Boryl-metal bonds facilitate cobalt/nickel-catalyzed olefin hydrogenation

Article abstract of DOI:10.1021/ja504667f

New approaches toward the generation of late first-row metal catalysts that efficiently facilitate two-electron reductive transformations (e.g., hydrogenation) more typical of noble-metal catalysts is an important goal. Herein we describe the synthesis of a structurally unusual S = 1 bimetallic Co complex, [(^CyPBP)CoH]₂(1), supported by bis(phosphino)boryl and bis(phosphino)hydridoborane ligands. This complex reacts reversibly with a second equivalent of H₂(1 atm) and serves as an olefin hydrogenation catalyst under mild conditions (room temperature, 1 atm H₂). A bimetallic Co species is invoked in the rate-determining step of the catalysis according to kinetic studies. A structurally related Ni^INi^Idimer, [(^PhPBP)Ni]₂(3), has also been prepared. Like Co catalyst 1, Ni complex 3 displays reversible reactivity toward H₂, affording the bimetallic complex [(^PhPBHP)NiH]₂(4). This reversible behavior is unprecedented for Ni^Ispecies and is attributed to the presence of a boryl-Ni bond. Lastly, a series of monomeric (^tBuPBP)NiX complexes (X = Cl (5), OTf (6), H (7), OC(H)O (8)) have been prepared. The complex (^tBuPBP)NiH (7) shows enhanced catalytic olefin hydrogenation activity when directly compared with its isoelectronic/isostructural analogues where the boryl unit is substituted by a phenyl or amine donor, a phenomenon that we posit is related to the strong trans influence exerted by the boryl ligand.

Full text of DOI:10.1021/ja504667f

Article
pubs.acs.org/JACS
Boryl−Metal Bonds Facilitate Cobalt/Nickel-Catalyzed Olefin
Hydrogenation
Tzu-Pin Lin and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S Supporting Information
*
ABSTRACT: New approaches toward the generation of late first-row
metal catalysts that efficiently facilitate two-electron reductive trans-
formations (e.g., hydrogenation) more typical of noble-metal catalysts is
an important goal. Herein we describe the synthesis of a structurally
Cy
unusual S = 1 bimetallic Co complex, [( PBP)CoH] (1), supported by
2
bis(phosphino)boryl and bis(phosphino)hydridoborane ligands. This
complex reacts reversibly with a second equivalent of H (1 atm) and serves as an olefin hydrogenation catalyst under mild
2
conditions (room temperature, 1 atm H ). A bimetallic Co species is invoked in the rate-determining step of the catalysis
2
I
I
Ph
according to kinetic studies. A structurally related Ni Ni dimer, [( PBP)Ni] (3), has also been prepared. Like Co catalyst 1, Ni
complex 3 displays reversible reactivity toward H , affording the bimetallic complex [( PBHP)NiH] (4). This reversible
2
Ph
2
2
I
behavior is unprecedented for Ni species and is attributed to the presence of a boryl−Ni bond. Lastly, a series of monomeric
tBu
PBP)NiX complexes (X = Cl (5), OTf (6), H (7), OC(H)O (8)) have been prepared. The complex (tBuPBP)NiH (7)
(
shows enhanced catalytic olefin hydrogenation activity when directly compared with its isoelectronic/isostructural analogues
where the boryl unit is substituted by a phenyl or amine donor, a phenomenon that we posit is related to the strong trans
influence exerted by the boryl ligand.
INTRODUCTION
Chart 1. Several Recent Examples of Co and Ni Olefin
Hydrogenation Catalysts
■
Catalytic hydrogenation of unsaturated hydrocarbons con-
stitutes one of the most broadly used methods in the
production of valuable chemicals. While these reactions are
1
often catalyzed by precious metals, a growing interest in the use
2
of earth-abundant first-row metals has emerged. Substituting
precious-metal catalysts with inexpensive base metals can be
hampered by the inherent tendency of 3d metals to undergo
one-electron reactions, thus bypassing desired two-electron3
processes such as oxidative addition or reductive elimination.
4
4f,5
Reflecting this challenge is the limited number of Co and Ni
homogeneous olefin hydrogenation catalysts reported to date.
4a
4b
Independent studies from Budzelaar and Chirik have shown
catalytic olefin hydrogenations by pyridyl diimine Co
Ar
complexes ( PDI)CoR (Chart 1). These results have
highlighted the role of redox-active ligands in multielectron
6
4c,g
5c
11
catalysis. Hanson recently reported Co
and Ni catalysts
reactions. Well-recognized for their strong trans influence,
Cy
supported by bis(phosphino)amido ( PNP) and bis-
boryl ligands have been shown to significantly alter the
electronic structure of transition metals. Nevertheless, the
utility of boryl functionalities has yet to be evaluated in the
context of many prototypical catalytic transformations. To
further understand the role of boryl ligands in catalytic
hydrogenation reactions, we herein describe the synthesis of a
series of boryl−Co/Ni complexes, some of which display
unusual reversible reactions with H through ligand−metal
cooperativity. Spectroscopic and kinetic data provide insight
12
Cy
(
phosphino)amino ( PNHP) ligands, respectively. Tetrame-
tBu
tallic Co and Ni complexes [( PN)M] (M = Co, Ni) have
4
13
been described by Stryker in the context of alkene hydro-
4
f
7
genation. As part of our ongoing interest in this field, we
recently reported the bis(phosphino)borane−Ni catalyst
Ph
5e
(
(
DPB)Ni and the bis(phosphino)boryl−Co catalyst
tBu
4d
PBP)Co(N2). Illustrating ligand−metal cooperativity in
2
8
small-molecule activation, both the borane−Ni and boryl−Co
9
into the mechanisms of catalytic olefin hydrogenation by these
subunits appear to cooperatively activate H via a net two-
2
1
0
electron process.
Historically, boryl−metal complexes have been extensively
studied because of their relevance in catalytic borylation
Received: May 9, 2014
©
XXXX American Chemical Society
A
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Journal of the American Chemical Society
Article
II
0
complexes. An efficient hydrogenation rate by one of the
boryl−Ni catalysts is highlighted by direct comparison with its
isoelectronic phenyl and amino congeners.
complex. A mixed-valent Co Co dimer is invoked according
to resonance form b. When the two Co centers are connected
with a dative bond (form a) or a covalent bond (form b), the
same electron counts (18 electrons for Co1, 16 electrons for
Co2) are attained according to the covalent bond classification
RESULTS AND DISCUSSION
■
17
Synthesis of Bimetallic Boryl−Co Complexes. Reaction
(CBC) method. Lastly, resonance form c depicts a bonding
situation in which two Co atoms are linked by a bridging
hydride with no direct Co−Co bond.
Complex 1 is paramagnetic with a solution magnetic moment
(S = 1) as a dimer or 1.99μ
monomer (298 K, C D , Evans method). The lack of an
Cy
I
of the PB(H)P ligand, whose synthesis was provided by
14
Yamashita and Nozaki, with CoI and sodium amalgam in
2
tetrahydrofuran (THF) afforded the bimetallic complex
Cy
1
[
( PBP)CoH] (1) in good yield (Scheme 1). Single-crystal
of μeff = 2.81μ
B
B
(S = /
2
) as a
2
18
6
6
observable electron paramagnetic resonance (EPR) signal of
complex 1 at room temperature (RT) or 77 K in 2-Me-THF is
consistent with an S = 1 spin state. The solution IR spectrum of
complex 1 in C H is essentially identical to the solid-state
Scheme 1. Synthesis of Complexes 1 and [2][PF6]
6
6
attenuated total reflectance (ATR) IR spectrum, as both display
−1
1
a broad B−H stretch at 1651 cm . H diffusion-ordered
spectroscopy (DOSY) NMR experiments [see the Supporting
Information (SI)] indicate that the diffusion coefficient of 1 in
C D is much smaller than that of the monomeric boryl−Ni
6
6
19
complex (vide infra). Taken collectively, these data indicate
that complex 1 exists as a dimer in solution.
The spin density plot shown in Figure 1 provides further
insight into the electronic structure of 1. The calculated spin
densities on Co1 and Co2 are, respectively, 0.41 and 1.39,
I
I
which is more consistent with a Co Co configuration
(
resonance forms a and c). To evaluate the bonding situation
in the Co−(μ -H)−Co moiety, we performed a natural orbital
2
Me
(
(
NO) analysis on a simplified model complex, [( PBP)CoH]
2
1′). This calculation indicated that no net Co−Co bond is
present and that the two Co atoms are involved in a three-
center two-electron interaction through the bridging hydride
X-ray diffraction analysis reveals two Co centers with distinct
coordination environments (Figure 1). The Co1 center is
(
(
resonance form c), reminiscent of that found in the Ni system
vide infra). The NO analysis also suggested that the two
unpaired electrons are ferromagnetically aligned in two
orthogonal π*(Co−Co) orbitals, leading to an S = 1 spin
state (see the SI).
The cyclic voltammogram trace acquired for 1 in THF shows
that the first oxidation and reduction waves are reversible with
E1/2 values of −1.34 and −2.91 V vs ferrocene/ferrocenium
+
(
Fc/Fc ), respectively (see the SI). Treatment of 1 with
[
[
1
Fc][PF ] in THF afforded the one-electron-oxidized product
6
Cy
( PBP)CoH] [PF ] ([2][PF ]) in moderate yield (Scheme
2
6
6
1
). The H NMR spectrum of [2][PF ] displays 52 resolved
6
resonances, indicating a highly unsymmetrical ground state (a
total of 72 H resonances are possible if all are resolved in the
Cy
1
Figure 1. (left) Solid-state structure of [( PBP)CoH] (1). Thermal
2
ellipsoids are drawn at the 50% probability level. Cyclohexyl groups are
drawn in wireframe. Solvent molecules and hydrogen atoms (except
for the bridging hydrides) have been omitted for clarity. Pertinent
metrical parameters can be found in Table 1. (right) Mulliken atomic
spin density surface of 1 (isovalue = 0.003). Hydrogen atoms have
been omitted for clarity.
unsymmetrical molecule). The solution magnetic moment of
1
μeff = 1.62μ (298 K, THF-d ) is consistent with an S = /
B
8
2
species. Accordingly, the EPR spectrum of [2][PF ] at 77 K in
6
2
-Me-THF exhibits a rhombic signal (g = [2.400, 2.210, 2.010];
Figure 2) showing anisotropic hyperfine coupling with one
59
7
31
1
Co (I = / , A = [50, 50, 170] MHz) and two P (I = / , A =
2
2
[
330, 120, 50] MHz) nuclei. An essentially identical spectrum
2
Cy
ligated by a boryl ligand (Co1−B1 2.049(2) Å) and an η -
was obtained for [( PBP)CoD] [PF ] ([2][PF ]-d )
prepared from [( PBP)CoD] (1-d ). No H hyperfine tensor
2 2
2 6 6 2
15
Cy
1
hydridoborane ligand (Co1−B2 2.006(2) Å). In addition, the
binuclear cobalt core is bridged by a hydride (Co1−H1 1.51(2)
Å, Co2−H1 1.65(2) Å). A short Co1−Co2 bond of 2.3542(3)
Å is measured, consistent with those observed for Co dimers
featuring bridging hydride ligands. On the basis of the
structure analysis, three Lewis representations out of numerous
possibilities were considered as most plausible for complex 1
could be resolved.
Slow diffusion of Et O into a THF solution of [2][PF ]
afforded brown crystals suitable for X-ray diffraction analysis
(Figure 3). Complex [2][PF ] crystallizes in space group P1
2
6
16
6
̅
with two independent yet nearly identical molecules in the
20
asymmetric unit. As in the case of 1, two bridging hydrides
between Co1 and Co2 as well as between Co1 and B2 were
located in the difference map. The most substantial structural
(
Scheme 1). In resonance form a, the bridging hydride is
I
I
covalently bound to Co2, resulting in a formally Co Co
B
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Journal of the American Chemical Society
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Figure 3 shows the Mulliken atomic spin density surface for
+
[
2] (spin density = −0.64 on Co1 and 1.75 on Co2), which
suggests that the two Co atoms are antiferromagnetically
coupled. To further investigate the electronic structure, an NO
Me
analysis was performed on a simplified complex, [( PBP)-
CoH]2+ ([2′] ). This analysis identified magnetic interactions
between two symmetry-related Co−Co bonding and antibond-
ing orbitals (see the SI), also in agreement with two
+
22
antiferromagnetically coupled Co centers. The unpaired
electron is located in an NO that is polarized toward Co2.
This observation is consistent with the experimentally obtained
EPR spectrum showing hyperfine coupling to only one Co
atom.
Cy
Reversible H Activation by [( PBP)CoH] (1). Exposure
2
2
Cy
of 1 to 1 atm H in C D immediately resulted in a color
change from brown to yellow. As monitored by H NMR
Figure 2. EPR spectra of [( PBP)CoH] [PF ] ([2][PF ], black) and
2
6
6
2
6
6
Cy
1
[
( PBP)CoD] [PF ] ([2][PF ]-d , red) in 2-Me-THF at 77 K (9.39
2
6
6
2
GHz). Dashed trace: simulation.
spectroscopy, clean conversion of 1 into a new bimetallic
species (hereafter denoted as 1-H ) was confirmed. The
2
solution magnetic moment of 1-H (μ = 2.56μ , 298 K,
2
eff
B
C D ) is consistent with an S = 1 spin state. An alternative
6
6
interpretation of the solution magnetic measurement is that 1-
1
H fully dissociates into two S = / species (μ = 1.81μB).
2
2
eff
1
However, such a monomeric S = / species is supported by
2
1
neither the H DOSY NMR experiments (see the SI) nor the
absence of an EPR signal (2-Me-THF, 77 K). The conversion
between 1 and 1-H is fully reversible. Subjecting a C D
2
6
6
solution of 1-H2 to a few freeze−pump−thaw cycles
quantitatively regenerated 1. The peak of HD was immediately
1
observed by H NMR spectroscopy when 1 was exposed to a
1
:1 mixture of H and D (1 atm), implying rapid and reversible
2 2 2
Figure 3. (left) Solid-state structure of one of the two independent
2 2
Cy
+
+
H /D activation. The ATR-IR spectrum of 1-H under an
atmosphere of H exhibited a broad B−H stretching band at
1738 cm . The same band disappeared under D . Multiple
attempts to crystallize 1-H under an atmosphere of H were
not fruitful. We hypothesize that a plausible identity of 1-H is a
molecules of [( PBP)CoH] ([2] ). Thermal ellipsoids are drawn at
2
the 50% probability level. Cyclohexyl groups are drawn in wireframe.
The counteranion, solvent molecules, and hydrogen atoms (except for
the bridging hydrides) have been omitted for clarity. Pertinent metrical
parameters can be found in Table 1. (right) Mulliken atomic spin
2
−1
2
2
2
2
+
density surface of [2] (isovalue = 0.003). Hydrogen atoms have been
Cy
bis(hydridoborane) Co−(μ -H) −Co complex, [( PBHP)-
2
2
omitted for clarity.
CoH] , resulting from addition of H across the boryl−Co
2
2
bond. Another likely scenario is bimetallic addition, leading to
changes upon oxidation are the elongation of the Co2−B1
16b
bridging or terminal Co−H species.
intact H ligand in 1-H appears to us a less likely scenario. This
assumption is based on (1) the lack of N binding by 1
The presence of an
+
contact (3.002(4) Å in [2] vs 2.375(2) Å in 1) and the Co2−
+
2
2
B2 contact (3.276(3) Å in [2] vs 2.563(2) Å in 1) (Table 1),
2
(
toluene, −100 °C) and (2) a structurally related bimetallic
Cy
Table 1. Selected Bond Lengths (Å) for [( PBP)CoH] (1)
2
boryl−Ni analogue (vide infra).
Cy
+
+
and [( PBP)CoH] ([2] )
Cy
2
Catalytic Olefin Hydrogenation by [( PBP)CoH] (1).
2
(CyPBP)CoH]
[( PBP)CoH]2+
Cy
The ability of 1 to serve as an olefin hydrogenation catalyst was
initially studied as follows. Exposure of a C D solution of 1 to
1 atm ethylene and 4 atm H resulted in the immediate
[
2
a
6
6
Co1−Co2
Co1−B1
Co1−B2
Co2−B1
Co2−B2
B2−H2
2.3542(3)
2.049(2)
2.006(2)
2.375(2)
2.563(2)
1.37(3)
2.4643(5)
a
2
1.994(4)
1
a
formation of ethane, as detected by H NMR spectroscopy. Full
2.290(3)
a
consumption of ethylene was observed after stirring at RT for 2
h. Next, we examined bulkier olefins and compared the
hydrogenation rate for 1 with that for the previously reported
monomeric complex ( PBP)Co(N2) under the standard
conditions (RT, 1 atm H , C D , 2 mol % catalyst; Table 2).
3.002(4)
a
3.276(3)
a
1.19(4)
tBu
4d
a
Average of two independent molecules.
2
6
6
Catalytic hydrogenation of styrene and 1-octene could be
exceeding the sum of covalent radii of the two elements (2.1
achieved by 1 (Table 2). However, the rates were about 2
2
1
tBu
Å) by 30−36%. A longer Co1−Co2 bond is measured
orders of magnitude lower than those measured for ( PBP)-
+
tBu
(
2.4643(5) Å in [2] vs 2.3542(3) Å in 1). Other noteworthy
Co(N ). Surprisingly, while ( PBP)Co(N ) could not
2
2
+
variations are the longer Co1−B2 contact (2.290(3) Å in [2]
catalyze the hydrogenation of internal olefins such as cis-
cyclooctene and norbornene, both alkenes were hydrogenated
using 1. The solution remained transparent during the catalysis,
and the reactions proceeded similarly in the presence of
vs 2.006(2) Å in 1) and the shorter B2−H2 bond (1.19(4) Å in
+
2
[
2] vs 1.37(3) Å in 1), hinting at a weakly ligated η -
hydridoborane. This statement is reinforced by the observation
+
of a higher-energy B−H stretching frequency in [2] (1775
elemental mercury. Hydrogenation of norbornene using D
showed syn addition at the exo positions.
2
−1
−1
23
cm ) than in 1 (1651 cm ).
C
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Journal of the American Chemical Society
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Table 2. Turnover Frequency (TOF) of Catalytic Olefin
Cy
Hydrogenation by [( PBP)CoH] (1) and
2
tBu
a
(
PBP)Co(N2)
Figure 5. (left) First-order plots for the depletion of cis-cyclooctene
(
43.7 mM) in its reaction with excess H (0.5−8.0 atm) catalyzed by
2
Cy
[( PBP)CoH] (1) (2.0 mM) in C D at 298 K. (right) Plot of k
2
6
6
obs
against the pressure of H2.
where k = 123 ± 6 M− s , Co denotes catalyst 1, and S
2
−1
2
a
represents the olefin.
Conditions: RT, 1 atm H , C D . Catalyst loading: 2 mol % relative
2
6
6
1
Three plausible mechanistic scenarios were considered in
light of the aforementioned rate law (Scheme 2). During the
to the olefin. The yield (given in parentheses) was determined by H
NMR spectroscopy using 1,3,5-trimethoxybenzene as an internal
standard. The turnover frequency (h ) was calculated at the end of
−1
the reaction when the starting olefin was not detectable.
Scheme 2. Mechanistic Scenarios Considered for Catalytic
Cy
Olefin Hydrogenation by [( PBP)CoH] (1) (Co =
2
2
a
Kinetics Studies and Proposed Mechanisms. To gain
catalyst 1, S = Olefin, P = Hydrogenated Product)
insight into the mechanism, kinetic studies were performed on
Cy
the hydrogenation of cis-cyclooctene by [( PBP)CoH] (1).
2
To this end, a series of cis-cyclooctene solutions (43.7 mM)
containing different amounts of catalyst 1 (0.84−8.43 mM)
were charged with a large excess of H (3.9 atm) in J. Young
2
NMR tubes. The progress of these reactions was then
1
monitored by H NMR spectroscopy. As shown in Figure 4,
a
The catalyst resting state is highlighted in blue. See the SI for the
derivations of the expected rate equations.
catalysis, the only observed form of the catalyst is 1-H (Co -
2
2
Figure 4. (left) First-order plots for the depletion of cis-cyclooctene
1
H ), detected in situ by H NMR spectroscopy, thus indicating
(
43.7 mM) in its reaction with excess H (3.9 atm) catalyzed by
2
2
Cy
a rapid equilibrium between Co and Co -H with a large
[
( PBP)CoH] (1) (0.84−8.43 mM) in C D at 298 K. (right) Plot
2
2
2
2
6
6
of k against the concentration of 1.
equilibrium constant (k /k ≫ 1). As such, the rate law
−1
1
obs
derived for mechanism A would show a zeroth-order
dependence on H (see the SI), which is inconsistent with
the olefin consumption reaction is first-order in olefin. A plot of
kobs against the concentration of the catalyst indicates that the
reaction is first-order in 1 (the same results were obtained by
the method of initial rates; see the SI).
2
the experimental observation. In turn, we propose that Co -H
2
2
is in equilibrium with the olefin adduct (Co -H -S), which then
2
2
reacts with another equivalent of H to yield the hydrogenated
2
product (P) and regenerate Co -H , as shown in mechanism B.
To determine the reaction order with respect to H , a series
2
2
2
Mechanism B would lead to an overall rate expression that is
of solutions of cis-cyclooctene (43.7 mM) and 1 (2.0 mM) in J.
