Ligand Design Principles to Enable New Reactivity with Iron and Cobalt

Abstract

Cobalt(II) dichloride complexes supported by a variety of neutral, tridentate pincer ligands were prepared and investigated for catalytic borylation. Stoichiometric experiments with HBPin and B2Pin2 provided insight into the nature and kinetic stability of the catalytically relevant species. When sufficiently electron donating pincers are present, Co(III) resting states were preferred and catalytic C-H borylation was efficient. Reducing the donating ability of the ligand disfavored C-H and B-H oxidative addition.

A new route to single component iron ethylene polymerization catalysts is described. Treatment of iron butadiene complexes with B(C6F5)3 generated betaine compounds which are active catalysts without the need for an MAO activator. Hemilability of the pendant donor ligand is demonstated to play a role in catalyst deactivation. MAO is demonstrated to enable [PNN] iron oligomerization catalysis in a unique fashion.

A family of bis(phosphino) pyridine iron(II) complexes relevant to cross-coupling is reported. Transmetallation with lithium reagents modify the chelate via deprotonation of the benzylic positions on the ligand. C-C reductive eliminatin was found to be disfavored upon thermolysis of (iPrPNP)Fe(Ph)2. Ligand induced reductive elimination was found to be ineffective for C-C bond formation. Chemical oxidation of (iPrPNP)Fe(Ar)2 yielded the desired C-C bond formation suggesting a preference for iron(III) in the bond forming process.

Hemi-labile phopshino bis-imine [PNN] chelates have been investigated for catalytic hydrofunctionalization chemistry with iron and cobalt. In iron catalysis, chelate arm dissociation was implicated in deactivation pathways and only hydrogenation was found comparable to [PDI] in activity. Introduction of a hemi-labile arm enabled cobalt catalyzed hydrogenation of trisubstituted olefins, reportedly an inaccessible class of olefins for [PDI] catalysts. Hydrosilylation suffered from competitive olefin isomerization pathways for both cobalt and iron. Cobalt alkyls were found to be effective pre-catalysts for the isomerization-hydroboration of olefins.

Model compounds were prepared to investigate intermediates in bis(imino)pyridine iron catalyzed polymerization of ethylene. The solid state structure of [(MePDI)Fe(OC(Ph)2(2-C3H5))][BArF24] was collected and afforded a thorough investigation of the compounds electronic structure. The electronic structure is best described as a neutral chelate associated to a high spin ferrous ion and demonstrates that the redox state of the chelate does not change upon olefin coordination.