Mono- and di-nuclear nickel(ii) complexes derived from NNO donor ligands: Syntheses, crystal structures and magnetic studies of dinuclear analogues

Article abstract of DOI:10.1039/c6ra02982e

The present report deals with the synthesis and structural characterisation of a mononuclear (1), a di(phenoxido)-bridged dinuclear (2) and a phenoxido/azide bridged dinuclear (3) nickel(ii) complexes derived from NNO donor Schiff base ligands. Structural

Full text of DOI:10.1039/c6ra02982e

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Monoꢀ and diꢀnuclear nickel(II) complexes derived from NNO
donor ligands: Syntheses, crystal structures and magnetic studies
of dinuclear analogues
a
b
c
Pravat Ghorai, Anindita Chakraborty, Anangamohan Panja, Tapan Kumar
a
a
Mondal, and Amrita Saha*
a
Department of Chemistry, Jadavpur University, Kolkataꢀ 700032, India
b
Molecular Materials Lab, Chemistry and Physics of Materials Unit, Jawaharlal Nehru
Centre for Advanced Scientific Research, Bangloreꢀ560064, India
c
Postgraduate Department of Chemistry, Panskura Banamali College, Panskura RS,
Purba Medinipur, West Bengal 721 152, India
*
Corresponding author
Eꢀmail: asaha@chemistry.jdvu.ac.in; amritasahachemju@gmail.com; Tel. +91ꢀ33ꢀ2457294
The present report deals with the synthesis and structural characterisations of a mononuclear
(1) and a di(phenoxido)ꢀbridged dinuclear (2) and a phenoxido/azide bridged dinuclear (3)
nickel(II) complexes derived from NNO donor Schiff base ligands. Structural studies reveal
that, in all complexes, the nickel(II) ions are hexaꢀcoordinated in a distorted octahedral
environment in which tridentate NNO ligand binds the metal centre in the meridional
configuration. The variableꢀtemperature (2–300 K) magnetic susceptibility measurements of
dinuclear analogues (2 and 3) show that the interaction between the metal centres is

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–1
–1
moderately ferromagnetic (J = 15.6 cm for 2 and J = 15.3 cm for 3). Broken symmetry
density functional calculations of exchange interaction have been performed on complexes 2
–1
and 3 and provide a good numerical estimate of J values (J = 10.31 cm for 2 and J = 17.63
–1
cm for 3) to support the experimental results. Most importantly, compound 2 is only the
second example where ferromagnetic coupling is operative in the class of di(phenoxido)ꢀ
bridged dinickel(II) complexes. The bridging Ni–O–Ni angle being close to the crossover
region would provide significant information to get better insight into the magnetoꢀstructural
correlation in these systems. On the other hand, compound 3 is an important addition to a
family of very few heteroꢀbridged (phenoxido/azide) discrete compounds of nickel(II).
Introduction
There has been extensive studies of the magnetic properties of diꢀ and polynuclear transition
1,2
metal complexes with exchangeꢀcoupled magnetic centres for the search of singleꢀmolecule
3–8
magnets (SMMs). The design of such moleculeꢀbased magnets relies on the presence of
both intraꢀ and interꢀmolecular coupling. Therefore, the most important systems to explore
fundamental aspects are discrete dinuclear exchangeꢀcoupled metal complexes because they
make possible the evaluation of the pairwise exchange interactions, knowledge of which at
least assess qualitatively the magnetic coupling in higher nuclearity systems. In fact, the most
of the experimental or theoretical magnetoꢀstructural correlations have been established from
the studies of dinuclear compounds to understand the basics of the magnetic coupling and to
9
–17
establish proper structureꢀproperty relationship.
A rational approach to synthesize discrete
exchangeꢀcoupled dinuclear metal complexes depends on synthetic strategy that involves the
use of particular transition metal ions, blocking ligands and flexidentate bridging ligands with
the objective to propagate specific magnetic interactions with interesting structures. The wellꢀ
−
−
−
−
−
−
−
known bridging anions like N
3 2 2 3 2 6 5
, NCS , N(CN) , CN ,C H O , C H O , and OH etc., are
1
8
often used to form such compounds. Among them, azido and hydroxo (phenoxide and

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alkoxide) bridges are the most versatile mediators of magnetic exchange interactions between
13,19
paramagnetic ions due to their different modes of coordination ability.
multinuclear Cu(II)ꢀcompounds featuring double
hydroxido/alkoxido/phenoxido and ꢁꢀX (X = azide, thiocyanate, cyanate, etc.) are quite
Interestingly, while
heteroꢀbridges ꢁꢀ
20
21–23
common in the literature, those of heteroꢀbridged compounds of other 3d metal ions
have been less investigated, and therefore, this area deserves more attention. Among the
transition metals, Ni(II) deserves special mention due to its large singleꢀion zeroꢀfield
splitting, and as a consequence numerous diꢀ and polynuclear Ni(II) complexes with
2
4,25
interesting magnetic properties have been derived.
Schiffꢀbase ligands have been extensively studied as blocking ligands in coordination
chemistry mainly due to their ease of synthesis, tremendous structural diversities and
26
immense possibilities towards different applications including molecular magnetism.
Common strategy to develop diꢀ and polynuclear transition metal complexes is to the use of
blocking ligands with lesser donor sites in combination with bridging ligands. Therefore,
NNO or NNN donor Schiff base ligands along with various polyatomic bridging anions are
excellent combinations to produce various structural architectures, and useful from the
2
7,28
magnetic coupling point of view.
aminoquinoline used as blocking ligands are scare in literature.
knowledge, only limited numbers of Cu(II)ꢀbinuclear complexes of this ligand have been
Schiff bases derived from salicylaldehydes and 8ꢀ
29,30
To the best of our
2
9,30
structurally characterized,
while none is reported for nickel. We are interested in
developing the coordination chemistry of nickel(II) derived from 8ꢀaminoquinoline and
salicyladehyde/oꢀvanillin to investigate whether it would cause any change in the structure
and magnetic properties of the resulting complexes. Accordingly, in this present report, we
have synthesized and structurally characterized one mononuclear, one homoꢀ and one heteroꢀ
1
2
bridged dinuclear Ni(II) complexes, Ni(L )2 (1), [Ni (L ) (SCN) (DMF) ] (2) and
2
2
2
2

