Synthesis, structure, and magnetic properties of [MnIII(salpn)NCS]n, a helical polymer

Article abstract of DOI:10.1021/ic0256617

Dimeric [Mn(salpn)NCS]₂ (1) and polymeric [Mn(salpn)NCS] _n (2) are formed by the reaction of Mn(CH₃CO₂)₂· 4H₂O, the schiff base, and thiocyanate. The formation of the two polymorphic forms is solvent and temperature dependent. 1: orthorhombic, space group Pbca, with a = 12.573(2) A, b = 13.970(7) A, c = 18.891(9) A, and Z = 8.2: orthorhombic, space group Pna2₁, with a = 12.5277(14) A, b = 11.576(2) A, c = 11.513(2) A, and Z = 4. The dimers in 1 are held together by weak noncovalent Smellip;π (phenyl) interactions leading to a chain along the a-axis. Each monomeric unit of the polymer in 2 is related to its adjacent ones by a 2-fold screw axis leading to a helix along the c-axis. The exchange coupling is nondetectable in the dimer. The magnetic susceptibility of the helical chain fits a classical chain law with J = -3.2 cm^-1 and shows a weak ferromagnetic ordering below 7 K due to spin canting effects.

Full text of DOI:10.1021/ic0256617

Inorg. Chem. 2003, 42, 180−186
Synthesis, Structure, and Magnetic Properties of [MnIII(salpn)NCS]n, a
Helical Polymer, and the Dimer [MnIII(salpn)NCS]2. Weak
Ferromagnetism in [MnIII(salpn)NCS]n Related to the Strong Magnetic
Anisotropy in Jahn−Teller Mn(III) (salpnH )
2
N,N′-Bis(salicylidene)-1,3-diaminopropane)
,†
S. Sailaja,† K. Rajender Reddy,† M. V. Rajasekharan,* C. Hureau,‡ E. Rivie`re,‡ J. Cano,‡ and
,‡
J.-J. Girerd*
School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 046, India, and Laboratoire de
Chimie Inorganique, UMR CNRS 8613, Institut de Chimie Mole´culaire d’Orsay,
UniVersite´ Paris-Sud, 91405 Orsay Cedex, France
Received April 18, 2002
Dimeric [Mn(salpn)NCS]2 (1) and polymeric [Mn(salpn)NCS]n (2) are formed by the reaction of Mn(CH CO ) ‚
3
2 2
4H O, the schiff base, and thiocyanate. The formation of the two polymorphic forms is solvent and temperature
2
dependent. 1: orthorhombic, space group Pbca, with a ) 12.573(2) Å, b ) 13.970(7) Å, c ) 18.891(9) Å, and
Z ) 8. 2: orthorhombic, space group Pna21, with a ) 12.5277(14) Å, b ) 11.576(2) Å, c ) 11.513(2) Å, and
Z ) 4. The dimers in 1 are held together by weak noncovalent S‚‚‚π (phenyl) interactions leading to a chain along
the a-axis. Each monomeric unit of the polymer in 2 is related to its adjacent ones by a 2-fold screw axis leading
to a helix along the c-axis. The exchange coupling is nondetectable in the dimer. The magnetic susceptibility of the
helical chain fits a classical chain law with J ) −3.2 cm-1 and shows a weak ferromagnetic ordering below 7 K
due to spin canting effects.
Introduction
terminal N-bonded thiocyanate ligands. The second one is
similar to the azide complex in having a helical polymeric
structure.
The combination of a pseudohalide ion and a tetradentate
Schiff base (SB) ligand is suitable for making neutral M(III)
complexes of the type M(SB)X. The structure and magnetic
properties of Mn(salpn)N3 and Fe(salpn)N3 have been
previously published,1 where the relevant literature on similar
Mn(III) complexes has been reviewed. The manganese
complex has a helical polymeric structure with a 1,3-azide
bridge, while the iron analogue has a dimeric structure having
cis-octahedrally coordinated salpn and a 1,1-azide bridge. It
is now shown that the analogous thiocyanate complex of
manganese can be obtained in two forms, depending upon
the solvent and crystallization temperature. One polymorph
is dimeric, having unsymmetrical phenolate bridges and
A compound having the same empirical formula as the
present one as well as a hydrate has been previously
prepared2,3 but not structurally characterized. Further, the
catalytic efficiency of this compound toward epoxidation of
alkenes has been investigated.4 The analogous complex of
salen, Mn(salen)NCS, the magnetic properties of which were
reported by Kennedy and Murray,5 has a monomeric five-
coordinate structure6 with weak axial perturbation by neigh-
boring phenolate oxygen (salenH2 ) N,N′-bis(salicylidene)-
1,2-diaminoethane). Several such dimeric complexes showing
ferromagnetic as well as antiferromagnetic coupling are now
(2) Kolawole, G. A. Synth. React. Inorg. Met.-Org. Chem. 1993, 23, 907.
