Palladium complexes of multidentate pyrazolylmethyl pyridine ligands: Synthesis, structures and phenylacetylene polymerization

Article abstract of DOI:10.1016/j.poly.2006.09.007

The compounds, 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine (_MeN?N?N) (L1) and 2,6-bis(3,5-ditertbutylpyrazol-1-ylmethyl)pyridine (_tBuN?N?N) (L2), react with either [Pd(NCMe)₂Cl₂] or [Pd(COD)ClMe] to form the mononuclear palladium complexes [Pd(_MeN?N?N)Cl₂] (1), [Pd(_MeN?N?N)ClMe] (2), [Pd(_tBuN?N?N)Cl₂] (3) and [Pd(_tBuN?N?N)ClMe] (4). Reactions of 1, 2 and 4 with the halide abstractor, NaBAr₄ (Ar = 3,5-(CF₃)₂C₆H₃), led to the formation of stable tridentate cationic species [Pd(_MeN?N?N)Cl]⁺(5), [Pd(_MeN?N?N)Me]⁺ (6) and [Pd(_tBuN?N?N)Cl]⁺ (7) respectively. The analogous carbonyl linker cationic species [Pd{(3,5-Me₂pz-CO)₂-py}Cl]⁺ (9) and [Pd{(3,5-^tBu₂pz-CO)₂-py}Cl]⁺ (10), prepared by halide abstraction of the neutral complexes [Pd{(3,5-Me₂pz-CO)₂-py}Cl₂] and [Pd{(3,5-^tBu₂pz-CO)₂-py}Cl₂] by NaBAr₄, were however less stable with t_1/2 of 14 and 2 days respectively. Attempts to crystallize 1 and 3 from the mother liquor resulted in the isolation of the salts [Pd(_MeN?N?N)Cl]₂[Pd₂Cl₆] (11) and [Pd(_tBuN?N?N)Cl]₂[Pd₂Cl₆] (12). Although when complexes 1-4 were reacted with modified methylaluminoxane (MMAO) or NaBAr₄, no active catalysts for ethylene oligomerization or polymerization were formed, activation with silver triflate (AgOTf) produced active catalysts that oligomerized and polymerized phenylacetylene to a mixture of cis-transoidal and trans-cisoidal polyphenylacetylene.

Full text of DOI:10.1016/j.poly.2006.09.007

Polyhedron 26 (2007) 851–861
www.elsevier.com/locate/poly
Palladium complexes of multidentate pyrazolylmethyl pyridine
ligands: Synthesis, structures and phenylacetylene polymerization
a
b
a,
c
Stephen O. Ojwach , Ilia A. Guzei , James Darkwa *, Selwyn F. Mapolie
a
Department of Chemistry, University of Johannesburg, Auckland Park Kingsway Campus, Auckland Park 2006, South Africa
b
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, USA
c
Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
Received 7 August 2006; accepted 14 September 2006
Available online 22 September 2006
Abstract
ˆ ˆ
The compounds, 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine (MeNNN) (L1) and 2,6-bis(3,5-ditertbutylpyrazol-1-ylmethyl)pyri-
ˆ ˆ
dine (tBuNNN) (L2), react with either [Pd(NCMe)2Cl2] or [Pd(COD)ClMe] to form the mononuclear palladium complexes
ˆ ˆ ˆ ˆ ˆ ˆ ˆ ˆ
[Pd(MeNNN)Cl2] (1), [Pd(MeNNN)ClMe] (2), [Pd(tBuNNN)Cl2] (3) and [Pd(tBuNNN)ClMe] (4). Reactions of 1, 2 and 4 with the halide
+
ˆ ˆ
abstractor, NaBAr4 (Ar = 3,5-(CF3)2C6H3), led to the formation of stable tridentate cationic species [Pd(MeNNN)Cl] (5),
+
+
ˆ ˆ
ˆ ˆ
[Pd(MeNNN)Me] (6) and [Pd(tBuNNN)Cl] (7) respectively. The analogous carbonyl linker cationic species [Pd{(3,5-Me2pz-CO)2-
py}Cl]+ (9) and [Pd{(3,5-tBu2pz-CO)2-py}Cl]+ (10), prepared by halide abstraction of the neutral complexes [Pd{(3,5-Me2pz-CO)2-
py}Cl2] and [Pd{(3,5-tBu2pz-CO)2-py}Cl2] by NaBAr4, were however less stable with t1/2 of 14 and 2 days respectively. Attempts to crystallize
ˆ ˆ
ˆ ˆ
1 and 3 from the mother liquor resulted in the isolation of the salts [Pd(MeNNN)Cl]2[Pd2Cl6] (11) and [Pd(tBuNNN)Cl]2[Pd2Cl6] (12).
Although when complexes 1–4 were reacted with modified methylaluminoxane (MMAO) or NaBAr4, no active catalysts for ethylene
oligomerization or polymerization were formed, activation with silver triflate (AgOTf) produced active catalysts that oligomerized
and polymerized phenylacetylene to a mixture of cis-transoidal and trans-cisoidal polyphenylacetylene.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: (Pyrazolylmethyl)pyridine; Bidentate palladium complexes; Tridentate ligand coordination; Cationic palladium complexes; Phenylacetylene;
Polymerization
1. Introduction
abstraction using silver or alkali metal salts of a very
weakly coordinating or non-coordinating counter ion. It
has been established that the presence of strongly coordi-
nating ligands compete with the monomer for the vacant
coordination site in the active catalyst [3]. It is therefore
essential to have a weakly coordinating ligand that would
not compete with the incoming monomer for the vacant
site of the metal and yet would protect the metal in the
absence of the substrate.
Recently we have used 2,6-bis(pyrazol-1-ylcarbonyl)pyr-
idine palladium dichloride complexes as catalyst precursors
for ethylene polymerization [4]. The ligands in these precur-
sors are potentially tridentate but only coordinate in a
bidentate mode through one pyrazolyl and the pyridine nitro-
gen atoms leaving the second pyrazolyl unit uncoordinated.
