Dinuclear palladium complex with a bridging dialkylsilylene ligand

Article abstract of DOI:10.1246/cl.2007.284

A novel dinuclear palladium complex with one bridging dialkylsilylene ligand 6 was synthesized by the reaction of isolable dialkylsilylene 7 (R ₂Si:) with two equiv of bis(tricyclohexyl)-phosphinepalladium (8). Complex 6 as a prototypical dipalladasilacyclopropane reacts with molecular hydrogen to give dihydrosilane 10 (R₂SiH₂) together with 8 and metallic palladium. Copyright

Full text of DOI:10.1246/cl.2007.284

284
Chemistry Letters Vol.36, No.2 (2007)
Dinuclear Palladium Complex with a Bridging Dialkylsilylene Ligand
Chieko Watanabe,1 Takeaki Iwamoto,ꢀ2 Chizuko Kabuto,2 and Mitsuo Kiraꢀ1
1Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578
2Research and Analytical Center for Giant Molecules, Graduate School of Science,
Tohoku University, Aoba-ku, Sendai 980-8578
(Received November 24, 2006; CL-061390; E-mail: iwamoto@mail.tains.tohoku.ac.jp, mkira@mail.tains.tohoku.ac.jp)
A novel dinuclear palladium complex with one bridging di-
Complex 6 was synthesized by the treatment of dialkyl-
silylene 7 with bis(tricyclohexylphosphine)palladium (8, 2.1
equiv) in toluene at room temperature (eq 1).7 Recrystallization
from toluene gave analytically pure 6 in 24% yield as air- and
moisture-sensitive dark blue-purple crystals with the decomposi-
tion temperature of 203 ꢁC. The structure of 6 was determined
by NMR spectroscopy, elemental analysis, and X-ray crystallog-
raphy.8,9
alkylsilylene ligand 6 was synthesized by the reaction of isolable
dialkylsilylene 7 (R2Si:) with two equiv of bis(tricyclohexyl)-
phosphinepalladium (8). Complex 6 as a prototypical dipallada-
silacyclopropane reacts with molecular hydrogen to give dihy-
drosilane 10 (R2SiH2) together with 8 and metallic palladium.
2.1 equiv (Cy3P)2Pd (8)
Multinuclear transition-metal complexes with bridging silyl
and/or silylene groups have received much attention because
they show unique structural and bonding properties and would
be key intermediates of transformation of silane mediated by
transition-metal complexes.1 Although various multinuclear
group-10 metal complexes with bridging silyl and/or silylene
units have been extensively studied, there have been few reports
on the corresponding dipalladium complexes. Osakada et al.
have synthesized [LnPd(ꢀ-SiH(R)Ph)]2 (1) as the first silyl-
bridged dimeric palladium complexes with three-center-two-
6
(1)
7
toluene, rt
(24% yield)
Figure 1 shows the molecular structure of 6 determined by
X-ray analysis. The Pd2 unit is disordered over two positions
with almost the same site occupancy factors [0.486(3) for
Pd1(Pd2) and 0.514(3) for Pd10(Pd20)]. The geometry around
the Si1–Pd1–Pd2 ring and Si1–Pd10–Pd20 ring are similar to each
other. The dialkylsilylene unit bridges over the two palladium
atoms and the silacyclopentane ring is nearly perpendicular to
the SiPd2 three-membered ring; the dihedral angle between
C1–Si1–C4 and Pd1–Si1–Pd2 (Pd10–Si1–Pd20) planes is
91.62ꢁ (92.28ꢁ). Each palladium atom is coordinated by one
phosphine ligand. The geometry around Pd1 and Pd2 atoms
are slightly different from each other. The Si1–Pd1–P1 angle
is 147.66(8), while Si1–Pd2–P2 angle is 171.15(6). The P1
and P2 atoms are slightly deviated from the central three-mem-
bered ring plane with the dihedral angles Si1–Pd2–Pd1–P1 and
electron Si–H–Pd bond.2 Furstner et al. and Herrmann et al. have
¨
reported dipalladium complexes with two bridging N-heterocy-
clic silylenes (2–4) that were prepared by the reactions of stable
N-heterocyclic silylenes with palladium(0) complexes.3,4
Suginome, Ito, et al. have synthesized dipalladium complex 5,
in which ꢀ-silylene and ꢀ-isocyanide ligands coordinate to
the two palladium atoms.5 In the present paper, we report the
synthesis, structure, and an interesting reaction of novel dinu-
clear palladium complex having one bridging silylene unit (6)
using isolable dialkylsilylene 7, which we have recently synthe-
sized as a silylene stabilized kinetically by a bulky 1,1,4,4-
tetrakis(trimethylsilyl)butane-1,4-diyl ligand.6 To the best of
our knowledge, complex 6 is the first mono(ꢀ-silylene) dipalla-
dium complex without Pd–H–Si bonding or other bridging li-
gands and, hence, will be useful as a prototypical dipalladasila-
cyclopropane to investigate the electronic structure and reactions
of the ring system (Chart 1).
