Copper(II) complexes with 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-methyl-6- phenylpyrimidine: Syntheses, crystal structures and catalytic activity in ethylene polymerization

Article abstract of DOI:10.1016/j.poly.2011.09.024

The copper(II) complexes CuLCl₂ (1) and CuLBr₂ (2), with the chelating pyrazolylpyrimidine ligand 4-(3,5-dimethyl-1H-pyrazol-1-yl)- 2-methyl-6-phenylpyrimidine (L), have been synthesized. A single crystal X-ray diffraction study reve

Full text of DOI:10.1016/j.poly.2011.09.024

Polyhedron 31 (2012) 235–240
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Copper(II) complexes with 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-methyl-6-phenyl-
pyrimidine: Syntheses, crystal structures and catalytic activity in ethylene
polymerization
c
Mark B. Bushuev a,b, , Viktor P. Krivopalov , Elizaveta V. Lider a,b, Nina V. Semikolenova d,
⇑
Natalia V. Pervukhina a, Dmitrii Yu. Naumov a, Ludmila G. Lavrenova a,b, Vladimir A. Zakharov d,
Stanislav V. Larionov a
a Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3, Akad. Lavrentiev Ave., Novosibirsk 630090, Russia
b Novosibirsk State University, 2, Pirogova Str., Novosibirsk 630090, Russia
c N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences, 9, Akad. Lavrentiev Ave., Novosibirsk 630090, Russia
d Boreskov Institute of Catalysis, Siberian Branch of Russian Academy of Sciences, 5, Akad. Lavrentiev Ave., Novosibirsk 630090, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 July 2011
Accepted 13 September 2011
Available online 7 October 2011
The copper(II) complexes CuLCl2 (1) and CuLBr2 (2), with the chelating pyrazolylpyrimidine ligand 4-(3,5-
dimethyl-1H-pyrazol-1-yl)-2-methyl-6-phenylpyrimidine (L), have been synthesized. A single crystal X-
ray diffraction study revealed that 1 and 2 have molecular mononuclear structures. The molecules of 1
and 2 form chains in the crystal structures of these compounds due to the formation of
p–p-stacking
interactions between the pyrimidine and the phenyl rings. The complexes, in combination with the co-
catalyst methylaluminoxane (MAO), reveal catalytic activity in ethylene polymerization, while the free
ligand L is inactive.
Keywords:
Copper(II)
Pyrazolylpyrimidines
Synthesis
Ó 2011 Elsevier Ltd. All rights reserved.
Supramolecular structure
Stacking
Polymerizations
1. Introduction
on these ligands have been obtained [8–10]. The role of 4,6-di
(pyrazolyl)pyrimidines and related bis(azolyl)azine derivatives as
4-Pyrazolylpyrimidines are of interest from various viewpoints.
They demonstrate biological activity as cytoprotectors [1,2] and
analogs of TNF-signaling modulators [3]. Some of them exhibit
analgesic properties [4]. Due to presence of nitrogen atoms, 4-pyr-
azolylpyrimidines act as chelating ligands forming mono-, di- or
oligo-nuclear complexes with various transition metals [5–24].
These complexes are of fundamental and practical importance
since, together with their structural diversity, they were shown
to demonstrate (i) redox properties (ruthenium(II) complexes)
[16], (ii) magnetic activity (dinuclear copper(II) complexes)
[11,24] and (iii) catalytic activity in ethylene polymerization (cop-
per(II) complexes in combination with co-catalysts) [24]. Another
field of potential application concerns supramolecular chemistry.
4,6-Di(pyrazolyl)pyrimidines were used for the design of supramo-
lecular self-assemblies and a series of [2 ꢀ 2] copper(I) grids based
supramolecular synthons was discussed in [25].
Previously, studying pyrazolylpyrimidines and their complexes
with metals, some of us unexpectedly observed the simultaneous
crystallization of three differently colored concomitant polymorphs
(green (G), emerald green (EG) and orange (O)) of a copper(II) chlo-
ride complex with 4-(3,5-dimethyl-1H-pyrazol-1-yl)-6-methyl-2-
phenylpyrimidine (3) [23]. The difference in color of the polymorphs
is likely to be due to the different p–p stacking. There is no stacking
in the G modification, but the EG and O modifications demonstrate
face-to-face and slipped stacking respectively.