(3) Ashmawy, F. M.; McAuliffe, C. A.; Parish, R. V.; Tames, J. Inorg.
Chim. Acta 1985, 103, 133.
(4) Agarwal, D. D.; Bhatnagar, R. P.; Jain, R.; Srivastava, S. J. Chem.
Soc., Perkin Trans. 2 1990, 989.
(5) Kennedy, B. J.; Murray, K. S. Inorg. Chem. 1985, 24, 1552.
(6) Mikuriya, M.; Yamato, Y.; Tokii, T. Bull. Chem. Soc. Jpn. 1992, 65,
1466.
* To whom correspondence should be addressed. E-mail: mvrsc@
uohyd.ernet.in (M.V.R); jjgirerd@icmo.u-psud.fr (J.-J.G.).
† University of Hyderabad.
‡ Universite´ Paris-Sud.
(1) Reddy, K. R.; Rajasekharan, M. V.; Tuchagues, J.-P. Inorg. Chem.
1998, 37, 5978.
180 Inorganic Chemistry, Vol. 42, No. 1, 2003
10.1021/ic0256617 CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/27/2002

Properties of [MnIII(Salpn)NCS]n
Table 1. Crystallographic Data for the Two Polymorphs of
Mn(salpn)NCS
known and the metric parameters of the phenolate bridge
and the exchange parameters have been reviewed in a recent
paper by Miyasaka et al.7 Crystallographic characterizations
of chain compounds of Mn(III) having pseudohalide bridges
are relatively rare.1,8-10 These compounds generally show a
magnetic phase transition at low temperatures.1,5,11 Detailed
magnetization measurements have been carried out to
characterize this spin-ordering phenomenon in the present
chain polymorph.
dimer
polymer
formula
fw
C18H16MnN3O2S
393.34
C18H16MnN3O2S
393.34
A
B
C
V
12.573(3) Å
13.970(7) Å
18.891(9) Å
3318(2) Å3
8
12.5277(14) Å
11.576(2) Å
11.513(2) Å
1669.6(4) Å3
4
Z
space group
Pbca
Pna21
T
λ
293(2) K
0.710 73 Å
1.575 g cm-3
9.38 cm-1
0.0523
293(2) K
0.710 73 Å
1.565 g cm-3
9.33 cm-1
0.0219
Experimental Section
Fcalcd
µ
[Mn(salpn)NCS]2. In a beaker open to the atmosphere, 229 mg
(1.88 mmol) of salicylaldehyde and 71 mg (0.96 mmol) of 1,3-
diaminopropane were stirred into 40 mL of ethanol. Mn(CH3-
CO2)2‚4H2O in an amount of 245 mg (1.00 mmol) was added, and
the stirring continued for about 1 h. To the resulting solution, 200
mg (2.06 mmol) of KNCS dissolved in a minimum amount of water
was added. The solution was allowed to stand for about 3 h to
complete the air oxidation of Mn(II). The filtered solution was then
kept aside at room temperature (∼35 °C) for 2-3 days, over which
time dark green crystals deposited. Yield: 330 mg (0.84 mmol,
89%). Anal. Calcd for MnC18H16N3O2S: C, 54.96; H, 4.10; N,
10.68. Found: C, 54.48; H, 4.13; N, 10.70. Important IR absorptions
(cm-1): 2043, 1601, 1543, 1468, 1443, 1396, 1310, 1148, 1124,
1076, 1030, 977, 895, 804, 754, 613, 463. When the ambient
temperature was lower (18-20 °C, for example, in winter) the
solution affords small amounts of the polymeric form along with
the dimer. Recrystallization from acetonitrile gives pure dimer over
a wide range of temperatures (5-30 °C).
