Structural similarities in dinuclear, tetranuclear, and pentanuclear nickel silyl and silylene complexes obtained via Si-H and Si-C activation

Article abstract of DOI:10.1021/om300120x

The reactions of (ⁱPr₃P)₂Ni⁰ precursors with Ph₂SiHCl, Ph₂SiH₂, PhSiH ₃, and Ph₃SiH provide mononuclear, dinuclear, tetranuclear, and pentanuclear complexes with silyl and silylene ligands. Reaction of the dinuclear Ni(0) dinitrogen complex [(ⁱPr ₃P)₂Ni]₂(μ-η¹: η¹-N₂) with Ph₂SiHCl afforded the thermally sensitive mononuclear complex (ⁱPr₃P)₂Ni(H) SiClPh₂ (1), which displays considerable hydridic character in the Ni-H-Si interaction. This species thermally converts to the dinuclear complex [(ⁱPr₃P)Ni(μ-SiHPh₂)]₂ (2), where the silyl ligand bridges via an agostic Si-H interaction. Alternate higher-yield routes to 2 include the rapid room-temperature reaction of Ph ₂SiH₂ with [(ⁱPr₃P) ₂Ni]₂(μ-η¹:η¹-N ₂) and the reaction of Ph₂SiH₂ with Ni(1,5-cyclooctadiene)₂ and ⁱPr₃P at elevated temperatures. Double Si-H activations are observed in the reaction of PhSiH ₃ with [(ⁱPr₃P)₂Ni] ₂(μ-η¹:η¹-N₂), which provides the tetranuclear C₃-symmetric complex (ⁱPr ₃P)Ni[(ⁱPr₃P)Ni(μ₃-SiHPh)] ₃ (3) as the major product and the pentanuclear S₄- symmetric complex Ni[(ⁱPr₃P)Ni(μ₃-SiHPh)] ₄ (4) as a minor product. Density functional theory (DFT) geometry optimizations of model complexes support the presence of agostic Ni-H-Si interactions within the tetra- and pentanuclear cores of 3 and 4. The reaction of Ph₃SiH with [(ⁱPr₃P)₂Ni] ₂(μ-η¹:η¹-N₂) results in Si-C bond cleavage to provide the asymmetric dinuclear complex [( ⁱPr₃P)₂Ni]₂(μ-C₆H ₅)(μ-SiHPh₂) (5), where the phenyl moiety is asymmetrically bridging. Complexes 2-5 all display similar coordination environments at one of the nickel centers, suggestive of formal oxidation state assignments of Ni(III)-Ni(I) in tetranuclear 3 and Ni(IV)-Ni(I) in pentanuclear 4.

Full text of DOI:10.1021/om300120x

Article
pubs.acs.org/Organometallics
Structural Similarities in Dinuclear, Tetranuclear, and Pentanuclear
Nickel Silyl and Silylene Complexes Obtained via Si−H and Si−C
Activation
Robert Beck and Samuel A. Johnson*
Department of Chemistry and Biochemistry, University of Windsor, Sunset Avenue 401, Windsor, Ontario N9B 3P4, Canada
S
* Supporting Information
ABSTRACT: The reactions of (iPr3P)2Ni0 precursors with
Ph2SiHCl, Ph2SiH2, PhSiH3, and Ph3SiH provide mono-
nuclear, dinuclear, tetranuclear, and pentanuclear complexes
with silyl and silylene ligands. Reaction of the dinuclear Ni(0)
dinitrogen complex [(iPr3P)2Ni]2(μ-η1:η1-N2) with Ph2SiHCl
afforded the thermally sensitive mononuclear complex
(iPr3P)2Ni(H)SiClPh2 (1), which displays considerable
hydridic character in the Ni−H−Si interaction. This species
thermally converts to the dinuclear complex [(iPr3P)Ni(μ-SiHPh2)]2 (2), where the silyl ligand bridges via an agostic Si−H
interaction. Alternate higher-yield routes to 2 include the rapid room-temperature reaction of Ph2SiH2 with [(iPr3P)2Ni]2(μ-
η1:η1-N2) and the reaction of Ph2SiH2 with Ni(1,5-cyclooctadiene)2 and iPr3P at elevated temperatures. Double Si−H activations
are observed in the reaction of PhSiH3 with [(iPr3P)2Ni]2(μ-η1:η1-N2), which provides the tetranuclear C3-symmetric complex
(iPr3P)Ni[(iPr3P)Ni(μ3-SiHPh)]3 (3) as the major product and the pentanuclear S4-symmetric complex Ni[(iPr3P)Ni(μ3-
SiHPh)]4 (4) as a minor product. Density functional theory (DFT) geometry optimizations of model complexes support the
presence of agostic Ni−H−Si interactions within the tetra- and pentanuclear cores of 3 and 4. The reaction of Ph3SiH with
[(iPr3P)2Ni]2(μ-η1:η1-N2) results in Si−C bond cleavage to provide the asymmetric dinuclear complex [(iPr3P)2Ni]2(μ-C6H5)(μ-
SiHPh2) (5), where the phenyl moiety is asymmetrically bridging. Complexes 2−5 all display similar coordination environments
at one of the nickel centers, suggestive of formal oxidation state assignments of Ni(III)−Ni(I) in tetranuclear 3 and Ni(IV)−
Ni(I) in pentanuclear 4.
not feature electronically, chelate, or sterically stabilized silylene
moieties.
INTRODUCTION
■
Transition-metal silyl and silylene complexes have attracted
attention because of their importance in transition-metal-
catalyzed transformations of silicon compounds1 as well as their
unique bonding and properties.2 Catalytic transformations
mediated by these species include hydrosilylation, dehydrocou-
pling, bis-silylation, silylcarbonylation, and metathesis reactions
and typically involve 4d and 5d transition metals.3 Less
attention has been given to nickel catalysts and complexes, but
they are of current interest due to the low cost of nickel in
comparison to that of the precious metals. Catalytic hydro-
silylation,4 dehydrogenative silylation of olefins,5 and dehy-
drocoupling of organosilanes6 to afford polysilanes have been
reported with nickel catalysts.
Our previous research has shown that with the appropriate
ancillary ligands and substrates Ni complexes can both
activate13 and catalytically functionalize C−H bonds.14 Herein
we report the reaction of Ni(0) precursors with a series of
silanes, which results in di-, tetra-, and pentanuclear organosilyl
or silylene nickel complexes with agostic Ni−H−Si interactions
derived from Si−H activation and Si−C bond breaking of
simple organosilanes from reactions with low-valent nickel
precursors. Despite the range of nuclearity in the structures
described, all but the mononuclear complex contain centers
that structurally resemble formal Ni(I) centers stabilized by
metal−metal bonding interactions.
