Mechanistic Investigations of the Hydrogenolysis of Diaryl Ethers Catalyzed by Nickel Complexes of N-Heterocyclic Carbene Ligands

Article abstract of DOI:10.1021/jacs.7b10537

Recent interest in the valorization of lignin has led to reactions involving the cleavage of strong aromatic C-O bonds. However, few experimental mechanistic studies of these reactions have been published. We report detailed mechanistic analysis of the hydrogenolysis of diaryl ethers catalyzed by the combination of Ni(COD)₂ (COD = 1,5-cyclooctadiene) and an N-heterocyclic carbene (NHC). Experiments on the catalytic reaction indicated that NaOt-Bu was necessary for catalysis, but kinetic analysis showed that the base is not involved in the rate-limiting C-O bond cleavage. The resting state of the catalyst is an NHC-Ni(η⁶-arene) complex. Substitution of the coordinated solvent with diaryl ether allowed isolation of a diaryl ether-bound Ni complex. Rate-limiting C-O bond cleavage occurs to generate a three-coordinate product of oxidative addition, a metallacyclic version of which has been prepared independently. Stoichiometric studies show that arene and phenol products are released following reaction with H₂. NaOt-Bu was found to deprotonate the phenol product and to prevent formation of inactive Ni^I dimers.

Full text of DOI:10.1021/jacs.7b10537

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Article
Mechanistic Investigations of the Hydrogenolysis of Diaryl Ethers
Catalyzed by Nickel Complexes of N–Heterocyclic Carbene Ligands
Noam Saper, and John F. Hartwig
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10537 • Publication Date (Web): 08 Nov 2017
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Journal of the American Chemical Society
Ni(COD)2 and SIPr under H2.20 They observed cleavage of
with H2 would generate t-BuOH and the arene product.
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the Caryl–O bond and 90% deuterium incorporation in the
newly formed arene (Scheme 1b), implying that reduction
occurs by β-deuteride elimination from a methoxide ligand.
Although revealing an important feature of the mechanism
of the reaction of aryl methyl ethers, these studies did not
reveal the mechanism of the reaction of diaryl ethers,
which lack the requisite β-hydrogen. In addition, this study
did not include the characterization of nickel complexes
along the pathway for reaction of SIPr-ligated complexes,
particularly the identity of the methoxide complex that
would form.22
Martin and coworkers reported silanolysis of the aro-
matic C–O bonds of aryl methyl ethers catalyzed by the
combination of Ni(COD)2 and PCy3.23 Detailed mechanis-
tic experiments supported the initial formation of a phos-
phine-ligated, NiI-silyl complex (Scheme 1c).21 This spe-
cies was proposed to undergo migratory insertion into the
napthyl C=C bond and elimination of MeOSiR3 to produce
a NiI aryl complex. Sigma-bond metathesis with silane was
proposed then to produce the arene product and regenerate
the active catalyst.
Thus, the mechanisms deduced by computation for hydro-
genolysis involve Ni0 and NiII complexes, whereas the
mechanism for silanolysis was concluded to involve NiI.
The mechanisms deduced by computation for hydrogenoly-
sis involve a neutral Ni complex in one case and an anionic
Ni0 complex in another.
To gain firm experimental information on the mechanism
of the hydrogenolysis of diaryl ethers, we have conducted a
series of studies to prepare potential reaction intermediates,
to obtain kinetic data on catalytic systems, and to study the
rates and outcomes of individual steps in possible cycles.
Here, we describe the outcome of these studies on the
mechanism of the hydrogenolysis of C–O bonds in diaryl
ethers catalyzed by a SIPr-ligated Ni complex. Catalytic
reactions and kinetic studies indicate that NaOt-Bu is not
involved in the rate-determining portion of the catalytic
cycle, even though it is necessary for the catalytic reactions
to occur in high yield. Spectroscopic studies show that
SIPr–Ni(η6–arene) is the resting state of the catalyst. Cata-
lytically relevant SIPr-ligated Ni complexes with bound
substrate and phenolic product have been synthesized inde-
pendently, and a three-coordinate Ni metallacycle that is
the formal product of oxidative addition of the C–O bond
of dibenzofuran to Ni0 has been isolated and fully charac-
terized. NaOt-Bu facilitates regeneration of the active cata-
lytic species by deprotonation of the phenolic product to
prevent the formation of inactive NiI phenoxide complexes
lying off of the catalytic cycle.
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Scheme 2. Mechanistic Proposals from DFT Calculations
on the Hydrogenolysis of Diaryl Ethers
Surawatanawong; 2014:
Chung; 2016:
SIPr
Ni
SIPr
Ph2O
Ni
SIPr
Ni
SIPr
Ni
O
t-Bu
O
O
O
t-Bu
O
RESULTS AND DISCUSSION
2
3
5
6
To gain experimental data on the mechanism of the hy-
drogenolysis of diaryl ethers catalyzed by an NHC-ligated
Ni complex, we analyzed in more depth the effect of base,
monitored the nickel species present in these reactions,
obtained kinetic data, and obtained information on the reac-
tivity of isolated, potential intermediates by single-turnover
studies. Studies on catalytic hydrogenolyses in the absence
and presence of NaOt-Bu revealed that NaOt-Bu is neces-
sary to achieve high yields in this reaction, and NMR and
UV-Vis spectroscopy revealed the resting state of the Ni
catalyst. Kinetic studies showed the order in which the in-
dividual steps of the catalytic cycle occur, and isolation of
discrete Ni0, NiI, and NiII complexes enabled each elemen-
tary step to be studied directly, including the step involving
hydrogen and a process that can lead to catalyst decomposi-
tion.
+ H2
- PhOH
- OPh
Direct oxidative addition
No discussion of NaOt-Bu
Lower barrier to
oxidative addition
from Ni-ate 4
SIPr
Ni
SIPr
Ni
H
4
O
7
t-Bu
Two studies have provided information on the mecha-
nism of the hydrogenolysis of aryl ethers by DFT computa-
tions. These two studies led to two different mechanisms to
be proposed for the hydrogenolysis of diaryl ethers by a
complex formed from the combination of Ni(COD)2 and an
NHC ligand (Scheme 2). In 2014, Surawantanawong and
coworkers proposed initial coordination of the diaryl ether
to Ni0 to form SIPr–Ni(η2–Ph–OPh) (2),24 followed by rate-
determining oxidative addition of the C–O bond to provide
low-coordinate arylnickel phenoxide species 3. Further
reaction with H2 was proposed to generate hydride 4, which
would undergo reductive elimination to form benzene and
bind substrate to close the catalytic cycle.
