Coordination chemistry to palladium(II) of pyridylbenzamidine ligands and the related reactivity with ethylene

Article abstract of DOI:10.1016/j.ica.2015.01.049

The coordination chemistry to palladium of three pyridylbenzamidines (N-N′) was investigated in detail. The studied pyridylbenzamidines are featured by the bulky 2,6-diisopropylphenyl substituent at the azomethine nitrogen atom of the amidine unit, and di

Full text of DOI:10.1016/j.ica.2015.01.049

Inorganica Chimica Acta xxx (2015) xxx–xxx
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Inorganica Chimica Acta
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Coordination chemistry to palladium(II) of pyridylbenzamidine ligands
and the related reactivity with ethylene
a
a
b
a
a
a,
⇑
Andrea Tognon , Vera Rosar , Nicola Demitri , Tiziano Montini , Fulvia Felluga , Barbara Milani
a
Department of Chemical and Pharmaceutical Sciences, University of Trieste, Via Licio Giorgieri, 1, 34127 Trieste, Italy
Elettra – Sincrotrone Trieste, S.S. 14 Km163.5 in Area Science Park, 34149 Basovizza, Trieste, Italy
b
a r t i c l e i n f o
a b s t r a c t
0
Article history:
The coordination chemistry to palladium of three pyridylbenzamidines (N-N ) was investigated in detail.
The studied pyridylbenzamidines are featured by the bulky 2,6-diisopropylphenyl substituent at the
azomethine nitrogen atom of the amidine unit, and differ in the substitution either at the amino atom,
which bears a 2-pyridyl or a 6-methyl-2-pyridyl group, or at the bridging N-atom that, in one case, is sub-
stituted by a methyl group, leading to a molecule reported herein for the first time. The accurate NMR
characterization of the free ligands points out the presence of dynamic phenomena in solution, due to
the interconversion of several possible isomers, including tautomers. The coordination chemistry to Pd(II)
Received 4 November 2014
Received in revised form 21 January 2015
Accepted 25 January 2015
Available online xxxx
Keywords:
Pyridylbenzamidine
Palladium
3 3 3 6
is studied using both [Pd(cod)(CH )Cl] and [Pd(cod)(CH )(CH CN)][PF ] as metal precursor. Depending on
the palladium precursor and on the pyridylbenzamidine, different coordination compounds are obtained,
demonstrating the capability of these molecules to act both as mono- and bidentate ligands. For the
pyridylbenzamidine substituted with the methyl group on the bridging N-atom, the C–H activation of
one of the isopropyl groups is observed with the formation of a six-membered palladacycle and methane.
None of the isolated complexes generates active catalysts either for ethylene homopolymerization or for
Coordination chemistry
Ethylene
ethylene/methyl acrylate copolymerization. When reacting with ethylene, the complexes lead to the for-
0
mation of propylene and the inactive dicationic [Pd(N-N )
2
][PF
6
]
2
complex.
Ó 2015 Elsevier B.V. All rights reserved.
1
. Introduction
of copolymer per gram of palladium per hour), a content of inserted
polar monomer in the range 4–25% depending on the substituent on
the aryl rings and with the polar monomer inserted at the end of the
branches [13,14]. A palladium complex with a cyclophane-modified
BIAN was reported to catalyze the same reaction with an activity of
1.6 g CP/g Pdꢀh and a content of incorporated MA up to 21.8% [15].
Starting from Brookhart’s discovery that Ni(II) and Pd(II) com-
plexes with
a-diimine ligands are extremely efficient catalysts
for alkene polymerization [1], the last two decades have witnessed
a huge development of complexes based on late transition metals
and nitrogen-donor ligands for application as catalysts in poly-
olefin synthesis [2,3].
a
-diimines with different skeletons were also studied leading, how-
ever, to poorly active catalysts [16]. The content of incorporated
This discovery opened the possibility for using late transition
metal complexes as catalysts for the ethylene/polar vinyl monomer
copolymerization to yield functionalized polyolefins, a highly chal-
lenging reaction [4–12]. In particular, focusing the discussion on
nitrogen-donor ligands, the catalytic systems reported in the lit-
acrylate was increased up to 37% by using Pd(II) complexes with
a
-diimines substituted by 1-naphthyl groups [17].
All the studied -diimines are characterized by the same aryl
rings on the donor atoms. Recently, we introduced a new nonsym-
metrically substituted -diimine having one aryl ring meta-substi-
a
a
erature are based on palladium(II) complexes with
a
-diimines hav-
tuted with an electron-withdrawing group and the other ring
ortho-substituted with an electron-releasing group (Fig. 1) [18].
The corresponding Pd(II) complex was found to be twice as pro-
ductive as the catalyst with the symmetrically ortho-substituted
ligand and leading to the ethylene/MA co-oligomer with a higher
content of incorporated polar monomer, thus pointing out the
positive effect of the subtle unbalance of the electronic and steric
properties of the nitrogen-donor atoms. The same effect was
previously observed by some of us in the CO/vinyl arene
ing an acenaphthene (BIAN) or a 1,4-diaza-1,3-butadiene (DAB)
skeleton and, on the iminic nitrogen atoms, aryl rings substituted
in ortho position with bulky groups [13,14]. These led to the forma-
tion of amorphous, branched ethylene/methyl acrylate (MA)
copolymers with an activity of 57 g CP/g Pdꢀh (g CP/g Pdꢀh = grams
⇑
Corresponding author. Tel.: +39 040 5583956.
E-mail address: milaniba@units.it (B. Milani).
http://dx.doi.org/10.1016/j.ica.2015.01.049
020-1693/Ó 2015 Elsevier B.V. All rights reserved.
0
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2
A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
copolymerization catalyzed by Pd(II) complexes with nonsymmet-
rically meta-substituted BIANs [19,20].
products. This method had previously proved to be successful in
the base mediated N-alkylation of the bridging nitrogen atom of
2,2 -dipyridylamine ligands [24] and of aza-bis(oxazoline)
0
With the aim to illustrate the beneficial effect of nonsymmetric
ancillary ligands, we have now investigated the coordination chem-
istry to palladium, and the catalytic behavior of related complexes
in the ethylene/MA copolymerization, of bidentate nitrogen-donor
ligands belonging to the family of pyridylbenzamidine molecules
compounds [29].
The structural characterization of 1–3 is not trivial as a number
of different isomeric forms are possible for these compounds. Con-
sidering only the E geometry of the C@N double bond, as the Z is
0
(Scheme 1). These ligands are characterized by one pyridinic and
known to be disfavored in N,N -disubstituted benzamidines [30],
one iminic nitrogen atom plus an additional nitrogen on the bridge
connecting the two moieties of the ligand. Upon coordination to pal-
ladium a six-membered palladacycle should result with structural
features, like the N–Pd–N bond angle close to the values of ideal
square planar complexes, that might have positive effects in cataly-
sis. Indeed, when studying the CO/styrene copolymerization we
found that moving from bis(oxazolines) (5-membered Pd-cycle) to
azabis(oxazolines) (6-membered Pd-cycle, with a nitrogen atom
bridging the two oxazoline rings) a remarkable increase in the cat-
alytic performance of the related Pd(II) complexes was achieved
together with an increase in catalyst stability [21].
Ligands 1 and 2 (Scheme 1) were previously reported as having
being applied to prepare both palladium(II) and nickel(II) complex-
es, which were tested as precatalysts for the Suzuki–Miyaura reac-
tion and ethylene oligomerization, respectively [22,23]. Pd(II) and
Ni(II) complexes with bis(pyridyl)-N-alkylaminato ligands were
reported to be active catalysts for ethylene dimerization to a mix-
ture of 1- and 2-butenes [24,25]. Ligand 3 is reported herein for the
first time.
tautomers stem from the presence of the secondary amino group,
while s-cis and s-trans conformations are the result of rotational
isomerism with respect to the C–Namino single bond, having a par-
tial double bond character. In addition, hindered rotation around
N-dip and the different relative position of the pyridinic N and
the C@N bond must be taken into account (Scheme 2). Complex
equilibria between isomers and tautomers in solution have been
already described for bulky non symmetric N,N -diarylbenzamidi-
nes [31–33]. Useful information about the structure of pyridylben-
zamidines 1–3 comes from the X-ray crystallographic structure of
0
0
0
N-dip-N -5-(nitropyridyl)benzamidine [23], and of N-phenyl-N -
pyridylbenzamidine [34], which showed that these compounds
crystallized in the expected E configuration of the C@N double
bond, with an anti relative position of the Npyridino and Nimino
atoms, and an s-cis conformation around the C–Namino bond, as
represented in Scheme 2.
