Steric influence of the 6-methyl group on the molecular and crystal structures of copper(II) chloride complexes with 2-(N-acetylamino)-6- methylpyridine

Article abstract of DOI:10.1016/j.poly.2014.04.016

New coordination compounds of copper(II) chloride with 2-(N-acetylamino)-6- methylpyridine (L) of the compositions [Cu₂L₂Cl ₄] (1), [CuL(H₂O)Cl₂]·L (1a), and [CuL₂Cl][CuLCl₃]·C₂H₅OH (2) have been prepared by the reactions between ethanol solutions of L and ethanol solutions of CuCl₂·2H₂O at different Cu:L ratios. The compounds have been characterized by a set of methods including elemental analysis, IR-spectroscopy, single-crystal X-ray diffraction analysis, and magnetic susceptibility measurements in the range 5-300 K (for 1). The crystal structures of 1 and 1a revealed distorted square-pyramidal geometry around Cu(II) centers, while in 2 both distorted square-pyramidal and trigonal-bipyramidal coordinations are present. L acts as a bidentate chelate ligand bound to the metal center through the pyridine N atom and the acetyl O atom. The results of the study demonstrate that the presence of 6-methyl substituent in L provides a high steric hindrance, precluding the formation of complexes with a distorted octahedral coordination of Cu(II).

Full text of DOI:10.1016/j.poly.2014.04.016

Polyhedron 77 (2014) 81–88
Contents lists available at ScienceDirect
Polyhedron
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Steric influence of the 6-methyl group on the molecular and crystal
structures of copper(II) chloride complexes
with 2-(N-acetylamino)-6-methylpyridine
Anton I. Smolentsev a,b, , Elisaveta V. Lider a,b, Ludmila G. Lavrenova a,b, Liliya A. Sheludyakova a,
⇑
Artem S. Bogomyakov c, Sergei F. Vasilevsky b,d
a Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, 3, Acad. Lavrentiev Ave., 630090 Novosibirsk, Russia
b Novosibirsk State University, 2, Pirogova Str., 630090 Novosibirsk, Russia
c International Tomography Center, Siberian Branch of the Russian Academy of Sciences, 3a, Institutskaya Str., 630090 Novosibirsk, Russia
d Institute of Chemical Kinetics and Combustion, Siberian Branch of the Russian Academy of Sciences, 3, Institutskaya Str., 630090 Novosibirsk, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
New coordination compounds of copper(II) chloride with 2-(N-acetylamino)-6-methylpyridine (L) of the
compositions [Cu2L2Cl4] (1), [CuL(H2O)Cl2]ÁL (1a), and [CuL2Cl][CuLCl3]ÁC2H5OH (2) have been prepared
by the reactions between ethanol solutions of L and ethanol solutions of CuCl2Á2H2O at different Cu:L
ratios. The compounds have been characterized by a set of methods including elemental analysis, IR-
spectroscopy, single-crystal X-ray diffraction analysis, and magnetic susceptibility measurements in
the range 5–300 K (for 1). The crystal structures of 1 and 1a revealed distorted square-pyramidal geom-
etry around Cu(II) centers, while in 2 both distorted square-pyramidal and trigonal–bipyramidal coordi-
nations are present. L acts as a bidentate chelate ligand bound to the metal center through the pyridine N
atom and the acetyl O atom. The results of the study demonstrate that the presence of 6-methyl substi-
tuent in L provides a high steric hindrance, precluding the formation of complexes with a distorted octa-
hedral coordination of Cu(II).
Received 20 March 2014
Accepted 9 April 2014
Available online 19 April 2014
Keywords:
Copper(II) complexes
2-(N-acetylamino)-6-methylpyridine
Steric effect
Synthesis
Crystal structure
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
anticancer agent. Besides, a number of 2-(N-acylamino)pyridine
ligands and their Ni(II) complexes were found to be suitable cata-
The pyridine derivatives are widely used in the practical appli-
cations. These compounds are known as important substrates for
the preparation of high-effective plants growth regulators; more-
over, they are good complexing agents for the extraction of metal
ions from waste water. The constant interest to the pyridine deriv-
atives can also be explained by the fact that many of them, natural
and synthetic, display significant physiological activity. A special
place in the chemistry of pyridine is occupied by 2-(N-acyla-
mino)pyridines. The research in this field can be exemplified by
many works, including [1] focused on the elucidation of influence
of some 2-(N-acylamino)pyridines on the growth and division of
cells. It is noteworthy that just the simplest member, unsubstitut-
ed 2-(N-acetylamino)pyridine, was found to be the most active
lysts for the ethylene oligomerization [2]. The perspectives of use
of 2-(N-acylamino)pyridine derivatives for the design of new
asymmetric binuclear metal complexes were demonstrated
recently [3].
