Tuning Nickel Electronics and Hydrogenation Reactivity with Rare Earth Metalloligands

Abstract

Industrially, many chemical transformations require the use of expensive precious metal catalysts to proceed. A major chemical pursuit aims at replacing these expensive metals with inexpensive, Earth-abundant transition metals. Unfortunately, Earth-abundant transition metals are often poor catalysts for challenging multi-electron processes. One strategy to circumvent this problem makes use of σ-accepting (or Z-type) ligands to control the electronic characteristics and reactivity of a metal center. However, a heavy focus on main group metals within this field has yielded a lack of diversity in the metals employed as Z-type ligands. In this vein, this dissertation investigates the use of rare earth metals as Z-type ligands to promote homogenous transition metal catalysis. A series of nickel–rare earth (Sc, Y, lanthanides) heterobimetallic complexes were synthesized using new phosphinoamide ligands. The complexes were characterized using a suite of spectroscopic, electrochemical, and computational methods. The electronic effects of the rare earth supporting metals poised the Ni metal center for the hydrogenation of olefins to alkanes as well as alkynes to (E)-alkenes. Furthermore, it was found that altering the coordination sphere of the rare earth support significantly impacts the resulting properties and catalytic activity of the active Ni metal center. By quantitatively comparing structure, redox properties, and mechanistic intermediates, the effects of the supporting metal on the Ni electronics, catalytic activity, and kinetics of the Ni−M complexes were elucidated. Collectively, this work demonstrates that modulating a transition metal center via an appended rare earth support metal can favorably alter the properties of inexpensive metals, thus promoting a new reactivity paradigm in homogenous transition metal catalysis.