuation of a solution of 1 in
This area has recently been reviewed and, apart from the
concentrated H SO4 results in disproportionation and the
CO-bridged dinuclear palladium(i) complex [Pd (m-CO) ]-
2
2
2
2‡
[
2a]
exclusive formation of cis-[Pt(CO) ] (solv) (2), but slow
(
SO F)2
and the unstable dinuclear mercury(i) carbonyl
2
3
2‡ [2b]
reformation of 1 occurs on addition of CO [Eq. (2)]. Both 1
and 2 are extremely sensitive to moisture, exposure results in
immediate decomposition to platinum metal.
cation [Hg (CO) ] , all the complexes are monomeric with
values of nÄ (CO) significantly higher than 2143 cm , the value
for free CO. In the 1970s the homoleptic, anionic platinum
�
carbonyl clusters [Pt (CO) ]
6 n
2
2
�
1
[
3]
2
[4a]
(n ˆ 1 ± 6, ꢀ10)
and
3
[4b]
4
�
[
Pt (CO) (m -CO) ] , in which the Pt atoms are bridged
1
9
12
2
10
by CO ligands, were reported. In 1993 the first homoleptic
carbonyl complex of platinum(ii), [Pt(CO) ][Pt(SO F) ], was
4
3
6
isolated from the incomplete reduction of Pt(SO F) with CO
The formulation of 1 as a dimer is shown from both 13C and
Pt NMR studies at natural abundance and 99% CO
3
4
[
5a]
195
13
at 258C. Subsequently, the complex [Pt(CO) ][Sb F ] was
4
2
11 2
prepared by solvolysis of cis-[Pt(CO) (SO F) ], which is
enrichment (Table 1). According to NMR simulations the
observed spectra are in agreement with the presence of a
2
3
2
[5b]
formed by complete reduction of Pt(SO F) with CO
in
3
4
[
5c]
195
195
liquid SbF .
Both the vibrational and NMR spectra of
T-shaped Pt(CO)3 group and the presence of
spin ± spin coupling (550.9 Hz).
Pt� Pt'
5
[
Pt(CO)4]2 and cis-[Pt(CO) (SO F) ] are entirely consistent
‡
2
3
2
with a square-planar coordination due to Pt adopting a ‡2
Furthermore, in the Raman spectrum of 1, there is clear
evidence for a very strong, sharp band due to nÄ (Pt�Pt) at
oxidation state.
Table 1. Spectroscopic properties of 1, 2, and related complexes.
nÄ (CO)IR [cm 1]
�
nÄ (CO)Raman [cm ]
� 1
d( C)
13
d(195Pt)
1J(Pt,C) [Hz]
Ref.
Compound
‡
2233, 2209, 2194, 2173[b]
2219, 2182
2218, 2181
2172, 2131
2289, 2267
158.7,[c] 166.3[d]
133.7
131
151.6
137
1595.7,[c] 1281.5[d,e]
[
{Pt(CO)
3
}
2
]2 1[a]
2195, 2186, 2174
2218, 2182
2219, 2185
2178, 2137
2244
� 211.0
[f]
[f]
[5b]
[10]
[5c]
]2 (solv) 2
‡
[a]
957.9
1907.3
2011
1576
1550
cis-[Pt(CO)
cis-[Pt(CO)
cis-[Pt(CO)
2
2
2
[
g]
(SO
Cl
3
F)
2
]
[h]
2
]
[
g]
[
Pt(CO)
4
][Sb
2
F
11
]
2
. [b] nÄ (Pt�Pt)Raman ˆ165 cm . [c] For 13C
� 1
; see Figure 1 for labeling scheme. [d] For 13C
[
a] In conc. H
2
SO
4
A
B
. [e] Other coupling constants [Hz]:
)ˆ19.8. [f] This paper. [g] In the solid state.
1
2
2
2
3
3
3
J(Pt,Pt')ˆ550.9, J(Pt,C
A
)ˆ� 26.2, J(Pt,C
B
)ˆ199.6, J(C
A
,C
B
)ˆ0, J(C
A
,C
A
)ˆ0, J(C
A
,C
B
)ˆ0, J(C
B B
,C
[
h] Spectroscopic data listed were obtained in benzene.