Multidentate nitrogen based ligands, such as 2,6-
bis(organylimino)pyridine and 2,6-bis(pyrazol-1-ylmethyl)-
pyridine late transition metal complexes, are good catalysts
for the oligomerization and polymerization of olefins [1].
Other nitrogen based ligands such as bidentate a-diimine
are also known to form cationic Ni and Pd complexes that
polymerize or oligomerize olefins depending on the steric
bulk of the ligand backbone [2]. These cationic a-diimine
Ni and Pd complexes are often prepared by direct halide
*
Corresponding author. Tel.: +27 11 489 2838; fax: +27 11 489 2819.
E-mail address: jdarkwa@uj.ac.za (J. Darkwa).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.09.007

852
S.O. Ojwach et al. / Polyhedron 26 (2007) 851–861
When the complexes are reacted with methylaluminoxane
(MAO) as co-catalysts, they form active catalysts for the
polymerization of ethylene to give high density polyethyl-
ene (HDPE) [4]. However, the expected activity in compar-
ison with our previous catalysts generated from 1,3-
bis(pyrazol-1-yl)benzene [5a] is considerably lower, possi-
bly due to the potential of 2,6-bis(pyrazol-1-yl-
carbonyl)pyridine to bind in a tridentate coordination
mode upon removal of a chloride (Scheme 1). This is likely
to result in the ethylene monomer competing with the sec-
ond pyrazolyl unit in coordinating to the vacant site on the
metal in the catalyst (Scheme 1, route B).
as a trinuclear complex with three ligand units. We found
2,6-bis(pyrazol-1-ylmethyl)pyridine to complex with palla-
dium dichloride fragments in a bidentate fashion; bonding
through one pyrazolyl nitrogen and the pyridine nitrogen,
with the second pyrazolyl unit uncoordinated. The latter
coordinates to the palladium metal upon chloride abstrac-
tion. These findings and attempts to use the palladium
complexes as ethylene and phenylacetylene polymerization
catalysts are described in this paper.
2. Experimental
The stability of the carbonyl linker catalysts was also
found to be lower than the bis(pyrazole)palladium(II) cat-
alysts [5b] and in an attempt to produce more stable cata-
lysts we have replaced the carbonyl linker with a methylene
group. Some of the chemistry of 2,6-bis(pyrazol-1-ylm-
ethyl)pyridine have earlier been explored by Steel et al.
[6] and others [7]. In a report by Steel et al. [6a] a palladium
dichloride complex with this ligand system was formulated
2.1. Materials and instrumentation
Synthetic and 1H NMR experimental manipulations
were performed under a nitrogen atmosphere using
standard Schlenk techniques and glove box. All solvents
were of analytical grade and were dried and distilled prior
to use. Toluene and dichloromethane were dried and dis-
tilled from sodium/benzophenone and P2O5 respectively.
R
+
N
N
O
R
N
polyethylene
R
R
N
Pd
Me
O
N
N
N
A
O
N
Cl
R
MAO
R
N
Pd
Cl
O
R
+
N
O
O
N
B
R
R = Me, tBu
R
R
N
Pd
Me
N
No polymer or
oligomer
N
N
R
R
2+
R
R
R
R
O
O
N
N
N
N
N
Cl
Cl
Pd
Pd
MAO
Cl
Me
Polyethylene
Cl
Cl
Pd
Pd
Cl
N
N
N
Me
O
O
R
R
R
R
R = Me, tBu
Scheme 1.

S.O. Ojwach et al. / Polyhedron 26 (2007) 851–861
853
2,6-Bis(chloromethyl)pyridine, tetrabutylammonium bro-
mide, silver triflate and phenylacetylene (98%) were
obtained from Sigma–Aldrich and used as received.
NaBAr4 (Ar = 3,5-(CF3)2C6H3) was obtained from Boul-
der Scientific and used as received. The starting materials
3,5-ditertbutylpyrazole [8], [Pd(COD)MeCl] [9,10] and
2,6-bis(3,5-dim-ethylpyrazol-1-ylmethyl)pyridine (L1) [6a]
were synthesized following the literature procedures. The
NMR spectra were recorded on a Varian Gemini 2000
instrument (1H at 200 MHz and 13C at 50.1 MHz) at room
temperature. The chemical shifts are reported in d (ppm)
and referenced to the residual CHCl3 in the NMR solvent.
Coupling constants are measured in Hertz (Hz). Elemental
analyses were performed by the micro analytical laboratory
at the University of Cape Town, South Africa, as a service.
Polymer molecular weights were determined by gel perme-
ation chromatography on a Waters 600E instrument
equipped with a Waters differential refractometer detector
(THF, at 30 ꢁC, rate = 1.0 mL/min) and PL–MIXED-Cðe
columns, using polystyrene standards.
to give a light yellow solid. Yield = 0.31 g (68%). 1H
NMR (CDCl3): d 0.97 (3H, s, CH3,Pd–Me); 2.34 (6H, s,
CH3, pz); 2.49 (6H, s, CH3, pz); 5.68 (2H, d, CH2,
2
2JHH = 15.4); 5.74 (d, 2H, CH2, JHH = 15.4); 5.91 (2H,
3
s, 4H-pz); 8.08 (1H, t, 4H-py, JHH = 8.4); 8.15 (2H, d,
3
3,5H-py, JHH = 8.2). 13C NMR (CDCl3): d À7.4; 12.3;
14.9; 52.6; 108.2; 125.3; 141.7; 143.2; 151.8; 152.0. Anal.
Calc. for C18H24N5PdCl Æ CH2Cl2: C, 42.53; H, 4.85; N,
13.06. Found: C, 42.31; H, 4.77; N, 11.64%.
ˆ ˆ
2.2.4. [Pd(tBuNNN)Cl2] (3)
To a solution of [Pd(NCMe)Cl2] (0.20 g, 0.78 mmol) in
CH2Cl2 (20 mL) was added L2 (0.36 g, 0.78 mmol). The
clear orange solution was stirred for 24 h after which an
equal volume of hexane was added and kept at À4 ꢁC to
afford compound 3 as a yellow solid. Single crystals suit-
able for X-ray analysis were obtained by slow evaporation
1
of CDCl3 used as H NMR solvent. Yield = 0.35 g (71%).
