Preparation, structure and catalytic properties of a binuclear Pd(0) complex with bridging silylene ligands

Article abstract of DOI:10.1039/b108132b

In contrast to the N-heterocyclic carbene (NHC) 1, the homologous N-heterocyclic silylene (NHS) 4 acts as a bridging ligand to Pd(0), giving rise to the dinuclear complex 5 which is catalytically active in Suzuki reactions.

Full text of DOI:10.1039/b108132b

View Article Online / Journal Homepage / Table of Contents for this issue
Preparation, structure and catalytic properties of a binuclear Pd(0)
complex with bridging silylene ligands
Alois Fürstner,* Helga Krause and Christian W. Lehmann
Max-Planck-Institut für Kohlenforschung, D-45470 Mülheim/Ruhr, Germany.
E-mail: fuerstner@mpi-muelheim.mpg.de; Fax: +49-208-306-2994
Received (in Cambridge, UK) 10th September 2001, Accepted 9th October 2001
First published as an Advance Article on the web 31st October 2001
In contrast to the N-heterocyclic carbene (NHC) 1, the
homologous N-heterocyclic silylene (NHS) 4 acts as a
bridging ligand to Pd(0), giving rise to the dinuclear complex
5 which is catalytically active in Suzuki reactions.
formation of 4 in high yield, but also gives much better
reproducible results and requires neither large excess of the
reducing agent nor the monitoring of the reaction progress by
NMR as recommended in the original publication.9
Having secured good access to multigram amounts of 4, this
ligand was reacted with Pd(PPh3)4 in THF at ambient
temperature. 31P NMR spectroscopy revealed the rapid release
of PPh3 accompanied by the virtually quantitative formation of
a single new product which was isolated as dark red, air-
sensitive crystals in 50% yield (Scheme 2).‡ Its structure in
solution can be deduced from the spectroscopic and analytical
data. Particularly diagnostic is the 29Si NMR signal which
appears as a triplet at d = 109.5 ppm (JP,Si = 21.5 Hz)
indicating a highly symmetrical structure containing two PPh3
ligands at degenerate positions. MS indicates the presence of a
dimeric species which was unambiguously identified by X-ray
crystallography. Fig. 1 depicts the molecular structure of 5 in
the solid state, showing that the silylene moieties act as bridging
ligands between two 14-electron Pd(0) centers.§
Applications of transition metal complexes bearing N-hetero-
cyclic carbene (NHC) ligands to catalysis have recently met
with considerable success,1,2 with olefin metathesis3 and cross
coupling chemistry being the most pertinent examples.4,5
Thereby the strong s-donor properties of the NHC account for
the observed gain in activity and stability, while the fact that
many catalysts can be conveniently prepared in situ from
readily accessible precursors renders the procedures partic-
ularly user-friendly. Some of the resulting complexes, however,
have also been isolated and structurally characterized. Thus, it
has been shown that the sterically hindered NHC 1 readily
displaces PAr3 from Pd(0), leading to the formation of the
homoleptic, mononuclear 14-electron complex 2 which is
catalytically competent for Suzuki cross-coupling reactions
(Scheme 1).6
As part of our project aiming at the design of novel metal
catalysts and their applications to advanced organic synthesis,7
we became interested in the properties of the homologeous N-
heterocyclic silylenes (NHS)8 as ancillary ligands. Although
they promise to exhibit a similarly favorable profile, the
catalytic performance of transition metal–NHS complexes is as
yet largely unexplored. Outlined below is an exploratory study
into palladium chemistry which resulted in the discovery that
NHS 4 leads to an entirely different structural motif than its
NHC-analogue 1, without compromising the catalytic activity
of the resulting Pd(0) template.
