The Role of Coulomb Interactions for Spin Crossover Behaviors and Crystal Structural Transformation in Novel Anionic Fe(III) Complexes from a π-Extended ONO Ligand

: To investigate the π -extension effect on an unusual negative-charged spin crossover (SCO) Fe III complex with a weak N 2 O 4 ﬁrst coordination sphere, we designed and synthesized a series of anionic Fe III complexes from a π -extended naphthalene derivative ligand. Acetonitrile-solvate tetramethylammonium (TMA) salt 1 exhibited an SCO conversion, while acetone-solvate TMA salt 2 was in a high-spin state. The crystal structural analysis for 2 revealed that two-leg ladder-like cation-anion arrays derived from π -stacking interactions between π -ligands of the Fe III complex anion and Coulomb interactions were found and the solvated acetone molecules were in one-dimensional channels between the cation-anion arrays. A desolvation-induced single-crystal-to-single-crystal transformation to desolvate compound 2’ may be driven by Coulomb energy gain. Furthermore, the structural comparison between quasi-polymorphic compounds 1 and 2 revealed that the synergy between Coulomb and π -stacking interactions induces a signiﬁcant distortion of coordination structure of 2 anion, and the 109th and 111th MOs for the HS [Fe(azp) 2 ] anion. Compared with the energy levels of the corresponding MOs between the [Fe(aznp) 2 ] and [Fe(azp) 2 ] anion, the π -extension of a ligand leads to slight stabilization of the MOs containing Fe d orbitals except the 135th MO containing π -orbital contribution from the naphtholate moiety. The stabilization of the MOs can be explained by similarly lowering the energies of an anionic π -ligand orbital by π -extension. On the other hand, the increase in 135th orbital energy suggests that the orbital interactions between Fe e g -like orbital and O π -orbital of the naphtholate moiety may be enhanced by π -extension. Although this leads to slight decrease in the ligand-ﬁeld splitting energy of the [Fe(aznp) 2 ] anion, a series of the [Fe(aznp) 2 ] complexes were still in the SCO region but their HS fractions were relatively increased as shown in the magnetic susceptibility section.


Introduction
Spin crossover (SCO) is one of a number of switching phenomena, namely a spin-state equilibrium or transition between high-spin (HS) and low-spin (LS) state found in a transition metal coordination complex.Since SCO can be induced by external stimuli such as temperature, pressure, magnetic field, and light, SCO complexes have aroused growing attention as externally-controllable molecular materials [1,2].Recently the applications of SCO complexes toward memory, display, and sensing devices as well as the development of multifunctional SCO compounds combining SCO with electronic properties such as electrical conductivity [3][4][5][6][7], magnetic property [8][9][10][11][12], and optical property [13,14] are actively studied.
A typical SCO complex is an octahedral six-coordinate complex whose metal center is a Fe II , Fe III , or Co II ion.Since SCO originates from competition between a spin-pairing and ligand-field splitting energy, the occurrence of SCO strongly depends on the number and kind of coordination donor atoms.Usual donor atom sets for SCO Fe II and Fe III complexes are N 6 and N 4 O 2 , respectively.This means that the ligand field of an SCO Fe III complex is weaker than that of an SCO Fe II complex.Note that the charge of an SCO complex is generally cationic or neutral and, thus, anionic mononuclear SCO complexes are very rare [15][16][17][18][19].
Recently, we discovered unusual anionic SCO Fe III complexes, (C + )[Fe III (azp) 2 ] [C + = monovalent cation, H 2 azp = (2'-hydroxyphenylazo)-2-hydroxybenzene] [20], whose first coordination sphere comprised a N 2 O 4 donor atom set assuming a weak ligand-field for Fe III complexes.The density functional theory (DFT) calculations for the [Fe III (azp) 2 ] complex suggested that orbital interactions between the Fe III atomic orbital and vicinity of frontier orbitals of an azp ligand may enhance the ligand-field splitting energy.To investigate the perturbation of molecular orbitals of a ligand and the introduction of intermolecular π-stacking interactions between Fe III complex anions, we focused on a π-extended naphthalene-substituted ligand, aznp dianion [H 2 aznp = (2'-hydroxyphenylazo)-2-hydroxynaphthalene].The synthesis and magnetic susceptibility of a series of monovalent quaternary onium salts with the [Fe III (aznp) 2 ] anion (1-5, Figure 1) revealed that tetramethylammonium (TMA) salts gave quasi-polymorphic acetonitrile-solvate SCO complex (1) and acetone-solvate HS complex (2).The X-ray crystal structure and thermal analysis for 2 revealed that it was successful to introduce π-stacking interactions between the Fe III anions and moreover a single-crystal-to-single-crystal (SC-SC) structural transformation from acetone-solvate complex 2 to non-solvate complex (2') was induced by desolvation.The structural comparison between the quasi-polymorphic compounds, HS 1, LS 1, 2, and 2' indicates that Coulomb interactions do not contribute to the cooperativity of SCO phenomenon, whereas the synergy between Coulomb interactions and intermolecular π-stacking interactions induces the distortion of coordination structures of the SCO complex ion, which results in the HS state for 2 and 2'.Although the role of Coulomb interaction has hardly been described in the studies of SCO complexes to date, the present study discloses the role of Coulomb interactions for charged SCO complexes.We, herein, report the synthesis, magnetic, and thermal property, and crystal structures of a series of the [Fe III (aznp) 2 ] complexes.
Crystals 2016, 6, 49 2 of 16 A typical SCO complex is an octahedral six-coordinate complex whose metal center is a Fe II , Fe III , or Co II ion.Since SCO originates from competition between a spin-pairing and ligand-field splitting energy, the occurrence of SCO strongly depends on the number and kind of coordination donor atoms.Usual donor atom sets for SCO Fe II and Fe III complexes are N6 and N4O2, respectively.This means that the ligand field of an SCO Fe III complex is weaker than that of an SCO Fe II complex.Note that the charge of an SCO complex is generally cationic or neutral and, thus, anionic mononuclear SCO complexes are very rare [15][16][17][18][19].
Recently, we discovered unusual anionic SCO Fe III complexes, (C + )[Fe III (azp)2] [C + = monovalent cation, H2azp = (2'-hydroxyphenylazo)-2-hydroxybenzene] [20], whose first coordination sphere comprised a N2O4 donor atom set assuming a weak ligand-field for Fe III complexes.The density functional theory (DFT) calculations for the [Fe III (azp)2] complex suggested that orbital interactions between the Fe III atomic orbital and vicinity of frontier orbitals of an azp ligand may enhance the ligand-field splitting energy.To investigate the perturbation of molecular orbitals of a ligand and the introduction of intermolecular π-stacking interactions between Fe III complex anions, we focused on a π-extended naphthalene-substituted ligand, aznp dianion [H2aznp = (2'-hydroxyphenylazo)-2-hydroxynaphthalene].The synthesis and magnetic susceptibility of a series of monovalent quaternary onium salts with the [Fe III (aznp)2] anion (1-5, Figure 1) revealed that tetramethylammonium (TMA) salts gave quasi-polymorphic acetonitrile-solvate SCO complex (1) and acetone-solvate HS complex (2).The X-ray crystal structure and thermal analysis for 2 revealed that it was successful to introduce π-stacking interactions between the Fe III anions and moreover a single-crystal-to-single-crystal (SC-SC) structural transformation from acetone-solvate complex 2 to non-solvate complex (2') was induced by desolvation.The structural comparison between the quasi-polymorphic compounds, HS 1, LS 1, 2, and 2' indicates that Coulomb interactions do not contribute to the cooperativity of SCO phenomenon, whereas the synergy between Coulomb interactions and intermolecular π-stacking interactions induces the distortion of coordination structures of the SCO complex ion, which results in the HS state for 2 and 2'.Although the role of Coulomb interaction has hardly been described in the studies of SCO complexes to date, the present study discloses the role of Coulomb interactions for charged SCO complexes.We, herein, report the synthesis, magnetic, and thermal property, and crystal structures of a series of the [Fe III (aznp)2] complexes.

