New atropisomeric N-N ligands for CO/vinyl arene copolymerization reaction

Article abstract of DOI:10.1039/b716656g

New atropisomeric bidentate bipyridine-based ligands (3,3′- (ethylenedioxy)-2,2′-bipyridine 3; 3,3′-(propylenedioxy)-2,2′- bipyridine 4; 3,3′-(butylenedioxy)-2,2′-bipyridine 5) containing a bridge between the 3,3′ positions of the aromatic rings have been prepared. Together with the previously reported analogous ligands ((R)-3,3′-(1-methylethylenedioxy)-2,2′-bipyridine) 1 and ((S,S)-3,3′-(1,2-dimethylethylenedioxy)-2,2′-bipyridine) 2, they were used to synthesize the corresponding bis-chelated dicationic complexes [Pd(N-N)₂][PF₆]₂. Crystal structures and comparison of the data obtained by X-ray analysis on four of these complexes is reported. These palladium compounds were used as precatalysts in the CO/styrene and CO/4-Me-styrene copolymerization reactions, where they showed that small variations in the ligand backbone remarkably affects the productivity of the catalytic system. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b716656g

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PAPER
www.rsc.org/dalton | Dalton Transactions
New atropisomeric N–N ligands for CO/vinyl arene copolymerization
reaction†
a
a
b
a
J e´ r oˆ me Durand,*‡ Ennio Zangrando, Carla Carfagna and Barbara Milani
Received 15th November 2007, Accepted 31st January 2008
First published as an Advance Article on the web 6th March 2008
DOI: 10.1039/b716656g
ꢀ
ꢀ
ꢀ
New atropisomeric bidentate bipyridine-based ligands (3,3 -(ethylenedioxy)-2,2 -bipyridine 3; 3,3 -(pro-
ꢀ
ꢀ
ꢀ
pylenedioxy)-2,2 -bipyridine 4; 3,3 -(butylenedioxy)-2,2 -bipyridine 5) containing a bridge between the
,3 positions of the aromatic rings have been prepared. Together with the previously reported analog-
ous ligands ((R)-3,3 -(1-methylethylenedioxy)-2,2 -bipyridine) 1 and ((S,S)-3,3 -(1,2-dimethylethylen-
edioxy)-2,2 -bipyridine) 2, they were used to synthesize the corresponding bis-chelated dicationic
ꢀ
3
ꢀ
ꢀ
ꢀ
ꢀ
complexes [Pd(N–N)
2
][PF ] . Crystal structures and comparison of the data obtained by X-ray analysis
6
2
on four of these complexes is reported. These palladium compounds were used as precatalysts in the
CO/styrene and CO/4-Me-styrene copolymerization reactions, where they showed that small
variations in the ligand backbone remarkably affects the productivity of the catalytic system.
ꢀ
ꢀ
Introduction
the 6 and/or 6 position of a 2,2 -bipyridine ligand leads to a
complete loss of activity in the CO/styrene copolymerization
reaction due to the steric congestion in proximity of the N-
The copolymerization of carbon monoxide with aromatic alkenes
such as styrene or its substituted derivatives, yielding to perfectly
alternated polyketones, have attracted a growing attention over the
13
donor atoms around the palladium center. Another strategy was
based on the introduction of axial chirality on the bipyridine
ligand, the rotation of the C(2)–C(2 ) axis being a potential
source of atropisomerism. Atropisomeric ligands have been widely
used in catalysis, among them the most famous being the
,1 -binaphthalene based BINOL, BINAP and BINAPHOS.
Recently, atropisomeric P–N ligands have been also used for
the palladium-catalyzed copolymerization of styrene with carbon
monoxide, as well as atropisomeric P–P ligands in the nickel-
catalyzed CO/ethene copolymerization.
Incorporation of the bipyridine framework into an atropiso-
meric system can be achieved by hindering the rotation around
the C(2)–C(2 ) chiral axis by bridging the 3,3 positions of the
heterocycles thus increasing the barrier of interconversion. Some
1
last past years. Catalytic systems based on palladium complexes
ꢀ
bearing bidentate nitrogen-donor ligands have shown to be the
best choice for this reaction. The fine tuning of the catalyst,
mainly achieved by ligand variation, allows the control of the
microstructure of the synthesized macromolecule leading to poly-
mers of different tacticity. For example, C
2
14
ꢀ
15
16
17
1
2
chiral, enantiomerically
3–5
pure ligands can induce the formation of isotactic polymers
18
even though in low yield. On the other hand, ligands such
19
ꢀ
as 2,2 -bipyridine (bipy) and 1,10-phenanthroline (phen) (and
their substituted derivatives) are much more efficient in terms of
productivity and molecular weight of the synthesized copolymers
and lead to the formation of macromolecules with a prevailingly
ꢀ
ꢀ
6
–11
syndiotactic microstructure.
In the latter case, the enantioface
20,21
examples of this approach have been previously reported
but to
selection is under chain-end control, whereas in the former one
the chirality around the metal is at the origin of the isotacticity of
the polymer.
the best of our knowledge, up to now, just two ligands were isolated
in an enantiomerically pure form that does not undergo racemiza-
tion at room temperature. Moreover, it has been established that
22
Our approach was to combine the high catalytic activity of
complexes containing a ligand with a bipyridine backbone with
the introduction of chirality on the ligand itself. In the literature,
many examples of bipyridine having a chiral substituent have
been reported as well as their use in asymmetric catalysis.
However, it is known that the introduction of substituents on
coordination to a transition metal of these ligands dramatically
lower the barrier of interconversion due to reduction of the
21,23
dihedral angle of the bipyridine backbone.
3
Nevertheless some
ꢀ
ꢀ
,3 -bridged 2,2 -bipyridine were used in asymmetric catalysis
where they displayed an asymmetric induction. Bridged chiral
bipyridine N,N -dioxides have also been successfully used in
asymmetric catalysis.
We previously reported the use of two new atropisomeric lig-
ands (R)-3,3 -(1-methylethylenedioxy)-2,2 -bipyridine 1 and (S,S)-
,3 -(1,2-dimethylethylenedioxy)-2,2 -bipyridine 2 (Fig. 1) in the
CO/styrene copolymerization catalyzed by the corresponding
[Pd(O CCF (N–N)] complexes. Ligand 2 was also successfully
12
24
ꢀ
25
a
Dipartimento di Scienze Chimiche, Universit a` di Trieste, Via Licio Giorgieri
, 34127, Trieste, Italy. E-mail: durand@chimie.ups-tlse.fr; Tel: +33 561 55
5 91
1
6
ꢀ
ꢀ
ꢀ
ꢀ
3
b
Istituto di Scienze Chimiche, Universit a` di Urbino, P.zza Rinascimento 6,
26
6
1029, Urbino, Italy
†
Electronic supplementary information (ESI) available: Additional spec-
2
)
3 2
troscopic data. CCDC reference numbers 667850–667855. For crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b716656g
applied to the enantioselective oxidation of stilbene catalyzed
27
by ruthenium complexes. These molecules were also used as
‡
Current address: Laboratoire H e´ t e´ rochimie Fondamentale et Appliqu e´ e,
28
the base skeleton to build up tetradentate N-donor ligands.
