Investigating Cyclometalation of Bis(imino)pyridine Cobalt Methyl Complexes

Abstract

Replacing costly precious metal catalysts with abundant base metal analogs has far-reaching implications across many different sectors of chemistry. Our lab has recently been exploring the high activity of bis(imino)pyridine (PDI) iron and cobalt catalysts. Initial data from hydrogenation, hydroboration, and hydrosilylation studies suggests that cyclometalation of the metal center with the PDI ligand could be an important competitor to catalysis. Thus, a range of cobalt methyl complexes were synthesized and a protocol was developed for measuring the rate of cyclometalation across a variety of pre-catalysts. A combination of \(^{1}\)H NMR and quantitative \(^{13}\)C NMR studies of the complexes after exposure to four atmospheres of deuterium identified two valid measures of cyclometalation rate, methane-d\(_{0}\) release and a combination of quantitative \(^{13}\)C and \(^{1}\)H NMR integrations. The resulting ranking of cyclometalation rate was: (\(^{Me}\)PDI)CoD < (4-Me\(_{2}\)N-\(^{iPr}\)PDI)CoD < (\(^{iPr}\)PDI)CoD ~ (\(^{tric}\)PDI)CoD ~ (\(^{iPr}\)EtPDI)CoD < (\(^{Et}\)PDI)CoD < (\(^{iPr(Tb)}\)PDI)CoD, indicating that there is no simple predictor such as steric bulk or electron-donating ability for cyclometalation rate. The cyclometalation of (\(^{tric}\)PDI)CoD was investigated more fully to reveal that deuteration occurs in a facially selective manner, exchanging solely with the hydrogen atoms adjacent to the methine position, but on the opposite face of the cyclopentyl ring from the methine. This suggests that converting primary C-H bonds into secondary C-H bonds may not have been an effective strategy for decreasing cyclometalation, but that decreasing the flexibility of the catalyst towards bond rotation may have the desired effect.