Ligands 1 and 2 were isolated as light yellow solids, and charac-
1
terized by ESI-MS, and NMR spectroscopy. The H NMR spectra
2 2
(CD Cl , r.t.) showed in the range of aliphatic protons two signals
for the isopropylic methyl groups, at around 0.9 and 1.1 ppm,
and two multiplets for the isopropyl CH protons at 3.0 and
3.3 ppm. These data are in agreement with those reported in the
literature [23,22], where the different resonances were generically
attributed to isomeric E,syn and E,anti mixtures. However, a deeper
insight into the mono- and bidimensional spectra of 1 and 2
showed that the isopropyl methyl group signal at around 1.1–
1.2 ppm was given, in both cases, by two overlapped doublets
and that the area of the multiplets due to isopropyl CH protons
did not correspond to one proton each, even though the relative
total area matched for the expected number of protons (Figs. S3
and S5). In the COSY spectra the higher frequency CH signal
2
. Results and discussion
2.1. Synthesis and characterization of ligands 1–3
N-pyridylbenzamidine ligands 1–3 share the presence of the
bulky 2,6-diisopropylphenyl (2,6-diisopropylphenyl = dip) sub-
stituent at the azomethine (Nimino) nitrogen atom of the amidine
unit. Compounds 1 and 2 differ in the nature of the substitution
at the amino (Namino) atom, which bears a 2-pyridyl and a 6-
methyl-2-pyridyl group, respectively; in compound 3 the acidic
N-hydrogen atom is replaced by a N-methyl group (Scheme 1).
The synthesis reported in the literature of 1 [23] and 2 [22] fol-
(3.3 ppm) correlated with both CH
quency CH (3.0 ppm) correlated only with the higher frequency
CH doublet (Figs. S4 and S6). NOESY analysis of 2 showed
3
doublets, while the lower fre-
0
lows a classical route for the preparation of N,N -disubstituted
3
amidines [26,27]. It proceeds through the reaction of the imi-
doylchloride with 2-aminopyridine or 6-methyl-2-aminopyridine
respectively, affording 1 and 2 in approximately 50% yield
exchange peaks for both the CH and the methyl signals (Fig. S7).
Finally, in the aromatic range, together with the well resolved
and intense peaks, large signals were also observed.
(
Scheme 1). This approach was extended to the synthesis of 3, by
Although it was not obvious from these spectroscopic evidence
which isomers were present, we assigned to 1 and 2 the exclusive
E,anti configuration, and a preferential s-cis conformation around
the C–Namino single bond, with the large signals in the aromatic
range and the multiple resonances for the methyl and CH hydrogen
of the isopropyl groups being caused by dynamic phenomena in
solution, such as hindered rotation around the Cipso–Nimino bond
and a slow equilibrium between tautomers.
using N-methylaminopyridine as the amine partner [28]. However,
the reaction required modification and optimization, as the sec-
ondary amine failed to react with the imidoylchloride under the
above conditions, probably due to steric reasons. The lack of reac-
tivity was circumvented by deprotonating the secondary amino
group in situ with NaH, added in excess to the reaction mixture,
leading to 3 in a 30% overall yield after purification (Scheme 1).
An alternative approach, involving deprotonation of the central
The expected anti position of the Npyridino and Nimino atoms was
3
N
aminic nitrogen atom of 1 with NaH and subsequent trapping of
substantiated by the H -pyridine chemical shift value (7.26 ppm)
the anion with CH I led to complex mixtures of unidentified
3
in 1 [35]; while the preference for the s-cis conformation was not
F3C
F3C
CF3
CF3
CF3
N
N
N
N
N
N
CF3
Fig. 1. The symmetric and nonsymmetric
a-diimines studied [18].
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A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
3
2
1
2
3
3
3
3
Scheme 1. Synthetic scheme for ligands 1–3.
R
N
H
H
rotation
Namino-py
N
N
N
N
N
R
Esyn
s-cis
Eanti
s-cis
rotation
C-Namino
R
R
N
N
N
N
H
tautomerism
N
N
H
Eanti
s-trans
Eanti
s-trans
Scheme 2. The equilibria among some of the possible isomeric forms in solution for E isomer of 1 and 2.
only suggested by the above mentioned crystallographic struc-
tures, but also supported by data concerning N,N -diphenylbenza-
midine [30].
at 6.65 ppm by correlation experiments, was lower than the one
observed in 1 and 2, suggesting a syn position of the pyridine nitro-
gen with respect to the C@Nimino, already shown to be the favorite
0
0
All these considerations were further confirmed by the
also in N-methyl-N-phenyl-N -(p-tolyl)benzamidine [30].
1
observed simplification in the H NMR spectrum of ligand 3, where
the tautomeric equilibrium is not possible. In fact only a single
multiplet for the CH proton at 3.00 ppm was observed, which cor-
related with both methyl doublets, at 0.95 and 1.12 ppm, originat-
ed by the N-dip hindered rotation.
2.2. Coordination chemistry of ligands 1–3 to Pd(II)
The coordination chemistry to Pd(II) of ligands 1–3 was studied
by using two palladium precursors, [Pd(cod)(CH
cod)(CH )(CH CN)][PF ], 5. The synthesis of the latter is reported
herein for the first time and it is based on the methodology applied
to obtain the precursor [Pd(cod)(CH )(OTf)] (OTf = triflate) [36],
consisting in the abstraction of the halide from [Pd(cod)(CH )Cl],
through the action of silver hexafluorophosphate in the presence
3
)Cl], 4, and [Pd(-
1
In the H NMR spectrum of 3, at room temperature, large signals
3
3
6
were observed, in particular for N–CH
ic and heteroaromatic protons. By lowering the temperature, up to
63 K, the resonances became sharp allowing their assignment
3
and for most of the aromat-
3
2
3
3
(Fig. S8). Significantly, the H -pyridine chemical shift, identified
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4
A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
3
of acetonitrile. The NMR characterization in solution of complex 5,
obtained as a gray solid, was in agreement with its proposed struc-
ture (Figs. S11 and S12).
When ligand 1 was reacted with complex 4 under the usual
reaction conditions [37,38,18], the coordination compound
low frequency of the H doublet, compared to the same signal in
the free ligand, confirming the E,anti conformation of 1 [35]; the
5.74 ppm shift to lower frequency of the broad signal of the bridg-
ing N–H due to the coordination; the presence of one multiplet and
3
two doublets for the CH and the CH groups of the isopropyl sub-
[
Pd(1)(CH
3
)Cl], 1a, was obtained as a yellow solid (Scheme 3a),
stituents, respectively, indicating the hindered rotation of the rele-
vant aryl ring around the N-dip bond. This also confirmed that the
two multiplets for the CH observed in the spectrum of 1 were
caused by tautomerism, which is no longer possible after
coordination to palladium; the singlet at 0.14 ppm due to the
which was characterized by solution NMR spectroscopy using CD
Cl as solvent, at room temperature. In the H NMR spectrum, the
2
number of signals and their integration indicated the presence in
solution of one species (Fig. 2). No signal due to the free ligand
was observed.
2
-
1
3
Pd–CH fragment. The lower chemical shift of the latter group with
1
The main features of the H NMR spectrum of 1a were: the reso-
respect to its typical value in analogous Pd–methyl complexes with
nonsymmetric N-donor ligands [40,19,20] suggested that the
methyl group experienced the shielding cone of the substituted
aryl ring cis to it [41]. Indeed, NOE experiment performed upon
6
nance of H remarkably shifted to a higher frequency with respect
to the same signal in the spectrum of free ligand, indicating that
the pyridyl ring is cis to the coordinated chloride ligand [39]; the
+ [PF ]-
H
H
N
6
Ph
N
Ph
AgPF
6
3
N
N
CH CN
N
N
Pd
Pd
H C
Cl
3
H CCN CH3
3
H
N
Ph
1
a
N
N
H
Ph
N
1b
(a)
1
N
N
CH3
2
Cl
CH3
Pd
4
CH3
N
Ph
N
N
3
+ [PF6]-
[PF ]-
H
N
+
6
Ph
HN
N
N
N
Ph
Pd
CH3
N
H3CCN
Pd
H
N
Ph
H C
3
N
N
1c
1b
(b)
H
N
Ph
1
+ [PF6]-
N
N
+
[PF6]-
CH3
HN
2
NCCH3
CH3
Ph
Pd
N
5
N
Pd
H3C
H C
3
CH3
N
2c
Ph
N
N
+
[PF ]-
+ [PF6]-
CH3
6
CH3
3
Ph
N
Ph
N
N
N
N
N
Pd
-
CH4
Pd
NCCH3
H CCN CH3
3
CH3
3d
3
b
3 3 3 6
Scheme 3. Coordination chemistry of 1, 2 and 3 to: (a) [Pd(cod)(CH )Cl]; (b) [Pd(CH )(CH CN)(cod)][PF ].
