Water-soluble complexes MX2L2 (M = Pd, Pt; L = PPh2(C6H4-p-SO3K)): Synthesis, stereoisomerism, and catalytic activities for aromatic cyanation in n-heptane/water biphasic solution

Article abstract of DOI:10.1016/j.jorganchem.2011.09.009

Reaction of (COD)MX₂ (M = Pd, Pt; X = Cl, I; COD = 1,5-cyclooctadiene) and P(C₆H₅)₂(C ₆H₄-p-SO₃K) afforded water-soluble complexes MX₂{P(C₆H₅)₂(C₆H ₄-p-SO₃K)}₂ (M = Pd; X = Cl (1), M = Pt; X = Cl (2), I (3)) in high yields. Complexes 1-3 were fully characterized by various spectroscopic methods (IR, ¹H-, ¹³C{¹H}- and ³¹P{¹H}-NMR spectroscopy) and elemental analyses. For 1 and 3, a mixture of the cis- and trans-isomer was produced from the reaction. For 2, however, only the cis-isomer was obtained. The stereochemistry of 1-3 can be assigned by the chemical shifts and the ¹J(Pt-P) values in ³¹P{¹H}-NMR spectral data. The ratios of the cis/trans isomers of 1 and 3 obtained from reactions in a range of solvents with various dielectric constants resulted in a little variation. However, addition of aqueous potassium halide solution to a DMSO-d₆ solution of 1 and 3 considerably increased the ratio of the cis/trans, respectively, indicating a strong intramolecular interligand Coulombic repulsion between the ionic phosphine ligands is present. Catalytic cyanation of aromatic iodide with KCN/ZnCl₂ in n-heptane/water biphasic system has been tested in the presence of 1-3 with base.

Full text of DOI:10.1016/j.jorganchem.2011.09.009

Journal of Organometallic Chemistry 696 (2012) 4173e4178
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Journal of Organometallic Chemistry
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Water-soluble complexes MX2L2 (M ¼ Pd, Pt; L ¼ PPh2(C6H4-p-SO3K)):
Synthesis, stereoisomerism, and catalytic activities for aromatic cyanation
in n-heptane/water biphasic solution
*
Young Ji Shim, Ho Jin Lee, Soonheum Park
Department of Chemistry, Dongguk University, Gyeongju 780-714, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of (COD)MX2 (M ¼ Pd, Pt; X ¼ Cl, I; COD ¼ 1,5-cyclooctadiene) and P(C6H5)2(C6H4-p-SO3K)
afforded water-soluble complexes MX2{P(C6H5)2(C6H4-p-SO3K)}2 (M ¼ Pd; X ¼ Cl (1), M ¼ Pt; X ¼ Cl (2),
I (3)) in high yields. Complexes 1e3 were fully characterized by various spectroscopic methods (IR, 1H-,
13C{1H}- and 31P{1H}-NMR spectroscopy) and elemental analyses. For 1 and 3, a mixture of the cis- and
trans-isomer was produced from the reaction. For 2, however, only the cis-isomer was obtained. The
stereochemistry of 1e3 can be assigned by the chemical shifts and the 1J(PteP) values in 31P{1H}-NMR
spectral data. The ratios of the cis/trans isomers of 1 and 3 obtained from reactions in a range of solvents
with various dielectric constants resulted in a little variation. However, addition of aqueous potassium
halide solution to a DMSO-d6 solution of 1 and 3 considerably increased the ratio of the cis/trans,
respectively, indicating a strong intramolecular interligand Coulombic repulsion between the ionic
phosphine ligands is present. Catalytic cyanation of aromatic iodide with KCN/ZnCl2 in n-heptane/water
biphasic system has been tested in the presence of 1e3 with base.
