Syntheses and structures of platinum siloxides bridged by a sulfur or selenium atom and a unique 1,3-aryl migration from silicon to platinum through the Si-O-Pt linkages

Article abstract of DOI:10.1021/om800210g

Platinum siloxides bridged by a sulfur or selenium atom, [Tbt(Mes)Si(μ-O)(μ-E)Pt(PPh₃)₂] (la: E = S, lb: E = Se), were synthesized by utilizing hydroxysilanechalcogenols, Tbt(Mes)Si(OH)(EH) (2a: E=S, 2b: E = Se), which were kinetically stabilized by an effective combination of steric protection groups, 2,4,6-tris[bis(trimethylsilyl)methyI] phenyl (Tbt) and mesityl (Mes), as key building blocks. Platinum siloxide 1 underwent a unique 1,3-migration of the Mes group from the silicon atom to the platinum center on treatment with chloride ion in the presence of a Bronsted acid, affording the corresponding chloro(hydroxysilane) chalcogenolatoplatinums, [TbtSi(OH)(Cl)(μ-E)Pt(Mes)(PPh₃) ₂] (3a: E=S, 3b: E = Se), in moderate yields.

Full text of DOI:10.1021/om800210g

2156
Organometallics 2008, 27, 2156–2158
Syntheses and Structures of Platinum Siloxides Bridged by a Sulfur
or Selenium Atom and a Unique 1,3-Aryl Migration from Silicon to
Platinum through the Si-O-Pt Linkages
Taro Tanabe, Yoshiyuki Mizuhata, Nobuhiro Takeda,† and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto UniVersity, Gokasho, Uji, Kyoto 611-0011, Japan
ReceiVed March 7, 2008
Chart 1. Overcrowded Silanedichalcogenols
Summary: Platinum siloxides bridged by a sulfur or selenium
atom, [Tbt(Mes)Si(µ-O)(µ-E)Pt(PPh3)2] (1a: E ) S, 1b: E )
Se), were synthesized by utilizing hydroxysilanechalcogenols,
Tbt(Mes)Si(OH)(EH) (2a: E ) S, 2b: E ) Se), which were
kinetically stabilized by an effectiVe combination of steric
protection groups, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl
(Tbt) and mesityl (Mes), as key building blocks. Platinum
siloxide 1 underwent a unique 1,3-migration of the Mes group
from the silicon atom to the platinum center on treatment with
chloride ion in the presence of a Brønsted acid, affording the
corresponding chloro(hydroxysilane)chalcogenolatoplatinums,
[TbtSi(OH)(Cl)(µ-E)Pt(Mes)(PPh3)2] (3a: E ) S, 3b: E ) Se),
in moderate yields.
reports on similar 1,3-migration reactions of metal siloxides,
i.e., silicon analogues of metal alkoxides, though several
catalytic processes are known to involve a 1,3-migration reaction
of a metal siloxide as a transmetalation step in their plausible
mechanisms, such as palladium(0)-catalyzed cross-coupling
reactions of organosilanols with aryl halides.5,6 On the other
hand, recently, we have succeeded in the synthesis and the first
structural characterization of overcrowded silanedichalcogenols,
such as silanedithiol [Tbt(Mes)Si(SH)2], hydroxysilanethiol [2a:
Tbt(Mes)Si(OH)(SH)], and hydroxysilaneselenol [2b: Tbt(Mes-
)Si(OH)(SeH)], bearing an effective combination of steric
protection groups on the silicon atom, i.e., 2,4,6-tris[bis(trim-
ethylsilyl)methyl]phenyl (Tbt7,8) and mesityl (Mes) (Chart 1).9
Furthermore, we have reported the application of the si-
lanedithiol in the synthesis of novel silanedithiolato complexes
of palladium and platinum.9b This finding prompted us to
investigate the utility of 2a and 2b in the synthesis of metal
The chemistry of late transition metal alkoxides has been
extensively studied by organic and inorganic chemists, and their
wide application to various organometallic reactions has been
reported.1 In particular, 1,3-migration reaction of a carbon
substituent in late transition metal alkoxides through their
C-O-M linkages is involved as an important step in various
catalytic processes.2,3 Hartwig and his co-workers have reported
the stoichiometric 1,3-aryl migration reaction of an isolated
Rh(I) alkoxide, that is, the intramolecular transmetalation from
carbon to rhodium through the C-O-Rh linkage.4a They have
also reported similar intramolecular transmetalation of a Rh(I)
boronate with 1,3-aryl migration from boron to rhodium through
the B-O-Rh linkage.4b Both of these systems afforded the
corresponding CdO or BdO double-bond products or their
oligomerization products. By contrast, there have been very few
(4) (a) Zhao, P.; Incarvito, C. D.; Hartwig, J. F. J. Am. Chem. Soc. 2006,
128, 3124. (b) Zhao, P.; Incarvito, C. D.; Hartwig, J. F. J. Am. Chem. Soc.
