Synthesis of a novel non-symmetric Pd(II) phosphinito-thiophosphinito PSCOP pincer compound

Article abstract of DOI:10.1016/j.ica.2010.02.003

The reaction of 3-mercaptophenol with diphenylchlorophosphine in a 1:2 ratio in the presence of NEt₃ as base, affords cleanly the non-symmetric ligand [C₆H₄-1-(SPPh₂)-3-(OPPh₂)] (1). The direct reaction of this ligand with PdCl₂ in refluxing toluene affords the non-symmetric phosphinito-thiophosphinito PSCOP pincer complex [PdCl{C₆H₃-2-(SPPh₂)-6- (OPPh₂)}] (2).

Full text of DOI:10.1016/j.ica.2010.02.003

Inorganica Chimica Acta 363 (2010) 1306–1310
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Note
Synthesis of a novel non-symmetric Pd(II) phosphinito–thiophosphinito PSCOP
pincer compound
*
Juan M. Serrano-Becerra, Simón Hernández-Ortega, David Morales-Morales
Instituto de Química, Universidad Nacional Autónoma de México, Cd. Universitaria, Circuito Exterior, Coyoacán 04510 D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of 3-mercaptophenol with diphenylchlorophosphine in a 1:2 ratio in the presence of NEt3 as
base, affords cleanly the non-symmetric ligand [C6H4-1-(SPPh2)-3-(OPPh2)] (1). The direct reaction of this
ligand with PdCl2 in refluxing toluene affords the non-symmetric phosphinito–thiophosphinito PSCOP
pincer complex [PdCl{C6H3-2-(SPPh2)-6-(OPPh2)}] (2).
Received 3 February 2010
Accepted 4 February 2010
Available online 10 February 2010
Ó 2010 Elsevier B.V. All rights reserved.
Dedicated to Prof. J.R. Dilworth
Keywords:
Cross coupling reactions
Suzuki–Miyaura couplings, palladium
complexes
Palladacycles, pincer compounds
Crystal structures
Catalysis
Pincer compounds represent a group of species with very par-
ticular and interesting properties among which, their high thermal
stability and unusual reactivities that confer to the metal com-
plexes they form stand out. It is due, to these characteristics of
robustness and thermal stability that pincer compounds have
heavily attracted the attention of the chemistry community for
multiple applications, particularly in homogeneous catalysis [1].
The very simple backbone exhibited by these compounds did not
anticipate the wide variety of possible functionalizations
(Scheme 1). And today, these ligands have been modified to in-
clude a plethora of different donor groups [2–7], including chiral
motifs allowing the synthesis of enantiomerically pure systems
successfully employed in asymmetric synthesis and enantioselec-
tive catalysis. The very same system can be modified to include
functional groups that enable these species to be anchored to solid
supports [8] or allowing further functionalization to afford dendri-
meric or nanostructured systems [9].
Additionally, the sole inclusion of different metals in the cavity
of the ligands offers an endless possibility of a very diverse chem-
istry according to the metal selected. Hence, nowadays pincer
compounds of many elements are known and their chemistry mo-
tif of continuous and numerous studies. In this sense, phosphinite
PCP pincer compounds have been an answer for the easy synthesis
of pincer compounds, maintaining the same characteristics of ther-
mal robustness and in many occasions enhanced reactivity when
compared to their phosphine counterparts. Moreover, the number
of examples of complexes including non-symmetric pincer type li-
gands is limited in comparison with those of their symmetric ana-
logs [10]. This is partly because their preparation is a considerable
challenge, being laborious and requiring a series of steps to intro-
duce different groups or donors. Additionally, complexes bearing
non-symmetric pincer ligands have shown enhanced and in many
cases markedly different reactivities, such as hemilability [11].
Thus, following our continuous interest in the development of
pincer chemistry [12], the present report describes a very simple
alternative for the synthesis of non-symmetric pincer compounds
by changing the spacer between the aromatic ring and the phos-
phorus donor atom, this time combining hard (oxygen) and soft
(sulfur) atoms.
