Synthesis and chemical characterization of CuII, NiII and ZnII complexes of 3,5-bis(2′-pyridyl)-1,2,4-oxadiazole and 3-(2′-pyridyl)5-(phenyl)-1,2,4-oxadiazole ligands

Article abstract of DOI:10.1016/j.ica.2011.03.057

The synthesis and structural characterization of Ni^II, Cu ^II and Zn^II complexes of two chelating 1,2,4-oxadiazole ligands, namely 3,5-bis(2′-pyridyl)-1,2,4-oxadiazole (bipyOXA) and 3-(2′-pyridyl)5-(phenyl)-1,2,4-oxadiazole (pyOXA), is here reported. The formed hexacoordinated metal complexes are [M(bipyOXA)₂(H ₂O)₂](ClO₄)₂ and [M(pyOXA) ₂(ClO₄)₂], respectively (M = Ni, Cu, Zn). X-ray crystallography, ¹H and ¹³C NMR spectroscopy and C, N, H elemental analysis data concord in attributing them an octahedral coordination geometry. The two coordinated pyOXA ligands assume a trans coplanar disposition, while the two bipyOXA ligands are not. The latter result is a possible consequence of the formation of H-bonds between the coordinated water molecules and the nitrogen atom of the pyridine in position 5 of the oxadiazole ring. The expected splitting of the d metal orbitals in an octahedral ligand field explains the observed paramagnetism of the d⁸ and d⁹ electron configuration of the nickel(II) and copper(II) complexes, respectively, as determined by the broadening of their NMR spectra.

Full text of DOI:10.1016/j.ica.2011.03.057

Inorganica Chimica Acta 373 (2011) 62–67
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and chemical characterization of CuII, NiII and ZnII complexes of
3,5-bis(20-pyridyl)-1,2,4-oxadiazole and 3-(20-pyridyl)5-(phenyl)-1,2,4-oxadiazole
ligands
b
d
Alessio Terenzi a, Giampaolo Barone a,c, , Antonio Palumbo Piccionello , Gianluca Giorgi ,
⇑
Annalisa Guarcello b, Andrea Pace b,c,
⇑
a Dipartimento di Chimica ‘‘S. Cannizzaro’’, Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy
b Dipartimento di Scienze e Tecnologie Molecolari e Biomolecolari, Sezione di Chimica Organica ‘‘E. Paternò’’, Università di Palermo, Viale delle Scienze, 90128 Palermo, Italy
c Istituto EuroMediterraneo di Scienza e Tecnologia, Via Emerico Amari 123, 90139 Palermo, Italy
d Dipartimento di Chimica, Università di Siena, Via Aldo Moro, 53100 Siena, Italy
a r t i c l e i n f o
a b s t r a c t
The synthesis and structural characterization of NiII, CuII and ZnII complexes of two chelating 1,2,
4-oxadiazole ligands, namely 3,5-bis(20-pyridyl)-1,2,4-oxadiazole (bipyOXA) and 3-(20-pyridyl)5-
(phenyl)-1,2,4-oxadiazole (pyOXA), is here reported. The formed hexacoordinated metal complexes are
[M(bipyOXA)2(H2O)2](ClO4)2 and [M(pyOXA)2(ClO4)2], respectively (M = Ni, Cu, Zn). X-ray crystallogra-
phy, 1H and 13C NMR spectroscopy and C, N, H elemental analysis data concord in attributing them an
octahedral coordination geometry. The two coordinated pyOXA ligands assume a trans coplanar disposi-
tion, while the two bipyOXA ligands are not. The latter result is a possible consequence of the formation
of H-bonds between the coordinated water molecules and the nitrogen atom of the pyridine in position 5
of the oxadiazole ring. The expected splitting of the d metal orbitals in an octahedral ligand field explains
the observed paramagnetism of the d8 and d9 electron configuration of the nickel(II) and copper(II)
complexes, respectively, as determined by the broadening of their NMR spectra.
