Synthesis and characterization of electronically varied XCX palladacycles with functional arene groups

Article abstract of DOI:10.1016/j.ica.2005.09.030

X-ray crystallographic studies show that varying the nature of the S-aryl ligands in SCS-Pd(II) pincer complexes and the electronic nature of the aryl substituent para to the Pd(II) group in PCP-Pd(II) pincer complexes do not lead to structural changes in

Full text of DOI:10.1016/j.ica.2005.09.030

Inorganica Chimica Acta 359 (2006) 1912–1922
www.elsevier.com/locate/ica
Synthesis and characterization of electronically varied XCX
palladacycles with functional arene groups
*
David E. Bergbreiter , Jonathon D. Frels, Jeffrey Rawson, Jun Li, Joseph H. Reibenspies
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012, United States
Received 20 August 2005; accepted 16 September 2005
Available online 2 November 2005
Dedicated to Professor Gerard van Koten in celebration of a scientific career of remarkable breadth.
Abstract
X-ray crystallographic studies show that varying the nature of the S-aryl ligands in SCS-Pd(II) pincer complexes and the electronic
nature of the aryl substituent para to the Pd(II) group in PCP-Pd(II) pincer complexes do not lead to structural changes in these palla-
dacycles that can be correlated with the changing nature of the ligands. While the original C2 symmetry for the S-aryl groups in SCS-
Pd(II) pincer complexes seen in the case of the 2,5-bis(thiophenylmethyl)phenylpalladium chloride pincer complex is also seen in other
SCS-Pd(II) pincer complexes, the relative stereochemistry of the S-aryl rings is not consistently maintained in 2,5-bis((4-dimethylamino-
thiophenyl)methyl)-phenylpalladium chloride.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Pincer complexes; Palladacycles; Structure; Catalysis; Synthesis
1. Introduction
plexes could similarly be attached to linear polymers.
Finally, our interest in palladacycles in particular was ex-
cited by a report by Milstein describing PCP-Pd(II) species
that could be used in cross-coupling chemistry in air at ele-
vated temperature [18]. While these species exact roles as
either catalysts or pre-catalysts in various reactions are
not always understood [12–15], their interesting chemistry,
their high stability, and the structural diversity inherent in
the pincer ligand framework of these palladacycles makes
them a subject of continuing interest.
Pincer-type transition metal complexes like 1 containing
2,6-disubstituted arenes rings with a r-bonded transition
metal bonded to C1 of the substituted arene are by now
well-known species [1–6]. These complexes are interesting
as versatile examples of stable r-bonded organometallic
derivatives and can be prepared using a variety of different
metals. Such complexes have useful applications as sensors
[2,7], in materials chemistry [8–10], and in catalysis as cat-
alysts or pre-catalysts [1,3–6,11–15]. Our interests in homo-
geneous catalysis and the many diverse examples, where
pincer complexes facilitate Kharasch couplings [16], C–H
activation [4,5,17], and various cross-coupling chemistry
[11,12,18,19] originally attracted our attention to these spe-
cies. This interest was heightened by the many successful
examples, where pincer complexes were attached to dendri-
mers [20,21] as this chemistry suggested that pincer com-
Metaln+
Ligand
Ligand
Substituent
1
*
As part of a broader program aimed at developing recy-
clable catalysts on soluble polymers [22,23], we have
Corresponding author. Tel.: +1 979 845 3437.
E-mail address: bergbreiter@tamu.edu (D.E. Bergbreiter).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.09.030

D.E. Bergbreiter et al. / Inorganica Chimica Acta 359 (2006) 1912–1922
1913
explored the synthesis of these complexes, preparing and
structurally characterizing a series of SCS- and PCP-type
Pd(II) complexes like 1 that contain a para-substituent to
immobilize a preformed metal complex onto a support.
An advantage of this approach is that we can fully structur-
ally characterize the complexes before immobilization
using X-ray crystallography. Here we describe some of
our work detailing the synthesis and structural details of
a selection of various SCS-Pd(II) and PCP-Pd(II) com-
plexes like 1. The ultimate goal of these studies was to even-
tually bind these complexes to soluble polymer and use
them in catalysis and we have previously described that ap-
proach in a number of different studies [24–30]. Below we
describe another aspect of these studies where we have pre-
pared and structurally characterized a series of SCS-Pd(II)
pincer complexes to study the effects of electronically vary-
ing S-aryl ligands on structure and the Pd(II) environment.
We also prepared a series of PCP-Pd(II) complexes and
structurally characterized them to probe how the electronic
nature of substituents para to the r-bonded Pd(II) species
effect the structure of PCP palladacycles.