It is known that the molecule of 2-(N-acylamino)pyridine has
one Lewis-acidic (NH group) and two Lewis-basic (pyridine N atom
and carbonyl O atom) coordination sites. The introduction of alkyl
substituents in the heterocycle affects electron density that can be
used to modify the donor properties of the ligand. On the other
hand, the presence of substituents in the a-position of the pyridine
ring can create a steric hindrance that blocks ligand access to the
metal ion, significantly changing the coordination geometry. Par-
ticularly, 2-(N-acylamino)pyridines, including their 6-methyl
derivatives, were used as convenient model compounds for the
investigation of steric influence of the alkyl groups on molecular
self-association in the solid state and in solution [4]. At the same
time, the structural data on the transition metal complexes with
simple (having only small acyl groups) 2-(N-acylamino)pyridines
are very limited. In one of the earliest work in this field [5], several
⇑
Corresponding author at: Nikolaev Institute of Inorganic Chemistry, Siberian
Branch of the Russian Academy of Sciences, 3, Acad. Lavrentiev Ave., 630090
Novosibirsk, Russia. Tel.: +7 383 330 9466; fax: +7 383 330 9489.
E-mail addresses: smolentsev@ngs.ru (A.I. Smolentsev), lisalider@ngs.ru
(E.V. Lider).
http://dx.doi.org/10.1016/j.poly.2014.04.016
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

82
A.I. Smolentsev et al. / Polyhedron 77 (2014) 81–88
complexes of Pd(II), Co(II), Ni(II), and Cu(II) with 2-(N-acetyl-
amino)pyridine were synthesized, but no attempts were made to
characterize them by X-ray diffraction methods. Some lanthanide
complexes with 2-(N-acetylamino)-4-methylpyridine were inves-
tigated spectroscopically [6]. To date, structural data are available
for the complexes of Ni(II), Zn(II) [7], Cu(II) [8], and Hg(II) [9] with
2-(N-acetylamino)pyridine, Cu(II) complex with 2-(N-acetyl-
amino)-4-methylpyridine [10], and Rh(II) complex with 2-(N-ace-
tylamino)-6-methylpyridine [11]. In view of this, it was of
interest to prepare a representative series of transition metal com-
plexes with sterically active 2-(N-acetylamino)-6-methylpyridine,
and to evaluate relationships between molecular and crystal
were commercially available products of reagent grade quality. IR
spectra of the complexes were recorded on Scimitar FTS 2000
and Vertex 80 spectrometers in the range of 4000–100 cmÀ1. The
samples were prepared as mineral and fluorinated oil mulls and
polyethylene pellets. The metal content of the compounds synthe-
sized was determined by complexonometry after the samples were
decomposed in a mixture of concentrated H2SO4 and HClO4 (1:2).
Elemental analyses of C, H, and N were performed using a Euro
EA 3000 analyzer using a standard procedure. All the compounds
show satisfactory elemental analyses (Table 1) and exhibit well-
resolved sharp IR bands confirming their purity.
structures formed under steric influence of the
heterocycle.
a-substituted
2.2. Synthesis of L
Thus, in this paper, we report the synthesis and characterization
of three complexes of Cu(II) chloride with 2-(N-acetylamino)-6-
methylpyridine (L), the full formulas of which are [Cu2L2Cl4] (1),
[CuL(H2O)Cl2]ÁL (1a), and [CuL2Cl][CuLCl3]ÁC2H5OH (2).
The synthetic procedure for the preparation of L is similar to
that reported previously [4] and [12]. However, it involves some
changes and adaptations and deserves to be described herein.
Solution of acetyl chloride (1.305 g, 16.5 mmol) in dry toluene
(30 mL) was added dropwise to the magnetically stirred mixture
of 2-amino-6-methylpyridine (1.62 g, 15 mmol) and triethylamine
(1.65 g, 16.5 mmol) in dry toluene (15 mL) at 5 °C. The reaction
mixture was allowed to reach room temperature and stirred for
2 h. To the reaction mixture, water (30 mL) was added, organic
layer was separated. The water layer was extracted once with
methylene chloride (20 mL), and the combined organic layers were
dried over Na2SO4, the solvent was evaporated at reduced pressure.
Recrystallization of crude product from hexane/ethyl acetate (8:1
v/v) mixture gave 2.1 g (93%) of 2-(N-acethylamino)-6-methylpyr-
idine with m.p. 87–88 °C (lit. m.p. 91–92 °C [4] or 85–88 °C [12]).
2. Experimental
2.1. Materials and instruments
CuCl2Á2H2O used for the synthesis of the title compounds was of
‘pure for analysis’ quality. All other reagents and solvents used
Table 1
Elemental analysis results for L and complexes 1 and 2 (%).
Results
L, C8H10N2O
1, C16H20Cl4Cu2N4O2
2, C26H36Cl4Cu2N6O4
2.3. Synthesis of [Cu2L2Cl4] (1)
C calcd.
Found
H calcd.
Found
N calcd.
Found
Cu calcd.
Found
64.0
64.0
6.7
6.7
18.4
18.6
–
33.3
33.8
3.6
3.5
9.6
9.8
22.0
22.3
41.5
40.8
4.9
L (0.30 g, 2 mmol) was dissolved in 5 mL of hot ethanol and
added to 5 mL of hot ethanol solution of CuCl2Á2H2O (0.34 g,
2 mmol). After evaporation of the excess of solvent up to one-third
of its original volume and cooling the solution in an ice bath, in
about an hour a light brown precipitate of 1 was formed. The pre-
cipitate was filtered off, washed twice with ethanol, and dried in
4.7
11.0
11.0
16.3
16.6
–
Table 2
Crystal data and structure refinement details for 1, 1a, and 2.
Compound
1
1a
2
Empirical formula
Formula weight
T (K)
C
16H20Cl4Cu2N4O2
C16H22Cl2CuN4O3
452.82
150(2)
C26H36Cl4Cu2N6O4
765.49
296(2)
569.24
296(2)
Crystal size (mm)
Crystal system
Space group
Z
0.20 Â 0.15 Â 0.04
0.30 Â 0.20 Â 0.12
monoclinic
P 21/n
0.20 Â 0.13 Â 0.02
monoclinic
P 21/c
triclinic
ꢀ
P1
1
4
4
a (Å)
b (Å)
c (Å)
7.5466(6)
8.9629(7)
9.7100(8)
106.033(3)
107.404(3)
103.783(3)
564.27(8)
1.675
7.4859(3)
32.8133(17)
8.4818(4)
7.8125(3)
29.2690(8)
14.5631(3)
a
(°)
b (°)
111.1300(10)
94.5200(10)
c
(°)
V (Å3)
1943.36(16)
1.548
1.422
3319.70(17)
1.532
1.644
Dcalc (g cmÀ3
)
l
(Mo K
a
) (mmÀ1
)
2.377
h Range (°)
2.52–27.52
2.48–27.50
1.98–27.52
h, k, l indices range
F(000)
À9 6 h 6 9, À11 6 k 6 11, À11 6 l 6 12
À6 6 h 6 9, À42 6 k 6 42, À11 6 l 6 10
À7 6 h 6 10, À38 6 k 6 38, À17 6 l 6 18
286
932
1568
Reflections collected
Unique reflections (Rint
Observed reflections
Parameters refined
5742
2589 (0.0198)
2334 [I > 2
129
R1 = 0.0249, wR2 = 0.0659
R1 = 0.0288, R2 = 0.0676
1.045
0.625, À0.224
11776
4436 (0.0482)
3873 [I > 2
247
R1 = 0.0593, R2 = 0.1071
R1 = 0.0701, wR2 = 0.1112
1.183
0.654, À0.557
22767
7373 (0.0522)
4727 [I > 2r(I)]
387
R1 = 0.0428, R2 = 0.0855
R1 = 0.0861, R2 = 0.0949
0.959
0.483, À0.368
)
r
(I)]
r
(I)]
R[F2 > 2
r
(F2)]
R(F2) (all data)
GOOF on F2
D
qmax
,
D
qmin (e ÅÀ3
)

A.I. Smolentsev et al. / Polyhedron 77 (2014) 81–88
83
Table 3
air. Yield: 0.42 g (70%). For the X-ray diffraction study, single crys-
Selected bond lengths (Å) and angles (°) around the Cu2+ ion in 1, 1a, and 2.
tals (brown plates) of 1 were obtained by slow crystallization dur-
ing a few days from ethanol solution (10 mL) of 1 mmol of
CuCl2Á2H2O (0.17 g) and 1 mmol of L (0.15 g) at room temperature.
As a by-product, single crystals (green–blue plates) of the com-
plex [CuL(H2O)Cl2]ÁL (1a) suitable for X-ray diffraction analysis
were obtained by slow crystallization from the mother liquor after
separation of the bulk of 1. Due to the lack of crystals of 1a, no
analyses were performed except for X-ray diffraction. The yield
of 1a could not be given by the same reason.
Bond/angle
1a
1
2, cation
2, anion
Cu(1)–N(1)
2.006(3)
2.0038(15)
2.015(2)
–
Cu(1)–N(3)
–
–
2.019(2)
–
Cu(2)–N(5)
–
–
–
2.029(3)
Cu(1)–O(1)
2.112(2)
2.1228(15)
2.051(2)
–
Cu(1)–O(2)
–
–
2.080(3)
–
Cu(2)–O(3)
–
–
–
2.158(2)
Cu(1)–O(1W)
1.957(3)
–
–
–
Cu(1)–Cl(1)
2.3629(10)
2.2809(6)
2.2580(11)
–
Cu(1)–Cl(2)
2.2794(10)
–
2.2814(5)
2.3677(6)
–
–
–
–
Cu(1)–Cl(2)i
2.4. Synthesis of [CuL2Cl][CuLCl3]ÁC2H5OH (2)
Cu(2)–Cl(2)
–
–
–
2.3755(10)
Cu(2)–Cl(3)
–
–
–
2.2448(11)
Cu(2)–Cl(4)
–
–
–
2.2972(10)
–
–
–
CuCl2Á2H2O (0.08 g, 0.5 mmol) was dissolved in 5 mL of ethanol
and added to 5 mL of ethanol solution of L (0.15 g, 1 mmol) at room
temperature with magnetic stirring. After a few days, the green
plates were filtered off, washed twice with ethanol, and dried in
air. Yield: 0.17 g (60%).
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–O(2)
N(3)–Cu(1)–O(1)
N(3)–Cu(1)–O(2)
N(5)–Cu(2)–O(3)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–O(1W)
N(1)–Cu(1)–Cl(1)
N(1)–Cu(1)–Cl(2)
N(1)–Cu(1)–Cl(2)i
N(3)–Cu(1)–Cl(1)
N(5)–Cu(2)–Cl(2)
N(5)–Cu(2)–Cl(3)
N(5)–Cu(2)–Cl(4)
O(1)–Cu(1)–Cl(1)
O(1)–Cu(1)–Cl(2)
O(1)–Cu(1)–Cl(2)i
O(2)–Cu(1)–Cl(1)
O(3)–Cu(2)–Cl(2)
O(3)–Cu(2)–Cl(3)
O(3)–Cu(2)–Cl(4)
O(1)–Cu(1)–O(2)
O(1W)–Cu(1)–Cl(1)
O(1W)–Cu(1)–Cl(2)
O(1)–Cu(1)–O(1W)
Cl(1)–Cu(1)–Cl(2)
Cl(1)–Cu(1)–Cl(2)i
Cl(2)–Cu(1)–Cl(2)i
Cl(2)–Cu(2)–Cl(3)
Cl(2)–Cu(2)–Cl(4)
Cl(3)–Cu(2)–Cl(4)
90.39(11)
–
–
–
–
–
178.01(13)
92.57(9)
89.49(8)
–
–
–
–
–
93.45(8)
108.91(8)
–
–
–
–
–
–
86.24(10)
91.01(9)
91.27(12)
157.54(4)
–
–
–
–
–
90.98(6)
–
–
–
–
–
–
93.24(5)
166.77(5)
86.72(5)
–
–
–
–
96.53(5)
99.92(4)
95.93(5)
–
–
–
–
–
86.58(9)
95.26(10)
89.26(9)
87.76(10)
–
–
88.32(10)
–
–
–
–
–
–
88.86(8)
171.50(8)
87.39(8)
–
–
–
–
90.88(7)
100.13(8)
100.75(8)
–
–
–
–
–
174.80(11)
–
91.84(9)
–
2.5. X-ray crystallography
–
The single crystals of compounds 1, 1a and 2 were selected
directly from the mother liquors and mounted on glass fibers using
epoxy resin. Single-crystal X-ray diffraction data were collected on
a Bruker-Nonius X8 APEX CCD diffractometer (graphite monochro-
89.93(8)
–
–
–
146.74(8)
–
–
124.33(8)
–
–
–
matized Mo Ka radiation, k = 0.71073 Å, u and x scans of narrow
frames) equipped with a 4 K CCD area detector at 150(2) K for 1a
and 296(2) K for 1 and 2. Absorption corrections were applied
using the SADABS program [13]. The crystal structures were solved
by direct methods and were refined by full-matrix least-squares
techniques with the use of the SHELXTL package [13]. Molecular
graphics was performed using standard means of SHELXTL package
and DIAMOND program [14]. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms of the water molecule in 1a were
found in the difference Fourier maps and refined isotropically with
O–H and HÁ Á ÁH distances restrained (using DFIX and DANG instruc-
tions) to 0.82(2) and 1.30(2) Å, respectively. Hydrogen atoms of the
2-(N-acetylamino)-6-methylpyridine ligands were placed in geo-
metrically calculated positions and treated as riding on their par-
ent atoms. Crystallographic data as well as details of data
collection and refinement for complexes 1, 1a and 2 are given in
Table 2. Selected bond distances and angles are given in Table 3.