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1
1
2
[
2 2 3 2
Ni (L ) (N ) ] (3), respectively, where HL and HL are the tridendentate Schiff base ligands
derived from 8ꢀaminoquinoline and oꢀvanillin/salicyladehyde, respectively, as shown in
Scheme 1. Magnetic characterization has been carried out for dinuclear complexes 2 and 3.
We also report here DFT calculations to provide a qualitative theoretical interpretation of
overall magnetic behaviour of these dinuclear analogues. Both experimental and theoretical
data suggest that the dinuclear nickel(II) analogues exhibit ferromagnetic interaction.
Experimental Section
Materials
Materials such as 8ꢀaminoquinoline, oꢀvanillin and salicylaldehyde were purchased from
SigmaꢀAldrich, India. The chemicals were of reagent grade and used without further
purification. All other chemicals and solvents were of reagent grate and used as received.
Caution! Azide 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 great care.
Synthesis of the Schiffꢀbase ligands
1
29
Tridentate Schiff base ligand (HL ) was prepared by the standard method. Briefly, 1.0
mmol of 8ꢀaminoquinoline (145 mg) was mixed with 1.0 mmol of oꢀvanillin (152 mg) in 20
ml of methanol. The resulting solution was heated to reflux for ca. 1 h, and allowed to cool.
2
Ligand HL was synthesized following the identical procedure using salicylaldehyde (1.0
mmol, 122 mg) instead of oꢀvanillin. The dark orange methanol solutions were used directly
for complex formation.
1
Synthesis of Ni(L )
2
(1)
2 2
NiCl ꢂ6H O (237 mg, 1.0 mmol) dissolved in 10 ml of methanol was added to a methanolic
1
solution (10 ml) of the ligand (HL , 2.0 mmol) with constant stirring for about 3 h. The
resulting solution was filtered, and the filtrate was left to stand in the air. X rayꢀquality red

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colour single crystals of complex 1 were obtained in several days upon slow evaporation of
the filtrate at ambient temperature. Yield: 430 mg (70%). Anal. calcd. for C34
H
26
N
4
O Ni: C
4
–1
6
6.65%, H 4.28%, N 9.15 %. Found: C 66.76 %, H 4.36 %, N 9.01 %.IR (cm , KBr):
ν(C=N) 1606 m; ν(C=N, py) 1533 m.
2
Synthesis of [Ni (L ) (SCN) (DMF) ] (2)
2
2
2
2
2
2 2
NiCl ꢂ6H O (237 mg, 1.0 mmol) and HL (1.0 mmol) were combined in a 20 ml methanol
and to the mixture 5 ml DMF solution of sodium thiocyanate (81 mg, 1.0 mmol) was added
with stirring. Finally few drops of triethylamine were added with stirring. The resulting
mixture was heated to reflux for 1 h during which time colour of the solution changed to dark
reddish brown. The reaction mixture was then filtered and kept at room temperature.
Analytically pure darkꢀbrown crystals suitable for Xꢀray diffraction were obtained from the
solution after several days, which was collected by filtration and washed with methanol/ether
36 8 4 2 2
and air dried. Yield: 670 mg (77%). Anal. calcd. For C40H N O S Ni : C 55.03%, H 4.16%,
–1
N 12.84 %. Found: C 54.87 %, H 4.27 %, N12.66 %. IR (cm , KBr): ν(C=N) 1608 m;
ν(C=N, py) 1581 m; ν(SCN¯) 2097.
1
Synthesis of [Ni
2
(L )
2
(N
3
)
2
] (3)
Complex 3 was synthesized from methanol/water solvent mixture following the very similar
1
2
procedure as described for complex 2 but HL and sodium azide were used instead of HL
and sodium thiocyanate, respectively. Colour: Dark brown, Yield: 650 mg (82%). Anal.
calcd. for C35 Ni : C 53.42%, H 3.84%, N 17.81 %. Found: C 53.58 %, H 3.72 %,
H
30
N
10
O
5
2
–
1
3
N17.62 %. IR (cm , KBr): ν(C=N) 1606 m; ν(C=N, py) 1537 m; ν(N ¯) 2057, 2032.
Physical Measurements
Elemental analyses for C, H and N were carried out using a Perkin–Elmer 240C elemental
–1
analyser. Infrared spectra (400–4000 cm ) were recorded from KBr pellets on a Nicolet