1H NMR (CDCl3): d 1.27 (9H, s, tBu, pz); 1.34 (9H, s, tBu,
pz); 1.41 (9H, s, tBu, pz); 1.75 (9H, s, tBu, pz); 5.72 (1H, d,
2
2
CH2, JHH = 18.6); 5.78 (1H, d, CH2, JHH = 15.4); 5.92
2.2. Synthesis of ligands and palladium complexes
(1H, s, 4H-pz); 5.97 (1H, s, 4H-pz); 6.23 (1H, d, CH2,
2
2JHH = 15.6); 7.01 (1H, d, CH2, JHH = 19.0); 7.44 (2H,
2.2.1. 2,6-{(3,5-tBu2pzCH2)2py} (L2)
d, 3,5H-py, JHH = 8.0); 7.69 (1H, t, 4H-py, JHH = 8.0).
13C NMR (CDCl3): d 30.1; 30.2; 30.6; 31.1; 31.4; 31.8;
32.0; 33.0; 56.4; 56.7; 100.8; 104.3; 122.9; 124.0; 140.4;
153.2; 154.0; 155.2; 162.0; 164.3; 164.6. Anal. Calc. for
C29H45N5PdCl2 Æ 0.5CH2Cl2: C, 52.33; H, 6.59; N, 10.17.
Found: C, 52.72; H, 7.08; N, 10.26%.
3
3
A mixture of 2,6-bis(bromomethyl)pyridine (1.00 g,
3.79 mmol) and 3,5-di-tert-butylpyrazole (1.36 g, 7.58
mmol) in benzene (40 mL), 40% aqueous NaOH (12 mL)
and 40% aqueous tetrabutylammonium bromide (10 drops)
was refluxed for 18 h. The organic layer was then sepa-
rated, and evaporated in vacuo. The crude product was
washed with water (40 mL) to afford an analytically pure
ˆ ˆ
2.2.5. [Pd(tBuNNN)MeCl] (4)
1
compound L2 as a white solid. Yield = 1.38 g (75%). H
Compound 4 was prepared according to the procedure
for 2 using L2 (0.21 g, 0.45 mmol) and [Pd(COD)MeCl]
(0.12 g, 0.45 mmol). Yield = 0.17 g (60%). 1H NMR
t
NMR (CDCl3): d 1.23 (18H, s, Bu, pz); 1.31 (18H, s,
tBu, pz); 5.55 (4H, s, CH2); 5.93 (2H, s, 4H-pz); 6.32
3
t
(2H, d, 3,5H-py, JHH = 8.0); 7.46 (1H, t, 4H-py,
(CDCl3): d 1.03 (3H, s, CH3, Pd–Me); 1.23 (9H, s, Bu,
3JHH = 8.0). Anal. Calc. for C29H45N5: C, 75.16; H, 9.72;
pz); 1.35 (9H, s, Bu, pz); 1.41 (9H, s, Bu, pz); 1.61 (9H,
t
t
t
2
N, 15.12. Found: C, 75.01; H, 9.55; N, 15.43%.
s, Bu, pz); 5.35 (1H, d, CH2, JHH = 18.6); 5.53 (1H, d,
CH2, JHH = 15.0); 5.93 (1H, s, 4H-pz); 5.95 (1H, s, 4H-
2
2
ˆ ˆ
2.2.2. [Pd(MeNNN)Cl2] (1)
pz); 6.11 (1H, d, CH2 JHH = 15.4); 6.87 (1H, d, CH2,
3
To a solution of L1 (0.11 g, 0.39 mmol) in CH2Cl2
(20 mL) was added [Pd(NCMe)2Cl2] (0.10 g, 0.39 mmol).
The pink solution was stirred for 12 h and the product
precipitated by addition of hexane (20 mL) to give a pink
solid. Yield = 0.10 g (56%). 1H NMR (CDCl3): d 2.45
(6H, s, CH3, pz); 2.51 (6H, s, CH3, pz); 5.85 (2H, d, CH2,
2JHH = 15.2); 5.93 (1H, s, 4H-pz); 6.17 (2H, d,
2JHH = 19.2); 7. 21 (2H, d, 3,5H-py, JHH = 8.0); 7.69
(1H, t, 4H-py, JHH = 8.0). 13C NMR (CDCl3): d 30.1;
3
30.4; 30.6; 31.1; 31.5; 32.6; 55.7; 100.3; 103.8; 121.8;
122.2; 138.5; 152.8; 153.7; 162.1. Anal. Calc. for
C30H48N5PdCl: C, 58.06; H, 7.80; N, 11.28. Found: C,
58.10; H, 8.32; N, 10.13%.
The synthesis of 6 and 7 are described as examples of
how the cationic complexes were prepared.
2
CH2, JHH = 15.0); 6.28 (1H, s, 4H-pz); 8.11 (2H, d,
3
3
3,5H-py, JHH = 8.4); 8.26 (1H, t, 4H-py, JHH = 8.4).
Anal. Calc. for C17H21N5PdCl2: C, 43.22; H, 4.44; N,
14.83. Found: C, 43.31; H, 4.10; N, 14.91%.
ˆ ˆ
2.2.6. [Pd(MeNNN)Me]BAr4 (6)
To a solution of 2 (0.05 g, 0.11 mmol) in CH2Cl2
(10 mL) was added a solution of NaBAr4 (0.10 g,
0.11 mmol) in CH2Cl2 (10 mL) and stirred for 10 min.
The resultant mixture containing Pd black was filtered over
Celite to give a clear solution. Hexane (20 mL) was added
to the filtrate and kept at À4 ꢁC to afford colourless single
crystals suitable for X-ray analysis. Yield = 0.15 g (30%).
1H NMR (CDCl3): d 1.09 (s, 3H, CH3, Pd–Me); 2.30
ˆ ˆ
2.2.3. [Pd(MeNNN)MeCl] (2)
To a solution of L1 (0.30 g, 1.07 mmol) in Et2O (20 mL)
was added a solution [Pd(COD)ClMe] (0.27 g, 1.07 mmol)
in Et2O (20 mL). A light yellow precipitate was formed
immediately. The mixture was stirred for 3 h, filtered and
the material isolated recrystallized from CH2Cl2–hexane

854
S.O. Ojwach et al. / Polyhedron 26 (2007) 851–861
(6H, s, CH3, pz); 2.35 (6H, s, CH3, pz); 5.14 (2H, d, CH2,
ized by of 1H NMR spectroscopy and gel permeation
chromatography.