1H NMR (DMSO-d6) 1.99 (s, 3H), 2.83 (s, 12H), 3.90 (s,
4H), 6.60 (dd, 4H), 6.78 (s, 1H), 7.17 (dd, 4H), 7.39 (s,
2H), 9.84 (s, 1H); 13C NMR (DMSO-d6) 24.0, 39.9, 40.4,
112.7, 117.8, 119.8, 124.1, 133.2, 138.6, 139.2, 149.7,
168.2; IR (KBr, cmꢀ1) 3292, 1664, 1594; HRMS (m/z)
[M + H+] Calcd. for C24H21O5NS2, 466.1987; Found,
466.1984.
2.3. 4-N-Acetyl-3,5-bis((p-dimethylaminophenyl)-
thiomethyl)phenylpalladium trifluoroacetate (10b)
In a 50-mL round-bottomed flask, dimethyl amino SCS
ligand 6b (100 mg, 0.215 mmol) was dissolved in 10 mL
acetone. Pd(TFA)2 (75 mg, 0.225 mmol) was dissolved in
10 mL acetone and added to the reaction via pipette. The
reaction stirred at room temperature for 4 h. The reaction
was filtered, the solvent removed and the resulting brown-
ish-red solid dried under vacuum (150 mg > 99%): m.p.
1
185 ꢁC (dec.); H NMR (DMSO-d6) 1.99 (s, 3H), 2.95 (s,
12H), 4.6 (s, 4H), 6.78 (dd, 4H), 7.20 (s, 2H), 7.66 (dd,
4H), 9.83 (s, 1H); 13C NMR (DMSO-d6) 24.6, 40.4, 52.1,
113.2, 114.2, 115.8, 134.4, 137.5, 150.5, 152.2, 168.9;
HRMS (m/z) (the spectrum was obtained using a dilute
acetic acid solution and compound was detected with an
acetate ligand) [M + H2O + H+] Calcd. for C28H36O4-
N3S2Pd, 648.1182; Found, 648.1085. Crystals were grown
by the slow diffusion of hexane into an acetone solution
of 10b.
2. Experimental
2.1. General procedures
Reagents and solvents were obtained from commercial
sources and were generally used without further purifica-
1
tion. H NMR spectra were recorded on Varian spectrom-
eters at 300 or 500 MHz. Chemical shifts are reported in
ppm using hexamethyldisiloxane (HMDS, 0.055 ppm) as
the internal standard. 13C NMR spectra were recorded at
75 or 125 MHz with CDCl3 (77.0 ppm) or DMSO-d6
(39.5 ppm) as the internal reference. Infrared spectra were
recorded as thin films between NaCl plates or as pressed
KBr pellets using a Mattson Galaxy 4021 FT-IR spectrom-
eter. Crystallographic data were collected on a Bruker
SMART 1000 X-ray three circle diffractometer and ren-
dered using Ortep for Windows. Melting points were deter-
mined with a Thomas-Hoover Unimelt capillary melting
point apparatus and were uncorrected.
2.4. Synthesis of N-Acetyl-3,5-
bis(dicyclopentylphosphinomethyl)aniline-borane (26)
A solution of 4.0 g (21.76 mmol) of dicyclopentylphos-
phine-borane in 40 mL of freshly distilled THF under N2
was prepared and then cooled to ꢀ78 ꢁC. Slow addition
of 20 mL of 1.6 M n-BuLi using a syringe formed a solu-
tion of the lithiated phosphine after stirring 2 h at
ꢀ78 ꢁC and then additional 2 h at room temperature.
The reaction solution was again cooled down to ꢀ78 ꢁC
and 10 mL of a THF solution of 2.32 g (10 mmol) of N-
acetyl-3,5-bis(chloromethyl)aniline was added using a syr-
inge. After 2 h of stirring at ꢀ78ꢁ, the reaction mixture
was allowed to warm to 25 ꢁC and to continue stirring
for 10 h. The solution was then concentrated to ca. 1/3 of
its original volume. A 100 mL of CH2Cl2 was added and
the solution was washed with 2 N NaOH (10 mL) and
brine (2 · 30 mL), dried over MaSO4, and the organic sol-
vent was removed at reduced pressure. The residue was
purified by silica gel chromatography (hexane/EtOAc, 1/
1) to yield 4.0 g of light yellow, air-stable crystals (76%
yield): 1H NMR (300 MHz, CDCl3) 0.23–0.60 (br, m,
6H), 1.55–2.13 (m, 36H), 2.18 (s, 3H), 2.98 (d, J =
11.4 Hz, 4H), 6.91 (s, 1H), 7.26 (s, 1H), 7.38 (s, 1H); 31P
NMR (CDCl3) 31.3; 13C NMR (CDCl3) 24.88, 26.10 (d,
J = 8.07 Hz), 26.50 (d, J = 9.13 Hz), 28.10 (d, J =
9.05 Hz), 30.88 (d, J = 29.19 Hz), 32.78 (d, J = 33.79 Hz),
119.16, 126.73, 134.67, 137.99, 168.26.
