Isolation of an [SNS]Pd(II) pincer with a water ladder and its Suzuki coupling activity in water

Article abstract of DOI:10.1039/b802043d

A water-compatible Pd(II) pincer with a hybrid [SNS]-donor set, [L ¹PdCl]Cl·2H₂O [L¹ = bis-(2-(i- butylsulfanyl)-ethyl)-amine] has been isolated and crystallographically characterized; its solid lattice at 223 K contains a

Full text of DOI:10.1039/b802043d

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Isolation of an [SNS]Pd(II) pincer with a water ladder and its Suzuki
coupling activity in waterw
Shi-Qiang Bai and T. S. Andy Hor*
Received (in Cambridge, UK) 5th February 2008, Accepted 26th March 2008
First published as an Advance Article on the web 30th April 2008
DOI: 10.1039/b802043d
A water-compatible Pd(II) pincer with a hybrid [SNS]-donor set,
[L1PdCl]Clꢀ2H2O [L1 = bis-(2-(i-butylsulfanyl)-ethyl)-amine]
has been isolated and crystallographically characterized; its
solid lattice at 223 K contains a ladder-like water polymer of
tetramers and extensive H-bonding exists among the cation,
anion and water cluster, and this water-soluble complex is active
in Suzuki–Miyaura coupling of phenylboronic acid and selected
aryl bromides in water at 75 1C.
Compound 1 was prepared from the reaction of K2[PdCl4]
with L1 [L1 = bis-(2-(i-butylsulfanyl)-ethyl)-amine] in an H2O/
EtOH mixture.z X-ray single-crystal structure analysisy at
223 K revealed a cationic mononuclear Pd(II) pincer complex
supported by a tridentate [SNS] chelating ligand (Fig. 1a). A
terminal chloride completes the square planar coordination
core. Each asymmetric unit comprises two lattice water mole-
cules and an uncoordinated Clꢁ ion. Its solid lattice shows that
the cationic fragments are stacked upon each other to form a
supramolecular network (Pdꢀ ꢀ ꢀPd 5.364 A) supported by van
der Waals forces between Pd and the skeletal methylene
proton of the ligand [C1–Hꢀ ꢀ ꢀPd1 3.87 & C7–Hꢀ ꢀ ꢀPd1
3.90 A]. Interactions between chloride ion and the methylene
[C3–Hꢀ ꢀ ꢀCl1 3.81 and C9–Hꢀ ꢀ ꢀCl1 3.78 A] and amine
[N1–Hꢀ ꢀ ꢀCl2 3.14 A] protons are also evident (Fig. 1c).
The chemistry of water-based palladium-catalysed C–C bond
formation reactions is a subject of topical interest.1 One of the
major challenges is the design of hydrophilic Pd(II) species of
high catalytic activity with good chemical, thermal and aquatic
stability. The works of Casalnuovo,2 Najera,3 Miyaura4 etc.
´
are examples of such. Sajiki et al.5 developed a ligand-free
heterogeneous Pd/C Suzuki catalyst. Ma and Xiao6 used
palladium on modified mesoporous silica support. A chroma-
tography-free approach was also reported by Ley et al.7
Catalysts that are more resistive to oxidation have also been
developed by Milstein et al. who used a PCP-type tridentate
ligand system for Heck-type vinylation of aryl halides.8 Other
hybrid ligand sets such as [NC], [SCS] and [NCN] donors are
accordingly appealing.9 We have also been exploring a range
of catalysts systems using, for example, metallomacrocycles,10
unsaturated species,11 hemilabile complexes,12 multidentate
hybrid-ligands,13 N,S-heterocyclic carbenes,14 nitrogen-rich
heterocycles,15 [PCP] pincers16 etc. Each of these systems has
its unique features and advantages. Our next target is to
develop single-molecular dual-character catalysts that may
inherit different traits and hence benefit across different cata-
lyst systems. In this communication, we highlight such a
possibility by the isolation of a water-compatible and ther-
mally stable Pd(II) pincer that is supported by a hybrid [SNS]
ligand, viz. [L1PdCl]Clꢀ2H2O (1) [L1 = bis-(2-(i-butylsulfa-
nyl)-ethyl)-amine], as well as its catalytic activity towards
Suzuki–Miyaura coupling in water. In its solid-state, this
complex shows an unusual ‘‘ladder-like’’ polymeric arrange-
ment of the hydrate. Such low-dimensional water/ice is the
structural intermediate of water clusters and bulk water.17 It
has important physical properties that are closely associated
with those of the latter.18
Fig. 1 (a) A perspective view of the cationic structure of 1 ignoring
the anion and water solvate (thermal ellipsoids at 30% level). (Pd:
purple; S: yellow; N: dark blue; Cl: green; O: red; H: sky blue). (Pd–S
2.300(1) and 2.301(1) A; Pd–N 2.016(4) A; Pd–Cl 2.293(1) A); Pd(1)
deviated 0.0064 A from the mean-square plane of the donor ligands
(S1, S2, N1, Cl1). (b) The H-bonding interactions in a zigzag water-
ladder structure of 1 at 223 K. (Symmetry codes: A, 1 ꢁ x, 1 ꢁ y, 1 ꢁ z;
B, 2 ꢁ x, 1 ꢁ y, 1 ꢁ z; C, 1 + x, y, z; D, x ꢁ 1, y, z; E, ꢁx, 1 ꢁ y, 1 ꢁ z.)
