H-bond assisted coordination bond formation in the 1D chains based on azido and phenoxido bridged tetranuclear Cu(II) complexes with reduced Schiff base ligands

Article abstract of DOI:10.1016/j.poly.2017.12.022

Two new 1D chains [(CuL^1R)₂Cu₂(N₃)₂(μ_1,1,3-N₃)₂]_n (1) and [{(CuL^2R)₂Cu₂(N₃)₂(μ_1,1,3-N₃)₂}·(CH₃)₂CO]_n (2) based on rare μ_1,1,3-N₃ bridged tetranuclear Cu(II) complexes, have been synthesized using [CuL^1R] and [CuL^2R] as “metalloligands” [where H₂L^1R = N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine and H₂L^2R = N,N′-bis(2-hydroxybenzyl)-1,2-ethylenediamine]. Both complexes have been characterized by elemental analysis, spectroscopic methods, single crystal XRD, and magnetic study. In case of chain 1, the basic building block is a centrosymmetric tetranuclear unit whereas for 2, it is an asymmetric tetranuclear unit containing two types of square pyramidal Cu(II) centers (terminal and central). The μ_1,1-N₃ bridged central copper atoms of one tetranuclear unit are connected weakly to the axial position of the terminal copper atoms of neibouring units via the azide ions forming a rare μ_1,1,3-N₃ bridged novel 1D polymeric chain structure. Variable temperature magnetic susceptibility measurements show the presence of an overall strong antiferromagnetic exchange interactions mediated through the double phenoxido bridges with J values of ?123.8 and ?144.6 cm^?1 for 1 and 2, respectively.

Full text of DOI:10.1016/j.poly.2017.12.022

Accepted Manuscript
H-bond assisted coordination bond formation in the 1D chains based on azido
and phenoxido bridged tetranuclear Cu(II) complexes with reduced Schiff base
ligands
Alokesh Hazari, Carmen Diaz, Ashutosh Ghosh
PII:
S0277-5387(17)30805-7
https://doi.org/10.1016/j.poly.2017.12.022
POLY 12979
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
11 August 2017
16 December 2017
Please cite this article as: A. Hazari, C. Diaz, A. Ghosh, H-bond assisted coordination bond formation in the 1D
chains based on azido and phenoxido bridged tetranuclear Cu(II) complexes with reduced Schiff base ligands,
Polyhedron (2017), doi: https://doi.org/10.1016/j.poly.2017.12.022
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H-bond assisted coordination bond formation in the 1D chains based on
azido and phenoxido bridged tetranuclear Cu(II) complexes with reduced
Schiff base ligands
a,b
Alokesh Hazari, Carmen Diaz , Ashutosh Ghosh *
c
a
aDepartment of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata-700
0
09, India. E-mail: ghosh_59@yahoo.com
bDepartment of Chemistry, Government General Degree College Singur, Jalaghata, Hooghly-712 409, India
cDepartament de Química Inorgànica, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11,
0
8028 Barcelona, Spain. E-mail: carme.diaz@qi.ub.es
ABSTRACT
Two new 1D chains [(CuL )
}·(CH CO]
1
R
2R
2
Cu
2
(N
3
)
2
(µ1,1,3-N
3
)
2
]
n
(1) and [{(CuL )
2
Cu
2
(N
3
) (µ1,1,3-
2
N
3
)
2
3
)
2
n
(2) based on rare µ1,1,3−N
3
bridged tetranuclear Cu(II) complexes, have been
1
R
2R
1R
synthesized using [CuL ] and [CuL ] as “metalloligands” [where H
2
L
= N,N′-bis(2-
2
R
hydroxybenzyl)-1,3-propanediamine and H
2
L
= N,N′-bis(2-hydroxybenzyl)-1,2-ethylenediamine].
Both complexes have been characterized by elemental analysis, spectroscopic methods, single crystal
XRD, and magnetic study. In case of chain 1, the basic building block is a centrosymmetric
tetranuclear unit whereas for 2, it is an asymmetric tetranuclear unit containing two types of square
pyramidal Cu(II) centres (terminal and central). The µ1,1-N
tetranuclear unit are connected weakly to the axial position of the terminal copper atoms of neibouring
units via the azide ions forming a rare µ1,1,3-N bridged novel 1D polymeric chain structure. Variable
3
bridged central copper atoms of one
3
temperature magnetic susceptibility measurements show the presence of an overall strong
antiferromagnetic exchange interactions mediated through the double phenoxido bridges with J values
-1
of -123.8 and -144.6 cm for 1 and 2, respectively.
Keywords: Reduced Schiff base; Cu(II); 1D Chain; H-bonding interaction; Antiferromagnetic
1
. Introduction
Since the last few decades, there is a growing interest in the synthesis, structural
characterization, and magnetic properties of polynuclear spin-coupled paramagnetic
transition metal complexes exhibiting ferro- and antiferromagnetic spin-exchange interactions
[
1]. There are various strategies to synthesize such polynuclear complexes; one of them is to
use the neutral chelate complex of a divalent transition metal ion with different N donor
2
O
2
tetradentate di-Schiff base ligands (e.g. salen = N,N'-ethylenebis(salicylideneimine), salpn =
N,N′-bis(salicylidene)-1,3-propanediamineetc...) [2]. The phenoxido oxygen atoms of the
1

coordinated Schiff base are capable of coordinating another transition metal ion to form
multinuclear complexes in which these chelates act as ‘‘metalloligand’’. Several homo- and
hetero-metallic complexes have been synthesized using this ‘‘metalloligands’’ synthetic
approach by different groups including ours [3]. Among the homo-nuclear Cu(II) complexes,
various species ranging from discrete dinuclear to hexanuclear as well as polymer are
reported with a number of anionic bridges e.g. cyanide, thiocyanate, isocyanate, dicyanamide,
azide, carboxylate, etc...[4]. Most of these bridges can mediate strong magnetic exchange
between the metal ions. Recently, we have found that complexes of reduced Schiff bases can
also be used as “metalloligands” and in some cases the resulting species showed interesting
structural variations compared to their unreduced analogues [5].
A special attention has been paid into the study of azido-bridged metal complexes in
the field of molecular magnetism [6]. The motivation in synthesizing new metal-azido
complexes comes from the versatility of bonding modes of this triatomic pseudohalide (e.g.
µ
1,3-N
3
(end-to-end), µ1,1-N
If we consider only the Cu(II) systems, there are a total of 822 Cu-complexes with terminal
azido ligands. The numbers of end-on (µ1,1-N ) and end-to-end (µ1,3-N ) bridged Cu(II) are
51 and 102, respectively. The complexes with other bridging modes are relatively less. For
examples, there are only 13 and 6 Cu(II) complexes with µ1,1,1-N bridges. All
3
(end-on), µ1,1,3-N
3
, µ1,1,1-N
3
, and also monodentate fashion) [6-7].
3
3
3
3
and µ1,1,3-N
3
these different bridging modes make azido bridge complexes as one of the most fascinating
subjects for studying structural variations and super-exchange pathways of magnetic
interactions [7]. In fact, more than one of these bonding modes often co-exists in the same
complex resulting discrete polynuclear complexes, one-dimensional (1D) chains, two-
dimensional (2D) layers and three-dimensional (3D) frameworks [8-10]. The primary ligands
which form the complex along with the azide ion are very important in determining the
2 2
nuclearity and connectivity of the structures. If we consider the N O donor salen type Schiff
base ligand, there are only nine azido bridged Cu(II) complexes and interestingly all of them
possess very similar discrete tetranuclear structure [11-16]. We are interested in synthesizing
2 2
similar complexes using N O donor reduced Schiff base ligands in order to see if there is any
variation in the structures and magnetic properties.
Herein, we report the syntheses, structure and magnetic properties of two novel azido
1
R
2R
bridged Cu(II) 1D chains, [(CuL )
2
2 3 2 3 2 n 2 2 3 2
Cu (N ) (µ1,1,3-N ) ] (1) and [{(CuL ) Cu (N ) (µ1,1,3-
1
R
N
3
)
2
}·(CH
3
)
2
CO]
n
(2), where H
2
L
(N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine) and
2

2
R
H
2
L (N,N′-bis(2-hydroxybenzyl)-1,2-ethylenediamine) are reduced salen-type Schiff bases.
In both structures, µ1,1-N and phenoxido bridged tetranuclear units are connected through
azide ion (µ1,1,3-N ) to form polymeric 1D chains. This type of chain is unprecedented with
donor ligands. Both chains exhibit overall antiferromagnetic exchange
interactions within the basic structural units.
3
3
2 2
salen type N O
2
. Experimental
2
.1. Starting Materials
The salicylaldehyde, 1,2-ethelenediamine, 1,3-propanediamine and sodium borohydride
were purchased from Lancaster and were of reagent grade. They were used without further
purification.
Caution! Azide salts and Perchlorate salts of metal complexes with organic ligands is
potentially explosive. Only a small amount of material should be prepared and it should be
handled with care.
2
.2. Synthesis of the reduced Schiff base ligand, N,N'-bis(2-hydroxybenzyl)-1,3-
1
R
1R
2
propanediamine (H L ) and “metalloligand” [CuL ]
The di-Schiff base ligand from 1,3-propanediamine and salicyldehyde was synthesized by a
reported method [17]. 5 mmol of 1,3-propanediamine (0.42 mL) was mixed with 10 mmol of
the salicylaldehyde (1.04 mL) in methanol (30 mL). The resulting solution was refluxed for
ca. 2 h and allowed to cool. Then 30mL (5 mmol) of this prepared yellow methanolic ligand
1
solution (H
2
L ) was cooled to 0°C, and solid sodium borohydride (570 mg, 15 mmol) was
added to this methanol solution with stirring. After completion of addition, the resulting
solution was acidified with concentrated HCl (12 mL) and then evaporated to dryness [18].
1
R
The reduced Schiff base ligand H
2
L
was extracted from the solid mass with methanol, and
this colourless methanol solution (ca. 30 mL) was added to a methanolic solution (10mL) of
1
R
4 2 2
Cu(ClO ) ·6H O (1.852 g, 5 mmol) to prepare the “metalloligand” [CuL ] [19].
2
.3. Synthesis of the reduced Schiff base ligand, N,N'-bis(2-hydroxybenzyl)-1,2-
2
R
2R
2
ethelenediamine (H L ) and “metalloligand” [CuL ]
3