Universit e´ Paul Sabatier, UMR CNRS 5069, B aˆ timent 2R1, 118, route de
Narbonne, 31062 Toulouse Cedex 9, France.
We present here a systematic investigation of this family of
This journal is © The Royal Society of Chemistry 2008
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(
1)
Attempts to prepare the analogue of 4 having two methyl
substituents on the central carbon of the bridge failed, since
conversion of the ditosylate derivative of the neopentyl diol
into the corresponding bis(bromopyridine) derivative remained
unsuccessful due to a total absence of reaction even after
prolonged reaction times. A similar behavior was observed for
Fig. 1
ligands, containing O(CH
2
)
n
O bridges of 4–6 atoms, including the
synthesis and characterization of the corresponding bis-chelated
palladium complexes [Pd(N–N) ][PF and their application in
2
]
6 2
the CO/styrene and CO/4-Me-styrene copolymerization.
ꢀ
the ditosylated product derived from 1,1 -binaphthol that does
not undergo the nucleophilic substitution when reacted with 2-
bromo-3-hydroxypyridine.
The newly synthesized ligands 3–5 were characterized in solu-
tion by UV-Visible and multinuclear NMR spectroscopies and in
solid state by X-ray diffraction.
Results and discussion
Synthesis and characterization of ligands 3–5
ꢀ
ꢀ
The new ligands 3–5 (namely 3,3 -(ethylenedioxy)-2,2 -bipyridine
13
C NMR spectra of 3–5 display five resonances in the aromatic
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
6,6
3
2
; 3,3 -(propylenedioxy)-2,2 -bipyridine 4; 3,3 -(butylenedioxy)-
5,5
region. The chemical shifts of C and C are in perfect agree-
ment with those of 3,3 -disubstituted bipyridines. The signals
corresponding to C are downfield shifted of ca. 20 ppm with
ꢀ
,2 -bipyridine 5, Scheme 1) were prepared from the corresponding
ꢀ
29
ꢀ
commercially available diols according to a three-step procedure
3,3
similar to that previously reported by us for the synthesis of 1 and
respect to dialkyl-substituted bipyridine due to the influence of the
26
ꢀ
ꢀ
4,4
2.
2,2
alkoxy substituents. C and C are shielded of ca. 10 ppm. The
1
aromatic region of the H NMR spectra of all the ligands presents
three different well-resolved signals (see ESI†). In all cases, the
ꢀ
6
,6
most downfield signal is assigned to H . The aliphatic region of
the spectrum of 3 and 5 shows, at room temperature, broad signals,
for the protons of the methylenic bridge. Variable temperature
◦
NMR experiments allow the observation of coalescence at 11 C
◦
for 3 and at 25 C for 5 (see ESI†). In the case of 4, the aliphatic
region presents, already at room temperature, the well-resolved
signals typical for an A
4
2
X system. Variable temperature NMR
experiments did not allow freezing the interconversion between
◦
the atropisomeric forms. Even at −90 C, though the resonance
of the OCH becomes slightly broader, the system is still far from
the coalescence.
2
Scheme 1 Synthetic route to atropisomeric ligands 3–5: a) TsCl, pyridine,
room temperature; b) 2-bromo-3-hydroxypyridine, NaOH, H O/toluene,
2
From these observations, it could be concluded that, at room
temperature, in the case of 3 and 5, interconversion between the
two atropisomers occurs at a rate comparable to the NMR time
scale, while for 4 the interconversion is fast on the NMR time
scale, thus evidencing an important influence of the length of the
bridge on the dynamic behavior of such atropisomeric molecules.
As we reported before, for 1 the resonances of the bridge are
rather broad whereas in the case of ligand 2 all the proton signals
◦
◦
[
NBu
4
]Br, 95 C; c) NiCl
2
·6H
2 3
O, Zn, PPh , DMF, 50 C.
The first step of the synthesis is the conversion of the diols
into their corresponding ditosylate derivatives. It is then followed
by a nucleophilic substitution to obtain the bis(2-bromopyridine)
derivatives 3a–5a, which are converted to the desired ligands by
intramolecular C–C coupling catalyzed by [Ni(PPh ) ] generated
3
4
26
in situ (Scheme 1).
are sharp at room temperature, thus showing also an important
influence of the presence of substituents on the bridge on the
conformational mobility of these ligands.
In the course of the synthesis of 3–5, formation of by-products
was observed. Though present in minor quantity in the case of 4
and 5, they were more abundant in the case of 3, thus explaining
the lower yield observed. The desired ligands were separated from
these by-products by column chromatography. Detailed analysis
ꢁ
The Gibbs free energy DG values for the interconversion
of 3 and 5 could be calculated using the data obtained from
−
1
the variable temperature NMR experiments: −13.3 kcal mol
1
−1
30
of their H NMR spectra allowed identifying them as products
for 3 and −14.2 kcal mol for 5. Note that separation
resulting from the mono or complete debromination of the starting
of enantiomers at room temperature becomes possible for
−
1
material 3a–5a. This was corroborated by comparison with
values exceeding 20 kcal mol . It was previously reported
that the length of the bridge in annulated bipyridine could
have a dramatic influence on the barrier to conformational
ꢀ
the NMR spectrum of 1,2-bis(3-pyridyloxy)ethane 3 prepared
independently by reaction of ethane-1,2-diol di-p-toluenesulfonate
with 3-hydroxypyridine (eqn (1)).
31
inversion as observed both experimentally and confirmed by
2
172 | Dalton Trans., 2008, 2171–2182
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3
2
ꢀ
theoretical studies. Conformational analysis performed on 3,3 -
Synthesis and characterization of complexes [Pd(N–N)
1c–5c
2
][PF
]
6 2
ꢀ
hydrocarbyl-bridged 2,2 -bipyridine derivatives evidenced that
ethano- and propano-atropisomers have a low interconversion
−
1
We previously reported ligands 1 and 2 and their use to prepare
the corresponding [Pd(O CCF (N–N)] complexes, 1b–2b, which
were tested as precatalysts for the CO/styrene copolymerization.
barrier (<10 kcal mol ) and that a bridge of at least four carbon
−
1
32
2
)
3 2
atoms is required to reach a value of 25 kcal mol .
26
In the case of bridged derivatives of biphenyl, UV-Visible
spectroscopy has been extensively used to evaluate the relationship
between the deviation from planarity of the two bound aromatic
rings and the degree of conjugative interaction between them.
Absorption maxima were found to shift toward shorter wave-
The development of research in the field evidenced that, from
a general point of view, mono- and dicationic complexes gen-
erate more efficient catalysts than the corresponding neutral
2
derivatives. Very recently, we have found that the monocationic
−
33
derivatives [Pd(Me)(MeCN)(N–N)][X] (X = PF
6
or triflate) with
lengths as the two rings become less coplanar. The same effect
3
1
N–N ligands 1 and 2 are active catalysts for the CO/styrene
was observed in the case of polymethylene-bridged bipyridine.
