Rare trinuclear NiII2MII complexes (MII?=?Mn, Fe and Co) with a reduced Schiff base ligand: Synthesis, structures and magnetic properties

Article abstract of DOI:10.1016/j.ica.2017.10.041

Three new trinuclear hetero-metallic Ni^II₂M^II complexes with M^II = Mn, Fe and Co have been synthesized using a [NiL^R] “metalloligand”, where H₂L^R = N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine. All complexes have been characterized by elemental analysis, spectroscopic methods, single crystal XRD and magnetic and electrochemical studies. In the three complexes, in addition to the double phenoxido bridges, the two terminal Ni^II atoms are linked to the central M^II [M = Mn(1), Fe(2) and Co(3)] ion by means of a bridging carboxylato co-anion, giving rise to a linear Ni^II-M^II-Ni^II structure. Variable temperature magnetic susceptibility measurements show the presence of weak ferromagnetic and antiferromagnetic exchange interactions mediated through the double phenoxido bridges with J values of +8.5 and ?3.0 cm^?1 for complexes 1 and 2, respectively. Compound 3 shows the presence of antiferromagnetic interactions. Cyclic voltammetry shows a common quasi-reversible one electron oxidation corresponding to the Ni(II)/Ni(III) process in 1–3 and an irreversible M(II)/M(III) oxidation for Mn(1) and Fe(2).

Full text of DOI:10.1016/j.ica.2017.10.041

Accepted Manuscript
Research paper
Raretrinuclear NiII 2MII complexes (M II = Mn, Feand Co) with a reduced
Schiff base ligand: Synthesis, structures and magnetic properties
Alokesh Hazari, Tanmoy Kumar Ghosh, Carlos J. Gómez-García, Ashutosh
Ghosh
PII:
S0020-1693(17)31518-9
https://doi.org/10.1016/j.ica.2017.10.041
ICA 17968
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
2 October 2017
24 October 2017
29 October 2017
Please cite this article as: A. Hazari, T. Kumar Ghosh, C.J. Gómez-García, A. Ghosh, Rare trinuclear NiII 2MII
complexes(M II =Mn,FeandCo)withareducedSchiff base ligand: Synthesis, structures and magnetic properties,
Inorganica Chimica Acta (2017), doi: https://doi.org/10.1016/j.ica.2017.10.041
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Rare trinuclear NiII MII complexes (MII = Mn, Fe and Co) with a reduced
2
Schiff base ligand: Synthesis, structures and magnetic properties
Alokesh Hazari,a,b Tanmoy Kumar Ghosh,a -García,c Ashutosh Ghosh*a
aDepartment of Chemistry, University College of Science, University of Calcutta, 92, APC Road, Kolkata-
700009, India
bDepartment of Chemistry, Government General Degree College Singur, Jalaghata, Hooghly-712409,
India
cInstituto de Ciencia Molecular (ICMol), ), Departmento de QuímicaInorgánica.
, 2, 46980 Paterna, Valencia, Spain
ABSTRACT
Three new trinuclear hetero-metallic NiII MII complexes with MII = Mn, Fe and Co have been
2
synthesized using a [NiLR] “metalloligand”, where H2LR = N,N'-bis(2-hydroxybenzyl)-1,3-
propanediamine. All complexes have been characterized by elemental analysis, spectroscopic
methods, single crystal XRD and magnetic and electrochemical studies. In the three complexes,
in addition to the double phenoxido bridges, the two terminal NiII atoms are linked to the central
MII [M = Mn(1), Fe(2) and Co(3)] ion by means of a bridging carboxylato co-anion, giving rise
to a linear NiII-MII-NiII structure. Variable temperature magnetic susceptibility measurements
show the presence of weak ferromagnetic and antiferromagnetic exchange interactions mediated
through the phenoxido bridges with J values of +8.5 and -3.0 cm-1 for complexes 1 and 2,
respectively. Compound 3 shows the presence of antiferromagnetic interactions. Cyclic
voltammetry shows a common quasi-reversible one electron oxidation corresponding to the
Ni(II)/Ni(III) process in 1-3 and an irreversible M(II)/M(III) oxidation for Mn(1) and Fe(2).
Keywords: Nickel(II), Manganese(II), Iron(II), Cobalt(II), Magnetic properties
1. Introduction
The chemistry of hetero-metallic polynuclear clusters of transition metals have attracted
considerable interest to the inorganic chemists not only for their attractive structural features but
also for their applications in various fields such as magnetism, photoluminescence and
multifunctional molecular materials [1-4]. There are various synthetic strategies to build hetero-
1

