Utilizing metallacrowns to develop new single -molecule magnets.

Abstract

The object of this dissertation addressed the use of metallacrowns to develop new single-molecule magnets (SMMs). The first example of the metallacrown linkage N-O leading to SMM was observed in the non-metallacrown cluster Mn<super> II</super>₂Mn<super>III</super>₂(pko)₂(pdol)(N₃)₆(CH₃OH)₂. The Mn<super>II</super>₂Mn<super>III</super>₂ complex exhibits a frequency dependent ac magnetic susceptibility signal, a characteristic of SMMs. Then the metallacrown complexes M<super>n+</super>[12-MC_{Mn<super>III</super>(N)shi}-4](M<super> n+</super> = Li<super>+</super>, Mn<super>II</super>, Gd<super>III</super>, Tb<super>III</super>, Dy<super>III</super>, and Y<super>III</super>) were examined to investigate the effect of the central metal cation on SMM behavior. Only the Mn<super>II</super> and Dy<super>III</super> 12-MC-4 complexes behave as SMMs. Therefore, the identity of the central cation plays a crucial role as to whether SMM behavior is observed as not all paramagnetic central ions lead to SMMs in 12-MC-4 complexes. In addition, two manganese metallacryptates behave as SMMs. The complex [Mn<super>II</super>₄Mn<super> III</super>₂₂(pdol)₁₂(mu₃-OCH₃)₁₂(mu₃-O)₆(mu₄-O)₁₀(OH)₂(H₂O)(OCH₃)₃]<super>&middot;</super>ClO₄ has a reduced symmetry when compared to the analogous compound Mn<super> II</super>₄Mn<super>III</super>₂₂(pdol)₁₂(mu₃-OCH₃)₁₂(mu₃-O)₆(mu₄-O)₁₀(N₃)₆. The reduced symmetry affects the Mn<super>III</super>₄O₆ core of each molecule. In the Mn₂₆ClO₄ complex the core has a lower symmetry and this reduction in symmetry leads to a higher ground spin state and more magnetoanisotropy for Mn₂₆ClO₄ than Mn₂₆N₃. In magnetic measurements the reduction in symmetry results in a higher effective blocking temperature (<italic>U</italic>_eff) for Mn₂₆ClO₄ (36.2 +/- 2.0 K; 25.1 +/- 1.4 cm<super>-1</super>) than Mn₂₆N₃ (16.5 +/- 0.7 K; 11.5 +/- 0.5 cm<super> -1</super>). Lanthanide-transition metal complexes related to metallacrowns also behave as SMMs. A series of Ln<super>III</super>₆Mn<super>III</super>₄Mn<super>IV</super>₂(H₂shi)₄(Hshi)₂(shi)₁₀(CH₃OH)₁₀(H₂O)₂ (Ln<super>III</super> = Gd<super>III</super>, Tb<super>III</super>, Dy<super>III</super>, Ho<super>III</super>, Er<super>III</super>, and Y<super> III</super>) compounds were studied for possible SMM behavior. Only the Dy<super> III</super>, Ho<super>III</super>, and Y<super>III</super> analogues exhibited SMM behavior. The Dy<super>III</super>Mn<super>III</super>₄Mn<super> IV</super>₂ has the highest blocking temperature of the investigated Ln<super>III</super> ions with a <italic>U</italic>_eff of 19.0 +/- 1.5 K (13.2 +/- 1.0 cm<super>-1</super>). A second series of Ln<super>III</super>₄Mn<super>III</super>₆(H₂shi)₂(shi)₆(sal)₂(O₂CCH₃)₄(OH)₂(CH₃OH)₈(Ln<super>III</super> = Gd<super>III</super>, Tb<super>III</super>, Dy<super>III</super>, Ho<super> III</super>, Er<super>III</super>, Lu<super>III</super>, and Y<super>III</super>) also show similar behavior to the Ln<super>III</super>₆Mn<super> III</super>₆ complexes with the Tb<super>III</super>, Dy<super> III</super>, Ho<super>III</super>, and Y<super>III</super> complexes behaving as SMMs. Ln<super>III</super>(NO₃)₃[15-MC_{Cu<super> II</super>(N)S-pheHA}-5] complexes (Ln<super>III</super> = Gd<super> III</super>, Tb<super>III</super>, Dy<super>III</super>, Ho<super>III</super>, Er<super>III</super>, and Y<super>III</super>) crystallize in two different polymorphs: dimer and helix. For both polymorphs only the Dy<super>III</super> and Ho<super>III</super> complexes behave as SMMs. The nature of the solid state arrangement has little to bear on the SMM behavior as both the dimer and helical polymorphs display SMM behavior. The helical chains may be considered one-dimensional chains of SMMs.