A new end-on azido bridged MnIII single-chain magnet and its dimeric single molecule magnet polymorph. Synthesis, structure and magnetic properties of [Mn(5-Clsalpn)N3]n and phenoxo bridged [Mn(5-Clsalpn)N3]2

Article abstract of DOI:10.1039/c3dt31966k

An end-on azido bridged Mn^III coordination chain polymer [Mn(5-Clsalpn)N₃]_n (complex 1) and its dimeric polymorph, [Mn(5-Clsalpn)N₃]₂ (complex 2), where 5-Clsalpn is N,N′-bis(5-chlorosalicylidene)-1,3-diaminopropane, were prepared. Complex 1, which is obtained as two concomitant polymorphs (1a and 1b) with nearly identical molecular structures but different crystal packing, has μ-1,1(end-on) azido bridges. This single atom bridge is unsymmetrical and leads to a -1 cm^-1 antiferromagnetic interaction among Mn ^III centers of the one-dimensional coordination polymer. Mn ^III anisotropy is manifested through Jahn-Teller elongation along the N(azido)-Mn-N(azido) axis, and results in single-chain magnet behavior. The phenoxo-bridged dimer (complex 2) has terminal azido ligands and a ferromagnetic interaction is mediated by the unsymmetrical bis-phenoxo bridge. Mn ^III anisotropy is manifested through Jahn-Teller elongation along an N-Mn-O axis, and results in single molecule magnet behavior. The dimers are organized into a two-dimensional network structure via Cl...Cl and Cl...H-phenyl interactions which are also ferromagnetic.

Full text of DOI:10.1039/c3dt31966k

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A New End-on Azido Bridged MnIII Single-chain Magnet and its Dimeric Single
Molecule Magnet Polymorph. Synthesis, Structure and Magnetic Properties of
[Mn(5-Clsalpn)N3]n and Phenoxo Bridged [Mn(5-Clsalpn)N3]2
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G. Bhargavi,a M. V. Rajasekharan,*a J.-P. Costesb,c and J.-P. Tuchagues*b,c
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______________________________________________________________________________
aSchool of Chemistry, University of Hyderabad, Hyderabad 500 046, AP, India. Eꢀmail:
mvrsc@uohyd.ernet.in
bCNRS : LCC (Laboratoire de Chimie de Coordination), 205, route de Narbonne, Fꢀ31077
Toulouse, France. Université de Toulouse, UPS, INPT, LCC, Fꢀ31077 Toulouse, France CCꢀ
Toulouse. Eꢀmail: jptuchagues@yahoo.fr
cUniversité de Toulouse, UPS, INPT, LCC, Fꢀ31077 Toulouse, France
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Electronic supplementary information (ESI) available : Crystallographic data in CIF format
CCDC 783108 (Compound 1a), CCDC 783109 (Compound 1b) and supplementary figures Fig.
S1ꢀS5.
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Abstract: An endꢀon azido bridged MnIII coordination chain polymer [Mn(5ꢀ
Clsalpn)N3]n (complex 1) and its dimeric polymorph, [Mn(5ꢀClsalpn)N3]2
(complex 2), where 5ꢀClsalpn is N,N’ꢀbis(5ꢀchlorosalicylidene)ꢀ1,3ꢀ
diaminopropane were prepared. Complex 1, which is obtained as two concomitant
polymorphs (1a and 1b) with nearly identical molecular structures but different
crystal packing has ꢁꢀ1,1(endꢀon) azido bridges. This single atom bridge is
unsymmetrical and leads to a –1 cm–1 antiferromagnetic interaction among MnIII
centers of the oneꢀdimensional coordination polymer. MnIII anisotropy is
manifested through JahnꢀTeller elongation along the N(azido)–Mn–N(azido) axis,
and results in singleꢀchain magnet behavior. The phenoxoꢀbridged dimer (complex
2) has terminal azido ligands and a ferromagnetic interaction is mediated by the
unsymmetrical bisꢀphenoxo bridge. MnIII anisotropy is manifested through Jahnꢀ
Teller elongation along an N–Mn–O axis, and results in single molecule magnet
behavior. The dimers are organized into a twoꢀdimensional network structure via
ClꢁꢁꢁCl and ClꢁꢁꢁHꢀphenyl interactions which are also ferromagnetic.
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Introduction
In recent years, the field of molecular magnetism has received renewed attention owing to
singleꢀmolecule magnets (SMMs) and singleꢀchain magnets (SCMs)1 which exhibit slow relaxation
of their magnetization at low temperatures. Applications of high technological interest have been
proposed for these molecules, like use in highꢀdensity magnetic storage media2 or quantum bits
(qbits) in quantum computing,3 owing to quantum tunneling of their magnetization (QTM).4 In
addition to these potential practical applications, research in singleꢀmolecule and singleꢀchain
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magnetism has provided the necessary impetus for fundamental research in the area of magnetism
and magnetic materials. Following the discovery of the SMM phenomenon on
[Mn12O12(O2CMe)16(H2O)4],5 which has been exhaustively studied since, numerous molecules
possessing the SMM property have been synthesized. These include homometallic complexes
containing Mn,1aꢀc,6 V,7 Fe,8 Co,9 Ni10 and Ln,11 as well as heterometallic 3dꢀ3d,12 or 3dꢀ4f13
complexes. Since the first discovery, SMMs, have been prepared with S values ranging from 214 to
83/215 and nuclearities ranging from 19c,9d,11 to 84.16 These molecules often exhibit complicated and
unexpected behaviors, which tested, and still test current theoretical models for magnetism. Fewer
SCMs have been characterized structurally and magnetically, including 3d,17 3d–radical,18 3d–3d,19
3d–5d,20 4f,21 3d–4f,22 4f–radical23 4fꢀ5d,24 and 3dꢀ4fꢀ5d25 While their SCM behavior is usually
associated with ferromagnetic or ferrimagnetic interactions, rare examples of antiferromagnetic CoII
and MnIII chain structures exhibiting SCM properties were recently reported.26
The MnIII ion being a particularly interesting spin carrier that exhibits easy axis magnetic
anisotropy given its large spin, S = 2, and the JahnꢀTeller elongation of its coordination octahedron,
we decided to explore the effect of different bridging units prone to generate MnIII chain structures,
and more particularly the azido one. Among pseudohalides, the azide ion is particularly known for its
versatile coordination modes.27 Besides the terminal binding mode, seven types of bridging modes
are known: ꢁꢀ1,1(endꢀon); ꢁꢀ1,3(endꢀend); ꢁ3ꢀ1,1,1; ꢁ3ꢀ1,1,3; ꢁ4ꢀ1,1,1,1; ꢁ4ꢀ1,1,3,3; ꢁ6ꢀ1,1,1,3,3,3. In
the case of MnIII complexes, the endꢀend mode has been observed in several instances,28 the endꢀon
mode is seen in two complexes,26d, 28h (alternating with the endꢀend mode in one of them28h), while
the other bridging modes have not been reported so far. The present paper reports the second example
of a MnIII chain polymer with only endꢀon azide bridges. Surprisingly, it was obtained as two
concomitant polymorphs in different space groups. The previously reported26d phenoxoꢀbridged
dimer was obtained as a third polymorph under different crystallisation conditions. The Schiff base
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used is N,N’ꢀbis(5ꢀchlorosalicylidene)ꢀ1,3ꢀdiaminopropane (5Clꢀsalpn) leading to [Mn(5ꢀ
Clsalpn)N3]n (1) and [Mn(5ꢀClsalpn)N3]2 (2) which augment the few polymeric complexes based on
ringꢀsubstituted salenꢀtype ligands.26d, 29 In compound 1, wherein the azido ligand is exclusively endꢀ
on bridging, the bridge is less symmetric than in the endꢀend bridged compounds. Compounds 1 and
2 show slow relaxation of their magnetization at low temperatures. Since the structure of the
phenoxoꢀbridged dimer (2) has been previously reported, its structure is only briefly mentioned in the
present paper to highlight the intraꢀdimer as well as interꢀdimer interactions, both of which are shown
to produce ferromagnetic coupling. While the previous report emphasised the role played by the
relative size of the chloro and bromo substituent in directing structure formation, it is now shown that
the chloro substituent favours both chain and dimeric polymorphs depending upon the conditions of
crystallisation. Moreover, we provide a more complete magnetic characterization which shows that
the dimeric polymorph exhibits not only ferromagnetic behavior but also SMM and QTM behavior.
Results
Crystal structure of [Mn(5-Clsalpn)N3]n (1a, 1b). The molecular structure of polymorphs 1a
and 1b differ only very marginally in their geometrical parameters. Therefore, the two structures are
described together. The MnIII ion assumes a distorted octahedral coordination through binding to one
5ꢀClsalpn in equatorial mode and two azido ions in axial positions as shown in Fig. 1. Selected bond
distances and angles are listed in Table 1. The corresponding bond distances in the MnIII octahedron
for 1a and 1b differ at most by 0.01 Å, except for the long axial Mn–N bonds which differ by 0.03 Å.
The bond angles differ by less than 1.4º. In the following discussion, the structural parameters for 1b
are given in parenthesis after the corresponding values for 1a. The basal atoms (N2O2, average bond
lengths, Mn–O,N of 1.963(1.963) Å) are coplanar (with rms deviation = 0.13(0.15) Å) even though
the central manganese atom is displaced by 0.11(0.11) Å towards the azido ligand. The two halves of
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the 5ꢀClsalpn ligand, excluding the methylene groups and chlorine atoms are individually planar (rms
deviations = 0.05(0.06), 0.05(0.04) Å), with the Mn atom deviating by 0.80(0.83) Å and 0.75(0.74) Å
from these planes, the planes themselves being inclined towards each other by 73º (74º).
Polymorph 1b
Polymorph 1a
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Fig. 1 ORTEP view of a portion of the coordination chain in [Mn(5ꢀClsalpn)]n. Hydrogen atoms are
omitted for clarity, and the thermal ellipsoids are represented at the 50% probability level. Symmetry
labels 1a: i = –x+1/2, y+1/2, –z+1/2; ii = –x+1/2, y–1/2, –z+1/2; 1b: i = x, –y+1/2, z+1/2; ii = x, –
y+1/2, z–1/2.