metallic coordination clusters. Among them, the use of previously synthesized metal complexes
with active bridging groups/atoms has shown to be a very efficient one [5-6]. Thus, neutral
transition metal chelates of tetradentate N2O2 donor salen type Schiff bases have been employed
as chelating bidentate “metalloligands” by different groups, including ours, since these metal-
containing ligands are well adapted for building poly-nuclear hetero-metallic clusters with
interesting structures and intriguing magnetic properties [7]. Recently, we have found that
complexes of reduced Schiff bases can also be used as “metalloligands” like their non-reduced
analogues [8]. Moreover, they may give rise to different structures, thanks to the higher
flexibility of the ligand backbone and the ability to form hydrogen bonds [8-9]. Since
“metalloligands” of Cu(II) or Ni(II) are the most stable ones among the divalent first row
transition metals, they have been widely used for this purpose leading to a vast library of hetero-
nuclear species with different combination of transition metal ions [10]. Among these, trinuclear
MCu2 [11] or MNi2 [12] (where M = any divalent hetero-metal atom) species are arguably the
most common ones but for M = CoII ion, the numbers of trinuclear structures are only 5 for NiII
and 3 for CuII chelates [13-14]. With the salen type Schiff base ligand, the number of MnIICu2
and MnIINi2 species are 9 and 13, respectively [13,15-16]. Surprisingly, there is no single
example of trinuclear MNi2 nor MCu2 complex for M = FeII with any type of Schiff base ligand.
One of the main reasons to synthesize phenoxido-bridged polynuclear complexes is to
study their magnetic properties. Accordingly, most of the species studied till date are homo-
nuclear first row transition metal ions such as CuII, NiII, FeII, FeIII, MnII/III/IV, and CrIII [17]. The
study of hetero-nuclear systems is relatively rare. Thus, there are only three NiII-CoII [13] and six
NiII-MnII [16] magnetically characterized hetero-metallic diphenoxido-bridged trinuclear species
with N2O2 donor salen type Schiff base ligands. Furthermore, there is no single report so far of
any magnetically characterized NiII-FeII species (either dinuclear, trinuclear or tetranuclear) with
a N2O2 donor salen type Schiff base ligand.
Herein, we report the synthesis, crystal structure, magnetic and electrochemical properties
of three new hetero-metallic complexes, [(NiLR)2Mn(CH3COO)2(H2O)2]2CH3OH (1)
[(NiLR)2Fe(CH3COO)2(H2O)2]3H2O (2) and [(NiLR)2Co(CH3COO)2(H2O)2]2CH3OH (3) with
the reduced N2O2 donor di-Schiff base ligand, H2LR (N,N'-bis(2-hydroxybenzyl)-1,3-
propanediamine). Among these, complex 2 is the first example of NiII FeII trinuclear cluster with
2
2

a tetradentate Schiff base ligand. Complex 3 is also a rare example of a trinuclear NiII CoII
2
cluster with a Schiff base ligand. The magnetic measurements show the presence of
ferromagnetic interactions in 1 and antiferromagnetic interactions in 2 and 3 between the NiII and
the respective hetero-metal ion (MnII, FeII and CoII for 1-3, respectively) within the trinuclear
cluster.
2. Experimental
2.1. Starting Materials
1,3-propanediamine, salicylaldehyde 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
(H2LR) “m g ” [N LR]
The di-Schiff base ligand derived from 1,3-propanediamine and salicyldehyde was
synthesized by a standard reported method [11(a)]: 1,3-propanediamine (5 mmol, 0.42 mL) was
mixed with salicylaldehyde (10 mmol, 1.04 mL) in methanol (30 mL). The resulting mixture was
refluxed for ca.1 h and allowed to cool. Then 30 mL (5 mmol) of the resulting yellow ligand
solution (H2L) was cooled to 0°C and solid sodium borohydride (456 mg, 12 mmol) was added
to this solution with constant stirring. The resulting solution was acidified with concentrated HCl
(15 mL) and then evaporated to dryness on a hot water bath [18]. The reduced Schiff base ligand
H2LR was extracted from the solid residue with methanol, and the resulting colourless solution
(ca. 30 mL) was added to a methanolic solution (20 mL) containing Ni(ClO4)2·6H2O (1.820 g, 5
mmol) and 10 mL of ammonia solution (20%) to prepare the “metalloligand” [NiLR] as a solid
green powder [19] that was filtered, washed with methanol and dried in a desiccator containing
anhydrous CaCl2 [Yield: 1.245 g, 69%].
2.3.
Syntheses
of
the
compounds
[(NiLR)2Mn(CH3COO)2(H2O)2]2CH3OH
(1)
[(NiLR)2Fe(CH3COO)2(H2O)2]3H2O (2) and [(NiLR)2Co(CH3COO)2(H2O)2]2CH3OH (3)
3

