Abstract
An iron(0) dinitrogen complex incorporating highly sterically encumbered bisphosphine ligands, Fe(N(2))(dibpe)(2) (dibpe = iBu(2)PCH(2)CH(2)PiBu(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.