A ferromagnetic methoxido-bridged Mn(III) dimer and a spin-canted metamagnetic μ1,3-azido-bridged chain

Article abstract of DOI:10.1021/ic300274n

Two new Mn(III) complexes of formulas [MnL¹(N ₃)(OMe)]₂ (1) and [MnL²(N₃) ₂]_n (2) have been synthesized by using two tridentate NNO-donor Schiff base ligands HL¹{(2-[(

Full text of DOI:10.1021/ic300274n

Article
pubs.acs.org/IC
A Ferromagnetic Methoxido-Bridged Mn(III) Dimer and a Spin-
Canted Metamagnetic μ -Azido-Bridged Chain
1
,3
Subrata Naiya,†,‡ Saptarshi Biswas, Michael G. B. Drew, Carlos J. Gomez-García,*,∥
†
§
́
,†
and Ashutosh Ghosh*
†
Department of Chemistry, University College of Science, University of Calcutta, 92, APC Road, Kolkata 700 009, India
Susil Kar College, Ghoshpur, Champahati, Baruipur, 24 Parganas(S), West Bengal 743 330, India
‡
§
∥
School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG 66AD, United Kingdom
́ ́ ́
Instituto de Ciencia Molecular (ICMol), Parque Científico, Universidad de Valencia, C/Catedratico Jose Beltran, 2. 46980 Paterna,
Valencia, Spain
*
S Supporting Information
ABSTRACT: Two new Mn(III) complexes of formulas
1
2
[
MnL (N )(OMe)] (1) and [MnL (N ) ] (2) have been
3 2 3 2 n
synthesized by using two tridentate NNO-donor Schiff base
1
ligands HL {(2-[(3-methylaminoethylimino)-methyl]-phe-
2
nol)} and HL {(2-[1-(2-dimethylaminoethylimino)methyl]-
phenol)}, respectively. Substitution of the H atom on the
secondary amine group of the N-methyldiamine fragment of
the Schiff base by a methyl group leads to a drastic structural
change from a methoxido-bridged dimer (1) to a single μ -
1,3
azido-bridged 1D helical polymer (2). Both complexes were
characterized by single-crystal X-ray structural analyses and
variable-temperature magnetic susceptibility measurements. The magnetic properties of compound 1 show the presence of weak
−1
ferromagnetic exchange interactions mediated by double methoxido bridges (J = 0.95 cm ). Compound 2 shows the existence
−
1
of a weak antiferromangetic coupling along the chain (J = −8.5 cm ) through the single μ -N bridge with a spin canting that
1,3
3
leads to a long-range antiferromagnetic order at T ≈ 9.3 K and a canting leading to a weak ferromagnetic long-range order at T
c
c
≈
8.5 K. It also exibits metamagnetic behavior at low temperatures with a critical field of ca.1.2 T due to the weak
antiferromagnetic interchain interactions that appear in the canted ordered phase.
INTRODUCTION
perpendicular to the original local moments) and generate a
ferromagnetic arrangement of small moments (weak ferro-
mangetism) that may give rise to canted single chain magnets
■
The study of magnetically ordered molecule-based materials
and their structure−function relationships is a very fast growing
area in chemistry. Among these materials, one of the most
widely investigated systems is the ferromagnetically coupled
polynuclear clusters of transition metal ions that behave like
1e,5
(
SCMs).
Besides the spin carrier, the judicious choice of bridging
atoms or groups is also crucial to the design of new molecular
magnets. Usually, ligands containing one to three atoms are
very efficient in transmitting the magnetic coupling. The azide
ion is such a ligand that has been used extensively for the
1
,2
magnets, i.e., single-molecule magnets (SMMs).
The
compounds containing high-spin Mn(III) are often found to
2a,c,d
behave like SMMs
because the Mn(III) ion possesses a
6
,7
2,3
construction of various magnetic systems for its versatile
bridging modes (Scheme 1). Moreover, the nature of the
magnetic coupling between the metal centers is exceedingly
large spin value (S = 2) and a pronounced single ion easy axis
anisotropy induced by Jahn−Teller elongation of the
coordination octahedron. However, ferromagnetic interactions
that would result in a high-spin ground state of the cluster are
6
−9
dependent on the bridging modes of azide.
Usually, the μ1,1
6
a,10
4
bridging mode transmits ferromagnetic,
and the μ
by far less common than the antiferromagnetic ones. A recent
1,3
a,7c,11
6
bridging mode transmits antiferromagneic couplings.
development in magnetism of 1D chain compounds reveals that
antiferromagnetic interactions between similar spins in
principle can also give rise to a magnetic behavior like
SMMs, provided that magnetic anisotropy is present and the
easy axes are not collinear. In such a case, the angle between
the AF-coupled moments is not exactly 180°, resulting in an
uncompensated small moment per magnetic center. These
small moments are all aligned in the same direction (almost
Consequently, numerous azido-bridged compounds of Cu(II),
Ni(II), and Mn(II) having different types of magnetic behavior,
6
a,10
6a,7c,11
e.g., ferromagnetic,
netic, spin-canting,
antiferromagnetic,
metamag-
12
4b,12,13
14
1,2
spin flop, etc. have been reported.
