Protonated metallocenes as extremely reactive PCET reagents

While the cyclopentadienyl ring has a venerable history in the development of organometallic chem., it is most commonly viewed as a robust ancillary ligand that does not itself participate in reaction chem. While there are of course many exceptions to this rule, an emerging and fascinating type of reactivity that is relevant to catalytic redns. of small mol. substrates (e.g., nitrogen) involves the in situ protonation of a metallocene ring to generate a highly reactive C-H bond. Such protonated metallocenes can serve as remarkably strong PCET reagents characterized by BDFE's well below 35 kcal/mol. Because of this, they afford an opportunity to initiate challenging redns. where the kick-off step is the generation of a new and very weak E-H bond (e.g., M(NNH)). Our lab has been exploring the role of decamethylcobaltocene as a participant in this type of reactivity in the context of Fe-mediated nitrogen fixation, and as such we have spent considerable effort characterizing the protonated form of this well-known metallocene reductant. In this talk I will describe our recent findings relating to spectroscopic characterization, exptl. and theor. elucidating phys. parameters relating to the BDFE values of the reactive C-H bonds formed upon metallocene protonation, and the reactivity patterns of such species