Novel phosphite palladium complexes and their application in C-P cross-coupling reactions

Article abstract of DOI:10.1016/j.jorganchem.2010.08.026

A mono- and a 1,3-bis-phosphite arene ligand based on 2,2′-biphenol have been synthesized in order to study the synthesis of the corresponding palladium(II) complexes starting from different Pd precursors. Novel bis-phosphite palladium complex 1 [PdCl₂(L)₂] (L = dibenzo[d,f][1,3,2]dioxaphosphepin, 6-phenoxy), C,P-chelate bonded monophosphite palladium complex 2 [Pd(κ₂-L)(μ-Cl)]₂, and PCP-pincer palladium complex 3 have been prepared from these ligands in promising to excellent yields (50-95%). Additionally, complexes 1 and 3 have been characterized by X-ray crystal structure determinations. The application of 2,6-bis-phosphite pincer palladium(II) complex 3 in C-P cross-coupling between diphenylphosphine-borane and a wide range of various aryl iodides under very mild conditions is reported. Kinetic investigations indicate that 3 merely acts as a pre-catalyst and that Pd nanoparticles are the actual catalytically active species.

Full text of DOI:10.1016/j.jorganchem.2010.08.026

Journal of Organometallic Chemistry 695 (2010) 2618e2628
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Novel phosphite palladium complexes and their application
in CeP cross-coupling reactions
,
Jie Li a, Martin Lutz b, Anthony L. Spek b, Gerard P.M. van Klink a 1, Gerard van Koten a,
,
Robertus J.M. Klein Gebbink a
*
a Chemical Biology and Organic Chemistry, Debye Institute for Nanomaterials Science, Faculty of Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
b Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
a r t i c l e i n f o
a b s t r a c t
A mono- and a 1,3-bis-phosphite arene ligand based on 2,20-biphenol have been synthesized in order to
study the synthesis of the corresponding palladium(II) complexes starting from different Pd precursors.
Novel bis-phosphite palladium complex 1 [PdCl2(L)2] (L ¼ dibenzo[d,f][1,3,2]dioxaphosphepin, 6-phe-
Article history:
Received 22 April 2010
Received in revised form
12 August 2010
Accepted 20 August 2010
Available online 6 September 2010
noxy), C,P-chelate bonded monophosphite palladium complex 2 [Pd(k2-L)(m-Cl)]2, and PCP-pincer
palladium complex 3 have been prepared from these ligands in promising to excellent yields (50e95%).
Additionally, complexes 1 and 3 have been characterized by X-ray crystal structure determinations. The
application of 2,6-bis-phosphite pincer palladium(II) complex 3 in CeP cross-coupling between diphe-
nylphosphine-borane and a wide range of various aryl iodides under very mild conditions is reported.
Kinetic investigations indicate that 3 merely acts as a pre-catalyst and that Pd nanoparticles are the
actual catalytically active species.
Keywords:
Palladation
Orthometallation
Palladacycle
Ó 2010 Elsevier B.V. All rights reserved.
Pincer complex
CeP cross-coupling
1. Introduction
Suzuki, Heck and allylation reactions [5] stimulated the further
development of these phosphite-derived pincer complexes.
In the past decades, phosphine ligands [1], as a major type of
phosphorus donor ligands, have received much attention due to
their excellent performance in transition metal catalyzed organic
transformations [2]. The development of non-oxygen sensitive,
electron-deficient phosphite ligands, and their corresponding
metallated complexes, have become of increasing interest as
supplements to phosphine ligands [3]. Phosphite ligands can be
prepared from commercially available PCl3 and a variety of different
alcohols, which allows easy access to a great variety of ligands with
various steric and electronic properties. For instance, Bedford et al.
recently reported the synthesis of a series of triarylphosphite
ligands P(OAr)3 and their facile ortho-metallation with late tran-
sition metals, along with their remarkably catalytic activity in CeC
bond formations [4]. Additionally, phosphite pincer complexes
have been recently reported [4]. The excellent catalytic activities in
Transition metal catalyzed cross-coupling reactions are partic-
ularly important and powerful tools to realize chemical bond
formations. In the past decades, numerous examples of CeC, CeN,
CeS and CeP cross-coupling reactions have been developed by
taking advantage of rationally designed ligands, appropriate tran-
sition metals and optimized reaction conditions [6]. Among the
large number of documented ligands bearing a variety of diversified
donors, phosphorus donor comprising ligands, especially, tertiary
phosphine ligands, have held a core position in the domain of
ligands design and catalysis [7]. Therefore, the development of new
methodologies to easily access tertiary phosphines or their deriv-
atives via direct PeC bond forming reactions became more and
more desirable [8]. For instance, a versatile Pd(OAc)2/1,10-bis
(diisopropylphosphino)ferrocene-catalyzed
cross-coupling
of
secondary phosphines with aryl halides was developed by Buch-
wald and Murata [9]. Zhao and coworkers reported an efficient
copper-catalyzed approach for the P-arylation of primary and
secondary organophosphorus compounds using commercially
available and inexpensive proline and pipecolinic acid as ligands.
This method provides an entry to arylphosphonates, arylphosphi-
nates and arylphosphine oxides [10]. To the best of our knowledge,
pincer complexes have not been studied in CeP cross-coupling
* Corresponding author. Tel.: þ31 30 253 1889; fax: þ31 30 252 3615.
E-mail address: r.j.m.kleingebbink@uu.nl (R.J.M. Klein Gebbink).
1
Present address: DelftChemTech, Faculty of Applied Sciences, Delft University of
Technology, Julianalaan 136, 2628 BL Delft, The Netherlands.
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.08.026

J. Li et al. / Journal of Organometallic Chemistry 695 (2010) 2618e2628
2619
O
Cl
Pd
O
P
O
O
P
O
O
O
O
O
O
P O
P
O
O
P
P
O
O
O
O
O
cis-PdCl (4)
Pd Cl
2
2
2
tBu
tBu
tBu
tBu
4
1
2
3
5
Fig. 1. Entitled phosphite Pd complexes 1e3 and phosphite ligands 4 and 5.
reactions for the preparation of tertiary phosphine-boranes,
though, their remarkable catalytic performances in Heck [11],
Suzuki [12] and Sonogashira [13] reactions have been carefully
studied and are well documented.
ligands do not override electronic effects, the weakly
chloride ligands are preferentially positioned in trans-positions
with respect to the phosphite ligands by taking advantage of their
strongly p-acidic feature [4d,e,14].
p
-basic
In this study, 2,20-biphenol was selected as an inexpensive and
commercially available building block for the preparation of mono-
dentate phosphite ligand 4 and bis-phosphite pincer ligand 5 (Fig. 1).