2007, 129.
* To whom correspondence should be addressed. Phone: +81-774-38-
3200. Fax: +81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp.
† Present address: Department of Chemistry and Chemical Biology,
Graduate School of Engineering, Gunma University, 1-5-1, Tenjin-cho,
Kiryu, Gunma 376-8515, Japan.
(1) For reviews, see: (a) Ryndza, H. E.; Tam, W. Chem. ReV. 1998, 88,
1163. (b) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry; Univer-
sity Science Books: Mill Valley, CA, 1989; pp 59-61. (c) Crabtree, R. H.
The Organometallic Chemistry of the Transition Metals, 2nd ed.; Wiley:
New York, 1994; pp 57-59.
(2) (a) Nishimura, T.; Uemura, S. Synlett 2004, 201. (b) Matsuda, T.;
Makino, M.; Murakami, M. Org. Lett. 2004, 6, 1257. (c) Matsuda, T.;
Makino, M.; Murakami, M. Angew. Chem., Int. Ed. 2005, 44, 4608. (d)
Terao, Y.; Wakui, H.; Satoh, T.; Miura, M.; Nomura, M. J. Am. Chem.
Soc. 2001, 123, 10407. (e) Terao, Y.; Wakui, H.; Nomoto, M.; Satoh, T.;
Miura, M.; Nomura, M. J. Org. Chem. 2003, 68, 5236. (f) Terao, Y.;
Nomoto, M.; Satoh, T.; Miura, M.; Nomura, M. J. Org. Chem. 2004, 69,
6942. (g) Wakui, H.; Kawasaki, S.; Satoh, T.; Miura, M.; Nomura, M J. Am.
Chem. Soc. 2004, 126, 8658, and references therein.
(5) (a) Denmark, S. E.; Sweis, R. F. J. Am. Chem. Soc. 2001, 123, 6439.
(b) Denmark, S. E.; Sweis, R. F. Org. Lett. 2000, 2, 565. (c) Denmark,
S. E.; Sweis, R. F. Org. Lett. 2001, 3, 61. (d) Denmark, S. E.; Sweis, R. F.
Chem. Pharm. Bull. 2002, 50, 1531. (e) Denmark, S. E.; Sweis, R. F. J. Am.
Chem. Soc. 2004, 126, 4865.
(6) For reviews on metal-catalyzed cross-coupling reaction of organo-
silicon compounds to organic halides, see: (a) Hiyama, T. In Metal-Catalyzed
Cross Coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH:
Weinheim, 1998; Chapter 10. (b) Hiyama, T.; Hatanaka, Y. Pure. Appl.
Chem. 1994, 66, 1471. (c) Denmark, S. E.; Sweis, R. F. Acc. Chem. Res.
2002, 35, 835.
(7) (a) Okazaki, R.; Unno, M.; Inamoto, N. Chem. Lett. 1987, 2293. (b)
Okazaki, R.; Tokitoh, N.; Matsumoto, T. In Synthetic Methods of Orga-
nometallic and Inorganic Chemistry; Herrmann, W. A., Ed.; Thieme: New
York, 1996; Vol. 2, pp 260-269.
(8) For recent reviews on the synthesis of highly reactive organosilicon
compounds kinetically stabilized by Tbt group, see:(a) Okazaki, R.; Tokitoh,
N. Acc. Chem. Res. 2000, 33, 625. (b) Tokitoh, N.; Okazaki, R. AdV.
Organomet. Chem. 2001, 47, 121. (c) Tokitoh, N. Acc. Chem. Res. 2004,
37, 86. (d) Tokitoh, N. Bull. Chem. Soc. Jpn. 2004, 77, 429. (e) Sasamori,
T.; Tokitoh, N. Organometallics 2006, 25, 3522.
