Dehydrogenative coupling of monohydrosilanes mediated by (2-phosphinoethyl)silylrhodium(I) complex

Article abstract of DOI:10.1246/cl.2001.952

Treatment of HSiMe₂Ph with a square-planar rhodium(I) complex Rh[(κ²-Si,P)-Me₂SiCH₂CH ₂PPh₂](PMe₃)₂ gave rise to selective dehydrogenative coupling of hydrosilanes to afford (SiMe₂Ph)₂ and RhH₂[(κ²-Si,P)-Me₂SiCH₂CH ₂PPh₂](PMe₃)₂.

Full text of DOI:10.1246/cl.2001.952

952
Chemistry Letters 2001
Dehydrogenative Coupling of Monohydrosilanes Mediated by
(2-Phosphinoethyl)silylrhodium(I) Complex
Masaaki Okazaki*, Shin Ohshitanai, Hiromi Tobita,* and Hiroshi Ogino
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578
(Received May 25, 2001; CL-010491)
Treatment of HSiMe2Ph with a square-planar rhodium(I)
is 2.3937(8) Å that is relatively longer than those of the previously
reported silylrhodium(I) complexes (Ph3Si)Rh(PMe3)3 [2.317(1)
Å]9a and (PhMe2Si)Rh(PMe3)3 [2.3804(10) Å].9c The change of the
Rh–Si bond lengths is attributable to the degree of the
M(dπ)–SiX(σ ) interaction.10 When more electron-withdrawing
groups are bonded to silicon, this interaction becomes stronger and,
as a result, the Rh–Si bond becomes shorter.3 The silyl ligand in 2
bears three electron-donating alkyl groups on the silicon atom which
makes the Rh–Si bond of 2 the longest among them. In agreement
with the strong trans-influence of the silyl ligand, the length of the
Rh–P3 bond is longer by 0.05 Å than that of the Rh–P2 bond.
complex Rh[(κ2-Si,P)-Me2SiCH2CH2PPh2](PMe3)2 gave rise to
selective dehydrogenative coupling of hydrosilanes to afford
(SiMe2Ph)2 and RhH2[(κ2-Si,P)-Me2SiCH2CH2PPh2](PMe3)2.
The empirical trans-influencing ability of silyl ligands has
been estimated to exceed both hydrido and alkyl ligands.1
Considering the strong σ-donor and the high trans-influencing abil-
ity of silyl ligands, the silyl ligands are expected to exhibit excep-
tionally strong trans-effect. Therefore, silyl ancillary ligands would
greatly accelerate the generation of the reactive unsaturated metal
center. Nevertheless, to our knowledge, little has been known about
the influence of silyl ancillary ligands on the reactivity of transition-
metal complexes.2 This comes mainly from the facile cleavage of
the metal–silicon bonds via reductive elimination, nucleophilic
attack at the silyl silicon atom, insertion, and σ-bond metathesis.3
To suppress the elimination of silyl ligands, five-membered (2-
phosphinoethyl)silyl chelate ligands have been employed.4,5 In this
communication, we report the synthesis, structure, and reactivity of
Rh[(κ2-Si,P)-Me2SiCH2CH2PPh2](PMe3)2.
Reaction of RhCl(PMe3)3 with HMe2SiCH2CH2PPh2 in
toluene at 50 °C gave RhCl(H)[(κ2-Si,P)-Me2SiCH2CH2PPh2]-
(PMe3)2 (1)6 in 65% yield (eq 1). The 1H, 29Si, and 31P NMR data
support the octahedral mer-geometry of complex 1. The iridium
analogue IrCl(H)[(κ2-Si,P)-Me2SiCH2CH2PPh2](PMe3)2 has been
synthesized by the thermal reaction of [Ir(CO)(PMe3)4]Cl with
HMe2SiCH2CH2PPh2.5c
The reaction 2 may proceed via formation of RhH(Me)[(κ2-
Si,P)-Me2SiCH2CH2PPh2](PMe3)2, but it was not detected spectro-
scopically in the course of the reaction. We previously reported the
synthesis of IrH(Me)[(κ2-Si,P)-Me2SiCH2CH2PPh2](PMe3)2 by the
reaction of IrCl(H)[(κ2-Si,P)-Me2SiCH2CH2PPh2](PMe3)2 with
MeLi.5c Thermolysis of the hydrido(methyl)iridium complex in
toluene at 55 °C led to the elimination of methane to give the
unidentified product(s) with an Ir–H bond. No formation of the
iridium analogue of 2 Ir[(κ2-Si,P)-Me2SiCH2CH2PPh2](PMe3)2 was
observed. This striking difference of the thermal stability between
Rh and Ir is consistent with the general trend: The iridium complex-
es with high oxidation state are more stable than the corresponding
rhodium complexes.11 Complex 2 represents the first example of a
coordinatively unsaturated trialkylsilylrhodium(I) complex. Due to
the strongly electron-releasing character of the trialkylsilyl moiety,
2 may be highly reactive toward various small molecules.
