Cyano-Bridged One-dimensional Compounds

Since the first observation of the single-chain magnet (SCM) behaviour in a CoⅡ-nitroxide radical chain in 2001, SCMs have attracted great interest in the field of molecular magnetism. For the construction of SCMs, the magnetic anisotropy of spin units, intrachain exchange interactions and interchain interactions are three most important factors. Compared with the former two ingredients which are the stronger the better, the design and control of the interchain magnetic interaction seems rather difficult and evokes people’s research interest. On the one hand, it was believed that negligible interchain coupling is necessary to prevent the three-dimensional magnetic ordering and to achieve the SCM behaviour. On the other hand, by taking advantage of the magnetic ordering, new types of high temperature SCM-based magnets can be achieved. This dissertation focus on the construction of cyano-bridged one-dimensionl compounds.Chapter 1 gives an outline of the development of single-chain magnets and its building strategy. The overview of the progress in cyano-bridged single-chain magnets is presented. Finally, the main research of this dissertation is introduced.Chapter 2 describes the synthesis, crystal structure, and the detailed magnetic characterization of a cyano-bridged homospin FeⅡ compound [Fe(LN5)(CN)][BF4] (1), which was synthesized from the anisotropic pentagonal bipyramidal FeⅡ starting material in the presence of excessive BF4- anion. In the crystal structure, FeⅡ cations were bridged by cyano groups to form a 1D chain running along the b axis. Linked by the obvious π…π stacking interactions (d=3.39 A) between the pyridine rings, these chains form two-dimensional layers. Detailed magnetic measurements revealed that this compound exhibits the rare coexistence of spin-canting, antiferromagnetic ordering, field-induced metamagnetism and single-chain magnet behaviour and is the first example of a cyano- bridged homospin system with the single-chain magnet dynamics.Chapter 3 presents the syntheses, crystal structures, and magnetic properties of a mononuclear Fe" compound [Fe(LN3O2)(H2O)2][MQ]2·H2O (2) and a cyano-bridged homospin compound [Fe(LN3O2)(CN)][ABSA]·3H2O (3) (LN3O2=2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene; MQ-= methyl orange; ABSA-= 4-aminoazobenzene-4’-sulfonic acid), which were prepared by recrystallization of [Fe(LN3O2)(CN)2] in the presence of the bulky anions. The structure of 2 contains a mononuclear Fe" cation and two bulky MQ- counter anions forming a hydrogen bonded 1D chain along the a direction. In the crystal structure of 3, well isolated cyano-bridged homospin FeⅡ chains were formed along the b direction and each chains were separated by the bulky ABSA- anions with the shortest Fe…Fe distances between the chains of 12.0 A along the a direction and 12.8 A along the c direction. Magnetic investigation of 2 reveals that the mononuclear FeⅡ compound exhibits field-induced slow magnetic relaxation with a negative D value of -3.7 cm-1. DC magnetic susceptibility measurements of 3 demonstrate the presence of intrachain antiferromagnetic coupling (J=-4.13 cm-1, g=2.13 (3)) between the Fe" centers bridged by cyano groups. In addition, ac magnetic susceptibilities as a function of both temperature and frequency reveal the chain compound exhibts slow relaxation of magnetization with an effective energy barrier of 26.1(5) K and a relaxation time of 8.3(6)×10-10 s. The systems further illustrate the AF ordering is advantageous to design high temperature SCM-based magnet in homospin systems and represent the first homospin systems switching between SCM and SCM-based magnet by anion tuning.In Chapter 4, utilizing the highly anisotropic hepta-coordinated CoⅡ precursors [Co(LN3O2)(H2O)2](ClO4)2 (4) with pentagonal bipyramid geometry to react with hexacyanometalate [Co(CN)6]3- building block, we have successfully synthesized a novel cyano-bridged coordination polymer [Co(LN3O2)6[Co(CN)6]4·26H2O (5) with a tubular topology featuring a hexagonal 1D channel. The magnetic behaviour of 4 was reported, displaying a high easy-plane magnetic anisotropy (D=+26.5 cm-1) and field-induced SIM behaviour with an energy barrier of Ueff=5.01 cm-1 and ro=2.24×10-5 s, which were comparable with its analogue. Magnetic investigation on 5 showed that the field-induced slow magnetic relaxation was also observed at low temperature with an similar energy barrier Ueff=6.4 cm-1 and r0=1.9×10-5 s, which results from the intrinsic magnetic anisotropy of the pentagonal bipyramidal CoⅡ centers. Specificlly,5 displays a potential void volume of 24.3% without water molecules and represents the largest canyo-bridged 1D molecular tube exhibiting field-induced slow magnetic relaxation, which is also the first time observed in the tube-like coordination compounds.