Synthesis, molecular structure, and spectral analysis of copper(II) complexes derived from pyridinediols

Article abstract of DOI:10.1080/00958972.2014.982111

A series of mononuclear copper(II) complexes was synthesized by reaction of different 2,6-disubstituted pyridines with elemental Cu in a CCl₄/DMSO solvent system. Physical properties were analyzed using IR, optical spectroscopy, mass spectromet

Full text of DOI:10.1080/00958972.2014.982111

This article was downloaded by: [Gazi University]
On: 06 January 2015, At: 09:44
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Coordination Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gcoo20
Synthesis, molecular structure,
and spectral analysis of copper(II)
complexes derived from pyridinediols
a
a
a
Guillermo M. Chans , Elizabeth Gómez , Virginia Gómez-Vidales ,
a
a
R. Alfredo Toscano & Cecilio Álvarez-Toledano
a
Instituto de Química, Universidad Nacional Autónoma de México,
México D.F., Mexico
Accepted author version posted online: 29 Oct 2014.Published
online: 21 Nov 2014.
Click for updates
To cite this article: Guillermo M. Chans, Elizabeth Gómez, Virginia Gómez-Vidales, R. Alfredo
Toscano & Cecilio Álvarez-Toledano (2015) Synthesis, molecular structure, and spectral analysis of
copper(II) complexes derived from pyridinediols, Journal of Coordination Chemistry, 68:2, 206-219,
DOI: 10.1080/00958972.2014.982111
To link to this article: http://dx.doi.org/10.1080/00958972.2014.982111
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“
Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Journal of Coordination Chemistry, 2015
Vol. 68, No. 2, 206–219, http://dx.doi.org/10.1080/00958972.2014.982111
Synthesis, molecular structure, and spectral analysis of copper
(
II) complexes derived from pyridinediols
GUILLERMO M. CHANS, ELIZABETH GÓMEZ*, VIRGINIA GÓMEZ-VIDALES,
R. ALFREDO TOSCANO and CECILIO ÁLVAREZ-TOLEDANO
Instituto de Química, Universidad Nacional Autónoma de México, México D.F., Mexico
(
Received 1 July 2014; accepted 5 September 2014)
A series of mononuclear copper(II) complexes was synthesized by reaction of different 2,6-disubsti-
tuted pyridines with elemental Cu in a CCl /DMSO solvent system. Physical properties were ana-
4
lyzed using IR, optical spectroscopy, mass spectrometry, and EPR. Single-crystal X-ray diffraction
for complexes 2a and 4a revealed that the molecular structure of 2a is composed of a six-coordinate
unit in which two ligands are linked to Cu(II) by oxygen and nitrogen donors. Conversely, in the
molecular structure of 4a, the Cu(II) has a slightly distorted trigonal bipyramidal arrangement, where
the coordination environment around the copper ion is five-coordinate. EPR spectra of Cu(II) com-
plexes were illustrated elaborately and some theoretical data were abstracted from EPR curves to
support the proposed structures.
Keywords: Synthesis; Pyridinediols; Copper(II) complexes; Mononuclear; X-ray diffraction
1. Introduction
Lying on the edge between the d-block metals and the main group elements, copper is one
of the most studied transition metals, owing to its electrical properties and commonness.
The structure and bond properties of copper(II) compounds are of continuous interest in
*
Corresponding author. Email: eligom@unam.mx
2014 Taylor & Francis
©

Copper(II) complex pyridinediols
207
inorganic chemistry [1, 2]. Recently, particular attention has been devoted to copper(II)
complexes of multifunctional ligands [3–10]. The ligand multifunctionality attracts much
attention owing to its chelating properties along with additional, not involved in the coordi-
nation, binding sites as potential linkers [11–14] and because of widespread applications in
technology and crystal engineering [15]. Investigation of these complexes has become an
interesting research field owing to relevance in bioinorganic and physical chemistry, in view
of biological [3, 16–24], thermal [25, 26], and luminescent properties [27], with potential
applications in materials. In addition, they have also been studied in the molecular magne-
tism area because they can act as building blocks for supramolecular architectures [28–31].
From the vast group of multidentate ligands, the so-called O,N,O-oxido pincer ligands
[
32–39] provide a scaffold to form Cu(II) complexes, which is interesting because the bind-
ing of this metal to O donors is likely to be weak; therefore, it is not surprising to find a
relatively small number of complexes of late transition metals derived from oxido pincer
ligands.
Copper complexes containing polypyridine ligands and their derivatives play a significant
role as pharmacological agents, since they exhibit numerous biological activities such as
antitumor [40], anticandida [41], antimicrobacterial [42], and antimicrobial activities
[
43, 44]. Pyridinediols are special cases of these metal receptors (figure 1). The combination
of two hydroxy groups and the pyridine ring nitrogen donor makes these systems suitable
for complexation of a variety of metals [45–52]. Moreover, the possibility of substituting
the protons on the methylene carbon allows interesting electronic and steric variations to be
introduced into ligands and their complexes. Such a study can give an insight into the influ-
ence of variable substitution to the geometry around Cu(II) or even to the packing of the
molecule.
On the other hand, direct synthesis employing metal powders with organic ligands
[
53, 54] is also of interest in view of the fascinating complexes of unusual stoichiometry
and structures that are formed in comparison with those obtained in conventional synthesis,
which usually involves a metal salt as a starting material [34, 35, 55, 56]. It has been
reported that copper metal is dissolved under extremely mild conditions by a solution of
dimethylsulfoxide (DMSO) containing CCl to yield dimethyl sulfide and CuCl (DMSO) ,
4
2
2
which reacts with the ligand [54]. This “one-step” synthesis of metal complexes starting
from elemental metals is an active research field, which has undergone rapid progress
[
57–62]. These facts prompted us to explore the direct synthesis of complexes derived from
pyridinediol and a zero-valent metal (Cu) as powder in a CCl /DMSO system, having in
4
mind the advantages that direct synthesis allows, i.e. easier and extremely simple reaction
conditions, some peculiar properties of the compounds obtained, and avoidance of impuri-
ties. Such advantages are particularly important with respect to this class of compounds, as
reported in the literature, involving long and/or tedious preparation of the reagents and
careful conditions [63–66].
R
R
N
N
R
R
R
R
R
R
OH
OH
OH HO
Figure 1. O,N,O-oxido pincer ligands.

2
08
G.M. Chans et al.
Herein, we report the synthesis of two mononuclear copper(II) complexes coordinated by
O,N,O-tridentate pyridyl alkoxy ligands, and their crystal structures by this straightforward
and expeditious method, with the aim to evaluate the influence of the substituents and the
chain length in the carbon α to the pyridyl ring in complex formation.
2
2
. Experimental
.1. Materials and methods
All reagents, solvents, and L1a were obtained from commercial suppliers and used without
purification. The reactions for ligand preparations were carried out under inert-gas
conditions.
2
2
.2. Preparation
.2.1. Synthesis of ligands. Synthesis of L1b,c: These ligands were synthesized according
to a modified literature procedure [46]. Thus, RLi 1 M (20.5 mM) was added dropwise to
dimethyl pyridine-2,6-dicarboxylate (1.0 g, 5.1 mM) in THF (50 mL), at −78 °C. After stir-
ring at reflux for 6 h, the mixture was filtered through Celite under reduced pressure. Upon
evaporation of the solvent and addition of water, the crude product was extracted with
CH Cl (30 mL). The organic layer was evaporated, dried over Na SO , and purified by
2
2
2
4
chromatography on silica gel using hexane/ethyl acetate (8 : 2) to give a white solid.
Ligands L3a,b were synthesized as described previously [67].
2
.2.2. Synthesis of complexes 2a–c and 4a,b. Metal powder (1 mM), the corresponding
ligand (1 or 2 mM), DMSO (0.5 mL), and CCl (3.5 mL) were placed in a flask and the
4
mixture was heated, at 65 °C with magnetic stirring until total dissolution of the metal was
observed (0.5−2 h). The reaction mixture was allowed to stand at room temperature, after
which a precipitate was formed. The precipitate was filtered off and dried, at room tempera-
ture. Suitable single crystals of 2a and 4a were obtained by allowing them to stand in a
hexane/methanol mixture.
2
.2.2.1. (L1a) CuCl (2a). Green solid, m.p. 168−169 °C (dec.), (C H Cl CuN O ,
2 2 14 18 2 2 4
−
1
M = 412.76 g M ) 0.186 g, 0.450 mM (75% yield). IR (ATR) ν 3010, 2814, 2748, 2699,
2
6
%
7
3
4
608, 1928, 1612, 1580, 1478, 1444, 1413, 1299, 1222, 1167, 1098, 1032, 957, 799, 695,
−
1
+
28, 530, 484 (Cu–O), 429, 376 (Cu–O), 315, 251 (Cu–N) cm . FAB MS m/z (rel. int.,
+
2 max
) 342 (21, [(L1a) Cu] ), 340 (40), 237 (12), 202 (52), 140 (100). UV−vis (λ , MeOH):
+
+
87 nm. HRMS (FAB ): m/z 340.0484 calcd for C H CuN O [(L1a) Cu] ; found
14 18 2 4 2
40.0487. Anal. Calcd for C H Cl CuF N O : C, 40.74; H, 4.40; N, 6.79. Found: C,
1
4
18
2
4 2 4
0.23; H, 4.74; N, 6.55.
2
.2.2.2. (L1b) CuCl (2b). Green solid, m.p. 185 °C (dec.), (C H Cl CuN O ,
2
2
22 34
2
2 4
−
1
M = 524.97 g M ) 0.180 g, 0.343 mM (65% yield). IR (ATR) ν 3295, 3451, 3058, 2987,
740, 2515, 1600, 1577, 1458, 1374, 1292, 1244, 1206, 1182, 1144, 1099, 1032, 954, 900,
2

Copper(II) complex pyridinediols
209
8
45, 825, 768, 718, 695, 590, 475 (Cu–O), 449 (Cu–N), 398, 373 (Cu–O), 338, 315, 261
−
1
+
+
(
(
Cu–N) cm . FAB MS m/z (rel. int., %) 454 (20, (L1b) Cu] ), 452 (39), 258 (26), 242
12), 196 (100), 155 (22), 79 (26), 77 (10). UV−vis (λmax, MeOH): 782 nm. HRMS calcd
2
+
for C H CuN O [(L1b) Cu–H] 452.1736; found 452.1747. Anal. Calcd for
2
2
34
2
4
2
C H Cl CuN O ·2C H OS: C, 45.84; H, 6.81; N, 4.11. Found: C, 46.06; H, 6.47;
2
2
34
2
2
4
2 6
N, 4.76.
2
.2.2.3. (L1c) CuCl (2c). Green solid, m.p. 190 °C (dec.), (C H Cl CuN O ,
2
2
62 50
2
2 4
−
1
M = 1021.52 g M ) 0.067 g, 0.066 mM (29% yield). IR (ATR) ν 3289, 3083, 3054, 3024,
2
1
918, 2801, 1729, 1652, 1569, 1489, 1439, 1405, 1364, 1316, 1256, 1206, 1169, 1092,
049, 1023, 1002, 946, 906, 849, 832, 809, 769, 755, 734, 698, 634, 593, 496 (Cu–O), 383
−
1
+
+
(Cu–O), 333, 270 (Cu–N) cm . FAB MS m/z (rel. int., %) 949 (2, [(L1c) Cu] ) 541 (39),
2
5
7
5
06 (9), 445 (100), 408 (100), 348 (34), 244 (25), 105 (53). UV−vis (λ , MeOH):
max
+
+
98 nm. HRMS (FAB ): m/z 506.1181 calcd for C H CuNO [(L1c)Cu] ; found
3
1
17
2
06.1182.
2
.2.2.4. L3aCuCl
(4a). Green
solid,
m.p.
179−183 °C,
(C H Cl CuNO ,
33 29 2 2
2
1
−
M = 606.04 g M ) 0.052 g, 0.086 mM (30% yield). IR (ATR) ν 3084, 3058, 3027, 2964,
2
7
911, 1603, 1572, 1493, 1463, 1444, 1344, 1259, 1088, 1032, 1009, 945, 872, 799, 776,
−
1
+
39, 605, 551, 510 (Cu–O), 444 (Cu–N), 383 (Cu–O), 323, 285, 271 (Cu–N) cm . FAB
+
+
MS m/z (rel. int., %) 569 (32, [L3aCu–Cl] ), 534 (24, [L3aCu–2 Cl] ), 472 (100), 436
(
8
27), 376 (9), 531 (51), 271 (16), 183 (13), 105 (43), 77 (38). UV−vis (λ , MeOH)
16 nm. HRMS (FAB ): m/z 569.1183; calcd for C H Cl CuNO [L3aCu–Cl] ), found
33 29 2 2
max
+
+
5
69.1194. Anal. Calcd for C H Cl CuF NO ·2C H OS: C, 53.11; H, 5.42; N, 1.84.
3
3
29
2
4
2
2 6
Found: C, 58.30; H, 5.25; N, 1.96.
2
.2.2.5. L3bCuCl (4b). Green solid, m.p. 150−155 °C, (C H Cl CuF NO ,
2
1
33 25
2
4
2
−
−1
M = 678.00 g M ) 0.460 g, 0.678 m M (48% yield). IR (ATR) ν 3256, 3060, 2994,
2
8
3
912, 2685, 1639, 1602, 1505, 1463, 1406, 1312, 1224, 1162, 1113, 1060, 1015, 996, 942,
88, 836, 794, 744, 662, 630, 588, 562, 548, 510 (Cu–O), 432 (Cu–N), 409, 342 (Cu–O),
−
1
+
+
11, 294, 263 (Cu–N) cm . FAB MS m/z (rel. int., %) 606 (2, [L3bCu–2 Cl] ), 544
(
100), 430 (5), 219 (5). UV−vis (λmax, MeOH): 820 nm. Anal. Calcd for C H Cl CuF -
33 25 2 4
NO ·1.5C H OS·2CH O: C, 53.11; H, 4.93; N, 1.63. Found: C, 52.66; H, 4.70; N, 1.65.
2
2
6
4
2
.3. Physical measurements
All compounds were characterized by IR spectra, recorded on a Bruker Tensor 27 spectro-
−
1
photometer by ATR technique, and all data are expressed in wavenumbers (cm ). UV–vis
−1
spectra were recorded in MeOH solution (0.8 g mL ) in a 1-cm path length quartz cell on
a Shimadzu U160 spectrophotometer. Melting points were obtained on a Melt-Temp II
1
13
apparatus, and are uncorrected. The H and C NMR spectra were recorded on a Bruker
1
13
Avance III, at 300 MHz ( H NMR) and 75 MHz ( C NMR) in chloroform-d. The MS-EI
spectra were obtained by JEOL JMS-AX505 HA using 70 eV as ionization energy, and for
MS-FAB, by JEOL JMS-SX102A using nitrobenzyl alcohol as matrix. EPR measurements
were made in a quartz tube at 77 K with a Jeol JES-TE300 spectrometer operating at
X-Band fashion at 100 kHz modulation frequency and a cylindrical cavity in mode TE011
.
The external calibration of the magnetic field was made with a precision Gauss meter Jeol

2
10
G.M. Chans et al.
ES-FC5, and microwave frequency with a frequency counter 5350B HP. The spectrometer
settings for all the spectra were as follows: center field, 300 mT; microwave power, 1 mW;
microwave frequency, 9.10 GHz; sweep width, ±75.0 mT; modulation width, 0.125 mT;
time constant, 0.1 s; amplitude, 160; sweep time, 120 s; and accumulation, 1 scan. Spectral
acquisition, manipulations, and simulation were performed using the program ES-IPRIT/
TE, v1.916. The EPR spectrum was recorded as a first derivate, and the main parameter
such as g-factor values were calculated according to Weil, Bolton, and Wertz [68]. All sam-
ples were measured in a DMSO/MeOH solution.
2
.4. Crystal structure determination
Single crystals of 2a and 4a suitable for X-ray diffraction studies were grown from its solu-
tion in a mixture of hexane:methanol or heptane:methanol (1 : 1 v/v). The crystals of each
compound were mounted on a glass fiber at room temperature and then placed on a Bruker
Smart Apex CCD diffractometer equipped with Mo Kα radiation; decay was negligible in
both cases. Details of crystallographic data collected on 2a and 4a are provided in table 1.
Systematic absences and intensity statistics were used in space group determination. The
structure was solved using direct methods [69].
Anisotropic structure refinements were achieved using full matrix least-squares technique
on all non-hydrogen atoms. All hydrogens were placed in idealized positions, based on
hybridization, with isotropic thermal parameters fixed at 1.2 times the value of the attached
atom. Structure solutions and refinements were performed using SHELXS-97 [70].
The interstitial solvent water molecule located in the channels of 2a is disordered over a
crystallographic twofold axis and was refined isotropically with partial fixed occupancy of
0
.125. No hydrogens were located for this moiety.
For 4a, both DMSO molecules display orientational disorder. In the first DMSO
(O3 S1 C35 C36), the sulfur and the methyl groups show orientational disorder which was
resolved over three orientations on approximately the same site and refined isotropically
with 0.487, 0.295, and 0.217 occupancies, while the second DMSO (O4 S2 C37 C38) is
disordered over two – almost equally populated 0.545(4)/0.455(4) – orientations and were
refined anisotropically. The geometries of the PARTs were kept similar using the SAME
restraint and displacement parameters in these disordered pieces were restrained using both
SIMU and DELU.
2+
R
R
N
Cu
N
R
R
HO
OH
R
R
Cu
2Cl-
2
N
R
R
CCl4, DMSO
HO
OH
OH
OH
Δ
R
R
R
R
L1a R = H
L1b R = Me
L1c R = Ph
2a-c
Scheme 1. Syntheses of 2a–c.

Copper(II) complex pyridinediols
211
N
R
H
R
Cu
R
N
R
R
R
R
R
CCl , DMSO
O
4
O
Cu
OH HO
Δ
H
Cl
Cl
L3a R = Ph
L3b R = 4-F-Ph
4a,b
Scheme 2. Syntheses of 4a,b.
3. Results and discussion
Pyridinediols L1b–c were synthesized by treating dimethyl 2,6-pyridinedicarboxylate with
the appropriate Grignard reagent or alkyl lithium as described previously [46], followed by
the reaction with elemental copper in CCl /DMSO [53] to afford the corresponding Cu com-
4
plexes (scheme 1). Compounds 2a,b have been described in the literature [34, 35, 55, 56].
Nevertheless, they were obtained by a different synthetic route using Cu(NO ) ·3H O and
3
2
2
CuCl ·2H O with 2 equivalents of the corresponding ligand.
2
2
Pyridinediols L3a,b were synthesized from lutidine and BuLi according to the literature
procedure [67]. Ligands were obtained in good yields and characterized by spectroscopic
methods. Complexes 4a,b were obtained by reaction of L3a,b with elemental copper in
equimolar quantities (scheme 2).
3
.1. Infrared spectral study
In table 2, Cu–N and Cu–O vibrations in the far-infrared area are shown. Based on data
−1
from earlier research [71, 72], we assign the bands at about 445 and 260 cm to the Cu–N
−1
vibration and the bands at approximately 480 and 375 cm to the Cu–O vibrations. The
in-plane deformation of the uncoordinated pyridine ring in the ligands is observed at
−
1
6
50 cm [73]. Nevertheless, the N-coordination of the pyridine ring is confirmed in all
−1
complexes by the position of the absorption bands at 700 cm . Additionally, bands at 429,
98, and 409 cm corresponding to out-of-plane deformation of the pyridine ring were
−
1
3
assigned to 2a, 2b, and 4b, respectively.
3
.2. Electronic absorption spectra
The UV−vis absorption spectra of L1a–c, L3a,b, and complexes 2a–c, 4a, and 4b were
recorded in MeOH. In the UV region, spectra of the five ligands display two bands from
2
[
7
00 to 240 nm (table 3). These bands are typical for pyridine ligands (π−π* transitions)
74]. The visible spectra of the complexes reveal the presence of a broad absorption from
80 to 820 nm, which could be due to the d–d transition band of Cu(II) [75] with a low
−
1
−1
−1
−1
molar extinction coefficient value (εmax 33–36 M cm for 2a–c and 97–113 M cm
for 4a, b). The comparison of band shifts observed from 200 to 240 nm for complexes with
their ligands indicates that the red shift is negligible.

2
12
G.M. Chans et al.

Copper(II) complex pyridinediols
213
−
1
Table 2. Selected FT-IR spectral data (cm ) for 2a–c and 4a,b.
a
Infrared data
Compound
ν(Cu−N)
ν(Cu−O)
Py ring
L1a
L1b
L1c
L3a
L3b
658s
623br
655 m
654 m
664 m
2
2
2
4
4
a
b
c
a
b
251s
449s, 261 m
270w
444w, 271 m
432w, 263 m
484w, 376w
475br, 373s
496 m, 383 m
510 m, 383br
510 m, 342w
695br, 429 m
695s, 398 m
698s
698s
695sh, 409w
a
s, strong; m, medium; w, weak; sh, shoulder; br, broad.
−
1
−1
−1
Table 3. Characteristic bands (λmax, in nm) and molar extinction coefficient (ε, M cm
in UV–vis spectra (recorded in MeOH solution) for 2a–c and 4a,b.
)
a
−1
Compound
λmax (nm)
Molar extinction coefficient (ε, M cm
)
3
L1a
L1b
L1c
208, 265
207, 262
208, 280
6.6 × 10
3
9.3 × 10
4
5.0 × 10
2
2
2
L3a
L3b
a
b
c
207, 264.0, 787br
207, 262.0, 782br
207, 266, 798br
209, 266
36
36
33
4.4 × 10
4.1 × 10
4
4
208, 265
4
4
a
b
207, 269, 816br
207, 266, 820br
113
97
a
br, broad.
3.3. Mass spectra
Mass spectra of 2a–b provide evidence for molecular formulas, owing to the loss of both
−
1
+
chlorides: 2a, C H CuN O , M = 342 g M ; and 2b, C H CuN O [(L1b) Cu–H] ,
1
4
18
2
4
22 34
2
4
2
M = 452; and whereas in 2c, the loss of one ligand is also observed (C H CuNO , [(L1c)
3
1
17
2
+
−1
Cu] , M = 506 g M ). On the other hand, the mass spectrum of 4a (C H Cl CuNO ,
M = 606 g M ) and 4b (C H Cl CuF NO , M = 678 g M ) did not show the molecular
3
3
29
2
2
−
1
−1
3
3
25
2
4
2
ion. Nevertheless, the loss of one (m/z 534) and two (m/z 569) chloride radicals is observed
for 4a and the loss of two chloride radicals (m/z 606) for 4b, confirming their formula
weights.
3
.4. EPR spectroscopy
Elongated and compressed Cu(II) octahedral complexes adopt different electronic ground
states, since their unpaired electrons occupy the d
2
2
or dz atomic orbitals, respectively.
2
x ꢀy
These orbitals yield very different EPR spectra, whose lineshapes give evidence about the
position of the singly occupied electron in either d-orbital. For a d
2
2
complex, an axial
x ꢀy
symmetry pattern in the EPR spectrum is expected (gzz >> gyy ≈ gxx > 2.0023), whereas a
2
dz configuration yields an inverse axial symmetry (gyy ≈ gxx >> gzz ≈ 2.0023) [76, 77].

