Phosphinodi(benzylsilane) PhP{(o -C6H4CH 2)SiMe2H}2: A versatile pSi 2Hx pincer-type ligand at ruthenium

Article abstract of DOI:10.1021/ic400703r

The synthesis of the new phosphinodi(benzylsilane) compound PhP{(o-C ₆H₄CH₂)SiMe₂H}₂ (1) is achieved in a one-pot reaction from the corresponding phenylbis(o- tolylphosphine). Compound 1 acts as a pincer-type ligand capable of adopting different coordination modes at Ru through different extents of Si-H bond activation as demonstrated by a combination of X-ray diffraction analysis, density functional theory calculations, and multinuclear NMR spectroscopy. Reaction of 1 with RuH₂(H₂)₂(PCy ₃)₂ (2) yields quantitatively [RuH₂{[η ²-(HSiMe₂)-CH₂-o-C₆H ₄]₂PPh}(PCy₃)] (3), a complex stabilized by two rare high order ε-agostic Si-H bonds and involved in terminal hydride/η²-Si-H exchange processes. A small free energy of reaction (Δ_rG₂₉₈ = +16.9 kJ mol^-1) was computed for dihydrogen loss from 3 with concomitant formation of the 16-electron species [RuH{[η²-(HSiMe₂)-CH ₂-o-C₆H₄]PPh[CH₂-o-C ₆H₄SiMe₂]}(PCy₃)] (4). Complex 4 features an unprecedented ²⁹Si NMR decoalescence process. The dehydrogenation process is fully reversible under standard conditions (1 bar, 298 K).

Full text of DOI:10.1021/ic400703r

Article
pubs.acs.org/IC
Phosphinodi(benzylsilane) PhP{(o‑C6H4CH2)SiMe2H}2: A Versatile
“PSi2Hx” Pincer-Type Ligand at Ruthenium
Virginia Montiel-Palma,*,† Miguel A. Munoz-Hernandez,† Cynthia A. Cuevas-Chavez,† Laure Vendier,‡,§
́
́
̃
Mary Grellier,‡,§ and Sylviane Sabo-Etienne*,‡,§
†Centro de Investigaciones Químicas, Universidad Auton
Cuernavaca, Morelos, C. P. 62209, Mexico
́
oma del Estado de Morelos, Avenida Universidad 1001, Col. Chamilpa,
́
‡CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France
§Universite
́
de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
S
* Supporting Information
ABSTRACT: The synthesis of the new phosphinodi-
(benzylsilane) compound PhP{(o-C6H4CH2)SiMe2H}2 (1) is
achieved in a one-pot reaction from the corresponding
phenylbis(o-tolylphosphine). Compound 1 acts as a pincer-
type ligand capable of adopting different coordination modes
at Ru through different extents of Si−H bond activation as
demonstrated by a combination of X-ray diffraction analysis,
density functional theory calculations, and multinuclear NMR
spectroscopy. Reaction of 1 with RuH2(H2)2(PCy3)2 (2)
yields quantitatively [RuH2{[η2-(HSiMe2)-CH2-o-
C6H4]2PPh}(PCy3)] (3), a complex stabilized by two rare
high order ε-agostic Si−H bonds and involved in terminal hydride/η2-Si−H exchange processes. A small free energy of reaction
(ΔrG298 = +16.9 kJ mol−1) was computed for dihydrogen loss from 3 with concomitant formation of the 16-electron species
[RuH{[η2-(HSiMe2)-CH2-o-C6H4]PPh[CH2-o-C6H4SiMe2]}(PCy3)] (4). Complex 4 features an unprecedented 29Si NMR
decoalescence process. The dehydrogenation process is fully reversible under standard conditions (1 bar, 298 K).
analogous PIII pincer species. In this context, the design of
INTRODUCTION
■
silicon pincer−type ligands featuring Si−H bonds for
“cooperating” effects seemed highly relevant. We have
previously explored the reactivity of two phosphinosilane
compounds with ruthenium precursors and observed a versatile
coordination chemistry. The phosphinobenzylsilane ligand
Ph2P(o-C6H4)CH2SiMe2H led to a complex displaying two
rare high-order ε-agostic Si−H bonds, which, upon stepwise H2
loss through C−H activation, produced a bis β-agostic Si−H
species with two carbon−metalated bonds.8 In the case of
Ph2PCH2OSiMe2H, we isolated a complex containing three
phosphinosilane ligands each adopting a different coordination
mode to Ru.13 Reported herein is the synthesis of the
phosphinodi(benzylsilane) compound PhP{(o-C6H4CH2)-
SiMe2H}2 (1) and its reactivity toward the bis(dihydrogen)
ruthenium complex RuH2(H2)2(PCy3)2 (2). The new
phosphinodibenzylsilane compound PhP{(o-C6H4CH2)-
SiMe2H}2 acts as a pincer-type ligand “PSi2Hx” capable of
adopting different coordination modes at ruthenium through
different extents of Si−H bond activation.