Article history:
Received 10 December 2010
Received in revised form 16 February 2011
Accepted 18 March 2011
Available online 31 March 2011
Keywords:
Copper(II) complexes
Crystal structure
Nickel(II) complexes
1,2,4-Oxadiazoles
Zinc(II) complexes
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
environment of living organisms [5–8]. Moreover, their complexes
with planar heterocyclic ligands are efficient DNA-binders and
Oxadiazole-based ligands have been recently used for complex-
ation of metals such as platinum, silver, palladium and copper
[1–3]. Recently, a series of PtII complexes of substituted 1,2,4-oxa-
diazole derivatives have shown cytotoxic activity towards human
ovarian cancer cell lines, as well as in colon and testicular cancer
cell lines and the results have been correlated to their DNA-binding
properties [4]. These heterocyclic systems are easily accessible and
the modulation of their properties is achievable through the intro-
duction of appropriate substituents on the central heterocycle. In
particular, 1,2,4-oxadiazole ligands can be designed so as to chelate
metal ions by the heteroatoms of the ring and of the side chain
substituents in the ring positions 3 and 5.
Based on the considerations above, we have started a research
project on the synthesis and characterization of novel 1,2,4-oxadi-
azole complexes of NiII, CuII and ZnII, with the aim to obtain metal
compounds with potential antitumor properties. The three metal
ions are present as essential elements in the biological intracellular
have a stronger affinity toward DNA than the corresponding iso-
lated ligands [9–11].
Recently, the first study of the biological activity of a copper(II)
complex of 1,2,4-oxadiazole ligands was reported [12]. In this re-
spect, biological assays showed that, despite the free ligand is
not being effective, its CuII complex reduces the vitality of human
hepatoblastoma and colorectal carcinoma cells in a dose- and
time-dependent manner [12]. Interestingly, the results of the bio-
logical assays receive a positive feedback in the DNA binding stud-
ies performed for the same complex [12].
The structure of [Cu(3,5-bis(20-pyridyl)-1,2,4-oxadiazole)2-
(H2O)2](ClO4)2 was characterized in the solid state by X-ray
crystallography [12]. Its structure consists of a discrete [Cu(3,5-
2+
ꢀ
bis(20-pyridyl)-1,2,4-oxadiazole)2(H2O)2] cation and two ClO4
anions. The CuII ion lies in a distorted octahedral environment,
where each 3,5-bis(20-pyridyl)-1,2,4-oxadiazole behaves as biden-
tate ligand and the two ligands are not coplanar. Two coordinated
water molecules occupy the cis positions in the equatorial plane.
The 1,2,4-oxadiazole ligand only uses two of its five potential
donor atoms, acting as a typical chelator and forming a five-
membered Cu–N–C–C–N cyclic moiety. The two non-coordinating
⇑
Corresponding authors.
E-mail addresses: gbarone@unipa.it (G. Barone), pace@unipa.it (A. Pace).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.03.057

A. Terenzi et al. / Inorganica Chimica Acta 373 (2011) 62–67
63
Fig. 1. Synthesis of bipyOXA and of its NiII, CuII and ZnII complexes.
Fig. 2. Synthesis of pyOXA and of its NiII, CuII and ZnII complexes.
ꢀ
ClO4 counter anions are involved in hydrogen bonds with the
coordinated water molecules. The water molecules are also in-
volved in intramolecular hydrogen bonding interactions with the
uncoordinated nitrogen of one pyridine moiety. The two pyridine
rings of 3,5-bis(20-pyridyl)-1,2,4-oxadiazole are not coplanar with
the oxadiazole ring and their orientation is forced by the coordina-
tion to CuII [12]. Only few CuII complexes of oxadiazole ligands, but
the 1,3,4 isomers, have been synthesised up to now [13–15].
The synthesis and characterization of the NiII and ZnII
complexes of 3,5-bis(20-pyridyl)-1,2,4-oxadiazole (here called
bipyOXA), and of the NiII, CuII and ZnII complexes of 3-(20-pyri-
dyl)5-(phenyl)-1,2,4-oxadiazole (called pyOXA), is reported in the
present paper. The two ligands differ in the substituent in position
5 of the oxadiazole ring (see Figs. 1 and 2), being a pyridine and a
phenyl in bipyOXA and pyOXA, respectively.
