Homo and heterometallic rhomb-like Ni4 and Mn2Ni 2 complexes

Article abstract of DOI:10.1016/j.poly.2013.12.031

Two new polynuclear complexes with hydroxyl-rich Schiff base ligand 3-[(2-Hydroxy-benzylidene)-amino]-propane-1,2-diol (H₃L), namely [Ni^II₂(HL)(H₂L)(SCN)]₂·DMF (1) and [Mn^III₂Ni^II₂(HL) ₂(L)₂] (2) have been synthesized and characterized by single crystal X-ray diffraction, elemental analyses, FTIR, UV-Vis spectroscopy and variable temperature magnetic susceptibility measurements. The X-ray refinements reveal that both compounds present defective rhomb-like dicubane central cores (Ni₄ in 1 and Mn₂Ni₂ in 2). Magnetic susceptibility measurements indicate the presence of overall antiferromagnetic exchange interactions in 1 along the side connected by a N and O atoms (J₁ = -43.6 cm^-1) and along the short diagonal of the rhomb (J₃ = -3.9 cm^-1) and a weak ferromagnetic coupling (J₂ = +5.8 cm^-1) along the side of the rhomb connected by a double O-bridge. In 2 the three exchange interactions are negative (J₁ = J₂ = -0.3 cm^-1 and J₃ = -0.7 cm^-1). A magneto-structural correlation with the ratio between the Ni-O-Mn bond angle and the Mn-O bond length is also discussed for double O-bridged Mn-Ni pairs.

Full text of DOI:10.1016/j.poly.2013.12.031

Polyhedron 70 (2014) 155–163
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Homo and heterometallic rhomb-like Ni4 and Mn2Ni2 complexes
Ritwik Modak a, Yeasin Sikdar a, Senjuti Mandal a, Carlos J. Gómez-García c, Samia Benmansour c,
Sudipta Chatterjee b, Sanchita Goswami a,
⇑
a Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India
b Department of Chemistry, Serampore College, West Bengal, India
c Instituto de Ciencia Molecular (ICMol), Universidad de Valencia, C/Catedrático José Beltrán, 2, 46980 Paterna, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new polynuclear complexes with hydroxyl-rich Schiff base ligand 3-[(2-Hydroxy-benzylidene)-
amino]-propane-1,2-diol (H3L), namely [NiII2(HL)(H2L)(SCN)]2ÁDMF (1) and [MnIII2NiII2(HL)2(L)2] (2) have
been synthesized and characterized by single crystal X-ray diffraction, elemental analyses, FTIR, UV–Vis
spectroscopy and variable temperature magnetic susceptibility measurements. The X-ray refinements
reveal that both compounds present defective rhomb-like dicubane central cores (Ni4 in 1 and Mn2Ni2
in 2). Magnetic susceptibility measurements indicate the presence of overall antiferromagnetic exchange
interactions in 1 along the side connected by a N and O atoms (J1 = À43.6 cmÀ1) and along the short
diagonal of the rhomb (J3 = À3.9 cmÀ1) and a weak ferromagnetic coupling (J2 = +5.8 cmÀ1) along the side
Received 23 October 2013
Accepted 28 December 2013
Available online 8 January 2014
Keywords:
Hydroxyl rich ligand
Defective dicubane core
XRD analysis
Homometallic NiII
of the rhomb connected by a double O-bridge. In 2 the three exchange interactions are negative (J1 = J2
=
4
À0.3 cmÀ1 and J3 = À0.7 cmÀ1). A magneto-structural correlation with the ratio between the Ni–O–Mn
bond angle and the Mn–O bond length is also discussed for double O-bridged Mn–Ni pairs.
Ó 2014 Elsevier Ltd. All rights reserved.
Heterometallic MnIII2NiII
2
Antiferromagnetic coupling
1. Introduction
compounds containing heterometallic Mn-Ni cores [21–26], the
number of mixed metal Mn–Ni rhomb-like cubane clusters is very
The design and synthesis of polynuclear transition-metal clus-
ters has been an active area of study for the last couple of decades
[1,2]. The presence of polynuclear transition-metal clusters in the
active sites of many enzymes has stimulated scientists to explore
this fascinating area of research [3]. Manganese and nickel are both
very important metal ions present in biological systems [4,5].
Apart from the biological importance, a growing interest in polynu-
clear transition-metal clusters originates from the prospects of
application in many other directions [6,7]. One of the most appeal-
ing ones is related to the presence of interesting magnetic proper-
ties in polynuclear transition-metal clusters as in molecule-based
magnets, whose possible applications in different areas as data
storage and magnetic refrigeration have encouraged an extensive
research interest in new transition-metal clusters [1,8–13]. Thus,
numerous research groups around the world are actively engaged
in the development of new synthetic procedures for the prepara-
tion of high nuclearity transition metals clusters. Among the differ-
ent strategies employed to reach this aim, the use of polyalkoxide
ligands [14–17] has become one of the most efficient strategy to
design and synthesize polynuclear transition-metal clusters.