[Mn(salpn)NCS]n. The procedure is the same as that for the
dimer described above, except that the final solution was kept in a
refrigerator (5 °C) for 8-10 days. The yield of dark green crystals
of the polymer in a typical experiment was 85%. Anal. Calcd for
MnC18H16N3O2S: C, 54.96; H, 4.10; N, 10.68. Found: C, 54.63;
H, 4.38; N, 10.65. Important IR absorptions (cm-1): 2068, 1609,
1541, 1466, 1400, 1300, 1150, 1126, 1067, 1028, 968, 903, 804,
750, 615, 459.
R(Fo )a
2
Rw(Fo )b
0.1303
0.0563
2
2
2
4
a R ) Σ|Fo| - |Fc|/Σ|Fo|. b Rw ) [Σw(Fo - Fc )2/ Σ(wFo )]1/2w-1
)
2
[σ(Fo ) + (AP)2 + BP], with A ) 0.0912 and B ) 0.00 for dimer and A )
2
2
0.0411 and B ) 0.15 for polymer. P ) [2Fc + Max(Fo , 0)] /3.
Table 2. Selected Bond Lengths (Å) and Angles (deg)a
Dimer Bond Distances
Mn-O(1)
Mn-N(1)
Mn-N(3)
1.844(3) Mn-O(2)
2.024(4) Mn-N(2)
2.154(5) Mn-O(2)#1
1.923(3)
2.025(4)
2.539(3)
Dimer Bond Angles
O(1)-Mn-O(2)
O(2)-Mn-N(1)
O(2)-Mn-N(2)
O(1)-Mn-N(3)
N(1)-Mn-N(3)
O(1)-Mn-O(2)#1
N(1)-Mn-O(2)#1
N(3)-Mn-O(2)#1
88.22(13) O(1)-Mn-N(1)
168.77(14) O(1)-Mn-N(2)
87.31(14) N(1)-Mn-N(2)
95.82(16) O(2)-Mn-N(3)
91.61(16) N(2)-Mn-N(3)
92.33(12) O(2)-Mn-O(2)#1
88.79(12) N(2)-Mn-O(2)#1
171.83(14)
91.47(15)
173.67(14)
92.04(15)
99.58(15)
89.35(16)
80.01(12)
82.48(13)
Polymer Bond Distances
1.880(2) Mn(1)-O(2)
2.037(2) Mn(1)-N(1)
2.270(3) Mn(1)-S(1)#1
Mn(1)-O(1)
Mn(1)-N(2)
Mn(1)-N(3)
1.897(2)
2.042(2)
2.7737(11)
Polymer Bond Angles
O(1)-Mn(1)-O(2)
O(2)-Mn(1)-N(2)
O(2)-Mn(1)-N(1)
O(1)-Mn(1)-N(3)
N(2)-Mn(1)-N(3)
85.37(9) O(1)-Mn(1)-N(2)
88.83(9) O(1)-Mn(1)-N(1)
176.06(8) N(2)-Mn(1)-N(1)
92.63(11) O(2)-Mn(1)-N(3)
91.43(10) N(1)-Mn(1)-N(3)
173.20(8)
90.70(9)
95.10(9)
95.13(10)
84.84(10)
Polymorphism. To study the effect of solvent, the dimer obtained
from the reaction in ethanol at 35 °C was recrystallized from various
solvents, viz., acetonitrile, dichloromethane, acetone, tetrahydro-
furan, methanol, and ethanol. In each case, crystallizations were
done at two temperatures, viz., 35 and 5 °C. The crystalline products
obtained after 8-10 days were identified as dimer, polymer, or a
mixture of the two by IR spectroscopy (vide supra).
O(1)-Mn(1)-S(1)#1 90.47(9) O(2)-Mn(1)-S(1)#1 95.80(8)
N(2)-Mn(1)-S(1)#1 86.57(8) N(1)-Mn(1)-S(1)#1 84.41(7)
N(3)-Mn(1)-S(1)#1 168.84(7)
a Symmetry transformations used to generate equivalent atoms: #1 -
x, -y + 1, -z for dimer and -x, -y, z + 1/2 for polymer.
Table 2. Powder diffractograms were measured on a model PW3710
Philips Analytical X-ray diffractometer.
X-ray Crystallography. X-ray data were collected for the two
polymorphs on an Enraf-Nonius CAD4 diffractometer at room
temperature using graphite monochromated Mo KR radiation. The
structures were solved by a combination of heavy atom and direct
methods with SHELX-8612 and refined with SHELXL-93/SHELXL-
97.13 The polymer was refined as a racemic twin. Crystal data are
in Table 1, and selected bond distances and angles are given in
Optical Spectroscopy. IR spectra were obtained with a Shi-
madzu FT-IR 8000 spectrometer. Reflectance spectra of powder
samples were measured by using a Shimadzu UV-3100 spectrometer
equipped with an ISR-3100 integrating sphere attachment.