The corresponding metal complexes 1e3 were obtained in
promising to good yields by reacting ligands 4 and 5 with different
metal precursors under various reaction conditions. Full details on
the synthesis and structural characterization of these ligands and
complexes are described. Furthermore, we report the first exam-
ples of the application of phosphite PCP-pincer complex in CeP
cross-coupling reactions.
In addition to cis-bis-phosphite Pd(II) complex 1, ortho-palla-
dated complex 2 was prepared in good yield by refluxing ligand 4 in
toluene overnight in the presence of a stoichiometric amount of
PdCl2 in accordance with the procedure published by Bedford et al.
[4a,b]. It is worth pointing out that other small size phosphite
ligands, such as triphenylphosphite, do not yield ortho-metallated
complexes under the same reaction conditions [4d,14]. 13C NMR
analysis showed a signal at
d 150.1 ppm, which is very typical for
the Cipso of CePd -bonds in ortho-metallated complexes [4,5,14].
s
MALDI-TOF MS data confirmed the formation of a dimeric complex.
31P NMR spectra in CDCl3 at 25 ꢀC showed complex 2 to be
a mixture of cis- and trans-isomers, as was also reported for other
ortho-palladated complexes [4,5,14]. Both 31P and 1H NMR spectra
showed rather broad peaks at 25 ꢀC, which could be due to the
facile interchange of cis- and trans-isomers and the conformational
flexibility of the seven-member ring in the phosphite ligand at
room temperature [4d]. The ratio of the two isomers in solution at
room temperature was estimated as 1:15 (cis:trans) by comparing
the 31P NMR signals, which is rather high with respect to reported
analogues [4e,14]. This is presumably due to the small and rigid
2,20-biphenol moiety that does not impose significant steric
congestions in the trans-isomer.
Interestingly, upon standing for 10 days a solution of complex 2
in CDCl3 gradually showed the formation of dimer 7 that contains
the non-orthometallated ligand. As shown in Fig. 3, the CePd bonds
in 2 have been split. It is well documented that palladacycles are
sensitive to protonolysis by HCl in CDCl3 solution in the presence of
light in air [4d].
2. Results and discussion
2.1. Synthesis and characterization of phosphite palladium
complexes 1 and 2
In order to synthesize monodentate ligand 4, 2,20-biphenol was
refluxed in freshly distilled PCl3 for 2 h to yield phosphorochloridite
6 in quantitative yield (Scheme 1). Without purification, 6 was
subsequently reacted with phenol in the presence of triethylamine
at reflux in toluene to afford the desired monodentate phosphite
ligand 4 in good yield. Ligand 4 was reacted with different Pd(II)
precursors under various conditions to give the corresponding
phosphite Pd complexes 1 and 2. For example, cis-PdCl2(4)2 (1) was
obtained in excellent yield (95%) by reacting 2 equivalents of
phosphite ligand
4 with 1.0 equivalent of PdCl2(MeCN)2 in
dichloromethane at room temperature. Suitable, colorless crystals
of 1 were obtained by slow vapor diffusion of methanol into
a dichloromethane solution of 1 at room temperature. The struc-
ture of 1 was determined by single crystal X-ray structure deter-
2.2. Synthesis and characterization of phosphite PCP-pincer
mination. As shown in Fig. 2, complex
1
adopts
a
typical
palladium complex 3
cis-configuration, in which two chloride ligands and two phosphite
ligands both occupy mutual cis-positions (ClePdeCl angles of 92.54
(3) and 91.92(3)ꢀ, and PePdeP angles of 95.36(3) and 95.74(3)ꢀ,
respectively). As expected, when steric effects of the phosphite
In contrast to the relatively straightforward synthesis of phos-
phine and phosphinite PCP-pincer ligands [15], the metallation via
direct CeH activation of phosphite pincer ligands in order to
iii
[cis-PdCl2(4)2]
1
O
O
O
O
OH
OH
i
ii
P
Cl
P
O
4
O
iv
Cl Pd
Cl
CDCl3
hυ.
O
O
P
Cl Pd Cl
2,2'-biphenol
4
6
Cl
Pd
4
2
2
7
Scheme 1. Synthesis of phosphite Pd complexes 1 and 2. Reaction conditions and reagents: i) PCl3, reflux, 2 h, quantitative yield; ii) phenol, Et3N, toluene, reflux, 2 h, 88%;
iii) PdCl2(MeCN)2, DCM, r.t., 1 h, 95%; iv) PdCl2, toluene, reflux, 16 h, 50%.

2620
J. Li et al. / Journal of Organometallic Chemistry 695 (2010) 2618e2628
ligand 5 with PdCl2(MeCN)2 in the presence of Et3N in dichloro-
ethane for 2 h at refluxing temperature smoothly yielded the
desired pincer complex 3 in good yield (65%). It seems that
the more hindered di-tert-butylresorcinol-based ligand 5 prevents
the formation of polymers and/or decreases the degree of poly-
merization upon heating as documented for the BINOL-derived
pincer ligand analogue [5b].
Colorless crystals of 3 were obtained by slow diffusion of
hexanes into a THF solution of 3 at room temperature. The structure
of 3 in the solid state was determined by single crystal X-ray
structure determination. As shown in Fig. 4, the complex adopts
a very typical mer-pincer configuration geometry. The palladium
atom is located at the central position of a slightly distorted square-
planar structure, with the chloride group occupying a trans-posi-
tion to the ipso-carbon atom. The two phosphorus donors are
mutually located in trans-positions, with PePdeP angles of 159.600
(17) and 159.313(17)ꢀ, respectively. The two independent mole-
cules in the crystal have opposite configurations on phosphorus,
pointing to the fluxional behavior of these stereocenters, as is also
indicated by solution NMR-data. A similar fluxional behavior is also
present in the other Pd complexes.
