Polyhedron 30 (2011) 3062–3066

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Polyhedron

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Crystal structure and magnetic property of spin crossover complexFeII(3-phenylpyridine)2[AuI(CN)2]2

Kazuki Yoshida a, Takashi Kosone a,c, Chikahide Kanadani b,c, Toshiaki Saito b,c, Takafumi Kitazawa a,c,⇑a Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japanb Department of Physics, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japanc Research Center for Materials with Integrated Properties, Toho University, Miyama, Funabashi, Chiba 274-8510, Japan

a r t i c l e i n f o

Article history:Available online 24 February 2011

Keywords:Bimetallic FeIIAuI coordination polymerSpin crossoverX-ray structure

0277-5387/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.poly.2011.02.033

⇑ Corresponding author at: Department of ChemisUniversity, 2-2-1 Miyama, Funabashi, Chiba 274-8510

E-mail address: kitazawa@chem.sci.toho-u.ac.jp (T

a b s t r a c t

A novel two-dimensional network bimetallic Fe Au spin crossover coordination polymer based on3-phenylpyridine-coordinated iron centers and linear gold cyanide bridges {Fe(3-phenylpyri-dine)2[Au(CN)2]2}n (1), has been synthesized. The compound is characterized by elemental analysis, IR,single-crystal X-ray analysis at 300 and 90 K and magnetic measurements. The FeII ions in 1 have octa-hedral FeIIN6 coordination geometries, which are linked by [Au(CN)2]� units at the equatorial plane toform a polymeric 2D sheet architecture. The two pyridine rings coordinate in axial position. Variable-temperature (2–300 K) magnetic susceptibility measurements of 1 were performed to determine the spintransition behavior. SQUID data show that high and low spin states exist in a 1:1 ratio at 90 K. However,only one kind of FeII atom is apparent crystallographically at 90 K, indicating that the high and low spinsites are disordered in the polymeric 2D framework.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Coordination polymer frameworks formed by self-assemblyconstitute a most useful and efficient process for building up nano-scale supramacromolecular architectures with unique networktopologies and potentially interesting magnetic properties.Cyanometalates are useful building blocks for various dimensionalcoordination polymeric networks as transition metal templates.Metal–organic framework (MOF) materials of coordination poly-mers have useful functional properties, and can act as clathratehosts [1] and molecular magnets [2], and they may also exhibitspin crossover phenomena [3–8]. Molecular self-assembly inducedby coordination to transition metal ions has become an importanttool in the investigation of the cooperative nature of spin crossovercoordination compounds which can change between low–spin (LS)and high-spin (HS) ground states because the strength of the ligandfield in the complexes is located at the borderline between the lowand high-spin states [9]. Usually, spin crossover phenomena can beinduced by external stimulation, such as heat, pressure, or light.The spin transition phenomena depend largely on intermolecularinteractions [10]. In the case that the magnitude of the intermolec-ular interactions exceeds a certain threshold value, the spin-cross-over phase transition proceeds cooperatively. In such a case, the

ll rights reserved.

try, Faculty of Science, Toho, Japan.

. Kitazawa).

spin transitions may not only be very abrupt but also may occurwith a hysteresis effect.

Recently, the FeII Hofmann-like spin crossover coordinationpolynuclear compounds have become a fascinating subject formuch research. Particularly, dicyanometalates [MI(CN)2]� (M = Agand Au) with linear coordination geometries have been used asbuilding blocks in Hofmann-like structures, because they canlink metal coordination centers, through the N atoms of the biden-tate CN groups. Examples of Hofmann-like coordination polymersincorporating dicyanometalates include {MII(py)2[M0I(CN)2]2}(M = Fe, Cd) (M0 = Ag, Au) (py = pyridine) [1a,4a,4c]. These com-plexes are isostructural with each other and form two-dimensional(2D) {MII[M0I(CN)2]2} sheet network structures, including the rhom-bus {MII

4[M0I(CN)2]4}. {Fe(py)2[AgI(CN)2]2} and {Fe(py)2[AuI(CN)2]2}undergo spin-crossover phase transitions. We have developed syn-thetic routes to prepare various Hofmann-like cyano-bridged coor-dination polymers with pyridine derivatives. Another interestingfeature of some silver(I) and gold(I) compounds are the metallo-philic interactions using ligands larger than pyridine but smallerthan phenylpyridine [1,5–8,11–14,16–20]. Metallophilicity is aclosed-shell intermolecular interaction between M(I) atoms(M = Au or Ag) [15]. The metallophilic interactions are comparableto hydrogen bond interactions [14,15,17–19]. In the case of gold(I)compounds, such attraction has been called an aurophilic interac-tion. Recently, an unprecedented three-step spin crossovertransition was found in Fe(4-methylpridine)2[Au(CN)2]2 [17], andtwo-step thermal spin-transitions and LIESST relaxation have beenreported for Fe(X-py)2[Ag(CN)2]2 compounds [20].