The use of multidentate ligands in transition-metal chemistry
offers widespread applications due to the possibility of varying
the anchoring points and thus modulating the properties of the
metal center. Recently, the design of “cooperating” ligands led
to remarkable improvements in catalysis as illustrated in
particular by aromatization−dearomatization processes.1,2
Silicon is a strong σ donor and exerts a strong trans influence,3
and these properties make it an excellent choice for
incorporation into the skeleton of multidentate ligands.4−6
Indeed, there is an increasing number of reports of
phosphorus−silicon multidentate ligands stemming from the
pioneering work of Stobart.7 These studies have demonstrated
different degrees of reactivity control resulting in unusual
activation or properties,8−18 together with enhanced catalytic
behavior.19−21 These systems feature only one Si in
combination with either one, two, or three phosphorus
atoms. However, the presence of two Si atoms in bis(silyl)
systems is known to induce specific properties with a major
influence in catalysis.22−24 Recently, Driess and Hartwig
reported how the presence of two Si in bis(silylene) SiCHSi
pincer ligands resulted in significant reactivity and selectivity
changes in catalytic C−H borylation of arenes with respect to
other bidentate nitrogen ligands in Pd25 and Ir26 systems. The
SiCSi ligand showed a stronger σ-donor capability than
Received: March 22, 2013
Published: August 13, 2013
© 2013 American Chemical Society
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RESULTS AND DISCUSSION
Scheme 2. Reactivity of 1 with 2 Generating 3 and 4 in
Subsequent Reaction Steps
■
Synthesis and Characterization of the Phosphinodi-
(Benzylsilane) PhP{(o-C6H4CH2)SiMe2H}2, (1) and of the
Ruthenium Complex [RuH2{[η2-(HSiMe2)-CH2-o-C6H4]2-
PPh}(PCy3)] (3). The new phosphinodi(benzylsilane) 1 was
prepared in 83% yield in a one-pot reaction from dilithiation of
PhP(o-tolyl)2 with nBuLi in the presence of TMEDA followed
by addition of ClMe2SiH (Scheme 1). Its main NMR
Scheme 1. Synthesis of the Phosphinodi(Benzylsilane)
Compound 1
1
spectroscopic features are H signals at δ 4.21 (multiplet) for
the H−Si and at δ 2.44 and δ 0.5 for the methylene and methyl
groups respectively. At 700 MHz, two diastereotopic environ-
ments are confirmed for the methyl hydrogens as two sets of
doublets of doublets with the larger constant due to Si−H
coupling (3JHH 3.5 Hz). The diastereotopicity of the methylene
hydrogens is barely distinguishable as two doublets separated
by less than 2 Hz with the resolved coupling also due to Si−H
coupling. The 31P and 29Si signals appear at δ −21.1 and δ
−11.5 (1JSi−H 196 Hz), respectively. Compound 1 reacts at
room temperature in benzene-d6, toluene-d8, or THF with the
bis(dihydrogen) complex 2 affording compound 3 as a result of
the formal substitution of the two dihydrogen and one
tricyclohexylphosphine ligands of 2 by the phosphinodi-
(benzylsilane) ligand (Scheme 2).
Crystals of 3 suitable for X-ray diffraction analysis were
grown under 3 bar of dihydrogen from concentrated benzene-
d6 solutions of in situ generated 3. Two highly similar
crystallographically independent molecules were found in the
unit cell. For simplicity, and because one molecule presents
positional disorder in one of the cyclohexyl rings of the
coordinated PCy3 ligand, only the structural features of the
nondisordered molecule will be further discussed. The
molecular structure of 3 is shown in Figure 1, and relevant
bond lengths and angles are presented in Table 1.
The ruthenium center adopts a pseudo-octahedral environ-
ment. The central phosphorus atom of the coordinated
phosphinodi(benzylsilane) ligand and the PCy3 ligand display
a rather acute P−Ru−P angle of 113.32(4)°. A cis disposition
for bulky phosphine ligands is not without precedent in
ruthenium chemistry. In particular, the disilane complexes
[RuH2[(η2-HSiMe2)2X](PCy3)2] (X = C6H4, CH2CH2,
OSiMe2O)27 generated from the reaction of 2 with the
corresponding disilanes (HSiMe2)2X exhibit cis-phosphines
with P−Ru−P angles close to 108° as a result of increased
stabilization owing to the establishment of secondary
interactions between the terminal hydrides and the Si atoms
(SISHA).28,29 In complex 3, the X spacer is now a
dibenzylphosphino group, allowing the formation of a pincer-
type ligand if one considers the phosphorus and the middle of
the two Si−H bonds coordinated to ruthenium. A large
distortion is imposed by the silicon atoms bending away from
each other to 112.36(5)°. To describe the binding mode of the
two Si and four H atoms around the Ru center, one could in
principle envisage three extreme structures. A structure
resulting from the oxidative addition of the two Si−H bonds
can be immediately discarded, as a tetrahydride(disilyl) Ru(VI)
formulation is unrealistic. Alternatively, the two Si−H bonds
coordinate to Ru in a nonclassical fashion, thus leading formally
to a dihydride bis(η2-Si−H) Ru(II) species 3a, or only one of
the Si−H bonds is η2-coordinated to Ru, whereas the second is
fully activated generating a trihydride mono(η2-Si−H) Ru(IV)
species 3b (Scheme 3). Bearing in mind the uncertainties in the
hydride location by X-ray diffraction, two elongated bonding
Si−H distances are found in the structure of 3: Si1−H100 and
Si2−H101 at 1.62(4) and 1.68(4) Å, respectively. The four
Ru−H bond distances are rather undifferentiated lying in the
range 1.53−1.59 Å. All but one of the hydrogens bound to Ru,
H102, can be accommodated in a plane together with the Ru
and the two silicon atoms; out-of-plane H102 lies almost trans
to the bridgehead P of the pincer ligand, while in-plane H103 is
approximately trans to PCy3. Additional secondary interactions
(SISHA) between the silicons and H103 can be evidenced by
distances in the typical range 1.9−2.4 Å. The distances
determined by X-ray diffraction are consistent with the 3a
formulation. To overcome the experimental ambiguities
brought by the X-ray location of the hydride ligands, DFT/
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Figure 1. (Left) X-ray diffraction structure of 3 with thermal ellipsoids at the 50% probability level; most hydrogen atoms were removed for clarity.