2.3. Bis-phosphite pincer palladium complex catalyzed CeP cross-
Fig. 2. Displacement ellipsoid plotof 1in the crystal (50% probability level). Hydrogen atoms
have been omitted for clarity. Only one of two independent molecules is shown. Charac-
coupling reactions
ꢀ
ꢀ
teristic bond distances (A) and bond angles ( ) in 1 [second molecule in square brackets]:
Pd1eCl11 ¼ 2.3388(7) [2.3246(7)], Pd1eCl12 ¼ 2.3298(7) [2.3303(7)], Pd1eP11 ¼ 2.2105(7)
[2.2154(7)], Pd1-P21 ¼ 2.2207(7) [2.2115(7)], Cl11ePd1eCl21 ¼92.54(3) [91.92(3)], P11-
Pd1-P21 ¼95.36(3) [95.74(3)].
Recently, secondary phosphine-borane adducts have been
widely used as versatile substrates to perform CeP cross-coupling
reactions due to their facile preparation, inertness towards oxygen
and moisture, and good reactivities [17,18]. Thus, diphenylphos-
phine-borane was employed as model phosphine substrate and
reaction optimization was carried out using a slight excess of Ph-I
(1.05 equivalent) in the presence of 2 equivalents of K2CO3 as base
and Pd-phosphite complexes 1e3 (5 mol%) as catalysts under
various reaction conditions (Table 1).
According to the experimental data showed in Table 2, the
chemical yields of the product PPh3$BH3 were highly solvent-
dependent when using 3 as the catalyst. The reaction in MeCN at
40 ꢀC proceeded with 50% chemical yield, whereas the reaction in
toluene did not furnish any conversion and the reaction merely gave
a poor yield of 10% in THF (Table 1, entry 3 vs entries 1 and 2).
Possibly, the reaction requires a polar and coordinating solvent to
prepare phosphite PCP-pincer complexes is rather difficult [5b]. A
small number of phosphite pincer palladium complexes have been
prepared by the oxidative insertion of a zerovalent Pd precursor (e.g
Pd2(dba)3.CHCl3) into CeI or CeBr bonds [5a,d]. Recently, Bedford
and coworkers found that the ortho-metallation via CeH activation
of phosphite PCP-pincer ligands could be dramatically enhanced by
using encumbered 4,6-di-tert-butylbenzene-1,3-diol as the key
building block [5b,16]. As elaborated in Scheme 2, a novel pincer
ligand 5 was prepared by following the same synthetic protocol as
mentioned above. Reacting phosphorochloridite 2 with 4,6-di-
tert-butylbenzene-1,3-diol in the presence of Et3N at 110 ꢀC in
toluene afforded pincer ligand 5 in high yield (88%). Reaction of
Fig. 3. Displacement ellipsoid plot of 7 in the solid state (50% probability level). Hydrogen atoms and co-crystallized CDCl3 have been omitted for clarity. Symmetry operation
i
ꢀ
ꢀ
i: 1 ꢁ x, 1 ꢁ y, 1 ꢁ z. Characteristic bond distances (A) and bond angles ( ) in 7: Pd1eCl1 ¼ 2.3133(4), Pd1eCl2 ¼ 2.2749(4), Pd1eCl1 ¼ 2.3944(4), Pd1eP1 ¼ 2.1783(4),
P1ePd1eCl1 ¼94.437(14), P1ePd1eCl2 ¼ 87.353(15), Cl2ePd1eCl1i ¼ 91.938(14).

J. Li et al. / Journal of Organometallic Chemistry 695 (2010) 2618e2628
2621
Cl
Pd
O
P
O
O
O
P
O
O
P
O
O
O
ii
i
O
P
Cl
O
O
tBu
tBu
tBu
tBu
6
5
3
Scheme 2. Synthesis of phosphite PCP-pincer complex 3. Reaction conditions and reagents: i) 4,6-di-tert-butylbenzene-1,3-diol, Et3N, toluene, reflux, 2 h, 88%; ii) PdCl2(MeCN)2,
Et3N, ClCH2CH2Cl, 80 ꢀC, 2 h, 65%.
Fig. 4. Displacement ellipsoid plot of 3 in the solid state (50% probability level). Hydrogen atoms and THF solvent molecules have been omitted for clarity. Only one of two
independent molecules is shown. Characteristic bond distances (A) and bond angles (ꢀ) in 3 [second molecule in square brackets]: Pd1eC11 ¼ 2.0089(17) [2.0080(16)],
ꢀ
Pd1eCl1 ¼ 2.3429(5) [2.3460(5)], Pd1-P11 ¼ 2.2539(4) [2.2437(4)], Pd1eP21 ¼ 2.2517(5) [2.2743(5)], C11ePd1eCl1 ¼176.11(5) [176.90(5)], P11ePd1eP21 ¼159.600(17) [159.313
(17)].
dissolve the inorganic base K2CO3 and to stabilize the catalytically
active palladium species. Interestingly, although both the diphenyl-
phosphine-borane substrate and the triphenylphosphine-borane
Table 1
product are resistant towards oxidation and hydrolysis in air, the
Pd-catalyzed CeP coupling between Ph2PH$BH3 and phenyl iodide; reaction
optimization.a
exclusion of air is crucial to afford conversion (Table 1, entry 4). Pd
dimer complex 2 gave slightly lower yield in contrast to pincer
complex 3 (Table 1, entry 3 vs. entry 5), while bis-phosphite palla-
dium complex 1 gave 90% yield (Table 1, entry 6). In this paper, we
focused on the investigation of the catalytic activity of pincer
complex 3. Elevated temperatures did not improve the yield when
using 3. In contrast, it led to an increased formation of many by-
products (Table 1, entry 7). A remarkable improvement was achieved
upon progressively increasing the loading of 3 (Table 1, entries 8 and
9). Eventually, complete conversion was obtained by using 10 mol%
loading of 3 in MeCN at 40 ꢀC in the presence of 2 equivalents of
K2CO3. It is worth pointing out that when Ph-Br and MePhH(BH3),
respectively, were tested as substrates under optimized conditions,
neither of them gave any conversion and starting materials were
fully recovered from the reaction mixtures. To illustrate the potential
of the catalytic procedure, a substrate library of 19 different aryl
iodides was evaluated by using the optimized conditions (Table 2).
