Control of Single-Molecule Magnetic Properties Using Metallacrowns.

[摘要] This dissertation studied controlling single-molecule magnetic (SMM) properties by using the metallacrown motif. Large mixed lanthanide-manganese complexes, LnIII6MnIII2MnIV2(shi)6(Hshi)4(H2shi)2(Hsal)4, where H3shi is salicylhydroxamic acid, H2sal is salicylic acid, LnIII = GdIII, TbIII, or DyIII, and LnIII4MnIII4(OH)2(O2C2H3)2(shi)4(H2shi)4(Hsal)4 LnIII = DyIII, HoIII, or ErIII, were produced, with the DyIII analog of each series showing slow magnetic relaxation, a hallmark of SMM behavior. A new family of LnIII 14-MCMnIIILnIII(μ-O)(μ-OH)N(shi)-5 complexes (LnIII = YIII, GdIII, TbIII, DyIII, HoIII, or ErIII) was prepared which oriented the anisotropy tensors of the MnIII ions in a single direction, perpendicular to the metallacrown plane. The complexes, with the exception of the YIII and GdIII analogs, showed slow magnetic relaxation, with the DyIII analog having a Ueff of 16.7 K and τ0 = 4.9 x 10-8 s, at the time, the third largest Ueff for mixed Mn/Ln complexes, despite using fewer metals than other examples. Using simpler, planar NiII(O2C2H3)2 12-MCMnIIIN(shi)-4 complexes revealed that lattice solvents affect whether SMM or single-chain magnetic (SCM) behavior is observed. The NiII 12-MC-4 isolated in methanol had a hydrogen-bond network, leading to SCM behavior; the complex isolated in dimethylformamide, lacking the hydrogen-bonding network, shows SMM behavior. To improve SMM properties, the more anisotropic DyIII ion was incorporated, forming DyIIIX4A 12-MCMnIIIN(shi)-4 (A = H3O+, Na+, K+; X = salicylate, benzoate, acetate). SMM behavior occurred only for the salicylate bridged complexes, regardless of counter ion. Slight structural changes due to ligand substitution explain this observation.Metallacrown single-ion magnet (SIMs) were examined using a series of LnIIIZn16 complexes (LnIII = GdIII, TbIII, DyIII, HoIII, ErIII, YbIII), with the ErIIIZn16 showing SMM behavior, as revealed by single-crystal SQUID magnetometry measurements. The ligand field around the LnIII ion dramatically affected the magnetic behavior, dependent on the shape of the occupied f orbitals. In conclusion, structures with controlled molecular anisotropy led to well isolated ground states, improving SMM behavior. It was found that for larger Mn/Ln complexes, it is difficult to obtain high Ueff without structural control. In addition, DyIII or ErIII ions proved to be better LnIII ions due to their intrinsic anisotropy and Kramers doublet ground state.

[发布机构] University of Michigan

[效力级别] Single-chain Magnetism

[关键词] Single-molecule Magnetism;Single-chain Magnetism;Coordination Chemistry;Chemistry;Science;Chemistry