Investigating N₂ Fixation Using a Bulky Fe(bisphosphine)₂ Framework

An iron(0) dinitrogen complex incorporating highly sterically encumbered bisphosphine ligands, Fe(N₂)(dibpe)₂ (dibpe = iBu₂PCH₂CH₂PiBu₂), ^{i Bu}1·N₂, has been prepared and thoroughly characterized, and its N₂ fixation reactivity assessed. ^{i Bu}1·N₂ is a more hindered analogue of Fe(N₂)(depe)₂ (depe = Et₂PCH₂CH₂PEt₂), ^Et1·N₂, which has previously been shown to be an efficient N₂ reduction catalyst with unusual selectivity for N₂H₄, and it was anticipated that greater bulk might make ^{i Bu}1·N₂ less prone to deleterious side reactivity, improving performance. The N₂ ligand in ^{i Bu}1·N₂ displays a similar degree of activation to ^Et1·N₂, and the two complexes can stoichiometrically fix N₂ with similarly high efficiency upon treatment with suitable acids, giving mixtures of NH₃ and N₂H₄. However, attempts to catalytically fix N₂ via treatment of ^{i Bu}1·N₂ with mixtures of excess acids and reductants led to poor results. Mechanistic investigations implicate a combination of more sluggish reaction kinetics and weaker binding of N₂ to the intermediate Fe(I) cation [Fe(dibpe)₂]⁺, [^{i Bu}1]⁺, whose study was aided by isolation as its BAr^F ₄ ^- salt (Ar^F = 3,5-bis(trifluoromethyl)phenyl), and which has also been fully characterized. This weak interaction hinders regeneration of the initial ^{i Bu}1·N₂ via reduction of [^{i Bu}1·N₂]⁺, which appears to be necessary to close the catalytic cycle.