Synthesis and Reactivity of Manganese and Iron Pincer Complexes Towards Hydroboration

In recent years, a growing number of 1st row transition metal complexes have been shown to be highly active for the hydroboration of alkenes and alkynes. The work discussed herein describes a series of Mn and Fe complexes supported by a pyrrole-based pincer ligand, RPNP (PNP = anion of 2,5-bis(dialkylphosphinomethyl)pyrrole, R = t Bu, Cy), and their applications in hydroboration catalysis. Chapter 2 discusses the synthesis of novel manganese complexes of the tBuPNP and CyPNP ligands as potential hydroboration catalysts. Reaction of [MnBr(CO)5] with either Na(CyPNP) or Li(tBuPNP) afforded a series of Mn(I) carbonyl complexes of the type [Mn(CO)n( RPNP)]. The compounds were found to be unreactive toward hydroboranes but did undergo clean ligand dehydrogenation in the presence of benzoquinone. In addition to the Mn(I) complexes, reduction of [Mn(CO)2( tBuPNP)] with KC8 generated a rare example of a stable Mn(0) pincer complex with a metal-based radical. Chapter 3 describes detailed mechanistic studies on the hydroboration of internal alkynes utilizing iron-hydride and iron-boryl complexes supported by tBuPNP. Key species such as [Fe(CHCH2)(tBuPNP)] and [Fe(Bcat)(tBuPNP)], as well as insertion products of alkynes into the Fe-B bond were isolated and fully characterized. Mechanistic studies revealed a pathway for hydroboration involving the iron-hydride as the active species. Chapter 4 details a series of iron-boryl complexes of the CyPNP and their reactivity toward hydroboration, nitrile de-cyanation, and alkene C-H borylation. The structure, spectroscopy, and reactivity of the compounds are presented and compared to related iron species thought to play a role in catalytic hydrofunctionalization protocols.