Cadmium(II) complexes containing N′-substituted N,N-bispyrazolyl ligands: The formation of 4- and 5-coordinated monomers versus 6-coordinated dimer

Article abstract of DOI:10.1016/j.inoche.2014.03.027

Novel Cd(II) bromide complexes [L_aCdBr₂], [L _bCdBr₂] and [L_cCd(μ-Br)Br]₂ have been synthesized and characterized. [L_aCdBr₂] and [L _bCdBr₂] were distorted tetrahedral and trigonal bipyramidal geometry around metal, respectively, depending on the coordination of the nitrogen atom of N′-substituted amine moiety and the cadmium center. However, [L_cCd(μ-Br)Br]₂ reveals a bromo-bridged 6-coordinated dimeric species. Specifically, the catalytic activity of [L_bCdBr₂] (1.13 × 10⁵ g PMMA/mol Cd·h) in the polymerization of methyl methacrylate (MMA) in the presence of modified methylaluminoxane (MMAO) at 60 °C was higher than that of [L_aCdBr₂] (5.03 × 10⁴ g PMMA/mol Cd·h) and reference complex [CdCl₂] (3.53 × 10 ⁴ g PMMA/mol Cd·h).

Full text of DOI:10.1016/j.inoche.2014.03.027

Inorganic Chemistry Communications 44 (2014) 164–168
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Cadmium(II) complexes containing N′-substituted N,N-bispyrazolyl
ligands: The formation of 4- and 5-coordinated monomers versus
6-coordinated dimer
Sunghye Choi a, Sunghoon Kim a, Ha-Jin Lee b,c, 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
Jeonju Center, Korea Basic Science Institute (KBSI), 634-18 Keumam-dong, Dukjin-gu, Jeonju City 561-180, Republic of Korea
Department of Chemistry, Chonbuk National University, Dukjin-gu, Jeonju City 561-756, Republic of Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel Cd(II) bromide complexes [LaCdBr2], [LbCdBr2] and [LcCd(μ-Br)Br]2 have been synthesized and character-
ized. [LaCdBr2] and [LbCdBr2] were distorted tetrahedral and trigonal bipyramidal geometry around metal,
respectively, depending on the coordination of the nitrogen atom of N′-substituted amine moiety and the
cadmium center. However, [LcCd(μ-Br)Br]2 reveals a bromo-bridged 6-coordinated dimeric species. Specifically,
the catalytic activity of [LbCdBr2] (1.13 × 105 g PMMA/mol Cd·h) in the polymerization of methyl methacrylate
(MMA) in the presence of modified methylaluminoxane (MMAO) at 60 °C was higher than that of [LaCdBr2]
(5.03 × 104 g PMMA/mol Cd·h) and reference complex [CdCl2] (3.53 × 104 g PMMA/mol Cd·h).
© 2014 Elsevier B.V. All rights reserved.
Received 31 January 2014
Accepted 21 March 2014
Available online 28 March 2014
Keywords:
Bispyrazolyl
Dimeric Cd(II) complex
Methyl methacrylate polymerization
Syndiotacticity
Transition metal complexes with pyrazole-based ligands have
attracted attention because of their efficient synthesis and modifi-
cation on a linker unit of two or three pyrazols [1–10]. Thus, pyrazole-
containing ligands can be designed to be coordinated to metals through
monodentate, bidentate, tridentate, and tetradentate binding modes
with various modifications of the ligand structure [11–19]. Accordingly,
their structural properties often meet specific stereo-chemical require-
ments for specific metal-binding sites during catalysis. Specifically N,N-
bidentate pyrazolyl-based chelating ligands were first reported by
Driessen [20], these complexes show structural stability and catalyt-
ic ability, and have been applied to the areas of synthesis, structural
design, and spectroscopy [3,8,9,12,14,16] and used for electronic
materials, supra-molecules for metal–organic frame (MOF) applications
[2,11], as catalysts for organic transformation [1,4,5,7], for biological
applications [6,10,13], and for olefin polymerization [17–19]. Some
recent studies have documented the use of transition metals, specifically
cadmium [21–23], as a catalyst of methyl methacrylate (MMA)
polymerization.