2
2JHH = 15.0); 5.70 (2H, d, CH2, JHH = 15.0); 5.95 (2H, s,
4H-pz); 7.22 (2H, d, 3,5H-py, 3JHH = 8.0). 7.55 (1H, t, 4H-
3
py, JHH = 8.2); 7.49 (4H, s, Hp, BAr4); 7.68 (8H, s, Ho,
2.4. X-ray crystallography
BAr4). Anal. Calc. for C50H36BF24N5Pd: C, 46.92; H,
2.73; N, 5.47. Found: C, 47.45; H, 2.48; N, 5.48%.
Crystal evaluation and data collection for 3, 5, 6, 11 and
12 were performed on a Bruker CCD-1000 diffractometer
with MoKa (k = 0.71073 A) radiation and the diffractom-
˚
ˆ ˆ
2.2.7. [(tBuNNN)PdCl]BAr4 (7)
This compound was synthesized according to the proce-
dure described for 6 using 3 (0.09 g, 0.13 mmol) and
NaBAr4 (0.12 g, 0.13 mmol) which gave 7 as a crystalline
eter to crystal distance of 4.9 cm. The initial cell constants
were obtained from three series of x scans at different start-
ing angles. The reflections were successfully indexed by an
automated indexing routine built in the SMART program.
The absorption correction was based on fitting a function
to the empirical transmission surface as sampled by multi-
ple equivalent measurements [11]. The structures were
solved by direct methods and refined by least-squares tech-
niques using SHELXTL program [11]. All non-hydrogen
atoms were refined with anisotropic displacement coeffi-
cients. All hydrogen atoms were included in the structure
factor calculation at idealized positions and were allowed
to ride on the neighbouring atoms with relative isotropic
displacement coefficients.
1
orange solid. Yield = 0.11 g (58%). H NMR (CDCl3): d
t
t
1.43 (18H, s, Bu, pz); 1.52 (18H, s, Bu, pz); 5.68 (2H, d,
2
CH2, JHH = 15.2); 6.08 (2H, s, 4H-pz); 6.21 (2H, d,
CH2, JHH = 15.2); 7.27 (2H, d, 3,5H-py, JHH = 8.0);
7.50 (1H, t, 4H-py, JHH = 8.0); 7.49 (4H, s, Hp, BAr4);
2
3
3
7.68 (8H, s, Ho, BAr4). Anal. Calc. for C59H57BF24N5PdCl:
C, 49.05; H, 3.98; N, 4.85. Found: C, 49.51; H, 4.00; N,
4.97%.
2.3. Phenylacetylene oligomerization and polymerization
In a typical experiment, a solution of AgOTf (0.05 g,
0.2 mmol) in a 20 mL mixture of CH2Cl2 and MeCN
(1:1) was added to a complex solution of 1 (0.05 g,
0.1 mmol) in CH2Cl2 (20 mL). A white precipitate of AgCl
formed immediately, the mixture was stirred for 5 min and
filtered to give a yellow solution of the active catalyst. To
this solution was added 50 equiv. of phenylacetylene mono-
mer (0.64 mL, 5 mmol). The yellow solution gradually
turned dark red and was stirred for a further 1 h. After
the reaction period, the solution was evaporated to dryness
to afford a dark brown crude product, which was dissolved
in minimum amount of CH2Cl2 and the polymer precipi-
tated from methanol (40 mL). This was dried and weighed
to obtain the percent polymer. The methanol solution was
allowed to evaporate to dryness to obtain the oligomer
fractions. Both the oligomers and polymers were character-
3. Results and discussion
3.1. Synthesis of bis(pyrazol-1-ylmethyl)pyridine palladium
complexes
Compound 2,6-bis(3,5-ditertbutylpyrazol-1-ylmethyl)-
ˆ ˆ
pyridine (tBuNNN) (L2) was prepared in good yields
(75%) via phase transfer catalyzed alkylation of 3,5-ditert-
butylpyrazole and 2,6-bis(bromomethyl)pyridine following
the literature procedure [6a] described for the synthesis of
ˆ ˆ
2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine (MeNNN)
(L1). Compounds L1 and L2 reacted with either
[Pd(NCMe)2Cl2]or [Pd(COD)ClMe] to produce the corre-
sponding complexes 1–4 in moderate to good yields
(Scheme 2).
R
R
H
N
N
2
R = Me, L = Cl (1)
R = Me, L = Me (2)
R = tBu, L = Cl (3)
R = tBu, L = Me (4)
N
N
N
N
R
Cl
Pd(NCMe)2Cl2
Pd(COD)MeCl
R
N
N
R
R
R
N
Pd
L
N
N
N
N
40% NaOH
Br
Br 40% [(n-Bu)4]Br
R
R
NaBAr4
R
-NaCl
Ar = 3,5-(CF3)2C6H3
R = Me (L1)
R = tBu (L2)
+
-
BAr4
R = Me, L = Cl (5)
R = Me, L = Me (6)
R = tBu, L = Cl (7)
R = tBu, L = Me (8)
N
R
R
N
N
Pd
L
N
N
R
R
Scheme 2.