25
first-order in Co , S, and H . However, in mechanism B we
Young tubes were charged with different H pressures (0.5−8.0
2
2
2
cannot rule out the possibility that a pre-equilibrium between
atm). These experiments indicate that the reaction is also first-
Co -H and Co -H occurs prior to olefin binding. Although
order with respect to H (Figure 5; see the SI for the data
2
2
2
4
2
the measured kinetic isotope effect (k /k ) of 1.4 appears to
obtained from the method of initial rates). On the basis of the
H
D
26
indicate H−H bond cleavage in the rate-determining step, we
note that the observed k /k value is a combination of the
above results, with the Henry’s constant for the concentration
24
of H in benzene, the overall experimental rate law is
H
D
2
isotope effects for all of the individual steps. Lastly, if Co -H
2
2
ν = k[Co ][S][H ]
(1)
2
2
predissociates into catalytically active monometallic Co−H
D
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Journal of the American Chemical Society
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2
9
30
species (mechanism C), the overall rate law should be half-
(2.559(2) Å), silyl (2.306−2.642 Å), phenyl (2.308−2.371
27
31
order with respect to Co2, again inconsistent with the kinetic
data. Taking these results collectively, we therefore propose
that the overall mechanistic outline B is a highly plausible
scenario. We hypothesize that the ligation of an olefin to Co2-
Å), or hydride (2.374−2.433 Å) ligands. Boryl−Ni complexes
are very rare, having been previously characterized only once in
3
2
the solid state.
Reversible H Activation by [( PBP)Ni] (3). Reaction of
Ph
2
2
H may require predissociation of a phosphine or a σ-borane
3 with 1 atm H in THF-d at 298 K led to the rapid and
2
2 8
Ph
ligand, given that Co -H is likely an electronically saturated
quantitative formation of complex [( PBHP)NiH] (4)
2
2
2
11
species (see the SI for plausible identities). That hydrogenation
should proceed by a bimetallic instead of a monometallic
boryl−Co catalyst is interesting and perhaps suggestive of
(Scheme 3). Complex 4 features a broad B resonance at
31
31.4 ppm as well as two coupled P resonances at 47.2 and
2
1
49.1 ppm ( J
= 60 Hz). As is evident in the H NMR
P−P
2
8
metal−metal cooperativity.
spectrum (Figure 7), complex 4 is unsymmetrical and features
Synthesis of Bimetallic Boryl−Ni Complexes. Reaction
Ph
14
of the ligand PB(H)P with Ni(COD) (COD = cyclo-
2
I
I
octadiene) in THF afforded the bimetallic Ni Ni complex
( PBP)Ni] (3) in moderate yield (Scheme 3). Complex 3 is
Ph
[
2
Scheme 3. Synthesis of Complexes 3 and 4
diamagnetic and is characterized by a broad 11B NMR signal
resonating at 42.7 ppm in THF-d , consistent with those
8
11c
31
observed for boryl−metal species. The P NMR spectrum
1
Ph
Figure 7. (top) H NMR spectrum of [( PBP)Ni] (3) in THF-d
2
8
of 3 displays a singlet at 46.4 ppm. The absence of a hydride
1
under 1 atm N2 at 298 K. (bottom) H NMR spectrum of
1
Ph
ligand in 3 is supported by the H NMR and ATR-IR spectra.
[
( PBHP)NiH] (4) in THF-d under 1 atm H at 298 K.
2 8 2
Complex 3 crystallizes in the P1 space group with two
20
independent molecules in the asymmetric unit (Figure 6).
four magnetically inequivalent methylene proton resonances
detected as doublets of doublets ( J
2
2
= 13 Hz, J
= 7 Hz).
H−H
P−H
Additionally, a broad peak (2H, 0.38 ppm) and a pseudoseptet
2
(
2H, −11.02 ppm, J
= 6.6 Hz, 13.2 Hz) were observed in
P−H
the hydridic region. The two resonances are assigned as B−H
1
11
1
31
and Ni−H, respectively, on the basis of H{ B} and H{ P}
NMR experiments (see the SI). Complex 4 is diamagnetic, thus
implying a dimeric species in solution. On the basis of these
I
observations, complex 4 is best formulated as a Ni −(μ -H) −
Ni dimer bridged by two η -hydridoborane ligands as a result
2
2
I
2
of the addition of 2 equiv of H across both boryl−Ni bonds in
2
3
. Accordingly, the measured T (min) values for both the B−H
1
resonance (124 ms, 283 K, 500 MHz) and the Ni−H resonance
(
338 ms, 283 K, 500 MHz) are longer than that expected for a
33
nonclassical H adduct (see the SI). When 3 was exposed to 1
2
Figure 6. Solid-state structure of one of the two independent
atm HD, no H−D coupling was observed for either the B−H
or Ni−H resonances, also arguing against the presence of a
Ph
molecules of [( PBP)Ni] (3). Thermal ellipsoids are drawn at the
2
5
0% probability level. Cyclohexyl groups are drawn in wireframe.
Hydrogen atoms have been omitted for clarity. Selected bond lengths
averaged, Å): Ni1−Ni2 2.2421(9), Ni1−B1 2.043(6), Ni1−B2
.368(6), Ni2−B1 2.328(6), Ni2−B2 2.046(6).
3
4
nonclassical H2 adduct.
Under 1 atm H , a peak
2
1
corresponding to free H was observed in the H NMR
(
2
2
spectrum at room temperature. When 1:1 H /D (1 atm) was
2
2
employed, a triplet corresponding to free HD was detected
immediately. The B−H and Ni−H stretching bands were not
Inspection of the crystallographic structure reveals the
observed in the ATR-IR spectrum of 4 under H .
2
unsymmetrical μ coordination mode of two boryl ligands
Subjecting a solution of 4 to a few freeze−pump−thaw cycles
cleanly regenerated 3 as the only detectable product, a process
which was monitored by multinuclear NMR spectroscopies.
2
(
Ni1−B1, 2.043(6) Å; Ni1−B2, 2.368(6) Å; Ni2−B1, 2.328(6)
Å; Ni2−B2, 2.046(6) Å), leading to two square-pyramidal Ni
centers. The average Ni1−Ni2 bond distance in 3 (2.2421(9)
Å) is shorter than those observed for related tetrakis-
I
Reversible oxidative addition of H2 to a Ni species is
unprecedented. Limberg reported the irreversible reaction of
I
I
I
II
II
35
(
phosphino) Ni Ni dimers bridged by two phosphido
a Ni complex with H , affording a Ni −(μ -H) −Ni dimer.
2
2
2
E
dx.doi.org/10.1021/ja504667f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Stryker also showed the irreversible reaction of [(tBuPN)Ni ]
Article
I
4
4f
(
Chart 1) with H , generating unknown hydride species. As
2
such, the present system emphasizes the significance of boryl−
metal cooperativity in facile and reversible H activation. We
2
speculate that the cooperative boryl−Ni framework helps to
circumvent irreversible one-electron reactions, as observed for
I
31a,35
other Ni species.
The re-formation of a covalent boryl−Ni
bond provides a thermodynamic driving force for the reverse
reaction. For the conversion of 3 to 4, plausible reaction
pathways include (1) concerted σ-bond metathesis and (2)
oxidative addition at Ni followed by reductive elimination at the
Ni−B bond. In terms of broader context, it is also interesting to
note that a mechanism involving oxidative addition of H by a
2
I
Ni intermediate has recently been proposed for the [NiFe]-
36
hydrogenase active site.
DFT Investigations. To gain more insight into the bonding
Ph
Ph
in [( PBP)Ni] (3) and [( PBHP)NiH] (4), simplified
2
2
Me
Me
model complexes [( PBP)Ni] (3′) and [( PBHP)NiH]
2
2
(
4′) featuring dimethyl substituents on the phosphorus atoms
were examined by density functional theory (DFT). The
optimized geometry of 3′ (Ni−Ni 2.273 Å) closely resembles
the crystallographic structure of 3 (Ni−Ni 2.242 Å). In
complex 3′, a Ni−Ni σ bond and a four-center four-electron
Ni−(μ -B) −Ni bonding interaction are supported by the
2
2
molecular orbitals (MOs) as well as an atoms in molecules
3
7
(
AIM) analysis (see the SI). The geometry optimization of 4′
yielded a C -symmetric structure (Figure 8). Inspection of the
2
LUMO reveals a σ*(Ni−H) orbital, thus suggesting the
Figure 8. (top left) DFT-optimized structure of [(MePBHP)NiH]2
(4′). Hydrogen atoms (except for the bridging hydrides) have been
omitted for clarity. Selected bond lengths (Å): Ni1−Ni2 2.631, Ni1−
B1 2.088, Ni1−H1 1.659, Ni1−H3 1.619, Ni1−H4 1.630, Ni2−B2
presence of a four-center bonding interaction in the Ni−(μ -
2
H) −Ni unit. The bonding situation in 4′ can be further
2
elucidated with the qualitative MO diagrams obtained from
linear combinations of symmetry-relevant atomic orbitals
2
.101, Ni2−H2 1.668, Ni2−H3 1.620, Ni2−H4 1.627. (top right)
(
Figure 8). All of the depicted MOs are supported by the
Plots of the LUMO and HOMO (isovalue = 0.05). (bottom)
DFT calculations, including the HOMO, which is a π*(Ni−Ni)
I
Qualitative MO diagrams illustrating the orbitals involved in the Ni−
orbital. Since the two electrons provided by the two Ni centers
(
μ -H) −Ni core.
2
2
are paired in the π*(Ni−Ni) orbital, no net Ni−Ni bond is
1
7b
present. We also note that the Ni−Ni separation in 4′ (2.631
Scheme 4. Synthesis of Complexes 5, 6, 7, and 8
I
I
Å) exceeds those measured for other related Ni −(μ -H) −Ni
2
2
31,38
species (2.374−2.441 Å)
in which a similar bonding
situation could be envisioned (see the SI). Lastly, the agostic σ-
borane coordination mode is in agreement with our
calculations (see the SI).
Synthesis of Monometallic Boryl−Ni Complexes. The
Ph
bimetallic complex [( PBP)Ni] (3) is not a precatalyst for
2
1
the hydrogenation of ethylene. As monitored by H NMR
spectroscopy, the complex’s resting state in the presence of
Ph
ethylene (1 atm) and H (4 atm) is [( PBHP)NiH] (4),
2
2
which is a diamagnetic, electronically saturated 19-electron/19-
electron species. For this reason, we pursued a monomeric
tBu
39
boryl−Ni system by reacting the ligand PB(H)P with
NiCl (DME) (DME = dimethoxyethane) in THF (Scheme 4).
2
This reaction proceeded cleanly to give the monomeric
tBu
complex ( PBP)NiCl (5) in moderate yield. Chloride
abstraction from 5 with AgOTf (OTf = trifluoromethanesul-
tBu
fonate) in THF gave ( PBP)NiOTf (6) in quantitative yield.
tBu
Synthesis of the boryl−Ni hydride species ( PBP)NiH (7)
i
was accomplished by treatment of 6 with Pr Mg in toluene.