The precursor complex [NiLR] (0.708 g, 2 mmol) was dissolved in methanol (20 mL) and
then an aqueous solution (1 mL) of Mn(ClO4)2·6H2O (0.364 g, 1 mmol), followed by an aqueous
solution (1 mL) of sodium acetate (0.164 g, 2 mmol), was added to this solution. The mixture
was stirred for one hour and then filtered. The filtrate was allowed to stand overnight to obtain
rhombic-shaped light violet coloured X-ray quality single crystals of 1. Needle-shaped red
crystals of 2 and square brown plates of 3 were prepared using a similar synthetic procedure with
the same stoichiometry of reactants as used for 1 but using Fe(ClO4)2·6H2O and
Co(ClO4)2·6H2O, for 2 and 3, respectively, instead of Mn(ClO4)2·6H2O. The crystals (1-3) were
washed with a methanol-water mixture and dried in a desiccator containing anhydrous CaCl2 and
then characterized by elemental analysis, spectroscopic methods, and X-ray diffraction.
Compound 1. Yield: 0.604 g, 63%. Anal. Calc. for C40H58MnN4Ni2O12: C 50.08, H 6.09,
N 5.84%. Found: C 50.12, H 6.35, N 5.65%. UV/vis: [λmax in nm (εmax in M-1 cm-1)] (MeCN) =
522(450), 290(18863) and 250(31168). IR (KBr) cm-1: ν(N-H) 3259, ν(CH3COO-) 1580. (Figs.
S1, S4 and S5)
Compound 2. Yield: 0.541 g, 57%. Anal. Calc. for C38H56FeN4Ni2O13: C 48.04, H 5.94, N
5.90%; Found: C 48.27, H 5.75, N 5.85%; UV/vis: [λmax in nm (εmax in M-1 cm-1)] (MeCN) =
470(1110), 279(16224), and 238(26192). IR (KBr) cm-1: ν(N-H) 3267, ν(CH3COO-) 1574. (Figs.
S2, S4 and S5)
Compound 3. Yield: 0.664 g, 69%. Anal. Calc. for C40H58CoN4Ni2O12: C 49.88, H 6.07, N
5.82%; Found: C 49.72, H 6.15, N 5.76%. UV/vis: [λmax in nm (εmax in M-1 cm-1)] (MeCN) =
591(297), 284(18190) and 249(30042). IR (KBr) cm-1: ν(N-H) 3261, ν(CH3COO-) 1587. (Figs.
S3-S5)
2.4. Physical Measurements
Elemental analyses (C, H and N) were performed using a Perkin-Elmer 2400 series II CHN
analyzer. IR spectra in KBr pellets (4000-400 cm-1) were recorded using a Perkin- Elmer RXI
FT-IR spectrophotometer. Electronic spectra in acetonitrile (800-200 nm) were recorded in a
Hitachi U-3501 spectrophotometer. The DC magnetic susceptibility measurements were carried
out in the temperature range 2-300 K with an applied magnetic field of 0.1 T on polycrystalline
samples of compounds 1-3 (with masses of 22.65, 16.37 and 16.25 mg, respectively) with a
Quantum Design MPMS-XL-5 SQUID susceptometer. The isothermal magnetization was
performed on the same samples at 2 K with magnetic fields up to 5 T. The susceptibility data
4

were corrected for the sample holders previously measured using the same conditions and for the
diamagnetic contributions of the salt as deduced by using Pascal´s constant tables ( = -529.6
dia
-6
-6
-6
-1
x 10 , -531.6 x 10 and -528.9 x 10 emu.mol , respectively) [20].
2.5. Electrochemical Measurements
The electrochemical measurements of the three complexes were performed using an
Epsilon Basi-C3 Cell instrument at a scan rate of 100-300 mV s-1 within the potential range of 0
to +2.00 V vs Ag/AgCl. Cyclic voltammograms were carried out using 0.1 M
tetrabutylammonium perchlorate (TBAP) as supporting electrolyte and 1.0 × 10-3 M solutions of
the complexes in acetonitrile previously deoxygenated with argon. The working electrode was a
glassy-carbon disk (0.32 cm2) that was polished with alumina solution, washed with absolute
acetone and acetonitrile and air-dried before each electrochemical run. The reference electrode
was Ag/AgCl, with platinum as the counter electrode. All experiments were performed in
standard electrochemical cells at 25 °C.
2.6. Crystallographic data collection and refinement
Well formed single crystals of each complex were mounted on a Bruker-AXS SMART
APEX II diffractometer equipped with a graphite monochromator and Mo Kα (λ = 0.71073 Å)
radiation. The crystals were positioned at 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 atom. 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 bonded to N or O was located in a difference Fourier map and refined with distance
constraints. The H-atoms of the three crystallization water molecules in 2 and of all the
coordinated water molecules in 1-3 could not be located. One of the crystallization water
molecules in 2 was refined isotropically using distance constraints with 50% occupancy.
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
5

program [21], while all calculations were made via SHELXS 97 [22], SHELXL 97 [23],
PLATON 99 [24], ORTEP-32 [25], WINGX system ver-1.64 [26] and refined using refined
using SHELXL-2014 [27]. Data collection, structure refinement parameters, and crystallographic
data for the three complexes are given in Table 1.
3. Results and discussion
3.1. Syntheses of the complexes
The reduced Schiff-base ligand (H2LR) and the corresponding metalloligand [NiLR] were
synthesized using the reported procedures [11a,19]. The metalloligand [NiLR] on reaction with
Mn(II) or Fe(II) or Co(II) and sodium acetate in a 2:1:2 molar ratio in MeOH:H2O 10:1(v/v)
resulted in three new trinuclear complexes: [(NiLR)2Mn(CH3COO)2(H2O)2]2CH3OH (1)
[(NiLR)2Fe(CH3COO)2(H2O)2]3H2O (2) and [(NiLR)2Co(CH3COO)2(H2O)2]2CH3OH (3),
respectively (Scheme 1).
Scheme 1. Syntheses of complexes 1-3.
3.2. Crystal Structures
[(NiLR)2Mn(CH3COO)2(H2O)2]·2CH3OH (1). The linear trinuclear structure of compound 1,
together with the atomic numbering scheme is shown in Fig. 1. The bond distances around the
metals are given in Table 2 and the bond angles in Table 3. The structure contains a
crystallographic inversion centre that sits on the central manganese atom, Mn(1). The
asymmetric unit consists of one terminal Ni(II) centre, half central Mn(II) ion, one reduced
6

