Synthesis and unexpected coordination of a silicon(II)-based sicsi pincerlike arene to palladium

Article abstract of DOI:10.1002/anie.201200632

SiCSi to pick up a metal: A bis(silylene) SiCSi pincerlike arene 1 (see scheme) has been synthesized through salt metathesis reaction of the dilithium salt of 2,4-di-tert-butyl-resorcinolate with the respective chloro silylene precursor [LSiCl]. Remarkably, 1 reacts with Pd(PPh₃)₄ in a molar ratio of 2:1 under loss of all phosphine ligands to give the pincer complex 2.

Full text of DOI:10.1002/anie.201200632

Angewandte
Chemie
DOI: 10.1002/anie.201200632
Coordination Chemistry
Synthesis and Unexpected Coordination of a Silicon(II)-Based SiCSi
Pincerlike Arene to Palladium**
Wenyuan Wang, Shigeyoshi Inoue,* Elisabeth Irran, and Matthias Driess*
Dedicated to Professor Akira Sekiguchi on the occasion of his 60th birthday
Chelate ligands can be important for steering the reactivity of
transition-metal complexes. Among multidentate ligands,
pincer arenes, consisting of a central aromatic backbone
linked to two-electron Lewis donor atoms (E) by different
spacers, are particularly attractive because of their feasible
structural tuning with many choices of donor groups.[1,2]
Recently, the chemistry of pincer-ligated transition-metal
complexes underwent rapid developments and has been used
for several intriguing chemical transformations.[2,3] The coor-
dination chemistry of the most common pincer ligands such as
NCN-, PCP-, and SCS-type ligands toward transition metals
in A (Figure 1) has been explored extensively.[3] Pincer arenes
are currently unknown, despite the fact that isolable metal-
lylenes[4] are no longer laboratory curiosities and can serve at
the same time as much stronger s-donor and p-acceptor
ligands toward transition metals than phosphine ligands.[5,6]
ꢀ
Up to now only a few isolable bis-silylenes with a Si Si bond
are known[7a–d] and a 4,4’-biphenyl-bis-silylene[7e] and Si2N2
four-membered ring bis-silylene[7f] have been reported. Very
recently we described the synthesis of an isolable bis-silylene
oxide and its nickel complex.[8] This encouraged us to attempt
the synthesis of a silicon(II)-based pincer ligand B and to test
its coordination ability towards a metal. We report herein the
synthesis and characterization of the first bis-silicon(II)-based
SiCSi pincer arene ligand 1 bearing amidinate chelate ligand
for stabilizing the SiII atom. Moreover, we describe the
unusual coordination ability of 1 towards tetrakis(triphenyl-
phosphine)palladium, which proceeds in the molar ratio of
2:1 to afford the unexpected bis-silylene-silyl(phenyl)-
palladium(II) complex 2 through insertion of Pd0 into the
ꢀ
C H bond in 2 position of the benzene backbone of 1 and
subsequent 1,2-hydride shift from palladium to a silicon atom
of 1.
The SiCSi pincer ligand 1 can be easily synthesized
following the protocol as shown in Scheme 1. Dilithiation of
4,6-di-tert-butylresorcinol 3 with nBuLi gives the correspond-
Figure 1. Pincer complexes A and currently unknown Group 14
metallylene-based ECE pincer arenes B for facile metallation of the
benzene backbone.
with stronger s-donor sites E than those provided by
Group 15 and 16 atoms are very attractive for the synthesis
of more electron-rich transition-metal complexes for activa-
tion of small molecules. Promising new ligands are pincer
arenes B having divalent heavier Group 14 elements which
ꢀ
could mediate facile C H activation in the 2 position of the
benzene ring by a metal (Figure 1). However, pincer arenes B
[*] Dipl.-Chem. W. Wang, Prof. Dr. S. Inoue, Dr. E. Irran,
Prof. Dr. M. Driess
Institute of Chemistry: Metalorganics and Inorganic Materials
Technische Universitꢀt Berlin
Straße des 17. Juni 135, Sekr. C2, 10623 Berlin (Germany)
E-mail: shigeyoshi.inoue@tu-berlin.de
matthias.driess@tu-berlin.de
Scheme 1. Synthesis of SiCSi pincer arene 1.
