Phenoxido mediated antiferromagnetic and azide mediated ferromagnetic coupling in two dinuclear ferromagnetic nickel(ii) complexes with isomeric Schiff bases: a theoretical insight on the pathway of magnetic interaction

Article abstract of DOI:10.1039/d0ce01861a

Two new dinuclear nickel(ii) complexes, [(H₂O)Ni(N₃)(L¹)(μ_1,1-N₃)Ni(L¹)] and [(H₂O)Ni(N₃)(L²)(μ_1,1-N₃)Ni(L²)]·MeOH, derived

Full text of DOI:10.1039/d0ce01861a

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Phenoxido mediated antiferromagnetic and azide
mediated ferromagnetic coupling in two dinuclear
ferromagnetic nickel(II) complexes with isomeric
Schiff bases: a theoretical insight on the pathway
of magnetic interaction†
Cite this: CrystEngComm, 2021, 23,
1942
ab
c
c
Samim Khan,
Tamal Dutta,a Miguel Cortijo,
Rodrigo González-Prieto,
Michael G. B. Drew,d Rosa M. Gomila,
e
Antonio Frontera f and Shouvik Chattopadhyay
a
*
Two new dinuclear nickel(II) complexes, [(H2O)Ni(N3)(L1)(μ1,1-N3)Ni(L1)] and [(H2O)Ni(N3)(L2)(μ1,1-N3)
Ni(L2)]·MeOH, derived from two isomeric Schiff base ligands, HL1 [2-{(2-(ethylamino)ethylimino)methyl}-6-
ethoxyphenol] and HL2 [2-{(2-(dimethylamino)ethylimino)methyl}-6-ethoxy-phenol], have been
synthesized and characterized. Variable temperature (2–300 K) magnetic susceptibility measurements
indicate the presence of moderate ferromagnetic exchange coupling between nickel(II) centers. In each
complex, antiferromagnetic exchange takes place through the phenoxido bridge and ferromagnetic
through the μ1,1-azido bridge. The competitive interactions therefore reduce the overall magnetic coupling.
In a theoretical complex, where the bridging azido ligand has been eliminated and the rest of the geometry
is kept frozen, the magnetic coupling becomes antiferromagnetic which suggests that the ferromagnetic
exchange occurs via the μ1,1-azido bridge. Mulliken population analysis and spin density plots clearly show
that the spin distributed spherically in the Ni centers is due to the presence of one unpaired electron in
Received 22nd December 2020,
Accepted 21st January 2021
both the dx
and dz2 orbitals. The shape of the spin density at the bridging O-atom and azide evidences
2–y2
the participation of their p orbitals in the magnetic coupling. The SOMO is basically constituted by the dz2
DOI: 10.1039/d0ce01861a
orbital of one nickel(II) center with the participation of the azide π-system. The SOMO−1 is constituted by
the dx
orbital of the other nickel(II), an oxygen atom and the azide π-system.
2–y2
rsc.li/crystengcomm
to their importance to bioinorganic chemistry and
Introduction
magnetochemistry.1 The key aspect of molecular magnetism
is understanding the mechanism of spin coupling and
determination of magneto-structural correlations.2 This can
be done by studying discrete molecules, and large clusters or
extended systems to obtain molecule-based magnetic
materials.3 The magnetic interactions in such complexes
mainly occur due to super exchange coupling between the
metal centers via bridging ligands, and the strength and
nature of this interaction, whether ferromagnetic or
antiferromagnetic, depend on the bridging moiety and its
subtended angle.4 Coordination complexes based on the
nickel(II) ion could have potential applications in molecular-
based ferromagnets, such as single molecule magnets (SMMs)
and single chain magnets (SCMs), and these are used as data
storage devices, nanoscale tools, and quantum computing
systems.5 Nickel(II) is a preferred spin carrier to prepare
molecular ferromagnets due to its large single-ion zero-field
splitting.6 In this regard, tridentate N2O donor Schiff base
ligands obtained from diamines and salicylaldehyde
The study of dinuclear coordination complexes of 3d metal
ions has attained special interest for the last two decades due
a Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata –
700032, India. E-mail: shouvik.chem@gmail.com
b Department of Chemistry, Aliah University, New Town, Kolkata 700156, India
c Departamento de Química Inorgánica, Facultad de Ciencias Químicas,
Universidad Complutense de Madrid, 28040, Madrid, Spain
d School of Chemistry, The University of Reading, P. O. Box 224, Whiteknights,
Reading RG6 6AD, UK
e Serveis Cientifico-Tècnics, Universitat de les Illes Balears, Crta de Valldemossa km
7.5, 07122 Palma de Mallorca, Baleares, SPAIN
f Departament de Química, Universitat de les Illes Balears, Crta de Valldemossa km
7.5, 07122 Palma de Mallorca, Baleares, SPAIN
† Electronic supplementary information (ESI) available: Fig. S1–S4 and Tables
S1–S4. CCDC 2049044 (1) and 2036426 (2) contain the supplementary
crystallographic data for complexes
1 and 2, respectively. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0ce01861a
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derivatives are widely used for synthesizing oxido or
phenoxido bridged nickel(II) complexes along with various
bridging anionic coligands such as azide, thiocyanate,
cyanate, etc.7 However, dinuclear nickel(II) complexes with two
dissimilar bridges of azide and phenoxido ligands are still
relatively less explored.8 In our previous work, we synthesized
Yield: 245 mg (68%). Anal. Calc. for C27H44N10Ni2O6 (FW =
722.10): C, 44.91; H, 6.14; N, 19.40%. Found: C, 44.7; H, 6.4; N,
19.2%. FT-IR (KBr, cm−1): 1627 (νCN); 2061 (νN ); 3051, 3267
3
(νNH ); 3446 (νOH). UV-VIS [λmax (nm)] [εmax (L mol−1 cm−1)]
2
(acetonitrile): 246 (1.73 × 104); 303 (5.4 × 103); 405 (1.4 × 102).