Synthesis of Ligand
The H2aznp molecule and its derivatives were known as azo-dyes.Two different procedures were reported to synthesize H2aznp.One is one-step azo-coupling reaction of 2-hydroxyphenyl diazonium salt with 2-naphtholate [21] and another is a two-step reaction, namely an azo-coupling reaction using phenyl diazonium salt followed by oxidative hydroxylation of 2-hydroxyphenylazo compound [22].When we tried to synthesize H2aznp according to the former procedure, it was not successful to obtain H2aznp.Since the formation of diazocyclohexadienone is possible [23], the

Synthesis of Ligand
The H 2 aznp molecule and its derivatives were known as azo-dyes.Two different procedures were reported to synthesize H 2 aznp.One is one-step azo-coupling reaction of 2-hydroxyphenyl diazonium salt with 2-naphtholate [21] and another is a two-step reaction, namely an azo-coupling reaction using phenyl diazonium salt followed by oxidative hydroxylation of 2-hydroxyphenylazo compound [22].When we tried to synthesize H 2 aznp according to the former procedure, it was not successful to obtain H 2 aznp.Since the formation of diazocyclohexadienone is possible [23], the hydroxyl group was protected to prevent 2-hydroxyphenyl diazonium salt from undesired isomerization.According to the synthesis of H 2 aznp derivatives [24], the synthesis of H 2 aznp was achieved by azo-coupling reaction of 2-methoxyphenyl diazonium salt followed by demethylation using AlCl 3 [25] in a moderate yield.

Synthesis of Complexes 1-5
The complexes 1-5 were prepared according to the literature procedure [20].The compositions were confirmed by microanalyses and crystal analyses.

Thermogravimetry-Differential Thermal Analysis (TG-DTA) for 1-3
To investigate thermal stability of the solvate compounds 1-3, TG-DTA were performed using a Rigaku TG8120 analyzer.The TG curves are shown in Figure 2. On heating compound 1, the weight loss was observed at around room temperature.However, consecutive decrease in weight was very gradual and the weight loss reached to 5.9% at 245 ˝C.Further heating resulted in abrupt decrease in weight, indicative of decomposition of compound 1.The weight loss ratio at 245 ˝C exactly corresponded to the weight ratio of one acetonitrile molecule (5.9%).On the other hand, the decrease in weight for 2 started at 75 ˝C and then the weight loss reached to 8.4% at 145 ˝C.The weight was almost constant up to 200 ˝C.The weight loss was in good agreement with the release of one acetone molecule (8.2%) from compound 2. The difference of the thermal behaviors between 1 and 2 will be discussed in the crystal structure description section.On heating 3, the weight decreased most rapidly.The weight loss at 120 ˝C was 8.5%, which was consistent with the loss of 1.5 acetonitrile molecules (8.0%).
Crystals 2016, 6, 49 3 of 16 hydroxyl group was protected to prevent 2-hydroxyphenyl diazonium salt from undesired isomerization.According to the synthesis of H2aznp derivatives [24], the synthesis of H2aznp was achieved by azo-coupling reaction of 2-methoxyphenyl diazonium salt followed by demethylation using AlCl3 [25] in a moderate yield.

Synthesis of Complexes 1-5
The complexes 1-5 were prepared according to the literature procedure [20].The compositions were confirmed by microanalyses and crystal analyses.

Thermogravimetry-Differential Thermal Analysis (TG-DTA) for 1-3
To investigate thermal stability of the solvate compounds 1-3, TG-DTA were performed using a Rigaku TG8120 analyzer.The TG curves are shown in Figure 2. On heating compound 1, the weight loss was observed at around room temperature.However, consecutive decrease in weight was very gradual and the weight loss reached to 5.9% at 245 °C.Further heating resulted in abrupt decrease in weight, indicative of decomposition of compound 1.The weight loss ratio at 245 °C exactly corresponded to the weight ratio of one acetonitrile molecule (5.9%).On the other hand, the decrease in weight for 2 started at 75 °C and then the weight loss reached to 8.4% at 145 °C.The weight was almost constant up to 200 °C.The weight loss was in good agreement with the release of one acetone molecule (8.2%) from compound 2. The difference of the thermal behaviors between 1 and 2 will be discussed in the crystal structure description section.On heating 3, the weight decreased most rapidly.The weight loss at 120 °C was 8.5%, which was consistent with the loss of 1.5 acetonitrile molecules (8.0%).After cooling the measured samples, compounds 1 and 3 turned to be a powder and amorphous solid, respectively.The DTA curve suggested that compound 3 melted above 150 °C.Interestingly, we found that compound 2 maintained a crystalline form after the TG measurement.We designate the desolvate crystal as compound 2' hereafter.To check the possibility of uptake of a small molecule into 2', the TG-DTA for the pristine sample 2' and samples exposed to vapors of common organic solvents revealed no weight loss up to 200 °C.Moreover, the crystalline form 2' unchanged after the exposure, suggesting that compound 2' may not have any pore to catch up a small molecule.

Magnetic Susceptibility for 1-5
The temperature variations of magnetic susceptibility for compounds 1-5 along with the desolvate compound 2' are shown in Figure 3.The χMT value for acetonitrile-solvate compound 1 at 300 K was 3.75 cm 3 K•mol −1 .On cooling the sample, the χMT values were gradually decreased and reached to 1.56 cm 3 K•mol −1 at 80 K, suggesting compound 1 exhibited a magnetic transition.This magnetic behavior of 1 was quite similar to that of the parent [Fe III (azp)2] compound [20].On the other hand, the χMT values below 80 K were higher than those of the parent compound.There are After cooling the measured samples, compounds 1 and 3 turned to be a powder and amorphous solid, respectively.The DTA curve suggested that compound 3 melted above 150 ˝C.Interestingly, we found that compound 2 maintained a crystalline form after the TG measurement.We designate the desolvate crystal as compound 2' hereafter.To check the possibility of uptake of a small molecule into 2', the TG-DTA for the pristine sample 2' and samples exposed to vapors of common organic solvents revealed no weight loss up to 200 ˝C.Moreover, the crystalline form 2' unchanged after the exposure, suggesting that compound 2' may not have any pore to catch up a small molecule.

Magnetic Susceptibility for 1-5
The temperature variations of magnetic susceptibility for compounds 1-5 along with the desolvate compound 2' are shown in Figure 3.The χ M T value for acetonitrile-solvate compound 1 at 300 K was 3.75 cm 3 K¨mol ´1.On cooling the sample, the χ M T values were gradually decreased and reached to 1.56 cm 3 K¨mol ´1 at 80 K, suggesting compound 1 exhibited a magnetic transition.This magnetic behavior of 1 was quite similar to that of the parent [Fe III (azp) 2 ] compound [20].On the other hand, the χ M T values below 80 K were higher than those of the parent compound.There are various possibilities regarding this magnetic transition, namely incomplete SCO conversion between the HS (S = 5/2) and LS (S = 1/2) state in an Fe III complex, that between the HS (S = 2) and LS (S = 0) state in an Fe II complex with one semiquinone-like radical (S = 1/2), or metal-to-ligand charge transfer (MLCT) transition from an HS Fe III complex (S = 5/2) to HS Fe II complex (S = 2) antiferromagnetically coupled to a semiquinone-like radical (S = 1/2).As discussed in Mössbauer spectroscopy and crystal structures sections, this magnetic transition proves to originate from the SCO conversion in the Fe III complex.The acetone-solvate compound 2 showed a different magnetic behavior.The χ M T value for 2 at 300 K was 4.11 cm 3 K¨mol ´1, indicating that 2 was in the HS state of an Fe III complex.On lowering temperatures, the χ M T values were almost constant down to 15 K and then slightly decreased, probably due to zero-field splitting, indicating compound 2 was in the HS state of the Fe III complex in the whole temperature range.Moreover, the magnetic susceptibility of 2' revealed that desolvation from 2 did not affect the spin-state of the compound.The difference of magnetic behaviors between quasi-polymorphic compounds 1, 2, and 2' is attributed to that of crystal packing effects.The magnetic behaviors for compounds 3-5 were less thermally dependent and nearly in the HS state of an Fe III complex.These observations imply that π-extension of ligands for the [Fe(azp) 2 ] compounds favors the HS state.various possibilities regarding this magnetic transition, namely incomplete SCO conversion between the HS (S = 5/2) and LS (S = 1/2) state in an Fe III complex, that between the HS (S = 2) and LS (S = 0) state in an Fe II complex with one semiquinone-like radical (S = 1/2), or metal-to-ligand charge transfer (MLCT) transition from an HS Fe III complex (S = 5/2) to HS Fe II complex (S = 2) antiferromagnetically coupled to a semiquinone-like radical (S = 1/2).As discussed in Mössbauer spectroscopy and crystal structures sections, this magnetic transition proves to originate from the SCO conversion in the Fe III complex.The acetone-solvate compound 2 showed a different magnetic behavior.The χMT value for 2 at 300 K was 4.11 cm 3 K•mol −1 , indicating that 2 was in the HS state of an Fe III complex.On lowering temperatures, the χMT values were almost constant down to 15 K and then slightly decreased, probably due to zero-field splitting, indicating compound 2 was in the HS state of the Fe III complex in the whole temperature range.Moreover, the magnetic susceptibility of 2' revealed that desolvation from 2 did not affect the spin-state of the compound.The difference of magnetic behaviors between quasi-polymorphic compounds 1, 2, and 2' is attributed to that of crystal packing effects.The magnetic behaviors for compounds 3-5 were less thermally dependent and nearly in the HS state of an Fe III complex.These observations imply that π-extension of ligands for the [Fe(azp)2] compounds favors the HS state.

