Reversible Substrate Activation and Catalysis at an Intact Metal-Metal Bond Using a Redox-Active Supporting Ligand

Article abstract of DOI:10.1021/jacs.5b03092

An electron rich Ni(I)-Ni(I) bond supported by a doubly reduced naphthyridine-diimine (NDI) ligand reacts rapidly and reversibly with Ph₂SiH₂ and Et₂SiH₂ to form stable adducts. The solid-state structures of these complexes reveal binding modes in which the silanes symmetrically span the Ni-Ni bond and exhibit highly distorted H-Si-H angles and elongated Si-H bonds. This process is facilitated by the release of electron density stored in the π-system of the NDI ligand. Based on this dinuclear mode of activation, [NDI]Ni₂ complexes are shown to catalyze the high-yielding hydrosilylation of alkenes, dienes, alkynes, aldehydes, ketones, enones, and amides. In comparative studies of alkyne hydrosilylations, the [NDI]Ni₂ catalyst is found to be significantly more active than its mononuclear counterparts for aryl-substituted substrates. (Chemical Equation Presented).

Full text of DOI:10.1021/jacs.5b03092

Article
pubs.acs.org/JACS
Reversible Substrate Activation and Catalysis at an Intact Metal−
Metal Bond Using a Redox-Active Supporting Ligand
Talia J. Steiman and Christopher Uyeda*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
S
* Supporting Information
ABSTRACT: An electron rich Ni(I)−Ni(I) bond supported by
a doubly reduced naphthyridine−diimine (NDI) ligand reacts
rapidly and reversibly with Ph2SiH2 and Et2SiH2 to form stable
adducts. The solid-state structures of these complexes reveal
binding modes in which the silanes symmetrically span the Ni−
Ni bond and exhibit highly distorted H−Si−H angles and
elongated Si−H bonds. This process is facilitated by the release
of electron density stored in the π-system of the NDI ligand. Based on this dinuclear mode of activation, [NDI]Ni2 complexes are
shown to catalyze the high-yielding hydrosilylation of alkenes, dienes, alkynes, aldehydes, ketones, enones, and amides. In
comparative studies of alkyne hydrosilylations, the [NDI]Ni2 catalyst is found to be significantly more active than its
mononuclear counterparts for aryl-substituted substrates.
be stored in the ligand, leading to Ni−Ni bonds that are stable
over an expanded range of oxidation states. Herein, these
complexes are shown to promote a rare example of a kinetically
facile and reversible substrate activation process across a
metal−metal bond. The [NDI]Ni2 complex 1 bearing a labile
C6H6 ligand coordinates secondary organosilanes to yield
bridging adducts. This dinuclear reactivity provides a
complementary pathway to mononuclear Si−H oxidative
addition10 as an entry into catalytic hydrosilylations (Scheme
1). Overall, these results provide the first demonstration that
redox-active ligands can enable prototypical organometallic
reactions to be catalyzed at an intact metal−metal bond.
INTRODUCTION
■
Mononuclear transition metal complexes catalyze a diverse
array of organic transformations through sequences of
elementary bond breaking and forming steps. Recently,
significant interest has been focused on engendering analogous
reactivity to complexes of higher nuclearity.1 These efforts are
in part inspired by mechanistic studies of biological and
heterogeneous redox catalysts that invoke substrate binding
through the direct participation of multiple metal centers.2 In
principle, synthetic catalysts that can capitalize on multimetallic
cooperativity effects may exhibit activity or selectivity profiles
that diverge from established mononuclear systems, providing
access to additional parameters for catalyst design and
optimization.
Scheme 1. Organosilane Activation Modes Relevant to
Catalytic Hydrosilylation Reactions
Discrete metal−metal bonds are documented to engage
organic substrates and small molecules in stoichiometric redox
reactions;1a,2e,f,3 however, their utility as active sites in catalysis
remains relatively unexplored.4,5 A contributing factor is the
limited stability of many metal−metal bonds toward redox
processes, causing speciation into mononuclear complexes
under turnover conditions.1a For example, while Pd(I) dimers
have been studied in cross-coupling reactions, in all reported
cases, the active species were found to be dissociated
monomers.6 Additionally, in Pd-catalyzed C−H functionaliza-
tions, the catalyst is proposed to traverse mononuclear Pd(II)
and dinuclear Pd(III)−Pd(III) states, requiring the formation
and cleavage of the metal−metal interaction during catalysis.1g,7
Thus, while group 10 metal−metal bonds can act as
precatalysts or transient intermediates, they generally lack the
redox flexibility to be maintained during a catalytic cycle.
In order to address this limitation, we recently reported the
synthesis of dinuclear Ni complexes using a chelating
naphthyridine−diimine (NDI) ligand.8 The redox-active
nature9 of the NDI π-system allows electron equivalents to
RESULTS AND DISCUSSION
■
Dinuclear Organosilane Complexes. The [i‑PrNDI]-
Ni2(C6H6) complex 1 is readily accessible by combining free
i‑PrNDI and Ni(COD)2 (2.0 equiv) in C6H6.8 Complex 1 reacts
rapidly with Ph2SiH2 (1.0 equiv) in C6D6 to form an
equilibrium mixture of the silane adduct 2-Ph (Figure 1a),
Received: March 24, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b03092
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
diamagnetic ground state with a thermally populated para-
magnetic excited state. For 2-Ph, the difference in energy
between the ground state and excited state is too small to
accurately determine a value by modeling the NMR data over
the accessible temperature range; however, for 2-Et, a singlet−
triplet gap of 2.8 kcal/mol was calculated by fitting the
naphthyridine chemical shifts from spectra acquired in the
range of +22 to −64 °C (Figure 3a). The accessibility of higher
Figure 1. Organosilane binding and solid-state structures of (a) 2-Ph
and (b) 2-Et (ellipsoids at 50% probability). i-Pr groups on the
[
i‑PrNDI] ligand are truncated for clarity.