R2
Si
RPhSi
LPd
H
PdL'
LPd
PdL
Si
H
SiPhR
R2
Figure 1. Molecular structure of 6. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level. Two Pd atoms and one cyclohexyl rings are
disordered. The site occupancy factor of Pd1 and Pd10 (and
Pd2 and Pd20) are 0.486(3) and 0.514(3), respectively. Selected
1a: R = Ph, L = L'= PMe3, PEt3,
PMePh2, PCy3
1b: R = Ph, L = PMe3, L' = (PMe3)2
2: SiR2 = Si[N(t-Bu)CH]2, L = PPh3
3a: SiR2 = Si[N(t-Bu)CH2]2, L = P(t-Bu)3
3b: SiR2 = Si[N(t-Bu)CH]2, L = P(t-Bu)3
4: SiR2 = L = Si[N(t-Bu)CH]2
1c: R = Me, L = L'= PMe3, PEt3
Me3Si
SiMe3
˚
Si
Me2Si
SiMe2
bond lengths (A) and angles (deg): Si1–Pd1 2.2952(15), Si1–Pd2
Me3Si
Me3Si
SiMe3
SiMe3
Me3Si
SiMe3
Si
2.3019(13), Si1–Pd20 2.3073(12), Si1–Pd10 2.3089(14), Pd1–
Pd2 2.6029(13), Pd10–Pd20 2.5833(13), C1–Si1–C4 96.56(12),
Pd1–Si1–Pd2 68.97(3), Pd20–Si1–Pd10 68.06(3), plane(C1–
Pd
Pd
Si
C
C
C
N
N
N
(Cy3P)Pd
Pd(PCy3)
Bu-t
Bu-t
7
Bu-t
6
5
Si1–C4)–plane(Pd1–Si1–Pd2)
plane(Pd10–Si1–Pd20) 92.28.
91.62,
plane(C1–Si1–C4)–
Chart 1.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.2 (2007)
285
Si1–Pd1–Pd2–P2 of ꢂ172:59ð18Þ and ꢂ169:50ð7Þꢁ. The sum of
the bond angles around the Pd1 and Pd2 atoms are 359.70(7) and
353.60(7)ꢁ, respectively. The Si–Pd distances of 6 (2.2952(15)–
H. Ogino, Dalton Trans. 2003, 493; N. J. Hill, R. West, J. Organo-
met. Chem. 2004, 689, 4165.
2
Y.-J. Kim, S.-C. Lee, J.-I. Park, K. Osakada, J.-C. Choi, T.
Yamamoto, Organometallics 1998, 17, 4929; Y.-J. Kim, S.-C.
Lee, J.-I. Park, K. Osakada, J.-C. Choi, T. Yamamoto, J. Chem.
Soc., Dalton Trans. 2000, 417; M. Tanabe, T. Yamada, K. Osakada,
Organometallics 2003, 22, 2190.
˚
2.3089(14) A) are significantly shorter than those of known silyl-
and silylene-bridged dipalladium complexes 1–5 (2.318(2)–
˚
˚
2.4543(5) A for Si–Pd distance, 2.332(2)–2.411(2) A for dis-
tances of Si–Pd bonds with a 3c-2e Pd–H–Si bond).2–5 The
3
4
A. Furstner, H. Krause, C. W. Lehmann, Chem. Commun. 2001,
¨
2372.
