A Molecular/Heterogeneous Nickel Catalyst for Suzuki-Miyaura Coupling

Article abstract of DOI:10.1021/acs.organomet.9b00082

A molecular/heterogeneous catalyst motif based on an earth abundant nickel catalyst and SiO₂ support has been designed and synthesized. Characterization and catalytic testing indicate that the molecular nickel catalyst [(2,2′:6′,2′′-terpyridine-4′-benzoic acid)Ni(II)]Cl₂ attached to a high surface area SiO₂ support is the active cross-coupling catalyst and increased product yields are a result of increased molecular stability of the catalyst while attached to the SiO₂ support. This molecular/heterogeneous motif is easily separated from reaction mixtures and can be recycled for multiple catalytic reactions. The catalyst is active for Suzuki-Miyaura catalysis at catalyst loadings as low as 0.1 mol %, and turnover numbers nearing 2000 have been achieved.

Full text of DOI:10.1021/acs.organomet.9b00082

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
A Molecular/Heterogeneous Nickel Catalyst for Suzuki−Miyaura
Coupling
Ryan J. Key,† John Meynard M. Tengco,‡ Mark D. Smith,† and Aaron K. Vannucci*,†
†Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States
‡Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States
S
* Supporting Information
ABSTRACT: A molecular/heterogeneous catalyst motif
based on an earth abundant nickel catalyst and SiO2 support
has been designed and synthesized. Characterization and
catalytic testing indicate that the molecular nickel catalyst
[(2,2′:6′,2′′-terpyridine-4′-benzoic acid)Ni(II)]Cl2 attached
to a high surface area SiO2 support is the active cross-coupling
catalyst and increased product yields are a result of increased
molecular stability of the catalyst while attached to the SiO2
support. This molecular/heterogeneous motif is easily
separated from reaction mixtures and can be recycled for multiple catalytic reactions. The catalyst is active for Suzuki−
Miyaura catalysis at catalyst loadings as low as 0.1 mol %, and turnover numbers nearing 2000 have been achieved.
alkyls,21,22 and various C−O bonds.23−25 Stereoselective
INTRODUCTION
■
Suzuki−Miyaura coupling with nickel catalysts bearing
Attaching molecular catalysts to heterogeneous supports can
generate a catalyst motif that merges the benefits of
homogeneous catalysis (high selectivity and activity) with
inexpensive ligands has also been achieved.26,27 High-valent
nickel complexes have even been isolated, showing that nickel
has the redox range typically only observed with palladium
heterogeneous catalysis (high stability/lifetime and ease of
catalysts.28−30 Mechanistic insight on nickel catalyzed C−C
separations). Many reports have been dedicated to the study of
cross-coupling has supported high-valent nickel intermediates
molecular/heterogeneous catalysts for cross-coupling reactiv-
in cross-coupling reactions.31,32 Furthermore, innovative routes
ity.1,2 While multiple successes have been reported, primarily
for nickel catalyzed cross-coupling have been reported.
with Pd catalysts, a critical review examined the stability of
molecular/heterogeneous catalysts.3 The review concluded
Reductive coupling between two carbon electrophiles has
expanded the scope of sp2−sp3 C−C coupling reactions.33−35
that many of the catalysts on oxide supports decomposed into
homogeneous catalysts or metal nanoparticles, and thus
extensive characterization of these catalytic systems is
necessary to confirm the nature of the catalysts.
Nickel catalysts have also found a prevalent role in photoredox
cross-coupling catalysis.36−39
While nickel catalysts offer clear advantages, controlled
reactivity and catalyst stability can still present issues.
Transition-metal catalyzed cross-coupling reactions, espe-
Compared to palladium catalysts, nickel catalysts exhibit a
cially those catalyzed by palladium, are extensively used in a
variety of synthesis routes.4,5 Of these reactions, the Suzuki−
Miyaura cross-coupling reaction is the most widely used for the
high degree of sensitivity to the choice of solvent and base
during Suzuki cross-couplings.40 In addition, Suzuki couplings
formation of C−C bonds.6−8 While palladium catalyzed
typically require upwards of 5−10 mol % of catalyst loadings
for efficient product formation.14 Higher catalyst loadings can
Suzuki−Miyaura coupling reactions have become textbook,9
increase the rate of bimolecular decomposition pathways for
development of new cross-coupling catalysts based on
nickel catalysts. Nickel dimers formed during bimolecular
abundant first row transition metals is a continual growing
research area.10−13 In particular, over the past decade, interest
decomposition pathways have been shown to be inactive
species formed during nickel catalyzed Suzuki coupling
has grown and great progress has been made in the field of
reactions.31 Increased concentrations of catalyst can also lead
to unwanted metal poisoning of products which is a major
concern for pharmaceutical development.41 In addition, a vast
majority of nickel catalyzed cross-coupling reactions are
performed in a homogeneous solution which makes post-
reaction separations and catalyst recycling difficult.