Figure 3. (a) Variable temperature 1H NMR chemical shifts (red
circles) for the two naphthyridine doublet signals of 2-Et. Data were
modeled by taking into account a Boltzmann population of a triplet
excited state (see Supporting Information for the fitting function).2g,12
From the best fits, a singlet−triplet gap of 2.8 kcal/mol and chemical
shifts for the diamagnetic state of 5.4 and 4.8 ppm were calculated. (b)
Calculated spin density plot for the triplet state of a model
dimethylsilane complex (B3LYP/6-311+G(d,p)).
1
free Ph2SiH2, and residual 1. At room temperature, the H
NMR spectrum exhibits distinct sets of signals for 1 and 2-Ph,
indicating that substitution of Ph2SiH2 for C6D6 is slow on the
NMR chemical shift time scale. In order to confirm the
reversibility of Ph2SiH2 binding, an equimolar mixture of 1 and
Ph2SiH2 at an initial concentration of 110 mM was diluted in
increments to a final concentration of 7.0 mM (Figure 2). At
spin states for the silane complexes as compared to 1, which is
diamagnetic at all examined temperatures (up to 80 °C), likely
reflects the weaker field strength of the silane ligand relative to
C6H6. Notably, this variable temperature behavior contrasts
with the temperature-independent paramagnetism observed by
Chirik for related (pyridine−diimine)Fe complexes.9b,11
Diffusion of pentane vapor into saturated solutions of the
[
i‑PrNDI]Ni2(C6H6) complex 1 dissolved in excess Ph2SiH2
yielded crystalline samples of the silane complex 2-Ph that were
suitable for X-ray diffraction analysis. The solid-state structure
(Figure 1a and Table 1) reveals that Ph2SiH2 symmetrically
Table 1. Selected Bond Metrics (Å) from Solid-State
Structures
a
2-Ph
2-Et
3
Figure 2. 1H NMR (22 °C, C6D6) spectra for the serial dilution of 1:1
Ni1−Ni2
Ni1−Si
Ni2−Si
Ni1−H1
Ni2−H2
Si−H1
2.6284(9), 2.5625(9)
2.258(1), 2.263(1)
2.228(2), 2.222(2)
1.51(4), 1.59(5)
1.53(4), 1.49(6)
1.55(3), 1.56(5)
1.65(5), 1.57(7),
2.5545(6)
2.268(1)
2.2559(9)
1.51(4)
2.4500(7)
2.335(1)
2.170(1)
1.56(3)
[
i‑PrNDI]Ni2(C6H6) (1) and Ph2SiH2: [Ni2] = 110 mM (bottom) to
7.0 mM (top). Signals assigned to 1 (red circles) and 2-Ph (blue
squares) are labeled.
1.54(4)
1.61(4)
1.66(3)
the highest concentrations, the equilibria favor the silane
complex 2-Ph, whereas 1 predominates under more dilute
conditions. Over the range of concentrations examined, a
Keq(Ph2SiH2) of 420 40 was calculated. Weaker binding is
observed using a more electron rich alkyl-substituted silane
Si−H2
1.67(4)
a
Two molecules in the asymmetric unit. Metrics for both are shown.
(Keq(Et2SiH2) = 44
and Et3SiH, did not interact to any detectable extent with
complex 1.
2). Tertiary silanes, including Ph3SiH
bridges the two Ni atoms13 with Ni−Si distances (2.243(2) Å
for two molecules in the asymmetric unit) that are within the
range of structurally characterized mononuclear Ni(silyl)
complexes.14 The μ-H atoms were located in the difference
map and refined to an average Ni−H distance of 1.53(5) Å and
Si−H distance of 1.58(5) Å. A remarkable structural feature of
2-Ph is the highly distorted H−Si−H angle of 156(3)°. An
analogous silane adduct was obtained using Et2SiH2 (Figure
1b). The weaker binding of Et2SiH2 to the [i‑PrNDI]Ni2
1
At room temperature, 2-Ph exhibits H NMR resonances
that are significantly shifted from diamagnetic reference values
for free NDI or complex 1. For example, the signals assigned to
the naphthyridine ring appear as broad singlets at −18.0 and
+10.6 ppm in THF-d8 at room temperature. These peaks shift
in a temperature-dependent fashion, suggesting that 2-Ph has a
B
DOI: 10.1021/jacs.5b03092
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
platform in solution is manifested in the solid state by longer
Ni−Si distances and a correspondingly shorter Ni−Ni distance
as compared to 2-Ph.
indicates a d9−d9 electron count for the Ni−Ni bond. Overall,
the calculations are most consistent with a [NDI]2− and Ni(I)−
Ni(I) oxidation state assignment for the C6H6 and silane
complexes. For the triplet state, the two unpaired electrons
occupy the two primarily ligand-centered π orbitals (HOMO
and LUMO in Figure 4b), and by Mulliken population analysis,
the spin density (Figure 3b) is nearly entirely localized on the
NDI ligand atoms.