(c) A lattice view along the a direction of the supramolecular network
showing H-bonding among cation, anion and the water cluster.
Department of Chemistry, National University of Singapore, 3 Science
Drive 3, 117543, Singapore. E-mail: andyhor@nus.edu.sg;
Fax: +65-6873-1324
w Electronic supplementary information (ESI) available: Crystal data
for 1 and 2. CCDC reference numbers 676819 and 676820. Materials
and physical measurements, Table S1, and Fig. S1 and S2. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b802043d
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Table 1 Suzuki–Miyaura coupling of arylbromide with arylboronic
acida
supramolecular layers of the pincer through H-bonding with
chloride ions which in turn are H-bonded with the amine and
methylene protons (Fig. 1c). These collectively give a 3-D
supramolecular framework stabilized extensively by H-bonds
across all three key entities of pincer cation, uncoordinated
Clꢁ ions and lattice hydrate (Table S1 and Fig. S1 in ESIw).
Thermal gravimetric analysis (Fig. S2 in ESIw) of a fresh
sample showed a weight-loss at 40–85 1C of 7.5%, which
corresponds to complete dehydration (calculated 7.9%). The
complex is stable up to B160 1C, beyond which decomposi-
tion occurs.
catꢁ1ð2 mol%Þ
Ar1 ꢁ Br þ Ar2 ꢁ BðOHÞ2 ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁ! Ar1 ꢁ Ar2
Na2CO3;H2O;75 ꢃC;6 h
Entry Ar1–Br
1
Ar2–B(OH)2
Product
Yieldb
(%)
100
2
3
68
The hydrophilic nature of 1 enables it to dissolve in water to
promote Suzuki–Miyaura coupling reactions of selected aryl
and heterocyclic bromides and phenylboronic (and heteroaryl)
acid at 75 1C. Some selected data are given in Table 1. For
4-bromoacetophenone (entry 1), it gives near-quantitative
yields at catalyst loadings of 2.0–0.4 mol% Pd (Fig. 2). The
yield drops to 33% when the catalyst loading reaches the limit
of 0.1 mol%. Towards unactivated 4-bromoanisole, 1 (2
mol%) gives 68% yield of 4-anisobiphenyl (entry 2). The
yields towards unactivated bromides are only moderate
(36–68%, entries 2–9).
62
4
5
45
61
Similar experiments using bis-(2-(benzylsulfanyl)-ethyl)-
amine (L2) as the ligand also yielded an analogous pincer
[L2PdCl]Cl (2) (Fig. 3). The multidentate pincer effect over-
comes the weak basicity of the thioether functions, to the
extent that the metal prefers a pincer with two fused 5-mem-
bered-rings to the alternative of a chelating [PdCl2(SNS)] with
a pendant thioether. There is no water cluster in 2. Instead, the
Clꢁ counter-anion is weakly linked to the amine, methylene
and benzyl protons. Complex 2 is not active towards activated
4-bromoacetophenone acid at 2 mol% under similar condi-
tions. This significant catalytic difference between 1 and 2 is
surprising. Current experiments are directed at the tuning of
the electronic character of the donor substituents and the
hydrophilic character of the complex.