nickel catalyzed cross-coupling reactions.14 Furthermore, the
ACS Pharmaceutical Round Table has called for increased use
of nickel catalysts and encouraged the development of catalyst
immobilization techniques.15
Nickel-based cross-coupling catalysts offer many advantages
over palladium catalysts beyond nickel being a less expensive/
more sustainable metal. Nickel catalysts have shown the ability
to activate less reactive carbon electrophiles such as aryl
chlorides,16−18 unactivated alkyl electrophiles,19,20 tertiary
Received: February 7, 2019
© XXXX American Chemical Society
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Molecular/heterogeneous catalysts based on molecular
nickel complexes42 and heterogeneous nickel catalysts in
solid polymer supports have been reported.43,44 An NHC-
containing nickel complex attached to Al2O3 nanoparticles
actually exhibited suppressed catalytic activity compared to the
analogous homogeneous catalyst in solution.42 Conversely, a
heterogeneous nickel-containing polystyrene-cross-linking bi-
sphosphine polymer catalytically outperformed the analogous
homogeneous Ni-BINAP catalysts (BINAP = 2,2′-bis-
(diphenylphosphino)-1,1′-binaphthyl).43 The homogeneous
Ni-BINAP catalyst resulted in moderate yields of desired
cross-coupled products due to bimolecular decomposition
pathways.45 Heterogeneous nickel polymers have also
exhibited greater product selectivity for the hydrosilyation of
alkynes compared to homogeneous catalysts.44 It is hypothe-
sized that the geometric confinement of the polymer network
led to the increased observed selectivity.
methane. Elemental Analysis: Calculated for [NiC22H15N3O2-
Cl2]0.5H2O: C 53.71%; H 3.07%; N 8.54%. Found: C 53.81%;
H 3.03%; N 8.32%.
COOH-Ni crystallizes in the monoclinic system and is
shown in Figure 2. The pattern of systematic absences in the
To attempt to build on the successes of molecular/
heterogeneous nickel catalysts and address the issues of high
catalyst loading in homogeneous solution (bimolecular
deactivation, product contamination, and difficult catalyst
recycling), we have designed and synthesized a molecular/
heterogeneous nickel-based catalyst system. This catalyst
system starts with the molecular catalyst [(2,2′:6′,2′′-terpyri-
dine-4′-benzoic acid)Ni(II)]Cl2, which is a direct analogue of
our previously reported (2,2′:6′,2′′-terpyridine)Ni(II) catalyst
that was shown to be an efficient catalyst for C−C and C−N
photoredox cross-coupling.20,46 The addition of the benzoic
acid moiety to the terpyridine ligand allows for this catalyst to
be attached to solid metal oxide supports for the generation of
a molecular/heterogeneous catalyst system. Acidic moieties,
such as COOH, have shown the ability to bind to metal oxides
in order to attach molecular catalysts to solid oxide supports.47
This system, illustrated in Figure 1, simplifies catalyst
Figure 2. Illustration of molecular/heterogeneous catalyst COOH-Ni|
SiO2.
intensity data was consistent with the space group P21/n. The
asymmetric unit consists of one NiCl2(H2O)(C22H15O2)
complex and one noncoordinated, partially occupied water
molecule. Around the nickel center, the chloride and
coordinated water ligands are disordered. Two sites were
modeled as a disordered mixture of Cl−/H2O; the third anion
site was modeled with a split chloride position. Further details
about the crystal structure can be found in the Supporting
Information. The distance between the nickel center and the
middle nitrogen atom of the (2,2′:6′,2′′-terpyridine-4′-benzoic
acid) ligand is 1.99 Å, and the distances from the nickel center
to the two nitrogen atoms in the equatorial plane are 2.08 Å
each. The average distance to the chloride atoms is 2.38 Å.
These distances are all within 0.02 Å when compared to the
previously reported crystal structure for the (2,2′:6′,2′′-
terpyridine)Ni(II) complex,20 indicating the benzoic acid
moiety does not significantly alter the electronic structure or
geometry of the nickel metal center.
Functionalization of high surface area SiO2 particles with the
molecular catalyst was achieved by soaking the SiO2 particles
in a methanol solution containing the catalyst. This catalyst
loading procedure is analogous to previously reported metal
oxide derivatization by molecular complexes.48 After soaking,
the particles were filtered from solution, rinsed with cold
methanol, dried, and then characterized. The resulting
molecular/heterogeneous catalyst (COOH-Ni|SiO2) was
characterized with ICP-MS and XRD. The paramagnetic
nature of the nickel complex prevented accurate character-
ization through solid-state NMR spectroscopy. ICP-MS
analysis indicated that COOH-Ni|SiO2 contained 0.56 wt %
nickel. This equates to 3.9 × 10−8 mol of catalyst per m2 of
silica support. X-ray diffraction analysis of COOH-Ni|SiO2
(Figure S2) did not show any evidence for crystalline nickel
particles when compared to fresh SiO2 particles. This result
suggests that the molecular nickel catalyst, and not metallic
Figure 1. Illustration of molecular/heterogeneous catalyst COOH-Ni|
SiO2.
separation from post-reaction mixtures and leads to good
catalyst recyclability. This molecular/heterogeneous catalyst
has been characterized and performs Suzuki−Miyaura cross-
coupling catalysis at catalyst loadings as low as 0.1 mol %.