As shown in Table 2, PCP-pincer complex 3 smoothly catalyzes
CeP cross-couplings of diphenylphosphine-boranes and a wide
scope of aryl iodide substrates with promising to excellent isolated
yields. The model substrate, Ph-I, gave 95% yield, whereas slightly
electron-enriched and ortho-substituted 1-iodonaphthalene
BH3
P
BH3
P
5 mol% [Pd], 1.05 eq Ph-I
H
Ph
Ph
Ph
Ph
Ph
2 eq K2CO3, solvent, T°C, 16h
Entry
Cat.
Condition (T ꢀC/Solvent)
Yieldb (%)
1
2
3
3
3
3
2
1
3
3
3
40/Toluene
40/THF
0
10
50
0
40
90
44
70
>95
3
40/MeCN
40/MeCN
40/MeCN
40/MeCN
82/MeCN
50/MeCN
40/MeCN
4c
5
6
7
8d
9e
a
Reaction conditions as follows unless otherwise stated: 0.105 mmol of Ph-I,
0.1 mmol of Ph2PH(BH3), 5 mol% catalyst, 0.2 mmol of K2CO3, 1 mL of solvent, 16 h,
under N2.
31P NMR yield with diethyl ethylphosphonate as internal standard.
b
c
In air.
d
7.5 mol% [Pd] catalyst loading.
10 mol% [Pd] catalyst loading.
e

2622
J. Li et al. / Journal of Organometallic Chemistry 695 (2010) 2618e2628
Table 2
Phosphite PCP-pincer complex 3 catalyzed CeP cross-coupling reactions.a
BH3
BH3
1.05 eq Ar-I,3 10mol%
Ph
P
H
Ph
Ph
P
Ph
2 eq K2CO3, 40°C, MeCN, 16h
Ar
Entry
Ar-I
Phosphine-borane
BH3
Yieldb (%)
95
Phosphine
Yieldc (%)
95
Referenced
PPh2
I
PPh2
1
PPh2
H3B PPh2
I
2
3
74
98
95
87
[19]
[20]
BH3
PPh2
O
O
O
O
O
I
PPh2
O
BH3
PPh2
O
O
O
I
PPh2
4
5
96
95
91
83
[21]
[22]
BH3
PPh2
PPh2
I
BH3
PPh2
PPh2
I
6
94
72
[23]
BH3
PPh2
PPh2
7
8
50
95
86
79
[23]
[24]
I
BH3
PPh2
MeO
MeO
PPh2
MeO
I
MeO
I
I
MeO
BH3
PPh2
9
87
84
[25]
PPh2
OMe
OMe
c
BH3
PPh2
PPh2
OMe
10
11
46
69
81
75
[26]
[27]
OMe
BH3
PPh2
HO
PPh2
HO
I
HO

J. Li et al. / Journal of Organometallic Chemistry 695 (2010) 2618e2628
2623
Table 2 (continued )
Entry
Ar-I
Phosphine-borane
Yieldb (%)
74
Phosphine
Yieldc (%)
89
Referenced
BH3
PPh2
Cl
PPh2
Cl
I
Cl
12
[28]
Cl
Cl
Cl
Cl
BH3
PPh2
13
71
90
[29]
PPh2
PPh2
I
I
BH3
PPh2
14
15
16
58
63
57
85
80
93
[30]
[31]
[32]
Cl
Cl
BH3
PPh2
Br
Br
PPh2
Br
I
Br
Br
Br
BH3
PPh2
PPh2
PPh2
I
BH3
PPh2
Br
17
17
95
95
88
95
95
[33]
[23]
[34]
I
Br
Br
BH3
Ph2P
BH3
PPh2
Ph2P
Ph2P
PPh2
I
I
18e
I
BH3
Ph2P
BH3
PPh2
PPh2
19e
I
a
Reaction conditions as follows unless otherwise stated: 0.105 mmol of Ar-I, 0.1 mmol of Ph2PH(BH3), 0.01 mmol of 3, 0.2 mmol of K2CO3, 1 mL of MeCN, 16 h, under N2.
Isolated yields of tertiary phosphine-boranes.
Isolated yields of tertiary phosphines.
References are provided for the original synthesis of the reaction products.
0.1 mmol of diiodobenzene, 0.21 mmol of Ph2PH(BH3) and 0.021 mmol of 3 were used.
b
c
d
e
decreased the yield by 20% (Table 2, entries 1 and 2). As anticipated,
chlorine atoms were evaluated in catalysis. The resulting products
bearing carbonehalogen bonds could be potentially valuable
targets for further organic transformations through subsequent
cross-coupling reactions. As shown in Table 3, chlorine function-
alized substrates in most cases outperformed the corresponding
bromine-functionalized ones under the same reactions conditions
(Table 2, entries 12 and 13 vs. entries 15 and 16), while the yields
of desired products were generally promising in both cases.
Moreover, meta- and para-functionalized substrates also out-
performed ortho-functionalized ones as mentioned before (Table
2, entries 12 and 13 vs. entry 14; entries 15 and 16 vs. entry 17).
Unfortunately, the yield was dramatically decreased to only 17%
for 1-bromo-2-iodo-benzene. To our delight, besides mono-
phosphine products, this method also works for the preparation of
diphosphine products. For example, reacting 2.1 equivalents of
diphenylphosphine-borane adducts to 1.0 equivalent of 1,3- or 1,4-
substrates comprising electron-withdrawing methyl acetate and
acetyl functional groups afforded excellent yields (Table 2, entries 3
and 4). Moreover, substrates functionalized with electron-donating
methyl and methoxyl groups on either meta- or para-positions
generally did not decrease the yields (Table 2, entries 5, 6, 8 and 9).
Nevertheless, ortho-functionalized substrates gave significantly
decreased yields of about 50% (Table 2, entries 7 and 10), which
could be due to steric congestion effects. Noticeably, the para-
hydroxy functionalized substrate gave much lower yield than the
para-methoxy group functionalized substrate (Table 2, entry 8 vs.
entry 11), presumably because the considerably acidic phenol
moiety reacts with K2CO3 and the resulting potassium phenolate
adduct is poorly soluble in MeCN.
