Magnetostructural examination of Mn(III) complexes [Mn(cyclam)X2]+ with strong axial ligands

Article abstract of DOI:10.1016/j.poly.2009.02.013

Three novel Mn(III) cyclam complexes, [Mn(cyclam)(NCBH₃)₂](CF₃SO₃), [Mn(cyclam)(NCBPh₃)₂](CF₃SO₃), and [Mn(cyclam)(NCSe)₂](CF₃SO₃) · H

Full text of DOI:10.1016/j.poly.2009.02.013

Polyhedron 28 (2009) 2087–2091
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Magnetostructural examination of Mn(III) complexes [Mn(cyclam)X2]+ with
strong axial ligands
*
Haruyuki Baba, Motohiro Nakano
Division of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 25 February 2009
Three novel Mn(III) cyclam complexes, [Mn(cyclam)(NCBH3)2](CF3SO3), [Mn(cyclam)(NCBPh3)2](CF3SO3),
and [Mn(cyclam)(NCSe)2](CF3SO3) ꢀ H2O, have been synthesized. These complexes are in the high-spin
state between 4 and 350 K, and show large zero-field splittings. The crystal structure of [Mn(cy-
clam)(NCBH3)2](CF3SO3) was determined where the axial elongation of Mn–N bonds is found to be the
largest among the homologue complexes. Ligand field in the [Mn(cyclam)X2]+ complex series was exam-
ined by angular-overlap model calculation.
Keywords:
Magnetic anisotropy
Ligand field
Angular-overlap model (AOM)
Manganese(III)
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
The magnetism of Mn(III) complexes receives much attention
recently because their large magnetic anisotropy serves useful pin-
ning potentials for magnetization reversal in various nanomagnets
[1] and their complicated d4 electron configurations provide a po-
tential for multiple bistabilities including spin-crossover and the
Jahn–Teller effects [2–5].
Commercially available solvents and chemicals were used with-
out further purification. The reaction procedures of 1 and 2 were
carried out in ambient atmosphere. The reaction procedure of 3
was performed under an argon atmosphere using standard Schrenk
technique. The complex [Mn(cyclam)(OH2)2](CF3SO3) ꢀ H2O was
prepared according to the literature method [8]. Elemental analy-
ses were carried out at the Laboratory for Instrumental Analysis,
Graduate School of Engineering, Osaka University. IR spectra were
recorded on a JASCO FT/IR-300E.
A
series of Mn(III) complexes, trans-[Mn(cyclam)X2]+ (cy-
clam = 1,4,8,11-tetraazacyclotetradecane,
X
ꢁ = axial anionic li-
gand), has been extensively studied, focusing on the electronic
structure and magnetic properties, which may take high-spin
(eg)1(t2g)3 or low-spin (t2g)4 electron configurations [6–11]. Within
the series, only one low-spin species is known with Xꢁ = CNꢁ [11],
and no spin-crossover complexes are reported yet. We are inter-
ested in the stronger side of a range of axial ligand field, and ob-
2.1. Synthesis
2.1.1. [Mn(cyclam)(NCBH3)2](CF3SO3) (1)
To an aqueous solution (3 mL) of [Mn(cyclam)(OH2)2](CF3-
SO3)3 ꢀ H2O (378 mg, 0.5 mmol) was added an aqueous solution
(2 mL) of NaNCBH3 (62.9 mg, 1.0 mmol) at room temperature
and the mixture was stirred. Sky-blue solid precipitated immedi-
ately was filtered off and dried in air (yield 135 mg, 56%). Infrared
ꢁ
ꢁ
tained three novel complexes with Xꢁ = NCBH3 (1), NCBPh3 (2),
and NCSeꢁ (3), exploring the spin-crossover boundary on the inter-
action parameter space. The crystal structure of trans-[Mn(cy-
clam)(NCBH3)2](CF3SO3) (1) was solved and reported in detail.
Several known complexes [8,9,11] were also prepared along with
novel ones and their magnetic susceptibilities were measured aim-
ing to determine the magnetic anisotropy parameter (uniaxial
zero-field splitting parameter D) as a guide scale of axial ligand
field. By applying angular-overlap model and extended Hückel
molecular orbital calculations, the relation between magnetic
anisotropy and electronic structure was discussed for a series of
[Mn(cyclam)X2]+ complexes.
spectrum (KBr disk, cmꢁ1): 2184 (
m{C„N}), 2350 (m{B–H}). Anal.
