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), iBu1·N2, has been prepared and thoroughly characterized, and its N2 fixation reactivity assessed. iBu1·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 iBu1·N2 less prone to deleterious side reactivity, improving performance. The N2 ligand in iBu1·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 iBu1·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]+, [iBu1]+, whose study was aided by isolation as its BArF4− salt (ArF = 3,5-bis(trifluoromethyl)phenyl), and which has also been fully characterized. This weak interaction hinders regeneration of the initial iBu1·N2 via reduction of [iBu1·N2]+, which appears to be necessary to close the catalytic cycle.