Binuclear Non-heme Iron Catalysts

Abstract

This project involved the investigation of the ability of non-heme iron metalloenzyme reactivity model complexes to catalyze the oxidation of alkane and arene molecules. The objectives were to synthesize a series of non-heme mononuclear and dinuclear iron complexes, characterize their electronic structure and reactivity properties, characterize intermediates formed during oxygen atom transfer chemistry, and elucidate the mechanisms and specificity of the reactions. These data also were to be compared to analogous heme-based chemistry.A series of diferrous, ferrous/ferric, and diferric complexes were synthesized from simple polyamide and polycarboxylate ligands and characterized by visible absorption, EPR and IR spectroscopy and by cyclic voltametry. The diferrous compounds, which are all powerful reductants, heterolytically decompose peracids and are capable of acting as oxygen atom transfer catalysts when reacted with oxygen atom donor compounds and simple olefins, yielding both allylic oxidation and epoxidation products. Lowtemperature optical spectroscopic studies provide evidence in support of an Fe(IV)=O intermediate, rather than an 0-donor adduct, as the reactive species. An isostructural dicobalt(I1) complex gives rise only to epoxidation products. These reactivity studies provide important new information of the chemistry of dinuclear nonheme iron systems.Publications resulting from this project:1. 2.3. 4."Fe2+-catalyzed heterolytic RO-OH bond cleavage and substrate oxidation: A fhctional synthetic non-heme iron monooxygenase system", Foster TL, Caradonna ' in anhydrous 1% methanol-acetonitrile under anaerobic conditions afforded @?e22+(Hzbamb)2(N-MeIm)2] (1)25a (60% yield). Isothermal distillation techniquesls indicate that 1 has a molecular weight of 4 100, consistent with its dimeric formulation. EPR spectra of 1 show a g H % 16 signal, suggesting a ferromagnetically coupled S = 4 core as previously reported for an analogous binuclear system.I9 Cyclic voltammetry experiments show two coupled le-oxidationheduction processes; a scanrate-dependent quasi-reversible process at -310 mV ("E) and an electrochemically reversible couple at -690 mV ("E), giving K , , = 2.7 x loa. Repetitive scans showed no significant decrease in either cathodic or anodic current; ligand centered redox behavior was observed outside this region of interest. The scan-rate dependence of AEp, attributed to kinetic effects, allowed the measurement of an intrinsic rate constant, k = 2 x assuming pseudo-first-order kinetics. This rate constant, thought to reflect a structural change, is independent of [I] interfaced to a "33% series U integrator. Mass spectral analyses were performed on a HP 5971 mass-selective detector attached to a HPS890 Series 2 GC. Authentic samples were used to verify all retention times. Expected reaction products were stable to GC conditions.(21) Studies using Fa12 and FeCla were performed and analyzed by GC and GUMS techniques using identical conditions as reported for 1-3 (Table 1) with catalyst:OIF'h:sub...