| Low-dimensional magnetic metal complexs are good candidates in designing the potential emerging molecular-based magnetic material. They basically are the complexes that point to zero-dimensional (0D) single-molecule magents (SMMs), one-dimensional (1D) single-chain magnets (SCMs) with quantum tunneling of magnetization type behavior. This paper focuses on controlled assembly of 3d metal based SMMs and SCMs by means of (1H-benzimidazol-2-yl)methanol and its derivatives for directional ligands, and modification for classical/quantum magnetic behavior of nanoscale molecular-based magnets by structure decoration of controlling ligands and alternation of different strength of magnetic super-exchange for cores and ancillary ligands for further work. This paper is divided into six chapters:In chapter 1, the background information of this paper and current research concentrations were accounted with the emphasize on the theoretical development and the latest situation of magnetic polynuclear clusters, SMMs, SCMs and spin-canted antiferromagnets, as well as the roles of crystal-engineering in preparing molecular-based magnets.In chapter 2, (1H-benzimidazol-2-yl)methanol and its 5-chlorinated and N-methyl substitute derivatives directional ligands, are synthesized to construct low-dimensional complexes. By varying the coligands, such as chloride, bromine, thiocyanate, formate, acetate, a series of varied symmetry and correlation interaction among intra/inter-cluster of cobalt, nickel cubane-based molecular clusters bridged by oxygen atoms are constructed in a controllable manner, and assembly methods and magneto-structural relationships are also studied. It is found that directional ligands and coligands have an effect on magnetic exchange distance, symmetry and crystal packing modes and strength of weak interaction among clusters, thus will influence bulk cooperative magnetic behavior of molecular-based clusters. The magnetic behavior of clusters has been changed from quantum tunneling of magnetization to spin glasses as strength of correlation interaction among Co4 clusters increase. Moreover, different spin carriers also affect the overall magnetic behaviors of the isostructural complexes. If M-O-M bond angles don’t change, and the magnetic coupling within clusters has been changed from ferromagnetic coupling to antiferromagnetic coupling as tetraclusters symmetry are reduced to Cv from S4. The above series of examples expand research area for assembling ferromagnetic cubane-based molecular magnets, according to the Godenough-Kanamori rule.In chapter 3, this work puts an emphasis on a single molecule magnetic, Co12 supercluster based on three Co4O4 cubanes, which constructed by nitrate templates and (1H-benzimidazol-2-yl)methanol directional ligands and acetate auxiliary ligands and perchlorate for new counter anions. More significantly, although the core skeletons of Co12 clusters remains unchanging, and the overall symmetry of the Co12 clusters are reduced to pseudo-C3 from the original S4 due to perchlorate counter anions replacement, helped doubling growth of energy barrier value. The move toward symmetry control ultimately did raise the value of blocking temperature by 0.4 K. Obviously, larger perchlorate as new counter anions, which embed in superclusters and further change the overall correlation interaction among clusters, leading to cooperative modification for quantum behavior of SMMs. Hence, cubane-based molecules can be further configured as a node to facilitate high ground state of superclusters by selecting the judicious bridged templates and counter ions. It also provides a new way to controlled-assembly of SMMs.In chapter 4, reactions of CoⅡwith (1H-benzimidazol-2-yl)methanol under solvent-thermal conditions give a disc like heptanuclear cluster, which shows six circular-arranged defective Co3O4 cubane sharing the neighboring faces one another, having ferromagnetic interaction. Disc like dodecanuclear clusters are gained by introducing azide groups to above reaction synstems, in which all azide groups adopt EO coordination modes. This Co12 cluster is the largest CoⅡ/N3-cluster, and the corresponding Ni12 is the second largest NiⅡ/N3-cluster. In particular, the core skeletons of Co12 clusters remains unchanging and leading to a slightly changed blocking temperature of SMMs by ther replacement of perchlorate. Hence, this research once again shows that counter anions can be adjusted cluster symmetry and crystal stacking modes, further modifed quantum tunneling of magnetization behavior. The "anion templates" method is confirmed to used to prepare similar core skeletons, and is also a new way of modification for quantum behavior of SMMs at the molecular, ion level.In chapter 5, as (1-methyl-1H-benzimidazol-2-yl)methanol organic ligands furnish a cubane-like Co4O4, Ni4O4 motif throughμ3/μ-O bridges, and usually this clusters are well separated by organic ligands. By introducing azide groups, azide bridged cubane-based one-dimensional compounds are constructed in a controllable manner. For a long time, Mn4O4 defected cubane-based one-dimensional compounds has been the uniquely study one, but the situation has been changed with the successful assembly of above cubane-based one-dimensional compounds. However, they all exhibit antiferromagnetic coupling, no SCMs behavior. It may be axis anisotropy of various metal ions cancel each other out within chains. Obviously, such simply bridged magnetic cluster approach for SCMs is somewhat unpredictable.In charpter 6, SS-1,2-bis(1H-benzimidazole-2-yl)-1,2-ethanediol (SS-H2bzimed) featuring rigid and flexible groups and CoⅡ, ZnⅡ、NiⅡgive four compounds by methods of alternative hydroponic and hydrothermal reactions. Room temperature reaction of SS-H2bzimed with CoⅡyielded mononuclear chiral compound. Using ZnⅡinstead of CoⅡ, under refluxed stirring, tetranuclear compound was obtained, showing a Zn4[μ3-O]4 cubic core. Under hydrothermal conditions, the SS-H2bzimed ligand unexpectedly changes to 1,2-bis(1H-benzimidazol-2-yl)-1,2-ethenol by an in situ intramolecular dehydration coupling reaction, and the neonatal ligands bridge metal systems into two kinds of rare single crystal microtubes constructed by a trigonal bipyramid M5[μ-O]6 (M=CoⅡ, NiⅡ) core. A discussion of a probable mechanism for in situ reaction of ligand and the formation of single crystal microtubes is provided. Magnetic studies show that Co5O6 exhibits spin-canting behavior below 10 K, whereas Ni5O6 shows only simple antiferromagnetic coupling. Such magnetic behavior of Co5O6 mainly arises from mixed geometries of pentameric CoⅡions, which can result in relatively noncompensated moments, according to different efficient spins of CoⅡat very low temperature and the Dzyaloshinski-Moriya interaction.Finally, the paper provides a brief summary of the work. |
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