Although there is a large number of homometallic cubane-like
Mn and Ni clusters [18–20] and there are also many coordination
limited [27]. Such clusters show interesting magnetic properties,
including single-molecule magnets [24]. Inspired by these studies,
research efforts have been made to synthesize homo- and hetero-
metallic mixed-metal clusters, using Mn and Ni as paramagnetic
units. In this context, we have chosen the hydroxyl-rich Schiff base
ligand 3-[(2-Hydroxy-benzylidene)-amino]-propane-1,2-diol (H3L)
[28–30] that presents four potential donor sites (one imine nitro-
gen, one phenoxido oxygen and two alkoxido oxygen atoms)
(Scheme 1). Gorden and co-workers utilized this ligand for the first
time and reported the syntheses and structural characterization of
four dinuclear uranyl complexes [28]. Recently the ligand has been
explored to construct polynuclear complexes with various core
structures, such as Cu4, Ni4, MnIII3MnIINa, Gd12Mo4 etc., [29–38].
Herein we describe the syntheses and characterization of two no-
vel defective rhomb-like dicubane clusters of the type Ni4 and Mn2-
Ni2:[NiII2(HL)(H2L)(SCN)]2.DMF (1) and [MnIII2NiII2(HL)2(L)2] (2),
prepared with the ligand H3L.
2. Experimental
2.1. Physical measurements
Elemental (C, H and N) analyses were performed on a Perkin–El-
mer 2400 II analyzer. IR spectra were recorded in the region 400–
4000 cmÀ1with a Bruker Optics Alpha-T spectrophotometer as KBr
⇑
Corresponding author. Tel.: +91 33 2350 9937; fax: +91 33 2351 9755.
E-mail address: sgchem@caluniv.ac.in (S. Goswami).
0277-5387/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.12.031

156
R. Modak et al. / Polyhedron 70 (2014) 155–163
Scheme 1. Synthetic route of formation 1 and 2.
pellets. NMR spectra were recorded with a Bruker AV 300 MHz
Supercon NMR system Dual Probe. Magnetic susceptibility mea-
surements 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 and 2 (with masses of 29.97 and 42.30 mg, respec-
tively) with a Quantum Design MPMS-XL-5 SQUID susceptometer.
The susceptibility data were corrected for the sample holders pre-
viously measured using the same conditions and for the diamag-
the crude product was washed with diethyl ether and dried under
vacuum (Yield: 74%).
2.3.2. Synthesis of [NiII2(HL)(H2L)(SCN)]2.DMF (1)
A methanolic solution (5 mL) of NaSCN (0.160 g, 2 mmol) was
added drop-wise to a methanolic solution (10 mL) of NiCl2Á6H2O
(0.951 g, 4 mmol). The resulting solution was stirred for one hour
and then a methanolic solution (5 mL) of H3L (0.780 g, 4 mmol)
was added to the mixture followed by 0.606 g (6 mmol) of Et3N.
The resulting green solution was stirred for 2.5 h and was evapo-
rated to dryness under reduced pressure. The resulting solid was
dissolved in DMF. Crystals suitable for X-ray studies were obtained
by slow diffusion of diethyl ether into this DMF solution.
´
netic contributions of the salt as deduced by using Pascals
constant tables (
1 and 2, respectively) [39].
v
dia = 662 Â 10À6 and 500 Â 10À6 emu molÀ1 for
Phase purity of polycrystalline samples 1 and 2 was established
by XRPD. Polycrystalline samples were lightly ground in an agate
mortar and pestle and filled into 0.7 mm borosilicate capillaries.
Data were collected at room temperature in the 2h range 5–40°
Complex 1: Yield: (ꢀ72%). Anal. Calc. for C42H46N6Ni4O12S2: C,
44.81; H, 4.12; N, 7.47. Found: C, 44.71; H, 4.09; N, 7.28%. Selected
IR data (KBr, cmÀ1
N) = 1632,
)
(Fig. S3):
m(OH) = 3424, m(SCN) = 2031,
on a Empyrean PANalytical powder diffractometer, using Cu K
a
m(C
@
m
(C–O,Phen)=1193, .
m
(C–O,Alco) = 1093 cmÀ1
radiation (k = 1.54177 Å). In both cases, the powder diffraction pat-
tern of the bulk sample was consistent with the pattern calculated
from single-crystal data (see Figs. S1 and S2).
2.3.3. Synthesis of [NiII2MnIII2(HL)2(L)2] (2)
A solution of NiCl2.6H2O (0.951 g, 2 mmol) in methanol (5 mL)
was added to a solution of H3L (0.901 g, 4 mmol) in methanol
(10 mL) followed by the addition of a methanolic solution (5 mL)
of MnCl2Á4H2O (0.394 g, 2 mmol). To the resulting solution, Et3N
(0.404 g, 4 mmol) was added dropwise. The resulting mixture
was stirred for 2 h and filtered to remove any insoluble material.