[
*] Dr. Q. Xu, Dr. Y. Souma
Osaka National Research Institute, AIST, MITI
�
1
1
65 cm , which is strongly polarized (1 ꢀ 0.25). An EXAFS
1-8-31, Midorigaoka, Ikeda, Osaka 563 ± 8577 (Japan)
study on 1 shows the presence of a Pt�Pt bond of 2.674 Š,
which is approximately 0.1 Š shorter than the Pt�Pt distance
in Pt foil.[ IR and Raman results of 1 (Table 1) are consistent
with a structure of D2d symmetry
Fax : (‡ 81)727-519-629
E-mail: xu@onri.go.jp
6]
Prof. B. T. Heaton, Dr. C. Jacob
Department of Chemistry
The University of Liverpool
Crown Street, Liverpool L69 7ZD (UK)
Fax : (‡ 44)151-794-3540
(
Figure 1), with only terminal
CO groups and a direct Pt�Pt
bond. According to calculations
at the B3LYP level the confor-
mation adopted by this molecule
has a dihedral angle of exactly
08.[ This structure is related to
the structure of [{PtCl (CO)} ]
which contains two slightly dis-
torted T-shaped PtCl (CO)
E-mail: amf@liverpool.ac.uk
Prof. K. Kanamori
Department of Chemistry
Faculty of Science, Toyama University
3190, Gofuku, Toyama 930-8555 (Japan)
6]
9
[
**] The authors acknowledge Prof. T. Eguchi for his continuous support
and helpful discussions. Q.X. is grateful to Drs. K. Mogi and Y.
Ichihashi for valuable help and discussions. B.T.H. thanks the EPSRC
for support and Dr. J. A. Iggo for helpful discussions.
Figure 1. Schematic structure
2
�
2
2
2‡
of [{Pt(CO)
3
}
2
]
(1), along
with the labeling scheme for
the carbonyl groups.
2
208
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
0570-0833/00/3901-0208 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2000, 39, No. 1

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groups with a dihedral angle of 608 between the two planes
and in which both CO groups are cisoid (length of the Pt�Pt
2: The IR spectrum of 1 in 96% H
2 4
SO was monitored with time under
�
1
12
evacuation (0.001 Torr). When the band for 1 (2174 cm for CO and
�
1
13
2
126 cm for CO) had disappeared (ca. 1 d), nitrogen was admitted to
[
7a]
bond 2.584 Š). Other related, crystallographically charac-
terized dinuclear platinum carbonyl species include
the solution, which contained only 2 and a black colloidal precipitate of Pt
metal. The solution of 2 was transferred under nitrogen by canula and used
for all the spectroscopic measurements. This solution is indefinitely stable
under a nitrogen atmosphere.
[
7b]
[7c]
[
{PtCl(CO)(PtBu Ph)} ]
and [{Pt(C F )(CO)(PPh )} ],
2
2
6 5 3 2
which have dihedral angles of 70.18 and 78.68, and Pt�Pt
distances of 2.628 and 2.599(Š), respectively. Preliminary
Received: July 29, 1999 [Z13795]
[
8]
reports have also appeared for [{PtCl(CO)(PPh )} ] and, in
3
2
all cases, the carbonyl groups are cis and the two phosphanes
are trans to the Pt�Pt bond.
[1] H. Willner, F. Aubke, Angew. Chem. 1997, 109, 2506 ± 2530; Angew.
Chem. Int. Ed. Engl. 1997, 36, 2403 ± 2425.
[2] a) C. Wang, M. Bodenbinder, H. Willner, S. Rettig, J. Trotter, F.
Aubke, Inorg. Chem. 1994, 33, 779 ± 786; b) M. Bodenbinder, G.
Balzer-Jöllenbeck, H. Willner, R. J. Batchelor, F. W. B. Einstein, C.
Wang, F. Aubke, Inorg. Chem. 1996, 35, 82 ± 92.
It has been shown that there is no correlation of d(Pt�Pt)
with 1J(Pt,Pt') in closely related dinuclear platinum com-
[
8]
1
plexes. It is worth noting that J(Pt,Pt') for 1 is 550.9 Hz,
2�
[9]
while the values for [{PtCl (CO)} ]
{PtCl(CO)(PPh )} ] (760 Hz) are widely different, although
(5250 Hz)
and
2
2
[
8]
[
[3] K. Nakamoto, Infrared Spectra of Inorganic Compounds and Coor-
dination Compounds, Wiley, New York, 1st ed., 1963, p. 72.
3
2
other related coupling constants are similar (see Table 1 and
refs. [8, 9]).
Prolonged evacuation of 1 results in disproportionation and
formation of 2 through loss of CO [see Eq. (2)]. In this case,
[
4] a) G. Longoni, P. Chini, J. Am. Chem. Soc. 1976, 98, 7225 ± 7231;
b) D. M. Washecheck, E. J. Wucherer, L. F. Dahl, A. Ceriotti, G.
Longoni, M. Manassero, M. Sansoni, P. Chini, J. Am. Chem. Soc. 1979,
101, 6110 ± 6112.
1
3
195
C and Pt NMR measurements on unenriched and 99%
[5] a) G. Hwang, M. Bodenbinder, H. Willner, F. Aubke, Inorg. Chem.
1993, 32, 4667 ± 4669; b) G. Hwang, C. Wang, M. Bodenbinder, H.
