Ethylene and styrene carbon monoxide copolymerization catalyzed by pyrazolyl palladium(II) complexes

Article abstract of DOI:10.1021/om300798y

The pyrazolyl pyridylimine ligands [2-(3,5-dimethylpyrazol-1-yl)ethyl] pyridin-2-ylmethyleneimine (L1) and [2-(3,5-di-tert-butylpyrazol-1-yl)ethyl] pyridin-2-ylmethyleneimine (L2) and pyrazolyl thienylimine ligands [2-(3,5-dimethylpyrazol-1-yl)ethyl]thiophen-2-ylmethyleneimine (L3), [2-(3,5-di-tert-butyl-pyrazol-1-yl)ethyl]thiophen-2-ylmethyleneimine (L4), [2-(3,5-dimethylpyrazol-1-yl)ethyl]-2-bromothiophen-2-ylmethyleneimine (L5), and [2-(3,5-di-tert-butyl-pyrazol-1-yl)ethyl]-2-bromothiophen-2-ylmethyleneimine (L6) were synthesized by condensation of the appropriate pyrazolylamine and the corresponding aldehyde. Reactions of L1-L6 with [PdCl(Me)(cod)] gave the corresponding palladium(II) complexes [PdCl(Me)(L)] (where L = L1 (1a), L2 (2a), L3 (3a), L4 (4a), L5 (5a), L6 (6a)) in very good yields. The pyridylimine ligands L1 and L2 were found to coordinate via the imine and pyridine nitrogen atoms, while the thienylimine ligands L3-L6 coordinate via imine and pyrazolyl nitrogen atoms to the palladium. The cationic complexes [Pd(Me)(A)]⁺ (where A = L1 (1b), L2 (2b), L3 (3b), L4 (4b), L5 (5b), L6 (6b)) were synthesized from 1a-6a, respectively, using Na[BAr₄] (Ar = 3,5-(CF₃)₂C₆H₃) in a 1:1 ratio. The cationic palladium complexes 1b and 2b were stabilized by the pyrazolyl nitrogen atom of ligands L1 and L2, respectively, while complexes 3b-6b were stabilized by NCMe. Attempts to activate 1a-6a with Na[BAr₄] to produce active catalysts for ethylene/CO and styrene/CO led to the formation of palladium black. Using the cationic complexes 1b-6b, only 2b and 3b were active in the copolymerization of ethylene/CO and styrene/CO.

Full text of DOI:10.1021/om300798y

Article
pubs.acs.org/Organometallics
Ethylene and Styrene Carbon Monoxide Copolymerization Catalyzed
by Pyrazolyl Palladium(II) Complexes
Collins Obuah,† Michael K. Ainooson,† Sibulele Boltina,† Ilia A. Guzei,†,‡ Kyoko Nozaki,§
and James Darkwa*,†
†Department of Chemistry, University of Johannesburg, Auckland Park Kingsway Campus, Auckland Park 2006, South Africa
‡Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States
§Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku
113-8656, Japan
S
* Supporting Information
ABSTRACT: The pyrazolyl pyridylimine ligands [2-(3,5-
dimethylpyrazol-1-yl)ethyl]pyridin-2-ylmethyleneimine (L1)
and [2-(3,5-di-tert-butylpyrazol-1-yl)ethyl]pyridin-2-ylmethyle-
neimine (L2) and pyrazolyl thienylimine ligands [2-(3,5-
dimethylpyrazol-1-yl)ethyl]thiophen-2-ylmethyleneimine (L3),
[2-(3,5-di-tert-butyl-pyrazol-1-yl)ethyl]thiophen-2-ylmethyle-
neimine (L4), [2-(3,5-dimethylpyrazol-1-yl)ethyl]-2-bromo-
thiophen-2-ylmethyleneimine (L5), and [2-(3,5-di-tert-butyl-
pyrazol-1-yl)ethyl]-2-bromothiophen-2-ylmethyleneimine (L6)
were synthesized by condensation of the appropriate pyrazolyl-
amine and the corresponding aldehyde. Reactions of L1−L6
with [PdCl(Me)(cod)] gave the corresponding palladium(II)
complexes [PdCl(Me)(L)] (where L = L1 (1a), L2 (2a), L3
(3a), L4 (4a), L5 (5a), L6 (6a)) in very good yields. The pyridylimine ligands L1 and L2 were found to coordinate via the imine
and pyridine nitrogen atoms, while the thienylimine ligands L3−L6 coordinate via imine and pyrazolyl nitrogen atoms to the
palladium. The cationic complexes [Pd(Me)(A)]+ (where A = L1 (1b), L2 (2b), L3 (3b), L4 (4b), L5 (5b), L6 (6b)) were
synthesized from 1a−6a, respectively, using Na[BAr4] (Ar = 3,5-(CF3)2C6H3) in a 1:1 ratio. The cationic palladium complexes
1b and 2b were stabilized by the pyrazolyl nitrogen atom of ligands L1 and L2, respectively, while complexes 3b−6b were
stabilized by NCMe. Attempts to activate 1a−6a with Na[BAr4] to produce active catalysts for ethylene/CO and styrene/CO led
to the formation of palladium black. Using the cationic complexes 1b−6b, only 2b and 3b were active in the copolymerization of
ethylene/CO and styrene/CO.
significant changes in reaction rate, natures of products, and
molecular weights of the polyketones.2
1. INTRODUCTION
Late-transition-metal catalysts containing nitrogen-donor atoms
have found applications in several polymerization reactions
such as homopolymerization of olefins1 and copolymerization
of olefins/CO.2 Palladium is the most commonly used metal to
catalyze copolymerization of olefin/CO.
There have been two types of palladium catalysts developed
so far for the copolymerization of olefin/CO. Both are salts of
general formulas [PdL(L′)2]2+(X−)2 (L = bidentate ligand, X =
weakly coordinating or noncoordinating anion, L′ = coordinat-
ing solvent) and [PdLRL′]+X− (L = bidentate ligand, R = alkyl
group, X = weakly coordinating or noncoordinating anion, and
L′ = coordinating solvent).2,3 These palladium(II) complexes
contain a wide array of bidentate ligands, including bidentate
phosphine donors (P∧P), mixed phosphino-phosphite (P∧O),
mixed bidentate phosphorus−nitrogen (P∧N) and bidentate
nitrogen donors (N∧N). Structural modification of the
bidentate ligands and diverse structures of α-olefins, results in
The most commonly used ligand systems for the
copolymerization of ethylene/CO are bisphosphines. However,
the bisphosphine palladium(II) catalysts have a drawback, in
that they easily degrade to inactive palladium(0) under
copolymerization conditions.4 For instance, (dppp)Pd−H+ is
known to be unstable in methanol and slowly undergoes
deprotonation to (dppp)Pd0, which either produces palla-
dium(0) and the free ligand or couples with (dppp)Pd2+ to
4
2+
form the catalytically inactive binuclear species [Pd(dppp)]2
,
although the inactive complex can be converted to catalytically
active Pd−H or Pd−OMe species by the addition of an oxidant
such as 1,4-benzoquinone. Furthermore, the use of bi-
sphosphine palladium complexes is limited to either ethyl-
ene/CO or aliphatic α-olefin/CO copolymerization. For
Received: August 22, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om300798y | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Chemical shifts for 1H NMR spectra are reported in δ (ppm)
downfield from tetramethylsilane (TMS) as an internal standard.
Infrared (IR) spectra for copolymers were recorded on a Shimadzu
FTIR-8400 spectrometer with a KBr plate. Size exclusion chromatog-
raphy (SEC) analyses were carried out at 40 °C with polystyrene as
internal standard, equipped with a GL Sciences instrument (Model PU
610 high-performance liquid chromatography pump, CO 631 liquid
chromatography column oven, and RI 713 refractive-index detector)
and with two columns (Shodex KF-804 L). The columns were eluted
with tetrahydrofuran (THF) at a rate of 1 mL min−1. Differential
scanning calorimetry (DSC) measurements were performed on a
Seiko DSC 7020 analyzer at a heating and cooling rate of 10 °C min−1,
and the reported Tg values were determined during the cooling
process. Thermogravimetric analyses (TGA) were performed on a
Seiko EXSTAR 6000 TG/DTA 6200 analyzer at a heating rate of 10
°C min−1.
instance, when styrene is used, only short-chain oligomers are
produced. As a result of these drawbacks, nitrogen-based
catalysts are becoming the preferred choice especially for
styrene/CO copolymerizations.
Over the past 10 years a variety of bidentate nitrogen-donor
metal complexes have been used as olefin/CO copolymeriza-
tion catalysts. Examples range from 2,2-bipyridine,5 1,10-
phenanthroline,6 oxazoline base,7 and α-diimine8 to pyrazoles.9
Incorporation of pyrazole in ligands is gaining ground due to
(i) the weaker donor ability of pyrazoles compared to other
nitrogen donors such as imines and pyridines that can render
metal centers in such pyrazolyl catalyst systems more
electrophilic and (ii) the ability to easily modify electronic
and steric properties of the catalysts, by adding different
substituents on the pyrazolyl unit. A recent review of pyrazolyl
metal complexes as catalysts for various carbon−carbon
coupling reactions10 demonstrate the extent to which the
above two properties of pyrazolyl metal complexes manifest
themselves in olefin/CO copolymerization and other areas of
catalysis.
There have been few reports on the use of terdentate
nitrogen-donor ligands in olefin/CO copolymerization catal-
ysis.11 Most of the reports are on the mechanistic studies.
Examples are terdentate nitrogen-donor ligands 2,2′:6′,2″-
terpyridine (terpy), 2,6-bis(2-pyrimidyl)pyridine, and 2,6-bis-
(N-pyrazolyl)pyridine.11b,12,13 An exception is a recent report
by Milani and co-workers,7b where the terdentate ligand 2-
oxazolinylphenanthroline is used as a support for a palladium
catalyst in styrene/CO copolymerization.
2.2. Syntheses of Ligands and Complexes. 2.2.1. Synthesis of
[2-(3,5-Dimethylpyrazol-1-yl)ethyl]pyridin-2-ylmethyleneimine (L1).
An EtOH solution (20 mL) of pyridine-2-carboxaldehyde (1.08 g, 0.01
mol) was added to a stirred EtOH solution (30 mL) of 3,5-
dimethylpyrazolylamine (1.40 g, 0.01 mol). The resulting mixture was
refluxed at 60 °C for 12 h to give an orange solution. This solution was
filtered, and the filtrate was evaporated. The orange sticky product was
redissolved in CH2Cl2, and the undissolved solid was filtered off. The
filtrate was evaporated, and the crude product was purified by
sublimation, affording L1 as a dark orange oil. Yield: 1.41 g (62%). 1H
NMR (CDCl3): δ 2.16 (s, 3H, CH3); 2.18 (s, 3H, CH3); 4.06 (t, 2H,
3
3JHH = 6.0 Hz, CH2); 4.32 (t, 2H, JHH = 6.0 Hz, CH2); 5.67 (s, 1H,
pz-H); 7.31 (t, 1H, 3JHH = 6.0 Hz, 2-py-H); 7.74 (t, 1H, 3JHH = 6.0 Hz,
3-py-H); 7.89 (d, 1H, 3JHH = 8.1 Hz, 4-py-H); 8.12 (s, 1H, CHN);
8.61 (d, 1H, 3JHH = 4.8 Hz, 1-py-H). 13C{1H} NMR (CDCl3): δ 10.5;
15.2; 63.3; 65.1; 108.3; 125.1; 127.5; 139.5; 145.9; 151.3; 155.1; 162.6;
167.7. IR (Diamond ATR, cm−1): 1649 ν(CHN) imine. HRMS
(ESI): [M + H]+ m/z 229.1450 (100%).
Here we report the synthesis of two classes of new terdentate
pyrazolyl ligands: one with a pyridinylimine group (L1 and L2)
and the other with a thienylimine group (L3−L6), their
methylpalladium chloride complexes (1a−6a), their cationic
palladium complexes (1b−6b), and application of these
pyrazolyl palladium complexes as catalysts for ethylene/CO
and styrene/CO copolymerizations.
Compounds L2−L6 were prepared following the same procedure as
described for compound L1, using the appropriate reagents.
2.2.2. Synthesis of [2-(3,5-Di-tert-butylpyrazol-1-yl)ethyl]pyridin-
2-ylmethyleneimine (L2). Pyridine-2-carboxaldehyde (0.09 g, 0.84
mmol) was reacted with 3,5-di-tert-butylpyrazolylamine (0.18 g, 0.84
mmol) to give a light yellow oily material, which solidified after 1
1
t
week. Yield: 0.15 g (75%). H NMR (CDCl3): δ 1.24 (s, 9H, Bu);
1.33 (s, 9H, tBu); 4.19 (t, 2H, 3JHH = 6.0 Hz, CH2); 4.50 (t, 2H, 3JHH
=
2. EXPERIMENTAL SECTION
6.6 Hz, CH2); 5.72 (s, 1H, pz-H); 7.30 (t, 1H, 3JHH = 6.3 Hz, 2-py-H);
2.1. Materials and Methods. Unless otherwise stated, all
manipulations were carried out under nitrogen atmosphere using
standard Schlenk techniques. Polymerization was carried out using a
50 mL stainless steel autoclave. All organic solvents were dried and
purified by distillation over standard reagents under argon prior to use.
The compounds [PdClMe(cod)]13 and alkylpyrazolylamine14 were
synthesized according to literature procedures. The compounds 3,5-
dimethylpyrazole, 2-bromoethylamine hydrochloride, thiophene-2-
carboxaldehyde, 5-bromothiophene-2-carboxaldehyde, pyridine-2-car-
boxaldehyde, 2,2,6,6-tetramethyl-3,5-heptadione, styrene, and
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP; caution! HFIP is a very
toxic compound) were purchased from Sigma-Aldrich while Na[BAr4]
(Ar = 3,5-(CF3)2C6H3) was purchased from Boulder Chemicals; all
chemicals were used as received.
Infrared (IR) spectra of ligands and complexes were recorded on a
Bruker Tensor 27 equipped with a Diamond ATR. Elemental analyses
were performed on a Vario Elementar III Microcube CHNS analyzer
at Rhodes University. The mass spectrometry unit at the University of
Stellenbosch performed the ESI-MS spectra on a Waters API Qualtro
micro spectrophotometer. NMR spectra were recorded on a Bruker
400 MHz instrument (1H at 400 MHz and 13C{1H} at 100 MHz). The
chemical shifts are reported in δ (ppm) and referenced to the residual
proton and carbon signals at 7.24 and 77.0 ppm, respectively, of
CDCl3 NMR solvent.
7.72 (t, 1H, 3JHH = 7.2, Hz 3-py-H); 7.89 (d, 1H, 3JHH = 7.5 Hz, 4-py-
3
H); 8.27 (s, 1H, CHN); 8.62 (d, 1H, JHH = 6.0 Hz, 1-py-H).
13C{1H} NMR (CDCl3): δ 31.3; 33.3; 33.7; 38.1; 64.3; 65.7; 108.7;
126.5; 128.8; 139.9; 146.9; 153.4; 156.6; 164.1; 168.3. IR (Diamond
ATR, cm−1): 1644 ν(CHN) imine. HRMS (ESI): [M + H]+ m/z
313.2350 (100%).
2.2.3. Synthesis of [2-(3,5-Dimethylpyrazol-1-yl)ethyl]thiophen-2-
ylmethyleneimine (L3). Thiophene-2-carboxaldehyde (0.81 g, 7.26
mmol) was reacted with 3,5-dimethylpyrazolylamine (1.01 g, 7.26
mmol) to give a dark purple oil. Yield: 1.35 g (80%). 1H NMR
(CDCl3): δ 2.15 (s, 3H, CH3); 2.18 (s, 3H, CH3); 3.94 (t, 2H, 3JHH
=
3
5.6 Hz, CH2); 4.26 (t, 2H, JHH = 5.6 Hz, CH2); 5.64 (s, 1H, pz-H);
3
7.02 (t, 1H, 3JHH = 3.6 Hz, 2-th-H); 7.17 (d, 1H, JHH = 3.2 Hz, 3-th-
3
H); 7.36 (d, 1H, JHH = 4.8 Hz, 1-th-H); 7.97 (s, 1H, CHN).