Received 22 August 2011
Received in revised form
13 September 2011
Accepted 14 September 2011
Keywords:
Water-soluble complexes
Pd(II) and Pt(II) complexes
Monosulfonated triphenylphosphine
complexes
Aromatic cyanation
Biphasic catalysis
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
solubilities of mTPPMS and mTPPTS with sodium salt in water are
80 g dmꢀ3 and 1100 g dmꢀ3, respectively [1,17]. Thus, the meta-
There has been a surge of interests in aqueous organometallic
chemistry and catalysis due to the principal advantages of eco-
friendly syntheses of fine chemicals along with relatively uncom-
plicated separation and regeneration of precious catalysts from
reaction mixtures [1,2]. The extensively explored water-soluble
complexes are those of tertiary phosphine derivatives of hydro-
philic ion-pair substituents on the phenyl ring, such as sulfonate
[3e6], carboxylate [7e9], phosphonate [10e12], or ammonium
functionalities [13e15]. The best knownprecedents of water-soluble
ligands are triphenylphosphine derivatives of sulfonate (SOꢀ3 ) group
at the meta-position on the phenyl ring: tris(meta-sulfonatophenyl)
phosphine (P(C6H4-m-SO3M)3 (Naþ or Kþ salt), mTPPTS) and
diphenyl(meta-sulfonatophenyl)phosphine (PPh2(C6H4-m-SO3M),
mTPPMS) (Fig. 1) [3e6,16]. The monosulfonated phosphine ligand
(mTPPMS) may offer a couple of merits over the trisubstituted one
(mTPPTS) in relatively less demanding synthetic manipulation, and
solubility toward reaction medium: phase-transfer property
(ambiphilicity) in a watereorganic biphasic system. For references,
substituted monosulfonated triphenylphosphine (mTPPMS) has
been extensively utilized as a supporting ligand for the synthesis of
water-soluble transition metal complexes [18e20]. In contrast, the
para-substituted monosulfonated triphenylphosphine (pTPPMS)
has attracted little attention as a supporting ligand in transition
metal complexes. The para-isomer (pTPPMS), however, provides
several advantages over the meta-isomer (mTPPMS), involving
enhanced crystallinity of the ligand and its metal complexes,
uncomplicated spectroscopic characteristics, and ease of prepara-
tion along with purification [21,22].
Reported in this paper are water-soluble palladium(II) and
platinum(II) complexes of the para-substituted monosulfonated
triphenylphosphine, MX2(PPh2(C6H4-p-SO3K))2 (M ¼ Pd; X ¼ Cl (1),
M ¼ Pt; X ¼ Cl (2), I (3)). Catalytic cyanation of aromatic iodide in n-
heptane/water biphasic system in the presence of the title
complexes has been investigated. Catalytic yields and selectivity of
aromatic cyanation largely varied depending not only on the
employed substrates of aromatic iodide and a cyanide source but
also on the applied base such as Zn-powder, NaBH4, NaOAc or
Na2CO3. The feasibility for catalytic cyanation of sterically
demanding 1,3-dichloro-2-iodobenzene in a biphasic system is also
described.
* Corresponding author. Tel.: þ82 54 770 2219.
E-mail address: shpark@dongguk.ac.kr (S. Park).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.09.009

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Y.J. Shim et al. / Journal of Organometallic Chemistry 696 (2012) 4173e4178
SO3Na(K)
p
p
p
SO3Na(K)
SO3Na(K)
SO3Na(K)
SO3Na(K)
mTPPTS
mTPPMS
pTPPMS
Fig. 1. Tri- and mono-sulfonated triphenylphosphines.
2. Experimental
NMR (DMSO-d6): d 126.1, 126.6, 129.2, 129.7, 131.7, 132.1, 134.8,
135.3. Anal. Calcd for C36H28Cl2K2O6P2S2Pd: C, 46.1; H, 3.01; S, 6.84.
Calcd for the monohydrate: C, 45.2; H, 3.16; S, 6.71. Found: C, 44.9;
H, 3.45; S, 6.50.
2.1. General methods and materials
All preparations of air sensitive compounds were performed on
a standard Schlenk line under nitrogen or argon atmosphere. THF
and diethyl ether were distilled from sodium/benzophenone ketyl.
n-Heptane was distilled from sodium/benzophenone ketyl in the
presence of tetraglyme (tetraethylene glycol dimethyl ether) and
stored on 4 Å molecular sieves under N2. DMF and MeOH were
distilled from MgSO4, for DMF at reduced pressure of ca. 20 mmHg,
and stored on 4 Å molecular sieves under N2. The used water was
doubly distilled under N2, and had been adequately purged with N2
prior to use. DMSO-d6 was purchased from Aldrich Chemical
Company, and used as supplied. ZnCl2 was recrystallized from 1,4-
dioxane [23]. PdCl2 and K2PtCl4 were supplied by Kojima Chemicals
Co., Ltd., and used without purification. PPh2Cl, 4-
fluorobenzenesulfonyl chloride, KHCO3 and NaBH4 were
purchased from Aldrich Chemical Company or Strem Chemicals. All
other chemicals were from various commercial companies.