Isolated hydridosilyl and silylene transition-metal complexes
provide insight into the intermediate stages of Si−H oxidative
addition and thus are indispensable for the development of new
catalytic methodologies.7 The majority of silylene complexes
feature second- and third-row transition metals. Although
mononuclear complexes have been accessed via double Si−H
activation,8 multinuclear complexes with bridging silylene
ligands are commonly formed from related reactions.8,9 Fewer
nickel complexes bearing silane,10 silyl,11 or silylene ligands12
have been reported; particularly scarce are complexes that do
RESULTS AND DISCUSSION
■
The reaction of [(iPr3P)2Ni]2(μ-η1:η1-N2)15 with 2 equiv of
Ph2SiHCl in n-pentane at −34 °C afforded the crystalline
mononuclear complex (iPr3P)2Ni(μ-H)(SiClPh2) (1) in 19%
yield (Scheme 1). The reaction occurs within 1 min, and the
modest yield is due to the thermal sensitivity of the complex. In
Received: February 14, 2012
Published: April 18, 2012
© 2012 American Chemical Society
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magnitude of the errors in the distances to the hydrogen atom
reflects the modest electron density associated with this atom in
comparison to that of the adjacent Ni and Si atoms. The
Ni(1)−H(1) distance of 1.41(3) Å is suitable for a nickel
hydride.13f,17 Although the Si(1)−H(1) distance of 1.78(3) Å is
significantly elongated in comparison to a typical Si−H bond,
which are more typically in the range of 1.4−1.5 Å, it is not as
long as the distances of 1.9−2.4 Å usually associated with
secondary interactions between a silicon atom and an adjacent
hydride.2l The relative position of the hydride and the silyl
substituents is consistent with an elongated η2-SiH σ complex,1
but related complexes with similar spectroscopic features but
slightly longer Si−H distances have been described as silyl
hydrido complexes.11c,l It should be noted that there is a
structural continuum between the limiting structures of an η2-
SiH σ-silane complex and complete oxidative addition to the
silyl hydrido complex.11c The molecular structure of 1 has a
near-planar nickel center, with a sum of bond angles around
nickel of 360(1)° for the two phosphine ligands, the silicon
atom, and the bridging hydrogen atom, with an internal Si(1)−
Ni(1)−H(1) angle of 52.6°. Two closely related complexes
with differing degrees of H−Si interactions have been described
with chelating bis-phosphine11c and nonchelating NHC
ligands.11l Both of these related complexes were reported as
stable at room temperature. The P(1)−Ni(1)−P(2) angle in
complex 1 is 115.96(3)°, whereas the related example11c which
utilizes 1,2-bis(di-tert-butylphosphine)ethane features an small-
er P−Ni−P angle of only 95.51(6)°; this suggests that the
decreased thermal stability of 2 is possibly due to increased
repulsion between the nonchelating phosphine donors. The
Ni(1)−Si(1) distance of 2.2211(9) Å and Si(1)−Cl(1) distance
of 2.1623(11) Å are similar to those reported for the 2-bis(di-
tert-butylphosphine)ethane analogue.11c Examples are known of
both a cationic analogue11b and Ni(0) silane complexes with
shorter Si−H interactions,10,11c as determined by X-ray
crystallography.
For mononuclear complex 1, the Ni−H−Si resonance is
observed at δ −10.75 as a triplet with a JPH value of 20.5 Hz.
The hydridic shift is suggestive of oxidative addition, so that an
alternate view of 1 is as a Ni(II) complex. The observation of a
single environment in the 121.5 MHz 31P{1H} NMR as low as
the freezing point of d8-toluene suggests a low barrier for
fluxional exchange from the approximately Cs-symmetric
structure observed in the solid state. The observed JPH value
is significantly smaller than coupling constants for trans-
disposed phosphines in Ni(II) hydrides, which are typically
near 70 Hz.13c−f The 29Si NMR shift was observed at δ 10.2 as a
triplet with a 2JSiP value of 62.5 Hz, but no resolved JSiH. The IR
spectrum of 1 features a strong absorption at 1887 cm−1 that
can be assigned as a stretching mode for the Ni−H moiety.
This was confirmed by a DFT calculation for 1 using the
B3LYP functional and the TZVP basis set in Gaussian 09,
which provided an unscaled Ni−H IR stretch at 1905 cm−1 as
the only vibration in that region.
Scheme 1
aliphatic or aromatic solvents 1 decomposes in less than 1 h at
20 °C. Despite the strength of the Si−Cl bond,16 the chlorine
substituent from Ph2SiHCl is abstracted to produce NiCl-
(PiPr3)2.15 The possible transient nickel(I) diphenylsilane
fragment (iPr3P)2Ni(SiHPh2) is not observed, but rather the
dinuclear complex [(iPr3P)Ni(μ-SiHPh2)]2 (2) is isolated in
low yield, from dimerization with loss of iPr3P. In contrast to 1,
complex 2 is stable under refluxing conditions in toluene. A
more synthetically useful route to 2 is via the reaction of
Ph2SiH2 with [(iPr3P)2Ni]2(μ-η1:η1-N2), which provides 2 in
86% yield. Similarly, the reaction of Ph2SiH2 with stoichio-
metric amounts of Ni(COD)2 in the presence of iPr3P provided
2 in 56% yield after heating at 55 °C for 16 h, obviating the
need for [(iPr3P)2Ni]2(μ-η1:η1-N2) in the synthesis of 2. These
various routes to 2 are summarized in Scheme 1.
The solid-state structure of 1 was determined by X-ray
crystallography, and an ORTEP depiction is shown in Figure 1.
The hydrogen atom originating from the silane was located in
the electron density map, and its position was refined; the
The solid-state structure of 2 was determined by X-ray
crystallography, and an ORTEP depiction is shown in Figure 2.
The molecule contains a crystallographic inversion center at the
midpoint of the Ni−Ni bond. The P, Ni, and Si atoms all lie
approximately within a plane. The bridging agostic interaction
is almost coplanar with the [Ni2Si2] core fragment, and the
agostic hydrogen is located in the pocket afforded by the wide
P(1)−Ni(1)i−Si(1) angle of 138.60(2)°, in comparison to the
narrower P(1)−Ni(1)−Si(1)i angle of 108.56(3)°. The Ni(1)−
Figure 1. Depiction of the solid-state molecular structure of 1 as
determined by X-ray crystallography with 30% probability ellipsoids.
Hydrogen atoms not attached to Si or Ni are omitted for clarity.
Selected bond distances (Å) and angles (deg): Ni(1)−Si(1),
2.2211(9); Ni(1)−H(1), 1.41(3); Si(1)−H(1), 1.78(3); Ni(1)−
P(1), 2.2045(8); Ni(1)−P(2), 2.2375(8); P(1)−Ni(1)−P(2),
115.96(3); P(1)−Ni(1)−Si(1), 109.93(3); P(2)−Ni(1)−Si(1),
134.07(3).
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3. These two reaction products are shown in eq 1. Complex 4
crystallizes as black needles that are insoluble in pentane and
Figure 2. Depiction of the solid-state molecular structure of 2 as
determined by X-ray crystallography with 30% probability ellipsoids.
Hydrogen atoms not attached to Si and Ni are omitted for clarity. The
superscript labels, e.g. Ni(1)i, indicate atom locations generated by a
crystallographic inversion center. Selected bond distances (Å) and
angles (deg): Ni(1)−Ni(1)i, 2.4878(6); Ni(1)−Si(1), 2.2654(7);
Ni(1)−Si(1)i, 2.2164(8); Ni(1)−H(1), 1.55(2); Si(1)−H(1), 1.64(2);
Ni(1)−P(1), 2.1681(6); Si(1)−Ni(1)−H(1), 46.5(8); P(1)−Ni(1)−
Si(1), 138.60(2); P(1)−Ni(1)−Si(1)i, 108.56(3); P(1)−Ni(1)−
Ni(1)i, 165.14(2); P(1)−Ni(1)−H(1), 92.9(8).
barely soluble in benzene. This complex is therefore readily
separated from the significantly more soluble 3, which is partly
soluble in pentane and soluble in benzene. The pentanuclear
compound could also be isolated by manual separation of the
dark brown crystalline needles of 4 from the large red crystals
of 3. The stoichiometry of the reaction for the two products is
slightly different. The generation of 3 requires 2 equiv of the
starting dinitrogen complex reacting with 3 equiv of PhSiH3,
which liberates 4 equiv of iPr3P and 3 equiv of H2 gas. Complex
4 is produced from 21/2 equiv of the starting dinitrogen
complex reacting with 4 equiv of PhSiH3 and produces 6 equiv
Ni(1)i distance of 2.4878(6) Å is relatively long compared to
the typical range of Ni−Ni single-bond distances of 2.36−2.54
Å.18 In comparison to 1, complex 2 features a 0.14(4) Å longer
Ni(1)−H(1) distance of 1.55(2) Å and a 0.14(4) Å shorter
Si(1)−H(1) distance of 1.64(2) Å, consistent with weaker
back-donation from a Ni(I) versus a Ni(0) species. The Ni(1)−
Si(1) distance for the agostic interaction of 2.2654(7) Å is
0.044(1) Å longer than that observed in 1, whereas the σ-
bound Ni(1)−Si(1)i distance of 2.2164(8) Å is a mere 0.005(1)
Å shorter. Related dimeric structures have been previously
described with nickel’s heavier congeners Pt and Pd,9b,19,20 and
a related Ni complex has recently been reported.11m
i
of Pr3P and 8 equiv of H2 gas. Attempts to isolate larger
amounts of 4 by the addition of up to 20 equiv of PhSiH3 to the
reaction mixture failed. Attempts to synthesize 3 using
Ni(COD)2 in lieu of [(iPr3P)2Ni]2(μ-η1:η1-N2) were only
modestly successful; small amounts of 3 were produced, as
1
characterized by H and 31P{1H} NMR spectroscopy, but only
when the reaction mixture was heated to 60 °C for extended
periods, and this was not a practical high-yield synthetic route
to 3.