The average Cd\Npyrazole bond lengths ranged from 2.217(6) Å to
2.382(8) Å for Cd(II) complexes. The bond distance of Cd\Namine in
[LaCdBr2] was 3.687 Å (calculated), indicative of a non-coordinative
interaction between the nitrogen atom of the N′-substituted amine
and the cadmium atom, forming a monomeric 4-coordinated complex.
However, the Cd\Namine bond distances were 2.524(9) Å and
2.739(10) Å for [LbCdBr2] and [LcCd(μ-Br)Br]2, respectively, reflecting
a coordination or a coordinative interaction between the nitrogen
atom of the N′-substituted amine and the cadmium atom, forming
monomeric 5-coordinated [LbCdBr2] and dimeric 6-coordinated
[LcCd(μ-Br)Br]2. The average bond lengths of Cd\Br for [LaCdBr2] and
[LbCdBr2] ranged from 2.5230(9) Å to 2.5848(13) Å. In [LcCd(μ-Br)Br]2,
the Cd\Brterminal bond [2.5810(16) Å] was shorter than that of the two
Cd–Brbridge bonds [2.7221(13) Å and 2.8415(13) Å]. The angles of
N
pyrazole\Cd\Npyrazole and Br(1)\Cd\Br(1) in [LaCdBr2] were
102.5(2)° and 119.54(4)°, indicative of a distorted tetrahedral geometry.
The geometry at the cadmium center in [LbCdBr2] was best described as a
distorted trigonal bipyramid through coordination of the nitrogen on the
cyclohexylmethanamine moiety to the cadmium metal center based on
the Cd\Namine bond length of 2.524(9) Å and the bond angles around
the cadmium metal. However, [LaCdBr2], which has the same ligand as
[LbCdBr2], showed a distorted tetrahedral structure without a coordina-
tive interaction of the nitrogen atom of the N′-substituted amine unit
and the cadmium atom. It is worth noting that the yz-plane of cyclohexyl
rings in the cyclohexylmethanamine unit of [LaCdBr2] was distorted by
approximately 90° with respect to the xy-plane of two pyrazole rings
and the cadmium metal center. However, the plane of the cyclohexyl
ring and the plane of pyrazole rings and fused metallocyclic rings were
The Cd(II) bromide complexes [LaCdBr2], [LbCdBr2] and [LcCd(μ-Br)
Br]2 were obtained from metal starting material [CdBr2·4H2O] with the
corresponding ligands [Ln] (L1–L3) in anhydrous ethanol with yields of
72%–85% (Scheme 1) [8,10,24].
The molecular structures of [LaCdBr2], [LbCdBr2], and [LcCd(μ-Br)
Br]2 are provided, and the selected bond distances and angles are listed
in the captions of Figs. 1, 2, and 3, respectively.
⁎
Corresponding author. Tel.: +82 53 950 5337; fax: +82 53 950 6330.
E-mail address: hyosunlee@knu.ac.kr (H. Lee).
http://dx.doi.org/10.1016/j.inoche.2014.03.027
1387-7003/© 2014 Elsevier B.V. All rights reserved.

S. Choi et al. / Inorganic Chemistry Communications 44 (2014) 164–168
165
Scheme 1. Synthesis of ligands and Cd(II) complexes.