PPh2(C6H4-p-SO3K) [22], (COD)PdCl2 [24] and (COD)PtCl2 [25] were
synthesized according to the literature methods.
2.3.2. cis-Pt(PPh2(C6H4-p-SO3K))2Cl2 (2)
A mixture of Pt(COD)Cl2 (200 mg, 0.534 mmol) and PPh2(C6H4-
p-SO3K) (447.2 mg, 1.18 mmol) in DMF (30 mL) was refluxed in DMF
at 60 ꢂC for 4 h. After cooling the reaction mixture to ambient
temperature, the solution volume was reduced to ca. 10 mL under
high vacuum. Addition of diethyl ether (30 mL) on the concentrated
solution gave off-white precipitates, which were isolated, washed
with diethyl ether (3 ꢁ 10 mL), and then dried in vacuo. Recrys-
tallization from MeOH/Et2O gave satisfactory microanalytical data.
Yield 521 mg (95%). IR (KBr):
n
(SO3) ¼ 1656, 1206, 1037 cmꢀ1 (vs,
br). 1H NMR (DMSO-d6):
d
7.43e7.97 m (Ph). 31P{1H}-NMR (DMSO-
d6):
d
14.6 s (1J(PteP) ¼ 3688 Hz). 13C{1H}-NMR (DMSO-d6):
d 125.8,
128.7, 128.8, 129.0, 131.9, 134.5, 135.1, 135.2, 150.4. Anal. Calcd for
C36H28Cl2K2O6P2S2Pt: C, 42.1; H, 2.75; S, 6.25. Calcd for the mono-
hydrate: C, 41.4; H, 2.89; S, 6.14. Found: C, 42.0; H, 3.21; S, 6.19.
2.3.3. Pt(PPh2(C6H4-p-SO3K))2I2 (3)
A similar procedure as for complex 2 using Pt(COD)I2 (50 mg,
0.0896 mmol) and PPh2(C6H4-p-SO3K) (75 mg, 0.1975 mmol) gave an
orange complex 3 asan isomeric mixture (trans/cis ¼ 1.3). Yield 98mg
2.2. Physical measurements
(90%). IR (KBr):
n
(SO3) ¼ 1656, 1206, 1038 cmꢀ1 (vs, br). 1H NMR
IR spectra were recorded on a Bomem (Michelson 100) or
a Bruker (Tensor 37) FT-IR spectrometer, as pressed KBr pellets. 1H-,
13C{1H}- and 31P{1H}-NMR spectra were measured on a Varian
Gemini-2000 spectrometer (1H (199.975 MHz), 13C{1H}
(50.288 MHz), 31P{1H} (80.950 MHz)), using the deuterium signal of
the solvent as an internal lock frequency. Chemical shifts for 1H-
(DMSO-d6):
d d 12.0 s
7.43e7.97 m (Ph). 31P{1H}-NMR (DMSO-d6):
(1J(PteP) ¼ 3494 Hz, cis-isomer (43%)),
d
12.6 s (1J(PteP) ¼ 2477 Hz,
trans-isomer (57%)). 13C{1H}-NMR (DMSO-d6):
d 125.7, 128.5, 128.6,
128.7, 129.0, 131.3, 131.6, 134.6, 135.0, 135.2, 135.6. Anal. Calcd for
C36H28I2K2O6P2S2Pt: C, 35.7; H, 2.33; S, 5.30. Calcd for the mono-
hydrate: C, 35.2; H, 2.46; S, 5.22. Found: C, 35.3; H, 2.85; S, 4.96.
