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

ABSTRACT An iron(0) dinitrogen complex incorporating highly sterically encumbered bisphosphine ligands, Fe(N 2 )(dibpe) 2 (dibpe = i Bu 2 PCH 2 CH 2 P i Bu 2 ), i Bu 1·N 2 , has been prepared and thoroughly characterized, and its N 2 fixation reactivity assessed. i Bu 1·N 2 is a more hindered analogue of Fe(N 2 )(depe) 2 (depe = Et 2 PCH 2 CH 2 PEt 2 ), Et 1·N 2 , which has previously been shown to be an efficient N 2 reduction catalyst with unusual selectivity for N 2 H 4 , and it was anticipated that greater bulk might make i Bu 1·N 2 less prone to deleterious side reactivity, improving performance. The N 2 ligand in i Bu 1·N 2 displays a similar degree of activation to Et 1·N 2 , and the two complexes can stoichiometrically fix N 2 with similarly high efficiency upon treatment with suitable acids, giving mixtures of NH 3 and N 2 H 4 . However, attempts to catalytically fix N 2 via treatment of i Bu 1·N 2 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 N 2 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·N 2 via reduction of i Bu 1·N 2 + , which appears to be necessary to close the catalytic cycle.