deprotonated di-Schiff base ligand [(LR)2-], one bridging acetate ligand and one coordinated
water molecule. The terminal nickel centre shows a distorted octahedral NiO4N2 geometry where
the two axial positions are occupied by an oxygen atom of the acetate ligand at a distance of
2.057(3) Å and by a coordinated water molecule at 2.181(3) Å. The basal plane is formed by the
two N atoms and two phenoxido O-atoms from the dianioinc tetradentate reduced Schiff-base
ligand [(LR)2-]. The basal Ni-N bond distances [2.075(4)-2.089(4) Å] are slightly greater than the
basal Ni-O ones [2.035(3)-2.046(3) Å] (Table 2). The four donor atoms in the equatorial plane
show root mean squared deviation (r.m.s.) of 0.045(1) Å with the nickel atom 0.035(1) Å shifted
from the mean plane towards O(2). The dihedral angle between two N-Ni-O planes [4.5(1)º] and
the range of cis [81.7(1)-95.2(2)º] and trans angles [172.2(1)-176.7(1)º] (Table 3) of the terminal
nickel centre also indicate the presence of a distortion from the ideal octahedral geometry. The
central manganese atom, Mn(1), presents an octahedral MnO6 geometry where the basal plane is
formed by the coordination of four µ2-phenoxido O-atoms from two dianionic reduced Schiff-
base ligand [(LR)2-] and the axial positions are occupied by two oxygen atoms from acetate
anions. The basal Mn-O bond distances are almost similar [2.120(3)-2.132(3) Å] but they are
slightly shorter than the axial one [2.245(3) Å] (Table 2).
a
Fig. 1. The structure of 1 with ellipsoids at 30% probability. Symmetry element = 2-x,1-y,1-z for 1. Methanol
solvent molecules outside the coordination sphere are also removed.
7

The range of cis angles [77.7(1)-86.4(1)º] (Table 3) around central Mn centre also
indicate a distortion from ideal octahedral geometry. The Ni(1)Mn(1) distance is 3.140(2) Å
and the two Ni-O-Mn bond angles are equal within experimental error: 97.79(11)º and
97.76(12)º.
[(NiLR)2Fe(CH3COO)2(H2O)2]·2H2O (2). The linear trinuclear structure of compound 2,
together with the atomic numbering scheme is shown in Fig. 2. The bond distances around the
metals are given in Table 2 and the bond angles in Table 3. This structure also contains a
crystallographic inversion centre at the central Fe atom. The structure of this compound is very
similar to that of previously described compound 1. The central Fe also has a FeO6 distorted
octahedral geometry where the two axial O atoms belong to two acetate ligands and the basal
plane is formed by four phenoxido oxygen atoms from two dianionic tetradentate reduced Schiff
base ligands [(LR)2-]. The basal Fe(1)-O bond lengths are shorter than the axial one (Table 2) and
the cis angles [80.1(1)-86.3(1)º] deviate from the ideal value (90º) (Table 3) indicating
distortions in the octahedral coordination geometry.
The basal plane of the terminal Ni atom is constructed by the coordination of two imine
nitrogen atoms and two oxygen atoms from the reduced Schiff base ligand [(LR)2-]. One oxygen
atom from a bridging acetate ion and one O-atom from a water molecule coordinate to the axial
positions to complete the octahedral NiO4N2 coordination environment. As in 1, the basal Ni-O
bond distances [2.034(3)-2.045(3)Å] are shorter than the Ni-N ones [2.089(3)-2.090(3) Å]. It is
noticeable that the Ni-O bond distances are shorter in 2 than those in 1 but the Ni-N distances
show the opposite trend (Table 2). This fact is due to a shortening of the two axial Ni-O bonds in
2 (since the basal Ni-O bonds are equal in 1 and 2, Table 2) probably due to the smaller size of
Fe(II) compared with Mn(II) that allows the acetato ligand to get closer to the Ni2M core in 2.
The distortion from the ideal octahedral geometry of the Ni atom is smaller in 2 than in 1 as
evidenced by the dihedral angle between the two N-Ni-O planes [3.2(1) and 4.5(1)º for 2 and 1,
respectively]. The r.m.s deviation [0.014(1) Å] of the four basal atoms from their mean plane and
the range of cis [80.2(1)-94.1(1)º] and trans [172.7(1)-176.8(1)º] angles (Table 3) also confirm
this fact. The Ni(1)Fe(1) distance is 3.075(2) Å and the two Ni-O-Fe bond angles are equal
within experimental error: 97.70(10)º and 97.79(10)º.
8

