Deprotonation, Chloride Abstraction and Dehydrohalogenation as Synthetic Routes to bis‐Pyrazolate Pyridyl Iron(II) Complexes

The process of removal of H+ and of Cl-, dehydrohalogenation, from (H₂L)FeCl₂ is investigated systematically, to understand the reactivity of the implied transient LFe(II). Reaction of (H₂L)FeCl₂ with 2 equivalents of LiN(SiMe₃)₂ yields LiClFe₂L₂, as its dimer with every iron 5-coordinate in an FeN₄Cl environment. To avoid Li+ cation derived from base, reaction of Na₂L with FeCl₂ gives a product Na₂[NaFe(HL)(L)]₂(LFeO) from addition of water, which reveals Na/pyrazolate Nβ interactions and a five coordinate oxo group in the OFe₃Na₂ core. Dehydrohalogenation in the presence of Ph₂PC₂H₄PPh₂ gives diamagnetic [LFe(κ²-dppe)]₂(μ-dppe) and LFe(κ²-dppe)(κ¹-dppe). Dehydrohalogenation in the presence of tBuNC gives diamagnetic LFe(CNtBu)₃. This is selectively methylated at both pyrazolate β-N to give LMeFe(tBuNC)₃²+ which is reduced with KC₈ to diamagnetic LMeFe(tBuNC)². Structure determination of these and IR data on isonitrile bands, show L²- to be a stronger donor than LMe. First installing triflate (to avoid the more persistent Cl-) facilitates access to LFe(Lewis base)₃²+ complexes, but this cation shows relatively weak binding of CO to LFeII, which implicates weak π basicity of that d6 species. Production of bis-pincer complexes (H₂L)₂Fe²+ and (LMe)₂Fe²+ in the presence of abundant Lewis base in polar medium is demonstrated, which illustrates a pincer ligand redistribution challenge to be kept in mind when trying to maintain a 1:1 Fe:pincer ratio, for highest reactivity.