Ferro- to antiferromagnetic crossover angle in diphenoxido- and carboxylato-bridged trinuclear NiII2-MnII complexes: Experimental observations and theoretical rationalization

Article abstract of DOI:10.1021/ic501425x

Three new trinuclear heterometallic Ni^II-Mn^II complexes have been synthesized using a [NiL] metalloligand, where H ₂L = N,N′-bis(salicylidene)-1,3-propanediamine. The complexes [(NiL)₂Mn(OCnn)₂(CH₃OH)₂] ·CH₃OH (1), [(NiL)₂Mn(OPh)₂(CH ₃OH)₂][(NiL)₂Mn(OPh)₂]·H ₂O (2), and [(NiL)₂Mn(OSal)₂(CH ₃OH)₂]·2[NiL] (3) (where OCnn = cinnamate, OPh = phenylacetate, OSal = salicylate) have been structurally characterized. In all three complexes, in addition to the double phenoxido bridge, the two terminal Ni^II atoms are linked to the central Mn^II by means of a syn-syn bridging carboxylate, giving rise to a linear structure. Complexes 1 and 2 with Ni-O-Mn angles of 97.24 and 96.43°, respectively, exhibit ferromagnetic interactions (J_Ni-Mn = +1.38 and +0.50 cm^-1, respectively), whereas 3 is antiferromagnetic (J_Ni-Mn = -0.24 cm^-1), having an Ni-O-Mn angle of 98.51°. DFT calculations indicate that there is a clear magneto-structural correlation between the Ni-O-Mn angle and J_Ni-Mn values, which is in agreement with the experimental results.

Full text of DOI:10.1021/ic501425x

Article
pubs.acs.org/IC
Ferro- to Antiferromagnetic Crossover Angle in Diphenoxido- and
Carboxylato-Bridged Trinuclear NiII −MnII Complexes: Experimental
2
Observations and Theoretical Rationalization
Piya Seth,† Albert Figuerola,*,‡,§ Jesus Jover,‡,∥ Eliseo Ruiz,‡,∥ and Ashutosh Ghosh*,†
́
†Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, India
‡
́
Departament de Quımica Inorgan
̀ ̀
ica, Universitat de Barcelona, Martí i Franques 1-11, 08028 Barcelona, Spain
§Institut de Nanocien
∥
̀
cia i Nanotecnologia (IN2UB), Universitat de Barcelona, Martí i Franques
̀
1-11, 08028 Barcelona, Spain
́
Institut de Recerca de Quımica Teor
̀
ica i Computacional, Universitat de Barcelona, Diagonal 645, E-08028 Barcelona, Spain
S
* Supporting Information
ABSTRACT: Three new trinuclear heterometallic NiII−MnII complexes have been synthesized using a [NiL] metalloligand,
where H2L = N,N′-bis(salicylidene)-1,3-propanediamine. The complexes [(NiL)2Mn(OCnn)2(CH3OH)2]·CH3OH (1),
[(NiL)2Mn(OPh)2(CH3OH)2][(NiL)2Mn(OPh)2]·H2O (2), and [(NiL)2Mn(OSal)2(CH3OH)2]·2[NiL] (3) (where OCnn =
cinnamate, OPh = phenylacetate, OSal = salicylate) have been structurally characterized. In all three complexes, in addition to the
double phenoxido bridge, the two terminal NiII atoms are linked to the central MnII by means of a syn-syn bridging carboxylate,
giving rise to a linear structure. Complexes 1 and 2 with Ni−O−Mn angles of 97.24 and 96.43°, respectively, exhibit
ferromagnetic interactions (JNi−Mn = +1.38 and +0.50 cm−1, respectively), whereas 3 is antiferromagnetic (JNi−Mn = −0.24 cm−1),
having an Ni−O−Mn angle of 98.51°. DFT calculations indicate that there is a clear magneto-structural correlation between the
Ni−O−Mn angle and JNi−Mn values, which is in agreement with the experimental results.
coupling constant J on the phenoxido bridging angle in
homometallic dinuclear CuII, NiII, and MnIII complexes have
been performed to quantify the respective contributions of ferro-/
antiferromagnetic interactions. From both theoretical calculations
and experimental results, it has been found that for dinuclear CuII
complexes ferromagnetism appears for a bridging Cu−O−Cu
angle lower than 92.4°.1a The critical phenoxido bridging angle is
93.5° for μ2-O-bridged dinuclear NiII complexes2a and 101° for
MnIII complexes.3d,e
INTRODUCTION
■
The rational design and synthesis of oxido- or phenoxido-
bridged molecular assemblies formed by a finite number of
exchange-coupled paramagnetic centers have attracted the
attention of inorganic chemists for a long time. The majority
of the species that have been studied are homometallic. Among
them, homonuclear CuII,1 NiII,2 and MnIII complexes derived
3
from di-Schiff base ligands and various bridging coligands are of
considerable interest because of their fascinating structural
features and intriguing magnetic properties. These compounds
also play a key role in deriving the magneto-structural
correlations for oxido- and/or phenoxido-bridged metal
complexes. From the experimental and theoretical results, it
can be said that the magnetic exchange interactions in this class
of compounds depend on several factors: e.g. M−O−M angle,
M−O distance, the effect of the asymmetry on the metal−
bridge bonds, and the hinge distortion of the M2O2 core.4
Detailed DFT calculations showing the dependence of the
The number of oxido-/phenoxido-bridged heterometallic
CuII, NiII, and MnIII complexes of salen type Schiff base ligands
is relatively less than that of the corresponding homometallic
complexes. Moreover, in most of these complexes the metal
5
ions are CuII−NiII and CuII−MnII.6 The number of NiII−MnII
complexes with this type of ligand is relatively scanty, and very
Received: June 17, 2014
© XXXX American Chemical Society
A
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commercial reagent grade and were used as received, without further
purification.
few of them have been magnetically characterized. A recently
updated CSD search reveals that the number of diphenoxido-
bridged NiII−MnII complexes with salen type Schiff base ligands
is 11,7−9 among which only 2 have been magnetically
characterized.7,8 Between these two complexes, one having an
Synthesis of the Schiff-Base Ligand N,N′-Bis(salicylidene)-
1,3-propanediamine (H2L). The di-Schiff base ligand H2L was
synthesized in our laboratory by standard methods.11 Salicylaldehyde
(1.05 mL, 10 mmol) was mixed with 1,3-propanediamine (0.42 mL,
5 mmol) in methanol (20 mL). The resulting mixture was refluxed for
ca. 1.5 h and cooled. The desired yellow crystalline ligand was filtered
off, washed with methanol, and dried in a vacuum desiccator
containing anhydrous CaCl2.
