Tuning the Spin States of Two Apical Iron ( II ) Ions in a Pentanuclear Iron ( II ) Cluster Helicate through the Choice of Anions

Spin-crossover clusters with iron(II) high nuclearity are rare. By using 3,5-bis(pyridin-2yl)-1,2,4-triazole (bptH) as the ligand with two bidentate chelating sites, we successfully obtained three pentanuclear iron(II) cluster helicate compounds [{Fe(μ-bpt)3}2Fe3(μ3-O)][Fe2(μ-Br)(μbpt)(NCS)4(H2O)]·2H2O·C2H5OH (1), [{Fe(μ-bpt)3}2Fe3(μ3-O)][Fe2(μ-bpt)2(NCS)4] (2), and [{FeII(μbpt)3}2Fe3(μ3-O)]I2·2C2H5OH (3). Research on single-crystal structure and magnetism has indicated that tuning the spin state of two iron(II) ions in axial direction is successfully realized by regulating the different counter anions: one apical [Fe(bpt)3] unit exhibits spin-crossover behavior while the other [Fe(bpt)3] unit remains in low spin state in 1, both apical ions are of high spin states in 2, and are of low spin states in 3.


Introduction
Spin-crossover (SCO), a spin-switching phenomenon that may exist in 3d 4 −3d 7 transition metals, has been extensively explored for potential use in data storage and display devices [1,2].Generally speaking, when an iron(II) ion with a d 6 electronic configuration is under an octahedral coordination environment, competition of electron pairing energy, and crystal field splitting-influenced arrangement of outer electrons, the iron(II) ion can adopt a low spin state (LS) or a high spin state (HS), thus influencing its magnetic behaviors.Hence, one key to preparing SCO compounds is to select a ligand that can provide appropriate cooperativity and field strength [3][4][5].However, it is difficult to design and regulate SCO phenomenon at a molecular level as the coordination environment of the metal ions in compounds and any tiny structural change will probably result in the radical disappearance of the whole compound's SCO behavior because the influence of the spin transition nature on un-coordinated solvent molecules [6][7][8][9], counterions [10][11][12], and polymorphisms [13][14][15] is quite sensitive.Hence, up to now, it is still very difficult to realize directed regulation of an iron(II) ion's spin state and to carry out more in-depth studies of the relationship between magnetism and structure so as to understand and regulate the influence of the supermolecular effect on SCO behavior.
So far, among the reported SCO compounds, most are mononuclear SCO compounds.From the concept of molecular design, taking the mononuclear spin transition compounds as structural units and fiber-bridging them as one molecule through covalent bonds can produce a multi-nuclear spin transition compound.By adjusting the local coordination environment of the multi-nuclear compound, we can optimize its magnetic behavior and in this way, the magnetic behavior of the step-by-step conversion of multi-nuclear metal ions can be realized.In addition, in a multi-nuclear compound system, we can probably capture a phenomenon combining SCO behavior and magnetic exchange;

Reagents and Instruments
All reagents and solvents were commercial and used without further purification.FT-IR spectroscopy was performed on a Bio-Rad FTS-7 spectrometer at room temperature in the range of 4000-400 cm −1 .Electrospray ionization mass spectrometry (ESI-MS) in methanol was carried out on a Thermo-Finniga LCQ DeCA XP quadrupole ion-trap mass spectrometer with an electrospray ionization source.Data were analyzed by the spectrometer software MassLynx NT (version 3.4).Elemental analysis was measured with an Elementar Vario-EL CHN elemental analyzer.The kinds and amounts of solvents were determined under N 2 atmosphere with a heating rate of 10 K min −1 by Thermogravimetric Mass Spectrometer, STA449 F3 Jupiter-QMS 403C aedo.