Accordingly, the Ni−H stretching band is observed at 1648
2
11
−1
−1
−1
The B NMR resonance of 7 (47.6 ppm) appears at lower field
compared with those for 5 (38.2 ppm) and 6 (31.6 ppm). The
same trend is also observed in the P NMR spectra of these
complexes (5, 85.9 ppm; 6, 83.6 ppm; 7, 122.5 ppm). The H
NMR spectrum of 7 features a triplet at −1.69 ppm ( J
cm and shifts to 1196 cm (calcd 1176 cm ) upon isotopic
labeling with deuterium.
3
1
II
The facile insertion of CO into a Ni −H bond is well-
2
1
40
II
known. While less hydridic Ni −H species featuring an amido
2
=
group trans to the hydride fail to react with CO , the insertion
P−H
2
II
40c−g
34.6 Hz) that is assigned to the terminal Ni−H resonance.
proceeds rapidly for phenyl and silyl Ni −H analogues.
F
dx.doi.org/10.1021/ja504667f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Since boryl ligands are known to engender a very strong trans
Catalytic Olefin Hydrogenation by (tBuPBP)NiH (7).
The capability of 7 to catalyze olefin hydrogenation was
examined as follows. A J. Young NMR tube containing a C D
1
2
influence, it was not surprising that 7 rapidly reacted with
tBu
CO (1 atm) to give the formate complex ( PBP)NiOC-
2
6
6
1
1
31
(
H)O (8). Complex 8 was characterized by B and
P
solution of an olefin and catalyst 7 was charged with 1 atm H .
2
1
resonances at 35.7 and 85.2 ppm, respectively. The C−H
resonance of the Ni-bound formate was detected at 9.11 ppm
as a singlet. The ATR-IR spectrum of 8 exhibited two bands at
The progress of the reaction was then monitored by H NMR
spectroscopy. The TOFs and yields are summarized in Table 4.
−1
1
619 and 1323 cm that are assigned to the antisymmetric and
Table 4. Turnover Frequency (TOF) of Catalytic Olefin
tBu
tBu
symmetric C−O stretches, respectively. The energetic differ-
Hydrogenation by ( PBP)NiH (7), ( PCP)NiH, and
( PNHP)NiH
−1
Cy
+
ence of 296 cm between these two stretches is consistent with
1
41
a κ -O-formate, as confirmed by the solid-state structure.
The solid-state structures of complexes 5−8 were deter-
mined by X-ray crystallography (Figure 9). In all cases, the Ni
a
Figure 9. Solid-state structures of (tBuPBP)NiCl (5), (tBuPBP)NiOTf
Conditions: 2 mol % catalyst relative to olefin, C D . The yield (given
6 6
1
tBu
tBu
in parentheses) was determined by H NMR spectroscopy using 1,3,5-
trimethoxybenzene as an internal standard. The turnover frequency
(
6), ( PBP)NiH (7), and ( PBP)NiOC(H)O (8). Thermal
ellipsoids are drawn at the 50% probability level. Solvent molecules
and hydrogen atoms (except for the Ni−H in 7 and the formate C−H
in 8) have been omitted for clarity. Pertinent metrical parameters can
be found in Table 3.
−1
(h ) was calculated at the end of the reaction when the starting olefin
b 5c
was not detectable. As reported by Hanson: 10 mol % catalyst
relative to olefin, THF-d8.
Under the standard conditions (RT, 1 atm H , C D , 2 mol %
2
6
6
centers adopt pseudo-square-planar geometries, as indicated by
the P1−Ni−P2 and B−Ni−X angles (X = Cl (5), O (6), H (7),
O (8); Table 3). The measured Ni−B distances in these
complexes are very similar (1.905−1.907 Å), seemingly because
of the rigid tridentate ligand scaffold. The strong trans influence
of the boryl group can be inferred by comparing the Ni−Cl
bond distance in 5 (2.2399(4) Å) with those measured for
catalyst), styrene was fully hydrogenated to ethylbenzene with a
−1
TOF of 25 h . A 64% yield was obtained for the
hydrogenation of 1-octene to n-octane in 1.3 h. The lower
yield is due to olefin isomerization, yielding internal alkenes
that were not hydrogenated at RT. The hydrogenation of
internal octenes could be achieved in 2 h by warming the
reaction mixture to 60 °C. 3,3-Dimethyl-1-butene (TOF = 5
II
bis(phosphino) Ni −Cl complexes featuring trans amino
−
1
−1
5
c
42
h ) and cis-cyclooctene (TOF = 0.6 h ) were fully
hydrogenated at lower rates under the standard conditions.
During the catalysis, the color of the solution remained yellow
and transparent. All of the hydrogenation reactions proceeded
similarly in the presence of elemental Hg.
(
2
Å)
2.169 Å), pyridinyl (2.160−2.164 Å), phenyl (2.181−
43 43c,44
.242 Å), alkyl (2.213−2.249 Å),
and silyl (2.242−2.265
4
0e,45
groups. The hydride ligand in 7 is located 1.53(4) Å
away from Ni. This Ni−H bond distance falls within those
II
43e,46
measured for related Ni −H complexes (1.37−1.60 Å).
Table 3. Selected Bond Lengths (Å) and Angles (deg) for (tBuPBP)NiCl (5), (tBuPBP)NiOTf (6), (tBuPBP)NiH (7), and
tBu
(
PBP)NiOC(H)O (8)
5
6
7
8
a
a
a
a
a
Ni−B
Ni−X
Ni−P
P1−Ni−P2
1.907(2)
2.2399(4)
2.2091(6)
1.907(4)
2.034(3)
2.236(1)
1.907(2)
1.905(3)
1.958(2)
2.217(1)
b
a
1.53(4)
a
a
2.153(1)
a
a
156.995(18)
178.20(6)
158.23(4)
160.24(2)
158.92(3)
b
a
a
B−Ni−X
173.21(16)
178.55(11)
173.54(12)
a
b
An averaged value. X = Cl (5), O (6), H (7), O (8).
G
dx.doi.org/10.1021/ja504667f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
While Ni is frequently employed as a heterogeneous
47
hydrogenation catalyst, homogeneous Ni-based olefin hydro-
4
f,5
genation catalysts remain rare, with some requiring high H
2
5a,b
pressure (50 bar) to achieve turnover.
To further evaluate
the effects of the boryl functionality in catalytic hydrogenation
tBu
46b
reactions, the phenyl analogue of 7, ( PCP)NiH,
was
independently prepared and subjected to the standard
tBu
hydrogenation conditions. As shown in Table 4, ( PCP)NiH
operates at a lower rate for styrene and is inactive for the other
olefins under the same conditions. Hanson recently synthesized
Cy
+
5c
a related amino analogue, ( PNHP)NiH . Compared with
Cy
+
those for 7, the reported rates for ( PNHP)NiH are lower
even at 80 °C under 4 atm H2.
Kinetics Studies and Proposed Mechanisms. A
plausible scenario for the hydrogenation reactions catalyzed
tBu
by ( PBP)NiH (7) is mechanism A in Scheme 5. Migratory
Scheme 5. Mechanistic Scenarios Considered for Catalytic
Figure 10. Reaction order determination by the method of initial rates
for the hydrogenation of cis-cyclooctene catalyzed by ( PBP)NiH
Olefin Hydrogenation by (tBuPBP)NiH (7) (Ni-H = catalyst
tBu
a
7, S = Olefin, P = Hydrogenated Product)
(7). (a) Plot of ln(rate) against ln[olefin] ([olefin] = 4.36−43.6 mM).
(
b) Plot of ln(rate) against ln[Ni-H] ([Ni-H] = 1.14−11.4 mM). (c)
Plot of ln(rate) against ln[H ] (pressure of H = 0.3−3.9 atm). (d)
Plot of 1/rate against 1/[H ].
2
2
2
pronounced for the C−H(D) bond than for the Ni−H(D)
48
bond, leading to a larger k when D is employed.
1
2
The other mechanism shown in Scheme 5 involves stepwise
hydrogen atom transfer from Ni-H to the olefin (mechanism
4
9
a
B). Turnover can be achieved by reaction of the two
The catalyst resting state is highlighted in blue.
generated Ni· species with H . However, under an atmosphere
of N , no reaction was observed between 7 (Ni-H) and excess
2
2
cis-cyclooctene over several days in C D , thus ruling out the
6
6
possibility of a hydrogen atom transfer pathway. Lastly, the
insertion of the olefin (S) into the Ni−H bond would give a
Ni−alkyl intermediate (Ni-H-S). This reaction is expected to
reaction of norbornene with D catalyzed by 7 gave exclusively
2
23
tBu
the syn addition product, which cannot rule out mechanism B
but is more consistent with mechanism A.
be reversible since the reaction of ( PBP)NiOTf (6) with
i
Pr Mg yields 7, presumably via β-hydride elimination from a
2
i
The mechanistic studies offer a reasonable explanation for
transiently formed Ni− Pr species. Ni-H-S would then react
with H to give the hydrogenated product (P) and regenerate
tBu
the observation that ( PBP)NiH (7) is a more active olefin
2
hydrogenation catalyst than its phenyl and amino analogues,
catalyst 7 (Ni-H). Since 7 is the only observable resting state of
the catalyst under this scheme, the derived rate law using the
steady-state approximation for Ni-H-S is
with the assumption that all these species follow a similar
50
mechanistic pathway. The strong trans influence of the boryl
12
group would be expected to destabilize the Ni−H and Ni−
alkyl bonds, thus leading to kinetically more favorable
migratory insertion (k ) and σ-bond metathesis (k ) steps.
k k [Ni‐H][S][H ]
1
2
2
v =
k
+ k [H ]
(2)
1
2
−
1
2
2
The presence of a weaker Ni−H bond in 7 is also supported by
In line with this rate expression, our kinetic studies indicate a
first-order dependence on both 7 (Ni-H) and cis-cyclooctene
the observation of an unusually low Ni−H stretching frequency
−
1
tBu
−1 46b
in 7 (1648 cm ) versus ( PCP)NiH (1754 cm )
(
and
Cy
+
−1 5c
(
Figure 10a,b). The reaction is expected to be first-order with
PNHP)NiH (1886 cm ).
respect to H for k ≫ k [H ] and zeroth-order for k
k [H ]. In turn, a fractional-order dependence on H is
≪
−1
2
−1
2
2
2
2
2
CONCLUSION
We have shown that a bis(phosphino)boryl auxiliary ligand,
originally introduced by Nozaki and Yamashita, facilitates rapid
■
observed (Figure 10c), thus indicating that neither k−1 nor
k [H ] is negligible within the range of H pressures employed
2
2
2
in these experiments (0.3−3.9 atm). The double-reciprocal
form of eq 2 can be rearranged as
and reversible H activation reactions in Co and Ni complexes,
2
for example, reversible H activation by the S = 1 bimetallic Co
2
Cy
1
v
k
1
complex [( PBP)CoH] (1). The ability of 1 to serve as an
2
−
1
=
+
olefin hydrogenation catalyst has been investigated, and kinetic
studies indicate that the rate-limiting step likely involves a
k k [Ni‐H][S][H ]
k1[Ni‐H][S]
(3)
1
2
2
A plot of 1/v against 1/[H ] (Figure 10d) yields a straight line.
