Utilizing metallacrowns to develop new single -molecule magnets.
Zaleski, Curtis M.
2005
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><sub>2</sub>Mn<super>III</super><sub>2</sub>(pko)<sub>2</sub>(pdol)(N<sub> 3</sub>)<sub>6</sub>(CH<sub>3</sub>OH)<sub>2</sub>. The Mn<super>II</super><sub> 2</sub>Mn<super>III</super><sub>2</sub> complex exhibits a frequency dependent ac magnetic susceptibility signal, a characteristic of SMMs. Then the metallacrown complexes M<super>n+</super>[12-MC<sub>Mn<super>III</super>(N)shi</sub>-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><sub>4</sub>Mn<super> III</super><sub>22</sub>(pdol)<sub>12</sub>(mu<sub>3</sub>-OCH<sub>3</sub>)<sub> 12</sub>(mu<sub>3</sub>-O)<sub>6</sub>(mu<sub>4</sub>-O)<sub>10</sub>(OH)<sub> 2</sub>(H<sub>2</sub>O)(OCH<sub>3</sub>)<sub>3</sub>]<super>·</super>ClO<sub> 4</sub> has a reduced symmetry when compared to the analogous compound Mn<super> II</super><sub>4</sub>Mn<super>III</super><sub>22</sub>(pdol)<sub>12</sub>(mu<sub> 3</sub>-OCH<sub>3</sub>)<sub>12</sub>(mu<sub>3</sub>-O)<sub>6</sub>(mu<sub> 4</sub>-O)<sub>10</sub>(N<sub>3</sub>)<sub>6</sub>. The reduced symmetry affects the Mn<super>III</super><sub>4</sub>O<sub>6</sub> core of each molecule. In the Mn<sub>26</sub>ClO<sub>4</sub> complex the core has a lower symmetry and this reduction in symmetry leads to a higher ground spin state and more magnetoanisotropy for Mn<sub>26</sub>ClO<sub>4</sub> than Mn<sub>26</sub>N<sub>3</sub>. In magnetic measurements the reduction in symmetry results in a higher effective blocking temperature (<italic>U</italic><sub>eff</sub>) for Mn<sub>26</sub>ClO<sub> 4</sub> (36.2 +/- 2.0 K; 25.1 +/- 1.4 cm<super>-1</super>) than Mn<sub>26</sub>N<sub>3</sub> (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><sub>6</sub>Mn<super>III</super><sub> 4</sub>Mn<super>IV</super><sub>2</sub>(H<sub>2</sub>shi)<sub>4</sub>(Hshi)<sub> 2</sub>(shi)<sub>10</sub>(CH<sub>3</sub>OH)<sub>10</sub>(H<sub>2</sub>O)<sub> 2</sub> (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><sub>4</sub>Mn<super> IV</super><sub>2</sub> has the highest blocking temperature of the investigated Ln<super>III</super> ions with a <italic>U</italic><sub>eff</sub> of 19.0 +/- 1.5 K (13.2 +/- 1.0 cm<super>-1</super>). A second series of Ln<super>III</super><sub>4</sub>Mn<super>III</super><sub>6</sub>(H<sub>2</sub>shi)<sub> 2</sub>(shi)<sub>6</sub>(sal)<sub>2</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sub> 4</sub>(OH)<sub>2</sub>(CH<sub>3</sub>OH)<sub>8</sub>(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><sub>6</sub>Mn<super> III</super><sub>6</sub> 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>(NO<sub>3</sub>)<sub>3</sub>[15-MC<sub>Cu<super> II</super>(N)S-pheHA</sub>-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.Subjects
Develop Magnetism Metallacrowns Metallamacrocycle New Single-molecule Magnets Utilizing
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