Four-coordinate Pd(II) complexes containing non-symmetric phosphorus ylides: Synthesis, characterization, and catalytic behavior towards Suzuki reaction

Article abstract of DOI:10.1016/j.jorganchem.2012.10.004

In this paper it is reported the synthesis of the complexes of the type {[Ph₂PCH₂PPh₂CHC(O)R]PdCl₂} (R = 4′-biphenyl (1), OCH₂Ph (2), 4-methylphenyl (3), 2-naphtyl (4), 2,4-dichlorophenyl (5), 3-nitro

Full text of DOI:10.1016/j.jorganchem.2012.10.004

Journal of Organometallic Chemistry 723 (2013) 207e213
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Journal of Organometallic Chemistry
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Four-coordinate Pd(II) complexes containing non-symmetric phosphorus ylides:
Synthesis, characterization, and catalytic behavior towards Suzuki reaction
,
a
a
b
Seyyed Javad Sabounchei a , Mohammad Panahimehr , Mohsen Ahmadi , Zahra Nasri ,
*
Hamid Reza Khavasi c
a Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65174, Iran
b Chemistry Department, University of Isfahan, Isfahan 81746-73441, Iran
c Department of Chemistry, Shahid Beheshti University, G.C., Evin, Tehran 1983963113, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper it is reported the synthesis of the complexes of the type {[Ph2PCH2PPh2CH]C(O)R]PdCl2}
(R ¼ 40-biphenyl (1), OCH2Ph (2), 4-methylphenyl (3), 2-naphtyl (4), 2,4-dichlorophenyl (5), 3-nitrophenyl
(6)) derived from the reactions of the dichloro(1,5-cyclooctadiene)palladium(II) with related phosphorus
ylides in dichloromethane under mild conditions. These compound have been characterized by, elemental
analysis and spectroscopic methods. The structure of complex 1 has been characterized crystallographi-
cally. This structure consists of five-membered ring formed by coordination of the ligand through the
phosphine group and the ylidic carbon atom to the metal center. The coordination geometry around the Pd
atoms in this complex can be defined as slightly distorted square planar. At a supramolecular level C
Received 6 July 2012
Received in revised form
25 September 2012
Accepted 2 October 2012
Keywords:
Palladium catalyst
Crystal structure
eH/Cl and CeH/p intermolecular interactions are responsible for the crystal packing. This complex
Suzuki cross-coupling reaction
has been found to act as efficient catalysts for the Suzuki cross-coupling reaction. Various aryl bromides
were coupled with aryl boronic acids in DMF, under air, in the presence of 0.2 mol% of the catalyst to afford
corresponding cross-coupled products in good to excellent yields.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
are various cross coupling reactions and among them the Suzuki
cross-coupling of aryl halides with aryl boronic acids is one of the
Phosphorus ylides are important reagents in organic chemistry,
especially in the synthesis of naturally occurring products with
biological and pharmacological activities [1]. Much of the interest
most useful method for synthesis of biaryl and hetero biaryl
derivatives [18e21]. The key advantages of the Suzuki coupling
compared to other coupling methods [22,23] are the low loading
catalyst [24,25] and the commercial availability of the diverse
boronic acids that are environmentally safer than the other
organometallic reagents [26e28]. In addition, the handling and
removal of boron-containing byproducts is easy when compared to
other organometallic reagents, especially in a large-scale synthesis.
in the coordination properties of
a-keto stabilized phosphorus
ylides stems from their coordination versatility due to the presence
of different functional groups in their molecular structure [2]. The
PeC, coordinated mode had been previously observed for Pd(II),
Pt(II), Rh(I), Hg(II) [3e12]. In addition, Rh(I) and Hg(II) were shown
to exhibit the P-bonding mode [5,12]. In this work, we have now
focused our attention in the study of the coordination mode
adopted by the resonance stabilized ylides when ligated to Pd(II).
Bidentate diphosphines are valuable ligands for metal-catalyzed
reactions. Metal-catalyzed cross-coupling reactions have gained
popularity over the past 30 years, in particular as convenient
techniques for the formation of carbonecarbon bonds [13].
Palladium-catalyzed CeC bond formation methodologies have
been proved as effective tools in organic synthesis [14e17]. There
2. Experimental
2.1. Physical measurements and materials
All synthetic procedures were performed using standard
Schlenk tube techniques under an atmosphere of dry nitrogen.
Starting materials were purchased from commercial sources and
used without further purification. Solvents were dried according to
the methods given in the literature and purified under inert
conditions [29]. The starting material dichloro(1,5-cyclooctadiene)
palladium(II) complex, [PdCl2(COD)] was prepared according to the
published procedures [30]. Melting points were measured on
* Corresponding author. Tel.: þ98 8118272072; fax: þ98 8118273231.
E-mail address: jsabounchei@yahoo.co.uk (S.J. Sabounchei).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.10.004

208
S.J. Sabounchei et al. / Journal of Organometallic Chemistry 723 (2013) 207e213
a SMP3 apparatus. IR spectra were recorded on a Shimadzu 435-U-
04 spectrophotometer from KBr pellets. Gas chromatography
carried out with a Shimadzu GC 14-A gas chromatograph and thin
layer chromatography on precoated silica gel fluorescent 254 nm
complex 1. Yellow solid. Anal. Calc. for C34H30Cl2OP2Pd: C, 58.85; H,
4.36. Found: C, 58.98; H, 4.39. Yield: 0.271 g (78%), m.p.199e201 ꢀC.
Selected IR absorption in KBr (cmꢁ1): 1621 ( C]O). 1H NMR (DMSO-
n
d6) dH (ppm): 4.70 (br, CH2), 5.94 (br, CH), 7.25e8.13 (m, 24H, Ph),
(CH3, merged with DMSO). 31P NMR (DMSO-d6) dP (ppm): 26.62
(bd, PPh2, 2JPP ¼ 47.83 Hz), 38.13 (d, PCH, 2JPP ¼ 48.03 Hz).
(0.2 mm) were used for monitoring the reaction progresses. 1H, 13
C
and 31P NMR spectra were recorded on 400 MHz Bruker and
90 MHz Jeol spectrometer in CDCl3 or DMSO-d6 as solvent at 25 ꢀC.
Chemical shifts (ppm) are reported according to internal TMS and
external 85% phosphoric acid. Coupling constants are given in Hz.
