An ORTEP drawing [21] of [Ni(L)(H-cpdc^–)₂] (1) with the atomic numbering scheme is shown in Figure 1. Selected bond distances and angles are listed in Table 1. The skeleton of the macrocyclic unit in 1 adopts the classical trans-III (R,R,S,S) conformation with two chair-form six-membered and two gauche-form five-membered chelate rings. The central nickel atom is located on an inversion center. The nickel atom and the four nitrogen atoms of the macrocycle are exactly in a plane. The nickel(II) ion exhibits a distorted octahedral coordination geometry with the four secondary amine nitrogen atoms of the macrocycle and two oxygen atoms of H-cpdc^– ligands in axial positions. The average Ni-N distance of 2.063(4) Å is significantly longer than in the square-planar geometry of [Ni(L)]Cl₂·2H₂O [1.948(4) Å] [22], but is similar to those observed for high-spin octahedral nickel (II) complexes with 14-membered macrocyclic ligands [15–20]. The Ni-O distance of 2.176(4) Å is similar to those previously reported values in closely related examples {[Ni(L)]₃[μ-btc]₂·8H₂O}_n (btc³⁻ = 1,3,5-benzenetricarboxylate, 2.193(4) and 2.163(4) Å) [15], {[Ni(L)(H-chtc)]·H₂O}_n (chtc³⁻ = 1,3,5-cyclohexanetricarboxylate, 2.176(6) and 2.152(6) Å) [16], in which these complexes are 2D coordination and hydrogen-bonded infinite chains. The N-Ni-N angles of the six-membered chelate rings are larger than those of the five-membered chelate rings. The dihedral angle between the plane of the carboxylate group and NiN₄ plane is 67.6(5)°. The closest intermolecular Ni···Ni distance between neighboring stands is 8.743(2) Å. The Ni-O(1) linkage is bent slightly off the perpendicular to the NiN₄ plane by 3.2–7.0°. The Ni-O(1)-C(11) and O(1)-C(11)-O(2) angles related to the H-cpdc⁻ ligand are 130.1(4) and 122.9(6)°, respectively. The deprotonated one among the two H-cpdc^– carboxylic groups is coordinated to the metal center. The secondary amines N(1) and N(2) of the macrocycle are intramolecular hydrogen bonded to the uncoordinated carboxylic oxygen O(2) and O(3) of the H-cpdc^– ligand [N(1)-H(17)^…O(2)#3 2.797(6) Å, 163(6)°; N(2)-H(18)^…O(3)#2 3.085(7) Å, 158(6)°; symmetry codes (#2) −x, −y + 1, −x + 1; (#3) x, y, z].

Interestingly, the protonated oxygen atom O(4) of the H-cpdc^– ligand forms intermolecular hydrogen bond to an adjacent uncoordinated carboxylic oxygen O(2) of the H-cpdc^– ligand [O(4)-H(19)^…O(2)#4 2.522(7) Å, 166(10)°; symmetry code (#4) −x, y − 1/2, −z + 3/2] (Figure 2 and Table 2). This interaction gives rise to a 1D hydrogen-bonded infinite chain, which is similar to that observed for the cyclo-aliphatic nickel(II) complexes [Ni(hatt)(H-chdc^–)₂] [19] and [Ni(L)(H-cbdc^–)₂] [20]. This fact may be due to the flexibility of the trans-1,2-cyclopentanedicarboxylate (H-cpdc^–) ligand as well as trans-1,2-cyclohexanedicarboxylate (H-chdc^–) and1,1-cyclobutanedicarboxylate (H-cbdc^–) ligands.

The IR spectrum of 1 shows a band at 3140 cm⁻¹ corresponding to the ν(NH) of the coordinated secondary amines of the macrocycle. Two strong bands exhibit ν_as(COO) stretching frequency at 1561 cm⁻¹ and ν_sym(COO) at 1396 cm⁻¹, respectively. The value of Δν (165 cm⁻¹) indicates that the carboxylate groups coordinated to the nickel(II) ion only as a monodentate ligand [23,24]. In addition, a sharp band at 3426 cm⁻¹ is associated to the ν(OH) stretching vibration of the hydroxyl group in the H-cpdc^– ligand. The UV-Vis spectrum of 1 is listed in Table 3. The UV spectrum of 1 in the water solution shows an absorption maximum in the region 260 nm attributed to a ligand-metal charge transfer associated with the nitrogen and oxygen donors [25]. As shown in Figure 3, the solid state electronic spectrum of 1 in the visible region shows three absorption bands at 340, 530, and 694 nm assignable to the ³B_1g → ³E_g^c, ³B_1g → ³E_g^b, ³B_1g → ³B_2g + ³B_1g → ³A_2g^a transitions, which is the characteristic spectrum expected for a high-spin d⁸ nickel(II) ion in a distorted octahedral environment [26,27]. However, the complex 1 dissolves in water and decomposes into the original compound [Ni(L)](ClO₄)₂ (459 nm) [28], which has a low-spin d⁸ nickel(II) ion in a square-planar environment. This fact can be understood in terms of the decomposition of the building block in water solution. The electronic spectrum for 1 clearly supports the structure determined by the X-ray diffraction study.

Cyclic voltammetric data for 1 in 0.10 M TEAP-DMSO solution are given in Table 4. Cyclic voltammogram of 1 in 0.1 M TEAP-DMSO solution is shown in Figure 4. The oxidation and reduction potentials for 1 give the irreversible and reversible one-electron processes at +0.66 and −1.23 V versus the Ag/AgCl reference electrode, assigned to the Ni^II/Ni^III and Ni^II/Ni^I couples, respectively. This fact may be attributed to the coordination of the axial H-cpdc⁻ ligand, which is in agreement with the crystal structure of 1.

All chemicals and solvents used in the syntheses were of reagent grade and were used without further purification. The complex [Ni(L)]Cl₂·2H₂O was prepared according to literature method [22]. IR spectra were recorded with a Perkin-Elmer Paragon 1000 FT-IR spectrophotometer using KBr pellets. Solution and solid electronic spectra were obtained on a JASCO Uvidec 610 spectrophotometer. Electrochemical measurements were accomplished with a three electrode potentiostat BAS-100BW system. A 3 mm Pt disk was used as the working electrode. The counter electrode was a coiled Pt wire and a Ag/AgCl electrode was used as a reference electrode. Cyclic voltammetric data were obtained in DMSO solution with 0.10 M tetraethylammonium perchlorate (TEAP) as supporting electrolyte at 20.0 ±0.1 °C. The solution was degassed with high purity N₂ prior to carrying out the electrochemical measurements. Elemental analyses (C, H, N) were performed on a Perkin-Elmer CHN-2400 analyzer.

To a methanol solution (20 cm³) of [Ni(L)]Cl₂·2H₂O (251 mg, 0.5 mmol) sodium trans-1,4-cyclohexanedicarboxylate was added (108 mg, 0.5 mmol) and the mixture was stirred for 30 min at room temperature. The solution was filtered to remove insoluble material. After the filtrate was allowed to stand at room temperature over a period of several days, violet crystals formed Crystals were collected by filtration and washed with diethyl ether. Anal. Calcd. for C₃₄H₅₈N₄NiO₈: C, 57.55; H, 8.24; N, 7.90. Found: C, 57.64; H, 8.32; N, 7.81%. IR (KBr, cm⁻¹): 3426(m), 3191(m), 3140(m), 2931(m), 2861(s), 1630(m), 1561(s), 1448(m), 1396(s), 1308(m), 1268(w), 1155(w), 1111(s), 1076(m), 997(m), 948(m), 897(m), 786(w), 719(w), 639(w), 552(w), 537(w).

Single crystal X-ray diffraction measurement for 1 was carried out on an Enraf-Nonius CAD4 diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Intensity data were measured at 293(2) K by ω-2θ technique. Accurate cell parameters and an orientation matrix were determined by the least-squares fit of 25 reflections. The intensity data were corrected for Lorentz and polarization effects. Empirical absorption correction was carried out using φ-scan [29]. The structure was solved by direct methods [30] and the least-squares refinement of the structure was performed by the SHELXL-97 program [31]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in calculated positions allowing them to ride on their parent C atoms with U_iso(H) = 1.2U_eq(C or N). Crystal parameters and details of the data collections and refinement are summarized in Table 4.