2.2. Synthesis of N-acetyl-3,5-bis((p-dimethylaminophenyl)-
thiomethyl)aniline (6b)
A 200-mL flask and attached condenser was purged with
nitrogen. Acetone (50 mL) was bubbled through with nitro-
gen for 15 min. Dimethylaminothiophenol (590 mg,
3.85 mmol), K2CO3 (604 mg, 4.38 mmol), and bis(benzyl
chloride) (5) (407 mg, 1.75 mmol) were combined in the
200 mL flask and dissolved in 100 mL acetone. The reaction
was protected from light and refluxed for 48 h. The reaction
was filtered and the solvent removed. The residue was dis-
solved in chloroform and washed with water and brine.
The solvent was evaporated and the residue purified by
chromatography on silica gel (ethyl acetate: CH2Cl2, 1:9)
to yield 574 mg (70%) of a white solid: m.p. 115–116.5 ꢁC;

1914
D.E. Bergbreiter et al. / Inorganica Chimica Acta 359 (2006) 1912–1922
2.5. Synthesis of 3,5-di(dicyclopentylphosphinomethyl)-
aniline-borane
2.7. Synthesis of 4-phthalimido-2,6-
bis(dicyclopentylphosphinomethyl)aniline-borane (30)
A solution of N-acetyl-3,5-di(dicyclopentylphosphinom-
ethyl)aniline-borane complex (2.45 g) in 10 mL of 95%
EtOH was allowed to react with NaOH (3.75 g) at reflux
for 20 h. After neutralization with concentrated HCl, the
majority of the solvent was removed under pressure. The
remaining residue was taken up in 50 mL of CH2Cl2,
washed with saturated Na2CO3 (3 · 10 mL), and dried over
MgSO4. The organic solvents were removed under pressure
to yield after chromatography (hexane/EtOAc 1/1) 1.3 g
(58% yield) of product: 1H NMR (300 MHz, CDCl3)
0.23–0.60 (br d, J = 114 Hz, 6H), 1.55–2.13 (m, 36H),
2.92 (d, J = 10.8 Hz, 4H), 3.48 (br, s, 2H), 6.46 (s, 2H),
6.50 (s, 1H); 31P NMR (CDCl3) 30.41; 13C NMR (CDCl3)
26.25 (d, J = 8.3 Hz), 26.75 (d, J = 9.18 Hz), 28.22 (d, J =
12.95 Hz), 30.80 (d, J = 29.05 Hz), 32.60 (d, J = 33.58 Hz),
115.14 (t, J = 3.02 Hz), 121.35, 135.16 (d, J = 1.51 Hz),
135.20 (d, J = 2.26 Hz), 146.70.
A
0.6-g sample 2,6-bis(dicyclopentylphosphinom-
ethyl)aniline-borane (29) (1.25 mmol) and 0.22 g
(1.5 mmol) of phthalic anhydride were added to 30 mL of
toluene and then refluxed for 24 h. The solvent was re-
moved under pressure, the resulting mixture was taken
up with 50 mL of dichloromethane, washed with 1 N HCl
(20 mL · 1), brine (20 mL · 2), and dried over MgSO4.
The solvent was removed under reduced pressure. Silica
gel chromatography (hexane/EtOAc 2/1) yielded 0.73 g of
product (96%): 31P NMR (CDCl3) 31.79; 1H NMR
(300 MHz, CDCl3) 0.40 (d, br, J = 103 Hz, 6H), 1.68 (m,
32H), 2.10 (m, 4H), 3.07 (d, J = 10.8 Hz, 4H), 7.20 (s,
3H), 7.79 (m, 2H), 7.94 (m, 2H).