and 13C{1H}-NMR are reported in ppm ( ) relative to TMS. For 31P
d
{1H}-NMR, chemical shifts were measured in ppm relative to
external 85% H3PO4 (in a sealed capillary). GC/MS analyses were
performed using an HP 6890 gas chromatograph equipped with an
HP 5973 MSD and an HP-Ultra 1 column (Crosslinked Methyl Sili-
2.4. A typical procedure for catalytic cyanation of aromatic iodide
in a biphasic system (n-heptane/H2O)
cone Gum, 50 m ꢁ 0.2 mm, 0.33
m
m film thickness). The injection
To a stirred solution of complex 1 (10 mg, 0.0125 mmol) in
amixed solventofn-heptane (1.5 mL)and water(1.5 mL) wereadded
KCN (10.6 mg, 0.1625 mmol), ZnCl2 (11.1 mg, 0.0825 mmol), NaBH4
(0.5 mg, 0.0125 mmol), and iodobenzene (26 mg, 0.125 mmol). The
reaction mixture was stirred at 100 ꢂC for 1 h under nitrogen
atmosphere. After cooling the reaction mixture in an ice-bath,
aliquots of the organic layer were transferred to a vial with a Pas-
teur pipette. Eluting the aliquots with diethyl ether on a short glass-
column (0.7 ꢁ 15 cm) packed with alumina (ca. 1 cm) resulted in
a clear yellowish solution which was analyzed with GC/MS.
temperature was 250 ꢂC, and the column temperature ramped 10ꢂ/
min from 40 ꢂC to 250 ꢂC. Elemental analyses were performed at
Korea Basic Science Institute in Seoul, Korea.
2.3. Synthesis
2.3.1. Pd(PPh2(C6H4-p-SO3K))2Cl2 (1)
A mixture of Pd(COD)Cl2 (50 mg, 0.175 mmol) and PPh2(C6H4-p-
SO3K) (166.5 mg, 0.438 mmol) in DMF (30 mL) was stirred for 4 h at
ambient temperature. The volume of the solution was reduced to
ca. 10 mL. Addition of diethyl ether (30 mL) on the concentrated
solution gave deep yellow precipitates, which were isolated,
washed with diethyl ether (3 ꢁ 10 mL), and then dried in vacuo.
3. Results and discussion
3.1. Synthesis of water-soluble complexes MX2L2
Yield 151 mg (92%). IR (KBr):
br). 1H NMR (DMSO-d6):
d6): 33.0 s (cis-isomer, 22%),
n
(SO3) ¼ 1656, 1206, 1038 cmꢀ1 (vs,
d
7.42e7.97 m (Ph). 31P{1H}-NMR (DMSO-
The ligand PPh2(C6H4-p-SO3K) was synthesized by the reaction
of potassium 4-fluorobenzenesulfonate with KPPh2 in THF
d
d
24.2 s (trans-isomer, 78%). 13C{1H}-

Y.J. Shim et al. / Journal of Organometallic Chemistry 696 (2012) 4173e4178
4175
trans:cis ratio being about 78:22). The respective 31P NMR chemical
shift values observed for complex 1 are in good comparison with
the reported values of Pd(II) complexes with the ion-paired phos-
M(COD)X 2
+
+
ML2X2
COD
2L
DMF
phine ligands, for PdCl2(mTPPTS)2 [32] at
(cis), and for PdCl2{PPh(C6H4-m-NHCNH2NMe2Cl)2}2 [33] at
(trans) and
d
25.3 (trans) and
d
34.3
28.8
M = Pd; X = Cl (1)
d
M = Pt; X = Cl (2), I (3)
d
35.9 (cis). For platinum complexes of the type PtL2X2,
the cis- and trans-isomers can be unambiguously assigned from the
respective 1J(PteP) value due to considerable variation of the trans-
influence of the ligands, halide and phosphine [34]. The observed
1J(PtePcis) value is significantly larger than that of the 1J(PtePtrans).