As a logical continuation of our work [23] we synthesized 4-(3,5-di-
methyl-1H-pyrazol-1-yl)-2-methyl-6-phenylpyrimidine (L) and used
it as a ligand for the synthesis of complexes with copper(II) chloride
and bromide. Earlier, the compound L (CAS Registry Number:
327100-59-4) was studied as a biologically active compound [26],
but details of its synthesis were not reported. The compounds 3 and
L are isomers with an identical NN-chelating fragment, but the phenyl
and the methyl groups are arranged in the inverse order (Scheme 1).
Thus, the goals of the present work can be formulated as fol-
lows: (i) to synthesize and study the crystallization of copper(II)
complexes with L, (ii) to answer the question on the influence of
⇑
Corresponding author at: Nikolaev Institute of Inorganic Chemistry, Siberian
Branch of Russian Academy of Sciences, 3, Akad. Lavrentiev Ave., Novosibirsk
630090, Russia.
E-mail addresses: mark.bushuev@gmail.com (M.B. Bushuev), krivic@nioch.
nsc.ru (V.P. Krivopalov).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.09.024

236
M.B. Bushuev et al. / Polyhedron 31 (2012) 235–240
1592 (m + d)ring, 1569 (m + d)ring, 1552 (m + d)ring, 1520 (m + d)ring,
1499, 1320, 1232, 864, 780, 741, 686, 647. 1H NMR (300.13 MHz,
CDCl3) d (ppm): 2.30 (s, 3H, CH3), 2.73 (s, 6H, 2 ꢀ CH3), 6.01 (s,
1H, 4-Hpyrazole), 7.42–7.53 (m, 3H, Ph), 8.09 (s, 1H, 5-Hpyrimidine),
8.07–8.16 (2H, m, Ph).
1
1
2
3
2
(3) R' = Me, R'' = Ph
R' = Ph, R'' = Me
2.3. Synthesis of CuLCl2 (1) and CuLBr2 (2)
L
A solution containing 0.5 mmol of the corresponding copper(II)
salt (0.09 g of CuCl2ꢂ2H2O or 0.11 g of CuBr2) in 5 ml of EtOH was
added to a solution of L (0.5 mmol, 0.13 g) in 10 ml of EtOH. A pre-
cipitate was formed immediately (yellow for CuLCl2, brown for
CuLBr2). The reaction mixture was stirred for 1 h, the precipitate
was filtered off, washed with EtOH and dried in the ambient air.
CuLCl2 Yield: 0.19 g (90%); Anal. Calc. for C16H16Cl2CuN4 (1): C,
48.2; H, 4.0; N, 14.0; Cu, 15.9. Found: C, 48.4; H, 4.1; N, 14.0; Cu,
Scheme 1. Representation of 3 and L.
the ligand structure on the crystal structure of the complexes and
(iii) to study the catalytic activity of the copper complexes with L
in ethylene polymerization.
A search for post-metallocene catalytic systems based on late
transition metals is a steadily growing area of modern chemistry
[27–30]. To-date, examples of copper catalysts for ethylene poly-
merization (or co-polymerization with polar monomers) are rather
scarce in the literature [24,31–36]. Recently, the work of Foley and
co-worker’s [37] initiated a discussion on the nature of the species
which are responsible for the polymerization. They have shown
that a ligand transfer from the copper(II) complexes to trimethyl-
aluminium takes place. As a result, it is an aluminium complex that
catalyzes ethylene polymerization instead of a copper(II) one.
However, lately Galletti et al. demonstrated that salicylaldiminates
of copper(II) in combination with co-catalysts manifest catalytic
activity in the homopolymerization of n-butyl metacrylate, while
the free salicylaldimine ligand is inactive [38]. In this context, we
have endeavored to answer the question do copper(II) catalysts
based on pyrazolylpyrimidine ligands exist, or is it free pyrazolyl-
pyrimidine in combination with a co-catalyst that catalyzes the
polymerization.