1
The H NMR spectrum of the dimeric complex 2 exhibits a
Si−H shift at δ −2.05 as a doublet with a 2JPH coupling constant
of 15.0 Hz. The 29Si NMR shift is observed at δ 123.7 as a
The molecular structure of 3 was determined by single-
crystal X-ray diffraction, and an ORTEP depiction is shown in
Figure 3. The Ni4Si3 core is composed of an alternating Ni3Si3
fragment with a cyclohexane ring chair conformation that is
capped by the unique apical Ni moiety that lies on the C3
symmetry axis. This apical nickel is bound to three symmetry-
related SiHPh moieties, with a Ni(2)−Si(1) distance of
2.2010(7) Å. The Ni3Si3 ring features a short Ni(1)−Si(1)
distance of 2.1935(8) Å alternating with a longer Ni(1)−Si(1)a
distance of 2.2645(8) associated with the Si−H- - -Ni
interaction. The bridging hydrogen ligand has a Ni(1)−H(1)
distance of 1.57(4) Å and a Si(1)−H(1) distance of 1.61(3) Å.
A comparison of the Ni−Si bonds in 3 with the single bonds
in 2, where the Ni−Si single-bond distance is 2.2164(8) Å and
the Si−H- - -Ni interaction has a Ni−Si distance of 2.2654(7)
Å, suggests that the Ni(1)−Si(1) and Ni(2)−Si(1) interactions
in 3 are single bonds; mononuclear NiSi double-bond
distances are significantly shorter (2.13 Å11j), although an
example of a heterocyclic mononuclear silylene features a
similar distance (2.20 Å12a). Recently Hillhouse et al. reported a
related mononuclear silylene coordination with an agostic Ni−
2
1
doublet with a JSiP value of 48.9 Hz. A JSiH value of 59.6 Hz
2
1
and a JSiH value of 20.3 Hz were observed in both the H and
29Si NMR spectra of 2; the observation of a large JSiH may be
2
1
due to the trans disposition of these nuclei. Typically, JSiH
coupling constants are negative and it is expected that JSiH
2
should be positive;2k interestingly, DFT predictions of NMR
coupling constants using the model complex [(Me3P)Ni(μ-
SiHPh2)]2 (2Me) suggest that both of the observed coupling
constants are negative. In 2 a stretching mode is observed for
the Ni−HSi moiety at 1582 cm−1. DFT calculations on the
model complex [(Me3P)Ni(μ-SiHPh2)]2 (2Me) predict un-
scaled asymmetric and symmetric Ni- - -H- - -Si IR stretches at
1565 and 1577 cm−1, respectively. Only the former is expected
to be IR active.
Treatment of dark red solutions of [(iPr3P)2Ni]2(μ-η1:η1-N2)
in n-pentane at 20 °C with stoichiometric amounts of PhSiH3
afforded the tetranuclear complex (iPr3P)Ni[(iPr3P)Ni(μ3-
SiHPh)]3 (3) in 54% yield as dark red crystals. The minor
pentanuclear byproduct Ni[(iPr3P)Ni(μ3-SiHPh)]4 (4) was
reproducibly isolated in 3−5% yield in multiple preparations of
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symmetry. The core structure of the pentanuclear complex
features the nickel center Ni(2) surrounded by a four Ni(1)
atoms in a square-planar arrangement, with a Ni(1)−Ni(2)
distance of 2.460(1) Å. The four bridging PhSi−H ligands are
coordinated around the central Ni(1) atom in a distorted
tetrahedron, with a 98.27(5)° Si(1)−Ni(2)−Si(1)b angle
bisected by the S4 axis and a 115.34(3)° Si−Ni(1)−Si(1)a
angle. Although the assignment of oxidation states in clusters
can be subjective, due to delocalized multicenter bonding, it is
interesting to consider what formal oxidation states could be
assigned in these highly symmetric complexes. The Ni(2)−
Si(1) bond lengths in 4 of 2.1743(2) Å are quite short. The
Ni(1)−Si(1) bond length of 2.2136(10) Å is significantly
longer, whereas the agostic Ni(1)−Si(1)a distance is
2.2901(10) Å. The Si(1)−H(1) distance is 1.65(4) Å, and
the Ni(1)−H(1)a distance is 1.55(4) Å. The coordination
environments for the Ni(1) centers in tetranuclear 3 and
pentanuclear 4 both resemble the Ni(I) centers in 2 (vide
infra); by this reasoning, one could dissect complex 3 as a
Ni(III) center surrounded by and bonded to three Ni(I)
centers and complex 4 as a Ni(IV) center surrounded by four
Ni(I) centers. The presence of Ni−Ni bonds, which are likely
significantly polarized, may aid in stabilizing such unusual
formal oxidation state combinations.23
Figure 3. Depiction of the solid-state molecular structure of 3 as
determined by X-ray crystallography with 30% probability ellipsoids.
Nickel−nickel bonds are shown as open bonds, and isopropyl
substituents and hydrogen atoms not attached to Si or Ni are omitted
for clarity. Selected bond distances (Å) and angles (deg): Ni(1)−
H(1), 1.57(4); Si(1)−H(1), 1.61(3); Ni(1)−Ni(2), 2.5704(5);
Ni(1)−Si(1), 2.1935(8); Ni(1)−Si(1)a, 2.2645(8); Ni(2)−Si(1),
2.2010(7); Ni(1)−P(1), 2.1495(7); Ni(2)−P(2), 2.2048(12);
Si(1)−Ni(2)−P(2), 108.90(2); P(1)−Ni(1)−Ni(2), 160.86(3);
P(2)−Ni(2)−Ni(1), 126.47(1).
H−Si interaction, where the bridging hydrogen participates in a
three-center−two-electron bond using the 1s H orbital and the
π orbital of the NiSi core to effectively give a protonated
NiSi double bond.11b In this example, the chelating
1,2-tBu2PCH2CH2PtBu2 ligand presumably prevents the mono-
nuclear species from cluster formation, in comparison to the
iPr3P-supported clusters 3 and 4. Silyl-containing clusters are
rare,21 and only recently was an unusual cluster with a Ni3Si2
core with six-coordinate silicon described.11k More closely
related to 3 and 4 are multinuclear Pd clusters, containing a
planar Pd4Si3 core20e and a Pd4Si5 framework.22 Both palladium
clusters show no agostic interactions.
1
The H, 31P{1H}, and 29Si{1H} NMR spectra support the
1
solid-state molecular structures of 3 and 4. The H NMR of 3
features a characteristic upfield-shifted24 doublet of doublets for
2
3
the agostic Si−H at δ −1.81 with JPH and JPH values of 12.0
and 1.8 Hz, respectively. In 4 a doublet at δ −2.27 ppm with
2JPH = 11.5 Hz was observed. In contrast, transition-metal
silylenes without agostic interactions typically feature downfield
Si−H 1H NMR resonances (δ 9.14,25 12.1,26 15.320e) in
comparison to the resonances of free organosilanes. The
31P{1H} NMR spectra of 3 features two 31P resonances in a 3:1
ratio. The resonance at δ 59.8 is a doublet, and the lower
intensity resonance as δ 68.7 is a quartet attributed to the apical
The minor product 4 shares some structural similarities to 3,
and a depiction of the solid-state molecular structure is shown
in Figure 4. The molecule has crystallographically imposed S4
3
P atom, with a mutual JPP coupling constant of 12.5 Hz. As
expected, one resonance was observed in the 31P{1H} NMR
spectrum of 4, at δ 61.8. The 29Si{1H} NMR spectra of 3 and 4
display characteristic downfield shifts for the silylene 29Si
resonances at δ 300.4 and 277.4, respectively. The former is a
2
doublet of doublets (2JPSi = 47.9 Hz, JPSi = 10.9 Hz), whereas
the latter is a doublet (2JPSi = 45.9 Hz). A related
diphenylsilylene bridging triangular triplatinum complex
features a similar 29Si{1H} NMR shift of δ 279.4.27 Downfield
29Si NMR spectroscopic shifts in the range of 250−350 ppm
are typically attributed to a paramagnetic shielding effect from
silylene bridging metal−metal bonds.2g The 1JSiH values of 45.0
and 35.9 Hz were observed for 3 and 4, respectively.