Single-Crystal X-ray Diffraction
Single-crystal X-ray diffraction data of compounds 1-3 were collected with Mo-Kα ray (λ = 0.71073 Å) on an R-AXIS SPIDER IP diffractometer produced by Rigaku.Compound 1 was measured first at 150 K and then the temperature was rapidly increased to 298 K while continuously measuring; compound 2 was measured at 298 K; compound 3 was measured at 298 K.All data went through absorption correction.SHELXTL [37] program was used to conduct an analysis of the initial structure model with a direct method; least square method was used to refine the structure; and non-hydrogen atom coordinates were determined through the difference value Fourier method.Displacement parameters of all non-hydrogen atoms in the compound went through anisotropic fine trimming.Hydrogen atoms were located at ideal geometric positions; therefore, they did not participate in fine trimming.Some unordered guest molecules were removed with the SQUEEZE [38] method during fine trimming.CCDC: 999096(1-150 K), 999097(1-298 K), 1495033 (2-298 K), 1495034 (3-298 K) contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Center at http://www.ccdc.cam.ac.uk/data_request/cif.

Magnetic Measurement
Temperature-changing molar susceptibility of the compounds was measured by a Quantum Design MPMS-XL7 SQUID instrument (Quantum Design, Inc., San Diego, CA, USA) under RSO pattern.Compound 1 was measured within the temperature range of 300-2 K. Test samples were wrapped in preservative film and diamagnetic correction was obtained with Pascal constant.The very low yield of 2 and 3 prevented the collection of reliable magnetic data.

