Modification of Cooperativity and Critical Temperatures on a Hofmann-Like Template Structure by Modular Substituent

In a series of Hofmann-like spin crossover complexes, two new compounds, {Fe(3-F-4-Methyl-py)2[Au(CN)2]2} (1) and {Fe(3-Methyl-py)2[Au(CN)2]2} (2) (py = pyridine) are described. The series maintains a uniform 2-dimentional (2-D) layer structure of {Fe[Au(CN)2]2}. The layers are combined with another layer by strong aurophilic interactions, which results in a bilayer structure. Both coordination compounds 1 and 2 at 293 K crystallize in the centrosymmetric space groups P21/c. The asymmetric unit contains two pyridine derivative ligands, one type of Fe2+, and two types of crystallographically distinct [Au(CN)2] units. Compound 1 undergoes a complete two-step spin transition. On the other hand, 2 maintains the characteristic of the high-spin state. The present compounds and other closely related bilayer compounds are compared and discussed in terms of the cooperativity and critical temperature. The bilayer structure is able to be further linked by substituent-substituent contact resulting in 3-dimentional (3-D) network cooperativity.


Introduction
An essential part for designing spin crossover (SCO) materials is to control and optimize the crystal structure [1].Especially, construction of the strong cooperative intermolecular interactions in the whole of a structure leads to steep transition behavior with a wide hysteresis loop, which is important for the practical materials [2].The cooperativity is now being investigated by a variety of coordination polymers.Coordination polymers are one of the interesting materials for constructing supramolecular networks.However, systematic designing of the networks is hard because of the structural diversity of coordination polymers.
Scheme 1.Molecular structure of the ligands of 3-Furuoro-4-methyl-py and 3-Methyl-py.Compound 1 at 293 K crystallizes in the monoclinic centrosymmetric space group P21/c.The asymmetric unit of the complex consists of the hetero-metal Fe II Au I unit (Figure 1a).The Fe II  The crystal structure of this state is similar to that for 293 and 90 K.However, as compared to the fully high spin (HS) state, the single crystals lose their qualities due to the narrow temperature region of the half transition phase.As a result, the crystal data at 180 K is not good enough (R = 0.1058 [I > 2sigma (I)]) for determining the correct structure (see Table S1).So, here we discuss in detail the structure at 293 K (fully high spin (HS) state) and 90 K (fully low spin (LS) state).The crystal structure of this state is similar to that for 293 and 90 K.However, as compared to the fully high spin (HS) state, the single crystals lose their qualities due to the narrow temperature region of the half transition phase.As a result, the crystal data at 180 K is not good enough (R = 0.1058 [I > 2sigma (I)]) for determining the correct structure (see Table S1).So, here we discuss in detail the structure at 293 K (fully high spin (HS) state) and 90 K (fully low spin (LS) state).

Thermal Dependence Magnetic Behavior of Compound 1
Figure 3a shows the thermal dependence of χMT for 1 with χM being the molar magnetic susceptibility and T the temperature.At room temperature, χMT is 3.65 cm 3 •K•mol −1 .This value is slightly higher than usual for paramagnetic Fe II compounds, possibly due to oxidation of the complex.Upon cooling, χMT remains almost constant down to 195 K; below this temperature, χMT undergoes a sharp decrease.The complex displays two-step spin transition with a characteristic plateau centered at around 50% conversion, and the warming mode reveals the occurrence of a hysteresis loop (second step).The SCO for this complex causes a reversible change of color from blue (HS) to purple (LS).The critical temperature (Tc) in the first step (Tc 1 ) is 188.5 K and the cooling (Tc 2 down ) and warming (Tc 2 up ) modes in the second step are 164.5 and 171.5 K, respectively, giving an approximately 7.0 K width hysteresis loop.