bimetallic Co species. Whereas 1 hydrogenates terminal olefins
2
−1
−1
tBu
The value of k (0.16 M s ) and the k /k ratio (0.005 M)
more slowly than its monomeric relative ( PBP)CoN , it is
1
−1
2
2
can therefore be estimated according to eq 3. The intermediacy
curiously more active toward internal olefins. The bimetallic Ni
Ph
of a Ni−alkyl species (Ni-H-S) is consistent with an overall k /
complex [( PBP)Ni] (3) featuring two unsymmetrical
H
2
k
value of 0.8. The inverse kinetic isotope effect could be
bridging boryl ligands has also been prepared. Reaction of 3
D
I
I
attributed to the difference in zero-point energy, which is more
with 1 atm H2 affords the Ni −(μ -H) −Ni complex
2
2
H
dx.doi.org/10.1021/ja504667f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Ph
[
( PBHP)Ni] (4) as a result of the addition of 2 equiv of H
After the tube was backfilled with H
with liquid nitrogen, and brought to the spectrometer. For a higher
pressure, the entire NMR tube was cooled to 195, 143, or 77 K and
2
, it was immediately sealed, frozen
2
2
2
across the boryl−Ni bonds. The facile and reversible H
I
activation by a Ni species was previously unknown and has
been herein attributed to boryl−Ni cooperativity. By contrast
to the bimetallic Co catalyst 1, the bimetallic Ni complex 3 is
then backfilled with 1 atm H , leading to a pressure of 1.5, 2.1, or 3.9
2
atm after the sealed tube warmed to 298 K. The progress of these
reactions was monitored by 1H NMR spectroscopy. In order to
inactive for olefin hydrogenation (e.g., ethylene, 4 atm H , RT).
2
achieve stirring, the tube was rotated at 10−15 rpm when spectra were
not being collected. To maintain a constant H pressure, the amount
However, monometallic boryl−Ni complexes show enhanced
2
catalytic activity. For example, the boryl−Ni hydride complex
of H was a large excess with respect to cis-cyclooctene.
2
tBu
(
PBP)NiH (7) can be exploited in the context of olefin
Crystallographic Measurements. The crystallographic measure-
ments were performed at 100(2) K using a Bruker APEX-II CCD area
detector diffractometer (Mo Kα radiation, λ = 0.71073 Å). In each
case, a specimen of suitable size and quality was selected and mounted
onto a nylon loop. The structures were solved by direct methods,
which successfully located most of the non-hydrogen atoms.
hydrogenation. Kinetic data on hydrogenation reactions of this
system are consistent with reversible olefin insertion into the
Ni−H bond followed by hydrogenolysis. It is interesting to
note that complex 7 is a more active olefin hydrogenation
catalyst compared with its isoelectronic/isostructural phenyl
and amino analogues. We attribute this enhanced catalytic
activity to the strong trans influence exerted by the boryl ligand.
The present study highlights the beneficial effects of boryl
52
Semiempirical absorption corrections were applied. Subsequent
refinement on F2 using the SHELXTL/PC package (version 6.1)
53
allowed the remaining non-hydrogen atoms to be located.
Computational Details. DFT structural optimizations were
ligands in reversible H activation chemistry and catalytic olefin
performed without any symmetry restraints using the Gaussian 03
2
5
4
55
hydrogenation. The use of a strong-field boryl ligand that
facilitates E−H bond activation across the metal−boryl bond
constitutes a promising approach for the design of base-metal
catalysts in multielectron reductive reactions.
suite of programs with the BP86 functional and a 6-31+G(d,p)
basis set on all atoms. No instability was found in the optimized wave
function with respect to relaxation of various constraints. Frequency
calculations performed on the optimized structures indicated the
absence of imaginary vibrational frequencies. The optimized structures
5
6
were plotted using Gaussview or Jimp 2. The topologies of the
EXPERIMENTAL SECTION
37
■
electron densities for 3′ and 4′ were subjected to an AIM calculation
General Considerations. Ligands PhPB(H)P, CyPB(H)P, and
PB(H)P were prepared according to the reported procedures.
57
using AIM2000.
tBu
14,39
Cy
Synthesis of [( PBP)CoH] (1). To a THF solution (10 mL) of
2
i
51
Pr Mg was prepared according to the known procedure. Solvents
CoI (280.3 mg, 0.896 mmol) was added a THF solution (5 mL) of
2
2
Cy
PB(H)P (482.6 mg, 0.896 mmol). The mixture was allowed to stir
at RT for 30 min, affording a greenish-yellow solution. To this mixture
was added a THF suspension of sodium amalgam (Na: 42.2 mg (1 wt
% Hg), 1.837 mmol). The mixture was subjected to rapid stirring for
24 h at RT, yielding a brown solution. The solvent was removed under
reduced pressure. The product was extracted into pentane (5 mL × 3)
and filtered through a plug of Celite. Recrystallization by slow
concentration of the pentane solution afforded dark crystals of 1,
which were dried under vacuum (430 mg, 80%). Crystals suitable for
were dried by passing them through an activated alumina column (n-
pentane, benzene, toluene, Et O, and THF). Deuterated solvents were
purchased from Cambridge Isotope Laboratories and were degassed
and stored over activated 3 Å molecular sieves prior to use. CoI , Na,
Hg, FcPF , Ni(COD) , NiCl (DME), and AgOTf were purchased
from Aldrich and used as received. Elemental analyses were performed
by Midwest Microlab (Indianapolis, IN).
Spectroscopic Measurements. Ambient-temperature NMR
spectra were recorded on Varian 300, 400, and 500 MHz NMR
spectrometers. Chemical shifts (δ) are given in parts per million and
2
2
6
2
2
X-ray diffraction analysis were obtained from diffusion of pentane into
a concentrated benzene solution of 1. H NMR (400 MHz, C
−11.78, −4.57, −3.82, −2.45, −1.84, −0.90, −0.25, −0.05, 0.27, 1.68,
2.04, 4.91, 5.80, 9.89, 10.44, 17.49, 21.90, 77.92. Solution magnetic
1
13
1
are referenced against residual solvent signals ( H, C) or external
6
D
6
): δ
1
1
19
31
BF −Et O ( B, F) and H PO4 ( P). T (min) values were
3
2
3
1
1
determined by fitting the pulse-recovery H spectra at various
−1
temperatures using the T calculation protocols in either Varian’s
moment (298 K, C
Elemental analysis calculated (%) for 1 (C64
64.33; H, 8.94; N, 4.69. Found: C, 64.04; H, 9.43; N, 4.37.
6
D
6
): 2.81μ
B
. ATR-IR: νB−H = 1651 cm .
1
VnmrJ software or Mestrelab Research S.L.’s Mestrenova version 6.2.1.
EPR spectra were recorded on a Bruker EMS spectrometer at ∼2 mM
concentrations. The ATR-IR measurements were obtained on a thin
film of the complex obtained from evaporating a drop of the solution
on the surface of a Bruker ALPHA ATR-IR spectrometer probe
H
106
B
2
Co N P ): C,
2 4 4
Cy
Reversible Reaction of [( PBP)CoH]
(1) with H . A C D
2
2 6 6
solution of 1 in a J. Young NMR tube was subjected to one freeze−
pump−thaw cycle and then backfilled with 1 atm H
spectrum of the resulting sample within 5 min confirmed the
1
. The H NMR
2
(
Platinum Sampling Module, diamond, OPUS software package) at 2
−1
Cy
cm resolution. Solution IR measurements were obtained in KBr
pellets using a Bio-Rad Excalibur FTS 3000 spectrometer with Varian
Resolutions Pro software.
quantitative consumption of 1 and the formation of [( PBHP)CoH]
(1-H ). Subjecting the resulting sample to three freeze−pump−thaw
cycles led to quantitative regeneration of 1. Characterizations obtained
2
2
1
Catalytic Hydrogenation of Olefins. A J. Young NMR tube was
charged with the olefin and the catalyst (8 mM, 2% mol relative to the
olefin) in C D . The tube was subjected to one freeze−pump−thaw
for 1-H : H NMR (400 MHz, C D ): δ −8.26, −3.33, −1.85, −1.24,
2
6
6
−0.88, −0.43, 0.00, 0.21, 0.30, 0.87, 1.12, 1.25, 1.42, 1.54, 1.60, 1.71,
2.05, 2.75, 3.27, 3.59, 4.32, 4.54, 5.00, 7.58, 8.46, 8.89, 10.96, 13.59,
6
6
cycle on a high-vacuum line. After the tube was backfilled with 1 atm
21.21, 26.51, 37.28, 55.03. Solution magnetic moment (298 K, C
6
D
6
):
. ATR-IR: νB−H = 1738 cm . Elemental analysis on this
compound was not performed because of the loss of H
−1
H , it was immediately sealed, frozen with liquid nitrogen, and brought
2.56μ
B
2
to the spectrometer. The progress of the reaction was then monitored
.
2
1
Cy
by H NMR spectroscopy. The yields were determined by integration
Synthesis of [( PBP)CoH] [PF ] ([2][PF ]). To a THF
2 6 6
1
of the olefinic H resonances against an internal standard (1,3,5-
suspension (5 mL) of FcPF (33.2 mg, 0.100 mmol) was added a
6
trimethoxybenzene). The identities of the hydrogenated products were
THF solution (5 mL) of complex 1 (120 mg, 0.100 mmol) at −78 °C.
The mixture was allowed to slowly warm to RT over a period of 2 h
and stirred at RT for 15 h, affording a brown solution. Removal of
1
13
established by H and C NMR spectroscopies as well as GC analysis.
Control experiments carried out under the same conditions in the
presence Hg showed essentially identical results. Control experiments
carried out in the absence of catalyst indicated no reactivity.
solvents gave a dark residue that was washed with Et O (5 mL × 3) to
2
give a red solid. THF (10 mL) was added, and the solution was filtered
Kinetic Studies of Catalytic Olefin Hydrogenation. A J. Young
NMR tube was charged with catalyst (1 or 7), cis-cyclooctene, and an
internal standard (1,3,5-trimethoxybenzene) in C D . The tube was
through a short plug of Celite. Slow Et O diffusion into the resulting
2
1
THF solution gave dark-red crystals of [2][PF ] (65 mg, 48%). H
6
NMR (400 MHz, THF-d ): δ −34.22, −29.10, −23.33, −21.25,
6
6
8
subjected to one freeze−pump−thaw cycle on a high-vacuum line.