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Magna IR 750 seriesꢀII FTIR spectrophotometer. DC magnetic susceptibility data of
polycrystalline powder samples of 2 and 3 were collected on a Vibrating Sample
Magnetometer, PPMS (Physical Property Measurement System, Quantum Design, USA) in
the temperature range of 2 K to 300 K with an applied field 1000 Oe. Field variation (−5 kOe
to 5 kOe) magnetization measurement was carried out at 2 K.
Xꢀray crystallography
Single crystal Xꢀray diffraction data of complexes 1–3 were collected on a Bruker SMART
APEXꢀII CCD diffractometer using graphite monochromated Mo/Kα radiation (λ = 0.71073
Å). The unit cells were determined from the setting angles of 36 frames of data. Data
processing, structure solution, and refinement were performed using the Bruker ApexꢀII suite
program. All available reflections to 2θmax were harvested and corrected for Lorentz and
3
1
polarization factors with Bruker SAINT plus. Reflections were then corrected for
31
absorption, interꢀframe scaling, and other systematic errors with SADABS. The structures
were solved by the direct methods and refined by means of full matrix leastꢀsquare technique
2
32
based on F with SHELXꢀ97 software package. All the nonꢀhydrogen atoms were refined
with anisotropic thermal parameters. All the hydrogen atoms belonging to carbon were
placed in their geometrically idealized positions, while hydrogen atom connected to oxygen
atom of DMF molecule was located on the difference Fourier map and all of them were
constrained to ride on their parent atoms. Crystal data and details of the data collection and
refinement for 1–3 are summarized in Table 1.
Results and Discussion
Syntheses and IR spectroscopy
1
In the synthesis of metal complexes 1–3, we used two different Schiff base ligands HL and
2
1
HL . HL was derived from 1:1 condensation of oꢀvanillin and 8ꢀaminoquinoline in

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2
methanolic solution, whereas ligand HL was synthesized using salicylaldehyde instead of oꢀ
1
vanillin. Mononuclear complex 1 was synthesized simply by allowing Schiff base HL to
2 2
react with NiCl ꢂ6H O in a methanol solution in a 2: 1 molar ratio. Complex 2 was obtained
by adding a methanolic solution of NiCl ꢂ6H O to a methanolic solution of Schiff base ligand
2
2
2
HL followed by DMF solution of NaSCN in a 1:1:1 molar ratio in the presence of few drops
of triethylamine. Complex 3 was synthesized adopting very similar procedure as that of 2, by
adding a methanolic solution of NiCl
2
ꢂ6H
2
O to a methanolic solution of Schiff base ligand
in a 1:1:1 molar ratio. A comparison of the
1
HL , followed by aqueous solution of NaN
3
powder XRD patterns of 2 and 3 (Figs. S1 and S2) with that of the simulated powder XRD
patterns of the single crystals clearly show the purity of the bulk samples which are in accord
with the elemental analyses results.
In the IR spectra of complexes 1, 2 and 3, a strong and sharp band due to azomethine ν(C=N)
–
1
appears at 1606, 1608 and 1606 cm , respectively. In the spectrum of complex 2, a strong
–
1
absorption band is observed at 2098 cm , which characterizes ν(C≡N) vibration of
28,33
–1
thiocyanate anions.
Complex 3 shows two sharp absorption bands, at 2057and 2032 cm ,
34
consistent with the presence of both endꢀon bridging and terminal azide in the structure.
Description of the crystal structures
1
Complex 1 crystallizes in the monoclinic space group P2 /n. The perspective view of the
mononuclear nickel(II) complex is shown in Fig.1. Selected bond distances and angles with
1
their estimated standard deviations are listed in Table 2. Two HL ligands bind the metal ion
2
in a N O fashion using a phenolate oxygen atom, imine nitrogen atom and amine nitrogen
atom, forming one five and one six membered chelate rings, and each of them occupies one
meridional position of the octahedral structure of nickel(II). The fiveꢀmembered chelate bite
angles N3–Ni1–N4 [79.92(11)] and N1–Ni1–N2 [79.35(12)°] are close but differ from other
two bite angles N4–Ni1–O3 [89.75(10)] and N2–Ni1–O1 [87.86(10)] which are considerably

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close to the ideal octahedral value (90°). The four coordinating atoms making up the basal
plane are two phenoxo–O atoms [O3 and O1] and two quinoloneꢀN atoms [N1 and N3] from
the two Schiff bases, while the axial sites are occupied by two imineꢀN atoms [N2 and N4].
From close inspection of bond distances it has been observed that Ni–N(quinoline) bond
distances are relatively longer compared to Ni–O(phenoxo) bond distances. The chelate bite
angles and the transoid angles strongly suggest that the complex has a distorted octahedral
structure. The solid state structure of 1 is stabilized by edgeꢀtoꢀedge π···π stacking, C–H···π
and nonconventional C–H···O hydrogen bonding interactions as shown in Fig. S3.
The crystal structure of 2 consists of a discrete diꢀ(phenoxido)ꢀbridged centro
2
symmetrical unit of formula [Ni
2
(L )
2
(SCN)
2
(DMF)
2
], (Fig.2). Selected bond lengths and
angles are presented in Table 2. In this complex, each nickel(II)centre is hexaꢀcoordinated
with a distorted octahedral geometry. Nickel(II) centre is bonded to three donor atoms of the
2
deprotonated Schiff base ligand HL through the quinone amine nitrogen atom N1, the imine
nitrogen atom N2, and the phenoxido oxygen atom O1 as well as to a bridging phenoxidoꢀ
oxygen atom O1a [a: 1–x,–y,–z] from the symmetryꢀrelated Schiff base ligand. Furthermore,
the metal centre is coordinated with a monodentate NCS¯ anion through N3 and a DMF
molecule through O2 centre to complete the coordinative requirement for the octahedral
geometry. As is usually found in this type of double phenoxidoꢀbridged Ni(II) dimers, the
Ni O core is slightly asymmetric, because each Ni(II) ion is closer to the phenoxido oxygen
2
2
2
atom of L [Ni1–O1 = 2.020(4), Table 2] than that of the phenoxido oxygen atom of the
symmetryꢀrelated Schiff base ligand [Ni1–O1a = 2.157(4)]. The two Ni atoms are separated
by 3.156(1) Å, and the Ni1–O1–Ni1a bridge angle is 98.08(16)°. The tridentate ligand
coordinates to the metal ion in the meridional configuration which is usual for this type of
rigid ligand. Note that Ni1–O1 and Ni1–N2 are the two shortest bonds, and bonds trans to N1
[
Ni1–O1a] are significantly lengthened in 2. The axial positions of the metal centres are