2.8. Synthesis of 4-amino-2,6-
bis(dicyclopentylphosphinomethyl)phenylpalladium
trifluoroacetate (33)
2.6. Synthesis of 4-N-acetamido-2,6-
di(dicyclopentylphosphinomethyl)phenylpalladium
trifluoroacetate
A solution of 80 mg of 4-phthalimido-2,6-bis(dicycl-
opentyl-phosphinomethyl)phenylpalladium trifluoroace-
tate and 2 mL (34.90 mmol) of hydrazine hydrate in
20 mL of ethanol was stirred at room temperature for
48 h. Afterwards the solvent was removed under reduced
pressure. Silica gel chromatography (EtOAc/hexane 1/1)
yielded 42 mg of product (60%): 31P NMR (CDCl3)
A solution of N-acetyl-3,5-bis(dicyclopentylphosphi-
nomethyl)aniline-borane (0.3 g, 0.57 mmol) in 10 mL of
freshly distilled THF and 10 mL of diethylamine was deox-
ygenated (freeze–pump–thaw, 5 cycles) and then heated to
60 ꢁC for 12 h. The solvents were carefully removed under
reduced pressure at room temperature. Then the free bis-
phosphine was dissolved in 10 mL of degassed THF and a
solution of Pd(O2CCF3)2 (189.5 mg, 0.57 mmol) in 10 mL
of degassed THF was added slowly. The reaction mixture
was stirred at room temperature for 12 h and then at
65 ꢁC for 48 h. The solvents were removed under reduced
pressure. Column chromatography using silica gel (hexane/
EtOAc 1/1) afforded 34 mg of product (10% yield): 1H
NMR (300 MHz, CDCl3): 1.52–2.02 (m, 32H), 2.11 (s,
3H), 2.42 (m, 4H), 3.13 (t, J = 4.5 Hz, 4H), 7.20 (s, 2H),
7.39 (s, 1H); 31P NMR (CDCl3, d): 52.56; 13C NMR
(CDCl3, d): 24.59, 26.36 (t, J = 4.53 Hz), 26.53(t, J =
3.55 Hz), 28.96, 29.31 (t, J = 3.01 Hz), 35.48 (t, J =
12.06 Hz), 114.40 (t, J = 11.09 Hz), 135.40, 150.60 (t,
J = 11.62 Hz), 153.89, 168.24.
1
52.12; H NMR (300 MHz, CDCl3) 1.49–2.06 (m, 32H),
2.40 (m, 4H), 3.07 (t, J = 4.5 Hz, 4 H), 3.38 (s, br, 2H),
6.41 (s, 2H); 13C NMR (CDCl3) 26.29 (t, J = 4.5 Hz),
26.53 (t, J = 3.54 Hz), 28.97, 29.32 (t, J = 3.02 Hz), 29.67,
35.26 (t, J = 12.60 Hz), 35.38 (t, J = 12.07 Hz), 110.58 (t,
J = 11.54 Hz), 143.26, 147.48, 150.97 (t, J = 11.09 Hz).
2.9. X-ray characterization of palladacycles
Crystals of the complexes 28, 32, and 34 were obtained
from a mixture of dichloromethane and hexane. In each
of these cases, the trifluoroacetate anion was replaced by
a chloride anion in the crystalline solids that were isolated
and in turn analyzed by X-ray crystallography. The details
of the crystal data for structures 10a, 10b, 10c, 25, 28, 32,
and 34 are provided in the supplementary material.
The same procedure using 4-phthalimido-2,6-bis(dicyclo-
pentylphosphinomethyl)aniline-borane 33 mg (8.5% yield)
4-phthalimido-2,6-bis(dicyclopentylphosphinomethyl)phe-
nylpalladium trifluoroacetate (31): 1H NMR (300 MHz,
CDCl3) 1.52–2.02 (m, 32H), 2.44 (m, 4H), 3.23 (t, J =
4.2 Hz, 4H), 7.07 (s, 2H), 7.80 (dd, J = 3.0 and 5.4 Hz,
2H), 7.95 (dd, J = 3.3 and 5.4 Hz, 2H); 31P NMR (CDCl3,
d) 52.88; 13C NMR (CDCl3) 26.28 (t, J = 4.5 Hz), 26.54 (t,
J = 3.5 Hz), 29.06, 29.30 (t, J = 2.5 Hz), 35.32 (t, J =
12 Hz), 35.41 (t, J = 12 Hz), 120.43 (t, J = 11 Hz),
123.60, 128.47, 131.78, 134.29, 150.88 (t, J = 11.5 Hz),
159.41, 167.52.
3. Results and discussion
The general route to the thioarene-containing ligands
used to prepare SCS-Pd(II) complexes is shown in Scheme
1. This route typically used commercially available thioa-
renes. In the case of 2,4-dimethoxythiophenol and 4-dime-
thylaminothiophenol, we had to prepare the thioarene
using established chemistry (Eqs. (1) and (2)). In all the
syntheses of bisthioarene ligands, the same general route
was used [13]. The starting 5-amino isophthalic acid was
esterified and the resulting ester reduced using LiAlH4,
forming the amino diol. The NH2 group was protected

D.E. Bergbreiter et al. / Inorganica Chimica Acta 359 (2006) 1912–1922
1915
CO2H
CO2H
CO2Et
CO2Et
OH
OH
LAH, THF
reflux, 8 h
Ac2O, TEA
H2SO4, EtOH
reflux, 16 h
H2N
H2N
H2N
THF, r.t., 3 h
2
3
S Ar
OH
OH
Cl
HSAr, K2CO3
DMF, 70 0C
SOCl2, pyridine
O
O
H3C
O
H
H
H
N
N
N
CH2Cl2, r.t., 12 h
H3C
H3C
S Ar
Cl
6
4
5
O
c:
d:
h:
b:
a:
e:
OMe
NMe2
OMe
N
H
MeO
g:
-SAr
O
f:
O
N
NO2
OH
Scheme 1. Synthesis of N-acetyl-3,5-bis(thioarylmethyl)aniline ligands.
