Molecular Assemblies and Spin-Crossover Behaviour of Cobalt(II) Complexes with Terpyridine Incorporating Different Nitrogen Positions in Pyridine Rings

Article abstract of DOI:10.1071/CH16556

Cobalt(ii) complexes with terpyridine-type ligands, [Co(n-pyterpy)₂](ClO₄)₂ (n=3 (1), 4 (2)), were prepared and characterised. Different positions of the nitrogen atom in the terpyridine ligands influenced their assembly properties in the crystal structures. Complex 1 showed a 2D network structure consisting of 1D chains connected by intermolecular NHC interactions. On the other hand, complex 2 consisted of two different cobalt ion sites (Co1 and Co2) with slightly different coordination environments. Complex 2 showed 1D chains with no interchain interactions. Such differences are discussed with the cooperativities estimated by their spin crossover behaviours.

Full text of DOI:10.1071/CH16556

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
http://dx.doi.org/10.1071/CH16556
Molecular Assemblies and Spin-Crossover Behaviour of
Cobalt(II) Complexes with Terpyridine Incorporating
Different Nitrogen Positions in Pyridine Rings
A
A
B
A
Risa Nakahara, Manabu Nakaya, Jong Won Shin, Ryo Ohtani,
A
, ,
A C D
Masaaki Nakamura, and Shinya Hayami
A
Department of Chemistry, Graduate School of Science and Technology, Kumamoto
University, 2-39-1 Kurokami, Kumamoto, 860-8555, Japan.
B
Daegu-Gyeongbuk Branch, Korea Institute of Science and Technology Information,
9
0 Yutongdanji-ro, Buk-gu, Daegu 41515, South Korea.
Institute of Pulsed Power Science (IPPS), Kumamoto University, 2-39-1 Kurokami,
Chuo-ku, Kumamoto, 860-8555, Japan.
C
D
Corresponding author. Email: hayami@sci.kumamoto-u.ac.jp
Cobalt(II) complexes with terpyridine-type ligands, [Co(n-pyterpy) ](ClO ) (n ¼ 3 (1), 4 (2)), were prepared and
2
4 2
characterised. Different positions of the nitrogen atom in the terpyridine ligands influenced their assembly properties
in the crystal structures. Complex 1 showed a 2D network structure consisting of 1D chains connected by intermolecular
N?HC interactions. On the other hand, complex 2 consisted of two different cobalt ion sites (Co1 and Co2) with slightly
different coordination environments. Complex 2 showed 1D chains with no interchain interactions. Such differences are
discussed with the cooperativities estimated by their spin crossover behaviours.
Manuscript received: 4 October 2016.
Manuscript accepted: 12 November 2016.
Published online: 14 December 2016.
Introduction
0
0
00
Since metal complexes consisting of 2,2 :6 ,2 -terpyridine
(terpy) ligands (or their derivatives) show various physical
Spin crossover (SCO) compounds show bistability in their
magnetic, optical, and structural properties, which have poten-
tial applications in switching and sensing devices. Bistability
is a necessary property in these compounds, which must be
[9]
[10]
properties such as redox activities and photoluminescence
[
1]
that depend on both metal ions and the nature of substituent
groups, we focussed on complexes with these ligands. A large
number of cobalt(II) SCO compounds consisting of terpy ligand
derivatives have produced a suitable ligand-field for SCO
phenomenon. Thus cobalt(II) compounds with terpy deriva-
tives incorporating various organic substituents such as
have been previously
reported. In addition, counter anions in metal complexes have
played an important role in controlling the cooperativity and
SCO behaviour.