88.86(11)
–
–
–
–
–
–
–
–
–
–
–
–
92.94(2)
167.54(2)
84.73(2)
–
–
–
–
–
90.08(4)
167.67(4)
91.89(4)
Symmetry code: (i) 1 À x, 1 À y, 1 À z.
2.6. Magnetic susceptibility measurements
O
Et3N
O
H3C
+
toluene
The magnetic susceptibility of the polycrystalline sample of
dimeric complex 1 was measured with a Quantum Design MPMSXL
SQUID magnetometer in the temperature range 5–300 K with mag-
netic field of up to 5 kOe. The complex showed no any field depen-
dence of molar magnetization at low temperatures. Diamagnetic
corrections were made using the Pascal constants. The effective
Cl
H3C
N
N
CH3
H3C
N
NH2
H
Scheme 1. Synthesis of L.
product was recrystallized from the hexane/ethyl acetate mixture.
Elemental analysis showed that the ligand was of enough purity to
be used in subsequent reactions.
magnetic moment was calculated as
leff(T) = [(3 k/NAl )
2B vT]1/
2
ꢀ (8
v .
T)1/2
The title copper(II) chloride complexes with 2-(N-acethylami-
no)-6-methylpyridine (L) were obtained by the reactions between
ethanol solutions of CuCl2Á2H2O and L at Cu:L ratios of 1:1 and 1:2,
as shown in the Scheme 2.
3. Results and discussion
3.1. Synthesis
The reaction performed with the initial Cu:L ratio of 1:1 and fol-
lowed by immediate evaporation of the excess solvent yielded the
compound 1 in the form of a polycrystalline precipitate. Scarce
crystals of 1a appeared on standing for a week of the mother liquor
left after precipitation of the bulk of 1. The reaction performed
with the same Cu:L ratio, but followed by slow (several days) crys-
tallization at room temperature yielded brown plate crystals of the
binuclear complex 1, in which each Cu2+ ion is bidentately coordi-
2-(N-Acethylamino)-6-methylpyridine (L) was prepared in
good yield through the condensation reaction of 2-amino-6-meth-
ylpyridine and acetyl chloride according to a modified procedure
[4] (Scheme 1). The reaction was smoothly performed in dry tolu-
ene at room temperature. The resulting mixture was quenched
with water and filtered. The white solid was thoroughly washed
with methylene chloride and dried at reduced pressure. The

84
A.I. Smolentsev et al. / Polyhedron 77 (2014) 81–88
.
-
Scheme 2. Synthesis of the complexes 1, 1a and 2.
nated by L and also bears one terminal and two bridging chloride
ions (CuNOCl3 coordination unit). Coordination unit of the complex
1a, CuNO2Cl2, includes one bidentate ligand L, one water molecule,
and two chloride ions, each coordinated to the Cu2+ ion. Finally, the
combination of Cu:L = 1:2 stoichiometry and slow crystallization of
the resulting solution led to the formation of the double complex
salt 2. The cationic part of 2 consists of the Cu2+ ion bidentately
coordinated by two L molecules and one chloride ion (CuN2O2Cl
coordination unit). The anionic part of 2 includes the Cu2+ ion coor-
dinated by one L molecule and three chloride ions; coordination
unit CuNOCl3. The nontrivial composition of 2 most likely indicates
the competition between ligands having a comparably strong
metal-coordination ability. On the other hand, the presence of
the 6-methyl group on the pyridine ring of L presumably prevents
the formation of the complexes trans-[CuL2Cl2], which were
reported previously for both unsubstituted 2-(N-acetylamino)pyr-
idine and its 4-methyl derivative. By this reason, at the Cu:L = 1:2
ratio the common distorted octahedral coordination with two
nearly coplanar L ligands is not possible; it rearranges into a trigo-
nal bipyramid with large (>50°) dihedral angles between the ligand
planes. Thus, although the cationic part of 2 includes two L mole-
cules per metal ion (i.e. the 1:2 stoichiometry is formally met),
the overall Cu:L ratio in the entire complex is lower, being 1:1.5,
which can be attributed to the steric hindrance provided by the
6-methyl group.
Fig. 1. Thermal ellipsoid representation (at the 30% probability level) of the
binuclear complex [Cu2L2Cl4]. Only the asymmetric unit is numbered. Unnumbered
atoms are related to numbered ones by an inversion center midway between the
bridging chloride ligands.
hydrogen bonds of 3.29 Å in such a way as to give infinite one-
dimensional ribbons parallel to the a axis (Fig. 2). Only non-bridg-
ing chloride ligands are involved in hydrogen bonding. The ribbons
are packed in the crystal lattice without any significant interac-
tions between them other than van der Waals forces.
