Spin-Crossover Hysteresis of [ FeII ( LHiPr ) 2 ( NCS ) 2 ] ( LHiPr = N-2-Pyridylmethylene-4-Isopropylaniline ) Accompanied by Isopropyl Conformation Isomerism

[FeII(LHiPr)2(NCS)2] (LHiPr = N-2-pyridylmethylene-4-isopropylaniline) showed an abrupt spin-crossover (SCO) at T1/2↓ = 154 K on cooling and at T1/2↑ = 167 K on heating. The thermal hysteresis with a width of 13 K is related with the structural solid-state phase transition. The space group was unchanged as P21/n with Z = 8, and there are two crystallographically independent molecules in a unit cell at 130 and 180 K. The two iron (II) sites synchronously underwent the SCO. The most drastic structural change across the SCO was found in the conformation isomerization of an isopropyl group. Namely, rotation around the C(sp2)–C(sp3) bond by ca. 120° takes place during the SCO. There is no structural disorder in the high-temperature phase. The thermal hysteresis probably originates in the bulk isomerization requiring considerable activation energy in the crystalline solid.


Scheme 1. Structural formulas.
We have already reported that [Fe(LH C16 )2(NCS)2] (LH C16 = 4-hexadecyl-N-(2-pyridylmethylene)aniline) exhibited SCO behavior with TC = 176 K only after annealing above 353 K, which can be interpreted in terms of a metastable phase [24].Another example has also been reported, where [Fe(LPy C16 )2(NCS)2] (LPy C16 = 4-hexadecyl-N-(di-2-pyridylmethylene)aniline) underwent SCO at TC↑ = 300 K and TC↓ = 210 K in the first thermal cycle only [25].These SCO phenomena are supposed to interact with a metaphase transition [24,25], and it is rationalized by thinking of the role of the long alkyl chains.Unfortunately, there is only evidence from powder X-ray diffraction and thermal analysis for the mesophase transition.In the present study, small and branched alkyl groups have been introduced to the [Fe(L)2(NCS)2] system, and single-crystal X-ray crystallographic analyses were attempted for a clue to possible structural phase transition.We will report here the SCO thermal hysteresis of [Fe(LH iPr )2(NCS)2] (LH iPr = N-2-pyridylmethylene-4-isopropylaniline) and discuss the origin of the hysteresis, based on the crystal structures determined in both LS and HS phases.

