Variation of nuclearity in NiII complexes of a Schiff base ligand: Crystal structures and magnetic studies

Article abstract of DOI:10.1039/c9ce00574a

Four new Ni^II complexes, [Ni(L¹)](NO₃)·1.5H₂O (1), [Ni(L¹)][Ni(HL²)](NO₃)₂·H₂O (2), [Ni₄(L²)₂(μ₃-OCH₃

Full text of DOI:10.1039/c9ce00574a

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21 April 2018
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Variation of nuclearity from mono-to-tetra-to-hexanuclear NiII
complexes of a Schiff base ligand: Crystal structures and
magnetic studies
Tanmoy Kumar Ghosh, Prithwish Mahapatra, Subrata Jana, Ashutosh Ghosh*
Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C.
Road, Kolkata 700 009, India, E-mail address: ghosh_59@yahoo.com
Abstract
Four new NiII complexes, [Ni(L1)](NO3)ꞏ1.5H2O (1), [Ni(L1)][(Ni(HL2)](NO3)2ꞏH2O (2),
[Ni4(L2)2(μ3-OCH3)2(NO3)](NO3)ꞏCH3OH (3), [(Ni6(L2)2(o-van)2(μ3-OH)4](NO3)2ꞏH2O (4)
where HL1 = 2-{[2-(2-aminoethylamino)ethylimino]methyl}-6-methoxyphenol and H2L2 =
N,N'-bis(3-methoxysalicylidene)diethylenetriamine), have been synthesized by changing the
reaction conditions and stoichiometry of the reactants. Single crystal X-ray diffraction study
reveals that complexes 1 and 2 are mononuclear with square planar geometry around NiII
centre whereas complexes 3 and 4 possess tetranuclear and hexanuclear structures,
respectively with octahedral NiII centres. The differences in nuclearity, coordination numbers
and geometry of NiII centres in the complexes have been explained considering flexible
coordination modes and denticity of the Schiff base ligands. The variable temperature molar
magnetic susceptibility measurements show that tetranuclear NiII complex (3) is
4
ferromagnetically coupled with two exchange couplings J1 = 0.64 cm−1 and J2 = 8.41 cm−1,
whereas hexanuclear NiII6 complex (4) is antiferromagnetically coupled with four exchange
couplings J1 = −5.60 cm−1, J2 = −9.39 cm−1, J3 = −5.18 cm−1 and J4 = 3.72 cm−1. The sign and
magnitude of these exchange couplings in complexes 3 and 4 are correlated with similar
types of reported cubane or defective cubane like NiII complexes. Electrochemical
measurements of these complexes reveal that the NiII centres are irreversibly reduce with one
reduction potential for 1, 3 and 4, whereas for 2 the NiII centres are irreversibly reduce with
two reduction potentials.
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Introduction
The interest for designed synthesis of discrete multinuclear metal clusters of transition metal
ions is primarily due to their aesthetic structural diversity, relevance to significant biological
processes in metalloenzymes1 and fascinating magnetic properties.2 The most challenging
aspect for the synthesis of such complexes is to control the nuclearity because it depends not
only upon the characteristics of metal ions and ligands but also on the metal ion to ligand
ratios, counter anions, reaction conditions, solvents used for synthesis etc.3 Discrete
polynuclear complexes of most of the 1st transition metal ions are well know. Several of these
species are of immense importance for mimicking biologically important reactions4 and for
understanding the fundamentals for magnetic coupling as well as single molecule magnet
behaviour.5 Among the ligands of different donor atoms, oxygen donor atoms seem to be the
most suitable for their flexible μ2- or μ3-bridging ability.6 For example, the Schiff base ligand,
N,N′-bis(3-methoxysalicylidene)diethylenetriamine) (H2L2 ) contains seven potential donor
atoms; three N and four O atoms. Among these four oxygen atoms, two are phenolic and can
coordinate to metal ions in monodentate mode or bridging μ2- or μ3-mode whereas two
methoxy oxygen atoms remain either non-coordinated or coordinated rather weakly in
monodentate mode. Exploiting this different coordination behaviours of oxygen atoms, quite
a few homo- and hetero-metallic transition metal and lanthanide complexes of various
nuclearity have been synthesised.7 Recently, during working on the synthesis of ZnII
complexes of this ligands, we serendipitously obtained di‐, tetra‐ and hexa-nuclear complexes
on changing the reaction conditions and ligand to metal ratios.8 We are curious to investigate
if tailored synthesis of such complexes with desired nuclearity is also possibly for other metal
ions under similar reaction conditions.