Elemental analysis for C, H and N atoms were performed using a
PerkineElmer 2400 series analyzer. All electrochemical measure-
ments were performed with a computer-controlled potentiostat,
Autolab electrochemical analyzer model PGSTAT30 (Eco Chemie,
Utrecht, The Netherlands) and a standard three electrode cell
consisting of GC working electrode, a platinum wire auxiliary
electrode and an Ag/AgCl (3 M KCl) reference electrode.
2.3.1.4. Data
for
{[Ph2PCH2PPh2CH]C(O)(C10H7)]PdCl2}
(4).
This compound was prepared by a similar procedure to that of
complex (1). Yellow solid. Anal. Calc. for C37H30Cl2OP2Pd: C, 60.88;
H, 4.14. Found: C, 60.72; H, 4.18%. Yield: 0.259 g (71%), m.p. 215e
217 ꢀC. Selected IR absorption in KBr (cmꢁ1): 1611 ( C]O). 1H
n
NMR (DMSO-d6) dH (ppm): 4.78 (br, CH2), 6.14 (s, CH), 7.29e9.05 (m,
27H, Ph and 2-naphtyl). 31P NMR (DMSO-d6) dP (ppm): 26.96 (bd,
PPh2, 2JPP ¼ 46.82 Hz), 38.17 (d, PCH, 2JPP ¼ 48.41 Hz).
2.3.1.5. Data for {[Ph2PCH2PPh2CH]C(O)(C6H3Cl2)]PdCl2} (5).
This compound was prepared by a similar procedure to that of
complex (1). Yellow solid. Anal. Calc. for C33H26Cl4OP2Pd: C, 52.94;
H, 3.50. Found: C, 53.04; H, 3.53%. Yield: 0.239 g (64%), m.p. 236e
2.2. X-ray crystallography
The single crystal X-ray diffraction data of suitable crystals of
1 were collected on a STOE IPDS-II diffractometer at 298 K,
using graphite monochromated Mo Ka radiation (
The data collection was performed using the
using the Stoe XAREA software package [31], while data reduc-
tion was carried out using the program X-RED [31]. The crystal
structures were solved by direct methods and refined by full-
matrix least squares on F2 using the programs SHELXS and
SHELXL, respectively [32], and using the XSTEP32 crystallo-
graphic software package [33]. The H atoms were included in
calculated positions and treated as riding atoms using SHELXL
[32] default parameters. Numerical absorption corrections were
applied for complex 1.
238 ꢀC. Selected IR absorption in KBr (cmꢁ1): 1624 ( C]O). 1H
n
ꢀ
l
¼ 0.71073 A).
NMR (DMSO-d6) dH (ppm): 4.75 (br, CH2), 5.67 (s, CH), 7.12e8.95
(m, 23H, Ph). 31P NMR (DMSO-d6) dP (ppm): 28.84 (bd, PPh2,
2JPP ¼ 41.34 Hz), 35.48 (d, PCH, 2JPP ¼ 43.49 Hz).
u
-scan technique
2.3.1.6. Data for {[Ph2PCH2PPh2CH]C(O)(C6H4NO2)]PdCl2} (6).
This compound was prepared by a similar procedure to that of
complex (1). Yellow solid. Anal. Calc. for C33H27Cl2NO3P2Pd: C,
54.68; H, 3.75; N, 1.93. Found: C, 54.46; H, 3.70; N, 1.89%. Yield:
0.297 g (82%), m.p. 192e194 ꢀC. Selected IR absorption in KBr
(cmꢁ1): 1623 ( C]O). 1H NMR (DMSO-d6) dH (ppm): 4.81(dd, CH2,
n
2JPH ¼ 12.46, 14.21 Hz), 6.15 (br, CH), 7.30e8.87 (m, 24H, Ph). 31P
NMR (DMSO-d6) dP (ppm): 26.69 (d, PCH, 2JPP ¼ 44.38 Hz), 38.13 (d,
PPh2, 2JPP ¼ 47.90 Hz).
2.3. Synthesis of compounds
The ligands Ph2PCH2PPh2CH]C(O)R [R ¼ 40-biphenyl, OCH2Ph,
4-methylphenyl, 2-naphtyl, 2,4-dichlorophenyl and 3-nitrophenyl]
were prepared based on the usual methods [34].
2.4. General experimental procedure for Suzuki cross-coupling
reactions
A mixture of an aryl halide (0.75 mmol), phenyl boronic acid
(1 mmol), complex 1 (0.2 mol%), K2CO3 (1.5 mmol), and DMF (2 ml)
was heated to 130 ꢀC for specified time. The reactions were
monitored by thin-layer chromatography. The reaction mixture was
then cooled to room temperature. The combined organic extracts
were washed with brine and dried over CaCl2. The solvent was
evaporated and a crude product was obtained and the product was
analyzed by 1H and 13C NMR. A small aliquot of the reaction
mixture was diluted in methanol for direct GC analysis. Yields were
calculated against consumption of the aryl halides.
2.3.1. Synthesis of Pd(II) halide complexes
2.3.1.1. Synthesis of {[Ph2PCH2PPh2CH]C(O)(C6H4Ph)]PdCl2} (1).
General procedure for complexes: To a [PdCl2(COD)] (0.142 g,
0.5 mmol) dichloromethane solution (5 ml),
a solution of
Ph2PCH2PPh2CH]C(O)C6H4Ph (0.289 g, 0.5 mmol) (5 ml, CH2Cl2)
was added dropwise. The resulting solution was stirred for 2 h at
room temperature and then concentrated to a ca. 2 ml in volume
and treated with n-hexane (ca. 25 ml) to afford a yellow solid. The
product was collected and dried under vacuum. Anal. Calc. for
C39H32Cl2OP2Pd: C, 61.96; H, 4.27. Found: C, 62.08; H, 4.23%. Yield:
0.257 g (68%), m.p. 189e191 ꢀC. Selected IR absorption in KBr
2.4.1. 4-Nitro-biphenyl (entry 1, Table 3)
(cmꢁ1): 1614 (
n
C]O). 1H NMR (DMSO-d6) dH (ppm): 3.74(br, CH2),
Mp: 112e113 ꢀC. 1H NMR (89.6 MHz, CDCl3, ppm):
d
¼ 7.43e8.35
6.12 (br, CH), 7.41e8.32 (m, 29H, Ph). 31P NMR (DMSO-d6) dP (ppm):
(m, phenyl, 9H). 13C NMR (100 MHz, CDCl3, ppm):
138.7, 129.1, 128.8, 127.7, 127.3, 124.0 [35,36].
d
¼ 147.5, 147.4,
23.54 (d, PPh2, JPP ¼ 45.11 Hz), 34.72 (d, PCH, 2JPP ¼ 46.16 Hz).