Silicon−Carbon Bond Cleavage. An unexpected Si−C
bond cleavage was observed when Ph3SiH was reacted with
stoichiometric amounts of [(iPr3P)2Ni]2(μ-η1:η1-N2) in n-
pentane, as shown in eq 2. Within 20 min at 20 °C this
reaction provided [(iPr3P)Ni]2(μ-C6H5)(μ-SiHPh2) (5), which
was isolated in 48% yield as brown needles from slow
evaporation of a solution in hexamethyldisiloxane/benzene.
Isolated 5 dissolved to give a pale green solution in C6H6.
The solid-state molecular structure of 5 was determined by
X-ray crystallography, and an ORTEP depiction is shown in
Figure 5. Complex 5 has a Ni(1)−Ni(2) bond length of
2.4075(9) Å, which is in the typical range for single bonds, but
much shorter than in 2 (2.4878(6) Å). The cleaved phenyl
Figure 4. Depiction of the solid-state molecular structure of 4 as
determined by X-ray crystallography with 30% probability ellipsoids.
Hydrogen atoms not attached to Si and Ni are omitted for clarity.
Selected bond distances (Å) and angles (deg): Ni(1)−Ni(2),
2.460(1); Ni(1)−Si(1), 2.214(1); Ni(1)−Si(1)a, 2.290(1); Ni(2)−
Si(1), 2.1780(9); Si(1)−H(1), 1.65(4), Ni(1)−H(1), 1.55(4); P(2)−
Ni(1)−Ni(2), 159.79(2); P(2)−Ni(1)−Si(1), 104.74(3); P(2)−
Ni(1)−Si(1), 145.64(3); Si(1)−Ni(2)−Si(1), 115.34(3); Si(1)−
Ni(2)−Si(1), 98.27(5).
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of strong back-donation from the Ni(I) center to the Si−H
moiety, which is supported by a short Ni(2)−H(1) distance of
1.46(6) Å, although the error is too large to draw a conclusion
from these data alone.
1
The H NMR spectrum of isolated 5 features a high-field-
shifted doublet resonance at δ −7.09 for the Si−H fragment
2
with a JPH coupling constant of 12.0 Hz in a range similar to
that for 1 but significantly less shielded in comparison to that
for dinuclear complex 2, suggesting a more hydridic character
in 5. In related Pt complexes of the type (R3P)2Pt(H)(μ-
SiR2)(μ-HSiR2)Pt(PR3), shifts of δ −4.9 to −7.05 have been
assigned as hydrides and thus formal Pt(III)−Pt(I) structur-
es.9b,20a,c In the 29Si{1H} NMR spectrum at δ 124.5 ppm a
2
doublet of doublets with JPSi = 82.5 and 3.2 Hz is observed,
which correlates in the 31P{1H} NMR spectrum to two
different phosphorus environments at δ 55.2 and 46.3 with 3JPP
= 24.3 Hz, respectively, suggesting that 5 is a C1-symmetric
complex. However, the more hydridic character suggested by
the upfield Ni−H shift only modestly correlates with the Si−
H−Ni stretch observed at 1635 cm−1, which is considerably
lower than the Ni−H stretching frequency observed for 1. Such
an observation is inconsistent with complete oxidative addition
of the Si−H bond. In comparison, related Pt and Pd complexes
are known with similar hydridic chemical shifts in the 1H NMR
spectrum that have been assigned as transition-metal hydrides
from complete oxidative addition largely on the basis of NMR
data.9b,19c,20a,c
Figure 5. Depiction of the solid-state molecular structure of 5 as
determined by X-ray crystallography with 30% probability ellipsoids.
Hydrogen atoms not attached to Si and Ni are omitted for clarity.
Selected bond distances (Å) and angles (deg): Ni(1)−Ni(2),
2.4075(9); Ni(1)−H(1), 1.46(6); Si(1)−H(1), 1.64(5); Ni(1)−
C(1), 1.892(5); Ni(2)−C(6), 2.439(5); Ni(2)−C(1) 1.958(5);
Ni(2)−Si(1), 2.2378(15); Ni(1)−Si(1), 2.2204(15); Ni(1)−P(1),
2.1966(15); Ni(2)−P(2), 2.1664(15); P(1)−Ni(1)−Ni(2),
153.19(5); P(1)−Ni(1)−Si(1), 145.47(6).
The cleavage of Si−C bonds by transition metals has
precedent, albeit the majority of examples feature the 4d and 5d
metals.28,29 Jones et al. were able to isolate several Si−C bond
coupling products from Si−C bond activation reactions of
alkynylsilanes and biphenylene with a Ni(0) precursor and
postulated a mononuclear nickel-mediated Si−C bond for-
mation process.30 We have recently shown that C−C coupling
group bridges the Ni centers in an asymmetric fashion, with σ
bonding to Ni(1) and an interaction of the π electrons to
Ni(2). The Ni(1)−C(1) σ bond of 1.892(5) Å is only 0.066(7)
Å shorter than the highly asymmetric Ni(2)−C(1) π-bonding
distance of 1.958(5) Å, with a longer Ni(2)−C(6) distance of
2.439(5) Å. A similar back-donation to a formally Ni(I) center
has been observed in a formally Ni(III)−Ni(I) complex.23 The
Ni(2)−Si(1) σ bond distance of 2.2378(15) Å is unexpectedly
longer than the Ni(1)−Si(1) distance of 2.2204(15) Å
associated with the agostic Si−H interaction. This is suggestive
i
of biphenylene with Ni(COD)2 in the presence of Pr3P
involves a formally dinuclear mixed-valent Ni(III)−Ni(I)
intermediate.23 Complex 5 suggests that dinuclear reactive
intermediates in Si−C coupling are also plausible. Group 10
complexes that undergo Si−C(sp2) bond cleavage have been
reported,31 and recently reversible intramolecular Si−C(sp3)
bond activation has been observed in a nickel-silyl pincer
complex11a and an amidodiphosphine complex of Ni(II).32
Figure 6. ORTEP representations selected fragments of 2−5 showing the similarity of approximately planar coordination environments to formal
Ni(I) centers, shown as the rightmost Ni(1) labeled in boldface in all four depictions. In all cases 30% probability ellipsoids are shown.
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Table 1. Summary of Experimental and Calculated Bond Length and NMR and IR Spectroscopy Data for Compounds 1−5
a
b
1
2
3
4
5
IR stretch (Ni−H−Si)
1887
1582
1578
1567
1551
1635
calcd IR stretch (Ni−H−Si)
1905
1584 (s, Au)
1548 (s, E)
1538 (w, A)
1556.3 (s, B)
1551.6 (m, B)
1613 (s)
1602 (s)
e
1595 (Ag)
e
1547 (A)
1H NMR shift for Si−H (δ)
−10.75
−10.4
10.16
54.8
−2.05
−2.3
123.7
−1.81
−0.5
300.4
−2.27
−0.8
277.4
−7.07
−7.7
124.5
130.0
50.3
d
1
calcd H NMR shift for Si−H (δ)
29Si NMR shift (δ)
calcd 29Si NMR shift (δ)
d
130.1
298.0
284.0
|JSiH| (Hz)
calcd JSiH (Hz)
<7
59.6, 20.0
−53.2, −14.7
45.0
35.9
c
70.8
−35.5, −11.2
−39.9, −9.0
−39.8
exptl Si−Ni distance (Å)
calcd Si−Ni distance (Å)
exptl agostic Si- - -Ni distance (Å)
calcd agostic Si- - -Ni distance (Å)
exptl Si−H distance (Å)
2.2211(9)
2.2708
n.a.