2
R
The reduced di-Schiff base ligand (H
described above for N,N'-bis(2-hydroxybenzyl)-1,3-propanediamine (H
ethelenediamine instead of 1,3-propanediamine. The colourless methanol solution (ca. 30
2
L ) was synthesized by the same procedure as
1
R
2
L ) using 1,2-
2
R
2R
mL) of H
2
L
was used for the preparation of a “metalloligand” (CuL ) following a
1
R
procedure similar to that for [CuL ].
1
R
[(CuL )
2
.4.
Syntheses
of
the
complexes
CO] (2)
2
Cu
2
(N
3
)
2
(µ1,1,3-N
3
)
2
]
n
(1)
and
2
{(CuL )
R
[
2
Cu
2
(N
3
)
2
(µ1,1,3-N
3
)
2
}·(CH
3
)
2
n
1
R
The precursor complex [CuL ] (0.348 g, 1 mmol) was dissolved in methanol (20 mL) and
then a water solution (2 mL) of Cu(ClO ·6H O (0.370 g, 1 mmol), followed by an aqueous
4
)
2
2
solution (1 mL) of sodium azide (0.130 g, 2 mmol), was added to this solution. The mixture
was stirred for 1 h and then filtered. The filtrate was allowed to stand over night when prism
shaped dark red coloured X-ray quality single crystals of 1 appeared at the bottom of the
beaker. Similarly, needle shaped single crystals of 2 were obtained by following the same
2
R
1R
procedure for 1, but using the “metalloligand” (CuL ) the instead of [CuL ]. The crystals (1
and 2) were re-crystallised from acetone-water mixture and dried in a desiccator containing
anhydrous CaCl
X-ray diffraction.
Complex 1. Yield: 0.366 g, 74%. Anal. Calc. [C34
found: C 41.32; H 4.25; N 22.85%; UV/vis: [λmax in nm (solid, reflectance)] = 645 and 398.
2
and then characterized by elemental analysis, spectroscopic methods, and
H40Cu
4
N
16
4
O ]: C 41.21; H 4.07; N 22.61;
−1
IR (KBr) in cm : ν(N-H) 3226, ν(N
Complex 2. Yield: 0.356 g, 68%. Anal. Calc. [C35
found: C 47.26, H 4.23, N 21.92%; UV/vis: [λmaxin nm (solid, reflectance)] = 541 and 399. IR
3
) 2069 and 2039.
4 16 5
H42Cu N O ]: C 47.17; H 4.15; N 21.95;
−1
(
3
KBr) in cm : ν(N-H) 3239, ν(N ) 2065 and 2039.
2
.5. Physical Measurements
Elemental analyses (C, H, and N) were performed using a Perkin-Elmer 2400 series II CHN
−1
analyzer. IR spectra in KBr pellets (4000−500 cm ) were recorded using a Perkin- Elmer
RXI FT-IR spectrophotometer. Electronic spectra in solid state (1000-300 nm) were recorded
in a Hitachi U-3501 spectrophotometer. The magnetic measurements were carried out in the
‘
‘Servei de Magnetoquímica (Universitat de Barcelona)” on polycrystalline samples with a
4

Quantum Design SQUID MPMSXL magnetometer in the temperature range of 2-300 K. The
experimental magnetic susceptibility data are correctedfor the diamagnetism estimate from
Pascal’s tables.
2
.6. Crystallographic data collection and refinement
Well formed single crystals of each complex was mounted on a Bruker-AXS SMART APEX
II diffractometer equipped with a graphite monochromator and Mo Kα (λ = 0.71073 Å)
radiation. The crystals were positioned 60 mm from the CCD, and frames (360) were
measured with a counting time of 5 s. The structures were solved using the Patterson method
through the SHELXS 97 program. Non hydrogen atoms were refined with independent
anisotropic displacement parameters, while difference Fourier synthesis and least-squares
refinement showed the positions of any remaining non-hydrogenatoms. The non-hydrogen
atoms were refined with anisotropic thermal parameters. The hydrogen atoms bound to
carbon atoms were included in geometric positions and given thermal parameters equivalent
to 1.2 (or 1.5 for methyl groups) times those of the atom to which they were attached.
Hydrogen atoms that bonded to N or O were located in a difference Fourier map and refined
with distance constraints. We squeezed the disorder solvent molecule acetone present in
complex 2. Hope, this revised cif is now of publication standard.Successful convergence was
indicated by the maximum shift/error of 0.001 for the last cycle of the least-squares
refinement. Absorption corrections were carried out using the SADABS program [20], while
all calculations were made via SHELXS 97 [21], SHELXL 97 [22], PLATON 99 [23],
ORTEP-32 [24], WINGX system ver-1.64 [25] and refined using SHELXL-2014 [26]. Data
collection, structure refinement parameters, and crystallographic data for the two complexes
are given in Table 1.
3
. Results and Discussion
3
.1. Syntheses of the Complexes
1
R
2R
1R
The Cu(II) complexes, [CuL ] and [CuL ] of the reduced Schiff-base ligands (H
2
L
and
2
R
1R
H L ) were synthesized using the reported procedures. Both the “metalloligands”, [CuL ]
2
2
R
and [CuL ], on reaction with copper perchlorate hexahydrate and sodium azide in 1 : 1 : 2
molar ratio in a MeOH-H
2
O medium (10 : 1, v/v) resulted in two new 1D Cu-azide chain
5

1
R
2R
complexes,
}·(CH
[(CuL )
2
Cu
2
(N
3
)
2
(µ1,1,3-N
3
)
2
]
n
(1)
and
2 2 3 2
[{(CuL ) Cu (N ) (µ1,1,3-
N
3
)
2
3
)
2
CO] (2) (Scheme 1)
n
Scheme 1. Syntheses of complexes 1 and 2.
3
.2. IR and UV−Vis Spectra
Besides elemental analysis, two complexes were initially characterized by the IR spectra. A
−1
moderately strong and sharp peak at 3226 and 3239 cm (due to a N−H stretching vibration)
−1
for complexes 1 and 2, respectively and absence of any peak at around 1620 cm , (Figs. S1
and S2) indicate that the imine group of the Schiff base is reduced. In addition, the presence
of azido ligands in both complexes is confirmed by the appearance of strong and sharp peaks
−1
−1
at 2069 and 2065 cm along with shoulders at 2039 cm in the spectra of 1 and 2
respectively (Figs. S1 and S2). The splitting of the band is indicative of the presence of two
different coordinated azido ions in agreement with their crystal structures.
The electronic spectra of these two compounds were recorded in the solid state (Fig. S3).
Compounds 1 and 2 show broad absorption bands at 645 and 541 nm, respectively, while the
6