S.O. Ojwach et al. / Polyhedron 26 (2007) 851–861
855
The 1H NMR spectra of L1 and L2 gave signature peaks
of the four CH2 linker protons as singlets at 5.28 and
5.55 ppm respectively and typical peaks of methyl and
tertbutyl at 2.25 and 2.51 ppm and 1.21 and 1.31 ppm
respectively. These peaks were diagnostic in the next step
to confirm that complexation of the ligands with the palla-
dium salts had occurred. On complexation of L1 and L2,
the CH2 linker protons appeared as AB quartets. In com-
plexes 1 and 2 these signals are between 5.68–6.17 ppm
(L1 singlet, 5.28 ppm) confirming the presence of diastereo-
topic protons in axial and equatorial positions. This
inequality arises from different chair and boat conforma-
tions that depict the existence of restricted rotations in
the complexes hence the protons could be subjected to dif-
ferent contributions from the ring currents from the pyra-
zole or pyridine groups on the NMR time scale. A
similar spectrum has been reported for the related Cu com-
plex, [Cu{(Me2pz-CH2)2py}(PPh3)]ClO4, [7a] in which the
CH2 linker protons were observed as AB quartets at 4.75
and 5.25 ppm.
ane (MMAO) in the presence of excess ethylene. We did
not observe the formation of oligomers or polymers of eth-
ylene as expected. In subsequent experiments performed on
a preparative scale (Scheme 2), 1H NMR spectroscopic
analyses suggested the formation of cationic species in
which the ligands bind in a tridentate fashion upon chlo-
+
ˆ ˆ
ride abstraction. These cationic species, [Pd(MeNNN)Cl]
+
+
ˆ ˆ
ˆ ˆ
(5), [Pd(MeNNNÀ)Me] (6) and [Pd(tBuNNN)Cl] (7), were
isolated as BAr4 salts and the structures of 5 and 6 (Figs.
2 and 3 respectively) confirmed the tridentate ligand coor-
dination deduced from the spectroscopic data.
Recently we reported that [Pd{(3,5-Me2pz-CO)2py}Cl2]
and [Pd{(3,5-tBu2pz-CO)2py}Cl2] can be activated with
methylaluminoxane (MAO) as co-catalyst to catalyze the
polymerization of ethylene [4]. The inability of the cationic
species generated from 2 and 4 to promote ethylene poly-
merization can be attributed to the strong coordination
of the ‘dangling’ arm of the pyrazolyl unit in 2 and 4, which
blocks the vacant coordination site created upon chloride
abstraction from the metal centre necessary for the coordi-
nation of the ethylene monomer. In the carbonyl linker
analogues used to catalyze the polymerization of ethylene
[4], it is likely that upon activation with MAO, similar tri-
dentate cationic species are formed but under a high ethyl-
ene pressure, one of the coordinated pyrazolyl units
dissociates from the Pd centre to allow the ethylene to
coordinate and undergo subsequent insertion to form poly-
ethylene (Scheme 1, route A). This behaviour is necessi-
tated by the weaker binding ability of the pyrazolyl units
in the carbonyl linker analogues. However, even when 6
and 7 were subjected to ethylene pressures up to 50 atm,
there was no formation of polyethylene; an observation
which highlights the strength of the Pd–N(pz) bonds in 6
and 7 relative to the binding affinity of the ethylene
monomer.
In complexes 3 and 4, the CH2 linker protons appeared
as four distinct AB quartets with geminal coupling con-
stants ranging from 15.4 to 19.2 Hz. This highlights the
increased restricted rotation in 3 and 4 arising from the
more sterically demanding ditertbutyl groups. The large
peak separation of the signals (Dd = 1.38 ppm), however,
is rather unusual. A smaller peak separation between the
CH2 linker protons (Dd = 0.70 ppm) of the related Ru
complex of L1, [Ru{(g6-C6H6)(Me2pz-CH2)2py}]PF6, [12]
has been reported. Recently Cavell and co-workers
reported even a smaller peak separation (Dd = 0.07 ppm)
for geminal coupling of the bridging methylene protons
ˆ ˆ
of [Pd(tBuCNC)Me]BF4; a feature attributed to the slow
inversion of the complex [13]. In contrast to our results,
Cavell and co-workers found that in the chloro analogue,
ˆ ˆ
[Pd(tBuCNC)Cl]BF4, the methylene protons appear as a
singlet at 5.69 ppm probably as a consequence of the lower
trans effect of the ClÀ, which reduces the Pd–N(py) bond
length and gives the ligand backbone greater ability to
3.3. Oligomerization and polymerization of phenylacetylene
The binding affinity of the monomer is important since
in experiments using phenylacetylene instead of ethylene,
we did observe the formation of oligomers and polymers
of phenylacetylene (Table 1). The active catalysts were gen-
erated by reacting complexes 1–4 with AgOTf as halide
abstractor in a mixture of CH2Cl2 and MeCN (3:1). The
ability of catalysts generated from 1 to 4 to oligomerize
and polymerize phenylacetylene therefore highlights the
significance of the reactivity and the binding affinity of
the incoming monomer in displacing or competing with
one of the Pd–N(pz) bonds prior to coordination to the
metal centre (Scheme 3, route A). However, the low per-
centage conversions observed for catalysts 1–4 (36–55%)
compared to 90% conversion [10] obtained for the carbonyl
linker and bis(pyrazole)palladium(II) complexes under
similar conditions suggests the possibility of a competing
deactivation pathway. This could arise from the formation
of the inactive cationic tridentate species (Scheme 3, route
B) which blocks monomer coordination. This argument
1
undergo unhindered rotation [13]. The H NMR spectra
of 3 and 4 could be used to diagnose the bidentate bonding
mode of compound L2 in these complexes. For example in
3 one set of peaks at 1.41 and 1.75 ppm for tertbutyl pro-
tons and 5.92 ppm for the pyrazolyl ring proton were
assigned to the bound pyrazolyl unit while peaks at 1.27
and 1.34 and 5.97 ppm were due to the ‘dangling’ pyrazolyl
unit. This assignment is consistent with the solid state
structure of 3 (vide infra). Similar spectra have been
reported
for
compounds
[Ru{(g6-C6H6)(Me2pz-
CH2)2py}]PF6 [12] and [Pd{(3,5-tBu2pz-CO)2py}Cl2] [4].
