Synthesis And Properties Of Complexes With Aromatic Carboxylate-Based Ligands

In recent years, coordination polymers draw considerable attention for their fascinating structures and potential applications as functional materials. Complexes with attractive topologies and particular properties such as magnetic, optical and adsorbing properties are reported continuously. The formation of these complexes depends mainly on the combinations of two factors: the coordination geometry of metal ions and the ligands. The pyridyl N-oxide ligands do not have the swing part like the flexible ones but the N-oxide group can bend to metal ions, which makes the coordinated character neither like the rigid one nor the like the flexible one. Generally, carboxylate ligands mediate antiferromagnetic coupling in the syn,syn and anti,anti modes and weak ferromagnetic (FM) or antiferromagnetic (AFM) coupling in the syn,anti mode.A series of new compound based on substituted carboxylate have been successfully obtained by hydrothermal or solvothermal method with the structures and magnetic properties fully investigated. The main points are summarized as follows:1. New divalent manganese complex with pyridine carboxylate N-oxide ligand have been synthesized and studied in magnetic properties. Two new manganese complexes, are [Mn₃(L¹)₄(NO₃)₂]_n (compound 2-A, HL¹ is nicotinate N-oxide acid) and [ Mn L2 Cl ]n (compound 2-B, HL2 is isonicotinate N-oxide acid)], have been hydrothermally synthesized and characterized by elemental analysis, IR and single-crystal X-ray diffraction. In 2-A, the L¹ ligands take two different coordinated modes bridging four and three MnII ions. The whole net can be viewed as a 3, 4, 6-connected 4-nodal net with Schl(o|¨)fli notation {4³}2{4⁴; 6²}4{4⁶; 6⁶; 8³}. Complex 2-B has a honeycomb layer mixed bridged by chlorine, N-oxide and carboxylate. The adjacent layers are linked by the phenyl ring of L2 ligand, giving a 3D framework with a {3⁴; 5⁴} {3²; 4; 5⁶; 6⁶} 4, 6-connect net. Magnetic studies indicate that 2-A is an antiferromagnet with low-dimensional characteristic , 2-B represent the character of the 2D honeycomb layer which results in a whole antiferromagnetic state.2. Two new CoII complexes, [Co(OCH₃)L] (3-A) and [Co₃(N₃)₄-(OCH₃)₂L₂] (3-B) (L = isonicotinate N-oxide), were synthesized by solvothermal reaction and magnetically characterized. The cobalt ions in 3-A and 3-B all exhibit distorted octahedral coordination geometry. In 3-A, there is aμ1,1-methoxy/syn,syn carboxylate mixed coordinated cobalt chain with mixed coordination, and a 3D structure is formed by the chains linked through the L ligands. In 3-B, the alternating CoII chains that are formed by linking the double end-on azide-bridged trimers are connected by the L ligands to form a 2D layer. Magnetic studies revealed that ferromagnetic coupling exists between the CoII ions in 3-A, and 3-A undergoes a ferromagnetic transition at 2.5 K, whereas alternating ferromagnetic (FM) and antiferromagnetic (AFM) interactions exist in the chain of 3-B.3. Diaquabis ( tetrazolo [1,5-a] pyridine– 8– carboxylate -κ²N¹, O ) cobalt (II) (Co(C₆H₃N₄O₂)₂(H₂O)₂]·2H₂O),have been synthesized and studied in structure. The CoII atom is located on an inversion center in a slightly distorted octahedral environment formed by the O atoms of two water molecules, and the N and O atoms of the chelating tetrazolo [1,5-a]pyridine-8-carboxylate anions. Hydrogen bonds of the O—H…O and O—H…N types result in a three-dimensional supramolecular network.