1
R
2R
band maxima for “metalloligands” [CuL ] and [CuL ] appear at 697 and 593 nm,
respectively. These bands are attributed to d–d transitions of Cu(II) ions. In addition, the
these two compounds show a sharp single absorption maximum near 398 and 399 nm for 1
1
R
2R
and 2, respectively, and the two “metalloligands” [CuL ] and [CuL ] show absorption
maxima at 401 and 394 nm, respectively, attributed to ligand-to-metal charge transfer
transitions.
3
3
.3. Description of the Structures
.3.1. Complex 1
The X-ray crystal structure shows that complex 1 is a rare
µ
1,1,3-N
3
bridged 1D
(N ) (µ1,1,3-
1
R
polymeric chain (Fig. 1) based on centrosymmetric tetranuclear [(CuL )
] unit. The basic tetranuclear unit is depicted in Fig. 2.
2
Cu
2
3
2
3 2
N )
Fig. 1. 1D supramolecular chain structure of complex 1. Chain formation of 1 is facilitated by N22−H22---N1,
H-bonding interactions. Other H-atoms are removed for clarity.
Selected bond lengths and angles of the repeating tetranuclear basic unit of the chain are
listed in Table 2. In this basic unit, the terminal copper centre [Cu(1)] has a distorted square
pyramidal geometry. The equatorial plane is formed by the two imine N-atoms and two
phenoxido O-atoms. The basal Cu−N bond distances are greater than the basal Cu−O
distances (Table 2). The axial position is weakly occupied by a nitrogen atom of a triply
bridging azido-anion (µ1,1,3-N ) from a neighboring unit at a distance of 2.592(7) Å. The
3
range of cis [81.7(2)−106.0(2)°] and trans angles [162.8(2)−171.5(2)°] (Table 2) around this
Cu center are indicative of the deviation from the ideal square pyramidal geometry. The
deviation of this geometry from square pyramid to trigonal bipyramid has been calculated by
the Addison parameter (τ) [27]. The value of τ is defined as the difference between the two
largest donor-metal-donor angles divided by 60, τ is 0 for the ideal square pyramid and 1 for
the trigonal bipyramid. The τ value of Cu(1) is 0.145, indicating that the geometry is slightly
7

distorted from its ideal square pyramidal geometry. The distortion around Cu(1) is also
apparent from the dihedral angle (17.1(1)°) between the two N–Cu–O planes and the r.m.s.
deviation (0.187Å) of the four equatorial donor atoms from their mean coordination plane.
Fig. 2. Ortep structure of the tetranuclear basic unit of chain complex 1 with ellipsoids at 30% probability.
a
b
c
Symmetry element = x,-1+y,z; = x,1+y,z; = 1-x,-y,1-z.
The central copper atom [Cu(2)] also presents a distorted square pyramidal geometry
like the terminal one. Here, the equatorial plane is formed by the two N atoms from µ-1,1,3
bridging azides, one N-atom of terminally coordinated azido ligand and one µ
2
−phenoxido
O˗atom; the axial position is occupied by another µ -phenoxido O−atom of tetradentate
2
reduced Schiff-base ligand. The equatorial Cu−N/O bond distances are smaller than the axial
Cu−O bond distance (Table 2). The range of cis [75.1(2)−108.9(2)°] and trans angles
[
158.2(2)−170.3(2)°] around central Cu atoms indicates that it suffers a distortion from the
ideal square pyramidal geometry like the terminal ones. The distortion from the ideal square
pyramidal geometry of Cu(2) is also apparent from the Addison parameter (τ = 0.202), the
r.m.s. deviation (0.152 Å) of the four equatorial donor atoms from their mean coordination
c
plane and the dihedral angle between O30−Cu2−N4 and N1−Cu2−N4 planes (20.8(1)°).
Along with the µ-1,1,3 azido bridge, the tetranuclear units are also connected by the strong
intermolecular H-bonding interaction (see Table 3) between one of the H-atom of reduced
imine moiety of one unit and the coordinated N-atom of the terminal azido ligand of another
unit (Fig. 1) reinforcing the formation of the 1D chain.
3
.3.2. Complex 2
Complex 2 having molecular formula [{(CuL )
2
R
2
2 3 2 3 2 3 2 n
Cu (N ) (µ1,1,3-N ) }·(CH ) CO] , also
possesses a polymeric chain structure (Fig. 3) in which the central copper atoms of one
8

