Ligand field tuned spin crossover for an iron(II)-di(diamine) system

Article abstract of DOI:10.1016/j.ica.2016.05.020

Investigation of spin-crossover in iron(II)-di(diamine) system, based on ligands L₁, 4,5-dimethy-2-(pyridine-2-yl)imidazole, has been presented in this paper. The methyl group at the adjacent sites of the donor atoms in ligand L₁ can generate the crossover situation for the iron(II) tris(diimine) system. While for the iron(II)-di(diamine) system with L₁, the results are complicated: complex 1 {Fe(L₁)₂(SCN)₂} displays paramagnetic behavior while 2 {Fe(L₁)₂(SeCN)₂} is in crossover situation. The difference was attributed to the various ligand field strength of SCN^- and SeCN^-, which also resulted to the blue-shift of IR and Raman spectra, as well as red-shift and decrease in molar absorptivity of UV-Vis absorption spectrum.

Full text of DOI:10.1016/j.ica.2016.05.020

Accepted Manuscript
Ligand Field Tuned Spin Crossover for an Iron (II)-di(diamine) System
Yang-Hui Luo, Lan Chen, Jing-Wen Wang, Ming-Xin Wang, Ya-Wen Zhang,
Bai-Wang Sun
PII:
S0020-1693(16)30249-3
http://dx.doi.org/10.1016/j.ica.2016.05.020
ICA 17056
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
6 January 2016
24 April 2016
9 May 2016
Please cite this article as: Y-H. Luo, L. Chen, J-W. Wang, M-X. Wang, Y-W. Zhang, B-W. Sun, Ligand Field Tuned
Spin Crossover for an Iron (II)-di(diamine) System, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/
j.ica.2016.05.020
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Ligand Field Tuned Spin Crossover for an Iron (Ⅱ)-di(diamine) System
Yang-Hui Luo, Lan Chen, Jing-Wen Wang, Ming-Xin Wang, Ya-Wen Zhang and Bai-Wang Sun*
School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 210096, P. R. China,
Fax: 86-25-52090614; Tel: 86-25-52090614; E-mail: chmsunbw@seu.edu.cn
Abstract: Investigation of spin-crossover in iron (Ⅱ)-di(diamine) system, based on ligands L
1
, 4,
5
-dimethy-2-(pyridine-2-yl)imidazole, has been presented in this paper. The methyl group at the adjacent
sites of the donor atoms in ligand L
1
can generate the crossover situation for the iron (Ⅱ) tris(diimine)
, the results are complicated: complex 1
system. While for the iron (Ⅱ)-di(diamine) system with L
Fe(L (SCN) } displays paramagnetic behaviour while 2 {Fe(L
1
{
1
)
2
2
1
)
2
(SeCN)
2
} is in crossover situation. The
-
-
difference was attributed to the various ligand field strength of SCN and SeCN , which also resulted to the
blue-shift of IR and Raman spectra, as well as red-shift and decrease in molar absorptivity of UV-vis
absorption spectrum.
1. Introduction
As is one of the most impressive examples of molecular bi-stability and potential application in
memory, molecular switch, sensor devices and intelligent contrast agents, the design and synthesis
of molecular architectures displaying spin crossover (SCO) properties has attracted great attention
1
5
over the past decade. The spin transition of a SCO system between high-spin (HS, T
2
) and
1
4
7
low-spin (LS, A
1
) electronic states of an octahedral 3d -3d transition metal complexes scan be
induced under such external perturbations as light irradiation, variation in temperature/pressure, or
2
magnetic/electric fields, and the resulted SCO behaviours are varied as complete, incomplete,