Calc. for C13H30B2F3MnN6O3S: C, 32.26; H, 6.25; N, 17.36. Found:
C, 31.77; H, 6.18; N, 17.35%. For X-ray crystallographic analysis,
an aqueous solution (1.5 mL) of NaNCBH3 (13.1 mg, 0.21 mmol)
was carefully layered on an aqueous solution (1 mL) of [Mn(cycla-
m)(OH2)2](CF3SO3)3 ꢀ H2O (76.1 mg, 0.10 mmol) to yield sky-blue
single crystals in 2 h.
2.1.2. [Mn(cyclam)(NCBPh3)2](CF3SO3) (2)
A mixture of KCN (130 mg, 2.0 mmol) and BPh3 (484 mg.
2.0 mmol) in ethanol (2.5 mL) at room temperature was stirred
for 1 h to give a clear solution. To an aqueous solution (2 mL) of
* Corresponding author. Tel./fax: +81 6 6879 4588.
E-mail address: moto@ch.wani.osaka-u.ac.jp (M. Nakano).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.02.013

2088
H. Baba, M. Nakano / Polyhedron 28 (2009) 2087–2091
[Mn(cyclam)(OH2)2](CF3SO3)3 ꢀ H2O (756 mg, 1.0 mmol) was added
the ethanolic solution and the mixture was stirred. Immediately
yellowish green solid was precipitated and filtered off. The solid
was washed by ethanol, and dried in air (yield 357 mg, 38%). Infra-
3. Results and discussion
3.1. Molecular structure of [Mn(cyclam)(NCBH3)2](CF3SO3) (1)
red spectrum (KBr disk, cmꢁ1): 2171 (
m{C„N}). Anal. Calc. for
Crystal structure was determined for the complex 1 which
crystallizes in the non-centrosymmetric space group P212121. The
molecular structure is depicted in Fig. 1. The counter anion
C49H54B2F3MnN6O3S: C, 62.57; H, 5.79; N, 8.93. Found: C, 62.30;
H, 5.82; N, 8.98%.
ꢁ
CF3SO3 links neighboring complex cations via weak hydrogen
2.1.3. [Mn(cyclam)(NCSe)2](CF3SO3) ꢀ H2O (3)
bonds OꢀꢀꢀH–N forming a chain structure along c-axis (Fig. 2).
The coordination environment around Mn(III) is elongated-
octahedron type. The equatorial Mn–N distances fall in a range of
0.2004–0.2034 nm. The axial Mn–N distances are significantly
longer than them, being 0.2209 and 0.2215 nm. These values are
the longest among axial Mn–N distances in the hitherto-reported
[Mn(cyclam)X2]+ complexes [6–11]. The Mn–N and N„C bonds
in MnꢀꢀꢀNCBH3 axial coordination are non-collinear with the Mn–
N–C angle of 169.4–173.7°, which gives rise to a small deviation
from tetragonal symmetry, while the axial Mn–N bond is almost
normal to the equatorial N4 plane with the tilt angle of 1.06° (Table
2).
To a solution of KSeCN (143 mg, 0.99 mmol) in acetonitrile
(10 mL) was added a solution of [Mn(cyclam)(OH2)2](CF3SO3)3 ꢀ
H2O (378 mg, 0.50 mmol) in the same solvent (5 mL) at room tem-
perature. The deep purple solution changed to a clear orange solu-
tion immediately after stirring. After 20 min, the solution was
completely evaporated in vacuo. In half volume of solvent evapo-
rated a dark-orange solid started to precipitate. The solid was
washed by ethanol and dried in vacuo (yield 86.7 mg, 28%). Infra-
red spectrum (KBr disk, cmꢁ1): 2055 (
m{C„N}). Anal. Calc. for
C13H26F3MnN6O4SSe2: C, 24.69; H, 4.14; N, 13.29. Found: C,
24.68; H, 3.78; N, 13.23%.
