Mechanistic Studies of First-Row Metal-Mediated and -Catalyzed Reactions

Abstract

First-row transition metals find growing applications in organic transformations due to their abundance and low cost. In addition, the difference in redox states of first-row metals to those of second or third row metals holds promise for unprecedented reactivity. The work in this thesis describes two mechanistic studies of first-row transition metal-mediated and -catalyzed reactions. Discussion throughout the thesis is focused on understanding electronic features of high-valent nickel and low-valent iron in relation to their observed reactivity. The data presented here are anticipated to contribute to the design of first-row metal complex for future applications. Chapter 1 describes a carbon–fluorine reductive elimination from an arylnickel(III) fluoride complex. One strategy for the synthesis of aryl fluorides is carbon–fluorine reductive elimination from high-valent metal centers. While carbon–fluorine reductive elimination from second- or third-row metals is well established, such a process from a first-row metal has not been observed. In chapter 1, the first direct evidence of a carbon–fluorine reductive elimination from a first-row transition metal fluoride complex is presented. Reductive elimination from the described nickel(III) complexes is faster than carbon–fluorine bond formation from any other characterized aryl metal fluoride complex. Throughout the chapter, the stereoelectronic requirements responsible for a facile carbon–fluorine reductive elimination are discussed. Chapter 2 describes the isolation and electronic structure determination of a well-defined, low-valent iron complex which is an active catalyst in the synthesis of cis,cis-1,5-cyclooctadiene (COD) from 1,3-butadiene. Spectroscopic and magnetic characterization establishes a high-spin Fe(I) center, which is supported by DFT studies, where partial metal–ligand antibonding orbital population is proposed to allow for facile ligand exchange during catalysis. Comparison of electronic structures of related iron complexes establishes the role of the spin state with respect to the catalytic activity. A detailed mechanistic study of the diene dimerization reaction was enabled by isolation of a competent catalyst and is also discussed.