tetranuclear unit (Fig. 4) are connected weakly to the terminal copper atom of another unit by
bridge like 1. However, unlike 1 the tetranuclear unit is asymmetric in 2.
µ
1,1,3-N
3
Fig. 3. 1D supramolcular chain structure of complex 2. H-bondings interactions (N1---H18−N18 and N10---
H39−N39) reinforce in the formation of 1D chain. Other H-atoms are removed for clarity.
The selected bond parameters of the basic tetranuclear unit are summarized in Table 2. The
terminal copper centres have the distorted square pyramidal geometry like 1. The basal plane
of each copper atom is formed by the two imine N-atoms and two phenoxido O-atoms from
2
R 2-
dianionic tetradentate reduced Schiff-base ligand [(L ) ] and the axial position is weakly
coordinated by a nitrogen atom of a bridged azido-anion (µ1,1,3-N ) at a distance,
3
Cu(1)/Cu(4)−N(9)/N(6) 2.779(9) Å / 2.782(9) Å which is slightly greater than that in
complex 1. The basal Cu−N bonds are larger than the Cu˗O ones like 1 (Table 2). The range
of cis [84.9(2)−94.8(4)° for Cu(1) and 84.1(2)−95.3(3)° for Cu(4)] and trans angles
[
174.6(4)−176.3(4)° for Cu(1) and 173.4(3)−177.3(3)° for Cu(4)] around these Cu centers,
the Addison parameters (τ = 0.028 and 0.065 for Cu(1) and C(4) respectively), the dihedral
angles between the two N–Cu–O planes (6.8(3) and 6.6(2)º for Cu(1) and Cu(4),
respectively) and r.m.s. deviation of the four equatorial donor atoms from their mean
coordination plane (0.015 and 0.052 Å for Cu(1) and Cu(4), respectively) of 2 show that here
the terminal metal centres are less distorted than that in 1.
9

Fig. 4. Ortep picture of the basic tetrameric building unit of chain 2 with ellipsoids at 30% probability. Acetone
solvent molecules are removed for clarity.
Both the central copper atoms [Cu(2) and Cu(3)] present a distorted square pyramidal
geometry like 1, where the equatorial plane is formed by the two N atoms from µ-1,1,3 azides,
one N˗atom of terminally coordinated azido ligand and one µ
2
˗phenoxido O˗atom; the axial
position is occupied by another µ ˗phenoxido O˗atom of tetradentate reduced Schiff-base
2
ligand. The axial Cu−O bond distances of each of the central Cu-atoms are considerably
longer than the equatorial Cu−O/N distances as in 1 (Table 2). The range of cis [72.7(2)−
1
08.6(3)° for Cu(2) and 75.1(2)−106.1(3)° for Cu(3)] and trans angles [160.1(3)−172.9(3)º
for Cu(2); 157.6(4)−173.8(3)º for Cu(3)], the Addison parameters, τ (0.2116 and 0.2733 for
Cu(2) and Cu(3) respectively), the r.m.s. deviations (0.130 and 0.155 Å for Cu(2) and Cu(3),
respectively) of the four equatorial donor atoms from their respective mean coordination
plane and the dihedral angles between two N–Cu–N and N−Cu−O planes (17.8(4) and
1
9.1(4)º for Cu(2) and Cu(3) respectively) indicate that these two copper centres have slightly
less distortion from the ideal square pyramidal geometry than that in 1.
Like 1, here also intermolecular H-bonding interactions (see Table 3) are observed
(
Fig. 3) between one of the H-atom of reduced nitrogen of one unit and N-atom of a terminal
azido ligand of another unit which facilitate the formation of the 1D chain.
Literature shows that nine tetranuclear Cu(II) complexes similar to the basic
tetranuclear units of the present complexes have been reported till date. It is interesting to
note that all these tetranuclear species are discrete and have been synthesized using
unreduced N
2
2
O donor Schiff base ligands. The bridging patterns in all these nine complexes
1
0

are the same: two central Cu(II) are connected with each other by double µ1,1-N
with terminal Cu(II) by double phenoxido bridge. For the synthesis of the present complexes,
we used reduced N donor Schiff base ligands. The H atom which is added to the N-atom
3
bridge and
2
O
2
on reduction of C=N moiety has the strong potential for H-bond formation. There are several
O and N atoms in the molecule which can act as acceptor for H-bond. However, the crystal
structures clearly show that H-bond is formed by the coordinated N-atom of the terminal
monodentate azide ion presumably because such orientation of the molecules facilitates the
coordination bond formation by the other terminal N-atom of the µ1,1-N
of the Cu(II). Thus, the µ1,1-N bridging mode of azide which remained unchanged in all nine
complexes of Schiff base ligands changes to µ1,1,3-N bridging mode in the present complexes
3
to the axial position
3
3
with the assistance of H-bond formation. This synergy between H-bond and coordinate bond
links the neighboring tetranuclear units by four bridges (two azido bridges and two H-
bonded) to stabilize the 1D structures of 1 and 2 (Figs. 1 and 3) .
3
.4. Magnetic properties
From structural point of view complexes, 1 and 2 are 1D coordination polymer
consisting of centrosymmetric tetranuclear units for 1 and asymmetric tetranuclear unit for 2.
The tetranuclear entities are linked through weakly coordinated asymmetric µ1,1,3-azido
bridges. In the tetranuclear units, the copper (II) ions are connected by pairs through a double
phenoxo bridge given dinuclear entities which are further connected by two µ1,1-azido bridge.
Two possible exchange pathways are present in the tetranuclear entity (Scheme 2): i) the
double phenoxo bridge in axial-axial position (J
axial-axial position (J ).
1
), and ii) the double µ1,1-azido bridge in
2
J
1
J
2
J
3
Scheme 2. Possible exchange pathways present in the tetranuclear unit.
1
1

II
A search in the CCDC data base shows that there exist nine Cu tetramers presenting similar
double µ-phenoxo and µ1,1-azido bridges [11-16]. Only four of them has been magnetically
characterized [11(a)-13].
Magnetostructural correlations in dinuclear copper(II) complexes bridged equatorially
by pairs of hydroxide or alkoxide [28,31] groups show that the major factor controlling spin
coupling between the S = 1/2 metal centres is the Cu-O(R)-Cu angle. Hatfield and Hodgson
found a linear correlation between the experimental exchange coupling constant and the
Cu−O−Cu bond angle, larger Cu−O−Cu angle than 97.6º favours large antiferromagnetic J
values [28]. Thompson et. al. found differences in µ-hydroxo, µ-alkoxo and µ-phenoxo [32],
the slope of the J vs Cu−O−Cu angle plots for the three cases are comparable, but the
absolute value of –J are larger for phenoxo than for alkoxo and this is larger than for
hydroxo. The main conclusion for phenoxo systems is that, generally, strong
antiferromagnetic exchange will dominate in these complexes. For complexes 1 and 2, the
coupling through the phenoxo groups should be antiferromagnetic having a Cu1-O-Cu2
average angle of 101.13º and 101.12° for 1 and 2, respectively.
Density functional calculations (DFT) show that the nature of coupling between two
copper ions with µ1,1-azido bridges depends on two parameters, the Cu-N-Cu angles and the
Cu-N distances [33,34]. If the Cu-N-Cu is in the 96º-104º range, the interaction is
ferromagnetic; otherwise is antiferromagnetic. The ferromagnetic coupling decreases as the
Cu-N distance increases, at distances larges than 2.05 Å the coupling becomes
antiferromagnetic. In complexes 1 and 2, the coupling through the double µ1,1-azido bridge
should be ferromagnetic having a Cu-N-Cu angles of 101.24º and of 102.38°, and Cu-N
average distances of 1.98 and 2.01 Å, for 1 and 2, respectively.
An additional exchange pathways exist between the tetranuclear entities (J ) through a
3
weakly coordinated asymmetric µ1,3-azido bridge in axial-equatorial fashion, that involved
two copper(II) ions of two neighbouring tetranuclear entities (Scheme 2), with Cu-N distance
equal to 2.592 Å for 1 and 2.779(9) Å / 2.782(9) for 2. According to the literature data
[
35,36], we can assumed, in a first approach, that the antiferromagnetic coupling through the
1,3-azido bridges are negligible as compared with the expected strong antiferromagnetic
coupling through the double phenoxo bridges, thus, the coupling scheme is reduced to the
two different coupling constants (J , J ) into the tetranuclear unit.
µ
1
2
1
2

Considering the structural parameter in the tetranuclear units, a strong antiferromagnetic
coupling (J ) through the double phenoxo bridge and a moderate ferromagnetic coupling (J
through the double µ1,1-azido bridge were expected. To study the correlation between both
exchange coupling a simulation of χ T vs T, using the PHI program [37], was performed,
value at 120 cm and the g value as 2.2 and varying the J value between 0 and
40 cm (Fig. 5).
1
2
)
M
-1
fixing the J
1
2
-1
+
1
1
1
1
0
0
0
0
0
.6
.4
.2
.0
.8
.6
.4
.2
.0
J2 = 0
J2 = +10
J2 = +20
J2 = +30
χ
J = +40
2
0
50
100 150 200 250 300
T /K
-
1
Fig. 5. Theoretical curves obtained by PHI program (see text) for different J
2
values considering J
1
= 120 cm ,
and g = 2.2.
The simulation was performed according to the following Hamiltonian:
H = -J (S +S )–J (S
T vs T with these ferromagnetic J
1
1
S
2
3
S
4
2
2 3
S )
As expected, the variation of χ
M
2
values is not noticeable.
Therefore the global exchange interaction in complexes 1 and 2 will be expected to depend
predominantly on the double µ-phenoxo bridge.
The temperature dependence of χ
copper ions) in an applied field of 10000 and 500 G in the temperature range of 2-300 K and
-30 K, respectively, is shown in Figs. 6 and 7, respectively.
M
T vs T for complexes 1 and 2 (referred to the two
2
3
-1
For complex 1, the χ
M
T value at 300 K, 0.76 cm mol K, which is slightly more than that
II
3
-1
expected for two uncoupled Cu ion with g = 2 (0.74 cm mol K). Upon cooling, the χ
M
T
3
-1
value decreases continuously, reaching a value of 0.023 cm mol K at 2 K. The χ
M
versus T
plot exhibits a maximum at 100 K (Fig. 6 inset), indicating an overall antiferromagnetic
interaction and the presence of paramagnetic impurities. Considering all the above
discussions, the experimental data were fitted to the Bleaney-Bowers expression for an
isotropically coupled pair of S = 1/2 ions [38], assuming the isotropic Hamiltonian H = -
1 1 2
J S S and introducing a ρ term to evaluate the paramagnetic impurity. The best-fit
1
3

1
parameters for reproducing satisfactorily the experimental data, as shown in fig. 6, are: J = -
-1
-5
2
2
1
23.8 cm , g =2.20, ρ= 2.6% and R = 3x10 , R = Σ
i
(χTicalc –χTiexp) /Σ
i
(χTiexp) .
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
0.008
0
0
0
0
.006
.004
.002
.000
χ
χ
0.1
.0
0
50 100 150 200 250 300
0
T / K
0
50
100 150 200 250 300
/ K
T
M
Fig. 6. Temperature dependence of χ T for complex 1. The solid lines represent the best-fit results with the
M
parameters described in the text. Inset: Temperature dependence of χ for complex 1.
3
-1
For complex 2, the χ
M
T value at 300 K, 0.72 cm mol K, which is than that expected for
II
3
-1
two uncoupled Cu ion with g = 2 (0.74 cm mol K). Upon cooling, the χ
M
T value decreases
3
-1
continuously, reaching a value of 0.012 cm mol K at 2 K. The χ
M
versus T plot exhibits a
maximum at 130 K (Fig. 7 inset),
0
0
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
.1
.0
0.006
0
.005
.004
0
0.003
χ
χ
0
.002
.001
0
0
50 100 150 200 250 300
T / K
0
50
100 150 200 250 300
T / K
Fig. 7. Temperature dependence of χ
M
T for complex 2. The solid lines represent the best-fit results, in the 80 K-
3
00 K range, with the parameters described in the text. Inset: Temperature dependence of χ
M
for complex 2.
indicating an overall antiferromagnetic interaction and the presence of paramagnetic
impurities. The X-ray experimental diffraction patterns of 2 and the calculate diffraction
patterns from the CIF file show the presence of some impurities, so the fit in the overall range
M
of χ T vs T values using the Bleaney-Bowers expression for an isotropically coupled pair of
S = 1/2 ions [38], do not fit well in the low temperature range. In order to evaluate the J
1
1
4

