Synthesis, structure, magnetic properties and DFT calculations of novel bis-(5-tetrazolyl)amine copper(II) complexes

Article abstract of DOI:10.1016/j.ica.2011.12.040

Four new multidimensional copper(II) complexes [Cu(BTA)(2,2′-bipy) H₂O] (1), [Cu(BTA)(1,10′-phen)] (2), [Cu₂(BTA) ₂(H₂O)₄] (3) and [Cu(BTA)(en)]_n (4) have been synthesized by conventional and hydrothermal reactions of the bis(5-tetrazolyl)amine (H₂BTA) with copper(II) salts in the presence of different ancillary ligands. X-ray diffraction studies on these compounds show 1 and 2 are mononuclear entities, in which the BTA ligand acts in a chelate coordination mode and the copper atoms exhibit a coordination environment intermediate between trigonal-bipyramidal and square-pyramidal for 1 and between square-planar and tetrahedral for 2. Complex 3 consists of centrosymmetric dinuclear molecules with the ligand acting in a μ₂-1,1′:2 chelate/bridging tridentate coordination mode and 4 is a linear zigzag neutral chain bearing a BTA bridging ligand in a μ₂-1,1′:3 chelate/bridging tridentate coordination mode. Complexes 3 and 4 exhibit antiferromagnetic and very weak ferromagnetic interactions, respectively. DFT calculations have been performed in order to explain their respective magnetic behaviors.

Full text of DOI:10.1016/j.ica.2011.12.040

Inorganica Chimica Acta 385 (2012) 73–80
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structure, magnetic properties and DFT calculations of novel
bis-(5-tetrazolyl)amine copper(II) complexes
⇑
Antonio Rodríguez-Diéguez , Marta E. López-Viseras, Jesús E. Perea-Buceta, Antonio J. Mota,
Enrique Colacio
Departamento de Quimica Inorganica, Universidad de Granada, 18071 Granada, Spain
a r t i c l e i n f o
a b s t r a c t
Four new multidimensional copper(II) complexes [Cu(BTA)(2,20-bipy)H2O] (1), [Cu(BTA)(1,100-phen)] (2),
[Cu2(BTA)2(H2O)4] (3) and [Cu(BTA)(en)]n (4) have been synthesized by conventional and hydrothermal
reactions of the bis(5-tetrazolyl)amine (H2BTA) with copper(II) salts in the presence of different ancillary
ligands. X-ray diffraction studies on these compounds show 1 and 2 are mononuclear entities, in which
the BTA ligand acts in a chelate coordination mode and the copper atoms exhibit a coordination environ-
ment intermediate between trigonal-bipyramidal and square-pyramidal for 1 and between square-planar
and tetrahedral for 2. Complex 3 consists of centrosymmetric dinuclear molecules with the ligand acting
Article history:
Received 12 October 2011
Received in revised form 20 December 2011
Accepted 21 December 2011
Available online 5 January 2012
Keywords:
BTA
Copper complexes
Magnetic properties
DFT calculations
in a
l
2-1,10:2 chelate/bridging tridentate coordination mode and 4 is a linear zigzag neutral chain bearing
a BTA bridging ligand in a
l
2-1,10:3 chelate/bridging tridentate coordination mode. Complexes 3 and 4
exhibit antiferromagnetic and very weak ferromagnetic interactions, respectively. DFT calculations have
been performed in order to explain their respective magnetic behaviors.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
copper salts. The aim of this work is to know how the great plastic-
ity of the copper coordination sphere influences the formation of
Tetrazole-derived ligands have attracted much attention in re-
cent years not only due to their rich coordination chemistry that
has provided a great variety of fascinating and intriguing structures
but also due to their interesting optical, magnetic and hydrogen
storage properties and potential energetic applications [1]. In
2001, Sharpless and Demko reported an environmentally friendly
process for the preparation of 5-substituted 1H-tetrazoles via me-
tal-assisted 1,3-dipolar cycloaddition of nitriles and azides [2]. After
that numerous tetrazole ligands have been prepared and structur-
ally characterized [3]. Recently, we and others showed the ability
of the versatile 5-pyrimidyl-tetrazolate (pmtz) and tetrazole-5-car-
boxylic acid (H2TzC) ligands to produce MII (CoII, FeII, CdII and CuII)
multidimensional coordination polymers with interesting mag-
netic [4], zeolitic [5] and photoluminescence properties [6]. These
ligands possess six and four nitrogen donor atoms, respectively,
and consequently exhibit a wide diversity of potential coordination
modes. In view of the above considerations, we have decided to pre-
pare a polytopic ligand, bis(5-tetrazolyl)amine (H2BTA) (Scheme I)
to perform a systematic study of its coordination chemistry with
different structures. Notice that only a few copper(II) complexes
bearing this ligand have been reported so far [7].
In principle, the BTA2ꢀ ligand is an ideal candidate for obtaining
polymeric frameworks with intriguing structures and interesting
physical properties because it has nitrogen donor atoms which
could provide a wide variety of potential coordination modes [8].
By changing the metal ions and their charges we could expect to
obtain coordination complexes with great structural diversity.
We report here on the preparation, structural characterization,
magnetic properties and some DFT calculations of the following
BTA-containing copper(II) complexes: [Cu(BTA)(2,20-bipy)H2O]
(1), [Cu(BTA)(1,100-phen)] (2), [Cu2(BTA)2(H2O)4] (3) and [Cu(B-
TA)(en)]n (4). 1 and 2 are mononuclear complexes, 3 is a centrosym-
metric dinuclear complex and 4 is a linear zigzag neutral chain.
2. Experimental
2.1. General
All analytical reagents were purchased from commercial
sources and used without further purification. Perchlorate salts
are potentially explosive and only should be handled in small
quantities.
Abbreviations: H2BTA, bis(5-tetrazolyl)amine; 2,20-bipy, 2,20-bipyridine; 1,100-
phen, 1,100-phenantroline; en, ethylenediamine.
⇑
Corresponding author. Tel.: +34 958240442.
E-mail address: antonio5@ugr.es (A. Rodríguez-Diéguez).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.040

74
A. Rodríguez-Diéguez et al. / Inorganica Chimica Acta 385 (2012) 73–80
Scheme I. H2BTA with three reversible types of protonated, deprotonated, triprotonated mode.
Scheme II. Synthesis of bis(5-tetrazolyl)amine (H2BTA).
2.2. Synthesis of bis(5-tetrazolyl)amine (H2BTA)
2.01; N, 50.29. Found: C, 9.53; H, 1.98; N, 50.27%. IR (KBr, cmꢀ1):
3335 (s), 3241 (m), 3139 (m), 2923 (m), 1629 (vs), 1531 (s),
m
Bis(5-tetrazolyl)amine was prepared according to the proce-
dure reported by Zubarev et al. [9] by refluxing dicyanamide and
sodium azide in dry N,N-dimethylformamide (Scheme II).
1485 (s).
2.3.4. [Cu(BTA)(en)]n (4)
A
mixture of Cu(ClO4)2ꢁ6H2O (0.183 g, 0.49 mmol), H2BTA
2.3. Preparation of the complexes
(0.100 g, 0.49 mmol), ethylenediamine (1 ml) and water (10 mL)
was added to a Teflon-lined stainless steal Parr acid digestion ves-
sel and heated at 150 °C for 4 h under autogenous pressure. After
slow cooling to room temperature, blue crystals of 4 were recov-
ered. Yield: 40%. Anal. Calc. for C4H9N11Cu: C, 17.49; H, 3.30; N,
56.08. Experimental: C, 17.43; H, 3.19; N, 55.78%. IR (KBr, cmꢀ1):
2.3.1. [Cu(BTA)(2,20-bipy)H2O] (1)
An aqueous solution of Cu(ClO4)2ꢁ6H2O (0.183 g, 0.49 mmol, in
20 mL H2O) was added under continuous stirring to an aqueous
solution of 2,20-bipyridine (0.077 g, 0.49 mmol, in 20 mL H2O).
After stirring the resulting suspension for 15 min, H2BTA (0.100 g,
0.49 mmol, in 20 mL H2O) was added. The resulting green solution
was stirred for 20 min and then it was left standing at room tem-
perature for 10 days. Green crystals of 1 formed, which were
filtered off and air dried. The yield was 15% based on copper. Anal.
Calc. for C12H11N11OCu: C, 37.07; H, 2.85; N, 39.62. Found: C, 36.95;
m
3328 (s), 3247 (m), 3131 (m), 2921 (m), 2817 (m), 1625 (vs),
1539 (s), 1491 (s), 1323 (m).
2.4. Physical measurements
Elemental analyses were carried out at the ‘‘Centro de Instru-
mentación Científica’’ (University of Granada) on a Fisons-Carlo
Erba analyser model EA 1108. The IR spectra on powdered samples
were recorded with a ThermoNicolet IR200FTIR by using KBr pel-
lets. Magnetization and variable temperature (1.9–300 K) magnetic
susceptibility measurements on polycrystalline samples were
carried out with a Quantum Design SQUID MPMS XL-5 device
operating at different magnetic fields. The experimental suscepti-
bilities were corrected for the diamagnetism of the constituent
atoms by using Pascal’s tables.
H, 2.80; N, 39.65%. IR (KBr, cmꢀ1):
1621 (vs), 1535 (s), 1489 (s).
m 3321 (s), 3138 (m), 2925 (m),
2.3.2. [Cu(BTA)(1,100-phen)] (2)
Hydrothermal treatment of a mixture containing Cu(ClO4)2ꢁ
6H2O (0.183 g, 0.49 mmol), 1,10-phenantroline (0.098 g,
0.49 mmol), H2BTA (0.100 g, 0.49 mmol) and water (10 mL) at
150 °C for four hours gave rise to a green-pale solution, which kept
at room temperature for 5 days, afforded green-dark crystals of 2.
The yield was 21% based on copper. Anal. Calc. for C28H18N22Cu2:
C, 42.59; H, 2.30; N, 39.02. Found: C, 42.13; H, 2.21; N, 38.97. IR
2.5. Single-crystal structure determination
(KBr, cmꢀ1):
m 3141 (m), 1617 (vs), 1531 (s), 1481 (s).
Suitable crystals of 1–4 were mounted on glass fiber and used
for data collection. Data were collected with a Bruker AXS APEX
CCD area detector equipped with graphite monochromated MoK
2.3.3. [Cu2(BTA)2(H2O)4] (3)
To a solution of copper acetate (0.099 g, 0.49 mmol in 20 mL of
water) was added a solution of H2BTA (0.100 g, 0.49 mmol in
20 mL H2O). The resulting green solution was additionally stirred
for 20 min. and then was left standing at room temperature for
5 days. Green crystals of 3 formed, which were filtered off and
air dried. Yield: 60%. Anal. Calc. for C4H10N18O4Cu2: C, 9.58; H,
radiation (k=0.71073 Å) by applying the
x-scan method. The data
were processed with APEX2 [10] and corrected for absorption using
SADABS [11]. The structures were solved by direct methods using
SIR97 [12], revealing positions of all non-hydrogen atoms. These
atoms were refined on F2 by a full matrix least-squares procedure

A. Rodríguez-Diéguez et al. / Inorganica Chimica Acta 385 (2012) 73–80
75
Table 1
Crystallographic data structural refinement details for 1–4.
Compound
1
C
2
3
4
Chemical formula
M/gmolꢀ1
T (K)
12H11N11OCu
C14H9N11Cu
394.86
293
C4H10N18O4Cu
501.38
293
C4H9N11Cu
274.76
293
388.86
293
k (Å)
0.71073
0.71073
0.71073
0.71073
monoclinic
P21/C
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
triclinic
triclinic
triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1
7.879(2)
11.621(3)
16.539(4)
85.749(3)
82.585(3)
80.756(3)
1480.0(6)
4
7.105(5)
9.091(5)
12.017(5)
76.156(5)
87.455(5)
70.722(5)
710.9(7)
2
5.475(5)
7.635(5)
9.229(5)
78.272(5)
81.872(5)
83.331(5)
372.4(5)
1
8.422(1)
11.607(2)
9.873(2)
90
95.724(2)
90
960.3(3)
4
1.901
2.269
9039
a
(°)
b (°)
c
(°)
V (A3)
Z
q
l
(g cmꢀ3
(mmꢀ1)
)
1.745
1.506
5811
1.845
1 565
6958
2.236
2.926
3709
Unique reflections
Rint
0.000
1.051
0.033
0.071
0.023
1.059
0.028
0.075
0.058
1.023
0.046
0.093
0.037
1.066
0.029
0.071
GOF on Fa
Rl [I > 2
wR2 [I > 2
r(I)]
r
(I)]
a
2
R(F) =
R
||Fo| ꢀ |Fc||/
R
|Fo|, wR(F2) ꢀ [
R
w(Fo ꢀ Fc2)2/
R .
wF4]1/2
using anisotropic displacement parameters [13]. All hydrogen
atoms were located in difference Fourier maps and included as
fixed contributions riding on attached atoms with isotropic ther-
mal displacement parameters 1.2 times those of the respective
atom. Final R(F), wR(F2) and goodness of fit agreement factors,
details on the data collection and analysis can be found in Table 1.
we used herein to determine the exchange coupling constants
has been described in detail elsewhere [19].
3. Results and discussion
3.1. Descriptions of crystal structures
2.6. Computational details
The structure of 1 consists of two neutral monomeric molecules
of [Cu(BTA)(2,20-bipy)H2O] (A and B) which are shown in Fig. 1. The
copper ion exhibits a CuN4O coordination environment which is
formed by the coordination of two nitrogen atoms from the biden-
tate bipy ligand and two nitrogen atoms (N1 and N6) belonging to
the dideprotonated BTA2ꢀ tetrazolate ligand. The fifth position is
occupied by a water molecule. Bond distances and angles are sim-
ilar in both mononuclear units. Cu–N(2,20-bipy) distances range
from 1.992(2) to 2.055(2) Å, whereas Cu–N(BTA) distances are in
the range 1.963(2)–1.994(2) Å. The Cu–O(w) axial distances are
2.180(2) and 2.236(2) Å for molecules A and B, respectively. The
All theoretical calculations were carried out at the DFT level of
theory using the hybrid B3LYP exchange-correlation functional
[14], as implemented in the GAUSSIAN 03 program [15]. A quadratic
convergence method was employed in the SCF process [16]. All-
electron, triple-f quality basis set proposed by Ahlrichs and co-
workers has been used for all atoms [17]. The calculations were
performed on the complexes 3 and 4 built from the experimental
geometries. The electronic configurations used as starting points
were created using Jaguar 7.6 software [18]. The approach that
Fig. 1. The structure of the [Cu(BTA)(2,20-bipy)H2O] (1). Thermal ellipsoids are drawn at the 35% probability level. Hydrogen atoms have been omitted for clarity.

76
A. Rodríguez-Diéguez et al. / Inorganica Chimica Acta 385 (2012) 73–80
Fig. 2. View of the molecular structure of complex 2. Thermal ellipsoids are drawn
at the 50% probability level. Hydrogen atoms have been omitted for clarity.
Fig. 4. Coordination environment of the CuII ion in 4, showing the atom numbering
scheme. Displacement ellipsoids are drawn at the 50% probability level, and H
atoms are shown as spheres of arbitrary radii. (Symmetry code: (i) ꢀ0.5 ꢀ x, 0.5 + y,
0.5 ꢀ z).
trans angles in the equatorial plane N1A–Cu1–N12C, N6A–Cu1–
N1C, N1B–Cu2–N12D, N6B–Cu2–N1D are of 141.99(9), 175.03(9),
173.29(9) and 143.94(9), respectively.
The coordination geometry around copper atoms is intermedi-
ate between trigonal-bipyramidal (TBPY5) and square-pyramidal
The copper(II) ion is coordinated by two nitrogen atoms belong-
ing to the BTA2ꢀ ligand (Cu–N1A = 1.951(2) Å and Cu–N6A =
1.950(2) Å) and two nitrogen atoms (Cu–N1B = 1.991(2) Å and
Cu–N14B = 2.016(2) Å) from the phenanthroline ligand. The cis
bond angles are in the range 82.00(8)–100.84(8)°, while the trans
bond angles have values of 154.77(9) and 165.39(8)°. Centrosym-
metric related molecules of 2 are hold together by a pair of comple-
mentary hydrogen bonds involving the N5 amine nitrogen atom of a
molecule and the N9 atom of the neighboring unit (donor–acceptor
distance of 2.867 Å) to form dinuclear species. The crystal packing of
the dinuclear copper complexes is stabilized by face-to-face interac-
tions between two phenantroline rings with centroid–centroid
distances near to 3.320 Å, ultimately leading to 1D chains along
the [011] direction.
(SPY5) (
TPY5 and
s
= 0.55 and 0.49 for Cu1 and Cu2, respectively,
= 0 for SPY5) [20].
s = 1 for
s
The coordinated water molecule in both molecules A and B is
involved in hydrogen bonds interactions with the nitrogen atoms
N4 and N8 from BTA ligands belonging to neighbor units of the
same type (A and B). These interactions give rise to the formation
of two 1D hydrogen bonded chains. Moreover, in these 1D systems
there are intrachain H-bonded interactions between N5 and N9.
The distances between the donor atoms involved in hydrogen bond
interactions are given in Table SI2. The crystal packing of the chains
. . .
is further stabilized by face-to-face
p p interactions involving
•
The structure of the compound [Cu2(BTA)2(H2O)4] (3) consists of
centrosymmetric dinuclear molecules [Cu2(BTA)2(H2O)4] with Ci
symmetry (Fig. 3) which are involved in an extensive network of
hydrogen bonds, ultimately affording a 3D structure. Within the
dinuclear unit, copper(II) atoms are bridged by two BTA2ꢀ anions
and each metal ion exhibits a square-pyramidal SPY5 CuN3O2 coor-
dination environment, which is formed by the coordination in the
same plane of the N1 and N6 tetrazolato nitrogen atoms in the cis
position of the same BTA ligand, giving rise to a stable six-
membered chelate ring, the N2 tetrazolate nitrogen atom belonging
to a symmetry related BTA2ꢀ anion, which is located in trans posi-
tion to the N6 nitrogen atom, one water molecule (O2 W) in trans
tetrazolate rings. Moreover, polymeric chains A and B are stacked
in such a way that centroid–centroid distances between bipyridine
rings are near to 3.340 Å.
The crystal structure of molecule 2 consists of discrete neutral
[Cu(BTA)(1,100-phen)] molecules. A perspective view of 2 is shown
in Fig. 2 and selected bond distances and angles are listed in
Table SI1. The CuN4 environment around each copper(II) atom is
close to being half way between square-planar (D4h) and tetrahe-
dral (Td) with a d value of 0.79 (d =
D/70.5, D being the difference
between the mean value of the trans N–Cu–N angles and the ideal
tetrahedral angle of 109.5°; d = 0 for the ideal tetrahedral geometry
and d = 1 for the ideal square planar geometry).
Fig. 3. View of the structure of 3. Thermal ellipsoids are drawn at the 50% probability level H-atoms are omitted for the sake of clarity.

A. Rodríguez-Diéguez et al. / Inorganica Chimica Acta 385 (2012) 73–80
77
Fig. 5. View of the chain along the c axis on the structure of 4. Thermal ellipsoids are drawn at the 50% probability level H-atoms are omitted for the sake of clarity. Copper(II),
nitrogen and carbon atoms are green, blue and gray, respectively.
positions to the N1 nitrogen atom and a molecule of water (O1 W) in
apical position. Therefore, the ligands exhibit a
2-1,10:2 chelating/
The Cu–N bond lengths involving the nitrogen atoms of the BTA
ligand with values of 1.966(2) and 1.975(2) Å (Table SI1), which are
slightly shorter than the Cu–N bond lengths involving the coordi-
nated nitrogen atoms pertaining to the ethylenediamine ligand
(2.010(2) and 2.024(2) Å) and are similar to those observed in com-
parable copper complexes containing tetrazolate or ethylenedia-
mine ligands [22]. The copper atom is not in the basal plane, but
it is located 0.230 Å out of the mean basal plane toward N3i.
l
bridging tridentate coordination mode, which generates a centro-
. . .
symmetric planar Cu(N–N)2Cu hexagon with Cu Cu distances of
•
4.012 Å, respectively. This structure is similar to the binuclear motif
of the chain [Zn(BTA)(NH3)2]ꢁ0.5H2O [8] and to that formed for the
dinuclear complex [Cu2(TzC)2(H2O)6] (TzC = 5-tetrazolate-carbox-
ylate dianion) [21].