Preparation of the “Metalloligand” [NiL]. A mixture of H2L
(1.432 g, 5 mmol) in methanol and ammonia solution (10 mL, 20%)
was added to a methanolic solution (20 mL) of Ni(ClO4)2·6H2O
(1.825 g, 5 mmol) to prepare the “ligand complex” [NiL] as previously
reported.12
Synthesis of Complexes 1−3. To a 20 mL methanolic solution
of [NiL] (0.321 g, 1 mmol) were added an aqueous solution (5 mL)
of Mn(OCnn)2 (0.175 g, 0.5 mmol), Mn(OPh)2·H2O (0.172 g,
0.5 mmol) and Mn(OSal)2 (0.165 g, 0.5 mmol) for 1−3, respectively,
and the mixtures were stirred for ca. 1 h at room temperature. A green
precipitate that separated from the reaction mixture in each case was
filtered and the respective filtrates were allowed to stand overnight.
Deep green X-ray-quality single crystals appeared on the wall of
the vessel on slow evaporation of the solvent in each case. The crystals
were isolated, washed with methanol, and dried in a desiccator
containing anhydrous CaCl2.
Ni−O−Mn angle of 86.38° is ferromagnetically coupled (JNi−Mn
=
+9.3 cm−1) while the other shows antiferromagnetic interactions
(JNi−Mn = −0.30 cm−1) with an Ni−O−Mn angle of 102.31°. The
results clearly suggest that a critical angle should exist in between,
and therefore it is worth to designing and synthesizing some NiII−
MnII heterometallic complexes with the phenoxido bridging angle
within the range 86−102°, so that one can have an idea of the
critical angle for this system. An efficient strategy for synthesizing
heterometallic complexes of salen type Schiff base ligands is to
employ the “metalloligand” approach. The use of various bridging
and/or terminally coordinating anionic coligands plays an
important role in controlling the phenoxido bridging angle and
consequently the magnetic coupling.10 Recently, we reported
some trinuclear CuII−MnII complexes using the [CuL] metal-
loligand (where H2L= N,N′-bis(salicylidene)-1,3-propanedi-
amine). Although the Cu−O−Mn bridging angles in these
complexes have been varied in a wide range of 92−101° by
suitably selecting the anionic coligands, the antiferro- to
ferromagnetic crossover angle was not detected.6e However, for
NiII−MnII complexes the experimental search for critical angle
should end in success by narrowing down the range, as both
ferromagnetically and antiferromagnetically coupled complexes are
known.7,8 The aim of the present investigation is therefore to
synthesize some NiII−MnII complexes with the variation of
phenoxido bridging angles and to study their magnetic properties.
We also want to substantiate the experimental magnetic coupling
with the help of DFT studies, which have rarely been done for
such heterometallic compounds.
Complex 1. Yield: 0.410
g (71%) Anal. Calcd for
C56H61N4O12Ni2Mn (1154.41): C, 58.26; H, 5.33; N, 4.85. Found:
C, 58.12; H, 5.29; N, 4.73. IR (KBr pellet, cm−1): 1638 ν(CN),
1578 νas(COO), 1468 νs(COO). λmax (MeOH, nm): 350, 409 599.
Complex 2. Yield: 0.388
g (74%). Anal. Calcd for
C51H50N4O10Ni2Mn (1051.26): C, 58.27; H, 4.79; N, 5.33. Found:
C, 58.18; H, 4.70; N, 5.26. IR (KBr pellet, cm−1): 1635 ν(CN),
1588 νas(COO), 1469 νs(COO). λmax (MeOH, nm): 356, 407, 592.
Complex 3. Yield: 0.297
g (68%). Anal. Calcd for
C84H82N8O16Ni4Mn (1749.28): calcd C, 57.67; H, 4.72; N, 6.41.
Found: C, 57.56; H, 4.65; N, 6.35. IR (KBr pellet, cm−1): 1625 ν(CN),
1541 νas(COO), 1463 νs(COO). λmax (MeOH, nm): 349, 408, 597.
Physical Measurements. Elemental analyses (C, H, and N) were
performed using a PerkinElmer 240C elemental analyzer. IR spectra in
KBr (4000−500 cm−1) were recorded using a PerkinElmer RXI FTIR
spectrophotometer. Temperature-dependent molar susceptibility measure-
ments of powdered polycrystalline samples of 1−3 were carried out at the
Herein we report the synthesis, crystal structure, and
magnetic properties of the three new complexes [(NiL)2Mn-
(OCnn)2(CH3OH)2]·CH3OH (1), [(NiL)2Mn(OPh)2-
(CH3OH)2][(NiL)2Mn(OPh)2]·H2O (2), and [(NiL)2Mn-
(OSal)2(CH3OH)2]·2[NiL] (3), where OCnn = cinnamate,
OPh = phenylacetate, and OSal = salicylate. Among these,
complexes 1 and 2 have average Ni−O−Mn angles of 97.24
and 96.43°, respectively, exhibiting ferromagnetic interactions
with JNi−Mn values of +1.38 and +0.50 cm−1. For 3 the Ni−O−
Mn angle is 98.51°, corresponding to antiferromagnetic
interactions (JNi−Mn = −0.24 cm−1). For the title heterometallic
complexes theoretical calculations support the experimentally
observed fact that the NiII−MnII systems have a large tendency
to show ferromagnetic coupling because of predominating
orthogonally oriented magnetic orbitals of the two metal ions,
and larger Ni−O−Mn angles (above 98°) are responsible for
antiferromagnetic spin exchange.
́
‘‘Servei de Magnetoquimica (Universitat de Barcelona)’’ in a Quantum
Design SQUID MPMSXL susceptometer with applied fields of 3000
and 198 G in the temperature ranges 2−300 and 2−30 K, respectively.
Computational Details. To calculate the exchange interactions,
a phenomenological Heisenberg−Dirac−van Vleck Hamiltonian was
used, excluding the terms relating to magnetic anisotropy, to describe
the exchange coupling in a general polynuclear complex:
̂ ̂
J S S
a b
ab
̂
H = −
∑
(1)
a<b
̂
̂
where Sa and Sb are the spin operators of the different paramagnetic
cations. The Jab parameters are the pairwise coupling constants
between the paramagnetic centers of the molecule. Basically, we need
to calculate the energy of n + 1 spin distributions for a system with n
different exchange coupling constants.13−17 These energy values allow
us to build up a system of n equations in which the J values are the
unknowns. In the present study, three calculations were performed in
order to obtain the two exchange coupling constants of the MnNi2
complexes. They correspond to the high-spin S = 9/2 state, one S =
1/2 wave function flipping the spin of the central manganese atom,
and finally one S = 5/2 state with the spin inversion of the two external
nickel atoms. Theoretical calculations were performed with the hybrid
B3LYP functional18 as implemented in Gaussian09 code19 using a
guess function generated with the Jaguar 7.0 code,20 which employs a
procedure that allows us to determine individually the local charges
EXPERIMENTAL SECTION
■
Synthesis of Manganese Carboxylates. The three metal
carboxylate salts, viz. Mn(OCnn)2 Mn(OPh)2·H2O, and Mn(OSal)2,
were prepared by following similar procedures. To an aqueous
solution (30 mL) of the corresponding carboxylic acid (148.16 mg of
cinnamic acid, 136.15 mg of phenylacetic acid, or 138.12 mg of
salicylic acid) was added manganese carbonate (60.0 mg) in small
portions with constant stirring with a glass rod until effervescence
ceased. Then the mixture was warmed on a water bath for 10−15 min
and filtered. The clear filtrate was kept over a water bath until a solid
started to separate. The solution was then cooled to room
temperature, and the crystalline solid product was filtered through
suction and dried under vacuum. All other chemicals are of
and multiplicities of the atoms, including the ligand field effects.21
triple-ζ all-electron Gaussian basis set was used for all atoms.22
A
B
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Table 1. Crystal Data and Structure Refinement of Complexes 1−3
1
2
3
formula
C56H61N4O12Ni2Mn
1154.41
C51H50N4O10Ni2Mn
1051.26
C84H82N8O16Ni4Mn
1749.28
formula wt
space group
P1
̅
P1
̅
P1
̅
cryst syst
triclinic
triclinic
triclinic
12.004(5)
12.166(5)
14.238(5)
89.107(5)
69.512(5)
88.037(5)
1946.6(13)
1
a/Å
10.8407(9)
14.1770(12)
18.5894(16)
90.611(4)
92.259(4)
104.275(3)
2766.0(4)
2
9.3692(13)
10.5173(14)
23.878(3)
90.311(2)
99.130(2)
90.807(2)
2322.8(5)
2
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
calcd density Dcalcd/g cm−3
abs coeff (μ)/mm−1
F(000)
1.386
1.503
1.492
0.962 (Mo Kα)
1204.0
1.135 (Mo Kα)
1090.0
1.181 (Mo Kα)
907
R(int)
0.030
0.021
0.025
θ range/deg
total no. of rflns
no. of unique rflns
no. of rflns with I > 2σ(I)
R1, wR2
1.1−28.4
35517
0.9−26.4
17569
1.5−26.5
24251
13444
8940
7783
10683
7044
6454
0.0438, 0.1341
293
0.0413, 0.1104
293
0.0358, 0.1031
293
temp/K
Crystal Data Collection and Refinement. Suitable single crystals
of complexes 1−3 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. A total of 360 frames were measured with a counting
time of 10 s. The structures were solved by the Patterson method
using SHELXS 97. Subsequent difference Fourier synthesis and least-
squares refinement revealed the positions of the remaining non-
hydrogen atoms, which were refined with independent anisotropic
displacement parameters. Hydrogen atoms were placed in idealized
positions, and their displacement parameters were fixed to be 1.2 times
larger than those of the attached non-hydrogen atom. Absorption
corrections were carried out using the SADABS program.23 All
calculations were carried out using SHELXS 97,24 SHELXL 97,25
PLATON 99,26 ORTEP-32,27 and WinGX system Ver-1.64.28 Data
collection and structure refinement parameters and crystallographic
data for the three complexes are given in Table 1.
Scheme 1. Formation of Complexes 1−3
597 nm for 1−3, respectively, which can be assigned to the d−d
transitions in NiII complexes.
RESULTS AND DISCUSSION
■
Description of Structures. The X-ray crystal structure of 1
reveals that it consists of two similar centrosymmetric trinuclear
units 1A,B having the same composition [(NiL)2Mn-
(OCnn)2(CH3OH)2]·CH3OH. An ORTEP diagram of 1A is
shown in Figure 1. Here, both the units contain a six-coordinate
MnII in a distorted-octahedral environment together with two
six-coordinated octahedral NiII atoms having equivalent geo-
metries. The manganese atom is situated at the center of
inversion and is bonded to four oxygen atoms from the two
ligands L, which form the basal plane of the MnII, while the
trans axial positions are occupied by the oxygen atom O(4)
(in 1A) and O(8) (in 1B) of the syn-syn bridging cinnamate
(bond distances are given in Table 2).
The two terminal nickel atoms are bonded to four donor
atoms (O(1), O(2), N(1), N(2) in 1A and O(6), O(7), N(3),
N(4) in 1B) of the ligand L, making up the equatorial plane.
One of the axial positions is occupied by an oxygen atom (O(3)
in 1A, O(9) in 1B) of the syn-syn bridging cinnamate. The
other axial positions of the nickel atoms are bonded to the
oxygen atom of the solvent methanol. The mean deviation of
Syntheses and IR and Electronic Spectra of the
Complexes. Three new NiII−MnII carboxylate complexes
derived from the Schiff base H2L were synthesized by following
similar procedures. For this purpose, we first prepared the
metalloligand [NiL] by a reported procedure.12 An aqueous
solution of the respective manganese carboxylate was then
mixed with a methanolic solution of this metalloligand [NiL] to
give the desired complexes (Scheme 1). In addition to
elemental analysis, all three complexes were characterized by
IR spectroscopy. For 1−3 a strong and sharp band appeared at
1638, 1635, and 1625 cm−1, respectively, due to azomethine
ν(CN). Other peaks due to asymmetric and symmetric
stretching of the carboxylate are observed at 1578, 1468 cm−1
(in 1) 1588, 1469 cm−1 (in 2), and 1541, 1463 cm−1 (in 3),
respectively. Electronic spectra of the three complexes in
methanol solvent are similar, having two strong peaks at 350,
409 nm (in 1), 356, 407 nm (in 2), and 349, 408 nm (in 3),
which correspond to ligand to metal charge transfer transitions.
