Synthesis, crystal structure and magnetic characterization of a cyanide-bridged Mo–Ni nanosized molecular wheel

2.1 Preparation of complex 1

A solution containing K₄[Mo(CN)₈] (0.1 mmol, 46 mg) in distilled water (5 mL) was placed on the bottom of a tube, upon which a mixture of water, CH₃CN and CH₃OH in a ratio of 1:1:1 was carefully layered. Then, a solution of [Ni(L)(H₂O)₂][ClO₄]₂ (0.2 mmol, 103.4 mg) in a mixture of 5 mL CH₃CN and CH₃OH each was carefully added to the top of the solvent layer. The tube was kept undisturbed in the dark, and needle-like single crystals suitable for X-ray diffraction were obtained about 1 week later, which were then collected by filtration and dried in air. Yield: 52.1 mg (ca. 48 %). – Anal. for C₂₃₀H₃₅₆Mo₆N₉₆Ni₁₂O₄₄: calcd. C 42.83, H 5.56, N 20.85; found C 42.59, H 5.14, N 21.28 %. – Selected IR frequencies: 2120 (s, v_C≡N), 2160 (s, v_C≡N) cm⁻¹.

2.2 X-ray structure determination

Single crystals of the complex with suitable dimensions were mounted on a glass rod, and the intensity data were collected on a Bruker SMART CCD diffractometer with a MoK_α sealed tube (λ = 0.71073 Å) at 293 K, using the ω scan mode. Final unit cell parameters were derived by global refinements of reflections obtained from integration of all the frame data. The collected frames were integrated by using the preliminary cell orientation matrix. CrysAlisPro (Agilent Technologies) [20] software was used for collecting the data frames, indexing reflections and determination of the lattice constants as well as for the integration of the reflection intensities. Multi-scan absorption corrections were applied with the SCALE3 ABSPACK algorithm of CrysAlisPro. The structure was solved by Direct Methods (Shelxs-97 [21, 22]) and refined by full-matrix least-squares on F² (Shelxl-97 [21, 22]). Anisotropic displacement parameters were used for the non-hydrogen atoms and isotropic parameters for the hydrogen atoms. Hydrogen atoms bonded to the C and N atoms were added geometrically and refined using a riding model. The SQUEEZE routine of Platon [23, 24] was used to remove the scattering from the disordered solvent molecules in the cavities. Some DFIX commands had to be used before the whole molecule was fixed by employing the DAMP command. The crystallographic data of complex 1 has been given in Table 1.

CCDC 1041767 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

3.1 Synthesis and general characterization

As has been known for a long time, the octacyanometalates are good candidates for assembling cyanide-bridged magnetic complexes with various structure types by using their rich coordination modes [7, 8]. The barrierless transformation between the three idealized basic geometries can lead to cyanide-bridged heterobimetallic systems with the octacyanometalates of molybdenum and tungsten acting as building blocks displaying all possible dimensionalities. Trinuclear and other one-dimensional cyanide-bridged Mo(W)–Ni complexes have been prepared by using the same nickel compound and [Mo(W)^V(CN)₈]³⁻ as assembly segments [11]. Our recent research has shown that the structures of the complexes assembled from the reaction of these types of cyanide precursors with the macrocyclic nickel complex used in this paper depend on the reaction conditions and methods. The solvent used to dissolve the cyanide precursor, the amount of water and the diffusion method used have an obvious influence on the structure type of the cyanide-bridged complexes formed [25]. Following this, we carried out the reaction of K₄[Mo(CN)₈] and [Ni(L)(H₂O)₂][ClO₄]₂ in a mixed solvent system and obtained an octadecanuclear cluster with the formula {[Ni(H₂O)(L)][Ni(L)][W(CN)₈]}₆ · 36H₂O · 2CH₃OH (1). The reason for the different structure type compared with those of reported complexes can be ascribed to the different oxidation state of the Mo(W) atom in the cyanide-precursor, the different charge of the cyanide-precursor and the different reaction method used. In the IR spectra of complex 1, two sharp peaks due to the cyanide stretching vibration were observed near 2120 and 2160 cm⁻¹, indicating the presence of bridging and non-bridging cyanide ligands in this complex.

3.2 Crystal and molecular structure of complex 1

Selected structural parameters for complex 1 are collected in Table 2. The coordination geometry and the wheel-like structure containing the polyhedra around the metal atoms are shown in Fig. 1, and a larger view of the crystal structure is shown in Fig. 2.

Fig. 1:

The coordination geometry of complex 1 (left) and its wheel-like structure showing the polyhedra of the metal atoms.

Fig. 2:

View of the crystal structure of 1 as shown down the crystallographic c axis.

Complex 1, crystallizing in the hexagonal system with space group R3̅, can be described as an octadecanuclear cluster containing a 36-membered macrocycle (Fig. 1). Each [Mo(CN)₈] module is linked to three [Ni(L)] units, two of which are bridged by [Mo(CN)₈] units and the other one hangs out of the ring, being terminal with the additional axial position occupied by a water molecule. The supramolecular structure of complex 1 with the elementary composition [Ni(L)]₁₂[Mo(CN)₈]₆ can be described as a large cyclohexane arrangement in a chair conformation comprising six [Mo(CN)₈] units linking in turn six [Ni(L)] units. The six sides or edges are formed by Mo–C≡N–Ni–N≡C–Mo linkages, and the six corners are occupied by the remaining six terminal [Ni(H₂O)(L)] acting as axial substituents. The two types of Ni(II) ions are six-coordinated, and the bond parameters around them show a slightly distorted octahedral geometry. The Ni–N_CN and Ni–O_water bond lengths are 2.055(4), 2.156(4), 2.111(4) and 2.231(3) Å, respectively, obviously longer than the average distance between the Ni atom and the equatorial N atoms with the value 2.022 Å. The angles between the bridging cyanide groups at the Mo atoms are 80.08(16)°, while the angles between the ring and the terminal groups are 144.36(16) and 124.02(16)°. The largest intramolecular Mo···Mo and Ni···Ni distances are 16.5 and 14.4 Å, respectively, which correspond to the major and minor axes of the ellipse, indicating that complex 1 is a nanosized molecular wheel. As can be seen in Fig. 2, the hexagonal crystal structure of complex 1 shows a highly ordered pattern with intermolecular hydrogen bond interactions between the O atom of the coordinated H₂O and the N atom of the uncoordinated cyanide group. The structure analysis reveals that there are solvent accessible voids with an approximate volume of 518 Å⁻³ in the crystal structure 1, implying potential applications as a porous material for gas absorption or even storage.

3.3 Magnetic properties of complex 1

The temperature dependence of the magnetic susceptibility of complex 1was measured in the range of 2–300 K under the external magnetic field of 2000 Oe (1 kOe = 7.96 × 10⁴ A m⁻¹) (Fig. 3). The χ_mT value at room temperature is 12.19 emu K mol⁻¹, slightly higher than the spin-only value of 12.0 emu K mol⁻¹ for 12 isolated Ni(II) ions (S = 1). With the temperature decreasing, the χ_mT value remains almost constant from 300 to about 20 K. Below this temperature, the χ_mT value starts to decrease rapidly and reaches the lowest value 7.22 emu K mol⁻¹. The magnetic susceptibility of complex 1 follows the Curie–Weiss law in the range of 2–300 K and gives the negative Weiss constant θ = −0.47 K and the Curie constant C = 12.66 emu K mol⁻¹. The negative Weiss constant and the tendency of the χ_mT–T line show that there exists weak antiferromagnetic interaction between the neighboring Ni(II) ions bridged by diamagnetic cyanide building blocks.

Fig. 3:

The χ_mT vs T and χ_m⁻¹ vs T curves for complex 1.