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(Cluster of Excellence “UniCat”, EXC 314). We thank the Alexander
von Humboldt foundation (S.I.) for support by the Sofja Kovalev-
skaja Program.
ing 1,3-dilithium resorcinolate 4. Its salt metathesis reaction
with the N-donor stabilized chloro silylene LSiCl [L(amidi-
nate) = PhC(NtBu)2] 5[9] in the molar ratio of 1:2 furnishes the
desired compound 1, which could be isolated as pale yellow
crystals in 79% yield (Scheme 1). The constitution and
Supporting information for this article, including full synthetic,
spectroscopic, and crystallographic details as well as references for
the DFT calculations, is available on the WWW under http://dx.doi.
org/10.1002/anie.201200632.
1
composition of 1 was established by H, 13C, and 29Si NMR
spectroscopy, and elemental analysis. The 1H NMR spectrum
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of 1 shows one singlet for the tBu groups and one set of
resonances for the Ph groups of the amidinate ligands. The
almost identical variable-temperature (VT) 1H NMR spectra
of 1 in C7D8 in the temperature range of 210 to 320 K indicates
ꢀ
relatively low rotation barriers of the Si O bonds on the
NMR time scale. In the 29Si NMR spectrum, a sharp singlet
was detected at d = ꢀ24.0 ppm, which is comparable to that
observed for alkoxy-substituted silylenes LSiOR (R = tBu,
iPr, Me).[10] The molecular structure of 1 was confirmed by
a preliminary single-crystal X-ray diffraction analysis (see the
Supporting Information); however the moderate crystal
quality disallows discussion of metric parameters. Neverthe-
less, the analysis proves that the two silylene-like moieties in
1 face in the almost same direction (Scheme 1). To obtain
detailed structural information of 1, DFT calculations
[B3LYP/6-31G(d)] were performed.[17] The geometry of
1 obtained by X-ray crystallographic analysis was used as
the initial structure. The optimized structure of 1 is diplayed
in Figure 2. The Si1-O1-C1 angle (141.798) is larger than that
Scheme 2. Formation of 2 from reaction of 1 with Pd(PPh3)4 in the
molar ratio of 2:1.
A different type of silylene–silyl metal complexes has been
described by Ogino and co-workers.[11] Compound 2 was fully
characterized by multinuclear NMR spectroscopy and single-
crystal X-ray diffraction analysis. The formation of 2 implies
that two equivalents of 1 were consumed. When a 1:1 molar
ratio of starting materials was applied, a smaller yield of 2
(< 40%) was obtained but no other product or intermediate
1
could be detected by H NMR spectroscopy. The molecular
structure of 2 is shown in Figure 3.[12] Remarkably, com-
pound 2 crystallizes as racemic mixture of its R and S en-
antiomers, of which only the S form is presented in Figure 3.
The PdII atom adopts a typical distorted square-planar
configuration defined by the two silylene SiII atoms, Si2 and
Si3, the silyl SiIV atom Si1, and the carbon atom C1 of the
central aryl ring. Interestingly, during the complexation, the
ꢀ
former Si1 N2 bond in 1 is disrupted because of the
ꢀ
formation of a Si H silyl group through 1,2-hydride shift
from palladium to silicon. Owing to the coordinative satu-
ration of the Pd center, one silylene subunit (Si4) remains
“free”. The bent Si3-Pd1-C1 angle of 167.83(12)8 in 2 is
contrary to the linear X-Pd-C (X = Cl, I, Ph, 4-fluorophenyl,
Figure 2. Optimized structures of bis(silylene)-like SiCSi pincer ligand 1
(left) and its rotational isomer 1’ (right) at the B3LYP/6-31G(d) level.