6
7
55
43
8
9
36
38
This work suggests that conventional organic-based pincer
complexes could be introduced to the field of water-based
catalysis. This is achieved by designing a supramolecular frame-
work that can incorporate water clusters or polymers. Key to this
strategy is the stacking of structurally compatible cations and
their electrostatic interaction with hydrophilic anion. The latter
serves as additional hydrophilic centers to attract hydrate
through H-bonding. The columnar arrangement also creates
a
Conditions: 0.5 mmol Ar1–Br, 0.6 mmol Ar2–B(OH)2, 1 mmol of
Na2CO3, and 0.01 mmol of cat-1 (2 mol%) in 5 mL H2O at 75 1C for
6 h. GC-MS yield.
b
A notable feature of the pincer lattice at 223 K is
the presence of a step-like water polymer propagated by
H-bonds
in
a
zig-zag
manner
(Fig.
1b).
If
O1Wꢀ ꢀ ꢀO2Wꢀ ꢀ ꢀO1WAꢀ ꢀ ꢀO2WA is viewed as a tetramer
(O1Wꢀ ꢀ ꢀO2W 2.82 A; O1Wꢀ ꢀ ꢀO2WA 2.90 A), this water
cluster can be treated as a polymer of interconnecting tetra-
mers (O1Wꢀ ꢀ ꢀO2WB 2.94 A). The average Oꢀ ꢀ ꢀO separation
(2.89 A) is slightly longer than those in liquid water tetramer
(2.78 A),19a liquid water (2.854 A),19b,c or ice (2.77–2.84 A).19d
This could be attributed to the multitude of H-bonds within
the polymer and with the chloride ions (O1W–Hꢀ ꢀ ꢀCl2 3.11
A). The dihedral angle between the two mean tetrameric
planes (O1Wꢀ ꢀ ꢀO2Wꢀ ꢀ ꢀ O1WAꢀ ꢀ ꢀO2WA and O1Wꢀ ꢀ ꢀ
O2Wꢀ ꢀ ꢀO1WBꢀ ꢀ ꢀO2WB) is 45.71, whereas the torsion angle
of O2WAꢀ ꢀ ꢀO1Wꢀ ꢀ ꢀO2WBꢀ ꢀ ꢀO1WC is 01. We are not aware
of similar step-alignment of polymeric tetra-aqua structures in
other hydrate systems. The aqua polymers are linked to the
Fig. 2 Yield of 4-acetylbiphenyl (’) and 4-anisobiphenyl (&) at
75 1C for 6 h in water as a function of catalyst (1) load.
ꢂc
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on a Bruker AXS CCD diffractometer with Mo-Ka radiation (l =
0.71073 A) at 223 K. The structures were solved by direct methods and
refined by a full matrix least squares technique based on F2 using
SHELXL 97 program.21 The hydrogen atoms on carbon were con-
strained. For complex 1, H atoms H1N, H1W, H2W, H3W, H4W
were located from difference map, the positions were refined with
constraints DFIX 0.90(2) for N–H, DFIX 0.85(2) for O–H, and DFIX
1.20(2) for Hꢀ ꢀ ꢀH of water, with thermal parameter at ꢁ1.2000 of the
relevant N or O. CCDC No.: 676819 (1) and 676820 (2).
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metallic Catalysis, Concepts and Applications, ed. B Cornils and
W. A. Herrmann, Wiley-VCH, Weinheim, 1998.
Fig. 3 A perspective view of the structure of 2 (ellipsoids at 30%
level). (Pd–S 2.2920(5) and 2.2929(5) A; Pd–N 2.030(2) A; Pd–Cl
2.3075(6) A); Pd(1) deviated 0.0185 A from the mean-square plane of
the donors (S1, S2, N1, Cl1). H-bonding interaction of N1–H1Nꢀ ꢀ ꢀCl2
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This work was supported by the Agency for Science,
Technology & Research (Singapore) and the National Uni-
versity of Singapore (143-000-277-305). We are grateful to
Dr L. L. Koh and Ms G. K. Tan for assistance in the X-ray
diffraction experiments and structural refinement.