15.7%. IR (KBr, cmꢁ1
+ d)ring, 1582 ( + d)ring, 1569 (
1310, 1256, 850, 779, 753, 702, 646, 325
254 (Cu–N); UV–Vis (diffuse reflectance, nm): 444, 763.
)
(selected data): 1603
(
m
m
+ d)ring
+ d)ring, 1490,
(Cu–Cl),
, 1595
(m
m
m
+ d)ring, 1533 (
m
(Cu–Cl), 309
m
m
CuLBr2 Yield: 0.21 g (86%). Anal. Calc. for C16H16Br2CuN4 (2): C,
39.4; H, 3.3; N, 11.5; Cu, 13.0. Found: C, 39.5; H, 3.5; N, 11.7; Cu,
12.8%. IR (KBr, cmꢁ1
)
(selected data): 1604
+ d)ring, 1569 ( + d)ring, 1535 (
1314, 1255, 848, 780, 753, 702, 646, 273
230 (Cu–Br); UV–Vis (diffuse reflectance, nm): 467, 575.
(
m
m
+ d)ring
+ d)ring, 1491,
(Cu–Br),
, 1595
(m
+ d)ring, 1583 (
m
m
m
(Cu–N), 250
m
m
2.4. X-ray crystallographic study
Single crystal data for 1 and 2 were collected at 150 K with
MoK radiation (k = 0.71073 Å) using a Bruker Nonius X8Apex
a
CCD diffractometer equipped with a graphite monochromator.
The SMART software was used for data collection and also for
indexing the reflections and determining the unit cell parameters;
the collected data were integrated using SAINT software [39]. The
structures were solved by direct methods and refined by full-ma-
trix least-squares calculations using SHELXTL software [40,41]. All
the non-H atoms were refined in the anisotropic approximation.
The crystallographic parameters of compounds 1 and 2 are given
in Table 1.
2. Experimental
2.1. General
All reagents and solvents were commercially available and were
used without additional purification. The copper content was
determined by complexometric titration, the samples were miner-
alized in a 1:1 mixture of concentrated H2SO4 and HClO4. Elemen-
tal analyses (C, H, N) were conducted using an Elemental Analyser
Carlo–Erba, and the results were found to be in good agreement
with the calculated values. The IR absorption spectra were
recorded on a Scimitar FTS 2000 spectrophotometer at 375–
4000 cmꢁ1 and on a BOMEM MB-102 spectrophotometer at 200–
400 cmꢁ1. Diffuse reflectance spectra of the complexes were
recorded on a UV-3101 PC Shimadzu spectrophotometer in the
range 400–800 nm. The 1H NMR spectra were measured on a
Bruker AV-300 spectrometer using the residual proton atom signal
of the solvent at 7.24 ppm (for CDCl3) with respect to TMS as the
internal standard. The mass spectra were obtained on a Finnigan
MAT-8200 spectrometer (EI, 70 eV).
2.5. General procedure for the polymerization of ethylene
The catalytic activity of the compounds in the ethylene poly-
merization reaction was monitored in a temperature-controlled
reactor (0.2 l) equipped with a stirrer. MAO (Crompton, a toluene
solution with a total Al concentration of 1.8 M) was used as a co-
catalyst. The polymerization conditions were as follows: 50 ml of
toluene, [Cu] = [L] = 3 ꢀ 10ꢁ5 mol/l, the molar ratio Alco-catalyst:Cu
(or L) = 500. In the course of the reaction (1 h), a constant temper-
ature (T = 35 °C) and pressure (Pethylene = 10 atm) were maintained.
The obtained polyethylene powder was dried in air and weighed.
Yield for 1: 1.00 g; for 2: 1.10 g.
