Bis(2-diphenylphosphinoxynaphthalen-1-yl)methane: Transition metal chemistry, Suzuki cross-coupling reactions and homogeneous hydrogenation of olefins

Article abstract of DOI:10.1039/b510589g

Transition metal complexes of bis(2-diphenylphosphinoxynaphthalen-1-yl) methane (1) are described. Bis(phosphinite) 1 reacts with Group 6 metal carbonyls, [Rh(CO)₂Cl]₂, anhydrous NiCl₂, [Pd(C₃H₅)Cl]₂/AgBF₄ and Pt(COD)I₂ to give the corresponding 10-membered chelate complexes 2, 3 and 5-8. Reaction of 1 with [Rh(COD)Cl]₂ in the presence of AgBF₄ affords a cationic complex, [Rh(COD){Ph₂P(-OC ₁₀H₆)(μ-CH₂)(C₁₀H ₆O-)PPh₂-KP,KP}]BF₄ (4). Treatment of 1 with AuCl(SMe₂) gives mononuclear chelate complex, [(AuCl){Ph ₂P(-OC₁₀H₆)(μ-CH₂)(C ₁₀H₆O-)PPh₂-KP,KP}] (9) as well as a binuclear complex, [Au(Cl){μ-Ph₂P(-OC₁₀H₆)(μ- CH₂)(C₁₀H₆O-)PPh₂-KP,KP}AuCl] (10) with ligand 1 exhibiting both chelating and bridged bidentate modes of coordination respectively. The molecular structures of 2, 6, 7, 9 and 10 are determined by X-ray studies. The mixture of Pd(OAc)₂ and 1 effectively catalyzes Suzuki cross-coupling reactions of a range of aryl halides with aryl boronic acid in MeOH at room temperature or at 60°C, giving generally high yields even under low catalytic loads. The cationic rhodium(I) complex, [Rh(COD){Ph₂P(-OC₁₀H₆)(μ-CH ₂)(C₁₀H₆O-)PPh₂-KP,KP}]BF ₄ (4) catalyzes the hydrogenation of styrenes to afford the corresponding alkyl benzenes in THF at room temperature or at 70°C with excellent turnover frequencies. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b510589g

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Bis(2-diphenylphosphinoxynaphthalen-1-yl)methane: transition metal
chemistry, Suzuki cross-coupling reactions and homogeneous hydrogenation
of olefins
Benudhar Punji,a Joel T. Magueb and Maravanji S. Balakrishna*a
Received 25th July 2005, Accepted 31st October 2005
First published as an Advance Article on the web 23rd November 2005
DOI: 10.1039/b510589g
Transition metal complexes of bis(2-diphenylphosphinoxynaphthalen-1-yl)methane (1) are described.
Bis(phosphinite) 1 reacts with Group 6 metal carbonyls, [Rh(CO)2Cl]2, anhydrous NiCl2,
[Pd(C3H5)Cl]2/AgBF4 and Pt(COD)I2 to give the corresponding 10-membered chelate complexes 2, 3
and 5–8. Reaction of 1 with [Rh(COD)Cl]2 in the presence of AgBF4 affords a cationic complex,
[Rh(COD){Ph2P(–OC10H6)(l-CH2)(C10H6O–)PPh2-jP,jP}]BF4 (4). Treatment of 1 with AuCl(SMe2)
gives mononuclear chelate complex, [(AuCl){Ph2P(–OC10H6)(l-CH2)(C10H6O–)PPh2-jP,jP}] (9) as
well as a binuclear complex, [Au(Cl){l-Ph2P(–OC10H6)(l-CH2)(C10H6O–)PPh2-jP,jP}AuCl] (10) with
ligand 1 exhibiting both chelating and bridged bidentate modes of coordination respectively. The
molecular structures of 2, 6, 7, 9 and 10 are determined by X-ray studies. The mixture of Pd(OAc)2 and
1 effectively catalyzes Suzuki cross-coupling reactions of a range of aryl halides with aryl boronic acid
in MeOH at room temperature or at 60 ◦C, giving generally high yields even under low catalytic loads.
The cationic rhodium(I) complex, [Rh(COD){Ph2P(–OC10H6)(l-CH2)(C10H6O–)PPh2-jP,jP}]BF4 (4)
catalyzes the hydrogenation of styrenes to afford the corresponding alkyl benzenes in THF at room
temperature or at 70 ◦C with excellent turnover frequencies.
num(0), nickel(II), palladium(II) and gold(I) complexes of 1 are also
described.
Introduction
Synthesis of novel bisphosphine ligand systems to stabilize plat-
inum metal chelates is considered to be a most challenging task
in view of their potential usefulness as homogeneous catalysts
Results and discussion
for a variety of organic transformations.1 The ligating properties
of these ligands depend to a large extent on the nature of the
spacer and also on the phosphorus substituents. In ligands such
as binap,2 restricted rotation makes them excellent ligands for
asymmetric synthesis. On the other hand, if the bulky groups
can rotate freely about a pivoting group, ring strain can be
induced that enhances the rate of dissociation of one of the
metal-phosphorus bonds so that it can perform as a better
catalyst. In contrast, in such cases, the increase in bite angle will
enhance the steric congestion around the metal centre, which
favours the less sterically demanding transition state that can
lead to selectivity in catalysis.3 In view of this, recently we have
synthesized and reported transition metal complexes of bis(2-
diphenylphosphinoxynaphthalen-1-yl)methane (1).4 As a continu-
ation of our work5 in designing new phosphorus based ligands for
exploring their organometallic chemistry and catalytic utility in or-
ganic synthesis, herein we report more transition metal complexes
of the large-bite bis(phosphinite) 1. As will be seen, we also show
that Pd(OAc)2/1 exhibits significant catalytic activity in Suzuki
cross-coupling reactions and the cationic rhodium complex in
catalytic hydrogenation of styrenes. X-Ray structures of molybde-
Our main objective initially was to explore the coordination
behavior of the large-bite bis(phosphinite) ligand (1) and to
explore its catalytic applications. Previously, we have described
the synthesis, derivatization and ruthenium(II), palladium(II) and
platinum(II) complexes of 1.4 In the present study, more chelate
complexes are described. The reaction of 1 with [Mo(CO)6]
or [W(CO)4(piperazine)2] in toluene affords the tetracarbonyl
derivatives, [M(CO)4{Ph2P(–OC10H6 )(l-CH2 )(C10H6O–)PPh2 -
jP,jP}] in good yield. The IR spectra of complexes 2 and 3 exhibit
strong absorptions in the range 1872–2035 cm−1 characteristic
of phosphine donors bound to a cis-M(CO)4 moiety.5b,6 The 31P
NMR spectra of complexes 2 and 3 show single resonances at
144.1 and 121.6 ppm respectively. The coordination shifts for 2
and 3 are 31.9 and 9.4 ppm respectively. The tungsten complex
1
shows JWP coupling of 285 Hz. The reaction of two equivalents
of 1 with [Rh(COD)Cl]2 in the presence of AgBF4 in a mixture of
methanol–dichloromethane affords a cationic chelate complex,
[Rh(COD){Ph2P(–OC10H6)(l-CH2)(C10H6O–)PPh2-jP,jP}]BF4
(4) as shown in Scheme 1. The 31P NMR spectrum of 4 shows a
doublet centered at 120.9 ppm with a 1JRhP coupling of 171 Hz. The
ESI mass spectrum of 4 shows m/z at 879 (100%) corresponding to
the cation. The 1H NMR spectrum of 4 shows a singlet at 4.81 ppm
for the bridging methylene group and the resonances due to COD
appear as multiplets in the region 2.32–2.72 and 6.02–7.25 ppm.
The reaction of 1 with [Rh(CO)2Cl]2 in a 2 : 1 ratio gives the
aDepartment of Chemistry, Indian Institute of Technology Bombay, Powai,
Mumbai, 400 076, India. E-mail: krishna@chem.iitb.ac.in; Fax: +91 22
2576 7152/2572 3480; Tel: +91 22 2576 7181
bDepartment of Chemistry, Tulane University, New Orleans, Lousiana,
70118, USA
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Scheme 1
with [AuCl(SMe2)] in a 1 : 1 ratio in dichloromethane gives a
chelate complex, [AuCl{Ph2P(–OC10H6)(l-CH2)(C10H6O–)PPh2-
jP,jP}] (9) whereas a similar reaction with two equivalents
of [AuCl(SMe2)] affords a dinuclear complex, [AuCl{l-Ph2P-
(–OC10H6)(l-CH2)(C10H6O–)PPh2-jP,jP}AuCl] (10) with ligand
expected mononuclear complex, [Rh(Cl)(CO){Ph2P(–OC10H6)(l-
CH2)(C10H6O–)PPh2-jP,jP}] (5), in quantitative yield. The IR
spectrum of 5 shows a strong CO absorption at 2037 cm−1, which
clearly indicates the trans disposition of CO ligand to one of
the phosphorus atoms rather than to chlorine.7 The 31P NMR
spectrum of 5 shows two doublet of doublets centered at 125.5 and
123.9 ppm due to the presence of two magnetically non-equivalent
1
exhibiting the bridged bidentate mode of coordination
(Scheme 2). The 31P NMR spectra of complexes 9 and 10 show
single resonances at 136.1 and 111.7 ppm, respectively. Further
evidence for the molecular composition of complexes 9 and 10
comes from the elemental analysis, mass spectrometry and the
single-crystal X-ray diffraction studies.
1
phosphorus centers. The corresponding JRhP couplings are 187
and 154 Hz respectively. The 2JPP coupling is 38 Hz. The reaction of
anhydrous NiCl2 with ligand 1 in THF at room temperature gives a
dark red colored square planar complex, [NiCl2{Ph2P(–OC10H6)-
(l-CH2)(C10H6O–)PPh2-jP,jP}] (6) in 52% yield. The 31P NMR
spectrum shows a broad singlet at 115.5 ppm. The reaction
The crystal and molecular structures of 2, 6, 7, 9 and 10
3
of 1 with the allyl-palladium dimer, [Pd(g -C3H5)Cl]2 in a 2 :
1 ratio in the presence of two equivalents of AgBF4 affords a
chelate complex, [Pd(C3H5){Ph2P(–OC10H6)(l-CH2)(C10H6O–)-
PPh2-jP,jP}]BF4 (7) in 76% yield. The 31P NMR spectrum
of 7 shows a single resonance at 121.8 ppm. The 1H NMR
spectrum and the microanalysis of complex 7 agree well with
the proposed molecular composition. The molecular structure
of 7 is also confirmed by a single crystal X-ray analysis. The
reaction of 1 with [Pt(COD)I2] in dichloromethane in a 1 : 1 ratio
gives cis-[PtI2{Ph2P(–OC10H6 )(l-CH2 )(C10H6O–)PPh2 -jP,jP}]
(8) in 88% yield. The 31P NMR spectrum of 8 exhibits a singlet
Perspective views of the molecular structures of compounds 2,
6, 7, 9 and 10 with the atom numbering schemes are shown in
Fig. 1–5 respectively. Crystal data and the details of the structure
determination are given in Table 1, while selected bond lengths
and bond angles appear below the corresponding Figures.
In the structure of 2, the molybdenum atom shows distorted
octahedral geometry with four terminal carbonyls and ligand 1
chelating in a cis fashion with a bite angle of 95.06(3)◦. There is no
significant difference in the Mo–C bond distances for carbonyl
groups having cis and trans dispositions with respect to the
1
˚
at 87.3 ppm with a large JPtP coupling of 3896 Hz, which is
phosphorus centers. The average C–O distance is 1.145(4) A. In
consistent with the proposed cis-geometry.4,5b The reaction of 1
the structure of 6, the nickel center adopts almost square planar
Scheme 2
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Table 1 Crystallographic data for 2, 6, 7, 9 and 10
2
6
7
9
10
Formula
M
Crystal system Triclinic
C49H34MoO6P2·CH2Cl2 C45H34Cl2NiO2P2·CH2Cl2 C48H39BF4O2P2Pd·2CH2Cl2 C45H34AuClO2P2·CH2Cl2 C45H34Au2Cl2O2P2
961.57
883.18
Monoclinic
P21/c (no. 14)
15.008(1)
10.949(1)
26.287(2)
90
1072.79
Monoclinic
P21/n (no. 14)
19.924(2)
10.741(1)
22.488(2)
90
986.05
1133.49
Monoclinic
P21/n (no. 14)
16.935(2)
12.521(2)
20.620(3)
90
Triclinic
P1 (no. 2)
¯
¯
Space group
P1 (no. 2)
˚
a/A
10.726(1)
11.460(1)
17.609(1)
77.918(1)
83.014(1)
88.284(1)
2100.8(2)
2
12.337(1)
12.453(1)
12.806(1)
84.941(1)
88.464(1)
78.992(1)
1923.6(3)
2
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
105.429(1)
90
4163.8(5)
4
108.071(1)
90
4575.1(7)
4
98.936(2)
90
4319.3(11)
4
3
˚
V/A
Z
Dc/g cm−3
l/mm−1
1.520
1.409
1.558
1.699
1.743
0.568
0.838
0.767
4.156
7.018
T/K
100
1.8, 28.3
0.0496
100
100
1.6, 28.3
0.0548
100
1.6, 27.5
0.0346
100
1.5, 24.7
0.0576
◦
h min, max
/
1.4, 28.3
0.0410
0.1061
a
R1
b
wR2
0.1120
0.1483
0.0920
0.1247
ꢀ
ꢀ
ꢀ
ꢀ
2
1/2
a R1 = Fo − Fc/ Fo. b wR2 = { w[(Fo − Fc2)2]/ [w(Fo2)2]}
.
Fig. 2 Molecular structure of 6·CH2Cl2. For clarity, solvent and all
hydrogen atoms have been omitted. Thermal ellipsoids are drawn at the
◦
˚
50% probability level. Selected bond distances (A) and angles ( ): Ni–Cl(1)
2.192(1), Ni–Cl(2) 2.177(1), Ni–P(1) 2.166(1), Ni–P(2) 2.169(1), P(1)–O(1)
1.636(1), P(2)–O(2) 1.620(1); Cl(1)–Ni–Cl(2) 91.33(2), Cl(1)–Ni–P(1)
81.38(2), Cl(1)–Ni–P(2) 173.06(2), Cl(2)–Ni–P(1) 169.39(2), P(1)–Ni–P(2)
95.96(2), C(22)–C(23)–C(24) 117.04(17).
Fig. 1 Molecular structure of 2·CH2Cl2. For clarity, solvent and all
hydrogen atoms have been omitted. Thermal ellipsoids are drawn at the
◦
˚
50% probability level. Selected bond distances (A) and angles ( ): Mo–P(1)
2.479(1), Mo–P(2) 2.520(1), P(1)–O(1) 1.640(2), P(2)–O(2) 1.644(2),
Mo–C(46) 2.008(3), Mo–C(48) 2.000(3), Mo–C(47) 2.063(3), Mo–C(49)
2.031(3), O(3)–C(46), O(5)–C(48), O(6)–C(49) 1.145(4), O(4)–C(47)
1.135(4); P(1)–Mo–P(2) 95.06(3), P(1)–Mo–C(46) 85.44(10), P(1)–Mo–
C(48) 169.39(9), C(46)–Mo–C(48) 84.56(13), C(22)–C(23)–C(24) 114.2(3).
Table 2 Bite angles (◦) in chelate complexes of 1 (LL)
Compound
Bite angle
Ref.
M(CO)4(LL)
CpRuCl(LL)
NiCl2(LL)
[(C3H5)Pd(LL)]BF4
PtCl2(LL)
P(1)–Mo–P(2)
P(1)–Ru–P(2)
P(1)–Ni–P(2)
P(1)–Pd–P(2)
P(1)–Pt–P(2)
P(1)–Au(1)–P(2)
95.06(3)
93.94(7)
95.96(2)
98.95(3)
95.37(9)
132.56(4)
This work
4
This work
This work
4
geometry and the bis(phosphinite) makes a twisted 10-membered
chelate ring with one of the naphthalene rings oriented almost
orthogonal to the plane of the ring. Similarly, the palladium
center in structure 7 also adopts a distorted square planar
geometry, where the bisphosphinite ligand shows chelating mode
of coordination with slight difference in Pd–P(1) and Pd–P(2)
bond lengths. The P–M–P bite angles for 6 and 7 are 95.96(2) and
98.95(3)◦, respectively, ◦which are comparable with◦those of the
ruthenium(II) (93.94(7) ) and platinum(II) (95.37(9) ) analogues4
(see Table 2). Interestingly, the P–O bond distances in the nickel
AuCl(LL)
This work
complex are comparable with those found in ruthenium complex,
whereas the bond distances shrink in the palladium complex and
further shrinking was observed in platinum complex, which may
be due to the size of metal atoms. The average O–C(aromatic) bond
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Fig. 5 Molecular structure of 10. For clarity, all hydrogen atoms have
been omitted. Thermal ellipsoids are drawn at the 50% probability
Fig. 3 Molecular structure of 7·2CH2Cl2. For clarity solvent and all
◦
˚
level. Selected bond distances (A) and angles ( ): Au(1)–Cl(1) 2.276(3),
Au(1)–P(1) 2.210(3), Au(2)–Cl(2) 2.287(3), Au(2)–P(2) 2.211(3), P(1)–O(1)
1.604(7); Cl(1)–Au(1)–P(1) 176.83(13), Cl(2)–Au(2)–P(2) 174.63(11),
Au(1)–P(1)–O(1) 116.2(3), C(22)–C(23)–C(24) 119.0(9).
hydrogen atoms have been omitted. Thermal ellipsoids are drawn ◦at
˚
the 50% probability level. Selected bond distances (A) and angles ( ):
Pd–P(1) 2.291(1), Pd–P(2) 2.288(1), Pd–C(46) 2.186(6), Pd–C(47) 2.163(8),
Pd–C(48) 2.185(4), P(1)–O(1) 1.623(3), P(2)–O(2) 1.628(3); P(1)–Pd–
P(2) 98.95(3), P(1)–Pd–C(46) 163.81(12), P(1)–Pd–C(47) 129.23(17),
P(1)–Pd–C(48) 96.61(11), C(22)–C(23)–C(24) 114.5(3).
from each other, preventing the formation of an aurophilic contact.
The geometry around the metals is almost linear, with P–Au–Cl
angles of 176.83(13) and 174.63(11)◦. The dinaphthol backbone is
almost perpendicular with a torsion angle of 106.19◦.
A close look at the way the molecules are packed in the
crystal shows the presence of intermolecular CH–Cl contacts (see
Fig. 6). Although, the two Au–Cl units of adjacent molecules are
situated in an antiparallel manner, there is no intermolecular Au–
˚
Cl contact (7.27 A).
Fig. 4 Molecular structure of 9·CH2Cl2. For clarity solvent and all hy-
drogen atoms have been omitted. Thermal ellipsoids are dr◦awn at the 50%
˚
probability level. Selected bond distances (A) and angles ( ): Au(1)–Cl(1)
2.506(1), Au(1)–P(1) 2.293(1), Au(1)–P(2) 2.294(1), P(1)–O(1) 1.639(3),
P(2)–O(2) 1.637(3); Cl(1)–Au(1)–P(1) 113.28(4), Cl(1)–Au(1)–P(2)
113.73(4), P(1)–Au(1)–P(2) 132.56(4), C(22)–C(23)–C(24) 122.2(3).
˚
Fig. 6 Intermolecular interactions in 10 [Cl(1) · · · H(15) 2.905 A (most H
atoms are omitted for clarity)].
˚
˚
distance is 1.40 A. The Ni–P distances (2.166(1) A) are the shortest
in theseries while theRu–Pbond distances are the longest(2.254(2)
Suzuki cross-coupling reactions
˚
A), which indicate the better p-donor capability of nickel center
as compared to the ruthenium center. In the structures of 9 and
10, the gold centers adopt trigonal planar and linear geometries
respectively. The angles around the gold center in 9 are 360◦ with
a P–Au–P angle of 132.56(4)◦. Because of this large bite angle the
angle at the bridgehead methylene carbon is 122.2(3)◦, which is
approximately 6–8◦ greater than those found in the other chelate
complexes of 1. In complex 10, the two Au–Cl units adopt anti
A mixture of Pd(OAc)2 and 1 effectively catalyzes the Suzuki cross-
coupling reactions of a variety of aryl halides and aryl boronic
acids affording the desired biaryls in remarkably high yields
(Table 3). For example, in a solution of Pd(OAc)2/1 (0.5 mol%)
in MeOH containing K2CO3 (2 equiv.), 4-bromobenzaldehyde
couples with phenyl boronic acid to give quantitative yield of
4-formylbiphenyl at room temperature after 1 h (entry 1). It is
notable that the use of deactivated, electron rich aryl bromides
˚
positions that situate the metal atoms at a longer distance (6.685 A)
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Table 3 Suzuki cross-coupling of aryl halides with boronic acids catalyzed by Pd(OAc)2/1
Entry
1
Aryl halide
Boronic acid
Product
Conditionsa
Conversionb (%)
100
0.5 mol%, R.T., 1 h
2
3
0.5 mol%, R.T., 1 h
0.5 mol%, R.T., 1 h
>99
97
4
5
0.5 mol%, R.T., 1 h
0.5 mol%, R.T., 1 h
79 (90)c
98
6
0.5 mol%, R.T., 1 h
87
7
8
9
0.5 mol%, R.T., 1 h
1 mol%, 60 ◦C, 2 h
1 mol%, 60 ◦C, 2 h
80 (99)c
92
57
10
11
1 mol%, 60 ◦C, 2 h
83
34
1 mol%, 60 ◦C, 1 h
a Aryl halide (0.5 mmol), aryl boronic acid (0.75 mmol), K2CO3 (1 mmol), MeOH (5 mL). b Conversion to coupled product determined by GC, based on
aryl halides; average of two runs. c At 60 ◦C, 1 h.
(e.g. entry 4) as well as activated, electron poor ones (e.g. entry 1)
also resulted in high yields. In general, this coupling can be
satisfactorily carried out at room temperature. When there is a
need to improve the yield, it can be carried out at elevated (60 ◦C)
temperature (see entries 4 and 7). We also studied the coupling of
aryl bromides with some substituted boronic acid (entries 5 and
6), which gave near-quantitative yields at room temperature. The
coupling of some heterocycles and sterically bulky aryl bromides
with boronic acids tend to give moderate yields (entries 8 and
10). For example, the coupling of 2-bromo-6-methoxynaphthalene
with phenyl boronic acid in the presence of 1 mol% of catalyst at
60 ◦C for 2 h, gives 2-phenyl-6-methoxynaphthalene in 83% yield
1326 | Dalton Trans., 2006, 1322–1330
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(entry 10) whereas 2,4-dimethoxybromobenzene resists coupling
with phenyl boronic acid (entry 11).
and 0.0078 mol% with a TON of 3905 and 4225, respectively.
These are indications of an effective catalytic system that merits
more downstream explorations.
The high efficiency of Pd(OAc)2/1 at room temperature or 60 ◦C
makes this a valuable catalyst for thermally sensitive substrates.
