[Fe₄] and [Fe₆] Hydride Clusters Supported by Phosphines: Synthesis, Characterization, and Application in N₂Reduction

Multiple iron atoms bridged by hydrides is a common structural feature of the active species that have been postulated in the biological and industrial reduction of N₂. In this study, the reactions of an Fe(II) amide complex with pinacolborane in the presence/absence of phosphines afforded a series of hydride-supported [Fe₄] and [Fe₆] clusters Fe₄(μ-H)₄(μ₃-H)₂{N(SiMe₃)₂}₂(PR₃)₄ (PR₃ = PMe₃ (2a), PMe₂Ph (2b), PEt₃ (2c)), Fe₆(μ-H)₁₀(μ₃-H)₂(PMe₃)₁₀ (3), and (η⁶-C₇H₈)Fe₄(μ-H)₂{μ-N(SiMe₃)₂}₂{N(SiMe₃)₂}₂ (4), which were characterized crystallographically and spectroscopically. Under ambient conditions, these clusters catalyzed the silylation of N₂ to furnish up to 160 ± 13 equiv of N(SiMe₃)₃ per 2c (40 equiv per Fe atom) and 183 ± 18 equiv per 3 (31 equiv per Fe atom). With regard to the generation of the reactive species, dissociation of phosphine and hydride ligands from the [Fe₄] and [Fe₆] clusters was indicated, based on the results of the mass spectrometric analysis on the [Fe₆] cluster, as well as the formation of a diphenylsilane adduct of the [Fe₄] cluster.