Novel Discoveries About the Spin-Crossover Transition in Schiff-Base Ferric Complexes

The high-spin (('6)A(,1)) to low-spin (('2)T(,2)) conversion has been studied in pseudo-octahedral ferric complexes {Fe('III) (X-SalEen)(,2)}Y. The ligand, X-SalEen, is obtained by the Schiff-base condensation of salicylaldehyde, 3-OCH(,3)-salicylaldehyde, or 5-OCH(,3)-salicylaldehyde with N-ethylethylenediamine. The anion, Y, used is NO(,3)('-), PF(,6)('-) or BPh(,4)('-). Magnetic susceptibility measurements revealed that, like other spin-crossover ferrous or ferric complexes, the effective magnetic moment per iron ((mu)(,eff)/Fe) vs. temperature curves are puzzling. Three types of (mu)(,eff)/Fe vs. temperature curves are obtained. The first exhibits a sharp transition from high-spin to low-spin; the second shows a gradual transition; the third and the most puzzling the (mu)(,eff)/Fe vs. temperature curves exhibits plateau regions. For the latter case the transition is incomplete and in a certain temperature range the (mu)(,eff)/Fe doe not change or changes very slowly. Mossbauer and EPR spectroscopies indicate that, regardless of the type of (mu)(,eff)/Fe vs. temperature curve, only molecules in the ('6)A(,1) and ('2)T(,2) states exist. No evidence for the intermediate spin state, ('4)T(,1), has been detected. It is found that unusual (mu)(,eff)/Fe vs. temperature curves could be brought about by grinding or doping the crystalline solids. For example, an unperturbed crystalline sample of {Fe (3-OCH(,3)-SalEen)(,2)}PF(,6) exhibits a sharp complete transition, but the same solid, ground with a mortar and pestle or in a ball mill, exhibits an incomplete transition, i.e., the (mu)(,eff)/Fe vs. temperature curve exhibits a plateau region. Moreover, the sharpness of the transition is lost. Doping the Fe(III) molecules into isostructural Co(III) or Cr(III) lattices leads to gradualness of the spin-transition. The effect of grinding doped solids is more dramatic than grinding pure solids. All of the above observations are discussed in terms of defects introduced into the unperturbed solids by grinding or doping. The mechanism of Nucleation and Growth of solid-solid phase transformations will be shown as best accommodating the experimental findings.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemistry

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2014

Author dc:creator

• Haddad, Muin Shawki

• Chemistry, Inorganic

Language dc:language

eng

Identifier

(UMI)AAI8026506

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/67230