2
2.3.1.2. Data for {[Ph2PCH2PPh2CH]C(O)(OCH2Ph)]PdCl2} (2).
This compound was prepared by a similar procedure to that of
complex 1. Yellow solid. Anal. Calc. for C34H30Cl2O2P2Pd: C, 57.53; H,
4.26. Found: C, 57.65; H, 4.31%. Yield: 0.266 g (75%), m.p. 195e
2.4.2. 4-Nitro-40-ethyl-biphenyl (entry 2, Table 3)
1H NMR (89.6 MHz, CDCl3):
d
¼ 7.28e8.34 (m, phenyl, 8H), 2.70
(q, CH2, 3J ¼ 7.5 Hz, 2H), 1.30 (t, CH3, 3J ¼ 7.4 Hz, 3H). 13C NMR
(22.5 MHz, CDCl3, ppm):
d
¼ 149.9, 148.6, 141.3, 128.8, 127.6, 127.4,
197 ꢀC. Selected IR absorption in KBr (cmꢁ1): 1638 (
n
C]O). 1H
126.9, 124.1, 28.69 (s, CH2), 15.43 (s, CH3).
NMR (DMSO-d6) dH (ppm): 3.79 (bt, CH2, 2JPH ¼ 11.97 Hz), 6.24 (br,
CH), 5.54 (s, 2H, CH2O) 7.33e7.87 (m, 25H, Ph). 31P NMR (DMSO-d6)
dP (ppm): 21.06 (bd, PPh2, 2JPP ¼ 11.20 Hz), 24.07 (br, PCH).
2.4.3. 4-Carboxaldehyde-biphenyl (entry 3, Table 3)
IR (KBr, cmꢁ1):
n
¼ 3057, 3031, 1699 (C]O), 1604, 761, 730, 697.
1H NMR (89.6 MHz, CDCl3, ppm):
¼ 9.91 (s, CHO, 1H), 7.30e7.86
(m, phenyl, 9H). 13C NMR (100 MHz, CDCl3, ppm):
¼ 191.9
(s, C]O), 147.0, 139.6, 135.0, 130.2, 128.9, 128.4, 127.6, 127.2 [37].
d
2.3.1.3. Data for {[Ph2PCH2PPh2CH]C(O)(C6H4Me)]PdCl2} (3).
This compound was prepared by a similar procedure to that of
d

S.J. Sabounchei et al. / Journal of Organometallic Chemistry 723 (2013) 207e213
209
2.4.4. 4-Carboxaldehyde-40-ethyl-biphenyl (entry 4, Table 3)
IR (KBr, cmꢁ1):
¼ 3024, 2965, 2936, 1682 (C]O), 1605, 881,
complexes around 1620 cmꢁ1 indicate coordination of the ylide
through ylidic carbon atom (Supporting information, Table S1).
Thus the IR absorption bands for the complexes at higher fre-
n
835, 808. 1H NMR (89.6 MHz, CDCl3, ppm):
d
¼ 10.04 (s, CHO, 1H),
7.24e7.59 (m, phenyl, 8H), 2.68 (q, CH2, 3J ¼ 7.9 Hz, 2H), 1.30 (t, CH3,
quencies indicate that C-coordination has occurred [11]. The n(CO),
3J ¼ 7.4 Hz, 3H). 13C NMR (100 MHz, CDCl3, ppm):
d
¼ 191.8 (s, C]
which is sensitive to complexation, occurs at around 1500 cmꢁ1 in
the parent ylides [34], as in the case of other resonance stabilized
ylides [40]. Coordination of the ylides through C-coordination
O), 147.0, 144.7, 136.8, 134.8, 130.1, 128.4, 127.3, 127.1.
2.4.5. 4-Methyl-biphenyl (entry 5, Table 3)
causes an increase in
n(CO) [41e43] while for O-coordination
Mp: 48e50 ꢀC. 1H NMR (89.6 MHz, CDCl3, ppm):
d
¼ 6.99e7.47
a lowering of (CO) is expected [44]. Thus, the IR data obtained
n
(m, phenyl, 9H), 2.27 (s, CH3, 3H). 13C NMR (100 MHz, CDCl3, ppm):
indicates that the interaction of the ligands with palladium(II)
chloride are through the ylidic carbon atom and PPh2 group [5,45],
as proved by the crystal structure of 1 (vide infra).
d
¼ 141.1, 138.3, 136.9, 129.4, 130.1, 128.6, 126.9, 21.0 [38].
2.4.6. 4-Methyl-40-ethyl-biphenyl (entry 6, Table 3)
The 31P chemical shift values for the complexes appear to be
shifted downfield with respect to the parent ylides, also indicating
that coordination of the ylides have occurred. The 31P NMR spectra
of complexes 1e6 result more informative and in the most of the
cases exhibit two doublets that indicate the presence of the PCHCO
and PPh2 groups in these molecules. In the 31P NMR spectra the
signal due to PCH appears as a broad signal for 2 and doublets for 1
and 3e5. The significant downfield shift of these signals from that
of the free ylides are in agreement with the C-bonding of the ylides
[43,45]. The coordination via the phosphine moiety is also clearly
evidenced from the strong downfield shifts of the signals due to
PPh2 group when compared to that of the same signal in the free
ylides [4,11]. Analogously, in the 1H NMR spectra of all complexes,
the signals due to the methine proton appear in the same region as
observed for the free ligands. In the 1H NMR spectra the signal due
to PCH appears as a broad signal for 1e3, 6 and singlet peak for 4
and 5. Similar behavior was observed earlier in the case of ylide
complexes derived from platinum(II) chloride [46]. The expected
lower shielding of both the 31P and 1H nuclei for the PC(H) group
upon complexation in the case of C-coordination was observed in
their corresponding spectra. Selected NMR data are reported in
Table S1.
