Characterization of the Electronic and Molecular Structures of Transition Metal Semiquinone and Catecholate Complexes

252 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1981.Quinones interact with transition metal ions in biological electron transfer processes such as photosynthesis and the respiratory electron transport chain. Two types of quinones exist, paraquinones and orthoquinones, however, only transition metal-orthoquinone complexes were studied. Orthoquinones are only known to bond to metals in a (sigma)-fashion through the oxygens. As a family of ligands, orthoquinones utilize three redox active forms when bonding to metals: {M(O)(quinone)}, {M(I) (semiquinone)}, and {M(II)(catecholate)}. The difference between these three formulations is related to the residence of electrons in either metal or ligand orbitals.Several new metal-quinone complexes were synthesized utilizing manganese, iron, cobalt, and nickel and the following two ligands: 9,10-phenanthrenequinone, (PhenQ, PhenSQ, and PhenCAT), and 3,5-di-tert-butyl-1,2-benzoquinone, (DBQ, DBSQ, and DBCAT). These complexes were studied by a variety of physical techniques, and these conclusions were drawn: (1) the semiquinone is a paramagnetic radical which can be stabilized by coordination to a metal, and (2) the electron distribution within these complexes is very sensitive to the specifics of the metal, quinone, and other ligands, to temperature, and to solvent media.An x-ray crystal structure was solved for one of the nickel complexes, {Ni(PhenSQ)(,2)(Py)(,2)}, and indicated that the molecule has a divalent nickel ion and coordinated semiquinone ligands. Magnetic susceptibility data for this complex, as well as other analogous nickel and cobalt complexes, support the presence of the semiquinone radical and suggest that an intramolecular antiferromagnetic exchange interaction is occurring. This interaction between the metal ion and semiquinones was gauged by a theoretical model.Two manganese complexes were studied, {Mn(,4)(DBSQ)(,8)} and {Mn(DBCAT)(,2)(Py)(,2)}, and their x-ray crystal structures are presented. They clearly illustrated by the differences in bond lengths within the quinone ligand and about the manganese ion that {Mn(DBCAT)(,2)(Py)(,2)} is a {Mn(IV)bis(catecholato)} species. The latter complex exhibits interesting thermochromic behavior when in solution. This behavior was characterized via both epr and electronic absorption spectroscopies.Several ion complexes of the following compositions were studied: {Fe(DBSQ)(DBCAT)(nitrogen base)} and {Fe(PhenSQ)(PhenCAT)(nitrogen base)}, where nitrogen base is bipyridine, phenanthrolene, ethylenediamine, pyridine, and dimethylbipyridine. ('57)Fe Mossbauer studies indicated the presence of high-spin ferric ions. The mixed-valence iron(III)(semiquinone)(catecholato) character of these complexes was substantiated by the observance of an intervalence transfer band in the electronic absorption spectra. {Fe(PhenSQ)(PhenCAT)(Bipy)}, when in solution, exhibits unusual solvent dependent behavior. When in solvents of low dielectric constant, the electronic absorption spectra show an intervalence transfer band while when in solvents of high dielectric constant, a dramatically different spectra is obtained. This change may be likened to the thermochromic behavior of the aforementioned manganese complex.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD