Palladium(II) complexes containing N,N′-bidentate N-cycloalkyl 2-iminomethylpyridine and 2-iminomethylquinoline: Synthesis, characterisation and methyl methacrylate polymerisation

Article abstract of DOI:10.1016/j.poly.2013.11.036

The reaction of [Pd(CH₃CN)₂Cl₂] with N-cyclopentyl-1-(pyridin-2-yl)methanimine (L₁), N-cyclohexyl-1- (pyridin-2-yl)methanimine (L₂), N-(piperidin-1-yl)-1-(pyridin-2-yl) methanimine (L₃) or

Full text of DOI:10.1016/j.poly.2013.11.036

Polyhedron 69 (2014) 149–155
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Palladium(II) complexes containing N,N0-bidentate N-cycloalkyl
2-iminomethylpyridine and 2-iminomethylquinoline: Synthesis,
characterisation and methyl methacrylate polymerisation
Sunghoon Kim a, Eunhee Kim a, Ha-Jin Lee b, Hyosun Lee a,
⇑
a Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, 1370 Sankyuk-dong, Buk-gu, Daegu-city 702-701, Republic of Korea
b Jeonju Center, Korea Basic Science Institute (KBSI), 634-18 Keumam-dong, Dukjin-gu, Jeonju 561-180, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of [Pd(CH3CN)2Cl2] with N-cyclopentyl-1-(pyridin-2-yl)methanimine (L1), N-cyclohexyl-1-
(pyridin-2-yl)methanimine (L2), N-(piperidin-1-yl)-1-(pyridin-2-yl)methanimine (L3) or N-cyclopentyl-
1-(quinolin-2-yl)methanimine (L4) in ethanol yields the bidentate (NN’) PdCl2 complexes [L1PdCl2],
[L2PdCl2], [L3PdCl2] and [L4PdCl2], respectively. The X-ray crystal structure of the Pd(II) complexes
revealed that the Pd atom in [LnPdCl2] (Ln = L1, L2, L3, L4) shows a distorted square planar geometry
involving two nitrogen atoms and two chloro ligands. The complexes [L1PdCl2] and [L4PdCl2] (of which
the ligands are N-cyclopentyl substituted) showed the highest catalytic activity for the polymerisation of
methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) with an activity of
1.45 ꢀ 105 g PMMA/mol Pd h at 60 °C and a PMMA syndiotacticity (characterized using 13C NMR
spectroscopy) of ca. 0.70.
Received 4 November 2013
Accepted 18 November 2013
Available online 5 December 2013
Keywords:
Schiff base
Pyridyl-imines
Palladium(II) complex
MMA polymerization
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
abundant pyridyl-imines and analogues, such as iminoquinoline
[44–50], in the transition metal complexes. Structural variations
Transition metal complexes containing ligands with an imine
moiety are common, including Schiff bases such as - and b-dii-
are observed in group 10 (Ni, Pd, Pt) pyridyl-imines; for example,
Ni complexes exist as dimers or two ligand units coordinated to
a
mines [1–8], 2,6-bis(imino)pyridines [9–16], pyridyl-imines and
N-substituted 2-iminoalkylpyridines [17–43]. In addition, these
transition metal complexes have been used in the areas of
synthesis and spectroscopy [17,18,20–23,27,28,30,35], photolumi-
nescence and photochemistry [1], as catalysts for organic
Ni, achieving a 5-coordinated trigonal–bipyramidal structure
[43,51–55]. In contrast, Pd and Pt complexes with pyridyl-imines
appear as monomeric square-planar structures [52–54,56–75].
Despite previous reports on transition metal complexes with
pyridyl-imine derivatives, little is known regarding palladium
complexes with bidentate N-substituted 2-iminoalkylpyridines
ligands as catalysts for methyl methacrylate (MMA)
polymerisation [76]. Thus, we report the synthesis of
bidentate N-cycloalkyl substituted 2-iminomethylpyridine and 2-
transformations
[5,16,24,25,36,37,43],
electrochemistry
[29,32,41], bioinorganic chemistry [19,26], supramolecular chem-
istry [33,38], molecular magnetism [34,39], microbial activity
[42] and olefin polymerisation [2–4,6–15,31,40]. Especially, transi-
tion metal complexes with
a-diimine ligands have attracted con-
iminomethylquinoline
ligands,
N-cyclopentyl-1-(pyridin-2-
siderable attention because of their catalytic activity, as
demonstrated by Brookhart et al. who reported that diimine com-
plexes of Ni(II) and Pd(II) act as catalysts for the polymerisation or
oligomerisation of olefins [6–8]. Specifically, the highly active com-
plexes with tridentate 2,6-bis(imino)pyridines, initially discovered
independently by the groups of Brookhart [9–11] and Gibson [13–
15], play important roles in cobalt and iron systems. Several struc-
tural variations in the diimines have been reported, and their steric
and electronic properties vary because of the polydentate charac-
teristics of these ligands. These variations include the presence of
yl)methanimine (L1), N-cyclohexyl-1-(pyridin-2-yl)methanimine
(L2), N-(piperidin-1-yl)-1-(pyridin-2-yl)methanimine (L3) and N-
cyclopentyl-1-(quinolin-2-yl)methanimine (L4), and their Pd(II)
complexes, as well as their X-ray crystal structures. Moreover,
the catalytic activity of the Pd(II) complexes for MMA polymerisa-
tion in toluene was investigated at 0–60 °C.
2. Experimental
2.1. Physical measurements
⇑
PdCl2, 2-picolylcarbaldehyde, quinoline-2-carbaldehyde, cyclo-
pentylamine, cyclohexylamine, 1-aminopiperidine, magnesium
Corresponding author. Tel.: +82 53 950 5337; fax: +82 53 950 6330.
E-mail address: hyosunlee@knu.ac.kr (H. Lee).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.11.036

150
S. Kim et al. / Polyhedron 69 (2014) 149–155
sulfate and methyl methacrylate (MMA) were purchased from
Aldrich, and anhydrous solvents, such as C2H5OH, DMF, diethyl
ether and dichloromethane, were purchased from Merck and used
without further purification. Modified methylaluminoxane
(MMAO) was purchased from the Tosoh Finechem Corporation as
6.9% weight aluminum, toluene solution and used without further
purification. Elemental analyses (C, H, N) of the prepared com-
plexes were carried out on an elemental analyzer (EA 1108;
Carlo-Erba, Milan, Italy). 1H NMR (400 MHz) and 13C NMR
(400 MHz) spectra were recorded on a Bruker Advance Digital
400 NMR spectrometer; chemical shifts were recorded in ppm
units (d) relative to SiMe4 as the internal standard. Infrared (IR)
spectra were recorded on a Bruker FT/IR-Alpha (neat) spectropho-
tometer and the data are reported in reciprocal centimeters. The
molecular weight and molecular weight distribution of the ob-
tained polymethylmethacrylate (PMMA) were measured using
gel permeation chromatography (GPC) (CHCl3, Alliance e2695;
Waters Corp., Milford, MA). The glass transition temperature (Tg)
was determined using a thermal analyzer (Q2000; TA Instruments,
New Castle, DE).
The product was obtained as a light orange oil (3.55 g, 94.0%). 1H
NMR (CDCl3, 400 MHz) d: 8.51 (d, 1H, J = 7.6 Hz, –NC5H4–), 7.82
(d, 1H, J = 8.0 Hz, –NC5H4–), 7.61 (t, 1H, J = 7.6 Hz, –NC5H4–), 7.59
(s, 1H, –N@CH–NC5H4–), 7.08 (t, 1H, J = 7.8 Hz, –NC5H4–), 3.22 (t,
4H, J = 5.6 Hz, –NC5H10–), 1.72 (m, 4H, –NC5H10–), 1.53 (m, 2H, –
NC5H10–). 13C NMR (CDCl3, 400 MHz) d: 155.85 (s, 1C, ipso-
NC5H4-), 148.99 (s, 1C, –NC5H4–), 136.13 (s, 1C, –N@CH–NC5H4–),
133.89 (s, 1C, –NC5H4–), 121.71 (s, 1C, –NC5H4–), 118.72 (s, 1C, –
NC5H4–), 51.65 (s, 2C, –NC5H10–), 25.05 (s, 2C, –NC5H10–), 24.00
(s, 1C, –NC5H10–). IR (neat liquid, cmꢁ1): 3056(w), 2934(s),
2854(s), 2811(w), 1643(s), 1566(s), 1436(s), 1357(s), 1291(s),
1254(s), 1162(s), 1006(s), 989(s), 887(s), 771(s), 666(s), 620(s).
2.2.4. N-cyclopentyl-1-(quinolin-2-yl)methanimine (L4)
L4 was prepared by an analogous method as described for L1,
except utilizing cyclopentylamine and quinoline-2-carbaldehyde.
The product was obtained as a light red oil (3.55 g, 96.0%). 1H
NMR (CDCl3, 400 MHz) d: 8.54 (d, 1H, J = 8.0 Hz, –NC9H6–), 8.17
(s, 2H, –N@CH–NC9H6–, –NC9H6–), 8.13 (d, 1H, J = 8.2 Hz, –
NC9H6–), 7.82 (s, 1H, –N@CH–NC9H6–), 7.74 (t, 1H, J = 7.8 Hz, –
NC9H6–), 7.58(t, 1H, J = 7.8 Hz, –NC9H6–), 3.94 (t, 4H, J = 5.6 Hz,
ipso-C5H9–), 1.94 (m, 4H, –C5H9–), 1.72 (m, 2H, –C5H9–). 13C
NMR (CDCl3, 400 MHz) d: 160.12 (s, 1C, ipso-NC9H6–), 156.33 (s,
1C, –N@CH–NC9H6–), 150.55 (s, 1C, –NC9H6–), 133.89 (s, 1C, –
NC9H6–), 140.86 (s, 1C, –NC9H6–), 135.86 (s, 1C, –NC9H6–),
133.84 (s, 1C, –NC9H6–), 132.84 (s, 1C, –NC9H6–), 130.57 (s, 1C, –
NC9H6–), 128.44 (s, 1C, –NC9H6–), 62.86 (s, 1C, ipso-C5H9–), 43.00
(s, 2C, –C5H9–), 25.81 (s, 2C, –C5H9–). IR (neat liquid, cmꢁ1):
3057(w), 2951(s), 2865(s), 1640(w), 1596(s), 1558(s), 1503(s),
1432(s), 1370(s), 1306(s), 1181(s), 1146(s), 1146(s), 1112(s),
1072(s), 960(s), 892(s), 831(s), 831(s), 751(s), 617(s).
2.2. Preparation of the ligands and the Pd(II) complexes
2.2.1. N-cyclopentyl-1-(pyridin-2-yl)methanimine (L1)
Although L1 and L2 [21] have been synthesized previously, here
we report a similar procedure as described in the literature with
complementary spectroscopic data [40,77–79]. Cyclopentylamine
(1.70 g, 0.020 mol) in dichloromethane (20.0 mL) was added to 2-
picolylcarbaldehyde (2.14 g, 0.020 mol) in dichloromethane
(20.0 mL). After 24 h of stirring at room temperature, water was re-
moved from the reaction solution. The dichloromethane solution
was dried over MgSO4 and the solvent was removed under reduced
pressure. The residue was vacuum distilled and dried to give a
brown oil (3.41 g, 98.0%). 1H NMR (DMSO-d6, 400 MHz) d: 8.61
(d, 1H, J = 7.8 Hz, –NC5H4–), 8.31 (s, 1H, –N@CH–NC5H4–), 7.92
(d, 1H, J = 8.6 Hz, –NC5H4–), 7.79 (t, 1H, J = 7.6 Hz, –NC5H4–), 7.38
(t, 1H, J = 7.4 Hz, –NC5H4–), 3.81 (m, 1H, ipso-C5H9–), 3.81 (m,
4H, –C5H9–), 1.59 (m, 4H, –C5H9–). 13C NMR (CDCl3, 400 MHz) d:
159.73 (s, 1C, ipso-NC5H4-), 154.89 (s, 1C, –N@CH–NC5H4–),
149.55 (s, 1C, –NC5H4–), 136.59 (s, 1C, –NC5H4–), 125.08 (s, 1C, –
NC5H4–), 120.98 (s, 1C, –NC5H4–), 71.69 (s, 1C, –ipso-C5H9–),
34.24 (s, 2C, –C5H9–), 24.75 (s, 2C, –C5H9–). IR (neat liquid,
cmꢁ1): 3058(w), 2952(s), 2866(s), 1644(w), 1578(s), 1517(s),
1465(s), 1374(s), 1319(s), 1226(s), 1178(s), 1051(s), 989(s),
933(s), 893(s), 772(s), 614(s).