The filtrate was left at room temperature overnight resulting in
the precipitation of brown block shaped crystals of compound 2.
2.2. X-ray crystallographic data collection and refinement of the
structures
The crystallographic data of the compounds are summarized in
Table 1. Diffraction data were collected on a Nonius APEX-II dif-
fractometer with CCD-area detector at 150 K using graphite-mono-
chromated Mo Ka radiation (k = 0.71073 Å). Crystal structure was
Complex 2: Yield: (ꢀ 62%). Anal. Calc. for C40H42Mn2N4Ni2O12
:
determined by direct methods and subsequent Fourier and differ-
ence Fourier syntheses, followed by full-matrix least-squares
refinements on F2 using SHELXL-97 [40]. Absorption correction was
done by SADABS method. Hydrogen atoms were located and refined
freely. The hydrogen atoms were refined isotropically, while the
non hydrogen atoms were refined anisotropically. The contribution
of the electron density associated with disordered solvent mole-
cules was removed by the SQUEEZE subroutine in PLATON [41].
C, 48.14; H, 4.24; N, 5.61. Found: C, 48.04; H, 4.10; N, 5.40%. Se-
lected IR data (KBr, cmÀ1) (Fig. S4):
(C–O,Phen) = 1208,
(C–O,Alco) = 1095 cmÀ1
m
(O–H) = 3407, m(C@N) = 1612,
.
m
m
3. Results and discussion
3.1. Syntheses and IR spectra of the complexes
The reaction of methanolic solution of NaSCN and NiCl2.6H2O
with H3L in presence of Et3N afforded the tetranuclear complex
[NiII2(HL)(H2L)(SCN)]2.DMF (1) by slow diffusion of diethyl ether
into DMF solution. When methanolic solutions of NiCl2Á6H2O and
2.3. Synthesis
All the chemicals were purchased from Sigma–Aldrich. All
chemicals were of reagent grade and were used as received.
MnCl2Á4H2O are used with H3L in presence of Et3N, a NiII2MnIII
2
based tetranuclear complex, [NiII2MnIII2(HL)2(L)2] (2) is formed.
The two complexes, 1 and 2 are characterized elemental micro-
analysis, infrared (IR) spectroscopy and single crystal X-ray crystal-
lography (Scheme 1).
IR spectra provided valuable information about the coordina-
tion environment of the complexes. In the IR spectra of 1 and 2,
the strong peaks at 1632 and 1612 cmÀ1 may be ascribed to the
presence of coordinated imine group. The spectrum of 1 reveals
2.3.1. Synthesis of the ligand3-[(2-Hydroxy-benzylidene)-amino]-
propane-1,2-diol (H3L)
The mono-condensed Schiff base (H3L) was prepared by a stan-
dard method [28]: 10 mmol of salicylaldehyde(1.05 mL) were
mixed with 10 mmol of 3-amino-1,2-propanediol (0.911 g) in
10 mLof absolute ethanol. The resulting mixture was refluxed for
one hour, after removal of the solvent under reduced pressure;

R. Modak et al. / Polyhedron 70 (2014) 155–163
157
Table 1
Crystallographic data and determination summery of the complexes 1 and 2.
1
2
Formula
C
42H46N6Ni4O12S2
C40H42Mn2Ni2N4O12
998.04
Molecular weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
635.97
trigonal
R-3
36.978(6)
36.978(6)
11.540(2)
90.00
90.00
120.00
13665(4)
18
1.391
triclinic
ꢀ
P1
10.947(4)
12.003(8)
12.012(5)
107.808(19)
116.010(13)
99.616(18)
1264.1(11)
1
a
(°)
b (°)
c
(°)
V (Å3)
Z
qc (g cmÀ3
)
1.311
1.276
l
(mmÀ1
)
1.353
F(000)
5940
512.0
Crystal size (mm3)
0.15 Â 0.13 Â 0.12
0.14 Â 0.13 Â 0.11
h (°)
1.10–25.13
1.90–24.51
Limiting indices
No. of reflections collected
No. of independent reflections (Rint
Completeness to h/%
No. of data/restraints/params
Goodness of fit (GOF) on F2
Final R indices (I > 2h(I))
R1
À43 6 h 6 44, À43 6 k 6 44, À13 6 l 6 13
À12 6 h 6 12, À13 6 k 6 14, À13 6 l 6 14
31286
13027
)
5358 (0.0402)
98.7
5430/0/357
1.083
4093 (0.0501)
97.4
4202/0/268
1.070
0.0504
0.1269
0.0693
0.2013
wR2
R indices (all data)
R1
0.0611
0.0892
wR2
0.1332
1.359, À1.048
0.2194
1.047, À0.461
Largest difference in peak, hole (e ÅÀ3
)
strong band at 2031 cmÀ1 suggesting the presence of isothiocya-
nate moiety.
