Coexistence of two distinct axial ligands and their thermo-induced substitution in trans-[FeL2(NCS)2][FeL2(CH 3OH)2](NCS)2 (L = 4-amino-3-(p-chlorophenyl)-5- (2-pyridyl)-1,2,4-triazole)

Article abstract of DOI:10.1016/j.inoche.2012.11.022

A novel iron(II) complex, trans-[FeL₂(NCS)₂][FeL ₂(CH₃OH)₂](NCS)₂ (1) with 4-amino-3-(p-chlorophenyl)-5-(2-pyridyl)-1,2,4-triazole (L) has been successfully synthesized and characterized. X-ray crystallography analysis shows that 1 is the first example in the mononuclear triazole-based complexes consisting of two distinct molecules: trans-[FeL₂(NCS)₂] and trans-[FeL₂(CH₃OH)₂](NCS)₂ with each octahedral iron(II) center coordinated axial by two NCS^- ions in Fe1 but two MeOH molecules in Fe2. Moreover, 1 can lose two MeOH molecules at 220 C to form trans-[FeL₂(NCS)₂] (2) which can be transformed to 1 when recrystallizing 2 in methanol. Both 1 and 2 are high-spin species in the range of 1.8-300 K.

Full text of DOI:10.1016/j.inoche.2012.11.022

Inorganic Chemistry Communications 28 (2013) 104–108
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Coexistence of two distinct axial ligands and their thermo-induced substitution
in trans-[FeL2(NCS)2][FeL2(CH3OH)2](NCS)2 (L =4-amino-3-(p-chlorophenyl)-
5-(2-pyridyl)-1,2,4-triazole)
Lei Wang a, Jing-Jing Jiang a, Lang Chen a, Xuan Shen a, Dun-Ru Zhu a,b,
⁎
a
College of Chemistry and Chemical Engineering, State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China
b
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel iron(II) complex, trans-[FeL2(NCS)2][FeL2(CH3OH)2](NCS)2 (1) with 4-amino-3-(p-chlorophenyl)-
5-(2-pyridyl)-1,2,4-triazole (L) has been successfully synthesized and characterized. X-ray crystallography
analysis shows that 1 is the first example in the mononuclear triazole-based complexes consisting of two
distinct molecules: trans-[FeL2(NCS)2] and trans-[FeL2(CH3OH)2](NCS)2 with each octahedral iron(II) center
coordinated axial by two NCS− ions in Fe1 but two MeOH molecules in Fe2. Moreover, 1 can lose two MeOH
molecules at 220 °C to form trans-[FeL2(NCS)2] (2) which can be transformed to 1 when recrystallizing 2 in
methanol. Both 1 and 2 are high-spin species in the range of 1.8–300 K.
Received 8 October 2012
Accepted 23 November 2012
Available online 12 December 2012
Keywords:
Syntheses
Triazole
Crystal structures
Iron(II) complex
Ligand's substitution
© 2012 Elsevier B.V. All rights reserved.
Over the past four decades, substituted 1,2,4-triazoles have
attracted considerable attention in coordination chemistry due to
their rich and versatile coordination modes [1,2]. Moreover, triazole
derivatives are known to often induce temperature-mediated spin-
crossover (SCO) in iron(II) complexes [3–10]. Among them, the
mononuclear complexes trans-[Fe(Trz)2(NCS)2] (Trz=1,2,4-triazole
derivative) are one of most interesting systems. For example, com-
plex trans-[Fe(ABPT)2(NCS)2] (ABPT=4-amino-3,5-bis(2-pyridyl)-
1,2,4-triazole) exhibits SCO properties associated with intriguing
polymorph behavior (red polymorph A, T1/2 =180 K [11]; orange
polymorph B, High-spin [12]; polymorph C, partial SCO at 86 K [13];
polymorph D, partial SCO at 162 K [13] and revealing a long-lived pho-
toinduced metastable state through NCS− linkage isomerization accom-
panied with a SCO [14]). In addition, complex trans-[Fe(MBPT)2(NCS)2]
shows an abrupt SCO with T1/2 =231 K, in contrast to its homologous
one cis-[Fe(mMBPT)2(NCS)2] still staying in a high-spin state [MBPT
(or mMBPT)=4-p(or m)-methylphenyl-3,5-bis(2-pyridyl)-1,2,4-
triazole] [5].
iron(II) complex, trans-[FeL2(NCS)2][FeL2(CH3OH)2](NCS)2 (1). Com-
plex 1 represents the first example in any mononuclear triazole-based
complexes consisting of two unique molecules: trans-[FeL2(NCS)2]
and trans-[FeL2(CH3OH)2](NCS)2 with each octahedral iron(II) center
coordinated axial by two NCS- ions in Fe1 but two MeOH molecules in
Fe2. Notable, when heated at 220 °C, complex 1 can be converted to
another new complex, trans-[FeL2(NCS)2] (2) accompanied with loss
of two coordinated methanol molecules. In turn, when recrystallized
in methanol, 2 can be transformed to 1. In addition, both 1 and 2 are
high-spin species in the range of 1.8–300 K.
Ligand L was solvothermally synthesized by anhydrous hydra-
zine and 1,4-dichloro-1-(2-pyridyl)-4-(p-chlorophenyl)-2,3-diaza-1,3-
butadiene (II) in a yield of 46.6% [25]. Compound II can be prepared
by treating N-(p-chlorophenylcarbonyl)-N′-(2-pyridylcarbonyl)hydra-
zine (I) [23] with thionyl chloride in anhydrous toluene (Scheme S1).
Ligand L reacting with iron(II) and KSCN in molar ratio of 2:1:2 under
an argon atmosphere produces complex 1 in a yield of 73.8% [26].
The X-ray structure analysis indicates that L crystallizes in the mono-
clinic space group C2/c (Table 1), and consists of one p-chlorophenyl
ring, one pyridyl group and one triazole ring (Fig. 1). These rings almost
share a common plane because the 1,2,4-triazole ring is oriented at
dihedral angles of 8.6(3)° and 5.7(3)° with the pyridyl ring and the
p-chlorophenyl ring, respectively. These values are similar to those
[3.28(10)° and 6.13(9)°] found in ABPT [27]. The intermolecular
N5\H5A⋯N2(N3)a hydrogen bonds link the L molecules to form a
1D chain along b axis (Fig. S1), which are further connected by
some intermolecular edge-to-face C\H⋯π and N5\H5A⋯π interactions
Rich magnetic properties of the trans-[Fe(Trz)2(NCS)2] complexes
prompted us to widen further the range of used ligands from sym-
metrical [15–19] to asymmetrical 3,5-disubstituted 1,2,4-triazole
[20–24]. Here we present the synthesis of a new asymmetrical triazole,
4-amino-3-(p-chlorophenyl)-5-(2-pyridyl)-1,2,4-triazole (L) and its
⁎
Corresponding author at: College of Chemistry and Chemical Engineering, State Key
Laboratory of Materials-oriented Chemical Engineering, Nanjing University of Technology,
Nanjing 210009, PR China. Tel.: +86 25 83587717; fax: +86 25 83172261.
E-mail address: zhudr@njut.edu.cn (D.-R. Zhu).
(Table S1) and
a face-to-face π⋯π stacking interaction between
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.11.022

L. Wang et al. / Inorganic Chemistry Communications 28 (2013) 104–108
105
trans-[FeL2(CH3OH)2]2+ cation (Fe2 site), together with two
non-coordinated NCS− anions (Fig. 2). The Fe1 site adopts one
distorted [FeN6] octahedron surrounded by four nitrogen atoms
from two L ligands in the equatorial plane and two NCS− in the
axial positions. While the Fe2 site is coordinated by four nitrogen
atoms from two L ligands in the basal plane and two oxygen atoms
from two trans-disposed CH3OH molecules to form another distorted
[FeN4O2] octahedral geometry. To the best of our knowledge, the
simultaneous presence of two trans-positions NCS− and CH3OH in a
mononuclear complex with triazole is unprecedented. The only
example of triazole-based complex with trans-coordinated NCS−
and CH3OH was reported by Tong et al. in a dinuclear cobalt(II)
complex, [Co2(bpt)2(NCS)2(MeOH)2] (bpt=3,5-bis(2-pyridyl)-1,2,4-
triazolato) [28]. Each L in 1 coordinates to iron(II) via N1 of pyridyl
and N2 of the triazole, similar to these observed in the related
complexes [5,11]. The average Fe1\N bond length is 2.157(3) Å
(Table S2), which falls in the typical range of an Fe(II)\N bond in a
high spin state. The spin-state assignment made on the basis of
Fe(II)\N bond distance is related with the octahedral distortion pa-
rameter (Σ), a parameter commonly used for the quantification of
the angular deviation of octahedron from an ideal octahedral geome-
try [29]. A smaller Σ value is generally associated with a stronger
ligand field and, therefore, a low spin state of the metal ion, while
the opposite suggests a high spin state [29,30]. In the present case,
the Σ value is 82.8°, suggesting that the Fe1 center is in a high
spin state at 293 K (vide post). The bond lengths in Fe2 center are
2.222(3) Å, 2.117(3) Å and 2.114(3) Å for Fe2\N7, Fe2\N8 and
Fe2\O1, respectively, which are similar to the characteristic values
of the [FeN4O2] core in a high spin state [31]. A distortion from the
[FeN4O2] octahedron was also induced from the inequivalence of
the bond lengths. Calculation of the octahedral distortion parameter,
Σ=64.8°, showed that the distortion of Fe2 center is smaller than
that of Fe1. In addition, the Fe\Ntrz bond lengths are always
shorter thanFe\Npy ones in two Fe(II) centers, consistent with
Table 1
Crystal data and structure refinement for L and 1.
Compounds
L
1
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
13H10N5Cl
C58H48Cl4Fe2N24O2S4
1494.94
Triclinic
271.71
Monoclinic
C2/c
28.68 (7)
6.041 (16)
14.68 (4)
90.00
103.90 (3)
90.00
2470 (11)
8
1.462
0.302
1120
0.16×0.12×0.10
1.46–25.00
8203
2175 [Rint =0.0757]
1436
2175/3/178
1.071
0.0682/0.1834
0.1027/0.1985
0.801 and −0.280
P-1
11.356 (4)
12.581 (4)
13.511 (4)
104.277 (4)
106.390 (4)
105.604 (4)
1671.4 (9)
1
γ (°)
V (Å3)
Z
Dc (g cm−3
)
1.485
0.781
764
μ (mm−1
F (000)
)
Crystal size (mm3)
0.20×0.16×0.08
1.79–25.00
11977
5820 [Rint=0.0337]
3651
5820/6/439
0.943
0.0444/0.1034
0.0892/0.1175
0.264 and −0.254
θ range (°)
Reflections collected
Independent reflections
Reflections observed [I>2σ(I)]
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices (I>2σ(I))
R indices (all data)
Largest difference peak and
hole (eÅ−3
)
p-chlorophenyl and triazole ring (the centroid–centroid distance=
3.817(3) Å and dihedral angle=5.7(3)°) to form a 2D network in the
bc plane (Fig. S2).
Complex 1 crystallizes in the triclinic space group P-1 (Table 1),
and its asymmetric unit contains two crystallographically distinct
mononuclear Fe(II) centers, trans-[FeL2(NCS)2] (Fe1 site) and
Fig. 1. Projection of the structure of L with the atomic labeling system. Hydrogen atoms are omitted for clarity.
Fig. 2. Projection of the structure of 1 with the atomic labeling system. Hydrogen atoms except CH3OH are omitted for clarity.

106
L. Wang et al. / Inorganic Chemistry Communications 28 (2013) 104–108
Fig. 3. The hydrogen bonding interactions involving NCS− in 1.
those observed in the analogous Fe(II) complexes [2,5,32–34]. The
pyridine–triazole moiety of L in 1 is nearly perfectly planar because
the dihedral angles are 4.2(2)° for Fe1 site and 3.2(2)° for Fe2 center,
showing that on coordination, the pyridyl–triazole twist angle be-
comes smaller than that in the free L ligand. However, the dihedral
angles (49.3(3)° for Fe1 and 32.5(4)° for Fe2) between the triazole
and the p-chlorophenyl ring are much larger than those in the free
L ligand. It is noticeable that in complex 1 there are four linear
NCS− groups [∠N6\C14\S1=178.4(3)° and ∠ N12\C28\S2=
178.0(4)°], two of which do not take part in coordination. The
non-coordinated NCS− group, nearer to the Fe2 center, acts not
only as a counter ion but also as an important bridge to link
the trans-[FeL2(NCS)2] and trans-[FeL2(CH3OH)2]2+ cations by the
intermolecular O1\H1B⋯N12 and N\H⋯S2 hydrogen bonds (Fig. 3
and Table S3). Finally, the molecules of 1 are further connected by
intermolecular N11\H11A⋯S1d hydrogen bonds and edge-to-face
C29\H29A⋯πc interactions to form a compact 3D supramolecular struc-
ture (Fig. S3).
The unusual structural feature of 1 encouraged us to further inves-
tigate the possible exchange interactions of axial ligands between the
trans-disposed CH3OH molecules and the non-coordinated NCS−
group on Fe2 site, which will be helpful to understand the competing
coordination of axial ligands in an octahedral complex in the process
of crystallization of 1. When 1 was heated at 220 °C under an argon
atmosphere, it can lose completely two coordinated CH3OH mole-
cules to produce a new complex, trans-[FeL2(NCS)2] (2) [35], which
was demonstrated by TGA, IR, elemental analysis and powder X-ray
diffraction (Fig. S4). TGA of 1 (Fig. 4) shows that the weight loss of
5.21% between 30 and 220 °C is due to the loss of two coordinated
CH3OH molecules (calculated, 4.29%), associated with an exothermic
peak at 218 °C in the DSC curve. As shown in Fig. 5, the IR spectrum
of 1 shows two strong bands at 2077 and 2057 cm−1, attributable
to C`N stretching vibrations of the trans-coordinated [5] and
non-coordinated NCS− groups, respectively [36]. Two medium
bands at 3284 and 3184 cm−1 are mainly due to N\H stretching vi-
brations of \NH2 groups. Two weak bands at 2973 and 2796 cm−1
can be assigned to C\H stretching vibrations of \CH3 groups of the
coordinated CH3OH molecules. In contrast, the IR spectrum of 2
(Fig. 5) exhibits only a strong band at 2077 cm−1 due to C`N
stretching vibrations of the trans-coordinated NCS− groups. Further-
more, lack of any bands in the region of 2700–3100 cm−1 in 2 also
reveals that the non-coordinated NCS− ions of 1 have replaced
completely the coordinated CH3OH molecules under the intense
heat conditions. It is worthwhile to note that 1 can be obtained
again when recrystallizing the powder of 2 in methanol solution
and its unit cell has been re-measured by single crystal X-ray diffrac-
tion [35]. So complexes 1 and 2 can be transformed each other in a
specific condition shown in Scheme 1.
Unfortunately, the magnetic property study (Fig. S5) revealed that
1 was not SCO active but kept in a high-spin state in the entire
observed temperature range, and this behavior can be interpreted
by Curie–Weiss law, χm =C/(T−θ), with a Curie value of 7.09 and
Weiss constant of −0.11(5) K. This result is in good agreement with
the X-ray crystallography analysis. Similarly, 2 is also a high-spin
species in the range of 1.8–300 K with C=3.16 and θ=−1.2(2) K
(Fig. S5).
In conclusion, a new ligand, 4-amino-3-(p-chlorophenyl)-5-(2-
pyridyl)-1,2,4- triazole (L) and its iron(II) complex, trans-[FeL2(NCS)2]
[FeL2(CH3OH)2](NCS)2 (1), have been successfully synthesized and
characterized by FT-IR, 1H NMR, ESI-MS, TGA, elemental analysis and
single crystal X-ray crystallography. The structural analysis indicates
that 1 is a rare example of coexistence of two trans-coordinated NCS−
218oC
10
TG
DSC
100
80
60
40
20
5
0
-5
1
2
-10
-15
3600 3400 3200 3000 2800 2600 2400 2200 2000 1800
Wavenumbers (cm-1)
100
200
300
400
500
600
700
Temperature (oC)
Fig. 5. IR spectra of 1 and 2 in the region of characteristic bands of NCS−, \NH2 and
CH3OH (3600–1800 cm−1).
Fig. 4. TGA/DSC curves of 1.

L. Wang et al. / Inorganic Chemistry Communications 28 (2013) 104–108
107
Scheme 1. The transformation of 1 and 2.
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anions and CH3OH molecules in a mononuclear triazole-based octahe-
dral complex. An interesting substitution of axial ligand NCS− for
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Acknowledgments
This work was funded by the National Nature Science Foundation
of China (nos. 20771059, 21171093) and by the State Key Laboratory
of Materials-oriented Chemical Engineering (KL11-03).
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Appendix A. Supplementary data
Crystallographic data for the structures reported in this paper
have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC 901929 (L) and
901930 (1). Copies of the data can be obtained, free of charge, on
application to the Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: +44-1223-336033 or email: deposit@ccdc.cam.ac.uk).
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2012.11.022.
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L suitable for X-ray diffraction study were obtained from

108
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transition around room temperature and light induced properties in FeII complexes
with N4O2 coordination sphere, Chem. Commun. 46 (2010) 2554.
for C13H10N5Cl (%): C, 57.47; H, 3.71; N, 25.78. Found (%): C, 57.65; H, 3.76; N,
25.62. FT-IR: 3283 (m), 3191 (m), 1615 (m), 1594 (s), 1467 (s), 1093 (s), 1012
(m), 966 (m), 834 (vs), 787 (m), 727 (m), 693 (m) cm−1
.
1H NMR δ: 6.44 (2H,
br), 7.39 (1H, t), 7.46 (2H, d), 7.87 (1H, d), 8.20 (2H, d), 8.35 (1H, d), 8.63 (1H,
s). ESI-MS: m/z=272.3.
[26] Synthesis of 1: A solution of KSCN (0.039 g, 0.4 mmol) in anhydrous MeOH
(2 mL) was added in a solution of FeSO4·7H2O (0.056 g, 0.2 mmol) in MeOH
(2 mL). The mixture was stirred for 30 min, then filtered. The K2SO4 precipitate
was washed with 2 mL of anhydrous MeOH. The methanolic fractions containing
Fe(SCN)2 were collected and then were poured into a methanol solution of the
corresponding ligand (0.108 g, 0.4 mmol). The resulting orange solution was
stirred for 2 h during which time an orange precipitate formed. The precipitate
was filtered and washed with H2O and dried in vacuo to give 0.11 g (73.8%) of
the complex. The orange single crystals suitable for X-ray diffraction were
obtained by evaporation from a methanol solution. Anal. Calcd for C58H48Cl4Fe2-
N24O2S4 (%): C, 46.60; H, 3.24; N, 22.49; S, 8.58%. Found: C, 46.48; H, 3.31; N,
22.37; S, 8.43%. FT-IR: 3421 (br, m), 3284 (m), 3184 (w), 2973 (w), 2796 (w),
2077 (vs), 2057 (vs), 1604 (m), 1485 (m), 1469 (m), 1297 (m), 1157 (m),
1093 (m), 1014 (m), 984 (m), 834 (m), 796 (m), 698 (w) cm−1. ESI-MS:
m/z=272.2, 656.5.
[27] M. Ramos Silva, J.A. Silva, N.D. Martins, A. Matos Beja, A.J.F.N. Sobral,
4-Amino-3,5-di-2-pyridyl-4H-1,2,4triazole, Acta Crystallogr. E64 (2008) o1762.
[28] M.-X. Peng, C.-G. Hong, C.-K. Tan, J.-C. Chen, M.-L. Tong, Syntheses and crystal
structures of two new cobalt(II) complexes derived from 3,5-bis(pyridine-
2-yl)-4-amino-1,2,4-triazole, J. Chem. Crystallogr. 36 (2006) 703.
[29] P. Guionneau, M. Marchivie, G. Bravic, J.F. Létard, D. Chasseau, Co(II) molecular
complexes as a reference for the spin crossover in Fe(II) analogues, J. Mater.
Chem. 12 (2002) 2546.
[32] J.A. Kitchen, G.N. Jameson, J.L. Tallon, S. Brooker, Spin crossover in co-crystallised
2:1 cis:trans [FeII(pldpt)2(NCS)2] occurs only in 1/3 of the iron centres, Chem.
Commun. 46 (2010) 3200.
[33] J. Klingele, D. Kaase, M.H. Klingele, J. Lach, S. Demeshko, Two-step spin crossover
in the mononuclear iron(II) complex [FeIIL2(NCS)2] (L=2,5-di(2-pyridyl)-
1,3,4-thiadiazole), Dalton Trans. 39 (2010) 1689.
[34] B. Li, R.-J. Wei, J. Tao, R.-B. Huang, L.-S. Zheng, Z.-P. Zheng, Solvent-induced trans-
formation of single crystals of a spin-crossover (SCO) compound to single crystals
with two distinct SCO centers, J. Am. Chem. Soc. 132 (2010) 1558.
[35] Synthesis of 2: After 1 (0.075 g, 0.5 mmol) was heated at 220 °C under an argon
atmosphere for 0.5 h, 2 was obtained quantitatively as an orange powder, 0.070 g
(97.8%). Anal. Calcd for C28H20Cl2FeN12S2 (%): C, 47.01; H, 2.82; N, 23.49; S, 8.96%.
Found: C, 47.25; H, 3.02; N, 23.23; S, 8.63%. FT-IR: 3350 (m), 3257 (w), 2077 (vs),
1629 (m), 1478 (m), 1467 (m), 1291 (m), 1252 (m), 1093 (m), 1014 (m), 977
(m), 835 (m), 796 (m), 692 (w) cm−1. When recrystallizing the powder of 2 in
methanol, complex 1 was obtained in a yield of 89% and its unit cell has been mea-
sured by single crystal X-ray diffraction as a=11.2515(9) Å, b=12.5804(10) Å,
c=13.3776(10) Å, α=104.0210(10)°, β=107.1340(10)°, γ=105.4930(10)°,
V=1634.4(2) Å3, and Z=1.
[36] W. Hage, A. Hallbrucker, E. Mayer, Increasing ion pairing and aggregation in
supercooled and glassy dilute aqueous electrolyte solution as seen by FTIR spec-
troscopy of alkali metal thiocyanates, J. Phys. Chem. 96 (1992) 6488.