For the chloro complex 2, the reaction yields exclusively the cis-
p
L =
SO3K
isomer judged from the 31P{1H} NMR resonance at
d 14.6 flanked
with the 195Pt satellites (1J(PteP) ¼ 3688 Hz). However, for the iodo
Scheme 1.
complex 3, a mixture of the cis- and trans-isomers was formed from
the reaction (cis (43%):
d
d
12.0 s, 1J(PteP) ¼ 3494 Hz, trans (57%):
12.6 s, 1J(PteP) ¼ 2477 Hz). The observed 1J(PteP) values for 2 and
according to the reported method [22]. The purity of the ligand was
confirmed by 31P{1H}-NMR (
ꢀ6.52 s, DMSO-d6), revealing no
d
3 compare well with those of PtX2L2 having neutral or ion-paired
phosphine ligands [35].
phosphine oxide present [26]. Water-soluble complexes of Pd(II)
and Pt(II), ML2X2 (L ¼ PPh2(C6H4-p-SO3K), M ¼ Pd; X ¼ Cl (1),
M ¼ Pt; X ¼ Cl (2), I (3)) were prepared by the reaction of M(COD)X2
with 2 equivalents of PPh2(C6H4-p-SO3K) in DMF (Scheme 1).
Complexes 1e3 were isolated as yellow crystalline solids in high
yields (90e95%), and fully characterized by IR and NMR(1H, 31P{1H},
13C{1H}) spectroscopy, and microanalyses. The complexes are fairly
soluble in DMF, DMSO and H2O but sparingly soluble in benzene,
acetone, THF, and chlorinated solvents such as CH2Cl2 and CHCl3.
All complexes 1e3 gave satisfactory microanalytical data for C, H,
and N (see Experimental). In the IR spectra of 1e3, the character-
Stereoisomerism in bisphosphine complexes of the type of
ML2X2 comprises a complicated combination of factors [36e40].
The trans/cis ratio varies depending not only on electronic and
steric features of constituents of metal complexes including phos-
phine and halide ligands, and metal but also on reaction mediums.
For complexes with ion-paired phosphine ligands, contributions
arising from either attractive or repulsive interactions between the
ionic functionalities have to be incorporated with the factors [41].
The ratio of the cis/trans isomers of MX2L2 complexes generally
increases as the dielectric constant or the dipole moment of the
solvent increases [42,43]. Thus, on reactions for PdCl2{PPh2(C6H4-
p-SO3K)}2, various solvents such as CH3CN, MeOH, DMF, DMSO and
H2O were employed, resulting in a little variation of the cis/trans
ratio. The formation of the cis-isomer barely increased from 19% (in
CH3CN) to 28% (in H2O) but was not predominant. For the Pt(II)
iodide 3, the cis/trans ratios are practically not much different from
the employed solvent, DMF to MeOH. These results are incompat-
ible with the previous studies for the type of PdCl2L2, on which the
cis/trans ratio considerably increases with the dielectric constant of
the employed solvent increases [33,41e43]. For sterical parameters
effecting the cis/trans ratio, the cone angle of the ligand PPh2(C6H4-
istic symmetric and anti-symmetric n(SO3) bands of the sulfonate
group on para-position of the phenyl substituent were observed at
1656, 1206, 1038 cmꢀ1 [27,28]. In the 1H NMR spectra of 1e3 in
DMSO-d6, phenyl protons resonate at
d 7.43e7.97 as multiplets,
respectively. The 13C{1H}-NMR spectral data for 1e3 were obtained,
though the fine structure patterns of 13C resonances (J(CeP)) for
assignments of stereoisomers were not well resolved due to a low
signal/noise ratio [29]. For complexes 1 and 3, a mixture of the cis-
and trans-isomers was obtained, while for complex 2, the cis-
isomer was exclusively formed. The stereochemistry of 1e3 can be
assigned from the respective 31P{1H}-NMR chemical shift value for
the palladium complex, and the spinespin coupling constants
(1J(PteP)) for the platinum complexes.
p-SO3K) (
q
¼ 137.7ꢂ) [28], comparing with those of other tertiary
arylphosphines (PPh3 (141.5ꢂ) [44], mTPPMS (177.6ꢂ) [45], mTPPTS
(170.0ꢂ)) [46,47] seems to be not responsible for the observed
results.
3.2. Stereoisomerism of MX2L2
For complexes of ion-paired phosphines, intramolecular inter-
ligand Coulombic interaction or repulsion has strong effect on the
thermodynamic stability of the respective isomers [36e39,48].