dimerizes via a triply bridging alkoxido oxygen atom (O3) and dou-
bly bridging phenoxido oxygen atom (O4) of the Schiff base ligand
as well as the nitrogen (N3) of the thiocyanate ligand to generate a
tetranuclear moiety that can be formulated as [Ni4(
l3-Oalkoxido)2
2-NCS)2]2+. The topology of the core can be de-
3.2. Description of the crystal structures
(l
2-Ophenoxido)2(
l
scribed as face sharing distorted double cubane with two missing
vertices. A search in the CCDC database (updated August 2013)
shows that this is the first Ni4 tetramer of this type with thiocya-
nate bridges. The Ni(II) atoms in 1 present a distorted octahedral
environment. The octahedral coordination around Ni1 center is
formed by two phenoxo oxygen atoms (O1, O4), two alkoxo oxygen
3.2.1. [NiII2(HL)(H2L)(SCN)]2.DMF (1)
Single crystal X ray diffraction analysis reveals that complex 1
crystallizes in the trigonal R-3 space group and that the asymmet-
ric unit is built up by a dinuclear Ni(II) fragment and one DMF sol-
vent molecule (Fig. 1). Selected bond distances and angles are
shown in Table S1. Fig. 2 shows that the dinuclear unit further
Fig. 1. (a) Structure of the [NiII/2(HL)(H2L)(SCN)]2 cluster in compound 1 (b) View of the vacant rhomb-like dicubaneNi4N2O4 central core in 1showing the coordination
environment of the four Ni(II) centers. Hydrogen atoms and the solvent molecule have been omitted for clarity. Color code: Ni = green, N = blue, O = red, S = yellow and
C = gray. (Colour online.)

158
R. Modak et al. / Polyhedron 70 (2014) 155–163
Fig. 2. Coordination environments in compound 1.
atoms (O3, O3#), one imine nitrogen (N2) and a thiocyanate nitro-
gen (N3). The Ni2 center is surrounded by one phenoxo oxygen
(O4), three alkoxo oxygen atoms (O2, O3, O5), an imine nitrogen
(N2) and a thiocyanate ligand (N3). The average Ni–Nimine and
Ni–NSCN distances are 1.9875 and 2.162 Å, respectively. The Ni–O
[MnNi] unit which undergoes a dimerization by means of a triply
bridging alkoxo oxygen group (O6). The valence states of the metal
ions are confirmed by charge balance considerations, bond valence
sum (BVS) calculations [43] and by a close examination of struc-
tural parameters [23,37,44] that show that the heteronuclear moi-
ety contains two Ni(II) and two Mn(III) centers (see Table S3). In
this moiety, the four metal ions are held together by means of
two triply deprotonated (L3À) and two doubly deprotonated Schiff
base ligands(HL2À). The deprotonated alkoxo oxygen atom O6
bridges the two Ni(II) centers with a Ni–O–Ni bond angle of
98.15°. The Mn(III) and Ni(II) centers are connected by the alkoxo
oxygen atoms O3 and O6 with Ni–O–Mn bond angles of 102.71°
and 97.74°, respectively. In the adjacent face, the phenoxo oxygen
atom O1 and the alkoxo oxygen atom O6 bridge the metals with
Ni–O–Mn bond angles of 97.05° and 100.12°, respectively. Thus,
as in 1, the overall arrangement takes the form of a face sharing
di-vacant, slightly distorted, rhomb-like double cubane where the
alkoxo oxygen atom O6 connects the three adjacent faces. All the
metal centers adopt a distorted octahedral environment (Fig. 5),
where three coordinating atoms O1, N1 and O3 of a Schiff base li-
gand coordinate each Ni(II) ion. The remaining sites are occupied
by two bridging alkoxo groups (O6 and O6a) and a terminal alkoxo
atom (O5) of another Schiff base ligand. The coordination geometry
of the Mn(III) ions is an elongated octahedral environment, as ex-
pected for high spin d4 ions with Jahn–Teller distortion. The equa-
torial plane if formed by a phenoxo oxygen atom (O4), imine
nitrogen (N2) and two alkoxo oxygen atoms (O3 and O6).The axial
positions are occupied by an alkoxo (O2a) and a phenoxo oxygen
atom (O1) with much longer Mn–O bond lengths (2.394(9) and
2.188(4) Å, respectively) compared to the equatorial ones (in the
range 1.856–2.001 Å, Table S3). The adjacent Mn2Ni2 entities are
held together by weak van der Waals interaction and propagate
along a axis to generate a one dimensional chain like form (Fig. S5).