Fig. 2. The structure of 2 with ellipsoids at 30% probability. Symmetry element a = 2-x,-y,2-z for 2. Water solvent
molecules outside the coordination sphere are also removed.
[(NiLR)2Co(CH3COO)2(H2O)2]·2 H3OH (3). Fig. 3 shows the trinuclear complex 3 with the
atomic numbering scheme. The bond distances around the metals are given in Table 2 and the
bond angles in Table 3. Compound 3 shows a structure similar to those of 1 and 2. Thus, 3 also
contains a crystallographic inversion centre, located at the central atom (Co in this case). The
asymmetric unit of 3 consists of a terminal Ni(II) atom, half a Co(II) atom located at the
inversion centre, one deprotonated reduced di-Schiff base ligand [(LR)2-], one bridging acetato
ligand and one coordinated water molecule. The terminal nickel centre [Ni(1)] presents a
distorted octahedral geometry as in 1 and 2. The basal plane of Ni(1) is formed by the two imine
N atoms and two phenoxido O atoms whereas the axial positions are occupied by one O-atom
from a bridging acetato ligand and a water molecule. The basal Ni-O bond distances are shorter
than the basal Ni-N and the axial Ni-O ones (Table 2), as is observed in 1 and 2. The range of cis
angles [80.3(7)-94.7(1)º] and trans angles [172.2(1)-176.2(1)º] around Ni(1) indicate that the
distortion of the coordination sphere from the ideal octahedral geometry in 3 is intermediate
between 1 and 2. This is confirmed by the root mean square deviations of the four basal atoms
from the mean plane which is 0.044(2) Å with the metal atom 0.026(1) Å shifted from this mean
plane towards O(2) and also from the dihedral angle between two N-Ni-O planes [4.0 (1)º].
9

Fig. 3. The structure of 3 with ellipsoids at 30% probability. The crystallization methanol molecules are omitted for
clarity. Symmetry element a = 1-x, 1-y,2-z.
The central Co(II) atom is bonded to four phenoxido oxygen atoms of two deprotonated
tetradentate reduced Schiff base ligands and two oxygen atoms from two bridging acetato
ligands as in 1 and 2. The basal Co-O bond distances are very similar, as observed in 1 and 2,
and are shorter than those of 1 but longer than those of 2. The central cobalt atom also shows a
slightly distorted octahedral geometry, as confirmed by the range of cis angles [79.0(1)-86.4(1)º].
The Ni(1)⋯Co(1) distance is 3.093(1) Å and the two Ni-O-Co bond angles are equal within
experimental error: 98.29(8)º and 98.23(8)º.
3.3. Electrochemistry
The cyclic voltammogram (CV) of the metallo-ligand, [NiLR] in acetonitrile shows a quasi-
reversible oxidation process with an anodic peak at Ea = +0.97 V with the corresponding
cathodic peak at +0.75 V in the reverse scan (Fig. S6) and a peak separation value (ΔEpa = Eanode
-
Ecathode) of +0.22 V. This quasi-reversible oxidation process is ascribed to the one electron
Ni(II)/Ni(III) oxidation process. On the other hand, complex 1 shows two peaks at E1 = +1.45 V
10

and E2 = +1.04 V on the anodic scan and only one peak at E2 = +0.52 V on the reverse scan with
a peak separation value, ΔEpa of +0.52 V (Fig. 4).
Fig. 4. Cyclic voltammograms (CVs) of 10-3 M solution of complex 1 in acetonitrile at room temperature and
different scan rates (100-300), using TBAP as supporting electrolyte.
Complex 2 also shows one irreversible and one quasi-reversible peaks at E1 = +1.12 V and
E2 = 0.78 V on the anodic potential scan and one peak at E2 = +0.55 V on the reverse scan (Figs.
S7 and S8) with a peak separation of +0.57 V, very similar to that of complex 1. Complex 3 only
shows one anodic peak at Ea = +1.12 V with the corresponding cathodic peak at Ec = +0.47 V
and with a peak separation of +0.65 V (Fig. S9). If we compare the cyclic voltammograms (CV)
of complexes 1-3 with that of the metalloligand [NiLR], we can see a common quasi-reversible
peak in all cases (at +1.04 in 1, +1.12 V in 2 and 3, and +0.97 V in [NiLR]). This common one-
electron process is, accordingly, attributed to the Ni(II)/Ni(III) oxidation in the metalloligand
[NiLR] and in complexes 1-3. The second irreversible wave observed in the anodic scan only in
complexes 1 and 2 (at E = +1.45 V in 1, +0.78 V in 2) can be attributed to the M(II)/M(III)
oxidation of the central ion with the corresponding irreversible complex rupture.
3.4. Magnetic properties
11

The thermal variation of the product of the molar magnetic susceptibility per Ni2Mn trimer
times the temperature (mT) for compound 1 shows a room temperature value of ca. 7.0 cm3 K
mol-1, close to the expected value for two isolated Ni(II) ions with g ≈ 2.2 plus a Mn(II) ion with
g ≈ 2 (Fig. 5). When the sample is cooled, the mT product increases and reaches a maximum of
ca. 9.1 cm3 K mol-1 at ca. 6.5 K. Below this temperature mT sharply decreases to reach a value
of ca. 5.7 cm3 K mol-1 at 2 K. The increase observed with cooling suggests the presence of
predominant ferromagnetic Ni-Mn exchange interactions in the trimer whereas the sharp
decrease at low temperatures can be attributed to the zero field splitting (ZFS) of the resulting S
= 9/2 ground spin state in the cluster. Given the centro-symmetric nature of the complex, we
have fitted the magnetic properties of compound 1 with a symmetric linear trimer model with
only one coupling constant (J, Fig. 6) using the Hamiltonian: H = -J (S1S2 + S2S3) with S1 = S3 =
1 and S2 = 5/2 with the MAGPACK programme [28,29]. This model reproduces very
satisfactorily the magnetic properties of compound 1 above the maximum with g = 2.239, J =
+8.5 cm-1 and a 8.7 % of paramagnetic impurity (solid line in Fig.5). The paramagnetic
impurities may be due to the presence of defective clusters or monomeric species formed from
the trimeric Ni2Mn cluster.
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
0
50 100 150 200 250 300
T (K)
Fig. 5. Thermal variation of the mT product per Ni2Mn trimer for compound 1. Solid line is the best fit to the model
(see text).
12