Table 1Crystal data and details of refinements for 1 at 300 and 90 K.

Crystal data 1 (300 K) 1 (90 K)

Empirical formula C26H18Au2FeN6 C26H18Au2FeN6

Formula weight 864.25 864.25T (K) 300 90

K. Yoshida et al. / Polyhedron 30 (2011) 3062–3066 3063

We report here the synthesis, crystal structures, and magneticproperties of a new bimetallic spin crossover coordination poly-mer, {FeII(3-phenylpyridine)2[AuI(CN)2]2}n (1). The new compound1 exhibits interesting spin crossover behavior and morecomplicated interactions compared to traditional Hofmann-likestructures.

Crystal system monoclinic monoclinicSpace group C2/c C2/cUnit cell dimensions

a (Å) 16.3062(16) 16.254(4)b (Å) 12.4944(13) 12.096(3)c (Å) 14.9240(15) 14.729(4)b (�) 118.925(2) 120.458(4)

V (Å3) 2661.3(5) 2496.1(11)Z 4 4Dcalc (Mg/m3) 2.157 2.3Absorption coefficient (mm�1) 11.561 12.326F(0 0 0) 1600 1600Crystal size (mm3) 0.17 � 0.15 � 0.14 0.16 � 0.15 � 0.14Reflections collected 9737 4896Independent reflections (Rint) 3304 (0.0222) 2403 (0.0678)Goodness of fit (GOF) on F2 0.737 1.838R1

a, wR2b 0.0243, 0.0634 0.1517, 0.3929

Largest difference in peak and hole(e �3)

1.012 and �0.477 7.414 and �9.295

a R1 = (R||Fo| � |Fc||)/R|Fo|.b wR2 ¼ f

PwðjFoj � jFcjÞ2=

PwjFoj2g1=2.

Table 2Selected bond lengths and angles.

1 (300 K) 1 (90 K)

Bond lengths (Å)Fe(1)–N(1): 2.200(3) Fe(1)–N(1): 2.08(3)Fe(1)–N(2): 2.154(3) Fe(1)–N(2): 1.982(18)Fe(1)–N(3): 2.159(4) Fe(1)–N(3): 1.989(17)Au(1)–C(12): 1.964(4) Au(1)–C(12): 1.98(2)Au(1)–C(13): 1.973(4) Au(1)–C(13): 1.96(2)

Bond angles (�)N(1)–Fe(1)–N(2): 90.93(13) N(1)–Fe(1)–N(2): 91.3(11)N(1)–Fe(1)–N(3): 91.61(14) N(1)–Fe(1)–N(3): 89.3(11)N(2)–Fe(1)–N(3): 91.61(14) N(2)–Fe(1)–N(3): 92.8(11)N(2)–Fe(1)–N(3)0: 88.39(14) N(2)–Fe(1)–N(3)0: 87.2(11)C(12)–N(2)–Fe(1): 162.8(4) C(12)–N(2)–Fe(1): 170.2(19)C(13)–N(3)–Fe(1): 160.9(4) C(13)–N(3)–Fe(1): 166(2)C(12)–Au(1)–C(13): 175.50(18) C(12)–Au(1)–C(13): 173.3(11)

2. Experimental

2.1. Materials

All the chemicals were purchased from commercial sources andused without any further purification.

2.2. Synthesis of {FeII(3-phenylpyridine)2[AuI(CN)2]2}n (1)

The compound was synthesized by the method of vial-in-vialslow diffusion using ethanol solution. An aqueous solution(10 mL) of FeSO4�(NH4)2SO4�6H2O (0.073 mmol, 28 mg), ascorbicacid (0.150 mmol, 26 mg), and K[Au(CN)2] (0.20 mmol, 57 mg)was placed inside a small vial. A few drops of 3-phenylpyridine(Scheme 1) were placed inside a large vial filled with ethanol.The smaller vial was then placed inside the larger vial and thelarger vial was then sealed. Colorless crystals suitable for X-raydiffraction of 1 were obtained over a period of 1 week. 1: Anal. Calc.for C26H18Au2FeN6: C, 36.13; H, 2.10; N, 9.72. Found: C, 35.27; H,2.36; N, 9.45%. IR (KBr method, cm�1): 2157 (mCN).

2.3. X-ray crystal structural determinations

It is very difficult to prepare suitable single crystals of 1 since3-phenylpyridine is a large ligand. The crystal structure of 1 wasdetermined using a BRUKER APEX SMART CCD area-detectordiffractometer with monochromated Mo Ka radiation (k = 0.71073Å) at 300 and 90 K. The diffraction data were treated using SMART

and SAINT, and absorption correction were performed using SADABS

[21]. The structure was solved by using direct methods with SHELXTL

[22]. All non-hydrogen atoms were refined anisotropically, and thehydrogen atoms were generated geometrically. The low quality ofcrystal data of 1 at 90 K is due to the occurrence of a sharp phasetransition that causes a notable increase of the mosaicity of thecrystals in the low temperature phase. The data of 1 at 90 K wererefined using DFIX. There are apparently possible satellite peaksin the X-ray diffraction patterns recorded in the CCD detector at90 K. There may a possibility of a larger cell with lower symmetry.Pertinent crystallographic parameters are displayed in Table 1 andselected metric parameters for the complex are presented in Table2.

N3-phenylpyridine

Scheme 1. Molecular structure of the ligand.

2.4. Magnetic measurements

The temperature dependence of the magnetic susceptibility ofthe complex in the temperature range of 2–300 K with a coolingand heating rate of 1 K min�1 in a 1 kOe field was measured on aMPMS-XL Quantum Design SQUID magnetometer.

3. Results and discussion

3.1. Preparation and IR spectra

Reaction of an aqueous solution of FeSO4�(NH4)2SO4�6H2O2 con-taining two equivalents of K[Au(CN)2] with a pyridine-derivativeligand, 3-phenylpyridine, by vial-in-vial slow diffusion affordedcrystals of the 2D CN-bridged coordination compound {FeII(3-phenylpyridine)2[AuI(CN)2]2}n (1).

The solid state IR spectrum of 1 had C„N bands at 2157 cm�1,which is at a higher wavenumber than that of free [Au(CN)2]�

(2135 cm�1), at room temperature. This suggests that the CNgroups of [Au(CN)2]� act as bidentate bridging ligands.

Fig. 2. Coordination structure of Fe and Au ions in 1 (300 K) containing itsasymmetric unit and atom numbering. The structure of 1 at 90 K is almost identicalto that of 1 at 300 K. Consequently, the same representation is used for thesecompounds. In this picture, hydrogen atoms are omitted for clarity. Atom code: Fe(purple), Au (yellow), N (blue), C (black). (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)

3064 K. Yoshida et al. / Polyhedron 30 (2011) 3062–3066

3.2. Crystal structure of 1 (300 and 90 K)

Coordination compound 1 crystallized in the centrosymmetricspace group C2/c at both 300 and 90 K. Framework structures of1 at 300 and 90 K are almost identical. The two-dimensionalcyano-bridged corrugated framework of 1 is shown in Fig. 1. Theframeworks are built-up in a stacked parallel arrangement. Thereis crystallographically one type of FeII ion which is coordinatedby four N atoms from bidentate cyano substituents in equatorialpositions. The complex has crystallographically one type of distinctalmost linear [Au(CN)2]� units as shown in Fig. 2. The angles for 1(300 K) and 1 (90 K) defined by C(12)–Au(1)–C(13) are 175.50(18)[173.3(11)], respectively. The pyridyl nitrogens of the two 3-phen-ylpyridine monodentate ligands coordinate in axial positions toFe(1) (Fig. 2). The two 3-phenylpyridine ligands in [FeII(3-phenyl-pyridine)2][AuI(CN)2]2 adopt a transoid conformation at Fe(1). TheFe� � �Fe distances in the Fe(1)–NC–Au(1)–CN–Fe(1) one-dimen-sional wave chain edges for 1 at 300 K are 10.116 Å [at 90 K:9.807 Å]. Fe–N(1), Fe–N(2) and Fe–N(3) bond distances for 1(300 K) and [1 (90 K)] are 2.200(3) [2.08(3)], 2.154(3) [1.982(18)]and 2.159(3) [1.989(17)] Å, respectively. These bond distances for1 at 300 K and 90 K correspond to the accepted values for FeII

100% HS states and 50% HS 50% LS states respectively [16,19]. Pos-sible satellite peaks at 90 K may be associated with disorder in thearrangement at the 50/50% high/low spin ratio.

The conformation of the 3-phenylpyridine ligand depends ontemperature. The phenyl–pyridine distance in the C(2)–C(3) for 1at 300 K is apparently 1.483 Å [1.346 Å at 90 K]. The torsion anglesfor C(1)–C(2)–C(3)–C(4) and C(9)–C(2)–C(3)–C(8) for 1 at 300 K (at90 K) are apparently 43.611 [43.455] and 45.661 [49.572], respec-tively. The differences are probably due to the rotation and thevibration of the 3-phenylpyridne.

As shown in Fig. 3 and 1 has single layer networks, while {FeII(3-bromo-4-picoline)2[AuI(CN)2]2}n contains bilayer networks [21].Although the structure of Fe(X-pyridine)2[Au(CN)2]2 comprises acorrugated 2D cyano boding array with strong aurophilic interac-tion (Au–Au distances = 3.224 Å (300 K), 3.073 Å (90 K)) [4b], 1does not have aurophilic interactions (Au–Au distances = 5.434 Å

Fig. 1. Cylinder drawing, wave-shaped 2-D structure of 1 (300 K). The 2-Dframework of 1 at 90 K is almost identical to that of 1 at 300 K. Consequently, thesame representation is used for these compounds. Atom code: Fe (purple), Au(yellow), N (blue), C (black). In this picture, hydrogen atoms and 3-phenylpyridineligands are omitted for clarity. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

(300 K), 5.463 Å (90 K)). Using larger ligands such as 3-phenylpyr-idine precludes the presence of strong aurophilic interactions andplays an important role in forming the single layer structure suchas found in {FeII(pyridine)2[AuI(CN)2]2}n

3.3. Magnetic properties

vMT versus T plots, with vM being the molar magnetic sus-ceptibility and T the temperature, for 1 are shown in Fig. 4. Atroom temperature, the magnetic behavior of the complexes 1is characteristic of Fe(II) compounds in the high-spin state,vMT = 3.55 and the value of leff is (leff = 2.828

p(vMT)) per one

FeII ion at 300 K is 5.35lB. The value for 1 is slightly higher than4.90 ( = g

p(S(S + 1)), g = 2, S = 4/2) expected for a pure spin only

system, though the values are similar to the leff value at 300 Kof other Hofmann-like spin crossover FeII compounds[3,4b,18b,18c]. Upon cooling, vMT remains almost constant downto 180 K; below this temperature, vMT undergoes a two-step50% transition. The step between 150 and 180 K consists of asubtlety of two steps and the subtle second step has a narrowhysteresis loop. The spin crossover behavior for this complexcauses a reversible change of color from colorless to purple.The critical temperature for the cooling (Tdown

c ) and warming(Tup

c ) modes of the second step are 159 and 162 K, giving riseto the approximately 3 K hysteresis loop. The decrease in the va-lue of vMT at lower temperature is due to typical zero-fieldsplitting (ZFS) effects of the metallic FeII centers in the residualHS (S = 2) species.

As shown in Fig. 4, there are at least three phases: high/low-spin ratios are 50/50%, 75/25%, and 100/0%. The order or disorderissue on distribution of the high/low-spin molecules in a crystal,for example, alternating, checkerboard, stripe, etc. or completelyrandom is very delicate. The crystal data at 90 K may indicate that

Fig. 3. Crystal packing showing the formation of a 3D supramolecular architecture for 1 (300 K). In this picture, hydrogen atoms are omitted for clarity. The structure of 1 at90 K is almost identical to that of 1 at 300 K. Consequently, the same representation is used for these compounds.

Fig. 4. Magnetic properties of 1. vMT vs T from 2 to 300 K.

K. Yoshida et al. / Polyhedron 30 (2011) 3062–3066 3065

the arrangement is truly disordered at ratios of 50/50% and 75/25%.Although 57Fe Mossbauer experiments would be helpful to charac-terize each phase, it is difficult to measure 57Fe Mossbauer spectrafor 1 since the Au atoms avoid Mossbauer measurements due tothe Ka of Au atoms.

4. Conclusions

We have prepared and characterized a new bimetallic spincrossover coordination polymer with the formula {FeII(3-phenyl-pyridine)2[AuI(CN)2]2}n (1). The quasi-linear bidentate [Au(CN)2]�

unit connects octahedral FeII ions. As a result, these units assem-ble forming huge {FeII

2[AuI(CN)2]4} rectangular mesh 2D networkarrays which are slightly wave-shaped. The spaces between the2D Fe(–NCAuCN–Fe1/4)4 host frameworks are occupied by 3-phenylpyridine ligands. The new complicated networks in singlelayers of 1, which are unprecedented in Hofmann-like cyano-bridged coordination polymers, arise due to the bulk of the 3-phenylpyridine ligand.

From measurements of the temperature dependence of themagnetic susceptibility, the profile of the magnetic behaviors ofthe complexes 1 includes a rapid 50% spin transition with a narrowhysteresis loop.

The present work shows that the new SCO compound 1 maybe a key material in the development of Hofmann-like Fe(II) com-pounds which have interesting magnetic properties such as aspin-crossover phase transition and molecular magnetism.

Acknowledgements

This work was supported by the ‘‘High Tech Research Center’’Project 2005–2009 of MEXT (Ministry of Education, Culture, Sports,Science and Technology), and the Research Center for Materialswith Integrated Properties, Toho University.

Appendix A. Supplementary data

CCDC 804787 and 804788 contain the supplementary crystallo-graphic data for 1 (300 K) and 1 (90 K). These data can be obtainedfree of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;or e-mail: deposit@ccdc.cam.ac.uk.

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