(Center/right) Views showing the close to cis disposition of the phosphorus and silicon atoms. The plane containing the two phosphorus and the
two hydrides is nearly perpendicular to that of the two η2(Si−H) ligands (right) allowing for SISHA interactions to take place. All carbon and most
hydrogen atoms were removed for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (°) for
Complexes 3 and 4 (X-ray) and Their Calculated
Counterparts (DFT/B3PW91)
by two rare high order ε-agostic Si−H interactions with
additional SISHA interactions.29
At 293 K, the 1H NMR spectrum of 3 in toluene-d8 displays
one broad signal in the hydride region integrating for four
hydrogen atoms at δ −9.02, which remains broad even at the
lowest accessible temperature (193 K). At 263 K, the HMQC
1H−29Si{31P} NMR spectrum shows a 29Si signal at δ −0.7
correlating with the 1H signal at δ −9.02 with an apparent JSi−H
coupling constant of 40 Hz. The JSi−H coupling constant value
is a highly valuable tool for assessing the extent of nonclassical
interactions, this value being very small for silyl hydrides,
typically fewer than 20 Hz, while between 40−80 Hz for
nonclassical η2-SiH interactions.23,29−31 However, due to sign
uncertainties, care must be taken when using these coupling
constants as indicators of the strength of the interaction
between the atoms.23,32 The T1 min value of 373 ms at 273 K
and 500 MHz is too long to postulate a dihydrogen
formulation. In the 31P{1H} NMR spectrum, two doublets
corresponding to an AX spin system pattern are observed at δ
29.7 for the phosphinodi(benzylsilane) and δ 68.3 for the
3 (X-ray)
3cis (DFT)
4 (X-ray)
4 (DFT)
Ru−P1
Ru−P2
Ru−Si1
Ru−Si2
2.376 (1)
2.392 (1)
2.542(2)
2.420(1)
1.62(4)
2.413
2.419
2.613
2.419
1.659
1.889
2.064
2.309
114.4
115.3
90.8
2.308(1)
2.346(1)
2.473(1)
2.324(1)
1.71(3)
2.353
2.375
2.523
2.352
1.763
2.800
Si1−H100
Si2−H101
Si2−H103
Si1−H103
P1−Ru−P2
Si1−Ru−Si2
P1−Ru−Si1
P1−Ru−Si2
P2−Ru−Si1
P2−Ru−Si2
1.68(4)
113.32(4)
112.36(5)
91.70(5)
95.16(5)
118.80(5)
119.14(5)
154.37(3)
104.50(4)
87.15(3)
88.66(3)
113.42(3)
100.02(3)
152.5
101.7
86.3
95.5
89.2
116.1
118.8
113.9
103.6
2
coordinated PCy3 with a JP−P coupling constant of 121 Hz.
B3PW91 calculations were carried out without any simplifica-
tion of the ligands. Two isomers, 3cis and 3trans have been
characterized on the potential energy surface. The lowest in
energy, 3cis, displays a geometry closely resembling that
determined by X-ray diffraction, and the results show an
excellent agreement between both methods (Table 1). In
particular, the two Si−H bond distances are typical of η2-Si−H
bonds within the range 1.6−1.9 Å, with the H atoms almost
trans to each other. Two nonbonding Si···H distances are
characteristic of SISHA interactions, Si2···H103 2.064 Å and
Si1···H103 2.309 Å. Finally, the Ru−Si distances are very good
indicators of the degree of activation of the Si−H bonds.