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A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
5
3
6
Pd-CH3
CH -iPr
3
4
5
1a
CH-iPr
H5
H3
H4
NH
H6
Fig. 2. 1H NMR spectrum of 1a, in CD
2 2
Cl , at 298 K, with the numbering scheme of the ligand. In the square the region of the aliphatic signals.
irradiation of this singlet demonstrated that this group was cis to
dip (Fig. S15), confirming the presence of a single species of the
two possible isomers differing in the relative position of the
Pd–CH fragment with respect to the two unequivalent halves of
3
the ligand (Fig. 3).
87.87(5)° is very close to the value for an ideal square planar geo-
metry. The dihedral angle between the square plane and the
plane containing the ortho-substituted aryl ring of 86.5° indicates
that the hindered aryl ring is orthogonal to the coordination
plane, a key feature in catalysts for alkene polymerization (Fig. 4).
The crystal structure of 1a can be almost perfectly superim-
posed onto the dichlorido variant [22]. The main difference
between 1a and the dichlorido complex is the replacement of a
chlorido ligand with a methyl in 1a coordination sphere. This sub-
stitution changes significantly the bond length between palladium
and the pyridinic nitrogen N1 of the pyridylbenzamidine ligand,
due to the trans influence of the methyl group (from 2.021(2) Å
to 2.130(1) Å in 1a) (Table 1).
In addition, in the solid state the cis isomer was the only species
found in the unit cell. Single crystals suitable for X-ray analysis
3
(
colorless plates, ꢁ0.5ꢀ0.5ꢀ0.2 mm in size) were obtained by slow
diffusion of n-hexane into a CD
2
Cl
2
solution of 1a at T = 277 K.
/a centrosymmetric
Compound 1a crystallized in the P2
1
monoclinic space group as a racemic mixture, with one complex
molecule in the crystal asymmetric unit (Fig. 4). The pyridylben-
zamidine moiety in structure 1a chelates the metal center with
the iminic N3 and the pyridinic N1 nitrogen atoms with a square
planar coordination geometry as previously reported for the ana-
When 1a was reacted with silver hexafluorophosphate in the
presence of acetonitrile under the reaction conditions typically
applied for the halogen abstraction [37,38,18], the expected mono-
0
logous complex dichlorido-(N -(2,6-diisopropylphenyl)-4-methyl-
N-(pyridin-2-yl)benzenecarboximidamide)-palladium [22]. The
coordination geometry locked the pyridylbenzamidine ligand in
cationic complex [Pd(CH
white solid (Scheme 3a). Its NMR characterization in CD
3
)(CH
3
CN)(1)][PF
6
], 1b, was obtained as a
Cl solu-
2
2
an E,syn,s-cis conformation. For amidines the
defined as
CN = d(C–N) ꢂ d(C@N), was introduced to assess the
extent of delocalization within the ‘‘–N@C–N–’’ fragment
31,32]. For 1a the CN of 0.085 Å indicated a fully localized sys-
D
CN parameter,
tion was in agreement with the expected structure. Only one spe-
cies was present in solution, but unlike the neutral derivative 1a,
the NOE experiment demonstrated that this species was the trans
D
[
D
isomer, having the Pd–CH
(Fig. S19). Low temperature NMR experiments (up to 253 K) per-
formed on the CD Cl
solutions of both 1a and 1b as well as 1
NMR spectra recorded in CD NO probed evidence of the presence
of one isomer only for each complex.
3
moiety trans to the Pd–Nimino bond
tem and allowed us to locate proton H2 on N2 (Table 1), as con-
firmed by inspection of electron density maps during structure
refinement. An analogous situation was reported for the unbound
2
2
H
3
2
0
N-phenyl-N -2-pyridylbenzamidine ligand, where N2 atom was
fully protonated (
D
CN = 0.12 Å) [34]. The bite angle N1–Pd–N3 of
Ligand 1 was reacted with the palladium precursor 5 under
mild conditions leading to the isolation of a white solid that was
characterized both in solid state, by X-ray analysis, and in CD Cl
2 2
solution, by NMR spectroscopy.
0/+
H
H
N
0/+
1
Ph
N
Ph
In the H NMR spectrum the signals typical of ligand 1 and 1,5-
cyclooctadiene, both coordinated to palladium, and of the Pd–CH
3
N
N
N
N
fragment were observed, while no resonance due to acetonitrile
was present, thus indicating that the latter was replaced by the
pyridylbenzamidine ligand. Whereas at room temperature, the sig-
nals of coordinated 1 were sharp, those of cod were broad and their
decoalescence was reached at T = 253 K showing four peaks for
Pd
Pd
L
H C
L
CH3
3
cis
trans
ꢂ
2
both the CH and the CH protons, thus indicating that cod was
3
Fig. 3. The possible cis and trans isomers of 1a and 1b (L = Cl , CH CN).
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A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
Fig. 4. X-ray structure of 1a with: (a) labeling scheme for Pd coordination sphere (ball and stick representation); (b) the angle between the palladium coordination sphere
light gray) and the ligand 2,6-diisopropylphenyl substituent planes (yellow plane) is 86.5°. (For interpretation of the references to color in this figure legend, the reader is
(
referred to the web version of this article.)
Table 1
2.98 Å suggesting a weak interaction between these two atoms
Table S3). The coordination geometry locked the pyridylbenza-
midine ligand in an E,s-trans conformation. Bond lengths related
to the N2–C7–N3 fragment indicated a change in bond orders com-
pared to 1a, resulting in a tautomeric rearrangement that shifted
Selected coordination distances (Å) and angles (°) for 1a and 1c (values averaged on
the two ASU molecules (ASU = asymmetric unit)) complexes. 1,5-cyclooctadiene bond
lengths and angles are reported as averages of values in trans (Cc ) and cis (Cc ) to
(
T
C
palladium-methyl group. Complete table is available as Supplementary Materials
(Tables S2 and S3).
the proton from N2 to N3. The
DCN value of 0.043 Å, smaller than
Bond distances (Å)
in 1a, indicated a lower degree of localization of these two bonds.
The expected asymmetry for the coordinated cyclooctadiene was
shown [34,42], which led to longer bond lengths for Pd-cod car-
1a
1c
Pd–C
2.053(2)
Pd–C
2.035(3)
2.093(2)
2.179(3)
2.349(3)
1.349(3)
1.306(4)
0.043
Pd–N1
Pd–N3
Pd–Cl
C7–N3
C7–N2
2.130(1)
2.077(1)
2.339(1)
1.294(2)
1.379(2)
0.085
Pd–N1
Pd–Cc
Pd–Cc
C7–N3
C7–N2
D
CN
3 3
bons trans to Pd–CH group. The Pd–CH bond distance, itself,
C
T
was also affected by the nature of the donor atom trans to it, being
remarkably shorter in 1c than in 1a (2.035(3) versus 2.053(2) Å,
respectively).
D
CN
Another significant difference among the structures of 1a and
1c is spotted on the 2,6-diisopropylphenyl substituent, which is
perpendicular to the palladium coordination plane in the first case
and almost parallel to it in the other model (Figs. 4b and 6b). This
change in conformation is due to different steric hindrance of dip
in the two coordination modes found (square planar and distorted
square pyramid) and changes drastically the accessibility to the
metal center in the two complexes, acting as a ‘‘cap’’ on the top
face of the palladium square plane in 1c. Finally, in the latter the
plane containing the N2–C7–N3 fragment is almost orthogonal to
the square plane (Fig. 6c).