Received: February 6, 2012
Published: April 11, 2012
©
2012 American Chemical Society
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Article
Scheme 1. Various Azido Bridging Modes
Scheme 2. Synthetic Routes of the Complexes
Azido-bridged complexes containing only Mn(III) are less
common, although a careful search in the CCDC database
1
2
resulting dark yellow solutions of the tridentate ligands HL and HL
were subsequently used for complex formation.
(
updated November 2011) shows that there are 12 discrete
complexes (with nuclearities from 2 to 13) with μ-N bridges:
seven with μ -N (Scheme 1c), three with μ -N (Scheme
1
3
Synthesis of [MnL (N )(OMe)] (1). At room temperature, 5 mmol
3 2
1
1
5
1,1
3
1,3
17
3
of methanolic solution of Schiff base ligand HL (10 mL) was added to
a 20 mL methanolic solution of Mn(OAc) ·4H O (1.225 g, 5 mmol)
1
6
1
a), one with both μ -N and μ -N bridges and one with
1
,1
3
1,3
3
2
2
18
μ1,1,1-N bridges (Scheme 1a), five chains formed by oxido-
followed by an aqueous solution (5 mL) of NaN (0.650 g, 10 mmol).
3
3
The solution turned red. The mixture was kept unstirred in a
refrigerator. The red colored, X-ray quality, prismatic shaped single
crystals started to separate within a few hours and were collected after
centered Mn O building units connected through μ -N
3
1,3
20
3
1
9
bridges, 12 Mn(III) chains (11 with μ -N bridges and
1
,3
3
21
22
one with μ -N bridges), and only one 3D compound.
1,1
3
2
days.
Most of these compounds (ca. 2/3) have been prepared with
Schiff-base ligands since, on reacting with such ligands, Mn(II)
can readily be oxidized in the air to Mn(III) and stabilized by
complex formation. Most of these Schiff-base ligands are
tetradentate N O or tridentate NO donors, but none present
Complex 1. Yield: 0.950 g. (74%). C H Mn N O (610.44)
22
32
2
10
4
calcd.: C, 43.29; H, 5.28; N, 22.95, Found: C, 43.21; H, 5.37; N, 23.07.
IR (KBr pellet): 2048 ν(NN) (terminal), 1615 ν(CN) cm . λ
−1
max
(methanol) = 397 nm.
2
Synthesis of [MnL (N ) ] (2). The procedure was the same as that
for complex 1, except that 5 mmol of a methanolic solution of the
Schiff base ligand HL (10 mL) was added instead of HL . The
solution turned red immediately. On storing the resulting solution in a
refrigerater, red colored, X-ray quality, prismatic shaped single crystals
were obtained after two days.
2
2
2
3 2 n
a tridentate N O donor set, as observed in our compound 2,
2
2
1
even though such tridentate N O Schiff-base ligands are very
2
common in Cu(II) and Ni(II) systems for the construction of
polynuclear complexes with interesting structures and magnetic
6a,10b,23
properties.
Complex 2. Yield: 0.950 g. (74%). C H Mn N O (660.46)
22
30
2
16
2
Here, we report the synthesis, crystal structure, and magnetic
calcd: C, 40.01; H, 4.58; N, 33.94. Found: C, 40.11; H, 4.57; N, 33.87.
1
properties of two Mn(III) complexes [MnL (N )(OMe)] (1)
3
2
IR (KBr pellet): 2065 ν(NN) (bridging), 2050 ν(NN)
2
1
−1
and [MnL (N ) ] (2) with the Schiff-base ligands HL = (2-
(3-methylaminoethylimino)-methyl]-phenol) and HL = (2-
1-(2-dimethylaminoethylimino)methyl]phenol) (Scheme 2).
(terminal), 1627 ν(CN) cm . λ (methanol) = 401 nm.
3
2 n
max
2
[
[
Physical Measurements. Elemental analyses (C, H, and N) were
performed using a Perkin-Elmer 240C elemental analyzer. IR spectra
−1
in KBr pellets (4500−500 cm ) were recorded using a Perkin-Elmer
In compound 1, the Mn(III) ions are ferromagnetically coupled
via a double methoxido bridge. Compound 2 is a Mn(III) chain
with single μ -N bridges that presents an antiferromagnetic
RXI FT-IR spectrophotometer. Electronic spectra in methanol (1000−
2
00 nm) were recorded in a Hitachi U-3501 spectrophotometer. The
1,3
3
DC magnetic susceptibility measurements were carried out in the
temperature range 2−300 K with different applied magnetic fields in
the range 0.01−5 T on polycrystalline samples of compounds 1 and 2
coupling with a long-range canted antiferromagnetic ordering
and metamagnetic behavior at low temperatures.