Another strong absorption band is observed at 599, 592, and
C
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(Figure 3). The octahedral manganese atom is bonded to four
oxygen atoms from the two ligands L, which form the basal
plane of the MnII, while the trans axial positions are occupied
by the oxygen atom O(4) of the syn-syn bridging salicylate. The
two terminal nickel atoms are bonded to four donor atoms
(O(1), O(2), N(1), and N(2)) of the ligand L, making up the
equatorial plane. One of the axial positions is occupied by the
bridging oxygen atom O(3) of the syn-syn bridging salicylate.
The other axial positions of the nickel atoms are also bonded to
the oxygen atom of the solvent methanol that completes a
distorted-octahedral environment around the Ni atom. The
mean deviation of four donor atoms in the basal plane from
their respective mean plane is 0.030 Å, while the Ni atom
deviates by 0.008(1) Å in the direction of the axial O(3). The
Ni···Mn distance is 3.171(1) Å, and the two Ni−O−Mn
bridging angles are 98.31(7) and 98.70(7)° (Table 2).
3B consists of a mononuclear [NiL] unit where four-
coordinated square-planar nickel is bonded to the donor atoms
O(6), O(7), N(3), N(4) of L (Figure 4). The Ni−O and Ni−N
distances (Table 2) are shorter than the corresponding
distances in 1A,B, 2A,B, and 3A, which is quite usual for a
square-planar geometry.30 The τ4 value for Ni(2) is 0.154,
which indicates a slightly distorted square-planar geometry
around it.31
Figure 1. ORTEP-3 view of the centrosymmetric trinuclear structure
of 1A with ellipsoids at the 30% probability level. There are two
trimers with equivalent structures; only one is shown.
four donor atoms in the basal plane from their respective mean
plane are 0.003 Å (in 1A) and 0.031 Å (in 1B), while the Ni
atom deviates by 0.024(1) Å in the direction of the axial O(3)
in 1A and 0.004(1) Å toward O(9) in 1B. The Ni···Mn
distance is 3.149(4) Å in 1A and 3.133(4) Å in 1B. The two
Ni−O−Mn bridging angles are 97.29(7), 97.74(7)° in 1A and
95.52(7), 98.36(8)° in 1B.
Complex 2 also consists of two centrosymmetric trinuclear
units [(NiL)2Mn(OPh)2(CH3OH)2]·H2O (2A) and [(NiL)2Mn-
(OPh)2] (2B), but the compositions of the units are different.
An ORTEP diagram of 2A is shown in Figure 2. Each of the
two units contains two terminal NiII atoms and a central MnII.
Each unit contains a six-coordinated manganese in a distorted-
octahedral environment at the center of inversion that is
bonded to four oxygens from the two ligands L, which form the
basal plane of the MnII. The trans axial positions are occupied
by the oxygen atom O(4) (in 2A) and O(8) (in 2B) of the
syn-syn bridging phenylacetate (bond distances are given in
Table 2).
The two terminal nickel atoms are bonded to four donor
atoms (O(1), O(2), N(1), N(2) in 2A and O(6), O(7), N(3),
N(4) in 2B) of the tetradentate ligand L, making up the
equatorial plane. The fifth coordination site is occupied by a
bridging oxygen atom (O(3) in 2A, O(9) in 2B) of the syn-syn
bridging phenylacetate. The other axial position of the nickel
atoms in 2A is bonded to the oxygen atom of the solvent
methanol, whereas this second axial position is vacant in 2B.
Thus, the Ni atoms are hexacoordinated with a distorted-
octahedral environment in 2A, whereas these atoms are
pentacoordinated in 2B with a geometry closer to square
pyramidal, as indicated by the Addison parameter (τ = 0.067).
τ = 0 for an ideal square pyramid and τ = 1 for a trigonal
bipyramid.29 The mean deviations of four donor atoms in the
basal plane from their respective mean plane are 0.005 and
0.042 Å in 2A,B, respectively. The Ni atom deviates by
0.040(1) Å in the direction of the axial O(3) in 2A and
−0.271(1) Å in the direction of the axial O(9) in 2B. The Ni···
Mn distance is 3.135(6) Å in 2A and 3.091(5) Å in 2B. Two
Ni−O−Mn bridging angles are 96.58(8), 97.34(8)° in 2A and
95.83(8), 95.87(7)° in 2B.
In this context it is to comparing the structural features of
similar kinds of reported diphenoxido- and carboxylato-bridged
linear trinuclear NiII −MnII complexes derived from N2O2
2
donor di-Schiff base ligands. There are three such complexes
in which the phenoxido bridging angle varies in the range
94−96°.9a,b,d When the carboxylato groups were replaced by
pseudohalides, bent trinuclear NiII −MnII complexes resulted
2
with a wider phenoxido bridging angle in the range 100−
103°.30a On the other hand, there is only one carboxylato-
bridged NiII −MnII complex of a tridentate N2O donor Schiff
2
base ligand, and the Ni−O−Mn angle in that compound is
∼102°.7 Another complex based on ha exadentate N3O3 donor
Schiff base ligand has been reported with an Ni−O−Mn
angle of ∼86°.8 It should be mentioned here that, in the
present complexes, the phenoxido bridging angle varies from
96.43 to 98.51°.