2. Experimental
2.1. Materials and methods
All chemicals and solvents were purchased from Sigma–Aldrich,
Fisher or Alfa Aesar and used as received, without further
purification.
1H NMR and 13C NMR spectra were recorded on a Bruker AC300
spectrometer, solvent residual peaks were used as reference. Flash
chromatography was performed by using silica gel (Merck, 0.040-
0.063 mm) and mixtures of ethyl acetate and petroleum ether
(fraction boiling in the range of 40–60 °C) in various ratios.
Mass spectrometry (MS) and MS/MS measurements of the com-
plexes have been carried out by using electrospray ionization (ESI)
with a LCQ-DECA ion trap (Thermo, Bremen, Germany). Operating

64
A. Terenzi et al. / Inorganica Chimica Acta 373 (2011) 62–67
conditions of the ESI source were as follows: spray voltage 4.5 kV;
capillary temperature 200 °C; sheath gas (nitrogen) flow rate ca.
0.75 L minꢀ1. Ultra-pure helium was the collision gas. CID collision
energy: 0.5ꢁ1.0 eV (laboratory frame). A methanol solution of the
complex (1 ꢂ 10ꢀ4 M) was introduced into the mass spectrometer
absolute ethanol and at room temperature. The mixture was left
under stirring for 12 h in the dark, the precipitate filtered, washed
by cold absolute ethanol and dried under vacuum (0.244 g; 83%).
The solid was recrystallized from acetonitrile.
MS-ESI (m/z): 604.9 ({[Ni(bipyOXA)2]ClO4}+); 380.9 ({[Ni(bipy-
OXA)]ClO4}+). Elemental Anal. Calc. for C24H20Cl2N8O12Ni: C,
38.85; H, 2.72; N, 15.10. Found: C, 38.67; H, 2.68; N, 15.07%. UV(-
using a syringe pump at a flow rate of 5 l .
L minꢀ1
Melting points were determined on a Reichart-Thermovar hot
stage apparatus and are uncorrected. IR spectra were registered
with a Shimadzu FTIR-8300 instrument.
Tris–HCl 1 mM, pH 7.5):
e
= 1.0 ꢂ 103 (k = 340 nm) ;
e
= 3.9 ꢂ 104
(k = 275 nm);
e
= 4.4 ꢂ 104 (k = 231 nm) cmꢀ1 Mꢀ1
.
2.3.2. PyOXA ligand and its CuII, ZnII and NiII complexes
2.2. X-ray crystallography
2.3.2.1. 3-(20-Pyridyl)5-(phenyl)-1,2,4-oxadiazole (pyOXA). 1 g of 2-
picolinamidoxime (2) (7.3 mmol) and 1.12 g of benzoyl chloride
(6, 8.03 mmol, d = 1.2 g/mL) were mixed in 120 mL of toluene.
0.64 mL of pyridine (0.63 g, 8.03 mmol, d = 0.98 g/mL) was added
and the mixture was left refluxing for 12 h. The residue was chro-
matographed yielding 0.60 g of 3-(20-pyridyl)5-(phenyl)-1,2,4-oxa-
diazole (70%) [20].
A Oxford-Diffraction Xcalibur Sapphire 3 diffractometer with a
graphite monochromated Mo K
a radiation (k = 0.71073 Å) was
used for data collection at 293(2) K. The [Cu(pyOXA)2(ClO4)2] com-
ꢀ
plex crystallizes in the triclinic crystal system, P1 space group. The
structure was solved by direct methods implemented in the SHELXS
-
97 program [16]. The refinement was carried out by full-matrix
anisotropic least-squares on F2 for all reflections for non-H atoms
by using the SHELXL-97 program [17].
