The Solid State Ferric Spin-Crossover Transition: Spectroscopic, Magnetic, and Structural Investigations (Moessbauer, Epr, High, Low)

313 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1985.A number of new ferric spin-crossover solids have been prepared and characterized by using variable temperature magnetic susceptibility, ('57)Fe Mossbauer spectroscopy, EPR spectroscopy, and single-crystal X-ray diffractometry. These solids were prepared and examined in order to determine the molecular and solid state features that may influence the bulk thermodynamics of the thermal low-spin-high-spin transformation and the kinetics of the intramolecular spin-state interconversion. The sample Fe(3-OEtSalBzen)(,2) BH(,4)(.)CH(,3)CN, where 3-OEtSalBzen is the monoanion of the Schiff-base condensate of 3-ethoxysalicylaldehyde and N-benzylethylenediamine, undergoes a thermally gradual spin-crossover transformation. Variable-temperature Mossbauer investigations indicate that the rate of intramolecular spin-state interconversion is less than (TURN)10('7) s('-1). Another sample, Fe(Hthpu)(thpu) , where H(,2)thpu is the Schiff-base condensate of pyruvic acid and thiosemicarbazide, was shown to undergo a thermally discontinuous spin-crossover transition (T(,c)(TURN)230K). Thermal hysteresis ((DELTA)T(,c)(TURN)12K) and the sensitivity of the transition to sample perturbations indicate that the spin-state transformation involves a first-order structural transition. A single-crystal X-ray structure of the analogous hydrated chromium(III) solid indicates that an extended intermolecular hydrogen bonding network may be responsible for the cooperative nature of the ferric spin-crossover transition. A different sample, Fe(3-OEtSalAPA)(,2) CIO(,4)(.)benzene, was found to exhibit rapid intramolecular spin-state interconversion rates (>10('7) s('-1)) on the Mossbauer spectroscopic time scale, but slow interconversion rates (<10('10) s('-1)) relative to the EPR time scale. For this sample, the ligand is formed by the Schiff-base condensation of 3-ethoxysalicylaldehyde and N-aminopropylaziridine. Despite the gradual nature of the thermal spin-crossover transformation, variable temperature (13K-300K) single-crystal X-ray structures of this sample reveal a first-order structural transition at (TURN)170K. Upon raising the sample temperature, the space group converts from P2(,1)/c to C2/c. This structural conversion corresponds to an ordering (or disordering) of the orientations of the benzene solvate, the perchlorate counterion, and the ethoxy substituents. It is possible that rapid solid state reorientations are responsible for the rapid spin-state interconversions.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD