CO and CO2 fixation by Se-Ru-CO hydride clusters

Article abstract of DOI:10.1021/ic500821z

The selective insertion of CO and CO₂ into the C-O and O-H bonds of alcohols by the Se-Ru-CO hydride clusters [(μ-H)Ru₄(CO) ₁₀Se₂]^- (1) and [(μ₃-H)Ru ₅(CO)₁₄Se]^- (2) was demonstrated by a cooperative effect of the protonic hydride, the electron-rich Ru atom, and the electronegative Se atom as well as the symmetry of the clusters. These reactions generated the first examples of Se-containing ruthenium carboxylate and alkylcarbonate clusters [{(μ-H)Ru₄(CO)₁₀Se ₂}₂{Ru₂(CO)₄(μ- η¹: η¹-OOCR)}]^3- (R = Me, 3; Et, 4) and [{(μ-H)Ru₄(CO)₁₀Se₂}₂{Ru ₂(CO)₄(μ- η¹: η¹-OOCOR)}] ^3- (R = Me, 5; Et, 6), respectively. These results disclosed herein provide a new avenue for the capture and storage of CO and CO₂ and useful synthetic routes to novel RCOO^-- and ROCOO^--bridged ruthenium selenide clusters.

Full text of DOI:10.1021/ic500821z

Communication
pubs.acs.org/IC
CO and CO2 Fixation by Se−Ru−CO Hydride Clusters
Minghuey Shieh,* Yen-Yi Chu, Li-Fing Jang, and Chia-Hua Ho
Department of Chemistry, National Taiwan Normal University (NTNU), Taipei 11677, Taiwan, Republic of China
S
* Supporting Information
unexplored.11 Besides, the cluster-like RuxSey nanoparticles have
ABSTRACT: The selective insertion of CO and CO2 into
the C−O and O−H bonds of alcohols by the Se−Ru−CO
hydride clusters [(μ-H)Ru4(CO)10Se2]− (1) and [(μ3-
H)Ru5(CO)14Se]− (2) was demonstrated by a cooperative
effect of the protonic hydride, the electron-rich Ru atom,
and the electronegative Se atom as well as the symmetry of
the clusters. These reactions generated the first examples
of Se-containing ruthenium carboxylate and alkylcarbonate
clusters [{(μ-H)Ru4(CO)10Se2}2{Ru2(CO)4(μ-η1:η1-
OOCR)}]3− (R = Me, 3; Et, 4) and [{(μ-H)-
Ru4(CO)10Se2}2{Ru2(CO)4(μ-η1:η1-OOCOR)}]3− (R =
Me, 5; Et, 6), respectively. These results disclosed herein
provide a new avenue for the capture and storage of CO
and CO2 and useful synthetic routes to novel RCOO−-
and ROCOO−-bridged ruthenium selenide clusters.
been known as efficient cathode materials in the direct MeOH
fuel and exhibit higher electrocatalytic activities than RuxSy and
RuxTey.12 Prompted by these, we have synthesized two Se−Ru−
CO hydride octahedral clusters, [(μ-H)Ru4(CO)10Se2]− (1) and
[(μ3-H)Ru5(CO)14Se]− (2), which were found to exhibit
surprising affinity toward CO and CO2 in ROH (R = Me, Et)
t o f o r m t h e a c t i v a t i o n p r o d u c t s [ { ( μ - H ) -
Ru4(CO)10Se2}2{Ru2(CO)4(μ-η1:η1-OOCR)}]3− (R = Me, 3;
Et, 4) and [{(μ-H)Ru4(CO)10Se2}2{Ru2(CO)4(μ-η1:η1-
OOCOR)}]3− (R = Me, 5; Et, 6), respectively. The present
study demonstrated the unprecedented selective insertion of CO
and CO2 into the C−O and O−H bonds of ROH by two
protonic hydride clusters and formation of the first examples of
carboxylato and alkylcarbonato Se−Ru complexes.