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occupied by the bridging phenoxoꢀO atom (O1a) from the tridentate ligand at a distance of
.157(4) Å, and the thiocyanate nitrogen atom N3 at a distance of 2.051(6) Å. The four donor
2
atoms namely O1, N2, N1 and O2 describe the basal plane around the Ni1 centre, and the
displacement of the metal centre from the mean square plane towards the axially coordinated
O1a atom is 0.012(3)Å. The equatorial plane is characterized by the bond lengths Ni1−O1 =
2.020(4), Ni1−N2 = 2.022(5), Ni1−N1 = 2.051(5) and Ni1−O2 = 2.061(4)Å. The solid state
architecture of 2 is stabilized by edgeꢀtoꢀedge π···π stacking, C–H···π and nonconventional
C–H···O hydrogen bonding interactions as shown in Fig. S4.
1
The crystal structure of [Ni (L ) (ꢁ ꢀN )(N )(CH OH)] (3) is shown in Fig. 3. The
2
2
1,1
3
3
3
1
structural analysis reveals that the dinickel(II) compound consists of two deprotonated L
1
ligands. Of the two phenoxo ꢀoxygen atoms of two L , one oxygen atom O3 bridges the two
metal centres (Ni1 and Ni2) while the second oxygen O1 acts as a monodentate ligand and is
1
coordinated to Ni1 centre. Again, of the two methoxy oxygen atoms of two L ligands, O4 is
coordinated to Ni1 while the second oxygen atom O2 is noncoordinated. The quinoline
1
nitrogen atoms (N1, N7) and imine nitrogen atoms (N2, N6) of each of the two L are
coordinated to each of the two metal centres. In addition to the phenoxo bridge, the metal
centres in the dinuclear core are also bridged by the nitrogen atom N3 of an endꢀon azide
ligand. The remaining coordination position of Ni2 is satisfied by the oxygen atom (O5) of a
methanol molecule. Thus, both metal centres are hexaꢀcoordinated, but the coordination
environments of Ni1 and Ni2 consist of a different set of donor atoms (N
3 3
O for Ni1 and
4 2
N O
for Ni2). The bond lengths and angles in the coordination environment of the metal
centres are listed in Table 2. In case of Ni1 centre, the bond distances involving the bridging
and monodentate phenoxoꢀoxygen atom and the imine nitrogen atom are shorter than other
_
three bond lengths, which follow the order Ni–O(methoxy)>Ni–N(ꢁ ꢀN ) >Ni N(quinoline).
1
,1
3
In contrast, the trend of bond distances in Ni2 is slightly different from the Ni1 centre

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because these are not similarly coordinated. For the Ni2 centre, the bond distances involving
the bridging phenoxoꢀoxygen atom, quinoline nitrogen atom and the imine nitrogen atom are
_
again shorter than other three bond lengths, which follow the order Ni O(methanol)>Ni–
N(ꢁ1,1ꢀN )>Ni–N(terminal N ). For Ni1, bridging phenoxoꢀoxygen atom (O3) and the imine
3
3
nitrogen (N2) occupy the axial positions, while the monodentate phenoxo(O1), methoxy
oxygen atom (O4), the bridging azide (N3) and quinoline nitrogen atom (N1) define the basal
plane. In the case of Ni2, the terminal azide (N8) and methanol oxygen (O5) occupy the axial
positions, while the bridging azide (N3), imine nitrogen atom (N6), quinoline nitrogen atoms
(N7), and bridging phenoxo oxygen (O3) define the basal plane. The range of the transoid
angles are 154.61(7)–174.65(7) for Ni1 and 169.28(7)– 177.92(7) for Ni2. The ranges of the
cisoid angles are 101.53(7)–74.37(6) for Ni1 and 81.88(8)–94.45(7) for Ni2 in 3. Clearly,
both the transoid and the cisoid angles deviate significantly from ideal values of an
octahedral geometry. The molecular packing of the complex is stabilized by nonconventional
hydrogen bonding interactions as shown in Fig, S5.
Magnetic properties
The plot of
χ
M
T versus T for 2 is displayed in Fig. 4. The room temperature
is 2.07 cm mol K. From room temperature to 7 K, there is a gradual increase in
and beyond this maximum,
interaction is evident from the nature of
χ
χ
M
T value
3
−1
M
T value
χ
M
T value gradually decreases. An overall ferromagnetic
T versus T plot. The decrease of T value
χ
M
χ
M
beyond 7 K is probably a consequence of the zeroꢀfield splitting arising from singleꢀion
anisotropy of Ni(II) ions and possible inter dimer interaction. To understand the nature of
magnetic exchange and to determine the value of the coupling constant (J), the
M
χ T data (in
the dominant ferromagnetic region 300K–22 K) were fitted to the isotropic spin Hamiltonian:
3
5
Ĥ = −JŜ Ŝ , for the Ni(II) dimer (S = S = 1). The temperature dependence of χM for such a
1
2
1
2
Ni(II) dimeric system can be written as following equation (1):