Thermal Dependence Magnetic Behavior of Compound 2
χMT versus T plots for 2 are shown in Figure 3b.At room temperature, the magnetic behavior of the complex 2 is characteristic of Fe(II) compounds in the HS state, χMT = 4.05 cm 3 •K•mol −1 .The value is slightly higher than that of a pure spin only system, whereas the value is similar to the values of

Thermal Dependence Magnetic Behavior of Compound 1
Figure 3a shows the thermal dependence of χ M T for 1 with χ M being the molar magnetic susceptibility and T the temperature.At room temperature, χ M T is 3.65 cm 3 •K•mol −1 .This value is slightly higher than usual for paramagnetic Fe II compounds, possibly due to oxidation of the complex.Upon cooling, χ M T remains almost constant down to 195 K; below this temperature, χ M T undergoes a sharp decrease.The complex displays two-step spin transition with a characteristic plateau centered at around 50% conversion, and the warming mode reveals the occurrence of a hysteresis loop (second step).The SCO for this complex causes a reversible change of color from blue (HS) to purple (LS).The critical temperature (T c ) in the first step (T c 1 ) is 188.5 K and the cooling (T c 2 down ) and warming
Cell volumes, SCO behavior type, and Tc for 1-6 are summarized in Table 1.It is noted that the cell volume clearly shows an expansion with the increase of the substituent bulk.The compounds from smallest to largest are as follows: 3-F-py (1915.7 Å 3 ) < 3-Methyl-py (1976.3Å 3 ) < 3-Br-py (1990.9Å 3 ) < 4-Methyl-py (2112.5 Å 3 ) < 3-F-4-Methyl-py (2155.2Å 3 ) < 3-Br-4-Methyl-py (2209.7 Å 3 ).In spite of the lattice expansion, these compounds completely maintain a bilayer structure.In terms of the magnetic properties of 1 and 2, 1 displays a steep two-step spin conversion with a hysteresis loop (second step), while 2 is fully HS in the whole range of temperatures.This result suggests that the Tc of 2 might be so low that no spin transition is observed in an ambient pressure.There is also no spin conversion observed for X = 3-Br.In this series, Tc increases in the following way: 3-Me-py, 3-Br-py (no transition) << 3-Br-4-Me-py (109.1 K) < 3-F-py (first step 147.9 K) < 3-F-4-Me-py (first step 188.5 K) < 4-Me-py (first step 210 K).The order of Tc must result from the difference in the ligand field strength Δo.It seems that the 4-Me substituent is the most effective for decreasing