RESULTS AND DISCUSSION
■
The previously unreported complex [(2,2′:6′,2′′-terpyridine-
4′-benzoic acid)Ni(II)]Cl2 (COOH-Ni) was synthesized by
dissolving a 1:1 ratio of the 2,2′:6′,2′′-terpyridine-4′-benzoic
acid ligand and nickel dichloride in ethanol and allowing the
solution to stir at reflux for 4 h. Post reflux, the green solid was
washed with diethyl ether and dried. Crystals suitable for X-ray
crystal analysis were grown by slow evaporation of dichloro-
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nickel particles, is present on the SiO2 support. It is worth
noting that the detection limit of the XRD instrument used is
1.0 nm crystalline particles.49,50
Further characterization of the COOH-Ni|SiO2 catalyst was
carried out through control reactions and catalytic optimiza-
tion reactions as summarized in Tables 1−3. The Suzuki−
the reaction solution. No cross-coupled products or dimer
formation was observed.
Metal oxides other than SiO2 were also examined. The
COOH-Ni molecular catalyst loaded onto TiO2, ZrO2, and
Al2O3 individually all lead to poor product formation. These
results are likely attributed to how the metal oxide reacts to the
dioxane solvent. Upon heating, the TiO2 particles immediately
clumped up into rock-like particles that could not be broken
up with vigorous stirring. These clumps greatly decreased the
exposed surface area of the particles, thus limiting catalysis.
Similar results were observed for Al2O3 particles which
immediately adhered to the walls of the round-bottom flask.
Lastly, ZrO2 oxide particles formed a milky solution that
turned yellow upon heating, indicating ZrO2 is not stable in
hot dioxane.
Table 1. Optimization of Suzuki−Miyaura Cross-Coupling
with COOH-Ni|SiO2 Catalyst
a
b
deviation from standard conditions
% yield
To examine the possible benefit of utilizing the molecular/
heterogeneous catalyst COOH-Ni|SiO2, homogeneous cata-
lytic reactions were performed using COOH-Ni as the catalyst
in solution, and those results are shown in Table 2. A reaction
none
6 h
18 h
no K3PO4
75
20
51
0
0.8 mmol K3PO4
3.5 mmol K3PO4
SiO2 no nickel catalyst
COOH-Ni|TiO2
COOH-Ni|ZrO2
COOH-Ni|Al2O3
72
43
0
5
0
Table 2. Catalytic Suzuki−Miyaura Cross-Coupling Results
from Homogeneous Catalysts
0
a
Standard conditions: 100 mg of COOH-Ni|SiO2 (1.2 × 10−6 mol of
COOH-Ni), 0.68 mmol of iodotoluene, 0.82 mmol of benzene
boronic acid, 1.7 mmol of K3PO4, 20 mL of dioxane. Yields
a
b
b
catalyst
% yield
determined by GC−MS.
0.1 mol % COOH-Ni homogeneous
10 mol % COOH-Ni homogeneous
Rainey Ni, 10 mol %
0
25
1
c
tpy2Ni, 10 mol %
0
Miyaura coupling between 4-iodotoluene and benzeneboronic
acid was chosen as the test reaction. Common reactions
conditionsequivalents of substrates, base used, and sol-
ventwere chosen for a general comparison to literature. The
chosen standard reaction conditions (row 1, Table 1)
contained 100 mg of COOH-Ni|SiO2, which is equivalent to
1.2 × 10−6 mol of the molecular COOH-Ni catalyst, 0.68
mmol of iodotoluene, 1.2 equiv of phenylboronic acid, and 2.5
equiv of K3PO4 base. A 24 h reaction in dioxane solvent heated
to reflux resulted in a 75% yield of the desired cross-coupled
product. Reactions of 6 and 18 h resulted in 20% and 51%
yields, respectively, while reactions performed for longer than
24 h did not result in a significant increase in product yield.
Next, the equivalents of base needed to promote the reaction
were varied. Bases are believed to be necessary to promote
transmetalation reactions between the catalyst and the boronic
acid substrates.51 Conversely, Lewis acids have been shown to
promote nickel catalyzed cross-coupling reactivity, and thus
the acidity of the SiO2 support may play a role in catalysis. As
can be seen in Table 1, complete removal of the K3PO4 base
resulted in no observed cross-coupled products. The use of
1.25 equiv of base led to 72% product yield, consistent with 2.5
equiv. Lastly, increasing the concentration of base up to 5.0
equiv had a detrimental effect on products yields. Excess base
has previously been shown to decrease nickel catalyzed cross-
coupled yields.40 The results for the base variance experiments,
therefore, have determined a range of base concentrations
suitable for this catalyst motif and indicate that the acidic sites
on SiO2 likely do not play a large role in promoting catalysis.
To further examine if SiO2 has any direct reactivity with the
substrates, a reaction was performed with just SiO2 added to
[(μ-Cl)Ni(tpy)]2, 10 mol %
SiO2 + 0.1 mol % COOH-Ni in situ
0
0
a
Standard conditions: 0.68 mmol of iodotoluene, 0.82 mmol of
b
benzene boronic acid, 1.7 mmol of K3PO4, 20 mL of dioxane. Yields
c
determined by GC−MS. tpy is 2,2′:6′,2′′-terpyridine.
performed with 0.1 mol % COOH-Ni, roughly equivalent to
the catalyst loading for COOH-Ni|SiO2, did not result in any
observed cross-coupled product. Furthermore, a mixture of
fresh SiO2 particles and 0.1 mol % COOH-Ni catalyst did not
result in any cross-coupled product formation, once again
indicating that the preformed COOH-Ni|SiO2 catalyst is
necessary for efficient Suzuki−Miyaura cross-coupling. In-
creasing the homogeneous catalyst loading to 10 mol % did
result in a 25% yield of cross-coupled product. These
homogeneous catalysis results indicate that the molecular
catalyst in homogeneous solution possesses the inherent
activity to promote Suzuki cross-coupling, but catalyst lifetime
may limit the overall product yields. The increased stability
imparted on the molecular catalyst by attaching it to a
heterogeneous support is a clear advantage that is obtained by
using the molecular/heterogeneous COOH-Ni|SiO2 catalyst.