Since we observed that bromobenzene is not active at all in this
catalytic process (vide supra), substrates bearing either bromine or

2624
J. Li et al. / Journal of Organometallic Chemistry 695 (2010) 2618e2628
Table 3
Selected crystallographic data for complexes 1, 3, and 7.
1
3
7
Formula
FW
C36H26Cl2O6P2Pd
793.81
C38H35ClO6P2Pd$0.5(C4H8O) þ disordered solvent
C36H26Cl4O6P2Pd2$2(CHCl3)
1209.84
827.50a
Crystal color
Crystal size [mm3]
Crystal system
Space group
Colorless
Colorless
Yellow
0.54 ꢃ 0.36 ꢃ 0.36
0.30 ꢃ 0.30 ꢃ 0.30
0.51 ꢃ 0.30 ꢃ 0.08
Triclinic
Triclinic
Triclinic
ꢁ
ꢁ
ꢁ
P 1 (no. 2)
P 1 (no. 2)
P 1 (no. 2)
ꢀ
a [A]
12.2961(2)
14.6736(3)
19.8556(4)
71.927(1)
72.144(1)
88.825(1)
3230.65(10)
4
14.6466(10)
16.1015(7)
20.4137(12)
99.657(2)
94.758(2)
112.789(2)
4317.2(4)
4
9.50019(17)
10.13286(18)
12.2159(2)
80.303(1)
73.343(1)
85.570(1)
1109.99(3)
1
ꢀ
b [A]
ꢀ
c [A]
a
b
g
[ꢀ]
[ꢀ]
[ꢀ]
3
ꢀ
V [A ]
Z
Dx [g/cm3]
1.632
1.273a
1.810
Refl. meas./unique
m
55,800/14,758
0.887
123,398/19,826
26,489/5077
1.529
[mmꢁ1
]
0.607a
Abs. corr.
Abs. corr. range
Param./restraints
Multi-scan
0.49e0.73
847/0
Multi-scan
0.61e0.83
922/0
Multi-scan
0.72e0.89
262/0
R1/wR2 [I > 2
R1/wR2 [all refl.]
S
s
(I)]
0.0333/0.0835
0.0444/0.0876
1.170
ꢁ1.18/0.62
0.0245/0.0705
0.0326/0.0736
1.063
ꢁ0.41/0.54
0.0187/0.0470
0.0208/0.0481
1.039
ꢁ0.32/0.64
3
ꢀ
Res. density [e/A ]
a
Derived parameters do not contain the contribution of the disordered solvent.
diiodobenzene under optimized conditions afforded the desired
1,3- or 1,4-phenylene-bis(diphenylphosphine-borane) products in
excellent yields (Table 2, entries 18 and 19). Although it requires
a relatively high catalyst loading (i.e., 10 mol%), the present cata-
lytic protocol can smoothly bring about the cross-coupling of
diphenylphosphine-borane and aryl iodides under very mild
conditions in comparison with several other protocols that rely on
bidentate phosphine ligands [17d,e].
observation strongly suggests that complex 3 might act as a pre-
catalyst, which decomposes under the reaction conditions to form
a catalytically active species. While Pd-black formation was not
observed during the entire procedure, the development of a deeply
orange color indicated the formation of Pd0 clusters.
To confirm this assumption, a standard poisoning test was
carried out. Crabtree and Anton [37a] and Whitesides et al. [37b]
independently reported that adding an excess of metallic
mercury leads to the amalgamation of the surface of heteroge-
neous metal particles thus poisoning them, whereas in most of the
cases this will not affect homogeneous catalysts. To ensure suffi-
cient time for the mercury droplets to capture Pd nanoparticles,
the experiments were carried out 10 times more diluted than the
previous profile study. As depicted in Fig. 6, completely sup-
pressed conversions were found after charging Hg at t ¼ 0 or
t ¼ 2.5 h.
According to reported procedures, the removal of the BH3
group can be easily accomplished by using strong acid [35] or
strong base [36] under mild conditions. Recently, Hiyama et al.
reported a novel benign deprotection method for phosphine-
boranes under neutral conditions [26]. The procedure afforded
free phosphines in excellent yields by refluxing phosphine-borane
ꢀ
adducts with activated molecular sieves (4 A) in an ethereal
solvent (THF/MeOH 7:3 (v/v)) for 96 h (Scheme 3). Accordingly, all
prepared phosphine-borane compounds were efficiently depro-
tected by using this procedure and all spectral data of the resulting
free phosphines were fully identical to those reported in the
literature (Table 2).
Besides the use of mercury as a poison, the protocol described by
Widegren and Finke [38] predicts that when catalytic activity can be
completely terminated with ꢂ1 equivalent of an external ligand,
100
90
80
70
60
50
40
30
20
10
0
2.4. Investigations into the nature of the catalytically active species
To understand the mechanism of the reaction and to identify the
catalytically active species, a reaction profile study along with
poisoning tests were carried out. By means of 31P NMR measure-
ments, a reaction profile curve of the chemical yield of the product
against time was obtained for the reaction of diphenylphosphine-
borane and Ph-I (Fig. 5).
Fig. 5 represents a S-shaped curve, which indicates that the
reaction rate varied during the catalytic process and included an
introduction period as well as an accelerating period. This
0
10 20 30 40 50 60 70 80 90 100
Time (minute)
BH3
MS 4 Å
THF/MeOH
P
Ph
Ph
Fig. 5. Reaction profile study of the CeP cross-coupling of Ph2PH(BH3) and Ph-I using 3
as catalyst. (Reaction conditions: 1 mmol of Ph2PH(BH3), 1.05 mmol of Ph-I, 2 mmol of
K2CO3, 0.1 mmol of complex 3, 4 ml of MeCN, 40 ꢀC, diethyl ethylphosphonate as
internal standard.)
P
Ar
Ph
Ar
Ph
Scheme 3. Deprotection of tertiary phosphine-boranes.

