Catalytic Nitrene Homocoupling by an Iron(II) Bis(alkoxide) Complex: Bulking Up the Alkoxide Enables a Wider Range of Substrates and Provides Insight into the Reaction Mechanism

Abstract

The reaction of HOR' (OR' = di-t-butyl-(3,5-diphenylphenyl)methoxide) with an iron(II) amide precursor forms the iron(II) bis(alkoxide) complex Fe(OR')(THF) (2). 2 (5-10 mol %) serves as a catalyst for the conversion of aryl azides into the corresponding azoarenes. The highest yields are observed for aryl azides featuring two ortho substituents; other substitution patterns in the aryl azide precursor lead to moderate or low yields. The reaction of 2 with stoichiometric amounts (2 equiv) of the corresponding aryl azide shows the formation of azoarenes as the only organic products for the bulkier aryl azides (Ar = mesityl, 2,6-diethylphenyl). In contrast, formation of tetrazene complexes Fe(OR')(ArNNNNAr) (3-6) is observed for the less bulky aryl azides (Ar = phenyl, 4-methylphenyl, 4-methoxyphenyl, 3,5-dimethylphenyl). The electronic structure of selected tetrazene complexes was probed by spectroscopy (field-dependent Fe Mössbauer and high-frequency EPR) and density functional theory calculations. These studies revealed that Fe(OR')(ArNNNNAr) complexes contain high-spin ( S = 5/2) iron(III) centers exchange-coupled to tetrazene radical anions. Tetrazene complexes Fe(OR')(ArNNNNAr) produce the corresponding azoarenes (ArNNAr) upon heating. Treatment of a tetrazene complex Fe(OR')(ArNNNNAr) with a different azide (NAr') produces all three possible products ArNNAr, ArNNAr', and Ar'NNAr'. These experiments and quantum mechanics/molecular mechanics calculations exploring the reaction mechanism suggest that the tetrazene functionality serves as a masked form of the reactive iron mono(imido) species.

Elucidation of the Reaction Mechanism of C2 + N1 Aziridination from Tetracarbene Iron Catalysts

“…78,79 While previous iron tetrazenes prepared by Riordan, 80 Holland, 79 and Chirik 81 were not reactive, some iron tetrazenes react to give the corresponding azoarenes (RNNR). 82 This reaction was studied by Cundari through DFT calculations on a nickel complex. 83 Our first-generation catalyst forms a metallotetrazene that can reform the catalyst and yield diazene.…”

“…Very little research on the mechanism of formation of metallotetrazenes has been conducted, 78,[84][85][86] and while a concerted pathway has been previously postulated, to our knowledge, the azide-based radical intermediate (stepwise pathway) has not been considered as an intermediate to a metallotetrazene. 82 To test the DFT results, we reacted p-tolyl azide with 1. This reaction formed the tetrazene, Figure S13).…”

Synthesis and structure of a new bulky bis(alkoxide) ligand on a terphenyl platform

“…As a result of their stereoelectronic properties, profoundly weak-field bulky alkoxides enable formation of reactive low-coordinate high-spin middle and late transition-metal centers (Bellow et al, 2016b;Grass et al, 2019b). We have previously reported bulky monodentate alkoxides that led to reactive chromium and iron nitrene-transfer catalysts, (Bellow et al, 2015;Wannipurage et al, 2021;Yousif et al, 2015Yousif et al, , 2018 and a series of low-coordinate cobalt carbene complexes capable of carbene transfer to isocyanides (Bellow et al, 2016a;Grass et al, 2019aGrass et al, , 2020. However, the lability of monodentate alkoxides affected catalyst stability and the substrate scope.…”

Structure, reactivity, and spectroscopy of nitrogenase-related synthetic and biological clusters

“…It is noteworthy to mention that EPR spectroscopy is much more sensitive to small perturbations in zero-field splitting than Mössbauer spectroscopy. [61][62][63][64] suggesting [L(Fe)-SAr*] 2À had a high-spin electronic configuration and [L(Fe)] À was low-spin. Reduction of [L(Fe)-SAr*] À by 2.4 equiv.…”