Electronic Structure Determination of Spin-Labile Manganese and Iron Complexes

This thesis describes a number of Mn(III), Fe(III) and Mn(II) complexes synthesised with hexadentate Schiff base ligands, with the aim of developing functional spin-labile systems and studying the fundamental aspects of their electronic structures. The complexes reported are formed by Schiff base condensation of functionalised salicylaldhydes and tetraamine backbones. In Chapter 1 a general overview of coordination chemistry and spin crossover is provided. In Chapter 2 a series of low-spin (S = 1/2) Fe(III) complexes were synthesised and characterised. Structural data for 53 of such complexes with varying ‘R’ group, charge balancing anion and/or lattice solvation are reported. Using bond lengths and structural distortion parameters we show all of these complexes, except one, are stabilised in the low-spin state. The outlier shows a gradual and incomplete spin crossover in the solid state, but persists in the low-spin state in solution. In Chapter 3 an isosymmetric order-disorder phase transition in one of the aforementioned Fe(III) complexes is presented. The complex undergoes an abrupt change in unit cell parameters at 171 K associated with the order-disorder transition in the perchlorate anion. In Chapter 4 a reversible proton sensitive magneto-responsive Fe(III) complex is presented. A change in magnetic susceptibility in solution was observed upon addition of protons which was fully reversible on addition of base. We attribute the effect to a coordination-induced spin state switching (CISSS) effect. The addition of a protochromic diethylamino group to the complex allows the facile detection of protons as the complex changes undergoes a colour change from blue-black to pink upon addition of acid. In Chapter 5 a series of Mn(III) complexes are reported which are sensitive to lattice solvation. The solvent-free complexes stabilise the low-spin (S = 1) state. Co-crystallisation of the samples with ethanol results in the stabilisation of the high-spin (S = 2) state. In Chapter 6 a Mn(III) complex which shows an abrupt and complete S = 1 ¿¿ S = 2 spin crossover at 51 K was studied using high-field electron paramagnetic resonance spectroscopy. High-field EPR was used to characterise both spin states, revealing easy-plane anisotropy with D = +21.23 cm-1 for the low-spin state and D = +5.66 cm-1 for the high-spin state. In Chapter 7 three Mn(III) complexes with heavy anions are reported. The ReO4- salt undergoes a two-stepped re-entrant symmetry breaking spin crossover in the sequence C2/c ¿ P21/c ¿ C2/c. The AsF6- salt undergoes an abrupt symmetry-breaking transition in the sequence C2/c ¿ P-1. The SbF6- salt undergoes a gradual S = 2 ¿ S = 1, S = 2 transition without any structural phase transition. Correlation of the volume of the anions, along with three other related complexes published in the literature reveal that an increase in the T1/2 value is coupled with an increase in the anion volume. In Chapter 8 spontaneous chiral resolution of enantiopure crystals of the delta and lambda forms of a Mn(III) spin crossover complex through conglomerate crystallisation is shown. Gradual spin crossover with a wide thermal hysteresis of 80 K is observed. The enantiopurity of the delta and lambda isomers was confirmed using single crystal X-ray diffraction and circular dichroism. In Chapter 9 modification of the tetraamine backbone with a central piperazine ring yielded Mn(II) complexes which readily adopted a trigonal prismatic geometry. A search of the Cambridge Structural Database reveals that only 2% of Mn(II) sites adopt such a geometry. Continuous shape measures (CShM) were used to quantify the coordination geometry of the Mn(II) centres. The rare geometry results in an atypical EPR spectrum for Mn(II), and a multi-frequency study using high fields allowed fitting of the zero-field splitting parameters, of D = 0.20 cm-1 and E/D = 0.33.