Among the complexes of 1st transition metal ions, those of NiII deserve special mention for
their structural diversity and tendency to form high nuclearity metal clusters, especially with
N,O donor ligands. The dinuclear NiII complexes are the most common but the complexes of
higher nuclearity i.e. Ni3, Ni4, Ni5 Ni6, Ni7, Ni11, Ni18, Ni20, Ni21, Ni32 etc. are also well
known.9 In addition to the structural diversity, magnetic properties of the high nuclearity NiII
complexes attract the potential interest of researchers. A variety of NiII complexes with
different geometries like; cubane, defective cubane, double cubane etc. are reported in
literature and their fascinating magnetic behaviours are investigated by various
groups.5a,5b,5d,5f It has been found that the magnetic interactions may change from antiferro-
2

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to ferromagnetic with the small changes in structural geometry, bridging mode and angle of
coordinating atoms of ligands.9c,9d,9f The nuclearity and nature of magnetic interactions are
found to be extremely important for the intriguing single molecule magnet properties
exhibited by several multinuclear NiII complexes. Therefore, our focus is to synthesize NiII
complexes with diverse but desired nuclearities and investigate the magnetic behaviours.
In the present work, we report synthesis and structural characterization of four
complexes, [Ni(L1)](NO3)ꞏ1.5H2O (1), [Ni(L1)][Ni(HL2)](NO3)2ꞏH2O (2), [Ni4(L2)2(μ3-
OCH3)2(NO3)](NO3)ꞏCH3OH (3) and [(Ni6(L2)2(o-van)2(μ3-OH)4](NO3)2ꞏH2O (4) where HL1
= 2-{[2-(2-aminoethylamino)ethylimino]methyl}-6-methoxyphenol and H2L2 = N,N'-bis(3-
methoxysalicylidene)diethylenetriamine). Complexes 2−4 have been synthesized by reacting
H2L2 with Ni(II) nitrate on varying the metal:ligand ratios and reaction conditions. The
presence of the NiII complex of monocondensed ligand HL1 as a component of cocrystalline
complex 2, encourages us to synthesize the complex 1 which is solely of the monocondensed
ligand. The electrochemical properties of all four complexes have been studied by cyclic
voltammetry measurements. The temperature dependent molar magnetic susceptibility and
isothermal magnetization measurements are performed for complexes 3 and 4 in which NiII
centres are paramagnetic. A magneto structural correlation has also been drawn considering
similar types of reported multinuclear NiII complexes.
Experimental Section
Starting materials
All chemicals and solvents were of reagent grade and were commercially available. They
were use without any further purification.
Synthesis of Schiff base ligand [N,N'-bis(3-methoxysalicylidene)diethylenetriamine]
(H2L2)
The Schiff base ligand H2L2 were prepared by following standard literature method:8 10
mmol (1.521 g) of o-vanillin was mixed with 5 mmol (0.540 mL) of diethylenetriamine in
methanol (20 mL). The resulting yellow methanolic solution was refluxed for 2 h. The
volume of the resulting solution was reduced to 5 mL on keeping upon a water bath. The
solution was cooled and then 10 mL diethyl ether was added to it. The resulting mixture was
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kept at −5 °C in a refrigerator for 2 days, when the desired Schiff base (H2L2) separated out
as yellow solid.
Synthesis of the complex [Ni(L1)](NO3)ꞏ1.5H2O (1)
Complex 1 was prepared by the reaction of two precursors, bis(o-vanillin)nickel(II) dihydrate
and bis(diethylenetriamine)nickel(II). Bis(o-vanillin)nickel(II)dihydrate was prepared
according to standard literature method;10 10 mmol (1.521 g) of o-vanillin was mixed with 5
mmol (1.45 g) of Ni(NO3)2ꞏ6H2O in methanol (30 mL) with constant stirring. Then
triethylamine 10 mmol (1.4 mL) was added drop wise to the above mixture and stirring was
continued for 30 min. A green precipitate separated out and was collected by filtration. The
precipitate was washed with methanol. The other precursor bis(diethylenetriamine)nickel(II)
was prepared 11 by mixing 10 mmol (1.08 mL) of diethylenetriamine with 5 mmol (1.45 g) of
Ni(NO3)2ꞏ6H2O in methanol (30 mL). The mixture was stirred for 30 min when a violet solid
separated out, which was collected by filtration. Then, bis(diethylenetriamine)nickel(II) (1.94
g, 5 mmol) was dissolved in 20 mL methanol and a methanolic solution (20 mL) bis(o-
vanillin)nickel(II) dihydrate (1.81 g, 5 mmol) was added to it and put under reflux for 5 h.
The colour of the solution was turned into deep red. The solution was filtered and left in open
atmosphere. The deep red X-ray quality single crystals were obtained after a few days on
slow evaporation of the solvent. The crystals were filtered out from the solution and air dried.
Complex 1: Yield 1.241g (65%) C24H38N8Ni2O13 (764.00). Calculated C, 37.73; H, 5.01; N,
14.67; Found C, 37.62; H, 5.10; N, 14.79; IR: ν(N–H) = 3170, 3185 cm–1, ν(C=N) = 1618 cm–1,
ν
(NO3) = 1384 cm–1.
Synthesis of complex [Ni(L1)][(Ni(HL2)](NO3)2ꞏH2O (2)
A methanolic solution (20 mL) of Ni(NO3)2ꞏ6H2O (1 mmol, 0.290 g) was added to the
methanolic solution (20 ml) of H2L2 (1 mmol, 0.371 g) with constant stirring. The resulting
solution was stirred for further 2 h. Diffraction quality red single crystals were obtained after
a few days on slow diffusion of diethyl ether into the solution.
