Halogen Substituent Effect on the Spin-Transition Temperature in Spin-Crossover Fe ( III ) Compounds Bearing Salicylaldehyde 2-Pyridyl Hydrazone-Type Ligands and Dicarboxylic Acids

Four Fe(III) spin-crossover (SCO) compounds, [Fe(HL1)2](HCl4TPA) (1-Cl), [Fe(HL1)2](HBr4TPA) (1-Br), [Fe(HL2)2](HCl4TPA) (2-Cl), and [Fe(HL2)2](HBr4TPA) (2-Br) (HL1 = 4-chloro-2-nitro-6-(1-(2-(pyridine-2-yl)hydrazono)ethyl)phenolate; HL2 = 4-bromo-2-nitro-6-(1-(2(pyridine-2-yl)hydrazono)ethyl)phenolate; HCl4TPA = 2,3,5,6-tetrachloro-4-carboxybenzoate; and HBr4TPA = 2,3,5,6-tetrabromo-4-carboxybenzoate), were synthesized to investigate the halogen substituent change effect in salicylaldehyde 2-pyridyl hydrazone-type ligands and dicarboxylic acids in SCO complexes to the spin-transition temperature. Crystal structure analyses showed that these compounds were isostructural. In addition, a one-dimensional hydrogen–bonded column was formed by the dicarboxylic acid anion and weak hydrogen bonds between the Fe(III) complexes. From Mössbauer spectroscopy and magnetic property measurements, these compounds were confirmed to exhibit gradual SCO. The spin-transition temperature can be shifted by changing the halogen substituent in the salicylaldehyde 2-pyridyl hydrazone-type ligands and dicarboxylic acids without changing the molecular arrangement in the crystal packing.


Introduction
Spin-crossover (SCO) complexes have attracted considerable attention as materials for developing memory devices because the spin state can be switched by external stimuli such as temperature, light, and pressure [1][2][3].In terms of practical applications, it is preferable for the spin-transition temperature to be near room temperature.Therefore, the effects of replacing anion and solvent molecules and the introduction of substituents into ligands on the spin-transition temperature have been actively studied for many types of SCO complexes [4][5][6][7][8][9][10][11][12][13][14][15][16].
As an approach toward shifting the spin-transition temperature, we focused on the effect of halogen substituent changes in the ligands and coexisting molecules, particularly in a carboxylic acid.In our previous study, we developed an Fe(II) SCO compound containing a neutral Fe(II) complex and a halogen-substituted dicarboxylic acid [17].The halogen substituent change on the dicarboxylic acid molecule was confirmed to affect the spin-transition temperature while maintaining the molecular arrangement of the complex.Fe(III) SCO complexes have a great advantage over Fe(II) SCO complexes in terms of in-air stability; thus, Fe(III) SCO complexes have attracted considerable attention and have been actively developed [18][19][20][21][22][23][24].In this study, we aim to introduce a halogen-substituted dicarboxylic acid into Fe(III) SCO compounds as an anion and confirm the effect of the halogen substituent change in the dicarboxylic acid to the spin-transition temperature.Simultaneously, the effect of the halogen substituent change in the ligand to the spin-transition temperature was also investigated.Salicylaldehyde 2-pyridyl hydrazone-type ligands can form 1+ cationic Fe(III) complexes [25][26][27][28].Those that bear 2-methoxy-6-(pyridine-2-ylhydrazonomethyl)phenol exhibit SCO properties [24].However, there are no reports on the effect of changing the halogen substituent on the salicylaldehyde 2-pyridyl hydrazone-type ligand to the spin-transition temperature because Fe(III) SCO complexes bearing such ligands are scarcely reported.In this study, we synthesized two ligands, 4-chloro-2-nitro-6-

Crystal Structure
The crystal structures of 1-Cl were determined at 123 and 330 K, and those of 1-Br, 2-Cl, and 2-Br were determined at 123 K.The crystallographic data of 1-Cl, 1-Br, 2-Cl, and 2-Br are summarized in Table 1.The coordination distances and intermolecular interactions of 1-Cl, 1-Br, 2-Cl, and 2-Br