Thermal Dependence Magnetic Behavior of Compound 2
χ M T versus T plots for 2 are shown in Figure 3b.At room temperature, the magnetic behavior of the complex 2 is characteristic of Fe(II) compounds in the HS state, χ M T = 4.05 cm 3 •K•mol −1 .The value is slightly higher than that of a pure spin only system, whereas the value is similar to the values of other Hofmann-like SCO Fe II compounds.The χ M T value is nearly constant in the range of 80-300 K.However, it decreases steeply as the temperature is lowered to less than 30 K. The decrease in the value of χ M T at lower temperatures is due to a typical behavior of zero-field splitting (ZFS) effects of the metallic Fe II centers in the residual HS (S = 2) species [22].
In terms of the magnetic properties of 1 and 2, 1 displays a steep two-step spin conversion with a hysteresis loop (second step), while 2 is fully HS in the whole range of temperatures.This result suggests that the T c of 2 might be so low that no spin transition is observed in an ambient pressure.There is also no spin conversion observed for X = 3-Br.In this series, T c increases in the following way: 3-Me-py, 3-Br-py (no transition) << 3-Br-4-Me-py (109.1 K) < 3-F-py (first step 147.9 K) < 3-F-4-Me-py (first step 188.5 K) < 4-Me-py (first step 210 K).The order of T c must result from the difference in the ligand field strength ∆ o .It seems that the 4-Me substituent is the most effective for decreasing ∆ o (see Scheme S1).This trend of T c is in the same order for [Fe(L) 2 Ag(CN) 2 ] (L = 3-F-py, 3-Br-py, 3-Me-py, and 4-Me-py) and is as follows: 3-Me-py [18], 3-Br-py (no transition) [11] << 3-F-py (first step 146.3 K) [11] < 4-Mepy (first step 189 K) [18].These orders of T c are opposite to the expected electronic effect according to the Hammet constants [23].In a previous related study, Real and co-workers discussed the effect of substituted pyridines for the analogous series of [Fe(X-py) 2 Ag(CN) 2 ] (X = halogen atoms) [11].The research says that T c may be dominantly influenced by crystal packing factor and polymeric structure.We interpret the former research as meaning that the Fe-N bond length is strongly influenced by the interlayer spaces.The steric effect from substituent bulk makes the space tight.In fact, 4-position is vertical to the layer.Thus, a 4-Methyl substituent gives rise to higher chemical pressure in the Fe-N bond.Consequently, the trend of T c can be explained by the substituent bulk.Electronic effects righteously coexist and compete with steric effects.
Both 1 and 3 have significantly similar transition behavior.However, each behavior has a different hysteresis width.In compound 3, intermolecular distance between F•••F is significantly shorter as compared to that of 1 (Scheme 2).The crystal structure of 3 shows the lowest cell volume which makes the interlayer spaces narrow.It results in shorter F•••F contact (closest distance of F•••F (HS) = 3.104 Å) between bilayers.Specifically, the half spin transition state (HS-LS) has extremely short F•••F contact (2.955 Å).Therefore, this complex must have higher cooperative networks which stabilize the HS-LS state.On the other hand, the F•••F contact of 1 is much longer than that of 3 (closest distance of F•••F (HS) = 3.717 Å).This longer distance of 1 is due to the 4-Me substituent which serves as bulk for the expansion along the a axis between the layers.Thus it generates weakened cooperativity.In fact, the thermal trapping of the HS-LS state for 3 is observed (see Figure S1), while this process cannot be seen for 1 at the same cooling/warming rate.This result is also strongly supported by the difference in the cooperativity of 1 and 3. SCO behavior types, hysteresis width, closest approach F•••F distance, and presence or absence of thermal trapping for 1 and 3 are summarized in Table 2.
In a previous related study, Real and co-workers discussed the effect of substituted pyridines for the analogous series of [Fe(X-py)2Ag(CN)2] (X = halogen atoms) [11].The research says that Tc may be dominantly influenced by crystal packing factor and polymeric structure.We interpret the former research as meaning that the Fe-N bond length is strongly influenced by the interlayer spaces.The steric effect from substituent bulk makes the space tight.In fact, 4-position is vertical to the layer.Thus, a 4-Methyl substituent gives rise to higher chemical pressure in the Fe-N bond.Consequently, the trend of Tc can be explained by the substituent bulk.Electronic effects righteously coexist and compete with steric effects.
Both 1 and 3 have significantly similar transition behavior.However, each behavior has a different hysteresis width.In compound 3, intermolecular distance between F•••F is significantly shorter as compared to that of 1 (Scheme 2).The crystal structure of 3 shows the lowest cell volume which makes the interlayer spaces narrow.It results in shorter F•••F contact (closest distance of F•••F (HS) = 3.104 Å) between bilayers.Specifically, the half spin transition state (HS-LS) has extremely short F•••F contact (2.955 Å).Therefore, this complex must have higher cooperative networks which stabilize the HS-LS state.On the other hand, the F•••F contact of 1 is much longer than that of 3 (closest distance of F•••F (HS) = 3.717 Å).This longer distance of 1 is due to the 4-Me substituent which serves as bulk for the expansion along the a axis between the layers.Thus it generates weakened cooperativity.In fact, the thermal trapping of the HS-LS state for 3 is observed (see Figure S1), while this process cannot be seen for 1 at the same cooling/warming rate.This result is also strongly supported by the difference in the cooperativity of 1 and 3. SCO behavior types, hysteresis width, closest approach F•••F distance, and presence or absence of thermal trapping for 1 and 3 are summarized in Table 2.

Materials
All the chemicals were purchased from commercial sources and used without any further purification.

Magnetic Measurements
Measurements of the temperature dependence of the magnetic susceptibility of the complexes 1 and 2 of the powdered samples in the temperature range of 2-300 K with a cooling and heating rate of 2 K•min −1 in a 1 kOe field were measured on a MPMS-XL Quantum Design SQUID magnetometer in the Cryogenic Research Center, the University of Tokyo.The diamagnetism of the samples and sample holders were taken into account.

X-ray Crystallography
Data collection was performed on a BRUKER APEX SMART CCD area-detector diffractometer at 293, 180 K, and 90 K for 1 and 2 with monochromated Mo-Kα radiation (λ = 0.71073 Å) (Bruker, Billerica, MA, USA).A selected single crystal was carefully mounted on a thin glass capillaly and immediately placed under a liquid N 2 cooled N 2 stream in each case.Crystal structures of the complexes 1 and 2 were determined using a BRUKER APEX SMART CCD area-detector diffractometer with monochrometed Mo Kα radiation (λ = 0.71073 Å).The diffraction data were treated using SMART and SAINT, and absorption correction was performed using SADABS [24].The structures were solved by using direct methods with SHELXTL [25].All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were generated geometrically.Pertinent crystallographic parameters and selected metric parameters for 1 and 2 are displayed in Tables S1 and S2.Crystallographic data have been deposited with Cambridge Crystallographic Data Centre: Deposition numbers CCDC-1559723 for compound 1 (293 K), CCDC-1559724 for 1 (90 K), and CCDC-1559832 for 2 (293 K).These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

Conclusions
In this paper, the 2-D bilayer structures of Hofmann-like coordination polymers {Fe II (X-py) 2 [Au I (CN) 2 ] 2 } were synthesized.It is noted that the substituents of pyridine derivatives have two steric effects in the whole of the crystal structure.One is the blocking interlayer spaces resulting in modification of the cooperativity.The other is the chemical pressure to the Fe II centers resulting in modification of the critical temperature.The two effects are opposite forces to each other, like action-reaction forces with a wall.
In general, the 3-D coordination bonding types, such as [Fe II (pyrazine)[M II (CN) 4 ]} (M = Ni, Pd, and Pt) display stable discontinuous spin transitions with strong cooperativity.On the other hand, most of the 2-D coordination types had weaker cooperativity than that of the 3-D type.
However, the present 2-D system shows a strong cooperativity that is as strong as the 3-D type due to the substituent-substituent interaction.
The 2-D coordination structure can be used in designs due to the unique properties, such as gate-opening behavior [26].Therefore, the 2-D structure would create novel and unique SCO material with strong cooperativity for practical applications.

Figure 1 .
Figure 1.(a) Coordination structure of compound 1 containing its asymmetric unit at 293 K; (b) View of the 2-D layer structure of 1; (c) Stacking of four consecutive layers of 1 at 293 K.In these pictures, hydrogen atoms are omitted for clarity.

Figure 1 .
Figure 1.(a) Coordination structure of compound 1 containing its asymmetric unit at 293 K; (b) View of the 2-D layer structure of 1; (c) Stacking of four consecutive layers of 1 at 293 K.In these pictures, hydrogen atoms are omitted for clarity.

Figure 1 .
Figure 1.(a) Coordination structure of compound 1 containing its asymmetric unit at 293 K; (b) View of the 2-D layer structure of 1; (c) Stacking of four consecutive layers of 1 at 293 K.In these pictures, hydrogen atoms are omitted for clarity.

Figure 2 .
Figure 2. (a) Coordination structure of compound 2 containing its asymmetric unit at 293 K; (b) View of the 2-D layer structure of 2; (c) Stacking of four consecutive layers of 2 at 293 K.In these pictures, hydrogen atoms are omitted for clarity.

Figure 2 .
Figure 2. (a) Coordination structure of compound 2 containing its asymmetric unit at 293 K; (b) View of the 2-D layer structure of 2; (c) Stacking of four consecutive layers of 2 at 293 K.In these pictures, hydrogen atoms are omitted for clarity.

Figure 3 .
Figure 3. (a) Thermal dependence of χMT for compound 1.The sample was cooled from 300 to 2 K (blue) and then warmed from 2 to 300 K (red).(b) Thermal dependence of χMT for compound 2.

Figure 3 .
Figure 3. (a) Thermal dependence of χ M T for compound 1.The sample was cooled from 300 to 2 K (blue) and then warmed from 2 to 300 K (red).(b) Thermal dependence of χ M T for compound 2.

Table 2 .FeScheme 2 .
Scheme 2. Representation of the lattice expansion with the addition of the 4-position substituent.Blue highlighted box shows the F•••F interactions (black and red dotted line).

Scheme 2 .
Scheme 2. Representation of the lattice expansion with the addition of the 4-position substituent.Blue highlighted box shows the F•••F interactions (black and red dotted line).

Table 2 .
Comparative hysteresis width, closest approach F•••F distances, and presence or absence of thermal trapping for compounds 1 and 3. SI = supplemental information.