Synthesis And Characterization Of Co^â…¡, Ni^â…¡ And Cu^â…¡ Complexes For Molecular Magnetic Materials

Molecular magnetic materials are not only one kind of functional materials, but also a new interdiscipline of chemistry, physics, material, biological science. Compared with traditional magnets which are atom based with d–(or f–) orbital–based spin sites and have extended network bonding in three dimensions, molecular magnetic materials are superior in many aspects, such as low density, flexibility, solubility, transport, tuning properties and so on. They have attracted much attention in recent years because of the meaningful contributory to magnetic theory and potential applications.The modern chemist can transform matter, not only for pleasure but for the design of new systems exhibiting desired magnetic properties. Such an ambition, when dealing with the magnetic properties of polynuclear systems, necessitates going many times through the arcs of a cycle: (i) the synthesis of a new compound which one links will exhibit desirable magnetic properties; (ii) the determination of its molecular and crystal structure; (iii) the investigation of its magnetic properties, i.e., the determination of the sign and the strength of the exchange interactions, the value of the spin ground state; (iv) the establishment of magnetostrtural correlations; and (v) the theoretical analysis of the magnetostructural correlations, so that it becomes possible in the following cycle to synthesize novel compounds whose magnetic properties will be even closer to what is desired or expected.In the present work, twelve new complexes have been synthesized based on pydirdine–2,6–dicarboxylate or isophthalate as bridging ligands and 2,4'–bipyridine as well as its derivative as pendants with different metal ions, under hydrothermal condition. The structures of these compounds have been characterized by X–ray crystallography, variable temperature magnetic measurements, infrared and thermal gravimetric analyses. The studies on synthetic conditions and rules for these new compounds, and the exploration of relationships between structures and magnetic properties for these compounds were carried out. This report may provide a promising strategy for the design and exploitation of new magnetic compounds for useful applications. The main contents in this thesis can be summarized as follows:1. The pyridine–2,6–dicarboxylic acid (H₂pdc) reacts as the bridging ligand under hydrothermal conditions, with divalent cobalt or nickel ions to form four 1D polymeric complexes [Co(pdc)(2,4′–bpy)2·H₂O]_n (1), [Co(pdc)(5–ph–2,4′–bpy)2·H₂O]_n (2), [Co₂(pdc)₂(5–(4–Br–ph)–2,4′–bpy)₄·2H₂O]_n (3) and [Ni(pdc)(5–ph–2,4′–bpy)₂·H₂O]_n (4) in the presence of 2,4′–bipyridine (2,4′–bpy) as well as its derivatives 5–phenyl–2,4′–bipyridine (5–ph–2,4′–bpy) or 5–(4–bromophenyl)–2,4'–bipyridine (5–(4–Br–ph)–2,4′–bpy) as the pendants. Complexes 1–4 show antiferromagnetic interactions between the metal centers bridged by the O–C–O bridge in a trans mode. The polymeric cobalt chains of 2 are severely twisted compared to those of 1 by the introducing of phenyl substituent on pendant, while for 3 half severely and half less twisted polymeric cobalt chains are ascribed to the bromophenyl substituent of the pendant. As a result, the steric hindrance of the substituent groups from neighboring chains results in the distorted structure of polymeric chains. It was found that antiferromagnetic interactions between CoII ions can be tuned by the substituent on the pendants.2. Three new mononuclear complexes [Cu(pdc)L(H₂O)]·xH₂O (L = 2,4′–bpy, x = 4 (5); L = 5–ph–2,4′–bpy, x = 2 (6); L = 5–Cl–2,4′–bpy, x = 0 (7)) were prepared in the reaction of H₂pdc with divalent copper ions under hydrothermal conditions in the presence of three bipyridyl ligands as pendants, respectively. The divalent copper ions adopted N2O3 five coordination with pyridine–2,6–dicarboxylate, one water, as well as 2,4′–bpy or its derivative such as 5–ph–2,4′–bpy or 5–chloro–2,4′–bipyridine (5–Cl–2,4′–bpy), where lattice water clusters possessing different configuration are formed for 5 and 6. In 5, (H₂O)₈ molecules adopting a chair-like conformation stack along the a–axis to form a water tape. Hexameric water cluster self–assembles forming a highly ordered comb-like infinite chain in 6. Unlike 5 and 6, no lattice water molecule was found in 7, except one coordination water molecule as found in 5 and 6. It was found that the antiferromagnetic interactions between the CuII ions can be tuned by the number of lattice water molecules for these three complexes.3. A series of complexes containing the identical bridging ligand and pendants and different metals such as Ni²⁺, Co²⁺, and Cu²⁺ was synthesized and characterized. In the nickel and cobalt complexes [M(L1)(L2)₂·H₂O]_n (M = Ni²⁺ (8) or Co²⁺ (3), L1 = pdc^2–, L2 =5–(4–Br)–ph–2,4′–bpy), the metal ions are hexa–coordinated with distorted octahedral geometries, and bridged by the L1 pdc^2– forming 1D backbone with L2 5–(4–Br)–ph–2,4'–bpy as pendants. However, in the copper complex [Cu₄(L1)₄(L2)₄]·2H₂O (9), the metal ions adopt a distorted square pyramidal geometry and the O–C–O bridge of pdc^2– adopts a basal–apical disposition between metal ions to give a square planar backbone with L2 pendants. Antiferromagnetic coupling between metal ions in the nickel and cobalt complexes has been observed, the copper complex possesses weakly ferromagnetic coupling due to its tetranuclear cluster structure. DFT calculations suggested that the O–C–O bridges in a basal–apical mode are responsible for intermetallic ferromagnetic exchange for the tetranuclear copper cluster. The successful transform of magnetic property from antiferromagnetic (8) or (3) to ferromagnetic coupling (9) can be simply traced back to the tuning of metal ions with investigated ligands. As a result, the metal ion plays the key role in the magnetic transform.4. Three new complexes {[Cu(ip)(2,4′–bpy)₂]·H₂O}_n (10), {[Co(ip)(2,4′–bpy)₂]·H₂O}_n (11) and [Ni(ip)(2,4′–bpy)₂(H₂O)]_n (12) were prepared in the reaction of isophthalic acid (H₂ip) with different metal ions such as Cu^II, Co^II, and Ni^II under hydrothermal conditions in the presence of 2,4′–bpy ligand. The metal ions are bridged by ip^2- anions to form 1D ribbon with 8–membered and 16–membered ring in complexes 10 and 11, while nickel ions are bridged by ip^2- anions and water molecules forming 1D ribbon with 8–membered and 20–membered ring in complex 12. It was found that antiferromagnetic interactions between metal ions can be tuned by the nature of metal center and/or the coordinated water molecule.