W. A. Herrmann, P. Harter, C. W. K. Gstottmayr, F. Bielert, N.
˚
Pd–Pd distance of 6 (2.5833(13)–2.6029(13) A) is remarkably
˚
shorter than those of 1–5 (2.6501(2)–2.751(3) A) and within
¨
¨
Seeboth, P. Sirsch, J. Organomet. Chem. 2002, 649, 141; A. Zeller,
F. Bielert, P. Haerter, W. A. Herrmann, T. Strassner J. Organomet.
Chem. 2005, 690, 3292.
M. Suginome, Y. Kato, N. Takeda, H. Oike, Y. Ito, Organometallics
1998, 17, 495.
the range of known Pd(I)–Pd(I) single bond lengths (2.500–
10
˚
3.311 A). The Pd–Si–Pd bond angles of 6 are 68.06(3)–
68.97(3)ꢁ, which are close to those of 1–5 (Pd–Si–Pd bond
5
6
7
angles: 66.784(10)–71.7(2)ꢁ).2–5
M. Kira, S. Ishida, T. Iwamoto, C. Kabuto, J. Am. Chem. Soc. 1999,
121, 9722.
All proton nuclei of four trimethylsilyl groups of complex 6
are equivalent to show a singlet signal in the 1H NMR spectrum,
indicating that complex 6 has a highly symmetric or fluxional
structure in solution. The 29Si NMR resonance of the silylene
In a Schlenk flask (200 mL) equipped with a magnetic stirrer bar, a
mixture of dialkylsilylene 7 (0.943 g, 2.53 mmol) and palladium
complex 8 (3.76 g, 5.63 mmol) in toluene (70 mL) was stirred at
room temperature for 65.5 h under argon atmosphere. Removal of
toluene in vacuo and then recrystallization at ꢂ35 ꢁC from toluene
twice gave pure 6 (709 mg, 0.618 mmol, 24% yield). 6: air-sensitive
dark blue-purple crystals; mp 203 ꢁC (dec.); 1H NMR (C6D6, ꢁ) 0.71
(s, 36H, SiCH3), 1.28–2.16 (m, 66H, Cy), 2.25 (s, 4H, CH2);
13C NMR (C6D6, ꢁ) 5.5, 26.8, 28.1, 31.7, 34.1, 35.0, 37.6; 29Si NMR
(C6D6, ꢁ) ꢂ0:70 (SiCH3), 303.5 (t, 2JSiP ¼ 78:5 Hz, Si:); 31P NMR
(C6D6, ꢁ) 28.3; UV–vis (hexane) ꢂmax/nm (") 254 (29000), 301
(38000), 497 (1900), 568 (1600); Anal calcd for C52H106Si5Pd2P2:
C, 54.47; H, 9.32%. Found: C, 54.23; H, 9.35%. Chemical shifts
are expressed in ppm down field from external tetramethylsilane
(1H, 13C, and 29Si) and 85% H3PO4 (31P).
2
silicon nucleus of 6 (ꢁ 304, t, J(Si–P) = 78.5 Hz) is upfield-
shifted from the corresponding resonance of 7 (ꢁ 567.4) but close
to that of [{Pt(ꢀ-SiPh2)(PMe3)}3] complex (ꢁ 279.4).11
A plausible mechanism for the formation of 6 is shown in
Scheme 1. The ligand-exchange reaction of dialkylsilylene 7
with Pd0 complex 8 will give initially (dialkylsilylene)(Cy3P)Pd
complex 9, which then reacts with another molecule of 8 to
form complex 6. The intermediary formation of complex 9
was confirmed by NMR spectroscopy.12
Me3Si
Me3Si
SiMe3
SiMe3
In the above reaction, the corresponding bis(ꢀ-dialkylsilylene)-
dipalladium complex did not form probably owing to severe steric
hindrance during the introduction of silylene 7 to 6.
(Cy3P)2Pd (8)
(Cy3P)2Pd (8)
Si
7
6
–Cy3P
–Cy3P
Pd(PCy3)
8
Single crystals of 6 suitable for X-ray diffraction study were ob-
tained by recrystallization from toluene at ꢂ35 ꢁC. Crystallographic
data for compound 6 have been deposited with Cambridge Crystal-
lographic Data Centre as supplementary publication no. CCDC
627230. Copies of the data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge, CB2
1EZ, UK; fax: +44 1223 336033). The final R1 and wR2 values were
0.0545 [I > 2ꢃðIÞ] and 0.0941 (for all data).
