Metathesis with Iron PCP Complexes

Olefin metathesis is one of the most versatile tools available to chemists. The ability to construct highly substituted alkenes from simple building blocks is highly appealing and has led to a vast number of significant discoveries in nearly every facet of chemistry. Despite this, the desire for olefin metathesis catalysts based on earth-abundant metals is highly appealing to alleviate a number of drawbacks of modern ruthenium catalysts. Iron, in particular, has been of elevated interest to replace ruthenium due to the economic efficiency and low toxicity. Despite this, no iron carbene complexes have shown to afford desired metathesis reactions with olefins.
The Iluc research group has previously synthesized a series of pincer iron carbenes, [{PC(sp2)P}Fe(L)(N2)] ((PC(sp2)P = bis(2-(diisopropylphosphino)phenyl)methylene, L = NCtBu or PMe3), which have shown novel 2p+2p cycloaddition reactivity with alkynes, implicating them as potential catalysts for iron-catalyzed olefin metathesis.
Efforts to expand the reactivity of the pincer iron carbenes towards olefin metathesis were undertaken. The reactivity explored with terminal olefins was consistent with previous results: allyl iron hydrides were primarily obtained by ß-hydride elimination from a metallacyclobutane intermediate. With a-methyl styrene, the metallacyclobutane undergoes a retro-2p+2p cycloaddition instead, which is then followed by a 1,2-hydride migration to afford an iron(0)-olefin complex instead. Despite the metathesis-divergent products, the reactivity observed is novel for iron carbene complexes.
Recent reports implicate that transient iron carbene complexes serve as catalytic species in ring-opening metathesis polymerization of norbornene. As a continuation of the work with terminal olefins, the pincer iron carbenes were reacted with norbornadiene and derivatives, leading to isolable metallacyclobutanes, which were structurally identified. These metallacyclobutanes, upon removal of an ancillary ligand, undergo a retro-2p+2p cycloaddition to afford isolable and structurally identified, ring-opened alkylidenes. These results are the first structurally characterized examples of olefin metathesis with iron, and showcase that iron olefin-metathesis adheres to the Chauvin mechanism, and under certain conditions, displays novel reversibility of the ring-opening process.
Expansion of this cycloaddition chemistry to cyclic diazenes led to the novel formation of imines and iron imido complexes. The iron imido moieties are generally unstable, leading to a variety of side reactions, dependent on the identity of the cyclic diazene. Despite this, the imido moieties can be stabilized by a one-electron oxidation, allowing for structural characterization.