almost parallel in [LbCdBr2], which contained four bulky methyl groups on
two pyrazole rings. Therefore, the parallel or perpendicular orientation of
the plane of the cyclohexyl ring with respect to the plane of the pyrazole
ring and the substituents attached to the pyrazole ring affected the coordi-
nation of the nitrogen of cyclohexylmethanamine to the cadmium metal,
thus achieving a 4-coordinated complex [LaCdBr2] and 5-coordinated
complex [LbCdBr2]. Because the Lc ligand has a coordinative methoxy
group in the 3-methoxypropylamine moiety, we expected a monomeric
6-coordinated complex. However, the geometry of [LcCd(μ-Br)Br]2 was
dimeric distorted octahedral, in which the cadmium metal was coordi-
nated with two pyrazole nitrogen atoms, two bridged bromide atoms,
and a terminal bromide atom. This was supported by the Cd\Namine
bond length of 2.739(10) Å, indicative of a coordinative interaction
between the nitrogen atom of the 3-methoxypropylamine moiety and
bonds around the cadmium center, achieving a 6-coordinated geometry
of distorted octahedron. However, monomeric [LbCdBr2], in which the
Lb ligand produces greater steric hindrance than Lc due to the four
methyls on the pyrazole rings, exists as a distorted trigonal bipyramid
resulting from interactions between the cadmium metal and the nitro-
gen atom of the cyclohexylmethylamine.
around the metal center by ligand and the steric factor of the substitu-
ents on the pyrazole rings. Syndiotacticity was not sufficiently high to
suggest a mechanism of coordination polymerization, but was similar
for all Cd(II) complexes (around 70%) and reference [CdCl2]. This also
indicated that syndiotacticity was only slightly affected by the substitu-
ents on the ligand.
We are now manipulating the substituents on the amine moiety and
pyrazole ring to control the formation of diverse coordination modes on
cadmium metal.
Acknowledgment
This research was supported by the National Research Foundation of
Korea (NRF), funded by the Ministry of the Education, Science, and
Technology (MEST) (Grant No. 2012-R1A2A2A01045730).
Appendix A. Supplementary material
CCDC 983313–983315 contains the supplementary crystallographic
data for [LaCdBr2], [LbCdBr2] and [LcCdBr2], 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, UK; fax: (+44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk. The supplementary spectroscopic
data for complexes can be obtained free of charge at http://dx.doi.org/
10.1016/j.inoche.2014.03.027.
Cd(II) complexes could be activated by cocatalyst modified methyl-
aluminoxane (MMAO) to polymerize methyl methacrylate (MMA),
producing poly(methylmethacrylate) (PMMA) [25–30]. The data of
PMMA such as tacticity, the number average of molecular weights
(Mw) and a glass transition temperature (Tg) are listed in Table 1
[31–33].
Relative to previously reported cobalt(II) complexes with N,N-
bis(1H-pyrazolyl-1-methyl)aniline [34], the Cd(II) complexes showed
higher molecular weights (1.20 × 106 g/mol for [LbCdBr2]) of
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S. Choi et al. / Inorganic Chemistry Communications 44 (2014) 164–168
Fig. 1. Molecular structure of [LaCdBr2]. Selected bond lengths (Å) and angles (°) are Cd(1)\N(3) 3.687, Cd(1)\N(1) 2.217(6), Cd(1)\N(5) 2.230(6), Cd(1)\Br(1) 2.5230(9), Cd(1)\
Br(2) 2.5377(10), N(1)\C(1) 1.337(10), N(1)\N(2) 1.353(8), N(2)\C(3) 1.357(10), N(2)\C(4) 1.471(10), C(1)\C(2) 1.372(13), N(1)\Cd(1)\N(5) 102.5(2), N(1)\Cd(1)\Br(1)
104.98(16), N(5)\Cd(1)\Br(1) 107.44(17), N(1)\Cd(1)\Br(2) 111.60(17), N(5)\Cd(1)\Br(2) 109.38(16), Br(1)\Cd(1)\Br(2) 119.54(4), C(1)\N(1)\N(2) 105.1(6),
C(1)\N(1)\Cd(1) 126.0(5), N(2)\N(1)\Cd(1) 128.6(5), N(1)\N(2)\C(3) 111.4(7).