ꢀ
copolymerization, giving, under optimized reaction conditions,
A bathochromic shift was observed when the bridging of the 2,2 -
bipyridine resulted in an increase of the rigidity and the planarity
as observed for example in the case of 4,5-diazafluorene. However,
as the length of the bridge increased, the absorption maxima
steadily shifted to shorter wavelengths (i.e. towards higher energy
transition) due to a decrease in the delocalisation resulting from an
increase of the bipyridine dihedral angle. In our case, the opposite
trend is observed as the absorption maxima are shifted towards
longer wavelength as the length of the bridge increases (see ESI,†
Table S1, comparison of ligands 3, 4 and 5). The same trend
is observed as the number of substituents present on the bridge
increases (ESI,† comparison of ligands 3, 1 and 2). These effects
are likely to be generated by the presence of the oxygen atoms
directly bound to the bipyridine backbone having an influence on
the conjugation of the aromatic rings and thus on the electronic
properties.
ꢀ
better results than their counterparts containing 2,2 -bipyridine,
both in terms of activity and of molecular weight of the synthesized
10
polyketones. In addition, it is known that the dicationic bis-
chelated complexes [Pd(N–N) ][PF
(N–N = bipy, phen and their
substituted derivatives) generate very efficient and stable catalysts,
2
]
6 2
8,9,34
even in the absence of 1,4-benzoquinone.
Thus, ligands 1–5
have now been used for the synthesis of the related bis-chelated
complexes 1c–5c (Scheme 2).
Scheme 2 Synthesis of the dicationic bis-chelated Pd(II) complexes 1c–5c;
a) i) [Pd(OAc) ], MeOH ii) CF COOH; b) i) N–N, MeOH ii) NaPF .
2 3 6
Single crystals of ligand 5 were obtained by recrystal-
lization from methanol. The X-ray structural determination
shows the free ligand molecule located on a two-fold axis bisecting
Complexes 1c–5c were prepared according to the procedure
ꢀ
34
the bipy C(5)–C(5 ) and the central butyl C–C bond (Fig. 2). The
developed by us starting from [Pd(O
2
CCF
3
)
2
(N–N)], 1b–5b,
◦
dihedral angle calculated between the py rings is 49.8(4) , which is
which, in turn, were obtained from [Pd(OAc)
2
26
], always following
◦
comparable to the value of 50.1 found in the two crystallographic
a method we previously reported (Scheme 2).
[Pd(O CCF
(N–N)] complexes 3b–5b (N–N = 3–5) were fully
characterized. H NMR spectra recorded in CD Cl , at room
26
independent molecules of 1. Although structural data are
available only for the cited free ligands, the different O(CH
2
3 2
1
)
2
)
n
O
2
2
bridge length does not appear to modify, to a large extent, the
pyridine interplanar angle.
temperature, present the three expected resonances in the aromatic
region having chemical shifts and coupling constants similar to
those obtained for 1b and 2b. Sharp well-resolved signals of the
expected multiplicity are observed in the aliphatic region for the
CH groups of the bridge except for 5b.
2
Crystals suitable for X-ray analysis were isolated upon addition
of diethyl ether to a dichloromethane solution of 3b. The
coordination geometry about Pd ion is square planar with the
metal lying in the mean N
N bond lengths average to 2.025(3) and 1.991(3) A, respectively,
and are comparable with those found in the strictly analogous
2
O plane (Fig. 3(a)). The Pd–O and Pd–
2
˚
26
complexes containing ligand 1 or 2. These Pd–N distances are
slightly shorter than those found in the bis-chelated species 2c–
−
5c (see below). The CF
3
CO
2
ligands assume a unidentate end-on
coordination and are oriented on the same side of the coordination
plane. The bipy derivative shows a conformation closer to that
observed in 4c and 5c rather than to the ligand geometry observed
ꢀ
in 3c. In fact the py rings are twisted along the C(2)–C(2 ) bond
◦
forming a interplanar angle of 18.5(2) . Thus, as already observed
26
in the crystal structures of 1b and 2b, due to the coordination
to palladium the two pyridine rings are forced towards planarity.
The molecules in the crystal are arranged head-to-tail about a
Fig. 2 Molecular structure of ligand 5 with labelling scheme of the
crystallographic independent part (py(N1)/py(N2) plane 49.8(4) ).
◦
˚
center of symmetry such that the metals are 3.443(1) A apart
This journal is © The Royal Society of Chemistry 2008
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◦
Fig. 3 Molecular structure of complex 3b with atom labelling scheme: (a) ORTEP drawing. Selected bond distances ( A˚ ) and angles ( ): Pd–N(1) 1.990(3),
Pd–N(2) 1.992(3), Pd–O(5) 2.022(2), Pd–O(3) 2.029(3); N(1)–Pd–N(2) 81.08(12), N(1)–Pd–O(3) 174.05(11), N(1)–Pd–O(5) 95.37(11), N(2)–Pd–O(3)
◦
9
3.15(11), N(2)–Pd–O(5) 176.30(11), O(3)–Pd–O(5) 90.37(11); interplanar angle py(N1)/py(N2) 18.5 . (b) complex pairing in the crystal packing.
◦
◦
◦
◦
Fig. 4 Variable temperature NMR of 2c in acetone-d
6
: (a) 20 C; (b) 0 C; (c) −30 C; (d) −60 C; aromatic and CH region (left); CH
3
signals (right).
◦
(
Fig. 3(b)), a feature already observed in the organometallic species
in the aromatic region at −60 C (Fig. 4). At this temperature,
10
[
Pd(CH
Complexes 1c–5c were characterized both in solid state and
in solution. The characterization in solution was performed by
3
)(CH
3
CN)(2)][PF
6
] and [Pd(Cl)
2
(2)].
different well-resolved signals are also observed for the CH and
the CH groups of the bridge. For 3c, containing the unsubstituted
four member bridge, lowering of the temperature results only in a
further broadening of the signals. For 4c, the spectrum recorded at
3
1
H NMR spectroscopy recording the spectra in acetone-d
6
. At
◦
room temperature, the spectra of all complexes 1c–5c have three
resonances in the region of the aromatic protons, indicating the
equivalence of both the two ligands and of the two halves of each
ligand (see ESI,† Table S2). All the aromatic signals, except for
H
ligand and no signal due to the latter is present. The aliphatic
protons give different signals depending on the ligand itself.
Since at room temperature most of the signals of both aromatic
and aliphatic protons are broad, NMR experiments at low
temperature were performed. While in the case of 1c lowering
the temperature only results in a further broadening of the signals,
−90 C presents two separated broad signals for the OCH
2
protons
of the bridge (Fig. 5). The aromatic and aliphatic signals are also
quite broad at this temperature. In the case of 5c, decreasing
temperature results in sharpening the signals of the aromatic
ꢀ
◦
6
,6
of 4c, are shifted to high frequency with respect to the free
protons, while at −80 C broad signals are observed for all the
CH groups of the bridge (Fig. 6).
