Exploring C(sp³)–C(sp³) reductive elimination from an isolable iron metallacycle

A six-coordinate iron metallacyclopentane, (phen)₂Fe(CH₂)₄, supported by two 1,10-phenanthroline (phen) ligands, has been synthesized and structurally and spectroscopically characterized. The complex is diamagnetic and an idealized octahedral geometry was observed in the solid state. The electronic structure of (phen)₂Fe(CH₂)₄ was determined by a combination of X-ray diffraction, Mössbauer spectroscopy, and DFT analyses and is best described as a low-spin Fe(III) center antiferromagnetically coupled to a radical anion delocalized equally over both phen ligands. The reactivity of (phen)₂Fe(CH₂)₄ under different conditions was explored. Thermolysis or photolysis promoted elimination reactions and mixtures of isomeric butenes and butane were observed. Reactions of (phen)₂Fe(CH₂)₄ with ethylene and isoprene yielded 3% and 11% of reductive elimination product cyclobutane, respectively, along with butane and butene isomers. Addition of π-accepting ligands such as carbon monoxide, maleic anhydride, or 1,4-benzoquinone to (phen)₂Fe(CH₂)₄ promoted C(sp³)-C(sp³) reductive elimination as judged by high selectivity for cyclobutane formation. Two electron oxidation of (phen)₂Fe(CH₂)₄ with two equivalents of ferrocenium tetraphenylborate also exclusively yielded cyclobutane in 95% yield. The electronic structure and reactivity of related bis(bipyridine) iron dialkyl compounds previously isolated by Kochi and co-workers were also revisited and their electronic structures revised based on structural, spectroscopic and computational data.