Crystal Structure
The crystal structures of 1-Cl were determined at 123 and 330 K, and those of 1-Br, 2-Cl, and 2-Br were determined at 123 K.The crystallographic data of 1-Cl, 1-Br, 2-Cl, and 2-Br are summarized in Table 1.The coordination distances and intermolecular interactions of 1-Cl, 1-Br, 2-Cl, and 2-Br are summarized in Table 2.The asymmetric unit of 1-Cl at 123 K shows half of the Fe(III) complex and half of the dicarboxylic acid molecule without any solvent molecules (Figure 1a).The half Fe(III) complex and the half dicarboxylic acid molecule in the asymmetric unit indicate that the two ligands in the Fe(III) complex are crystallographically identical and the two carboxyl groups in the dicarboxylic acid molecule are also crystallographically identical.The molecular arrangement of the Fe(III) complexes and the dicarboxylic acid molecules is illustrated in Figure 1b.The dicarboxylic acid molecules were confirmed to form a one-dimensional (1D) column through hydrogen bonds between each carboxyl group.Because a hydrogen bond is formed between each dicarboxylic acid molecule, it was predicted that (i) one of the two carboxyl groups in each H 2 Cl 4 TPA molecule was deprotonated and (ii) 1-Cl is comprised of one HCl 4 TPA − anion and one [Fe III (HL type hydrogen bond found between each HCl 4 TPA molecule is generally referred to as a "charge-assisted hydrogen bond", and such types of hydrogen bond tend to be strong and very short [30].Indeed, the hydrogen bond distance of O5•••O5A was 2.458 Å.This value can be recognized as a considerably strong and short hydrogen bond.The two ligands in the Fe(III) complexes were linked to a 1D hydrogen-bonded column through the hydrogen bond between N2 in the ligand and O4 in the carboxyl group.The distance of N2•••O4 was 2.892 Å.This value is classified as a weak hydrogen bond.Overall, each hydrogen-bonded column occupied the area surrounded by Fe(III) complexes and extended along the c-axis (Figure 2).The dependence of coordination distances around the Fe(III) atom and temperature in 1-Cl are also listed in Table 1.At 123 K, the distances between Fe1 and N1, N3, and O1 were 1.975(3), 1.925(3), and 1.857(2) Å, respectively.These values indicated that the Fe(III) complex in 1-Cl at 123 K was in the low-spin (LS) state.Meanwhile, the same coordination distances at 330 K were 2.098( 3 are summarized in Table 2.The asymmetric unit of 1-Cl at 123 K shows half of the Fe(III) complex and half of the dicarboxylic acid molecule without any solvent molecules (Figure 1a).The half Fe(III) complex and the half dicarboxylic acid molecule in the asymmetric unit indicate that the two ligands in the Fe(III) complex are crystallographically identical and the two carboxyl groups in the dicarboxylic acid molecule are also crystallographically identical.The molecular arrangement of the Fe(III) complexes and the dicarboxylic acid molecules is illustrated in Figure 1b.The dicarboxylic acid molecules were confirmed to form a one-dimensional (1D) column through hydrogen bonds between each carboxyl group.Because a hydrogen bond is formed between each dicarboxylic acid molecule, it was predicted that (i) one of the two carboxyl groups in each H2Cl4TPA molecule was deprotonated and (ii) 1-Cl is comprised of one HCl4TPA − anion and one [Fe III (HL type hydrogen bond found between each HCl4TPA molecule is generally referred to as a "charge-assisted hydrogen bond", and such types of hydrogen bond tend to be strong and very short [30].Indeed, the hydrogen bond distance of O5•••O5A was 2.458 Å.This value can be recognized as a considerably strong and short hydrogen bond.The two ligands in the Fe(III) complexes were linked to a 1D hydrogen-bonded column through the hydrogen bond between N2 in the ligand and O4 in the carboxyl group.The distance of N2•••O4 was 2.892 Å.This value is classified as a weak hydrogen bond.Overall, each hydrogen-bonded column occupied the area surrounded by Fe(III) complexes and extended along the c-axis (Figure 2).The dependence of coordination distances around the Fe(III) atom and temperature in 1-Cl are also listed in Table 1.At 123 K, the distances between Fe1 and N1, N3, and O1 were 1.975(3), 1.925(3), and 1.857(2) Å, respectively.These values indicated that the Fe(III) complex in 1-Cl at 123 K was in the low-spin (LS) state.Meanwhile, the same coordination distances at 330 K were 2.