Willner, F. Aubke, J. Fluorine Chem. 1994, 66, 159 ± 166; c) G. Hwang,
C. Wang, F. Aubke, H. Willner, M. Bodenbinder, Can. J. Chem. 1993,
1
3
CO-enriched 2 show that there are two magnetically
1
95
195
equivalent carbonyl groups per platinum with no Pt� Pt
coupling; 2 is thus a monomer. We presently favor a square-
planar platinum(ii) center with two CO ligands in a cis
configuration, consistent with both IR and Raman measure-
ments; the CO stretching frequencies are close to those for
7
1, 1532 ± 1536.
[6] K. Mogi, Y. Ichihashi, Q. Xu, unpublished results.
7] a) A. Modinos, P. Woodward, J. Chem. Soc. Dalton Trans. 1975, 1516 ±
520; b) C. Couture, D. H. Farrar, D. S. Fisher, R. R. Gukathasan,
Organometallics 1987, 6, 532 ± 536; c) R. Us o n, J. Forni e s, P. Espinet,
[
1
[
5b]
cis-[Pt(CO) (SO F) ],
[
but higher than those for cis-
2
3
2
Â
C. Fortu nÄ o, M. Tomas, A. J. Welch, J. Chem. Soc. Dalton Trans. 1989,
[
10]
Pt(CO) Cl ]. It is difficult to be sure whether the other
1583 ± 1587.
2
2
two coordination sites on platinum are occupied by a
[8] N. M. Boag, J. Browning, C. Crocker, P. L. Goggin, R. J. Goodfellow,
bidentate SO42 or by two monodentate SO4 /HSO4 groups
�
2�
�
M. Murray, J. L. Spencer, J. Chem. Res. (M) 1978, 2962 ± 2983.
[9] N. M. Boag, P. L. Goggin, R. J. Goodfellow, I. R. Herbert, J. Chem.
since it is impossible to obtain any useful IR or Raman data in
Soc. Dalton Trans. 1983, 1101 ± 1107.
the sulfato region. However, when 1 is in concentrated H SO ,
2
4
[10] J. Browning, P. L. Goggin, R. J. Goodfellow, M. G. Norton, A. J. M.
Rattray, B. F. Taylor, J. Mink, J. Chem. Soc. Dalton Trans. 1977, 2061 ±
2067.
it seems more probable that the other two sites are occupied
�
by monodentate HSO groups as recently found for sil-
4
[
11]
[11] D. B. DellꢁAmico, F. Calderazzo, F. Marchetti, Chem. Mater. 1998, 10,
ver(i).
5
24 ± 530.
The discovery of 1 suggests that homoleptic cationic
carbonyl complexes of late transition metals in low oxidation
states can be formed in media which are less acidic than the
superacids that have been used previously. We found that this
unusual dinuclear platinum carbonyl complex, 1, exhibits high
[
12] Q. Xu, Y. Souma, Top. Catal. 1998, 6, 17 ± 26.
[
12]
catalytic activity for the carbonylation of olefins;
future
Enantioselective Total Synthesis of
Epothilone A Using Multifunctional
Asymmetric Catalyses**
studies will investigate the detailed reaction mechanism of
this catalytic activity and attempts will be made to obtain 1 as
a crystalline salt.
Daisuke Sawada and Masakatsu Shibasaki*
Experimental Section
Epothilones (see Scheme 1 for epothilones A (1) and B (2))
show potent antitumor activity by binding and stabilizing
microtubules in the same way as taxol, and they are promising
drug candidates. Epothilones A and B were isolated from the
Standard canula transfer techniques were used for all sample manipula-
tions. NMR spectra were recorded in D
2
SO
4
at room temperature on a
1
3
Bruker AMX 200. The C chemical shifts were referenced to external
1
95
tetramethylsilane (TMS), and Pt chemical shifts were referenced to
2.8 MHz at such a magnetic field that the protons in external TMS
4
resonate at exactly 200 MHz. NMR simulations were carried out using
gNMR 4.1 (Cherwell Scientific, Oxford, UK). Infrared spectra were
obtained on thin films between two silicon discs on a JASCO FT/IR-230
spectrometer. Raman spectra were recorded on a Nicolet FT-Raman 960
spectrometer.
[*] Prof. Dr. M. Shibasaki D. Sawada
Graduate School of Pharmaceutical Sciences
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax: (‡81)3-5684-5206
E-mail: mshibasa@mol.f.u-tokyo.ac.jp
1
: PtO
2
1
(2 mmol) in 96% H
2
SO
4
(10 mL) was vigorously stirred for 2 weeks
2
under CO at constant pressure (1 atm), whereupon the dark colloidal
suspension became colorless. The resulting solution is very moisture
[**] This study was financially supported by CREST. We thank Dr.
Motomu Kanai, Yoshitaka Hamashima, and Rie Namme for their
support and fruitful discussion.
1
3
sensitive. Complex 1 with 99% CO was prepared similarly.
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209