2.2. X-ray structure determination
3.2. Magnetic properties
Single crystal structure determination was performed on the
compound 1 at 123 K using a Rigaku RAXIS RAPID imaging-plate
Isofield magnetizations of 1 and 2 were measured from 2 to
350 K, and the ones of 3 were from 4 to 350 K. All the compounds
area detector with graphite monochromated Mo K
a radiation
showed effective magnetic moments
l
eff of 4.9–5.2
lB at room tem-
(k = 0.071073 nm). The structure was solved by direct methods
[12] and expanded using Fourier techniques [13]. The positions
of all non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were included and their positions were refined using a rid-
ing model. All calculations were performed using the CRYSTALSTRUC-
TURE crystallographic software package [14,15]. Detail on the data
collections and refinements are summarized in Table 1.
perature, which agree well with the spin-only value of 4.9
lB (S = 2)
expected for a high-spin d4 electron configuration of Mn(III)
(Fig. 3). The leff drops observed at low temperature should be
attributable to the zero-field splitting accompanying with tetrago-
nal coordination environment, rather than to antiferromagnetic
intermolecular interactions. The uniaxial zero-field splitting
2.3. Magnetic susceptibilities
Magnetic measurements of 1 and 2 were carried out on a Quan-
tum Design MPMS-XL5 SQUID magnetometer equipped with recip-
rocating sample option (RSO) at magnetic fields of 1.0 and 5.0 T.
Magnetic measurement of 3 was carried out on a Quantum Design
MPMS-2 SQUID magnetometer at a field of 1.0 T. Polycrystalline
samples were mounted in calibrated gelatin capsules held at the
center of a polypropylene straw fixed to the end of the sample rod.
Table 1
Crystal data and structure refinement for 1.
trans-[Mn(cyclam)(NCBH3)2](CF3SO3)
Formula
C13H30B2F3MnN6O3S
484.03
orthorhombic
P212121
8.6989(5)
13.3506(8)
19.3131(10)
90.0
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a
(°)
b (°)
90.0
c
(°)
90.0
2242.9(2)
4
V (Å3)
Z
Dcalc (g cmꢁ3
)
1.433
Crystal color, habit
Crystal dimensions (mm)
F(000)
blue, prism
0.15 ꢂ 0.10 ꢂ 0.10
1008.00
7.315 cmꢁ1
0.0717
l
(Mo K
R1 [I > 1.50
wR2 [I > 1.50
a)
r
(I)]
(I)]
r
0.0823
Fig. 1. Molecular structure of trans-[Mn(cyclam)(NCBH3)2](CF3SO3) (1). All H atoms
are shown as open circles, and thermal ellipsoids for heavier atoms are drawn at the
50% probability level.
Goodness of fit
Flack parameter [16]
1.070
0.49

H. Baba, M. Nakano / Polyhedron 28 (2009) 2087–2091
2089
6.0
5.0
4.0
3.0
2.0
1.0
0.0
5.0 T
1.0 T
0
50
100
150
200
250
300
350
T / K
Fig. 2. Chain structure of trans-[Mn(cyclam)(NCBH3)2](CF3SO3) with weak hydro-
gen bonds along c-axis.
Fig. 3. The temperature dependence of effective magnetic moment of a polycrys-
talline sample of trans-[Mn(cyclam)(NCBH3)2](CF3SO3) from 2 to 350 K. At temper-
atures lower than 50 K, the effective magnetic moment shows a drop caused by the
zero-field splitting.
parameter D of 1, 2 and 3 were estimated by assuming random ori-
entation of crystallites (Table 3).
3.3. Magnetic anisotropy analysis on the basis of AOM
Table 3
Uniaxial zero-field splitting parameter D of trans-[Mn(cyclam)X2]+ complexes.
