Nickel(II) and Palladium(II) Complexes Bearing an Unsymmetrical Pyrrole-Based PNN Pincer and Their Norbornene Polymerization Behaviors versus the Symmetrical NNN and PNP Pincers

Article abstract of DOI:10.1021/acs.inorgchem.8b03562

Unsymmetrical pincers have been shown to be better than the corresponding symmetrical pincers in several catalysis reactions. A new unsymmetrical PNN propincer, 2-(3,5-dimethylpyrazolylmethyl)-5-(diphenylphosphinomethyl)pyrrole (1), was synthesized from pyrrole through Mannich bases in a good yield. In addition, the new byproduct 2-(3,5-dimethylpyrazolylmethyl)-5-(dimethylaminomethyl)-N-(hydroxymethyl)pyrrole was also isolated. The reaction of 1 with [PdCl₂(PhCN)₂] and Et₃N in toluene yielded [PdCl{C₄H₂N-2-(CH₂Me₂pz)-5-(CH₂PPh₂)-κ³P,N,N}] (2). The analogous reaction between 1 and [NiCl₂(DME)] or NiX₂ (X = Br, I) in the presence of NEt₃ in acetonitrile afforded [NiX{C₄H₂N-2-(CH₂Me₂pz)-5-(CH₂PPh₂)-κ³P,N,N}] (3; X = Cl, Br, I). All complexes were structurally characterized. The norbornene polymerization behaviors of the unsymmetrical pincer complexes 2 and 3 in the presence of MMAO or EtAlCl₂ were compared with those of the symmetrical pincer complexes chloro[2,5-bis(3,5-dimethylpyrazolylmethyl)pyrrolido]palladium(II) (NNN), chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]palladium(II), and chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]nickel(II) (PNP) at different temperatures. The PNN and NNN complexes exhibited far greater activity on the order of 10⁷ g of PNB/mol/h, with quantitative yields in some cases, in comparison to the PNP pincer palladium and nickel complexes. This trend was also supported by the ⁱPr group substituted PNP nickel and palladium pincer complexes. These polymerization behaviors are explained using steric crowding around the metal atom with the support of NMR studies and suggested that the activity increases as the N_pyrazole donor increases. Polymers were characterized by ¹H NMR, IR, TGA, and powder XRD methods.

Full text of DOI:10.1021/acs.inorgchem.8b03562

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Nickel(II) and Palladium(II) Complexes Bearing an Unsymmetrical
Pyrrole-Based PNN Pincer and Their Norbornene Polymerization
Behaviors versus the Symmetrical NNN and PNP Pincers
Sanghamitra Das, Vasudevan Subramaniyan, and Ganesan Mani*
Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India 721 302
S
* Supporting Information
ABSTRACT: Unsymmetrical pincers have been shown to be
better than the corresponding symmetrical pincers in several
catalysis reactions. A new unsymmetrical PNN propincer, 2-
(3,5-dimethylpyrazolylmethyl)-5-(diphenylphosphinomethyl)-
pyrrole (1), was synthesized from pyrrole through Mannich
bases in a good yield. In addition, the new byproduct 2-(3,5-
dimethylpyrazolylmethyl)-5-(dimethylaminomethyl)-N-
(hydroxymethyl)pyrrole was also isolated. The reaction of 1
with [PdCl2(PhCN)2] and Et3N in toluene yielded [PdCl-
{C4H2N-2-(CH2Me2pz)-5-(CH2PPh2)-κ3P,N,N}] (2). The
analogous reaction between 1 and [NiCl2(DME)] or NiX2 (X = Br, I) in the presence of NEt3 in acetonitrile afforded
[NiX{C4H2N-2-(CH2Me2pz)-5-(CH2PPh2)-κ3P,N,N}] (3; X = Cl, Br, I). All complexes were structurally characterized. The
norbornene polymerization behaviors of the unsymmetrical pincer complexes 2 and 3 in the presence of MMAO or EtAlCl2
were compared with those of the symmetrical pincer complexes chloro[2,5-bis(3,5-dimethylpyrazolylmethyl)pyrrolido]-
palladium(II) (NNN), chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]palladium(II), and chloro[2,5-bis-
(diphenylphosphinomethyl)pyrrolido]nickel(II) (PNP) at different temperatures. The PNN and NNN complexes exhibited
far greater activity on the order of 107 g of PNB/mol/h, with quantitative yields in some cases, in comparison to the PNP pincer
i
palladium and nickel complexes. This trend was also supported by the Pr group substituted PNP nickel and palladium pincer
complexes. These polymerization behaviors are explained using steric crowding around the metal atom with the support of
NMR studies and suggested that the activity increases as the Npyrazole donor increases. Polymers were characterized by 1H NMR,
IR, TGA, and powder XRD methods.
reported that the benzoquinoline-based NCP pincer iridium
complex catalyzes more efficiently the transfer hydrogenation
of alkene using ethanol in comparison to the [(PCP)Ir(H)-
(Cl)] complex.10 Shubina and co-workers have described the
superior catalytic behavior of a PCN pincer iridium complex
containing a pyrazole N-donor relative to the PCP pincer in
the amine−borane dehydrogenation reaction.11
We have been working on pyrrole-based symmetrical
NNN12 and PNP13 pincers having pyrazole and diphenyl-
phosphine units on either side of the pyrrole ring. Since these
reports, its been our goal to synthesize unsymmetrical pincer
with the pyrrole ring. Remarkably, the pyrrole-based pincer
ligands have become famous owing to their abilities in
stabilizing a variety of metal complexes14 and their catalytic
properties.15 Herein, we report a follow-up study detailing the
synthesis of the first pyrrole-based unsymmetrical pincer
containing mixed donor atoms (N and P) derived from the
diphenylphosphine and pyrazole units and its Pd(II) and
Ni(II) complexes. We also report their efficiencies at different
temperatures in catalyzing norbornene (NB) to polynorbor-
INTRODUCTION
■
Pincer ligands often coordinate to metal atoms in a tridentate
fashion and are classified on the basis of the ligating atoms
such as PNP, PCP, and NCN among others.1 When the donor
atoms are different, typically soft and hard donors, they are
usually called unsymmetrical pincers such as PCN, PCO, and
PCS.2 In addition, pincers containing the same donor atoms
but differ by substituents or having different groups on the
periphery of ligand are also called unsymmetrical.3 The main
attractive feature of a unsymmetrical pincer is the hemilability:
that is, its ability to showcase the bidentate binding mode with
a spectator arm.4 This coordination flexibility is one of the key
factors by which metal complexes can be judged to be potential
precatalysts and to exhibit unique reactivities.5 This, therefore,
became a driving force, and a variety of unsymmetrical pincers,
their metal complexes, and catalytic properties have been
studied.6 Milstein and co-workers have demonstrated the
better catalytic activity of the unsymmetrical PNN pincer in
the dehydrogenation of primary alcohols to esters and H27 and
in the hydrogenation of esters to alcohols. Gebbink, Szabo,
8
́
and co-workers have shown higher turnover numbers for the
PCS unsymmetrical pincer in comparison to PCP and SCS
ligands in aldol reactions.9 Huang and co-workers have
Received: December 21, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b03562
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Scheme 1. Synthesis of the Unsymmetrical PNN Pincer 6 and New Byproducts Formed in the Course of Its Synthesis
Scheme 2. Synthesis of Unsymmetrical PNN Pincer Pd(II) and Ni(II) Complexes
nene (PNB) to demonstrate that the unsymmetrical PNN
pincer is better than the PNP pincer.
spectrum. The 31P NMR spectrum showed a singlet at δ 49.9
ppm, which is shifted downfield in comparison to δ 33.8 ppm
for the symmetrical pincer palladium complex [PdCl{C4H2N-
2,5-(CH2PPh2)2-κ3PNP}].13 The coordination-induced chem-
ical shift change is 65.3 ppm in comparison to the free ligand
(δ −15.4 ppm), which is larger than that of 50.1 ppm of the
symmetrical pincer palladium complex.
RESULTS AND DISCUSSION
■
Synthesis of the Unsymmetrical PNN Pincer. The
synthesis of the unsymmetrical PNN pincer 6 involves first the
preparation of the mono-Mannich base 2 from pyrrole
followed by the synthesis of 2-(3,5-dimethylpyrazolylmethyl)-
pyrrole (3) and 2-(dimethylaminomethyl)-5-(3,5-
dimethylpyrazolylmethyl)pyrrole (4). Following the synthetic
procedure of 2,5-bis(diphenylphosphinomethyl)pyrrole
(PNP),13 treatment of 4 with 1 equiv of Ph2PH in toluene
under reflux conditions afforded 6 in 60% yield after column
chromatographic separation (Scheme 1). In addition, the
Mannich reaction of 3 also gave the N-hydroxymethyl
derivative 5 as a minor product along with 4. The yield of 5
was improved to 76% when 3 was treated with 2 equiv of
HCHO. 4−6 are new compounds and were characterized by
spectroscopic and HRMS methods. The methylene groups
present in 4−6 feature strikingly distinct peaks in their 1H and
13C NMR spectra. The 31P NMR spectrum of 6 in CDCl3
showed a singlet at δ −15.4 ppm, which is slightly shifted
downfield by 0.9 ppm in comparison to the symmetrical
diphosphine ligand 2,5-bis(diphenylphosphinomethyl)pyrrole.