Since intramolecular interligand Coulombic repulsion is dimin-
ished at higher ionic strengths due to shielding of the charges
[41,49], further experiments on the stereoisomers were performed
in potassium halide solution of DMSO. Treatment of DMSO-d6
solution of PdCl2{PPh2(C6H4-p-SO3K)}2 (the trans/cis ¼ ca. 8/2) with
It is well documented that in square planar d8-Pd(II) complexes,
a type of PdL2X2, the 31P chemical shift (
d) of the trans-isomer
generally appears upfield from that of the cis-isomer [30,31]. In the
31P{1H}-NMR spectrum of 1 in DMSO-d6, two resonances display at
d
24.2 and 33.0 as a single peak, assignable to trans- and cis-
PdCl2{PPh2(C6H4-p-SO3K)}2, respectively. In DMF solution the
trans-isomer was predominantly formed from the reaction (the
SO3K
SO3K
Cl
KCl / d6-DMSO
Pd
KO3S
P
P
SO3K
P
P
Cl
Pd
Cl
Cl
cis
trans
Scheme 2.

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Y.J. Shim et al. / Journal of Organometallic Chemistry 696 (2012) 4173e4178
a saturated aqueous KCl solution gave the trans/cis ratio of ca. 2/8 in
a short period of time (30 min) (Scheme 2). For PtI2{PPh2(C6H4-p-
SO3K)}2 with KI, the trans/cis ratio considerably changed from 6/5 to
3/7. These results are consistent with the recent report that the
trans/cis ratio decreases with increasing the ionic strength of
solution [49]. Thus the observed results for the stereoisomeric ratio
may be attributed to a strong intramolecular interligand Coulombic
repulsion between the ionic phosphine ligands, which disfavors the
cis-isomer. In the title complexes, intramolecular interligand
interaction and repulsion are likely to be finely balanced, except for
the Pt(II) chloride in which the two ionic phosphine ligands occupy
a cis configuration with respect to each other.
X
X
ML2X2, Na2CO3
I
+
KCN / ZnCl
or
CN
2
n-heptane/H2O
X
X
100°C, 1h
Zn(CN)2
X = H, Cl
Scheme 4.
an alternative cyanide source comparable with KCN/ZnCl2 [51e54].
Our results are consistent with earlier studies that excess cyanide
anions released from KCN deactivate the palladium catalyst,
forming inactive palladium species of cyanide [55,56]. As the
applied base, NaBH4 or Na2CO3 was found to be more effective than
Zn-powder. In the absence of base, no conversion of iodobenzene to
benzonitrile was observed. The base apparently involves in the
formation of reactive Pd(0) species. However, high concentration of
NaBH4 (40 mol %) diminished the catalytic activity, generating
hydrodehalogenation product. In an immiscible biphasic solvent,
complete conversion of iodobenzene to benzonitrile by utilizing
KCN/ZnCl2 or Zn(CN)2 as a cyanide source was observed in the
presence of catalytic amount of 1 along with base such as Na2CO3 or
NaBH4. Although catalytic efficiency with the title complex is
comparable with precedents of lipophilic catalysis [56], the present
protocol has an advantage of uncomplicated separation of organic
product from catalyst and reactants soluble in water-phase. Our
results compare well with earlier investigation on catalytic cyana-
tion of aromatic iodide in a biphasic system with water-soluble
catalysts [50]. The complexes with the monosulfonated ligands
pTPPMS and mTPPMS revealed higher catalytic activity than those
with the carboxylate and ammonium derivatives PPh2(C6H4-m-
CO2Na) and PPh2(C6H4-m-CH2NMe3Cl), respectively. The higher
catalytic efficiency with the complexes of the monosulfonated
ligands is likely attributed to advantageous counter phase-transfer
property in a biphasic system.
