Reactivity of a Ruthenium(0) Complex Bearing a Tetradentate Phosphine Ligand: Applications to Catalytic Acrylate Salt Synthesis from Ethylene and CO2

Article abstract of DOI:10.1021/acs.organomet.8b00789

By using a zerovalent, electron-rich ruthenium complex bearing a tetradentate phosphine ligand, a five-membered ruthenalactone was generated by oxidative cyclization of ethylene and CO₂. This ruthenalactone underwent thermal, reversible β-H elimination to afford an acrylato(hydrido)ruthenium(II) complex, which liberated acrylate salt on treatment with a base. The reaction was successfully applied to the first ruthenium-catalyzed synthesis of an acrylate salt from ethylene and CO₂. This study demonstrated the feasibility of the electron-rich ruthenium(0) complex as a catalyst in CO₂-fixation chemistry.

Full text of DOI:10.1021/acs.organomet.8b00789

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Reactivity of a Ruthenium(0) Complex Bearing a Tetradentate
Phosphine Ligand: Applications to Catalytic Acrylate Salt Synthesis
from Ethylene and CO2
Tatsuyoshi Ito, Kohei Takahashi, and Nobuharu Iwasawa*
Department of Chemistry, School of Science, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8551, Japan
*
S Supporting Information
ABSTRACT: By using a zerovalent, electron-rich ruthenium
complex bearing a tetradentate phosphine ligand, a five-
membered ruthenalactone was generated by oxidative
cyclization of ethylene and CO . This ruthenalactone
2
underwent thermal, reversible β-H elimination to afford an
acrylato(hydrido)ruthenium(II) complex, which liberated
acrylate salt on treatment with a base. The reaction was
successfully applied to the first ruthenium-catalyzed synthesis
of an acrylate salt from ethylene and CO . This study
2
demonstrated the feasibility of the electron-rich ruthenium(0) complex as a catalyst in CO -fixation chemistry.
2
9
uthenium complexes are some of the most extensively
complex 2 from RuH(OAc)(PPh3)3 (Scheme 1). Treatment
R
investigated catalysts for organic synthesis, and five-
of this complex with the tetradentate ligand PP at 60 °C in
3
membered ruthenacycles generated by oxidative cyclization of
unsaturated molecules are some of the key intermediates in
catalytic reactions. However, the formation of ruthenacycles
has been mostly limited to olefinic and acetylenic substrates
Scheme 1. Preparation of Ethylene Ruthenium(0) 2 having
a Tetradentate Phosphine Ligand
II
1
using CpRu species or Ru(0) carbonyls, and oxidative
cyclization containing polar double bonds such as CO and
CN has been limited partially because these Ru(II) or
Ru(0) complexes have relatively weak reducing ability.
Actually, a limited number of papers have been reported on
the hetero-Pauson−Khand-type [2 + 2 + 1] cycloaddition and
oxidative cyclization of dienes with carbonyl compounds. In
2
3
these reactions, Ru(0) carbonyls such as Ru (CO) and
THF gave the ligand-exchanged complex 1 in 87% yield. Then
3
12
t
Ru (CO) −phosphine systems were usually employed, and
KO Bu-mediated elimination of acetic acid was found to
3
12
ruthenium(0) complexes having only electron-donating ligands
such as phosphines have not been extensively utilized for these
catalytic reactions partially because these complexes are rather
proceed smoothly by carrying out the reaction at 60 °C in
DMA (N,N-dimethylacetamide) under an ethylene atmos-
phere to give the ethylene-coordinated Ru(0) complex 2 in
10,11
4
,5
71% isolated yield.
unstable and their preparative methods are limited.
We have chosen CO as a substrate containing a polar
In the present paper, we report utilization of an electron-rich
Ru(0) complex having a tetradentate phosphine ligand for
oxidative cyclization of ethylene and CO2 to give a
ruthenalactone, which gave an acrylate salt on treatment with
base. The reaction was further applied to the catalytic
preparation of an acrylate salt for the first time except for
2
double bond, because conversion of CO2 into valuable
12
commodities is an important topic in chemistry. In our
initial studies, the reaction of ethylene ruthenium complex 2
with CO was investigated. On treatment of 2 with CO (0.1
2
2
MPa) in DMA at 60 °C, the five-membered ruthenalactone 3
was found to be produced smoothly in 71% yield (Scheme 2a).
Formation of five-membered metallalactones by oxidative
6
,7
group 10 transition-metal complexes.
We thought of preparing the known ruthenium(0) complex
having a tetradentate phosphine ligand, Ru(PP )(C H )
cyclization of ethylene and CO has been studied to realize
2
2
3
2
4
8
acrylic acid synthesis employing various transition-metal
(
PP = P(CH CH PPh ) ), expecting high reducing ability
3 2 2 2 3
6
,7,13−22
complexes.
However, the isolation and identification
and stability of the complex. As the literature methods for the
preparation of this class of complexes usually necessitate the
use of sodium metal or metal hydride to reduce Ru Cl
precursors, we first devised a convenient method for the
of the resulting five-membered metallalactone intermediates
II
2
4
Received: October 28, 2018
preparation of the tetradentate phosphine coordinated Ru(0)
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00789
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Scheme 2. Formation and Reactivity of Ruthenalactone 3
and cis-Acrylato(hydrido)ruthenium 5
are two possible isomers for the ruthenalactone with respect to
the geometry of the ruthenium center, the isomer in which the
oxygen atom (O1) of the cyclic carboxylato ligand is oriented
trans to the bridgehead phosphorus atom (P2) was selectively
formed under these reaction conditions. In comparison to the
20b
structures of the reported square-planar nickelalactone
or
7a
palladalactone, the C−Ru−O angle (79.82(16)°) is more
reduced (86.4° for C−Ni−O, 83.08(19)° for C−Pd−O) and
the Ru−O bond length (2.180(3) Å) is longer than those of
Ni−O (1.88 Å) and Pd−O (2.06 Å).
On the basis of the expectation that the longer Ru−O bond
length might facilitate the cleavage of the metallalactone, the
reactivity of 3 was studied. Although 3 was stable at 60 °C in
DMA, it was found that by simple heating at 100 °C for 4 h
without any additive, the cis-acrylato(hydrido)ruthenium(II)
complex 5 was formed (24% yield, 46% conversion), as shown
in Scheme 2b. It should be noted that, in the case of the
2
1
square-planar nickelalactone, addition of a methylating agent,
Lewis acid,
20b,22
6
or base was required for β-H elimination, and
further isomerization to four-membered nickelalactone oc-
curred. In contrast, the cis-acrylato(hydrido)ruthenium(II)
complex 5 was generated by simply heating ruthenalactone 3
26
and was stable for isolation and identification. Furthermore,
under these reaction conditions, ethylene ruthenium(0)
complex 2 and carbonatoruthenium(II) complex 4 were also
observed as byproducts, suggesting that oxidative cyclization of
27
2
with CO was reversible. The octahedral structure of 5 was
2
confirmed by an X-ray analysis (Figure 1b). The acrylato
1
ligand is κ O-coordinated and oriented trans to the bridgehead
phosphorus atom (P2) of the tetradentate ligand.
During the examination of the reactivity of the ruthenium
complexes, it was found that the cis-acrylato(hydrido)
ruthenium(II) complex 5 was prepared by the reaction of
the ethylene ruthenium(0) complex 2 with acrylic acid in THF
at room temperature (Scheme 2c). Moreover, the cis-
acrylato(hydrido) ruthenium complex 5 quantitatively iso-
merized to ruthenalactone 3 in DMA at 60 °C. This is the first
example of direct observation of interconversion between the
five-membered metallalactone and the acrylato hydrido
complex. Because of this reversibility of β-H elimination,
ruthenalactone 3 was not completely converted to the cis-
acrylato(hydrido)ruthenium complex 5 as shown in Scheme
were limited to Ti, Zr, V, and Ni6,19−22 complexes, and
1
3
14
15
the catalytic synthesis of an acrylate salt from ethylene and
CO was only recently achieved by using a Ni or Pd catalyst.
This is the first example of the formation and isolation of a
ruthenalactone via oxidative cyclization involving CO2. As a
byproduct, the corresponding carbonatoruthenium(II) 4 was
observed in 7% yield.
The octahedral structure of the ruthenalactone 3 was
confirmed by an X-ray analysis (Figure 1a). Although there
6
,7
2
23
2
4,25
28
2
b. On the basis of these findings, direct synthesis of
ruthenalactone 3 from the ethylene ruthenium(0) complex 2
and acrylic acid was also achieved by carrying out the reaction
29−31
in DMA at 60 °C.
To apply these ruthenium complexes to the catalytic
synthesis of an acrylate salt, reductive liberation of acrylate
salt from the cis-acrylato(hydrido)ruthenium(II) complex 5
was examined. The expected reaction was found to proceed on
treatment of 5 with KOEt in DMA at 60 °C under C H (0.1
2
4
MPa) to afford potassium acrylate along with the ethylene
ruthenium(0) complex 2 in a manner similar to the reaction of
RuH(OAc)(PP ) (1) (Scheme 2d). Concerning the reductive
3
liberation of acrylate salt, it was reported that n-BuLi was
employed for the reaction of the acrylato hydrido complexes of
Figure 1. (a) ORTEP drawing of ruthenalactone 3 at the 50%
probability level. Hydrogen atoms and a solvent molecule are omitted
for clarity. Selected bond lengths (Å) and angle (deg): Ru−O1,
16b
group 6 metals.
The cis-acrylato(hydrido)ruthenium com-
plex 5 reacted with milder base probably due to its lower
oxophilicity and higher reduction potential as a group 8
transition-metal complex.
2
.180(3); Ru−C1, 2.207(4); C1−Ru−O1, 79.82(16). (b) ORTEP
drawing of cis-acrylato(hydrido) ruthenium 5 at the 50% probability
level. Hydrogen atoms except for Ru−H are omitted for clarity.
Selected bond lengths (Å): Ru−O1, 2.2083(17); C1−C2, 1.304(4);
C2−C3, 1.496(4).
As the elementary steps of the formation of acrylate salt from
ethylene and CO were found to proceed stoichiometrically,
2
we next investigated the catalytic activitiy of ruthenium
B
DOI: 10.1021/acs.organomet.8b00789
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
complex 2 for the carboxylation of ethylene (Table 1). To our
delight, when a DMA solution of 2 was heated at 140 °C under
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Table 1. Screening of Ruthenium Catalyst
AUTHOR INFORMATION
■
Corresponding Author
*
E-mail for N.I.: niwasawa@chem.titech.ac.jp.
ORCID
Nobuharu Iwasawa: 0000-0001-6323-6588
Notes
The authors declare no competing financial interest.
a
entry
Ru
TON
1
2
3
4
5
2
2
<1
<1
<1
<1
RuCl (PP ) (6)
2
3
Ru (CO)
3
12
ACKNOWLEDGMENTS
Ru(CO) (PPh )
[Cp*Ru(MeCN) ]PF
2
3
3
■
3
6
This research was supported by JST ACT-C Grant Number
JPMJCR12Y3 and JSPS KAKENHI Grant Numbers
15H05800 and 17H06143.
b
6
2
6
a
1
b
t
Determined by H NMR. Conditions: 2 (10 μmol), KO Bu (250
equiv), 12 h.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00789.
■
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Experimental procedures and spectral data and crystallo-
graphic data for 1−3 and 5 (PDF)
Accession Codes
CCDC 1876653−1876656 contain the supplementary crys-
tallographic 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
C
DOI: 10.1021/acs.organomet.8b00789
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
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(
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(18) (a) Aresta, M.; Quaranta, E. Synthesis, characterization and
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1
991, 30, 279−287. (b) Osman, R.; Pattison, D. I.; Perutz, R. N.;
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4
l
Bianchini, C.; Casares, J. A.; Peruzzini, M. Photochemistry of
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2
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(19) (a) Burkhart, G.; Hoberg, H. Oxanickelacyclopentene
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9) Mitchell, R. W.; Spencer, A.; Wilkinson, G. Carboxylato-
8
(
triphenylphosphine Complexes of Ruthenium, Cationic Triphenyl-
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D
DOI: 10.1021/acs.organomet.8b00789
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Organometallics
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kohlendioxid und ethylen sowie mono- oder di-substituierten alkenen.
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(28) The β-H elimination and interconversion between complexes 3
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Burkhart, G.; Kru
verknupfung zwischen kohlendioxid und alkinen sowie alkenen. J.
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(see Table S1 and Scheme S7 in the Supporting Information).
(29) For the formation of a five-membered nickelalactone or
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(30) For the formation of a cyclic π-allyl carboxylato ruthenium
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T.; Yamamoto, A. Preparation and Reactions of η -
(
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(31) For formation of an unsaturated five-membered ruthenalactone
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3
2
Komiya, S. Preferential Bond Activation of sp C−H over sp C−H in
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CO −Ethylene Coupling to Acrylates. Chem. - Eur. J. 2014, 20, 3205−
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211.
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21) (a) Bruckmeier, C.; Lehenmeier, M. W.; Reichardt, R.; Vagin,
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2
202. (b) Lee, S. Y. T.; Cokoja, M.; Drees, M.; Li, Y.; Mink, J.;
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A. A.; D’Elia, V.; Cokoja, M.; Herrmann, W. A.; Basset, J. M.; Kuhn,
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22) Jin, D.; Schmeier, T. J.; Williard, P. G.; Hazari, N.;
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Ethylene. Organometallics 2013, 32, 2152−2159.
(
23) Darensbourg and co-workers attempted the oxidative coupling
of ethylene and CO2 involving ruthenium carbonyls, but it was
unsuccessful: Li, B.; Kyran, S. J.; Yeung, A. D.; Bengali, A. A.;
Darensbourg, D. J. Acrylic Acid Derivatives of Group 8 Metal
Carbonyls: A Structural and Kinetic Study. Inorg. Chem. 2013, 52,
5
(
438−5447.
24) When the reaction was carried out at 100 °C, the ruthenium(0)
carbonyl complex bearing the tetradentate ligand was also formed as a
byproduct. Concerning reductive disproportionation of CO , see:
2
(
a) Allen, D. L.; Green, M. L. H.; Bandy, J. A. Reactions of Carbon
Dioxide with Electron-rich Trimethylphosphine Compounds of
Rhenium and Tungsten: Crystal Structure of [W(PMe ) H (CO )].
3
4
2
3
J. Chem. Soc., Dalton Trans. 1990, 541−549. (b) Allen, O. R.;
Dalgarno, S. J.; Field, L. D. Reductive Disproportionation of Carbon
Dioxide at an Iron(II) Center. Organometallics 2008, 27, 3328−3330.
(
25) The carbonato complex 4 could be synthesized alternatively
from RuCl (PP ) (6) with K CO in THF/H O (see the Supporting
2
3
2
3
2
Information). For similar carbonato ruthenium complexes, see:
Hartwig, J. F.; Bergman, R. G.; Andersen, R. A. Structure and
Reactions of Oxametallacyclobutanes and Oxametallacyclobutenes of
Ruthenium. Organometallics 1991, 10, 3344−3362.
(
26) For β-H elimination of five-membered ruthenacycles, see:
a) Fujiwhara, M.; Nishikawa, T.; Hori, Y. Ruthenium-Catalyzed
Regioselective Codimerization of Enol Acylates with 2-Substituted-
,3-butadienes. Org. Lett. 1999, 1, 1635−1637. (b) Hirano, M.;
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,5-Bis(methoxycarbonyl)ruthenacyclopentane by Oxidative Cou-
(
1
2
pling of Methyl Acrylate on Ruthenium(0) as an Active Intermediate
for Tail-to-Tail Selective Catalytic Dimerization. Organometallics
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E
DOI: 10.1021/acs.organomet.8b00789
Organometallics XXXX, XXX, XXX−XXX