Although one of the two Si is closer to Ru (X-ray: 2.420(1) vs
2.542(2) Å, DFT: 2.419 vs 2.613 Å), the two bond distances lie
within the range generally accepted for nonclassical η2-Si−H
bonds.23 These findings are in accordance with the nonclassical
character of the two Si−H bonds, and the complex can be
formulated in the solid state as the dihydride species
[RuH2{[η2-(HSiMe2)-CH2-o-C6H4]2PPh}(PCy3)] stabilized
This latter value is significantly larger than those obtained for
[RuH2{(η2-HSiMe2)2X}(PCy3)(PR3)] (with X = C6H4,
(CH2)2, (CH2)3, OSi(Me2)O and R = Ph, pyl), which are in
the range 20−30 Hz.27 It is however considerably smaller than
that reported for [RuH2{η4-HSiMe2(CHCHMe)}(PCy3)2]
(2JP−P 206 Hz) related to a P−Ru−P angle of 145.30(6)°.33
The diminished 2JP−P coupling constant is in accordance with a
stereochemistry in which the phosphorus atoms are inter-
mediate between cis and trans disposition. In addition, the 31P
NMR spectrum of a reaction mixture of in situ generated 3 and
PCy3 at 293 K (from the reaction mixture of 1 and 2 in a 2:1
mol equivalent ratio in a Young’s tap NMR tube) shows
exchange of the Ru-coordinated PCy3 ligand with free PCy3,
which is halted by lowering the temperature. The high
fluxionality of 3 is in contrast with the observed behavior of
the aforementioned disilane complexes [RuH2[(η2-
HSiMe2)2X](PCy3)2] (X = C6H4, CH2CH2, OSiMe2O), all of
which exhibit two resonances in the hydride region at ambient
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Scheme 3. Possible Arrested Structures for 3 and 4
Figure 2. X-ray diffraction structure of 4 with thermal ellipsoids at the 50% probability level. Most hydrogen atoms were removed for clarity (left).
View showing the square planar geometry around Ru when one considers the middle of the η2-(Si1−H100) ligand as a single coordination point.
The vacant coordination site lies trans to Si2. Most hydrogen and carbon atoms were removed for clarity (right).
temperature.27 The dynamics of hydride/η2-Si−H exchange
have been previously described as accounting for stereo-
chemical nonrigidity of the systems. In our case, the value of 40
Hz measured for the JSi−H coupling in conjunction with the
observed fluxionality of 3 in solution points to the existence of
nonclassical η2-SiH character in at least one but probably the
two Si moieties, therefore supporting formulation 3a. The rapid
exchange between the four hydride ligands (Ru−H and Ru−
H−Si) of 3 in solution, evidenced by their equivalency on the
NMR time scale at all accessible temperatures, is thus
associated with a very low activation barrier. However, the
situation is not clear-cut, and the contribution of structure 3b
cannot be utterly ruled out. This is reminiscent of what we have
shown previously in the case of the phosphinosilane ligand
PPh2CH2OSiMe2H, which can coordinate to ruthenium via two
different extreme modes: full oxidative addition and non-
classical sigma Si−H. A continuum between these two modes
was proposed to be present in solution.13
Dihydrogen Loss Leading to [RuH{[η2-(HSiMe2)-CH2-o-
C6H4]PPh[CH2-o-C6H4SiMe2]}(PCy3)] (4). NMR Observa-
tion of a 29Si Decoalescence Process. Despite complex 3
being stable over prolonged periods in solution at T ≤ 288 K,
attempts at vacuum drying of benzene-d6 solutions resulted in
partial formation of a new complex 4. Indeed, when pure
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1
Figure 3. High field region of the H NMR spectrum (toluene-d8, 400 MHz) of 4 at varying temperatures showing decoalescence at 273 K.
crystals of 3 were exposed for 15 min to dynamic vacuum and
redissolved, a 3:4 molar ratio was determined by NMR
integration as 1:0.5. Complex 3 was quantitatively regenerated
at ambient temperature under a pressure of 1−3 bar H2.
Keeping THF solutions of 3 at 308 K over 7 days yielded
complex 4 in 90% isolated yield from 3. Slow hexane diffusion
into THF solutions of either in situ generated 3 kept at 308 K
or of mixtures containing 3 and 4 in an initial 1:0.5 molar ratio
at 238 K resulted in the growth of crystals of 4 suitable for X-
ray diffraction analysis.
Thus, complex 3 undergoes reversible loss of dihydrogen
leading to the formation of an orange 16-electron Ru(II)
complex 4, [RuH{[η2-(HSiMe2)-CH2-o-C6H4]PPh[CH2-o-
C6H4SiMe2]}(PCy3)] (Scheme 2), as characterized by X-ray
diffraction and multinuclear NMR spectroscopy. To depict 4,
three arrested structures could be envisaged (Scheme 3). In 4a,
the two Si−H bonds would be coordinating to Ru in a
nonclassical fashion giving rise to a formally 16-electron Ru(0)
compound; in 4b, oxidative addition of one Si−H bond would
generate a hydrido(silyl)(η2-Si−H) Ru(II) species prone to
exhibiting SISHA interactions; finally, in 4c, full oxidative
addition of both Si−H bonds would render a Ru(IV)
dihydride(disilyl) formulation. The molecular structure of 4 is
shown in Figure 2, and relevant bond distances and angles are
presented in Table 1. It confirms a structure relatively close to
that of complex 3 bearing only two hydrogen atoms in the
coordination sphere of ruthenium. The findings discussed
below in the solid state are in agreement with a structure nearer
to proposed formulation 4b in which one hydride is terminal,
while the other one is involved in a η2-Si−H interaction.