Magnetic Measurements. The two polymorphs obtained from
the reaction in ethanol were used for magnetic measurements. The
purity of the polymorphs used for magnetic studies were checked
by using IR and by comparing their X-ray powder diffraction
patterns with those calculated14 based on crystal structure. The
samples were ground and pressed into pellets to avoid orientation
effects. Magnetic data were recorded on a MPMS5 magnetometer
(Quantum Design Inc.). The calibration was made at 298 K by using
a palladium reference sample furnished by Quantum Design Inc.
(7) Miyasaka, H.; Cle´rac, R.; Ishii, T.; Chang, H.-C.; Kitagawa, S.;
Yamashita, M. J. Chem. Soc., Dalton Trans. 2002, 1528.
(8) Li, H.; Zhong, Z. J.; Duan, C.-Y.; You, X.-Z.; Mak, T. C. W.; Wu, B.
Inorg. Chim. Acta 1998, 271, 99.
(9) Stults, B. R.; Marianelli, R. S.; Day, V. W. Inorg. Chem. 1975, 14,
722.
(10) Stults, B. R.; Day, R. O.; Marianelli, R. S.; Day, V. W. Inorg. Chem.
1979, 18, 1847.
(11) Gregson, A. K.; Moxon, N. T. Inorg. Chem. 1982, 21, 586.
(12) Sheldrick, G. M. SHELXS-86. Acta Crystallogr. 1990, A46, 467.
(13) Sheldrick, G. M. SHELXL-93; University of Go¨ettingen: Germany,
1993; SHELXL-97; University of Go¨ettingen: Germany, 1997.
(14) Kraus, W.; Nolze, G. Powder Cell for Windows version 2.3; Federal
Institute for Materials Research and Testing, Berlin: Germany, 1999.
Inorganic Chemistry, Vol. 42, No. 1, 2003 181

Sailaja et al.
Figure 1. ORTEP view of one complex cation and its two bonded
thiocyanate anions along the 1D chains of [Mn(salpn)NCS]n in polymer.
Hydrogen atoms are omitted for clarity, and the thermal ellipsoids are
represented at 50% probability level. ′ ) -x, -y, z + 1/2
Figure 3. ORTEP view of the Mn(salpn)NCS dinuclear complex molecule
in the dimer. Hydrogen atoms are omitted for clarity, and the thermal
ellipsoids are represented at the 50% probability level. ′ ) -x, -y + 1, -z
thiocyanate acts as an end-to-end bridge. Each monomeric
unit is related to its adjacent ones by a 2-fold screw axis,
leading to a helix propagating along the crystallographic
c-axis. While the gross structure is similar to that of the azide,
there are significant differences in the coordination geometry.
The two in-plane Mn-N distances in the azide complex were
unequal, which was attributed to the strain involved in the
spiral formation. In contrast, the thiocyanate has equal in-
plane Mn-N distances. The long Mn-S bond apparently
relieves any strain in the polymer chain formation. The
Jahn-Teller axis of Mn(III) corresponds approximately to
the N3-Mn-S direction. We will see later the impact of
this distortion on the magnetic properties. The inclination
between the mean planes of the two halves of the SB ligand
(excluding the methylene groups) is now 37.2(1)° as opposed
to 44.0(1)° in the azide complex.
Besides the previously mentioned azide bridged com-
plexes,1,8-10 several other polymeric chain compounds of Mn-
(III), most of them containing acetate bridges, are known.16-20
The unique feature of the chains in Mn(salpn)N3 and Mn-
(salpn)NCS is that the adjacent repeat units are related by a
crystallographic screw axis leading to a helical chain
crystallizing in a chiral space group.
Structure of [Mn(salpn)NCS)]2. In the dimer also (Figure
3) the salpn ligands adopt the equatorial coordination mode;
however, the two halves of each salpn are now more coplanar
than in the helical chain, the interplanar angle being 10.2-
(1)°. The thiocyanate ions remain terminal and N-bonded.
Six coordination around the manganese atoms is completed
by two highly unsymmetrical (Mn-O 1.923(3), 2.539(3) Å)
Figure 2. Perspective view of helical chains in the polymer. Atom codes:
Large shaded spheres, Mn; small shaded spheres, O; small footballs, N;
large open circles, S; small open circles, C. The labels pertain to the
discussion on canting geometry.