2
These NMR data clearly indicate the presence of dynamic
processes in solution, which is a typical phenomenon for the
bis-chelated complexes [Pd(N–N)
bipyridine ligands, and represents a way for these complexes to
relieve the steric hindrance, observed in the solid state, between
the a hydrogens of the two facing molecules of ligand bound
2
][PF
6
2
] with phenanthroline and
9,35
◦
even at −97 C; in the case of 2c, well resolved signals are observed
2
174 | Dalton Trans., 2008, 2171–2182
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◦
Table 1 Relevant bond lengths ( A˚ ) and angles ( ) and geometrical data
of compounds 2c–5c
a
2
c
3c
4c
5c
Pd–N(1)
Pd–N(2)
Pd–N(3)
Pd–N(4)
1.856(4)
2.116(5)
1.880(4)
2.116(5)
2.026(3)
2.044(3)
—
2.054(4)
2.043(5)
2.031(5)
2.047(5)
2.023(5)
2.046(5)
2.041(5)
2.022(5)
—
N(1)–Pd–N(2)
N(3)–Pd–N(4)
N(1)–Pd–N(3)
N(1)–Pd–N(4)
N(2)–Pd–N(3)
N(2)–Pd–N(4)
73.3(3)
73.4(2)
179.4(4)
106.5(3)
106.8(2)
179.5(2)
—
78.77(11)
—
—
—
—
—
81.68(19)
81.35(18)
164.06(19)
101.84(18)
100.24(19)
161.74(18)
—
80.57(19)
80.4(2)
161.3(2)
100.57(19)
104.04(19)
163.03(19)
—
ꢀ
N(1)–Pd–N(2 )
101.23(11)
Pd · · · F ( A˚ )
3.275
3.365
3.091
3.030
a
Metal located on a symmetry center.
to envisage that the dynamic process is the equilibrium between
two conformers of the complex.
Single crystals for 2c, 3c, 4c and 5c were obtained upon addition
of diethyl ether to a solution of the complex in acetone or
by recrystallization from DMSO/MeOH using the double layer
method. The X-ray structural determination of compounds 2c–5c
evidences the expected square planar geometry for the palladium
atom and the ORTEP drawings of the complex cations are depicted
in Fig. 7. The Pd–N bond distances fall in a close range from
◦
◦
6
Fig. 5 Variable temperature NMR of 4c in acetone-d : (a) 20 C; (b) 0 C;
◦
◦
(
c) −40 C; (d) −90 C.
˚
2
.022(5) to 2.054(4) A, with the exception of compound 2c where
the trans located coordination distances are remarkably different,
˚
.116(5) and 1.868(4) A (mean values; Table 1).
2
2
+
The overall conformation of [Pd(N–N)
2
]
bis-chelated com-
plexes can be rationalised taking into account the interplanar
angle descriptors we introduced a decade ago to illustrate the
ligand distortions in bis(bipy) and bis(phen) species (or their
35
alkyl substituted bipy and phen derivatives): a, between the
MN planes; b, between the MN and NCCN mean planes; c ,
2
2
between the two pyridyl rings. These angles, together with a
selection of geometrical data, are reported in Table 2. Basically
two limiting conformations that relieve the crowding between the
a-hydrogens of the two chelating ligands can be identified in the
solid state: twist and bow-step. Fig. 8 illustrates the conformations
of complexes 2c (3c) and 4c (5c). Complexes 2c and 3c, differing
for the methyl groups on the O–C–C–O bridge, have a close
comparable conformation with dihedral angle a close to zero
◦
◦
Fig. 6 Variable temperature NMR of 5c in acetone-d
6
: (a) 20 C; (b) 0 C;
◦
◦
(
c) −40 C; (d) −80 C; aromatic region (left); OCH
2
signals (right).
to palladium. The nature of these dynamic processes depends
on the ligand itself. For instance, in the case of the bis-chelated
complexes with symmetrical phenanthrolines or bipyridines it was
attributed to the establishment of an exchange process between
free and coordinated chelating nitrogen ligand, whereas for
the bis-chelated complexes with non symmetric phenanthrolines,
like the 3-alkyl substituted 1,10-phenanthrolines (3-R-phen), an
equilibrium between syn and anti isomers was proposed. For the
present complexes it is difficult to recognize the origin of this
dynamic behavior. The H NMR spectrum, at the decoalescence
◦
and b in between 25–30 . This arrangement is very close to the
conformation we indicated as bow-step (Fig. 8). On the other hand
the coordination geometry of Pd in both the cations of 4c and 5c is
35
◦
Table 2 Relevant angles ( ) describing the distortions around palladium
9
Angle
2c
0.7(4)
29.3(4)
30.3(3)
36.4(4)
34.3(3)
8.6(9)
3c
4c
5c
1
PdN
PdN
PdN
2
2
2
/PdN
/N1–CC–N2 b
/N3–CC–N4 b
2
a
0.0
26.3(2)
2.8(4)
4.9(3)
31.0(3)
27.9(2)
27.4(7)
24.1(7)
27.7(2)
4.8(3)
5.1(3)
27.8(2)
26.7(2)
23.6(7)
25.2(7)
25.0(2)
◦
temperature (−60 C), of the solution of complex 2c, does
not evidence signals due to the free ligands, all the aromatic
signals have the same integration and similar chemical shifts
indicating that the protons should experience a similar chemical
environment. On the basis of these considerations, it is reasonable
py(N1)/py(N2) c
py(N3)/py(N4) c
N(1)–C(5)–C(6)–N(2)
N(3)–C(15)–C(16)–N(4)
34.6(2)
± 17.2(4)
13.0(8)
This journal is © The Royal Society of Chemistry 2008
Dalton Trans., 2008, 2171–2182 | 2175

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Fig. 7 Molecular structure of the cations of complexes: (a) 2c; (b) 3c; (c) 4c; (d) 5c.
tetrahedrally distorted from square planar with a dihedral angle,
the bischelated Pd-complexes these ligands preserve part of their
atropisomerism, as confirmed by the values of the torsion angles
N–C–C–N (Table 2).
◦
◦
a, of ca. 27 and correspondently b averaging to 4.4 , indication
of a twist conformation (Fig. 8). As far as c angle is concerned,
◦
36
the values fall in a range from 26.7(2) to 36.4(4) (regardless of the
The absolute configuration of the atropisomeric ligands is MM
type of described deformation) which seems in contrast with the
strong correlation between the angles b and c found in bis-chelated
in 2c, MP in 3c, while both atropisomers, MM and PP, are present
in the centrosymmetric unit cell of 4c and 5c.
(
bipy and phen) metal complexes: bow-step conformations are
The chelating N–Pd–N angle also appears to be affected by
the observed conformations, and slightly narrower values were
featured by large b and c values; whereas twist conformations are
characterized by small b and c values; being c angles close to 0
indicative of coplanar pyridine rings. In the present complexes
◦
◦
measured for 2c and 3c (78.77(11) and 73.3(3) , respectively) when
35
◦
compared with 4c and 5c (mean of 81 ) (Table 1).
c values smaller than those typical of bis(bipy) square planar
The molecular packing shows weak interactions between the
complexes in the bow-step conformation are measured for 2c and
metal and the counterion with short Pd · · · F distances of about
◦
˚
3c, but large c values (from 26.7 to 31.0 ) are also found for
c and 5c (twist conformation). Actually, the latter values are
3.0 A (Table 1) in 4c and 5c, while similar contacts in 2c and 3c
˚
4
are longer (about 3.3 A). This feature is not novel in the solid
indicative of the tilting between the pyridine rings, and are related
to the atropisomerism of the ligands. Thus, upon coordination in
state, and formation of ion pairs for similar complexes was also
demonstrated in solution via NMR studies.
