Hydrolytic cleavage of both CS2 carbon-sulfur bonds by multinuclear Pd(II) complexes at room temperature

Article abstract of DOI:10.1038/NCHEM.2637

Developing homogeneous catalysts that convert CS₂ and COS pollutants into environmentally benign products is important for both fundamental catalytic research and applied environmental science. Here we report a series of air-stable dimeric Pd complexes that mediate the facile hydrolytic cleavage of both CS₂ carbon-sulfur bonds at 25 °C to produce CO2 and trimeric Pd complexes. Oxidation of the trimeric complexes with HNO3 regenerates the dimeric starting complexes with the release of SO2 and NO2. Isotopic labelling confirms that the carbon and oxygen atoms of CO2 originate from CS₂ and H2O, respectively, and reaction intermediates were observed by gas-phase and electrospray ionization mass spectrometry, as well as by Fourier transform infrared spectroscopy. We also propose a plausible mechanistic scenario based on the experimentally observed intermediates. The mechanism involves intramolecular attack by a nucleophilic Pd-OH moiety on the carbon atom of coordinated μ-OCS₂, which on deprotonation cleaves one C-S bond and simultaneously forms a C-O bond. Coupled C-S cleavage and CO2 release to yield [(bpy)3Pd3(μ3-S)2](NO3)2 (bpy, 2,2'-bipyridine) provides the thermodynamic driving force for the reaction.

Full text of DOI:10.1038/NCHEM.2637

ARTICLES
PUBLISHED ONLINE: 24 OCTOBER 2016 | DOI: 10.1038/NCHEM.2637
Hydrolytic cleavage of both CS2 carbon–sulfur
bonds by multinuclear Pd(II) complexes at
room temperature
Xuan-Feng Jiang1,2, Hui Huang2 , Yun-Feng Chai3, Tracy Lynn Lohr4, Shu-Yan Yu1 , Wenzhen Lai5 ,
*
*
*
Yuan-Jiang Pan3 , Massimiliano Delferro4 † and Tobin J. Marks4
*
*
*
Developing homogeneous catalysts that convert CS2 and COS pollutants into environmentally benign products is important
for both fundamental catalytic research and applied environmental science. Here we report a series of air-stable dimeric
Pd complexes that mediate the facile hydrolytic cleavage of both CS2 carbon–sulfur bonds at 25 °C to produce CO2 and
trimeric Pd complexes. Oxidation of the trimeric complexes with HNO3 regenerates the dimeric starting complexes with
the release of SO2 and NO2. Isotopic labelling confirms that the carbon and oxygen atoms of CO2 originate from CS2 and
H2O, respectively, and reaction intermediates were observed by gas-phase and electrospray ionization mass spectrometry,
as well as by Fourier transform infrared spectroscopy. We also propose a plausible mechanistic scenario based on the
experimentally observed intermediates. The mechanism involves intramolecular attack by a nucleophilic Pd–OH moiety on
the carbon atom of coordinated µ-OCS2, which on deprotonation cleaves one C–S bond and simultaneously forms a C–O bond.
Coupled C–S cleavage and CO2 release to yield [(bpy)3Pd3(µ3-S)2](NO3)2 (bpy, 2,2′-bipyridine) provides the thermodynamic
driving force for the reaction.
he cleavage of carbon–sulfur (C–S) bonds by transition metals factors, the very large bond-dissociation enthalpy of the remaining
is of great interest from both fundamental catalytic and C–S bond (171.5 kcal mol−1 for free CS)22,26. For these reasons,
T
environmental standpoints1,2. Hydrodesulfurization, the cata- well-understood approaches to efficient CS2 desulfurization are
lytic technology used to remove sulfur from hydrocarbons during highly desirable.
refining, is one of the two most important steps in petroleum
processing3. CS2 and COS are major chemical feedstocks, particularly
Results
for cellophane and viscose rayon production4,5, but when released
Reaction of CS2 with dimeric Pd complexes. Here we report a
into the environment form photochemical tropospheric sulfur
aerosols6,7 to yield fog, haze and acid rain, which are harmful to
human health and the environment8,9. Conventional CS2 abatement
technologies, such as adsorption10,11, thermal or catalytic com-
bustion12, and catalytic hydrolysis13 over supported metal oxides
(Al, Ti, Zr and Nb) typically require high temperatures (>250 °C)
(ref. 14) along with absorbent regeneration and challenging
utilization of the dithiocarbamate products14–16. Some microbial
extremophiles can convert CS2 into H2S and CO2 via the rapid
cleavage of both C–S bonds under mild conditions (70–88 °C);
however, the mechanistic details remain obscure and hence the
active sites are difficult to simulate17,18. At transition-metal
centres, CS2 is known to undergo a variety of reactions, including
insertion, dimerization and disproportionation19–21. Nevertheless,
catalytic cleavage of both C–S bonds by a homogeneous transition-
metal complex has, to the best of our knowledge, not been achieved.
Cleaving the first C–S bond of CS2 (ΔHdissoc = 94.0 kcal mol−1 for
free CS2)22 to produce LnM(S) + LnM(C–S) species (Ln, ancillary
ligand array) has been reported23; however, cleavage of the second
C–S bond is rare24,25, which presumably reflects, among other
synthetic and mechanistic investigation of the hydrolytic
desulfurization of CS2 mediated by the air-stable dimeric Pd
complexes, [(N^N)2Pd2(NO3)2](NO3)2 (N^N, 2,2′-bipyridine,
4,4′-dimethylbipyridine or 1,10-phenanthroline) at room
temperature, to generate CO2 and trinuclear [(N^N)3Pd3(µ3-S)2]
(NO3)2 clusters. Furthermore, the [(N^N)2Pd2(NO3)2](NO3)2
dimers are quantitatively regenerated by the addition of HNO3 to
yield a Pd-dimer-mediated aqueous cycle that converts CS2 and
HNO3 into CO2 + SO2 + NO2 (Fig. 1). Also, HNO3 can be
hydrolytically regenerated from NO2 in the Oswald process for
HNO3 production27, and the addition of Ba2+ and HNO3
precipitates BaSO4 to yield an overall cycle for CS2 conversion
into CO2 and the environmentally benign BaSO4. Although di-
and polynuclear Pd complexes are known to react
stoichiometrically with CS2, which involves CS2 insertion into, or
coordination to, the Pd–Pd bond27–31, the present results indicate
that dinuclear metal complexes provide a platform for hydrolytic
C–S bond cleavage that involves both C–S moieties and,
ultimately, a promising strategy for neutralizing CS2, COS and
other toxic organosulfur compounds.
1Beijing Key Laboratory for Green Catalysis and Separation, Laboratory for Self-Assembly Chemistry, Department of Chemistry and Chemical Industry,
College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China. 2College of Materials Science and
Opto-electronic Technology and Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049, China. 3Department of
Chemistry, Zhejiang University, Hangzhou 310027, China. 4Department of Chemistry and the Center for Surface Science and Catalysis, Northwestern
University, Evanston, Illinois 60208, USA. 5Department of Chemistry, Renmin University of China, Beijing 100872, China. †Present address: Chemical
Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, USA. e-mail: selfassembly@bjut.edu.cn; huihuang@ucas.ac.cn;
*
panyuanjiang@zju.edu.cn; delferro@anl.gov; t-marks@northwestern.edu
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2637
ARTICLES
N
N
N
N
+
2
1a–3a
1b–3b
a
–
2NO3
N
N
N
N
N
N
Pd1
ONO2
ONO2
Pd
Pd
3
Pd
N
N
6
O
O
O
O
1a
1c–3c
N
N
1a–1c
N
N
O
2a–2c
O
+ 4 H2O
2 CS2
SO2
NO2 H O
+
2
+
S
+
N
2
Excess HNO3
–
2 CO2
2NO3
+ 8 HNO3
N
N
N
Pd
Pd
Pd
2
N
S1
N
Pd1
S
Pd3
3a–3c
Pd2
S2
2.5
2.4
2.3
2.2
2.1
2.0
b
150
120
90
60
30
0
3a
c
Overall reaction:
2 CS2 + 24 HNO3
2 CO2 + 4 SO2 + 24 NO2 + 12 H2O
ΔHorxn = –248 kcal mol–1
0
200
400
600
800
1,000
Time (min)
Figure 1 | Converting CS2 into CO2 using dimeric palladium complexes in water. a, Two sequential steps show the hydrolysis of CS2 to generate CO2 in
the presence of aqueous 2a·2NO3–2c·2NO3 to form 3a·2NO3–3c·2NO3, and subsequent regeneration of 2a·2NO3–2c·2NO3 with HNO3 after the isolation
of 3a·2NO3–3c·2NO3. Attempts to make the reaction catalytic in HNO3(aq) were not successful. The X-ray core structures of 1a and 3a are shown as
ball-and-stick diagrams (hydrogen atoms (1a and 3a) and PF6 anion (3a) are omitted for clarity). Selected bond lengths (Å) and angles (°) for 1a, Pd1–O4,
2.040(3); Pd1–O1, 2.018(2); Pd1–N1, 1.987(3); Pd1–N2, 2.000(3); O4–Pd1–O1, 87.13(9); O4–Pd1–N2, 98.2(1); O1–Pd1–N1, 92.9(1); and for 3b, Pd1–S1, 2.2618(7);
Pd1–S2, 2.2825(7); Pd2–S1, 2.2912(8); Pd2–S2, 2.3045(9); Pd3–S1, 2.2898(8); Pd3–S2, 2.311(1); Pd1–S1–Pd, 90.72(2); Pd1–S1–Pd3, 89.80(2); Pd2–S1–Pd3,
77.05(2); P1–S2–Pd2, 98.86(2); Pd1–S2–Pd3, 88.75(2); Pd2–S2–Pd3, 76.37(2); S1–Pd2–S2, 76.06(2); S1–Pd1–S2, 77.07(2); S1–Pd3–S2, 75.95(2). b, CO2
concentration and pH values versus time for the reaction of 2a·2NO3 with CS2 and H2O at 22 °C. c, Reaction enthalpy for converting CS2 and HNO3 into CO2,
NO2, SO2 and H2O. rxn, reactions.
Experimental characterization of reaction mechanism. Regarding by 1H and 13C NMR spectroscopy, electrospray ionization mass
the reaction pathway, the control reaction CS2 + HNO3 (dilute or spectrometry (ESI–MS), elemental analysis and single-crystal
concentrated) at 22 °C in water does not produce significant X-ray diffraction. The 1H NMR spectrum exhibits downfield
amounts of CO2, COS, NO2 or SO2. Furthermore, a negative signals at δ = 8.95 ∼ 7.78 ppm assignable to coordinated bpy
mercury-drop assay32 argues that Pd nanoparticles play no role in ligands in a symmetrical environment, and the 13C NMR exhibits
the double C–S cleavage process (see Supplementary Information). five signals at δ = 155.47, 151.04, 140.76, 127.61 and 123.79 ppm,
Stirring an aqueous solution of [(bpy)2Pd2(NO3)2](NO3)2 which correspond to the five bpy ring carbon nuclei
(2a·2NO3; bpy, 2,2′-bipyridine) with excess CS2 at 22 °C for two hours (Supplementary Fig. 2). Furthermore, the formation of 3a2+ is
quantitatively yields the trinuclear cluster [(bpy)3Pd3(µ3-S)2]2+(NO3−)2 supported by the ESI–MS feature at the mass-to-charge ratio
(3a·2NO3), CO2 and HNO3. The PF6− salt of 3a is isolated in 94% (m/z) of 425.56 (Supplementary Fig. 3). The quasi-D3h 3a·2PF6
yield by adding a tenfold excess of KPF6 to aqueous 3a·2NO3 (see structure was confirmed by X-ray diffraction (Fig. 1a), and
Supplementary Information). Complex 3a·2PF6 was characterized features
a
triangular M3 core33. In the distorted trigonal
2
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2637
ARTICLES
a
100
75
50
25
0
324.0
+
2NO3
–
H2O
N
N
Pd
S
O
S
Pd
250
500
750
1,000
1,250
m/z
N
N
b
100
75
50
25
0
324.2
758.3
760.3
762.3
756.3
1,083.1
1,081.1
1,079.1
1,077.1
4
426.3
m/z = 758.9 (z = +1)
1,085.1
1,087.1
754.3
764.3
752.3
752 756 760 764
758.3
1,076 1,080 1,084 1,088
1,083.1
+
–
3NO3
H2O
N
N
250
500
750
1,000
1,250
m/z
Pd
N
Pd
S
N
c
100
75
50
25
0
O
S
425.9
Pd
N
N
10
m/z = 1083.9 (z = +1)
250
500
750
1,000
1,250
m/z
Figure 2 | Monitoring the reaction of CS2 with 0.05 M aqueous 2a·2NO3 by ESI–MS as a function of time. Both 4 and 10 are observed over the course of
the reaction. The m/z peaks at 324.0 and 425.9 correspond to [2a·NO3]+ and [3a]2+, respectively. a, t = 0.0 min. b, t = 8 min. c, t = 35 min.
bipyramidal Pd3S2 framework, all the Pd atoms are coordinated to a ion at m/z 45 that corresponds to 13CO2 is observed by MS for the
bpy ligand and bridged by two µ3-S2– ligands to form a Pd3 isosceles reaction of 2a·2NO3 with 13CS2 and H2O, which indicates the CO2 is
triangle with two long edges and one short edge. The Pd–Pd generated from CS2. Furthermore, 13C18O2 (m/z 49) is also observed
distances Pd1···Pd2, Pd2···Pd3 and Pd3···Pd1 are 3.2396(5), 2.8535(7) when 2a·2NO3 is reacted with 13CS2 and H218O, which confirms
and 3.2128(6) Å, respectively, which indicates relatively weak that the CO2 oxygen originates from the H2O. Consistent with the
metal–metal interactions. The angles Pd1–Pd2–Pd3, Pd2–Pd1–Pd3 stoichiometry in Fig. 1b, the reaction mixture pH continuously falls
and Pd1–Pd3–Pd2 are 63.27(1)°, 52.49(1)° and 64.24(1)°, respectively. as the reaction progresses, indicative of HNO3 generation.
Neighbouring bpy units stack in an almost parallel manner with
centroid···centoid distances of 4.62(8) and 4.48(2) Å for pyridyl of an aqueous 2a·2NO3 solution with CS2 was monitored by
pairs, which suggests weak intermolecular π···π interactions30,31,34–36
Fourier-transform infrared spectroscopy (FTIR), gas-phase MS
To probe further the mechanism of CS2 hydrolysis, the reaction
.
Regarding the transformation shown in Fig. 1c, the yield of CO2 and on-line ESI–MS. Three days after a methanolic CS2 solution
generated by the 2a·2NO3-mediated hydrolysis of the CS2 C–S was added to aqueous 2a·2NO3 it exhibited strong absorption fea-
bonds is 57% within five minutes (Supplementary Information). tures in the FTIR spectrum at 2,350 and 666 cm−1, which corre-
The aqueous CO2 concentration was monitored with a CO2-sensitive sponds to CO2 (refs 37,38), along with a strong CS2 mode at
electrode and, as shown in Fig. 1b, the CO2 concentration increases 1,530 cm−1 and signals at 2,070 and 2,050 cm−1 attributable to
as the reaction proceeds until aqueous/dissolved and gaseous CO2 COS (Supplementary Fig. 14)38, implicating COS as a reaction inter-
reach equilibrium in the closed vessel. A molecular peak at m/z 44 mediate or by-product37,38. After ten days, the disappearance of the
in the gas-phase MS supports CO2 generation. To confirm that COS and CS2 peaks is accompanied by growth of the CO2 signal
CS2 hydrolysis is the origin of the CO2, experiments were carried intensity, which suggests that COS is formed in the reaction, but
out with 13CS2 and H218O (Supplementary Figs 11–13). A molecular is then consumed by reaction with 2a·2NO3 to yield CO2.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2637
ARTICLES
a
CS2+H2O+2a
– COS
0.0
TS2
–14.9
7
–28.2
4
2
+
[Pd(bpy)]
–11.4
TS3
–45.7
–
TS1
NO3
6
H2O
–32.6
HNO3
8
– HNO3
–
–36.0
5
NO3
–44.2
–
9
NO3
–40.4
11
–53.4
12
–53.0
10
TS4
–52.4
–54.1
–59.8
TS5
–87.3
HNO3
13
–77.9
Pd
Pd
S
S
S
Pd
Pd
Pd
Pd
O
O
O
S
S
Pd
14
–96.0
S
S
O
Pd
Pd
Pd
Pd
Pd
Pd
Pd
O
O
O
– CO2
S
S
Pd
S
O
– CO2
RDS
3a
–127.2
13
14
3a
b
+
+
+
+
2
3
2
3
6
5
3a
(ESI–MS, X-ray)
4
(ESI–MS)
+
+
+
+
2
4
4
4
7
10
8
9
(ESI–MS)
Pd
S
+
+
+
+
3
3
2
2
C
O
N
H
12
13
11
14
Figure 3 | Proposed mechanism for the reaction of 2a2+ with CS2 from DFT calculations. a, DFT-computed pathway for the reaction of 2a2+ with CS2.
Computed relative free energies (kcal mol–1) are given in green. b, DFT optimized molecular structures for the compounds in a. Selected hydrogen atoms
removed for clarity.
The same reaction of an aqueous solution of 2a·2NO3 with CS2
Finally, clusters 3a·2NO3–3c·2NO3 were treated with concen-
was also investigated by gas-phase MS and on-line ESI–MS. The trated HNO3 to yield the corresponding complexes 1a–1c, SO2
former exhibits a minor ion at m/z 60, which confirms the presence and NO2. The latter gases were confirmed by strong absorption
of COS (Supplementary Fig. 11) as a reaction product. Analysis by peaks at 2,900 and 1,610 cm−1 in the FTIR analysis
ESI–MS before CS2 addition reveals an ion at m/z 324, corre- (Supplementary Fig. 25), and an MS ion peak at m/z 46 (ref. 35).
sponding to 2a·2NO3 (Fig. 2a). On CS2 addition, low-intensity In addition to a gas-phase MS signal at m/z 64, assignable to
features at m/z 1,083.9 and 758.9 were observed after eight minutes SO2, treating the solution with BaCl2 yields a white solid, identified
and are assignable to fragmentation ions (([(bpy)Pd]3(µ- as BaSO4 by FTIR and powder X-ray diffraction (Supplementary
OCS2)(H2O))(NO3)3)+ and (([(bpy)Pd]2(µ-HOCS2)(H2O))(NO3)2)+, Figs 26 and 27)28,29 confirming SO2 generation by HNO3
respectively (Fig. 2b). After 35 minutes these peaks are absent, oxidation of the Pd3S2 clusters. The structure of the mononuclear
whereas the signal of the final product ([(bpy)Pd]3(µ3-S))2+ is detected square-planar Pd complex 1a (Fig. 1a) was verified by single-crystal
at m/z 425.3 (Fig. 2c). Next, the independent reaction of COS in X-ray diffraction (Supplementary Fig. 30)39. Recrystallization of 1a
methanol with an aqueous 2a·2NO3 solution was monitored by from water regenerates the active dimer 2a, also verified by X-ray
on-line ESI–MS. An ion at m/z 742 is observed, assignable to the diffraction, and verifies that 1a and 2a are in equilibrium. The pos-
([(bpy)Pd]2(OCSOH)2(NO3))+ fragment of ([(bpy)Pd]3(OCSOH)2)(NO3)4 ition of the monomer–dimer equilibrium mainly depends on the
−
(Supplementary Fig. 1). Furthermore, the reaction of 2a with COS concentration of NO3 (the monomeric structure is favoured at
−
−
produces 3a·2NO3 and CO2, the identities of which were confirmed high NO3 concentrations and the dimeric structure at low NO3
by single-crystal X-ray diffraction and MS, respectively.
concentrations) and the pH values (the monomer is favoured
4
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2637
ARTICLES
in the strong acid solution). Regenerated monomeric complexes
Conclusions
1a–1c again mediate C–S bond cleavage in CS2 to form clusters It is shown here that air- and moisture-tolerant dimeric Pd2 complexes
3a·2NO3–3c·2NO3, presumably via dimeric 2a·2NO3–2c·2NO3. quantitatively cleave the C–S bonds of CS2 in aqueous solution to yield
Interestingly, both NO2 (m/z 46) and SO2 (m/z 64) are detected Pd3(µ3-S)2 clusters. The complexes can be regenerated fully with
throughout the reaction of aqueous 2a with CS2 (Supplementary strong oxidants at room temperature to complete the elements of a cat-
Figs 11 and 13), which indicates that the small amounts of alytic cycle. To our knowledge, this is the first process to cleave both
HNO3 formed throughout the reaction oxidize the Pd3S2 (3a) clus- C–S bonds under ambient conditions, and opens new possibilities
ters in situ, consistent with slow catalytic turnover.
for neutralizing CS2 and COS pollutants.
Received 23 February 2016; accepted 5 September 2016;
published online 24 October 2016
Computational analysis of CS2 cleavage by dimeric Pd complexes.
Based on the intermediates observed experimentally (see above),
density functional theory (DFT) calculations were performed to
probe the unusual hydrolysis mechanism of CS2 mediated by
2a·2NO3 (Fig. 3 and Supplementary Information). From evidence
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afford the trimetallic complex 3a2+ (Fig. 3). In the process of CS2
hydrolysis, COS can be formed as
a by-product from 6
via transition state TS2 with a barrier of +21.1 kcal mol–1. This
slightly higher barrier than that of the rate-limiting step from
13 to 14 is consistent with the minor amounts of COS observed.
Overall, the computed steps in the proposed mechanism (Fig. 3)
are found to be both kinetically and thermodynamically accessible
at 25 °C.
To explore the scope of dimeric Pd complexes competent for CS2
cleavage, Pd2 complexes [(dmbpy)Pd2](NO3)4 (2b·2NO3) (ref. 41)
and [(phen)Pd]2(NO3)4 (2c·2NO3) (ref. 42) (dmbpy, 4,4′-dimethylbi-
pyridine, phen, 1,10-phenanthroline) were also investigated for CS2
cleavage in aqueous solution. Significantly, the results of ESI–MS,
pH changes, [CO2](aq) assay and single-crystal X-ray diffraction
show that both C–S bonds of CS2 are cleaved by 2b·2NO3 and
2c·2NO3 at 25 °C to generate CO2 and the Pd3S2 clusters 3b·2NO3
and 3c·2NO3 (Supplementary Figs 15–20 and 31). Not surprisingly,
Pd intermediates similar to species 9 derived from 2a·2NO3 are
detected by on-line ESI–MS as signals at m/z 1,165.5 and 1,155.5,
which correspond to (([(dmbpy)Pd]3(µ-OCS2)(H2O))(NO3)3)+
(Supplementary Fig. 15) and (([(phen)Pd]3(µ-OCS2)(H2O))(NO3)3)+
(Supplementary Fig. 16), respectively. These are observed in the
reactions that involve 2b·2NO3 and 2c·2NO3, and this argues that
Pd complexes with different ancillary ligands operate via a mechanism
similar to that of 2a·2NO3.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2637
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Acknowledgements
This research was supported by National Natural Science Foundation of China (Grant
numbers 21471011, 21574135, 91127039, 51303180, 21203245, 21532005, 21327010), the
Beijing Natural Science Foundation (Grant 2162043), the One Hundred Talents Program,
the Chinese Academy of Sciences, the Importation and Development of High-Caliber
Talents Project of the Beijing Municipal Institution and Beijing Postdoctoral Research
Foundation (Grant 2015M570017) and the Beijing Synchrotron Radiation Facility for
crystal structure determination using synchrotron radiation X-ray diffraction analysis. We
also thank H. Ma (Beijing Institute of Technology) for the X-ray crystallography. Research
at Northwestern University is supported by the US National Science Foundation under
grant CHE-1464488. The Northwestern CleanCat Core Facilityacknowledges funding from
the US Department of Energy (Grant DE-FG02-03ER15457) used for the purchase of the
portable universal gas analyser. We thank N. M. Schweitzer for supporting the
mass spectroscopy.
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Author contributions
S.-Y.Y. and H.H. conceived the study. H.H., S.-Y.Y., M.D., T.L.L. and T.J. M. planned the
research. X.-F.J., H.H. and T.L.L. synthesized and characterized the compounds. Y.-J.P. and
Y.-F.C conducted the on-line ESI experiments and analysis. W.L. conducted the DFT
calculations. H.H., X.-F.J., T.L.L., S.-Y.Y., W.L., M.D. and T.J.M. prepared the manuscript.
All the authors commented on the manuscript.
Additional information
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Supplementary information and chemical compound information are available in the
online version of the paper. Reprints and permissions information is available online at
www.nature.com/reprints. Correspondence and requests for materials should be addressed to
H.H., S.-Y.Y., W.L., Y.-J.P., M.D. and T.J.M.
39. Adrian, R. A., Zhu, S., Klausmeyer, K. K. & Walmsley, J. A. Synthesis and X-ray
crystal structure determination of monomeric cis-Pd(2,2′-bipyridine)(NO3)2.
Inorg. Chem. Commun. 10, 1527–1530 (2007).
Competing financial interests
The authors declare no competing financial interests.
6
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