Complex 2: Yield 0.325 g (65%) C32H44N8Ni2O13 (866.13). Calculated C, 44.37; H, 5.12; N,
12.94; Found C, 44.21; H, 5.20; N, 12.89; IR: ν(N–H) = 3190 cm–1, ν(C=N) = 1625 cm–1, ν(NO3)
1384 cm–1.
=
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Synthesis of the complex [Ni4(L2)2(μ3-OCH3)2(NO3)](NO3)ꞏCH3OH (3)
A methanolic solution (20 mL) of Ni(NO3)2ꞏ6H2O (2 mmol, 0.580 g) was mixed with
methanolic solution (20 mL) of H2L2 (1 mmol, 0.371 g) with constant stirring. Then the
triethylamine (2 mmol, 0.28 mL) was added drop wise to the mixture. The resulting solution
was stirred for another 2 h. Green single crystals for X-ray diffraction were obtained by slow
diffusion of diethyl ether into the mother liquor.
Complex 3: Yield 0.379 g (62%) C43H52N8Ni4O17 (1187.69). Calculated C, 43.48; H, 4.41; N,
9.43; Found C, 43.38; H, 4.59; N, 9.48; IR: ν(N–H) = 3200 cm–1, ν(C=N) = 1615 cm–1, ν(NO3)
1384 cm–1.
=
Synthesis of the complex [(Ni6(L2)2(o-van)2( μ3-OH)4](NO3)2ꞏH2O (4)
A methanolic solution (25 mL) of Ni(NO3)2ꞏ6H2O (3 mmol, 0.870 g) was added to the
methanolic solution (15 mL) of H2L2 (1 mmol, 0.371 g). To this solution triethylamine (2
mmol, 0.28 mL) and o-vanillin (1 mmol, 0.152 g) were added. The resulting solution was
refluxed for 3 h, cooled and filtered. The X-ray diffraction quality green single crystals were
obtained on slow diffusion of diethyl ether into the filtrate.
Complex 4: Yield 0.480 g (60%) C56H64N8Ni6O25 (1601.41). Calculated C, 42.00; H, 4.03; N,
7.00; Found C, 42.09; H, 3.94; N, 6.95; IR: ν(O–H) = 3400 cm–1,ν(N–H) = 3210 cm–1, ν(C=N)
1638 cm–1, ν(NO3) = 1384 cm–1.
=
Physical measurements
Elemental analyses (C, H and N) were performed using a Perkin-Elmer 2400 series II CHN
analyzer. IR spectra in KBr pellets (4000–500 cm1) were recorded using a Perkin-Elmer RXI
FTIR spectrophotometer. Temperature-dependent magnetic measurements of powdered
samples of 3 and 4 were performed by a superconducting quantum interference device
vibrating sample magnetometer (SQUID-VSM) with an applied field of 500 Oe in the
temperature range 2–300 K. Pascal’s constants were used to quantify diamagnetic corrections
to the molar paramagnetic susceptibility and a correction was applied for the sample holder.
Field dependent magnetic measurements (0–5 Tesla) were carried out at 2 K for 3 and at 2.5
K for 4.
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Electrochemical measurements
The electrochemical studies of all four complexes 1−4 were performed using a Basi-Epsilon
C3 Cell instrument at a scan rate of 100 mVs–1 within the potential range of 0 to –1.80 V vs.
Ag/AgCl. Cyclic voltammograms were carried out using 0.1 M TBAP as supporting
electrolyte and 1.0×10–3 M of complexes in acetronitrile solution which are deoxygenated by
argon purging. The working electrode was a glassy carbon disk (0.32 cm2) which was
polished with alumina solution, washed with absolute acetone and accetronitrile, and air dried
before each electrochemical run. The reference electrode was Ag/AgCl, with platinum as
counter electrode. All experiments were performed in standard electrochemical cells at 25 C.
X-ray Crystallographic data collection and refinement
Suitable single crystals of complexes 1−4 were mounted on a Bruker-AXS SMART APEX II
diffractometer equipped with a graphite monochromator and Mo-K ( = 0.71073 Å)
radiation. The structure was solved by direct methods and refined by full-matrix least squares
on F2 using the SHELXL-2014 package12 and Olex2 program.13 Non-hydrogen atoms were
refined with anisotropic thermal parameters. The hydrogen atoms bonded to carbon were
included in geometric positions and given thermal parameters equivalent to 1.2 times those of
the atom to which they were attached. All other hydrogen atoms were placed in their
geometrically idealized positions and constrained to ride on their parent atoms except for the
hydrogen atoms of the interstitial two water molecules in complex 1 and one water molecule
in complex 4, and a methanol solvent molecule in complex 3 that could not be located in the
Fourier map. The interstitial one water molecule for complexes 1 and 4, and one nitrate ion in
complex 1 were refined isotropically. The methanol molecule in complex 3 was disordered
and a model was refined in which two different sites were located for the carbon and oxygen
atoms. 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.14 Data collection, structure refinement parameters, and crystallographic
data for complexes 1−4 are given in Table 1.
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Table 1: Crystal data and structure refinement of complexes 1−4.