13C{1H} NMR (CDCl3): δ 10.5; 13.1; 51.3; 53.6; 106.9; 126.9; 128.0;
130.8; 137.9; 139.7; 149.5; 152.5. IR (Diamond ATR, cm−1): 1633
ν(CHN) imine. HRMS (ESI): [M + H]+ m/z 234.0082 (100%).
2.2.4. Synthesis of [2-(3,5-Di-tert-butylpyrazol-1-yl)ethyl]-
thiophen-2-ylmethyleneimine (L4). Thiophene-2-carboxaldehyde
(0.51 g, 4.57 mmol) was reacted with 3,5-di-tert-butylpyrazolylamine
(1.02 g, 4.57 mmol) to give a yellow oil material, which solidified after
1 week at room temperature. Yield: 1.18 g (82%). 1H NMR (CDCl3):
t
t
3
δ 1.28 (s, 9H, Bu); 1.30 (s, 9H, Bu); 4.11 (t, 2H, JHH = 6.0 Hz,
3
CH2); 4.42 (t, 2H, JHH = 6.0 Hz, CH2); 5.69 (s, 1H, pz-H); 7.01 (t,
NMR spectroscopy data for copolymers were recorded on a JEOL
JNM-ECS 400 instrument, which has 1H at 400 MHz and 13C{1H} at
101 MHz with digital resolutions of 0.11 and 0.96 Hz, respectively.
1H, 3JHH = 3.6 Hz, 2-th-H); 7.15 (d, 1H, 3JHH = 3.6 Hz, 3-th-H); 7.34
(d, 1H, 3JHH = 5.2 Hz, 1-th-H); 8.09 (s, 1H, CHN). 13C{1H} NMR
(CDCl3): δ 31.4; 33.1; 33.5; 37.7; 47.8; 59.5; 105.5; 127.2; 134.5;
B
dx.doi.org/10.1021/om300798y | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
136.9; 138.4; 139.9; 150.2; 164.2. IR (Diamond ATR, cm−1): 1635
ν(CHN) imine. HRMS (ESI): [M + H]+ m/z 318.2004 (100%).
2.2.5. Synthesis of [2-(3,5-Dimethylpyrazol-1-yl)ethyl]-2-bromo-
thiophen-2-ylmethyleneimine (L5). 5-Bromothiophene-2-carboxalde-
hyde (0.96 g, 5.00 mmol) was reacted with 3,5-dimethylpyrazolyl-
amine (0.69 g, 5.00 mmol) to give a dark purple oil. Yield: 1.25 g
105.9; 126.9; 134.0; 136.8; 137.9; 139.7; 149.5; 162.5. IR (Diamond
ATR, cm−1): 1622 ν(CHN) imine. Anal. Calcd for C13H18ClN3PdS:
C, 40.01; H, 4.65; N, 10.77; S, 8.20. Found: C, 40.05; H, 4.75; N,
10.48; S, 7.74.
2.2.10. Synthesis of [2-(3,5-Di-tert-butyl-pyrazol-1-yl)ethyl]-
thiophen-2-ylmethyleneiminemethylpalladium(II) Chloride (4a).
Compound L4 (0.14 g, 0.44 mmol) was reacted with [PdCl(Me)-
(cod)] (0.12 g, 0.44 mmol) to give 4a as a light orange solid. Yield:
0.16 g (82%). 1H NMR (CDCl3): δ 0.87 (s, 3H, Pd-CH3); 1.33 (s, 9H,
tBu); 1.45 (s, 9H, tBu); 5.76 (s, 1H, pz-H); 7.11 (t, 1H, 3JHH = 4.4 Hz,
1
(80%). H NMR (CDCl3): δ 2.15 (s, 3H, CH3); 2.17 (s, 3H, CH3);
3
3
3.90 (t, 2H, JHH = 5.6 Hz, CH2); 4.24 (t, 2H, JHH = 5.6 Hz, CH2);
3
5.65 (s, 1H, pz-H); 6.89 (d, 1H, JHH = 3.2 Hz, 3-th-H); 6.97 (d, 1H,
3JHH = 3.2 Hz, 2-th-H); 7.82 (s, 1H, CHN). 13C{1H} NMR
3
3
(CDCl3): δ 10.9; 13.7; 51.6; 54.1; 105.2; 127.1; 127.2; 131.2; 136.5;
137.1; 147.1; 151.3. IR (Diamond ATR, cm−1): 1632 ν(CHN)
imine. HRMS (ESI): [M − H]+ m/z 310.2019 (40%), [M − Br]+ m/z
231.1604 (100%).
2-th-H); 7.69 (d, 1H, JHH = 3.2 Hz, 3-th-H); 7.72 (d, 1H, JHH = 4.8
Hz, 1-th-H); 8.32 (s, 1H, CHN). 13C{1H} NMR (CDCl3): δ 10.1;
28.5; 29.1; 30.0; 32.0; 45.1; 49.3; 135.5; 138.5; 148.1; 164.2. IR
(Diamond ATR, cm−1): 1620 ν(CHN) imine. Anal. Calcd for
C19H30ClN3PdS: C, 48.10; H, 6.37; N, 8.86; S, 6.74. Found: C, 48.13;
H, 6.66; N, 8.66; S, 6.40.
2.2.6. Synthesis of [2-(3,5-Di-tert-butylpyrazol-1-yl)ethyl]-2-bro-
mothiophen-2-ylmethyleneimine (L6). 5-Bromothiophene-2-carbox-
aldehyde (0.87 g, 4.57 mmol) was reacted with 3,5-di-tert-
butylpyrazolylamine (1.02 g, 4.57 mmol) to give a yellow oil, which
2.2.11. Synthesis of [2-(3,5-Dimethylpyrazol-1-yl)ethyl]-2-bromo-
thiophen-2-ylmethyleneiminemethylpalladium(II) Chloride (5a).
Compound L5 (0.14 g, 0.43 mmol) was reacted with [PdCl(Me)-
(cod)] (0.11 g, 0.43 mmol) to give 5a as a light brown solid. Yield:
0.16 g (80%). 1H NMR (CDCl3): δ 0.73 (s, 3H, Pd-CH3); 0.94 (s, 3H,
Pd-CH3iso); 2.18 (s, 3H, CH3); 2.26 (s, 3H, CH3iso); 2.27 (s, 3H,
CH3iso); 2.36 (s, 3H, CH3); 5.69 (s, 1H, pz-H); 5.81 (s, 1H, pz-Hiso);
1
solidified after 1 week at room temperature. Yield: 1.48 g (82%). H
t
t
NMR (CDCl3): δ 1.28 (s, 9H, Bu); 1.30 (s, 9H, Bu); 4.08 (t, 2H,
3JHH = 6.0 Hz, CH2); 4.40 (t, 2H, JHH = 6.0 Hz, CH2); 5.69 (s, 1H,
3
3
3
pz-H); 6.86 (d, 1H, JHH = 3.2 Hz, 3-th-H); 6.96 (d, 1H, JHH = 3.2
Hz, 2-th-H); 7.93 (s, 1H, CHN). 13C{1H} NMR (CDCl3): δ 31.2 ;
33.2; 33.5; 37.9; 47.5; 58.2; 105.2; 125.1; 133.4; 134.1; 138.2; 138.9;
149.1; 162.5. IR (Diamond ATR, cm−1): 1632 ν(CHN) imine.
HRMS (ESI): [M + 2H]+ m/z 398.1072 (40%), [M − Br]+ m/z
316.2032 (100%).