3.2. Description of crystal structures
The crystals of compound 1 are triclinic; space group P–1 and
the unit cell contains 1 formula unit. The asymmetric unit consists
only half of the binuclear complex [Cu2L2Cl4] with all atoms occu-
pying general positions. The entire [Cu2L2Cl4] molecule is gener-
ated by the inversion center located at (1/2, 1/2, 1/2). The
coordination environment of the Cu2+ ion includes N and O atoms
of the bidentate ligand L, and three chloride ions, two of which are
The compound 1a crystallizes in the monoclinic space group
P21/n with 4 formula units per unit cell. All atoms comprising the
asymmetric unit, [CuL(H2O)Cl2] neutral complex and uncoordi-
nated L molecule, are located in general positions. The coordination
environment of the Cu2+ ion in the [CuL(H2O)Cl2] complex includes
N and O atoms of the bidentate ligand L, two chloride ions, and O
atom of the water molecule. It can be described as either a dis-
torted trigonal bipyramid with the equatorial plane defined by
O(1), Cl(1), and Cl(2) atoms, or as a distorted tetragonal pyramid
with the basal plane defined by N(1), O(1W), Cl(1), and Cl(2) atoms
(Fig. 3). The latter representation is more appropriate since the Cl–
Cu–Cl angle is rather wide, 157.54(4)°, while both Ow–Cu–Cl angles
are close to 90° (Table 3). The Cu–N, Cu–O, Cu–Ow and two Cu–Cl
bond lengths are 2.006(3), 2.112(2), 1.957(3), and 2.3629(10) and
2.2794(10) Å, respectively, which are in good agreement with the
l2-bridging (Fig. 1). These atoms form a distorted tetragonal pyr-
amid with the basal plane defined by N(1), Cl(1), Cl(2), and symme-
try-generated Cl(2)i atoms ((i): 1 – x, 1 – y, 1 – z). The Cu–N and
Cu–O distances are 2.0038(15) and 2.1228(15) Å, respectively.
Three Cu–Cl bond lengths are 2.2809(6), 2.2814(5) and
2.3677(6) Å; there is no clear dependence on the coordination
mode of the particular chloride ion. The CuÁ Á ÁCu separation within
the binuclear unit is 3.4356(5) Å. Each binuclear unit is further
connected to two adjacent units through pairs of Namide–HÁ Á ÁCl

A.I. Smolentsev et al. / Polyhedron 77 (2014) 81–88
85
Fig. 2. Fragment of the one-dimensional ribbon along the a axis, generated by Namide–HÁ Á ÁCl hydrogen bonds (dashed lines) between the [Cu2L2Cl4] molecules in the structure
of 1. For clarity, only those H atoms engaged in hydrogen bonding are shown.
Fig. 3. Thermal ellipsoid representation (at the 50% probability level) with the atom labeling scheme for (a) [CuL(H2O)Cl2] complex and (b) uncomplexed L molecule in 1a.
values typical of the Cu(II) complexes of similar composition. The
interesting feature of 1a is the presence of extra L molecule, which
is not involved in coordination to the metal ion and has a some-
what different geometry as compared to the coordinated one. Spe-
cifically, it has the acetylamino group rotated by almost 180° with
respect to the parent pyridine ring. The C–Namide–C angle in unco-
ordinated L molecule is expectedly lower, being 128.6(3)° versus
131.8(3)° in the coordinated one. Other angles in the acetylamino
group are also slightly lower in the case of uncoordinated ligand.
In the structure, the [CuL(H2O)Cl2] complexes are connected in
the chains along the a axis through the Namide–HÁ Á ÁCl hydrogen
bonds of 3.32 Å in length (hereafter the DÁ Á ÁA distances are given).