1H NMR (300 MHz, CD3CN) d (ppm): 8.77 (dd, J = 11.8, 7.8 Hz,
1H), 8.30–8.15 (m, 3H), 8.04–7.91 (m, 1H), 7.76–7.59 (m, 3H),
7.59–7.49 (m, 1H). 13C NMR (75 MHz, CD3CN) d (ppm): 177.1 (C),
169.8 (C), 151.3 (CH), 147.4 (C), 138.3 (CH), 134.1 (CH), 130.4
(CH), 128.9 (CH), 126.8 (CH), 125.1 (C), 124.3 (CH). UV(CH3CN):
2.3. Synthesis and characterization
e
= 2.1 ꢂ 104 (k = 259 nm);
e
= 1.8 ꢂ 104 (k = 234 nm) cmꢀ1 Mꢀ1
.
2.3.1. BipyOXA ligand and its CuII, ZnII and NiII complexes
2.3.1.1. 3,5-Bis(20-pyridyl)-1,2,4-oxadiazole (bipyOXA1).00 g of 2-cya-
nopyridine (1) (8.14 mmol) and 0.56 g of 2-picolinamidoxime (2) [18]
(4.07 mmol) were mixed in a sealed tube and heated at 120 °C for 8 h.
The residue was chromatographed yielding 0.59 g of 3,5-bis(20-
pyridyl)-1,2,4-oxadiazole (65%): mp 173–176 °C (from EtOH) (lit.
173–175 °C) [19]; 1H NMR (300 MHz, CDCl3) d (ppm): 7.44–7.58 (m,
2H); 7.87–7.98 (m, 2H); 8.28–8.31 (d, 1H); 8.42–8.44 (d, 1H); 8.84–
8.89 (m, 2H).
2.3.2.2. [Cu(pyOXA)2(ClO4)2], 9a. A light blue solution of Cu-
(ClO4)2ꢃ6H2O (0.15 g, 0.4 mmol) in absolute ethanol was added
drop wise and under constant stirring to a colorless solution of
pyOXA (0.178 g, 0.8 mmol) in absolute ethanol and at room tem-
perature. The mixture was left under stirring for 12 h, the precipi-
tate filtered, washed by cold absolute ethanol and dried under
vacuum (0.230 g; 81%). The solid was recrystallized from acetoni-
trile. After 1 week at 4 °C, blue crystals of Cu(pyOXA)2(ClO4)2 suit-
able for X-ray crystallographic analysis, were obtained.
1H NMR (300 MHz, CD3CN) d (ppm): 8.96–8.70 (m, 2H), 8.34 (dt,
J = 7.9, 1.0 Hz, 1H), 8.21 (dt, J = 7.9, 1.1 Hz, 1H), 8.13–7.91 (dtd,
J = 17.3, 7.8, 1.8 Hz, 2H), 7.64 (ddd, J = 7.7, 4.8, 1.2 Hz, 1H), 7.56
(ddd, J = 7.7, 4.8, 1.2 Hz, 1H). 13C NMR (75 MHz, CD3CN) d (ppm):
176.1 (C), 170.0 (C), 151.5 (CH), 151.3 (CH), 147.3 (C), 144.5 (C),
138.7 (CH), 138.4 (CH), 128.1 (CH), 126.9 (CH), 125.3 (CH), 124.4
MS-ESI (m/z): 607.8 ([Cu(pyOXA)2ClO4]+); 385.1 ([Cu(pyOX-
A)ClO4]+). Elemental Anal. Calc. for C26H18Cl2N6O10Cu: C, 44.05;
H, 2.56; N, 11.85. Found: C, 44.19; H, 2.35; N, 11.83%. UV(CH3CN):
e
= 4.0 ꢂ 104 (k = 259 nm);
e
= 4.2 ꢂ 104 (234 nm) cmꢀ1 Mꢀ1
.
(CH). UV(MeOH):
e
.
= 1.7 ꢂ 104 (k = 274 nm);
e
= 1.5 ꢂ 104
(k = 231 nm)cmꢀ1 Mꢀ1
UV (Tris–HCl, pH 7.5): 1.5 ꢂ 104
2.3.2.3. [Zn(pyOXA)2(ClO4)2], 9b. A solution of Zn(ClO4)2ꢃ6H2O
(0.149 g, 0.4 mmol) in absolute ethanol was added drop wise and
under constant stirring to a colorless solution of pyOXA (0.179 g,
0.8 mmol) in absolute ethanol and at room temperature. The mix-
ture was let under stirring for 12 h in the dark but no precipitate
was observed. The solution was let for 6 h at ꢀ18 °C the precipitate
was filtered, washed by cold absolute ethanol and dried under vac-
uum (0.20 g; 70%). The solid was recrystallized from acetonitrile.