Hydrogen atoms are omitted for clarity. Selected bond lengths [ꢁ] and
angles [8] of 1 and 1’: compound 1, Si1–O1 1.7056, Si2–O2 1.71905; Si1-
O1-C1 141.79, Si2-O2-C3 132.17; compound 1’, Si1–O1 1.7294, Si2–O2
1.7294; Si1-O1-C1 130.28, Si2-O2-C3 130.28.
of Si2-O2-C3 (132.178). While the dihedral angle of Si1-O1-
C1-C2 is 24.378, the dihedral angle of Si2-O2-C3-C2 is 0.398.
ꢀ
The Si O single bond distances of 1.7056 and 1.7190 ꢀ are
little longer than those of bis-silylene oxide LSiOSiL [1.641(2)
and 1.652(2) ꢀ][8] and alkoxy silylenes LSiOR (R = tBu, iPr)
[1.6442(3) and 1.6501(2) ꢀ] owing to steric congestion.[10] We
could also identify the C2-symmetric rotational isomer 1’ as
stable form on the hyperpotential energy surface which is only
1.8 kcalmolꢀ1 less stable than 1. Interestingly, the Si O
ꢀ
distance of 1’ is even slightly longer than those of 1. The
ꢀ
bond rotation barrier of the Si O bonds in 1 and 1’ are similar
and estimated to be + 8.0 kcalmolꢀ1 by theoretical calcula-
tion.
Figure 3. Molecular structure of 2. The compound crystallizes as
racemic mixture of its S and R enantiomers. Only the S form is shown.
Thermal ellipsoids are drawn at a probability level of 30%. Hydrogen
atoms, tBu groups, and phenyl groups are omitted for clarity, except
for the H1 atom. Selected bond lengths [ꢁ] and angles [8]: Pd1–C1
2.125(3), Pd1–Si1 2.3561(12), Pd1–Si2 2.3271(12), Pd1–Si3 2.3038(11),
Si1–O1 1.694(3), Si1–N1 1.789(4), Si2–O2 1.675(3), Si2–N5 1.862(3),
Si2–N6 1.840(4), Si3–O3 1.662(3), Si3–N7 1.888(3), Si3–N8 1.860(3),
Si4–O4 1.704(3), Si4–N3 1.893(4), Si4–N4 1.892(4); Si1-Pd1-Si2
155.03(4), Si1-Pd1-C1 78.61(11), Si2-Pd1-C1 76.81(11), Si3-Pd1-C1
167.83(12).
Although bis-silylene 1 bearing an amidinate ligand may
not serve as a p acceptor, bis-silylene 1 could work as a strong
s-donor ligand. To probe the pincer-type coordination ability
of 1, its reactivity towards phosphine palladium(0) complexes
was investigated. Treatment of 1 with 0.5 equivalents of
tetrakis(triphenylphosphine)palladium, Pd(PPh3)4, in hexane
at room temperature afforded the unprecedented bis-silylene-
silyl(phenyl)palladium(II) complex 2 as sole product, which
could be isolated as orange crystals in 81% yield (Scheme 2).
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OCOCF3, etc.) angles of reported palladium(II)
complexes supported by a PCP type pincer arene
ligand.[3,13] The Si1-Pd1-Si2 angle of 155.03(4)8 devi-
ates from the linear alignment, and is smaller than the
respective P-Pd-P angle of the PCP pincer-type
palladium complex.[3,13] The Si2 Pd1 and Si3 Pd1
bond lengths (2.3271(12) and 2.3038(11) ꢀ) of 2 are
ꢀ
ꢀ
ꢀ
shorter than that of the Si1 Pd1 distance
(2.3561(12) ꢀ), reflecting the difference between
a silyl ligand (Si1) with a SiIV Pd single bond vs.
