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

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 Na-2[NaFe(HL)(L)](2)(LFeO), which reveals Na/pyrazolate N beta 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(kappa(2)-dppe)](2)(mu-dppe)] and [LFe(kappa(2)-dppe)(kappa(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 beta-nitrogen atom to give [(LFe)-Fe-Me(tBuNC)(3)](2+) which shows rich cyclic voltammetry, and which is reduced, with KC8, to diamagnetic [(LFe)-Fe-Me(tBuNC)(2)]. Structure determination of some of these, together with IR data on isocyanide stretching frequencies, show L2- to be a stronger donor than L-Me. 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 basicity of that d(6) species. Production of paramagnetic bis-pincer complexes [(H2L)(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.