Synthesis of a sulfur-bridged diphosphine ligand and its unique complexation properties toward palladium(II) ion

Article abstract of DOI:10.1246/cl.2008.418

Sulfur-bridged phenol dimer 3₂ is converted into P-S-P ligand 5, mono- and dinuclear palladium complexes of which are selectively synthesized and characterized by X-ray crystallography. Copyright

Full text of DOI:10.1246/cl.2008.418

418
Chemistry Letters Vol.37, No.4 (2008)
Synthesis of a Sulfur-bridged Diphosphine Ligand
and Its Unique Complexation Properties toward Palladium(II) Ion
Naoya Morohashi,Ã1 Yuki Akahira,1 Shinya Tanaka,1 Kensuke Nishiyama,1 Takashi Kajiwara,2 and Tetsutaro HattoriÃ1
1Department of Environmental Studies, Graduate School of Environmental Studies, Tohoku University,
6-6-11 Aramaki-Aoba, Aoba-ku, Sendai 980-8579
2Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aoba, Aoba-ku, Sendai 980-8578
(Received December 17, 2007; CL-071394; E-mail: morohashi@orgsynth.che.tohoku.ac.jp)
Y
Y
Sulfur-bridged phenol dimer 32 is converted into P–S–P
S
32: Y = OH
6: Y = OTf
7: Y = P(O)Ph2
5: Y = PPh2
ligand 5, mono- and dinuclear palladium complexes of which
are selectively synthesized and characterized by X-ray crystal-
lography.
i) 99 %
ii) 66 %
iii) 63 %
But
But
Scheme 1. Reagents and conditions: i) Tf2O, pyridine, rt, 2 h,
i
ii) Ph2P(O)H, Pd(OAc)2, dppb, Pr2EtN, DMSO, 120 ꢀC, 2.5 h,
iii) HSiCl3, Et3N, toluene, reflux, 2 h.
Diphosphines constitute a large ligand family useful for
a variety of transition-metal-catalyzed transformations. Steric
and electronic properties of a ligand often influence the rate
and selectivity of catalysis, as well as the stability of its com-
plexes. Recently, diphosphine ligands with a heteroatom linker
between the two phosphino moieties have been developed and
applied to catalysts. Of particular interest among such entities
are Xantphos (1) and its derivatives, which were designed to
enforce a large phosphorus–metal–phosphorus angle and have
proven to be successful in tuning the activity and selectivity in
various metal-catalyzed reactions.1 A variety of coordination
modes, i.e., bidentate cis and trans coordination and tridentate
P–O–P coordination, have been achieved by tuning the substitu-
ents on the phosphine ligand and/or replacing the other ligands
coordinating to the metal center.1b,1c,1f On the other hand, little
attention has been paid to the corresponding P–S–P ligands.1b,1f,2
Recently, Ohba et al. and we independently reported that sulfur-
bridged phenol oligomers 3n exhibit high extractability toward
soft metal ions in solvent extraction experiments, while their
methylene-bridged counterparts 4n per se did not extract
any metal ions, implying a vital role of the soft sulfur atoms in
binding to metal ions.3 We reasoned that all or a part of the hy-
droxy groups of 3n would be easily replaced with phosphino
groups via a transition-metal-catalyzed Ar–O bond-cleavage re-
action after conversion into triflate esters. This provides an easy
access to a series of novel phosphine ligands with sulfur donors.
As the first effort in this line, we report herein the preparation
of P–S–P ligand 5 and its varied coordination modes, the latter
being revealed by stepwise and quantitative introduction of
PdCl2 cores, taking advantage of good resistance of the Pd–Cl
bonds toward substitution reactions.
5
PdCl2(CH3CN)2
(1 mol equiv)
79%
PdCl2(CH3CN)2
(2 mol equiv)
76%
Cl
Cl
Cl
But
Cl
Pd
S
Pd
Ph2P
PPh2
Ph2P
PdCl2(CH3CN)2
(1 mol equiv)
S
83%
PPh2
Pd
But
But
But
Cl
10
Cl
OTf
8
Cl
AgOTf
Ph2P
Pd
S
PPh2
But
(1 mol equiv)
88%
But
9
Scheme 2.
HSiCl3 to afford diphosphine 5 in a good yield after purification
by column chromatography (Scheme 1).
Treatment of diphosphine
5 with 1.0 mol equiv of
PdCl2(CH3CN)2 in refluxing benzene gave red complex 8, the
1H NMR spectrum of which showed one singlet for the But
protons, indicating that this complex has a symmetric structure
(Scheme 2).
The solid-state structure was determined by X-ray diffrac-
tion of a single crystal, which was obtained by slow diffusion
of acetone into a dichloromethane solution of complex 8. It
was revealed that complex 8 has a mononuclear structure; the
palladium(II) ion is centered on a distorted square plane defined
by the two phosphorus atoms and two chloride ions (Figure 1).
Ligand 5 coordinates to the metal center in a cis fashion with
OH
OH
PPh2
PPh2
PPh2
PPh2
H
X
X
S
n-1
But
3n: X = S
4n: X = CH2
But
But
But
1: X = O
2: X = S
5
˚
Sulfur-bridged phenol dimer 32 was prepared by the reaction
of p-tert-butylphenol with SCl2 according to the literature proce-
dure.4 Esterification of 32 with triflic anhydride gave bistriflate 6,
