Self-Assembly of a 1D Hydrogen-Bonded Polymer from a Hexamethyltetraaza Macrocyclic Nickel(II) Complex and Isophthalic Acid

The compound [Ni(L)(isoph)2][Ni(L)]·8H2O (1; L = C-meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; H2-isoph = isophthalic acid) has been synthesized and structurally characterized. Complex 1 exhibits a geometrically symmetric core with a {4/6} coordination number set. The coordination environment around the Ni(1) ion is a distorted octahedron, while the geometry around the four-coordinate Ni(2) is depicted as square planar in 1D hydrogen-bonded infinite chain. The compound crystallizes in the triclinic system P-1 with a = 8.602(2), b = 10.684(7), c = 16.550(3) Å, α = 91.04(4), β = 94.09(2), γ = 111.09(4)°, V = 1413.9(10) Å3, Z = 1. The cyclic voltammogram of 1 undergoes one-electron wave corresponding to NiII/NiI process. The electronic spectra, electrochemical and TGA behavior of the complex are significantly affected by the nature of the hexamethyltetraaza macrocycle and the axial isoph2− ligand.


Chemical Properties
The IR spectrum of 1 shows a band at 3148 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 1606 cm −1 and ν sym (COO) at 1357 cm −1 , respectively. The value of Δν (249 cm −1 ) indicates that the carboxylate groups are coordinated to the nickel(II) ion only as monodentate ligands [22,23]. In addition, a sharp band at 3420 cm −1 is associated to the ν(OH) stretching vibration of the water molecule. The UV-Vis spectral data of 1 are listed in Table 3. The solid reflectance spectrum of 1 is also shown in Figure 4. 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 [23]. As shown in Figure 4, the solid state electronic spectrum of 1 in the visible region shows three absorption bands at 325(sh), 474, and 696 nm assignable to the 3 B 1g → 3 E g (T 1g (P)), 1 B 2g → 1 B 1g , 3 B 1g → 3 B 2g (T 2g (F)) transitions [3,24]. The transitions at 325 and 696 nm arise from the tetragonal complex while the transition at 474 nm results from the square planar species. However, the visible spectrum of 1 in water solution displays a broad band 458 nm, which has a low-spin d 8 nickel(II) ion in a square-planar environment [Ni(L)](ClO 4 ) 2 (463nm) [5]. This fact can be understood in terms of the decomposition of the hydrogen bonding or the building block in water solution. The electronic spectrum for 1 clearly support 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 5. The H 2 -isoph ligand is electrochemically active in the range of potential studied. Cyclic voltammetry of H 2 -isoph shows two irreversible one-electron reductions at −1.33 and −1.61 V vs. the Ag/AgCl reference electrode. The reduction potential for 1 gives the irreversible one-electron process at −0.76 V vs. the Ag/AgCl refernce electrode, assigned to the Ni II /Ni I process. The reduction potentials for 1 are considerably more positive than that for the square-planar [Ni(L)](ClO 4 ) 2 (−1.29V) [5]. This fact may be due to the coordination of the axial isoph 2− ligand, which is in agreement with the crystal structure of 1. Table 4. Cyclic voltametric data a .   Figure 6. The compound was heated in the temperature range 50-1,000 °C in nitrogen gas. The first weight loss is observed from 30 to 139 °C, which is due to the loss of six water molecules (observed 8.1%, calculated 9.3%). A second weight loss corresponding to the macrocycle (observed 51.8%, calculated 49.1%) is found in the temperature range 382-405 °C. On further heating, 1 lost weight between 405 and 756 °C corresponding to two isophthalate ligands (observed 26.7%, calculated 28.3%). Further weigh loss is not observed in the temperature range 756 to 1000 °C corresponding to the greenish black residue of NiO (observed 13.3%, calculated 12.9%).

Materials and Physical Measurements
The chemicals and solvent NiCl 2 ·6H 2 O (99.99%), isophthalic acid (99%), and acetonitrile (99.93%) were purchased from Aldrich (Milwaukee, WI, USA) and used without futher purification. The C-meso -5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane ligand (L) was prepared according to literature method [4,5]. IR spectra were recorded with a Jasco FT/IR-4100 spectrophotometer using KBr pellets. 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 an Ag/AgCl electrode was used as a reference electrode. Cyclic voltammetric data were obtained in DMSO solution using 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. The DSC and TGA were performed under flowing nitrogen at a heating rate of 10 °C mim −1 using a SDT 2960 Thermogravimetric Analyzer. Elemental analyses (C, H, N) were performed on a Perkin-Elmer CHN-2400 analyzer.

X-ray Crystallography
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 ψ-scan [25]. The structure was solved by direct methods and the least-squares refinement of the structure was performed by the SHELXL-97 program [26]. All the non-hydrogen atoms were refinded anisotropically. The hydrogen atoms were placed in calculated positions, allowing them to ride on their parent C and N atoms with U iso (H) = 1.2U eq (C or N). A summary of the data collections and details of the structure refinement is given in Table 5.
Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC No. 846954 for 1. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2, 1EZ, UK (fax: +44-1223-336033; e-mail: deposit@ccdc.cam.uk or http://www.ccdc.cam.ac.uk).

Conclusions
Complex 1 reveals a geometrically symmetric core with a {4/6} coordination number set. The coordination environment around the Ni(1) ion is a distorted octahedron, while the geometry around the four-coordinate Ni(2) is depicted as square planar in 1D hydrogen-bonded infinite chain. The solid state electronic spectrum of 1 in the visible region shows three absorption bands assignable to the tetragonal and the square planar species. The cyclic voltammogram of 1 displays a one-electron wave corresponding to the Ni II /Ni I redox process. The reduction potentials for 1 are considerably more positive than that for the square-planar [Ni(L)](ClO 4 ) 2 . The TGA behavior of the complex 1 is significantly affected by the nature of the hexamethyltetraaza macrocycle and the axial isoph 2− ligand.