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 [(H$_2$L)FeCl$_2$] is investigated systematically, to understand the reactivity of the implied transient LFeII. Reaction of [(H$_2$L)FeCl$_2$] with 2 equiv. of LiN(SiMe$_3$)$_2$ yields the “‐ate” complex LiClFe$_2$l$_2$, as its dimer with every iron five‐coordinate in an FeN$_4$Cl environment. To avoid Li$^+$ cation derived from LiN(SiMe$_3$)$_2$, reaction of Na$_2$L with FeCl$_2$ gives a product from addition of water, paramagnetic Na$_2$[NaFe(HL)(L)]$_2$(LFeO), which reveals Na/pyrazolate Nβ interactions and a five coordinate oxo group in the OFe$_3$Na$_2$ core of this aggregate. Abstraction of chloride in [(H$_2$L)FeCl$_2$] with NaBArF$_4$ in THF gives paramagnetic [(H$_2$L)Fe(THF)$_2$]$^{2+}$, which fails to react with CO. Dehydrohalogenation in the presence of Ph$_2$PC$_2$H$_4$PPh$_2$, dppe, gives both [LFe(κ$^2$‐dppe)]$_2$(µ‐dppe)] and [LFe(κ$^2$‐dppe)(κ$^1$‐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 [L$^{Me}$Fe(tBuNC)$_3$]$^2$+ which shows rich cyclic voltammetry, and which is reduced, with KC$_8$, to diamagnetic [L$^{Me}$Fe(tBuNC)$_2$. Structure determination of some of these, together with IR data on isocyanide stretching frequencies, show $L^{2–}$ 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 LFe$^{II}$, which implicates weak π basicity of that d$^6$ species. Production of paramagnetic bis‐pincer complexes [(H$_2$L)$_2$Fe]$^{2+}$ and [(L$^{Me}$)$_2$Fe]$^{2+}$ 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.