THE IRON(II)-DIIMINE COMPLEX [FE(CH3N=CHCH=NCH3)3]2+ - ITS STRUCTURE AND ITS SOLVATION AND REACTIVITY IN AQUEOUS-ORGANIC SOLVENT MIXTURES

Tris (N,N'-dimethylglyoxal diimine)iron(II) (Fe(gmi)3(2+)) salts have been prepared, and the structure of the tetrafluoroborate salt has been determined by an X-ray diffraction study. Fe(gmi)3(BF4)2, C12H24N6B2F8Fe, is rhombohedral and crystallizes in space group R3cBAR, and there are six molecules per cell. In addition the structure of [Fe(bmi)3](ClO4)2 (C18H36N6O8Cl2Fe, trigonal, space group P3c1BAR, two molecules per cell), where the bmi ligand (N,N'-dimethyl-2,3-butanedione diimine) is the dimethyl analogue of gmi, has been determined so that together with available structural data for other iron(II)-diimine complexes the general reactivity of the Fe(gmi)3(2+) cation on the basis of structural information may be ascertained. Dissociation of Fe(gmi)3(2+) in the presence of hydroxide ion has been studied spectrophotometrically. The second-order rate constant, k2 (the reaction is first order in both reactants under the conditions studied), was determined for several aqueous mono-ol mixtures and in aqueous acetone. The trend of a sharp increase in k2 with increasing organic cosolvent content was observed; destabilization of hydroxide is a significant contributing factor. Solubility measurements were made using the perchlorate salt of the complex ion. These results, in conjunction with the TATB extrathermodynamic assumption, afforded the transfer chemical potentials for the initial state, and in combination with appropriate kinetic data those for the transition state. The reactivity trends are controlled more or less equivalently by an increasing destabilization of the overall initial state and a stabilized transition state with increasing cosolvent for aqueous ethanol, 2-propanol, and tert-butyl alcohol mixtures. Kinetic measurements at pressures up to 1 kbar yielded DELTA-V* of + 16 cm3 mol-1 for dissociation by hydroxide ion in aqueous solution. DELTA-V* is lowered by all cosolvents added, the value declining to + 4 cm3 mol-1 at the highest practicable cosolvent concentration.