J. Li et al. / Journal of Organometallic Chemistry 695 (2010) 2618e2628
2625
3. Conclusion
In summary, we have developed a series of novel phosphite
palladium complexes 1e3 by using a straightforward synthetic
route. Complexes 1 and 3 were characterized by X-ray crystal
structure determinations. The application of phosphite PCP-pincer
palladium complex 3 in CeP cross-coupling reactions was opti-
mized with Ph-I and Ph2PhH(BH3) as model substrates and
complete conversion was obtained by using 10 mol% loading of 3 in
MeCN at 40 ꢀC in the presence of 2 equivalents of K2CO3. Following
the optimized procedure, a series of tertiary phosphine-borane
compounds were synthesized in promising to excellent yields with
a very wide functional group tolerance and under mild conditions.
The reaction profile and poisoning studies with metallic mercury
and PPh3 indicated the involvement of heterogeneous Pd0 species
as the actual catalyst. Pincer complex 3, therefore, merely acts as
a pre-catalyst which decomposes to generate catalytically active Pd
nanoparticles that are able to catalyze CeP coupling reactions at
very mild conditions.
Fig. 6. Poisoning tests with Hg and PPh3 as the inhibitors. (Reaction conditions:
0.1 mmol of Ph2PH(BH3), 0.105 mmol of Ph-I, 0.2 mmol of K2CO3, 0.01 mmol of
complex 3, 4 mL of MeCN, 40 ꢀC, diethyl ethylphosphonate as internal standard.)
4. Experimental section
such as PPh3, that it is highly suggestive of the presence of a hetero-
geneous catalyst. In the case of 3, complete conversion was obtained
when0.05 equivalentofPPh3 was added at t ¼ 0whichdoes not agree
with the reported poisoning protocol for a heterogeneous catalyst. To
understand this observation, complex 3 and 0.05 equivalent of PPh3
were mixed in an NMR tube and the 31P NMR spectrum of the
resulting mixture was recorded at 40 ꢀC. A new single peak at
24.3 ppmwas observed, which corresponds to the formation of trans-
dichlorobis(triphenylphosphine)palladium, which might be an active
catalyst too for the cross-coupling of aryl iodides and secondary
phosphine-boranes. Nevertheless, completely suppressed catalytic
activity was observed right after adding PPh3 at t ¼ 3.5 h (Fig. 6),
which is in accordance with a catalytic species of heterogeneous
nature. Noticeably, PCP- [15a] and SCS- [11b,12b,39] pincer palladium
complexes have been reported to decompose upon heating (e.g.
160 ꢀC) to form active Pd nanoparticles. Most importantly, in this
study the active Pd species were formed atrelativelylow temperature
(i.e. 40 ꢀC).
According to these kinetic investigations, the present CeP cross-
coupling process presumably proceeds via a pathway as shown in
Scheme 4 [40a,b]: i) complex 3 decomposes under the reaction
conditions to form catalytically active Pd0 nanoparticles, which can
be oxidized to Pd2þ after reacting to aryl iodides; ii) deprotonation
of the secondary phosphine-borane occurs in the presence of K2CO3
and the generated phosphorus anion attacks the palladium atom to
afford Ar-Pd-PPh2BH3; iii) after reductive elimination, the desired
tertiary phosphine-borane product forms with concomitant
reduction of Pd2þ back to Pd0.
4.1. General remarks
All reactions were performed under a dry N2 atmosphere using
standard Schlenk techniques. CH2Cl2, ClCH2CH2Cl, MeCN, and Et3N
were distilled over CaH2 in a conventional heating bath before use.
Toluene was freshly distilled over Na sand prior to use. PCl3 was
freshly distilled and stored under N2 before use. All other reagents
were purchased and used as received. 1H NMR (1H 400.0 MHz), 13
C
NMR (13C 100.6 MHz) and 31P NMR (161.9 MHz) spectra were
recorded at room temperature in CDCl3 on a Varian 400 MHz
INOVA spectrometer. Chemical shift values are reported in ppm (d)
relative to (CH3)4Si (1H and 13C NMR) or a capillary containing 85%
H3PO4 in D2O (31P{1H} NMR). Flash chromatography was performed
using ACROS silica gel, 0.06e0.200 mm, pore diameter ca. 6 nm. MS
measurements were carried out on an Applied Biosystems Voyager
DE-STR MALDI-TOF MS. Elemental microanalyses were performed
by Dornis und Kolbe, Mikroanalytisches Laboratorium, Mülheim
a/d Ruhr, Germany.
4.2. General procedure for the preparation of phosphite ligands
2,20-Biphenol (2 mmol, 0.3730 g) was azeotropically dried over
toluene in vacuum three times and was suspended in freshly
distilled PCl3 (5 mL) at room temperature. The resulting mixture
was heated at refluxing temperature for 2 h with vigorously stir-
ring. During this procedure, HCl gas evolved and was trapped by
a saturated aqueous NaOH solution. After cooling, the remaining
PCl3 was removed under reduced pressure and the trace amounts
of PCl3 were further azeotropically evaporated with dry toluene
(3 ꢃ 5 mL). The resulting colorless oily product was directly used
in the next step without purification. The oily product was
redissolved in toluene (25 mL) and was added to a solution of
azeotropically dried aryl alcohol (2 mmol of phenol; 1 mmol of
resorcinol or 2,4-tert-butylbenzene-1,3-diol) in toluene (10 mL) at
0 ꢀC (ice bath). To this mixture, a solution of Et3N (0.31 mL,
2.2 mmol) in toluene (5 mL) was added drop wise over 5 min. The
reaction mixture was then heated to 110 ꢀC for 2 h under N2. After
cooling, the resulting white suspension mixture was filtered over
a short plug of silica gel and the filter cake was washed with
toluene (3 ꢃ 10 mL). The combined toluene layers were concen-
trated to dryness and a white solid was obtained. The analytic
sample was obtained after further purification via flash chroma-
tography with EtOAC/hexanes (1:1, v/v).
Ph2PHBH3
+ K2CO3
K[Ph2P(BH3)]
Ar-Pd-I
Ar-I
Cl
Pd
KI
Ar-Pd-PPh2BH3
O
O
P
P
O
O
Pd (0)
O
O
active Pd
tBu
tBu
nanoparticles
BH3
3
P
Ph
Ar
Ph
tertiary phosphine-borane
Scheme 4. Proposed reaction mechanism for the CeP coupling process.