which was submitted to palladium-catalyzed phosphorylation,5,6
followed by reduction of the resulting phosphine oxide 7 with
the two phosphorus atoms. The Pd–P (av 2.261 A) and Pd–Cl
˚
(av 2.349 A) bond lengths are within the range of normal values.
The sulfur atom is forced into the apical position and a weak
Pd–S bonding interaction seems to be present, judging from
˚
the interatomic distance between Pd–S (2.987 A), as well as
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.4 (2008)
419
by the treatment of complex 8 with 1.0 mol equiv of PdCl2-
(CH3CN)2. It seems that the conformational flexibility of ligand
5, with the aid of the strong coordination ability of the
epithio group to palladium(II) ion, enabled the transformation
of mono- into dinuclear complexes, which may open an access
to heterodinuclear complexes by a stepwise treatment of 5 with
two different metal ions.
In conclusion, we have shown here synthesis and unique
complexation properties of ligand 5. It serves as both bidentate
P–P and tridentate P–S–P ligands, depending on the electron
density of the palladium center, by changing the coordination
mode of the two phosphino moieties between cis and trans
fashions. The epithio linkage can coordinate to two palladium
ions by using the two lone pairs to form a dinuclear complex,
in which the two metal ions are placed close to each other. These
properties are of special interest for the catalytic application
of these palladium complexes.
Figure 1. X-ray structures of a) complex 8 and b) complex 9.
Hydrogen atoms and triflate ion are omitted for clarity.
the geometry of the complex.1b,8 It has been reported that
bis[2-(diphenylphophino)ethyl]sulfide (PSP) coordinates to a
platinum(II) ion in a tridentate fashion to form complexes
formulated as [Pt(PSP)X]X (X = ClÀ, BrÀ, and IÀ).2c It has also
been reported that Thioxantphos 2 forms [(Thioxantphos)-
Pd(CH3)]Cl and [(Thioxantphos)Pd(C6H4-p-CN)]Br, in which
2 serves as a tridentate ligand as evidenced by molar conductiv-
ity measurements.1b Therefore, it was a surprise to find that
the soft sulfur atom did not expel one of the chloride ions to
coordinate to the soft palladium center.
Interestingly, ionic complex 9 could be easily prepared by
the treatment of 8 with 1.0 mol equiv of AgOTf under refluxing
conditions in benzene (Scheme 2). Crystals suitable for X-ray
structural analysis were obtained by slow diffusion of pentane
into a dichloromethane solution of 9. In the crystal, the complex
has a square-planar geometry, in which the two phosphorus
atoms coordinate in a trans fashion with P1–Pd1–P2 angle of
167.52(10)ꢀ and the sulfur atom coordinates trans to the chloride
References and Notes
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˚
ligand [Pd–S distance = 2.2685(30) A].
On the other hand, the reaction of diphosphine 5 with
PdCl2(CH3CN)2 in the molar ratio of 1:2 afforded yellow
complex 10, the 1H NMR spectrum of which was clearly differ-
ent from that of complex 8 (Scheme 2). X-ray structural analysis
of a single crystal, prepared by slow diffusion of diethyl ether in-
to a dichloromethane solution of complex 10, revealed that it is a
dinuclear complex, in which the sulfur, one of the two phospho-
rus atoms, and two chloride ions coordinate to each palladium
ion, forming a square-planar geometry (Figure 2). It is notewor-
thy that the sulfur atom takes part in the coordination to both the
palladium ions by using the two lone pairs. The interatomic dis-
2
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Detailed synthetic methods, spectral data, and crystallographic
data of the compounds reported in this manuscript were depos-
ited in Supporting Information.7 Crystallographic data have
been deposited with Cambridge Crystallographic Data Centre
as supplementary publication Nos. CCDC-670856–670858.
The data can be obtained free of charge via http://www.ccdc.
cam.ac.uk/conts/retrieving.html.
˚
tance between Pd1–Pd2 (3.809 A) indicates the absence of bond-
ing interaction. To the best of our knowledge, such dinuclear
metal complexes have not been reported for xanthene-type li-
gands 1 and 2. Interestingly, complex 10 could also be prepared
4
5
6
7
8
Supporting Information is available electronically on the CSJ
Journal Web site, http://www.csj.jp/journals/chem-lett/.
M. J. Green, G. J. P. Britovsek, K. J. Cavell, B. W. Skelton,
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Figure 2. X-ray structure of compound 10. Hydrogen atoms,
solvents, and disordered carbon atoms are omitted for clarity.
Published on the web (Advance View) March 8, 2008; doi:10.1246/cl.2008.418