The magnetic susceptibility data were collected over a temperature
range of 2-300 K at a magnetic field of 2000 G for [Mn(salpn)-
NCS]n and 500 G for [Mn(salpn)NCS]2 and were corrected for
diamagnetism using Pascal’s constants. In the case of [Mn(salpn)-
NCS]n, the øMT versus T curve was fitted, in the range 15-300 K,
by using the classical S ) 2 chain law,15 based on the Heisenberg
model.
Results and Discussion
(16) Aurangazeb, N.; Hulme, C. E.; McAuliffe, C. A.; Pritchard, R. G.;
Watkinson, M.; Gracia-Deibe, A.; Bermejo, M. R.; Sousa, A. J. Chem.
Soc., Chem. Commun. 1992, 1524.
(17) Davies, J. E.; Gatehouse, B. M.; Murray, K. S. J. Chem. Soc., Dalton
Trans. 1973, 2523.
Structure of [Mn(salpn)NCS]n. The structure of the
polymer (Figures 1 and 2) is analogous to that of the azide
complex.1 The salpn binds in the equatorial mode and
(18) Akhtar, F.; Drew, M. G. B. Acta Crystallogr. 1982, B38, 612.
(19) Bonadies, J. A.; Kirk, M. L.; Lah, M. S.; Kessissoglou, D. P.; Hatfield,
W. E.; Pecoraro, V. L. Inorg. Chem. 1989, 28, 2037.
(20) Kirk, M. L.; Lah, M. S.; Raptopoulou, C.; Kessissoglou, D. P.; Hatfield,
W. E.; Pecoraro, V. L. Inorg. Chem. 1991, 30, 2037.
(15) Fisher, M. E. Am. J. Phys. 1964, 32, 343. The Hamiltonian, in zero-
field, is of the form H ) - JΣSi Si+1. The anisotropic terms are not
included as they do not significantly influence the magnetic suscep-
tibility above 15 K.
182 Inorganic Chemistry, Vol. 42, No. 1, 2003

Properties of [MnIII(Salpn)NCS]n
Figure 4. RASMOL drawing of 1D chain of dimers linked by S‚‚‚ring
interaction along the a-axis. Color code for atoms: blue, N; red, O; yellow,
S; gray, C; Mn atoms are hidden.
phenoxide bridges. The two halves of the dimer are related
by a crystallographic inversion center. SCN-Mn-Obridge
appears to be the Jahn-Teller elongation axis, even though
the terminal thiocyanate ion is more strongly bound com-
pared to the bridging thiocyanate in the polymer. The bridge
angle Mn-Obridge-Mn′ is 100.00(11)°, while the Mn-Mn′
distance is 3.4406(11) Å. The plane defining the MnO2Mn′O2′
bridge makes an angle of 85.0(1)° with the equatorial
coordination plane defined by the atoms Mn, N1, N2, O1,
and O2.
Figure 5. Powder diffuse reflectance spectra of polymeric and dimeric
Mn(salpn)NCS.
amount of polymer being more at LT. Both methanol and
ethanol give the dimer at RT and the polymer at LT. In the
case of ethanol the LT product is contaminated with a small
amount of the dimer. It turns out that the dimer is the
preferred form at RT in all solvents studied. Pure polymer
is obtained only from methanol at LT. It is to be noted that
even after complete evaporation of the solvent, there is no
noticeable amount of dimer in the product obtained at LT
from methanol, implying that dimer is kinetically not favored
at this temperature in this solvent. The two polymorphs can
be readily distinguished based on the IR absorption frequency
corresponding to the N-C stretching vibration of the
thiocyanate ion: 2043 cm-1 dimer; 2068 cm-1 polymer. The
rest of the IR region (3000-400 cm-1) is virtually identical
for the two compounds. This is consistent with the general
observation that the N-bonded thiocyanate ions have a lower
N-C stretching frequency than the bridging thiocyanate.21
The different coordination environments in the two
compounds is also reflected in the d-d absorption region:
band maxima at 15 690 and 16 710 cm-1 for dimer, 16 750
cm-1 for polymer (Figure 5). The solution spectra in
acetonitrile has several features and agree closely with the
previously reported spectrum in aqueous solution:3 16 750
(220), 21 100 (1200), 25 640 (5000), 36 100 (17 600) cm-1
(L mol-1 cm-1). The species present in solution is, in all
probability, solvent perturbed Mn(salpn)NCS. The bands at
21 100 and 25 640 cm-1 are probably due to ligand-to-metal
charge-transfer transition. The weak band at 16 750 cm-1 is
assigned to d-d transitions and may include in its envelope
the transitions from the split components of the 5E state. The
splitting is more prominent in the reflectance spectrum of
the dimer, which has only a weak axial interaction with the
phenolate oxygen atoms.
The uncoordinated S has a significant role in stabilizing
the crystal structure. The most remarkable feature is the S-π
(phenyl) interactions which connect the dimers into one-
dimensional chains running along the crystallographic a-axis
(Figure 4). The distance between the S atom and the centroid
of the phenyl ring is 3.66 Å, while the C-centroid-S angle
is 90 ( 11° for the six ring C atoms. A search in the
Cambridge Crystallographic Database for metal complexes
having N-coordinated NCS- ion interacting with the π-cloud
of phenyl rings resulted in 57 hits. Restricting the C-cen-
troid-S angle to 90 ( 2° reduces the number of structures
to 14. As expected, the S-π (phenyl) interaction is a “soft
bond”, being influenced by other crystal packing consider-
ations. The four symmetry-related chains are linked into a
three-dimensional network through the C-H‚‚‚S interaction,
the S‚‚‚H distance being in the range 3.0-3.4 Å. The
bridging through the phenolate oxygen brings the phenyl
rings on the two halves of the dimer into close proximity,
resulting in four C‚‚‚C contacts, on each side, in the range
3.37-3.50 Å. By contrast, the coordination in Mn(salen)-
NCS has been described as essentially a five-coordinate
square pyramidal,6 with a rather long Mn-Obridge distance
of 2.750 Å.7 It has been generally observed that the most
symmetrical bridge geometry results when water is the axial
ligand.7 Thiocyanate and chloride ligands were found to
produce much weaker interaction with the bridging oxygen
atom. The present structure is an exception, since the Mn-
Obridge distance is comparable to that in the aquo complexes.
Magnetic Properties. Magnetic susceptibility of the chain
compound [Mn(salpn)(NCS)]n was measured between 300
and 2 K. The results are represented in Figure 6 as the
product øMT per Mn as a function of T. At 300 K, øMT )
2.82 cm3 mol-1 K. By itself, this value, which is lower than
Polymorphism and Optical Spectra. Crystallization leads
to one or the other polymorph or a mixture of the two
depending upon the solvent and temperature. Acetonitrile
and dichloromethane give only the dimer at both room
temperature, 35 °C (RT), and low temperature, 5 °C (LT).
The situation is similar in tetrahydrofuran, except that a small
amount of polymer is also obtained at RT along with the
dimer. Acetone produces a mixture at both temperatures, the
(21) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 3rd ed.; John Wiley & Sons: New York, 1977;
p 270.
Inorganic Chemistry, Vol. 42, No. 1, 2003 183

Sailaja et al.
Figure 6. Product of the magnetic susceptibility per mole of manganese
and temperature for the chain compound [Mn(salpn)NCS]n (dotted line).
Solid line corresponds to the classical spin S ) 2 chain model with g )
1.99, J ) -3.2 cm-1 using data above 15 K.
Figure 8. Hysteresis in the ordered phase of the chain compound [Mn-
(salpn)NCS]n at 2 K.
Figure 9. Temperature dependence of the magnetization per mole of
manganese for the chain compound [Mn(salpn)NCS]n within a field of 30
G: (9) ZFC; (O) FC. The remnant magnetization was calculated as FC -
ZFC (4).
Figure 7. Magnetization per mole of manganese for the chain compound
[Mn(salpn)NCS]n as a function of magnetic field.
and field cooled (FC) magnetization. The sample was cooled
under zero field down to 4.5 K; at this temperature the field
was set to 30 G and T was increased to 10 K. The sample
was then cooled again under 30 G down to 4.5 K to get FC
magnetization. As expected the FC curve is above the ZFC
curve. The difference is indicative of the remnant magnetiza-
tion, which is found to vanish at Tc ) 6.8 K. It has to be
noted that the saturation magnetization is only a fraction of
the magnetization of a high-spin Mn(III) (4â), as expected
for weak ferromagnetism.
Magnetic susceptibility of [Mn(salpn)(NCS)]2 was mea-
sured between 300 and 2 K. The results are represented in
Figure 10 as the product øMT as a function of T. At 300 K
the product øMT was found to be equal to 6.01 cm3 mol-1
K. corresponding to the expected value for two isolated high-
spin Mn(III)s. No exchange coupling is detected from these
data, the drop in øMT below ∼5 K being due to zero field
splitting. This is to be contrasted with the situaton in the
salen series,7 where it was observed that (i) in MnIII2(SB)2X2,
when X ) NCS- or Cl-, the trans Mn-Obridge is long (3.44-
3.76 Å), whereas when X ) OH2, it is short (2.43-2.66 Å),
and (ii) short bridges result in stronger ferromagnetic
that for one isolated high spin Mn(III), indicates the presence
of an antiferromagnetic coupling in the chain. This is
confirmed by the decrease of øMT as T decreases. At low
temperature, an anomaly is observed in øMT (see below).
The data above 15 K are well-fitted by the law for a chain
of classical spins S ) 2 with J ) -3.2 cm-1 and g ) 1.99.
The anomaly in øMT was found in fact to be related to a
spin ordering phenomenon. First, when we recorded the
magnetization at 2 K versus magnetic field, we obtained a
curve (Figure 7) exhibiting two regimes. At low field,
magnetization increases steeply with magnetic field; at higher
field magnetization increases at a slower rate and in an almost
linear fashion. This is strongly reminiscent of weak ferro-
magnetism.22,23 An hysteresis (Figure 8) was detected in the
ordered phase at 2 K with a coercive field of 2500 Oe.
The phenomenology of the spin ordering was explored in
detail (Figure 9) by measuring the zero field cooled (ZFC)
(22) Carlin, R. L.; Van Duyneveldt, A. J. Magnetic properties of Transition
Metal Compounds; Springer-Verlag. Inc.: New York, 1977; Vol. 2,
p 184.
(23) Bakalbassis, E.; Bergerat, P.; Kahn, O.; Jeannin, S.; Jeannin, Y.;
Dromzee, Y.; Guillot, M. Inorg. Chem. 1992, 31, 625.
184 Inorganic Chemistry, Vol. 42, No. 1, 2003

Properties of [MnIII(Salpn)NCS]n
As a consequence, the uncompensated moment per Mn can
be estimated as
Ms ) M0 sin[(bu,bv)/2] ) 4â sin(55/2) ) 1.84â
This is almost 10 times larger than the saturation moment,
which can be estimated from measurements at 2 K (Figure
7). We assume that the low value observed is due to a
combination of antiferromagnetic coupling between chains
with a canting effect. This would need further studies to be
ascertained.
Long-range ordering characterized by an anomalous rise
in susceptibility at low-temperatures has been observed also
in some other antiferromagnetically coupled Mn(III) chain
-
compounds, viz, Mn(acac)2X (X ) N3 , NCS-),11 Mn(acen)-
1
N3,5 and Mn(salpn)N3 (acacH ) acetylacetone, acenH2 )
N,N′ -ethylenebis(acetylacetone imine)). As explained by
Gregson and Moxon,11 the magnetic ordering requires a
negative zero field splitting, which is guaranteed by the
tetragonally elongated coordination of Mn(III) in all the
above chain compounds.
Figure 10. Product of the magnetic susceptibility per mole of manganese
and temperature for the dimer compound [Mn(salpn)NCS]2.
interaction (J ∼ 3.0 cm-1). In Mn2(salpn)2(NCS)2, the Mn-
Obridge is short, but J ∼ 0. There is no doubt that the Mn-
Obridge distance in the observed range is to some extent
influenced by crystal packing considerations. However, the
disagreement of the present dimer with the observed
magneto-structural correlation in the salen series is more
difficult to explain.
Origin of the Weak Ferromagnetism of the Chain
Compound. The antiferromagnetic nature of the NCS--
mediated coupling in the chain compound is quite clear from
above. It is also well-known that Mn(III) complexes exhibit
a large anisotropy (typically D ) -4 cm-1 for an elonga-
tion).24,25 From the crystal structure, it was observed that the
Mn(III) ions are in an elongated environment with the Jahn-
Teller (JT) elongation axis oriented toward the NCS- anion.
The fourth unpaired electron is then in a dz2 orbital with the
z-axis along the JT axis. As a consequence the axial
anisotropy energy component must be negative, which means
the spins will tend to orient along the JT axis.25
Conclusion
Two factors related to the formation of helical chain are
the flexibility of the salpn ligand, which allows considerable
deviation from coplanarity of the two halves of this ligand
(inclination angle 37° in the thiocyanate complex, 44° in the
azide complex1), and the Jahn-Teller elongation along the
pseudohalide bridge. In the case of the present thiocyanate
complex, a third factor comes into play, viz., the expected
low affinity of Mn(III) for coordination with S. This factor
would contribute to the relative stability of the dimer despite
the highly unsymmetrical nature of the phenolate bridges.
An important feature of the dimer is the interaction between
the phenyl ring and the free S atom of the thiocyanate ion.
While such interactions are present in other crystal structures
as shown by the database search, the importance of this
noncovalent interaction has not yet been fully appreciated.
Although the preferred crystallization of the dimer from most
solvents is certainly related to the above factors, it is not
clear why the polymer is the kinetically preferred product
from methanol and ethanol.
The thiocyanate-mediated exchange along the helical chain
is weakly antiferromagnetic. However, below 7 K a magnetic
ordering with weak ferromagnetic coupling sets in. It is
shown that, in the present case, the weak ferromagnetism
arises through large magnetic anisotropy in Mn(III) and
resultant spin-canting with respect to the chain direction. The
low value of the observed saturation moment compared to
the expected value based on the canting geometry may be
due to antiferromagnetic interaction between chains. Since
there are no noncovalent bonds between chains, one would
speculate that this coupling arises from nonbonded contacts
in the lattice and would require more detailed studies in this
and other systems. It may be further noted that S ) 2 infinite
chains are of interest in the context of the Haldane
conjecture,26 experimental verification of which requires
single-crystal measurements on 1D chains of integer spin
ions.
It has to be remarked that the JT axis is tilted relative to
the chain axis by 23° if we take the MnnMnn+2Sn angle and
by 34° if we take MnnMnn+2Nn angle. Let us calculate the
canting angle, i.e., the angle between two successive spins
S1 and S2 along the chain. We will assume that S1 and S2 are
along the respective bisecting vectors. We define
bu ) 0.5(Mn1S1 + N1Mn1)
bv ) 0.5(Mn2S2 + N2Mn2)
where S1, Mn1, N1 and S2, Mn2, N2 belong to adjacent (screw
related) units in Figure 2.
From the X-ray structure we determined
(bu, bv) ) 55°
(24) Barra, A.-L.; Gatteschi, D.; Sessoli, R.; Abbati, G. L.; Cornia, A.;
Fabretti, A. C.; Uytterhoeven, M. G. Angew. Chem., Int. Ed. Engl.
1997, 36, 2329.
(25) Gerritsen, H. J.; Sabisky, E. S. Phys. ReV. 1963, 132, 1507.
Inorganic Chemistry, Vol. 42, No. 1, 2003 185

Sailaja et al.
The structure and magnetic properties of the dimer are at
variance with the recently observed trends in phenolate-
bridged Mn(III) dimers of salen-type Schiff base ligands.7
The present dimer has a short bridge despite the strong axial
ligand (NCS-) and magnetic interaction is absent. With
regard to structure, while it is reasonable to expect the axial
ligand to have the major role in controlling the structure of
the dimer that may form in solution (and more definitely in
the gas phase), in the crystal lattice the phenolate interaction
should not be considered in isolation but should be taken
together with other possible noncovalent interactions, espe-
cially the S-π(phenyl) interaction and stacking interactions.27
With regard to magnetic coupling, the net exchange param-
eter has contributions from several exchange pathways
involving the four magnetic orbitals on each d4 ion and may
be influenced by terminal ligands as well as bridge geometry.
More phenolate bridged systems need to be studied to
establish the correlations.
Acknowledgment. This work was supported by CSIR,
India.
Supporting Information Available: One crystallographic file,
in CIF format, is available on the Internet only. Access information
is given on any current masthead page.
IC0256617
(27) Several noncovalent interactions, though generally weaker than the
coordinate bonds, can sometimes strongly influence the coordination
geometry and conformation in crystals. Swarnabala, G.; Rajasekharan,
M. V. Polyhedron 1996, 15, 3197. Sailaja, S.; Swarnabala, G.;
Rajasekharan, M. V. Acta Crystallogr. 2001, C57, 1162.
(26) Haldane, F. D. M. Phys. Lett. 1983, 93A, 464; Phys. ReV. Lett. 1983,
50, 1153.
186 Inorganic Chemistry, Vol. 42, No. 1, 2003