Besides, each [CuL(H2O)Cl2] unit form Ow–HÁ Á ÁNpyridine, Ow–HÁÁÁO
and weak Namide–HÁ Á ÁCl hydrogen bonds (2.83, 2.71 and 3.51 Å,
respectively) with two uncoordinated L molecules through which
the adjacent chains are extended into a layer parallel to the ac
plane (Fig. 4). Only one of two available chloride ligands, Cl(1), is
involved in hydrogen bonding. In total, each [CuL(H2O)Cl2] com-
plex forms five hydrogen bonds. Moreover, the pyridine rings of
the adjacent [CuL(H2O)Cl2] complexes are partially overlapped
plex cation [CuL2Cl]+. The second geometry is formed around the
Cu(2) center by one coordinated L molecule and three chloride
ligands, yielding the complex anion [CuLCl3]À. The Cu–N and
Cu–O bond lengths fall in the intervals 2.015(2)–2.029(3) and
2.051(2)–2.158(2) Å, respectively. It is seen that the Cu–O dis-
tances are noticeably (up to ꢁ0.1 Å) shorter in the case of trigo-
nal–bipyramidal coordination, while the Cu–N distances are
almost the same in both cases. Comparison of the bond angles in
the copper metallocycles indicates that the L molecule is more
strained in the case of tetragonal–pyramidal geometry. Particu-
larly, transition from the [CuLCl3]À anion to the [CuL2Cl]+ cation
is accompanied by the decrease of the C–Namide–C angle from
131.2(3)° to 128.3(3)° and 129.7(3)°. The latter values are compa-
rable with 128.6(3)° observed in free L molecule in 1a. The Cu–Cl
bond lengths vary in the wide range 2.2448(11)–2.3755(10) Å,
showing no dependency on the coordination geometry. Summariz-
ing, it is worth to note that for many copper(II) halide complexes
with unidentate substituted pyridine-based ligands the same steric
influence of the a-substituents is well documented, which, in most
cases, makes it impossible for the Cu2+ ions to achieve coordination
numbers higher than 4 (square-planar coordination) [15–17].
An analysis of the structure of 2 reveals a crucial role of the
amide H atoms and chloride ligands in the hydrogen bond forma-
tion, similar to that in the above described structures of 1 and 1a.
In particular, the [CuLCl3]À anions are connected in chains along
the a axis via the Namide–HÁ Á ÁCl hydrogen bonds of 3.37 Å in length.
To each member of these chains, the [CuL2Cl]+ cation is attached
through the Namide–HÁ Á ÁCl hydrogen bond of 3.25 Å. Using the
due to the
p-stacking interactions with an interplanar distance of
about 3.2 Å.
The compound 2 crystallizes in the monoclinic system with the
space group P21/c and 4 formula units per unit cell. The asymmet-
ric unit contains the full formula unit. All atoms are in general posi-
tions. The structure of 2 is characterized by the simultaneous
presence of both possible for Cu2+ ion five-coordinate geometries,
a distorted trigonal bipyramid and a tetragonal pyramid (Fig. 5).
The first one is achieved through the coordination of two bidentate
L ligands and one chloride ligand to Cu(1) center, giving the com-
other amide H atom, the [CuL2Cl]+ cation forms strong Namide
–
HÁ Á ÁO hydrogen bond of 2.78 Å with the lattice ethanol molecule,

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A.I. Smolentsev et al. / Polyhedron 77 (2014) 81–88
Fig. 4. Fragment of the two-dimensional layer parallel to the ac plane in the structure of 1a, showing Ow–HÁ Á ÁNpyridine, Ow–HÁ Á ÁO and Namide–HÁ Á ÁCl hydrogen bonds (dashed
lines) between the [CuL(H2O)Cl2] and L molecules. For clarity, only those H atoms engaged in hydrogen bonding are shown.
Fig. 5. Thermal ellipsoid representation (at the 30% probability level) with the atom labeling scheme for (a) [CuL2Cl]+ cation and (b) [CuLCl3]– anion in 2.
whereby the joining of the chains into a layer parallel to the ab
vibrations m(CO), which are observed in the IR spectrum of L [4].
plane is achieved (Fig. 6). The layers are further interconnected
Amide group, along with the stretching-bending vibrations of the
heterocyclic ring, contributes to the intensive absorption bands
at 1598, 1582, and 1545 cmÀ1. The vibrations of NH-groups
involved in hydrogen bonding are observed as a wide structured
through the
p–p interactions between the pyridine rings with
interplanar distances ranging from about 3.3 to 3.8 Å.
3.3. IR spectroscopy
band in the range 3240–3100 cmÀ1, while the
m
(CH) bands are
observed in the range 3050–2780 cmÀ1
In the spectra of complexes 1 and 2, the
.
It is well-known that the amide group of the ligand L can form
intermolecular hydrogen bonds of two types: Namide–HÁ Á ÁNpyridine
and Namide–HÁ Á ÁO = C. According to [18,19] only hydrogen bonds
NHÁ Á ÁOC are realized in 2-(N-acetylamino)-6-methylpyridine; in
this case the band in the range 1660 cmÀ1 corresponds to the
m(NH) bands are shifted
(as compared to the spectrum of free L) to higher wavenumbers
(3313–3170 cmÀ1), which obviously can be attributed to the
changing of the hydrogen bond pattern. In addition, the effect of
ligand coordination and changes in the hydrogen bond network

A.I. Smolentsev et al. / Polyhedron 77 (2014) 81–88
87
Fig. 6. Projection along the c axis of the two-dimensional layer in the structure of 2, showing O–HÁ Á ÁCl, Namide–HÁ Á ÁO and Namide–HÁ Á ÁCl hydrogen bonds (dashed lines)
between the [CuL2Cl]+ cations, [CuLCl3]À anions and solvate ethanol molecules. For clarity, only the H atoms involved in hydrogen bonding are shown.