Complex
1
2
C32H44N8Ni2O13
866.13
Monoclinic
P21/c
3
C43H52N8Ni4O17
1187.69
Monoclinic
P21/c
4
C56H64N8Ni6O25
1601.29
Monoclinic
C2/c
20.212(11)
22.656(11)
16.048(8)
121.342(10)
6276(6)
4
Chemical formula C24 H38 N8Ni2 O13
Formula weight
Crystal system
Space group
a(Å)
764.00
Monoclinic
P21/n
5.1874(4)
34.715(3)
18.1886(15)
90.275(2)
3275.4(5)
4
22.233(4)
7.3217(14)
23.045(4)
101.296(4)
3678.7(12)
4
1.564
299
1.100
1808.0
13.838(4)
29.260(9)
14.867(4)
116.548(4)
5385(3)
4
1.465
296
1.450
2456.0
0.072
b (Å)
c (Å)
β (ᵒ)
V (Å)3
Z
ρcalc(g cm-3)
T (K)
1.549
292
1.224
1592.0
0.081
1.695
296
1.853
3296.0
µ (Mo Kα) (mm-1)
F(000)
R(int)
0.049
0.102
Total reflections
Unique reflections
104175
8159
86877
6675
37759
9473
13643
4749
Reflections with I
5916
5907
6223
2408
> 2σ(I)
[I>2σ(I)]R1 , wR2
a
b
0.0806,0.2110
0.1166
0.038,0.099
0.0448
0.0924,0.2922
0.1299
0.0617,0.2121
0.1349
R(all data)
GOF c
1.145
1.115
1.087
1.008
Residual electron
density, e/Å-3
-0.54,1.27
-0.45,0.52
-0.89,0.1.26
-0.60,1.12
2
2
4 ½
2
½
aR1 = ||Fo|−|Fc|||Fo|,bwR2 (Fo ) = [[w(Fo − Fc2)2w Fo ] and cGOF = [[w(Fo − Fc2)2(Nobs– Nparams)]
Results and discussion
Syntheses of the complexes
The multi‐dentate Schiff base ligand H2L2 was prepared by 1:2 condensation of
diethylenetriamine and o-vaniline in methanol.8 The reaction of H2L2 with Ni(NO3)2ꞏ6H2O in
different stiochiometric ratios produces complexes 2−4 (Scheme 1). When ligand H2L2 was
reacted with Ni(NO3)2ꞏ6H2O in 1:1 molar ratio at room temperature in methanol solution,
complex 2 was formed. It was a cocrystal of two mononuclear NiII units, [NiL1]+ and
[(Ni(HL2)]+. Addition of triethylamine (equimolar amount) to the solution did not make any
difference in the composition of the resulted complex. The monocondensed ligand HL1 was
formed during the course of reaction via hydrolysis of dicondensed ligand H2L2. Previously,
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the complexes of such ligands derived from the monocondensation of diethylenetriamine and
an aromatic carbonyl compound with various metal ions were synthesized simply by 1:1
condensations of diethylenetriamine and o-vaniline in methanol and then reacting the ligand
with corresponding metal salt in 1:1 molar ratio.15 We tried to prepare complex 1 following
this method but the attempt was unsuccessful as the product was found to be complex 2.
Hence, for preparation of complex 1, we developed here a new method, analogous to the
monocondensation of 1,3-propanediamine with aromatic carbonyl compounds3b,6a,16 (detailed
method is given in the experimental section).
Scheme 1. Syntheses of complexes 1−4.
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In fact, the method was a NiII template synthesis where the tetra-coordinated square planar
geometry of the resulting complex [NiL1]+ was exploited. Complexes 3 and 4 were prepared
by the reaction of H2L2 with Ni(NO3)2ꞏ6H2O in 1:2 and 1:3 molar ratio, respectively in
presence of triethylamine. In complex 4, two o-vanillin molecules which were formed by the
hydrolysis of H2L2 during complex formation, were also coordinated to the NiII centers along
with Schiff base ligand H2L2. As a result, the yield was very low. However, when
stoichiometric amount of o-vanillin was added to the reaction mixture, the yield of complex 4
was increased considerably (detailed method is given in the experimental section).
Description of the structures
Fig. 1: ORTEP diagram of complex 1A with 20% ellipsoid probability. Nitrate anions and
solvent waters are not shown for clarity. The structure of 1B is equivalent.
Complex 1
The asymmetric unit of complex 1 contains two independent cationic complex units (called A
and B), two nitrate ions that balance the charge and three solvent water molecules. A
perspective view of molecule A together with selective atomic numbering scheme is shown
in Fig. 1. and that of B is presented in S1, ESI. Both units have equivalent structure with
slight difference in bond angles and bond lengths. The NiII center possesses tetra-coordinated
distorted square planar geometry being coordinated by two amine nitrogen atoms (N2A, N3A
for Ni1 and N2B, N3B for Ni2), one imine nitrogen atom (N1A and N1B for Ni1 and Ni2,
respectively) and one phenoxido oxygen atom (O1A for Ni1 and O2B for Ni2) of the Schiff
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base ligand, HL1. The NiN and NiO bond lengths are in the ranges of 1.829(4)1.917(4) Å
and 1.826(3)−1.830(4) Å, respectively. The selected bond lengths and angles are listed in
Table S1, ESI. The r.m.s. (root mean squared) deviations of the four coordinated atoms in the
basal planes are 0.115 and 0.096 Å around Ni1 and Ni2, respectively. The geometry around
the NiII atom is confirmed by the (4) index.17 The 4 values of NiII atoms are 0.126 and 0.113
for Ni1 and Ni2, respectively, indicating that the geometries are slightly distorted square
planar for both NiII atoms. The mononuclear units are linked to each other through several
hydrogen bonds (Table S5, ESI) to form a one-dimensional chain as shown in S2, ESI.