2.2164(8)
2.2499
2.1935(8)
2.23044
2.2645(8)
2.2815
2.214(1)
2.22961
2.290(1)
2.3072
2.2378(15)
2.25375
2.2204(15)
2.2552
f
2.2654(7)
2.3263
f
n.a.
1.78(3)
1.869
1.41(3)
1.486
n.a.
1.64(2)
1.597
1.61(3)
1.685
1.65(4)
1.672
1.64(5)
1.698
calcd Si−H distance (Å)
exptl Ni−H distance (Å)
calcd Ni−H distance (Å)
exptl Ni−Ni distance (Å)
calcd Ni−Ni distance (Å)
1.55(2)
1.661
1.57(4)
1.618
1.55(4)
1.621
1.46(6)
1.604
2.4878(6)
2.5704(5)
2.6013
2.442(3)
2.4075(9)
2.4472
n.a.
2.5390
2.5094
a
b
c
d
Calculations on model complex 3Me. Calculations on model complex 3Me. Calculated JSiH for model complex 2Me. Relative to Me4Si and scaled;
e
f
see text for details. Predicted to be IR inactive. The Ni−Si distance associated with the agostic Si−H moiety.
Examples of unstrained Si−C bond activation within the nickel
coordination sphere are limited, and dinuclear Ni(I) complexes
such as 5 from such processes are unprecedented. It is
noteworthy that the Si−C bond activation that produces 5
occurs even in the presence of a typically more reactive Si−H
bond.
A comparison of bond lengths and spectroscopic parameters
for complexes 1−5 is shown in Table 1. Due to the inherent
difficulties in locating hydrogen atoms by X-ray crystallography,
as well as the significant errors in bond lengths to hydrogen,
bond lengths from DFT calculations performed with the
B3LYP functional and the TZVP basis set are included to
support the experimental data. Optimized structures with zero
imaginary stretching frequencies were used for all complexes.
For 3 and 4 the model complexes (Me3P)Ni[(Me3P)Ni(μ3-
SiHPh)]3 (3Me) and Ni[(Me3P)Ni(μ3-SiHPh)]4 (4Me) were
used for computational efficiency. Similarly, the model complex
[(Me3P)Ni(μ-SiHPh2)]2 (2Me) was used for the calculated
NMR coupling constants for complex 2. All the optimized
structures gave complexes with the same geometrical features
and connectivity predicted from the X-ray crystallographic data,
thus confirming the location of the silicon-bound hydrogen
atoms in these complexes.
A comparison of the IR stretching frequencies in complexes
1−5 reveals that complexes 2−5 feature Si−H−Ni stretching
frequencies that range from 1551 to 1635 cm−1. The unique
complex is 1, which displays a stretching frequency at 1887
cm−1 appropriate for a Ni−H moiety. The DFT calculated
stretch of 1905 cm−1 for 1 revealed that the motion of the H
moiety in this vibration occurs nearly coincident with the Ni−
H bond vector. In complexes 2−5 the calculated motion of the
hydrogen atoms for these strong absorptions lies between the
Ni−H and H−Si vectors. The highest stretching frequency for
the formally Ni(I) complexes is for 5, suggestive of stronger
back-bonding to the Si−H moiety in this complex. DFT
calculations predict that high-intensity lower frequency
vibrations associated with the Ni−H−Si moiety should be
observed at 1326, 1207, 1270, and 1158 cm−1 for 2, 3Me, 4Me,
and 5, respectively, but the experimental spectra all contained
too many peaks in this region to allow assignment.
Comparison of Formally Ni(I) Centers. Complexes 2 and
5 are both formally Ni(I)−Ni(I) species, with a Ni−Ni bond,
albeit with varying degrees of back-donation to the bound Si−
H, so that 5 could be viewed as nearing a formal Ni(I)−Ni(III)
oxidation state assignment. The formal oxidation states of the
nickel centers in 3 and 4 are not absolutely defined, although by
connectivity they can be assigned as formally Ni(III)−Ni(I)23
and Ni(IV)−Ni(I) complexes, respectively, where the central
atom in each complex has the high formal oxidation state.
Although in many clusters oxidation state assignments are not
particularly useful due to electron delocalization, in some cases
geometries combined with other spectroscopic parameters
allows a useful assignment to be made. A comparison of
complexes 2−5 shows similarities in both geometries and
spectroscopic parameters for the formally Ni(I) centers.
Significant polarization of the Ni−Ni bonds may stabilize
such species with apparently incompatible oxidation states.23
Figure 6 shows a comparison of the Ni(I) bonding environ-
ments in these molecules, using fragments of the solid-state
structures of 2−5. The views are shown so that the similarities
between the rightmost Ni(I) centers, labeled in boldface, are
evident. The donor environment around each of these nickel
centers is approximately planar and features a Ni−Ni bond, a
Si−H interaction suggestive of significant back-donation, a
phosphine donor, and a σ bond to either Si or C. The silicon
centers all feature an approximately tetrahedral arrangement of
σ bonds. The Ni−Ni−P bond is always significantly less than
180° and is always bent away from the adjacent bound Si−H
moiety.
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1
The H NMR shift of the Si−H−Ni moiety supports the
donors around the central atom is a distorted tetrahedron in
both complexes. The bonding in 4 is particularly interesting
from an orbital overlap point of view because of the perfect
square-planar arrangement of bonded nickel atoms in S4
symmetry. The formation of four bonds to the nickel atoms
requires molecular orbitals of A, B, and E symmetry on the
central atom. The pairs of both px/py and dxz/dyz orbitals on the
central nickel have E symmetry; however, due to the square-
planar arrangement of nickel centers, the actual overlap with
the dxz/dyz pair is nearly 0. Thus, nickel−nickel metal bonds can
be expected to have a relatively large p orbital contribution
from the central atom. As a result, the bonds from the central
atom to the silylene moieties in 4 should feature significantly
more d orbital character.
greater degree of back-donation to the Si−H moiety in 5
relative to 2−4, consistent with the IR data. Complex 1 displays
a strongly upfield shifted resonance at δ −10.75. In contrast, the
1
shifts in 2−4 are in the range of δ −1.81 to −2.27. The H
NMR shift of the Si−H−Ni moiety in 5 at δ −7.07 is
significantly shifted toward the range expected for a nickel
hydride. Most notably, clusters 3 and 4 do not feature 1H NMR
Si−H−Ni shifts that are significantly different from those of the
verifiably Ni(I) complex 2. DFT calculations approximate the
trends in the chemical shifts reasonably well. The 29Si NMR
shifts are nearly identical for the Ph2Si−H silyl moieties in 2
and 5, but the PhSi−H silylene moieties of 3 and 4 are clearly
1
distinct. The experimental JSiH values range from 35.5 to 59.6
Hz in complexes 2−5; however, there appears to be no strong
correlation between the value of this coupling constant and the
degree of back-donation to the Si−H moiety. Clusters 3 and 4
do not feature unusual coupling constants in comparison to the
verifiably Ni(I) centers in 2 and 5. DFT calculations predict
that these observed coupling constants are negative, as
CONCLUSIONS
■
In summary, a series of mono-, di-, tetra-, and pentanuclear
nickel complexes (1−5) with bridging organosilyl/silylene
ligands and agostic Ni−H−Si interactions have been obtained
under relatively mild reaction conditions by reaction of
[(iPr3P)2Ni]2(μ-η1:η1-N2) with a series of silanes. The
syntheses of these complexes include some remarkable
transformations, including the cleavage of a Si−C bond in the
presence of a typically more reactive Si−H bond. Silylene
complexes of nickel prepared from simple sterically unencum-
bered silanes are uncommon. The tetranuclear and pentanu-
clear complexes 3 and 4 with Si−H agostic interactions are
without precedent. Furthermore, the peripheral nickel coordi-
nation environments in these complexes are strikingly similar to
those of the verifiable Ni(I) complexes 2 and 5. The similarity
of these environments suggests that complexes 3 and 4 can be
formally described as mixed-valent Ni(I)−Ni(III) and Ni(I)−
Ni(IV) complexes, though the polarization of Ni−Ni bonds in
these complexes undoubtedly allows for the presence of such
apparently incompatible oxidation states, and the electronic
structures of the central atoms in these complexes are not
expected to be the same as those of mononuclear organo-
metallic Ni(III) and Ni(IV) complexes.12i,34 Regardless, the
silicon−nickel bonds to these formally high-valent central
nickel atoms are interesting to compare to other related high-
valent congeners, such as Pd.35 The reactivities of these
bridging silylene nickel clusters are of continuing interest in our
studies.
