Development of homogeneous first row metal catalysts (Fe, Mn, Co) for organic transformations and bond activation

ABSTRACT

Transition metals have been extensively employed to address various challenges

related to catalytic organic transformations, small molecule activation, and energy storage

over the last few decades. Inspired by recent catalytic advances mediated by redox noninnocent

pyridine diimine (PDI) and α-diimine (DI) ligand supported transition metals,

our group has designed new PDI and DI ligands by modifying the imine substituents to

feature donor atoms. My doctoral research is focused on the development of PDI and DI

ligand supported low valent first row metal complexes (Mn, Fe, Co) and their application

in bond activation reactions and the hydrofunctionalization of unsaturated bonds.

First two chapters of this dissertation are centered on the synthesis and

application of redox non-innocent ligand supported low valent iron complexes. Notably,

reduction of a DI-based iron dibromide led to the formation of a low valent iron

dinitrogen compound. This compound was found to undergo a sequential C-H and C-P

bond activation processes upon heating to form a dimeric compound. The plausible

mechanism for dimer formation is also described here.

Inspired by the excellent carbonyl hydrosilylation activity of our previously

reported Mn catalyst, (Ph2PPrPDI)Mn, attempts were made to synthesize second generation

Mn catalyst, which is described in the third chapter. Reduction of (PyEtPDI)MnCl2

furnished a deprotonated backbone methyl group containing Mn compound

[(PyEtPDEA)Mn] whereas reduction of (Ph2PEtPDI)MnCl2 produced a dimeric compound,

[(Ph2PEtPDI)Mn]2. Both compounds were characterized by NMR spectroscopy and XRD

analysis. Hydrosilylation of aldehydes and ketones have been studied using

[(PyEtPDEA)Mn] as a pre-catalyst. Similarly, 14 different aldehydes and 6 different

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formates were successfully hydrosilylated using [(Ph2PEtPDI)Mn]2 as a pre-catalyst.

Encouraged by the limited number of cobalt catalysts for nitrile hydroboration, we

sought to develop a cobalt catalyst that is active for hydroboration under mild conditions,

which is discussed in the last chapter. Treatment of (PyEtPDI)CoCl2 with excess NaEt3BH

furnished a diamagnetic Co(I) complex [(PyEtPDIH)Co], which exhibits a reduced imine

functionality. Having this compound characterized, a broad substrate scope for both

nitriles and imines have been investigated. The operative mechanism for nitrile

dihydroboration has been investigated based on the outcomes of a series of stoichiometric

reactions using NMR spectroscopy.