Fig. 6. View of the Ni2M linear complexes in 1-3 showing the magnetic exchange scheme.
In order to explain the ferromagnetic coupling observed in complex 1 we must take into
account that previous magneto structural correlations for double oxido bridges connecting
different transition metal ions (mainly Cu) show that the coupling passes from ferro- to
antiferromangetic when the M-O-M angles overpass a certain value [16,30-38]. This value
depends on the metal ions and also on other structural parameters as the MO2M dihedral angle
[30-38]. As far as we know, there are only seven magnetically and structurally characterized
Ni2Mn complexes with a double oxido and a syn-syn carboxylato bridges [16d-e,38]. These
compounds present very weak antiferromangetic (J = -0.60, -0.30 and -0.24 cm-1) [16d-e,38] or
ferromagnetic (J = +2.0, +1.1, +0.50 and +1.38 cm-1) [16d-e] Ni-Mn couplings, mainly
depending on the Ni-O-Mn bond angle with a crossing angle of ca. 98º. Thus, for angles below
ca. 98º the coupling is ferromagnetic and when the angle is above ca. 98º the coupling is
antiferromagnetic. This value is further confirmed by compound 1 where the two Ni-O-Mn bond
angles are identical within experimental error (97.78(11)º and 97.77(11)º) and slightly below 98º
and, thus, a ferromagnetic coupling is expected. Accordingly, compound 1 presents a weak
ferromagnetic coupling. Note that the larger value of J found in 1 may be related to the larger
dihedral NiO2Mn angle found in 1 (157.7º), compared with the other four ferromagnetic
compounds (where the dihedral angles are 154.1º, 153.9º, 151.8º and 151.3º). These lower values
simply a reduction of the orbital overlap and, accordingly, of the magnetic coupling, in
agreement with the experimental results. Of course, the presence of an additional syn-syn
carboxylate bridge is expected to increase the antiferromagnetic coupling in all cases unless
counter-complementarity takes place.
The mT value at room temperature per Ni2Fe trimer in 2 is ca. 5.0 cm3 K mol-1, close to
the expected value for two isolated Ni(II) ions plus a Fe(II) ion (Fig. 7). When the sample is
13

cooled, mT decreases to reach a value of ca. 1.2 cm3 K mol-1 at 2 K. This behaviour indicates
the presence of an antiferromagnetic coupling of the two S = 1 Ni(II) ions and the central S = 2
Fe(II) ion in the trimer leading to a S = 0 ground spin state. As in 1, given the symmetric nature
of complex 2, we have fitted the magnetic properties of compound 2 with the same symmetric
linear trimer model with one coupling constant (J, Fig. 6) using the Hamiltonian: H = -J (S1S2 +
S2S3) but now with S1 = S3 = 1 and S2 = 2 with the MAGPACK programme [28,29]. This model
reproduces very satisfactorily the magnetic properties of compound 2 in the whole temperature
range with g = 2.101, J = -3.0 cm-1 and a 4.1 % of paramagnetic impurity (solid line in Fig.7). As
in compound 1, the paramagnetic impurities may be attributed to the presence of defective
clusters or monomeric species formed from the trimeric Ni2Fe cluster.
6
5
4
3
2
1
0
0
50 100 150 200 250 300
T (K)
Fig. 7. Thermal variation of the mT product per Ni2Fe trimer for compound 2. Solid line is the best fit to the model
(see text).
Since there is no other magnetically and structurally characterized NiII FeII trimers with a
2
double phenoxido and a syn-syn carboxylato bridges, we cannot establish any magneto-structural
correlation to explain the weak antiferromagnetic coupling found in compound 2. Since the
Ni-O-Fe bond angles (97.87(10)º and 97.77(11)º) and the dihedral NiO2Fe angle (160.2º) in
compound 2 are similar to those observed in compound 1 (where the coupling is weak but
ferromagnetic), we can conclude that the main difference between 1 and 2 (besides the central
metal atom) is the much shorter M-Oacetato bond distances in 2 (Table 2). These shorter bond
distances are expected to give rise to a larger (in module) antiferromagnetic contribution in 2,
resulting in an overall antiferromagnetic coupling, as observed experimentally.
14