Fig. 2: ORTEP diagram of complex 2 with 20% ellipsoid probability. Nitrate anions and
solvent water are not shown for clarity.
Complex 2
The asymmetric unit of complex 2 contains two independent mononuclear NiII units of two
different Schiff base ligands HL1 and H2L2, two nitrate ions and one water molecule. Both the
Ni(II) centers have square planar geometry. The Ni1 center is coordinated by two imine
nitrogen atoms (N1 and N3), one amine nitrogen atom (N2) and one phenoxido oxygen atom
(O1) of singly deprotonated dicondensed Schiff base ligand (HL2)−. The Ni2 center is bonded
by two amine nitrogen atoms (N5 and N6), one imine nitrogen atom (N4) and one phenoxido
oxygen atom (O5) of monocondensed Schiff base ligand HL1. Here, the NiN bond lengths
are in the range 1.837(2)1.922(2) Å. The NiO(phenoxido) bond lengths are 1.824(2) and
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1.828(2) Å . The selected bond lengths and angles of both the complex units of 2 are listed in
Table S2 (ESI). The r.m.s. deviations of the four coordinated atoms in the basal planes
around the Ni1 and Ni2 centres are 0.037 and 0.038 Å, respectively. Here the 4 values of NiII
atoms are 0.079 and 0.075 for Ni1 and Ni2, respectively, indicating that the geometries are
nearly square planar. The two mononuclear units form a three dimensional network as shown
in Fig. S3 (ESI) via several intermolecular hydrogen bonding interactions (Table S5, ESI).
Fig. 3: ORTEP diagram of complex 3 with 20% ellipsoid probability. Hydrogen atoms,
nitrate anion and solvent methanol are not shown for clarity.
Complex 3
Complex 3 has a tetranuclear structure with a defective cubane core where four NiII centers
are bridged via two µ3-methoxido ions. The asymmetric unit of this complex consists of two
deprtotonated Schiff base ligands (L2)2−, two methoxido ions and one chelating nitrato co-
anion along with four NiII ions. One non-coordinating nitrate ion and a solvent methanol
molecule remain outside of the coordination sphere. The structure of complex 3 is shown in
Fig. 3 together with selective atomic numbering scheme. Here, all the NiII ions have a hexa-
coordinated distorted octahedral geometry. The Ni1 centre is bonded by the four donor atoms
of the two Schiff base ligands (O1, N1 and O5, N4) and two µ3-bridged oxygen atoms of
methoxido ions (O9 and O10). Each of the Ni2 and Ni3 centers are coordinated by three
donor atoms of one Schiff base ligand (O2, N2, N3 for Ni2 and O6, N5, N6 for Ni3) and two
oxygen atoms of another Schiff base ligand (O5, O7 for Ni2, and O1, O3 for Ni3) and one µ3‐
bridged oxygen atom of methoxido ion (O9 for Ni2 and O10 for Ni3). The Ni4 center is
coordinated by two phenoxido oxygen atoms (O2 and O6) of two Schiff base ligands, two µ3-
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methoxido oxygen atoms (O9 and O10) and the two oxygen atoms (O11 and O12) of the
chelating nitrato ion. The NiN and NiO bond lengths are within the ranges of 1.972(9)
2.142(9) Å and 1.925(7)2.134(6) Å, respectively. The selected bond lengths and angles of
complex 3 are listed in Table S3, ESI. The complex shows several intermolecular hydrogen
bonding interactions (Fig. S4, ESI).
Fig. 4: ORTEP diagram of complex 4 with 20% ellipsoid probability. Hydrogen atoms,
nitrate anions and solvent water are not shown for clarity.
Complex 4
Complex 4 possesses a discrete hexa-nuclear Ni6 structure with two fused defective cubane
cores, having a crystallographic C2 axis passing through the Ni1, Ni4 centers. All six NiII
centers are hexa-coordinated with distorted octahedral geometry. The asymmetric unit
contains four NiII ions, one deprotonated Schiff base ligand (L2)2−, one deprotonated o-
vaniline co-ligand (o-van) and one nitrate ion. A non-coordinating water molecule remains
outside the coordination core. The structure of complex 4 is shown Fig. 4 together with
selective atomic numbering scheme. The Ni1 center is coordinated by two symmetry related
phenoxido oxygen (O1, O1*), two methoxido oxygen (O3, O3*) of two ligand H2L2 and two
µ3-bridged hydroxido oxygen atoms (O9, O9*) where symmetry element * = 1-x,y,3/2-z.
Similarly, Ni4 center is coordinated by two symmetry related phenoxido oxygen (O2, O2*)
and four µ3-bridged hydroxido oxygen atoms (O8, O8* and O9, O9*). The Ni2 center is
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coordinated by one nitrogen atom and five oxygen atoms; N1 and O1 from H2L2, O5 and O6
from o-vaniline, O9 and O8 of two µ3-bridged hydroxido ions. On the other hand, Ni3 center
is coordinated by two nitrogen atoms and four oxygen atoms; N2, N3 and O2 from H2L2, O6
and O7 from o-vaniline, and O8 from µ3-bridged hydroxido ion. The NiN and NiO bond
lengths are in the range of 1.959(9)2.120(1) Å, and 1.954(7)2.126(5) Å, respectively. The
selected bond lengths and angles of complex 4 are listed in Table S4, ESI. The hydrogen
bonding interactions present in complex 4 is shown in Fig. S5 (ESI). Details of the hydrogen
bonding interactions are given in Table S5, ESI.
A
CSD
search
reveals
that
the
ligand,
N,N′-bis(3-
methoxysalicylidene)diethylenetriamine) (H2L2) has been used mostly for the synthesis of
lanthanide complexes. The homo-metallic Ln-complexes are usually dinuclear7a,7g but one
mono‐7h and two dodeca−nuclear7i complexes are also known where as the heterometallic
complexes containing LnIII are found to be tri-, hexa‐ and poly−nuclear.7b,7c,7d,7f In contrast,
this ligand has rarely been used for the synthesis of transition metal complexes and till date,
only one each of dinuclear MnII, di- and tetranuclear CuII complexes are reported.7e,7j In
addition, three dinuclear ZnII and two trinuclear CdII complexes are also known.18 Recently,
we used this ligand for the synthesis of ZnII complexes and obtained a di-, a tetra- and a hexa-
nuclear complexes.8 Thus complexes 2, 3 and 4 of present paper are the first report of NiII
complexes of this ligand. On the other hand, only one NiII and one CuII complexes of the
mono-condensed ligand, HL1 is reported till date and both of these complexes are
mononuclear.15,19
Magnetic Properties
Among the four complexes, NiII centres are diamagnetic with square planar geometry in
complexes 1 and 2. Therefore, the temperature dependent molar magnetic susceptibility
measurements were performed only for complexes 3 and 4. Thermal variations of the product
of molar magnetic susceptibility with temperature (χMT) for complexes 3 and 4 are shown in
Fig. 5. The room temperature χMT value for complex 3 is 5.40 cm3 K mol−1, which is higher
than the value of magnetically noninteracting NiII4 system of 4.0 cm3 K mol−1 for gNi = 2.1 at
300 K; whereas for complex 4, the χMT value is 5.55 cm3 K mol−1, which is slightly lower
than the value of magnetically noninteracting NiII6 system of 6.0 cm3 K mol−1 for gNi = 2.1 at
300 K.21 The temperature dependence of χMT is different for complexes 3 and 4. In case of
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complex 3, the χMT value gradually increases upon cooling down to 50 K and below this
temperature, it sharply increases and reaches the maximum of 8.82 cm3 K mol−1 at 6 K. On
further cooling, the χMT value starts to decrease and reaches to 7.89 cm3 K mol−1 at 2 K. This
increasing nature of χMT value on lowering the temperature, indicates overall intramolecular
ferromagnetic coupling between the metal ions in complex 3. For complex 4, the χMT value
gradually decreases upon cooling to 60 K and below this temperature it sharply decreases and
reaches to1.25 cm3 K mol−1 at 2.5 K. Here, the decreasing nature of χMT value with
decreasing temperature indicates the intramolecular antiferromagnetic interaction between the
metal ions. The field dependent molar magnetizations values (M/Nβ) up to 5 T magnetic
fields are 6.91 μB for complex 3 at 2 K and 2.62 μB for complex 4 at 2.5 K (Fig. S6, ESI). The
magnetization values of both complexes increase with increasing the field but the nature of
enhancement indicates that the magnetic coupling in 3 is ferromagnetic where as that in 4 is
antiferromagnetic.20,21
The structure of tetranuclear complex 3 is defective cubane, where NiII centres are connected
via both phenoxido and μ3−methoxido oxygen atoms (Fig. 6, left). In this complex, two
different types of bridging are observed between the NiII centres: i) bridging via one
phenoxido and one μ3-methoxido oxygen atoms and ii) bridging via two μ3-methoxido
oxygen atoms. Therefore, for simulation of the magnetic data of complex 3, we have
considered two exchange couplings; J1 for both phenoxido and μ3-methoxido bridged NiII (S
= 1) centres and J2 for double μ3-methoxido bridged NiII (S = 1) centres. The Hamiltonian
used for the simulation of χMT data of complex 3 is written as H = −J1(S1S2 + S1S3 + S2S4 +
S3S4) − J2(S2S3), where S1 = S2 = S3 = S4 = SNi. The interaction between two NiII centres, S1
and S4 are taken as zero due to the long intranuclear NiII....NiII distance. Complex 4 has a
hexanuclear NiII6 structure, where two defective cubane like cores are symmetrically fused
together perpendicularly (Fig. 6, right). In this complex, NiII centres are connected via both
phenoxido and μ3-hydroxido oxygen atoms and a crystallographic C2 axis is passing through
the two NiII atoms by which two defective cubanes are fused to each other. Here also, two