2.2.5. N-cyclopentyl-1-(pyridin-2-
yl)methanimine(dichloro)palladium(II) ([L1PdCl2])
A solution of L1 (0.087 g, 0.50 mmol) in anhydrous ethanol
(10.0 mL) was added to a solution of anhydrous Pd(CH3CN)2Cl2
[80,81] (0.13 g, 0.50 mmol) in dried ethanol (10.0 mL) at room
temperature. Precipitation of a yellow material occurred while stir-
ring at room temperature for 12 h. The yellow powder was filtered
and washed with ethanol (50.0 mL), followed by washing with
diethyl ether (50.0 mL) (0.15 g, 88%). X-ray quality crystals of [L1-
PdCl2] were obtained within three days from diethyl ether
(10.0 mL) diffusion into a DMF solution (10.0 mL) of [L1PdCl2]
(0.050 g). Anal. Calc. for C11H14N2Cl2Pd: C, 37.58; H, 4.01; N, 7.97.
Found: C, 37.41; H, 4.02; N, 8.12%. 1H NMR (DMSO-d6, 400 MHz)
d: 8.97 (d, 1H, J = 7.6 Hz, –NC5H4–), 8.54 (s, 1H, –N@CH–NC5H4–),
8.33 (t, 1H, J = 7.6 Hz, –NC5H4–), 8.12 (d, 1H, J = 8.0 Hz, –NC5H4–),
7.83 (t, 1H, 8.0 Hz, –NC5H4–), 4.65 (m, 1H, –C5H9–), 2.05 (m, 2H,
–C5H9–), 1.89 (m, 2H, –C5H9–), 1.74 (m, 2H, –C5H9–), 1.64 (m, 2H,
–C5H9–). 13C NMR (DMSO-d6, 400 MHz) d: 168.10 (s, 1C, ipso-
NC5H4–), 157.07 (s, 1C, –N@CH–NC5H4–), 150.19 (s, 1C, –NC5H4–
), 141.58 (s, 1C, –NC5H4–), 128.77 (s, 1C, –NC5H4–), 128.63 (s, 1C,
–NC5H4–), 68.62 (s, 1C, ipso-C5H9–), 32.73 (s, 2C, –C5H9–), 23.63
(s, 2C, –C5H9–). IR (solid, cmꢁ1): 3038(w), 2951(s), 2872(s),
1915(w), 1835(s), 1747(s), 1693(s), 1649(s), 1596(s), 1517(s),
1472(s), 1319(s), 1107(s), 1045(s), 994(s), 936(s), 850(s), 772(s),
657(s).
2.2.2. N-cyclohexyl-1-(pyridin-2-yl)methanimine (L2)
L2 [21]was prepared by an analogous method as described for
L1, except utilizing cyclohexylamine and 2-picolylcarbaldehyde.
The product was obtained as a light orange oil (3.76 g, 98.0%). 1H
NMR (DMSO-d6, 400 MHz) d: 8.63 (d, 1H, J = 7.8 Hz, –NC5H4–),
8.41 (s, 1H, –N@CH–NC5H4–), 7.97 (d, 2H, J = 7.6 Hz, –NC5H4–),
7.71 (d, 1H, J = 8.4 Hz, –NC5H4–), 7.27 (t, 1H, J = 8.0 Hz, –NC5H4–),
3.29 (m, 1H, ipso-C6H11–), 1.81 (m, 4H, –C6H11–), 1.62 (m, 4H, –
C6H11–), 1.27 (m, 2H, –C6H11–). 13C NMR (DMSO-d6, 400 MHz) d:
159.72 (s, 1C, ipso-NC5H4–), 154.78 (s, 1C, –N@CH–NC5H4–),
149.59 (s, 1C, –NC5H4–), 136.98 (s, 1C, –NC5H4–), 125.21 (s, 1C, –
NC5H4–), 120.69 (s, 1C, –NC5H4–), 68.78 (s, 1C, ipso-C6H11–),
34.23 (s, 2C, –C6H11–), 25.54 (s, 1C, –C6H11–), 24.39 (s, 2C, –
C6H11–). IR (neat liquid, cmꢁ1): 3058(w), 2926(s), 2854(s),
1646(w), 1579(s), 1519(s), 1447(s), 1377(s), 1341(s), 1298(s),
1145(s), 1069(s), 967(s), 890(s), 854(s), 772(s), 672(s), 617(s).
2.2.6. N-cyclohexyl-1-(pyridin-2-
yl)methanimine(dichloro)palladium(II) ([L2PdCl2])
[L2PdCl2] was prepared according to a similar procedure as de-
scribed for [L1PdCl2]. The yellow powder was filtered and washed
with ethanol (50.0 mL), followed by washing with diethyl ether
(50.0 mL) (0.16 g, 90%). X-ray quality crystals of [L2PdCl2] were ob-
tained within five days from diethyl ether (10.0 mL) diffusion into
a DMF solution (10.0 mL) of [L2PdCl2] (0.05 g). Anal. Calc. for
2.2.3. N-(piperidin-1-yl)-1-(pyridin-2-yl)methanimine (L3)
L3 was prepared by an analogous method as described for L1,
except utilizing 1-aminopiperidine and 2-picolylcarbaldehyde.