More recently, Chung and coworkers proposed a mecha-
nism that directly involved NaOt-Bu in the catalytic cycle
for the hydrogenolysis of diaryl ethers.25 They computed
that the barrier to oxidative addition of the C–O bond to
anionic Ni-alkoxide 5 to form the anionic NiII complex 6
would be lower than that for oxidative addition to a neutral
complex, such as 2. Dissociation of phenoxide from Ni-ate
6 would give the neutral Ni-t-butoxide 7. Further reactions
Dependence of Yield on NaOt-Bu: The published nick-
el-catalyzed cleavage of diaryl ethers was conducted with
added NaOt-Bu.18 NaOt-Bu was initially added to the sys-
tem to deprotonate the carbene precursor SIPrŸHCl (1) to
generate the free carbene SIPr.26 However, the role of
NaOt-Bu is more extensive than simply generating the free
carbene from the hydrochloride salt.
As initially reported,18 the hydrogenolysis of diaryl ether
8 catalyzed by the combination of Ni(COD)2 and SIPrŸHCl
(1) with an excess of NaOt-Bu formed arene 10 in 81%
yield and fully converted diaryl ether 8 (Table 1, Entry 1).18
To bypass the role of NaOt-Bu in generating the free car-
bene, we conducted reactions catalyzed by the combination
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Page 2 of 12
lyst. Under the basic conditions of our hydrogenolysis of
Table 1. Effect of Base on the Hydrogenolysis of Diaryl
Ether 8a
1
2
3
4
5
6
7
8
C–O bonds, the product phenol 9 would be converted to the
corresponding phenoxide salt. Thus, we tested whether the
effect of NaOt-Bu resulted from forming NaOAr salts that
would react at some point of the catalytic process. To do
so, we conducted the reaction of ether 8 with H2 catalyzed
by the combination of Ni(COD)2 and free SIPr with NaOPh
in place of NaOt-Bu. This reaction occurred to only 28%
conversion after 18 hours (Table 1, Entry 4).
9
The same effect of NaOt-Bu on the conversion of diaryl
ethers was observed when the reactions were initiated with
the preformed Ni0 complex Ni(SIPr)2 (11).27 The reaction
catalyzed by di-ligated complex 11 with 2.5 equivalents of
NaOt-Bu formed arene 10 in 69% yield, but the reaction
catalyzed by di-ligated complex 11 without added base
occurred in only 12% yield (Table 2, Entry 1–2). This body
of results on the reactions with free carbene as ligand or the
pre-formed, NHC-ligated Ni0 complex 11 as catalyst
demonstrate that NaOt-Bu is necessary for the hydrogenol-
ysis catalyzed by a Ni complex ligated by SIPr.
Identification of the Resting State of the Catalyst: To
gain further insight into the mechanism of the Ni-catalyzed
hydrogenolysis of diaryl ethers we identified the catalyst
resting state by spectroscopic techniques. Catalytic reac-
tions containing Ni(SIPr)2 changed color from purple to red
when heated to 100 °C. This color change indicated that the
active Ni species in solution was not the bis-NHC complex
11. Instead, the major species is SIPr–Ni(η6–C6D6) (12) or
an analogous h6-arene complex, depending on the arene
solvent. In non-aromatic solvents, SIPr–Ni0 binds diaryl
ether substrates to generate η6 bound Ni complexes (vide
infra). Complex 12 was prepared in this manner by heating
11 in C6D6 (Figure 1, top) at 100 °C for 1 hour. This reac-
tion gave a mixture that consisted of an approximately 1:1
mixture of C6D6-ligated 12 and free ligand 13 (Figure 1,
bottom). Mononuclear Ni-NHC complexes of the general
structure NHC–Ni(η6–arene) have been reported previously
by Ogoshi in 2014.28 The complete conversion of 11 to 12
in C6D6 at 100 °C shows that ligand dissociation from bis-
NHC complex 11 and association of the aromatic solvent
occurs under conditions analogous to those we reported for
hydrogenolysis of diaryl ethers.18
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aReactions were conducted on a 0.15 mmol scale in a sealed
Schlenk tube under H2 atmosphere. See the Supporting Infor-
b
mation for further experimental details Conversions and yields
were determined by 19F-NMR spectroscopy against an internal
standard.
of Ni(COD)2 and the free carbene SIPr in the presence and
absence of NaOt-Bu. The reaction catalyzed by 20 mol%
Ni(COD)2 and 40 mol% carbene without added NaOt-Bu
gave only 19% yield of arene 10 (Table 1, Entry 2). In con-
trast, the reaction catalyzed by these components in the
presence of NaOt-Bu gave full conversion of ether 8 and
82% yield of arene 10 (Table 1, Entry 3). If NaOt-Bu were
simply forming the free carbene from SIPrŸHCl, then the
reaction of ether 8 catalyzed by the combination of
Ni(COD)2 and free SIPr as ligand in the absence of NaOt–
Bu should have occurred to full conversion and formed
arene and phenol in high yield. Thus, NaOt-Bu participates
Table 2. Catalytic Reactivity of Ni complexes 11, 14, 16
and 18 in the Hydrogenolysis of Diaryl Ether 8a
To probe the identity of the resting state of the Ni cata-
lyst directly by NMR spectroscopy, aliquots from the reac-
tion of Ph2O with H2 catalyzed by the combination of
Ni(COD)2 and NHC 1 with excess NaOt-Bu at room tem-
perature and 100 °C were monitored by NMR spectroscopy
(see the Supporting Information for further experimental
details). The only NHC–ligated Ni species observed in the
catalytic hydrogenolysis of diphenyl ether by NMR spec-
troscopy was SIPr–Ni(η6–C6D6) (12). This complex ac-
counted for 96% of the initially added nickel complex, as
determined by integration of Ni complex 12 vs diphenyl
ether. The 13C-NMR spectrum of C6D6-ligated 12 is partic-
ularly diagnostic of its structure, containing a 1:1:1 triplet
aReactions were conducted on a 0.15 mmol scale in a sealed
Schlenk tube under H2 atmosphere. See the Supporting Information
for further experimental details. bConversions and yields were
determined by 19F-NMR spectroscopy against an internal standard.
at 89.3 ppm (J1
= 25.4 Hz) that corresponds to the car-
C–D
d
cReaction was heated for 6 h. SIPrŸHCl (1, 40 mol%) was added
bons of coordinated C6D6; this 13C-NMR signal is observed
clearly in the 13C NMR spectrum of an aliquot of the reac-
tion mixture for the catalytic hydrogenolysis of diphenyl
ether.
e
as ligand. The reaction mixture was allowed to stir at room tem-
perature for 10 min prior to heating.
in the catalytic system after generation of the active cata-
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Ni(SIPr)2 (11)
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9
Catalytic conditions
SIPr
SIPr
Ni
1.5
Ni
SIPr
1
11
R
R = H/Me
SIPr
Ni
SIPr
Ni
0.5
100 °C, 45 min
♦
♦
+
SIPr
••
C6D6
SIPr
D6
♦
•
0
11
300
400
500
600
700
10
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Wavelength (nm)
♦
•
12 (
)
13 ( )
•
♦
Figure 2. UV–Vis spectrum (toluene, 298K) of Ni(SIPr)2
(11, 0.13 µM, purple trace), an aliquot from a typical hy-
drogenolysis reaction mixture under H2 prior to heating
([Ni] = 0.17 µM, black trace, see the Supporting Infor-
mation for further experimental details), and of SIPr–
Ni(η6–C6H6) (14, 0.17 µM, red trace).
1
Figure 1. H-NMR spectra of Ni(SIPr)2 (11, top) and the
thermolysis of Ni(SIPr)2 (11) in C6D6 at 100 °C (bottom).
Ni complex 11 undergoes ligand exchange with C6D6 to
provide SIPr–Ni(η6–C6D6) (12, ¨) and free SIPr (13, l).
Kinetic Data and Experimental Rate Law: To gain
quantitative kinetic data on the hydrogenolysis reaction
catalyzed by the isolated arene complex, we measured the
rate dependence of each component of the hydrogenolysis
of ether 8 in the presence of NaOt-Bu catalyzed by SIPr–
Ni(η6–C6H6) (14) (Scheme 3). The data were acquired by
the method of initial rates by 19F-NMR spectroscopy in
toluene under an atmosphere of H2 at 100 °C in sealed
tubes. The kinetic data on this reaction were obtained by
the method of initial rates, rather than a full kinetic profile,
to ensure sufficient mass transport of H2, to avoid kinetic
effects of the consumption of H2 as the reaction proceeds,
and to circumvent the long reaction times needed to
achieve high conversions of starting material.
Because NMR silent Ni complexes could be formed in
the catalytic reaction mixture, we also monitored these re-
actions by UV-vis spectroscopy. We compared these spec-
tra of the catalytic reaction to those of discrete isolated
nickel complexes. The UV-Vis spectrum of a solution of
Ni(SIPr)2 (11) in toluene contains three distinctive bands
(Figure 2, purple trace). In contrast, the UV-Vis spectrum
of a catalytic hydrogenolysis reaction mixture (Figure 2,
black trace) lacked these signals. Instead, the major absorb-
ance of the catalytic reaction mixture was at 365 nm, and
this absorption is identical to that of independently pre-
pared SIPr–Ni(η6–C6H6) (14) (red trace, Figure 2). This
result shows the absence of large amounts of NMR silent
Ni complexes in the catalytic reaction mixture and is con-
sistent with the high conversion of the nickel catalyst pre-
cursor to SIPr–Ni(η6–arene), as determined by NMR spec-
troscopy.
Catalytic Competence of the Catalyst Resting State:
The catalytic competence of SIPr–Ni(η6–C6H6) (14) was
assessed by running the hydrogenolysis of diaryl ether 8
with 14 as catalyst in a sealed Schlenk tube under 1 atm of
H2. The reaction of ether 8 with H2 catalyzed by SIPr–
Ni(η6–C6H6) (14) with added NaOt-Bu occurred to 98%
conversion and gave 79% yield of arene 10 (Table 2, Entry
3). The yield of arene 10 from the reaction catalyzed by
SIPr–Ni(η6–C6H6) (14) with added NaOt-Bu is comparable
to the yield of arene 10 from the hydrogenolysis of ether 8
catalyzed by the combination of Ni(COD)2, ligand, and
NaOt-Bu (Table 1, Entries 1,3). In addition, the hydrogen-
olysis of ether 8 catalyzed by the combination of
Ni(COD)2, ligand, and NaOt-Bu proceeds at approximately
the same rate as the hydrogenolysis catalyzed by SIPr–
Ni(η6–C6H6) (14) with added NaOt-Bu. After 2 hours, these
reactions occurred with 71% and 74% conversion of ether 8
respectively (see the Supporting Information for full reac-
tion profiles).
Scheme 3. Kinetic Reaction Orders of the Hydrogenolysis
of Ether 8.
SIPr-Ni(η6-C6H6) (14)
O
O
H
H
NaOt-Bu
H2
+
CF3
CF3
toluene/toluene-d8
(1:1); 100 °C
8
9
10
•1st order in diaryl ether 8
•1st order in SIPr-Ni(η6-C6H6) (14)
•Zero order in NaOt-Bu
rate = k[8][14]
•Zero order in H2 pressure
The dependence of the initial rate on the concentration of
ether 8 and the concentration of Ni complex 14 was first
order, and the dependence on the concentration of NaOt-Bu
and the pressure of H2 was zero order (see the Supporting
Information for further experimental details). These kinetic
data for the hydrogenolysis are consistent with a mecha-
nism by which cleavage of the C–O bond in the diaryl ether
by a Ni0 complex is rate determining, and the product of
this step reacts with hydrogen and base in steps with transi-
tion-state energies that are lower than that for the reaction
of the Ni0 species with the diaryl ether. This evidence for
rate-limiting cleavage of the C–O bond is also consistent
with the observed Ni0 resting state.
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0.11 s-1. The bound and unbound aryl ring in 15* also ex-
change rapidly at room temperature, as deduced by 2D
Scheme 4. Kinetic Isotope Effect Experiment
OH
1
2
3
4
5
6
7
8
EXSY, with a rate constant of 0.45 s-1. The characterization
and isolation of complex 15 is strong evidence that the sol-
vent-bound Ni0 resting state binds diaryl ether prior to the
next step of the reaction. Computations by Surawantana-
wong support this ligand exchange but suggest further
isomerization of h6-diaryl ether 15 to h2-diaryl ether 2 pre-
cedes oxidative addition.29 Although this mechanism could
be occurring, our observation of chemical shifts character-
istic of an h6-arene interaction imply that neither the major
observed species 15 nor the minor species 15* are this h2-
adduct.
10 mol % 14
2.5 equiv NaOt-Bu
O
H2 (1 atm)
or
D2 (1 atm)
+
+
CF3
toluene/toluene-d8 (1:1)
100 °C
CF3
8
H/D
kH/kD =1.0 ± 0.1
for separate reactions
By measuring the rate of the reaction under an atmos-
phere of D2, rather than H2, the kinetic isotope effect of the
hydrogenolysis of ether 8 was found to be 1.0 ± 0.1
(Scheme 4). This value agrees with the lack of dependence
of rate on the pressure of H2 and indicates that cleavage of
the H–H bond is not involved during the rate–limiting step.
Substrate Binding: Prior to oxidative addition of the C–
O bond in the substrate to a SIPr-ligated Ni0 complex, the
substrate would likely replace the coordinated arene solvent
in the resting state. The reaction of Ni complex 14 with
excess Ph2O in C6D12 formed a mixture of the starting
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Independent Generation of NHC-Ligated Arylnickel
Phenoxide Complexes: Few three-coordinate T-shaped
NiII species have been reported. All of them possess a
metallacyclic core and, with one exception,30 are stabilized
by a sterically hindered NHC ligand.30-33 Although one
could imagine that the direct synthesis or observation of
three-coordinate SIPr–Ni(Ar)(OAr) could be accomplished
by the salt metathesis of SIPr–Ni(Ar)(X) and phenoxide,
Matsubara reported that the products from the direct oxida-
tive addition of aryl halides to NHC–ligated Ni0 are dinu-
clear NiI complexes containing bridging halides or aryl
groups.34 To our knowledge, monomeric NHC–Ni(Ar)(X)
complexes containing a single NHC ligand have not been
reported previously. A similar NiII species was isolated as a
cationic arylnickel complex stabilized by solvent with an
outer sphere iodide.35 This adduct was proposed to exist in
equilibrium with NHC–Ni(Ar)(X). Stable, four-coordinate
Ni complexes of the general form (NHC)2Ni(Ar)(X) bear-
ing less sterically demanding NHC ligands than SIPr have
been reported.36-40
complex 14 and SIPr–Ni(η6–Ph–OPh) (15). The H-NMR
1
spectrum of 15 contains resonances that are shifted signifi-
cantly upfield (~ 4.8-5.3 ppm) of typical aromatic signals,
consistent with coordination of one of the phenyl rings in
1
Ph2O to Ni. The peaks in the H-NMR spectrum corre-
sponding to the second aromatic ring are not shifted from
the aromatic region, indicating that the nickel is bound to
only one of the two aryl rings of the ether.
Scheme 5. Isolation of Substrate Bound Ni Complex 15
SIPr
SIPr
5 equiv Ph2O
Ni
Ni
HMDSO
0.5 h, r.t.
O
Scheme 6. Synthesis of NiII Aryl Phenoxide 16
15
14
N
N
N
N
NaOPh
THF
+ 15*
Ni
Ni
Cl
O
h6-Diaryl ether 15 was isolated from the reaction of Ni
complex 14 and excess Ph2O in hexamethyldisiloxane sol-
vent (Scheme 5). Complex 15 was characterized by single-
crystal X-ray diffraction and by multi-dimensional NMR
spectroscopy (see the Supporting Information for detailed
assignment). Although the quality of the crystallographic
data was not suitable for deducing detailed information on
the bond lengths and angles, the data did confirm the as-
signed connectivity as η6-arene complex 15. The bound and
unbound aryl rings in complex 15 exchange rapidly at room
temperature, as deduced by 2D exchange spectroscopy
(EXSY), with a rate constant of 0.35 s-1.
The isolated material contained a less abundant Ni com-
plex 15* containing a bound diaryl ether (3.2:1 ratio of 15
to 15* in the isolated material at room temperature in
C6D12; see the Supporting Information for further discus-
sion and spectral data). Although we have not determined
its precise structure, 15* clearly contains an h6-bound and
an unbound aryl unit of the diaryl ether, like 15, and com-
pounds 15 and 15* rapidly equilibrate at room temperature,
as deduced by 2D EXSY, with an exchange rate constant of
16
71%
Considering the lack of NHC–Ni(Ar)(X) complexes, we
prepared TMEDA–Ni(o–Tolyl)(OPh) (16) in which the
TMEDA ligand might be displaced by an NHC to form the
formal product of oxidative addition of the C–O bond in a
diaryl ether to Ni0. In addition, we prepared a metallacyclic
aryl NiII phenoxide complex ligated by SIPr.
TMEDA-ligated 16 was prepared by the reaction of
TMEDA-Ni(o–Tolyl)Cl41 with NaOPh. Complex 16 was
isolated from this reaction in 71% yield, and its structure
was confirmed by X-ray crystallography (Scheme 6). The
hydrogenolysis of ether 8 catalyzed by Ni complex 16 in
the absence of external carbene provided <5% conversion
(Table 2, Entry 5). However, the reaction of ether 8 with H2
catalyzed by complex 16 in the presence of SIPrŸHCl (1)
and NaOt-Bu gave arene 10 in 62% yield (Table 2, Entry
6). We hypothesize that displacement of the TMEDA in
complex 16 by SIPr generates the catalytically active, tran-
sient SIPr–Ni(o–Tolyl)(OPh). Treatment of 16 with SIPr 13
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to form observable quantities of SIPr–Ni(o–Tolyl)(OPh)
phosphine ligated arylpalladium phenoxide,47 and this
smaller angle leads to a geometry that is more distorted
from a T-shape.
The NHC ligand in complex 18 is located trans to the
phenoxide group. This configuration is consistent with a
larger trans influence of a phenyl ligand than of a phenox-
ide ligand.48 Consistent with this assertion are the relative
trans influences shown by the lengths of the Ni–N bonds in
TMEDA complex 16. The Ni–N bond that is trans to the
phenoxide ligand is 0.063(2) Å shorter than the Ni–N bond
that is trans to the aryl ligand.
1
2
3
4
5
6
7
8
led to no reaction at lower temperatures and to decomposi-
tion at elevated temperatures. To prepare a stable arylnickel
phenoxide complex ligated by SIPr, we targeted the prod-
uct of oxidative addition of a cyclic diaryl ether, such as
dibenzofuran. We envisioned that metallacycle 18 could be
formed independently by initial oxidative addition of aryl
bromide 17 containing a pendant phenoxide to Ni0 to form
the NiII bromide 19. Rapid intramolecular cyclization of the
pendant phenoxide in bromide 19 would provide complex
18. The rapid intramolecular displacement of bromide
would avoid the need to isolate an unstable arylnickel hal-
ide complex.
As designed, the three-coordinate complex 18 that would
result from oxidative addition of a dibenzofuran C–O bond
was synthesized independently by the reaction of phenox-
ide 17 with SIPr–Ni(η6–C6H6) (Scheme 7). The deep purple
Ni complex 18 was isolated in 79% yield. The structure of
18 was suggested to be a three-coordinate metallacycle
from multi-dimensional 1H and 13C-NMR spectroscopy
experiments and was confirmed by single crystal X-ray
diffraction (Figure 3). Intramolecular trapping of the ar-
ylnickel halide was crucial to the isolation of an NHC-
ligated arylnickel phenoxide. The reaction of SIPr–Ni(η6–
C6H6) (14) with bromobenzene and NaOPh did not give
products of this structural type (vide infra). Instead, bi-
phenyl was observed by GC/MS and an unidentified Ni
species was formed. In addition, no reaction of SIPr–Ni(η6–
C6H6) (14) and dibenzofuran was observed below 80 °C,
and decomposition was observed at 120 °C.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 7. Synthesis of Three-Coordinate Ni Metallacycle
18 Through Intramolecular Substitution
Figure 3. ORTEP diagram of metallacycle 18 (thermal
ellipsoids at the 50% probability level). For clarity, one
molecule in the asymmetric unit and all hydrogen atoms
have been omitted. Values reported are averaged values of
the 2 molecules in the asymmetric unit. Selected bond
lengths (Å): Ni1–C1 = 1.859(2), Ni1–O1 = 1.793(1), and
Ni1–C28 = 1.849(2). Selected bond angles (°): C1–Ni1–O1
= 162.33(7), O1–Ni1–C28 = 97.87(6), C1–Ni1–C28 =
99.55(7).
Br
SIPr
Ni
SIPr
Br Ni
ONa
SIPr
Ni
17
via:
C6H6; -196 °C → r.t.
O
O
14
18
19
Reactivity of Metallacycle 18: To assess the relevance
of metallacycle 18 to the catalytic hydrogenolysis, we con-
ducted the reaction of dibenzofuran with H2 catalyzed by
the NHC-ligated h6-arene complex 14. The reaction of this
diaryl ether catalyzed by complex 14 with added NaOt-Bu
occurred at 140 °C to provide the corresponding phenol
after acidic workup in 87% yield by GC analysis (see the
Supporting Information for further experimental details).
To assess initially whether the metallacycle 18 is compe-
tent to be an intermediate in this catalytic reaction, the hy-
drogenolysis of ether 8 catalyzed by 18 with excess NaOt-
Bu was conducted. This reaction occurred to 94% conver-
sion to form arene 10 in 63% yield (Table 2, Entry 7). The
hydrogenolysis of dibenzofuran catalyzed by metallacycle
18 with added base gave 49% conversion with 49% yield
of the corresponding phenol after acidic workup (see the
Supporting Information for further experimental details).
Having established the catalytic competence of 18, we
conducted reactions of 18 with hydrogen and base to de-
79%
Metallacycle 18 was found to be thermally sensitive;
heating a solution of metallacycle 18 in C6D6 at 120 °C for
1 hour provided a complex mixture of products with full
conversion of metallacycle 18. We did not observe diben-
zofuran resulting from C–O bond-forming reductive elimi-
nation in significant quantities; this result is in agreement
with Hillhouse’s original findings that reductive elimina-
tion of C–O bonds from NiII alkoxide complexes occurs
faster and in higher yield after oxidation of an alkyl or ar-
ylnickel alkoxide complex to a NiIII species.42-44
The solid-state structure of 18 consists of a SIPr-ligated
metallacycle in which the NiII atom adopts a distorted T-
shaped geometry (C1–Ni1–O1 161.15(7)°, O1–Ni1–C28
97.91(6)°, C1–Ni1–C28 100.63(7)°).45-46 These bond an-
gles are similar to those of the other three-coordinate NiII
complexes.30-33 The C1–Ni1–O1 angle in metallacycle 18 is
12° smaller than the P–Pd–O angle of 173.1° in a T-shaped,
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Page 6 of 12
termine how the product of oxidative addition of the C–O
of SIPr–Ni0 and phenol 23 (Figure 4). The phenolic hy-
1
2
3
4
5
6
7
8
bond of a diaryl ether would be converted to the free organ-
ic product and the starting Ni0 species. The reaction with all
components in C6D6 proceeded with full conversion of 18
to give 96% yield of C6D6-ligated 12, as determined by 1H-
NMR spectroscopy, and 83% yield of phenoxide 20, as
determined by GC analysis of the corresponding phenol
after neutralization of the reaction (Scheme 8a).
droxyl group was found to coordinate to Ni in a bidentate
fashion by a dative O–Ni interaction and an η2–arene inter-
action. Although the phenolic hydrogen was not located by
X-ray diffraction, its presence was confirmed by a sharp
stretch in the IR spectrum of 22 at 3572 cm-1. In addition,
1
the broad peak in the H-NMR spectrum of 22 at 2.12 ppm
that was assigned to the phenolic hydrogen was absent after
addition of D2O.