Magnetic Properties. Magnetic measurements in com-
plexes 1 and 2 show clear ferromagnetic interactions between
NiII and MnII ions within the complexes, while antiferromag-
netic exchange becomes obvious in complex 3. Temperature-
dependent molar susceptibility measurements on polycrystal-
line samples of 1−3 were carried out in an applied field of 0.3 T
in the temperature range 1.9−300 K. The data are shown in the
χMT versus T plot in Figure 5, where χM is the molar magnetic
susceptibility and T is the absolute temperature. The room-
temperature values of χMT for compounds 1−3 are 6.67, 6.61,
and 6.63 cm3 mol−1 K, respectively, slightly higher than the
6.4 cm3 mol−1 K value expected for noninteracting NiII−MnII−
NiII trinuclear units. For complexes 1 and 2, the χMT values
increase with decreasing temperature until they reach a
maximum of 9.47 cm3 mol−1 K at 4.5 K for 1 and 7.43 cm3
mol−1 K at 6 K for 2. Below these temperatures, the value of
χMT drops sharply. On the other hand, the χMT values
measured for complex 3 remain approximately constant down
to 50 K; below that temperature, they suddenly drop. In order
to quantitatively interpret these data, simulations of the
experimental curves were carried out by using the MAGPACK
program as shown in Figure 5.32 A Hamiltonian of the type H =
−J[S1S2 + S1S3], where S1 = SMn and S2 = S3 = SNi, was used for
The X-ray crystal structure analysis shows that 3 contains
two units, namely [(NiL)2Mn(OSal)2(CH3OH)2] (3A) and
[NiL] (3B). Of these, 3A has a trinuclear centrosymmetric
structure containing two terminal NiII atoms and a central MnII
D
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Table 2. Dimensions in the Metal Coordination Spheres in 1A,B, 2A,B, and 3A,B (Distances in Å, Angles in deg)
1A
2A
3A
1B
2B
3B
Ni(1)−O(1)
Ni(1)−O(2)
Ni(1)−O(3)
Ni(1)−O(5)
Ni(1)−N(1)
Ni(1)−N(2)
Mn(1)−O(2)
Mn(1)−O(4)
Mn(1)−O(1)
Mn(1)−O(4)
Mn(1)−O(1)
Mn(1)−O(2)
O(1)−Ni(1)−O(2)
O(1)−Ni(1)−O(3)
O(1)−Ni(1)−O(5)
O(1)−Ni(1)−N(1)
O(1)−Ni(1)−N(2)
O(2)−Ni(1)−O(3)
O(2)−Ni(1)−O(5)
O(2)−Ni(1)−N(1)
O(2)−Ni(1)−N(2)
O(3)−Ni(1)−O(5)
O(3)−Ni(1)−N(1)
O(3)−Ni(1)−N(2)
O(5)−Ni(1)−N(1)
O(5)−Ni(1)−N(2)
N(1)−Ni(1)−N(2)
O(2)−Mn(1)−O(4)
2.028(2)
2.023(2)
2.055(2)
2.153(2)
2.024(2)
2.035(2)
2.170(2)
2.210(2)
2.158(2)
2.210(2)
2.151(2)
2.170(2)
82.31(7)
92.50(7)
88.99(8)
90.59(8)
173.18(8)
94.05(7)
88.07(10)
172.82(8)
91.03(8)
177.56(10)
87.28(8)
89.44(8)
90.77(11)
89.30(9)
96.04(9)
92.15(7)
180.00
2.016(2)
2.022(2)
2.054(3)
2.153(3)
2.021(3)
2.016(3)
2.175(2)
2.199(2)
2.158(2)
2.199(2)
2.158(2)
2.175(2)
83.30(8)
91.61(8)
87.55(10)
90.70(11)
172.45(11)
95.22(9)
89.63(9)
173.73(11)
89.58(11)
174.95(10)
86.75(13)
91.52(10)
88.29(13)
89.93(12)
96.33(13)
92.77(7)
180.00
2.016(2)
2.006(2)
2.095(2)
2.185(2)
2.037(2)
2.042(2)
2.171(2)
2.185(2)
2.174(2)
2.185(2)
2.174(2)
2.171(2)
80.04(6)
91.40(7)
91.51(7)
90.78(7)
170.18(8)
95.05(7)
89.53(8)
170.70(7)
90.42(8)
174.94(8)
86.59(8)
87.13(7)
89.23(9)
90.70(8)
98.82(9)
90.04(6)
180.00
Ni(2)−O(6)
Ni(2)−O(7)
Ni(2)−N(3)
Ni(2)−N(4)
Ni(2)−O(9)
Ni(2)−O(10)
Mn(2)−O(6)
2.017(2)
2.020(2)
2.030(2)
2.022(2)
2.052(2)
2.147(3)
2.212(2)
2.212(2)
2.120(2)
2.120(2)
2.225(2)
2.225(2)
97.25(9)
89.76(9)
172.74(8)
82.14(7)
90.91(8)
171.72(8)
94.77(8)
87.64(8)
86.08(9)
92.90(7)
89.53(10)
89.01(9)
175.75(9)
88.84(10)
92.01(11)
180.00
2.002(2)
1.999(2)
2.017(2)
2.014(2)
1.995(2)
1.845(3)
1.862(2)
1.889(3)
1.900(5)
2.161(2)
2.161(2)
2.163(2)
2.163(2)
2.138(2)
2.138(2)
95.50(9)
89.68(8)
165.00(8)
81.91(8)
88.84(8)
160.97(8)
100.08(8)
93.28(8)
96.92(9)
101.33(8)
b
Mn(2)−O(6)
Mn(2)−O(7)
Mn(2)−O(7)
Mn(2)−O(8)
Mn(2)−O(8)
a
a
a
b
b
N(3)−Ni(2)−N(4)
O(7)−Ni(2)−N(4)
O(7)−Ni(2)−N(3)
O(6)−Ni(2)−O(7)
O(6)−Ni(2)−N(3)
O(6)−Ni(2)−N(4)
O(7)−Ni(2)−O(9)
O(9)−Ni(2)−N(3)
O(9)−Ni(2)−N(4)
O(6)−Ni(2)−O(9)
O(6)−Ni(2)−O(10)
O(7)−Ni(2)−O(10)
O(9)−Ni(2)−O(10)
O(10)−Ni(2)−N(3)
O(10)−Ni(2)−N(4)
94.16(14)
92.36(12)
168.91(11)
80.89(10)
93.86(13)
169.27(13)
a
a
b
O(7)−Mn(2)−O(7)
O(6)−Mn(2)−O(7)
O(6)−Mn(2)−O(8)
O(6)−Mn(2)−O(6)
O(6)−Mn(2)−O(7)
O(6)−Mn(2)−O(8)
180.00
O(4)−Mn(1)−O(4)
75.48(7)
91.84(7)
180.00
74.65(7)
86.74(7)
180.00
a
a
a
a
O(1) −Mn(1)−O(2)
76.19(6)
86.29(7)
87.85(7)
93.71(7)
93.71(7)
92.15(7)
76.19(6)
86.29(7)
180.00
76.53(7)
85.62(7)
87.23(7)
94.38(7)
94.38(7)
92.77(7)
76.53(7)
85.62(7)
180.00
73.08(6)
90.73(7)
89.97(6)
89.27(7)
89.27(7)
90.04(6)
73.08(6)
90.73(7)
180.00
a
b
b
b
b
O(1) −Mn(1)−O(4)
a
O(2) −Mn(1)−O(4)
104.52(7)
88.16(7)
93.66(7)
88.16(7)
93.66(7)
180.00
105.35(7)
93.26(7)
93.37(7)
93.26(7)
93.37(7)
180.00
a
O(1)−Mn(1)−O(4)
a
O(1) −Mn(1)−O(4)
O(7)−Mn(2)−O(8)
a
b
O(2) −Mn(1)−O(4)
O(6) −Mn(2)−O(8)
b
O(1)−Mn(1)−O(2)
O(1)−Mn(1)−O(4)
O(1)−Mn(1)−O(1)
O(1)−Mn(1)−O(2)
O(2)−Mn(1)−O(2)
O(7) −Mn(2)−O(8)
b
O(8)−Mn(2)−O(8)
a
b
b
O(6) −Mn(2)−O(7)
75.48(7)
91.84(7)
86.34(7)
86.34(7)
104.52(7)
95.52(7)
98.36(8)
74.65(7)
86.74(7)
86.63(7)
86.63(7)
105.35(7)
95.83(8)
95.87(7)
a
b
b
103.81(6)
180.00
103.47(7)
180.00
106.92(6)
180.00
O(6) -Mn(2)−O(8)
a
b
b
O(7) −Mn(2)−O(8)
O(2)−Mn(1)−O(4)
87.85(7)
103.81(6)
97.74(7)
97.29(7)
87.23(7)
103.47(7)
97.34(8)
96.58(8)
