Syntheses, structures and magnetic properties of two mononuclear nickel(II) complexes based on bicarboxylate ligands

2.1 Preparation of tri-aqua-pyridine-2,6-dicarboxylato-nickel(II), [Ni(pydc)(H₂O)₃] (1)

Ni(CH₃COO)₂·4H₂O (Sinopharm Chemical Reagent Co., Ltd, Shanghai, P. R. China) (0.5 mmol, 0.125 g) and H₂pydc (0.5 mmol, 0.084 g) were dissolved in 20 mL of methanol–H₂O (Tianjin Fuyu Fine Chemical Co., Ltd, Tianjin, P. R. China) (v/v, 1:3) and then the pH of the mixture was adjusted with KOH (Tianjin De‘en Chemical Reagent Co., Ltd, Tianjin, P. R. China) (0.5 mol L^–1) to pH = 7.5. The solution was filtered after being stirred for 4 h at 80 °C. Four days later, green block-shaped crystals had grown from the filtrate by slow evaporation. Yield: 57 mg (41 % based on Ni). – Anal. for C₇H₉NiNO₇ (%): calcd. C 30.23, H 3.24, N 5.04; found: C 30.28, H 3.17, N 5.12.

2.2 Preparation of tri-aqua-2-(carboxylatomethoxy)benzoato-nickel(II), [Ni(cma)(H₂O)₃] (2)

Ni(CH₃COO)₂·4H₂O (0.5 mmol, 0.125 g) and H₂cma (0.5 mmol, 0.098 g) were dissolved in 20 mL of methanol–H₂O (v/v, 1:1) and then the pH of the mixture was adjusted with KOH (0.5 mol L^–1) to pH = 8.0. The solution was sealed in a 25 mL Teflon reactor and kept under autogeneous pressure at 130 °C for 4 days. After cooling to room temperature at a rate of 6 °C h^–1, green block-shaped crystals suitable for X-ray diffraction were grown from the filtrate by slow evaporation. Yield: 74 mg (48 % based on Ni). – Anal. for C₉H₁₂NiO₈ (%): calcd. C 35.19, H 3.91; found: C 35.25, H 3.84.

2.3 Single-crystal X-ray structure determinations

Two green block-shaped crystals with dimensions of 0.41 × 0.36 × 0.28 mm³ (1) and 0.41 × 0.35 × 0.29 mm³ (2) were selected for measurement. Diffraction data of 1and 2were collected at 296(2) K on a Bruker SMART APEX II CCD diffractometer (Germany) with graphite-monochromatized MoK_α radiation (λ = 0.71073 Å). A total of 5805 (1) and 6250 (2) reflections were collected in the range of 3.20 ≤ θ ≤ 28.25° (1) and 2.93 ≤ θ ≤ 25.50° (2) by using ω-2θ scans. Of these reflections, 2418 (1) and 2136 (2) were unique with R_int = 0.0154 (1) and 0.0494 (2), respectively. The structures were solved by direct methods [11, 12] and refined by full-matrix least squares on F² using the SHELXL-97 software [11, 13]. All non-hydrogen atoms were refined anisotropically. Both crystal structures were tested for higher metrical symmetry with the program Platon [14]. Indications for C-centering were found for 1 which could not be verified by subsequent tests including the routine ADDSYM in PLATON. The hydrogen atoms of the coordinated water molecules in 1 were added from difference Fourier syntheses and needed restraints for their bond lengths in the course of the further refinement. Altogether there were four least-squares restraints. The hydrogen atoms of 2 were generated geometrically and treated by a mixture of independent and constrained refinement. The crystal data and data collection and refinement parameters for 1 and 2 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 917854 (1) and 960696 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

3.1 Structure description for [Ni(pydc)(H₂O)₃] (1)

The molecular structure determination revealed that compound 1 is a mononuclear species and the asymmetric unit is comprised of one Ni^II ion, one fully deprotonated pyridine-2,6-dicarboxylate ligand (pydc²ˉ) and three coordinated water molecules.