by treatment with acetic anhydride to provide an acetam-
ido diol. The alcohols were changed to chlorides by stirring
the diol with
The thioarenes prepared in Scheme 1 were subsequently
palladated in an electrophilic aromatic substitution reac-
tion using a Pd(II) electrophile (e.g., Pd(O2CCF3)2 or
Pd(NCPh)2Cl2) to form an SCS palladacycle. Examples
of palladacycles prepared by this palladation chemistry
are shown in Scheme 2. The procedures for these pallada-
tions have been previously reported [13,19], and the cata-
lytic chemistry seen when these and related SCS-Pd(II)
palladacycles are used in cross-coupling reactions has been
discussed extensively. This chemistry is not discussed here.
However, in three cases we also were able to isolate X-ray
quality crystals of palladated complexes. These studies al-
lowed us to study the effect of changing the electronic nat-
ure of the S-aryl ligands on the pincer complex structure. A
discussion of some these structures are presented below.
O
NH2
SH
EtO
S
OCH3
OCH3
1. KOH, aq. EtOH
1. NaNO2, HCl
OCH3
2. benzene, Zn, HCl
2. EtOCS2K, H2O
OCH3
NMe2
OCH3
OCH
ð1Þ
Zn, HCl
Me2N
C N
NH4SCN
SH
Me2N
S
50 o
C
Br2, HOAc
10 oC, 1 h
ð2Þ
S
OMe
OMe
OMe
OMe
S
NMe2
NMe2
O
O
N
H
Pd Cl
N
H
Pd TFA
S
S
10a
10b
MeO
O
R
N
H
S
S
O
O
HN
O
N
Pd TFA
N
Pd TFA
S
H
H
H
N
S
10d
10c
O
S
S
MeO
Ar
Ar
O
6a - 6f
S
S
O
O
OH
OH
O
N
Pd OAc
S
N
Pd Cl
S
H
H
10e
10f
Scheme 2. SCS-Pd(II) palladacycles prepared from N-acetyl-3,5-bis(thioarene) ligands. SOCl2. Subsequent treatment of this dichloride with thiophenol
and K2CO3 in DMF yielded the acetamido SCS ligands 6a–6f.

1916
D.E. Bergbreiter et al. / Inorganica Chimica Acta 359 (2006) 1912–1922
Fig. 3. Crystal structure of SCS palladacycle 10c.
in a pseudo C-2 fashion as was seen in an earlier structure
of an SCS-Pd(II) palladacycle prepared by our group [19].
While our initial impression on isolation and characteriza-
tion of that earlier structure was that these S-aryl substi-
tuted palladacycles would have structures where one ÔupÕ
aromatic ring would force the other ring ‘‘down’’, this
assumption appears not to be general based on a compar-
ison of the structures 10a, 10b, and 10c. Based on the crys-
tal data for these three complexes and the earlier structure,
the thioaryl rings may be spaced too far apart to interact
significantly with one another. This stereochemical ambi-
guity could be significant as it might explain the relative
ineffectiveness of these SCS-Pd(II) complexes in asymmet-
ric catalysis [31]. SCS-Pd(II) palladacycles with bis(isopro-
pylsulfoxide) ligands (cf. 11 in Fig. 4) too exist in both the
rac and meso forms [32].
Fig. 1. Crystal structure of SCS palladacycle 10a.
The structures of 10a–c are shown in Figs. 1–3. The
structures of complexes 10a and 10c are similar with pseu-
do C-2 symmetry for the S-arene substituents relative to the
X–Pd–Cipso axis. The crystal structure of palladacycle 10b
differs from these other structures in the alignment of the
S-aryl rings. The thioaryl rings of 10b are positioned on
the same face of the compound, forming a cup-shaped
arrangement. One thioaryl group sits at an angle of 102ꢁ
and the other at 135ꢁ relative to the plane defined by the
central, cyclopalladated ring. The thioaryl rings of 10a
and 10c are positioned on opposite faces of the central ring
Other features of the SCS complexes 10a, 10b, and 10c
were more similar. Data comparing the four structures
are assembled in Tables 1 and 2. For easy comparison,
the data are normalized to the SCS substructure shown
in Fig. 5. The length of the C–Pd bond ranges from
˚
˚
1.971 A for 10b to 1.98 A for 10a. The distances between
each sulfur atom and the palladium center range from
˚
2.297 to 2.336 A. The S–Pd–S atom trio is nearly collinear
in all the complexes. The S–Pd–S angle in 10a is 171ꢁ, in
10b the angle is 168ꢁ, and in 10c the angle is 167ꢁ. The
S–Pd–S axis is slightly out of alignment with the plane
formed by the central aromatic ring. This twisting angle
is approximately 10ꢁ for S-phenyl 10a, 17ꢁ for bis(dime-
thoxy) 10c, and 14ꢁ for bis(dimethylamino) 10b.