−15.65, −14.73, −14.08, −9.52, −9.22, −8.10, −6.55, −4.30, −4.05,
I
dx.doi.org/10.1021/ja504667f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
−
3.96, −3.83, −3.45, −2.73, −2.65, −2.35, −2.25, −2.22, −2.16,
1.98, −1.49, −0.90, −0.57, −0.19, 0.36, 0.54, 1.09, 1.23, 1.44, 1.59,
of a pentane solution of 6 via vapor diffusion into hexamethyldisilox-
1
3
−
ane. H NMR (400 MHz, C D ): δ 1.20 (t, 36H, J = 6.8 Hz), 3.32
6
6
P−H
1
1
.67, 1.80, 2.06, 2.20, 2.60, 3.25, 3.39, 3.56, 4.00, 5.35, 6.48, 8.67, 8.95,
2.72, 14.28, 16.56, 26.20, 59.11, 99.85. Solution magnetic moment
(s, 4H), 6.77 (dd, 2H, JH−H = 5.7 Hz, JH−H = 3.1 Hz), 7.05 (dd, 2H,
J
H−H = 5.7 Hz, JH−H = 3.1 Hz). 31P{ H} NMR (161.8 MHz, C
1
D
): δ
): δ 31.6. F{ H} NMR
): δ
6
6
−1
11
1
19
1
(
298 K, THF-d ): 1.62μ . ATR-IR: ν = 1775 cm . Satisfactory
83.6. B{ H} NMR (128.4 MHz, C D
6 6
8
B
B−H
13
1
elemental analysis could not be obtained for this compound. MS
(376.2 MHz, C
6
D
6
): δ −76.7. C{ H} NMR (100.5 MHz, C D
6 6
+
+
+
(
ESI ): calcd for [2] (C H B Co N P ) 1194.6, found 1194.6.
29.52 (t, JP−P = 2.6 Hz), 34.97 (t, JP−P = 4.8 Hz), 38.99 (t, JP−P = 18.1
Hz), 108.86, 119.34, 138.51 (t, JP−P = 6.8 Hz). Elemental analysis
64
106
2
2
4 4
Ph
Synthesis of [( PBP)Ni] (3). To a THF solution (5 mL) of
2
Ni(COD) (57.1 mg, 0.208 mmol) was added a THF solution of
ligand PB(H)P (106.8 mg, 0.208 mmol) dropwise at RT. The
resulting mixture was allowed to stir at RT for 12 h, leading to a
homogeneous red solution. On this THF solution was carefully layered
calculated (%) for 6 (C25
H44BF N NiO P S): C, 46.83; H, 6.92; N,
3 2 3 2
2
Ph
4.37. Found: C, 46.66; H, 7.01; N, 4.36.
tBu
Synthesis of ( PBP)NiH (7). To a toluene solution (5 mL) of 6
(326.6 mg, 0.509 mmol) was added a toluene solution (3 mL) of
i
10 mL of pentane. After 12 h, dark-red crystals of 3 were obtained by
Pr Mg (73.2 mg, 0.662 mmol) at −78 °C. The mixture was slowly
2
filtration, washed with pentane (5 mL × 3), and dried under vacuum
warmed to RT over 1 h and stirred at RT for 12 h. Solvents were
removed under vacuum. The resulting solid was triturated with
pentane three times. The product was extracted into pentane (15 mL)
and filtered through Celite. The resulting filtrate was concentrated to
1
(
88 mg, 74%). When the reaction was carried out in C D , a H
6 6
resonance corresponding to free H was observed within 10 min in the
2
1
1
H NMR spectrum. H NMR (400 MHz, THF-d ): δ 3.60 (bs, 4H),
8
∼
4 mL and allowed to stand at −30 °C for 24 h, affording yellow
4
.01 (bs, 4H), 6.61 (bs, 4H), 6.67 (bs, 4H), 6.94 (bs, 16H), 7.09−7.30
1
31 1
crystals of 3 (190 mg, 76%). H NMR (400 MHz, C D ): δ −1.69 (t,
(
m, 16H), 7.42 (bs, 8H). P{ H} NMR (161.8 MHz, THF-d ): δ
6
6
8
2
3
6.4. 11B{ H} NMR (128.4 MHz, THF-d ): δ 42.7 (the B resonance
1
11
NiH, 1H, J
4H), 7.05 (dd, 2H, JH−H = 5.4 Hz, J
= 34.6 Hz), 1.27 (t, 36H, J
P−H
= 6.3 Hz), 3.80 (s,
4
P−H
= 2.7 Hz), 7.19 (dd, 2H, JH−H
8
13
1
shows the same line width). C{ H} NMR (100.5 MHz, THF-d ): δ
H−H
8
= 2.7 Hz). 31P{ H} NMR (161.8 MHz, C D ): δ 122.5.
1
=
5.4 Hz, J
107.51, 117.41, 128.31, 128.85, 129.82, 131.20, 134.30, 137.39, 140.65.
H−H 6 6
11
1
13
1
B{ H} NMR (128.4 MHz, C D ): δ 47.6. C{ H} NMR (100.5
Elemental analysis calculated (%) for 3 (C H B N Ni P ): C, 67.19;
6
6
64
56
2
4
2 4
MHz, C D ): δ 29.96 (t, J = 3.1 Hz), 34.65 (t, JP−P = 6.3 Hz), 42.07
H, 4.93; N, 4.90. Found: C, 66.64; H, 5.65; N, 4.72.
6
6
P−P
Ph
(t, JP−P = 16.6 Hz), 109.28, 118.39, 140.36 (t, JP−P = 7.8 Hz).
Elemental analysis calculated (%) for 7 (C H BN NiP ): C, 58.46;
Synthesis of [( PBHP)NiH]2 (4). A J. Young NMR tube
containing a THF-d solution of 3 was subjected to one freeze−
24 45
2
2
8
H, 9.20; N, 5.68. Found (from two independently prepared samples):
pump−thaw cycle and then backfilled with 1 atm H . Rapid and
2
C, 57.91; H, 9.11; N, 5.37 and C, 57.98; H, 9.15; N, 5.43.
quantitative formation of 4 was confirmed by multinuclear NMR
tBu
1
Synthesis of ( PBP)NiOC(H)O (8). Method A (in situ
spectroscopies. H NMR (400 MHz, THF-d ): δ −11.02 (tt, NiH, 2H,
8
2
2
generation): A J. Young NMR tube containing a C D solution of 7
J
= 6.6 Hz, 13.2 Hz), 0.38 (s, BH, 2H), 3.78 (dd, 2H, J
= 13.2
6
6
P−H
Hz, J
H−H
2
2
2
was subjected to one freeze−pump−thaw cycle and then backfilled
= 6.9 Hz), 3.91 (dd, 2H, J
= 13.2 Hz, J
= 6.9 Hz),
P−H
H−H
P−H
2
2
2
with 1 atm CO . Quantitative formation of 8 was observed
2
4
1
4
4
.12 (dd, 2H, J
= 13.2 Hz, J
= 6.9 Hz), 5.06 (dd, 2H, J
=
H−H
H−H
P−H
2
2
immediately by multinuclear NMR spectroscopies. Method B: To a
flask containing 7 (90 mg, 0.183 mmol) was added 5 mL of benzene.
The flask was subjected to one freeze−pump−thaw cycle and
3.2 Hz, J
= 6.9 Hz), 6.50−6.59 (m, 8H), 6.65 (t, J
= 7.8 Hz,
= 7.7 Hz,
P−H
P−H
2
H), 6.74−6.91 (m, 20H), 7.03−7.12 (m, 6H), 7.19 (t, J
P−H
2
2
H), 7.27 (t, J
= 7.2 Hz, 2H), 7.72 (t, J
= 7.5 Hz, 4H).
= 60 Hz), 49.1
P−P
P−H
P−H
3
1
1
2
backfilled with 1 atm CO . Removal of solvents under reduced
2
P{ H} NMR (161.8 MHz, THF-d ): δ 47.2 ( J
8
pressure gave 8 as a yellow solid (96 mg, 98%). Yellow crystals of 8
were obtained by slow concentration of a pentane solution of 8 under
2
11
1
(
J
= 60 Hz). B{ H} NMR (128.4 MHz, THF-d ): δ 31.4.
P
−
P
8
1
3
1
C{ H} NMR (100.5 MHz, THF-d ): δ 108.22 (d, J
= 36.1 Hz),
P−P
1
8
an atmosphere of N at RT. H NMR (400 MHz, C D ): δ 1.30 (t,
2
6
6
1
1
17.92 (d, JP−P = 27.8 Hz), 127.54, 127.60, 127.85, 128.00, 128.03,
28.06, 128.35, 128.39, 128.45, 128.54, 128.68, 128.81, 129.55 (d, JP−P
3
3
36H, J
= 6.5 Hz), 3.50 (t, 4H, J
= 1.9 Hz), 6.88 (dd, 2H, JH−H
P−H
P−H
= 5.7 Hz, JH−H
= 3.3 Hz), 7.12 (dd, 2H, J
= 5.7 Hz, JH−H = 3.3 Hz),
H−H
=
21.2 Hz), 130.34, 130.39, 130.44, 132.57 (d, JP−P = 12.6 Hz), 134.03
31
1
9.11 (s, NiOC(H)O, 1H).
P{ H} NMR (161.8 MHz, C D ): δ 85.2.
6 6
(
d, JP−P = 13.2 Hz), 134.65 (d, JP−P = 14.0 Hz), 135.62 (d, JP−P = 26.2
11
1
13
1
B{ H} NMR (128.4 MHz, C D ): δ 35.7. C{ H} NMR (100.5
6
6
Hz), 138.97 (d, JP−P = 23.8 Hz), 140.79 (t, JP−P = 19.1 Hz), 142.16 (d,
JP−P = 24.1 Hz). Elemental analysis on this compound was not
MHz, C D ): δ 29.58 (t, J = 2.9 Hz), 35.16 (t, JP−P = 5.0 Hz), 39.84
6
6
P−P
(t, JP−P = 17.8 Hz), 108.67, 118.88, 139.26 (t, JP−P = 7.5 Hz), 167.61.
Elemental analysis calculated (%) for 8 (C H BN Ni O P ): C,
performed because of the loss of H .
2
25
45
2
2
2 2
tBu
Synthesis of ( PBP)NiCl (5). THF (15 mL) was added to a flask
containing ligand PB(H)P (1.596 g, 3.675 mmol) and NiCl (DME)
5
5.91; H, 8.45; N, 5.22. Found: C, 55.89; H, 8.24; N, 5.22.
tBu
2
(
0.848 g, 3.859 mmol) at RT. The reaction mixture was stirred for 2 h,
ASSOCIATED CONTENT
leading to the formation of a green solid suspended in a yellow
solution. The green solid was removed by filtration. Removal of
solvent from the filtrate gave 5 as a yellow solid that was washed with
cold pentane (5 mL × 3) and dried under vacuum (1.2 g, 62%).
Yellow crystals of 5 were obtained by slow concentration of a pentane
solution of 5. H NMR (400 MHz, C D ): δ 1.67 (t, 36H, J = 6.7
Hz), 3.54 (t, 4H, J
3
(
■
*
S
Supporting Information
Spectroscopic characterizations, crystallographic analyses
CIF), kinetic details, and computational data. This material
(
is available free of charge via the Internet at http://pubs.acs.org.