MS-ESI (m/z): 608.9 ([Zn(pyOXA)2ClO4]+); 386.1 ([Zn(pyOX-
A)ClO4]+). Elemental Anal. Calc. for C26H18Cl2N6O10Zn: C, 43.94; H,
2.55; N, 11.82. Found: C, 42.48; H, 2.32; N, 11.37%. UV(CH3CN):
(k = 275 nm);
e
= 1.7 ꢂ 104 (k = 231 nm) cmꢀ1 Mꢀ1
.
2.3.1.2.
[Zn(bipyOXA)2(H2O)2](ClO4)2,
5b
.
A
solution
of
Zn(ClO4)2ꢃ6H2O (0.14 g, 0.4 mmol) in absolute ethanol was added
drop wise and under constant stirring to a colorless solution of
3,5-bis(20-pyridyl)-1,2,4-oxadiazole (0.179 g, 0.8 mmol) in abso-
lute ethanol and at room temperature. The mixture was left under
stirring for 12 h in the dark, the white precipitate filtered, washed
by cold absolute ethanol and dried under vacuum (0.215 g; 72%).
The solid was recrystallized from acetonitrile.
e
= 4.4 ꢂ 104 (k = 259 nm);
e
= 3.9 ꢂ 104 (k = 236 nm) cmꢀ1 Mꢀ1
.
MS-ESI (m/z): 610.9 ({[Zn(bipyOXA)2]ClO4}+); 386.9 ({[Zn(bipy-
OXA)]ClO4}+). Elemental Anal. Calc. for C24H20Cl2N8O12Zn : C,
38.50; H, 2.69; N, 14.97. Found: C, 38.31; H, 2.63; N, 15.06%. UV(-
1H NMR (300 MHz, CD3CN) d (ppm): 9.02 (d, J = 5.23 Hz, 1H),
8.47–8.25 (dd, J = 4.46, 8.32 Hz, 2H), 8.14–7.93 (dd, J = 4.91,
9.41 Hz, 1H), 7.88–7.60 (m, 3H), 7.49 (t, J = 7.80, 7.80 Hz, 2H). 13C
NMR (75 MHz, CD3CN) d (ppm): 178.7 (C), 165.3 (C), 150.2 (CH),
143.3 (CH), 141.6 (C), 135.1 (CH), 130.3 (CH), 130.1 (CH), 129.5
(CH), 124.8 (CH), 121.2 (C).
Tris–HCl, 1 mM, pH 7.5):
e
= 3.9 ꢂ 104 (k = 273 nm);
e
= 4.2 ꢂ 104
(k = 231 nm) cmꢀ1 Mꢀ1
.
1H NMR (300 MHz, CD3CN) d (ppm): 8.69–8.37 (m, 4H), 8.31 (t,
J = 7.8 Hz, 1H), 8.18 (t, J = 7.8 Hz, 1H), 7.89–7.68 (m, 2H). 13C NMR
(75 MHz, CD3CN) d (ppm): 176.3 (C), 166.5 (C), 150.9 (CH), 150.0
(CH), 142.5 (CH), 142.1 (C), 140.8 (C), 140.6 (CH), 130.3 (CH),
130.0 (CH), 127.2 (CH), 125.1 (CH).
2.3.2.4. [Ni(pyOXA)2(ClO4)2], 9c. A light green solution of Ni(ClO4)2 ꢃ
6H2O (0.15 g, 0.4 mmol) in absolute ethanol was added drop wise
and under constant stirring to a colorless solution of pyOXA
(0.179 g, 0.80 mmol) in absolute ethanol and at room temperature.