In support of the calculated electronic structure model, the
experimental bond metrics associated with the NDI ligand of 2-
Et exhibit typical features of ligand-centered reduction:
elongated imine C−N and napthyridine Cipso−N distances
and contracted Cipso−Cimine distances. Notably, the degree of
NDI reduction is highly responsive to the identity of the
substrate bound to the Ni−Ni bond (Figure 5). For example,
In both 2-Ph and 2-Et, the μ-H atoms are nearly equidistant
between Ni and Si, suggesting that these adducts are
intermediate structures between the limiting cases of a σ-
complex and a μ-silylene dihydride derived from a double
oxidative addition process (Scheme 1). The Ni−H−Si bonding
arrangement found in these complexes is reminiscent of similar
three-center-two-electron bonds described by Hillhouse for a
mononuclear (diphosphine)Ni(μ-H)(SiMes2)+ complex.14d
Electronic Structure and Computational Models. In
order to gain further insight into the nature of the interaction
between the Ni−Ni bond and secondary organosilanes, DFT
calculations were conducted on a model S = 0 Me2SiH2
complex (B3LYP/6-311+G(d,p)). The triplet state was also
calculated and found to be 8.3 kcal/mol higher in energy. The
computationally optimized structure closely reproduces the
symmetrical silane binding mode and experimental bond
metrics for 2-Et (see Supporting Information for a comparison
of the bond metrics).
Orbitals with significant Ni−Si and Ni−H bonding character
were located at HOMO−15 and HOMO−17 respectively
(Figure 4a). These orbitals arise from mixing between the dxy−
Figure 5. Summary of [i‑PrNDI] bond metrics sensitive to ligand-
centered reduction. In all structures, the bond distances observed for
the two halves of the ligand are approximately equivalent. Average
values are plotted. The numbering scheme is shown in Figure 1.
the NDI ligand appears to be in a more oxidized state in 2-Ph
than in 2-Et. For both silane complexes, the NDI ligand is more
oxidized than that for the C6H6 adduct 1. Taken together, these
structural trends demonstrate that donation of electron density
stored in the NDI π-system plays a significant role in promoting
the binding and activation of organosilanes at the Ni2 fragment.
Overall, the degree of ligand-centered reduction is inversely
correlated with the measured binding preference of Ph2SiH2 >
Et2SiH2 > C6H6.
Hydride Transfer Reactivity. Under the crystallization
conditions described above for 2-Ph, an additional species 3
was identified by XRD (Figure 6). The isomeric complex
features a silyl ligand bound to one Ni center with a secondary
agostic interaction of the Si−H bond to the adjacent Ni. By 1H
NMR, a sharp singlet, sensitive to labeling with Ph2SiD2, is
Figure 4. Selected Kohn−Sham orbitals (B3LYP/6-311+G(d,p)) for a
model dimethylsilane complex (S = 0) highlighting (a) interactions
between the Ni2 and Me2SiH2 fragments and (b) frontier orbitals.
dxy π orbital of the Ni−Ni bond and the HOMO and the
LUMO for the silane in its distorted geometry (A1 symmetry in
C2v). The calculated net Ni−H and Si−H bond orders
according to the natural bond orbital (NBO) Wiberg bond
index are 0.41 and 0.47, respectively, corresponding to an
approximately symmetrical bonding of the μ-H to Ni and Si.
This analysis implicates back-donation from the filled Ni−Ni π-
bonding orbital into the unfilled H−Si−H antibonding orbital
as a significant contributor to the silane binding energy. The
stronger binding affinity of Ph2SiH2 and the longer Ni−Ni
distance in 2-Ph as compared to 2-Et are rationalized by this
model.
The calculated HOMO is a primarily ligand-centered π
orbital, and the Ni−Ni σ* orbital (LUMO + 1) is unoccupied
(Figure 4b). The electronic configuration by NBO analysis
Figure 6. Solid-state structure of 3 (ellipsoids at 50% probability). i-Pr
groups on the [i‑PrNDI] ligand are truncated for clarity.
C
DOI: 10.1021/jacs.5b03092
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
observed at −16.1 ppm (THF-d8). Dearomatization of one of
the rings in the naphthyridine system is evident in the
dihydropyridine-like distortions in the C−C and C−N
distances. Additionally, the two hydrogens bound to the sp3-
hybridized carbon were located in the difference map. It is
noteworthy that this isomeric product was not formed in
reactions between 1 and Et2SiH2.
with the lack of a catalyst induction period, this observation
suggests that 3 is likely not relevant to the mechanism of
catalysis.
Substrate Scope for Catalytic Hydrosilylations. Com-
plex 1 exhibits a broad substrate scope for the catalytic
hydrosilylation of carbon−carbon and carbon−oxygen multiple
bonds (Table 2). For aliphatic alkenes such as 1-octene, the
Complex 3 is generated when 1 is stored in neat Ph2SiH2;
however, in C6D6 solution, this reaction does not occur even
after heating at 60 °C over 24 h. This concentration
dependence is highly suggestive that 3 does not arise from an
intramolecular rearrangement, but rather, a higher kinetic order
intermolecular process. When stored as a C6D6 solution, 3
reverts back to 1 over the course of days at room temperature.
The formation of 3 provides evidence that this system facilitates
hydride transfer from a silane to an organic fragment.
Table 2. Substrate Scope for Catalytic Hydrosilylations with
1
a
Catalytic Hydrosilylation of 1-Octene. Hydrosilylations
catalyzed by group 10 transition metal complexes are generally
proposed to proceed by variants of the Chalk-Harrod
mechanism (Scheme 1), where oxidative addition of a Si−H
bond leads to a Si−M−H intermediate. While such oxidative
additions at Pt are well-precedented15indeed, Pt complexes
are often used as industrial hydrosilylation catalystsexamples
at Ni are relatively uncommon. Reported cases either utilize
strongly donating ligands, such as alkylphosphines14c,16 and
NHCs,14b or ligands that incorporate pendant Lewis acidic
functionality to assist in the Si−H bond cleavage.17 The facile
silane activation observed by the [NDI]Ni2 platform demon-
strates that Ni complexes of less electron rich N-donor ligands
can nonetheless engage organosilane substrates by taking
advantage of binding across multiple metal centers.