Considering the centroid of the η2-Si−H bond as a
coordination point, the ruthenium is in a distorted square
planar pyramid with the silyl in the apical position (Si2), the
phosphorus atoms in the square planar base approximately
trans to each other, exhibiting a P−Ru−P angle of 154.37(3)°
much larger than that found in 3 of 113.32(5)°. The Si−Ru−Si
angle is rather acute at 104.50(4)° by comparison to the
corresponding angle in 3 accounting for 112.36(5)°. One of the
silicon atoms exhibits an elongated Si1−H100 bond distance of
1.71(3) Å as well as a Ru−Si1 distance of 2.473(1) Å, both
consistent with the η2-Si−H formulation.23,29,31,34 The other
silicon atom bond distance to ruthenium, Ru−Si2, is by
comparison shorter at 2.324(1) Å in agreement with a silyl
formulation. It is noteworthy that the distances between Si2
and the hydrogen atoms H100 and H101 are nonbonding
(>2.6 Å). The vacant site is trans to the strongest trans director
Si2. Differential functional theory (DFT) calculations show an
excellent agreement with the experimental values. In particular,
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Figure 4. Region of the HMQC 1H−29Si{31P}NMR experiment at 324 K showing correlation of the silicon signal to the hydride resonance (H and
H−Si), the two silicon, and the two hydrogen atoms around the ruthenium in complex 4 being in fast exchange (right). Region of the HMQC
1H−29Si{31P}NMR experiment at 213 K showing correlation of both silicon signals of 4 to only one high field 1H resonance (H−Si) with the same
JSi−H of 37 Hz (left).
two different Ru−Si bond distances were computed Ru−Si1
2.523 Å (exptl. 2.473(1) Å) and Ru−Si2 2.352 Å (exptl.
2.324(1) Å) as well as different extents of Si−H bond
activation: the Si1−H100 bond distance, 1.763 Å, is typical of
an η2-Si−H bond, notably shorter than the nonbonding Si2···
H100 and Si2···H101 distances (>2.6 Å). Thus, in the solid
state, 4 appears to be formally a 16-electron Ru(II) species
close to proposed structure 4b.
Figure 4 left). For each silicon atom, the magnitude of the
apparent JSi−H coupling constant is 37 Hz. In contrast, the
hydride signal at δ −2.82 does not correlate to any of the two
silicon signals. It was impossible to measure directly any 29Si
signals from 293 to 213 K due in particular to small T2 values.
This prevented the determination of the 29Si coalescence
temperature, which should be close to room temperature. To
our knowledge, the observation of a 29Si NMR decoalescence is
unprecedented in silane complexes.23
Furthermore, DFT calculations gave the free energy of
reaction ΔrG = +16.9 kJ mol−1 (298 K) for the conversion of 3
to 4 + H2. Such a low value is in accordance with the facile
transformation under experimental conditions. The process is
assisted by a very facile isomerization of 3cis (the DFT model
of 3) to a species 3trans featuring trans phosphines (P1−Ru−
P2 150.5°) only 3 kJ mol−1 higher in energy than the cis isomer
(see Supporting Information, Table S2). From 3trans, H2
release leading to 4, featuring also the two phosphorus in
trans disposition, is then facile.
At room temperature, the 1H NMR spectrum of 4 in toluene-
d8 solution shows in the hydride region a broad signal at δ
−4.34 (Figure 3). Decoalescence is observed at 273 K and two
2
signals in a 1:1 integration ratio are seen at δ −2.82 (dd, JPH
39.6 and 14 Hz) and δ −5.47 (br) at the low temperature limit
(213 K) (Figure 3). These rather low chemical shift values
might be surprising but are in the range found for trans
dihydride ruthenium species.35,36 In our system, the hydride
and the hydrogen attached to silicon (H−Si) are roughly in
trans position as determined in the solid state by X-ray
diffraction as well as by DFT. The exchange between the two
types of hydrogen, Ru−H and Ru−η2-Si−H, is thus blocked
and characterized by a ΔG‡ = 48.5 kJ mol−1, a value close to
those previously reported for Si−H/Ru−H exchanges in
bis(silane) complexes (47.5 kJ mol−1 to 68.4 kJ mol−1).27
The 31P{1H } NMR spectrum shows two inequivalent
phosphines giving rise to an AX spin pattern with two doublets
at δ 35 (phosphinodi(benzylsilane) ligand) and δ 53
The exchange of the two hydrogen atoms around the
ruthenium (Ru−H and Ru−H−Si) is characterized by a barrier
of 48.5 kJ mol−1, as determined by 1H NMR at 273 K. A much
more unusual observation of a decoalescence process by 29Si
NMR could be demonstrated. Hydride exchange is blocked at
213 K, and decoalescence of the 29Si signal observed at δ 32 at
324 K gives rise to two signals at δ 53 and δ 16. Most
noteworthy, both these two signals correlate with only the
2
(coordinated PCy3) exhibiting a JP−P coupling constant of
1
177 Hz. This value is by comparison 46% larger than that of 3
and in accordance with a significantly larger P−Ru−P angle,
which compensates for the unsaturated electronic character of
4. This increased coupling is a strong indicator of the
preservation in solution of the solid state structure for the
heavy atoms.