3.3. Three-coordinated palladium(II) species PdCl2L
Separation of the trans- and cis-isomers from an isomeric
mixture of PdCl2{PPh2(C6H4-p-SO3K)}2 from column chromatog-
raphy on silica gel with various eluents was attempted without
success, resulting in the practically same ratio (trans/cis ¼ ca. 7/3) of
the trans- and cis-isomers along with an additional complex which
shows a broad signal at
d
31.2 in the 31P{1H}-NMR spectrum. Since
the amount of the complex resulted from column chromatography
was significant (43%), we conducted further investigation on the
complex. This complex is verified as a three-coordinated Pd(II)
species PdCl2{PPh2(C6H4-p-SO3K)} by its independent synthesis
from the reaction of Pd(COD)Cl2 with one equivalent of PPh2(C6H4-
p-SO3K) (Scheme 3). Thus the formation of PdCl2{PPh2(C6H4-p-
SO3K)} resulted from column chromatography can be apparently
explained by a strong intermolecular dissociative interaction
between the ion-paired phosphine ligand and silica, liberating one
of the coordinated phosphine ligands from the palladium center.
When an excess amount of PPh2(C6H4-p-SO3K) was added into
a DMSO-d6 solution of PdCl2{PPh2(C6H4-p-SO3K)}, the mono-
phosphine complex converts into the bisphosphine complex 1,
being supported by observation of disappearing the broad peak at
d
31.2 and appearing two sharp peaks at d P
24.2 and 33.0 in the 31
{1H}-NMR spectrum, corresponding to trans- and cis-
PdCl2{PPh2(C6H4-p-SO3K)}2 (trans/cis ¼ 83/17), respectively. It is
noteworthy that in the reaction, no significant variation of the
isomeric ratio (trans/cis ¼ ca. 8/2) was observed in the presence of
excess PPh2(C6H4-p-SO3K), even for a prolong period of time (48 h).
Table 1
Catalytic cyanation of iodobenzene with MX2L2.a
3.4. Aromatic cyanation in n-heptane/water biphasic system
The title complexes 1e3 have been tested for catalytic cyanation
of aromatic iodide with a number of cyanide source (KCN, KCN/
ZnCl2 or Zn(CN)2) in n-heptane/water biphasic system (Scheme 4).
An additive base such as Zn-powder, NaBH4, Na2CO3 or NaOAc has
been varied for the best results. In Table 1, the obtained results for
catalytic cyanation of iodobenzene to produce benzonitrile are
summarized. The palladium complex 1 shows higher catalytic
activity than the analogous platinum complexes 2 and 3. When
ZnCl2 was employed as a supporting agent along with KCN, the
catalytic activity was considerably enhanced [50]. Zn(CN)2 displays
Comp.
CN Source
ZnCl2 (equiv)b
Base (equiv)c
Conversion (%)d
1
1
1
1
1
1
1
2
2
2
3
3
KCN
KCN
KCN
KCN
e
Zn (1)
Zn (1)
<4
14
32
16
35
100f
100f
3
4
3
5
4
0.5
0.5
0.5
0.6
0.6
e
NaBH4 (1)
NaBH4 (4)
Na2CO3 (4)
Na2CO3 (4)
NaBH4 (1)
NaBH4 (1)
Na2CO3 (4)
NaBH4 (1)
NaBH4 (1)
NaBH4 (1)
KCN
KCNe
Zn(CN)2
Zn(CN)2
KCN
e
0.5
0.5
0.5
e
KCN
KCN
Zn(CN)2
- COD
DMF
- L
PdL2Cl2
PdLCl2
Pd(COD)Cl 2
L
+
SiO2
a
Reaction conditions: iodobenzene (0.125 mmol), KCN (0.163 mmol), Zn(CN)2
(0.0815 mmol), MX2L2 (0.0125 mmol).
L
b
Molar equivalent to KCN.
Molar equivalent to MX2L2.
GC-based yield for 1 h at 100 ꢂC.
c
d
PdL2Cl2 (trans/cis = 8/2)
e
0.250 mmol of KCN was employed.
f
Scheme 3.
Conversion obtained after 24 h.

Y.J. Shim et al. / Journal of Organometallic Chemistry 696 (2012) 4173e4178
4177
Table 2
particles formed from the reaction mixture after 48 h reaction time
could be conspicuous for decomposed catalysts.