Based on these observations, we next examined the
propensity of complex 1 to catalyze a model hydrosilylation
reaction (Scheme 2). While other examples of dinuclear Ni
Scheme 2. Catalytic Hydrosilylation of 1-Octene with
Ph2SiH2
complexes bearing bridging silane or silyl ligands have been
reported, their relevance to catalytic hydrosilylation has not
been explored.13,18 At 5 mol % loading of 1, the hydrosilylation
of 1-octene with Ph2SiH2 proceeds to >95% conversion in 22 h
at room temperature, producing the hydrosilylated product in
77% yield. Et2SiH2 reacts more slowly, yielding only 35% of the
product after the same reaction time with significant unreacted
1-octene. The relative rate of hydrosilylation with Ph2SiH2 and
Et2SiH2 presumably reflects the weaker binding constant for
Et2SiH2. Tertiary silanes, which do not bind to complex 1, were
unreactive in hydrosilylation reactions even after heating at 60
°C for 24 h.
a
Reactions were conducted in C6D6 with 1.0 equiv of the substrate, 1.1
equiv of Ph2SiH2, and 5 mol % of 1. Yields were averaged over two
runs and determined by 1H NMR integration against an internal
b
c
standard. Reaction was conducted with 1 mol % of 1. Reaction was
conducted with 4.0 equiv of Ph2SiH2.
anti-Markovnikov product is formed exclusively. By contrast,
styrene yields an approximately 1:1 mixture of regioisomers.
Dienes undergo highly selective 1,4-hydrosilylation to form the
corresponding allyl silane products. The reaction of isoprene is
particularly efficient, proceeding to complete conversion in 1 h
at room temperature with 1 mol % loading of catalyst 1.
Internal alkynes containing alkyl or aryl substituents are
hydrosilylated in a syn fashion to yield (E)-alkene products.
More polar substrates, including aldehydes and ketones, are
also hydrosilylated by catalyst 1. Cyclic and acyclic enones
undergo 1,4-hydrosilylation. In the case of methyl vinyl ketone,
the E stereoisomer is favored. Finally, N,N-dimethylbenzamide
is reductively deoxygenated to N,N-dimethylbenzylamine in
77% yield using an excess of Ph2SiH2.
During catalytic hydrosilylations between 1-octene and
Ph2SiH2 in C6D6, the silane complex 2-Ph can be directly
1
observed by H NMR as the primary catalyst resting state. As
Ph2SiH2 is depleted at higher conversions, the equilibrium shifts
toward 1. During the reaction course, there is no change in the
total catalyst concentration (the sum of 1 and 2-Ph), indicating
minimal catalyst decomposition. The presence of elemental Hg
also does not cause inhibition of the reaction. The isomeric
complex 3 is not formed within the limit of detection. Coupled
D
DOI: 10.1021/jacs.5b03092
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
The scope of substrates that are efficiently hydrosilylated by
the dinuclear complex 1 is noteworthy in comparison to
reported mononuclear Ni catalysts.17,19 In particular, common
side products arising from dehydrogenative silylation or
silylative C−C coupling were not formed using catalyst
1.19a,i−l In cases where the yields are less than quantitative,
products arising from Ph2SiH2 redistribution to PhSiH3 and
Ph3SiH were observed. For example, with 1-octene, Ph3SiH,
and (C8H17)2PhSiH were identified by GC/MS. In a majority
of cases, however, hydrosilylation is sufficiently rapid to
outcompete the redistribution process. Tetrasubstituted silane
products are not formed for any substrate class, consistent with
the lack of reactivity with tertiary silanes.
Catalyst Nuclearity Effects in Alkyne Hydrosilylations.
Motivated by the fast rate and high efficiency of alkyne
hydrosilylations using 1, we undertook a comparative study
with analogous mononuclear Ni catalysts containing chelating
N-donor ligands. The relatively electron rich 2,2′-bipyridine
complex 4 and the more hindered and electron deficient α-
diimine complex 5 were selected as comparison compounds.
For 2-butyne (Table 3, entries 1−3), [bpy]Ni(COD) (4)
CONCLUSIONS
■
In summary, metal−metal bonds supported by a redox-active
ligand are shown to be viable platforms for stoichiometric bond
activations and for catalytic processes. The doubly reduced
[NDI]2− ligand in complex 1 affords a sufficiently electron-rich
Ni(I)−Ni(I) bond that secondary organosilanes coordinate
reversibly to form symmetrically bridging species. These
adducts fall on the continuum between a σ-complex and a
product of double Si−H oxidative addition. The lack of
reactivity with tertiary organosilanes suggests that interactions
with both Ni atoms are required for this binding to be
favorable. Structural studies in combination with relative
binding constant measurements are consistent with a significant
release of electron density from the NDI π-system upon silane
coordination.
This dinuclear mode of silane activation provides a
complementary mechanism to mononuclear Si−H oxidative
additions as a means of promoting catalytic hydrosilylation.
Thus, complex 1 catalyzes the high-yielding addition of
Ph2SiH2 to a broad range of organic substrates. Preliminary
data comparing the dinuclear complex to analogous mono-
nuclear complexes in alkyne hydrosilylations reveal distinct rate
advantages for aryl-substituted substrates. Overall, these results
provide a promising indication that nuclearity is a parameter
that can be exploited for catalyst optimization. Ongoing studies
are aimed at elucidating the origin of these effects and
extending these principles to other catalytic processes.
Table 3. Comparison of Mononuclear and Dinuclear Ni
a
Catalysts for the Hydrosilylation of Alkynes
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out using
standard Schlenk or glovebox techniques under an atmosphere of N2
unless otherwise noted. Solvents were dried and degassed by passage
through a column of activated alumina and sparging with Ar gas.