(3) For recent reviews on transition-metal-catalyzed transformations that
involve 1,3-migration of the carbon substituents, see: (a) Satoh, T.; Miura,
M. Top. Organomet. Chem. 2005, 14, 1. (b) Jun, C.-H.; Moon, C. W.; Lee,
D.-Y. Chem.-Eur. J. 2002, 8, 2422. (c) Murakami, M.; Ito, Y. In ActiVation
of UnreactiVe Bonds and Organic Synthesis; Murai, S., Ed.; Springer: New
York, 1999; pp 97-129.
(9) (a) Takeda, N.; Tanabe, T.; Tokitoh, N. Bull. Chem. Soc. Jpn. 2006,
79, 1573. (b) Tanabe, T.; Takeda, N.; Tokitoh, N. Eur. J. Inorg. Chem.
2007, 1225. (c) Takeda, N.; Tokitoh, N. Synlett 2007, 2483.
10.1021/om800210g CCC: $40.75
2008 American Chemical Society
Publication on Web 04/12/2008

Communications
Organometallics, Vol. 27, No. 10, 2008 2157
Scheme 1. Synthesis of Platinum Siloxides Bridged by a
Sulfur (1a) or Selenium (1b) Atom
When 1a or 1b was treated with 1 molar equiv of acetic acid,
the protonation of their oxo units was confirmed by NMR
spectroscopy, but the resulting oxonium salts remained un-
changed. Tatsumi et al. have recently reported a similar
reactivity of germanium analogues, where they have isolated
the corresponding oxonium salts.12 The addition of LiCl to the
THF solutions of the reaction mixtures of 1a or 1b with acetic
acid resulted in the formation of the corresponding chloro(hy-
droxy)silanechalcogenolatoplatinum complexes 3a,b, in which
the mesityl group had migrated from the silicon atom to the
platinum metal center. The resulting products were formed in
moderate yield (Scheme 2). Similarly, the reaction of 1b with
1 molar equiv of anhydrous HCl (Et2O solution) followed by
the addition of an excess of water produced directly the
correspoding dihydroxysilaneselenolatoplatinum complex 4b,
the formation of which is most likely rationalized in terms of
the hydrolysis of the initially generated 3b.13 The structures of
3a,b and 4b were confirmed by NMR and mass spectroscopy
together with elemental analysis, and that of 4b was determined
by X-ray crystallographic analysis (Figure 2).14
Although the formation of 3a,b from 1a,b can be explained
as the result of 1,3-migration of the mesityl group from the
silicon to platinum atom through the Si-O-Pt linkages,15 it
was found that treatment of 1 with a Brønsted acid and chloride
ion is indispensable for the transformation of 1a,b to 3a,b. In
general, the 1,3-migration reaction of a hydrogen atom or carbon
substituent in late transition metal alkoxides is known to produce
the corresponding carbonyl compounds as the elimination
products.2–4 However, 1a,b were found to be thermally and
photochemically stable, and they did not undergo any trans-
metalation or hydrolysis under the conditions used even after
the workup procedure with water. Therefore, this result ruled
out the reaction pathway via the initial 1,3-migration (thermal
or photochemical) of the mesityl group giving the corresponding
siloxides bridged by a sulfur or selenium atom with a view to
elucidate their reactivities as metal siloxides. We describe here
the syntheses and structural characterization of the novel
platinacycles [Tbt(Mes)Si(µ-O)(µ-E)Pt(PPh3)2] (1a: E ) S, 1b:
E ) Se), in which we utilized stable hydroxysilanechalcogenols,
Tbt(Mes)Si(OH)(EH) (2a: E ) S, 2b: E ) Se),9b as key
reagents. In addition, we have found a unique 1,3-migration
reaction of the Mes group from the silicon to the platinum atoms
in the reactions of 1 with chloride ion in the presence of a
Brønsted acid.
The reactions of 2a or 2b with 2 molar equiv of sodium
hydride followed by the addition of a THF solution of
cis-[PtCl2(PPh3)2] afforded the corresponding platinum siloxides
bridged by a sulfur or selenium atom, 1a (89%) or 1b (90%),
respectively, as air- and moisture-stable pale yellow crystals
(Scheme 1). When the reactions were performed by utilizing
n-BuLi instead of sodium hydride, the same products were
obtained in similar yields.