1H NMR (89.6 MHz, CDCl3, ppm):
d
¼ 7.21e7.82 (m, phenyl, 8H),
2.71 (q, CH2, 3J ¼ 7.9 Hz, 2H), 2.45 (s, CH3, 3H), 1.30 (t, CH3 (Ethyl),
3J ¼ 8.1 Hz, 3H). 13C NMR (100 MHz, CDCl3, ppm):
d
¼ 143.0, 138.4,
138.2, 136.6, 129.4, 128.2, 126.85, 126.80, 28.4 (s, CH2), 21.0 (s, CH3),
15.6 (s, CH3 (Ethyl)).
2.4.7. 1-(4-Ethylphenyl)naphthalene (entry 8, Table 3)
1H NMR (89.6 MHz, CDCl3, ppm):
d
¼ 7.11e8.50 (m, phenyl, 11H),
2.83 (q, CH2, 3J ¼ 7.6 Hz, 2H), 1.45 (t, CH3, 3J ¼ 7.3 Hz, 3H). 13C NMR
(100MHz,CDCl3, ppm):
d
¼ 143.1,140.2,137.9,133.7,131.6,129.9,128.2,
127.7,127.3,126.8,126.0,125.8,125.6,125.3, 28.6 (s, CH2),15.5 (s, CH3).
2.4.8. 4-Ethyl-40-biphenyl carboxylic acid (entry 12, Table 3)
IR (KBr, cmꢁ1):
n
¼ 3025, 3014, 2984, 1722 (C]O), 1604, 792,
765, 739. 1H NMR (89.6 MHz, CDCl3, ppm):
d
¼ 10.4 (s, COOH, 1H),
6.92e7.81 (m, phenyl, 8H), 2.68 (q, CH2, 3J ¼ 6.5 Hz, 2H),1.30 (t, CH3,
3J ¼ 7.8 Hz, 3H).
2.4.9. 2-(4-Ethylphenyl)thiophene (entry 14, Table 3)
1H NMR (89.6 MHz, CDCl3, ppm):
d
¼ 6.91e7.80 (m, phenyl and
thiophene, 7H), 2.62 (q, CH2, 3J ¼ 8.1 Hz, 2H),1.16 (t, CH3, 3J ¼ 7.2 Hz,
3H). 13C NMR (100 MHz, CDCl3, ppm):
126.8, 125.9, 124.2, 122.5, 28.5 (s, CH2), 15.5 (s, CH3).
d
¼ 131.8, 128.3, 128.2, 127.8,
3.3. X-ray crystallography
3. Results and discussion
Complex 1 was recrystallized by slow evaporation over several
days from dimethylsulfoxide solutions. The molecular structures are
shown in Fig.1. Relevant parameters concerning data collection and
refinement are given in Table S2 and selected bond distances and
angles are collected in Table 1. Fractional atomic coordinates and
equivalent isotropic displacement coefficients (Ueq) for the non-
hydrogen atoms of this complex are available as Supplementary
material.
The X-ray analysis in complex 1 reveals the P, C-chelated mode of
coordination of the ligand Ph2PCH2PPh2CH]C(O)C6H4Ph to Pd(II)
atom in this complex. The Pd atom is surrounded by one P atom of
the phosphine group, one ylidic carbon and two chlorine atoms
leading to a slightly distorted square-planar geometry around the
metal. The angles subtended by the ligand at the Pd(II) center in 1
vary from 89.58(4)ꢀ to 90.87(4)ꢀ (sum of the angles is 360.14) and
176.89(13)ꢀ to 177.48(5)ꢀ) indicating the distortion from the square-
planar environment. The Pd(1)eP(2) bond length in this complex
3.1. Synthesis
The reaction of [PdCl2(COD)] with the non-symmetric phos-
phorus ylides Ph2PCH2PPh2CH]C(O)R [R ¼ 40-biphenyl, OCH2Ph, 4-
methylphenyl, 2-naphtyl, 2,4-dichlorophenyl and 3-nitrophenyl] in
dichloromethane in a 1:1 M ratio, at room temperature afforded, in
all cases the PeC chelated complexes (see Scheme 1). These species
being fairly soluble in halogenated solvents such as dichloro-
methane but insoluble in non-polar solvents such as n-hexane.
3.2. Spectroscopy
The coordination of the ylide through carbon or oxygen causes
a significant increase or decrease, respectively, in the
frequency [39]. The infrared absorption bands observed for the six
n(CO)
PPh2
CH
(1): R= 4'-biphenyl
(2): R= OCH2Ph
(3): R= 4-methylphenyl
(4): R= 2-naphtyl
(5): R= 2,4-dichlorophenyl
(6): R= 3-nitrophenyl
CH
C
O
R
Ph2P
-COD
Ph2P
C
O
R
PdCl2(COD)
+
Pd
PPh2
Cl
Cl
Scheme 1.

210
S.J. Sabounchei et al. / Journal of Organometallic Chemistry 723 (2013) 207e213
Fig. 1. Molecular structure of 1 with ellipsoids at the 50% probability level. Hydrogen
atoms are omitted for reasons of clarity.
ꢀ
[2.2289(11) A] is comparable to analogous distances in palladium
complexes such as [PdCl2(dppe)] (dppe ¼ 1,2-(diphenylphosphino)
ethane) [47] and {[(Ph2P(CH2)2PPh2CH]C(O)C6H4Cl)] PdCl2} [45].
ꢀ
The PdeC(ylide) bond length in complex 1 (2.116(4) A is similar to
that found in [PdCl2{PPh2C(H)C(O)CH2PPh2C(H)}] (2.099(6) and
ꢀ
2.104(6) A) [9]. The stabilized resonance structure for the parent
ylide is destroyed by the complex formation, thus the C(13)eC(14)
ꢀ
bond length in 1 (1.484(6) A) is significantly longer than the corre-
sponding distance found in the uncomplexed parent ylide
Fig. 2. Best view of 1 revealing the supramolecular packing. Intermolecular CeH/Cl
and CeH/ bonds are shown as blue dashed lines. Other hydrogen atoms are omitted
for clarity. (For interpretation of the references to colour in this figure legend, the
ꢀ
Ph2PCH2PPh2CH]C(O)C6H4Ph (1.253(3) A) [34]. On the other hand,
p
ꢀ
the bond length of PeC(H) in the parent ylide is 1.722(3) A [34]
reader is referred to the web version of this article.)
which shows that the corresponding bonds are considerably elon-
gated to 1.776(4) in 1. An interesting structural feature, common to
the complex 1, concerns the crystal packing, where intermolecular
interactions of molecules are observed. The crystal packing of the
with phenylboronic acid in presence of various solvents and K2CO3
base. In all instances the solvent was used as obtained commercially
without further purification and the reactions were performed in air.
