On the ability of nickel complexes derived from tripodal aminopyridine ligands to catalyze arene hydroxylations

Article abstract of DOI:10.2533/CHIMIA.2020.489

The development of catalysts for the selective hydroxylation of aromatic C-H bonds is an essential challenge in current chemical research. The accomplishment of this goal requires the discovery of powerful metal-based oxidizing species capable of hydroxyl

Full text of DOI:10.2533/CHIMIA.2020.489

NoN-Noble Metals iN Catalysis
CHIMIA 2020, 74, No. 6 489
doi:10.2533/chimia.2020.489
Chimia 74 (2020) 489–494 © E. Masferrer-Rius, R. M. Hopman, J. van der Kleij, M. Lutz, R. J. M. Klein Gebbink
On the Ability of Nickel Complexes
Derived from Tripodal Aminopyridine
Ligands to Catalyze Arene Hydroxylations
a
a
a
b
Eduard Masferrer-Rius , Raoul M. Hopman , Jishai van der Kleij, Martin Lutz, and Robertus J. M.
Klein Gebbink *
a
Abstract: The development of catalysts for the selective hydroxylation of aromatic C–H bonds is an essential
challenge in current chemical research. The accomplishment of this goal requires the discovery of powerful
metal-based oxidizing species capable of hydroxylating inert aromatic bonds in a selective manner, avoiding the
generation of non-selective oxygen-centered radicals. Herein we show an investigation on the ability of nickel(ii)
complexes supported by tripodal tetradentate aminopyridine ligands to catalyze the direct hydroxylation of
benzene to phenol with H O as oxidant. We have found that modifications on the ligand structure of the nickel
2
2
complex do not translate into different reactivity, which differs from previous findings for nickel-based arene
hydroxylations. Besides, several nickel(ii) salts have been found to be effective in the oxidation of aromatic C–H
bonds. The use of fluorinated alcohols as solvent has been found to result in an increase in phenol yield; however,
showing no more than two turn-overs per nickel. These findings raise questions on the nature of the oxidizing
species responsible for the arene hydroxylation reaction.
Keywords: Aromatic C–H oxidation · Fluorinated solvents · Hydrogen peroxide · Nickel complexes · Phenols
Eduard Masferrer-Rius received his classes of chemicals. Nowadays, many improvements have been
Bachelor and Master degrees in chemistry made in the development of different oxidation catalysts; how-
from the University of Girona (UdG). Since ever, the selective oxidation of organic substrates, such as aro-
2
016 he took part in a Marie Skłodowska- matic compounds, still represents a critical challenge in modern
CurieITNAction(NoNoMeCatproject)and chemical research.
started his PhD in the Organic Chemistry &
Phenols are essential intermediates in the generation of a broad
Catalysis group at Utrecht University, under range of products such as pharmaceuticals and polymers.[
3–6]
the guidance of Prof. Bert Klein Gebbink. Currently, the industrial production of phenol from benzene is
His interests are based on the development carried out via the cumene process, which suffers overall from low
of transition-metal complexes for arene hy- efficiencies in product yield.[ The direct introduction of a hy-
droxyl functionality through activation of an aromatic C–H bond
7]
droxylation reactions.
is difficult because of the high stability of aromatic compounds
Robertus (Bert) J.M. Klein Gebbink ob- and the high bond dissociation energy of an aromatic C–H bond
–
1
[8]
tained a PhD in supramolecular and bioin- (112 kcal mol ). To overcome this challenge, the generation
organic chemistry under the guidance of of highly reactive and selective metal-oxygen species is neces-
Roeland Nolte in 1998. After postdoctoral sary. However, often phenol products are more easily oxidized
studies at Stanford University (USA), than non-oxidized aromatic compounds, causing a chemoselectiv-
he accepted a senior postdoctoral posi- ity issue. In addition, a lack of discrimination between different
tion at Utrecht University (NL) with Prof. oxidation sites results in a regioselectivity issue, especially when
Gerard van Koten in 1999. Climbing the substituted benzenes are used in which the oxidation of benzylic
ranks, he was promoted to full professor of C–H bonds can be preferred over oxidation at the aromatic ring.
Homogeneous and Bioinspired Catalysis at
Utrecht in 2006. In the past years he acted radicals, are well known to oxidize aromatic compounds; how-
as head of the chemistry department at Utrecht and as the coor- ever, poor selectivity is usually observed due to the non-discrim-
On the one hand, hydroxyl radicals, as well as hydroperoxyl
dinator of the European initial training network NoNoMeCat on inative reactivity of oxygen-centered radicals.[
9,10]
On the other
Non-Noble Metal Catalysis.
. Introduction
hand, metal-based oxidants are known to lead to more selective
hydroxylation reactions.[ Along this line, some progress has
been made in the direct hydroxylation of benzene to phenol using
11]
1
Oxidations of organic compounds are essential reactions and H O as the benign oxidant catalyzed by homogeneous catalysts
2
2
[12–19]
have been intensively studied in academia as well as in the chemi- (Fig. 1), providing some mechanistic insights.
cal industry.[ Interest stems from the fact that oxygenated or-
1,2]
Over the last years, much interest has been devoted to the study
ganic molecules can be subsequently used to produce different of bioinspired iron complexes, which are minimalistic models of
*
Correspondence: Prof. Dr. R. J. M. Klein Gebbink, E-mail: r.j.m.kleingebbink@uu.nl
a
Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG Utrecht, The Netherlands;
b
Crystal and Structural Chemistry, Bijvoet Centre for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands

490 CHIMIA 2020, 74, No. 6
NoN-Noble Metals iN Catalysis
Fig. 1. Examples of metal com-
plexes previously used in catalytic
arene hydroxylation reactions with
H O .
N
N
N
2
2
N
N
OTf
OTf
OTf
OTf
N
N
OTf
OTf
N
Fe
N
Fe
N
Fe
N
N
[
Fe(tpa)(OTf)2]
[Fe(bpmen)(OTf)2]
[Fe(bpbp)(OTf)2]
(BPh4)
(ClO4)3
(OTf)2
N
N
N
O
N
N
N
Cu
N
N
O
O
Cu
Ni
N
H
N
Fe
N
N
N
N
N
N
N
N
Fe-iminopyridine
[Cu (µ-OH)(6-hpa)](ClO )
[Ni(tepa)(OAc)]BPh4
2
4 3
natural oxygenase enzymes.[20] These systems have been extensive- 10 mol% loading with respect to the benzene substrate only 21%
ly studied for the oxidation of aliphatic C–H groups and epoxidation phenol (2.1 turnovers per nickel) was formed in 5 h reaction time
[21]
reactions with H O , whereas hydroxylation of aromatic com- at 60 ºC. Based on the previous work from Itoh and co-workers,
2
2
pounds has remained challenging. The main problem is that the phe- another recently reported study shows improved nickel-based
nol products bind irreversibly to the iron center, which prevents cat- catalysts for the selective oxidation of benzene to phenol through
alytic turnover.[
22–24]
For instance, several studies on the use of iron modifications of aminopyridine ligands by introduction of elec-
complexes supported by the tpa and bpmen aminopyridine ligands tron-rich pyridines, affording phenol with up to 820 TON in 5 h
[19]
(tpa = tris(2-pyridylmethyl)amine) and bpmen = N,N'-dimethyl- at 60 ºC using 0.05 mol% loading of catalyst.
N,N'-bis(2-picolyl)ethylenediamine) showed that these complexes
Many efforts have focused on the development of highly selec-
were capable of oxidizing aromatic C–H bonds, but do not allow tive catalyst systems for phenol formation. These studies paral-
for catalytic turnover.[
ported by the bpbp type ligands (bpbp = N,N'-bis(2-pyridylmethyl)- aliphatic C–H bonds to the corresponding alcohols, avoiding the
,2'-bipyrrolidine) were found to be active for the hydroxylation of generation of overoxidized ketone products. In an effort to create
aromatic substrates with H O , but with poor selectivities due to the more selective catalysts, several of the latter studies have described
22–24]
Recently, a series of iron complexes sup- lel the development of catalysts for the selective hydroxylation of
2
2
2
[12–14]
generation of overoxidation products.
Non-heme imine-based the use of fluorinated alcohol solvents, i.e. 2,2,2-trifluoroethanol
iron complexes have also been investigated in the field of hydroxyl- (TFE) or 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) in hydroxyl-
ation reactions.[ For instance, an iminopyridine iron(ꢀꢀ) complex ation reactions, showing improved selectivities for the first-formed
25]
[
26–30]
prepared in situ by self-assembly of commercially starting materials hydroxylation product and avoiding overoxidation reactions.
was found to be active for the hydroxylation of aromatic rings us- Inspired by previous studies on arene hydroxylation catalyzed
ing H O as the oxidant, likely through a metal-based electrophilic by nickel complexes.[
18,19]
we studied the effect of different tripo-
2
2
[15]
aromatic substitution mechanism.
dal aminopyridine ligands for the direct hydroxylation of benzene
Other first-row transition metals have also been shown to be to phenol with H O by nickel under mild reaction conditions. Our
2
2
capable of performing arene hydroxylation reactions with H O findings show that the use of different tripodal tetradentate ami-
2
2
as benign oxidant. Kodera and co-workers reported a dinuclear nopyridine ligand designs in the nickel complexes does not lead
copper complex stabilized by the 6-hpa ligand (6-hpa = 1,2-bis{2- to a different reactivity in arene hydroxylation reactions, showing
[
bis(2-pyridylmethyl)aminomethyl]-6-pyridyl}ethane) for the that this type of ligand does not play an important role in catalysis.
selective hydroxylation of benzene to phenol with H O , show- Besides, we show that the oxidation of benzene can be achieved
2
2
[
16]
ing high activity for phenol formation. Another remarkable using simple nickel salts with high chemoselectivity. The effect
example is the selective hydroxylation of benzene catalyzed by of solvents, such as fluorinated alcohols, is found to produce and
a [Ni(tepa)(OAc)]BPh (tepa = tris(2-pyridylethyl)amine)/H O enhance activity for phenol formation, highlighting the use of this
4
2
2
system, which was reported to work through a metal-based mech- kind of solvents on oxidation processes. On the basis of these
anism, affording a maximum of 749 turnover numbers (TON) in results, we discuss some mechanistic considerations for arene
2
16 h at 60 ºC for phenol production when using an 10000-fold hydroxylations using molecular nickel complexes derived from
[
18]
excess of benzene with respect to the catalyst. The authors aminopyridine ligands.
found that among a series of nickel complexes supported by tripo-
dal tetradentate aminopyridine ligands, the one supported by the 2. Results and Discussion
tepa ligand is able to chemoselectively catalyze the hydroxylation
of benzene and alkylbenzenes at high H O loadings, without the 2.1 Aminopyridine Ligands and Nickel Complexes
2
2
[18]
formation of substantial amounts of over-oxidized products.
For our study, we investigated several nickel complexes sup-
Remarkably though, when this complex was used in catalysis in ported by aminopyridine ligands as arene hydroxylation catalysts

NoN-Noble Metals iN Catalysis
CHIMIA 2020, 74, No. 6 491
with H O as the benign oxidant under mild reaction conditions over-oxidized by-product was not observed. Thus, all complexes
2
2
(
Fig. 2). Our aim was to investigate if small modifications in the tested show chemoselectivity for phenol formation, as was re-
[18,19]
structure of the aminopyridine ligand would influence the reactiv- ported previously for similar Ni/H O systems.
Furthermore,
2
2
ity of the complexes in arene hydroxylation reactions.
biphenyl was detected after analysis of the crude mixtures. The
Based on the work of Itoh and co-workers,[ we focused formation of biphenyl seems to originate from the tetraphenylbo-
on the use of tripodal tetradentate aminopyridine ligands, play- rate counterion of the complex, since it is known that biphenyl can
ing with the length of the arms. First, we synthesized the parent form through radical decomposition of tetraphenylborate.[
tpa ligand, containing three methylene arms, and the tepa ligand, Similar oxidation experiments without benzene substrate afford
which contains three ethylene arms. Of interest was also the biphenyl as well, corroborating that benzene is not the source of
pmea ligand (pmea = (2-(2-pyridylethyl))bis(2-pyridylmethyl) the biphenyl formation. Interestingly, acetamide was detected
amine), and the pmap ligand (pmap = bis(2-(2-pyridylethyl))- as the main product in the crude reaction mixtures. Acetamide
18]
35,36]
2
-pyridylmethylamine). Finally, we envisioned an enhancement may form through the oxidation of triethylamine, which can be
[37,38]
of efficiency towards aromatic C–H oxidations by introducing oxidized in the presence of H O .
However, the amounts of
2
2
electron-donating substituents into some of the pyridines, such acetamide obtained were higher than the amount of triethylamine
as the 6-Me -tpa and 4-OMe-3,5-Me-pmap ligands (6-Me -tpa used, suggesting that the acetonitrile solvent is hydrated to acet-
2
2
=
bis(6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine, and amide under our experimental conditions.
4
-OMe-3,5-Me-pmap = bis(2-(2-pyridylethyl))-(4-methoxy-3,5-
dimethyl)-2-pyridylmethylamine). The benefit of such electron-
rich pyridine donors has been shown for several C–H and C=C
oxidation reactions with non-heme iron and manganese complex-
[
31–34]
es,
complexes.[
The corresponding nickel complexes were synthesized by
as well as for arene hydroxylation reactions with nickel
19]
Scheme 1. Catalytic hydroxylation of benzene to phenol catalyzed by
mixing nickel acetate tetrahydrate with the different aminopyri-
dine ligands; subsequent addition of sodium tetraphenylborate led
to precipitation of the final complex. Several of the complexes
were analyzed by X-ray single crystal structure determination,
nickel(ii) complexes with H O under mild reaction conditions.
2
2
Overall, these first catalytic experiments using acetonitrile as
showing a mononuclear nickel(ꢀꢀ) species exhibiting a distorted solvent provided poor phenol yields, ranging from 9.0 to 10.5%
octahedral geometry. Details of the synthesis and characterization (Table 1). These findings compare quite well with the initial ex-
of the ligands and complexes can be found in the Supplementary periments performed by Itoh and co-workers, for which they re-
Information.
ported 2 TON for the oxidation of benzene catalyzed by Ni(tepa)
under the same experimental conditions. Not only do our re-
sults represent a single turn-over per nickel, the differences in
[18]
2
.2 Screening of Complexes
Next, we focused on the oxidation of benzene as model sub- phenol yields between the different complexes lie within the ex-
strate to selectively screen for aromatic oxidation (Scheme 1). perimental error of our GC analysis. Accordingly, these results
Catalytic experiments were carried out using 40 µmol of benzene, indicate that no particular complex in the series of complexes
4
0 µmol of triethylamine as a base, and 5.7 mmol of H O (142 tested catalyzes benzene hydroxylation more effectively than an-
2
2
equiv.) in acetonitrile as solvent, with 10 mol% of catalyst; fol- other complex. Our results might even indicate that the aminopyr-
[
18]
lowing the initial conditions described by Itoh. Reactions were idine ligand plays no important role in performing the oxidation
run under air, at 60 °C, for 5 h using a closed reaction vessel. reaction. A control experiment without any complex as catalyst
Crude mixtures were analyzed by GC, detecting mainly phenol as showed that no phenol product formation occurs in the absence of
oxidized product, whereas formation of para-benzoquinone as an a nickel complex. Thus, we can confidently conclude that a nickel
Fig. 2. Nickel(ii) complexes sup-
BPh4
BPh4
BPh4
ported by tripodal tetradentate
aminopyridine ligands employed
in this work.
N
N
N
O
N
N
O
O
N
O
O
Ni
O
Ni
Ni
OH2
N
N
N
N
N
N
Ni(tpa)
Ni(tepa)
Ni(pmea)
O
BPh4
BPh4
BPh4
N
N
N
O
O
O
N
N
O
N
Ni
N
Ni
N
Ni
N
O
N
O
N
N
Ni(6-Me -tpa)
Ni(4-OMe-3,5-Me-pmap)
2
Ni(pmap)

492 CHIMIA 2020, 74, No. 6
NoN-Noble Metals iN Catalysis
Table 1. Direct hydroxylation of benzene to phenol employing Ni(ii) com-
plexes in acetonitrile.
From these results, we concluded that the fluorinated alcohol
HFIP is the best solvent to perform the oxidation of benzene to
phenol catalyzed by the Ni(tepa) complex. Next, we tried high-
er amounts of H O , which did not afford an increase in phenol
Entry
Catalyst
Ni(tpa)
Phenol Yield [%]
2
2
formation (Table 2, entry 9). We also envisioned an increase in
catalytic activity by delivering H O slowly during catalysis, as
1
2
3
4
5
6
9.6
9.9
2
2
has been shown for other C–H hydroxylation reactions using this
Ni(tepa)
Ni(pmea)
Ni(pmap)
oxidant.[
39,40]
However, no improvement was observed, and phe-
9.7
nol was formed in a much lower yield, highlighting that the dis-
proportionation of H O is not product-limiting, probably due to
2
2
9.0
the high-excess conditions (Table 2, entry 10).
Next, we screened all nickel complexes for catalysis in the
HFIP solvent (Table 3). Overall, we observed that for all com-
plexes phenol yields increase when HFIP is used as the solvent
compared to the use of acetonitrile, as was initially shown for the
Ni(tepa) complex. While this observation highlights the use of a
fluorinated alcohol solvent in arene hydroxylation catalysis by
nickel(ꢀꢀ) complexes, the overall yields still represent an average
Ni(6-Me -tpa)
10.5
n.d.
2
-
a
n.d. = non-detected. Reaction conditions: benzene (40 µmol), H O
2
2
(
5.7 mmol), Ni complex (4 µmol), and NEt (40 µmol) at 60 ºC for 5 h in
3
CH CN.
3
1
.5 TON per nickel. In addition, no distinctive differences in cata-
complex is involved in the oxidation reaction.
.3 Different Reaction Solvents
lytic efficiencies are observed between the different complexes.
Even the nickel complexes Ni(6-Me -tpa) and Ni(4-OMe-3,5-
2
2
Me-pmap) supported by electron-rich tripodal ligands did not
Since the phenol yields were low, we decided to screen differ- afford substantial changes in reactivity (Table 3, entries 5 and 6).
ent solvents for the hydroxylation of benzene with the Ni(tepa) Control experiments with HFIP as solvent showed that the reac-
catalyst (Table 2). With acetonitrile, we could detect 4.7% phenol tion does not work in absence of a catalyst (Table 3, entry 7). Even
yield after 2.5 h, which increased to 9.9% when the reaction was though a solvent screening did result in some increase in catalytic
run for 5 h (Table 2, entries 1 and 2). Next, fluorinated alcohols efficiency, the small differences in product yields attainable with
were tested, which have been reported to be suitable solvents in the current, yet limited, set of tripodal tetradentate aminopyridine
[
26–30]
different hydroxylation reactions.
TFE provided 7.6% phe- ligands in our view does not allow for a rationalized ligand modi-
nol formation after 2.5 h, which slightly increased to 8% when fication toward improved catalyst efficiency.
the reaction was run for 5 h (Table 2, entries 3 and 4). Another
Interestingly, the simple salts nickel nitrate hexahydrate and
fluorinated alcohol, HFIP, was also tested as solvent, providing nickel chloride hexahydrate led to 6.7 and 3.0% phenol yield, re-
an enhanced activity, with 11.8 and 15.3% phenol yield after 2.5 spectively, using our current conditions (Table 3, entries 8 and 9).
and 5 h of reaction time, respectively. Methanol afforded poor However, the hydroxylation reaction did not work when nickel
phenol formation (3.8% yield), whereas acetone did not provide acetate tetrahydrate was used (Table 3, entry 10). Remarkably,
any phenol product (Table 2, entries 7 and 8); showing that these these results show that aromatic C–H oxidations can be done us-
two solvents are not suitable to perform arene hydroxylation reac- ing some simple commercial nickel(ꢀꢀ) salts with high chemose-
tions with nickel complexes.
lectivity, albeit in low efficiencies.
Table 2. Screening of solvents for the direct hydroxylation of benzene to
a
phenol catalyzed by Ni(tepa).
Table 3. Direct hydroxylation of benzene to phenol employing Ni(ii) com-
plexes in HFIP.
Entry
Solvent
Reaction
Time [h]
Phenol
Yield [%]
Entry
Catalyst
Ni(tpa)
Phenol Yield [%]
1
2
3
4
5
6
7
8
CH CN
2.5
5
4.7
9.9
3
1
2
3
4
5
6
7
8
9
14.9
15.3
14.6
16.4
15.0
17.4
n.d.
6.7
CH CN
3
Ni(tepa)
Ni(pmea)
Ni(pmap)
TFE
TFE
2.5
5
7.6
8.0
HFIP
HFIP
2.5
5
11.8
15.3
3.8
Ni(6-Me -tpa)
2
Ni(4-OMe-3,5-Me-pmap)
CH OH
5
3
-
(CH ) CO
5
n.d.
14.8
7.2
3
2
Ni(NO ) ·6H O
3
2
2
b
9
HFIP
HFIP
5
NiCl ·6H O
3.0
2
2
c
1
0
2.5
1
0
Ni(CH CO ) ·4H O
n.d.
3
2
2
2
a
Reaction conditions: benzene (40 µmol), H O (5.7 mmol), Ni complex
2
2
b
a
(
4 µmol), and NEt (40 µmol) at 60 ꢀC. 500 equivalents of H O were
n.d. = non-detected. Reaction conditions: benzene (40 µmol), H O
2 2
3
2
2
c
used. H O was added slowly within 1 h with the use of a syringe pump.
n.d. = non-detected.
(5.7 mmol), Ni complex (4 µmol), and NEt
3
(40 µmol) at 60 ꢀC for 5 h in
2
2
HFIP.

NoN-Noble Metals iN Catalysis
CHIMIA 2020, 74, No. 6 493
2
.4 Mechanistic Considerations
On the other hand, we believe that some kind of deactivation
Finally, our efforts have been devoted to the understanding of the of the catalyst occurs during catalysis, which prevents efficient
mechanism of the aromatic hydroxylation catalyzed by nickel com- and catalytic turnover. To further investigate catalyst stability, we
plexes. In previous studies by Itoh and co-workers with the Ni(tepa) have studied the formation of phenol over time using the Ni(tepa)
complex, it has been proposed that oxidation of benzene proceeds complex with H O , triethylamine and acetonitrile as the solvent,
2
2
through a metal-based mechanism in which, after activation of H O , which showed that the phenol yield increases in the first few hours
2
2
a dinickel(ꢀꢀꢀ) bis(µ-oxo) species is formed as the active oxidant of reaction, to then come to a stop. Reaction analysis after 5 h
Scheme 2).[ However, no direct evidence for the involvement of provided us with 10% phenol yield, which remained the same for
18]
(
such species has been shown. A recent work by Mayilmurugan and the next 24 h. Addition of an extra portion of catalyst and allow-
co-workers on arene hydroxylations catalyzed by nickel complexes ing the reaction to run for another 5 h resulted in an increase in
supported by similar tripodal tetradentate aminopyridine ligands, phenol yield to 32%. Besides, attempts to obtain high turnover
postulates the same dinuclear nickel species as the oxidant respon- numbers by reproducing the same conditions described by Itoh
sible for the oxidation of the aromatic ring.[ Only recently, Itoh and co-workers did not afford results that are consistent with the
and co-workers have shown an example of a dinickel(ꢀꢀꢀ) bis(µ-oxo) ones reported in the literature.[ Therefore, our results clearly
species with the dpema ligand (dpema = N,N-di-[2-pyridine-2-yl] differ from those reported by Itoh and co-workers, in which they
ethyl)methylamine) displaying oxygenation reactivity towards ex- describe up to 749 TON for phenol formation after 216 h of reac-
ternal hydrocarbon substrates, however, no reactivity of such spe- tion time using the Ni(tepa) catalyst with H O , triethylamine and
19]
18]
2
2
cies towards external aromatics was reported.[
41]
[18]
acetonitrile.
Scheme 2. Proposed catalytic
mechanism for the direct hydrox-
ylation of benzene to phenol with
nickel(ii) complexes based on pre-
[
18]
vious studies.
Here, we considered the role of triethylamine as a base, and 3. Conclusions
how this component could affect catalysis. In these oxidation re-
We have presented a study on the effect of changes in the
actions triethylamine is thought to activate H O and facilitate its tripodal tetradentate aminopyridine ligand in nickel complexes
2
2
reaction with the mononuclear nickel complex. Aqueous H O used for the direct one-step hydroxylation of benzene to phenol
2
2
[
42]
solutions are acidic (pK of H O = 11.62); therefore, the pres- in combination with H O as benign oxidant under mild reaction
a
2
2
2
2
ence of a base in the catalytic reactions could help in the activa- conditions. Our results show that these modifications in the li-
tion of H O by deprotonating it. Indeed, we carried out some gand structure do not translate into different activities in phenol
2
2
catalytic experiments using Ni(tepa) in acetonitrile without tri- formation, which differs from previous studies on nickel-based
ethylamine, where we could observe a slight decrease in phenol arene hydroxylations using similar tetradentate aminopyridine li-
formation (6.9% yield) in comparison with experiments using the gands.[18,19] Remarkably, the oxidation of benzene could also be
base (compare with Table 1, entry 2); indicating the positive role achieved using nickel(ꢀꢀ) salts without the use of sophisticated
of the amine in activating the oxidant.
ligand designs and using a fluorinated alcohol solvent, which
However, the fluorinated solvent HFIP is rather acidic (pK of seemed to improve phenol yields for the nickel complexes. The
a
HFIP = 9.3)[ and could consequently affect the role of triethyl- phenol yields obtained with the salts is significantly lower than
43]
[
44]
amine (pK for the conjugate acid = 10.75) in the deprotonation with the complexes. The product selectivity obtained throughout
a
of H O . Carrying out a catalytic reaction with Ni(pmea) in HFIP our study, with no formation of catechols, hydroquinones or ben-
2
2
and in the absence of triethylamine as a base gave a 15.4% phenol zoquinones, points towards a metal-based mechanism, with no
yield, which is similar to the yield obtained when triethylamine involvement of oxygen-centered radicals.
was employed (14.6%, see Table 3,). Thus, this observation made
Our studies do corroborate the earlier findings of Itoh et al.
us conclude that triethylamine does not have an essential role in concerning the catalytic performance of the nickel complexes at
aromatic oxidations catalyzed by nickel complexes when HFIP is high catalyst loadings.Yet, attempts to reproduce the high turnover
used as a solvent. Indeed, this fluorinated alcohol itself is known numbers achieved in the literature at low catalyst loadings were
to activate H O , as has been shown in some selective oxidation not successful. Together with the findings that catalytic perfor-
2
2
[
45]
reactions such as the epoxidation of alkenes.
mance is rather insensitive towards ligand structure variation and
Overall, we believe that the reaction might proceed through a that simple nickel salts also are able to form phenol, the molecular
metal-based mechanism, since high chemoselectivity for the for- nature of the catalyst involved in benzene hydroxylation by the
mation of phenol is observed. Generation of overoxidized prod- nickel complexes can be questioned. Other than the involvement
[
18,19]
ucts, such as hydroquinones or benzoquinones, easily occurs when of a dinickel(ꢀꢀꢀ) bis(µ-oxo) species proposed earlier,
we be-
However, lieve that some kind of decomposition of the nickel complexes
such products have not been observed in the current and in pre- occurs under the experimental conditions, possibly leading to the
[
46–49]
oxygen-centered radicals are involved in catalysis.
vious studies using aminopyridine-based nickel complexes.[
18,19]
formation of nickel-based nanoparticles that are involved in ca-

494 CHIMIA 2020, 74, No. 6
NoN-Noble Metals iN Catalysis
talysis. Our future efforts will therefore be focused on identifying [20] L. Que Jr, W. B. Tolman, Nature 2008, 455, 333.
[
[
21] G. Olivo, O. Cussó, M. Costas, Chem. Asian J. 2016, 11, 3148.
22] N. Y. Oh, M. S. Seo, M. H. Lim, M. B. Consugar, M. J. Park, J.-U. Rohde, J.
Han, K. M. Kim, J. Kim, L. Que Jr, Chem. Commun. 2005, 5644.
23] S. Taktak, M. Flook, B. M. Foxman, L. Que Jr, E. V. Rybak-Akimova, Chem.
Commun. 2005, 5301.
24] O. V. Makhlynets, E. V. Rybak-Akimova, Chem. Eur. J. 2010, 16, 13995.
25] G. Olivo, O. Lanzalunga, S. Di Stefano, Adv. Synth. Catal. 2016, 358, 843.
26] V. Dantignana, M. Milan, O. Cussó, A. Company, M. Bietti, M. Costas, ACS
Cent. Sci. 2017, 3, 1350.
the actual nature of the hydroxylation catalyst and on the develop-
ment of more active and stable catalyst systems based on nickel
that allow for the high-turnover direct hydroxylation of benzene
to phenol.
[
[
[
[
Acknowledgements
The European Commission is acknowledged for financial support
through the NoNoMeCat project (675020-MSCA-ITN-2015-ETN). We
also thank Utrecht University. The X-ray diffractometer has been fi-
nanced by the Netherlands Organization for Scientific Research (NWO).
[
27] R.V. Ottenbacher, E. P. Talsi, T.V. Rybalova, K. P. Bryliakov, ChemCatChem
2018, 10, 5323.
28] D. Wang, W. G. Shuler, C. J. Pierce, M. K. Hilinski, Org. Lett. 2016, 18,
[
3
826.
[29] E. Gaster, S. Kozuch, D. Pappo, Angew. Chem. Int. Ed. 2017, 56, 5912.
[30] A. M. Adams, J. Du Bois, Chem. Sci. 2014, 5, 656.
[31] O. Cussó, I. Garcia-Bosch, X. Ribas, J. Lloret-Fillol, M. Costas, J. Am.
Chem. Soc. 2013, 135, 14871.
Supplementary Information
Supplementary information is available on https://www.ingentacon-
nect.com/content/scs/chimia
[
[
[
[
[
32] O. Cussó, M. Cianfanelli, X. Ribas, R. J. Klein Gebbink, M. Costas, J. Am.
Chem. Soc. 2016, 138, 2732.
33] O. Cussó, I. Garcia-Bosch, D. Font, X. Ribas, J. Lloret-Fillol, M. Costas,
Org. Lett. 2013, 15, 6158.
Received: April 20, 2020
34] M. Milan, G. Carboni, M. Salamone, M. Costas, M. Bietti, ACS Catal. 2017,
[
1] H. Arakawa, M. Aresta, J. N. Armor, M. A. Barteau, E. J. Beckman, A. T.
Bell, J. E. Bercaw, C. Creutz, E. Dinjus, D. A. Dixon, K. Domen, D. L.
DuBois, J. Eckert, E. Fujita, D. H. Gibson, W. A. Goddard, D. W. Goodman,
J. Keller, G. J. Kubas, H. H. Kung, J. E. Lyons, L. E. Manzer, T. J. Marks, K.
Morokuma, K. M. Nicholas, R. Periana, L. Que, J. Rostrup-Nielson, W. M.
H. Sachtler, L. D. Schmidt, A. Sen, G. A. Somorjai, P. C. Stair, B. R. Stults,
W. Tumas, Chem. Rev. 2001, 101, 953.
2] T. Punniyamurthy, S. Velusamy, J. Iqbal, Chem. Rev. 2005, 105, 2329.
3] M. Weber, M. Weber, M. Kleine-Boymann, in ‘Ullmann’s Encyclopedia of
Industrial Chemistry’, Wiley-VCH, Weinheim, Germany, 2004, p. 503.
4] S. Fukuzumi, K. Ohkubo, Asian J. Org. Chem. 2015, 4, 836.
7
, 5903.
35] S. N. Mlondo, P. O’Brien, P. J. Thomas, M. Helliwell, J. Raftery, D. J.
Procter, Chem. Commun. 2008, 2456.
36] P. J. Grisdale, J. L. Williams, M. Glogowski, B. Babb, J. Org. Chem. 1971,
3
6, 544.
[
[
[
[
37] S. D. Ross, J. Am. Chem. Soc. 1946, 68, 1484.
38] J. Gee, R. Williamson, J. Am. Oil Chem. Soc. 1997, 74, 65.
39] M. S. Chen, M. C. White, Science 2007, 318, 783.
40] K. Suzuki, P. D. Oldenburg, L. Que Jr, Angew. Chem. Int. Ed. 2008, 47,
[
[
1
887.
[
[
[
[
41] Y. Morimoto, Y. Takagi, T. Saito, T. Ohta, T. Ogura, N. Tohnai, M. Nakano,
5] J. Genovino, D. Sames, L. G. Hamann, B. B. Touré, Angew. Chem. Int. Ed.
S. Itoh, Angew. Chem. 2018, 130, 7766.
42] D. R. Lide, ‘Handbook of Chemistry and Physics’, CRC Press, Boca Raton,
FL, 2005.
2
016, 55, 14218.
[
6] R. A. Sheldon, J. K. Kochi, in ‘Metal-Catalyzed Oxidations of Organic
Compounds’, Academic Press, New York, 1981, p. 315.
[
[
43] R. Filler, R. M. Schure, J. Org. Chem. 1967, 32, 1217.
44] J. Riddick, W. Bunger, T. Sakano, in ‘Techniques of Chemistry’, 4th ed.,
Volume II: Organic Solvents, New York, NY, John Wiley and Sons, 1985, p.
[
[
7] R. J. Schmidt, Appl. Catal. A 2005, 280, 89.
8] Y.-R. Luo, ‘Comprehensive Handbook of Chemical Bond Energies’, CRC
Press, Boca Raton, FL, 2007.
9] C. Yuan, Y. Liang, T. Hernandez, A. Berriochoa, K. N. Houk, D. Siegel,
6
38.
[
[
[
[
[
[
45] K. Neimann, R. Neumann, Org. Lett. 2000, 2, 2861.
46] D. I. Metelitsa, Russ. Chem. Rev. 1971, 40, 563.
47] C. Walling, R. A. Johnson, J. Am. Chem. Soc. 1975, 97, 363.
48] A. Kunai, S. Hata, S. Ito, K. Sasaki, J. Am. Chem. Soc. 1986, 108, 6012.
49] T. Kurata, Y. Watanabe, M. Katoh, Y. Sawaki, J. Am. Chem. Soc. 1988, 110,
Nature 2013, 499, 192.
[
[
[
10] J. P. Hage, A. Llobet, D. T. Sawyer, Bioorg. Med. Chem. 1995, 3, 1383.
11] M. Costas, K. Chen, L. Que Jr, Coord. Chem. Rev. 2000, 200, 517.
12] O.Y. Lyakin, A. M. Zima, N. V. Tkachenko, K. P. Bryliakov, E. P. Talsi, ACS
Catal. 2018, 8, 5255.
7
472.
[
[
[
[
[
[
[
13] N. V. Tkachenko, R. V. Ottenbacher, O. Y. Lyakin, A. M. Zima, D. G.
Samsonenko, E. P. Talsi, K. P. Bryliakov, ChemCatChem 2018, 10, 4052.
14] A. M. Zima, O. Y. Lyakin, D. P. Lubov, K. P. Bryliakov, E. P. Talsi, Mol. License and Terms
Catal. 2020, 483, 110708.
This is an Open Access article under the
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License CC BY_NC 4.0. The material may
not be used for commercial purposes.
15] G. Capocasa, G. Olivo, A. Barbieri, O. Lanzalunga, S. Di Stefano, Catal. Sci.
Technol. 2017, 7, 5677.
16] T. Tsuji, A. A. Zaoputra, Y. Hitomi, K. Mieda, T. Ogura, Y. Shiota, K.
Yoshizawa, H. Sato, M. Kodera, Angew. Chem. Int. Ed. 2017, 129, 7887.
17] S. Muthuramalingam, K. Anandababu, M. Velusamy, R. Mayilmurugan,
Inorg. Chem. 2020, doi.org/10.1021/acs.inorgchem.9b03676.
18] Y. Morimoto, S. Bunno, N. Fujieda, H. Sugimoto, S. Itoh, J. Am. Chem. Soc.
The license is subject to the CHIMIA terms and conditions: (http://
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The definitive version of this article is the electronic one that can be
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2
015, 137, 5867.
19] S. Muthuramalingam, K. Anandababu, M. Velusamy, R. Mayilmurugan,
Catal. Sci. Technol. 2019, 9, 5991.