2.2. Synthesis of 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-methyl-6-
phenylpyrimidine (L)
3. Results and discussion
3.1. Synthetic aspects (Part 1)
A
solution of 4-hydrazinyl-2-methyl-6-phenylpyrimidine
(7.5 mmol, 1.5 g) and acetylacetone (11 mmol, 1.12 g,) in EtOH
(10 ml) was refluxed for 6 h. The solid obtained after cooling the
mixture in a refrigerator was filtered off, washed with cold EtOH
and dried in vacuo to give 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-
methyl-6-phenylpyrimidine (L). Yield: 1.82 g (92%). High-resolu-
tion mass spectrum (EI, 70 eV), m/z: Calc. for C16H16N4 264.1375;
The ligand L was obtained by condensation of 4-hydrazinyl-2-
methyl-6-phenylpyrimidine with acetylacetone. The complex
compounds CuLCl2 (1) and CuLBr2 (2) were synthesized by the
reaction of copper(II) chloride and bromide, respectively, with L
in ethanol solutions at room temperature. Taking into account
the possible bidentate chelating function of L as well as the poten-
tial formation of molecular mono-chelate copper(II) compounds,
Found 264.1374. IR (KBr, cmꢁ1) (selected data): 1600 sh (
m + d)ring,

M.B. Bushuev et al. / Polyhedron 31 (2012) 235–240
237
Table 1
to those obtained previously during the crystallization from EtOH
and CHCl3/EtOH; no formation of polymorphs was detected.
The complexes were obtained in good yields (90% and 86% for 1
and 2, respectively). They are stable in ambient air. Thus, the
following transformations were carried out (Scheme 2).
Crystal data and structure refinement for 1 and 2.
Compound
1
2
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
C
16H16Cl2CuN4
C16H16Br2CuN4
487.69
Orthorhombic
P212121
398.77
Orthorhombic
P212121
3.2. Structural aspects
7.2389(5)
13.0482(11)
17.5510(16)
1657.8(2)
4
7.2647(8)
13.3904(15)
17.8668(18)
1738.0(3)
4
b (Å)
c (Å)
Compounds 1 and 2 are isostructural. They have molecular
mononuclear structures, and the structure of 1 is shown as an
example (Fig. 1). The bond lengths and angles for the coordination
cores of the complexes are given in Table 2. The molecules of L are
coordinated to the copper atoms through the N2(pyrazole) and
N3(pyrimidine) atoms, adopting thus a bidentate chelating coordi-
nation mode. As a result of the ligand coordination, the practically
planar CuCN3 (and practically symmetrical regarding to the Cu–N
distances) chelate cycles are closed. The copper atom deviates from
the mean chelate plane by 0.02(2) and ꢁ0.02(2) Å for 1 and 2,
respectively. The environment around the copper atoms is dis-
torted tetrahedral and is formed by two chlorine or bromine atoms
and two nitrogen atoms of the L molecule. The (hetero)aromatic
rings are nearly planar in all the reported compounds. The bond
distances as well as the valence angles in the L molecules coordi-
nated by the copper atoms in 1 and 2 are in a good agreement with
the data [42]. Since complexes 1 and 2 are tetrahedral compounds,
this results in the formation of two enantiomeric forms of the
complex molecules [CuLCl2] (for 1) and [CuLBr2] (for 2), which
are present in the crystals of these compounds (racemic mixture).
