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.

Electrophile Recruitment as a Structural Element in Bis‐Pyrazolate Pyridine Complex Aggregation

“…Our oxidative goal was based on the fact that high spin Fe II in octahedral LFe(DMAP) 3 ( 1 H NMR δ range +60 to – 20 ppm) has two electrons in the high lying e g orbitals, and thus might be ideal for two ‐electron oxidation. [11a] Indeed, anodic cyclic voltammetry of LFe(DMAP) 3 shows multiple quasi‐reversible oxidative processes (Figure S18). Reaction of Ag(OTf) (OTf = CF 3 SO 3 – ) with LFe(DMAP) 3 in a 2:1 mol ratio occurs rapidly in THF at 25 °C to give, after filtration, a red solution that yields a red solid whose 1 H NMR spectrum in CD 2 Cl 2 indicates paramagnetism, and symmetry equivalence of the two pincer arms; signals for two equivalent DMAP ligands per pincer were also seen.…”

“…We were also interested in how reactivity would change if the oxidizable metal were initially five coordinate, so we studied LCo(PEt 3 ) 2 . [11d] In practice, the loss of phosphine here will play a key role in the product formed.…”

“…In that report, it was noted that a mixture of products was obtained in the absence of LiCl. [11h] Accordingly, oxidation of LCo(PEt 3 ) 2 with AgOTf (in the absence of LiCl) in Et 2 O with 1, 2, and even 3 equiv. of AgOTf resulted in a mixture of products, containing both diamagnetic Co III and some unreacted paramagnetic Co II .…”

“…The 31 P NMR spectrum shows one signal, and coupling to Ag isotopes can be resolved, similar to our previous report. [11h] The product is soluble in both benzene and chloroform; the former is unusual for a presumed [LCo III ]OTf salt.…”

“…Our studies of the bis ‐pyrazolate pyridine ligand L (Scheme ) seeking low coordinate complexes of the later 3d elements has revealed that deprotonation of the pyrazolate β site attracts electrophiles there: alkali metal ions or a second transition metal ion (Scheme ), leading to the frequent occurrence of bridging pyrazolates . This becomes a pervasive structural element among complexes containing the unit LM because M is unsaturated.…”