3
3
7.01 (d, 1H, JHH = 3.2 Hz, 2-th-Hiso); 7.07 (d, 1H, JHH = 3.2 Hz, 2-
3
th-H) 7.29 (d, 1H, JHH = 2.0 Hz, 3-th-H); 8.10 (s, 1H, CHNiso);
8.18 (s, 1H, CHN). 13C{1H} NMR (CDCl3): δ 11.0; 11.56; 13.0;
47.9; 58.9; 107.1; 125.5; 131.6; 137.8; 138.1; 139.3; 149.1; 160.0. IR
(Diamond ATR, cm−1): 1626 ν(CHN) imine. Anal. Calcd for
C13H17BrClN3PdS: C, 33.28; H, 3.65; N, 8.95; S, 6.82. Found: C,
33.24, H, 3.13; N, 7.87; S, 6.72.
2.2.7. Synthesis of [2-(3,5-Dimethylpyrazol-1-yl)ethyl]pyridin-2-
ylmethyleneiminemethylpalladium(II) Chloride (1a). A Et2O sol-
ution (20 mL) of L1 (0.98 g, 4.33 mmol) was added to a suspension of
[PdCl(Me)(cod)] (1.15 g, 4.33 mmol) in Et2O (10 mL) with stirring.
The resulting suspension was stirred for 12 h at 25 °C. The reaction
mixture was filtered, and the solid was isolated, washed three times
with 20 mL of Et2O, and dried in air to give an analytically pure yellow
2.2.12. Synthesis of [2-(3,5-Di-tert-butylpyrazol-1-yl)ethyl]-2-
bromothiophen-2-ylmethyleneiminemethylpalladium(II) Chloride
(6a). Compound L6 (0.17 g, 0.44 mmol) was reacted with
[PdCl(Me)(cod)] (0.12 g, 0.44 mmol) to give 6a as a light brown
solid. Yield: 0.20 g (82%). 1H NMR (CDCl3): δ 0.88 (s, 3H, Pd-CH3);
1
powder of 1. Yield: 1.17 g (70%). H NMR (CDCl3): δ 1.00 (s, 3H,
t
t
Pd-CH3); 2.19 (s, 3H, CH3); 2.36 (s, 3H, CH3); 5.59 (s, 1H, pz-H);
1.33 (s, 3H, Bu); 1.46 (s, 3H, Bu); 5.77 (s, 1H, pz-H); 7.07 (d, 1H,
3
3
2JHH = 4.0 Hz, 2-th-H); 7.35 (d, 1H, JHH = 3.2 Hz, 3-th-H); 8.20 (s,
2
4.23 (t, 2H, JHH = 5.2 Hz, CH2); 4.28 (t, 2H, JHH = 5.2 Hz, CH2);
7.41 (t, 1H, 3JHH = 7.6 Hz, 2-py-H); 7.46 (s, 1H, CHN) 7.62 (t, 1H,
3JHH = 5.2 Hz, 3-py-H); 7.93 (d, 1H, 3JHH = 7.6 Hz, 4-py-H); 9.02 (d,
1H, CHN). 13C{1H} NMR (CDCl3): δ 9.1; 29.1; 29.8; 30.4; 31.6;
45.1; 49.3; 121.5; 135.1; 146.2; 163.1. IR (Diamond ATR, cm−1):
1619 ν(CHN) imine. Anal. Calcd for C19H29BrClN3PdS: C, 41.24;
H, 5.28; N, 7.59; S, 5.78. Found: C, 41.36; H, 5.20; N, 7.21; S, 6.11.
2.2.13. Synthesis of [2-(3,5-Dimethylpyrazol-1-yl)ethyl]pyridin-2-
ylmethyleneiminemethylpalladium(II) Tetrakis(3,5-
trifluoromethylphenyl)borate (1b). A CH2Cl2 (20 mL) of complex
1a (0.07 g, 0.18 mmol) was added to an NCMe solution (10 mL) of
Na[BAr4] (0.16 g, 0.18 mmol). The resulting mixture was stirred at 25
°C for 2 h to afford a light yellow mixture, which was filtered through a
plug of Celite, and the filtrate was evaporated in vacuo to give an oily
material. The oily material was dried under high vacuum overnight,
after which a foamy light orange solid formed. Yield: 0.21 g (97%). 1H
NMR (CDCl3): δ 1.04 (s, 3H, Pd-CH33); 2.28 (s, 6H, CH3); 3.65 (t,
1H, JHH = 4.8 Hz, 1-py-H). 13C{1H} NMR (CDCl3): δ 11.4; 13.5;
3
15.2; 47.19; 59.19; 104.8; 125.5; 128.2; 138.7; 141.1; 148.7; 149.4;
151.2; 168.5. IR (Diamond ATR, cm−1): 1589 ν(CHN) imine.
Anal. Calcd for C19H28Cl2N4Pd: C, 43.65; H, 4.97; N, 14.54. Found:
C, 44.10; H, 4.80; N, 14.37.
Complexes 2a−6a were prepared following the same procedure as
described for complex 1a, using the appropriate reagents.
2.2.8. Synthesis of [2-(3,5-Di-tert-butylpyrazol-1-yl)ethyl]pyridin-
2-ylmethyleneiminemethylpalladium(II) Chloride (2a). Compound
L2 (0.31 g, 1.00 mmol) was reacted with [PdCl(Me)(cod)] (0.27 g,
1.00 mmol) to give 2 as a yellow solid. Yield: 0.35 g (75%). 1H NMR
(CDCl3): δ 0.99 (s, 3H, Pd-CH3); 1.26 (s, 9H, tBu); 1.29 (s, 9H, tBu);
3
3
3
4.45 (d, 2H, JHH = 4.0 Hz, CH2); 4.60 (d, 2H, JHH = 4.0 Hz, CH2);
2H, JHH = 5.2 Hz, CH2); 4.30 (t, 2H, JHH = 5.2 Hz, CH2); 6.03 (s,
3
3
5.72 (s, 1H, pz-H); 7.36 (t, 1H, JHH = 5.2 Hz, 2-py-H); 7.59 (t, 1H,
1H, pz-H); 7.39 (d, 1H, JHH = 7.6 Hz, 2-py-H); 7.49 (s, 4H, BAr4);
3JHH = 7.6 Hz, 3-py-H); 7.87 (s, 1H, CHN); 8.55 (d, 1H, 3JHH = 5.2
7.69 (s, 8H, BAr4); 7.94 (s, 1H, CHN); 7.97 (t, 1H, 3JHH = 8.0 Hz,
Hz, 4-py-H); 9.00 (d, 1H, JHH = 4.8 Hz, 1-py-H). 13C{1H} NMR
3
3
3-py-H); 8.50 (d, 1H, JHH = 5.2 Hz, 1-py-H). 13C{1H} NMR
(CDCl3): δ 11.9; 29.4; 37.2; 36.5; 49.3; 58.51; 105.2; 127.9; 130.1;
139.7; 144.51; 150.8; 151.4; 153.2; 169.5. IR (Diamond ATR, cm−1):
1592 ν(CHN) imine. Anal. Calcd for C20H31ClN4Pd: C, 50.77; H,
6.73; N, 12.02. Found: C, 51.18; H, 6.66; N, 11.94.
(CDCl3): δ 2.1; 11.7; 14.7; 48.3; 55.8; 108.8; 117.5; 120.4; 123.1;
125.8; 127.6; 128.9; 134.7; 140.1; 143.0; 149.7; 153.0; 155.0; 162.4. IR
(Diamond ATR, cm−1): 1609 ν(CHN) imine. Anal. Calcd for
C46H31N4PdBF24: C, 45.50; H, 2.55; N, 4.61. Found: C, 46.01; H,
2.55; N, 4.64. Positive ion ESI-MS: m/z (%) = 349.1 (100%) [M]+.
Negative ion ESI-MS: m/z (%) 863.0 (100%) [M]−.
