Unusual site selection of NCS?in trinuclear complexes of Cu(II) and Ni(II) with a reduced N2O2donor Schiff base: Structural, theoretical and magnetic studies

Article abstract of DOI:10.1016/j.poly.2016.07.035

Four new trinuclear linear homo-metallic complexes [(CuL^R)₂Cu(NCS)₂]·(CH₃)₂CO (1), [(CuL^R)₂Cu(NO₃)₂] (2), [(NiL^R)₂Ni(NCS)₂·(CH₃CN)₂] (3), [(NiL^R)₂Ni(NO₃)₂·((CH₃)₂CO)₂] (4) have been synthesized using [CuL^R] and [NiL^R] as metalloligands (where H₂L^R?=?N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine) along with polyatomic anions, thiocyanato or nitrato. All four complexes have been characterized by elemental analysis, spectroscopic methods, single crystal XRD, DFT and magnetic study. In all complexes, two units of metalloligands, [CuL^R] or [NiL^R] coordinate to the central copper or nickel atom through double phenoxido bridges forming a linear trinuclear structure. In the structures of 1 and 3, the N-bonded thiocyanato ion coordinates to the terminal metal centers of the trinuclear unit to complete the distorted square pyramidal and octahedral geometry, respectively; the central metal ions remain square planar in both structures. The coordination of thiocyanato to terminal Cu(II) and Ni(II) instead of central metal ion is unusual compared to the complexes of analogous unreduced species. On the other hand, complexes 2 and 4 possess usual structures in which the anionic coligand, nitrato bridges the central and terminal metal centers. The geometries of central metal ions are octahedral in both structures and those of terminal metal ions are square pyramidal in 2 and octahedral in 4. DFT calculations show that experimentally obtained structure 1 is the most stable compared with the possible other hypothetical isomers but complex 3 is energetically slightly disfavored and possibly packing forces stabilize it in the solid state. Variable temperature magnetic susceptibility measurements show the presence of strong antiferromagnetic exchange interactions mediated through the phenoxido bridges with J values of ?328.9, ?474.8, and ?11.13?cm^?1for 1, 2, and 4, respectively. In complex 3, the central Ni(II) ion is diamagnetic, in agreement with its square planar geometry; however small antiferromagnetic interaction is found between the terminal Ni(II) ions with a J value equal to ?5.36?cm^?1.

Full text of DOI:10.1016/j.poly.2016.07.035

Accepted Manuscript
−
Unusual site selection of NCS in trinuclear complexes of Cu(II) and Ni(II)
with a reduced N O donor Schiff base: Structural, theoretical and magnetic
2
2
studies
Alokesh Hazari, Sanjib Giri, Carmen Diaz, Ashutosh Ghosh
PII:
S0277-5387(16)30332-1
DOI:
http://dx.doi.org/10.1016/j.poly.2016.07.035
POLY 12129
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
17 June 2016
20 July 2016
25 July 2016
Please cite this article as: A. Hazari, S. Giri, C. Diaz, A. Ghosh, Unusual site selection of NCS − in trinuclear
complexes of Cu(II) and Ni(II) with a reduced N O donor Schiff base: Structural, theoretical and magnetic studies,
2
2
Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.07.035
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−
Unusual site selection of NCS in trinuclear complexes of Cu(II) and Ni(II)
with a reduced N O donor Schiff base: Structural, theoretical and magnetic
2
2
studies
a,b
a
c
Alokesh Hazari, Sanjib Giri, Carmen Diaz * and Ashutosh Ghosh *
a
aDepartment of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata-700 009,
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
Abstract
Four new trinuclear linear homo-metallic complexes [(CuL )
R
2
Cu(NCS)
2
]⋅(CH
3
)
2
CO (1),
R
R
R
[
(CuL )
2
Cu(NO
3
)
2
] (2), [(NiL )
2
Ni(NCS)
R
2
⋅(CH
3
R
CN)
2
] (3), [(NiL )
2
Ni(NO
3
)
2
⋅((CH
3
)
2
CO) ] (4)
2
R
have been synthesized using [CuL ] and [NiL ] as metalloligands (where H
2
L = N,N′-bis(2-
hydroxybenzyl)-1,3-propanediamine) along with polyatomic anions, thiocyanato or nitrato. All
four complexes have been characterized by elemental analysis, spectroscopic methods, single
R
crystal XRD and magnetic study. In all complexes, two units of metalloligands, [CuL ] or
R
[
NiL ] coordinate to the central copper or nickel atom through double phenoxido bridges
forming a linear trinuclear structure. In the structures of 1 and 3, the N-bonded thiocyanato ion
coordinates to the terminal metal centers of the trinuclear unit to complete the distorted square
pyramidal and octahedral geometry, respectively; the central metal ions remain square planar in
both structures. The coordination of thiocyanato to terminal Cu(II) and Ni(II) instead of central
metal ion is unusual compared to the complexes of analogous unreduced species. On the other
hand, complexes 2 and 4 possess usual structures in which the anionic coligand, nitrato bridges
the central and terminal metal centers. The geometries of central metal ions are octahedral in
both structures and those of terminal metal ions are square pyramidal in 2 and octahedral in 4.
DFT calculations show that experimentally obtained structure 1 is the most stable compared with
the possible other hypothetical isomers but complex 3 is energetically slightly disfavoured and
possibly packing forces stabilize it in the solid state. Variable temperature magnetic
susceptibility measurements show the presence of strong antiferromagnetic exchange
interactions mediated through the phenoxido bridges with J values of -328.9, -474.8, and -11.13
-1
cm for 1, 2, and 4, respectively. In complex 3, the central Ni(II) ion is diamagnetic, in
agreement with its square planar geometry; hence its magnetic behaviour is typical of
paramagnetic Ni(II) ions.
Keywords: Homometallic complexes; Copper(II); Nickel(II); DFT study; Magnetic study.
1

1
. Introduction
The coordination chemistry of salen type di-Schiff base complexes of transition metal
ions is of continuous interest for the past several decades due to their fascinating structural
features, interesting photochemical, electrochemical and magnetic properties and various
applications covering catalysis in organic synthesis, magnetic materials in electronic devices and
model compound in important biological processes [1-6]. Among the transition metal ions, the
complexes of copper(II) and nickel(II) are arguably the most widely studied not only because of
their various applications and interesting properties [7-8] but also due to their facile synthesis
[
9]. Moreover, the neutral mononuclear complexes of these divalent metal ions can be used as
metalloligand as they are quite stable and the two oxygen atoms can coordinate to another metal
ion to produce homo- or hetero-polynuclear complexes [10]. Recently, exploiting the
coordinating ability of these mononuclear complexes [ML] [where H
2
L = N,N′-bis(salicylidene)-
or N,N′-
1
,3-propanediamine or N,N′-bis(α-methylsalicylidene)-1,3-propanediamine
bis(salicylidene)-1,4-butanediamine; M = Cu(II)/Ni(II)], we synthesized quite a few homo- and
hetero-metallic trinuclear complexes [11]. These trinuclear species are conformationally very
flexible and can exist in various shapes depending upon the coordination modes of anionic
coligands and geometry of the coordination sphere of the central metal ion [12]. For example,
when the anionic coligands bridge the central octahedral metal ion to the terminal
metalloligands, the complexes are invariably linear [13]. On the other hand, if the anionic
coligands are monodentate the molecule can be bent or linear depending upon the coordination
geometry of the central metal ion; a linear molecule is obtained if the central metal ion is square
planar [14,17b] but bent molecule is produced if it is octahedral [15]. Very recently, in order to
increase the flexibility of the chelating ligand and also to increase the potential for hydrogen
bonding ability, we used the metalloligand of reduced Schiff bases for the synthesis of hetero-
metallic trinuclear complexes [16]. Our study shows that the metalloligands of reduced Schiff
bases can produce complexes having very different shapes and bonding compared to their
unreduced analogues [16]. It has also been found that for ambivalent coordinating ligand such as
−
SCN , even the coordination mode can be changed on reduction of the Schiff base [16]. A survey
of literature shows that although a plethora of trinuclear homo-metallic complexes of Cu(II) and
Ni(II) with N
2
2
O donor Schiff bases and various anions are reported, [11] such species with
analogous reduced Schiff bases are scanty [17]. We would like to synthesize such homo-metallic
2

trinuclear complexes of Cu(II) and Ni(II) with reduced Schiff bases in order to investigate the
structural variations and coordination behaviour of the anionic coligands with respect to their
unreduced analogues.
Herein, we report the synthesis, crystal structure and magnetic properties of four linear trinuclear
R
R
complexes,
[(CuL )
2
Cu(NCS)
2
]⋅(CH
3
)
2
CO
(1),
⋅((CH
[(CuL )
2
Cu(NO
3
)
2
]
(2),
R
R
R
[
(NiL )
2
Ni(NCS)
2
⋅(CH CN)
3
2
] (3) and [(NiL )
2
Ni(NO
3
)
2
3
)
2
CO) ] (4) [where H
2
2
L = N,N′-
bis(2-hydroxybenzyl)-1,3-propanediamine] with thiocyanato or nitrato as coligands. The
complexes of Cu(II) or Ni(II) with nitrato anion are similar to those of analogous unreduced
Schiff-base ligand, in which nitrato ions form a bridge between central and terminal metal
centers. However, the thiocyanato ions which generally coordinate to the central metal atom as is
found in many such reported trinuclear complexes [11a-d] are bonded rather unusually to the
terminal metal centers in the present complexes. Although there are few reports where halide ion
like chloride [17a-d] or iodide [17g] coordinates to the terminal metal ion but for thiocyante
anion this type of coordination behaviour is very rare [11g]. A theoretical study has been
performed using DFT optimization to rationalize the structures of the experimentally obtained
complexes 1 and 3 considering the relative stability of various hypothetical isomeric compounds,
concerning to our.
2
. Experimental
2
.1. Starting Materials
The salicylaldehyde, 1,3-propanediamine and sodium borohydride were purchased from
Lancaster and were of reagent grade. They were used without further purification.
Caution! Perchlorate salts of metal complexes with organic ligands are 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-propanediamine
R
R
R
2
(H L ) and its “metalloligand” [CuL ] and [NiL ]
The di-Schiff base ligand of 1,3-propanediamine and salicyldehyde was synthesized by a
standard method [18]. 5 mmol of 1,3-propanediamine (0.42 mL) was mixed with 10 mmol of the
3

salicylaldehyde (1.04 mL) in methanol (30 mL). The resulting solution was refluxed for ca. 2 h
and allowed to cool. Then 30 mL (5 mmol) of this yellow methanolic solution of ligand (H L)
2
was cooled to 0°C, and solid sodium borohydride (456 mg, 12 mmol) was added to it with
stirring. After completion of addition, the resulting solution was acidified with concentrated HCl
(
10 mL) and then evaporated to dryness on a heating water bath [19]. The reduced Schiff base
R
ligand (H
2
L ) was extracted from the solid mass with methanol, and this colourless solution (ca.
3
0 mL) was added to a methanolic solution (10 mL) of Cu(ClO
4
)
2
·6H
2
O (1.852 g, 5 mmol) to
R
R
prepare the “metalloligand” [CuL ] as reported earlier [19]. The other metalloligand [NiL ] was
prepared by the similar method using Ni(ClO
4
)
2
·6H
2
O (1.820 g, 5 mmol) and 10 mL of
ammonia solution (25%) as reported earlier [20].
R
.3. Synthesis of the complexes, [(CuL )
R
CO(1) and[(CuL )
2
2
Cu(NCS)
2
]⋅(CH
3
)
2
2
Cu (NO
3
)
2
] (2)
R
The precursor complex [CuL ] (0.718 g, 2 mmol) was dissolved in methanol (20 mL) and then a
water solution (1 mL) of Cu(ClO
4
)
2
·6H O (0.370 g, 1 mmol), followed by an aqueous solution
2
(
1 mL) of ammonium thiocyanate (0.159 g, 2 mmol), was added to this solution. The mixture
was stirred for 1 h and then filtered. The filtrate was allowed to stand overnight when green
crystalline product found at the bottom of the vessel. The crystalline product is recrystallized
from acetone solution in a beaker when rhombic shaped bright green X-ray quality single
crystals of 1 appeared at the bottom of the beaker. Prismatic brown coloured crystals of 2 were
prepared using similar synthetic procedure as for 1 except NaNO
The crystals (1 and 2) were washed with a methanol-water mixture and dried in a desiccators
containing anhydrous CaCl and then characterized by elemental analysis, spectroscopic methods
and X-ray diffraction.
Complex 1: Yield: 0.714 g, 72%. Anal. Calc. for C42
3
was used in place of NH
4
SCN.
2
H52Cu
3
N
6
6
O S
2
: C 50.87, H 5.29, N 8.47%.
−
1
−1
Found: C 50.72, H 5.35, N 8.65%. UV/vis: [λmax in nm (εmax in M cm )] (MeCN) = 614(321),
3
89(3436), 326(3538) and 271(15000) and λmax in nm (solid, reflectance) = 615 and 405. IR
−
1
(
KBr) cm : ν(N–H) 3242, ν(SCN) 2066.
Complex 2: Yield: 0.671 g, 76%. Anal. Calc. for C34
H40Cu
3
N
6
O10: C 46.23, H 4.56, N 9.51.
−
1
−1
Found: C 46.37, H 4.45, N 9.75%; UV/vis: [λmax in nm (εmax in M cm )] (MeCN) = 593(209),
4