Hence, the reaction of 3-mercaptophenol with two equivalents
of diphenylchlorophosphine in the presence of triethylamine as
base under toluene reflux conditions affords ligand [C6H4-1-
(SPPh2)-3-(OPPh2)] (1) as a white solid in good yields (Scheme 2).1
1
Synthesis of [C6H4-1-(SPPh2)-3-(OPPh2)] (1). In a Schlenk flask 3-mercaptophenol
(6 mmol, 0.64 mL) and triethylamine (6.6 mmol, 1.8 mL) were stirred during a few
minutes until a white solid was formed. Addition of toluene (60 mL) followed by
chlorodiphenylphosphine (12 mmol, 2.25 mL) resulted in a whitish solution, which
was set to reflux for 18 h. During this time precipitation of Et3NꢀHCl occurred as a
crystalline solid. The ammonium salt was filtered off through a short plug of CeliteÒ
on a glass frit and then washed with anhydrous toluene (10 mL). The resulting
solution was concentrated under vacuum affording compound (1) as a white solid
after being washed with anhydrous hexane (2 ꢁ 10 mL). 31P{1H} NMR (CDCl3, d ppm):
112.5 (s, PO), 32.0 (s, PS).
* Corresponding author. Tel.: +52 55 56224514; fax: +52 55 56162217.
E-mail address: damor@unam.mx (D. Morales-Morales).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.02.003

J.M. Serrano-Becerra et al. / Inorganica Chimica Acta 363 (2010) 1306–1310
1307
Fig. 1. An ORTEP representation of the structure of [PdCl{C6H3-2-(SPPh2)-6-(OPPh2)}] (2) at 50% of probability showing the atom labeling scheme. Selected bond lengths (Å):
Cl–Pd 2.3734(11), Pd–C(2) 2.022(5), Pd–C(2)#1 2.022(5), Pd–P(1)#1 2.2711(8), Pd–P(1) 2.2711(8), S(1)–C(1) 1.777(7), S(1)–P(1) 2.162(2), C(3)–O(1) 1.394(9), O(1)–P(1)#1
1.558(5), P(1)–O(1)#1 1.558(5). Selected Bond Angles (°):C(2)–Pd–C(2)#1 9.8(7), C(2)–Pd–P(1)#1 79.2(3), C(2)#1–Pd–P(1)#1 88.6(3), C(2)–Pd–P(1) 88.6(3), C(2)#1–Pd–P(1)
79.2(3), P(1)#1–Pd–P(1) 167.84(4), C(2)–Pd–Cl 175.1(3), C(2)#1–Pd–Cl 175.1(4), P(1)#1–Pd–Cl 96.08(2), P(1)–Pd–Cl 96.08(2), C(1)–S(1)–P(1) 97.9(3), C(3)–O(1)–P(1)#1
115.7(5), O(1)#1–P(1)–Pd 107.2(3), S(1)–P(1)–Pd 104.68(9).
- Steric and electronic modulation
via donor groups
- Chirality
- Anchoring site
- Remote electronic
modulations
R
R
D
Y
E
MmLn
D
- Cavity for metal binding
with tunable accesibility
- Sites for counter anions or
ancillary ligands
- Chirality
- Steric modulation
Scheme 1. Versatility of the pincer backbone and potential sites for modification.
Analysis of this ligand by 1H NMR is not very informative and
Toluene
only signals due to the aromatic fragments are observed in the typ-
ical region for aromatic protons. However, analysis of ligand (1) by
31P{1H} NMR results very illustrative, revealing the presence of two
sharp singlets, one located at d 112.5 ppm due to the phosphinito
moiety, and the other, placed at higher field at d 32.0 ppm due to
the presence of the thiophosphinito fragment [13]. This analysis
O
PPh2
S
PPh2
1) NEt3, 2) ClPPh2
HO
SH
(1)
Scheme 2. Synthesis of the ligand [C6H4-1-(SPPh2)-3-(OPPh2)] (1).

1308
J.M. Serrano-Becerra et al. / Inorganica Chimica Acta 363 (2010) 1306–1310
Hence, the equimolar reaction of ligand [C6H4-1-(SPPh2)-3-
(OPPh2)] (1) with PdCl2 under reflux condition in toluene affords
complex [PdCl{C6H3-2-(SPPh2)-6-(OPPh2)}] (2) in an almost quan-
titative yield (Scheme 3).2
PdCl2
O
PPh2
S
PPh2
O
Ph2P
S
PPh2
toluene, reflux
Pd
Cl
(2)
Complex [PdCl{C6H3-2-(SPPh2)-6-(OPPh2)}] (2) was obtained as
microcrystalline whitish powder. Analysis of this compound by 1H
NMR exhibits signals due to the aromatic fragments between d
6.99 and d 8.02 ppm. However, is the 31P{1H} analysis that provides
fundamental information about the structure of the complex. Thus,
the spectrum of (2) shows two doublets at d 141.4 (doublet, PO,
J2P—P = 473 Hz) and 60.4 (doublet, PS, JP2—P = 472 Hz). Characteriza-
tion by 13C{1H} NMR afforded a set of signals (d 164.1, 152.8,
148.7, 133.3, 132.6, 132.2, 132.1, 132.0, 131.6, 131.4, 129.0,
128.0, 117.7, 110.1 ppm) which are consistent with the proposed
structure. Analysis by EI-MS of (2) reveals a similar fragmentation
to other POCOP pincer compounds [14], showing the molecular ion
[M+] at 636 (35%) m/z and the loss of the anion chloride being ac-
counted by the peak at [M+–Cl] = 599 (55%) m/z. Elemental analy-
sis results are also coherent with the proposed formulation.