distances range from 1.990 to 2.139 Å. Each l3-Oalkoxidogroup pre-
sents one short and two longer bonds to the nickel centers (Ni2–
O3 = 1.993 Å, Ni1–O3 = 2.052 Å and Ni1#–O3 = 2.055 Å). These
Ni–N and Ni–O bond lengths are in the normal range of those re-
ported for other six coordinate Ni(II) complexes.[30,42,43] The
thiocyanate ligand shows an almost linear geometry, with a N–
C–S bondangle of 178.95°. The bridging angles between Ni ions
range from 88.93° to 101.81° (Ni1–NSCN–Ni2 = 88.93°, Ni1–Ophenox-
ido–Ni2 = 94.60°, Ni1–Oalkoxido–Ni2 = 101.81°). The alkoxo oxygen
atom (O2) and the DMF solvent molecule participate actively in
hydrogen bonding interactions (O2–H20Á Á ÁO7 = 2.608 Å) resulting
in a 3D H-bonded structure (Fig. 3).
3.2.2. [NiII2MnIII2(HL)2(L)2] (2)
ꢀ
Complex 2 crystallizes in the triclinic space group P1 and the se-
lected bond distances and angles are listed in Table S2. The molec-
ular structure of
2 contains the discrete cluster moiety
[MnIII2NiII2(L3À)2(HL2À)2] as depicted in Fig. 4. As in 1, here the
asymmetric unit also contains a crystallographically independent
3.3. Magnetic properties
The thermal variation of the molar magnetic susceptibility per
Ni(II) tetramer times the temperature (vmT) for compound 1 shows
a value of ca. 4.8 emu K molÀ1 at room temperature. This value is
the expected one for four non interacting Ni(II) S = 1 ions with
g ꢁ 2.2 (Fig. 6). When the sample is cooled,
sive decrease and reaches a smooth plateau of ca. 2.4 emu K molÀ1
between ca. 10 and 5 K. Below 5 K mT shows a more abrupt de-
vmT shows a progres-
v
crease to reach a value of ca. 1.4 emu K molÀ1 at 2 K. This behavior
indicates that complex 1 presents predominant antiferromagnetic
interactions between the Ni(II) ions. The structure of 1 shows the
presence of a rhomb-like Ni(II) tetramer with Ni–Ni exchange
interactions along the sides of the rhomb and along the short diag-
onal. From the symmetry of the cluster (Scheme 2), the exchange
Fig. 3. Crystal packing diagram of 1.

R. Modak et al. / Polyhedron 70 (2014) 155–163
159
Fig. 4. (a) Structure of the [NiII2MnIII2(HL)2(L)2]cluster in compound 2 (b) View of the vacant rhomb-like dicubane Ni2Mn2O6 central core in 2 showing the coordination
environment of the two Ni(II) and two Mn(III) centers. Hydrogen atoms and the solvent molecule have been omitted for clarity. Color code: Ni = green, Mn = Orange, N = blue,
O = red and C = gray. (Colour online.)
Fig. 5. Coordination environments in compound 2.
5
4
3
2
1
0
Scheme 2. Magnetic exchange scheme in compound 1.
i and j is written as ÀJSiSj. The fit obtained reproduces very satisfac-
torily the magnetic properties of compound 1 in the whole temper-
ature range with the following parameters: g = 2.292,
J1 = À43.6 cmÀ1, J2 = +5.8 cmÀ1, J3 = À3.9 cmÀ1 and a paramagnetic
monomeric S = 1 impurity of 0.7% (solid line in Fig. 6).
0
50 100 150 200 250 300
T (K)
Fig. 6. Thermal variation of the
Solid line represents the best fit to the model (see text).
vmT product per Ni(II) tetramer for compound 1.
The thermal variation of
plex 2 shows at room temperature a value of ca. 8.3 emu K molÀ1
slightly above the expected value for two isolated Mn(III) S = 2 ions
(6 emu K molÀ1) plus two isolated Ni(II) S = 1 ions (2 emu K molÀ1
v
mT per MnIII2NiII tetramer for com-
2
,
between Ni atoms 1–2 and 3–4 is the same (J1, through the O3 and
N3 bridge) and the exchange between Ni atoms 2–3 and 1–4 is also
identical (J2, through the O3 and O4 bridge). Finally, we have in-
cluded a third exchange constant along the short diagonal of the
rhomb (J3, through the double O3 bridge).
Accordingly, we have fitted the magnetic properties of 1 to a
spin model that takes into account the structural features of this
Ni4 cluster using the package MAGPACK [45,46]. In this model,
the Hamiltonian describing the exchange interaction between sites
)
with g = 2 (Fig. 7). When the temperature is lowered
vmT remains
constant down to ca. 25 K and below this temperature
vmT shows a
progressive decrease to reach a value of ca. 6.0 emu K molÀ1 at 2 K.