As mentioned previously, nickel dimers formed from
monomeric nickel catalysts have been shown to be inactive
for Suzuki cross-couplings.31 Potential nickel catalyst decom-
position products were thus tested for possible catalytic activity
with the results in Table 2. The [(μ-Cl)Ni(tpy)]22+ dimer was
examined in homogeneous solution, but no cross-coupled
product was observed. In addition, [tpy2Ni]Cl2 was also
examined, and no cross-coupled products were obtained;
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however, deborylation products were observed. Rainey nickel
was tested to examine if metal nickel particles exhibited
catalytic activity. The metallic nickel produced little to no
desired cross-coupled products, but some homocoupled
product of the boronic acid substrate was observed. Thus,
the monomeric COOH-Ni catalyst appears to be the only
active Ni species for the desired Suzuki coupling, and
preventing of the decomposition of this catalyst is essential
for prolonged catalytic activity.
The results shown in Table 1 and Table 2 indicate that the
COOH-Ni|SiO2 catalyst isolates individual COOH-Ni mole-
cules and helps prevent bimolecular decomposition of the
catalyst, which can occur in homogeneous solution. Preventing
the bimolecular decomposition of the catalyst increases catalyst
lifetime, thus increasing cross-coupled product yields. Alternate
decomposition routes, such as reduced nickel nanoparticle
formation, or ligand-free nickel salt adhering to the SiO2
support could possibly occur. Furthermore, these ligand-free
nickel species could be active cross-coupling catalysts.3 With
this possibility in mind, a series of control reactions were
performed to gain further insight into the active form of the
nickel catalyst, and the results of these experiments are shown
in Table 3.
comparable cross-coupled product yields. Assuming molecular
integrity of the catalyst during the cross-coupling reactions,
these results show that catalysts of this molecular/heteroge-
neous motif can be easily synthesized from mild loading
procedures.
To examine the molecular integrity and possibility of ligand-
free nickel species being active catalysts, a series of further
experiments were performed. Table 2 shows that the
homogeneous nickel catalyst and possible molecular decom-
position products do not result in product yields nearly as high
as using the COOH-Ni|SiO2 catalyst; however, nonmolecular
nickel particles could leach from the catalyst into solution. To
test if any leached species could be the active catalyst, post-
reaction solutions were tested for catalytic activity. To perform
this test, a catalytic cross-coupling reaction was performed
using COOH-Ni|SiO2 as the catalyst. After the initial reaction,
the solid COOH-Ni|SiO2 catalyst was filtered out of the
solution and product yield was determined with GC−MS.
Then extra substrates were added to the filtrate, and a second
reaction was performed without the presence of solid COOH-
Ni|SiO2. No additional products were obtained, indicating that
the post-reaction filtrate did not contain any active cross-
coupling species. This result is consistent with the catalyst
recycle studies shown in Table 4 and discussed later.
The catalytic activity of nickel particles on the surface of the
SiO2 support was also explored. To examine reduced nickel
nanoparticles as possible catalysts, we synthesized a 0.5 wt %
nickel nanoparticle catalyst on the SiO2 support using charge
enhanced dry impregnation (CEDI).52 The CEDI particles
were then tested under the standard conditions from Table 1
and resulted in a 12% yield of the desired cross-coupled
catalyst. Thus, reduced nickel nanoparticles can catalyze Suzuki
cross-coupling reactivity, but the yields are 5× lower than what
is observed for the molecular/heterogeneous COOH-Ni|SiO2
catalyst. To gain further insight into the reactivity of the CEDI
particles, a mercury drop test was performed, as mercury has
been shown to poison heterogeneous nanoparticle catalysts.53
This test was carried out by placing 2 g of mercury into a
reaction containing 100 mg of the CEDI catalyst. Table 3
shows that the addition of a mercury drop to the CEDI sample
completely inhibited catalysis and no product yield was
obtained. Conversely, adding mercury to a reaction containing
the molecular/heterogeneous COOH-Ni|SiO2 catalyst did not
result in a significant change in observed product yield (75% vs
71%). The difference between the CEDI samples and the
COOH-Ni|SiO2 catalyst in response to mercury strongly
suggests that nickel nanoparticles are not present as active
catalysts during reactions catalyzed by COOH-Ni|SiO2.
Ligand-free nickel chloride salt was then examined for
catalytic activity. The first experiment involved loading NiCl2
salt onto the SiO2 support prior to the reaction. Following the
standard overnight loading procedure, the NiCl2 loaded SiO2
particles contained 5.4 wt % nickel compared to only 0.56 wt %
nickel of the COOH-Ni|SiO2 catalyst. The lower nickel weight
percent loading of the COOH-Ni|SiO2 catalyst could arise
from the geometrically larger 2,2′:6′,2′′-terpyridine-4′-benzoic
acid ligand occupying more space on the SiO2 support
compared to NiCl2 salt. Following the standard catalyst testing
reaction, the NiCl2|SiO2 particles did give a 17% cross-coupled
product yield. Once again, this result shows that ligand-free
nickel can act as a moderate catalyst for Suzuki cross-coupling,
but the yields are not as high as the COOH-Ni|SiO2 catalyst,
even with greater nickel content in the NiCl2|SiO2 catalyst.
