Spin Crossover and Light-induced Excited Spin-state Trapping Observed
for an Iron(II) Complex Chelated with Tripodal Tetrakis(2-pyridyl)methane
Naoki Hirosawa, Yuya Oso, and Takayuki Ishida*
Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585
(Received March 4, 2012; CL-120187; E-mail: ishi@pc.uec.ac.jp)
A novel iron(II) complex [Fe(py
4
C)
2
][Fe(py
4
C)(NCS)
3
]
2
was synthesized, where py
4
C stands for tetrakis(2-pyridyl)-
methane. Spin crossover occurred at 161 K (T
1/2
). Light-induced
excited spin-state trapping was observed when irradiated with
532 nm at 10 K, and the conversion was 89%. The relaxation
took place at 58 K on heating.
Spin crossover (SCO) is a reversible low-spin-high-spin
transition by external stimuli such as heat and light.
1,2
Although
various bis-tripodaliron(II) SCO complexes involving hydro-
tris(1-pyrazolyl)borate (pz
3
BH
¹
) have well been studied,
3
the
corresponding tris(1-pyrazolyl)methane (pz
3
CH) compounds are
rather rare (Scheme 1).
4
To our knowledge, there has been only
one report on tris(2-pyridyl)methane (py
3
CH) complex toward
development of SCO compounds
5
(the 2-pyridyl group is
abbreviated as py hereafter).
Oda and co-workers have exploited a unique skeleton
tetrakis(2-pyridyl)methane (py
4
C),
6
which can serve as bi- and
tridentate chelating and/or bridging ligands, and actually the
Ag
+
, Cu
2+
, Co
2+
, and Fe
2+
complexes were synthesized.
7
The
Co
2+
complex exhibited SCO behavior.
7b
We have reported
py
4
C-bridged dinuclear Mn
2+
and Ni
2+
complexes
8
from the
magnetic interest owing to the spiro-junctioned D
2d
symmetry.
For the development of SCO materials, [Fe(py
4
C)
n
]
2+
com-
pounds will be an attractive target. We will report here distinct
SCO and LIESST (light-induced excited spin-state trapping)
9
behavior on a novel iron(II) coordination compound [Fe-
(py
4
C)
2
][Fe(py
4
C)(NCS)
3
]
2
(1).
Compound 1 was synthesized by simplymixing methanol
solutions (50 mL in total) containing FeCl
2
¢4H
2
O (31 mg, 0.16
mmol), LiNCS (60%, 56 mg, 0.52 mmol), and py
4
C (62 mg,
0.19 mmol) in the presence of L-ascorbic acid (5 mg) under
nitrogen. The mixture was allowed to stand in a refrigerator for
three days, giving 1 as reddish orange polycrystalline precip-
itates. The products were separated on a filter and dried under
reduced pressure. The yield was 61%. The elemental analysis
supported the formula, and the IR spectrum showed absorptions
characteristicof thiocyanate and py
4
C.
10
The X-ray crystal structure of 1 was determined at
100 K.
11
Figure 1 shows the molecular structures of the [Fe-
(py
4
C)(NCS)
3
]
¹
and [Fe(py
4
C)
2
]
2+
moieties in 1. The Fe1 ion is
surrounded by the sixnitrogen atoms from one tridentate py
4
C
ligand and three NCS anions. The Fe1-Ndistances are 1.924(3)-
1.984(3) ¡, which are characteristic of a low-spin Fe
2+
N
6
coordination structure.
12
The Fe2 ion in [Fe(py
4
C)
2
]
2+
is located
at the crystallographic inversion center and surrounded by the
sixnitrogen atoms from two py
4
C ligands. The six py rings are
approximately related with an S
6
symmetry, but the actual space
group is P
1 owing to the peripheral py group. The Fe2-N bond
lengths (1.952(3)-1.979(3) ¡) fell in a typical range of a low-
spin Fe
2+
N
6
octahedron.
12
The hydrogen atoms were exper-
imentally found, and the uncoordinated py groups were proven
to show no disorder.
After being heated to 240 K, the X-ray crystallographic
analysis on the same crystal was also successful, thanks to
asingle-crystal-to-single-crystal structural transformation. The
bond lengths are somewhat elongated in the same space group;
Scheme 1. Structuralformulas of pz
3
BH
¹
, pz
3
CH, py
3
CH, and
py
4
C.
(a)
(b)
Figure 1. X-ray crystal structure of (a) [Fe(py
4
C)(NCS)
3
]
¹
and (b) [Fe(py
4
C)
2
]
2+
moieties in 1 measured at 100 K. Thermal
ellipsoids are drawn at the 50% probability level. Hydrogen
atoms are omitted. Selected bond distances at 100 K: Fe1-N1,
1.957(3); Fe1-N2, 1.984(3); Fe1-N3, 1.924(3); Fe1-N5,
1.960(4); Fe1-N6, 1.973(3); Fe1-N7, 1.963(3); Fe2-N8,
1.979(3); Fe2-N9, 1.964(3); Fe2-N10, 1.952(3) ¡. At 240 K:
Fe1-N1, 2.195(4); Fe1-N2, 2.286(4); Fe1-N3, 2.133(4); Fe1-
N5, 2.075(5); Fe1-N6, 2.148(4); Fe1-N7, 2.094(4); Fe2-N8,
1.980(4); Fe2-N9, 1.961(3); Fe2-N10, 1.949(3) ¡.
Published on the web June 30, 2012 716
doi:10.1246/cl.2012.716
© 2012 The Chemical Society of Japan Chem. Lett. 2012, 41, 716-718 www.csj.jp/journals/chem-lett/