Thermally Stable Redox Noninnocent Bathocuproine-Iron Complex for Cycloaddition Reactions

In this work, we aim to formally design iron(0) complexes combined with a phenanthroline-type ligand (phen) and investigate their utility in cycloaddition catalysis. Owing to the strong noninnocence of the phen scaffold, its ligation to reduced iron oxidation states classically affords particularly unstable species. The reported examples of such well-defined coordination complexes are thus particularly scarce. We demonstrate herein that a strategic steric protection of the C4 and C7 positions of the phen ring leads to neutral (N,N)2Fe species, which exhibits an unprecedented thermal and kinetic stability, amenable to its easy use as an in situ generated precursor in catalytic processes. The electronic structure of this noninnocent complex has been fully rationalized, and its promising catalytic activity in alkyne [2 + 2 + 2] cyclizations is discussed. Given its intrinsic thermal stability due to the noninnocent behavior of the (N,N) ligand, (N,N)2Fe appears to be an efficient dormant state of the catalytic process, precluding deactivation of iron as nonreactive aggregates.