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2
2
χ
M
= (2N
1)
In this expression, N,
temperatureꢀindependent paramagnetism which is assumed to be 200 × 10 cm mol for two
nickel(II) ions. The T data (300 K– 10 K) were fitted using equation (1) and the bestꢀfit
parameters are J = 15.6 cm with g = 2.01, and R = 4.7 ×10 {R = ∑[(
−
β g /kT) {[exp(J/kT) + 5 exp(3J/kT)]/ [1 +3 exp(J/kT) + 5 exp(3J/kT)]} + Nα
(
β
, k and g have their usual meanings and the last term (Nα) is the
ꢀ6
3
–1
χ
M
−
1
−4
obs
χ
M
)
calc 2
obs 2
(χ
M
)
M
] /[(χ ) ] } (Fig. 4). The value of J suggests ferromagnetic interaction, which is also
validated by the positive value of Weiss constant θ of 20.4 K obtained from Curie–Weiss
fitting of the plot of 1/χM vs T in the temperature range 300 – 140 K (Fig. S6). The maximum
value of magnetization at 2 K reaches of 3.54Nβ (Fig. 5), which is slightly less than the
expected value for ferromagnetically coupled Ni(II) dimers.
Fig. 6 shows the variableꢀtemperature magnetic susceptibility of 3 measured at 1000
3
–1
Oe. At 300 K, the χMT value is 2.8 cm mol K, typical for two Ni(II) ions with g >2.00.
3
−1
Upon cooling,
and then finally decreases with temperature. Similar to compound 2, a dominant
ferromagnetic interaction is suggested by the nature of T versus T plot, while the decrease
χ
M
T value increases and reaches a maximum value of 3.74 cm mol at 17 K,
χ
M
of χMT value beyond the maximum is result of the zeroꢀfield splitting as well as the possible
inter dimer antiferromagnetic interaction. The best fit parameters using equation (1) show J =
−1
−4
obs
calc 2
obs 2
1
5.3 cm with g = 2.32, and R = 2.6 ×10 {R = ∑[(
χ
M
)
− (χ
M
)
M
] /[(χ ) ] } (Fig. 6).
The positive value of J suggests the existence of a ferromagnetic coupling within the Ni(II)
dimer. The temperature dependence of the reciprocal susceptibility plot above 120K (Fig. S7)
follows the Curie–Weiss law with a Weiss constant θ of 11.25 K, which is also consistent
with the ferromagnetic coupling between Ni(II) ions. The M vs H curve at 2 K reaches a

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maximum value of 3.8 N
β (Fig. 7), which is very close to the expected value for
ferromagnetically coupled Ni(II) dimer.
DFT study and magnetoꢀstructural correlations
In order to get better insight into the magnetic exchange phenomena in the binuclear
nickel(II), phenoxoꢀO and azidoꢀN bridged complexes 2 and 3, “brokenꢀsymmetry” density
3
6
functional theory(DFT) calculations have been carried out using the hybrid B3LYP
3
7
38
exchangeꢀcorrelational functional in Gaussian 09 program. All elements except nickel
have been assigned the 6ꢀ31+G(d) basis set. The LanL2DZ basis set with effective core
3
9
potential has been employed for the nickel atoms. The brokenꢀsymmetry procedure can be
applied to a system of two nickel(II) ions, each containing two unpaired spins. First a selfꢀ
consistent field (SCF) calculation is carried in quintet state (HS) of the molecule. In the next
stage, another SCF calculation is performed taking two spins up on a nickel atom and two
spins down on the other nickel atom, which is referred to as the brokenꢀsymmetry (BS)
solution. Finally, the magnetic exchange coupling constant (J) value is evaluated using the
4
0
2
2
Yamaguchi formula, J = (EBS – EHS) / (〈S 〉HS – 〈S 〉BS), where EHS and EBS are the energies,
2
2
〈
S 〉 and 〈S 〉 are the spinꢀsquared operator in the highꢀspin (HS) and broken symmetry
HS
BS
–1
(
BS) state. The calculated J value for complex 3 is found to be 17.63 cm and is comparable
–1
to the experimental value 15.3 cm (Table 3), while for complex 2 the theoretical value 10.31
–1
–1
cm is found to be lower compare to the experimental value 15.6 cm . In both the cases the
positive J value indicates, there is ferromagnetic interactions between the nickel centres. The
spin density plots in highꢀspin and brokenꢀsymmetry states are shown in Fig. 8 and Fig. 9 for
complexes 2 and 3, respectively. The spin density plots suggest, in addition to the nickel
centres, the spin density is well distributed in bridging azideꢀN and phenoxoꢀO atoms and so
the magnetic exchange phenomena have been taking place through the bridging atoms in the
complexes.