Mössbauer Spectroscopy
The variable-temperature Mössbauer spectra of 1-Cl were measured to identify the spin states and valence of Fe at 123 and 300 K (Figure 3).The spectrum at 123 K shows a single quadrupole doublet.The isomer shift (δ) and quadrupole splitting (∆E Q ) were 0.179(1) and 2.178(2) mm•s −1 , respectively.The δ and ∆E Q at 123 K were assigned to the Fe(III) LS isomer.These results were consistent with the prediction from the crystal structure at 123 K.At 300 K, there was a singlet peak without quadrupole splitting.The isomer shift was 0.281(9) mm•s −1 , which was assigned to the Fe(III) HS isomer, and this result was also consistent with the spin-state predictions from the crystal structure at 330 K. Overall, the valence of iron in 1-Cl was confirmed to be Fe(III) (1-Br, 2-Cl, and 2-Br were isostructural to 1-Cl and should therefore also be Fe(III)), and the change of coordination distances around Fe with temperature change in the crystal structures originates from the SCO.

Mössbauer Spectroscopy
The variable-temperature Mössbauer spectra of 1-Cl were measured to identify the spin states and valence of Fe at 123 and 300 K (Figure 3).The spectrum at 123 K shows a single quadrupole doublet.The isomer shift (δ) and quadrupole splitting (ΔEQ) were 0.179(1) and 2.178(2) mm•s −1 , respectively.The δ and ΔEQ at 123 K were assigned to the Fe(III) LS isomer.These results were consistent with the prediction from the crystal structure at 123 K.At 300 K, there was a singlet peak without quadrupole splitting.The isomer shift was 0.281(9) mm•s −1 , which was assigned to the Fe(III) HS isomer, and this result was also consistent with the spin-state predictions from the crystal structure at 330 K. Overall, the valence of iron in 1-Cl was confirmed to be Fe(III) (1-Br, 2-Cl, and 2-Br were isostructural to 1-Cl and should therefore also be Fe(III)), and the change of coordination distances around Fe with temperature change in the crystal structures originates from the SCO.

Magnetic Property
The magnetic susceptibility measurements of 1-Cl and 1-Br were performed over 100-400 K, whereas those of 2-Cl and 2-Br were performed over 150-400 K (Figure 4).At 123 K, the χmT value of 1-Cl was 0.32 cm 3 •K•mol −1 , indicating that the Fe(III) complex in 1-Clwas in the LS state at 123 K.This result is in agreement with the spin-state predictions from the crystal structure and Mössbauer spectra at the same temperature.Additionally, the χmT values of 1-Br, 2-Cl, and 2-Br at 150 K were 0.40, 0.32, and 0.36 cm 3 •K•mol −1 , respectively; these values also show that 1-Br, 2-Cl, and 2-Br were in the LS state at 150 K.The χmT value of 1-Cl gradually increased during the heating process as a result of the SCO and ultimately reached 3.72 cm 3 •K•mol −1 at 400 K.Meanwhile, 1-Br showed a gradual two-step SCO between 150 and 240 K and finally reached 4.07 cm 3 •K•mol −1 at 400 K.The spin-transition temperature T1/2 in the first and second steps were 188 and 213 K, respectively.However, there was no clear plateau in the two-step SCO in 1-Br, so the crystal structure of the intermediate phase could not be determined.
2-Cl and 2-Br showed gradual SCO near room temperature and finally reached 2.80 and 3.72 cm 3 •K•mol −1 , respectively.The χmT value for 2-Cl at 400 K was clearly lower than the expected value for the HS Fe(III) state (S = 5/2).Additionally, the χT-T plot of 2-Br still did not reach the plateau region, even at 400 K.For this reason, the SCO of 2-Cl and 2-Br were considered incomplete at 400 K.In addition, 2-Cl and 2-Br showed a T1/2 dependence on the halogen substituent in the dicarboxylic acid, similar to 1-Cl and 1-Br.The χmT values of 2-Cl and 2-Br in the HT phase were assumed to reach the χmT values of 1-Cl (3.72 cm 3 •K•mol −1 ) and 1-Br (4.07 cm 3 •K•mol −1 ) in the HS state, respectively.It was estimated that the T1/2 of 2-Cl and 2-Br were 373 and 320 K, respectively.The difference of T1/2 between 1-Cl and 1-Br, or 2-Cl and 2-Br clearly showed that the T1/2 in SCO complexes coexisting with halogen-substituted dicarboxylic acids can be controlled by changing the halogen substituent in

