Synthesis, structure and magnetic properties of pyrazolate-bridged dinuclear complexes [{M(NCS)(4-Phpy)}2(μ-bpypz)2] (M = Co2+ and Fe2+)

Article abstract of DOI:10.1016/j.poly.2005.03.047

The synthesis and magnetic characterization of pyrazolato-bridged dinuclear complexes [{M(NCS)(4-Phpy)}₂(μ-bpypz)₂] (Hbpypz = 3,5-bis(2-pyridyl)-pyrazole; 4-Phpy = 4-phenylpyridine; M = Co²⁺ (1) and Fe²⁺ (2)) are described together with the X-ray crystal analysis of the cobalt complex. The structure of 1 shows that the desired coordination has been achieved with the cobalt atoms being coordinated to two bpypz to form the dimer. The X-ray diffraction patterns show 1 and 2 to be isomorphous at room temperature. 2 displays a single spin-crossover transition between the [HS-HS] and [LS-LS] states with T_c = 150 K.

Full text of DOI:10.1016/j.poly.2005.03.047

Polyhedron 24 (2005) 2437–2442
www.elsevier.com/locate/poly
Synthesis, structure and magnetic properties of pyrazolate-bridged
dinuclear complexes [{M(NCS)(4-Phpy)}2(l-bpypz)2]
(M = Co2+ and Fe2+)
Ko Yoneda, Keisaku Nakano, Junji Fujioka, Koichi Yamada, Takayoshi Suzuki,
*
Akira Fuyuhiro, Satoshi Kawata, Sumio Kaizaki
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
Received 5 October 2004; accepted 31 October 2004
Available online 19 May 2005
Abstract
The synthesis and magnetic characterization of pyrazolato-bridged dinuclear complexes [{M(NCS)(4-Phpy)}2(l-bpypz)2]
(Hbpypz = 3,5-bis(2-pyridyl)-pyrazole; 4-Phpy = 4-phenylpyridine; M = Co2+ (1) and Fe2+ (2)) are described together with the
X-ray crystal analysis of the cobalt complex. The structure of 1 shows that the desired coordination has been achieved with the
cobalt atoms being coordinated to two bpypz to form the dimer. The X-ray diffraction patterns show 1 and 2 to be isomorphous
at room temperature. 2 displays a single spin-crossover transition between the [HS–HS] and [LS–LS] states with Tc = 150 K.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Dinuclear complex; Spin-crossover; 3,5-Bis(2-pyridyl)-pyrazole; 4-Phenylpyridine
1. Introduction
be enhanced by controlling both intra- and intermolecu-
lar interactions if polynuclear compounds are employed
[5–8]. Thus, the cooperativity can be achieved by using
suitable bridging ligands. A series of dimeric and poly-
meric iron(II) spin-crossover compounds were synthe-
sized and magnetically characterized along this line
while the crystal structures were not determined. Real
et al. [9–14] investigated the LIESST effect on the dinu-
clear iron(II) compounds and pointed out a form of syn-
ergy between magnetic interaction and spin conversion
in the presence of light. However, there are no X-ray
analysis for dinuclear complexes until the first X-ray
structure of the spin-crossover dinuclear diiron(II) com-
plex was very recently reported for [{Fe(NCSe)-
(l-pzpy)(pzpyH)}2] (pzpyH = 2-pyrazolylpyridine) by
Murray and co-workers [15]. Shortly later, we studied
the X-ray structure of [{Fe(NCBH3)(py)}2(l-bpypz)2]
(Hbpypz = 3,5-bis(2-pyridyl)-pyrazole) and the mag-
netic properties of [{Fe(NCX)(py)}2(l-bpypz)2] (X = S,
The spin-crossover between low-spin and high-spin
states is a type of molecular magnetism that occurs in
some six-coordinate, first row transition metal com-
plexes [1,2]. This phenomenon is one of the most specta-
cular examples of molecular bistability driven by
external constraints leading to molecular switch or
memory [3,4]. The fundamental origin of the spin-cross-
over phenomenon is molecular, but the magnetic behav-
ior strongly depends on intermolecular interaction. Up
to now, the major source of information on spin transi-
tion systems comes from mononuclear iron(II) com-
pounds where molecular distortions spread through
the whole crystal by means of intermolecular interac-
tions. However, the cooperativity in the systems would
*
Corresponding author. Tel.: +81668505409; fax: +81668505408.
E-mail address: kaizaki@chem.sci.osaka-u.ac.jp (S. Kaizaki).
0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.03.047

2438
K. Yoneda et al. / Polyhedron 24 (2005) 2437–2442
BH3) [16,17]. We found the changes of the spin-
crossover temperature Tc with S/BH3 substitution
depends on the enthalpy, presumably arising from the
ligand-field strength. To get the further insight into
the cooperativity between dinuclear complexes and the
synergy between the intra-dimer magnetic exchange cou-
pling and the spin-crossover phenomenon, we report
here the synthesis and magnetic characterization of
[{M(NCS)(4-Phpy)}2(l-bpypz)2] (M = Co2+ and Fe2+).
As the additional axial pyridine derivatives and the me-
tal ions are replaceable for the bpypz-bridged dinuclear
complexes, the role of these items on the spin-crossover
is investigated.
2.1.3. [{Fe(NCS)(4-Phpy)}2(l-bpypz)2]
Hbpypz (0.074 g, 0.033 mmol) was dissolved in
36 mL of a methanol solution of 4-Phpy after deproto-
nating with a 10% methanol solution of tetra-n-butylam-
monium hydroxide (n-Bu)4NOH (0.87 g, 0.34 mmol).
To this solution was added 15 mL of a methanol solu-
tion of trans-[Fe(NCS)2(py)4] (0.16 g, 0.33 mmol) under
an N2 stream. A yellow precipitate was obtained and
after stirring this reaction mixture for 2 h, the precipitate
was filtered off and washed with methanol. The
solid was then dried in vacuo. Yield 71%. Calc. for
C50H36N12S2Fe2: C, 61.23; H, 3.70; N, 17.14. Found:
C, 60.10; H, 3.52; N, 16.72%.
2.2. Physical measurements
2. Experimental
Magnetic susceptibility data were recorded over the
temperature range from 2 to 300 K in the presence of
a magnetic field between 50 and 5000 G with a SQUID
susceptometer (Quantum Design, San Diego, CA). All
data were corrected for diamagnetism, which was calcu-
lated from Pascalꢀs tables. Least squares fitting of the
magnetic susceptibility to appropriate equations with
Heisenberg Hamiltonian (H = ꢀJSi Æ Si) were performed
for the cobalt complex. X-ray powder diffraction data
were collected on a Rigaku MultiFlex diffractometer
by using Cu Ka radiation.
2.1. Syntheses
2.1.1. Preparation of the starting complexes
The starting complex trans-[Fe(NCS)2(py)4] was ob-
tained by a method given in the literature [18]. trans-
[Co(NCS)2(py)4] was obtained by the analogous method
to trans-[Fe(NCS)2(py)4]; to an aqueous solution
(250 mL) containing 14.58 g of Co(NO3)2 Æ 6H2O and
pyridine (20 mL) was slowly added an aqueous solution
(100 mL) containing 8.04 g of NaNCS. The pink precip-
itate obtained was filtered off and washed with 10 mL
portions of ethanol–pyridine (9:1). After drying in
vacuo, a pink powder was obtained.