S. Kim et al. / Polyhedron 69 (2014) 149–155
151
C12H16N2Cl2Pd: C, 39.42; H, 4.41; N, 7.66. Found: C, 39.58; H, 4.45;
N, 8.02%. 1H NMR (DMSO-d6, 400 MHz) d: 8.98 (d, 1H, J = 7.6
Hz, –NC5H4–), 8.57 (s, 1H, –N@CH–NC5H4–), 8.33 (t, 1H,
J = 7.8 Hz, –NC5H4–), 8.09 (d, 1H, J = 7.8 Hz, –NC5H4–), 7.83 (t, 1H,
7.8 Hz, –NC5H4–), 4.07 (m, 1H, –C6H11–), 2.15 (m, 2H, –C6H11–),
1.81 (m, 2H, –C6H11–), 1.65 (m, 1H, –C6H11–), 1.40 (m, 2H, –
C6H11–), 1.29 (m, 2H, –C6H11–), 1.15 (m, 1H, –C6H11–). 13C NMR
(DMSO-d6, 400 MHz) d: 168.96 (s, 1C, ipso-NC5H4–), 157.14 (s,
1C, –N@CH–NC5H4–), 150.23 (s, 1C, –NC5H4–), 141.65 (s, 1C, –
NC5H4–), 128.72 (s, 1C, –NC5H4–), 65.08 (s, 1C, ipso-C6H11–),
32.84 (s, 2C, –C6H11–), 25.37 (s, 2C, –C6H11–). IR (solid, cmꢁ1):
3035(w), 2931(s), 2852(s), 1753(w), 1691(s), 1597(s), 1539(s),
1446(s), 1388(s), 1315(s), 1237(s), 1193(s), 1108(s), 1050(s),
972(s), 890(s), 844(s), 779(s), 659(s).
equipped with a graphite-monochromated Mo K
a
(k = 0.71073 Å)
radiation source and a nitrogen cold stream (ꢁ100 °C). Data
collection and integration were performed with SMART (Bruker,
2000) and SAINT-Plus (Bruker, 2001) [82]. Semi-empirical absorption
corrections based on equivalent reflections were applied by SADABS
[83]. The structures were solved by direct methods and refined by
full-matrix least-squares on F2 using SHELXTL [84]. All the
non-hydrogen atoms were refined anisotropically and hydrogen
atoms were added in their geometrically ideal positions. The crystal
and structure refinement data for all the structures are
summarized in Table 1. The final cycle of the refinement
converged with R1 [I > 2
[L1PdCl2], R1 [I > 2 (I)] = 0.0511, wR2 [I > 2
[L2PdCl2], R1 [I > 2 (I)] = 0.0566, wR2 [I > 2 (I)] = 0.1195 for [L3-
(I)] = 0.0394, wR2 [I > 2 (I)] = 0.0774 for
r
(I)] = 0.0480, wR2 [I > 2
r
(I)] = 0.1315 for
r
r(I)] = 0.1270 for
r
r
PdCl2] and R1 [I > 2
r
r
2.2.7. N-(piperidin-1-yl)-1-(pyridin-2-
[L4PdCl2].
yl)methanimine(dichloro)palladium(II) ([L3PdCl2])
[L3PdCl2] was prepared according to a similar procedure as de-
scribed for [L1PdCl2]. The yellow powder was filtered and washed
with ethanol (50.0 mL), followed by washing with diethyl ether
(50.0 mL) (0.16 g, 88%). X-ray quality crystals of [L3PdCl2] were ob-
tained within five days from diethyl ether (10.0 mL) diffusion into
a DMF solution (10.0 mL) of [L3PdCl2] (0.05 g). Anal. Calc. for C11
H15N3Cl2Pd: C, 36.04; H, 4.12; N, 11.46. Found: C, 35.58; H, 4.08;
N, 11.53%. 1H NMR (CDCl3, 400 MHz) d: 8.87 (d, 1H, J = 7.2 Hz, –
NC5H4–), 8.14 (s, 1H, –N@CH–NC5H4–), 8.21 (t, 1H, J = 7.2 Hz, –
NC5H4–), 7.91 (d, 1H, J = 7.8 Hz, –NC5H4–), 7.63 (t, 1H, J = 7.4 Hz,
–NC5H4–), 3.23 (t, 4H, J = 5.6 Hz, –NC5H10–), 1.68 (m, 4H, –
NC5H10–), 1.49 (m, 2H, –NC5H10–). 13C NMR (DMSO-d6, 400 MHz)
d: 156.14 (s, 1C, ipso-NC5H4–), 150.37 (s, 1C, –N@CH–NC5H4–),
144.93 (s, 1C, –NC5H4–), 139.97 (s, 1C, –NC5H4–), 125.26 (s, 1C, –
NC5H4–), 124.26 (s, 1C, –NC5H4–), 55.45 (s, 2C, –NC5H10–), 32.84
(s, 2C, –NC5H10–), 25.37 (s, 1C, –NC5H10–). IR (solid, cmꢁ1):
3060(w), 2939(s), 2835(s), 2529(w), 1691(s), 1651(s), 1604(s),
1463(s), 1341(s), 1279(s), 1172(s), 1103(s), 911(s), 843(s), 770(s),
730(s), 672(s).
2.4. Catalytic activity for MMA polymerization
In a Schlenk line, the complex (15 mol, 5.3 mg for [L1PdCl2],
l
5.5 mg for [L2PdCl2], 5.5 mg for [L3PdCl2] and 6.0 mg for [L4PdCl2])
was dissolved in dried toluene (2.3 mL) followed by the addition of
modified methylaluminoxane (MMAO) (3.25 mL, 7.50 mmol) as a
cocatalyst. The solution was stirred for 20 min at 0, 25 and 60 °C.
The MMA (5.0 mL, 47.1 mmol) was added to the above reaction
mixture and stirred for 10 min–2 h to obtain a viscous solution.
Methanol (50.0 mL) was added to terminate the polymerization.