2.2.9. Synthesis of [2-(3,5-Dimethylpyrazol-1-yl)ethyl]thiophen-2-
ylmethyleneiminemethylpalladium(II) Chloride (3a). Compound L3
(0.10 g, 0.43 mmol) was reacted with [PdCl(Me)(cod)] (0.11 g, 0.43
Complexes 2b−6b were prepared in a manner similar to that
described for 1b.
1
mmol) to give 3a as a violet solid. Yield: 0.14 g (80%). H NMR
(CDCl3): δ 0.70 (s, 3H, Pd-CH3); 0.93 (s, 3H, Pd-CH3) 2.18 (s, 3H,
2.2.14. Synthesis of [2-(3,5-Di-tert-butylpyrazol-1-yl)ethyl]-
pyridin-2-ylmethyleneiminemethylpalladium(II) Tetrakis(3,5-
trifluoromethylphenyl)borate (2b). Complex 2a (0.07 g, 0.16
mmol) was reacted with NCMe and Na[BAr4] (0.14 g, 0.16 mmol)
CH3); 2.26 (s, 3H, CH3); 2.36 (s, 3H, CH3); 5.68 (s, 1H, pz-H); 5.80
3
(s, 1H, pz-Hiso); 7.05 (t, 1H, JHH = 4.0 Hz, 2-th-Hiso); 7.11 (t, 1H,
3JHH = 4.4 Hz, 2-th-H); 7.56 (d, 1H, 3JHH = 3.6 Hz, 3-th-Hiso); 7.58 (d,
1
3
3
1H, JHH = 3.2 Hz, 3-th-H); 7.65 (d, 1H, JHH = 4.8 Hz, 1-th-Hiso);
7.74 (d, 1H, 3JHH = 4.8 Hz, 1-th-H); 8.23 (s, 1H, CHNiso); 8.32 (s,
1H, CHN). 13C{1H} NMR (CDCl3): δ 11.0; 11.8; 13.1; 46.8; 60.4;
to give 2b as a light orange solid. Yield: 0.19 g (90%). H NMR
(CDCl3): δ 1.11 (s, 3H, Pd-CH3); 1.37 (s, 8H, tBu); 1.51 (s, 8H, tBu);
3.56 (t, 1H, 2JHH = 11.7 Hz, CH2); 3.76 (d, 1H, 2JHH = 10.5 Hz, CH2);
C
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a
Scheme 1. Syntheses of Pyrazolylimine Ligands and Their Palladium Complexes
a
Reagents: (a) pyridine-2-carboxaldehyde; (b) thiophene-2-carboxaldehyde; (c) [PdCl(Me)(cod)]; (d) Na[BAr4].
4.67 (t, 1H, 2JHH =10.5 Hz, CH2); 4.68 (d, 1H, 2JHH = 15.3 Hz, CH2);
C53H45N4PdSBF24: C, 47.35; H, 3.35; N, 4.17; S, 2.38. Found: C,
47.42; H, 3.33; N, 4.09; S, 2.75. Positive ion ESI-MS: m/z (%) 423.0
(15%) [M − Me − NCMe]+. Negative ion ESI-MS: m/z (%) 863.0
(100%) [M]−.
3
6.15 (s, 1H, pz-H); 7.35 (d, 1H, JHH = 7.6 Hz, 2-py-H); 7.48 (s, 4H,
BAr4); 7.62 (t, 1H, 3JHH = 6.4 Hz, 3-py-H); 7.89 (s, 8H, BAr4); 7.92 (t,
3
1H, 3JHH = 8.0 Hz, 4-py-H); 7.97 (s, 1H, CHN); 8.50 (d, 1H, JHH
= 5.2 Hz, 1-py-H). 13C{1H} NMR (CDCl3): δ 3.1; 32.1; 33.4; 33.8;
37.0; 49.5; 57.1; 109.4; 119.2; 125.1; 124.5; 124.9; 128.3; 130.0; 135.1;
143.8; 144.3; 150.0; 154.2; 156.1; 169.1. IR (Diamond ATR, cm−1):
1627 ν(CHN) imine. Anal. Calcd for C52H43N4PdBF24: C, 48.10;
H, 3.31; N, 4.31. Found: C, 47.73; H, 3.59; N, 4.44. Positive ion ESI-
MS: m/z (%) 433.0 (100%) [M]. Negative ion ESI-MS: m/z (%)
863.0 (100%) [M]−.
2.2.17. Synthesis of [2-(3,5-Dimethylpyrazol-1-yl)ethyl]-2-bromo-
thiophen-2-ylmethyleneiminemethylpalladium(II) Tetrakis(3,5-
trifluoromethylphenyl)borate (5b). Complex 5a (0.12 g, 0.27
mmol) was reacted with Na[BAr4] (0.24 g, 0.27 mmol) to afford 5b
as an orange solid product. Yield: 0.33 g (90%). 1H NMR (CDCl3): δ
0.70 (s, 3H, Pd-CH3); 0.94 (s, 3H, Pd-CH3iso); 2.04 (s, 6H, CH3iso);
2.11 (s, 3H, CH3); 2.19 (s, 3H, CH3); 2.27 (s, 3H, Pd-NC-CH3); 3.63
2
3
(t, 1H, JHH = 10.0 Hz, CH2); 4.24 (t, 1H, JHH = 4.8 Hz CH2); 4.30
(t, 1H, 2JHH = 11.6 Hz, CH2); 5.39 (t, 1H, 3JHH = 4.0 Hz, CH2); 5.72
(s, 1H, pz-H); 5.84 (s, 1H, pz-Hiso); 7.07 (d, 1H, 3JHH = 4.8 Hz, 2-th-
Hiso); 7.13 (d, 1H, 3JHH = 3.6 Hz, 2-th-H); 7.37 (d, 1H, 3JHH = 3.6 Hz,
2.2.15. Synthesis of [2-(3,5-Dimethylpyrazol-1-yl)ethyl]thiophen-
2-ylmethyleneiminemethylpalladium(II) Tetrakis(3,5-
trifluoromethylphenyl)borate (3b). Complex 3a (0.20 g, 0.52
mmol) was added to an NCMe solution of Na[BAr4] (0.46 g, 0.52
mmol) to give 3b as a light orange solid product. Yield: 0.61 g (93%).
1H NMR (CDCl3): δ 0.70 (s, 3H, Pd-CH3); 0.94 (s, 3H, Pd-CH3iso);
3
3-th-Hiso); 7.48 (s, 4H, BAr4); 7.60 (d, 1H, JHH = 3.2 Hz, 3-th-H);
7.69 (s, 8H, BAr4); 8.00 (s, 1H, CHN); 8.19 (s, 1H, CHNiso). IR
(Diamond ATR, cm−1): 1620 ν(CHN) imine. Anal. Calcd for
C47H32BrN4PdSBF24: C, 40.47; H, 2.38; N, 3.93; S, 2.24. Found: C,
40.04, H, 2.04; N, 3.40%; S, 2.82. Positive ion ESI-MS: m/z (%) 433.0
(18%) [M − NCMe]+, 460.0 (10%) [M − Me]+. Negative ion ESI-
MS: m/z (%) 863.0 (100%) [M]−.
2.04 (s, 6H, CH3iso); 2.11 (s, 3H, CH3); 2.19 (s, 3H, CH3); 2.25 (s,
3H, Pd-NC−-CH3); 3.63 (t, 1H, 2JHH = 10.0 Hz, CH2); 4.30 (m, 2H,
3
CH2); 5.39 (t, 1H, JHH = 4.0 Hz, CH2); 5.72 (s, 1H, pz-H); 5.84 (s,
1H, pz-Hiso); 7.07 (t, 1H, 3JHH = 4.0 Hz, 2-th-H); 7.13 (d, 1H, 3JHH
=
3
3.6 Hz, 2-th-Hiso); 7.37 (d, 1H, JHH = 3.6 Hz, 3-th-H); 7.48 (s, 4H,
BAr4); 7.60 (d, 1H, 3JHH = 3.2 Hz, 4-th-Hiso); 7.69 (s, 8H, BAr4); 7.76
(d, 1H, 3JHH = 4.8 Hz, 1-th-H); 7.87 (d, 1H, 3JHH = 4.8 Hz, 1-th-Hiso);
8.00 (s, 1H, CHN); 8.19 (s, 1H, CHNiso). IR (Diamond ATR,
cm−1): 1623 ν(CHN) imine. Anal. Calcd for C47H33N4PdSBF24: C,
44.79: H, 2.62; N, 4.45; S, 2.54. Found: C, 44.13; H, 2.21; N, 4.21; S,
3.33. Positive ion ESI-MS: m/z (%) 354.0 (10%) [M − NCMe]+,
380.0 (10%) [M − Me]+. Negative ion ESI-MS: m/z (%) 863.0
(100%) [M]−.