37
2
176 | Dalton Trans., 2008, 2171–2182
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Fig. 8 Two perspective views, showing the conformations of the complexes and the atropisomerism of the ligands (the black moieties are those pointing
towards the observer: (a) bow-step conformation in 3c (same arrangement in 2c); (b) twist conformation in 4c (same arrangement in 5c). Upper part
evidencing the c angle, lower part the a angle.
a
2 6 2
Table 3 CO/vinyl arene copolymerization: effect of 1,4-benzoquinone and of alkene. Precatalyst: [Pd(N–N) ][PF ] 1c–5c
b
Precatalyst
Alkene
[BQ]/[Pd]
g of CP/g of Pd
M
w
(M
w
/M
n
)
R.U.
[
Pd(bipy)
2
2
2
][PF
][PF
][PF
6
6
6
]
]
]
2
2
2
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
4-Me-styrene
4-Me-styrene
4-Me-styrene
4-Me-styrene
4-Me-styrene
40
40
40
40
40
40
0
0
0
0
0
0
0
0
0
0
0
3709
2715
1300
1350
698
1300
1931
783
368
656
285
826
4839
2018
828
2693
474
109 000 (2.1)
52 000 (2.3)
48 000 (2.5)
53 000 (2.8)
30 500 (2.3)
41 000 (2.2)
115 500 (2.4)
58 000 (2.9)
31 000 (2.2)
45 000 (2.2)
34 000 (3.2)
46 500 (2.8)
198 000 (2.3)
161 000 (4.6)
73 000 (3.2)
113 000 (2.3)
41 000 (3.0)
825
393
363
401
231
311
874
439
235
340
257
352
1354
1101
499
773
280
1
2
3
4
5
c
c
c
c
c
[
Pd(bipy)
1
2
3
4
5
c
c
c
c
c
[
Pd(bipy)
1
2
3
4
c
c
c
c
a
−5
Reaction conditions: nPd = 0.54 × 10 mol; vinyl arene V = 30 mL, [styrene]/[Pd] = 48 000, [4-Me-styrene]/[Pd] = 42 100, solvent: TFE V = 20 mL;
◦
b
T = 50 C; t = 24 h; PCO = 40 bar. R.U. = number of repetitive units inserted in the polymeric chain.
Catalytic behavior of complexes [Pd(N–N)
2
][PF
6
]
2
1c–5c
All the complexes generate active species that show a produc-
tivity lower than that of the bipy-containing catalyst regardless of
the presence of benzoquinone (BQ) as oxidant and of the nature
of the alkene (Table 3). This contrasts with what observed when
using monochelated species containing ligands 1 and 2 as catalysts
since, under optimised conditions, they appear to show better
performances than the analogous complexes containing the bipy
Complexes 1c–5c were used as precatalysts for the CO/styrene and
CO/4-Me-styrene copolymerizations, carrying out the reactions
in 2,2,2-trifluoroethanol (TFE), at 40 bar of carbon monoxide
◦
and at T = 50 C. All the solids isolated at the end of the runs
were perfectly alternating polyketones. Their stereochemistry was
1
3
10
characterized by C NMR spectroscopy recording the spectra
in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) and CDCl . The mi-
ligand.
3
The nature of the ligand backbone affects both the productivity
of the system and the molecular weight of the synthesized
polyketones. Ligand 4 leads to the least productive catalyst
among those tested. The effect of the ligand on the productivity
is more pronounced when the reactions are carried out with
no addition of the oxidant. In particular, when styrene is the
comonomer, the productivity increases in the order: 4c < 2c <
3c < 1c < 5c < bipy; while when 4-Me-styrene is used, 1c
shows a productivity lower than that of 3c. Under these reaction
crotacticity was determined by integration of the signals assigned
to the ipso-carbon atom for the CO/styrene copolymers and to
the methylenic protons for the CO/4-Me-styrene copolymers.
This analysis proved that all the synthesized polyketones have
a syndiotactic microstructure with the triads distribution (uu:
8
0%; ul, lu: 20%) typical for copolymers synthesized with bipy-
2
containing catalysts: thus no effect of the ligand backbone on the
microstructure of the synthesized macromolecules was observed.
This journal is © The Royal Society of Chemistry 2008
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conditions extensive decomposition of the catalyst to inactive
palladium metal is observed, as evidenced by the grey colour of
the synthesized copolymers. Thus, the important variations found
for the productivity are mainly attributed to a decrease of the
stability of the active species. It appears reasonable to exclude that
such differences might be originated by electronic effects, which
are expected to be very similar for the ligands studied. More likely
they have a steric nature, related to the distortions created by the
introduction of the bridge between the two pyridine rings.
Actually, if for the discussion of the catalytic data the precata-
lysts are separated into two classes, it seems that the productivity
data are better related to the solution behavior of the ligand
itself rather than to that of the precatalysts. One class includes
complexes 3c, 2c and 1c with ligand dioxy bridge of the same length
and differing for the number of methyl groups on the methylenic
carbons; the other comprises complexes 3c, 4c and 5c having the
ligands with the bridge of different length, but with no substituents
on the methylenic carbons. For both series of complexes the least
productive catalyst contains the ligand that shows the dynamic
process with a high rate on the NMR time scale, that is 2 and 4,
respectively.
copolymer with the highest molecular weight values inside the
series regardless of the presence of the oxidant and of the nature
39
of the alkene; the lowest M
w
values are obtained with 4c. The
M
w
values and the number of repetitive units inserted into the
polymer chains for the CO/4-Me-styrene copolymers are higher
than those of the CO/styrene polyketones, thus indicating that the
enhancement of productivity might be due to both an increase in
the length of the chains and in their number.
Unexpectedly and in contrast with all the literature data
5,6,11,40
reported up to now,
the polyketones prepared in the absence
of the oxidant have similar molecular weight values, and in few
cases (entries related to precatalysts 2c and 3c) even lower, to
those of the copolymers synthesized with [BQ]/[Pd] = 40, thus
indicating that the enhancement of productivity, observed when
the benzoquinone is present, is mainly due to an increase in
the number of the polymeric chains and not of their length.
These results suggest that in the present catalytic system neither
benzoquinone nor hydroquinone are involved in chain transfer
reactions, as for instance observed in the catalytic system based
5
10,41
on bisoxazoline ligands. In agreement with the literature,
the main role played by benzoquinone should be related to the
transformation of the Pd–H intermediate, formed through the b-
hydrogen elimination reaction on the growing polymer chain, into
the Pd-alkoxy derivative (Scheme 4), which is one of the active
species of the catalytic cycle, thus preventing the catalyst from
its decomposition and explaining the observed increase in the
productivity.
It is known from the literature, that the activation step of the
bis-chelated complexes requires the complete, or the partial, disso-
ciation of one of the two molecules of N–N ligand from palladium
(
Scheme 3(a)). During the catalytic process the dissociated N–N
molecule can bind again palladium (Scheme 3(b)), preferentially
38
in a monodentate fashion, contributing to protect the catalyst
toward its decomposition. The role of the second molecule of
N–N in the Pd-phen catalytic system as stabiliser of the catalyst
11
has been recently demonstrated. On the basis of this analysis
and by taking into account the solution behavior of the ligands,
the catalytic results suggest that on decreasing the rigidity of the
bridge, less stable catalysts are generated. The lower rigidity of the
ligand might disfavour its re-coordination to the metal center.
3,9,11
In agreement with most of the literature,
4-Me-styrene
Scheme 4 The step of the catalytic cycle involving BQ.
was found to be a more active alkene than styrene, reaching
productivities at least double than those obtained with styrene.
Experimental
Ethane-1,2-diol di-p-toluenesulfonate, propane-1,3-diol di-p-
4
2
43
toluenesulfonate and butane-1,4-diol di-p-toluenesulfonate
were prepared according to published procedures. Ligands 1 and 2
26
and complexes 1b and 2b were prepared as previously reported.
p-Toluenesulfonyl chloride and triphenylphosphine (Fluka), 2-
bromo-3-hydroxypyridine (Aldrich) and NiCl O (Carlo Erba)
are commercial and were used as received. Pyridine was dried over
sodium hydroxide prior to use.
2
·6H
2
2
,2,2-Trifluoroethanol, styrene and 4-Me-styrene (Aldrich) for
the catalytic reactions were used as received without any further
purification. [Pd(OAc) ] was a gift of Engelhard Italiana.
2
Carbon monoxide (CP grade, 99.9%) was supplied by SIAD.
Proton and carbon NMR spectra were recorded at 400 and
100.5 MHz respectively on a JEOL EX 400; the resonances were
3
Scheme 3 (a) The formation of the active species and (b) two key
intermediates of the catalytic cycle: b1: the Pd-alkyl species; b2: the Pd-acyl
species.
referenced to the solvent peak versus TMS (CDCl at 7.26 ppm,
CD
2
Cl
2
at 5.33 ppm and acetone at 2.04 ppm for the proton; CDCl
Cl at 53.8 ppm for the carbon). Attributions
3
The nature of the modified-bipy also affects the molecular
weight values of the synthesized polyketones (Table 3). The trend
of molecular weight with the nature of the ligand roughly reflects
that of productivity: the bipy-containing catalyst leads to the
at 77.0 ppm and CD
2
2
1
3
for the C NMR spectra of 3–5 and attribution of the resonances
1
for H spectra were made respectively on the basis of heteronuclear
correlation experiments and of two dimensional homonuclear
2
178 | Dalton Trans., 2008, 2171–2182
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correlation experiments (COSY) obtained with the automatic
program of the instrument. C NMR spectra of the polymers
were recorded in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) with a
5.61), 4.17 (4H, t, OCH
(2H, dd, J 4.64), 7.98 (2H, dd).
2
), 7.15 (2H, dd, J 1.71 and 8.06), 7.21
1
3
small amount of CDCl
3
for locking purposes.
General procedure for the Ni catalyzed coupling of bis[3-(2-bromo)-
pyridine] derivatives
UV-Visible spectra were obtained on a Jasco V-500 UV-Vis.
Spectrophotometer equipped with a Peltier temperature con-
troller, using 1.0 cm path-length quartz cuvettes.
Elemental analyses (C, H, N), performed at Dipartimento di
Scienze Chimiche (Universit a` di Trieste), were in perfect agreement
with the proposed stoichiometry.
The molecular weights (M
weight distributions (M /M
chromatography versus polystyrene standards. The analyses were
Ligands 3–5 were synthesized by a procedure similar to that
26
reported for 1 and 2 and purified by column chromatography.
ꢀ
ꢀ
3
,3 -(Ethylenedioxy)-2,2 -bipyridine 3. Obtained from 3a
44%). (Found: C, 66.9; H, 4.8; N, 13,6%. Calc. C12 : C,
Si) 4.32
(
6
H
, Me
10
N
2
O
2
w
) of copolymers and the molecular
) were determined by gel permeation
7.28; H, 4.71; N, 13.08%). d
4H, br, OCH
, J 1.47), 8.63 (2H, dd, H ). d
1.8 (CH
H
(400 MHz, CD
2
Cl
2
4
5,5ꢀ
w
n
(
H
7
1
2
), 7.39 (2H, dd, H , J 4.64 and 8.30), 7.50 (2H, dd,
4,4ꢀ
6,6ꢀ
C
4,4ꢀ
(100 MHz, CD
2
Cl
2
, Me Si)
4
recorded on a Knauer HPLC (K-501 Pump, K-2501 UV-detector)
5,5ꢀ
6,6ꢀ
2,2ꢀ
2
3,3
), 124.9 (C ), 130.2 (C ), 146.0 (C ), 148.5 (C ),
4
˚
ꢀ
with a PLgel 5 lm 10 A GPC column and chloroform as solvent
54.7 (C ).
−
1
(
flow rate 0.6 mL min ). CO/styrene samples were prepared
ꢀ
ꢀ
as follows: 2 mg of the copolymer was solubilized with 120 lL
of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and chloroform was
added up to 10 mL; instead, CO/4-Me-styrene copolymers were
directly soluble in chloroform. The statistical calculations were
performed using the Bruker Chromstar software program.
3,3 -(Propylenedioxy)-2,2 -bipyridine 4. Obtained from 4a
(63%). (Found: C, 68.8; H, 4.9; N, 11,9%. Calc. C13
H
12
N
2
2
O : C,
68.41; H, 5.30; N, 12.27%). d (400 MHz, CD Cl , Me
H
2
2
4
Si) 2.07
ꢀ
5
,5
(2H, quint, CH
2
, J 5.2), 4.40 (4H, t, OCH
2
), 7.31 (2H, dd, H , J
ꢀ
ꢀ
4
,4
6,6
4.75 and 8.26), 7.44 (2H, dd, H , J 1.2), 8.49 (2H, dd, H ).
d
C
(100 MHz, CD
), 125.2 (C ), 144.5 (C ), 149.6 (C ), 154.0 (C ).
2
Cl
2
, Me
4
Si) 29.8 (CH
2
2,2
), 71.2 (OCH ), 123.9
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
3,3
5
,5
4,4
6,6
(
C
CAUTION!
ꢀ
ꢀ
3
,3 -(Butylenedioxy)-2,2 -bipyridine 5. Obtained from 5a
67%). (Found: C, 69.3; H, 5,6; N, 11,8%. Calc. C14 : C,
9.41; H, 5.82; N, 11.56%). d (400 MHz, CD Cl , Me
br, CH ), 4.35 (4H, very br, OCH
HFIP is a very volatile and highly toxic solvent, so proper
protection should be used when it is handled.
(
6
H
14
N
2
O
2
H
2
2
4
,5
Si) 1.94 (4H,
ꢀ
5
2
2
), 7.31 (2H, dd, H , J 4.64 and
Synthesis of ligands 3–5: General procedure for the synthesis of bis-
ꢀ
ꢀ
4
,4
6,6
8
.30), 7.40 (2H, dd, H , J 1.22), 8.47 (2H, dd, H ). d
C
(100 MHz,
), 123.6 (C ), 124.2 (C ),
[3-(2-bromo)pyridine] derivatives 3a–5a
ꢀ
ꢀ
5
,5
4,4
CD
2
Cl
2
, Me
4
Si) 27.5 (CH
2
), 71.8 (OCH
2
ꢀ
ꢀ
ꢀ
6
,6
2,2
3,3
Bis[3-(2-bromo)pyridine] derivatives were prepared according to
the modified procedure reported by us previously: NaOH (75
mmol) and 2-bromo-3-hydroxypyridine (37.5 mmol) were stirred
in 18 mL of water for 1 h. Then 40 mL of toluene, the diol di-p-
toluenesulfonate derivative (9.4 mmol) and tetrabutylammonium
bromide (12.4 mmol) were added and the resulting solution was
143.3 (C ), 147.6 (C ), 154.6 (C ).