The reaction mixture was poured into a large quantity of MeOH
(500 mL), and 35% HCl (5.0 mL) was injected to remove the
remaining co-catalyst (MMAO). PMMA was obtained by filtration
and repeated washing with methanol, and then dried under vac-
uum for 24 h.
3. Results and discussion
3.1. Synthesis and chemical properties
2.2.8. N-cyclopentyl-1-(quinolin-2-
yl)methanimine(dichloro)palladium(II) ([L4PdCl2])
Scheme 1 shows the synthesis of the ligands and Pd(II) com-
plexes. The ligands were obtained i yields of 98% (L1), 98% (L2),
94% (L3) and 96% (L4) from the condensation reaction between
the appropriate X-amine (X = cyclopentyl, cyclohexyl, N-1-piperi-
dine) and 2-picolylcarbaldehyde or quinoline-2-carbaldehyde in
dichloromethane. The ligands L1 and L2 have been reported previ-
ously and have been applied to rhenium and copper complexes
[21,77]. The [L1PdCl2] (88%), [L2PdCl2] (90%), [L3PdCl2] (88%) and
[L4PdCl2] (89%) complexes were obtained from the corresponding
ligands with [Pd(CH3CN)2Cl2] in anhydrous ethanol. 1H NMR, 13C
NMR, and elemental analyses were consistent with the ligands
and Pd(II) complex formulation. The 1H NMR peaks of the Pd(II)
complexes were shifted to low field by approximately d 0.1–
0.5 ppm as compared with the ligands, while the 13C NMR peaks
of the Pd(II) complexes were shifted to low field by approximately
d 2–10 ppm as compared with the ligands. In addition, an absorp-
tion band at around 1650 cmꢁ1 for the imine moiety was identified.
[L4PdCl2] was prepared according to a similar procedure as de-
scribed for [L1PdCl2]. The yellow powder was filtered and washed
with ethanol (50.0 mL), followed by washing with diethyl ether
(50.0 mL) (0.17 g, 89%). X-ray quality crystals of [L4PdCl2] were ob-
tained within five days from diethyl ether (10.0 mL) diffusion into
a DMF solution (10.0 mL) of [L4PdCl2] (0.05 g). Anal. Calc. for C15-
H16N2Cl2Pd: C, 44.86; H, 4.02; N, 6.98. Found: C, 45.03; H, 4.08;
N, 6.53%. 1H NMR (CDCl3, 400 MHz) d: 8.64 (d, 1H, J = 8.0 Hz, –
NC9H6–), 8.51 (s, 1H, –N@CH–NC9H6–), 8.51 (s, 1H, –NC9H6–),
8.17 (d, 1H, J = 8.2 Hz, –NC9H6–), 7.95 (d, 1H, –N@CH–NC9H6–),
7.83 (t, 1H, J = 7.8 Hz, –NC9H6–), 7.64(t, 1H, J = 7.8 Hz, –NC9H6–),
2.17 (t, 4H, J = 5.6 Hz, ipso-C5H9–), 1.71 (m, 4H, –C5H9–), 1.67 (m,
2H, –C5H9–). 13C NMR (CDCl3, 400 MHz) d: 162.15 (s, 1C, ipso-
NC9H6–), 158.34 (s, 1C, –N@CH–NC9H6–), 153.65 (s, 1C, –NC9H6–
), 134.77 (s, 1C, –NC9H6–), 139.76 (s, 1C, –NC9H6–), 134.46 (s, 1C,
–NC9H6–), 130.64 (s, 1C, –NC9H6–), 129.74 (s, 1C, –NC9H6–),
127.37 (s, 1C, –NC9H6–), 124.39 (s, 1C, –NC9H6–), 65.76 (s, 1C,
ipso-C5H9–), 42.80 (s, 2C, –C5H9–), 27.83 (s, 2C, –C5H9–). IR (solid
neat; cmꢁ1): 3059(w), 2951(s), 2858(s), 1836(w), 1746(s),
1695(s), 1649(s), 1517(s), 1462(s), 1365(s), 1209(s), 1148(s),
1100(s), 1049(s), 976(s), 925(s), 868(s), 819(s), 757(s), 707(s),
656(s).
3.2. Crystal structures
The ORTEP drawings of the complexes are shown in Fig. 1 ([L1-
PdCl2]), Fig. 2 ([L2PdCl2]), Fig. 3 ([L3PdCl2]) and Fig. 4 ([L4PdCl2]).
Selected bond lengths and angles are listed in Table 2. Single
crystals suitable for X-ray crystallography were obtained from
diethyl ether (10.0 mL) diffusion into DMF solutions (10.0 mL) of
the complexes. The coordination geometry around the Pd(II) centre
of the synthesized complexes can be described as a slightly
2.3. X-ray crystallographic studies
In each case, a colorless cubic-shaped crystal was picked up with
paraton oil and mounted on a Bruker SMART CCD diffractometer

152
S. Kim et al. / Polyhedron 69 (2014) 149–155
Table 1
Crystal data and structure refinement for the Pd(II) complexes.