Further, treatment of 6 with aqueous H2O2 in toluene afforded
the oxidized compound 7, which gives a singlet at δ 31.2 ppm
in the 31P NMR spectrum recorded in CDCl3.
The κ3-PNN bonding mode is confirmed by the X-ray
structure of 8 given in Figure 1 along with selected bond
lengths and angles. The geometry around the palladium atom
is a distorted square plane formed by two different pallada-
cycles, five- and six-membered, containing P and N donors
trans to each other. Interestingly, the twist angle between the
pyrrole ring and the palladium square planes is 26.2°, which
Metalation and Structural Characterizations. The
equimolar reaction of 6, [PdCl2(PhCN)2], and Et3N in
toluene afforded the new air-stable unsymmetrical pincer
palladium complex 8 in good yield (Scheme 2). The same
reaction in the absence of Et3N also gave complex 8, as shown
by the 31P NMR spectrum, in which the product is formed by
the deprotonation of the pyrrolic NH with concomitant
Figure 1. ORTEP diagram of 8 with 50% probability ellipsoids. H
atoms are omitted for clarity. Selected bond lengths (Å) and angles
(deg): P1−Pd1 2.2274(18), N3−Pd1 1.983(5), N1−Pd1 2.133(5),
Cl1−Pd1 2.3174(18); N1−Pd1−N3 87.4(2), N1−Pd1−P1
164.87(15), N3−Pd1−P1 79.73(15), N1−Pd1−Cl1 95.74(15),
N3−Pd1−Cl1 172.84(16), P1−Pd1−Cl1 97.96(7).
1
formation of HCl. In the H NMR spectrum of 8 in CDCl3,
one of the methylene groups appeared as a doublet at δ 3.65
ppm owing to the coupling with the phosphorus atom
(2J(P,H) = 13.2 Hz), which was absent in the free ligand
B
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lies between the values found for the symmetrical NNN
(38.9°)12 and PNP (13.9°)13 pincer palladium complexes. The
P1−Pd−N1 angle of 164.87(15)° also remains between the
values found for the symmetrical complexes (172.7(3)°
(NNN)12 and 162.3(1)° (PNP)).13 The same trend is seen
in the reported analogous Pd(II) complexes. The P−Pd−N
angle in the unsymmetrical [(PCN)PdCl] complex containing
one phosphinite and one pyrazole donor is 166.6°,16 which is
greater than the value (∼160.3°) found in the symmetrical
PCP-bis(phosphinite) palladium complexes17 but is lower than
the values found in NCN-bis(pyrazolylmethyl)benzene
(177.3°)18 and bis(dimethylpyrazolylmethyl)pyridine
(175.8°)19 Pd(II) complexes. In addition, the Pd−Npyrazole
bond distance of 2.133(5) Å is slightly longer than that
(2.041(5) Å) found in the NNN pincer palladium complex.
Conversely, the Pd−P distance of 2.2274(18) Å is slightly
shorter than that (2.3074(16) Å) of the PNP pincer complex,
indicating a stronger bond in 8. This bond length difference
could be due to the strong trans effect of P in comparison to
N.4a,20 The pyrrolide N−Pd distance of 1.983(5) Å is in the
range reported for the symmetric complexes [(PNP)PdCl]
(1.981(4) Å),13 [(NNN)PdCl] (1.958(7) Å),12 and [2,5-
bis(N-cyclohexyl-1-(p-tolyl)methanimine)pyrrolide)PdCl]
(1.879(3) Å).21 It is close to that of an isoindolide nitrogen
coordinated palladium complex (1.972(3) Å).22 However, it is
shorter than the amide N−Pd distance found in the analogous
pincer palladium(II) complex [PdCl{N(SiMe2CH2PPh2)2}]
(2.063(2) Å),23 indicating the strong bond formed by the
pyrrolide nitrogen atom.
Figure 2. ORTEP diagram of 10 with 50% probability ellipsoids. H
atoms and crystal lattice diethyl ether are omitted for clarity. Selected
bond lengths (Å) and angles (deg): P1−Ni1 2.1613(11), N3−Ni1
1.861(3), N1−Ni1 1.969(3), Br1−Ni1 2.3215(7); N1−Ni1−N3
91.12(13), N1−Ni1−P1 166.06(11), N3−Ni1−P1 82.77(10), N1−
Ni1−Br1 97.50(10), N3−Ni1−Br1 162.47(10), P1−Ni1−Br1
91.85(4).
11. In addition, both twist angles are larger than those (13.8°)
found in [(PNP)NiX] (X = Br, I) complexes containing the
symmetric PNP pincer ligand.13 Interestingly, Ni−P distances
(2.1613(11) Å (10) and 2.1647(19) Å (11)) are shorter than
the distances (2.253(1)−2.1955(9) Å) found in the nickel(II)
pincer complexes of type [(PNP)NiX] (X = halides)
containing symmetrical PNP ligands,13,14c,f,15a,24 owing to the
strong trans effect of the P atom in comparison to the N atom.
Further, the pyrrolide N−Ni distances (1.861(3) Å (10) and
1.847(6) Å (11)) are shorter than the amide N−Ni distances
found in the analogous pincer nickel(II) complexes (1.924(2)
Å in [NiCl{N(SiMe2CH2PPh2)2}]23 and 1.895(3) and
1.912(5) Å in [NiX{N(o-C6H4PPh2)2}] (X = Cl, Br)),25
indicating the strong bond formed by the pyrrolide N atom.
Vinyl Polymerization of Norbornene. The vinyl
norbornene polymer has several applications and has a cost
advantage over related materials.26 Hence, a variety of
precatalyst metal complexes have been developed, among
which palladium27 and nickel28 complexes have been shown to
possess the highest activities29 and even catalyze polymer-
ization of polar group substituted norbornene and copoly-
merization reactions.30 Remarkably, the polymerization of
polar norbornenes by the cationic allyl palladium complex in
the absence of any cocatalyst has also been reported.31
Recently, we reported the highly active palladium complexes of
o-bis(3,5-dimethylpyrazolylmethyl)phenolate for the vinyl
norbornene polymerization reactions.32
The norbornene polymerization behaviors of the unsym-
metrical PNN pincer complexes 8−11 were studied and
compared with those of symmetrical NNN and PNP pincer
palladium and nickel complexes to demonstrate the difference
caused by a change in the donor atoms. The polymerization
reaction with 1 μmol of the precatalyst 8 in the presence of a
1000 molar excess of MMAO in toluene yielded 28% of PNB
(Table 1, entry 1). A dramatic improvement in the yield of
PNB (97%) was obtained when the amount of catalyst 8 was
increased to 2 μmol (entry 2). In addition, further improved
activity was found when the concentration of NB was doubled
under these conditions (entry 3). In contrast, when the
The analogous 1:1 reaction between 6 and [NiCl2(DME)]
or NiX2 (X = Br, I) in acetonitrile in the presence of NEt3
under reflux conditions afforded the pincer nickel complexes
9−11 in good yields (Scheme 2). These complexes are
diamagnetic, air stable, and soluble in toluene, CH2Cl2, CHCl3,
ether, THF, and acetone and have been characterized by both
1
spectroscopic and X-ray methods. Their H spectra featured a
doublet around δ 3.4 ppm owing to the coupling with the
phosphorus atom for the phosphorus-CH2 protons, whereas
the pyrazole-CH2 protons appeared as a singlet. The 31P NMR
spectra of 9−11 displayed singlets at δ 30.6, 35.0, and 44.6
ppm, respectively, which are, unlike the palladium complex 8,
close to the values (δ 30.6, 35.0, and 42.6 ppm) shown by their
corresponding symmetrical PNP nickel complexes.13
The X-ray structure of 10 is given in Figure 2 together with
selected bond distances and angles, and that of 11 is given in
Figure S36 in the Supporting Information. The X-ray structure
revealed the typical pincer structures containing two fused
rings created by the pyrazole and phosphorus arms. The
geometry around the nickel atom in both complexes 10 and 11
is not a perfect square plane; it is distorted toward a
tetrahedron, as shown by their trans angles ranging from
160.6(2) to 166.2(2)°. Thus, the nickel atom is located 0.041
and 0.064 Å above the nickel square plane defined by the
coordinated atoms in 10 and 11, respectively, owing to the
steric conflict between the pyrazole methyl and halide atoms.