3
3
80(2691), 327(2934), 270(9500) and 238(21625) and λmax in nm (solid, reflectance) = 607 and
−
1
98. IR (KBr) cm : ν(N–H) 3222, ν(NO
3
) 1261, 1384 and 1487.
R
[(NiL )
2
.4.
Syntheses
of
the
CO) ] (4)
complexes
2
Ni(SCN)
2
⋅(CH
3
CN)
2
]
(3)
and
R
(NiL )
[
2
Ni(NO
3
)
2
⋅((CH
3
)
2
2
Complexes 3 (niddle shaped green coloured) and 4 (hexagonal green coloured) were prepared
using the same synthetic methods (and in the same stoichiometries) as described for 1 and 2
R
R
respectively, but using [NiL ] “metalloligand” instead of [CuL ]. The crystalline compound of 3
was recrystallized from acetonitrile solution instead of acetone.
Complex 3: Yield: 0.745 g, 79%. Anal. Calc. for C40
H
46
N
8
Ni
3
O
4
2
S : C 50.94, H 4.92, N 11.88%.
−
1
−1
Found: C 50.72, H 5.05, N 11.76%. UV/vis: [λmax in nm (εmax in M cm )] (MeCN) = 671(34),
−
1
2
80(1085), and 236(2985) and λmax in nm (solid, reflectance) = 606 and 363. IR (KBr) cm :
ν(N–H) 3259, ν(SCN) 2108.
Complex 4: Yield: 0.669 g, 68%. Anal. Calc. for C40
H
52
N
6
Ni O12: C 48.78, H 5.32, N 8.53%.
3
−
1
−1
Found: C 48.66, H 5.45, N 8.45%; UV/vis: [λmax in nm (εmax in M cm )] (MeCN) = 584(190),
−
1
2
79(12566), and 237(29532) and λmax in nm (solid, reflectance) = 598 and 390 IR (KBr) cm :
ν(N–H) 3268, ν(NO ) 1285, 1384 and 1484.
3
2
.5. Physical Measurements and crystallographic data collection and refinement
Elemental analyses (C, H, and N) were performed using a Perkin-Elmer 2400 series II CHN
−
1
analyzer. IR spectra in KBr pellets (4000−400 cm ) were recorded using a Perkin-Elmer RXI
FT-IR spectrophotometer. Electronic spectra in acetonitrile (800−200 nm) and in solid state
(
800−200 nm) were recorded by a Hitachi U-3501 spectrophotometer. The magnetic
measurements were carried out in the “Servei de Magnetoquímica (Universitat de Barcelona)” on
polycrystalline samples with a Quantum Design SQUID MPMSXL magnetometer in the
temperature range of 2-300 K. The experimental magnetic susceptibility data are corrected for
the diamagnetism estimate from Pascal’s tables.
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.
5

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-hydrogen atoms. 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. 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 [21],
while all calculations were made via SHELXS 97 [22], SHELXL 97 [23], PLATON 99 [24],
ORTEP-32 [25], and WINGX system ver-1.64 [26]. Data collection, structure refinement
parameters, and crystallographic data for the four complexes are given in Table 1.
2
.6. Theoretical methods
To study the relative stabilization energy, we used the DFT calculations with full geometry
optimization for the relevant structures. The calculations were performed at the B3LYP basis
function [27] with LANL2DZ basis [28] and core potential for metal ions (Cu or Ni) and 6–
3
1G(d) for rest of the atoms [29]. The popular polarized Pople’s basis set, 6-31G(d) with d
functions to C, N and O atoms and LANL2DZ basis set with effective core potential for metal
ions have been used for all the calculations. The coordinate of initial structures are obtained from
the modification of previously reported analogous hetero-metallic compounds [16b]. In few
cases, a pre-optimization has also been performed in order to avoid convergence failure. All the
calculations have been performed by using the Gaussian 09 program [30] and for the geometry
optimization; we did not impose any symmetry restriction. The final energies of optimized
structure were calculated with tight SCF convergence criteria.
3
. Results and discussion
6

3
.1. Syntheses of the complexes
R
R
R
The reduced Schiff-base ligand (H
2
L ) and their metalloligands, [CuL ] and [NiL ] were
R
synthesized using the reported procedures. The “metalloligand”, [CuL ] on reaction with copper
perchlorate hexahydrate and ammonium thiocyanate or sodium nitrate in 2:1:2 molar ratios in
MeOH-H
2
O medium (10:1, v/v) resulted in two new trinuclear Cu complexes,
R
R
[
(CuL )
2
Cu(SCN)
2
]⋅(CH
3
)
2
CO (1) and [(CuL )
2
Cu(NO
3
)
2
] (2), respectively. Similarly, when the
R
R
R
“
metalloligand” [NiL ] was used instead of [CuL ], complexes 3 [(NiL )
2
Ni(SCN)
2
]⋅(CH
3
CN)
2
R
and 4 [(NiL )
2
Ni(NO
3
)
2
]⋅((CH
3
)
2
CO) were produced under the similar reaction conditions as
2
for 1 and 2 (Scheme 1).
Scheme 1. Syntheses of complexes 1-4.
3
.2. IR and UV−Vis Spectra
Besides elemental analysis, all four complexes were initially characterized by the IR spectra. The
−
1
appearance of a moderately strong and sharp peak in the range of 3220−3270 cm (due to a N−H
7

−
1
stretching vibration) and the absence of any peak around 1620 cm , for all the complexes (Figs.
S1-S4) indicate that the imine group of the Schiff base is reduced [16,31]. In addition, the
presence of thiocyanato ligand in complexes 1 and 3 is confirmed by the strong and sharp peaks
−
1
−1
at 2066 and 2108 cm , respectively [32]. Peaks at 1261, 1384 and 1487 cm for complex 2 and
−
1
at 1285, 1384 and 1484 cm for complex 4 are due to nitrato anions [33].
The electronic spectra of these compounds were recorded in acetonitrile solutions (Figs. S5 and
S6) and in solid state (Fig. S7). The complexes show a broad absorption band in the visible
region at 614, 593, 671 and 584 nm in acetonitrile (Fig. S6) and 615, 607, 606 and 598 nm in the
solid state (Fig. S7) for 1-4 respectively, attributable to d−d transitions of the respective metal
ions. Moreover, all the four complexes show a sharp absorption band near 271, 270, 280 and 279
nm in acetonitrile (Fig. S5) and 405, 398, 363 and 390 nm in the solid state (Fig. S7) for 1-4,
respectively, for the ligand-to-metal charge transfer transitions.
3
3
.3. Description of Structures
.3.1. Complex 1
The
linear
trinuclear
structure
of
1
having
molecular
formula
R
[
(CuL )
2
Cu(SCN) ]⋅(CH
2
3 2
)
CO is shown in Fig. 1 together with the atomic numbering scheme.
Dimensions of the metal coordination sphere of 1 are given in Table S1 in the Supporting
Information. The structure contains a crystallographic inversion center (symmetry: −x, −y, −z)
passing through the central copper atom [Cu(2)]. Thus, the asymmetric unit consists of two
R 2-
Cu(II) centers (terminal and central), one reduced deprotonated di-Schiff base ligand [(L ) ] and
one isothiocyanato coligand. The terminal copper center has a five-coordinated distorted square
−
pyramidal geometry. The axial position is occupied by the nitrogen atom of SCN anion at a
distance of 2.161(3) Å. The basal plane is formed by the two N atoms and two phenoxido O-
R 2−
atoms from dianioinc tetradentate reduced Schiff-base ligand [(L ) ]. The basal Cu−N bond
distances are slightly greater than Cu−O distances (Table S1). The four donor atoms in the
equatorial plane show r.m.s. deviations of 0.082Å for Cu(1) with the copper atom 0.255(1) Å
shifted from the plane. The dihedral angle between two N−Cu−O planes [22.29(8)°] and the
range of cis [78.18(8)−105.65(10)°] and trans angles [159.04(8)−167.29(9)°] also indicate the
distortion from the ideal square pyramidal geometry. The distortion has been calculated by the
8

