| In the past two decades, single-molecule magnets (SMMs) have been of intense interest due to their novel properties and potential applications in magnetic information storage, quantum computation and spintronics. In 2003, Ishikawa et al. reported the first mononuclear, lanthanide ion-based SMM, which is subsequently referred to as a single ion magnet (SIM). More recently SIMs based on transition metal ions have attracted much attention. For the transition metal ions, a low coordinate number would split the d orbitals with a small separation between the electronic ground state and the excited states or within a narrow energy gap, facilitating the spin-orbital coupling to enhance the magnetic anisotropy. Indeed most of the quest for transition metal-based SIMs is focused on low-coordinate metal species with coordination numbers from two to six. In this work, the magnetic properties of mononuclear transition metal complexes with the high-coordination number have been investigated, and the relationship between structure and the magnetic property has been explored. The content includes four parts as follow:1. Single-ion magnets based on eight-coordinate cobalt(Ⅱ) complexes with two 12-crown-4 ligandsWe synthesized three cobalt complexes [CoⅡ(12C4)2](l3)2(12C4) (1), [CoⅡ(12-crown-4)2](ClO4)2·2(CH3CN) (2) and [CoⅡ(12C4)2](BPh4)2 (3) with similar structure based on 12-crown-4 ligand and characterized their structures and magnetic properties. Complex 1-3 possess the distorted square antiprism geometry. However, the structure of complex 2 is disordered, and the asymmetrical unit for complex 3 includes two molecules with different structural distortion. In accordance with variable-field magnetization data, high-field electron paramagnetic resonance (HFEPR) spectroscopy and theoretical calculation reveal the presence of large negative D in complex 1. Fitting the variable-field magnetic susceptibility data suggest that complexes 2 and 3 have large negative D value. Magnetic measurements show field-induced, slow magnetic relaxation in all complexes, and the spin effective energy barrier are 17.0 cm-1,20.7 cm-1 and 36.1 cm-1, respectively. The work here presents the first example of the eight-coordinate, mononuclear,3d metal complex exhibiting the slow magnetic relaxation.2. Single-ion magnets based on mononuclear seven-coordinate cobalt(Ⅱ) complexes(1) Two mononuclear seven-coordinate cobalt(Ⅱ) complexes [Co(L)3(NO3)2] (L =4-tert-butylpyridine,4; L=isoquinoline,5) were prepared and structurally analyzed by single-crystal X-ray crystallography. The coordination spheres of 4 and 5 exhibit the distorted pentagonal bipyramid geometry. Analysis of their direct-current magnetic data reveals the easy plane anisotropy (D>0) with small transverse anisotropy (E), which was further confirmed by high-field electron paramagnetic resonance (HFEPR) spectroscopy. Field-induced slow magnetic relaxations were observed under the applied dc field of 500 Oe in complexes 4 and 5 by alternating-current magnetic susceptibility measurements.(2) The magnetic properties of a seven-coordinated mononuclear cobalt(Ⅱ) complex [Co(12-crown-4)(NO3)2] (6) with a 4:3 geometry have been studied. The Co(Ⅱ) ion is sandwiched between the plane from the four polyether oxygen atoms and that from three bonded nitrate oxygen atoms, and the two planes are almost parallel to each other. Magnetic studies revealed that this complex exhibits field-induced slow magnetic relaxation with a negative D. The spin effective energy barrier Ueff= 18.4 cm-1 was obtained.3. Influence of the different counter-cations on the coordination geometries and magnetic dynamics of [Co(NO3)4]2-Three mononuclear cobalt(Ⅱ)-tetranitrate complexes with tetraphenylarsonium (7), methyltriphenylphosphonium (8) and tetraphenylphosphanium (9) as the counter-cations, have been synthesized and structurally characterized. The coordination sphere for complex 7 is established by the eight oxygen atoms. However, the Co atoms in complexes 8 and 9 locate in seven-coordinated distorted geometry. Complexes 8 and 9 show distinct field-induced slow magnetic relaxation. However, slow magnetic relaxation for 7 is not observed under external dc fields.4. Single-ion magnets based on mononuclear six-coordinate cobalt(Ⅱ) complexesWe report the dynamic magnetic properties of two octahedral mononuclear Co(Ⅱ) complexes [Co(Li)3(NO3)2]·(Li)2 (L1=pyridine,10) and [Co(L2)3(NO3)2] (L2= 3-methylpyridine,11). Magnetic studies revealed that two complexes with the easy plane anisotropy exhibit field-induced slow magnetic relaxation. For complex 10, the regions of high temperature, middle temperature and low temperature are likely dominated by Orbach, Raman and direct relaxation processes, respectively.5. Single-ion magnets based on mononuclear six-coordinate manganese(Ⅲ) complexesWe report the dynamic magnetic properties of three mononuclear Mn(Ⅲ) complexes with the chelating dibenzoylmethanide (dbm-) ligand, [Mn(dbm)3] (12), [Mn(dbm)2(L)2](ClO4) (L=DMSO=dimethyl sulfoxide,13; L=py=pyridine,14). The zero-field splitting parameters D for 12-14 were determined by fitting the magnetic susceptibility and the magnetization. The large, negative D values consist with the Jahn-Teller axis elongation of octahedral geometry. Complexes 12-14 show the frequency-dependence of ac susceptibility under the applied dc field. The out-of-phase maxima of out-of-phase ac susceptibility signals are observed in 3. The effective spin-reversal barrier of Ueff= 18.5 cm-1 for 3 is higher than those of the reported Mn(Ⅲ)-based SIMs. |
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