Table 3. Molecular/Heterogeneous Catalyst Identity
Experiments
a
b
catalyst
% yield
c
COOH-Ni|SiO2
COOH-Ni|SiO2
reaction filtrate
75
80
0
d
Ni nanoparticles|SiO2 (CEDI)
12
0
Ni nanoparticles|SiO2 (CEDI) + Hg drop
d
COOH-Ni|SiO2 + Hg drop
71
17
15
18
0
NiCl2|SiO2
NiCl2|SiO2 + Hg drop
10 mol % NiCl2 no SiO2
tpyNi|SiO2
a
Standard conditions: 100 mg of SiO2 supported catalyst unless
otherwise noted, 0.68 mmol of iodotoluene, 0.82 mmol of benzene
b
boronic acid, 1.7 mmol of K3PO4, 20 mL of dioxane. Yields
c
determined by GC−MS. Two-step loading procedure as described in
d
the experimental procedure. One-step loading procedure as
described in the experimental procedure.
Initially, two different procedures for loading the nickel
catalyst onto the SiO2 support, described in the Experimental
Section, were examined. Row 1 of Table 3 is identical to row 1
of Table 1 and shows the results of the two-step loading
procedure where the ligand was first bound to the SiO2 surface,
followed by addition of NiCl2 salt to synthesize the COOH-Ni|
SiO2 catalyst. This synthesis procedure resulted in 0.56 wt % of
nickel on SiO2 as confirmed by ICP-MS. Row 2 of Table 3
shows the results for the one-step loading method. This one-
step method was performed by first isolating the molecular
nickel catalyst, followed by loading the catalyst onto the
support to also generate COOH-Ni|SiO2. This loading
procedure resulted in 1.05 wt % of nickel on SiO2. As can
be seen from Table 3, both loading procedures resulted in
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A mercury drop test was also performed on the NiCl2|SiO2
catalyst, as shown in Table 3. In contrast to the CEDI nickel
nanoparticles, mercury did not have an effect on the catalytic
activity of NiCl2|SiO2, suggesting that the NiCl2 salt may leach
from the SiO2 support during catalysis. Therefore, a reaction
mixture containing 10 mol % NiCl2 (approximately equivalent
to the nickel mol % loading for 100 mg of NiCl2|SiO2) in the
absence of SiO2 was tested for catalytic activity. Statistically,
the same yield of cross-coupled product was obtained from
NiCl2 in solution compared to NiCl2|SiO2. This result also
shows that nearly 100× the concentration of NiCl2 salt is
required to obtain 5× less product compared to the molecular/
heterogeneous COOH-Ni|SiO2 catalyst.
five reactions and filtrations hits a critical point by the fifth
reaction and product formation begins to drop due to low
nickel catalyst loadings.
Post-reaction analysis of the molecular/heterogeneous
COOH-Ni|SiO2 catalyst was performed with ICP-MS and
XRD. Once again, XRD patterns showed no evidence of Ni
nanoparticle formation on the surface of the SiO2 support.
This result, in conjunction with the results from Tables 2 and
3, supports that the molecular COOH-Ni catalyst attached to
the SiO2 support is responsible for the observed catalytic
activity. ICP-MS analysis of the used catalyst showed that only
78 mg/g of the original 687 mg/g of nickel remained on the
SiO2 support. This leaching of the catalyst is not ideal and is
the likely cause for eventual catalyst deactivation. Though this
result also corroborates that the molecular catalyst on the SiO2
support is the active catalyst and molecular nickel catalyst in
homogeneous solution quickly deactivates.
Table 4. Cross-Coupled Product Yields from Consecutive
Reactions Using Recycled COOH-Ni|SiO2
To ensure that the molecular/heterogeneous catalyst
COOH-Ni|SiO2 is generally capable of Suzuki−Miyaura
cross-coupling reactions, a substrate scope for this catalyst
was examined. Figure 3 shows that coupling of 4-tert-butyl-
a
experiment
% yield
1
2
3
4
5
76
74
72
72
37
a
Standard conditions used as described in Table 1.
Lastly, the need for the benzoic acid moiety of the ligand
was examined. Acidic moieties, such as COOH, have shown
the ability to bind to metal oxides in order to attach molecular
catalysts to solid oxide supports;47 however, nickel catalysts
have been shown to bind to activated silica through the nickel
metal.54 Following the standard overnight loading procedures,
the solid SiO2 support was soaked in a solution of (2,2′:6′,2′′-
terpyridine)Ni(II) (tpyNi). These particles were then
examined for cross-coupling reactivity, but no catalysis or
product formation was observed. This result indicates that the
benzoic acid moiety is necessary to adhere the molecular
catalyst to the surface of SiO2, and this result is consistent with
the low loading homogeneous catalyst results in Table 1.