This behavior indicates that in complex 2 the magnetic coupling is
also antiferromagnetic but significantly weaker than in 1. Since the
structure of 2 from the magnetic point of view is similar to 1

160
R. Modak et al. / Polyhedron 70 (2014) 155–163
These five examples show coupling constants ranging from
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
moderate antiferromagnetic (À19 cmÀ1) to moderate ferromag-
netic (20 cmÀ1) (Table 3). Unfortunately, in two of these five com-
plexes there is an additional bridge (a water molecule in one case
[61] and a carboxylate bridge in the other [52]) and in two of the
three remaining examples, one of the Ni(II) ions presents a square
pyramidal structure. Therefore, only in one case the two nickel
atoms are octahedrally coordinated as in 1. This lack of examples
similar or close to complex 1 precludes any attempt to establish
a magneto-structural correlation in this kind of double bridge
formed by an O-bridge plus a l-(j
N)-SCNÀ bridge (J1). Fortunately,
the two other coupling constants found in 1 are more easy to ex-
plain since previous magneto-structural correlations have estab-
lished that double O-bridges give rise to ferromagnetic coupling
between Ni(II) ions when the Ni-O-Ni bond angle is below ca.
0
50 100 150 200 250 300
T (K)
98-99° [20,47,48,54–60]. In compound
1 the J2 interaction
(+5.8 cmÀ1) corresponds to a double O-bridge along the side of
the rhomb with an average Ni–O–Ni bond angle of 96.9°, close,
but inside the range for ferromagnetic interaction. In contrast,
the J3 interaction (À3.9 cmÀ1) corresponds to a double O-bridge
along the short diagonal of the rhomb with an average Ni–O–Ni
bond angle of 101.8°, out of the ferromagnetic region. Note that
although the bond lengths also play an important role in determin-
ing the strength of the interaction, in compound 1 these are similar
to those found in many other Ni clusters (Table 3).
Fig. 7. Thermal variation of the
2. Solid line represents the best fit to the model (see text).
v
mT product per MnIII2NiII2 tetramer for compound
The coupling constants found in compound 2 are weak and
antiferromagnetic, in the normal range found in the eight magnet-
ically and structurally reported Mn2Ni2O6 clusters similar to 2,
where the coupling constant values expand from ca. À20 to
20 cmÀ1 (Table 4) [21,23–26,27,61]. As in 1, here again there is
no magneto-structural correlation because only in three of the
eight cases the authors have considered a model with two different
coupling constants along the sides. If we look at these three exam-
ples (MAJZON, SUDWOD and WEFZAK, Table 4) we can see that the
first one presents ferromagnetic NiÁ Á ÁMn interactions whereas in
the other two, these are antiferromagnetic. The average Ni–O–
Mn bond angles in the ferromagnetic cluster MAJZON (97.9° and
99.3°) are very similar to those of the antiferromagnetic cluster
DUSWOD (97.3° and 99.4°) but very different to those of the other
antiferromagnetic cluster WEFZAK (93.2° and 102.6°). These val-
ues, together with the large dispersion observed in other structural
parameters, as the Ni–O and Mn–O bond lengths, preclude the
establishment of any simple magneto-structural correlation be-
tween the J values and the Ni–O–Mn bond angles nor the Ni–O
or Mn–O bond lengths in this kind of tetranuclear clusters. The
main reason for this lack of clear simple correlation may be the
existence of the additional coupling through the short diagonal
of the rhomb (J3) and the presence of a zero field splitting and int-
erclusters interactions (which are included in some of them). Both
parameters may be strongly correlated to the J values, especially
when these are weak, precluding any accurate determination of
their values.
If we do not limit to tetranuclear Mn2Ni2O6 clusters, we can find
up to seven magnetically and structurally characterized additional
examples presenting a Ni-Mn interaction through a double O-
bridge (Table 5) [62–64]. Surprisingly, the magnetic coupling in
these seven complexes ranges from weak antiferromagnetic
(À10.8 cmÀ1) to moderate ferromagnetic (32 cmÀ1). If we analyze
the structural parameters of these bridges we can observe that
the main parameter governing the magnetic coupling seems to
be the Mn–O bond lengths and the Mn–O–Ni bond angles. Thus,
the three compounds presenting ferromagnetic Ni-Mn interactions
are those presenting the largest Ni-O-Mn bond angles (above 101°)
whereas in the four compounds with antiferromagnetic Ni–Mn
couplings these angles are smaller (below 100°). On the other
hand, the three ferromagnetic compounds present short Mn-O
Scheme 3. Magnetic exchange scheme in compound 2.
(except for the change of two Ni(II) ions by two Mn(III) ions), we
have used a similar magnetic scheme (Scheme 3) to fit the mag-
netic properties of 2 using the package MAGPACK [45,46]. The fit
so obtained reproduces very satisfactorily the magnetic properties
of compound 2 in the whole temperature range with the following
parameters: g = 2.032, J1 = J2 = À0.3 cmÀ1 and J3 = À0.7 cmÀ1 (solid
line in Fig. 7). Note that J2 was allowed to vary free but the result-
ing value was equal to J1.