Deuterated solvents were purchased from Cambridge Isotope
Laboratories, Inc., degassed, and stored over activated 3 Å molecular
sieves prior to use. All other reagents and starting materials were
purchased from commercial vendors and used without further
purification unless otherwise noted. Liquid reagents were degassed
and stored over activated 3 Å molecular sieves prior to use. Elemental
analyses were performed by Midwest Microlab (Indianapolis, IN). The
entry
substrate
catalyst
yield of 7
1
2
3
4
5
6
6-Me
6-Me
6-Me
6-Ph
6-Ph
6-Ph
1
4
5
1
4
5
93%
77%
<1%
93%
<1%
<1%
[
i‑PrNDI]Ni2(C6H6) complex 1 was prepared according to previously
reported procedures.8
1
2
Physical Methods. H and H NMR spectra were collected on
Varian INOVA 300 MHz and Varian INOVA 600 MHz NMR
spectrometers. 13C NMR spectra were collected at room temperature
on Bruker ARX400 and Bruker AV-III-500-HD spectrometers.
a
Reactions were conducted in C6D6 with 1.0 equiv of the alkyne
substrate, 1.1 equiv of Ph2SiH2, and 5 mol % of the catalyst. For 6-Me,
the reaction was run for 1.5 h, and for 6-Ph, the reaction was run for
3.5 h. Yields were averaged over two runs and determined by 1H NMR
integration against an internal standard.
1
Variable temperature H NMR spectra were collected on both Varian
INOVA 300 MHz and Bruker DRX500 spectrometers. H and 13C
1
NMR spectra are reported in parts per million relative to
tetramethylsilane, using the residual solvent resonances as an internal
standard. GC/MS data was collected on a Shimadzu GC-2010/MS-
QP2010 spectrometer containing a mini-bore capillary GC column
and single quad EI detector. ATR-IR data were collected on a Thermo
Nicolet Nexus spectrometer containing a MCT* detector and KBr
beam splitter with a range of 350−7400 cm−1. UV−vis measurements
were acquired on a Cary 100 UV/vis spectrophotometer using a 1 cm
two-window quartz cuvette.
X-ray Crystallography. Single-crystal X-ray diffraction studies
were carried out at the Purdue X-ray crystallography facility on either a
Nonius KappaCCD or Rigaku Rapid II diffractometer. Data were
collected at 150 or 200 K using Mo Kα (λ = 0.71073 Å) or Cu Kα (λ
= 1.54178 Å) radiation. Structures were solved using direct methods
using SHELXT and refined against F2 on all data by full-matrix least-
squares.
exhibits a similar reaction rate to 1 with a slightly diminished
yield due to the competing formation of tetrasubstituted silanes
and hydrogenated alkyl silane side products.
For diphenylacetylene, the dinuclear catalyst 1 is significantly
more active than either mononuclear catalyst. After 3.5 h, 93%
yield of the hydrosilylated product is obtained with 1. There is
no detectable conversion using 4 under otherwise identical
conditions (approximately 40% yield after 11 d at room
temperature). While the origin of this effect requires more
detailed mechanistic studies, preliminary observations suggest
that 4 forms a stable alkyne complex20 that persists under
catalytic conditions. A plausible rationale for this low activity is
the unfavorable alkyne dissociation that would be required for
silane activation.
Computational Methods. DFT calculations were performed with
the Gaussian 09 software package. The geometry of a model
E
DOI: 10.1021/jacs.5b03092
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
1
[
MeNDI]Ni2(Me2SiH2) and isomeric [MeNDI-H]Ni2(Me2SiH) com-
against mesitylene. H NMR (300 MHz, 22 °C, C6D6) δ 7.61−7.54
(m, 4 H), 7.20−7.15 (m, 6 H), 5.14 (t, J = 3.6 Hz, 1 H), 1.59−1.49
(m, 2 H), 1.43−1.22 (m, 10 H), 1.21−1.09 (m, 2 H), 0.95 (t, J = 6.7
Hz, 3 H).
plex was fully optimized at the B3LYP/6-311+G(d,p) level of DFT21
using the XRD coordinates of 2-Et and 3 as a input geometries. The
stationary points were verified by frequency analysis. A comparison of
the calculated and experimental bond distances from the XRD
structure for 2-Et is included in the Supporting Information.
Comparison of Catalyst Activity in Alkyne Hydrosilylations.
The alkyne (0.13 mmol, 1.0 equiv), Ph2SiH2 (0.14 mmol, 1.1 equiv),
i‑PrNDI]Ni2(Ph2SiH2) (2-Ph) and [i‑PrNDI-H]Ni2(Ph2SiH) (3). A 20
[
i‑PrNDI]Ni2(C6H6) (1) (0.0065 mmol, 5.0 mol %), and mesitylene
[
mL vial was charged with [iPrNDI]Ni2(C6H6) (1) (25 mg, 0.034
mmol, 1.0 equiv) and Ph2SiH2 (6.4 μL, 0.034 mmol, 1.0 equiv).
Diethyl ether (2.0 mL) was added, and the reaction was stirred until a
golden brown homogeneous solution was obtained. The solvent was
removed under reduced pressure. The residue was redissolved in
diethyl ether (2.0 mL), and the resulting solution was concentrated to
dryness under vacuum to produce 29 mg of an inseparable mixture of
(0.13 mmol, 1.0 equiv) in C6D6 (0.5 mL) were allowed to react at
room temperature. After 1.5 h for 2-butyne or 3.5 h for
diphenylacetylene, yields of the hydrosilylated products were
1
determined by H NMR integration against mesitylene.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, spectra, crystallographic details, and
calculated structures. The Supporting Information is available
free of charge on the ACS Publications website at DOI:
10.1021/jacs.5b03092.
