Reactivity of Bis(imino)pyridine Cobalt Complexes in C-H Bond Activation and Catalytic C-C and C-Si Bond Formation

Abstract

The reactivity of aryl-substituted bis(imino)pyridine (ArPDI = 2,6-(Ar=NCMe)2C5H3N) cobalt complexes was explored in C-H activation and catalytic C-C and C-Si bond formation. Thermolysis of cobalt azide compounds, in attempts to generate a cobalt nitride complex, resulted in formation of cyclometallated amide and imine products from the insertion of the putative nitride into the benzylic C-H bond of the alkyl group on the imine aryl substituent. Reaction of cobalt dinitrogen complexes with O2 or N2O also failed to furnish the corresponding cobalt oxo compound but resulted in formation of a cobalt hydroxide complex and a cobalt dinitrogen complex where one of the imine methyl groups has lost an equivalent of hydrogen. This reactivity is attributed to the presence of unpaired electrons in the partially filled pi* orbitals of the Co=X (X = N or O) moiety.

The synthesis and characterization of cationic (ArPDI)Co alkyl complexes were targeted to model the active species in the (ArPDI)CoCl2/MAO-catalyzed (MAO = methylaluminoxane) ethylene polymerization. Experimental and computational studies established the electronic structure of this class of compounds as a low-spin Co(II) with a neutral chelate. Each of the cationic cobalt alkyl complexes prepared was an active single-component catalyst for ethylene polymerization and exhibited significantly improved activities over cationic cobalt(I) compounds previously proposed as the active species in the reaction.

A series of (ArPDI)Co compounds was evaluated for catalytic C-Si bond formation and found to be active pre-catalysts for the dehydrogenative silylation of terminal olefins to generate allylsilanes. Using (MesPDI)CoMe (MesPDI = 2,6-(2,4,6-Me3-C6H2N=CMe)2C5H3N) as representative pre-catalyst, it was demonstrated that linear internal olefins can be isomerized and silylated to the terminal allylsilane. This type of reactivity was utilized in the crosslinking of liquid polysilylhydrides with -olefins to produce solid gels. Mechanistic investigations established that the pre-catalyst is activated by the silylhydride to generate a cobalt silyl complex. The olefin then inserts into the Co-Si bond and subsequent beta-hydrogen elimination liberates the allylsilane product. The resulting cobalt hydride reacts with a second equivalent of olefin and forms a cobalt alkyl which in turn reacts with the silylhydride to regenerate the cobalt-silyl complex.