89.97(6)
106.92(6)
98.31(7)
98.70(7)
O(7)−Mn(2)−O(8)
a
b
O(1) −Mn(1)−O(2)
O(6) −Mn(2)−O(7)
Ni(1)−O(1)−Mn(1)
Ni(1)−O(2)−Mn(1)
Ni(2)−O(6)−Mn(2)
Ni(2)−O(7)−Mn(2)
a
b
Symmetry element 2 − x, −y, −z in 1A, −x, −3 − y, −3 − z in 2A, and 2 − x, 1 − y, 2 − z in 3A. Symmetry element 2 − x, 1 − y, 1 − z in 1B and
−x, −4 − y, −4 − z in 2B.
the simulations. In the model, the crystallographic equivalence
of the two NiII ions in the trinuclear unit was considered by
assigning one single g value for that ion. Additionally, a single
set of magnetic parameters was deduced for each of the studied
compounds, regardless of the presence of two nonequivalent
Ni2Mn trinuclear molecules in the unit cell in 1 and 2.
Simulations were carried out including a zero-field splitting (D)
value for the two NiII ions and considering that the exchange
JNi−Mn = 1.38 cm−1, and zJ′ = −0.06 cm−1 for complex 1; gNi
=
2.10, gMn = 2.00, DNi = 4.0 cm−1, JNi−Mn = 0.50 cm−1, and zJ′ =
−0.06 cm−1 for complex 2; gNi = 2.10, gMn = 2.00, DNi = 4.0 cm−1,
JNi−Mn = −0.24 cm−1, and zJ′ = −0.03 cm−1 for complex 3. The
D parameter was fixed in the simulations and assumed to have
the same value for complexes 1−3 in order to avoid
overparametrization. Moreover, the intermolecular magnetic
coupling (zJ′) and the D parameter are very closely related and
their independent contributions cannot be easily accounted for.
This result indicates that, in addition to the intramolecular
coupling, the zero-field splitting and the intermolecular coupling
are present but their correct evaluation is not possible, given their
close relation, as was also found in various Ni(II) complexes.2c,d
coupling between these two terminal ions was zero (JNi−Ni
=
0 cm−1). Moreover, a term accounting for intermolecular inter-
actions (zJ′) was also included. The best agreement between
experimental and simulated curves was obtained with the follow-
ing sets of parameters: gNi = 2.10, gMn = 2.00, DNi = 4.0 cm−1,
E
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Figure 2. ORTEP-3 view of the centrosymmetric trinuclear structure
of 2A with ellipsoids at the 30% probability level. There are two
trimers with equivalent structures; only one is shown.
Figure 5. Thermal dependence of the χMT values for complexes 1−3.
Symbols represent experimental data, while solid lines represent the
simulations obtained from the parameters indicated in the main text.
accurately determined for μ2-phenoxido-bridged CuII homo-
nuclear compounds.1a However, there is still a lack of empirical
information on the effect of this angle in coordination
complexes other than those based on CuII ions, not to mention
in the heteropolynuclear compounds. Nevertheless, there is the
possibility that other metallic ions or the combination of
different metallic ions with a similar M−O−M′ skeleton might
push this angle to higher values in order to obtain ferromagnetic
complexes with a larger variety of geometries. In fact, and
keeping this in mind, we decided to compare the value of this
angle for the very limited amount of μ2-phenoxido-bridged
Ni2Mn compounds reported so far that might also present syn-
syn carboxylate bridges to fulfill the coordination sphere of their
paramagnetic ions; hence, they are easily comparable in all
terms. syn-syn carboxylate bridges have often been observed to
lead to antiferromagnetic interactions in homonuclear com-
plexes.33 However, the heterometallic nature of the complexes
studied in this work can effectively weaken such antiferro-
magnetic exchange, as explained later, and thus it has not been
considered for simplicity. Table 3 shows the main structural
features of those compounds, including those reported in this
work, ordered by decreasing value of the magnetic exchange
constant obtained from either the fit or the simulation of the
experimental data.
Figure 3. ORTEP-3 view of the centrosymmetric trinuclear structure
of 3A with ellipsoids at the 30% probability level.
As can be seen from Table 3, the number of μ2-phenoxido
bridges between the NiII and MnII centers is directly correlated
with the Ni−O−Mn angle, 86.38° being the lowest in the case
where there are three bridges and 102.31° the largest when only
one phenoxide is bridging the magnetic ions. Our complexes,
in which the two ions are linked by two monodentate
phenoxide bridges, show very similar angles with an average
value of 97.24° for 1, 96.43° for 2, and 98.51° for 3, these three
values being between those previously mentioned. Concerning
the value of the magnetic exchange constant (JNi−Mn), this
seems to follow a similar trend, by which low values of the
angle induce ferromagnetic coupling while the largest values
lead to antiferromagnetic exchange. A linear fit showing a
possible dependence of the magnetic exchange constant JNi−Mn
on the Ni−O−Mn angle is shown in Figure 6. The fit with
an adjusted R2 value of 0.88 indicates that the critical angle
for μ2-phenoxido-bridged NiII−MnII complexes could be
around 99°.
Figure 4. ORTEP-3 view of 3B with ellipsoids at the 30% probability
level.