(
with masses of 43.39 and 38.20 mg, respectively) with a Quantum
EXPERIMENTAL SECTION
■
Design MPMS-XL-5 SQUID susceptometer. AC suscetibility measure-
ments were performed for sample 2 in the frequency range 0.1−10
kHz with an alternating field of 16 mT in the presence of DC fields in
the range 0.01−2 T with a Quantum Design PPMS-9 system. The
isothermal magnetizations were performed on the same samples in the
temperature range 2−14 K with magnetic fields up to 5 or 8 T. The
susceptibility data were corrected for the sample holders previously
measured using the same conditions and for the diamagnetic
contributions of the salts, as deduced by using Pascal’s constant tables
1
2
Materials. The two tridentate Schiff base ligands, HL and HL ,
have been synthesized in our laboratory according to the methods
described below. The diamines and salicyldehyde were purchased from
Lancaster Chemical Co. The chemicals were of reagent grade and used
without further purification.
Caution! Although no incidents were recorded in this study, azido salts
of metal complexes with organic ligands are potentially explosive. Only a
small amount of material should be prepared, and it should be handled
with care.
−6
−6
−1
(χdia = −137.76 × 10 and −147.78 × 10 emu mol for 1 and 2,
The ligands HL1 and HL2 were prepared by refluxing
salicylaldehyde (0.5 mL, 5 mmol) with N-methylethane-1,2-diamine
respectively).
24
Crystal Data Collection and Refinement. A total of 4011 and
8236 independent reflection data were collected with Mo Kα radiation
at 150 K using the Oxford Diffraction X-Calibur CCD System. The
(
0.3 mL, 5 mmol) and N,N-dimethylethane-1,2-diamine (0.55 mL, 5
mmol), respectively, in methanol (30 mL) for 30 minutes. The
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Article
crystals were positioned at 50 mm from the CCD. A total of 321
frames were measured with a counting time of 10 s. Data analyses were
2 corroborate the presence of two types of azido ligands
(terminal and μ1,3 bridges). For complex 1, a N−H stretching
mode is observed at 3244 as a sharp band. The electronic
spectra in methanolic solution of the two complexes display a
single absorption band at 397 and 401 nm for complexes 1 and
2, respectively. The positions of these bands are consistent with
25
carried out with the CrysAlis program. The structures were solved
26
using direct methods with the Shelxs97 program. The non-hydrogen
atoms were refined with anisotropic thermal parameters. The
hydrogen atoms bonded to carbon were included in geometric
positions and given thermal parameters equivalent to 1.2 times those
of the atom to which they were attached. Absorption corrections were
4b
those reported in hexacoordinated Mn(III) complexes.
Description of Structures of the Complexes.
27
1
carried out using the ABSPACK program. Crystal data and structure
[MnL (N )(OMe)] (1). The structure of compound 1 is formed
3
2
refinement parameters of complexes 1 and 2 are given in Table 1.
by a centrosymmetric methoxido-bridged Mn(III) dimer as
shown in Figure 1. Bond distances and bond angles are given in
Table 2.
Table 1. Crystal Data and Structure Refinement of
Complexes 1 and 2
1
2
formula
M
C H Mn N O
C H Mn N O
2
2
2
32
2
10
4
22 30
2
16
610.46
660.50
cryst syst
space group
a/Å
monoclinic
monoclinic
P2 /c
1
P2 /c
1
9.036(5)
11.590(14)
13.339(15)
96.52(10)
1388(2)
2
6.7361(3)
19.6039(14)
21.5481(17)
95.929(5)
2830.3(3)
4
b/Å
c/Å
β/deg
V/Å3
Z
temp (K)
150
150
−
3
D /g cm
1.461
1.550
c
μ/mm−
F (000)
R(int)
1
0.957
0.946
632
1360.0
0.099
0.072
total reflns
unique reflns
I > 2σ(I)
8588
19740
Figure 1. The structure of the centrosymmetric dimer 1 with ellipsoids
at 30% probability.
4011
8236
1691
3356
R1, wR2
0.0474, 0.0983
0.0536, 0.1068
Table 2. Bond Distances (Å) and Angles (deg) in the Metal
Coordination Spheres of Compound 1
RESULTS AND DISCUSSION
■
Synthesis of the Complexes. The two new Mn(III)
1
2
complexes [MnL (N )(OMe)] (1) and [MnL (N ) ] (2)
atoms
distance
3
2
3 2 n
1
have been prepared with the tridentate Schiff base ligands HL
Mn1−O11
Mn1−O31
Mn1−N19
Mn1−N22
Mn1−N1
Mn1−O31
atoms
1.887(3)
1.894(3)
1.992(4)
2.098(3)
2.174(4)
2.233(4)
angle
2
and HL , respectively, following a similar method: manganese-
II) acetate was allowed to react with the corresponding Schiff
(
base and sodium azide in 1:1:2 molar ratios in methanol under
ambient conditions to yield compounds 1 and 2 as red
crystalline solids within a few hours, on keeping the solutions in
a refrigerater. However, the compositions and structures of the
compounds are very different. In 2, one deprotonated
tridentate Schiff base ligand and two azide ions coordinate to
the Mn(III) centers, to provide a neutral complex. Of these two
azides, one remains terminally coordinated while the other acts
a
O11−Mn1−O31
O11−Mn1−N19
O31−Mn1−N19
O11−Mn1−N22
O31−Mn1−N22
N19−Mn1−N22
O11−Mn1−N1
O31−Mn1−N1
N19−Mn1−N1
N22−Mn1−N1
O11−Mn1−O31
O31−Mn1−O31
N19−Mn1−O31
N22−Mn1−O31
92.07(13)
91.32(14)
171.09(10)
172.51(11)
94.52(14)
81.66(14)
95.32(13)
96.61(15)
91.28(15)
87.46(13)
90.45(12)
78.42(14)
93.32(14)
86.40(12)
172.54(9)
as a μ -bridge to produce the one-dimensional chain structure.