9
Scheme 8. Stoichiometric Reactivity of Metallacycle 18
with a) NaOt-Bu and H2 and b) with Et3SiH and c) with H2.
Complex 22 was prepared independently by the reaction
of SIPr–Ni(η6–C6H6) (14) with 2-hydroxy-1,1-biphenyl
(23). The reaction of SIPr–Ni(η6–C6H6) (14) with phenol
23 in pentane formed Ni complex 22 in 45% isolated yield.
(Scheme 9). The formation of adduct 22 is specific to the
reaction of metallacycle 18 to form hydroxybiphenyl 23
because this phenol can chelate Ni0 through the arene and
hydroxyl groups.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
SIPr
Ni
a)
2.5 equiv NaOt-Bu
H2 (1 atm)
ONa
+
Ph
C6D6, r.t
16 h
D6
12
96%
20
83%
Scheme 9. Isolation of Phenol-Bound Ni Complex 22
SIPr
Ni
Si
SIPr
Ni
OH
SIPr
SIPr
O
excess Et SiH
3
b)
Ph
H
+
Ph
+
Ni
Ni
C6D6, r.t
15 min
O
pentane; 2 h, r.t.
D6
O
12
93%
21
96%
18
14
23
22
45%
SIPr
Ni
SIPr
Ni
H2 (1 atm)
c)
This phenol fragment could form in the hydrogenolysis
process by addition of H2 across the Ni–O or Ni–C bond in
metallacycle 18 to form an O–H or C–H bond. The result-
ant Ni–hydride could then form the O–H or C–H bond by
reductive elimination to generate hydroxybiphenyl 23,
which would bind to Ni0 to produce complex 22. We could
not distinguish between these pathways experimentally, but
it is clear that NaOt-Bu is not needed for the reaction of the
arylnickel phenoxide with hydrogen.
H
+
O
C6D6, r.t
16 h
D6
12
19%
22
56%
This reaction of metallacycle 18 to form C6D6-ligated 12
and phenoxide 20 could occur by several paths. To deter-
mine if the reaction requires only H2 or both H2 and NaOt-
Bu, a series of reactions were conducted with the metallay-
cle and NaOt-Bu, H2 surrogates, and H2 itself. First, we
treated metallacycle 18 with NaOt-Bu. Exposure of metal-
lacycle 18 to 2.5 equivalents of NaOt-Bu at room tempera-
1
ture did not form any new species by H-NMR spectrosco-
py; starting material 18 was recovered from this reaction in
87% isolated yield. Therefore, it seems likely that metal-
lacycle 18 reacts with H2 before it reacts with base.
Second, as a surrogate for H2, a solution of metallacycle
18 was allowed to react with excess Et3SiH in the absence
of NaOt-Bu (Scheme 8b). The reaction mixture immediate-
ly changed from the purple color indicative of metallacycle
18 to a light red color indicative of a SIPr–Ni0 complex;
C6D6-ligated 12 and silyl ether 21 were formed in 93% and
96% yield, respectively. The identity of silyl ether 21 was
confirmed by independent synthesis, as well as GC/MS
analysis of the reaction mixture.
Third, the stoichiometric reaction of metallacycle 18 was
performed with 1 atm of H2 (Scheme 8c). This reaction was
more complex than the reaction with silane. C6D6-complex
12 formed in 19% yield. In addition, the h2, k1 hydroxybi-
phenyl Ni0 complex 22 formed in 56% yield. The identity
of complex 22 was confirmed by independent synthesis
(see below), and the solid-state structure of complex 22
was determined by X-ray crystallography to be an adduct
Figure 4. ORTEP diagram of Ni complex 22 (thermal el-
lipsoids at the 50% probability level). For clarity, one mol-
ecule of phenol 23 in the asymmetric unit and all hydrogen
atoms including the phenolic hydrogen have been omitted.
Selected bond lengths (Å): Ni1–C1 = 1.932(4), Ni1–O1 =
2.002(2), Ni1–C28 = 2.282(4), and Ni1–C29 = 2.141(4).
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Selected bond angles (°): C1–Ni1–O1 = 110.1(1), C1–Ni1–
C28 = 152.4(1), C1–Ni1–C29 = 158.5(2).
mental details).
The H-NMR spectrum of dimer 24 comprises 10 broad
1
1
2
3
4
5
6
7
8
peaks at chemical shifts suggesting that the complex is par-
amagnetic. Indeed, the magnetic moment of dimer 24 is
2.89 µΒ, indicating that complex 24 has 2 unpaired elec-
trons and is most likely a NiI/NiI dimer. We propose that
dimer 24 forms by initial oxidative addition of Ni0 to the
O–H bond in phenol to form SIPr–Ni(OPh)(H). Dispropor-
tionation of 2 molecules of SIPr–Ni(OPh)(H) would then
provide SIPr–Ni(OPh)2 and a Ni dihydride. Reductive
elimination of H2 from the latter complex would afford a
Ni0 complex, which could react with SIPr–Ni(OPh)2 to
form 24.
