Research On Nitronyl Nitroxide Bridged Molecule-Based Magnetic Complexes

Nitronyl nitroxide(NIT)radicals are a family of stable radicals under ambient conditions with its single electron delocalizes on the five atoms of O-N-C-N-O.Both of their-NO groups can coordinate with metal ions.As paramagnetic bridging ligands,nitronyl nitroxide radicals can efficiently transfer magnetic interactions between metal ions.In addition,various auxiliary ligands can be conveniently introduced to modify their structures.On one hand,the coordination mode of these radical can be increased,and on the other hand,their electronic effects and steric effects can be regulated.Based on these interesting characteristics,nitronyl nitroxide radicals have been widely used in the area of molecular magnets to construct materials of long-range magnetic ordering,single-molecule magnets and single-chain magnets.At present,researches on the nitronyl nitroxide radicals mainly focuses on the monoradicals.Studies on the nitronyl nitroxide diradicals and polyradicals are still very limited.This dissertation mainly focuses on nitronyl nitroxide diradicals and triradicals.The research work mainly includes the following four parts:In part one,we designed and synthesized a new nitronyl nitroxide diradical based on pyrazine ring,named BNITPz(1).Magnetic measurements revealed that existence of weak antiferromagnetic coupling between the two radicals in 1 through the pyrazine ring.Subsequent treatment of this diradical with Ln(hfac)3·2H2O resulted in an unexpected reduced radical ligand BHIMPz.Six new dinuclear compounds[Ln(hfac)3]2(BHIMPz)(H20)2[Ln = Gd(2Gd),Dy(3Dy),Ho(4Ho),Er(5Er),Tm(6Tm),Yb(7Yb)]were obtained based on the reduced form of the ligand.The structural analysis shows that these compounds are isostructural where the rare earth ions are eight-coordinated with a distorted square antiprism geometry(D4d).Magnetic studies showed that compounds 3Dy and 7Yb behave as field-induced single-molecule magnet.We have elucidated,by theoretical calculations for this series of compounds,the different magnetic kinetic behavior exhibited by the compounds 3Dy-7Yb and the reason why the compound 7Yb has a slower magnetic relaxation process than 3Dy.In the second part,we synthesized a nitronyl nitroxide diradical based on thiophene ring,named BNITTp(8).Subsequent treatment of this diradical with Ln(hfac)3·2H2O resulted in three binuclear compounds[Ln(hfac)3]2(BNITTp)(H2O)2,Ln = Sm(9sm),Eu(10Eu),Dy(11Dy).Single crystal structure analysis showed that compounds 10Eu and 11Dy are isostructural,while compound 9sm is in a different configuration.All rare earth ions of these complexes are eight-coordinated with a double-cap trigonal prism.The local symmetry is D2d.Magnetic studies showed that there is a possible antiferromagnetic interaction between Sm?(or Eu?)and the radical,and there is a ferromagnetic interaction within the compound 11Dy.The ac measurements revealed that none of these three compounds behave as single-molecule magnets.In the third part,we synthesized a nitronyl nitroxide triradical based on benzene ring,named BTR(12).Subsequent treatment of this triradical with Ln(hfac)3·2H2O resulted in four binuclear penta-spin compounds[Ln(hfac)3]2BTR,Ln = Pr(13Pr),Nd(14Nd),Gd(15Gd),Dy(16Dy).The single-crystal structure analysis showed that the four compounds were isostructural in the monoclinic P2/c space group.Rare-earth ions are all eight-coordinated,the Pr3+ and Nd3+ coordination environments can be approximated as square antiprism and the local symmetry is D4d;Gd3+ and Dy3+ is slightly prone to C2v than D4d due to the increase in atomic radius.Magnetic dynamics studies showed that the compound 16Dy has a frequency-dependent behavior of ac susceptibilities under zero dc field,indicating the presence of slow relaxation of magnetization.Theoretical calculations for compounds 15Gd and 16ny showed that Ln3+-radical interactions were found to be antiferromagnetic,and radical-radical interactions are ferromagnetic.The energy barrier of 16Dy is found to originate from the exchange states,rather than the isolated Dy3+ center.In the fourth section,we synthesized a nitronyl nitroxide diradical based on 2,5-dimethoxy-1,4-benzenedicarboxaldehyde,named BNITOMe(17).Subsequent treatment of this diradical with Co(hfac)2·2H2O resulted in a binuclear compound[Co(hfac)2]2(BIMOMe)(18co).In this compound,we found that diradical was reduced to one-armed IM radical during the reaction.The single crystal structure analysis showed that it crystallized in the monoclinic P2/c space group.The Co? ion is six-coordinated and the coordination environment is a distorted octahedron(Oh).Magnetic dynamics studies showed that compound 18co behaves as a field-induced single-molecule magnet.In addition,we also obtained a Co?-based compound[Co(hfac)2]2(BNITPz)(19co)using biradical 1.The compound 19co is in a monoclinic C2/c space group.The central Co? ion is also six-coordinated,and the coordination environment is also a distorted octahedron(Oh).Magnetic studies showed that there is a strong antiferromagnetic interaction within the system,and compound 19co also behaves as a field-induced single-molecule magnet.