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

Abstract

The process of removal of protons and chloride, dehydrohalogenation, from [(H2L)FeCl2] is investigated systematically, to understand the reactivity of the implied transient LFeII. Reaction of [(H2L)FeCl2] with 2 equiv. of LiN(SiMe3)2 yields the “‐ate” complex LiClFe2L2, as its dimer with every iron five‐coordinate in an FeN4Cl environment. To avoid Li+ cation derived from LiN(SiMe3)2, reaction of Na2L with FeCl2 gives a product from addition of water, paramagnetic Na2[NaFe(HL)(L)]2(LFeO), which reveals Na/pyrazolate Nβ interactions and a five coordinate oxo group in the OFe3Na2 core of this aggregate. Abstraction of chloride in [(H2L)FeCl2] with NaBArF4 in THF gives paramagnetic [(H2L)Fe(THF)3]2+, which fails to react with CO. Dehydrohalogenation in the presence of Ph2PC2H4PPh2, dppe, gives both [LFe(κ2‐dppe)]2(µ‐dppe)] and [LFe(κ2‐dppe)(κ1‐dppe)], diamagnetic saturated species, which can be separated by pentane extraction. Dehydrohalogenation in the presence of tBuNC gives diamagnetic [LFe(CNtBu)3]. This is selectively methylated at both pyrazolate β‐nitrogen atom to give [LMeFe(tBuNC)3]2+ which shows rich cyclic voltammetry, and which is reduced, with KC8, to diamagnetic [LMeFe(tBuNC)2]. Structure determination of some of these, together with IR data on isocyanide stretching frequencies, show L2– to be a stronger donor than LMe. First installing triflate (to avoid the more persistent chloride ligand) facilitates access to [LFe(Lewis base)3]2+ complexes, but this cation still shows relatively weak binding of CO to LFeII, which implicates weak π basicity of that d6 species. Production of paramagnetic bis‐pincer complexes [(H2L)2Fe]2+ and [(LMe)2Fe]2+ 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.