A New Systematic Construction of Novel Three-Dimensional Spin Crossover Coordination Polymers Based on the [AgI 2(CN)3] Building Unit.

New three-dimensional spin crossover (SCO) coordination polymers systematically constructed by the novel building unit [AgI 2(CN)3], FeII(3-Br-5-CH3pyridine)2[AgI 2(CN)3][AgI(CN)2] (1), FeII(3-Br-5-Clpyridine)2[AgI 2(CN)3][AgI(CN)2] (2), and FeII(3,5-Brpyridine)2[AgI 2(CN)3][AgI(CN)2] (3), have been synthesized and characterized. The bismonodentate binuclear [Ag2(CN)3]- and mononuclear [AgI(CN)2]- units and FeII atoms assemble to form a 3D network structure. The structures of 1-3 are crystallographically identical, which made up the triply interpenetration combined with complicated intermolecular interactions including Ag···Ag, Ag···X (pyridine substituents) and π-stacking interactions. Magnetic and differential scanning calorimetry studies were performed for 1-3. These compounds display a similar SCO behavior, while the critical temperatures (T c) are shifted by the substituent effect. Due to the identical structures of 1-3, the order of T c clearly corresponds with the Hammett constant.

Introduction

Coordination chemistry associated with d- and f-block complexes attracts much attention from various forms of chemistry such as photochemistry, electrochemistry, catalyst chemistry, radiochemistry, and magnetochemistry.[1] In particular, the Fe(II) spin crossover (SCO) phenomenon for developing practical molecular magnetic devices is one of the most attractive research areas. It is necessary to construct a strong cooperative interaction between Fe centers and optimize the ligand field strength of the FeN6 coordination environment.[2] Particularly, the strong cooperativity creates a hysteresis loop between the high-spin (HS) state and low-spin (LS) state, which is the most important part of the memory effect. Therefore, the design and synthesis of new SCO coordination polymers (CPs) are fundamental steps to develop the practical SCO materials. Up to now, a huge number of two-dimensional (2D) and three-dimensional (3D) SCO CPs have been designed. Furthermore, the 3D polymeric structure can add a nanoporous function.[3]

As stated above, the Hofmann-like CP series that formulated M1(L)[M2(CN)4] and M1(L)[M3(CN)2]2 (M1 = Mn(II), Fe(II), Co(II), Ni(II), Cd(II), Zn(II), M2 = Ni(II), Pd(II), Pt(II),[4] M3 = Ag(I) or Au(I),[5] L = pyridine derivatives, x = 1 or 2) is the most convenient motif, which strongly defined the dimensionality depending on using monodentate or bidentate pyridine ligands. This motif consists of an octahedral metal ion (M1) through N atoms of the cyanometalate unit at the equatorial position, building up an almost square mesh layer structure woven by −M1–N–C–M2,3–C–N–M1– infinite chains and monodentate pyridine (py) ligands at axial positions where it is vertical to the layer. A gap inside the square is penetrated by py ligands from the upper and lower layers, resulting in a parallel stacking array. The 3D structure can be also formed by using a pillared bidentate ligand. One of the best discoveries of the 3D Hofmann-like structure is Fe(pyrazine)[M2(CN)4]. This has a novel SCO property and trim nanoporous space, in which several interesting functionalities and applications were reported.[6] Such an excellent structural consistency gives us controlling dimensionality and functionality. Particularly, since 2000, many 2D bilayer structures FeII(L)2[M3(CN)2]2 have been developed. This series has an outstandingly similar crystal structure. However, the applicable monodentate ligands for the Hofmann-like 2D layer system are still determinative, which are only three- and/or four-substituent pyridine derivatives. So far, the 3-X-5-Ypyridine ligand produces a different unexpected structure. For example, SCO CP Fe(3,5-CH3py)2[Ag2(CN)3][Ag(CN)2] has been reported.[7] It is noted that the unique [Ag2(CN)3]− dimeric building block is generated by the dissociation equilibrium from the solution of K[Ag(CN)2] as follows:

As a result, existing two types of cyanometalate moieties are simultaneously assembled, demonstrating a new type of framework. In recent years, some examples by using the [Ag2(CN)3]− unit were reported.[8] Although an isostructural series of FeII(L)2[Au(CN)2]2 and FeII(L)2[Ag(CN)2]2 have been much reported, the [Au2(CN)3]− unit is still not found. Thus, the [Ag2(CN)3] species is a rare building unit. The chain longer than [Ag(CN)2] can be utilized for a new variety of cyano-bridged motifs. These new prototypical structures show the variety of the SCO behavior. However, how to systematically design and control its structure such as the traditional Hofmann-like structure is still not found. In this paper, we report the synthesis, crystal structures, and physical properties of three novel CPs incorporating the [AgI2(CN)3] building unit based on the 3-Br-5-Xpyridine derivatives (X = Cl, Br, and CH3; see Scheme S1), formulating FeII(3-Br-5-CH3py)2[AgI2(CN)3][AgI(CN)2] (1), FeII(3-Br-5-Clpy)2[AgI2(CN)3][AgI(CN)2] (2), and FeII(3,5-Brpy)2[AgI2(CN)3][AgI(CN)2] (3).

Results

Overview of the Structure for 1–3

These 3D frameworks of Fe(3-Br-5-Xpy)2[Ag2(CN)3][Ag(CN)2] are almost isostructural (Figure and Figures S1 and S2). 1–3 in the HS and LS state crystallize in the monoclinic centrosymmetric space group C2/c. For 1–3, the asymmetric unit consists of the FeN6 coordination environment. Two 3-Br-5-Xpy ligands coordinate to the axial positions. The 3-Br and 5-X substituents are disordered to each other at the 3,5-position. Thus, the two 3-Br-5-Xpy ligands are crystallographically equivalent. The equatorial positions are occupied by two types of cyanometalate units, [Ag(CN)2]− and [Ag2(CN)3]− (Figure (HS state) and Figure S3 (LS state)). In [Ag2(CN)3]− units, the crystallographic discrimination of the central CN linking group between Ag ions in the N–C–Ag–(CN)–Ag–C–N unit is impossible due to the inversion center between them. Thus, the central cyano substituent coexists in Ag(2)–CN–Ag(2) and Ag(2)–NC–Ag(2) conformations, which must be a static disorder. Both [Ag(CN)2]− and [Ag2(CN)3]− have linear coordination geometries, forming shorter chains −Fe–N–C–Ag–C–N–Fe– and longer chains −Fe–N–C–Ag–(CN)–Ag–C–N–Fe–, respectively. The shorter and longer chains formed the rectangular motifs {Fe[Ag2(CN)3]}2{Fe2[Ag(CN)2]2}2. The Fe···Fe distance in the shorter side of the rectangular moiety is ca. 15 Å, while the longer side is ca. 20 Å. The rectangles are perpendicularly connected to form a 3D network just like a piled brick wall (Figure ). The intraframework spaces are occupied by two other identical but independent frameworks, which interpenetrate to each other (Figure S4). The Fe–N bond lengths for 1–3 correspond to the usual values for the FeII 100% HS state and 100% LS state (average Fe–N bond lengths for 1 = 2.164(4) Å (296 K), 1.956(3) Å (90 K), for 2 = 2.158(4) Å (275 K), 1.957(3) Å (90 K), and for 3 = 2.155(3) Å (275 K), 1.969(3) Å (90 K)). Close Ag···Ag distances between each frameworks indicate strong argentophilic interactions (see Figure ), which is less than the sum of the van der Waals radius of Ag (ca. 3.60 Å). The py rings are stacked parallel to one another, which exhibits π-stacking interactions with displaced a stacking arrangement (Figure ). The shortest contact Cpy···Cpy distances are 3.361(8) [3.271(6)] Å at 296 K [90 K] for 1, 3.359(10) [3.280(4)] Å at 275 K [90 K] for 2, and 3.345(7) [3.273(4)] Å at 275 K [90 K] for 3. Furthermore, significantly smaller distances between Ag and X(Br) are also observed (Figure ). These additional interactions increase dimensionality. The distances are gathered in Table .