■
S
[
i‑PrNDI]Ni2(Ph2SiH2) (2-Ph) (94% yield) and [i‑PrNDI-H]-
Ni2(Ph2SiH) (3) (5% yield). Elemental analysis data for the mixture
of isomers is shown below. A crystalline sample suitable for XRD
analysis was obtained by slow diffusion of pentane vapor into the
mixture of products dissolved in THF (approximately 100 μL
containing one drop of Ph2SiH2). The crystalline material obtained
1
AUTHOR INFORMATION
Corresponding Author
*cuyeda@purdue.edu
by this procedure was analyzed by H NMR and determined to be a
■
1:1.2 ratio of 2-Ph and 3. NMR and UV−vis data for 2-Ph were
obtained by combining 1 and Ph2SiH2 in a 1:1 ratio in THF. NMR
data for 3 were obtained from the crystalline mixture of 2-Ph and 3.
Data for 2-Ph: 1H NMR (300 MHz, 22 °C, THF-d8) δ 10.64 (s, 2 H),
8.31 (d, J = 7.4 Hz, 4 H), 7.13−7.00 (m, 6 H), 6.98−6.86 (m, 4 H),
6.01 (t, J = 7.5 Hz, 2 H), 5.45 (s, 6 H), 4.10 (s, 4 H), 1.16 (d, J = 6.1
Hz, 12 H), 0.98 (d, J = 5.6 Hz, 12 H), −17.96 (s, 2 H). UV−vis
(THF): λ (nm) {ε, cm−1 M−1} 453 {860}, 363 {18 000}, 271
{27 000}. Data for 3: 1H NMR (300 MHz, 22 °C, C6D6) δ 7.55 (d, J =
7.0 Hz, 4 H), 7.15−7.07 (m, 6 H), 7.05−6.92 (m, 6 H), 6.38 (d, J =
7.2 Hz, 1 H), 6.09 (d, J = 7.1 Hz, 1 H), 4.87 (t, J = 4.2 Hz, 1 H), 3.58
(sept, J = 6.7 Hz, 2 H), 3.39−3.24 (m, 4 H), 1.38 (s, 3 H), 1.16 (d, J =
6.7 Hz, 6 H), 1.11 (d, J = 6.7 Hz, 6 H), 1.09 (s, 3 H), 1.02 (d, J = 6.9
Hz, 6 H), 0.98 (d, J = 6.8 Hz, 6 H), −15.59 (s, 1 H). Anal. Calcd for 2-
Ph and 3 (C48H56N4Ni2Si): C, 69.09; H, 6.76; N, 6.71. Found: C,
69.12; H, 6.64; N, 6.64.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was generously supported by Purdue University. We
thank Dr. Phillip Fanwick and Ian Powers for assistance with X-
ray crystallography.
REFERENCES
■
(1) For examples of reactivity studies with group 10 metal−metal
bonds: (a) Velian, A.; Lin, S.; Miller, A. J. M.; Day, M. W.; Agapie, T. J.
Am. Chem. Soc. 2010, 132, 6296−6297. (b) Lin, S.; Herbert, D. E.;
Velian, A.; Day, M. W.; Agapie, T. J. Am. Chem. Soc. 2013, 135,
15830−15840. (c) Lin, S.; Agapie, T. Synlett 2011, 2011, 1−5.
(d) Keen, A. L.; Johnson, S. A. J. Am. Chem. Soc. 2006, 128, 1806−
1807. (e) Beck, R.; Johnson, S. A. Chem. Commun. 2011, 47, 9233−
9235. (f) Powers, D. C.; Ritter, T. Acc. Chem. Res. 2011, 45, 840−850.
(g) Powers, D. C.; Ritter, T. Nat. Chem. 2009, 1, 302−309.
(h) DeLaet, D. L.; Del Rosario, R.; Fanwick, P. E.; Kubiak, C. P. J.
Am. Chem. Soc. 1987, 109, 754−758. (i) Laskowski, C. A.; Hillhouse,
G. L. Chem. Sci. 2011, 2, 321−325.
(2) For select recent discussions of multinuclear functional models of
metalloenzymes: (a) Chang, C. J.; Loh, Z.-H.; Shi, C.; Anson, F. C.;
Nocera, D. G. J. Am. Chem. Soc. 2004, 126, 10013−10020.
(b) Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu, L. Chem.
Rev. 2004, 104, 561−588. (c) Kanady, J. S.; Tsui, E. Y.; Day, M. W.;
Agapie, T. Science 2011, 333, 733−736. (d) Park, Y. J.; Ziller, J. W.;
Borovik, A. S. J. Am. Chem. Soc. 2011, 133, 9258−9261. (e) Krogman,
J. P.; Foxman, B. M.; Thomas, C. M. J. Am. Chem. Soc. 2011, 133,
14582−14585. (f) Powers, T. M.; Betley, T. A. J. Am. Chem. Soc. 2013,
135, 12289−12296. (g) Rittle, J.; McCrory, C. C. L.; Peters, J. C. J.
Am. Chem. Soc. 2014, 136, 13853−13862.
(3) (a) Thomas, C. M.; Napoline, J. W.; Rowe, G. T.; Foxman, B. M.
Chem. Commun. 2010, 46, 5790−5792. (b) Powers, T. M.; Fout, A. R.;
Zheng, S.-L.; Betley, T. A. J. Am. Chem. Soc. 2011, 133, 3336−3338.
(c) Hruszkewycz, D. P.; Wu, J.; Hazari, N.; Incarvito, C. D. J. Am.
Chem. Soc. 2011, 133, 3280−3283.
(4) (a) Gibson, S. E.; Stevenazzi, A. Angew. Chem., Int. Ed. 2003, 42,
1800−1810. (b) Magnus, P.; Principe, L. M. Tetrahedron Lett. 1985,
26, 4851−4854. (c) Cooper, B. G.; Napoline, J. W.; Thomas, C. M.