To our knowledge, there have been only a few reports on efficient
Suzuki cross-coupling reactions of aryl bromides at room temper-
ature, most of which are with electron rich phosphine systems:
e.g. use of the Pd(OAc)2/aminophosphine and Pd(OAc)2/(o-
biphenyl)P(tBu)2 by Buchwald et al.8 and Pd2(dba)3/P(tBu)3 by
Fu et al.9 However, reports on Pd/phosphinite systems are rarely
seen in the literature.
Hydrogenation reactions
More interestingly, we found that rhodium complex 4 can be em-
ployed in the catalytic hydrogenation of styrene and its derivatives
such as 4-methylstyrene and a-methylstyrene (Table 5). For all
three substrates, the hydrogenation reactions were performed at
25 ◦C as well as at 70◦C under 100 psi of H2 pressure in the presence
of triethylamine as a base and THF as solvent. The catalyst was
found to be considerably more efficient in terms of conversion
under mild conditions than the ruthenium(II) catalyst, [RuCl2(p-
cymene)(PPh2Py)], employed by Moldes, for the hydrogenation
of styrenes.13 As shown in Table 5, the catalyst exhibits a good
conversion of styrene and 4-methylstyrene to yield ethyl benzene
and 4-methyl ethyl benzene, respectively. The reaction rate is
marginally temperature dependent. Thus, at 25 ◦C, the styrene
hydrogenation shows a TOF value of 333.3 h−1 and on increasing
the temperature to 70 ◦C, TOF increases to only 388 h−1. Interest-
ingly, 4-methylstyrene is hydrogenated with TOF values of 500 and
1000 h−1 at 25 and 70 ◦C, respectively. However, the hydrogenation
of a-methylstyrene at 25 ◦C is slow with a TOF value of only
89.9 h−1 with 54% conversion after 6 h. When the temperature
The activity of Pd(OAc)2/1 is found to be superior to
that of PdCl2(dppf), which catalyzes the reaction between 4-
bromoacetophenone and phenyl boronic acid in toluene at 70 ◦C
in 94% yield.10 In comparison with the other phosphinite based
palladium catalysts, the catalytic activity of Pd(OAc)2/1 is similar
to that found with the palladium bis(phosphinite), PCP-p◦incer sys-
tem, whose reactions were carried out in toluene at 130 C,1◦1 and
an orthopalladated phosphinite complex carried out at 110 C.12
It is important to achieve good yields using minimum amounts
of catalysts. We therefore examined the effect of catalyst loading on
a convenient coupling between 4-bromobenzaldehyde and phenyl
boronic acid (Table 4). High yields are maintained from normal
catalyst loads down to a level of 0.0312 mol%. A moderate yield (61
and >33%) is achieved even at catalyst loading as low as 0.0156
Table 4 Influence of low catalyst loading on the coupling reaction
Entry
Amount of catalyst Pd(OAc)2/1 (mol%)
Conversion (%)
TONa
1
2
3
4
5
6
7
0.5
0.25
0.125
0.0625
0.0312
0.0156
0.0078
100
99
99
99
91
61
33
200
396
792
1584
2912
3905
4225
a In units of mol product (mol Pd)−1
.
Table 5 Catalytic olefin hydrogenation reactionsa
Substrate
T/◦C
t/h
Conversion (%)
TOFb/h−1
R1 = R2 = H
25
70
25
70
25
70
70
3
2.5
2
1
6
2
6
100
97
100
100
54
97
6
333.3
388
500
1000
89.9
485
R1 = R2 = H
R1 = Me, R2 = H
R1 = Me, R2 = H
R1 = H, R2 = Me
R1 = H, R2 = Me
R1 = H, R2 = Me (no catalyst)
9.9
a Conditions: The molar ratio of substrate to base (Et3N) to catalyst was 1000 : 50 : 1; p(H2) = 100 psi. Solvent THF (20 mL). b TOF expressed as moles
of substrate converted to product per mole of catalyst per hour. Products were ascertained by GC analysis (based on conversion of styrenes).
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was increased to 70 ◦C, it showed good conversion with TOF
value of 485 h−1. One run was performed on a-methylstyrene at
70 ◦C without catalyst which showed only 6% conversion after
6 h. The homogeneous nature of the catalysis was checked by
the classical mercury test.14 Addition of a drop of mercury to the
reaction mixtures did not affect the yields of the hydrogenations
thus showing them to be truly homogeneous systems.
From the reaction profile, we can deduce that, 4-methylstyrene
is hydrogenated faster than styrene or a-methylstyrene. Probably,
the para substituents on the benzene ring enhanced the reaction
rate whereas the substituents on olefinic carbons controlled the
reaction rate due to the steric attributes. It would be interesting
to examine the regioselectivity in the hydrogenations of a less
substituted olefin site in the presence of highly substituted olefin
centers. The work in this direction is underway.
Synthesis of [Mo(CO)4{g2-Ph2P(–OC10H6)(l-CH2)-
(C10H6O–)PPh2-jP,jP}] (2)
A mixture of the ligand 1 (0.106 g, 0.16 mmol) and Mo(CO)6
(0.04 g, 0.15 mmol) in 12 mL of toluene was heated at 90 ◦C for 10 h
to give a clear yellowish solution. The solvent was removed under
reduced pressure to give a sticky residue. The residue was dissolved
in CH2Cl2 (2 mL), layered with petroleum ether (1 mL; bp 60–
80 ◦C) and was cooled to −25 ◦C to afford colourless crystals
of 2. Yield: 49% (0.064 g). Mp 138 ◦C (decomp.). Anal. Calc. for
C49H34MoO6P2: C, 67.13; H, 3.91%. Found: C, 66.93; H, 3.84%. FT
IR (KBr disk) cm−1: mCH: 3058 s; mCO: 2034 s, 1922 vs, 1912 vs, 1892
vs. 1H NMR (400 MHz, CDCl3): d 8.13 (d, 2H, Ar) 7.69 (d, 2H,
Ar), 7.49 (br s, 2H, Ar), 7.17–7.35 (m, 4H, Ar, 20H, OPPh2), 6.79
1
(d, 2H, Ar), 5.25 (s, 2H, Ar–CH2–Ar). 31P{ H} NMR (161.9 MHz,
CDCl3): d 144.1 (s).
Summary
Synthesis of [W(CO)4{g2-Ph2P(–OC10H6)(l-CH2)-
(C10H6O–)PPh2-jP,jP}] (3)
In conclusion, the bis(phosphinite) 1 is a versatile and electron-
rich bidentate ligand which forms complexes with most of
the transition metals. With gold(I) derivatives it forms both
mononuclear and binuclear complexes with the ligand exhibiting
chelating and bridged bidentate modes of coordination. The
use of Pd(OAc)2/1 demonstrated that we can achieve very high
yields under mild conditions in Suzuki cross-coupling for both
activated and inactivated aryl bromides. The cationic rhodium(I)
complex (4) shows excellent catalytic activity in the hydrogenation
of styrenes in as low as 0.1 mol% concentrations. The ease of
synthesis, moderate stability towards moisture and flexible ligand
skeleton and excellent turnover numbers in the hydrogenation
reactions are promising features of this large-bite ligand system. It
has also encouraged us to examine other catalytic systems beyond
the hydrogenation and Suzuki coupling reactions. Further utility
of this ligand in several other hydrogenation processes and C–C
coupling reactions of stubborn aryl chlorides is under active
investigation in our laboratory.
A
mixture of the ligand 1 (0.075 g, 0.112 mmol) and
[W(CO)4(NHC5H10)2] (0.05 g, 0.107 mmol) in 12 mL of toluene
was heated at 90 ◦C for 18 h, which was then allowed to come to
room temperature and filtered. The solvent was removed under
vacuum to give yellow residue, which was washed with petroleum
ether thrice and dried. The residue was dissolved in CH2Cl2 (3 mL),
and layered with petroleum ether (1 mL), which on cooling to
−25 ◦C gave analytically pure, crystalline product of 3. Yield: 83%
(0.086 g). Mp 168 ◦C (decomp.). Anal. Calc. for C49H34WO6P2:
C, 61.01; H, 3.55%. Found: C, 60.92; H, 3.45%. FT IR (KBr
disk) cm−1: mCH: 3053 s; mCO: 2025 s, 1949 vs, 1912 vs, 1873 vs.
1H NMR (400 MHz, CDCl3): d 8.23 (d, 2H, Ar), 8.14 (d, 2H,
Ar), 7.12 (d, 2H, Ar), 7.62–7.81 (m, 4H, Ar), 7.18–7.57 (m, 20H,
OPPh2), 6.78 (d, 2H, Ar), 4.74 (s, 2H, Ar–CH2–Ar). 31P{ H} NMR
1
(161.9 MHz, CDCl3): d 121.6 (s), 1JWP = 285 Hz.
Synthesis of [Rh(COD){g2-Ph2P(–OC10H6)(l-CH2)(C10H6O–)-
PPh2-jP,jP}]BF4 (4)
Experimental
All experimental manipulations were carried out under an atmos-
phere of dry nitrogen or argon using Schlenk techniques. Solvents
were dried and distilled prior to use by conventional methods.
Bis(phosphinite) 1,4 [W(CO)4(NHC5H10)2],15 [Rh(COD)Cl]2,16
AgBF4 (0.021 g, 0.11 mmol) was added to the stirred solution
of [Rh(COD)Cl]2 (0.025 g, 0.051 mmol) in 8 mL of methanol.
After 30 min, a solution of the ligand 1 (0.074 g, 0.11 mmol)
in 6 mL of CH2Cl2 was added dropwise and stirred for 4 h.
The suspension obtained was filtered to remove AgCl, and the
yellowish-orange filtrate was concentrated to 5 mL. Cooling this
solution to −25 ◦C, gave 4 as an analytically pure yellow crystalline
solid. Yield: 66% (0.07 g). Mp 174 ◦C (decomp.). Anal. Calc. for
C53H46BF4O2P2Rh: C, 65.85; H, 4.79%. Found: C, 65.88; H, 4.63%.
1H NMR (400 MHz, CDCl3): d 8.03 (d, 2H, Ar), 7.77 (d, 2H, Ar),
7.75 (br s, 2H, Ar), 7.69 (br s, 2H, Ar), 7.41–7.56 (m, 20H, OPPh2),
7.38 (m, 2H, Ar), 7.29 (m, 2H, Ar), 6.02–7.25 (m, 4H, (COD)),
3
[Rh(CO)2Cl]2,17 [(g -C3H5)PdCl]2,18 [Pt(COD)I2],19 [AuCl(SMe2)],20
were prepared according to the published procedures. The 1H and
1
31P{ H} NMR (d in ppm) spectra were obtained on a Varian VXR
300 or VRX 400 spectrometer operating at frequencies of 300 or
400 and 121 or 162 MHz, respectively. The spectra were recorded
in CDCl3 (or DMSO-d6) solutions with CDCl3 (or DMSO-d6)
as an internal lock; TMS and 85% H3PO4 were used as internal
1
1
and external standards for H and 31P{ H} NMR, respectively.
Positive shifts lie downfield of the standard in all of the cases.
Infrared spectra were recorded on a Nicolet Impact 400 FTIR
instrument as a KBr disk or Nujol mull. Microanalyses were
carried out on a Carlo Erba Model 1106 elemental analyzer. Q-
Tof Micromass experiments were carried out using Waters Q-Tof
micro(YA-105). Melting points of all compounds were determined
on Veego melting point apparatus and were uncorrected. GC
analyses were performed on a Perkin Elmer Clarus 500 GC fitted
with packed column.
1
4.81 (s, 2H, Ar–CH2–Ar), 2.32–2.72 (m, 8H, (COD)). 31P{ H}
NMR (161.9 MHz, CDCl3): d 120.9 (d, 1JRhP = 171 Hz). ESI MS:
879 (M+ − BF4).
Synthesis of [Rh(CO){g2-Ph2P(–OC10H6)(l-CH2)(C10H6O–)-
PPh2-jP,jP}Cl] (5)
A solution the ligand 1 (0.105 g, 0.158 mmol) in 8 mL of
CH2Cl2 was added dropwise to a solution of [Rh(CO)2Cl]2 (0.03 g,
1328 | Dalton Trans., 2006, 1322–1330
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0.077 mmol), also in CH2Cl2 (6 mL). The reaction mixture was
heated to reflux for 4 h, after which the solution was allowed
to come to room temperature. The yellow colored solution was
concentrated to 3 mL, layered with diethyl ether (1 mL), and
cooled to −25 ◦C to give5 as an analytically pure, yellow crystalline
solid. Yield: 94% (0.12 g). Mp 136 ◦C (decomp.). Anal. Calc. for
C46H34ClO3P2Rh: C, 66.16; H, 4.10%. Found: C, 65.96; H, 3.93%.
IR (Nujol, mCO, cm−1): 2037 s. 1H NMR (400 MHz, CDCl3): d 8.02
(d, 2H, Ar), 7.75–7.80 (m, 4H, Ar), 7.25–7.72 (m, 6H, Ar and 20H,
and dried. The residue was redissolved in CH2Cl2 (3 mL), and
layered with 1 mL of petroleum ether, which on cooling to −25 ◦C
gave analytically pure, yellow crystalline product of 8. Yield: 88%
(0.071 g). Mp 204 ◦C (decomp.). Anal. Calc. for C45H34I2O2P2Pt: C,
48.36; H, 3.07%. Found: C, 48.33; H, 2.92%. 1H NMR (400 MHz,
DMSO-d6): d 8.08 (br s, 2H, Ar), 7.81 (d, 2H, Ar), 7.59–7.62 (m,
4H, Ar), 7.41–7.51 (m, 20H, OPPh2), 7.38–7.40 (m, 4H, Ar), 5.10
1
(s, 2H, Ar–CH2–Ar). 31P{ H} NMR (161.9 MHz, DMSO-d6): d
87.3 (s), 1JPtP = 3896 Hz.
1
OPPh2), 4.79 (s, 2H, Ar–CH2–Ar). 31P{ H} NMR (121.4 MHz,
CDCl3): d 125.5 (dd, 1JRhP = 187 Hz), d 123.9 (dd, 1JRhP = 154 Hz),
2JPP = 38 Hz.
Synthesis of [Au(Cl){g2-Ph2P(–OC10H6)(l-CH2)(C10H6O–)-
PPh2-jP,jP}] (9)
A solution of the ligand 1 (0.068 g, 0.102 mmol) in 8 mL
of CH2Cl2 was added dropwise to a solution of [AuCl(SMe2)]
(0.03 g, 0.102 mmol), also in CH2Cl2 (6 mL) and the mixture was
stirred for 6 h at room temperature. The colourless solution was
concentrated to 3 mL and layered with 1 mL of petroleum ether,
which on cooling to −25 ◦C gave 9 as analytically pure, white
crystals. Yield: 78% (0.072 g). Mp 170–172 ◦C. Anal. Calc. for
C45H34ClO2P2Au: C, 59.98; H, 3.80%. Found: C, 59.87; H, 3.76%.
1H NMR (400 MHz, CDCl3): d 8.21 (d, 2H, Ar), 8.03 (d, 2H, Ar),
7.68–7.73 (m, 2H, Ar), 7.48–7.58 (m, 20H, OPPh2), 7.43 (t, 2H,
Ar), 7.34 (d, 2H, Ar), 6.84 (d, 2H, Ar), 4.79 (s, 2H, Ar–CH2–Ar).
Synthesis of [NiCl2{g2-Ph2P(–OC10H6)(l-CH2)(C10H6O–)-
PPh2-jP,jP}] (6)
A suspension of the ligand 1 (0.107 g, 0.16 mmol) and NiCl2
(0.02 g, 0.15 mmol) in 10 mL of THF was stirred at room
temperature for 20 h. The unreacted NiCl2 was removed by
filtration. The solvent was removed under reduced pressure to
afford a sticky residue. The residue was dissolved in CH2Cl2 (3 mL),
and layered with petroleum ether (1 mL), which on slow evapo-
ration gave red colored crystals of 6 suitable for X-ray analysis.
Yield: 52% (0.062 g). Mp 216 ◦C (decomp.). Anal. Calc. for
C45H34Cl2O2P2Ni: C, 67.70; H, 4.29%. Found: C, 67.78; H, 4.33%.
1H NMR (400 MHz, CDCl3): d 8.22 (d, 2H, Ar), 8.09 (d, 2H, Ar),
7.81 (br s, 2H, Ar), 7.48–7.73 (m, 20H, OPPh2), 7.43 (br s, 2H, Ar),
7.34 (br s, 2H, Ar), 6.84 (br s, 2H, Ar), 4.79 (s, 2H, Ar–CH2–Ar).
1
31P{ H} NMR (161.9 MHz, CDCl3): d 136.1 (br). ESI MS: 865.6
(M+ − Cl).
Synthesis of [AuCl{Ph2P(–OC10H6)(l-CH2)(C10H6O–)-
PPh2}AuCl] (10)
1
31P{ H} NMR (121.4 MHz, CDCl3): d 115.5 (br s).