In both structures, the pyrazole arm lies well above the pyrrole
ring mean plane with an N1−N2−C6−C7 torsion angle of
51.3° (10), while the Ph2P arm remains slightly below the
pyrrole ring plane with a P1−C11−C10−N3 torsion angle of
−24.0° (10), owing to the steric hindrance among the
phosphorus phenyl, pyrazole methyl, and the halide groups.
The twist angle between the mean pyrrole ring and the nickel
square planes in 10 is 26.1°, higher than that (27.2°) found in
C
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Table 1. Polymerization of Norbornene Using the
Palladium Complex 8 in the Presence of Cocatalyst
MMAO
Table 2. Polymerization of Norbornene Using the Nickel
Complexes 9−11 in the Presence of MMAO
entry cat. (μmol) [Al]/[cat.] PNB (g) yield (%) activity (×107)
a
a
b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
10 (1.0)
10 (1.0)
10 (1.0)
10 (1.0)
10 (1.0)
10 (1.0)
10 (2.0)
10 (3.0)
10 (1.0)
10 (0.5)
10 (1.0)
9 (1.0)
500
1000
1000
1000
1500
2000
500
1.543
1.614
1.085
0.881
1.537
1.21
1.497
1.486
0.749
1.452
0.495
1.270
1.266
1.491
0.863
77
81
54
44
77
60
75
74
37
73
25
63
63
74
43
9.3
9.7
6.5
5.3
9.3
7.3
4.7
3.0
4.5
17.5
3.0
7.6
15.3
9.0
10.4
c
d
NB
(g)
PNB
(g)
yield
(%)
activity
d
entry 8 (μmol)
[Al]/[8]
(×107)
333
200
b
1
2
3
4
5
6
7
8
9
1.0
2.0
2.0
2.0
2.0
1.0
2.0
2.0
2.0
2.0
1
1
2
2
2
2
2
3
2
2
1000
1000
1000
500
0.284
0.968
1.679
0.254
0.036
0.370
1.97
2.256
0.727
0.264
28
97
84
13
2
19
99
75
36
13
0.1
0.6
1.7
0.8
0.1
2.2
5.9
7.0
2.2
0.8
1000
1000
1000
1000
1000
1000
b
b
b
b
c
e
9 (0.5)
11 (1.0)
11 (0.5)
250
1000
1000
1000
500
c
a
c
All reactions were carried out with 2 g of NB in 5 mL of CH2Cl2 at
b
room temperature for 1 min unless specified otherwise. In units of g
c
c
d
e
of PNB mol−1 h−1. 10 mL of CH2Cl2. 20 mL of CH2Cl2. In
toluene.
c
10
250
a
All reactions were carried out at room temperature for 1 min, except
entries 1−3, for which the reaction times were 15, 5, and 3 min,
Table 3. Polymerization of Norbornene Using the
b
c
respectively. Reaction carried out in 5 mL of toluene. Reaction
Palladium Complex 8 and Nickel Complexes 9−11 with
d
carried out in 5 mL of CH2Cl2. In units of g of PNB mol−1 h−1.
a
EtAlCl2 as a Cocatalyst
d
entry
cat. (μmol)
8 (2.0)
8 (2.0)
9 (1.0)
9 (1.0)
10 (1.0)
10 (1.0)
11 (1.0)
11 (1.0)
[(NNN)PdCl] (2.0)
[(PNP)PdCl] (2.0)
[(PNP)NiBr] (1.0)
[(iPrPNP)NiBr] (1.0)
PNB (g) yield (%) activity (×107)
aluminum ratio was decreased, the yield of polymer was also
decreased, which could be due to a decrease in chain transfer
rate (vide infra). When the polymerization was carried out in
CH2Cl2, higher yield and activity were obtained (Table 1,
entries 6−10). The reactions in CH2Cl2 showed the same
trend of decrease in activity with a decrease in the aluminum
content, consistent with the case for the toluene reactions.
Notably, the reaction in CH2Cl2 gave a near-quantitative yield
(99%) of polynorbronene with an activity of 5.9 × 107 g of
PNB mol−1 h−1 (entry 7). This is greater than the 84% yield
with activity 1.7 × 107 g of PNB mol−1 h−1 from the toluene
reaction.
b
1
0.893
1.446
1.418
0.897
1.872
0.565
1.876
0.864
1.976
0.471
1.174
0.972
45
72
71
45
94
28
94
43
99
24
59
49
2.7
4.4
8.5
5.4
11.3
3.4
11.3
5.2
5.9
1.4
7.1
c
2
b
3
c
4
b
5
c
6
b
7
c
8
c
9
c
e
e
e
10
b
11
b
12
5.8
The polymerization behavior of the nickel complexes 9−11
is summarized in Table 2. The reaction with the aluminum to
complex 10 mole ratio of 500 in CH2Cl2 gave a 77% yield of
PNB with an activity of 9.3 × 107 g of PNB mol−1 h−1 (entry
1). When the aluminum ratio is increased to 1000, both the
yield and activity are increased slightly (entry 2). However, a
further increase in the aluminum content (ratios of 1500 and
2000) resulted in decreased yields and activities. In addition,
the yields and activities decreased with an increased amount of
complex 10 (entries 7 and 8) or a lower amount of aluminum
([Al]/[M] = 200) (entry 9) or lower concentration of
norbornene (entries 3 and 4). Gratifyingly, an excellent activity
of 1.7 × 108 g of PNB mol−1 h−1 (entry 10) was obtained with
0.5 μmol of complex 10, indicating its highly active nature.
Conversely, the polymerization reaction in toluene gave poor
yield and activity, as observed with palladium pincer complex
8. In addition, as shown in Table 2, although similar
polymerization behaviors were obtained with the chloride 9
and iodide complex 11, the bromide complex 10 remains a
better catalyst in terms of greater yields and activities.
a
All reactions were carried out with 2 g of NB and [MMAO]/[cat.] =
b
c
1000 at room temperature for 1 min. In 5 mL of CH2Cl2. In 5 mL
d
e
of toluene. In units of g of PNB mol−1 h−1. gel-like solid dried in
open air only.
and norbornene in CH2Cl2, the solution turned black, probably
due to palladium metal particles, and hence gave lower activity
in comparison to the MMAO reactions discussed above.
Conversely, in the cases of nickel complexes, the addition of
EtAlCl2 resulted in a color change to yellow and yielded
polymer with >90% monomer conversion in CH2Cl2 (entries 5
and 7). Both the palladium- and nickel-catalyzed reactions in
toluene afforded relatively lower yields. In addition, the activity
shown by [(NNN)PdCl] is greater than those of PNN and
PNP palladium complexes (vide infra).
Having observed excellent activities of the unsymmetrical
pincer Pd(II) and Ni(II) complexes, we became interested in
studying the effect of the pincer framework on the norbornene
polymerization reactions. The catalytic performances of the
symmetrical pincer complexes chloro[2,5-bis(3,5-
dimethylpyrazolylmethyl)pyrrolido]palladium(II) ([(NNN)-
PdCl]), chloro[2,5-bis(diphenylphosphinomethyl)pyrrolido]-
palladium(II) ([(PNP)PdCl]), chloro[2,5-bis-
Using the optimized conditions obtained with MMAO
reactions, the polymerization reactions with EtAlCl2 as a
cocatalyst were carried out, and the results are given in Table 3.
When EtAlCl2 was added to the mixture of palladium complex
D
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Table 4. Comparison of the Catalytic Norbornene Polymerization Performances of the Symmetrical (NNN and PNP) as Well
a
as Unsymmetrical (PNN) Pyrrole-Based Pincer Palladium(II) and Nickel(II) Complexes
b
entry
cat.
solvent
time (min)
quantity of cat. (μmol)
PNB (g)
yield (%)
activity (×107)
1
2
3
4
5
6
7
8
[(NNN)PdCl]
[(NNN)PdCl]
[(NNN)PdCl]
[(PNN)PdCl]
[(PNN)PdCl]
[(PNN)PdCl]
[(PNP)PdCl]
[(iPrPNP)PdCl]
[(PNN)NiCl]
[(PNP)NiCl]
[(PNN)NiBr]
[(PNP)NiBr]
[(PNP)NiBr]
[(iPrPNP)NiBr]
[(iPrPNP)NiBr]
[(PNN)NiI]
[(PNP)NiI]
CH2Cl2
CH2Cl2
toluene
CH2Cl2
CH2Cl2
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
1
1
1
1
1
3
1
1
1
1
1
1
10
1
10
1
1
1
10
1
1.0
2.0
2.0
1.0
2.0
2.0
2.0
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.808
1.998
1.572
0.370
1.97
90
100
79
19
99
84
14
11
64
16
81
24
42
9
38
75
34
27
34
2
10.9
6.0
4.7
2.2
5.9
1.7
0.8
0.7
7.6
1.9
9.7
2.8
0.5
1.1
0.5
0.9
4.1
3.3
0.4
0.5
0.3
0.9
2.5
2.1
1.679
0.281
0.221
1.270
0.314
1.614
0.473
0.849
0.189
0.768
1.491
0.689
0.542
0.680
0.087
0.451
0.156
0.408
0.354
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
[NiCl2(DME)]
[NiCl2(DME)]
NiBr2
NiBr2
PdCl2
10
1
1
22
8
20
[PdCl2(PhCN)2]
[PdCl2(COD)]
1
18
a
b
All reactions were carried out with 2 g of NB in 5 mL of solvent with [MMAO]/[cat.] = 1000 at room temperature. In units of g of PNB mol−1
h−1.