Catalytic cyanation of 1,3-dichloro-2-iodobenzene in the presence of 1.a
In the present catalytic reaction, the mechanism may follow the
typical cycle for a palladium-catalyzed cross-coupling reaction with
oxidative addition and reductive elimination [56]. Reactive Pd(0)
species Pd{PPh2(C6H4-p-SO3K)}2 presumably generated from
reduction of PdCl2{PPh2(C6H4-p-SO3K)}2 in the presence of base
would drive the catalytic reaction via facile oxidative addition of
aryl iodide and followed by ligand substitution with cyanide to lead
PdAr(CN){PPh2(C6H4-p-SO3K)}2, finally undergoing reductive
elimination to produce aromatic nitrile along with regeneration of
CN Source
ZnCl2 (equiv)b
Base (equiv)c
A (%)d
B (%)d
active palladium(0) species.
depicted in Scheme 5.
A plausible reaction pathway is
KCN
KCN
KCN
KCN
e
Zn (1)
e
38
e
e
Na2CO3 (1)
NaOAc (1)
NaBH4 (1)
Na2CO3 (1)
NaOAc (1)
Na2CO3 (4)
Na2CO3 (1)
e
e
e
e
0.5
0.5
0.5
0.5
e
<1.4
9
16
4. Conclusion
KCN
<0.1
<0.1
e
KCN
7
KCNe
Zn(CN)2
8f
We have prepared water-soluble complexes of Pd(II) and Pt(II),
MX2L2 (L ¼ PPh2(C6H4-p-SO3K), X ¼ Cl, I). For PdCl2L2 and PtI2L2,
a mixture of the cis- and trans-isomer was obtained while for
PtCl2L2, the cis-isomer was produced, exclusively. The ratios of the
cis/trans isomers of 1 and 3 obtained from reactions in a range of
solvents with various dielectric constants resulted in a little vari-
ation. However, addition of aqueous potassium halide solution to
a DMSO-d6 solution of 1 and 3, respectively, resulted in a significant
increase of the ratio of the cis/trans, indicating a strong intra-
molecular interligand Coulombic repulsion between the ionic
phosphine ligands is present. The title complexes were tested for
catalytic cyanation of aromatic iodide in n-heptane/water biphasic
system. The palladium complex revealed better catalytic activity
than the platinum complexes for aromatic cyanation of iodo-
benzene to benzonitrile. However, catalytic cyanation of sterically
restrained 1,3-dichloro-2-iodobenzene was found to be consider-
ably retarded, leading to motif for alteration of the ligand-
framework of catalysts to overcome the limited catalytic activity
for steric tolerance of substrates.
11f
<0.1
a
Reaction conditions: PdCl2L2 (0.0125 mmol), 1,3-dichloro-2-iodobenzene
(0.125 mmol), KCN (0.163 mmol), Zn(CN)2 (0.0815 mmol).
b
Molar equivalent to KCN.
Molar equivalent to PdCl2L2.
GC-based yield for 1 h at 100 ꢂC.
0.250 mmol of KCN was employed.
Conversion obtained after 24 h.
c
d
e
f
Selective cyanation of 1,3-dichloro-2-iodobenzene was also
investigated in the presence of complex 1. As shown in Table 2, no
cyanation reaction proceeded in the absence of the additive ZnCl2.
When Zn-powder was applied as a reducing agent, a considerable
amount of hydrodeiodination derivative 1,3-dichlorobenzene was
exclusively produced. The hydride source may be attributed to H2O
generating molecular hydrogen by the reaction with Zn catalyzed
by Pd species [57,58]. For sterically hindered 1,3-dichloro-2-
iodobenzene, catalytic cyanation remarkably retarded at the same
reaction condition, comparing with that of iodobenzene. No
significant increase of the cyanation product was observed with
increasing amount of KCN or Zn(CN)2 even for a prolonged reaction
time (>48 h). An excess amount of cyanide source or base rather
decreases the reactivity. The restricted catalytic activity of complex
1 for aromatic cyanation of 1,3-dichloro-2-iodobenzene could be
ascribed to a limited thermal barrier in an aqueous biphasic system
and decomposition of palladium species, generating metal particles
which could be inactive for catalysis. The observation of black
Acknowledgments
This work was supported by the Dongguk University Research
Fund of 2011.
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