The thermal variation of mT for compound 3 shows a room temperature value of ca.
5.0 cm3 K mol-1, close to the expected value for two isolated Ni(II) ions (ca. 2 cm3 K mol-1) plus
one isolated Co(II) ion with an orbital contribution (ca. 2.8-3.4 cm3 K mol-1). When the sample is
cooled mT shows a progressive decrease reaching a minimum of ca. 4.3 cm3 K mol-1 at ca. 25
K. Below 25 K mT increases and reaches a maximum of ca. 4.5 cm3 K mol-1 at ca. 8 K followed
by a sharp decrease to ca. 3.9 cm3 K mol-1 at 2 K (Fig. 8). This behaviour suggests the presence
of antiferromagnetic Ni-Co exchange interactions in the trimer, leading to a ferri-magnetic
exchange since the Ni(II) and Co(II) spins cannot cancel each other. The sharp decrease
observed at very low temperatures can be attributed to the presence of a zero field splitting on
the ground spin state and/or weak inter-trimer antiferromagnetic interactions.
5.5
5.0
4.5
4.0
3.5
0
50 100 150 200 250 300
T (K)
Fig. 8. Thermal variation of the mT product per Ni2Co trimer for compound 3
The ferri-magnetic Ni-Co coupling found in compound 3 cannot be easily rationalized
since there are no magneto-structural correlations in other Ni-Co clusters with similar bridges.
Nevertheless, in a similar trinuclear Ni2Co complex (A) with a double oxido and a syn-syn
carboxylato bridges, the coupling found was weak and ferromagnetic [38]. This result contrast
with the weak antiferromagnetic coupling observed in 3. In compound A the two oxido bridges
are quite different (one is a phenoxido and the other is a carboxylato) and the Ni-O-Co bond
angles are also very different (102.110(4)º and 94.380(4)º with an average value of 98.245º). In
contrast, in 3 the two phenoxido bridges are very similar and the Ni-O-Co bond angles are equal
within experimental error (98.24(7)º and 98.26(7)º) and also equal to the average value in A.
Besides the different nature of the double oxido bridge, the main differences between 3 and A
15

are: (i) the dihedral angle in the NiO2Co unit (157.1º in A and 159.9º in 3) and (ii) the M-Oacetato
bond distances which are significantly longer in compound 3 (Co-O = 2.183(2) Å and Ni-O =
2.048(2) Å) than in A (Co-O = 2.045(4) Å and Ni-O = 2.064(4) Å) [38]. These two differences
play opposite roles (in 3 the larger dihedral angle is expected to increase the antiferromagnetic
coupling but the longer Co-O bond distance is expected to decrease it). Since the coupling is
weak antiferromagnetic in 3 and weak ferromagnetic in A, we can conclude that the dihedral
angle in the NiO2Co unit may be the dominant factor. Nevertheless, we cannot exclude the
possibility that the double phenoxido/carboxylato bridge in 3 shows complementarity (i.e., the
antiferromagnetic interactions add each other) in contrast to compound A where the counter-
complementarity of the triple oxido/phenoxido/carboxylato bridge was pointed as the origin of
the weak ferromagnetic coupling found in A [38].
4. Conclusions
The metalloligand synthetic approach has proved once again to be a very useful and
simple synthetic tool to synthesize polynuclear hetero-metallic clusters. Here we have shown
how the use of a [NiLR] complex [H2LR = tetradentate reduced di-Schiff base] as a “metallo-
ligand” towards divalent first row transition metal ions, MII= Mn, Fe and Co has yielded three
new trinuclear linear NiII-MII-NiII clusters where the central MII ion is coordinated to the terminal
[NiLR] via a double phenoxido bridge along with a syn-syn carboxylato bridge. Complexes 1-3
are among the very few known Ni2MII complexes with double oxido bridges and N2O2 donor
Schiff bases. Even more, complex 2 (NiII FeII) is the first example of a trinuclear NiII-FeII-NiII
2
metal cluster with a salen type Schiff base ligand. In complexes 1-3 the double phenoxido and
syn-syn carboxylato bridges mediate ferromagnetic (in 1), antiferromagnetic (in 2) and
ferrimagnetic (in 3) interactions between the terminal NiII ions and the central MII one (MII = Mn
(1), Fe (2) and Co (3)). Despite the lack of magneto-structural correlations for this kind of
clusters, given the very reduced number of known examples, the magnetic interactions could be
rationalized with the help of previous studies on related examples.
16

Acknowledgements
The authors thank the Department of Science and Technology (DST), New Delhi, India, for
financial support (SR/S1/IC/0034/2012). TKG is thankful to the University Grants Commission
(UGC), New Delhi, for the Junior Research Fellowship [Sr. no. 2061510191, Ref. No.-
21/06/2015 (I) EU-V, Dated 15/12/2015], Council of Scientific and Industrial Research (CSIR).
We also acknowledge the Generalitat Valenciana (PrometeoII/2014/076 and ISIC Programs) for
financial support.
Appendix A. Supplementary data
CCDC 1565583-1565585 contains the supplementary crystallographic data for (1–3). These data
can be obtained free of charge via 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. Supplementary data (Discussion of IR and
UV–Vis spectroscopy (Figs. S1-S5), CV Plots (Figs. S6-S9) associated with this article can be
found, in the online version.
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Highlights:
 Trinuclear hetero-metallic NiII MII complexes (MII = Mn, Fe and Co) have been
2
synthesized using a [NiLR] “metalloligand” s nthetic approach.
 NiII FeII trinuclear complex is the first example of a NiII-FeII-NiII metal cluster with a
2
reduced salen type Schiff base ligand.
 The double phenoxido and syn-syn carboxylato bridges mediate ferromagnetic in 1,
antiferromagnetic in 2 and ferrimagnetic in 3 exchange interaction between the terminal
NiII ions and the central MII one (MII = Mn (1), Fe (2) and Co (3))
20