2
14
G.M. Chans et al.
Figure 2. EPR spectrum (X band) of frozen solution of 2a in DMSO at 77 K, showing an inverse axial
symmetry.
Figure 3. EPR spectrum of 4a in frozen solution of DMSO.
a
Table 4. EPR data for 2ª–c and 4a,b.
b
Compound
g
zz
g
yy
g
xx
g
iso/ave
A
zz
A
yy
A
xx
Spectral symmetry Molecular symmetry
2
2
2
4
4
a
b
c
a
b
2.01 2.21 2.21 2.14
2.01 2.22 2.22 2.15
2.34 2.07 2.07 2.16
2.27 2.13 2.01 2.13
2.30 2.12 2.01 2.14
154.3 8.0
153.8 7.8
147.5 8.2
8.0
7.8
8.2
8.8
7.8
Axial compressed
Axial compressed
Axial elongated
Rhombic
OC
OC
OE
TBP
TBP
108.8
110.3
–
–
Rhombic
a
−1
4
Constants (A) are given in cm and multiplied by a factor 10 .
b
Symmetry assignment based on EPR spectroscopy, OC = octahedral compressed, OE = octahedral elongated, and TBP = trigonal
bipyramidal.
The DMSO frozen glass (77 K) EPR spectra of the Cu(II) coordination compounds stud-
ied herein are shown in figures 2 and 3. EPR spectra display a signal in the center field,
obtained for a system with S = 1/2, wherein the hyperfine lines correspond to copper

Copper(II) complex pyridinediols
215
unpaired electron coupling with metal ion nuclei (ICu = 3/2), characteristic of monomeric
copper(II) complexes; no nitrogen hyperfine structure is observed, possibly because of the
line broadening effects [78]. In particular, the EPR spectra obtained for 2a and 2b display
an inverse axial symmetry with gyy ≈ gxx >> gzz ≈ 2.0023 (figure 2) and show hyperfine
coupling. The magnetic parameters obtained by simulation are listed in table 4, in agree-
ment with a dz ground state related to a compressed octahedral symmetry. Only a few
2
structural studies with this geometry have been found in the literature [76, 79]. This struc-
tural feature had been also confirmed by the single-crystal structure analysis described
below. On the contrary, 2c, which has bulkier substituents (R = Ph), imposes an octahedral
distortion to the copper(II) complex, showing an ill-resolved axial spectra. The g-values
(
gzz > gyy ≈ gxx > 2.0023) confirm that the unpaired electron on copper(II) corresponds to
−1
that geometry [80]. The gzz and Azz parameters (gzz = 2.34, Azz = 147.5 cm ) show a slight
decrease of gzz and an increase of A with respect to the elongated octahedral geometry
zz
−
1
(
gzz = 2.42, Azz = 124.0 cm ) [77, 81], indicating that the octahedron is slightly distorted
towards square planar geometry. This behavior is due to variations in the orbital energy dur-
ing the elongation.
The DMSO/MeOH frozen EPR spectra of 4a and 4b exhibit rhombic character as reflected
by the g parameters, since gzz > gyy > g , and show hyperfine coupling (I = 3/2) (figure 3).
xx
Cu
The spectral patterns suggest a huge distortion of the copper site, as a pseudo-trigonal bipyra-
midal geometry [80, 82, 83]. This information is consistent with the crystal structure of 4a.
3
.5. Structural determination
Whereas Cu(II) environments have been well studied in unidentate or three bidentate
ligands [18, 84], studies of symmetrical systems involving two tridentate ligands are rela-
tively few [55, 56, 85]. The molecular structures of 2a and 4a were established by single-
crystal X-ray diffraction analysis. ORTEP views of the complexes are shown in figure 4. In
table 1 are gathered structural data; selected bond lengths and angles are given in table 5.
Compound 2a was crystallized in the orthorhombic space group Cmca from hexane/
methanol and consists of a crystal structure with a tridentate O,N,O-donor ligand and cop-
per(II), in a discrete monomeric species. 2a is isostructural and polymorphous to those
structures obtained with CuCl and Cu(NO ) [34, 35, 55]. Complex 2a possesses a dis-
2
3 2
torted compressed octahedral geometry, where Cu(II) is six-coordinate, surrounded by two
nitrogens of the pyridyl groups and four oxygens of the alcohol arms. The Cu–N bond
Figure 4. ORTEP view of 2a and 4a. Thermal ellipsoids at 30% probability.

2
16
G.M. Chans et al.
Table 5. Selected bond lengths (Å) and angles (°) for 2a and 4a.
2a
4a
Bond lengths
Cu(1)‒O(1)
Cu(1)‒O(2)
Cu(1)‒N(1)
Bond lengths
Cu(1)‒O(1)
Cu(1)‒O(2)
Cu(1)‒N(3)
Cu(1)‒Cl(1)
Cu(1)‒Cl(2)
2.178(3)
2.151(3)
1.935(3)
2.167(3)
2.144(3)
1.998(3)
2.261(1)
2.215(1)
Bond angles
Bond angles
O(2)‒Cu(1)‒O(1)
O(2)‒Cu(1)‒N(1)
O(2)‒Cu(1)‒O(2A)
O(2)‒Cu(1)‒O(1A)
O(2)‒Cu(1)‒N(1A)
O(1)‒Cu(1)‒N(1)
O(1)‒Cu(1)‒O(1A)
O(1)‒Cu(1)‒N(1A)
N(1)‒Cu(1)‒O(2A)
N(1)‒Cu(1)‒N(1A)
156.5(1)
78.3(1)
94.2(2)
92.7(2)
101.3(1)
78.3(1)
89.9(2)
102.2(1)
101.3(1)
179.4(2)
O(2)‒Cu(1)‒O(1)
O(2)‒Cu(1)‒N(1)
O(2)‒Cu(1)‒Cl(1)
O(2)‒Cu(1)‒Cl(2)
O(1)‒Cu(1)‒N(1)
O(1)‒Cu(1)‒Cl(1)
O(1)‒Cu(1)‒Cl(2)
N(1)‒Cu(1)‒Cl(1)
N(1)‒Cu(1)‒Cl(2)
Cl(2)‒Cu(1)‒Cl(1)
93.7(1)
86.0(1)
124.4(1)
91.7(1)
81.0(1)
140.7(1)
90.8(1)
91.5(1)
171.3(1)
96.7(1)
lengths in the apical positions (Cu(1)–N(1) = 1.935(3) Å) are shorter than the Cu–O bond
lengths in the equatorial positions (Cu(1)–O(1) = 2.151(3), Cu(1)–O(2) = 2.178(3) Å),
clearly indicating a Jahn–Teller compression [35]. A comparison between Cu(1)–O(2) bond
lengths of 2a with those described in the literature indicates that it is slightly shorter
[
(
34, 35, 55]. However, the remaining values of bond lengths and angles around the copper
II) in the complex do not show significant differences.
The OH groups are H-bonded to chlorides with H⋯Cl distances between 2.27 and
2
.15 Å, bridging complex cations, showing a 2-D layer parallel to the (0 1 0) plane in the
crystal packing, which is stabilized by weak hydrogen bonds (O1–H1⋯Cl1), as shown in
the Supplemental Data. These H-bonds link neighboring Cu(II) monomers along the b-axis
and their existence rationalizes the high thermodynamic stability of 2a. The two pyridine
rings are oriented almost perpendicular to each other, since the mean plane formed by pyri-
dine ring atoms N(1)–C(2)–C(3)–C(4)–C(5) is 75.2 (0.1)°.
Compound 4a crystallizes in the monoclinic system. The structure was solved in the
P2 /c space group. The coordination environment around copper is five-coordinate, consist-
1
ing of one pyridyl nitrogen, two hydroxyl oxygens, and two chlorides. The molecular struc-
ture of 4a has a distorted trigonal bipyramidal geometry, in which equatorial positions are
occupied by Cl(1), O(1), and O(2) and the axial positions by the pyridyl N(1) and one
chloride Cl(2). These results show opposite behavior to the preference of electronegative
groups for the axial position on the TBP on similar silicon complexes [86–88].
Weak H-bonding interactions between the solvent and the complexes, or between the sol-
vent molecules were determined. The shortest contacts are observed between the O(1)–H(1)
proton of 4a with the O(5) of water with an H⋯O distance of 1.95(4) Å [O⋯O 2.66(1) Å]
and between the hydroxyl protons O(2)–H(2) of the complex with the DMSO oxygen O(3)
(H⋯O 1.87(4) Å [O⋯O 2.63(1) Å]) (see the crystal packing in the Supplemental Data) [89].
A marked difference can be observed in the orientation of the chloride co-ligands. Thus,
two chloride ligands complete a trigonal bipyramidal arrangement around copper (figure 4).
The polyhedron is distorted, with the Cl–M–Cl angle of 96.7°. The apical positions
(N–M–Cl) are separated by an angle of 171.3°.

Copper(II) complex pyridinediols
217
The angular structural parameter τ (index of trigonality) has been used for distinguishing
between a trigonal bipyramidal (TBP) and square pyramidal (SP) geometry in five-coordinate
ꢁ
complexes, s ¼ ðb ꢀ aÞ=60 where α and β are the two largest angles [21, 90]. τ = 0 for an
ideal square pyramid (α and β = 180°) and τ = 1 for an ideal trigonal bipyramid (α = 120°
and β = 180°). Using this criterion, the τ value is 0.51, indicating that the complex is
distorted towards a trigonal bipyramidal geometry.
4
. Conclusion
The present study shows that the reaction of 2,6-bis-disubstituted pyridine derivatives with
elemental copper produces monomeric compounds where one or two ligands are coordi-
nated to copper. The magnetic behavior is fully in line with the assumption that the species
2
2
a–c exhibit a distorted octahedral geometry. Complexes 2a,b show gyy ≈ gxx >> gzz
.0023, consistent with a compressed symmetry, whereas 2c displays gzz > gyy
≈
≈
gxx > 2.0023, typical for an elongated symmetry. In the case of 4a–b, g parameters suggest
a pseudo-trigonal bipyramidal geometry. These geometries have been proved by single-crys-
tal X-ray analysis.
The preference for formation of six-coordinate 2a–c could be associated to the formation
of two five-membered rings from the pyridine ligands that stabilize a cationic complex.
However, in the case of 4a,b, they are formed through the complexation of the pyridine
ligands with the elemental copper. The formation of these five-coordinate complexes seems
to be highly favored owing to the six-membered rings, which are probably stabilized due to
increasing steric factor on the pyridine carbons.
Supplementary material
CCDC-1001885 (2a) and CCDC-1001886 (4a) contain the supplemental crystallographic
data for this paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Center via www.ccdc.ac.uk/data_request/cif.
Acknowledgements
We gratefully acknowledge financial support from Instituto de Química, UNAM. One of
the authors, G.M.C., is thankful to The World Academy of Sciences (TWAS) and the
Consejo Nacional de Ciencia y Tecnología (CONACYT) for awarding a post-doctoral
research fellowship. We also appreciate the technical assistance of Rocío Patiño, María de
la Paz Orta Pérez, Alejandra Núñez Pineda, Luis Velasco, and Javier Pérez.
Supplemental data
Supplemental data for this article can be accessed here [http://dx.doi.org/10.1080/00958972.2014.982111].