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S. Choi et al. / Inorganic Chemistry Communications 44 (2014) 164–168
167
Fig. 2. Molecular structure of [LbCdBr2]. Selected bond lengths (Å) and angles (°) are Cd(1)\N(3) 2.524(9), Cd(1)\N(1) 2.380(8), Cd(1)\N(5) 2.382(8), Cd(1)\Br(2) 2.5309(12),
Cd(1)\Br(1) 2.5848(13), N(1)\C(1) 1.329(12), N(1)\N(2) 1.362(11), N(2)\C(3) 1.351(13), N(2)\C(4) 1.451(12), C(1)\C(2) 1.414(15), N(1)\Cd(1)\N(5) 138.9(3), N(1)\
Cd(1)\N(3) 69.3(3), N(5)\Cd(1)\N(3) 70.3(3), N(1)\Cd(1)\Br(2) 101.77(19), N(5)\Cd(1)\Br(2) 101.37(18), N(3)\Cd(1)\Br(2) 135.68(17), N(1)\Cd(1)\Br(1) 102.0(2),
N(5)\Cd(1)\Br(1) 94.95(19), N(3)\Cd(1)\Br(1) 103.87(17), Br(2)\Cd(1)\Br(1) 120.37(5).

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S. Choi et al. / Inorganic Chemistry Communications 44 (2014) 164–168
Fig. 3. Molecular structure of [LcCd(μ-Br)Br]2. Selected bond lengths (Å) and angles (°) are Cd(3)\N(3) 2.739(10), Cd(1)\N(1) 2.295(8), Cd(1)\N(5) 2.338(9), Cd(1)\Br(2)
2.5810(16), Cd(1)\Br(1) 2.7221(13), Cd(1)\Br(1#) 2.8415(13), N(1)\C(1) 1.328(13), N(1)\N(2) 1.341(13), N(2)\C(3) 1.356(14), N(2)\C(4) 1.459(14), C(1)\C(2) 1.410(16),
N(1)\Cd(1)\N(5) 90.6(3), N(1)\Cd(1)\Br(2) 97.2(2), N(5)\Cd(1)\Br(2) 94.6(2), N(1)\Cd(1)\Br(1) 156.1(2), N(5)\Cd(1)\Br(1) 90.7(2), Br(2)\Cd(1)\Br(1) 106.42(4),
N(1)\Cd(1)\Br(1#) 84.6(2), N(5)\Cd(1)\Br(1#) 161.8(2), Br(2)\Cd(1)\Br(1#) 103.44(4), Br(1)\Cd(1)\Br(1#) 86.77(4).
Table 1
Polymerization of MMA by Cd(II) complexes in the presence of MMAO.
d
Entry
Catalysta
Yieldb(%)
Activityc × 104 (g/mol C at·h)
Tg (°C)
%mm
Tacticiy %mr
%rr
Mw × 105 (g/mol)
Mw/Mn
1
2
3
4
5
[CdCl2]e
22.6
8.97
32.3
72.4
66.2
3.53
1.40
5.03
11.3
10.3
123
120
121
124
123
5.60
37.2
6.90
6.20
6.70
30.3
10.9
22.9
25.5
24.8
64.1
51.9
70.2
68.3
68.5
4.72
6.78
7.73
12.0
8.75
1.49
2.09
2.16
1.46
2.00
MMAOf
[L1CdBr2]
[L2CdBr2]
[L3Cd(μ-Br)Br]2
a
[Cd(II) catalyst]0 = 15 μmol, [MMA]0/[MMAO]0/[Cd(II) catalyst]0 = 3100:500:1.
Yield defined a mass of dried polymer recovered/mass of monomer used.
Activity is calculated as (g PMMA)/(mol Cd·h) at 60 °C.
Determined by gel permeation chromatography (GPC) eluted with THF at room temperature by filtration with polystyrene calibration.
It is a blank polymerization in which anhydrous [CdCl2] was also activated by MMAO.
It is a blank polymerization which was done solely by MMAO.
b
c
d
e
f