Single-Crystal Structure
The structure of compound 1 was determined at 150 and 298 K, that of compound 2 was determined at 298, and that of compound 3 was determined at 298 K.For compounds 2 and 3, the temperature change did not cause a phase change or spin transition; therefore, we only report their structural data under room temperature.Even though 1-3 had different counter anions and solvent molecules, they had a similar {Fe 5 } cation cluster ([{Fe(µ-bpt) 3 } 2 Fe 3 (µ 3 -O)] 2+ ) and therefore only 1 and 2 are discussed in detail.
Single-crystal X-ray diffraction measurements reveal that 1 crystallizes in the triclinic P-1 space group at 298 and 150 K (Table 1).At first glance, the asymmetric unit at 298 K contains one [{Fe II (µ-bpt) 3 } 2 Fe II 3 (µ 3 -O)] 2+ cation and one [Fe II 2 (µ-Br)(µ-bpt)(NCS) 4 (H 2 O)] 2− anion and one ethanol molecule (Figure 1).The disordered water was removed by SQUEEZE [38] to achieve reasonable refinements, which were determined by thermogravimetric analysis (Figure S1) and elemental analysis [36].The geometry of the pentanuclear cation is similar to that of our previous work [35,36] in which two terminal triple-stranded [Fe II (bpt) 3 ] − units wrap a rare planar [Fe II  3 (µ 3 -O)] 4+ core to form a bis(triple-helical) complex with trigonal bipyramidal topology.The rigid bis-bidentate bpt − ligand links one apical and one equatorial Fe II ion in a cis bridging mode.Six bpt − ligands wrap around the pentanuclear cluster with offset face-to-face π-π interactions, giving rise to two homochiral [Fe II (bpt) 3 ] − units.The apical Fe1 and Fe2 ions are located in the distorted octahedral environments with six N atoms from three bpt − ligands.The equatorial Fe3~Fe5 ions are all in the distorted N 4 O trigonal pyramidal geometries, where four N atoms come from the remaining coordination sites of two bpt − ligands.The values of the trigonality index τ [= (θ 1 − θ 2 )/60] [39,40]-where θ 1 , θ 2 are defined as the two largest L-Fe-L angles in the coordination geometry-are 0.59, 0.57, and 0.68 for Fe3~Fe5 at 298 K, respectively (Table 2).Theoretically, the τ values for perfect trigonal bipyramid and square pyramid are 1 and 0, respectively [39,40].Fe6 and Fe7 ions of the dinuclear anion have different coordination numbers and are bridged by the bpt − ligand and a chloride ion.Fe6 ion is located in a distorted N 4 Br square pyramidal environment (τ = 0.48 at 298 K) formed by one bridging chloride ion, one bpt − ligand, and two NCS − ligands.Meanwhile, the Fe7 ion adopts an N 4 BrO octahedral geometry defined by one water and one NCS − ligand at the axial positions; the other NCS − ligand and the bridging bpt − and Br − ligands are in the equatorial plane.The details of the bond distance and distortion parameter around the octahedral Fe II ion are effective to construct the magneto-structural relationships (Table 3) [41].At 150 K, the average Fe1-N and Fe2-N bond distances are 1.98 and 1.99 Å, respectively, corresponding to the LS state.At 298 K, they increase to 2.00 and 2.18 Å, respectively, suggesting LS and HS states.It clearly indicates that the Fe2 ion undergoes an SCO process while Fe1 ion remains in the LS state.The values of the octahedral distortion parameter Σ, defined as the sum of absolute deviations between real and ideal values of 12 cis N-Fe-N angles [42], change from 59.9° to 96.6° for Fe2 ion as the temperature is increased from 150 to 298 K.Meanwhile, it displays relatively small Σ values for Fe1 ion.They are consistent with the formation of a more regular octahedron in the LS state.Two dinuclear anions are linked by offset face-to-face π-π interactions between two bpt − ligands and hydrogen bonding between bpt − and NCS − ligands to form an anion dimer.Meanwhile, a pair of cationic enantiomers that have opposite chirality is present on either side of the anion dimer to form a racemic tetramer unit (Figure 1c and S2).The pentanuclear cation links one anion through hydrogen bonding and offsets face-to-face π-π interactions between two bpt − ligands, and the other anion through hydrogen bonding contact (Table S1).The tetramer units are further packed into a three-  The details of the bond distance and distortion parameter around the octahedral Fe II ion are effective to construct the magneto-structural relationships (Table 3) [41].At 150 K, the average Fe1-N and Fe2-N bond distances are 1.98 and 1.99 Å, respectively, corresponding to the LS state.At 298 K, they increase to 2.00 and 2.18 Å, respectively, suggesting LS and HS states.It clearly indicates that the Fe2 ion undergoes an SCO process while Fe1 ion remains in the LS state.The values of the octahedral distortion parameter Σ, defined as the sum of absolute deviations between real and ideal values of 12 cis N-Fe-N angles [42], change from 59.9 • to 96.6 • for Fe2 ion as the temperature is increased from 150 to 298 K.Meanwhile, it displays relatively small Σ values for Fe1 ion.They are consistent with the formation of a more regular octahedron in the LS state.Two dinuclear anions are linked by offset face-to-face π-π interactions between two bpt − ligands and hydrogen bonding between bpt − and NCS − ligands to form an anion dimer.Meanwhile, a pair of cationic enantiomers that have opposite chirality is present on either side of the anion dimer to form Crystals 2018, 8, 119 6 of 12 a racemic tetramer unit (Figure 1c and Figure S2).The pentanuclear cation links one anion through hydrogen bonding and offsets face-to-face π-π interactions between two bpt − ligands, and the other anion through hydrogen bonding contact (Table S1).The tetramer units are further packed into a three-dimensional racemic supramolecular structure through intermolecular hydrogen bonding and π-π interactions (Figure S3) [36].