Each azido ion functions as a ꢂꢀ1,1 bridge with similar metric parameters in 1a (2.22Å, 2.47Å,
131º) and 1b (2.21Å, 2.44Å, 132º). The bridges produce oneꢀdimensional polymeric zigꢀzag chains
along the bꢀaxis in 1a and along the cꢀaxis in 1b (Fig. 2). Along the chain the dihedral angle between
the equatorial N2O2 planes of adjacent molecules is 48º (47º). JahnꢀTeller elongation is seen along the
N3–Mn–N3i axis, with bond distances Mn–N3, 2.22(2.21) Å, Mn–N3i 2.47(2.44) Å. The intraꢀchain
and the shortest interꢀchain MnꢁꢁꢁMn distances are 4.27(4.24) Å and 9.52(9.53) Å. The intraꢀchain
ClꢁꢁꢁCl distances are 6.60(6.58) and 6.71(6.62) Å, and the interꢀchain ClꢁꢁꢁCl distances are 4.93(6.31)
and 5.28(7.30) Å, which are longer than the sum of the van der Waals radii (3.5 Å). It may be noted
that the ꢂꢀ1,1 bridge is less symmetrical than previously reported ꢂꢀ1,3 bridges.28
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Polymorph 1b
Polymorph 1a
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Fig. 2 Perspective view of zigꢀzag chain in [Mn(5ꢀClsalpn|)N3]n along the bꢀaxis for 1a (i = –x+1/2,
y+1/2, –z+1/2) and along the cꢀaxis for 1b (i = x, –y+1/2, z+1/2). Colour code for atoms: blue, N;
red, O; grey C; purple, Mn, green Cl.
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Though the coordination chains are practically identical in both polymorphs, the polymorphs
differ in the way they are packed in the crystal (Fig. S1). There are no significant interꢀchain contacts
in either type of packing. As stated above, the interꢀchain ClꢁꢁꢁCl distances differ by as much as 2 Å
between 1a and 1b. However, these distances are too long (≥ 5 Ǻ) in both polymorphs to have a
significant influence on their magnetic properties. Repeated preparations showed that the initial
precipitate as well as the crystallized sample always contained both polymorphs with form 1b as the
major component (as seen from the powder Xꢀ ray diffraction pattern, Fig. S2).
Table 1 Selected bond lengths (Å) and angles (°) for [Mn(5-Clsalpn)N3]n
Polymorph 1a
MnꢀO1
1.8821(12)
1.8970(12)
1.204(2)
MnꢀN1
2.0398(15)
2.0328(15)
1.149(2)
MnꢀN3
MnꢀN3i
2.2161(15)
2.4715(15)
MnꢀO2
MnꢀN2
N3ꢀN4
N4ꢀN5
O1ꢀMnꢀO2
O1ꢀMnꢀN2
89.81(6)
O2ꢀMnꢀN1
O2ꢀMnꢀN2
172.62(6)
88.79(6)
O1ꢀMnꢀN3i
O2ꢀMnꢀN3i
85.53(5)
85.08(5)
174.10(6)
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O1ꢀMnꢀN1
O1ꢀMnꢀN3
N2ꢀMnꢀN3
MnꢀN3ꢀ N4
N3ꢀN4ꢀ N5
88.66(6)
91.78(6)
94.03(6)
122.85(12)
178.5(2)
O2ꢀMnꢀN3
93.60(6)
92.00(6)
93.67(6)
105.86(11)
N1ꢀMnꢀN3i
N2ꢀMnꢀN3i
N3ꢀMnꢀN3i
MnꢀN3ꢀMnii
87.60(6)
N2ꢀMnꢀN1
N1ꢀMnꢀN3
MnꢀN3iꢀN4i
88.64(6)
177.00(3)
131.29(7)
(i) –x+1/2, y+1/2, –z+1/2. (ii) –x+1/2, y–1/2, –z+1/2.
Polymorph 1b
MnꢀO1
1.8975(16)
1.8862(16)
1.209(3)
89.64(7)
172.77(7)
88.74(7)
93.61(7)
93.50(7)
122.3(2)
178.5(2)
MnꢀN1
2.0326(19)
2.0369(19)
1.154(3)
174.45(7)
88.91(7)
91.38(7)
92.03(8)
94.02(7)
106.0(1)
MnꢀN3
MnꢀN3i
2.2112(18)
2.4359(18)
MnꢀO2
MnꢀN2
N3ꢀN4
N4ꢀN5
O1ꢀMnꢀO2
O1ꢀMnꢀN2
O1ꢀMnꢀN1
O1ꢀMnꢀN3
N2ꢀMnꢀN3
MnꢀN3ꢀ N4
N3ꢀN4ꢀ N5
O2ꢀMnꢀN1
O2ꢀMnꢀN2
O2ꢀMnꢀN3
N2ꢀMnꢀN1
N1ꢀMnꢀN3
MnꢀN3iꢀN4i
O1ꢀMnꢀN3i
O2ꢀMnꢀN3i
N1ꢀMnꢀN3i
N2ꢀMnꢀN3i
N3ꢀMnꢀN3i
MnꢀN3ꢀMnii
84.76(7)
85.76(7)
88.80(7)
88.07(7)
176.71(5)
131.69(9)
(i) x, –y+1/2, z+1/2. (ii) x, –y+1/2, zꢀ1/2.
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Crystal structure of [Mn(5-Clsalpn)N3]2 (2). Since this compound (Fig. 3) has been previously
crystallographically characterised26d, we only mention the features relevant for the discussion of the
magnetic properties.
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Fig. 3 ORTEP view of [Mn(5ꢀClsalpn)N3]2 in 2. Atoms are shown as 30% thermal ellipsoids.
Hydrogen atoms are omitted for clarity. i = –x, –y, –z+1.