Bond angles (Å)
1a
1c
C
C–Pd–Cl
87.26(4)
93.24(6)
91.85(4)
87.87(5)
Cc –Pd–C
91.73(13)
93.58(10)
88.44(11)
86.46(12)
C
C–Pd–N3
N1–Pd–Cl
N1–Pd–N3
Cc –Pd–N1
N1–Pd–C
C
T
Cc –Pd–Cc
bonded to palladium in a nonsymmetric environment (Fig. 5). It is
worthwhile to mention that both H and one of the protons of a
3
CH
2
group of cod resonated at an unusually low frequency
By combining the solid state and the solution structures it
was possible to explain some features of the NMR spectrum:
the cod was coordinated in a nonsymmetric environment, thus
generating different signals for each of its protons; the H3 of
the pyridine ring fell in the shielding cone of the unsubstituted
phenyl ring, thus it resonated at a low chemical shift; one of
(
6.22 ppm and 1.09 ppm, respectively), suggesting that they expe-
rienced a shielding cone. In addition, both the CH and the methyl
groups of the isopropyl substituents, that at room temperature
originated one signal each, at T = 253 K led to two different reso-
nances, suggesting that the hindered rotation of dip was slowed
down at low temperature.
2
the protons of one of the CH groups of cod experienced the
On the basis of this NMR analysis the solid isolated from the
shielding cone of dip upon it, thus it also resonated at a low che-
mical shift; finally, the two aryl rings were far away from each
other and so at room temperature there was no restricted rota-
tion of the substituted aryl ring around the N-dip bond. More-
over, the signal of the proton bonded to the nitrogen atom
progressively shifted to low frequency going from the free ligand
(13.18 ppm), to 1a (7.44 ppm) to 1c (6.88 pm), thus suggesting
that its chemical shift values might be used as a probe for the
nature of the observed tautomer.
3 6
reaction of 1 with 5 was formulated as [Pd(cod)(CH )(1)][PF ], 1c
(
Scheme 3b), with the cod ligand involved in a dynamic process
at a rate intermediate on the NMR time scale, at room temperature.
The X-ray analysis of single crystals (colorless rods,
3
ꢁ
0.3ꢀ0.4ꢀ0.2 mm ), grown by slow diffusion of diethyl ether in a
2 2
CD Cl solution of 1c at T = 277 K, was in agreement with the struc-
ture depicted by the NMR data (Fig. 6).
Compound 1c crystallized in the orthorhombic space group
Pbc2
1
with two complex and two diethyl ether solvent molecules
When the reaction of 1 with 5 was followed over time by
reacting in the NMR tube a 10 mM dichloromethane solution of 5
with 1 equivalent of 1, at room temperature, the species formed
immediately after the mixing of the compounds was 1c, which
in the ASU. The two crystallographically independent molecules
in 1c crystals were almost perfectly superimposable. It was a race-
mic mixture with crystals affected by inversion twinning. The pal-
ladium adopted a heavily distorted square pyramid geometry, with
the pyridylbenzamidine ligand bonded to it in a monodentate fash-
ion through the pyridinic N1 atom that fell in the plane containing
the methyl and cyclooctadiene ligands, whereas the N2 atom
occupied an apical position with a Pdꢀ ꢀ ꢀN2 distance of about
3 3 6
over time slowly evolved to 1b, [Pd(CH )(CH CN)(1)][PF ] with
the dissociation of cod, the coordination of 1 as a bidentate ligand,
while the acetonitrile reentered in the first coordination sphere of
palladium (Figs. S25 and S26; Scheme 3b). However, even after
1 day 1c remained the main species in solution.
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A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
7
Pd-CH3
a’ + CH -iPr
3
CH -iPr
3
H4
CH-iPr
NH
H5
H3
H6
d’+a’
c
ad
b
b’
c’
Fig. 5. 1H NMR spectrum of 1c, in CD
Cl
, at 253 K, with the numbering scheme of the ligands. ÇDiethyl ether.
2
2
Fig. 6. X-ray structure of 1c with: (a) labeling scheme for Pd coordination sphere (ball and stick representation); (b) the angle between palladium coordination sphere (light
gray) and ligand 2,6-diisopropylphenyl substituent planes (yellow plane) is 16.5°; (c) the angle between palladium coordination sphere (light gray) and ligand planes
(
containing N2, C7 and N3 – red plane) is 82°.
When ligands 2 and 3 were reacted with [Pd(cod)(CH
3
)Cl] under
the solid in CDCl
temperature. They were analogous to those observed in the
NMR spectrum of 1c at 253 K indicating that the obtained product
was [Pd(cod)(CH )(2)][PF ], 2c (Fig. S27). However, in contrast to
3
, sharp signals were already observed at room
1
the well established reaction conditions [37,18], no reaction took
place (Scheme 3b). Instead, both ligands reacted with the palladi-
um precursor 5 but leading to different complexes.
H
3
6
From the reaction of 2 with 5, under the same conditions of
1c, when the reactivity of 2 with 5 was studied by in situ NMR
experiments, the immediate formation of 2c was observed, but
ligand 1, a white solid was isolated. In the 1H NMR spectrum of
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8
A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
no further transformation within 72 h occurred, thus indicating
that 2c was the only species formed and it did not transform to
from the NMR analysis were the integration of the doublets of
the methyl groups of the isopropyl substituents corresponding to
9 instead of the expected 12 protons (Fig. 8), and the presence of
the
Scheme 3b).
Upon slow diffusion of n-pentane into a CDCl
corresponding
[Pd(CH
3
)(CH
3
CN)(2)][PF
6
]
complex
(
a CH
2
fragment (Fig. S39), indicating that one of the two isopropyl
(CH)CH moiety, bonded
3
solution of 2c,
substituents was transformed into a CH
3
2
single crystals suitable for X-ray analysis were obtained (Figs. S1
and S2, Tables S2 and S3).
to palladium, forming the six-membered metallacycle, 3d
(Scheme 3b).
Compound 2c crystallized, as a racemic mixture, in the
monoclinic space group P2 /n with one complex molecule in the
1
The palladacycle 3d was obtained from 3b through the C–H
activation of one methyl group of one of the two isopropyl sub-
crystal asymmetric unit and with a centrosymmetric packing. Its
crystal structure was the same as that observed for 1c, with the
stituents that gave its proton to the Pd–CH
methane that left the palladium coordination sphere. The fourth
coordination site on the metal was occupied by the CH (CH)CH
2
3
fragment, originating
nitrogen-donor ligand in its tautomeric form (the
D
CN value was
3
the same found for 1c) and bonded to palladium in a monodentate
fashion through the pyridinic nitrogen atom. The cyclooctadiene
molecule was coordinated in a nonsymmetric way due to the trans
moiety. 3b had the proper geometry for this reaction, having the
acetonitrile, the more labile ligand, in cis to the substituted aryl
ring. The activation of C–H bond of the ortho isopropyl substituents
influence of the Pd–CH
Fig. S1).
As found for 1c, the structure of 2c in solid state corresponded
3
fragment trans to one C–C double bond
on the aryl ring of
[Pd(CH )(OEt )(iso-DAB)][BArF] having diethyl ether as labile
ligand (OEt = diethyl ether; iso-DAB = bis(2,6-diisopropyl-phe-
nyl)-1,4-diaza-1,3-butadiene); BArF = [B(3,5-(CF ]) [43],
a-diimines was observed by Brookhart in
(
3
2
2
with the structure in solution.
3 2 6 4 4
) C H )
Instead, no solid was isolated from the reaction of 3 with 5.
Thus, the latter reaction was studied by in situ NMR spectroscopy
while the corresponding acetonitrile derivative was found to be
stable for weeks in dichloromethane solution. In addition, the acti-
vation of C–H bonds of ligands having alkyl substituents in close
proximity to palladium was reported for Pd(II) complexes with
2,2 -bipyridines bearing alkyl groups in ortho position with respect
to the nitrogen-donor atoms [38,44,45].
by adding almost 1 equivalent of 3 to a 10 mM CD
2 2
Cl solution of
5, at room temperature. In the spectrum recorded immediately
0
after mixing the two compounds, the signals of a new palladium
species (I) together with those of free cyclooctadiene were
observed, while no signal due to the free pyridylbenzamidine 3
was detected (Fig. 7). The reaction was followed with time, and
in the spectrum recorded after 1 h the resonances of a second pal-
ladium species (II) were observed together with the singlet of
methane (0.21 ppm). After one day, the first species formed disap-
peared in favor of the second one that became the major product
2.3. Reactivity with ethylene of Pd(II) complexes 1b, 1c, 2c
Complexes 1b, 1c and 2c were tested as precatalysts for the
ethylene/methyl acrylate copolymerization by carrying out the
reaction in a glass pressure reactor, in 2,2,2-trifluoroethanol
(Figs. S31 and S32).