3
abrupt, gradual, one-step, multi-step transitions, or hysteretic. To date over 200 SCO systems, many
II
4
II
of them are interesting mononuclear Fe complexes with N-heterocyclic ligands or polymeric Fe
5
III 6
II 7
III 2b
II 8
complexes with triazole or pyrazine ligands, as well as some Fe , Co , Mn
and Cr
complexes, have been reported. Most of these reported SCO studies were centred on detailed
structural analysis along with magnetic studies duo to the key relationship between solid state
structural features (intra- and intermolecular interactions) and lattice cooperativity (the extent to
which spin-state changes are propagated in the solid-state). For instance, the intermolecular
9a
interactions promoting effective cooperativity,
the existence or absence of solvent molecules
9
b
turning the spin transition behaviour of the metal centre, and the counter anions triggering SCO
9c
transition through supramolecular host-guest binding.
Scheme 1. Molecular structures of complexes 1 and 2.
The iron (Ⅱ) tri(diimine) system, which act as one of the most common models for spin crossover
behavior due to their important feature that they can be modified readily to tune the ligand-field strength,
10
have attracted much attention in the past years.
While among the diimine system, 2-(pyridin-2-yl)
imidazole-related ligands has occupied a special position as ideal candidate for probing and understanding
11
2+
the fundamental science of spin crossover.
The [FeN
6
]
derivative of 2-(pyridin-2-yl)imidazole was
1
2
shown continuous above room temperature spin transition both in solid state and in solution, which was
1
3
relatively early on to be a crossover system, thus, an appropriate variation in either their σ-donor and/or
2
+
π-acceptor character of this ligand may result in crossover situation in this [FeN
6
]
systems. In our other

1
4
work, we introduced methyl group at the adjacent sites of the donor atoms ligand 2-(pyridin-2-yl)imidazole,
which generated ligand L (4, 5-dimethy-2 -(pyridine-2-yl)imidazole) and we have made a complete
complexes formation of ligand L with different Fe (Ⅱ) salts (Fe(ClO , Fe(BF , Fe(PF and Fe(SbF ).
PPMS results revealed that these [FeN were all shown crossover situation, which display
counter-anions-dependent transition temperatures due to interactions between counter-anions and L
1
1
4
)
2
4
)
2
4
)
2
)
4 2
2
+
6
] systems with L
1
1
. Hence
-
in this work, we investigated the iron (Ⅱ) di(diimine) system with L
1
by introduce of ligands field of SCN
-
and SeCN , and we have obtained two new complexes {Fe(L
1
)
2
(SCN) } (1) and {Fe(L (SeCN)
2
1
)
2
2
} (2)
(
Scheme 1). PPMS results revealed that the 1 displays paramagnetic behaviour while 2 is in complete
crossover situation. Single-crystal X-ray diffraction, PXRD, TGA, Raman and IR spectra as well as UV-vis
absorption measurements (Supporting Information) were performed.
Figure 1. Crystal structures and connecting motif of complex 1, hydrogen atoms are omitted for clarity: (a)
II
The ASU of complex 1; (b) The “di-Fe ” sites packed by C-H…π and π…π contacts; (c) The 1D connecting
motif of complex 1.
2. Results and discussion
2
.1 Crystal structure of 1 and 2
Complex
crystallized as yellowish-brown micro-crystals. The crystal structures of complexes
characterized by single-crystal X-ray diffraction, and the phase purity of complex
by powder X-ray diffraction. The crystal data and structure refinement for complex
1
was crystallized from methanol solution as yellow block crystals, complex
2
was
was
1
2
was confirmed
was listed in
1

II
Table S1. Complex
1
crystallizes in triclinic P-1 space group, the Fe ion adopts a distorted
-
octahedral geometry formed by four N atoms of the ligands
L
1
and two N atoms of SCN , and it
II
displays a “batterfly-like” motif (Figure 1a). Every two “batterfly” motif form “di-Fe ” sites through
C-H…π (C…C distance 3.312 Ǻ) and π...π (centre…centre distance 3.854 Ǻ) contacts between
II
adjacent
L
1
ligands (Figure 1b). The “di-Fe ” sites further packed into 1D chains through C-H…π
and π...π contacts (Figure 1c), and the 1D chains then stacked into3D motif (Figure S1). For the
“
batterfly-like” unit, the average Fe-N distance is 2.18 Ǻ (Table 1), and the distortion parameters
1
5
II
∑
is 88.05° (Table 2), typical values for a HS Fe atom.
Table 1. The bond length (Ǻ) of the distorted octahedral geometry for 1
Fe1-N1
.197
Fe1-N2
2.229
Fe1-N3
2.129
Fe1-N4
2.202
Fe1-N5
2.113
Fe1-N6
2.212
Mean
2
2.18033333
Table 2. The bond angles (º) and distortion parameter (∑/º)of the distorted octahedral geometry for 1
N1-Fe1-N2 N1-Fe1-N3 N1-Fe1-N4 N1-Fe1-N5 N2-Fe1-N3 N2-Fe1-N4 N2-Fe1-N6
1
06.52
N3-Fe1-N5 N3-Fe1-N6 N4-Fe1-N5 N4-Fe1-N6 N5-Fe1-N6
8.42 87.84 90.41 106.36 91.3
90.25
75.8
87.3
93.13
82.11
∑
75.29
9
88.05
Figure 2. PXRD profiles of complexes 1 and 2.

Shown in Figure 2 were the PXRD profiles of complex
1 and 2 in 5-40° range, which revealed
that these two complexes shown almost identical crystal lattice, demonstrating that the different
-
-
magnetic properties between
1 and 2 may attribute to various ligand field of SCN and SeCN rather
than the crystal lattice or intermolecular interactions.
Figure 3. The χ
m
T versus T plots of the direct-current (dc) magnetic measurement for complexes 1 and 2
over the temperature range 400-5 K.
2
.2 Magnetic properties
The variable-temperature direct-current (dc) magnetic-susceptibility measurements for crystalline
samples of complexes and were performed on a PPMS under 2000Oe in temperature range
00-5 K. Shown in Figure 3 were the χ shows χ T = 3.11
1
2
4
m
T versus T plots of them. Complex
1
m
3
-1
II
cm mol K at the room temperature, typical value for a HS Fe atom. Upon cooling, this value kept
3
-1
constant until 50 K, then it undergo a gradual decrease to χ T = 2.92 cm mol K at 25 K, followed
m
3
-1
by a drop to 1.82 cm mol K at 3 K owing to the combined effect of spin-orbit coupling and
II
zero-field splitting of the HS Fe ion. The χ
m
T versus T plots of complex 2 shows a significant
-
-
different behaviour with
1
when the SCN was changed to SeCN , the latter possess a larger ligand
3
-1
field strength than the former one. Complex
2
shows χ T = 3.05 cm mol K at the upper
m

II
temperature limit, typical value for a HS Fe atom. Upon cooling, the χ T versus T curve displays a
m
complete one-step spin transition. The χ
m
T values undergo a gradual decline to reach a plateau close
3
-1
II
to 0cm mol K at around 120 K, indicating a complete LS Fe sites. The spin transition temperature
-
T
1/2 for complex
2
is 214 K. Thus, the combination of ligand
L
1
with SeCN can provided appropriate
II
ligand-field strength for a SCO mononuclear Fe site.
3. Conclusions
-
In conclusion, the combination of ligand L
1
(4, 5-dimethy-2-(pyridine-2-yl)imidazole) with ligand SeCN can
generated the crossover situation for iron (Ⅱ)-di(diamine) complex 2 {Fe(L
1
)
2
(SeCN) }, while the
2
-
combination of ligand L
1
with ligand SCN cant’ provide appropriate ligand-field strength for a SCO
II
mononuclear Fe site thus resulted paramagnetic behaviour for complex 1{Fe(L
1
)
2
(SCN) }. The stronger
2
-
-
ligand field strength of SeCN than SCN shown pronounced effect on the spin transition, blue-shift of
Raman spectra as well as the red-shift and decrease in molar absorptivity of UV-vis absorption spectrum for
complexes 1 and 2.
4. Experimental
4
.1 Materials and physical measurements
All syntheses were performed under ambient conditions. Fe(ClO
4
)
2
·6H O, KSCN and KSeCN were all
2
obtained commercially from Aldrich and used as received. All syntheses were performed under ambient
conditions. Elemental analyses were performed by a Vario-EL III elemental analyzer for carbon, hydrogen,
and nitrogen for complexes 1 and 2. Temperature-dependent magnetization (M-T) of the complexes 1 and 2
were measured in the temperature range 305-3 K and 305-50 K using a quantum design vibrating sample
magnetometer in a physical property measurement system. Measurements were performed on ground
polycrystalline samples of 15-20 mg. The magnetic data were corrected from the sample holder and the
diamagnetic contributions.

4
.2 Synthesis of L (4, 5-dimethy-2-(pyridine-2-yl)imidazole)
1
1
6
The synthetic routes of the ligand L
1
are illustrated in Scheme S1. 1 ml oxalylchloride (10 mmol) was
added drop-wise to a CH Cl solution (100 ml) with 1.5 g N-methylacetamide (20 mmol) at 0 °C under
2
2
heavy gas evolution. After stirred for about 30 min, 2-pyridine carboxamide (2.74 g, 10 mmol) was added.
The obtained mixture was stirred for 3 h at rt, and the volatiles were removed under reduced pressure. The
residue was dissolved in an aqueous solution of NaHCO
The water phase was extracted three times with CHCl
re-crystallized from MeOH gave the ligand L . Yield: 1.14 g, ca. 78%. M.p. 116-118°C. Elemental analysis
%) for L (C10 ): calcd: C 69.34, H 6.40, N 24.25, found: C69.27, H6.44, N 24.19. IR (KBr, cm ):
048, 2980, 2180, 1750, 1560, 1562, 1533, 1482, 1340, 1269, 1235, 1120, 1040, 1015, 987, 960, 798, 747,
3
(60 ml) and refluxed for a further 2 h at 100 °C.
3
. The organic phase was concentrated and
1
−
1
(
H
11
N
3
3
7
1
13
21. H NMR δ: 2.67-2.75 (d, 6H), 7.36 (m, H), 7.85 (m, 1H), 8.38 (d, 1H), 8.59 (d, 1H), 13.00 (s, 1H). C
NMR δ: 12.6, 43.5, 124.5, 125.6, 131.6, 137.2, 143.4, 149.2, 155.1, 165.9.
4
.3 Synthesis of complexes 1 and 2
A methanol solution (10 mL) of ligand L
1
(0.8 mmol) were added dropwise to an methanol solution (5 mL)
of Fe(ClO ·4H O (0.4 mmol), Potassium thiocyanate (Potassium Selenocyanate) (0.8 mmol) and a small
4
)
2
2
amount of ascorbic acid (to avoid oxidation of Fe (II)). The resulting brown solution was filtered after stirred
for about 30 minutes. Crystals suitable for X-ray single-crystal diffraction or PXRD were obtained within
two weeks and collected by filtration. Yield: ca. 85%. Elemental analysis (%) for 1 (C22
C 51.17, H 3.90, N 21.69; found: C 51.43, H 3.78, N 21.52. 2 (C22 Se ): calcd: C 43.30, H 3.30, N
8.36; found: C 43.51, H 3.46, N 18.44.
H20FeN
8
S
2
): calcd:
H20FeN
8
2
1
X-ray crystallographic study
The single-crystal X-ray diffraction data of complex 1 was collected with graphite-monochromated Mo Ka
1
7
radiation (λ = 0.071073 nm). A Rigaku SCXmini diffractometer with the v-scan technique was used. The

lattice parameters were integrated using vector analysis and refined from the diffraction matrix, the
absorption correction was carried out by using Bruker SADABS program with the multi-scan method. The
structures were solved by full-matrix least-squares methods on all F2 data. The SHELXS-97 and
1
8
SHELXL-97 programs were used for structure solution and structure refinement, respectively. The
1
9
molecular graphics were prepared by using the mercury program.
Acknowledgements
This material is based on work supported by the Natural Science Foundation of China (Grant No. 21371031).
Y. H. Luo thanks the Scientific Research Foundation of Graduate School of Southeast University (No.
YBPY1409) and B. W. Sun thanks the International S&T Cooperation Program of China (No.
2
015DFG42240) for financial support.
Electronic Supplementary Information (ESI) available: [crystal data and structure refinement for compounds
, TGA profiles, Raman and IR spectra as well as UV-vis absorption spectra for complexes 1and 2]. See
1
DOI: 10.1039/b000000x/.
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Graphical abstract
Investigation of spin-crossover in iron ( Ⅱ )-di(diamine) system, based on ligands L , 4,
1
5
-dimethy-2-(pyridine-2-yl)imidazole, has been presented. The methyl group at the adjacent sites of the
donor atoms in ligand L can generate the crossover situation for the iron (Ⅱ) di(diimine) system. For
1
complex 1 {Fe(L
1
)
2
(SCN)
2
}, it displays paramagnetic behaviour, while for 2 {Fe(L
1
)
2
(SeCN)
2
}, it is in
-
complete crossover situation. The difference was attributed to the various ligand field strength of SCN and
-
SeCN .

Highlights
>
>
>
> Two new complexes from 4, 5-dimethy-2-(pyridine-2-yl)imidazole and NCX have been presented.
> Influences of NCX on spin-crossover have been investigated.
> The vibrational and absorption spectra of these two complexes have been quantificationally visualized.