New Iron ( II ) Spin Crossover Complexes with Unique Supramolecular Networks Assembled by Hydrogen Bonding and Intermetallic Bonding

Two spin crossover (SCO) coordination polymers assembled by combining FeII octahedral ion, 4-cyanopyridine (4-CNpy) and [Au(CN)2] liner unit are described. These compounds, Fe(4-CNpy)2[Au(CN)2]2·1/2(4-CNpy) (1a) and {Fe(4-CNpy)2[Au(CN)2]2}-{Fe(H2O)2[Au(CN)2]2} (1b), present quite different supramolecular networks that show different magnetic behaviors. Compound 1a crystallizes in the centrosymmetric space group Pbcn. The asymmetric unit contains two 4-CNpy, one type of Fe2+, and two types of crystallographically distinct [Au(CN)2] units which form Hofmann-like two dimensional layer structures with guest spaces. The layers are combined with another layer by strong gold-gold intermetalic interactions. Compound 1b crystallizes in the centrosymmetric space group Pnma. The bent bismonodentate [Au(CN)2] units and FeII ions form a complicated interpenetrated three dimensional structure. In addition, 1b exhibits ferromagnetic interaction.


Introduction
The designing of supramolecular networks is essential for practical spin crossover (SCO) materials [1][2][3][4].The networks enhance the cooperativity in the entire crystal structure.Strong cooperativity leads to steep spin transition with a wide hysteresis loop [5,6].From the viewpoint of constructing supramolecular networks, coordination polymers are useful material.However, systematic designing of networks is still hard because of the unexpected occurrence of supramolecular isomerism in the process of self-assembling.On the other hand, this structural diversity can result in unanticipated and interesting materials.Therefore, control of structural diversity represents fundamental research in crystal engineering.Since we reported the first Hofmann like two-dimensional (2-D) SCO coordination polymer {Fe(py) 2 [Ni(CN) 4 ]} n (py = pyridine) [7], many 2-D layers of {Fe II (L) 2 [M I (CN) 2 ] 2 } n [8][9][10][11][12][13][14][15][16] (M I = Ag, or Au, L = monodentate pyridine derivatives) have been developed.These compounds show an almost similar bilayer structure because of their strongly determinate self-assembly process in which they link octahedral metal centers through the N atoms of the bidentate [Au(CN) 2 ] − unit with strong aurophilic interaction between layers.This structural constancy enables us to precisely modify its crystal structure and properties.However, the applicable ligands for this system are still determinative.For instance, 3-cyano pyridine (3-CNpy) displays three different polymorphs [8].A strong polarity of cyano substituent must cause variations of the supramolecular networks, which strongly affects SCO properties.Therefore, more applicable ligands for this structural system must be investigated.Vice versa, cyano substituent offers new interesting networks and properties in cyano-bridged coordination polymers.Here, we report new supramolecular isomers of the general formula Fe(4-CNpy) 2 [Au(CN) 2 ] 2 • 1/2(4-CNpy) (1a) and  {Fe(4-CNpy) 2

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

Preparation of Compound 1b
Complex 1b was prepared by the same procedure as 1a.The reaction mixture was allowed to stand undisturbed for 2 days.After forming yellow single crystals (1a), orange crystals (1b) slowly grew.The crystalline sample for SQUID measurement was picked up using a binocular lens.The samples were checked by XRPD data (Figure S1).Impurity of the samples was observed as almost absent.Due to the small amount of sample picked, the background of the diffraction data were very high.Elem.Anal.Calcd for C 20 H 11 Au 4 Fe 2 N 12 O 2 : C, 17.77;H,0.89;N,12.43. Found: C,17.75;H,1.13;N,12.15.IR (cm −1 ): 2250 (νCN (4-CNpy)), 2169 (νCN)

X-ray Crystallography
Data collection was performed on a BRUKER APEX SMART CCD area-detector diffractometer for 1a and 1b with Monochrometed Mo-Kα radiation (λ = 0.71073 Å) (Bruker, Billerica, MA, USA).A selected single crystal was carefully mounted on a thin glass capillary and immediately placed under liquid N 2 cooled N 2 stream in each case.The diffraction data were treated using SMART and SAINT, and absorption correction was performed using SADABS [17].The structures were solved by using direct methods with SHELXTL [18].All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were generated geometrically.Pertinent crystallographic parameters and selected metric parameters for 1a and 1b are displayed in Tables 1-3.Diffraction data of 1a in high spin (HS) state was measured at 150 K in order to suppress the thermal motion of the guest molecules.When the sample was sufficiently cooled, both compounds showed a drastic and reversible change of color from yellow (1a) or orange (1b) to purple.The crystal structure of 1a in low spin (LS) state could not be determined.The low-quality data at 90 K was likely due to the occurrence of a sharp phase transition that provoked a notable increase of the mosaicity of the whole crystal structure.We described here the only HS state at 150 K. Crystallographic data have been deposited with Cambridge Crystallographic Data Centre: Deposition numbers CCDC-1869343 for compound 1a (150 K), CCDC-1869342 for 1b (298 K), and CCDC-1869341 for 1b (90 K).These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

Magnetic Measurements
Measurements of the temperature dependence of the magnetic susceptibility of the complexes 1a and 1b of the powdered samples in the temperature range 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.The diamagnetism of the samples and sample holders were taken into account.[19].This compound formed a flat monolayer structure.4-(3-pentyl) substituent was apparently of lager bulk than that of the 4-CN substituent.This much larger bulk caused bilayer interaction to break.

Crystal Structure of Compound 1b (T = 298 K)
Compound 1b at 298 K crystallized in the orthorhombic centrosymmetric space group Pnma.The asymmetric unit also consisted of the cyano bridged hetero-metal coordination (Figure 2a).There were two crystallographically different octahedral Fe II ions.2b).The rectangular moieties were penetrated by the other frameworks, which gave rise to a triply interpenetrated structure (Figure 2c).In addition, the closest approach between Au•••Au suggested the presence of aurophilic interactions (Au( 1

Thermal Analysis
The thermal analysis of 1a showed the three step weight loss between 380 K and 580 K corresponded to the loss of the two coordinated molecules of 4-CNpy and 0.5 solvent molecules (observed loss: 18.0% (first step) = ca.1.5 molecules, 6.4% (second step) = ca.0.5 molecule, 6.7% (third step) = ca.0.5 molecule) (Figure S2).This result was consistent with the elemental analysis.K•mol −1 at around 20 K, indicative of a ferromagnetic interaction between the Fe(II) centers.On the other hand, the isostructural former reported that compound 1b' showed weak antiferromagnetic interaction between two Mn II (HS state, S = 5/2).Although in a similar coordination environment, 1b showed ferromagnetic interaction.In terms of the spin state, the X-ray structural analysis of 1b at 90 K gave evidence of the arrangement of a•••Fe(HS)-Fe(LS)•••pair.On the other hand, 1b showed the different arrangement of•••Mn(HS)-Mn(HS)•••.Thus, 1b apparently much further distorted structure.Consequently, the magnetic structure of the residual HS site of Fe( 2) could cause the different magnetic coupling.In fact Fe(2) ions of HS site were close to each other in the supramolecular networks.At even lower temperature, the decrease in the value of χ M T was similar to 1a due to ZFS effects.

Conclusions
The new supramolecular networks designed by the components of Hofmann-like frameworks with 4-CNpyridine were reported.
These compounds showed unique multi-dimensional supramolecular networks involving hydrogen interactions and strong metallophilic interactions.Specifically, 1b exhibited the two magnetic functions of a SCO and a ferromagnetic transition.The diversity of the self-assembly process offered both unexpectedly interesting structure and properties.

4 4Figure 1 .
Figure 1.(a) Coordination structure of 1a containing its asymmetric unit at 150 K. (b) View of the

4 10Figure 2 .
Figure 2. (a) Coordination structure of 1b containing its asymmetric unit at 293 K involved in

Figure 2 .
Figure 2. (a) Coordination structure of 1b containing its asymmetric unit at 298 K involved in hydrogen bonding interactions as indicated by blue and white lines; (b) bent rectangular [Fe II Au I (CN) 2 ] 4 mesh structure; (c) cylinder drawing of triply interpenetrated 3D-networks of 1b involved in hydrogen bonding interactions as indicated by red and white lines; (d) perspective view of the crystal structure along a axis.In these pictures, hydrogen atoms are omitted for clarity.

1 .
Figure3ashows the thermal dependence of χ M T for 1a with χ M being the molar magnetic susceptibility and T the temperature.At room temperature, χ M T was 4.21 m 3 •K•mol −1 .Upon cooling, χ M T remained almost constant down to 120 K; below this temperature, χ M T underwent a sharp decrease to around 50% conversion with approximately 1 K hysteresis loop (T c down = 111 K, T c up = 112 K).The decrease in the value of χ M T at lower temperature was due to the typical behavior of zero-field splitting (ZFS).
Author Contributions: Data curation, I.T.; Formal analysis, D.A. and T.S.; Investigation, T.K. and I.T.; Project administration, T.K.; Supervision, T.K. Funding: This work was financially supported by KAKENHI (JSPS/15K05485 and 18K04964) and the Yashima Environment Technology Foundation.Part of this work was supported by the Ministry of Education, Culture,

Table 2 .
Selected bond lengths and angles for