[L1PdCl2]
[L2PdCl2]
[L3PdCl2]
[L4PdCl2]
Empirical formula
Formula weight
T (K)
C
11H14Cl2N2Pd
C12H16Cl2N2Pd
365.57
200(2)
C11H15Cl2N3Pd
366.56
200(2)
C15H16Cl2N2Pd
401.60
200(2)
351.54
200(2)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
0.71073
monoclinic
C2/c
0.71073
monoclinic
C2/c
0.71073
0.71073
monoclinic
P2(1)/C
triclinic
ꢀ
P1
35.1316(17)
5.3706(3)
13.5589(7)
90
104.0850(10)
90
2481.4(2)
8
38.799(11)
5.3558(15)
13.516(4)
90
101.295(6)
90
2754.2(13)
8
8.1462(9)
9.3693(10)
9.6650(10)
112.804(2)
95.506(2)
97.174(2)
666.23(12)
2
7.9134(8)
9.1944(10)
20.525(2)
90
94.036(2)
90
1489.7(3)
4
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (Mg/m3)
Absorption coefficient (mmꢁ1
F(000)
1.882
1.899
1392
1.763
1.715
1456
1.827
1.774
364
1.791
1.594
800
)
Crystal size (mm3)
h (°)
0.25 ꢀ 0.16 ꢀ 0.11
0.31 ꢀ 0.19 ꢀ 0.16
0.25 ꢀ 0.19 ꢀ 0.17
2.31–28.32
ꢁ10 6 h 6 10, ꢁ8 6 k 6 12,
ꢁ12 6 l 6 11
4865
0.20 ꢀ 0.14 ꢀ 0.12
1.20–28.27
1.07–28.31
1.99–28.33
Index ranges
ꢁ46 6 h 6 46, ꢁ7 6 k 6 7,
ꢁ12 6 l 6 17
8454
ꢁ51 6 h 6 51, ꢁ6 6 k 6 7,
ꢁ18 6 l 6 16
9460
ꢁ10 6 h 6 10, ꢁ12 6 k 6 11,
ꢁ27 6 l 6 26
10785
Reflections collected
Independent reflections (Rint
Completeness to h = 28.30° (%)
Absorption correction
)
3025 (0.0299)
98.7
none
3387 (0.0380)
99.2
none
3227 (0.0122)
97.5
none
3714 (0.0503)
99.8
none
Refinement method
full-matrix least-squares on
full-matrix least-squares on
full-matrix least-squares on F2 full-matrix least-squares on F2
F2
F2
Data/restraints/parameters
3025/0/145
1.197
3387/6/154
1.203
3227/0/155
1.288
3714/0/181
1.193
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0480, wR2 = 0.1315
R1 = 0.0766, wR2 = 0.2397
1.637 and ꢁ2.335
R1 = 0.0511, wR2 = 0.1270
R1 = 0.0877, wR2 = 0.2201
1.694 and ꢁ3.035
R1 = 0.0566, wR2 = 0.1195
R1 = 0.1119, wR2 = 0.2071
2.657 and ꢁ4.879
R1 = 0.0394, wR2 = 0.0774
R1 = 0.0904, wR2 = 0.1396
1.627 and ꢁ2.337
Largest difference in peak and
hole (e Åꢁ3
)
R
NH2
Pd(CH3CN)2Cl2
H
+
CH2Cl2
rt, 24 hr
+
EtOH
rt, 12 hr
R
N
N
H
O
H
Pd
Cl
Cl
N
R
N
N
L1 (R = cyclopentyl)
L2 (R = cyclohexyl)
[L1PdCl2] (R = cyclopentyl)
[L2PdCl2] (R = cyclohexyl)
[L3PdCl2] (R = 1-piperidine)
L
3 (R = 1-piperidine)
R
NH2
Pd(CH3CN)2Cl2
+
H
CH2Cl2
rt, 24 hr
+
EtOH
H
O
rt, 12 hr
R
N
N
Pd
H
N
R
N
N
Cl
Cl
L4 (R = cyclopentyl)
[L4PdCl2] (R = cyclopentyl)
Scheme 1. Synthesis of the ligands and Pd(II) complexes.
distorted square plane, consisting of the two N atoms and two Cl
atoms.
approximately 0.02 Å, ranging in size from cyclopentyl ([L1-
PdCl2]) < cyclohexyl ([L2PdCl2]) < 1-piperidinyl ([L3PdCl2]) < -
cyclopentyl ([L4PdCl2]). The Pd–Npyridine bond lengths are shorter
than those of Pd–Nimine due to the different basicity for the imine
and pyridine groups. The Pd–Cl bond lengths ranged from
2.284(3)–2.303(2) Å. The double imine N(2)–C(6) distances of
1.304(10) ([L1PdCl2]), 1.293(11) ([L2PdCl2]) and 1.322(13) Å
The Pd–Npyridine and Pd(1)–N(1) bond lengths in [LnPdCl2]
(Ln = L1, L2, L3, L4) are in the range 2.020(7)–2.043(8) Å, while those
of Pd–Nimine and Pd(1)–N(2) ranged from 2.023(8)–2.094(5) Å, sim-
ilar to the Pd–N bond length of square planar imine Pd(II) com-
plexes [52,85]. The Pd–Nimine bond length increased by

S. Kim et al. / Polyhedron 69 (2014) 149–155
153
Fig. 1. ORTEP drawing of [L1PdCl2] with thermal ellipsoids at 50% probability. All
hydrogen atoms are omitted for clarity.
Fig. 4. ORTEP drawing of [L4PdCl2] with thermal ellipsoids at 50% probability. All
hydrogen atoms are omitted for clarity.
angles in [LnPdCl2] (Ln = L1, L2, L3) are almost 90°, but this angle
is 86.99(7)° for [L4PdCl2]. This trend was observed in related pyri-
dyl-imine palladium [52,61,62,73,86] and platinum [67,68] sys-
tems. Interestingly, the plane of the N-cyclopentyl group and the
plane of palladium and pyridine are perpendicular in [L1PdCl2].
Moreover, the plane of the N-cyclopentyl group and the plane of
palladium and pyridine in [L4PdCl2] are slightly twisted by ca.
30° rather than being perpendicular (90°) for [L1PdCl2]. The N-
cyclohexyl and N-piperidine groups in [L2PdCl2] and [L3PdCl2]
are in the stable chair formation. Basically, both rings (N-cyclo-
hexyl or N-piperidine group and palladium and pyridine) are lo-
cated in the same plane.
Fig. 2. ORTEP drawing of [L2PdCl2] with thermal ellipsoids at 50% probability. All
hydrogen atoms are omitted for clarity.
3.3. MMA polymerisation
All the Pd(II) complexes were activated by MMAO to polymerise
methyl methacrylate (MMA) [87–89], yielding PMMA with Tg rang-
ing from 119 to 132 °C. The polymers were isolated as white solids
and characterized by GPC in THF using standard polystyrene as the
reference. The triad microstructure of PMMA was analyzed using
1H NMR spectroscopy. The polymerisation results, including tactic-
ity based on the isotactic (mm), heterotactic (mr) and syndiotactic
(rr), and polydispersity index (PDI), which represents the average
degree of polymerisation in terms of the number of structural units
and molecules, are summarized in Table 3.
