Preparation of Ni(cod)2 Using Light as the Source of Energy

Article abstract of DOI:10.1021/acs.organomet.9b00057

A convenient method to prepare Ni(cod)₂ from Ni(acac)₂ using light as the source of energy is reported. In the first step of this process, xanthone is reductively dimerized upon irradiation of solar or LED light in 2-propanol to form a vicinal diol possessing a highly sterically congested C-C bond. In the second step, a ketyl radical derived from the diol reacts with Ni(acac)₂, ultimately reducing nickel(II) to nickel(0), which is bound by 1,5-cyclooctadiene (COD) to produce Ni(cod)₂. This new method obviates the need for hazardous reductants such as diisobutylaluminum hydride (DIBAH) and sodium.

Full text of DOI:10.1021/acs.organomet.9b00057

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Preparation of Ni(cod) Using Light as the Source of Energy
2
Naoki Ishida,* Yoshiki Kamae, and Masahiro Murakami*
Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan
*
S Supporting Information
ABSTRACT: A convenient method to prepare Ni(cod) from Ni(acac)
2
2
using light as the source of energy is reported. In the first step of this process,
xanthone is reductively dimerized upon irradiation of solar or LED light in 2-
propanol to form a vicinal diol possessing a highly sterically congested C−C
bond. In the second step, a ketyl radical derived from the diol reacts with
Ni(acac) , ultimately reducing nickel(II) to nickel(0), which is bound by 1,5-
2
cyclooctadiene (COD) to produce Ni(cod) . This new method obviates the
2
need for hazardous reductants such as diisobutylaluminum hydride (DIBAH)
and sodium.
ight is a readily available, clean, and traceless source of
energy. Utilization of the energy of light, particularly
steps. In the first step, xanthone is irradiated with solar or LED
L
light in 2-propanol to effect an endergonic reductive
dimerization reaction, producing a sterically congested diol.
Thus, light energy is “stored” chemically in the diol in the form
of the strain energy that arises due to steric congestion. In the
second step, the strain energy inherent in the diol is released by
breaking the highly crowded central C−C bond to liberate
energy that is used to reduce nickel(II) to nickel(0).
Conceptually, the key to this transformation is that the energy
to chemically reduce nickel originates from light. Of note is
that natural sunlight as well as an LED lamp can be used as the
source of light. From a practical standpoint, the present
method is far less harmful in comparison to existing methods
since the diol that is employed as a reductant is air-stable,
water-stable, bench-stable, and recyclable.
sunlight, has gained much attention from every science and
technology. It is possible to store the energy of light in a form
of a strained C−C bond through photoinduced endergonic
1
reactions. For example, two molecules of benzophenone
2
couple upon irradiation of sunlight, forming a diol, which
contains a sterically congested, high-energy C−C bond. A base
3
induces its cleavage to convert the intrinsic energy into
electric potential so that the resultant ketyl radical acts as a
single-electron donor. It is an important challenge to exploit
the energy originating from light for chemical transformations.
Nickel complexes catalyze numerous transformations of
4
organic compounds under homogeneous conditions. Typi-
cally, phosphorus-, nitrogen-, or carbene-based ligands, which
donate two electrons, are bound to the nickel center. The
10
Diol 2 was prepared from xanthone (1), by simply
bis(1,5-cyclooctadiene)nickel(0) complex, Ni(cod) , is the
following the procedure described in the literature for the
2
synthesis of benzopinacol from benzophenone using sunlight
most frequently employed nickel(0) complex because the 1,5-
cyclooctadiene ligands can readily exchange with more strongly
electron donating ligands such as those based on the elements
2
(
Scheme 1). Thus, to a 2-propanol solution of 1 in a Pyrex
flask was added 1 drop of acetic acid, and the flask was capped
5
,6
mentioned above. The most conventional laboratory method
7
to prepare Ni(cod) uses DIBAH to reduce Ni(II) salts.
Scheme 1. Photodimerization of Xanthone (1)
2
DIBAH is a hazardous pyrophoric organometallic reductant
often requiring that reaction temperatures be kept at −78 °C
during reduction. In addition, DIBAH readily decomposes
upon exposure to air and moisture, necessitating rigorously
anhydrous conditions for its storage. Although there are
alternative methods for the reduction of Ni(II) complexes that
8
9
employ triethylaluminum or metallic sodium as the
reductant, they are no less hazardous. A more convenient
and less hazardous method to prepare Ni(cod) will be of great
2
benefit.
In this Communication, we report a new method to prepare
Ni(cod) . It utilizes light as the ultimate source of energy for
Received: January 28, 2019
2
the reduction. Mechanistically, the method consists of two
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00057
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
with a rubber septum. The resulting solution was deaerated by
sparging with argon gas under sonication. The resulting
solution was then irradiated with sunlight at ambient
temperature, during which white solids of diol 2 precipitated.
After 3 days of irradiation, ca. 75% of 1 had been consumed.
The resulting precipitate was collected by filtration. The filtrate
was again deaerated and subjected to sunlight irradiation. After
6-31G(d), 298.15 K, 1 atm, PCM: 2-propanol). Light offers
energy to drive the dimerization reaction forward, thereby
converting the energy of light into chemical energy, and the
energy serves as a driving force for the reduction of Ni(acac)2.
The chemical energy stored in 2 manifests itself in its crystal
structure. Columnar crystals of 2 grew from its dichloro-
methane solution, and its structure was delineated by X-ray
crystallography, as shown in Chart 1. The distance between the
1
was fully consumed (ca. 2 more days), the volatiles were
removed under reduced pressure, leaving a white solid in the
a
flask. The combined solids were washed with Et O to afford
Chart 1. Molecular Structure of 2
2
11
analytically pure diol 2 in 89% yield.
Natural sunlight could be replaced with UV LEDs (λmax 365
nm, 18 W), which accelerated the rate of photodimerization.
Using 1.0 g (5.0 mmol) of 1, diol 2 (0.94 g, 2.4 mmol, 94%
yield) was obtained within 1 h.
In the second step of the overall reduction process, the
chemical energy stored in the diol 2 was used to reduce
nickel(II) to nickel(0) (Scheme 2). In an illustrative run,
Scheme 2. Synthesis of Ni(cod)2
a
Thermal ellipsoids are given at 50% probability; hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): C(10)−C(10′)
1.590(2), C(9)−C(10) 1.518(1), C(10)−C(11) 1.516(1).
3
two tertiary sp carbons is 1.59 Å, which is significantly longer
3
3
than a typical C(sp )−C(sp ) bond length (ca. 1.54 Å). Each
xanthene core is bent back into a butterfly shape (Chart 1b).
The angle between the two mean planes of the two juxtaposing
benzene rings is 35.6°, which is in sharp contrast to the planar
xanthene core of less strained 9-arylxanthen-9-ols. These
structural features of 2 indicate that a severe repulsive force is
operative between the two diarylcarbinol units.
anhydrous Ni(acac) (1.54 g, 6.0 mmol), diol 2 (3.31 g, 8.4
2
mmol, 1.4 equiv), and COD (4.05 mL, 3.57 g, 33 mmol, 5.5
equiv) were placed in a flask under a nitrogen atmosphere.
THF (100 mL) was added in the flask to give an emerald green
solution. t-BuOK (2.02 g, 18 mmol, 3.0 equiv) was added to
the emerald green solution at room temperature, turning the
15
3
solution deep blue, indicating the formation of a ketyl radical.
Diol 2 in the solid state could be stored for months under
ambient conditions without any deterioration. Once it is
deprotonated by a base such as t-BuOK, however, the repulsive
force confined in the congested structure is liberated to
generate two molecules of the corresponding anionic ketyl
radicals (the reduced state of xanthone 1) through homolytic
After the solution was stirred for 24 h, MeOH (20 mL) was
added to quench the remaining ketyl radical, and the color
changed to yellow. All volatiles were removed under reduced
pressure. Pentane (300 mL) was added to the residue. The
resulting suspension was vigorously stirred, and then it was
passed through a short pad of Celite under a nitrogen
atmosphere to remove insoluble substances, which included
xanthone (1) and potassium acetylacetonate. The filtrate was
concentrated under reduced pressure to provide a residue
which was recrystallized from toluene containing 10 vol % of
COD. Bright yellow flakes precipitated out of the toluene
3
3
3
cleavage of the elongated central C(sp )−C(sp ) bond. UV−
vis absorption spectroscopy confirmed that the blue color
observed in the step reducing nickel(II) to nickel(0) arose
red
from the anionic ketyl radical. The reduction potential E
1
/2
16
of xanthone (1) (−1.65 V vs SCE in MeCN) was more
red
1/2
negative than that of Ni(acac) (E
= −1.3 V vs SCE in
2
17
18
solution. Ni(cod) was isolated as yellow flakes (1.22 g, 4.4
MeCN). The ketyl radical serves as a single-electron donor
toward Ni(acac)2 to generate a nickel(I) species, which
2
mmol, 74% yield) with >99% purity by filtration under a
nitrogen atmosphere. On the other hand, xanthone (1) was
eluted out from a short pad of Celite using ethyl acetate as the
eluent (2.88 g, 14.7 mmol, 87% recovery based on 2).
Recovered 1 was pure enough to utilize in another photo-
dimerization reaction.
19
disproportionates into nickel(II) and nickel(0). Meanwhile,
the anionic acetylacetonate ligands on nickel are replaced by
the COD ligands, which accept the electron density of the
reduced nickel(0) center by back-donation.
It was also possible to use benzopinacol, which is prepared
2
Alcohols such as 2-propanol are known to serve as a
by photodimerization of benzophenone and possesses a
1
2
13
20
reductant for palladium(II), rhodium(III), and iridium-
(
sterically strained C−C bond in the center, as the reductant
1
4
III), presumably via (i) formation of the corresponding
in place of 2. However, the reduction reaction was less efficient
metal alkoxide, (ii) β-hydrogen elimination, and (iii) reductive
elimination. We attempted to reduce Ni(acac)2 using 2-
propanol as the reductant and t-BuOK as the base but failed,
demonstrating the superior reducing ability of the diol 2.
The photoinduced dimerization reaction of 2 equiv of 1 with
to yield Ni(cod) in 14% yield. On the other hand, the use of
less-strained 1,1,2,2-tetramethylethylene glycol totally failed to
produce Ni(cod)2.
2
Sodium hydroxide (NaOH) was strong enough to generate
the anionic ketyl radical from 2, which was confirmed by UV−
2
-propanol to form the diol 2 and acetone is energetically
vis absorption spectroscopy. In fact, Ni(cod) was obtained
2
uphill (ΔG = +19.5 kcal/mol, ΔH = +5.0 kcal/mol, ωB97XD/
when reduction was performed using NaOH in place of t-
B
DOI: 10.1021/acs.organomet.9b00057
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
BuOK under otherwise identical reaction conditions. On the
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(e) Ishida, N.; Masuda, Y.; Uemoto, S.; Murakami, M. A Light/
other hand, other bases such as NEt(i-Pr) and K CO failed
2
2
3
Ketone/Copper System for Carboxylation of Allylic C−H Bonds of
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2
Alkenes with CO . Chem. - Eur. J. 2016, 22, 6524−6527.
2
too weak to generate the anionic ketyl radical. Thus,
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to be crucial to effect the reduction of Ni(acac) to Ni(cod) .
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In conclusion, we have developed a convenient and safe
method to synthesize Ni(cod) using light as the ultimate
2
source of energy. The overall process consists of two steps: (1)
light-induced endergonic reductive dimerization of xanthone to
furnish the corresponding diol, which is a C−C bond forming
process, and (2) reduction of a Ni(II) salt with the diol, which
is a C−C bond breaking process. This new method obviates
the need for hazardous reductants such as sodium and DIBAH,
and all of the operations are performed at ambient temper-
ature. Application of the present protocol using the diol to the
synthesis of other low-valent metal complexes such as
platinum(0) and metal-catalyzed reductive organic trans-
formations are underway in our laboratory.
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met.9b00057.
(
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6
0
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2
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AUTHOR INFORMATION
Corresponding Authors
E-mail for N.I.: naisida@sbchem.kyoto-u.ac.jp.
E-mail for M.M.: murakami@sbchem.kyoto-u.ac.jp.
■
Kratzert, D.; Weis, P.; Wernet, M.; Peter, A.; Yassine, Z.; Schmitz, D.;
Scheidt, E.-W.; Scherer, W.; Weber, S.; Feuerstein, W.; Breher, F.;
Higelin, A.; Krossing, I. Synthesis, Characterisation and Reactions of
*
I
*
Truly Cationic Ni -Phosphine Complexes. Chem. - Eur. J. 2018, 24,
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b) Semmelhack, M. F. Formation of Carbon−Carbon Bonds via π-
ORCID
(
Naoki Ishida: 0000-0003-2162-3605
Notes
The authors declare no competing financial interest.
̈
́
̈
(
ACKNOWLEDGMENTS
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■
This work was supported by JSPS KAKENHI Grant Numbers
JP15H05756 (M.M.) and JP18H04648 (N.I.) (Hybrid
Catalysis).
(
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2
(
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1
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D
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