3.2. Reactions of complexes 1–4 with NaBAr4
In attempts to generate catalysts for the oligomerization
or polymerization of ethylene, 8 mg of complexes 2 and 4
were reacted with either stoichiometric equivalent of
NaBAr4 or 1000-fold excess of modified methylaluminox-

856
S.O. Ojwach et al. / Polyhedron 26 (2007) 851–861
Table 1
Phenylacetylene oligomerization and polymerization dataa
% Conversionb
Mw
Mn
Mw/Mn
% cisd
c
c
c
Entry
Catalyst
1
2
3
4
5
6
7
8
a
1
51
47
36
38
35
55
47
12
2552
3444
2303
2062
1575
1893
1466
1350
1.62
1.82
1.57
1.52
1.72
1.48 (1.00)
1.65 (1.01)
1.01
44
80
56
74
2
3
4
1e
1f
2g
2h
2716
1697
2230 (620)
3613 (684)
860
1523 (619)
2187 (678)
797
Conditions: Time = 60 min unless stated otherwise; amount of monomer = 0.64 mL; solvent: CH2Cl2:MeCN (40 mL); 3:1, temp = 25 ꢁC;
[Pd] = 1.0 · 10À3 mol; phenylacetyelene:[Pd] = 50:1.
b
Determined by the total mass of the crude product as a percentage of monomer used.
c
Determined by room temperature GPC using polystyrene standards.
Determined by 1H NMR analysis: cis % = [A5.82/(Atotal/6)] · 100 or cis % A5.82 · 104/Atotal Æ 16.66 (A5.82 is the integrated peak area of the vinyl proton
d
in the cis isomer and Atotal is the total integrated peak area of the polymer spectrum).
e
Time = 30 min.
Time = 120 min.
Crude product (mixture of oligomer and polymer).
Oligomer fraction.
f
g
h
+ BAr4
-
R
N
N
R
N
R
+ BAr4-
R
Ph
R
N
Pd
Me
polymer or oligomer
Ph
N
N
N
A
B
N
Cl
R
AgOTf
R
N
Pd
Me
R
N
Ph
N
R
R = Me, tBu
R
N
Pd
Me
N
N
Ph
N
No polymer or oligomer
R
R
Scheme 3.
also supports the slow initiation process observed with sys-
tems 1–4 as opposed to rapid initiation observed with sim-
ple pyrazole and carbonyl linker pyrazolyl palladium
complexes [14].
behaviour. As a reference, even ca. 1% changes in the value
of a ligand solid angle can affect the ligand’s coordination
mode [16]. However, it is interesting to note the size of the
j2-(tBuNNN) ligand in 3; the shielding percentage of this
ˆ ˆ
ˆ ˆ
Generally catalysts 1 and 2 bearing methyl substituents
on the pyrazolyl ligand showed higher activity than the
tertbutyl analogues 3 and 4 (Table 1, entries 1–4). It is
likely that the bulkier tertbutyl group hinders monomer
coordination resulting in low product yields. We have
examined the structurally characterized complexes 3, 5, 6,
11 and 12 from the point of view of the ligand solid angles
expressed in percentage of the metal coordination sphere
shielded by each ligand. Such an approach provides a
quantitative measure of the steric bulk of the ligands [15].
The tridentate ligands in 5, 6 and 11 shield on average
59.6(10)% of the palladium coordination sphere. The j3-
ligand is only 51.0%, and thus the flexibility of (tBuNNN)
can play a major role in the kinetics of the system. Time
dependent polymerization experiments with catalyst 1
(Table 1, entries 1, 5 and 6) showed little variation in the
percent conversion of monomer after 60 min. This is a typ-
ical feature involving catalyst decomposition with time [17]
and in our case would suggest active catalysts forming tri-
dentate species via route B in Scheme 3.
The stereochemistry of the polymers was determined by
1
1H NMR spectroscopy. A typical H NMR spectrum of
polymers obtained showed a sharp singlet at 5.85 ppm
for the vinylic protons, and broad singlets at 6.65 and
6.91 ppm corresponding to the phenyl ortho and meta
protons respectively. From the 1H NMR data, it is evident
that all the polymers were a mixture of cis-transoidal and
ˆ ˆ
tBuNNN) ligand in 12 shields an enormous 70.1% of the
(
central metal, and the difference of over 10% from its
methyl analogue is very significant in terms of dynamic

S.O. Ojwach et al. / Polyhedron 26 (2007) 851–861
857
trans-cisoidal [18] with the percentage cis content ranging
3.4. Stability of cationic and neutral species of 1–4
from 44 to 80 (Table 1, entries 1–4). The cis and trans con-
tent of the polyphenylacetylene formed were established by
the NMR method reported by Perec et al. [18a] and the for-
mula to calculate the cis and trans content of the polymer is
included as a footnote in Table 1. However, there was no
clear dependence of the polymer stereochemistry with
respect to catalyst structure.
GPC analysis showed the presence of both oligomers
(Mw = 620–860) and low molecular weight polymers
(Mw = 2062–3444) (Table 1, entries 1–4, 7 and 9). For
example analysis of the crude products from catalysts 2
(Table 1, entry 7) gave a bimodal distribution GPC trace
corresponding to oligomers of average Mw of 684 and poly-
mers of average Mw of 3613. Upon purification of this
product, the oligomers (methanol soluble fraction) and
polymers (methanol insoluble) gave average Mw of 860
and 3444 respectively.
Following the attempts to establish the ability of 2 and 4
to produce active catalysts for ethylene polymerization as
described above, we investigated the stability of the cat-
ionic species with CH2 and CO linkers using complexes
1–4, [Pd{(3,5-Me2pz-CO)2py}Cl2] and [Pd{(3,5-tBu2pz-
CO)2py}Cl2]. In a typical experiment, 1 (6 mg, 0.012 mmol)
and NaBAr4 (12 mg, 0.012 mmol) were placed in a
J-Young NMR tube and about 0.4 mL CDCl3 added and
the reaction followed by 1H NMR spectroscopy. In all
cases the cationic species of 1–4 were formed within
10 min but reactions of the CO linker analogues took up
to 4 h to go to completion. The stability of the cationic
+
+
ˆ ˆ
ˆ ˆ
species [Pd(MeNNN)Cl]
(5), [Pd(MeNNN)Me]
(6),
+
+
ˆ ˆ
ˆ ˆ
[Pd(tBuNNN)Cl] (7), [Pd(tBuNNN)Me] (8), [Pd{(3,5-
Me2pz-CO)2py}Cl]+ (9) and [Pd{(3,5-tBu2pz-CO)2py}Cl]+À
(10) also varied. By using the proton peaks of the BAr4
counter ion in these salts as internal standard, we estab-
lished that species 5–8 were stable for 30 days without
any signs of decomposition while the carbonyl linker coun-
terparts, 9 and 10, had t1/2 of 14 and 2 days respectively
(Table 2). The low stability of 9 and 10 may arise from
the effect of the carbonyl group in reducing the donor abil-
ity of the pyrazolyl nitrogen atoms in 9 and 10 compared to
the cations with methylene linkers, 5–8. The stability of the
cationic species 5–8 might explain why reactions of 1–4
with either MMAO or NaBAr4 did not result in the pro-
duction of active ethylene polymerization catalysts.