Physical measurements. Elemental analyses (carbon,
hydrogen and nitrogen) were performed using a Perkin Elmer
240C elemental analyzer. IR spectra in KBr (4500–500 cm−1)
a
similar copper(II) complex with antiferromagnetically
coupled copper(II) centers with J = −46.18 cm−1.9 Herein, we
report that replacement of the Cu sites in [(H2O)Cu(L1)(μ1,1
-
were recorded with
spectrophotometer. Electronic spectra in acetonitrile were
recorded on Perkin Elmer Lambda 35 UV-visible
a
Perkin Elmer Spectrum Two
N3)Cu(L1)]ClO4 with Ni results in an isostructural complex
[(H2O)Ni(N3)(L1)(μ1,1-N3)Ni(L1)] (1) that exhibits ferromagnetic
coupling. We have also replaced the HL1 by a blocking ligand
HL2 to prepare [(H2O)Ni(N3)(L2)(μ1,1-N3)Ni(L2)]·MeOH (2) to
check its effect on the overall magnetic behaviour. Density
functional theory (DFT) combined with the broken symmetry
approach has also been reported to provide a qualitative
theoretical interpretation on the overall magnetic behaviour
of complexes 1 and 2.
a
spectrophotometer. Powder X-ray diffraction was performed on
a Bruker D8 instrument with Cu Kα radiation. In this process,
both complexes were ground with a pestle and mortar to
prepare fine powder. The powder was then dispersed with
ethanol onto a zero background holder (ZBH). The ethanol was
allowed to evaporate to provide a nice, even coating of powder
adhered to the sample holder. The variable temperature
magnetization measurements of the complexes were carried out
at 1 T in the 2–300 K range using a Quantum Design MPMSXL
SQUID magnetometer and 51.11 mg of 1 and 54.42 mg of 2.
The data were corrected for the diamagnetic contribution of the
sample holder and the intrinsic contributions on the basis of
Pascal's constants.
Experimental section
All other chemicals were of reagent grade and used as
purchased from Sigma-Aldrich without further purification.
X-ray crystallography. Single crystals suitable for diffraction
were used for data collection using an Oxford Diffraction
X-Calibur System diffractometer for 1 at 150 K and a Bruker
SMART APEX II diffractometer for 2 at 296 K equipped with a
graphite-monochromated Mo-Kα radiation source (λ = 0.71073
Å). The structures were solved by direct methods using Shelxs
and refined by full-matrix least squares on F2 using the
Shelx2016/6 package.10 Non-hydrogen atoms were refined with
anisotropic thermal parameters. Hydrogen atoms were placed in
their geometrically idealized positions and constrained to ride
on their parent atoms. In 2, the solvent methanol molecule was
disordered over three overlapping sites and refined accordingly.
However, as it was not possible to see clear electron-density
peaks in difference maps which would correspond with
acceptable locations for the various disordered methanol H
atoms, the refinement was completed with no allowance for
these methanol H atoms in the model. Empirical absorption
corrections were applied for 1 using the ABSPACK program11
and for 2 using SADABS.12 Other programs used included
PLATON13 and ORTEP.14 Crystallographic data and refinement
details of both complexes are given in Table S1, ESI.†
Hirshfeld surfaces. Hirshfeld surface analysis was explored
to evaluate the structural flexibility and magnitude of
interchain interactions in both complexes. Hirshfeld
surfaces15 and associated 2D-fingerprint16–18 plots were
calculated using Crystal Explorer.19 This analysis is useful for
the evaluation of closest intermolecular atomic contacts, even
in complex crystal structures.20
Theoretical methods. The study of the magnetic behaviour
of the complexes was performed at the B3LYP/6-31+G* level
of theory21 and using the crystallographic coordinates by
means of the Gaussian-16 program.22 To calculate the
Caution!!