1
3
6
6
P−H
2
= 2.0 Hz), 6.91 (dd, 2H, JH−H = 5.6 Hz, J
=
H−H
P−H
= 3.3 Hz). 3 P{ H} NMR
1
1
AUTHOR INFORMATION
.3 Hz), 7.13 (dd, 2H, JH−H = 5.6 Hz, J
■
H−H
11
1
161.8 MHz, C D ): δ 85.9. B{ H} NMR (128.4 MHz, C D ): δ
Corresponding Author
jpeters@caltech.edu
6
6
6
6
8.2. 13C{ H} NMR (100.5 MHz, C D ): δ 29.85 (t, J
1
= 2.5 Hz),
3
3
1
6
6
P−P
5.54 (t, JP−P = 5.1 Hz), 39.98 (t, JP−P = 17.0 Hz), 108.84, 118.88,
39.28 (t, JP−P = 7.4 Hz). Elemental analysis calculated (%) for 5
Notes
(
C H BClN NiP ): C, 54.64; H, 8.41; N, 5.31. Found: C, 54.63; H,
The authors declare no competing financial interest.
24
44
2
2
8.38; N, 5.33.
Synthesis of (tBuPBP)NiOTf (6). To a THF solution (10 mL) of 5
ACKNOWLEDGMENTS
■
(
(
627.5 mg, 1.190 mmol) was added a THF solution (5 mL) of AgOTf
320.9 mg, 1.249 mmol) at RT. The mixture was stirred for 30 min.
This material is based upon work performed by the Joint
Center for Artificial Photosynthesis, a DOE Energy Innovation
Hub supported through the Office of Science of the U.S.
Department of Energy under Award DE-SC0004993. We thank
Dr. David C. Leitch, Dr. Yichen Tan, and Dr. Charles C.
Solvents were removed under reduced pressure. The product was
dissolved in benzene, and silver salts were removed by filtration
through Celite. Lyophilization of the filtrate gave 6 as a yellow solid
(
759 mg, 99%). Yellow crystals of 6 were grown by slow concentration
J
dx.doi.org/10.1021/ja504667f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
McCrory for insightful discussions on kinetics. We also thank
Prof. Gerard Parkin for insightful discussions on bonding.
G. J.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.;
Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem. Rev.
1
998, 98, 2685−2722. (d) Braunschweig, H.; Colling, M. Coord. Chem.
Rev. 2001, 223, 1−51. (e) Cho, J.-Y.; Tse, M. K.; Holmes, D.;
Maleczka, R. E.; Smith, M. R. Science 2002, 295, 305−308. (f) Dang,
L.; Lin, Z.; Marder, T. B. Chem. Commun. 2009, 3987−3995.
REFERENCES
■
(
1) The Handbook of Homogeneous Hydrogenation; de Vries, J. G.,
Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, Germany, 2007.
(12) (a) Iverson, C. N.; Smith, M. R. J. Am. Chem. Soc. 1995, 117,
(
2) Catalysis without Precious Metals; Bullock, R. M., Ed.; Wiley-VCH:
Weinheim, Germany, 2010.
3) Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to
Catalysis; University Science Books: Sausalito, CA, 2010.
4) (a) Knijnenburg, Q.; Horton, A. D.; van der Heijden, H.;
Kooistra, T. M.; Hetterscheid, D. G. H.; Smits, J. M. M.; de Bruin, B.;
Budzelaar, P. H. M.; Gal, A. W. J. Mol. Catal. A: Chem. 2005, 232,
4
403−4404. (b) Dai, C.; Stringer, G.; Marder, T. B.; Baker, R. T.;
Scott, A. J.; Clegg, W.; Norman, N. C. Can. J. Chem. 1996, 74, 2026−
(
2
4
9
031. (c) Sakaki, S.; Kai, S.; Sugimoto, M. Organometallics 1999, 18,
825−4837. (d) Zhu, J.; Lin, Z.; Marder, T. B. Inorg. Chem. 2005, 44,
(
̈
384−9390. (e) Braunschweig, H.; Brenner, P.; Muller, A.; Radacki,
K.; Rais, D.; Uttinger, K. Chem.Eur. J. 2007, 13, 7171−7176.
(
(
13) van der Vlugt, J. I. Angew. Chem., Int. Ed. 2010, 49, 252−255.
14) Segawa, Y.; Yamashita, M.; Nozaki, K. Organometallics 2009, 28,
1
51−159. (b) Monfette, S.; Turner, Z. R.; Semproni, S. P.; Chirik, P. J.
J. Am. Chem. Soc. 2012, 134, 4561−4564. (c) Zhang, G.; Scott, B. L.;
Hanson, S. K. Angew. Chem., Int. Ed. 2012, 51, 12102−12106. (d) Lin,
T.-P.; Peters, J. C. J. Am. Chem. Soc. 2013, 135, 15310−15313. (e) Yu,
R. P.; Darmon, J. M.; Milsmann, C.; Margulieux, G. W.; Stieber, S. C.
E.; DeBeer, S.; Chirik, P. J. J. Am. Chem. Soc. 2013, 135, 13168−13184.
6
(
(
234−6242.
15) Pandey, K. K. Coord. Chem. Rev. 2009, 253, 37−55.
16) (a) Dapporto, P.; Midollini, S.; Sacconi, L. Inorg. Chem. 1975,
14, 1643−1650. (b) Fryzuk, M. D.; Ng, J. B.; Rettig, S. J.; Huffman, J.
C.; Jonas, K. Inorg. Chem. 1991, 30, 2437−2441. (c) Kersten, J. L.;
Rheingold, A. L.; Theopold, K. H.; Casey, C. P.; Widenhoefer, R. A.;
Hop, C. E. C. A. Angew. Chem., Int. Ed. Engl. 1992, 31, 1341−1343.
(17) (a) Green, M. L. H. J. Organomet. Chem. 1995, 500, 127−148.
(b) Green, J. C.; Green, M. L. H.; Parkin, G. Chem. Commun. 2012,
48, 11481−11503.
(
f) Camacho-Bunquin, J.; Ferguson, M. J.; Stryker, J. M. J. Am. Chem.
Soc. 2013, 135, 5537−5540. (g) Zhang, G.; Vasudevan, K. V.; Scott, B.
L.; Hanson, S. K. J. Am. Chem. Soc. 2013, 135, 8668−8681.
(
̈
h) Gartner, D.; Welther, A.; Rad, B. R.; Wolf, R.; Jacobi von
Wangelin, A. Angew. Chem., Int. Ed. 2014, 53, 3722−3726.
(
1
5) (a) Angulo, I. M.; Kluwer, A. M.; Bouwman, E. Chem. Commun.
998, 2689−2690. (b) Angulo, I. M.; Bouwman, E. J. Mol. Catal. A:
(18) Evans, D. F. J. Chem. Soc. 1959, 2003−2005.
Chem. 2001, 175, 65−72. (c) Vasudevan, K. V.; Scott, B. L.; Hanson,
S. K. Eur. J. Inorg. Chem. 2012, 4898−4906. (d) Wu, J.; Faller, J. W.;
Hazari, N.; Schmeier, T. J. Organometallics 2012, 31, 806−809.
(19) Evans, R.; Deng, Z.; Rogerson, A. K.; McLachlan, A. S.;
Richards, J. J.; Nilsson, M.; Morris, G. A. Angew. Chem., Int. Ed. 2013,
52, 3199−3202.
(
5
(
2
e) Harman, W. H.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 5080−
(20) Given the subtle differences that exist between these two
molecules, an averaged value is adopted for the following discussion.
(21) Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.;
Echeverria, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans.
082.
6) (a) Ray, K.; Petrenko, T.; Wieghardt, K.; Neese, F. Dalton Trans.
007, 1552−1566. (b) Chirik, P. J. Inorg. Chem. 2011, 50, 9737−9740.
c) Luca, O. R.; Crabtree, R. H. Chem. Soc. Rev. 2013, 42, 1440−1459.
7) (a) Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474−7485.
b) Fong, H.; Moret, M.-E.; Lee, Y.; Peters, J. C. Organometallics 2013,
2, 3053−3062.
8) (a) Bonanno, J. B.; Henry, T. P.; Wolczanski, P. T.; Pierpont, A.
(
2
(
(
(
1
(
2
008, 2832−2838.
22) (a) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
b) Green, M. T. J. Am. Chem. Soc. 1999, 121, 7939−7940.
23) Marchand, A. P.; Marchand, N. W. Tetrahedron Lett. 1971, 12,
365−1368.
24) Luehring, P.; Schumpe, A. J. Chem. Eng. Data 1989, 34, 250−
52.
25) (a) Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J.
Chem. Soc. A 1966, 1711−1732. (b) Sanchez-Delgado, R. A.; Rosales,
M. Coord. Chem. Rev. 2000, 196, 249−280.
26) Gomez-Gallego, M.; Sierra, M. A. Chem. Rev. 2011, 111, 4857−
963.
27) (a) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks,
(
(
3
(
W.; Cundari, T. R. Inorg. Chem. 2007, 46, 1222−1232. (b) Alcaraz, G.;
Helmstedt, U.; Clot, E.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc.
2
008, 130, 12878−12879. (c) Yang, J. Y.; Bullock, R. M.; Shaw, W. J.;
Twamley, B.; Fraze, K.; DuBois, M. R.; DuBois, D. L. J. Am. Chem. Soc.
009, 131, 5935−5945. (d) He, T.; Tsvetkov, N. P.; Andino, J. G.;
Gao, X.; Fullmer, B. C.; Caulton, K. G. J. Am. Chem. Soc. 2009, 132,
10−911. (e) Miller, A. J. M.; Labinger, J. A.; Bercaw, J. E. J. Am.
Chem. Soc. 2010, 132, 3301−3303. (f) Gunanathan, C.; Milstein, D.
Acc. Chem. Res. 2011, 44, 588−602. (g) Wade, C. R.; Gabbaï, F. P.
Angew. Chem., Int. Ed. 2011, 50, 7369−7372. (h) Tsoureas, N.; Kuo,
Y.-Y.; Haddow, M. F.; Owen, G. R. Chem. Commun. 2011, 47, 484−
86. (i) Lin, T.-P.; Gabbaï, F. P. J. Am. Chem. Soc. 2012, 134, 12230−
2238. (j) Pan, B.; Bezpalko, M. W.; Foxman, B. M.; Thomas, C. M.
Dalton Trans. 2012, 41, 9083−9090. (k) Podiyanachari, S. K.;
Frohlich, R.; Daniliuc, C. G.; Petersen, J. L.; Muck-Lichtenfeld, C.;
Kehr, G.; Erker, G. Angew. Chem., Int. Ed. 2012, 51, 8830−8833.
l) Gutsulyak, D. V.; Piers, W. E.; Borau-Garcia, J.; Parvez, M. J. Am.
Chem. Soc. 2013, 135, 11776−11779. (m) Boone, M. P.; Stephan, D.