ꢀ
1,2-Bis(3-pyridyloxy)ethane 3
A procedure analogous to that used for the synthesis of 3a was used
with 3-hydroxypyridine instead of 2-bromo-3-hydroxypyridine.
(400 MHz, CD
and H ), 8.27 (2H, dd, H , J 1.9 and 4.0), 8.38 (2H, d,
). d
24.3 (C ), 138.6 (C ), 143.1 (C ), 155.3 (C ).
◦
d
H
H
H
2
Cl
2
, Me
4
Si) 4.41 (4H, s, OCH
2
), 7.26 (4H, m,
heated at 95 C. The reaction was monitored by TLC. After
ꢀ
ꢀ
ꢀ
4
,4
5,5
2,2
completion, the reaction mixture was allowed to come to room
temperature and 50 mL of water were added. The organic phase
was decanted and washed with a saturated solution of NaCl (3 ×
,6ꢀ
5,5
ꢀ
6
C
4
(100 MHz, CD
2
Cl
2
, Me
4
2,2
Si) 67.4 (OCH
2
), 121.6 (C ),
ꢀ
ꢀ
ꢀ
ꢀ
3,3
,4
6,6
1
5
0 mL). Filtration afforded a solid that was dried under reduced
Synthesis of [Pd(O
2
CCF
3
)
2
(N–N)] complexes 1b–5b
pressure and recrystallized from hot acetone.
Synthesis of complexes 1b and 2b were already reported
previously. 3b–5b were prepared according to a similar proce-
dure.
1
,2-Bis[3-(2-bromo)pyridyloxy]ethane 3a. Obtained from
26
ethane-1,2-diol di-p-toluenesulfonate (81%). (Found: C, 38.1; H,
.58; N, 7.49%. Calc. for C12
.49%). d (400 MHz, CDCl
2
7
(
H
10Br
2
N
2
O
2
: C, 38.53; H, 2.69; N,
H
3
, Me
4
Si) 4.50 (4H, s, OCH ), 7.25
2
[
Pd(O
2
CCF
3
)
2
(3)] 3b. (57%). (Found: C, 35.1; H, 1.58; N,
Pd: C, 35.15; H, 1.84; N, 5.12%).
2H, dd, J 4.64 and 8.18), 7.37 (2H, dd, J 1.46), 8.04 (2H, dd).
6
.64%. Calc. for C16
H
10
F
6
N
2
O
6
d
H
H
5
(400 MHz, CD
, J 5.4 and 8.6), 7.83 (2H, dd, H , J 1.4), 8.02 (2H, dd, H ).
2
Cl
2
, Me
4
Si) 4.42 (4H, s, OCH
2
), 7.56 (2H, dd,
1
,3-Bis[3-(2-bromo)pyridyloxy]propane 4a. Obtained from
propane-1,3-diol di-p-toluenesulfonate (92%). (Found: C, 40.0;
H, 1.30; N, 7.19%. Calc. for C13 : C, 40.24; H, 3.12;
N, 7.22%). d (400 MHz, CDCl , Me Si) 2.41 (2H, quint, CH , J
.8), 4.32 (4H, t, OCH ), 7.22 (4H, m), 7.99 (2 H, dd, J 2.46 and
.82).
ꢀ
ꢀ
ꢀ
,5
4,4
6,6
H
12Br
2
N
2
O
2
[Pd(O CCF ) (4)] 4b. (67%). d (400 MHz, CD Cl , Me Si)
2
3
2
H
2
2
4
H
3
4
2
2.46 (2H, quint, CH , J 5.9), 4.58 (4H, t, OCH ), 7.53 (2H, m,
H ), 7.89 (4H, m, H and H ). d (400 MHz, CDCl , Me Si)
2.45 (2H, quint, CH , J 6.1), 4.58 (4H, t, OCH ), 7.51 (2H, dd,
H
2
2
ꢀ
ꢀ
ꢀ
6,6
5
,5
4,4
5
3
2
H
3
4
2
2
ꢀ
ꢀ
ꢀ
6,6
5
,5
4,4
, J 5.4 and 8.5), 7.82 (2H, dd, H , J 1.0), 7.96 (2H, dd, H ).
1
,4-Bis[3-(2-bromo)pyridyloxy]butane 5a. Obtained from
butane-1,4-diol di-p-toluenesulfonate (87%). (Found: C, 41.9;
H, 3.36; N, 6.92%. Calc. C14 : C, 41.82; H, 3.51; N,
.97%). d (400 MHz, CDCl , Me Si) 2.14 (4H, quint., CH
, J =
[Pd(O CCF (5)] 5b. (63%). (Found: C, 37.8; H, 1.15; N,
2
)
3 2
H
14Br
2
N
2
O
2
4.87%. Calc. for C18 Pd: C, 37.62; H, 2.46; N, 4.87%).
d
H
14
F
6
N
2
O
6
6
H
3
4
2
H
(400 MHz, CD Cl Si) 2.09 (4H, br, CH ), 4.40 (4H, br,
2
2
, Me
4
2
This journal is © The Royal Society of Chemistry 2008
Dalton Trans., 2008, 2171–2182 | 2179

View Article Online
5
,5ꢀ
4,4ꢀ
OCH
2
), 7.53 (2H, dd, H , J 5.4 and 8.7), 7.80 (2H, dd, H , J
CO/4-Me-styrene copolymer: d
CO 206.5, Cipso 136.3, C–CH 133.8, Carom 129.0, 128.0, CH 53.0,
CH 44.0, 43.2, CH 21.0.
C
(100.5 MHz, HFIP + CDCl )
3
ꢀ
6
,6
1
.0), 7.89 (2H, dd, H ).
3
2
3
General procedure for the synthesis of [Pd(N–N)
c–5c
2
][PF
6
2
] complexes
1
Crystallographic data collection and refinement
These complexes were obtained starting from the corresponding
Pd(O CCF (N–N)] derivatives 1b–5b according to a procedure
developed previously in our laboratory for similar complexes.
To a suspension of 0.15 mmol of [Pd(O CCF (N–N)] in
mL of methanol at room temperature was added 0.18 mmol
of the corresponding N–N ligand. The solution obtained is
filtrated on paper. NaPF (0.33 mmol) dissolved in the minimum
amount of methanol is then added to the solution. The immediate
precipitation of the desired product is observed. After 15 min it
was filtered, washed with cold methanol and dried under vacuum.
Diffraction data for the structures reported were collected at
room temperature on a Nonius DIP-1030H system with Mo-
[
2
)
3 2
34
˚
Ka radiation (k = 0.71073 A). Cell refinement, indexing and
2
)
3 2
scaling of the data sets were carried out using programs Denzo,
6
4
4
45
Scalepack, Mosflm and Scala. All the structures were solved by
46
direct method or Patterson and subsequent Fourier analyses and
refined by the full-matrix least-squares method based on F with
all reflections. In 3c the PF anion was found to be disordered
6
2
46
6
over two positions about a F–P–F axis with refined occupancies
of 0.54/0.46. In 5 two residuals on the DF map were interpreted
as a methanol molecule at half occupancy (H atoms not located).
