Crystal-to-Crystal Transformation from K₂[Co(C₂O₄)₂(H₂O)₂]·4H₂O to K₂[Co(μ-C₂O₄)(C₂O₄)]

Abstract

Crystal-to-crystal transformation is a path to obtain crystals with different crystal structures and physical properties. K₂[Co(C₂O₄)₂(H₂O)₂]·4H₂O (1) is obtained from K₂C₂O₄·2H₂O, CoCl₂·6H₂O in H₂O with a yield of 60%. It is crystallized in the triclinic with space group P1 and cell parameters: a = 7.684(1) Å, b = 9.011(1) Å, c = 10.874(1) Å, α = 72.151(2)°, β = 70.278(2)°, γ = 80.430(2)°, V = 670.0(1) ų, Z = 2 at 100 K. 1 is composed of K⁺, mononuclear anion [Co(C₂O₄)₂(H₂O)₂²⁻] and H₂O. Co²⁺ is coordinated by two bidentated oxalate anion and two H₂O in an octahedron environment. There is a hydrogen bond between mononuclear anion [Co(C₂O₄)₂(H₂O)₂²⁻] and H₂O. K₂[Co(μ-C₂O₄)(C₂O₄)] (2) is obtained from 1 by dehydration. The cell parameters of 2 are a = 8.460(5) Å, b = 6.906 (4) Å, c = 14.657(8) Å, β = 93.11(1)°, V = 855.0(8) ų at 100 K, with space group in P2/c. It is composed of K⁺ and zigzag [Co(μ-C₂O₄)(C₂O₄²⁻]_n chain. Co²⁺ is coordinated by two bisbendentate oxalate and one bidentated oxalate anion in trigonal-prism. 1 is an antiferromagnetic molecular crystal. The antiferromagnetic ordering at 8.2 K is observed in 2.

1. Introduction

The change of the weak interaction of guest molecules, coordination geometry distortion, and coordination number in coordination compounds can effectively modulate the physical properties as magnetism, absorption, and chirality, so the dynamic molecular crystals have received great attention for their potential applications in molecular devices, as molecular sensors and switches become a powerful method for obtaining a specific compound with the yield of 100% by crystal-to-crystal transformation in crystal engineering [1,2,3,4,5,6,7]. The crystal-to-crystal transformations were observed between different dimensional coordination units as zero-dimensional (0D), one-dimensional chain (1D), two-dimensional (2D) layer, and three-dimensional (3D) coordination frameworks [8,9,10]. We are interested in dynamic crystals of MX₂–(1,4-dioxane)–H₂O system, and 0D to 2D, 1D to 2D, 1D to 3D crystal-to-crystal transformations were found [11,12,13]. Oxalate (C₂O₄²⁻) is one of most popular used three-atoms ligands in the study of molecular-based magnet, its versatile abilities and intermediating efficient magnetic coupling among transition atoms have constructed 1D, 2D, and 3D magnetic materials [14,15,16,17,18,19,20,21]. However, the research on oxalate-based dynamic crystal is limited. Herein, we present a crystal-to-crystal transformation from 0D mononuclear compound K₂[Co(C₂O₄)₂(H₂O)₂]·4H₂O (1) into a reported 1D coordination compound K₂[Co(μ-C₂O₄)(C₂O₄)] (2) accompanied by changes in crystal color, cell parameters, space group, coordination environment, crystal structure, and magnetic property.

2. Experiment and Discussion

1 was obtained from K₂C₂O₄·2H₂O, Co(NO₃)₂·6H₂O in H₂O with yield of 60%.

When 1 was heated at an elevated temperature (at 120 °C for three minutes), it transferred to 2 after dehydration with a mass loss of 25.7%, crystal structure changed from mononuclear to one-dimensional chain (Scheme 1) and the crystal color changed from orange to pink. 2 remained stable until 300 °C (Figure 1). This is the second method to obtain 2 except the solvothermal method. The IR bands (Figure S1) between 1 and 2 is the strong broad band above 3000 cm⁻¹ υ(O-H) as from H₂O in 1. The existence weak broad band above 3000 cm⁻¹ means 2 is unstable to air as reported [17].