Table 1
Crystal data and structure refinement of complexes 1-3.
1
2
3
Formula
M
C40H58Mn1N4Ni2O12 C38H54Fe1N4Ni2O13 C40H58Co1N4Ni2O12
959.23
950.10
963.23
Crystal System
Space Group
Temperature (K)
a/Å
Triclinic
Triclinic
Triclinic
P
P
P
1
1
1
293
10.178(5)
10.438(5)
10.946(5)
67.583(5)
80.703(5)
87.614(5)
1060.6(9)
1
293
9.927(5)
10.470(5)
10.845(5)
68.184(5)
81.311(5)
86.255(5)
1034.4(9)
1
293
10.117(5)
10.412(5)
10.903(5)
67.669(5)
81.390(5)
87.835(5)
1050.2(9)
1
b/Å
c/Å
/
/
/
V/Å3
Z
Dc/g cm-3
µ/mm-1
1.498
1.238
1.512
1.314
1.521
1.345
F (000)
499
488
501
R(int)
0.069
0.029
0.025
Total Reflections
Unique reflections
I>2(I)
R1a, wR2b
GOFc on F2
R (all)
12221
3739
2622
8904
3608
3050
11681
3690
3172
0.0509, 0.1188
1.05
0.0377, 0.1286
1.07
0.0308, 0.0926
1.06
0.0785
0.0462
0.0375
2
2
4 ½
aR1 = ||Fo| − |Fc|||Fo|, bwR2 (Fo ) = [[w(Fo − Fc2)2w Fo ] ;
2
½
cGOF = [[w(Fo − Fc2)2(Nobs-Nparams)]
21

Table 2
Bond distances (Å) for complexes 1-3.
atoms
1
2
3
Ni(1)-O(10) 2.046(3) 2.045(3) 2.032(2)
Ni(1)-O(30) 2.035(3) 2.034(3) 2.032(2)
Ni(1)-N(18) 2.089(4) 2.090(3) 2.096(2)
Ni(1)-N(22) 2.075(4) 2.089(3) 2.077(3)
Ni(1)-O(1) 2.180(3) 2.149(3) 2.162(2)
Ni(1)-O(2) 2.057(3) 2.046(3) 2.048(2)
M(1)*-O(10) 2.120(3) 2.039(2) 2.057(2)
M(1)*-O(30) 2.132(3) 2.045(3) 2.060(2)
M(1)*-O(3) 2.245(3) 2.143(2) 2.183(2)
* M = Mn, Fe or Co in 1-3, respectively
Table 3
Bond angles (º) for complexes 1-3.
atoms
1
2
3
O(10)-Ni(1)-O(30)
O(10)-Ni(1)-N(18)
N(18)-Ni(1)-N(22)
O(30)-Ni(1)-N(22)
O(1)-Ni(1)-O(10)
O(1)-Ni(1)-O(30)
O(1)-Ni(1)-N(18)
O(1)-Ni(1)-N(22)
O(2)-Ni(1)-N(18)
O(2)-Ni(1)-N(22)
O(2)-Ni(1)-O(10)
O(2)-Ni(1)-O(30)
81.7(1)
91.2(1)
95.2(2)
92.0(1)
88.3(1)
91.0(1)
89.6(1)
87.3(2)
89.2(1)
89.7(2)
94.8(1)
90.6(1)
80.2(1)
93.0(1)
94.1(1)
92.6(1)
91.0(1)
90.5(1)
86.8(1)
88.0(1)
90.4(1)
90.7(1)
90.8(1)
92.5(1)
80.3(7)
92.0(8)
94.7(1)
93.1(1)
89.5(8)
92.4(1)
88.9(1)
86.7(1)
89.4(1)
90.1(1)
93.9(1)
89.7(1)
O(10)-Ni(1)-N(22) 172.2(1) 172.8(1) 172.2(1)
O(30)-Ni(1)-N(18) 172.8(1) 172.7(1) 172.2(1)
O(1)-Ni(1)-O(2)
176.7(1) 176.8(1) 176.2(1)
O(10)-M(1)*-O(30) 77.7(1)
80.1(1)
86.3(1)
85.9(1)
79.0(1)
85.3(1)
86.4(1)
O(3)-M(1)*-O(30)
O(3)-M(1)*-O(10)
85.2(1)
86.4(1)
Ni(1)-O(10)-M(1)* 97.79(11) 97.70(10) 98.29(8)
Ni(1)-O(30)-M(1)* 97.76(12) 97.89(10) 98.23(8)
* M = Mn, Fe or Co in 1-3, respectively
22

Highlights:
 Trinuclear hetero-metallic NiII MII complexes (MII = Mn, Fe and Co) have been
2
synthesized using a [NiLR] “metalloligand” synthetic approach.
 NiII FeII trinuclear complex is the first example of a NiII-FeII-NiII metal cluster with a
2
reduced salen type Schiff base ligand.
 The double phenoxido and syn-syn carboxylato bridges mediate ferromagnetic in 1,
antiferromagnetic in 2 and ferrimagnetic in 3 exchange interaction between the terminal
NiII ions and the central MII one (MII = Mn (1), Fe (2) and Co (3))
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Graphical Abstract
Rare trinuclear NiII MIIcomplexes (MII = Mn, Fe and Co) with a reduced
2
Schiff base ligand: Synthesis, structures and magnetic properties
Alokesh Hazari,a,b Tanmoy Kumar Ghosh,a -García,c Ashutosh Ghosh*a
Three new trinuclear NiII MII structures (M = Mn, Fe and Co) with reduced salen-type Schiff
2
base ligand show ferromagnetic, antiferromagnetic and ferrimagnetic exchange interactions
respectively, within the trimer.
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