2
18
G.M. Chans et al.
References
[
1] B.P. Murphy. Coord. Chem. Rev., 124, 63 (1993).
[
2] B.J. Hathaway, In Comprehensive Coordination Chemistry: The Synthesis, Reactions, Properties, and
Applications of Coordination Compounds, G. Wilkinson, R.D. Gillard, J.A. McCleverty (Eds), Pergamon
Press, Oxford (1987).
[
[
3] L.-D. Wang, K. Zheng, Y.-T. Li, Z.-Y. Wu, C.-W. Yan. J. Mol. Struct., 1037, 15 (2013).
4] G. Psomas, A. Tarushi, E.K. Efthimiadou, Y. Sanakis, C.P. Raptopoulou, N. Katsaros. J. Inorg. Biochem.,
100, 1764 (2006).
[
[
[
[
[
5] Z. Zude, Z. Tao, X. Xingyou, C. Hua, L. Qingliang. J. Coord. Chem., 48, 265 (1999).
6] D.J. Hutchinson, L.R. Hanton, S.C. Moratti. Inorg. Chem., 49, 5923 (2010).
7] F. Yu. Cryst. Struct. Commun., 67, m331 (2011).
8] J. Ran, X. Li, Q. Zhao, Z. Qu, H. Li, Y. Shi, G. Chen. Inorg. Chem. Commun., 13, 526 (2010).
9] L.-L. Zheng, W.-X. Zhang, L.-J. Qin, J.-D. Leng, J.-X. Lu, M.-L. Tong. Inorg. Chem., 46, 9548 (2007).
[
[
[
[
[
[
10] A.W. Hanson. Cryst. Struct. Commun., 42, 501 (1986).
11] D.W. Min, S.S. Yoon, D.Y. Jung, C.Y. Lee, Y. Kim, W.S. Han, S.W. Lee. Inorg. Chim. Acta, 324, 293 (2001).
12] B.O. Patrick, C.L. Stevens, A. Storr, R.C. Thompson. Polyhedron, 22, 3025 (2003).
13] S. Noro, S. Kitagawa, T. Wada. Inorg. Chim. Acta, 358, 423 (2005).
14] L. Xue, F. Luo, Y.-X. Che, J.-M. Zheng. J. Mol. Struct., 832, 132 (2007).
15] S. Fukuzumi, H. Kotani, H.R. Lucas, K. Doi, T. Suenobu, R.L. Peterson, K.D. Karlin. J. Am. Chem. Soc.,
132, 6874 (2010).
[
[
[
16] J.J. Martínez Medina, M.S. Islas, L.L. López Tévez, E.G. Ferrer, N.B. Okulik, P.A.M. Williams. J. Mol.
Struct., 1058, 298 (2014).
17] P.S. Lopes, D.A. Paixão, F.C.S. de Paula, A.M.D.C. Ferreira, J. Ellena, S. Guilardi, E.C. Pereira-Maia, W.
Guerra. J. Mol. Struct., 1034, 84 (2013).
18] B. Kupcewicz, M. Ciolkowski, B.T. Karwowski, M. Rozalski, U. Krajewska, I.-P. Lorenz, P. Mayer, E.
Budzisz. J. Mol. Struct., 1052, 32 (2013).
[
[
[
[
[
[
[
[
[
[
[
[
[
[
19] S. Sarkar, T. Mukherjee, S. Sen, E. Zangrando, P. Chattopadhyay. J. Mol. Struct., 980, 117 (2010).
20] S.O. Bahaffi, A.A. Abdel Aziz, M.M. El-Naggar. J. Mol. Struct., 1020, 188 (2012).
21] M. Choudhary, R.N. Patel, S.P. Rawat. J. Mol. Struct., 1060, 197 (2014).
22] A.L. Abuhijleh. J. Mol. Struct., 980, 201 (2010).
23] I. Ali, W.A. Wani, K. Saleem, M.-F. Hsieh. J. Coord. Chem., 67, 2110 (2014).
24] M. Kose, M. Digrak, I. Gonul, V. McKee. J. Coord. Chem., 67, 1746 (2014).
25] A. Trzesowska-Kruszynska. J. Mol. Struct., 1017, 72 (2012).
26] A. Beheshti, V. Nobakht, L. Carlucci, D.M. Proserpio, C. Abrahams. J. Mol. Struct., 1037, 236 (2013).
27] S. Mandal, R. Modak, S. Goswami. J. Mol. Struct., 1037, 352 (2013).
28] M. Outirite, B. Mernari, F. Bentiss, F. Capet, M. Lagrenée. J. Mol. Struct., 989, 60 (2011).
29] I.F. Santos, G.P. Guedes, L.A. Mercante, A.M.R. Bernardino, M.G.F. Vaz. J. Mol. Struct., 1011, 99 (2012).
30] B.L. Solomon, C.P. Landee, M.M. Turnbull, J.L. Wikaira. J. Coord. Chem., (2014). doi: .
31] Y. Wu, Y. Mu, L. Bai, S. Guo, J. Zhao, D.-S. Li. J. Coord. Chem., 67, 1629 (2014).
32] A.M.S. Silva, L.M.P.M. Almeida, J.S. Cavaleiro, C. Foces-Foces, A.L. Llamas-Saiz, C. Fontenas, N.
Jagerovic, J. Elguero. Tetrahedron, 53, 11645 (1997).
[
33] W.-J. Gu, B.-X. Wang. Acta Crystallogr. Sect. E, 65, o233 (2009).
[
34] G.C. Vlahopoulou, D.I. Alexandropoulos, C.P. Raptopoulou, S.P. Perlepes, A. Escuer, T.C. Stamatatos.
Polyhedron, 28, 3235 (2009).
[
[
[
[
[
[
[
[
35] S. Winter, W. Seichter, E. Weber. Z. Anorg. Allg. Chem., 630, 434 (2004).
36] R. Vafazadeh, R. Esteghamat-Panah, A.C. Willis, A.F. Hill. Polyhedron, 48, 51 (2012).
37] R.N. Patel, V.L.N. Gundla, D.K. Patel. Polyhedron, 27, 1054 (2008).
38] R.N. Patel, N. Singh, V.L.N. Gundla. Polyhedron, 25, 3312 (2006).
39] A. Klein, K. Butsch, J. Neudörfl. Inorg. Chim. Acta, 363, 3282 (2010).
40] J.D. Ranford, P.J. Sadler, D.A. Tocher. J. Chem. Soc., Dalton Trans., 3393 (1993).
41] M. Geraghty, V. Sheridan, M. McCann, M. Devereux, V. McKee. Polyhedron, 18, 2931 (1999).
42] D.K. Saha, U. Sandbhor, K. Shirisha, S. Padhye, D. Deobagkar, C.E. Anson, A.K. Powell. Bioorg. Med.
Chem. Lett., 14, 3027 (2004).
[43] M.A. Zoroddu, S. Zanetti, R. Pogni, R. Basosi. J. Inorg. Biochem., 63, 291 (1996).
[44] R.N. Patel, N. Singh, K.K. Shukla, V.L.N. Gundla, U.K. Chauhan. Spectrochim. Acta, Part A, 63, 21 (2006).
[45] E. Gómez, Z. Hernández, C. Alvarez-Toledano, R.A. Toscano, V. Santes, P. Sharma. J. Organomet. Chem.,
648, 280 (2002).
[
[
[
[
46] E. Gómez, V. Santes, V. de la Luz, N. Farfán. J. Organomet. Chem., 622, 54 (2001).
47] B. Kaptein, G. Barf, R.M. Kellogg, F. Van Bolhuis. J. Org. Chem., 55, 1890 (1990).
48] H. Mack, M.S. Eisen. J. Chem. Soc., Dalton Trans., 917 (1998).
49] J.J.H. Edema, R. Libbers, A.M. Ridder, R.M. Kellogg, F. van Bolhuis, H. Kooijman, A.L. Spek. J. Chem.
Soc., Chem. Commun., 625 (1993).