1 crystallizes in triclinic with space group of P1: a = 7.684(1) Å, b = 9.011(1) Å, c = 10.874(1) Å, α = 72.151(2)°, β = 70.278(2)°, γ = 80.430(2)°, V = 670.0(1) ų, Z = 2. 1 is composed of K⁺, mononuclear coordination anion Co(C₂O₄)₂(H₂O)₂²⁻ and H₂O (Figure 2a). There are two K⁺, one Co²⁺, two oxalate (C₂O₄²⁻), and six H₂O in an independent unit. K1 is surrounded by five O from three oxalato and four H₂O, K2 is surrounded by five O from three oxalato and four H₂O. K column formed by K1 and K2 host the vacancy of H-bond network formed by oxalate and H₂O along the b axis. K1⋯K2 distances are 3.872(2) Å and 5.630(2) Å alternatively, and K1⋯K1 and K2⋯K2 distances are 9.011(1) Å. Each Co²⁺ is coordinated by two oxalate anions with Co-O 2.078(2)~2.099(2) Å on the equatorial plane, and two H₂O with Co-O 2.114(4) Å~2.120(3) Å to fulfill the octahedron environment. O-Co-O angles are among 88.4(2)~92.3(1)° from H₂O to equatorial plane and 176(1)° between two H₂O atoms. Viewed along the b axis, Co⋯Co distances are of 9.011(1) Å and 7.684(2) Å alternatively along the a axis. There are hydrogen bonds between anions and coordinated H₂O: O9-H1⋯O8(-x,1-y,1-z) 2.839(5) Å/170°, O10-H4⋯O1(-x,2-y,-z) 3.178(5) Å/127°; between anions and solvent H₂O: O14-H12⋯O1 2.795 Å/163°, O13-H9⋯O6(1-x,y,z) 2.740 Å/169°, O9-H2⋯O14(-x,1-y,-z) 2.760 Å/179°, O12-H8⋯O2(1-x,1-y,z) 2.820 Å/167°, O11-H5⋯O8(-x,1-y,1-z) 2.826 Å/156°, O10-H3⋯O12(-x,1-y,z) 2.755 Å/176°, O11-H6⋯O3(-x,1-y,-z) 2.709 Å/178°, O13-H10⋯O7 2.803 Å/164°.

The crystal structure of 2 is the same as reported isostructural of K₂[Fe(μ-C₂O₄)(C₂O₄)] [17]. It consists of K⁺ and zigzag chain [Co(μ-C₂O₄)(C₂O₄)²⁻]_n (Figure 2b). There are one and two half K⁺, one Co²⁺, one and two half oxalato in an independent unit. Co²⁺ is trigonal-prismatic coordinated by three oxalate anions with Co-O distance 2.058(3)~2.151(4) Å. K⁺ is in the vacant formed by Co(C₂O₄)₂²⁻ chain.

Depending on the extensive hydrogen bonds in 1 and zigzag chained structure of 2, the magnetic properties of them were investigated. The transformation from 1 to 2 is irreversible. The sample was checked and remained the same before and after magnetic experiments.

1: χT is 3.41 cm³ K mol⁻¹ at 300 K. It is significantly larger than the value of 1.875 cm³ K mol⁻¹ expected for an isolated, spin-only ion with S = 3/2 and g = 2.00. This suggests a strong spin-orbit coupling. [20,21,22] The χT value decreased upon cooling and reached 1.30 cm³ K mol⁻¹ at 2 K. The susceptibility data above 50 K fit the Curie–Weiss law well, giving Curie and Weiss constants of C = 3.613(6) cm³ K mol⁻¹ and θ = −21.2(2) K, respectively, with R = 3.74 × 10⁻⁵ (Figure 3). The negative Weiss constant means the antiferromagnetic interaction between Co²⁺ ions through hydrogen bonds. At 2 K, the isothermal magnetization is 2.24 Nβ at 65 kOe (Figure 4). No long-range magnetic ordering was observed in 1.

2: On χ versus T plot, a broad maximum of 0.031 cm³ mol⁻¹ was observed around 50 K, which is similar to reported oxalate-bridged one-dimensional compounds [15,17,20]. Then χ value decreased upon cooling smoothly, it is 0.0070 cm³ mol⁻¹ at 2 K. At 300 K, χT is 3.24 cm³ K mol⁻¹, this means a strong spin-orbit coupling of Co²⁺ as 1. The χT value decrease upon cooling and reach 0.014 cm³ K mol⁻¹ at 2 K. The data above 120 K were fitted with Curie–Weiss law, giving Curie and Weiss constant C = 3.66(2) cm³ K mol⁻¹, θ = −35(1) K, R = 4.96 × 10⁻⁵. Field-cooled magnetization (FCM) and zero-field-cooled magnetization (ZFCM) measurements under a field of 10 Oe show a magnetic ordering at 8.2 K (Figure 3, inset). At 2 K, the isothermal magnetization increases smoothly and reaches 0.072 Nβ at 65 kOe. The Hysteresis loop (Hc) is 500 Oe.

3. Conclusions

Orange 1 transfer to pink 2 by dehydration. 1 is composed of K⁺, mononuclear coordination anion Co(C₂O₄)₂(H₂O)₂⁻ and H₂O with extensive hydrogen bond between anion and H₂O, H₂O, and H₂O. 2 is consisted of K⁺ and zigzag chain anion [Co(μ-C₂O₄)(C₂O₄)²⁻]_n. The antiferromagnetic interaction in 1 from hydrogen bonds is weaker than oxalate-bridge in 2 [23]. 2 shows antiferromagnetic ordering at 8.2 K.