(diphenylphosphinomethyl)pyrrolido]nickel(II) ([(PNP)-
NiCl]), bromo[2,5-bis(diphenylphosphinomethyl)pyrrolido]-
nickel(II) ([(PNP)NiBr]), and iodo[2,5-bis-
(diphenylphosphinomethyl)pyrrolido]nickel(II) ([(PNP)-
NiI]) together with those of the pincer complexes
[(iPrPNP)PdCl]24a and [(iPrPNP)NiBr],14f containing the
diisopropylphosphinomethyl group , are given in Table 4.
The quantitative conversion of norbornene to polynorbor-
nene was observed with 2 μmol of the symmetrical NNN
pincer palladium complex in CH2Cl2 (entry 2, Table 4).
Although a similar yield (99%) was obtained with the
unsymmetrical PNN palladium complex 8, the yield of
polynorbornene was very low (14%) when the symmetrical
PNP palladium complex was used under the same conditions.
The activity difference is then attributed to the number of
pyrazole N donors present in the pincer complexes. The
activity decreases as the pyrazole ring is replaced with a PPh2
group in the NNN pincer ligand. This decreasing trend was
also found with the corresponding nickel pincer complexes.
The yields of polymer (64, 81, and 75%) given by the
unsymmetrical nickel complexes 9−11, respectively, are greater
than those (16, 24, and 34%) given by the symmetrical PNP
nickel pincer complexes. Hence, the activity decrease with the
PNP- or PPh2-substituted pincer metal complexes can be
explained by steric crowding around the active metal atom.
The substituents attached to the phosphorus atom have a
three-dimensional orientation, which effectively shields the
active metal center; as a result, the sterically encumbered
norbornene monomer finds it difficult to bind for subsequent
transformations, resulting in the decreased activity. On the
other hand, the pyrazole rings in either the NNN or PNN
ligand are oriented in one direction only; hence, this creates
more space for the monomer binding and facilitates chain
growths. Therefore, greater activities and yields were obtained
with the pyrazole-substituted pincer metal complexes. In order
to confirm the role of steric factors in causing the activity
difference, we synthesized the reported complexes chloro[2,5-
bis(diisopropylphosphinomethyl)pyrrolido]palladium(II)
( [ ( i P r P N P ) P d C l ] ) a n d b r o m o [ 2 , 5 - b i s -
(diisopropylphosphinomethyl)pyrrolido]nickel(II)
([(iPrPNP)NiCl]) and carried out the polymerization reac-
tions. It is found that the yield (11%) and activity (0.7 × 107 g
of PNB mol−1 h−1) of the isopropyl-substituted pincer
palladium complex (entry 8) are relatively lower than the
phenyl-substituted PNP pincer palladium complex activity (0.8
× 107 g of PNB mol−1 h−1) and yield (14%) (entry 7). This is
further supported by the nickel complex reactions. The phenyl-
substituted pincer nickel complex gave a 23% yield of PNB,
i
whereas the Pr group substituted nickel complex yielded a
negligible amount of polymer. This activity difference is
i
conceivable, given the Tolman cone angle (θ) of Pr (114°)
being greater than the phenyl group’s cone angle (105°).33 In
addition, a similar trend was also observed for EtAlCl2
cocatalyst reactions (see Table 3). Furthermore, polymer-
izations using [NiCl2(DME)], NiBr2, PdCl2, and
[PdCl2(PhCN)2] in CH2Cl2 gave poor yields of polynorbor-
nene under the same conditions, indicating the important role
of the pincer ligands in stabilizing active metal centers involved
in the course of polymerization cycles.
Effect of Temperature on NB Polymerization. The
effect of temperature on NB polymerization is summarized in
Table 5. In the cases of [(PNN)PdCl]- and [(NNN)PdCl]-
catalyzed reactions, the yield and activity of PNB decreased as
the temperature was increased from room temperature to 60
E
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
lower than those given by PNN and NNN pincer complexes at
room temperature. In these palladium-catalyzed high-temper-
ature reactions, the solution turned black, probably indicating
decomposition of the catalyst and formation of palladium
metal particles, which accounts for the lower yield and activity
for the complexes containing the more N donating PNN and
NNN pincers. In contrast to this, the more P donating PNP
pincer palladium complex was able to stabilize adequately the
active species formed at 60 °C to give improved activity. In the
case of [(PNN)NiBr], the yield and activity of PNB increased
as the temperature was increased from 0 to 100 °C for the
given aluminum mole ratio (Figure 3B). The same trend was
observed when the aluminum content was increased from a
100 to 1000 molar ratio for a given temperature (100 °C). In
addition, the [(PNP)NiBr] pincer also showed an increased
activity with temperature, and its activity at 100 °C is quite
close to that of the unsymmetrical PNN complex, as given in
Figure 3B.
NMR Study and Rationalization of Polymerization.
These observations can be explained if the PNN and NNN
pincers are regarded as sterically less encumbered in
comparison to the PNP pincer and are readily able to form
an equilibrium between the precatalyst and MMAO to give an
activated complex formed via the alkyl−halide exchange
reaction, as indicated in Chart 1. A similar equilibrium was
suggested to explain the increased activity in the presence of a
large excess of MAO or with an increase in temperature for NB
or olefin polymerization reactions.34
Table 5. NB Polymerization at Varied Temperatures
b
entry [Al]/[cat.] PNB (g) yield (%) activity (×106)
T (°C)
[cat.] = [(PNN)PdCl] (2.0 μmol)
1
2
3
4
5
6
1000
1000
1000
500
400
400
1.218
1.755
0.578
0.397
0.390
0.147
61
88
29
20
19
7
2.4
3.5
1.2
0.8
0.8
0.3
0
25
60
60
60
100
[cat.] = [(NNN)PdCl] (2.0 μmol)
7
8
9
10
1000
1000
1000
1000
1.608
1.998
0.384
0.114
80
>99
19
3.2
4.0
0.8
0.2
0
25
60
6
100
[cat.] = [(PNP)PdCl] (2.0 μmol)
11
12
13
14
1000
1000
1000
1000
0.214
0.382
0.820
0.566
11
19
41
28
0.4
0.8
1.6
1.1
0
25
60
100
[cat.] = [(PNN)NiBr] (1.0 μmol)
15
16
17
18
19
20
21
22
23
24
1000
1000
1000
1000
100
100
200
300
400
0.515
0.695
0.974
1.335
0.405
0.559
0.688
0.950
1.231
1.310
26
35
49
67
20
28
34
47
61
65
2.1
2.8
3.9
5.3
0.4
0.6
2.7
3.8
4.9
5.2
0
25
60
100
60
100
100
100
100
100
The 31P NMR spectrum of a mixture of 10 and MMAO (10
or 100 equiv) in toluene-d8 showed only one peak around δ
38.0 ppm, and when the amount of MMAO is 4 equiv, it
appeared at δ 33.5 ppm. This is in the range found for 10,
[(PNP)NiBr], and its alkyl derivatives.14c Further, its 1H NMR
spectrum featured a doublet around δ 3 ppm for the P-bound
methylene protons with the coupling constant J(PH) = 11.6
Hz similar to the 1H NMR spectrum of 10, in addition to shifts
of other protons in the PNN ligand. Furthermore, the 31P
NMR spectrum of a mixture of PNN and MMAO (10 equiv)
in toluene-d8 showed a peak at δ −11.2 ppm, which did not
appear for the above mixtures. This suggests that the nickel
pincer complex did not decompose in the presence of MMAO
500
[cat.] = [(PNP)NiBr] (1.0 μmol)
25
26
27
28
29
30
1000
1000
1000
1000
400
0.318
0.451
0.895
1.286
0.389
0.881
16
22
45
64
19
44
1.3
1.8
3.6
5.1
1.5
3.5
0
25
60
100
60
100
400
a
All reactions were carried out with 2 g of NB in 5 mL of toluene for
b
15 min. In units of g of PNB mol−1 h−1.
and 100 °C for the given aluminum ratio (400 or 1000 molar
excess), as shown in Figure 3A. Conversely, [(PNP)PdCl]
exhibited increased yield and activity at 60 °C in comparison
to its room-temperature activity. However, the yield is
decreased considerably at 100 °C and its best activity is still
1
to give the free PNN. Similar H and 31P NMR spectra were
obtained for a mixture of 8 and MMAO (10 equiv). These
strongly suggest that the chelation bonding mode of PNN
Figure 3. Effect of temperature on polymerization behaviors of NNN, PNN, and PNP pincer Pd and Ni complexes with an [Al]/[M] ratio of 1000.