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As already described in the structural section, 2 is a Ni(II) dimer with a diꢀphenoxo
bridging unit. Experimental and theoretical magnetoꢀstructural correlations carried out by
several groups have clearly shown that the major factor controlling the exchange coupling in
24
planar alkoxidoꢀ and phenoxidoꢀNi(ꢁꢀO) Ni complexes is the bridging Ni–O–Ni angle (θ).
2
According to these magnetoꢀstructural correlations, a ferromagnetic coupling is expected
when Ni–O–Ni angle is close to 90°. As Ni–O–Ni angle deviates from 90°, the ferromagnetic
coupling decreases and above the critical Ni–O–Ni angle of ∼96–98°, the coupling becomes
24
antiferromagnetic. The bridging Ni–O–Ni angle of 98.089(16)° in 2 suggests that magnetic
coupling between two Ni(II) centres could be either weak antiferromagnetic of even
ferromagnetic as the angle falls in the crossover region. Moreover, it has also been seen from
the theoretical studies that the outꢀofꢀplane displacement of the phenyl carbon atom directly
linked to the phenoxo oxygen atom from the Ni
2 2
O plane (τ) is of as much importance as
bridging angle θ in determining the sign and direction of the J value. It has been shown from
2
4
the theoretical studies that the AF interaction decreases with increase in τ value. Although
the bridging angle falls in the crossover region but the larger τ angle of 39.5º in 2 prevent an
adequate pathway for super exchange (weakening antiferromagnetic coupling), and thus it is
reasonable to think that the ferromagnetic interaction between the Ni(II) ions observed for 2
is not unexpected. It should be noted that, to the best of our knowledge, 2 is the second
example of a structurally and magnetically characterized bis(ꢀꢀalkoxido or phenoxido)
bridged dinuclear nickel(II) complex which shows ferromagnetic coupling between two metal
.
centres. Careful inspection of the literature results shows that all the reported diꢀ(alkoxido or
phenoxido)ꢀbridged dinuclear nickel(II)complexes show strong antiferromagnetic interactions
except in one case where the ferromagnetic coupling is observed in which bridging angle was
41
found to be 95.6º. As can be seen from the magnetoꢀstructural parameters for the reported
41–53
diꢀ(alkoxido or phenoxido)ꢀbridged dinuclear nickel(II)complexes in Table 4,
in most of

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the antiferromagnetically coupled systems, bridging angles are found to be above 99º.
Interestingly, there is one Ni(II) dimer presenting a very similar bridge to that of compound 2
42
with Ni–O–Ni bond angle of 98.28º but in that case antiferromagnetic coupling is observed.
Magnetoꢀstructural correlation data clearly suggest how slight variation of bridging angle
affects magnitude of the magnetic coupling. In addition to this difference in bridging angle,
several other structural dissimilarities perhaps play the significant role in the opposite sign of
the magnetic coupling in these complexes: (i) 8ꢀaminoquinoline part of the ligand imposes
geometrical restraint in the present case but the ligand used in the previous study was flexible
2
enough, (ii) Structural rigidity enforces the deprotonated ligand L to bind with the metal
center in meridional fashion while it is facially coordinated in the previous complex and (iii)
finally, a major difference is that the coordination environment in the correlated complex
consists of a N
2
O
4
donor set in which donor atoms from tridentate ligands exclusively occupy
in complex 2 and one of the apical position is occupied
the axial positions, while it is N
3
O
3
‒
42
by exogenous SCN ligand. All these above facts strongly suggest that as a large number of
structural parameters such as bond angles, bond lengths, out of plane displacement, dihedral
angles and the remaining coordination of planes need to be considered to account for the
variation of the exchange interaction in this type of complexes. The bridging angle of the
most of the reports compounds in these systems is above 99º, and thus a large number of
examples on such systems covering a wide range of bridging angles (especially at the region
of crossover angle) are required to develop proper magnetoꢀstructural correlation.
Unlike to diꢀ(phenoxo)ꢀbridged complexes, ꢁꢀphenoxoꢀꢁ1,1ꢀazide bridged complexes
possess dissimilar bridges, and thus there are many parameters which can affect the
magnitude of J. Here, the exchange interaction propagated mainly through the phenoxo and
endꢀon azido bridges and the structural parameters related to these bridges should be taken
into account to understand the magnetic behaviour of a compound having a ꢁꢀphenoxoꢀꢁ ꢀ
1
,1

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azide bridging moiety. As we mentioned earlier, when the Ni–phenoxido–Ni bridge angle is
greater than 96–98°, antiferromagnetic behaviour is expected. In contrast, the interaction is
predicted to be ferromagnetic for the endꢀon (EO) azideꢀbridged Ni(II) complexes, with J
increasing upon increasing the bridging angle, yielding a maximum at around 104°. In
addition to the bridging angle, it has also been established theoretically that the extent of
ferromagnetic interaction in EO azideꢀbridged Ni(II) complexes decreases linearly with the
Ni–N bond lengths. These three governing structural parameters of the ꢁꢀ
alkoxido/phenoxido–ꢁꢀazide nickel(II) systems along with their exchange integrals are listed
15,55–58
in Table 5.
Of them, it is clear from the magnetoꢀstructural correlation table that the
parameters relevant to the azide bridging are the key factor governing the sign and strength of
the magnetic interaction between two metal centres. In fact, all the reported ꢁꢀ
alkoxido/phenoxidoꢀꢁꢀazide nickel(II) compounds exhibit ferromagnetic interaction. All
these structural parameters of
3
are very similar to previously reported ꢁꢀ
alkoxido/phenoxido–ꢁꢀazide nickel(II) systems, and thus nature and magnitude of
interactions observed in 3 is reasonable.
Conclusion
We have synthesized and structurally characterized one mononuclear (1) and one
di(phenoxido)ꢀbridged dinuclear (2) and one ꢁꢀ phenoxido/ꢁꢀazide bridged dinuclear (3)
nickel(II) complexes derived from NNO donor Schiff base ligands. Structural studies reveal
that the metal centres in all complexes are hexaꢀcoordinated in a distorted octahedral
geometry in which the rigid tridentate ligands span around the metal centre in the meridional
position. Both complexes exhibit ferromagnetic interaction at lower temperatures.
Interestingly, the observed J values for both compounds 2 and 3 are well matched with J
values obtained from broken symmetry density functional calculations. It is important to note
that the di(phenoxido)ꢀbridged dinickel(II) complex (2) is only the second example where