(
TFE), at T = 308 K and Pethylene = 2.5 bar, for 24 h. During the cata-
The first palladium complex formed (I) was identified as
lysis the formation of inactive palladium metal was evident. After
the work up of the catalytic mixture only traces of palladium inter-
mediates were obtained with no formation of the ethylene/MA
copolymer and/or of higher alkenes. Notably, the GC–MS analysis
of a small portion of the reaction mixture evidenced only the pres-
ence of very small amounts of butenes. This behavior is in contrast
to that reported in the literature for the relevant Ni(II) complexes,
[
2
3
Pd(CH
3
)(CH
73 K to slow down its transformation into the second species.
b was detected as the trans isomer, featured by the Pd–CH group
3 6
CN)(3)][PF ], 3b, by full NMR characterization at
3
trans to the Pd–Nimino bond (Scheme 3b).
The nature of the second palladium species formed was unam-
biguously identified by NMR spectroscopy. The main evidences
CH -iPr
3
c)
N-CH3
CH-iPr
CH -cod
2
free
CH -iPr(I)
3
Pd-CH (I)
3
N-CH (I)
3
Pd-NCCH3
CH-iPr(I)
b)
Pd-NCCH3
Pd-CH3
CH -cod
2
a)
Fig. 7. 1H NMR spectra in CD
Cl
2
, at 298 K of: (a) 5; (b) 5 + ꢁ1 equiv. of 3; (c) 3. Region of aliphatic protons.
2
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A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
9
H3
H5
H4
H6
3d
CH - cod
Pd-NCCH3
2
free
N-CH3
CH CN
3
free
CH3d
CH e
3
CH- codfree
CH4
CHb
CHc CHa
CHx
Fig. 8. 1H NMR spectrum in CD
2 2
Cl , at 298 K of 5 + ꢁ1 equiv. of 3 after 1 day.
CH -iPr
3
CH CN
3
free
H5
CH-iPr
6
4
3
H H H
NH
Fig. 9. 1H NMR spectrum in CD
Cl , at 283 K of 1b + ethylene at t = 20 min. dPropylene.
2 2
that were found to be catalysts for ethylene oligomerization [23].
On the other hand, the formation of butenes in trace amount has
been previously reported using Pd–carbene complexes having hin-
dered substituents [46].
thus demonstrating that the polar monomer had no tendency to
substitute acetonitrile in the palladium coordination sphere, likely
for steric reasons.
The reactivity with ethylene was studied by bubbling it into a
With the aim to understand the reason for the inactivity of the-
se complexes, the reactivity of 1b with the two comonomers, ethy-
lene and methyl acrylate, was studied by in situ NMR experiments.
2 2
CD Cl solution of 1b for 5 min, at room temperature. In the spec-
trum recorded immediately after the addition of the alkene, the
signals of the precatalyst were still present together with the
singlet of free acetonitrile and some new signals, including those
of propene (Fig. S40). After 20 min the formation of palladium met-
al was evident, but no signal due to the free ligand was observed
The addition of 2 equivalents of MA to a CD
room temperature, did not result in either any coordination or
migratory insertion of the polar monomer into the Pd–CH bond,
2 2
Cl solution of 1b, at
3
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1
0
A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
(
a)
(
(
b)
c)
Scheme 4. Proposed reaction mechanism for the reactivity of 1b with ethylene.
1
and the H NMR spectrum indicated that only traces of 1b were
equilibria among different isomers of the ligands, that might affect
their coordinating capability. Indeed, despite their similarity, the
three ligands show remarkably different coordinating behavior to
Pd(II): 1 is able to act both as a bidentate and a monodentate
ligand, in the latter mode it prefers its tautomeric form; 2 acts only
as a monodentate ligand through its tautomer, suggesting that the
introduction of one methyl group in position 6 of the pyridine ring
encumbers too much the first coordination sphere of palladium to
generate a stable square plane complex; 3 does not show the
predicted coordinating behavior leading to a palladacycle through
the C–H activation of one isopropyl group of dip, suggesting
that the tautomerism, that is not possible due to the introduction
present, together with a new palladium species identified as the
dicationic, bischelated complex [Pd(1) ][PF ] (Fig. 9). No reso-
2 6 2
nance due to higher alkenes, like butenes, hexenes, etc., was
observed.
On the basis of this NMR analysis it is possible to propose a
hypothesis for the mechanism of the reaction between 1b and
ethylene. According to the literature [1], the activation reaction
of the Pd–CH
ladium through the substitution reaction of acetonitrile; after-
wards its immediate migratory insertion into the Pd–CH bond
takes place, followed by the b-H elimination that leads to propene
and to the Pd-hydride intermediate (Scheme 4a).
3
precatalyst involves the alkene coordination to pal-
3
3
of the methyl group on the sp nitrogen, is of fundamental
The Pd-hydride species is the real catalyst; on it the coordina-
tion and migratory insertion of ethylene can take place starting
the growth of a polymeric chain. However, it also represents the
starting point for catalyst deactivation. It may dissociate the hydro-
gen atom as a proton, leading to a Pd(0) intermediate that, by dis-
sociation of the nitrogen-donor ligand, gives palladium black and
the free ligand (Scheme 4b) [47].
importance for the coordination to palladium.
Even though the structural features of the synthesized complex-
es, like two different N-donors, the six-membered palladacycle, the
bridging nitrogen atom with a methyl group, the orthogonal orien-
tation of the substituted aryl ring with respect to the square planar
plane, were in favor of a catalytic activity, none of the tested com-
plexes generated active species for either the ethylene/MA copoly-
merization or the ethylene homopolymerization. The in situ NMR
investigation probed evidence that neither methyl acrylate nor
ethylene were prone to give rise to their migratory insertion reac-
tion into the Pd–alkyl bond. In fact, the activation of the precatalyst
With the precatalysts herein reported, in place of the reso-
nances of the free ligand, those of the dicationic palladium com-
plex [Pd(1)
additional deactivation pathway is operative. For Pd(II) complexes
with -diimines applied as catalysts for both ethylene homopoly-
merization and ethylene/MA copolymerization, the formation of
inactive dicationic species [Pd(N-N) ][X] has been recently pro-
2
][PF
6
]
2
were evident, thus indicating that an
a
led to the Pd-hydride intermediate that evolved towards the inac-
0
tive dicationic complex [Pd(N-N )
incoming monomer molecule.
2 2
][X] instead of reacting with an
2
2
posed as a new deactivation pathway [16]. The mechanism is
unclear, but it has been suggested to be a bimolecular process with
one molecule of the palladium complex abstracting the ligand from
another molecule to produce the dicationic species and palladium
black. In the complexes under investigation, it is reasonable to
assume that the proton and the pyridylbenzamidine ligand
dissociated from the Pd–H intermediate attack, respectively, the
4
. Experimental
4.1. General procedures and materials
All complex manipulations were performed using standard Sch-
lenk techniques under argon. Anhydrous dichloromethane was
obtained by distillation over CaH under argon. Ligands 1 [23]
and 2 [22], [Pd(CH )Cl(cod)] 4, the neutral Pd complexes and the
acetonitrile derivative were synthesized according to literature
procedures [37]. [Pd(OAc) ] was a donation from Engelhard Italia
3
Pd–CH bond, yielding methane, and the two so created vacant
coordination sites of another precatalyst molecule (Scheme 4c).
2
3
3
. Conclusions
2
and used as received. Ethylene (purity P99.9%) supplied by SIAD
and methyl acrylate (99.9%, with 0.02% of hydroquinone mono-
methyl ether) supplied by Aldrich were used as received. Deuterat-
ed solvents, Cambridge Isotope Laboratories, Inc., were stored as
recommended by CIL. All the other reagents and solvents were pur-
chased from Sigma–Aldrich and used without further purification
for synthetic, spectroscopic and catalytic purposes.