Figure 1

Crystal structure of 1 at 300 K. Short Ag···Ag contacts, as indicated by gray sticks. In this picture, hydrogen atoms are omitted for clarity.

Figure 2

Molecular fragment of 1 at 275 K (a), 2 at 275 K (b), and 3 at 275 K (c). In these pictures, hydrogen atoms are omitted for clarity. The image showing Ag(2)–C–C–Ag(2) in the [Ag2(CN)3] unit means the disorder model in −C–N– and −N–C– conformations.

Figure 3

Schematic showing the 3D framework. Green lines indicate the one rectangular face of {Fe[Ag2(CN)3]}2{Fe2[Ag(CN)2]2}2.

Figure 4

Representation of intermolecular short contacts of Cpy···Cpy (red cylinder lines) and Ag···X (blue cylinder lines) of 2. Black lines show the shortest distances between the centroid of the rings. In this picture, hydrogen atoms are omitted for clarity.

Table 2

Selected Close Ag···Ag, Cpy···Cpy, and Ag···X Distances [Å] of 1–3

	1 (293 K)	1 (90 K)	2 (275 K)
Ag···Ag	Ag(2)···Ag(2): 3.2546(16)	Ag(2)···Ag(2): 3.0995(13)	Ag(2)···Ag(2): 3.2751(22)
Cpy···Cpy	C(2)···C(4): 3.480(10)	C(2)···C(4): 3.399(6)	C (2)···C(4): 3.459(11)
	C(3)···C(3): 3.361(8)	C(3)···C(3): 3.271(6)	C(3)···C(3): 3.359(10)
Ag···X	Ag(1)···Br(1): 3.5223(19)	Ag(1)···Br(1): 3.4582(16)	Ag(1)···Cl(1): 3.5142(24)

Interestingly, Ag···Ag contacts are also drastically changed by the spin transition. The changes of Ag···Ag distances upon the spin transition are 0.1551(10) (1), 0.1540(13) (2), and 0.1517(96) (3) Å. On the other hand, Ag···X and Cpy···Cpy distances are slightly changed.

Magnetic Behaviors

Figure shows the thermal dependence of χMT for 1–3. The spin transition behaviors of 1–3 are almost similar. For 1, at room temperature, χMT was 3.52 cm3·K·mol–1. Upon cooling, χMT remains almost constant down to 280 K; below this temperature, χMT decreases to 0.42 cm3·K·mol–1 at 212 K (Tc = 240 K), exhibiting an almost complete one-step spin transition. The observed second decrease in the residual value of χMT at a lower temperature of around 20–2 K is due to the typical behavior of zero-field splitting (ZFS). χMT values for 2 and 3 were 2.85 and 3.53 cm3·K·mol–1 at room temperature, respectively. Like in 1, 2 and 3 also show a sharp one-step spin transition. The critical temperatures are Tc = 249 K (2) and 250 K (3).

Figure 5

Temperature dependence of the magnetic susceptibilities for 1 (blue), 2 (red), and 3 (black).

DSC Measurement

Spin transition behaviors were also confirmed by differential scanning calorimetry (DSC) measurement, as shown in Figure . DSC curves for 1–3 show anomalous peaks (Tc↓ = 235.0 K (1), 246.7 K (2), and 247.5 K (3) and Tc↑ = 239.3 K (1), 250.5 K (2), and 250.8 K (3)). The average enthalpy and entropy variations associated with the cooling and heating modes obtained from these DSC curves, ΔH = 19.5 kJ·mol–1 (1), 18.0 kJ·mol–1 (2), and 21.1 kJ·mol–1 (3) and ΔS = 82.5 J·K–1·mol–1 (1), 72.1 J·K–1·mol–1 (2), and 85.1 J·K–1·mol–1 (3), are in the range of typical values for cooperative SCO FeII compounds.

Figure 6

DSC curves of 1 (blue line), 2 (red line), and 3 (black line).

Discussion

The previously reported compound of Fe(3,5-CH3)2[Ag2(CN)3][Ag(CN)2] (4) is crystallographically identical (see Table S2) to 1–3, also discussed in this paper. These compounds are only different in substituents. In terms of the dissociation equilibrium, although the precursor [Ag(CN)2] is thermodynamically more stable than [Ag2(CN)3], the [Ag2(CN)3] species is the main product. The reason for the main product of Fe(L)2[Ag(CN)2][Ag2(CN)3] is due to the differences of stability of the crystal structure. In the case of Fe(L)2[Ag(CN)2]2, the width of the 3,5-position substituents collides with the edge of the “square” window of {Fe2[Ag(CN)2]2}4. On the other hand, the wider “rectangular” window of {Fe[Ag2(CN)3]}2{Fe2[Ag(CN)2]2}2 easily penetrates the rectangle window. As a result, the crystal packing is stabilized. Therefore, the dissociation equilibrium (AgCN + [Ag(CN)2]− ↔ [Ag2(CN)3]−) is shifted to the right side.

Cell volumes, Tc, and related substituent parameters (the Hammett constant and Tolman cone angle) for 1–4 are listed in Table . The order of cell volumes is as follows: 2 (2373.7(3) Å3) < 4 (2381.0(3) Å3) < 3 (2388(3) Å3) < 1 (2394.5(2) Å3). In the view point of the substituent size, for example, considering the Tolman cone angles[9] (−CH3 (90°) < −Cl (102°) < −Br (105°)), it seems that there are no relationships between the cell volume and substituent size. On the other hand, the series of the traditional 2D Hofmann layer structure of Fe(L)2[M3(CN)2]2 clearly shows a lattice expansion with the increase of the substituent size. The difference between the 2D layer type and the present 3D type is that there might be a margin of void space for 1–4 to accept the bulky substituents, while the interlayer space of the 2D layer type is relatively tight. In the view point of the electronic effect, the order of Tc has considerable relationships according to the Hammett constants[10] as follows: 4 (235 K) < 1 (240 K) < 2 (249 K) ≈ 3 (250 K) (Figure S5). The electron-donating substituent group (σp < 0) enhances the d−π interaction, decreasing Δo. Therefore, the −CH3 substituent results in a relatively lower Tc. On the other hand, electron-withdrawing substituents (σp > 0) induce a higher Tc. The σp values of −Cl and −Br have the same positive values; therefore, the Tc values for 2 (3-Br-5-Cl-) and 3 (3-Br-5-Br-) are actually almost the same. Due to the complete isostructures of 1–4, the order of Tc must be only considered and directly reflected to the substituent effect inducing electron-accepting/electron-donating properties.[11] In the previous papers, the synthesis and characterization of related compounds by using [Ag2(CN)3], {Fe(L)[Ag(CN)2][Ag2(CN)3]}·nG (L = pyrimidine (L1)[8a], 1,4-di(pyridin-4-yl)benzene (L2)[8b], 2,6-naphthyridine (L3)[8c], and 1,2,4,5-tetrakis(pyridin-4-ylethynyl)benzene (L4)[8d]), have been reported. These compounds utilize bridging bi(L1–3)- or tetra(L4)-dentate ligands, forming quite different crystal structures to each other. On the other hand, 1–4 have monodentate ligands with a similar shape, forming an isostructure. Tc values for 1–4 are generally higher than previous compounds, L1: Tc1 = 185 K, Tc2↑ = 147.5, Tc2↓ = 146 K, L2 (G = DMF·EtOH): Tc1↑ = 245 K, Tc1↓ = 218 K, L2 (G = partially desorbed): Tc1↑ = 245 K, Tc1↓ = 218 K Tc1 = 185 K, Tc = 150 K, L2 (desorbed): Tc = 250 K, L3: Tc↑ = 195 K, Tc↓ = 189 K, and L4: Tc = 187 K. The ligands L1–4 are relatively longer and bulky and have a complicated shape. Therefore, the higher Tc for 1–4 would be due to the denser crystal packing than those of L1–4. Consequently, the dense packing induces a higher chemical pressure, increasing the ligand field strength Δo.

Table 3

Cell Volumes (HS State) and Tc of {Fe(3-Y-5-Xpy)2[AgI(CN)2]2} and the Hammett Constant

substituents (Y, X)	ref	cell volume (Å3)	average Tc (K)a
X, Y = −CH3	(6)	2381.0(3)	235 K
X = −CH3, Y = −Br	this work	2394.5(2)	240 K
X = −Cl, Y = −Br	this work	2373.7(3)	249 K
X = −Br, Y = −Br	this work	2388(3)	250 K
	–CH3	–Cl	–Br
Hammet constant σp9	–0.17	0.23	0.23

From the magnetic measurement.

Conclusions

New 3D network coordination polymers were reported. The present 3D structure was strongly defined by the substituent position, which would systematically design crystal structures and physical properties. Although this complicated 3D network includes various intermolecular interactions, there are no apparent hysteresis loops. It may be caused by longer chain −Fe–N–C–Ag–(CN)–Ag–C–N–Fe–, which separated the Fe···Fe distance and weakened cooperativities. However, the new series of spin crossover family as the Hofmann-like structure is still useful for investigating and modifying the spin crossover phenomena.

Experimental Section

Synthesis of 1–3

Freshly prepared single crystals of 1–3 were synthesized by slow diffusion of two solutions, one of which contained a mixture of Fe(BF4)2·6H2O (33.7 mg, 1.00 × 10–4 mol) and each contained pyridine ligands (2.00 × 10–4 mol) in 1 mL of a water/ethanol mixed solvent. The others contained a water solution of K[Ag(CN)2] (20.0 mg, 1.00 × 10–4 mol) in 2 mL of water. The two solutions filled the glass tube. Yellow (1 and 2) and brown (3) single crystals suitable for single-crystal X-ray diffraction were formed over 2–4 days. The crystalline sample was picked up using a binocular lens. Due to the very small amount of crystals and mixed few impurities in the glass tube for 1 and 2, the powder samples for a superconducting quantum interference device (SQUID), X-ray powder diffraction (XRPD), and elemental analysis were also prepared. For preparing the powder sample, one of these contained a mixture of FeCl2·4H2O (119.3 mg, 6.00 × 10–4 mol) and ascorbic acid (105.67 mg, 6.00 × 10–4 mol) in 5 mL of water. The others contained pyridine ligands (1.20 × 10–3 mol) in 1 mL of ethanol. The two solutions were communicated. Then, the solution of K[AgI(CN)2] (240.00 mg, 1.20 × 10–3 mol) in 5 mL of water was added to the previous solution. Yellow powder samples of 1 and 2 were formed immediately. The powder samples were washed with a large amount of ethanol (ca. 500 mL). The powder sample was checked by XRPD data (see Figures S6–S8). No impurity and isomers were observed. Elem. Anal. Calcd for C17H12FeN7Ag3Br2 (1): C, 23.92; H, 1.42; N, 11.48. Found: C, 23.68; H, 1.60; N, 11.31. IR (cm–1): 2173 (νC≡N). Calcd for C15H6Fe N7Ag3Br2Cl2 (2): C, 20.14; H, 0.68; N, 10.96. Found: C, 19.86; H, 0.86; N, 10.75. IR (cm–1): 2171 (νC≡N). Anal. Calcd for C15H6FeN7Ag3Br4 (3): C, 18.32; H, 0.62; N, 9.97. Found: C, 17.70; H, 0.85; N, 9.48. IR (cm–1): 2171 (νC≡N).

Structural Analysis

Data collection was performed on a BRUKER APEX SMART CCD area detector diffractometer for 1–3 with monochromated MoKα 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 a liquid-cooled N2 stream. The diffraction data were treated using SMART and SAINT, and absorption correction was performed using SADABS.[12] The structures were solved using direct methods with SHELXTL.[13] All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were generated geometrically. Pertinent crystallographic parameters, bond lengths, and angles for 1–3 are displayed in Table and Table S1. Crystallographic data were deposited at the Cambridge Crystallographic Data Centre (CCDC): deposition numbers CCDC-2062871 for compound 1 (296 K), CCDC-2062872 for 1 (90 K), CCDC-2062873 for 2 (275 K), CCDC-2062874 for 2 (90 K), CCDC-2062875 for 3 (275 K), and CCDC-2062876 for 3 (90 K). These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

Table 1

Crystallographic Parameters for 1–3a

	1 (296 K)	1 (90 K)	2 (275 K)	2 (90 K)	3 (275 K)	3 (90 K)
empirical formula	C17H12Ag3FeBr2N7	C17H12Ag3FeBr2N7	C15H6Ag3FeBr2Cl2N7	C15H6Ag3FeBr2Cl2N7	C17H12Ag3FeBr4N7	C15H6Ag3FeBr4N7
FW	853.62	853.62	894.418	894.418	983.37	983.37
crystal system	monoclinic	monoclinic	monoclinic	monoclinic	monoclinic	monoclinic
space group	C2/c	C2/c	C2/c	C2/c	C2/c	C2/c
a (Å)	12.7439(7)	12.4157(6)	12.7896(11)	12.4285(11)	12.806(11)	12.4670(14)
b (Å)	13.6462(8)	13.2373(7)	13.5068(11)	14.328(4)	13.584(11)	13.3156(15)
c (Å)	14.0322(8)	13.6566(7)	13.9946(12)	14.652(4)	13.973(11)	13.6789(15)
β (°)	101.1156(10)	100.2930(10)	100.9217(15)	100.2040(10)	100.781(14)	100.331(2)
V (Å3)	2394.5(2)	2208.35(19)	2373.7(3)	2190.9(3)	2388(3)	2234.0(4)
Z	4	4	4	4	4	4
final R indices [I > 2sigma(I)]	R1 = 0.0447	R1 = 0.0303	R1 = 0.0337	R1 = 0.0272	R1 = 0.0272	R1 = 0.0261
	wR2 = 0.1965	wR2 = 0.0647	wR2 = 0.1219	wR2 = 0.0520	wR2 = 0.0679	wR2 = 0.611
R indices (all data)	R1 = 0.0557	R1 = 0.0323	R1 = 0.0444	R1 = 0.0381	R1 = 0.0357	R1 = 0.0325
	wR2 = 0.1913	wR2 = 0.0652	wR2 = 0.1112	wR2 = 0.0496	wR2 = 0.0651	wR2 = 0.0591

R = (Σ||Fo| – |Fc||)/Σ|Fo| wR = {Σw(|Fo| – |Fc|)2/Σw|Fo|2}1/2.

Cation

These cif files show the alerts A “short inter D···A Contact” (PLAT430), which was caused by the crystallographic discrimination of the central CN linking group between the CN–Ag–()–Ag–CN unit. Each C(N) and N(C) are generated by the symmetry operation. Therefore, this “short inter D···A Contact” does not exist.

Physical Measurements

We measured the temperature dependence of the magnetic susceptibility of complexes 1–3 in the temperature range of 2–300 K with a cooling and heating rate of 2 K·min–1 in a 1 kOe field on a Quantum Design MPMS-XL SQUID magnetometer (Quantum Design, Inc., Pacific Center Court, San Diego, CA, USA). The diamagnetism of the samples and sample holders was considered. Calorimetric measurements were carried out using a differential scanning calorimeter Mettler Shimadzu DSC-60 in the temperature range of 215–285 K with a cooling and heating rate of 10 K·min–1.