F
DOI: 10.1021/acs.inorgchem.8b03562
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Chart 1. Proposed Activation Equilibrium of Precatalyst in the Presence of MMAO To Give an Active Catalyst
a
The symbol □ denotes a vacant site for NB binding and subsequent transformations.
using the phosphine arm (κ2P,Npyrrole) is retained in the
presence of an excess amount of MMAO.
The 31P NMR spectrum of a mixture of 10, MMAO (100
equiv), and NB (2 equiv) in toluene-d8 showed two peaks, δ
37.7 (s) and −4.9 (br s) ppm. A similar spectrum was obtained
even after the same mixture was heated at 60 °C. Hence, the
new broad peak around −5 ppm could be due to a new nickel
complex containing both NB and PNN pincer formed in the
course of catalysis. Its 1H NMR spectrum showed mainly peaks
due to MMAO protons, as it is in a very large excess amount
and the bound NB protons could not be identified. Conversely,
the 31P NMR spectrum of the palladium complex 8 in the
presence of 100 equiv of MMAO and 2 equiv of NB showed
more peaks, indicating decomposition. Although it is difficult
to suggest the coordination of the pyrazole N to the aluminum
atom in MMAO from the change in the chemical shift value of
PNN, the M−Npyrazole bond distances in both Ni and Pd
complexes are longer than those found in the symmetrical
complexes and the pyrazole arm forms a six-membered ring,
indicating their labilities in 8−11. Therefore, the M−Npyrazole
bond can also break in the presence of a strong Lewis acid
(Al3+ in MMAO) and the activated complex decomposes to
give lower yield and activity at high temperatures for palladium
complexes.
In addition, the polymerization of NB in CH2Cl2 in the
absence of MMAO using the in situ generated [(PNN)Ni-
(Me)] or [(PNP)Ni(Me)] from the reaction between
[(PNN)NiBr] or [(PNP)NiBr] and MeMgBr in THF,
respectively, gave no precipitate of polymer. Hence, MMAO
is required to activate the nickel or palladium complex to form
an active metal complex having the structure [(PNN-
κP,N)Ni(R)] (Chart 1) containing one vacant site for NB
binding and subsequent transformations.
Figure 4. TGA profiles of the polynorbornene obtained with
[EtAlCl2]/[(PNN)PdCl] (red line), [EtAlCl2]/[(PNN)NiCl] (light
blue line), [EtAlCl2]/[(PNN)NiBr] (black line), [MMAO]/[(PNN)-
PdCl] (magenta line), and [MMAO]/[(PNN)NiCl] (dark blue line)
in the presence of a 1000 molar excess of cocatalyst in CH2Cl2.
about 58% and 36%, respectively (Figure 4). The powder XRD
pattern of polymers obtained with MMAO reactions featured
two broad peaks at 2θ = 9−11° and 18−19°, whereas that
formed from EtAlCl2 gave one merged broad peak around
17.5°, indicating the noncrystalline nature as reported in the
literature (see Figures S45−48 in the Supporting Informa-
tion).27b,c
CONCLUSIONS
■
In conclusion, 6 years after the report of the symmetrical NNN
and PNP pincers, the first unsymmetrical pyrrole-based PNN
pincer was synthesized in an analogous manner but in a
stepwise fashion. Their palladium(II) and nickel(II) complexes
were synthesized and structurally characterized. A systematic
study of norbornene vinyl polymerization was carried out using
both symmetrical and unsymmetrical pyrrole-based pincer
metal complexes to check their catalytic efficiencies at different
temperatures. The PNN pincer palladium and nickel
complexes gave very high yields with activities in the range
of 107 g of PNB mol−1 h−1, which remains close to those of the
NNN pincer complex. Interestingly, the symmetrical N-donor
pincer complex showed a far greater activity and quantitative
yield in comparison to the symmetrical P-donor pincer
complexes. This suggests that, as the number of N-donors
increases, the yield and activity increase, following the order
NNN > PNN > PNP at room temperature. At higher
temperatures, these Pd and Ni complexes behaved differently.
At 60 °C, the palladium complex showed the reverse order:
NNN < PNN < PNP. The observed variation in the yield and
Polymer Characterization. All MMAO-cocatalyzed re-
actions gave powdery materials which are insoluble in several
solvents such as THF, CH2Cl2, CH3CN, MeOH, and
CH3COOEt. However, polymers formed by nickel catalysts
1
are slightly soluble in CDCl3 and their H NMR spectra
showed no peak around 5.5 ppm. Therefore, polymers were
characterized by ATR, TGA, and powder XRD methods. ATR
spectra showed no band in the region 1620−1680 cm−1 and a
band at 941 cm−1 for polymers formed at room temperature as
well as at high temperatures, suggesting the vinyl type
polynorbornene. In addition, TGA showed one major
degradation temperature at around 421 °C with a mass loss
of 89% (Figure 4). The same type of TGA was observed for
the vinyl polynorbornene reported previously by us32 and
others.35 Conversely, polymers obtained with EtAlCl2 reactions
were of a gel type material that became very hard upon drying
under vacuum and then in open air. This polymer is also
insoluble in the aforementioned solvents. Its TGA showed two
degradation points around 122 and 420 °C with mass losses of
G
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Inorganic Chemistry
Article
and the residue was loaded onto a silica gel column chromatograph.
Elution using methanol/chloroform (10/90 v/v) afforded 4 as a
colorless oily compound after the solvents were removed under
vacuum (7.100 g, 30.56 mmol, 99%). 1H NMR (CDCl3, 400 MHz): δ
2.18 (s, 6H, CH3), 2.20 (s, 3H, CH3), 2.21 (s, 3H, CH3), 3.34(s, 2H,
CH2), 5.07 (s, 2H, CH2), 5.76 (s, 1H, pyrazole CH), 5.88 (d, J(H,H)
= 5.6, 1H, pyrrole β-CH), 5.98 (d, J(H,H) = 4.8, 1H, pyrrole β-CH),
8.87 (br s, 1H, NH). 13C NMR (CDCl3, 100.6 MHz): δ 11.5, 13.1,
45.0, 46.4, 56.8, 105.8, 106.7, 107.7, 126.7, 130.1, 139.0, 148.0. ATR-
IR (cm−1): ν 3197 (m), 3123 (m), 3100 (m), 3000 (m), 2984 (m),
2964 (m), 2942 (m), 2922 (m), 2851 (m), 2809 (s), 2780 (m), 2761
(s), 2718 (m), 1657 (w), 1592 (w), 1547 (s), 1460 (vs), 1437 (vs),
1418 (vs), 1388 (s), 1356 (vs), 1304 (m), 1272 (s), 1256 (s), 1214
(w), 1197 (s), 1172 (m), 1142 (m), 1123 (w), 1097 (w), 1039 (s),
1020 (vs), 994 (s), 955 (w), 935 (w), 848 (vs), 825 (s), 777 (vs), 744
(s), 722 (w), 683(s), 667 (w), 634 (m), 621 (w), 469 (w). HRMS
(+ESI): calcd m/z for [M + H+] C13H21N4 233.1761, found
233.1775.
activity of different pincer frameworks is explained using steric
factors with the support of Tolman cone angles and NMR
studies. Studies of other olefin and copolymerization reactions
using these pincer metal complexes are in progress.