Although no problems were encountered in this work,
organic ligands in the presence of azides are potentially
explosive. Only a small amount of the material should be
prepared and they should be handled with care.
Preparation of complex [(H2O)Ni(N3)(L1)(μ1,1-N3)Ni(L1)] (1).
A methanol solution (10 mL) of 3-ethoxysalicylaldehyde (1
mmol, 0.166 g) and N-ethyl-1,2-diaminoethane (1 mmol,
0.105 mL) was refluxed for 1 h to prepare a tetradentate N2O2
donor Schiff base, HL1. The Schiff base was not isolated and
was used directly for preparation of the complex. A methanol
solution (10 mL) of nickel(II) acetate tetrahydrate (1 mmol,
0.250 g) was added to the methanol solution of the Schiff
base followed by the addition of a methanol solution (5 mL)
of sodium azide (1 mmol, 0.65 g) with constant stirring. The
stirring was continued for 2 h. Diffraction quality single
crystals were obtained after a few days upon slow evaporation
of a dark green solution of the compound in an open
atmosphere.
Yield: 248 mg (72%). Anal. Calc. for C26H40N10Ni2O5 (FW =
690.06): C, 45.25; H, 5.84; N, 20.30%. Found: C, 45.4; H, 5.6;
N, 20.5%. FT-IR (KBr, cm−1): 1638 (νCN); 2064 (νN ); 3258
3
(νNH ). UV-VIS [λmax (nm)] [εmax (L mol−1 cm−1)] (acetonitrile):
2
244 (1.73 × 104); 304 (4.76 × 103); 405 (1.14 × 103).
Preparation
of
complex
[(H2O)Ni(N3)(L2)(μ1,1-N3)
Ni(L2)]·MeOH (2). Complex 2 was prepared in a similar
method to that of complex 1, except that N,N′-dimethyl-1,2-
diaminoethane (1 mmol, 0.10 mL) was used instead of
N-ethyl-1,2-diaminoethane. Single crystals, suitable for X-ray
diffraction, were obtained upon slow evaporation of the
solution after 5 days.
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coupling constant (J) of the dinuclear nickel complexes, two
energy levels are evaluated, corresponding to the high-spin
(Ehs) and broken-symmetry (Ebs) states. Subsequently, the J
values used in this work were obtained from the equation
and methodology proposed by Ruiz et al.23 The plots of the
spin density and SOMOs were generated using Gauss View
software v. 6.0.16 (ref. 24) for the HS configuration.
coordination) and another molecule of the Schiff base shows
a tridentate coordination mode (keeping the alkoxy part
pendant). Similarly, one azide is used to bridge two nickel(II)
centers and another azide acts as a terminal ligand [vide
infra]. This synthetic procedure may be extended in the
future to prepare a series of mixed bridged dinuclear
nickel(II) complexes.
Results and discussion
Structural description
Synthesis
[(H2O)Ni(N3)(L1)(μ1,1-N3)Ni(L1)] (1). Single crystal X-ray
diffraction analysis reveals that complex 1 crystallizes in the
orthorhombic space group P212121 with Z = 4. The molecular
structure is built from isolated dinuclear molecules of [(H2O)
Ni(N3)(L1)(μ1,1-N3)Ni(L1)], in which both nickel(II) centers are
hexacoordinated. A perspective view of the complex is shown
in Fig. 1a. The dinuclear complex contains two deprotonated
Schiff bases, one acting as a tetradentate ligand and the
other as a tridentate one.
Ni(1) has a distorted octahedral geometry, in which an
amine nitrogen atom, N(22), one imine nitrogen atom, N(19),
one phenoxido oxygen atom, O(11), of a Schiff base ligand
and one nitrogen atom, N(1), of a bridged azide constitute
the equatorial plane. One oxygen atom, O(1), of a water
molecule and a nitrogen atom, N(4), of a terminal azide
coordinate in the axial positions to complete the distorted
octahedral geometry. The deviations of the coordinating
atoms, N(1), O(11), N(19) and N(22), in the basal plane from
their least-squares mean plane are −0.041(1), 0.051(1),
−0.050(2), and 0.040(1) Å, respectively. The deviation of Ni(1)
from the same plane is 0.068(2) Å.
The potential tetradentate Schiff base ligands HL1 and HL2
were prepared by the condensations of N-ethyl-1,2-
diaminoethane and N,N′-dimethyl-1,2-diaminoethane with
3-ethoxysalicylaldehyde following
a
literature method.25a
Methanol solutions of the Schiff base ligands, thus prepared,
were added to methanol solutions of nickel(II) acetate
tetrahydrate, and stirred for 2 h followed by the addition of
sodium azide under constant stirring conditions for 2 h to
produce complexes 1 and 2. Formation of both complexes is
shown in Scheme 1.
