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

Andrew D. Crawford

doi:10.1002/chem.202502688

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

2025-11-01

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DOI：10.1002/chem.202502688

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An iron(0) dinitrogen complex incorporating highly sterically encumbered bisphosphine ligands, Fe(N2)(dibpe)2 (dibpe = iBu2PCH2CH2PiBu2), iBu1N2, has been prepared and thoroughly characterized, and its N2 fixation reactivity assessed. iBu1N2 is a more hindered analogue of Fe(N2)(depe)2 (depe = Et2PCH2CH2PEt2), Et1N2, 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 iBu1N2 less prone to deleterious side reactivity, improving performance. The N2 ligand in iBu1N2 displays a similar degree of activation to Et1N2, 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 iBu1N2 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]+, [iBu1]+, whose study was aided by isolation as its BArF 4 - salt (ArF = 3,5-bis(trifluoromethyl)phenyl), and which has also been fully characterized. This weak interaction hinders regeneration of the initial iBu1N2 via reduction of [iBu1N2]+, which appears to be necessary to close the catalytic cycle.

Keywords:

ammonia

hydrazine

iron

nitrogen fixation

phosphine ligands

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

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