Addison parameter (τ) [34a]. 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 values of Cu(1) is 0.137, showing that the geometry is distorted slightly from
its ideal square pyramidal geometry.
The central copper atom [Cu(2)] presents a square planar geometry constructed by the
R 2−
coordination of four µ −phenoxido O atoms from two dianionic Schiff-base ligand [(L ) ]. The
2
Cu−O bond distances are considerably smaller than those around the terminal Cu (Table S1).
a
Fig. 1. The structure of 1 with ellipsoids at 30% probability. Symmetry element = −x, −y, −z.
The range of cis angle [81.64(8)−98.36(8)°] around the Cu center is indicative of non-ideal
square planar geometry. The so-called τ
tetrahedron (τ = 1) and a perfect square planar geometry (τ
α + β)]/ 141°, with α and β (in °) being the two largest angles around the central metal in the
complex [34b]. The τ value for Cu(2) is 0 as it sits on the centre of inversion.
4
index that measures the distortion between a perfect
4
4
= 0) with the formula: τ = [360° −
4
(
4
3
.3.2. Complex 2
The discrete trinuclear unit of complex 2, [(CuL )
R
2
Cu(NO ) ] is shown in Fig. 2 and the
3
2
bond parameters are given in Table S1. This structure also contains a crystallographic inversion
centre [symmetry: −x, −y, −z] at central Cu atom like 1. However, unlike 1, the central Cu has a
six-coordinated octahedral geometry being bonded to two oxygen atoms from two nitrato ligands
at axial positions and four phenoxido oxygen atoms from two dianionic tetradentate reduced
R -2
Schiff base ligands [(L ) ] at basal plane. As usual, the basal Cu(2)−O bond lengths are shorter
9

than the axial ones (Table S1). The cis angles [77.19(8)–102.81(8)°] are deviated from the ideal
value (90°). The central Cu−O distances are smaller than those of the terminal distances like 1
(
Table S1), but the differences are less compared to those in 1.
The terminal Cu atom is bonded with the two nitrogen atoms and two oxygen atoms of
R 2−
the reduced Schiff base ligand [[L ] ] and one oxygen atom from bridging nitrato ion, forming a
square pyramidal coordination environment. The Cu−O bond distances of complex 2 are smaller
than those of the complex 1, whereas the Cu−N bond lengths of 2 are similar with those of 1.
(
Table S1). The distortions from ideal square pyramidal geometry of terminal Cu is smaller
compared to that of complex 1 as is indicated by the Addison parameter (0.012), the dihedral
angle between two N−Cu−O planes [8.25(8)°], the r.m.s deviation [0.018 Å] of the four basal
atoms from their mean plane and the ranges of cis [76.43(8)– 99.36(8)°] and trans [167.34(10)–
1
68.07(9)°] angles. The Cu(1)⋅⋅⋅Cu(2) distance [3.005 Å] is slightly greater than that of complex
1
[Table S4].
a
Fig. 2. The structure of 2 with ellipsoids at 30% probability. Symmetry element = −x, −y, −z.
3
.3.3. Complex 3
R
The structure of 3, [(NiL )
2
Ni(NCS)
2
]⋅(CH
3
CN) is shown in Fig. 3 together with the
2
atomic numbering scheme. Bond lengths and angles in the metal coordination spheres are given
in Table S2. Complex 3 also contains a crystallographic inversion center (symmetry: −x, −y, −z)
at the central nickel atom [Ni(2)] like 1 and 2. The asymmetric unit of 3 consists of two Ni(II)
R
centers (terminal and central), one reduced di-Schiff base ligand [H
2
L ], one thiocyanato anion
and a coordinated acetonitrile molecule. The terminal nickel center [Ni(1)] has a distorted
10

octahedral geometry. The basal plane of terminal Ni(II) ion is formed by the two imine nitrogen
atoms and two phenoxido oxygen atoms. These four donor atoms in the basal plane show root-
mean-squared deviation from their mean plane of 0.040 Å and the metal atom is displaced 0.01
Å from the mean plane in the direction of axial coordinating acetonitrile molecule. The basal
Ni−O bond distances are slightly smaller than the Ni−N distances (TableS2). The axial positions
−
are occupied by the nitrogen atoms from a terminally N coordinated NCS anion (like 1) and a
nitrogen atom of a coordinated acetonitrile molecule. The ranges of the cis [79.47(2)–96.4(2)°]
and trans [170.3(2)–173.8(2)°] angles around the terminal nickel centre and the dihedral angle
between the two N−Ni−O planes [5.52(2)°] are the indicative of a slight deviation from the ideal
octahedral geometry. The central nickel atom [Ni(2)] presents an almost perfect (as the metal ion
sits on the center of inversion, the τ
4
value is 0) square planar geometry formed by the four
R 2−
µ −phenoxido O atoms from the two dianionic Schiff-base ligands [(L ) ]. The Ni−O bond
2
distances around central Ni atom are considerably smaller compared to those of the terminal
distances (Table S2). The range of cis angles is [85.94(2)−94.06(2)°] also pointing out the almost
perfect square planar geometry of Ni(2). The Ni(1)⋅⋅⋅Ni(2) distance is 2.932 Å.
a
Fig. 3. The structure of 3 with ellipsoids at 30% probability. Symmetry element = −x, −y, −z.
3
.3.4. Complex 4
The discrete trinuclear unit of complex 4, [(NiL )
R
2
Ni(NO
3
)
2
]⋅((CH
3
)
2
CO) is shown in
2
Fig. 4 and the bond parameters are given in Table S2. Like other three structures described
above, this structure also contains a crystallographic inversion centre at the central Ni(2) atom
11

