Theoretical Investigations On The Bistable Magnetic Complexes

Large investigations suggest that the bistable materials are of important in fundamental research and have an extensive prospect in next-generation molectronics devices. The thermodynamics study about bistable complex suggested that except for deciding the property of the system itself, many important characteristics such as mutation degree and the hysteresis breadth et al. are related to the concertedness (the intermolecular interaction), but the thermodynamics models can not provide the information about the mechanism of intermolecular interaction in the bistable complex. Molecular magnetism theory is a foundation on explaining physical and chemical phenomena involved in molecular magnetism and on rational syntheses of novel molecular-based magnetic materials. It is one of the frontier subjects in theoretical chemistry. As a result, it is important to investigate the interaction mechanism among the spin sites and the magnetic-structural correlation of the bistable magnetic complex by theoretical chemistry method.In the present work, for assessing the accuracy of different functional theory methods, in calculating exchange-coupling constants of transition-metal complexes by comparing them with available data in the literature obtained by fitting magnetic susceptibility data to a spin Hamiltonian, the magnetic-structural correlation in typical ferromagnetic complex [L₁Ni₂(N₃)](NO₃)₂ and antiferromagnetic complex [L₂Ni₂(N₃)](ClO₄)₂(L- is pyrazolate-based compartmental ligand) have been investigated by various unrestricted density functional theory (UDFT) combined with the broken symmetry (BS) approach. On the basis of the result, the magnetic-structural correlation in two kinds of magnetic switchable complexes [LNi₂(N₃)3] and (BDTA)₂[Co(mnt)₂] which possess two spin sites has been investigated. The purpose of this study is to gain deep insight into the understanding of the magnetic-structural correlation and the key role that affecting the magnetic properties of bistable complex and to facilitate experimental investigations on these promising new materials. The main contents in this thesis can be summarized as follows:1. The magnetic properties of ferromagnetic complex [L₁Ni₂(N₃)](NO₃)₂ (1) and antiferromagnetic complex [L₂Ni₂(N₃)](ClO4)₂ (2) have been investigated on the basis of various unrestricted density functional theory (UDFT) or HF combined with the broken symmetry (BS) approach. The computational results of the hybrid density functional theory (B3P86, B3LYP, B3PW91, and PBE0) is well agreement with the experimental one, i.e., HDFT can describe the magnetic properties of the two complexes correctly. The small energy splitting for SOMOs shows the orbital degeneracy nearly and is responsible to ferromagnetic coupling in complex 1, while the antiferromagnetic coupling in complex 2 is also ascribed to the large energy deference for SOMOs. SOMO distribution pattern of 1 show that the perpendicular magnetic orbital for nitrogen atoms on azido and the metal, which result in ferromagnetic, and the p orbital overlap for the nitrogen atoms on the pyrazolate bridge groups, corresponding to antiferromagnetic coupling behavior. As a result, the complex 1 exhibits ferromagnetic interaction. SOMO distribution pattern of the complex 2 display the p orbital overlap for nitrogen atoms on azido and the pyrazolate as bridge groups in two interaction paths, corresponding to antiferromagnetic coupling behavior. Compared with the hybrid density functional theory, the local spin density approach (SVWN, SVWN5) method and the generalized gradient approximations (BP86, BPW91, BLYP, and PBE) have overestimated the spin delocalization, and Hartree-Fock has overestimated the spin localization.2. The broken symmetry (BS) approach within density functional theory (DFT) was applied to investigating magnetic exchange interactions in dinickel(II)-azide magnetic bistable complex. Our calculated exchange coupling constants of complex [LNi₂(N₃)₃] are in good agreement with the experimental phenomena that the material exhibits a transition from strong antiferromagnetic to only weak antiferromagnetic with the temperature increasing. The antiferromagnetic interaction between the Ni(II) ions is mainly due to the large energy difference of the singly occupied molecular orbitals (SOMOs), and the p orbital overlap for nitrogen atoms on azido and the pyrazolate bridge groups. The analysis of the spin density distribution reveals that both the spin polarization and spin delocalization contribute to the antiferromagnetic interaction. The spin polarization effect and the spin delocalization effect compensate with each other and favor for the total antiferromagnetic interaction. The large energy deference for SOMOs is also responsible to the strong antiferromagnetic coupling in low temperature phase with small Ni-NNN-Ni dihedral angleÏ„, while the decreasing of energy splitting for SOMOs results in the weak antiferromagnetic coupling in high temperature phase due to large Ni-NNN-Ni dihedral angle. The detail investigation for exchange coupling constant with the variation of Ni-NNN-Ni dihedral angle suggests clearly that the conformational change ofμ1,3-N₃ bridge to be the key factor in the different magnetic exchange interactions found in this bistable system. So the abrupt modulation of the magnitude of Ni-NNN-Ni dihedral angle in the [LNi₂(N₃)₃] complex by external perturbations provides new possibilities for the design of molecular magnetic switching devices.3. Two spin sites bistable complex (BDTA)₂[Co(mnt)₂] has been investigated by BS-DFT. The computational results by UB3PW91/LANL2DZ are in good accordance with the experimental results, i.e., in the decreasing of the temperature, the phase transition occur and the coupling interaction is changed from very week antiferromagnetic to the strong one. During the phase transition, the system is accompanied by theÏ€-Ï€andÏ€-d charge transfer, the coordination bond between Co and S is formed. The SOMO distribution pattern display the S-S orbital overlap for the layers between Co(mnt)₂ and (BDTA), lead to the antiferromagnetic interaction. While in the high temperature, the orbital located in the layer of Co(mnt)₂ and (BDTA) itself. The large energy deference for SOMOs is responsible to the strong antiferromagnetic coupling in low temperature phase, while the distinct decreasing of energy splitting for SOMOs suggest orbital degeneracy feckly and results in the very weak antiferromagnetic coupling in high temperature phase. The analysis of the spin density distribution reveals that both the spin polarization and spin delocalization contribute to the antiferromagnetic interaction.