[Fe 4 ] and [Fe 6 ] Hydride Clusters Supported by Phosphines: Synthesis, Characterization, and Application in N 2 Reduction.

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 N ₂ . 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 [Fe ₄ ] and [Fe ₆ ] clusters Fe ₄ (μ-H) ₄ (μ ₃ -H) ₂ {N(SiMe ₃ ) ₂ } ₂ (PR ₃ ) ₄ (PR ₃ = PMe ₃ (2a), PMe ₂ Ph (2b), PEt ₃ (2c)), Fe ₆ (μ-H) ₁₀ (μ ₃ -H) ₂ (PMe ₃ ) ₁₀ (3), and (η ⁶ -C ₇ H ₈ )Fe ₄ (μ-H) ₂ {μ-N(SiMe ₃ ) ₂ } ₂ {N(SiMe ₃ ) ₂ } ₂ (4), which were characterized crystallographically and spectroscopically. Under ambient conditions, these clusters catalyzed the silylation of N ₂ to furnish up to 160 ± 13 equiv of N(SiMe ₃ ) 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 [Fe ₄ ] and [Fe ₆ ] clusters was indicated, based on the results of the mass spectrometric analysis on the [Fe ₆ ] cluster, as well as the formation of a diphenylsilane adduct of the [Fe ₄ ] cluster.