The pyrazole and pyrimidine rings of the coordinated L molecules
are not coplanar, the torsion angles between the planes of the
pyrazolyl and pyrimidine rings are 6.9(3)° (1) and 7.1(7)° (2), while
the angle between the pyrimidine and phenyl planes are 31.0(3)°
(1) and 31.3(7)° (2). In the structures of both 1 and 2, double
Volume (Å3)
Z
Dcalc (Mg/m3)
Absorption coefficient
1.598
1.643
1.864
5.858
(mmꢁ1
)
Crystal size (mm3)
Theta range for data
collection (°)
0.05 ꢀ 0.05 ꢀ 0.025
0.114 ꢀ 0.021 ꢀ 0.013
1.94–25.00
1.90–24.99
Index ranges
ꢁ7 6 h 6 8
ꢁ15 6 k 6 15
ꢁ20 6 l 6 20
13685
2928 (Rint = 0.0433)
100.0
ꢁ8 6 h 6 8
ꢁ15 6 k 6 13
ꢁ15 6 l 6 21
7431
3016 (Rint = 0.0604)
99.8
Reflections collected
Independent reflections
Completeness to h = 25.00°
(%)
Absorption correction
SADABS
SADABS
l
Maximum and minimum
transmission
Refinement method
1 and 0.724144
1 and 0.337248
Full-matrix least-
squares on F2
2928/0/211
1.058
R1 = 0.0339,
wR2 = 0.0761
R1 = 0.0456,
wR2 = 0.0822
ꢁ0.004(16)
Full-matrix least-
squares on F2
3016/0/211
0.956
R1 = 0.0425,
wR2 = 0.0548
R1 = 0.1167,
wR2 = 0.0670
0.031(16)
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2
r(I)]
R indices (all data)
Absolute structure
parameter
Largest diff. peak and hole
0.337 and ꢁ0.179
0.394 and ꢁ0.418
(e/Å3)
p–p interactions between the phenyl and the pyrimidine rings
are present, resulting in the formation of a column of complex mol-
ecules running along a axis, and the centroid–centroid distances
within each (phenylꢂ ꢂ ꢂpyrimidine)n chain for 1 are 3.722(4) and
3.813(4) Å, those for 2 are 3.684(7) and 3.777(8) Å (Figs. 2 and
3). It ought to be noted that in both cases the pyrazole ring is
[CuLHal2], the Cu:L = 1:1 molar ratio was applied for the synthesis
of the complexes. As follows from the chemical analysis data, com-
pounds having the CuLHal2 stoichiometry were obtained in both
cases, suggesting thus at a first glance a molecular mononuclear
structure of both complexes. X-ray diffraction data (see below)
revealed that they do have molecular mononuclear structures,
[CuLHal2]. In contrast to previous data on the crystallization of a
copper(II) chloride complex with 3, during the crystallization of
complex 1 from the mother liquor (in EtOH) we did not observe
the phenomenon of the simultaneous crystallization of different
polymorphic modifications. This situation does not change with a
change in solvent, and the crystallization of 1 from a CHCl3/
EtOH = 4/1 mixture does not produce any other polymorphic mod-
ifications. Even for experiments carried out under the same condi-
tions as were reported in [23], i.e., crystallization of 1 from an
iPrOH/MeCN = 2/1 mixture, afforded only the crystals of 1 identical
not involved in p–p stacking. Thus, the packing of the isostructural
complex compounds 1 and 2 has no analogs among the poly-
morphs of the copper(II) chloride complex with ligand 3, where
for the O polymorph the pyrazole and pyrimidine rings participate
in
p–p stacking, and for EG polymorph – the pyrimidine rings. A
possible explanation of this observation could be that in the struc-
tures of the EG and O polymorphs of the copper(II) chloride com-
plex with 3 the phenyl group is attached at C2 position of the
pyrimidine ring (i.e., in an a-position to the aza-atom participating
in coordination) and a short Cuꢂ ꢂ ꢂH(Ph) intramolecular contact is
formed. Such a contact, of course, influences the angle between
the phenyl and pyrimidine groups: these values are 38.7(3)° and
35.2(3)° for the O and EG modifications respectively. This can
obstruct the involvement of the phenyl group in the formation of
Scheme 2. Synthesis of L and CuLHal2 (Hal = Cl, Br).

238
M.B. Bushuev et al. / Polyhedron 31 (2012) 235–240
Fig. 1. Molecular structure of 1.
Table 2
Selected bond lengths (Å) and angles (°) for 1 and 2.
Fig. 3. Intermolecular interactions in the structure of 2.
Bond
d
Angle
x
Compound 1
Cu(1)–Cl(1)
Cu(1)–Cl(2)
Cu(1)–N(3)
Cu(1)–N(12)
the phenyl and pyrimidine groups in 1 and 2 (see above). Interest-
ingly, the metastable G polymorph has practically the same value
of the angle between the phenyl and pyrimidine groups, 31.0(3)°,
but in fact its structure is the most distorted among the structures
of the discussed complexes (for example, the torsion angle
between the planes of the pyrazolyl and pyrimidine rings is
2.212(1)
2.198(1)
2.087(3)
1.961(3)
Cl(2)Cu(1)Cl(1)
N(3)Cu(1)Cl(1)
N(3)Cu(1)Cl(2)
N(12)Cu(1)Cl(1)
N(12)Cu(1)Cl(2)
N(12)Cu(1)N(3)
98.88(5)
136.73(9)
109.61(8)
97.80(9)
142.49(10)
79.05(11)
Compound 2
Cu(1)–Br(1)
Cu(1)–Br(2)
Cu(1)–N(3)
Cu(1)–N(12)
13.1(3)°) and it does not show p–p stacking at all.