After completion of the reaction, the biphenyl was isolated from the
reactionmixture. Theresultsof theabovereactionsaresummarizedin
Table 2. In the present study non-polar solvent like toluene gave
moderate yield of 63% conversion (entry 3), whereas polar solvents
such as methanol or DMF are found to be more efficient for the
conversion of biaryl compounds to 71% and 97% respectively (entries
2, 4). On the other hand polar aprotic solvents (entries 1 and 5) gave
comparatively less yields. The lowest yield (25%) was obtained with
THF as the solvent. In water the reaction is not progressing in room
complex is stabilized by intermolecular CeH/Cl and CeH/
interactions (Cl2eH3 ¼ 2.921 A and C8eH31 ¼ 2.825 A). Crystal
packing of complex 1 is reported in Fig. 2.
p
ꢀ
ꢀ
3.4. Suzuki coupling reactions of aryl halides
Firstly, we carried out a model reaction to optimize the reaction
conditions. Various parameters including solvents and bases were
investigated. To study the effects of different solvents in our catalytic
system, we have chosen the reaction between 4-bromobenzaldehyde
Table 1
ꢀ
ꢀ
Table 2
Selected bond lengths (A) and bond angles ( ) for 1.
Optimization of base and solvent for Suzuki cross-coupling reaction.a
Bond distances
Pd(1)eC(14)
Entry
Base
Solvent
Temp (ꢀC)
Time (h)
Yield (%)b
2.116(4)
Pd(1)eP(2)
Pd(1)eCl(1)
Pd(1)eCl(2)
O(1)eC(13)
P(2)eC(27)
P(1)eC(14)
C(13)eC(14)
Bond angles
C(14)ePd(1)eP(2)
C(14)ePd(1)eCl(1)
C(14)ePd(1)eCl(2)
P(2)ePd(1)eCl(2)
P(2)ePd(1)eCl(1)
Cl(1)ePd(1)eCl(2)
2.2289(11)
2.3716(11)
2.3494(10)
1.226(5)
1.852(4)
1.776(4)
1
2
3
4
5
6
7
8
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
Na2CO3
Cs2CO3
NaOAc
NaF
Dioxane
DMF
Toluene
Methanol
THF
H2O
DMF
DMF
DMF
130
130
130
65
8
2
6
12
24
24
24
4
68
97
63
71
25
e
60
100
130
130
130
130
130
1.484(6)
Trace
72
68
76
54
90.03(11)
89.66(11)
176.89(13)
89.58(4)
177.48(5)
90.87(4)
9
10
11
5
8
8
DMF
DMF
a
Reaction conditions: 4-bromobenzaldehyde (0.75 mmol), phenylboronic acid
(1 mmol), base (1.5 mmol), solvent (2 ml), catalyst (0.2 mol%), 130 ꢀC.
b
GC yield.

S.J. Sabounchei et al. / Journal of Organometallic Chemistry 723 (2013) 207e213
211
Table 3
temperature as well as in reflux condition. Further, the effect of
different mineral bases on this reaction was investigated by using the
coupling of 4-bromobenzaldehyde with phenylboronic acid as a test
case. First the reaction was conducted without any base and no
reaction was observed. In presence of NaOAc and NaF base shows
a 76% and 54% yield, respectively (entries 10,11). Similarly Na2CO3 and
Cs2CO3 did not significantly affect the reactions carried out at 130 ꢀC
(Table 2). Among the different bases used for our studies, the K2CO3
wasthebestchoiceofbase, andtheyieldofproductcouldbeincreased
to 97% (entry 2). After an optimization process, we found that our
complex could perform the coupling of 4-bromobenzaldehyde with
phenylboronic acid using 0.2 mol% catalyst loading at 130 ꢀC in very
good yieldin acouple ofhours, whenusingtechnicalgradeDMFasthe
solvent (Table 2, entry 2).
Suzuki cross-coupling reaction of aryl halides with aryl boronic acids catalyzed by
complex 1.a
Entry AreX
Ar0eB(OH)2
Product
Time (h) Yield
(%)b
1
2
3
4
5
6
7
8
p-O2NePheBr PhB(OH)2
p-O2NePhePh
2
89
85
97
95
83
76
81
79
80
73
91
82
75
74
76
63
p-O2NePheBr p-EtePhB(OH)2 p-O2NePhePhep-Et
p-OHCePheBr PhB(OH)2 p-OHCePhePh
p-OHCePheBr p-EtePhB(OH)2 p-OHCePhePhep-Et
2
2
2
p-MeePheBr
p-MeePheBr
C10H7eBrc
C10H7eBr
PheBr
PhB(OH)2
p-EtePhB(OH)2 p-MeePhePhep-Et
PhB(OH)2 C10H7ePh
p-EtePhB(OH)2 C10H7ePhep-Et
PhB(OH)2 PhePh
p-EtePhB(OH)2 p-EtePhePh
p-HO2CePhePh
p-MeePhePh
3.5
3.5
5
5
10
10
4
4
9
10
11
12
13
14
15
16
PheBr
p-BrePheCO2H PhB(OH)2
P-BrePheCO2H p-EtePhB(OH)2 p-HO2CePhePhep-Et
The Suzuki reaction, involving the coupling of an organic halide
or pseudo-halide with an organoboron reagent in the presence of
a base and a Pd catalyst, is arguably the most used cross-coupling
procedure for the formation of CeC bonds [21,48,49]. Under the
optimized reaction conditions in hand, we have taken a series of
aryl halide, aryl boronic acids and the yield of the coupled products
are given in Table 3. The complex 1 was tested as catalyst in the
Suzuki reaction of aryl halides with aryl boronic acids. Following
optimization experiments we found that the use of 0.2 mol% of 1
with K2CO3 as the base at 130 ꢀC in DMF appeared to be best. Under
these conditions, aryl halides react with phenylboronic acid in
average to excellent yields (Table 3). In palladium catalyzed
carbonecarbon bond formation reactions, it is commonly believed
that better yields are achieved for aryl halides with electron-
withdrawing rather than electron donating substituent [50]. The
reaction of electron deficient aryl bromides with arylboronic acids
was transformed efficiently to the coupling products in >82% yield