Extended Hückel molecular orbital calculations [18] were made
Axial ligand X
Spin state
D/hc (cmꢁ1
)
References
to figure out one-electron orbital energies for the complexes with
ꢁ
X
ꢁ = Clꢁ, Brꢁ, Iꢁ, NCBH3ꢁ, NCOꢁ, NCSꢁ, N3ꢁ, CNꢁ, ClO4ꢁ, NO3
,
I
Br
HS
HS
HS
HS
HS
HS
LS
0.82*
ꢁ1.67
this work
[17]
and NO2ꢁ using their crystallographic coordinates [6–11]. The orbi-
tal energies of five d-character molecular orbitals are depicted in
Fig. 4. It is obvious that the dz2 orbitals are strongly affected by
NCBH3
NCBPh3
NCSe
NCO
CN
ꢁ4.65*
ꢁ5.76*
ꢁ9.05*
ꢁ10.26*
51.02*
this work
this work
this work
this work
this work
r
-donor character of axial ligands, while the orbital energy of
the dx2 orbitals are dominated by the equatorial ligand cyclam
ꢁy2
and kept almost unchanged at around ꢁ8.2 eV throughout the ser-
*
Apparent value estimated by fitting of magnetic susceptibility data.
ies. Non-bonding t2g orbitals found below ꢁ12 eV are slightly lifted
by p-donor/acceptor character of the axial ligands, providing a sin-
gle and a doubly degenerate levels. In spite that the splitting in the
t2g orbitals are much smaller than that in eg orbitals, this small
splitting plays an essential role in the low-spin electronic configu-
ration under a very strong ligand field.
One-electron orbital energies obtained from extended Hückel
molecular orbital calculations are useful to estimate ligand-field
parameters. The angular-overlap model (AOM), instead of a con-
ventional cubic harmonics expansion, was adopted to describe
the ligand fields in [Mn(cyclam)X2]+ complexes, since it is more
convenient to attribute the effect of individual ligand to the AOM
parameters representing localized
r and p donors (e and e ,
r p
respectively). For an octahedral complex trans-[ML4X2] of D4h sym-
metry, the orbital-energy relations:
Eðx2 ꢁ y2Þ ¼ 3e ðLÞ;
ð1Þ
ð2Þ
ð3Þ
ð4Þ
r
Eðz2Þ ¼ e ðLÞ þ 2e ðXÞ;
r
r
EðyzÞ ¼ EðzxÞ ¼ 2e ðLÞ þ 2e ðXÞ;
p
p
Fig. 4. 3d-levels for trans-[Mn(cyclam)X2]+/3+ estimated by extended Hückel
calculations. Anions noted on the abscissa stand for the axial ligand X in the
complex.
EðxyÞ ¼ 4e ðLÞ ffi 0;
p
Table 2
Comparison of structural data for trans-[Mn(cyclam)X2]+/3+ complexes.
Axial ligand X
Spin state
Coordination sphere
Average Mn–X distance (Å)
Average Mn–Ncyclam distance (Å)
s
(°)a
References
I
Br
Cl
HS
HS
HS
HS
HS
HS
HS
HS
HS
HS
HS
LS
4N, 2I
4N, 2Br
4N, 2Cl
4N, 2O
4N, 2O
4N, 2O
4N, 2O
6N
6N
6N
6N
4N, 2C
2.9416(2)
2.689(1)
2.527(1)
2.187(8)
2.1909(9)
2.221(4)
2.188(12)
2.175(3)
2.166(17)
2.212(7)
2.148(4)
2.007(4)
2.028(2)
2.029(6)
2.035(3)
2.037(6)
2.0280(7)
2.036(7)
2.034(3)
2.041(3)
2.038(4)
2.020(9)
2.043(4)
2.029(4)
3.71
3.28
2.50
7.80
8.71
ꢁ4.49
ꢁ5.90
3.76
1.48
1.06
1.40
1.30
[8]
[6]
[6,7]
[8]
[8]
[6]
[8]
[10]
[6]
this work
[9]
OH2
ClO4
NO3
NO2
N3
NCS
NCBH3
NCO
CN
[11]
a
Tilt angle
s is defined as the angle between the normal to the equatorial MnN4 plane and the Mn–X bond [8].

2090
H. Baba, M. Nakano / Polyhedron 28 (2009) 2087–2091
parameters: Racah’s parameters B/hc = 1140 cmꢁ1 and C/B = 4.3
[21]; spin-orbit coupling f/hc = 355 cmꢁ1 [19]; Stevens’ orbital
reduction factor k = 0.8 [22]; and the AOM parameter of equatorial
Table 4
AOM parameters of trans-[Mn(cyclam)X2]+/3+ extracted from extended Hückel energy
levels.
(X)/hc (cmꢁ1
)
e (X)/hc (cmꢁ1
)
e )
(L)/hc (cmꢁ1
r
p
cyclam ligands e (L)/ hc = 10000 cmꢁ1. The red broken curve on
Axial ligand X
e
r
r
the map is a spin-crossover boundary separating high-spin (S = 2)
I
Br
Cl
3860
5660
7930
4220
4640
4070
5700
5900
7830
7290
9100
800
970
960
420
490
490
410
860
410
840
710
ꢁ260
11600
11650
11200
11440
11600
11410
11410
11160
11820
11340
11330
11640
and low-spin (S = 1) region. On this map, e (X) and e (X) values
r
p
of several complexes summarized in Table 4 are also plotted. This
map suggested that the high-spin complex 1 does not have strong
enough ligand field to break Hund’s rule, and the cyanohydrobo-
OH2
ClO4
NO3
NO2
N3
NCBH3
NCS
NCO
CN
rate anion behaves as not a p-acceptor expected but a weak p-do-
nor for Mn3+. Given the comparison of D-value in Table 4,
complexes 2 and 3 appear to be located between 1 and [Mn(cy-
clam)(NCO)2]+. Although the D map was obtained on the basis of
rather naïve AOM model, the agreement with experimental values
is satisfactory in semiquantitative level. Comparing these plots
with the spin-crossover boundary curve, it is revealed that the
attainment of spin-crossover Mn(III) cyclam complexes requires
17660
are known for
r-donor equatorial ligand L and r,p-donor axial li-
stronger
have.
r-donor and p-acceptor ligands than the complexes 1–3
gand X, where E(d) stands for the one-electron orbital energy of
d-character orbital [19]. These relations were used to extract AOM
parameter values compiled in Table 4. If e (X) is negative, i.e.
p
4. Conclusion
E(yz), E(zx) < E(xy), the axial ligand behaves as a
than a
p-acceptor rather
p
-donor. In this case, the possible low-spin electronic config-
The manganese(III) complexes 1–3 were newly prepared and
found to be in the high-spin state. The magnetic measurements
of them revealed very large uniaxial zero-field splittings D in 1–
3. The crystal structures of 1 and analogous complexes were uti-
lized in extended Hückel and AOM calculations, providing useful
information of the spin-crossover boundary of [Mn(cyclam)X2]+
series.
uration is orbitally-degenerate {(dyz)(dzx)}3(dxy)1, not (dxy)2(-
dyz)1(dzx)1, which is subject to the Jahn–Teller instability. It may
be responsible for the tilting of axial ligands observed in Xꢁ = CNꢁ
[11].
Although there is a report that the magnetic anisotropy in the
high-spin [Mn(cyclam)I2]+ complex may be dominated by MLCT
excited states mixing into the high-spin 5B1 ground state [20],
the contribution of such low-energy MLCT excited states is very
rare and reliably ignored in most of the [Mn(cyclam)X2]+ com-
plexes. Thus, the magnetic anisotropies of them were simulated
taking only d-orbitals into account. Fig. 5 shows the zero-field
splitting parameter (D) of [Mn(cyclam)X2]+ series mapped on an
Supplementary data
CCDC 705754 contains the supplementary crystallographic data
for complex 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
interaction parameter plane e (X)–e (X), which was calculated
r
p
from the longitudinal Zeeman splitting simulations based on the
AOM in conjunction with typical values for Mn(III) electronic
Fig. 5. Zero-field splitting parameter D of [Mn(cyclam)X2]+ complexes as a function of axial ligand AOM parameters e (X) and e (X). Several points from Table 4 are also
r
p
plotted on the map. The spin-crossover boundary is shown as a red dotted curve.

H. Baba, M. Nakano / Polyhedron 28 (2009) 2087–2091
2091
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Acknowledgments
The authors would like to thank Prof. Takayoshi Kuroda-Sowa
in Kinki University and Dr. Tetsuya Takeuchi at Low Temperature
Center in Osaka University for magnetic measurements. We also
appreciate Dr. Nobuko Kanehisa for X-ray crystal structure
determinations.
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