To confirm the catalytic activity of the MMA polymerisation, a
blank polymerisation of MMA was performed with [Pd(CH3CN)2-
Cl2] and MMAO at specific temperatures. The catalytic activities
of the Pd(II) complexes increased with the decreasing ring size of
the N-substituted cycloalkyl group (cyclohexyl, ꢂ 1-piperidi-
nyl < cyclopentyl) at 60 °C and with increasing temperatures
(0 < 25 < 60 °C). The complexes [L1PdCl2] and [L4PdCl2], which
have a perpendicular and twisted N-cyclopentyl group towards
the plane of the iminopyridine-Pd residue, thus endow steric hin-
drance around the Pd metal center during the MMA
Polymerization.
Moreover, the catalytic activity of [L4PdCl2], containing an imi-
noquinoline ligand, was much higher than that of [L2PdCl2] and
[L3PdCl2], which contained an iminopyridine ligand. Presumably,
the electron-rich cloud around the Pd metal in [L4PdCl2] provides
increased activity compared to the electronic effect of [L2PdCl2]
and [L3PdCl2]. Thus, the activity of these Pd(II) complexes toward
MMA polymerisation is influenced by both steric effects and metal
electronics. This result is comparable with previous reported
Fig. 3. ORTEP drawing of [L3PdCl2] with thermal ellipsoids at 50% probability. All
hydrogen atoms are omitted for clarity.
([L3PdCl2]), and the corresponding imine N(2)–C(10) distance of
1.272(8) Å ([L4PdCl2]) are in the range of accepted carbon–nitro-
gen double bonds. The C(5)–C(6) bond distances of the complexes
ranged from 1.446(14)–1.452(12) Å, reflecting delocalised
p-elec-
trons. The bond lengths of the synthesized palladium complexes
are slightly affected by the N-cycloalkyl substituent group. The
N(1)–Pd(1)–Cl(2) and N(2)–Pd(1)–Cl(1) angles for the complexes
[LnPdCl2] (Ln = L1, L2, L3) are nearly linear, being in the range
172.8(2)–175.98(17)°. However, the N(1)–Pd(1)–Cl(2) and N(2)–
Pd(1)–Cl(1) angles for complex [L4PdCl2] are 173.44(14) and
167.45(16)°, respectively, showing a more distorted planarity than
[LnPdCl2] (Ln = L1, L2, L3). The average N(1)–Pd(1)–N(2) bond angle
of the five membered rings range from 80.1(2)° to 80.7(3)° and are
slightly affected by the substituted rings. The Cl(1)–Pd(1)–Cl(2)

154
S. Kim et al. / Polyhedron 69 (2014) 149–155
Table 2
Selected bond lengths (Å) and angles (°) of all the Pd(II) complexes.
[L1PdCl2]
[L2PdCl2]
[L3PdCl2]
[L4PdCl2]
Bond lengths
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–Cl(1)
Pd(1)–Cl(2)
N(1)–C(5)
N(2)–C(6)
N(2)–C(7)
C(5)–C(6)
2.021(6)
2.023(8)
2.300(2)
2.292(2)
1.369(9)
1.304(10)
1.470(10)
1.452(10)
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–Cl(1)
Pd(1)–Cl(2)
N(1)–C(5)
N(2)–C(6)
N(2)–C(7)
C(5)–C(6)
2.020(7)
2.044(6)
2.286(2)
2.303(2)
1.360(10)
1.293(11)
1.463(11)
1.452(12)
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–Cl(1)
Pd(1)–Cl(1)
N(1)–C(5)
N(2)–C(6)
N(2)–N(3)
C(5)–C(6)
2.043(8)
2.061(9)
2.284(3)
2.284(3)
1.354(13)
1.322(13)
1.360(11)
1.446(14)
Pd(1)–N(2)
Pd(1)–N(1)
Pd(1)–Cl(2)
Pd(1)–Cl(1)
N(1)–C(9)
N(2)–C(10)
N(1)–C(11)
C(9)–C(10)
2.030(5)
2.094(5)
2.2726(19)
2.2984(17)
1.331(8)
1.272(8)
1.478(8)
1.437(9)
Bond angles
N(1)–Pd(1)–N(2)
N(1)–Pd(1)–Cl(2)
N(2)–Pd(1)–Cl(2)
N(1)–Pd(1)–Cl(1)
N(2)––Pd(1)–Cl(1)
Cl(1)–Pd(1)–Cl(2)
C(6)–N(2)–C(7)
C(6)–N(2)–Pd(1)
C(8)–C(7)–C(11)
C(8)–C(7)–N(2)
C(11)–C(7)–N(2)
80.6(3)
175.98(17)
95.5(2)
N(1)–Pd(1)–N(2)
N(1)–Pd(1)–Cl(2)
N(2)–Pd(1)–Cl(2)
N(1)–Pd(1)–Cl(1)
N(2)–Pd(1)–Cl(1)
Cl(1)–Pd(1)–Cl(2)
C(6)–N(2)–C(7)
C(6)–N(2)–Pd(1)
C(8)–C(7)–C(12)
C(8)–C(7)–N(2)
C(12)–C(7)–N(2)
80.7(3)
175.87(18)
95.34(19)
93.42(19)
173.75(19)
90.46(8)
121.8(7)
113.1(5)
111.9(8)
114.6(7)
109.6(7)
N(1)–Pd(1)–N(2)
N(1)–Pd(1)–Cl(2)
N(2)–Pd(1)–Cl(2)
N(1)–Pd(1)–Cl(1)
N(2)–Pd(1)–Cl(1)
Cl(1)–Pd(1)–Cl(2)
C(6)–N(2)–N(3)
C(6)–N(2)–Pd(1)
C(7)–N(3)–C(11)
C(7)–N(3)–N(2)
C(11)–N(3)–N(2)
80.5(3)
172.9(3)
97.6(2)
N(1)–Pd(1)–N(2)
N(1)–Pd(1)–Cl(2)
N(2)–Pd(1)–Cl(2)
N(1)–Pd(1)–Cl(1)
N(2)–Pd(1)–Cl(1)
Cl(1)–Pd(1)–Cl(2)
C(10)–N(2)–C(11)
C(10)–N(2)–Pd(1)
C(12)–C(11)–C(15)
C(12)–C(11)–N(2)
C(15)–C(11)–N(2)
80.1(2)
173.44(14)
93.65(12)
99.54(14)
167.45(16)
86.99(7)
121.9(5)
112.1(4)
102.1(5)
117.7(6)
110.6(5)
93.32(18)
173.7(2)
90.59(8)
121.7(7)
114.2(5)
102.5(7)
115.5(7)
109.0(7)
92.6(2)
172.8(2)
89.10(11)
122.2(9)
112.8(7)
112.7(9)
116.7(8)
111.7(8)
Table 3
Polymerization of MMA by the Pd(II) complexes in the presence of MMAO.