highest field H signal (δ −5.47) and with the same JSi−H (37
Hz). We propose that this behavior is consistent with the
intermediacy of a Si---H---Si bond interaction through a σ-
CAM mechanism.37 A simple rationalization would be a
structure in which only one hydrogen lies in between the two
silicons with the other being too far away to be involved in a σ-
CAM process. The RuH/Ru−H−Si positions could be easily
interchanged until the process is frozen at low temperature as
The 29Si{31P}{1H} NMR (99.4 MHz) spectrum shows a
broad singlet at δ 33 at 293 K for the two silicon atoms. At 324
K, broadening is reduced and a HMQC 1H−29Si{31P}
experiment shows clearly the correlation between the 29Si
1
demonstrated both by H and 29Si NMR. The rather different
chemical shifts observed in the 29Si NMR spectra at the low
temperature limit, namely, δ 54 and δ 16, are consistent with an
arrested structure with two Si atoms in different chemical
environments as shown in the solid state by X-ray diffraction.
We have recently reported a series of silazane complexes
exhibiting a range of multicenter Ru−H−Si bonds, and in
1
signal at δ 33 and the H signal at δ −4.4 (Figure 4 right). At
the low temperature limit (213 K), the unique 29Si signal gives
1
rise to two resonances at δ 54 and δ 16. A HMQC H−29Si-
{31P} experiment at 213 K shows the correlation of both silicon
1
signals with only the H resonance at highest field (δ −5.47,
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distillation under a reduced pressure. The oily residue was redissolved
in 50 mL of a 9:1 hexane/toluene mixture and transferred by cannula
to a frit containing silica. The filtrate was dried under a vacuum
yielding an opaque viscous liquid. Yield 83% from PhP{(o-C6H4)-
several cases the Si···H···Si interaction was evidenced by
different techniques but the silicon atoms could never be
discriminated by NMR.38 It should also be noted that our
findings are reminiscent of the transition metal−free poly-
agostic Si−H−Si interactions reported in a series of silylium
ions derived from polysilyl-substituted benzenes.39
1
CH3}2. H NMR (C6D6, 700 MHz, 293 K): δ [{0.04 (dd, JHH = 3.5
Hz, 1.4 Hz, SiMe2) + 0.05 (dd, JHH = 3.5 Hz, 1.4 Hz, SiMe2)} = 12H];
[{2.44 (d, 3JHH = 3.5 Hz, CH2) + 2.44 (d, 3JHH = 3.5 Hz, CH2)} = 4H],
4.22 (m, 2H, 1JSiH = 196 Hz, 3JHH = 3.5 Hz, SiH), 6.82 (t, J = 7 Hz, 2H,
aromatic), 7.02 (m, 9H, aromatic), 7.35 (tm, J = 7 Hz, 2H, aromatic).
31P{1H} (C6D6, 80.96 MHz, 293K): δ −21.1 (s). 13C{1H} (C6D6,
176.008 MHz, 293K): δ −4.27 (d, JCP = 2.1 Hz, SiCH3), −4.18 (d, JCP
= 2.1 Hz, SiCH3), 23.22 (d, JCP = 20.4 Hz, CH2), 124.98 (s, CHarom),
128.44 (d, JCP = 3.3 Hz, CHarom), 128.49 (s, CHarom), 128.75 (s,
CHarom), 128.99 (d, JCP = 4.9 Hz, CHarom), 134.09 (d, JCP = 19.5 Hz,
CONCLUSIONS
■
We have synthesized a novel pincer-like phosphinodi-
(benzylsilane) ligand that allows for the isolation of ruthenium
complexes exhibiting terminal hydride/η2-Si−H exchange
processes. Complex 3 is best formulated as an 18-electron
species stabilized by two rare high order ε-agostic Si−H
interactions with additional SISHA interactions. 3 can readily,
and reversibly, lose dihydrogen to produce the 16-electron
complex 4, featuring a 29Si NMR decoalescence, which to the
best of our knowledge has never been reported in silane
complex chemistry. The free energy ΔrG298 of the reaction of 3
to 4 + H2 is only +16.9 kJ mol−1 accounting for the facile
conversion observed experimentally at 308 K. Our study shows
that the phosphinodi(benzylsilane) can act as a “cooperating”
ligand accommodating different coordination modes at a metal
center through different extents of Si−H bond activation.
These first results show great promise for the development of a
new coordination chemistry with this pincer-like phosphinodi-
(benzylsilane) ligand, and we are currently exploring its
reactivity with other metal precursors. Catalytic studies are
also underway and will be reported in due course.