All the calculations were performed using the WinGX System,
[
Pd(1)
Calc. for C26
(400 MHz, acetone-d
2
][PF
6
]
H
2
1c. (85%). (Found: C, 35.5; H, 2.63; N, 6.46%.
Pd: C, 36.62; H, 2.84; N, 6.57%).
, Me Si) 1.48 (6H, d, CH , J 6.3), 4.43
24
F
12
N
4
O
4
P
2
47
Ver 1.64.05. Crystal data and details of data collections and
refinements for the structures reported are summarized in Table 4.
d
H
6
4
3
(
8
2H, m, OCH
2
), 4.61 (2H, m, OCH
2
), 5.02 (2H, br, CH(CH
3
)),
ꢀ
ꢀ
5
,5
4,4
.00 (4H, dd, H , J 5.3 and 8.5), 8.25 (4H, dd, H ), 8.71 (4H,
ꢀ
,6
6
br, H ).
Conclusions
[
Pd(2)
Calc. for C28
(400 MHz, acetone-d
2
][PF
6
]
H
2
2c. (63%). (Found: C, 38.4; H, 2.73; N, 6.30%.
In this paper we have reported a systematic investigation on
a new series of atropisomeric bipyridine ligands featured by a
bridge of different length or with different substituents, on the 3,3
positions. The nature of the bridge affects the dynamic behavior
of these molecules in solution, due to the interconversion of the
two atropisomers.
These ligands were used to synthesize both mono-chelated and
bis-chelated palladium complexes. The X-ray characterization of
some components of the two series of complexes evidences that the
coordination of the ligands in the mono-chelated complexes results
in the loss of most of their atropisomerism, whereas when they are
coordinated in the bischelated derivatives remarkable distortions
involving the ligand itself and the whole complex are observed
and part of the atropisomerism is preserved. In addition, the bis-
chelated complexes show markedly different deformations with
28
F
12
N
4
O
4
P
2
Pd: C, 38.18; H, 3.20; N, 6.36%).
d
H
6
, Me
4
Si) 1.44 (12H, d, CH
3
, J 6.03), 4.80
ꢀ
ꢀ
ꢀ
5
,5
4,4
(
8
4H, br, OCH), 7.93 (4H, br s, H ), 8.17 (4H, br d, H , J 8.09),
.62 (4H, br s, H ).
ꢀ
6
,6
[
Pd(3)
Calc. for C24
(400 MHz, acetone-d
2
][PF
6
]
2
3c. (62%). (Found: C, 32.4; H, 2.26; N, 6.29%.
H
20
F
12
N
4
O
4
P
2
Pd: C, 34.95; H, 2.44; N, 6.79%).
d
H
6
, Me
4
Si) 4.64 (8H, br, OCH
2
), 7.96 (4H,
ꢀ
ꢀ
ꢀ
5
,5
4,4
6,6
br, H ), 8.21 (4H, br d, H ), 8.70 (4H, br, H ).
[
Pd(4)
2
][PF
6
]
2
4c. (Found: C, 35.5; H, 2.63; N, 6.46%. Calc. for
Pd: C, 36.62; H, 2.84; N, 6.57%). d (400 MHz,
C
26
H
24
F
12
N
4
O
4
P
2
H
acetone-d
OCH
.29 (4H, d, H ).
6
, Me
4
Si) 2.48 (4H, quint, CH
2
, J 5.8), 4.73 (8H, t,
ꢀ
ꢀ
5
,5
4,4
2
), 7.80 (4H, dd, H , J 4.9 and 8.30), 7.87 (4H, d br, H ),
ꢀ
6
,6
8
2
+
respect to those previously observed in [M(N–N)
2
]
derivatives
[
Pd(5)
2
][PF
6
]
2
5c. (Found: C, 37.5; H, 3.53; N, 6.67%. Calc. for
Pd: C, 38.18; H, 3.20; N, 6.36%). d (400 MHz,
(
M = Pd, Pt), especially those occurring between the bipy py
C
28
H
28
F
12
N
4
O
4
P
2
H
rings. These differences are reasonably induced by the strain of the
O(CH ) O bridge.
acetone-d
6
5
, Me
4
Si) 2.16 (4H, br, CH
2
), 4.67 (4H, br, OCH
2
6,6
), 8.06
ꢀ
ꢀ
ꢀ
,5
4,4
(
4H, br, H ), 8.41 (4H, d br, H , J 9.0), 8.72 (4H, br, H ).
2 n
Instead, the chemical behavior in solution of complexes 1c–5c is
similar to that of other bis-chelated complexes with phenanthro-
lines and bipyridines, showing the presence of a dynamic process
that renders equivalent the two molecules of ligand and the two
halves of them.
Copolymerization reactions
These reactions were carried out in a 150 mL stainless-steel auto-
clave equipped with a Teflon liner, magnetic stirrer, heating mantle
and temperature controller. The complex [Pd(N–N)
2
][PF
6
]
2
, 1,4-
Complexes 1c–5c generate active species for the CO/vinyl
arene copolymerization yielding to the corresponding syndiotactic
polyketones. The introduction of the bridge between 3 and 3
benzoquinone when used, the alkene (styrene or 4-Me-styrene)
and the solvent were introduced in the autoclave which was then
ꢀ
◦
pressurised to 40 bar of CO and heated to 50 C. At the end of the
positions of the pyridine rings does not lead to more produc-
tive catalysts than that obtained from the Pd-bipy precatalyst.
However, small variations in the backbone of the ligand affects
both the productivity and the molecular weight of the synthesized
polyketones. When these ligands are used benzoquinone is an
important component of the catalytic system to protect the catalyst
from its decomposition and the role of the oxidant is mainly
related to the transformation of the Pd–H intermediate into the
catalysis (24 h) and after cooling and releasing the residual gas, the
solution from the autoclave was poured into 200 mL of methanol
and stirred for 1 h. The copolymer was then filtrated, washed with
methanol and dried under vacuum.
CO/styrene copolymer: d
10.4, Cipso 136.1, 135.8, 135.3, Carom 129.4, 128.3, 128.0, CH 54.0,
CH 42.3.
C
(100.5 MHz, HFIP + CDCl ) CO
3
2
2
2
180 | Dalton Trans., 2008, 2171–2182
This journal is © The Royal Society of Chemistry 2008

View Article Online
Pd-carboalkoxy species, while no effect on the molecular weight is
observed.
Acknowledgements
This work was supported by the European Network “PALLA-
◦
DIUM” (FP5, contract N HPRN-CT-2002–00196) and by the
ꢀ
ꢀ
Ministero dell Istruzione, dell Universit a` e della Ricerca (MIUR-
◦
Rome; PRIN N 2005035123). Engelhard Italiana is gratefully
acknowledged for a generous gift of [Pd(OAc) ].
2
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182 | Dalton Trans., 2008, 2171–2182
This journal is © The Royal Society of Chemistry 2008