Bond distances around Cu(II) atoms are shown in the Table SI1
with values of 1.984(4), 1.981(4), 1.953(4), 1.981(5) and 2.330(5) Å
for Cu–N1, Cu–N2, Cu–N6, Cu–O2 W and Cu–O1 W bond distances,
respectively. Trans N2–Cu–N6 and N1–Cu–O2 W bond angles are
significantly deviated from linearity (168.45(17)° and 172.8(2)°)
and consequently the trans-coordinated water molecule (O2 W)
is 0.186 Å out from the equatorial plane. It should be noted that
this complex displays asymmetric double tetrazolato bridges, as
the Cu–N1–N2 and Cu–N2–N1 angles are different (130.1(3)° and
132.6(3)°). In this compound, with the exception of the coordi-
nated nitrogen atoms, N3 and N7, all donor atoms of the BTA2ꢀ li-
gand are involved in hydrogen bonds with the coordinated water
molecules and the N5–H group of neighboring dinuclear mole-
cules, giving rise to a intrincate 3D network (Fig. SI3). It should
be noted that the shortest hydrogen bond is established between
the water molecules O1 W and O2 W with a value of 2.758 Å.
In the chain compound [Cu(BTA)(en)]n (4) each Cu(II) ion is five
coordinate (Fig. 4) and exhibits an almost perfect CuN5 square-pyra-
Chains of 4 are oriented in such way that there exist face-
. . .
to-face
p p interactions between centrosymmetric related tet-
•
razolate rings with centroid–centroid distances of about 3.480 Å
and hydrogen bond interactions involving the N5 and N4 nitrogen
atoms of centrosymmetric related BTA2ꢀ ligands with donor–
acceptor distances of 2.947 Å. These interactions generate a com-
plicated 3D network.
3.2. Magnetic properties
The magnetic properties of the complex [Cu2(BTA)2(H2O)4] (3)
in the form
vMT versus T (vM is the molar magnetic susceptibility
per dinuclear copper(II) unit) are shown in Fig. 6.
The
v
MT product at room temperature of 0.91 cm3 K molꢀ1 is
close to the expected value for two uncoupled CuII atoms with
g = 2.18 (0.89 cm3 K molꢀ1). On lowering the temperature, the
v
MT product steadily decreases until 50 K and then decreases dee-
ply tends toward zero at 2 K. This behavior indicates the existence
of a weak antiferromagnetic interaction through the tetrazolato
bridging ligand leading to a S = 0 ground state. The susceptibility
data were analyzed using the Bleaney–Bowers expression for two
magnetically interacting spin doublets derived from the isotropic
Hamiltonian:
midal coordination geometry (s = 0.036). The basal plane is formed
by the nitrogen atoms N1A and N6A belonging a BTA ligand, and the
nitrogen atoms from the coordinated ethylenediamine molecule. A
nitrogen atom from a neighbor molecule (N3Ai) occupies the apical
position (symmetry code: (i) ꢀ0.5 ꢀ x, 0.5 + y, 0.5 ꢀ z) at longer
distance of 2.256(2) Å, and connects [Cu(BTA)(en)] units to form a
zig-zag chain (Fig. 5).
^
^ ^
^ ^
H ¼ ꢀJS1S2 þ gbHðS1S2Þ
Fig. 6. Temperature dependence of
vMT for 3 under an applied magnetic field of 0.5
Fig. 7. Temperature dependence of vMT for 4 under an applied magnetic field of 0.5
T. Solid lines represent the best fit of the date with the model described in the text.
T. Solid lines represent the best fit of the date with the model described in the text.

78
A. Rodríguez-Diéguez et al. / Inorganica Chimica Acta 385 (2012) 73–80
Fig. 8. The units calculated for complex 3.
Least-squares best-fit results were: J = ꢀ1.13(2) cmꢀ1
,
the case of compound 4, that exhibits an almost perfect square-
pyramidal geometry, the observed ferromagnetic behavior is
therefore not unexpected.
g = 2.135(2). Therefore, the antiferromagnetic interaction medi-
ated by the bridging tetrazolate ligand is very weak.