The dimer is situated upon a crystallographic inversion
center. The distance between the Pd atoms (2.65005(17) Å) is
shorter than that of Pd(0) in the bulk metal (2.7511 Å), but
slightly longer than the distances observed in structurally
related, bridged dinuclear Pd( )-complexes with a linear R3P–
I
Pd–Pd–PR3 entity.12 The co-ordination of the Pd atoms is
planar, the sum of the Pd bond angles is 359.8°. The 1,3-diaza-
2-silacyclopentene is also essentially planar (with a mean
deviation of 0.005 Å) and is oriented perpendicular to the Pd1,
Si1, Pd1*, Si1* plane (interplanar angle 94.1°). Its geometry
reflects the silylene character of this ligand, with the N–Si–N
For the preparation of the required NHS, the procedure
described by West et al. has been modified.9 Rather than
refluxing the dichlorosilane 3 in THF with a large excess of
potassium chunks for extended periods of time, its treatment
with C8K10,11 in THF at ambient temperature turned out highly
beneficial (Scheme 2).† Not only does this method lead to the
Scheme 1 Preparation of a homoleptic Pd(0)–NHC complex.6a
Fig 1 Molecular structure of 5. Anisotropic displacement parameters are
shown at 50% probability level, hydrogen atoms have been omitted for
clarity. Selected bond lengths [Å] and angles [°]: Pd1–P1 2.2706(3), Pd1–
Si1 2.4154(4), Pd1–Si1* 2.3996(3), Pd1–Pd1* 2.65005(17), Si1–N1
1.7582(11), Si1–N2 1.7630(12), N1–C1 1.3930(17), N2–C2 1.3938(16),
C1–C2 1.344(2), Pd1*–Si1–Pd1 66.784(10), Si1*–Pd1-Si1 113.217(10),
N1–Si1–N2 88.80(5), C1–N1–Si1 111.97(9), C2–N2–Si1 111.87(9), C2–
C1–N1 113.75(12), C1–C2–N2 113.60(12), symmetry transformation used
to generate equivalent atoms: * 2x + 1, 2y, 2z
Scheme 2 Reagents and conditions: [a] C8K (2.2 equiv.), THF, rt, 18 h,
67%; [b] Pd(PPh3)4, rt, 1 h, 50%.
2372
Chem. Commun., 2001, 2372–2373
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b108132b

View Article Online
squares based on F2 using SHELXL-97, final R = 0.026, wR = 0.071,
Ωrmax 0.7 eÅ23. CCDC 168806. For crystallographic files in .cif or other
electronic format see http://www.rsc.org/suppdata/cc/b1/b108132b/
¶ In this context it is noteworthy that treatment of Ni(CO)4, Ni(COD)2 or
Pt(PPh3)4 with 4 or its benzoanellated analogues is reported to give
monomeric rather than bridged dinuclear complexes, cf. ref. 8, 13, 16.
1 A. J. Arduengo, Acc. Chem. Res., 1999, 32, 913.
2 (a) W. A. Herrmann and C. Köcher, Angew. Chem., Int. Ed. Engl., 1997,
36, 2162; (b) D. Enders and H. Gielen, J. Organomet. Chem., 2001,
617–618, 70.
3 (a) T. M. Trnka and R. H. Grubbs, Acc. Chem. Res., 2001, 34, 18; (b) A.
Fürstner, Angew. Chem., Int. Ed., 2000, 39, 3012.
Scheme 3 Reagents and conditions: [a] complex 5 (5 mol%), DME, K2CO3,
80 °C.
4 (a) C. Zhang, J. Huang, M. L. Trudell and S. P. Nolan, J. Org. Chem.,
1999, 64, 3804; (b) A. Fürstner and A. Leitner, Synlett, 2001, 290; (c) S.
Lee and J. F. Hartwig, J. Org. Chem., 2001, 66, 3402; (d) G. A. Grasa
and S. P. Nolan, Org. Lett., 2001, 3, 119; (e) W. A. Herrmann, M.
Elison, J. Fischer, C. Köcher and G. R. J. Artus, Angew. Chem., Int. Ed.,
1995, 34, 2371; (f) J. Huang and S. P. Nolan, J. Am. Chem. Soc., 1999,
121, 9889 and literature cited therein.
angle being even more acute than in dicarbonyl-bis(1,3-diaza-
2-silacyclopentene)nickel13 and the Si–N distances being also
slightly longer.
