A new binuclear Ni^II complex with tetrafluorophthalate and 2,2′-bipyridine ligands: synthesis, crystal structure and magnetic properties

2.1 Preparation of bis(μ₂-tetrafluorophthalato-κO,O′)-tetra-aqua-bis(2,2′-bipyridine-κN,N′)-dinickel (II), [Ni₂(tfpa)₂(bipy)₂(H₂O)₄] (1)

Ni(CH₃COO)₂·4H₂O (0.5 mmol, 0.126 g), H₂tfpa (0.5 mmol, 0.119 g) and bipy (0.5 mmol, 0.088 g) were dissolved in 30 mL of ethanol-H₂O (1:1, v/v), and the mixture was neutralized with KOH (0.2 mol L⁻¹) to pH 7.5. The solution was filtered after being stirred for 4 h at 80°C. Three weeks later, green block-shaped crystals had grown from the filtrate by slow evaporation. Yield: 26 mg (11% based on Ni). –C₃₆H₂₄F₈N₄Ni₂O₁₂ (%): calcd. C 44.35, H 2.46, N 5.75; found C 44.39, H 2.40, N 5.69.

2.2 Single-crystal X-ray structure determination

One green crystal with dimensions of 0.25×0.22×0.18 mm³ was selected for measurement. A total of 6817 reflections were collected in the range of 2.51°≤θ≤25.50° by using the ω-2θ scan mode, of which 3295 were unique with R_int=0.0112. The structure of 1 was solved by Direct Methods [29], [30] with Shelxs-1997 and refined by full-matrix least-squares on F² using the Shelxl-97 software [31], [32]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were generated geometrically and treated by a mixture of independent and constrained refinement. The crystal data and collection parameters for 1 are summarized in Table 1. Selected bond lengths and bond angles are given in Table 2. Hydrogen bond parameters are listed in Table 3.

CCDC 1574809 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 Structure description for [Ni₂(tfpa)₂(bipy)₂ (H₂O)₄] (1)

Single-crystal X-ray diffraction analysis has revealed that the asymmetric unit of compound 1 consists of two Ni^II cations, two tetrafluorophthalate anions (tfpa²⁻), two bipy ligands as well as four coordinated water molecules.

The Ni1 center is coordinated by two carboxylate oxygen atoms (O1 and O4A) of two different tfpa²⁻ dianions, two nitrogen atoms (N1 and N2) of a bipy ligand together with two H₂O molecules (O5 and O6), exhibiting a distorted NiN₂O₄ octahedral geometry (Table 2 and Fig. 1). The N1, N2, O1 and O6 atoms define the equatorial plane, while the O5 and O4A atoms occupy the axial positions. Ni1 deviates from the equatorial least-squares plane N1/N2/O1/O6 by 0.0113 Å toward O4A, with an O4A–Ni1–O5 angle of 176.53(3)°. The tfpa²⁻ dianions act as bis-monodentate linkers connecting two Ni^II cations related by crystallographic inversion symmetry forming the binuclear structure of 1.

Fig. 1:

The molecular structure of 1, showing the atom numbering scheme. Displacement ellipsoids are shown at the 30% probability level. The H atoms have been omitted for clarity. Dashed lines show the π–π stacking between the benzene rings of tfpa²⁻ and the pyridine rings of bipy. Symmetry operations used to generate equivalent atoms: A 1−x, −y, 1−z.

In the structure of 1, there are offset face-to-face π–π stacking interactions between the benzene rings of tfpa²⁻ and the pyridine rings of bipy (Fig. 1): C1/C2/C3/C4/C5/C6 and N2A/C13A/C14A/C15A/C16A/C17A, C1A/C2A/C3A/C4A/C5A/C6A and N2/C13/C14/C15/C16/C17. The dihedral angle, average ring-ring distance and ring-centroid separation are 4.36°, 3.303 Å and 4.542 Å, respectively.

The coordinated H₂O ligands are involved as donors in intermolecular hydrogen bonding interactions with coordinated and uncoordinated carboxylate oxygen atoms of the tfpa²⁻ anions (Table 3 and Fig. 2). The dimeric units of 1 are associated into a layer, stabilized by π–π stacking and O–HLO hydrogen bonds (Fig. 2).

Fig. 2:

Layer formed by hydrogen bonding interactions in 1.

Compound 1 is isostructural with [Co₂(tfpa)₂(bpy)₂ (H₂O)₄] (1-Co) [16]. As Ni and Co are neighboring 3d elements and the ionic radius of Ni^II (0.69 Å) is much smaller than that of Co^II (0.75 Å), the corresponding bond lengths of 1 are significantly shorter than those in 1-Co (Table 2), but the NiLNi separation (5.107 Å) is longer than that of CoLCo (5.074 Å) in the cobalt analogue.

Comparing 1 with other binuclear species [Co₂(tfpa)₂ (bipy)₂(H₂O)₄] (1-Co) [16], {[Cu₂(tfpa)(Htfpa)₂(bipy)₂](H₂O)₂] (CH₃OH)} (1-Cu) [17] and [Cd₂(tfpa)₂(bipy)₂(H₂ O)₂] (1-Cd) [17] constructed with the same H₂tfpa and bipy tectons (Scheme 2), some important features and differences between them can be found as follows: (i) the number of H₂tfpa ligands. There are three H₂tfpa in 1-Cu, but two in 1, 1-Co and 1-Cd. (ii) The degree of deprotonation of H₂tfpa. In order to balance the corresponding positive charges of metal ions, two H₂tfpa molecules have become mono-deprotonated as Htfpa⁻, while one is doubly deprotonated as tfpa²⁻ in 1-Cu. By contrast, H₂tfpa is fully deprotonated in 1, 1-Co and 1-Cd. (iii) The coordination number of the central metal ions. As the ionic radii of the 3d elements (Ni^II, Cu^II and Co^II) are smaller than those of the 4d ones (Cd^II), the coordination numbers in 1-Cu (five), 1 and 1-Co (six) are smaller than in 1-Cd (seven), displaying square-pyramidal (1-Cu), octahedral (1 and 1-Co) and pentagonal-bipyramidal (1-Cd) coordination environments, respectively. (iv) The coordination mode of H₂tfpa. Although the H₂tfpa entities in the four structures all show similar μ₂-bridging fashion, distinct coordination differences are observed. In 1-Cu, only one carboxyl group participates in coordination, and the other remains uncoordinate. In detail, one Htfpa⁻ and tfpa²⁻ adopt a μ₂-η²:η⁰ pattern, and the other Htfpa⁻ shows a μ₂-η¹:η¹ connectivity. In contrast, the two carboxyl groups of tfpa²⁻ are all involved in coordination in 1, 1-Co and 1-Cd. They are bis-monodentate in 1 and 1-Co, but bis-bidentate chelating in 1-Cd.

Scheme 2:

The formulae for 1-Co [16], 1-Cu and 1-Cd [17] as compared to compound 1.

3.2 Magnetic properties

The magnetic susceptibility of compound 1 was measured in the 2–300 K temperature range as shown in Fig. 3.

Fig. 3:

Temperature dependence of χ_MT (squares) and χ_M⁻¹ (circles) for 1.

The experimental χ_MT value per (Ni^II)₂ is 2.42 cm³ K mol⁻¹ (μ_eff=4.40 μ_B) at 300 K, which is slightly larger than the theoretical value 2.00 cm³ K mol⁻¹ (μ_eff=4.00 μ_B) for two spin-only uncoupled Ni^II ions with g=2.1–2.3 [33], which indicates that orbital contributions are involved. The χ_MT value increases gradually upon cooling from 300 K and reaches the maximum value of 2.72 cm³ K mol⁻¹ at 10 K and finally drops quickly to 1.84 cm³ K mol⁻¹ at 2 K. The increase in χ_MT with decreasing temperature clearly indicates ferromagnetic coupling interactions between the Ni^II centers.

The Heisenberg spin Hamiltonian model for the isotropic magnetic exchange interaction in the binuclear (Ni^II)₂ unit has been given in the following expression, where the numbering 1 and 2 represent the marks of Ni1 and Ni1A as shown in Fig. 1.

H^=–2J S^1 S^2

where Ŝ₁=Ŝ₂=1. The exchange constant J presents magnetical coupling within the carboxylate-bridged dinuclear unit [34]. The experimental data were fitted based on the above Hamiltonian equation. The data in the temperature range of 10–300 K were considered in the fit, and the best-fit parameters obtained with this computing model yielded J=0.21 cm⁻¹, g=2.09 and R=8.90×10⁻⁹. The small and positive J value suggests weak ferromagnetic exchange between Ni^II centers in 1. These parameters agree with the long NiLNi separation of 5.107 Å in complex 1 and are comparable to values reported in related cases [8], [35].

The cryomagnetic behavior of 1 obeys the Curie-Weiss law in the temperature range of 10–300 K, yielding C_m=2.42 cm³ K mol⁻¹, θ=3.56 K. The positive θ value further reveals the presence of intramolecular ferromagnetic coupling.