Results and Discussion
The specimen [Fe(LH iPr )2(NCS)2] was prepared according to the conventional method [21][22][23][24][25].After the LH iPr was isolated and characterized, the complex formation was conducted in methanol using FeCl2 and LiNCS as ion sources.The dark blue polycrystalline compound was isolated on a filter, and the product was subjected to X-ray crystal structure analysis and physical measurements without further purification.The elemental and spectroscopic analyses supported the composition formula of [Fe(LH iPr )2(NCS)2].
The magnetic susceptibilities of polycrystalline specimens of [Fe(LH iPr )2(NCS)2] were measured on a SQUID magnetometer in a temperature range of 10-300 K.As Figure 1 shows, the χmT value was 3.5 cm 3 K•mol −1 at 300 K, being compatible with the S = 2 HS state of the iron(II) ion.On cooling, the χmT value abruptly decreased to practically null χmT at T1/2↓ = 154 K, and this phase is assigned to the S = 0 LS state.The present complex showed thermal hysteresis.On heating, the χmT profile exhibited an upsurge at T1/2↑ = 167 K and traced the cooling curve above 170 K. Thus, the hysteresis width was 13 K.This SCO behavior was reproduced on repeated thermal cycles.The X-ray crystallographic analysis on [Fe(LH iPr )2(NCS)2] was successful below and above the SCO temperature (Figure 2), but owing to the microcracks occurred during the transition the final R factors in the low-temperature phase were slightly unsatisfactory (Table 1).The space group was unchanged as P21/n with Z = 8, and there are two crystallographically independent molecules in a unit cell at 130 and 180 K.The unit cell was somewhat deformed on cooling, as indicated with the change of β (102.153(9)° at 130 K vs. 105.247(9)°at 180 K).The cell volume was shrunk by 3.0%, but interestingly the b length was elongated by 2.5%.This issue will be discussed later.
The asymmetric units determined at 130 and 180 K are drawn in Figure 2.Each iron (II) is coordinated with two diimines and two thiocyanates, affording an Fe II N6 coordination sphere.The molecular structures are similar to each other, and two thiocyanate groups are located at a cis position, as often seen in the [Fe(L)2(NCS)2] family [21][22][23][24][25].The two molecules are arranged so that two LH iPr fragments are pancake-type stacked with respect to the throughout benzylideneaniline -conjugate system.The shortest interatomic distances are 3.510(10) Å (C24Ph•••C57Ph), 3.435(11) Å (C18py•••C50py), and 3.358(10) Å (C21imine•••C53imine) at 130 K, which are comparable to twice the van der Waals radius of carbon (3.4 Å) [26].At 180 K, the shortest distances are 3.490(5) Å (C24Ph•••C57Ph), 3.500(5) Å (C17py•••C50py), and 3.460(4) Å (C21imine•••C53imine).The Fe-N coordination bonds at 130 K are shortened by 9.9% and 7.9% on average for Fe1 and Fe2, respectively, compared with those measured at 180 K (Table 2).This finding evidences the LS phase at 130 K and HS phase at 180 K. Furthermore, the SCO behavior at two iron (II) sites is synchronized, as the spin transition was quite sharp and no intermediate structure could be found inbetween.The - interaction seems to bring about the cooperative SCO at Fe1 and Fe2.One may point out that the bond angles around the Fe ions are relatively bent at 180 K. Actually, the trans-N-Fe-N(CS) bond angles at 180 K are smaller than those at 130 K by ca.9° (Table 3).On elongating the Fe-N bonds, the coordination polyhedra are enlarged, and the Fe-N(CS) bond may be forced to bend and partially relieve the van der Waals interaction around the terminal sulfur atoms.The Fe-N-C(S) angles at 180 K are found to be bent as well, possibly owing to the same reason.The dislocation of the atomic positions is basically small across the SCO, but from a close look at the N1-N2-involving LH iPr ligand, we can find that the phenyl ring is largely twisted.The torsion angles (ϕ) between the pyridine and benzene rings in the N1-N2 LH iPr are 99.0(3)° at 130 K while 51.7(1)° at 180 K.In three other LH iPr ligands, ϕ are 62.8(3), 55. 6(3), and 53.7(3)° at 130 K and 56.8(1), 46.5(1), and 49.8(1)° at 180 K, for the N3-N4, N7-N8, and N9-N10 ligands, respectively.As a result, Δϕ are 47, 6, 9, and 4°, respectively, for four LH iPr ligands.In short, only one LH iPr ligand experiences meaningful conformation change.The most drastic difference was found in the conformation of the C14-C13-C15 isopropyl group, while three other isopropyl groups hardly moved.The conformation is regulated with the steric effect from two methyl and one hydrogen groups at C13, and the rotation around the C(sp 2 )-C(sp 3 ) bond by ca.120° would give at least three possible energy minima.Thus, the X-ray crystallographic analysis clarifies that the present spin transition is accompanied by the structural solid-state phase transition.It is reasonable that the thermal hysteresis would usually be found in the first-order phase transition.
The conformation energy profile of a mother compound isopropylbenzene (kumene) has been well investigated [27][28][29][30].Theoretical computations suggested the conformation of the lowest energy as that in which the methine C-H group is situated coplanar to the phenyl ring, and agreed well with many experiments [27,28].An apparent barrier of 6.5 kJ•mol −1 to rotation about the C(sp 2 )-C(sp 3 ) bond was characterized, determined by means of 1 H NMR technique in solution [28,29].The microwave spectroscopy indicated that the barrier is only 0.8 kJ•mol −1 in a gas phase [30].The experimental barriers for rotation around the C(sp 2 )-C(sp 3 ) bond seem controversial.However, it may be acceptable that the barrier depends on the phase where the measurements performed.The notable difference between the present work and the references [27][28][29][30] is that experimentally observed conformations (the bottom of Figure 2) violate the theoretical prediction.In the solid-state case like the present study, the barrier would be severely influenced by van der Waals interaction from surrounding molecules.
From the present experiments on [Fe(LH iPr )2(NCS)2], we found two stable conformers, as schematically expressed in the bottom of Figure 2. Since they are approximately in a mirror image, the preference of a stable conformer is determined from the intermolecular interaction in the crystalline solid.The C (sp 2 )-C(sp 3 ) bond of the C14-C13-C15 isopropyl group is arranged almost parallel in the b direction (Figure 3).This isopropyl group seems to accommodate the steric congestion increased on cooling, being responsible for the negative shrinkage factor of the b axis (see above).The C13 (methine)•••S3 # interatomic distance of 4.38(1) Å at 130 K may afford a clue to the present steric effect (the symmetry operation code for # is 1 − x, 1 − y, 1 − z).The two methyl groups C14 and C15 are located away from S3 # because of the van der Waals repulsive interaction.At 180 K, the C13•••S3 # interatomic distance becomes as long as 5.100(6) Å, and a methyl group approaches S3 # with the C15•••S3# distance of 3.824(5) Å, possibly owing to the van der Waals or hydrogen bond attractive interaction with S3 # .Such intermolecular interactions must be much stronger than the internal one.We have to stress that there is no disorder in the molecular structures determined.In the HT phase, attempts at the population analysis of scrambling two methyl and one hydrogen groups were unsuccessful.The residual electron density in the differential Fourier map has a maximum 1.70 e − /Å 3 and the residues are located near the iron and sulfur atoms.The present transition does not belong to a category of the disorder-order transition.Conformation isomerism is schematically explained in Figure 4, in connection with the energy profile.There have been increasing reports on hysteretic SCO materials coupled with order-disorder transition [16,17,[31][32][33] An n-alkyl conformation disorder was found in the SCO of [Fe(ptz)6](BF4)2 with a hysteresis width of 7 K (ptz = 1-propyltetrazole) [32].A very unique multi-stable SCO material was reported on [Fe(nBu-im)3(tren)](PF6)2, which involves both disorders of n-butyl groups and counter anion PF6 and showed 14 and 41 K hysteresis for two polymorphs ((nBu-im)3(tren) = tris[2-(1-n-buty-1H-limidazol-2ylmethyleneamino)ethyl]amine) [33].These compounds involve short n-alkyl groups in common and attract attention as a prototype of higher alkyl analogs.On the other hand, [Fe(LH iPr )2(NCS)2] has a methyl-branched short alkyl group, and the factor of conformation preference is different from trans/gauche-type energy profile.The present case is rare, because methyl configuration is utilized and there is completely no disorder in the HS state of [Fe(LH iPr )2(NCS)2].
As described in the introduction, when completely irreversible mesophase transitions were coupled with the spin equilibrium system, irreversible spin-transition behavior was observed [24,25].On the other hand, complete reversible spin transition systems belong to an opposing limit, giving spin equilibrium systems.We can imagine typical systems free from intermolecular interaction like solution systems [34][35][36] for the latter category.The present compound has a moderately bulky alkyl group and intermediate intermolecular interaction between the two limits.In other words, an activation energy just suitable for the structural phase transition seems to afford a hysteretic SCO system.

Crystallographic Analysis
X-ray diffraction data of [Fe(LH iPr )2(NCS)2] at 130 and 180 K were collected on Mercury CCD (Rigaku, Tokyo, Japan) and R-axis Rapid IP diffractometers (Rigaku, Tokyo, Japan) with graphite monochromated MoKα radiation (λ = 0.71073 Å).The structures were directly solved by a heavy-atom method and expanded using Fourier techniques in the CRYSTALSTRUCTURE version 4.0 program package (Rigaku/MSC, The Woodlands, TX, USA).Selected crystallographic data are listed in Table 1.CCDC 1434210 and 1434211 for the data at 130 and 180 K, respectively, include the experimental details and full geometrical parameter tables.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

Magnetic Measurements
The direct-current magnetic susceptibility of a polycrystalline specimen of [Fe(LH iPr )2(NCS)2] was measured on a SQUID magnetometer (MPMS-XL7, Quantum Design, San Diego, CA, USA) in a temperature range 10-300 K.The sample was held in a gelatin capsule.The magnetic response was corrected with diamagnetic blank data of the sample holder measured separately, and the diamagnetism of the sample itself was corrected using Pascal's constants [38].

Conclusions
The abrupt SCO with the hysteresis width of 13 K was recorded for [Fe(LH iPr )2(NCS)2].The crystal structures were determined at 130 and 180 K for the LS and HS phases, respectively.Although the space group and Z value were unchanged at both temperatures, the conformation isomerization of one out of four independent isopropyl groups was clearly demonstrated, where the rotation around the C(sp 2 )-C(sp 3 ) bond occurs during the SCO.Detailed temperature dependent crystallographic analysis afforded a clue to the thermal hysteresis.Therefore, such a model compound carrying relatively small alkyl group may be helpful to comprehend mechanisms of SCO hysteresis.

Figure 2 .
Figure 2. X-ray crystal structures of the asymmetric unit of of [Fe(LH iPr )2(NCS)2] in (a) the low-temperature phase measured at 130 K and (b) the high-temperature phase measured at 180 K. Schematic drawings of the conformation around the C14-C13-C15 isopropyl and phenyl groups are also shown.

Figure 3 .
Figure 3. Molecular arrangement in the crystal of [Fe(LH iPr )2(NCS)2] at 130 K, viewed along the a axis.The C14-C13-C15 isopropyl groups are highlighted.Selected intermolecular atomic distances are shown with dotted lines.