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ferromagnetic coupling is operative inꢀbetween two nickel(II) centres. The bridging Ni–O–Ni
angle being close to the crossover region would provide significant information to get better
insight into the magnetoꢀstructural correlation in these systems. As there are very few heteroꢀ
bridged discrete compounds of dinickel(II), compound 3 is an important addition in the
family of such systems.
Acknowledgements
A. S. gratefully acknowledges the financial support of this work by the DST, India (Sanction
no. SB/EMEQꢀ159/2013 dated 10/10/2013). A. P. also gratefully acknowledges the financial
support of this work by the CSIR, New Delhi, India (Sanction no. 01(2834)/15/EMRꢀII dated
02/06/15). Crystallography was performed at the DSTꢀFIST, India, funded single crystal
diffractometer facility at the Department of Chemistry, Jadavpur University, India.
†
Electronic supplementary information ESI) available: The electronic supplementary
information file contains Figs. S1–S7. CCDC 1449944–1449946 contain the supplementary
crystallographic data for complexes 1 and 2, respectively. For ESI and crystallographic data
in CIF or other electronic format see DOI: To be filled by the publisher.
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Scheme 1. Drawing of the ligands used in this study

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Table 1.Crystal data and structure refinement of complexes 1–3.
Compound
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
1
2
3
C
34
H
26
N
4
O
4
Ni
C
40
H
36
N
8
O
4
SNi
2
C
35 30 10 5 2
H N O Ni
613.30
874.31
788.11
298 (2)
298 (2)
0.71073
Triclinic
Pꢀ1
298 (2)
0.71073
0.71073
Monoclinic
P2 /n
Orthorhombic
1
P
bca
12.5834(7)
10.2983(6)
21.547(1)
90
9.488(3)
18.6631(5)
18.8121(5)
19.0739(5)
90
b (Å)
10.734(4)
11.133(4)
73.787(7)
68.999(7)
84.003(7)
1016.4(6)
1
c (Å)
α(°)
β(°)
98.706(3)
90
90
γ(°)
90
3
Volume (Å )
2760.1(3)
4
6696.7(3)
8
Z
ꢀ3
D
calc (g cm )
1.476
1.428
1.563
ꢀ1
Absorption coefficient (mm ) 0.752
F(000) 1272
Range for data collection (°) 1.77ꢀ 27.24
1.080
1.185
452
3248
θ
1.98ꢀ27.50
9071
1.87ꢀ 25.04
91058
Reflections collected
41174
Independent reflection / Rint
Data / restraints / parameters
3699/ 0.0711
6084/0/ 388
1.022
2902 / 0.0583
4260 / 0 / 253
1.072
4660 / 0.0492
5912 / 0 / 472
1.021
2
Goodnessꢀofꢀfit on F
Final R indices [I>2σ(I)]
R indices (all data)
R1 = 0.0922,
wR2 = 0.1316
R1 = 0.1101,
wR2 = 0.1598
R1 = 0.1037,
wR2 = 0.2515
R1 = 0.1338,
wR2 = 0.2853
R1 = 0.0287,
wR2 = 0.0642
R1 = 0.0447,
wR2 = 0.0717

Page 25 of 34
RSC Advances
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DOI: 10.1039/C6RA02982E
Table 2. Bond distances (Å) and bond angles (º) of 1–3.
For complex 1
Ni1–N1
Ni1– N2
Ni1– N3
2.099(3)
2.055(3)
2.102(3)
Ni1– O1
Ni1– O3
Ni1– N4
2.056(2)
2.032(2)
2.039(3)
N1–Ni1–N2
79.35(12)
N3–Ni1–N4
79.92(11)
N1–Ni1–O1
N2–Ni1–N4
N2–Ni1–O1
167.16(11)
173.78(11)
87.86(10)
N3–Ni1–O3
N4–Ni1–O3
169.58(10)
89.75(10)
For Complex 2
Ni1–N1
Ni1–N2
Ni1–N3
Ni1–O1
2.051(5)
2.022(5)
2.051(6)
2.020(4)
Ni1–O2
Ni1–O1a
S1–C17
N3–C17
2.061(4)
2.157(4)
1.630(7)
1.165(9)
N2–Ni1–N1
O1–Ni1–N2
O1–Ni1–N1
81.2(2)
N2–Ni1–O2
N3–Ni1–O1a
Ni1–O1–Ni1a
172.92(18)
172.93(19)
98.08(16)
90.40(17)
168.52(19)
Symmetry code: a = 1ꢀx,ꢀy,ꢀz
For complex 3
Ni1–N1
Ni1– N2
Ni1–N3
Ni1–O1
Ni1–O3
Ni1–O4
N3–N4
2.0436(18)
Ni2–N3
Ni2–N6
Ni2–N7
Ni2–N8
Ni2–O3
Ni2–O5
N8–N9
2.1333(19)
2.0253(19)
2.0434(18)
2.1197(19)
2.0077(14)
2.1619(17)
1.189(3)
2.0127(19)
2.1611(18)
1.9784(15)
2.0127(15)
2.2562(16)
1.207(3)