in short reaction times (Table 3, entries 1e4 and 11e12). The
reaction of 4-bromobenzaldehyde with phenylboronic acid shows
excellent yield (entry 3, 97%). In the electron rich or deactivated p-
bromotoluene the yield are good (Table 3, entry 5, 83% and entry 6,
76%). The electronically neutral bromobenzene (entry 1) produced
good amount of desired product when coupled with aryl boronic
acids. As expected, very satisfactory results were obtained with the
2-bromothiophene and 1-bromonaphthalene of the arylboronic
acid (entries 7e8, 13e14) with good yields. Several homocoupling
reactions of thiophene derivatives have been shown to proceed
with a transition-metal catalyst [51]. Especially, the homocoupling
reactions have been monitored, when bromothiophene was
employed. The CeH homocoupling of bromothiophene derivatives
was found to take place under mild conditions and use of the
activator system, such as AgF [51]. Thus homocoupling reactions in
presence of dichloropalladacycle complex 1 as catalyst were not
observed. Trying to apply aryl chloride as an efficient substrate was
successful (Table 3, entries 15e16). The main drawback of the Pd-
mediated Suzuki cross-coupling reaction is that only aryl iodides
C4H3SeBrd
C4H3SeBr
PheCl
PhB(OH)2
p-EtePhB(OH)2 C4H3SePh-p-Et
PhB(OH)2 PhePh
p-EtePhB(OH)2 p-EtePhePh
C4H3SePh
5
5
12
12
PheCl
a
Reaction conditions: aryl halide (0.75 mmol), boranic acid (1 mmol), K2CO3
(1.5 mmol), DMF (2 ml), catalyst (0.2 mol%), 130 ꢀC.
b
GC yields based on aryl halide.
1-Bromonaphthalene.
2-Bromothiophene.
c
d
and aryl bromides can be used efficiently. Recent progress on
increasing the reaction rate of aryl chlorides permitted to overcome
this problem [52e54]. The stronger CeCl bond is in fact responsible
of the slower reaction rate of aryl halides, because the oxidative
addition step was suggested being the rate determining step in
cross-coupling catalytic cycles [55]. The catalytic activity of the
present complex is good when compared to cyclopalladated
complexes [56,57]. The CeC cross coupling reactions was con-
ducted under air and without degassing or drying the solvents. This
protocol represents a great advantage with regard to the standard
protocols with conventional palladiumephosphine complexes
[58e60].
Although several catalytic systems have been reported to
support Suzuki CeC coupling reaction, a catalyst of this type is novel
for its P and CH of phosphorus ylide environment. The high effi-
ciency of this catalyst in low catalyst loading and short reaction time
make them valuable. The comparison of conditions of palladacycle
bisphosphine monoylide, monophosphine and bisphosphine are
presented inTable 4. These data show the efficiency of newcatalyst 1
toward the Suzuki reactions. Of note is that the best results for the
coupling of aryl boronic acids with aryl halides were obtained with
phosphapalladacycles or with palladacycles modified with carbenes
or phosphorus containing ligands (see entries 1, 3, 4 and 7 of
Table 4). This might be rationalized by the extra stability provided by
these ligands for the stabilization of the low-ligated catalytically
active Pd(0) species involved in the main catalytic cycle [61]. In all of
Table 4
Comparison catalytic activity of some palladacycles complexes that promote the Suzuki coupling between phenylboronic acid and aryl halides.
Entry
AreX
Palladacycles
complexes
[Pd] (mol%)
Base
Solvent
Conditions
Yield (%)
Ref.
1
2
3
4
5
6
7
8
9
4-MeOPhBr
4-MeOPhBr
PhBr
PhBr
4-OHCPhBr
PhI
4-MeOCPhBr
PhBr
PhCl
[PdCl2(P ˇ P)]
[PdCl2(P ˇ S)]
[PdCl2(P ˇ C)]
[PdCl2(P ˇ P)]
[PdCl2(P ˇ N)]
[PdCl2(P ˇ O)]
[PdCl2(P ˇ P)]
[PdCl2(P ˇ N)]
[PdCl2(P ˇ N)]
[PdCl2(P ˇ C)]
0.04
0.04
0.5
1
0.1
Et3N
Et3N
CH3CN
CH3CN
Dioxane
Dioxane
Toluene
THF
Dioxane
Toluene
Toluene
DMF
25 ꢀC, 24 h
25 ꢀC, 24 h
80 ꢀC, 16 h
100 ꢀC, 16 h
100 ꢀC, 50 min
130 ꢀC, 20 h
100 ꢀC, 24 h
100 ꢀC, 24 h
110 ꢀC, 2 h
130 ꢀC, 2 h
88
87
81
100
95
35
86
79
53
97
[62]
[62]
[63]
[59]
[64]
[65]
[66]
[67]
Cs2CO3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
0.8
0.01
0.00025
0.00025
0.2
[67]
This work
10
4-OHCPhBr

212
S.J. Sabounchei et al. / Journal of Organometallic Chemistry 723 (2013) 207e213
Appendix A. Supplementary material
CCDC 888457 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Appendix B. Supplementary material
Supporting information related to this article can be found at
http://dx.doi.org/10.1016/j.jorganchem.2012.10.004.
References
Fig. 3. Cyclic voltammograms of 1 mM of phosphorus ylide and complex 1 in aceto-
nitrile containing 0.1 M TBATFB as supporting electrolyte, at GC electrode and scan rate
of 100 mV/s.
[1] G. Wittig, Angew. Chem. 92 (1980) 671e675.
[2] E.P. Urriolabeitia, Dalton Trans. (2008) 5673e5686.
[3] Y. Oosawa, H. Urabe, T. Saito, Y. Sasaki, J. Organomet. Chem. 122 (1976) 113e121.
[4] S.M. Sbovata, A. Tassan, G. Facchin, Inorg. Chim. Acta 361 (2008) 3177e3183.
