Control of Single-Molecule Magnetic Properties Using Metallacrowns.

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.PhDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/93971/1/ttboron_1.pd