The magnetic susceptibility of the [Cu(BTA)(en)]n (4) complex
has been measured over the temperature range of 300–2 K. The
3.3. DFT calculations
temperature dependence of
vMT (vM being the susceptibility for
copper(II) atom) is shown in Fig. 7.
DFT calculations have been performed on complexes 3 and 4 in
order to evaluate the corresponding theoretical J values. For com-
pound 3, just the dimer 3-calc. (Fig. 8) has been initially considered,
for which it has been determined a J value of +4.8 cmꢀ1, which dif-
The
v
MT product at room temperature of 0.445 cm3 K molꢀ1 is
slightly larger than the spin-only value of 0.37 cm3 K molꢀ1 ex-
pected for a single copper(II) ion (S = 1/2), assuming g = 2.00. Upon
cooling down
vMT increases continuously from room temperature
fers in sign from the experimental value of ꢀ1.13 cmꢀ1
.
until reaching a value of 0.51 cm3 K molꢀ1 at 2 K. This behavior
support the existence of a weak ferromagnetic interaction between
the paramagnetic centres. To estimate the magnitude of the ferro-
magnetic coupling, the magnetic data were fitted using the expres-
sion proposed by Baker et al. for a ferromagnetic chain with S = 1/2
[23]
When intermolecular interactions through hydrogen bonds
(involving coordinated water molecules) are, however, taken into
account by calculating the dimer 3b-calc. (Fig. 8), the J value dimin-
ishes to +3.9 cmꢀ1, and an inter-dinuclear Jinter coupling constant of
ꢀ0.10 cmꢀ1 can be found. Since many intermolecular interactions
can be established by a single dinuclear unit, it would be expected
that these interactions were responsible for the very weak antifer-
romagnetic behavior of 3. It should be noted at this point that in
the case of molecules formed by fragments with well localized
charges (for instance, the BTA anion of 3) there exist some prob-
lems for the theoretical calculation of the energy, and therefore
for the evaluation of J. Because of this, the calculated J value could
actually be much smaller than +3.9 cmꢀ1. If so, the set of weak
interdinuclear AF magnetic interactions transmitted by the hydro-
gen bonds pathways could overcome the interdinuclear F one and
an overall AF magnetic coupling would be observed in good agree-
ment with the experimental results. This fact is also present in
compound [Cu2(TzC)2(H2O)6] [21].
nꢀ1
X
H ¼ ꢀJ
SAiSAiþ1
i¼1
From the best fit to the following parameters J = +0.233(9) cmꢀ1
,
g = 2.180(2).
The magnetic coupling in this compound takes place through
the tetrazolate bridging ligand involving basal(short)-axial (long)
positions on neighboring copper(II) atoms. This exchange pathway
always leads to very small coupling (either antiferro- or ferromag-
netic) regardless of the structural parameters of the bridge. This is
due to the long axial Cu–N distance and to the fact that the spin
density of the unpaired electron at axial position is, if any, negligi-
ble (the magnetic orbitals on the Cu(II) atoms of the dx2 ꢀ y2 type
are mainly located in the equatorial plane and directed to the
nitrogen atom of the ethylenediamine and tetrazolate ligands). In
The J values can be expressed as the sum of the ferro- and anti-
ferromagnetic contributions, JF and JAF, respectively [24]. When the
AF contribution is smaller than the F one, a ferromagnetic coupling
can be achieved. Given that the AF contribution can be expressed
Fig. 9. Symmetric and antisymmetric SOMOs calculated for 3-calc.

A. Rodríguez-Diéguez et al. / Inorganica Chimica Acta 385 (2012) 73–80
79
Fig. 10. The fragments calculated for complexes 4 and 5, showing the following J scheme.
as a function of the square of the SOMOs gap (
D
), the calculated J
toward the neighbor nitrogen atoms. The calculated spin densities
on the copper centres are +0.6038, +0.6262, +0.6222 and +0.6404
for 4-calc., and +0.6499, –0.6442, +0.6423 and ꢀ0.6328 for 5-calc.
