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|Title:||Structure and Reactivity of Paramagnetic Iron Dinitrosyl Complexes|
|Authors:||Bryar, Traci R.|
|Abstract:||<p>Paramagnetic iron dinitrosyl complexes have been investigated since 1968. The compounds are not easy to isolate and the majority of studies have concentrated on solution EPR measurements. As a result ambiguities remain, which concern both the geometries and electronic structures of these complexes. More recently, their catalytic properties in reactions involving the polymerization and oligimerization of olefinic compounds have been reported. Questions remain regarding the effectiveness of the complexes as catalysts, the mechanism of catalysis and the variation in catalytic activity with changes in the ligands.</p> <p>This thesis reports research aimed at clarifying both the structural and catalytic chemistry of this class of compounds. The major tool used in the structural work is solid state EPR spectroscopy, although this has been supplemented by infra-red spectroscopy and X-ray crystallography. The conclusions reached favour a description of the electronic configuration which differs from that adopted by most previous researchers. The emphasis in the catalytic work has been the determination of the mechanism of polymerization. The effect of electron-withdrawing or -donating substituents on the rate of polymerization was studied. In addition, the tacticity of the polymeric products was characterized using high resolution ¹³C NMR spectroscopy. Different microstructures are predicted for polymers produced using a metal-coordinated catalyst or using free-radical, anionic or cationic initiators.</p> <p>It was determined that the paramagnetic Fe(NO)₂ compounds are best described as 17-electron complexes with a d⁹, rather than a d⁷, electronic configuration on iron and a distorted tetrahedral geometry. The nature of the distortion is dependent on the type of ligands bonded to iron. When the Iigands include hard, nonpolarizable donor atoms such as oxygen or fluoride, the complex distorts towards a square-planar geometry. The spin-containing molecular orbital is predominantly dᵪ²y². When atoms such as sulfur and phosphorus are bonded to iron the geometry of the complex can be described as a trigonal bipyramid with one axial ligand missing. The dz² orbital now contains the unpaired electron. When the ligands are halides or N-bonded species, the distortions from tetrahedral are less extreme and the spin-containing MO is comprised of a mixture of d orbitals. The crystal structure of [Fe(NO)₂I₂]⁻ has been determined to be tetrahedral with a slight distortion towards the tbp geometry. This is the first example of an unrestricted complex with a tetrahedral geometry. The factors influencing these structural changes will also be discussed.</p> <p>The paramagnetic Fe(NO)₂ complexes are labile in solution and one complex can often be converted to another simply by addition of excess ligand to the reaction mixture. The reactivity of these compounds is dependent on the availability of a vacant coordination site on iron, therefore the more labile complexes are more reactive, while complexes missing one ligand have significantly greater reactivity than those which are four-coordinate. The paramagnetic dinitrosyl complexes effectively initiate the polymerization of styrene. The efficiency of initiation does not depend markedly on the geometry of the complex. A mechanism is suggested in which the alkene coordinates to iron; this is followed by electron transfer which formally reduces the iron. This process creates an organic ligand which bears a formal positive charge and which subsequently participates in a cationic polymerization mechanism. This mechanism is unusual in that a paramagnetic complex is involved, but the mechanism is not a free radical one. Although the 17-electron complexes react quite well with alkenes they do not react with cyclooctatetraene or dienes such as norbomadiene and isoprene.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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