Accepted Manuscript: Inorganica Chimica Acta
Accepted Manuscript: Inorganica Chimica Acta
Accepted Manuscript: Inorganica Chimica Acta
Yang-Hui Luo, Lan Chen, Jing-Wen Wang, Ming-Xin Wang, Ya-Wen Zhang,
Bai-Wang Sun
PII: S0020-1693(16)30249-3
DOI: http://dx.doi.org/10.1016/j.ica.2016.05.020
Reference: ICA 17056
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
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
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
A complex ion (over molecule) consists of one central atom (ion) and a number of ligands that are fully
supported by the central atom (ion). The relative number of components in a stable complex is followed by
very special stoichiometry, although this cannot be transferred in the classical definition of the valence
concept. The central atom is characterized by integers, a round number, which indicates the number of
ligands (monodentates) which can form stable complexes with one central atom. For instance, the
low-spin (LS, 1A1) electronic states of an octahedral 3d 4-3d7 transition metal complexes scan be
intermolecular
magnetic/electric fields, 2 and the resulted SCO behaviours are varied as complete, incomplete,
abrupt, gradual, one-step, multi-step transitions, or hysteretic. 3 To date over 200 SCO systems, many
complexes with triazole or pyrazine ligands, 5 as well as some FeIII,6 CoII 7, MnIII 2b
and CrII 8
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
turning the spin transition behaviour of the metal centre, 9b and the counter anions triggering SCO
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
the fundamental science of spin crossover. The [FeN6]2+ derivative of 2-(pyridin-2-yl)imidazole was
shown continuous above room temperature spin transition both in solid state and in solution,12 which was
relatively early on to be a crossover system,13 thus, an appropriate variation in either their σ-donor and/or
π-acceptor character of this ligand may result in crossover situation in this [FeN6]2+ systems. In our other
work,14 we introduced methyl group at the adjacent sites of the donor atoms ligand 2-(pyridin-2-yl)imidazole,
which generated ligand L1 (4, 5-dimethy-2 -(pyridine-2-yl)imidazole) and we have made a complete
complexes formation of ligand L1 with different Fe (Ⅱ) salts (Fe(ClO4)2, Fe(BF4)2, Fe(PF4)2 and Fe(SbF4)2).
PPMS results revealed that these [FeN6]2+ systems with L1 were all shown crossover situation, which display
counter-anions-dependent transition temperatures due to interactions between counter-anions and L1. Hence
in this work, we investigated the iron (Ⅱ) di(diimine) system with L1 by introduce of ligands field of SCN-
and SeCN-, and we have obtained two new complexes {Fe(L1)2(SCN)2} (1) and {Fe(L1)2(SeCN)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
Figure 1. Crystal structures and connecting motif of complex 1, hydrogen atoms are omitted for clarity: (a)
The ASU of complex 1; (b) The “di-FeII” sites packed by C-H…π and π…π contacts; (c) The 1D connecting
motif of complex 1.
Complex 1 was crystallized from methanol solution as yellow block crystals, complex 2 was
characterized by single-crystal X-ray diffraction, and the phase purity of complex 2 was confirmed
by powder X-ray diffraction. The crystal data and structure refinement for complex 1 was listed in
Table S1. Complex 1 crystallizes in triclinic P-1 space group, the FeII ion adopts a distorted
octahedral geometry formed by four N atoms of the ligand s L 1 and two N atoms of SCN-, and it
displays a “batterfly-like” motif (Figure 1a). Every two “batterfly” motif form “di-FeII” sites through
C-H…π (C…C distance 3.312 Ǻ) and π...π (centre…centre distance 3.854 Ǻ) contacts between
adjacent L1 ligands (Figure 1b). The “di-FeII” 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
Table 1. The bond length (Ǻ) of the distorted octahedral geometry for 1
Table 2. The bond angles (º) and distortion parameter (∑/º) of the distorted octahedral geometry for 1
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
Figure 3. The χm T versus T plots of the direct-current (dc) magnetic measurement for complexes 1 and 2
over the temperature range 400-5 K.
samples of complexes 1 and 2 were performed on a PPMS under 2000Oe in temperature range
400-5 K. Shown in Figure 3 were the χm T versus T plots of them. Complex 1 shows χm T = 3.11
cm 3mol-1K at the room temperature, typical value for a HS Fe II atom. Upon cooling, this value kept
by a drop to 1.82 cm 3mol-1K at 3 K owing to the combined effect of spin-orbit coupling and
zero-field splitting of the HS FeII ion. The χmT 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
field strength than the former one. Complex 2 shows χm T = 3.05 cm 3mol-1K at the upper
temperature limit, typical value for a HS FeII atom. Upon cooling, the χm T versus T curve displays a
complete one-step spin transition. The χm T values undergo a gradual decline to reach a plateau close
to 0cm 3mol-1K at around 120 K, indicating a complete LS FeII sites. The spin transition temperature
T1/2 for complex 2 is 214 K. Thus, the combination of ligand L 1 with SeCN- can provided appropriate
3. Conclusions
In conclusion, the combination of ligand L1 (4, 5-dimethy-2-(pyridine-2-yl)imidazole) with ligand SeCN- can
generated the crossover situation for iron ( Ⅱ)-di(diamine) complex 2 {Fe(L1)2(SeCN)2}, while the
combination of ligand L1 with ligand SCN- cant’ provide appropriate ligand-field strength for a SCO
mononuclear FeII site thus resulted paramagnetic behaviour for complex 1{Fe(L1)2(SCN)2}. The stronger
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
All syntheses were performed under ambient conditions. Fe(ClO4)2·6H2O, KSCN and KSeCN were all
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
polycrystalline samples of 15-20 mg. The magnetic data were corrected from the sample holder and the
diamagnetic contributions.
4.2 Synthesis of L1 (4, 5-dimethy-2-(pyridine-2-yl)imidazole)
The synthetic routes of the ligand L1 are illustrated in Scheme S1.16 1 ml oxalylchloride (10 mmol) was
added drop-wise to a CH2Cl2 solution (100 ml) with 1.5 g N-methylacetamide (20 mmol) at 0 °C under
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 NaHCO3 (60 ml) and refluxed for a further 2 h at 100 °C.
The water phase was extracted three times with CHCl3. The organic phase was concentrated and
re-crystallized from MeOH gave the ligand L1. Yield: 1.14 g, ca. 78%. M.p. 116-118°C. Elemental analysis
(%) for L (C10H11N3): calcd: C 69.34, H 6.40, N 24.25, found: C69.27, H6.44, N 24.19. IR (KBr, cm−1):
3048, 2980, 2180, 1750, 1560, 1562, 1533, 1482, 1340, 1269, 1235, 1120, 1040, 1015, 987, 960, 798, 747,
721. 1H 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). 13
C
NMR δ: 12.6, 43.5, 124.5, 125.6, 131.6, 137.2, 143.4, 149.2, 155.1, 165.9.
A methanol solution (10 mL) of ligand L1 (0.8 mmol) were added dropwise to an methanol solution (5 mL)
of Fe(ClO4)2·4H2O (0.4 mmol), Potassium thiocyanate (Potassium Selenocyanate) (0.8 mmol) and a small
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 (C22H20FeN8S2): calcd:
C 51.17, H 3.90, N 21.69; found: C 51.43, H 3.78, N 21.52. 2 (C22H20FeN8Se2): calcd: C 43.30, H 3.30, N
The single-crystal X-ray diffraction data of complex 1 was collected with graphite-monochromated Mo Ka
radiation (λ = 0.071073 nm). A Rigaku SCXmini diffractometer with the v-scan technique was used.17 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
18
SHELXL-97 programs were used for structure solution and structure refinement, respectively. The
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.
Electronic Supplementary Information (ESI) available: [crystal data and structure refinement for compounds
1, TGA profiles, Raman and IR spectra as well as UV-vis absorption spectra for complexes 1and 2]. See
DOI: 10.1039/b000000x/.
References
[1] a) Spin Crossover in Transition Metal Compounds I–III, ed. P. Gutlich and H. A. Goodwin, Top. Curr.
Chem. (2004) vol. 233-235; b)P. Gutlich, Y. Garcia, H. A. Goodwin, Chem. Soc. Rev. 29 (2000) 419;
c) M. A. Halcrow, Chem. Soc. Rev. 40 (2011) 4119.
[2] a) A. Lennartson, A. D. Bond, S. Piligkos, C. J. McKenzie, Angew. Chem. Int. Ed. 51 (2012) 11049; b)
P.N. Martinho, B. Gildea, M. M. Harris, T. Lemma, A. D. Naik, H. Mller-Bunz, T. E. Keyes, Y. Garcia,
G. G. Morgan, Angew. Chem. Int. Ed. 51 (2012) 12597; c) H.-J. Kruger, Coordination Chemistry
Reviews, 253 (2009) 2450.
[5] a) M. M. Dirtu, C. Neuhausen, A. D. Naik, A. Rotaru, L. Spinu, Y. Garcia, Inorg. Chem. 49 (2010)
5723; b) A. Grosjean, N. Daro, B. Kauffman, A. Kaiba, J. F. Ltard, P. Guionneau, Chem. Commun. 47
(2011) 12382; c) P. D. Southon, L. Liu, E. A. Fellows, D. J. Price, G. J. Halder, K.W. Chapman, B.
Moubaraki, K. S. Murray, J. F. Ltard, C. J. Kepert, J. Am. Chem. Soc. 131 (2009) 10998; d) Y. H.
Luo, Q. L. Liu, L. J. Yang, Y. Ling, W. Wang, B. W.Sun, Journal of Solid State Chemistry, 222 (2015)
76.
[6] a) S. Hayami, Z. Z. Gu, H. Yoshiki, A. Fujishima, O. Sato, J. Am. Chem. Soc. 123 (2001) 11644; b) S.
Hayami, K. Hiki, T. Kawahara, Y. Maeda, D. Urakami, K. Inoue, M. Ohama, S. Kawata, O. Sato, Chem.
Eur. J. 15 (2009) 3497; c) P. N. Martinho, Y. Ortin, B. Gildea, C. Gandolfi, G. McKerr, B. O_Hagan, M.
Albrecht, G. G. Morgan, Dalton Trans. 41 (2012) 7461.
[7] a) M. G. Cowan, J. Olgun, S. Narayanaswamy, J. L. Tallon, S. Brooker, J. Am. Chem. Soc. 134 (2012)
2892; b) S. Hayami, K. Murata, D. Urakami, Y. Kojima, M. Akita, K. Inoue, Chem. Commun. (2008)
6510; c) S. Hayami, Y. Shigeyoshi, M. Akita, K. Inoue, K. Kato, K. Osaka, M. Takata, R. Kawajiri, T.
Mitani, Y. Maeda, Angew. Chem. 117 (2005) 4977; Angew. Chem. Int. Ed. 44 (2005) 4899.
1859.
[17] Rigaku, CrystalClear, Version 14.0. Rigaku Corporation, Tokyo, Japan, (2005).
[18] M. Sheldrick, SHELXS97: Programs for Crystal Structure Analysis, University of Gottingen, Germany,
(1997).
[19] Mercury 2.3 Supplied with Cambridge Structural Database, CCDC: Cambr idge, UK, (2003-2004).
Graphical abstract
5-dimethy-2-(pyridine-2-yl)imidazole, has been presented. The methyl group at the adjacent sites of the
donor atoms in ligand L1 can generate the crossover situation for the iron (Ⅱ) di(diimine) system. For
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.