2
anticipated for one-bond Si−H coupling. The smaller JSi−H
value observed for 2 is also predicted to be negative. It should
be noted that the basis sets used are not optimized to describe
electron density near the nucleus and thus should provide only
approximate NMR coupling constants. The largest error is for
1, where a large positive coupling constant is predicted, but no
1
JSiH value is resolved in either the H or 29Si spectrum.
The σ-bound Ni−Si bond distances do not seem to strongly
correlate to the proposed oxidation state. The Ni−Si distance
in the formally Ni(II) complex 1 is 2.2211(9) Å, which is
similar to those in the formally Ni(I) containing complexes 2−
4, which have distances in the range of 2.1935(8)−2.2164(8) Å.
The Ni−Si distance of 2.2378(15) Å in 5 is the longest and
may be due to a more electron rich Ni center, due to the poorer
π-accepting ability of the aryl substituent in comparison to a
Si−H moiety in 2. The agostic Ni- - -Si interactions fall in the
range of 2.228(15)−2.290(1) Å. The shortest of these
interactions are in the same range as the Ni−Si σ bonds,
suggesting that the Si−H moiety is a very good acceptor. As
previously noted, in complex 5 the degree of back-donation
could be interpreted as nearing a formally Ni(I)−Ni(III)
compound.23
Calculations on the model complexes 2, 3Me, 4Me, and 5
suggest that back-donation is greatest in 5, with a Si−H bond
distance of 1.698 Å. By this measure, complexes 3Me and 4Me
feature similar degrees of back-donation, with Si−H bond
distances of 1.685 and 1.672 Å, respectively. The least activated
complex is 2. The complexes with the longest calculated Si−H
distances also feature the shortest Ni−H distances. The Ni−H
distance of 1.604 Å in 5 is only ∼0.1 Å longer than typical
crystallographically determined nickel−hydride bond distan-
ces.13f Similar short distances of 1.621 and 1.618 Å are
calculated for 3Me and 4Me, whereas 2 features the longest
calculated Ni−H distance of 1.661 Å.
These parameters all suggest that complexes 3 and 4 possess
Ni(I) sites similar to those in 2 and 5. The degree of back-
bonding suggested from the IR and NMR data does not
correlate exactly with the calculated lengthening of the Si−H
bond; similar observations have been noted before in σ
complexes of silanes.2k,33
EXPERIMENTAL SECTION
■
General Procedures. Unless otherwise stated, all manipulations
were performed under an inert atmosphere of nitrogen using either
standard Schlenk techniques or an MBraun glovebox. Dry, oxygen-free
solvents were employed throughout. Anhydrous pentane, toluene, and
THF were purchased from Aldrich, sparged with dinitrogen, and
passed through activated alumina under a positive pressure of nitrogen
gas; toluene and hexanes were further deoxygenated using using a
Grubbs type column system.1 Benzene-d6 was dried by heating at
reflux with Na/K alloy in a sealed vessel under partial pressure andthen
trap-to-trap distilled and freeze−pump−thaw degassed three times.
Toluene-d8 was purified in an analogous manner by heating at reflux
over Na. THF-d8 was purified in an analogous manner by heating at
1
reflux over K. H, 31P{1H}, and 13C{1H} spectra were recorded on a
Bruker AMX spectrometer operating at 300 or 500 MHz with respect
to proton nuclei. All chemical shifts are recorded in parts per million,
and all coupling constants are reported in hertz. 1H NMR spectra were
referenced to residual protons (C6D5H, δ 7.16; C7D7H, δ 2.09;
C4D7HO, δ 1.73) with respect to tetramethylsilane at δ 0.00. 31P{1H}
NMR spectra were referenced to external 85% H3PO4 at δ 0.00.
The central Ni atoms in complexes 3 and 4 also feature
interesting bonding environments. Excluding bonding inter-
actions with the peripheral nickel atoms, the arrangement of
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s, 698 vs, 653 s, 614 m, 496 s, 479 s, 429 m cm−1. Anal. Calcd for
C42H64Ni2P2Si2 (804.46): C, 62.71; H, 8.02. Found: C, 62.35; H, 8.67.
Synthesis of (iPr3P)Ni[(iPr3P)Ni(μ3-SiHPh)]3 (3) and Ni[(iPr3P)-
Ni(μ3-SiHPh)]4 (4). To an ice-cold solution of [(iPr3P)2Ni]2(μ-N2)
(250 mg, 0.31 mmol) in 10 mL of n-pentane was added a solution of
phenylsilane (68 mg, 0.62 mmol) in 5 mL of n-pentane. Immediately,
the solution turned from dark red to brown and gas evolution (N2)
was observed. After the mixture was stirred for 1 h at room
temperature, the mixture was put aside for crystallization. Over the
course of 5 days at room temperature, dark red-brown cubic crystals
precipitated. The crystals were filtered off and the mother liquor
reduced by half its volume under vacuum, and a second crop of 3 was
obtained. The combined yield afforded between 102 and 115 mg (54−
61%) of an analytically pure sample of 3. Black needlelike crystals of 4
(13 mg, 5% yield) were manually separated from the accompanying
crystals of 3. Complex 4 could also be isolated by dissolving the more
soluble 3 in benzene and filtering.
13C{1H} NMR spectra were referenced relative to solvent resonances
(C6D6, δ 128.0; C7D8, δ 20.4; C4D8O, δ 25.37). 29Si{31P} NMR
spectra were referenced to an external sample of 50% Si(CH3)4 in
C6D6 at δ 0.0. Infrared spectra (IR) were recorded on a Bruker Tensor
27 instrument operating from 4000 to 400 cm−1. Elemental analyses
were performed at the Center for Catalysis and Materials Research,
Windsor, Ontario, Canada. [Ni2N2(PiPr3)4] was prepared by the Mg
reduction of (PiPr3)2NiCl in THF, followed by extraction into and
crystallization from pentanes. Ni(COD)2 (COD = 1,5-cyclooctadiene)
was synthesized according to literature methods. The compounds
benzene-d6, toluene-d8, and THF-d8 were purchased from Cambridge
Isotope Laboratory. The compounds nickel dichloride, chlorodiphe-
nylsilane, phenylsilane, diphenylsilane, and triphenylsilane were
purchased from Aldrich and used as received. Triisopropylphosphine
was purchased from Strem and used as received.
Synthesis and Characterization of (iPr3P)2Ni(H)SiClPh2 (1).
Chlorodiphenylsilane (100 mg, 0.45 mmol) dissolved in 2 mL of n-
pentane was slowly added to a stirred solution of [(iPr3P)2Ni]2(μ-N2)
(175 mg, 0.22 mmol) in 2 mL of n-pentane at −34 °C. The low
temperature was maintained, and after 1 min, the reaction mixture
turned green-brown. Stirring was stopped and pale yellow cubic
shaped crystals formed. The crystals were separated from the solution
by decantation, and the residual solvent was removed using vacuum.
