Spectroscopic, electrochemical and reactivity studies of oxoiron and oxoruthenium complexes containing nitrogen atom donor ligands

Abstract

Metal-oxo species constitute an important type of metal-ligand multiple-bonded complexes and play a key role in metal-catalyzed/mediated oxidation reactions including oxidation processes in biological systems. This thesis describes the synthesis, characterization, electrochemistry and reactivity or catalytic properties of a number of iron and ruthenium complexes supported by nitrogen atom donor neutral multidentate ligands, with particular reference to the formation, spectroscopic characterization and reactivity studies of iron- and ruthenium-oxo species including an oxoiron(IV) ligand cation radical species and a cis-dioxoruthenium(VI) complex.

Firstly, a series of iron(III) complexes supported by a combination of neutral tridentate and mono-anionic bidentate ligands were synthesized, and their catalytic properties for oxidation of alkanes with Oxone were examined. [(Me3tacn)FeIII(Cl-acac)Cl]ClO4 (Me3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane; Cl-acac = 3-chloro-acetylacetonate) catalyzed the oxidation of ethane and propane with Oxone at room temperature for 30 min to give oxidized products with total turnover number (TON) of 3.0 and 20.4 respectively; oxidation of cyclohexane afforded cyclohexanol and cyclohexanone with total TON up to 41 for a 5-min reaction. 18O-Labeling experiments, kinetic isotope effects on cyclohexane oxidation, and log krel vs. CH bond dissociation energy plot suggest the involvement of a high-valent oxoiron intermediate. A reaction mechanism on alkane oxidation reminiscent of the Cytochrome P-450 chemistry is proposed, which involves hydrogen atom abstraction followed by rapid oxygen atom rebound. Various spectrometric/spectroscopic studies by ESI-MS, EPR and UV-Vis, and also DFT calculations, on the reaction intermediate point to [(Me3tacn)FeIV({Cl-acac}•+)(O)]2+ as the active species in the catalytic oxidation of hydrocarbons.

Secondly, a new method for the synthesis of ruthenium complexes bearing pyridylmethyl- and/or quinolyl-amine is described. This method has been utilized to synthesize the cis-diaquoruthenium(II) complexes of different chiral ligands including BQCN (N,N-dimethyl-N,N-di(quinolin-8-yl)cyclohexane-1,2-diamine) and its derivative Me2BQCN. Electrochemical studies show that cis-[(PDP)RuVI(O)2]2+ (PDP = 1,1-bis(pyridin-2-ylmethyl)-2,2-bipyrrolidine) is a strong oxidant with Ef = 1.13 V vs. SCE and a strong H-atom abstractor with a calculated DO–H = 90.8 kcal/mol. The catalytic activity of Ru(PDP) complexes toward oxidation of water, alkanes or alkenes was examined. In alkene oxidation with sodium periodate, reaction monitoring by ESI-MS revealed facile C=C bond cleavage. Using cerium(IV) ammonium nitrate as the oxidant, cis-[(PDP)RuII(OH2)2](OTs)2 catalyzed the oxidation of unactivated C–H bonds (e.g. ethane) with bond dissociation energies up to 100.5 kcal/mol.

Finally, the electrochemistry of [(L-N4Me2)FeIIICl2]ClO4 (L-N4Me2 = N,N-dimethyl-2,11-diaza[3.3](2,6)pyridinophane), in both aqueous and non-aqueous media, has been established. In aqueous solution at pH 1, an irreversible oxidation wave at Epa = 1.18 V vs. SCE is attributed to the [(L-N4Me2)FeIV(O)(OH2)]2+/[(L-N4Me2)FeIII(OH)(OH2)]2+ redox process. In non-aqueous medium, [(L-N4Me2)FeIIICl2]ClO4 shows activity as a CO2 reduction electrocatalyst. Electrolysis of [(L-N4Me2)FeIIICl2]ClO4 in CO2-saturated DMF solution produced formate as the major product (70 %) along with small amount of CO (5 %). Extension of the studies to cobalt complexes revealed that electrolysis of [(PDP)CoIICl2] in CO2-saturated acetonitrile produced CO with high current efficiency (up to 96 %). The electrochemistry of Ru(L-N4Me2) complexes has been studied with a vertical comparison to the iron analogs, and [(L-N4Me2)RuIV(O)(OH2)]2+/[(L-N4Me2)RuIII(OH)(OH2)]2+ was observed at E1/2 = 1.03 V vs. SCE (aqueous solution, pH 1).