Developing new, reactive sources of low oxidation state group 9 metal catalysts: Progress towards C-H bond activation and functionalization
Singer, Laci M.
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The use of 1st row transition metals as catalysts in C-H bond activation processes is described. Specifically, we have employed cobalt as the catalytically active metal center in a series of triple-decker arene complexes, which allows access to reactive Co (I) species in solution. Although some of these complexes are known (originally prepared by Schneider and co-workers), we report a vastly improved synthesis and report on their efficacy in catalytic C-H activation processes. The triple-decker complexes are able to dissociate into a monomeric species, providing access to the desired 14-electron Cp* Co (I) species. We have examined the effect of a variety of arenes on catalysis. It was our goal to use the π-system of the arene as an alkene donor, which could then facilitate the loss of the ‘active’ fragment with concomitant regeneration of the aromaticity of the arene. The strong driving force from the added stability of regenerating aromaticity in the arene would encourage this fragmentation and allow the Co (I) species to be accessed in solution. In order to fully examine the catalytic efficacy of the triple-decker complexes, we chose to examine both the scope of substrates tolerated and how varying the catalyst structure could affect the catalysis. By changing the electronic factors in the substrates, one can use the [(Cp*Co)2-μ-( η4-η4-toluene)] complex to facilitate C-H bond activation in environments other than the well studied silyl piperidine and its analogues. Secondly, we modified the catalyst structure by changing the arene both electronically and sterically (relative to the parent toluene complex) to probe how these changes would affect the ability of the arene to dissociate from cobalt and thus promote C-H bond functionalization. Lastly, in preliminary investigations, C-H activation is being studied using an in-situ activation of a Co(II) precatalyst, [Cp*CoCl]2. Finally, I intend to marry the approaches summarized above in a new catalyst architecture: 1) the success of pincer-M (M = Ir, Rh) complexes in C-H bond activation processes make these ligand complexes attractive targets, and 2) the need to transition away from costly precious metals. To achieve this, the preparation of catalytically active Co(I) centers by synthesizing pincer-Co(I) complexes to realize our desire to move towards abundant 1st row metal catalysts has been undertaken. Recently, Heinekey and coworkers reported the preparation of an interesting POCOP-Co(I)(μ2-H2) complex. This complex remains the sole example of a POCOP-Co system yet reported. However, the low oxidation state cobalt(I) was thermally unstable and decomposed even below room temperature.17 Given our own success in preparing thermally robust POCOP-Ir(I)(PR3) complexes, we believe the use of a labile phosphine ligand, in preference to the weakly binding H2 will give rise to a more stable Co(I) species.