Notes and references
z Preparation: the SNS ligands L1 and L2 were prepared by literature
methods.20 Complexes 1 and 2 were prepared by a common procedure
as follows: an aqueous solution of K2[PdCl4] (326 mg, 1 mmol) was
mixed with an ethanol solution of L1 (249 mg, 1 mmol) or L2 (318 mg,
1 mmol). Upon standing for B2 weeks, yellowish prismatic crystals
suitable for X-ray diffraction experiments were collected. For 1, yield:
260 mg (56%). 1H-NMR (300 MHz, CDCl3, 25 1C): d = 1.05–1.08 (m,
12H, CH3), 1.82 (s, 4H, H2O), 1.95–2.13 (m, 2H, CH2CH(CH3)2),
2.81–3.49 (m, 12H, CH2). 13C-NMR (75.47 MHz, CDCl3, 25 1C): d =
21.1 (s, CH3), 22.0 (s, CH3), 27.8 (s, CH2CH(CH3)2), 39.5 (s,
CH2CH2S), 49.7 (s, SCH2CH), 54.9 (s, NHCH2CH2). Elemental
analysis calcd C12H31Cl2NO2PdS2 (462.8): C, 31.14; H, 6.75; N,
10. Z. Q. Weng, S. H. Teo, L. L. Koh and T. S. A. Hor, Organo-
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3.03. Found: C, 31.53; H, 6.57; N, 3.20%. Selected IR data (cmꢁ1
,
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S. R. Batten, P. Chen and J. Zheng, Eur. J. Inorg. Chem., 2007,
1925; (d) S. R. Choudhury, A. D. Jana, E. Colacio, H. M. Lee,
G. Mostafa and S. Mukhopadhyay, Cryst. Growth Des., 2007, 7, 212.
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KBr): 3493 v, 3441 v, 3000 m, 2963 v, 2846 v, 1621 m, 1465 m, 1421 m,
1382 m, 1259 m, 496 m. For 2, yield: 350 mg (70%). 1H-NMR (300
MHz, (CD3)2SO, 25 1C): d = 2.81–3.13 (m, 8H, NHCH2CH2S),
4.44–4.58 (m, 4H, SCH2Ph), 7.40–7.56 (m, 10H, Phenyl H). 13C-
NMR (75.47 MHz, (CD3)2SO, 25 1C): d = 36.1 (s, SCH2Ph), 54.4
(s, NHCH2CH2S), 128.6, 129.2, 129.9, 133.7 (s, Ar-C). Elemental
analysis calcd C18H23Cl2NPdS2 (494.79): C, 43.69; H, 4.68; N, 2.83.
Found: C, 43.49; H, 4.61; N, 2.82%. Selected IR data (cmꢁ1, KBr):
3448 b, 2973 m, 2919 m, 2816 m, 2735 v, 1492 m, 1455 m, 1420 m, 1240
m, 1073 m, 1024 m, 1004 m, 799 m, 770 v, 700 v, 636 m. General
procedure for Suzuki–Miyaura coupling reactions: a 15 mL vial was
charged with activated or deactivated aromatic bromide (0.5 mmol),
aryl boronic acid (0.6 mmol), Na2CO3 (1 mmol), SNS–Pd catalyst (1),
and H2O (5 mL), the mixture was stirred at 75 1C in air for 6 h. The
products were assayed by GC-MS.
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y Crystal data for 1: formula C12H31Cl2NO2PdS2, yellow crystal,
triclinic space group P 1; a = 5.3641(7), b = 13.405(2), c =
ꢀ
15.300(2) A; a = 67.357(2), b = 84.856(2), g = 80.298(2)1; V =
1000.5(2) A3; Z = 2; crystal size: 0.40 ꢄ 0.10 ꢄ 0.06 mm3; GOF =
1.087; reflections collected: 8989; independent reflections: 3524 [Rint
=
0.0333]; R1 = 0.0446; wR2 = 0.1161. Crystal data for 2: formula
C18H23Cl2NPdS2, yellow crystals, monoclinic space group P21/n; a =
12.3815(6), b = 5.6441(3), c = 28.793(2) A; b = 99.376(1)1; V =
1985.2(2) A3; Z = 4; crystal size: 0.70 ꢄ 0.24 ꢄ 0.10 mm3; GOF =
1.027; reflections collected: 13376; independent reflections: 4540 [Rint
= 0.0270]; R1 = 0.0249; wR2 = 0.0621. Data 1 and 2 were collected
21. G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Gottingen, Germany, 1997.
¨
ꢂc
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3174 | Chem. Commun., 2008, 3172–3174