[Fe4] and [Fe6] Hydride Clusters Supported by Phosphines: Synthesis, Characterization, and Application in N2 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(mu-H)(4)(mu(3)-H)(2){N(SiMe3)(2)}(2)(PR3)(4) (PR3 = PMe3 (2a), PMe2Ph (2b), PEt3 (2c)), Fe-6(mu-H)(10)(mu(3)-H)(2)(PMe3)(10) (3), and (eta(C7H8)-C-6)Fe-4(mu-H)(2){mu-N(SiMe3)(2)}N(SiMe3)(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(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 [Fe-4] and [Fe-6] clusters was indicated, based on the results of the mass spectrometric analysis on the [Fe-6] duster, as well as the formation of a diphenylsilane adduct of the [Fe-4] cluster.