Dynamics and Mechanism of Spin-State Interconversion in Iron Spin-Crossover Complexes

Abstract

The work described in this dissertation is directed at a better understanding of the dynamics and mechanism of the so-called spin-crossover phenomenon. Several iron spin-crossover systems, including ferrous and ferric complexes, are investigated with various spectroscopic techniques and single-crystal X-ray structure determination.

In the first part the cooperativity of ferric spin-crossover complexes in the solid state and the influence of lattice solvation and metal dilution upon these properties is studied. The complex Fe(3-OMeSalAPA)$\sb 2$ClO$\sb 4$, where 3-OMeSalAPA is the Schiff base ligand condensed from one mole of 3-methoxylsalicylaldehyde with N-aminopropylaziridine, and its various halobenzene solvated forms were studied with $\sp{57}$Fe Mossbauer and EPR spectroscopies, as well as magnetic susceptibility, DSC, single-crystal structure determination and variable temperature XRD techniques. It has been found from the experimental results that the ferric complex Fe(3-OMeSalAPA)$\sb 2$ClO$\sb 4$ and its solvated forms are spin-crossover systems of the gradual type.

In the second part of this thesis, an investigation of the correlation between the onset of solvate molecule dynamics and the spin-crossover phase transition is investigated. The complex Fe(dppen)$\sb 2$Cl$\sb 2$ with lattice solvate molecules of CH$\sb 3$COCH$\sb 3$, CD$\sb 3$COCD$\sb 3$, CH$\sb 2$Cl$\sb 2$, CD$\sb 2$Cl$\sb 2$, CHCl$\sb 3$, CDCl$\sb 3$ were investigated by the variable-temperature solid-state $\sp{2}$H NMR technique. Dppen stands for cis-1,2-bis(diphenylphosphano)ethylene.

In the third part of this thesis, an investigation on the dynamics of the spin-state interconversion in solution and other media is presented. The spin-state interconversion rate of the ferrous complexes series (Fe(6-Me-py)$\sb{\rm n}$(py)$\sb{\rm 3-n}$tren) (ClO$\sb 4)\sb 2$ (n = 0,1,2,3), where the ligand is the Schiff base of triethylaminotetraamine(tren) was measured by the means of the laser-flash photolysis technique. The temperature dependence of the relaxation constant of (Fe(6-Me-py)$\sb 2$(py)tren) (ClO$\sb 4)\sb 2$ doped in PSS polymer film from 300K-4.2K can be fit well to quantum-mechanical tunneling theory. It is concluded that the spin-state relaxation process of this complex occurs by a quantum-mechanical tunneling mechanism. The inverse relationship between the zero-energy separation and the apparent activation energy was observed in the complex series. This observation suggests that the spin-state interconversion process of these complexes is in the "normal" region, not the inverted region. (Abstract shortened with permission of author.)