(
symmetry: −x, −y, −z). The central Ni(2) is six-coordinated being bonded to two oxygen atoms
from two nitrato ligands, and four phenoxido oxygen atoms from the two reduced Schiff base
ligands. The four phenoxido oxygen atoms define the basal plane and the two oxygen atoms of
nitrato ions coordinate to the slightly elongated axial positions (Table S2). The range of cis
angles [80.97(8)−99.03(8)°] around the Ni center shows a slight distortion from the ideal
octahedral geometry.
The terminal nickel center is hexa-coordinated with a distorted octahedral geometry. The
basal plane is formed by the two imine N atoms and the two phenoxido O atoms, of the dianionic
2
- R
reduced tetradentate Schiff-base ligand [(L ) ] and the two axial positions are occupied by an
oxygen atom of a coordinating acetone molecule and an O-atom of a bridging nitrato ion. Ni−N
bond distances are longer than the Ni−O bond distances like complex 3 [Table S2]. The r.m.s
deviation of the four equatorial donor atoms [0.003 Å] is quite smaller than that of the complex
3
, representing a very slight distortion from the ideal octahedral geometry. The slight distortion
is also indicated by the ranges of cis [80.67(8)−94.28(11)°] and trans [172.69(10)−175.29(11)°]
angles around Ni(1) and the dihedral angle [4.42(7)°] between two N−Ni−O planes [Table S3].
The Ni(1)⋅⋅⋅⋅Ni(2) distance [3.010 Å] is slightly greater than that of complex 3 [Table S4].
a
Fig. 4. The structure of 4 with ellipsoids at 30% probability. Symmetry element = −x, −y, −z.
A literature survey for homo- and hetero-metallic trinuclear complexes of Cu(II) and
Ni(II) with salen type ligand reveals that there are plenty of structures with nitrato coligand [10a-
12

f] where it bridges the central and terminal metal centers as in the present complexes (2 and 4).
There are also quite a few complexes with monodentate anion like thiocyanato [10h,i,11a-d] and
in all the structures it binds to central metal atom unlike complexes 1 and 3. A CSD search on
reduced metalloligand containing a deprotonated tetradentate Schiff base ligand reveals that only
ten such structures with copper and seven with nickel have been reported so far. Among them, in
six chlorido [17a-d], in one iodo [17g] and in two methanol [17e] (with non-coordinated
perchlorate anions) ligands are coordinated to the terminal Cu(II) centers. In the remaining one,
acetate [17f] ion bridges the central and terminal metal centers. On the other hand, all seven
trinuclear nickel complexes are linear with bridging carboxylate ions e.g. two with formate
[
17h,i], four with acetate, [17j-m] and one with benzoate [17n]. No structures with monodenate
anions like chloride, bromide, iodide or thiocyanato etc are reported with such nickel complexes.
Therefore, we may notice that the structures 1 and 3, in the present study are rare examples
where the thiocyante ion binds to the terminal metal centers of a trinuclear structure; only one
such example (hetero-metallic one) is reported very recently with unreduced Schiff base [11g].
3
.4. DFT studies of the complexes
As discussed in the previous section, complexes 1 (Cu
3
) and 3 (Ni ), have coordinated
3
−
SCN ions at the terminal metal centers, which is quite unusual. Therefore, it is worth to study
the stabilization energy for hypothetical isomeric structures of complexes 1 and 3 taking into
−
account the versatile coordination mode of SCN as reported in the literature for analogous
complexes [16b]. To accomplish it, we adopted DFT calculations and computed the lowest
energy of concerning isomers. A series of isomeric hypothetical complexes are obtained from the
result of fully optimization process as shown in Figs. S8 and S9, in the Supporting Information.
Regarding the copper complex 1, we have found four isomeric structures (1b, 1c, 1d and 1e)
−
with a significant change in metal coordination sphere as well as SCN coordination as shown in
Fig. S8. The results show that the experimentally obtained complex 1 is the most stabilized
compared to other isomers. In case of solvated nickel complex, we have successfully optimized
two hypothetical isomers along with complex 3 as shown in Fig. S9. Interestingly, the linear
trimer with µ1,1-NCS (3g) has higher stability than the experimentally obtained complex 3.
Moreover, we have also optimized the possible nickel trimers without solvent molecule. Their
13

structures and relative energies are indicated in Fig. S9. Again, the results show that Ni-trimer
−
with SCN ions coordinated to terminal Ni centers (3j) is slightly destabilized compared with the
−
hypothetical linear Ni trimer having µ1,1-NCS ions (3i). Thus, isolation of complex 3 is unlikely
on the basis of molecular stability but it is found experimentally in the solid state, possibly due to
packing forces. In summary, the results of calculations show that for copper complexes, the most
stable isomer in which the thiocyanato ion is bonded to the terminal metal center is obtained
experimentally. Conversely, the stable isomer for nickel complex is the one in which thiocyanato
ion bridges the central and terminal metal centers via µ1,1-NCS coordination mode. The disparity
in the most stable structure for copper and nickel complex is likely due to the dissimilarity in
crystal field stabilization energy (CFSE). It is well known that the linear trinuclear complexes
which possess additional covalent bonds through bridging of anionic coligands are endowed with
an extra stability due to additional bond formation [13-15]. This assumption seems hold good in
case of nickel isomers 3 and 3g (as well as 3i and 3j). On the contrary, for the copper isomers, 1
with lesser number of covalent bonds is more stable than 1b. The less stability of 1b might be
associated with the relatively less CFSE in octahedral coordination than that in square planar
geometry of central copper ion under the defined ligand field. According to crystal field theory,
II
Cu ion can gain an extra CFSE (−1.21∆
o
+0.6∆
o
=−0.61∆ ) through the transformation of
o
octahedral to square planar geometry. Therefore, the reverse transformation (square planar to
octahedral) is unfavourable with an equivalent amount of energy. Although such transformation
would lead to additional bond formation, the bond is expected to be weak due to Jahn-Teller
distortion and thus the additional bond formation energy would not be sufficient to compensate
the loss of CFSE. As a result, square planar to octahedral transformation is energetically
unfavourable in case of copper isomers (1 to 1b).
3
.5. Magnetic Measurements of the Complexes
3
.5.1. Complexes 1 and 2
Temperature-dependence molar susceptibility measurements of powder samples 1 and 2 were
carried out in an applied field of 10000 and 500 G in the temperature range of 2-300K and 2-30
M
K, respectively. The global feature of the χ T versus T curve for complexes 1 and 2 are shown
in Figs. 5 and 6, respectively, in both cases χ
M
T being the magnetic susceptibility per Cu unit. In
3
14