[5] M.M. Ebrahim, H. Evans, K. Panchanatheswaran, Polyhedron 26 (2007) 3491e
3495.
the successful reactions, the 1H and 13C NMR spectra of the products
indicated no Suzuki coupling byproducts.
[6] L.R. Falvello, S. Fernandez, R. Navarro, E.P. Urriolabeitia, Inorg. Chem. 39
(2000) 2957e2960.
[7] A. Spannenberg, W. Baumann, U. Rosenthal, Organometallics 19 (2000) 3991e
3993.
3.5. Electrochemical studies
[8] R. Uson, J. Fornies, R. Navarro, A.M. Ortega, J. Organomet. Chem. 334 (1987)
389e397.
[9] L.R. Falvello, M.E. Margalejo, R. Navarro, E.P. Urriolabeitia, Inorg. Chim. Acta
347 (2003) 75e85.
[10] H. Takahashi, Y. Oosawa, A. Kobayashi, T. Saito, Y. Sasaki, Chem. Lett. (1976)
15e16.
[11] M.M. Ebrahim, K. Panchanatheswaran, A. Neels, H. Stoeckli-Evans, J. Organo-
met. Chem. 694 (2009) 643e648.
[12] D. Saravanabharathi, T.S. Venkatakrishnan, M. Nethaji, S.S. Krishnamurthy,
Proc. Indian Acad. Sci. (Chem. Sci.) 115 (2003) 741e749.
[13] E. Negishi, F. Liu, A. Suzuki, S. Brase, A.D. Meijere, Metal-catalysed Cross-
coupling Reactions, in: F. Diederich, P.J. Stang (Eds.), Palladium-catalyzed
Coupling of Organyl Halides to Alkenes-the Heck Reaction, Wiley-VCH,
Weinheim, 1998, pp. 99e166.
[14] K. Sonogashira, Y. Tohda, N. Hagihara, Tetrahedron Lett. 16 (1975) 4467e
4470.
[15] D.L. Martins, H.M. Alvarez, L.C.S. Aguiar, Tetrahedron Lett. 51 (2010) 6814e
6817.
[16] N. Miyaura, T. Yanagi, A. Suzuki, Synth. Commun. 11 (1981) 513e519.
[17] D. Milstein, J.K. Stille, J. Am. Chem. Soc. 101 (1979) 4992e4998.
[18] N.E. Leadbeater, M. Marco, J. Org. Chem. 68 (2003) 888e892.
[19] B. Basu, P. Das, M.M.H. Bhuiyan, S. Jha, Tetrahedron Lett. 44 (2003) 3817e
3820.
[20] A. Suzuki, J. Organomet. Chem. 576 (1999) 147e168.
[21] N. Miyaura, A. Suzuki, Chem. Rev. 95 (1995) 2457e2483.
[22] J. Hassan, M. Se’vignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev. 102 (2002)
1359e1470.
Electrochemical studies of phosphorus ylide as ligand and Pd (II)
complex (1) have been carried out by cyclic voltammetry (CV) in
acetonitrile solution, containing 0.1 M tetrabutylammonium tet-
rafluoroborate (TBATFB) as supporting electrolyte using glassy
carbon (GC) working electrode and Pt wire counter electrode. All
the potentials were referenced to Ag/AgCl reference electrode.
Cyclic voltammograms of GC electrode in acetonitrile containing
0.1 M TBATFB and also in the solution of 1 mM of phosphorus ylide
and 1 mM of complex 1 in acetonitrile, containing 0.1 M TBATFB at
scan rate of 100 mV/s are shown in Fig. 3. For the complex 1 the
cathodic potential peak corresponds to the redox pair of Pd(II)/Pd(I)
appeared at ꢁ0.76 V vs. Ag/AgCl. That reduction was due to one
electron transfer process. When the potential was scanned further in
the cathodic direction, the second cathodic potential peak at ꢁ1.15 V
was observed, which corresponds to the redox pair of Pd(I)/Pd(0).
There were no anodic peaks observed on the reverse scan [68]. This
behavior is similar tothat reported previously in cyclic voltammetric
study of various types of Pd(II) complexes in aprotic solvents [69,70].
This irreversibility may be due to the reaction of a Pd(0) complex
with adventitious O2 or other components of the solution. The CV
data reveal the potential of 1 for catalytic oxidation.
[23] K. Tamao, K. Sumitani, M. Kumada, J. Am. Chem. Soc. 94 (1972) 4374e4376.
[24] J. Zhang, L. Zhao, M. Song, T.C.W. Mak, Y. Wu, J. Organomet. Chem. 691 (2006)
1301e1306.
4. Conclusions
[25] K. Karami, C. Rizzoli, M. Mohamadi Salah, J. Organomet. Chem. 696 (2011)
940e945.
[26] A. Suzuki, Pure Appl. Chem. 57 (1985) 1749e1758.
[27] A. Suzuki, Pure Appl. Chem. 66 (1994) 213e222.
[28] A.R. Martin, Y. Yang, Acta Chem. Scand. 47 (1993) 221e230.
[29] A.J. Gordon, R.A. Ford, The Chemist’s Companion: A Handbook of Practical
Data, Techniques and References, Wiley-Interscience, New York, 1972.
[30] D. Drew, J.R. Doyle, Inorg. Synth. 13 (1972) 52e55.
[31] Stoe & Cie, X-AREA and X-RED, Version 1.30: Program for Data Reduction and
Absorption Correction, Stoe & Cie GmbH, Darmatadt, Germany, 2005.
[32] G.M. Sheldrick, Acta Crystallogr. A64 (2008) 112e122.
[33] Stoe & Cie, X-STEP32, Version 1.07b: Crystallographic Package, Stoe & Cie
GmbH, Darmstadt, Germany, 2000.
In this paper, we have shown that Palladium(II) chloride reacts
with new bidentate phosphorus ylides to form P, C-chelated
complexes. On the basis of the physicochemical and spectroscopic
data we have proposed that ligands employed herein exhibit
a chelate PeC coordination behavior to the metal center affording
a five-membered chelate rings, which is further confirmed by the
X-ray crystal structure of the complex 1. In this work, we used
palladium(II) complexes as an efficient catalyst for the Suzuki
reaction of various aryl halides. The complex 1 is reported of
a highly active and efficient catalyst for promoting the Suzuki cross-
coupling reaction of various aryl halides to produce the corre-
sponding products in average to excellent yields.