ꢀ
a silylene ligand (Si2 and Si3) with a SiII!Pd dative
ꢀ
ꢀ
bond. However, the Si2 Pd1 and Si3 Pd1 distances
in 2 are similar with those of other silylene!
palladium complexes.[14,15] Furthermore, the Pd1
ꢀ
C1(aryl) bond length of 2 (2.125(3) ꢀ) is longer than
those of reported PCP pincer-type palladium(II)
complexes, presumably because of steric conges-
tion.[3,13]
Scheme 3. DFT-derived relative energies of model compounds 6–10
=
[L=1,3-m2-PhC(NtBu)2, L’=m1-(NtBu)C(Ph) NtBu].
Consistent with its crystallographic analysis, the
1H and 13C NMR spectra of 2 show twelve sets of
nonequivalent tBu groups. Moreover, the resonance
ꢀ
for the Si H proton is observed at d = 6.59 ppm. The
29Si NMR spectrum of 2 in C6D6 displays four signals at d =
ꢀ8.7 (L(Si:), Si4), 39.7 (SiH, Si1), 62.3 and 65.8 ppm (L-
(Si:)Pd, Si2 and Si3), respectively. The chemical shift of the
resulting in the formation of more stable 16-electron com-
plexes. Accordingly, the calculated relative energies of P!Pd
complex 9 stabilized by PPh3 and Si!Pd complex 10 stabi-
lized by methoxy silylene LSiOMe (L = PhC(NtBu)2) as
model silylene donor are + 9.1 and ꢀ4.1 kcalmolꢀ1, respec-
tively. Since SiII of intramolecular N-donor stabilized silylenes
is a stronger s donor than PIII of phosphines, complex 10 is
13.3 kcalmolꢀ1 more stable than 9. Overall, the stepwise
transformation of 6 into 10 in the presence of PPh3 and the
model silylene LSiOMe is exothermic by 4.2 kcalmolꢀ1.
In conclusion, we have reported the synthesis and
isolation of the first SiII-based SiCSi pincer arene 1 and its
unusual coordination behavior towards palladium. The enor-
mous electron-rich Lewis donor character of SiII in pincer
arene 1 gives facile access to new oligosilylene silyl(phenyl)
transition-metal complexes with potential application in
catalytic chemical transformations. Further investigations
using 1 for the synthesis of SiCSi pincer complexes with
other transition metals and their use in homogeneous
catalysis are currently in progress.
Si H SiIV atom (Si1) is shifted upfield in comparison to those
ꢀ
of a related HSi!Pd complex reported by Kira, Iwamoto and
co-workers because of the electronegative N and O atoms
bound to the Si1 atom.[14] The 29Si resonances of the two SiII
atoms, Si2 and Si3, coordinated to the palladium center are
comparable to those of other amidinate-based silylene com-
plexes[16] and significantly shifted to higher field in compar-
ison to those reported for donor-free silylene palladium
complexes owing to the additional N-donor coordination of
the amidinate ligand to the SiII atom. In addition, the infrared
ꢀ
spectrum of 2 shows a weak Si H stretching vibration band at
2135 cmꢀ1. The observed stretching frequency of 2 is higher
than those of other reported palladium hydridosilyl com-
plexes (2008–2121 cmꢀ1).[13c,g,14]
To rationalize the peculiar reactivity of 1 towards Pd-
(PPh3)4, quantum chemical calculations using density func-
tional theory (DFT) at the B3LYP level using 6-31G(d) basis
set for H, C, N, O, P, and Si atoms and the LANL2DZ level for
the Pd atom with the Gaussian 03 program for model
compounds 6–10 have been performed.[17] A schematic
representation of the potential energy surface of the proposed
pathway for the formation of 2 is given in Scheme 3. We
assume that 6 is formed as initial intermediate, resulting from
Received: January 23, 2012
Published online: March 1, 2012
Keywords: chelates · pincer ligands · silicon ·
silylene complexes · transition metals
.
stepwise dissociation of phosphine ligands of Pd(PPh3)4 and
0
ꢀ
insertion reaction of Pd into the pincer C H bond of the
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