Divergent Reactivity of Azides and Diazoalkanes Toward Ferrous Complexes and Isolation of a FeIII Carbene Radical Complex

Abstract

Treatment of [(^tBupyrr₂pyr)Fe(OEt₂)] (1-OEt₂) (^tBupyrr₂pyr^2– = 3,5-^tBu₂-bis(pyrrolyl)pyridine) with trimethylsilyl azide (N₃SiMe₃) and subsequent photolysis at 390 nm results in clean formation of [(^tBupyrrpyrpyrrNHSiMe₃)Fe] (2) as the result of a nitrene being inserted into a tert-butyl C–H bond of the ^tBupyrr₂pyr ligand. Treatment of 1-OEt₂ with azidotrimethyltin (N₃SnMe₃), however, results in isolation of the γ-bound azide adduct ferrous complex, [(^tBupyrr₂pyr)Fe(N₃SnMe₃)] (1-N₃SnMe₃). When treated with diphenyl diazomethane (Ph₂CN₂), complex 1-OEt₂ converts to the iron carbene complex [(^tBupyrr₂pyr)FeCPh₂] (1-CPh₂), and the X-ray structure revealed a Fe–CPh₂ bond length of 1.964(3) Å. A room temperature magnetic moment of 1-CPh₂ indicates an S = 2 spin state, consistent with a high-spin Fe^III center antiferromagnetically coupled to a carbene radical anion (CPh₂^•–). Zero-field ⁵⁷Fe Mössbauer spectroscopy and Fe K-edge X-ray absorption spectroscopy confirm this assignment. In solution, complex 1-CPh₂ rearranges to [{^tBupyrrpyrC(═CPh₂)-C(CMe₃)═CH–C(CMe₃)═N}Fe]₂ (3) resulting from carbene insertion into the 1-position of the pyrrolide arm of the ^tBupyrr₂pyr ligand and dimerization. Complex 3 possesses two high-spin Fe^II centers according to ⁵⁷Fe Mössbauer spectroscopy, with antiferromagnetic coupling between the spin centers. Monitoring the conversion of 1-CPh₂ to 3 by UV–vis spectroscopy reveals this process to be first order in 1-CPh₂ with a highly ordered transition state evidenced by activation parameters: ΔS^‡ = −87.6 ± 25.8 J·mol^–1·K^–1 and ΔH^‡ = 63.1 ± 8.3 kJ·mol^–1.