The phenoxido group has been known to have a tendency
to bridge two or more metal centres for a long time.25b We
have therefore used ‘salen’ or ‘half salen’ type ligands to
prepare many di- and polynuclear complexes of different
transition metals for the last decades.25d–g Pseudo-halides,
e.g. azide, thiocyanate, cyanate, etc., also have the ability to
bridge metal centres thereby producing multimetallic
complexes.25c,h,i In the present study, we have used two
potential tetradentate (isomeric) half-salen type Schiff base
ligands and the azide co-ligand to prepare two dinuclear
nickel(II) complexes. In each complex, one Schiff base
molecule shows a tetradentate coordination mode (where the
amine, imine, phenoxido and alkoxy groups participate in
Similarly, Ni(2) also has a distorted octahedral geometry,
where an imine nitrogen atom, N(39), of one Schiff base
ligand, one phenoxido oxygen atom, O(11), one ethoxy oxygen
Scheme 1 Synthetic route to complexes 1 and 2.
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Fig. 1 ORTEP views of complexes (a) 1 and (b) 2 with 30% thermal ellipsoid probability. Hydrogen atoms have been omitted for clarity. The lattice
methanol molecule in 2 has also not been shown.
atom, O(131), of the other Schiff base ligand and a nitrogen
atom, N(1), from a bridging azide constitute the equatorial
plane. One phenoxido oxygen atom, O(31), and an amine
nitrogen atom, N(42), of one Schiff base ligand coordinate in
the axial positions to complete the coordination. The
deviations of the coordinating atoms, O(11), O(131), N(1) and
N(39), in the basal plane from the mean plane passing
through them are −0.011(2), 0.009(1), 0.009(1), and −0.007(1)
Å, respectively. The deviation of Ni(2) from the same plane is
0.011(2) Å. The equatorial planes from the two metal centers
intersect at an angle of 18.2(1)°. Selected bond lengths and
bond angles are listed in Tables S2 and S3 (ESI†),
respectively.
Saturated five membered chelate rings [Ni(1)–N(19)–C(20)–
C(21)–N(22)] and [Ni(2)–N(39)–C(40)–C(41)–N(42)] have
envelope and half-chair conformations with puckering
parameters q = 0.454(3) Å; ϕ = 106.5(3)° and q = 0.427(3) Å; ϕ
= 90.7(3)° respectively.26
The hydrogen atom, H(22), attached to the nitrogen atom,
N(22), participates in hydrogen bonding interaction with the
symmetry-related
oxygen
atom,
O(331)c
{symmetry
c
transformation = 1/2 + x, 3/2 − y,−z} (Table S4, ESI†). The
hydrogen atom, H(42), attached to the nitrogen atom, N(42),
also participates in an intramolecular hydrogen bonding
interaction with nitrogen atom N(6). Similarly, hydrogen
atoms, H(1) and H(2), attached to the oxygen atom, O(1),
participate in strong hydrogen bonding interaction with the
oxygen atom, O(31), and symmetry-related nitrogen atom,
N(4)a {symmetry transformation a = −1/2 + x, 3/2 − y,−z} (Table
S4, ESI†). All these hydrogen bonding interactions lead to the
formation of a supramolecular chain (Fig. 2).
A nitrogen atom, N(1), of an azide ligand and a phenoxido
oxygen atom, O(11), of a Schiff base ligand bridge two
nickel(II) centers. The bridging angles Ni(1)–O(11)–Ni(2) and
Ni(1)–N(1)–Ni(2) are 103.06(10)° and 94.82(11)° respectively.
The distance between nickel(II) centers is 3.165(4) Å.
Fig. 2 One-dimensional hydrogen bonded chain structure of complex 1. Selected hydrogen atoms and ethyl group have been omitted for clarity.
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The deviation of Ni(1) from the same plane is 0.037(1) Å.
Selected bond lengths and bond angles are listed in Tables
S2 and S3 (ESI†), respectively.
Ni(2) has a distorted octahedral geometry, where an imine
nitrogen atom, N(39), one phenoxido oxygen atom, O(11), an
ethanoate oxygen O(131) of a Schiff base ligand and one
nitrogen atom, N(1), of a bridged azide constitute the
equatorial plane. One phenoxy oxygen atom, O(31), and an
amine nitrogen atom, N(42), of a terminal azide coordinate
in the axial positions to complete its distorted octahedral
geometry. The deviations of the coordinating atoms, N(1),
O(11), N(39) and O(131), in the basal plane from their least-
squares mean plane are −0.018(1), 0.022(1), 0.012(1), and
−0.018(1) Å, respectively. The deviation of Ni(2) from the
plane is 0.028(1) Å. A nitrogen atom, N(1), of an azide ligand
and a phenoxido oxygen atom, O(11), of a Schiff base ligand
bridge two nickel(II) centers. The bridging angles Ni(1)–
O(11)–Ni(2) and Ni(1)–N(1)–Ni(2) are 103.44(8)° and
95.90(10)° respectively. The distance between nickel(II)
centers is 3.129(1) Å. Saturated five membered chelate rings
Fig. 3 3D supramolecular assembly via hydrogen bonding interactions
in complex 2. Selected hydrogen atoms have been omitted for clarity.
[(H2O)Ni(N3)(L2)(μ1,1-N3)Ni(L2)]·MeOH (2). Complex
2
shows a similar dinuclear molecular structure to complex 1
but crystallizes in the monoclinic space group P21/n with Z =
4. The structure of [(H2O)Ni(N3)(L2)(μ1,1-N3)Ni(L2)] also
contains two hexacoordinated nickel(II) centers. A perspective
view of complex 2 is shown in Fig. 1b. It also contains two
deprotonated Schiff bases, one acting as a tetradentate ligand
and the other as a tridentate one. Ni(1) adopts a distorted
octahedral geometry, where an imine nitrogen atom N(19),
an amine nitrogen atom, N(22), of one Schiff base ligand,
one phenoxido oxygen atom, O(11), and a nitrogen atom,
N(1), from a bridging azide constitute the equatorial plane.
An oxygen atom of a water molecule, O(1), and a terminal
azide nitrogen atom, N(4), coordinate in the axial positions
to complete the distorted octahedron. The deviations of the
coordinating atoms, N(1), O(11), N(19) and N(22), in the basal
plane from the mean plane passing through them are
−0.041(1), 0.049(1), −0.047(1), and 0.039(1) Å, respectively.
[Ni(1)–N(19)–C(20)–C(21)–N(22)]
and
[Ni(2)–N(39)–C(40)–
C(41)–N(42)] have envelope and half-chair conformations
with puckering parameters q = 0.301(4) Å; ϕ = 300.9(6)° and q
= 0.442(3) Å; ϕ = 300.9(6)° respectively.26
Thus, the structure of 2 is basically the same as that found
in complex 1, although as is apparent from Table S2,† there
are significant differences in the dimensions between the two
structures. For example, the Ni(2)–O(131) bond length is
much longer in 2 at 2.472(2) Å, than in 1 at 2.238(2) Å, but
there is no obvious explanation for the differences.
The water hydrogen atoms, H(1) and H(2), attached to
O(1) participate in an intermolecular hydrogen bond with the
terminal azide N(3)c (c =1/2 − x, −1/2 + y, 1/2 − z) and a strong
intramolecular hydrogen bond with O(31). In addition,
Fig. 4 Hirshfeld surfaces mapped with dnorm (left), shape index (middle), and curvedness (right bottom) for complexes 1 (above) and 2 (below).
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although their hydrogen atoms were not located, it seems
clear that all three alternative oxygen positions for the
disordered methanol solvent molecule form intermolecular
hydrogen bonds with the terminal azide. All these hydrogen
O⋯H and H⋯O interactions comprise 3.5% and 5.3% of the
Hirshfeld surfaces for each molecule of complexes 1 and 2,
respectively. The C⋯H/H⋯C {12.9% (1) and 13.6% (2)} and
N⋯H and H⋯N {21.4% (1) and 13.6% (2)} interactions also
contribute to the overall Hirshfeld surfaces of both
complexes (Fig. 5).
bonding interactions lead to the formation of
supramolecular architecture (Fig. 3).
a 3D
IR and electronic spectra. In the IR spectra of both
complexes, distinct bands corresponding to the azomethine
(CN) stretching vibration appear at around 1640 cm−1.27
Distinct bands at around 2065 cm−1 are indicative of the
presence of the EO azido group in both the complexes.28 The
bands in the range of 2974–2878 cm−1 may be assigned to
alkyl C–H bond stretching vibrations.29 A moderately strong,
sharp peak at around 3251 cm−1, may be attributed to the
N–H stretching vibration.30 Broad bands at around 3450 cm−1
clearly indicate the presence of water molecules in the
complex.31
Electronic spectra of both complexes in acetonitrile
display absorption bands at around 405 nm.32 Absorption
bands at around 305 nm are also observed and may be
attributed to ligand to metal charge transfer transitions.33
The intense absorption bands at a wavelength of 245 nm may
be assigned as π → π* transitions.34 The electronic spectra of
complexes 1 and 2 are shown in Fig. S1 and S2 (ESI†)
respectively.
X-ray powder diffraction pattern. The experimental powder
X-ray diffraction pattern of the bulk product agrees well with
the simulated XRD pattern generated from cif. This indicates
the purity of the bulk samples. Fig. S3, ESI† shows the
experimental and simulated XRD patterns of complex 1.