Catalysta
Temp.(time)
Yieldb
(g)
Activityc
Tg
Tacticity
Mw
Mw/Mn
d
e
f
Entry
(°C)
(g/mol-Cat h) ꢀ 104
(°C)
%mm
%mr
%rr
(g/mol) ꢀ 105
g
g
g
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Pd(AN)2Cl2
MMAOh
60(2hr)
60(2hr)
60(30 min)
60(2hr)
60(2hr)
60(30 min)
25(2hr)
25(2hr)
25(2hr)
25(2hr)
25(2hr)
25(2hr)
0(2hr)
0.88
0.42
1.09
0.73
0.81
1.06
0.55
0.14
1.09
1.18
0.35
0.86
0.38
0.12
0.18
0.27
0.29
0.26
2.93
1.40
14.5
2.43
2.70
14.1
1.83
0.47
3.63
3.93
1.17
2.87
1.27
0.40
0.60
0.90
0.97
0.87
131.47
119.61
129.24
129.51
126.08
128.30
129.93
125.29
129.80
128.32
128.33
130.11
129.05
129.50
130.65
128.88
126.97
128.27
7.60
37.2
7.70
8.30
8.10
8.10
8.80
15.4
7.90
8.00
10.1
8.50
9.20
11.6
13.7
11.4
12.6
10.1
22.8
10.9
22.5
23.6
22.9
22.7
19.7
28.4
17.8
19.0
18.1
22.8
18.9
28.8
21.6
22.3
21.0
26.8
69.6
51.9
69.8
68.1
69.0
69.2
71.5
56.2
74.3
73.0
71.8
68.7
71.9
59.6
64.7
66.3
66.4
63.1
0.66
0.61
2.45
1.72
8.42
6.47
0.72
1.12
1.38
0.43
1.36
0.63
9.93
3.25
8.13
10.11
2.17
0.67
2.90
2.20
3.71
4.31
1.92
5.18
2.97
3.95
3.09
2.99
4.16
1.17
1.70
2.56
2.10
1.71
1.44
1.17
[L1PdCl2]
[L2PdCl2]
[L3PdCl2]
[L4PdCl2]
Pd(AN)2Cl2
MMAOh
[L1PdCl2]
[L2PdCl2]
[L3PdCl2]
[L4PdCl2]
Pd(AN)2Cl2
MMAOh
0(2hr)
0(2hr)
0(2hr)
0(2hr)
[L1PdCl2]
[L2PdCl2]
[L3PdCl2]
[L4PdCl2]
0(2hr)
a
b
c
[Pd (II) catalyst]0 = 15 lmol and [MMA]0/[MMAO]0/[Pd (II) catalyst]0 = 3100:500:1.
Yield defined as the mass of dried polymer recovered/mass of monomer used.
Activity is g of PMMA/(mol-Pd h).
Tg is the glass transition temperature, which is determined with a thermal analyzer.
Determined by gel permeation chromatography (GPC) eluted with THF at room temperature by filtration with polystyrene calibration.
Mn refers to the number average of molecular weights of PMMA.
AN refers to CH3CN in Pd(AN)2Cl2. It is a blank polymerization in which Pd(AN)2Cl2 was also activated by MMAO.
It is a blank polymerization which was done solely by MMAO.
d
e
f
g
h
palladium complexes containing the bispyridylamine ligand N,N-
di(2-picolyl)cycloheptylamine [90].
prepared by substitution reaction of Pd(CH3CN)2Cl2 with the corre-
sponding iminopyridine and iminoquinoline ligands. The coordina-
tion geometry around the Pd(II) centres in the iminopyridine-Pd(II)
complexes were square-planar. The catalytic activity of complexes
[L1PdCl2] and [L4PdCl2] towards the polymerisation of methyl
methacrylate (MMA) in the presence of modified methylaluminox-
ane resulted in an activity of 1.45 ꢀ 105 g PMMA/mol Pd h at 60 °C,
as well as moderate syndiotacticity, irrelevant of the polymerisa-
tion temperature.
The tacticity of PMMA was determined in the range around syn-
diotactic (d 0.85), heterotactic (d 1.02) and isotactic (d 1.21), based
on 1H NMR [91]. The syndiotacticity of PMMA was around 70%,
which is similar for all [LnPdCl2] (Ln = L1, L2, L3, L4), regardless of
the polymerisation temperature. Although the moderate syndio-
tacticity was not sufficient to confer a mechanism of coordination
polymerisation, it clearly shows the steric and electronic effects in
[LnPdCl2] (Ln = L1, L2, L3, L4) during the MMA polymerisation.
4. Conclusions
Acknowledgments
We investigated the synthesis and X-ray crystallographic struc-
tures of [L1PdCl2], [L2PdCl2], [L3PdCl2] and [L4PdCl2], which were
This research was supported by the National Research Founda-
tion of Korea (NRF) and funded by the Ministry of the Education,

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Appendix A. Supplementary data
CCDC 970000–970003 contains the supplementary crystallo-
graphic data for [L1PdCl2], [L2PdCl2], [L3PdCl2] and [L4PdCl2],
respectively. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
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