2626
J. Li et al. / Journal of Organometallic Chemistry 695 (2010) 2618e2628
4.2.1. Ligand 4
148.2, 137.1, 135.8, 132.6, 130.2 (d, J ¼ 23 Hz), 129.5, 127.1, 125.6,
0.5430 g, 1.76 mmol; Yield: 88%. 1H NMR (CDCl3, 400 MHz):
122.6, 35.0, 30.0. 31P{1H} NMR (CDCl3, 161.9 Hz):
d 146.2 (s). MS
3
d
7.54 (d, JHeH ¼ 1.5 Hz, 1 H, BIPOLH, biphenyl-H2), 7.52 (d,
(MALDI-TOF): m/z calcd for C38H35O6P2Pd: 756.05 [M ꢁ Cl]þ;
found: 756.04; Anal. Calcd for C38H35ClO6P2Pd: C, 57.66; H, 4.46; P,
7.83; Pd, 13.45; found: C, 57.71; H, 4.48; P, 7.79; Pd, 13.42.
3JHeH ¼ 1.5 Hz, 1 H, BIPOLH, biphenyl-H3), 7.42e7.31 (m, 4 H,
BIPOLH, biphenyl-H4, 40 and H5, 50 þ 2 H, ArH), 7.26e7.17 (m, 2
H, BIPOLH, biphenyl-H20 and H30 þ 3H, PhH). 13C{1H} NMR (CDCl3,
100.6 Hz):
d
131.7, 130.5, 129.9, 129.8, 128.4, 127.1, 126.2, 125.3,
4.6. General procedure of synthesis of tertiary phosphine-boranes
121.7, 120.4. 31P{1H} NMR (CDCl3, 161.9 Hz):
d 145.0 (s). Anal.Calcd
for C, 70.13; H, 4.25; P, 10.05; found: C, 70.10; H, 4.27; P, 10.07.
To a flame dried Schlenk tube, Ph2PH(BH3) (20.0 mg, 0.1 mmol),
aryl iodide (0.105 mmol), K2CO3 (28.0 mg, 0.2 mmol), complex 3
(7.9 mg, 0.01 mmol), and MeCN (1 mL) were added. This resulting
mixture was warmed to 40 ꢀC for 16 h under N2. After cooling,
water (2 mL) was added and the mixture was extracted with EtOAc
(3 ꢃ 5 mL). The combined organic layers were concentrated on
a rotary evaporator and the residue was subjected to flash chro-
moatography (EtOAc/hexanes 3:7 (v/v)) to obtain the entitled
tertiary phosphine-boranes as white solids. The spectral data of
these compounds were identical to those reported in the literature
(see Table 3, references).
4.2.2. Ligand 5
0.5730 g, 0.88 mmol; Yield: 88%. 1H NMR (CDCl3, 400 MHz):
d
7.52(m, 4H, BIPOLH, biphenyl-H2, 20), 7.26e7.42(m, 12H, BIPOLH,
biphenyl-H3, 30, H4, 40 and H5, 50), 7.13 (s, 1H, ArH), 7.04e7.07
(m, 1H, ArH), 1.42 (s, 12H, tBu), 1.40 (s, 6H, tBu). 13C{1H} NMR (CDCl3,
100.6 Hz):
d
135.4, 131.6,129.9, 129.3, 127.2,125.7, 122.4, 121.9,121.7,
145.0 (s). Anal.
117.0, 35.0, 30.4. 31P{1H} NMR (CDCl3, 161.9 Hz):
d
Calcd for C, 70.15; H, 5.58; P, 9.52; found: C, 70.17; H, 5.63; P, 9.49.
4.3. Complex 1
4.7. General procedure of deprotection of tertiary phosphine-
boranes
A mixture of [PdCl2(NCMe)2] (0.1 mmol, 0.0263 g) and ligand 4
(0.2 mmol, 0.062 g) in 10 mL dichloromethane was stirred at room
temperature for 1 h. The reaction mixture was then concentrated
under reduced pressure and a white solid was precipitated from the
solution by the addition of hexanes. The solid was collected by
centrifugation and further washed with 2 ꢃ 10 mL hexanes. The
According to reported procedure [26], to a Schlenk tube equip-
ped with condenser were added activated molecular sieves
ꢀ
(250 mg, 4 A), phosphine-boranes (0.05 mmol), THF (3.5 mL) and
MeOH (1.5 mL). The reaction mixture was then refluxed for 96 h
under N2. After cooling, the reaction mixture was filtered through
a pad of Celite and the filter cake was washed with THF (3 ꢃ 5 mL).
All volatiles were removed on a rotary evaporator and the resultant
waxy product was re-crystallized from MeOH/EtOAc at ꢁ30 ꢀC to
yield analytically pure samples as white solids. The spectral data of
these compounds were identical to those reported in the literature
(see Table 3).
product was obtained as
a
fine white powder (0.0755 g,
7.49e7.60
0.095 mmol). Yield: 95%. 1H NMR (CDCl3, 400 MHz):
d
(m, 8H, BIPOLH, biphenyl-H2, 20, H3, 30), 7.40e7.45 (m, 8H, BIPOLH,
biphenyl-H4, 40, H5, 50), 7.10 (d, 3JHeH ¼ 7.2 Hz, 6H, PhH), 6.84e6.92
(m, 4H, PhH). 13C{1H} NMR (CDCl3, 100.6 Hz):
d 149.3, 148.0, 130.5,
130.3, 129.6, 129.3, 127.2, 126.5, 122.6, 121.6. 31P{1H} NMR
(CDCl3, 161.9 Hz): 111.7 (s). Anal. Calcd for C36H26Cl2O6P2Pd: C,
d
54.47; H, 3.30; P, 7.80; Pd, 13.41; found: C, 54.49; H, 3.37; P, 7.86; Pd,
13.45.