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Page 26 of 34
View Article Online
DOI: 10.1039/C6RA02982E
N4–N5
1.152(3)
N9–N10
1.173(3)
O1–Ni1–N2
N2–Ni1–N1
O3–Ni1–O4
N6–Ni2–N7
O3–Ni2–N6
N2–Ni1–O3
O1–Ni1–N1
91.37(7)
81.36(7)
74.37(6)
81.88(8)
89.47(7)
174.65(7)
172.29(7)
N6–Ni2–N3
169.28(7)
154.57(7)
170.96(7)
177.92(7)
102.67(6)
93.94(7)
N3–Ni1–O4
O3–Ni2–N7
N8–Ni2–O5
Ni2–O3–Ni1
Ni2–N3–Ni1
Table 3. Experimental and DFT calculated magnetic exchange coupling constants for 2 and
3
.
2
2
ꢀ1
ꢀ1
Complexes EHS (a.u.)
EBS (a.u.)
〈S 〉
〈S 〉
J
theo (cm )
J
exp (cm )
HS
BS
2
3
ꢀ3419.3024026 ꢀ3419.3022145 6.011 2.007 10.31
ꢀ2613.420067 ꢀ2613.4197459 6.015 2.011 17.63
15.6
15.3

Page 27 of 34
RSC Advances
Table 4. Magnetic and structural parameters of the double phenoxido or alcoxidoꢀbridged octahedral dinuclear nickel (II) complexes
a
‒1)
Compound
θ (º)
J (cm
10.1
15.3
MꢂꢂꢂM(Å)
3.005
3.156
3.053
3.077
3.066
3.085
3.071
3.085
3.105
3.124
3.100
3.119
3.156
3.101
3.176
3.1538
3.135
3.203
3.237
Ref
41
This Work
42
43
43
44
45
46
47
48
47
49
48
45
50
51
47
exp
[
Ni (L)(CH CO ) ]·10H O
95.6
2
3
2 2
2
2
98.08
98.28
98.7
98.92
99.15
99.16
99.2
99.2
99.31
99.5
99.75
100.0
100.15
100.58
100.6
101.3
101.5
105.43
a
[
[
[
[
[
[
[
Ni (L) (NO ) ]·CH Cl ·C H OH·2H O
Ni(L)(OOCCF )(CH OH)]
2
Ni(L)(OOCCF )(H O)]
2
–5.26
–19.2
–19.5
–18.8
–24.6
–14.8
–21.3
–20.34
–17.0
–24.27
–25.25
–29.6
–36.18
–66.4
–29.5
−12.6
‒69.7
2
2
2 2
2
2
2
5
2
3
3
3
2
Ni (L)(N ) (H O) ]·CH3CN
2
3 2
2
2
Ni (Hemp)(H O) ](F CCOO) ·2H O
2
2
4
3
2
2
{Ni(Hsalhyph)Cl(H O)} ]·2DMF
2
2
a
Ni L(NCS) (H O) ] ·2DMF
2
2
2
2
Ni (L) (NO ) ]
2
2
3 2
[
[
Ni L(H O) ](ClO ) . 4NH CONH
2
2
2
4
4 2
2
Ni (L) (NO ) ]
2
2
3 2
Ni (L) (NO ) ]
2
2
2 2
[
[
[
[
[
[
Ni (Hemp)(H O) ](ClCH COO) ·2H O
2
2
4
2
2
2
Ni L (OAc) ]
2
2
2
(NiL) ] ·CH OH·4H O
2
3
2
a
Ni L(MeOH) (ClO ) ] ·2NHEt ClO
4
2
2
4 2
3
Ni (ꢁꢀL)(acac) (H O)]·CH CN
52
53
2
2
2
3
Ni (symꢀhmp) ](BPh ) ·3.5DMF·0.5(2ꢀPrOH)
2
2
4 2
a = average bridging angle

RSC Advances
Page 28 of 34
Table 5.Magnetic and Structural Parameters of ꢁꢀPhenoxoꢀꢁ ꢀAzide Nickel(II) Compounds
1
,1
Compound
Jexp
NiꢀOꢀNi NiꢀNꢀNi average
average
Asymmetry
ref
ꢀ
1
(
cm )
(deg)
102.68
103.0
104.7
106.9
106.7
102.3
(deg)
93.92
98.0
97.0
96.3
96.5
95.6
NiꢀO(Å) NiꢀN(Å) in NiꢀN (Å)
3
15.6
16.6
16.9
5.0
2.01
2.145
2.11
0.03
0.06
0.01
0.12
0.02
0.03
0.05
This work
[
[
[
[
[
[
Ni (L) (ꢁ ꢀN )(N )(H O)] ·CH CH OH
2.035
2.005
1.99
54
54
55
15
56
57
2
2
1,1
3
3
2
3
2
Ni (L) (ꢁ ꢀN )(CH CN)(H O)](ClO ) ·H O·CH CN
2.115
2.15
2
2
1,1
3
3
2
4
2
3
Ni (HL)3(ꢁ ꢀN )]·3H O
2
1,1
3
2
1
Ni(L)( ꢁ ꢀN )Ni(L )(N )(OH )]·H O
25.6
2.85
6.69
1.985
2.02
2.14
1
,1
3
3
2
2
Ni (L)(N )(H O)]·CH OH·H O
2.125
2.125
2
3
2
3
2
Ni (HL) (ꢁ ꢀN )(oꢀvanillin)]·H O
2.006
2
2
1,1
3
2
2
8

Page 29 of 34
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DOI: 10.1039/C6RA02982E
Fig.1. Crystal structure of 1 showing atom numbering scheme. Hydrogen atoms are omitted
for clarity and ellipsoids are drawn at 30% probability.
Fig.2. Molecularl structure of 2 showing atom numbering scheme. Hydrogen atoms are
omitted for clarity and ellipsoids are drawn at 30% probability.