In fact, the reaction of 2 with 6 equiv of HSiMe2Ph proceeded
spontaneously at room temperature to give the hydrido(silyl)rhodi-
um(III) complex 312 quantitatively. After 20 h at room temperature,
the mixture of 3 and HSiMe2Ph was converted to dihydrido com-
plex 413 and (SiMe2Ph)2 in 91% yield each. The 31P{1H} NMR
data established the fac-geometry of 3. The RhH signal in the 1H
NMR spectrum appears at –10.30 ppm as pseudo double quartets
Treatment of 1 with 1 equiv of MeLi at room temperature led
to the almost quantitative formation of a coordinatively unsaturated
silylrhodium(I) complex Rh[(κ2-Si,P)-Me2SiCH2- CH2PPh2]-
(PMe3)2 (2)7 (eq 2). Workup of the resulting solution and recrystal-
lization from toluene at –75 °C gave orange crystals of 2 in 35%
isolated yield. The molecular structure and selected bond lengths
and angles of 2 are shown in Figure 1.8 Complex 2 is among few
silyl rhodium(I) complexes9 and adopts a slightly distorted square
planar geometry. The angles P1–Rh–P2 and P3–Rh–Si are
167.65(3) Å and 176.58(2) Å, respectively. The Rh–Si bond length
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
953
Dedicated to Prof. Hideki Sakurai on the occasion of his 70th
birthday.
References and Notes
1
a) J. Chatt, C. Eaborn, and S. Ibekwe, Chem. Commun., 1966, 700. b)
R. N. Haszeldine, R. V. Parish, and J. H. Setchfield, J. Organomet.
Chem., 57, 279 (1973).
2
3
4
H. Tobita, K. Hasegawa, J. J. G. Minglana, L-S. Luh, M. Okazaki, and
H. Ogino, Organometallics, 18, 2058 (1999).
T. D. Tilley, “The Chemistry of Organic Silicon Compounds,” ed. by S.
Patai and Z. Rappoport, Wiley, New York (1989), p 1415.
(J(HPtrans) = 123.0 Hz, J(HRh) = J(HPcis) = 16.0 Hz). Other NMR
data also support the structure of 3. The dihydrido complex 4 was
independently synthesized by treatment of 1 with LiAlH4 in THF
and characterized based on the results of elemental analysis and
spectroscopic data. Scheme 1 shows a plausible mechanism for the
formation of 4 and (SiMe2Ph)2. The first step is the dissociation of
PMe3 from 3 and then oxidative addition of HSiMe2Ph takes place
to give a silylrhodium(V) intermediate RhH2(SiMe2Ph)2[(κ2-Si,P)-
Me2SiCH2CH2PPh2](PMe3) (A). A similar silylrhodium(V) com-
plex to A has been reported by Nagashima et al.14 Complex A
undergoes reductive elimination of disilane and subsequent ligation
of PMe3 to Rh gives 4.
a) M. J. Auburn, R. D. Holmes-Smith, and S. R. Stobart, J. Am. Chem.
Soc., 106, 1314 (1984). b) M. J. Auburn, S. L. Grundy, S. R. Stobart,
and M. J. Zaworotko, J. Am. Chem. Soc., 107, 266 (1985). c) M. J.
Auburn and S. R. Stobart, Inorg. Chem., 24, 318 (1985).
a) M. Okazaki, Y. Kawano, H. Tobita, S. Inomata, and H. Ogino, Chem.