The structures of platinacycles 1a and 1b were determined
by NMR spectroscopic data, elemental analyses, and X-ray
crystallographic analyses (Figure 1). The core four-membered
rings of 1a and 1b were found to have a puckered geometry,
and their central platinum atoms deviated slightly from tetrago-
nal planar geometry, which is typical for neutral, four-
coordinated Pt(II) complexes10 [sum of the interior bond angles
of the central four-membered rings: for 1a, 356.20°; for 1b,
355.98°; sum of the bond angles around the platinum atoms:
for 1a, 351.68°; for 1b, 361.01°; dihedral angles between the
E(1)-Pt(1)-O(1) and P(1)-Pt(1)-P(2) planes: for 1a (E )
S), 9.99(11)°; for 1b (E ) Se), 10.97(12)°]. The lengths of the
Pt-P bonds [1a: 2.2327(9) Å; 1b: 2.2324(12) Å] situated
opposite the Pt-O bonds are slightly shorter than those situated
opposite the Pt-S [for 1a: 2.2758(10) Å] or Pt-Se [for 1b:
2.2835(12) Å] bonds, resulting from the trans influence of
chalcogen atoms. Therefore, their 31P{1H} NMR spectrum
silicon-oxygen
double-bond
species,
[TbtSi(dO)-
EPt(Mes)(PPh3)2] (5), followed by hydrolysis. On the other
hand, the prerequisite activation of a silicon atom for the
transmetalation of organosilanes is known to generate a five-
coordinate silicate by the addition of some activator such as
fluoride ion.16 Taking this information into account, the
transformation of 1 to 3 should be interpreted in terms of the
intermediacy of five-coordinate silicates generated by the
nucleophilic attack of chloride ion on the silicon atom of the
initially formed oxonium complex. Then, the Mes group on the
silicon atom should migrate to the electron-deficient platinum
center to afford the stable, neutral platinum complexes 3. Since
the significant breakthrough of the palladium-catalyzed orga-
nosilicon cross-coupling reactions that takes advantage of the
addition of a nucleophilic promoters developed by Hiyama and
1
showed the two signals with apparently different JPPt values
[1a: 6.9 (1JPPt ) 3532 Hz), 25.1 (1JPPt ) 3209 Hz) ppm; 1b:
6.2 (1JPPt ) 3537 Hz), 24.7 (1JPPt ) 3206 Hz)]. In both cases,
(12) (a) Matsumoto, T.; Tatsumi, K. Chem. Lett. 2001, 964. (b)
Matsumoto, T.; Nakaya, Y.; Tatsumi, K. Organometallics 2006, 25, 4835.
(13) Isolated 3b was converted into 4b in 90% yield by reaction with
an excess of H2O in diethyl ether.
(14) In contrast to the highly strained geometry around the platinum
atom in the structures of 1a,b, the platinum atom of 4b has an almost planar
geometry due to the release of the ring strain and steric repulsion, which
naturally resulted in the increase of the Si-Se-Pt angle of 4b [111.96(10)°]
as compared with that of 1b [75.32(10)°].
(15) Although there have been some reports on the catalytic reactions
involving 1,3-migration of carbon substituents, such as an alkenyl group,
from a silicon atom to a transition metal center in metal siloxides,
stoichiometric reactions with such a migration have not been reported so
far. See ref 5.
(16) Pioneering studies by Kumada and Hiyama have shown that the
addition of an activator such as fluoride ion activates organosilicon
compounds to afford the corresponding five-coordinate silicate species,
which promote the transition-metal-catalyzed cross-coupling reactions with
various organic halides. See ref 6.
1
the signals at higher field have larger JPPt coupling constants
than those at lower field, and the former are assigned to the
phosphorus atoms situated in trans position to the oxygen atom
and the latter to those in trans position to the sulfur or selenium
atom. In this system, the trans influence of the sulfur and
selenium atoms on the silicon atom was similar, but larger than
that of the oxygen atom. This tendency is essentially consistent
with that reported for some related platinum chalcogenides.11
(10) (a) Hartley, F. R. The Chemistry of Platinum and Palldium; Applied
Science: London, 1973. (b) Hartley, F. R. ComprehensiVe Organometallic
Chmistry; Pergamon Press: Oxford, 1982; Vol. 6, p 471.
(11) (a) Xu, C.; Siria, J. W.; Anderson, G. K. Inorg. Chim. Acta 1993,
206, 123. (b) Nagata, K.; Takeda, N.; Tokitoh, N. Bull. Chem. Soc. Jpn.
2003, 76, 1577.