The fitting of the magnetic properties of 1 shows that one of
the exchange coupling constants along the sides of the rhomb
and the one along the short diagonal are antiferromagnetic
(J1 = À43.6 cmÀ1 and J3 = À3.9 cmÀ1) whereas the coupling along
the other side of the rhomb is ferromagnetic (J2 = +5.8 cmÀ1). In
order to rationalize these values, we have searched in the CCDC
database all the reported magnetically characterized Ni4 tetramers
with a similar Ni4O4N2core (Scheme 2, Table 2) [18–20,47–49]. In
the seven cases found, the coupling along the sides (J1 and J2)
and along the short diagonal (J3) of the rhomb are all ferromag-
netic, in contrast to the antiferromagnetic J1 and J3 coupling
constants found in 1. The reasons for this discrepancy can be ex-
plained from the differences in the structures of complex 1 and
of all the other Ni4 tetramers. Thus, in the seven reported Ni4
À
tetramers the N atoms belong to
l
1,1-N3 bridges, which are well
known to give rise to ferromagnetic interactions [50]. In contrast,
in compound 1 the N atom belongs to a -(
l
j
N)-SCNÀ bridge and,
therefore, compound 1 is the first example of a Ni4O4N2 core with
SCN- bridging ligands.
Since there are no precedents in this kind of Ni4 complexes, in
order to estimate the sign and magnitude of such a Ni–Ni interac-
tion through an O-bridge and a
l-(j
N)-SCNÀ bridge, we have
searched in the CCDC database this kind of bridge in Ni clusters
with any nuclearity. There is a total of five magnetically character-
ized Ni complexes containing this kind of bridge (Table 3) [51–53].

R. Modak et al. / Polyhedron 70 (2014) 155–163
161
Table 2
Table 4
Magnetic and structural parameters (average values) of all the reported Ni4 tetramers
Magnetic and structural parameters (average values) of all the reported Mn2Ni2
with Ni4O4N2 core similar to compound 1.
tetramers with the same Mn2Ni2O6 core than compound 2.
CCDC code
JAZRUY
J (cmÀ1
)
Ni–O (Å)
Mn–O (Å)
Ni–O–M (°)
Refs.
[25]
CCDC code
AQUFUO
J (cmÀ1
)
Ni–X (Å)
Ni–X–Ni (°)
Refs.
[20]
J1 = 1.5
J2 = 1.5
J3 = 20
J1 = À3.6
J2 = À3.6
J3 = À6.8
J1 = 7.2
2.039
2.084
2.105
2.050
2.052
2.086
2.053
2.059
2.098
2.106
2.099
2.180
2.004
2.018
2.087
2.098
2.077
2.078
2.080
2.122
2.051
2.048
2.072
2.097
2.090
2.082
2.131
2.062
2.089
2.139
2.121a
1.936
96.2 (Mn)
100.3 (Mn)
96.1 (Ni)
98.9 (Mn)
96.5 (Mn)
98.2 (Ni)
97.3 (Mn)
101.0 (Mn)
97.9 (Ni)
97.9 (Mn)
99.3 (Mn)
101.7 (Ni)
98.3 (Mn)
99.5 (Mn)
97.3 (Mn)
97.3 (Mn)
97.8 (Ni)
J1 = 15.32
2.042 (O)
2.122 (N)
2.030 (O)
2.101 (O)
2.030 (O)
2.104 (N)
2.029 (O)
2.071 (O)
2.040 (O)
2.114 (N)
2.041 (O)
2.092 (O)
2.112 (O)
2.093 (N)
2.072 (O)
2.056 (O)
2.120(O)
2.066(N)
2.081 (O)
2.069 (O)
2.037 (O)
2.103 (N)
2.040 (O)
2.098 (N)
2.037 (O)
2.041 (O)
2.081 (O)
2.073 (O)
2.118 (O)
2.088 (N)
2.078 (O)
2.060 (O)
2.025 (O)
2.157 (N)
2.044 (O)
2.054 (O)
102.2 (O)
97.0 (N)
93.5 (O)
98.4 (O)
101.9 (O)
97.1 (N)
93.2 (O)
99.4 (O)
102.0 (O)
97.2 (N)
93.7 (O)
97.4 (O)
100.6 (O)
101.8 (N)
94.9 (O)
99.2 (O)
99.6(O)
103.2(N)
94.8 (O)
99.0 (O)
102.3 (O)
97.9 (N)
102.3 (O)
98.5 (N)
93.1 (O)
92.7 (O)
99.2 (O)
99.6 (O)
100.4 (O)
102.4 (N)
95.5 (O)
99.9 (O)
96.9 (O)
88.9 (N)
96.0 (O)
101.8 (O)
J2 = 4.51
J3 = 4.51
J1 = 15.53
LICVID
LICVOJ
MAJZON
