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.
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