value, the fit was made in the 300-80 K range. The best-fit parameters for reproducing
-
1
satisfactorily the experimental data, as shown in fig. 7, are: J
1
= -144.6 cm , g = 2.18, and R
-5
2
2
=
4 x 10 , R = Σ
i
(χTicalc –χTiexp) /Σ
i
(χTiexp) . The J values of complexes 1 and 2 are
according with their analogous structural data.
4
. Conclusions
We have synthesized and characterized two new Cu(II) azide complexes with reduced
salen type Schiff base ligands in order to see if the reduction of the C=N bond has any effect
on the structures and magnetic properties of the resulting complexes. The structures of these
species are found to be unusual 1D chains based on phenoxido and azido bridged (µ1,1,3-N
tetranuclear units in contrast to the discrete phenoxido and azido bridged (µ1,1-N
3
)
)
3
tetranuclear units found in all the reported complexes of unreduced Schiff base ligands. The
H atom which is added on the N-atom of reduced Schiff base not only forms strong
intermolecular H-bonds between the tetranuclear units but also facilitates the change of most
3 3
common µ1,1-N bridging mode of azide to the very rare µ1,1,3-N in Cu(II). Hence, in the
present complexes, linking of the tetranuclear units takes place with the synergic formation of
H-bond and coordination bond to result in the 1D chain which is unprecedented in similar
complexes of unreduced Schiff base ligands. The overall magnetic coupling is dominated by
strong antiferromagnetic interaction between the double phenoxo bridged Cu(II) ions for both
the complexes (1 and 2) like the complexes of unreduced ligands, in agreement with their
phenoxido bridging angles, Addison parameters and the dihedral angle in the Cu
2
2
O core.
Acknowledgements
Crystallography was performed at the DST-FIST, India, funded Single Crystal
Diffractometer Facility at the Department of Chemistry, University of Calcutta. The authors
also thank the Department of Science and Technology (DST), New Delhi, India, for financial
support (SR/S1/IC/0034/2012)and also the Spanish Government (Grant CTQ2015-63614-P).
Appendix A. Supplementary data
CCDC 1565581-1565582 contains the supplementary crystallographic data for 1–2.
These data can be obtained free of charge via http://dx.doi.org..., or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
1
5

3
36-033; or e-mail: deposit@ ccdc.cam.ac.uk. Supplementary data associated with this article
can be found, in the online version, at http://dx.doi.org...
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Table 1
Crystal data and structure refinement of complexes 1 and 2.
1
2
Formula
M
C
34
H
990.97
40Cu
4
N
16
O
4
35 4 16 5
C H42Cu N O
1020.99
Crystal System
Space Group
a/Å
Monoclinic
P21/n
13.863(5)
Monoclinic
Pn
7.970(5)
b/Å
c/Å
α/°
β/°
γ/° 3
V/Å
Z
8.001(5)
18.718(5)
90
104.877(5)
90
13.722(5)
20.223(5)
90
91.350(5)
90
2006.6(15)
2
2211.1(17)
2
-3
D
c
/g cm
1.640
2.151
1008
0.057
8719
1.545
1.958
1041
0.055
15415
6660
-
1
µ/mm
F (000)
R(int)
Total Reflections
Unique reflections
3204
1
8