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The two halves of the 5ꢀClsalpn ligand, excluding the methylene groups and chlorine atoms are
nearly planar (rms deviations = 0.06, 0.04 Å) with an interplanar angle of 11º. The two halves of the
ligand are unsymmetrically disposed about the basal coordination plane with interplanar angles of 37
and 26º. The intraꢀdimer MnꢁꢁꢁMn distance is 3.44 Å, and the shortest interꢀdimer MnꢁꢁꢁMn distance is
8.29 Å. The plane defining the Mn2O2 bridge makes an angle of 86.6º with the basal coordination
plane, MnN2O2. The overall coordination geometry of each MnIII ion is distorted octahedral with the
JahnꢀTeller elongation seen along the N3–Mn–O1i axis. In the crystal, there are three types of interꢀ
dimer contacts: =CHꢁꢁꢁNazido (C7H/N5, 2.65 Å), phenylꢀHꢁꢁꢁCl (C3H/Cl1, 2.83 Å) and ClꢁꢁꢁCl
(Cl1/Cl2, 3.47Å). These interactions organize the dimers into a 2Dꢀnetwork (Fig. 4).
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Fig. 4 A view of the 2Dꢀnetwork of dimers in compound 2.
Magnetic Properties. The temperatureꢀdependence of the magnetic susceptibility χM, of 1 and 2 was
measured in the temperature range 1.98ꢀ300 K, under applied magnetic fields of 0.5 (1) and 0.3 (2) T.
As discussed in the previous section, compound 1 consists of two polymorphs. Polymorphs can often
have significantly different magnetic properties due to differences in the nature of the coordination
bridges in the polymorphs as well as due to differing intermolecular contacts.30 Sheu et al.31 describes
an Fe(II) complex in which one polymorph is paramagnetic while the other one exhibits spinꢀ
crossover behaviour. The crystal structures are greatly different in this case, the spinꢀcrossover
compound consisting of two independent Fe(II) complex molecules while the paramagnetic
compound consists of only one molecule in the unit cell. Another example of large differences in
coordination geometries and solid state arrangement is described by Liu et al.32 for a Cu(II) azido
compound. The two polymorphs exhibit greatly different magnetic exchange parameters though both
have ferromagnetic interactions. Weng et al.33 reported a Cu(II) chloro complex the two polymorphs
of which differ in crystal packing and hydrogen bonding patterns. The values of the magnetic
parameters differ by about 9% between the two polymorphs. Pinkowicz et al.34 described polymorphs
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of a cyano bridged Mn(II)ꢀNb(IV) compound differing only in their crystal packing via hydrogen
bonds and showing very similar magnetic coupling. In all the above examples the polymorphs are
either obtained pure in different preparations, or they have different morphologies and colour when
formed together in the same preparation. In the present case, the polymorphs 1a and 1b have nearly
identical coordination environments. They differ in crystal packing. However, the intermolecular
ClꢁꢁꢁCl distances are too long to have any significant impact on the magnetic behaviour. Therefore, no
attempt was made to separate the two concomitant polymorphs which in any case is virtually
impossible due to similar morphologies of the crystals and the predominance of 1b in the crystallized
samples as seen from Xꢀray powder diffractograms (Fig. S2). The room temperature χM (cm3 mol–1)
and χMT (cm3 mol–1 K) values for complex 1 (Fig. 5) per magnetic ion are 9.5 × 10–3 cm3 mol–1 and
2.84 cm3 mol–1 K at 300 K, respectively (close to the expected spinꢀonly values). Upon decreasing
temperature, there is no significant change down to about 75 K, from where χM increases
monotonously to 0.11 cm3 mol–1 and χMT decreases to 1.10 cm3 mol–1 K at 9.8 K. Below this
temperature, while χM increases abruptly to 0.22 cm3 mol–1, χMT further decreases smoothly to 0.433
cm3 mol–1 K at 1.99 K.
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Fig. 5 Temperature variation of magnetic susceptibility (squares) and χMT product (diamonds) per
magnetic ion for [Mn(5ꢀClsalpn)N3]n (1) at 0.5 T applied magnetic field. The solid lines result from a
leastꢀsquare fit to the data (15ꢀ300 K range) of the theoretical values calculated with the Heisenberg
chain equation. Inset: Plots of χM versus T at 0.01, 0.1, 0.3, 0.7, 2 and 4 T applied magnetic fields.
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The decrease in χMT is characteristic of an antiferromagnetic exchange interaction between
adjacent highꢀspin MnIII ions. In addition, the slope change and sharp increase in χM beetwen 9.8 and
5.8 K may indicate long range ordering in the oneꢀdimensional (1D) chains of 1. In order to
substantiate such a possibility, fieldꢀcooled magnetic susceptibilities as a function of temperature
were collected for several applied magnetic fields (0.01, 0.1, 0.3, 0.7, 2, 4 and 5 T). Upon cooling, a
broad maximum is observed at ~9 K under applied magnetic fields above 1 T, substantiating
operation of antiferromagnetic exchange interaction between adjacent highꢀspin MnIII ions; on the
other hand, an abrupt rise in χM occurs at T < 7 K and H < 1 T, which is a signature of spin canting
(Fig. 5, insert).26c,26d,35,36
The data (15ꢀ300 K range) were fitted by using a Heisenberg chain equation37 with the following
results: g = 1.997, J = – 0.98(2) cmꢀ1, θ set to zero and L. S. Error = 1.7 × 10–4. The zeroꢀfield part of
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the Hamiltonian has the form − 2J S Si+1 . Most previously reported MnIII chains29 involve multiꢀ
∑
i
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atom bridges including endꢀend azido and have |J| < 4 cm–1. The quite unsymmetrical single atom
bridge in the present chain is similar to that reported for [Mn(5ꢀBrsalpn)N3]n,26d and it generates a
similar value of approximately –1 cm–1 for the antiferromagnetic interaction. The significant axial
elongation along the Nazido–Mn–Nazido axis, suggests that the dz
2
orbitals oriented along this axis
contribute predominantly to the magnetic interactions.
