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

• Published in: Journal of the American Chemical Society Vol. 129; no. 26; pp. 8112 - 8121
• Main Authors: Sadique, Azwana R, Gregory, Elizabeth A, Brennessel, William W, Holland, Patrick L
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 04-07-2007
• Subjects: Iron - chemistry; Kinetics; Models, Molecular; Molecular Structure; Nitrogen - chemistry; Nitrogenase - chemistry; Nitrogenase - metabolism
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja069199r

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, LtBuFeH (LtBu = bulky β-diketiminate), reacts with PhNNPh to completely cleave the N−N double bond, giving LtBuFeNHPh. 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.