Although the propensity of silylenes to act as bridging
ligands in a variety of stable binuclear complexes is well
established,14 we are not aware of any precedence involving
Pd(0). Moreover, the synthesis of 5 favorably compares with
that of most other silylene bridged metal(0) complexes¶ which
are usually prepared by less convenient indirect routes. Finally,
complex 5 seems to be the first case of a NHS–metal complex
used as catalyst in organic synthesis. Specifically, it was found
to effect Suzuki reactions15 of aryl boronic acids with
bromoarenes in high yield. Two representative examples are
depicted in Scheme 3.
5 For further applications see: (a) A. Fürstner and H. Krause, Adv. Synth.
Catal., 2001, 343, 343; (b) H. M. Lee, T. Jiang, E. D. Stevens and S. P.
Nolan, Organometallics, 2001, 20, 1255; (c) A. C. Chen, L. Ren, A.
Decken and C. M. Crudden, Organometallics, 2000, 19, 3459; (d) J.
Louie and R. H. Grubbs, Chem. Commun., 2000, 1479.
6 (a) V. P. W. Böhm, C. W. K. Gstöttmayr, T. Weskamp and W. A.
Herrmann, J. Organomet. Chem., 2000, 595, 186; (b) L. R. Titcomb, S.
Caddick, F. G. Cloke, D. J. Wilson and D. McKerrer, Chem. Commun.,
2001, 1388.
7 (a) A. Fürstner, L. Ackermann, B. Gabor, R. Goddard, C. W. Lehmann,
R. Mynott, F. Stelzer and O. R. Thiel, Chem. Eur. J., 2001, 7, 3236; (b)
L. Ackermann, A. Fürstner, T. Weskamp, F. J. Kohl and W. A.
Herrmann, Tetrahedron Lett., 1999, 40, 4787; (c) A. Fürstner, O. R.
Thiel, L. Ackermann, H.-J. Schanz and S. P. Nolan, J. Org. Chem.,
2000, 65, 2204; (d) A. Fürstner, O. R. Thiel, N. Kindler and B.
Bartkovska, J. Org. Chem., 2000, 65, 7990; (e) A. Fürstner, M. Liebl,
C. W. Lehmann, M. Picquet, R. Kunz, C. Bruneau and P. H. Dixneuf,
Chem. Eur. J., 2000, 6, 1847; (f) A. Fürstner, H. Krause, L. Ackermann
and C. W. Lehmann, Chem. Commun., 2001, 2240
8 For pertinent reviews see: (a) B. Gehrhus and M. F. Lappert, J.
Organomet. Chem., 2001, 617–618, 209; (b) M. Haaf, T. A. Schmedake
and R. West, Acc. Chem. Res., 2000, 33, 704.
9 (a) M. Denk, R. Lennon, R. Hayashi, R. West, A. V. Belyakov, H. P.
Verne, A. Haaland, M. Wagner and N. Metzler, J. Am. Chem. Soc.,
1994, 116, 2691; (b) M. Haaf, A. Schmiedl, T. A. Schmedake, D. R.
Powell, A. J. Millevolte, M. Denk and R. West, J. Am. Chem. Soc., 1998,
120, 12714.
10 (a) A. Fürstner and H. Weidmann, J. Organomet. Chem., 1988, 354, 15;
(b) A. Fürstner, Angew. Chem., Int. Ed. Engl., 1993, 32, 164; (c) M.
Kira, S. Ishida, T. Iwamoto and C. Kabuto, J. Am. Chem. Soc., 1999,
121, 9722.
11 In his review (ref. 8a), Lappert also mentions the advantages of C8K in
the preparation of other NHS derivatives as unpublished results.
12 (a) R. Vilar, D. M. P. Mingos and C. J. Cardin, J. Chem. Soc., Dalton
Trans., 1996, 4313; (b) M. Sommovigo, M. Pasquali, P. Leoni and U.
Englert, Inorg. Chem., 1994, 33, 2686; (c) P. Leoni, M. Sommovigo, M.
Pasquali, P. Sabatino and D. Braga, J. Organomet. Chem., 1992, 423,
263; For a detailed discussion see: (d) J. Krause, R. Goddard, R. Mynott
and K.-R. Pörschke, Organometallics, 2001, 20, 1992; (e) See also: H.