We have previously reported 1D terpy cobalt(II) complexes,
n
stable in two distinct electronic states. Numerous d (n ¼ 4–7)
transition metal compounds exhibiting SCO have been reported
and their spin state changes can be induced by the variation of
[11]
[
2]
temperature, pressure, or light illumination. SCO phenomena
are often observed in cobalt(II), iron(II), and iron(III) com-
pounds. SCO behaviours of numerous cobalt(II) complexes
between low spin (LS) (S ¼ 1/2) and high spin (HS) (S ¼ 3/2)
states have been observed, which, accompanied by entropy
[
8]
[9]
alkyl chain, ferrocene and so on
[12]
[12,13]
changes, gives the following equation, DS ¼ R[ln(2S þ 1)
spin
ꢀ
1
ꢀ1 [3]
0
0
0
HS ꢀ ln(2S þ 1)LS] ¼ 5.8 J K mol
.
is smaller than SCO compounds of iron(II) (13.4 J K mol
This entropy change
)
[Co(pyterpy)X ]ꢁsolvent (pyterpy ¼ 4 -(pyridin-4-yl)-2,2 :6 ,
2
ꢀ1
ꢀ1
0
2 -terpyridine; X ¼ Cl or Br; solvent ¼ MeOH or 2H O), which
2
ꢀ
1
or iron(III) (9.1 J K mol ).
ꢀ1 [4]
have shown spin state change depending on the solvent mole-
[
14]
The gradual or abrupt spin transitions are generally observed
in the solid state and depend on cooperativity from intermole-
cules. The mononuclear cobalt(II) complex, [Co(pyterpy)2]
(PF ) , also exhibited SCO behaviour with thermal hysteresis
6
2
[
5]
[15]
cular interactions. Cooperativities in the compounds play an
important role to control the magnetic behaviour. Molecules that
are arranged by intermolecular interactions such as hydrogen
arising from a phase transition.
Herein, we focussed on the nitrogen position in the pyridine
ring and investigated the assembly nature and the effect
of cooperativities on the magnetic properties of two terpy
cobalt(II) complexes. The cobalt(II) compounds, [Co(n-pyterpy)2]
(ClO ) (n ¼ 3 (1) or 4 (2)) were prepared and their single-
[6]
[7]
[8]
bonds, p–p interactions, fastener effects, and so on lead to
an increase of the cooperativities and produce unique magnetic
behaviour. Thus, controlling the assembly of SCO compounds
would be an important key to obtain cooperative behaviour in
SCO compounds.
4
2
crystal X-ray structure determined as a basic method of
characterisation.
Journal compilation � CSIRO 2016
www.publish.csiro.au/journals/ajc

B
R. Nakahara et al.
Results and Discussion
cobalt(II) ion sites, of which Co1 and Co2 site are dimerised by
CH–p interactions between pyridine rings in the terpyridine
Both terpyridine ligands, 3-pyterpy (L1) and 4-pyterpy (L2) were
prepared by literature methods (Fig. 1; see Experimental). The
complexes, [Co(3-pyterpy) ](ClO ) (1) and [Co(4-pyterpy) ]
˚
ligands (2.59 A, PrH (in Co1 site)–Py (in Co2 site)). Co–N bond
lengthsin theCo1site are Co(1)–N(1) ¼ 2.147(2), Co(1)–N(2) ¼
2
4 2
2
1
.921(2), Co(1)–N(3) ¼ 2.144(2), Co(1)–N(5) ¼ 1.989(2),
˚
(ClO ) (2), formed crystals suitable for structure determination
4 2
Co(1)–N(6) ¼ 1.859(2), and Co(1)–N(7) ¼ 1.993(2) A, whereas
by slow diffusion by using a H-tube at room temperature. Brown
block shaped crystals of 1ꢁ2MeOH and 2ꢁMeOH were obtained
following diffusion in MeOH and were fully characterised
by elemental analyses and single crystal X-ray diffraction
those in the Co2 site are Co(2)–N(9) ¼ 2.066(2), Co(2)–N(10) ¼
1
.891(2), Co(2)–N(11) ¼ 2.074(2), Co(2)–N(13) ¼ 2.070(2),
˚
Co(2)–N(14) ¼ 1.889(2),
and
Co(2)–N(15) ¼ 2.057(2)A.
Obtaining a non-solvated single crystal of 2 was also not
successful for X-ray structure determination due to the same
reason as for complex 1. The two cobalt ion sites in 2ꢁMeOH had
different assembling natures. At the Co1 site, intermolecular
NꢁꢁꢁHC bonds between the pyridine substituent and the middle
(XRD) determinations. Single crystal XRD measurements for
1
ꢁ2MeOH and 2ꢁMeOH were carried out at 100 K (Figs 2 and 3)
and their crystallographic data are listed in the Experimental.
Compound 1ꢁ2MeOH was crystallised in the monoclinic
space group P2 and its molecular structure is shown in Fig. 2a.
1
˚
pyridine ring in the terpyridine ligand were observed (2.48 A),
The Co–N bond lengths are Co–N(1)¼ 2.022(4), Co–N(2) ¼
while the side pyridine ring formed a NꢁꢁꢁHC bond with the
1
1
.877(3), Co–N(3) ¼ 2.019(4), Co–N(5) ¼ 2.113(4), Co–N(6) ¼
˚
pyridine substituent in the Co2 site (2.65 A) (Fig. 3b). In the
˚
.908(3), and Co–N(7) ¼ 2.118(4) A. It was not possible to
crystal packing of 2, the two molecules containing Co1 and Co2
sites produced independent 1D columns. Each 1D chain came
into short contact by pyridine rings along the b-axis (Fig. 3c), and
the 1D chains were arrayed along the a-axis, in which the 1D
chains of Co1 and Co2 sites arranged along the a-axis consisting
obtain a non-solvated single crystal of 1, since it was easily
cracked at room temperature. In compound 1, intermolecular
˚
NꢁꢁꢁHC bonds between pyridine substituents (2.58 A) were
found (Fig. 2b). The NꢁꢁꢁHC bond networks were extended
along the c-axis, in which 1 formed 1D network structures
(
of layers. MeOH molecules existed betweenindividual 1D chains
ꢀ
Fig. 2c). Furthermore, the 1D chains were assembled by p–p
and ClO counter anions existed between the layers (Fig. 3d).
4
interactions at the terpyridine cores and hence they formed 2D
assemblies consisting of 1D chains. The 2D assemblies were
stacked along the b-axis through CH–p interactions (Fig. 2d).
2ꢁMeOH was crystallised in the monoclinic space group P-1
and its molecular structure is shown in Fig. 3a. It consists of two
Since complexes 1 and 2 show different molecular assem-
blies which affects the cooperativity influencing their SCO
behaviour, their magnetic susceptibility measurement was per-
formed using a superconducting quantum interference device
(SQUID) and the cooperative factors, C, also estimated. The
x T versus T curves (x , molar magnetic susceptibility; T,
m
m
N
N
temperature) for 1ꢁ2MeOH and 2ꢁMeOH are shown in Fig. 4.
Both compounds showed gradual SCO behaviour. In compound
1
ꢁ2MeOH, stepwise SCO behaviour was observed around 320 K
upon heating. The solvent molecules in the crystal were
removed during the cooling process but the stepwise SCO
behaviour was still observed. In compound 2ꢁMeOH, stepwise
SCO behaviour was also observed around 350 K upon heating
although it was incomplete and gradual. On cooling, however, 2
showed gradual SCO behaviour. This stepwise behaviour of
2ꢁMeOH seems to be observed due to the removal of solvent
N
N
N
N
N
N
3-pyterpy (L1)
4-pyterpy (L2)
Fig. 1. Schematic structure of the ligands, L1 and L2.
(a)
(b)
N7
Co
N1
N8
N2
N6
N4
2.580 Å
N3
N5
(c)
(d)
o
a
o
c
b
a
ꢀ
4
counter anions, and solvent molecules are omitted for clarity. Colour
Fig. 2. (a) Molecular structures of 1ꢁ2MeOH. Hydrogen atoms, ClO
code: Co, magenta; N, blue; C, grey. (b) NꢁꢁꢁHC bonds between neighbouring pyridine substituents. (c) 2D structure of 1 at the ac plane.
ꢀ
(
d) Stacking structures of 2D assemblies of 1. Blue coloured molecules are MeOH and pink ClO
4
, respectively.

Molecular Assemblies in Terpyridine Cobalt Complex
C
(a)
(b)
Co1 site
Co2 site
N9
N13
2.483 Å
N14
N10
N16
N12
Co2
N15
N5
N11
N3
N6
2.651 Å
N2
Co1
N1
N8
N4
N7
(c)
(d)
o
a
o
c
b
b
ꢀ
4
counter anions and solvent molecules are omitted for clarity. Colour code:
Fig. 3. (a) Molecular structure of 2ꢁMeOH. Hydrogen atoms, ClO
Co, magenta; N, blue; C, grey. (b) Dimerisation of molecule 2. (c, d) Molecular packing structure of 2. Red coloured molecules show Co1 sites
ꢀ
and green coloured molecules Co2 sites. Blue coloured molecules are MeOH and pink ClO
4
, respectively.
(a)
2.0
1.5
1.0
0.5
0
(b) 2.0
1.5
1.0
0.5
0
0
100
200
300
400
0
100
200
300
400
Temperature [K]
Temperature [K]
Fig. 4.
x
modes, respectively. Black solid lines indicate the fit by the regular solution models.
m
T versus T plots for the compound 1ꢁ2MeOH (a) and compound 2ꢁMeOH (b). Red and blue circles show heating and cooling
molecules from the crystals, while the solvent molecules in
1
Table 1. DH, DS, T1/2, and C values obtained from the regular solution
model
ꢁ2MeOH did not influence the SCO behaviour.
To estimate the cooperativity of SCO behaviour, the experi-
mental x T versus T curves were simulated by applying the
ꢀ1
ꢀ1
DS [J K ]
Compound
DH [J mol
]
T
1/2 [K]
C
m
regular solution model (see Experimental). The simulated para-
meters for the compounds before and after removal of solvent
molecules are shown in Table 1. The cooperative factors in
1
1
2
2
ꢁ2MeOH
3261.5
1904.8
5390
9.79
5.54
333
344
345
300
0.78
0.75
0.62
0.15
ꢁMeOH
15.6
43.2
1
ꢁ2MeOH and 1 were almost the same. 2D networks consisting
12962.6
of 1D chains of 1 were stacked through CH–p interactions,
which revealed that the removal of MeOH solvent did not
influence the cooperativity of 1. Since molecules of 2 experi-
enced intermolecular interactions through MeOH, it seems
they could not interact with each other after removal of MeOH.
ꢀ
The ClO counter anions also prevent interactions between the
4
Co1 and Co2 sites. Therefore, the cooperative factor seems to
significantly decrease after removal of solvents.

D
R. Nakahara et al.
Conclusion
distance was 63 mm, omega scan; Dv ¼ 18, exposure time was
[
17]
1
s per frame) and HKL3000sm (ver. 703r) was used for cell
Cobalt(II) compounds with a different nitrogen position in the
pyridine ring, [Co(3-pyterpy) ](ClO ) (1) and [Co(4-pyterpy) ]
refinement, reduction, and absorption correction. XRD data for
single crystals of 2ꢁMeOH were collected with a Rigaku R-AXIS
RAPID 191R diffractometer. Crystal evaluation and data col-
lection were performed using Cu Ka radiation (l 1.54187 A˚ )
with a detector-to-crystal distance of 1.91 cm. The crystal
2
4 2
2
(ClO ) (2), showed different molecular assemblies depending
4 2
on the pyridine ring. Complex 1 showed a 2D assembly con-
sisting of 1D chains, while 2 was formed as individual 1D chain
assemblies with each Co1 and Co2 site separated by ClO4
anions. The cooperativities estimated by their magnetic beha-
viours reflected the assembly natures, in which the nitrogen
position in the pyridine substituent has an important role con-
trolling the molecular assemblies. The slight differences, such as
nitrogen position in a molecule, seem to play an important role in
the construction of supramolecules.
ꢀ
[18]
structures were solved by direct methods, and refined by full-
matrix least-squares refinement using the SHELXL-2014 com-
[19]
puter program. The positions of all non-hydrogen atoms were
refined with anisotropic displacement factors. All hydrogen
atoms were placed using a riding model, and their positions were
constrained relative to their parent atoms using the appropriate
HFIX command in SHELXL-2014.
X-ray crystallographic data for 1ꢁ2MeOH at 100 K
Experimental
(C H O N Cl Co): FW 942.62; brown block crystals, mono-
clinic, space group P2 , a 8.5800(17), b 12.097(2), c 19.706(4) A,
42 36 10 8 2
The preparation of ligands L1 and L2 and compounds 1 and 2
were essentially carried out according to the literature with
slight modifications.
able and used without further purification.
˚
ꢀ1
1
˚
3
ꢀ3
b 99.22(3)8, V 2018.9 A , Z 2, D 1.551 g cm , m 0.453mm
,
[14]
calc
All reagents were commercially avail-
R 0.0528 for I . 2s(I), R 0.0627, and wR 0.1375 for all data.
1
1
1
X-ray crystallographic data for 2ꢁMeOH at 100 K
(
C H O N Cl Co): FW 910.59; brown block crystals, triclin-
1
4
32
9
8
2
Synthesis
¯
˚
ic, space group Pı, a 10.6786(8), b 17.909(2), c 21.752(1) A, a
3
-Pyterpy (L1)
˚
02.569(4)8, b 90.237(3)8, g 107.244(3)8, V 3867.2 A , D
calc
1.564 g cm , m 0.652 mm , R 0.0399 for I . 2s(I), R 0.0747,
3
1
ꢀ3
ꢀ1
2-Acetylpyridine (4.8 g, 40 mmol) was added to a solution of
pyridine-3-carbaldehyde (2.2 g, 20 mmol) in EtOH (10 mL).
1
and wR 0.1062 for all data.
NaOH (2.0 g, 50 mmol in 10 mL H O) and ammonium acetate
2
(2.3 g, 30 mmol in 10 mL H O) were slowly added to the mixture
2
Elemental Analysis
and stirred overnight at ambient temperature. The off-white
solid was collected by filtration and washed with H O
Elemental analyses were carried out with a J-SCIENCE LAB-
ORATORY JM10 analyser at the Instrumental Analysis Centre
of Kumamoto University.
2
(
3 ꢂ 15 mL) and EtOH (3 ꢂ 15 mL). d (500 MHz, CDCl3)
H
9
7
.03 (d, 1H), 8.62–8.55 (m, 7H), 8.09 (d, 1H), 7.79 (t, 2H),
.34 (t, 1H), 7.25 (t, 2H).
Magnetic Susceptibility Measurements
Temperature-dependent magnetic susceptibilities for 1ꢁ2MeOH
and 2ꢁMeOH were measured on a SQUID magnetometer at field
strengths of 1 T with a sweep mode in the temperature range 5 to
4
-Pyterpy (L2)
L2 was prepared by a similar method as described for L1,
except for using of pyridine-4-carbaldehyde instead of pyridine-
4
00 K. Crystalline samples were put into a gelatine capsule,
3
2
-carbaldehyde. dH (500 MHz, CDCl ) 8.71 (d, 4H), 8.68 (d,
3
mounted inside the straw, and then fixed to the end of the sample
transport rod.
H), 8.62 (d, 2H), 7.86 (t, 2H), 7.57 (d, 2H), 7.30 (t, 2H).
[
Co(3-pyterpy) ](ClO ) (1)
Regular Solution Model
2
4 2
ꢀ5
L1 (20 mg, 6.5 ꢂ 10 mol) was placed into one sidearm of an
Cooperativity was estimated from the measured x T versus T
m
ꢀ
5
H-tube. Co(NO ) ꢁ6H O (7.2 mg, 3.3 ꢂ 10 mol) and NaClO4
curves (x ; molar magnetic susceptibility, T; temperature) by
m
3
2
ꢀ
2
5
[20]
applying the regular solution model (Eqn 1), where DH, DS,
(
7.9mg, 6.5 ꢂ 10 mol) were placed into the other side, and
MeOH was gently layered over both sides to fill the H-tube
and the system left at ambient temperature to produce brown
crystals suitable for X-ray structure determination. Anal. Calc.
for [Co(3-pyterpy) ](ClO ) ꢁ2MeOH (C H CoN Cl O ):
and G are the enthalpy and the entropy variations, and the
parameter accounting for cooperativity based on SCO, respec-
tively. The HS molar fraction, gHS, is shown as a function of the
magnetic susceptibility through Eqn 2, where (x T) is the x T
2
4 2
42 36
8
2
10
m
m
m
C 53.52, H 3.85, N 11.89. Found: C 53.30, H 3.74, N 11.52 %.
value at any temperature, (x T) and (x T) are the pure LS
m HS m LS
ꢀ
1
and HS states, respectively. R is the gas constant, 8.314 J K
ꢀ
1
mol . The cooperativity value, C, is given by Eqn 3.
[
Co(4-pyterpy) ](ClO ) ꢁ(2)
2
4 2
Compound 2 was prepared by the same method as 1 except
using L2 instead of L1. Anal. Calc. for [Co(4-pyterpy)2]
ln½ð1 ꢀ gHSÞ=ðgHSÞꢃ ¼ ½fDH þ Gð1 ꢀ 2gHSÞg=RTꢃ ꢀ DS=R ð1Þ
(
1
ClO ) ꢁMeOH (C H CoN Cl O ): C 54.08, H 3.54, N
4
2
41 32
8
2
9
ꢀ
ꢁ
2.31. Found: C 54.24, H 3.35, N 12.49 %.
gHS ¼ ðwmTÞm ꢀ ðwmTÞLS =½ðwmTÞHS ꢀ ðwmTÞLSꢃ
ð2Þ
ð3Þ
Single Crystal Structural Analysis
C ¼ G=ð2RT1=2Þ; T ¼ DH=DS
1=2
A crystal of 1ꢁ2MeOH was coated with paratone-N oil and the
diffraction data measured at 100(2) K with synchrotron radia-
˚
tion (l 0.63000 A) on an ADSC Quantum-210 detector at 2D
SMC with a silicon (111) double crystal monochromator (DCM)
Acknowledgements
This work was supported by a JSPS Grant-in-Aid for Young Scientists (B)
15K17833 and a JSPS Grant-in-Aid for Scientific Research on Innovative
Areas (Dynamical Ordering & Integrated Functions) 16H00777. This work
at the Pohang Accelerator Laboratory, Korea. The PAL BL2D-
[
SMDC program
16]
was used for data collection (detector

Molecular Assemblies in Terpyridine Cobalt Complex
E
was also supported by a KAKENHI Grant-in-Aid for Scientific Research (B)
(b) S. Hayami, R. Moriyama, A. Shuto, Y. Maeda, K. Ohta, K. Inoue,
Inorg. Chem. 2007, 46, 7692. doi:10.1021/IC700754S
(c) Y. Komatsu, K. Kato, Y. Yamamoto, H. Kamihata, Y. H. Lee,
A. Fuyuhiro, S. Kawata, S. Hayami, Eur. J. Inorg. Chem. 2012, 2769.
doi:10.1002/EJIC.201101040
26288026. This work was partially supported by the Cooperative Research
Program of ‘Network Joint Research Centre for Materials and Devices’. M.N.
isgratefultoJSPSResearchFellowshipsfor YoungScientists(No. 15J11333).
[
9] K. Takami, R. Ohtani, M. Nakamura, T. Kurogi, M. Sugimoto, L. F.
Lindoy, S. Hayami, Dalton Trans. 2015, 44, 18354. doi:10.1039/
C5DT02592C
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