4.8. Procedure of reaction profile study
4.4. Complex 2
A flame dried Schlenk tube equipped with a condenser was
charged with Ph2PH(BH3) (1 mmol), Ph-I (1.05 mmol), K2CO3
(2 mmol), complex 3 (0.1 mmol) and MeCN (4 mL). This reaction
mixture was then warmed to 40 ꢀC with vigorous stirring. Small
A mixture of ligand 4 (0.1 mmol) and PdCl2 (0.0180 g, 0.1 mmol)
in 5 mL of toluene was heated at 110 ꢀC for overnight under N2.
After cooling, the reaction mixture was filtered over a pad of Celite,
all volatiles were removed under vacuum, and the residue crys-
tallized from CH2Cl2/MeOH (1:9, v/v) to afford an orange solid
aliquot (50 mL) was taken at alloted time and diluted in EtOAc
(1 mL). The resulting organic mixture was washed with water
(1 mL) and the aqueous layer was extracted with EtOAc (3 ꢃ1 mL).
The combined organic layers were dried over MgSO4. Removal of all
the volatiles under reduced pressure gave an oily orange mixure,
which was redissovled in CDCl3 and transfered into an NMR tube.
The chemical conversion was determined by integration of corre-
sponding 31P resonances in the 31P NMR spectra. Diethyl ethyl-
(0.0450 g, 0.05 mmol). Yield: 50%. 1H NMR (CDCl3, 400 MHz):
d
7.55
3
(br d, JHeH ¼ 8 Hz, 4H, PhH), 7.35e7.50 (very br m, 16H, BIPOLH),
7.21e7.28 (very br s, 4H, PhH). 13C{1H} NMR (CDCl3, 100.6 Hz):
d
150.1, 148.0 (d, J ¼ 12 Hz), 131.8, 130.4, 129.1, 129.0, 127.5, 127.2,
126.8, 122.6, 121.5 (d, J ¼ 4.5 Hz), 116.4. 31P{1H} NMR (CDCl3,
161.9 Hz):
d
80.1 (br s) minor isomer, 81.4 (br s) major isomer. Anal.
phosphonate (10
mL) was added to the respective samples as an
Calcd for C36H24Cl2O6P2Pd2: C, 48.14; H, 2.69; P, 6.90; Pd, 23.69
found: C, 48.22; H, 2.79; P, 6.88; Pd, 23.65.
internal standard.
4.9. Procedure of poisoning tests
4.5. Complex 3
A flame dried Schlenk tube equipped with a condenser was
charged with Ph2PHBH3 (0.1 mmol), Ph-I (0.105 mmol), K2CO3
(0.2 mmol), complex 3 (0.01 mmol) and MeCN (4 mL). This reaction
mixture was warmed to 40 ꢀC with vigorous stirring. Metallic
mercury (300 equivalents) or PPh3 (0.05 equivalent) were added to
A
mixture of the ligand
[PdCl2(NCMe)2] (0.0264 g, 0.1 mmol) in 2 mL of 1,2-dichloroethane
was treated with NEt3 (14
L, 0.10 mmol) and then heated at 80 ꢀC
5
(0.0650 g, 0.1 mmol) and
m
for 2h. After cooling, the reaction mixture was filtered over a pad of
Celite, all volatiles were removed under vacuum and the residue
crystallized from CH2Cl2/pentane to afford a slightly orange solid
the reaction mixture at alloted time. Small aliquot (50 mL) was
taken at alloted time and diluted in EtOAc (1 mL). The resulting
organic mixture was washed with water (1 mL) and the aqueous
layer was extracted with EtOAc (3 ꢃ1 mL). The combined organic
layers were dried over MgSO4. Removal of all the volatiles under
reduced pressure gave an oily orange mixture, which was dissolved
(0.0515 g, 0.065 mmol). Yield: 65%. 1H NMR (CDCl3, 400 MHz):
3
d
7.55(d, JHeH ¼ 7.6 Hz, 4H, BIPOLH, biphenyl-H2, 20), 7.35e7.43
(m, 12H, BIPOLH, biphenyl-H3, 30, H4, 40 and H5, 50), 7.22 (s, 1H,
ArH), 1.34 (s, 18H, tBu). 13C{1H} NMR (CDCl3, 100.6 Hz):
d 152.6,

J. Li et al. / Journal of Organometallic Chemistry 695 (2010) 2618e2628
2627
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was determined by integration of corresponding 31P resonances in
the 31P NMR spectra. Diethyl ethylphosphonate (10
to the respective samples as an internal standard.
mL) was added
(b) K.N. Gavrilov, O.G. Bondarev, A.V. Korostylev, A.I. Polosukhin, V.N. Tsarev,
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ꢀ
(
l
¼ 0.71073 A) on a Nonius KappaCCD diffractometer with rotating
anode at a temperature of 150 K up to a resolution of (sin
q/
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ꢀꢁ1
¼ 0.65 A . Integration of the intensities was performed with
l
)
max
EvalCCD [41] and absorption correction with SADABS [42]. The
structures were solved with Direct Methods (program SHELXS-97
[43] for complexes 1 and 3; program SIR-97 [44] for complex 7).
Refinement was performed with SHELXL-97 [43] against F2 of all
reflections. Non hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms were located in differ-
ence Fourier maps (complexes 1 and 7) or introduced in calculated
positions (complex 3) and refined with a riding model. Geometry
calculations and checking for higher symmetry was performed
with the PLATON program [45]. Further details are given in Table 1.
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With the matrix (1,0,ꢁ2/ꢁ1,0,0/0,1,0) the triclinic cell parame-
ters can be transformed into a pseudo-monoclinic C-centered cell.
The a-axis of the triclinic cell then becomes the b-axis of the
pseudo-monoclinic cell. The two independent molecules in the
triclinic cell are related by an approximate twofold rotation roughly
about the a-axis. The reflection intensities only support a triclinic
symmetry and there is no indication for twinning.
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Besides the ordered THF molecule, the crystal structure contains
3
ꢀ
a large void (852.4 A /unit cell), filled with disordered THF solvent
molecules. Their contribution to the structure factors was secured
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program PLATON [45], resulting in 60 electrons/unit cell.
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5562e5563.
Acknowledgements
We gratefully thank Utrecht University for financial support.
This work was supported in part (ML, ALS) by the Council for the
Chemical Sciences of the Netherlands Organization for Scientific
Research (CW-NWO).
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