Catal. Rev. 2012, 54, 1−40.
(5) For examples of carbene transfer using Rh2 catalysts: (a) Doyle,
M. P.; Duffy, R.; Ratnikov, M.; Zhou, L. Chem. Rev. 2010, 110, 704−
724. (b) Davies, H. M. L.; Morton, D. Chem. Soc. Rev. 2011, 40, 1857−
1869. (c) Kornecki, K. P.; Berry, J. F.; Powers, D. C.; Ritter, T. In
[
i‑PrNDI]Ni2(Et2SiH2) (2-Et). A 20 mL vial was charged with
[
i‑PrNDI]Ni2(C6H6) (1) (20 mg, 0.028 mmol, 1.0 equiv) and Et2SiH2
(50 μL, 0.39 mmol, 14 equiv). Diethyl ether (2.0 mL) was added, and
the reaction was stirred at room temperature for 30 min to produce a
golden brown homogeneous solution. The solvent was removed under
reduced pressure. To ensure complete ligand exchange, the residue
was redissolved in diethyl ether (2.0 mL) and a second portion of
Et2SiH2 (50 μL, 0.39 mmol, 14 equiv) was added. After stirring at
room temperature for 30 min, the solution was concentrated to
dryness under vacuum to produce [i‑PrNDI]Ni2(Et2SiH2) (2-Et) (20
mg, 99% yield). Single crystals suitable for XRD were obtained by
diffusion of pentane vapor into a concentrated solution of 2-Et in
1
THF. H NMR (300 MHz, 22 °C, THF-d8) δ 7.22−7.16 (m, 4 H),
6.82 (t, J = 7.7 Hz, 2 H), 6.14 (d, J = 7.6 Hz, 2 H), 3.62−3.49 (m, 4
H), 1.77 (s, 6 H), 1.32 (d, J = 6.8 Hz, 12 H), 1.16 (d, J = 6.8 Hz, 12
H), 1.13−0.99 (m, 6 H), 0.66 (t, J = 7.4 Hz, 6 H). Anal. Calcd for 2-Et
(C40H56N4Ni2Si): C, 65.07; H, 7.64; N, 7.59. Found: C, 64.91; H,
7.79; N, 7.31.
Equilibrium Constant Measurements. A 1:1 ratio of [iPrNDI]-
Ni2(C6H6) (1) and R2SiH2 were combined in C6D6 (0.5 mL) at initial
concentrations of 110 mM for R = Ph and 165 mM for R = Et. The 1H
NMR spectra were recorded, and 2-fold dilutions were performed by
removing 0.25 mL of the solution and adding 0.25 mL of C6D6.
Solutions were diluted to final concentrations of 7.0 mM for R = Ph
and 10 mM for R = Et. Equilibrium constants were determined from
the ratios of 1 and 2. Values at each concentration were averaged:
Keq(Ph2SiH2) = 420 40 and Keq(Et2SiH2) = 44 2.
Catalytic Hydrosilylation of 1-Octene with Ph2SiH2. 1-Octene
(15 mg, 0.13 mmol, 1.0 equiv), Ph2SiH2 (27 mg, 0.14 mmol, 1.1
equiv), [i‑PrNDI]Ni2(C6H6) (1) (4.8 mg, 0.0067 mmol, 5.0 mol %),
and mesitylene (16 mg, 0.13 mmol, 1.0 equiv) in C6D6 (0.5 mL) were
allowed to react for 22 h at room temperature. The yield of
octyldiphenylsilane was determined to be 77% by 1H NMR integration
F
DOI: 10.1021/jacs.5b03092
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Progress in Inorganic Chemistry; Karlin, K. D., Ed.; John Wiley & Sons,
Inc.: New York, 2014; Vol. 58, pp 225−302. (d) Kornecki, K. P.;
Berry, J. F. Eur. J. Inorg. Chem. 2012, 2012, 562−568. (e) Sarkar, M.;
Daw, P.; Ghatak, T.; Bera, J. K. Chem.Eur. J. 2014, 20, 16537−
16549.
(6) (a) Barder, T. E. J. Am. Chem. Soc. 2005, 128, 898−904.
(b) Hruszkewycz, D. P.; Balcells, D.; Guard, L. M.; Hazari, N.; Tilset,
M. J. Am. Chem. Soc. 2014, 136, 7300−7316. (c) Christmann, U.;
Pantazis, D. A.; Benet-Buchholz, J.; McGrady, J. E.; Maseras, F.; Vilar,
R. J. Am. Chem. Soc. 2006, 128, 6376−6390.
(7) Powers, D. C.; Geibel, M. A. L.; Klein, J. E. M. N.; Ritter, T. J.
Am. Chem. Soc. 2009, 131, 17050−17051.
(8) Zhou, Y.-Y.; Hartline, D. R.; Steiman, T. J.; Fanwick, P. E.;
Uyeda, C. Inorg. Chem. 2014, 53, 11770−11777.
Maciejewski, H.; Rosenthal, U. J. Organomet. Chem. 1994, 484, 147−
151. (j) Marciniec, B.; Maciejewski, H.; Kownacki, I. J. Mol. Catal. A
1998, 135, 223−231. (k) Maciejewski, H.; Marciniec, B.; Kownacki, I.
J. Organomet. Chem. 2000, 597, 175−181. (l) Chaulagain, M. R.;
Mahandru, G. M.; Montgomery, J. Tetrahedron 2006, 62, 7560−7566.
(m) Cornella, J.; Gom
2013, 135, 1997−2009.
(20) (a) Rosenthal, U.; Nauck, C.; Arndt, P.; Pulst, S.; Baumann, W.;
́
ez-Bengoa, E.; Martin, R. J. Am. Chem. Soc.
Burlakov, V. V.; Gorls, H. J. Organomet. Chem. 1994, 484, 81−87.