Field dependent magnetization data were collected at 2 K: at low applied magnetic field,
magnetization increases steeply with magnetic field while it increases at a slower rate at higher field.
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In line with operation of canted antiferromagnetism,26c,26d,35,36 the magnetization value per MnIII at 5
T and 2 K (~0.8 Nβ) is smaller than the theoretical saturation value and a critical field of ~0.4 T
could be spotted from the M(H) raw data and from the corresponding dM/dH plot (Fig. S3). No
hysteresis loop was observed at 2 K.
In order to check whether compound 1 behaves as a single chain magnet (SCM), its ac
susceptibility was measured in the temperature range 2ꢀ5 K at zero dc field, 3 G ac field, and
different frequencies. The observation of frequency dependent inꢀphase (
χ
׳) and outꢀofꢀphase (
χ
״)
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magnetic susceptibilities are shown in Fig. 6. The temperatures at the maxima in the χ׳ and χ″ curves,
Tmax, shift toward higher temperatures with increasing oscillating frequency (f).
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Fig. 6 Thermal variation of the inꢀphase (χ׳, left) and outꢀofꢀphase (χ״, right) components of ac
magnetic susceptibility for complex 1 at zero dc field. Solid lines are shown as a guide to the eye.
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From these data, the ratio ꢀTmax/[ Tmaxꢀ(logf)], is estimated to be 0.24, which lies in the range
associated with a superparamagnet.17,26c,26d,37,38 Taking into account that a unique relaxation process
occurs in 1 with a single relaxation time, the blocking temperature TB can be deduced experimentally,
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from the maximum of the χ″ vs. T curves. A leastꢀsquares fit to the Arrhenius equation τ =
τ0exp(ꢀ/kBT), in which kB is the Boltzmann constant,
τ
= 1/2πf, and ꢀ/kB = activation energy (Ueff),
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yielded a value of 21.4 K for Ueff and 2.97×10–8 s for
τ0, at zero static magnetic field (Fig. S4). The
obtained preꢀexponential factor and energy barrier for the reversal of the magnetization are in line
with values previously observed for SCMs.17ꢀ26
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220
Under an applied magnetic field of 0.3 T, the room temperature χMT value of the dinuclear
compound 2 is 6.13 cm3 mol–1 K, which is comparable to the expected spinꢀonly value of 6.00 cm3
mol–1 K per dinuclear MnIII unit. When temperature is lowered, χMT increases continuously to reach a
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Fig. 7 Temperature variation of the χMT product (per dimeric unit) for [Mn(5ꢀClsalpn)N3]2 (2). The
solid lines result from a leastꢀsquare fit to multi field data (see text) of the theoretical values
calculated through exact diagonalization of the spin Hamiltonian of the S1 = S2 = 2 dimer including
axial zfs and intermolecular interactions assessed within the mean field approximation.
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maximum value of 13.59 cm3 mol–1 K at 2.8 K (Fig. 7). Since the expected value for the S = 4 ground
state is 10.00 cm3 mol–1 K, this result suggests operation of ferromagnetic interactions not only
within the dimer, but also between dimeric units.
In order to unambiguously characterize the magnetic behavior of 2, magnetization data were
collected in the 2ꢀ8 K temperature range with applied fields of 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5,
4.0, 4.5 and 5.0 T. The magnetic susceptibility was calculated through exact diagonalisation of the
spin Hamiltonian including axial Zeeman and zfs terms and isotropic exchange40 and simultaneously
fitted to the 13 sets of experimental magnetic data (including the 2.8ꢀ300 K data at 0.3 T) by leastꢀ
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squares techniques. The zeroꢀfield part of the Hamiltonian was − 2JS1 ⋅ S2 + D(Sz21 + Sz22 ) and
intermolecular interactions were taken into account through the meanꢀfield Hamiltonian41
H = −2zj' Sz Sz yielding the best set of parameters: J = 0.70(2) cm–1, D = – 3.00 (5) cm–1, zj' =
ˆ
ˆ
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0.025(2) cm–1 g = 2.022 and L. S. Error = 4.5 × 10–4 (Fig. 7). Even though the intermolecular
interactions appear to be quite small compared to J and D, all parameters J, D and zj' are required to
obtain a reasonable fit, indicating that there is no overparametrization of the system.
In order to check whether compound 2 behaves as a single molecule magnet (SMM), the ac
susceptibility was measured in the temperature range 2ꢀ6 K at different frequencies in the absence
and presence (0.3 T) of an applied static magnetic field. The observation of frequency dependent inꢀ
phase (χ׳) and outꢀofꢀphase (χ״) magnetic susceptibilities are shown in Fig. 8.
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255
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Fig. 8 Temperature variation of the inꢀphase (χ׳, left) and outꢀofꢀphase (χ״, right) components of the
ac magnetic susceptibility for complex 2 at a dc field of 0.0 T and 0.3 T. Solid lines between the
experimental data points are shown as a guide to the eye, they do not result from fitting.
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The temperatures at the maxima in the
χ׳ and χ″ curves shift toward higher temperatures with
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increasing oscillating frequency (f). Upon applying a dc field of 0.3 T the maxima in the
χ
׳ and χ″
curves shifted toward higher temperature. Taking into account that a unique relaxation process occurs
in 2 with a single relaxation time, the blocking temperature TB can be deduced experimentally, from
the maximum of the χ″ vs. T curves. Leastꢀsquares fits to the Arrhenius equation
yielded a value of 18.2 K for the activation energy (Ueff) and 5.77 × 10ꢀ8 s for
τ
=
τ
0exp(ꢀ/kBT)
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τ
0, at zero static
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magnetic field. The corresponding values under a 0.3 T static field are 26.0 K and 4.05 × 10ꢀ8 s,
respectively. Arrhenius type plots are shown in Fig. S5.
Discussion
Synthesis and Polymorphism. Reaction of a methanolic solution of manganese(II) acetate and
sodium azide with a methanolic solution of the 5Clꢀsalpn Schiff base formed in situ from 5ꢀ
chlorosalicylaldehyde and 1,3ꢀdiaminopropane yielded the Mn(5ꢀClsalpn)N3 coordination compound
as three polymorphs according to the reaction conditions. Two concomitant polymorphs (compounds
1a (C2/c) and 1b (P 21/c)) having almost identical endꢀon azido bridged 1Dꢀpolymeric structures were
obtained by recrystallizing the initial precipitate from methanol. Infraꢀred spectra of these crystalline
polymorphs are identical to that of the initial precipitate. The absence of any significant interꢀchain
contact in either type of packing is remarkable. It is then reasonable to infer that these chains do not
admit closer packing and the absence of interꢀchain interactions leads to polymorphism. To the best
of our knowledge, this is the second example of exclusively endꢀon bridged MnIIIꢀazido coordination
chain. A third polymorph (compound 2 (P21/n)) was obtained when the methanol solution was vaporꢀ
diffused with hexane over a period of several weeks. In agreement with a previous report,26d the
crystals of this polymorph consist of phenoxoꢀbridged dimeric molecules organized into a 2Dꢀ
network via interꢀdimer interactions. Slow growth of crystals of compound 2 through vapor diffusion
and its significantly higher density as well as absence of interꢀmolecular interactions in polymorphs
1a and 1b indicate that the dimeric compound is the thermodynamically favored product while the
chain polymers are formed under kinetic control. Similar observations have been previously made
with regard to polymorphs of Mn(salpn)NCS.36
Obtaining three polymorphs of the same coordination compound through a unique reaction
pathway is quite unusual. Previous report26d mentioned that the bulkiness of the bromine substituent
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favours the formation of a 1ꢀdimensional chain rather than phenoxoꢀbridged dimer. We showed in
this report that the chloro substituent yields both the dimeric motif and oneꢀdimensional linear chains.
The Cl...Cl distance in the dimer is 4.13 Å which exceeds twice the van der Waal’s radius of Cl by
about 0.53 Å. In the case of the bromo compound the difference would have been only 0.23 Å
(assuming the same gross structure for the, as yet unknown, dimer).
Structural comparison of [Mn(5ꢀClsalpn)N3]n, 1, and [Mn(5ꢀBrsalpn)N3]n,26d shows that the
latter compound is isomorphous to polymorph 1b (P21/c). Except for the 5ꢀXsalpn substituent, the
constituting [Mn(5ꢀBrsalpn)N3] and [Mn(5ꢀClsalpn)N3] molecules are virtually identical: the average
bond lengths for the basal N2O2 donor set, Mn–O,N are 1.963(1a), 1.963(1b) and 1.9626d Å; each
azido ion functions as a ꢂꢀ1,1 bridge and JahnꢀTeller elongation is seen along the Nazido–Mn–Niazido
axis, with Mn–Nazido(Mn–Niazido) bond distances of 2.216(2.472)(1a), 2.211(2.436)(1b) and
2.215(2.475)26d Å; the intraꢀchain and the shortest interꢀchain MnꢁꢁꢁMn distances are 4.27(9.52)(1a),
4.24(9.53)(1b) and 4.29(9.64)26d Å.
Magneto-structural correlations. The JahnꢀTeller (JꢀT) elongation along the N3–Mn–N3i axis
of the octahedron around MnIII in the 1D Heisenberg chains of 1 induces a predictable orientation of
the easy axis of the single ion magnetic anisotropy. Two symmetryꢀrelated local axes alternate, as
shown in Figure 2. Both JꢀT axes form an angle
of 1a(1b). Projection of these axes on the ac(ab) plane forms an angle
axis. If the JꢀT axis is taken as the easy anisotropy axis, the angle = 47.9(47.3)° between the JꢀT
θ
of 23.9°(23.7) with the b(c) crystallographic axis
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φ
of ±88.2(88.8)° with the a
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δ
axes of neighbouring Mn sites will be the canting angle in 1a(1b). Consequently, the low temperature
increase of the magnetic susceptibility of compound 1 is solely due to the 1D shortꢀrange order
correlation of the uncompensated moments in these canted structures.
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The magnetic anisotropy may be estimated to be of the order of magnitude of that evaluated
(D = – 3.00 cm–1) for the phenoxoꢀbridged dimeric polymorph (2). This value is in a 3/1 ratio against
the isotropic superexchange interaction along the well insulated 1D Heisenberg chains of 1. Indeed, it
has been shown that, in the presence of Heisenberg interaction and easy axis single ion anisotropy,
the domain wall becomes infinitely sharp, i.e., contains only one unit cell, when the ratio |D/J|
exceeds 4/3.42 In the present case, where | D/J| ~3, this condition is well fulfilled. Compound 1 is thus
one of the very few examples illustrating that SCM behavior can be observed in Heisenberg AF
chains provided that single ion anisotropy exceeds the exchange interaction and that the local easy
axes of adjacent spins are not collinear.26 Then, the canted moment should be 4 sin
θ sinφ =
1.62(1.61) BM along b(c) of a single crystal of 1a(1b). This value is much higher than the
experimental value (0.8 BM). However, the assumption that the spins are aligned along their local
anisotropy axis is only valid for |D/J|→ ∞. When this limit is not reached, the energy is minimized
with a smaller canting than that expected from structural considerations. We shall also take into
account that we have assumed a ~3 cm–1 value for |D| based on the D value computed for the
dinuclear polymorph 2, where the Mn coordination sphere is different from that in 1 and the JꢀT axis
is along a different set of ligands (NꢁꢁꢁO for 2 instead of NꢁꢁꢁN for 1): thus the actual D value in 1 may
be somewhat different and this ~3 cm–1 value assessed for |D| in 1 shall be considered as a rough
approximation. A similar observation has been reported for [Mn(TPP)O2PHPh]ꢁH2O26c: as suggested
by the authors, comparison of systems with different |D/J| ratios would be useful to clarify this point.
In this contribution, it is also shown that the third polymorph, structurally identical to the
phenoxoꢀbridged dimer reported in ref. 26d, exhibits not only intradimer superexchange
ferromagnetic interactions through the diphenoxo bridge26d but also weak ferromagnetic interꢀdimer
interactions extended throughout the 2Dꢀnetwork resulting from the =CHꢁꢁꢁNazido, phenylꢀHꢁꢁꢁCl and
ClꢁꢁꢁCl contacts. These ferromagnetic interꢀdimer interactions, gauged through the meanꢀfield
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approximation41 to be 0.025 cm–1, are supported by the χMT values larger than 10 cm3 mol–1 K at low
temperatures. More importantly, we show that the JahnꢀTeller elongation along the N3–Mn–O1i axis
of the octahedron around MnIII is associated with significant single ion magnetic anisotropy gauged
by a zfs parameter D (–3 cm–1) larger than the isotropic exchange parameter J (0.7 cm–1). As a result
of these newly evidenced magnetic characteristics of [Mn(5ꢀClsalpn)N3]2 (2) an SMM behavior could
be anticipated and subsequently evidenced for 2. Based on the energy diagram resulting from the
simultaneous fit of the calculated magnetic susceptibility to the 13 sets of experimental magnetic
data, the energy barrier ꢀ/kB = Ueff between the mS = ± 4 levels was estimated to be 16 cm–1 (23 K).
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This value is larger than the experimental activation energy value ꢀ/kB deduced from fitting the
Arrhenius equation to the experimental temperatures TB corresponding to the maximum of the χ″ vs.
T curves at zero static magnetic field, Ueff = 18.2 K. This may result from operation of quantum
tunneling which shortens the magnetic relaxation time at zero dc field.6b,6c,43 In agreement with this
hypothesis, the energy barrier deduced from fitting the Arrhenius equation to the experimental
temperatures TB corresponding to the maximum of the χ″ vs. T curves at 0.3 T static field was 26.0
K: the applied static dc field blocks the tunneling process thus increasing the activation energy ꢀ/kB
2
349
and the magnetic relaxation time
τ
=
τ
0exp(ꢀ/kBT). Since ꢀ/kB = |DMn2|ST /kB, we deduce DMn2/kB = –
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1.44 K. This value, as well as the values of Ueff and
τ compare quite well with those reported for the
previously published phenoxoꢀbridged dimanganese(III) compounds.6b,6c,44 Also, the quantum
tunneling behavior described above was reported for some phenoxoꢀbridged dimanganese(III)
compounds.6b,6c,44 Intermolecular interactions can influence the magnetic relaxation in SMM systems.
Such effects have been reported for a oneꢀdimensional chain of dimers linked by hydrogen bonds
having weak interꢀdimer antiferromagnetic interactions,45 and for tetranuclear manganese (Mn4)
based SMM compounds in which twoꢀdimensional (antiferromagnetic)46 or threeꢀdimensional
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(antiferromagnetic as well as ferromagnetic)47 networks are formed. The weak interdimer
ferromagnetic interactions organizing 2 in a two dimensional network are most probably responsible
for the quantum tunneling of its magnetic relaxation at zero applied static magnetic field.
To conclude, it is remarkable to notice that the three polymorphs of an unique coordination
compound, Mn(5ꢀClsalpn)N3, all exhibit slow relaxation of their magnetization at low temperature
(2ꢀ3 K range) with similar characteristic parameters (Ueff = 21.4 K and τ0 = 2.97 × 10ꢀ8 s (0.0 T) for
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1; Ueff = 18.2 K and τ τ
0 = 5.77 × 10ꢀ8 s (0.0 T), and Ueff = 26.0 K and 0 = 4.05 × 10ꢀ8 s (0.3 T) for 2)
regardless of their SCM (1) or SMM (2) nature.
Experimental Section
Synthesis. All chemicals used for synthesis were reagent grade. The Schiff base was formed in
situ in the presence of the appropriate metal salt. Caution: Azide compounds are potentially
explosive and should be prepared only in small quantities and handled with care.
[Mn(5-Clsalpn)N3] (1). In a beaker open to the atmosphere, Mn(CH3COO)2.4H2O (0.245g, 1.00
mmol) and NaN3 (0.130g, 2.00 mmol) were stirred into 25 mL of methanol. 5ꢀChlorosalicyladehyde
(0.313g, 2.00 mmol) and 1,3ꢀdiaminopropane (0.071g, 1.0 mmol) were stirred in 30 mL of methanol
and added to the above solution and stirring was continued for about 40 minutes to complete the
aerial oxidation of MnII. The green coloured precipitate formed was separated by filtration and dried.
Yield: 0.361g (0.809 mmol, 80.9%). It was recrystallized from CH3OH at 5°C to obtain green Xꢀray
quality crystals. Anal. Calcd. for MnC17H14Cl2N5O2: C, 45.76; H, 3.16; N, 15.70. Found: C, 45.76; H,
3.12; N, 15.96. Important IR absorptions(cmꢀ1): νa(N3) 2046(s), νs(N3) 1377(s), δ(N3) 707(s), νschiff
base(C=N) 1620(s). The precipitate and crystals exhibit identical IR spectra. Xꢀray examination
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revealed that the crystals consist of two polymorphic forms belonging to different space groups, 1a
(C2/c) and 1b (P21/c), with essentially the same molecular structure.
[Mn(5-Clsalpn)N3]2 (2). The precipitate of [Mn(5ꢀClsalpn)N3] obtained as in the preparation of 1
was dissolved in methanol and vapor diffused with nꢀhexane in a desiccator. Darkꢀgreen Xꢀray
quality crystals were collected after three weeks. Anal. Calcd. Mn2C34 H28Cl4N10O4: C, 45.76; H,
3.16; N, 15.70. Found: C, 45.69; H, 3.18; N, 15.81. Important IR absorptions (cmꢀ1): νa(N3) 2035(s),
νs(N3) 1373(s), δ(N3) 707(s), νschiff base(C=N) 1620(s).
Physical Measurements. IR spectra were obtained in KBr pellets using a Shimadzu FTꢀIR 8000
spectrometer. Elemental analyses of the complexes were performed on a FLASH EA SERIES CHNS
analyzer. The magnetic susceptibility was measured for 1 and 2 in the 1.98ꢀ300 K temperature range
using a Quantum Design MPMS SQUID susceptometer. The samples (in the case of compound 1, it
consisted of both polymorphs 1a and 1b) were pressed into pellets to avoid orientation effects of the
microcrystals during magnetic measurements. Diamagnetic corrections were applied using Pascal’s
constants.48 The magnetic susceptibilities were computed by using the Heisenberg chain equation of
Koenig et al.37 for 1, and exact diagonalization of the spin Hamiltonian with a general program for
axial symmetry for 2.40 Leastꢀsquares fittings were accomplished with an adapted version of the
functionꢀminimization program MINUIT.49
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X-ray Crystallography. Xꢀray data were collected for 1a, 1b and 2 on a Bruker SMART APEX
CCD Xꢀray diffractometer, using graphiteꢀmonochromated MoꢀKα radiation (λ = 0.71073 Å).
Data were reduced using SAINTPLUS50 and a multiꢀscan absorption correction using SADABS51
was performed. The structure was solved and refined against F2 using SHELXꢀ97.52 All ring
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hydrogen atoms were assigned on the basis of geometrical considerations and were allowed to
ride upon the respective carbon atoms. All hydrogen atoms were assigned fixed Uiso values, equal
to 1.2Ueq of the parent atom. Crystallographic data are presented in Table 2. The results for
compound 2 are very similar to those previously reported (CCDCꢀ715714)26d and hence not
included in the Table.
Table 2 Crystallographic data for [Mn(5ꢀClsalpn)N3]n (1)
Polymorph 1a
Polymorph 1b
Formula
Formula weight
Crystal system
a (Å)
C17H14Cl2MnN5O2
446.17
C17H14Cl2MnN5O2
446.17
Monoclinic
24.035(4)
8.5326(14)
19.386(3)
90
Monoclinic
12.5241(8)
17.6241(11)
8.4720(5)
90
b (Å)
c (Å)
α (°)
β (°)
114.162(8)
90
107.7210(10)
90
γ (°)
V (Å3)
3627.4(10)
C2/c
1781.26(19)
P21/c
Space group
Z
8
4
T (K)
100(2)
100(2)
ρcalcd (g cmꢀ3)
ꢁ (mmꢀ1)
θ range (°)
h/k/l indices
1.634
1.664
1.046
1.065
1.86 – 28.25
–31, 32/ –11, 11/ –25, 25
1.71 – 28.25
–16, 16/ –22, 22/ –11, 11
20425
Reflections collected 20426
Unique reflection, Rint 4344, 0.0273
4268, 0.0433
1.117
GooF
1.053
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R1[I > 2σ(I)]
wR2[all data]
0.0381
0.0968
0.0477
0.1140
Weight. scheme(A, B) 0.0491, 3.50
0.0574, 1.11
409
410
411
Acknowledgments. This work was supported by DRDO, India. G.B. thanks the University of
Hyderabad for grant of a DSTꢀPURSE postꢀdoctoral fellowship.
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413
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