The results summarized above in Tables 1−3 all indicate
that the molecular/heterogeneous cross-coupling catalyst
COOH-Ni|SiO2 is much more active compared to ligand-
free nickel species and exhibits a greater lifetime than the
COOH-Ni catalyst in homogeneous solution. The extended
lifetime and solid nature of the COOH-Ni|SiO2 catalyst should
therefore be easily separated from reaction mixtures and be
recyclable. In relation to the first aspect, the solid COOH-Ni|
SiO2 catalyst was easily separated from product mixtures
through filtration without the need for column chromatog-
raphy. To test the recyclability, the used, filtered catalyst was
rinsed, dried, and added to a fresh reaction solution. This
procedure was repeated for multiple reactions, and Table 4
shows the results of those trials. The molecular/heterogeneous
COOH-Ni|SiO2 catalyst exhibited consistent catalyst activity
and product formation over the course of four consecutive
reactions. The percent yields reported in Table 4 equate to
1700 catalytic turnovers before catalyst deactivation is
observed in the fifth reaction. These experiments were
repeated, and catalyst deactivation was once again observed
during the fifth reaction. We hypothesize that loss of nickel
catalyst from the surface of the SiO2 support over the course of
Figure 3. Catalytic Suzuki−Miyaura cross-coupling aryl iodides and
aryl boronic acids. 34 mM aryl iodide, 41 mM boronic acid in 20 mL
of 1,4-dioxane. % yields are isolated yields and based on limiting
reagent.
benzeneboronic acid with 4-iodotoluene gave comparable
yields to the coupling of benzeneboronic acid with 4-
iodotoluene. A strong electron donating methoxy group on
the boronic acid increased yields of the cross-coupled product
up to 92%. This catalyst was also amenable to coupling boronic
acids with an electron withdrawing CF3 group. The coupling
reaction involving 4-fluorobenzeneboronic acid resulted in
80% yield, showing this catalyst exhibits functional group
tolerance to pharmaceutically important fluorine atoms. No
coupling at the fluoro position was observed. Boronic acids
with a functional group ortho to the acid were also successfully
coupled as product 6 was obtained in 50% yield.
Naphthylboronic acids and aryl iodides were also tolerated
as evidenced by products 7 and 8.
Next, the cross-coupling of various aryl halides with aryl
boronic acids was examined. Entry 9 (Figure 4) shows the
general trend of reactivity for COOH-Ni|SiO2 toward aryl
halides. Aryl iodides were generally more efficient coupling
partners compared to aryl bromides, while no reactivity was
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EXPERIMENTAL SECTION
■
Materials. All reagents were used as received from the
manufacturer and were used without any further purification. [(μ-
Cl)Ni(tpy)]2 was prepared by previously reported methods.20 The
silica oxide support used throughout was Aerosil 300 (A300, Evonik).
A300 is amorphous SiO2 with a 300 m2/g surface area and an average
particle size of 20 nm.
Instrumentation. X-ray diffraction (XRD) analysis was carried
out with a benchtop powder X-ray diffractometer (Rigaku Miniflex-II
with a silicon strip detector, D/teX Ultra, capable of detecting
nanoparticles down to 1.0 nm) with Cu Kα radiation (λ = 1.5406 Å),
operated at 15 kV and 30 mA. Powder samples were loaded on a zero-
background holder, and scans were made from the 20−80° 2θ range,
with a scan rate of 3.0° 2θ/min. Inductively coupled plasma-mass
spectrometry (ICP-MS, Finnigan ELEMENT XR double focusing
magnetic field) analysis was used for the analysis of nickel present on
silica with rhenium as internal standard. A quartz torch and injector
(Thermo Fisher Scientific) and a 0.2 mL/min Micromist U-series
nebulizer (GE, Australia) were used for sample introduction. GC−MS
(Shimadzu QP-2010S) analysis was performed with a 30 m long Rxi-5
ms (Restek) separation column with a 0.25 mm id, and the oven
temperature program was 40 °C for 0.5 min, followed by a 10 °C/min
ramp to 280 °C and held for 2 min. Mass spectrometer electron
ionization was at 70 eV, and the spectrometer was scanned from 1000
to 50 m/z at low resolution. NMR spectroscopy was performed using
a Bruker Advance III HD 300. Data were processed using Bruker
TopSpin software. 1H NMR spectroscopy was performed using a 300
MHz instrument using deuterated chloroform or deuterated
methylene chloride as the solvent with a calibrated peak at 7.26 or
5.32 ppm, respectively. 13C NMR was conducted on a 300 MHz
instrument set to a frequency of 75 MHz using chloroform or
methylene chloride as the solvent and a calibrated solvent peak at
77.33 or 54.00 ppm. Details of the single crystal X-ray crystallography
can be found in the Supporting Information.
Synthesis of [(2,2′:6′,2′′-Terpyridine-4′-benzoic acid)Ni(II)]-
Cl2. To a round-bottom flask, 0.100 g (0.28 mmol) of 2,2′:6′,2′′-
terpyridine-4′-benzoic acid was dissolved in 100 mL of ethanol and
heated to 60 °C. To this solution, 0.067 g of NiCl2·6H2O (0.28
mmol) was added and stirred for 3 h. The solution was then placed on
the rotary evaporator and evaporated to dryness. The solid was
washed several times with ethanol and diethyl ether. The resulting
green solid was then isolated and dried in an oven overnight. Yield:
0.125 g, 0.113 mmol. Elemental Analysis for [NiC22H15N3O2Cl2]-
0.5H2O: Calculated: C 53.71%; H 3.07%; N 8.54%. Actual: C:
53.81%; H: 3.03%; N: 8.32%.