The structural data for SCS-Pd(II) pincer complexes
10a–c are similar to the SCS-Pd(II) chloride pincer com-
plex we described previously [19]. The C–Pd bond lengths
˚
(ca. 1.95–2.01 A) in a collection of other terdentate chlor-
Fig. 2. Crystal structure of SCS palladacycle 10b.
opalladium(II) SCS pincer complexes (Fig. 5) that have

D.E. Bergbreiter et al. / Inorganica Chimica Acta 359 (2006) 1912–1922
1917
Cl
R
R
CH3
Cl
H3C
Cl
H3C
CH3
CH3
S
Pd
S
O
S
Pd
S
O
CH3
CH3
Pd
S
S
H3C
H3C CH3
12: R = -C(CH3)3
13: R = -CH2CH3
15
14: R = -CH2C6H5
Cl
Cl
Ph
Ph
11
Cl
Pd
Cl
Pd
S
S
S
Pd
S
P
Ph
Ph
P
Ph
S
Ph
S
S
Ph
Ph
Ph
Ph
S
CH3
17
Pd Cl
Cl
Pd
S
H3C
CH3
Cl
S
S
S
Ph
Ph
Pd
S
S
CH3
Ph
Ph
H3C
Pd
Pd
Cl
CH3
H3C
S
S
O
Cl
18
Ph
Ph
Cl
(H3C)3C
C(CH3)3
16
Cl
Pd
S
S
S
Pd
S
NR2
R2N
S
Pd
Cl
S
S
Pd
Cl
S
CH3
C(CH3)3
(H3C)3C
19
21: R = - CH3
22: R = - CH2CH2CH2CH2-
O
O
HO2CH2CH2CH2COCHN
O
20
H3CO2C
CO2CH3
S
Pd
Cl
S
S
Pd
S
Cl
24
23
Fig. 4. Structurally characterized SCS-Pd(II) palladacycles, 11 [32], 12 [33], 13 [34], 14 [34], 15 [35], 16 [36], 17 [37], 18 [38], 19 [39], 20 [40], 21 [41], 22 [42],
23 [19], and 24 [34].
been structurally characterized by other groups are consis-
tent with our results that show that a variation of the elec-
tronic character of ligands in SCS palladacycles have only
small structural effects on the C–Pd bond [19,32–42].
Although SCS-Pd(II) complexes serve as precatalysts in
formation of very efficient Pd catalysts for cross-coupling
reactions and can be modified both at the position para to
the Pd and at positions in the arene ring of the S-aryl li-
gand, the reactions carried out with these complexes as a
source of the catalyst are not effective for cross coupling
of less active arenas-like bromobenzene. Since Milstein re-
ported that PCP-Pd(II) species are effective with these less

1918
D.E. Bergbreiter et al. / Inorganica Chimica Acta 359 (2006) 1912–1922
Table 1
Selected bond lengths of SCS palladacycles
Bond
10f
10b
10c
C1–Pd
C5–S1
C7–S1
C4–S2
C6–S2
Pd–S1
Pd–S2
C2–C4
C3–C5
1.971(7)
1.808(8)
1.774(7)
1.838(7)
1.784(7)
2.291(2)
2.297(2)
1.497(9)
1.508(9)
1.98(1)
1.82(1)
1.74(1)
1.85(1)
1.82(1)
2.336(4)
2.336(4)
1.49(1)
1.55(1)
1.977(3)
1.831(3)
1.769(3)
1.847(3)
1.777(3)
2.308(1)
2.309(1)
1.499(4)
1.505(4)
Table 2
Selected bond angles of SCS palladacycles
Atoms
10f
10b
10c
C3–C5–S1
C2–C4–S2
S1–Pd–S2
C4–S2–Pd
C5–S1–Pd
111.1(5)
110.8(5)
170.5(1)
102.0(3)
100.2(2)
115.1(8)
107.2(9)
167.6(1)
98.5(4)
108.0(2)
107.4(2)
166.8(1)
99.6(1)
99.5(4)
97.6(1)
Fig. 6. X-ray crystal structure of 2,6-bis(dicyclopentylphosphinomethyl)-
phenylpalladium trifluoroacetate palladacycle (25).
chloride salts were characterized by crystallography (see
Fig. 6).
6
S
4
2
1
2
While the synthetic route used to prepare the PCP-
Pd(II) complexes paralleled the route used to prepare
SCS-Pd(II) complexes, the synthesis of the SCS-Pd(II) spe-
cies was simplified by the fact that the intermediate thioe-
thers unlike the phosphine intermediates were stable to
ordinary handling in air. In the case of the PCP complex
syntheses, oxidation of intermediate phosphine species
was problematic because both phosphines must be present
to form the PCP complex. In the syntheses of 28, 32, and 34
below, this problem was minimized by using borane com-
plexes of the phosphine.
1
X
Pd
3
S
7
5
Fig. 5. Substructure of SCS palladacycles.
active substrates [18], we explored synthetic routes to p-
amino and p-amido PCP-Pd(II) complexes that we could
also couple to polymers. The syntheses of these complexes
mirrored those used for the SCS-Pd(II) complexes de-
scribed above as these syntheses too proceed through a
3,5-bis(chloromethyl)aniline derivative. Variations in the
synthesis occurred with the introduction of the phosphine
groups and the palladium. First we prepared an unsubsti-
tuted 2,6-bis(dicyclopentylphosphino-methyl)phenylpalla-
dium trifluoroacetate (25) pincer complex. Next, we
prepared a set of three PCP-Pd(II) palladacycles that
had substituents with varying electronic character para
to the Pd(II) substituent. These palladacycles included
4-acetamido-2,6-bis(dicyclopentylphosphinomethyl)phenyl-
palladium chloride (28), 4-phthalimido-2,6-bis(dicyclopen-
tylphosphinomethyl)phenyl-palladium chloride (32), and
4-amino-2,6-bis(dicyclopentylphosphinomethyl)phenyl-
palladium chloride (34). While these last three palladacy-
cles were prepared as trifluoroacetates, recrystallization
yielded the palladacycle crystals as chloride salts and the
Originally, we thought the changing electronic nature of
the para substituent might affect the structure of these com-
plexes and, if the complexes were catalysts instead of pre-
catalysts, the catalytic activity of the palladacycle. Thus,
we characterized crystallographically a set of PCP com-
plexes containing different types of substituents para to
the Pd(II) substituent. First we prepared the palladacycle
25 with only a hydrogen para to the Pd(II) group of the
PCP palladacycle. Then we introduced a electronically var-
iable para nitrogen substituent. The first of these para-N-
substituted palladacycles was prepared from 5 using the
chemistry shown in Eq. (3). The p-acetamido group in 27
was then hydrolyzed to form an –NH2 group (Eq. 4). Pal-
ladation of this compound was not successful. Therefore,
we converted this –NH2 group into a phthalimido group
and palladated this protected phthalimido diphosphine to
form 30 (Eq. 5). The palladacycle containing an –NH2
group para to the Pd(II) center was then obtained from
31 by hydrazinolysis (Eq. 6).

D.E. Bergbreiter et al. / Inorganica Chimica Acta 359 (2006) 1912–1922
1919
Cl
1.Et2NH/THF(1/1,v/v)
P
55 OC-65 OC 20h
2
AcHN
+
BH3
BuLi
2.Pd(OCOCF3)2
AcHN
60 OC, 48h
5
hexanes
-78 o
THF
H3B
BH3
Cl
C
P
H
P
26
2
ð3Þ
P
Pd
P
P
2
2
2
recrystallization
AcHN
Cl
AcHN
Pd
P
OCOCF3
hexane
CH2Cl2
28
27
2
P
P
2
2
BH3
BH3
BH3
NaOH
AcHN
H2N
ð4Þ
ð5Þ
BH3
P
95% EtOH
reflux
P
26
2
29
2
1. Et2NH/THF
(1/1,v/v)
65 oC, 24 h
P
O
O
2
BH3
BH3
toluene
+ 29
N
O
2. Pd(OCOCF3)2
THF
60 oC, 48 h
reflux
48 h
P
O
O
30
2
O
P
O
P
2
2
2
recrystallization
hexane-CH2Cl2
N
Pd OCOCF3
P
Cl
N
Pd
P
O
O
32
31
2
O
P
P
2
2
2
NH2NH2
EtOH
N
Pd OCOCF3
P
H2N
Pd OCOCF3
P
48 h
25 oC
O
33
31
2
ð6Þ
P
2
2
recrystallization
hexane-CH2Cl2
Cl
H2N
Pd
P
34
The three complexes 27, 31, and 33 had to be recrys-
tallized from a hexane–CH2Cl2 mixture to form crystals
suitable for X-ray analysis as shown in the equations
above. This led to the structures 28, 32, and 34 shown
in Figs. 7–9. While we had intended to compare these
structures to each other and to the unsubstituted PCP
palladacycle 25, the conversion of the palladiumÕs triflu-
oroacetate ligand into a chloride ligand presumably
either by chloride abstraction from the solvent or by
reaction of traces of HCl in the solvent with the initial
trifluoroacetate palladacycle salts focused our attention
on the structural effects of electronic variation of the
para nitrogen substituent in the three chloropalladium
palladacycles.