Copper(II) complex pyridinediols
50] J.M. Berg, R.H. Holm. J. Am. Chem. Soc., 106, 3035 (1984).
51] T.K. Prakasha, A. Chandrasekaran, R.O. Day, R.R. Holmes. Inorg. Chem., 35, 4342 (1996).
52] J.M. Berg, R.H. Holm. J. Am. Chem. Soc., 107, 917 (1985).
219
[
[
[
[
53] R. Gutiérrez, J. Vázquez, R.A. Vázquez, Y. Reyes, R.A. Toscano, M. Martinez, C. Álvarez. J. Coord. Chem.,
4, 313 (2001).
5
[
54] Y. Tezuka, M. Miya, A. Hashimoto, K. Imai. J. Chem. Soc., Chem. Commun., 1642 (1987).
[
55] A. Klein, K. Butsch, S. Elmas, C. Biewer, D. Heift, S. Nitsche, I. Schlipf, H. Bertagnolli. Polyhedron, 31,
649 (2012).
[
[
[
[
[
[
[
[
[
[
[
[
[
56] A. Klein, S. Elmas, K. Butsch. Eur. J. Inorg. Chem., 2009, 2271 (2009).
57] D.G. Alexander, B.I. Kharisov, G. Gojon-Zorrilla, D.A. Garnovskii. Russ. Chem. Rev., 64, 201 (1995).
58] S.R. Petrusenko, V.N. Kokozay, O. Yu Vassilyeva, B.W. Skelton. J. Chem. Soc., Dalton Trans., 1793 (1997).
59] V.N. Kokozay, A.V. Sienkiewicz. Polyhedron, 14, 1547 (1995).
60] V.V. Skopenko, V.N. Kokozay, V.R. Polyakov, A.V. Sienkiewicz. Polyhedron, 13, 15 (1994).
61] V. Kokozay, A. Sienkiewicz. Polyhedron, 12, 2421 (1993).
62] G. Nifontova, I. Lavrentiev. Transition Met. Chem., 18, 27 (1993).
63] E. Boschmann, L.M. Weinstock, M. Carmack. Inorg. Chem., 13, 1297 (1974).
64] J.T. Wrobleski, G.J. Long. Inorg. Chim. Acta, 30, 221 (1978).
65] S.F. Mason, R.D. Peacock. J. Chem. Soc., Dalton Trans., 226 (1973).
66] S.N. Choi, R.D. Bereman, J.R. Wasson. J. Inorg. Nucl. Chem., 37, 2087 (1975).
67] B. Koning, J. Buter, R. Hulst, R. Stroetinga, R.M. Kellogg. Eur. J. Org. Chem., 2000, 2735 (2000).
68] J.A. Weil, J.R. Bolton, J.E.Wertz, Electron Spin Resonance. Elementary Theory Practical Applications,
p. 568, Wiley, New York, NY (1994).
[
69] A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M.C. Burla, G. Polidori, M. Camalli. J. Appl.
Crystallogr., 27, 435 (1994).
[
[
[
[
70] G. Sheldrick. Acta Crystallogr., Sect. A, 64, 112 (2008).
71] G.A. Van Albada, M.T. Lakin, N. Veldman, A.L. Spek, J. Reedijk. Inorg. Chem., 34, 4910 (1995).
72] G.A. van Albada, W.J.J. Smeets, A.L. Spek, J. Reedijk. Inorg. Chim. Acta, 260, 151 (1997).
73] K. Nakamoto. In Part B Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th Edn,
Wiley, New York, NY (1997).
[74] K.R. Kyle, C.K. Ryu, P.C. Ford, J.A. DiBenedetto. J. Am. Chem. Soc., 113, 2954 (1991).
[75] Z. Wei, X. Xie, J. Zhao, L. Huang, X. Liu. Inorg. Chim. Acta, 387, 277 (2012).
[76] J.M. Holland, X. Liu, J.P. Zhao, F.E. Mabbs, C.A. Kilner, M. Thornton-Pett, M.A. Halcrow. J. Chem. Soc.,
Dalton Trans., 3316 (2000).
[
77] S.K. Hoffmann, J. Goslar, S. Lijewski, K. Basiński, A. Gąsowska, L. Łomozik. J. Inorg. Biochem., 111, 18
(
2012).
[
[
[
[
[
[
[
[
[
[
[
[
[
78] Y. Nonaka, T. Tokii, S. Kida. Bull. Chem. Soc. Jpn., 47, 312 (1974).
79] B. Kozlevčar, A. Golobič, P. Strauch. Polyhedron, 25, 2824 (2006).
80] D. Kroczewska, K. Bogusz, B. Kurzak, J. Jezierska. Polyhedron, 21, 295 (2002).
81] G. Tabbì, A. Giuffrida, R.P. Bonomo. J. Inorg. Biochem., 128, 137 (2013).
82] B. Kurzak, A. Kamecka, K. Bogusz, J. Jezierska. Polyhedron, 26, 4345 (2007).
83] D. Kroczewska, B. Kurzak, J. Jezierska. Polyhedron, 25, 678 (2006).
84] M. Sarma, B. Mondal. Dalton Trans., 41, 2927 (2012).
85] J. Pinart, C. Petitfaux, J. Faucherre. Bull. Soc. Chim. Fr., 4534 (1972).
86] A. Chandrasekaran, R.O. Day, R.R. Holmes. Organometallics, 17, 5114 (1998).
87] A. Chandrasekaran, R.O. Day, R.R. Holmes. Organometallics, 15, 3182 (1996).
88] C. Breliere, F. Carre, R.J.P. Corriu, M. Poirier, G. Royo. Organometallics, 5, 388 (1986).
89] T. Steiner. Angew. Chem. Int. Ed., 41, 48 (2002).
90] A.W. Addison, T.N. Rao, J. Reedijk, J. van Rijn, G.C. Verschoor. J. Chem. Soc., Dalton Trans., 1349 (1984).