complexes 1 and 2 the copper ions are bridged by a double µ-phenoxo and an additional µ-
nitrato ligand bridge the central and the terminal Cu(II) pairs in complex 2 (Scheme 2). In both
II
complexes, the trinuclear array is lineal with large distances, between the terminal Cu ions,
5
.879 Å for 1 and 6.011 Å for 2.
O31
O32
O30
O10
O10
O30
Cu2
O10
O30
Cu1
Cu1
Cu1
Cu1
O30
Cu2
O10
Complex 1
Complex 2
O32
O31
Scheme 2. Trinuclear linear structure of 1 and 2.
-1
3
-1
The χ T value at 300 K, 0.71 emu mol K for 1 and 0.52 emu mol K for 2, are significantly
M
II
-1
lower than that expected for three uncoupled Cu ion with g = 2 (1.125 emu mol K) thus
II
indicating the presence of strong antiferromagnetic interaction between the Cu ions. The χ
M
T
product shows a continuous decrease with decreasing temperature to reach a plateau around 75-
-
1
1
0
8 K for 1 (χ
M
T between 0.47 and 0.46 emu mol K) and 90-20 K for 2 (χ
1
M
T between 0.43 and
-
.44 emu mol K) which in both cases is a little above of the expected value for S = 1/2 ground
-1
state (0.375 emu mol K for g = 2). From these temperatures to 2K, χ T clearly decreases to
M
-1
-1
0
.43 emu mol K for 1 and to 0.39 emu mol K for 2. The shape of the curves from room
II
temperature to around 20 K is typical of antiferromagnetic linear trinuclear Cu complexes. The
M
reduction of the χ T value in the low-temperature region is indicative of intermolecular
interactions through hydrogen bonds and or saturation effects.
15

M
Fig. 5. Plot of the χ T vs T for complex 1, the solid line represents the best fit. Inset plot of the reduced
magnetization.
0.54
0.52
0.50
0.48
0.46
0.44
0.42
0.40
0.38
1
.2
1.0
0.8
0.6
0.4
0.2
µ
0.0
0
12000 24000 36000 48000
H / G
χ
0
50
100 150 200 250 300
T / K
M
Fig. 6. Plot of the χ T vs T for complex 2¸ the solid line represent the best fit. Inset plot of the reduced
magnetization.
In keeping with the centrosymmetric structures of 1 and 2, the spin Hamiltonian describing the
low-lying levels at zero-field can be written as:
′
ꢁ
ꢁ ꢁ
ꢁ ꢁ
ꢁ ꢁ
ꢁ
ꢁ ꢁ
ꢈ ꢈ
ꢀ
=ꢂ−ꢃ ꢄꢅ ꢅ + ꢅ ꢅ ꢆ − ꢃ ꢄꢅ ꢅ ꢆ + ꢇβꢅ ꢀ − ꢈꢃ ꢉꢅ ꢊꢅ
1
2
2
3
1
3
ꢈ
1
2
The first and second term of this Hamiltonian include isotropic nearest neighbour and next-
nearest neighbour interactions, respectively. J
1
correspond to the exchange parameter between
II
II
neighbouring Cu ions and J
2
the exchange parameter between the terminal Cu ions. The third
term corresponds to the Zeeman term, for which a single isotropic g tensor has been considered
for the two different Cu(II) ions. The fourth term takes into account intermolecular interactions
in the mean field approximation. From the above Hamiltonian, the following equations for the
thermal dependence of the magnetic susceptibility are obtained:
ꢌꢇ2β2
′
χ =ꢂ
ꢏ(ꢎ)
ꢋ
4
ꢍꢎ
16

ꢗ
1
1
+ ꢐꢑꢒꢓ(ꢃ ꢃ )/ꢍꢎ] + 10ꢂꢐꢑꢒꢓ3ꢃ /2ꢍꢎ]
ꢔꢕ ꢖ ꢔ
+ expꢂꢓ(ꢃ ꢃ )/ꢍꢎ] + 2ꢂꢐꢑꢒꢓ3ꢃ /2ꢍꢎ]
ꢔꢕ ꢖ ꢔ
ꢏ(ꢎ) =
′
ꢋ
χ
χꢋꢂ=
′
′
ꢘ1 − (2ꢈꢃ /0.2625)ꢙ χ
ꢋ
In complexes 1 and 2, where the trinuclear array is perfect linear, the dominant interaction is
II
strong antiferromagnetic. The terminal-terminal Cu distance is large, indicating that no direct
exchange mechanism is likely to contribute to the magnetic coupling, thus J can be neglected.
2
Therefore, the strong antiferromagnetic coupling is ascribed solely to superexchange
II
mechanisms within the central-terminal Cu pairs. The best set of parameters obtained for
-1
-1
complexes 1 and 2 (full lines in Figs. 5 and 6) were: J = -328.9 cm , and J′ = -0.56 cm , g =
1
-5
-1
-1
-6
2
.23 and R = 2.68 x 10 for 1; and J = -474.8 cm , J′ = -0.56 cm , g = 2.14, and R = 3.9 x 10
1
2
2
for 2 (R = Σ
i
(χTi,calc-χTi,exp) /Σ
i
(χTi,exp) ).
A further confirmation of the antiferromagnetic coupling in complexes 1 and 2 and the S = 1/2
ground state resulting from this antiferromagnetic coupling is provide by the isothermal
magnetization at 2 K (Figs. 5 and 6, inset), that show a saturation value close to 1 µ
and 0.99 µ for 2).
B
(1.06 µ for
B
1
B
In the centrosymmetric complexes 1 and 2, the oxygen atoms O(10) and O(30) of the two
phenoxo groups bridge the copper centers in equatorial-equatorial fashion (Scheme 2).
Additionally, in complex 2 two oxygen atoms, O(31) and O(32), of the nitrato ligand bridge the
copper centers in axial-axial sites, being the Cu(2)−O(32) and the Cu(1)−O(31) distances equal
to 2.663 Å and 2.353Å, respectively which are significantly longer than the normal coordination
bond. The magnetic orbital describing the single electron in the three copper ions mainly consist
2
2
of a dx -y contribution so the delocalization of the electronic density through the bridge ligands in
axial positions is poor, and extremely weak interaction should be expected from the µ-nitrato in
complex 2, so the magnetic interaction, in both complexes, occurs principally via the µ-phenoxo
bridged ligand. Although a number of linear trinuclear copper(II) complexes exists, only few
examples of structurally and magnetically characterized bis(diphenoxo)-bridged trinuclear
complexes have been reported so far [35-39, 17c,d,e,g]. According to the literature data the
exchange coupling in dinuclear complexes with Cu
2
O
2
core depends on several factors.
Magnetostructural correlations in dinuclear copper(II) complexes bridged equatorially by pairs of
17

hydroxide [40,41] or alkoxide [42,43] groups show that the major factor controlling spin
coupling between the S = ½ metal centers 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 favours large AF J values [40]. Thompson et al.
found differences in µ-hydroxo, µ-alkoxo and µ-phenoxo [44], 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 strong antiferromagnetic exchange will dominate in these complexes. However, recently
three ferromagnetic bis-phenoxo-bridged copper(II) complexes have been reported and
theoretically studied [45-47].
II
Another important structural factors in penta coordinate Cu complexes are: the Addison
parameter (τ) [34a], the increasing of τ diminished the antiferromagnetic coupling; and the
dihedral angle in the Cu
2
2
O entity, when this dihedral angle is not zero the ferromagnetic term
increases [44]. Other factors that can also influence the coupling, even though in small measure,
are: the Cu−O bridged ligand distance, the Cu⋅⋅⋅Cu separation and the geometries around the
II
copper ions and the presence of electronegative atoms directly linked to the Cu ion. Complex 1
and 2, have different structural parameters (see Table 2). This feature explains their different J
-1
values (-328.9 and -474.8 cm for 1 and 2 respectively). The parameter τ for 2 is lower (0.01)
than for complex 1 (0.14), indicating that for 2 the magnetic orbital is almost exclusively dx2-y2
perfectly directed to the Cu−O−Cu bonds. The AF character is, thus, enhance for 2. Furthermore,
the high value of the Cu−O−Cu angle of 100.74º (average) for 2 and 97.61º (average) for 1 and
relative smaller dihedral angle for 2 (21.28º) than for 1 (21.99º) also favour the more AF
character of 2.
3
.5.2. Complexes 3 and 4
Temperature-dependent molar susceptibility measurements of powder samples of complexes 3
and 4 were carried out in and applied magnetic field of 5000 and 300 G in the temperature ranges
2
-300 and 2-30 K, respectively. In the two complexes the nickel ions are bridged by a double
II
pheonoxo groups and an additional nitrato ligand bridged the central and terminal Ni ions in
complex 4. In complex 3, the central Ni(2) ion lays in a square planar geometry and the terminal
II
Ni(1) ions in a distorted octahedral geometry, while in complex 4, the three Ni ions lays in a
18

distorted octahedral geometry (Scheme 3). In both complexes, the trinuclear array is linear with
II
large distances, between the terminal Ni ions, 5.864 Å for 3 and 6.019 Å for 4.
O32
O31
O10
Ni1
O10
Ni2
Ni1
O30
O30 Ni2
Ni1
Ni1
O30
O30
O10
O10
O31
O32
Complex 3
Complex 4
Scheme 3. Trinuclear linear structure of 3 and 4.
The magnetic properties of 3 in the form of χ T vs T plots are shown in Fig. 7, these
M
II
representing the magnetic susceptibility per two Ni ions (the central one is diamagnetic). The
-1
II
χ
M
T value is 2.54 emu mol K at 300 K which is a typical value for two Ni ions with g > 2.00.
-1
χ
M
T vs T values are practically constant to 100 K and then decreases to 0.11 emu mol K at 2 K.
II
The experimental data can be fitted using the following spin Hamiltonian for dinuclear Ni ions:
2
ꢈ
1
3
ꢁ
ꢁ ꢁ
ꢁ
ꢁ
ꢁ
ꢈ
ꢀ
= −ꢃꢅ ꢅ + ꢚ(ꢂꢅ − ꢄꢅ + 1ꢆ) + ꢇꢛꢀꢅ
1
2
In which the single-ion anisotropy D and the g value are assumed to be the same for the two
II
terminal Ni ions and J the magnetic coupling between the terminal Ni(II) ions. The best set of
-1
parameters obtained for complex 3 (full line in Fig. 7) were: J = -5.36 cm , g = 2.26, |D| = 3.98
-1
-4
2
2
cm and R = 0.69x10 (R =Σ
i
(χTi,calc-χTi,exp) /Σ
i
(χTi,exp) . The magnetic susceptibility data treatment
was made using the PHI program [48]. The magnetic coupling find indicate that complex 3
shows a weak magnetic coupling between the terminal Ni(II) ions. In general, the difficulty for
calculating the D values lies in the strong correlations between the intermolecular coupling with
the zero-field splitting parameter at low temperature and such calculation usually led to
19

unrealistic values. In the present case, the reduced magnetization (M/Nµ
B
) at 2 K is involved into
the data analysis and fit very well with the model used (see inset in Fig.7).
2
2
1
1
0
0
.5
.0
.5
.0
.5
.0
1.0
0.8
0.6
0.4
0.2
0.0
0
12000 24000 36000 48000
H / G
0
50
100
150
T / K
200
250
300
M
Fig. 7. Plot of the χ T vs T for complex 3¸ the solid line represent the best fit. Inset plot of the reduced
magnetization, the sòlid line represent the best fit.
The magnetic susceptibility of 4, expressed in the form of χ
M
T vs T, where χ
Μ
is the molar
susceptibility per Ni unit and T the temperature are depicted in Fig. 8. The χ
3
M
T at 300 K is 3.58
-1
II
emu mol K, a value higher than that expected for the contribution of three non-interacting Ni
-1
S = 1 spin centers (the expected spin only value is 3.0 emu mol K, g = 2.0). From 300 to 75 K
the χ T values steadily decreases. Below this temperature χ T rapidly decreases to reach a value
Μ
Μ
-1
of 0.40 emu mol K at 2 K. The shape of the curve indicates antiferromagnetic coupling,
together with zero field splitting and or antiferromagnetic interdimer interactions. Experimental
II
data can be fitted using the following spin Hamiltonian for a linear Ni
3
:
3
3
2
ꢈꢌꢜ,ꢜ
2
3
ꢁ
ꢀ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢅ
= −ꢃꢂꢅ ꢝꢅ + ꢅꢌꢜ3ꢞ + ꢟ ꢚ ꢠꢅ
− ꢡ + ꢟꢜ=1ꢛ ꢀꢇꢌꢜ,ꢜ
ꢌꢜ2 ꢌꢜ1
ꢜ
ꢌꢜ,ꢜ
ꢜ=1
II
With an approximation that the magnetic interaction between terminal Ni ions could be
neglected, the zero-field splitting parameter D and the g values are assumed to be the same for
II
the three Ni ions. The best set of parameters obtained for complex 4 (full line in Fig. 8) were: J
-1
-1
-4
2
2
=
-11.13 cm , g = 2.22, |D| = 10.1 cm and R = 3.6x10 (R = Σ
i
(χTi,calc-χTi,exp) /Σ
i
(χTi,exp) . The
magnetic coupling find indicate that complex 4 shows a weak magnetic coupling. The magnetic
susceptibility data treatment was made using the PHI program [48]. The magnetization
measurements at 2 K corroborates the effect of the D parameter, i.e. the reduced magnetization
(
M/Nβ) at 5 T is consistent with 1.62 electrons instead of two electrons which should be the
20

