Mononuclear five- and six-coordinate iron hydrazido and hydrazine species

This article describes the synthesis and characterization of several low-spin iron(II) complexes that coordinate hydrazine (N₂H ₄), hydrazido (N₂H₃^-), and ammonia. The sterically encumbered tris(di-meta-terphenylphosphino)borate ligand, [PhBP^mter₃]^-, is introduced to provide access to species that cannot be stabilized with the [PhBP^Ph ₃]^- ligand ([PhBP^R₃]^- = PhB(CH₂PR₂)₃^-). Treatment of [PhBP^mter₃]FeMe with hydrazine generates the unusual 5-coordinate hydrazido complex [PhBP^mter₃] Fe(η²-N₂H₃) (1), in which the hydrazido serves as an L₂X-type ligand. Upon coordination of an L-type ligand, the hydrazido shifts to an LX-type ligand, generating [PhBP^mter ₃]Fe(L)(η²-N₂H₃) (L = N ₂H₄ (2) or NH₃ (3)). In contrast, treatment of [PhBP^Ph₃]FeMe with hydrazine forms the adduct [PhBP ^Ph₃]Fe(Me)(η²-N₂H₄) (5). Complex 5 is thermally unstable to methane loss, generating intermediate [PhBP^Ph₃]Fe(η²-N₂H₃), which undergoes bimolecular coupling to produce {[PhBP^Ph ₃]Fe}₂(μ-η¹:η¹-N ₂H₄)(μ-η²:η²-N ₂H₂). The oxidation of these and related hydrazine and hydrazido species is also presented. For example, oxidation of 1 or 5 with Pb(OAc)₄ results in disproportionation of the N₂H _x ligand (x = 3, 4), and formation of [PhBP^R ₃]Fe(NH₃)(OAc) (R = Ph (9) and mter (11)).