EXPERIMENTAL SECTION
■
All reactions and manipulations were carried out using standard
Schlenk-line techniques under a nitrogen atmosphere or nitrogen-
filled glovebox. Petroleum ether (bp 40−60 °C) and other solvents
were distilled under an N2 atmosphere according to the standard
procedures. Norbornene, modified methylaluminoxane (MMAO-12,
7 wt % in toluene), and EtAlCl2 (25 wt % in toluene) were purchased
from Aldrich and used as received, except for norbornene, which was
distilled over sodium under vacuum. Other chemicals were obtained
from commercial sources and used as received. All solvents were dried
and distilled under an N2 atmosphere using standard procedures. 2-
(Dimethylaminomethyl)pyrrole, 3 6 [PdCl2 (COD)],3 7
[PdCl2(PhCN)2],38 [NiCl2(DME)],39 Ph2PH,40 chloro[2,5-bis-
(diisopropylphosphinomethyl)pyrrolido]palladium(II),24a and
bromo[2,5-bis(diisopropylphosphinomethyl)pyrrolido]nickel(II)14f
were prepared by following the reported procedures. Other chemicals
were obtained from commercial sources and were used without
Synthesis of 2-(Dimethylaminomethyl)-5-(3,5-dimethylpyr-
azolylmethyl)-N-hydroxymethylpyrrole (5). Formaldehyde (40
wt % aqueous solution, 0.86 mL, 11.4 mmol) and dimethylamine
hydrochloride (0.465 g, 5.70 mmol) were placed in a round-bottom
flask, cooled to 0 °C in an ice bath, and stirred for 30 min. To this
1
solution was added slowly a solution of 2-(3,5-
further purification. H NMR (200, 400, and 500 MHz) and 13C
dimethylpyrazolylmethyl)pyrrole (1.000 g, 5.707 mmol) in methanol
(60 mL), and the mixture was stirred at room temperature for 48 h.
The reaction was quenched by adding a 20% aqueous solution of
NaOH (0.250 g, 6.25 mmol). After 1 h the volatiles were removed
under vacuum and the resulting residue was extracted into
dichloromethane (3 × 30 mL). The solvent was removed again,
and the residue was loaded onto a silica gel column chromatograph.
Elution using methanol/chloroform (10/90 v/v) afforded 5 as a
colorless oily compound after the solvents were removed under
vacuum (1.140 g, 4.345 mmol, 76%). 1H NMR (CDCl3, 400 MHz): δ
2.18 (s, 6H, CH3), 2.23 (s, 6H, CH3), 3.44 (s, 2H, CH2), 5.17 (s, 2H,
CH2), 5.48 (s, 2H, CH2), 5.76 (s, 1H, pyrazole CH), 5.94 (d, J(H,H)
= 3.2, 1H, pyrrole β-CH), 6.01 (d, J(H,H) = 3.2, 1H, pyrrole β-CH).
13C NMR (CDCl3, 50.3 MHz, ppm): δ 11.5, 14.0, 44.5, 45.0, 55.7,
67.1, 105.5, 108.4, 109.9, 128.3, 130.6, 139.3, 147.5. FT-IR (KBr,
cm−1): ν 3201 (s), 3133 (s), 3107(s), 3036 (s), 2984 (s), 2922 (s),
2871 (s), 2786 (s), 2731 (m), 2606 (w), 1709 (w), 1657 (m), 1592
(m), 1553 (s), 1479 (s), 1462 (s), 1424 (s), 1382 (s), 1301(vs), 1262
(m), 1220 (w), 1149 (m), 1061 (s), 1029 (vs), 1007 (vs), 981 (m),
806 (s), 776 (vs), 735 (s), 702 (m), 660 (m). HRMS (+ESI): calcd
m/z for [M + H+] C14H23N4O 263.1866, found 263.1857.
Synthesis of 2-(3,5-Dimethylpyrazolylmethyl)-5-
(diphenylphosphinomethyl)pyrrole (6). To a solution of 2-
(dimethylaminomethyl)-5-(3,5-dimethylpyrazolylmethyl)pyrrole
(1.000 g, 4.304 mmol) in toluene (30 mL) was added Ph2PH (0.75
mL, 0.802 g, 4.31 mmol), and the mixture was heated at 115 °C for
24 h. The reaction mixture was cooled to room temperature, and the
solvent was removed under vacuum. The resulting residue was
dissolved in a minimum amount of diethyl ether and subjected to
flash silica gel column chromatography. Elution was first with
petroleum ether and then with diethyl ether. The solvent was
removed to give 6 as a colorless solid, which was washed with pentane
(2 × 20 mL) and dried under vacuum (0.970 g, 2.60 mmol, 60%).
Mp: 109 °C. 1H NMR (CDCl3, 400 MHz): δ 2.16 (s, 3H, CH3), 2.21
(s, 3H, CH3), 3.32 (s, 2H, CH2), 4.98 (s, 2H, CH2), 5.70 (br s, 1H,
pyrrole β-CH), 5.76 (s, 1H, pyrazole CH), 5.90 (br s, 1H, pyrrole β-
CH), 7.30 (m, 10H, C6H5), 8.30 (br s, 1H, NH). 13C NMR (CDCl3,
125.7 MHz): δ 11.0, 13.6, 28.0 (d, J(C,P) = 15 Hz, CH2), 46.0, 105.6,
107.1 (d, J(C,P) = 5.0 Hz, pyrrole β-CH), 107.2, 126.0, 128.2 (d,
J(C,P) = 8.8 Hz, pyrrole α-CH), 128.5 (d, J(C,P) = 6.2 Hz, phenyl),
128.8, 132.8 (d, J(C,P) = 17.6 Hz, phenyl), 138.5 (d, J(C,P) = 15.1
Hz, phenyl), 138.8, 147.6. 31P{1H}NMR (CDCl3, 161.9 MHz): δ
−15.4 (s). FT-IR (KBr, cm−1): ν 3435 (s), 3197 (s), 3141 (s), 3003
(s), 2934 (s), 1590 (w), 1545 (s), 1478 (s), 1460 (s), 1434 (vs), 1378
(m), 1351 (m), 1300 (m), 1271 (s), 1206 (m), 1182 (m), 1115 (m),
1089 (m), 1030 (m), 994 (m), 921 (m), 801 (s), 773 (s), 739 (s),
NMR (50.3, 100.6, 125.7, and 150.9 MHz) spectra were recorded on
Bruker ACF200, AV400, AV500, and AV600 spectrometers. Chemical
shifts were referenced with respect to the chemical shifts of the
residual protons present in deuterated solvents. H3PO4 (85%) was
used as an external standard for 31P NMR measurements. FTIR and
ATR spectra were recorded using a PerkinElmer Spectrum Rx
instrument. High-resolution mass spectra (ESI) were recorded using a
Xevo G2 Tof mass spectrometer (Waters). Thermogravimetric
analysis (TGA) was recorded on a TG 209 F3 Tarsus instrument
(Netzsch) in which polymer samples were heated from room
temperature to 800 °C at a rate of 10 or 20 K min−1 under a
nitrogen atmosphere. Powder XRD data were collected by using a
Bruker AXS D8 diffractometer.
Synthesis of 2-(3,5-Dimethylpyrazolylmethyl)pyrrole (3). To
a xylene solution (30 mL) of 2-(dimethylaminomethyl)pyrrole (5.000
g, 40.26 mmol) was added 3,5-dimethylpyrazole (3.800 g, 39.53
mmol), and the solution was refluxed at 145 °C for 15 h. The solution
was then cooled to room temperature and the solvent was removed
under vacuum. The resulting residue was loaded onto a silica gel
column and eluted with an ethyl acetate/petroleum ether mixture (1/
2 v/v). The solvent was removed from the first fraction to give 3 as a
white fluffy solid (4.200 g, 23.97 mmol, 59.5% or 60%). Mp: 132−134
1
°C. H NMR (CDCl3, 400 MHz): δ 2.20 (s, 3H, CH3), 2.22 (s, 3H,
CH3), 5.11 (s, 2H, CH2), 5.77 (s, 1H, pyrazole CH), 6.10 (d, J(H,H)
= 2.4, 2H, pyrrole β-CH), 6.71 (t, J(H,H) = 1, 1H, pyrrole α-CH),
9.07 (br s, 1H, NH). 13C NMR (CDCl3, 100.6 MHz): δ 11.2, 13.6,
45.7, 105.5, 106.9, 108.1, 119.0, 126.9, 139.3, 148.0. FT-IR (KBr,
cm−1): ν 3169 (vs), 3130 (vs), 3101 (vs), 2971 (s), 2938 (s), 1570
(m), 1560 (w), 1548 (s), 1508 (w), 1458 (s), 1420 (s), 1380 (m),
1348 (m), 1301 (m), 1274 (s), 1209 (m), 1138 (s), 1097 (m), 1031
(s), 986 (m), 957 (w), 927 (w), 882 (m), 866 (w), 836 (m), 813 (m),
796 (s), 738 (vs), 726 (vs), 682 (m), 632 (w), 609 (m), 470 (w).
HRMS (+ESI): calcd m/z for [M + H+] C10H14N3 176.1182, found
176.1185.
Synthesis of 2-(Dimethylaminomethyl)-5-(3,5-
dimethylpyrazolylmethyl)pyrrole (4). Formaldehyde (40 wt %
aqueous solution, 2.30 mL, 30.6 mmol) and dimethylamine
hydrochloride (2.514 g, 30.83 mmol) were placed in a round-bottom
flask, cooled to 0 °C in an ice bath, and stirred for 30 min. To this
solution was added slowly
a solution of 2-(3,5-
dimethylpyrazolylmethyl)pyrrole (5.400 g, 30.82 mmol) in methanol
(60 mL), and the mixture was stirred at room temperature for 48 h.