1,3
On the other hand, in 1, a deprotonated methanol coordinates
to the metal center instead of an azide ion and forms a double
methoxido-bridged Mn(III) dimer. (Scheme 2).
IR and UV−vis Spectra of the Complexes. Spectroscopic
data and their assignments are given in the Experimental
Section. The IR spectra of the two complexes are very similar
and show a strong and sharp peak corresponding to ν(CN)
a
a
a
a
−1
at 1615 and 1627 cm for complexes 1 and 2, respectively,
confirming the presence of the Schiff base. In the spectrum of
−1
a
complex 1, a strong band at 2048 cm is indicative of the
presence of the N-coordinated terminal azide. While two
overlapping but distinct peaks at 2050−2065 cm for complex
N1−Mn1−O31
−
1
a
Symmetry element 1 − x, −y, 2 − z.
5
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Figure 2. The structure of 2 with ellipsoids at 30% probability.
The metal atom occupies a distorted octahedral coordination
sphere being meridionally coordinated to the L ligand through
Table 3. Bond Distances (Å) and Angles (deg) in the Metal
Coordination Spheres of Compound 2
1
O11, N19, and N22, together with a terminal azide N1 and two
a
atoms
distance
atoms
distance
methoxido ligands, O31 and O31 , acting as bridges between
Mn1−O11
Mn1−N7
Mn1−N19
Mn1−N22
Mn1−N3
Mn1−N12
atoms
1.878(2)
1.941(3)
1.991(3)
2.113(3)
2.280(3)
2.297(3)
angle
Mn2−O31
Mn2−N4
Mn2−N39
Mn2−N42
Mn2−N1
Mn2−N10
atoms
1.883(2)
1.945(3)
1.980(3)
2.110(3)
2.275(3)
2.277(3)
angle
the two Mn(III) metal ions. The two bridging Mn−O bond
distances are significantly different Mn1−O31 = 1.894(3) Å
a
and Mn1−O31 = 2.233(4) Å. The short bond is trans to the
azido ligand, whereas the longer one is trans to an imino
nitrogen atom of the ligand. The manganese−azide bond is also
longer than usual (Mn1−N1 = 2.174(4) Å). The azido ligand is
quasi-linear, with a N−N−N angle of 176.7(4)°. Bond lengths
to the tridentate ligand are in the normal range: Mn1−O11 =
b
O11−Mn1−N7
O11−Mn1−N19
N7−Mn1−N19
O11−Mn1−N22
N7−Mn1−N22
N19−Mn1−N22
O11−Mn1−N3
N7−Mn1−N3
N19−Mn1−N3
N22−Mn1−N3
O11−Mn1−N12
N7−Mn1−N12
N19−Mn1−N12
N22−Mn1−N12
93.92(10)
91.66(10)
174.06(12)
173.79(10)
91.55(11)
82.77(11)
88.70(10)
90.71(11)
91.44(10)
94.16(10)
89.62(10)
92.57(11)
85.43(11)
87.22(10)
176.41(11)
O31−Mn2−N4
O31−Mn2−N39
N4−Mn2−N39
O31−Mn2−N42
N4−Mn2−N42
N39−Mn2−N42
O31−Mn2−N1
N4−Mn2−N1
N39−Mn2−N1
N42−Mn2−N1
O31−Mn2−N10
N4−Mn2−N10
N39−Mn2−N10
N42−Mn2−N10
N1−Mn2−N10
91.65(11)
91.86(11)
176.49(12)
175.54(10)
92.40(11)
84.08(11)
90.11(11)
94.82(11)
85.37(11)
91.40(11)
90.23(11)
94.45(11)
85.33(11)
87.61(11)
170.71(11)
1.887(3) Å, Mn1−N19 = 1.992(4) Å, and Mn1−N22 =
2.098(3) Å. The four donor atoms in the equatorial plane
(
O11, N19, N22, and O31) are nearly coplanar with an RMS
deviation of 0.044(2) Å. The manganese atom is 0.041(1) Å
from this plane in the direction of N19. Within the dimer, the
Mn−Mn separation is 3.205(1) Å, and the distance between
the two bridging oxygen atoms is 2.622(4) Å. The μ-O bridge
in this complex forms a perfectly planar although asymmetric
Mn (μ-O) ring since the dimer sits on a crystallographic
2
2
b
inversion center. The Mn−O−Mn angle is 101.59(13)°. The
b
phenyl rings are almost perpendicular to the planar Mn (μ-O)
2
2
b
b
core with a dihedral angle of 88.99(8)°. Two phenyl rings are
coplanar to each other. The donor H22 atom of the imine
nitrogen of a dinuclear compound participate in intermolecular
b
N3−Mn1−N12
b
c
Symmetry element x, 0.5 − y, −0.5 + z.
hydrogen bonding with an acceptor N3 atom of a terminal
azido of a neighboring dimer (H···A = 2.49 Å, D···A = 3.276(7)
Å, D−H···A angle =145°; c = x, 1/2 − y, 1/2 + z) to form the
also noteworthy that the bonds to the terminal azido bridges,
Mn1−N7 = 1.941(3) Å and Mn2−N4 = 1.945(3) Å, are
significantly shorter than the bonds to the bridging azido ones,
2D supramolecular architecture shown in Figure S1.
2
[
MnL (N ) ] (2). The structure of compound 2 is formed by
3 2 n
b
a linear Mn(III) chain, as shown in Figure 2. Bond distances
and angles are given in Table 3.
Mn1−N3 = 2.280(3) Å, Mn1−N12 = 2.297(3) Å, Mn2−N1 =
2.275(3) Å, and Mn2−N10 = 2.277(3) Å, as a consequence of
the Jahn−Teller distortion in the Mn(III) ion. The RMS
deviations of the four equatorial atoms from the mean plane
passing through them are 0.009 and 0.015 Å for Mn1 and Mn2,
respectively. The metal atoms are 0.041(1) and 0.017(1) Å
from this plane toward N3 and N1 atoms for Mn1 and Mn2,
respectively. The dihedral angle between these two equatorial
planes is 58.5(1)°. The Mn−Mn separation within the chain is
5.958(1) Å. It is interesting to note that each single μ -N
There are two metal atoms in the asymmetric unit, but they
have identical coordination spheres. Both present an octahedral
2−
geometry with a meridionally bonded tridentate ligand L ,
together with one terminal and two μ -N bridges. The three
1
,3
3
donor atoms of the Schiff base, along with the coordinated
nitrogen atom of the terminal azido ligand, constitute the
equatorial planes in both Mn atoms. The bridging azides (N1
and N10 atoms in Mn2 and N3 and N12 atoms in Mn1) are
coordinated in the two trans axial positions. Bond lengths to
1
,3
3
bridge between Mn1 and Mn2 induces helicity in the chain
along the c direction. However, as the compound crystallizes in
an achiral space group, there is an equal number of right and
left handed helical chains (Figure S2). This kind of helical chain
propagating through single azido-bridged metal ions is reported
2
the ligand L are equivalent for the two Mn atoms (Mn1−O11
=
1.878(2) Å, Mn1−N19 = 1.991(3) Å, and Mn1−N22 =
2.113(3) Å, very close to those of Mn2: Mn2−O31 = 1.883(2)
Å, Mn2−N39 = 1.980(3) Å, and Mn2−N42 = 2.110(3) Å). It is
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2
0a,f,g,28
in the literature.
The 1D chains are interconnected by
H = −JS S ; solid line in Figure 4). Note that the presence of a
1
2
weak C13−H13···N2 and C421−H42F···N6 hydrogen bonds
to produce a 2D supramolecular structure (Figure 3 and Table
weak zero field splitting cannot be excluded. In fact, a low D
value has been observed in similar Mn(III) dimers with this
2a,c,d
kind of asymmetric oxido bridge.
Nevertheless, the values
of both parameters are similar and closely correlated,
precluding a reliable simultaneous fit of both parameters.
The weak ferromagnetic coupling observed in compound 1
can be easily rationalized from the structural parameters of the
double asymmetric oxido bridge (see above). This double
bridge shows a long (2.233(4) Å) and a short (1.894(3) Å)
Mn−O bond distance due to the presence of a Jahn−Teller
4
distortion in the d Mn(III) center along one of the Mn−O
bridging bonds, leading to a poor overlap of the magnetic
orbitals that results in a very weak coupling, as observed
experimentally. This result agrees with the weak ferromagnetic
couplings found in similar Mn(III) dimers with double
2
a,c,d,29,30
asymmetric oxido bridges.
2
Complex [MnL (N ) ] (2). The thermal variation of the
3
2 n
molar magnetic susceptibility per Mn(III) ion times the
Figure 3. C−H···N hydrogen bonded 2D network of compound 2.
temperature (χ T) for compound 2 shows at room temper-
ature a value of ca. 2.5 emu K mol (Figure 5), slightly below
m
−1
Table 4. Hydrogen Bond Interactions in Complex 2
D−H
H···A
(Å)
∠D−H···A
D−H···A
(Å)
D···A (Å)
(deg)
symmetry
C13−
0.93
2.57
3.481(4)
165
1 + x, y, z
H13···N2
C421−
H42F···N6
0.96
2.60
3.522(5)
160
1 + x, y, z
4
) with distances of 3.481(4) and 3.522(5) Å, respectively.
These interactions seem to have an important effect on the
Figure 5. Thermal variation of the product of the molar magnetic
magnetic properties of complex 2, which are discussed below.
susceptibility per Mn(III) ion times the temperature, χ T, for
1
m
Magnetic Properties. Complex [MnL (N )(OMe)] (1).
3
2
compound 2 at 0.1 T. Inset shows the low temperature region.
Solid line represents the best fit to the model (see text).
The thermal variation of the molar magnetic susceptibility per
Mn(III) dimer times the temperature (χ T) for compound 1
m
−1
shows at room temperature a value of ca. 5.9 emu K mol , very
close to the expected one for two S = 2 Mn(III) isolated ions
6.0 emu K mol for g = 2). When the temperature is
the expected one for one isolated Mn(III) S = 2 ion with g =
−1
(
2.0. When the temperature is lowered, the χ
progressive decrease and reaches a value of ca. 0.05 emu K
m
T product shows a
decreased, χ T remains constant down to ca. 150 K, and below
m
−1
this temperature, χ T increases to reach a value of ca. 9.6 emu
mol at 2 K. A close inspection of this plot shows a small hint
m
K mol− at ca. 2 K (Figure 4). This behavior is indicative of
predominant ferromagnetic exchange interactions inside the
Mn(III) dimer present in this compound. Accordingly, we have
used a simple S = 2 dimer model, which reproduces very
satisfactorily the magnetic data in the whole temperature range
1
at ca. 10 K, indicative of a transition to a long-range ordering
(inset in Figure 5). The decrease observed in the χ T product
m
indicates the presence of predominant antiferromagnetic
exchange interactions in compound 2. Given the chain
structure shown by this compound and the structural similitude
−1
with g = 1.982 and J = 0.95 cm (the Hamiltonian is written as
of the μ1,3-N
used the simple S = 2 regular antiferromagnetic chain model
3
bridges connecting the Mn(III) ions, we have
31
derived by Ficher to fit the magnetic properties:
2
2
Ng β S(S + 1) 1 + u
χchain
=
=
with u
3kT
1 − u
⎡
coth⎢
⎣
⎤
⎡
⎤
⎥
JS(S + 1)
kT
⎥ − ⎢
kT
⎦
⎣ JS(S + 1)⎦
where J is the intrachain coupling constant and S = 2 in this
case.
This simple model reproduces very satisfactorily the
magnetic properties of compound 2 above ca. 15 K (solid
−
1
line in Figure 5) with g = 1.967 and J = −8.5 cm (the
Figure 4. Thermal variation of the χ T product for compound 1. Inset
m
shows the thermal variation of χ . Solid line represents the best fit to
Hamiltonian is written as H = −JS S ). Note that although the
two intrachain bridges are not identical, they are very similar,
i i+1
m
the model (see text).
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Inorganic Chemistry
Article
and in order to reduce the number of adjustable parameters, we
have assumed that they are equivalent. The very satisfactory fit
obtained with the model confirms that this assumption is
acceptable.
chains when the ions present a tetragonal elongated environ-
ment with four short basal bonds and two longer axial
4
b,35−37,40,41
ones.
In fact, in compound 2, there are two
crystallographically independent Mn(III) centers that present
axial directions not colinear with the helical chain axis (see
above). Interestingly, a close look at the DC measurement
The small kink observed in the χ T product at ca. 10 K can
m
be more clearly seen in the χ plot (Figure 6) as a sharp peak at
m
(
lower inset in Figure 6) shows that the peak observed at ca. 9.3
K also presents a shoulder at ca. 8.5 K corresponding to the
canted antiferromagnetic ordering. We can, therefore, assume
that the ordering process takes place in two distinct steps: first,
all the spins along the chain align antiparallelly and order
antiferromagnetically at 9.3 K, and in a second step, the single
ion anisotropy slightly rotates the spins to give a canted
antiferromagnetic order, responsible for the frequency-
independent peak at 8.5 K.
In order to study the unusual frequency dependence of the
intensity of the χ ″ peak at 8.5 K, we have performed AC
m
measurements at many different frequencies (inset in Figure 7).
These measurements show that the intensity decreases as the
frequency increases in a double linear way with the logarithm of
the frequency (left inset in Figure 7). Interestingly, similar
behavior has very recently been observed in a related Mn(III)
Figure 6. Thermal variation of the molar magnetic susceptibility per
Mn(III) ion for compound 2 at 0.1 T. The upper inset shows the low
temperature region. Lower inset shows a detail of the peak at ca. 9.3 K
and the shoulder at ca. 8.5 K. Solid line represents the best fit to the
model (see text).
4
b
canted antiferromagnetic chain and in the magnetic
Nd Fe Al alloys, where this behavior is attributed to the
60
30 10
presence of a distribution of relaxation times, τ, and to the fact
9
.3 K, suggesting the presence of antiferromagnetic long-range
42
that τ is larger than 1/ν. Note that this frequency dependence
of the intensity of the χ ′ and χ ″ peaks cannot be attributed to
ordering taking place at 9.3 K. At lower temperatures, the χm
plot shows a Curie tail arising from the presence of monomeric
paramagnetic impurities resulting from crystal defects and
vacancies in the chain.
m
m
the movement of the domain walls along the chain since its
position is frequency independent.
Further proof of the canted antiferromagnetic long-range
order is provided by the isothermal magnetization measure-
ments at different temperatures above and below the transition
temperature (Figures 8 and 9). These measurements show a
In order to investigate the exact nature of the peak observed
in the susceptibility plot at ca. 9.3 K, we have performed AC
susceptibility measurements at different frequencies (Figure 7).
These measurements show a frequency-independent maximum
only observed in the in-phase signal (χ ′) at ca. 9.3 K, also
m
observed in the DC measurements, corresponding to the
antiferromagnetic long-range ordering. Besides this peak, the in
phase (χ ′) and out of phase (χ ″) signals show a narrow peak
m
m
at lower temperatures (ca. 8.5 K) whose position is frequency-
independent but with an intensity that increases as the
frequency is decreased. This narrow peak corresponds to either
a ferromagnetic or a canted antiferromagnetic (weak
32−34
ferromagnetic) long-range ordering.
Since the coupling
observed in 2 is antiferromagnetic (see above), this peak has to
be attributed to the presence of a long-range canted
antiferromagnetic order, as already observed in several other
4b,22,35−41
Mn(III) chain compounds.
This canted antiferromag-
Figure 8. Isothermal magnetization of 2 at 2 K. Insets show the low
field region and the metamagnetic transition near 1.0 T.
netic long-range order appears when the single ion anisotropy
axes are not parallel, as is commonly observed in Mn(III)
Figure 7. (a) Thermal variation of the in-phase (χ ′, filled circles, left scale) and out-of-phase (χ ″, empty circles, right scale) susceptibilities for
m
m
compound 2 at different frequencies. Inset shows the dependence of the maximum of the intensity of the χ ″ signal with the Ln ν. (b) χ ″ signal at
m
m
different frequencies in the 10−10 000 Hz range.
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Inorganic Chemistry
Article
Figure 9. Isothermal magnetization of 2 at different temperatures.
Inset shows the magnetic field dependence of the derivative of the
magnetization with the magnetic field near the critical field.
Figure 10. Phase diagram of 2 showing the antiferromagnetically
coupled phase at high temperatures and low fields, the canted
antiferromagnetic phase at low temperatures and low fields, and the
canted ferromagnetic phase at high fields and low temperatures.
linear increase of the magnetization with increasing the
magnetic field and a very low value at high fields (ca. 0.2 μB
hysteretic behavior. At temperatures below the critical point,
the metamagnetic transition presents hysteresis and is a first
order transition, whereas at temperatures above the TCP, the
at 5 T, well below the expected value of ca. 4.0 μ per Mn(III)
B
ion), in agreement with the antiferromagnetic coupling
observed in this compound. Furthermore, the magnetization
measurements show a hysteresis cycle with a coercive field of
ca. 20 mT at 2 K (right inset in Figure 8) that confirms the
weak ferromagnetic long-range ordering in compound 2. This
coercive field decreases as the temperature increases and
vanishes above ca. 7 K. At 2 K, the magnetization shows an
metamagnetic transition is second order and does not present
43,44
hysteresis.
The area where the hysteresis takes place
corresponds to the region of bistability where the two states
may exist depending on the conditions.
In order to better study the field dependence on the
magnetic ground state in 2, we have performed a more
complete study of the DC and AC magnetic susceptibilities as a
function of the applied DC magnetic field. These measure-
ments show that the DC magnetic susceptibility shows a peak
at ca. 9.3 K for fields below ca. 1.0 T (Figure 11), but for fields
abrupt increase at ca. 1.2 T with an increase from ca. 0.04 μ at
B
1
.1 T to ca. 0.10 μ at 1.5 T (left inset in Figure 8). For fields
B
above 1.5 T, the magnetization shows again a linear behavior
with a similar slope to the one observed at low fields. This jump
in the magnetization at ca. 1.25 T suggests the presence of a
metamagnetic transition with a critical field of ca. 1.25 T.
However, at 2 K the critical field is different in the increasing
and decreasing field scans (left inset in Figure 8), with values of
ca. 1.24 and 1.08 T, respectively, as observed in the derivatives
of the magnetization with the magnetic field (inset in Figure 9).
A detailed study of the thermal variation of the critical field of
the metamagnetic transition (Figure 9) shows that (i) the
hysteresis in the critical field decreases very fast as the
temperature is increased and vanishes above ca. 6 K, (ii) the
critical field also decreases, although less abruptly, when the
temperature is increased and disappears above ca. 8 K (Figure
Figure 11. Thermal variation of the molar susceptibility of compound
2 with different applied magnetic fields.
9
), and (iii) the metamagnetic transition has two very close
critical fields in both scans in the temperature range 3−7 K
inset in Figure 9). The difference between both critical fields is
(
above ca. 1.0 T the peak shifts to lower temperatures and
disappears above ca. 1.4 T. This behavior indicates that the
interchain antiferromagnetic coupling is canceled by magnetic
fields above ca. 1.4 T, as observed in the isothermal
magnetization measurements. The lack of saturation of the
susceptibility at low temperatures confirms the presence of a
spin canting in the ferromagnetic phase.
The AC measurements also show a very different behavior
when a DC magnetic field is applied simultaneously to the AC
oscillating field (Figure 12). Thus, the peak observed at ca. 8.5
close to 0.1 T at any temperature, suggesting that they might be
related to the presence of two different interchain antiferro-
magnetic interactions corresponding to the two directions
perpendicular to the chain axis (see above).
The study of the variation of the critical field with the
temperature leads to the phase diagram shown in Figure 10.
This diagram shows that compound 2 is a canted
antiferromagnet at low fields and low temperatures, as a result
of a canted antiferromagnetic intrachain coupling (through the
μ -N bridges). These chains present a weak interchain
K in the χ ″ signal disappears for any applied magnetic field
1,3
3
m
antiferromagnetic coupling (through weak H bonds along the x
and y directions, see Table 4) that can be overpassed with a
moderate magnetic field of ca. 1.2 T at 2 K, corresponding to
the observed metamagnetic transition. The tricritical point
above 0.01 T (inset in Figure 12), indicating that the small
canting angle observed in the antiferromagnetic ordered phase
is suppressed by the application of a weak magnetic field. As
expected, the application of higher DC magnetic fields, near the
metamagnetic transition, leads to the appearance of a novel χm′
signal, now at lower temperatures, ca. 7.0 K, that corresponds
to the interchain long-range ferromagnetic ordering induced by
the DC magnetic fields above the critical field of ca. 1.2−1.3 T
(Figure 13). This new in-phase signal is also accompanied by
(
TCP) in this diagram, where the three phases coexist (the
canted antiferro- and ferromagnetic and the nonordered
antiferrommagnetic ones), belongs to the H = f(T) line. In
c
our case, this point is located at ca. 7 K, which is the
temperature at which the metamagnetic transition loses the
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Inorganic Chemistry
Article
−1
weak ferromagnetic coupling (J = 0.95 cm ). Compound 2 is a
Mn(III) chain compound with μ -azido bridges that presents a
1,3
−1
moderate (J = −8.5 cm ) antiferromagnetic coupling. At ca.
.3 K, 2 shows an antiferromagnetic long-range order (as
shown by the presence of a peak in χ and in χ ′ but not in χ ″
9
m
m
m
at ca. 9.3 K). Albeit, the local anisotropy of the Mn(III) ions
that present an elongated tetragonal environment) prevents a
(
complete antiparallel alignment of the spins. This canting angle
of the local spins leads to a weak ferromagnetic long-range
order that appears at ca. 8.5 K (as shown by the presence of a
frequency-independent peak in χ ″ at ca. 8.5 K). Below this
m
temperature, the residual moments on the chains are
antiferromagnetically coupled with those of the neighboring
chains (in both directions perpendicular to the chain). This
interchain coupling is overcome with the application of a
magnetic field above ca. 1.1−1.2 T, leading to a long-range
ferromagnetic order of the resulting magnetic moments on the
chains. Therefore, 2 presents a metamagnetic transition at low
temperatures with a critical field of ca. 1.1−1.2 T that changes
the sign of the interchain interaction from antiferromagnetic to
ferromagnetic. The critical field presents a hysteresis below ca.
Figure 12. Thermal variation of χ ′ of compound 2 at 10 kHz with
different applied DC fields.
m
6
K (corresponding to the tricritical point) that leads to the
phase diagram depicted in Figure 10. Similar behavior has been
observed in a cyanide bridged Cr−Ni metamagnet. Note also
that similar, although not identical, behavior has already been
43
3
6,38,45,46
observed in other Co(II) and Mn(III) chain systems.
Figure 13. Thermal variation of χ ″ of compound 2 at 10 kHz with
m
different applied DC fields.
ASSOCIATED CONTENT
Supporting Information
Hydrogen-bonded 2D supramolecular structure of complex 1,
helical structure of complex 2, and a crystallogrphic information
file. This material is available free of charge via the Internet at
http://pubs.acs.org.
■
the corresponding out-of-phase signal, which confirms the long-
range ferromagnetic ordering observed at ca. 7 K.
Confirmation of the presence of two close critical fields for
the metamagnetic transition comes from the measurement of
the AC magnetization as a function of the applied DC field at a
fixed temperature (Figure 14). These measurements show that
*
S
AUTHOR INFORMATION
Corresponding Author
E-mail: ghosh_59@yahoo.com.
■
*
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank CSIR, India for awarding a senior research fellowship
Sanction No.09/028 (0732)/2008-EMR-I] to S.B. We also
■
[
thank EPSRC and the University of Reading for funds for the
X-Calibur system, the Spanish Ministerio de Economıa
́
y
Competitividad (Projects Consolider-Ingenio in Molecular
Nanoscience CSD2007-00010 and CTQ-2011-26507), and
the Generalitat Valenciana (Project Prometeo 2009/95).
Figure 14. Isothermal (in phase, χ ′) AC magnetization of compound
m
2
at 10 kHz and different temperatures. Inset shows the out-of-phase
(χm″) AC magnetization at the same temperatures.
REFERENCES
■
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m
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■
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