The pale yellow crystals were washed with cold n-pentane and dried
under vacuum to provide 1 (yield 52 mg, 19%). Complex 1 is
thermally sensitive, and solutions of 1 cannot be handled for extensive
Characterization of 3. 1H NMR (298 K, C6D6, 500 MHz): δ −1.81
(dd, JPH = 12.0 Hz, 1.8 Hz, 3H, Ni−H−Si), 0.90 and 0.93 (dd, 3JPH
=
=
3
3
13.0 Hz, JHH = 7.0 Hz, 9H each, P(CH(CH3)2)3), 1.19 (dd, JPH
3
3
12.0 Hz, JHH = 7.0 Hz, 27H, P(CH(CH3)2)3), 1.35 (dd, JPH = 13.5
3
3
Hz, JHH = 7.0 Hz, 27H, P(CH(CH3)2)3), 1.47 (sept, JHH = 7.0 Hz,
3H, P(CH(CH3)2)3), 2.10 (sept, 3JHH = 7.0 Hz, 9H, P(CH(CH3)2)3),
3
3
7.19 (t, JHH = 7.0 Hz, 3H, para-H), 7.36 (apparent t, JHH = 7.5 Hz,
6H, Ar-H), 8.38 (apparent t, 3JHH = 7.0 Hz, 6H, Ar-H). 13C{1H} NMR
(298 K, C6D6, 75.5 MHz): δ 19.5 (s, 9C, CH3), 20.2 (s, 3C, CH3),
20.5 (s, 3C, CH3), 21.2 (d, 9C, 1JPC =4.5 Hz, CH3), 25.9 (d, JPC =18.2
Hz, P-CH), 126.6 (s, Ar-CH), 128.6 (s, Ar-C), 136.6 (s, Ar-CH), 150.4
1
periods at room temperature. H NMR (298 K, C6D6, 500 MHz): δ
2
3
−10.75 ppm (t, JPH = 20.5 Hz, 1H, Ni−H−Si), 1.11 (dd, JHH = 7.0
(s, Ar-C). 31P{1H} NMR (298 K, C6D6, 121.5 MHz): δ 68.7 (q, 3JPP
=
3
3
Hz, JPH = 13.0 Hz, 36H, P(CH(CH3)2)3), 2.15 (sept, JHH = 7.0 Hz,
12.5 Hz, 1P, NiPiPr3), 59.8 (d, JPP = 12.5 Hz, 3P, NiPiPr3). 29Si{1H}
3
3
6H, P(CH(CH3)2)3), 7.07 (t, JHH = 7.0 Hz, 2H, para-H), 7.28
2
(apparent t, 3JHH = 6.5 Hz, 4H, Ar-H), 8.10 (apparent t, 3JHH = 7.0 Hz,
4H, Ar-H). 31P{1H} NMR (298 K, C6D6, 121.5 MHz): δ 47.3 (s).
29Si{1H} NMR (298 K, C6D6, 59.6 MHz): δ 10.16 (t, 2JPSi = 62.5 Hz).
NMR (298 K, C6D6, 59.6 MHz): δ 300.4 (dd, JSiP = 47.9 Hz, 10.9
3
Hz). 29Si NMR (298 K, C6D6, 59.6 MHz): δ 300.4 (ddd, JSiH = 45.0
Hz). IR (Nujol, KBr): 3157 w, 3125 w, 2032 w, 1946 w, 1887 w, 1819
w, 1578 (Ni−H−Si), 1359 s, 1298 m, 1245 s, 1156 s, 1086 s, 1053 s,
1043 s, 1023 s, 997 s, 960 w, 925 m, 880 w, 852 vw, 733 vs, 697 vs, 653
vs, 607 s, 563 m, 521 vs, 481 vs, 456 s, 410 s cm−1. Anal. Calcd for
C54H102Ni4P4Si3 (1194.31): C, 54.31; H, 8.61. Found: C, 54.17; H,
8.99.
IR (Nujol, KBr): 3130 m, 1887 s (br, Ni−H), 1645 w, 1583 w, 880 s,
1297 w, 1240 s, 1184 s, 1156 w, 1090 s, 1053 s, 1029 s, 966 w, 922 m,
802 s, 735 s, 572 m, 564 m, 522 s, 494 vs, 475 vs, 431 s cm−1. Anal.
Calcd for C30H53ClNiP2Si (597.92): C, 60.26; H, 8.93. Found: C,
59.86; H, 8.76.
Characterization of 4. 1H NMR (298 K, C6D6, 500 MHz): δ −2.27
Synthesis and Characterization of [(iPr3P)Ni(μ-SiHPh2)]2
(2). Method a. To a solution of [(iPr3P)2Ni]2(μ-N2) (190
mg, 0.24 mmol) in 5 mL of n-pentane was added a solution of
diphenylsilane (88 mg, 0.48 mmol) in 2 mL of n-pentane at 20
°C. Gas evolution was observed immediately. After the mixture
was stirred for 10 min at room temperature, precipitation was
induced by cooling the solution overnight at −33 °C. The
crystals were separated by decanting off the solution, followed
by washing the solid with 1 mL of ice-cold n-pentane and
drying under vacuum to afford red rhombic crystals of 2. A
second crop from the mother liquor delivered a combined yield
of 165 mg (86%) of the product.
2
(d, JPH = 11.5 Hz, 4H, Ni−H−Si), 1.17 and 1.20 (m, 24 H, CH3),
3
1.83 (sept, JHH = 7.0 Hz, 12H, P(CH(CH3)2)3), 6.97 (m, 4H, para-
H), 7.40 (m, 8H, meta-H), 7.80 (dd, 3JHH = 7.8 Hz, 4JHH = 2.1 Hz, 8H,
ortho-H). 31P{1H} NMR (298 K, C6D6, 121.5 MHz): δ 61.8 (s).
29Si{1H} NMR (298 K, C6D6, 59.6 MHz): δ 277.4 (d, 2JSiP = 45.9 Hz).
1
29Si{1H} NMR (298 K, C6D6, 59.6 MHz): δ 277.4 (dd, JSiH = 35.9
Hz). Anal. Calcd for C60H108Ni5P4Si4 (1359.20): C, 53.02; H, 8.01.
Found: C, 53.12; H, 7.52. IR (Nujol, KBr): 3217 vw, 3061 m, 3046 m,
2027 vw, 1939 vw, 1882 vw, 1805 vw, 1567 m, 1551 m, 1427 m, 1297
w, 1246 m, 1179 m, 1157 w, 1092 s, 1045 s, 964 vw, 928 w, 883 m, 824
vw, 729 s, 695 vs, 654 vs, 636 m, 612 vs, 557 m, 522 vw, 495 vs, 458 s
cm−1.