Synthesis of tpy2Ni. To a round-bottom flask, 150 mg of (2,2′:
6′,2″-terpyridine)Ni was dissolved in 20 mL of acetonitrile. To this
solution, 23 mg of potassium tert-butoxide was added, and the
solution was stirred at room temperature for 24 h. The resulting
brown solution was dried on the rotary evaporator to produce a
brown solid. Yield: 0.105 g, 0.20 mmol. ESI MS two most prominent
peaks 166 m/z (C17H14N4Ni2+, tpyNi(MeCN)) and 262 m/z
(C30H22N6Ni2+, tpy2Ni2+), matching previously reported data for
tpy2Ni.56
Preparation of COOH-Ni|SiO2. Two-step method: To a 500 mL
media bottle, 1.00 g of Aerosil A300 SiO2 and 100 mL of methanol
were mixed. To this solution, 0.100 g of 2,2′:6′,2″-terpyridine-4′-
benzoic acid and 0.067 g of NiCl2·6H2O were dissolved. The solution
was allowed to settle overnight. The solid was then vacuum filtered,
rinsed with hexanes, and dried before characterization or catalytic
reactions.
Figure 4. Catalytic Suzuki−Miyaura cross-coupling aryl boronic acids
and aryl halides. 34 mM aryl halide, 41 mM boronic acid in 20 mL of
1,4-dioxane. % yields are isolated yields and based on limiting reagent.
observed with aryl chlorides. This reactivity trend is consistent
with previously reported nickel catalysts containing tridentate
polypyridyl ligands.20 Good yields were, however, obtained
from aryl bromide coupling partners with electron withdrawing
groups in both the para and meta positions (entries 10−12).
Aryl iodides containing the electron donating methoxy group
were also successfully coupled to form products 13 and 14.
Although, electron donating groups on the boronic acid are
more tolerated by this catalyst compared to electron donating
groups on the aryl halide. Product 13 was only obtained in 58%
yield starting with phenylboronic acid and 4-methoxy-
iodobenzene, while a closely related product (entry 3, Figure
3) was obtained in 92% yield through coupling of 4-methoxy-
phenylboronic acid and 4-iodotoluene. Coupling involving
iodonaphthalene was also successful (product 15).
CONCLUSIONS
■
A molecular/heterogeneous catalyst system for Suzuki−
Miyaura cross-coupling has been designed, synthesized, and
characterized. The catalyst is composed of an earth-abundant
Ni molecular catalyst and an inexpensive amorphous SiO2
support. The catalyst exhibits activity at low catalyst loadings
(approximately 0.1 mol % based on the molecular nickel
catalyst) and good functional group tolerance. Control
reactions tested possible homogeneous and heterogeneous
catalyst decomposition species for catalytic activity. All of the
control reactions showed that potential catalyst decomposition
species were either not as active or not as robust as COOH-Ni|
SiO2, and these decomposition species could not account for
the observed catalytic activity of COOH-Ni|SiO2. Eventual
catalyst leaching from the surface of the support leads to
catalyst deactivation, though the catalyst is easily recycled and
turnover numbers of 1700+ were observed before deactivation.
This loss of the molecular catalyst from the solid support
shows the need to design new molecular/heterogeneous
systems with stronger immobilization of the catalyst onto the
support.55 Obtaining good yields of cross-coupled products
from a low-loading of a nonprecious metal catalyst, however,
shows promise for future studies for this catalyst motif for
cross-coupling reactions.
One-step method: 0.100 g of (2,2′:6′,2′′-terpyridine-4′-benzoic
acid)NiCl2 (COOH-Ni) was dissolved in 100 mL of deionized water.
To this solution, NaBF4 was added, resulting in green colored solid to
precipitate out of solution. NaBF4 was added until no additional
precipitate formation was observed. The solid was filtered from the
solution and dried. A 0.1 mmol solution of the resulting (2,2′:6′,2′′-
terpyridine-4′-benzoic acid)Ni(BF4)2 complex was generated in
methanol, and 1.00 g of Aerosil A300 was added to the solution
F
DOI: 10.1021/acs.organomet.9b00082
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
and allowed to sit overnight. The solid was then vacuum filtered,
rinsed with hexanes, and dried before catalytic reactions.
(3) Phan, N. T. S.; Van Der Sluys, M.; Jones, C. W. On the Nature
of the Active Species in Palladium Catalyzed Mizoroki−Heck and
Suzuki−Miyaura Couplings − Homogeneous or Heterogeneous
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Metal (Pd,Ni,Fe)-Catalyzed Cross-Coupling Reactions Using Alkyl-
organometallics as Reaction Partners. Chem. Rev. 2011, 111, 1417−
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Decades of Innovative Catalyst Design for Palladium-Catalyzed
Cross-Couplings. Organometallics 2015, 34, 5497−5508.
(6) Miyaura, N.; Suzuki, A. Palladium-Catalyzed Cross-Coupling
Reactions of Organoboron Compounds. Chem. Rev. 1995, 95, 2457−
2483.
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Synthesis; John Wiley & Sons Inc.: New York, 2002.