A solution of the ligand 1 (0.045 g, 0.067 mmol) in 8 mL of
CH2Cl2 was added dropwise to a solution of [AuCl(SMe2)] (0.04 g,
0.136 mmol), also in CH2Cl2 (6 mL) and stirred for 6 h at room
temperature. The colourless solution was concentrated to 3 mL
and layered with 1 mL of petroleum ether, which on cooling to
−25 ◦C gave 10 as analytically pure, white crystals. Yield: 86%
(0.065 g). Mp 176–178 ◦C. Anal. Calc. for C45H34Cl2O2P2Au2: C,
47.68; H, 3.02%. Found: C, 47.46; H, 2.98%. 1H NMR (400 MHz,
CDCl3): d 8.03 (d, 2H, Ar), 7.67–7.73 (m, 2H, Ar), 7.61 (d, 2H,
Ar), 7.48–7.51 (m, 2H, Ar), 7.40–7.48 (m, 20H, OPPh2), 7.25–7.35
Synthesis of [(g3-C3H5)Pd{g2-Ph2P(–OC10H6)(l-CH2)(C10H6O–)-
PPh2-jP,jP}]BF4 (7)
3
A solution of [(g -C3H5)PdCl]2 (0.02 g, 0.055 mmol) in 6 mL of
CH2Cl2 was added dropwise to a stirred solution of the ligand 1
(0.075 g, 0.112 mmol), also in CH2Cl2 (8 mL) at room temperature.
After 2 h, AgBF4 (0.022 g, 0.112 mmol) was added to the pale
yellow solution and stirring was continued for an additional
30 min. The suspension was filtered through a pad of Celite, the
solution was concentrated to 3 mL under reduced pressure, and
the product precipitated with petroleum ether. The light yellow
product was dissolved in 5 mL of CH2Cl2, and layered with
petroleum ether (1 mL), which on slow evaporation gave 7 as
analytically pure, light-yellow crystals. Yield: 76% (0.075 g). Mp
182 ◦C (decomp.). Anal. Calc. for C48H39BF4O2P2Pd: C, 63.84; H,
4.35%. Found: C, 63.73; H, 4.32%. 1H NMR (400 MHz, CDCl3):
d 8.01 (br s, 2H, Ar), 7.78 (d, 2H, Ar), 7.59 (d, 2H, Ar), 7.34–7.39
(m, 20H, OPPh2), 7.03–7.26 (br, 4H, Ar), 6.96 (d, 2H, Ar), 5.90–
5.92 (m, 1H, allyl), 5.05 (d, 4H, allyl), 4.95 (s, 2H, Ar–CH2–Ar).
1
(m, 2H, Ar), 7.08 (d, 2H, Ar), 4.70 (s, 2H, Ar–CH2–Ar). 31P{ H}
NMR (161.9 MHz, CDCl3): d 111.7 (s). ESI MS: 1097.9 (M+ −
Cl), 865.0 (M+ − AuCl2).
General procedure for the coupling reactions of aryl halides with
boronic acids
In a two-necked round bottom flask under an atmosphere of
nitrogen were placed the appropriate amount of Pd(OAc)2 (0.5
or 1 mol%) and 1.2 equivalent of ligand 1, and 5 mL of methanol
was added to it. After stirring for 5 min, aryl halide (0.5 mmol),
aryl boronic acid (0.75 mmol) and K2CO3 (0.138 g, 1 mmol) were
introduced into the reaction flask. The mixture was stirred at
1
31P{ H} NMR (161.9 MHz, CDCl3): d 121.8 (s).
Synthesis of [PtI2{g2-Ph2P{-OC10H6)(l-CH2)(C10H6O-}-
PPh2-jP,jP}] (8)
◦
room temperature or 60 C for 1 or 2 h under an atmosphere of
A solution of the ligand 1 (0.05 g, 0.075 mmol) in 8 mL of
CH2Cl2 was added dropwise to a solution of [Pt(COD)I2] (0.04 g,
0.072 mmol) also in CH2Cl2 (6 mL) at room temperature. The
reaction mixture was stirred for 6 h to give a clear yellow solution.
The solvent was removed under reduced pressure to afford a
yellow residue, which was washed thrice with petroleum ether
nitrogen (the course of reaction was monitored by GC analysis).
Then the solvent was removed under reduced pressure. The
resultant residual mixture was diluted with H2O (10 mL) and Et2O
(10 mL), followed by extraction twice (2 × 6 mL) with Et2O. The
combined organic fraction was dried (MgSO4), stripped of the
solvent under vacuum and the residue was redissolved in 5 mL
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of dichloromethane. An aliquot was taken with a syringe and
subjected to GC analysis. Yields were calculated vs. aryl halides or
dodecane as an internal standard.
analytical data and J. T. M. thanks the Louisiana Board of Regents
through grant LEQSF(2002-03)-ENH-TR-67 for purchase of the
CCD diffractometer and the Chemistry Department of Tulane
University for support of the X-ray laboratory.
Hydrogenation reactions (general procedure)
A combination of 5 mg (0.0051 mmol) of [Rh(COD){Ph2P-
(–OC10H6)(l-CH2)(C10H6O–)PPh2-jP,jP}] [BF4], 5.1 mmol of
substrate and 0.035 mL (0.255 mmol) of triethylamine was
dissolved in 20 mL of THF and was introduced into a 50 mL
glass vessel. The glass vessel containing the reaction mixture was
placed in the steel autoclave and the reactor was sealed. The vessel
was purged three times with hydrogen and then the autoclave was
pressurized with 100 psi of hydrogen. The reaction mixture was
stirred at the desired temperature for the required time. The extent
of conversion was determined by periodic GC analysis until the
desired conversion (partial or complete) was attained.
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For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b510589g
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We are grateful to the Department of Science and Technol-
ogy (DST), New Delhi for funding through grant SR/S1/IC-
05/2003. B. P. thanks CSIR for Research Fellowship (J. R. F.
and S. R. F.). We also thank RSIC, Mumbai, Department of
Chemistry Instrumentation Facilities, Bombay, for spectral and
1330 | Dalton Trans., 2006, 1322–1330
This journal is
The Royal Society of Chemistry 2006
©