In this paper we have investigated the coordination chemistry
to Pd(II) of three pyridylbenzamidine ligands, 1–3, differing in
the presence of a methyl group either on the pyridine ring or on
3
the sp nitrogen atom bridging the two moieties of the molecule.
Ligand 3 is reported herein for the first time. An in-depth NMR
study in solution probed evidence of the presence of several
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A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
11
3
NMR spectra of ligands, complexes, and of the in situ reactivity
investigations were recorded on a Varian 500 spectrometer at the
following frequencies: 500 MHz ( H) and 125.68 MHz ( C); the
6.61 (d, 1H, H ), 3.69 (s, 3H, N-CH
3
), 2.97 (m, 2H, CH-iPr), 1.11 (d,
6H, CH
3
-iPr), 0.90 (d, 6H, CH
C NMR (125.68 MHz, CD
(3-Cpy), 117.44 (5-Cpy), 28.99 (CH-iPr), 28.68 (CH-iPr), 24.20 (Py-
CH ), 25.52 (CH -iPr), 24.38 (CH -iPr), 21.85 (CH -iPr).
ESI-MS 394.2 [M+Na] , 372.1 [M+H] , 264.1 [MꢂPyNH] .
3
-iPr).
1
13
13
2
Cl , 298 K) d = 138.32 (4-Cpy), 123.82
2
resonances are reported in ppm (d) and referenced to the residual
1
13
solvent peak versus Si(CH
3
)
4
: CDCl
3
at d 7.26 ( H) and d 77.0 ( C),
3
3
3
3
1
13
1
+
+
+
2 2 3 2
CD Cl at d 5.32 ( H) and d 54.0 ( C), CD NO at 4.33 ( H). NMR
experiments were performed employing the automatic software
parameters; in the case of NOESY experiments a mixing time of
4
4
2
.3. Synthesis of Pd(II) complexes
5
00 ms was used.
IR spectra were recorded in Nujol on a Perkin Elmer System
000 FT-IR. Elemental analyses were performed in the analytical
.3.1. Synthesis of [Pd(cod)(CH
To a solution of 0.26 mmol (70 mg) of [Pd(cod)(CH
mL anhydrous dichloromethane in a Schlenk tube under inert
(1.2 equiv., 80.5 mg, 0.32 mmol)
3 3 6
)(CH CN)][PF ], 5
2
3
)Cl], 4, in
laboratories of Department of Chemistry of University of Bologna.
GC-MS analysis of the possible products of cooligomerization were
performed using a Agilent 7890 GC equipped with a 5975C mass
spectrometer. The separation was achieved with a DB-225 ms col-
atmosphere, a solution of AgPF
6
in 2 mL of anhydrous acetonitrile was added under stirring. The
reaction mixture was protected from light and stirred for 1 h, then
it was filtered over Celite ; the desired solid was precipitated from
the mother liquor upon addition of diethyl ether, then filtered,
washed thoroughly with diethyl ether and dried under vacuum.
Yield: 60%.
umn (30 m, ID 0.25 mm, 0.25
analysis, a small portion (200
tion experiments was diluted with MeOH (1 mL).
l
m film) using He as carrier. For the
Ò
lL) of solution after cooligomeriza-
ꢂ1
ꢂ
6
IR:
6
mmax = 844.08 cm (PF ). Anal. Calc. for PdC11H18NPF : C,
1.79; H, 4.16; N, 3.27. Found: C, 31.58; H, 4.12; N, 3.13%.
4.2. Synthesis of ligands 1–3
3
1
0
H NMR (500 MHz, CD
.43 (broad, 2H, CH-cod), 2.68 (broad, 4H, CH
H, CH -cod), 2.42 (s, 3H, Pd-NCCH ), 1.21 (s, 3H, Pd-CH
C NMR (500 MHz, CDCl , 298 K): d = 123.96 (CH-cod), 104.53
(CH-cod), 30.78 (CH -cod), 27.72 (CH -cod), 13.15 (Pd-CH ), 2.97
(Pd-NCCH ).
2
Cl
2
, 298 K) d = 5.88 (broad, 2H, CH-cod),
-cod), 2.55 (broad,
).
N-(2,6-diisopropyl)phenyl-N -2-pyridylbenzamidine 1 was pre-
5
4
2
pared according to the literature [23] and was obtained as a yellow
2
3
3
solid in a 45% yield.
1
3
1
3
2 2
H NMR (500 MHz, CD Cl , 298 K) d = 13.18 (s, 1H, NH), 8.29 (d,
6
4
3
2
2
3
1
7
H, H ), 7.70 (m, 1H, H ), 7.40 (d, 1H, Ph), 7.26 (m, 1H, H ), 7.20–
.08 (m and br signals, 7H, ArH), 6.95 (m, 1H, H ), [3.25 (m, CH-
5
3
3
iPr), 3.04 (m, CH-iPr) total area 2H], [1.13 (d, CH -iPr), 0.94 (d,
CH -iPr), total area 12H].
3
4
.3.2. Synthesis of [Pd(1)(CH
To a stirred solution of [Pd(CH
3
)Cl], 1a
)Cl(cod)] (0.36 mmol) in dichlor-
1
3
C NMR (125.68 MHz, CD
2
Cl
2
, 298 K) d = 29.12 (CH-iPr), 28.74
-iPr).
3
(
CH-iPr), 24.85 (CH -iPr), 21.69 (CH
3
3
omethane (1 mL), a solution of ligand 1 (1.1 equiv.) in dichloro-
methane (2 mL) was added. After 5 h of stirring at room
temperature, the reaction mixture was concentrated and the pro-
duct precipitated upon addition of cold diethyl ether. Yield: 74%.
+
+
+
ESI-MS 381.1 [MH+Na] , 358.1 [MH] , 264.0 [MꢂPyNH] .
0
N-(2,6-diisopropyl)phenyl-N -(6-methyl)-2-pyridylbenza-
midine 2 was prepared according to the literature [22] and was
obtained as a yellow solid in a 50% yield.
Anal. Calc. for PdC25
30
H N
3
Cl: C, 58.37; H, 5.98; N, 7.97. Found: C,
1
2 2
H NMR (500 MHz, CD Cl , 298 K) d = 13.28 (br, 1H, NH), 7.62
5
8.24; H, 6.02; N, 7.76%.
4
(
bt, 1H, H ), 7.40–7.15 (m and br signals, 7H, ArH) 7.09 (m, 1H,
1
6
H NMR (500 MHz, CD
2
Cl
2
, 298 K) d = 9.44 (d, 1H, H ), 7.78 (m,
3
5
H ), 6.84 (d, 1H, H ), [3.28 (m, CH-iPr), 3.04 (m, CH-iPr) total area
H], 2.46 (s and br, 3H, Py-CH ), [1.14 (2d, CH -iPr), 0.96 (d, CH
iPr), total area 12H].
4
5
1
7
H, H ), 7.44 (s, 1H, NH), 7.35–7.25 (aromatic H), 7.15 (m, 1H, H ),
2
3
3
3
-
3
.1–6.95 (aromatic H), 6.94 (d, 1H, H ), 3.33 (m, 2H, CH-iPr), 1.39
(
d, 6H, CH
3
-iPr), 0.82 (d, 6H, CH
C NMR (125.68 MHz, CD Cl
4-Cpy), 127.77 (5-Cpy), 114.20 (3-Cpy), 28.30 (CH-iPr), 24.18 (CH
-iPr), 2.13 (Pd-CH ).
H NMR (500 MHz, CD NO , 298 K) d = 9.35 (d, 1H, H ) 8.25 (s,
H, NH), 7.88 (m, 1H, H ), 7.40–7.28 (aromatic H), 7.17 (m, 1H,
3
-iPr), 0.14 (s, 3H, Pd-CH
3
).
1
3
C NMR (125.68 MHz, CD
3-Cpy), 117.44 (5-Cpy), 28.99 (CH-iPr), 28.68 (CH-iPr), 24.20
Py-CH ), 25.52 (CH -iPr), 24.38 (CH -iPr), 21.85 (CH -iPr).
ESI-MS 394.1 [M+Na] , 372.1 [M+H] , 264.0 [MꢂPy(CH
2
Cl
2
, 298 K) d = 138.32 (4-Cpy), 123.82
13
2
2
, 298 K) d = 150.3 (6-Cpy), 139.57
(
(
(
3
-
3
3
3
3
iPr), 23.56 (CH
3
3
+
+
)NH]+.
3
1
6
3
4
2
0
0
N-(2,6-diisopropyl)phenyl-N -methyl-N -2-pyridylbenzamidine
1
3.
5
3
H ), 7.12 (d, 1H, H ), 7.07–7.04 (aromatic H), 3.41 (m, 2H, CH-
iPr), 1.40 (d, 6H, CH -iPr), 0.90 (d, 6H, CH -iPr), 0.05 (s, 3H, Pd-CH ).
2
-(N-methylamino)pyridine was prepared according to the lit-
3
3
3
erature [28].
To a 10 mL THF solution of the intermediate imidoylchloride
[
2
23] (0.6 g, 2.0 mmol) and 2-(N-methylamino)pyridine (0.21 g,
.0 mmol), NaH (0.16 g, 4.0 mmol, 60% dispersion in mineral oil)
3 3 6
4.3.3. Synthesis of [Pd(1)(CH )(CH CN)][PF ], 1b
To a solution of 0.14 mmol (70 mg) of 1a in 8 mL anhydrous
was added portionwise at ꢂ10 °C under inert atmosphere. After
the addition was complete, the mixture was left stirring to reach
the room temperature, then heated to reflux for 7 h. The course
of the reaction was monitored by TLC (petroleum ether/ethyl acet-
dichloromethane in a Schlenk tube under inert atmosphere, a solu-
tion of AgPF (1.2 equiv., 41.7 mg, 0.16 mmol) in 1.5 mL of anhy-
6
drous acetonitrile was added under stirring. The reaction mixture
was protected from light and stirred for 1 h, then filtered over
Ò
ate 8:2). After consumption of the amine, CH
20 mL) and the solution extracted with dil. HCl (10 mL).
The organic layer was dried (Na SO ), and the solvent removed
under reduced pressure to give an oil which was purified on col-
umn removing also the mineral oil (eluent: petroleum ether/ethyl
acetate 98/2).
2
Cl
2
was added
Celite ; the desired solid was precipitated from the mother liquor
(
upon addition of diethyl ether, then filtered, washed thoroughly
with diethyl ether and dried under vacuum. Yield: 60%.
2
4
ꢂ1
ꢂ
6
IR:
33 4 6
mmax = 844.08 cm (PF ). Anal. Calc. for PdC27H N PF : C,
48.77; H, 5.00; N, 8.23. Found: C, 48.61; H, 5.11; N, 8.19%.
1
6
2 2
H NMR (500 MHz, CD Cl , 298 K) d = 8.30 (d, 1H, H ), 8.08 (s,
4
3
Ligand 3 was obtained as a yellow solid in a 30% yield.
1H, NH), 7.96 (m, 1H, H ), 7.45 (aromatic H), 7.35 (m, 1H, H ),
7.25 (aromatic H), 7.18 (m, 1H, H ), 7.15–7.00 (aromatic H), 3.16
1
6
5
H NMR (500 MHz, CD
2
Cl
2
, 263 K) d = 8.27 (dd, 1H, H ), 7.30 (td,
4
1
H, H ), 7.09–6.99 (m, 5H, aromatic H), 6.93–6.91 (m, 2H, aromatic
H), 6.88–6.85 (m, 1H, aromatic H), 6.82–6.80 (m, broad, 1H, H ),
3 3
(m, 2H, CH-iPr), 1.67 (s, 3H, Pd-NCCH ), 1.41 (d, 6H, CH -iPr),
5
0.84 (m, 9H, CH -iPr + Pd-CH ).
3
3
Please cite this article in press as: A. Tognon et al., Inorg. Chim. Acta (2015), http://dx.doi.org/10.1016/j.ica.2015.01.049

1
2
A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
13C NMR (125.68 MHz, CD
4-Cpy), 129.69 (3-Cpy), 120.93 (5-Cpy), 28.86 (CH-iPr), 25.04 (CH
iPr), 23.55 (CH -iPr), 2.69 (Pd-NCCH ), 2.21 (Pd-CH ).
H NMR (500 MHz, CD NO , 298 K) d = 8.48 (s, 1H, NH), 8.41 (d,
Cl
, 298 K) d = 150.07 (6-Cpy), 141.60
c
a
2
2
3
(s, 3H, Pd-NCCH ), 2.16 (m, 1H, CH ), 1.85 (m, 1H, CH ), 1.46 (d,
e
d
d
(
3
-
3H, CH
3
), 0.69 (d, 3H, CH
C NMR (125.68 MHz, CD
(4-Cpy), 116.07 (3-Cpy), 123.13 (5-Cpy) 41.10 (CH ), 3.61 (Pd-
3
), 0.64 (d, 3H, CH
3
).
1
3
3
3
3
2
Cl , 298 K) d = 148.71 (6-Cpy), 141.62
2
1
b
3
2
6
4
3
3 3 3 3
NCCH ), 24.08 (CH ), 20.23 (CH ).
), 18.13 (CHe
d
d
1
7
3
H, H ), 8.02 (m, 1H, H ), 7.47–7.37 (aromatic H), 7.30 (m, 1H, H ),
.25 (m, 1H, H ), 7.17 (aromatic H), 3.28 (m, 2H, CH-iPr), 1.79 (s,
5
H, Pd-NCCH
3
), 1.46 (d, 6H, CH
).
3
-iPr), 0.90 (d, 6H, CH
3
-iPr), 0.84
4.5. Crystallographic data of complexes 1a, 1c and 2c
(
s, 3H, Pd-CH
3
Data collections were performed at the X-ray diffraction beam-
line (XRD1) of the Elettra Synchrotron, Trieste (Italy), with a Pilatus
2 M image plate detector. Complete datasets were collected at 100 K
(nitrogen stream supplied through an Oxford Cryostream 700) with
a monochromatic wavelength of 0.700 Å through the rotating
0
4
.3.4. Synthesis of [Pd(cod)(CH
0
3
)(N-N )][PF
6
], 1c, 2c
.18 mmol (75 mg) of 5 were dissolved in 8 mL anhydrous
dichloromethane in a Schlenk tube under inert atmosphere; a solu-
tion of 1.2 equiv. of ligand (0.20 mmol; ligand 1 for 1c, ligand 2 for
2
c) in 3 mL of anhydrous dichloromethane was added under stir-
3
crystal method. The crystals of 1a ([Pd(1)(CH )Cl]), 1c ([Pd(1)
ring. The reaction mixture was protected from light and stirred
for 40 min, then the desired solid was precipitated upon addition
of diethyl ether, after which it was filtered, washed thoroughly
with diethyl ether and dried under vacuum. Average yield: 80%.
(cod)(CH )][PF ]) and 2c ([Pd(2)(cod)(CH )][PF ]) were dipped in
3
6
3
6
N-paratone and mounted on the goniometer head with a nylon loop.
The diffraction data were indexed, integrated and scaled using XDS
[48]. A complete dataset for the monoclinic crystal forms of com-
pound 1a was obtained by merging two different data collections
done on the same crystal with different orientations. The structures
were solved by direct methods using SIR2011 [49], Fourier analyzed
ꢂ1
ꢂ
6
1
c – IR:
42 3 6
mmax = 844.08 cm (PF ). Anal. Calc. for PdC33H N PF :
C, 55.24; H, 5.98; N, 5.64. Found: C, 55.36; H, 6.01; N, 5.61%.
1
6
H NMR (500 MHz, CD
2 2
Cl , 253 K) d = 8.22 (d, 1H, H ), 7.55–7.33
4
5
2
(
6
aromatic H), 7.34 (m, 1H, H ), 6.94 (m, 1H, H ), 6.88 (s, 1H, NH),
.22 (d, 1H, H ), 5.53 (broad, 1H, CH -cod), 5.20 (broad, 1H, CH -
cod), 5.12 (broad, 1H, CH -cod), 4.83 (broad, 1H, CH -cod), 3.18
and refined by the full-matrix least-squares based on F implement-
3
c
a
ed in SHELXL-2013 [50]. The Coot program was used for modeling
[51]. Anisotropic thermal motion modeling was then applied to all
atoms and hydrogen atoms were included at calculated positions
with isotropic Ufactors = 1.2 Ueq or Ufactors = 1.5 Ueq for methyl
d
b
0
2
c
(
broad, 2H, CH-iPr), 2.74 (broad, 2H, CH
-cod), 2.28 (broad, 3H, CH
-iPr), 1.26 (m,
-iPr), 0.94 (s, 3H, Pd-CH
2
0
0
0
d +a
b
-
cod), 1.91 (broad, 2H, CH
2
-cod), 1.34 (d, 3H, CH
3
0
a
6
H, CH
3
-iPr), 1.09 (m, 4H, CH
2
-cod + CH
Cl
CH -cod), 122.59 (CH -cod), 119.63 (3-Cpy), 118.79 (5-Cpy), 107.10
3
3
).
groups. The structure of 1c has been refined as an inversion twin,
1
3
ꢂ
C NMR (125.68 MHz, CD
2
2
, 253 K) d = 148.77 (6-Cpy), 122.96
and disorder on PF
6
counterion has been modeled. Essential crystal
d
a
(
(
(
(
and refinement data (Table S1), together with selected bond
distances and angles, are reported in the manuscript and in
Supplementary Materials (Tables 1, S2 and S3).
0
c
c
b
CH -cod), 100.41 (CH -cod), 32.20–32.16 (CH
2
-cod), 29.07
-cod), 27.20–26.71
-iPr), 9.98 (Pd-CH ).
). Anal. Calc. % for PdC34
: C, 53.43; H, 5.98; N, 5.33. Found: C, 53.47; H, 5.90; N, 5.29%.
0
0
d
-cod), 28.74 (CHb
CH-iPr), 28.96 (CH
2
2
0
a
CH
2
-cod), 25.18–22.06 (CH
3
ꢂ1
3
ꢂ
6
2
c – IR:
m
max = 844.08 cm (PF
H
44
N
3
-
4.6. Ethylene/methyl acrylate copolymerization
PF
6
1
H NMR (500 MHz, CDCl
3
, 298 K) d = 7.53–7.28 (m, 8H, Ph), 7.28
All catalytic experiments were carried out in a Büchi ‘‘tinyclave’’
reactor equipped with an interchangeable 50 mL glass vessel. The
vessel was loaded with the desired complex (21 lmol), TFE
(21 mL) and methyl acrylate (1.13 mL). The reactor was then
placed in a preheated oil bath, connected to the ethylene tank
and pressurized. The reaction mixture was stirred at constant tem-
perature and pressure. After 24 h, the reactor was cooled to room
4
5
3
(
(
4
1H, H ), 6.87 (d, 1H, H ), 6.75 (br, 1H, NH), 6.05 (d, 1H, H ), 5.65
m, 1H, CH -cod), 5.30 (m, 1H, CH -cod), 5.10 (m, 1H, CH -cod),
.78 (m, 1H, CH -cod), 3.28 (m, 1H, CH-iPr (I)), 3.17 (m, 1H, CH-
iPr (II)), 2.86 (m, 4H, Py-CH
c
a
d
b
0
2
0
c
c
3
+ CH
-cod), 2.73 (m, 1H, CH
2
-cod),
-cod), 1.40 (d, 3H,
-iPr (I)+
0
0
0
2
0
d +b
a +b
2
CH
CH
.33 (m, 3H, CH
2
-cod), 1.90 (m, 2H, CH
3
-iPr (II)), 1.30 (d, 6H, CH
3
-iPr (I + II)), 1.15 (d, 3H, CH
).
C NMR (125.68 MHz, CDCl
3
0
a
2
1
-cod), 0.99 (s, 3H, Pd-CH
3
temperature and vented. An aliquot (200
lL) of the reaction mix-
3
3
, 298 K) d = 139.42 (4-Cpy), 131.71–
ture was withdrawn and diluted in CH OH (1 mL) for GC–MS ana-
3
a
d
1
1
(
2
24.07 (Ph), 123.00 (CH -cod), 122.52 (CH -cod), 119.05 (5-Cpy),
lyses. The reaction mixture was poured in a 50 mL round flask,
together with the dichloromethane (3 ꢃ 2 mL) used to wash the
glass vessel. Volatiles were removed under reduced pressure and
the residual oil was dried at constant weight and analyzed by
NMR spectroscopy.
c
b
16.19 (3-Cpy), 106.82 (CH -cod), 99.82 (CH -cod), 32.81 + 32.67
0
0
b
c
CH
2
-cod), 29.39 (CH-iPr (II)), 28.91 (CH-iPr (I)), 28.51 (CH
2
-cod),
0
2
0
0
2
0
a +b
-cod), 27.46 (CHa
-iPr (I + II)), 24.07 (CH
).
8.16 (CH
cod), 25.35 + 22.27 (CH
-cod), 26.23 (CH
3
-Py), 26.07 (CH
3
-iPr (II)), 22.30
2
-
3
(
CH
3
-iPr (I)), 8.62 (Pd-CH
3
4.7. In situ reactivity of complex 1b with ethylene
3 3 6
4.4. In situ reactivity of ligand 3 with [Pd(cod)(CH )(CH CN)][PF ], 5
In a 10 mM solution of complex 1b in CD Cl in NMR tube, ethy-
2
2
1
To a 10 mM solution of complex 5 (0.008 mmol, 3.32 mg) in
.80 mL CD Cl in NMR tube, 1 equiv. of ligand 3 (0.008 mmol,
.97 mg) was added at room temperature. The reaction was fol-
lene was bubbled for 5 min, then a H NMR spectrum was recorded
0
2
2
2
at 298 K immediately after mixing and after 20 min.
1
2 2
H NMR (500 MHz, CD Cl , 298 K) d = 9.67 (s, 1H, NH), 8.35 (d,
6
4
3
5
lowed with time via NMR.
1H, H ), 8.01 (t, 1H, H ), 7.72 (d, 1H, H ), 7.53 (t, 1H, H ), 2.91 (sept,
1
6
3
b – H NMR (500 MHz, CD
2
Cl
2
, 273 K) d = 8.33 (m, 1H, H ), 8.11
3 3
2H, CH-iPr), 1.12 (d, 6H, CH -iPr), 0.89 (d, 6H, CH -iPr).
4
3
5
(
m, 1H, H ), 7.5–6.8 (aromatic H), 7.41 (d, 1H, H ), 7.29 (m, 1H, H ),
3
1
.23 (s, 3H, N-CH
.41 (d, 6H, CH -iPr), 1.00 (d, 6H, CH
C NMR (125.68 MHz, CD Cl , 273 K) d = 151.04 (6-Cpy), 141.78
4-Cpy), 115.53 (3-Cpy), 41.85 (N-CH ), 28.58 (CH-iPr), 2.92 (Pd-
NCCH ), 24.74 (CH -iPr), 23.28 (CH -iPr), 2.85 (Pd-CH ).
d – H NMR (500 MHz, CD Cl , 298 K) d = 8.32 (m, 1H, H ), 8.13
m, 1H, H ), 7.62 (d, 1H, H ), 7.45 (m, 1H, H ), 7.4–6.7 (aromatic H),
3
), 2.83 (sept, 2H, CH-iPr), 1.82 (s, 3H, Pd-NCCH
3
),
Acknowledgements
3
3
-iPr), 0.84 (s, 3H, Pd-CH ).
3
1
3
2
2
This work was financially supported by Università degli Studi di
Trieste – Finanziamento di Ateneo per progetti di ricerca scientifica
– FRA 2011, FRA 2012 and FRA 2013. Fondazione CRTrieste and
Fondazione Benefica Kathleen Foreman Casali are gratefully
acknowledged for the generous donations of a Varian 500 MHz
spectrometer and a Büchi tinyclave glass reactor, respectively.
(
3
3
3
3
3
1
6
3
2
2
4
3
5
(
x
b
3
3
.36 (s, 3H, N-CH ), 3.12 (m, 1H, CH ), 2.46 (m, 1H, CH ), 2.35
Please cite this article in press as: A. Tognon et al., Inorg. Chim. Acta (2015), http://dx.doi.org/10.1016/j.ica.2015.01.049

A. Tognon et al. / Inorganica Chimica Acta xxx (2015) xxx–xxx
13
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Appendix A. Supplementary material
3
CCDC 1015153, 1015009 and 1015008 contain the supplemen-
tary crystallographic data for complexes 1a, 1c and 2c, respective-
ly. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif. Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.ica.
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Please cite this article in press as: A. Tognon et al., Inorg. Chim. Acta (2015), http://dx.doi.org/10.1016/j.ica.2015.01.049