CHarom), 134.11 (s, Cipso arom), 134.22 (s, CHarom), 136.95 (d, JCP
=
10.9 Hz, Cipso arom), 145.17 (d, JCP = 26.9 Hz, Cipso arom). 29Si{1H}
NMR (CDCl3, 79.46 MHz, 293K): δ −11.5 (s). IR (C6D6): 2118
cm−1 (m, νSiH), 1259 cm−1 (s, νSiMe2). Anal. Calcd for C24H31PSi2:
C 70.88, H 7.68. Found: C 71.21, H 7.84. MS (EI+): m/z 405 (M+ −
1H, 10), 391 (M+ − CH3, 22), 256 (M+ − C6H4CH2SiMe2H − 1H,
70).
Synthesis of [RuH2{[η2-(HSiMe2)-CH2-o-C6H4]2PPh(PCy3)], 3.
Compound 1 (130 mg, 0.32 mmol) was dissolved in approximately 1
mL of either C6D6 or toluene-d8 and added to complex 2 (255 mg,
0.38 mmol) at 298 K in a Fischer−Porter vessel inside the glovebox.
Immediate evolution of gas was observed, and the sample was left to
stir for 1 h. The contents were then put under 3 bar of H2 gas and left
at room temperature. Crystals suitable for X-ray diffraction were
obtained after one week in these conditions. Yield 90%. For
spectroscopic characterization they were filtered off and dried under
1
a current of dihydrogen gas. H NMR (C6D6, 300 MHz, 293K): δ
−9.02 (br, 4H, Ru−H/σSi−H), 0.48 (br s, 6H, CH3), 0.64 (br s, 6H,
CH3), 1.08−1.95 (m, 33H, PCy3), 2.41 (br s, 4H, CH2), 6.59−7.57
(m, 11H, arom), 8.03 (m, 2H, arom). Diastereotopic methylene
protons were undistinguishable. 31P{1H} NMR (C6D6, 121.44 MHz,
EXPERIMENTAL SECTION
■
General Considerations. All experiments were performed under
argon atmosphere using standard Schlenk methods or in MBraun
glove boxes. THF, toluene, hexane, and pentane were either dried and
distilled from sodium using benzophenone ketyl as indicator or
purified over a MBraun column system. In either case, they were
degassed prior to use. Benzene-d6 and toluene-d8 were degassed via
three freeze−pump−thaw cycles and stored over molecular sieves.
Compound 2 was synthesized according to reported procedures.40
Nuclear magnetic resonance spectra were recorded on Bruker AV 300,
400, 500, Varian Inova 400 MHz, and Varian NMRS-700 MHz
spectrometers. Infrared spectra were recorded on a Bruker Alpha FT-
IR spectrometer equipped with a Platinum single reflection ATR
module. Microanalyses were performed at the Laboratoire de Chimie
de Coordination on a Perkin-Elmer 2400 Series II analyzer or at
UAEM on an Elementar Vario EL III instrument.
2
2
293 K): δ 29.6 (d, JPP = 121 Hz, PCy3), δ 68.3 (d, JPP = 121 Hz, P
ligand 1). 13C{1H} (toluene-d8, 125.81 MHz, 263K): δ 11.25 (s,
SiCH3), 26.68 (s, PCy3, CH2), 27.75 (d, JPC = 9.2 Hz, PCy3, CH2),
30.36 (s, PCy3, CH2), 34.90 (br, PSi2, CH2), 35.18 (br, PCy3, CH),
124.17 (d, CHarom), 129.02 (d, JCP = 3 Hz, CHarom), 129.26 (br s,
CHarom), 130.14 (br s, CHarom), 130.94 (br, CHarom), 134.90 (br,
Cipso arom), 136.50 (d, JCP = 1.3 Hz, Cipso arom), 136.83 (s, CHarom),
136.91 (s, CHarom), 145.53 (br, Cipso arom). 29Si{31P} NMR (toluene
-d8, 99.325 MHz, 273K): δ −0.7 (JSi−H app = 40 Hz). IR: 1997 cm−1 (w,
νRuHSi and RuH), 1873 cm−1 (m, νRuHSi and RuH), 1811 cm−1 (w,
νRuHSi and RuH).
Synthesis of [RuH{[η2-(HSiMe2)-CH2-o-C6H4]PPh[CH2-o-
C6H4SiMe2](PCy3)], 4. The reaction of compound 1 (130 mg, 0.32
mmol) and complex 2 (260 mg, 0.39 mmol) was performed in a
mixture of THF (approximately 1 mL) and hexane (approx. 0.3 mL).
The contents were degassed by two cycles of freeze−pump−thaw with
liquid nitrogen and kept under a vacuum at 298 K in a Schlenk tube
for ca. two weeks. The orange crystals were filtered off, washed three
times with cold hexane, and dried under a vacuum. Yield: 76% from 2.
Alternatively, keeping THF solutions of 3 at 308 K over 7 days yielded
complex 4 in 90% isolated yield. 1H NMR (toluene-d8, 500 MHz, 293
K): −4.34 (br, 2H, RuH and RuHSi); 0.40 (br, 12H, SiMe2); [{0.85−
2.1 (m, PCy3, overlapping with CH2) + 2.07 (m, overlapping with
PCy3, CH2)}= 35H]; 2.22 (dd, J = 5, 10 Hz, 2H, CH2), 6.89−7.96 (m,
13H, arom). Diastereotopic methyl groups were undistinguishable. 1H
NMR (toluene-d8, 500 MHz, 213 K): δ −2.82 (dd, 2JH‑PCy3 = 39.6 and
Synthesis of PhP{(o-C6H4CH2)SiMe2H}2, 1. The phosphine
PhP(o-tolyl)2 was made in house from the reaction of 2 equiv of
BrMg(o-C6H4CH3) and 1 equiv of PhPCl2. Indeed, 10 g (58.5 mmol)
of Br(o-C6H4CH3) in THF was added dropwise to activated Mg (1.45
g, 58.5 mmol), and the reaction mixture was left to react at reflux
temperature until the full consumption of the activated magnesium.
Addition of Cl2PPh (4 mL, 29.2 mmol) and stirring for 24 h left a
yellow solution and a white precipitate of MgBrCl which was separated
by filtration. To the solution, ice (2 g) was added followed by 100 mL
of a 0.5 M aqueous NH4Cl solution. The product was extracted twice
with 50 mL of ethylic ether using a separating funnel. The ethereal
phase was collected, dried over MgSO4, and finally evaporated to
dryness. The yellowish powder was further purified by recrystallization
from hot ethanol solutions at 273 K to yield a white solid in 75%, its
purity verified by mp and 1H and 31P NMR. PhP{(o-C6H4)CH3}2 (2 g,
6.9 mmol) was dissolved in 30 mL of hexane, TMEDA (2.1 mL, 13.8
mmol) and titrated nBuLi (13.8 mmol) were added. After being stirred
for 16 h, the bright orange reaction mixture was cooled to −78 °C, and
ClSiMe2H (3 mL, 27.5 mmol) was added via syringe. The off-white
suspension was allowed to warm up to room temperature and kept
under stirring for 12 h, after which the solvent was removed by
2
2JH‑PSi2 = 14 Hz, 1H, RuH101), −5.47 (pseudo t, JH‑PCy3 = 14 Hz,
2JH‑PSi2 = 6.7 Hz, 1H, RuH100Si). The assignment was made upon
selective 31P NMR decoupling experiments. 31P{1H } NMR (toluene-
d8, 202.54 MHz, 213 K): δ 35.0 (d, 2JPP = 177 Hz, PSi2), 53.1 (d, 2JPP
=
177 Hz, PCy3). 29Si{31P}(toluene-d8, 99.40 MHz, 324 K): δ 32 (br,
correlation with the −4.4 1H signal). 29Si{31P}(toluene-d8, 99.40 MHz,
213 K): δ 54 (br, JH100−Si = 37 Hz), 16 (br, JH100−Si = 37 Hz). 13C{1H}
(toluene-d8, 125.81 MHz, 293 K): δ 8.38 (s, SiCH3), 9.62 (s, SiCH3),
26.45 (s, PCy3, CH2), 27.41 (d, JCP = 10.1 Hz, PCy3, CH2), 29.64 (s,
9804
dx.doi.org/10.1021/ic400703r | Inorg. Chem. 2013, 52, 9798−9806

Inorganic Chemistry
Article
3
PCy3, CH2), 34.17 (d, JCP = 21.4 Hz, PSi2, CH2), 35.94 (br, PCy3,
material is available free of charge via the Internet at http://
pubs.acs.org.
CH), 127.88 (br s, CHarom), 128.16 (br s, CHarom), 129.02 (s,
CHarom), 129.43 (s, CHarom), 129.99 (d, JCP = 6.29 Hz, CHarom),
130.20 (s, CHarom), 134.19 (s, Cipso arom), 134.82 (d, JCP = 7.5 Hz,
Cipso arom), 147.48 (d, JCP = 15.1 Hz, Cipso arom). IR: 1808 cm−1 (w,
νRuHSi and RuH). Anal. Calcd for C42H64P2Si2Ru: C 64.02, H 8.19.
Found: C 63.90, H 8.11.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: vmontielp@uaem.mx (V.M.P.), sylviane.sabo@lcc-
toulouse.fr (S.S.E.).
X-RAY DATA
■
Notes
Important crystallographic data are given in Table 2; the
experimental procedure and relevant data are in the Supporting
Information as CIF data (also deposited at the CCDC: No.
929314 (3), No. 929315 (4)).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by CONACYT (Project 105762 and
sabbatical grants), CNRS and Universite
professorship to V.M.P. and M.A.M.H.). Computer time was
given by the HPC resources of CALMIP (Toulouse, France)
́
Paul Sabatier (invited
Table 2. Crystal Data for Compounds 3 and 4
3
4
under the allocation 2012-[P0909]. Ms. Hanit Trevino is
̃
formula
C84H132P4Ru2Si4
C42H64P2RuSi2·
2(C4H8O)
thanked for laboratory assistance and Dr. Yannick Coppel for
recording 29Si NMR spectra.
formula weight
cryst syst
space group
a, Å
1580.28
932.33
monoclinic
P21/c (No. 14)
37.6275(15)
10.4778(3)
20.726(5)
90
triclinic
P1 (No. 2)
̅
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COMPUTATIONAL DETAILS
■
DFT calculations were performed with the GAUSSIAN 03 series of
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