2.2.18. Synthesis of [2-(3,5-Di-tert-butylpyrazol-1-yl)ethyl]-2-
bromothiophen-2-ylmethyleneiminemethylpalladium(II) Tetrakis-
(3,5-trifluoromethylphenyl)borate (6b). Complex 6a (0.23 g, 0.42
mmol) was reacted with Na[BAr4] (0.37 g, 0.42 mmol) to afford 6b as
1
a foamy light orange solid. Yield: 0.54 g (90%). H NMR (CDCl3): δ
0.88 (s, 3H, Pd-CH3); 1.28 (s, 9H, tBu); 1.31 (s, 9H, tBu); 2.24 (s, 3H,
2
Pd-NC-CH3); 3.63 (t, 1H,2JHH =11.6 Hz, CH2); 4.16 (d, 1H, JHH
=
13.6 Hz, CH2); 4.74 (d, 1H, 2JHH =15.2 Hz, CH2); 5.87 (s, 1H, pz-H);
5.99 (t, 1H, JHH = 16.0 Hz, CH2); 7.11 (d, 1H, JHH = 3.6 Hz, 2-th-
2
3
2.2.16. Synthesis of [2-(3,5-Di-tert-butylpyrazol-1-yl)ethyl]-
thiophen-2-ylmethyleneiminemethylpalladium(II) Tetrakis(3,5-
trifluoromethylphenyl)borate (4b). Complex 4a (0.06 g, 0.13
mmol) and Na[BAr4] (0.12 g, 0.13 mmol) were reacted to give 4b
3
H); 7.22 (d, 1H, JHH = 4.0 Hz, 3-th-H); 7.50 (s, 4H, BAr4); 7.68 (s,
8H, BAr4); 8.07 (s, 1H, CHN). IR (Diamond ATR, cm−1): 1626
ν(CHN) imine. Anal. Calcd for C53H44BrN4PdSBF24: C, 44.72; H,
3.09; N, 3.94; S, 2.25. Found: C, 44.43; H, 3.08; N, 3.73; S, 2.55.
Positive ion ESI-MS: m/z (%) 502.0 (100%) [M − Me − NCMe]+.
Negative ion ESI-MS: m/z (%) 863.0 (100%) [M]−.
1
as a foamy orange solid. Yield: 0.17 g (95%). H NMR (CDCl3): δ
t
0.85 (s, 3H, Pd-CH3); 1.30 (s, 18H, Bu); 2.22 (s, 3H, Pd-NC-CH3);
3
5.86 (s, 1H, pz-H); 7.15 (t, 1H, JHH = 4.4 Hz, 2-th-H); 7.49 (s, 4H,
3
BAr4); 7.55 (d, 1H, JHH = 3.6 Hz, 3-th-H); 7.67 (s, 8H, BAr4); 7.78
2.3. Molecular Structures of 1a and 5a. Single-crystal X-ray
diffraction data for compounds 1a and 5a were collected on a Bruker
APEXII diffractometer with Mo Kα (λ = 0.71073 Å) radiation and a
3
(d, 1H, JHH = 4.8 Hz, 1-th-H); 8.23 (s, 1H, CHN). IR (Diamond
ATR, cm−1): 1627 ν(CHN) imine. Anal. Calcd for
D
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detector to crystal distance of 4.00 cm. The initial cell matrix was
obtained from three series of scans at different starting angles. Each
series consisted of 12 frames collected at intervals of 0.5° in a 6° range
with an exposure time of about 10 s per frame. The reflections were
successfully indexed by an automated indexing routine built in the
APEXII program suite. The data were collected using the full sphere
data collection routine to survey the reciprocal space to the extent of a
full sphere to a resolution of 0.75 Å. Data were harvested by collecting
2982 frames at intervals of 0.5° scans in ω and φ with exposure times
of 10 s per frame.15 A successful solution by the direct methods of
SHELXS97 provided all non-hydrogen atoms from the E map. All
non-hydrogen atoms were refined with anisotropic displacement
coefficients. All hydrogen atoms except those on the solvent water
molecules were included in the structure factor calculations at idealized
positions and were allowed to ride on the neighboring atoms with
relative isotropic displacement coefficients.16
2.4. Copolymerization Procedure for Styrene and Carbon
Monoxide. In a stainless steel autoclave (50 mL) containing the
catalyst precursor (0.01 mmol) and stirrer was transferred 2.5 mL of
styrene and 1 mL of CH2Cl2 under an argon atmosphere. In the case
of neutral complexes, Na[BAr4] was added. The system was then
charged with CO. The reaction mixture was stirred at 800 rpm at
various temperatures, pressures, and time. After the reaction the
autoclave was cooled under running tap water, the gas was vented, and
the reaction mixture was quenched with MeOH. The crude product
(viscous) was purified in CH2Cl2/MeOH, and the white polyketone
was dried under vacuum overnight.
5a,b. These isomers only exist in solution, since the solid-state
structure of 5a has only the trans isomer in the unit cell. A
typical 1H NMR spectrum that depicts the presence of the trans
and cis is given for compound 3a in Figure S1 (Supporting
Information). Here signals for the imine proton18 were found at
8.32 ppm (trans) and 8.23 ppm (cis). Furthermore, Pd-CH3
signals were observed at 0.70 ppm (trans) and 0.93 ppm (cis).
All the signature peaks for the methyl and thiophene showed
similar trans and cis peaks. The peaks for complexes 3b and
5a,b showed similar peak patterns.
The CH2CH2 linker protons in all the palladium complexes
were not always well resolved at room temperature. Typical
examples are illustrated by complexes 2b and 6a. At 25 °C
complex 2b showed four broad peaks between 3.56 and 4.86
ppm, while no signal was observed for 6a. However, at −50 °C,
these protons appeared as two triplets and two doublets in both
2b and 6a (Figures 1 and 2, respectively). Our inability to
2.5. Copolymerization Procedure for Ethylene and Carbon
Monoxide. In a stainless steel autoclave (50 mL) containing the
catalyst precursor (0.01 mmol) and stirrer was transferred 5 mL of
CH2Cl2 under an argon atmosphere. In the case of neutral complexes,
Na[BAr4] was added. The system was then charged with ethylene and
CO. The reaction mixture was stirred at 800 rpm at various
temperatures, pressures, and time. After the reaction, the autoclave
was cooled under running tap water, the gas was vented, and the dark
solid crude product was purified in 1,1,1,3,3,3-hexafluoro-2-propanol
(HFIP). The polyketone was in soluble in common organic solvents,
and this characteristic was attributed to the high molecular weight of
the copolymer.
Figure 1. 1H NMR spectra of 2b at (a) 25 °C and (b, insert) −50 °C.
3. RESULTS AND DISCUSSION
3.1. Syntheses of Ligands and Complexes. Compounds
L1−L6, used as ligands to prepare palladium complexes, were
prepared by the condensation of the appropriate aldehyde and
alkylpyrazolylamine (Scheme 1) and purified by sublimation to
remove unreacted starting materials. Compounds L1−L6 were
isolated as oils in yields ranging from 60 to 82%. Reactions of
L1−L6 with [PdCl(Me)(cod)] afforded the corresponding
complexes 1a−6a in high yields (70−82%) (Scheme 1).