Lett., 1995, 1005. b) M. Okazaki, H. Tobita, and H. Ogino, Chem. Lett.,
1996, 477. c) M. Okazaki, H. Tobita, and H. Ogino, Organometallics,
15, 2790 (1996). d) M. Okazaki, H. Tobita, and H. Ogino, Chem. Lett.,
1997, 437. e) M. Okazaki, H. Tobita, and H. Ogino, J. Chem. Soc.,
Dalton Trans., 1997, 3531. f) M. Okazaki, H. Tobita, and H. Ogino,
Chem. Lett., 1998, 69. g) M. Okazaki, H. Tobita, Y. Kawano, S.
Inomata, and H. Ogino, J. Organomet. Chem., 553, 1 (1998).
5
6
7
1
2
Data for 1: H NMR (300 Hz, C6D6) δ –9.32 (pseudo dq, J(HPtrans) =
2
1
158.1 Hz, J(HPcis) = J(HRh) = 15.3 Hz, 1H, RhH). 31P{1H} NMR
(121.5 MHz, C6D6) δ –25.1 (broad m, PMe3 (trans to RhH)), –7.6 (ddd,
2J(PPtrans) = 364.3 Hz, J(PRh) = 107.5 Hz, J(PPcis) = 31.5 Hz, PMe3
(trans to PPh2)), 55.5 (ddd, 2J(PPtrans) = 364.3 Hz, 1J(PRh) = 109.7 Hz,
2J(PPcis) = 26.2 Hz, PPh2). 29Si{1H} NMR (59.6 MHz, C6D6) δ 40.4
(dddd, 1J(SiRh) = 26.1 Hz, 2J(SiPcis) = 10.5, 8.2, 6.9 Hz). Anal. Calcd
for C22H39SiP3ClRh: C 46.94, H 6.98%. Found: C 47.21, H 6.99%.
Data for 2: 1H NMR (300 MHz, toluene-d8, –40 °C) δ 0.80 (d, 2J(HP)
= 4.7 Hz, 9H, PMe3), 0.86 (s, 6H, SiMe2), 1.32 (d, 2J(HP) = 5.7 Hz, 9H,
PMe3), 2.43 (m, 2H, PCH2), 6.95–7.07, 7.95 (m, 10H, PPh2). 31P{1H}
1
2
1
NMR (121.5 MHz, toluene-d8, –40 °C) δ –21.1 (pseudo dt, J(PRh) =
112.9 Hz, 2J(PPcis
)
35.9 Hz, PMe3 (trans to SiMe2)), –9.0 (ddd,
2J(PPtrans) = 296.0 Hz, J(PRh) = 148.0 Hz, J(PPcis) = 36.6 Hz, PMe3
(trans to PPh2)), 70.5 (ddd, 2J(PPtrans) = 296.0 Hz, 1J(PRh) = 157.2 Hz,
2J(PPcis) = 35.1 Hz, PPh2). Anal. Calcd for C22H38SiP3Rh: C 50.19, H
7.28%. Found: C 49.77, H 7.15%.
1
2
Osakada et al. reported the reaction of RhCl(PMe3)3 with
HSiAr3.15 The reaction proceeded at room temperature to give mer-
[RhCl(H)(SiAr3)(PMe3)3] (5) (eq 4). In the reaction, the Si–Si
reductive elimination products such as RhCl(H)2(PMe3)3 and
(SiAr3)2 were not observed.16 The dramatic difference of reactivity
between 3 and 5 is of great interest. The difference is attributable to
the exceptionally strong trans-effect of the silyl ligand in 3. Further
reaction of 3 and 5 requires the dissociation of the phosphine ligand,
which allows it to form bis(silyl)rhodium(V) intermediate. In com-
plex 3, the silyl moiety of (2-phosphinoethyl)silyl chelate ligand
would accelerate the dissociation of the trans-PMe3 ligand, while
such an effect is not operative in complex 5.
8
9
Crystallographic data for 2: C22H38SiP3Rh, Mr = 526.45, triclinic, space
group P1 (variant No.2), a = 11.4349(6) Å, b = 12.9113(9) Å, c =
9.5714 Å, α = 100.379(5)°, β = 104.093(2)°, γ = 106.562(3)°, V =
1265.5(1) Å3, T = –123.0 °C, Z = 2, Dc = 1.38 g cm–3, µ(Mo Kα) = 9.61
cm–1, R = 0.068, Rw = 0.189 for 5596 unique reflections.
a) D. L. Thorn and R. L. Harlow, Inorg. Chem., 29, 2017 (1990). b) P.
