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 (N_2H_4), hydrazido (N_2H_3^–), and ammonia. The sterically encumbered tris(di-meta-terphenylphosphino)borate ligand, [PhBP^(mter)_3]^−, is introduced to provide access to species that cannot be stabilized with the [PhBP^(Ph)_3]^− ligand ([PhBP^R_3]^− = PhB(CH_2PR_2)_3^–). Treatment of [PhBP^(mter)_3]FeMe with hydrazine generates the unusual 5-coordinate hydrazido complex [PhBP^(mter)_3]Fe(η^2-N_2H_3) (1), in which the hydrazido serves as an L_2X-type ligand. Upon coordination of an L-type ligand, the hydrazido shifts to an LX-type ligand, generating [PhBP^(mter)_3]Fe(L)(η^2-N_2H_3) (L = N_2H_4 (2) or NH_3 (3)). In contrast, treatment of [PhBP^(Ph)_3]FeMe with hydrazine forms the adduct [PhBP^(Ph)_3]Fe(Me)(η^2-N_2H_4) (5). Complex 5 is thermally unstable to methane loss, generating intermediate [PhBP^(Ph)_3]Fe(η^2-N_2H_3), which undergoes bimolecular coupling to produce {[PhBP^(Ph)_3]Fe}_2(μ-η^1:η^1-N_2H_4)(μ-η^2:η^2-N_2H_2). 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 N_2H_x ligand (x = 3, 4), and formation of [PhBP^R_3]Fe(NH_3)(OAc) (R = Ph (9) and mter (11))