Discussion. In all complexes, NiII and MnII centers are
linked through two monodentate oxygen atoms coming from
the deprotonation of two phenol groups that belong to the
same ligand L and simultaneously by a bidentate carboxylate
group coordinated to the metal ions in a syn-syn mode. One of
the parameters that has been observed to have a crucial effect
on the magnetic coupling between ions is the M−O−M′ angle
from the monodentate bridge, M and M′ being the same or
different metallic ions. There seems to be a critical value of the
angle above which the superexchange is antiferromagnetic and
below which the interaction becomes ferromagnetic, and this is
called the critical angle. Actually, the critical angle has been very
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Table 3. Selected Structural Parameters of μ2-Phenoxido-Bridged Ni2Mn Complexes, Ordered by Decreasing Value of the
Magnetic Exchange Constant
compound
[MnII(NiIIL)2]·2CH3OH
JNi−Mn(exptl)/cm−1
JNi−Mn(calcd)/cm−1
+8.3 (−0.3)
no. of μ2-phenoxido bridges
Ni−O−Mn angle/deg
ref
+9.30
3
86.38
8
a
complex 1
av
1
+1.38
+0.50
+1.0
2
2
2
97.24
96.89
97.61
this work
this work
+1.2 (−0.3)
+0.8 (−0.3)
2
a
complex 2
av
+0.25
2
2
2
2
1
96.43
95.87
96.99
98.51
102.31
1
−0.4 (−0.3)
+0.9 (−0.3)
−1.4 (−0.2)
−3.4 (−0.1)
2
complex 3
[MnII(NiIIL)2(OAc)4(H2O)2]
−0.24
−0.30
this work
7
a
Two nonequivalent molecules in the unit cell.
Figure 7. Critical angles for five different families of μ2-phenoxido-
bridged M−M′ polynuclear compounds. For homometallic poly-
nuclear complexes, these values have been extracted from refs 1a, 3d, e,
and 33. The values of the critical angles associated with heterometallic
polynuclear complexes have been tentatively assigned on the basis of
correlations established in this work and in ref 6e.
Figure 6. Variation of the magnetic coupling (JNi−Mn) in trinuclear
Ni2Mn double-phenoxido-bridged complexes with the average Ni−O−
Mn bond angle. Data are extracted from complexes 1−3 of the present
work and from complex 1 of ref 8 and complex 2 of ref 7.
However, complex 2 shows the lowest Ni−O−Mn angle
among the complexes reported in this work and thus the
strongest ferromagnetic exchange is expected for complex 2 in
comparison to complexes 1 and 3. Nevertheless, complex 2
shows an intermediate JNi−Mn value, lower than that of complex
1. The crystal structure of complex 2 shows two nonequivalent
molecules in the structure, one of which presents a
coordination number of 5 for its two NiII ions, as opposed to
all other NiII centers in complexes 1−3, where they are 6-
coordinated. This fact strongly affects the NiII−MnII magnetic
exchange interaction in this complex, and it is the reason for
such disagreement in the magnetostructural correlation, as will
be shown later by means of theoretical calculations. The critical
angles for five different families of μ2-phenoxido-bridged M−
M′ polynuclear compounds have been depicted in Figure 7,
where the critical angle is defined as the M−O−M′ angle
formed by the monodentate phenoxido-based bridging ligand.
While for homometallic polynuclear complexes of CuII, NiII, or
MnIII ions these values have been extracted or assigned from
the results reported in the literature by other authors,1a,3d,e,34
the values of the critical angles associated with heterometallic
polynuclear complexes such as NiII−MnII and CuII−MnII
systems have been tentatively assigned on the basis of
correlations established with Schiff base trinuclear complexes
studied by us in this work and in ref 6e, respectively.
Apparently, among the five different families of polynuclear
compounds compared, the heterometallic family based on
CuII−MnII ions shows the lowest critical angle of all, restricting
the range of angles that lead to ferromagnetic exchange in
comparison to more established homometallic compounds
made of CuII, NiII, or MnIII ions. On the other hand, the family
of heterometallic complexes based on NiII−MnII ions could
represent an increase of this value up to ca. 99°, comparable to
the value reported for μ2-phenoxido-bridged MnIII-based
homometallic complexes and significantly higher than that
reported for analogous NiII-based homometallic complexes.
Still, Mn(III)-based homometallic complexes show the highest
critical angle at 101°,3d,e thus remaining as the family of
μ2-phenoxido-bridged polynuclear complexes with the largest
range of angles available for ferromagnetic exchange. Despite
this, similar heterometallic compounds based on MnIII ions
have been rarely studied from this point of view and definitely
deserve more attention due to the potential they hold.
Theoretical Results. The calculated exchange coupling
constants (see Computational Details) of the three synthesized
Ni2Mn complexes (1−3) together with those of two systems
previously reported are collected in Table 3 and depicted in
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Figure 8.7,8 As shown in Figure 8, there is a clear magneto-
structural correlation between the Ni−O−Mn angle and the
JNi−Mn values, in agreement with what was observed
experimentally (Figure 6). The coordination number of the
NiII cations is 6, with the exception of one of the molecules of
complex 2 (black circle in Figure 8 and antiferromagnetic
Figure 9. Spin density distribution for complex 1 corresponding to the
S = 9/2 ground state. The isodensity surface represented corresponds
to a value of 0.005 e/bohr3 (positive and negative values are
represented as blue and white surfaces, respectively).
cation, the spin distribution is almost spherical, while in the NiII
Figure 8. Dependence between the calculated JNi−Mn values and the
average Ni−O−Mn angle of the μ2-phenoxido bridging ligands. For
molecule 1 of complex 2, with a Ni−O−Mn angle of 95.87°, an
antiferromagnetic coupling with JNi−Mn = −0.4 cm−1 was found
experimentally (black circle), since NiII centers are five-coordinated. A
JNi−Mn value of +2.2 cm−1 is calculated (white circle) by adding a
methanol molecule to give the same coordination number as for the
other systems.