[34] Sabounchei et al., unpublished results.
[35] Z. Zhang, Z. Wang, J. Org. Chem. 71 (2006) 7485e7487.
[36] J. Ma, X. Cui, B. Zhang, M. Song, Y. Wu, Tetrahedron 63 (2007) 5529e5538.
[37] J.H. Li, Q.M. Zhu, Y.X. Xie, Tetrahedron 62 (2006) 10888e10895.
[38] L. Liang, P. Chien, M. Huang, Organometallics 24 (2005) 353e357.
[39] M. Kalyanasundari, K. Panchanatheswaran, W.T. Robinson, H. Wen,
J. Organomet. Chem. 491 (1995) 103e109.
Acknowledgments
[40] J. Vicente, M.T. Chicote, M.C. Lagunas, P.G. Jones, J. Chem. Soc. Dalton Trans.
(1991) 2579e2583.
[41] S.J. Sabounchei, A. Dadrass, M. Jafarzadeh, S. Salehzadeh, H.R. Khavasi,
J. Organomet. Chem. 692 (2007) 2500e2507.
[42] S.J. Sabounchei, H. Nemattalab, S. Salehzadeh, M. Bayat, H.R. Khavasi,
H. Adams, J. Organomet. Chem. 693 (2008) 1975e1985.
We are grateful to the Bu-Ali Sina University for a grant and Mr.
Zebarjadian for recording the NMR spectra. Our gratitude also goes
to Miss. Behranj for all cooperation.

S.J. Sabounchei et al. / Journal of Organometallic Chemistry 723 (2013) 207e213
213
[43] S.J. Sabounchei, S. Samiee, S. Salehzadeha, Z. Bolboli Nojini, E. Irran,
J. Organomet. Chem. 695 (2010) 1441e1450.
[58] L. Chahen, B. Therrien, G. Süss-Fink, J. Organomet. Chem. 691 (2006) 4257e
4264.
[44] H. Koezuka, G. Matsubayashi, T. Tanaka, Inorg. Chem. 15 (1976) 417e421.
[45] S.J. Sabounchei, S. Samiee, D. Nematollahia, A. Naghipour, D.M. Morales, Inorg.
Chim. Acta 363 (2010) 3973e3980.
[59] R.C. Smith, C.R. Bodner, M.J. Earl, N.C. Sears, N.E. Hill, L.M. Bishop, N. Sizemore,
D.T. Hehemann, J.J. Bohn, J.D. Protasiewicz, J. Organomet. Chem. 690 (2005)
477e481.
[46] J.A. Teagle, J.L. Burmeister, Inorg. Chim. Acta 118 (1986) 65e72.
[47] W.L. Steffen, G.J. Palenik, Inorg. Chem. 15 (1976) 2432e2439.
[48] O. Navarro, S.P. Nolan, CeC Bond Formation By Cross-coupling, in:
R.H. Crabtree, D.M.P. Mingos (Eds.), Comprehensive Organometallic Chemistry
III, Elsevier, Amsterdam, 2007, pp. 1e37.
[60] P. Das, U. Bora, A. Tairai, C. Sharma, Tetrahedron Lett. 51 (2010) 1479e
1482.
[61] V.V. Dunina, O.N. Gorunova, Russ. Chem. Rev. 73 (2004) 309e350.
[62] K. Saikia, B. Deb, B.J. Borah, P.P. Sarmah, D.K. Dutta, J. Organomet. Chem. 696
(2012) 4293e4297.
[49] A. De Mejiere, P.J. Stan, Metal-catalyzed Cross-coupling Reactions, second ed.,
Wiley-VCH, Weinheim, 2004.
[50] N. Kataoka, Q. Shelby, J.P. Stambuli, J.F. Hartwig, J. Org. Chem. 67 (2002) 5553e
5566.
[51] M. Takahashi, K. Masui, H. Sekiguchi, N. Kobayashi, A. Mori, M. Funahashi,
N. Tamaoki, J. Am. Chem. Soc. 128 (2006) 10930e10933.
[52] V.P.W. Böhm, C.W.K. Gstöttmayr, T. Weskamp, W.A. Herrmann, J. Organomet.
Chem. 595 (2000) 186e190.
[53] C. Zhang, M.L. Trudell, Tetrahedron Lett. 41 (2000) 595e598.
[54] A. Fürstner, A. Leitner, Synlett (2001) 290e292.
[55] S.R. Chemler, D. Trauner, S.J. Danishefsky, Angew. Chem. Ed. Engl. 40 (2001)
4544e4568.
[56] K. Matos, L.L. de Oliveira, C. Favero, A.L. Monteiro, M. Hörner, J.F. Carpentier,
M.P. Gil, O.L. Casagrande Jr., Inorg. Chim. Acta 362 (2009) 4396e4402.
[57] S.M. Nobre, A.L. Monteiro, J. Mol. Catal. A Chem. 313 (2009) 65e73.
[63] H.M. Lee, P.L. Chiu, J.Y. Zeng, Inorg. Chim. Acta 357 (2004) 4313e4321.
[64] T. Mahamo, M.M. Mogorosi, J.R. Moss, S.F. Mapolie, J. Chris Slootweg,
K. Lammertsma, G.S. Smith, J. Organomet. Chem. 703 (2012) 34e42.
[65] K. Nishide, H. Liang, S. Ito, M. Yoshifuji, J. Organomet. Chem. 690 (2005) 4809e
4815.
[66] J.C. Hierso, A. Fihri, R. Amardeil, P. Meunier, Organometallics 22 (2003) 4490e
4499.
[67] A. Scrivanti, M. Bertoldini, U. Matteoli, S. Antonaroli, B. Crociani, Tetrahedron
65 (2009) 7611e7615.
[68] R.J. Batchelor, F.W.B. Einstein, I.D. Gay, J. Gu, B.M. Pinto, X. Zhou, Inorg. Chem.
35 (1996) 3667e3674.
[69] N.R. Champness, P.F. Kelly, W. Levason, G. Reid, A.M.Z. Slawin, D. Williams,
Inorg. Chem. 34 (1995) 651e657.
[70] A.J. Downard, A.M. Bond, A.J. Clayton, L.R. Hanton, D.A. McMorran, Inorg.
Chem. 35 (1996) 7684e7690.