Werner, Adv. Organomet. Chem., 1981, 19, 155.
Generous financial support by the Deutsche Forschungs-
gemeinschaft (Leibniz award program) and the Fonds der
Chemischen Industrie is gratefully acknowledged. We thank
Professor K. Pörschke for a valuable discussion.
Notes and references
† Preparation of 4: To a stirred solution of dichloride 3 (6.04 g, 22.6 mmol)
in THF (100 mL) under Ar is added C8K (6.7 g, 49.7 mmol)10 in several
portions. The resulting suspension is stirred at ambient temperature for 18
h during which its color changes from bronze to black. The insoluble
residues are filtered off under Ar and are repeatedly rinsed with THF, the
combined filtrates are evaporated and the crude product is purified by
sublimation in vacuo (bath temperature 60–75 °C, 1021 torr) thus affording
silylene 4 as a colorless solid (2.97 g, 67%). Its spectroscopic and analytical
data are in full agreement with those reported in the literature.9
‡ Preparation of complex 5: To a solution of silylene 4 (345 mg, 1.75 mmol)
in THF (40 mL) under Ar is added a solution of Pd(PPh3)4 (1.69 g, 1.46
mmol) in THF (60 mL) over a period of 30 min and stirring is continued for
another 30 min after the addition is complete. The resulting dark red
solution is evaporated, the residue is suspensed in hexane (15 mL), the
precipitate is filtered off under Ar and is carefully rinsed with hexane in
several portions. Drying in vacuo affords complex 5 as a dark-red solid (824
mg, 50%). 1H NMR (THF-d8): d = 7.21–7.44 (m, 30H), 6.86 (s, 4H), 1.25
(s, 36H); 13C NMR (THF-d8): d = 139.8, 139.7, 139.5, 135.2, 135.1, 134.8,
134.7, 129.2, 129.0, 128.8, 128.7, 128.6, 128.4, 121.3, 54.6, 33.8; 31P NMR
(THF-d8): d = 31.4; 29Si NMR (THF-d8): 109.5 (t, J = 21.5 Hz); MS (EI):
m/z (rel. intensity) 1130 (0.5, [M+]), 350 (10), 263 (20), 262 (100), 261 (14),
184 (13), 183 (58), 154 (14), 121 (14), 108 (23), 84(4), 77(3), 57(7),
51(4).
13 M. Denk, R. K. Hayashi and R. West, J. Chem. Soc., Chem. Commun.,
1994, 33.
14 Reviews: (a) T. D. Tilley, Comments Inorg. Chem., 1990, 10, 37; (b) W.
Petz, Chem. Rev., 1986, 86, 1019; (c) C. Zybill, Top. Curr. Chem., 1990,
160, 1; (d) P. D. Lickiss, Chem. Soc. Rev., 1993, 21, 271; (e) J. Y. Corey
and J. Braddock-Wilking, Chem. Rev., 1999, 99, 175.
§ Crystal data for 5: C56H70N4P2Pd2Si2, M = 1130.12 g mol21, red–brown,
¯
crystal dimensions 0.17 3 0.13 3 0.09 mm, triclinic P1 (no. 2), at 100 K a
= 10.2277(2), b = 10.8525(2), c = 13.8201(2) Å, a = 66.897(1), b =
73.515(1), g = 89.888(1)°, V = 1342.51(4) Å3, Z = 1, r = 1.398 Mgm23
,
15 A. Suzuki, J. Organomet. Chem., 1999, 576, 147.
m = 0.814 mm21, l = 0.71073 Å. w-Scans covering reciprocal space up to
qmax 33.18° with 93.7% completeness, total of 13730 reflections (9616
unique) with Rint = 0.034. Structure solution SHELXS-97, full matrix least-
16 (a) B. Gehrhus, P. B. Hichcock, M. F. Lappert and H. Maciejewski,
Organometallics, 1998, 17, 5599; (b) T. A. Schmedake, M. Haaf, B. J.
Paradise, D. Powell and R. West, Organometallics, 2000, 19, 3263.
Chem. Commun., 2001, 2372–2373
2373