W. J. Am. Chem. Soc. 2013, 135, 8508−8511. (n) Gloaguen, Y.; Jacobs,
W.; de Bruin, B.; Lutz, M.; van der Vlugt, J. I. Inorg. Chem. 2013, 52,
(
́
2
(
4
(
́
9
T. J. J. Am. Chem. Soc. 1985, 107, 8111−8118. (b) Zuffa, J. L.; Blohm,
M. L.; Gladfelter, W. L. J. Am. Chem. Soc. 1986, 108, 552−553.
(28) (a) Reed, S. A.; White, M. C. J. Am. Chem. Soc. 2008, 130,
4
1
3
8
316−3318. (b) Powers, D. C.; Ritter, T. Acc. Chem. Res. 2012, 45,
40−850. (c) Per
́
ez-Temprano, M. H.; Casares, J. A.; Espinet, P.
Chem.Eur. J. 2012, 18, 1864−1884. (d) Radlauer, M. R.; Day, M.
̈
̈
W.; Agapie, T. J. Am. Chem. Soc. 2012, 134, 1478−1481.
(e) Mazzacano, T. J.; Mankad, N. P. J. Am. Chem. Soc. 2013, 135,
(
1
(
7258−17261.
29) Jones, R. A.; Norman, N. C.; Seeberger, M. H.; Atwood, J. L.;
Hunter, W. E. Organometallics 1983, 2, 1629−1634.
30) (a) Tanabe, M.; Yumoto, R.; Osakada, K. Chem. Commun. 2012,
(
1
̈
682−1684. (o) Ostapowicz, T. G.; Merkens, C.; Holscher, M.;
4
8, 2125−2127. (b) Nova, A.; Suh, H.-W.; Schmeier, T. J.; Guard, L.
Klankermayer, J.; Leitner, W. J. Am. Chem. Soc. 2013, 135, 2104−2107.
M.; Eisenstein, O.; Hazari, N.; Maseras, F. Angew. Chem., Int. Ed. 2014,
53, 1103−1108.
(31) (a) Bach, I.; Goddard, R.; Kopiske, C.; Seevogel, K.; Porschke,
̈
(
p) Sgro, M. J.; Stephan, D. W. Chem. Commun. 2013, 49, 2610−2612.
9) Two plausible reaction pathways that were considered include
1) concerted σ-bond metathesis and (2) oxidative addition at the
(
(
metal followed by hydride migration to boron. See refs 10a and 10b.
10) (a) Zeng, G.; Sakaki, S. Inorg. Chem. 2013, 52, 2844−2853.
b) Harman, W. H.; Lin, T.-P.; Peters, J. C. Angew. Chem., Int. Ed.
014, 53, 1081−1086.
11) (a) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457−2483.
b) Waltz, K. M.; Hartwig, J. F. Science 1997, 277, 211−213. (c) Irvine,
K.-R. Organometallics 1999, 18, 10−20. (b) Li, T.; García, J. J.;
Brennessel, W. W.; Jones, W. D. Organometallics 2010, 29, 2430−
2445.
(32) Adhikari, D.; Huffman, J. C.; Mindiola, D. J. Chem. Commun.
2007, 4489−4491.
(33) Morris, R. H. Coord. Chem. Rev. 2008, 252, 2381−2394.
(
(
2
(
(
K
dx.doi.org/10.1021/ja504667f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(
34) (a) Gru
Etienne, S.; Chaudret, B. J. Phys. Chem. A 1999, 103, 4752−4754.
b) Gelabert, R.; Moreno, M.; Lluch, J. M.; Lledos, A.; Pons, V.;
Heinekey, D. M. J. Am. Chem. Soc. 2004, 126, 8813−8822.
35) Pfirrmann, S.; Yao, S.; Ziemer, B.; Stosser, R.; Driess, M.;
Limberg, C. Organometallics 2009, 28, 6855−6860.
36) Lill, S. O. N.; Siegbahn, P. E. M. Biochemistry 2009, 48, 1056−
066.
37) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford
University Press: Oxford, U.K., 1994.
38) (a) Barnett, B. L.; Kruger, C.; Tsay, Y.-H.; Summerville, R. H.;
̈
ndemann, S.; Limbach, H.-H.; Buntkowsky, G.; Sabo-
(52) Sheldrick, G. M. SADABS, version 2007/4; Bruker Analytical X-
ray Systems: Madison, WI, 2007.
(53) Sheldrick, G. M.; SHELXTL, version 6.1; Bruker Analytical X-
(
́
ray Systems: Madison, WI, 2000.
(
̈
(54) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.
J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(
1
(
(
̈
Hoffmann, R. Chem. Ber. 1977, 110, 3900−3909. (b) Fryzuk, M. D.;
Clentsmith, G. K. B.; Leznoff, D. B.; Rettig, S. J.; Geib, S. J. Inorg.
Chim. Acta 1997, 265, 169−177. (c) Cornella, J.; Gom
Martin, R. J. Am. Chem. Soc. 2013, 135, 1997−2009.
39) Segawa, Y.; Yamashita, M.; Nozaki, K. J. Am. Chem. Soc. 2009,
31, 9201−9203.
40) (a) Darensbourg, D. J.; Darensbourg, M. Y.; Goh, L. Y.; Ludvig,
M.; Wiegreffe, P. J. Am. Chem. Soc. 1987, 109, 7539−7540.
b) Darensbourg, M. Y.; Ludwig, M.; Riordan, C. G. Inorg. Chem.
́
ez-Bengoa, E.;
(
1
(
(
1
989, 28, 1630−1634. (c) Chakraborty, S.; Zhang, J.; Krause, J. A.;
Guan, H. J. Am. Chem. Soc. 2010, 132, 8872−8873. (d) Schmeier, T. J.;
Hazari, N.; Incarvito, C. D.; Raskatov, J. A. Chem. Commun. 2011, 47,
(
55) (a) Perdew, J. P. Phys. Rev. B 1986, 33, 8822−8824. (b) Becke,
A. D. Phys. Rev. A 1988, 38, 3098−3100.
56) (a) Hall, M. B.; Fenske, R. F. Inorg. Chem. 1972, 11, 768−775.
b) Manson, J.; Webster, C. E.; Perez, L. M.; Hall, M. B. http://www.
chem.tamu.edu/jimp2/index.html.
57) Konig, F. B.; Schonbohm, J.; Bayles, D. J. Comput. Chem. 2001,
2, 545−559.
1
824−1826. (e) Suh, H.-W.; Schmeier, T. J.; Hazari, N.; Kemp, R. A.;
(
Takase, M. K. Organometallics 2012, 31, 8225−8236. (f) Chakraborty,
(
́
S.; Patel, Y. J.; Krause, J. A.; Guan, H. Polyhedron 2012, 32, 30−34.
(
g) Chakraborty, S.; Zhang, J.; Patel, Y. J.; Krause, J. A.; Guan, H.
(
2
Inorg. Chem. 2013, 52, 37−47. (h) Venkanna, G. T.; Tammineni, S.;
Arman, H. D.; Tonzetich, Z. J. Organometallics 2013, 32, 4656−4663.
(
(
41) Gibson, D. H. Coord. Chem. Rev. 1999, 185−186, 335−355.
42) (a) van der Vlugt, J. I.; Lutz, M.; Pidko, E. A.; Vogt, D.; Spek, A.
L. Dalton Trans. 2009, 1016−1023. (b) Vogt, M.; Rivada-Wheelaghan,
O.; Iron, M. A.; Leitus, G.; Diskin-Posner, Y.; Shimon, L. J. W.; Ben-
David, Y.; Milstein, D. Organometallics 2013, 32, 300−308.
(
43) (a) Benito-Garagorri, D.; Bocokic,
́
V.; Mereiter, K.; Kirchner, K.
mez-Benítez, V.;
n, O.; Herrera-Alvarez, C.; Toscano, R. A.;
Morales-Morales, D. Tetrahedron Lett. 2006, 47, 5059−5062.
c) Pandarus, V.; Zargarian, D. Organometallics 2007, 26, 4321−
334. (d) Boro, B. J.; Dickie, D. A.; Goldberg, K. I.; Kemp, R. A. Acta
Organometallics 2006, 25, 3817−3823. (b) Go
Baldovino-Pantaleo
́
́
́
(
4
Crystallogr., Sect. E 2008, 64, m1304. (e) Chakraborty, S.; Krause, J. A.;
Guan, H. Organometallics 2009, 28, 582−586. (f) Zhang, J.; Medley, C.
M.; Krause, J. A.; Guan, H. Organometallics 2010, 29, 6393−6401.
(
2
g) Duncan, D.; Hope, E. G.; Singh, K.; Stuart, A. M. Dalton Trans.
011, 40, 1998−2005.
44) Castonguay, A.; Sui-Seng, C.; Zargarian, D.; Beauchamp, A. L.
Organometallics 2006, 25, 602−608.
45) Mitton, S. J.; McDonald, R.; Turculet, L. Angew. Chem., Int. Ed.
009, 48, 8568−8571.
46) (a) Liang, L.-C.; Chien, P.-S.; Lee, P.-Y. Organometallics 2008,
(
(
2
(
2
7, 3082−3093. (b) Boro, B. J.; Duesler, E. N.; Goldberg, K. I.; Kemp,
R. A. Inorg. Chem. 2009, 48, 5081−5087. (c) Steinke, T.; Shaw, B. K.;
Jong, H.; Patrick, B. O.; Fryzuk, M. D. Organometallics 2009, 28,
2
830−2836. (d) Martínez-Prieto, L. M.; Melero, C.; del Río, D.;
́
Palma, P.; Cam
́
pora, J.; Alvarez, E. Organometallics 2012, 31, 1425−
1438. (e) Sgro, M. J.; Stephan, D. W. Organometallics 2012, 31, 1584−
1587.
(
47) Keim, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 235−244.
(
48) (a) Threlkel, R. S.; Bercaw, J. E. J. Am. Chem. Soc. 1981, 103,
2
650−2659. (b) Parkin, G.; Bunel, E.; Burger, B. J.; Trimmer, M. S.;
Van Asselt, A.; Bercaw, J. E. J. Mol. Catal. 1987, 41, 21−39. (c) Cheng,
T.-Y.; Bullock, R. M. J. Am. Chem. Soc. 1999, 121, 3150−3155.
(
d) Jones, W. D. Acc. Chem. Res. 2003, 36, 140−146.
(
49) Bullock, R. M.; Samsel, E. G. J. Am. Chem. Soc. 1990, 112,
886−6898.
50) A similar mechanism has been proposed for the catalyst
6
(
Cy
+
(
PNHP)NiH by Hanson. See ref 5c.
51) Bohm, V. P. W.; Schulze, V.; Bro
Hoffmann, R. W. Organometallics 2003, 22, 2925−2930.
(
̈
̈
̈
nstrup, M.; Muller, M.;
L
dx.doi.org/10.1021/ja504667f | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX