Crystallographic identification of an unusual homoleptic palladium citrate [Na(OH2)6]·{[Na3(OH2) 8]3[NaPd3(C6H4O 7)3]2}·(H2O) stabilised by intermetallic aggregation with sodium and heavy hydration

Article abstract of DOI:10.1039/c0dt00273a

A rare precious metal binary citrate represented by palladium in the form of a water-rich Na-Pd intermetallic aggregate has been isolated and crystallographically characterised. It shows a penta-anionic [NaPd ₃(citrate)₃]^5- core balanced by [Na ₃]³⁺ and Na⁺ hydrates in a 3-D coordination network stabilised exclusively by citrate bridges and fortified by extensive H-bonding.

Full text of DOI:10.1039/c0dt00273a

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Crystallographic identification of an unusual homoleptic palladium citrate
Na(OH ) ]·{[Na (OH ) ] [NaPd (C H O ) ] }·(H O) stabilised by
[
2
6
3
2 8 3
3
6
4
7 3 2
2
intermetallic aggregation with sodium and heavy hydration†
a
a
a
a,b
Parag Gunari, Suresh s/o Krishnasamy, Shi-Qiang Bai and T. S. Andy Hor*
Received 7th April 2010, Accepted 11th August 2010
DOI: 10.1039/c0dt00273a
A rare precious metal binary citrate represented by palladium
in the form of a water-rich Na–Pd intermetallic aggregate has
been isolated and crystallographically characterised. It shows
PdCl
and NaHCO
yield of a water-rich intermetallic ionic complex [Na
1 (Scheme S1, ESI†) which is sufficiently stable to be isolated
as a solid at r.t. and remains stable for about 5 days. Its
single-crystal X-ray crystallographic analysis revealed a 3-D ionic
intermetallic complex network heavily fortified by hydrate viz.
2
slowly dissolves in an aqueous mixture of citric acid
in a 1 : 2 : 6 molar ratio to give near-quantitative
Pd(citrate)]
3
2
n
5-
3+
a penta-anionic [NaPd
3
(citrate)
3
]
core balanced by [Na
3
]
+
and Na hydrates in a 3-D coordination network stabilised
exclusively by citrate bridges and fortified by extensive H-
bonding.
[
Na(OH
2
)
6
]·{[Na
3
(OH
2
)
8
]
3
[NaPd
3
(C
6
H
4
O
7
)
3
]
2
}·(H
2
O).‡ Its asym-
metric unit (formula PdNa
2
C
6
H
43/3
O73/6) comprises one Pd, one
Citric acid (C
6
H
8
O
7
) is a natural and abundant a-hydroxy
1
citrate, four Na moieties (with occupancies 1/3, 1/2, 1 and 1/6 for
Na1, Na2, Na3 and Na4, respectively), five coordinated hydrate,
and one lattice hydrate with 1/6 occupancy.
Complete deprotonation of tetra-basic citric acid gives a
citrate with maximum coordinative functions in the form of tri-
carboxylates and alkoxide. They bring together three discrete
tricarboxylic acid. It binds to a range of metals, notably those
that are biologically active. Its coordination behavior towards
2
the precious metals is much less developed, probably because
3
the complexes formed are mostly unstable. For example, pure
gold citrates are difficult to be isolated. Ironically, the ready
decomposition of these precious metal citrates enables them to
be widely used as molecular precursor to nano-metal or its
entities, namely, C
3
symmetric NaPd
3
cubane, linear Na cation
3
+
4
aggregate, and mononuclear hydrated Na cation ([Na(OH
(Fig. 1 and S1, ESI†). The entire intermetallic network is
supported by oxygen donors from the citrate and water. The
2
) ] )
6
composites. Much of the attention in the citrates of platinum
5
metals is focused on their nano applications whereas very little
is known on the coordination behavior and structural chemistry.
6
We are interested in structurally defined metal citrates of metals
that are normally considered “incompatible” with citrates, such
as gold and palladium. The stabilisation, isolation and structural
understanding of their molecular networks enable better design
7
of molecular substrates as precursors to materials. The use of
binary citrate complexes also provides an avenue for water-based
8
catalysis in lieu of the water amenability of citrate. Its presence
in the core of a catalytically active metal such as palladium
provides an ideal opportunity for such investigation. In this
communication, we highlight the self-assembly of an intermetallic
three-dimensional (3-D) supramolecular Na/Pd citrate of high
symmetry that is water soluble and catalytically active. Amidst
9
a large variety of structurally established metal citrates, this
is one of the rare examples of precious metal citrates that are
crystallographically identified. It offers a structural insight on
1
0
2+
citrate serving as a “capping agent” in the conversion of Pd to
11
palladium nanocrystals. We reveal herein how citrate can bring to
+
2+
proximity two inherently different metals (Na and Pd ) within a
molecular network from which one could explain the stabilisation
of a normally unstable precious metal binary citrate.
a
Department of Chemistry, National University of Singapore, 3 Science Drive
3
, Singapore 117543. E-mail: andyhor@nus.edu.sg; Fax: (+)65-6873-1324
b
Institute of Materials Research and Engineering, Agency for Science,
Technology and Research, 3 Research Link, Singapore 117602
†
Electronic supplementary information (ESI) available: Fig. S1, experi-
2
Fig. 1 (a) Na-capped trinuclear palladium core of 1 omitting lattice H O
mental and X-ray crystallography. CCDC reference number 692934. For
ESI and crystallographic data in CIF or other electronic format see DOI:
+
3+
and [Na(OH
2
)
6
] for clarity. (b) Linear [Na]
3
chain connecting the NaPd
3
1
0.1039/c0dt00273a
nodes in the 3-D network of 1. (Pd: green, Na: blue, O: red, H: cyan)
9
462 | Dalton Trans., 2010, 39, 9462–9464
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Fig. 2 XPS spectrum of a mixture of Pd and PdO formed from decomposition of 1.
heterometals (Na/Pd) are bridged exclusively by the citrate. The
two bridging alkoxo (O7) and two carboxylate (O1 and O5) oxygen
donors form the coordination sphere of square planar Pd(II)
(Fig. 2). This substantiates the important role of H-bonding in this
hydrate-saturated aggregate. Further experiments are under way
to examine the metallic palladium distributions under different
solvent or cation conditions.
(
Fig. 1). The [NaPd
oxygen atoms (O7). The distorted octahedral Na (Na1) on the
axis is supported by six O donors from citrates (three O3 and
three O7). The [NaPd ] frame thus constitutes a cubane with
a missing vertex (“defective cubane”). The central symmetric Na
3
] core is held together by three capping alkoxo
+
The stability and isolability of the titled homoleptic intermetallic
citrate is unusual but can be understood from the structural
revelations. The absence of any b-hydride in a citrate is notable
since Pd(II) alkoxyl complexes are prone to such elimination and
C
3
3
O
3
3
14
chain comprises a distorted octahedral Na2 sandwiched by two
seven-coordinated Na3 with doubly-bridging aqua ligands and
co-bridged by m-O atom (O4) of the carboxylate. The external
decomposition.
15
There is also evidence of weak anagostic interaction between
˚
the g hydrogen of one of the CH
2
groups (H ◊ ◊ ◊ Pd 2.80 A;
◦
Na3 atoms are connected to the [NaPd
carboxylate groups (O5–O6). Effectively, this Na
as a central unit to connect to all the four NaPd
3
(C
6
H
4
O
7
)
3
] cores via the
entity serves
aggregates.
∠C3–H3B ◊ ◊ ◊ Pd 117 with downfield d
H
shift of 0.72 ppm) with
3
the metal (Fig. 1(a)). This sufficiently perturbs and distinguishes
1
3
the CH protons ( H-NMR). Although b-hydride elimination is
2
(
Fig. 1(b)) Each nano-sized [NaPd
different Na blocks to give the present 3-D coordination network
Fig. 1 and Fig. S1, ESI†). In addition, the octahedral [Na(OH
is sandwiched by two C symmetric cavities in this network with
3
] node is in turn linked to six
commonly found in transition metal alkyls in the hydrocarbon
3
reforming process, nickel has shown a preference for a-hydride
+
16
(
2
)
6
]
elimination whereas platinum also yields g-hydride elimination.
3
Such presence of anagostic g-hydride interaction may explain our
insofar futile attempts to prepare the stable Pt(II) analogue of 1.
six intermolecular H-bonding interactions (O1S–H ◊ ◊ ◊ O5S 2.83
˚
3+
+
A) between the bridging aqua (O1S) of [Na
3
]
and coordinated
The presence of oxophilic Na provides an enthalpic drive for the
+
hydrate (O5S) of [Na(OH
found in the malate (mal)-stabilised Na
which is stabilised by malate. ESI-MS analysis reveals the
2
)
6
] (Fig. S1, ESI†). A related system is
aggregate formation, which is further assisted by the extensive H-
bond formation within the network. Citrate, as a skeletally flexible
multidendate oxygen donor and a ligand that is well suited for
multidirectional H-bonding, is an ideal ligand as it meets all the
structural and bonding demands. Without a formal Pd ◊ ◊ ◊ Pd (3.23
12a
3
[Cu
3
(mal)
3
(H
2
O)]·8H
2
O
+
palladium citrate species ([Pd
2
Na(citrate)(H
similar iron-citrate species has been
Formation of this complex network illustrates
symmetric molecules
for supramolecular recognition, new materials, and asymmetric
2
O)
4
]
(m/z 495,
8
5%)) in water. A
12b,c
˚
˚
reported.
A) or Pd ◊ ◊ ◊ Na (3.40 A) bond, this coordination network of 1 is
the challenges in the assembly of C
3
best treated as a metal aggregate, not cluster, in which the major
molecular entities are held together by citrates (in NaPd
or in conjunction with hydrate (in Na blocks). Taking advantage
of the water-rich nature of the intermetallic complex and it being a
3
nodes)
13
catalysis.
3
The citrate ligand supplies the oxygen donors for both Na(I)
and Pd(II) and the bridging framework to connect the inherently
different metals. It also attracts hydration and provides the media
for extensive H-bonding in the network. These enable the rare
isolation of 1 among a large number of unstable precious metal
citrates. The sodium ion provides a site for hydration and fortifies
the network through inter-ionic H-bonding. It also plays a unique
2
+
ready source of active Pd , we have examined its Suzuki coupling
activity of phenylboronic acid at r.t. with a selected range of
aryl bromides and chlorides in aqueous-methanolic media [H O–
2
MeOH, 5 : 1] and compare the alkali metal effect [M/Pd, M = Na
(1a), K (1b), Cs (1c), Li (1d)] (Table 1). The catalyst is conveniently
generated in situ from PdCl
2
, citric acid and MHCO
3
or M
2
CO .
3
role to bridge, connect and stabilise the Pd
3
core intra- and inter-
The yields for bromides (50–93%) are understandably higher than
those of chloride (20–67%). The highest activities are observed
for M = Na, which also gives the most stable intermetallic citrate.
The poorest performance is found in M = Li when the complex
is also the least stable. Other catalytic possibilities are currently
being examined. Preliminary data suggest that 1 is active towards
molecularly. This unprecedented form of sodium ion arrangement
within the palladium network offers a mechanism to stabilise the
usually unstable precious metal homoleptic citrates. Accordingly,
+
+
+
+
when Na is replaced by other alkaline metals (Li , K , Cs ) in
the synthesis, the orange solids isolated are significantly less stable
with rapid decomposition. Such decomposition in the solid state
or in water is not apparent in 1. However, in ethanol or upon
sonication, the Na/Pd aggregate readily decomposes to give a
solid comprises mainly PdO and Pd, as verified by the XPS analysis
hydroamination of 1,3-cyclohexadiene with 4-nitroaniline in H
THF (5 : 1) at 40 C but with poor yield (27%).
2
O–
◦
This work offers a molecular level understanding of how citrate
can bind to palladium and prevent it from oxidative etching
This journal is © The Royal Society of Chemistry 2010
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3
Table 1 Suzuki coupling results of phenylboronic acid with selected aryl
halides catalysed by an in situ mixture of PdCl , citric acid and MHCO
M = Na (1a) and K (1b)) or M CO (M = Cs (1c) and Li (1d))
0.14 ¥ 0.08 mm ; independent reflections: 2452 [R = 0.0409]; reflections
int
2
3
collected: 11 706; GOF = 1.302; R [I > 2s(I)] = 0.0579, wR (all data) =
0.1123. CCDC number: 692934.
1
2
a
(
2
3
1
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Jin, N. Zhu, X. Hou, F. Deng and H. Sun, J. Am. Chem. Soc., 2003, 125,
Entry
Ar–X (X = Br or Cl)
Catalyst
Isolated yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
4-Bromotoluene
1a
1b
1c
1d
1a
1b
1c
1d
1a
1b
1c
1d
1a
1b
1c
1d
1a
1b
1c
1d
93
75
75
75
92
87
66
51
80
70
63
50
88
76
67
60
67
40
34
20
1
2408; (d) D. W. Hartley, G. Smith, D. S. Sagatys and C. H. L. Kennard,
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9
17; (f) T. L. Feng, P. L. Gurian, M. D. Healy and A. R. Barron, Inorg.
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4-Bromonitrobenzene
4-Bromoanisole
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3
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Wang, Inorg. Chem. Commun., 2005, 8, 274; (d) T. A. Hudson, K. J.
Berry, B. Moubaraki, K. S. Murray and R. Robson, Inorg. Chem., 2006,
0
1
2
3
4
5
6
7
8
9
0
4
5, 3549.
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Fulton, A. W. Holland, D. J. Fox and R. G. Bergman, Acc. Chem. Res.,
4-Chlorobenzonitrile
2
002, 35, 44; (c) J.-E. B a¨ ckvall, E. E. Bj o¨ rkman, L. Pettersson and P.
Siegbahn, J. Am. Chem. Soc., 1984, 106, 4369; (d) J.-E. B a¨ ckvall, E. E.
Bj o¨ rkman, L. Pettersson and P. Siegbahn, J. Am. Chem. Soc., 1985, 107,
7265; (e) H. E. Bryndza, J. C. Calabrese, M. Marsi, D. C. Roe, W. Tam
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a
Reaction conditions: phenylboronic acid (1.2 mmol); aryl halide
1 mmol); H O–MeOH 5 : 1 (6 mL); PdCl (0.03 mmol), citric acid
0.06 mmol) and MHCO or M CO (7 mmol); r.t. for 24 h.
(
(
2
2
3
2
3
1
0538; (i) G. M. Kapteijn, D. M. Grove, H. Kooijman, W. J. J. Smeets,
A. L. Spek and G. van Koten, Inorg. Chem., 1996, 35, 526; (j) M. A.
and eventually allow the formation of nano-materials such as
palladium icosahedra. It demonstrates the potential of citrate
17
Jalil, T. Nagai, T. Murahashi and H. Kurosawa, Organometallics, 2002,
2
1, 3317.
2
+
to support water-based catalysis of Pd . The isolated anionic
4
5
(a) B.-K. Pong, H. I. Elim, J.-X. Chong, W. Ji, B. L. Trout and J.-Y.
Lee, J. Phys. Chem. C, 2007, 111, 6281; (b) R. Zhang and X. Wang,
Chem. Mater., 2007, 19, 976; (c) S. H. Y. Lo, Y.-Y. Wang and C.-C.
Wan, J. Colloid Interface Sci., 2007, 310, 190.
2
+
Pd aggregate supported solely by carboxylate type donors is
reminiscent of Pd(OAc) which is among the most popular catalyst
precursors, and also one that is known to lead to catalytically
2
(a) O. M. Wilson, X. Hu, D. G. Cahill and P. V. Braun, Phys. Rev. B:
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2007, 17, 1002.
18
active anionic Pd(II) or Pd(0). The higher aqueous compatibility
of citrate (compared to common mono- or di-carboxylates) and
its higher denticity and donor flexibility would make citrate
and related ligands good candidates for water-based reactions
promoted by precious metal catalysts. Isolation of the titled
aggregate has fuelled the optimism that such catalyst precursors
can be easily prepared, and stable enough to be isolated and
structurally characterised. The structural insights thus obtained
would be key to future catalyst design.
We are grateful to L. L. Koh and G. K. Tan for X-ray
diffractometry measurement assistance. This work was sup-
ported by the National University of Singapore, the Ministry
of Education (R-143-000-361-112) and the Agency for Science,
Technology and Research (R-143-000-364-305). Several school
students in the H3/SRP programs, notably J. Chen, L. Yeh, V. Lim,
D. Lim and S. Lim have contributed in the form of experimental
assistance.
6
7
8
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4
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9
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1
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000, 13, 91.
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4 T. Yoshida, T. Okano and S. Otsuka, J. Chem. Soc., Dalton Trans.,
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Notes and references
‡
1
1
Crystal data of 1: empirical formula: C
6
H
43/3Na
2
O
73/6Pd, formula weight:
16 F. Zaera, Appl. Catal., A, 2002, 229, 75.
¯
300.66; crystal system: trigonal; space group: R3; a = 15.8632(6), b =
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˚
◦
5.8632(6), c = 28.772(2) A; a = 90, b = 90, g = 120 ; V = 6270.1(6)
˚
3
-3
A ; Z = 18; r(calcd) = 2.067 Mg m ; F(000) = 3882; crystal size: 0.14 ¥
9
464 | Dalton Trans., 2010, 39, 9462–9464
This journal is © The Royal Society of Chemistry 2010