Preparation of Reduced Nickel Nanoparticles on SiO2. The
silica (Aerosil 300, Evonik) supported Ni nanoparticle catalyst was
synthesized using the method of charge enhanced dry impregnation
(CEDI),52 using parameters for strong electrostatic adsorption (SEA)
of hexaamminenickel(II) complex on a silica surface.57 An
impregnating solution was prepared by dissolving Ni(NO3)2·6H2O
in 14 N NH4OH solvent. The amount of solvent required was based
on pore filling of the support to incipient wetness (3.0 mL/g), and the
amount of salt added was determined from the desired Ni loading of
0.5% by weight for the final catalyst. The solution was added dropwise
to the silica powder with intense agitation to ensure distribution into
the pores. The resulting cake was dried in an oven (120 °C, 12 h) and
subsequently ground up. The powder was subjected to reduction in a
horizontal tubular furnace, under 250 sccm flow of 20% H2, with He
comprising the remaining balance, ramping the temperature at 5 °C/
min to 500 °C and holding the temperature for 1 h, before being
cooled.
General Catalytic Reaction Scheme. In a round-bottom flask,
0.100 g of COOH-Ni|SiO2, 0.68 mmol of aryl halide, 0.82 mmol of
boronic acid, and 1.7 mmol of K3PO4 were added to 20 mL of
dioxane. This solution was heated to 115 °C and kept at reflux for 24
h. The resulting solution was concentrated, and the product was
isolated by preparative scale TLC.
For recycle experiments, the solid catalyst was collected post
reaction via suction filtration, rinsed with hexanes, and dried. The
collected catalyst was then placed in a new reaction following the
general procedure described above.
(10) Netherton, M. R.; Fu, G. G. Nickel-Catalyzed Cross-Couplings
of Unactivated Alkyl Halides and Pseudohalides with Organometallic
Compounds. Adv. Synth. Catal. 2004, 346, 1525−1532.
(11) Crockett, M. P.; Tyrol, C. C.; Wong, A. S.; Li, B.; Byers, J. A.
Iron-Catalyzed Suzuki−Miyaura Cross-Coupling Reactions between
Alkyl Halides and Unactivated Arylboronic Esters. Org. Lett. 2018, 20,
5233−5237.
ASSOCIATED CONTENT
* Supporting Information
(12) Anderson, T. J.; Jones, G. D.; Vicic, D. A. Evidence for a NiI
Active Species in the Catalytic Cross-Coupling of Alkyl Electrophiles.
J. Am. Chem. Soc. 2004, 126, 8100−8101.
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00082.
(13) Sherry, B. D.; Furstner, A. The Promise and Challenge of Iron-
̈
Catalyzed Cross Coupling. Acc. Chem. Res. 2008, 41, 1500−1511.
(14) Ananikov, V. P. Nickel: The “Spirited Horse” of Transition
Metal Catalysis. ACS Catal. 2015, 5, 1964−1971.
X-ray crystallography details, 1H and 13C NMR data, and
references to previously reported products (PDF)
(15) Bryan, M. C.; Dunn, P. J.; Entwistle, D.; Gallou, F.; Koenig, S.
G.; Hayler, J. D.; Hickey, M. R.; Hughes, S.; Kopach, M. E.; Moine,
G.; Richardson, P.; Roschangar, F.; Steven, A.; Weiberth, F. J. Key
Green Chemistry research areas from a pharmaceutical manufacturers’
perspective revisited. Green Chem. 2018, 20, 5082−5103.
(16) Liu, N.; Wang, L.; Wang, Z.-X. Room-temperature nickel-
catalysed cross-couplings of aryl chlorides with arylzincs. Chem.
Commun. 2011, 47, 1598−1600.
Accession Codes
CCDC 1896060 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(17) Jones, K. D.; Power, D. J.; Bierer, D.; Gericke, K. M.; Stewart, S.
G. Nickel Phosphite/Phosphine-Catalyzed C−S Cross-Coupling of
Aryl Chlorides and Thiols. Org. Lett. 2018, 20, 208−211.
(18) Gatien, A. V.; Lavoie, C. M.; Bennett, R. N.; Ferguson, M. J.;
McDonald, R.; Johnson, E. R.; Speed, A. W. H.; Stradiotto, M.
Application of Diazaphospholidine/Diazaphospholene-Based Bi-
sphosphines in Room-Temperature Nickel-Catalyzed C(sp2)−N
Cross-Couplings of Primary Alkylamines with (Hetero)aryl Chlorides
and Bromides. ACS Catal. 2018, 8, 5328−5339.
(19) Xi, Z.; Liu, B.; Chen, W. Room-Temperature Kumada Cross-
Coupling of Unactivated Aryl Chlorides Catalyzed by N-Heterocylic
Carbene-Based Nickel(II) Complexes. J. Org. Chem. 2008, 73, 3954−
3957.
(20) Paul, A.; Smith, M. D.; Vannucci, A. K. Photoredox-Assisted
Reductive Cross-Coupling: Mechanistic Insight into Catalytic Aryl−
Alkyl Cross-Couplings. J. Org. Chem. 2017, 82, 1996−2003.
(21) Zultanski, S. L.; Fu, G. C. Nickel-catalyzed carbon-carbon
bond-forming reactions of unactivated tertiary alkyl halides: Suzuki
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AUTHOR INFORMATION
Corresponding Author
*E-mail: vannucci@mailbox.sc.edu.
■
ORCID
Aaron K. Vannucci: 0000-0003-0401-7208
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors gratefully acknowledge support for this work by
the University of South Carolina.
■
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