One-Dimensional Hydrogen-Bonded Infinite Chain from Nickel(II) Tetraaza Macrocyclic Complex and 1,2-Cyclopentanedicarboxylate Ligand

The reaction of [Ni(L)]Cl2·2H2O (L = 3,14-dimethyl-2,6,13,17-tetraazatricyclo [14,4,01.18,07.12]docosane) with trans-1,2-cyclopentanedicarboxylic acid (H2-cpdc) yields a 1D hydrogen-bonded infinite chain with formula [Ni(L)(H-cpdc−)2] (1). This complex has been characterized by X-ray crystallography, spectroscopy and cyclic voltammetry. The crystal structure of 1 exhibits a distorted octahedral geometry about Ni atom with four nitrogen atoms of the macrocycle and two oxygen atoms of the H-cpdc− ligand at the axial position. Compound 1 crystallizes in the monoclinic system P21/c with a = 8.7429(17), b = 10.488(2), c = 18.929(4) Å, β = 91.82(2), V = 1734.8(6) Å3, Z = 2. Electronic spectrum of 1 reveals a high-spin octahedral environment. Cyclic voltammetry of 1 undergoes two waves of a one-electron transfer corresponding to NiII/NiIII and NiII/NiI processes.


Chemical Properties
The IR spectrum of 1 shows a band at 3140 cm −1 corresponding to the (NH) of the coordinated secondary amines of the macrocycle. Two strong bands exhibit  as (COO) stretching frequency at 1561 cm −1 and  sym (COO) at 1396 cm −1 , respectively. The value of  (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 (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 3 B 1g → 3 E g c , 3 B 1g → 3 E g b , 3 B 1g → 3 B 2g + 3 B 1g → 3 A 2g a transitions, which is the characteristic spectrum expected for a high-spin d 8 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)](ClO 4 ) 2 (459 nm) [28], which has a low-spin d 8 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.   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. Table 4. Cyclic voltammetric data a .

Materials and Methods
All chemicals and solvents used in the syntheses were of reagent grade and were used without further purification. The complex [Ni(L)]Cl 2 · 2H 2 O 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 N 2 prior to carrying out the electrochemical measurements. Elemental analyses (C, H, N) were performed on a Perkin-Elmer CHN-2400 analyzer.

Synthesis of [Ni(L)(H-cpdc  ) 2 ] (1)
To a methanol solution (20 cm 3 ) of [Ni(L)]Cl 2 · 2H 2 O (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 C 34 H 58 N 4

X-ray Crystallography
Single crystal X-ray diffraction measurement for 1 was carried out on an Enraf-Nonius CAD4 diffractometer using graphite-monochromated Mo-K radiation ( = 0.71073 Å). Intensity data were measured at 293(2) K by -2 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 -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.

Conclusions
The reaction of [Ni(L)]Cl 2 · 2H 2 O (L = 3,14-dimethyl-2,6,13,17-tetraazatricyclo[14,4,0 1.18 ,0 7.12 ]docosane) with trans-1,2-cyclopentanedicarboxylic acid (H 2 -cpdc) yields a 1D hydrogen-bonded infinite chain, which exhibits a distorted octahedral geometry with four nitrogen atoms of the macrocycle and two oxygen atoms of the trans-1,2-cyclopentanedicarboxylate ligand at the axial position. The hydrogen-bonding interactions of 1 play a significant role in aligning the polymer stands. Solid state electronic absorption spectrum of 1 reveals a high-spin d 8 nickel(II) ion in a distorted octahedral environment. Cyclic voltammetry of 1 undergoes two waves of a one-electron transfer corresponding to Ni II /Ni III and Ni II /Ni I processes. This complex makes the oxidation of Ni(II) to Ni(III) easier and the reduction to Ni(I) more difficult. This fact may be attributed to the coordination of the axial H-cpdc − ligand, which is in agreement with the crystal structure of 1.