Hofmann, C. Meier, W. Hiller, M. Heckel, J. Riede, and M. U. Schmidt,
J. Organomet. Chem., 490, 51 (1995). c) M. Aizenberg, J. Ott, C. L.
Eisevier, and D. Milstein, J. Organomet. Chem., 551, 81 (1998).
10 K. Hübler, P. A. Hunt, S. M. Maddock, C. E. F. Rickard, W. R. Roper,
D. M. Salter, P. Schwerdtfeger, and L. J. Wright, Organometallics, 16,
5076 (1997).
11 “Advanced Inorganic Chemistry,” ed. by F. A. Cotton and G.
Wilkinson, Wiley, New York (1988), p 777.
12 Isolation of 3 was unsuccessful due to its thermal instability. Data for 3:
1H NMR (300 MHz, toluene-d8, –20 °C) δ –10.30 (pseudo dq 1J(HPtrans
)
1
2
= 123.0 Hz, J(HRh) = J(HPcis) = 16.0 Hz, 1H, RhH). 31P{1H} NMR
(121.5 MHz, toluene-d8, –20 °C) δ –28.3 (ddd, 1J(PRh) = 80.0 Hz,
2
1
2J(PPcis) = 32.0 Hz, J(PPcis) = 28.6 Hz, PMe3), –18.2 (ddd, J(PRh) =
2
2
103.0 Hz, J(PPcis) = 28.6 Hz, J(PPcis) = 24.8 Hz, PMe3), 60.1 (ddd,
1J(PRh) = 82.0 Hz, 2J(PPcis) = 32.0 Hz, 2J(PPcis) = 24.8 Hz, PPh2).
13 Data for 4: 1H NMR (300 MHz, toluene-d8) δ –10.57 (pseudo dq,
2J(HPtrans) = 135.0 Hz, 2J(HPcis) = 1J(HRh) = 18.0 Hz, 1H, RhH), –9.27
(pseudo dq, 2J(HPtrans) = 135.0 Hz, 2J(HPcis) = 1J(HRh) = 21.0 Hz, 1H,
RhH), 0.61, 0.83 (s, 3H, SiMe). 31P{1H} NMR (121.5 MHz, toluene-d8)
1
2
2
δ –20.0 (ddd, J(PRh) = 87.7 Hz, J(PP) = 28.6 Hz, J(PP) = 19.1 Hz,
It has been known that the dehydrogenative coupling of mono-
hydrosilanes requires the considerably severe conditions and is
accompanied by the scrambling of substituents on the silicon
atoms.17 It should be noted that the reaction 3 proceeds at room tem-
perature to give the silicon–silicon coupling product exclusively.
1
2
PMe3), –11.3 (dt, J(PRh) = 101.1 Hz, J(PP) = 26.7 Hz, PMe3), 68.2
(dt, 1J(PRh) = 101.1 Hz, 2J(PP) = 21.9 Hz, PPh2). Anal. Calcd for
C22H40SiP3Rh: C 50.00, H 7.63%. Found: C 49.29, H 7.44%.
14 H. Nagashima, K. Tatebe, T. Ishibashi, J. Sakakibara, and K. Itoh,
Organometallics, 14, 2868 (1995).
15 a) K. Osakada, T. Koizumi, S. Sarai, and T. Yamamoto,
Organometallics, 16, 3973 (1997). b) K. Osakada, T. Koizumi, S. Sarai,
and T. Yamamoto, Organometallics, 17, 1868 (1998).
16 The reaction of RhCl(PMe3)3 with HSiMe2Ph gave the Si–H oxidative
addition product quantitatively. Even at 70 °C, no formation of
(SiMe2Ph)2 was observed.
17 a) J. Y. Corey, “Advances in Slilicon Chemistry,” ed. by G. Larson, JAI
Press, Inc., Greenwich, (1991), Vol. 1, p 327. b) M. D. Curtis and P. S.
Epstein, Adv. Organomet. Chem., 19, 213 (1981).
This work was supported by Grants-in-Aid for Scientific
Research (Nos. 11740367, 12640533, and 13029012) from the
Ministry of Education, Cullture, Sports, Science and Technology of
Japan. The authors wish to thank NISSAN SCIENCE FOUNDA-
TION and Inoue Foundation for Science for financial supports.