While we were successful in generating a set of structur-
ally characterized PCP complexes with electronically varied
para substituents, the results showed no consistent substi-
tuent effects. The C–Pd bond length, the Pd–P bond
lengths, and the Pd–Cl bond length in these species did
not vary in any consistent way with the nature of the para
substituent (Table 3). For easy comparison, the data in
Tables 3 and 4 are normalized to the PCP substructure

1920
D.E. Bergbreiter et al. / Inorganica Chimica Acta 359 (2006) 1912–1922
Fig. 7. X-ray crystal structure of 4-acetamido-2,6-bis(dicyclopentylphos-
phinomethyl)phenyl-palladium chloride palladacycle (28).
Fig. 9. X-ray crystal structure of 4-amino-2,6-bis(dicyclopentylphosphi-
nomethyl)phenyl-palladium chloride palladacycle (34).
Table 3
Selected bond lengths of PCP palladacycles
Bond
25
28
32
34
C1–Pd
C5–P1
C7–P1
C6–P1
C4–P2
C8–P2
C9–P2
Pd–P1
Pd–P2
C2–C4
C3–C5
Pd–Cl
2.003(4)
1.838(3)
1.831(3)
1.829(3)
2.010(2)
1.826(3)
1.826(3)
1.821(2)
1.841(3)
1.827(3)
1.821(3)
2.272(1)
2.281(1)
1.508(3)
1.513(3)
2.396(1)
2.055(7)
1.883(7)
1.848(7)
1.839(8)
1.849(7)
1.890(7)
1.852(7)
2.318(2)
2.316(2)
1.563(9)
1.538(9)
2.430(2)
2.022(2)
1.839(2)
1.827(2)
1.830(2)
1.826(2)
1.822(2)
1.831(2)
2.279(1)
2.285(1)
1.509(2)
1.513(2)
2.409(1)
2.304(1)
1.514(5)
Fig. 8. X-ray crystal structure of 4-phthalimido-2,6-bis(dicyclopentyl-
phosphinomethyl)phenyl-palladium chloride palladacycle (32).
Table 4
Selected bond angles of PCP palladacycles
shown in Fig. 10. The most electron-donating substituent –
the para-NH2 group – had a C–Pd bond length that was
intermediate in length. Similarly, there were no clear corre-
lations between the Pd–P or the Pd–Cl bond lengths and
the electron-donating ability of the nitrogen group para
to the Pd(II) group. van Koten in earlier work has pointed
out that various para substituents on similar palladacycles
generally have minimal effects on catalysis, an effect that
has been seen both in experimental studies and in theoret-
ical calculations [43].
Atoms
25
28
32
34
C3–C5–P1
C2–C4–P2
P1–Pd–P2
C4–P2–Pd
C5–P1–Pd
114.1(2)
106.8(2)
109.1(2)
162.9(1)
104.1(1)
103.0(1)
107.8(4)
109.6(5)
165.4(1)
104.1(2)
104.2(2)
109.4(1)
108.2(1)
163.92(2)
102.94(6)
104.79(6)
163.8(1)
100.4(1)
A survey of the Cambridge Crystallographic Database
(Feb 2005) for C–Pd(P)(P)–Cl structures matched 149
compounds and resulted in 182 observed C–Pd bond dis-
tances. The C–Pd mean bond length of 2.03(5) A for these
observations is similar to the mean deviation of the C–Pd
Bond angles were similarly not affected by the N-substi-
tuent in any predictable way (Table 4). The bond length
and angle differences were well with experimental error.
˚

D.E. Bergbreiter et al. / Inorganica Chimica Acta 359 (2006) 1912–1922
1921
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Acknowledgements
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Experimental details for synthesis of thioarenes and some
of the intermediates used in the syntheses of the palladacy-
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the structures reported have been deposited with the Cam-
bridge Crystallographic Data Centre (281740-281746).
Copies of this information can be obtained free of charge
from the Director, CCDC, 12 Union Road, Cambridge,
CB2 1EZ, UK (fax: +44 1223 336 033; email: depos-
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