When K2SeO3 was treated with Ru3(CO)12 in refluxing
MeOH, cluster 1 was obtained in good yield. X-ray analysis
showed that cluster 1 consisted of an octahedral Ru4Se2
geometry with a hydride across one of the Ru−Ru bonds (see
the Supporting Information (SI), Figure S1a). The resonance for
the hydride of 1, δ = −4.48 ppm, was substantially shifted
downfield compared with those of the related octahedral
ruthenium hydride clusters,13 which implied the acidic character
of the hydride. This acidity was also confirmed by the treatment
of 1 with NaH, forming the deprotonated product
[Ru4(CO)10Se2]2− (7; see the SI, Figure S2) with the release
ixations of CO and CO2 have recently attracted an extensive
F
amount of attention mainly because they are key or
potentially useful C1 feedstocks for the production of valuable
C-containing molecules.1−3 Along these lines, the utilization of
transition-metal complexes bearing appropriate ligands to
facilitate CO and CO2 activation has become an increasingly
desirable target. Transition-metal hydrides are known to exhibit
significant activity for the reduction of CO and CO2.3a−d,f,4,5 In
contrast with most metal hydride complexes coordinated by
electron-donating ligands, hydride complexes equipped with π-
accepting ligands such as CO are known to have hydrides with
enhanced acidity.6 The function of the acidity of metal hydrides
with regard to CO and CO2 activation has become intriguing in
light of limited studies. The most noted example is found in
HCo(CO)4, which is catalytically active for carbonylation of
methanol (MeOH) to acetic acid under high pressure and
temperature.7 On the other hand, CO2 activation by protonic
metal hydride complexes has rarely been observed because of the
formation of unstable metallocarboxylic acid species.8 Addition-
ally, carboxylation of MeOH by metal carbonyl complexes has
only been reported in the case of W(CO)(N2)(dppe)2 (dppe =
Ph2PCH2CH2PPh2) to form the hydridomethylcarbonato
complex WH(η1-OCOOMe)(CO)(dppe)2.9 To date, no
examples of the insertion of CO2 into alcohols by protonic
hydride metal carbonyl complexes have been demonstrated.
Apart from the CO ligand, the electronegative main-group
elements could also fine-tune the electronic properties of metal
hydride complexes and thereby exert an effect on their acidity
and reactivity patterns.10 While chalcogen-containing metal
carbonyl hydrides have been widely reported, the cooperative
effect of the hydride and chalcogen elements, as well as the
transition metal for CO and CO2 activation, has remained
1
of H2 (4.60 ppm in the H NMR spectrum). It was of great
interest that when [Et4N][1] was mixed with Ru3(CO)12 in the
presence of Et4NBr/NaBr and heated under an atmosphere of
CO in MeOH/MeCN solutions at specifically 70 °C, the novel
carboxylate-bridged di-HRu4Se2 cluster [Et4N]3[{(μ-H)-
Ru4(CO)10Se2}2{Ru2(CO)4(μ-η1:η1-OOCMe)}] ([Et4N]3[3])
was formed in 68% yield (Scheme 1). Complex 3 was obtained in
trace amounts in the absence of a CO atmosphere, indicating the
capture of CO in this reaction.
X-ray analysis showed that 3 consisted of two 1 clusters linked
by a Ru2(CO)4 fragment that was further bridged by a MeCOO−
1
group (Figure 1). The H NMR spectrum of 3 gave a single
hydride resonance at δ = −12.12 ppm, which was shifted upfield
compared with that for cluster 1 owing to the charge effect. Its IR
spectrum also showed a diagnostic band at 1551 cm−1, which was
attributable to the νasym(COO) mode of the carboxylato bridge,
and a weaker band at 1395 cm−1, which was due to νsym(COO).
To gain insight into the generation of the key fragment
MeCOO− in 3, a MeOH solution of Ru3(CO)12 was placed
under an atmosphere of CO and refluxed under controlled
reaction conditions followed by the addition of 1. However, this
Received: April 8, 2014
Published: April 17, 2014
© 2014 American Chemical Society
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Communication
1, the HRu5Se cluster 2 was inert toward CO. Surprisingly,
despite the thermodynamic stability of CO2, we found that when
[PPh4][2] in the MeOH solution was bubbled with CO2 at 80 °C
in the presence of PPh4Br, the methylcarbonate cluster
[PPh4]3[{(μ-H)Ru4(CO)10Se2}2{Ru2(CO)4(μ-η1:η1-OO-
COMe)}] ([PPh4]3[5]) was formed as a green solid in 76% yield
(Scheme 1). This reaction was significantly influenced by the
choice of the countercation, for which [PPh4]+ salt proved to be
the best because of its better reactivity and solubility in MeOH.
The IR spectrum showed that cluster 5 had a CO absorption
pattern similar to those of 3 and 4 but with different bridging
MeOCOO− stretching bands at 1580 and 1438 cm−1, indicating
that cluster 5 exhibited a core geometry similar to that of clusters
3 and 4. X-ray analysis (Figure 2) and 1H NMR (δ = −12.13 ppm
Scheme 1. Formation of CO- and CO2-Inserted Clusters 3−6
Figure 1. ORTEP of anion 3 at 30% probability.
Figure 2. ORTEP of anion 5 at 30% probability.
reaction failed to yield the activation product and resulted in the
recovery of 1. Thus, formation of the carboxylate “MeCOO−” in
cluster 3 was presumed to occur via protonation of MeOH by the
for the hydrides) further confirmed that cluster 5 possessed two
“HRu4(CO)10Se2” octahedral cores linked by a MeOCOO−-
bridged Ru2(CO)4 moiety. According to the Cambridge
Crystallographic Data Centre, there were very few examples of
polynuclear complexes coordinated with monoalkylcarbonate.16
Cluster 5 represents the first structurally characterized cluster
equipped with a MeOCOO− ligand in the μ-η1:η1-bonding
mode. Notably, the reaction between [PPh4][2] and CO2 is also
sensitive to the substituent of the alcohols. With EtOH, the
reaction proceeded similarly to afford the analogous ethyl-
carbonate cluster [PPh4]3[{(μ-H)Ru4(CO)10Se2}2{Ru2(CO)4-
(μ-η1:η1-OOCOEt)}] (PPh4]3[6]) according to elemental
analysis as well as IR and 1H NMR spectroscopic methods.
Formation of the bridging alkylcarbonate ROCOO− groups in
5 and 6 could be considered as a result of the uptake of CO2 by 2
accompanied by the nucleophilic attack of ROH onto the C atom
of CO2 (vide infra). These reactive alkylcarbonates, which were
trapped by 5 and 6, may serve as useful materials for the
preparation of symmetrical and unsymmetrical dialkyl carbo-
nates, which are important precursors for pharmaceuticals,
agrochemicals, and lubricants.2,17
DFT calculations were performed in order to elucidate the
carbonylation and carboxylation of alcohols by clusters 1 and 2 to
form clusters 3−6. Analysis showed that the lowest unoccupied
molecular orbital of 1 (see the SI, Figure S4a) received a
significant contribution from the d orbitals of the Ru atoms.
Hence, we postulated that two molecules of 1 readily underwent
a Ru−Ru edge addition of the reactive “[Ru2(CO)4(μ-η1:η1-
OOCR)]−” (R = Me, Et) derived from Ru3(CO)12 with
RCOO−,18 which was produced from CO insertion into the
C−O bond of ROH, which was induced by the five-membered
intermolecular interaction of ROH with the protonic hydride of
1 (natural charge 0.32+; see the SI, Figure S5a) and the lone-pair
electrons of the Se atom19 (see the SI, Figure S6a). This led to the
formation of trianionic clusters 3 and 4 (R = Me, 3; Et, 4). On the
other hand, the highest occupied molecular orbital of 2 had a
major contribution from the d orbitals of the apical Ru atom (see
the SI, Figure S4b). In addition, natural population analysis20
+
protonic hydride of 1 to form a reactive “MeOH2 ” species with
the assistance of Br− and a subsequent CO insertion [discussed
later with density functional theory (DFT) calculations], similar
to HI-promoted MeOH carbonylation.14 This hypothesis was
also related to MeOH protonation by the acidic complex
+
HCo(CO)4 to form MeOH2 , which was potentially followed by
carbonylation.15 The speculation concerning a halide-involved
mechanism in our reaction was further supported by the fact that
the yield of 3 was significantly increased by the addition of NaBr
salts in the course of the reactions, which was supposed to
stabilize the intermediate “Me+” (from MeOH2 ). It was noted
+
that the aprotic polar solvent MeCN significantly facilitated these
reactions because of the increased acidity of 1. The same
reactivity pattern was also observed in the reaction of [Et4N][1]
with Ru3(CO)12/Et4NBr/NaBr under an atmosphere of CO in
EtOH/MeCN solutions at 80 °C, affording the EtCOO−-
bridged cluster [Et4N]3[{(μ-H)Ru4(CO)10Se2}2{Ru2(CO)4(μ-
η1:η1-OOCEt)}] ([Et4N]3[4]) in 55% yield (Scheme 1),
confirming that CO inserted into the C−O bond of ROH.
Cluster 4 was isomorphous with 3 on the basis of X-ray
crystallography (see the SI, Figure S3) and spectroscopic
methods. These results of the formation of RCOO−-bridged
clusters 3 and 4 motivated us to evaluate the catalytic activity of
cluster 1 toward CO in MeOH. In a preliminary study, cluster 1
with NaI was treated with an atmosphere of CO and refluxed in
MeOH/CD3CN. 1H NMR analysis revealed that 6% of MeOH
was converted to acetic acid with turnover number 6.44,
indicating that carbonylation of MeOH did occur, although the
efficiency was significantly lower than that of the known group 9
systems.14 Further studies are needed to improve the catalytic
performances of 1.
On the other hand, if cluster 1 was treated with excess
Ru3(CO)12 in superheated MeOH solutions under a N2
atmosphere, a Ru3-capped hydrido octahedral cluster 2 (see
the SI, Figure S1b) was obtained (Scheme 1). The hydride of 2
could be abstracted by NaH, but contrary to the HRu4Se2 cluster
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Inorganic Chemistry
Communication
2007, 107, 2365−2387. (f) Cokoja, M.; Bruckmeier, C.; Rieger, B.;
showed that the hydride of 2 carried a positive charge of 0.37+
and that the apical Ru atom possessed a negative charge of 0.72−
(see the SI, Figure S5b). The space-filling model also revealed
that the hydride-capped Ru3 plane of 2 was less hindered and
therefore susceptible to the incoming CO2 (see the SI, Figure
S7). Because ROH cannot be deprotonated by cluster 2, it was
reasonable to postulate that the Ruapical−H bond of 2 might serve
as a kind of “Lewis pair” that would polarize the incoming CO2
molecule first5d,21 (see the SI, Figure S6b), and then the
electrophilic C of CO2 would be attacked by ROH, resulting in
the formation of a ROCOO− moiety accompanied by the
breakage of Ru−Ru bonds to release the Ru(CO)x fragments,
followed by the combination of resultant metal fragments to give
rise to clusters 5 and 6 (R = Me, 5; Et, 6). These results indicated
that the asymmetric cluster 2 plays a key role in the pronounced
affinity toward CO2, in contrast with 1, which has a lower
molecular polarizability.
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ASSOCIATED CONTENT
* Supporting Information
Experimental and computational details. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
(12) (a) Grothausmann, R.; Zehl, G.; Manke, I.; Fiechter, S.;
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AUTHOR INFORMATION
Corresponding Author
*E-mail: mshieh@ntnu.edu.tw.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Ministry of Science and
Technology of Taiwan (Grant 101-2113-M-003-005-MY3 to
M.S.). We are also grateful to the National Center for High-
Performance Computing, which provided the Gaussian package
and computer time. Our gratitude also goes to the Academic
Paper Editing Clinic, NTNU.
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