2158 Organometallics, Vol. 27, No. 10, 2008
Communications
Figure 1. ORTEP drawings of [1a · hexane] (left) and [1b · 2(benzene)] (right) (50% probability). The hydrogen atoms and solvents were
omitted for clarity. Selected bond lengths [Å] and angles [deg]: for 1a, Si(1)-S(1) 2.1708(13), Si(1)-O(1) 1.636(3), Pt(1)-S(1) 2.3495(9),
Pt(1)-O(1) 2.076(2), Pt(1)-P(1) 2.2397(9), Pt(1)-P(2) 2.2758(10), S(1)-Si(1)-O(1) 97.07(10), Si(1)-O(1)-Pt(1) 100.02(12), Si(1)-S(1)-Pt(1)
78.34(4), S(1)-Pt(1)-O(1) 80.77(7), S(1)-Pt(1)-P(1) 87.48(3), P(1)-Pt(1)-P(2) 99.30(4), P(2)-Pt(1)-O(1) 93.27(7); for 1b, Si(1)-Se(1)
2.3152(12), Si(1)-O(1) 1.635(3), Pt(1)-Se(1) 2.4565(5), Pt(1)-O(1) 2.067(3), Pt(1)-P(1) 2.2324(12), Pt(1)-P(2) 2.2835(12), Se(1)-Si(1)-O(1)
95.82(11), Si(1)-O(1)-Pt(1) 103.37(14), Si(1)-Se(1)-Pt(1) 75.32(3), Se(1)-Pt(1)-O(1) 81.47(8), Se(1)-Pt(1)-P(1) 87.00(3), P(1)-Pt(1)-P(2)
99.71(4), P(2)-Pt(1)-O(1) 92.83(8).
are very important, as they provide the first experimental
demonstration of the transmetalation of a carbon substituent
from a silicon atom to a transition metal center in a metal
siloxide, a process that is postulated as a plausible mechanism
for some silicon-containing catalytic systems, such as the
palladium(0) catalyzed cross-coupling reaction of aryl- or
alkenylsilanols with aryl or vinyl iodides in the presence of
tetrabutylammonium fluoride, reported by Denmark and his co-
workers.5 The key to isolate the 1,3-migration products as
silicon-containing platinum complexes is the use of chalcogen-
bridged platinum siloxides as starting materials.
In summary, we have succeeded in the syntheses of novel
platinacycles 1a,b, by taking advantage of hydroxysilanechal-
Figure 2. ORTEP drawing of 4b (30% probability). The hydrogen
cogenols 2a,b as key building blocks. The structures of these
atoms were omitted for clarity. Selected bond lengths [Å] and angles
complexes were characterized by NMR spectroscopic data and
[deg]: Si(1)-O(1) 1.637(9), Si(1)-O(2) 1.653(8), Si(1)-Se(1)
X-ray crystal structural analysis. Furthermore, unique 1,3-aryl
2.236(4), Pt(1)-Se(1) 2.5009(13), Pt(1)-C(1) 2.043(12), Pt(1)-P(1)
migration reactions of the metal siloxides through their Si-O-Pt
linkages giving chloro(hydroxy)silanechalcogenolato platinums
3a,b were experimentally demonstrated and achieved for the
first time. Further investigation on the reaction mechanism and
applications toward the catalytic systems are currently under
way.
2.259(3), Pt(1)-P(2) 2.352(3), Si(1)-Se(1)-Pt(1) 111.96(10),
Se(1)-Pt(1)-C(1)89.7(3),C(1)-Pt(1)-P(1)89.0(3),P(1)-Pt(1)-P(2)
97.11(11), P(2)-Pt(1)-Se(1) 169.41(9).
Scheme 2. 1,3-Migration Reactions of the Mes Group from
1a,b
Acknowledgment. This work was partially supported by
Grants-in-Aid for Creative Scientific Research (No. 17GS0207)
and the Global COE Program (B09, “Integrated Material
Science”) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Supporting Information Available: X-ray crystallographic data
of 1a,b and 4b in CIF format, experimental procedures, and spectral
data. These materials are available free of charge via the Internet
at http://pubs.acs.org. Crystallographic data for 1a, 1b, and 4b have
also been deposited with the Cambridge Crystallographic Data
Centre, as CCDC Nos. 673348, 673349, and 673347, respectively.
his co-workers, wide ranging investigations of various func-
tionalized organosilicon compounds have been described.6 It
should be noted that the 1,3-migration reactions described here
OM800210G