9
Scheme 1. A stepwise formation of 6 from 7.
Complex 6 reacts slowly with an excess amount of molecu-
lar hydrogen under atmospheric pressure in benzene-d6 to give
dialkylsilane 1013 in 85% yield (eq 2). Because no reaction oc-
curs between silylene 7 with molecular hydrogen under similar
conditions, the hydrogenation should occur on the palladium
metals of complex 6. A possible mechanism involves the oxida-
tive addition of hydrogen molecule to one of the palladium
atoms followed by 1,2-hydride migration from palladium to
silicon.
9
When the refinement is carried out regardless of disorder at Pd
atoms, bond lengths and angles showed different values with stretch-
ed ellipsoids of Pd atoms. The final R1 and wR2 values were 0.0825
˚
[I > 2ꢃðIÞ] and 0.1481 (for all data). Selected bond lengths (A) and
angles (deg) regardless of disorder is below: Pd1–Pd2 2.5676(9);
Si1–Pd1 2.2891(12); Si1–Pd2 2.2934(12); C1–Si1–C4 96.64(19);
Pd1–Si1–Pd2 68.15(4); plane (C1–Si1–C4)–plane (Pd1–Si1–Pd2)
92.01.
Me3Si
Me3Si
SiMe3
SiMe3
excess H2
6
+
+
(2)
8
Pd metal
Si
C6D6, rt, 39 h
H2
10 The PdI–PdI distances are obtained from the Cambridge Crystallo-
graphic Database (http://www.ccdc.cam.ac.uk).
10
(85%)
11 K. Osakada, M. Tanabe, T. Tanase, Angew. Chem. 2000, 39,
4053.
This work was supported by the Ministry of Education,
Culture, Sports, Science and Technology of Japan (Specially
Promoted Research (No. 17002005)).
12 By monitoring the reaction of 7 with 2 equiv of 8 by NMR spectros-
copy, we observed that (dialkylsilylene)(Cy3P)Pd (9) was formed at
the early stage of the reaction and then complex 9 slowly converted
to complex 6. Complex 9 formed as an air-sensitive purple oil in
a good yield by the reaction of 7 with one equivalent of 8, but
the isolation of 9 was failed because of the difficult separation from
tri(cyclohexyl)phosphine produced during the reaction. 9: 1H NMR
(C6D6, ꢁ) 0.52 (s, 36H, SiMe3), 2.12 (s, 4H, CH2); 29Si NMR (C6D6,
ꢁ) ꢂ1:15 (SiMe3), 413.5 (d, 2J(Si–P) = 132.3 Hz); 31P NMR
(C6D6, ꢁ) 35.2. The 1H NMR resonances due to cyclohexyl moiety
of 9 were unseparable from the resonances of tri(cyclohexyl)-
phosphine.
References and Notes
1
T. D. Tilley, in The Chemistry of Organic Silicon Compounds, ed.
by S. Patai, Z. Rappoport, John Wiley & Sons, New York, 1989,
Chap. 24; P. D. Lickiss, Chem. Soc. Rev. 1992, 271; M. S. Eisen,
in The Chemistry of Organic Silicon Compounds, ed. by Z.
Rappoport, Y. Apeloig, John Wiley & Sons, New York, 1998,
Chap. 35; H. Tobita, H. Ogino, Adv. Organomet. Chem. 1998, 42,
223; J. Y. Corey, J. Braddock-Wilking, Chem. Rev. 1999, 99, 175;
B. Gehrhus, M. F. Lappert, J. Organomet. Chem. 2001, 617–618,
209; H. Ogino, Chem. Rec. 2002, 2, 291; M. Okazaki, H. Tobita,
13 M. Kira, T. Hino, Y. Kubota, N. Matsuyama, H. Sakurai, Tetra-
hedron Lett. 1988, 29, 6939.
Published on the web (Advance View) January 18, 2007; doi:10.1246/cl.2007.284