The coordination sphere of the Ni^II center is a slightly distorted octahedron comprised of two carboxylate O (O4, O5) atoms and one pyridyl N (N1) atom of one pydc²ˉ dianion and three H₂O ligands. As it is induced by the pydc²ˉ ligand, both sets of coordinating atoms are arranged in a meridional fashion (Table 2 and Fig. 1). The pydc²ˉ ligand binds to the Ni^II center in a tridentate chelate mode forming two five-membered chelate rings. If the atoms N1, O4, O3, O5 define an equatorial plane, the atoms O1, O2 occupy the axial positions. Ni1 deviates from the equatorial least-squares plane N1/O4/O3/O5 by 0.0297 Å toward O1, with an O1–Ni1–O2 angle of 172.39(5)°.

Fig. 1:

The molecular structure of 1 with the crystallographic atom numbering scheme adopted. Displacement ellipsoids are shown at a 30 % probability level.

The Ni–N and Ni–O(COO) bond lengths of 1 are in the range of 1.903–2.040 Å, in agreement with those of the previously published triclinic complex [Ni(pydc)(H₂O)₂] (1.903–2.006 Å) [15] which contains penta-coordinated Ni^II in an approximately square-pyramidal geometry. The Ni–O(H₂O) bond lengths of 1(2.385 Å, 2.509 Å) are much longer than those in [Ni(pydc)(H₂O)₂] (2.150 Å) which reflects the higher coordination number in 1 as compared to [Ni(pydc)(H₂O)₂].

The coordinated H₂O ligands are involved as donors in intermolecular hydrogen bonding interactions with coordinated and uncoordinated carboxylate O atoms of pydc²ˉ and another coordinated H₂O as acceptors (Table 3). The adjacent mononuclear units of 1 are stabilized and constructed into a three-dimensional architecture by these O–H···O hydrogen bonds (Fig. 2).

Fig. 2:

3D supramolecular architecture formed by hydrogen bonding interactions between molecular units of 1.

Compound 1 is another hydrate closely related to the complex [Ni(pydc)(H₂O)₂] (see above) [15], but with three H₂O ligands instead of two. The formation of 1 and [Ni(pydc)(H₂O)₂] can be ascribed to the differences in the synthesis conditions: the starting reagents (Ni(CH₃COO)₂ for 1, NiCl₂ for [Ni(pydc)(H₂O)₂]), the solvent (presence or absence of methanol) and the preparative methods (conventional solution reaction for 1, hydrothermal reaction for [Ni(pydc)(H₂O)₂]). It proves again that the synthesis conditions may have a decisive influence on slight differences in the composition of complexes.

Compound 1should also be compared to the complex cation of the bimetallic compound [Ni(H₂pydc)(H₂O)₃][Ce(pydc)₃]·3H₂O [16]. In [Ni(H₂pydc)(H₂O)₃][Ce(pydc)₃]·3H₂O, the H₂pydc ligand is fully protonated at Ni, while it is fully deprotonated at Ce. The Ni–N and Ni–O(COO) bond lengths in 1 (1.903–2.040 Å) are shorter than those in [Ni(H₂pydc)(H₂O)₃][Ce(pydc)₃]·3H₂O (1.994–2.178 Å), while the Ni–O(H₂O) bond lengths in 1(2.385, 2.509 Å) are much longer than those in [Ni(H₂pydc)(H₂O)₃][Ce(pydc)₃]·3H₂O (2.030, 2.049 Å). The corresponding bond angles in 1 also show larger differences as compared with those in [Ni(H₂pydc)(H₂O)₃][Ce(pydc)₃]·3H₂O.

3.2 Structure description for [Ni(cma)(H₂O)₃] (2)

Compound 2 is also mononuclear and the asymmetric unit contains one Ni^II ion, one deprotonated 2-(carboxymethoxy)benzoate dianion (cma²ˉ) and three water ligands. It is isostructural with the analogous Co^II complex [Co(cma)(H₂O)₃] [17].

The Ni^II ion is coordinated by two carboxylate O (O5, O6) atoms and one ether O (O7) atom of one cma²ˉ ligand and three water molecules, exhibiting a distorted NiO₆ octahedral environment (Table 2 and Fig. 3). The cma²ˉ anion coordinates to the Ni^II center in a tridentate chelate pattern forming one five-membered and one six-membered chelate ring. The O3, O5, O6, O7 atoms are in the equatorial plane and the O1, O2A atoms occupy the axial sites. The two sets of ligating atoms (O atoms of the cma²ˉ ligand and the three water oxygens) are arranged meridionally each. Ni1 deviates from the equatorial least-squares plane O3/O5/O6/O7 by 0.0116 Å toward O1, with an O1–Ni1–O2 angle of 179.27(9)°. The bond lengths are in the range from 1.986(2) to 2.097(2) Å, and agree well with that of the isostructural cobalt complex [Co(cma)(H₂O)₃] [17].

Fig. 3:

The molecular structure of 2 with the atom numbering scheme adopted. Displacement ellipsoids are shown at a 30 % probability level.

The coordinated water molecules serve as donors in intermolecular hydrogen bonds with coordinated and uncoordinated carboxylate O atoms of cma²ˉ as acceptors (Table 3). Neighboring mononuclear units of 1 are stabilized and joined into a three-dimensional network structure via these O–H···O hydrogen bonds (Fig. 4).

Fig. 4:

3D network structure formed by hydrogen bonding interactions in 2.

3.3 Magnetic properties

The magnetic susceptibility of complexes 1 and 2is measured at 1 T in the temperature range of 2–300 K. The plots of χ_MT versus T and 1/χ_M versus T are shown in Figs. 5 and 6 with χ_M being the molar magnetic susceptibility and T the temperature.

For 1, the molar magnetic susceptibility, χ_M, increases gradually as the temperature is lowered. It reaches the maximum of 0.056 cm³ mol⁻¹ at 5 K, and then decreases sharply. The χ_MT value increases slowly from 300 K to 75 K. As the temperature is lowered continuously, it sharply declines to 0.099 cm³ K mol⁻¹ at 2 K. Weak ferromagnetic exchange interactions are evident at lower temperatures. The thermal variation of the molar susceptibility obeys the Curie–Weiss law (1/χ_M = (T–θ)/C_m) in the temperature range of 25–250 K (inset in Fig. 5). The values of the Curie and Curie–Weiss constants are θ = 1.54 K, C_m = 0.69 cm³ K mol⁻¹. At 300 K, χ_MT = 0.66 cm³ K mol⁻¹, the magnetic moment (μ_eff) per Ni^II, which is determined by the equation μ_eff = 2.828(χ_MT)^1/2, is 2.30 μ_B. This value is smaller than that expected for an isolated divalent high-spin Ni^II system with μ_eff = 2.83 μ_B.

Fig. 5:

Temperature dependence of χ_M (triangle block) and χ_MT (square block) for 1.

Fig. 6:

Temperature dependence of χ_M (triangle block) and χ_MT (square block) for 2.

For 2, χ_M remains nearly constant from 300 to 25 K. As the temperature descends continuously, it sharply increases to 0.57 cm³ mol⁻¹ at 2 K. The χ_MT value increases gradually when the temperature drops. It reaches the peak value of 1.32 cm³ K mol⁻¹ at 6 K, and then decreases sharply down to 2 K. The thermal variation of the molar susceptibility obeys the Curie–Weiss law over the whole range (2–300 K) (inset in Fig. 6). The values of the Curie and Curie–Weiss constants are θ = 1.92 K, C_m = 1.19 cm³ K mol⁻¹. The positive θ value supports the presence of intermolecular ferromagnetic interactions in 2. At 300 K, χ_MT = 1.20 cm³ K mol⁻¹, the μ_eff of per Ni^II is 3.10 μ_B. This value is higher than that expected for a spin-only isolated Ni^II ion in the ground state with μ_eff = 2.83 μ_B.