(in both cases, on going from the left to the right in Fig. 10 or
Fig. SI5).
value would point to a low value for the SOMOs splitting. In effect,
the energy difference between both single-occupied orbitals in
3-calc. (Fig. 9) is only 852 cmꢀ1 (about 0.1 eV), just on the lower-
limit for an antiferromagnetic coupling, following the criterion of
Hay et al. for dicopper(II) complexes [24,25] and, therefore, a weak
ferromagnetic interaction could appear, supporting our theoretical
findings.
4. Conclusions
The corresponding calculated spin densities for 3-calc. and 3b-
calc. are depicted in Fig. SI4, showing that the main spin density
is, as expected, located at the dx2 ꢀ y2 copper(II) orbitals (+0.6737
e in 3-calc., and |0.6784| and |0.6709| e for each dinuclear unit in
We present the syntheses and structures of four novel cop-
per(II) complexes containing the bis(5-tetrazolyl)amine ligand
(H2BTA), which show a wide range of structural diversity. More-
over, studies about magnetic properties and DFT calculations of
two of these complexes have been performed. Dinuclear and chain
coordination complexes exhibit antiferromagnetic and very weak
ferromagnetic interactions, respectively. We have used DFT calcu-
lations to theoretically determine the sign and value of magnetic
couplings exhibited in these compounds. The magnetic exchange
couplings for 4 and the analogous complex [Cu(BTA)(NH3)2]n (that
exhibits a similar structure and weak AF interaction between the
copper(II) ions) have been compared and the differences in mag-
netic behavior between these compounds have been ascribed to
different degree of distortion of their respective CuN5 coordination
environments.
3b-calc.) accompanied of a significant
r-type spin density delocal-
isation toward the neighbor nitrogen atoms.
For complex 4, a quintuplet ground state with a J value of
+2.0 cmꢀ1 has been calculated on considering the fragment 4-calc.
(Fig. 10) formed by four copper(II) centers. This value has been
evaluated for the two central copper atoms, which are the more
chain-like owing to their inner positions. The two peripheral cou-
pling constants of the model, namely J0 and J00 (Fig. 10) are +1.0
and +2.3, respectively. All these values are in perfect agreement
with the experimental one (+0.23 cmꢀ1). As indicated above, these
results are quite in line with those obtained for the chain [Cu(B-
TA)(NH3)2]n (5), 7 which experimentally showed a weak antiferro-
magnetic coupling (J = ꢀ7.82 cmꢀ1
)
based on the same
Acknowledgments
considerations of orthogonality of the single occupied copper(II)
d orbitals.
This work was supported by the MEC (Spain; Project CTQ-2008-
02269/BQU), the Junta de Andalucía (FQM-4228). We would like to
thank the Centro de Supercomputación de la Universidad de Gra-
nada (UGRGRID) for computational resources.
However, this value seems to be too high from the calculations
carried out by us. Thus, taking as model the tetranuclear unit 5-
calc. (Fig. 10), which corresponds with the same fragment as for
compound 4, we have calculated a J value of ꢀ0.6 cmꢀ1 for the
two inner copper centers, and peripheral coupling constants J0
and J00 of ꢀ0.9 and ꢀ0.2 cmꢀ1. Therefore, calculations demonstrate
that in both cases the coupling constants are close to zero, since the
exchange pathways are poor mediator of the magnetic exchange
interaction. The difference in sign between the two BTA-based
chains, 4 and 5, could be due to the different distortion of their
respective CuN5 coordination environments. Thus while 4 exhibits
an almost perfect square-pyramid coordination geometry, com-
pound 5 has a geometry that is intermediate between square-pyra-
Appendix A. Supplementary data
Supplementary material CCDC 790132–790135 contains the
supplementary crystallographic data for the complexes 1–4. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via 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.2011.12.040.
midal and trigonal bipyramidal (s = 0.36). Since for this latter
geometry the magnetic orbital is of the dz2 type, which has 2/3
of its spin density directed to axial positions, there will be some de-
gree of overlapping between the magnetic orbitals on neighboring
copper atoms through the tetrazole bridging group. This overlap
would lead to a significant antiferromagnetic contribution that
would predominate over the ferromagnetic one and, therefore, a
overall antiferromagnetic coupling, even though small, would be
observed.
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