Table 2
Selected 1H NMR signals and half lives (t1/2) of the cationic complexes
5–10 in CDCl3
Compound 1H NMR (CDCl3, ppm)
t1/2
(days)
t
CH2 4H-pz Rpz (R = Me or Bu) 4H-py
5
6
5.25, 5.95 5.99
5.14, 5.70 5.95
5.74, 6.31 6.07
5.68, 6.21 6.08
6.38
2.35, 2.57
2.30, 2.35
1.41, 1.66
1.41, 1.52
2.57, 2.67
1.17, 1.76
7.27
7.55
7.40
7.50
8.04
8.01
>30
>30
>30
>30
14
7
8
Another interesting observation made during attempts
to grow crystals of complexes 1–4 is that the species that
crystallized depended on the procedure used. Attempts to
9
10
6.57
2
Table 3
Crystal data and structure refinement parameters for compounds 3, 5, 6, 11 and 12
Parameter
3
5
6
11
12
Formula
Fw
Temperature (K)
Wavelength (A)
Cryst system
Space group
C
879.74
100(2)
0.71073
triclinic
31H47Cl8N5Pd
C50H35BCl3F24N5Pd
1385.39
100(2)
0.71073
monoclinic
P21/n
15.498(2)
9.2217(12)
38.010(5)
90
100.563(2)
90
5340.5(12)
4
1.723
0.624
2752
C50H36BF24N5Pd
1280.05
100(2)
0.71073
monoclinic
P21/n
10.8623(16)
17.848(3)
26.862(4)
90
96.444(2)
90
5174.7(13)
4
1.643
0.487
2552
C38H46Cl20N10Pd4
1777.45
100(2)
0.71073
triclinic
C64H102Cl20N10Pd4
2146.16
100(2)
0.71073
triclinic
˚
ꢀ
ꢀ
ꢀ
P1
P1
P1
˚
A (A)
10.7180(15)
12.6687(19)
15.490(2)
87.557(3)
70.102(2)
81.527(3)
1956.05(4)
2
9.6999(17)
9.7901(17)
17.018(3)
87.410(3)
89.354(3)
64.335(3)
1455.1(4)
1
10.0902(10)
14.1549(14)
15.7395(15)
88.201(2)
85.368(2)
77.909(2)
2190.7(4)
1
˚
B (A)
˚
C (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
Z
Dcalc. (Mg/m3)
1.494
1.050
900
2.028
2.176
868
1.627
1.461
1080
Absorption coefficient (mmÀ1
F(000)
)
Final R indices (R1)
Reflections collected
Completeness to theta (%)
Goodness-of-fit on F2
0.0509
9611
95.2
1.030
0.0640
41777
99.6
1.067
0.0409
42250
99.8
1.097
0.0626
16921
92.8
0.977
0.0355
13905
89.3
1.036
À3
˚
Largest difference peak and hole (e A
)
1.220 and À0.600
3.243 and À1.802
1.353 and À1.309
1.275 and À1.660
0.835 and À0.636

858
S.O. Ojwach et al. / Polyhedron 26 (2007) 851–861
grow crystals of 1 and 3 directly from the mother liquor
(reaction mixture) gave crystals of tridentately bound cat-
ionic species with [Pd2Cl6]2À as counter ion (11 and 12)
within 7 days. On the other hand, when 3 was precipitated
out initially from the reaction mixture and subsequently
re-dissolved in CH2Cl2, crystals of 3 were obtained. Crys-
tals of 3 were dissolved in CDCl3 and monitored for
21 days by 1H NMR but showed no signs of forming
the cationic species 12. It is not clear what causes the
formation of 11 and 12 from the reaction mixture which
does not occur when 1 and 3 are first isolated and re-
dissolved in CH2Cl2. However, both 11 and 12 crystal-
lized with solvent molecules and rapidly lost solvent when
crystals were removed from the mother liquor and became
insoluble. It was therefore difficult to further characterize
these materials.
3.5. Molecular structure determination by single crystal
X-ray crystallography
Single crystals suitable for X-ray analysis of complexes 3
and 5 were grown by slow evaporation of the dichloro-
methane solvent at room temperature while crystals of 6
were grown by slow diffusion of hexane into dichlorometh-
ane at À4 ꢁC. In other attempts to obtain crystals of 1 and
ˆ ˆ
3, cationic tridentate palladium complexes 2[(MeNNN)-
PdCl]+ (11) and 2[(tBuNNN)PdCl] (12) with [Pd2Cl6]2À
+
ˆ ˆ
counter ions were obtained as described earlier.