Role of NaOt-Bu in the Ni-Catalyzed Hydrogenolysis
of C–O Bonds: To regenerate the catalytic resting state,
h6-arene complex 12 (if C6D6 is the reaction solvent), the
bound arene must be replaced by C6D6. The reaction of 22
with excess NaOt-Bu in C6D6 occurred at room tempera-
ture to give a 1:3.75 mixture of starting 22 and η6–C6D6 12
(Scheme 10). Deprotonation by NaOt-Bu to provide t-
butanol and phenoxide 20 may provide a driving force for
the generation of benzene-complex 12 from hydroxybi-
phenyl complex 22. The more electron rich phenoxide 20
should not bind to the electron-rich Ni0 as well as the neu-
tral arene solvent or substrate. This assertion is supported
by the reported high Keq for replacement of toluene with
1,3,5-tris(trifluoromethyl)benzene in IPr–Ni(η6–C7H8). In
contrast, the more electron rich 1,3,5-trimethoxybenzne
does not give any product from displacement of toluene.28
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 10. Proposed Equilibrium of Phenol 22 and the
Role of NaOt-Bu
SIPr
SIPr
Ni
ONa
C6D6
H
Ph
+
NaOt-Bu
+
+
HOt-Bu
Ni
O
D6
H
22
12
20
To probe more generally how the phenol or phenoxide
products could interact with the Ni0 product from reaction
of H2 with an arylnickel phenoxide complex, we conducted
reactions of phenol and phenoxide with h6–arene Ni0 com-
plex 14. No reaction was observed between Ni0 14 and
NaOPh. This result shows that binding of a neutral arene to
the NHC-Ni0 fragment is favored over binding of an anion-
ic phenoxide, as would be expected for this electron-rich
Ni0 unit.
Figure 5. ORTEP diagram of dimer 24 (thermal ellipsoids
at the 50% probability level). For clarity, all hydrogen at-
oms have been omitted. Selected bond lengths (Å): Ni1–
Ni1’ = 2.9710(7), Ni1–C1 = 1.880(2), Ni1–O1 = 2.020(1),
Ni1–N1 = 1.354(2), and Ni1–N1 = 1.369(3). Selected bond
angles (°): C1–Ni1–O1’ = 156.12(8), C1–Ni1–O1 =
120.43(8), and O1–Ni1–O1’ = 82.69(6).
Scheme 11. Reaction of Ni0 with Phenol to generate NiI
Dimer 24
Ph
OH
The reaction of 14 with phenol to generate the NiI dimer
suggests an important effect of NaOt-Bu. Under base-free
reaction conditions for C–O bond hydrogenolysis (such as
Table 1, Entry 2), the phenol product generated would react
with the Ni0 catalyst to produce dimeric NiI phenoxides like
24. Despite the precedent for the intermediacy of NHC–Ni1
dimers in cross-coupling and cycloaddition reactions,34, 50
our results on the hydrogenolysis of diaryl ether 8 in the
absence of NaOt-Bu (Table 1, Entry 2) indicate that for-
mation of dimer 24 leads to a loss of catalytic activity. The
reaction of diaryl ether 8 with H2 catalyzed by dimer 24
without added base proceeds to only 9% conversion,
whereas the same reaction with added NaOt-Bu proceeds to
full conversion of diaryl ether and formed arene 10 in 67%
yield. Moreover, the hydrogenolysis of diaryl ether 8 with
both H2 and NaOt-Bu catalyzed by dimer 24 occurs at ap-
proximately the same initial rate as the hydrogenolysis cat-
alyzed by Ni0 14 (see the Supporting information for fur-
ther experimental details). This result suggests that dimer
24 can re-enter the catalytic cycle when both H2 and NaOt-
SIPr
O
+
Ni
SIPr Ni
Ni SIPr
pentane; 4 h, r.t.
O
Ph
24
14
82%
SIPr-Ni(OPh)(H)
SIPr
SIPr
Ni
Ni
H2
+
+
O
O
14
In contrast, Ni0 14 reacted with PhOH. This reaction did
not produce a p-arene complex; rather it formed the new,
orange, phenoxide bridged dimer 24 shown in Scheme 11.
This complex was identified by single crystal X-ray dif-
fraction (Figure 5) and was independently synthesized by
the reaction of Sigman’s NiI dimer49 [(SIPr)Ni(µ–Cl)]2 with
NaOPh (see the Supporting Information for further experi-
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Scheme 12. Overall Proposed Mechanism for the Ni-Catalyzed Hydrogenolysis of Diaryl Ethers with NHC as Ligand
Ni(COD)2
1
2
3
4
SIPr • HCl
NaOt-Bu
ONa
H2
-cyclooctane
R1
R3
+ HOt-Bu
+
R2
5
6
7
8
Substrate Binding
F
G
O
SIPr
Ni
R2
R3
R1
9
Catalyst
Turnover
With NaOt-Bu
R1
R1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
A
Resting State
SIPr
Ni
SIPr
Ni
OH
+
R3
O
R2
R2
Rate-Limiting C-O Bond
Oxidative Additon
D
E
Reaction with
H2
B
SIPr
Ni
R3
R2
H2
O
R3
C
15
R2 = R3 = H
24
18
Catalyst Decompositon
Without NaOt-Bu
Bu are present. The same reaction in the absence of
NaOt-Bu generates phenol, and the phenol reacts with Ni0
to produce the catalytically inactive dimer 24. The stoi-
chiometric reaction of dimer 24 with H2 and NaOt-Bu in
C6D6 generated C6D6-bound 12. We did not observe any
intermediate species in this reaction providing insight into
the mechanism for this transformation.
NaOt-Bu and the catalytic activity of 18 are consistent
with the intermediacy of arylnickel phenoxides in the
hydrogenolysis reactions. Complex C reacts with hydro-
gen to provide phenol D and arene-ligated E. This reac-
tion presumably proceeds through a Ni–H intermediate.
In the absence of NaOt-Bu, phenol D reacts with SIPr–
Ni0 to generate Ni1 dimers with bridging phenoxides. Di-
mer 24 has been prepared from the reaction of SIPr–
Ni(η6–C6H6) (14) with phenol and structurally character-
ized. If NaOt-Bu is present, phenol D reacts with the base
to form phenoxide G, which does not react with the Ni0
species. Replacement of the arene product with solvent
would then regenerate resting state A and close the cata-
lytic cycle. Replacement of the arene product with sub-
strate can also occur, but the arene solvent is present in
higher concentrations and binds more strongly to Ni0 than
the aryl ether substrate.
2. Comparison of Experimental and Computational
Mechanisms for the Ni-Catalyzed Cleavage of C–O
Bonds: The main steps of the mechanism of the catalytic
hydrogenolysis in Scheme 12 most closely resemble the
steps computed by Surawantanawong.29 Surawantana-
wong calculated the barrier for oxidative addition of the
C–O bond in diaryl ether 8 to SIPr-Ni0 to be 26.2
kcal/mol from Ni(COD)2. This value is consistent with
our kinetic data showing that this step is rate limiting. Our
kinetic data show that the overall catalytic hydrogenolysis
occurs with a DG‡ of 26.0 kcal/mol when SIPr–Ni(η6–
C6H6) (14) was the catalyst.
CONCLUSION
1. Mechanism of Ni-Catalyzed Hydrogenolysis of
Diaryl Ethers: On the basis of the results described
above, we conclude that the Ni catalyzed hydrogenolysis
of diaryl ethers occurs by the mechanism shown in
Scheme 12. Deprotonation of NHC salt 1, coordination to
Ni0, hydrogenation of the cyclooctadiene ligand, and as-
sociation of the aromatic reaction solvent proceeds under
ambient conditions to provide η6–arene complex A.
Complex A is the resting state of the catalytic reaction.
The resting state A reacts reversibly with the diaryl ether
substrate to form η6–diaryl ether complex B. The unsub-
stituted analog of B, η6–Ph–OPh complex 15, has been
structurally characterized. Oxidative addition likely oc-
curs from the η2–diaryl ether intermediate, rather than the
η6 isomer, as deduced by computation.29
Kinetic studies show that reaction of the diaryl ether
with SIPr–Ni0(η6–arene) to cleave the C–O bond is rate
limiting. The direct oxidative addition of the C–O bond of
the diaryl ether forms the three-coordinate complex C.
The metallacyclic arylnickel phenoxide 18 has been pre-
pared and represents the product of oxidative addition of
dibenzofuran to Ni0 during catalytic hydrogenolysis of the
diaryl ether C–O bond in this substrate. The reactivity of
the three-coordinate complex 18 with hydrogen and
Computations reported by Chung and coworkers sug-
gested that the oxidative addition of the C–O bond to ani-
onic Ni-alkoxide 5 proceeded with a lower barrier than
the oxidative addition of the C–O bond to a neutral spe-
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Journal of the American Chemical Society
cies. We have not been able to isolate or observe anionic
perimental studies in similar systems suggested Ni1 in-
termediates,21 our data indicate that oxidative addition of
aryl ethers occurs to a neutral Ni0 species in the catalyst
system we investigated containing a hindered NHC lig-
and. Overall, we hope that exploration of mechanisms for
Ni-catalyzed reactions will lead to new catalyst design for
the cleavage of strong C–O bonds.
1
2
3
4
5
6
7
8
Ni complexes with compositions like that of 5. In addi-
tion, our kinetic experiments show that the initial rate of
the reaction is zero-order in NaOt-Bu and first order in
diaryl ether. These data, coupled with the observed rest-
ing state, show clearly that reaction of the diaryl ether
does not occur to an anionic species formed by the com-
bination of the neutral resting state and NaOt-Bu.
At this time, we cannot explain why our experimental
results do not match with Chung’s computations of a
lower-energy pathway through anionic Ni0 complexes.
The catalytic cycle computed by Chung starts from
Ni(SIPr)2 (11) and did not include the formation of SIPr–
Ni(η6–arene). Our experimental data indicates that the
catalytic resting is SIPr–Ni(η6–arene) and not Ni(SIPr)2
(11) at elevated temperatures in aromatic solvents.
Surawantanawong and coworkers proposed two ener-
getically feasible pathways for the reaction of three-
coordinate C with H2.29 A Ni dihydride species was not
located computationally. Instead, complex C could bind
H2 and react directly through sigma-complex assisted
metathesis29 to produce phenol D and a Ni–hydride. Re-
ductive elimination of the hydride would then provide the
arene product. Alternatively, three-coordinate C could
isomerize to a Ni complex with the aryl ligand trans to the
NHC. This species would then undergo sigma-complex
assisted metathesis to form the C–H bond and reductive
elimination would form the O–H bond. In both cases, O–
H bond formation has the higher barrier of approximately
19 kcal/mol. We have not experimentally observed any
intermediate Ni species in the reaction of metallacycle 18
with H2 to distinguish between these two mechanistic
possibilities.
The mechanism for the hydrogenolysis of aryl ethers by
a Ni catalyst varies substantially with ligand, reductant,
and substrate. Martin carefully examined the mechanism
of the silanolysis of C–O bonds in aryl methyl ethers cata-
lyzed by a phosphine-ligated Ni complex and concluded
that the catalytic pathway involves NiI intermediates.21 In
this case, the barrier to direct oxidative addition of the C–
O bond to a Ni0 complex to form a NiII complex was cal-
culated to be 40.4 kcal/mol from Ni(COD)2 and free
phosphine. The authors concluded that the largest ener-
getic barrier in a catalytic cycle involving phosphine-
ligated NiI species would be only 31.6 kcal/mol. A cata-
lytic cycle involving NiI species was also consistent with
their observed kinetic and EPR spectroscopic data.
ASSOCIATED CONTENT
Additional experimental details and procedures and spec-
tra for all new compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
jhartwig@berkeley.edu
Notes
The authors declare no competing financial interests
ACKNOWLEDGEMENTS
This work was supported by the Director, Office of Sci-
ence, of the US Department of Energy under contract no.
DE-AC02-05CH11231, and by the National Science
Foundation (graduate research fellowship to N.I.S.). We
thank Cynthia Hong and Dr. Antonio DiPasquale for X–
ray crystallographic analysis (NIH Shared Instrumenta-
tion Grant S10–RR027172). N.I.S. thanks Dr. Hasan Ce-
lik and David M. Peacock for assistance with NMR ex-
periments. N.I.S. is thankful to Yumeng Xi and Sophie
Arlow for insightful discussions.
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diaryl ethers as substrate. The direct oxidative addition of
the diaryl ether to Ni0 without involvement of hydrogen
or base in this system is supported by our kinetic data.
In general, this work on NHC-ligated nickel complexes
that cleave the C–O bonds in diaryl ethers complements
Martin’s detailed analysis of the mechanism for silanoly-
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mechanism that can be caused by small modifications in a
catalytic system and reagent. Although computational
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NaOt-Bu
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•Kinetic Analysis
•Isolation of Catalytic Intermediates
•Stoichiometric Reactivity of Ni Complexes
•Role of Base
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