Method b. Under vigorous stirring, a mixture of Ni(COD)2 (220
mg, 0.8 mmol) in the presence of 1 equiv of iPr3P (128 mg, 0.8 mmol)
in 5 mL of toluene was combined dropwise with a solution of
diphenylsilane (147 mg, 0.8 mmol) in 5 mL of toluene at 20 °C. After
the reaction mixture was heated for 16 h at 55 °C, the volatiles were
removed under vacuum and the reddish residue was extracted with n-
pentane (2 × 5 mL) and filtered through a sintered-glass disk. Crystals
were isolated by cooling the solution to −33 °C, affording 161 mg
Synthesis and Characterization of [(iPr3P)Ni]2(μ-C6H5)(μ-
SiHPh2) (5). A solution of [Ni2N2(PiPr3)4] (198 mg, 0.25 mmol) in
5 mL of n-pentane was combined with triphenylsilane (65 mg, 0.25
mol) dissolved in 5 mL of n-pentane. After the mixture was stirred for
20 min, the volatiles were removed under vacuum, which afforded a
dark brown oil. The oil was taken up in a mixture of 4 mL of
hexamethyldisiloxane/benzene (100/1). Brown needles of 5 suitable
for analysis by X-ray crystallography precipitated via slow evaporation
over 4 days and were isolated and dried under vacuum (84 mg, 48%
yield). 1H NMR (298 K, C6D6, 500 MHz): δ −7.07 (d, 2JPH = 13.2 Hz,
1
2
(56%) of 2. H NMR (298 K, C6D6, 500 MHz): δ −2.05 (d, JPH
=
15.0 Hz, 2H, Ni−H−Si), 0.87 (dd, 3JPH = 13.0 Hz, 3JHH = 7.0 Hz, 36H,
P(CH(CH3)2)3), 1.79 (sept, 3JHH = 7.0 Hz, 6H, P(CH(CH3)2)3), 7.19
3
3
3
3
(t, JHH = 7.0 Hz, 2H, Ar-H), 7.30 (t, JHH = 7.0 Hz, 4H, Ar-H), 8.38
Ni−H−Si), 0.84 (dd, JHH = 6.9 Hz, JHP = 6.0 Hz, 18H, CH3), 0.97
3 3 3
(t, JHH = 7.0 Hz, 4H, Ar-H). 13C{1H} NMR (298 K, C6D6, 75.5
(dd, JHH = 6.9 Hz, JHP = 6.0 Hz, 18H, CH3), 1.55 (sept, JHH = 6.9
Hz,, 3H, PCH), 1.78 (sept, JHH = 6.9 Hz, 3H, PCH), 6.69 (m, 1H,
3
1
3
3
MHz): δ 19.9 (s, Ni−P(CH(CH3)2)3), 25.87 ppm (vt, JPC + JPC
=18.2 Hz, Ni−P(CH(CH3)2)3), 127.4 (s, Ar-CH), 127.7 (s, Ar-C),
136.3 (s, Ar-CH), 144.11 (s, Ar-C). 31P{1H} NMR (298 K, C6D6,
121.5 MHz): δ 57.8 (s, 2P, NiPiPr3). 29Si{1H} (298 K, C6D6, 59.6
Ni−C−H), 7.16−7.22 (m, 4H, Ar-H), 7.30−7.32 (m, 6H, Ar-H),
8.10−8.12 (m, 4H, Ar-H). 13C{1H} NMR (298 K, C6D6, 75.5 MHz):
1
δ 19.9 (s, CH3), 19.9 (s, CH3); 24.5 (d, JPC = 17.6 Hz, Ni−
MHz): δ 123.7 (d, JSiP = 48.9 Hz). 29Si NMR (298 K, C6D6, 59.6
PCH(CH3)2)3), 25.0 (d, 1JPC =16.5 Hz, Ni−P(CH(CH3)2)3), 120.7 (s,
Ar-CH), 125.6 (s, Ar-CH), 127.2 (s, Ar-CH), 127.7 (s, Ar-C), 129.7
(s, Ar-CH), 136.0 (s, Ar-CH), 144.3 (s, Ar-CH), 183.5 (d, 2JPC = 17.6
2
MHz): δ 123.7 (ddd, 2JSiP = 48.9 Hz, JSiH = 59.6, 20.3 Hz). IR (Nujol,
KBr): 3045 w, 1582 m, 1425 m, 1243 m, 1090 m, 1043 w, 881 w, 735
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Article
Table 2. Crystal Data and Structure Refinement Parameters for Complexes 1−5
1
2
3
4
5
empirical formula
fw
C30H53ClNiP2Si
597.91
C42H64Ni2P2Si2
804.47
C54H102Ni4P4Si3
1194.35
cubic
C60H108Ni5P4Si4
1359.25
cubic
C36H58Ni2P2Si
698.27
cryst syst
a (Å)
monoclinic
18.741(2)
11.195(1)
32.174(3)
90
monoclinic
11.713(2)
17.723(4)
10.674(2)
90
monoclinic
15.297(2)
15.285(2)
17.426(2)
90
29.199(3)
29.199(3)
29.199(3)
90
15.158(1)
15.158(1)
14.868(2)
90
b (Å)
c (Å)
α (deg)
β (deg)
101.875(1)
90
108.04(3)
90
90
90
117.461(1)
90
γ (deg)
V (Å3)
90
90
6605.6(1)
C2/c
2106.8(10)
P21/c
24894(5)
3416.0(7)
3615.6(7)
P21/c
space group
I43d
̅
I4
̅
Z
8
2
16
2
4
Dcalcd (g/cm3)
μ(Mo Kα) (mm−1
temp (K)
2θmax (deg)
total no. of rflns
1.202
1.268
1.275
1.321
1.280
)
0.818
1.054
1.384
1.551
1.186
173(2)
55
173(2)
173(2)
56
173(2)
55
173(2)
50
53
34 367
22 104
133 036
5059; 0.0403
multiscan
0.67−0.57
4833
19 186
3871; 0.0219
multiscan
0.88−0.74
3722
34 464
no. of unique rflns; Rint
abs cor
7456; 0.040
multiscan
0.90−0.88
6281
4315; 0.0236
multiscan
0.77−0.64
3931
6371; 0.119
multiscan
0.91−0.82
4198
transmissn factors
no. of rflns with I ≥ 2σ(I)
no. of variables
332
227
266
252
386
rflns/params
GOF on F2
22.4
17.3
19.0
15.4
16.5
1.09
1.048
1.087
1.071
1.040
R1; wR2 (all data)
residual density (e/ Å3)
0.063; 0.108
0.433; −0.352
0.0295; 0.077
0.529; −0.208
0.0349; 0.0822
0.867; −0.349
0.029; 0.0685
0.672; −0.308
0.110; 0.1406
0.974; −0.513
Hz Ni−C). 31P{1H} NMR (298 K, C6D6, 121.5 MHz): δ 47.9 (d, 3JPP
= 24.5 Hz, 1P, NiPiPr3), 56.8 (d, 3JPP = 24.5 Hz, 1P, NiPiPr3). 29Si{1H}
via the NMR=spinspin keyword. Energies and optimized geometries
are provided in the Supporting Information.
2
2
NMR (298 K, C6D6, 59.6 MHz): δ 124.5 (dd, JPSi = 82.5 Hz, JPSi
=
3.2 Hz). 29Si (298 K, C6D6, 59.6 MHz): 124.5 (dd, JPSi = 82.5 Hz,
2JSiH = 50.3 Hz). IR (Nujol, KBr): 3037 w, 2718 w, 1950 w, 1889 w,
1818 w, 1635 s(br), 1555 s, 1526 w, 1460 vs, 1425 s, 1404 m, 1297 m,
1238 s, 1181 w, 1156 m, 1091 s, 1046 s, 1028 s, 996 m, 964 m, 925 m,
882 vs, 803 w, 727 vs, 697 vs, 655 vs, 613 s, 592 s, 506 vs, 469 s, 444
m, 423 s cm−1. Anal. Calcd for C36H58Ni2P2Si (698.3): C, 61.92; H,
8.37. Found: C, 62.08; H, 8.61.
2
ASSOCIATED CONTENT
* Supporting Information
■
S
Tables giving details of DFT calculations and CIF files giving X-
ray crystallographic data for 1−5. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sjohnson@uwindsor.ca. Fax: (+1) 519-973-7098.
X-ray Crystallography. The X-ray structures were obtained at low
temperature, with the crystals being covered in Paratone and placed
rapidly into the cold N2 stream of a Kryo-Flex low-temperature device.
The data were collected using the SMART36 software on a Bruker
APEX CCD diffractometer using a graphite monochromator with Mo
Kα radiation (λ = 0.710 73 Å). A hemisphere of data was collected
using a counting time of 10−30 s per frame. A summary of crystal data,
data collection, and structure refinement is given in Table 2. Data
reductions were performed using the SAINT37 software, and the data
were corrected for absorption using SADABS.38 The structures were
solved by direct or Patterson methods using SHELXS-9739 and refined
by full-matrix least squares on F2 with anisotropic displacement
parameters for the non-H atoms using SHELX-9739 and the WinGX40
software package, and thermal ellipsoid plots were produced using
ORTEP32.41 Complex 3 featured a 2-fold rotational disorder of two of
the phosphine ligand isopropyl substituents. Complex 4 featured a
disorder of the molecule about the S4 axis that was modeled and
refined to account for 10.2(1)% of the molecules. The agostic
hydrogen of this disordered fragment was omitted from the model.
Details of the structure solution and refinement are provided as
Supporting Information for 1−5.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Sciences and Engineering Research
Council (NSERC) of Canada, the Shared Hierarchical
Academic Research Computing Network (SHARCNET), and
the donors of the American Chemical Society Petroleum
Research Fund for their support of this research.
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