The reaction was quenched by adding a 20% aqueous solution of
NaOH (1.250 g, 31.25 mmol). After 1 h, the volatiles were removed
under vacuum and the resulting residue was extracted into
dichloromethane (3 × 30 mL). The solvent was removed again,
H
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716 (m), 699 (vs), 625 (w), 533 (w), 503 (s), 464 (m), 418 (m).
HRMS (+ESI): calcd m/z for [M + H+] C23H25N3P 374.1781, found
374.1768.
OCH2), 5.09 (s, 2H, CH2), 5.80 (br s, 1H, pyrrole β-CH), 5.83 (s,
1H, pyrazole CH), 5.93 (br s, 1H, pyrrole β-CH), 7.43−7.96 (m,
10H, C6H5). 13C NMR (CDCl3, 125.7 MHz): δ 11.7, 15.5, 16.2, 32.6
(d, J(C,P) = 25.1 Hz, CH2), 45.9, 66.0, 103.1 (d, J(C,P) = 11.3 Hz,
pyrrole β-CH), 107.7, 128.0, 129.0 (d, J(C,P) = 8.8 Hz, phenyl),
129.3, 129.7, 131.3, 132.9 (d, J(C,P) = 7.5 Hz, phenyl), 136.7, 140.0,
152.2. 31P{1H} NMR (CDCl3,161.9 MHz): δ 30.6 (s). ATR-IR
(cm−1): ν 3074 (w), 3052 (w), 2964 (w), 2929 (w), 2858 (w), 1964
(w), 1550 (m), 1486 (m), 1463 (m), 1434 (m), 1421 (m), 1395 (m),
1382 (m), 1379 (m), 1346 (m), 1308 (w), 1272 (m), 1253 (m), 1181
(w), 1159 (w), 1104 (s), 1075 (m), 1052 (m), 1036 (m), 1023 (m),
997 (m), 939 (w), 916 (w), 842 (m), 812 (m), 777 (w), 738 (vs),
706 (s), 689 (vs), 676 (s), 634 (m), 631 (m), 605 (m), 576 (w), 553
(w), 524 (vs), 479 (s), 473 (m). Anal. Calcd for C23H23ClN3NiP: C,
59.21; H, 4.97; N, 9.01. Found: C, 59.18; H, 4.903; N, 9.01.
Synthesis of 2-(Diphenylphosphorylmethyl)-5-(3,5-
dimethylpyrazolylmethyl)pyrrole (7). To a solution of 2-
(diphenylphosphinomethyl)-5-(3,5-dimethylpyrazolylmethyl)pyrrole
(0.100 g, 0.268 mmol) in toluene (20 mL) was added aqueous H2O2
(0.20 mL, 1.8 mmol, 30% (w/w) in water). The solution was stirred
for 16 h, and the solvent was removed under vacuum to give 7 as a
1
colorless sticky solid (0.075 g, 0.19 mmol, 71%). H NMR (CDCl3,
400 MHz): δ 2.04 (s, 3H, CH3), 2.09 (s, 3H, CH3), 3.46 (d, J(H,P) =
12, 2H, CH2), 4.89 (s, 2H, CH2), 5.53 (s, 1H, pyrrole β-CH), 5.64 (s,
1H, pyrazole CH), 5.74 (s, 1H, pyrrole β-CH), 7.26−7.47 (m, 10H,
C6H5), 9.60 (br s, 1H, NH). 13C NMR (CDCl3, 150.9 MHz): δ 11.2,
13.7, 30.2 (d, J(C,P) = 69.4, CH2), 46.1, 105.6, 107.3, 108.8 (d,
J(C,P) = 7.5, pyrrole β-CH), 121.8(d, J(C,P) = 9, pyrrole α-CH),
127.5, 128.7 (d, J(C,P) = 12.1, phenyl), 131.1 (d, J(C,P) = 9, phenyl),
131.8 (d, J(C,P) = 99.6, phenyl), 132.1 (d, J(C,P) = 3, phenyl), 138.9,
147.7. 31P{1H} NMR (CDCl3, 161.9 MHz): δ 31.2 (s). FT-IR (KBr,
cm−1): ν 3385 (br m), 3028 (m), 2963 (vs), 2871 (m), 1687 (m),
1610 (m), 1511 (s), 1466 (s), 1404 (m), 1386 (m), 1361 (m), 1229
(br s), 1115 (m), 1063 (m), 1016 (s), 902 (m), 829 (vs), 760 (w),
713 (w), 572 (s).
Synthesis of [PdCl{C4H2N-2-(CH2Me2pz)-5-(CH2PPh2)-
κ3P,N,N}] (8). To a solution of [PdCl2(PhCN)2] (0.100 g, 0.261
m m o l ) a n d 2 - ( d i p h e n y l p h o s p h i n o m e t h y l ) - 5 - ( 3 , 5 -
dimethylpyrazolylmethyl)pyrrole (6; 0.100 g, 0.268 mmol) in toluene
(20 mL) was added dropwise triethylamine (0.08 mL, 0.574 mmol)
with stirring at room temperature. The solution was stirred for 24 h,
and the solvent was removed under vacuum. The resulting residue
was dissolved in dichloromethane (10 mL), washed with water (3 ×
30 mL), and dried over anhydrous Na2SO4. The solution was then
filtered, and the solvent was evaporated again under vacuum to give a
deep yellow residue (0.094 g, 0.18 mmol, 69%). Suitable single
crystals for an X-ray diffraction study were obtained by slow
evaporation of a solution of 8 in dichloromethane/petroleum ether
(20/80 v/v). Mp: >150 °C. 1H NMR (CDCl3, 400 MHz): δ 2.31 (s,
3H, CH3), 2.64 (s, 3H, CH3), 3.67 (d, J(H,H) = 13.2, 2H, CH2), 5.00
(s, 2H, CH2), 5.86 (s, 1H, pyrazole CH), 5.93 (d, J(H,H) = 1.6, 1H,
pyrrole β-CH), 6.03 (d, J(H,H) = 1.6 Hz, 1H, pyrrole β-CH), 7.45−
7.89 (m, 10H, C6H5). 13C NMR (CDCl3, 100.6 MHz): δ 11.9, 15.5,
34 (d, J(C,P) = 34.2 Hz, CH2), 46.8, 102.7 (d, J(C,P) = 14.1 Hz,
pyrrole β-CH), 106.4, 107.2, 127.9, 128.8, 129.1 (d, J(C,P) = 11 Hz,
phenyl), 131.8 (d, J(C,P) = 2 Hz, phenyl), 133.2 (d, J(C,P) = 11 Hz,
phenyl), 136.0 (d, J(C,P) = 5 Hz, phenyl), 140.2, 152.3. 31P{1H}
NMR (CDCl3, 161.9 MHz): δ 49.9 (s). ATR-IR (cm−1): ν 1720 (w),
1548 (m), 1463 (m), 1434 (m), 1408 (m), 1393 (m), 1373 (m),
1350 (m), 1278 (m), 1109 (m), 1067 (m), 1047 (m), 998 (m), 940
(w), 834 (s), 798 (m), 747 (s), 732 (s), 716 (s), 684 (vs), 636 (m),
517 (vs), 487 (s), 477 (s). HRMS (+ESI): calcd m/z for [M − Cl]+
C23H23N3PPd+ 478.0659, found 478.0674. Anal. Calcd for
C23H23ClN3PPd: C, 53.71; H, 4.51; N, 8.17. Found: C, 53.91; H,
4.50; N, 7.79.