different types of bridging are observed between the NiII centres: i) bridging via one
phenoxido and one μ3-hydroxido oxygen atoms, and ii) bridging via two μ3-hydroxido
oxygen atoms. For simulation of the magnetic data of complex 4 we have considered four
exchange couplings depending on the bridging angles between NiII centres; J1 and J2 for both
phenoxido and μ3-hydroxido bridged NiII (S = 1) centres, and J3 and J4 for double μ3-
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hydroxido bridged NiII (S = 1) centres. The Hamiltonian used for the simulation of χMT data
of complex 4 is written as H = −J1(S1S2 + S5S6) −J2(S1S3 + S2S4 +S3S6 + S4S5) −J3(S2S3 + S3S5)
−J4(S3S4), where S1 = S2 = S3 = S4 = S5 = S6 = SNi. The other exchange interactions between
NiII centres, S1−S4, S1−S6 and S4−S6 are taken as zero due to the long intranuclear NiII.....NiII
distances. The simulations of experimental χMT data of both complexes 3 and 4 were carried
out using the PHI program taking into account the molecular field approximations.22 The best
fittings were obtained using the following set of parameters: J1 = 0.64 cm−1, J2 = 8.41 cm−1,
gNi = 2.28, R = 6.56 × 10−4 for complex 3; J1 = −5.60 cm−1, J2 = −9.39 cm−1, J3 = −5.18 cm−1,
J4 = 3.72 cm−1, R = 1.41 × 10−4 for complex 4, where R = Σ[(χMT)exp.
−
(χMT)calcd.]2/Σ((χMT)exp)2. The g value of Ni is fixed at gNi = 2.20 for complex 4 during
simulation.
Figure 5: Thermal variations of the χMT products for complexes 3 and 4 (left to right
respectively). Solid red line represents the best fitting to the appropriate model for 3 and solid
blue line represents the best fitting to the appropriate model for 4.
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Figure 6: Basic tetranuclear core of complex 3 (left) and hexanuclear core of complex 4
(right) with different magnetic exchange couplings (J).
Magneto-structural correlations
A lot of di-phenoxido, di-alkoxido and di-hydroxido bridged NiII complexes which have been
characterized both structurally and magnetically are reported and their magneto-structural
correlations are well-established.23 The factors affecting the exchange interaction in these
complexes are: Ni–O–Ni bridging angle (θ), deviation of phenyl/alkyl carbon from the Ni2O
plane (τ), Ni...Ni distances and the hinge angle between two O–Ni–O planes (β). Among
these, the most important factor is the Ni–O–Ni bridging angle (θ) as the sign and magnitude
of exchange coupling is very much dependant on this angle.24 From the reported magneto-
structural correlations, it is found that the ferro- to antiferromagnetic crossover angle for such
complexes is ~96–98°. Although it is also found that the antiferromagnetic coupling increases
with decrease in τ and β angles, a nearly linear relationship is observed between exchange
coupling and average Ni–O–Ni angle for such complexes, where antiferromagnetic coupling
increases with increase in Ni–O–Ni angle.24
Complex 3 shows two types of exchange pathways; coupling constants J1 and J2 are assigned
to μ-phenoxido/μ3-methoxido bridge and double μ3-methoxido bridge, respectively. The four
average μ-phenoxido/μ3-methoxido bridging angles (97.97°, 98.38°, 98.34° and 98.57°) are
very close, therefore same exchange coupling (J1) is considered for these angles. The average
Ni–O–Ni bridging angles for J1 and J2 are 98.3° and 97.57° respectively. Complex 4
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possesses a C2 symmetry and here for the two μ-phenoxido/μ3-hydroxido bridges two
different exchange coupling constants, J1 and J2 are considered as the average Ni–O–Ni
bridging angles (98.13° and 99.56°) for the two bridges are different. Similarly for the
double μ3-hydroxido bridges, two different coupling constant J3 and J4 are considered as the
average Ni–O–Ni bridging angles are 98.62° and 96.79°, respectively. Fig. 7 (left) shows an
approximate linear dependence of exchange coupling (J) with corresponding mixed μ-
phenoxido/μ3-OR (R = Me/H) bridged Ni–O–Ni angles in some previously reported defective
cubane like NiII complexes (Table S6, ESI), where antiferromagnetic coupling increases with
increase in average Ni–O–Ni angle. From this plot as shown in Fig. 7 (left), it is apparent that
the crossover angle is ~97° for this type of bridging. The sign and magnitude of exchange
couplings; J1 in 3 and J1 and J2 in 4 show good agreement with their corresponding average
Ni–O–Ni bridging angles in this correlation. Fig. 7 (right) shows similar linear dependence of
exchange coupling with corresponding double μ3-OR (R = Me/H) bridged Ni–O–Ni angles in
some earlier reported defective cubane or cubane like NiII complexes (Table S6, ESI). The
sign and magnitude of exchange couplings; J2 in 3 and J3 and J4 in 4 agree well with the
general trend of linear dependence with their corresponding average Ni–O–Ni bridging
angles. In case of μ3-OR (R = Me/H) bridged cubane like NiII complexes, it is noticed that the
crossover angle is ~99° for such type of bridging.25 However, one should be cautious on
mentioning the exact crossover angle as other factors like τ and β can affect the exchange
interactions and the points are highly scattered.