theoretical value for and S = 1 ground spin state resulting of the antiferromagnetic coupling in
this compounds (see inset in Fig. 8).
4
3
3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
.5
.0
2.0
1
1
0
0
.5
.0
.5
.0
µ
χ
0
12000
24000
H / G
36000
48000
0
50
100 150 200 250 300
T / K
M
Fig. 8. Plot of the χ T vs T for complex 4¸ the solid line represent the best fit. Inset plot of the reduced
magnetization.
II
Unlike magnetostructural correlations for hydroxo and alkoxo-bridged Cu dimers are well
II
established, for analogous Ni complexes these studies are less common due to fewer numbers of
known complexes and to the large number of structural parameters that affect the exchange
II
mechanism in Ni systems. Correlations of the superexchange coupling in centrosymmetric
Ni−O−Ni dinuclear complexes made by Nanda et al. [49], and a quantum chemical ab initio
study by Wang et al. [50] show that the value of the exchange coupling constant is directly
proportional to the Ni−O−Ni bridge angle and ferromagnetic coupling takes place for angles
smaller than 97º. Later Bu et al. [51] found that this relation is not linear, and the ferromagnetic
coupling appears for angles smaller than 93.5º. The Ni−O−Ni angles observed in complex 4 are
-1
9
7.89º and 98.11º, thus the magnitude of the obtained J value of -11.13 cm between the nickel
centers is fully in the expected range. A search in the CCDC data for double phenoxo-bridge and
II
an additional nitrato-bridge Ni complexes show six related centrosymmetric linear trinuclear
complexes [52-54,10a,b,d], and two related dinuclear complexes [55,56]. Unfortunately, only
one trinuclear [54] and one dinuclear reported to date have been magnetically characterized [55].
In both cases, the Ni−O−Ni angles dominate the value of the magnetic interaction.
4
. Conclusion
21

We have synthesized and characterized four homo-metallic complexes (1−4) (two each of
copper(II) and nickel(II)) with salen type reduced Schiff base ligands. Complexes 2 (Cu ) and 4
Ni ) are usual linear trinuclear species with bridging nitrato ions while the structures of
complexes 1 (Cu ) and 3 (Ni ) in which the thiocyanato ions coordinate to the terminal metal
3
(
3
3
3
centers of the trinuclear units instead of the central one, are rather unusual compared to the
analogous complexes of unreduced salen type Schiff base ligands. Thus, the reduction of Schiff
base moiety lead to the formation of unusual structures (1 and 3) in these homo-metallic
trinuclear complexes. From DFT calculations, it is found that among the analogous isomeric
copper complexes, the experimentally found one (1) is the most stable. However, for the nickel
complexes, the stable isomer is not the experimentally isolated species (3) but the one in which
thiocyanato ion bridges the central and terminal metal centers via end-on coordination mode. The
magnetic coupling between the double phenoxo bridged Cu(II) ions is very strong
antiferromagnetic for both the complexes (1 and 2), in agreement with their phonoxo bridging
angles, Addison parameters and the dihedral angle in the Cu
2
2
O core. Among the two Ni
complexes, in complex 3 the central Ni(II) ions is diamagnetic because of its square planar
8
geometry (d configuration) and the other one (4) shows moderate antiferromagnetic coupling
between terminal and central Ni(II) centers in conformity with the phenoxo bridging angle.
Appendix A. Supplementary data
Crystallographic data have been deposited with the CCDC. The supplementary information file
contains the IR spectras, UV-Vis spectras and Optimized geometries (Figs. S1–S9) as well as
tables for bond parameters (Tables S1 and S2), dihedral angles (Table S3), metal-metal distances
(
Table S4). CCDC (1481700−1481703) contains the supplementary crystallographic data for (1–
). These data can be obtained free of charge via
4
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgements
22

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 (GrantCTQ2015-63614-P). S.G is
grateful to the UGC, for Dr. D.S. Kothari fellowship (F.4-2/2006(BSR)/CH/13-14/0069).
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Table 1
Crystal data and structure refinement of complexes 1−4.
1
2
3
4
Formula
M
Crystal System Monoclinic
C
42
H
52Cu
3
N
6
O
6
S
2
C
34
H
40Cu
3
N
6
O
10
C
40
H
46Ni
3
N
8
O
4
S
2
40 3 6 12
C H52Ni N O
991.67
883.37
Monoclinic
943.04
Monoclinic
984.95
Monoclinic
26

Space Group
P21/n
P21/c
P21/c
P21/n
a/Å
b/Å
c/Å
α/°
β/°
γ/°
12.634(5)
11.354(5)
16.449(5)
90.000(5)
104.519(5)
90.000(5)
2284.2(15)
2
9.946(5)
11.350(5)
16.011(5)
90.000(5)
101.834(5)
90.000(5)
1769.0(13)
2
11.0366(18)
14.356(2)
15.2403(17)
90.000(5)
119.262(8)
90.000(5)
2106.6(5)
2
10.091(5)
15.927(5)
13.670(5)
90.000(5)
98.995(5)
90.000(5)
2170.0(15)
2
3
V/Å
Z
-3
D
c
/g cm
1.442
1.526
1.658
1.854
1.487
1.476
1.507
1.356
-
1
µ/mm
F (000)
R(int)
Total
1026
906
980
1028
0.037
15068
0.050
19409
0.028
9590
0.039
14486
Reflections
Unique
4019
3148
3729
3758
reflections
I>2σ(I)
3187
0.0326, 0.0897
1.04
0.0466
293
2586
0.0297, 0.0737
1.03
0.0403
293
2312
0.0643, 0.2293
1.04
0.1004
296
2876
0.0362, 0.0898
1.05
0.0541
293
a
b
R1 , wR2
GOF on F
R (all)
c
2
Temp (K)
a
b
2
2
2 2
c
4 ½
c
2
2 2
½
R1 = ∑||F
o
| − |F
c
||/∑|F
o
|, wR2 (F
o
) = [∑[w(F
o
− F
) /∑w F
o
o c
] and GOF = [∑[w(F − F ) /(Nobs– Nparams)]
Table 2
Selected structural parameters for complexes 1 and 2 related to their magnetic data.
1
-328.9
0.14
2
-
1
J value [cm ]
-474.8
0.01
101.01/100.47
21.28
τ (Addison parameter)
Angles (Cu−O−Cu) [º]
Dihedral Cu O angle [º]
2 2
97.96/97.26
21.99
Average Cu-O distances [Å]
1.953
1.951
Cu⋅⋅⋅Cu distance [Å]
2.939
3.005
27

Graphical Abstract (Synopsis)
−
Unusual site selection of NCS in trinuclear complexes of Cu(II) and Ni(II)
with a reduced N O donor Schiff base: Structural, theoretical and magnetic
2
2
studies
a,b
Alokesh Hazari, Sanjib Giri, Carmen Diaz * and Ashutosh Ghosh *
a
c
a
Structural analyses and DFT studies of four new homo-metallic compounds and other related
species established that reduction of Schiff-base can alter the selection of the coordination site by
the thiocyanate ion. Magnetic studies show antiferomagnetic interaction in three complexes.
28

Research highlights:
ꢀ
ꢀ
ꢀ
ꢀ
Homo-metallic complexes of Cu(II) or Ni(II) with reduced salen-type Schiff base ligand.
All four complexes with thiocyanato or nitrato as coligand are linear trinuclear.
DFT study shows Ni(II) complex with thiocyanato anion is experimentally unlike.
Three complexes exhibit antiferromagnetic interaction and the rest one is diamagnetic.
29