RABJIPb
2.153a
1.920
[21]
[21]
[24]
[23]
AQUGAV
AQUGEZ
CUFTUS01
HOWQUF
IGIGAHa
[20]
[20]
[40]
[41]
[18]
2.111a
1.928
J2 = 4.32
J3 = 4.32
J1 = 17.86
J2 = 7.2
J3 = À15.6
J1 = 9.0
2.004
1.924
J2 = 8.6
J2 = 4.68
J3 = 4.68
J1 = 5.6
J3 = À15.8
J1 = À5.8
2.070a
2.081a
1.930
1.933
J2 = À5.8
J3 = À5.6
J2 = 5.6
J3 = 11.8
J1 = 18.8
97.4 (Ni)
SUDWOD
TEBMOE
WEFZAK
2
J1 = À3.0
J2 = À3.9
J3 = À5.2
J1 = À10.5
J2 = À10.5
J3 = 16.2
J1 = À23.7
J2 = À12.9
J3 = 0.7
2.122a
2.139a
99.4 (Mn)
97.3 (Mn)
98.7 (Ni)
102.4 (Mn)
94.2 (Mn)
98.1 (Ni)
102.6 (Mn)
93.2 (Mn)
100.3 (Ni)
98.6 (Mn)
100.2 (Mn)
98.1 (Ni)
[54]
J2 = 6.9
J3 = 1.3
J1 = 29.5
2.106a
1.927
[26]
2.097a
1.923
[28]
J2 = 11.6
J3 = 11.6
J1 = 7.07
J1 = À0.3
J2 = À0.3
J3 = À0.7
2.069a
1.933
This work
RICQAW
[42]
J2 = 7.08
J3 = 11.4
J1 = À43.6
a
This length includes the Jahn–Teller ellongation in Mn(III).
This compound contains two independent Mn2Ni2 tetramers.
b
1
This work
J2 = +5.8
J3 = À3.9
a
ment with the very weak antiferromagnetic coupling observed in
this compound). Note that the Ni-O bond lengths do not show
any clear tendency. From this observations we can deduce that if
we represent the J value as a function of the ratio between the
There is an additional carboxylate bridge.
bond lengths (below 2.0 Å) whereas the four antiferromagnetic
compounds show much longer Mn-O bond lengths (above 2.1 Å,
except compound 2 that presents an intermediate value, in agree-
Ni-O-Mn bond angle (a) and the Mn-O bond length (R) we expect
to observe a linear trend (Fig. 8). Note also that the so obtained lin-
ear relation, J (cmÀ1) = À270 + 5.556⁄(
a/R) (solid line in Fig. 8), fits
Table 3
Magnetic and structural parameters of all the reported Ni complexes with an O-bridge and a
j
-N-SCNÀ bridge connecting two Ni ions.
CCDC code
IXUDUA
J (cmÀ1
À3.7
)
Ni–O (Å)
Ni–N (Å)
Ni–O–Ni (°)
Ni–N–Ni (°)
Refs.
[44]
2.049
2.003
2.097
2.090
2.027
2.042
2.014
2.044
1.992
2.058
1.993
2.056
2.207
2.250
2.277
2.142
2.131
2.172
2.039
2.170
2.077
2.197
2.129
2.195
110.5
96.6
NIWZAUa
NOBNUNb
NOBPEZ
NOBPID
1
À19.0
À3.4
+20
85.6
80.1
88.1
96.5
95.1
88.9
[45]
94.7
[46]
101.4
102.4
96.9
[46]
+20
[46]
À43.6
This work
a
This complex presents and additional carboxylate bridge.
This complex presents an additional H2O bridge.
b

162
R. Modak et al. / Polyhedron 70 (2014) 155–163
Table 5
Magnetic and structural parameters (average values) of all the reported complexes with Mn and Ni connected through a double O-bridge as in compound 2.
CCDC code
J (cmÀ1
)
Ni–O (Å)
Mn–O (Å)
Ni–O–Mn (°)
a
/Ra
Refs.
GIQFOC
IBOTUQ
MIYXOI
MIYXUO
MIYYAV
À10.4
17.0
À10.8
À10.4
À10.2
2.057
2.087
2.052
2.067
2.048
2.059
2.053
2.056
2.076
2.171
1.936
2.127
2.111
2.135
2.139
1.889
1.889
2.001
99.1
101.1
99.6
99.3
99.7
45.65
52.22
46.83
47.04
46.70
46.70
53.52
53.47
49.68
[55]
[56]
[55]
[55]
[55]
99.9
OVISES
OVISIW
2
32.0
29.2
À0.3
101.1
101.0
99.4
[57]
[57]
This work
a
a
is the Ni–O–Mn angle (°) and R is the Mn–O bond length (in Å).
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: de-
posit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2013.12.031.
40
OVISES
OVISIW
30
20
IBOTUQ
10
2
MIYYAV
GIQFOC
0
-10
-20
MIYXUO
MIYXOI
44
46
48
50
52
54
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