I>2σ(I)
2160
4322
a
b
R1 , wR2
0.0475, 0.1285
1.05
0.0521, 0.1157
0.98
c
2
GOF on F
R (all)
0.0822
293
0.0865
293
Temp (K)
a
b
2
2
2 2
4 ½
c
2
2 2
½
R1 = ∑||F
o
|−|F
c
||/∑|F
o
|, wR2 (F
o
) = [∑[w(F
o
− F
c
) /∑w F
o
o c
] and GOF = [∑[w(F − F ) /(Nobs– Nparams)]
Table 2
Bond distances (Å) and angles (°) for complexes 1 and 2.
1
2
1
2
Cu(1)–O(10)
Cu(1)–O(30)
Cu(1)–N(18)
Cu(1)–N(22)
Cu(1)–N(6) / N(9)
Cu(2)–O(10)
1.902(4)
1.969(4)
2.011(6)
1.987(6)
2.592(7)
1.985(6)
1.965(4)
1.997(5)
1.961(5)
2.007(5)
…
1.890(7)
1.877(6)
1.954(10)
1.970(11)
2.779(9)
2.244(13)
2.285(6)
1.972(9))
2.018(7)
…
N(1)–Cu(2)–O(10)
N(4)–Cu(2)–O(30)
N(4) –Cu(2)–O(30)
95.1(2)
170.3(2)
91.6(2)
108.9(2)
…
75.1(2)
…
93.4(2)
…
158.2(2)
78.8(2)
106.7(2)
…
93.7(3)
100.0(3)
…
95.1(3)
77.7(3)
72.7(2)
108.6(3)
160.1(3)
172.9(3)
…
c
N(4)–Cu(2)–O(10)
N(4)–Cu(2)–N(7)
O(30)–Cu(2)–O(10)
N(7)–Cu(2)–O(30)
N(1)–Cu(2)–N(4)
N(7)–Cu(2)–O(10)
b
Cu(2)–O(30)
Cu(2)–N(1)
Cu(2)–N(4)C
c
Cu(2)–N(4)
N(1)–Cu(2)–N(4)c
Cu(3)–O(31)
Cu(3)–O(51)
Cu(3)–N(4)
Cu(3)–N(7)
Cu(3)–N(10)
Cu(4)–O(51)
Cu(4)–O(31)
Cu(4)–N(39)
1.971(6)
2.247(7)
2.009(7)
2.018(8)
1.922(9)
1.894(6)
1.957(7)
1.970(7)
2.004(9)
84.9(2)
85.8(4)
94.1(3)
94.8(4)
174.6(4)
176.3(4)
…
N(4)–Cu(2)–N(4)
…
…
c
…
…
O(10)–Cu(2)–N(4)
N(7)–Cu(3)–N(4)
77.5(3)
97.5(3)
96.4(3)
103.9(3)
90.3(3)
95.3(3)
75.1(2)
173.8(3)
157.6(4)
106.1(3)
84.1(2)
87.3(3)
93.2(3)
95.3(3)
177.3(3)
173.4(3)
…
…
N(7)–Cu(3)–O(51)
N(7)–Cu(3)–O(31)
N(10)–Cu(3)–O(51)
N(10)–Cu(3)–N(4)
N(10)–Cu(3)–O(31)
O(51)–Cu(3)–O(31)
N(4)–Cu(3)–O(31)
N(7)–Cu(3)–N(10)
N(4)–Cu(3)–O(51)
O(31)–Cu(4)–O(51)
N(39)–Cu(4)–N(43)
N(39)–Cu(4)–O(31)
N(43)–Cu(4)–O(51)
N(39)–Cu(4)–O(51)
N(43)–Cu(4)–O(31)
…
…
…
…
…
…
…
...
…
…
Cu(4)–N(43)
O(10)–Cu(1)–O(30)
N(18)–Cu(1)–N(22)
N(22)–Cu(1)–O(30)
N(18)–Cu(1)–O(10)
N(18)–Cu(1)–O(30)
N(22)–Cu(1)–O(10)
O(30)–Cu(1)–N(6)
N(18)–Cu(1)–N(6)
N(22)–Cu(1)–N(6)
N(1)–Cu(2)–O(30)
81.7(2)
92.7(3)
92.4(2)
94.7(2)
162.8(2)
171.5(2)
106.0(2)
90.6(2)
86.6(2)
95.2(2)
…
…
…
…
…
…
b
…
…
b
…
…
b
…
99.7(3)
92.9(3)
N(1)–Cu(2)–N(7)
Symmetry element = x,1+y,z and = 1-x,-y,1-z for 1
…
b
c
Table 3
Hydrogen Bond parameters in Complexes 1 and 2.
Complex 1
D−H⋅⋅⋅A
D−H(Å)
H⋅⋅⋅A(Å)
D⋅⋅⋅A(Å)
∠D−H⋅⋅⋅A(°)
N(22)–H(22)⋯N(1)
0.88(7)
2.23(6)
3.04(8)
154.51(6)
Complex 2
H⋅⋅⋅A(Å)
D−H⋅⋅⋅A
D−H(Å)
0.91(8)
0.92(7)
D⋅⋅⋅A(Å)
3.12(4)
3.28(1)
∠D−H⋅⋅⋅A(°)
149.50(6)
N(39)–H(39)⋯N(10)
N(18)–H(18)⋯N(1)
2.30(2)
2.45(6)
151.88(3)
1
9

Highlights:
3
i) Intermolecular H-bonds facilitates the change of common µ1,1-N bridging mode to rare
(
µ
1,1,3-N .
3
(
(
ii) Synergic formation of H-bond and coordination bond (µ1,1,3-N
3
) to result in the 1D chains
1 and 2).
(
iii) Strong antiferromagnetic exchange interactions between two Cu(II) centers are observed
for both 1 and 2.
2
0

Graphical Abstract (Pictogram)
H-bond assisted coordination bond formation in the 1D chains based on
azido and phenoxido bridged tetranuclear Cu(II) complexes with reduced
Schiff base ligands
a,b
Alokesh Hazari, Carmen Diaz , Ashutosh Ghosh *
c
a
aDepartment of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata-700
0
09, India. E-mail: ghosh_59@yahoo.com
bDepartment of Chemistry, Government General Degree College Singur, Jalaghata, Hooghly-712409
cDepartament de Química Inorgànica, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11,
0
8028 Barcelona, Spain. E-mail: carme.diaz@qi.ub.es
Graphical Abstract (Synopsis)
H-bond assisted coordination bond formation in the 1D chains based on
azido and phenoxido bridged tetranuclear Cu(II) complexes with reduced
Schiff base ligands
a,b
Alokesh Hazari, Carmen Diaz , Ashutosh Ghosh *
c
a
aDepartment of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata-700
0
09, India. E-mail: ghosh_59@yahoo.com
bDepartment of Chemistry, Government General Degree College Singur, Jalaghata, Hooghly-712409
2
1

cDepartament de Química Inorgànica, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11,
8028 Barcelona, Spain. E-mail: carme.diaz@qi.ub.es
0
Structural analysis reveals that the synergic formation of H-bond and a very rare µ1,1,3-N
3
bridging bond in Cu(II) tetramers resulting in two unusual 1D chains. Magnetic study shows
an overall antiferromagnetic exchange interaction in both the chains.
2
2