Mechanistic Insight into NN Cleavage by a Low-Coordinate Iron(II) Hydride Complex

Abstract

The reaction pathways of high-spin iron hydride complexes are relevant to the mechanism of N₂ reduction by nitrogenase, which has been postulated to involve paramagnetic iron-hydride species. However, almost all known iron hydrides are low-spin, diamagnetic Fe(II) compounds. We have demonstrated that the first high-spin iron hydride complex, L^tBuFeH (L^tBu = bulky β-diketiminate), reacts with PhNNPh to completely cleave the N−N double bond, giving L^tBuFeNHPh. Here, we disclose a series of experiments that elucidate the mechanism of this reaction. Crossover and kinetic experiments rule out common nonradical mechanisms, and support a radical chain mechanism mediated by iron(I) species including a rare η²-azobenzene complex. Therefore, this high-spin iron(II) hydride can break N−N bonds through both nonradical and radical insertion mechanisms, a special feature that enables novel reactivity.