in 1 and 2 are reflected in the range 1660–1540 cmÀ1. Particularly,
the
m(CO) band appears as split into two components, 1682 and
1619 cmÀ1 for 1, and 1684 and 1623 cmÀ1 for 2. The vibrations
of the ring are shifted to the values 1536, and 1577 and
1536 cmÀ1 for 1 and 2, respectively. These facts confirm that pyri-
dine N atom and carbonyl O atom of L are involved in coordination
to the Cu(II) ion.
The comparison of the spectra of 1 and 2 with the spectrum of L
in the low-frequency regions makes it possible to attribute the
bands at 376 (for 1) and 374 cmÀ1 (for 2) to the
m(Cu–O) vibrations.
The presence of Cu–Cl bonds of different lengths are observed in
the spectra as wide bands at 284 (for 1) and 274 cmÀ1 (for 2),
which correlate with the literature data [20]. The bands at 263
and 257 cmÀ1 correspond to the vibrations
m(Cu–N).
3.4. Magnetic properties of 1
The temperature plot of leff for compound 1 is shown in Fig. 7.
Fig. 7. Temperature dependence of the leff for 1. Solid line corresponds to the
In the room-temperature region, the value of leff is 2.69 lB and
remains almost constant up to 50 K. Below 50 K the leff value
decreases, reaching 0.54 lB at 5 K. The room-temperature values
of leff are slightly higher than the theoretical value of 2.45 lB for
two almost non-interacting Cu2+ ions with the g-factor 2. As the
temperature is lowered, the leff values decrease, which indicates
the predominant antiferromagnetic exchange interactions in 1.
The observed magnetic behavior is typical of compounds whose
exchange clusters consist of even number of the paramagnetic cen-
ters with identical spin. In the case of 1, according to the chemical
analysis and X-ray diffraction data, the cluster is a dimer, the
exchange channel of which is formed by two bridging ClÀ ions.
The optimal values of the effective g-factor and the exchange
parameter J/k were found to be 2.220( 0.003) and À12.9( 0.1) K,
theoretical approximation.
4. Conclusion
In the present work three new complexes of Cu(II) chloride with
2-(N-acetylamino)-6-methylpyridine have been isolated and char-
acterized. The crystal structures of the compounds revealed an
important role of the
N
amide–HÁ Á ÁCl intermolecular hydrogen
bonds, which force the structural units to organize in one- and
two-dimensional associates. The steric hindrance of the 6-methyl
group presumably precludes the formation of the octahedral com-
plexes of the type trans-[CuL2Cl2] previously reported for unsubsti-
tuted 2-(N-acetylamino)pyridine and its 4-methyl derivative.
Instead, the Cu2+ ion in the title complexes takes either a distorted
square-pyramidal or a distorted trigonal–bipyramidal coordination
respectively. The theoretical curve
leff(T) corresponding to these
values is shown in Fig. 7 by solid line.

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A.I. Smolentsev et al. / Polyhedron 77 (2014) 81–88
geometry. It was demonstrated that copper(II) chloride complexes
with L exhibiting the Cu:L ratios other than 1:1 and 2:3 could not
be prepared. The observations from this work can be used to design
new coordination compounds having solid-state organization
adjusted in a predictable way through the artful use of steric
effects.
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Acknowledgments
This research was financially supported by the Russian
Foundation for Basic Research (Grants No. 12-03-31343 mol_a
and 14-03-31539 mol_a), by GC No. 02.740.11.0628 of FFP ‘‘SSESIR
2009–2013’’, and by the Ministry of Education and Science of the
Russian Federation.
The authors would also like to thank Dr. Alexander A. Stepanov
(Institute of Chemical Kinetics and Combustion SB RAS) for help in
the synthesis of the ligand, and Olga V. Lundovskaya (Nikolaev
Institute of Inorganic Chemistry SB RAS) for help in the synthesis
of the compounds.
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Appendix Supplementary. data
Crystallographic data for the structures of the compounds 1, 1a
and 2 have been deposited at the Cambridge Crystallographic Data
Center under reference numbers CCDC 987921, 987922, 987923,
respectively. Copies of this information may be obtained free of
charge from the CCDC, 12 Union Road, Cambridge, CB2 1 EZ,
UK (fax: +44-1223-336033; http://www.ccdc.cam.ac.uk/conts/
retrieving.html).