Guanidine-based 3d/4f Single Molecule Magnets And Spin-Frustrated Systems

Single molecule magnets(SMMs)are superparamagnetic molecules that show magnetic bistability at low temperature owing to the presence of an effective energy barrier(Ueff)for magnetisation reversal.The paramagnetic nature of lanthanide ions arises from their large number of unpaired 4f electrons.The single molecule magnetic behavior of these ions stems from a combination of their intrinsic paramagnetism with large magnetic anisotropy.These types of molecular-based materials have been receiving considerable attention towards high-density data storage devices,spintronics and quantum computing.In this regard,we have developed numerous Dysprosiumbased single ion magnets(Dy-SIMs)by using for the first time a guanidine-based ligand.Furthermore,we have also designed couple of spin-frustrated triangular units by coordinating this guanidine-based ligand with CuⅡ ions and have obtained very promising results.All the complexes synthesized during this study are fully characterized structurally,magnetically and theoretically as follows:1.A family of four mononuclear DyⅢ complexes of guanidine-based ligand,tris(2-hydroxybenzylidene)triaminoguanidine(L)were successfully prepared by fluctuating the reaction conditions and their structures were measured by X-ray single crystal measurement.Magnetic investigations showed that Complex 1 did not show single molecule magnetic behavior while complexes 2-4 are single-ion magnets(SIMs)under zero applied DC field with distinict effective energy barriers(Ueff).The different types of coordinated solvent molecules and counter anions bring changes in the intermolecular interactions and the coordination geometries,which severely affect their magnetic dynamics.The magnetic behaviors of these complexes were investigated by means of complete-active space self-consistent field(CASSCF)calculations with inclusion of spin-orbit effects.Calculations reveal that the measured differences in magnetic behaviors originate mainly from intermolecular and crystal-packing effects as the isolated complexes 1-4 have almost identical electronic and magnetic properties.2.Altering the synthesis strategies and further tuning the magnetic dynamics of SIMs are critical tasks for chemists.Based on the counter anion-induced effect two mononuclear DyⅢ complexes 6 and 7 were successfully synthesized by using the same ligand L(see above).Structural and magnetic investigations reveal that different counter anions play an important role in dynamic magnetic behaviors of 6 and 7.Both complexes the DyⅢ centers are SIMs under zero dc applied field with distinct Ueff.A comparison of the structural parameters shows that the small but significant changes at axial positions in bond lengths and bond angles affect the axial ligand field which in turn is mainly responsible for distinct magnetic properties of both complexes.3.Two C3-symmetric guanidine-based triangles were bridged through acetate and chloride anions in cis and trans manner,respectively,to give rise to two novel antiferromagnetically coupled hexanuclear CuⅡ complexes,Cu6 and Cu6Cl,respectively.The magnetic investigations of the titled complexes were analyzed experimentally as well as theoretically.A relatively good agreement was obtained with the experimental data when using wavefunction theory(CASSCF)in combination with DFT(B3LYP)calculations for the very strong antiferromagnetic coupling within Cu3 triangles,leading to spin-frustrated systems.It is worth mentioning that the electronic structure of each CuⅡ centers remains very similar in each complex with a Kramers ground state well separated from the first excited state and weakly anisotropic.