Synthesis, Crystal Structures, and Magnetic Properties of One-dimensional Lanthanide(III)-Octacyanomolybdate(V) Assemblies with 3,4,7,8-Tetramethyl-1,10-phenanthroline as a Blocking Ligand

Experimental Section

Materials and Physical Measurements. The reagent 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen) was purchased from Aldrich and used without further purification. Cs3[W(CN)8]·2H2O was prepared according to literature.17 All other reagents were commercial available and used as received. Infrared spectra were obtained within the 4000-400 cm−1 as KBr disks on a VECTOR 22 spectrometer. Elemental analyses were performed on a Perkin Elmer 240C elemental analyzer. Magnetic measurements on microcrystalline sample were carried out on a Quantum Design MPMP-XL7 superconducting quantum interference device (SQUID) magnetometer. Diamagnetic corrections were made for both the sample holder as the background and the compound estimated from Pascal’s constants.18

Synthesis of Complexes 1-2. To a solution of Tb(NO3)3·6H2O (0.1 mmol, 45.3 mg) or Dy(NO3)3·6H2O (0.1 mmol, 45.6 mg) and Cs3[Mo(CN)8]·4H2O (15.5 mg, 0.02 mmol) in H2O (5 mL), a solution of tmphen (4.7 mmol, 0.02 mmol) in CH3CN (2 mL) was added dropwise with gentle stirring. The yellow precipitate was dissolved using ca. 2 mL of DMF. The resulting solution mixture was allowed to stand in the dark without disturbance for several weeks and red prism single crystals suitable for X-ray analysis were obtained.

Complex 1: Yield 8.9 mg (41.1％). Calculated for C46H46N14O2MoTb: C, 51.07％; H, 4.29％; N, 18.13％. Found: C, 51.15％; H, 4.30％; N, 18.32％, IR stretching cyanide (KBr)/cm−1: 2118, 2160.

Complex 2: Yield 9.1 mg (41.9％). Calculated for C46H46N14O2MoDy: C, 50.90％; H, 4.27％; N, 18.07％. Found: C, 50.84％; H, 4.26％; N, 17.91％, IR stretching cyanide (KBr)/cm−1: 2121, 2154.

Structural Determination and Refinement. The crystal structures were determined on a Siemens (Bruker) SMART CCD diffractometer using monochromated Mo-Kα radiation (λ = 0.71073 Å) at room temperature. All absorption corrections were performed by using the SADABS program.19 Structures were solved by direct methods using the program SHELXL-97.20 All non-hydrogen atoms were located in difference Fourier maps and refined anisotropically. All H atoms were refined isotropically, with the isotropic vibration parameters related to the non-H atom to which they are bonded. A summary of the structural determination and refinement for the title complexes 1-2 is listed in Table 1 and the selected bond distances and angles are shown in Tables 2-3.

Table 1.R1= Σ| |Fo|−|Fc| |/Σ|Fo|. ωR2 = Σ[w(Fo2−Fc 2)2]/Σ[w(Fo 2)2]1/2

Table 2.Symmetry transformations used to generate equivalent atoms: 1 −x+3/2, y+1/2, z; #2 −x+3/2, y−1/2, z.

Table 3.Symmetry transformations used to generate equivalent atoms: #1 −x+3/ 2, y+1/2, z; #2 −x+3/2, y−1/2, z.

 

Results and Discussions

Crystal Structures of Complexes 1-2. Complexes 1-2 have the same structural skeleton with the orthorhombic space group Pbca, we only depict the crystal structure of complex 1 in detail. As shown in Figure 1, the asymmetric unit of complex 1 consists of a [Tb(tmphen)2(DMF)2]3+ and a [Mo(CN)8]3−. The eight coordination environment around Tb ion is composed of two tmphen, two DMF molecules and two bridging cyanide ligands forming a distorted square anti-prism. Two tmphen and two DMF molecules coordinate to Tb ion on two sides, and the tmphen molecules stabilize the structure by intramolecule π-π stacking effect. The Tb-Ntmphen bond lengths range from 2.546(4) to 2.559(4) Å, the Tb-NCN from 2.476(4) to 2.478(4) Å, and the Tb-ODMF distances are slightly shorter in the range of 2.313(4)-2.329(4) Å. The Tb-N≡C linkages are poorly linear with the angles of 166.7(4) and 176.8(5) Å. In all complexes 1-2, the bond lengths and angles related to lanthanide ions are in good agreement with those in the reported literature.15b MoV(CN)8 takes a slightly distorted square anti-prism geometry, in which two cyanides (C1N1 and C4N4) connect to the two neighboring [Tb(tmphen)2(DMF)2]3+ units by cisoid mode to form a chain structure, while the other cyanides are terminal. The Mo-CN bond lengths range from 2.145(5) to 2.169(7) Å, the C≡N from 1.132(8) to 1.151(8) Å, and the Mo-C≡N linkages are almost linear from 176.0(5)° to 179.3(7)°. For complex 2, the Mo-CN bond lengths range from 2.142(8) to 2.165(7) Å, the C≡N from 1.143(9) to 1.162(9) Å, and the Mo-C≡N linkages are from 175.9(6)° to 179.9(8)°. All bond lengths and angles based Mo(CN)8 in complexes 1-2 are comparable with those in reported literatures.3-6

Figure 1.ORTEP drawing of the asymmetrical unit of 1 with atomic labeling for metal ions and donor atoms. Displacement ellipsoids are drawn at 30％ probability level. Hydrogen atoms have been omitted for clarity.

In complex 1, [Tb(tmphen)2(DMF)2]3+ and [Mo(CN)8]3− ions are linked in an alternating fashion to form a 1D cyanobridged chain as shown in Figure 2. In a unit cell, four equivalent chains separate from each other with a minimum intermetallic distance between Tb and Mo of 11.348 Å. The intramolecular distances Tb–Mo is 5.719 Å (Tb-C1≡N1-Mo) and 5.758 Å (Tb-C4≡N4-Mo). The Mo1–Tb–Mo1 angle is 140.99° for 1, which is an indication of the zig-zag chain structure. In the solid state, the neighboring {TbIIIMoV}n chains weakly interact through the face-to-face π-π stacking of the tmphen aromatic rings, leading to the 2D supramolecular folded layer. However, no hydrogen bonding is observed, so the structure of the solid state for complex 1 is stabilized mainly by van der Waals forces.

Figure 2.The 1D infinite structure of 1 along bc plane. Hydrogen atoms have been omitted for clarity.

Magnetic Properties of Complex 1-2. In lanthanide complexes, the spin-orbital coupling leads to the 4fn configuration splitting into 2S+1LJ states, and further into Stark components under the crystal-field perturbation. So, usually, the variable-temperature magnetic behavior of lanthanidebased complexes mainly arises from the significant orbital contributions of LnIII ions (with the exception of GdIII). Above room temperature, all of the Stark levels are populated, but as the temperature decreases, the effective magnetic moment of the lanthanide ion will change as a result of thermal depopulation of the Stark sublevels.

For 1, χMT slowly decreases from 12.28 cm3 K mol−1 at 300 K (spin-only values of 11.815 cm3 K mol−1 for isolated TbIII (J = 6, g = 3/2) and 0.375 cm3 K mol−1 for MoV (S = 1/2, g = 2)) to a minimum of 9.40 cm3 K mol−1 at 5 K, which is mainly ascribed to the depopulation of the Stark levels of the terbium 7F6 ground state. Upon cooling, the value increases to a maximum of 10.06 cm3 K mol−1 at 1.8 K, which indicates that the coupling interaction between metal ions overcomes the depopulation of the ground state leading to a net spin along the field, but cannot draw a conclusion about the magnetic coupling nature. The variable-field magnetic properties show that the magnetization undiversified increases with the external field and goes up to 5.69 NμB mol−1 at 7 T which is consistent with the ferromagnetic ground state spin based on a spin of S = 1/2 with g = 2 for MoV ion and an effective spin of S = 1/2 with g|| = 10 and g⊥ = 0 for TbIII ion.612-14 So, it is suggested that the cyano-bridge mediates the ferromagnetic interaction between MoV and TbIII ions.

Figure 3.Temperature dependence of the χMT product for 1 at 100Oe. The inset shows the magnetization versus the applied magnetic field at 1.8 K.

For 2, the χMT value is 14.32 cm3 K mol−1 at the room temperature, slightly lower than the sum of the spin-only values of 14.145 cm3 K mol−1 expected for isolated DyIII and 0.375 cm3 K mol−1 for MoV. Upon cooling, χMT value invariantly decreases and reaches 7.12 cm3 K mol−1 at 1.8 K (Figure 4). The variable-field magnetic properties show that the magnetization monotonously increases with the external field and goes up to 6.44 NμB mol−1 at 7 T, indicating that the cyano-bridge also mediates the ferromagnetic interaction between MoV and DyIII ions (1 + 5.23) NμB.21 No increase of χMT observed at low temperature like that of complex 2 implies that the ferromagnetic interaction is very weak relative to the depopulation of the Stark levels of the DyIII ion.

Figure 4.Temperature dependence of the χMT product for 2 at 100Oe. The inset shows the magnetization versus the applied magnetic field at 1.8 K.

In conclusion, we report here the synthesis, structures, and magnetic characterization of two new one-dimensional cyano-bridged coordination polymers [Ln(tmphen)2(DMF)2-Mo(CN)8]∞ (Ln = Tb(1) and Dy(2)). Two complexes are isostructural and crystallize in the orthorhombic space group Pbca. Magnetic investigations show that the ferromagnetic interaction exists in complexes 1-2.

Supplementary Material. CCDC-836416(1) and 836411 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK; Telephone: (44) 01223 762910; Facsimile: (44) 01223 336033; E-mail: deposit@ccdc.cam. ac.uk].