The single-crystal structure of compound 2 was determined at 298 K. 2 was crystallized in monoclinic space group P2/c. 2 contains [{Fe(µ-bpt) 3 } 2 Fe 3 (µ 3 -O)] 2+ cations and one [Fe II  2 (µbpt) 2 (NCS) 4 ] 2− anion.The disordered solvated molecules at 298 K were removed by SQUEEZE [37] to achieve reasonable refinements.The structure of the cation and anion clusters are shown in Figure 2. The structure of [{Fe(µ-bpt) 3 } 2 Fe 3 (µ 3 -O)] 2+ is quite similar to that in compound 1.Five iron(II) ions are wrapped by six ligands, thus forming a pentagonal bipyramid configuration, and µ 3 -O further bridges three iron(II) ions in the equatorial plane and completes their trigonal bipyramid coordination geometries.However, different from 1, the spin state of the two iron(II) ions on axial top do not change and the average bond length of the six Fe-N bonds is 2.20 Å (Table 3), which indicates that Fe II is under the HS state.It is worth noting that pentanuclear cation clusters form 1D chains (as shown in Figure 3a) among the cation clusters through supermolecular acting force π-π stacking (distance is about 3.40 Å) (Table S2).dimensional racemic supramolecular structure through intermolecular hydrogen bonding and π-π interactions (Figure S3) [36].The single-crystal structure of compound 2 was determined at 298 K. 2 was crystallized in monoclinic space group P2/c. 2 contains [{Fe(μ-bpt)3}2Fe3(μ3-O)] 2+ cations and one [Fe II 2(μbpt)2(NCS)4] 2− anion.The disordered solvated molecules at 298 K were removed by SQUEEZE [37] to achieve reasonable refinements.The structure of the cation and anion clusters are shown in Figure 2. The structure of [{Fe(μ-bpt)3}2Fe3(μ3-O)] 2+ is quite similar to that in compound 1.Five iron(II) ions are wrapped by six ligands, thus forming a pentagonal bipyramid configuration, and μ3-O further bridges three iron(II) ions in the equatorial plane and completes their trigonal bipyramid coordination geometries.However, different from 1, the spin state of the two iron(II) ions on axial top do not change and the average bond length of the six Fe-N bonds is 2.20 Å(Table 3), which indicates that Fe II is under the HS state.It is worth noting that pentanuclear cation clusters form 1D chains (as shown in Figure 3a) among the cation clusters through supermolecular acting force π-π stacking (distance is about 3.40 Å) (Table S2).The structure of the anion cluster is different from that in compound 1 as shown in Figure 3.The dinuclear anions have one crystallographically independent Fe4 site, which is located in the N6 octahedral environment provided by four nitrogen atoms from two bpt − ligands and two nitrogen atoms from two NCS − anions (Figure 2b).Binuclear anion clusters form 1D chains among anion clusters through supermolecular acting force π-π stacking (Figure 3b) (Table S2).A supermolecularacting force between each anion 1D chain and pentanuclear cation chain is mainly π-π stacking between a pyridine ring and a triazole ring on the ligand, thus making it extend into a 2D layer (Figure 3c).dimensional racemic supramolecular structure through intermolecular hydrogen bonding and π-π interactions (Figure S3) [36].
The single-crystal structure of compound 2 was determined at 298 K. 2 was crystallized in monoclinic space group P2/c. 2 contains [{Fe(μ-bpt)3}2Fe3(μ3-O)] 2+ cations and one [Fe II 2(μbpt)2(NCS)4] 2− anion.The disordered solvated molecules at 298 K were removed by SQUEEZE [37] to achieve reasonable refinements.The structure of the cation and anion clusters are shown in Figure 2. The structure of [{Fe(μ-bpt)3}2Fe3(μ3-O)] 2+ is quite similar to that in compound 1.Five iron(II) ions are wrapped by six ligands, thus forming a pentagonal bipyramid configuration, and μ3-O further bridges three iron(II) ions in the equatorial plane and completes their trigonal bipyramid coordination geometries.However, different from 1, the spin state of the two iron(II) ions on axial top do not change and the average bond length of the six Fe-N bonds is 2.20 Å(Table 3), which indicates that Fe II is under the HS state.It is worth noting that pentanuclear cation clusters form 1D chains (as shown in Figure 3a) among the cation clusters through supermolecular acting force π-π stacking (distance is about 3.40 Å) (Table S2).The structure of the anion cluster is different from that in compound 1 as shown in Figure 3.The dinuclear anions have one crystallographically independent Fe4 site, which is located in the N6 octahedral environment provided by four nitrogen atoms from two bpt − ligands and two nitrogen atoms from two NCS − anions (Figure 2b).Binuclear anion clusters form 1D chains among anion clusters through supermolecular acting force π-π stacking (Figure 3b) (Table S2).A supermolecularacting force between each anion 1D chain and pentanuclear cation chain is mainly π-π stacking between a pyridine ring and a triazole ring on the ligand, thus making it extend into a 2D layer (Figure 3c).The structure of the anion cluster is different from that in compound 1 as shown in Figure 3.The dinuclear anions have one crystallographically independent Fe4 site, which is located in the N 6 Crystals 2018, 8, 119 7 of 12 octahedral environment provided by four nitrogen atoms from two bpt − ligands and two nitrogen atoms from two NCS − anions (Figure 2b).Binuclear anion clusters form 1D chains among anion clusters through supermolecular acting force π-π stacking (Figure 3b) (Table S2).A supermolecularacting force between each anion 1D chain and pentanuclear cation chain is mainly π-π stacking between a pyridine ring and a triazole ring on the ligand, thus making it extend into a 2D layer (Figure 3c).
The single-crystal structure of compound 3 was determined at 298 K. 3 was crystallized in triclinic space group P-1, consisting of unordered solvent water molecules among [{Fe II (µ-bpt) 3 } 2 Fe 3 (µ 3 -O)] 2+ pentanuclear cation, I − ion, and clusters.The structure of the cation cluster is shown in Figure 4.The structure of [{Fe(µ-bpt) 3 } 2 Fe 3 (µ 3 -O)] 2+ is quite similar to those in compounds 1 and 2 as shown in Figures 1a and 2a.Five iron(II) ions are wrapped by six ligands to form a pentagonal bipyramid configuration, and µ 3 -O further bridges three iron(II) ions in the equatorial plane and completes their trigonal bipyramid coordination geometries.However, different from 1, the spin states of the two iron(II) ions on axial top do not change, and the average bond length of the six Fe-N bonds is 1.98 Å (Table S3), which indicates that the iron(II) ions are under an LS state.
The structure of anion is different from that in compounds 1 and 2 as shown in Figure S4.The anions have two crystallographically independent I − anions; I1 ion forms three pairs of hydrogen bonds through two ethanol molecules and one pyridine from one bpt − ligand, while the I2 ion forms four pairs hydrogen bonds through four pyridines from four bpt − ligands.There is no obvious π-π stacking in 3.  S3), which indicates that the iron(II) ions are under an LS state.
The structure of the anion is different from that in compounds 1 and 2 as shown in Figure S4.The anions have two crystallographically independent I − anions; I1 ion forms three pairs of hydrogen bonds through two ethanol molecules and one pyridine from one bpt − ligand, while the I2 ion forms four pairs hydrogen bonds through four pyridines from four bpt − ligands.There is no obvious π-π stacking in 3.

Magnetism Study
To investigate the presence of spin-crossover in compound 1, variable-temperature magnetic susceptibility data were collected in the temperature range of 300-2 K on crystalline samples (Figure 5).The χMT value is 15.77 cm 3 K mol −1 at 300 K, which is smaller than the value (18 cm 3 K mol −1 ) expected for six free HS ions and one LS Fe II ion.On the whole, the χMT value smoothly decreases upon cooling with two little bulges around 210 and 70 K, and then descends more steeply to 0.76 cm 3 K mol −1 at 2 K, suggesting obvious antiferromagnetic coupling.Such magnetic behavior is repeatable in the subsequent heating mode (Figure S5).As mentioned above, X-ray crystal structure analyses clearly indicate that only Fe2 ions undergo the SCO process.To make this SCO behavior apparent, the χMT contributions of [Fe II 3(μ3-O)] 4+ core and dinuclear anion can be subtracted by the following two steps.First, the experimental χMT value of [Fe II 3(μ3-O)] 4+ core can be obtained from a similar cluster [{Fe II (μ-bpt)3} 2 Fe II 3(μ3-O)](NCS)2•10H2O, in which two apical Fe II ions are in the LS state [35].Second, after the above subtraction, the magnetic susceptibility plot between 35 K and 100 K (Fe1 and Fe2 are in the LS state) can be fitted by a simple dinuclear S = 2 model [43].The fitted curve is then

Magnetism Study
To investigate the presence of spin-crossover in compound 1, variable-temperature magnetic susceptibility data were collected in the temperature range of 300-2 K on crystalline samples (Figure 5).The χ M T value is 15.77 cm 3 K mol −1 at 300 K, which is smaller than the value (18 cm 3 K mol −1 ) expected for six free HS ions and one LS Fe II ion.On the whole, the χ M T value smoothly decreases upon cooling with two little bulges around 210 and 70 K, and then descends more steeply to 0.76 cm 3 K mol −1 at 2 K, suggesting obvious antiferromagnetic coupling.Such magnetic behavior is repeatable in the subsequent heating mode (Figure S5).As mentioned above, X-ray crystal structure analyses clearly indicate that only Fe2 ions undergo the SCO process.To make this SCO behavior apparent, the χ M T contributions of [Fe II 3 (µ 3 -O)] 4+ core and dinuclear anion can be subtracted by the following two steps.First, the experimental χ M T value of [Fe II 3 (µ 3 -O)] 4+ core can be obtained from a similar cluster [{Fe II (µ-bpt) 3 } 2 Fe II 3 (µ 3 -O)](NCS) 2 •10H 2 O, in which two apical Fe II ions are in the LS state [35].
Second, after the above subtraction, the magnetic susceptibility plot between 35 K and 100 K (Fe1 and Fe2 are in the LS state) can be fitted by a simple dinuclear S = 2 model [43].The fitted curve is then extrapolated to the whole temperature region to obtain the contribution of the dinuclear anion.Since the intramolecular magnetic coupling between the SCO-active Fe2 ion and central [Fe II 3 (µ 3 -O)] 4+ core and intermolecular magnetic interactions between anions and cations are very weak [35], they can be omitted for simplification.Moreover, the Fe1 ion remains in an LS state in the whole temperature region.Thus, the magnetic susceptibility data of only Fe2 ion is obtained and shown in Figure 6.The χ M T value at 300 K is 4.07 cm 3 K mol −1 , corresponding to the HS state of the Fe2 ion.Upon cooling, it clearly shows a gradual and complete SCO behavior with a spin transition temperature T 1/2 = 198 K.As comparison examples, the bulk magnetic susceptibility of compounds 1 and the chloro-analog [{Fe II (µ-bpt OH compounds were determined in the 2-300 K temperature range.The results are represented in Figure S6.A similar SCO behavior was observed at much lower temperatures (near 193 K) for the chloro-analog with an approximate decrease of 5 K.In other words, T 1/2 (Cl) < T 1/2 (Br).Similar results were reported in [44].
Crystals 2018, 8, 118 8 of 12 extrapolated to the whole temperature region to obtain the contribution of the dinuclear anion.Since the intramolecular magnetic coupling between the SCO-active Fe2 ion and central [Fe II 3(μ3-O)] 4+ core and intermolecular magnetic interactions between anions and cations are very weak [35], they can be omitted for simplification.Moreover, the Fe1 ion remains in an LS state in the whole temperature region.Thus, the magnetic susceptibility data of only Fe2 ion is obtained and shown in Figure 6.The χMT value at 300 K is 4.07 cm 3 K mol −1 , corresponding to the HS state of the Fe2 ion.Upon cooling, it clearly shows a gradual and complete SCO behavior with a spin transition temperature T1/2 = 198 K.

Discussion
The spin state of the two iron(II) ions in the axial direction was successfully realized by regulating different counter anions: one apical [Fe II (bpt)3] − unit exhibits spin-crossover behavior while  4+ core and intermolecular magnetic interactions between anions and cations are very weak [35], they can be omitted for simplification.Moreover, the Fe1 ion remains in an LS state in the whole temperature region.Thus, the magnetic susceptibility data of only Fe2 ion is obtained and shown in Figure 6.The χMT value at 300 K is 4.07 cm 3 K mol −1 , corresponding to the HS state of the Fe2 ion.Upon cooling, it clearly shows a gradual and complete SCO behavior with a spin transition temperature T1/2 = 198 K.

Discussion
The spin state of the two iron(II) ions in the axial direction was successfully realized by regulating different counter anions: one apical [Fe II (bpt)3] − unit exhibits spin-crossover behavior while

Discussion
The spin state of the two iron(II) ions in the axial direction was successfully realized by regulating different counter anions: one apical [Fe II (bpt) 3 ] − unit exhibits spin-crossover behavior while the other [Fe II (bpt) 3 ] − unit remains in a low spin state in 1.Both apical ions are of high spin states in 2 and are of low spin states in 3.Although these two apical iron(II) ions are coordinated by the same three bpt − ligands, previous work has already shown that their spin states could be tuned by different counterions.Both of them are in the HS states with a [Fe 2 OCl 6 ] 2− anion and in the LS states with NCS − , ClO 4 − , or I − anions [35].The main reason might be a variation of the supermolecular structure coordination environment of the {Fe 5 } cation cluster caused by anion replacement.Firstly, among the three compounds, hydrogen bond interactions exist between the {Fe 5 } cation cluster and the anion cluster.Different acceptor atoms of the hydrogen bonds and differences in the hydrogen bonds would probably influence the electronic configuration in the metal center, thus affecting the magnetic behavior [45,46] (see in Tables S1-S3).In addition, the π-π stacking effect between neighboring ion clusters is observed in both 1 and 2, and the π-π effect in 2 is obviously stronger than in 1.This effect forces the coordination sphere around the Fe(II) to accommodate this exterior constraint.Apparently, such a rearrangement would require too much energy to overcome the rigidity of the π-π stacking and is, therefore, not possible at low temperatures.In consequence, this molecular response leads to a highly distorted coordination octahedron as described above, favoring compound 2 to remain in the HS state over the whole temperature range.A similar effect was also observed recently in another iron(II) complex with the pyrene groups of the ligands and showing steadily the HS state [47].There is no obvious π-π stacking in 3. Similar observations have been previously reported [35].Previous work has already shown that two apical iron(II) ions are in the LS state with I − anions.Secondly, geometric distortion degrees that the two axial iron(II) ion coordination in the three compounds are different, which could be certified by the octahedral distortion parameter Σ (see in Table 3).For example, in compound 2, distortion of the octahedral configuration of the two axial iron(II) ions is the most serious, which means that the electrons in the coordination atoms cannot flow into the d track of the center metal atoms very well [43], and this exerts a stabilizing effect on the HS state.The relatively large Σ values of 60.5 and 61.2 for the two apical iron(II) ions in 3 suggest that the iron(II) ions are in the LS state (Table 3).Interestingly, the SCO at the Fe2 site is associated with a more regular coordination environment about the Fe1 site, as evidenced by the increase in Σ from 59.5 to 96.6 • , which also indicates that only Fe2 experiences spin transition.Similar observations have been previously reported [8,36].Finally, a slightly shorter trans Fe2−N bond length (Table 3) should play a role in its transition, taking into account that the ligand field strength [48], ∆ 0 , is proportional to 1/R 6 (Fe−N bond distances); a decrease of ca.9.5% can be estimated for the Fe2 site with respect to the Fe1 site.These indicate that the observation of SCO in 1 relies on the presence of the Fe2 configuration providing an optimized ligand field.

Conclusions
In summary, by introducing an unsymmetrical binuclear anion [Fe II 2 (µ-Br)(µ-bpt)(NCS) 4 (H 2 O)] 2- to the pentanuclear cluster helicate, different magnetic behaviors of two apical [Fe II (bpt) 3 ] − units in the cationic cluster with trigonal bipyramidal topology can be obtained, in which one exhibits the SCO property while the other remains in the LS state; by introducing [Fe II 2 (µ-bpt) 2 (NCS) 4 ] 2− and I − counterions to the pentanuclear {Fe 5 } cluster compounds, the spin states of two apical iron(II) ions in the cationic pentanuclear {Fe 5 } clusters can be tuned in an HS and LS state, respectively.The origin of the different spin states can ascribe to their different strengths of intermolecular interactions.Then, to our knowledge, 1 represents the second case of SCO cluster helicate, which provides an excellent example to introduce SCO properties into cluster helicates to explore multifunctional materials.S1: Geometrical Parameters of the Hydrogen Bonds in Compound 1; Table S2: Geometrical Parameters of the Hydrogen Bonds in Compound 2; Table S3: Geometrical Parameters of the Hydrogen Bonds in Compound 3.

Figure 1 .
Figure 1.Structure illustrations of 1: coordination geometries of the pentanuclear cation (a) and binuclear anion (b) and tetramer in the structure of 1 (c).

Figure 1 .
Figure 1.Structure illustrations of 1: coordination geometries of the pentanuclear cation (a) and binuclear anion (b) and tetramer in the structure of 1 (c).

Crystals 2018, 8 , 118 7 of 12 Figure 3 .
Figure 3. Structure illustrations of 2: π-π interactions between the pentanuclear iron(II) cluster (a), ππ interactions between the binuclear anion (b), π-π interactions between the pentanuclear cation and binuclear anion (c).The single-crystal structure of compound 3 was determined at 298 K. 3 was crystallized in triclinic space group P-1, consisting of unordered solvent water molecules among [{Fe II (μbpt)3}2Fe3(μ3-O)] 2+ pentanuclear cation, I − ion, and clusters.The structure of the cation cluster is shown in Figure 4.The structure of [{Fe(μ-bpt)3}2Fe3(μ3-O)] 2+ is quite similar to those in compounds 1 and 2 as shown in Figures 1a and 2a.Five iron(II) ions are wrapped by six ligands to form a pentagonal bipyramid configuration, and μ3-O further bridges three iron(II) ions in the equatorial plane and completes their trigonal bipyramid coordination geometries.However, different from 1, the spin states of the two iron(II) ions on axial top do not change, and the average bond length of the six Fe-N bonds is 1.98 Å (TableS3), which indicates that the iron(II) ions are under an LS state.The structure of the anion is different from that in compounds 1 and 2 as shown in FigureS4.The anions have two crystallographically independent I − anions; I1 ion forms three pairs of hydrogen bonds through two ethanol molecules and one pyridine from one bpt − ligand, while the I2 ion forms four pairs hydrogen bonds through four pyridines from four bpt − ligands.There is no obvious π-π stacking in 3.

Figure 4 .
Figure 4. Structure illustrations of 3: coordination geometries of the pentanuclear cation and I − anion.Hydrogen atoms are omitted.

Figure 4 .
Figure 4. Structure illustrations of 3: coordination geometries of the pentanuclear cation and I − anion.Hydrogen atoms are omitted.

Figure 6 .
Figure 6.The χMT value of the Fe2 ion is obtained by subtracting the contributions of the [Fe II 3(μ3-O)] 4+ core and the dinuclear anion from 1.

Figure 6 .
Figure 6.The χMT value of the Fe2 ion is obtained by subtracting the contributions of the [Fe II 3(μ3-O)] 4+ core and the dinuclear anion from 1.

Figure 6 .
Figure 6.The χ M T value of the Fe2 ion is obtained by subtracting the contributions of the [Fe II 3 (µ 3 -O)] 4+ core and the dinuclear anion from 1.

Figure S3 :
Figure S3: Packing diagram of 1; Figure S4.Structural illustrations of 3: the hydrogen bond interactions.Figure S5: Plots of χ M T vs. T for 1; Figure S6: Plots of χ M T vs. T for 1 and Cl-analogue.TableS1: Geometrical Parameters of the Hydrogen Bonds in Compound 1; TableS2: Geometrical Parameters of the Hydrogen Bonds in Compound 2; TableS3: Geometrical Parameters of the Hydrogen Bonds in Compound 3.