The mixture was left under stirring for 12 h in the dark but no pre-
cipitate was observed. The solution was let for 6 h at -18 °C the
precipitate was filtered, washed by cold absolute ethanol and dried
2.3.1.3. [Ni(bipyOXA)2(H2O)2](ClO4)2, 5c. A light green solution of
Ni(ClO4)2ꢃ6H2O (0.15 g, 0.4 mmol) in absolute ethanol was added
drop wise and under constant stirring to a colorless solution of
3,5-bis(20-pyridyl)-1,2,4-oxadiazole (0.179 g, 0.80 mmol) in

A. Terenzi et al. / Inorganica Chimica Acta 373 (2011) 62–67
65
Table 1
Crystallographic data for [Cu(pyOXA)2(ClO4)2].
Formula
C26H18Cl2N6O10Cu
708.90
Formula weight
Crystal color, habit
Crystal dimensions (mm)
Crystal system
Space group
a (Å)
Light blue, prism
0.5 ꢂ 0.3 ꢂ 0.3
triclinic
ꢀ
P1
8.103(5)
8.647(5)
10.033(5)
86.736(5)
86.273(5)
78.892(5)
687.6(7)
1
b (Å)
c (Å)
a
(°)
b (°)
c
V
Z
(°)
T (K)
293(2)
1.71
359
Dcalc (g cm-3
F (0 0 0)
)
l
(mmꢀ1
)
3.125
Number of observations (all reflections)
Final R (all data)
4748
Fig. 4. Anion–p and OꢃꢃꢃH(–C) interaction motif in [Cu(pyOXA)2(ClO4)2] (9a).
R1 = 0.087, wR2 = 0.115
44.35, H, 2.58, N, 11.94; found: C, 44.05, H, 2.74, N, 11.22.
= 4.3 ꢂ 104 (k = 259 nm);
e
= 4.0 ꢂ 104 (k = 236 nm)
Table 2
UV(CH3CN):
e
Selected bond lengths (Å) and angles (°) for
[Cu(pyOXA)2(ClO4)2].
cmꢀ1Mꢀ1
.
Bond lengths
(Å)
3. Results and discussion
Cu–N2
Cu–N4
Cu–O4
2.007(3)
2.023(3)
2.354(3)
3.1. Synthesis and structural characterization
Bond angles
N2A–Cu–N4*
N2A–Cu–N4
N2A–Cu–O4
N2A–Cu–O4*
N4–Cu–O4
N4–Cu–O4*
C5–N4–Cu
(°)
The ligand bipyOXA was synthesised through a solvent-free
reaction between amidoxime 2 (Fig. 1) and 2-cyanopyridine (1).
The same amidoxime, together with benzoyl chloride (6, Fig. 2)
was used to synthesize the ligand pyOXA. The reaction was con-
ducted in toluene at 130 °C (see Fig. 2). BipyOXA and pyOXA were
characterized by elemental analysis, ESI mass spectrometry exper-
iments and NMR (see also Supplementary Information).
All the metal complexes have been synthesised in absolute eth-
anol by mixing the hydrate metal perchlorate salt and the ligand in
a 1:2 molar ratio (see Figs. 1 and 2 and Section 2).
100.03(12)
79.97(12)
89.34(10)
90.66(10)
87.45(10)
92.55(10)
113.5(2)
139.3(2)
130.05(15)
O3–N4–Cu
Cl1–O4–Cu
*
Symmetry transformations used to generate equiva-
lent atoms: ꢀx,ꢀy,ꢀz + 1.
Due to their paramagnetism, it was not possible to acquire
noiseless NMR spectra for the copper(II) and the nickel(II) com-
plexes, both presenting an octahedral coordination geometry lead-
ing to open shell electron configurations. Suitable crystals for X-ray
diffraction analysis were obtained only for the two copper com-
plexes. The crystal structure of the copper(II)–bipyOXA complex
under vacuum (0.210 g; 75%). The solid was recrystallized from
acetonitrile.
MS-ESI (m/z): 602.9 ([Ni(pyOXA)2ClO4]+); 379.9 ([Ni(pyOX-
A)ClO4]+). Elemental analysis calcd (%) for C26H18Cl2N6O10Ni: C,
Fig. 3. Crystal structure of the [Cu(pyOXA)2(ClO4)2] (9a) complex. Unlabeled atoms are generated by symmetry (ꢀx,ꢀy,ꢀz + 1).

66
A. Terenzi et al. / Inorganica Chimica Acta 373 (2011) 62–67
Fig. 5. Partial 1H NMR (CD3CN, 300 MHz) of bipyOXA (4) (upper) and of its ZnII (5b) complex.
was recently reported [12], while that of the copper(II)–pyOXA
complex is described in the next paragraph. Nevertheless, elemen-
tal analysis data confirm that the three metal complexes of
bipyOXA contain two hydration water molecules, presumably
coordinated as in the structure of the copper(II)–bipyOXA complex.
phenyl ring (d O(4)(x,y,z)ꢃꢃꢃH(3b)(1 ꢀ x,1 ꢀ y,1 ꢀ z) = 2.61 Å), O(6)
with the pyridyl H–C(4a) (d O(6)(x,y,z)ꢃꢃꢃH(4a)(2 ꢀ x,ꢀ1 ꢀ y,1 ꢀ
z) = 2.66 Å) and H–C(3a) (d O(6)(x,y,z)ꢃꢃꢃH(3a)(2 ꢀ x,ꢀ1 ꢀ y,1 ꢀ
z) = 2.63 Å), and with H–C(2b) of the phenyl ring (d O(6)(x,y,z)ꢃꢃꢃH-
(2b)(x,ꢀ1 + y,z) = 2.67 Å), while O(7) interacts with H–C(5a) of the
pyridine ring (d O(7)(x,y,z)ꢃꢃꢃH(5a)(x,y,1 + z) = 2.55 Å).
3.2. X-ray crystallography
3.3. NMR spectroscopy
Single crystals of [Cu(pyOXA)2(ClO4)2] (9a) were obtained by
crystallization from acetonitrile. Crystallographic data and selected
geometrical parameters are reported in Tables 1 and 2, respec-
tively. Differently than the complex with the ligand bipyOXA 4
[12], in [Cu(pyOXA)2(ClO4)2] copper has no coordinated water mol-
ecules (Fig. 3). The metal ion lies at an inversion center and PyOXA
acts as a bidentate ligand. Copper is coordinated to two symmetry-
related ligands that are in trans disposition and to two perchlorate
molecules occupying the two axial positions of the octahedral
structure. This results in a neutral complex with a consistently
lower water solubility compared to [Cu(bipyOXA)2(H2O)2](ClO4)2.
The ligand forms a five-membered Cu–N–C–C–N cyclic moiety
with a N–Cu–N angle equal to 79.97(12)°. The Cu–N(pyridine)
bond distances are equal to 2.007(3) Å, while Cu–N(oxadiazole)
and Cu–O(perchlorate) bond lengths are 2.023(3) and 2.354(3) Å,
respectively.
Fig. 5 shows the comparison between the 1H NMR spectra of the
ligand bipyOXA and of its ZnII complex. The complexation of the li-
gand with the metal center can be argued by the upfield shift of the
protons H1 and H10 and by the downfield shift of the remaining
protons.
Analogously, the comparison between the 1H NMR spectra of
the ligand pyOXA, 8, and its ZnII complex 9b is shown in Fig. 6. It
can be noticed that all the pyridine protons (H1–H4, Fig. 6) are
shifted in the left part of the spectrum (downfield shift) while
the phenyl protons are slightly shifted in the right part of the spec-
trum (upfield shift) or remain essentially in the same position. Tak-
ing into account that, according to the crystal structure of its CuII
complex (see Fig. 3), the coordinated pyOXA ligands are coplanar,
the change in the chemical shift of the pyridine protons to lower
fields can be attributed only to zinc coordination. In fact, a low field
shift indicates that the electron density around the pyridine pro-
tons decreases. The observed chemical shift values of [Zn(bipyOX-
A)2(H2O)2]2+, 5b, compared to those of the isolated ligand, are more
difficult to rationalize, because the two coordinated bipyOXA li-
gands are not coplanar, in analogy with the structure of [Cu(bipy-
OXA)2(H2O)2]2+, 5a (see Figs. 1 and 5). In this case, non-bonding (or
through space) interactions could indeed contribute to determine
the observed shifts. As a matter of fact, concentration dependent
1H NMR spectra of 5b in acetonitrile showed an effect on the
chemical shift values (see Fig. S2) that suggest the occurrence of
displacement of coordinated water by solvent molecules.
A non-conventional intramolecular hydrogen bond occurs be-
tween the endocyclic oxygen of the 1,2,4-oxadiazole moiety and
H–C(3a) of the pyridine ring (d O(3)(x,y,z)ꢃꢃꢃH(3a)(2 ꢀ x,ꢀy,1 ꢀ
z) = 2.46 Å), thus forming a six-membered chelate ring.
The crystal packing reveals interesting features (Fig. 4). The oxa-
diazole ring is involved in an anion–p interaction with the perchlo-
rate, thus suggesting a -acidic character of this heterocyclic ring.
p
The distance O(5)ꢃꢃꢃring centroid (CpOxa) is 2.985(4) Å and the angle
Cl–O(5)ꢃꢃꢃCpOxa is 174.8(3)°. The crystal packing also includes inter-
molecular hydrogen bonding interactions involving the perchlorate
anion and different C–H groups: O(4) interacts with H–C(3b) of the

A. Terenzi et al. / Inorganica Chimica Acta 373 (2011) 62–67
67
H6, H7, H8
H5, H9
H4
H3
H2
H1
H6
H7
H8
H5
H3
H1
H4
N
N
H9
H2
O
N
H6, H8
O3ClO
O
Zn
OClO 3
H7
N
N
N
9.1
9.0
8.9
8.8
8.7
8.6
8.5
8.4
8.3
8.2
8.1
8.0
7.9
7.8
7.7
7.6
7.5
7.4
7.3
7.2
Fig. 6. Partial 1H NMR (CD3CN, 300 MHz) of bipyOXA (8) (upper) and of its ZnII (9b) complex.
3.4. ESI mass spectrometry
Appendix A. Supplementary material
The ESI(+) mass spectrum of a methanolic solution of [M(bipyOX-
A)2(H2O)2](ClO4)2 shows the presence of different ions attributed to
{[M(bipyOXA)2](ClO4)}+, {[M(bipyOXA)2]Cl}+, [MI(bipyOXA)2]+ and
{[M(bipyOXA)](ClO4)}+, respectively and confirmed by the analysis
of their isotopic clusters and by tandem mass spectrometry experi-
ments. Methanolic solutions of [M(pyOXA)2(ClO4)2] show ions due
to [M(pyOXA)2(ClO4)]+ and to [M(pyOXA)(ClO4)]+, together with
some adducts containing methanol. These data suggest that (i) the
species formed in the gas phase are mainly singly positively charged
ions while multiply charged species are not detectable; (ii) the two
coordinated water molecules of the copper(II)–bipyOXA complex
are easily lost; (iii) the copper ion(II) can undergo a reduction pro-
cess with the formation of CuI.
CCDC 804195 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via http://www.ccdc.
cam.ac.uk/data_request/cif. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.ica.2011.03.057.
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4. Conclusions
The novel complexes [M(bipyOXA)2(H2O)2](ClO4)2 (M = NiII, and
ZnII) and [M(pyOXA)2(ClO4)2] (M = NiII, CuII, and ZnII), were syn-
thesised and structurally characterized in the solid state and in
solution.
The pyOXA ligand and its NiII, CuII and ZnII complexes are con-
sistently less soluble in water solution than the same metal com-
plexes of bipyOXA.
The agreement among X-ray crystallography, NMR spectros-
copy and elemental analysis measurements allow to assign an
octahedral coordination geometry to all the investigated metal
complexes. However, the two coordinated pyOXA ligands assume
a trans-coplanar disposition, while the two bipyOXA ligands are
in an almost orthogonal arrangement.
Considerations based on the splitting of the d metal orbitals in
an octahedral ligand field allow to explain the observed paramag-
netism of the d8 electron configuration of the nickel(II) complexes
of both bipyOXA and pyOXA ligands, as determined by the
broadening of their NMR spectra.
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