Figure 7: Variation of the magnetic exchange coupling (J) in cubane and defective cubane
like NiII complexes with mixed μ-phenoxido/μ3-OR (R = Me/H) bridged (left) and double μ3-
OR (R = Me/H) bridged average Ni–O–Ni angles (right).
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Electrochemistry
Cyclic voltammograms (CVs) of all complexes 1−4 were recorded in acetonitrile with respect
to an Ag/AgCl electrode at a scan rate of 100 mV/sec. On forward cathodic scan (0 to −1.8
V) only one wave appeared in the cyclic voltammograms of complexes 1, 3 and 4 (Fig. 8) for
the reduction process at the electrode surface. The peaks were observed at Ec = −1.43 V for
complex 1, Ec = −1.65 V for complex 3 and Ec = −1.48 V for complex 4. The appearance of
one peak on forward scan in each of these complexes suggests the single step reduction of
NiII to NiI in these complexes. In case of cyclic voltammogram of complex 2, two waves
appeared on forward cathodic scan. Here, the two peaks were observed at Ec = −1.44, and Ec
= −1.55 V. The potential difference (ΔE12 = E1−E2)26 between two consecutive peaks was
−0.11 V for complex 2. The appearance of two peaks in the cyclic voltammogram of complex
2 indicates the presence of two different Ni(II) species in the solution. Complex 2 contains
two NiII species i.e. Ni(L1)+ and Ni(HL2)+ of two different ligands.27 Therefore, during
reduction of NiII to NiI two consecutive peaks are observed in the cyclic voltammogram of
complex 2. On reverse scan, cyclic voltammograms of all complexes 1−4 do not show any
prominent peak for the oxidation of NiI to NiII, which implies that the reduction processes of
NiII species are irreversible for each of the complexes.
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Figure 8: Cyclic voltammograms of complexes 1−4 which are recorded in actronitrile (0.1 M
TBAP) with a glassy carbon working electrode at the scan rate of 100 mVs–1.
Conclusions
We have reacted the flexidentate ligand, N,N′-bis(3-methoxysalicylidene)diethylenetriamine)
(H2L2) having seven potential donor atoms with nickel nitrate by changing the metal:ligand
ratios and obtained one each of mono-, tetra- and hexanuclear complexes. These are the first
report of the synthesis of NiII complexes with the ligand H2L2. The mononuclear complex
[NiHL2] was isolated as cocrystal along with another complex [NiL1]. HL1 is the
monocondensed product of diethylenetriamine and o-vaniline. It was formed by the
hydrolysis of H2L2 during the synthesis. To obtain pure [NiL1], we successfully developed a
new template method, exploiting the square planar geometry of NiII. In this report, by
isolating mono-, tetra- and hexa-nuclear complexes, we have also shown that the nuclearity
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of the resulting NiII complexes of the ligand (H2L2) can be controlled simply by monitoring
the proportion of the reactants and the reaction conditions. The magnetic characterization
reveals that the NiII ions were ferromagnetically coupled in complex 3 whereas the coupling
was antiferromagnetic in complex 4. Thus, these two complexes are unique examples
showing the reversal of magnetic coupling on changing the nuclearity of the complex and
bridging ligand from μ3-methoxido to μ3-hydroxido. Careful inspection of the structural
parameters revealed that these changes brought about slight difference in the bridging angles.
As the bridging angles in these complexes are close to the crossover angle, these changes,
albeit very small, are good enough for this reversal of magnetic coupling.
Acknowledgements
T. K. G. is thankful to the University Grants Commission (UGC), New Delhi, for Senior
Research Fellowship [Sr. no. 2061510191, Ref. no. 21/06/2015 (I) EU-V, dated 15/12/2015].
P. M. is thankful to the Council of Scientific and Industrial Research (CSIR), Government of
India for the Senior Research Fellowship [Sanction no. 09/02(0935)/2014-EMR-I]. A. G.
thanks CRNN, University of Calcutta, Kolkata, India for the magnetic measurement. We
thank USIC, The University of Burdwan, West Bengal, India, for providing the single-crystal
X-ray diffractometer facility.
Supporting Information (see footnote on the first page of this article): ORTEP diagram of
complex 1B, Description of IR spectra, pictures of hydrogen bonding interactions, table of
bond length and bond angles parameters and hydrogen bonding parameters of complexes 14
as well as magnetization plots of complexes 34. CCDC 1907763 (for 1), 1907764 (for 2),
1907765 (for 3) and 1907766 (for 4) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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