Crystallographic data for 3, 5, 6, 11 and 12 are presented
in Table 3, while selected bond lengths and angles are given
in Table 4. Solid state structures of 3, 5, 6, 11 and 12 are
shown in Figs. 1–5 respectively. The five palladium com-
plexes contain the central metal in a slightly distorted
Table 4
˚
Selected bond lengths (A) and bond angles (ꢁ) for 3, 5, 6, 11 and 12
3
5
6
11
X = Cl(1)
12
X = Cl(1)
X = Cl(2)
X = Cl(1)
X = C(1)
Bond lengths
Pd(1)–N(1)
Pd(1)–N(3)
Pd(1)–N(5)
Pd(1)–X
2.026(3)
2.042(3)
2.007(4)
2.031(4)
2.023(4)
2.2803(13)
1.366(6)
1.367(6)
2.028(2)
2.128(2)
2.043(2)
2.029(3)
1.369(3)
1.370(3)
2.011(5)
2.010(5)
2.012(5)
2.2838(18)
1.381(7)
1.356(7)
2.029(2)
1.9925(19)
2.028(2)
2.2872(6)
1.381(3)
1.378(3)
2.3125(5)
1.371(4)
1.371(4)
N(1)–N(2)
N(4)–N(5)
Bond angles
N(1)–Pd(1)–N(3)
N(3)–Pd(1)–X
N(1)–Pd(1)–X
N(1)–Pd(1)–N(5)
N(3)–Pd(1)–N(5)
84.00(13)
173.92(10)
94.36(10)
87.39(17)
177.63(12)
91.07(12)
175.85(17)
88.57(17)
86.07(9)
175.95(11)
93.73(15)
172.90(9)
88.03(9)
88.81(2)
175.1(1)
92.13(11)
175.1(2)
86.9(2)
84.87(10)
178.69(6)
94.59(9)
170.77(8)
85.94(8)
Fig. 1. Molecular structure of complex 3 shown with 50% probability ellipsoids. The hydrogen atoms are omitted for clarity.

S.O. Ojwach et al. / Polyhedron 26 (2007) 851–861
859
Fig. 2. Molecular structure of cation of 5 shown with 50% probability ellipsoids. The boron counter ion and hydrogen atoms are omitted for clarity.
Fig. 3. Molecular structure of cation of 6 shown with 50% probability ellipsoids. The boron counter ion is omitted for clarity.
square planar geometry. In the structure of 3 (Fig. 1) the
nitrogen atom, N5, of the free pyrazolyl unit is directed
away from the palladium metal. It is therefore evident that
the rotation about the CH2 linker in the j2-2,6-
The average bond lengths of the Pd–N(pz) of 2.026(3),
2.015(11), 2.026(11), 2.012(5) and 2.029(2) A of complexes
˚
3, 5, 6, 11 and 12 respectively are all slightly shorter than
˚
the average Pd–N(pz) bond length of 2.06(9) A obtained
{(3,5-tBu2pzCH2)2py} ligand yields
a
j3-2,6-{(3,5-
by averaging 607 bonds in 229 relevant complexes reported
to the Cambridge Structural Database (CSD) [19] but the
difference is not statistically significant because of the high
uncertainties for the average Pd–N(pz) bond length of
tBu2pzCH2)2py} ligand in the cationic species 12 (Fig. 5).
The [Pd2Cl6]2À in the structures of 11 and 12 occupies a
crystallographic inversion centre with the two cationic pal-
ladium complexes residing on each side of the anion. The
spatial arrangement of the cationic palladium moieties in
11 and 12 is such that the Pd–Cl bonds of the cationic
complexes are facing away from the central [Pd2Cl6]2À unit.
˚
2.06(9) A.
˚
The average Pd–N(pz) distance of 2.015(11) A in 5 is
somewhat shorter than that in 6 (2.036(11) A), however
˚
the difference is not statistically significant. On the other

860
S.O. Ojwach et al. / Polyhedron 26 (2007) 851–861
Fig. 4. Molecular structure of compound 11 shown with 50% probability ellipsoids. The hydrogen atoms are omitted for clarity.
Fig. 5. Solid state structure of solvated complex 12 shown with 50% probability ellipsoids. The hydrogen atoms are omitted for clarity.
hand, the Pd–N(py) distance in 6 is substantially longer
than those in 5, 11 and 12, and that is attributed to the
high trans influence of the methyl group as compared to
the chloro ligand. A shorter Pd–N(py) bond length
those reported for the carbene methylene bridged cationic
ˆ ˆ
Pd complex [Pd(CNC)Me]BF4 of 2.0154(9) and
˚
2.044(1) A respectively [20]. The Pd–Cl bond distances of
˚
˚
˚
˚
2.3125(5) A (3), 2.2803(13) A (5), 2.2838(18) A (11) and
˚
˚
2.128(2) A for 6 and 2.031(4) A for 5. A shorter Pd–N(py)
2.2872(6) A (12) in the mononuclear complexes agree well
˚
˚
bond length of 2.036(3) A has been recently reported for
with the average bond distances of 2.33(5) A determined
ˆ ˆ
the chloro complex, [Pd(CNC)Cl]BF4 compared to
by averaging 2055 Pd–Cl bonds in 1268 relevant complexes
reported to the CSD [19]. Interestingly, the average
˚
ˆ ˆ
2.116(3) A of the methyl analogue, [Pd(CNC)Me]BF4
˚
˚
˚
[20]. The average bond length of Pd–N(py) of 2.028(2) A
and Pd–C of 2.044(1) A of 6 are significantly shorter than
Pd–N(pz) bond distances of 2.026(3) A (3) and 2.029(2) A
˚
˚
(12) and Pd–Cl bond distances of 2.3125(5) A (3) and

S.O. Ojwach et al. / Polyhedron 26 (2007) 851–861
861
˚
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metal in 12 is more electrophilic and is expected to have
shorter bond distances. Surprisingly, the longer Pd–N(pz)
˚
distance of 2.012(5) A is observed for 11 than for 12
˚
(2.028(2) A) that contains a bulkier tridentate ligand.
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4. Conclusions
The potentially tridentate bis(pyrazolylmethyl)pyridine
compounds form monometallic palladium complexes with
one uncoordinated pyrazolyl unit when complexed with
either [PdCl2(NCMe)2] or [Pd(COD)MeCl]. Coordination
of the dangling pyrazolyl unit upon chloride abstraction
in an attempt to generate catalysts for ethylene oligomeri-
zation or polymerization results in the formation of inac-
tive cationic tridentate species. The same palladium
complexes however, do form catalysts that catalyze the
oligomerization and polymerization of the more reactive
phenylacetylene producing a mixture of oligomers and
low molecular weight polyphenylacetylene. Competitive
coordination of phenylacetylene and the second pyrazolyl
unit to the Pd metal centre appears to control the catalysis
process. The polydentate ligands were also characterized
with the use of the ligand solid angles.
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Funding for this project was provided by National Re-
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*
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