Cationic complexes 1b−6b were prepared using Na[BAr4] to
abstract chlorides from complexes 1a−6a (Scheme 1). Ligands
L1 and L2 behave either as N∧N donors in 1a and 2a or
N∧N∧N donors in 1b and 2b. Ligands L3−L6 behave as
bidentate N∧N donors in 3a−6a and 3b−6b.
1
Figure 2. H NMR spectrum of 6a at (a) 25 °C and (b, insert) −50
°C.
observe linker protons in the 1H NMR spectra of these
complexes could be attributed to the rapid molecular motion of
the linker protons at 25 °C, which slows down considerably at
−50 °C.
The ligands and their corresponding palladium complexes
both exhibited strong IR absorption bands between 1589 and
1692 cm−1, typical of an imine functionality.17 1H NMR spectra
of the ligands and complexes also provided useful information
1
The H NMR spectra of complexes 3b−6b was a singlet
between 2.22 and 2.27 ppm, assigned to the methyl of NCMe,
indicative of the NCMe stabilizing the palladium center in 3b−
6b. In the absence of NCMe, there is instant formation of
palladium black, suggesting the sulfur atom in the thiophene is
unable to stabilize the palladium center in 3b−6b without a
coordinating solvent.
1
in the structural elucidation of all compounds. The H NMR
spectra of complexes 3a,b and 5a,b showed two sets of signals
for each proton, representing two isomers, but the rest of the
complexes showed no isomerization. The isomers were in a
ratio of 3:1 (based on integration) representing trans and cis
geometries. Complexes with no isomerization are trans.
Isomers with the CH3 group trans to the imine nitrogen are
defined as trans in 3a,b and 5a,b, and those with the CH3 group
trans to the pyrazolyl nitrogen are defined as cis in 3a,b and
Positive and negative ion mass spectroscopic data were
collected for complexes 1b−6b. Molecular ions of the positive
species of complexes 1b and 2b were obtained at m/z 349.1
(100%) (Figure S2, Supporting Information) and m/z 433.0
E
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(100%), respectively. Complexes 3b−6b did not show the
molecular ion but gave various fragments of the molecule,
which can be attributed to the individual complexes. The
negative ion mass spectroscopic data of complexes 1b−6b
showed the counterion molecular ion at m/z 863.0 (100%)
(Figure S3, Supporting Information). The mass spectral data
confirmed the presence of both the cationic species and the
counteranion [BAr4]−.
3.2. Molecular Structures of 1a and 5a. Single crystals
suitable for X-ray analysis of complexes 1a and 5a were grown
by slow diffusion of hexane into a CH2Cl2 solution at −4 °C.
Crystallographic data and structural refinement parameters are
given in Table S1 (Supporting Information). It must be noted
that in the crystal structure of 1a there is compositional
disorder at the C1 site. Namely, 88% of the time this
coordination site is occupied by the C1 methyl group and 12%
of the time by a Cl atom. Thus, two compounds with different
compositions cocrystallized in the crystal selected for the
analysis. Hereafter we will concentrate on the discussion of the
major component of complexes 1a and 5a.
Molecular structures of 1a and 5a are shown in Figures 3 and
4, respectively, while selected bond lengths and angles of these
complexes are given in the figure captions. The Pd coordination
geometry in both complexes is distorted square planar. The
observed bond angles for N(1)−Pd(1)−C(1) are 96.68(19)° in
1a, and 91.80(2)° in 5a, showing deviations from planarity and
ideal geometry. The Pd−CH3 bond length is 2.018(5) Å in 1a
Figure 4. Molecular structure of 5a, drawn with 50% probability
ellipsoids. Selected bond lengths (Å) and angles (deg): Pd(1)−N(1),
2.020(5); Pd(1)−N(3), 2.173(6); Pd(1)−Cl(1), 2.3217(13); Pd(1)−
C(1), 2.034(6); N(2)-(N(3), 1.362(7); N(1)−Pd(1)−C(1), 91.8(2);
N(1)−Pd−N(3), 81.48(19); N(1)−Pd−Cl(1), 177.65(14); C(1)−
Pd(1)−N(3), 172.5(2); N(3)−Pd(1)−Cl(1), 96.96(13); C(1)−Pd−
Cl(1), 89.60(17).
and 2.034(6) Å in 5a, and this difference is probably statistically
significant. Interestingly, in both structures the Pd−N distance
to the imine N atom situated in the middle of the ligand is
substantially shorter (∼0.07 and ∼0.12 Å in 1a and 5a) than
that to the N ligand at the pyrazole or pyridine ring. The
observed Pd−CH3 bond lengths in 1a and 5a are in good
agreement with similar bond lengths (2.055(38) Å) averaged
for 351 relevant palladium complexes reported to the
Cambridge Structural Database (CSD).19 The Pd−Cl bond
length was recorded as 2.267(8) Å in 1a and 2.3217(13) Å in
5a, which are statistically different but are in the range (2.242−
2.516 Å) reported for 1776 relevant palladium complexes in the
Cambridge Structural Database (CSD).19 The different pallada-
cycle sizes in 1a and 5a expectedly result in different N−Pd−N
angles: 79.31(5)° for the five-membered cycle in 1a and a much
less strained 81.48(19)° for the six-membered cycle in 5a.
3.3. Copolymerization Reaction of Olefins/CO Cata-
lyzed by Complexes 2b and 3b. Complexes 1a−6a were
screened as catalyst precursors for the reactions of ethylene/
CO and styrene/CO in a 1:1 mixture of CH2Cl2 and NCMe
when equimolar amounts of Na[BAr4] were added to each
precursor. Invariably, these attempts resulted in the production
of palladium black. Subsequently the cationic species 1b−6b
were prepared in stoichiometric reactions as precursors to the
olefin/CO copolymerization reactions above. With the
exception of 2b and 3b, (Scheme 2) all of the other cationic
species had very low activities, invariably decomposing to
palladium black. Even for 2b and 3b there were significant
amounts of palladium black formation, hence their moderate
activities.
Figure 3. Molecular structure of 1a, drawn with 50% probability
ellipsoids. Selected bond lengths (Å) and angles (deg): Pd(1)−N(1),
2.1252(15); Pd(1)−N(2), 2.0573(14); Pd(1)−Cl(1), 2.267(8);
Pd(1)−C(1), 2.018(5); N(2)−C(8), 1.462(2); N(1)−C(6),
1.354(2); C(8)−C(9), 1.523(2); C(1)−Pd(1)−N(2), 96.68(19);
C(1)−Pd(1)−N(1), 174.9(2); N(2)−Pd(1)−N(1), 79.31(5);
C(1)−Pd(1)−Cl(1), 89.8(2); N(2)−Pd(1)−Cl(1), 169.86(4);
N(1)−Pd(1)−Cl(1), 94.61(4); C(2)−N(1)−C(6), 118.63(14);
N(1)−Pd(1)−Cl(1), 94.61(4); N(1)−C(6)−C(7), 114.71(14).
