Coordination chemistry of mu-oxo diiron(III) compounds containing polyimidazole/pyridine ligands: Inorganic and bioinorganic perspectives

This study has attempted to correlate the physical and electronic properties of several mononuclear, and symmetric and unsymmetric dinuclear oxo-bridged iron(III) complexes with the steric and electronic properties of a series of imidazole/pyridine tetradentate tripod ligands: tris((1-methylimidazolyl)-methyl) amine (tmima), bis((1-methylimidazole)(2-pyridyl)-methyl) amine (bipa), bis((2-pyridyl)(1-methylimidazole)-methyl) amine (bpia) and tris(2-pyridylmethyl)amine (tpa). In addition, a new sterically hindered polyimidazole ligand, tris((1-ethyl-4-methylimidazol-2-yl)methyl) amine (temima), has been prepared and found to stabilize unsymmetrical oxo-bridged diiron(III) complexes. The {dollar}mu{dollar}-oxo diiron(III) unit is a structural unit found in a number of non-heme metalloproteins. The iron(III) complexes described herein have been characterized by X-ray crystallography, UV-visible and {dollar}sp1{dollar}H NMR spectroscopy, variable-temperature magnetism, and electrochemical methods.; All of the dinuclear compounds display strong antiferromagnetic exchange coupling. The dinuclear oxo iron complexes display broad proton signals between 4-40 ppm, depending on the nature of the tripod ligand, and the magnitude of the antiferromagnetic exchange interaction. Several of the compounds displayed quasireversible electrochemical behavior. The Fe(III)/Fe(II) redox couples for the mononuclear compounds, (Fe(L)Cl{dollar}sb2rbracksp{lcub}+{rcub}{dollar} (where L = tmima, bipa, bpia, and tpa) shift progressively to more cathodic potentials. Several of the unsymmetrical oxo bridged diiron(III) complexes displayed quasireversible electrochemical behavior, and the symmetrical oxo bridged complexes were found to electrochemically irreversible.; We studied the correlation between ligand properties and metal coordination environment in oxo-bridged diiron complexes, and how the steric and electronic properties of the ligand control the coordination environment of the metal. Ligands containing pyridines provide a stronger ligand field than ligands containing imidazoles. Due to the different steric bulk of the two aromatic rings, the coordination environments of the two irons are changing from symmetric to asymmetric along the series.; We successfully applied this knowledge to the ligand design of temima in the process of synthesizing coordination asymmetric ({dollar}mu{dollar}-oxo)diiron(III) metalloprotein model compound. Temima's diiron(III) complex serves as a synthon to a series of mixed-ligand compounds. Two of these complexes are the first examples of a ({dollar}mu{dollar}-oxo)({dollar}mu{dollar}-carboxylato) diiron(In{9B}) complex containing two different polyimidazole/pyridine ligands and display many of the structural and physical properties reported for non-heme diiron proteins. We have also developed a general step-wise synthetic procedure for preparing unsymmetrical {dollar}mu{dollar}-oxo diiron(III) complexes by exploiting the steric properties of a new tetradentate polyimidazole ligand. (Abstract shortened by UMI.)...