Mössbauer Spectroscopy for 1
To confirm the valence and spin states of the Fe ion for compound 1, the temperature valuable Mössbauer spectra for 1 were recorded at 10.5, 30, 70, 180 and 293 K (Figure 4).The spectrum at 293 K consisted of mainly a broad singlet with a small shoulder, suggesting the overlapping of a doublet spectrum.On lowering temperatures, a quadrupole doublet was developed at 180 K and then the doublet spectra unchanged at 70 K and below.Assuming the observed spectra at 180 and 293 K from a broad singlet and quadrupole doublet, the fitting parameters for isomer shift (IS) and quadrupole splitting (QS) are listed in Table 1.The IS value for a broad singlet is typical of an HS Fe III or LS Fe II complex.If compound 1 consists of LS Fe II center with one semiquinone-like radical, the χMT value of 3.75 cm 3 K•mol −1 at 300 K cannot be explained.Therefore, a broad singlet spectrum is ascribed to an HS Fe III complex.The IS value for a quadrupole doublet is typical of an LS Fe III complex.Consequently, the temperature variations in Mössbauer spectra clearly revealed that the magnetic conversion in compound 1 originates from the SCO transition of the Fe III complex.However, it was very difficult to fit the observed spectra from a broad HS singlet and LS quadrupole doublet at 70 K and below.This is probably because an HS singlet is too broad to fit the spectra.The magnetic relaxation due to a weak spin-spin interaction were known to result in broadening or magnetic splitting of a spectrum for magnetically-diluted HS Fe III complexes [26,27].Therefore, we simulated each observed spectrum by fixing the isomer shift for HS species at 70 K and below.As compared with the parent [Fe(azp)2] compound [20], the Mössbauer parameters for

Mössbauer Spectroscopy for 1
To confirm the valence and spin states of the Fe ion for compound 1, the temperature valuable Mössbauer spectra for 1 were recorded at 10.5, 30, 70, 180 and 293 K (Figure 4).The spectrum at 293 K consisted of mainly a broad singlet with a small shoulder, suggesting the overlapping of a doublet spectrum.On lowering temperatures, a quadrupole doublet was developed at 180 K and then the doublet spectra unchanged at 70 K and below.Assuming the observed spectra at 180 and 293 K from a broad singlet and quadrupole doublet, the fitting parameters for isomer shift (IS) and quadrupole splitting (QS) are listed in Table 1.The IS value for a broad singlet is typical of an HS Fe III or LS Fe II complex.If compound 1 consists of LS Fe II center with one semiquinone-like radical, the χ M T value of 3.75 cm 3 K¨mol ´1 at 300 K cannot be explained.Therefore, a broad singlet spectrum is ascribed to an HS Fe III complex.The IS value for a quadrupole doublet is typical of an LS Fe III complex.Consequently, the temperature variations in Mössbauer spectra clearly revealed that the magnetic conversion in compound 1 originates from the SCO transition of the Fe III complex.However, it was very difficult to fit the observed spectra from a broad HS singlet and LS quadrupole doublet at 70 K and below.This is probably because an HS singlet is too broad to fit the spectra.The magnetic relaxation due to a weak spin-spin interaction were known to result in broadening or magnetic splitting of a spectrum for magnetically-diluted HS Fe III complexes [26,27].Therefore, we simulated each observed spectrum by fixing the isomer shift for HS species at 70 K and below.As compared with the parent [Fe(azp) 2 ] compound [20], the Mössbauer parameters for HS species in 1 are in good agreement of those in the parent compound, while the IS values for LS in 1 are slightly smaller than those in the parent compound.  Isomer shift. 2 Quadrupole splitting. 3Linewidth. 4High-spin. 5Low-spin.

Crystal Structures of 1 and 2
Single crystal X-ray structural analyses for 1, 2, and 2' were performed using a Bruker AXS APEXII Ultra diffractometer (Bruker AXS, Yokohama, Japan).Fortunately, it was successful to determine the crystal structure of the desolvate compound 2' despite a poor quality of crystal.

Crystal Structures of 1 and 2
Single crystal X-ray structural analyses for 1, 2, and 2' were performed using a Bruker AXS APEXII Ultra diffractometer (Bruker AXS, Yokohama, Japan).Fortunately, it was successful to determine the crystal structure of the desolvate compound 2' despite a poor quality of crystal.Crystallographic data Crystals 2016, 6, 49 6 of 16 are listed in Table 2.The crystal structures for 1 at 90 and 273 K were isostructural and belonged to orthorhombic system with Pbca, whereas the crystal structures of 2 and 2' belonged to triclinic P-1.All asymmetric units contained one TMA cation and one [Fe(aznp) 2 ] anion and, additionally, one solvent molecule for 1 and 2.Although a positional disorder of the TMA cation was observed in 1, there was no orientational disorder of π-ligands in all [Fe(aznp) 2 ] compounds unlike the parent [Fe(azp) 2 ] compounds [20].The π-ligand molecule was coordinated to a central Fe atom as a tridentate chelate ligand and, thus, two coordinated ligand molecules were arranged in an almost perpendicular manner (Figure 5).Although both azo nitrogen atoms are possible to coordinate to a Fe atom, the nitrogen atom bonding to the phenyl ring was uniquely bound to the Fe atom, resulting in the formation of five-membered and six-membered chelating structures for the phenolate and naphtholate moieties, respectively.This suggests that the steric repulsion between the lone pair of an uncoordinated nitrogen atom and peri-hydrogen atom of a naphthalene ring governs the formation of a coordination structure in the [Fe(aznp) 2 ] anion.
The selected coordination bond lengths and distortion parameters are listed in Table 3.As compared with the coordination bond lengths between 90 and 273 K in 1, the average differences in the Fe-O bond lengths for the six-membered and five-membered chelate side were 0.086(3) and 0.043(3) Å, respectively.On the other hand, the average difference in the Fe-N bond lengths was 0.164(3) Å.The difference of the Fe-O bond lengths was very similar to that of the parent [Fe III (azp) 2 ] compound [20], whereas that of the Fe-N bond lengths was smaller than that of the parent compound.This observation was in good agreement with larger change in the HS/LS ratio in the parent compound.The distortion parameters of Σ, Θ, and φ in 1 were largely changed between 90 and 273 K, which corresponded to the HS-LS conversion, whereas the dihedral angles of θ were unchanged.Although there is a possibility that an MLCT may occur in 1 as described in the magnetic susceptibility section, the differences in the C-O, C-N, and N=N bond lengths between 90 and 273 K were less than 0.01 Å.This observation clearly indicates that the magnetic conversion in 1 originates from the SCO not MLCT transition.The selected coordination bond lengths and distortion parameters are listed in Table 3.As compared with the coordination bond lengths between 90 and 273 K in 1, the average differences in the Fe-O bond lengths for the six-membered and five-membered chelate side were 0.086(3) and 0.043(3) Å, respectively.On the other hand, the average difference in the Fe-N bond lengths was 0.164(3) Å.The difference of the Fe-O bond lengths was very similar to that of the parent [Fe III (azp)2] compound [20], whereas that of the Fe-N bond lengths was smaller than that of the parent compound.This observation was in good agreement with larger change in the HS/LS ratio in the parent compound.The distortion parameters of Σ, Θ, and φ in 1 were largely changed between 90 and 273 K, which corresponded to the HS-LS conversion, whereas the dihedral angles of θ were unchanged.Although there is a possibility that an MLCT may occur in 1 as described in the magnetic susceptibility section, the differences in the C-O, C-N, and N=N bond lengths between 90 and 273 K were less than 0.01 Å.This observation clearly indicates that the magnetic conversion in 1 originates from the SCO not MLCT transition.
Both Fe-O and Fe-N bond lengths of 1 at 273 K were a little shorter than those of 2 at 90 K and 2' at 296 K, suggesting that the HS fractions of 2 and 2' were larger than that of 1.This is consistent with the magnetic susceptibility data.Note that two kinds of distortion parameters of Σ and Θ for 2 and 2' were much larger than those for 1 at 273 K.This suggests that some crystal packing effects may induce the distortion of a coordination structure.1 The sum of the absolute differences of bite angles from 90°. 2 The sum of the absolute differences of all the angles of triangle surfaces of an coordination octahedron from 60°. 3 The angles of N1-Fe1-N3. 4Dihedral angles between the mean planes of π-ligands in the [Fe(aznp)2] anion.   (1 80.24 (11) 99.67 (10) 91.4(4) Θ 2 / ˝46.72 (13)  151.24 (9) 185.12(9) 172.3(3) φ 3 / ˝172.44 (15)  168.96 (10) 168.30(9) 165.0(4) θ 4 / ˝87.60 87.51 88.97 87.6 Both Fe-O and Fe-N bond lengths of 1 at 273 K were a little shorter than those of 2 at 90 K and 2' at 296 K, suggesting that the HS fractions of 2 and 2' were larger than that of 1.This is consistent with the magnetic susceptibility data.Note that two kinds of distortion parameters of Σ and Θ for 2 and 2' were much larger than those for 1 at 273 K.This suggests that some crystal packing effects may induce the distortion of a coordination structure.

Crystal Description of 1
The molecular arrangement for 1 at 273 K is depicted in Figure 6.No notable intermolecular interaction between [Fe(aznp) 2 ] anions and acetonitrile molecules was found in 1.Short cation¨¨¨anion distances between the nitrogen atoms of the TMA cation and iron atoms of [Fe(aznp) 2 ] anion were 5.549 (a), 5.724 (b), and 5.736 Å (c) at 273 K (Table 4), which formed two-dimensional (2D) honeycomb Coulomb interaction networks parallel to the ab plane (Figure 6a).On the other hand, the shortest distance along the c axis was 11.760 Å (e) at 273 K. Therefore, the crystal of 1 mainly constructed 2D networks based on Coulomb interactions between cations and anions.At 90 K there was no remarkable difference in the corresponding cation¨¨¨anion distances.This suggests that the Coulomb interactions does not largely affect the SCO behavior.4), which formed two-dimensional (2D) honeycomb Coulomb interaction networks parallel to the ab plane (Figure 6a).On the other hand, the shortest distance along the c axis was 11.760 Å (e) at 273 K. Therefore, the crystal of 1 mainly constructed 2D networks based on Coulomb interactions between cations and anions.At 90 K there was no remarkable difference in the corresponding cation•••anion distances.This suggests that the Coulomb interactions does not largely affect the SCO behavior.4.   4. The shortest distances between the Fe centers were 8.6246(g) and 8.5622(g) Å at 273 and 90 K, respectively.These values were a little longer than 5.832 Å for [Fe(pap) 2 ](ClO 4 )¨H 2 O [28] and 7.063 Å for [Fe(asal) 2 ](NCSe)CH 2 Cl 2 [29] as similar mononuclear SCO Fe III complexes from π-ligands.These observations might lead to the magnetic relaxation found in Mössbauer spectra in 1.

Crystal Description of 2
The molecular arrangement for 2 at 90 K is shown in Figure 7a,c,e.Interestingly, π-stacking interactions between neighboring Fe III complex anions were observed (Figure 7a, green double arrows).The π-plane distance between the naphthyl ring and the centroid of phenyl ring was 3.44 Å (Table 4).The intermolecular π-stacking interactions construct a one-dimensional (1D) Fe anion array along the a axis.The short cation¨¨¨anion distances between the nitrogen atoms of TMA cation and iron atoms of [Fe(aznp) 2 ] anion along the 1D Fe anion array were 4.982 (a) and 5.649 Å (b) and therefore the TMA cation and [Fe(aznp) 2 ] anion were alternately arranged along the a axis.Moreover, the short cation¨¨¨anion distances along the c axis were calculated to be 5.767 (c) and 10.299 Å (d) between the Crystals 2016, 6, 49 9 of 16 1D Fe anion arrays, whereas those along the b axis were 11.381 (e) and 13.357 Å (f) (Figure 7c, Table 4).Consequently, two-leg ladder-like cation¨¨¨anion arrays were formed by π-stacking and Coulomb interactions in compound 2. Note that solvate acetone molecules existed in the cavity between the two-leg ladder-like arrays and then 1D channels perpendicular to the arrays were found along the b axis (Figure 7e).However, the thermal stability of compound 2 observed in the TG measurement is not consistent with this channel structure.One possible reason is that the diameter size of 1D channels are smaller than the molecular size of acetone.Another is electrostatic and hydrogen bonding interactions between acetone molecules and TMA cations, because all acetone molecules were arranged in a manner directing the oxygen atom to the TMA cation and the distance between the nitrogen atom of the cation and the oxygen atom of acetone molecule was 3.785 Å, and the C(TMA cation)¨¨¨O distances were 3.343 and 3.366 Å.
π-stacking interactions between neighboring Fe III complex anions were observed (Figure 7a, green double arrows).The π-plane distance between the naphthyl ring and the centroid of phenyl ring was 3.44 Å (Table 4).The intermolecular π-stacking interactions construct a one-dimensional (1D) Fe anion array along the a axis.The short cation•••anion distances between the nitrogen atoms of TMA cation and iron atoms of [Fe(aznp)2] anion along the 1D Fe anion array were 4.982 (a) and 5.649 Å (b) and therefore the TMA cation and [Fe(aznp)2] anion were alternately arranged along the a axis.Moreover, the short cation•••anion distances along the c axis were calculated to be 5.767 (c) and 10.299 Å (d) between the 1D Fe anion arrays, whereas those along the b axis were 11.381 (e) and 13.357 Å (f) (Figure 7c, Table 4).Consequently, two-leg ladder-like cation•••anion arrays were formed by π-stacking and Coulomb interactions in compound 2. Note that solvate acetone molecules existed in the cavity between the two-leg ladder-like arrays and then 1D channels perpendicular to the arrays were found along the b axis (Figure 7e).However, the thermal stability of compound 2 observed in the TG measurement is not consistent with this channel structure.One possible reason is that the diameter size of 1D channels are smaller than the molecular size of acetone.Another is electrostatic and hydrogen bonding interactions between acetone molecules and TMA cations, because all acetone molecules were arranged in a manner directing the oxygen atom to the TMA cation and the distance between the nitrogen atom of the cation and the oxygen atom of acetone molecule was  The molecular arrangement for 2' at 293 K is shown in Figure 7b,d.The TG measurement and single crystal structural analysis for 2' revealed that an SC-SC structural transformation from 2 to 2' occurred upon desolvation.The crystal system and space group were unchanged accompanying the SC-SC transformation.The SC-SC transformation resulted in the shortening of 1.6 Å along the b axis  4. The molecular arrangement for 2' at 293 K is shown in Figure 7b,d.The TG measurement and single crystal structural analysis for 2' revealed that an SC-SC structural transformation from 2 to 2' occurred upon desolvation.The crystal system and space group were unchanged accompanying the SC-SC transformation.The SC-SC transformation resulted in the shortening of 1.6 Å along the b axis and 0.8 Å along the c axis, whereas the lengthening of 0.8 Å along the a axis.These changes in the cell parameters were rationalized by the motion to fill in the cavities given by the release of acetone molecules as compared between Figure 7c,d.The crystal structure of 2' maintained the two-leg ladder-like molecular arrays in 2. The short cation¨¨¨anion distances in the two-leg ladder-like arrays (a, b, c) were slightly changed, whereas those between them (e, f) were significantly shortened.Note that π-stacking interactions between the neighboring two-leg ladder-like molecular arrays appeared along both the b and c axis (Figure 7d, Table 4).Consequently, 2D π-stacking interaction networks parallel to the ac plane in 2' were connected by the Coulomb interactions, affording 3D closed-packing structure.Since the Coulomb interaction and π-stacking interaction are long-distance and short-distance interactions, respectively, the SC-SC transformation may be driven by the Coulomb energy gains due to the shrinkage of cation¨¨¨anion distances.

Structural Comparison between 1, 2, and 2'
As compared with the crystal structures of 1 and 2, the crystal of 1 comprised 2D honeycomb Coulomb interaction networks, whereas that of 2 consisted of two-leg ladder-like arrays based on Coulomb and π-stacking interactions.Very interestingly, the nearer cation¨¨¨anion distances for compounds 1 and 2 were found to be almost similar in a range of 4.9-5.8Å with a coordination number of 3, and moreover those between the interaction networks were also similar in a range of 11-13 Å (Table 4).Furthermore, the short cation¨¨¨anion distances for the parent compound are 5.426, 5.777, and 5.812 Å [20].These observations imply that Coulomb interactions contribute to similar magnitude of lattice energies for the quasi-polymorphic compounds 1 and 2 as well as the parent compound.The difference in the crystal structures for 1 and 2 arises from the competition between π-stacking interactions and crystal packing effects including different solvate molecules.Note that a considerable distortion of the coordination structure of [Fe(aznp) 2 ] anion was found in 2, whereas the two-leg ladder-like arrays remained after undergoing the SC-SC transformation from 2 to 2'.These indicate that Coulomb interactions as well as π-stacking interactions play a key role in the formation of two-leg ladder-like arrays in 2 and 2'.On the other hand, only the similar magnitude of Coulomb interactions observed in 1 and the parent compound resulted in no significant distortion of the coordination structure and, moreover, do not contribute to the cooperativity of SCO.Therefore, the synergistic energy gains from Coulomb interactions and π-stacking interactions may lead to a considerable distortion of the coordination structure of [Fe(aznp) 2 ] anion, leading to the HS state for compounds 2 and 2'.

Density Functional Theory (DFT) Calculations
To provide an insight into the π-extension effect on a ligand field splitting energy, the DFT calculations were carried out by using the hybrid B3LYP functional with the Wachters-Hay basis set for Fe and the 6-31 + G(d) basis set for C, H, N, and O.The atomic coordinates for the HS and LS states of an [Fe(aznp) 2 ] anion were taken from the single crystal structural data of 1.The optimized coordination structures for the [Fe(aznp) 2 ] anions were in good agreement with the corresponding crystal structures for 1.The energy level diagrams of the occupied α-spin molecular orbitals (MOs) and the selected MO surfaces for the HS [Fe(aznp) 2 ] anion along with those of the HS [Fe(azp) 2 ] anion [20] are depicted in Figure 8.The Fe e g -like orbitals were contributed to the 150th and 147th MOs for the HS [Fe(aznp) 2 ] anion, which corresponded to the 123th and 121st MOs for the HS [Fe(azp) 2 ] anion, respectively.The Fe t 2g -like orbitals were observed in the 135th and 134th MOs for the HS [Fe(aznp) 2 ] anion, and the 109th and 111th MOs for the HS [Fe(azp) 2 ] anion.Compared with the energy levels of the corresponding MOs between the [Fe(aznp) 2 ] and [Fe(azp) 2 ] anion, the π-extension of a ligand leads to slight stabilization of the MOs containing Fe d orbitals except the 135th MO containing π-orbital contribution from the naphtholate moiety.The stabilization of the MOs can be explained by similarly lowering the energies of an anionic π-ligand orbital by π-extension.On the other hand, the increase in 135th orbital energy suggests that the orbital interactions between Fe e g -like orbital and O π-orbital of the naphtholate moiety may be enhanced by π-extension.Although this leads to slight decrease in the ligand-field splitting energy of the [Fe(aznp) 2 ] anion, a series of the [Fe(aznp) 2 ] complexes were still in the SCO region but their HS fractions were relatively increased as shown in the magnetic susceptibility section.
anion [20] are depicted in Figure 8.The Fe eg-like orbitals were contributed to the 150th and 147th MOs for the HS [Fe(aznp)2] anion, which corresponded to the 123th and 121st MOs for the HS [Fe(azp)2] anion, respectively.The Fe t2g-like orbitals were observed in the 135th and 134th MOs for the HS [Fe(aznp)2] anion, and the 109th and 111th MOs for the HS [Fe(azp)2] anion.Compared with the energy levels of the corresponding MOs between the [Fe(aznp)2] and [Fe(azp)2] anion, the π-extension of a ligand leads to slight stabilization of the MOs containing Fe d orbitals except the 135th MO containing π-orbital contribution from the naphtholate moiety.The stabilization of the MOs can be explained by similarly lowering the energies of an anionic π-ligand orbital by π-extension.On the other hand, the increase in 135th orbital energy suggests that the orbital interactions between Fe eg-like orbital and O π-orbital of the naphtholate moiety may be enhanced by π-extension.Although this leads to slight decrease in the ligand-field splitting energy of the [Fe(aznp)2] anion, a series of the [Fe(aznp)2] complexes were still in the SCO region but their HS fractions were relatively increased as shown in the magnetic susceptibility section.

Physical Measurements
Variable temperature direct current magnetic susceptibilities of polycrystalline samples (ca.20 mg) fold in an aluminum foil were measured on a Quantum Design MPMS-XL magnetometer under a field of 0.5 T at a sweep speed of 2 K¨min ´1 in the temperature range of 2-300 K.The sample magnetization data were obtained by the subtraction of background magnetization data for an aluminum foil from the measured data, and then the magnetic susceptibilities were corrected for diamagnetic contributions estimated by Pascal constants [30].
The Mössbauer spectra were recorded on a constant acceleration spectrometer with a source of 57 Co/Rh in the transmission mode.The measurements at low temperature were performed with a closed-cycle helium refrigerator (Iwatani Co., Ltd.Tokyo, Japan).Velocity was calibrated by using an α-Fe standard.The obtained Mössbauer spectra were fitted with symmetric Lorentzian doublets by the least squares fitting program (MossWinn).

Crystal Structure Determinations
A needle crystal was mounted in a polyimide loop.A nitrogen gas flow temperature controller was used for the temperature variable measurements.All data were collected on a Bruker APEX II CCD area detector with monochromated Mo-Kα radiation generated by a Bruker Turbo X-ray Source coupled with a Helios multilayer optics.All data collections and calculations were performed using the APEX2 crystallographic software package (Bruker AXS).The data were collected to a maximum 2θ value of 55.0 ˝.A total of 720 oscillation images were collected.The APEX II program was used to determine the unit cell parameters and for data collection.Data were integrated by using SAINT.Numerical absorption correction was applied by using SADABS.The structures at all temperatures were solved by direct methods and refined by full-matrix least-squares methods based on F 2 by using the SHELXTL program.All non-hydrogen atoms were refined anisotropically.Hydrogen atoms were generated by calculation and refined using the riding model.CCDC 1471055-1471058 (1, 2, 2') contains the supplementary crystallographic data for this paper.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk).Preliminary crystal analysis data for 3, 4¨C 6 H 5 Cl, and 5 includes the CIFs in the supporting information.

DFT Calculations
All theoretical calculations were performed using the Gaussian 09 program package [31].All calculations of the compounds were carried out at the B3LYP functional [32,33].The Wachters-Hay basis set [34,35] for Fe atoms and the 6-31 + G(d) basis set [36] for H, C, O, and N atoms were used.No imaginary frequencies were found in the optimized structures.Cartesian coordinates of HS [Fe(aznp) 2 ] and LS [Fe(aznp) 2 ] anions calculated by the B3LYP level of theory are summarized in Tables S1 and S2.

Conclusions
We report here the synthesis, crystal structures, thermal and magnetic properties of a series of the [Fe(aznp) 2 ] compounds to investigate the π-extension effect on the novel negative-charged SCO [Fe III (azp) 2 ] complex.Since the occurrence of the SCO transition in the acetonitrile-solvate TMA salt was evidenced by the temperature variations in magnetic susceptibility, Mössbauer spectrum, and crystal structure, the SCO phenomenon found in the parent [Fe(azp) 2 ] compound was not specific, and the N 2 O 4 coordination sphere for the Fe III ion can provide a ligand-field splitting energy in the SCO region.The DFT calculations suggested the π-extension effect reduced the ligand-field splitting energy.Further investigations on either substitutions on the azp ligand or ligand substitutions to ligands having different kinds of N and/or O donor atoms from the phenolate and azo group are needed to clarify the SCO conditions for the Fe III N 2 O 4 coordination sphere.
Crystals 2016, 6, 49 5 of 16HS species in 1 are in good agreement of those in the parent compound, while the IS values for LS in 1 are slightly smaller than those in the parent compound.

Figure 4 .
Figure 4. Temperature dependence of Mössbauer spectra for 1. Gray lines are observed spectra.Red and blue lines indicate the simulated HS and LS spectra, respectively.Green lines are the sum of the simulated HS and LS spectra.

Figure 4 .
Figure 4. Temperature dependence of Mössbauer spectra for 1. Gray lines are observed spectra.Red and blue lines indicate the simulated HS and LS spectra, respectively.Green lines are the sum of the simulated HS and LS spectra.

Figure 6 .
Figure 6.Crystal structure of 1 at 273 K. (a) Cation-anion arrangement parallel to the ab plane; and (b) cation-anion arrangement perpendicular to the ab plane.Acetonitrile molecules are omitted for clarity.Orange and cyan double arrows indicate neighboring Fe•••N and Fe•••Fe distances, respectively.Orange dash lines indicate neighboring Fe•••N distances shorter than 6 Å.The letters correspond to those in Table4.

Figure 6 .
Figure 6.Crystal structure of 1 at 273 K. (a) Cation-anion arrangement parallel to the ab plane; and (b) cation-anion arrangement perpendicular to the ab plane.Acetonitrile molecules are omitted for clarity.Orange and cyan double arrows indicate neighboring Fe¨¨¨N and Fe¨¨¨Fe distances, respectively.Orange dash lines indicate neighboring Fe¨¨¨N distances shorter than 6 Å.The letters correspond to those in Table4.

Figure 7 .
Figure 7. Crystal structures parallel to two-leg ladder-like cation¨¨¨anion arrays for 2 (a) and 2' (b).Acetone molecules are omitted for clarity; Crystal structures perpendicular to the two-leg ladder-like cation¨¨¨anion arrays for 2 (c) and 2' (d); (e) 1D channel structure along the b axis in 2. Orange, cyan, and green double arrows indicate neighboring Fe¨¨¨N, Fe¨¨¨Fe distances, and π-stacking interactions, respectively.Orange dash lines indicate Fe¨¨¨N distances shorter than 6 Å.The letters correspond to those in Table4.

Table 1 .
Temperature variations of Mössbauer parameters for

Table 1 .
Temperature variations of Mössbauer parameters for

Table 3 .
Selected coordination bond lengths and distortion parameters for 1