̈
(b) Sgro, M. J.; Stephan, D. W. Dalton Trans. 2010, 39, 5786−5794.
(21) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee,
C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789.
(c) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724−728. (d) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys.
1972, 56, 2257−2261. (e) Hariharan, P. C.; Pople, J. A. Theor. Chim.
Acta 1973, 28, 213−222.
(9) (a) Bouwkamp, M. W.; Bowman, A. C.; Lobkovsky, E.; Chirik, P.
J. J. Am. Chem. Soc. 2006, 128, 13340−13341. (b) Bart, S. C.; Chłopek,
K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.; Neese, F.; Wieghardt,
K.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13901−13912. (c) Lu, C.
C.; Bill, E.; Weyhermuller, T.; Bothe, E.; Wieghardt, K. J. Am. Chem.
̈
Soc. 2008, 130, 3181−3197.
(10) (a) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16−21.
(b) Glaser, P. B.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125, 13640−
13641.
(11) (a) Bart, S. C.; Bowman, A. C.; Lobkovsky, E.; Chirik, P. J. J.
Am. Chem. Soc. 2007, 129, 7212−7213. (b) Bart, S. C.; Lobkovsky, E.;
Bill, E.; Wieghardt, K.; Chirik, P. J. Inorg. Chem. 2007, 46, 7055−7063.
(c) Trovitch, R. J.; Lobkovsky, E.; Bill, E.; Chirik, P. J. Organometallics
2008, 27, 1470−1478.
(12) Pfirrmann, S.; Limberg, C.; Herwig, C.; Knispel, C.; Braun, B.;
Bill, E.; Stosser, R. J. Am. Chem. Soc. 2010, 132, 13684−13691.
̈
(13) (a) Elder, M. Inorg. Chem. 1970, 9, 762−767. (b) Takao, T.;
Yoshida, S.; Suzuki, H.; Tanaka, M. Organometallics 1995, 14, 3855−
3868. (c) Takao, T.; Yoshida, S.; Suzuki, H. Chem. Lett. 2001, 30,
1100−1101. (d) Ohki, Y.; Kojima, T.; Oshima, M.; Suzuki, H.
Organometallics 2001, 20, 2654−2656. (e) Tanabe, M.; Ito, D.;
Osakada, K. Organometallics 2007, 26, 459−462. (f) Heiden, Z. M.;
Zampella, G.; De Gioia, L.; Rauchfuss, T. B. Angew. Chem. 2008, 120,
9902−9905. (g) Alvarez, M. A.; Alvarez, M. P.; Carreno, R.; Ruiz, M.
̃
A.; Bois, C. J. Organomet. Chem. 2011, 696, 1736−1748.
(14) (a) Beck, R.; Johnson, S. A. Organometallics 2012, 31, 3599−
3609. (b) Zell, T.; Schaub, T.; Radacki, K.; Radius, U. Dalton Trans.
2011, 40, 1852−1854. (c) Iluc, V. M.; Hillhouse, G. L. Tetrahedron
2006, 62, 7577−7582. (d) Iluc, V. M.; Hillhouse, G. L. J. Am. Chem.
Soc. 2010, 132, 11890−11892.
(15) (a) Mullica, D. F.; Sappenfield, E. L.; Hampden-Smith, M. J.
Polyhedron 1991, 10, 867−872. (b) Simons, R. S.; Sanow, L. M.; Galat,
K. J.; Tessier, C. A.; Youngs, W. J. Organometallics 2000, 19, 3994−
3996. (c) Chan, D.; Duckett, S. B.; Heath, S. L.; Khazal, I. G.; Perutz,
R. N.; Sabo-Etienne, S.; Timmins, P. L. Organometallics 2004, 23,
5744−5756. (d) West, N. M.; White, P. S.; Templeton, J. L.; Nixon, J.
F. Organometallics 2009, 28, 1425−1434.
(16) (a) Chen, W.; Shimada, S.; Tanaka, M.; Kobayashi, Y.; Saigo, K.
J. Am. Chem. Soc. 2004, 126, 8072−8073. (b) Tsay, C.; Peters, J. C.
Chem. Sci. 2012, 3, 1313−1318.
(17) MacMillan, S. N.; Hill Harman, W.; Peters, J. C. Chem. Sci.
2014, 5, 590−597.
(18) Fryzuk, M. D.; Rosenberg, L.; Rettig, S. J. Organometallics 1996,
15, 2871−2880.
(19) (a) Tamao, K.; Miyake, N.; Kiso, Y.; Kumada, M. J. Am. Chem.
Soc. 1975, 97, 5603−5605. (b) Bheeter, L. P.; Henrion, M.; Brelot, L.;
Darcel, C.; Chetcuti, M. J.; Sortais, J.-B.; Ritleng, V. Adv. Synth. Catal.
2012, 354, 2619−2624. (c) Porter, T. M.; Hall, G. B.; Groy, T. L.;
Trovitch, R. J. Dalton Trans. 2013, 42, 14689−14692. (d) Zheng, J.;
Darcel, C.; Sortais, J.-B. Catal. Sci. Technol. 2013, 3, 81−84. (e) Miller,
Z. D.; Li, W.; Belderrain, T. R.; Montgomery, J. J. Am. Chem. Soc.
2013, 135, 15282−15285. (f) Chakraborty, S.; Krause, J. A.; Guan, H.
Organometallics 2009, 28, 582−586. (g) Tran, B. L.; Pink, M.;
Mindiola, D. J. Organometallics 2009, 28, 2234−2243. (h) Postigo, L.;
Royo, B. Adv. Synth. Catal. 2012, 354, 2613−2618. (i) Marciniec, B.;
G
DOI: 10.1021/jacs.5b03092
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX