Novel iron, molybdenum, and tungsten complexes as models for nitrogenase and other metalloenzymes

Iron(IV) is proposed to be a key oxidation state in many metalloenzymes that catalyze the selective oxidation of hydrocarbons and related compounds using dioxygen. This work reports the designed synthesis of stable, diamagnetic, trigonal bipyramidal iron(IV) complexes starting from either iron(II) or iron(III) chloride. For instance, the reaaion of the tris-thiolate ligand, P(C$\sb6$H$\sb3$-3-SiMe$\sb3$-2-S$\sp-)\sb3$), PS$\sb3\sp{3-}$ (generated in situ from H$\sb3$PS$\sb3$ and Et$\sb3$N) with FeCl$\sb2\cdot$n H$\sb2$O (n = 0 or 4) in CH$\sb3$CN under a dinitrogen atmosphere produces an emerald green solution which contains (Fe$\sp{\rm II}$(PS$\sb3$)) $\sp-.$ Following removal of CH$\sb3$CN and addition of CH$\sb2$Cl$\sb2$ or CHCl$\sb3$, the color changes to brown. This brown material can be oxidized using ferrocenium ion or air (dioxygen) to produce an intense purple-colored complex, (Fe$\sp{\rm IV}$Cl(PS$\sb3$)), which is the first diamagnetic iron(IV) complex and the first trigonal bipyramidal iron(IV) complex to be structurally characterized. A series of experiments have led to the conclusion that the reaction sequence that produces (Fe$\sp{\rm IV}$Cl(PS$\sb3$)) from FeCl$\sb2$ involves: (1) ligand coordination to form (Fe$\sp{\rm II}$(PS$\sb3$)) $\sp-$, (2) chlorine-atom abstraction from chlorinated solvent to form (Fe$\sp{\rm III}$Cl(PS$\sb3$)) $\sp-$, and (3) oxidation to form (Fe$\sp{\rm IV}$Cl(PS$\sb3$)). Recent reactions indicate the existence of a competing reaction sequence, which involves (1) ligand coordination to form (Fe$\sp{\rm II}$(PS$\sb3$)) $\sp-$, (2) addition of the anion X$\sp-$ from (Et$\sb3$NH) X, formed as a co-product in the initial ligand coordination step, to form (Fe$\sp{\rm II}$(X)(PS$\sb3$)) $\sp{2-}$, and (3) oxidation to form (Fe$\sp{\rm IV}$X(PS$\sb3$)) where X = Br, Cl. A more general synthesis of complexes of the type (Fe$\sp{\rm IV}$(X)(PS$\sb3$)) has been designed, which includes: (1) coordination of the ligand to Fe$\sp{\rm III}$Cl$\sb3$ in CH$\sb3$CN to produce (Fe$\sp{\rm III}$(PS$\sb3$)), (2) reaction with Et$\sb4$NX (X = Cl, CN) to form (Fe$\sp{\rm III}$(X)(PS$\sb3)\rbrack\sp-$, and finally (3) oxidation using ferrocenium ion or air (dioxygen) to form the stable (Fe$\sp{\rm IV}$(X)(PS$\sb3$)). In this dissertation, the mechanistic details of these reactions, the identity of iron-containing co-products, and the physical and chemical properties of the complexes are discussed