Crystals suitable for single crystal X-ray diffraction analysis
were obtained and thus complex (2) was unequivocally character-
ized.3 For compound [PdCl{C6H3-2-(SPPh2)-6-(OPPh2)}] (2), the Pd
center is located into a slightly distorted square planar environment,
flanked by the two different phosphorus donor atoms in a trans
arrangement, and completing the coordination sphere the character-
(1)
Scheme 3. Synthesis of compound [PdCl{C6H3-2-(SPPh2)-6-(OPPh2)}] (2).
Table 1
Crystal data and structure parameters for [PdCl{C6H3-2-(SPPh2)-6-(OPPh2)}] (2).
Identification code
(2)
Formula
Formula weight
C30H23Cl1O1P2Pd1S1
635.33
T (K)
298(2)
0.71073
0
k (AÅ)
Crystal system
Space group
Unit cell dimensions
monoclinic
C2/c
0
17.195(2)
10.839(1)
16.528(2)
a (AÅ)
0
b (AÅ)
0
c (AÅ)
a
(°)
90
istic Pd–C
r bond and the chloride ligand. In one of this case, the
b (°)
119.801(2)
90
2673.1(5)
4
1.579
1.015
1280
c
(°)
strain of one of the 5 member metallocycles is partially alleviated
by the larger size of the sulfur atom. In all other respects this struc-
ture resembles other PCP and POCOP Pd(II) pincer structures.
V (Å3)
Z
DCalc (Mg/m3)
Absorption coefficient (mmꢂ1
F(0 0 0)
)
Crystal size
h (°)
Index ranges
0.33 ꢁ 0.22 ꢁ 0.14 mm
2.32–25.35
ꢂ20 6 h 6 20, ꢂ13 6 k 6 13,
ꢂ19 6 l 6 19
10 821
2
Synthesis of [PdCl{C6H3-2-(SPPh2)-6-(OPPh2)}] (2). PdCl2 (6 mmol, 1064 mg) was
placed in the collecting Schlenk flask of the filtered solution containing ligand (1)
(vide supra). The reaction mixture was then set to reflux for 16 h. After the prescribed
reaction time precipitation of the product was observed as a white powder. The solid
was filtered off, washed with toluene (10 mL) and hexane (3 ꢁ 20 mL) affording
compound 3 as a microcrystalline white powder stable to moisture and air in almost
quantitative yield (99%, based on PdCl2). Compound (2) was further purified by flash
chromatography to eliminate any possible impurities employing a dichloromethane/
pentane solvent mixture (2:1) as eluent, affording compound 2 in its pure form as a
white microcrystalline powder. 1H NMR (CDCl3, d ppm): 8.02–7.95 (m, 8H, PhHortho),
Reflections collected
Independent reflections (Rint
Absorption correction
Maximum and minimum
transmission
)
2451 (0.0485)
analytical
0.8893 and 0.7191
Refinement method
Full-matrix least-squares on F2
2451/60/200
0.894
R1 = 0.0311, wR2 = 0.0614
R1 = 0.0430, wR2 = 0.0642
0.576 and ꢂ0.457
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
7.51–7.44 (m, 12H, Ph–Hmeta, Ph–Hpara), 7.00 (d, 1H, JHH = 4 Hz, Ar–Hmeta), 6.99
Final R indices [I > 2
R indices (all data)
r(I)]
(apparent q, 1H, JHH = 4.4 Hz, Ar–Hpara). 13C{1H} NMR (CDCl3, d ppm): 164.1 (dd, COP,
J = 12.2, 3 Hz), 152.8 (dd, CSP, J = 18.6, 2.5 Hz), 148.7 (t, J = 1.6), 133.3, 133.3 (dd,
J = 13.6, 1.8), 132.6 (d, J = 6.5), 132.2 (d, J = 2.5), 132.1 (d, J = 3.6), 132.0, (dd, J = 14.3,
1.5 Hz), 131.6 (d, J = 3.8), 131.4 (d, J = 2.4), 129.0 (d, J = 10.9), 128.0, 117.7 (d,
J = 17.5 Hz), 110.1 (d, J = 17.5 Hz). 31P{1H} NMR (CDCl3, d ppm): 141.4 (d, J = 473 Hz,
PO) 60.4 (d, J = 472 Hz, PS). MS-FAB+ (m/z): 601 [M+–Cl]. IR (KBr, cmꢂ1): 1430, 1103,
907, 693. Elemental Anal. Calc. for C30H23ClOP2PdS (635.39): C, 56.71; H, 3.65; S, 5.05.
Found: C, 56.69; H, 3.71; S, 4.53%.
Largest difference peak and hole
(e Åꢂ3
)
3
Data Collection and refinement for [PdCl{C6H3-2-(SPPh2)-6-(OPPh2)}] (2). Crys-
also reveals the ligand to be pure enough to be used in the follow-
ing process, the metallation step. For most of the pincer ligands,
the metallation process involves an aromatic C–H activation reac-
tion. Although this process for aromatic protons is easier than that
for aliphatic C–H bonds, very often this step still requires activated
starting materials in low oxidation states or activation of the C–H
bond (e.g. lithiation) or harsh conditions that often render in the
partial or total decomposition of the starting materials leading to
low or null yields. Thus, we have explored the direct synthesis of
pincer complexes from readily available un-activated starting
materials e.g. PdCl2, these reactions resulting to be very easy to
perform, facilitating and speeding up the synthesis of PCP pincer
compounds. Moreover, this direct reaction results very interesting
since PdCl2 is not soluble in the used solvent toluene and one can
easily follow the advance of the reaction by the disappearance of
the metal salt and the increasing in the color tonality of the reac-
tion mixture, for the present case colorless-yellowish, thus turning
these reactions into an auto-indicative process.
talline colorless prisms of [PdCl{C6H3-2-(SPPh2)-6-(OPPh2)}] (2) were grown by slow
evaporation of a CH2Cl2/n-heptane solvent system, and mounted in random orien-
tation on a glass fiber. The X-ray intensity data were measured at 298 K on a Bruker
SMART APEX CCD-based three-circle X-ray diffractometer system using graphite
mono-chromated Mo K
distance of 4.837 cm from the crystals in all cases. A total of 1800 frames were
collected with a scan width of 0.3° in and an exposure time of 10 s/frame. The
a (k = 0.71073 Å) radiation. The detector was placed at a
x
frames were integrated with the Bruker SAINT software package [15] using a narrow-
frame integration algorithm. The integration of the data was done using a monoclinic
unit cell to yield a total of 10 821 reflections for 2 to a maximum 2h angle of 50.00°
(0.93 Å resolution), of which 2451(2) were independent. Analysis of the data showed
negligible decays during data collection. The structure was solved by Patterson
method using SHELXS-97 [16] program. The remaining atoms were located via a few
cycles of least squares refinements and difference Fourier maps, using C2/c space
group, with Z = 4. Hydrogen atoms were input at calculated positions and allowed to
ride on the atoms to which they are attached. Thermal parameters were refined for
hydrogen atoms on the phenyl groups with Uiso(H) = 1.2 Ueq of the parent atom. The
final cycle of refinement was carried out on all non-zero data using SHELXL-97 [17] and
anisotropic thermal parameters for all non-hydrogen atoms. The details of the
structure determination are given in Table 1. The numbering of the atoms is shown in
Fig. 1 (ORTEP) [18].

J.M. Serrano-Becerra et al. / Inorganica Chimica Acta 363 (2010) 1306–1310
1309
[3] See for instance: (a) D. Morales-Morales, C. Grause, K. Kasaoka, R. Redón, R.E.
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745;
Having complex [PdCl{C6H3-2-(SPPh2)-6-(OPPh2)}] (2) on hand,
we decided to explore its catalytic activity4 on the Suzuki–Miyaura
(C–C) cross coupling reaction of bromobenzene and phenylboronic
acid as a bench mark experiment to compare its reactivity against
that of [PdCl{C6H3-2,6-(OPPh2)2}] (3) [3a,3c].
(c) D. Morales-Morales, R. Redón, C. Yung, C.M. Jensen, Chem. Commun. (2000)
1619;
(d) O.A. Wallner, K.J. Szabó, Org. Lett. 6 (2004) 1829;
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(2006) 3817;
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(2006) 3233;
From the results attained compound [PdCl{C6H3-2-(SPPh2)-6-
(OPPh2)}] (2) resulted to be
a better, faster catalyst than
[PdCl{C6H3-2,6-(OPPh2)2}] (3) (compare 100% versus 76% yield
even by doubling the reaction time) for the Suzuki–Miyaura cross
coupling reactions under the very same conditions.
There has been a considerable debate in the literature about the
oxidation states of the species involved in the catalytic cycle using
pincer compounds, with Pd(IV)/Pd(II) and Pd(II)/Pd(0) both being
proposed at various times [19]. Although we generally favor the
participation of Pd(II)/Pd(IV) species, in this case the presence of
the thiophosphinito bond may led the behavior of ligand (1) to a
hemilabile pincer compound (2) in solution [20] (a possibility that
can not be ruled out). However, the presence of the sulfur on the
pincer structure may just lead to a stronger electronic factors
affecting the reactivity of complex (2) and thus perhaps assisting
the oxidative addition process. Experiments, aimed to shed further
light to support or decline these theories and the further applica-
tion of this an other transition metal derivatives e.g. Ni, Pt, Ru, Ir,
etc. of ligand (1) in other relevant organic transformations are cur-
rently under study in our laboratories.
(k) D. Morales-Morales, R. Redón, C. Yung, C.M. Jensen, Inorg. Chim. Acta 357
(2004) 2953;
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(c) K. Yu, W. Sommer, M. Weck, C.W.S. Jones, J. Catal. 226 (2004) 101;
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(e) D.E. Bergbreiter, P.L. Osburn, J.D. Frels, Adv. Synth. Catal. 347 (2005) 172;
(f) M. Arroyo, R. Cervantes, V. Gómez-Benítez, P. López, D. Morales-Morales, H.
Torrens, R.A. Toscano, Synthesis (2003) 1565.
[5] See for instance: H. Nishiyama, Chem. Soc. Rev. 36 (2007) 1133. and references
cited therein.
[6] See for instance: (a) D. Morales-Morales, The Chemistry of Pincer Compounds,
in: D. Morales-Morales, C. Jensen (Eds.), Elsevier, Amsterdam, 2007, pp. 151–
180;
Acknowledgments
The generous financial support of this research by CONACYT
(F58692) and DGAPA-UNAM (IN227008) is gratefully acknow-
ledged.
(b) D. Morales-Morales, Mini-Rev. Org. Chem. 5 (2008) 141. and references
therein;
(c) J.M. Serrano-Becerra, D. Morales-Morales, Curr. Org. Synth. 6 (2009) 169.
and references cited therein.
[7] See for instance: (a) I.G. Jung, S.U. Son, K.H. Park, K. Chung, J.W. Lee, Y.K. Chung,
Organometallics 22 (2003) 4715;
Appendix A. Supplementary material
(b) M.Q. Slagt, G. Rodríguez, M.M.P. Grutters, R.J.M.K. Gebbink, W. Klopper,
L.W. Jenneskens, M. Lutz, A.L. Spek, G. van Koten, Chem. Eur. J. 10 (2004) 1331;
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CCDC 664411 (2) contains the free supplementary crystallo-
graphic data for this paper. These can be obtained free of the Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.02.003.
(h) M.S. Yoon, R. Ramesh, J. Kim, D. Ryu, K.H. Ahn, J. Organomet. Chem. 691
(2006) 5927;
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4
General procedure for the Suzuki cross-coupling of bromobenzene with phenyl
boronic acid. Bromobenzene (4 mmol), phenylboronic acid (6 mmol, 731 mg), K2CO3
(8 mmol, 1105 mg, 0.1 mol% of the palladium catalyst and toluene (12 mL) were
charged in the open air into a Schlenk tube equipped with a magnetic stirrer. The tube
was sealed and fully immersed in a 100 °C silicon oil bath under stirring. After the
prescribed reaction time, the tube was taken out of the oil bath and allowed to reach
room temperature. Samples taken from the organic phase, which was previously
filtered trough a short plug of CeliteÒ, were diluted with dichloromethane (1 mL) and
analyzed by gas chromatography–mass spectrometry (GC–MS) techniques.

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J.M. Serrano-Becerra et al. / Inorganica Chimica Acta 363 (2010) 1306–1310
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