Magnetic properties of complex 1. The temperature
dependence of both the molar magnetic susceptibility (χM)
and the product of the molar magnetic susceptibility and the
temperature (χMT) of 1 is shown in Fig. 6. The χM value
increases continuously upon cooling, as what usually occurs
for a paramagnetic compound. The value of the χMT product
at room temperature is 2.68 cm3 K mol−1, which is slightly
larger than the spin-only value (2.35 cm3 K mol−1) expected
for a system with two isolated S = 1 ions assuming g = 2.17
(see below). An increase of the χMT product, which suggests
the existence of predominant ferromagnetic interactions, is
observed upon cooling. As a result, the χMT product reaches a
maximum value of 4.26 cm3 K mol−1 at 5.7 K. Further cooling
of the sample leads to a decrease of the χMT value, which is
3.44 cm3 K mol−1 at 2.0 K. This decrease is probably due to
the existence of a zero-field splitting (D) which is typical of
these types of Ni(II) complexes, and/or antiferromagnetic
interactions. This behavior is similar to that found in other
μ-phenoxido–μ1,1-azide dinickel(II) complexes.8
Hirshfeld surface analysis. Hirshfeld surfaces of both
complexes, mapped over dnorm (range of −0.1 to 2.5 Å), shape
index and curvedness, are illustrated in Fig. 4. The
fingerprint plots can be decomposed to highlight particular
atom pair close contacts. This decomposition enables the
separation of contributions from different interaction types,
which overlap in the full fingerprint. The proportions of
Experimental magnetic susceptibility data have been fitted
using a modification of the model described by Prushan
Fig. 5 2D fingerprint plots: full; O⋯H/H⋯O, C⋯H/H⋯C and N⋯H/H⋯N interactions contributed to the total Hirshfeld surface area of complexes
1 (above) and 2 (below).
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Fig.
6 Temperature dependence of the molar susceptibility χM
Fig. 7 Temperature dependence of the molar susceptibility χM (circles)
and χMT (squares) for complex 2. Solid lines show the best fit to the
data as described in the text.
(circles) and χMT (squares) for complex 1. Solid lines show the best fit
to the data as described in the text.
et al.35 This model considers the existence of intramolecular
interactions (J), and zero-field splitting (D) of the Ni(II) ions
by applying the following Hamiltonian:
The magnetic behaviour of this compound is analogous to
that observed in complex 1. Predominant ferromagnetic
interactions are deduced due to the increase upon cooling
observed in the χMT value, which reaches a maximum value
of 5.04 cm3 K mol−1 at 7.5 K and then falls to a value of 3.45
ꢀ
ꢁ
Â
Ã
1
3
H ¼ −2 JS1S2 þ D M2S − SðS þ 1Þ
(1)
̂
̂
cm3
K K caused by antiferromagnetic
mol−1 at 2.0
Because the sign of D cannot be unequivocally determined
from magnetization measurements of powder samples, the
absolute value of this zero-field splitting parameter has been
interactions and/or the presence of a zero-field splitting (D).
Therefore, the magnetic data were fitted using the same
model used for complex 1. Applying this model, the best fit
of the experimental data yielded the following results (Fig. 7):
g = 2.31, |D| = 1.26 cm−1, J = 13.24 cm−1, zJ = 0.84 cm−1, TIP =
1.54 × 10−13 cm3 mol−1 and σ2 = 2.03 × 10−3. The TIP value is
negligible for this compound. Indeed, the fitting of the data
without considering this parameter yields almost the same
results (g = 2.30, |D| = 1.28 cm−1, J = 13.60 cm−1, zJ = 0.85
cm−1, σ2 = 2.00 × 10−3). The values of D8b,g,38 and g8,35 are
comparable to those observed in similar Ni(II) complexes..
The differences found in the coordination environments of
the Ni(II) centres between complexes 1 and 2, like the Ni(2)–
O(131) bond length (mentioned above in the “Structural
description” section), can explain the small variations in their
values of g and D.
considered in the equation.
A temperature-independent
paramagnetism (TIP) term is also included in this model. In
addition, intermolecular interactions (zJ) have been also
considered using the molecular field approximation.36 Thus,
the parameters obtained from the best fit of the magnetic
data are g = 2.17, |D| = 0.96 cm−1, J = 11.35 cm−1, zJ = 0.61
cm−1, TIP = 5.42 × 10−4 cm3 mol−1 with σ2 = 1.48 × 10−3. Fig. 6
shows the fit of the experimental data using this model.
Comparable g8,35 and D8b,g,37,38 values have been obtained for
similar dinickel(II) complexes.
The small zJ value obtained from the fit of the data (0.61
cm−1)
indicates
weak
ferromagnetic
intermolecular
interactions. In spite of this small value, it has been
necessary to include this parameter in the model in order to
obtain good quality fit results. These interactions may take
place through the hydrogen bonds that form the 1D
supramolecular assembly, as shown in Fig. 2. This kind of
magnetic interaction through hydrogen bonds has been
previously reported.39
Magnetic properties of complex 2. Fig. 7 shows the
variation with the temperature of the molar magnetic
susceptibility (χM) and the product of the molar magnetic
susceptibility and the temperature (χMT) of 2. There is a
continuous increase of the χM value with decreasing
temperature, showing the usual paramagnetic behaviour. The
room temperature value of χMT (2.90 cm3 K mol−1) is slightly
higher than the spin-only value (2.67 cm3 K mol−1) expected
for a g = 2.31 system with two isolated S = 1 Ni(II) ions (see
below).
Slight ferromagnetic intermolecular interactions are
present in complex 2 as indicated by the low zJ value (0.84
cm−1) obtained from the fit of the experimental data. Similar
to compound 1, they may occur through the hydrogen bonds
that form the 3D supramolecular structure shown in Fig. 3.
Magneto-structural correlation. The J value of complex 1 is
11.35 cm−1. Similar positive values (2.8–25.6 cm−1),
J
indicating the existence of intramolecular ferromagnetic
interactions between the two Ni(II) ions, have been obtained
from the fit of the magnetic data of analogous heterobridged
μ-phenoxido–μ1,1-azide dinickel(II) complexes.8f It has been
described in the literature that the magnitude of the J value
in these compounds depends on different structural
parameters such as the Ni–O–Ni and Ni–N–Ni angles, the
Ni–O and Ni–N distances and the asymmetry of the two Ni–N
bond lengths.8d,f The high number of parameters that affect
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Table 1 Experimental magnetic coupling constant (Jexp) and structural parameters of 1, 2 and selected analogous μ-phenoxido–μ1,1-azide dinickel(II)
complexes
Asymmetry
Ni–N [Å]
Complex
Jexp [cm−1
]
Ni–O–Ni [°]
Ni–N–Ni [°]
Ni–O [Å]
Ni–N [Å]
Ref.
1
11.35
15.60
5.84
13.24
16.9
103.1(1)
94.8(1)
2.037(2), 2.005(2)
2.013(1), 2.008(1)
2.037(2), 2.052(1)
2.003(2), 1.984(2)
1.994(2), 2.017(2)
2.185(3), 2.113(3)
2.161(2), 2.133(2)
2.095(2), 2.208(2)
2.105(2), 2.110(2)
2.125(3), 2.115(3)
0.072
0.028
0.113
0.003
0.010
This work
37
38
This work
8d
[Ni(L1)2(N3)(N3)]
[Ni(L2)2(N3)(N3)(H2O)]
2
102.67(6)
102.30(7)
103.44 (8)
104.68(9)
93.94(7)
95.43(9)
95.9(1)
[Ni2(L3)2(N3)(CH3CN)(H2O)]
(ClO4) H2O·CH3CN
[Ni2(L4)2(N3)(CH3CN)(CH3OH)]
(ClO4)·CH3CN
97.02(11)
18.0
104.55(12)
97.58(15)
2.004(3), 2.000(3)
2.119(3), 2.091(4)
0.028
8f
L1, L2, L3 and L4 are the Schiff base ligands indicated in the corresponding references.
the exchange coupling constant makes it very difficult to
predict the value of J only according to these structural
Ni–N bond lengths shown by these compounds is in the
range 0.010–0.028 Å, but complex 2 exhibits two almost
identical Ni–N distances. Their J values however do not
exhibit the usual tendency for higher ferromagnetic coupling
constants to be found in complexes with no Ni–N
difference.8d,e However, it is obvious that other structural
parameters also influence the magnitude of the
ferromagnetic interaction, although the differences between
these parameters in complex 2 and in the previously cited
complexes do not seem very significant.8,37
parameters. However,
a
comparison with analogous
complexes with similar parameters is possible. For example,
compounds [Ni(L1)2(μ1,1-N3)(N3)]37 and [Ni(L2)2(μ1,1-N3)(N3)
(H2O)],38 where L1 and L2 are Schiff base ligands, show very
similar structural parameters to those of the title
compounds, apart from the asymmetry in the Ni–N bond
lengths (Table 1). There is a difference of 0.028 Å between
the two Ni–N bond lengths and a coupling constant of 15.6
cm−1 in the former compound, while the J in the latter is 5.84
cm−1, for a difference of 0.113 Å.
This is in accordance with the observed trend of stronger
ferromagnetic interactions in compounds with two almost
equal Ni–N distances.8d,e This difference in compound 1 is
intermediate between these two extremes at 0.072 Å, which is
consistent with a ferromagnetic interaction of intermediate
strength (J = 11.35 cm−1).
DFT study. To better understand the magnetic behavior of
the complexes, we have obtained the J values of complexes 1
and 2 theoretically, which are in good agreement with the
experimental results and confirm the ferromagnetic nature of
the intramolecular couplings. They are 11.70 cm−1 for 1 and
20.57 cm−1 for 2, and are in acceptable agreement with the
experimental findings (11.35 and 13.24 cm−1 for 1 and 2,
respectively). Since the theoretical J value for compound 2
seems overestimated, we have also computed it using a
higher level of theory (B3LYP/def2-TZVP instead of B3LYP/6-
31+G*) but the resulting J value (20.36 cm−1) remains a poor
The J value of 13.24 cm−1 obtained for complex 2 is
comparable with values observed in complexes with
analogous structural parameters: [Ni(L1)2(μ1,1-N3)(N3)] (J =
15.6 cm−1);37 [NiII2(L3)2(μ1,1-N3)(CH3CN)(H2O)](ClO4) H2-
O·CH3CN (J = 16.9 cm−1);8d and [NiII2(L4)2(μ1,1-N3)(CH3CN)
(CH3OH)](ClO4)·CH3CN (J = 18.0 cm−1),8f where L1, L3, and L4
are Schiff base ligands (Table 1). The difference between
fit to the experimental value of 13.24 cm−1
.
It has been previously demonstrated8 that, in mixed
bridged (μ-oxo, μ1,1-azido) dinuclear Ni(II) complexes,
antiferromagnetic exchange takes place through the
Fig. 8 (a) Graphical representation of spin density (contour 0.004 a.u.) at the ground state (high spin) configuration. (b and c) Pictorial
representation of the SOMO involving the dz2 and dx orbitals of nickel(II) for the high spin state of complex 1.
2–y2
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CrystEngComm
Table 2 The spin densities on selected atoms for compound 1 at the
UB3LYP/6-31G* level of theory. See Fig. 8 for labelling
center with the participation of the π-system of azide. The
2
2
SOMO−1 is constituted by the dx –y orbital of the other
nickel(II) and the π-system of azide.
Atom
Spin density (HS)
Spin density (LS)
Ni(1)
Ni(2)
O(11)
N(1)
1.66
1.67
0.08
0.06
0.04
0.08
0.07
0.01
0.05
0.01
0.07
0.07
−1.68
1.67
Conclusions
0.005
0.011
−0.03
−0.07
−0.07
−0.01
0.05
0.02
0.08
0.07
Herein, we report two relatively rare mixed phenoxido and
azide bridged dinuclear nickel(II) complexes. Both complexes
show strong intermolecular hydrogen bonding interactions to
form a supramolecular chain structure in 1 and a 3D network
in 2. Variable temperature (2–300 K) magnetic susceptibility
measurements indicate the presence of ferromagnetic
exchange coupling between nickel(II) centers (J = 11.35 and
13.24 cm−1 for complexes 1 and 2). The experimental findings
were further checked and rationalized using broken-
symmetry DFT calculations, and spin density and SOMO
plots, which clearly support the presence of ferromagnetic
coupling which is transmitted through the azide bridging
ligand and compensates for the antiferromagnetic
communication via the phenoxido bridge.
N(4)
N(19)
N(22)
O(1)
O(31)
O(131)
N(39)
N(42)
phenoxido bridge and ferromagnetic through the μ1,1-azido
bridge. The competitive interaction in this type of complex
reduces the overall magnetic coupling. The low experimental
values of J in compounds 1 and 2 suggest the existence of
this compensating effect. To further corroborate this
explanation, we have determined the J in compound 1 using
a theoretical complex where the bridging azido ligand has
been eliminated and the rest of the geometry is kept frozen.
Conflicts of interest
There are no conflicts to declare.
As
a
result, the magnetic coupling changes to
antiferromagnetic J = −5.70 cm−1, thus evidencing that the
ferromagnetic exchange occurs via the μ1,1-azido bridge.
To examine the magnetic coupling mechanism, the spin
density distribution has been analyzed in both complexes 1
and 2 (see Fig. S4, ESI† for compound 2). The spin density of
compound 1 in the high spin (HS) state is represented in
Fig. 8a and the spin density values are summarized in
Table 2 for both high spin (HS) and low spin (LS) states,
where positive and negative signs denote α and β spin states,
respectively. The Mulliken spin population analysis (HS)
indicates that a significant spin (ca. 1.33 e) is delocalized
through the ligands, and the rest (2.77 e) is carried by the
central nickel atoms. The spin carried by the phenoxido
oxygen atom is ca. 0.08 e in the high-spin state and only
0.005 e in the broken-symmetry state of complex 1 indicating
a polarization competition between the two nickel atoms with
α and β spin density, respectively. The spin carried by the
bridging N-atom of azide is 0.059 e for the high spin and
0.011 e for the low spin, thus showing a similar behaviour.
Acknowledgements
S. K. thanks CSIR, India, for awarding
a
Research
Associateship [Sanction No. 09/1157 (0005) 2K19-EMR-I]. S. C.
wishes to thank the UGC-CAS II programme, Department of
Chemistry, Jadavpur University, for financial support under
the head [Chemicals/Consumables/Glassware]. M. C. and R.
G.-P. thank the Spanish Ministerio de Economía
y
Competitividad (project CTQ2015-63858-P, MINECO/FEDER)
and Comunidad de Madrid (project B2017/BMD-3770-CM) for
financial support. M. G. B. D. thanks the EPSRC and the
University of Reading for funds for the diffractometer.
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