2.345(1)
2.326(1)
2.073(5)
1.933(6)
Br(2)Cu(1)Br(1)
N(3)Cu(1)Br(1)
N(3)Cu(1)Br(2)
N(12)Cu(1)Br(1)
N(12)Cu(1)Br(2)
N(12)Cu(1)N(3)
98.64(4)
134.8(2)
111.2(2)
98.6(2)
141.1(2)
79.1(2)
In the crystal packing the presence of intramolecular C–Hꢂ ꢂ ꢂHal
contacts should be outlined (Cl(1)ꢂ ꢂ ꢂC(131) 3.548(4) Å,
Cl(2)ꢂ ꢂ ꢂC(21) 3.462(4) Å for 1; Br(1)ꢂ ꢂ ꢂC(131) 3.691(8) Å,
Br(2)ꢂ ꢂ ꢂC(21) 3.552(7) Å for 2). Molecules of [CuLHal2], running
along b axis, are linked into chains by means of C-Hꢂ ꢂ ꢂHal hydrogen
bonds (Cl(1)ꢂ ꢂ ꢂC(14)0 3.684(4) Å, Cl(1)ꢂ ꢂ ꢂC(64)0 3.584(4) Å for 1;
Br(1)ꢂ ꢂ ꢂC(14)0 3.811(7) Å, Br(1)ꢂ ꢂ ꢂC(64)0 3.708(9) Å for 2). In addi-
tion, there are C-Hꢂ ꢂ ꢂHal hydrogen bonds between adjacent chains
of complex molecules (Cl(1)ꢂ ꢂ ꢂC(62)0 3.560(4) Å, Cl(2)ꢂ ꢂ ꢂC(131)0
3.619(4) Å, Cl(2)ꢂ ꢂ ꢂC(151)0 3.859(4) Å for 1; Br(1)ꢂ ꢂ ꢂC(62)0
3.701(8) Å, Br(2)ꢂ ꢂ ꢂC(131)0 3.831(8) Å, Br(2)ꢂ ꢂ ꢂC(151)0 4.032(7) Å
for 2 (Fig. 3)).
3.3. IR and electronic spectra
The positions of the stretching-deformation vibrations of the
heteroaromatic rings, (m + d)ring, in the IR spectra of complexes 1
and 2 are changed in comparison with the spectrum of L. The
far-IR spectra of the complexes exhibit stretching vibration bands
of the Cu–N and Cu–Hal bonds. For the
m(Cu–Hal) bands, the fre-
quency ratios (Cu–Br)/ (Cu–Cl) are close to the literature data
m
m
(0.77–0.74) [43]. These observations are consistent with coordina-
tion of the heteroaromatic rings of the L molecule as well as with
coordination of halogen atoms.
The electronic diffuse reflectance spectra of 1 and 2 contains
two bands with
m
max = 22520 and 13100 cmꢁ1 for 1 and 21410
Fig. 2. Double
structure of 1.
p–p interactions between the phenyl and the pyrimidine rings in the
and 17390 cmꢁ1 for 2. Although the assignment of these bands
without quantum chemical calculations seems not to be fully
unambiguous, it is natural to assume that the high-energy band
can be assigned to charge transfer while the low-energy one seems
to be due the d–d transition between the components of the terms
2E and 2T. This is supported by the data of Lever [44] that pseudo-
tetrahedral copper(II) complexes show relatively low energy tran-
sitions between the components of the terms 2E and 2T. For
double p–p stacking. In contrast to this situation, in the structures
of 1 and 2 the phenyl group is at the C6 position of the pyrimidine
ring and thus can be regarded as less hindered to participate in the
formation of double
dine. This is also reflected by the lesser values of the angle between
p–p interactions together with the pyrimi-

M.B. Bushuev et al. / Polyhedron 31 (2012) 235–240
239
pseudotetrahedral chromophores CuA4 (A = Cl, Br, N, O) these tran-
sitions are usually observed in the range 9000–18000 cmꢁ1 [44]. At
the same time, charge transfer bands for copper(II) complexes are
observed above 20000 cmꢁ1 [44].
To answer these questions we synthesized 4-(3,5-dimethyl-1H-
pyrazol-1-yl)-2-methyl-6-phenylpyrimidine (L) and two copper(II)
complexes, CuLCl2 and CuLBr2, with this ligand. All attempts to obtain
different polymorphic modifications ofthese complexes bycrystalliz-
ing from various solvent failed, only one modification for each com-
pound, with double p–p stacking interactions between the phenyl
3.4. Catalytic activity
and pyrimidine rings, was obtained. This packing has no analogs
among the polymorphs of the copper(II) chloride complex with 4-
(3,5-dimethyl-1H-pyrazol-1-yl)-6-methyl-2-phenylpyrimidine. This
could be due to the fact that in the structures of CuLCl2 and CuLBr2
the phenyl group is at the C6 position of the pyrimidine ring and is
In the presence of the co-catalyst MAO, both CuLCl2 and CuLBr2
exhibit weak catalytic activity of 48 and 52 kg/(molCu h atm) in
ethylene polymerization, whereas the free ligand L was found to
be inactive. To support this experimental observation we have
checked the catalytic behavior of free 2-(3,5-diphenyl-1H-pyra-
zol-1-yl)-4,6-diphenylpyrimidine, a compound which was used
earlier as a ligand for the synthesis of copper(II) chloride and bro-
mide complexes, which were found to be active in ethylene poly-
merization [36]. Like L, this compound is also inactive. Thus, it
should be underlined that, catalytic activity in ethylene polymeri-
zation, although not high, is observed only in the presence of the
copper(II) complexes with pyrazolylpyrimidine ligands, but not
in the case when the free pyrazolylpyrimidines were used. In addi-
tion, to gain an insight into the nature of the intermediates, an
electron paramagnetic resonance (EPR) analysis of the mixture
CuLCl2/MAO = 1/200 (CCu = 5 ꢀ 10ꢁ3 mmol/ml) was performed.
Similarly to the results obtained by Galletti et al. [38], it evidenced
the absence of copper(II) species, thus suggesting the formation of
an EPR-silent copper(I) species in the reaction mixture.
not hindered to participate in the formation of double p–p interac-
tions together with the pyrimidine ring. This leads to the fast self-
assembly of CuLHal2 (Hal = Cl, Br) molecules and to the formation of
a double stacked structure. To answer the second question we have
checked the catalytic behavior of free L and the CuLCl2 and CuLBr2
complexes in the presence of the co-catalyst MAO, and found that
only the complexes exhibit catalytic activity in ethylene polymeri-
zation, whereas the free ligand L is inactive.
Acknowledgements
We are grateful to E.V. Peresypkina and A.V. Virovets for X-ray
diffraction experiments, to L.A. Sheludyakova for recording IR spec-
tra of the compounds, to E. G. Boguslavskii for ESR measurements
and to I.V. Yushina for recording diffuse reflectance spectra.
Appendix A. Supplementary data
3.5. Synthetic aspects (Part 2)
CCDC 827148 and 827149 contain the supplementary crystallo-
graphic data for 1 and 2. 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, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: deposit@
ccdc.cam.ac.uk.
After describing the observations made during the crystalliza-
tion experiments and after analysis of the packing of complexes,
a question arises: why despite numerous experiments on the crys-
tallization of complex 1 have we not observed the formation of
polymorphs, and why does only one modification with double
p–
p
stacking interactions between the phenyl and pyrimidine rings
crystallize? The brief answer seems to be clear. It is an interplay
between kinetics and thermodynamics that affects the process of
crystallization and affords this modification. But can we make an
attempt to deduce the microscopic origin for this? A possible
explanation can be as follows: it is natural to assume that the mod-
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