Synthesis of [NiBr{C4H2N-2-(CH2Me2pz)-5-(CH2PPh2)-
κ3P,N,N}] (10). The above procedure was followed with NiBr2
(0.058 g, 0.26 mmol) to give complex 10 as crystals upon
crystallization from ether (0.068 g, 0.12 mmol, 46%). Mp: >150
1
°C. H NMR (CDCl3, 400 MHz): δ 1.21 (t, J(H,H) = 7 Hz, 3H,
OCH2CH3), 2.26 (s, 3H, CH3), 2.72 (s, 3H, CH3), 3.43 (d, J(H,H) =
11.6, 2H, CH2), 3.49 (q, J(H,H) = 7 Hz, 4H, OCH2), 5.11 (s, 2H,
CH2), 5.82 (d, J(H,H) = 3.2 Hz, 1H, pyrrole β-CH), 5.84 (s, 1H,
pyrazole CH), 5.96 (d, J(H,H) = 3.2 Hz, 1H, pyrrole β-CH), 7.43−
7.97 (m, 10H,C6H5). 13C NMR (CDCl3, 100.6 MHz): δ 11.7, 15.5,
17.4, 33.7 (d, J(C,P) = 31.2 Hz, CH2), 45.9, 66.0, 102.9 (d, J(C,P) =
10.0 Hz, pyrrole β-CH), 107.7, 127.9, 128.9 (d, J(C,P) = 10 Hz,
phenyl), 129.8, 130.3, 131.2 (d, J(C,P) = 3 Hz, phenyl), 133.1 (d,
J(C,P) = 9 Hz, phenyl), 136.6 (d, J(C,P) = 7 Hz, phenyl), 140.2,
152.2. 31P{1H} NMR (CDCl3,161.9 MHz): δ 35.0 (s). ATR-IR
(cm−1): ν 3071 (w), 3052 (w), 2964 (w), 2919 (w), 2858 (w), 1984
(w), 1715 (w), 1592 (w), 1573(w), 1550 (m), 1486 (m), 1466 (m),
1434 (m), 1392 (m), 1375 (m), 1340 (m), 1272 (m), 1259 (m),
1230 (w), 1185 (w), 1155 (w), 1104 (s), 1071 (m), 1049 (m), 1029
(m), 1020 (m), 997 (m), 942 (m), 832 (m), 780 (m), 731 (s), 706
(m), 689 (vs), 657 (m), 631 (m), 612 (m), 518 (vs), 482 (s), 466
(m). Anal. Calcd for C23H23BrN3NiP: C, 54.06; H, 4.54; N, 8.22.
Found: C, 54.37; H, 4.75; N, 7.88.
Synthesis of [NiI{C4H2N-2-(CH2Me2pz)-5-(CH2PPh2)-κ3P,N,N}]
(11). The above procedure was followed with NiI2 (0.081 g, 0.26
mmol) to give complex 11 as crystals upon crystallization from ether
1
(0.076 g, 0.12 mmol, 46%). Mp: >150 °C. H NMR (CDCl3, 500
MHz): δ 1.22 (t, J(H,H) = 7 Hz, 3H, OCH2CH3), 2.28 (s, 3H, CH3),
2.77 (s, 3H, CH3), 3.46−3.51 (m, 6H, OCH2 and 2H, CH2), 5.12 (s,
2H, CH2), 5.85 (d, J(H,H) = 3 Hz, 1H, pyrrole β-CH), 5.86 (s, 1H,
pyrazole CH), 6.01 (d, J(H,H) = 2.5 Hz, 1H, pyrrole β-CH), 7.43−
7.94 (m, 10H, C6H5). 13C NMR (CDCl3, 125.7 MHz): δ 11.7, 15.5,
19.7, 35.2 (d, J(C,P) = 28.9 Hz, CH2), 46.0, 66.1, 102.6 (d, J(C,P) =
10.0 Hz, pyrrole β-CH), 107.8 (d, J(C,P) = 10.0 Hz), 127.8, 128.7 (d,
J(C,P) = 11.3 Hz, phenyl), 131.3, 133.5 (d, J(C,P) = 8.8 Hz), 136.6
(d, J(C,P) = 7.5 Hz, phenyl), 140.5, 152.1. 31P{1H} NMR (CDCl3,
161.9 MHz): δ 44.6 (s). ATR-IR (cm−1): ν 3045 (w), 2961 (w), 2925
(w), 2903 (w), 2861 (w), 2165 (w), 1715 (m), 1553 (m), 1466 (m),
1434 (m), 1401 (m), 1392 (m), 1379 (m), 1343 (m), 1308 (w), 1269
(m), 1259 (m), 1178 (w), 1159 (w), 1117 (w), 1097 (m), 1071 (m),
1055 (w), 1016 (w), 1000 (m), 945 (m), 916 (w), 877 (w), 842 (m),
783 (m), 738 (vs), 709 (m), 689 (vs), 673 (m), 628 (m), 608 (m),
521 (vs), 492 (s), 486 (s), 473 (m), 463 (m). Anal. Calcd for
C23H23IN3NiP: C, 49.51; H, 4.15; N, 7.53. Found: C, 49.92; H, 4.50;
N, 7.09.
General Procedure for Norbornene Polymerization. The
nickel or palladium complex was placed in a 100 mL Schlenk flask and
was dried under vacuum for 20 min. To this was added CH2Cl2 or
toluene followed by norbornene. The polymerization reaction began
after addition of MMAO-12 using a syringe at room temperature.
Precipitation occurred usually within 1 min. The reaction was stopped
by adding an MeOH/HCl (10 mL/1 mL) mixture. The precipitate
was filtered, washed with MeOH several times, and then dried under
vacuum at 80 °C for 4 h to give a powdery polynorbornene polymer.
Synthesis of [NiCl{C4H2N-2-(CH2Me2pz)-5-(CH2PPh2)-
κ3P,N,N}] (9). To a solution of 2-(diphenylphosphinomethyl)-5-
(3,5-dimethylpyrazolylmethyl)pyrrole (0.100 g, 0.268 mmol) in
acetonitrile (20 mL) was added triethylamine (0.08 mL, 0.574
mmol) with stirring at room temperature. To this solution was added
[NiCl2(DME)] (0.058 g, 0.26 mmol), and the mixture was refluxed
for 18 h. The solution was cooled to room temperature, and then the
solvent was removed under vacuum. The resulting residue was
dissolved in dichloromethane (20 mL). The solution was washed with
water (3 × 10 mL), dried over anhydrous Na2SO4, and then filtered
over Celite. The solvent was removed again under vacuum to give an
orange-red residue which was extracted into diethyl ether. Upon
cooling to −4 °C for 1−2 days, complex 9 was obtained as deep
orange crystals containing one ether molecule (0.078 g, 0.14 mmol,
54%). Mp: >150 °C. 1H NMR (CDCl3, 400 MHz): δ 1.21 (t, J(H,H)
= 7 Hz, 3H, OCH2CH3), 2.25 (s, 3H, CH3), 2.66 (s, 3H, CH3), 3.41
(d, J(H,H) = 11.6 Hz, 2H, CH2), 3.48 (q, J(H,H) = 7 Hz, 4H,
I
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In the case of polymerization at higher temperature, the precatalyst
and NB in toluene were placed in a preheated oil bath and then
MMAO was added. In the nickel-catalyzed reactions, the reaction
mixture turned to a viscous liquid until 15 min, whereas a colorless
precipitate began to form after 2 min in the case of palladium-
catalyzed reactions. An MeOH/HCl (10 mL/1 mL) mixture was
added, and solid polymers were dried as above.
In the case of EtAlCl2 as cocatalyst, the reaction mixture is a
suspension and gel-like precipitate is formed after quenching with
MeOH/HCl mixture. The precipitate was initially dried under
vacuum at 50 °C. The resulting polymer still appears to be a gel-
like material which became a very hard one piece of chunk slowly
upon drying in open atmosphere at room temperature.
X-ray Crystallography. Single crystals suitable for X-ray
diffraction for 8, 10 and 11 were obtained from the solvents
mentioned in their respective synthetic procedures. Data collections
were performed using a Bruker APEX-II or D8 Venture APEX3 CCD
diffractometer with graphite-monochromated Mo Kα radiation (λ =
0.71073 Å). The space group for each structure was obtained by the
XPREP program. The structures were then solved by SIR-9241 or
SHELXT42 available in WinGX, which successfully located most of
the non-hydrogen atoms. Subsequently, least-squares refinements
were carried out on F2 using SHELXL-2018/1 (Sheldrick, 2018)43 to
locate the remaining non-hydrogen atoms. Non-hydrogen atoms were
refined with anisotropic displacement parameters.
All hydrogen atoms were fixed in calculated positions. In the
structure of 8, one of the phenyl rings is disordered over two positions
with occupation factors of 56 and 44%. Diethyl ether in the crystal
lattices of structures 10 and 11 is disordered. This disorder was
successfully handled with EADP and SADI restraints. The structure
refinement data are given in Table S1.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03562.
■
S
NMR, IR, crystal structure and refinement, TGA,
powder XRD, and crystallographic data (PDF)
Accession Codes
CCDC 1875470−1875472 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: gmani@chem.iitkgp.ac.in (G.M.). Tel: +91 3222
282320. Fax: +91 3222 282252.
ORCID
Sanghamitra Das: 0000-0003-4890-9849
Vasudevan Subramaniyan: 0000-0001-8130-6789
Ganesan Mani: 0000-0002-0782-6484
■
Notes
The authors declare no competing financial interest.
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Leitus, G.; Milstein, D. Electron−Rich PNP− and PNN−Type
ACKNOWLEDGMENTS
■
We are thankful to the DST, New Delhi, India, for financial
support, and DST FIST Level-II grant (No. SR/FST/CSII-
026/2013) for the 500 MHz NMR instrument. We are also
thankful to Prof. M. C. Das, Department of Chemistry, IIT-
Kharagpur, for a TGA discussion.
J
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