Magnetic Property
The magnetic susceptibility measurements of 1-Cl and 1-Br were performed over 100-400 K, whereas those of 2-Cl and 2-Br were performed over 150-400 K (Figure 4).At 123 K, the χ m T value of 1-Cl was 0.32 cm 3 •K•mol −1 , indicating that the Fe(III) complex in 1-Clwas in the LS state at 123 K.This result is in agreement with the spin-state predictions from the crystal structure and Mössbauer spectra at the same temperature.Additionally, the χ m T values of 1-Br, 2-Cl, and 2-Br at 150 K were 0.40, 0.32, and 0.36 cm 3 •K•mol −1 , respectively; these values also show that 1-Br, 2-Cl, and 2-Br were in the LS state at 150 K.The χ m T value of 1-Cl gradually increased during the heating process as a result of the SCO and ultimately reached 3.72 cm 3 •K•mol −1 at 400 K.Meanwhile, 1-Br showed a gradual two-step SCO between 150 and 240 K and finally reached 4.07 cm 3 •K•mol −1 at 400 K.The spin-transition temperature T 1/2 in the first and second steps were 188 and 213 K, respectively.However, there was no clear plateau in the two-step SCO in 1-Br, so the crystal structure of the intermediate phase could not be determined.the dicarboxylic acid while keeping the molecular crystal packing.The effect of the halogen substituent change in salicylaldehyde 2-pyridyl hydrazone-type ligands to T1/2 was also observed.The T1/2 values of 2-Cl and 2-Br were clearly shifted to higher temperatures compared with those of 1-Cl and 1-Br as the halogen substituent changed from Cl to Br.The shift of T1/2 to higher temperatures with the change of halogen substituent in the ligand from Cl to Br was considered to have originated from the electron withdrawing effect of halogen substituents.It is reported that the T1/2 of [Fe(X-sal2trien R )] + that possesses the phonoxide part, which is similar to our ligands, tends to increase with the change of the substituent on the phenoxide part from the OCH3 substituent possessing electron donor character to the NO2 possessing electron withdrawing character [31].It is known that the σp Hammett parameter of Br is larger than that of Cl.Thus, the larger electron withdrawing effect of Br compared with that of Cl is considered to induce the shift of T1/2 to a higher temperature, which is the same as the case with [Fe(X-sal2trien R )] + .

Compound Synthesis
3.1.1.Synthesis of Ligand HL1 The 5′-Chloro-2′-hydroxy-3′-nitroacetophenone (2.2 g; 10 mmol) and 2-hydrazinopyridine (1.1 g; 10 mmol) were dissolved in ethanol (100 mL).The mixture was refluxed for 5 h and left to stand at 2-Cl and 2-Br showed gradual SCO near room temperature and finally reached 2.80 and 3.72 cm 3 •K•mol −1 , respectively.The χ m T value for 2-Cl at 400 K was clearly lower than the expected value for the HS Fe(III) state (S = 5/2).Additionally, the χT-T plot of 2-Br still did not reach the plateau region, even at 400 K.For this reason, the SCO of 2-Cl and 2-Br were considered incomplete at 400 K.In addition, 2-Cl and 2-Br showed a T 1/2 dependence on the halogen substituent in the dicarboxylic acid, similar to 1-Cl and 1-Br.The χ m T values of 2-Cl and 2-Br in the HT phase were assumed to reach the χ m T values of 1-Cl (3.72 cm 3 •K•mol −1 ) and 1-Br (4.07 cm 3 •K•mol −1 ) in the HS state, respectively.It was estimated that the T 1/2 of 2-Cl and 2-Br were 373 and 320 K, respectively.The difference of T 1/2 between 1-Cl and 1-Br, or 2-Cl and 2-Br clearly showed that the T 1/2 in SCO complexes coexisting with halogen-substituted dicarboxylic acids can be controlled by changing the halogen substituent in the dicarboxylic acid while keeping the molecular crystal packing.The effect of the halogen substituent change in salicylaldehyde 2-pyridyl hydrazone-type ligands to T 1/2 was also observed.The T 1/2 values of 2-Cl and 2-Br were clearly shifted to higher temperatures compared with those of 1-Cl and 1-Br as the halogen substituent changed from Cl to Br.The shift of T 1/2 to higher temperatures with the change of halogen substituent in the ligand from Cl to Br was considered to have originated from the electron withdrawing effect of halogen substituents.It is reported that the T 1/2 of [Fe(X-sal 2 trien R )] + that possesses the phonoxide part, which is similar to our ligands, tends to increase with the change of the substituent on the phenoxide part from the OCH 3 substituent possessing electron donor character to the NO 2 possessing electron withdrawing character [31].It is known that the σ p Hammett parameter of Br is larger than that of Cl.Thus, the larger electron withdrawing effect of Br compared with that of Cl is considered to induce the shift of T 1/2 to a higher temperature, which is the same as the case with [Fe(X-sal 2 trien R )] + .

Synthesis of 1-Cl
H 2 L1 (72 mg; 0.20 mmol) was dissolved in methanol (100 mL), and FeCl 3 •6H 2 O (27 mg; 0.10 mmol) was added to the solution.The mixture was stirred for 30 min.Subsequently, H 2 Cl 4 TPA (30 mg; 0.10 mmol) was added, and the mixture was stirred for 30 s.Then, trimethylamine (14 µL; 0.10 mmol) was added to the solution.The solution was left to stand for several days.

Conclusions
In brief, we synthesized four Fe(III) SCO compounds that contained a halogen-substituted salicylaldehyde 2-pyridyl hydrazone-type ligand complex with a dicarboxylic acid as the anion.Crystal structures confirmed that all the compounds were isostructural.Furthermore, the dicarboxylic acid anion formed a 1D hydrogen-bonded column, and the Fe(III) complex formed weak hydrogen bonds with the 1D hydrogen-bonded column.Mössbauer spectroscopy for 1-Cl demonstrated that the valence of the iron center was Fe(III), and these compounds showed SCO.The magnetic property measurements of these compounds showed that they exhibited gradual SCO, and the spin-transition temperature shifted to higher values without changing the molecular crystal packing when the halogen substituent in the dicarboxylic acid changed from Br to Cl.Additionally, it was confirmed that the change of halogen substituent in the salicylaldehyde 2-pyridyl hydrazone-type ligands from Cl to Br shifts T 1/2 to a higher temperature while still maintaining the molecular crystal packing.The shift of
), 2.112(3), and 1.882(3) Å, respectively.The increment of coordination distances around Fe1 from 123 to 330 K indicates that the Fe(III) complexes in 1-Cl exhibited SCO from the LS state (S = 1/2) to the high-spin (HS) state (S = 5/2) with increasing temperature.We also determined the crystal structures of 1-Br, 2-Cl, and 2-Br at 123 K.It was confirmed that the crystal structures of 1-Br, 2-Cl, and 2-Br were isostructural to 1-Cl.The coordination distances around Fe(III) in 1-Br, 2-Cl, and 2-Br at 123 K were characteristic of the LS Fe(III) complex.The hydrogen bond distances between the dicarboxylic acid and Fe(III) complex and between each dicarboxylic acid in 1-Cl, 1-Br, 2-Cl, and 2-Br were barely different from each other.
098(3), 2.112(3), and 1.882(3) Å, respectively.The increment of coordination distances around Fe1 from 123 to 330 K indicates that the Fe(III) complexes in 1-Cl exhibited SCO from the LS state (S = 1/2) to the high-spin (HS) state (S = 5/2) with increasing temperature.We also determined the crystal structures of 1-Br, 2-Cl, and 2-Br at 123 K.It was confirmed that the crystal structures of 1-Br, 2-Cl, and 2-Br were isostructural to 1-Cl.The coordination distances around Fe(III) in 1-Br, 2-Cl, and 2-Br at 123 K were characteristic of the LS Fe(III) complex.The hydrogen bond distances between the dicarboxylic acid and Fe(III) complex and between each dicarboxylic acid in 1-Cl, 1-Br, 2-Cl, and 2-Br were barely different from each other.

Figure 2 .
Figure 2. View of the crystal structure of 1-Cl along the c-axis.

Figure 2 .
Figure 2. View of the crystal structure of 1-Cl along the c-axis.

Figure 3 .
Figure 3. Mössbauer spectra of 1-Cl at (a) 123 K and (b) 300 K. Open circles and lines are measurement data and fitting curves, respectively.

Figure 3 .
Figure 3. Mössbauer spectra of 1-Cl at (a) 123 K and (b) 300 K. Open circles and lines are measurement data and fitting curves, respectively.

Table 1 .
Crystallographic data of 1

Table 2 .
Coordination and hydrogen bond distances in 1

Table 2 .
Coordination and hydrogen bond distances in 1