Both types of polyketones were characterized by a
combination of NMR, IR, GPC, and thermal analyses. A
typical 13C{1H} NMR spectrum (Figure S4, Supporting
Information) of the styrene/CO copolymer showed a
methylene carbon at 43 ppm, a tertiary carbon at 53 ppm,
phenyl carbons between 126 and 128 ppm, ipso carbon peaks
F
dx.doi.org/10.1021/om300798y | Organometallics XXXX, XXX, XXX−XXX

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Scheme 2. Copolymerization Reaction of Styrene/CO and Ethylene/CO
a
Table 1. Styrene and Carbon Monoxide Copolymerization Catalyzed by 2b
b
c
c
c
d
entry
temp, °C
time, h
pressure, MPa
yield, mg
activity, g mol−1 h−1
Mw
Mn
PDI
TON
selectivity ll:(ul:lu):uu
1
2
3
4
5
6
7
8
25
40
60
40
40
40
40
40
24
24
24
3
1
1
1
1
1
1
3
5
100
190
110
120
140
220
50
420
790
23200
28400
8400
21900
16 100
5900
1.06
1.76
1.42
1.10
1.69
1.47
1.15
1.39
0.46
1.17
1.84
1.30
1.54
0.92
1.40
0.95
0:9:18:73
0:11:11:78
0:11:11:78
0:11:11:78
0:11:11:78
0:11:11:78
0:11:11:78
0:11:11:78
460
4000
1170
460
10100
15400
35300
4100
9200
12
48
3
9100
24000
3500
1670
250
24
60
8 700
6300
a
b
c
Conditions: catalyst precursor, 0.01 mmol; styrene:Pd = 2100:1; CH2Cl2 volume, 1.0 mL. Yield after purification. Determined by GPC.
d
Determined by 13C{1H} NMR spectroscopy.
a
Table 2. Ethylene and Carbon Monoxide Copolymerization Catalyzed by 2b and 3b
b
entry
cat.
temp, °C
time, h
ethylene pressure, MPa
CO pressure, MPa
yield, mg
activity, g mol−1 h−1
1
2b
2b
3b
3b
2b
2b
3b
3b
2b
2b
2b
3b
3b
3b
2b
2b
3b
3b
2b
3b
25
40
25
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
60
60
24
24
24
24
3
2
2
2
2
2
3
2
3
4
4
4
4
4
4
4
3
4
3
4
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
3
2
3
1
1
110
210
95
460
880
2
3
395
4
184
60
767
5
2000
3330
1700
2767
4670
1380
1060
3667
1275
944
6
3
100
51
7
3
8
3
83
9
3
140
165
190
110
153
170
40
10
11
12
13
14
15
16
17
18
19
20
12
18
3
12
18
3
1330
1330
1330
1 330
3670
3333
3
40
3
40
3
40
3
110
100
3
4
a
b
Conditions: catalyst precursor, 0.01 mmol; CH2Cl2 volume, 5.0 mL. Yield after purification.
at 136, 137, and 138 ppm indicative of three stereoisomers, and
a carbonyl carbon peak at 206.4 ppm. The stereoslectivity of
C1-symmetry ligands such as those reported here are hardly
predictable in comparison to enantiopure C2 or C2v symmetry
ligands, which as a rule of thumb give optically active isotactic
and syndiotactic polyketones, respectively.20 The stereo-
chemistry for the polyketone produced with our catalysts
showed that three ipso carbon peaks, which were assigned as uu
(unlike−unlike), ul (unlike−like), and lu (like-unlike). The uu
isomer was dominant (78%); hence, the stereochemistry of the
copolymer is syndiotactic.21,22
IR spectra of the styrene/CO copolymers showed a strong
absorption band in the range 1695.3−1708.8 cm−1, assigned to
the carbonyl functionality, a methylene band in the range
2906.5−2910.4 cm−1, and monosubstituted benzene bands at
696 and 748 cm−1 (Figure S6, Supporting Information). These
are indicative of CO and styrene as pendant groups on the
polyketone backbone.
Differential scanning calorimetry (DSC) of styrene/CO
copolymers gave Tg values in the range 112.2−116.5 °C.
However, no Tm values were observed. This suggests that the
copolymer could be either amorphous or semicrystalline. The
polyketone decomposed above 360.6 °C, as shown by TGA
experiments (Figure S7, Supporting Information).
1
The 13C{1H} NMR and H NMR (Figure S5, Supporting
Information) spectra showed that the polyketones formed are
perfectly alternating.
The reaction parameters of catalyst 2b for styrene/CO
copolymerization are shown in Table 1. Temperature variation
G
dx.doi.org/10.1021/om300798y | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
from 25 to 60 °C showed activity and molecular increase from
25 to 40 °C but decreased at 60 °C (Table 1, entries 1 and 2)
with no significant change in stereochemistry. This suggests
that at 60 °C there is some deactivation of catalyst and the
solubility of CO in the liquid phase is low. Similar observations
have been reported.23 Furthermore, an increase in temperature
above 60 °C resulted in the formation of polystyrene, which
was attributed to free radical polymerization of styrene at high
temperatures.24
Time variation from 3 to 48 h showed a decrease in activity
and increase in molecular weight, suggesting catalyst
deactivation with time (Table 1, entries 4−6). An increase in
CO pressure resulted in a decrease in activity and molecular
weight (Table 1, entries 7 and 8). This is because at high CO
pressure CO coordinates strongly to the palladium center,
therefore retarding the coordination−insertion of olefin.
Catalyst 2b was observed to have optimum reaction conditions
at 1 MPa of CO, 40 °C, and 3 h which gives an activity of 4000
g mol h−1.
decomposes at temperatures above 60 °C and also gradually
deactivates with time.
4. CONCLUSION
Pyrazolylimine hemilable ligands and their palladium complexes
were prepared. These ligands show that the pyrazolyl nitrogen
atom is a weak donor in comparison to the pyridine nitrogen
atom but a stronger donor than thiophene sulfur. In situ
generation of the active catalysts with Na[BAr4] under
copolymerization conditions resulted in very low activities
and the formation of palladium black. However, presynthesized
cationic species showed moderate olefin/CO copolymerization
activities. Comparing the activities of 2b and 3b to those of
other nitrogen-based palladium complexes such as 2,2′-
bipyrindine and 1,10-phenanthroline reported in the literature
revealed that 2b and 3b have low activities.
ASSOCIATED CONTENT
* Supporting Information
■
S
Complexes 2b and 3b catalyzing the ethylene/CO
copolymerization reaction produced alternating copolymers,
which were insoluble in common organic solvents. 13C{1H}
(Figure S8, Supporting Information) and 1H NMR (Figure S9,
Supporting Information) spectroscopic data of the copolymers
were obtained in both HFIP and CDCl3. The 13C{1H} NMR
spectra showed methylene carbons and carbonyl carbons in a
ratio of 2:1 at 35.4 and 212.9 ppm, respectively. This and the
1H NMR spectrum, which showed only a singlet at 2.78 ppm,
suggest that the ethylene/CO copolymer was a perfectly
alternating polyketone. IR spectrum (Figure S10, Supporting
Information) also showed a strong carbonyl peak at 1693 cm−1
and a methylene peak at 2912 cm−1, indicating the presence of
CO and saturated ethylene in the polyketone backbone.
DSC experiments performed on the copolymers gave Tg
values in the range 41.9−45.2 °C and Tm values in the range
263−268 °C. The observation of aTm value showed that the
copolymer was crystalline. The polyketones decomposed above
325.5 °C.
Figures, CIF files, and a table giving additional characterization
data and crystallographic data. This material is available free of
charge via the Internet at http://pubs.acs.org. Crystallographic
data for structural analysis have also been deposited with the
Cambridge Crystallographic Data Centre: CCDC 885488 and
885489. Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road, Cambridge
CB2 1EZ, U.K. (fax, +44−1223−336063; e-mail, deposit@
ccdc.cam.ac.uk; web, http://www.ccdc.cam.ac.uk).
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
Funding for this project was provided by the National Research
Foundation and the University of Johannesburg (South Africa).
We thank one of the reviewers for a number of very helpful
comments and suggestions.
Catalysis data (Table 2) showed that the activities of 2b and
3b increased with temperature from 25 to 40 °C (Table 2,
entries 1−4) but decreased at 60 °C (Table 2, entries 19 and
20) with no change in the nature of the polyketone. However,
increasing the temperature above 60 °C gave no activity. This
suggests that both catalysts deactivate or decompose at higher
temperatures.
A time study from 3 to 18 h showed activities decreasing as
the reactions were run for longer times (Table 2, entries 9−14),
suggesting catalyst deactivation with time. Increasing the CO
pressure decreased the activities of both catalysts (Table 2,
entries 15−18). Furthermore, an increase in ethylene pressure
resulted in an increase in activity for both catalysts (Table 2,
entries 5−8), which agrees with previous reports.2,3 Optimum
conditions for both 2b and 3b of the catalytic reaction are 40
°C, 1 MPa of CO, 3 h, and 4 MPa of ethylene, giving activities
of 4670 and 3667 g mol−1 h−1, respectively (Table 2, entries 9
and 12).
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