2.3. Crystallographic data collection and refinement of the
structure
2.1.2. [{Co(NCS)(4-Phpy)}2(l-bpypz)2]
Data collection of 1 was carried out by a Rigaku
Mercury charge coupled device (CCD) system with
graphite-monochromated Mo Ka radiation. Crystallo-
graphic data are given in Table 1. The structure was
solved by a standard direct method (TEXSAN crystallo-
graphic software package of the Molecular Structure
A
methanolic solution (2 mL) of trans-[Co-
(NCS)2(py)4] (3 mmol Lꢀ1) was transferred to a glass
tube, and then a methanolic solution of 2 mL of Hbpypz
(3 mmol Lꢀ1) and 4-Phpy (10 mmol Lꢀ1) was poured
into the glass tube without mixing the solutions. Orange
crystals began to form at ambient temperature in 2
weeks. One of these crystals was used for X-ray crystal-
lography. Physical measurements were conducted on a
polycrystalline powder that was synthesized as follows:
Hbpypz (0.022 g, 0.099 mmol) was dissolved in 12 mL
of a methanol solution of 4-Phpy (0.022 g, 0.14 mmol)
with a 10% methanol solution of tetra-n-butylammo-
nium hydroxide (n-Bu)4NOH (0.26 g, 0.10 mmol). To
this solution was added 15 mL of a methanol solution
of trans-[Co(NCS)2(py)4] (0.049 g, 0.10 mmol) under
an N2 stream. An orange precipitate was obtained and
after stirring this reaction mixture for 2 h, the precipitate
was filtered off and washed with methanol. The solid
was then dried in vacuo. Yield 71%. Calc. for
C50H36N12S2Co2: C, 60.85; H, 3.68; N, 17.04. Found:
C, 59.61; H, 3.48; N, 17.54%. The homogeneity of the
powder sample was confirmed by comparison of the ob-
served powder diffraction pattern with that calculated
from the single-crystal data.
Table 1
Crystallographic data for 1
Formula
C50H36Co2N12S2
986.90
Molecular weight
Crystal system
Space group
Z
triclinic
ꢀ
P1 (No. 2)
1
200
T (K)
˚
a (A)
˚
9.271(4)
10.657(4)
12.968(5)
93.71(1)
108.02(1)
105.69(1)
1157.6(8)
1.416
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
Dcalc (g cmꢀ3
l (cmꢀ1
R1/wR2
)
)
a
8.57
0.0362/0.1094
P
P
P
P
a
R1 = ||Fo| ꢀ |Fc||/ |Fo|; wR2 = [ w(|Fo| ꢀ |Fc|)2/ |Fo|2]1/2
.

K. Yoneda et al. / Polyhedron 24 (2005) 2437–2442
2439
Corp. and Rigaku). Full-matrix least-squares refine-
ments were carried out with anisotropic thermal param-
eters for all non-hydrogen atoms. All the hydrogen
atoms were placed in the calculated positions and fixed.
CCDC reference number is CCDC 251440.
being coordinated to two bpypz to form a dinuclear com-
plex [19]. The dinuclear unit is centrosymmetric, thus, the
only one kind of cobalt and ligands exist in the molecule.
Each cobalt atom is bound to the four nitrogen atoms of
two pyridyl and two pyrazolate moieties in bpypz in the
basal plane and two nitrogen atoms of 4-Phpy and NCSꢀ
˚
anion at the axial positions (2.190(4) A (Co–N(1)),
˚ ˚ ˚
2.059(3) A (Co–N(2)), 2.046(3) A (Co–N(3)), 2.216(4) A
3. Results and discussion
˚
˚
(Co–N(4)), 2.168(4) A (Co–N(6)), 2.095(4) A (Co–
N(21))) forming a distorted octahedral environment as
clearly seen from the top and side views of the molecule.
The dihedral angle between pyridyl and phenyl rings in
4-Phpy is 20°, the value of which is smaller than
those of [Ta(NMe2)Cl3(4-Phpy)] (40°) [20] and [Ni-
(4-Phpy)2(DBM)2] (DBM = dibenzoylmethane) (30°)
[21]. The efficient packing presumably compensates for
3.1. Structure of [{Co(NCS)(4-Phpy)}2(l-bpypz)2]
(1)
An ORTEP drawing of the structure around the co-
balt ions in 1 with the atom numbering scheme is shown
in Fig. 1(a). The structure of 1 shows that the desired
coordination has been achieved with the cobalt atoms
Fig. 1. (a) ORTEP drawing of 1 with atom numbering scheme. Thermal ellipsoids are set at 50% probability and hydrogen atoms have been omitted
˚
for clarity. Selected bond distances (A): Co–N1 2.190(4), Co–N2 2.059(3), Co–N3* 2.046(3), Co–N4* 2.216(4), Co–N6 2.168(4), Co–N21 2.095(4).
˚
˚
(b) Emphasizing the p–p stacking interactions between two bpypz ligands of consecutive 1 dimers: a (C2–C12) = 3.46 A, b (C4–C7) = 3.42 A.
4-Phenylpyridine ligands and NCSꢀ anions have been omitted for clarity. (c) Emphasizing p–p stacking interactions between consecutive
˚
4-phenylpyridine ligands: a (C15–C36) = 3.51 A, b (C34–C36) = 3.73 A.
˚

2440
K. Yoneda et al. / Polyhedron 24 (2005) 2437–2442
any unfavorable H—H contact that results in the coordi-
nated 4-Phpy (vide infra). Interestingly, 1 forms a layer
by the two types of p–p stacking interactions between
the adjacent bpypz ligands of the neighbor dimers (Fig.
1(b)): (1) between pyridyl rings and (2) between pyridyl
and pyrazole rings on the adjacent complexes. The layers
are connected by additional p–p stacking interactions be-
tween adjacent 4-Phpy ligands (Fig. 1(c)) to form a three-
dimensional structure.
3.2. Structure of [{Fe(NCS)(4-Phpy)}2(l-bpypz)2] (2)
Despite many efforts, we did not succeed in growing
single crystals of 2 suitable for single crystal X-ray anal-
ysis and we used powder diffraction pattern to determine
the structure. Fig. 2 shows the diffractograms of com-
pounds 1 and 2 accompanied with the simulated pattern
obtained from the crystal structure of 1. The X-ray dif-
fraction patterns show 1 and 2 to be isomorphous at
room temperature, and thus, the dinuclear iron(II)
structure is confirmed to be formed.
Fig. 3. Temperature dependence of vMT plots for 1 (s) and 2 (h). The
solid lines are the fit of the data; see the text for the fitting parameters.
ions with a low-lying spin singlet as the ground state.
Thus we have attempted to fit the essential feature of
the experimental susceptibility by using the simple clas-
sical spin approach (H = ꢀJSA Æ SB), to give a fitting
with J = ꢀ8.78 cmꢀ1 and g = 2.49. No spin transition
occurs in 1 while J value correlates well to the related
high spin cobalt(II) dinuclear complexes reported [22].
On the other hand, compound 2 shows a spin-cross-
over phenomenon. The substitution of the metal ion
from cobalt to iron leads to the emergence of the spin-
crossover. This may be due to the difference in the stabil-
ity of the low spin phases. The temperature dependence
of the vMT value is reversible in both the cooling and
warming mode with no hysteresis. The vMT value de-
creases on lowering the temperature, and then it remains
nearly constant at 0.6 emu K molꢀ1, the value of which
corresponds to a mixture of low spin and high spin ions,
with the molar fractions 0.17 and 0.83, respectively. Be-
low 40 K, the vMT value slightly decreases, reaching a
value of 0.2 emu K molꢀ1 at 2 K. This decrease in the
low temperature range may be attributed to the antifer-
romagnetic coupling of the residual HS sites as seen in
the cobalt complex shown above. The spin-crossover
temperature Tc (150 K) of complex 2 is higher than that
of [{Fe(NCS)(py)}2(l-bpypz)2] (127 K) [23]. The de-
tailed comparison in spin-crossover behavior between
these two complexes could be made by the least-squares
fitting of the magnetic susceptibility using the regular
solution model through Eq. (1) [24].
3.3. Magnetic properties
Fig. 3 shows temperature dependence of the magnetic
susceptibilities of the compounds 1 and 2. The vMT va-
lue of 1 in the higher temperature region is about
5.4 emu K molꢀ1, which is larger than the spin only va-
lue for two Co(II) site (3.752 emu K molꢀ1). This behav-
ior is typical of the high spin Co(II) ion in a distorted
octahedral environment, and a large orbital contribu-
tion to the magnetic moment should be considered.
However, at lower temperature the vMT value dramati-
cally decreases in a manner indicative of intramolecular
antiferromagnetic coupling between high spin cobalt(II)
ln½ð1 ꢀ cHSÞ=cHSꢁ ¼ DHHL þ Cð1 ꢀ 2cHSÞ=RT ꢀ DSHL=R;
ð1Þ
where cHS is the high spin fraction, C is an interaction
parameter, and DHHL and DSHL are the enthalpy and
entropy change associated with the spin transitions.
Fig. 2. Powder X-ray diffraction patterns of 1 (a) and 2 (b). (c) shows
the simulated pattern obtained from the crystal structure of 1.

K. Yoneda et al. / Polyhedron 24 (2005) 2437–2442
2441
The cooperative factor of the spin transition is defined
by C = C/2RTc. A good simulation of the magnetic
as exemplified in the comparison between [{Fe(NCX)-
(4-Phpy)}2(l-bpypz)2] since it can be affected by the syn-
ergy between the inter- and intramolecular interactions.
Therefore, further exploration into this series of com-
pounds could give a clue to deeper understanding
spin-crossover phenomena in dinuclear complexes.
susceptibility is achieved with DHHL = 7.18 kJ molꢀ1
,
DSHL = 47.9 J Kꢀ1 molꢀ1, C = 1.08 kJ molꢀ1 and C =
0.433 as indicated by the solid curve in Fig. 3. DHHL
for 2 is larger than that for [{Fe(NCS)(py)}2(l-bpypz)2]
(5.96 kJ molꢀ1
) whereas DSHL is similar to each
other (DSHL = 46.6 J Kꢀ1 molꢀ1 for [{Fe(NCS)(py)}2-
(l-bpypz)2]) [23]. Thus, the difference of Tc with the
substitution of pyridine moiety for [{Fe(NCS)(X-py)}2-
(l-bpypz)2] is attributed in their enthalpy difference.
This isꢀ similar to the replacement of NCSꢀ to
NCBH3 moiety for [{Fe(NCX)(py)}2(l-bpypz)2], but
is different from the case for the series of compounds
[Fe(PM-L)2(NCX)2] in which DSHL is significantly af-
fected by the S/Se substitution of X in NCXꢀ [24]. Inter-
estingly, the cooperative factor C of 2 is smaller than
that of [{Fe(NCS)(py)}2(l-bpypz)2] indicating that the
spin-crossover behavior for 2 is more gradual than that
for [{Fe(NCS)(py)}2(l-bpypz)2] [23]. This may reflect
the difference in the crystal packing structure due to
the substitution from py to 4-Phpy. Moreover, the
spin-crossover in the present dinuclear complex occurs
apparently in terms of a single molecular basis rather
than by intermolecular interactions as claimed for cis-
[Fe(NCBH3)2(phen)2] [25]. On the other hand, we have
very recently revealed that [{Fe(NCBH3)(4-Phpy)}2-
(l-bpypz)2] shows the non-stepwise direct two step
spin-crossover through the intermediate magnetic state
with the coexistence of [HS–HS] and [LS–LS] dinuclear
spin states while the X-ray diffraction patterns show
[{Fe(NCX)(4-Phpy)}2(l-bpypz)2] (X = S, BH3) to be
isomorphous at room temperature [17]. Therefore, the
intermediate magnetic state can be affected by the subtle
change in the molecular structure or the intermolecular
interactions such as p–p stacking interactions.
Acknowledgments
This research was supported by a Grant-in-Aid for
Scientific Research (No. 15550050), and by a Grant-
in-Aid for Scientific Research on Priority Area (No.
417) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
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