2
2
2
centers it is the square of the sum of the dz and dx −y magnetic
orbitals. Due to the presence of unpaired electrons in all of the
antibonding metal−ligand orbitals (those of t2g symmetry
assuming Oh symmetry), there is a predominance of the spin
delocalization due to the strong mixing of metal−ligand
orbitals.38,39 This fact results in all the atoms coordinated to
the metals having the same sign of spin population. Small white
lobes in some of the carbon atoms of the terminal ligands
indicate that the spin polarization mechanism prevails in the
atoms of the second coordination sphere.
coupling of −0.4 cm−1 in Table 3). However, the inclusion of a
methanol molecule in this structure to reach the coordination
number of 6 present in all the other cases results in a
ferromagnetic coupling of +2.2 cm−1 (white circle in Figure 8),
improving the magnetostructural correlation expressed by a
linear regression with an adjusted R2 value of 0.97. The
calculation of next-nearest-neighbor JNi−Ni constants shows that
all cases correspond to weak antiferromagnetic couplings.
Futhermore, these JNi−Ni values become slightly less anti-
ferromagnetic when the Ni−O−Mn angle is increased, just the
opposite behavior of the JNi−Mn couplings.
CONCLUSIONS
■
Three new NiII−MnII complexes derived from a salen type
Schiff base ligand along with various syn-syn bridging
carboxylate coligands, viz. cinnamate, phenylacetate, and
salicylate, have been synthesized and characterized in order to
determine the crossover angle experimentally. All three have
similar trinuclear structures comprised of a central octahedral
MnII and two terminal octahedral or square-pyramidal NiII
atoms having slight variations in Ni−O−Mn angles (96.43−
98.51°). A linear dependence was found in these NiII−MnII
diphenoxido-bridged complexes between the JNi−Mn value and
the Ni−O−Mn angle of the monodentate phenoxido bridges,
from which a high value of the crossover angle from ferro- to
antiferromagnetic exchange could be assigned. This value
corresponds approximately to 98° and apparently indicates that
the family of NiII−MnII complexes is that which supports
ferromagnetic interactions for a higher angle among the various
known MM′ diphenoxido-bridged complexes, with the
exception of the MnIIIMnIII compounds. Theoretical calcu-
lations also indicate that an incomplete octahedral coordination
sphere for these ions can significantly alter this correlation, and
thus it is only valid for six-coordinated species.
As pointed out in Figure 7, the interaction between NiII and
MnII centers needs a relatively large Ni−O−Mn critical angle to
become antiferromagnetic. In order to justify this fact, we can
apply a simple rule based on the Kahn−Briat model.35−37 By
considering the number and symmetry of interactions of pairs
of magnetic orbitals for the NiII−MnII systems, we can expect
respectively two antiferromagnetic and eight ferromagnetic
contributions. For instance, only one of the five d orbitals of
MnII will have the right symmetry to give an antiferromagnetic
contribution in its interaction with each one of the two orbitals
bearing the unpaired electrons of the NiII center. Due to the
relatively high number of ferromagnetic contributions, the
NiII−MnII systems have a large tendency to show ferromagnetic
coupling, and only large Ni−O−Mn angles (above 98°) can
induce antiferromagnetism.
ASSOCIATED CONTENT
The calculated spin density distribution for the ground state
of complex 1 showing ferromagnetic coupling between NiII and
MnII centers (see Table 3) is represented in Figure 9. As
expected, due to the d5 electronic configuration of the MnII
■
S
* Supporting Information
CIF files giving crystallographic data for 1−3. This material is
available free of charge via the Internet at http://pubs.acs.org.
H
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Chim. Sin. 2007, 65, 1129−1134. (c) Brychcy, K.; Jens, K.-J.; Tilset,
AUTHOR INFORMATION
Corresponding Authors
*E-mail for A.F.: albert.figuerola@qi.ub.edu.
*E-mail for A.G.: ghosh_59@yahoo.com.
Notes
■
̈
M.; Behrens, U. Chem. Ber. 1994, 127, 991−995. (d) Oz, S.; Kurtaran,
̈
R.; Arici, C.; Ergun, U.; Kaya, F. N. D.; Emregul, K. C.; Atakol, O.;
̈
̈
Ulku, D. J. Therm. Anal. Calorim. 2010, 99, 363−368. (e) Seth, P.;
̈
Ghosh, S.; Figuerola, A.; Ghosh, A. Dalton Trans. 2014, 43, 990−998.
(7) Sharma, A. K.; Lloret, F.; Mukherjee, R. Inorg. Chem. 2007, 46,
5128−5130.
The authors declare no competing financial interest.
(8) Kobayashi, T.; Yamaguchi, T.; Ohta, H.; Sunatsuki, Y.; Kojima,
M.; Re, N.; Nonoyama, M.; Matsumoto, N. Chem. Commun. 2006,
1950−1952.
ACKNOWLEDGMENTS
■
We thank the DST-FIST of India funded Single Crystal
Diffractometer Facility at the Department of Chemistry,
University of Calcutta, Kolkata, India. The authors thank the
Department of Science and Technology (DST), New Delhi,
India, for financial support (SR/S1/IC/0034/2012). P.S. is
grateful to the UGC for a research fellowship (UGC/52/Jr.
Fellow (Enhancement) dated 17.01.2013). A.F. acknowledges
financial support from the Spanish MINECO through
(9) (a) Sari, M.; Atakol, O.; Svoboda, I. Anal. Sci.: X-Ray Struct. Anal.
Online 2005, 21, x205−x206. (b) Ercan, F.; Atakol, O. Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun. 1998, 54, 1268−1270. (c) Ercan, F.;
Atakol, O.; Arici, C.; Svoboda, I.; Fuess, H. Acta Crystallogr., Sect. C:
Cryst. Struct. Commun. 2002, 58, m193−m196. (d) Atakol, O.; Nazir,
H.; Durmus, Z.; Svoboda, I.; Fuess, H. Anal. Sci. 2002, 18, 493.
(e) Chen, H.; Ma, C. B.; Yuan, D. Q.; Hu, M.-Q.; Wen, H.-M.; Liu, Q.-
T.; Chen, C.-N. Inorg. Chem. 2011, 50, 10342−10352.
(10) (a) Biswas, S.; Naiya, S.; Drew, M. G. B.; Estarellas, C.;
Frontera, A.; Ghosh, A. Inorg. Chim. Acta 2011, 366, 219−226.
(b) Das, L. K.; Park, S.-W.; Choc, S. J.; Ghosh, A. Dalton Trans. 2012,
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CTQ2012-32247 and for a Ramon y Cajal Fellowship (RYC-
2010-05821). J.J. and E.R. thank the Spanish Ministerio de
Economia y Competitividad (CTQ2011-23862-C02-01) and
the regional Generalitat de Catalunya authority (2009SGR-
1459). The authors gratefully acknowledge the computer
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